Engineering topics

Kundu, M. R.

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

At least 19 records

NASA NTRS2011

Microflares and the Statistics of X-Ray Flares

This review surveys the statistics of solar X-ray flares, emphasising the new views that Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has given us of the weaker events (the microflares). The new data reveal that these microflares strongly resemble more energetic events in most respects; they occur solely within active regions and exhibit high-temperature/nonthermal emissions in approximately the same proportion as major events. We discuss the distributions of flare parameters (e.g., peak flux) and how these parameters correlate, for instance via the Neupert effect. We also highlight the systematic biases involved in intercomparing data representing many decades of event magnitude. The intermittency of the flare/microflare occurrence, both in space and in time, argues that these discrete events do not explain general coronal heating, either in active regions or in the quiet Sun.

Hannah, I. G.↗

NASA NTRS2011

The Relationship Between Solar Radio and Hard X-Ray Emission

This review discusses the complementary relationship between radio and hard Xray observations of the Sun using primarily results from the era of the Reuven Ramaty High Energy Solar Spectroscopic Imager satellite. A primary focus of joint radio and hard X-ray studies of solar flares uses observations of nonthermal gyrosynchrotron emission at radio wavelengths and bremsstrahlung hard X-rays to study the properties of electrons accelerated in the main flare site, since it is well established that these two emissions show very similar temporal behavior. A quantitative prescription is given for comparing the electron energy distributions derived separately from the two wavelength ranges: this is an important application with the potential for measuring the magnetic field strength in the flaring region, and reveals significant differences between the electrons in different energy ranges. Examples of the use of simultaneous data from the two wavelength ranges to derive physical conditions are then discussed, including the case of microflares, and the comparison of images at radio and hard X-ray wavelengths is presented. There have been puzzling results obtained from observations of solar flares at millimeter and submillimeter wavelengths, and the comparison of these results with corresponding hard X-ray data is presented. Finally, the review discusses the association of hard X-ray releases with radio emission at decimeter and meter wavelengths, which is dominated by plasma emission (at lower frequencies) and electron cyclotron maser emission (at higher frequencies), both coherent emission mechanisms that require small numbers of energetic electrons. These comparisons show broad general associations but detailed correspondence remains more elusive.

White, S. M.↗

NASA NTRS2005

RHESSI and Radio Imaging Observations of Microflares E2.3-0011-04

We present the analysis of five microflares, three observed simultaneously by RHESSI in hard X-rays and Nobeyama RadioHeliograph (NoRH) in microwaves (17 GHz) and two observed by RHESSI and Nancay RadioHeliograph (NRH) at metric wavelengths (150-450 MHz). Since we have no radio imaging telescopes simultaneously operating at microwave and meter wavelengths in the same time zone, we are obliged to use a different set of microevents for comparison with metric wavelength counterparts in contrast to that used for comparison with metric wavelength counterparts in contrast to that used for comparison with microwave events. This is because we are interested in using the locations and other imaging characteristics of the events from both RHESSI and Nancay instead of just temporal correlation. Here we describe the properties of five events -- three in microwaves and two at metric wavelengths.

Kundu, M. R.↗

NASA NTRS1997

A multi-wavelength analysis of the February 6/7, 1997 coronal mass ejection

The coronal mass ejection (CME) event on 6 to 7 February 1997 that originate from the southwest quadrant of the sun are analyzed. The CME is accelerated from a projected speed of 170 km/s to about 650 km/s at 25 solar radii. The CME is an arcade eruption followed by bright prominence core structures. The CME was accompanied by an arcade formation. The X-ray and extreme ultraviolet observations suggest that the reconnection proceeded from the northwest and the southeast end of a filament channel. The CME event caused interplanetary effects that produced a medium size geomagnetic storm on earth. Issues relating to the origin and propagation of geo-effective solar disturbances are addressed.

Plunkett, S. P.↗

NASA NTRS1995

Metric type III bursts from a flaring X-ray bright point near an active region

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.

Kundu, M. R.↗

NASA NTRS1995

Intense radio outburst from the supermassive star Eta Carinae

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.

Duncan, R. A.↗

NASA NTRS1994

Transient microwave brightenings in solar active regions: Comparison between VLA and Yohkoh observations

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.

Gopalswamy, N.↗

NASA NTRS1994

Polarization features of solar radio emission and possible existence of current sheets in active regions

We show that it is possible to account for the polarization features of solar radio emission provided the linear mode coupling theory is properly applied and the presence of current sheets in the corona is taken into account. We present a schematic model, including a current sheet that can explain the polarization features of both the low frequency slowly varying component and the bipolar noise storm radiation; the two radiations face similar propagation conditions through a current sheet and hence display similar polarization behavior. We discuss the applications of the linear mode coupling theory to the following types of solar emission: the slowly varying component, the microwave radio bursts, metric type U bursts, and bipolar noise storms.

Gopalswamy, N.↗

NASA NTRS1994

Detection of 17 GHz radio emission from X-ray-bright points

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.

Kundu, M. R.↗

NASA NTRS1994

The radio source around Eta Carinae

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.

White, S. M.↗

NASA NTRS1994

Nonthermal processes in flaring X-ray-bright points

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.

Kundu, M. R.↗

NASA NTRS1994

Nonthermal radio emission associated with a coronal disconnection event

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.

Gopalswamy, N.↗

NASA NTRS1994

Non-radial magnetic field structures in the solar corona

We report on the structure and geometry of coronal magnetic fields inferred from the observations of meter-decimeter type 3 and moving type 4 radio bursts, associated with an H(alpha) flare. This is the first report of type 3 radio bursts from the Nancay radioheliograph after it acquired the two dimensional multifrequency capability. Dispersion of the radio source positions with frequency suggests that open and closed field lines are considerably inclined to the radial direction which is consistent with the connectivity observed in the magnetogram. We suggest that multiple arch systems are involved in the type 4 emission. From the polarization and dispersion characteristics of the type 4 source, we infer that the emission is due to fundamental plasma emission.

Gopalswamy, N.↗

NASA NTRS1994

Radio constraints on coronal models for dMe stars

Radio data are used to test coronal models for dMe stars. Specifically, we show that photospheric magnetic field observations imply that the low corona of a dMe star should be saturated by magnetic fields with an average strength in excess of 1 kG. In such fields the hot component of the corona detected in X-ray observations (temperature of order 2 x 10(exp 7) K) would be optically thick at least up to 15 GHz due to thermal gyroresonance opacity. The resulting emission would easily be detectable by radio observations and should have a radio spectrum rising in the microwave range. We have carried out observations to test this prediction, and in the majority of cases find that the observed fluxes at 15 GHz are too low to be consistent with the assumptions. In the few cases where the stars were detected at 15 GHz, the evidence indicates that the observed emission is nonthermal. These results imply that the hot component of the X-ray-emitting plasma in the corona is not coincident with the strong magnetic fields in the lower corona. Because the hot plasma must still be confined by closed magnetic field lines, it is likely to be restricted to heights of the order of a stellar radius above the photosphere. The results seem to imply a different genesis for the two components of the X-ray-emitting corona of flare stars: the hot component may be cooling flare plasma, while the cooler component (temperature of order 3 x 10(exp 6) K) is associated with a more conventional coronal heating mechanism.

White, S. M.↗

NASA NTRS1994

Observing stellar coronae with the Goddard High Resolution Spectrograph. 1: The dMe star AU microscopoii

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.

Maran, S. P.↗

NASA NTRS1994

Millimeter, microwave, hard X-ray, and soft X-ray observations of energetic electron populations in solar flares

We present comparisons of multiwavelength data for a number of solar flares observed during the major campaign of 1991 June. The different wavelengths are diagnostics of energetic electrons in different energy ranges: soft X-rays are produced by electrons with energies typically below 10 keV, hard X-rays by electrons with energies in the range 10-200 keV, microwaves by electrons in the range 100 keV-1 MeV, and millimeter-wavelength emission by electrons with energies of 0.5 MeV and above. The flares in the 1991 June active period were remarkable in two ways: all have very high turnover frequencies in their microwave spectra, and very soft hard X-ray spectra. The sensitivity of the microwave and millimeter data permit us to study the more energetic (greater than 0.3 MeV) electrons even in small flares, where their high-energy bremsstrahlung is too weak for present detectors. The millimeter data show delays in the onset of emission with respect to the emissions associated with lower energy electrons and differences in time profiles, energy spectral indices incompatible with those implied by the hard X-ray data, and a range of variability of the peak flux in the impulsive phase when compared with the peak hard X-ray flux which is two orders of magnitude larger than the corresponding variability in the peak microwave flux. All these results suggest that the hard X-ray-emitting electrons and those at higher energies which produce millimeter emission must be regarded as separate populations. This has implications for the well-known 'number problem' found previously when comparing the numbers of non thermal electrons required to produce the hard X-ray and radio emissions.

Kundu, M. R.↗

NASA NTRS1993

Spectral observations of active region sources with RATAN-600 and WSRT

We present spectral observations of neutral line and sunspot associated sources obtained with the RATAN-600 radio telescope and the WSRT in the wavelength range of 2 to 6 cm. Sources associated with large sunspots have flat spectra, while neutral line sources have very steep spectra. In the case of a large spot we estimated the magnetic field to be at least 2700 G at the base of the transition region and 1800 G in the low corona. We consider possible interpretations of the radio emission above the neutral lines. Gyroresonance emission at the fourth harmonic is inadequate, whereas emission from a small population of nonthermal electrons (total number 10 exp 30 to 10 exp 31) with a delta = 3 power law distribution seems to be sufficient.

Alissandrakis, C. E.↗

NASA NTRS1993

Thermal and nonthermal emissions during a coronal mass ejection

Observations were made of thermal and nonthermal radio emissions from a coronal mass ejection (CME) at meter-decameter wavelengths. The speed of the CME was found to be approximately 450 km/sec. It was possible to observe the thermal structure of the CME in radio due to the absence of nonthermal radio emission in the beginning of the event, and the weakness of the following event. Several minutes after the onset of the CME, type III bursts and a nonthermal continuum began. Radio and optical observations are used to show that the CME was not driven by the flare. The thermal structure and geometry of the mass ejection in radio is investigated and compared with the optical evidence. Lastly, a schematic model of the event is developed to show that particle acceleration high in the corona is possible.

Gopalswamy, N.↗