Dual frequency radio sounding of the Martian nighttime ionosphere was carried out during the exits from behind the planet of the Mars-4 spacecraft on February 2, 1974 and the Mars-5 spacecraft on February 18, 1974. In these experiments, the spacecraft transmitter emitted two coherent monochromatic signals in decimeter and centimeter wavelength ranges. At the Earth receiving station, the reduced phase difference (or frequencies) of these signals was measured. The nighttime ionosphere of Mars measured in both cases had a peak electron density of approximately 5 X 1,000/cu cm at an altitude of 110 to 130 km. At the times of spacecraft exit, the solar zenith angles at the point of occultation were 127 deg and 106 deg, respectively. The height profiles of electron concentration were obtained assuming spherical symmetry of the Martian ionosphere

Samovol, V. A.

Samoznaev, L. N.

Savich, N. A.

Shtern, D. Y.

Sidorenko, A. I.

Vasilyev, M. B.

Vyshlov, A. S.

English

NASA Technical Reports Server

THE NIGHTTIME IONOSPHERE OF MARS FROM MARS-4 AND MARS-5 
RADIO OCCULTATION DUAL-FREQUENCY MEASUREMENTS 
N. A. Savich, V. A. Samovol, M. B. Vasilyev, A. S. Vyshlov, 
L. N. Samoznaev, A. I. Sidorenko, and D. Ya. Shtern 
Institute o f  Radio Engineering and Electronics, Academy of Sciences 
Moscow, USSR 
ABSTRACT 
Dual-frequency radio sounding of the Martian nighttime ionosphere was carried out during 
the exits from behind the planet of the Mars-4 spacecraft on February 2, 1974 and the 
Mars-5 spacecraft on February 18, 1974. In these experiments, the spacecraft transmitter 
emitted two coherent monochromatic signals in decimeter (A, % 32 cm) and centimeter (h, 
x 8 cm) wavelength ranges. At the Earth-receiving station, the reduced phase difference (or 
frequencies) of these signals was measured. 
The nighttime ionosphere of Mars measured in both cases had a peak electron density of 
-5 X 1 O3 /cm3 at an altitude of 1 10 to 130 km. At the times of spacecraft exit, the solar 
zenith angles at the point of occultation were 127' and 106", respectively. The height pro- 
files of electron concentration were obtained assuming spherical symmetry of the Martian 
ionosphere. 
Moreover, the results obtained allowed the conclusion that above the main maximum there 
is an additional one and also that plasma possibly exists at low altitudes above the planetary 
surface. However, these conclusions require experimental confirmations. 
INTRODUCTION 
Dual-frequency radio sounding of the Martian nighttime atmosphere above the dark surface 
of the planet was carried out during exits from behind the planet of the Mars4 and Mars-5 
spacecraft on February 10 and on February 18, 1974, respectively. The main aim of these 
experiments was the detection of the Martian nighttime ionosphere and the determination of 
the height profile of electron concentration. 
At exit, the solar zenith angle at the point of contact of the radio beam with the planetary 
surface was x % 127"; the areographic coordinates of this point were latitude O % 9"S, 
longitude A x 236"W, and local time -3h30m. The season in this Martian region was 
autumn. The second exit was in spring at a solar zenith angle of 106" with coordinates 
O a 38"N, A % 214"W, and local time 4h30m. 
https://ntrs.nasa.gov/search.jsp?R=19760018040 2020-03-22T14:07:09+00:00Z
METHODOLOGY 
During these experiments, the spacecraft transmitters emitted two coherent monochromatic 
signals in decimeter (h, 32 cm) and centimeter (A2 = 8 cm) wavelength ranges where the 
frequency ratio is f2/fl = 4. Each signal was separately received at the Earth-based point 
and was processed by two independent systems of a dispersion interferometer [ 1,2] which 
were essentially modified for increase of reliability and accuracy of measurements. In the 
system [ 1 1, the reduced phase difference of received signals was recorded and measured by 
the analog technique: 
where $, and $, are the total phases of decimeter and centimeter signals, respectively. In 
the system, the two received signals are initially recorded on magnetic tape and then processed 
by a digital computer. In this case, after digital filtering in the band Af, % 0.8 Hz, the fre- 
quency of each signal was measured and a reduced frequency difference was calculated as 
where il and 4, are the time derivatives of the total phases of the received signals. 
For decreasing the fluctuation enors of measurements, values of A\k(t) and Af(t) were 
smoothed by a running average method over 11 points. Five minutes later, after exit of the 
station from behind the planet, in the control part of the mission free from the influence 
of the Martian ionosphere, the mean values and nfcaused by electron concentration chan- 
ges along the radio wave propagation path were measured. For separation of the effects 
caused only by the Martian ionosphere, the mean values were extrapolated backward to the 
surface and subtracted from the results of the measurements. The measured values of A\k(t) 
and Af(t) as a function of the height, h, of the radio beam above the Martian surface were 
obtained from the trajectory data and from the diffraction pattern of the change of signal 
amplitude which was found at exit of the spacecraft from behind the planet. 
RESULTS 
The variation of the values A\k and Of with time, measured from the moment of exit of 
Mars-4 on February 10, 1974 and with height h [31, are given in figure 1. The curve of Af is 
of the characteristic S-shape, and the absolute value Af does not exceed 0.01 6 Hz. The initial 
value, Aq(O), at the moment of exit was assumed to be equal to zero. Since A\k is in pro- 
portion to the integral electron concentration, the height shape A\E(h) shows a change of 
AN(h) in the ionosphere sampled. The maximum increase, AN, ,, - 6 X 10" /cm2, cor- 
responds to a maximum change of A\kmax - 320'. On a control part of the mission, the 
mean-square fluctuation enors of measurement of the values A* and Af caused by receiver 
noise and by electron concentration variations along the path of communication were esti- 
mated to be a,, x 14', and a,, % 0.003 Hz. 
Figure 1. Dependence of reduced frequency (a) and phase (b) differences on the distance 
between the radio beam and the Martian surface. 
The height profile of the electron concentration, N(h), in the nighttime Martian ionosphere 
is given in figure 2. It was calculated from the data obtained using the assumption of 
spherical symmetry of the ionosphere sampled. The mean-square error, o,, is slightly de- 
pendent upon the height and is about 250/cm3. A possible displacement of the curve N(h), 
due to errors of approximation of the mean shape of the integral electron concentration 
changes along the path of communication at the time of radio sounding, was estimated to be 
AN x +500/cm3. 
The profile N(h) at the exit, February 10, 1974, has the main peak at a height of -1 1.0 km 
above the surface with the electron concentration Nm x 4.6 X 103/cm3. A regular profile 
shape is followed up to the height of -250 km, where the measured values become compara- 
ble with errors of measurements. An additional ionization peak on this profile, at the height 
of -1 80 km with concentration -2.2 X 1 O3 /cm3 and a formally-obtained value of plasma 
concentration -103/cm3 in the 0- to 80-km height range, are seen in figure 2. 
The second exit was under unfavorable meteorological conditions. The reception of signals 
in the centimeter wave range was followed by deep amplitude fluctuations, almost up to the 
noise level. Recording of signals was accomplished only by a system with magnetic tape. 
The technique of processing the recorded information is similar to that described above. 
The results of this measurement are less reliable than those of the previous one but consi- 
dering them together, one can draw some conclusions. 
February 10,1974 
February 18,1974 
Figure 2. Distribution of electron concentration, N(h), in the nighttime 
ionosphere of Mars. 
The profile N(h) obtained for this exit is also given in figure 2. The electron concentration 
for the main peak is located at a height of -130 km and is -5 X 103/cm3. At the level of 
-2 10 km, a secondary peak is noticed with an electron concentration -1.5 X 1 O3 /cm3. 
At heights higher than 210 km, the concentration sharply decreases and higher than 220 km, 
the regular profile shape was not practically followed. In the 0- to 80-km height interval, 
the data obtained also show the possible existence of a plasma with a concentration of 
-1.9 X 103/cm3. 
DISCUSSION 
Martian Ionosphere in the Nighttime 
Given the above experimental data for the two sets of measurements, the existence of a 
Martian ionosphere in the nighttime is indicated. The maximum values of the electron 
concentration in both cases are almost equal (-4.6 X 103/cm3 and -5 X 103/cm3), but 
the heights of these maxima are different (1 10 and 130 km). Repeated checking of calcula- 
tions of height determination has confirmed this height separation. 
It can be proven that the nighttime ionosphere of Mars cannot be considered as a part of the 
daytime ionosphere. Really, the process of decrease of electron concentration in the iono- 
sphere after sunset is characterized by the time constant T = l/aNo, where No is the value of 
the electron concentration at the moment of "switching-ofr' the source of ionization and 
a = 2.55 X 1 0-"/cm3 s is the effective coefficient of recombination. If we take the mini- 
mum value of No - 104/cm3 for an estimate, T is 400 s. It is obvious that with this value, 
the charged particles of any ionospheric plasma must practically be recombined completely 
within several hours after sunset. Apparently it is necessary to suppose the presence of an 
additional source of ionization, not connected directly with solar radiation, to explain the 
fact of the existence on Mars of a nighttime ionosphere. 
Additional Maxima of Ionization 
The additional maxima of ionization, located at the heights of -190 and -210 km, are 
seen on both profiles. Besides the high-altitude separation between them and the lower alti- 
tude main maxima, the difference of 20 km coincides with the height distance between the 
main maxima. In our opinion, these circumstances increase the probability of the supposi- 
tion about the reality of the existence of the additional maxima. This conclusion, however, 
needs further confirmation. 
Presence of Near-Surface Plasma 
The conclusion of the possible presence of plasma in the 0- to 80-km height interval was 
made on the basis of the formal inversion using a spherically-symmetric approximation. 
This effect, however, may occur due to other reasons. One of them is the fact that a random 
increase of integral electron concentration along the path of communication has the effect 
of the apparent presence of plasma, for the solution of the inverse problem could take place 
with radio sounding of the region of small heights. However, the possibility of the occur- 
rence of that variation at the necessary time during two exits is unlikely. The similarity of 
both these profiles in the 0- to 80-km interval shows apparently the generality of this effect. 
Such a cause can be the asymmetry of the ionosphere sampled which is special during the 
exit on February 18, 1974. However, quantitative consideration of this problem becomes 
complicated due to the absence of a model, not only of the nighttime, but also the daytime 
Martian ionosphere, without which such an analysis is rather difficult to perform. 
CONCLUSIONS 
Experiments on dual-frequency radio sounding of the atmosphere above the dark Martian 
surface gave us the possibility to detect reliably the nighttime ionosphere with an electron 
concentration -4.6 X 1 O3 /cm3 for the main peak located at a height of 1 10 km above the 
planet's surface. 
Perhaps an additional source of ionization is responsible for the formation of the Mars night- 
time ionosphere. The profiles obtained permit us to suggest conclusions about the presence 
of the additional peak of ionization above the main one and the possible existence of plasma 
at heights below 80 km. These, however, require new experiments in order to confirm the 
tentative conclusions. 
REFERENCES 
1. Savich, N. A. et al., 1973, Kossmicheskiye Issledovaniya, 11, p. 756. 
2. Kolosov, M. A. et al., 1973, Radiotekhnika i Electronika, 18, p. 2009. 
3. Vasilyev, M. B. et al., 1974, DAN USSR, 218, p. 1298. 
4. Ivanov-Kholodyni, G. S. et al., 1973, Uspekhi Phys. Nauk, 11 1, p. 373. 
QUESTIONS 
Sizvich/Ness: How many measurements of the nightside ionosphere were made? Since the 
spacecraft operated for more than two revolutions, as indicated by the data from the mag- 
netometer and plasma devices, I would have thought you'd have more data. 
Savich: We have two measurements of the nightside ionosphere on February 10 and 18. 
Ow instrument was not operating during the other orbits. 
Savich/Bauer: Did you obtain any dayside profiles of the Martian ionosphere? Is the lower- 
most part of the Martian ionosphere uncontaminated by effects from the neutral atmosphere? 
Savich: Yes. We have two profiles of the evening Martian ionosphere with zenith angles of 
about 82" and 72", respectively. The dual-frequency method, to first approximation, ex- 
cludes the effects of the neutral atmosphere on the parameters being measured. 


The nighttime ionosphere of Mars from Mars-4 and Mars-5 radio occultation dual-frequency measurements

The nighttime ionosphere of Mars from Mars-4 and Mars-5 radio occultation dual-frequency measurements

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