- Email: [email protected]
On electromagnetic phenomena in Mercury’s magnetosphere
Advances in Space Research 33 (2004) 2161–2165 www.elsevier.com/locate/asr
On electromagnetic phenomena in MercuryÕs magnetosphere L.G. Blomberg
a,*
, J.A. Cumnock
a,b
a
b
Alfven Laboratory, Royal Institute of Technology, SE-10044 Stockholm, Sweden William B. Hanson Center for Space Sciences, University of Texas at Dallas, Richardson, TX 75083, USA Received 16 December 2002; received in revised form 21 February 2003; accepted 4 March 2003
Abstract Mercury has a small but intriguing magnetosphere. In this brief review, we discuss some similarities and differences between MercuryÕs and EarthÕs magnetospheres. In particular, we discuss how electric and magnetic field measurements can be used as a diagnostic tool to improve our understanding of the dynamics of MercuryÕs magnetosphere. These points are of interest to the upcoming ESA-JAXA BepiColombo mission to Mercury. Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Mercury; Magnetosphere; Magnetospheric currents; Field-aligned currents; Planetary magnetism
1. Introduction Earth and Mercury are the only Terrestrial planets to possess proper magnetospheres (i.e., magnetospheres set up mainly because of an intrinsic planetary magnetic field rather than magnetospheres induced by the solar wind flow, like at Venus and Mars). There are, however, significant differences between the magnetospheres of Mercury and the Earth. Mercury is located much closer to the Sun and has a much more eccentric orbit than the Earth. The proximity to the Sun means that the (dayside) temperature is higher, that the solar wind pressure is higher and that the planet photo-emits electrons at a much higher rate. The latter effect is also due to the lack of an atmosphere at Mercury, which has interesting consequences for the closure of field-aligned currents. In addition, Mercury has a much weaker intrinsic magnetic field than the Earth, with an equatorial surface strength of a few hundred nT, corresponding to a magnetic dipole moment of 3 1019 Am2 (e.g., Ness et al., 1975). This, together with the higher solar wind pressure, makes the magnetosphere much smaller. Depending on where the magnetic field is anchored, i.e., where the *
Corresponding author. Tel.: +46-8-790-7697; fax: +46-8-24-54-31. E-mail addresses: [email protected] (L.G. Blomberg), [email protected] (J.A. Cumnock).
dynamo is located, the magnetosphere will be more or less sensitive to fluctuations in the solar wind pressure. Possibly, the ‘‘anchor depth’’ is frequency dependent, cf. Blomberg (1997). Using the values in Table 1 and the simple assumption of pressure balance (in planar geometry) at the magnetopause between the dynamic pressure of the solar wind and the magnetic pressure of the planetary field, stand-off distances of 1.05 RM at perihelion and 1.20 RM at aphelion result. (For comparison, calculating the stand-off distance at Earth in a similar way yields 7.2 RE . Thus, the values tend to be slight under-estimates.) More sophisticated models have been employed (e.g., Slavin and Holzer, 1979, 1981). They give slightly higher values for the stand-off distance. For the stand-off distance to be less than unity (i.e., the solar wind reaches the planetary surface), the product n v2sw must exceed 2 1019 (m3 (m/s)2 ). The combinations of density and velocity for which this happens are illustrated in Fig. 1, using our simple model. Earlier estimates predict that the solar wind reaches MercuryÕs dayside surface approximately 6–7% of the time (e.g., Goldstein et al., 1981; Russell et al., 1988). More recent MHD simulations by Kabin et al. (2000) indicate that the solar wind hits the Hermean surface if either the solar wind speed exceeds its nominal value by a factor of 2.5 or the solar wind density is increased by a
0273-1177/$30 Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S0273-1177(03)00449-6
2162
L.G. Blomberg, J.A. Cumnock / Advances in Space Research 33 (2004) 2161–2165
Table 1 Average interplanetary conditions at the Terrestrial planets (adapted from Slavin and Holzer (1981)) Planet
R (AU)
vsw (km s1 )
Np cm3
jBj (nT)
Tp (eV)
Te (eV)
Mercury Venus Earth Mars
0.31 0.47 0.72 1.00 1.52
430 430 430 430 430
73 32 14 7 3.0
46 21 10 6 3.3
17 13 10 8.0 6.1
22 19 17 15 13
Scaling
–
R0
R2
ðR2 þ 1Þ1=2 =R
R2=3
R1=3
Fig. 1. Solar wind density and velocity values for which the solar wind nominally reaches the planetary surface. For all points above the curve the solar wind hits the planet, according to a simple pressure balance model.
factor of 9 (assuming that the other parameter is nominal). These numbers are within a factor of 2 of the numbers we arrived at in Fig. 1 using a simplistic model. Thus, the simple model seems remarkably good. We are, in this paper, mainly concerned with some of the many electromagnetic phenomena taking place in MercuryÕs magnetosphere. Several of these may be addressed with the upcoming ESA-JAXA mission BepiColombo (e.g., Anselmi and Scoon, 2001; Grard and Balogh, 2001). BepiColombo consists of three elements, one of which is an orbiter with a comprehensive instrument payload focused on plasma measurements. Undoubtedly, BepiColombo will make a major contribution to our understanding of planetary magnetospheres in general and MercuryÕs magnetosphere in particular.
2. Shielding of the interplanetary magnetic field From the Terrestrial magnetosphere we know that at least 10–20% of the y-component of the IMF penetrates into the magnetotail. One of the consequences is a rotation of the central current sheet. If the penetration were equally effective at Mercury, the tail would be severely twisted rather close to the planet, since the intrinsic Hermean magnetic field is significantly weaker
than the terrestrial one and the IMF is stronger at 0.4 AU than at 1 AU. Thus, either Mercury has a magnetotail whose topology is strongly dependent on the IMF, or the shielding of the IMF from the magnetosphere is significantly more effective at Mercury than at Earth. In either case, further study of the Hermean magnetotail will yield interesting results that will improve our understanding of solar wind-magnetosphere interactions in general. Examples of the effect of IMF penetration on the Terrestrial magnetosphere are found in Kullen and Blomberg (1996). In situ measurements of the magnetic field together with measurements of the DC electric field will provide a clear picture of both the magnetic topology of the tail and the plasma flows within the tail.
3. Mirror point asymmetry, particle trapping, radiation belts Radiation belts occur around a planet when there is a mechanism for injecting energetic charged particles into regions with a magnetic field geometry that allows (quasi) stable trapping. We will, in the following, examine to what extent stable trapping is possible in MercuryÕs magnetic field, without considering the particle injection mechanism(s). The details of MercuryÕs internal magnetic field are currently debated. The only consensus seems to be that a centered dipole is not a good approximation. Models of MercuryÕs magnetic field have been presented and discussed by several authors (e.g., Bergan and Engle, 1981; Connerney and Ness, 1988; Jackson and Beard, 1977; Ness et al., 1974, 1975, 1976; Ng and Beard, 1979; Whang, 1977, 1979). The models have been constructed either by directly fitting the Mariner 10 magnetometer data to a magnetic field model, or by fitting the Mariner 10 data to a magnetospheric model from which the planetary magnetic field is then deduced. Using a magnetospheric model has the advantage that the fit is done to the global structure of the magnetosphere rather than to a local magnetic field along the spacecraft trajectory which may contain local stray fields. The disadvantage is that additional model assumptions are necessary which introduces uncertainties.
L.G. Blomberg, J.A. Cumnock / Advances in Space Research 33 (2004) 2161–2165
We examine here the case of a dipole offset along the axis some 0.2 RM toward north. This is a reasonable approximation to the average reported field. The mirror points of the trapped particles are located at different altitudes in the two hemispheres, and the drift velocities of the particles will differ between the hemispheres, which will affect a possible ring current at Mercury. Because of the asymmetries in the mirror points, all charged particles escaping from the planetary surface in the northern hemisphere will impact on the surface in the southern hemisphere in the absence of collisions and field-aligned potentials. However, not all particles leaving the southern hemisphere will reach the conjugate surface. Rather, some of them will be reflected well above the surface and return (roughly) to the point from where they were emitted. If there is an asymmetry in the net flux of escaping particles between positive and negative charges, a field-aligned potential will build up gradually. The effect of this will be to neutralize the charge imbalance by attracting more particles of the ‘‘under-represented’’ polarity to the southern hemisphere. Thus, net plasma transport from the northern to the southern hemisphere results. Whether this transport can be sustained depends on the surface conductivity, or possibly on the current-conducting capacity of the photo-electron cloud over the dayside surface of the planet. The maximum kinetic energy of the particles that can be trapped in MercuryÕs magnetic field can be estimated by requiring the gyro radius be less than some fraction of the size of the magnetosphere. The upper limit to the energy of trapped protons is found to be of the order of 100 keV with a fairly large uncertainty depending on the assumptions made both about the size of the magnetosphere, the relevant fraction of its size, and the magnitude of the magnetic field (which may vary significantly over the gyro path). For heavier ion species the upper energy limit is lower, in inverse proportion to the square root of the atomic mass. Electrons can be trapped at energies up to several hundred MeV. Whether radiation belts exist at Mercury is controversial. If they exist they are probably not present at all times, but rather an occasional phenomenon. A possible related ring current would most likely be only partial, not reaching around all longitudes. Closure of a partial ring current would require field-aligned currents, which present further problems, discussed below. Radiation belts at Mercury have also been discussed by, for example, Baker et al. (1987), Orsini et al. (2001), and Russell et al. (1988).
4. Low-frequency waves, field line resonances In the Terrestrial magnetosphere (damped) standing Alfven waves are quite frequently generated by a Kelvin–Helmholtz instability at the flanks of the magneto-
2163
Fig. 2. Standing Alfven waves observed by Mariner 10. The waves were assumed to be the fourth harmonic of a field-line resonance. After Russell (1989).
sphere as the solar wind streams past. A similar effect is conceivable at Mercury and there have been some indications that it does indeed occur (Russell, 1989), see Fig. 2. This class of waves is often referred to as field line resonances, since they involve a large-scale fluctuating motion along the entire length of a set of magnetic field lines. Depending on the conducting properties of the planet and its immediate environment, the waves will be reflected either above, at, or below the surface. Also, depending on whether the conductance at the reflection boundary is greater or less than the waveguide conductance either the magnetic field or the electric field of the wave will change phase when it is reflected. Mariner 10 did not measure the electric field and, thus, no firm conclusions regarding the nature of the waves observed can be drawn. Combined, low-frequency electric and magnetic field measurements on BepiColombo will enable us, for the first time, to properly diagnose standing Alfven waves at Mercury, as well as tell us more about the conductive properties of the reflective boundary at low altitude. They will also tell us more about the conductive properties across the magnetic field close to the planetary surface (cf. Blomberg, 1997).
5. Field-aligned currents Field-aligned currents are a fundamental mediator of energy and momentum between electrically conducting regions. In EarthÕs environment these currents transfer energy from the outer magnetosphere into the ionosphere. In the regions where the field-aligned currents close across the magnetic field, momentum is exchanged. Because of the difficulties predicted at Mercury with current closure across B, the exact role (and even
2164
L.G. Blomberg, J.A. Cumnock / Advances in Space Research 33 (2004) 2161–2165
Fig. 3. Field-aligned current signatures observed by Mariner 10. After Slavin et al. (1997).
existence) of field-aligned currents in the Hermean magnetosphere is unclear. On the dayside some current closure through a photoelectron cloud near the surface may be possible and some closure may be provided by the planetary surface itself. On the nightside, current closure is much more difficult, since there is neither any photoelectron cloud nor any obvious mechanism to release electrons from the surface. Quite possibly, fieldaligned currents at Mercury may be transient rather than stationary, especially on the nightside. Fig. 3 illustrates field-aligned current signatures recorded by the Mariner 10 magnetometer. Because of the motion of the spacecraft it was not possible to conclude whether these currents were stationary or transient. Statistics of magnetometer data from several orbits will provide significant additional information. This will be available from Messenger as well as BepiColombo. Combined with in situ measurements of the DC and low-frequency electric field, as on BepiColombo, unambiguous determination of the nature of the magnetic field signatures should be possible in many cases.
6. Current closure The various possible mechanisms for low-altitude current closure across the magnetic field at Mercury are illustrated in Fig. 4. Photoelectrons are only available on the dayside of the planet and current closure is thus even more difficult on the nightside. To what extent the photoelectrons can close field-aligned currents is presently unclear, although recent estimates predict it to be a not very effective mechanism (Grard et al., 1999). The current-carrying capacity of the planetary surface is likewise unclear, as are the coupling mechanisms between the plasma and the surface. On the nightside there is a potential problem with electron release. Some electrons will likely be released due to particle impact, al-
Fig. 4. Schematic of various possible mechanisms for current closure at or near the Mercury surface. After Grard (1997).
though it is not clear whether these electrons are sufficient to carry other than transient currents. To some extent ions should be able to carry field-aligned current, but their contribution to possible current circuits is probably relatively minor. Once again, the combined low-frequency electric and magnetic field measurements on BepiColombo will provide much needed information on the current closure mechanisms. Different diagnostic techniques were discussed by Blomberg (1997).
7. Summary MercuryÕs magnetosphere is the stage for electromagnetic phenomena both similar to and different from those encountered in the Terrestrial magnetosphere. We have discussed some of the phenomena that can be addressed with the field and wave instrumentation on BepiColombo once it reaches Mercury. Many of them can be addressed, thanks to the DC and low-frequency electric field measurements planned on BepiColombo. The effects of the dipole offset on a possible ring current and on inter-hemispheric current flow along the magnetic field lines have not been discussed much in the literature so far. Neither have the shielding of the IMF and its effect on the magnetotail topology. We plan to study these phenomena more carefully in the near future.
Acknowledgements The authors thank the two referees for their constructive comments.
References Anselmi, A., Scoon, G.E.N. BepiColombo, ESAÕs Mercury cornerstone mission. Planet. Space Sci. 49, 1409–1420, 2001.
L.G. Blomberg, J.A. Cumnock / Advances in Space Research 33 (2004) 2161–2165 Baker, D.N., Borovsky, J.E., Burns, J.O., Gistler, G.R., Zeilik, M. Possible calorimetric effects at Mercury due to solar windmagnetosphere interactions. J. Geophys. Res. 92, 4707–4712, 1987. Bergan, S., Engle, I.M. MercuryÕs magnetic field revisited. J. Geophys. Res. 86, 1617–1620, 1981. Blomberg, L.G. MercuryÕs magnetosphere, exosphere and surface: low-frequency field and wave measurements as a diagnostic tool. Planet. Space Sci. 45, 143–148, 1997. Connerney, J.E.P., Ness, N.F. MercuryÕs magnetic field and interior, in: Mercury. University of Arizona Press, pp. 494– 513, 1988. Goldstein, B.E., Suess, S.T., Walker, R.J. Mercury: magnetospheric processes and the atmospheric supply and loss rates. J. Geophys. Res. 86, 5485–5499, 1981. Grard, R. Photoemission on the surface of Mercury and related electrical phenomena. Planet. Space Sci. 45, 67–72, 1997. Grard, R., Balogh, A. Returns to Mercury: science and mission objectives. Planet. Space Sci. 49, 1395–1407, 2001. Grard, R., Laakso, H., Pulkkinen, T.I. The role of photoemission in the coupling of the Mercury surface and magnetosphere. Planet. Space Sci. 47, 1459–1463, 1999. Jackson, D.J., Beard, D.B. The magnetic field of Mercury. J. Geophys. Res. 82, 2828–2836, 1977. Kabin, K., Gombosi, T.I., DeZeeuw, D.L., Powell, K.G. Interaction of Mercury with the solar wind. Icarus 143, 397–406, 2000. Kullen, A., Blomberg, L.G. The influence of IMF By on the mapping between the EarthÕs magnetotail and the ionosphere. Geophys. Res. Lett. 23, 2561–2564, 1996.
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Ness, N.F., Behannon, K.W., Lepping, R.P., Whang, Y.C., Schatten, K.H. Magnetic field observations near Mercury: preliminary results from Mariner 10. Science 185, 151–160, 1974. Ness, N.F., Behannon, K.W., Lepping, R.P., Whang, Y.C. Magnetic field of Mercury confirmed. Nature 255, 204–205, 1975. Ness, N.F., Behannon, K.W., Lepping, R.P., Whang, Y.C. Observations of MercuryÕs magnetic field. Icarus 28, 479–488, 1976. Ng, K.H., Beard, D.B. Possible displacements of MercuryÕs dipole. J. Geophys. Res. 84, 2115–2117, 1979. Orsini, S., Milillo, A., De Angelis, E., Di Lellis, A.M., Zanza, V., Livi, S. Remote sensing of MercuryÕs magnetospheric plasma environment via energetic neutral atoms imaging. Planet. Space Sci. 49, 1659–1668, 2001. Russell, C.T. ULF waves in the Mercury magnetosphere. Geophys. Res. Lett. 16, 1253–1256, 1989. Russell, C.T., Baker, D.N., Slavin, J.A. The Magnetosphere of Mercury, in: Mercury. University of Arizona Press, pp. 514–561, 1988. Slavin, J.A., Holzer, R.E. The effect of erosion on the solar wind standoff distance at Mercury. J. Geophys. Res. 84, 2076–2082, 1979. Slavin, J.A., Holzer, R.E. Solar wind flow about the Terrestrial planets 1. Modeling bow shock position and shape. J. Geophys. Res. 86, 11401–11418, 1981. Slavin, J.A., Owen, J.C.J., Connerney, J.E.P., Christon, S.P. Mariner 10 observations of field-aligned currents at Mercury. Planet. Space Sci. 45, 133–141, 1997. Whang, Y.C. Magnetospheric magnetic field of Mercury. J. Geophys. Res. 82, 1024–1030, 1977. Whang, Y.C. Model magnetosphere of Mercury. Phys. Earth Planet. Interiors 20, 218–230, 1979.
On electromagnetic phenomena in MercuryÕs magnetosphere L.G. Blomberg
a,*
, J.A. Cumnock
a,b
a
b
Alfven Laboratory, Royal Institute of Technology, SE-10044 Stockholm, Sweden William B. Hanson Center for Space Sciences, University of Texas at Dallas, Richardson, TX 75083, USA Received 16 December 2002; received in revised form 21 February 2003; accepted 4 March 2003
Abstract Mercury has a small but intriguing magnetosphere. In this brief review, we discuss some similarities and differences between MercuryÕs and EarthÕs magnetospheres. In particular, we discuss how electric and magnetic field measurements can be used as a diagnostic tool to improve our understanding of the dynamics of MercuryÕs magnetosphere. These points are of interest to the upcoming ESA-JAXA BepiColombo mission to Mercury. Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Mercury; Magnetosphere; Magnetospheric currents; Field-aligned currents; Planetary magnetism
1. Introduction Earth and Mercury are the only Terrestrial planets to possess proper magnetospheres (i.e., magnetospheres set up mainly because of an intrinsic planetary magnetic field rather than magnetospheres induced by the solar wind flow, like at Venus and Mars). There are, however, significant differences between the magnetospheres of Mercury and the Earth. Mercury is located much closer to the Sun and has a much more eccentric orbit than the Earth. The proximity to the Sun means that the (dayside) temperature is higher, that the solar wind pressure is higher and that the planet photo-emits electrons at a much higher rate. The latter effect is also due to the lack of an atmosphere at Mercury, which has interesting consequences for the closure of field-aligned currents. In addition, Mercury has a much weaker intrinsic magnetic field than the Earth, with an equatorial surface strength of a few hundred nT, corresponding to a magnetic dipole moment of 3 1019 Am2 (e.g., Ness et al., 1975). This, together with the higher solar wind pressure, makes the magnetosphere much smaller. Depending on where the magnetic field is anchored, i.e., where the *
Corresponding author. Tel.: +46-8-790-7697; fax: +46-8-24-54-31. E-mail addresses: [email protected] (L.G. Blomberg), [email protected] (J.A. Cumnock).
dynamo is located, the magnetosphere will be more or less sensitive to fluctuations in the solar wind pressure. Possibly, the ‘‘anchor depth’’ is frequency dependent, cf. Blomberg (1997). Using the values in Table 1 and the simple assumption of pressure balance (in planar geometry) at the magnetopause between the dynamic pressure of the solar wind and the magnetic pressure of the planetary field, stand-off distances of 1.05 RM at perihelion and 1.20 RM at aphelion result. (For comparison, calculating the stand-off distance at Earth in a similar way yields 7.2 RE . Thus, the values tend to be slight under-estimates.) More sophisticated models have been employed (e.g., Slavin and Holzer, 1979, 1981). They give slightly higher values for the stand-off distance. For the stand-off distance to be less than unity (i.e., the solar wind reaches the planetary surface), the product n v2sw must exceed 2 1019 (m3 (m/s)2 ). The combinations of density and velocity for which this happens are illustrated in Fig. 1, using our simple model. Earlier estimates predict that the solar wind reaches MercuryÕs dayside surface approximately 6–7% of the time (e.g., Goldstein et al., 1981; Russell et al., 1988). More recent MHD simulations by Kabin et al. (2000) indicate that the solar wind hits the Hermean surface if either the solar wind speed exceeds its nominal value by a factor of 2.5 or the solar wind density is increased by a
0273-1177/$30 Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S0273-1177(03)00449-6
2162
L.G. Blomberg, J.A. Cumnock / Advances in Space Research 33 (2004) 2161–2165
Table 1 Average interplanetary conditions at the Terrestrial planets (adapted from Slavin and Holzer (1981)) Planet
R (AU)
vsw (km s1 )
Np cm3
jBj (nT)
Tp (eV)
Te (eV)
Mercury Venus Earth Mars
0.31 0.47 0.72 1.00 1.52
430 430 430 430 430
73 32 14 7 3.0
46 21 10 6 3.3
17 13 10 8.0 6.1
22 19 17 15 13
Scaling
–
R0
R2
ðR2 þ 1Þ1=2 =R
R2=3
R1=3
Fig. 1. Solar wind density and velocity values for which the solar wind nominally reaches the planetary surface. For all points above the curve the solar wind hits the planet, according to a simple pressure balance model.
factor of 9 (assuming that the other parameter is nominal). These numbers are within a factor of 2 of the numbers we arrived at in Fig. 1 using a simplistic model. Thus, the simple model seems remarkably good. We are, in this paper, mainly concerned with some of the many electromagnetic phenomena taking place in MercuryÕs magnetosphere. Several of these may be addressed with the upcoming ESA-JAXA mission BepiColombo (e.g., Anselmi and Scoon, 2001; Grard and Balogh, 2001). BepiColombo consists of three elements, one of which is an orbiter with a comprehensive instrument payload focused on plasma measurements. Undoubtedly, BepiColombo will make a major contribution to our understanding of planetary magnetospheres in general and MercuryÕs magnetosphere in particular.
2. Shielding of the interplanetary magnetic field From the Terrestrial magnetosphere we know that at least 10–20% of the y-component of the IMF penetrates into the magnetotail. One of the consequences is a rotation of the central current sheet. If the penetration were equally effective at Mercury, the tail would be severely twisted rather close to the planet, since the intrinsic Hermean magnetic field is significantly weaker
than the terrestrial one and the IMF is stronger at 0.4 AU than at 1 AU. Thus, either Mercury has a magnetotail whose topology is strongly dependent on the IMF, or the shielding of the IMF from the magnetosphere is significantly more effective at Mercury than at Earth. In either case, further study of the Hermean magnetotail will yield interesting results that will improve our understanding of solar wind-magnetosphere interactions in general. Examples of the effect of IMF penetration on the Terrestrial magnetosphere are found in Kullen and Blomberg (1996). In situ measurements of the magnetic field together with measurements of the DC electric field will provide a clear picture of both the magnetic topology of the tail and the plasma flows within the tail.
3. Mirror point asymmetry, particle trapping, radiation belts Radiation belts occur around a planet when there is a mechanism for injecting energetic charged particles into regions with a magnetic field geometry that allows (quasi) stable trapping. We will, in the following, examine to what extent stable trapping is possible in MercuryÕs magnetic field, without considering the particle injection mechanism(s). The details of MercuryÕs internal magnetic field are currently debated. The only consensus seems to be that a centered dipole is not a good approximation. Models of MercuryÕs magnetic field have been presented and discussed by several authors (e.g., Bergan and Engle, 1981; Connerney and Ness, 1988; Jackson and Beard, 1977; Ness et al., 1974, 1975, 1976; Ng and Beard, 1979; Whang, 1977, 1979). The models have been constructed either by directly fitting the Mariner 10 magnetometer data to a magnetic field model, or by fitting the Mariner 10 data to a magnetospheric model from which the planetary magnetic field is then deduced. Using a magnetospheric model has the advantage that the fit is done to the global structure of the magnetosphere rather than to a local magnetic field along the spacecraft trajectory which may contain local stray fields. The disadvantage is that additional model assumptions are necessary which introduces uncertainties.
L.G. Blomberg, J.A. Cumnock / Advances in Space Research 33 (2004) 2161–2165
We examine here the case of a dipole offset along the axis some 0.2 RM toward north. This is a reasonable approximation to the average reported field. The mirror points of the trapped particles are located at different altitudes in the two hemispheres, and the drift velocities of the particles will differ between the hemispheres, which will affect a possible ring current at Mercury. Because of the asymmetries in the mirror points, all charged particles escaping from the planetary surface in the northern hemisphere will impact on the surface in the southern hemisphere in the absence of collisions and field-aligned potentials. However, not all particles leaving the southern hemisphere will reach the conjugate surface. Rather, some of them will be reflected well above the surface and return (roughly) to the point from where they were emitted. If there is an asymmetry in the net flux of escaping particles between positive and negative charges, a field-aligned potential will build up gradually. The effect of this will be to neutralize the charge imbalance by attracting more particles of the ‘‘under-represented’’ polarity to the southern hemisphere. Thus, net plasma transport from the northern to the southern hemisphere results. Whether this transport can be sustained depends on the surface conductivity, or possibly on the current-conducting capacity of the photo-electron cloud over the dayside surface of the planet. The maximum kinetic energy of the particles that can be trapped in MercuryÕs magnetic field can be estimated by requiring the gyro radius be less than some fraction of the size of the magnetosphere. The upper limit to the energy of trapped protons is found to be of the order of 100 keV with a fairly large uncertainty depending on the assumptions made both about the size of the magnetosphere, the relevant fraction of its size, and the magnitude of the magnetic field (which may vary significantly over the gyro path). For heavier ion species the upper energy limit is lower, in inverse proportion to the square root of the atomic mass. Electrons can be trapped at energies up to several hundred MeV. Whether radiation belts exist at Mercury is controversial. If they exist they are probably not present at all times, but rather an occasional phenomenon. A possible related ring current would most likely be only partial, not reaching around all longitudes. Closure of a partial ring current would require field-aligned currents, which present further problems, discussed below. Radiation belts at Mercury have also been discussed by, for example, Baker et al. (1987), Orsini et al. (2001), and Russell et al. (1988).
4. Low-frequency waves, field line resonances In the Terrestrial magnetosphere (damped) standing Alfven waves are quite frequently generated by a Kelvin–Helmholtz instability at the flanks of the magneto-
2163
Fig. 2. Standing Alfven waves observed by Mariner 10. The waves were assumed to be the fourth harmonic of a field-line resonance. After Russell (1989).
sphere as the solar wind streams past. A similar effect is conceivable at Mercury and there have been some indications that it does indeed occur (Russell, 1989), see Fig. 2. This class of waves is often referred to as field line resonances, since they involve a large-scale fluctuating motion along the entire length of a set of magnetic field lines. Depending on the conducting properties of the planet and its immediate environment, the waves will be reflected either above, at, or below the surface. Also, depending on whether the conductance at the reflection boundary is greater or less than the waveguide conductance either the magnetic field or the electric field of the wave will change phase when it is reflected. Mariner 10 did not measure the electric field and, thus, no firm conclusions regarding the nature of the waves observed can be drawn. Combined, low-frequency electric and magnetic field measurements on BepiColombo will enable us, for the first time, to properly diagnose standing Alfven waves at Mercury, as well as tell us more about the conductive properties of the reflective boundary at low altitude. They will also tell us more about the conductive properties across the magnetic field close to the planetary surface (cf. Blomberg, 1997).
5. Field-aligned currents Field-aligned currents are a fundamental mediator of energy and momentum between electrically conducting regions. In EarthÕs environment these currents transfer energy from the outer magnetosphere into the ionosphere. In the regions where the field-aligned currents close across the magnetic field, momentum is exchanged. Because of the difficulties predicted at Mercury with current closure across B, the exact role (and even
2164
L.G. Blomberg, J.A. Cumnock / Advances in Space Research 33 (2004) 2161–2165
Fig. 3. Field-aligned current signatures observed by Mariner 10. After Slavin et al. (1997).
existence) of field-aligned currents in the Hermean magnetosphere is unclear. On the dayside some current closure through a photoelectron cloud near the surface may be possible and some closure may be provided by the planetary surface itself. On the nightside, current closure is much more difficult, since there is neither any photoelectron cloud nor any obvious mechanism to release electrons from the surface. Quite possibly, fieldaligned currents at Mercury may be transient rather than stationary, especially on the nightside. Fig. 3 illustrates field-aligned current signatures recorded by the Mariner 10 magnetometer. Because of the motion of the spacecraft it was not possible to conclude whether these currents were stationary or transient. Statistics of magnetometer data from several orbits will provide significant additional information. This will be available from Messenger as well as BepiColombo. Combined with in situ measurements of the DC and low-frequency electric field, as on BepiColombo, unambiguous determination of the nature of the magnetic field signatures should be possible in many cases.
6. Current closure The various possible mechanisms for low-altitude current closure across the magnetic field at Mercury are illustrated in Fig. 4. Photoelectrons are only available on the dayside of the planet and current closure is thus even more difficult on the nightside. To what extent the photoelectrons can close field-aligned currents is presently unclear, although recent estimates predict it to be a not very effective mechanism (Grard et al., 1999). The current-carrying capacity of the planetary surface is likewise unclear, as are the coupling mechanisms between the plasma and the surface. On the nightside there is a potential problem with electron release. Some electrons will likely be released due to particle impact, al-
Fig. 4. Schematic of various possible mechanisms for current closure at or near the Mercury surface. After Grard (1997).
though it is not clear whether these electrons are sufficient to carry other than transient currents. To some extent ions should be able to carry field-aligned current, but their contribution to possible current circuits is probably relatively minor. Once again, the combined low-frequency electric and magnetic field measurements on BepiColombo will provide much needed information on the current closure mechanisms. Different diagnostic techniques were discussed by Blomberg (1997).
7. Summary MercuryÕs magnetosphere is the stage for electromagnetic phenomena both similar to and different from those encountered in the Terrestrial magnetosphere. We have discussed some of the phenomena that can be addressed with the field and wave instrumentation on BepiColombo once it reaches Mercury. Many of them can be addressed, thanks to the DC and low-frequency electric field measurements planned on BepiColombo. The effects of the dipole offset on a possible ring current and on inter-hemispheric current flow along the magnetic field lines have not been discussed much in the literature so far. Neither have the shielding of the IMF and its effect on the magnetotail topology. We plan to study these phenomena more carefully in the near future.
Acknowledgements The authors thank the two referees for their constructive comments.
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