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Title: Imaging and Spectral Studies of Solar type I Noise Storms at Metric Wavelengths
Authors: Shanmugha Sundaram, G A
Advisors: Subramanian, K R
Submitted Date: Feb-2004
Publisher: Indian Institute of Science
Abstract: Type I noise storms constitute a sizeable fraction of the active-Sun component of solar radiation at meter wavelengths. The storms occur over a prolonged duration as intense, narrow-band spikes, superposed on a low-intensity broadband continuum, in the 30-400 MHz frequency range. Either component of the noise storm radiation has a very high degree of ordinary-mode circular polarization (~ 100 %), and is widely believed to be generated by the plasma emission mechanism. Existence of large sunspot groups or active region complexes, with a high degree of complexity and strength in the associated magnetic topology, have a remarkable spatio-temporal correlation to the occurance of noise storms. Hence type I noise storm events were employed as tracers in estimating the solar cycle activity, with specific relevance to resolving the mimimum-phase occuring between solar cycles 22 and 23, to a precise yet unique epoch of 1998 October,in consonance with the monthly average sunspot count and the 10.7 cm solar radio flux data. The latter have a proven close link with associated sunspot activity. Spatio-temporal correlation of energetic eruptive event termed Coronal Mass Ejection ( CME ), with the type I noise storm events has been convincingly established, from the point of view of ``Space Weather'' prediction. A particular aspect of this study has been the choice of limb-event CMEs as against the halo ones; this criterion has aided in an unambiguous position-angle determination for the eruption- site of the CMEs. Noise storms are found to be the precursors, temporally succeeded by the CME events over a time-scale of 1 to 14 hours. Large scale reconfiguration of the photospheric magnetic field topology, by way of reconnection, merging, cancellation or submergence, in the ambience of pre-existing lines of magnetic flux, triggered by the shearing and twisting motion of the footpoints, and leading to the sigmoid-to-arcade evolution of coronal magnetic loops, traces the event-specific linkage involving the type I noise storms and CMEs. Phenomena that occur at metric wavelengths in the solar corona, and vary on short time scales, are investigated, with the aid of a prototype, custom-built antenna-receiver system to the Gauribidanur RadioHeliograph ( GRH ). The GRH is a transit-mode instrument, while the time-delay control implemented on the prototype enables a radio source to be followed, as it traverses the sky at the sidereal rate. The delay-tracking scheme has been implemented on the front-end electronics, as this would eliminate the otherwise cumbersome task of mechanical-steering of the log-periodic dipole array, while also permitting radio observations over a significantly wider frequency band. The performance of the Gauribidanur Prototype Tracking System ( GPTS ) has been validated on the basis of exhaustive tests, in order to characterise its attenuation, phase, and pointing-accuracy, and optimised for solar observations at 77.5 MHz. Continuous Sun observation was performed with the GPTS, over a period from 24th of June, 2002 to 20th of August, 2002. The daily schedule involved solar observations at seven beam-positions on either side of the local meridian,spaced apart by ~ 9 deg., for about four hours each day. Absolute radio flux calibration was performed by following an identical observing schedule for the intense, unresolvable radio source Cygnus A. Periods of enhanced radio emission, corresponding to global rise in the solar radio radiation, were detected on several days. On each of those days of enhanced activity, the absolute deviation from the mean-flux, corresponding to the various beam-positions for that day, was determined. From this, the intra-day quasi-periodicity in solar radio flux was found to be 110 minutes, with the fluctuation in flux being 3 (+/- 1.5) sfu. Positional information from the Nancay (sic) Radioheliograph data, and features of the causative Active Regions of the underlying photospheric disk from the full-disk H-alpha images of the Big Bear Solar Observatory, along with the radio-spectral data published in the Solar Geophysical Data Reports led to conclusion that, heightened flux emission had been the result of the type I noise storms, known for their sharply defined directivity characteristics. The continuum component of type I noise storms is studied for variation in the frequency-dependent flux characteristics. Swept-frequency data from the Gauribidanur Radio Spectrograph, on 26th and 27th September 2000, in the 30-80 MHz band, were analyzed. The quiet-sun and burst components in the acquired data were excised-out. Absolute flux calibration was performed from spectral observations of Cygnus A in the same band. The spectral-index of the continuum was found to be ~ +3.7 . From a knowledge of the continuum source-size at various other spot frequency imaging observations, the source-size of the particular event was estimated, from interpolation and curve-fitting, to be 13.2 +/- 1.2 arcmin. From a knowledge of the continuum radio flux and its source-size, the brightness-temperature was found to vary from 1.07 x 1e7 to 1.96 x 1e7 K, in the 50 - 80 MHz band. Plasma emission is widely believed to bethe radiation mechanism for the continuum. The excitation of plasma waves by trapped, energetic electron beams moving along the coronal magnetic loops, and their coalescence with the low-frequency ion-acoustic waves or upper-hybrid waves excited due to shock-waves generated by magnetic reconnections above the active region complexes, at sites of coronal density inhomogeneities, are the cause for the noise storm radiation. From knowing the brightness-temperature in the source-region, the supra-thermal density in the electron-beam is estimated. Corroborative evidence, in the form of complementary observations for source-size, extent of the active region complexes, and the associated variations in strength and polarity of magnetic flux on the photosphere, the density enhancement over that of the tenuous coronal density, as per the Newkirk's model, above such active regions, the emission-measure, density, and brightness-temperature in the large-scale coronal loops interconnecting the trans-equatorial active regions in this case, in extreme uv and soft X Ray wavelengths, is applied to validate the assumptions, and estimations on various parameters involved in this plasma emission phenomenon. The burst component of type I noise storms is studied with the newly commissioned high temporal and spectral resolution spectrograph at the Gauribidanur Radio Observatory. The bursts reveal themselves as narrow-band, spiked events on the dynamic spectral records, and their occurance is of a stochastic nature. Isolated Type I bursts were chosen based on their bandwidth (2-2.5 MHz ), fractional-bandwidth ( 1.5 ), lifetime ( 1.5 seconds ), and their radio flux (~ 20-40 sfu ) distribution. The dynamic~- spectrum was calibrated from galactic background observations towards the direction of the North and the South Galactic poles. The flux calibration scheme is ideally suited for those radio telescopes capable of a low spatial resolution, wherein the predominant contribution to the system temperature arrives from the galactic background radiation. The frequency and time profiles of the bursts were analyzed on a case-by-case basis. The results of the study reveal that, a majority of the frequency profiles show a remarkable gaussian symmetrical distribution as compared to the less significant assymmetry in either the ascending or the descending limb ( which appear as enhanced tail-like features ) of the corresponding gaussians. This, in consonance with their narrow emission bandwidth, endorses the view that, the source region for Type I bursts are in a state of extreme homogeneity, as regards their plasma density and temperature. The time profiles on the other hand show a greater level of asymmetry on either their ascending or descending segments; deviations from the gaussian fit, to each of the bursts' time profiles, reveal a higher incidence in abrupt rise or fall on either of the limbs, to cases where the profiles conform to a symmetric gaussian. Since the rise and decay in the time profiles correspond to growth of plasma instabilities and damping of the plasma waves, respectively, they portray a region of the turbulent corona that is replete with magnetic reconnections contributing to the energetics of plasma waves.
URI: http://etd.iisc.ernet.in/handle/2005/46
Appears in Collections:Physics (physics)

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