Engineering topics

Kundu, M. R.

Publications and source records attributed to Kundu, M. R..

At least 109 records · Page 6

NASA NTRS1984

Recent advances in solar radio physics

High spatial resolution microwave observations of solar active regions, coronal loops and flares are reviewed. Observations of preflare active regions are presented; in particular the interpretations of reversal of polarization at the flare site and the role of newly emerging flux in triggering the onset of flares are considered. The spatial locations of microwave burst emitting regions are discussed; loops or arcades of loops appear to be the sites of flare energy release in microwave bursts. Direct observational evidence of magnetic reconnection as the primary cause of acceleration of electrons in microwave bursts is provided.

Kundu, M. R.↗

NASA NTRS1984

Sharp edges in solar microwave spectra - Neutral current sheets or cyclotron lines?

Two solar active regions have been mapped using the VLA at three closely spaced frequencies (4496, 4716, and 4996 MHz) in an attempt to determine the origin of the steep spectra (index gamma equal to about -5 to -8) sometimes observed with large single telescopes. One of the regions observed indeed shows an anomalously large slope (gamma equal to about -6) compared to the usual (gamma equal to about -2 to -2.5). The other region shows a similar slope (gamma equal to about -5) but with a larger range of statistical error. Two possible explanations for such steep edges in solar spectra are (1) transmission effects of neutral current sheets, and (2) the appearance of cyclotron lines. The internal evidence of the microwave maps and simultaneous optical observations favor an explanation in terms of cyclotron lines.

Schmahl, E. J.↗

NASA NTRS1984

The spatial distribution of 6 centimeter gyroresonance emission from a flaring X-ray loop

Simultaneous high resolution soft X-ray, and 6-cm images of the decay phase of an M3 X-ray flare in Hale Region 16413 have been compared. The X-ray images were obtained with a sounding rocket flown on November 7, 1979 and the 6-cm observations were made with the VLA. A large loop system approximately 1.3 arc min in length, with an average temperature of about 8,000,000 K made up the X-ray flare structure. The peak 6-cm emission seemed to originate from a region below the X-ray loop, and its predicted flux due to thermal bremsstrahlung was approximately one order of magnitude less than observed. The expected gyroresonance absorption is examined through a loop geometry model, and it is found that thermal gyroresonance emission requiring large azimuthal or radial field components, or non-thermal gyrosynchrotron emission which involves continuous electron acceleration, may explain the observations.

Kahler, S. W.↗

NASA NTRS1984

VLA observations of fine structures in a solar active region at 6 centimeter wavelength

A complex solar active region at 6 cm has been observed with the VLA. Radio images of the region have been constructed and compared with simultaneous optical data from the region. It is found that all radio sources are associated with either sunspots or regions of the corona where strong transverse magnetic fields are likely, that the transverse field sources are brighter than the sunspot sources, and that some of the sunspot sources have a polarized ring structure. The observations are interpreted in terms of thermal gyroresonance emission, with the radiation from the transverse field sources being at the third harmonic of the gyrofrequency and predominantly at the second harmonic from the sunspot sources.

Mcconnell, D.↗

NASA NTRS1984

Microwave, soft and hard X-ray imaging observations of two solar flares

A set of microwave and hard X-ray observations of two flares observed simultaneously with the Very Large Array (VLA) and the Solar Maximum Mission Hard X-ray Imaging Spectrometer (SMM-HXIS) are presented. The LVA was used at 6 cm to map the slowly varying and burst components in three neighboring solar active regions (Boulder Nos. 2522, 2530, and 2519) from approximately 14:00 UT until 01:00 UT on June 24-25, 1980. Six microwave bursts less than 30 sfu were observed, and for the strongest of these, two-dimensional 'snapshot' (10 s) maps with spatial resolution of 5 in. were synthesized. HXIS data show clear interconnections between regions 2522 and 2530. The X-ray observations present a global picture of flaring activity, while the VLA data show the complexity of the small magnetic structures associated with the impulsive phase phenomena. It is seen that energy release did not occur in a single isolated magnetic structure, but over a large area of intermingled loop structures.

Kundu, M. R.↗

NASA NTRS1984

Interpretation of microwave active region structures using SMM soft X-ray observations

A new kind of sunspot-associated microwave source was detected by Alissandrakis and Kundu (1982). The observed emission came predominantly from a ring structure with a size between that of the umbra and the penumbra. It was thought that the newly discovered structure could be the product of the low gyroresonance opacity when the angle between the magnetic field and the line of sight is close to zero. The present investigation has the objective to study the roles played by the geometrical effect of gyroresonance absorption and by the low-temperature flux tubes above the relevant sunspot, taking into account 6 cm observations made with the aid of a radio telescope and X-ray observations conducted with the X-ray Polychromator (XRPO on the Solar Maximum Mission) (SMM) satellite.

Strong, K. T.↗

NASA NTRS1984

A study of the evolution of energetic electrons in a solar flare

A study of the impulsive microwave and hard X-ray emissions from the June 25, 1980 solar flare notes that its light curves are consistent with the flaring of the regions in unison rather than sequentially, or in an uncorrelated manner. The maximum 6-cm flux, which occurs 1.4 min after the maximum in the integrated 28-498 keV X-ray emission, and coincides with a secondary peak in the X-ray light curve, is explained by the observation that the X-ray spectrum, and hence that of the electrons, is harder at the time of the 6-cm flux maximum. The results show a clear correlation between the X-ray flux greater than 100 keV and the microwave flux. The steep, low-frequency spectrum at the time of the 6-cm maximum indicates that the emission at this time is nonthermal. The spectral behavior of the hard X-ray emission is not consistent with the betatron acceleration mechanism.

Holman, G. D.↗

NASA NTRS1984

Radio and visible-light observations of a coronal arcade transient

Simultaneous visible-light and radio observations of a coronal transient that occurred on April 9, 1980 are discussed. Visible-light observations of the transient and the associated erupting prominence were available from the Coronagraph/Polarimeter carried aboard SMM, the P78-1 coronagraph, and from the Haleakala Observatory. Radio observations of the related type III-II-IV bursts were available from the Clark Lake and Culgoora Observatories. The transient was extremely complex; it is suggested that an entire coronal arcade rather than just a single loop participated in the event. Type III burst sources observed at the beginning of the event were located along a nearby streamer, which was not disrupted, but was displaced by the outmoving loops. The type II burst showed large tangential motion, but, unlike such sources usually do, it had no related herringbone structure. A moving type IV burst source can be associated with the most dense feature of the white-light transient.

Gergely, T. E.↗

NASA NTRS1984

Observations of solar radio bursts from meter to kilometer wavelengths

New results obtained with the Clark Lake multifrequency radioheliograph at meter-decameter wavelengths and from satellite multifrequency directive observations at hectometer and kilometer wavelengths are reviewed. Evidence is presented that type III electrons propagate in dense coronal streamers and that frequently observed microbursts (presumably type III) at meter-decameter wavelengths are due to plasma radiation. Observations of hectometer and kilometer type III radio storms which reveal information about active region structures, the interplanetary magnetic field configuration, and solar wind acceleration are discussed. Kilometer type II bursts and interactions between type III electrons and interplanetary shocks are examined, and some new results on shock-associated events are presented.

Kundu, M. R.↗

NASA NTRS1984

A multiple type-II burst associated with a coronal transient and its MHD simulation

A large coronal transient took place on May 8, 1981. The transient was related to an M7.7/2B flare and was associated with at least two coronal type-II bursts. The velocities of the type-II bursts were in the range 1100-1800 km/s, in excess of the transient velocity of 1000 km/s. Two dimensional positions of the type-II radio sources are available from both the Clark Lake and the Culgoora Radio Observatories. Two dimensional MHD simulations of the event are carried out, taking into account the observed velocity, position, and size of the type-II bursts. The multiple shocks observed during the event and their interaction are simulated, and results of the simulation are discussed.

Gergely, T. E.↗

NASA NTRS1983

VLA observations of a solar active region and coronal loops

In May 1981, observations of a solar active region at 6 cm and 20 cm were conducted with the aid of a Very Large Array (VLA). It was found that the 6 cm emission was associated with sunspots and transverse magnetic fields between spots. The 20 cm emission came from elongated sources connecting regions of opposite magnetic polarity. The sources of the emission at 20 cm could be identified as coronal loops. A model for hydrostatic loops proposed by Rosner et al. (1978) was employed to determine a likely mechanism for the emission at 20 cm. It was found that a two-component model of bremsstrahlung emission from the feet and gyroresonance emission from the top of the loops can satisfactorily explain the observations.

Mcconnell, D.↗

NASA NTRS1983

X-ray and microwave observations of active regions

Coordinated high-resolution (1-3 arcsec) observations of two active solar regions (H 421 and H 419) on November 16, 1979, are reported: soft-X-ray filtergrams from a sounding rocket flight, VLA total-intensity and circular-polarization microwave (6-cm) radio maps, KPNO full-disk photospheric magnetograms, and BBSO H-alpha data. The images were converted to 4.8-arcsec/mm-scale transparencies and coaligned on the basis of sunspot positions for comparison. The two active regions are characterized in detail, and intensity, size, and polarization data for the brightest microwave components (BMC) are listed. It is found that 19 of the 32 BMC are farther than 5 arcsec from any sunspot, and that X-ray-emitting structures only rarely correspond to sunspots, or BMC. About one third of the BMC are located at the feet or legs of coronal loops smaller than about 50,000 km. The limitations implied by these obervations for proposed thermal-bremsstrahlung, thermal-gyro-resonance, and nonthermal microwave-emission mechanisms are discussed.

Webb, D. F.↗

NASA NTRS1983

A coronal transient associated with a high-speed type II burst

Several characteristics of a white light transient observed by the ATM Coronagraph experiment aboard Skylab are discussed. The transient was associated with a very high speed type II radio burst. The observed plane of the sky speed of the burst was 4900 km/s, 6-7 times higher than the speed of the transient itself. Based on the relative speeds of the transient and the radio burst, it is argued that the type II disturbance must have propagated in a still undisturbed corona, at an approximately 35-deg angle to the preexisting streamer and along an essentially open field configuration. The electron density gradients derived from the radio and the white light observations are compared, and it is shown that they are in agreement within the limits of the observations. The assumption that the type II was a weak MHD shock leads to large values for the coronal magnetic field. The possibility that the type II radition originated in a strong shock, with a magnetic Mach number greater than 2, is considered.

Gergely, T. E.↗

NASA NTRS1983

Ground based solar radio observations during solar maximum mission

The Very Large Array (VLA) and the Westerbork Synthesis Radio Telescope (WSRT) were used for making aperture synthesis maps of solar active and flaring regions. Observations of the Flare buildup in the form of increased intensity and polarization were made. Ring structure associated with sunspots were interpreted as due to the existence of cool material above the spot. Model computations were performed to explain the total intensity and polarization structures of a continuous set of active region maps.

Kundu, M. R.↗

NASA NTRS1983

First results from the Clark Lake Multifrequency Radioheliograph

The characteristics of the Clark Lake Multifrequency Radioheliograph, which is presently being used for solar research in the frequency range 20-125 MHz, are examined and several solar maps obtained with this instrument are presented. Two-dimensional images of the sun's corona at several frequencies within this range are examined, which were obtained with time resolution as good as 1.28 s. It is found that the shape of the sun's outer corona as observed at meter-decameter wavelengths appears to change in time periods as short as a few seconds.

Kundu, M. R.↗

NASA NTRS1983

Observations of H-alpha and microwave brightening caused by a distant solar flare

Synthesized maps with integration times of 10 and 30 sec, based on the observation of three subflares at 6 cm and H-alpha 6563 A, indicate that most of the 6 cm burst emission originated in 10-15 arcsec features coincident with, or adjacent to, H-alpha flare kernels. During the onset of one of the subflares, 6 cm emission was discovered in a loop stretching over 100,000 km from the primary flare site in association with H-alpha flare-like brightness at the remote footpoint of the loop. Assuming a primary flare site origin for the energy of the distant brightening, about 4 x 10 to the 24th ergs/sec propagated along the connecting magnetic loop at a velocity of more than 6000 km/sec. It is suggested that the energy may have been carried by electrons originating in the high energy tail of the electron thermal velocity distribution, escaping from the primary flare site.

Kundu, M. R.↗

NASA NTRS1983

Thermal and nonthermal phenomena in solar flare loops at 20 cm wavelength and in X-rays

X-ray images from the P78-1 satellite are presented for a long-lasting burst at 20 cm wavelength mapped with the Very Large Array on May 19, 1979. The decimeter wave observations were originally interpreted in terms of two models, one invoking thermal electrons radiating at low harmonics of the gyrofrequency, and the other invoking mildly relativistic electrons emitting gyrosynchrotron radiation. If the 20 cm source is thermal, it should also be visible in soft X-rays, while if it is nonthermal, the soft X-ray emission should be weak or spatially or temporally distinct from the 20 cm burst. It is found that only one of the three 20 cm sources was approximately co-spatial with the soft X-ray source, and that it was only partially thermal. The 20 cm burst is therefore primarily decimeter type IV emission from mildly relativistic electrons of the post-flare phase. The over 2 hr lifetime and smooth temporal variation of the burst belie its nonthermal nature and suggest continuous acceleration as well as long-term storage of energetic electrons.

Schmahl, E. J.↗

NASA NTRS1982

Advances in solar radio astronomy

The status of the observations and interpretations of the sun's radio emission covering the entire radio spectrum from millimeter wavelengths to hectometer and kilometer wavelengths is reviewed. Emphasis is given to the progress made in solar radio physics as a result of recent advances in plasma and radiation theory. It is noted that the capability now exists of observing the sun with a spatial resolution of approximately a second of arc and a temporal resolution of about a millisecond at centimeter wavelengths and of obtaining fast multifrequency two-dimensional pictures of the sun at meter and decameter wavelengths. A summary is given of the properties of nonflaring active regions at millimeter, centimeter, and meter-decameter wavelengths. The properties of centimeter wave bursts are discussed in connection with the high spatial resolution observations. The observations of the preflare build-up of an active region are reviewed. High spatial resolution observations (a few seconds of arc to approximately 1 arcsec) are discussed, with particular attention given to the one- and two-dimensional maps of centimeter-wavelength burst sources.

Kundu, M. R.↗