On the nature of the chromospheric fine structure

II. Intensity and velocity oscillations of dark mottles and grains

¹ National Observatory of Athens, Institute for Space Applications and Remote Sensing, Lofos Koufos, 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: pierre.mein@obspm.fr

Received: 11 February 2003
Accepted: 10 May 2004

Abstract

We study periodicities of dark mottles and grains observed in high spatial and temporal resolution  2-D intensity and velocity images, obtained with the Multichannel Subtractive Double Pass (MSDP) spectrograph at THEMIS in Tenerife, Canary Islands. Intensity and velocity variations of individual mottles and grains, as well as of large regions containing a substantial number of these structures are examined with a wavelet analysis which provides the temporal distribution of periods and with a randomization method that gives the respective probabilities associated with them. The wavelet analysis, which shows that velocity variations are the dominant ones, results in a wide range of periods, from 100 to 500 s, with variable probabilities. However, it strongly indicates the presence of a predominant period of ~5 min seen in both dark mottle and grain oscillations. We suggest that differences in periods found in individual mottles as well as in averages of regions containing these structures are due to the topology of the local magnetic field which can affect oscillatory processes. We report for the first time oscillations of dark grains which are more likely vertical structures “anchored" in the photosphere. The analysis supports further the suggestion that dark mottles and grains are the same structures, the only difference between them being their respective inclinations to the line-of-sight.