The Comparison of Simultaneous SOI/MDI and Mt. Wilson 60-foot Tower Power Spectra and p-Mode Parameters
Abstract
We present the results of the first detailed comparison between the 1996 SOHO SOI/MDI Dynamics Run program of full-disk Ni I Dopplergram observations and a simultaneous time series of ground-based observations obtained in the Na D lines at the 60-Foot Solar Tower of the Mt. Wilson Observatory (MWO). Specifically, we will compare sets of simultaneously-observed SOHO/MDI and MWO power spectra and the high-degree p-mode frequencies, frequency splittings, widths, and power densities which we obtained by fitting these two sets of power spectra. Beginning on May 23, 1996, the SOI/MDI experiment began its first high duty cycle run of 1024x1024 pixel images. this was the 1996 Dynamics Run. On all but three of the days of this 60.75-day time series a second time series of simultaneous 1024x1024 pixel full-disk Dopplergrams was obtained at MWO. From these simultaneous MWO observations we have computed 601 sets of zonal, tesseral, and sectoral power spectra which covered the degree range of 0 to 600. These sets of power spectra were then analyzed in two different ways to yield both frequencies and frequency splittings. First, estimates of the frequency splittings were computed for the frequency range of 1800 to 4800 microhertz at each degree and these n-averaged splittings were employed to compute an average power spectrum for that degree. Estimates were then made of the frequencies, frequency uncertainties, widths, peak power densities, and background power densities of the set of peaks in these 601 average power spectra. As is described in our companion paper on the MDI ridge-fit frequency measurements (Rhodes et al.,1998), we also had to correct our raw MWO ridge-fit frequencies for the effects the merger of individual p-mode peaks and sidelobes into ridges. We could also directly compare the frequency dependence of the observed power density in both the MDI and MWO power spectra. We have found that there is a systematic difference such that the power density in the chromospheric-level power spectra from MWO tends to be below that of the photosheric-level MDI spectra at low frequencies and to rise above the MDI power density as the frequency increases. All but the l = 0 MWO power spectra were also processed to yield estimates of the rotational splitting coefficients for individual p-mode ridges for every degree between l = 4 and 600. We will compare these splittings with both the previously-published splittings from the 1996 SOI/MDI Medium-l Program (Kosovichev et al., 1997) and with the MDI high-degree splittings presented in our companion MDI paper.
- Publication:
-
Structure and Dynamics of the Interior of the Sun and Sun-like Stars
- Pub Date:
- 1998
- Bibcode:
- 1998ESASP.418..311R