Magnetosheath fluctuations at Venus for two extreme orientations of the interplanetary magnetic field



[1] Using the magnetosheath crossings of Venus Express on two consecutive days, we investigate magnetic fluctuations in the same locations for two extreme interplanetary magnetic field orientations, i.e., nearly along and nearly perpendicular to the solar wind flow. It is shown that the properties of the fluctuations are drastically different at basically the same location of the spacecraft in the magnetosheath. The strength and properties of the fluctuations are strongly controlled by the types of the upstream bow shock. The magnetic fluctuations behind a quasi-parallel bow shock are quite strong and turbulent, having a strongly variable angle αeB between maximum variance direction of the fluctuations and the direction of the magnetic field, which may be convected from the upstream waves in the foreshock. The magnetic fluctuations behind a quasi-perpendicular bow shock are less intensive and wave-like, showing a less perturbed angle αeB, which are probably generated locally.

1. Introduction

[2] Unlike Earth, Venus lacks an intrinsic magnetic field. The upper atmosphere is partially ionized by solar extreme ultraviolet radiation to form the ionosphere. This highly conducting layer excludes the interplanetary magnetic field and the field lines are draped to form an induced magnetosphere [e.g., Zhang et al., 2007]. It acts as an obstacle to the supersonic solar wind, which results in the formation of a detached bow shock. The region in between these two surfaces, known as the magnetosheath, is filled with various magnetic field fluctuations. These fluctuations in magnetosheath play an important role in the process of solar wind interactions with a planet. A Large number of studies about the waves in the Earth's magnetosheath have been performed, which were reviewed by Schwartz et al. [1996], Song and Russell [1997] and Lucek et al. [2005]. However, the fluctuations in the Venusian magnetosheath have not been investigated as widely as at Earth. Previous work was based on data obtained by the Pioneer Venus Orbiter (PVO). Luhmann et al. [1983] found that the magnetic field in the Venusian magnetosheath behind a quasi-parallel bow shock fluctuated more intensively. Oscillations with a frequency of 0.05 Hz observed in the magnetosheath were believed to be convected from the upstream waves identified by Hoppe and Russell [1982]. Proton cyclotron waves were reported within Venusian magnetosheath [Russell et al., 2006]. Recently, the success of the Venus Express mission [Titov et al., 2006] provides a new opportunity to investigate the interaction of the solar wind with Venus. Mirror mode waves were identified in the Venusian magnetosheath, with a ∼1/10 duration and frequency of that found in Earth's magnetosheath [Volwerk et al., 2008]. Using a wavelet technique, Vörös et al. [2008] found 1/f fluctuations in the Venusian magnetosheath, large-scale structures near the terminator and more developed turbulence further down stream in the wake.

[3] Previous studies at Earth indicate that the magnetic fluctuations in magnetosheath are strongly controlled by the property of the upstream bow shock [Luhmann et al., 1986; Song and Russell, 1997]. The shock property is usually represented by the angle θBn between the upstream interplanetary magnetic field (IMF) and the shock normal. At Venus, the spacecraft is within the magnetosheath without another one monitoring the upstream IMF. Therefore, to investigate the effect of θBn on magnetic fluctuations in the Venusian magnetosheath, only cases with quite steady IMF should be selected. In this paper, we select two magnetosheath crossings of Venus Express during two consecutive days. For these two crossings, the Venus Express orbits are nearly the same and the IMF directions are nearly parallel and perpendicular to the Sun-Venus line, respectively. They are very ideal cases to investigate the effect of the upstream IMF on Venusian magnetosheath fluctuations. We examine the properties of magnetic fluctuations in the different types of magnetosheath by using the angle between the maximum variance and average magnetic field direction that was used by Lucek et al. [1999] and Volwerk et al. [2008].

2. Observations

[4] Venus Express, launched on November 9, 2005, is the first European mission to Venus and went into orbit around Venus on April 11, 2006. The spacecraft is in a near-polar orbit with periapsis at 250–300 km altitude and therefore can cross the Venusian magnetosheath at each orbit. In this study, magnetic field data obtained by the Venus Express magnetometer [Zhang et al., 2006] are used with a sampling rate of 1 Hz.

[5] Two magnetosheath crossings were selected for our study: One case is on June 26, 2006 and the other is on June 27, 2006. Their orbits are shown in Figure 1 in Venus Solar Orbital coordinates (VSO), where the XY plane coincides with the Venus orbit plane, X is the direction to the Sun, Y is the direction opposite to planetary motion, Z is perpendicular to the orbit plane of Venus and positive toward ecliptic north. The bow shock location is shown by a dashed line for average conditions of the solar wind. The black line is the orbit on June 26 and the gray one is the orbit on June 27. On the two consecutive days, the events occurred about mostly the same orbit, almost in the dawn-dusk plane. The spacecraft moved across the terminator from dusk (−Y) to dawn (+Y) and explored the Venusian magnetosheath.

Figure 1.

The orbits of the Venus Express on June 26 (black line) and June 27 (gray line), 2006. The lower left and right plot show the projections of orbits on the YZ and XY plane in Venus Solar Orbital (VSO), respectively. The labels 01, 02, and 03 are the universal time in unit of hours and the correspondent points represent the spacecraft positions at that time.

[6] The magnetic field data with a time resolution of 1 s for these two crossings are shown in Figure 2. The bow shock crossings are identified by the sudden change in the magnitude of the magnetic field. The spacecraft moved across the bow shock at about 0129 UT and 0230 UT on June 26, and about 0128 UT and 0232 UT on June 27. Since the interplanetary conditions were quite steady when the spacecraft were in the solar wind region, the average IMF before and after the crossing calculated over the intervals with a length of 30 min are regarded as the constant IMF. For the case on June 26, the two intervals are 0045-0115UT and 0233-0303UT, and on June 27 they are 0051-0121UT and 0233-0303UT. The angles between the direction of IMF measured before and after the crossing are about 13° for June 26 and 14° for June 27. Therefore, the IMF can basically be considered as constant, equal to the average values of IMF measured before and after the crossing. On June 26 the IMF is (−5.30, 1.12, −0.91) nT with the cone angle ∼15° which is the angle between the IMF vector and X axis. Correspondingly, on June 27 the IMF is (0.34, 8.04, 0.57) nT with the cone angle ∼88°. These cases give the examples of magnetosheath conditions under two extreme IMF orientations, i.e., approximately parallel and perpendicular to the Sun-Venus line which is also the assumed solar wind flow direction. Because the spacecraft explored nearly the same regions of the Venusian magnetosheath during these days, we can use these two events to investigate the behavior of the magnetic fluctuations in the same regions under two extreme IMF conditions.

Figure 2.

Magnetic field measurements on June 26 and 27, 2006. Missing data are due to the data cleaning processes. The dashed lines indicate the crossing of the bow shock. The solid vertical lines show the time of periapsis.

3. Data Analysis

[7] To represent the properties of upstream bow shock when Venus Express was inside the magnetosheath, the streamlines must be known at first. In previous work, an analytical description of the streamlines was employed [Russell et al., 1983; Luhmann et al., 1983, 1986]. It was based on the theory of fluid flow around the obstacle, which was assumed to have the shape of the ionopause derived from PVO plasma data [Theis et al., 1980]. Zhang et al. [1991] realized that the effective obstacle was not the ionopause, but the magnetic barrier above it in the inner magnetosheath, which was also considered as an induced magnetosphere in analogy to the Earth's magnetosphere [Luhmann et al., 2004; Kallio et al., 2008; Zhang et al., 2008a]. At solar minimum, the magnetopause location is at an altitude of about 1000 km at the terminator and 300 km at the subsolar point [Zhang et al., 2008b]. Therefore, with this magnetopause as the obstacle, we used the gas dynamic simulation described by Spreiter and Stahara [1980] to calculate the streamlines in the magnetosheath. By tracing along them from the spacecraft, the shock normal angle θBn at the point of intersection with the bow shock can then be calculated as a function of time as the spacecraft moves across the magnetosheath. We have used the parameters δBc and δBt to represent the compressional and transverse field fluctuations following the approach by Luhmann et al. [1986]. In this study, the ratios δBc/∣B∣ and δBt/∣B∣ are used as the estimations of the strength of fluctuations. The angle αeB between the maximum variance and average magnetic field direction is also calculated. Because the magnetic field fluctuations on Jun 26th 2006 have a quasi-period of 30 seconds, all these values are calculated for sliding windows of 90 seconds width and 1 second shift.

[8] These parameters are shown in Figure 3. To compare them easily, the same range of axes are used to plot for the two cases. For the June 26 case, it can be seen that there is an interval with high fluctuations from about 0132 UT to 0140 UT. The field fluctuates very intensively and the transverse component is much greater than the compressional component. The value of θBn for the inbound magnetosheath is below 30°, and θBn is above 45° during the outbound (i.e., 0210–0230 UT). The magnetosheath fluctuations are expected to be stronger during the inbound which is downstream of a quasi-parallel shock than that during the outbound which is downstream of a quasi-perpendicular shock. There are some waves observed in the magnetosheath which are at the same frequency (∼0.03 Hz) as the fluctuations in the foreshock. It is generally thought that the fluctuations in the foreshock are convected into the magnetosheath [Luhmann et al., 1983]. It is easy to see that the observation agree with both points.

Figure 3.

Statistical parameters calculated during the time period as plotted in Figure 2. From the bottom to top plots, shown are magnetic field magnitude BT, the relative standard deviation of field δB/∣B∣ (black line for the compressional component δBc/∣B∣, and gray one for the transverse component δBt/∣B∣), θBn and the angle αeB between the maximum variance and average magnetic field directions, respectively. The solid vertical lines show the time of periapsis.

[9] For the June 27 case, most of the magnetosheath is behind a quasi-perpendicular bow shock as we can see from θBn. Though the spatial positions where the measurements were made are almost the same as those on the previous day, the magnetosheath fluctuations are mostly weaker than the previous day, because of the larger θBn and also different IMF direction respected to solar wind flow. From the June 27 case of Figure 3, we see that the magnetosheath is quieter with smaller δBc/∣B∣ and δBt/∣B∣. From 0146 UT to 0200 UT, the spacecraft was inside the induced magnetosphere, and the magnetosheath is the region between bow shock and magnetopause. Moreover, δBt/∣B∣ is always greater than δBc/∣B∣, indicating that the transverse fluctuations dominant in the magnetosheath. These magnetosheath fluctuations are more likely to be generated locally than to be convected from the foreshock, due to large θBn. These fluctuations decrease very quickly approaching the magnetopause (near 0146 UT and 0200 UT, respectively) as defined by Zhang et al. [2008a]. During 0130–0135 UT and 0209–0221 UT, the transverse power spectrums have enhanced powers near the proton gyrofrequency about 0.3 Hz. Our analysis based on the method of Means [1972] shows that the waves are left-handed, elliptically polarized, and propagating parallel to the magnetic field (with propagation angles of 5° and 8°, respectively). They are possibly ion cyclotron waves due to planetary pickup as reported by Russell et al. [2006]. Other regions in the magnetosheath have power spectrums with enhanced power over a much broader band which is possibly due to mixing of several types of waves. More detailed analysis about these magnetosheath waves will be discussed in another paper in preparation.

[10] In the top plots for both cases, the behavior of αeB can represent the properties of magnetic fluctuations to some extent, which was used to identify mirror mode waves by Lucek et al. [1999] and Volwerk et al. [2008]. Using the value of αeB, we can compare the properties of fluctuations in the different regions. For the quasi-parallel magnetosheath from about 0130 UT to 0140 UT on June 26, the data points of αeB scatter between 0° and 90°, and there is no distinctive feature. However, for the quasi-perpendicular magnetosheath from about 0214 UT to 0224 UT on June 26, all of the data points of αeB concentrate in the range above 60°. With the upstream quasi-perpendicular bow shock on June 27, the value of αeB for most parts of magnetosheath is also concentrated above 60° except for the regions near the bow shock. The value of αeB in quasi-perpendicular magnetosheath, which is generally above 60°, is much more regular than that in quasi-parallel magnetosheath. While αeB < 30° was used as the criterion to identify mirror waves, αeB > 60° indicates that the fluctuations seem to be some transverse wave-like signal. As mentioned in the last paragraph, these fluctuations during 0130–0135 UT and 0209–0221 UT on June 27 are identified as ion cyclotron waves. This indicates that the fluctuations in “quasi-parallel” and “quasi-perpendicular” magnetosheaths are generated from different sources. The former may be upstream waves convected from the foreshock, and the latter are probably generated locally.

[11] Luhmann et al. [1986] gave the observed spatial distribution of magnetic field fluctuations in the Earth's magnetosheath under the different IMF orientations. The quasi-parallel shock with its foreshock is a powerful wave generation source. Waves growing in the foreshock are convected by the solar wind flow into the magnetosheath and produce fluctuating magnetic fields. Shevyrev and Zastenker [2005] also showed that both ion flux and magnetic field fluctuations steadily decrease with increasing θBn. Most waves downstream of a quasi-perpendicular shock are generated at or downstream of the bow shock [Song and Russell, 1997]. Comparing our results for Venus with the above results for Earth, it can be seen that the effect of the IMF orientation on the magnetosheath fluctuations at Venus is the same as that at Earth. Although the scale of the Venus magnetosheath is about 1/10 that of Earth's magnetosheath, the fluctuations in them have similar properties.

4. Conclusions

[12] Using two special crossings of Venus Express on two consecutive days, the effect of the IMF on the fluctuations in the magnetosheath is investigated. For these two cases, The IMF has two distinct directions: one is nearly along the Sun-Venus line and the other is perpendicular to it. The strength and properties of the magnetic field fluctuations in the same spatial location are strongly controlled by the types of the upstream bow shock. The magnetic field in a “quasi-parallel” magnetosheath fluctuates more intensively than in a “quasi-perpendicular” magnetosheath. In a quasi-parallel magnetosheath the angles αeB are very scattered, while in a quasi-perpendicular magnetosheath they mainly concentrate in the range above 60°. The quasi-perpendicular magnetosheath is filled with some transverse wave-like fluctuations except for the regions near the magnetopause and bow shock, which are probably ion cyclotron waves. Therefore, the fluctuations in quasi-parallel and quasi-perpendicular magnetosheaths should originate from different sources.The former may be convected from the upstream waves in foreshock, and the later is probably generated locally by pickup of new born exospheric ions. Finally, we found that in spite of the different scales of Venus' and Earth's magnetosheath, the fluctuations in them have the same response to the upstream IMF orientation.


[13] The authors are grateful to H. Lichtenegger for providing the Spreiter's model run of streamlines, and thank the anonymous referees for their suggested corrections. This work in China was supported by the Specialized Research Fund for State Key Laboratories, Chinese Academy of Sciences grant KJCX2-YW-T13 and the National Science Foundation of China (NSFC) grants 40621003 and 40628003.