Mid-term periodicities in solar radio emission corresponding to sunspot number during solar cycle 23

Abstract

We present a systematic time-series analysis of solar radio emission in nine different frequencies to compare with that of daily sunspot number (SSN) during Solar Cycle 23 (1996-2009). Owing to the contribution from quiet-sun emission, the total solar fluxes in microwaves do not decrease as significantly as the sunspot number does during 2006 to 2009. Lomb-Scargle (LS) and wavelet analysis techniques are employed to infer the various periodicities present in the time-series data. False alarm probability (FAP) levels are estimated by the use of background mean power spectrum in the global wavelet spectrum. The LS periodogram contains resolved period peaks, some of which are below FAP levels, for example a well-known rotational period. These peaks are assessed with global significance levels of the wavelet analysis. In all the data sets, the period for solar rotational modulation (26-31 days) is present. The periodogram for the SSN presents Riéger type periods (130-180 days), mid-term periods (300-400 days) and long-term periods (430-850 days). These periods in north and south are not similar, especially long term periods are missing in SSN data of the southern hemisphere. Corresponding to the SSN periodicities, Riéger and near Riéger type of oscillations (130-180 days), quasi-biennial periodicities in the range of 1.2 to 3 years were detected in the time-series data of radio frequencies. Several of these detected periods fall in the range of the periods that are suggested to be connected with magneto-Rossby wave spherical harmonics. Our analysis found reduced power levels in the LS periodograms of low frequencies because of the fact that these low frequency emissions originate higher up in the corona with diminishing contrast to small scale structures.