First identification of mirror mode waves in Venus' magnetosheath?

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X - 6VOLWERK ET AL.: VENUS’ MIRROR MODE WAVES4.1. Event 1: 5 May 2006On 5 May 2006 VEX entered from the solar wind (SW) through the bowshock (BS)into the magnetosheath (MS). At the beginning of the event the spacecraft was locatednear (1.69, -0.05, 0.48) R V in VSO coordinates and the solar wind magnetic field wasB SW = (−2.04, −4.73, 1.37) nT. Immediately after crossing the bowshock large amplitudecompressional waves occur as shown in shaded region I in Fig. 1. In the two bottom panels∆B/B and the angles θ Bmv and β Bmv are shown. Just before the bowshock crossing ∼ 0113UT, ∆B/B starts to increase, which is an artifact due to the difference between the lowpassfiltered and non-filtered data. However, after the bowshock crossing in the shadedregion I, it is clearly visible that there are strong oscillations of the magnetic field withlarge amplitude. In the bottom panel of Fig. 1, the angle θ Bmv between the mean magneticfield and the maximum variance direction of the non-filtered data drops well below 20 ◦ ,indicating that the waves are mainly compressional. At the same time that θ Bmv dropsbelow 20 ◦ , the minimum variance direction angle with the ambient magnetic field, β Bmv ,increases to well above 80 ◦ . This means that after the bowshock crossing there is a clearindication of compressional waves with a period of ∼ 5 sec propagating perpedicular tothe ambient magnetic field.Spectral analysis of the waves, in a mean field-aligned coordinate system and the twotransverse components transformed into right- and lefthanded polarized components,shows a broad peak in the compressional component at ∼ 0.2 Hz, which agrees withthe observed waves (see Fig. 2, top panel).Slighty later, ∼ 0117 - 0118:30 UT, with the spacecraft deeper into the magnetosheath,there is another region (shaded region II in Fig. 1) displaying compressional waves prop-D R A F T February 13, 2008, 4:35am D R A F T
VOLWERK ET AL.: VENUS’ MIRROR MODE WAVES X - 7agating almost perpendicular to the ambient magnetic field. These waves, in contrast toshaded region I, now have a period of ∼ 15 sec.4.2. Event 2: 2 October 2006On 2 October 2006 VEX passed through pericenter just before ∼ 0433 UT, beinginside the magnetopause [see Zhang et al., 2007].At the beginning of the event thespacecraft was located near (-0.08, 0.09, 1.06) R V in VSO coordinates. VEX then crossedthe magnetopause at ∼ 0433:30 UT (when the rotations in B y and B z end) and thecompressional waves start at ∼ 0434 UT, shown by the shaded box I in Fig. 3.Thesolar wind conditions before VEX crossed the bowshock into the magnetosheath showeda slightly increased magnetic field strength B m ≈ 9 nT, whereas after VEX crossed thebowshock out from the magnetosheath the solar wind magnetic field strenght was B m ≈ 6nT. The waves are of larger size in this case, as can be seen in Fig. 2 bottom panel. Thewaves have a period of ∼ 10 sec and are strongly compressional, with minimum variancedirection almost perpendicular to the ambient magnetic field. Spectral analysis of thefirst interval of interest shows that there is a shoulder at ∼ 0.1 Hz, which agrees with theobserved waves.There is also a second region from 0436:30 till 0438:30 UT (shaded area II in Fig. 3)where strong compressional waves occur. The waves during this interval are basically thesame as in the interval I, also with a period of 10 seconds.5. DiscussionThis paper set out to find evidence of MM waves in Venus’ magnetosheath. Based onexperiences in the Earth’s magnetosheath, two regions were investigated, one near theD 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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