Whistler-mode waves in near-equatorial THEMIS measurements: reconstruction of magnetic field spectra from electric field and plasma measurements

TL;DR

This paper presents a validated method to reconstruct magnetic field spectra from electric field and plasma data in THEMIS spacecraft measurements, enabling better analysis of whistler-mode waves despite instrument limitations.

Contribution

A novel technique for reconstructing magnetic field spectral density from electric and plasma measurements in THEMIS data, compensating for instrument failures.

Findings

Reconstructed magnetic field spectral density is within a factor of ~1.5 of actual measurements.

The technique is validated using measurements from THEMIS A with full 3D data.

Enables inclusion of E and D spacecraft data in whistler-mode wave studies.

Abstract

Electromagnetic whistler-mode waves are a natural emission in the outer radiation belt and the Earth's magnetotail. The resonant interaction of these waves and energetic electrons are responsible for electron acceleration and losses, thus coupling the magnetosphere and ionosphere. Near-equatorial spacecraft use search-coil magnetometers for whistler-mode wave measurements, and one of the largest (covering the longest period of time) dataset of such waves has been collected by the THEMIS mission operating in the near-Earth magnetosphere within 2008-2025. However, after 2017, the search-coil magnetometers on two THEMIS spacecraft, THEMIS E and D, experienced problems with their signal along the spacecraft spin axis and were only able to detect the spin plane components of the wave vector. This significantly reduces our ability to detect the total wave amplitude wave magnitudes and limits our ability to incorporate the THEMIS E, D datasets into investigation of whistler-mode waves. In this technical report, we propose and validate a technique for reconstruction of magnetic field spectral density for Fast Fourier transform data product collected during Fast-Survey mode hereafter referred to as the fff dataset collected by THEMIS E and D. We use measurements of the electric field instrument and cold plasma dispersion relation to evaluate the whistler-mode magnetic field spectral density. Verification of this technique by comparison with THEMIS A measurements (which retained their 3D measurement capability intact) confirms that restored magnetic field spectral density is within a factor of ~1.5 of the actually measured magnitudes.