Infrared helioseismology: detection of the chromospheric mode

Abstract

Observations of velocity waves from the 5-min p-mode oscillations in the solar sub-photospheric cavity have permitted a number of important inferences about physical conditions and differential rotation in the solar interior¹. Models also predict the existence of an overlying acoustic cavity, defined by reflections between the temperature minimum and the sharp rise from chromospheric to coronal temperatures^2–5. Observations of chromospheric emission features^6,7, the cores of strong absorption lines^8–11 and the far-infrared continuum¹² have established that chromospheric oscillatory power is shifted to higher frequencies than is the case for the sub-photospheric cavity. However, it remains to be established that this oscillatory power is sufficiently localized to indicate the presence of resonant modes at frequencies close to the predicted eigenfrequencies. We have obtained time-series observations of an infrared (11-µm) solar OH absorption line profile, on two consecutive days, using a laser heterodyne spectrometer to view a 2-arc-s portion of the quiet Sun at disk centre. A power spectrum of the line-centre velocity shows the well-known photospheric p-mode oscillations very prominently, but also shows a second feature near 4.3 mHz. A power spectrum of the line intensity shows only the 4.3-mHz feature, which we identify as the fundamental (n = 1) p-mode resonance of the solar chromosphere. The frequency of the mode is observed to be in substantial agreement with the eigenfrequency of current chromospheric models. A time series of two beam difference measurements shows that the mode is present only for horizontal wavelengths > 19 Mm. The period of a chromospheric p-mode resonance is directly related to the sound travel time across the chromosphere, which depends on the chromospheric temperature and geometric height. Thus, detection of this resonance will provide an important new constraint on chromospheric models.