First identification of mirror mode waves in Venus' magnetosheath?

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X - 8VOLWERK ET AL.: VENUS’ MIRROR MODE WAVESquasi-perpendicular bowshock and one deep inside the magnetosheath near the magnetopausein times of increased solar wind pressure. In both events, large amplitude (∆B/B)compressional waves (θ Bmv ≤ 20 ◦ ) that propagate perpendicular to the ambient magneticfield (β Bmv ≥ 80 ◦ ) were found. Therefore, these waves are considered to most likely beMM waves. Due to the lack of plasma data, there cannot be a definite proof that thesewaves are MM. However, the analysis technique used to identify MM without plasma datahas been proven successful by Lucek et al. [1999a] and been confirmed with plasma databy Rae et al. [2007].The first observed event takes place in a region where MM waves are expected tooccur, near the bowshock, the location of the spacecraft and the solar wind magneticfield direction indicate quasi-perpendicular shock conditions. This is one of the situationsmentioned above, in which ions can be (multiple) reflected at the bowshock, creating anenergized ion population with T ⊥ > T ‖ . Note that this is only a very short interval ≤ 1min in which the waves occur, when compared with the data from Lucek et al. [1999a]where intervals of more than 10 mins were found. However, at the subsolar point Venus’bowshock is located at ∼ 1.3R V and magnetopause at ∼ 1.1R V [Zhang et al., 2007],whereas for the Earth the bowshock is located at ∼ 12 − 15R E and the magnetopause at∼ 10R E . This would mean that the MM region near the quasi-perpendicular shock scalesrather well with the size of the magnetosheath at Venus.In Fig. 4 the data from Equator-S and VEX are shown for comparable regions in themagnetosheath, just after passing the bowshock. The difference is clear, on Earth theperiod of the waves is ∼ 30 sec., whereas for VEX the period is ∼ 5 sec. The amplitudeof the waves, at the begining of interval I of 5 May 2006 is similar to that of Equator-S,D R A F T February 13, 2008, 4:35am D R A F T
VOLWERK ET AL.: VENUS’ MIRROR MODE WAVES X - 9but drops off quite quickly and the period of the wave seems to change too. Interestingly,the results by Lucek et al. [1999a] show that the peak in the power spectrum shifts tolower frequencies as the spacecraft moves deeper into the magnetosheath, similar to whatis found for VEX in this paper.The second event occurs further inside the magnetosheath, near the magnetopause. Inthis case, it was mentioned above that the energization of ions can be produced throughcompression of the magnetosphere. Available ACE solar wind data for that time (notshown in this paper) reveal that the solar wind density was increased during this intervalas well as the solar wind speed.This indicates an increased ram pressure on Venus’induced mangetosphere and thus a compression. This compression of the magnetospherecan also be seen when comparing the data with the next orbit. For 2 October the magneticfield strength at periapsis is B m ≈ 50 nT, wheras on 3 October the magnetic field B m ≈ 40nT.The fact that the MM waves are non-continuous through the data for both events, butappear in small patches, is in agreement with Equator-S observations, where it was foundthat the MM waves appear in separate bursts.6. ConclusionsFor the first time, mirror-mode waves have been identified in Venus’ magnetosheathat two different locations. Comparing these waves with those found in the Earth’s magnetosheathshows that there are many similarities in the characteristics of these waves.Therefore, it can be concluded that Venus’ magnetosheath is much like the Earth’s, onlyscaled down by a factor ∼ 10.D R A F T February 13, 2008, 4:35am D R A F T
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First identification of mirror mode waves in Venus' magnetosheath?

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