Cavitons Associated with Ion-Acoustic-Like Waves in Foreshock Transients

Abstract

Foreshock transients upstream of the Earth's bow shock, such as foreshock bubbles and hot flow anomalies, are often characterized by reduced-density cores and strong plasma fluctuations. These conditions provide environments where electrostatic wave activity and localized density structures can coexist. Using high-time-resolution measurements from the Magnetospheric Multiscale (MMS) mission, we investigate the relationship between bursty electrostatic wave activity and localized electron density depletions within foreshock transients. A representative case study reveals a clear scaling between wave activity and density depletion, and a statistical analysis across multiple events shows that this scaling persists when the wave activity, with characteristics consistent with ion-acoustic-like waves, is represented in terms of electrostatic potential fluctuations normalized by electron temperature. In contrast, representations based on electric field amplitude, even when similarly normalized, exhibit substantial event-to-event variability. These results provide observational evidence for a causal relationship between ion-acoustic-like electrostatic wave activity and cavitons in foreshock plasmas.

Figures (3)

• Figure 1: Overview of a representative foreshock transient observed by MMS1. (a) Magnetic field magnitude and components in the Geocentric Solar Ecliptic (GSE) coordinate system. (b) Ion and electron number densities from FPI together with the electron density inferred from the spacecraft potential. (c) Electric field components in GSE coordinates. (d) Band-pass filtered electron density (1–30 Hz) derived from the spacecraft potential, highlighting localized density depletions. (e–f) Dynamic power spectra of the electric and magnetic fields, respectively. The horizontal reference lines indicate, from low to high frequency, the lower-hybrid frequency, electron cyclotron frequency, and ion plasma frequency. (g) Scatter plot showing the relationship between the electric-field envelope amplitude and the density-depletion amplitude on logarithmic scales, with a linear fit overlaid.
• Figure 2: Statistical relationship between electric-field amplitude and density depletion for all selected foreshock transient events. Scatter plots combine data from all events using different parameterizations: (a) $\log_{10}(|\mathbf{E}|)$ versus $-\log_{10}(|\delta n_e|)$, (b) $\log_{10}(|\mathbf{E}|)$ versus $-\log_{10}(|\delta n_e/n_e|)$, (c) $\log_{10}(e|\mathbf{E}|\lambda_D/T_e)$ versus $-\log_{10}(|\delta n_e|)$, and (d) $\log_{10}(e|\mathbf{E}|\lambda_D/T_e)$ versus $-\log_{10}(|\delta n_e/n_e|)$. Here $|\mathbf{E}|$ is the envelope amplitude of the high-pass-filtered electric field, $\lambda_D$ is the electron Debye length, and $T_e$ is the background electron temperature. The quantity $|\delta n_e|$ denotes the depletion-only density amplitude derived from band-pass-filtered spacecraft-potential-based electron density. Colored symbols denote individual events, with thin colored lines showing event-wise linear fits. The thick black line indicates the linear fit to the combined dataset in logarithmic space; Pearson and Spearman correlation coefficients and the RMSE are shown in each panel.
• Figure 3: Same format as Figure 2, except that $|\mathbf{E}|$ is replaced by $\delta \Phi$.