Dual-line spectral and phase analysis of sunspot oscillations

¹ National Observatory of Athens, Institute for Space Applications and Remote Sensing, 15236 Palea Penteli, Greece e-mail: [kostas;georgia]@space.noa.gr
² Observatoire de Paris, Section de Meudon, LESIA, 92195 Meudon Principal Cedex, France e-mail: [Nicole.Mein;Pierre.Mein]@obspm.fr

Received: 18 September 2006
Accepted: 8 November 2006

Abstract

Context.Sunspots exhibit a wide range of oscillatory phenomena within their umbrae and penumbrae.

Aims.We investigate the behavior of intensity and Doppler velocity oscillations in the umbra and the penumbra to study sunspot oscillations and their associations.

Methods.Simultaneous, high-cadence (8 s), two-dimensional,  8542  Å and 6563 Å observations are used. Doppler velocity and intensity variations are studied with a wavelet spectral, phase difference and coherence analysis, both at distinct positions and within the whole umbra and the penumbra.

Results.The analysis reveals the presence of several umbral flashes (UFs) that seem to fill the whole umbra. The spectral analysis indicates oscillating elements of size 2.5″ to 5″ within the umbra with periods around the 3-min band and oscillation periods around the 5-min band within the penumbra. Two remarkable jumps of the oscillation period and the intensity-velocity phase difference are present at both umbra-penumbra and penumbra-superpenumbra boundaries reflecting a drastic change in physical and/or magnetic conditions. The intensity-velocity phase analysis shows a delay of the intensity response to the velocity variations in accordance with the physics of the observed sawtooth velocity behavior. Most of the UFs oscillate incoherently, while the calmest umbral area seems to be associated with velocity spreading from neighboring UFs. The derived incoherency among UFs in conjunction with the existence of coherently oscillating elements within the umbra suggests the presence of umbral areas with slightly different physical and/or magnetic field conditions.

Conclusions.The presented analysis provides further important constraints for realistic models and theoretical interpretations describing sunspot oscillations.