The last dance of the bashful ballerina?

Abstract

Aims. The heliospheric magnetic field (HMF) has long been hemispherically asymmetric so that the field in the northern hemisphere is weaker and the area larger than in the south. This asymmetry, also called the bashful ballerina, has existed during roughly three-year intervals of the late declining to minimum phase of solar cycles 16-22. We study the HMF and its hemispheric asymmetry during the exceptional solar cycle 23. Methods. We use NASA National Space Science Data Center OMNI database, which contains all solar wind and HMF observations at the Earth's orbit, and coronal field predictions by Wilcox Solar Observatory. We present a new method to study the global hemispheric asymmetry by using the power n of the radial decrease of the radial field from the coronal source surface to 1 AU. Results. We find that the HMF is exceptional at low latitudes in solar cycle 23: while the typical latitudinal variation was attained in the north in 2008, it did not take place in the south until Spring 2009. Thus, the Rosenberg-Coleman rule is abnormally delayed or broken for the first time in 50 years. The n-values verify the clear northern dominance in cycles 21-22. However, the low-latitude observations depict a considerably smaller asymmetry in cycle 23, although Ulysses observations at high latitudes show an equally large asymmetry in 2007 and in 1994-1995. We argue that the weak low-latitude visibility of the asymmetry in cycle 23 is due to the exceptionally weak polar fields, leading to large tilt angle and a wide current sheet. Conclusions. We note that the exceptional properties of cycle 23 (weak dynamo, large tilt, small asymmetry) agree with the long-term evolution of hemispheric asymmetry viewed at the Earth. The active Sun is seen as more asymmetric at the Earth than the quiet Sun because the polar coronal holes with unipolar fields extend closer to the equator, allowing their asymmetry to be viewed even at low latitudes. We suggest that, after the period of weak activity and small asymmetry at 1 AU that started with cycle 23, the hemispheric asymmetry will again, with the increasingly active cycles, become better visible at 1 AU but the asymmetry will be oppositely oriented, including a northward shifted current sheet, and larger areas but weaker intensities in the south. Thus, the ballerina should no longer be systematically bashful for some 100-150 years.

Photospheric and coronal magnetic fields in six magnetographs

“…There is also strong evidence of a southward shift during SC20 and SC24. Thus, the phenomenon of the bashful ballerina (Mursula & Hiltula 2003) is consistently valid since 1974 according to six magnetographs in agreement with results based on heliospheric observations (Mursula & Virtanen 2012) and heliospheric-solar comparison (Mursula & Virtanen 2011).…”

“…During the minima after active solar cycles (SC), such as SC21 and SC22, the polar fields are strong, leading to a flat and almost nontilted HCS. However, when polar fields are weak, for example, during SC23, the HCS tends to remain tilted over most of the declining and minimum phase (Richardson & Paularena 1997;Mursula & Virtanen 2011). …”

Comparing Coronal and Heliospheric Magnetic Fields Over Several Solar Cycles

“…Similarly to the previous solar cycle there is another peak in polarity match percentage in the late declining phase, before the polarity match minimum of solar minimum in [2008][2009]. Also, similar to that described earlier, these changes in polarity match are related to the HCS asymmetry, although the asymmetry in cycle 23 was overall less systematic at 1 au than in earlier cycles (Mursula & Virtanen 2011).…”

“…A fairly similar cyclic variation is found for all four source surface distances. The solar cycle variation in p is caused by the structural changes in the coronal magnetic field and, in particular, by the mean location of the heliospheric current sheet (HCS) over the course of the solar cycle, as explained by Mursula & Virtanen (2011). During solar minimum the HCS is flat and the Earth is long located close to the HCS, so the source latitude of the observed HMF is close to the neutral line on the source surface.…”

“…In particular, the flatness of the HCS and the proximity of the HMF field source to the HCS produce the minima of p during sunspot minima (Mursula & Virtanen 2011). The PFSS model gives low field values close to the neutral line because the field goes continuously to zero before changing its sign (Zhao & Hoeksema 1995).…”