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Astron. Astrophys. 362, 379-382 (2000)

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1. Introduction

The main solar magnetic field demonstrates clear 22-year periodicity: at the beginning of the cycle (near the activity minimum), the magnetic field is predominantly dipolar, and the dipole is almost aligned with the Sun's rotation axis. The relative role of higher harmonics (quadrupole, octupole, etc.) increases with solar activity, resulting in total destruction of the initial regular dipolar magnetic field configuration after 5-6 years, near the solar maximum. On the decline phase of the solar cycle, the dipole field is restored with the opposite direction of the dipole. Thus, the main solar magnetic field becomes predominantly dipolar again (being rotated by 180 o) after approximately 11 years. During the subsequent 11 years the dipole performs the next turn and arrives at the initial configuration (e.g. Babcock 1961).

This cycle is accompanied by variations of signatures of solar activity like sunspots, solar flares and coronal mass ejections. Obviously, the solar wind parameters are also affected by changing initial/boundary conditions of its outflow in the solar atmosphere. The dependence of the solar wind spatial/temporal structure on the solar magnetic field is investigated in a number of papers (e.g., King 1979, Slavin et al. 1984, 1986; Winterhalter et al. 1990, McComas et al. 1992, Neugebauer et al. 1998). However, it remains obscure whether a feedback exists between the solar wind and the solar magnetism. In other words, does the solar wind affect the solar cycle?

There are at least two reasons to ask this question. First, the magnetic field produced by the heliospheric current system is not negligibly small near the Sun and, hence, it may be necessary to take it into consideration in the problem of solar magnetic field generation. Indeed, the magnetic field, produced near the Sun by the heliospheric current sheet, can be estimated as


where [FORMULA] is the azimuthal component of the electric current surface density in the sheet, and [FORMULA] is the distance at which the current sheet starts. The electric current surface density is [FORMULA], where [FORMULA] is the radial component of IMF above (or below) the current sheet. Since [FORMULA] and equals [FORMULA] G at 1 AU, [FORMULA] G, i.e. several percent of the global solar intrinsic magnetic field.

The second reason, which we believe to be more important and which we will address in this note is as follows. It is not clear in advance, that the interplanetary electric field (IEF) associated with the solar wind plasma flow [FORMULA] is a potential field. Of course, the instantaneous values of [FORMULA] may be non zero because of the variability of the solar wind. Discussing the potentiality of the IEF, we mean [FORMULA] values averaged over time scales larger than the period of the Sun's rotation (but much smaller than the solar cycle period). Usually, in modeling it is implicitly assumed that [FORMULA] which means that the solar wind plasma flows along the IMF lines (in the frame of reference rotating with the Sun) (e.g. Pneumann & Kopp 1971, Cuperman et al. 1990, Linker et al. 1996). We will check the validity of this assumption using the available data on the long term solar wind monitoring.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100