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

2. The GOLF signal

The GOLF instrument is a sodium resonant scattering spectrophotometer which measures the integrated photometric signal of the solar disc on either the blue or red wing of the Na D lines. The GOLF sodium cell is positioned in a gauss magnetic field with an additional magnetic field modulation of gauss from an electromagnet (Ulrich et al. 1998). This allows two photometric measurements on one wing of the Na D line profiles. The signal variation observed by GOLF is primarily a result of velocity and only 15% of the signal in the 5-minute band can be attributed to an intensity-like signal (e.g. Pallé et al. 1999). Due to a malfunction of the rotating mechanism for the linear polarizer and the quarter wave plate, the GOLF instrument observed in the blue wings of the Na D lines for the period investigated here.

The GOLF signal used in this work is an average of the four calibrated velocities using the S method (Ulrich et al. 1998). This calibration method is based on an accurate velocity scaling over the period data acquisition. Four signals are observed since GOLF measures the intensity from two magnetic modulations with two photometers. The four calibrated GOLF signals are filtered using a non-recursive digital filter with a band pass between 30 and 8100 µHz and then resampled into the MDI Doppler data time reference with a cadence of 60s. The GOLF signal coverage is approximately 99% for the period studied in this paper.

2.1. The GOLF velocity response

Following the notation of Ulrich et al. (1999), the signals observed by GOLF in the blue wing of the Na D line profiles, , can be represented as

where

In the above relations, the solar surface line-of-sight velocity field and a small surface element projected onto the observing plane are represented by and dA respectively. The superscript indicates the two electromagnet states. The velocity sensitivity, or response, function, , is derived from Na line profiles observed from the Mt. Wilson 150-foot solar tower at different solar center-to-limb positions and is discussed in detail by Ulrich et al. (1999).

The velocity response function is dependent on the observed velocity, which is the sum of the line-of-sight oscillation velocity and the effective offset velocity signal. During the course of a year the sun-SOHO line-of-sight offset velocity signal, which includes the orbital, gravitational and convective shift velocities, ranges between approximately 100 and 1100 m/s. The computed GOLF velocity sensitivity function is defined here as

where v and are the line-of-sight velocity the conversion factor from fractional intensity change to velocity respectively. In Eq. (3), the GOLF observed intensity from the Na D line profiles in the blue wing at a given solar disc position and the average over the disc is represented by and respectively.

Due to the change in the observed slope along the wings of the Na line profiles the velocity response function is asymmetric across the solar disc and depends on the effective offset velocity signal. This effect is illustrated in Figs. 1 and 2 for three offset velocities. The GOLF instrument is less sensitive to velocities in the solar disc area rotating away from SOHO when the offset velocity signal is greater than 600 m/s (see Fig. 2). In addition to sampling different parts of the wing due to Doppler shifts of the solar line, the Na D₁ line profile shape is observed to vary due to magnetic activity and center-to-limb position (e.g. Ulrich et al. 1993 and Ulrich et al. 1999). Note that the single wing mode of observing is a photometric measurement, thus the signal includes both intensity and velocity fluctuations. In Eq. (1) the effects of magnetic activity and temperature perturbations are ignored for this preliminary work. The details about calculating are fully developed in Ulrich et al. (1999).

Fig. 1. Examples of the GOLF velocity sensitivity function, , illustrating the extremes with the offset velocities of 100 and 1100 m/s (top row ). Also shown, is an example of a MDI LOI-proxy velocity image with the day average of each bin subtracted to remove large scale spatial gradients (bottom left ). Binned using the LOI-proxy mask, an example GOLF velocity sensitivity function image for an offset velocity of 600 m/s is shown bottom right. White represents the maximum value and black is the minimum.

Fig. 2. Example equatorial slices of the GOLF velocity sensitivity function, , for effective offset velocities of: 100 m/s (solid), 600 m/s (dot) and 1100 m/s (dash). The central meridian distance angle is represented by . The response function slices are normalized by their individual maximum.

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999
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