A new development in the Frost-Dennis paradigm

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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 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]). Very recently a kind of Rosetta Stone among solar flares has appeared, as described in the next section, and this promises to clarify the physics. Figure 1 shows the original (1969) observation.

Figure 1: The original Frost & Dennis hard X-ray observation, from the OSO-5 satellite: SOL1969-03-30. (Ref. [1]).

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 processes in the large-scale corona of course come from long-wavelength radio astronomy, with its zoo of non-thermal phenomena that began to be discovered 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 (see Ref. [2]).

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 (Konus) light curves for this event.

Figure 2: Comparison of microwave and hard X-ray light curves for SOL2014-09-01, showing the close agreement typical of the Frost-Dennis events.

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 as high as 3 GeV and with durations up to many hours. Most of these events have been on the visible hemisphere of the Sun, yet the great puzzle has been their identification with other solar phenomena. Without identification in terms of high-resolution optical or EUV images we cannot readily understand the physical nature of these events. Now Ref. [3] has noted 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.

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

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. A third, SOL2013-10-11 also did show the signatures, and the Fermi/LAT Consortium is pursuing detailed analysis. Still, the lack of identification in SOL2014-01-06 is unambiguous, and so this says simply that we still have some unknowns at these highest energies - more physics to figure out, probably.

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"

[3] "Fermi Large Area Telescope observations of high-energy gamma-ray emission from behind-the-limb solar flares"