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Particle Densities within the Acceleration Region of a Solar Flare  

Säm Krucker   Submitted: 2013-10-31 10:35

The limb flare SOL2012-07-19T05:58 (M7.7) provides the best example of a non-thermal above-the-loop-top hard X-ray (HXR) source with simultaneous observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamic Observatory (SDO). By combining the two sets of observations, we present the first direct measurement of the thermal proton density and non-thermal electron density within the above-the-loop-top source where particle acceleration occurs. We find that both densities are of the same order of magnitude of a few times 1e9 cm-3, about 30 times lower than the density in the underlying thermal flare loops. The equal densities indicate that the entire electron population within the above-the-loop-top source is energized. While the derived densities depend on the unknown source depth and filling factor, the ratio of these two densities does not. Within the uncertainties, the ratio is one for a low energy cut-off of the non-thermal electron spectrum between 10 and 15 keV. RHESSI observations only constrain the cut-off energy to below ~15 keV, leaving the spectral shape of the electrons within the above-the-loop-top source at lower energies unknown. Nevertheless, these robust results strongly corroborate earlier findings that the above-the-loop-top source is the acceleration region where a bulk energization process acts on all electrons.

Authors: Sam Krucker, Marina Battaglia
Projects: RHESSI,SDO-AIA

Publication Status: in press
Last Modified: 2013-11-01 09:06
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Solar flares at submillimeter wavelengths  

Säm Krucker   Submitted: 2013-03-26 19:01

We discuss the implications of the first systematic observations of solar flares at submillimeter wavelengths, defined here as observing wavelengths shorter than 3~mm (frequencies higher than 0.1 THz). The events observed thus far show that this wave band requires a new understanding of high-energy processes in solar flares. Several events, including observations from two different observatories, show during the impulsive phase of the flare a spectral component with a positive (increasing) slope at the highest observable frequencies (up to 405 GHz). To emphasize the increasing spectra and the possibility that these events could be even more prominent in the THz range, we term this spectral feature a ``THz component''. Here we review the data and methods, and critically assess the observational evidence for such distinct component(s). This evidence is convincing. We also review the several proposed explanations for these feature(s), which have been reported in three distinct flare phases. These data contain important clues to flare development and particle acceleration as a whole, but many of the theoretical issues remain open. We generally have lacked systematic observations in the millimeter-wave to far-infrared range that are needed to complete our picture of these events, and encourage observations with new facilities.

Authors: Krucker et al.
Projects: None

Publication Status: published; The Astronomy and Astrophysics Review, Volume 21, Issue 1
Last Modified: 2013-03-27 12:16
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Measurements of the Coronal Acceleration Region of a Solar Flare  

Säm Krucker   Submitted: 2010-04-30 12:53

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Nobeyama Radioheliograph (NoRH) are used to investigate coronal hard X-ray and microwave emissions in the partially disk-occulted solar flare of December 31, 2007. The STEREO mission provides EUV images of the flare site at different viewing angles, establishing a two-ribbon flare geometry and occultation heights of the RHESSI and NoRH observations of ~16 Mm and ~25 Mm, respectively. Despite the occultation, intense hard X-ray emission up to ~80 keV occurs during the impulsive phase from a coronal source that is also seen in microwaves. The hard X-ray and microwave source during the impulsive phase is located ~6 Mm above thermal flare loops seen later at the soft X-ray peak time, similar in location to the above-the-loop-top source in the Masuda flare. A single non-thermal electron population with a power-law distribution (with spectral index of ~3.7 from ~16 keV up to the MeV range) radiating in both bremsstrahlung and gyrosynchrotron emission can explain the observed hard X-ray and microwave spectrum, respectively. This clearly establishes the non-thermal nature of the above-the-loop-top source. The large hard X-ray intensity requires a very large number (>5e35 above 16 keV) of suprathermal electrons to be present in this above-the-loop-top source. This is of the same order of magnitude as the number of ambient thermal electrons. We show that collisional losses of these accelerated electrons would heat all ambient electrons to superhot temperatures (ten's of keV) within seconds. Hence the standard scenario, with hard X-rays produced by a beam comprising the tail of a dominant thermal core plasma, does not work. Instead, all electrons in the above-the-loop-top source seem to be accelerated, suggesting that the above-the-loop-top source is itself the electron acceleration region.

Authors: Krucker, Hudson, Glesener, White, Masuda, Wuelser, Lin
Projects: RHESSI,STEREO

Publication Status: The Astrophysical Journal, Volume 714, Issue 2, pp. 1108-1119 (2010)
Last Modified: 2010-05-02 13:51
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Subject will be restored when possible  

Säm Krucker   Submitted: 2008-02-04 15:56

Observations of solar flares partially occulted by the solar limb provide diagnostics of coronal hard X-ray (HXR) emissions in the absence of generally much brighter emissions from footpoints of flare loops. In this paper, a statistical survey of 55 partially occulted flares observed by the Reuven RamatyHigh-Energy Solar Spectroscopic Imager (RHESSI) is presented, revealing the existence of two different components of coronal HXR emissions. Below15 keV thermal HXR emission with a gradual time profile is generally dominant, while at higher energies an additional component is seen in 50 out of 55 events. This additional component shows faster time variations in the order of tens of seconds and is most prominent during the rise of the thermal emission. Acomparison of the centroid positions of these two emissions shows that they are most often cospatial within ~2000 km, although for a few events clear separations are observed as well. The spectra of the high-energy component show a rather steep (soft) power law with indices mostly between 4 and 6. Thin target emission in the corona from flare-accelerated electrons is discussed as a possible origin of the fast time variation component.

Authors: Säm Krucker & R.P. Lin
Projects: RHESSI

Publication Status: ApJ 673, 1181, 2008 (February 1)
Last Modified: 2008-02-05 09:59
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Säm Krucker   Submitted: 2007-10-31 16:44

This letter presents X-ray observations of a partially limb-occulted solar flare taken by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the X-ray telescope (XRT) onboard Hinode. Thermal emission originates from a simple loop at the western limb that rises slowly (~7 km s-1) until the flare peak time. Above 18 keV, faint non-thermal emission with a hard/flat spectrum (gamma ~ 4) and fast time variations (of the order of tens of seconds) is seen that comes from a loop slightly above (~2000 km) the thermal loop, if compared at the same time. However, the non-thermal loop agrees well in altitude with the thermal flare loop seen later, at the soft X-ray peak time. This is consistent with simple flare models where non-thermal electrons in a flare loop produce thin target hard X-ray emission in the corona as they travel to the loop footpoints. There they lose all their energy and heat chromospheric plasma that fills the loop earlier seen in non-thermal hard X-rays. This suggests that electron acceleration in solar flares occurs in the corona.

Authors: Säm Krucker, I.G. Hannah, R.P. Lin
Projects: Hinode,RHESSI

Publication Status: accepted
Last Modified: 2007-11-01 07:23
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Säm Krucker   Submitted: 2007-10-31 14:25

High resolution hard X-ray observations provided by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are used to study the spectral evolution of ~50-200 keV non-thermal electron bremsstrahlung emissions of five X-class flares observed during the January 2005 solar storm events. Four of these flares show progressive spectral hardening during at least some hard X-ray peaks, while only one event shows the otherwise more commonly observed soft-hard-soft behavior. Imaging observations reveal that ~50-100 keV non-thermal electron bremsstrahlung emissions originate from footpoints of flare loops at all times, also during time of progressive spectral hardening, indicating that the spectral hardening component is produced by precipitating electrons, and not by electrons trapped in the corona. The four flares with progressive spectral hardening are all related to Solar Energetic Particle (SEP) events, while the only X-class flare with soft-hard-soft behavior is not. This finding is consistent with earlier studies (Kiplinger 1995), suggesting that electron acceleration and transport in flares is somehow linked to the production of SEPs escaping into interplanetary space.

Authors: R. Saldanha, S. Krucker, R.P. Lin
Projects: RHESSI

Publication Status: accepted
Last Modified: 2007-11-01 07:33
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Säm Krucker   Submitted: 2007-10-31 14:09

One of the largest solar hard X-ray (HXR) flares and solar energetic particle (SEP) events recorded by the Mars Odyssey mission while orbiting Mars occurred on 2002 October 27 and is related to a very fast (∼2300 km s-1 ) coronal mass ejection (CME). 1 From the Earth, the flare site is 40.4 degree behind the solar limb and only emissions from the high corona at least 150 000 km radially above the main flare site can be seen. Nevertheless, the Earth-orbiting Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observed HXR emission up to 60 keV with a relatively flat, nonthermal spectrum (g between 3 and 3.5) that has an onset simultaneous with the main HXR emission observed above 60 keV by the Gamma-Ray Spectrometer (GRS) orbiting Mars. While GRS records several smaller enhancements after the main peak, the high coronal source observed by RHESSI shows a long exponential decay (135 s) with progressive spectral hardening. The emissions from the high corona originate from an extended source (∼150 000 km in diameter) that expands (390 km s-1) and moves upwards (750 km s-1) in the same direction as the CME. These observations reveal the existence of energetic electrons in the high corona in closed magnetic structures related to the CME that are accelerated at the same time as the main energy release in the flare. Although the number of energetic electrons in the high corona is only a small fraction of the total accelerated electrons, about 10% of all electrons in the high coronal source are nonthermal (110 keV).

Authors: Säm Krucker, S. M. White, and R. P. Lin
Projects: RHESSI

Publication Status: published
Last Modified: 2007-11-01 07:34
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Particle Densities within the Acceleration Region of a Solar Flare
Solar flares at submillimeter wavelengths
Measurements of the Coronal Acceleration Region of a Solar Flare
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