Correlation of Bow Shock Plasma Wave Turbulence with Solar Wind Parameters

Abstract

The rms field strengths of electrostatic and electromagnetic turbulence in the earth's bow shock, measured in the frequency range 20 Hz to 200 kHz with the Imp 6 satellite, are found to correlate with specific solar wind parameters measured upstream of the bow shock. The largest rms field strengths of electrostatic turbulence (200 Hz to 4 kHz) occur when the upstream electron to proton temperature ratio Te/Tp is large and when the proton temperature Tp is small, an indication that the mechanism for generating electrostatic turbulence in the bow shock is more efficient when lower upstream proton temperatures occur. No substantial correlation is found between the rms field strengths of electrostatic turbulence and three upstream parameters commonly used to classify the magnetohydrodynamic structure of the turbulent bow shock: the Alfvén Mach number MA, the ratio of particle pressure to magnetic field pressure β, and the shock normal angle . The strong correlation with Te/Tp and Tp and the lack of strong correlation with MA,β, and indicate that the strength of electrostatic turbulence in the bow shock is determined by the kinetic properties of the solar wind plasma rather than by its fluid properties. The largest rms field strengths for electromagnetic turbulence (20 Hz to 4 kHz) occur when the upstream particle density N is large and when the shock normal angle is closer to 90°, this result supporting a previous conclusion that whistler waves comprise the electromagnetic turbulence in the bow shock. Electric field turbulence, composed of both electrostatic and electromagnetic fluctuations, correlates with the upstream parameters Te/Tp, Tp, and in such a way as to imply that mode coupling occurs between electrostatic and electromagnetic waves. A broad spectrum of high‐frequency (3–30 kHz) electrostatic turbulence typically observed in the leading edge of the bow shock is interpreted as indicating the region of electron heating. Deeper within the shock transition the intensity of low‐frequency (<3 kHz) electrostatic turbulence greatly increases to form a broad peak, centered between 200 and 800 Hz, and is interpreted as corresponding to the region of maximum proton heating. The characteristic development of the electric field spectrum through the shock transition indicates that strong coupling exists between the electron and proton heating processes.

A comparative study of plasma heating by ion acoustic and modified two‐stream instabilities at subcritical quasi‐perpendicular shocks

“…Certainly, if the measured values of the shock widths are to be believed, one is below the threshold for the ion acoustic instability, and thus one does not expect appreciable growth of the waves. The fact that the measured wave intensities, albeit with a limited number of data points, show strong dependence on upstream Te/Ti, consistent with the results of Rodriguez and Gurnett [1976] but contrary to the theoretical results based on saturated ion acoustic waves, again suggests that the waves have not reached saturation. In short, the wave data alone tend to indicate that the modified two-stream instability most likely drives the dissipation at these shocks, while the ion acoustic turbulence level is much too low to do significant heating.…”

“…The higher-frequency waves, due to instabilities such as the ion acoustic, are capable of heating the plasma [e.g., Boyd, 1977], but the applicability of this process is limited by the fact that the ion acoustic instability has a very high threshold unless T e >> T/. Nevertheless, these higher-frequency modes are still present at the shock when T e _• T i in the solar wind upstream from the shock [Rodriguez and Gurnett, 1976]. The lower hybrid modes, particularly the lower hybrid drift instability and the modified two-stream instability, on the other hand, are attractive because they heat both electrons and ions, are not suppressed when T e • T•, and have lower thresholds than the ion acoustic instability [Lemons and Gary, 1978].…”

Electromagnetic effects on the modified two-stream instability

“…The linear theory of the ion acoustic instability gives good agreement with laboratory experiments in which Te >> T• [Daughney et al, 1970;Muraoka et al, 1973' Craig et al, 1974, and this instability probably plays a major role in generating electrostatic turbulence within the earth's bow shock [Rodriguez and Gurnett, 1976]. However, as is often the case in the solar wind [Feldman et al, 1977], in the earth's bow shock [Feldman et al, 1973, Table 2], and in laboratory plasmas [Keilhacker and Steuer, 1971], Te/Tt is not large enough to allow the ion acoustic instability to grow.…”

“…Growth rates for the electrostatic modified two stream (•e = 0.01 ) agree with previously published growth rates in Figure 3 [Rodriguez and Gumerr, 1976, Table 2] are used, IV is of the order of 10 -•'2 erg cm -3. This value falls below the mean energy density of electric field fluctuations, 10 -•8 erg cm -3, observed in the earth's bow shock [Rodriguez and Gurnett, 1976, Table 2] by several orders of magnitude. This suggests that these fluctuating fields are a result of plasma instabilities.…”

Waves and Instabilities in Collisionless Shocks

“…Even after nearly twenty years of study the factors that control the intensities are only partially understood. The only systematic investigation of the relation between the upstream parameters and the wave intensities is the study carried out by Rodriguez and Gurnett [1976]. In their study, a total of 96 shocks detected by IMP-6 were used to determine the correlation between three plasma wave intensities and twelve upstream plasma parameters.…”