On the reconciliation of simultaneous microwave imaging and hard X-ray observations of a solar flare     

N. Nitta  S. M. White  E. J. Schmahl  M. R. Kundu 《Solar physics》1991,132(1):125-136

We have compared microwave imaging data for a small flare with simultaneous hard X-ray spectral observations. The X-ray data suggest that the power-law index of the energy distribution of the radiating electrons is 5.3 (thick-target) which differs significantly from the estimate ( = 1.4) from a homogeneous optically-thin gyrosynchrotron model which fits the radio observations well. In order to reconcile these results, we explore a number of options. We investigate a double power-law energy spectrum for the energetic electrons in the flare, as assumed by other authors: the power law is steep at low energies and much flatter at the higher energies which produce the bulk of the microwaves. The deduced break energy is about 230 keV if we tentatively ignore the X-ray emission from the radio-emitting electrons: however, the emission of soft photons by the flat tail strongly contributes to the observed hard X-ray range and would flatten the spectrum there. A thin-target model for the X-ray emission is also inconsistent with radio data. An inhomogeneous gyrosynchrotron model with a number of free parameters and containing an electron distribution given by the thick-target X-ray model could be made to fit the radio data.  相似文献   

Imaging observations of radio bursts at meter-decameter wavelengths     

Mukul R. Kundu 《Solar physics》1982,113(1-2):87-94

In this article, we review some of the recent results obtained with imaging observations of the Sun at meter-decameter wavelengths, using the Clark Lake multifrequency radioheliograph. We briefly discuss the use of imaging observations to study the large scale structure of the upper corona. We discuss non-flare associated type II/type IV bursts associated with a coronal streamer disruption event associated with a slow (100 Km/s) CME. We discuss meter-decameter microbursts, which occur at coronal heights, often without any surface activity. Finally, we discuss a correlated type III burst whose emission originates almost simultaneously from two widely separated ( 10⁵ Km) locations.  相似文献   

Spatial Structure of Solar Coronal Magnetic Loops Revealed by Transient Microwave Brightenings     

Zhang  J.  Gopalswamy  N.  Kundu  M. R.  Schmahl  E. J.  Lemen  J. R. 《Solar physics》1998,180(1-2):285-298

We present the measurement of magnetic field gradient in magnetic loops in the solar corona, based on the multi-wavelength Very Large Array observations of two transient microwave brightenings (TMBs) in the solar active region 7135. The events were observed at 2 cm (spatial resolution 2=) and 3.6 cm (spatial resolution 3=) with a temporal resolution of 3.3 s in a time-sharing mode. Soft X-ray data (spatial resolution 2.5=) were available from the Soft X-ray Telescope on board the Yohkoh satellite. The three-dimensional structure of simple magnetic loops, where the transient brightenings occurred, were traced out by these observations. The 2-cm and 3.6-cm sources were very compact, located near the footpoint of the magnetic loops seen in the X-ray images. For the two events reported in this paper, the projected angular separation between the centroids of 2 and 3.6-cm sources is about 2.3= and 3.1=, respectively. We interpret that the 2 and 3.6-cm sources come from thermal gyro-resonance emission. The 2-cm emission is at the 3rd harmonic originating from the gyro-resonance layer where the magnetic field is 1800 G. The 3.6-cm emission is at the 2nd harmonic, originating from the gyro-resonance layer with a magnetic field of 1500 G. The estimated magnetic field gradient near the footpoint of the magnetic loop is about 0.09 G km=¹ and 0.12 G km=¹ for the two events. These values are smaller than those observed in the photosphere and chromosphere by at least a factor of 2.  相似文献   

Microbursts at meter-decameter wavelengths     

G. Thejappa  N. Gopalswamy  M. R. Kundu 《Solar physics》1990,127(1):165-183

We study the characteristics of microbursts using a large data base obtained with the multifrequency radioheliograph of the Clark Lake Radio Observatory. Most of the new observations were made during July 29, 1985 to August 2, 1985; we also include for statistical studies the microburst data used in our earlier studies. We perform a statistical analysis of many characteristics such as frequency drift, source size and brightness temperature and compare them with the properties of normal type III bursts. We investigate the coronal structures and surface activities associated with some of the events. We find that (i) the brightness temperature is in the range 6 × 10⁵ K to 6 × 10⁷ K; (ii) the drift rate of the microbursts is slightly smaller than that of normal type III bursts, implying electron beams with speeds 0.2c.We explore various theoretical interpretations of the observed low brightness temperatures. We show that the microbursts can be explained as due to spontaneously emitted Langmuir waves by electron beams whereas normal type III bursts are due to coherently emitted plasma waves in a two-stream instability. We estimate the range of number densities for electron beams responsible for microbursts.On leave from Indian Institute of Astrophysics, Bangalore, India.  相似文献   

UVSP and VLA observations of the 24 June 1980 flare: Asymmetric or isotropic beaming?     

M. R. Kundu  C. C. Cheng  E. J. Schmahl 《Solar physics》1990,129(2):343-349

Observations of the 15:22 UT flare of 24 June 1980 were made using the Very Large Array (VLA) at 6 cm wavelength simultaneously with the Hard X-ray Imaging Spectrometer (HXIS) aboard the Solar Maximum Mission. It was found that at the peak of the impulsive phase, the brightest microwave point appeared to lie between the soft (3.6–8.0 keV) and hard (22–30 keV) X-ray maxima, which were themselves separated by 20 (Kundu et al., 1984). Since the publication of these results, we have analyzed the imaging data from the Ultraviolet Spectrometer Polarimeter (UVSP) with the goal of narrowing the possible interpretations of the event. Like the VLA and HXIS, the UVSP observations provide information about the location of the primary electrons; the observations taken together suggest that the fast electrons were symmetrically distributed within the flare loop.  相似文献   

Filament eruption and storm radiation at meter-decameter wavelengths     

M. R. Kundu  N. Gopalswamy 《Solar physics》1990,129(1):133-152

We report the study of a weak noise storm observed by the Clark Lake multifrequency radioheliograph at four frequencies. The noise storm onset was associated with a filament eruption and a gradual rise and fall in soft X-rays. We compare the noise storm emission with related emissions in other wavelengths to develop a composite scenario of the event. Using the properties of the quiet corona inferred from the simultaneously observed quiet-Sun radiation, we estimate the brightness temperature of the storm continuum, which seems to be consistent with the observations reported in Solar Geophysical Data. Superthermal particles with a temperature that is ten times the coronal electron temperature and a density of 10^–3 times the coronal density are adequate to explain the observed radiation. Since the noise storm observations were made at four frequencies, we were able to obtain a brightness temperature spectrum of the storm radiation. If the storm radiation is affected in the same way as the quiet-Sun emission by inhomogeneities, the observed spectrum can be interpreted as due to propagation effects. Since the Clark Lake instrument can observe both quiet Sun and weak bursts simultaneously, we were able to estimate the propagation effects from the quiet-Sun observations and use it to correct the brightness temperature of storm radiation.  相似文献   

3.5mm Depression features associated with Hα ‘disparitions brusques’     

M. R. Kundu  P. Lantos 《Solar physics》1977,52(2):393-396

The characteristics of 3.5 mm depression features associated with two disparition brusques observed in H are discussed. The millimeter depressions still exist, although reduced in strength, after the disappearance of the H filament. The two depressions correspond to temperatures of 600 and 450 K before and to 200 and 250 K after the H filament disappearance.  相似文献   

VLA observations of ‘Compact’ sources in CTB 80     

T. Velusamy  M. R. Kundu 《Journal of Astrophysics and Astronomy》1983,4(4):253-260

High resolution (3 arcsec-4 arcsec) maps of three compact sources in CTB 80, observed with the Very Large Array at 6 and 20 cm wavelengths are presented. The central core consists of a limb-brightened shell (of diameter 30 arcsec) superimposed on a diffuse emission of size 75 arcsec x 45 arcsec. The compact sources 1949 + 324 and 1952 + 332 located near the tips of the southwest and northeast ridges are rather remarkable and possibilities for these to be fragments ejected by the SN are examined. 1949 + 324 is fully resolved by the 4 arcsec beam and shows structure with multiple components oriented perpendicular to the ridge. 1950 + 326 is a background radio source.  相似文献   

Simultaneous microwave observations of solar flares at 6 and 20 cm wavelengths using the VLA     

M. Melozzi  M. R. Kundu  R. K. Shevgaonkar 《Solar physics》1985,97(2):345-361

Using the Very Large Array, solar burst observations have been carried out simultaneously at 6 and 20 cm. Structural changes and preheating have been observed in the flaring regions on time scales of minutes to tens of minutes before the onset of the burst impulsive phase. The 6 cm burst sources are located close to the neutral line, or near the legs of a flaring loop. The 20 cm burst sources show complex and extended structures spatially separated from both the preburst emission and the gradual decay phase of the burst. We interpret the observations in terms of a two-component flare model (bulk heating as well as acceleration of particles) and derive the physical parameters of the burst sources.On leave of absence from Indian Institute of Astrophysics, Bangalore, India.  相似文献   

Twin tropical cyclones in the Indian Ocean: the role of equatorial waves     

Sujata K. Mandke  A. K. Sahai 《Natural Hazards》2016,84(3):2211-2224

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