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Moreover, the large range of heliocentric distances in its orbit allows us to study the wave occurrence variability in a way that is not possible elsewhere in the solar system unless we utilize a comparative planetology approach. Mercury’s plasma environment properties are unique in the solar system, as the solar wind Alfvén Mach number is small but sufficient for the development of a foreshock. However, the SW Alfvénic Mach number at Mercury’s heliocentric distance range is expected to be above the critical value (~2–3), thus particle reflection at the bow shock is not negligible 16, 23, 30. Mercury constitutes an ideal natural laboratory to test this theory and previous reports, as the solar wind around Mercury is characterized by relatively low SW Alfvénic Mach number (~3–6) and beta that vary with the planet’s heliocentric distance. In general, ion reflection is a fundamental property of relatively high Mach number collisionless shocks 3, 21, 22, 23, 24, although it is not the only variable affecting it. In particular, it was shown that the reflected ion current (proxy for a controlled backstreaming ion current) increases linearly with it, after a critical value is surpassed. As shown by laboratory experiments, the Alfvénic Mach number (ratio between the solar wind velocity and the Alfvén speed) can have a strong impact on the presence of foreshock ULF waves. Ultra-low frequency electromagnetic plasma (ULF) waves associated with backstreaming protons have been reported in several solar system planetary foreshocks. These waves and the turbulent energy cascade rate contribute to restore the thermodynamical equilibrium of the plasma. Such plasma instabilities are responsible for the presence of electromagnetic waves with magnetic field spectral power above the turbulent solar wind spectrum.
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Waves 9 mercury free#
As the backstreaming particles move along the Interplanetary Magnetic Field (IMF) away from the planet, they constitute a source of free energy for various plasma instabilities. Due to the presence of both ion populations, the local proton velocity distribution function is therefore non-Maxwellian. The latter are produced by reflection of SW protons at the bow shock or leakage of plasma from downstream of the shock in the magnetosheath. As a result of this magnetic connection, the foreshock is filled with SW protons coexisting with a secondary population of backstreaming ions flowing upstream. For this reason, low frequency waves play a fundamental role in the transfer of energy and linear momentum between charged particles, especially in collissionless environments such as the SW or near a planetary body in a region known as the foreshock.Ī planetary foreshock is the spatial region upstream of, but magnetically connected to, a planet’s bow shock. These waves can be the cause or effect of non-Maxwellian plasma velocity distribution functions observed in the pristine SW, as well as in solar system planetary magnetosheaths, ionospheres and foreshocks. Low frequency waves are ubiquitous in space plasmas and have been observed in drastically different environments throughout the solar system. This state can be disturbed by low frequency waves, in association with additional plasma populations. The solar wind (SW) can ideally be thought of as a plasma that is Maxwellian. These results are relevant for planetary magnetospheres throughout the solar system, and the magnetospheres of exoplanets, and provide knowledge of particle acceleration mechanisms occurring inside foreshocks. Detection of these waves throughout Mercury’s highly eccentric orbit suggests the conditions for backstreaming protons are potentially present for all of Mercury’s heliocentric distances, despite the relatively low solar wind Alfvén Mach number regime. Consequently, we find that the wave occurrence rate increases with Mercury’s heliocentric distance. These conditions are consistent with larger foreshock size and reflection of solar wind protons, their most likely source. Here, we report that 0.05–0.41 Hz quasi-monochromatic waves are mostly present under quasi-radial and relatively low intensity Interplanetary Magnetic Field, based on 17 Mercury years of MESSENGER Magnetometer data. However, an open question remains in regards to understanding favorable conditions for these planetary foreshocks waves.
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Studies of Mercury’s foreshock have analyzed in detail the properties of ultra-low frequency waves.