A Study of Velocity Fields in the Hα Chromosphere by Means of Time-Lapse Doppler Movies

Citation

Title, Alan Morton (1966) A Study of Velocity Fields in the Hα Chromosphere by Means of Time-Lapse Doppler Movies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7FCG-0448. https://resolver.caltech.edu/CaltechETD:etd-08282008-085815

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The technique of Leighton for making Doppler spectroheliograms has been extended to movies. Doppler movies made in H [alpha] from the core show a class of upgoing features with a 113 ± 3.4 second mean lifetime and a definite velocity history. The velocity reaches its maximum in less than 30 seconds, then declines for the next 90 seconds. In the case of 27 percent of these features a similar upflow event occurred soon after the first had died away. The average lifetime of these "double" events is 236 ± 8 seconds, approximately twice the lifetime of a single event. No region has been observed to repeat more than twice in the time a region could be followed (about 15 m), nor has any showed a tendency to repeat after a lapse of more than 60 seconds. Moreover, the region on the sun where an upflow event has occurred tendsto show no downflow for at least fifteen minutes afterward. The prominent upgoing features occur in the rosette structure seen in the H [alpha] wings. Further, they are often visible as absorbing features in the red as well as the violet wing. This suggests that their profiles are broadened with respect to the mean profile. A statistical study of a time series of high dispersion H [alpha] spectra has shown that there is a positive correlation between the average speed of upgoing features and their profile width. The measured increase in width is sufficient to cause features with speeds greater than 1.5 km/second to appear in absorption on both sides of the line. Lifetimes of upflow features have been measured using the time series of spectra and the Doppler movies. The results of these measurements are consistent with there being a single predominate form of upflow that is visible in the H [alpha] wings further than .4 […] from the core with an average lifetime of a few minutes. The mean lifetime of downflow regions is six to nine minutes. However, about 20 percent of the downflow regions are observed to persist for fifteen minutes or more. A few downflow regions have lasted the entire length of a Doppler movie, which is about forty minutes. Downflow typically occurs in the central regions of rosettes. As with upflow features, downflow features often are visible as absorbing features in both wings of H [alpha]. However, the statistical study of spectra has shown that although the average profile width increases with average speed for downflow features, approximately 50 percent of the downflow features have profiles that do not tend to increase with feature speed. Individual upflow or downflow features did not have a periodic nature on a time scale longer than a few minutes. However, plots of the total upflow or total downflow areas as a function of time in regions 30 x 10[…] km square did show systematic variations. The auto-correlation functions made from the plots of up and downflow area versus time had secondary peaks spaced by about 550 seconds thus indicating a periodicity in both the total upflow and total downflow with a period of 550 seconds. Cross-correlation of the plots of upflow and downflow areas for six different regions indicated that the phase relation between upflow and downflow is random.

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