Long period oscillations in sunspots

¹ Centre for Fusion, Space and Astrophysics, Physics Department, University of Warwick, Coventry CV4 7AL, UK e-mail: n.d.y.chorley@warwick.ac.uk
² Radiophysical Research Institute, 25 B. Pecherskaya Street, Nizhny Novgorod, 603950, Russia e-mail: rinbak@mail.ru
³ Central Astronomical Observatory at Pulkovo, Russian Academy of Sciences, Pulkovskoe chaussee., 65/1, St. Petersburg, 196140, Russia

Received: 17 November 2009
Accepted: 23 January 2010

Abstract

Long period oscillations of the gyroresonant emission from sunspot atmospheres are studied. Time series data generated from the sequences of images obtained by the Nobeyama Radioheliograph operating at a frequency of 17 GHz for three sunspots have been analysed and are found to contain significant periods in the range of several tens of minutes. Wavelet analysis shows that these periods are persistent throughout the observation periods. The presence of the oscillations is confirmed by several methods (periodogram, wavelets, Fisher randomisation and empirical mode decomposition). Spatial analysis using the techniques of period, power, correlation and time lag mapping reveals regions of enhanced oscillatory power in the umbral regions. Also seen are two regions of coherent oscillation of about 25 pixels in size, that oscillate in anti-phase with each other. Possible interpretation of the observed periodicities is discussed, in terms of the shallow sunspot model and the leakage of the solar g-modes.