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Location of Decimetric Pulsations in Solar Flares  

Arnold Benz   Submitted: 2011-03-28 08:01

This work investigates the spatial relation between coronal X-raysources and coherent radio emissions, both generally thought to besignatures of particle acceleration. Two limb events were selectedduring which the radio emission was well correlated in time with hardX-rays. The radio emissions were of the type of decimetric pulsationsas determined from the spectrogram observed by Phoenix-2 of ETHZurich. The radio positions were measured from observations with theNançay Radioheliograph between 236 and 432 MHz and compared to theposition of the coronal X-ray source imaged with RHESSI. The radiopulsations originated at least 30 - 240 Mm above the coronal hardX-ray source. The altitude of the radio emission increases generallywith lower frequency. The average positions at different frequenciesare on a line pointing approximately to the coronal hard X-ray source.Thus, the pulsations cannot be caused by electrons trapped in theflare loops, but are consistent with emission from a current sheetabove the coronal source.

Authors: Arnold O. Benz, Marina Battaglia and Nicole Vilmer
Projects: RHESSI

Publication Status: Solar Physics (in press)
Last Modified: 2011-03-28 08:23
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Are there Radio-quiet Solar Flares?  

Arnold Benz   Submitted: 2007-01-19 04:57

Some 15% of solar flares having a soft X-ray flux above GOES class C5 are reported to lack coherent radio emission in the 100 - 4000 MHz range (type I - V and decimetric emissions). A detailed study of 29 such events reveals that 22 (76%) of them occurred at a radial distance of more than 800'' from the disk center, indicating that radio waves from the limb may be completely absorbed in some flares. The remaining seven events have statistically significant trends to be weak in GOES class and to have a softer non-thermal X-ray spectrum. All of the non-limb flares that were radio-quiet > 100 MHz were accompanied by metric type III emission below 100 MHz. Out of 201 hard X-ray flares, there was no flare except near the limb (R>800'') without coherent radio emission in the entire meter and decimeter range. We suggest that flares above GOES class C5 generally emit coherent radio waves when observed radially above the source.

Authors: Arnold O. Benz, Roman Brajsa, and Jasmina Magdalenic
Projects: None

Publication Status: Solar Physics, in press
Last Modified: 2007-01-19 10:43
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Microflares and Hot Component in Solar Active Regions  

Arnold Benz   Submitted: 2002-09-30 01:56

Open-shutter RHESSI observations of 3-15 keV X-rays are found to exhibit active region transient brightenings and microflares at a rate of at least 10 per hour occurring even during the periods of lowest solar activity so far in the mission. A thermal component fitted by temperatures of 6-14 MK dominates from 3 keV to about 9 keV, but can be traced up to 14 keV in some cases, and has an average duration of 131+-103 seconds at 7-8 keV. The duration increases with decreasing photon energy. The peak count rate defined by cross-correlation is delayed at low energies. The temperature peaks early in the event and then decreases, whereas the emission measure increases throughout the event. The properties are consistent with thermal conduction dominating the evolution. In some of the bigger events, a second component was found in the 11-14 keV range extending down to 8 keV in some cases. The duration is typically 3 times shorter and ends near the peak time of the thermal component consistent with the Neupert effect of regular flares. Therefore the second component is suggested to be of non-thermal origin, presumably causing the beam-driven evaporation of the first component. The two components can be separated and analyzed in detail for the first time. Low-keV measurements allow a reliable estimate of the energy input by microflares necessary to assess their relevance for coronal heating. See also CD-ROM material at http://www.astro.phys.ethz.ch/papers/benz/RHESSImicroflares/BenzGrigis.htm

Authors: Benz,A.O. and Grigis, P.C.
Projects: RHESSI

Publication Status: Solar Phys. 210, 431 - 444 (2002)
Last Modified: 2005-11-17 01:38
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Location of narrowband spikes in solar flares  

Arnold Benz   Submitted: 2001-12-14 02:07

Narrowband spikes of the decimeter type have been identified in dynamic spectrograms of Phoenix-2 of ETH Zurich and located in position with the Nancay Radioheliograph at the same frequency. The spike positions have been compared with the location of hard X-ray emission and the thermal flare plasma in soft X-rays and EUV lines. The decimetric spikes are found to be single sources located some 20' to 400' away from the flare site in hard or soft X-rays. In most cases there is no bright footpoint nearby. In at least two cases the spikes are near loop tops. These observations do not confirm the widely held view that the spike emission is produced by some loss-cone instability masering near the footpoints of flare loops. On the other hand, the large distance to the flare sites and the fact that these spikes are all observed in the flare decay phase make the analyzed spike sources questionable sites for the main flare electron acceleration. They possibly indicate coronal post-flare acceleration sites.

Authors: A.O. Benz, P. Saint-Hilaire and N. Vilmer
Projects: Yohkoh-SXT

Publication Status: A+A, 383, 678-684 (2002)
Last Modified: 2005-11-17 01:39
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Bidirectional Type III Solar Radio Bursts  

Arnold Benz   Submitted: 1999-11-04 13:02

Bidirectional coronal type III bursts are modeled by combining a model of coronal electron heating and beam generation via time-of-flight effects with semiquantitative estimates of quasilinear relaxation. Electromagnetic emissivit- emissivities are estimated by extending the recently developed theory of interplanetary type III bursts to coronal emissions, including its features of stochastic Langmuir-wave growth and three-wave interactions. The results are investigated for heating on open and closed coronal field lines and are compared with observations of normal, reverse-slope, bidirectional, and inverted-J and -U coronal type III radio bursts. Harmonic emission is predicted to dominate at plasma frequencies above roughly 100 MHz where the efficiency of fundamental emission falls off steeply, while its free-free reabsorption rises. The model also explains the observed trends in the likelihood of occurrence of normal, reverse-slope, and bidirectional coronal type III bursts.

Authors: P. A. Robinson and A. O. Benz
Projects: None

Publication Status: Solar Physics, 194, 371 - 391 (2000)
Last Modified: 2005-11-17 01:40
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Heating Events Observed in the Quiet Corona  

Arnold Benz   Submitted: 1999-11-04 13:01

The emission measure of the quiet corona, defined by the plasma hotter than one million degrees, has been observed by EIT on SoHO to fluctuate in a alrge majority of the 1900km x 1900km pixels during the observing time of 42 minutes. In the average, the larger the emission measure in a pixel, the more it fluctuates. Increases in emission measure constitute a major energy input into the corona, suggesting that the lower corona is not just heated, but continuous- continuously replenished by chromospheric material heated to coronal temperatur- temperatures. We test the hypothesis that the coronal heat input may be largely by chromospheric evaporation. ESA Special Publication, in press

Authors: Arnold O. Benz and Säm Krucker
Projects: Soho-EIT

Publication Status: To appear in: Magnetic Fields and Solar Processes}, 9th European Meeting on Solar Physics, Florence, Italy, 12 - 18 September 19
Last Modified: 2005-11-17 01:40
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Are Heating Events in the Quiet Solar Corona Small Flares? - Multiwavelength Observations of Individual Events  

Arnold Benz   Submitted: 1999-11-04 13:00

Temporary enhancements of the coronal emission measure in a quiet region have been shown to constitute a significant energy input. Here some relatively large events are studied for simultaneous brightenings in transition region lines and in radio emission. Associated emissions are discussed and tested for characteri- characteristics known from full-sized impulsive flares in active regions. Heating events and flares are found to have many properties in common, including (i) associated polarized radio emission, which usually precedes the emission measure peak (Neupert effect) and sometimes has a non-thermal spectrum, and (ii) associated and often preceding peaks in O V and He I emission. On the other hand, heating events also differ from impulsive flares: (i) In half of the cases, their radio emission at centimeter waves shows a spectrum consistent with thermal radiation, (ii) the ratio of the gyro-synchrotron emission to the estimated thermal soft X-ray emission is smaller than in flares, and (iii) the associated emission in the O V transition region line shows red shifts and blue shifts, indicating upflows in the rise phase and downflows in the decay phase, respectively. Nevertheless, the differences seem to be mainly quantitative, and the analyzed heating events with thermal energies around 1026 erg may in principle be considered as microflares or large nanoflares, thus small versions of regular flares.

Authors: Säm Krucker and Arnold O. Benz
Projects: Soho-EIT

Publication Status: Solar Physics,191, 341 - 385 (2000)
Last Modified: 2005-11-17 01:41
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Abstracts by Author
Location of Decimetric Pulsations in Solar Flares
Are there Radio-quiet Solar Flares?
Microflares and Hot Component in Solar Active Regions
Location of narrowband spikes in solar flares
Bidirectional Type III Solar Radio Bursts
Heating Events Observed in the Quiet Corona
Are Heating Events in the Quiet Solar Corona Small Flares? -- Multiwavelength Observations of Individual Events

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