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Register a new userRapid pulsations in sub-THz solar bursts
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A new solar burst emission spectral component has been found showing sub-THz fluxes increasing with frequency, spectrally separated from the well known microwave component. Rapid pulsations are found present in all events observed at the two frequencies of the solar submillimeter-wave telescope (SST): 212 and 405 GHz. They were studied in greater detail for three solar bursts exhibiting the new THz spectral component. The pulse amplitudes are of about 5-8% of the mean flux throughout the bursts durations, being comparable for both frequencies. Pulsations range from one pulse every few seconds to 8-10 per second. The pulse repetition rates (R) are linearly proportional to the mean burst fluxes (S), following the simple relationship S = k R, suggesting that the pulsations might be the response to discrete flare particle accelerator injections quantized in energy. Although this result is consistent with qualitative trends previously found in the GHz range, the pulse amplitude relative to the mean fluxes at the sub-THz frequencies appear to be nearly ten times smaller than expected from the extrapolation of the trends found in the GHz range. However there are difficulties to reconcile the nearly simultaneous GHz and THz burst emission spectrally separated components, exhibiting rapid pulsations with considerably larger relative intensities in the GHz range.
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Impulsive 30 THz continuum bursts have been recently observed in solar flares, utilizing small telescopes with a unique and relatively simple optical setup concept. The most intense burst was observed together with a GOES X2 class event on October 27, 2014, also detected at two sub-THz frequencies, RHESSI X-rays and SDO/HMI and EUV. It exhibits strikingly good correlation in time and in space with white light flare emission. It is likely that this association may prove to be very common. All three 30 THz events recently observed exhibited intense fluxes in the range of 104 solar flux units, considerably larger than those measured for the same events at microwave and sub-mm wavelengths. The 30 THz burst emission might be part of the same spectral burst component found at sub-THz frequencies. The 30 THz solar bursts open a promising new window for the study of flares at their origin
Observations of solar flares at sub-THz frequencies (mm and sub-mm wavelengths) over the last two decades often show a spectral component rising with frequency. Unlike a typical gyrosynchrotron spectrum decreasing with frequency, or a weak thermal component from hot coronal plasma, the observations can demonstrate a high flux level (up to ~10^4 s.f.u. at 0.4 THz) and fast variability on sub-second time scales. Although, many models has been put forward to explain the puzzling observations, none of them have clear observational support. Here we propose a scenario to explain the intriguing sub-THz observations. We show that the model, based on free-free emission from the plasma of flare ribbons at temperatures 10^4-10^6 K, is consistent with all existing observations of frequency-rising sub-THz flare emission. The model provides a temperature diagnostic of the flaring chromosphere and suggests fast heating and cooling of the dense transition region plasma.
We studied a solar flare with pronounced quasi-periodic pulsations detected in the microwave, X-ray, and radio bands. We used the methods of correlation, Fourier, and wavelet analyses to examine the temporal fine structures and relationships between the time profiles in each wave band. We found that the time profiles of the microwaves, hard X-rays and type III radio bursts vary quasi-periodically with the common period of 40-50 s. The average amplitude of the variations is high, above 30% of the background flux level and reaching 80% after the flare maximum. We did not find the periodicity in either the thermal X-ray flux component or source size dynamics. Our findings indicate that the detected periodicity is likely to be associated with periodic dynamics in the injection of non-thermal electrons, that can be produced by periodic modulation of magnetic reconnection.
We study THz-emission from a plasma driven by an incommensurate-frequency two-colour laser field. A semi-classical transient electron current model is derived from a fully quantum-mechanical description of the emission process in terms of sub-cycle field-ionization followed by continuum-continuum electron transitions. For the experiment, a CEP-locked laser and a near-degenerate optical parametric amplifier are used to produce two-colour pulses that consist of the fundamental and its near-half frequency. By choosing two incommensurate frequencies, the frequency of the CEP-stable THz-emission can be continuously tuned into the mid-IR range. This measured frequency dependence of the THz-emission is found to be consistent with the semi-classical transient electron current model, similar to the Brunel mechanism of harmonic generation.
The term ultraviolet (UV) burst is introduced to describe small, intense, transient brightenings in ultraviolet images of solar active regions. We inventorize their properties and provide a definition based on image sequences in transition-region lines. Coronal signatures are rare, and most bursts are associated with small-scale, canceling opposite-polarity fields in the photosphere that occur in emerging flux regions, moving magnetic features in sunspot moats, and sunspot light bridges. We also compare UV bursts with similar transition-region phenomena found previously in solar ultraviolet spectrometry and with similar phenomena at optical wavelengths, in particular Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic reconnection events occurring in the low atmosphere, at photospheric and/or chromospheric heights. Their intense emission in lines with optically thin formation gives unique diagnostic opportunities for studying the physics of magnetic reconnection in the low solar atmosphere. This paper is a review report from an International Space Science Institute team that met in 2016-2017.
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