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Adv. Space Res. Vol. 26, No. IO. PP. 1599-1608.2000 0 2000 COSPAR. Published by Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0273-I 177/00 $20.00 + 0.00

PII: SO273-1177(00)00099-5

OUR CURRENT UNDERSTANDING OF THE IONOSPHERE OF MARS

H. Shinagawa

Solar-Terrestrial Environment Laboratory, Nagoya University, 3-13 Honohara, Toyokawu 442-8507,

Japan

AIXI’RACT

Despite a number of observations near Mars, the structure and dynamics of the ionosphere of Mars have

not been fully understood mainly because of an insufficient amount of magnetic field data within the

ionosphere. In 1997, Mars Global Surveyor (MGS) successfully observed magnetic fields within the

ionosphere for the first time. MGS discovered fairly strong (B I 1600 nT) but localized magnetic fields

in various regions on the Martian surface. Those magnetic fields are interpreted as crustal magnetic

anomalies. At the same time, strength of a global intrinsic magnetic field of core origin was found to be

smaller than about 5 nT at the surface of Mars. A Venus-like ionospheric magnetic field induced by the

solar wind was also seen in the ionosphere of Mars. The results suggest that the Martian ionosphere is

controlled both by solar wind interaction and by the crustal magnetic field. Therefore, the nature of the

Martian ionosphere is probably different from any other planetary ionospheres, and is likely to be most

complicated among the planetary ionospheres. Current understanding of the ionospheres of Mars is

reviewed, and outstanding problems with the Martian ionosphere are pointed out. o 2000 COSPAR.

Published by Elsevier Science Ltd. All rights reserved.

INTRODUCTION

While a number of electron density profiles of the Martian ionosphere have been obtained by early radio

occultation measurements (Kliore et al., 1972, 1973; Kliore, 1992; Zhang et al., 1990), measurements

of the ionospheric magnetic field had not been made until the Mars Global Surveyor (MGS) arrived at

Mars in 1997 (AcuAa et al., 1998). Several pre-MGS studies had already shown that the topside

ionospheric thermal pressure (Pi) is not large enough to hold the average solar wind dynamic pressure

(P,,), suggesting that either an intrinsic or a solar wind-induced magnetic field must be present in the

ionosphere of Mars to compensate for the missing pressure (Hanson and Mantas, 1988). Although a lot

of efforts have been made to determine the source of the ionospheric magnetic field at Mars, no

conclusive result had been obtained because of lack of in situ measurement of magnetic field within the

ionosphere.

1599

1600 H. Shinagawa

The magnetometer and electron reflectometer (MAG/ER) on MGS have obtained magnetic field and

plasma data throughout the near-Mars environment, from the solar wind to the bottom of the ionosphere

(- 110 km). It was found that the surface magnetic field of core origin is likely to be smaller than about 5

nT, indicating that the global intrinsic magnetic field at Mars should not play an important role in the

solar wind interaction with Mars. Instead, MGS detected fairly strong (B I 1600 nT) magnetic

anomalies which are considered to be of crustal origin (Acufia et ul., 1999). It is likely that the

ionosphere of Mars is significantly affected both by the crustal magnetic field as well as by solar wind

interaction.

In the following sections, observations and theoretical studies on (1) ion and electron densities, (2)

dynamics, (3) energetics, and (4) ionospheric magnetic field, are briefly reviewed and discussed based

on pre-MGS studies and the initial reports of the MGS mission (Acufia et al., 1998, 1999).

ION AND ELECTRON DENSITIES

Electron densities on the dayside ionosphere of Mars were measured by the radio occultation

measurements of a series of Mariner and Mars missions, and the Viking 1 and 2 orbiters. Pre-MGS

studies on the electron density profiles (cf. Kliore et al., 1973; Whitten and Colin, 1974; Zhang et al.,

1990) show: (i) the peak electron density and the plasma scale height increase with solar activity, (ii)

most of the electron density profiles exhibit relatively constant scale height with altitude, (iii) the peak

altitude of the electron density increases with solar zenith angle (SZA), (iv) the peak electron density

altitude at the time of the Mariner 9 primary mission was higher by about 20 km than the other periods,

suggesting heating of the lower atmosphere by the global dust storm, and (v) the electron density

decreases rapidly with SZA near the terminator and attain very small values (-103 cm-3). Obviously,

some of the above characteristics of the ionosphere are associated with variation of the thermosphere.

The only ion composition data of the Martian ionosphere were obtained by in situ measurements of the

Retarding Potential Analyzer (RPA) on the Viking 1 and 2 landers (Hanson et aZ., 1977). The profiles

of the measured densities of O+, Oz+, COT+, and electron obtained by Viking 1 are shown in Figure 1

along with the electron densities obtained by theoretical models. During the daytime, the major ion is

02+ throughout the ionosphere, and 0+ becomes comparable near 300 km. Like the ionosphere of

Venus, CO2+ is produced by the photoionization of COT, and COT+ is quickly converted to 02+ through

the reaction: CO2+ + 0 + 02+ + CO. Therefore, the major ion is 02+ near the ionospheric peak. The

observed ion densities in the lower region (< 200 km) can be explained by photochemical equilibrium, if

the atomic oxygen density (not measured) is chosen appropriately (Hanson et al., 1977; Chen et al.,

1978; Fox, 1993).

Above about 200 km, the photochemical model breaks down. According to the Viking observations,

02+ is still dominant even at high altitudes, while in photochemical equilibrium 0+ should become the

major ion above 250 km. Although the production rate of 02 + is practically negligible compared with

the production rate of 0+ above 250 km, the 02+ density is still larger than the 0+ density even in the

topside ionosphere. This fact suggests that there is an upward ion transport from the lower ionosphere

to the upper ionosphere. Indeed, good agreement between the observed and calculated profiles of the

Ionosphere of Mm 1601

ion densities in the topside region can be obtained only when a large upward diffusive transport is

assumed (Chen et al., 1978; Fox, 1993). A one-dimensional (1-D) ionospheric model of Mars

developed by Shinagawa and Cravens (1989, 1992) roughly reproduces the observed ion density

profiles, and the results suggested that an intrinsic magnetic field at Mars is insignificant. However,

there are a few discrepancies in ion composition at high altitudes: (1) in their model, the O+ density is

larger than the 02+ density at above 250 km, (2) the calculated O+ density is larger than the observed

O+ density at above 250 km, (3) when artificial ion loss through divergence of horizontal convection is

introduced, 02+ and electron densities tend to become significantly smaller than the observed density.

.

‘, Ne (diff. eq.)

DENSITY (cme3)

Fig. 1. Major ion densities (solid lines) and electron density (long dashed line) in the ionosphere of

Mars obtained by the Viking 1 lander. The electron density profile obtained by a 1-D model in diffusive

equilibrium at the upper boundary is indicated as a dotted line. The electron density profiles at SZA=45”

under the condition of P,, > Pi, obtained by a 2-D MHD model (Shinagawa and Bougher, 1999) and by

a 3-D MHD model of Tanaka (1998) under the same conditions, are shown for comparison. The

original profile of Tanaka (1998), which is given by normalized values, is resealed so that the peak

electron density agrees with the observed value.

1602 H. Shinagawa

Shinagawa and Bougher [ 19991 studied the ionosphere of Mars, using a two-dimensional (2-D) single-

fluid MHD model of the solar wind interaction with Mars. The calculated electron density decreases

rapidly with altitude, which disagrees with the typical electron density profile observed at Mars (Figure

1). Most of the observed electron density profiles of Mars show relatively extended ionosphere, and no

clear ionopause-like structures have been found except a few cases (cf. Kliore, 1992; Zhang et al.,

1990). MGS also did not observe large gradient in the electron density (Acufia et al., 1998).

Tanaka (1998) studied the solar wind interaction with the ionospheres of non-magnetized planets for two

different solar wind conditions using a three-dimensional (3-D) MHD model including two-component

plasma, i.e., the solar wind ions and the ionospheric ions. It was found that a fairly sharp ionopause is

formed at a certain altitude. The height of the ionopause depends on Psw as well as SZA. When Psw is

larger than Pi, the ionopause is located at low altitudes. The result of Tanaka (1998) is basically

consistent with the result obtained by a high-resolution 2-D MHD model of Shinagawa and Bougher

(1999) (Figure 1). Although the MHD models could reproduce the Venus ionosphere reasonably well,

both 2-D and 3-D MHD models fail to reproduce the observed electron density of Mars. The results

suggest that dynamics of the Martian ionosphere are different from dynamics of the Venus ionosphere.

Since the crustal magnetic field discovered by MGS might affect ion transport process, and signilicantly

alter the ion and electron density profiles, future modeling studies need to include the effect of the crustal

magnetic field in a realistic manner.

DYNAMICS

Since the plasma velocity in the ionosphere of Mars has not been measured, dynamics of the ionosphere

have been investigated only theoretically on the assumption that the Martian ionosphere is basically the

same as the Venus ionosphere. If the solar wind interaction with Mars is analogous to the solar wind

interaction with Venus, a strong convection is likely to be driven by the pressure gradient force or the

electromagnetic forces driven by the solar wind. The magnitude of the convection depends on solar

wind and ionospheric conditions. When the solar wind dynamic pressure exceeds the ionospheric

thermal pressure, i.e., Psw > Pi, the convection velocity in the topside ionosphere is expected to be

larger than 10 km/s (e.g., Tanaka, 1998). Even when P sw is less than Pi, a day-to-night convection of

as much as a few km/s could be driven at high altitudes by the pressure gradient of the plasma pressure.

Under the condition of Psw > Pi, 2-D and 3-D MHD models predict that vertical motion is downward in

most of the dayside region and changes to upward near the terminator region (Tanaka, 1998; Shinagawa

and Bougher, 1999).

However, the Venus-type scenario might not be applicable to the Martian ionosphere. The complex

magnetic field morphology at Mars could signilicantly affect the ionospheric dynamics. There might be

interaction between the ionospheric plasma, the crustal magnetic field, and the solar wind-induced

magnetic field. Furthermore, global thermospheric wind, and tidal and gravity waves from the lower

atmosphere might also influence the ionospheric structure and dynamics through ion-neuual drag and

possibly through dynamo electric field. In order to better understand the dynamics of the ionosphere, it

is necessary to study the effects of: (1) solar wind, (2) crustal magnetic tield, (3) rotation of Mars, (4)

neutral density and composition, and (5) thermospheric wind.

Ionosphere of Mars I603

ENERGETICS

Two very similar daytime ion temperature profiles were obtained by RPA on the Viking landers (Hanson

et al., 1977). The electron temperature profiles have been also obtained by analyzing the RPA data

(Hanson and Mamas, 1988). The ion and electron temperature profiles were also theoretically calculated

by Chen et ~1. (1978). The results indicate that a heat inflow into the ion gas from the topside

ionosphere, which is probably related to the solar wind-ionosphere interaction, needs to be assumed to

reproduce the observed ion and electron temperature profiles. Johnson (1978) calculated ion and

electron temperatures assuming the presence of an ionospheric magnetic field, and showed that the

observed ion temperature profile could be explained by introducing a heat source at high altitudes and a

magnetic field with small dip angle (-10 deg). Rohrbaugh et al. (1979) constructed a model of the ion

and electron temperatures including the effect of energetic 02+ as well as a nearly horizontal magnetic

field. They demonstrated that the thermalization of the energetic 02+ can greatly increase the ion

temperature above 200 km. The combination of the heating by the energetic 02+ and the effect of the

magnetic field could provide a fairly good agreement with the Viking 1 measurements below 250 km. A

magnetic field with small dip angle (-10 deg) could provide electron temperatures as high as the

observed values because of the resulting reduction of thermal conductivity. Previous studies indicate

that plasma temperatures of the Martian ionosphere are likely to be influenced by the heat flux from the

topside region and by the ionospheric magnetic field. The energetics of the ionosphere need to be

reexamined including effects of the crustal magnetic tield.

IONOSPHERIC MAGNETIC FIELD

Although the magnetic field in the vicinity of Mars has been measured a number of times, no irr sim

measurements in the ionosphere had been made until MGS measurements (Acuiia rt al., 1998, 1999).

The magnetic field structure observed by MGS implies that the solar wind is interacting directly with the

Martian ionosphere. Draping of the interplanetary magnetic field around the ionospheric obstacle

presented by Mars is clearly seen. The decrease in magnetic field magnitude within the ionosphere and

the low-altitude magnetic field has been also observed, which is a typical characteristic of the solar wind-

Venus interaction. On many occasions, a very small magnetic field (B 5 5 nT) was observed below the

ionospheric peak, indicating that any global-scale magnetic field of core origin is less than an equatorial

field strength of about 5 nT. This result represents an upper limit for a Mars dipole moment of -2x1021

G cm3, about a factor of 5 or 10 smaller than the most recent estimates derived from the Phobos results

(Riedler et al., 1989).

Several MHD models have been developed assuming that the solar wind-Mars interaction is the same as

the solar wind-Venus interaction. Figure 2 shows the magnetic field profiles obtained by three kinds of

MHD models, i.e., 1-D model (Shinagawa and Cravens, 1989), 2-D model (Shinagawa and Bougher,

1999), and 3-D model (Tanaka, 1998). The magnetic field profiles appear to be similar to one obtained

by the MGS magnetometer (Acuna et al., 1998), although the MGS data is obtained near the terminator.

Presence of a magnetic field in the lower ionosphere suggests that a magnetic flux is transported

downward by convection and diffusion processes, which is the case with the Venus ionosphere.

1604 H. Shinagawa

Recent analysis of the MGS magnetic field data shows fairly strong multiple magnetic anomalies of

small spatial scale at Mars (Acufia et al., 1999). The magnitude of the magnetic tield reaches nearly

1600 nT in several regions in the southern hemisphere, while the crustal magnetic field appears to be

insignificant in the northern hemisphere. Such complicated magnetic fields could strongly intluence the

global structure and dynamics of the Martian ionosphere. There might be interaction between the crustal

magnetic field and the solar wind-induced magnetic field. The ionospheric magnetic field is also coupled

with the plasma dynamics. The interaction process could modify the structure of the magnetic field,

which changes the ionospheric dynamics. Further analysis of the magnetic field data as well as realistic

modeling of the ionosphere are necessary to understand the ionospheric structure.

360 -

320 -

$ - 280 -

k!i

? i=

240 -

d 200 -

160 -

0 10 20 30 40 50 60 70 60

Fig. 2. Ionospheric magnetic fields obtained by three kinds of MHD models. 1-D model (Shinagawa

and Cravens, 1989), 2-D model (Shinagawa and Bougher, 1999), and 3-D model (Tanaka, 1998). A

larger magnetic field at the upper boundary is assumed in the 1 -D model. The original result of Tanaka

(1998), which is given by normalized values, is resealed so that the magnetic field at 400 km becomes

about 30 nT. It is noted that magnetic diffusion at the bottom of the ionosphere is not included in the 3-

D model, which results in an enhanced magnetic filed below 150 km.

Ionosphere of Man 1605

DISCUSSION

It has been recognized that the ionospheric peak plasma pressure at Mars observed by the Viking landers

is smaller than the average Psw, indicating that the ionospheric magnetic field must be present to

compensate for the missing pressure at least under the Viking conditions, i.e., solar minimum (Hanson

and Mamas, 1988). Since the surface magnetic field of a global intrinsic magnetic field is smaller than

about 5 nT (Acuiia er al., 1998), strong convection is expected to be driven in the ionosphere if the

crustal magnetic field does not make any contribution to the dynamics. In such a case, the ionopause is

created by combination of downward motion and horizontal transport process, as was known for the

Venus ionosphere (Cravens and Shinagawa, 1991; Shinagawa, 1996; Tanaka and Murawski, 1997;

Tanaka, 1998). Krimskii et al. (1995) also pointed out that the scale height of the Martian ionosphere

should be considerably smaller than the observed scale height, if the Venus-like convection is driven in

the Martian ionosphere. Indeed, the vertical electron density profiles of the Martian ionosphere obtained

by 2-D and 3-D numerical models tend to show a fairly sharp ionopause (Tanaka, 1998; Shinagawa and

Bougher, 1999). However, the radio occultation measurements made by the Mariner and Viking

spacecraft indicate that most of the observed electron density profiles have fairly constant and relatively

large plasma scale height at all altitudes, exhibiting no clear ionopause structure (Kliore, 1992; Zhang et

al., 1990). MGS also did not observe large gradient in the electron density (Acufia et ~1.. 1998).

solar wind

convection downward convection

nernendicular to B ‘--%-&EE%g..;:n.B?

> o+

\ 2 ‘, reconnection?

atmosphefic waveS :.:.:.:.: ..................................................... . .................................

....... ..:.:.:.: ............. .................. . MARS :.:

..................... .:.:.:.:.:,:.:.:.:.:.:.:.:.:.:.:.:.:.:.:., ................. 1.; .............................. . ......... . .... .........................................................

. .... . .... . .... . .... . ......

................................................................................. ..........................................................................

................................................................................. ....................................... ...............................................

ion outtlow?

nightside ionosphere?

Fig. 3. Schematic picture of possible processes in the ionosphere of Mars.

As was mentioned earlier, ion composition and the electron density of the topside ionosphere cannot be

reproduced by MHD models which assume interaction of the solar wind-Venus ionosphere. In the

I606 H. Shinagawa

Venus ionosphere under the condition of Psw > Pi, magnetic field lines of the solar wind are transported

from the topside region down to the bottom of the ionosphere. If the magnetic field lines are purely

horizontal in the ionosphere, the upward ion transport suggested by Chen et al. (1978) should be

significantly inhibited. However, if the magnetic field lines have some vertical component, a modest

amount of upward ion transport along the magnetic field lines might be possible (Figure 3). In such a

case, 02+ density dominates the O+ density even at high altitudes, and the electron density in the upper

ionosphere becomes larger than the one for the case with the purely horizontal magnetic field.

There is a possibility that the crustal magnetic field significantly alters the configuration of the

ionospheric magnetic field at least in some regions. A global magnetic field composed of the

accumulated effect of a large number of anomalies could also become important (Acufia et al., 1999),

and rotation of Mars would make the magnetic structure even more complicated. Besides the processes

mentioned above, the ionosphere of Mars might be influenced by gravity waves and tidal waves from the

lower atmosphere (Bougher et al., 1993, 1997; Bougher and Shinagawa, 1999; Keating et ul., 1998).

The magnetic field configuration at Mars appears to be very complicated, and more sophisticated

numerical models are necessary to examine the relative importance of various processes.

SUMMARY

MGS provided us with vety important information on the structure of the Martian ionosphere. What we

learned about ionospheric processes from the observations of MGS are as follows:

1. The global intrinsic magnetic field is very small, and does not play an important role in the Martian

ionosphere,

2 _ The solar wind-induced magnetic field is present in the ionosphere of Mars,

3. A number of fairly strong small-scale magnetic structures, which are thought to be of crustal origin,

are present in the ionosphere in various regions of Mars.

Taking the above results into account, the following major problems need to be solved in the future:

1. What kinds of processes control the dynamics of the Martian ionosphere‘!

2. Why is a Venus-like ionopause rarely observed in the Martian ionosphere’!

3. Does the crustal magnetic field significantly affect the ionosphere?

4. What kinds of physical processes control the plasma temperatures?

5. How does rotation of Mars affect the ionosphere‘?

6. What are the effects of lower atmospheric processes on the ionosphere‘!

7. Does the thermospheric wind significantly affect the ionosphere‘!

8. Is there any nightside ionosphere at Mars. 7 If so, what is a primary ionization source on the

nightside?

Obviously, more data of the upper atmosphere of Mars as well as various kinds of numerical models are

necessary. According to the present plan of Mars exploration, NOZOMI (PLANET-B) and Mars

Express will arrive at the Mars orbit around 2004, and will extensively observe the Martian plasma

environment and the upper atmosphere. These missions will give us a lot of information on the

ionosphere of Mars, and hopefully, will lead to solution of the above problems.

Ionosphere of Mars 1607

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