An impulsive geomagnetic effect from an early-impulsive flare

TL;DR

This paper reports a unique case of an impulsive geomagnetic effect caused by an early-impulsive solar flare with exceptionally hard radiation, highlighting the importance of short-timescale solar physics phenomena.

Contribution

It introduces the observation of an early-impulsive component in a solar flare that deviates from typical patterns, emphasizing the need to study impulsive responses in solar-terrestrial interactions.

Findings

Detected an early-impulsive, gamma-ray-rich component preceding typical flare emissions.

Characterized the spectral distribution of the early-impulsive radiation in detail.

Observed a standard gradual SFE during the main phase of the flare.

Abstract

The geomagnetic "solar flare effect" (SFE) results from excess ionization in the Earth's ionosphere, famously first detected at the time of the Carrington flare in 1859. This indirect detection of a flare constituted one of the first cases of "multimessenger astronomy," whereby solar ionizing radiation stimulates ionospheric currents. Well-observed SFEs have few-minute time scales and perturbations of >10 nT, with the greatest events reaching above 100 nT. In previously reported cases the SFE time profiles tend to resemble those of solar soft X-ray emission, which ionizes the D-region; there is also a less-well-studied contribution from Lyman-alpha. We report here a specific case, from flare SOL2024-03-10 (M7.4), in which an impulsive SFE deviated from this pattern. This flare contained an "early impulsive" component of exceptionally hard radiation, extending up to gamma-ray energies above 1 MeV, distinctly before the bulk of the flare soft X-ray emission. We can characterize the spectral distribution of this early-impulsive component in detail, thanks to the modern extensive wavelength coverage. A more typical gradual SFE occurred during the flare's main phase. We suggest that events of this type warrant exploration of the solar physics in the "impulse response" limit of very short time scales.