Statistical Characteristics of Sunspot Umbra Oscillations in the Ca Bandtwo

Abstract

A group of successive evolutionary images of a sunspot were observed on 4th March 2014 at the Ca II H (396.85nm) line by the telescope on board of the Hinode satellite. The calculated oscillation period of the sample point in the umbra is 149.75s by using the Jurkevich (JK) method. A proper averaged value of the intensity profile is selected as the steady background radiation, then the distribution of the extreme values of oscillations in the umbra is analyzed by a statistical method. The results show that the valley values follow a normal distribution, while the peak values satisfy a partial normal distribution. The distribution of these extreme values indicates that the 3min oscillation of the umbra may be caused by the propagation of a traveling wave with a period of about 5min in the umbra. The variation of light intensity caused by the traveling wave may be proportional to the square of the displacement of the traveling wave, resulting in a 3min period of the light intensity, which is one half of the period of traveling wave.