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Keywords:

  • Venus Express;
  • magnetosheath;
  • mirror mode waves

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Venus Express
  5. 3. Mirror-Mode Identification
  6. 4. Data
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References

[1] In this paper first time observations of mirror mode like structures in Venus' magnetosheath are presented. Using magnetometer data from the Venus Express spacecraft it is shown that in two regions in the Venusian magnetosheath strong compressional waves exist, which propagate nearly perpendicular to the ambient magnetic field. They are most likely mirror-mode waves. The waves have periods between 5 and 15 sec, depending on the location in the magnetosheath. These waves show up just behind the quasi-perpendicular bow shock, and near the magnetopause during compression of the magnetosheath due to increased solar wind pressure. The characteristics of the waves are similar to mirror mode waves found in the Earth's magnetosheath, however, they are down-scaled in duration and frequency by a factor of 10, comparable with the difference in size of Venus' and Earth's magnetosheath.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Venus Express
  5. 3. Mirror-Mode Identification
  6. 4. Data
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References

[2] The mirror instability has been discussed for space plasmas with strong temperature asymmetries, i.e., with perpendicular temperature higher than the parallel temperature [Hasegawa, 1969; Gary et al., 1993; Southwood and Kivelson, 1993]. The MM instability generates compressional waves that grow preferentially in the direction perpendicular to the ambient magnetic field [see Treumannn and Baumjohann, 1996, section 3.5]. Such waves have been found in various objects, for example, the Earth's magnetotail [Rae et al., 2007], the magnetosheath (MS) of the Earth [e.g., Tsurutani et al., 1982; Baumjohann et al., 1999; Lucek et al., 1999a; Constantinescu et al., 2003] of Jupiter [Erdös and Balogh, 1993; Joy et al., 2006] and Saturn [Bavassano Cattaneo et al., 1998], in the tail [e.g., Russell et al., 1987] and magnetic pile up boundary [Glassmeier et al., 1993] of a comet and in the ion pick-up region near Io [e.g., Huddleston et al., 1999].

[3] Up until Venus Express, mirror mode (MM) waves have not been identified in Venus' MS. Luhmann [1995] discussed the (non)-existence of MM waves and found mainly transverse waves. However, this is most likely an artifact of the data analysis, where the investigators mainly concentrated on quasi-parallel BS conditions, whereas the MM occurs mainly during quasi-perpendicular BS conditions. In order to find out how similar Venus' MS is compared to the Earth's, the investigation of MM waves is necessary.

[4] The instability criterion for 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 that the growth rate 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 MSs. Also, the presence of He-ions suppresses the growthrate for proton IC waves [Gary et al., 1993].

[5] At frequencies below the IC frequency, the MM behaves in such a way that the perpendicular pressure p of the plasma will be in anti-phase with compressional variations in the magnetic field [Hasegawa, 1969]. In a bi-Maxwellian plasma with perpendicular temperature T and parallel temperature T this means that

  • equation image

which leads to the instability criterion:

  • equation image

[6] The temperature asymmetry in the (Earth's) MS can be created by two mechanisms [Lucek et al., 1999b]: (1) gyratory motion of the ions caused by (multiple) reflection(s) at the BS under quasi-perpendicular conditions before entering the MS; or (2) compression of the MS close to the magnetopause. In this paper two events in Venus' MS are presented which show that the same mechanism probably creates MM. For each of these mechanisms an example is presented.

2. Venus Express

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Venus Express
  5. 3. Mirror-Mode Identification
  6. 4. Data
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References

[7] Magnetometer data from the Venus Express mission (VEX) [Svedhem et al., 2007] are used; the spacecraft is in a polar orbit around Venus with periapsis at ∼300 km and therefore will enter deeply into Venus' induced magnetosphere, as shown by Zhang et al. [2007].

[8] Magnetic field data from VEXMAG [Zhang et al., 2006] are used with a sampling rate of 1 Hz. During the nominal mission of VEX data are also available at 32 Hz sampling rate (and for short intervals also at 128 Hz). The waves in question have periods 4 ≤ T ≤ 15 sec as will be shown below. The plasma data from ASPERA [Barabash et al., 2007] have a resolution of ∼3 min for ions and 4 sec for electrons. Unfortunately, this means that the ion plasma data cannot be used for identification of the waves, but could give information on the density and the temperature asymmetry of the MS plasma. Also, as this paper is written, plasma data are not yet available.

3. Mirror-Mode Identification

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Venus Express
  5. 3. Mirror-Mode Identification
  6. 4. Data
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References

[9] In the absence of high-resolution ion data, MM waves need to be identified from the magnetic field data only. This situation is similar to that by Lucek et al. [1999a, 1999b] for Equator-S, and in this paper the same identification method will be adopted. MM waves are identified as having large strengths ΔB/B, and small angles θBmv between the maximum variance and the magnetic field direction θBmv ≤ 30° [Price et al., 1986].

[10] These two quantities are determined for sliding windows of 30 sec width and 1 sec shift. The mean magnetic field is determined by a low-pass filter with a shortest period of 1.5 min; the amplitude of the waves in the data is then determined as the maximum difference 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 variance direction and the mean magnetic field is determined. Additionally, the angle between the minimum variance direction and the magnetic field, βBmv, is determined, which is expected to be nearly perpendicular for MM waves, where for this study a limit is set that βBmv ≥ 80°.

4. Data

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Venus Express
  5. 3. Mirror-Mode Identification
  6. 4. Data
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References

[11] Two events are discussed: the first takes place during an inbound pass, where the spacecraft moves from the solar wind, through the BS, into the MS; the second takes place during an outbound pass, where the spacecraft moves from periapsis, through the magnetopause, into the MS.

4.1. Event 1: 5 May 2006

[12] On 5 May 2006 VEX entered from the solar wind (SW) through the bow shock (BS) into the magnetosheath (MS). At the beginning of the event the spacecraft was located near (1.69, −0.05, 0.48) RV in VSO coordinates implying a local time of 1200, noon, and the solar wind magnetic field was BSW = (−2.04, −4.73,1.37) nT. The angle θBn between the solar wind magnetic field and the local shock normal is calculated by approximating the BS normal as the radial vector from the planetary centre to VEX. This means that the BS is approximated by a semi-sphere on the day side, which leads to minor errors when the spacecraft moves towards the terminator. The actual best fit for the BS is given by Zhang et al. [2007]. For this event it is found that θBn ≈ 106°, implying a quasi-perpendicular BS. Immediately after crossing the BS large amplitude compressional waves occur as shown in shaded region I in Figure 1. In Figures 1e and 1f ΔB/B and the angles θBmv and βBmv are shown. Just before the BS crossing ∼0113 UT, ΔB/B starts to increase, which is an artifact due to the difference between the low-pass filtered and non-filtered data. However, after the BS crossing in the shaded region I, strong oscillations of the magnetic field with large amplitude are clearly visible. In Figure 1f, the angle θBmv between the mean magnetic field 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 θBmv drops below 20°, the minimum variance direction angle βBmv with the ambient magnetic field increases to well above 80°. This is a clear indication of compressional waves after the BS crossing with a period of ∼5 sec propagating perpendicular to the ambient magnetic field.

image

Figure 1. The magnetic field data for 5 May 2006. The spacecraft moves from the solar wind (SW), through the BS (BS) into the magnetosheath (MS). (e) and (f) The fluctuation of the magnetic field ΔB/B and the angle θBmv between the maximum variance direction and the mean magnetic field (dots), and the angle βBmv between the minimum variance direction and the mean magnetic field (pluses).

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[13] Spectral analysis of the waves, in a mean field-aligned coordinate system and the two transverse components transformed into right- and lefthanded polarized components, shows a broad peak in the compressional component at ∼0.2 Hz, which agrees with the observed waves (see Figure 2 (top)).

image

Figure 2. Power spectra for intervals I ((top) 5 May and (bottom) 2 October)) of mirror mode waves for the compressional (solid), right- (dotted) and left-handed (dashed) polarized components of the magnetic field. The observed compressional waves are marked with a vertical line.

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[14] Slightly later, ∼0117–0118:30 UT, with the spacecraft deeper into the MS, there is another region (shaded region II in Figure 1) displaying compressional waves propagating almost perpendicular to the ambient magnetic field. These waves, in contrast to shaded region I, now have a period of ∼15 sec. VEX approaches Venus' magnetopause, with increasing magnetic field strength and most likely plasma pile up, changing the magnetoplasma environment and thus the wave characteristics.

4.2. Event 2: 2 October 2006

[15] On 2 October 2006 VEX passed through pericenter just before ∼0433 UT, being inside the magnetopause [see Zhang et al., 2007]. At the beginning of the event the spacecraft was located near (−0.08, 0.09, 1.06) RV in VSO coordinates, which implies a local time of 0300, however, VEX is located almost above the north pole and local time does not have much meaning in this event. VEX then crossed the magnetopause at ∼0433:30 UT (when the rotations in By and Bz end) and the compressional waves start at ∼0434 UT, shown by the shaded box I in Figure 3. The solar wind conditions before VEX crossed the BS into the MS showed a slightly increased magnetic field strength Bm ≈ 9 nT, whereas after VEX crossed the BS out from the MS the solar wind magnetic field strength was Bm ≈ 6 nT. The angle between the magnetic field and the BS normal θBn ≈ 97°, implying a quasi-perpendicular BS. The waves are of larger size in this case, as can be seen in Figure 2 (bottom). The waves have a period of ∼10 sec and are strongly compressional, with minimum variance direction almost perpendicular to the ambient magnetic field. Spectral analysis of the first interval of interest shows that there is a shoulder at ∼0.1 Hz, which agrees with the observed waves.

image

Figure 3. The magnetic field data for 2 October 2006. The spacecraft moves from periapsis in Venus' magnetosphere (MSp), through the magnetopause (MP) into the magnetosheath (MS). (e) and (f) The fluctuation of the magnetic field ΔB/B; the angle θBmv between the maximum variance direction and the mean magnetic field (dots), and the angle βBmv between the minimum variance direction and the mean magnetic field (pluses).

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[16] There is also a second region from 0436:30 till 0438:30 UT (shaded area II in Figure 3) where strong compressional waves occur. The waves during this interval are basically the same as in the interval I, also with a period of 10 sec.

5. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Venus Express
  5. 3. Mirror-Mode Identification
  6. 4. Data
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References

[17] This paper set out to find evidence of MM waves in Venus' MS. Based on experiences in the Earth's MS, two regions were investigated, one near the quasi-perpendicular BS and one deep inside the MS near the magnetopause in times of increased solar wind pressure. In both events, large strength (ΔB/B) compressional waves (θBmv ≤ 20°) that propagate perpendicular to the ambient magnetic field (βBmv ≥ 80°) were found. Therefore, these waves are considered to be MM waves. Due to the lack of plasma data, there cannot be a definite proof that it are MM. However, the analysis technique used to identify MM without plasma data has been proven successful by Lucek et al. [1999a] and been confirmed with plasma data by Rae et al. [2007].

[18] The first observed event takes place in a region where MM waves are expected to occur, near the BS, the location of the spacecraft and the solar wind magnetic field direction indicate quasi-perpendicular shock conditions. This is one of the situations mentioned above, in which ions can be (multiple) reflected at the BS, creating an energized ion population with T > T. Note that this is only a very short interval ≤ 1 min 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' BS is located at ∼1.3 RV and magnetopause at ∼1.1 RV [Zhang et al., 2007], whereas for the Earth the BS is located at ∼12–15 RE and the magnetopause at ∼10 RE. This indicates that the MM region near the quasi-perpendicular shock scales rather well with the size of the MS at Venus.

[19] In Figure 4 the data from Equator-S and VEX are shown for comparable regions in the MS, just after passing the BS. The difference is clear: on Earth the period of the waves is ∼30 sec, whereas for VEX the period is ∼5 sec. The amplitude of the waves, at the beginning of interval I of 5 May 2006 is similar to that of Equator-S, but drops off quite quickly and their period seems to change too. Interestingly, the results by Lucek et al. [1999a] show that the peak in the power spectrum shifts to lower frequencies as the spacecraft moves deeper into the MS, similar to what is found for VEX in this paper.

image

Figure 4. Comparison of (top) the MM waves in the Earth's magnetosheath, measured by the Equator-S magnetometer (reproduced from Lucek et al. [1999a]) and (bottom) the compressional waves measured in a similar region in Venus' magnetosheath by the VEX magnetometer during event 1. Both panels show the total magnetic field strength Bm.

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[20] The second event occurs further inside the MS, near the magnetopause. In this case energization of ions can be produced through compression of the magnetosphere. Available ACE solar wind data for that time (not shown here) reveal that the solar wind density was increased during this interval as well as the solar wind speed, indicating an increased ram pressure on Venus' induced magnetosphere and thus a compression. Such compression of the magnetosphere can also be seen when comparing the data with the next orbit. For 2 October the magnetic field strength at periapsis is Bm ≈ 50 nT, whereas on 3 October the magnetic field Bm ≈ 40 nT.

[21] The fact that the MM waves are non-continuous through the data for both events, but appear in small patches, is in agreement with Equator-S observations, where it was found that the MM waves appear in separate bursts.

[22] Recent statistical investigations of MM waves in the Earth's MS [Tátrallyay and Erdös, 2005] and Jupiter's MS [Joy et al., 2006] have shown some of the general characteristics of these waves. It was shown that the MM waves originate at the BS and move towards the magnetopause, with increasing period when moving away from the BS. This is also found here during event 1, where the period is ∼5 sec just behind the BS and ∼15 sec further into the MS. A statistical study of MM waves in Venus' MS is presented by M. Volwerk et al. (Mirror mode like structures in Venus' induced magnetosphere, submitted to Journal of Geophysical Research, 2008).

6. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Venus Express
  5. 3. Mirror-Mode Identification
  6. 4. Data
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References

[23] For the first time, mirror-mode waves have been identified in Venus' MS at two different locations. Comparing these waves with those found in the Earth's MS shows that there are many similarities in the characteristics of these waves. Therefore, it can be concluded that Venus' MS is much like the Earth's, only scaled down by a factor ∼10.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Venus Express
  5. 3. Mirror-Mode Identification
  6. 4. Data
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References

[24] The authors thank Simon Pope at the University of Sheffield for preparing the data. The work by ZV is supported by the Austrian Wissenschaftsfonds under grant P20131-N16.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Venus Express
  5. 3. Mirror-Mode Identification
  6. 4. Data
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References