RHESSI and quasi-periodic pulsations
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|1st Author:||Claire Foullon|
|Published:||16 August 2005|
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Astronomers often have to rely on images and spectra of unchangingstructures - changing, perhaps, but because of their huge sizes, ontime scales too slow for us to perceive readily.In solar physics, by contrast, we have the luxury of observing objects close enough that we can actually make movies. Thus time variations become an important tool, and we can study physical processes in their development.
A variation that is regular in time can be characterized by a period,and its power spectrum shows the equivalent of an emission line; a tall thin peak at a particular frequency.Less regular variations can show quasi-periodicities, and these mayhave interesting physical interpretations.Solar flares with"quasi-periodic pulsations" (QPP) in their emissionshave been known since the early 1970s. With RHESSI, we can now investigate the origin of QPPin X-rays with better spatial and temporal resolution than ever achieved before.
QPP have periods ranging from fractions of seconds to several minutes. In the 80s, the coarse images from HXIS/SMM indicated that long periods (12-29 min) were seen to originate at the foot of, or below, large-scale coronal loops. Similar phenomena exist in stellar flares, where no spatial resolution is available.
First observations of long periods at high spatial resolution
<img width="256px" src="images/hsi_20030206_013700_corrected_rate.png">
On February 5-6 2003, two sequences of QPP of large-amplitude and with periods around 8-12 min were recorded by RHESSI (see above). These new events were investigated by Foullon et al. (2005). Supported by complementary data at other wavelengths from space-based and ground-based telescopes, they give us a number of extra clues to understand what is happening. Each of the two sequences of QPP originates from the top of a small flaring loop. But the two sequences do not originate from the same region. Two principal active sources are present: one in the Northern hemisphere (first QPP), and one in the Southern hemisphere (second QPP).
Possible MHD resonator: transequatorial loop
The characteristic periods we observe could result from a resonance occurring in the magnetic forces acting on the coronal matter.We typically use [] (magnetohydrodynamics) theory, a standard and well-proven approximation, to study the magnetic forces in the corona.Several clues suggest the presence of a large-scale transequatorial loop connecting the two oscillating regions: * the long periods, explained in terms of magnetohydrodynamic (MHD) oscillation modes of a large-scale loop;* the similarity of periodicities found at opposite sites; * for each QPP, some aperiodic sympathetic activity observed in the oppositely located and quieter region;* evidence of the transequatorial loop in the EUV.
The sketch above shows how flare-induced long-period QPP can be explained as modulations of emission in a small flaring loop due to fast kink MHD waves in a magnetically linked and large-scale loop. The large-scale loop is a transequatorial coronal loop connecting the flaring regions in the Northern and Southern hemispheres. The kink oscillations are transverse to the direction of the dominant magnetic field component along the transequatorial loop axis.
These findings suggest that QPP can be interpreted as a periodic pumping of electrons in a compact flaring loop, modulated by oscillations in a magnetically linked and larger loop acting as a long-period MHD resonator. This study links together solar flare pulsations and a MHD kink oscillation that has been widely exploited in recent years through coronal seismology. Such studies, resolving QPP on the solar disk with RHESSI, combined with complementary data at other wavelengths, in particular high-resolution EUV images from instruments such as TRACE or the future EIS/Solar-B, have the potential to provide remote diagnostics of solar plasma and broaden our understanding of physical processes operating in solar and stellar flares.
Biographical note: Claire Foullon is a physics researcher at Britain's University of Warwick.