ABSTRACT. We study the temporal behavior of the intensity and velocity chromospheric umbral oscillations, applying wavelet analysis techniques to four sets of observations in the Hα line and one set of simultaneous observations in the Hα and the nonmagnetic Fe I (5576.099 Å) line. The wavelet and Fourier power spectra of the intensity and the velocity at chromospheric levels show both 3 and 5 minute oscillations. Oscillations in the 5 minute band are prominent in the intensity power spectra; they are significantly reduced in the velocity power spectra. We observe multiple peaks of closely spaced cospatial frequencies in the 3 minute band (5-8 mHz). Typically, there are three oscillating modes present: (1) a major one near 5.5 mHz, (2) a secondary near 6.3 mHz, and (3) oscillations with time-varying frequencies around 7.5 mHz that are present for limited time intervals. In the frame of current theories, the oscillating mode near 5.5 mHz should be considered as a fingerprint of the photospheric resonator, while the other two modes can be better explained by the chromospheric resonator. The wavelet spectra show a dynamic temporal behavior of the 3 minute oscillations. We observed (1) frequency drifts, (2) modes that are stable over a long time and then fade away or split up into two oscillation modes, and (3) suppression of frequencies for short time intervals. This behavior can be explained by the coupling between modes closely spaced in frequency or/and by long-term variations of the driving source of the resonators.

Subject headings: Sun: chromosphere; Sun: oscillations; sunspots

^* Based on observations performed on the National Solar Observatory/Sacramento Peak Observatory Richard B. Dunn Solar Telescope (DST) and on the Big Bear Solar Observatory Harold Zirin Telescope.

Print publication: Issue 1 (2003 July 1)
Received 2003 January 23, accepted for publication 2003 March 10