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Astron. Astrophys. 346, 626-632 (1999)

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1. Introduction

The solar mean magnetic field (hereafter SMMF) has been recorded in integrated sunlight (i.e., the Sun seen as a star) with solar magnetographs at Crimea, Mount Wilson, Stanford and Sayan for a long time (Severny et al. 1970; Scherrer et al. 1977a,b; Grigoryev & Demidov 1987).

As most field regions on the Sun are bipolar, the SMMF is small, less than [FORMULA] 1 G, and changing with the magnetic cycle (Livingston et al. 1991), reaching a typical value of about 0.15 G during solar minimum.

SMMF is interesting in many problems of solar physics. The determination of differential rotation, meridional circulation and magnetic field diffusion coefficients has been possible by comparison with numerical simulations (Sheeley et al. 1985; DeVore et al. 1985; Sheeley & DeVore 1986a,b). Also, its influence on the interplanetary magnetic field, measured from Earth, has been studied (Severny et al. 1970; Scherrer 1977a,b), showing a high correlation between the two (after accounting for the 4 day solar wind transit time to Earth).

The pattern of solar magnetic fields can be described in terms of its spherical harmonic components as in the case of the Doppler velocities. Stenflo & Vogel (1986) have analyzed a modal power spectrum of 25 years of synoptic maps, showing that the modes behave differently when odd or even [FORMULA] modes are considered (Stenflo & Güdel 1988). When the SMMF is measured, we filter out the modes of high degree as with the Doppler velocities. These kinds of modes have not been unambiguously detected until now. However, the existence of rapid time variations in the SMMF, that cannot be explained directly by the solar rotation, has been pointed out by several authors (for example: Severny 1971; Ioshpa et al. 1973; Kotov 1990; Demidov 1995) giving periods ranging from 5 to 300 minutes. This has been a very difficult task due to the expected low amplitudes of these variations. The problem becomes more complicated by the fact that the signal of the magnetographs, used to obtain the SMMF, depends on many factors such as the brightness, the spectral line profile parameters and the determination of the zero level. Due to this fact, a very high sensitivity measurement has been included as a part of the GOLF experiment on-board the SOHO mission.

In this paper, we first describe in Sect. 2, the instrument and the data series used. In Sect. 3, we describe two independent methods to derive the SMMF and perform tests applied to numerical simulations (Sect. 4). After computing the theoretical sensitivity of GOLF to the magnetic field measurement (Sect. 5), we analyze the real GOLF data series showing the resultant power spectrum and discussing the results (Sects. 6 and 7).

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© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999