Spectropolarimetric Observations of Solar Noise Storms at Low Frequencies

Abstract

A new high-resolution radio spectropolarimeter instrument operating in the frequency range of 15 – 85 MHz has recently been commissioned at the Radio Astronomy Field Station of the Indian Institute of Astrophysics at Gauribidanur, 100 km north of Bangalore, India. We describe the design and construction of this instrument. We present observations of a solar radio noise storm associated with Active Region (AR) 12567 in the frequency range of ${\approx}\,15\,\mbox{--}\,85~\mbox{MHz}$ during 18 and 19 July 2016, observed using this instrument in the meridian-transit mode. This is the first report that we are aware of in which both the burst and continuum properties are derived simultaneously. Spectral indices and degree of polarization of both the continuum radiation and bursts are estimated. It is found that

• i) Type I storm bursts have a spectral index of ${\approx}\,{+}3.5$,

• ii) the spectral index of the background continuum is ${\approx}\,{+}2.9$,

• iii) the transition frequency between Type I and Type III storms occurs at ${\approx}\,55~\mbox{MHz}$,

• iv) Type III bursts have an average spectral index of ${\approx}\,{-}2.7$,

• v) the spectral index of the Type III continuum is ${\approx}\,{-}1.6$, and

• vi) the degree of circular polarization of all Type I (Type III) bursts is ${\approx}\,90\%$ ($30\%$).

The results obtained here indicate that the continuum emission is due to bursts occurring in rapid succession. We find that the derived parameters for Type I bursts are consistent with suprathermal electron acceleration theory and those of Type III favor fundamental plasma emission.