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We describe a flare of GOES class M5.7 that was observed simultaneously by RHESSI (Ramaty High Energy Solar Spectroscopic Imager) and NoRH (Nobeyama Radio Heliograph). The flare occurred in AR 9866 located near the disk center. The hard X-ray (HXR), microwave, EIT, and TRACE 195 A observations indicate that the flaring region consisted of a complex of multiple loops. In the microwave domain the source morphology, the timing, the polarization characteristics, and the photospheric magnetic fields clearly indicate that it is of a class characterized as a "double loop" configuration, meaning two systems of magnetic flux, each consisting of many smaller loops. The observations suggest the existence of a small loop system created by the emergence of new flux, which interacts with an old flux system, and of a remote flare site that is observed primarily in radio. The former is the main flare site where we observe microwave, HXR, and EUV emissions. In HXR there are two main identifiable loop systems. The first is an elongated one filled with energetic electrons primarily emitting lower energy (12-25 keV) HXR with a colocated microwave source; this source has distinct footpoints at higher X-ray energies. The second loop system is implied by compact HXR sources in opposite magnetic polarities separated by a distance greater than the length of the first loop system. Spectroscopic analysis of the RHESSI data shows that the spectrum can be fitted with a thick-target model with a thermal component and a broken power-law component of the electron energy distribution. This model is used to address the thermal/nonthermal and radio/HXR electron number problems.

We discuss a filament eruption/coronal mass ejection (CME) event associated with a flare of GOES class M2.8 that occurred on 2001 November 17. This event was observed by the Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz. NoRH observed the filament during its eruption both as a dark feature against the solar disk and a bright feature above the solar limb. The high cadence of the radio data allows us to follow the motion of the filament at high time resolution to a height of more than half a solar radius. The filament eruption shows a very gradual onset and then a rapid acceleration phase coincident with the launch of a fast halo CME. Soft X-ray and extreme-ultraviolet (EUV) images show heating in a long loop underneath the filament prior to the flare. The NoRH height-time plot of the filament shows a roughly constant gradual acceleration for 1 hr, followed by a very abrupt acceleration coincident with the impulsive phase of the associated flare, and then a phase of constant velocity or much slower acceleration. This pattern is identical to that recently found to occur in the motion of flare-associated CMEs, which also show a sharp acceleration phase closely tied to the impulsive phase of the flare. When the rapid acceleration occurs in this event, the flare site and the filament are separated by ~0.5 R☉, making it unlikely that a disturbance propagates from one location to the other. Models in which a disruption of the large-scale coronal magnetic field simultaneously permits the acceleration of the filament and the flare energy release seem to be a better explanation for this event.

We present an overview of the radio observations of the X1.5 flare of 2002 April 21 and complementary data from other wavelengths. This flare was fairly well observed by the Ramaty High Energy Spectroscopic Imager (RHESSI) spacecraft and fully observed by the Nobeyama Radioheliograph (NoRH) at 17 and 34 GHz. This long-duration event lasted more than 2 hr and featured a beautiful arcade of rising loops on the limb visible at X-ray, EUV, and radio wavelengths. The main flare was preceded by a small event 90 minutes earlier showing a long EUV loop connecting well-separated radio and hard X-ray sources. The main flare itself starts with a compact radio and hard X-ray source at the eastern end of the region that develops into emission close to the solar surface (and well inside the solar limb) over a large region to the northwest. As the flare proceeds, a large set of loops is seen to rise well above the solar limb. Distinct regions of radio emission with very different time behavior can be identified in the radio images, and, in particular, a peculiar nonthermal source seen in radio and hard X-rays low in the corona at the base of the arcade is seen to turn on 30 minutes after the start of the impulsive phase. At about the same time, an extremely intense burst of coherent radio emission is seen from 500 to 2000 MHz; we speculate that this lower-frequency burst is produced by electrons that are accelerated in the nonthermal source at the base of the arcade and injected into the loop system where they radiate plasma emission in the 1010 cm-3 density plasma at the top of the arcade of loops. This event is striking as a demonstration of the many ways in which a flare can produce radio emission, and the combined data at different wavelengths reveal a diversity of energy release and nonthermal acceleration sites.

We present the first comparison between radio images of high-energy electrons accelerated by a solar flare and images of hard X-rays produced by the same electrons at photon energies above 100 keV. The images indicate that the high-energy X-rays originate at the footpoints of the loops dominating the radio emission. The radio and hard X-ray light curves match each other well and are quantitatively consistent with an origin in a single population of nonthermal electrons with a power-law index of around 4.5-5. The high-frequency radio spectral index suggests a flatter energy spectrum, but this is ruled out by the X-ray spectrum up to 8 MeV.

We report the multiwavelength investigations of an eruptive flare event that occurred on 2001 April 2 at about 11 UT. The manifestations associated with this flare event have been studied from the near-Sun region to about 0.5 AU. The H-alpha images from the Meudon Spectroheliograph reveal a fast spectacular eruption of plasmoids from the flare site to the west and a Moreton wave disturbance propagating toward the south, A bright, fast, wide coronal mass ejection (CME) associated with this eruptive event was imaged by SOHO/LASCO and the remote-sensing interplanetary scintillation technique. The timings and positions of the Type II radio bursts, H-alpha eruption, and CME onset as well as the magnetic field configuration suggest a release of energy at the null point. The results seem to support the "breakout" scenario proposed by Antiochos and coworkers, and they are also suggestive that the energy release is followed by magnetic reconnection between the low-lying loops near the separatrix and the loop system above them.

We studied a microwave burst that showed deep quasi-periodic pulsations using imaging observations at 17 and 34 GHz. Strongly modulated pul sations appear in radio images from the eastern end of a long loop an d in hard X-rays from the western end of the loop. Radio modulations are seen at the western end of the loop but at a level some 20 times weaker than at the eastern end; these radio modulations at the western end of the loop, like the hard X-ray modulations at the same locatio n, appear to lead the modulations at the eastern end by about 0.5 s, but all have the same period. The period of the modulation can be exp lained by MHD oscillations of the loop approximately 120" long connecting the sources.

We use high-resolution radio observations to study the physical parameters of a flaring loop. The loop is visible at radio wavelengths because of gyrosynchrotron emission by nonthermal electrons (energies typically above several hundred keV) accelerated by the flare. We are able to measure the loop thickness and length with a precision on the order of 1''. We find that the loop length increases from about 60'' initially to about 80'' in the decay phase of the event. The loop (averaged along its length) initially is no more than 3'' wide. The soft and hard X-ray data obtained with the Soft X-Ray Telescope and Hard X-Ray Telescope on the Yohkoh satellite are consistent with the same loop as observed at radio wavelengths (although the soft X-ray morphology has some small differences early in the event). This event was accompanied by a coronal mass ejection and a coronal dimming visible in Solar and Heliospheric Observatory Extreme-Ultraviolet Imaging Telescope images, so it involved a very large volume of the corona, yet the radio observations clearly indicate that much of the energy release in the low corona was restricted to a region apparently no more than 2000 km across. As the event proceeds, the loop develops a bright feature at the loop top in both the radio and soft X-ray images that cannot be reproduced in gyrosynchrotron loop models in which the electron distribution has relaxed by pitch angle scattering to fill the loop. This prevents us from using the flare properties to measure the magnetic field strength and variation along the loop. The bright loop-top source may require that trapping of electrons take place at the loop top late in the event.

We use extensive multiwavelength data to study a flare belonging to the interacting-loop class of events identified by Hanaoka. The class of flares is identified morphologically from the presence of two well-separated radio sources in 17 GHz images (in this event, 160'' apart), with only one source showing soft X-ray emission. This event shows many of the other properties apparently shared by this class of flares: a gradual rise showing many subsidiary peaks in both radio and hard X-ray light curves with a quasi-oscillatory nature, the presence of a bright compact X-ray-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, higher circular polarization in the radio emission of the remote source than in the main source, and stronger photospheric magnetic field in the remote source. The new results of our analysis are that we are able to show, using a sequence of magnetograms, that the magnetic field in the main flare site changes sharply at the time of the flare, and further we argue that the remote site is magnetically connected to the main flare site only up to the time of the main impulsive phase, at which point we believe the magnetic connection to the remote site was broken and further flare manifestations are largely confined to the main flare site. This severing of the magnetic connection between two well-separated active regions may be an intrinsic part of the energy release in this flare. The region around the main flare site also exhibits rotation in the magnetogram in the period leading up to the flare. Radio and hard X-ray oscillations with periods of order 5-10 s are observed in the rise phase of this event. If they are due to transverse oscillations of the flare loop at the Alfven speed, then the density in the loop is inferred to be of order 1011 cm-3 and to increase with time as expected.

The Sun's corona, which is composed of plasma at a temperature of a few millions of degrees, can be best viewed in two electromagnetic domains, one from wavelengths of a few angstroms to hundreds of angstroms (in the soft X-ray and EUV domain), the other from wavelengths of a few centimeters to several tens of centimeters (in the radio domain). In this paper, we present a quantitative comparison of coronal observations made in these two domains with high spatial resolution over the full disk of the Sun. The EUV observations were taken with the EIT (Extreme-Ultraviolet Imaging Telescope) on board SOHO (Solar and Heliospheric Observatory), and the radio observations were taken with the VLA (Very Large Array). The two sets of images show very similar morphologies, indicating that the different wavelengths originate from common solar features. We predict radio fluxes using the temperature and emission measure of the corona calculated from EIT observations, adopting Meyer's table of coronal abundances for the calculations. In each of the seven observations investigated, there always exists a good linear correlation in the pixel-by-pixel correlation plot between the predicted and the observed radio flux for coronal features over a wide range of flux variation. Nevertheless, the predicted radio flux is systematically larger than that observed by a factor of 2.0 ± 0.2, on average. We attribute the difference to the underestimation of the abundance of Fe relative to H in the abundances adopted by Meyer. On this basis, we place the absolute Fe abundance in the corona at 7.8 × 10-5, which has an enrichment factor of 2.4 relative to the accepted photospheric Fe abundance.

In this paper we report the first detection of metric/decimetric radio emission associated with two soft X-ray plasmoid ejecta events that occurred during two limb flares observed by the Yohkoh SXT. In the first event a loop started to rise slowly (~10 km s-1) before the beginning of the hard X-ray impulsive phase of the flare. At about the onset of the impulsive flare, there was acceleration of the ejecta, resulting in a speed of 130 km s-1 and finally to ~200 km s-1. The associated radio emission was observed with the Nancay radioheliograph (NRH) in the frequency range of 230-450 MHz. It was an unpolarized continuum that lasted 8-10 minutes. The 410 MHz source was located close to the height where the plasmoid was last identified in the SXT images. In the second event an eruption resulted in the expansion of a large-scale, looplike feature and the development of two plasmoid ejecta which moved in different directions. The speed of the ejecta was 60-100 km s-1. In this event, the associated radio emission was a long-lasting (about 2 hr) continuum observed from 450 to 164 MHz. The onset of the low-frequency emission was delayed with respect to the onset of the high-frequency emission. In both cases the radio sources were located above the soft X-ray ejecta in the general direction of the prolongation of the ejecta movement. In both cases the radio emission comes from nonthermal electrons which are accelerated in close relationship with the propagation of the X-ray plasmoid: as the plasmoid reaches higher altitudes, it interacts with increasingly more extended magnetic field lines and new coronal sites of production of nonthermal electrons are created.

The temporal relationship between coronal mass ejections (CMEs) and associated solar flares is of great importance to understanding the origin of CMEs, but it has been difficult to study owing to the nature of CME detection. In this paper, we investigate this issue using the Large Angle and Spectrometric Coronagraph and the EUV Imaging Telescope observations combined with GOES soft X-ray observations. We present four well-observed events whose source regions are close to the limb such that we are able to directly measure the CMEs' initial evolution in the low corona (~ 1-3 R☉) without any extrapolation; this height range was not available in previous space-based coronagraph observations. The velocity-time profiles show that kinematic evolution of three of the four CMEs can be described in a three-phase scenario: the initiation phase, impulsive acceleration phase, and propagation phase. The initiation phase is characterized by a slow ascension with a speed less than 80 km s-1 for a period of tens of minutes. The initiation phase always occurs before the onset of the associated flare. Following the initiation phase, the CMEs display an impulsive acceleration phase that coincides very well with the flares' rise phase lasting for a few to tens of minutes. The acceleration of CMEs ceases near the peak time of the soft X-ray flares. The CMEs then undergo a propagation phase, which is characterized by a constant speed or slowly decreasing in speed. The acceleration rates in the impulsive acceleration phase are in the range of 100-500 m s-2. One CME (on 1997 November 6, associated with an X9.4 flare) does not show an initiation phase. It has an extremely large acceleration rate of 7300 m s-2. The possible causes of CME initiation and acceleration in connection with flares are explored.

In this paper we seek a self-consistent model for three strong limb flares observed at 17 and 34 GHz by the Nobeyama radioheliograph and also in soft X-rays and hard X-rays by the Yohkoh SXT (Soft X-Ray Telescope) and HXT (Hard X-Ray Telescope) instruments. Additional radio spectral data were provided by the Nobeyama polarimeter. The flare geometry is simple, with one well-defined flaring loop in each event. The 17 and 34 GHz emissions are optically thin gyrosynchrotron radiation from energetic electrons that outlines the flaring loops and peaks close to the loop tops. We infer that the variation of magnetic field along the loops is very small. We try to reproduce the observed radio morphologies and fluxes using a model gyrosynchrotron loop. The results of our modeling rely on the model magnetic field geometry that we choose. Although the exact loop geometry cannot be constrained from a two-dimensional snapshot, we choose for simplicity a line-dipole magnetic field, and the model field lines are circular. The SXT/HXT images are used to provide the physical parameters of the model loops. The high-frequency polarimeter data give the energy spectral index of the radio-emitting electrons. We could not reconcile the observed radio morphologies and fluxes using classic dipole magnetic field models. The best-fit model that uses the same input parameters for both frequencies and partly reconciles the observed 17 and 34 GHz morphologies and fluxes is produced when we invoke a magnetic field with constant strength along the model loop. These model loops have uniform thickness. The derived densities of the radio-emitting electrons are (1-6) × 104 cm-3 with energy limits between 60 and 5000 keV. These models are the best fits we can get under the best assumptions we can justify, but they do not in fact match the radio morphologies very well; their problems and limitations are discussed.

We present the results of a radio study of the evolution of an active region through its flare productivity. The radio study was carried out with data obtained by the Nobeyama Radio Heliograph at 17 GHz. We chose the active region AR 7515, which appeared at the east limb on 1993 May 23 and then evolved during its passage across the disk. We followed its evolution until June 2. This region produced many small flares. We consider this region to be a typical active region in the sense that it did not produce any large flares, but a large number of weak flares. We investigate the optical and magnetic development of the region and show how this affects the locations of the flaring activity. We discuss a number of events in detail in order to investigate the roles of nonthermal and thermal radio emission in the flares. The nonthermal gyrosynchrotron emission generally occurs in regions of strong magnetic fields, is generally circularly polarized, and often varies rapidly in time. On the other hand, gradual radio components tend to be thermal and only weakly polarized, if at all. An interesting aspect of evolution of the flares in this region is that many of the flares in the early phase of the evolution show strong but brief nonthermal radio emission in the impulsive phase followed by gradual thermal emission, whereas in the last 3 days more gradual events without a strong spike of radio emission in the impulsive phase tend to be seen. Correspondingly, the flare images suggest that the radio sources are more compact during the early phases and more extended in the last half of the period covered. The most dominant component of the preflare region is often not the component that undergoes immediate flaring. Sometimes a number of components in the preflare region participate in the flare process together. We speculate that these component sources are unresolved compact bipolar loops that flare in sequence. Loop-loop interactions occurring at many different sites at the same time seems to be a less plausible explanation of these events.

We present the results of a study of spatial structure of sources of microwave and millimeter bursts with simple spiky time profiles at 17 and 34 GHz, similar to those found to be common at 3 mm wavelength. These bursts are of short duration, with fast 2-4 s rise time to peak, followed by a rapid exponential decay. When mapped at high spatial resolution with the Nobeyama Radio Heliograph (NoRH), the radio images show direct evidence that the radio sources are compact bipolar loops: source sizes are less than 5'' and three of the five events studied show closely spaced oppositely polarized components in the circular polarization maps. All five events are located directly over magnetic neutral lines in the photosphere. The soft X-ray behavior is not entirely consistent with the Neupert effect in these events, since all five events show a rise in the soft X-ray flux well before any nonthermal electrons are present in the corona and the ratio of peak soft X-ray flux to peak radio 17 GHz flux may vary by many orders of magnitude from one event to the next. The abrupt time profiles of these events and their physical properties are consistent with a single-loop scenario in which magnetic energy release and acceleration of nonthermal electrons are confined to a compact localized region.

We present analyses of two solar flares observed with high spatial resolution at 86 GHz with the BIMA millimeter-wavelength telescope and at 17 and 34 GHz with the Nobeyama radioheliograph. The flares were observed on 1998 November 24 and 1999 May 1. At millimeter wavelengths these are impulsive events, and therefore they must be produced by MeV-energy electrons. The present study using simultaneous observations of two flares at 86, 34, and 17 GHz provides an excellent opportunity to study high-energy electrons with high spatial resolution observations at three optically thin frequencies. The morphology of millimeter emission can reveal both the properties of the MeV-energy electrons and the nature of the coronal magnetic field lines where they radiate. One of the two events we present is the first clear case of a λ = 3 mm source in which both footpoints of a loop are detected. In the second event the polarization image at 17 GHz also suggests a bipolar or looplike morphology. Such morphological observations can be used to constrain the nature of the magnetic field in the solar corona.

Using Yohkoh SXT and Nobeyama 17 GHz data, we have studied the soft X-ray and microwave emission above several stable, large sunspots near central meridian passage. Our study confirms the well-known fact that soft X-ray emission is depressed above sunspots. It also shows that the distribution of their soft X-ray intensity is not uniform; usually the darkest pixels are associated with the umbra or the far edges of the leading part of the penumbra while the following part of the penumbra may contain higher intensity pixels associated with brighter loops. For the first time, we present a systematic survey of the temperatures and emission measures of the soft X-ray material above sunspots. Sunspots always contain the lowest temperatures and emission measures in the active regions. The mean umbral temperature is 1.8 × 106 K, and the mean penumbral temperature is 2.4 × 106 K. The mean umbral and penumbral emission measures are log EM = 26.60 cm-5 and log EM = 27.00 cm-5, respectively. The differences between the umbral and penumbral plasma temperatures are physically significant. The higher penumbral values imply that the loops associated with the penumbrae are generally hotter and denser than the loops associated with the umbrae. The highest sunspot temperatures and emission measures are still lower than the average active region parameters but higher than the quiet-Sun plasma parameters. The coronal radiative energy loss rate above the umbrae is 15% higher than the radiative loss rate of the quiet-Sun plasma but a factor of 8.3 lower than the typical active region radiative loss rate. The radio emission comes from the gyroresonance mechanism, and, as expected, it is sensitive to the magnetic field rather than the soft X-ray-emitting plasma.

Simultaneous images of a flaring loop at two frequencies are used to model the magnetic structure of the loop and the energy distribution of the radiating electrons. The imaging data were obtained with the VLA at 5 and 15 GHz. Additional spectral data were provided by the Owens Valley Radio Observatory (OVRO) solar array at several frequencies between 2 and 15 GHz. At 15 GHz, the flare emission was optically thin and came from the footpoints of the flaring loop, while at 5 GHz the loop itself was outlined. Most of the 5 GHz emission was optically thick, and its spatial maximum was close to the loop top. A striking feature of the observations is that the 5 GHz emission does not reach down to the 15 GHz footpoints. We compare the observations with calculations of gyrosynchrotron emission from an inhomogeneous magnetic loop in order to determine the conditions in the flaring loop. The best fit to the OVRO fluxes was reached with a model flaring loop with photospheric footpoint magnetic field strength of 870 G. The thickness of the model loop was small compared with its footpoint separation. The energy spectral index of the energetic electrons was 3.7, and their number density was 7.9 × 107 cm-3. The low- and high-energy cutoffs of the nonthermal electrons were 8 and 210 keV, respectively. The 5 GHz emission in this model is at low harmonics (3-7), and harmonic effects are responsible for the weak 5 GHz emission at the footpoints. The absence of electrons above 210 keV is necessary in this model to explain why no emission is observed from the loop top at 15 GHz. That model reproduced well the high-frequency part of the OVRO flux spectrum as well as the VLA spatial structure. Thus, comparisons between the spatially resolved observations and models reveal the three-dimensional structure of the loop geometry.

We present a method for deriving a two-thermal-component approximation to the differential emission measure distribution of plasma in the Sun's corona in the temperature range to which the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) spacecraft is sensitive. EIT takes high-resolution full-disk coronal images in three of its four optimized channels by observing emission lines of highly ionized Fe whose formation temperatures overlap and cover the range from 0.7 to 2.8 MK. It is straightforward to show that the traditional single-temperature models based on the ratio of a pair of EIT images at different wavelengths are not able adequately to represent the plasma contributing to all three wavelength ranges. In this paper, we develop a modified image-ratio method that results in a two-thermal-component model for the plasma producing the coronal emission observed by EIT. The products of this method are two temperature and two emission measure full-disk maps of the Sun's corona, with the full resolution of the EIT telescope, in two temperature regimes: one from 0.8 to 1.6 MK and the other from 1.6 to 2.6 MK. The two-component solutions are tested using a series of model differential emission measures (DEMs) from the CHIANTI atomic database package. This method appears to produce realistic results in all regions of the Sun's atmosphere with the exception of coronal holes, where very cool Si VII/Mg VII lines (

We study a metric continuum burst observed on 1993 February 18, and its X-ray signatures from imaging observations in radio and X-rays using the Nancay radioheliograph and the Yohkoh Soft X-Ray Telescope (SXT). The event in question was associated with weak type III bursts; these were detected at only one frequency (164 MHz), except for one burst (at 10:58:05 UT), which was observed over a broad frequency range (164-435 MHz). We believe that the early metric continuum burst is an extension of the microwave continuum which was observed at frequencies as high as 5 GHz, and its onset at ~10:50 UT is associated with the development of an X-ray-emitting diffuse loop system which appears to advance with a speed of ~50-100 km s-1. The observed type III bursts seem to correspond to the repeated occurrence/appearance of a collimated jet emanating from the loop system that is responsible for the continuum burst. A few minutes prior to the main continuum onset there is a soft X-ray ejection from the main flare region. The main continuum has a brightness temperature greater than 108 K; it is unpolarized, and it shows dispersion in position with frequency and moves with speeds of ~50 km s-1 at 236-410 MHz. The SXT images reveal that this initially ejected soft X-ray-emitting hot plasma seems to gradually fill up the loop system with hot material. This hot plasma must contain enough energetic electrons of energy greater than several tens of keV, which are responsible for producing the metric continuum burst by plasma radiation mechanism.

We analyze a small flare using imaging data at millimeter, microwave, and soft X-ray wavelengths and microwave and hard X-ray spectral observations. The remarkable aspect of this flare is evidence for the presence of MeV-energy electrons, which are responsible for the nonthermal millimeter emission, at a time when no hard X-rays from lower energy electrons are detected. This occurs during a smoothly varying phase, which is seen at radio wavelengths to last several minutes and is the brightest phase at millimeter wavelengths but is undetected in hard X-rays: it follows a brief spike of emission at flare onset, which has the more usual properties of impulsive events and features nonthermal microwave, millimeter, and hard X-ray emission. We interpret the phase that is brightest at millimeter wavelengths as being due to efficient trapping of a relatively small number of nonthermal electrons, whereas during the hard X-ray emission, trapping is much less efficient, and the decay time is much shorter at all energies, which leads to a larger ratio of hard X-ray flux to radio flux. As in many previous events studied at millimeter wavelengths, there is a discrepancy between the electron energy spectral indices inferred from the milllimeter and hard X-ray data during the impulsive phase when both are detected: again it appears that the energy spectrum at 1 MeV must be significantly flatter than at several hundred keV and below. However, there are problems in reconciling quantitatively the energy spectra for the hard X-ray-emitting and radio-emitting components: based on the most plausible parameters, the radio-emitting electrons should produce most of the hard X-rays. One solution to this contradiction is to invoke a coronal magnetic field stronger than seems likely based on the photospheric magnetic field.

We present the results of a search for radio-selected transient brightenings (TBs) in the solar atmosphere as a complement to the more common X-ray-selected surveys. The Sun was generally quiet during the observations, making these data sensitive to weak TBs both in and outside active regions. Five small impulsive events were identified in a set of VLA observations at 4.5, 1.5, and 0.33 GHz and compared with soft X-ray images from Yohkoh and EUV images from SOHO/EIT. Four of the events were located at the edges of an active region, but one was located 100'' away in a quiet region of the atmosphere. Possible emission mechanisms for these brightenings are investigated. The time profiles of the radio TBs show impulsive peaks, while the corresponding soft X-ray profiles are gradual. The impulsive radio peaks were up to 35% polarized. Our data favor an interpretation in terms of gyrosynchrotron radiation from mildly relativistic electrons. A small number of nonthermal electrons with spectral index 3 can explain the observed properties of the TBs. Thus, nonthermal TBs can be found away from active regions. Two of the microwave TBs also show evidence for type III radio emission at 327 MHz.

As models for the physical properties of the corona above solar active regions grow more sophisticated, we will require better means for testing them. In this paper we discuss and apply such a test to a magnetic field model for an active region. This test is based on the expectation that the temperatures at different points on a given magnetic field line should be well correlated because of the rapid transport of heat along field lines in the corona. We use radio observations of an active region to measure the temperatures on field lines as they cross two isogauss surfaces (at 430 and 750 G) in the corona. The field lines and isogauss surfaces are derived from a coronal magnetic field model obtained via a nonlinear force-free field extrapolation of a photospheric vector magnetogram; for comparison, we also investigate a potential-field extrapolation of the same magnetogram. In a region in which strongly sheared fields are present, the nonlinear force-free field model does indeed show a good correlation between the temperatures in the two surfaces at points on the same field line, while the potential-field model does not. This diagnostic acts both as a test of the magnetic field model as well as of the interpretation of the radio data, and we show how this test can also aid in understanding the radio data.

We investigate the structure of the solar chromosphere and transition region using full Sun images obtained with the Extreme-Ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory spacecraft. The limb seen in the EIT coronal images (taken in lines of Fe IX/X at 171 A, Fe XII at 195 A, and Fe XV at 284 A) is an absorption limb predicted by models to occur at the top of the chromosphere where the density of neutral hydrogen becomes significant (~1010 cm-3). The transition-region limb seen in He II λ304 images is an emission limb. We find that (1) the limb is higher at the poles than at the equator both in the coronal images (by 1300±650 km) and the 304 A images (by 3500±1200 km), and (2) the 304 A limb is significantly higher than the limb in the coronal images. The height difference is 3100±1200 km at the equator and 6600±1200 km at the poles. We suggest that the elevation of the 304 A limb above the limb in the coronal images may be due to the upper surface of the chromosphere being bumpy, possibly because of the presence of spicules. The polar extension is consistent with a reduced heat input to the chromosphere in the polar coronal holes compared with the quiet-Sun atmosphere at the equator.

We compare microwave images of a solar active region with state-of-the-art fully nonlinear force-free extrapolations of the photospheric fields in order to study the link between coronal currents and heating of the corona. This extrapolation fully takes into account the nonuniform distribution of electric currents observed in the photosphere and its role in the coronal magnetic structure. We carry out the comparison for AR 6615, a complex region observed with the VLA on 1991 May 7. Under the assumption that the microwave emission is dominated by optically thick gyroresonance radiation, we may use the radio images to infer the temperature of the corona at different heights and locations. This is then compared with heating models based on the observed current distribution. We are able to reproduce the radio images remarkably well with a model in which temperature is structured along magnetic field lines, depends on the current on the field line, and increases with height in a manner similar to that inferred from static heated loop models. This result implies a direct link between electric currents and coronal heating.

Transient microwave brightenings (TMBs) are small-scale energy releases from the periphery of sunspot umbrae, with a flux density two orders of magnitude smaller than that from a typical flare. Gopalswamy et al (1994) first reported the detection of the TMBs and it was pointed out that the radio emission implied a region of very high magnetic field so that the emission mechanism has to be gyroresonance or nonthermal gyrosynchrotron, but not free-free emission. It was not possible to decide between gyroresonance and gyrosynchrotron processes because of the low time resolution (30 s) used in the data analysis. We have since performed a detailed analysis of the Very Large Array data with full time resolution (3.3 s) at two wavelengths (2 and 3.6 cm) and we can now adequately address the question of the emission mechanism of the TMBs. We find that nonthermal processes indeed take place during the TMBs. We present evidence for nonthermal emission in the form of temporal and spatial structure of the TMBs. The fast time structure cannot be explained by a thermodynamic cooling time and therefore requires a nonthermal process. Using the physical parameters obtained from X-ray and radio observations, we determine the magnetic field parameters of the loop and estimate the energy released during the TMBs. The impulsive components of TMBs imply an energy release rate of 1.3 x 10^22 erg/s so that the thermal energy content of the TMBs could be less than 10^24 erg., 15 pages (Latex), 4 figures (eps). ApJ Letters in press (1997)

We present evidence of the detection of microwave emission at 17 GHz in association with coronal X-ray jets. We present two typical cases—one on the disk (1995 March 31) and the other at the limb (1992 August 25). For the disk event we see 17 GHz emission from the upper part of the jet base (active region loop or loops), but no emission from the collimated X-ray jet itself, implying that it must be optically thin at 17 GHz. For the limb event, we see the base of the jet as well as the bottom part of the jet itself, implying that the optical depth is higher at the bottom part (obviously because of higher electron density) than at the top. We believe that the 17 GHz emission is thermal, because it is gradual and unpolarized, and that the heating process that gives rise to the jet X-ray plasma also results in the 17 GHz emission. The calculated 17 GHz flux densities seem to agree with the observed values within a factor of 2. We consider this disagreement to be quite reasonable in view of the various uncertainties involved in computing the emission in both radio and X-rays.

On 1994 July 31, a fast (900 km s-1) eruptive structure was observed in X-rays, followed by a slower plasmoid (180 km s-1). They were associated with a coronal mass ejection, prominence eruption, and a host of metric radio bursts. The X-ray structure seems to be a part of a white light coronal mass ejections (CME), as inferred from the white light images of July 30 and 31. A type II burst was observed at the leading edge of the X-ray eruption, while a type IV burst was spatially associated with the detached plasmoid. The type III radio bursts occurred on thin overdense structures associated with the eruption. We detected the rise of plasma levels because of mass addition to the type III burst sources as a result of the eruption. This event further clarifies the manifestation of a CME in X-rays. We identify the X-ray eruption as the driver of the coronal shock wave. This provides answer to the long-standing question regarding the origin of coronal and interplanetary shock waves. We have also found evidence to support the idea that herringbone bursts are produced when the coronal shock wave crosses open magnetic field lines.

We report on the observations of an X-ray coronal mass ejection (CME) with its three part structure: frontal loop, coronal cavity, and the eruptive prominence core. The prominence core was observed in microwaves, and the frontal loop was observed in X-rays. A coronal volume much larger than that occupied by the prominence seems to be affected by the eruption. Formation of an arcade structure was also observed beneath the erupting prominence. X-ray enhancement at the arcade persisted for several hours similar to long decay events. At the apex of the arcade there was a bright knot, which we interpret as the reconnection region from which the filament gets detached. We determined the trajectories of the frontal loop and the prominence core and found them to have very different characteristics. The CME showed an extremely small acceleration, while the prominence had a linear motion in the beginning followed by an exponential rise. However, during the several hours of simultaneous observation, the prominence did not catch up with the frontal loop. We determined the evolution of the CME mass, which increased by a factor of 4 during our observations. We discuss the implications of the observations in the general context of coronal mass ejections.

In this paper, we report the detection of a metric continuum burst source at the top of a coronal loop observed in soft X-rays. The continuum burst was probably a flare continuum that lasted more than 1 hr. This is the first observation of such metric continuum emission produced by energetic electrons with high-spatial resolution imaging instruments in both X-rays and radio. The nonthermal radio emission appears to be associated with the rupture of a part of the loop top and the ejection of soft X-ray plasma at the top of the coronal loop. We have also identified X-ray coronal structures in which type III emitting electron beams propagate. The metric continuum is most likely caused by second-harmonic plasma emission, and the electron density in the soft X-ray structure where type IIIs are observed is close to the critical plasma density derived from radio observations.

We present the first high spatial resolution images of a solar flare at millimeter wavelengths. On 1994 August 17, a GOES soft X-ray class M1 flare was observed by the Berkeley-Illinois-Maryland Array at 86 GHz by the Nobeyama 17 GHz array and by the Yohkoh spacecraft. The flare displayed both a prominent impulsive phase in microwaves and a gradual phase that lasted over 30 minutes. The millimeter data were taken only during the gradual phase. The millimeter images show a source with a size of ~8'', a peak brightness temperature of ~106 K, and maximum optical depth of 0.09. At both X-ray and radio wavelengths, the emitting region appeared to be compact (20''). In soft X-ray, the images are resolved into two sources: one located at a footpoint and the other at the top of the flaring loop. The millimeter emission is consistent with the predicted free-free flux from an isothermal temperature (~14 MK) loop-top source, a multitemperature footpoint source with a hot (~22 MK), and a cold (~12 MK) component. Most (80%) of the millimeter flux density originates from the top of the magnetic loop, and the footpoint contribution is only 20%.

We report on the detection of a large-scale radio structure and plasma flow associated with a bright point flare observed on 1993 July 11. The bright point (BP) flare was simultaneously imaged by the Nobeyama radioheliograph at 17 GHz and the Soft X-Ray Telescope on board the Yohkoh mission. The microwave emission consists of a large-scale structure and a compact moving source. The large-scale component seems to be the radio counterpart of large-scale loop structures sometimes observed in association with BP flares in X-rays. The compact source moved from the location of the X-ray BP flare with a speed of about 60 km s-1, which suggests a plasma flow. Spatial comparison between X-ray and radio data shows that the BP flare had different manifestations in the two wavelength domains. The emission peaks in the two wavelength domains did not coincide, which suggests cool plasma flow along the large-scale radio structure. We were able to determine the temperature and emission measure of the BP flare plasma from the X-ray data, and thus we computed the expected radio flux from the X-ray-emitting plasma. We found that the computed radio flux was much smaller than the total observed radio flux.

X-ray bright points (XBPs) are known to show variability on a number of timescales, including impulsive X-ray brightenings. The relationship between these XBP 'flares' and normal flares is poorly known. A fundamental question is whether nonthermal acceleration of particles takes place in XBP flares. We address this issue by seravhing for nonthermal radio emission at metric wavelengths from flaring XBPs identified in Yohkoh/SXT data. Unequivocal evidence for type III-like radio bursts usually attributed to beams of nonthermal electrons on open field lines was found recently by Kundu et al. This suggests that XBP flares are similar to normal flares and can indeed accelerate nonthermal populations of energetic particles. Here we provide further evidence of metric type III bursts from flaring XBPs located near active regions.

On five occasions between 1992 June 29 and 1994 May 3, we have used the Australia Telescope Compact Array (ATCA) to image Eta Carinae at a wavelength of 3 cm and a resolution of 1 arcsec. These observations have revealed remarkable activity. Since 1992 June, the total flux density has increase from 0.8 to 2.2 Jy, and the original single compact source has grown to a complex of sources spread over an area of about 16 sq arcsec. Strong hydrogen recombination-line spectral emission has appeared at the site of the strongest of these new sources. This recombination emission has the largest spectral width ever observed from a star, +/- 250 km/s, and reveals gas with turbulent velocities as great as 250 km/s approaching us at an average velocity of about 200 km/s. We believe that this radio outburst has been caused by a more than threefold increase of ultraviolet luminosity, and consequent ionization of previously neutral gas clouds.

We report observations of transient microwave (2 cm) brightenings and their relationship with brightenings in soft X-rays. The peak flux of the microwave brightenings observed by the Very Large Array (VLA) is smaller than the previously reported fluxes by two orders of magnitude. The microwave sources were highly polarized (up to 100%) and were situated on the periphery of a sunspot umbra. Among the many transients observed in X-rays by Yohkoh, two were observed simultaneously in microwaves. The microwave sources were found to be closer to the umbra of the sunspot than were the X-ray loops. It seems that the microwave sources are located at the footpoints of the looplike X-ray transients. Using the combined VLA, Yohkoh, and Mees data set, we determine the physical parameters of the loop in which the brightenings occur. We find that an increase in emission measure accompanied by small-scale heating can account for the X-ray brightening. The microwave emission can be interpreted as thermal gyroresonance or nonthermal gyrosynchrotron processes during the X-ray brightening. The magnetic field in the microwave-source region is found to be 1200-1800 G. The observations also provide evidence for temperature gradient in the coronal loops.

Using observations made with the Nobeyama radio heliograph (NRH) at 17 GHz and the Yohkoh/SXT experiment, we report the first detection of 17 GHz signatures of coronal X-ray-bright points (XBPs). This is also the first reported detection of flaring bright points in microwaves. We have detected four BPs at 17 GHz out of eight identified in SXT data on 1992 July 31, for which we looked for 17 GHz emission. For one XBP located in a quiet mixed-polarity region, the peak times at 17 GHz and X-rays are very similar, and both are long-lasting-about 2 hr in duration. There is a second BP (located near an active region) which is most likely flaring also, but the time profiles in the two spectral domains are not similar. The other two 17 GHz BPs are quiescent with fluctuations superposed upon them. For the quiet region XBP, the gradual, long-lasting, and unpolarized emission suggests that the 17 GHz emission is thermal.

We present high spatial resolution radio observations of the peculiar southern star Eta Carinae, made with the Australian Telescope. The images, at 8 and 9 GHz with a resolution of 1.0 arcsec show a source of dimension 10 arcsec and total flux of 0.7 Jy dominated by a strong central peak. The radio emission is unpolarized and offers no support to models which invoke degenerate stars or more exotic objects within the core of Eta Car. In these data we find no evidence for more than one energy source in the core with arcsecond separations as some infrared observations have suggested. Several levels of structure are evident in the radio image, which shows symmetry on the larger scales. Conventional formulae for stellar wind radio sources give a mass loss rate of order 3 x 10(exp -4) Solar Mass/yr based on the radio flux in the central peak, which yields a wind momentum flux of order 20% of the momentum flux available from the star's radiation field. The radio emission at these frequencies is consistent with thermal emission from gas flowing away from a 'luminous blue variable' star (LBV) Eta Car is probably the brightest thermal stellar wind radio source in the sky.

X-ray-bright point (XBPs) are known to show variability on a number of timescales, including impulsive X-ray brightenings. The relationship between these XBP 'flares' and normal solar flares is poorly known. A fundamental question is whether nonthermal acceleration of particles takes place in XBP flares. We address this issue by searching for nonthermal radio emission at metric wavelengths from flaring XBPs identified in Yohkoh soft x-ray telescope (SXT) data. Unequivocal evidence for type III-like radio bursts, usually attributed to beams of nonthermal electrons on open field lines, is found. This suggests that XBP flares are similar to normal flares and can indeed accelerate nonthermal populations of energetic particles.

We have found possible radio signatures of a coronal disconnection event observed by the Solar Maximum Mission Coronagraph/Polarimeter. The radio emission seems to be produced by charged particles accelerated during the same reconnection process which is responsible for the disconnection event. We discuss the implications of the radio emission characteristics to the reconnection process.

We study the nonlinear dynamics of particle acceleration in solar flares by analyzing the time series of various quasi-periodic radio signatures during flares. In particular we present the radio and hard X-ray data of three flares which support the following tentative conclusions:1.Particle acceleration and injection into magnetic structures occurs intrinsically in a pulsed mode (with a typical period of 1-2 s), produced by a single, spatially coherent, nonlinear system, rather than by a stochastic system with many spatially independent components (“statistical flare” produced by a fragmented primary energy release).2.The nonlinear (quasi-periodic) mode of pulsed particle acceleration and injection into a coronal loop can be stabilized by phase locking with an MHD wave (oscillation) mode, if both periods are close to each other.3.Pulsed injection of electron beams into a coronal loop may trigger nonlinear relaxational oscillations of wave-particle interactions. This is particularly likely when the limit cycles of both systems are similar.Subject headings: acceleration of particles— Sun: corona — Sun: flares — Sun: radio radiation — Sun: X-rays, gamma rays

We report on an observation of AU Mic taken with the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope. The data consist of a rapid sequence of spectra covering the wavelength range 1345-1375 A with a spectral resolution of 10,000. The observations were originally intended to search for spectral variations during flares. No flares were detected during the 3.5 hr of monitoring. A method of reducing the noise while combining the individual spectra in the time series is described which resulted in the elimination of half of the noise while rejecting only a small fraction of the stellar signal. The resultant spectrum was of sufficient quality to allow the detection of emission lines with an integrated flux of 10(exp -15) ergs/sq cm(sec) or greater. Lines of C I, O I, O V, Cl I, and Fe XXI were detected. This is the first indisputable detection of the 1354 A Fe XXI line, formed at T approximately = 10(exp 7) K, on a star other than the Sun. The line was well resolved and displayed no significant bulk motions or profile asymmetry. From the upper limit on the observed line width, we derive an upper limit of 38 km/s for the turbulent velocity in the 10(exp 7) K plasma. An upper limit is derived for the flux of the 1349 A Fe XII line, formed at T approximately = 1.3 x 10(exp 6) K. These data are combined with contemporaneous GHRS and International Ultraviolet Explorer (IUE) data to derive the volume emission measure distribution of AU Mic over the temperature range 10(exp 4)-10(exp 7) K. Models of coronal loops in hydrostatic equilibrium are consistent with the observed volume emission measures of the coronal lines. The fraction of the stellar surface covered by the footprints of the loops depends upon the loop length and is less than 14% for lengths smaller than the stellar radius. From the upper limit to the estimated width of the Fe XXI line prfile we find that the we cannot rule out Alfven wave dissipation as a possible contributor to the required quiescent loop heating rate.

We obtained simultaneous images of solar plage on 1991, May 7 with SERTS, the VLA,4 and the NASA/National Solar Observatory spectromagnetograph at the NSO/Kitt Peak Vacuum Telescope. Using intensity ratios of Fe XVI to Fe XV emission lines, we find that the coronal plasma temperature is (2.3-2.9) x 10 exp 6 K throughout the region. The column emission measure ranges from 2.5 x 10 exp 27 to l.3 x 10 exp 28 cm exp -5. The calculated structure and intensity of the 20 cm wavelength thermal bremsstrahlung emission from the hot plasma observed by SERTS is quite similar to the observed structure and intensity of the 20 cm microwave emission observed by the VLA. Using the Meyer (1991, 1992) revised coronal iron abundance, we find no evidence either for cool absorbing plasma or for contributions from thermal gyroemission. Using the observed microwave polarization and the SERTS plasma parameters, we calculate a map of the coronal longitudinal magnetic field. The resulting values, about 30-60 G, are comparable to extrapolated values of the potential field at heights of 5000 and 10,000 km.

We report the results of a radio survey of the Hyades open stellar cluster carried out with the Very Large Array at 1.5 GHz. Seventeen fields containing over 150 catalogued stars were mapped down to a limiting sensitivity ranging from 0.3 mJy at the centers of the fields to 0.9 mJy at a distance of 20' from field centers. Only two stars were detected as radio sources: the evolved spectroscopic binary V471 Tau, consisting of a white dwarf and a red dwarf; and the apparently pre-main-sequence G+K star spectroscopic binary HD 27130. The lack of detection of any single stars in the Hyades is consistent with other observations showing that most dwarf stars are no longer active by the time they reach the age of the Hyades, presumably because they have spun down

The mass of a coronal mass ejection (CME) is estimated using meter-decametric observations obtained with the Clark Lake multifrequency radioheliograph. Mass estimates in the past were made using coronagraph and white-light photometer observations. Since the radiation at radio and optical wavelength regimes has different physical origins, the radio method can provide an independent check on the mass estimates. The estimate of the 1986 February 16 CME using the radio method is close to the average value of CME masses reported in the literature.

High dynamic range, multifrequency radio observations of a solar active region are presented. The evolution of the region is followed at 5 GHz as it rotates from the limb to disk center, and when it is at disk center, observations at 0.33, 1.5, 5, 8.4, and 15 GHz are used to analyze the distribution of density and magnetic field within the active region. A dynamic range of up to 1500 (at 8.4 GHz) was achieved because these data were well suited to the technique of self-calibration

Observations of an impulsive solar radio burst from three observatories are presented. The striking observational aspects or this flare are that the time profile was identical throughout at 8.6, 15, and 86 GHz, that the spectrum was apparently flat from 15 to 86 GHz, and that there was a sharp cutoff in the spectrum between 5.0 and 8.6 GHz. The simplest interpretation of the cutoff, namely as a plasma frequency effect, leads to the conclusion that there was exceptionally high-density material in the solar corona (∼5 × 10 11 cm -3 )

Results are reported from an experimental study of the characteristics of large-scale coronal structures such as active regions, plages, filaments, and coronal holes using data obtained with the VLA at 1.5 GHz during the period September 11-17, 1988. The radio data were supplemented with He 10830- A, H-alpha, and Calcium-K spectroheliograms. A statistical analysis of some of the characteristics of the active regions is performed. Most of the active region sources were found to be about 100 arcsec in size, with bridges between regions common; lower brightness temperature regions showed a higher degree of polarization in general. The maximum polarization was found at the edge of active regions but well within the associated plages. The degree of polarization from bright active regions was small (not more than 20 percent), in agreement with previous results. Evidence was found for compression of preexisting flux by the emerging flux from a new region, which took place in the apparent absence of magnetic reconnection.