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Publications and source records attributed to Kundu, M. R..
An introduction to the field of solar millimeter interferometry is given. Solar observations using the BIMA at Hat Creek since 1989 are reviewed along with other solar millimeter observations. The radiation mechanisms relevant to solar millimeter astronomy are discussed, and observations in the millimeter and microwave ranges are compared. The use of closure phases in arrays containing a small number of elements is examined.
The millimeter, microwave, and soft X-ray emission from a number of solar flares is compared in order to determine the properties of the HF radio emission of flares. The millimeter observations use a sensitive interferometer at 86 GHz which offers much better sensitivity and spatial resolution than most previous high-frequency observations. The 86-GHz emission onset appears often to be delayed with respect to the microwave onset. Even in large flares the millimeter-wavelength emission can arise in sources of only a few arc sec dimension. The millimeter emission in the impulsive phase does not correlate with the soft X-ray emission, and thus is unlikely to contain any significant thermal bremsstrahlung component. The electron energy distributions implied by the millimeter observations are much flatter (spectral indices of 2.5 to 3.6) than is usual for microwave or hard X-ray observations.
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-spatial-resolution observations of the sun which reflect on the role of mode coupling in the solar corona, and a number of new observations are presented. It is shown that typically, polarization inversion is seen at 5 GHz in active region sources near the solar limb, but not at 1.5 GHz. Although this is apparently in contradiction to the simplest form of mode coupling theory, it remains consistent with current models for the active region emission. Microwave bursts show no strong evidence for polarization inversion. Bipolar noise storm continuum emission is discussed in some detail, utilizing recent VLA observations at 327 MHz. It is shown that bipolar sources are common at 327 MHz. Further, the trailing component of the bipole is frequently stronger than the leading component, in apparent conflict with the 'leading-spot' hypothesis. The observations indicate that, at 327 MHz, mode coupling is apparently strong at all mode-coupling layers in the solar corona. The 327 MHz observations require a much weaker magnetic field strength in the solar corona to explain this result than did earlier lower-frequency observations: maximum fields are 0.2 G. This is a much weaker field than is consistent with current coronal models.
We have carried out high-spatial-resolution millimeter observations of solar flares using the Berkeley-Illinois-Maryland Array (BIMA). At the present time, BIMA consists of only three elements, which is not adequate for mapping highly variable solar phenomena, but is excellent for studies of the temporal structure of flares at millimeter wavelengths at several different spatial scales. We present BIMA observations made during the Gamma Ray Observatories (GRO)/Solar Max 1991 campaign in Jun. 1991 when solar activity was unusually high. Our observations covered the period 8-9 Jun. 1991; this period overlapped the period 4-15 Jun. when the Compton Telescope made the Sun a target of opportunity because of the high level of solar activity.
Recent results of radio observations of the solar and stellar coronae are reviewed. Attention is given to the results obtained on quiet-sun fine structures; the active region observations, with particular consideration given to the soft X-ray and radio comparisons of observations made during the Coronal Magnetic Structures Observing Campaign and the strength of the magnetic fields in the corona; the radio observations of solar flares; and the radio observations of stellar coronae of the RS Canum Venaticorum binaries, M dwarf stars, pre-main-sequence stars, and chemically-peculiar B stars. Also discussed are current issues in stellar radiophysics and perspectives in solar radio physics.
Flare blast waves and shocks piston driven by coronal mass ejections (CMEs) have been proposed to be responsible for generating type II radio bursts in the solar corona. The idea for piston-driven shocks came primarily from temporal association of shocks and CMEs. Our compilation of CME events with simultaneous radio observations with positional information supports idea of flare blast waves.
High-resolution observations are presented of two active regions at 6 and 20 cm over a period of 5 days, together with H-alpha and photospheric magnetic fields. The large-scale emission at 20 cm is associated with the H-alpha plage. In one region the strongest source was over the neutral line, near the tip of an active-region filament, which indicates that the emission probably originated in small-scale coronal loops. In the second region the peak of the emission was near a well-developed sunspot. Neither region showed evidence of large-scale loops joining their preceding and following parts. Several other sources were observed at 20 cm; a source associated with an H-alpha plage region crossed by a filament and one associated with a small bipolar region are briefly discussed. The 6-cm emission from a well-developed spot showed clearly the characteristics expected from gyroresonance model computations.
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.
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Microwave imaging data for a small flare with simultaneous hard X-ray spectral observations are compared. The X-ray data suggest that the power-law index delta of the energy distribution of the radiating electrons is 5.3 (thick-target) which differs significantly from the estimate (delta = 1.4) from a homogeneous optically-thin gyrosynchrotron model which fits the radio observations well. In order to reconcile these results, a double power-law energy spectrum is investigated 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 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 inhomogenous gyrosychrotron 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.
The first high-spatial-resolution interferometric observations of solar flares at millimeter wavelengths are presented. They are of high sensitivity, and events ranging from subflares to X-class flares were detected. One to three baselines with fringe spacings of 2 to 5 arcsec were available, which demonstrated that generally source sizes were in excess of 2 arcsec, but in some events the sources may be about 1 arcsec.
Observations of an X4 flare of May 19, 1984 using the VLA and the SMM X-ray Polychromator are discussed. The 6 cm radio source remained stationary throughout the decay phase. Combined with other evidence, this indicates that the flare was magnetically confined and did not disrupt the complex structure of its region. The observed structures may be associated with a neutral sheet connecting the two bipoles.
The first high spatial resolution interferometric observations of solar flares at millimeter wavelengths, carried out with the Berkeley-Illinois-Maryland Array are presented. The observations were made at 3.3 mm wavelength during the very active periods of March 1989, using one or three baselines with fringe spacings of 2-5 arcsec. The observations represent an improvement of an order of magnitude in both sensitivity and spatial resolution compared with previous solar observations at these wavelengths. It appears that millimeter burst sources are not much smaller than microwave sources. The most intense bursts imply brightness temperatures of over 10 to the 6th K and are due to nonthermal gyrosynchrotron emission or possibly thermal free-free emission. If the emission in the flash phase is predominantly due to gyrosynchrotron emission, thermal gyrosynchrotron models can be ruled out for the radio emission because the flux at millimeter wavelengths is too high.
The peak times of impulsive microwaves bursts are compared with those of shock-associated (SA) kilometric radio events. The first peaks in these two frequency regimes are usually well-correlated in time, but the last peaks of the SA events observed at 1 MHz occur an average of 20 min after the last impulsive microwave peaks. In some cases, the SA events overlap in time with the post-burst increases of microwave bursts; sometimes there is general correspondence in their intensity time profiles. These observations suggest that the earlier components of the SA events are usually caused by electrons accelerated in or near the microwave source region. The possibility that the later components of some SA events could be associated with nonthermal electrons responsible for microwave post-burst increases, although they have traditionally been attributed to electrons accelerated at type II burst producing shocks in the upper corona is discussed.
Measurements of magnetic fields in the sun's atmosphere using radio observations are discussed. Radio measurements of magnetic fields in the chromosphere and the corona are indirect. In the chromosphere, these are based upon the measurement of polarization of active regions at millimeter wavelengths. In the transition region and the corona, the properties of gyroresonance radiation of active regions at centimeter wavelengths are used. Total intensity and polarization maps and modeling are used to determine which harmonic (second or third) is predominant in the active region emission. A direct method of measuring coronal magnetic fields using cyclotron line emission is discussed, and the relevant data as presently available are presented.
Preliminary results of high-spatial-resolution millimeter observations of recent solar flares carried out with the Berkeley-Illinois-Maryland Array are presented. Findings obtained from multifrequency observations using the VLA are reported. The time profiles of several bursts during March 1989 are presented. Most of the bursts were impulsive and short-lived, and a number of longer-duration bursts were observed as well. All the bursts were fairly weak, with none exceeding 1 sfu of correlated amplitude. A number had rise times less than the available resolution of 10 sec. VLA observations in September 1988 showed that there is exceptionally good correspondence between the 1.5-GHz radio sources and the regions which are dark in He 10830 images and bright in Ca images.
Preliminary results of solar observations at 0.333 and 1.5 GHz made with the VLA during the September 11-17, 1988 period are presented. Generally, there are few structural changes in the active region sources from one day to the next, suggesting that structural evolution is relatively slow. Contour maps at 1.5 GHz are presented for each of the four days. Two noise storms were present at 0.33 GHz all week and were highly polarized.