DIAGNOSING THE MERCURY PLASMA ENVIRONMENT USING LOW-FREQUENCY ELECTRIC FIELD MEASUREMENTS

The magnetosphere ofMercury is most intriguing because of its extreme nature, with scale sizes vastly different from the corresponding terrestrial ones, and a harsh environment at a comparatively s ...


Introduction
Mercury's magnetosphere is quite different from that of the Earth making it interesting to study in its own right, yet the two magnetospheres are similar enough to allow comparative studies which are highly likely to provide new knowledge and improved understanding of them both.The electric field describes the plasma transport in a magnetosphere.The electric field also plays an important role in the interaction of the solar wind with the planetary magnetosphere and in the interaction of the magnetosphere with the underlying ionosphere, in the case of Earth, or, most likely, the underlying planetary surface or its immediate environment, in the case of Mercury.Finally, it is an important parameter for the energy transport between the different regions.
BepiColombo MMO (Mercury Magnetospheric Orbiter) will make the first observations ever of the low-frequency electric field in the Mercury environment.The spacecraft is equipped with two pairs of sensors for the electric field, WPT 1 (Wire Probe anTenna) and MEFISTO 1,2 (Mercury Electric Field In-Situ TOol).The two pairs are orthogonally mounted in the spacecraft spin plane and will measure the electric field vector in the spin plane over a wide frequency range.

Scientific Topics
We here briefly review a number of scientific topics related to the lowfrequency electric field in the plasma environment of planet Mercury.For a more thorough overview of the Mercury environment the reader is referred to, e.g., Cumnock and Blomberg. 3

Plasma Convection and Cross-Polar Potential
Plasma convection in a magnetosphere is controlled by the DC electric and magnetic fields.Thus, to determine plasma circulation and plasma transport across boundaries the static component of the electric field needs to be measured.The electric field and plasma flow inside the magnetosphere also give important information on the interaction of the solar wind with the magnetosphere.
In Earth's magnetosphere there is a saturation effect so that the potential drop inside the magnetopause does not increase linearly with an increasing potential drop across a similar distance in the solar wind. 4,5,6,7Hill's explanation of the saturation effect is based on the idea that field-aligned currents are set up that create a magnetic field opposing the field at the magnetopause and thus weakening reconnection, eventually limiting it at some fixed plateau value.The efficiency of this feedback process depends on the ionospheric conductivity.However, at Mercury it is difficult to conceive of a similar process given the low conductivity near the planet and the typically corresponding low ability of its magnetosphere to sustain any appreciable field-aligned currents.Thus, for strong solar wind electric fields, a significantly higher fraction may penetrate into the Hermean magnetosphere than would penetrate into the Terrestrial one, as further discussed elsewhere. 8In-situ measurements are needed to clarify the saturation mechanism at Mercury.

ULF Pulsations
There are indications from Mariner 10 magnetometer data of the existence of field-line resonances at Mercury 9 .A field-line resonance is a fundamental response of the planetomagnetic field to the solar wind's interaction with the magnetopause.At Mercury they are interesting also because they depend on the (electromagnetic) reflective properties of the surface and thus may be used to estimate the surface conductivity.This requires simultaneous measurements of the electric and the magnetic fields at low frequency.Some of the "surface" conductivity may reside in a photoelectron cloud just above the surface.By comparing observations in different local time sectors the two components can be separated.In the terrestrial magnetosphere (damped) standing ULF waves are quite frequently generated by a Kelvin-Helmholtz instability at the flanks of the magnetosphere as the solar wind streams past.A similar effect is conceivable at Mercury, and there have been some indications that it does indeed occur, 9 see Figure 1.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 lowfrequency electric and magnetic field measurements on BepiColombo will enable us, for the first time, to properly diagnose standing ULF 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. 10

Field-aligned Currents
Mariner 10 observed signatures of field-aligned currents. 11Whether these were persistent of transient is not known, although the generally poorly conducting low-altitude region may make persistent currents unlikely.Studying these with simultaneous measurements of the electric and the magnetic fields will shed additional light not only on the surface conductivity issue but also on the more general, and largely unsolved, question of current closure. 12,13epending on the details of the low-altitude closure mechanism the correlation between the electric and the magnetic "disturbance fields" will occur for different components, thus enabling diagnostics (Section 2.5).

Exo-ionospheric Conductivity
A conducting layer above the surface of Mercury could, at least in principle, come about in several ways.If the atmosphere were sufficiently dense it would be a plasma created mainly by photo-ionization of the neutral atmosphere.However, another possibility is that of a cloud of electrons, similar to an electron-rich cathode sheath, made up of electrons photo-emitted from the surface.

Current Closure
A spacecraft overflying a static structure of field-aligned currents closed by Pedersen current at low altitude will see a correlation between orthogonal components of the electric and magnetic vectors transverse to the main magnetic field (Blomberg 10 and references therein).Assuming also homogeneous conductivity, orthogonal components of the transverse E and B are correlated.Such a correlation is often observed when overflying structures in the terrestrial ionosphere.If photo-emission from the surface should indeed be an important source of charge carriers in the near-Hermean environment, there would be a net negative charge present, i.e., there would be a cathode sheath rather than a plasma.In this case, because of the absence of collisions, the current flowing may be a Hall current arising from ExB drifting electrons.With this assumption the correlation is seen for the same components of E and B rather than for the orthogonal ones. 10

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

Mirror Point Asymmetry
Because of the expected dipole offset from the centre of the planet, causing differently wide loss-cones in the two hemispheres, interhemispheric charge flow may arise along Mercury's magnetic field. 13Such a charge flow will lead to weak field-aligned currents connecting the hemispheres or to a parallel electric potential distributed along the field lines, or both.

Particle Trapping
The maximum kinetic energy of the particles that can be trapped in Mercury's 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 likely be trapped at energies up to several hundred MeV (i.e., beyond the energy range of all magnetospheric electrons).Whether radiation belts exist at Mercury is controversial.If they exist they are likely not present at all times, but rather an occasional phenomenon.Possible radiation belts at Mercury have also been discussed by, for example, Russell et al. 14 and Orsini et al. 15

LF and MF Waves
Energy transport within the Hermean magnetosphere is partly governed by plasma wave activity, in particular Alfvénic activity.Alfvén waves arise where dynamic processes occur.They can transport energy, in the form of electromagnetic energy, large distances before dissipating it through kinetic, inertial, or wave breaking processes.BepiColombo will be the first mission to Mercury where the detailed physics can be studied, i.e., energy transport, acceleration processes, and dissipation.A number of wave emission processes are expected.Hermean Kilometric Radiation (HKR) well below 1 MHz may be emitted from the "auroral" acceleration regions that are known to exist at Mercury, and have a similar cause as the Auroral Kilometric Radiation (AKR) near Earth.It is interesting to note that radio emissions originating near Mercury below about 50-80 kHz will be trapped inside (reflected back into) the Hermean magnetospheric cavity due to the dense ambient solar wind (30-70 cm -3 ).Further, the possible existence of radiation belts around Mercury may be inferred from synchrotron radiation emissions with a peak around a few MHz. 8

Solar Activity
By monitoring bursts of solar radio emissions that indicate the solar disturbance level, it is possible to investigate the whole chain of events from the Sun to the processes directly responsible for magnetospheric storms and substorms at Mercury.Specific activity on the Sun can then be related to specific activity in the Hermean magnetosphere.The time delay is somewhat less than one day for, for example, a solar coronal mass ejection to reach Mercury.It will be possible to compare space weather effects on Mercury with those on Earth, both directly when Earth and Mercury are close to each other in heliospheric longitude, and indirectly when they are separated in longitude.

Summary
Mercury's plasma environment in general and its magnetosphere in particular are exciting targets for future exploration.Since Mariner 10 did not and Messenger will not measure the low-frequency electric field, BepiColombo provides the first opportunity to make direct measurements of the electric fields around Mercury.

Fig. 1 .
Fig. 1.ULF pulsations recorded at Mercury by the Mariner 10 magnetometer, assumed to be the fourth harmonic of a standing wave (after Russell 9 ).
Figure 2 illustrates some of the possible current closure mechanisms at Mercury.