Electrostatic Solitary Waves in Venus’ Magnetosphere

Abstract

Electrostatic solitary waves (ESWs), also known as “Broadband Electrostatic Noise", are associated with regions carrying field-aligned currents, i.e. electron and ion beams, and have been shown to play a particular role in particle acceleration via wave-particle interactions and magnetic reconnection [Malaspina, 2020]. In space plasmas, bipolar electric field variations of short duration travelling largely parallel to the magnetic field are treated as the signature of these solitary waves in electric field (E− field) data, in addition to monopolar pulses also (but less often) recorded in the electric field data. While the former (bipolar E-field structures) are mostly interpreted as ion/electron- acoustic solitary waves, monopolar structures are interpreted as the double layers, i.e. transitions between regions of constant (finite) potential.

ESWs have long been observed and extensively investigated in near-Earth environments. They have been detected in various regions of the magnetosphere, the magnetosheath [Pickett, 2005], magnetospheric boundary layers, and also in the magnetopause [Cattell, 2002], the polar cap boundary layer [Tsurutani, 1998], and the bow shock [Bale, 2003]. They’ve also been observed in powerful currents like those associated with the auroral acceleration area, where magnetically aligned electron and ion beams are accelerated and pierce the Earth’s upper atmosphere (the Aurora) [Temerin, 1982]. Asides in the Earth’s magnetosphere, double layers have been recorded in no other planetary magnetosphere. Electric field structures are very likely to be present in induced planetary magnetospheres and play a prominent role in the physics of magnetic field-aligned currents and plasma homogenization, but they have remained undetected due to the small number of observations capable of detecting them. Very recently, Malaspina et al., [Malaspina, 2020] reported the first-ever observation of a plasma double layer in a distant (from the Earth) space environment, in observations of electric field structures at the induced magnetosphere of Venus. Their presence is related to various plasma processes such as slowing, deflection, and the heating of solar wind particles at the induced magnetosphere of Venus. Inspired by those measurements, we have investigated the implications of electron populations in the generation of DLs in Venus’ magnetosphere and their interplay with electrostatic solitary waves.

Based on a multi-fluid plasma model, we have interpreted the significance of cooler electron concentration in the formation of electrostatic solitary wave structures in the induced magnetosphere of Venus. Our proposed theory not only explains the E-field data’s particular morphology, but it also provides a generic interpretation for observed localized structures, thus connecting the dots between the mathematical description of coherent nonlinear structures and satellite observations. Our model aims at establishing a comprehensive framework for localised pulses in E-field data recorded during satellite expeditions, which in turn will be significant in defining the microphysics of Venus’ magnetosphere and presumably other similar planetary environments. Interestingly enough, a recent study has focused for the first time on modeling ESW structures recorded by the MAVEN mission on Mars’s induced magnetosphere [Kakad et al, 2022] and those observations qualitatively resemble the recordings from Venus.

References

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