First identification of mirror mode waves in Venus' magnetosheath?

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. ???, XXXX, DOI:10.1029/,First identification of mirror mode waves in Venus’magnetosheath?M. Volwerk 1 , T.L. Zhang 1 , M. Delva, 1 , Z. Vörös 2 , W. Baumjohann 1 andK.-H. Glassmeier 3M. Volwerk, Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042Graz, Austria (martin.volwerk@oeaw.ac.at)1 Space Research Insitute, AustrianAcademy of Sciences, 8042 Graz, Austria2 Institute for Astro- and Particle Physics,University of Innsbruck, Austria3 Institut für Geophysik undextraterrestrische Physik, TechnischeUniversität, 38106 Braunschweig, GermanyD R A F T February 13, 2008, 4:35am D R A F T

X - 2Abstract.VOLWERK ET AL.: VENUS’ MIRROR MODE WAVESMagnetometer data from the Venus Express spacecraft obtainedin Venus’ magnetosheath are investigated, to search for evidence for the existenceof mirror-mode waves. In two regions in the Venusian magnetosheathstrong compressional, waves which propagate nearly perpendicular to the ambientmagnetic field, have been found. They are most likely mirror-mode waves.These waves show up just behind the quasi-perpendicular bow shock, andnear the magnetopause during compression of the magnetosheath due to increasedsolar wind pressure.D R A F T February 13, 2008, 4:35am D R A F T

VOLWERK ET AL.: VENUS’ MIRROR MODE WAVES X - 31. IntroductionThe mirror instability has been discussed for space plasmas with strong temperatureasymmetries, i.e. with perpendicular temperature higher than the parallel temperature[Hasegawa, 1969; Gary et al., 1993; Southwood and Kivelson, 1993]. These waves havebeen found in various objects, e.g. the Earth’s magnetotail [Rae et al., 2007], the magnetosheathof the Earth [e.g., Tsurutani et al., 1982; Baumjohann et al., 1999; Lucek et al.,1999a; Constantinescu et al., 2004] and of Jupiter [Erdös and Balogh, 1993], in the tail[e.g. Russell et al., 1987] and magnetic pile up boundary [Glassmeier et al., 1993] of acomet and in the ion pick-up region near Io [e.g., Huddleston et al., 1999].The instability criterion for mirror mode (MM) waves, i.e. high-β plasma and T ⊥ > T ‖ ,however, is shared with another wave mode, namely the ion cyclotron (IC) instability[see Delva et al., 2008, for IC waves at Venus].Gary et al. [1993] have shown thatthe growthrate for IC is usually greater than for MM. Southwood and Kivelson [1993]have proposed that the inhomogeneities in the background plasma, created by the MM,will inhibit the IC growth in planetary magnetosheaths. The MM instability generatescompressional waves that grow preferentially in the direction perpendicular to the ambientmagnetic field [see section 3.5 in Treumannn and Baumjohann, 1997].At frequencies below the IC frequency, the MM behaves in such a way that the perpendicularpressure p ⊥ of the plasma will be in anti-phase with compressional variationsin the magnetic field [Hasegawa, 1969]. In a bi-maxwellian plasma with perpendiculartemperature T ⊥ and parallel temperature T ‖ this means thatD R A F T February 13, 2008, 4:35am D R A F T

X - 4VOLWERK ET AL.: VENUS’ MIRROR MODE WAVESwhich leads to the instability criterion:(δp ⊥ = 2p ⊥ 1 − T )⊥ δBT ‖ B , (1)(1 + β ⊥ 1 − T )⊥< 0. (2)T ‖The temperature asymmetry in the (Earth’s) magnetosheath can be created by twomechanisms [Lucek et al., 1999b]: (1) gyratory motion of the ions caused by (multiple)reflection(s) at the bowshock under quasi-perpendicular conditions before entering themagnetosheath; or (2) compression of the magnetosheath close to the magnetopause. Inthis paper two events in Venus’ magnetosheath are presented which show that the samemechanism could create MM there. For each of these mechanisms an example is presented.2. Venus ExpressMagnetometer data from the Venus Express mission (VEX) [Svedhem et al., 2007] areused; the spacecraft is in a polar orbit around Venus with periapsis at ∼ 300 km andtherefore will enter deeply into Venus’s induced magnetosphere, as shown by Zhang et al.[2007].Magnetic field data from VEXMAG [Zhang et al., 2006] are used with a sampling rateof 1 Hz. During the nominal mission of VEX data are also available at 32 Hz samplingrate (and for short intervals also at 128 Hz). The plasma data from ASPERA [Barabashet al., 2007] have a resolution of ∼ 3 min for ions and 4 sec for electrons. The wavesin question have periods 4 ≤ T ≤ 10 sec as will be shown below. Unfortunately, thismeans that the ion plasma data cannot be used for identification of the waves, but couldD R A F T February 13, 2008, 4:35am D R A F T

VOLWERK ET AL.: VENUS’ MIRROR MODE WAVES X - 5give information on the density and the temperature asymmetry of the magnetosheathplasma. Also, as this paper is written, plasma data with the required accuracy are notyet available.3. Mirror-Mode IdentificationIn the absence of high-resolution ion data, MM waves need to be identified from themagnetic field data only. This situation is similar to that by Lucek et al. [1999a, b] forEquator-S, and in this paper the same identification method will be adopted. MM wavesare identified as having large amplitudes ∆B/B, and having small angles θ Bmv betweenthe maximum variance and the magnetic field direction θ Bmv ≤ 30 ◦ [Price et al., 1986].These two quantities are determined for sliding windows of 30 sec width and 1 secshift. The mean magnetic field is determined by a low-pass filter with a shortest period of1.5 minutes; the amplitude of the waves in the data is then determined as the maximumdifference between the data and mean field. For each window a minimum variance analysis[Sonnerup and Scheible, 1998] is performed and the angle between the maximum variancedirection and the mean magnetic field is determined. Additionally, the angle betweenthe minimum variance direction and the magnetic field, β Bmv , is determined, which isexpected to be nearly perpendicular for MM waves, where a limit is set that β Bmv ≥ 80 ◦ .4. The DataTwo events are discussed:the first takes place during in inbound pass, where thespacecraft moves from the solar wind, through the bowshock, into the magnetosheath; thesecond takes place during an outbound pass, where the spacecraft moves from periapsis,through the magnetopause, into the magnetosheath.D R A F T February 13, 2008, 4:35am D R A F T

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

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

X - 10VOLWERK ET AL.: VENUS’ MIRROR MODE WAVESAcknowledgments. The authors would like to thank Simon Pope at the Universityof Sheffield for preparing the data. The work by ZV is supported by the Austrian Wissenschaftsfondsunder grant number P20131-N16.ReferencesBarabash, S., Sauvaud, J.-A., Gunell, H., Andersson, H., Grigoriev, A., Brinkfeldt, K.,Holmström, M., Lundin, R., Yamauchi, M., Asamura, K., Baumjohann, W., Zhang,T., Coates, A., Linder, D., Kataria, D., Curtis, C., Hsieh, K., Sandel, B., Fedorov, A.,Mazelle, C., Thocaven, J.-J., Grande, M., Koskinen, H., Kallio, E., Säles, T., Riihela, P.,Kozyra, J., Krupp, N., amd J. Luhmann, J. W., McKenna-Lawlor, S., Orsini, S., Cerulli-Irelli, R., Mura, M., Milillo, M., Maggi, M., Roelof, E., Brandt, P., Russell, C., Szego,K., Winningham, J., Frahm, R., Scherrer, J., Sharber, J., Wurz, P., and Bochsler, P.:The analyser of space plasmas and energetic atoms (ASPERA-4) for the Venus Expressmission, Planet. Space Sci., 55, 1772 – 1792, doi:10.1016/j.pss.2007.01.014, 2007.Baumjohann, W., Treumann, R. A., Georgescu, E., Haerendel, G., Fornaçon, K.-H., andAuster, U.: Waveform and packet structure of lion roars, Ann. Geophys., 17, 1528 –1534, 1999.Constantinescu, O. D., Glassmeier, K.-H., Treumann, R., and Fornaçon, K.-H.: Magneticmirror structures observed by Cluster in the magnetosheath, Geophys. Res. Lett., 30,1802, doi:10.1029/2003GL017313, 2004.Delva, M., Zhang, T. L., Volwerk, M., Russell, C. T., and Wei, H. Y.: First upstreamproton cyclotron wave observations at Venus, Geophys. Res. Lett., 000, 000, doi:000,2008.D R A F T February 13, 2008, 4:35am D R A F T

VOLWERK ET AL.: VENUS’ MIRROR MODE WAVES X - 11Erdös, G. and Balogh, A.: Statistical properties of mirror mode structures observed byUlysses in the magnetosheath of Jupiter, J. Geophys. Res., 101, 1 – 12, 1993.Gary, S. P., Fuselier, S. A., and Anderson, B. J.: Ion anisotropy instabilities in themagnetosheath, J. Geophys. Res., 98, 1481 – 1488, 1993.Glassmeier, K. H., Motschmann, U., Mazelle, C., Neubauer, F. M., Sauer, K., Fuselier,S. A., and Acuña, M.: Mirror modes and fast magnetoacoustic waves near the magneticpileup boundary of Comet P/Halley, J. Geophys. Res., 98, 20 955 – 20 964, 1993.Hasegawa, A.: Drift mirror instability in the magnetosphere, Phys. Fluids, 12, 2642 –2650, 1969.Huddleston, D. E., Strangeway, R. J., Blanco-Cano, X., Russell, C. T., Kivelson, M. G.,and Khurana, K. K.: Mirror-mode structures at the Galileo-Io flyby: Instability criterionand dispersion analysis, J. Geophys. Res., 104, 17 479 – 17 489, 1999.Lucek, E. A., Dunlop, M. W., Balogh, A., Cargill, P., Baumjohann, W., Georgescu, E.,Haerendel, G., and Fornaçon, K.-H.: Mirror mode structures observed in the dawn-sidemagnetosheath by Equator-S, Geophys. Res. Lett., 26, 2159 – 2162, 1999a.Lucek, E. A., Dunlop, M. W., Balogh, A., Cargill, P., Baumjohann, W., Georgescu, E.,Haerendel, G., and Fornaçon, K.-H.: Identification of magnetosheath mirror modes inEquator-S magnetic field data, Ann. Geophys., 17, 1560 – 1573, 1999b.Price, C. P., Swift, D. W., and Lee, L.-C.: Numerical simulation of nonoscillatory mirrorwaves at the Earth’s magnetosheath, J. Geophys. Res., 91, 101 – 112, 1986.Rae, I., Mann, I., Watt, C., Kistler, L., and Baumjohann, W.: Equator-S observationsof drift mirror mode waves in the dawnside magnetosphere, J. Geophys. Res., A11203,doi:10.1029/2006JA012064, 2007.D R A F T February 13, 2008, 4:35am D R A F T

X - 12VOLWERK ET AL.: VENUS’ MIRROR MODE WAVESRussell, C. T., Riedler, W., Schwingenschuh, K., and Yeroshenko, Y.: Mirror instabilityin the magnetoshpere of Comet Halley, Geophys. Res. Lett., 14, 644 – 647, 1987.Sonnerup, B. U. Ö. and Scheible, M.: Minimum and maximum variance analysis, in:Analysis Methods for Multi-Spacecraft Data, edited by Paschmann, G. and Daly, P.,pp. 185–220, ESA, Noordwijk, 1998.Southwood, D. J. and Kivelson, M. G.: Mirror instability: 1. Physical mechanism of linearinstability, J. Geophys. Res., 98, 9181 – 9187, 1993.Svedhem, H., Titov, D. V., Taylor, F. W., and Witasse, O.: Venus as a more Earth-likeplanet, Nature, 450, 629 – 632, doi:10.1038/nature06432, 2007.Treumannn, R. and Baumjohann, W.: Advances Space Plasma Physics, Imperical CollegePress, London, UK, 1997.Tsurutani, B. T., Smith, E. J., Anderson, R. R., Ogilvie, K. W., Scudder, J. D., Baker,D. N., and Bame, S. J.: Lion roars and nonoscillatory drift mode mirror waves in themagnetosheath, J. Geophys. Res., 87, 6060 – 6072, 1982.Zhang, T., Baumjohann, W., Delva, M., Auster, H.-U., Balogh, A., Russell, C., Barabash,S., Balikhin, M., Berghofer, G., Biernat, H., Lammer, H., Lichtenegger, H., Magnes, W.,Nakamura, R., Penz, T., Schwingenschuh, K., Vörös, Z., Zambelli, W., Fornaçon, K.-H.,Glassmeier, K.-H., Richter, I., Carr, C., Kudela, K., Shi, J., Zhao, H., Motschmann,U., and Lebreton, J.-P.: Magnetic field investigation of the Venus plasma environment:expected new results, Planet. Space Sci., 54, 1336 – 1343, 2006.Zhang, T. L., Delva, M., Baumjohann, W., Auster, H.-U., Carr, C., Russell, C., Barabash,S., Balikhin, M., Kudela, K., Berhofer, G., Biernat, H. K., Lammer, H., Lichtenegger,H., Magnes, W., Nakamura, R., Schwingenschuh, K., Volwerk, M., Vörös, Z., Zam-D R A F T February 13, 2008, 4:35am D R A F T

VOLWERK ET AL.: VENUS’ MIRROR MODE WAVES X - 13belli, W., Fornaçon, K.-H., Glassmeier, K.-H., Richter, I., Balogh, A., Schwartzl, H.,Pope, S. A., Shi, J. K., Wang, C., Motschmann, U., and Lebreton, J.-P.: Little orno solar wind enters Venus atmosphere at solar minimum, Nature, 450, 654 – 656,doi:doi:10.1038/nature06026, 2007.D R A F T February 13, 2008, 4:35am D R A F T

X - 14VOLWERK ET AL.: VENUS’ MIRROR MODE WAVESFigure 1.The magnetic field data for 5 May 2006. The spacecraft moves from thesolar wind (SW), through the bowshock (BS) into the magnetosheath (MS). The twobottom panels show the fluctuation of the magnetic field ∆B/B and the angle betweenthe maximum variance direction and the mean magnetic field θ Bmv (black dots), and theangle between the minimum variance direction and the mean magnetic field β Bmv (redplusses).Figure 2.Power spectra for intervals I (top panel 5 May, bottom panel 2 October)of mirror mode waves for the compressional (solid) , and right- (dotted) and left-handed(dashed) polarized components of the magnetic field. The obsevered compressional wavesare marked with a vertical line.Figure 3.The magnetic field data for 2 October 2006. The spacecraft moves fromperiapsis in Venus’ magnetosphere (MSp), through the magnetopause (MP) into themagnetosheath (MS). The two bottom panels show the fluctuation of the magnetic field∆B/Band the angle between the maximum variance direction and the mean magneticfield θ Bmv (black dots), and the angle between the minimum variance direction and themean magnetic field β Bmv (red plusses).D R A F T February 13, 2008, 4:35am D R A F T

VOLWERK ET AL.: VENUS’ MIRROR MODE WAVES X - 15Figure 4.Comparison of the MM waves in the Earth’s magnetosheath, measuredby the Equator-S magnetometer (top) [reproduced from Lucek et al., 1999a] and thecompressional waves measured in a similar region in Venus’ magnetosheath by the VEXmagnetometer. (bottom)D R A F T February 13, 2008, 4:35am D R A F T

502006−5−5−inSW BS IMSIIBx0−5050By0−5050Bz0−506040Bm2003∆ B / B21θ , β08060402000112 0115 0118time (dh)

10 3 Freq. (Hz)Power (nT 2 /Hz)10 210 110 010 −110 −2 10 −1 10 010 4 Freq. (Hz)Power (nT 2 /Hz)10 310 210 110 010 −2 10 −1 10 0

2006−10−2−out50IIIBx0−5050By0−5050Bz0−506040Bm2003∆ B / B21θ , β08060402000430 0433 0436 0439 0442time (dh)

6050Bm Equator−S4030201000900 0905 0910 0915 0920UTBm VEX60504030201000112 0113 0114 0115UT