Observations of Running Penumbral Waves Emerging in a Sunspot

Abstract

We present results from the investigation of 5 minute umbral oscillations in a single-polarity sunspot of active region NOAA 12132. The spectra of TiO, Hα, and 304 Å are used for corresponding atmospheric heights from the photosphere to lower corona. Power spectrum analysis at the formation height of Hα -0.6 Å to the Hα center resulted in the detection of 5 minute oscillation signals in intensity interpreted as running waves outside the umbral center, mostly with vertical magnetic field inclination >15°. A phase-speed filter is used to extract the running wave signals with speed v ph >4kms −1 , from the time series of Hα -0.4 Å images, and found twenty-four 3 minute umbral oscillatory events in a duration of one hour. Interestingly, the initial emergence of the 3 minute umbral oscillatory events are noticed closer to or at umbral boundaries. These 3 minute umbral oscillatory events are observed for the first time as propagating from a fraction of preceding running penumbral waves (RPWs). These fractional wavefronts rapidly separate from RPWs and move toward the umbral center, wherein they expand radially outwards suggesting the beginning of a new umbral oscillatory event. We found that most of these umbral oscillatory events develop further into RPWs. We speculate that the waveguides of running waves are twisted in spiral structures and hence the wavefronts are first seen at high latitudes of umbral boundaries and later at lower latitudes of the umbral center.

The dynamics of 3-min wavefronts and their relation to sunspot magnetic fields

“…It is assumed that the development of wavefronts can be caused by a thin magnetic structure in the umbra, including umbral dots and light bridges, where the cutoff frequency may be inherently different. It has been assumed that the waveguides of running waves are twisted into local spiral structures [14]. A model for the appearance of spiral wavefronts was proposed in relation to their propagation in radial magnetic fields [15].…”

Where do running penumbral waves emerge in chromosphere?

“…Also, it is observed that the wavefronts which are part of the already emerged RPWs detaches itself from the RPWs and jumps into the center of the umbra where it appears to expand radially and starts a new oscillations as shown in Fig. 1 (see Priya et al 2018 for more details). Along the edges of the wavefront, the periods of running waves show a large spread.…”

Propagation Properties of Sunspots Umbral Oscillations in Horizontal and Vertical Directions

“…In the horizontal direction, Felipe et al (2019) noted out that the spiral-arm structure rotates counterclockwise to the umbral-penumbral boundary and evolves into RPWs. Su et al (2016a) and Priya et al (2018) have slightly different views; they argued out that the wavefront spiral-arm structure of the umbral oscillations splits when it reaches the umbral boundary; a part returns to the umbra to excite the next round of umbral oscillations, and the other part enters the penumbral to form the RPWs. However, in sunspots with multiple umbrae, the propagation mechanism of oscillations between different umbrae, as well as the possibility of their sharing a common origin in the sub-photosphere, have not been definitively established.…”