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Observations of a Radio-quiet Solar Preflare  

Arnold O. Benz   Submitted: 2017-09-25 01:20

The preflare phase of the flare SOL2011-08-09T03:52 is unique in its long duration, its coverage by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Nobeyama Radioheliograph, and the presence of three well-developed soft X-ray (SXR) peaks. No hard X-rays (HXR) are observed in the preflare phase. Here we report that also no associated radio emission at 17 GHz was found despite the higher sensitivity of the radio instrument. The ratio between the SXR peaks and the upper limit of the radio peaks is larger by more than one order of magnitude compared to regular flares. The result suggests that the ratio between acceleration and heating in the preflare phase was different than in regular flares. Acceleration to relativistic energies, if any, occurred with lower efficiency.

Authors: Arnold O. Benz, Marina Battaglia, Manuel G?del
Projects: None

Publication Status: Accepted by Solar Physics
Last Modified: 2017-09-25 10:57
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Physical Processes in Magnetically Driven Flares on the Sun, Stars, and Young Stellar Objects  

Arnold O. Benz   Submitted: 2010-09-10 09:12

The first flare on the Sun was observed exactly 150 years ago. During most of the long history, only secondary effects have been noticed, so flares remained a riddle. Now the primary flare products, high-energy electrons and ions, can be spatially resolved in hard X-rays (HXRs) and gamma rays on the Sun. Soft X-rays (SXRs) are observed from most stars, including young stellar objects. Structure and bulk motions of the corona are imaged on the Sun in high temperature lines and are inferred from line shifts in stellar coronae. Magnetic reconnection is the trigger for reorganization of the magnetic field into a lower energy configuration. A large fraction of the energy is converted into nonthermal particles that transport the energy to higher density gas, heating it to SXR-emitting temperatures. Flares on young stars are several orders of magnitude more luminous and more frequent; they significantly ionize protoplanetary disks and planetary ionospheres.

Authors: Arnold O. Benz and Manuel G?del
Projects: None

Publication Status: Published in Annual Reviews of Astronomy and Astrophysics
Last Modified: 2010-09-10 13:12
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High Spectral Resolution Observation of Decimetric Radio Spikes Emitted by Solar Flares - First Results of the Phoenix-3 Spectrometer  

Arnold O. Benz   Submitted: 2009-09-23 01:27

A new multichannel spectrometer, Phoenix-3, is in operation having capabilities to observe solar flare radio emissions in the 0.1 - 5 GHz range at an unprecedented spectral resolution of 61.0 kHz with high sensitivity. The present setup for routine observations allows measuring circular polarization, but requires a data compression to 4096 frequency channels in the 1 - 5 GHz range and to a temporal resolution of 200 ms. First results are presented by means of a well observed event that included narrowband spikes at 350 - 850 MHz. Spike bandwidths are found to have a power-law distribution, dropping off below a value of 2 MHz for full width at half maximum (FWHM). The narrowest spikes have a FWHM bandwidth less than 0.3 MHz or 0.04% of the central frequency. The smallest half-power increase occurs within 0.104 MHz at 443.5 MHz, which is close to the predicted natural width of maser emission. The spectrum of spikes is found to be asymmetric, having an enhanced low-frequency tail. The distribution of the total spike flux is approximately an exponential.

Authors: Benz, A.O., Monstein, C., Beverland, M., Meyer, H., Stuber, B.
Projects: None

Publication Status: Solar Physics, in press
Last Modified: 2009-09-23 06:36
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Subject will be restored when possible  

Arnold O. Benz   Submitted: 2008-02-26 11:45

Solar flares are observed at all wavelengths from decameter radio waves to gamma-rays at 100 MeV. This review focuses on recent observations in EUV, soft and hard X-rays, white light and radio waves. Space missions such as RHESSI, Yohkoh, TRACE, and SOHO have enlarged widely the observational base. They have revealed a number of surprises: Coronal sources appear before the hard X-ray emission in chromospheric footpoints, major flare acceleration sites appear to be independent of coronal mass ejections (CMEs), electrons and ions may be accelerated at different sites, and basic characteristics vary from small to large flares. Recent progress also includes improved insights into the flare energy partition, on the location(s) of energy release, tests of energy release scenarios and particle acceleration. The interplay of observations with theory is important to deduce the geometry and to disentangle the various processes involved. There is increasing evidence supporting reconnection of magnetic field lines as the basic cause. While this process has become generally accepted as the trigger, it does not explain the huge energy involved, nor the impulsive acceleration of charged particles. Flare-like processes may be responsible for large-scale restructuring of the magnetic field in the corona as well as for its heating. Large flares influence interplanetary space and substantially affect the Earth's lower ionosphere. While flare scenarios have slowly converged over the past decades, every new observation still reveals major unexpected results, demonstrating that solar flares, after 150 years since their discovery, remain an unexplained problem of astrophysics.

Authors: Arnold O. Benz
Projects: None

Publication Status: published in Living Reviews in Solar Physics (2008)
Last Modified: 2008-09-23 21:22
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Subject will be restored when possible  

Arnold O. Benz   Submitted: 2008-02-14 03:55

Radio spectrometers of the CALLISTO type to observe solar flares have been distributed to 9 locations around the globe. The instruments observe automatically. Their data is collected every day via internet and stored in a central data base. A public web-interface exists through which data can be browsed and retrieved. The 9 instruments form a network called e-CALLISTO. It is still growing in the number of stations, as redundancy is desirable for full 24 hour coverage of the solar radio emission in the meter and low decimeter band. The e-CALLISTO system has already proven to be a valuable new tool for monitoring solar activity and for space weather research.

Authors: A. O. Benz , C. Monstein, H. Meyer, P. K. Manoharan, R. Ramesh, A. Altyntsev, A. Lara, J. Paez, and K.-S. Cho
Projects: None

Publication Status: in press at Earth, Moon, and Planets (2008)
Last Modified: 2008-02-14 09:53
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Radio Emission of Solar Flare Particle Acceleration  

Arnold O. Benz   Submitted: 2005-11-21 03:44

The solar corona is a very dynamic plasma on time scales of decades to a few milliseconds. Radio missions provide diagnostic tools particularly suited for the analysis of non-thermal electron distributions, enhanced levels of various kinds of plasma waves and plasma phenomena related to electron acceleration in flares. Very intense coherent emissions are observed at frequencies below about 3 GHz, weaker ones up to 9 GHz. They are caused by plasma instabilities driving various wave modes that in turn may emit observable radio waves. The focus here is on type III and stationary type IV bursts from about 0.2 to 4 GHz. Type III bursts can be traced back in the corona to the acceleration region of electron beams. Less known are radio emissions from magnetically trapped electrons driving loss-cone unstable waves. This is the interpretation usually given to type IV emission. It is a very powerful radiation probably also observed in stars and possibly related to acceleration after the main flare energy release phase. The comparison of the radio emissions with hard X-rays reveals surprisingly that the two emissions often do not correlate in time and thus must originate from different electron acceleration processes. In combination with other wavelengths and their recent imaging capabilities, exciting new possibilities may soon open for radio diagnostics.

Authors: Arnold O. Benz
Projects: RHESSI

Publication Status: Sixth International Workshop on Planetary and Solar Radio Emissions Graz, April 20 - 22, 2005, (eds. H.O. Rucker, W.S. Kurth, G. Mann), in press (2005)
Last Modified: 2005-11-21 03:44
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CALLISTO - A New Concept for Solar Radio Spectrometers  

Arnold O. Benz   Submitted: 2004-10-19 06:44

A new radio spectrometer, CALLISTO, is presented. It is a dual-channel frequency-agile receiver based on commercially available consumer electronics. Its major characteristic is the low price for hardware and software, and the short assembly time, both two or more orders of magnitude below existing spectrometers. The instrument is sensitive at the physical limit and extremely stable. The total bandwidth is 825 MHz, and the width of individual channels is 300 kHz. A total of 1000 measurements can be made per second. The spectrometer is well suited for solar low-frequency radio observations pertinent to space weather research. Five instruments of the type were constructed until now and put into operation at several sites, including Bleien (Zurich) and NRAO (USA). First results in the 45 - 870 MHz range are presented. Some of them were recorded in a preliminary setup during the time of high solar activity in October and November 2003.

Authors: Benz, A.O., Monstein, C., and Meyer, H.
Projects:

Publication Status: Solar Physics, 226, 143 - 151 (2005)
Last Modified: 2005-11-21 03:23
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Survey on solar X-ray flares and associated coherent radio emissions  

Arnold O. Benz   Submitted: 2004-10-19 06:40

The radio emission during 201 X-ray selected solar flares was surveyed from 100 MHz to 4 GHz with the Phoenix-2 spectrometer of ETH Zurich. The selection includes all RHESSI flares larger than C5.0 jointly observed from launch until June 30, 2003. Detailed association rates of radio emission during X-ray flares are reported. In the decimeter wavelength range, type III bursts and the genuinely decimetric emissions (pulsations, continua, and narrowband spikes) were found equally frequently. Both occur predominantly in the peak phase of hard X-ray (HXR) emission, but are less in tune with HXRs than the high-frequency continuum exceeding 4 GHz, attributed to gyrosynchrotron radiation. In 10% of the HXR flares, an intense radiation of the above genuine decimetric types followed in the decay phase or later. Classic meter-wave type III bursts are associated in 33% of all HXR flares, but only in 4% they are the exclusive radio emission. Noise storms were the only radio emission in 5% of the HXR flares, some of them with extended duration. Despite the spatial association (same active region), the noise storm variations are found to be only loosely correlated in time with the X-ray flux. In a surprising 17% of the HXR flares, no coherent radio emission was found in the extremely broad band surveyed. The association but loose correlation between HXR and coherent radio emission is interpreted by multiple reconnection sites connected by common field lines.

Authors: Benz, A.O., Grigis, P.C., Csillaghy, A. and Saint-Hilaire, P.
Projects: RHESSI

Publication Status: Solar Physics,226, 121 - 142 (2005)
Last Modified: 2005-11-21 03:24
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SOLAR FLARE ELECTRON ACCELERATION: COMPARING THEORIES AND OBSERVATIONS  

Arnold O. Benz   Submitted: 2003-02-25 01:12

A popular scenario for electron acceleration in solar flares is transit-time damping of low-frequency MHD waves excited by reconnection and its outflows. The scenario requires several processes in sequence to yield energetic electrons of the observed large number. Until now there was very little evidence for this scenario, as it is even not clear where the flare energy is released. RHESSI measurements of bremsstrahlung by non-thermal flare electrons yield energy estimates as well as the position where the energy is deposited. Thus quantitative measurements can be put into the frame of the global magnetic field configuration as seen in coronal EUV line observations. We present RHESSI observations combined with TRACE data that suggest primary energy inputs mostly into electron acceleration and to a minor fraction into coronal heating and primary motion. The more sensitive and lower energy X-ray observations by RHESSI have found also small events (C class) at the time of the acceleration of electron beams exciting meter wave Type III bursts. However, not all RHESSI flares involve Type III radio emissions. The association of other decimeter radio emissions, such as narrowband spikes and pulsations, with X-rays is summarized in view of electron acceleration.

Authors: Benz, A. O. and Saint-Hilaire, P.
Projects: RHESSI

Publication Status: Adv. Space Res. 32, 2415 - 2423 (2003)
Last Modified: 2005-11-21 03:27
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Energy Budget and Imaging Spectroscopy of a Compact Flare  

Arnold O. Benz   Submitted: 2002-09-30 02:04

We present the analysis of a compact flare that occurred on 2002/02/26 at 10:26 UT, seen by both RHESSI and TRACE. The size of the nearly circular hard X-ray source is determined to be 4.7(+-1.5)' from the modulation profiles of the RHESSI collimators. The power-law distribution of non-thermal photons is observed to extend down to 10 keV without flattening, and to soften with increasing distance from the flare kernel. The former indicates that the energy of the precipitating flare electron population is larger than previously estimated: it amounts to 2.6(+-0.8) 1030 erg above 10 keV, assuming thick-target emission. The thermal energy content of the soft X-ray source (isothermal temperature of 20.8(+-0.9) MK) and its radiated power were derived from the thermal emission at low energies. TRACE has observed a low-temperature ejection in the form of a constricted bubble, which is interpreted as a reconnection jet. Its initial energy of motion is estimated. Using data from both satellites, an energy budget for this flare is derived. The kinetic energy of the jet bulk motion and the thermal and radiated energies of the flare kernel were more than an order of magnitude smaller than the derived electron beam energy. A movie is available on the CD-ROM accompanying this volume http://www.astro.phys.ethz.ch/papers/benz/RHESSIebudget/RHESSIebudget.htm

Authors: Saint-Hilaire, P. and Benz, A.O.
Projects: RHESSI

Publication Status: Solar Phys., 210, 287 - 306 (2002)
Last Modified: 2005-11-21 03:31
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Energy Distribution of Micro-events in the Quiet Solar Corona  

Arnold O. Benz   Submitted: 2001-09-03 07:54

Recent imaging observations of EUV line emissions have shown evidence for frequent flare-like events in a majority of the pixels in quiet regions of the solar corona. The changes in coronal emission measure indicate impulsive heating of new material to coronal temperatures. These heating or evaporation events are candidate signatures of 'nanoflares' or 'microflares' proposed to interpret the high temperature and the very existence of the corona. The energy distribution of these micro-events reported in the literature differ widely, and so do the estimates of their total energy input into the corona. Here we analyze the assumptions of the different methods, compare them by using the same data set and discuss their results. We also estimate the different forms of energy input and output, keeping in mind that the observed brightenings are most likely secondary phenomena. A rough estimate of the energy input observed by EIT on the SoHO satellite is of the order of 10% of the total radiative output in the same region. It is considerably smaller for the two reported TRACE observations. The discrepancy can be explained partially by different thresholds for flare detection. There is agreement on the slope and the absolute value of the distribution if the same method were used and a numerical error corrected. The extrapolation of the power law to unobserved energies that are many orders of magnitude smaller remains questionable. Nevertheless, these micro-events and unresolved smaller events are currently the best source of information on the heating process of the corona.

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

Publication Status: ApJ 568, 413-421 (2002)
Last Modified: 2005-11-21 03:31
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The Source Regions of Impulsive Solar Electron Events  

Arnold O. Benz   Submitted: 2001-06-28 01:35

Low-energy (2 - 19 keV) impulsive electron events observed in interplanetary space have been traced back to the Sun, using their interplanetary type III radiation and metric/decimetric radio-spectrograms. For the first time we are able to study the highest frequencies and thus the radio signatures closest to the source region. All the selected impulsive solar electron events have been found to be associated with an interplanetary type III burst. This allows to time the particle events at the 2 MHz plasma level and identify the associated coronal radio emissions. Except for 5 out of 27 cases, the electron events were found to be associated with a coronal type III burst in the metric wavelength range. The start frequency yields a lower limit to the density in the acceleration region. We also search for narrowband spikes at the start of the type III bursts. In about half of the observed cases we find metric spikes or enhancements of type I bursts associated with the start of the electron event. If interpreted as the plasma emission of the acceleration process, the observed average frequency of spikes suggests a source density of the order of 3 108 cm-3 consistent with the energy cut-off observed.

Authors: Arnold O. Benz, Robert P. Lin, Olga A. Sheiner, Säm Krucker and Joe Fainberg
Projects: None

Publication Status: Solar Physics 203, 131 -144 (2001)
Last Modified: 2005-11-21 03:32
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On the reliability of peak-flux distributions, with an application to solar flares  

Arnold O. Benz   Submitted: 2001-06-28 01:30

Narrow-band radio spikes have been recorded during a solar flare with unprecedented resolution. This unique example allows us to study the effect of low resolution in previously published peak-flux distributions of radio spikes. We give a general, analytical expression for how an actual peak-flux distribution is changed in shape if the peaks are determined with low temporal and/or frequency resolution. It turns out that, generally, low resolution tends to cause an exponential behavior at large flux values if the actual distribution is of a power-law shape. The distribution may be severely altered if the burst-duration depends on the peak-flux. The derived expression is applicable also to peak-flux distributions derived at other wavelengths (e.g. soft and hard X-rays, EUV). We show that for the analyzed spike-event the resolution was sufficient for a reliable peak flux distribution. It can be fitted by generalized power-laws or by an exponential. Keywords: Acceleration of particles - Methods: statistical - Sun: flares - Sun: corona - Sun: radio radiation

Authors: H. Isliker and A.O. Benz
Projects: None

Publication Status: A&A 375, 1040 - 1048(2001)
Last Modified: 2005-11-21 03:33
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High-sensitivity observations of solar flare decimeter radiation  

Arnold O. Benz   Submitted: 2000-12-05 01:40

A new acousto-optic radio spectrometer has observed the 1 - 2 GHz radio emission of solar flares with unprecedented sensitivity. The number of detected decimeter type III bursts is greatly enhanced compared to observations by conventional spectrometers observing only one frequency at the time. The observations indicate a large number of electron beams propagating in dense plasmas. For the first time, we report weak, reversed drifting type III bursts at frequencies above simultaneous narrowband decimeter spikes. The type III bursts are reliable signatures of electron beams propagating downward in the corona, apparently away from the source of the spikes. The observations contradict the most popular spike model that places the spike sources at the footpoints of loops. Conspicuous also was an apparent bidirectional type U burst forming a fish-like pattern. It occurs simultaneously with an intense U-burst at 600-370 MHz observed in Tremsdorf. We suggest that it intermodulated with strong terrestrial interference (cellular phones) causing a spurious symmetric pattern in the spectrogram at 1.4 GHz. Symmetric features in the 1 - 2 GHz range, some already reported in the literature, therefore must be considered with utmost caution.

Authors: Arnold O. Benz, Peter Messmer and Christian Monstein
Projects: None

Publication Status: Astronomy and Astrophysics, 366, 326 - 330 (2001)
Last Modified: 2005-11-21 03:34
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Heating the Quiet Corona by Nanoflares: Evidence and Problems  

Arnold O. Benz   Submitted: 2000-12-05 01:32

The content of coronal material in the quiet Sun is not constant as soft X-ray and high-temperature EUV line observations have shown. New material, probably heated and evaporated from the chromosphere is occasionally injected even in the faintest parts above the magnetic network cell interiors. Assuming that the smaller events follow the pattern of the well observed larger ones, we estimate the total energy input. Various recent analyses are compared and discussed. The results using similar EUV data from EIT/SOHO and TRACE basically agree on the power-law exponent when the same method is used. The most serious deviations are in the number of nanoflares per energy unit and time unit. It can be explained only partially by different thresholds for flare detection.

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

Publication Status: APS Conference Series, IAU Symposium 203, 471 - 474 (2001)
Last Modified: 2005-11-21 03:36
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Abstracts by Author
Observations of a Radio-quiet Solar Preflare
Physical Processes in Magnetically Driven Flares on the Sun, Stars, and Young Stellar Objects
High Spectral Resolution Observation of Decimetric Radio Spikes Emitted by Solar Flares - First Results of the Phoenix-3 Spectrometer
Subject will be restored when possible
Subject will be restored when possible
Radio Emission of Solar Flare Particle Acceleration
CALLISTO - A New Concept for Solar Radio Spectrometers
Survey on solar X-ray flares and associated coherent radio emissions
SOLAR FLARE ELECTRON ACCELERATION: COMPARING THEORIES AND OBSERVATIONS
Energy Budget and Imaging Spectroscopy of a Compact Flare
Energy Distribution of Micro-events in the Quiet Solar Corona
The Source Regions of Impulsive Solar Electron Events
On the reliability of peak-flux distributions, with an application to solar flares
High-sensitivity observations of solar flare decimeter radiation
Heating the Quiet Corona by Nanoflares: Evidence and Problems

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