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Gopalswamy, N.

Publications and source records attributed to Gopalswamy, N..

105 records · Page 6
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NASA NTRS1992

Estimation of the mass of a coronal mass ejection from radio 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.

Gopalswamy, N.↗
NASA NTRS1992

Millimeter and hard x ray/gamma ray observations of solar flares during the June 1991 GRO campaign

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.

Kundu, M. R.↗
NASA NTRS1992

Are coronal type II shocks piston driven?

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.

Gopalswamy, N.↗
NASA NTRS1991

Large-scale features of the sun at 20 centimeter wavelength

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.

Gopalswamy, N.↗
NASA NTRS1990

First high spatial resolution interferometric observations of solar flares at millimeter wavelengths

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.

Kundu, M. R.↗
NASA NTRS1990

The sun at the VLA's metric and decimetric wavelengths

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.

White, S. M.↗
NASA NTRS1989

The radio signatures of a slow coronal mass ejection - Electron acceleration at slow-mode shocks?

The evolution of a coronal mass ejection (CME) event observed on February 17, 1985 is studied using two-dimensional radio imaging observations along with simultaneously obtained coronagraph observations. This event shows that a slow CME can be associated with type II and type IV radio bursts. The implications of the spatial association of the radio bursts with the CME are discussed. It is argued that the CME is due to an instability of the large-scale magnetic field in a helmet streamer and that the radio bursts are some of the responses to this instability. The new feature of this event is the clear association of the moving type IV burst with a CME traveling slower than the coronal Alfven speed. The structure of slow shocks driven by such a CME is discussed, and it is shown that shock drift and diffusive acceleration are ineffective. An acceleration mechanism involving current-driven lower hybrid waves is proposed.

Kundu, M.↗
NASA NTRS1988

Millimeter wavelength observations of solar flares for Max 1991

The Hat Creek millimeter-wave interferometer (to be known as the Berkeley-Illinois-Maryland Array, BIMA) is being upgraded. The improved array will become available during the coming solar maximum, and will have guaranteed time for solar observing. The Hat Creek millimeter-wave interferometer is described along with the improvements. The scientific objectives are briefly discussed.

Kundu, M. R.↗
NASA NTRS1987

Two dimensional imaging observations of meter-decameter bursts associated with the February 1986 flare activity

An analysis is presented of the two dimensional imaging observations of a flare observed on 3 Feb. l986 using the Clark Lake Multifrequency Radioheliograph. The flare produced almost all types of Meter-decimeter radio emission: enhanced storm radiation, type III/V bursts, II and IV and flare continuum. The flare continuum had early (FCE) and late (FC II) components and the type II occurred during the period between these two components. Comparing the source positions of type III/V and FCE it was found that these bursts must have occurred along adjacent open and closed field lines, respectively. The positional analysis of type II and FC II implies that the nonthermal electrons responsible for FC II need not be accelerated by type II shock and this conclusion is further supported by the close association of FC II with a microwave peak. Using the positional and temporal analysis of all these bursts and the associated hard X-ray and microwave emissions, a schematic model is developed for the magnetic field configuration in the flaring region in which the nonthermal particles responsible for these bursts are confined or along which they propagate.

Gopalswamy, N.↗
NASA NTRS1987

Propagation of electrons emitting weak type III bursts in coronal streamers

The observations of weak type III bursts at 73.8, 57.5, 50.0, and 38.5 MHz from Clark Lake Radio Observatory on four days are reported and their characteristics are discussed. In addition to Clark Lake data, the magnetogram and sunspot/active region data and the coronal streamer data obtained by HAO's Coronagraph/Polarimeter aboard SMM satellite are used to study the location of the burst sources with respect to the coronal streamers emanating from active regions. It is shown that the bursts occur within or close to the edge of dense coronal streamers implying that the coronal streamers contain open magnetic field lines along which the electrons generating the bursts propagate. The positional analysis of the bursts is used to estimate the variation of coronal electron density with radial distance.

Gopalswamy, N.↗
NASA NTRS1987

Imaging observations of the evolution of meter-decameter burst emission during a major flare

The results of a study of the evolution of the February 3, 1986 flare at meter-decameter wavelengths are presented, using the two-dimensional imaging observations made with the Clark Lake multifrequency radioheliograph. The flare was complex and produced various types of meter-decameter bursts. The preflare activity was observed in the form of type III bursts some tens of minutes prior to the impulsive onset. From the positional analysis of the preflare and impulsive phase type III bursts and other measured characteristics, the characteristics of energy release and possible magnetic field configurations in the vicinity of the energy release region are discussed. From positional and temporal studies of the flare continuum and type II burst in relation to the microwave and hard X-ray emissions, the possible magnetic field structures in which the accelerated particles are confined or along which they propagate are discussed; a schematic model of the flaring region is developed, based upon this study.

Gopalswamy, N.↗
NASA NTRS1987

A study of solar preflare activity using two-dimensional radio and SMM-XRP observations

A study of type III activity at meter-decameter wavelengths in the preflare phase of the February 3, 1986 flare is presented, using data obtained with the Clark Lake Multifrequency Radioheliograph. This activity is compared with similar type III burst activity during the impulsive phase, and it is found that there is a displacement of burst sources between the onset and end times of the activity. A comparison of this displacement at three frequencies suggests that the type III emitting electrons gain access progressively to diverging and different field lines relative to the initial field lines. The energetics of the type III emitting electrons are inferred from observations and compared with those of the associated hard X-ray emitting electrons. The soft X-ray data from SMM-XRP show enhanced emission measure, density, and temperature in the region associated with the preflare type III activity.

Kundu, M. R.↗
NASA NTRS1987

Simultaneous radio and white light observations of the 1984 June 27 coronal mass ejection event

Two-dimensional images of radio bursts in the 25-50-MHz frequency range obtained with the SMM Coronagraph/Polarimeter and Mauna Loa K-coronameter during a coronal mass ejection event on June 27, 1984 are discussed. The present event is shown to be spatially and temporally associated with precursors in the form of meter-decameter type III bursts, soft X-ray emission, and a H-alpha flare spray. The results suggest gyrosychrotron emission from a plasmoid containing a magnetic field of about 2.5 G and nonthermal electrons with a number density of about 100,000/cu cm and energy of greater than about 350 keV.

Gopalswamy, N.↗