Observations of Oscillatory Motions in the Solar Atmosphere

Citation

Noyes, Robert Wilson (1963) Observations of Oscillatory Motions in the Solar Atmosphere. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/31J4-0X44. https://resolver.caltech.edu/CaltechTHESIS:09302010-103625645

Abstract

This thesis presents observations of the macroscopic line-of-sight velocity field in the solar atmosphere. The observations were made at Mt. Wilson Observatory, primarily in 1960 and 1961. A quasi-periodic vertical oscillatory motion has been detected in the upper photosphere and low chromosphere. The "average period" of this oscillation has been determined with some accuracy to be about 300 sec.; there appears to be a slight decrease of period with increasing altitude. The mean life of a single oscillation is about 400 sec. The “velocity elements" which partake of the oscillation have an appearance very similar to the photospheric granulation of low levels, and are probably identical to it. The vertical velocity amplitude is about 1/2 km/sec at these levels, and increases with altitude. The horizontal component of velocity is nearly as large as the vertical at low levels, but decreases with altitude, until at higher levels the velocities are substantially vertical only. The mean diameter of the velocity elements is only slightly larger than the photospheric granulation at low levels, but increases to many times that size at higher levels. We also report in this thesis on observations of macroscopic intensity fields, made concurrently with the velocity observations. A correlation coefficient between intensity and velocity has been found which appears to decrease monotonically with altitude, from about +0.5 at the lowest elevations observed to about -0.2 at the highest observed elevations. An oscillatory behavior of the intensity field is found in the chromosphere, with roughly the same period as the velocity oscillation, and evidently coupled to it. This oscillation may be followed to considerable altitudes in the chromosphere. It is apparently absent in the upper photosphere, however, even though the velocity oscillation is quite strong there. Also discussed are some possible interpretations of the observations and what they might imply about the structure of the solar atmosphere. We find that the observed period is close to the "critical period" separating propagating and standing acoustic waves in a gravitating atmosphere with the solar values for temperature and gravitational field. Several possible explanations for the decrease of period with altitude are suggested. The amplitude and phase of the temperature fluctuations accompanying a wave propagating in an atmosphere with radiative leakage are discussed; this leads to a possible explanation of the presence of an intensity oscillation at high levels and its absence at low levels, and of the reversal with altitude of the sign of the correlation coefficient between intensity and velocity. Some observational problems are also discussed, both in the body of the thesis (Parts I and II) and in appendices.

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