It is shown that the interaction between the solar wind flux and the cometary ionosphere leads to the excitation of ion sound, whistler, electron-cyclotron, low hybrid, and magnetohydrodynamic waves. We investigated the frequency spectrum and found linear-increasing increments and lengths of excited waves

Churyumov, Klim I.

Kotsarenko, N. YA.

Lizunov, G. V.

Verkhoglyadova, O. P.

English

NASA Technical Reports Server

Asteroids, Comets, Meteors 1991, pp. 125-128 S_'. _3 -- _
Lunar and Planetary Institute, Houston, 1992
PLASMA-BEAM INSTABILITIES IN COMETARY IONOSPHERES f. _/
K. I. Churyumov, N. Ya. Kotsarenko, G. V. Lizunov, and O. P. Verkhoglyadova
Department of Astronomy of Kiev Shevchenko University
ABSTRACT
It is shown that the interaction between the solar wind flux and the cometary ionosphere leads to
the excitation of ion sound, whistler, electron-cyclotron, low hybrid and magnetohydrodynamic waves.
We investigated the frequency spectrum and found linear-increasing increments and lengths of excited
waves.
INTRODUCTION
A cometary ionosphere is separated from the circumfluent solar wind flux around it by a large interim
region (105 km thick) within which the solar wind plasma and that of a comet are mutually interpenetrating.
This system develops plasma-beam instabilities exciting plasma waves of different types.
A wide spectrum of these waves was detected by the ICE-probe that encountered on September 11,
1985 the comet P/Giacobini-Zinner (1984e) plasma tail (Scarfet al., 1986) and ICE, Vega-l,2, Sakigake-
probes that passed through P/Halley's head in March 1986 (TsurutanL 1991). The spectra obtained
are of a specific type and they are shown by dashes in Fig. 1 (TsurutanL 1991). The maximum on
the right corresponds to the ion acoustic waves at the ion Langmuir frequency. Lower in frequencies
are whistler mode waves with an intensity maximum at the Low Hybrid Frequency. Finally, ion-cyclotron
and magnetohydrodynamic waves materialize as regular fluctuations of the magnetic field are registered
in the lowest frequency range. The total energy density of the waves in question can be estimated
by measurement data (Scarfet al., 1986) as 1-2 "10-13erg/cm 3, which is 0.01% of the solar wind kinetic
energy density.
The energy source and the exciting mechanism of the waves in question were investigated by many
authors. The plasma-beam instability was studied by Chernikov (1974), Gold, rein and Wong (1987),
Gary and Madland (1988), Price and Lee (1988), and in a number of more recent papers presented
in Tsurutani (1991). The authors investigated, rather thoroughly, applying analytical and numerical
methods, the excitation of ion-cyclotron and magnetohydrodynamic waves that are important for cometary
ions acceleration and that can affect shock structure and the cometary ionosphere as a whole. Instability
of the remaining higher frequency plasma waves has not been investigated, except by Chernikov (1974).
He investigated the ion acoustic instability, yet he did not study the strongest hydrodynamic mechanism
of instability excitation. The above work focuses the purpose of the present paper, which deals with
the whole plasma-beam instability frequency spectrum that occurs under the solar wind and cometary
plasma interaction.
https://ntrs.nasa.gov/search.jsp?R=19930009952 2020-03-17T07:33:14+00:00Z
126 Asteroids, Comets, Meteor$1991
THE INITIAL MODEL
We examined an extended region of space where the solar wind plasma and the comet plasma are
mutually penetrating. Scarf et al. (1986), Bame et al. (1986), and Ogilvie et al. (1986) indicate that
this region consists of the proper transition sheet and the comet sheet whose total thickness (for P/
Giacobini-Zinner -105 km) is much higher than the visual comet cross-section. The plasma parameters
and the magnetic field configuration gradually change in the region in question, the cometary plasma
density no greatly surpassing the solar wind density nb, throughout the entire region except the outer
edge of the transition sheet, with t_ = nb/np _ 1. Since the characteristic scale of plasma heterogeneities
is much greater than the pertinent wavelengths (from tens of meters for ion-acoustic waves up to hundreds
of kilometers for magnetohydrodynamic waves), heterogeneity that can affect the development of
instabilities can be neglected. Further, we investigated a simple model in which a small density homo-
geneous plasma beam (the solar wind) interacts with homogeneous cometary plasma.
It should be noted that the relative drift velocity of the two plasmas, Vo, agrees with the relation
VTi _ Vo _ VTe where VTi,e are the ion and electron heat velocities. Thus we deal with a hydrodynamic
flux only of the ion plasma component, while the t0tal velocity distribution function of electrons is
of a one-hump character. This condition enables us to elaborate a linear theory of the plasma-beam
instability.
RESULTS OF CALCULATIONS
A general idea of the results obtained is shown in Fig. ! (the dependence of instability growth rate
3/ o n frequency to). We shall consider the spectrum of excitation_wa, es, - moving from high frequency
waves to low frequeney0nes. _ _:: :__ _ ;:: _. .... ___ .... __ :
a) Langmuir Oscillations: Plasma-beam instabilities do not excite the oscillations in question, for
under the given conditions, instabilities of all types (electron kinetic and hydrodynamic) are inhibited
bec-ause ofhigia temperature ot; the flux ......
' b) Ion-Acoustic Oscillations: Their instability is due to the interaction of the solar wind and cofiaetar 3, '
plasma ion-acoustic waves. There are waves with the frequency 0_ topi [where topi --- (471"e2?lp/M) 1/2 !
is the cometary ion plasma Langmuir frequency] and the wave number k _ topi/Vo. This brings about
the maximum instability growth rate equal to 3, _ X/'3kVootl/3/24/3 (Mikhailovsky, 1975). For the
characteristic values of plasma parameters nb = 50 cm -3, no -- 5 cm -3, V o -- 100 km/s we shall obtain
to _ 4.2 "103 s q, 3, _ 1.3 "10 3 s -I. The ion sound wave length is h _ 150 km.
c) The Whistler Range Waves (toni < to < torte, where tom,e are ion and electron gyration frequen- .[
cies): In this range and in lower frequency ones the dispersion plasma properties are essentially affected
by the presence of the outer magnetic field J_o, "frozen-in" the solar wind.
Electron cyclotrgn_0sci!!ations with the d isper_sio_n_ law p _ toHeCOSO [wh_e_re 0_ -- (_',Vo), with roll
/'70] can be excited due to kinetic instability of the electron flux, as was studied by Akhiezer (1974,
6.2). After transforming the equations given for the maximum growth rate, one can get a simple equation
3, _ ax/--_-/2to_ccosOsinO/(3k4V3eVo), valid for n-/2-O > x/--_e/Mi. Simultaneously k = (4/3)(tone/Vo). :
With 0 _ rr/2 the electronic oscillation excitation is more effective on account of the ionic flux
hydrodynamic instability, with the growth rate maximum at the Low Hybrid Frequency to _ toLU --
(toHetoH) I/2 and equal to 3' _ atoLrt with k = toHdVo. Assuming Ho = 20 nT, we obtain to _ 20 s -l, =
3, _ 2 s -1, h _ 360 m. The dependency ",/(to) is given by curve 2.
In this frequency range, faster waves can be excited, whistlers with an electromagnetic character
that interact with solar wind ionic acoustic waves. The dependency "/(to) tabulated from the accurate
formula taken from Akhieser (1974) ",/(to) is shown in Fig. 1 by curve 3.
Asteroids, Comets, Meteors 1991 127
d) Magnetohydrodynamic (MHD) Waves: Their excitation effectively occurs under cyclotron res-
onance of the ionic flux (with to _ kVocosO-torti). A fast magnetosonic wave is unstable (Akhieser,
1974) with the dispersion law to = kVA [where VA = Ho/(4rrMinp) I/2 is the Alfven velocity], the wave
number k = toHi/(VocosO-VA) and the growth rate 3' _ al/2toHi (see curve 4, Fig. 1). The order of
the values gives the following: to _ 0.4 s q, 3" _ 0.7 s q, h _ 520 km.
The results obtained give good account of the spectral composition of the radiation observed (see
Fig. 1). Moreover, the absolute values of the instability increment of the whistler and MHD-waves
are in good agreement with the results by Price and Lee (1988) and Brinca and Tsurutani (1988), who
investigated the excitation of waves by numerical methods.
CONCLUSIONS
The interaction of the solar wind plasma and the cometary ionosphere gets a wide spectrum of the
waves that were excited (Fig. I), namely: ion-acoustic ones with the ion Langmuir frequency, electron-
cyclotron ones with the intensity maximum of low hybrid, whistlers and fast magnetic acoustic waves.
With this wave energy of about 0.01% the solar wind kinetic energy is produced.
REFERENCES
Akhieser A. I., ed. (1974) Electrodinamika Plazmy. Nauka, Moscow. 720 pp.
Bame S. J. et al. (186) Science, 232, 356-361.
Brinca A. L. and Tsurutani B. T. (1988) J. Geophys. Res., 93, 48-58.
Chernikov A. A. (1974) Astronomicheskiy J., 51, 852-858.
Goldstein M. L. and Wong H. K. (1987) J. Geophys Res., 92, 4695-4700.
Mikhailovsky A. B. (1975) Teorriya Plazmennykh Neustojchibostey, 1, Nauka, Moscow. 272 pp.
Ogilvie K. W. et al. (1986) Science, 232, 374-377.
Price C. P. and Lee L. C. (1988) Astrophys. J., 324, 606-620.
Scarf F. L. et al. (1986) Science, 232, 377-381.
Tsurutani B. T. (1991) Cometary plasma waves and instabilities. In Comets in the Post-Halley Era,
Vol. 2 (R. L. Newburn, M. Neugebauer, and J. Rahe, eds.), pp. 1171-1210. Kluwer Academic Publishers.
Fig. 1. The dependence of instability growth rate 3" on frequency to. Curve 1 -- an ion-acoustic wave,
2 -- electron-cyclotron wave, 3 -- whistler, 4 -- magnetosonic wave. The dashed line shows the spectral
density I(to) of radiation that was observed experimentally (Tsurutani, 1991).
128 Asteroids, Comets, Meteors 1991
m
t£2
0
IX)
Ig,y
i
.., 0
I !
CO
I
6r0 ° S
Ig I(_)
_11 " j lb='-_ I I
I
I I
I I
f
/(
S
J
J
f
J
t"
,, ,L
I _ I w I
_ ".4 CO


Plasma-beam instabilities in cometary ionospheres

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server