A remarkable, but confused, coronal hard X-ray source

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Nugget
Number: 325
1st Author: Alexandra Lysenko
2nd Author: Hugh Hudson
Published: 25 June 2018
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Contents

Introduction

Large-scale coronal hard X-ray sources have become very interesting, in this era of RHESSI, especially now with the arrival of microwave imaging spectroscopy. The traditional way of studying the hard X-rays has been to arrange for the event to occur behind the Sun, so that the solar limb occults the bright footpoint sources and allows event a Sun-as-a-star instrument to isolate the coronal contributions.

The occultation technique long ago revealed a strikingly different morphology from the non-thermal properties of ordinary flares (Ref. [1]). Some of the properties of these events include very flat hard X-ray spectra, smooth time variations, high altitudes (as inferred from the occultation geometry) and microwave spectra with characteristically low peak frequencies. These events commonly occur in conjunction with other coronal disturbances, such as [CMEs], type II/IV radio events, and [SEPs], all of which relate strongly to particle acceleration. Recently the pace of discovery in this area has sharply increased because of much-improved instrumentation, such as RHESSI hard X-ray/gamma-ray imaging, and especially the [Fermi] observations of "sustained" gamma-ray emission.

As a taste of the remarkable properties of these events, please see Figure 1, which shows the time variation of the hard X-ray and gamma-ray continuum spectrum as observed by the [Konus/WIND] instrument in SOL2014-09-01. The power-law spectral index remains remarkably constant at a suggestive slope (is it really exactly 2, and why would that ever happen?). It would be fair to say that no simple model has yet been proposed for this.

Figure 1: The remarkable lack of spectral variation during SOL2014-09-01, not the subject of this Nugget but an event that was well observed by RHESSI and which must be closely related to the one we describe here.

These events appear to be fairly common, occurring many times per year at a rate no doubt limited by the difficulty of the observations and their limited sensitivity even in the modern era. In this Nugget we describe an occulted event from two decades ago, which presented the difficult problem of [source confusion] in that multiple flaring events were simultaneously present on the Sun. The observations of this event, SOL1999-06-04T07:14 This problem of confusion necessarily comes from Sun-as-a-star observation and will greatly improve with true hard X-ray imaging observations become possible.

SOL1999-06-04T07:14

This remarkable event could be localized well by the [Siberian Cross] microwave array, whose relatively low frequencies fit well with the characteristic spectra of these events. Figure 2 shows the time series from our "new" event, not really new but not previousy described in the terms we use here.

Figure 2: A time-series overview of SOL1999-06-05, as labeled. The [GOES] soft X-ray time derivative (red in the upper panel) shows the timing of the flare energization as measured by the growth of the coronal soft X-ray source (the [Neupert effect]). This derivative is artificially noisy after about 06:59:30 because of inadequate sampling in the original time series.

One can see a striking difference with the event in Figure 1, in that the hard X-ray spectrum is not quite so hard and not quite so invariant with time. On the other hand, the new event does give the impression of a slowly-varying component added to a rapid component. We would describe the variations prior to about 06:59:30 as "soft-hard-soft", the pattern invariably seen in impulsive flare emission. In these "normal" flares the spectral hardness tends strongly to peak at the time of peak hard X-ray emission. The slow component here would be a "soft-hard-harder" pattern, typically seen during major CME/SEP flares with coronal disturbances. The microwave time series (from the [Nobeyama Radio Polarimeters] also has complexity suggesting some sort of combination.

In this case we have identified the confusing source. It is not the flare itself, even though these two characteristic patterns often do coincide spatially. In this case the confusing source is quite far away, and likely not to be related at all, as shown in Figure 3.

Figure 3: Images showing the limb event (green contours from SSRT microwave imaging and [LASCO] CME structure in the corona; red contours showing another active region on the disk of the Sun, responsible for the M-class flare SOL1999-06-05T07:03.

Note that the GOES soft X-ray data also are Sun-as-a-star, so it seems likely that this event's classification as M3.7 may mix up the two events. The SSRT microwave imaging distinguishes them.

Conclusion

We report a probable occulted event in 1999 that had gone un-noticed previously, and could identify it spatially with SSRT as well as characterize its hard X-ray spectral evolution with [Konus/WIND]. It may be difficult to do much quantitative analysis on this particular event because of source confusion and the lack of RHESSI or even [Yohkoh] hard X-ray imaging, but this event adds one more case to the limited catalog of occulted solar hard X-ray coronal events.

References

[1] "Evidence from Hard X-Rays for Two-Stage Particle Acceleration in a Solar Flare"

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