MEASUREMENTS OF THE CORONAL ACCELERATION REGION OF A SOLAR FLARE

Krucker, Sä,m,Hudson, H. S.,Glesener, L.,White, S. M.,Masuda, S.,Wuelser, J.-P.,Lin, R. P. IOP Publishing 2010 The Astrophysical journal Vol.714 No.2

<P>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 2007 December 31. 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 similar to 16 Mm and similar to 25 Mm, respectively. Despite the occultation, intense hard X-ray emission up to similar 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 similar to 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 similar to 3.7 from similar to 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 (>5 x 10(35) above 16 keV for the derived upper limit of the ambient density of similar to 8 x 10(9) cm(-3)) 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 (tens 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.</P>

<i>RHESSI</i>IMAGING SURVEY OF γ-RAY BREMSSTRAHLUNG EMISSION IN SOLAR FLARES

Ishikawa, S.,Krucker, Sä,m,Takahashi, T.,Lin, R. P. IOP Publishing 2011 The Astrophysical journal Vol.728 No.1

<P>We present a high-energy (>150 keV) imaging survey of all solar gamma-ray flares observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to study bremsstrahlung emission from relativistic electrons. Using RHESSI rear segment data, images in the energy range from 150 to 450 keV integrated over the total duration of the impulsive phase of the flare are derived. Out of the 29 gamma-ray peaks in 26 RHESSI flares, we successfully obtained images for 21 gamma-ray peaks in 20 flares. The remaining eight peaks have >150 keV fluences of less than a few hundred photons per cm(2) and counting statistics are too poor for detailed imaging. The flux ratio of the footpoint sources is found to be similar at 50 keV and above 150 keV, indicating that relativistic electrons are present in both footpoints of the flare loop. No correlation between the footpoint separation and the fluence ratio of the 2.2 MeV line and the >300 keV photons is found. This indicates that the relative efficiency of proton to electron acceleration does not depend on loop length, as could have been expected from stochastic acceleration models. As previously reported, the three flares with the best counting statistics show not only footpoint emission, but also a coronal gamma-ray bremsstrahlung source. For events with lower counting statistics, no coronal source could be identified. However, instrumental limitation could easily hide a coronal source for events with lower statistics, suggesting that coronal gamma-ray bremsstrahlung sources are nevertheless a general feature of gamma-ray flares.</P>

ON THE RELATION OF ABOVE-THE-LOOP AND FOOTPOINT HARD X-RAY SOURCES IN SOLAR FLARES

Ishikawa, S.,Krucker, Sä,m,Takahashi, T.,Lin, R. P. IOP Publishing 2011 The Astrophysical journal Vol.737 No.2

<P>We report on the most prominent example of an above-the-loop hard X-ray source in the extensive solar flare database of RHESSI. The limb flare of 2003 October 22 around 20 UT resembles the famous Masuda flare, except that only one of the footpoint sources is visible with the other one occulted. However, even for this very prominent event, the above-the-loop source is only visible during one of the four hard X-ray peaks, highlighting the rare occurrence of above-the-loop sources that are equally bright as footpoint sources. The relative timing between the above-the-loop and footpoint sources shows that the coronal source peaks about 10 s before the footpoint source and decays during the time the footpoint source is most prominent. Furthermore, the derived number of non-thermal electrons within the above-the-loop source is large enough to provide the needed number of precipitating electrons to account for the footpoint emission over the duration of the hard X-ray peak. Hence, these observations support the simple scenario where bulk energization is accelerating all electrons within the above-the-loop source and precipitating electrons are emptying out of the above-the-loop source to produce the footpoint emissions.</P>

HARD X-RAY OBSERVATIONS OF A JET AND ACCELERATED ELECTRONS IN THE CORONA

Glesener, Lindsay,Krucker, Sä,m,Lin, R. P. IOP Publishing 2012 The Astrophysical journal Vol.754 No.1

<P>We report the first hard X-ray observation of a solar jet on the limb with flare footpoints occulted, so that faint emission from accelerated electrons in the corona can be studied in detail. In this event on 2003 August 21, RHESSI observed a double coronal hard X-ray source in the pre-impulsive phase at both thermal and nonthermal energies. In the impulsive phase, the first of two hard X-ray bursts consists of a single thermal/nonthermal source coinciding with the lower of the two earlier sources, and the second burst shows an additional nonthermal, elongated source, spatially and temporally coincident with the coronal jet. Analysis of the jet hard X-ray source shows that collisional losses by accelerated electrons can deposit enough energy to generate the jet. The hard X-ray time profile above 20 keV matches that of the accompanying Type III and broadband gyrosynchrotron radio emission, indicating both accelerated electrons escaping outward along the jet path and electrons trapped in the flare loop. The double coronal hard X-ray source, the open field lines indicated by Type III bursts, and the presence of a small post-flare loop are consistent with significant electron acceleration in an interchange reconnection geometry.</P>

<i>SUZAKU</i>/WAM AND<i>RHESSI</i>OBSERVATIONS OF NON-THERMAL ELECTRONS IN SOLAR MICROFLARES

Ishikawa, Shin-nosuke,Krucker, Sä,m,Ohno, Masanori,Lin, Robert P. IOP Publishing 2013 The Astrophysical journal Vol.765 No.2

<P>We report on hard X-ray spectroscopy of solar microflares observed by the Wide-band All-sky Monitor (WAM), on board the Suzaku satellite, and by RHESSI. WAM transient data provide wide energy band (50 keV-5 MeV) spectra over a large field of view (similar to 2 pi sr) with a time resolution of 1 s. WAM is attractive as a hard X-ray solar flare monitor due to its large effective area (similar to 800 cm(2) at 100 keV, similar to 13 times larger than that of RHESSI). In particular, this makes it possible to search for high energy emission in microflares that is well below the RHESSI background. The WAM solar flare list contains six GOES B-class microflares that were simultaneously observed by RHESSI between the launch of Suzaku in 2005 July and 2010 March. At 100 keV, the detected WAM fluxes are more than similar to 20 times below the typical RHESSI instrumental background count rates. The RHESSI and WAM non-thermal spectra are in good agreement with a single power law with photon spectral indices between 3.3 and 4.5. In a second step, we also searched the RHESSI microflare list for events that should be detectable by WAM, assuming that the non-thermal power-law emission seen by RHESSI extends to >50 keV. From the 12 detectable events between 2005 July and 2007 February, 11 were indeed seen by WAM. This shows that microflares, similar to regular flares, can accelerate electrons to energies up to at least 100 keV.</P>

Observational Aspects of Particle Acceleration in Large Solar Flares

Raymond, John C.,Krucker, Sä,m,Lin, Robert P.,Petrosian, Vahé Springer-Verlag 2012 Space science reviews Vol.173 No.1

A STATISTICAL STUDY OF SOLAR ELECTRON EVENTS OVER ONE SOLAR CYCLE

Wang, Linghua,Lin, R. P.,Krucker, Sä,m,Mason, Glenn M. IOP Publishing 2012 The Astrophysical journal Vol.759 No.1

<P>We survey the statistical properties of 1191 solar electron events observed by the WIND 3DP instrument from <1 keV to greater than or similar to 300 keVfor a solar cycle (1995 through 2005). After taking into account times of high background, the corrected occurrence frequency of solar electron events versus peak flux exhibits a power-law distribution over three orders of magnitude with exponents between -1.0 and -1.6 for different years, comparable to the frequency distribution of solar proton events, microflares, and coronal mass ejections (CMEs), but significantly flatter than that of soft X-ray (SXR) flares. At 40 keV (2.8 keV), the integrated occurrence rate above similar to 0.29 (similar to 330) cm(-2) s(-1) sr(-1) keV(-1) near 1 AU is similar to 1000 year(-1) (similar to 600 year(-1)) at solar maximum and similar to 35 year(-1) (similar to 25 year(-1)) at solar minimum, about an order of magnitude larger than the observed occurrence rate. We find these events typically extend over similar to 45 degrees in longitude, implying the occurrence rate over the whole Sun is similar to 10(4) year(-1) near solar maximum. The observed solar electron events have a 98.75% association with type III radio bursts, suggesting all type III bursts may be associated with a solar electron event. They have a close (similar to 76%) association with the presence of low-energy (similar to 0.02-2 MeV nucleon(-1)), He-3-rich (He-3/He-4 >= 0.01) ion emissions measured by the ACE ULEIS instrument. For these electron events, only similar to 35% are associated with a reported GOES SXR flare, but similar to 60% appear to be associated with a CME, with similar to 50% of these CMEs being narrow. These electrons are often detected down to below 1 keV, indicating a source high in the corona.</P>

KAPPA DISTRIBUTION MODEL FOR HARD X-RAY CORONAL SOURCES OF SOLAR FLARES

Oka, M.,Ishikawa, S.,Saint-Hilaire, P.,Krucker, S.,Lin, R. P. IOP Publishing 2013 The Astrophysical journal Vol.764 No.1

<P>Solar flares produce hard X-ray emission, the photon spectrum of which is often represented by a combination of thermal and power-law distributions. However, the estimates of the number and total energy of non-thermal electrons are sensitive to the determination of the power-law cutoff energy. Here, we revisit an 'above-the-loop' coronal source observed by RHESSI on 2007 December 31 and show that a kappa distribution model can also be used to fit its spectrum. Because the kappa distribution has a Maxwellian-like core in addition to a high-energy power-law tail, the emission measure and temperature of the instantaneous electrons can be derived without assuming the cutoff energy. Moreover, the non-thermal fractions of electron number/energy densities can be uniquely estimated because they are functions of only the power-law index. With the kappa distribution model, we estimated that the total electron density of the coronal source region was similar to 2.4 x 10(10) cm(-3). We also estimated without assuming the source volume that a moderate fraction (similar to 20%) of electrons in the source region was non-thermal and carried similar to 52% of the total electron energy. The temperature was 28 MK, and the power-law index delta of the electron density distribution was -4.3. These results are compared to the conventional power-law models with and without a thermal core component.</P>

A diffusive description of the focused transport of solar energetic particles : Intensity- and anisotropy-time profiles as a powerful diagnostic tool for interplanetary particle transport conditions⋆

Artmann, S.,Schlickeiser, R.,Agueda, N.,Krucker, S.,Lin, R. P. EDP Sciences 2011 Astronomy and astrophysics Vol.535 No.-

ESTIMATES OF DENSITIES AND FILLING FACTORS FROM A COOLING TIME ANALYSIS OF SOLAR MICROFLARES OBSERVED WITH<i>RHESSI</i>

Baylor, R. N.,Cassak, P. A.,Christe, S.,Hannah, I. G.,Krucker, Sä,m,Mullan, D. J.,Shay, M. A.,Hudson, H. S.,Lin, R. P. IOP Publishing 2011 The Astrophysical journal Vol.736 No.1

<P>We usemore than 4500 microflares from the RHESSI microflare data set to estimate electron densities and volumetric filling factors of microflare loops using a cooling time analysis. We show that if the filling factor is assumed to be unity, the calculated conductive cooling times are much shorter than the observed flare decay times, which in turn are much shorter than the calculated radiative cooling times. This is likely unphysical, but the contradiction can be resolved by assuming that the radiative and conductive cooling times are comparable, which is valid when the flare loop temperature is a maximum and when external heating can be ignored. We find that resultant radiative and conductive cooling times are comparable to observed decay times, which has been used as an assumption in some previous studies. The inferred electron densities have a mean value of 10(11.6) cm(-3) and filling factors have a mean of 10(-3.7). The filling factors are lower and densities are higher than previous estimates for large flares, but are similar to those found for two microflares by Moore et al.</P>