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Gopalswamy, N.

Publications and source records attributed to Gopalswamy, N..

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS2005

Microsat and Lunar-Based Imaging of Radio Bursts

No present or approved spacecraft mission has the capability to provide high angular resolution imaging of solar or magnetospheric radio bursts or of the celestial sphere at frequencies below the ionospheric cutoff. Here, we describe a MIDEX-class mission to perform such imaging in the frequency range approx. 30 kHz to 15 MHz. This mission, the Solar Imaging Radio Array (SIRA), is solar and exploration-oriented, with emphasis on improved understanding and application of radio bursts associated with solar energetic particle (SEP) events and on tracking shocks and other components of coronal mass ejections (CMEs). SIRA will require 12 to 16 micro-satellites to establish a sufficient number of baselines with separations on the order of kilometers. The constellation consists of microsats located quasi-randomly on a spherical shell, initially of approx. 10 km diameter. The baseline microsat is 3-axis stabilized with body-mounted solar arrays and an articulated, earth pointing high gain antenna. The constellation will likely be placed at L1, which is the preferred location for full-time solar observations. We also discuss briefly follow-on missions that would be lunar-based with of order 10,000 dipole antennas.

interferometry↗
NASA NTRS2003

Phenomena Associated With EIT Waves

We discuss phenomena associated with "EIT Wave" transients. These phenomena include coronal mass ejections, flares, EUV/SXR dimmings, chromospheric waves, Moreton waves, solar energetic particle events, energetic electron events, and radio signatures. Although the occurrence of many phenomena correlate with the appearance of EIT waves, it is difficult to mfer which associations are causal. The presentation will include a discussion of correlation surveys of these phenomena.

Thompson, B. J.↗
NASA NTRS2002

Phenomena Associated with EIT Waves

We discuss phenomena associated with 'EIT Wave' transients. These phenomena include coronal mass ejections, flares, EUV/SXR dimmings, chromospheric waves, Moreton waves, solar energetic particle events, energetic electron events, and radio signatures. Although the occurrence of many phenomena correlate with the appearance of EIT waves, it is difficult to infer which associations are causal. The presentation will include a discussion of correlation surveys of these phenomena.

Thompson, B. J.↗
NASA NTRS2000

Space VLBI at Low Frequencies

At sufficiently low frequencies, no ground-based radio array will be able to produce high resolution images while looking through the ionosphere.

Space VLBI Interferometry Low Frequency↗
NASA NTRS2000

Shock Wave and EUV Transient During a Flare

A metric type II burst and a 'brow' type enhancement in EUV were observed during the hard X-ray flare of 1997 April 15 from a newly emerging region, AR 8032. The position of the type II burst obtained from the Nancay radioheliograph coincided with the EUV transient. The type II burst and the EUV transient were in the equatorial streamer region to the north of the flaring region. This observation suggests that the EUV transient may be the manifestation of the MHD shock responsible for the type II burst.

Gopalswamy, N.↗
NASA NTRS1999

"Driverless" Shocks in the Interplanetary Medium

Many interplanetary shocks have been detected without an obvious driver behind them. These shocks have been thought to be either blast waves from solar flares or shocks due to sudden increase in solar wind speed caused by interactions between large scale open and closed field lines of the Sun. We investigated this problem using a set of interplanetary shock detected {\it in situ} by the Wind space craft and tracing their solar origins using low frequency radio data obtained by the Wind/WAVES experiment. For each of these "driverless shocks" we could find a unique coronal mass ejections (CME) event observed by the SOHO (Solar and Heliospheric Observatory) coronagraphs. We also found that these CMEs were ejected at large angles from the Sun-Earth line. It appears that the "driverless shocks" are actually driver shocks, but the drivers were not intercepted by the spacecraft. We conclude that the interplanetary shocks are much more extended than the driving CMEs.

Gopalswamy, N.↗
NASA NTRS1999

Microwave Enhancement in Coronal Holes: Structure, Variability and Magnetic Nature

The microwave enhancement in coronal holes in comparison with the quiet Sun is a distinct and easily observed signature related to the magnetic activity. This has proven to be a new tool to study the solar atmospheric layer where the fast solar wind originates. We have developed a catalog of a large number of coronal holes using images obtained by SOHO's Extreme-ultraviolet Imaging Telescope in EUV and by the Nobeyama radioheliograph in microwaves. We also have high resolution longitudinal magnetograms obtained by SOHO's Michelson Doppler Imager. We present the statistical properties of the microwave enhancements and discuss the small-scale dynamics as revealed by the magnetograms and radioheliograms. We also discuss the geoeffectiveness of these coronal holes.

Gopalswamy, N.↗
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 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

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

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

Simultaneous observations of solar plage with the solar extreme ultraviolet rocket telescope and spectrograph (SERTS), the VLA, and the Kitt Peak magnetograph

We obtained simultaneous images of solar plage on 1991, May 7 with SERTS, the VLA,4 and the NASA/National Solar Observatory spectromagnetograph at the NSO/Kitt Peak Vacuum Telescope. Using intensity ratios of Fe XVI to Fe XV emission lines, we find that the coronal plasma temperature is (2.3-2.9) x 10 exp 6 K throughout the region. The column emission measure ranges from 2.5 x 10 exp 27 to l.3 x 10 exp 28 cm exp -5. The calculated structure and intensity of the 20 cm wavelength thermal bremsstrahlung emission from the hot plasma observed by SERTS is quite similar to the observed structure and intensity of the 20 cm microwave emission observed by the VLA. Using the Meyer (1991, 1992) revised coronal iron abundance, we find no evidence either for cool absorbing plasma or for contributions from thermal gyroemission. Using the observed microwave polarization and the SERTS plasma parameters, we calculate a map of the coronal longitudinal magnetic field. The resulting values, about 30-60 G, are comparable to extrapolated values of the potential field at heights of 5000 and 10,000 km.

Brosius, Jeffrey W.↗
NASA NTRS1993

A new investigation of microbursts at meter-decameter wavelengths

The results of a new investigation of microbursts at meter-decameter wavelengths are reported. Several properties of microbursts reported earlier are confirmed, and some new properties, such as time profile characteristics, flux density, and energy spectra are studied in order to compare the properties of microbursts with those of normal type III bursts. Electron beams of similar characteristics are found to generate both microbursts and type III bursts. Plasma waves generated by the electron beams due to beam-plasma instability are quickly isotropized as they scatter on the density fluctuations in the corona, leading to the low brightness temperature of microbursts. The resulting low levels of plasma waves produce transverse radiation of low brightness temperature. As a result of the isotropization, the second harmonic plasma emission dominates the fundamental, so that microbursts are expected to be primarily a harmonic plasma emission.

Subramanian, K. R.↗
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.↗
NASA NTRS1993

Plasma emission from isotropic Langmuir turbulence - Are radio microbursts structureless?

The brightness temperature of radio emission through the fundamental and second harmonic plasma processes is determined for isotropic Langmuir waves of low-energy density in order to account for the microbursts at meter-dekameter wavelengths. The probable cause for low levels of Langmuir turbulence is the presence of isotropic density fluctuations in the corona which isotropize the beam-generated Langmuir waves. We determined the energy density of Langmuir waves attainable from the beam-plasma instability in the presence of isotropic density fluctuations. Since the electron density fluctuations isotropize the beam-generated plasma waves, the head-on collision of plasma waves becomes efficient to produce the second harmonic plasma emission. For reasonable beam parameters, the brightness temmperature of the fundamental never exceeds 10 exp 6 K, while the second harmonic covers the observed range of microburst brightness temperatures. Thus, the microbursts are predominantly at second harmonic. This leads to an important conclusion that the microbursts are structureless, similar to a population of normal type III bursts of low polarization with no fundamental-harmonic structure.

Gopalswamy, N.↗