Torben Leifsen

FIRST RESULTS FROM VIRGO, THE EXPERIMENT FOR HELIOSEISMOLOGY AND SOLAR IRRADIANCE MONITORING ON SOHO

First results from the VIRGO experiment (Variability of solar IRradiance and Gravity Oscillations) on the ESA/NASA Mission SOHO (Solar and Heliospheric Observatory) are reported. The observations started mid-January 1996 for the radiometers and sunphotometers and near the end of March for the luminosity oscillation imager. The performance of all the instruments is very good, and the time series of the first 4 - 6 months are evaluated in terms of solar irradiance variability, solar background noise characteristics and -mode oscillations. The solar irradiance is modulated by the passage of active regions across the disk, but not all of the modulation is straightforwardly explained in terms of sunspot flux blocking and facular enhancement. Helioseismic inversions of the observed -mode frequencies are more-or-less in agreement with the latest standard solar models. The comparison of VIRGO results with earlier ones shows evidence that magnetic activity plays a significant role in the dynamics of the oscillations beyond its modulation of the resonant frequencies. Moreover, by comparing the amplitudes of different components of -mode multiplets, each of which are influenced differently by spatial inhomogeneity, we have found that activity enhances excitation.

Sunspot transition region oscillations in NOAA 8156

Based on observations obtained with the Solar and Heliospheric Observatory joint observing program for velocity fields in sunspot regions, we have detected 3 minute transition region umbral oscillations in NOAA 8156. Simultaneous recordings of O V λ629 and N V λ1238, λ1242 with the SUMER instrument give the spatial distribution of power in the 3 minute oscillations, both in intensity and in line-of-sight velocity. Comparing loci with the same phase, we find that the entire umbral transition region oscillates. The observed maxima in peak line intensity are nearly in phase with the maxima in velocity directed toward the observer. We discuss the suggestion that the waves are upward-propagating acoustic waves.

New light on solar infrared intensity oscillations

The detection of large-amplitude infrared solar intensity oscillations in the 5-min region is reported. Using a broad-band multichannel photometer, the peak-to-peak intensity variation at 2.23 μm is found to be as high as 2.4% for a circular aperture of 1 arc min and 0.8% in the full disk observations, i.e., remarkably higher than at the other four observed wavelength regions.The spatially-integrated power spectrum shows the 5-min oscillation plus a strong feature near 4 mHz. This feature coincides in frequency with the fundamental p-mode resonance of the chromosphere. However, a power-spectrum autocorrelation as well as a second-order Fourier transform of the data suggest that a high-frequency tail of the 5-min power spectrum is a more likely interpretation of this feature.

The limb effect of the Ki resonance line, 769.9 n m

Low-noise observations have been obtained to search for a possible limb effect in the K i 769.9 nm resonance line. The observations were carried out along the north/south diameter of the solar disc. The data were individually corrected for the effects of straylight on the velocity measurements. A small, but significant limb effect is detected. The total shift in the line core from center to limb corresponds to 125 m s−1 with an uncertainty of < 30 m s−1.

Solar 5-Minute Oscillations at 2.23 μm

Large amplitude solar 5-min intensity oscillations have recently been detected at 2.23 μm using broad band(650 A FWHM) photometry (Leifsen and Maltby, 1990). Large intensity amplitudes in a broad range in the near infrared was unexpected, and several questions concerning the source of the high amplitudes were raised. In an attempt to study the nature of these oscillations, time series of spectra have been obtained with the Fourier Transform Spectrometer (FTS) of the McMath telescope at National Solar Observatory at Kitt Peak. We present preliminary results from a 10 day long run in May 1991 in support for the suggestion that the results may be useful in both hello-and asteroseismological investigations.