arXiv subjects

Arnold O. Benz

Publications and source records attributed to Arnold O. Benz.

44 records · Page 3Linked to original sources

arXiv2003

Micro-events in the active and quiet solar corona

The content of hot material in the corona is not constant. Soft X-ray and high-temperature EUV line observations show that new material, apparently heated and evaporated from the chromosphere, is frequently injected into the corona both in active and quiet regions. Active regions are found to exhibit transient brightenings, termed here microflares, due to such enhancements in emission measure. They appear at a rate of up to 10 per hour in RHESSI observations of 3--15 keV X-rays, occurring even during the periods of lowest solar activity so far in the mission. The RHESSI observations combined with measurements at other wavelengths yield estimates of the energy input into the corona. These observations suggest that the models for coronal heating must be complemented with respect to continuous replenishing the lower corona by chromospheric material heated to coronal temperatures. The observed micro-events are secondary phenomena and do not represent the primary energy release, nor its total amount. Nevertheless, they are an interesting source of information on the heating process(es) of the corona. The micro-events are compared to events in quiet regions, termed here nanoflares, which seem to be a different population, well separated in temperature and emission measure from microflares.

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arXiv2001

Location of Narrowband Spikes in Solar Flares

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.

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arXiv2001

Energy Distribution of Micro-events in the Quiet Solar Corona

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.

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arXiv2000

Heating the Quiet Corona by Nanoflares: Evidence and Problems

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.

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arXiv2000

High-sensitivity observations of solar flare decimeter radiation

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.

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arXiv2000

Electron Firehose instability and acceleration of electrons in solar flares

An electron distribution with a temperature anisotropy T_par/T_perp > 1 can lead to the Electron Firehose instability (Here par and perp denote directions relative to the background magnetic field B_0). Since possible particle acceleration mechanisms in solar flares exhibit a preference of energizing particles in parallel direction, such an anisotropy is expected during the impulsive phase of a flare. The properties of the excited waves and the thresholds for instability are investigated by using linearized kinetic theory. These thresholds were connected to the pre-flare plasma parameters by assuming an acceleration model acting exclusively in parallel direction. For usually assumed pre-flare plasma conditions the electrons become unstable during the acceleration process and lefthand circularly polarized waves with frequencies of about the proton gyrofrequency are excited at parallel propagation. Indications have been found, that the largest growth rates occur at oblique propagation and the according frequencies lie well above the proton gyrofrequency.

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arXiv1999

The Minimum Bandwidth of Narrowband Spikes in Solar Flare Decimetric Radio Waves

The minimum and the mean bandwidth of individual narrowband spikes in two events in decimetric radio waves is determined by means of multi-resolution analysis. Spikes of a few tens of millisecond duration occur at decimetric/microwave wavelength in the particle acceleration phase of solar flares. A first method determines the dominant spike bandwidth scale based on their scalegram, the mean squared wavelet coefficient at each frequency scale. This allows to measure the scale bandwidth independently of heuristic spike selection criteria, e.g. manual selection. The major drawback is a low resolution in the bandwidth. To overcome this uncertainty, a feature detection algorithm and a criterion for spike shape in the time-frequency plane is applied to locate the spikes. In that case, the bandwidth is measured by fitting an assumed spike profile into the denoised data. The smallest FWHM bandwidth of spikes was found at 0.17 % and 0.41 % of the center frequency in the two events. Knowing the shortest relevant bandwidth of spikes, the slope of the Fourier power spectrum of this two events was determined and no resemblance to a Kolmogorov spectrum detected. Additionally the correlation between spike peak flux and bandwidth was examined.

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arXiv1999

Are Heating Events in the Quiet Solar Corona Small Flares? - Multiwavelength Observations of Individual Events

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 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 10^26 erg may in principle be considered as microflares or large nanoflares, thus small versions of regular flares.

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