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Planet. Space Sci., Vol. 45, NO. 1, 143-148, 1997 pp. 0 1997 Elsevier Science Ltd Pergamon PII: SOO32-0633(96)00092-X Printed in GreatBritain. All rights reserved 0032-0633/97 $17.00+0.00 Mercury’s magnetosphere, exosphere and surface : low-frequency field and wave measurements as a diagnostic tool Lars G. Blomberg AlfvCn Laboratory, Royal Institute of Technology, Stockholm, Sweden Received 25 October 1995; revised 7 May 1996; accepted 7 May 1996 Introduction Mercury is by far the least explored of the terrestrial planets. Hard to observe from the Earth and visited by only one spacecraft, Mariner 10, which made two close and one distant fly-by in 1974-75, our knowledge of this planet is limited. Because of its unique location in the solar system, as an “end-member” of the terrestrial planets, it may hold the key to many of the questions that remain unanswered concerning the formation of the solar system in general and of the terrestrial planets in particular. A general overview of Mercury’s magnetosphere based on the Mariner 10 observations is found in Ness (1979). One of the findings of Mariner 10 was that Mercury, contrary to expectations, does possess a relatively sig- nificant intrinsic magnetic field (e.g. Ness et al., 1975; Jackson and Beard, 1977). The details of the field are not known. The magnetic moment is estimated to be 2- 6 x 10” T m3, although it is not clear whether a dipole is a good approximation. It may be that the quadrupole moment is significant. The existence of an intrinsic field is surprising both because of the very slow planetary rotation and because of the lack of moons. Thus, more detailed information on this intrinsic field will teach us more about planetary magnetism. The intrinsic magnetic field is normally strong enough to deflect the solar wind such that a magnetosphere is formed. It is, however, believed that under extreme con- ditions the solar wind may in fact impinge directly upon the planet. The typical (planetocentric) magnetopause stand-off distance has been estimated from Mariner 10 data to be 1.35) 0.2 RH (cf. Slavin and Holzer, 1979). Mariner 10 also detected magnetic fluctuations in the Hermean magnetosphere which were interpreted as being due to standing AlfvCn waves (Russell, 1989). This implies that either Mercury has a conducting ionosphere or it has a planetary surface which is a relatively good conductor or alternatively a relatively efficient insulator. The con- temporary view is that Mercury does not have any sig- nificant atmosphere and thus no significant ionosphere. Mercury’s near-environment is thought to be exospheric in nature, consisting mainly of hydrogen and helium accreted from the solar wind, and of oxygen, sodium, and potassium released from the planetary surface by meteoroid impacts or photo-sputtering (e.g. Ip, 1986a). If there is no conducting ionosphere, the standing AlfvCn waves are actually bouncing on the (conducting or insulating) planetary surface itself. There may be other possibilities as well, as discussed below. Unfortunately, Mariner 10 did not measure the electric field and it is therefore impossible to know in any detail the true nature of these fluctuations. The compressibility of the magnetosphere under fluc- tuations is also related to the conductivity of the planet. If the surface and body are non-conducting the field lines are anchored at the centre of the planet (or, more precisely, in the dynamo region), whereas with a perfectly con- ducting surface they would be anchored at the surface, thus reducing the magnetospheric compressibility. For the intermediate case the anchor depth would be frequency- dependent and approximately given by the condition that the magnetic Reynold’s number be about unity. This is yet another reason for trying to understand the nature of Mercury’s conductivity.

Mercury's magnetosphere, exosphere and surface: low-frequency field and wave measurements as a diagnostic tool

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Page 1: Mercury's magnetosphere, exosphere and surface: low-frequency field and wave measurements as a diagnostic tool

Planet. Space Sci., Vol. 45, NO. 1, 143-148, 1997 pp. 0 1997 Elsevier Science Ltd Pergamon

PII: SOO32-0633(96)00092-X

Printed in GreatBritain. All rights reserved 0032-0633/97 $17.00+0.00

Mercury’s magnetosphere, exosphere and surface : low-frequency field and wave measurements as a diagnostic tool

Lars G. Blomberg

AlfvCn Laboratory, Royal Institute of Technology, Stockholm, Sweden

Received 25 October 1995; revised 7 May 1996; accepted 7 May 1996

Introduction

Mercury is by far the least explored of the terrestrial planets. Hard to observe from the Earth and visited by only one spacecraft, Mariner 10, which made two close and one distant fly-by in 1974-75, our knowledge of this planet is limited. Because of its unique location in the solar system, as an “end-member” of the terrestrial planets, it may hold the key to many of the questions that remain unanswered concerning the formation of the solar system in general and of the terrestrial planets in particular. A general overview of Mercury’s magnetosphere based on the Mariner 10 observations is found in Ness (1979).

One of the findings of Mariner 10 was that Mercury, contrary to expectations, does possess a relatively sig- nificant intrinsic magnetic field (e.g. Ness et al., 1975; Jackson and Beard, 1977). The details of the field are not known. The magnetic moment is estimated to be 2- 6 x 10” T m3, although it is not clear whether a dipole is a good approximation. It may be that the quadrupole moment is significant. The existence of an intrinsic field is surprising both because of the very slow planetary rotation and because of the lack of moons. Thus, more

detailed information on this intrinsic field will teach us more about planetary magnetism.

The intrinsic magnetic field is normally strong enough to deflect the solar wind such that a magnetosphere is formed. It is, however, believed that under extreme con- ditions the solar wind may in fact impinge directly upon the planet. The typical (planetocentric) magnetopause stand-off distance has been estimated from Mariner 10 data to be 1.35) 0.2 RH (cf. Slavin and Holzer, 1979).

Mariner 10 also detected magnetic fluctuations in the Hermean magnetosphere which were interpreted as being due to standing AlfvCn waves (Russell, 1989). This implies that either Mercury has a conducting ionosphere or it has a planetary surface which is a relatively good conductor or alternatively a relatively efficient insulator. The con- temporary view is that Mercury does not have any sig- nificant atmosphere and thus no significant ionosphere. Mercury’s near-environment is thought to be exospheric in nature, consisting mainly of hydrogen and helium accreted from the solar wind, and of oxygen, sodium, and potassium released from the planetary surface by meteoroid impacts or photo-sputtering (e.g. Ip, 1986a).

If there is no conducting ionosphere, the standing AlfvCn waves are actually bouncing on the (conducting or insulating) planetary surface itself. There may be other possibilities as well, as discussed below. Unfortunately, Mariner 10 did not measure the electric field and it is therefore impossible to know in any detail the true nature of these fluctuations.

The compressibility of the magnetosphere under fluc- tuations is also related to the conductivity of the planet. If the surface and body are non-conducting the field lines are anchored at the centre of the planet (or, more precisely, in the dynamo region), whereas with a perfectly con- ducting surface they would be anchored at the surface, thus reducing the magnetospheric compressibility. For the intermediate case the anchor depth would be frequency- dependent and approximately given by the condition that the magnetic Reynold’s number be about unity. This is yet another reason for trying to understand the nature of Mercury’s conductivity.

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144 L. G. Blomberg: Mercury’s magnetosphere, exosphere and surface

The present paper discusses the diagnostics made poss- ible by measuring the magnetic as well as the electric fields at low frequencies (DC to tens of Hz) on a future Mercury Orbiter mission. Such measurements may yield important information on plasma convection, field-aligned currents, conductivity of the ionosphere or planetary surface and body, magnetosphere-atmosphere interaction as well as on possible substorm related phenomena. Fundamental concepts of the data analysis are briefly discussed.

Spatial or temporal structures?

With spacecraft-based measurements there is always a spatial-temporal ambiguity. What is measured is the rate of change of certain parameters along the trajectory of the spacecraft, i.e. a total derivative. To determine the contribution to this total derivative from temporal and spatial variations, respectively, is fundamentally imposs- ible. However, often one can make educated assumptions which enable a reasonable interpretation of the data at hand. Also, by making statistics, one can find which assumptions give the most consistent picture. This is one major advantage of an orbiter compared to a limited number of fly-bys. When it comes to field measurements it is also a fact that the availability of both the magnetic and the electric field components limits the ambiguity in the interpretation significantly, as will be elaborated on below.

Plasma convection

With a rotation period of 59 (Earth) days and a surface magnetic field strength of some 400nT, the equatorial co-rotation electric field close to the surface would be 1.2 ~Vrn-‘, decreasing with the square of the plane- tocentric distance. Even if the plasma were co-rotating out to some distance, the associated electric field is clearly below the threshold of current instrument technology. (The peripheral velocity of Mercury is only 3 m s-l, so co- rotation could not be detected with a particle instrument either.)

Hill et al. (1976) estimated the upper limit to the con- vection potential available across the Hermean mag- netosphere to be 13 kV for typical solar wind conditions. This corresponds to an average electric field of the order of 2mVm-‘. Russell and Walker (1985) inferred from marine Mariner 10 data a reconnection voltage of 5-25 kV across Mercury’s magnetopause, corresponding roughly to I-5mVm-‘. Convection electric fields below about 0.5 mVm-’ will not be detectable, or rather, dis- tinguishable from background noise. This corresponds to convection velocities of at least 1 kms-‘, depending on planetocentric distance.

However, plasma convection is known to play a sig- nificant role in the dynamics of the terrestrial mag- netosphere. Therefore, it is highly interesting to look for strong convective flows in the Hermean magnetosphere. Such strong flows would be expected in relation to possible substorms on Mercury.

Transient induced electric fields integrating to hundreds

of kV across the tail have been suggested by several authors (e.g. Baker et al. (1986) and references therein) as a mechanism for particle acceleration.

Another place where convective electric fields are of prime interest is in the magnetopause boundary layer, typically located less than a planetary radius above the surface, where they will tell us more about the solar wind- magnetosphere interaction processes operating. Mariner 10 observations have indicated that these processes at Mercury are similar to those at Earth (e.g. Christon, 1989).

A problem with detecting small electric fields in the terrestrial environment is the electric field induced by the spacecraft motion. In the Hermean case, an upper limit to this field is approximately 400nT times a periherm velocity of the spacecraft of, say 4 kms-’ which yields 1.6 mVm-‘. To resolve the weak convection fields, proper v x B subtraction is necessary. For this purpose a spa- cecraft attitude knowledge (all three axes) better than a couple of degrees at all times is needed.

Magnetic-field-aligned currents and electric fields

Using the infinite-sheet approximation a field-aligned cur- rent gives rise to a magnetic field variation of 1.3 nT km-’ across the sheet for a current density of 1 PArn-‘. By careful analysis of the spatial variation of all components of the magnetic vector, information about the actual geometry of the current system may be found. For geo- metries which are not well approximated by an infinite sheet, other quantitative relations hold. Also, along a flux- tube the current density scales inversely with the flux-tube cross-sectional area, or in other words, it scales directly with the magnetic field strength, assuming that there is no “leakage” but that all current flows parallel to the mag- netic field. Thus, the inferred currents must be scaled to a common altitude for comparison or statistics.

Field-aligned currents are a fundamental mediator of energy and momentum between different conducting regions. In the case of the Earth, field-aligned currents transfer energy from the magnetopause through the mag- netosphere and into the ionosphere. In the regions where these currents close across B momentum is exchanged. Whether field-aligned currents play a similar role in the Hermean environment depends on the conductive proper- ties of the exosphere or planetary surface. If Mercury has an electrically conducting layer at low altitude, be it an ionosphere or the planetary surface itself, then field-alig- ned currents are likely to close in this conducting layer. Of course, if the conductivity is infinite then no Ohmic dissipation will occur, but this is not realistically the case. If there is no (or little) Ohmic dissipation it is more likely that the reason is that the planetary surface is a good insulator, in which case there will be no (or weak) field- aligned currents.

Whether field-aligned currents can close at low altitude or not also has important consequences for possible aur- ora at Mercury, assuming, of course, that there are enough atmospheric particles that can be excited. At Earth, for example, magnetic-field-aligned electric fields are estab- lished to maintain current continuity in the coupled mag- netosphere-ionosphere system. These parallel fields are

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L. G. Blomberg: Mercury’s magnetosphere, exosphere and surface

necessary to have the charge carriers overcome the mag- netic mirror force presented by a converging magnetic field, and thus allow sufficient current to flow. At the same time as maintaining the current flow by pitch-angle redistribution, the particles (electrons) are also energized to typical aurora1 energies. If there is a conducting layer at low altitude on Mercury a similar process is conceivable. However, if the conductivity should be very low, no par- allel currents would flow, and thus, no parallel electric fields would be necessary for current continuity.

The existence of parallel fields in the Hermean mag- netosphere is a question that could be answered by an electric field instrument on an orbiter. The parallel com- ponent is difficult to measure directly, but by making statistics of the perpendicular field it should be possible to determine the existence of U-shaped potential structures. Depending on the low-altitude current closure mech- anism, very strong electric fields similar to the ones recently found by Freja in the terrestrial ionosphere (Marklund et al., 1995) may exist also at Mercury.

145

surface itself, no pronounced day-night asymmetry is to be expected, assuming that the coupling between the sur- face and the surrounding medium does not depend stron- gly on photo-processes. This is not necessarily true.

A spacecraft overflying a static structure of field-aligned currents closed by Pedersen current at low altitude will see a correlation between orthogonal components of the electric and magnetic vectors transverse to the main mag- netic field (e.g. Sugiura et al., 1982; Sugiura, 1984). The current produces locally a transverse magnetic field and when closing at low altitude it drives an electric field which projects along magnetic field lines to spacecraft altitude (in aurora1 acceleration regions this is true only if the spacecraft is below the region of parallel electric field). The ratio of magnetic to electric field variations is a direct measure of the (height-integrated in the case of an iono- sphere) Pedersen conductivity. A further indication of this phenomenon is that the variations of the electric and magnetic fields are in phase. This is also true, as discussed below, for a travelling Alfven wave. However, a travelling Alfven wave should have an electric to magnetic field ratio that equals the phase (Alfven) velocity.

If the field-aligned currents close in an ionosphere, the basic equation relating the electric field to the magnetic variations caused by the field-aligned current reads :

Conductivity

A conducting layer above the surface of Mercury could, at least in principle, come about in several ways. If the atmosphere were sufficiently dense it would be a plasma created mainly by photo-ionization of the neutral atmo- sphere. However, another possibility is that a cloud of electrons, similar to an electron-rich cathode sheath, made up of electrons photo-emitted from the surface. Photo- emission from the surfaces of celestial bodies has been considered earlier in the literature (e.g. Manka, 1973). Willis et al. (1973) estimated the photo-emission of the Lunar surface from Apollo 14 and 15 samples to be 4.5 PArn-‘. The emission scales as (1 AU)2/d2, where d is the heliocentric distance (e.g. Mendis et al., 1981). Thus, the Hermean photo-emission would be roughly an order of magnitude greater than that found by Willis et al. (1973), assuming that the surface emissivity of Mercury is not dramatically different from that of the Moon. In addition, secondary emission may play a role as well. More quantitative work concerning Mercury will be the subject of a future study. There may be a difficulty with current closure in the case of a plasma. If collisions are infrequent, the plasma would mainly drift as an uncharged cloud in the E x B direction, thus, not giving rise to any current flow.

Diagnostics using electric and magnetic field measure- ments yield information on the conductivity at the mag- netosphere-planet interface. The principle holds whether the conducting region is found above the planetary surface or not. By looking at differences between the dayside and the nightside it should be possible to determine whether Mercury has a conducting ionosphere or whether it is the surface itself that is conducting. If the conductivity “resides” in an ionized (or charged) atmosphere it should exhibit large variations between the dayside and the night- side. This is particularly true at Mercury. Because of its proximity to the Sun, photo-ionization is, by far, the dominant ionization process. If, on the other hand, the conductive properties reside in or below the Hermean

where j,, is the field-aligned current, H the magnetic field, and JL the height-integrated transverse current. Sub- scripts P and H refer to the Pedersen and Hall components, respectively. When applied to the Earth’s ionosphere the normal assumption is that the field-aligned current is closed by a Pedersen current, i.e. the Hall current is divergence-free (e.g. Sugiura et al., 1982 ; Sugiura, 1984). With this assumption, and using Ohm’s law, J = XE, equation (1) can be written as

where & is the height-integrated Pedersen conductivity, and E the electric field. A particular solution to this equa- tion is that both parentheses vanish separately. Assuming also homogeneous conductivity, orthogonal components of E and H are correlated, and C, = HJE, = - HJE,,. Such a correlation is often observed when overflying struc- tures in the terrestrial ionosphere.

This will apply also to Mercury provided that the clos- ure is indeed effectuated by a Pedersen current. Another important condition is, of course, that there are enough charge carriers in the magnetosphere to keep magnetic field lines roughly at constant potential, since otherwise the electric field seen at spacecraft altitude will differ from the field driving the closure current. Let us assume for the moment that this is the case.

If photo-emission from the surface should indeed be an important source of charge carriers in the near-Hermean environment, there would be a net negative charge present, i.e. there would be a cathode sheath rather than a plasma. In this case, because of the absence of collisions,

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146 L. G. Blomberg: Mercury’s magnetosphere, exosphere and surface

the current flowing may be a Hall current arising from E x B-drifting electrons. With this assumption (i.e. a non- existent, and, thus, manifestly divergence-free Pedersen current) equation (1) can be re-written as

&&f&E,)- +L+Z,,E,) = 0 (3)

where Cn is the height-integrated Hall conductivity. Here, the correlation is seen for the same components of E and H rather than for the orthogonal ones.

The details of the current conduction in a possible col- lisionless electron cloud depends on the ratio of gyro period to lifetime for the electrons before impacting on the surface. If the lifetime is short the electrons will move in the direction of the electric field, and, thus, give rise to a Pedersen current.

Studying the correlation between the transverse com- ponents of the electric and magnetic fields may yield infor- mation on the mechanism closing the field-aligned currents. Most likely the actual closure current is a com- bination of Pedersen and Hall currents. Using the tech- nique described above it should be possible to estimate their relative importance.

Electrostatic charging of the Hermean surface

Supposing that the dayside photo-emission of electrons from Mercury’s surface exceeds the random electron cur- rent from the surrounding plasma (cf. Manka, 1973; Willis et al., 1973), the dayside surface would be (weakly) positively charged in order to attract electrons and to maintain current continuity. This situation is analogous to that of a spacecraft which normally assumes an equi- librium potential different from that of the surrounding plasma. It has also been suggested (Ip, 1986b) that the nightside of the Hermean surface might be negatively charged (up to a kV, or so) because of the different thermal velocities of ions and electrons. If this is the case and if the planetary surface is not too good a conductor, a horizontal electric field would be set up directed from the dayside to the nightside, possibly projecting along the magnetic field into the magnetosphere. Although small, such a field may be non-negligible. Ip (1986b) also dis- cusses the effect of nightside electrostatic charging on dust transport. Although a possible dayside charging would be small and of the opposite polarity compared to the nightside one, it might affect the dust transport as well.

Field line resonances

In the terrestrial magnetosphere, field line resonances, i.e. standing Alfven waves bouncing between the hemi- spheres, are observed rather frequently. They are thought of as being generated by a Kelvin-Helmholtz instability close to the magnetopause as the solar wind streams past (e.g. Potemra et al., 1989). They bounce between the hemi- spheres because of a mismatch between the Alfvtn con- ductance in the flux-tube and the Pedersen conductances in the two ionospheres. They are never perfectly standing, there is always some dissipation in the ionosphere. The

phase shift between the electric and the magnetic field is directly dependent on the reflection coefficient at the interface. For a purely travelling wave the electric and magnetic components vary in phase. For a purely standing wave they are completely out of phase, while for a partially reflected wave the phase shift is between 0 and 90 deg. The situation may be further complicated by gradual partial reflection over an extended distance.

Additional complication may be introduced by inter- fering or obliquely propagating waves. For a recent expose of these waves with application to Viking data from the terrestrial magnetosphere see Aikio et al. (1995).

In the case of non-travelling (purely or partly standing) waves, the ratio of electric to magnetic field is unrelated to their phase velocity. Rather, it may give information on the actual location relative to the nodes of the wave envelope. In all cases some assumption regarding spatial versus temporal variations is needed.

This type of waves at Mercury was inferred by Russell (1989) from Mariner 10 magnetometer data (Fig. 1). The bounce period was estimated to be roughly 8 s while the observed wave period was 2s. Thus, if the waves were indeed standing AlfvCn waves it was not the fundamental but rather some harmonic of the fundamental that was observed. Furthermore, the magnetic oscillation was found in the meridional component rather than in the azimuthal one, which is normally the case in the terrestrial magnetosphere. Probably, the wave polarization would give additional information on the reflection mechanism at low altitude if also the electric field were known. Unfor- tunately, Mariner 10 did not measure the electric field. Therefore, it is impossible to determine reflection and conductivity. In fact, it is not even possible to confirm that what was observed were indeed field line resonances.

It is quite clear that combined electric and magnetic field measurements may add significantly to the under- standing of wave propagation in general and field line resonances in particular in the Hermean magnetosphere. In addition, studying standing waves is yet another way of understanding the nature of Mercury’s conductivity, which, as discussed above, is of crucial importance to the understanding of solar wind-magnetosphere-exosphere- surface interactions. An important requirement for these studies is that the periherm of the spacecraft orbit is rather low.

Summary

We believe that there are ample reasons for including a comprehensive instrument package for measuring the low-frequency magnetic and electric fields on a future Mercury Orbiter mission. Low frequency in this context means DC up to a few tens of Hz. Such instrumentation will give significant information on the interaction of the solar wind and magnetosphere with the conducting near- surface region, whether the conductivity is found in an ionized or charged atmosphere or in the planetary surface itself. The nature of Mercury’s conductive properties is intriguing since it may affect the compressibility and thus the shape of the entire magnetosphere.

Field measurements on a Mercury Orbiter will also tell

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L. G. Blomberg: Mercury’s magnetosphere, exosphere and surface

March 29, 1974

147

Mariner 10

2-

-2 I I I 2045:OO :09 :19 :28 2045:38

Universal time

Fig. 1. Mariner 10 40 ms magnetic field records from an altitude of 822 km expressed in radial, east, and north coordinates. The average magnetic field was subtracted. After Russell (1989) (their Fig. 3)

us more about possible aurora1 processes and their nature. Co-rotation is out of reach because it would be very slow, but possible substorm-related strong plasma convection is detectable, as well as convective electric fields and possible flux transfer events associated with the energy transfer from the solar wind to the magnetosphere. Very strong transient inductive electric fields related to particle accel- eration have been predicted in the literature. Such fields would also be measurable. By making statistics of the electric field observations, aurora1 potential structures could be detected if they exist. It should be noted that in most cases knowledge of the electric as well as the mag- netic field is crucial.

Finally, we would like to conclude that the wealth of unique physical processes that may take place in the Her- mean environment makes a mission to Mercury within the foreseeable future highly desirable. This applies to the magnetospheric, pIasma physical as well as to the planetological aspects.

Acknowledgements. The author is grateful to Lars Block, Nils Brenning, Per Carlqvist, Carl-Gunne Falthammar, Herbert Gunell, Tomas Karlsson, Per-Arne Lindqvist and G&an Marklund for valuable discussions.

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fluctuations in the aurora1 zone. J. Geophys. Res. (sub- mitted).

Baker, D. N., Simpson, J. A. and Eraker, J. H. (1986) A model of impulsive acceleration and transport of energetic particles in Mercury’s magnetosphere. J. Geophys. Res. 91, 8742.

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