Mariner 10 Observations of Field-Aligned Currents at Mercury

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Mariner 10 observations of field-aligned currents at Mercury

J. A. Slavin,’ J. C. J. Owen,’ J. E. P. Connerney’ and S. P. Christon ‘Laboratory for Extraterrestrial Physics, NASA/GSFC, Greenbelt, MD 20771, U.S.A. ‘Astronomy Unit, Queen Mary and Westfield College, London, U.K. 3Department of Physics, University of Maryland, College Park, MD 20742, U.S.A.

Received 5 October 1995; revised 11 April 1996; accepted 13 April 1996

magnetic field at Mercury with a dipole moment of about 300nT R& (see review by Connerney and Ness (1988)). This magnetic field is sufficient to stand-off the average solar wind at an altitude of about 1 RM as depicted in Fig. 1 (see review by Russell et al. (1988)). For the purposes of comparison the diameter of the Earth is indicated at the bottom of the figure (i.e. 1 RE = 6380 km ; 1 RM = 2439 km). Whether or not the solar wind is ever able to compress and/or erode the dayside magnetosphere to the point where solar wind ions could directly impact the surface remains a topic of considerable controversy (Siscoe and Christopher, 1975 ; Slavin and Holzer, 1979 ; Hood and Schubert, 1979; Suess and Goldstein, 1979; Goldstein et al., 1981). Overall, the basic morphology of this small magnetosphere appears quite similar to that of the Earth. The magnetic field investigation observed clear bow shock and magnetopause boundaries along with the lobes of the tail and the cross-tail current layer (Ness et al., 1974, 1975, 1976). The colatitude of the polar cap was estimated on the basis of these measurements to be about 25” as compared with a typical value of 16” at Earth. The plasma investigation was hampered by a deployment failure which kept the ion portion of the instrument from making any useful measurements. However, the electron portion of the instrument recorded the bow shock, the low density, cool plasma in the high latitude magnetosphere and the hotter electrons in the plasma sheet (Ogilvie et al., 1974, 1977). As expected from the large fraction of the magnetosphere occupied by the planet, the cosmic ray experi- ment did not detect any trapped radiation belts at Mercury (Simpson et al., 1974). However, intense energetic particle events were observed in the tail during the first Mariner 10 encounter and, as will be discussed later, they have Introduction provided strong evidence for terrestrial-type substorm activity at this planet (Simpson et al., 1974; Siscoe et al., One of the major discoveries made by NASA’s early plan- 1975). etary missions was the existence of a modest intrinsic In considering Fig. 1, and the analysis to follow, it is useful to note that a scaling in which 1 RM at Mercury corresponds to -6 RE at Earth produces good cor- Correspondence to: J. A. Slavin respondence between the boundaries and magnetospheric 134 J. A. Slavin et al.: Field-aligned currents at Mercury

BOW / MAGNETOSHEATH

/ SOLAR WIND

DIAMETER OF THE EARTH

Fig. 1. A schematic view of the bow shock, magnetopause, and some field lines within the magnetosphere of Mercury. The rendering is approximately to scale relative to the diameter of Mercury. For comparison, the diameter of the Earth is also indicated

regions at these two planets (Ogilvie et al., 1977). Hence, axis completes the right-handed orthogonal system and the 2 R, distance from the center of Mercury to the sub- is positive toward the “north”. The first encounter was solar magnetopause maps to about 12& at the Earth planned to pass through the wake of the planet and survey which is, indeed, close to the average magnetopause nose its interaction with the solar wind. The closest approach distance. Similarly, the surface of Mercury would cor- to the surface of the planet during this passage was near respond roughly to geosynchronous distance within the 700 km and a peak magnetic field intensity of about 100 nT Earth’s magnetosphere and rule out the possibility of a was observed (Ness et al., 1974). The second encounter plasmasphere or trapped radiation belts even if the planet was targeted well upstream of the planet to return images possessed the sources and the rotation rate to create such in full sunlight and did not intersect the magnetosphere. a region (N.B. Mercury’s rotational and orbital periods The third encounter was intended to achieve higher lati- are 59 and 88 days, respectively). Finally, the observed tudes and lower altitudes in order to confirm the intrinsic near-tail diameter of 4-5 RM scales to about 24-30 RE nature of the magnetic field. This effort was successful which is close to the typical near-tail diameter at Earth. with a closest approach altitude of only 327 km and a In the sections to follow a concise review of the Mariner peak magnetic field intensity of about 400 nT (Ness et al., 10 observations will be presented as they apply to our 1976). primary purpose ; the search for FACs electrodynamically A radial projection of the two Mariner 10 trajectories linking Mercury’s magnetosphere to its weak ionosphere through the nightside magnetosphere onto the surface of or regolith. After presenting Mariner 10 magnetometer this planet are shown in Fig. 3 (adapted from Lepping et measurements which strongly suggest such FACs do al. (1979)). As discussed by Ness et al. (1974, 1975) and sometimes exist in the near-tail tail of Mercury, we will Ogilvie et al. (1974, 1977), the first encounter saw the discuss their implications for Mercury’s plasma environ- spacecraft traverse the near-tail starting in the south lobe ment and for the dynamics of its magnetosphere. of the tail, crossing the plasma sheet, and eventually exit- ing through the north lobe of the near-tail (see also Christon (1987)). The third encounter, in contrast, took Mariner 10 encounters the spacecraft through the northern high latitude magnetosphere where it observed quiet magnetic fields and Following its launch on November 2, 1973 the Mariner the “horns” of a cooler, quiet-time plasma sheet (Ness et 10 spacecraft had three close encounters with the planet al., 1976; Ogilvie et al., 1977). Mercury on March 29, 1974, September 21, 1974, and Our re-examination of the Mariner 10 observations dur- March 16,1975. All three fly-bys occurred at a heliocentric ing the third encounter confirmed the quiescent condition distance of 0.46AU. This compares with perihelion and of Mercury’s magnetosphere during this interval reported aphelion distances for Mercury of 0.31 and 0.47AU, by the original investigators (see review by Russell et al. respectively. The spacecraft trajectories during the first (1988)). The interplanetary magnetic field was directed and third encounters, which took it through Mercury’s northward both before and after the encounter (Ness et small magnetosphere, are displayed in Fig. 2 in Mercury al., 1976). Hence, conditions were unfavorable for dayside centered solar ecliptic coordinates (adapted from Ness et reconnection and the input of large amounts of energy al. (1976)). In these “ME” coordinates the X,, axis is into the magnetosphere. No major energetic particle directed toward the Sun and the YNIEaxis is in the plane of events were detected (Eraker and Simpson, 1986) and the the ecliptic, perpendicular to X,, and directed oppositely passage through the horns of the plasma sheet revealed from the direction of planetary orbital motion. The Z,, relatively cool plasma electrons characteristic of the quiet- J. A. Slavin et al.: Field-aligned currents at Mercury 135

- MERCURY I 29 MARCH ‘74 -----MERCURY IU 16 MARCH’75 SCALED WITH My = 5.6 X lO22 I’ cm3

MAGNETOPAUSE (A) -3 BOW t I SHOCK (III)

‘ME Fig. 2. The Mariner 10 trajectories during the first (solid line) and third (dashed line) encounters are displayed relative to model magnetopause and bow shock surfaces (adapted from Ness et al. (1976)) time plasma sheet at the Earth (Ogilvie et al., 1977). and magnetospheric convection, in general, decrease gre- Despite a spacecraft trajectory which passed across the atly in intensity or even disappear during extended inter- polar cap and “aurora1 oval” regions where kilovolt elec- vals of northward IMF (e.g. Hoffman et al., 1988), it is tron precipitation produce auroras at the Earth (see Fig. not possible to infer whether the lack of FACs during the 3), we found no evidence for FAC signatures. Since FACs third encounter is associated with the northward IMF or

TO SUN

Fig. 3. Projections of the Mariner 10 first (left) and third (right) encounter trajectories upon the surface of the planet between the inbound and outbound magnetopause crossings. The time interval between solid dots is 2 min and the distance of the spacecraft from the center of the planet is indicated (adapted from Lepping et al. (1979)) 136 J. A. Slavin et al.: Field-aligned currents at Mercury MARCH 29, 1974 II

100 I I B Y 0-T I II- I II -100 I, I I I I I I DIPOLARIZATION

UT Fig. 4. The Mariner 10 magnetic field observations (1.2 s averages) taken during the first encounter are displayed in Mercury centered solar ecliptic coordinates. Vertical dashed lines call out the inbound and outbound magnetopause crossings as well as the point of closest approach (CA). The clear signatures of a substorm dipolarization in the B, component and a field-aligned current (FAC) in the BYcomponent are also identified some other factor peculiar to Mercury. Accordingly, the of studies (Siscoe et al., 1975; Baker et al., 1986; Eraker Mariner 10 observations collected during this third and Simpson, 1986). The Christon et al. (1987) analysis encounter will not be considered further in this study. of both the magnetic field and energetic particle signatures for this interval clearly showed that this event is quali- tatively identical to commonly observed substorm “injec- Substorm observations tions” at altitudes between geosynchronous orbit and the inner edge of the plasma sheet at the Earth. Such energetic Figure 4 displays 1.2s averages of the magnetic fields particle and magnetic field signatures have been alter- observed during the first Mariner 10 encounter in Mercury natively referred to as “dipole collapse” or “dipolar- centered solar ecliptic coordinates. Vertical dashed lines ization” events (Heppner et al., 1967), “injection fronts” mark the inbound and outbound magnetopause crossings (Moore et al., 1981), or “current sheet disruption” events as well as the point of closest approach to the planet. The (Sauvaud, 1992) depending upon the theoretical interpret- tail-like nature of the magnetic field during the inbound ations of the various researchers. Attempts to understand passage is very evident with B, >I By, B,. During the out- the underlying physical processes causing this phenom- bound passage the magnetic field variations are extremely enon are still at the cutting edge of magnetospheric physics dynamic. These large amplitude variations in the total today. What is important for our present study of Mer- field intensity have a number of different sources. The cury’s magnetosphere is that clear and readily identifiable large dip in field intensity just after closest approach substorm signatures were present between 20:48 and coincided with the spacecraft becoming immersed in a 20 :49 UT during the outbound Mariner 10 passage of the hot plasma sheet (Ogilvie et al., 1977). Hence, it is most first encounter. probably diamagnetic in nature (see Christon, 1989). The very short duration of the substorm signature in Less than a minute after Mariner 10 entered the plasma Fig. 5, about 1 min, as compared typically with 1 h at the sheet, i.e. the strong dip in magnetic field intensity begin- Earth was first addressed by Siscoe et al. (1975). Although ning around 20 :47 UT, there is a sharp increase in the B, much remains to be understood concerning substorm pro- field component. As displayed in Fig. 5, the initial sudden cesses, it is clear that they correspond to intervals of gre- B, increase and subsequent quasi-periodic increases are atly enhanced magnetospheric convection which involves nearly coincident with strong enhancements in the flux of the dissipation of large amounts of stored magnetic energy > 35 keV electrons observed by the cosmic ray telescopes from the tail lobes through the acceleration of energetic (Simpson et al., 1974; Eraker and Simpson, 1986; Chri- particles, the driving of intense FACs linking the tail and ston, 1987). This energetic particle signature, and several ionosphere and the generation of strong bulk flows and weaker events observed later in the outbound pass, were heating in the central plasma sheet. Siscoe et al. argued interpreted as evidence for substorm activity by a number that the timescale of isolated substorms at the Earth is J. A. Slavin et al.: Field-aligned currents at Mercury 137 t I I I / 1 7, , , / , / , 3

MARINER 10-MERCURY 1

event

B 65.

2047:42 :48 :54 2u48:oo :06 :12 :18 :24 :30 :36 :42 :48 34 2D49.m :06 H20 DO88 Y1974 (seconds) Fig. 5. High time resolution measurements of the Mariner 10 energetic electron (> 35 keV) flux (0.6 s) and magnetic field (0.04s) for the substorm dipolarization event identified in Fig. 4. The vertical dashed lines mark fine structure increases in larger energetic particle event (adapted from Christon et al. (1987)) determined by the time necessary for plasma to convect electric potential which the solar wind can maintain across across the polar cap or, equivalently, from the outer the magnetosphere. The lower the electrical conductivity boundary of the tail down to the mid-plane where recon- in the polar cap region, the closer the potential drop nection can take place. They estimated, using typical solar across the magnetosphere can approach to the maximum wind and magnetotail parameters, that the time necessary possible which is just the width of the magnetosphere to “cycle” all of the flux in the tail lobes at the Earth is times the solar wind -V x B electric field. At Mercury about 1 h which is indeed comparable to a time span of a Hill et al. assumed that any ionosphere would be unim- typical substorm. Relative to conditions at 1 AU, the portant and suggested that the low conductivity of the larger solar wind dynamic and static pressures and regolith (about 0.1 mho based upon lunar sample analysis) enhanced interplanetary - V x B electric field associated would make line-tying negligible and the full solar wind with the intense ambient IMF at 0.46 AU result in a much imposed cross-magnetosphere potential drop would then more rapid magnetic flux cycle time than that found in contribute to the short magnetotail flux cycle times sug- the tail of the Earth. The value calculated by Siscoe et al. gested by Siscoe et al. (1975). Detailed discussions of what was approximately 1 min, in close agreement with the is known regarding Mercury’s tenuous neutral atmo- duration of the energetic particle event in Fig. 5. sphere/ionosphere and its implications for the mag- Hill et al. (1976) took these concepts a step further netosphere are given in Potter and Morgan (1985), Cheng by pointing out the limiting effect that the conductive et al. (1987), Ip (1987), Hunten et al. (1988), and Killen ionosphere is thought to have on magnetospheric con- and Morgan (1993). vection (e.g. Coroniti and Kennel, 1973). This effect, usu- ally referred to as “line-tying”, occurs because ions con- vecting in response to the electric field which the Field-aligned current observations magnetosphere attempts to impose on the ionosphere experience a net drag force as a result of collisions with Two minutes after the energetic particle injection event, thermospheric neutral species. These collisions effectively the BY component of the magnetic field went through a “short-out” part of the magnetospheric electric field. The very pronounced bipolar variation with a full amplitude greater the ionospheric conductivity, the lower the total of about 60 nT. As shown in Fig. 4, this bipolar variation 138 J. A. Slavin et al.: Field-aligned currents at Mercury

MARINER10 MARCH 29. 1974

-40 I I I I I II I 1 I I I I I FAC 40

20:48 20:49 20:50 20:51 20: 52 20:53 20:s UT Fig. 6. Six minutes of magnetic field measurements centered on the field-aligned current event identified in Fig. 4 are displayed. Note the presence of weaker downward current layers on either side of the larger upward current sheet

in the By, i.e. approximately the east-west component, and after as indicated by arrows in Fig. 6. The occurrence around 20 : 5 1 UT was the largest perturbation in this field of multiple current sheets is particularly common on the component recorded during either of the Mariner 10 nightside of the aurora1 oval during substorms (Iijima and passes through Mercury’s magnetosphere. At this time the Potemra, 1978). spacecraft location in ME coordinates was approximately What do these currents correspond to in the Earth’s (-0.95&, - I .56 RM, 0.23 R,). As can be seen in Fig. 3, magnetosphere? Since the strong central current sheet was this By signature occurred at a distance from the center of directed upward at the spacecraft and therefore down- the planet of about 1.84 RM and a local time of approxi- ward into the planet’s aurora1 zone at a point well east of mately 03 :50 (i.e. 58” from the midnight meridian). midnight, it could be associated with the Region 1 currents In the expanded scale view of Fig. 6, it can be seen which flow into the poleward edge of the aurora1 zone in that this BY signature was accompanied by only small the dawn hemisphere at the Earth (Iijima and Potemra, variations in the 3, and B, components. The background 1976). Alternatively, dawnside current down into the aur- field at the time of this bipolar event was mostly in the oral zone could also be associated with the east-most leg X,,-Z,, plane. The average Bz around this event was of the substorm current wedge (McPherron et al., 1973). about twice the magnitude of B, as would be expected for This latter interpretation is of interest because of the sub- a pass through the near-tail region of the terrestrial-type storm energetic particle injection observed a couple of magnetosphere at a distance approaching geosyn- minutes earlier. However, experience at the Earth has chronous orbit at the Earth. shown that, especially in the near-tail, it is often difficult The 20 : 50 : 55-20 : 5 1 : 18 UT magnetic field signature in to uniquely associate a single current sheet event with a Fig. 6 is well known to the magnetospheric community specific current system (e.g. Elphic et al., 1987). All of and can be readily interpreted as being due to the spa- these “field-aligned” currents couple the magnetosphere cecraft traversing a central quasi-planar current sheet in to the ionosphere, or in the case of Mercury, perhaps also which the current flow is nearly aligned with the ambient to the regolith. magnetic field (e.g. Iijima and Potemra, 1976). Given the Minimum variance analysis performed upon this field Mariner 10 trajectory in Fig. 2, the main gradient in B, perturbation yielded a well determined normal direction from negative to positive is indicative of an upward FAC to the main current sheet, (-0.625, -0.364,0.691), which at the spacecraft. This upward current sheet is largely corresponds to a direction within 30” of the Z,, axis balanced by two smaller downward current sheets before consistent with the current sheets being quasi-aligned with J. A. Slavin et ~2.:Field-aligned currents at Mercury 139 constant L-shells. The tilt of the current sheet normal is to disappear more readily as the IMF turns northward and consistent with the FAC being observed on distorted, the driving voltage applied ultimately via reconnection at closed field lines north of the cross-tail current layer. the dayside magnetopause subsides. Under the assumption that the observed magnetic field For these reasons, the Mariner 10 observation of what perturbation is caused by an infinite current sheet, the appears to be a strong FAC signature in the near-tail sheet current density corresponding to a total field per- following a substorm energetic particle injection and just turbation of 60nT in Fig. 6 is about 50mAm-‘. If this prior to the spacecraft’s exit into a southward IMF is current sheet were indeed quasi-aligned with a constant extremely interesting. Once additional measurements L-shell, and given the Mariner 10 trajectory, the average become available from a Mercury orbiter mission, it may current intensity in the central current sheet would be be found that the total FAC estimated from this single approximately 0.7 PA m-*. This calculation assumes that Mariner 10 event, 1.4 x lo6 A, is excessive due to the errors the current layer was stationary during the very oblique inherent in our analysis of this event. However, it may crossing by Mariner 10. A more likely scenario is that also be the case that once the low altitude neutral atmo- some dynamic event caused the current sheet to move and sphere of Mercury is more fully explored, that sufficient precipitate a rapid crossing at the observed time. Still, charge carriers may exist in the ionosphere to allow for both the sheet current intensity and current density significant current closure at very low altitudes. For exam- inferred in this manner from the Mariner 10 observations ple, Cheng et af. (1987) have calculated a Pederson elec- lie within the range of values observed at ionospheric to trical conductance appropriate to the currents driven by near-tail altitudes (e.g. Iijima and Potemra, 1976 ; Elphic the photo-ionization and the pick-up of sodium ions by et al., 1987). magnetospheric convection in Mercury’s magnetosphere Although the current in the aurora1 oval generally varies which is 2-3 times greater than the 0.1 mho previously as a function of local time, solar wind, and substorm suggested for the regolith below. Only comprehensive, conditions, it is of interest to estimate what the total low altitude in situ particles and fields measurements will current flowing into the aurora1 oval might be on this resolve this issue. occasion. To make the estimate, we simply assume that the sheet current intensity extends along a circular path about the Z,, axis with a constant value. The total current into the Mercurian equivalent of the Earth’s aurora1 oval Future Mercury orbiter missions is then just JT = 2nR,J, where R, is the distance of Mari- ner 10 from the normal to the ecliptic plane at the center Mercury is of particular importance to the mag- of the planet (i.e. the Z,, axis). Inserting the sheet current netospheric physics community because it is at once so intensity yields 1.4 x lo6 A. This value for the total current similar to Earth’s magnetosphere and yet so different in lies within the l-3 x lo6 A range typically reported for the key aspects (e.g. ionospheric conductance or the ratios of total Region 1 or 2 currents at the Earth (Iijima and magnetospheric scale lengths to characteristic ion and Potemra, 1976). electron gyro radii). Hence, Mercury may be an excep- tional fertile testing ground for theories regarding critical processes in the Earth’s magnetosphere. For example, the very tenuous nature of its atmosphere should make Discussion internal plasma sources far less significant than at Earth or any other of the known magnetospheres. This fact In recent years much attention has been devoted to should make Mercury ideal for quantitative modeling of improving our understanding the electrodynamics of the how and to what degree solar wind plasma enters a ter- Earth’s coupled solar wind-magnetosphere-ionosphere restrial-type magnetosphere. Mercury’s location in the system. For example, experimental studies have shown inner heliosphere also exposes this magnetosphere to solar that for a given cross-polar cap potential, the greater the wind conditions which are relatively rare at 1 AU. In ionospheric conductivity, the larger the FACs which are particular, it has been suggested that the lower Alfvenic driven between the ionosphere and magnetosphere (Fujii Mach numbers and more intense IMF at 0.3-0.5 AU will et al., 1981; Robinson, 1984). In this case the solar wind result in reconnection rates at the dayside magnetopause is regarded, by analogy to circuit theory, as acting over which are a factor of 3 greater than at the Earth. Again, large-scale lengths as a “voltage source”. Thus, if the measurements at Mercury could be pivotal to the testing ionosphere were negligible and the electrical conductance of existing models of the reconnection and lead to dra- appropriate for currents closing in the regolith at Mercury matic advances in our understanding of this important were only 0.1 mho, as suggested by Hill et al. (1976), magnetospheric process. Finally, and perhaps most then the FACs within Mercury’s magnetosphere would importantly, observations from a Mercury orbiter may be expected to be much weaker than at Earth where height lead to great advances in our understanding and modeling integrated conductances of l-10mhos are typical (e.g. capability with respect to magnetospheric substorms. Kamide et al., 1986). Thus, even when the stronger IMF With substorm durations of only about 1 min, it should and enhanced solar wind electric field at 0.5 AU are con- be possible to gather definitive substorm statistics and sidered, the FACs at Mercury should be weaker by a observe complete substorms at a variety of locations factor of 2-50 with the larger differences occurring during throughout the magnetosphere without the motion of the substorms when precipitation at the Earth greatly spacecraft aliasing the in situ measurements as commonly enhances ionospheric conductivity. Accordingly, FACs at happens at Earth where substorms typically last l-2 h. Mercury would be expected to be generally weaker and Furthermore, many theories of substorms at the Earth 140 J. A. Slavin et al.: Field-aligned currents at Mercury differ as to the critical role of “feedback” between con- (HSTX Corp.) with computational aspects of this work is also ditions at high altitudes and the electrical conductivity in acknowledged. Finally, we wish to thank J. Clemmons and N. the ionosphere at the foot of the magnetospheric flux Fox for useful comments and suggestions regarding this work. tubes which becomes greatly enhanced due to pre- cipitating energetic electrons. At this point it appears doubtful, but not impossible, that some similar feedback mechanism is operating at Mercury with its very weak References atmosphere. Additional examples of scientific questions of great interest to the magnetospheric community which Baker, D. N., Simpson, J. A. and Eraker, J. H. (1986) A model would directly benefit from measurements at Mercury of impulsive acceleration and transport of energetic particles in Mercury’s magnetosphere. J. Geophys. Res. 91,8742. are described in Mercury Orbiter : Report of the Science Cheng, A. F., Johnson, R. E., Krimigis, S. M. and Lanzerotti, L. Working Team (199 1). J. (1987) Magnetosphere, exosphere and surface of Mercury. Icarus 71,430. Christon, S. P. (1987) A comparison of the Mercury and Earth Summary magnetospheres : electron measurements and substorm time scales. Icarus 71,448. Christon, S. P. (1989) Plasma and energetic electron flux vari- In this study we have re-examined the Mariner 10 mag- ations in the Mercury 1 C event: evidence for a magnetic field observations for evidence of FACs. No FAC netospheric boundary layer. J. Geophys. Res. 94, 648 1. signatures were detected during the third encounter Christon, S. P., Feynman, J. and Slavin, J. A. (1987) Dynamic despite its low periapsis and high latitude trajectory across substorm injections : similar magnetospheric phenomena at Mercury’s polar cap. However, this negative result could Earth and Mercury. In Magnetotail Physics, ed. A. T. Y. Lui, be attributed to the northward IMF observed both before pp. 393400. Johns Hopkins University Press, Baltimore. Connerney, J. E. P. and Ness, N. F. (1988) The magnetosphere and after the magnetospheric passage without recourse to of Mercury. In Mercury, ed. C. Chapman, pp. 494-513. Uni- any factors intrinsic to Mercury. The reason is that FACs versity of Arizona Press, Tucson. are known to weaken and largely disappear at Earth dur- Coroniti, F. V. and Kennel, C. F. (1973) Can the ionosphere ing extended intervals of northward B, as magnetospheric regulate magnetospheric convection? J. Geophys. Res. 78, convection decreases in intensity. In contrast, exam- 2837. ination of the first Mariner 10 encounter showed a clear Elphic, R. C., Kelly, T. J., Spence, H. E., Walker, R. J., Russell, example of a FAC sheet in the near tail with two weaker C. T. and Sugiura, M. (1987) Aurora1 zone field-aligned adjacent current sheets of lesser intensity and opposite currents observed in the magnetotail and at intermediate altitudes : an ISEE perspective. In Magnetotail Physics, ed. polarity. Despite the weakness of any ionosphere and the A. T. Y. Lui, pp. 4146. Johns Hopkins University Press, expected resistive nature of the underlying regolith, the Baltimore. total current estimated from this single substorm-related Eraker, J. H. and Simpson, J. A. (1986) Acceleration of charged FAC event, 1.4 x 106A, is comparable to typical values particles in Mercury’s magnetosphere. J. Geophys. Res. 232, for the terrestrial magnetosphere. This surprising result 9973. may be due to the inherent uncertainties associated with Fujii, R., Iijima, T., Potemra, T. A. and Sugiura, M. (1981) estimating total current from a single point measurement Seasonal dependence of large-scale Birkeland currents. or it may reflect higher than anticipated substorm electric Geophys. Res. Lett. 8, 1103. Gary, J. B., Heelis, R. A., Hanson, W. B. and Slavin, J. A. fields in this small magnetosphere and/or the existence of (1994) Field-aligned Poynting flux observations in the high unexpected charge carriers and transport mechanisms at latitude ionosphere. J. Geophys. Res. 99, 11417. very low altitudes. However, what is clear is that the FACs Goldstein, B. E., Suess, S. T. and Walker, R. J. (1981) Mercury : identified in this study add yet one more item to the magnetospheric processes and the atmospheric supply and extensive list of features common to both magnetospheres. loss rates. J. Geophys. Res. 86, 5485. To achieve closure on these and other issues related Heppner, J. P., Sugiura, M., Skillman, T. L., Ledley, B. G. and to Mercury’s magnetosphere an orbiter mission which is Cambel, M. (1967) OGO-A magnetic field observations. J. adequately instrumented to conduct the necessary par- Geophys. Res. 72, 5417. Hill, T. W., Dessler, A. J. and Wolf, R. A. (1976) Mercury and ticles and fields investigations will be required. For exam- Mars : the role of ionospheric conductivity in the acceleration ple, to complement the vector magnetometer, a vector of magnetospheric particles. Geophys. Res. 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University of Arizona Press, Tucson. Iijima, T. and Potemra, T. A. (1976) The amplitude of field- Acknowledgements. We wish to thank all of the personnel who aligned currents at northern high latitudes observed by Triad. contributed to the great success of the Mariner 10 mission. The J. Geophys. Res. S&2165. National Space Science Data Center and the Principal Inves- Iijima, T. and Potemra, T. A. (1978) Large-scale characteristics tigators for the magnetic field (N. Ness), plasma (H. Bridge) and of field-aligned currents associated with substorms. J. cosmic ray (J. Simpson) investigations. The assistance of S. Jones Geophys. Res. 83, 599. J. A. Slavin et al.: Field-aligned currents at Mercury 141

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