Interpretation and implications of diameter and differential radius observations of the 160 minute period solar oscillation

An oscillation is observed with a period of 160.06 ± 0.08 minutes in the 1978 diameter observations, and with a period of 159.98 ± 0.03 minutes in the 1979 differential radius observations. This oscillation is classified as the well-known 160 minute period oscillation. A hypothesis is examined that the primary physical mechanism by which this long-period oscillation is detected via Doppler shifts involves temperature perturbations conjoined with solar surface rotation, and not physical displacement of the solar surface. This hypothesis is found to be viable for observations on the 160 minute oscillation. The hypothesis was engendered by recognition of the existence of approximately 2 orders of magnitude discrepancy between the amplitudes found in differential velocity and those in radius observations when the traditional interpretation of the Doppler shifts is employed. The formalism is developed for computing the apparent Doppler shifts from the temperature perturbations obtained in diameter and differential radius observations. Calculations take into account opacity effects based on a non-LTE analysis of the Fe I λ5124 transition used in the differential velocity observations, and a symmetric line profile is not assumed. The predicted apparent velocities are obtained with uncertainties of + 120%, -40%, and, for physically plausible values of l, the predicted values are consistent with observed velocities. The observed velocities for the Crimean and Stanford observations of the 160 minute oscillation are 0.58 and 0.17 m s-1, respectively. If l = 1, the predicted apparent velocities are 0.33 and 0.13 m s-1, respectively. If l=2, the predicted apparent velocities are 0.26 and 0.14 m s-1,respectively. Good agreement is also obtained when comparing phases if l = 2. The observed phase of the differential velocity observations relative to that for the differential radius observations is -82° ± 30° the predicted values are either 0° or 180° for l = 1, ∣m∣ = 1 and ±90° if l = 2, ∣m∣ = 2, with an uncertainty of ± 18°. Detection of the oscillation by an apparent velocity shift favors m ≠ 0; the magnitude of the observed phase is only consistent with l = 2, and the sign of the relative phase restricts m to be positive for l = 2. The 160 minute period is thus classified as an l = 2, m = 2 mode. The apparent-velocity hypothesis has important implications for gravity mode classification and for the design of instruments to detect gravity modes.