Abstract
Abstract. Because of the small planetary magnetic field as well as proximity to the Sun that leads to enhanced solar wind pressure as compared to Earth, the magnetosphere of Mercury is very dynamical and at times subjected to prominent compression. We investigate the dynamics of magnetospheric ions during such compression events. Using three-dimensional single-particle simulations, we show that the electric field induced by the time varying magnetic field can lead to significant ion energization, up to several hundreds of eVs or a few keVs. This energization occurs in a nonadiabatic manner, being characterized by large enhancements of the ion magnetic moment and bunching in gyration phase. It is obtained when the ion cyclotron period is comparable to the field variation time scale. This condition for nonadiabatic heating is realized in distinct regions of space for ions with different mass-to-charge ratios. During compression of Mercury's magnetosphere, heavy ions originating from the planetary exosphere may be subjected to such an abrupt energization, leading to loading of the magnetospheric lobes with energetic material.
Highlights
The early measurements of Mariner-10 in 1974–1975 revealed that Mercury possesses a weak intrinsic magnetic field that leads to the formation of a miniature magnetosphere (e.g., Ness, 1979) with typical time scales much smaller than those at Earth (e.g., Russell and Walker, 1985)
We examine the dynamics of magnetospheric ions during such compression events
For ion species with cyclotron period comparable to the field variation time scale, prominent nonadiabatic heating may be achieved under the effect of the electric field induced by the magnetic field reconfiguration
Summary
The early measurements of Mariner-10 in 1974–1975 revealed that Mercury possesses a weak intrinsic magnetic field that leads to the formation of a miniature magnetosphere (e.g., Ness, 1979) with typical time scales much smaller (by about a factor 20) than those at Earth (e.g., Russell and Walker, 1985). The recent observations of MESSENGER during flybys of Mercury confirm this result, the internal field being in first approximation dipolar with an estimated moment of ∼250 nT RM3 (RM denoting Mercury’s radius) (Anderson et al, 2008). These observations provided evidences of a substantial contribution of ionized material originating from the planetary exosphere (e.g., Na+-Mg+, S+-O+2 , K+-Ca+) to the inner magnetotail (Zurbuchen et al, 2008). Given the small magnitude of the internal field and the enhanced solar wind pressure due to proximity with the Sun, the magnetosphere of Mercury is expected to be very dynamical and subjected to frequent reconfigurations. For ion species with cyclotron period comparable to the field variation time scale, prominent nonadiabatic heating may be achieved under the effect of the electric field induced by the magnetic field reconfiguration
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