Characteristic boundaries of the Hermean magnetosphere and energetic particles close to the planet

Planet. space Sci., Vol. 45, No. 1, pp. 167-180, 1997 0 1997 Elsevier Science Ltd Printed in GreatBritain. All rights reserved 0032-0633/97 $17.00+0.00

Pergamon

PII: SOO32-0633(96)OOllS

Characteristic boundaries of the Hermean magnetosphere and energetic particles close to the planet Susan M. P. McKenna-Lawlor Space Technology Received

Ireland,

29 September

St. Patrick’s

1995; revised

College, Maynooth,

11 July 1996; accepted

Co. Kildare, 25 July 1996

Ireland

168

S. M. P. McKenna-Lawlor:

Whiter, Et is expected inthis

connection thst outside, but close to, the Her-mean ~MagnetospherE~ the &pr&rnits of the relatively utlevolved-solar wind flow c&d be s&died from a unique vantagepoint using the proposed Energetic Partiele Detector. Q 1997 Elsevier Science Ltd. All rights reserved

1. Introduction In the approximately 38 years since Sputnik I heralded the dawn of the space age, only one mission has hitherto been directed to planet Mercury, namely NASA’s Mariner 10 spacecraft, which was launched from Cape Kennedy on 3 November 1973. Through the making of suitable navigational choices, multiple encounters of this spacecraft with the planet were technically achievable, and spacecraft resources were sufficient to enable useable data to be sent back to Earth during three separate fly-bys, in March and September 1974 and in March 1975. During Encounter I, Closest Approach on 29 March 1974 was at approximately 700 km above the planet on its dark side. During Encounter II, Mariner 10 crossed the dayside hemisphere well upstream of the Bow Shock, with Closest Approach at 50,000 km. The spacecraft came closest to Mercury during Encounter III, on 16 March 1975, when it reached an altitude of 327 km above the planet on its dark side. In the present paper, only data secured during Encounter I and Encounter III are discussed. These records comprise mainly magnetic field, plasma and energetic particle observations. However, investigations to detect an atmosphere and ionosphere at Mercury are also described having regard to future investigations of possible links between magnetospheric and exospheric dynamics.

2. Instrumentation 2.1. The magnetometer The magnetometer flown by Ness et al. (1974) provided the first flight of a dual triaxial fluxgate system (allowing, thereby, the inflight determination of any contamination produced due to the presence of onboard magnetic fields coupled with associated data correction, to achieve high quality measurements). During planet encounters, the instrument operated continuously in a range where each axis covered f 128 G. Vector measurements at intervals of 40 ms with 10 bit precision yielded quantization step sizes of 0.26 G. The r.m.s. noise level of each sensor over the O-12.5 Hz band pass ranged between 0.03 and 0.06 G (which was significantly less than the digitization value).

2.2. The low energy plasma experiment Studies of the low energy plasma were originally planned to include investigations of both positive ions and electrons. In the event, due to technical difficulties, only plasma electrons were recorded (Oglivie et al., 1977). The

Characteristic

boundaries

of the Hermean

Magnetosphere

instrument consisted of a hemispherical electrostatic analyser that accepted electrons in the energy range 13.4687 eV. This range was divided into 15 energy windows of width AE/E = 6.6%, equally spaced on a logarithmic scale. This instrument was stepped continuously through the energy sequence, measuring the electron flux at each step for 0.4s so that an energy spectrum was obtained every 6 s. The field of view was fan-shaped, with angular extent + 3.5” (FWHM) in the scan plane, and 13.5” perpendicular to that plane. Every 84 s the deflection potentials applied to the analyser plates were set to zero for 6 s to obtain a measurement of the instrumental background. At the output slit of the analyer, which was elongated in the direction of the radius of curvature of the plates, two channel electron multipliers were mounted. These were operated in a saturated mode and were provided with charge-sensitive preamplifiers separate (sensitivity lo-l3 C) and amplifiers. Either output could be selected by ground command and passed to the body mounted electronics box, in which was sited control, logarithmic compression, data interfacing and command decoding functions. Constant sensitivity was maintained by appropriately varying the voltage settings throughout the flight. Quantitative analysis of the data recorded by this instrument was complicated by the significant and varying electric potential of the spacecraft (Oglivie et al., 1974, 1977).

2.3. The energetic particle experiment The energetic particle experiment on Mariner 10 flown by Simpson et al. (1974) consisted of two charged particle telescopes. The first of these was a low energy telescope designed to measure protons in two energy ranges 0.531.9 and 1.9-8.9 MeV, with an efficiency for single electron counting of I lop4 for all electron ranges. The second was a telescope which had a low energy counting rate which responded to protons and helium in the energy range 0.62-10.3 MeVnucleon-‘, and to electrons with an efficiency for single electron counting of 8% at 170 keV ; 50% at 300 keV and a maximum of 70% at 840 keV. The philosophy of particle identification is treated in Simpson et al. (1974) and references therein. Ambiguities now recognized to be possible in the interpretation of the data from this instrument were pointed out by Armstrong et al. (1975) and by Criston et aE. (1979) (see also Sections 4.1 and 4.2).

3. Magnetic observations 3. I. Magnetic

data (Encounter

I)

Figure 1 (bottom four panels) presents 6 s averaged magnetic held data recorded during Encounter I in spacecraft centred solar ecliptic coordinates. F is the magnetic field strength. The r.m.s. parameter for the averages determined represents the Pythagorean mean of the X, Y, 2 component r.m.s. values, and it is coordinate system independent. The latitude of the magnetic field vector is represented by 0 and the longitude by 4. Figure 2 provides the

S. M. P. McKenna-Lawlor:

Characteristic boundaries of the Hermean Magnetosphere Plasma-magnetic field-charged particles - Mariner 10

electron

flux level

BS - Bow shock MP - Magnetopause CA - Closest approach

UT 29 March

1974

Fig. 1. Simultaneous

charged particle (panels l-3 from top), plasma pressure (panel 4) and 6s averaged magnetic field data (panels S-S), recorded in spacecraft centred solar ecliptic coordinates aboard Mariner 10 during Encounter I with Mercury. F (panel 5) represents the magnetic field magnitude. The r.m.s. parameter for the measurement averages (panel 6) represents the Pythagorean mean of the X, Y, 2 component r.m.s. values. The longitude of the magnetic field vector is represented by 4 (panel 7) and the latitude (panel 8) by 8. Inferred crossings of the inbound and outbound Bow Shock and Magnetopause boundaries are designated by BS and MP, respectively. The time of Closest Approach to Mercury is indicated by CA (following Ness (1979))

orthogonal components of the magnetic field observations (B,, B,, B,), which also incorporate 6 s averaging. Figures 1 and 2 indicate the times, identified by Ness et al. (1974) of the inbound and outbound Bow Shock crossings (BS), of Mercury by Mariner 10; of the corresponding Magnetopause traversals (MP) and the time of Closest Approach (CA). Recognition of the BS and MP signatures by the experimenters was based on an existing heritage of interpreting BS and MP crossings in similar data recorded in the Earth’s magnetosphere. (Details of the arguments in support of these identifications are contained in the associated literature.) The crossing (inbound) of the BS was accompanied by a pronounced rise in the magnetic field strength to > 40 G, see Fig. 1. In all, three crossings were made in the interval 20.27-20.28 UT due to the effect of BS “flapping” (compare Fig. 1 with the records of the X and Z components displayed in Fig. 2). The spacecraft then encountered a disturbed magnetic field regime and, here, the steady

decrease to a value of 30 G in field magnitude recorded previous to 20.37UT is in accord with what would be expected inside a steady state Magnetosheath. At 20.37UT a sharp boundary was traversed by Mariner 10, as evidenced by an increase in the magnitude of the magnetic field to > 40 G, accompanied by a decrease in the level of field fluctuations. Meanwhile, the field direction (#) changed abruptly by 135”. See also the welldefined discontinuity in the X and Y components shown in the data of Fig. 2 at 20.37 UT. The magnetic field next increased steadily to reach a maximum value of 98 G near CA at 20.47 UT. The direction of the magnetic field was then mainly parallel to the Mercury-Sun line, with a polarity sense away from the planet. A smooth, but small, variation in the orientation of the field occurred during this period. Following CA, a distinct change in the character of the magnetic field was recorded, such that large amplitude variations over a wide range of timescales occurred. As

S. M. P. McKenna-Lawlor:

170 NASA-GSFC

Characteristic

magnetic field experiment Bow shock

Mariner

boundaries

Magnetosphere

10 Tail sheet

Magnetopause

of the Hermean

Magnetopause

Bow shock 10

40-

20 I .20 -40

.40

-60 -

.60

-80

.80

20 3 .20

50 $0 20 3 -20 2010

2020

2030

2040

UT 29 March

1974

Fig. 2. Orthogonal components of the magnetic field observations at Mercury during Encounter I (incorporating 6 s averages), see also the designations used in the caption to Fig. 1 (following Ness

(1979) shown in Fig. 1, a large field depression with a minimum of 17 G occurred precipitously just after CA. The field magnitude then increased rapidly to attain a value of approximately 70 G. Subsequently, the field magnitude showed other significant variations. Meanwhile, the field direction changed steadily until it pointed northwards relative to the ecliptic. Variability in the field magnitude was not matched by comparable variability in the field direction. Fluctuations in the field were present in all three components, see Fig. 2. However, the 2 component, which showed the largest variations, maintained the same polarity, so that the general topology of the magnetic field was preserved during these events, despite the large amplitudes of individual perturbations. From 20.54 to 20.55 UT the MP was crossed outbound. This crossing was less distinct than that recorded inbound, and an associated directional change in the field was essentially from northward to southward. Within the Magnetosheath, the magnetic fields were highly variable in both direction and magnitude. The outbound BS crossing is not well defined in Fig. 1 although, using data with higher resolution, the experimenters recognized that this traversal took place in the interval 20.57-20.59UT and, briefly thereafter again due to BS ‘Yapping”, at 21 .OOUT. It was deduced that a quasi-perpendicular shock was traversed inbound while a quasi-parallel shock was traversed outbound. It is noted (not shown) that right-handed, pol-

arized whistler mode waves were detected up to a frequency of I 10 Hz, inbound, upstream of the shock (Fairfield and Behannon, 1976).

3.2. Observations

during Encounter

III

Magnetic field observations (6 s averaged) recorded during Encounter III are presented in the lower four panels of Fig. 3, where the symbols used are the same as those adopted in Fig. 1. The Bow Shock (BS) and Magnetopause (MP) crossings, and the position of Closest Approach (CA) identified by Ness et al. (1974) are each marked on the record. During this fly-by, in contrast to the situation during Encounter I (see above), the orientation of the magnetic field was such that a quasi-parallel shock was recorded inbound while a quasi-perpendicular shock was traversed outbound. The magnetosphere was relatively quiet throughout the transit. The maximum magnetic field measured in the near polar region of the planet was approximately 400 G.

3.3. Scaling of the Magnetospheric

boundaries

The relative positions of the BS and MP surfaces recorded along the Mariner 10 trajectories during Encounters I and

S. M. P. McKenna-Lawlor:

200 -

Characteristic boundaries of the Hermean Magnetosphere

171

BS - Bow shock MP - Magnetopause CA - Closest approach

UT 16 March

1975

Fig. 3. Energetic electron observations

(top panel) and 6 s averaged magnetic field data recorded at Mercury during Encounter III, see also the designations used in the caption to Fig. 1 (following Ness (1979))

III with Mercury are summarized in Fig. 4 (right). The slashes along individual trajectories define 1 min intervals. In cases where the identification of a particular boundary surface was ambiguous due to multiple crossings, a general transition region along the trajectory is indicated. The spacecraft flew by the planet at 11 km SK’ so that the entire passage from BS to BS required less than 35 min. Two curves, representing a scaled (see below) MP and BS, both obtained from theoretical studies of the solar wind interaction with the geomagnetic field, are shown superimposed on the trajectory plots of Fig. 4 (right). The shape of the MP was computed for the case of the solar wind incident on a Hermean centred magnetic dipole orthogonal to the solar wind flow (assumed to be along X,,) and to the plane of the figure. The BS depicted is in accord with a sonic Mach number of 10 and an Alfven Mach number of 20 at the subsolar point. The values used

correspond approximately (see below) to contemporary measured values of the interplanetary magnetic field, plasma density and velocity. The theoretical BS position was first solved for the case of aligned flow (in which the upstream magnetic field and solar wind velocity were deemed to be parallel). A somewhat better correspondence with the observations was then obtained by taking into account the effect of aberration due to planetary heliocentric motion. This involved taking the true solar wind flow to be from YE of the solar direction rather than parallel to the Mercury-Sun line (see Fig. 4 left). Cylindrical symmetry in the shape of the BS and MP boundaries about the direction of solar wind flow was, in every case, assumed. A characteristic length scale of any magnetosphere is the distance from the centre of the planet to the subsolar point of the MP. This length is 11 RE ( = 7 x lo4 km) for

172

S. M. P. McKenna-Lawlor:

Characteristic

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of the Hermean

Magnetosphere

2220 UT P

XlME

&SE

5” Aberration (S.W. radial).

No aberration (S.W. 5” from east)

Fig. 4. (Right) Relative locations of characteristic boundaries of Mercury’s Magnetosphere based on magnetic data secured during Encounter I and Encounter III. Superimposed on the diagram are the average positions of the appropriately scaled (see the text) terrestrial Magnetopause and Bow Shock surfaces, taking into account prevailing interplanetary conditions at Mercury. (Left) A correction for the indicated aberration associated with solar wind flow is also included. Cylindrical symmetry in the shave of both boundary surfaces about the direction of solar wind flow is assumed in each case (following Ness et al. (1975))

the Earth and it was deduced, using the Mariner 10 data, to be approximately 1.4 &,,, ( = 3.4 x lo3 km) for Mercury (Ness, 1979). Compared to the magnetosphere of the Earth, the magnetosphere of Mercury is thus smaller by approximately a factor of 8 in relation to the size of the planet, and by a factor of about 20 in absolute units. Mariner 10 observations made in the Hermean magnetosphere at a distance of xR, can thus be compared with corresponding observations in the Terrestrial Magnetosphere at a distance of 8xR,. Using this scaling (see Fig. 4), even though no attempt was made to adjust the scaled curves to fit all the observed boundary traversals. The MP and BS locations at Mercury and the Earth matched quite well along the spacecraft trajectory and, on the basis of these comparisons, the magnetic field patterns in both magnetospheres appeared to be nearly the same. It is noted that, while the geometrically determined distance to the MP stagnation point of solar wind flow was taken to be 1.45 5 0.15 R, at Mercury by Ness (1979) in constructing Fig. 4, this distance would, in actuality, be expected to be somewhat larger on average (i.e. 1.8 kO.2 R,), due to solar wind variability and to the highly eccentric orbit of Mercury (Siscoe and Christopher, 1975). Also, the comparisons made in the figure with terrestrial surfaces assume that identical conditions of solar wind flow and the same interplanetary magnetic field configuration characterized Encounters I and III. In fact, the environmental values adopted in the calculations were based on a convolution of the temporal variations that actually occurred. As a result of fitting suitably averaged experimental data to a centred, tilted magnetic dipole (Ni = I), while assuming that external terms could vary linearly across the magnetosphere (Ni = 2) Ness et al. (1975) calculated

the internal dipole moment of Mercury to be 5.1 k 0.3 x 10z2G cm3, oriented at a solar ecliptic latitude of - 80” k 5” and a longitude of +285” &-10”. Also, the polarity of the Hermean dipole moment was found to be in the same sense as that of the terrestrial magnetic field. The magnetic moment thus calculated compares well with that deduced from the positions of the MP and BS boundaries, and with the inferred magnetic moment responsible for the local deflection of the solar wind. On the basis of the estimate made of the intrinsic planetary field, and utilizing 42 s data averages, Ness (1979) computed the perturbation magnetic field pertaining during the magnetospheric traversal of Mercury by Mariner 10 in the course of Encounter I. Figure 5 provides a projection of this perturbation magnetic field on the Y-Z and X-Y solar ecliptic planes. The diagrams indicate that the magnetic field, as viewed in the Y-Z plane, showed a predominantly southward directed orientation, and a magnitude which varied relatively smoothly from MP, to CA, to MP. The sense of the field is exactly what would be expected from a Magnetic Tail Current Sheet on the night-side of the planet. The orientation of the field, as viewed in the X-Y plane, shows a characteristic change in direction as the spacecraft passed from below the magnetic equatorial plane to above it. This change in the perturbation field direction corresponds with what would be observed in the Terrestrial Magnetosphere on passing from the southward lobe of the Magnetic Tail to its northward lobe (near X = - 8-12 RE). Measurements of the magnetic field made just after entry into the magnetosphere, indicate that the tail field BT is 30-40 G, and that the radius of the tail RT is 2.3 +- 0.3 R,. Assuming that the magnetic flux in the polar regions connects with the tail, and neglecting merging across the

S. M. P. McKenna-Lawlor:

173

Characteristic boundaries of the Hermean Magnetosphere

View from Sun

Centered

ZSE

A

@I

=

dipole

5.1 x 10z2 r cm3

Magnetopause

111 Bow shock obs.

8 = -80” l) = 285”

Dipole axis I I

uiet”

M 3 YSE

M

5

=

5.6 x lo** r cm3

Scaled

Magnetopause

XSE

Bow shock

to Sun

Mariner 10 Mercury I encounter 29 March

1914

Fig. 5. Projections of the perturbation magnetic field (I&) on the Y-Z and X-Y solar ecliptic planes. While Bz is mainly negative, especially close to the planet, the BxmY field shows a characteristic rotation by 90” as the Mariner 10 spacecraft crosses the neutral sheet-equatorial region of the Hermean Magnetosphere (following Ness (1979))

neutral sheet, the size of the Polar Cap (or,) can be estimated (Ness, 1979). This leads to the result (&) = 17”26”, which is approximately twice the value known to pertain in the Earth’s Polar Cap. Direct entry of the solar wind plasma to the Hermean Magnetosphere can thus occur more efficiently at Mercury than is the case at the Earth and is estimated that the penetration of plasma through the dayside cusps can reach down to latitudes of 50-60” (Ness (1979) and references therein).

3.4. Discussion

of Mevcury’s

magnetic field

A historical review of various attempts to determine the magnetic moment of Mercury is provided by Russell (1979). The estimates range from 1.8 x 10” to 6 x 102’ G cm3. Some authors have attempted to derive

multipole moments for Mercury and, among these, Whang (1977) provided a dipole value of 2.4 x 10” G cm3, tilted at 2.3” from the normal to the planetary orbital plane, having the same directional sense as that of the Earth and with the dipole, quadrupole and octupole moment intensities in the approximate ratios 1:0.4 :0.3, respectively. The quadrupole and octupole modelling studies have been criticized by Ness (1979) who indicate that they do not improve the dipole calculation, and that the apparent moments are due to spatial harmonic aliasing since there was not, in fact, sufficient planetary coverage to allow higher moments to be calculated. (The value estimated, see Section 3.3, for Mercury’s dipole moment from the Mariner 10 measurements is 5.1 f 0.3 x 1O22G cm3.) Owing to the high average density of Mercury, the planet is likely to have a large iron-nickel core with a

174

S. M. P. McKenna-Lawlor:

radius which is approximately 2/3 that of the planetary radius. Since this ratio substantially exceeds that pertaining at the Earth, the planetary surface field of Mercury can be surmised to be relatively distorted. In addition, the presence of sizeable higher order moments can be expected. Existing planetary field models suggest that a solar wind dynamic pressure of 25-150 times the “normal” value would be required to push the MP down to the planetary surface, due to the fact that, since the core is a significant fraction of the pIanetary radius, the magnetic field is effectively “stiffened” against compression (Russell (198 1) and references therein). Thus, see Ness (1979) and also references therein, the percentage of time that the solar wind might directly impact the surface of Mercury should be less than that calculated, for example, by Siscoe and Christopher (1975) on the basis of a relatively large value for the planetary magnetic moment. It may indeed be the case that the dynamic pressure is not, in general, sufficient to drive the solar wind flow directly into the planet. The origin of Mercury’s magnetic field is unclear. It may be due either to intrinsic magnetization of sub-Curie point material in the outer layers of the planet, or it may be due to an active dynamo in the interior (Ness, 1979; Russell, 1981). In recent years, a number of models have been devised of the stable current system needed for the formation of the observed magnetic field and most of these contain, not only a dipole term, but significant higher order terms as well. A complete mapping of the planetary field is presently required to provide observational constraints on these various models and, thereby, derive information concerning Mercury’s interior and its thermal history.

4. Energetic particle measurements 4.1. Measurements

during Encounter

I

Figure 1 allows a comparison to be made between charged particle counts (top three panels), electron plasma pressure P (panel 4) and magnetic field data (panels 5-8) simultaneously recorded during Encounter I. These records indicate that, in general, in the lead up to CA, all of these data were much less disturbed than was the case thereafter. In the disturbed regime, the large amplitude fluctations already noted in the magnetic field record (see Section 3.1) were accompanied by four intense bursts of energetic electrons (see the top panel of Fig. 1 where these latter events are labelled A, B, C and D). Consideration of the relative velocity of the spacecraft, the electron gyroradius, the count rate increases and the magnetic field data lead Simpson et al. (1974) to the conclusion that the particle events recorded were transients in time and not spatial structures. As shown in Fig. 6, taken from Criston et al. (1987) (see also Section 4.2), the A, B and C events occurred in the magnetosphere of Mercury, whereas event D occurred in the dawn Magnetosheath. No other energetic particle enhancements were observed in the vicinity of the planet. A preliminary analysis of the energetic particle data by Simpson et al. (1974) indicated that protons with energies

Characteristic boundaries of the Hermean Magnetosphere

up to approximately 550 keV and electrons with energies up to about 300 keV were present in the Hermean Magnetosphere. Later, Armstrong et aZ. (1975) and Criston et al. (1979) reconsidered these records and demonstrated that the energetic particle data cannot be interpreted uniquely, but are simultaneously consistent with the presence of (a) simultaneous fluxes of energetic electrons and protons ; and (b) intense fluxes of electrons only, the latter exhibiting very steep differential energy spectra. Electron pile up was suggested to be the most plausible explanation for the apparent detection of protons.

4.2. Discussion data

of the (Encounter

I> energetic particle

Comparative scaling of the Magnetosphere of Mercury to that of the Earth (see Section 3.3) shows that Mercury occupies a relatively large fraction of its magnetosphere. The permanent existence of a trapped charged particle radiation belt would not, given this configuration, be expected, and was not detected, but the presence during Encounter I of intense bursts of energetic particles in the magnetosphere indicates that a local acceleration process was active. The question as to whether simultaneous proton and electron bursts, or electron bursts alone, produced the energetic particle signatures recorded during Encounter I (see Figs 1 and 6), remains unresolved. The latter interpretation is, at the present time, generally adopted for interpretative purposes but the possible additional presence of protons can, nevertheless, not be excluded. In this connection it is noted that, over the years 1974-1980, there have been several occasions when the Helios-2 spacecraft has been upstream of Mercury and connected by interplanetary magnetic field lines to its Magnetosphere. In considering these data, Kirsch and Richter (1985) reported an instance, in May 1979, when statistically significant increases (2-50) in both proton (E > 80 keV) and electron (E > 60 keV) fluxes were recorded at Helios-2. These protons and electrons, which emanated from Mercury, were propagating towards the Sun, and a plausible interpretation of the data is that these particles originated in substorm activity in the Hermean Magnetosphere. (It was demonstrated that solar wind particles accelerated and reflected at the BS of Mercury could not have been responsible for the measured fluxes.) It is, associatively, interesting to recall the detection by the SLED instrument on the Phobos Mission to Mars and its Moons of ion beams with energies up to at least 55 keV travelling down the Martian Magnetotail (McKennaLawlor et al., 1993). These particles, which were travelling out along open field lines, were suggested to have originated in reconnection events similar to those reported by Sarris et al. (1976) to occur in the geomagnetic tail. Several authors, have pointed out that the transient magnetic field variations and particle disturbances (Events A-D), see Fig. 1, top, recorded during Encounter I in the Magnetic Tail of Mercury, closely resemble substorm related phenomena in the Earth’s Magnetotail. In this connection, Siscoe et al. (1975) suggested that a change in the Magnetosheath field from northward to southward

S. M. P. McKenna-Lawlor:

Characteristic

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of the Hermean

175

Magnetosphere

Mariner 10 - Mercury I DO88 Y 1974

View from north

View toward Sun

Fig. 6. Mariner 10 trajectory plot in Mercury Orbital coordinates, showing the relative locations of characteristic magnetosnheric boundaries and energetic particle events (A-D). For details see the text (following Cristoi et aZ:(1987))

while the spacecraft was deep within the magnetosphere would have resulted, according to a mechanism already recognized to act at the Earth, in the onset of substorm activity. This argument was shown to be consistent with the observation that the field of the Magnetosheath was indeed directed northward just prior to spacecraft entry to, and southward just after spacecraft exit from, the Hermean Magnetosphere. These authors also estimated that the spatial and temporal scales for magnetospheric phenomena at Mercury are smaller and shorter by factors of approximately 7 and approximately 50 respectively than is the case at the Earth. Thus, although the observed encounter interval at Mercury was short, a substorm frequency of four events in 17 min is reasonable when compared with the corresponding substorm frequency observed at the Earth. However, it was stressed that the absence of an appreciable ionosphere at Mercury precludes the making of direct comparisons with the terrestrial situation, and the interpretation that similar substorms occur at Mercury is only a tentative one. Eraker and Simpson (1986) reconsidered the data and suggested that strong similarities exist between the energetic electron B and C events at Mercury (see Fig. 1, top), and substorm injections at the Earth. They also argued that the B, C and D events are sequences of particle bursts, with repetition rates of 5-6 s and decay times of approximately 1.5 s decade-‘, associated with fast (6 s), recurrent neutral line formations and associated episodes of electron acceleration. More recently, Criston et al. (1987) re-examined the relationships between energetic electrons (> 35 and

> 175 keV), plasma electrons (13-688 eV) and magnetic fields recorded during Mercury Encounter I, and compared them with the pattern of Earthward propagating dynamic injections in the near Earth Magnetotail, to provide both agreement and disagreement with the work of Eraker and Simpson (1986). In their analysis, Criston et al. (1987) recognized differences in character between certain of the particle events (event B was divided into three components, B, B’ and BTailsee Fig. 6), which had not been previously recognized, for details see the original paper. Evidence was provided that well documented substorm injections at the Earth are similar in nature to the dynamic particle enhancements B, B’, and that these events, in analogy with the results of Eraker and Simpson (1986) (see above), can be interpreted to represent two separate consecutive dynamic injections at Mercury, perhaps corresponding to multiple onset substorms at the Earth. A marked correlation was noted between relatively hot plasma electrons and the > 35 keV electrons present in the B and B’ events, which were individually separated by 18 s of significantly cooler plasma electrons. Also, in the case of the complex B event, a positive correlation between > 35 keV electron increases and increases in the Z component of the local magnetic field throughout the first 30 s of the event, was taken to support an interpretation that the disturbances manifest in B and B’ were compressional waves. The suggestion of Eraker and Simpson (1986) that the fluctuations represent sequences of neutral line formations and associated bouts of electron acceleration on 6 s timescales, was rejected. Event C was shown to be different in nature to the

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Characteristic boundaries of the Hermean Magnetosphere

104 103 % s

102

\o

8 rJJ10’ ‘ct: e 2 100 : u

2

104

z 103 102

I ,

0 1

I

I I

, \ L\

104

103 102

10’ 100

103 103 102 -20

-10

C.A.

10

20

30

40

Minutes

Fig. 7. Observations of the low energy electron plasma close to Mercury recorded in two separate channels during Encounter III (top two panels) and Encounter I (bottom two panels), respectively. See the text for details (following Hartle et al. (1975)) complex B event and was argued to represent the near dawn signature of the injection process. Event A was demonstrated not to be a dynamic injection. Overall, there was a correlation between increases in the 2 component of the ambient magnetic field and event A, the complex B event and the C event. At the Earth such changes often indicate a restructuring of magnetospheric current systems when the inflated tail field relaxes to a lower tail flux. This finding supports in general the similarity between the Hermean and Terrestrial Magnetosvheres. However, the question of the occurrence of substorm activity at .Mer&ry remains, in general, controversial.

4.3. Measurements

during Encounter

A feature of the corresponding plasma electron observations shown in Fig. 7, which was first noted by Hartle et al. (1975) and later by Oglivie et al. (1977), is a significant drop in low energy electron flux near CA in the Polar Cap region of Mercury. The figure provides differential energy flux measurements from two characteristic channels of the Plasma Electron Spectrometer recorded during Encounter III (top two panels) and Encounter I (bottom two panels). The dearth of plasma in the polar region is evident. Again, the locations of the characteristic BS and MP boundaries are individually indicated on these plots. The observations display remarkable similarities to complementary data recorded in the terrestrial magnetosphere.

III 5. The atmosphere and ionosphere at Mercury

As shown in the top panel of Fig. 7, a burst of electrons with energies up to approximately 300 keV appeared in the polar regions as a spike during Encounter III. This transient burst was suggested by Criston et al. (1979) to be indicative of electron flux which is mirrored in the high latitude regions.

The possible existence, composition and structure of an atmosphere on Mercury was investigated by the UV Spectrometer experiment on Mariner 10, which searched for airglow at preselected wavelengths corresponding to the positions of the strong lines of He+, He, Ne, Ar, Xe, H,

S. M. P. McKenna-Lawlor:

0 and C (Broadfoot et al., 1974). This experiment detected a neutral atmosphere (an exosphere) in which neutral helium (surface density - 600 cm-3), and atomic hydrogen (surface density - 8 cme3), are the main constituents. Only upper limits were obtained for the other components. In addition, the He 584A airglow intensity showed an asymmetry from the morning to the evening terminators (Kumar, 1976). Possible sources for the development of the observed Exosphere include, solar wind accretion ; radioactive decay of uranium and thorium in the planetary crust and an “anomalous” supply of He due to the closeness of the Sun. Further research needs to be carried out to determine the contributions of these individual processes to the nature and origin of the Exosphere and to determine the state of reduction of the Hermean surface. Ultraviolet radiation from the Sun can release sodium from the surface rocks of Mercury. Recent observations made from the Earth indicate that, not only are there dayto-day variations in the distribution of sodium emissions, but also excess sodium emissions occur in localized regions at high northern and southern Hermean latitudes which sometimes extend into a “tail” in the antisunward direction. These phenomena suggest that there may be a significant link between magnetospheric and exospheric dynamics. Thus, studies of the sodium exosphere can potentially provide a means to remotely sense such processes as sputtering of surface minerals in polar regions during magnetic substorms, and transport of sodium ions along magnetic fields within the magnetosphere. In the case of the ionosphere, dispersive frequency measurements by the Mariner 10 dual frequency (S and X band) Radio Occultation Experiment, yielded only an upper limit to the dayside electron density of 103cmp3 (Fjeldbo et al., 1976). The Mariner 10 experiment recorded within the polar region, and on an open magnetic flux tube, an electron density of 0.1 cm3, whereas, elsewhere in the magnetosphere, the density was 1 cm3. For an ionosphere composed largely of He+ and electrons, the electron density would be 1.6 x lo-‘cm3 if the observed atmospheric values for He are used (Oglivie et al., 1977). If the Mariner 10 observation of 0.1 cm3 is correct it must be asked ; (a) is this ionosphere solar wind induced or of atmospheric origin? and (b) what is its ionic composition?

6. Problems for study on a future “Mercury Orbiter” 6.1. The magnetic$eld

177

Characteristic boundaries of the Hermean Magnetosphere

at Mercury

There are two possible sources for an intrinsic field, permanent or remanent magnetization of the crust, and regenerative dynamo action in a liquid core. The effectiveness of these sources at Mercury depends on the present thermal state of the planet and, in the case of remanence, also on its thermal history. In recent years, a number of models have been devised to provide the stable current system needed for the formation of the observed magnetic field and most of these contain, not only a dipole term, but significant higher order terms as well. A complete mapping of the planetary field is clearly now required to replace the presently available (very limited) Mariner 10

data set, so as to provide observational constraints on these various models and, thereby, gain insights into the current state of Mercury’s interior and elucidate its previous history. A candidate mission during which such measurements could be realized is the Mercury Orbiter, currently under consideration by the European Space Agency (ESA). The Assessment Study Report “Mercury Orbiter” presented by Balogh et al. (1994) to ESA (based on a proposal submitted to the Agency a year earlier in response to a Call for Mission Proposals), includes a Magnetometer in the proposed “strawman payload”.

6.2. Magnetospheric

dynamics at Mercury

The Mariner 10 particles and fields observations indicate that the magnetic field of Mercury, while weaker than that of the Earth, is nevertheless sufficiently strong to present an obstacle to the Solar Wind flow, thus creating a Magnetosphere which differs in several important respects from that of the Earth. The Hermean Magnetosphere consequently provides an environment in which magnetospheric phenomena can be studied on time and length scales dramatically different from those pertaining in other planetary magnetospheres. In this connection, the very tenuous atmosphere and ionosphere provide boundary conditions which result in significantly different patterns of current flow and energy transfer from the solar wind to the magnetosphere than those characterizing other such systems. The pressure balance between the planetary magnetic field and the impinging solar wind provides the basic scaling law for the size of the magnetosphere of Mercury. The magnetic field is weaker than that of the Earth by three orders of magnitude and, while the solar wind velocity is not greatly different at Mercury than at the Earth, its density is, on the average, about eight times larger than at 1 AU. Consideration of the available data by Ness (1979) indicates that, at the times of Encounters I and III, Mercury possesssd a magnetosphere which was smaller by a factor of about 8 than the (average) terrestrial magnetosphere (see Section 3.3). The nature of the substorm generation process is currently a matter of debate. This is because the lack of a significant ionosphere at Mercury rules out mechanisms invoking magnetosphereionosphere coupling as a key element. Even so, solar wind interaction must lead to the development of a magnetospheric current system. How such an electric current system would flow, and how it should connect with the planetary surface (as a function of local electrical conductivities), is presently unknown and awaits the making of detailed magnetic field and plasma investigations by state-of-the-art instrumentation on board the future Mercury Orbiter.

6.3. Proposal tojy an Energetic the Mercury Orbiter

Particle Detector

on

The suggested strawman payload for the Mercury Orbiter includes an Ion Spectrometer (E/q range : 10 eV-30 keV)

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and an Electron Analyser (range : 5 eV-30 keV) (Balogh et al., 1994). These capabilities would not allow the substorm process to be investigated over a sufficiently broad range of energies since particles up to several hundred keV were found to be present in the Hermean Magnetosphere during Encounter I (see Section 4). Also, these instruments would not allow the controversial question as to whether energetic protons are present in the Hermean Magnetosphere to be answered. It is recommended that, in addition to the Magnetometer and Low Energy Particle Detectors already foreseen for the payload, an Energetic Particle Detector be flown on the Mercury Orbiter, designed to provide “clean” ion and electron measurements in the approximate range 30 keV to several tens of MeV (the upper limit is to allow Cruise Phase studies of solar cosmic rays to also be mounted, see Section 6.5). This instrument would provide intensities, energy spectra and, if more than one set of telescopes could be accommodated, anisotropies (streaming directions) of ion and electron fluxes in the Hermean Magnetosphere as a function of time and location.

6.4. Scientific objectives of an Energetic Detector at Mercury The proposed tives : l

l

l

l

l

l

l

l

instrument

Particle

would have the following

objec-

To study the occurrence of transient intense bursts of relativistic electrons in the Magnetotail of Mercury and search for possible positive ion acceleration. To evaluate the characteristic timescales for energy storage in the Hermean Magnetotail and its release through electron, and possibly positive ion, acceleration. To investigate the origin and nature of dynamic particle processes in the Hermean Magnetosphere, and also their interaction and association with the interplanetary medium. To search for the possible quasi-trapping of energetic particles in the Hermean Magnetosphere. To search for gradient curvature drift and the loss cone effect, which should be more important at Mercury than at the Earth. To investigate the penetration of interplanetary electrons and positive ions of various energies into the Hermean Magnetosphere. This behaviour should differ significantly from that observed at the Earth, because Mercury occupies a significantly larger portion of its magnetosphere and has larger Polar Cusp regions than the Earth. Also, its rotational electric field is much less than the solar wind connection electric field, so that particles accelerated inside this Magnetotail should have direct access to the entire Hermean surface. To study the structure and dynamics of the magnetospheric boundary layers. To examine the interaction of the solar wind with the overall magnetosphere of Mercury. This should be different from .the situation at the Earth because of (a) the much smaller intrinsic magnetic field of Mercury and (b) the higher solar wind pressure, as well as the

l

l

l

Characteristic boundaries of the Hermean Magnetosphere more pronounced temporal and azimuthal variations in the solar wind parameters pertaining at 0.3 AU than at 1 AU. To study the nature and shape of the BS of Mercury and the possibility of associated electron and positive ion acceleration by shock reflection, and/or the firstand second-order Fermi acceleration mechanisms. To use the signatures of energetic particles to determine the shape, size and length of the Hermean Magnetotail. Characteristic gamma- and X-ray emissions produced at the surface of Mercury due to interactions with energetic particle radiation would be detected by the Gamma-ray and X-ray experiments aboard the Orbiter. These data could be usefully correlated with concomitant energetic proton and electron measurements.

6.5. Scientljic objectives of an Energetic Particle Detector during the Cruise Phase to Mercury The Mercury Orbiter will spend a considerable amount of time in the solar wind, so that an instrument flown to detect energetic particles in the Hermean magnetosphere could also, during the Cruise Phase, be used to monitor energetic particles present in the interplanetary medium. It is then appropriate to add to the scientific objectives of the proposed energetic particle instrument the following aims : l

l

l

Investigation, while in orbit outside the magnetosphere of Mercury, of the dynamics of the relatively unevolved solar wind. Study of expanding and travelling solar atmospheric structures and phenomena with respect to their composition and structural evolution. Measurements of the propagation, acceleration and modulation of suprathermal and energetic solar and galactic cosmic rays.

7. Conclusions An overview has been presented of published results concerning “particles and fields” data, and of attempts to detect an atmosphere and ionosphere at Mercury, using instruments flown aboard Mariner 10 during two encounters with the planet on March 1974 and March 1975, respectively. To date, Mariner 10 is the only spacecraft to have made in situ observations at Mercury. The relative locations of BS and MP signatures recorded at Mercury in magnetic and plasma electron data recorded during the two encounters considered, correlate well with the calculated positions of the same boundaries in the Earth’s magnetosphere, when equivalent interplanetary conditions are assumed, when aberration in the solar wind flow is taken into account and when observations made in the Hermean Magnetosphere at a distance of xRM are compared with corresponding observations in the Terrestrial Magnetosphere at a distance of 8xR,. The BS and MP signatures identified in the magnetic field, plasma and energetic particle data at Mercury are in

S. M. P. McKenna-Lawlor:

Characteristic boundaries of the Hermean Magnetosphere

accord with the geometry which is expected for interaction between a planet centred magnetic dipole and the solar wind. Although the origin of the magnetic field is not uniquely established, its dipole moment is estimated to be 5.lkO.3 x 1022Gcm3 (or about 0.04-0.09% that of the Earth). The geometrically determined distance to the MP stagnation point of Solar Wind flow is 1.45 fO.15 RM. This distance, however, can be expected to be somewhat larger on the average (reaching a value of I .8 f 0.2 RM), due to solar wind variability and to the highly eccentric orbit of Mercury. Right-hand polarized whistler modes (< 10 Hz) were recorded upstream of the Hermean BS. Also, an extended Polar Cusp region, about twice the size of that pertaining at the Earth was identified, as well as a Current Sheet and an extended Magnetotail. The impulsive acceleration of particles in the Tail suggests similarities with terrestrial substorm events. It is not presently known if keV electrons alone, or keV electrons and protons participate in the acceleration process. The restricted nature of the magnetic measurements made during the March 1974 and March 1975 fly-bys, as well as the effect of technical problems suffered by the instruments recording plasma and energetic particle data in limiting some of the deductions that can presently be made concerning the Hermean Magnetosphere, require the mounting of a further expedition to Mercury. A candidate for such a mission is the “Mercury Orbiter” which is currently under study by the European Space Agency. Considering only the fields and particles aspect of this proposed mission, since the origin of the Hermean magnetic field is presently in doubt, extended observations made from a spacecraft orbiting at a low altitude are required to determine higher order multiple moments of the planetary magnetic field, and to derive the topology of this field inside the magnetosphere. Such data would provide insights into the present nature of the interior of Mercury and elucidate its thermal history. In addition to instrumentation already foreseen to monitor the low energy plasma environment at Mercury, it is recommended to include in the payload of the Mercury Orbiter an Energetic Particle Detector to investigate transient bursts of relativistic electrons (and possibly also of protons), in the Magnetic Tail with energies of several hundred keV. These studies will involve evaluating the characteristic timescales for energy storage and release in a magnetosphere which has significantly different boundary conditions at its interior surface than is the case at the Earth. The mechanisms whereby, in the absence of a significant ionosphere, solar wind interaction leads to the development of a magnetospheric current system which connects with the planetary surface, awaits detailed investigation and interpretation. Studies will also be made of the basic mechanism/s whereby particles are accelerated in a Magnetic Tail-Neutral Sheet configuration. The Energetic Particle Detector could also support Cruise Phase studies carried out on board the Mercury Orbiter. It is expected in particular that outside, but close to, the Hermean Magnetosphere, the dynamics of the relatively unevolved solar wind flow could be investigated from a unique vantage point using the proposed instrument.

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Characteristic

boundaries

of the Hermean

Magnetosphere

Siscoe, G. L. and Christopher, I. (1975) Variations in the solar wind stand-off distance at Mercury. Geophys. Res. Lett. 2, 158-160. Siscoe, G. L., Ness, N. F. and Yeates, C. M. (1975) Substorms on Mercury? J. Geophys. Res. 80(3 l), 4359-4363. Whang, Y. C. (1977) Magnetospheric magnetic field of Mercury. J. Geophys. Res. 82(7), 10241030.