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1 COMPARISON OF THE CALIBRATED MDI AND SIGNALS. II. POWER SPECTRA C.J. Henney, L. Bertello, R.K. Ulrich, R.S. Bogart, R.I. Bush, A.H. Gabriel, R.A. Garca, G. Grec, J.-M. Robillot 6, T. Roca Cortes 7, S. Turck-Chieze Department of Physics & Astronomy, UCLA, Los Angeles, CA , USA Stanford University, CSSA-HEPL, Stanford, CA 90-08, USA Institut d'astrophysique Spatial, Universite Paris XI, 90 Orsay Cedex, France Service d'astrophysique, CEA/DSM/DAPNIA, CE Saclay, France Obsevatoire de la C^ote d'azur, Laboratoire Cassini, 060 Nice, France 6 Observatoire de l'universite Bordeaux, BP 89, 70 Floirac, France 7 Instituto de Astrofsica de Canarias, 80 La Laguna, Tenerife, Spain ABSTRACT The and MDI instruments on SOHO utilize dierent spectral lines: the Na D lines for and the Ni I 676:8nm line for MDI. The two instruments also detect the solar surface motions utilizing dierent techniques: full-disc integrated intensity measurements on one wing of the lines for and a series of ltergrams with peak transmission tuned to four wavelengths spanning the Ni line for MDI. Presented here are preliminary steps to understanding the nature of the two helioseismology observations by comparing the power spectra from an identical 69-day period. We hope to capitalize on the dierences between the two instruments to enhance the ability to detect low frequency modes. Key words: power spectra; velocity; helioseismology; MDI;. and emissivity variations than either MDI or in a two-wing mode. In addition, the instrument does not detect all parts of the solar surface with equal sensitivity. Since the instrument is not uniformly sensitive to velocity spatially across the solar disc, we plan to separate the MDI images into a -like component and a non- component. The cross spectrum between the MDI simulated signal, hereinafter referred to as sim, and the non- signal can be used to conrm the solar origin of potential low frequency oscillation candidates. In addition, the cross spectrum between the and sim signals will reduce the eects of instrument and photon noise. In this preliminary work, we present the cross spectra between the and sim signals.. RESULTS AND DISCUSSION. INTRODUCTION The and MDI instruments dier in many respects. The instrument measures the coherent signals of the integrated solar disc on the blue wing of the doublet Na D lines. The MDI instrument utilizes a dierent spectral line, Ni I 676:8nm, and it is designed to measure the photospheric manifestations of solar oscillations using a pair of tunable Michelson Doppler Interferometers. These dierences cause the two instruments to respond to solar phenomena with diering sensitivity. For example the sodium lines are formed near the temperature minimum where acoustic modes may have a larger amplitude due to the solar atmospheric density gradient. The supergranulation is generally conned to the photospheric layers and may contribute less incoherent velocity variation to an instrument like deriving its signal from the temperature minimum. The instrument in its current one-wing mode of operation may be more sensitive to temperature The MDI velocity-like signal used here is from MDI LOI-proxy velocity images integrated over the solar disc. For shortness of notation, this time series is referred to as MDI hereafter. The MDI velocity signal used in this comparison has nearly 97% coverage. In addition to the MDI velocity signal, a second data series is created by multiplying the LOI-proxy velocity images by a velocity response function. The utilization of this velocity response mask is to produce a simulated velocity from the MDI data which responds like the actual data to the solar oscillation spectrum. Details of this procedure are given in the rst paper of this series (Henney et al., 998). The S calibration of the velocity-like signal used in this work is discussed in Ulrich et al. (998). A unique feature of the experiment is the use of two redundant photomultiplier tubess, each working at two dierent magnetic modulations. Consequently, a total of four quasi-independent counting rate measurements are provided. These four measurements are independently calibrated and averaged

2 000 Amplitude squared (mm/s) 00 0 _sim MDI Figure. Power spectra for (lled circles), sim (open circles) and MDI (lled squares). Each power spectra is from 69 days of data, begining May, 996 and ending February, 998. Ratio of power spectra /MDI /_sim Figure. Ratios of power spectra obtained from 69 days of data, begining May, 996 and ending February, 998. The ratio of relative to sim (open circles) and MDI (lled squares) is shown.

3 : one wing (V S ) vs. two wings (V R ) data Ratio of power spectra V S / V R Figure. The ratio of power spectra observed in the two-wing mode (V R ) and the blue-wing mode (V S ). The details of each time series are outlined in Ulrich et al. (998). to provide the single velocity signal used in this comparison. Since April 996, the instrument has provided stable and reliable data with nearly 00% coverage and continuity. A comparison among the power spectra calculated from the three investigated time series is shown in Figure. The power spectra ratios relative to are shown in Figure. For both instruments the time series is from a 69 day period beginning May 996 and ending February 998. Each individual periodogram has been properly normalized to give an amplitude square spectrum. Examination of Figure shows that the amplitudes of oscillations in the ve-minute band are enhanced in the signal compared to the MDI integrated velocity. The utilization of a velocity response function to produce the sim velocity signal from the MDI velocity data moves the spectrum towards that of the actual data but by only a small fraction of the dierence. Since the instrument is operating in a single wing mode, it is expected to be sensitive to rst order changes in intensity variations due to solar oscillations (e.g. Ulrich et al., 998). Thus some of enhanced power in the one-wing signal relative to MDI is most likely a result of the contribution of intensity. Dierences between the and the MDI instruments, in terms of their response to the solar oscillation signal, is emphasized by taking the ratio of the power spectra. Figure reveals the dierences in the solar background prole as observed by MDI and. Contributions from intensity in the spectrum and the dierence in the signal to noise ratio between the data sets account for these prole dierences. It is important to note that the signal to noise ratio in the MDI velocity signal is greatly improved if a zero-sum mask is used with the integrated MDI signal (e.g. Scherrer, 998). However, an eect of the zero-sum mask is to signicantly reduce the contribution from the `=0 modes. For consistency with the analysis performed in this series, a MDI velocity signal using a zero-sum mask is not considered here. The sensitivity to granulation since observing in the one-wing mode is illustrated in Figure. The power spectra bulge centered near 70 Hz, shown in Figure, is most likely due to the intensity contribution from granulation overshoot. We note that the comparison in Ulrich et al. (998) between in dierent operating modes shows that a shift in going from two-wing operations to one-wing operations is similar to that seen in going from the MDI simulation of to one-wing (see Figure ). The substantial dierence in the power ratio near 000 Hz, shown in Figure, could be the result of the intensity contribution from chromospheric oscillations (e.g. Harvey et al., 998). This -minute broad band power is also seen in the blue-wing to twowing ratio in Figure. Examination of the ne structure of individual power spectrum peaks, displayed in Figure, shows that the match is better between the and the sim signals than between the and the straight MDI signal. In Figure, notice the enhance-

4 MDI LOI-proxy _sim Cross Amplitude Figure. Comparison of velocity power spectra between MDI LOI-proxy (top), sim (second from top) and (second from bottom) for a period of 69 days: May, 996 through February, 998 Also shown is the the cross amplitude spectrum between sim and (bottom). Notice the enhancement of the ` = mode (near.7 mhz) and the ` = mode (near.8 mhz) in the sim signal relative to MDI alone. ment of the ` = mode (near.7 mhz) and the ` = mode (near.8 mhz) in the sim signal relative to MDI and. This enhancement is a result of the velocity response function acting as a spatial lter with preference to ` = and. Example response functions are shown in Henney et al. (998). Figure shows a detailed section of the p-mode spectrum around the multiplet l =, n = 8. For the 69-day period investigated here, this multiplet is the lowest p-mode clearly identiable for either or MDI. Notice that the signal to noise is substantially improved in the cross amplitude spectrum of sim and compared to MDI or alone. Using the power spectrum presented here, individual mode frequencies and splittings for the three signals are compared in paper III of this series (Bertello et al., 998a). In addition, the p-mode energies observed from MDI and are presented in paper IV (Bertello et al., 998b).. CONCLUSION We nd that the observed velocity power spectrum is up to. times greater than the power observed in the MDI velocity signal in the frequency band.0-.0 mhz. The and MDI signals are in better agreement in the frequency range below 0. mhz. The indirect comparison of two-wing data, shown in Ulrich et al. (998), to the MDI signal suggests that the amplitudes for these two signals are similar. Consequently, the enhanced amplitude of the one-wing signal probably results from an intensity component of the signal rather than the dierence in altitude of formation of the two spectral lines. Thus, the enhancement of the signal in the frequency band 0. to.8 mhz is most likely due to intensity contributions from granulation overshoot. Additionally, the signal enhancement in the.0-.0 mhz band is believed to be the result of intensity contributions from chromospheric oscillations. In addition, we nd that the sim signal matches the `= and `= mode ne structure observed by better than MDI velocity images alone. Furthermore, the cross correlation between and sim resulted in an improved signal to noise ratio compared to the MDI or power spectra alone.

5 MDI LOI-proxy _sim Cross Amplitude Figure. Detailed comparison of velocity power spectra around the ` = ; n = 8 mode between MDI LOI-proxy (top), sim (second from top) and (second from bottom) for a period of 69 days: May, 996 through February, 998. Also shown is the the cross amplitude spectrum between sim and (bottom), which shows an improved signal to noise ratio compared to the MDI or spectra alone. ACKNOWLEDGMENTS The instrument has been constructed by a consortium of French and Spanish institutes, supported by a large number of scientic investigators from many countries. SOHO is a mission of international cooperation between ESA and NASA. This research is supported by a NASA subcontract to UCLA through Stanford University. REFERENCES Nice, France, (eds.) J. Provost and F.X. Schmider, p 77 Henney, C.J., Ulrich, R.K., Bertello, L. et al. 998, I. time series, These Proceedings Scherrer, P.H. 998, Search for the 60-minute signal in the MDI data, These Proceedings Scherrer, P.H., Bogart, R.S., Bush, R.I. et al. 99, Solar Physics, 6, 9 Ulrich, R.K., Bertello, L., Boumier, P. et al. 998, Calibration of the velocity signal, These Proceedings Bertello, L., Henney, C.J., Ulrich, R.K. et al. 998a, III. p-mode frequencies and splittings, These Proceedings Bertello, L., Ulrich, R.K., Henney, C.J. et al. 998b, IV. p-mode energy budget, These Proceedings Gabriel, A.H., Grec, G., Charra, J., et al. 99, Solar Physics, 6, 6 Harvery, J., Jeeries, S., Duvall, T. et al. 998, Sounding Solar and Stellar Interiors, IAU Symposium 8, September 0 - October, Nice, France, Observatoire de la Cote d'azur and Universite de