A new development in the Frost-Dennis paradigm
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Much of our research on solar hard X-rays focuses on one of two paradigms, | Much of our research on solar hard X-rays focuses on one of two paradigms, | ||
one of which (the | one of which (the | ||
- | [nanoflares]) | + | [https://en.wikipedia.org/wiki/Nanoflares nanoflares]) |
remains somewhat intangible at present. | remains somewhat intangible at present. | ||
- | As | + | As regards "proper" [https://en.wikipedia.org/wiki/Solar_flare flares], we have a well-documented paradigm that arose in |
- | the 1950s and 60s, and | + | the 1950s and 60s, and has become an excellent framework for interpreting many flare effects. |
- | effects. | + | This paradigm works well for X-rays, which were only barely observed at those times. |
- | This paradigm works well for X-rays, which were only barely observed at those | + | |
- | times. | + | |
Briefly we can summarize the standard | Briefly we can summarize the standard | ||
- | [flare | + | [https://en.wikipedia.org/wiki/Solar_flare flare] paradigm |
as a sudden transfer of energy from the coronal magnetic field into | as a sudden transfer of energy from the coronal magnetic field into | ||
particle acceleration, with a loss of hydrostatic equilibrium as a result. | particle acceleration, with a loss of hydrostatic equilibrium as a result. | ||
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parent flare was occulted by the edge of the Sun. | parent flare was occulted by the edge of the Sun. | ||
This and a few later, similar episodes gave the following list of properties: | This and a few later, similar episodes gave the following list of properties: | ||
+ | |||
+ | PROPERTIES OF FROST-DENNIS EVENTS | ||
Coronal origin | Coronal origin | ||
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features of a Frost-Dennis event with high-energy gamma-ray observations | features of a Frost-Dennis event with high-energy gamma-ray observations | ||
from the | from the | ||
- | [Fermi/LAT] detector. | + | [https://www-glast.stanford.edu/instrument.html Fermi/LAT] detector. |
These gamma rays point to extremely high photon energies, which would be | These gamma rays point to extremely high photon energies, which would be | ||
very consistent with the hard X-ray and microwave signatures of a Frost-Dennis | very consistent with the hard X-ray and microwave signatures of a Frost-Dennis | ||
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These latter presumably produce the Frost-Dennis signature from within the | These latter presumably produce the Frost-Dennis signature from within the | ||
same coronal volume as the high-energy gamma rays. | same coronal volume as the high-energy gamma rays. | ||
+ | |||
+ | |||
+ | [[File:268fx.png|600px|thumb|center|Figure 3: | ||
+ | High-energy data for SOL2014-09-01, showing (upper) the hard X-ray time series, | ||
+ | and (lower) the 100 MeV gamma-ray counts, both from the Fermi satellite (Ref. [3]) | ||
+ | The peak at 11:10 is that of Figure 2, but note the high signal-to-noise ratio of the | ||
+ | Fermi/LAT observations at 100 MeV, which allow the event to be seen for hours, | ||
+ | rather than minutes.]] | ||
== Conclusions | == Conclusions |
Revision as of 10:54, 20 February 2016
Nugget | |
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Number: | 268 |
1st Author: | Hugh Hudson |
2nd Author: | Melissa Pesce-Rollins and Larisa Kashapova |
Published: | 22 February 2016 |
Next Nugget: | A meritorious index |
Previous Nugget: | Dynamic Ionosphere |
List all |
Contents |
Introduction
Much of our research on solar hard X-rays focuses on one of two paradigms, one of which (the nanoflares) remains somewhat intangible at present. As regards "proper" flares, we have a well-documented paradigm that arose in the 1950s and 60s, and has become an excellent framework for interpreting many flare effects. This paradigm works well for X-rays, which were only barely observed at those times.
Briefly we can summarize the standard flare paradigm as a sudden transfer of energy from the coronal magnetic field into particle acceleration, with a loss of hydrostatic equilibrium as a result. The structures and dynamics we observe as a consequence include matters such as chromospheric evaporation thick-target hard X-rays and the [ihttps://en.wikipedia.org/wiki/Neupert_effect Neupert effect]. Flares on all magnitude scales exhibit these properties, which also appear to apply to stellar flares.
The Frost & Dennis paradigm
Reference [1] introduced a different paradigm, which has slowly developed in the intervening decades (e.g., Ref. [2]). Now a kind of Rosetta Stone among solar flares has appeared, as described in the next section. Figure 1 shows the original (1969) observation.
The Figure shows the event to have had a remarkably smooth time variation (in the usual flare paradigm, the hard X-rays are impulsive), a very flat spectrum (also different), and an origin high in the corona, since the parent flare was occulted by the edge of the Sun. This and a few later, similar episodes gave the following list of properties:
PROPERTIES OF FROST-DENNIS EVENTS
Coronal origin No soft X-ray (GOES) event Smooth hard X-ray time profile Flat hard X-ray spectrum Flat gyrosynchrotron radio spectrum, with a very low peak frequency Little to no spectral evolution for hard X-rays or microwaves Meter-wave type II burst and diffuse continuum radiation
All of these properties distinguish what was happening in the middle corona (heights of 20,000-50,000 km), associated with a major flare, with what typically happens in the low corona. Other hints at some fundamentally different process in the large-scale corona of course come from long-wavelength radio astronomy, with its zoo of non-thermal processes that began in the late 1940s. In addition we can note the hint given by Alan Kiplinger's discovery of a relationship between anomalous hard X-ray spectral behavior and CME occurrence.
A flare that is a Rosetta Stone?
Now Ref. [3] has pointed out a very nice event that combines all of the features of a Frost-Dennis event with high-energy gamma-ray observations from the Fermi/LAT detector. These gamma rays point to extremely high photon energies, which would be very consistent with the hard X-ray and microwave signatures of a Frost-Dennis event. In that sense SOL2014-09-01 is like a Rosetta Stone that bridges a previously irritating gap in our knowledge.
Figure~2 shows the exceedingly close match between microwave RSTN and hard X-ray light curves Konus for this event.
The new and very important aspect of this event is its coincidence with the relatively rare high-energy gamma-ray event. Many of these events have been detected by the Fermi/LAT instrument, at photon energies reaching one GeV and with durations up to many hours. The great puzzle has been their identification with other solar phenomena, without which we cannot readily understand their physical origin. Ref. [3] notes three such events associated with flares that occurred behind the limb, as with the original Frost & Dennis event, and now we have found that for one of them there is a close match. Apparently the hard X-ray and microwave signatures of this event arise in the extreme particle acceleration needed to produce the high-energy gamma rays. We believe that these gamma-rays result from the production of pions as secondary particles, which then decay both into gamma-rays (if neutral) and into positrons or electrons (if charged). These latter presumably produce the Frost-Dennis signature from within the same coronal volume as the high-energy gamma rays.
== Conclusions
In this Nugget we have decribed a possible connection between a well-known but infrequent solar high-energy phenomenon (Frost & Dennis) and a novel but ill-understood process (the Fermi/LAT long-duration events). This kind of identification may lead to a better physical understanding of these phenomena, crucial in importance to flare research and space weather because of the extreme particle acceleration they require.
Unfortunately we still do not have optical or EUV identification of these sources, and worse yet there is complexity: another of the three events described in Ref.[3], namely SOL2014-01-06, did not show the Frost-Dennis signatures. Still, the identification in SOL2014-09-01 is unambiguous, and so this says simply that we still have some unknowns at these highest energies.
References
[1] "Evidence from Hard X-Rays for Two-Stage Particle Acceleration in a Solar Flare"
[2] "Hard X-ray emission from the solar corona"
RHESSI Nugget Date | 22 February 2016 + |
RHESSI Nugget First Author | Hugh Hudson + |
RHESSI Nugget Index | 268 + |
RHESSI Nugget Second Author | Melissa Pesce-Rollins and Larisa Kashapova + |