Abstract
Abstract. The observational rate of mirror mode waves in Venus's magnetosheath for solar maximum conditions is studied and compared with previous results for solar minimum conditions. It is found that the number of mirror mode events is approximately 14 % higher for solar maximum than for solar minimum. A possible cause is the increase in solar UV radiation, ionizing more neutrals from Venus's exosphere and the outward displacement of the bow shock during solar maximum. Also, the solar wind properties (speed, density) differ for solar minimum and maximum. The maximum observational rate, however, over Venus's magnetosheath remains almost the same, with only differences in the distribution along the flow line. This may be caused by the interplay of a decreasing solar wind density and a slightly higher solar wind velocity for this solar maximum. The distribution of strengths of the mirror mode waves is shown to be exponentially falling off, with (almost) the same coefficient for solar maximum and minimum. The plasma conditions in Venus's magnetosheath are different for solar minimum as compared to solar maximum. For solar minimum, mirror mode waves are created directly behind where the bow shock will decay, whereas for solar maximum all created mirror modes can grow.
Highlights
Mirror mode (MM) waves are a key ingredient of the wave activity in planetary and cometary magnetosheaths
The waves are generated by a temperature asymmetry and Hasegawa (1969) showed that for a bi-Maxwellian plasma the instability criterion is given by 1 + β⊥
At crossing the quasi-perpendicular bow shock the ions are mainly heated in the perpendicular direction, with respect to the background magnetic field, when compared to the parallel direction, increasing the already existing temperature asymmetry of the ring-beam distribution
Summary
Mirror mode (MM) waves are a key ingredient of the wave activity in planetary and cometary magnetosheaths (see, e.g. Tsurutani et al, 1982; Erdös and Balogh, 1993; Glassmeier et al, 1993; Bavassano Cattaneo et al, 1998; Baumjohann et al, 1999; Lucek et al, 1999; Joy et al, 2006; Schmid et al, 2014; Volwerk et al, 2014; Soucek et al, 2015). The growth rate for MM waves was estimated by Gary (1991) to be proportional to the proton cyclotron frequency γ ∝ 0.1ωc,p; Tátrallyay et al (2008) with spacecraft observations have shown that this is an overestimation Another driver for MM waves is magnetic field line draping (see Tsurutani et al, 2011; Volwerk et al, 2008b), Published by Copernicus Publications on behalf of the European Geosciences Union. Tsurutani et al (2002) and Remya et al (2014), showed that during a period of exceptionally low solar wind plasma-β (∼ 0.35), the magnetosheath can be prone to a high occurrence rate of ion cyclotron waves (see Czaykowska et al, 2001). A discussion about the differences and similarities between the two states of solar activity is performed and the paper ends with some conclusions and concluding remarks
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