The dimensions of the obstacle forming the shock wave of Mars are estimated by use of electron trap data from Mars 2, 3, and 5. The mean altitude of the obstacle at the subsolar point can be convincingly explained if the obstacle is the magnetosphere of Mars. On the assumption that Mars has its own dipole magnetic field, the magnetic moment of Mars is estimated, Mm approximately equal to 2 x 10 to the twenty second power gs x cubic cu

Bezrukikh, V. V.

Breus, T. K.

Gringauz, K. I.

Remizov, A. P.

Verigin, M. I.

English

NASA Technical Reports Server

The Magnetic Field of Mars Estimated from the Data
of Plasma Measurements by Soviet Artificial
Satellites of Mars
K. I. Gringauz, V. V. Bezrukikh, T. K. Breus,
M. I. Verigin, and A. P. Remizov
Moscow, U.S.S.R.
Institution for Space Research
The dimensions of the obstacle forming the shock wave of Mars are estimated by use of
electron trap data from Mars 2, 3, and 5. The mean altitude of the obstacle at the
subsolar point can be convincingly explained if the obstacle is the magnetosphere of Mars.
On the assumption that Mars has its own dipole magnetic field, the magnetic moment
of Mars is estimated, M™ s 2 x 10" gs x cm'.
The spacecraft Mars 2, Mars 3, and Mars
5 were injected into orbit around Mars on
November 27 and December 2, 1971, and on
February 12, 1974, respectively. The space-
craft used identical electron traps to measure
the characteristics of the electron component
of the solar wind plasma, while Mars 5 also
used an ion trap to measure the ionic com-
ponent (ref. 1). During each revolution of
these satellites around Mars, a bow shock
was recorded, the existence of which was
first suggested after a single fly-by of Mars
by the American Mariner 4 spacecraft
(ref. 2).
Figures la and Ib show ion spectra and
electron retardation curves, respectively, for
the solar wind and the transition zone behind
the front of the bow shock recorded by Mars
5. We can see that both ions and electrons
behind the front of the bow shock are ther-
malized and their concentrations increase.
The characteristic changes in the spectra also
serve as the criterion for determining the
front of the bow shock.
The position of the front of the bow shock
depends on the dimensions and form of the
obstacle which stops the solar wind. There-
fore, experimental determination of the posi-
tion of the front of the bow shock near a
planet allows us to estimate the dimensions
of the obstacle and, consequently, to some ex-
tent to gain an idea of its nature.
Figure 2 shows sections of the orbits of the
Mars 2, Mars 3, and Mars 5 spacecraft where
they crossed the bow shock front. The char-
acteristics of the plasma measured closest to
the planet (fig. 2) are typical of the transi-
tion zone behind the bow shock (fig. Ib),
while at the remotest points the measure-
ments are characteristic of solar wind un-
disturbed by a planet (fig. la). The length
of the orbital sections shown depends on the
mode of operation of the instrument (2, 10,
or 20 minutes) and the rate of movement of
the satellite in orbit. Estimates of the dimen-
sions of the obstacle were made by calculat-
ing the position of the front of the bow shock,
on the assumption that the obstacle has the
form of an extended body of rotation like
that of the terrestrial magnetosphere (ref.
3). The solid line in figure 2 shows the shape
and dimensions of the obstacle, and the
dashed line shows the position of the front
of the bow shock for Mach number MM = 8
859
https://ntrs.nasa.gov/search.jsp?R=19780005041 2020-03-22T06:50:03+00:00Z
860 COSMOCHEMISTRY OF THE MOON AND PLANETS
£0 40 60 JO 100 SO U,l
b
20
Figure 1.—Ion spectra and electron retardation curves are shown for Mars 5 data. Figure la is for the solar
wind, Ib is for the transition zone, and Ic corresponds to entry of Mars 5 into the obstacle.
and y = 5/3, calculated according to Spreiter
and Alksne (ref. 3). The dimensions of the
obstacle were selected such that the sum of
squares of distances from both ends of the
trajectory sectors shown in figure 2 to the
front of the bow shock formed by the ob-
stacle was minimal (ref. 4).
The areocentric distance to the subsolar
point of the obstacle is (4.6 ±0.8) X 10s
km, and to the subsolar point of the shock
wave (5.7 ± 1) X 103 km; or, respectively
(1.2 ± 0.8) X 103 km and (2.3 ±1) X 103
km from the surface of Mars. The relative
dispersion in areocentric distances to the ob-
stacle and shock wave front (about 17 per-
cent) for Mars corresponds to the charac-
teristic relative values of changes in geocen-
tric distances to the subsolar point of the
bow shock front and the magnetosphere of
the Earth (about 20 percent), for example,
due to changes in the dynamic pressure of
the solar wind (for example, see reference
5). The high relative dispersion of the height
of the obstacle and front of the bow shock
above the surface of Mars is related to the
fact that on Mars the dimensions of the ob-
stacle, expressed in planetary radii, are al-
most an order of magnitude less than on
Earth.
With the use of Mars 2 and Mars 3 data,
the minimum height of the obstacle was es-
timated from intersection of the bow shock
front by the Mars 2 spacecraft on May 12,
1972 (see fig. 2) as about 600 km. From
Mars 5 to the minimum estimate of height
of the obstacle is apparently about 500 km,
based on intersection of the front by the
spacecraft in the February 20, 1974, session
THE MAGNETIC FIELD OF MARS 861
Mars 2
i
z
3
17.02.7i
8. fll 72
/Z OS. 72
MOT 3
4
5
5
7
8
9
«>
«
15. 12.71
9. Oi. 72.
21. 01. 72
210112
MirsS
««2.W
20. 02. 71,
22.02. »
2*. 02 74
0 -2 -6
Figure 2.—Sections of the orbits of the Mars 2, 3,
and Mars 5 spacecraft are shown where they
crossed the bow shock front. The length of the
orbital sections depends on the mode of operation
of the instrument (2, 10, or %0 min.) and the rate
of movement of the satellite in orbit. The solid
line shows the shape and dimensions of the ob-
stacle, and the dashed line shows the position of
the front of the bow shock for Mach number M« =
8 and y = 513, calculated according to Spreiter and
Alksne (ref. 3).
with 2-minute intervals between measure-
ments. It should be noted that in the Feb-
ruary 13, 1974, session, with 10-minute
intervals between measurements, the esti-
mate of the height of the obstacle based on
the closest approach to the planet (fig. 2)
yields lower values, while the estimate based
on the farthest point in the trajectory sector
yields higher values than in the February 20,
1974, session. However, since the dynamic
pressure of the solar wind in this "10-
minute" session (determined by the ion trap,
with data produced before the bow shock
front was intersected) was less than in the
"2-minute" session of February 20, 1974, the
front of the bow shock was apparently inter-
sected on February 13, 1974, by the Mars 5
spacecraft no closer to the planet than on
February 20, 1974. One confirmation of the
influence of changes in dynamic solar wind
pressure on the position of the bow shock
front is obtained by comparison of the meas-
urement sessions of February 20 and 22,
1974. Between these sessions the solar wind
pressure decreased and, correspondingly, the
boundaiy of the obstacle rose to an altitude
of about 900 km.
The size of the obstacle determined by our
data, even with its minimum size of about
500 km, does not agree with the idea that
solar wind is stopped by the ionosphere of
Mars (ref. 6). As follows from radio occul-
tation (refs. 7, 8, and 9) data on the altitude
distribution of electron concentrations in the
ionosphere of Mars, the maximum iono-
spheric ionization at Mars is at about 140-
km altitude; and by about 300-350 km the
pressure of the Martian ionosphere is some
two orders of magnitude less than the dy-
namic pressure of the solar wind.
Weisberg and Bogdanov (ref. 10) consider
that there is an "ion cushion" (on the day-
side) and a "boundary layer" (on the night-
side) between the ionosphere of Mars,
bounded by the ionopause, and the transition
zone behind the bow shock front; and that
these layers form the obstacle creating the
bow shock. This boundary layer, according
to Weisberg and Bogdanov (ref. 10), is
formed as a result of the viscous interaction
of the incident flux (that is, obviously, the
flux in the transition zone behind the front
of the bow shock) with particles dissipating
from the ionosphere of Mars. Obviously, the
pressure of particles dissipating from the
ionosphere is less than the pressure within
the ionosphere or at the ionopause. Weisberg
and Bogdanov do not present quantitative es-
timates explaining how the balance of pres-
sures at the boundary of the obstacle is
realized.
The results of observations, however, can
explain if the obstacle is the magnetosphere
of Mars. Assuming that the primary contri-
bution to Mars' magnetic field is made by a
dipole term, we can estimate the value of
the magnetic moment of Mars using the
following formula (ref. 3): Mm = D3
862 COSMOCHEMISTRY OF THE MOON AND PLANETS
\727rfcpooVoo, where k = 0.88, px and Vx are
the density and velocity of the solar wind,
and D is the areocentric distance to the sub-
solar point of the magnetosphere. For D =
4.6 X 103 km (dimensions of the obstacle es-
timated above), proton concentration px ^
3 cm-3 and Vx ^ 400 km/s, the value of the
magnetic moment of Mars Mm ~ 2 X 1022
gauss cm3.
The magnetic measurements performed on
the Mars 2 and Mars 3 satellites by Dolginov
et al. (ref. 11) led the authors of these mea-
surements to the conclusion that Mars might
have its own magnetic field, and our estimates
of the value of magnetic moment do not con-
tradict the estimates of Mm made by Dol-
ginov et al. (ref. 11) on the basis of the
magnetic measurements.
One factor in favor of the existence of a
Martian magnetosphere is the characteristic
variations in the plasma and magnetic field
as the spacecraft enter the obstacle. As the
Mars 2 and Mars 3 spacecraft crossed the
boundary of the obstacle, they observed an
increase in the magnetic field modulus along
with a drop in the electron flux. This be-
havior of the plasma and magnetic field is
characteristic of the magnetosphere of Earth
and non-characteristic of Venus which, ap-
parently, has no magnetic field of its own
(the plasma fluxes and magnetic field
changed synchronously as Venera 4 ap-
proached the surface of Venus (ref. 12).
Let us examine changes in the charact-
eristics of the plasma as Mars 5 entered the
obstacle. Figure 3 shows the trajectory sec-
tors of Mars 5 near the planet in coordinates
X, \/Y2 + Z2 during two successive revolu-
tions of the satellite around the planet (Feb-
ruary 13 and 14, 1974). The roman numerals
and various arbitrary symbols show the
three characteristic zones intersected by the
spacecraft, for which typical spectra are
shown in figure 1. The first two zones cor-
respond to the solar wind (fig. la) and the
transition zone (fig. Ib). Zone III (fig. Ic)
corresponds to entry of Mars 5 into the ob-
stacle. For Zone III, a sharp drop in ion cur-
rents in comparison with Zones I and II and
a decrease in the fluxes of electrons and their
temperatures in comparison with Zone II are
characteristic. However, the electron currents
and temperatures in Zone III are higher than
in the unperturbed solar wind in Zone I.
Simultaneous magnetic measurements on
Mars 5 (ref. 13) also showed the presence
of three zones with mutually different char-
acteristics, for which the boundaries of the
zones according to our measurements and
the magnetic measurements coincide. On the
basis of magnetic data, there were no strong
fluctuations of the magnetic field in Zone III,
as characteristic of Zone II, and the pre-
dominate component of the magnetic field is
directed along the Mars-Sun line.
X,/oV< 'i Z 0 -2 -<> -6 -8 -JO -12
0 -2 -1! -6 -8 -10 -12X.V
Figure 3.—The trajectory sectors of Mars 5 near the
planet in coordinates X, V Y* + Z' for two succes-
sive revolutions of the satellite around the planet
(13 and 14 February 1974). The roman numerals
and various arbitrary symbols show the three char-
acteristic zones intersected by the spacecraft, for
which typical spectra are shown in figure 1.
THE MAGNETIC FIELD OF MARS 863
Thus, variations in plasma and magnetic
field recorded by Mars 5 also indicated that
the obstacle at Mars is a magnetosphere, al-
though Mars 5, due to its orbit, apparently
recorded the tail of the Martian magneto-
sphere.
Plasma Zone III bounded by the transition
zone behind the bow shock front, may be
similar either to the boundary layer in the
tail of the terrestrial magnetosphere (ref.
14) or its plasma layer. Comparison of the
electron and ion spectra indicates that in
this zone the ions are apparently highly iso-
tropic. This allows us to consider Zone III
to be more probably a plasma layer in the
magnetosphere of Mars rather than its boun-
dary layer. In the boundary layer, as we know
from observations near Earth, the essential
antisolar component of the directed velocity
of the ions is retained. Of course, one should
keep in mind the possibility that terrestrial
analogues have, in general, little application
to the physical phenomena observed on Mars
in these experiments, due to essential differ-
ence in the relative role of internal and ex-
ternal sources in the Martian magnetic field.
12. DOLGINOV, SH. SH., L. N. ZHUZGOV, AND YE. G.
YEROSHENKO, Kosmich. Issled., Vol. 6, 1968,
p. 561.
13. DOLGINOV, SH. SH., YE. G. YEROSHENKO, L. N.
ZHUZGOV, AND V. A. SHAROVA, The Magnetic
Field of Mars According to the Data of the
Mars 5 Spacecraft. This collection, paper 256.
14. ALASOFU, S. I., AND E. W. HONES, JR., ET AL., J.
Geophys. Res., Vol. 78, 1973, p. 7257.
References
1. GEINGAZ, K. L, AND V. V. BEZRUKIKH ET AL.,
Kosmich. Issled., Vol. 12, 1974, p. 430.
2. DRYER, M., AND G. R. HECKMAN, Solar Phys.,
Vol. 2, 1967, p. 112.
3. SPREITER, J. R., AND A. Y. ALKSNE, Rev. of
Geophys., Vol. 7, 1969, p. 11.
4. GRINGAUZ, K. I., AND V. V. BEZRUKIKH ET AL.,
Kosmich. Issled., Vol. 12, No. 4., 1974. In
press.
5. HESS, W. N., The Radiation Belt and Magneto-
sphere, 1972, p. 216.
6. SPREITER, J. R., AND A. W. RIZZI, Planet. Sp.
Sci., Vol. 20, 1972, p. 205.
7. KLIORE, A. J., AND D. L. CAIN ET AL., Science,
Vol. 149, 1965, p. 1234.
8. KLIORE, A. J., AND G. FJELDBO ET AL., Science
Vol. 175, 1972, p. 313.
9. KOLOSOV, M. A., AND N. A. SAVICH ET AL., Radio-
tekhn. I Electron., Vol. 18, 1973, p. 2009.
10. WEISBERG, 0. L., AND A. V. BOGDANOV, Kosmich.
Issled., Vol. 12, 1974, p. 296.
11. DOLGINOV, SH. SH., YE. G. YEROSHENKO, AND
L. N. ZHUZGOV, DAN SSSR, Vol. 207, 1972,
p. 1296.


The magnetic field of Mars estimated from the data of plasma measurements by Soviet artificial satellites of Mars

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