GECAM Discovery of Peculiar Oscillating Particle Precipitation Events

TL;DR

This paper reports the detection and analysis of rare, oscillating charged particle precipitation events by GECAM during a geomagnetic storm, revealing peculiar spectral behaviors and challenging existing explanations, suggesting they may be a new phenomenon.

Contribution

It presents the first detailed observations of oscillating particle precipitation events with spectral analysis, proposing they could be a new type of particle precipitation or Lightning-induced Electron Precipitation.

Findings

Two events showed significant periodicity.

One event exhibited spectral lag transition from 'softer-earlier' to 'softer-later'.

No link to lightning activity was found.

Abstract

Charged particle precipitation typically manifests as a gradual increase and decrease of flux observed by space detectors. Cases with rapidly flux variation are very rare. Periodic events are even more extraordinary. These oscillating particle precipitation (OPP) events are usually attributed to the bounce motion of electrons, which are induced by lightning. Owing to the observation limitations, there has been debate regarding whether these oscillations originate from temporal flux evolution or spatial structure evolution. Here we report three peculiar charged particle precipitation events detected by GECAM during a geomagnetic storm on March 21, 2024, with two exhibiting significant periodicity. These events were observed around the same region during three consecutive orbits. Through comprehensive temporal and spectral analyses, we revealed that one of the OPP events exhibited a transition in spectral lag of mini-pulses, shifting from "softer-earlier" to "softer-later" while showing no significant time evolution in overall frequency characteristics. And there is no association found between these two OPP events and lightning activity. Several possible scenarios are discussed to explain these charged particles with a life time of more than 3.5 hours, but the nature of these three events remains an enigma. We suggest that these GECAM-detected OPP events may represent a new type of particle precipitation event or a peculiar Lightning-induced Electron Precipitations (LEPs).