Subsecond Spikes in Solar Flare X-ray Flux as Seen by Fermi GBM

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Nugget
Number: 407
1st Author: Trevor KNUTH
2nd Author: Lindsay GLESENER
Published: 19 April 2021
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Contents

Introduction

Powerful solar flares have an early phase in which particle acceleration dominates the conversion of magnetic energy into observable forms. Hard X-ray (HXR) bremsstrahlung and microwave gyrosynchrotron emissions make it clear that the acceleration of electrons readily produces particle energies of 10s to 100s keV energy, and γ-rays show that (sometimes) ions can reach GeV energies. But how does this particle acceleration happen, and what does it have to do with the global instability of the overall flare/CME development, given that the total energies involved far outweigh "heating" at these times?

One important clue lies in the time scales of the bremsstrahlung, and (Ref. [1]) suggested time scales below one second. Now our study (Ref. [2]), backed up by independent work reported in Ref.[3], puts this conclusion on a firm footing.

The Fermi Gamma-ray Burst Monitor

The Fermi γ-ray burst monitor (GBM) has great sensitivity and reports data with high time resolution in order to track cosmic γ-ray bursts (of cosmological importance). This instrument responds to radiation from any direction in the sky and therefore can't help detecting solar flares (of physical importance, if not cosmological). The spectral/temporl signatures of flares and cosmic gamma-ray burst differ greatly and so in most cases they will not be confused, even though GBM provides no directional information.

Two impulsive solar flares

n Ref. [2] we studied two GBM transients associated with solar flares, SOL2011-07-30 and SOL2011-08-04. as shown in Figure 1. Both exhibit strong variability, especially at higher energies. This is expected because the lower-energy X-rays tend to come from large-scale coronal structures, with correspondingly long time scales. The HXR, on the other hand, mainly come from the dense chromosphere on basically unresolved spatial scales.


Figure 1: X-ray time profiles of 2 M9.3 solar flares as observed by GOES, Fermi GBM, and RHESSI. The dashed gray boxes indicate where subsecond spikes were observed.

Spikes were identified applying a 4-second moving boxcar average to the time profiles (as seen by the black lines in Figure 1). Any 3 σ variation from this smoothed curve that persisted for three timebins across two energy bins was identified as a spike. Figure 2 shows a period with subsecond spikes observed in the SOL2011-08-04 M9.3 flare, as seen in GBM data.

Figure 2: Subsecond spikes for the SOL2011-08-04 M9.3 flare, as seen in the residuals of GBM data. The dashed line shows the 3 σ detection level.

These spikes were analyzed by fitting Gaussians, cross correlating across energy bins, and performing fast Fourier transforms (FFTs) to the intervals with spikes using the data directly (rather than the residuals). Spikes as short as 0.1 s were detected, with an average of 0.49 and 0.38 s for the two flares. Cross correlations determine if burst peak times depend on energy band. While the lag was relatively small (0.01 to 0.1 seconds), we found that the higher energy flux consistently peaked earlier than the lower energy flux. As noted in Ref. ]4], such a result may imply a coronal particle acceleration mechanism wherein the higher energy particles are able to reach the footpoints and emit sooner than the low energy particles. Finally, our FFTs of the spiking intervals showed no periodicity for two of the intervals, but there was significant power at the 1.7 0.1 Hz frequency in the early interval for SOL2011-08-04 (the one shown in Figure 2). A periodicity may indicate a periodic energy release mechanism such as the ``leaky faucet" mechanism wherein magnetic tension increases until a critical limit is reached, releasing the energy in a rapid burst.

Conclusions

Ultimately this study was to develop a robust method for identifying and measuring spikes in Fermi GBM flux. Now that it has been demonstrated, the plan is to expand the study to a much greater set of flares. Questions of spike prevalence, scaling with flare size, spectral properties of flares, as well as the variability of the aforementioned spike parameters will be the focus of such a study.

References

[1] "Millisecond time variations in hard X-ray solar flares"

[2] "Subsecond Spikes in Fermi GBM X-Ray Flux as a Probe for Solar Flare Particle Acceleration"

[3] "Rapid Variability in the SOL2011-08-04 Flare: Implications for Electron Acceleration"

[4] "Electron Time-of-Flight Differences in Solar Flares"

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