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Astron. Astrophys. 348, 627-635 (1999)

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1. Introduction

Instrument details and project goals of the Global Oscillations at Low Frequency (GOLF) and the Michelson Doppler Imager (MDI) instruments aboard the Solar and Heliospheric Observatory (SOHO) are outlined in Gabriel et al. (1995) and Scherrer et al. (1995) respectively. Preliminary results from GOLF are presented by Gabriel et al. (1997) and Turck-Chièze et al. (1997). Additionally, initial results from the MDI instrument are presented by Kosovichev et al. (1997) and Duvall et al. (1997). One of the mission objectives of both the GOLF and the MDI instruments on SOHO is the detection of new low frequency globally coherent oscillation modes. After more than two years of nearly continuous observation of the sun by both instruments, the clear detection of modes below 1000 µHz has still proven to be elusive. The search for new modes may be aided by combining the high coverage of GOLF with the spatial resolution and various data products of MDI. By combining the two data sets, a signal enhancement is anticipated since both instruments provide a low noise data stream and their sources of solar and instrumental noise are expected to be different from each other.

The GOLF and MDI instruments differ in many respects. For the period investigated here, the GOLF instrument measured the intensity of the integrated solar disc on the blue wing of the doublet Na D lines. The MDI instrument utilizes a mid-photospheric absorption line, Ni I 676.8nm, and it is designed to measure the intensity at five positions on or near the line profile using a pair of tunable Michelson interferometers. These differences cause the two instruments to respond to solar phenomena with differing 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. Supergranulation is mostly confined to the photospheric layers and may contribute less incoherent velocity variation to an instrument like GOLF deriving its signal from the temperature minimum.

Ideally we want to synthesize a GOLF-simulated velocity signal from the spatially resolved MDI velocity images and compare that directly to the observed GOLF signal. A GOLF-simulated signal may be useful in comparing the instrumental performances of both GOLF and MDI. Since the GOLF instrument is sensitive to solar magnetic activity, the MDI magnetic proxy data may be functional in removing active region effects from the observed GOLF signal (e.g. Ulrich et al. 1993). Correspondingly, the sodium cell design of the GOLF instrument allows for an atomic wavelength reference, whereas the MDI instrument relies on a thermal system to remain optically stable. Thus, both data sets may be beneficial to each other in terms of calibration.

Previous MDI and GOLF velocity signal comparisons showed excellent agreement in the 5-minute band (e.g. Toutain et al. 1997, Henney et al. 1998a and Pallé et al. 1999). In the range 1300 to 4000 µHz, the low degree ([FORMULA]) acoustic mode frequency differences between the two instruments are found to be within the error of the measurements (Toutain et al. 1997 and Bertello et al. 1998). In addition, a GOLF-simulated velocity signal similar to the one presented in this paper showed a slight improvement in the correlation with the GOLF signal relative to the raw integrated full-disk MDI velocity signal (Henney et al. 1998a).

In the following sections the procedure used to create the MDI masked velocity signals is outlined, along with power spectra comparison between MDI and GOLF signals. The observed GOLF signal and the modeled GOLF velocity response is discussed in Sect. 2. Signal processing and spatial masking of MDI images is described in Sect. 3. The signal-to-background ratios for low frequency acoustic modes of the MDI and GOLF signals are presented and discussed in Sect. 4.

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© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999
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