| Infrared techniques were used to observe continuum emission from the solar chromosphere near temperature minimum in order to model the thermal response of the atmosphere to compressions due to 5-minute solar oscillations. Using one airborne and two ground-based observatories, simultaneous infrared intensity (temperature) and visible Doppler velocity measurements were acquired at several heights in the chromosphere, thus allowing comparisons between the motions of the atmospheric gas and the thermal fluctuations. While 5-minute oscillations in the lower chromosphere are frequently thought to be evanescent, so that the compression of the gas is in phase at all heights, the temperature changes due to these oscillations are found to vary in phase with altitude, implying the gas behaves non-adiabatically in this region.; The phases between the velocities of the gas and the temperature fluctuations were determined at several heights near temperature minimum. The chromosphere was then modelled as a planar gravitationally-stratified gas with thermal relaxation toward an equilibrium, isothermal temperature permitted in the equation describing temperature change with compression. The rates of thermal relaxation at different altitudes were estimated from the observed phases between the infrared and visible data. The relaxation times were found to vary from 30 seconds at an altitude of 350 km above the photosphere to roughly 200 seconds at 600 km altitude. The effects of compression on the opacity of the gas were also studied, in order to predict the consequences of a non-isothermal atmosphere on the continuum observations. An estimate of the energy lost from solar oscillations due to thermal relaxation is calculated for the altitudes observed, and it appears that solar oscillations may be partially responsible for heating the lower chromosphere by thermal relaxation. |
