The origin of molecular ions in the Martian magnetosphere

The origin of molecular ions in the Martian magnetosphere

Adw. @cc Res. Vol. IS. No. 4, PP. (4)17_(4)20, 199s 1995 COSPAR Printed in Great Britain. 0273-l 177(94)0006i-1 0273-I 177/M$9.50+ 0.00 THE ORIGIN ...

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Adw. @cc Res. Vol. IS. No. 4, PP. (4)17_(4)20, 199s 1995 COSPAR Printed in Great Britain.

0273-l 177(94)0006i-1

0273-I 177/M$9.50+ 0.00

THE ORIGIN OF MOLECULAR IONS IN THE MARTIAN MAGNETOSPHERE K. K. Mahajan, J. Kar and R. Kohli National Physical Laboratory, New De&l

IO 012, India

ABSTRACT Hot plasma measurements from the instrument ASPERA on the Soviet Phobos-2 spacecraft indicate a large density of molecular ions in the Alartisn magnetosphere and thus a high outflow of the molecular species. This is an unexpected result since it ia generally molecular ions in the flartian ionosphere are in photochemical believed that the equilibrium and these remain undisturbed by the 8olar wind. On the basis of knowledge gained from the Radio occul+etion measurements on the Pioneer Venus during solar minimum, profiles observed by the Viking landers at flora have been we demonstrate that the 0 This erosion takes place down to an altitude of 160 groasly eroded by the aola?r wind. km and thus contributes to the high outflow’of the molecular species. INTROOUCTION The Soviet Phobos-2 spacecraft carried a three dimensional plasma composition experiment ASPERA, the Automatic Space Plasma Experiment with a Rotating Analyser /l/. This lnatrument measured the composition , energy and angular distribution of ions with energies 0.5 eV/e to 24 keV/e and electrons with energies 1 eV to 50 keV. The spacecraft was in elliptical orbit around flora during the first four orbits with pericenter at 870 km and an orbit period of 77.6 hours. Later it was injected into a circular orbit just outside the Phoboa moon. Fig.1, taken from Lundin et al./Z/ shows the main features ,of the Nartian charged particle environment during orbit 2 as measured by ASPERA. This orbit uaa characterized by a moderate solar wind pressure and an expanded magnetosphere /2/. The major boundary crossings are shown by vertical lines. The bow-shock was crossed near the subsolar point at an altitude of about 1800 km and the magnetopause was crossed near the location of the pericenter (860 km). Other important features of the Portion magnetosphere as described by Lundin et al. /2/,are the strong reduction in the H+ ion dfnsitiea and in the flow velocities +and a corresponding large _FJIcrease of energized 0 and molecular ions. One can note 0 densities as high as 6 cm even in the distant tail. The mass resolution of ASPCRA is n:t sufficient to distinguish between the molecular ion species lik: N2’, 0 and CO . However, the molecular ions have been interpreted to be mainly O2 ions /2 3 .

Fig. ions by

1.

in the

Number density of O* and molecular the Plars magnetosphere measured ion comporition experiment ASPERA

on Phobor-2. molecular ion ty /from 11.

(4)17

Notice density

that exceeds

at times the the 0’ densi-

K. K. Msbajsc ct al.

(4)18

on the svarega, was found to be twit! the molecular lon density, Although 0+ density, yet st times molsculsr ions were found to dominate ove+r the 0 ions. It is quite apparent in the Asrtian magnetosphere. On 1 that t+hsrs is significant amount of 02 from Fig. ions in the Nertisn ionosphers have generally been believed to be the other hand, 02 in photo-chemical undisturbed by the solar wind. Thsrefore, sq+uilibrium /e.g. 3 A 4/, ions in the Alsrtisn msgnetosphere wss an unexpected result. In the existence of 02 thls psper, wa argue that the solar wind interectfo+n at the flsrtisn ionosphere extends densltfss meesured by the Retsrding down to an altitude of about 160 km. By using the 0 we st all demonsttste that these densities Potential Analyser on the Viking landers /3/, have been grossly eroded by the solar wlnd interaction. The 02 ions seen in the Nsrtisn magnetosphere sre due to this erosion. THE NARS IONOSPHERE

AN0 SOLAR WIND INTERACTION

The direct measurements of the Nartian lonosphsrs made by the Rstsrdlng Potential +Rnslyssr on the Viking landers /3/ are shown in Fig. 2. The major ionic species is O2 , which is formed by+ the photolo+nirstion of CO , O2 and 0 end is produc+ed primarily by the + CO and O+ ‘+ CO -a 0 + + CO. The 0 reactions: CO + 0 -b 0 ions sra destroyed 4/ have pfoduced 02+ profiles by dissocistivf racombinst 1 on. Photochemical ‘models ‘/3, which have agreed fairly well with those observed on the Viking landers. This agreement

Fig.

2.

Ion

concentration

by the Viking-l /from 3/.

snd

2

profiles spacecrafts

measured at

Fig.

flsrs

3. Diffusion (Tg) time constants (Tc) to Viking 1 measurements.

end chemical corresponding

hsvs not bsen profiles in the flsrtian ionosphere has left the impression that the O,* it is now known that mars has little or affected by the solar wind. On the -other hand, directly with ths ionosphere no intrinsic magnetic field and the solar wind lnterects From the works of Shinsgawe and Cravens /5/ and Rshsjsn and msyr /8/. It is of mars. quits apparent that the solar wind lntsrsction with the mars ionosphere extends down to altitudes where photochemclal equlllrbium is expected to exist. This interaction induces horizontal and vertical flows in ths plasma and produces a modified ionosphere. lhis modified ionosphere can be mistaken ss photo-chemically controlled, as the scale ionosphere and the modif led solar wind distributions (ViZ. two heights of the photochemical

ionosphere)

are

quite

close

/6/a

the mars Ionosphere is only limited the photochamical equilibrium in In our opinion, have checked this by calculating the chemical and to altitudes below about 160 km. We by using standard axpresions /e.g. 7, also 61. The diffusion time constants of 0 ’ chemical snd tha diffusion tima’constsnts calculetsd for Viking 1 messuremants era shown It can be immedietsly noted that diffusion becomes important above sboui in Fig. 3. For Viking 2 this altitude is found to be somewhat lower. The expected 0 160 km. Thts above 160 km is shown in Fig. 4. distribution under diffusive equilibrium Since distribution is very sensitlvla to Te and Ti values and thelr altitude gradients. from 150 to 200 km and in Ti from 200 to there are large+temparstura gradients in Te (but not as steeply as observed on concentration falls vary steeply 300 km /6/, O2 Above about 300 km, no measurements Viking) upto about 300 km, as can be Sean in Fig. 4. However, of Ta and Ti exist. thsoretlcal calculations /9/, It used a value therefora, we have, 300

from

experimental that at appears of Te + Ti I 6000

maesuramnts and above 300 K to calculate

/6/, as well km+Te & Ti g distribution .02

as from 3000 K. above

km.

It can be immediately to higher altitudss, concentration expected altitude of 1000 km.

inferred from Fig. 4 thet 02’ concentration, when extrspolste$ On the other hand, O2 would nearly vanish above about 400 km. is quite significant eyen at an under diffusive equilibrium, profiles We believe that the expected diffusiva equilibrium O2

MolecularIons In he Martian h4agoeto+spbere he3 been eroded by the solar wind interaction end her finelly resulted into the would need very high diffusive outflow This erosion, of course, 0 profile. hfqh horizontal velocities induced by solar wind interaction /q/. Rahsjan and velocities to be of the order of several kilometers per second. estimated these

Fig. 4. The expected 0 + profile under diffusive aquilib?ium above 160 km for Viking 1 end the observed 0 profile (marked ae Viking 1). TEe expected O+ profile has been grossly eroded *by the solar wind interaction.

Fig. 5. and solar profile5 The solar expected equilibrium

(4)19 observed or very fleyr /6/

minimum Typical solar maximum electron density at Venus /f ram minimum O+ (W Ne) pro::i; under diffusive is also shown.

A situation similar to Rers has been seen to occur at Venus during solar minimum. At As the Venus Venus, diffusion takes over photochemistry at altitudes above about 200 km. wind dynamic pressure exceeds the ionosphere is weak during solar minimum and solar the solar wind interaction penetrates down to an altitude ionospheric plasma pressure, of 200 km. Fig. 5 shows typicsl solar minimum and solar msximum electron density profiles The O+ ( +rNe) profile expected under diffusive at Venus reported by Knudsen et el. /lo/. It can be noted that the solsr msximum equilibrium for soler minimum is sleo shown /ll/. ae the high ionospheric plasma No profile has not been affected by the solar wind, pressure stends off the soler+wind at altitude5 well lbove 600 km. On the other hand, ( 4 Ne) profile expected under diffusive equilibrium for as in the csee of flars, the 0 resulting in the observed solar minimum, has been grossly eroded by the solar wind, scale height and can be Ne profile. The modified (observed) profile has II smaller mistaken fror the photo-chemical profile /IT/. DISCUSSION Thf difference between the observed and the expec+trd (diffusive equilibrium) Viking 1 which has flown away into the flars profile, seen in Fig. 4, is the amount of 0 O2 magnstoephere. Although this difference is observ%d at ionospheric altitudes, yet the flow need not be exactly horizontal. The plesme can be diverted to .high altitude5 by the denser regions of the near tsrminator ionosphere when the rslstivsly high dtnaity dayside plasma flows towards the low pressure nighteide plrama /12/. Further 0 ions ten etay in the megnetosphere for lonqer times bsforo being lost by disso a ietive recombination, since the loss rats ie low beceuse of the low density of the iona. The outflow of ionospheric plasma into the flars m5QnetOsphete has recently been explained by Lundin and Oubinin /13/. on the basis of ion - momentum considoretions. They have argued that the solar wind interaction with the Mars ionosphere results in transfer of energy and momentum from the solar wind to the ionospheric ions. In this process, the ions get heated and ten escape from the planetary ionosphere thereby resulting in the loss of etmosphero flso. The escape vefocity on Mars is 5 km/s+ which corrsaponds to about 0.13 eV for H ions, 2.1 5V for 0 ions and 4.2 elf for 0 ions. On the basis of simple momentum considerations, Lundin and Dubinin have shown 2 hat the maximum escape flux for e solar wind velocity of 400 km/a becomes SD times the solar wind flux at flare, if it is assumed thst the interrctinq ions hsve the same maes. For the D* and 0 2 l ions, this flux would be 5 and 2.5 times the solar wind flux. JASR15:4-c

(420

K. K. Mabajae et al. of solar wind momentum exchanqa la primarily the altltuda interval floss Loading Boundary, IAL6 and the “maqnatopauaa” /13/. If this target one can than calculate the ascapa rata of particles. Lundin and Oubinin,

lying area

The reqion between the

is the

known,

Phoboa-2 measurements mated this target

ions/a

of

FlL6 and tha area to be about

Oubinin in their t;;,G?~a:,~;;p.

(,‘,:oe

using terminator,

the

of

;;;;;inqf::rt;plas

ara 0 ions, the maximum which is equivalent to

2 ) atmosphere/s

near

4x10 They obtain an H* aacapa flux momentum transfer with _ylar wind protona. Lundin and assumed a density of +5 cm and flow valcity of 400 km/s

ss a result calculations

i:

altitudea

escape escape

r:;e,:ou.L;

/13/. CONCLUSIONS

The solar wind interaction extends down to an altitude+ of about if mars. Viking RPA measurements indicate that the O2 ions are in an outflow of ionospheric ions into the flars magnetosphere.

160 km in the ionosphere grossly eroded resulting

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