THEMIS Observations of Electrostatic Waves in Context with Ion Foreshock Plasma Structures

The terrestrial foreshock region plays an essential role in preprocessing the undisturbed solar wind en route to Earth's bow shock and magnetopause. Such preprocessing involves a rich array of plasma structures observed within the terrestrial ion foreshock, including short-duration large-amplitude magnetic structures (SLAMS), hot flow anomalies (HFAs), foreshock cavities and recently examined density holes. Much work has been done to characterize the macroscopic structure of these objects, as revealed in plasma and DC fields data, and to study higher frequency waves using wave spectra. Little has been done to date to examine high-frequency waveform data in the foreshock, and to place such measurements into context of the sub-structure of the features observed there. Here we present case studies of foreshock electrostatic waves observed by THEMIS, from a few tens to few thousand Hz. The THEMIS/EFI, FGM and SCM instruments provide long duration three-axis measurements of electric and magnetic field waveforms from DC to 8000 Hz, allowing us to assess how these waves are organized within the substructure of various foreshock phenomena. Preliminary analysis indicates large (20-100 mV/m) amplitude oscillatory electrostatic waves from a few tens of Hz to few thousand Hz. Such waves are often but not always observed in association with fine-scale currents embedded within foreshock structures, which is suggestive of different generation mechanisms. These oscillatory waves appear to be consistent with short-scale, ion-acoustic like waves observed in the foreshock reported in the literature. Notably, we also see large amplitude, solitary-like electrostatic waveforms embedded within ion acoustic turbulence, which is suggestive of counter-streaming particles. We discuss the characteristics of the electrostatic waves, such as orientation with respect to magnetic field, wavelength, and relation to fine structure in the magnetic fields and plasma density.