User talk:Jiongqiu

From RHESSI Wiki

Revision as of 20:17, 29 December 2012

Introduction

Solar flares are characterized by impulsively enhanced emissions most pronounced in hard X-ray and microwave light curves. These light curves in even a simple, single, short-lived flare usually exhibit quite complicated structures consisting of numerous fast-varying bursts, unlike the rather smooth and gradual soft X-ray emissions. Ground-based and space missions in pre-RHESSI era have revealed the shortest time scales, from a few seconds to several tens of milli-seconds, of these bursts. A few studies correlating high cadence yet spatially unresolved hard X-ray observations with high cadence high resolution optical observations have further shown that hard X-ray bursts are correlated with optical emission on time scales of about one second, suggesting that these fast-varying bursts are thick-target emissions, and their size is within one arc-second. This is, in general, consistent with recent EUV observations showing that the size of a flare loop is about one arc-second. It may be noted that these quoted values are likely upper bounds of the tempo-spatial scales as limited by the resolving capability of instruments.

Today we commonly recognize that a flare is a collection of multiple episodes of energy release on some fundamental scales. For example, the bursts' time scales may depend on the characteristic size of the elementary flux tubes (e.g., Sturrock et al. , 1984; LaRosa & Moore , 1993), or the turbulent dynamics of the reconnecting current sheets (eg., Litvinenko , 1996). As these fast-varying bursts are typically reported in hard X-ray and microwave light curves, the observed time scales are also a product of convolution with timescales of acceleration and transport of non-thermal particles in flare environment. Whereas it is yet difficult to disentangle these different physical mechanisms involved in a single burst, there are basic observational questions to be addressed: what are the temporal, spatial, and spectral properties of these bursts, when compared with the gross properties of a flare?

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI Lin et al. , 2002) launched in early 2002 has provided unprecedented advantage to hopefully answer these questions. This nugget studies a sample of hard X-ray flares observed by RHESS using a demodulation algorithm that allows analysis of hard X-ray light curves with sub-second time resolution. It should be noted that the study focuses on the most prominent fast-varying bursts standing out of, rather than making up, the entire flare emission. We call them "hard X-ray spikes".

Temporal and Spectral Properties of Hard X-ray Spikes

Having searched the entire flare catalog in 2002, it turns out only a small fraction of flares exhibit hard X-ray spikes that can be detected at up to 100 keV. The spikes usually have symmetric rise and decay, and have durations below one second, which is independent of the photon energy. These results are consistent with the findings by SMM three decades ago.

In terms of the spectral property of the spikes, the spectrum of integrated counts of greater than 20 keV can be fitted to a power-law distribution. Compared with underlying components, spikes have slightly harder spectrum. These are generally recognized signatures of non-thermal emission. A fraction of the spikes also exhibit energy-dependent time lag of either kind: the low-energy emission lagging high-energy emission, usually interpreted as reflecting time-of-flight of direct precipitation electrons, or high-energy emission lagging low-energy emission thought to be indicative of Coulomb collision effects in the trap.

Figure 1: Maps of the Bastille Day flare, superposed on an array of positive (P) and negative (N) poles used to characterize the solar magnetic field. Left: hard X-rays; right: UV ribbons time-coded according to the bar at the bottom. Note that both emissions tend to have EW elongations lying in opposite polarities.

Where Are the Spikes?

A few different ways have been attempted to locate ten spikes out of the studied sample. Direct mapping of the spikes, as confirmed by coordinated UV and optical images whenever available, shows that the spikes at above 25 keVs are most likely thick-target emissions at conjugate foot-points of flare loops, and observed loop length is consistent with estimate from time-of-flight analysis. The spikes are basically inside or very closely attached to the sources of underlying components, indicating that reconnection and acceleration events that produce hard X-ray spikes take place in the same magnetic environment of the underlying sources.

Figure 2: UV time variations in individual pixels.

Conclusions

RHESSI capabilities have allowed us to continue the decades-long effort to study rapidly evolving hard X-ray bursts. These observations may hold the key to uncover fundamental scales of energy release in solar flares. The most prominent hard X-ray spikes from a sample of RHESSI flares exhibit similar temporal properties as those discovered in SMM flares in the 70s. The additional advantage of RHESSI spectral and imaging capability, combined with other imaging instruments, further

Given their generally harder spectrum, hard X-ray spikes are the most energetic population of elementary bursts. For the ten samples that have been mapped, they are almost certainly thick-target emissions, mostly from double foot-points of flare loops. They are produced in nearly the same location and magnetic environment as the less impulsive and less energetic bursts in the neighborhood. Should this mean that a peculiar dynamic perturbation in the same macroscopic current sheet which is the master board of clusters of energy release events? Are we at the position to directly associate the observed scales with a "fundamental" scale of flare, and what does this "fundamental" scale mean?

Acknowledgements

This Nugget is based on a presentation at the recent Solar Physics Division meeting by the author with Wenjuan Liu, Nicholas Hill, and Maria Kazachenko.

[Category: Nugget]]