At the end of last year, the Sun’s large-scale magnetic field suddenly strengthened, reaching its highest value in over two decades. Here, Neil Sheeley and Yi-Ming Wang (both of the Naval Research Laboratory) propose an explanation for why this happened and what it predicts for the next solar cycle.
Until midway through 2014, solar cycle 24 — the current solar cycle — was remarkably quiet. Even at its peak, it averaged only 79 sunspots per year, compared to maximums of up to 190 in recent cycles. Thus it was rather surprising when, toward the end of 2014, the Sun’s large-scale magnetic field underwent a sudden rejuvenation, with its mean field leaping up to its highest values since 1991 and causing unprecedentedly large numbers of coronal loops to collapse inward.
Yet in spite of the increase we observed in the Sun’s open flux (the magnetic flux leaving the Sun’s atmosphere, measured from Earth), there was not a significant increase in solar activity, as indicated by sunspot number and the rate of coronal mass ejections. This means that the number of sources of magnetic flux didn’t increase — so Sheeley and Wang conclude that flux must instead have been emerging from those sources in a more efficient way! But how?
Aligned ActivityThe authors show that the active regions on the solar surface in late 2014 lined up in such a way that the emerging flux was enhanced, forming a strong equatorial dipole field that accounts for the sudden rejuvenation observed.
Interestingly, this rejuvenation of the Sun’s open flux wasn’t just a one-time thing; similar bursts have occurred shortly after the peak of every sunspot cycle that we have flux measurements for. The authors find that three factors (how the active regions are distributed longitudinally, their sizes, and the contribution of the axisymmetric component of the magnetic field) determine the strength of this rejuvenation. All three of these factors happened to contribute optimally in 2014.
As a final note, Sheeley and Wang suggest that the current strength of the axisymmetric component of the magnetic field can be used to provide an early indication of how active the next solar cycle might be. Using this method, they predict that solar cycle 25 will be similar to the current cycle in amplitude.
N. R. Sheeley Jr. and Y.-M. Wang 2015 ApJ 809 113. doi:10.1088/0004-637X/809/2/113