Velocity Fields in Magnetically Disturbed Regions of the Hα Chromosphere

Citation

Roberts, Phillip Howard, Jr. (1970) Velocity Fields in Magnetically Disturbed Regions of the Hα Chromosphere. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/BVQF-BY60. https://resolver.caltech.edu/CaltechETD:etd-08282008-095104

Abstract

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A cancellation machine has been constructed to automatically produce the cancellation required by the method of Leighton in making Doppler movies. Various adaptations of the machine allow rapid data reduction to the 16 mm print required for cinema viewing. The preliminary data gathered in 1966 and 1967 has been reduced. These data consist mostly of He Doppler movie pairs, and some Zeeman movie pairs, of sunspot regions. A new calibration technique for inferring Doppler velocities from cancellation densities has been developed. This technique extends the calibration to large Doppler shifts with no less in accuracy.

Doppler movies taken in Hα,.070 nm from the core, show a long lived flow region connecting spots of opposite magnetic polarity in newly developing sunspot regions. The flow regions consist of a series of parallel arched filaments about 2 Mm thick by 23 ± 8 Mm long which are found also in the Hα core superimposed upon bright plage. The material in the arches is shown to flow downward along the legs of the arch from its center. The average height of the arches is 2 to 4 Mm depending upon the method of calculation. Velocities of 35 km/s inferred in the arches are consistent with those expected from frictionless descent from the top of the arch. An ascent velocity of about 5 km/s of the arch center is also consistent with the data. The material flow is not a mass flow from one leg of the arch to another as previously reported by other observers. A discussion of other minor discrepancies of this work with that of previous observers points out certain possible sources of error in both their work and this.

A physical model for the arches is proposed, whereby they consist of material trapped by magnetic lines of force which are emerging from the photosphere in the forming sunspot region. This material then flows downward along the arched lines of force as they rise. The subsurface magnetic field energy is shown to be sufficient to raise the material to the heights inferred for the arches, but insufficient to raise the temperature of the photosphere at the feet of the arches by more than about 7° K. The average lifetime of an arch is about thirty minutes yet the rise and decay times indicated by the data are shown to be about five minutes. Two possible explanations are offered for this discrepancy, one involving a great optical depth and the other a series of arches superimposed. Several suggestions are made for further persuing this phenomenon.

The study of Tile of velocity features in the Ha chromosphere of the quiet sun has been extended to active regions. The results for network regions near the sunspots corroborate those of Title. A mean lifetime of 215 ± 9 s was found for upflow events and 371 ± 30 s for downflow. In the regions covered by bright plage in the H[alpha] core photographs the mean lifetime in a majority of both upflow and downflow events was 217±71 s and 211 ± 91 s respectively.

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