Statistical imaging of the Venus foreshock using VLF wave emissions

Abstract

Abstract. Strong VLF wave emissions have been shown to be an intrinsic property of the Venus foreshock and foreshocks in general. In this work, we use these measured wave emissions to constmct statistical "images" of the Venus foreshock. This analysis method allows us to develop a macroscopic picture of the wave properties, the inferred particle distributions, and their evolution as a function of position within the foreshock as well as compare our observations to Earth. The electron foreshock emissions at Venus are parallel polarized Langrnuir mode waves with the same peak amplitude at the foreshock boundary as terrestrial emissions (10 mV/m). However, the wave characteristics differ markedly between the upstream and downstream foreshocks for near Parker spiral interplanetary magnetic field (IMF) orientations (-35 ø for Venus). Additionally, there is a dramatic decrease in wave intensity for distances beyond --15 Rv from the point of tangency along the foreshock boundary. These characteristics in the wave emissions provide strong observational evidence supporting reflection and energization of solar wind electrons at the shock as the dominant source for providing upstream electrons, not leakage. In the ion foreshock the wave emissions consist of parallel polarized ion acoustic-like waves with similar intensifies and spectral characteristics to terrestrial emissions. However, these waves are situated much deeper in the foreshock than expected from terrestrial observations. Surprisingly, no emissions are observed in regions where field aligned ion distributions are expected. Rather, the emissions are confined to a region where diffuse ion distributions are expected.

Characteristics of quasi‐monochromatic ULF waves in the Venusian foreshock

“…In a previous study used the mapping of the standard deviations of the magnetic field components and magnitude from more than 700,000 time intervals within nominal Parker spiral region (IMF cone angle from 25° to 45°) to infer the ULF wave activity, which appear confined to the nose of the shock [ Crawford et al ., , their plate 2]. But their study was not limited to quasi‐monochromatic ULF waves and can also introduce many other solar wind or foreshock transients, which can impact the variance.…”

Spatial distribution of Langmuir waves observed upstream of Saturn's bow shock by Cassini

“…Although the spatial resolution is low (4 × 4 R S ), individual orbits can be recognized. This figure shows relatively poor spatial coverage of the upstream region compared to explorations of the Venusian or terrestrial foreshock [e.g., Crawford et al, 1998;Kasaba et al, 2000]. Due to the Cassini trajectory, the region upstream of the subsolar position of the bow shock (10-12 LT) is not covered.…”

“…The first systematic study of Langmuir wave activity was done for Venus' foreshock [Crawford et al, 1993]. Measurements from the Pioneer Venus Orbiter showed the highest wave intensity close to the leading foreshock boundary and strong decrease of the wave intensity for distances beyond ∼15 R V from the tangent point along the foreshock boundary [Crawford et al, 1998]. An extensive mapping of the terrestrial foreshock 10.1002/2016JA022912 was done using ISEE-3 [Fitzenreiter, 1995], Geotail [Kasaba et al, 2000], and STEREO [Malaspina and Ergun, 2008] observations.…”

Statistical Study of the Solar Wind Deceleration in the Earth's Foreshock Region

“…To construct two-dimensional coordinates the influence from such a latitude factor should be excluded. In the study we map all the data and also the bow shock profiles at different latitudes to be on the ecliptic plane using the method developed by Crawford et al [19] . Then two-dimensional coordinates called the foreshock coordinates are build, in which all the data including the solar wind velocity, satellites' location and also the IMF are on the X-Y plane.…”