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M. S. Sundararajan

Bio: M. S. Sundararajan is an academic researcher from Indian Institute of Astrophysics. The author has contributed to research in topics: Spectral resolution & Spectrograph. The author has an hindex of 4, co-authored 7 publications receiving 173 citations.

Papers
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Journal ArticleDOI
TL;DR: In this article, a radio heliograph for obtaining two-dimensional images of the solar corona sequentially at many frequencies in the range 40-150 MHz has been built by the Indian Institute of Astrophysics at the Gauribidanur Radio Observatory (lat. 13°36′12″ N and long. 77°27′07″ E) about 100 km north of Bangalore, India.
Abstract: A new radio heliograph for obtaining two-dimensional images of the solar corona sequentially at many frequencies in the range 40–150 MHz has been built by the Indian Institute of Astrophysics at the Gauribidanur Radio Observatory (lat. 13°36′12″ N and long. 77°27′07″ E) about 100 km north of Bangalore, India. This paper describes various aspects of the antenna system, receiver front end, digital hardware, the data acquisition and the calibration procedure. The performance of the instrument is illustrated with maps of the continuum emission from the undisturbed corona at different frequencies.

97 citations

Journal ArticleDOI
TL;DR: A new digital spectrograph for obtaining a dynamic spectrum of radio burst emission from the Sun in the frequency range 30{80 MHz has been recently commissioned at the Gauribidanur Radio Observatory (Lat: 1336 0 12 00 N and Long: 7727 0 07 00 E), about 100 km north of Bangalore, India as discussed by the authors.
Abstract: A new digital spectrograph for obtaining a dynamic spectrum of radio burst emission from the Sun in the frequency range 30{80 MHz has been recently commissioned at the Gauribidanur Radio Observatory (Lat: 1336 0 12 00 N and Long: 7727 0 07 00 E), about 100 km north of Bangalore, India. This paper describes various aspects of the antenna system, frontend receiver and digital hardware of the spectrograph. Some of the initial results obtained with the instrument are also presented.

37 citations

Journal ArticleDOI
TL;DR: An interferometer antenna system to observe polarized radio emission from the solar corona at different frequencies in the range 30 − 110 MHz has been commissioned recently by the Indian Institute of Astrophysics at the Gauribidanur Radio Observatory (latitude 13°36′12′N and longitude 77°27′07′E), about 100 km north of Bangalore.
Abstract: An interferometer antenna system to observe polarized radio emission from the solar corona at different frequencies in the range 30 – 110 MHz has been commissioned recently by the Indian Institute of Astrophysics at the Gauribidanur Radio Observatory (latitude 13°36′12′′N and longitude 77°27′07′′E), about 100 km north of Bangalore (http://www.iiap.res.in/centres_radio.htm). This paper describes the antenna system, associated analog/digital receiver setup, calibration scheme, and preliminary results.

31 citations

01 Jun 2007
TL;DR: The Gauribidanur radio array solar spectrograph (GRASS) as discussed by the authors uses 8 log periodic dipoles, a spectrum analyzer, and a data acquisition system.
Abstract: We describe the Gauribidanur radio array solar spectrograph (GRASS) and its various systems. The system consists of an array of 8 log periodic dipoles, a spectrum analyzer and a data acquisition system. The spec- trograph normally operates in the frequency range of 30 - 150 MHz with a frequency resolution of 250 KHz and a time resolution of 43 msec. The Gau- ribidanur radio array solar spectrograph operates approximately from 04:00 UT to 10:00 UT each day. We illustrate the working of the spectrograph with a few observations.

20 citations

Journal ArticleDOI
TL;DR: In this paper, an East-West, one-dimensional radio interferometer array consisting of five parabolic dish antennas has been set up at Cachoeira Paulista (longitude 45°0′20″ W, latitude 22°41′19″ S) for observations of the Sun and some of the strong sidereal sources by the Instituto Nacional de Pesquisas Espaciais (INPE), Brazil.
Abstract: An East – West, one-dimensional radio interferometer array consisting of five parabolic dish antennas has been set up at Cachoeira Paulista (longitude 45°0′20″ W, latitude 22°41′19″ S) for observations of the Sun and some of the strong sidereal sources by the Instituto Nacional de Pesquisas Espaciais (INPE), Brazil. This is Phase-I of the proposed Brazilian Decimetric Array and can be operated at any frequency in the range 1.2 – 1.7 GHz. The instrument has been in operation since November 2004 onwards at 1.6 GHz. The angular and temporal resolutions at this frequency are ∼3′ and 100 ms, respectively. Details of the array, analog/digital receiver system, and a preliminary East – West one-dimensional solar image at the 1.6 GHz are presented in this paper.

4 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, the authors reviewed the contribution of radio observations to the understanding of solar and solar-terrestrial physics from the first discovery of the radio emissions to present days, focusing on the radio observations of phenomena linked to solar activity.
Abstract: This paper will review the input of 65 years of radio observations to our understanding of solar and solar–terrestrial physics. It is focussed on the radio observations of phenomena linked to solar activity in the period going from the first discovery of the radio emissions to present days. We shall present first an overview of solar radio physics focussed on the active Sun and on the premices of solar–terrestrial relationships from the discovery to the 1980s. We shall then discuss the input of radioastronomy both at metric/decimetric wavelengths and at centimetric/millimetric and submillimetric wavelengths to our understanding of flares. We shall also review some of the radio, X-ray and white-light signatures bringing new evidence for reconnection and current sheets in eruptive events. The input of radio images (obtained with a high temporal cadence) to the understanding of the initiation and fast development in the low corona of coronal mass ejections (CMEs) as well as the radio observations of shocks in the corona and in the interplanetary medium will be reviewed. The input of radio observations to our knowledge of the interplanetary magnetic structures (ICMEs) will be summarized; we shall show how radio observations linked to the propagation of electron beams allow to identify small scale structures in the heliosphere and to trace the connection between the Sun and interplanetary structures as far as 4AU. We shall also describe how the radio observations bring useful information on the relationship and connections between the energetic electrons in the corona and the electrons measured in-situ. The input of radio observations on the forecasting of the arrival time of shocks at the Earth as well as on Space Weather studies will be described. In the last section, we shall summarize the key results that have contributed to transform our knowledge of solar activity and its link with the interplanetary medium. In conclusion, we shall indicate the instrumental radio developments at Earth and in space, which are from our point of view, necessary for the future of solar and interplanetary physics.

191 citations

Journal ArticleDOI
TL;DR: In this article, the authors synthesize the current state of knowledge of the properties of CMEs, along with some specific observed characteristics that illuminate the physical processes occurring during CME eruption.
Abstract: CMEs have been observed for over 30 years with a wide variety of instruments. It is now possible to derive detailed and quantitative information on CME morphology, velocity, acceleration and mass. Flares associated with CMEs are observed in X-rays, and several different radio signatures are also seen. Optical and UV spectra of CMEs both on the disk and at the limb provide velocities along the line of sight and diagnostics for temperature, density and composition. From the vast quantity of data we attempt to synthesize the current state of knowledge of the properties of CMEs, along with some specific observed characteristics that illuminate the physical processes occurring during CME eruption. These include the common three-part structures of CMEs, which is generally attributed to compressed material at the leading edge, a low-density magnetic bubble and dense prominence gas. Signatures of shock waves are seen, but the location of these shocks relative to the other structures and the occurrence rate at the heights where Solar Energetic Particles are produced remains controversial. The relationships among CMEs, Moreton waves, EIT waves, and EUV dimming are also cloudy. The close connection between CMEs and flares suggests that magnetic reconnection plays an important role in CME eruption and evolution. We discuss the evidence for reconnection in current sheets from white-light, X-ray, radio and UV observations. Finally, we summarize the requirements for future instrumentation that might answer the outstanding questions and the opportunities that new space-based and ground-based observatories will provide in the future.

81 citations

Journal ArticleDOI
TL;DR: In this article, a coronal shock associated with a CME and type II radio burst was found to be located at the expanding flank of the CME, where the shock geometry is quasi-perpendicular with θ Bn ~ 70°.
Abstract: Context. Type II radio bursts are evidence of shocks in the solar atmosphere and inner heliosphere that emit radio waves ranging from sub-meter to kilometer lengths. These shocks may be associated with coronal mass ejections (CMEs) and reach speeds higher than the local magnetosonic speed. Radio imaging of decameter wavelengths (20–90 MHz) is now possible with the Low Frequency Array (LOFAR), opening a new radio window in which to study coronal shocks that leave the inner solar corona and enter the interplanetary medium and to understand their association with CMEs.Aims. To this end, we study a coronal shock associated with a CME and type II radio burst to determine the locations at which the radio emission is generated, and we investigate the origin of the band-splitting phenomenon.Methods. Thetype II shock source-positions and spectra were obtained using 91 simultaneous tied-array beams of LOFAR, and the CME was observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) and by the COR2A coronagraph of the SECCHI instruments on board the Solar Terrestrial Relation Observatory(STEREO). The 3D structure was inferred using triangulation of the coronographic observations. Coronal magnetic fields were obtained from a 3D magnetohydrodynamics (MHD) polytropic model using the photospheric fields measured by the Heliospheric Imager (HMI) on board the Solar Dynamic Observatory (SDO) as lower boundary.Results. The type II radio source of the coronal shock observed between 50 and 70 MHz was found to be located at the expanding flank of the CME, where the shock geometry is quasi-perpendicular with θ Bn ~ 70°. The type II radio burst showed first and second harmonic emission; the second harmonic source was cospatial with the first harmonic source to within the observational uncertainty. This suggests that radio wave propagation does not alter the apparent location of the harmonic source. The sources of the two split bands were also found to be cospatial within the observational uncertainty, in agreement with the interpretation that split bands are simultaneous radio emission from upstream and downstream of the shock front. The fast magnetosonic Mach number derived from this interpretation was found to lie in the range 1.3–1.5. The fast magnetosonic Mach numbers derived from modelling the CME and the coronal magnetic field around the type II source were found to lie in the range 1.4–1.6.

66 citations

Journal ArticleDOI
TL;DR: In this paper, an estimation of the electron density modulation index for the first time using solar type IIIb radio burst observations is presented, which shows a power-law dependence on $r$676 with a power law index of approximately 0.006/pm 0.002$676.
Abstract: We present an estimation of the electron density modulation index ( $\frac{\delta{N_{\mathrm{e}}}}{N_{\mathrm{e}}}$ ) for the first time using solar type IIIb radio burst observations. The mean value of $\frac{\delta{N_{\mathrm {e}}}}{N_{\mathrm{e}}}$ is calculated to be ${\approx}\,0.006\pm0.002$ over the heliocentric distance range $r \approx1.6\,\mbox{--}\,2.2~\text{R}_{\odot}$ . The estimated $\frac {\delta{N_{\mathrm{e}}}}{N_{\mathrm{e}}}$ shows a power law dependence on $r$ with a power law index ${\approx}\,0.31\pm0.10$ . The wavenumber ( $k$ ) spectrum for the electron density fluctuation $({\frac{\delta{N_{\mathrm{e}}}}{N_{\mathrm {e}}}})^{2}$ values shows a Kolmogorov-like behavior. Using $\frac{\delta{N_{\mathrm{e}}}}{N_{\mathrm{e}}}$ and the Kolmogorov turbulence index, we estimated the amplitude of density turbulence [ $C_{n}^{2}{(r)}$ ] in the aforementioned range of $r$ .

49 citations

Journal ArticleDOI
TL;DR: In this article, the authors examined multiple type-II bursts and compared their kinematics with those of a CME occurring near the time of the bursts, and suggested that a single shock in the leading edge of the CME could be the source of the multiple type II bursts.
Abstract: Aims. Two or more type II bursts are occasionally observed in close time sequence during solar eruptions, which are known as multiple type II bursts. The origin of the successive burst has been interpreted in terms of coronal mass ejections (CMEs) and/or flares. Detailed investigations of the relationship between CMEs and the bursts enable us to understand the nature of the multiple type II bursts. In this study, we examine multiple type II bursts and compare their kinematics with those of a CME occurring near the time of the bursts. Methods. To do this, we selected multiple type II bursts observed by the Culgoora radiospectrographs and a limb CME detected in the low corona field of view (1.4−4 R s ) of a STEREO/SECCHI instrument on December 31, 2007. To determine the 3D kinematics of the CME, we applied the stereoscopic technique to the STEREO/SECCHI data.Results. Our main results are as follows: (1) the multiple type II bursts occurred successively at ten minute intervals and displayed various emission structures and frequency drifting rates; (2) near the time of the bursts, the CME was observed by STEREO and SOHO simultaneously, but no evidence of other CMEs was detected; (3) inspection of the 3D kinematics of the CME using the stereoscopic observation by STEREO/SECCHI revealed that the CME propagated along the eastward radial direction as viewed from the Earth; (4) very close time and height associations were found between the CME nose and the first type II burst, and between CME-streamer interaction and the second type II burst. Conclusions. On the basis of these results, we suggest that a single shock in the leading edge of the CME could be the source of the multiple type II bursts and support the notion that the CME nose and the CME-streamer interaction are the two main mechanisms able to generate the bursts.

49 citations