Investigation of Long-Period Oscillations of Sunspots with Ground-Based (Pulkovo) and SOHO/MDI Data

Abstract

We applied special data-processing algorithms to the study of long-period oscillations of the magnetic-field strength and the line-of-sight velocity in sunspots. The oscillations were investigated with two independent groups of data. First, we used an eighthour-long series of solar spectrograms, obtained with the solar telescope at the Pulkovo Observatory. We simultaneously measured Doppler shifts of six spectral lines, formed at different heights in the atmosphere. Second, we had a long time series of full-disk magnetograms (10 -34 hour) from SOHO/MDI for the line-of-sight magnetic-field component. Both ground-and space-based observations revealed long-period modes of oscillations (40 -45, 60 -80, and 160 -180 minutes) in the power spectrum of the sunspots and surrounding magnetic structures. With the SOHO/MDI data, one can study the longer periodicities. We obtained two new significant periods (> 3σ ) in the power spectra of sunspots: around 250 and 480 minutes. The power of the oscillations in the lower frequencies is always higher than in the higher ones. The amplitude of the long-period magnetic-field modes shows magnitudes of about 200 -250 G. The amplitude of the line-of-sight velocity periodicities is about 60 -110 m s −1 . The absence of low-frequency oscillations in the telluric line proves their solar nature. Moreover, the absence of low-frequency oscillations of the lineof-sight velocity in the quiet photosphere (free of magnetic elements) proves their direct connection to magnetic structures. Long-period modes of oscillation observed in magnetic elements surrounding the sunspot are spread over the meso-granulation scales (10 -12 ), while the sunspot itself oscillates as a whole. The amplitude of the long-period mode of the line-of-sight velocity in a sunspot decreases rapidly with height: these oscillations are clearly visible in the spectral lines originating at heights of approximately 200 km and fade away in lines originating at 500 km. We found a new interesting property: the low-frequency oscillations of a sunspot are strongly reduced when there is a steady temporal trend (strengthening or weakening) of the sunspot's magnetic field. Another important result is that the frequency of long-period oscillations evidently depends on the sunspot's magnetic-field strength.

Long-period oscillations of millimeter emission above sunspots

“…As result, this conclusion could support the shallow sunspot model suggested by Solov'ev & Kirichek (2008). But the main harmonic of global sunspot oscillations is about 800−900 min (12−15 h) (Efremov et al 2010), so it is evident that far sub-harmonics of this mode (10−100 min) could be weak and unstable.…”

“…The mechanism responsible for the generation of the long-period component in sunspot oscillations is under discussion. These oscillations have been observed in the variation of the magnetic-field strength and the line-of-sight velocity (Efremov et al 2010). The theoretical interpretation of long quasi-periodic oscillations in sunspots is related to the so-called "shallow" model of sunspots, suggested by Solov'ev & Kirichek (2008).…”

Period persistence of long period oscillations in sunspots

“…Efremov et al (2010) detected periods in the range 40−180 min in wavelengths formed between 190 km and 535 km (periods of 250 and 480 min were found in the line-ofsight magnetograms from space-borne SOHO/MDI data).…”

Artifacts of SDO/HMI data and long-period oscillations of sunspots

“…These oscillations were detected in the optical range as the time variations of the line-of-sight velocities, magnetic fields and spot areas (Efremov et al 2007(Efremov et al , 2010(Efremov et al , 2012. Also, long-period oscillations were revealed in microwavemillimeter radio waves as the time-variations of the radio emission from the sources connected with sunspots (Bakunina et al 2009;Chorley et al 2010Chorley et al , 2011Smirnova et al 2011).…”