Solar Cycle Variation of the Sun’s Low-Order Magnetic Multipoles: Heliospheric Consequences

Abstract

The Sun’s dipole and quadrupole components play a central role in the solar cycle evolution of the interplanetary magnetic field (IMF). The long-term variation of the radial IMF component approximately tracks that of the total dipole moment, with additional contributions coming near sunspot maximum from the quadrupole moment and from CMEs. The axial and equatorial components of the dipole vary out of phase with each other over the solar cycle. The equatorial dipole, whose photospheric sources are subject to rotational shearing, decays on a timescale of ∼1 yr and must be continually regenerated by new sunspot activity; its fluctuating strength depends not only on the activity level, but also on the longitudinal phase relationships among the active regions. During cycles 21–23, the equatorial dipole and IMF reached their peak strength ∼2 yrs after sunspot maximum; conversely, large dips or “Gnevyshev gaps” occurred when active regions emerged longitudinally out of phase with each other. The ¹⁰Be-inferred phase shift in the IMF variation during the Maunder Minimum may be explained by a decrease in the amplitude of the equatorial dipole relative to the axial dipole, due either to a systematic weakening of the emerging bipoles or to an increase in their tilt angles. In mid-2012, during the polarity reversal of cycle 24, the nonaxisymmetric quadrupole component became so dominant that the heliospheric current sheet (HCS) split into two cylindrical components. Hemispheric asymmetries in sunspot activity give rise to an axisymmetric quadrupole component, which has combined with the axial dipole to produce a systematic southward displacement of the HCS since cycle 20.