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Investigations of long-period oscillations of sunspots using ground-based observations (Pulkovo) and instrumental MDI (SOHO) data
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Slava Efremov Submitted: 2009-09-22 07:29
The special algorithms for data processing were applied to study the long-period oscillations of magnetic field strength and the line-of-sight velocity magnitude in sunspots.
The oscillations were revealed and investigated using two independent groups of data:
1. 8 hours-long series of solar spectrograms. The data were obtained by the Solar telescope at Pulkovo Observatory. Doppler shifts of six spectral lines, originated at different heights, were measured simultaneously.
2. The long-term series of full-disk magnetorams (10 through 34 hour) from the MDI (SOHO) for line-of-sight magnetic field component.
Both the ground-based and space observations have been revealed the evident presence of long-period modes of oscillations (the spectral bands with periods about 40-45, 60-80 and 160-180 minutes) in power spectrum of sunspots and surrounding magnetic structures. Using the MDI (SOHO) data, one can study the observed data for the lower periodicities. We have obtained two new significant periods (>3σ) in the power spectra of sunspots, around 250 and 480 min. The power of oscillations on the lower frequencies is always larger than on the higher ones . The amplitude of long-term modes of magnetic field shows the magnitudes of about 200-250G. The amplitude of the line-of-sight velocity periodicities occurred to be of about 60-110 m/s.
The absence of low-frequency oscillations in telluric line uniquely points out on their solar nature. Moreover, the absence of low-frequency oscillations of line-of-sight velocity in a quiet photosphere (free from magnetic elements) proves their direct connection to the magnetic structures.
Long-period modes of oscillations 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-term mode of line-of-sight velocity in a sunspot decreases rapidly with a height: these oscillations are clearly seen in spectral lines originated at heights of approximately 200 km, and are fading away in lines originated at 500 km.
A new interesting physical effect was found: the low-frequency oscillations of a sunspots are strongly reduced when the steady time-trend (strengthening or weakening) of sunspot magnetic field takes place. An another important result is that the frequency of long-period oscillations is evidently depended on sunspot magnetic field strength.
Authors: Efremov V. I., Parfinenko l. D., Solov'ev A. A.
Projects: SoHO-MDI
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Publication Status: Solar Physics (submitted)
Last Modified: 2009-09-22 07:39
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