Prolonged nonthermal emission from solar flares and the Neupert effect

We analyze the observations of the hard (ACS SPI, > 150 keV) and soft (GOES, 1–8 Å) X-ray emissions and the microwave (15.5 GHz) emission in the solar flares on September 7, 2005 and December 6 and 13, 2006. The time profiles of the nonthermal emission from these flares had a complex structure, suggesting that active processes in the flare region continued for a long time (more than an hour). We have verified the linear relationship between the nonthermal flux and the time derivative of the soft X-ray flux (the Neupert effect) in the events under consideration. In the first two cases, the Neupert effect held at the time of the most intense nonthermal emission peak, but not at the decay phase of the soft X-ray emission, when the intensity of the nonthermal emission was much higher than the background values. At the same time, the hard X-ray emission was suppressed compared to the main peak, while the microwave emission remained approximately at the same level. In the December 13, 2006 event, the prolonged hard X-ray emission was difficult to observe due to the fast arrival of solar protons, but the Neupert effect did not hold for its main peak either. At comparable intensities of the microwave emission on December 6 and 13, the intensity of the hard X-ray emission on December 13 at the time of the main peak was suppressed approximately by an order of magnitude. These observational facts are indicative of several particle acceleration and interaction episodes under various physical conditions during one flare. When the Neupert effect did not hold, the interaction of electrons took place mainly in a low-density medium. An effective escape of accelerated particles into interplanetary space rather than their precipitation into dense layers of the solar atmosphere may take place precisely at this time.     

Solar flares registered by the IRIS spectrometer onboard the CORONAS-F satellite: Peculiarities of the X-ray emission

The X-ray spectrometer IRIS was designed to register the fluxes of quanta with energies ranging from 2 to 250 keV in various time-resolution modes: 0.01, 1.0, and 2.5 s in 4, 64, and 12 channels, respectively. Owing to the high instrument sensitivity, individual spikes of tens of milliseconds can be distinguished in the time structure of the X-ray flare emission. The time spectral analysis carried out for the X-ray emission of a number of registered flares points to the presence of a quasiperiodic structure with characteristic times of the same order. For the flares of December 19, 2001, and August 10, 2002, the process of energy release has been considered with a 1-s time resolution, and its periodic character has been revealed. For the flare of December 19, 2001, the energy spectra of the hard X-ray emission have been simulated by thermal and nonthermal models. It has been shown that the both models can describe this emission.     

Time dependent studies of the aurora—II. Spectroscopic morphology

The temporal morphology of auroral spectral emission features is investigated. For a given energy distribution of bombarding electrons but a time varying flux magnitude, the emission rates of various auroral radiations exhibit a nonlinear time response due to the variety of reactions that contribute to excitation. Absolute intensities and intensity ratios of various spectroscopic features, therefore, can vary solely with atmospheric interaction effects, indicating the importance of considering the time history of a precipitation event when attempting to infer the characteristics of auroral electrons from optical measurements.     

Energetics and morphology of powerful impulsive solar flares

We have studied the energetics of two impulsive solar flares of X-ray class X1.7 by assuming the electrons accelerated in several episodes of energy release to be the main source of plasma heating and reached conclusions about their morphology. The time profiles of the flare plasma temperature, emission measure, and their derivatives, and the intensity of nonthermal X-ray emission are compared; images of the X-ray sources and magnetograms of the flare region at key instants of time have been constructed. Based on a spectral analysis of the hard X-ray emission from RHESSI data and GOES observations of the soft X-ray emission, we have estimated the spatially integrated kinetic power of nonthermal electrons and the change in flare-plasma internal energy by taking into account the heat losses through thermal conduction and radiation and determined the parameters needed for thermal balance. We have established that the electrons accelerated at the beginning of the events with a relatively soft spectrum directly heat up the coronal part of the flare loops, with the increase in emission measure and hard X-ray emission from the chromosphere being negligible. The succeeding episodes of electron acceleration with a harder spectrum have virtually no effect on the temperature rise, but they lead to an increase in emission measure and hard X-ray emission from the footpoints of the flare loops.     

Fundamental and Harmonic Emission in Type III Solar Radio Bursts – I. Emission at a Single Location or Frequency

A model of type III solar radio bursts is developed that incorporates large-angle scattering and reabsorption of fundamental emission amid ambient density fluctuations in the corona and solar wind. Comparison with observations shows that this model accounts semiquantitatively for anomalous harmonic ratios, the exponential decay constant of bursts, burst rise times, and the directivity of fundamental emission. It is concluded that the long emission tail on interplanetary type III bursts is mostly fundamental emission, while much of the anomalous time delay of fundamental relative to harmonic emission from a given location must be ascribed to other causes.     

Green corona and solar sector structure

Analysis of the green line corona for the interval 1947–1970 suggests the existence of largescale organization of the emission. The green line emission at high northern latitudes (≈ 40°–60°) is correlated with the emission at high southern latitudes 6, 15 and 24 days later, while the low latitude green corona seems to be correlated on both sides of the equator with no time lag. These coronal features are recurrent with a 27-day period at all latitudes between ± 60 °, and we associate these large-scale structures with the solar magnetic sector structure. The high correlation between northern and southern high-latitude emission at 15 days time lag is explained as a signature of a two-sector structure, while four sectors are associated with the 6 and 24 day peaks.     

Waiting Time Distribution of Emissions in Complex Coronal Mass Ejections

The waiting time distribution of emissions in Coronal Mass Ejections (CMEs) with several emissions is examined. We define the waiting time as the time interval between the commencement of an emission and the commencement of the next emission considered as parts of a unique CME. The distribution seems to follow a power-law. Two classes of CMEs several emissions are considered: “close” and “separate” depending on angular distance between emissions.     

空间等离子体中相干射电辐射过程的粒子模拟研究

射电辐射机制,尤其是射电暴发现象的相干辐射机制,是天体物理中最复杂、争议最多的电磁辐射机制.由于受到多重物理因素相互牵连的复杂影响,相干射电辐射机制的理论研究存在很大的难度,长期以来在等离子体辐射和电子回旋脉泽辐射这两类相干辐射机制间争议一直不断.近年来,人们开始尝试将粒子数值模拟方法应用于相干射电辐射机制的研究,并已经取得了一些积极的进展.本文将着重介绍近年来的粒子模拟研究工作及其取得的主要进展,并对现存的一些问题和困难进行简要评述.     

A hard X-ray observation of a solar flare with 100 ms time resolution

A solar flare which occurred on 4 July 1974 was observed in hard X-rays with a balloon-borne detector. When analyzed with a time resolution of 100 ms, four 2 s long spikes are observed, which are correlated with decimetric emission. Spectral analysis shows that the hardest X-rays were produced during the decay phase of the burst, when the microwave emission reached its peak. It is argued that the fine time structure could either be a bounce time effect, or that it could be due to the electron acceleration mechanism.     

Semi-empirical models of the white-light flare on October 24, 1991

On October 24, 1991, a white-light flare was observed both from space and from the ground. A multi-waveband spectral analysis shows that the peak time of the continuum emission coincides well with that of a radio burst at 2840 MHz and with the hard X-ray emission. Three semi-empirical models, corresponding to the pre-flare condition and to the peak time of continuum emission both with and without non-thermal excitation and ionization of hydrogen by an electron beam, have been obtained. The results indicate that there is fast heating both in the chromosphere and the photosphere. Some evidence is given that this WLF is very likely a result of bombardment by an electron beam. By taking into account non-thermal effects, the chromospheric temperature of the semi-empirical model is significantly reduced.     

Stokes imaging of the AM Her system ST LMi

The Stokes imaging technique of Potter, Hakala & Cropper is applied to the polarized emission from the AM Her system ST LMi. For the first time, the cyclotron emission region on the surface of the white dwarf is mapped in terms of optical depth/density in an analytical and objective manner. The region is found to consist of a less dense region leading a higher density region in orbital phase. It is demonstrated that the emission region needs to have a multi-temperature structure in order to explain the spectral slope and the general morphology of the cyclotron humps observed in the IR during the bright phase in ST LMi.
Furthermore, it is shown that a secondary emission region, fed by the same magnetic field lines that feed the main emission region, could be responsible for the positive circular polarization in the IR, the position angle variation and the excess flux during the faint phase of the white light observations which cannot be accounted for with a single emission region.     

A Search for Tev Emission From X-ray Selected Agn with Milagro

A TeV emission component has been detected from several active galactic nuclei (AGN). The majority of TeV detections of AGN are for nearby (z < 0.13) X-ray selected BL Lacs in flaring states. The variability of these sources makes continuous monitoring necessary to detect flaring activity. The Milagro gamma-ray observatory is a wide-field (∼2 sr) instrument that operates continuously allowing daily observations of the Northern hemisphere. No significant detections of unidentified sources have been made to date. The observations are used to set limits on the time-integrated behavior of a set of X-ray selected BL Lacs. Costamante and Ghisellini [A&A 384 (2002) 56] present a selection of BL Lacs with X-ray and radio flux levels favorable for TeV emission and make predictions for the TeV flux levels. Twenty-seven objects selected from this sample are the subjects of a search for TeV emission on time scales from 8 days to 2.8 years in data taken between December 15, 2000, and September 8, 2003. None of the selected BL Lacs are detected on any of the time scales. Flux limits will be presented for each galaxy and time scale and comparisons will be made with the predicted emission. For the Milagro Collaboration.     

Determination of differential emission measure from X-ray solar spectra registered by RESIK aboard CORONAS-F

The differential emission measure (DEM) describes the temperature distribution of the emitting plasma. The DEM distribution allows one to study the physical conditions and the energy of flares in detail (including the mean temperature and the total emission measure). In this paper, we analyze the time changes of the DEM distributions for a selected flare, which has been observed with the RESIK instrument. To calculate the differential emission measure, we used the Withbroe-Sylwester (W-S) iterative algorithm corresponding to the maximum likelihood procedure. The required emission functions were calculated with the CHIANTI package. We calculated the DEM for four available estimates of the ionization equilibrium and coronal composition of plasma.     

Time delays in large and small loop thermal models for hard X-ray bursts

The time histories of the emission at 10, 30, and 100 keV averaged over the loop from small and large loop thermal models of hard X-ray emission are studied. The small (15000 km) loop cases show a characteristic delay in the peak of the 100 keV emission relative to the 30 keV emission of about 1.5 s which should be detectable. The large (47 000 km) loop cases show no delay, but in the case of a continuous energy input, the 30 keV emission has a peak at 9.5 s whereas the 100 keV emission rises monotonically. Again, this difference should be detectable to the extent that it is not washed out by a dominant beam or escaping tail component which is not considered in this paper. A large loop case where only classical and saturated heat conduction are allowed is considered. The 30 keV emission has a peak at 7.5 s whereas the 100 keV emission rises monotonically. The peak temperature reached is 8 × 10⁷ K and the probability of finding examples in the data uncontaminated by a dominant beam or escaping tail component should be considerably higher than in the cases with higher rates of energy input.     

Quiescent times in gamma-ray bursts – I. An observed correlation between the durations of subsequent emission episodes

Although more than 2000 astronomical gamma-ray bursts (GRBs) have been detected, the precise progenitor responsible for these events is unknown. The temporal phenomenology observed in GRBs can significantly constrain the different models. Here we analyse the time histories of a sample of bright, long GRBs, searching for the ones exhibiting relatively long (more than 5 per cent of the total burst duration) 'quiescent times', defined as the intervals between adjacent episodes of emission during which the gamma-ray count rate drops to the background level. We find a quantitative relation between the duration of an emission episode and the quiescent time elapsed since the previous episode. We suggest here that the mechanism responsible for the extraction and the dissipation of energy has to take place in a metastable configuration, such that the longer the accumulation period, the higher the stored energy available for the next emission episode.     

Time delays in solar bursts measured in the mm-cm range of wavelengths

Various solar bursts have been analysed with high sensitivity (0.03 sfu, rms) and high-time resolution (1 ms) at two frequencies in the millimeter wave range (22 GHz and 44 GHz), and with moderate time resolution (100 ms) by a patrol telescope at a frequency in the microwave range (7 GHz). It was found that, in most cases, burst maximum emission is not coincident in time at those frequencies. Preceding maximum emission can be either at the higher or at the lower frequency. Time delays ranged from about 3 s to near coincidence, defined within 10 ms. Some complex bursts presented all kinds of delays among different time structures, and sometimes nearly uncorrelated time structures.Large time delays favour the association of the dynamic effects to shock wave speeds. Directional particle acceleration in complex magnetic configuration could be considered to explain the variety of the dynamic effects. Fastest burst rise times observed, less than 50 ms at 44 GHz and at 22 GHz, might be associated to limiting formation times of emission sources combined with various absorption mechanisms at the source and surrounding plasma.In memoriam, 1942–1981.INPE operates Itapetinga Radio Observatory and CRAAM.     

On some observations of the thermal emission of iron flakes produced in a grinding wheel

A report is given of some laboratory experiments on the thermal emission of glowing iron flakes. Clear effects of polarization are found sometimes in flakes of small size, indicating polarization of a kind similar to that appearing in the thermal emission from narrow metallic filaments. Sudden flashes of light appear in the thermal emission from flakes produced in a grinding wheel. At the same time the flake splits into two parts. It is suggested that the flash is due to tribo-thermoluminescence. It seems possible that the infrared radiation of the solar corona may contain a faintly polarized component due to thermal emission from dust particles.Paper dedicated to Professor Hannes. Alfvén on the occasion of his 70th birthday, 30 May, 1978.     

Observations of non-LTE emission at 4-5 microns with the planetary Fourier spectrometer abord the Mars Express mission

We report on PFS-MEX (Planetary Fourier Spectrometer on board Mars Express) limb observations of the non-Local Thermodynamic Equilibrium emission by CO and CO₂ isotopic molecules. The CO emission is observed peaking at altitudes lower than the CO₂ emission peak. Two orbits have been considered, which explore latitudes from 75 to 15° N, located in local time at 11:30 and 06:40, and with Ls=138° and 168°, respectively. In general in the season considered (northern summer) the emission intensity increases going to lower latitudes. The peak emission height is also decreasing with decreasing latitude. The CO₂ isotopic molecules are emitting radiance out of proportion with respect to the normal isotopic abundance, which surely indicates a strong contribution from a large number of much weaker CO₂ bands, a result that will demand careful theoretical modeling. By comparison with Hitran data base we can identify, among the emitting bands, the second hot band for the 626 and 636 molecule, while for the 628 and 627 emission from the third hot bands are very possible. Other minor bands or lines are also observed in emission for the first time in Mars. In one of the two orbits considered, the orbit 1234 of MEX, we also observe at altitudes 80-85 km scattered radiation, with indication of CO₂ ice aerosols as scattering centers. At the same altitude the Pathfinder descending measurements show a temperature that allows CO₂ condensation. Pathfinder measurements were at 03:00 local time, while our observations are for orbit 1234 showing CO₂ ice signature at 11:30 local time. These non-LTE limb emissions, with their unprecedented spectral resolution in this portion of the near infrared and their sensitivity and geographical coverage, will represent in our opinion an excellent data set for testing current theoretical models of the martian upper atmosphere.     

致密射电源短时标变化的相对论效应

本文讨论相对论激波在喷流中传播时,由照明不均匀性所引起的同步加速辐射问题中的某些相对论效应.研究激波的结构和厚度以及不均匀性尺度对于辐射变化的时标和变幅的关系.结果表明,由于激波辐射区的厚度对于光学辐射和射电辐射的不同,可能引起射电变化相对于光学变化的时间迟延,从而对某些观测现象提供了一种解释.     

Emission stars in Cyg OB7. new flare stars. III

A continued search for emission objects in three new fields within the association Cyg OB7 yielded eight new emission stars in one of these fields. No emission stars were detected in the other two fields, apparently because we have come to the boundary of the T association on this side. Two UV Cet-type flare stars have been detected in this region for the first time.