1. Cosmic Plasma Fundamentals

 

 

 

 

 

 

1.1 Plasma

Plasma consists of electrically charged particles thai respond collectively to electromagnetic forces. The charged particles are usually clouds or beams of electrons or ions, or a mixture of electrons and ions, but also can be charged grains or dust particles. Plasma is also created when a gas is brought to a temperature that is comparable to or bigher than that in the interior of stars. At these temperatures, all light atoms are stripped of theirele‹:tums, and the gas is induced to its constituent parts: positively charged bare nuclei and negatively charged free electrons. The name plasma is also pmperly applied to ionized gases at lower temperatures where a considerable fraction of neutral atoms or molecules are present

While all maneris subject to gravitational forces,the positively charged nuclei, orions, and the negatively charged electrons of plasmas react strongly to electromagnetic forces, as formu- lated by Oliver Heaviside(1850--1925),' but now called Maxwell’s Equations, after James Gerk Maxwell (1831—1879),'

 

 

(1.2)

 

(1. 3)

 

(1.4)

 

 

and the equation of motion due to Hendrik Antmn Lorentz (1853-1928),

 

 

(1.5)

 

The quantiées D= eE and B = mH are the constitutive relations lx:tween the electric field E and the displacement D and the magnetic induction B and magnetic intensity H, in and e are the permeability and peimittivity of the medium, respectively, and r and j  mm M cfcand current

 

2           1. Cosmic Plasma Fundamentals

 

 

densities, n:spectively.’ The mass and charge of the particle obeying the force law(1.5) are mand q, respectively.

Because of their strong interaction withelectromagnetism, plasmas display a complexity in stricture and motion that far exceeds that found in matter in the gaseous, liquid, or solid states. For this reason, plasmas, especially their electrodynamic properties, are far from understood:L Irring Langmuir (1881—l957),theelectricalengineerandNobelchemist,coinedthe mnnplasma in 1923,probably lx›rrowingthetermfrommedical sciencetodescribe the collective motions that gave an almost lifelike behavior to the ion and electron regions with which he experimented. Langmuir was also the first to note the separation of plasma into cell-like regions separated by charged particle sheathes. Today, this cellular structure is observed wherever plasmas with dif- ferent densities, temperatures, or magnetic field strengths come in contact.

Plasmasneednotbeneutra1(i.e., balanced innumberdensitiesofelectronsand ions). Indeed, the study ofpureelectronplasmasand evenpositronplasmas,as well as the electric fieldsthatform when electr‹ins and ions separate, are among the most interesting topics in plasma research today. In addition to cellular morphology, plasmas often display a filamentary structure. This structure derives from the fact that plasma, because of its free electrons, is a good conductor of electricity, far exceeding the conducting pmpeities of metals such as copper or gold. Wherever charged particles flow in a neutralizing medium, such as free electrons in a background of ions, the charged particle flow or current pnxluces aring of magnetic field around the current, pinching

the current into filamentaiy strands of conducting currents.

Maneriniheplasmastatecanrangeintempeiaturefrnmhundiedsofthousandsofelectronvolts (1 eV= 11,605 degreesabsolute)tojustone-hundredthofanelectmnvolt. Indensity,plasmasmay be tenuous, with just a few electmns present in a million cubic centimeters, or they may be dense, with more than llf° electrons packed per cubic centimeter (Figure 1.1).

Nearly all the matter in the universe exists in the plasma state, occurring predominantly in this form in the Sun and stars and in interstellar space. Auroras, lighming, and welding arcs are also plasmas. Plasmasexist in neon and fluorescent tubes, in the seaofelectrons that moves freely within energy bands in the crystalline structure of metallic solids, and in many other objects.

Plasmas are prodigious producers of electromagnetic radiation.

 

1.2 The Physical Sizes and Characteristics of Plasmas in the Universe

 

 

On the earth, plasmas are found with dimensions of microns to meters, that is, sizes spanning six orders of magnitude. The magnetic fields ass‹x:iated with these plasmas range fmm about 0.5 gauss (the eanh's ambient field) to megagauss field strengths. Plasma lifetimes on earth span 12 to 19 orders of magnitude: Laser pnxluced plasmas have pmpeities measurable in picoseconds, pulsed power plasmas have nanosecond to microsecond lifetimes(Figure 1.2), and magnetically confined fusion orientedplasmas persist for appreciable fractions of a second. Quiescent plasma sources, including fluorescent light sources, continuously produce plasmas whose lifetimes may be measured in hours, weeks, or years, depending on the cleanliness of the ionization system or the integrity of the cathode and anode discharge surfaces.

 

1. Cosmic Plasma Fundamentals 3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10-'

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1o-1          1          10'        10'       10'       10’       10'

Electron energy in electronvolts

TEMPERATURE

 

 

Figare 1.1. The remarkable range of temperatures aixl densities of plasmas is illustrated by this chan. In comparison, solids, liquids, and gases exist over a very small range of temperatures and pressures. In “solid” metals, the elections that carry an electric current exist as a plasma within the more rigid crystal structure.

 

 

Lighting is a natural plasma resulting from electrical discharges in the earth’s lower tropo- sphere(Figure 1.3). Such flashmareusuallyassix:iatedwithcumu1onimbusclouds butalso‹xx:ur insnowandduststomis,activevolcanos,nuclearexplosions,andgroundfracturing.The maximum time duration ofa lightning flash is about 2 sin which peak currents as high as 2IX1 kA can cxx:ur. The conversion frnmair molecules to a singly ionizedplasma ‹xx:urs in a few micmseconds, with hundreds of megajoules of energy dissipated and plasma temperatures teaching 3 eV. The dis- charge channelavalanches ataixiut one-tenth the speedoflight, and the high current carrying core expands to a diameter of a few centimeters. The total length of the discharge is typically 2—3 km, although cloud-tn-cloud discharges can be appreciably longer. Lighming has been observed on Jupiter, Saturn, Uianus, and Venus [Borucki 1989]. The energy released in a single flash on earth, Venus, and Jupiter is typically 6 x 10• I, 2 x 10'° I, and 2.5 x 10" J, respectively.

Nuclear driven atmospheric plasmas were a notable exception to the generally short-lived energetic plasmas onearik For example, the 1.4 megaton (5.9 x 10" I) Starfish detonation, 4tXl kmabnve JohnstnnIsland, onJuly9, 1962,generatedplasmafrom which artificial VanAllen 6effs of electrics circulating the earth were cieatecL These electrons, lxiund at about 1.2 earth--radii in a0.175 Gfield, produoedsynchrotron radiation whose decay constantexceeded HMI days(Figure 1.4).

 

4   1. Gosmic Plasma Fundamentals

 

 

 

 

Figure ID. Filamentaiy plasnu structure produced by “exploding" titanium wires within a 10terawaa pulsod- jx›wcr generator. The photograph was taken with a pin-hole X my camera

 

 

• Near-Earth Plasmas

 

be ’z ionosphere magnetosphere co t m a cosmic plasma system that is readily available for extensive and detailed in situ observation and even active experimentation Its usefulness as a source of understanding of cosmic plasmas is enhanced by the fact that it contains

 

1. Cosmic Plasma Fundamentals 5

 

 

 

Figure ID. Goud to ground lightning on earth (5 min ezjxisure). The discharge channel may be 2096 ionized plasma. The potential difference between the lower portion of the cloud and earth is in excess of 10 MV. The bright line at the lower right is the Santa Fe-Los Alamos highway (courtesy of Henry Ortega, Santa Fe, New

 

 

 

 

a rich variety of plasma populations with densities ranging fmm more than 10•cm‘° to less than 1(P' cm-', and temperatures flom aix›ut0.1 eV to more than 10 keV.

Theearth’smagnetospheie isthatregionofspace defined by the interactionofthe solar wind withtheearth’sdipole-likemagneticfiel‹IItextendsfiomapproximately10t1kmabovetheearth’s surface, where the protnn neutral atom collision frequency is equal to the proton gyrofrequency, to alxiut ten earth talii (-63,8t1£l km) in the sunward direction and to several hundred earth radii in the anti-sunward direction. It is shown schematically in Figure 1.5.

First detected by radio waves and then by radar, the ionospheie is a layered plasma region closest to the surface of the earth whose properties change continuously during a full day(Figure

 

6          1. Cosmic Plasma Fundamentals

 

 

 

 

Figure 1•4. Staifish event. Arfificial aurom produced by plasma particles streaming along the earth’s magnetic field lines. Picture taken from a Los Alamos KC-135 aircraft 3 min after the July 9, 1962, 1.4--megaton 40fi- km-altitude nuclear detonation above fohnston Island. The event produced a degradation of radio communi- cations oxer large areas of the Pacific and an intense equatorial tube of synchrotron emitting electrons having a decay constant of l0fl days. The brightest background object (mark) at the top, left-hand comer is the star Antares, while the right-hand-most object is B-Centauri. The burst point is two-thirds of the way up the lower

 

 

 

 

1.6).Firsttobe identified wasalayerofmolecularionization, ca1ledtheElayer.Thisregionextends over a height range of 90--140 km and may have a nominal density of IU cm—’ during periods of low solaractivity. AD mgion underlies this with a nominal daytime density of 1€Pcm—'. Overlying M E region is the F layer of ionization, the major layer of the ionosphere, starting at about 140 km. In the heightrange IOT—l5Okm, strong electric currents are generatedbyapnx:essanalogous tothatofaconventionalelectric generator, ordynamo. The region, in consequence, isoftentemied thedynamoregionandmayhavedensitiesof10•cm—°. Thef1ayermayexiend 1,lXDkminaltitude where it eventually merges with the plasmas of the magnetopause and solar wind.

 

1. Cosmic Plasma Fundamentals 7

Magnetosheath

Plasma Mantle

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bow Shock

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(Magnetopause

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Magnetospheric

BoundaryLayer

 

 

Figure 1.5. Earth’s magnetosphere. Ions and electrons in the solar wind (left) impinge against the magneto- sphere, distorting the field lines and creating a bow shock and the various regions shown.

 

 

 

The interaction of the supersonic solar wind with the intrinsic dipole magnetic field of the earth forms the magnetosphere whose boundary, called the magnetopause, separates interplan- etaiy and geophysical magnetic fields and plasmaenvironments. Upstream of the magnetopause a collisionless bow sh‹x:k is fotmed in the solar wind-magnetosphere interaction process. At the bow sh‹x:k the solar wind Incomes thetinalized and subsonic and continues its flow around the magnetosphere as magnetosheath plasma, ultimately rejoining the undisturbed solar wind.

In the anti-solar direction, observations show that the earth’s magnetic field is stretched out in an elongated geomagnetic tail to distances of several hundred earth radii. The field lines of the geomagnetic tail intersect the earth at high latitudes(-D3°—75°) in both the northern and southern hemisphere (polar horns), near the geomagnetic poles. Topologically, the geomagnetic tail roughly consists of oppositely directed field lines separated by a "neutral" sheet of nearly zero magnetic field. Surrounding the neutral sheet is a plasma of“hot" particles having a temperature of 1—10 keV, density of -D.01—1 cm—', and a bulk flow velocity of a few tens to a few hundreds

 

Deepwidiinthemagnetnsphereistheplasmasphere,apopulationofcold(A l eV)iono-spheric ions and electrons combating with the earth. Table 1.1 lists some typical values of parameters in the earth’s magnetosphere.

 

 

100,000

 

 

 

«› 10,000

 

 

 

 

 

 

 

 

10                                                                                 6

0

Particles per cubic centimeter

 

Figure 1.6. Daytime ionosplieie at low latitudes and low solar acévity.

 

 

 

Thespaceenvironmentaroundthevariousplanetarysatellitesandrings inthesolar system is filled withplasmasuchasthe solar wind, so1arandgalwticcosmicrays(highenergychargedparticles), and particles trapped in the planetary magnetospheres. Thefirstin sim observationsofplasmaand energetic particle populations in the magnetospheres of Jupiter, Satuni, Uranus, Neptune, and Titan were made by the Voyager I and 2 spacecraft from 1979 to 1989. Interplanetaiy spacecraft have identified magnetospheres around Mercury, Venus, Jupiter, Saturn, Uranus, and Neptune (e.g., Figure 1.7).

Comets also have“magnetmpheies” as depicted in Figure 1.8. The cometosheath, a legion extending aix›ut 1.1 x 10• km (for Comet Halley), consists of decelerated plasma of density ID < n < 4x ID and temperature T, -1.5eV. The peak magnetic fieldstrength found in Comet Halley was 7IXI--8IXI mG.

Excluding the Sun, the largest organized stnictures found in the solar system are the plasma

tort around Jupiter and Saturn. The Jupiter-lo plasma toms (Figure 1.9) is primarily filled «a

sulphur ions at a density of 3 x lfP cm—' (Section 4.6.2) An immense weakly ionized hydrogen plasmatorus has been found to encircle Saturn, with an outer diameter 25 times the radius of the planet and an inner diameter of about fifteen Saturn radii.

 

Tablt 1.1. Typical values of some parameters in the earth’s magnetosphere (adapted from A.T.Y. Lui)

 

Magnetosficath Magnctopausc       inner side of" Plasma mantle         Tail lobs         Plasma-sheet            Central

boundarY  laYcr                                              boundary lavc‹   plasma sheet

 

 

 

 

 

magnetic field ccaric field

 

1. I

0.05

5(nT)         5

x(mY/in)         1

20

0.8

0.08

4

0.15

40

O.l

0.05

1. 1

0.05

25

0.1

 

0.5

20

0.5                   1

5

1

2

0.01                   6

 

 

Debyc lcngth                kg (in)          18                  10                                       40                 400                 4O)                 280                200

plasma skin dtpth          Zg (in)    2.4 x TO                                                    5.3 • it          5.3 • ID          1.7 • io4          7.s • ip3          i.3 • it

cleyron gyroradius          r(z        )      4.9 x ID         1.1 x ID         I.I x ID        9.8 x l(1—5         9.8 x lfi          3.9 s TO                    3                   3

ion gyroiadius               r¿¡     )  3.0x lW          2.2 x lfi          2.5 x l(U'        5.9 x 10-3        5.9 x lfi         2.3 x lfi          1.0 x lfi          5.2 x 10-1

 

 

 

 

clcction plasma              /y   (Hz)     2.0 « 10a          4.0 • io4          s.0  jQ3           9.0 • io3           9.0 • i0*          2.8 x   3          6.3 x 103          9.0 x 10

ion plasma                           fp;         )        4.7 x 10'              1.0 x 103               2.0 x  10'               2. 1 x 10                 2.1 x 101               6.6 x   p1                1.5 x jp2                 2.1 x 10

frequency ratio                                           1.4 « io2              5.7 • 10'               1.5 x lo*               1.3 x 10                 1.3 x 100              5.0 x 10                 4.5 x 10                 1.6 x 10

 

 

ionsound                   cy(km/s)           83

Alfrén                     vp (kin/s)           49

electron themal              vthc (km/s)             2,800 ion thermal              v Jj¡ (kin/s)                               91

°°"•° *'°"                •E  M/•)         200

122

 

277

250

870

 

619

50

83

550

2,800

91

83

5,500

2,800

91

4

 

 

8,?00

290

5

370              3.7 n 10

300              4.4 x 10

12•*o4    £2**o4

640        é4x )0*

1                      5.0 x l0

 

 

 

themal/magnetic cncrgy ratio

6.9

t0.0

1.0

0.056            5.6 x ID            0.087                 8.8                430

 

magnetic energy dmsity  (J in   )    9.9 xlo-'

2.5 x i  16       2.5 x IQ—16       i.6 s 1   16       4.0 • 1   l7       i.s • i()—18

 

10         1. Cosmic Plasma Fundamentals

 

 

 

Flgnre 1.7.Jupiter’s magnetosphere. Io’s plasma terns contains energetic sulfur and oxygen ions arising fmm the m‹xin’s ctive discharges.

 

 

 

• Transition Regions in the Solar System

 

Examples of transition regions include the boundary layers found in planetary and comet mag- netospheres(Table 1.2).Transition regions between plasmas of different densities, temperatures, magnetization, and chemical composition offer a rich variety of plasma phenomena in the solar

 

 

ldN Solar, Stellar, and Interstellar Plasmas

 

The nuclear core of the Sun is a plasma at aIx›ut a temperature of TO keV. Beyond this, our knowledge aix›ut the Sun’s interior is highly uncertain. Pnxesses which govern the abuixlance ofeleoients,nuclearieactions,andthegeneiationmechanismandstiengthoftheinteriormagnetic fields. are incompletely known.

We do have intonation about the Sun’s surface atmospheres thatare‹Jelineatedas followsi thephotosptiere,the chmmosphere, and the inner corona. These plasma layers are supeiposed on the Sun like onion skins. The phoiosphere T-0.5 e  iz only a very weakly ionized atmosphere,

 

1. Cosmic Plasma Fundamentals 11

Table 1.2.    Plasma transition regions within the solar system

Earth’s Magnetosphere

plasmapause bow shix:k	5 - t0fXJ 10 - 50	1 - 1t3£D N  -  IQQ (ioftS)		50 - 300
magnetopause and	0.5 - 30	5IXl - 16IXl (ions)	25 - 350
		zs - zixi («  )
plasma sheet boundary	o.oz - i	3oo - zixo (ions)	o - isixi	in - son
i•yc		30 - 500 (e )
Other Transition Regions
Jovian boundary layer	0.01 - 1	10 - 30 LXV (IOIIS)	l       - 😯 i	1 - 15
cometaiy boundary layer	10 - ltXi	1 - 50	20 -300	50-300
heliopause	0.tXD1 - I	0. 1 - ttXl	10 -1tI£t	0.1-20

* Temperature associated with 1 eV = 1.602 x 10      J = 11,60) °K. b } liG = 0.1 nT.

 

 

 

 

 

 

Figure ID Comet KohouteL (January 1974). The filamentaiy structure of the tail is typical of the structure of

 

12 1. Cosmic Plasma Fundamentals

 

 

 

Figure 1.9. The Jupiter-lo plasma terns. The diagram shows the megaampere Birkeland cuneiiu flowing between Jupiter and Io.

 

 

the degree of ionization being lf •—ID’ in the quiet regions and perhaps 1fP•—ID’ in the vicinity

of sunspots. The chmmosphere T - 4 eV) extends 5,tXD km above the photosphere and is a

transition region to the tunics The highly ionized iiu•cicoionaexiends son  5   above

the photospheie. From a plasma physics point of view, the corona is perhaps the most interesting region of the Sun. The corona is the sight of explosively unstable magnetic-field configurations, X my emission (Figure 1.10), and plasmatemperatures in the range 70--263 eV(Table 1.3).’ The source of this heating is uncertain.'

 

 

 

 

at centrr

 

 

 

 

in climinorplicre in corona

(I      cm")        g/cm (io7     —3) jg-16 $/  3

 

1.5 keV

0.5 eV

0.37 eV

0.38 - 4.5 eV 70cV - 263 cY

 

3.826 x 1o°6

 

 

 

 

 

1. Cosmic Plasma Fundamentals 13

 

 

 

 

Figore1.10.Ultraviolet image showing eruption of a solar prominence fmm the sun(Skylab 1973). The Loop- like so-refutes may be caused by electric cui-rents. The solar “north pole” is to the left.

 

14         1. Cosmic Plasma Fundamentals

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Local (LISM)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1.11. The heliosphere.

 

 

 

Solar flaiesiesultingfmmcornnal instabilitioraisetemperaturesto 1G-30keV and produce relativistic stieamsofelectmns and protons. Particles acceleiatedouiward pnxluceradio interfer- ence at the earth. Photons accelerated inward collide with ions in the Sun’satmospheretoproduce nuclear inactions, whose gamma rays and neutmns have been detected from spacecraft Solar flares cousin of plasma at a temperature of about I keV to 10keV. Although flares represent the most intense energy dissipation of any form of solar activity, releasing energy in the form of gamma rays, X rays, and microwaves, the active sun has many other plasma manifestations.

These include sunspots, photospheric faculae, chmmospheric and transition region plages, large coronal l‹xips, and even larger scale coronal streamers and ‹x:casional comnal mass ejec- tions. In addition, pmminences (referred to as filaments when seen in Hix absorption on the disk) frequently form between oppositemagnetic polarifies inactiveregionsofthe sum These phenom- enaare dynamic, on time scales ranging fmm seconds to the complete solar magnetic cycle of 22

y       -

Theoutercoronaandsolarwindformtheheliospheie(Figure 1.11).Atoneastronomicalunit the solar wind hasaplasmadensity 5 fi n fi 60 cm 3 andavel‹x:i(yof2tX1 fi V  fi 8tXl km S‘ '

Its temperature can be as high as 50 eV, while B may reach 2IXl;iG. The outer heliosphere has a plasma density lW fi n 0 ID * cm ’, a temperature 0. 1 < T < 10 eV, and a magnetic field strength - 1JiG. The l‹x:al interstellar medium is characterized by lQ- 2   ' it fi  1 °>    , a tem-

 

1. Cosmic Plasma Fundamentals 15

 

Table 1.4.   Solar	system plasmas
	solar wind	outer	l‹x:al interstellar	planetary boundary
Parameter	at I AU	heliosnher«	medium	lavers
ni =    (cm=')	5 - 60	0.011 - 0.1	0.01 - 1	0.05 - 50
kT ;,z (eV)		0.1 - 10	0.1 - 10
	2tXt - 8tXl	10 -1fO	1 - 30

 

 

 

 

perature of order 1eV, and magnetic field strength 1 < B < 20 JPG. Table 1.4 lists some of the parameter values found in the solar system.

Therotatingsun,coupledwithitscontinualradialejectionofplasma,twistsitsmagneticfield (that isreferred teas the interplanetary magnetic fielder IMF) into a classical Archimedean spiral, as depicted in Figure 1.12a. Measurements have confirmed thai the interplanetary magnetic field is directed toward the sun in certain regions of the solar system and away from the sun in other regions. These regions are separated by a very sharp boundary layer that is interpreted as a current layer. This layer is depicted in Figure 1.12b which shows“ripples" in the sheet. In this situation, the planets find themselves sometimes in a region where the field has a strong northward com- ponent and sometimes where it has a strong southward component.

Stellar plasmas have not only the dimension of the star, 0.3 x 10•km to 1£P km, but also the stellarmagnetospheres,aremnantoftheinierstellarp1asmathatthestaranditssatellitescondensed out of. The surface temperatures vary from alxiut 0.3 to 3 eV. Estimates of the magnetic fields range from a few gauss to tens of kilogauss or more for magnetic variable stars.

Stellarwinds‹x:curinstarsof manytypes, withwindpropertiesprobablyconnectedwithstellar magnetism (Table 1.5).

 

 

 

Tsble lS. Pumcwrs msociamd withMW ingOrs:rlIarmcdium

 

 

16        1. Gosmic Plasma Fundamentals

 

 

 

 

 

Figure 1.t2. (a) Magnetic field lines of the sun which are bent into Archimedean spirals owing to the rotation of the sun aixi the radial ciiimision of plasma (which carries the magnetic field). This is a view over the nonh

;x›le of the sun 1‹x›i‹:ing downward at the field lines slightly above the sun’s equatoriiil plane. Gcse below the equatorial plane, the field lines have the same geometry but opposite direction. The shapes of the associated current lines are also shown. (b)An artist’s view of the solar current sliert in the equatorial plane. Alfv‹in has proposed that the sheet develops ripples like the skin of a ballerina. As this system rotates. the planets find themselves in diffcmt regions so that the interplanetary magnetic field can assume a variety of orientations, e.g., directed toward or away from the sun and northward or southward (courtesy of T. Poiemra).

 

 

 

Dark clouds within our Galaxy have dimensions of lff km and microgauss strength magnetic fields (Figure 1.13).

The Galactic plasma has an extent e9ual to the dimensions of our Galaxy itself; -35 kpc or HP' m. The most salient feature of th Galactic plasma are 1O’G Jx›loidal---toioidalp1asma fila- ments extending sly 250light years(60 jx:, 1.8 x 10“in) at the Galix:tic center(Figure 1.14). The vast regions of nearly neuoal hydrogen(HI regions) found in the Galaxy and other galaxies are weakly ionized plasmas. These legions extend acinss the entire width of the galaxy and are sometimesfoundbetweeninteim:tinggalaxies.Theyaredetectedbythe21cmradiationtheyemit.

Galaxies may havebulkplasma‹Iensitiesof1W' cm-'; groups of galaxies, 3 x ID'cm-'; and

rich clusters of galaxies, 3 x lfF' cm-'.

By far the single largest plasmas detectedinthe Universe arethose of doubk ratio galaxies. In size, these souix:es extend hundreds of kiloparsecs (1tF'—10° in) tea few mega-parsers(10°— Itf° in). Double radio galaxies are thought to have densities of 1W'cm-' and magnetic fields of the order of 1O‘G (Figure 1.15).

 

1. Cosmic Plasma Fundamentals 17

 

 

 

 

 

 

 

 

ID Regions of Applicability of Plasma Physics

Thedegreeofionization ininterplanetaiy space and in other cosmic plasmas may vaiy over a wide range, fmm fully ionized to degrees of ionization of only a fraction of a pement.• Even weakly ionizedplasmaieactsstnmglytoelectiomagnetic fieldssincetheiatiooftheelectminagnetic force to the gravitational fome is 39 orders of magnitude. For example, although the solar photospheric plasma has a degree of ionization as low as ID’, the major pan of the condensable components is still largely ionized. The “neutral” hydrogen (Hi) regions around galaxies are also plasmas, althoughthedegieeofionizationisonly ID’.Mostofourknowledgeaixiutelecttomagnetic  waves in plasmas derives from laboratory plasma experiments where the gases used have a low degree of ionization, 1£r°—1£r•.

Because electmmagnetic fields play six:h an important role in the electnxlynamics ofplas- mas, and because the dynamics of plasmas are often the soumes of electnxnagnetic fields, it is desimbletodetenninewherewithintheuniverseaplasmaapproachisnecessaiy.Wefirstconsider the magnetic field. The criterion for neglecting magnetic effects in the treatment of a problem in gas dynamics is that the Lundquist parameter L (Section 2.4.2) is much less than unity,

 

 

18         1. Cosmic Plasma Fundamentals

 

 

 

 

Figure 1.13. The Veil nebula in Cygnus.

 

1. Cosmic Plasma Fundamentals 19

 

 

Telile 1.6. Characteristic quantities for laboratory and cosmic plasmas [adapted from Alfv6n and Ftilthammar 19633

 

		B
Laboratory experimerus						lb   1p7 3 x 10 3 xi0°* 10 3 x10 z xio
	0.1
	0.1
Cosmic plasmas
s»nh’s i»«,ior	t ré	io—3('?)	it	8 x ion	to—2
Solar granuiaao»	lb	1m2	lb	8 ›‹ i(›3	1(i3
Magneticvariablestars	10	1	ID(?)	8 x 10	30
i»      >11<«	1#	I   ’('	lii-°' (?)	8 •102(i)	3 •1(i4
Interplanetary space	10' '	ID	g-20	8 x 10‘(’	ID
S‹›lafcorona	10	lb(?)	io- *(?)	8 x i0* (?)	3 x i(i6
Daikclouds		10	g-17	§ g { 2	3o

 

 

 

 

(1.6)

 

who I is a characteristic length of the plasma and p « ’is bemuse did.M be conductivity of known plasmas generally varies only over about four orders of magnitude, from IU to 10‘ siemens/m, the value ofL is largely dependent on the strength of 11 in the plasma as delineated in Table 1.6.

The variation of 21 in plasmas can be 18 orders of magnitude, from microgauss strengths in intergalactic spacetoperhapsteragauss levels inthemagnetospheresofneutronsources. Onearth, magnetic field strengths can be found from about 0.5 gauss (0.5 x ID’ T) to 107 gauss (lfI' T) in pulsed-powerexperiments;theouterplanetshave magnetic fields teaching many gauss, while the

magnetic fields of stars ate 30-40kG(3--4 T). Large scale magnetic fields have also beendiscov- ered in ‹hstant cmmic objects. The center of the Cialaxy has milligauss magnetic field strengths stretching 60 pc in lengik Similar strengths are inferred from polarization measurements of radiation recouped for double radio galaxies. No rotating object in the universe, thai is devoid of a magnetic field, is known.

In cosmic problems involving planetary, interplanetaiy, interstellar, galactic, and extraga- actic phcnomena,L g is usually of the order 10"—III°(Figure 1.16). In planetary ionospheies Lg alls below unity in the Elayer. Neglecting lighming, planeiaryatmospheres andhydrospheresare W only domains in the universe wliereanonhydrnmagnetic tieannent of fluid dynamic problems

 

20     1. Cosmic Plasma Fundamentals

 

 

 

£'ï$arc 1.14. VLA radiograph of Ïarge-scala, 20-cm radio cmission features within pc (20 azc mia) of the Galactic nucleus. The gale:tic plane nine fiomtop-left to bottom-right, though the Iowerdiffusestriu:ture. The contrast is chosen to bring out radio features brighter than IS mJy per beam area (the half-power beam width is 5 x 9 arc s) (Courtesy F. Yusef-Zadeh).

 

 

 

1.4 Power Generation  and TYunsmission

On earth, power is generated by nuclear and nonnuclear fuels, hydto and solar energy, aixl to a much lesser extent, by geothermal sources and magrietohydnxlynamic generators. Always, the l‹x:ation of the supply is not the l‹x:ation of major power usage or dissipation. Transmission lines are used to convey the power generated to the load region. As an example, abundanthydrmlectric iesoumes in the Pacific Northwest of the United States pnxluce jxiwer(- 1,501h'IW) that is then wasnbmdmL n tcs, t d3Obmaway,via8DkVI# -slfic erydc ssionhs.N optical and infraiedemissi‹ m, only the load region, Los Angeles, is visible ftomthe light and heat it dissipates in power usage. The transmission line is invisible fig 1.17)

This situation is also true in space. With the coming of the spec age and the subsequent discoveiyofmagnetnspheric-ionosphericelectricalcimuits, Kirchoff’scitcuitlaws(ApperuliyA) were six1‹Ienly catapulted to dimensions eight orders of magnitude larger than that previously investigatedinthe laboratniyandnear1yfourordersOfmagnituiJegreaterthanthatas .•«x ated with the longest power distribution systems on earth.

 

1. Cosmic Plasma Fundamentals 21

 

 

 

 

Figure 1.15.Photographic representation ofCygnus A at6 cm wavelength with 0."4 resolution. The east-west extent of the radio emission is 127”. The bars beneath each radio lobe denote the receiver sensitivityi The left- hand lobe is at about 5 mJy/pixel, while the right-hand lobe is 1.5 niJy/pixel. The plasma associated with this radio galaxy is thought to have n = 2—3xlW' cm-’ and B -- I-2x IW’ G.

 

 

 

On earth, transmission lines consist of metallic conductors or waveguides in which energy ismade to flow via the motion of freeelectrons(currents) in the metal or in displacement currents in a time varying electric field. Often strong currents within the line allow the transmission of power many orders of magnitude stronger than that possible with weak currents. This is because acurientass‹x:iated with the flow ofelectronspr‹xlucesaself-magnetic field that helps to confine orpinch the particle flow.4 fagneiic-izisii f‹ztion is commonly used in pulsed-power technology to transmitlargeamountsofpower  from the generator to the load without suffering abreakdown due

to teaI‹:age  currmts caused by high electric  potmtials.

There is a tendency for charged particles to follow magnetic lines of force and this forms the basisoftransmissionlinesinspace(Appendix    A).lnthemagnetosphere-ionosphere,atransmission line 7--8 earth radii in length (It,= 6,350 km) can convey tens of terawatts of power, that derives fmmthe solarwind--magnetosphere coupling’, to the lower atmosphere. The transmission line is theeanh’s dipole magnetic field lines along whichelectmns and ions are constrained to flow. The driving potential is solar-wind induced plasma moving acmss the magnetic field lines at large radii. The result is an electrical circuit in which electric currents cause the fomiation of aumras at high latitude in the upper atmosphere on earth. Tbis aurora mechanism is observed on Jupiter, Io, Saturn, Uranus, and is thought to have been detected on Neptune and perhaps, Venus.

Only die aurora discharge is visible at optical wavelengths to an observer. The soume and transmissionlineaieinvisible(Figural.18).Beforethecomingofspa:e  bes,insituineasurenient was impossible andexoticexplanations were often given of auroras. Thisisprobablytnie of other noninsituc‹mnicplasmastoday. Theexistence ofamegaampere thin tube ofcunent, connecting theJoviansatel1iteIotoitsmotherplanet(Figural   .9), wasverifiedwiththepassageofthe Voyages

 

 

 

 

% x D T ED E i  A n U

°=°**°°*°*°*°*°*°*°*°*°*°*

Hneur dlm•nson cm

Figural.16. The Lundquist parameter L vs linear dimension fora variety of laboratory, space, and astronomi-

 

 

 

1.5 Electrical Discharges in Cosmic Plasma

 

An electrical discharge is a sudden release of electric or magnetic stnied energy. This generally

‹x:curs when the electmmagnetic stress exceeds some threshold for breakdown that is usually determined by small scale properties of the energy transmission medium. As such, discharges are l‹x:al phenomena and aieusually accompanied by violent pr‹x:esses such as rapidheating, ioniza- tion, the creation of pinched and filamentary conduction channels, particle acceleration, and the genemtion ofprixligious amounts of electiiimagnetic  radiation.

As an example, multi-terawan pulsed-power generators on earth rely on strong electrical discharges to produce intense particle beams, X rays, and microwaves. Megajoules of energy are electrically stored incapacitnr banks, whose volume may encompass 250m'. This energy is then transferred to a discharge region, located many meters fmm the source, via a tiansmissi‹m . The discharge region, or load, encompasses at most a few cubic centimeters of space, and is the site of high-variability, intense, electromagnetic radiation (Figure 1.2).

On earth, ligliming is another example of the discharge mechanism at work where electm- static energy is stored in clouds whose volume may be of the out:r of 3,tItO km'. This energy is released in a few cubic meters of the discharge channel.

The anion is a discharge caused by the bombardment ofatnms in the upper atmosphere by 1—20 keV elections and 2fD keV ions spiralling down the earth’s magnetic field lines at high latitudes. Here. the electric field accelerating the charged particles derives fmm plasma moving across the earth’s dipole magnetic field lines many eanh radii into the magnetosphere. The

 

1. Cosmic Plasma Fundamentals 23

 

 

jxitentialenergygeneratedbytheplasmamoéonisfedtotheupperatmospheiebymulti-megaampeie Birkeland currents (Chapter 2) that comprise a transmission line, 50,IXD kilometers in length, as they flow intoandoutofthe dischargeregions at the polar hems(Figure 1.5).Thegeneratorregion may encompass 10"—10" km’ while the total discharge volume can be 1£P—10" km’. The stored or generated and radiated energies and powers versus linear dimension(approximately, the cube tix›t of the volume) of several cosmic plasma discharge objects are shown in Figure 1.19.

 

1.6 Particle Acceleration in Cosmic Plasma

• Acceleration of Electric Charges

 

The acceleration of a charged particle q in in electromagnetic field is mathematically described by the Loientz equation Eg.(1.5),

 

F = ma = q(E + v x B)                                                                  (1.7)

 

The electric field vector E can arise from a number of piv›cesses (Chapters 4 and 5) that include the motion of plasma with velocity v across magnetic fields lines B, charge separation, and time varying magnetic fields via Eg.(1.1).

Acceleration of charged paiticles in laboratory plasmas is achieved by applying a potential gradientbetweenmetallicconductors(cathodesandanodes); by producing time varying magnetic fields such as in betatrons; by radio fmquency (RF) fields applied to accelerating cavities as in linear accelerators (LINACS); and by beat frequency oscillators or wake-field accelerators that use either the electric field of lasets or charged particle lx:cms to accelerate particles.

The magnetospheric plasma is essentially collisionless. In such a plasma, electric fields aligned along the magnetic field direction (Chapter 3) freely accelerate panicles. Electrons and ions are accelerated in opposite directions, giving rise to a current along the magnetic field lines

 

 

• Collective Ion Acceleration

 

The possibility of producing electric fields by the space-charge effect to accelerate positive ions to high energies was first discussed by Alfvén and Wemholm(1952). They were unsuccessful in their attempt to experimentally accelerate ions in the collective field of clouds of electrons, pmbably because of the low intensity of electron beam devices available then. However, proof ofprinciplecame in 1961 whenPlyuttorepnriedthefirstsuccessfulexperimentinwhichions were collectively accelerateck By 1975, the collective acceleration of ions had become a wide-spread areaofresearch. Luce(1975)reportedco11ectivelyacceleratingboth1ightandheavyionstomulti- MeV energies, pr‹x1ucing an intense burst of O—D neutmns and nuclear reactions leading to the identification of several radioisotopes. Luce used a plasma-f‹x:us device (Section 4.6.2) and attributed the collective beam to intense current vortex laments in the pinched plasma(Section 1.7). Subsequently, Destler, Hoeberling, Kim,and Bostick(1979)col1ectivelyacceleratedcarlxm ions to energies in excess of 170 MeV using a 6 MeV electmn beam.

 

 

 

 

 

Flgure 1.17. White light view of North America on the evening of 13 Mai*-                     Y Department of intense F9 metmrology satellitc. Visible ight i>>a*i                                                                                            fi"es    J<              >            •      ° other land features are illuminated by mixmlight. The white bond at                                                                                            t°P of the phoiograph

 

 

 

 

 

parent hull of this composite image was obiaimd amund4:ID Universal time on l4 Marcb, about three hours after the auroml peak display. The western half was inken one orbit (roughly ltD min) later, during wbich the

• ••••« intensity had decreased sulntaniially (courtesy of Frederick Rich, Air Force Geophysics Lalmmmiy).

 

26         1. Cosmic Plasma Fundamentals

 

 

 

 

 

Figure 1.18. (top) The aurora photographed from the space shunle Challenger (photo taken by Don Lind).

(bottom) The auiora imiealis photographed in ultraviolet light by the Dynamic Explorer satellite.

 

 

Individual ion energies up to several GeV using pulsed-power generators have teen sug- gested in particle-in-cell simulations of collective ion acceleration prix:esses [FaeH and Godfrey 1978; Shanahan and Fmh1 1981].

Cohm0ve iem0on in a mechanism for creating high energy ions in astrophysical plas- mas were investigated by Bostick (1986).

 

• Plasma Pinches and Instabilities
  • The Bennett Pincb

 

In cosmic plasma the perhaps most imponant constriction mechanism is the electromagnetic attraction between parallel currents. A manifestation of this mechanism is the piriCh effect as fifst

 

1. Cosmic Plasma Fundamentals 27

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ilnear   d Imenslon,  m

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(a)

 

 

 

40

 

 

 

30

 

 

 

20

 

 

10

 

 

 

 

                                               

 

Figure 1.19. (a) Siomd and radiated energies from man-made and natural plasmas versus linear dimension. Mankind’stoia1energy consumption in 1989 is shown for comparison. (b)Oenerated and radiated power versus linrar dimension.

 

28          1. Gosmic Plasma Fundamentals

 

 

studied by Bennett(1934). Phenomena of this general type also exist on a cosmic scale and lead to a bunching of curientsaixl magnetic fields to filaments. This bunching is usually accompanied bytheaccumu1ationofmatter,anditmayexplaintheobservationalfactthatcosmicmatterexhibits an abundance of filamentaiy strictures.

Consider a fully ionized cylindrical plasma column of radius r, in on H electric field E„ that produces an axial current densityj,. Associated withj, is an azimuthal magnetic fieldB$. The current flowing across its own magnetic field exerts ajx B, radially inward, pinch force. In the steady-state, the balance of forces is

 

                                                                              (1.8)

 

ByemployingEq.(1.2),V x B = p j.andtheperfectgaslawp = N k T , wearriveatthe Bennea

 

 

 

 

(1.9)

 

where Nis the number of electrons per unit length along the beam, T,and T,       be electron and ion temperatures, f is the total beam current, and k is Boltzmann’s constant

• The Force-Free Configuration

 

 

Sheared magnetic fields(V 

isconsideiedanonpotentialfield(V

0$ are a characteristic of most plasmas. Here,the sheared field

0)thatiscausedbyshearflowsofplasmaAnnnpotential

 

field tends to settle into a particular configuration called a “force-free" field, namely

 

                                                                                                  

 

 

sincej = V x B jz9 ,showing thattheelectriccurrenttendstoflow along B. SubstiNting(1.11) into(1.8) gives F = Vp = 0, hence the name“force-free". Force-free fields tend to have atwisled or“sheaied” appearance. Examples of force-free fields are chmmospheric fibrils and penumbral stnictures near active suns{xits (Sects. 3.7.2 and 5.6.2).

The erudition(1.10) can be satisfied in three ways: B = 0(trivial), P X B = 0 (i.e., j=0), or

 

                                                                                             (1.12)

 

1. Cosmic Plasma Fundamentals 29

 

 

wherethescalara--a(r)  ingemms.                    of aforce-fieefield is simply that electric currents flow parallel to magnetic fieldlines. Suchcunentsareoftencalled“field-aligned”currents(Chap-

 

The force-free fields with constant ‹x represent the lowest state of magnetic energy that a closed system may attain. This has two important consequences. It proves the stability of force- free fields with constant o, and shows that in a system in which the magnetic forces are dominant and in which there is a mechanism to dissipate the fluid motion, force-free fields with constant ‹x ate the nanirat end configuration. In astrophysical plasmas, thedissipationmechanismmaybeihe acceleration of charged particles to cosmic ray energies.

• The Diocotron Instability

 

One of the outstanding problems in the propagation of electron teams along an axial magnetic field is the breakup of the team into discrete vortex-like current bundles when a threshold deter- minedbyeithertheteamcurrentordistanceofpropagationissurpassed.Thephenomenaobserved, closelyiesemblesthatass‹x:iaied with the Kelvin—Helmholtz fluid dynamical shearinstability, in which vortices develop thmughout a fluid when a critical vel‹x:ity in the flow is exceeded, with a large increase in the resistance to flow [Chandrasekhar 19613.

While structural changes in the azimuthal direction are observed in solid, annular, or sheet beams, it is with thin electron teams that the vortex phenomenon is most pronouixxxL Since thin anniilarbeamsareeasilyproducedandarecapableofconductingintensecurrents,theyhavefound widespread application in micmwave generation and accelerators. Conversely, in many applica- tions a cold beam is desired and the heating of the beomby the onset of instabilities is an undesired

 

The instability leading to the lamentation of the beam is known as the“slipping steam” or “di‹x:otmn’* and ‹xx:urs when charge neutrafity is not locally maintained, for example, when electrons and ions separate. In the steady state Eg.(1.3) in cylindrical coordinates is

 

r‘                    —— n,— n;)

 

which gives rise to a shear in the drift vel‹x:ity. The shear (precisely, the axial component of the

vorticity) is given by

 

 

(n — n¡) —— q try(1 —f )                                                                                                            (1.14)

 

where

 

 

 

                                                                                    

 

30  1. Cmmic Plasma Fundamentals

 

 

is called the crnss-field electron beam parameter [Buneman ct al. 1966], nip = n e° / in       z , cij=‹iibJ y= —e B / in y , where his the Lorentz factor r= ta — ft') ' ’', and /I, = v J c for a beam of axial vel‹x:ity v, .Thefactorf, = z»; I », agents M &          ofchaigeneutialization.

For strong magnetic field“low-density” teams (q < 0.1) of thickness Qtr, the instability ‹x:curs at long  wavelengths

 

                                                                                         (1. 16)

or at wavelengths about eight times the beam thickness.

Equations (1.13)-(1.15) are exactly the magnetron equaéoiis(c.f. “Buneman Small Ampli- tudeTheoiy”inCollins, Microw'ave lagnetroris), exceptthatn,=0inaproperlyvacuum-pumped magnetron.

Figure 1.20 depicts both the vortices of a 90kA electron beam etched onto a carbon witness plate and those of a 58 JiA electron beam detected by a sensitive fluorescent screen. Therefore, in the laboratory, well-defined vortices are found to occur over some 12 orders of magnimde in beam current. This mechanism was first introduced to explain aumral cunains (Figure 1.21) by Alfvén(1950). Thedi‹x:otroninstabi1ity as the cause of the aumral curtains is discussed in Section 2.9.8.

 

• Critical Ionization Velocity

 

Fboth a plasma and a neutral gas are so thin that collisional momentum exchange is negligible, onewouldexpectthemtomovethrougheach other without appreciable interaction. That this need not be so was suggested by Alfvén (1942) in his theory of the origin of the planets and satellites. He introduced the hypothesis that if the relative velocity exceeds a certain critical value, a strong interaction and rapid ionization of the neutral gas would take place. He funher assumed that this “criticalvel‹x:ity"v was given by the vel‹x:ityat which the neutml gas particles with mass M have a kinetic energy equal to their ionization energy ed,:

 

 

= eV ;                                                                                                        (1.17)

 

Table 1.7 summarizes the critical ionization vel‹x:ity parameters for many elements while Figure 1.22illustratesthegravitationalpotentialenergyversusionizationpoiential[Alfvénand Anhenius 19763.

At the time Eg.(1.17) was proposed, there was no known reason why such a relation should hold. Nevenheless, the hypothesis was confirmed later in a laboratory experiment [Fahleson 19613. Formanyyearsitiemainedamystery,butexperimentshavenowc1arifiedthephenomenon at least in general tennis. What is involved is an instability that transfers energy fmm ions to elections, so that they become capable of ionizing. Still, important questions remain (Burning and Axniis 1988J.

Meanwhile, critical ionization velcx:ity phenomena have been observed in space plasma

[I-Iaerendel 1982, Torbert 19883. The phenoniermn has been invoked in severer cosmical appli-

 

1. Cosmic Plasma Fundamentals 31

 

 

 

Figure 1.20.(top) Vortices of a 90-kA election beametched onto a carbon witness plate(courtesy ofH. Davis).

(bottom) Vortices of a 58-pA election beam photographed on a fluorescent screen(counesy of H.F. Webster).

 

 

cations, such as the formation of an ionospheie at the Jovian satellite Io [Cloudier 1978], the interacti‹m of the solar wind with gas clouds (Lindeman ct al. 1974. Gold and Soter 19763. with comets [Haerendel 19g6, Galeev ct at. 19863, with planetary atmospheres [Luhmann 1988], and withtheintersiellarmedium[Petelskietal. 1980, Petelski 19813.Thus,thephenomenonmayhave imponant astmphysical implications. However, these cannot be evaluated in detail until a full understanding of the phenomenon has been achieved, which is the goal of nx:ket experiments.

 

32         1. Cosmic Plasma Fundamentals

 

 

Table 1.7.   Parameters ass‹x:iated with the critical ionization velocity

 

	Ionization potential	Average atomic mass	Gravitational potential	Atomic abundance	Critical velocity
Element°			gj g¿$y	b	(  ()5 cm/sec)	Band
	volts	amu	flock e/cm	'i = lo‘	fmm/usecl
H	13.5	1.0	20.29	2x ] 10	50.9	I
He	24.5	4.0	19.94	2 »io9	y,3	I
Ne	21.5	20.2	19.18	2x ] 6	14.3	II
N	14.5	14.0	19.18	gp](j6	14.1
C	li.2	12.0	19.1i	i «io7	i3.4
O	13.s	i6.0	19.08	z• io7	iz.7
(F)	17.42	19.0	19.11	4x ] 3	13.3
(B)	8.3	10.8	19.08	] yj 2	12.1
[Be)	9.32	9.0	19.18	8xl0	14.1
/i]	5.39	6.9	19.04	5xl0	12.2
	15.8	40.0	18.78	I xl05	8.7	III
	10.5	3i.0	18.70	i •io4	8.1	III
	10.3	32.1	18.70	5x10	7.8
	7.6	24.3	18.60	]   ](j6	7.7
	8.1	28.1	18.60                 j  6             7.4
	s.i2	23.0	18.3o            6 • io4             6.5
	5.97	27.0	18.48            8 • io4             6.5
	6.09	40.1	18.30            7x ]p4             5.4
Fe	7.8	55.8	18.30	9x  (;5	5.2
	7.4	54.9	18.30	i • io4	$,{
Cr	6.8	52.1	18.so	} y { 4	5.0
Ni	7.6	58.7	18.30	5x ] 4	s.o
(CI)	13.0	35.5	18.70	2x10	8.4
M)	4.3	39.1	18.30	2 x t0!	4.6

a ]),jinor elements (abundance 1fi'-- ID) are indicated by parentheses; trace elements (abundance < ID) are indicated by brackets.

b The very fact that sepamtion processes are active in interstellar and ciicumstellar space makes it difficult to specify relative abundances of elements except by ordeiof magnitude and for specific environments (such as the solar photosphere. the solar wind at a given point in time, the lunar crust). Tbe abundances are the averages estimated by Urey (1972). Most values are based on carbonaceous chondrites of Type II which form a particularly well analyzed set, apparently unaffected by the type of differentiation which is characteristic of planetary interiors. Supplementary data for volatile elements are based on estimates for the solar photosphcre and

trapped solar wind. All data are normalized to silicon, arbitrarily set at ID.

 

 

Because the existence of this anomalous interaction has beensodefinite1ydix:umented plus the fact that it has been so successfully applied to the cosmogonic ptix:ess, should make it interesting to keep in mind in cmmological contexts, too.

 

1. Cosmic Plasma Fundamentals 33

 

 

 

Figure1.21. Auroral vortices observed in the magnetic zenith, College, Alaska, 31 January 1973.The exposure time is 0.1 s and the field of view is 12x 16°. The image was obtained from a low-light level television system with a broad-band red-light filter (counesy T. Hallinan).

 

 

1.8 Diagnosing Cosmic Plasmas

ID.1 The Electromagnetic Spectrum

 

For millennia our knowledge of the universe has been based on information received in the visual ix:tave,4O1--8£Dnm,supplemeniedduringthelasthalf-centuryby infiaiedandradioobserrations (Figure 1.23). During the 1970s and 1980s, however, space teseamhhasopenedthefullspectnim, including the entite infiaiediegionand the ultraviolet, X ray,    7 y legions(Figure 1.24). The

fu1lelectmmagneticspectnimisdelineatedinto6‹izids, which generally aredmignatedas follows:

 

Gamma Rayaiid Xray.Mostemissionatthesewavelengths islikelytobe producedbyelectrons with energies in excess of 1fP eV. We know that pixx:esses in magnetized plasmas, especially conoemingelectric fieldsalignedby magnetic fields, accelerate aumralelectmnsto keV energies. Similarplasniapixx:essesinsolarflaresprix1uceeneigiesof1—10GeV. Under cosmic oqpditions, relativistic dcaible layers (Chapter 5) may generate even higher energies in magnetized cosmic

 

34         1. Cosmic Plasma Fundamentals

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0         /         1'0          1/        /0        2/

l0niZati0ls  p0tglstial  g¡  n (V0ltS)

 

Figure ld2. The gravitational energy lip and ionization potential of the most abundant elements. Roman numemls tefer to row in the periodic table, with “III” including the fourth role. All elements in a band have approximately the same gravitational energy and ionization potential as discussed by Alfvén and Arrhenius (1976).

 

 

Therefore we can assume with some confidence that the X rays and 7 ys we observe derive mainly from magnetized plasmas with energies in excess of IU'- eV. Therefore, we call the picture we get from these wavelengths the high-energy-plasma universe, or simply, beplasma universe.

HighenergymagnetizedplasmasnotonlyemitXraysand ys,butalsosynchtotionradiation that often falls in lower energy bands, including the optical and radio regions.

The energy densities of radiation in the tray and X ray bands are -ID" J m-' and -III" J m—',respectively,andmayarisefromthetotalcontributionotdiscretesoumes(Section 6.7J).The isotropy of the X ray background is dTlT < 1£r'

 

Figuz'e ID.(Opposite) Cross section through the earth’s atmosphere showing the aItitu‹lc and the approximate wavelength coverage of the different spacecraft. r‹x:kets. bal1‹:x:ins, and ground-based observatories that make the observations. The solid white line shows tbe altitude as a function of wavelength where the intensity of the solar radiation is reduced to half its original value(from Max 91/courtesy of R. Banfield and B. DennisfNASA- GSFC).

36     I. CksNéCP1MFMaFuMdaWCWtdS

 

 

 

 

O    radlo

+ uhf/microwave

 

 

• x-ray/g-ray

 

 

 

 

 

 

 

 

 

 

 

 

 

 

“

 

 

 

 

frequency, Hz

 

 

Figure ID.(t•p) Spectium of the cosmic backgrouixl radiation fmin 10‘ Hz to llf' M (bottoms) The ap- proximate year when new technology made possible observation in the diffeientregions of the electromagnetic spccoum. As slaowo, most of tI›c specoum was noc accessible cntil the mid-t910s.

 

Ultraviolet. Ultiavioletastronomy satellites have discoveiedvarious circumstellarplasmadistri-

biltiixis tit aS5‹x:iatiOfi Witfi :iVariety of stellar Objects, flofR protOStBiS tO highly evOlved red giants.

The energy density of the cosmic diffuse ultraviolet background is estimated to be -lW" J m—'.

 

VisibA Visible light comes from solid b‹x1ies such as planets, but to a much larger extent comes

fmmstellarphotospheres,whichaietypica1lyplasinaswithlowenergies—    lessthan10eV. Hence

 

1. Cosmic Plasma Fundamentals 37

 

 

M visual universe is almost synonymous with the low-energy plasma universe. The energy density of visible light in the universe is -IO" J m—’.

 

Infrareit. Infrared radiation is emitted from the photospheies of stais. For example, 527o of the electromagnetic radiation emitted by the Sun falls between lOl and 1mm. The energy density of radiation in the infraredapproaches lO" Jm-? The infrared background radiation is a component of the cosmic background radiation that also includes the submillimeter and microwave back-

unds.

 

Submillimeter and Microwave. High-power microwave generation on earth belongs exclu- sively to devices using relativistic electron beams. The microwaves derive from naturally occur- ring beam instabilities or from electromagnetic-induced beam instabilities caused by cavities or slow-wave structures placed nearihe beam. For example, the di‹x:otron instability(Section 1.7.3) isresponsibleformicrowavegenerationinmagnetronsandmagnetically-insulated-transmission- line oscillators. Microwaves from beam interactions with slow-wave structures or cavities is the mechanismusedbybackward-waveoscillatorsandrelativistic-klystrnnoscillators(Section 2.7.2). In addition high-powermicrowavesaregeneialed viathe Baikhausen-Kurz, orreflex, mechanism wheneverthecurrentcarriedby therelativisticelectronsexceedsthe space charge limiting current (Section 2.5.3) causing the formation of virtual cathodes or virtual anodes(double layers) [Peratt 1985]. A ielativistic electron team that does not produce microwave radiation is unknown.

These same basic mechanisms are likely to have their namral analogs in cosmic plasmas. Coronal loopsconductingelectric currentsonthe Sun pnxluce microwaves, as doelectric currents inthelobesofdoub1eradiogalaxies.Thenucleiofspiialgalaxiesradiaieatmicmwavefmquencies and are even sites of MASER (microwave amplification by stimulated emission of radiation) action [Moran 1984]. In our Galaxy water-vapor masers ‹x:cur in the dusty plasma surrounding newly formed massive stars.

Measurements in the frequency range 4IX1 MHz to 6tXl GHz show a cosmic microwave be:I:ground(CMB) that can befittedtoablack bodyspectrum(Section 7.3)attempeiaiureof2.73 K (Figure 1.25).

In the Rayleigh—Jeans region (< 120 GHz) the radiation is isotropic with a precision ap- pix»z:hing(andinsomecasesbetterthan)z1 T J T < 3 x l0— * inascaleof1°at 3 = 7.6 cm[Berlin

etal. 19833. The isotropy is Zl T I r <  io— 4    ver all angular scales’(Figure 1.26) [Pariiskii and

Korolkov 1986].

The energy density of microwaves in the universe is -4.5 x 1O'• J m—'.

 

Radio Wave.The radio wave portion of the electromagnetic spectnim is further divided into sub-

 

Like Jupiter and Saturn, the earth is an intense source of long(kilometer) wavelength radio waves. These waves were discovered by satellites above the ionosphere. Since their frequency c<<zti„ where ci, is the maximum angular plasma frequency in the ionosphere, they cannot penetrate the ionosphere and socannot be detectedatground level. The radiation derives from the polar aurnra at high altitudes (animal kilometric radiation orAKR). The mean intensity of AKR fmm earth is ltEl MW, with peak intensities as high a gigawatC

Both Jupiter and Sanim radiate at kilometric, hectnmetric, and de‹:ametric wavelengths via the synchmiron prix:ess (Chapter 6). The average Jxiwer radiated by Jupiter between 51Xl and 40

 

35         t. Cosixiic Plasma Fundamentals

 

 

Frequency (GHz)

 

Figure 1.25. Spectral brightness vs frequency for the cosmic microwave background measured by COBE at the north galactic pole. The solid curve ts a 2.735 I 0.06 K blackbody spectium.

 

'  I I ' I ' I '    I      I I    ' I I I      I                         Dipole   -                                                 - -                           -                                       '   ' ' '“ O ' ' 300 ' 1800

1

 

1 0-3

1.0

 

10-4

 

 

10-

 

Angular scale

 

0.01

 

 

Figure 1.f6. Isotmpy observations in i:he cosmic microwave background on all angular scales. Except for the dipole effect at 180° all results are upprr levels. Symbols indicate the sources of the data: (closed triangles) Very Large Array interfurometer. S‹x:orro, New Mexicoi (closed squares) 140’ telescope, Green Bank. West Virginia; (open circles) RATAN-6tXI telescope in Zelenchukskaya, USSR; (cmsses) Testa Grigia alpine station, Italy; (open triangles) balloon launched fmm Sicily; (closed cimles) bal1oims1auiu:lied fxom                                                                                         , Texas. and She José dos Campos, Brazil.

 

 

 

Figure 1.27. Map of southern sky at 144 meters wavelength(2.085 MHz). This map shows a bright background (corresponding to 3.5x10‘ K black body) with dimming at the center of the Galaxy and along the Milky Way (courtesy of G. Reber).

 

 

 

 

 

 

 

kl-Iz is 6GW while Saturn produces an average power of 1GW between 3 kHzand 1.2MHz. The planetary radio emissions are superimposed onto a cosmic radio background.

At VHF frequencies(178 MHz), the radio background radiation has a nonthermal, isotropic cnmponentwhosebrightnesstemperature(Section 7.3) is 65:k5 K [Bridle 1967].Reber(1986) has surveyed the southern sky at 2.085 MHz (144 in) and reports a bright background corresponding to a 3.5x10• K blackbody (Figure 1.27).

Because of the uncertainties in radio intensity at ultra-low fmquencies, the energy density of radio waves in the universe is unknown but, nevertheless, appreciable.

 

1.8.2 In Situ Space Probes

 

Until the early 1970s, almost everything we knew about the universe had been obtained from information brought to the observer by electmmagnetic radiation. Only a very small part of our knowledge stemmed fmm material information carriers, be they in the form of meteorites hitting the surface of the earth, cosmic ray particles, or material collected by manned or unmanned satellites or fmm lunar and planetary landings.

With the advent of earth and inierplanetary space probes, this knowledge has been aug- mentedby insitumeasureinents inourownsolar system. Thesemeasurementshaveoftenresulted in discoveries that were unsuspected or misinterpreted from information contained in the electro- magnetic spectrum alone. For example, prior to space probe measurements, it was universally

 

40         1. Cosmic Plasma Fundamentals

 

 

assumed that the outer magnetosphere was populated by hydrogen plasma from the solar win‹t and therefore ultimately from the sun. In contrast to this, we now know thai the magnetosphere is sometimes dominated by oxygen plasma originating in the earth’s own atmosphere. Electric fields within the near-earth plasma were generally not thought possible until space probes mea- suiedthemdirectly.Theirexisiencewaseithernotinferablebymeansofelectromagneticradiation or they have radiation signatures at frequencies far below that currently measurable on earth.

It is a sobering fact that even after hundreds of satellites had circled the earth, the generally accepted picture of our space environment was fundamentally wrong in aspects as basic as the originandchemicalcompositionofmatterintheearth'sownneighborhoodandtheexistenceand role of electric fields in the magnetosphere. This must inspire caution in making assertions about the composition and properties of other invisible cosmic objects, whether they be stellarinteriors, interstellar plasma, pulsar magnetospheres, or intergalactic cosmic rays. The danger of miscon- ception is panicularly great for distant astrophysical objecn that will forever remain inaccessible to in situ observation. To avoid this danger, it is essential to utilize the empirical knowledge of plasma behavior that has been, and will continue to be, gathered from plasmas in the laboratory and accessible regions of space.

 

Notes

 

• Oliver Heaviside was the first to reduce Maxwell’s 20 equations in 20 variables to the two equations(1.1) and(1.2) in vector field For some years Eqs. (1.1)--(1.4) were known as M Hertz--Heaviside Equations, and later A. Einstein called them the Maxw'ell--Hertz Equations. Today, only Maxwell’s name is mentioned [Nahin 1988].
• These are “rewritten" in update form ideal for This also emphasizes the causality correctly: V x E is the cause of changes in B, V x H is the cause of changes in D.
• In free space c= e,= 8542 x lW" farad m—' and jz = yr, = 4it x 1   7 henry m-'.

One of the Sun’s outstanding problems is the temperature of the corona. The temperature rises steadily in the chmmosphere, then jumps abniptly in the comna to a level 3IXl times hotter than the surface. That the Sun is a plasma and not just a hot gaseous object is illustrated by the fact that the temperature increases away from its surface, rather than cmling as dictated by the thermodynamic principle for maner in the nonplasma state.

For decades the preferred explanation has been thai energy flows fmm the Sun’s surface to the comnainthe form of sound waves generated by convective upswelling motions. However, space-based ultraviolet observations pmved that sound waves do not carry energy as high as the cc One mechanism that may pnxluce coronal heating is electron beams pnxluced in double layersincoronall‹xips(Chapter5).Theseareexpectedtoacce1erateelectnmstoenergiescomparable    to those in the corona. Generally, thetermaccelerationrefers to the preferential gain of energy by a Jxipulation of electrons and ions, while heating is definedasthe bulkenergizationoftheambient plasma Paraphrasing Kirchoff diat “heating is a special kind of acceleration,” one may argue, since heating and acceleration are always present in flares and in laboratory relativistic elm:tron beams, thai electron beam instat›i1ities (Section 1.7.3) may be the soume of comnal heating.

* The degree of ionization is defined as n /(no + n ) whem zz is the plasma density and »

is the density of neutral particles.

 

1. Cosmic Plasma Fundamentals 41

 

 

7 It is not known how the energy carried by the solar wind is transformed into the energy of the as It has been demonstrated thai the southward-directed interplanetaiy magnetic field is an essential ingredientincausingauroralsubstormssothatenergytransformationappearsto ‹x:cur through interactions between the interplanetary and geomagnetic fields [Akasofii 1981].

• The term diocotron derives from the Greek dt‹»iziv, meaning “pursue.”
• A dipole anisotropy in the cosmic microwave background, because of the net motion of

the solar system through the CMB, is measured at the ID' level [Biecker 1984].