MESSENGER: Exploring Mercury’s Magnetosphere

Abstract

Abstract. The MESSENGER mission to Mercury offers our first opportunity to explore this planet's miniature magnetosphere since the brief flybys of Mariner 10. Mercury's magnetosphere is unique in many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands off the solar wind only -1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic particles and, hence, no radiation belts. The characteristic time scales for wave propagation and convective transport are short and kinetic and fluid modes may be coupled. Magnetic reconnection at the dayside magnetopause may erode the subsolar magnetosphere allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury's interior may act to modify the solar wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects may be an important source of information on the state of Mercury's interior. In addition, Mercury's magnetosphere is the only one with its defining magnetic flux tubes rooted in a planetary regolith as opposed to an atmosphere with a conductive ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived, -1-2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury's magnetic tail. Because of Mercury's proximity to the sun, 0.3 -0.5 AU, this magnetosphere experiences the most extreme driving forces in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and re-cycling of neutrals and ions between the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury's magnetosphere are expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection at the magnetopause and in the tail, and the pick-up of planetary ions all driving field-aligned electric currents.However, these field-aligned currents do not close in an ionosphere, but in some other manner. In addition to the insights-into magnetospheric physics offered by study of the solar wind -Mercury system, quantitative specification of the "external" magnetic field generated by magnetospheric currents is necessary for accurate determination of the strength and multi-polar decomposition of Mercury's intrinsic magnetic field.MESSENGER'S highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin of Mercury's magnetic field and the acceleration of charged particles in small magnetospheres. In. this article, we review what is known about Mercury's magnetosphere and describe the MESSENGER science team's strategy for obtaining answers to the outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere.2

Intense energetic electron flux enhancements in Mercury's magnetosphere: An integrated view with high‐resolution observations from MESSENGER

“…The particle acceleration we have documented in this study can be well accounted for, we argue, by the inductive electric fields associated with collapsing, rapidly reconfiguring fields illustrated in Figure [ Slavin et al , ]. As discussed recently in detail by Birn et al [], the changing magnetic field creates powerful inductive electric fields [see also Baker et al , ; Hoshino , ; Drake et al , ] that can readily accelerate electrons to energies of hundreds of keV on very short time scales.…”

Energetic Electron Acceleration and Injection During Dipolarization Events in Mercury's Magnetotail

“…Closed drift paths, as well as the potential for Mercury's magnetosphere to host radiation belts, have been controversial subjects since Mariner 10's flybys (e.g., Baker et al, ). While the large loss cones and small magnetopause standoff distance prevent permanent radiation belts like at Earth (Slavin et al, ), an increasing number of observations at Mercury suggest that “quasi‐trapped” populations of electrons are able to execute multiple drifts about the planet before being lost to surface precipitation or magnetopause shadowing (e.g., Baker et al, ; Ho et al, ). Analytic simulations also suggest the possibility of closed drift paths at Mercury, although they appear to be more Shabansky‐like in nature (Walsh et al, ).…”

“…While Mercury's intrinsic magnetic field forms a terrestrial‐like magnetosphere when it interacts with the solar wind (e.g., Alexeev et al, ; Anderson et al, ), its magnetospheric dynamics operate on significantly smaller spatial scales and shorter temporal scales than the Earth's due to the many differences between the two magnetospheres (see, e.g., Slavin et al, , , ). The small physical scales limit the time an energetic particle can gain energy in Mercury's magnetosphere before being lost to surface precipitation or magnetopause shadowing, leaving little possibility for trapped radiation belts (Slavin et al, ) and constraining possible acceleration mechanisms (Zelenyi et al, ). While electrons behave adiabatically, the magnetosphere's small size can result in nonadiabatic ion behavior and strong finite gyroradius effects (Delcourt et al, ).…”

Paraboloid model of Mercury's magnetosphere

“…The duration of a substorm at Mercury was found by Mariner 10 to be only a few minutes as compared to about 1 hr on Earth [ Siscoe et al , 1975]. However, it remains to be determined whether these disturbances are directly driven by dayside reconnection as suggested by Luhmann et al [1998] or associated with the storage of energy in this planet's tail and sporadic, explosive release as seen in the Earth's magnetosphere [ Slavin et al , 2007].…”