VLF Waves in the Foreshock

R. J. Strangeway* and G. W. Crawford**

*Institute of Geophysics and Planetary Physics,
University of California at Los Angeles

**Radio Atmosphere Science Center
Kyoto University at Kyoko 611, Japan
Now at SRI International, Menlo Park, California 94025, U. S. A.

Adv. Space Res., vol 15, (8/9)29-(8/9)42, 1995
Copyright 1995 by COSPAR

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VLF Emissions at Venus: Examples

      In order to understand VLF wave phenomena in the Venus foreshock it useful to make use of a coordinate system first derived for terrestrial observations /17, 19/, shown in Figure 3 for the Venus bow shock /20, 21/. This figure shows the IMF, solar wind, bow shock, and the observing spacecraft (PVO) in the aberrated Venus Solar Orbital (VSO) coordinate system. (VSO coordinates are analogous to Geocentric Solar Ecliptic Coordinates.) In Figure 3 we have assumed that the plane containing the solar wind and IMF, the B-v plane, also contains the center of the planet. For convenience we refer to this as the equatorial plane. In general, the observing spacecraft need not lie in the equatorial plane, but the geometry is similarly defined, except that the B-v plane containing the spacecraft intersects the bow shock in a plane parallel to the equatorial plane. The tangent field line then intersects the shock further downstream from the point of tangency in the equatorial plane.

Fig. 3. Foreshock coordinate system at Venus (after /20, 21/).

      Depending on the phenomenon of interest, various coordinates systems can be chosen. However, given the discussion in the previous section, we use the distance along the tangent field line and the depth downstream from the tangent field line to the point of observation as the foreshock coordinate system. For a reflected particle the distance depends primarily on the parallel reflection velocity, while the depth is governed by the solar wind flow. An alternative coordinate system could use the time-of-flight angle (), and the distance traveled along the time-of-flight velocity vector, indicated by the dashed line. Other parameters of interest include the shock normal direction (bn) as measured at the point of intersection on the bow shock of the field line passing through the spacecraft, and the distance along the field line from the bow shock to the spacecraft. Note that as drawn in Figure 3 the shock normal lies in the B-v plane, but this is not the case when the B-v plane containing the spacecraft lies above or below the equatorial plane.

      An example of the VLF emissions observed in the foreshock of Venus is shown in Figure 4 /20, 21/. This figure shows 1 hr 45 min of data acquired by PVO when it was in the solar wind, some 5 Rv behind and the terminator and about 7 Rv from the Venus-Sun line. The top four panels show wave electric field intensity, measured at 30 kHz, 5.4 kHz, 730 Hz, and 100 Hz, which are the four frequency channels of the Orbiter Electric Field Detector (OEFD). The OEFD was restricted to these four frequencies because of the power, weight, and telemetry restrictions of the Pioneer Venus Orbiter /22, 23, 24/. Because the OEFD antenna is so short, 0.76 m, the wave instrument suffers from high levels of interference when the spacecraft is in sunlight and the plasma Debye length is large, as occurs when the spacecraft is in the solar wind. The 100 Hz channel is most susceptible to this interference. However, in Figure 4 we have applied a noise removal scheme based on Bayesian statistical methods /25/ to the data, and much of the noise has been removed. The middle four panels in Figure 4 show the magnetic field components in VSO coordinates and total field strength. The bottom two panels show depth behind the tangent field line, and bn at the bow shock intersection point of the field line passing through the spacecraft. When the depth is negative the spacecraft is upstream of the tangent field line, and bn is not defined. On the other hand, depth is not defined if the magnetic field becomes sufficiently close to radial that there is no field line that is tangent to the bow shock, although bn is defined. The changes in depth and bn are mainly due to changes in the IMF orientation, rather than spacecraft motion.

Fig. 4. Example of VLF emissions observed in the foreshock at Venus (after /20, 21/).

      In Figure 4 waves are observed at 30 kHz for the first 20 min of data shown. At this time depth is small and positive, while bn is > 45°, indicating that the spacecraft is in the electron foreshock. Around 0605 UT there is a rotation in the IMF, and from 0605 UT to 0627 UT most of the wave activity occurs in the 5.4 kHz and 730 Hz channels. Depth is generally larger than before this time, and the waves are most intense when bn drops below 45°. This suggests that at this time the spacecraft is in the ion foreshock. Also some ULF waves are present when the VLF waves are most intense, again indicative of the ion foreshock. After 0630 UT depth is mainly negative until 0645 UT, at which time the rotation in the IMF causes the electron foreshock to rapidly sweep over the spacecraft, and we observe a brief burst of 30 kHz noise. From 0650 UT until the end of the data the spacecraft is deep in the ion foreshock, and intense 5.4 kHz, 730 Hz, and ULF waves are observed. Throughout most of the interval after 0650 UT the spacecraft is upstream of a quasi-parallel bow shock.

      Figure 4 provides a succinct overview of the types of wave phenomena observed by PVO in the foreshock at Venus. The VLF waves observed in the foreshock of Venus have been analyzed in terms of polarization, intensity as a function of location within the foreshock, and dependence on solar wind plasma density /20, 21, 26/. The 30 kHz wave intensity peaks at the tangent field line, with a peak amplitude around 10 mV/m /26/, comparable to plasma oscillations detected at the Earth /17/. Using the variation in plasma density to scan in frequency, the 30 kHz wave intensity is centered on the local plasma frequency /26/, and the waves are polarized parallel to the magnetic field /21, 26/. It is hence clear that the waves generated at the tangent field line are indeed plasma oscillations. The 5.4 kHz and lower frequency waves, on the other hand, tend to be observed further downstream from the tangent field line /21/. Comparisons with terrestrial observations shows a similar spectral shape, with the wave power extending up to 10 times the ion plasma frequency, with intensities comparable to those observed by the ISEE-2 spacecraft /21/. Through comparison of wave power as observed on both the ISEE-l and -2 spacecraft it was concluded that the wavelength of the VLF waves in the terrestrial foreshock was >30 m (the length of the ISEE-2 antenna), but less than 215 m (ISEE-l) /27/. The PVO antenna is even shorter than the ISEE-2 antenna, but since the wavelength is greater than the antenna length in both cases, the observed wave power is independent of wavelength. At Venus the 5.4 kHz waves were found to be parallel polarized /21/, which contradicts a terrestrial study /28/ that used wave interference patterns to show that the wave vector direction was typically 40° away from the magnetic field. This apparent contradiction has yet to be resolved, but may be a consequence of the spin averaging used for determining wave polarization in the PVO studies.

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