Precise timing of flare footpoint sources from mid-infrared observations

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Nugget
Number: 457
1st Author: Paulo SIMÕES et al.
2nd Author:
Published: October 23, 2023
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Contents

Introduction

The mid-infrared wavelengths of the solar spectrum (the 30 THz or 10 μ band) represent a new frontier, with only the skimpiest spectroscopic or imaging data yet available for research on solar flares. We remember Pierre Kaufmann here for his inspirational work in this area. Good ground-based observations in the mid-IR require special atmospheric conditions (low amounts of precipitable water vapor), plus special kinds of detector. Good image resolution also requires a big telescope because of diffraction. These requirements all came together at the iconic McMath-Pierce telescope at Kitt Peak for the flare SOL2014-09-24T17:50 (C7.0), as described in Ref. [1].


Surprisingly, the mid-IR directly impacts our understanding of flare hard X-ray emissions, an essential aspect of the energy release This Nugget explains the connection here. For reference, the mid-IR in our observations means wavelengths 10x those of visible light, and so the radiation processes may be quite different.

Footpoint emissions

The observations of the 2014 flare revealed two footpoints, a characteristic of hard X-ray observations. Singularly bright double-footpoint emissions also appear in white light, following the pattern set in 1859 by the Carrington event. That this pattern should appear so clearly in the mid-IR was somewhat unexpected, but this offered a very valuable asset: time resolution. Figure 1 shows the footpoint character of the mid-IR in SOL2014-09-24.

Figure 1: The broadband character of flare footpoint sources: from left to right, two mid-IR continuum bands, AIA 1700Å, slightly saturated, and RHESSI hard X-ray contours at 50-70 keV with AIA 335Å as the base image.

The IR observations have high cadence, sampled in this event at 1 s intervals. This is required for mid-IR imaging because the warm telescope creates a very high background level at these long wavelengths.

Footpoint timing

We take advantage of this high time resolution to ask the important question: How well synchronized are the twin footpoints? This characteristic of "conjugacy" has immediate implications for energy transport, and up until now the best knowledge had come from Ref. [2], exploiting Yohkoh hard X-ray imaging. This work, across multiple flares, established conjugacy to a fraction of a second, limited both by the time resolution of data sampling and by the image processing needed to deconvolve the images; we note that RHESSI data have similar limitations.

The mid-IR timing results for the longer-wavelength band, centered at 8.2μ, are here in Figure~2. They show the S footpoint to lead the N footpoint by 0.754±0.066 s.

Figure 2: The time variability of N and S footpoint sources at 8.2μ, with a correlation plot in the upper-right inset. The result is a lag of -0.754±0.066 s. The 8.2μ band confirms this but with poorer SNR.

The quantitative results disagree with exact conjugacy and thus exhibit some tension with the earlier HXR results, but the emission mechanisms and other characteristics of the observations make this tension somewhat model-dependent. Among these, for example (and this applies to the HXR observations as well) is that the observations do not resolve the actual sources, which we suspect to have very fine spatial scales. This means that we cannot really be sure about strict magnetic conjugacy. In any case Ref. [1] deals with these issues comprehensively.

Conclusions

Double-footpoint flare structure appears very cleanly in the mid IR, and in the analysis of a single flare we have taken advantage of the high time resolution to test for conjugacy, important in the context of the theory of flare energy transport. The finding favors a relatively slow mechanism, as opposed to transport by beamed semi-relativistic electrons. This is less important (it's only one flare!) than the general conclusion that mid-IR observations are quite useful and should be pursued as energetically as possible. We note that ground-based observations at still longer wavelengths (the 20μ band) should also be possible.

References

[1] "Spectral and Imaging Observations of a White-light Solar Flare in the Mid-infrared"

[2] "Characteristics of hard X-ray double sources in impulsive solar flares"

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