Multiple wavelength observations of flaring active regions

We present observations of flaring active regions with the Very Large Array (V.L.A. at 6 cm and 20 cm wavelengths) and the Westerbork Synthesis Radio Telescope (W.S.R.T. at 6 cm wavelength). These are compared with photospheric magnetograms (Meudon) and with Hα and offband Hα photographs (Big Bear and Ottawa River Solar Observatories). The 6 cm radiation of these active regions marks the legs of dipolar loops which have their footpoints in lower-lying sunspots. The intense, million degree radiation at 6 cm lies above sunspot umbrae in coronal regions where the longitudinal magnetic field strength H_? = 600 Gauss and the height above the sunspot umbrae h = 3.5±0.5 × 10⁹ cm. Circularly polarized horseshoe structures at 6 cm ring the sunspot umbrae. The high degree of circular polarization (?_c = 95%) of the horseshoes is attributed to gyroresonant emission above sunspot^? penumbrae. The 20 cm radiation of these active regions exhibits looplike coronal structures which extend across regions of opposite magnetic polarity in the underlying photosphere. The 20 cm loops are the radio wavelength counterparts of the X-ray coronal loops. We infer semilengths L = 5 × 10⁹ cm, maximum electron temperatures T_e(max) = 3 × 10⁶ K, emission measures ∫N_e²d$\text{l}$ = 10²⁸ cm^?5, and electron densities N_e = 10⁹ cm^?3 (or pressures p = 1 dyn cm^?2) for the 20 cm bremsstrahlung. A total of eight solar bursts were observed at 6 cm or 20 cm wavelength with second-of-arc angular resolution. The regions of burst energy were all resolved with angular sizes between 5″ and 30″, brightness temperatures between 2 × 10⁷ K and 2 × 10⁸ K, and degrees of circular polarization between 10% and 90%. The impulsive phase of the radio bursts are located near the magnetic neutral lines of the active regions, and between the flaring Hα kernels which mark the footpoints of magnetic loops. In one case there was preburst heating in the coronal loop in which a burst occurred. Snapshot maps at 10 s intervals reveal interesting burst evolution including rapid changes of circular polarization and an impulsive burst which was physically separated from both the preburst radio emission and the gradual decay phase of the burst.     

Collaborative VLA,SOHO and RHESSI observations of evolving sources of energy release in the corona above active regions

Very Large Array (VLA) observations at 20 and 91 cm wavelength are compared with data from the SOHO (EIT and MDI) and RHESSI solar missions to investigate the evolution of decimetric Type I noise storms and Type III bursts and related magnetic activity in the photosphere and corona. The combined data sets provide clues about the mechanisms that initiate and sustain the decimetric bursts and about interactions between thermal and nonthermal plasmas at different locations in the solar atmosphere. On one day, frequent, low-level hard X-ray flaring observed by RHESSI appears to have had no clear affect on the evolution of two closely-spaced Type I noise storm sources lying above the target active region. EIT images however, indicate nearly continuous restructuring of the underlying EUV loops which, through accompanying low-level magnetic reconnection, might give rise to nonthermal particles and plasma turbulence that sustain the long-lasting Type I burst emission. On another day, the onset of an impulsive hard X-ray burst and subsequent decimetric burst emission followed the gradual displacement and coalescence of a small patch of magnetic magnetic polarity with a pre-existing area of mixed magnetic polarity. The time delay of the impulsive 20 and 91 cm bursts by up to 20 min suggests that these events were unlikely to represent the main sites of flare electron acceleration, but instead are related to the rearrangement of the coronal magnetic field after the main flare at lower altitude. Although the X-ray flare is associated with the decimetric burst, the brightness and structure of a long-lasting Type I noise storm from the same region was not affected by the flare. This suggests that the reconfiguration of the coronal magnetic fields and the subsequent energy release that gave rise to the impulsive burst emission did not significantly perturb that part of the corona where the noise storm emission was located.     

Asymmetric precipitation in a coronal loop as explanation of a singular observed spectrum

Almost 10 years of solar submillimeter observations have shown new aspects of solar activity, such as the presence of rapid solar spikes associated with the launch of coronal mass ejections and an increasing submillimeter spectral component in flares. We analyse the singular microwave–submillimeter spectrum of an M class solar flare on 20 December, 2002. Flux density observations measured by Sun patrol telescopes and the Solar Submillimeter Telescope are used to build the radio spectrum, which is fitted using Ramaty’s code. At submillimeter frequencies the spectrum shows a component different from the microwave classical burst. The fitting is achieved proposing two homogeneous sources of emission. This theoretical fitting is in agreement with differential precipitation through a magnetically asymmetric loop or set of loops. From a coronal magnetic field model we infer an asymmetric magnetic structure at the flare location. The model proposed to quantify the differential precipitation rates due to the asymmetry results in a total precipitation ratio Q₂/Q₁≈10⁴–10⁵, where Q₁(Q₂) represents the total precipitation in the loop foot with the high (low) magnetic field intensity. This ratio agrees with the electron total number ratio of the two sources proposed to fit the radio spectrum.     

Distinctive spatial configuration of a class ofmicrowave flaring sources

We discuss a class of microwave flares whose source regions exhibit a distinctive spatial configuration; the primaryenergy release in these flares results from the interaction between emerging magnetic flux and an existing overlying region. Such events typically exhibit radio, X-ray and EUV emission at the main flare site (the site of interaction) and in addition radio emission at a remote site up to 1 × 10⁵ km away in another active region. We have identified and studied more than a dozen microwave flares in this class, in order to arrive at some general conclusions on reconnection and energy release in such solar flares. Typically, these flares show a gradual rise showing many subsidiary peaks in both radio and hard X-ray light curves with a quasi-oscillatory nature with periods of 5–6 seconds, a bright compact X-ray & EUV emitting loop in the main flare source, a delay of the radio emission from the remote source relative to the main X-ray-emitting source. The magnetic field in the main flare site changes sharply at the time of the flare, and the remote site appears to be magnetically connected to the main flare site.     

An analysis of solar radio burst events on December 1, 2004

We present the analysis of the radio observations of December 1, 2004 from 07:00 UT to 07:40 UT in the 1.100–1.340 GHz band by Solar Broadband Radio Dynamic Spectrometer (SBRS) in Huairou Station. There are three groups of radio fine structures during the impulsive phase of this flare denoted by N1, Z2, and Z3. N1 has several emission lines with mixed fast and slow frequency drift rate which may reflect the conditions of flare loop and fast flows out from reconnection site; Z2 and Z3 are zebra patterns. The radio observations combined with hard X-ray and other observations show that the fine structures are connected with energetic particles. The information about magnetic field and energetic particle during the burst are also estimated based on our model.     

On the effects of electron-cyclotron masers during flares

First recognized by Wu and Lee (Ap. J. 230, 621, 1979), electron-cyclotron masers can be activated under very mild conditions. Large growth rates can occur even for relatively mild anisotropies in the electron velocity distribution, e.g., the one-sided loss cones that commonly occur when electrons with small pitch angles precipitate into high density regions at the footpoints of flaring loops while others are reflected in the converging field in the corona. Maser action can plausibly occur at the second harmonic of the local gyrofrequency and so explain certain very bright (? 10¹⁰ K) microwave bursts from the sun and other stars. However, the preponderance of the energy is at the first harmonic.We suggest that masers operating at the local gyrofrequency in a flaring loop generate radiation at decimeter wavelengths that is a significant fraction of the total energy of the flare, in fact (and not coincidentally) comparable with the energy in electrons associated with hard X-ray bursts. Essentially all of the radio energy is trapped in the corona and serves to produce localized heating in a volume large compared with the energy release region. Thus it can transfer energy by radiation from one magnetic loop to another, possibly inducing further instabilities, and spreading the course of the flare. Eventually the energy probably escapes the corona as soft X-rays. The electron-cyclotron maser saturates by extracting the perpendicular energy of the electrons, thereby diffusing them into the loss cone at the maximum possible rate; the enhanced precipitation into the footpoints can produce bright emission in hard X-rays, EUV and Hα and remove any necessity for directive acceleration in the energy release region.Details of the proposed mechanism and effects are contained in two papers by Melrose and Dulk (Ap. J. 259, 1982).This work was sponsored by NASA under grants NAGW-91 and NSG-7287 to the University of Colorado.     

1980年11月5日耀斑的硬、软X射线及微波的象与Hα等光度象的比较

本文对1980年11月5日22点25分开始的1B/M1-M4的Hα耀斑进行了图象处理,绘制了等光度图;与硬、软X射线象,微波象进行了比较.结果表明:1.耀斑的第一次极大,高能电子没有穿透到色球.Hα耀斑主要是由T=107—108K(产生软硬X射线的热区)等离子体向下传导到色球而形成.2.Hα耀斑的第二次极大,是由高能电子轰击色球而形成,Hα耀斑滞后数秒(小于5秒).3.耀斑闪光相,Hα面积与Hα强度同步增长.4.从耀斑前后的横向磁场变化(Hα短纤维的变化),估计磁能释放～1031尔格.      

Relative positions of microwave and hard X-ray burst sources

Simultaneous microwave and hard X-ray imaging observations of 12 bursts show that it is difficult to discern a general pattern between microwave and hard X-ray burst locations. In general, the microwave source is displaced from the hard X-ray source. The commonly believed behavior of the microwave source being located near the top and hard X-ray source near the footpoints of a loop appears to be true in some cases but not all. If the burst source is simple, both may be located near loop tops. Sometimes when the hard X-ray source has two components, one weak and one strong, the microwave source is not located over a neutral line (loop top) but close to a sunspot where the magnetic field is strongest. It appears that more than one loop or arcade may sometimes be involved in the microwave and hard X-ray emission. This is particularly true when several interacting loops trigger the onset of a flare.     

Origin and location of chromospheric evaporation in flares

Observation of two flares obtained with the Solar Maximum Mission spectrometers indicate that at flare onset the emission in soft (3.5 – 8 keV) and hard (16 – 30 keV) X-rays is predominant at the footpoints of the flaring loops. Since, at the same time, blue-shifts are observed in the soft X-ray spectra from the plasma at temperature of 10⁷ K, we infer that material is injected at high velocity into the coronal loops from the footpoints. These areas are also the sites of energy deposition, since their emission in hard X-rays is due to non-thermal electrons penetrating in the denser atmosphere. Hence, chromospheric evaporation occurs where energy is deposited. During the impulsive phase, the configuration of the flare region changes indicating that the flaring loop is progressively filled by hot plasma.     

The relationship between coronal mass ejections and solar flares

X-ray observations show that at a time consistent with a coronal mass ejection onset there is a small, soft X-ray burst (precursor). Generally this is followed some 20–30m later by a more significant flare. At the onset time there is frequently simultaneous activity from widely separated points on the Sun (>10⁵km). We present a model which accounts for the relationship between the coronal mass ejection and the precursor using 10²–10³ keV protons as the energy transfer agent. The protons (1) heat the high coronal loop. Inferred from the simultaneous activity, destabilizing the pressure balance to produce the ejection and (2) are guided by the magnetic field to below the transition region where they heat the chromospheric plasma to produce the precursor X-rays. High correlation between these events and a subsequent flare suggests that there may be a feedback mechanism operating from the coronal mass ejection.     

Microwave imaging observation of an electron stream in a solar flare

We report a Nobeyama Radioheliograph (NoRH) microwave observation of a propagating feature of non thermal emission in a solar flare. The flare had a very extended source well resolved by NoRH. In the rising phase of the microwave burst, a non-thermal gyrosynchrotron source was observed by the high-rate (10 images per second) observations to propagate from one end of the loop to the other with a speed of 9 × 10⁴ km s⁻¹. We interpret this non-thermal propagating source is emitted from streaming electrons.     

Multi-wavelength study of the short term TeV flaring activity from the blazar Mrk 501 observed in June 2014

In this work, we study the short term flaring activity from the high synchrotron peaked blazar Mrk 501 detected by the FACT and H.E.S.S. telescopes in the energy range 2–20 TeV during June 23–24, 2014 (MJD 56831.86–56831.94). We revisit this major TeV flare of the source in the context of near simultaneous multi-wavelength observations of $\gamma$–rays in MeV-GeV regime with Fermi-LAT, soft X-rays in 0.3–10 keV range with Swift-XRT, hard X-rays in 10–20 keV and 15–50 keV bands with MAXI and Swift-BAT respectively, UV-Optical with Swift-UVOT and 15 GHz radio with OVRO telescope. We have performed a detailed temporal and spectral analysis of the data from Fermi-LAT, Swift-XRT and Swift-UVOT during the period June 15–30, 2014 (MJD 56823–56838). Near simultaneous archival data available from Swift-BAT, MAXI and OVRO telescope along with the V-band optical polarization measurements from SPOL observatory are also used in the study of giant TeV flare of Mrk 501 detected by the FACT and H.E.S.S. telescopes. No significant change in the multi-wavelength emission from radio to high energy $\gamma$–rays during the TeV flaring activity of Mrk 501 is observed except variation in soft X-rays. The varying soft X-ray emission is found to be correlated with the $\gamma$–ray emission at TeV energies during the flaring activity of the source. The soft X-ray photon spectral index is observed to be anti-correlated with the integral flux showing harder-when-brighter behavior. An average value of 4.5$\%$ for V-band optical polarization is obtained during the above period whereas the corresponding electric vector position angle changes significantly. We have used the minimum variability timescale from the H.E.S.S. observations to estimate the Doppler factor of the emission region which is found to be consistent with the previous studies of the source.     

Proton versus electron heating in solar flares

Proton and electron heating of a flaring atmosphere is compared in a kinetic approach for the particles ejected from a non-neutral reconnecting current sheet (RCS) located above the top of reconnected flaring loops in a two-ribbon flare. Two kinds of high-energy particles are considered: particles accelerated by a super-Dreicer electric field and those ejected from the reconnection region as neutral outflows, or separatrix jets. The beam electrons are assumed to deposit their energy in Coulomb collisions and Ohmic heating of the ambient plasma particles by the electric field induced by the precipitating beams. The protons are assumed to deposit their energy in generation of kinetic Alfvén waves (KAWs), which, in turn, dissipate due to Cherenkov resonant scattering on the ambient plasma electrons. The beam electrons are found to provide a fast (within a few tenth of a second) heating of the atmosphere that is well spread in depth from the corona to the lower chromosphere. The protons are shown to precipitate to the lower atmosphere much slower (up to few seconds for beam and up to 10–20 s for slow jets). Slow jet protons provide heating of the two compact regions: the first located at the top of a flaring loop just below the RCS, and the second one appearing at the transition region (TR) and upper chromosphere; fast beam protons deposit their energy in the TR and chromosphere only.     

The evolution of a complex solar radio burst corresponding to special configuration of microwave sources

A complex radio burst associated with periodic (∼1 and 6 min) pulsations and several kinds fine structures, e.g., normal- and reverse-drifting type III bursts, zebra patterns, and slowly drifting structure was observed with the radio spectrometers (1.0–2.0, 2.6–3.8, 5.2–7.6, and 0.65–1.5 GHz) at the National Astronomical Observatories of China (NAOC) in Beijing and Yunnan on 19 October 2001. In combination with the images of 17 and 34 GHz from NoRH and the magnetograms from MDI we reveal the existence and evolution of preexisting and new emerging sources, and find the horseshoe-shaped structure of microwave sources intensity during the late phase of the burst. Through the detailed comparison of the evolution of each source with the time profiles of radio bursts corresponding to these sources we indicate that the intimate correlation between the microwave sources evolution and the generation of the radio burst associated fine structures. Some fine structures can be considered as the MHD turbulence and plasma emission mechanism, based on the anisotropic beam instability and hybrid waves generations. From the characteristics of observations we may presume that the coronal magnetic structures should contain an extended coronal loop system and multiple discrete electrons acceleration/injection sites. The mechanisms of this complex radio burst are deal with the incoherent gyrosynchrotron emission from the trapped electrons and the coherent plasma emission from the non trapped electrons.     

Modulations of broad-band radio continua and X-ray emissions in the large X-ray flare on 03 November 2003

The GOES X3.9 flare on 03 November 2003 at ∼09:45 UT was observed from metric to millimetric wavelengths by the Nançay Radioheliograph (NRH), the Radio Solar Telescope Network (RSTN) and by radio instruments operated by the Institute of Applied Physics (University of Bern). This flare was simultaneously observed and imaged up to several 100 keV by the RHESSI experiment. The time profile of the X-ray emission above 100 keV and of the radio emissions shows two main parts, impulsive emission lasting about 3 min and long duration emission (partially observed by RHESSI) separated in time by 4 min. We shall focus here on the modulations of the broad-band radio continua and of the X-ray emissions observed in the second part of the flare. The observations suggest that gyrosynchrotron emission is the prevailing emission mechanism even at decimetric wavelengths for the broad-band radio emission. Following this interpretation, we deduce the density and the magnetic field of the decimetric sources and briefly comment on possible interpretations of the modulations.     

The radio signature of a kink unstable loop

Based upon multi-wavelength observations outlined by Huang et al. [Solar Phys. 213 (2003) 341], especially the dynamic spectrum at 4.5–7.5 GHz, we study the physical nature of the radio fine structures (FS) during the flare on August 25, 1999 in AR 8674 (S28E21). The main results are: (1) the helical loop of the event related to the FS is unstable for m = 1 kink mode; (2) the time interval between the beginning of reconnection and relaxation of the unstable loop, inferred from the observation, is quantitatively consistent with the results of the numerical simulations on kink unstable loop; (3) the magnetic field strength estimated from the fast kink standing wave is basically of the same order as that estimated from the photospheric magnetic field, which provides strong support to our analysis.     

一个亚耀斑的精细结构与演化

本文对1981年5月8日16h13mUT开始的-N级Hα耀斑进行了图象处理,画出了等光度图。在此基础上,采用二种光度测量法:一是定点法,相对于日面上稳定的定点(～1″),测出50个亮点的位置及其光度变化;另一是亮点跟踪法,测出每一时刻最亮点的位置,看位置的变动。从而得到耀斑的精细结构与演化过程。      

A solar type U burst in the decimetric range

This paper analyzes a Type U burst at 1.0 – 2.8 GHz which occurred between 12:36:26 – 12:36:32 UT on 1992 August 22, observed by Ond ejov Observatory, Czech Republic. This may be the first example of Type U bursts in the decimetric range, as far as we know. From analysis we came to the following conclusion: (1) The frequency drift rates of the ascending and descending branches are 1.25 and 0.225 GHz/s, respectively, and the velocities of the electron beam are 0.38 c and 0.26 c, respectively; (2) The burst decay of the ascending branch is larger than that of the descending branch; (3) The variations of the maximum frequencies of instantaneous spectra with time appear as from the highest (1.92 GHz) to the lowest (1.0 GHz), then toward higher frequency (1.53 GHz) (this is consistent with that predicted by plasma emission theory); (4) The bandwidths of the ascending branch are about twice that of the descending branch (this may be caused by the larger drift rates of the ascending branch); (5) The temperature of the coronal loop apex is 6.3 × 10⁶ K; (6) The magnetic field at the top of the loop is greater than 9.2 G; (7) This U burst emission is plasma radiation at the second harmonic.     

RadioAstron reveals super-compact structures in the bursting H2O maser source G25.65+1.05

Water masers are well-known to be variable on a variety of time scales, but only three Galactic H₂O masers are known to flare to the level of 10⁵–10⁶ Jy (${\text{T}}_{\text{B}}\sim {10}^{17}$ K): Orion KL, W49N, and the recently discovered G25.65+1.05. Recently detected flaring activity of H₂O maser in the massive star-forming region G25.65+1.05 gave us a unique opportunity to study the fine structure of H₂O maser emission in the bursting state with extremely high space VLBI angular resolution. Observation of the source was carried out with ∼9 Earth diameter space-ground baseline within the framework of the RadioAstron project. H₂O maser emission from two spectral features, including the bursting one, was detected in the experiment. Only ∼1% of the bursting H₂O maser emission was detected on the space-ground baselines: it indicates the presence of a very compact spatial structure with a size of ∼25 $\mathrm{\mu }$as, which corresponds to 0.05 AU or ∼5 solar diameters at the distance to the source of 2.08 kpc, and the brightness temperature of ∼3 × 10¹⁶ K. Analysis of the flux density as a function of the baseline length for the bursting H₂O maser feature in the source shows that most of the emission comes from an extended “halo” structure, while the core of emission is very compact and has an extreme brightness temperature. These results are in agreement with the model of interacting maser clouds considered as the likely explanation of the nature of the burst in the source. Under the assumption of such a model, the beam size of maser emission is reduced while the brightness temperatures similar to the highest observed values are produced.     

The temperature map of the loop and coronal sources of an occulted solar flare

An occulted solar flare occurred at about 06:07 UT on 2002, November 2. The RHESSI X-ray images show two separate parts. The lower part consists of a complete loop and the upper part a coronal source which well extends above the solar limb. The loop source shrank for about 3 min with a speed of ∼24 km s⁻¹ during the early impulsive phase and then expanded at ∼7 km s⁻¹, while the coronal source presented an upward motion at about 6 km s⁻¹. We obtained the temperature map of the loop source from RHESSI image spectrum. The temperature of the loop increases with altitude, indicating that the reconnection X-point of this flare is located above the loop source. However, the apparent coronal source is the top of another independent large-scale loop.