Observations of the Sun's magnetic field during the recent solar maximum

Abstract

We present new observations and analyses of the Sun's magnetic field and coronal holes. Using magnetic field observations from the Wilcox Solar Observatory, we present a simple means whereby the tilt angle of the current sheet can be calculated. We use a data set covering the last 26 years, which shows for the first time how the dipole component rotated once during a full 22‐year solar cycle. We show how this influenced the current sheet. At solar minimum, the Sun's coronal magnetic field was essentially dipolar and aligned parallel to the spin axis. As a result, the heliospheric current sheet was flat and had very little warp. Around solar maximum, the dipole was perpendicular to the spin axis, and the ratio of quadrupole to dipole strength was high for much of the time. This meant that the current sheet was tilted and highly warped, and reached up to high latitudes. Surprisingly, there were also times close to solar maximum when the quadrupole/dipole ratio was low, and the current sheet was relatively flat, but still highly inclined. We apply for the first time to solar magnetic data a method, which quantitatively analyses the quadrupole component of the magnetic field. From the terms of the expansion of the observed photospheric magnetic field, we compute the position of the poles of the magnetic field. We combine for the first time over an extended period of time magnetic field data from the Wilcox Solar Observatory with coronal hole positions taken from the National Solar Observatory/Kitt Peak. We find that the position of the coronal holes followed the motion of the poles of the magnetic field as the poles moved over the surface of the Sun and that the polar coronal holes broke up into groups of smaller like‐polarity holes as the poles approached the midlatitude regions and the quadrupole became more important. We discuss the implications for energetic particle observations at Ulysses.