Sunspot activity and the long‐term variation of the Sun's open magnetic flux

Abstract

[1] The interplanetary magnetic field (IMF) originates in open magnetic regions of the Sun (coronal holes), which in turn form mainly through the emergence and dispersal of active region fields. The radial IMF strength is proportional to the total open flux È open , which can be estimated from source surface extrapolations of the measured photospheric field, after correction for magnetograph saturation effects. We derive the long-term variation of È open during 1971-2000 and discuss its relation to sunspot activity. The average value of È open was $20-30% higher during 1976-1996 than during 1971-1976 and 1996-2000, with major peaks occurring in 1982 and 1991. Near sunspot minimum, most of the open flux resides in the large polar coronal holes, whereas at sunspot maximum it is rooted in relatively small, low-latitude holes located near active regions and characterized by strong footpoint fields; since the decrease in the total area occupied by holes is offset by the increase in their average field strengths, È open remains roughly constant between activity minimum and maximum, unlike the total photospheric flux È tot . The long-term variation of È open approximately follows that of the Sun's total dipole strength, with a contribution from the magnetic quadrupole around sunspot maximum. Global fluctuations in sunspot activity lead to increases in the equatorial dipole strength and hence to enhancements in È open and the IMF strength lasting typically $1 year. We employ simulations to clarify the role of active region emergence and photospheric transport processes in the evolution of the open flux. Representing the initial field configuration by one or more bipolar magnetic regions (BMRs), we calculate its subsequent evolution under the influence of differential rotation, supergranular convection, and a poleward bulk flow. The initial value of È open is determined largely by the equatorial dipole strength, which in turn depends on the longitudinal phase relations between the BMRs. As the surface flow carries the BMR flux to higher latitudes, the equatorial dipole is annihilated on a timescale of $1 year by the combined effect of rotational shearing and turbulent diffusion. The remaining flux becomes concentrated around the poles, and È open approaches a limiting value that depends on the axisymmetric dipole strengths of the original BMRs. The polar coronal holes thus represent the longlived, axisymmetric remnant of the active regions that emerged earlier in the cycle.

Open solar flux estimates from near-Earth measurements of the interplanetary magnetic field: comparison of the first two perihelion passes of the Ulysses spacecraft

“…With an improved latitudedependent saturation correction factor, Wang and Sheeley (1995) were able to match to the radial field seen at Earth during solar cycles 20 and 21, again using the assumption that B r is independent of latitude in the heliosphere, as found from the Ulysses observations. Recently, Wang and Sheeley (2002) have shown that this result holds for cycles 22 and 23 as well. Thus, the work of Sheeley (1995, 2002) gives strong evidence that the Ulysses result on the uniformity of the radial heliospheric field is valid throughout cycles 21-23.…”

“…(1). The PFSS procedure, as applied by Wang and Sheeley (2002) were used. The first perihelion pass took place during the interval September 1994 until July 1995 and thus, the [F s ] PFSS value in Table 1 , Lockwood et al 1999aLockwood, 2001Lockwood, , 2002b, agrees well with the predictions of the modelling by Solanki et al (2000) and Lean et al (2002).…”

Surface Evolution of the Sun's Magnetic Field: A Historical Review of the Flux-Transport Mechanism

“…Mackay et al concluded that something important must be missing from the model and suggested that the problem might lie with the source-surface component. However, the source-surface model seems to be an unlikely culprit because it correctly reproduces the observed variation of open flux when it is applied to the observed photospheric field (as opposed to the field simulated from the doublet sources) (Wang et al, 2000a;Wang and Sheeley Jr, 2002).…”

The Sun's global magnetic field

“…The author pointed out that the correction factor used by Wang & Sheeley [50] was only correct for the use on MWO data and did not apply to WSO data. It has its own correction factor of 1.85 [52].…”

“…In the study by Wang & Sheeley [50], a combination of synoptic magnetograms and PFSS models were used to compute the open flux from 1971 to 1998. The synoptic magnetograms originated from either Wilcox Solar Observatory (WSO 1976(WSO -1995 or Mount Wilson Observatory (MWO 1971(MWO -1976(MWO , 1995(MWO -1998.…”