A remarkable, but confused, coronal hard X-ray source
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|number = 325 | |number = 325 | ||
|first_author = Alexandra Lysenko | |first_author = Alexandra Lysenko | ||
- | |second_author = Larisa Kashapova | + | |second_author = Larisa Kashapova, Hugh Hudson |
|publish_date = 25 June 2018 | |publish_date = 25 June 2018 | ||
|next_nugget= TBD | |next_nugget= TBD | ||
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== Introduction == | == Introduction == | ||
- | Large-scale coronal hard X-ray sources have | + | Large-scale coronal hard X-ray sources have drawn special attention in this era of RHESSI, particularly now with the arrival of microwave imaging spectroscopy. |
- | era of RHESSI, | + | The traditional way of studying the hard X-rays from corona 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" (no imaging) instrument to isolate the coronal contributions. |
- | 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" (no imaging) instrument to isolate | + | |
- | the coronal contributions. | + | |
- | The occultation technique long ago revealed a strikingly different morphology | + | The occultation technique long ago revealed a strikingly different morphology of coronal sources from the non-thermal properties of ordinary flares (Ref. [1]). |
- | from the non-thermal properties of ordinary flares (Ref. [1]). | + | The properties of some 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. | |
- | smooth time variations, high altitudes (as inferred from the occultation | + | 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. |
- | 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, | + | 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 http://www.ioffe.ru/LEA/kwsun/] instrument in SOL2014-09-01. |
- | which shows the time variation of the hard X-ray and gamma-ray continuum | + | The power-law spectral index remains remarkably constant at a suggestive slope (is it really exactly 2, and why would that ever happen? AL: Yes, it varies between ~1.9 and ~2.1). |
- | spectrum as observed by the [Konus/WIND] instrument in SOL2014-09-01. | + | It would be fair to say that no simple model has yet been proposed for this. |
- | 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. | + | |
[[File:325f1.png|400px|thumb|center|Figure 1: | [[File:325f1.png|400px|thumb|center|Figure 1: | ||
Line 46: | Line 30: | ||
]] | ]] | ||
- | These events appear to be fairly common, occurring many times per year at a | + | These events appear to be fairly common (really? i know only few events like 2014-09-01), 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. |
- | rate no doubt limited by the difficulty of the observations and their | + | In this Nugget we describe an occulted event from two decades ago, which presents the difficult problem of [source confusion] as multiple flaring events were simultaneously observed on the Sun, and this question will greatly improve with true hard X-ray imaging observations become |
- | limited sensitivity even in the modern era. | + | |
- | In this Nugget we describe an occulted event from two decades ago, which | + | |
- | + | ||
- | flaring events were simultaneously | + | |
- | + | ||
- | + | ||
- | and will greatly improve with true hard X-ray imaging observations become | + | |
possible. | possible. | ||
== SOL1999-06-04T07:14 == | == SOL1999-06-04T07:14 == | ||
- | + | 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 shows the time series from our "new" event, not really new but | + | |
- | not previousy described in the terms we use here. | + | |
[[File:325f2.png|600px|thumb|center|Figure 2: | [[File:325f2.png|600px|thumb|center|Figure 2: | ||
Line 74: | Line 48: | ||
]] | ]] | ||
- | One can see a striking difference with the event in Figure 1, in that the | + | 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. |
- | hard X-ray spectrum is not quite so hard and not quite so invariant with | + | On the other hand, the new event does give the impression of a slowly-varying component added to a rapid component. |
- | time. | + | We would describe the variations prior to about 06:59:30 as "soft-hard-soft", the pattern invariably seen in impulsive flare emission. |
- | On the other hand, the new event does give the impression of a slowly-varying | + | In these "normal" flares the spectral hardness tends strongly to peak at the time of peak hard X-ray emission. |
- | component added to a rapid component. | + | The slow component here would be a "soft-hard-harder" pattern, typically seen during major CME/SEP flares with coronal disturbances. |
- | We would describe the variations prior to about 06:59:30 as "soft-hard-soft", | + | The microwave time series (from the [Nobeyama Radio Polarimeters] also has complexity suggesting some sort of combination. |
- | 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. | + | |
- | + | This remarkable event could be localized well by the [Siberian Cross] microwave array, which identified the confusing sources: the first one above the western solar limb and the second located on the disc. Thus these two characteristic patterns likely do not coincide spatially. | |
- | + | The gradually varying hard X-ray component could be related with coronal source above the solar limb, while the component with soft-hard-soft spectral evolution probably was related with "ordinary" on-disc flare. | |
- | + | ||
- | + | ||
- | related | + | |
[[File:325f3.png|500px|thumb|center|Figure 3: | [[File:325f3.png|500px|thumb|center|Figure 3: | ||
Line 101: | Line 65: | ||
]] | ]] | ||
- | Note that the GOES soft X-ray data also are Sun-as-a-star, so it seems | + | 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. |
- | likely that this event's classification as M3.7 may mix up the two events. | + | |
The SSRT microwave imaging distinguishes them. | The SSRT microwave imaging distinguishes them. | ||
== Conclusion == | == Conclusion == | ||
- | We report a probable occulted event in 1999 that had gone un-noticed | + | 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]. |
- | previously, and could identify it spatially with SSRT as well as characterize | + | 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. |
- | 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 == | == References == | ||
[1] [http://adsabs.harvard.edu/abs/1971ApJ...165..655F "Evidence from Hard X-Rays for Two-Stage Particle Acceleration in a Solar Flare"] | [1] [http://adsabs.harvard.edu/abs/1971ApJ...165..655F "Evidence from Hard X-Rays for Two-Stage Particle Acceleration in a Solar Flare"] |
Revision as of 11:42, 30 June 2018
Nugget | |
---|---|
Number: | 325 |
1st Author: | Alexandra Lysenko |
2nd Author: | Larisa Kashapova, Hugh Hudson |
Published: | 25 June 2018 |
Next Nugget: | TBD |
Previous Nugget: | A white-light flare caveat |
List all |
Contents |
Introduction
Large-scale coronal hard X-ray sources have drawn special attention in this era of RHESSI, particularly now with the arrival of microwave imaging spectroscopy. The traditional way of studying the hard X-rays from corona 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" (no imaging) instrument to isolate the coronal contributions.
The occultation technique long ago revealed a strikingly different morphology of coronal sources from the non-thermal properties of ordinary flares (Ref. [1]). The properties of some 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 http://www.ioffe.ru/LEA/kwsun/] 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? AL: Yes, it varies between ~1.9 and ~2.1). It would be fair to say that no simple model has yet been proposed for this.
These events appear to be fairly common (really? i know only few events like 2014-09-01), 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 presents the difficult problem of [source confusion] as multiple flaring events were simultaneously observed on the Sun, and this question will greatly improve with true hard X-ray imaging observations become possible.
SOL1999-06-04T07:14
Figure 2 shows the time series from our "new" event, not really new but not previousy described in the terms we use here.
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.
This remarkable event could be localized well by the [Siberian Cross] microwave array, which identified the confusing sources: the first one above the western solar limb and the second located on the disc. Thus these two characteristic patterns likely do not coincide spatially. The gradually varying hard X-ray component could be related with coronal source above the solar limb, while the component with soft-hard-soft spectral evolution probably was related with "ordinary" on-disc flare.
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"
RHESSI Nugget Date | 25 June 2018 + |
RHESSI Nugget First Author | Alexandra Lysenko + |
RHESSI Nugget Index | 325 + |
RHESSI Nugget Second Author | Larisa Kashapova, Hugh Hudson + |