Variation of Whistler-Mode Wave Characteristics Along Magnetic Field Lines: Comparison of Near-Equatorial THEMIS and Middle-Latitude ERG Observations

TL;DR

This study compares whistler-mode wave intensities observed by near-equatorial THEMIS and off-equatorial ERG spacecraft, revealing distinct latitudinal variation patterns and implications for radiation belt modeling.

Contribution

It provides a direct comparison of wave intensities at the same MLT and time, identifying two categories of wave behavior and enhancing understanding of wave distribution in the radiation belt.

Findings

Confirmed similar wave spectrum characteristics between THEMIS and ERG.

Identified two categories of wave intensity variation: predictable and rapid decay.

Discussed implications for radiation belt models based on observed wave behaviors.

Abstract

The latitudinal distribution of whistler-mode wave intensity plays a crucial role in determining the efficiency and energy of electrons scattered by these waves in the outer radiation belt. Traditionally, this wave property has mostly been derived from statistical measurements of off-equatorial spacecraft, which collect intensity data at various latitudes under different geomagnetic conditions and at different times. In this study we examine a set of events captured by both the near-equatorial THEMIS spacecraft and the off-equatorial ERG spacecraft. Specifically, we compare the whistler-mode wave intensity from THEMIS and ERG measurements at the same MLT and time sectors. Similar wave spectrum characteristics confirm that THEMIS and ERG indeed observed the same wave activity. However, upon closer examination of the wave intensity variations, we identify two distinct categories of events: those that follow the statistically predicted variations in wave intensity along magnetic latitudes, and those that exhibit rapid wave intensity decay away from the equatorial plane. We analyze main characteristics of events from both categories and discuss possible implications of our analysis for radiation belt models.