What the Sudden Death of Solar Cycles Can Tell us About the Nature of the Solar Interior

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Monday, July 1, 2019

We observe the abrupt end of solar activity cycles at the Sun’s Equator by combining almost 140 years of observations from ground and space. These "terminator" events appear to be very closely related to the onset of magnetic activity belonging to the next solar cycle at mid-latitudes and the polar-reversal process at high latitudes.

Graph depicting 140 year record of solar filaments
The 140 year record of solar filaments as observed in H-alpha observations from three sites: Arcetri Astrophysical Observatory (AO; 1880-1929), Meudon Observatory (MO; 1919-1989), and the Kislovodsk Observatory (KO; 1980-2018). Vertical dashed lines show the times of the terminator event as derived from the activity bands of McIntosh et al. 2014 and the sunspot area proxy. The terminator events correspond to the rapid growth of sunspot and low-latitude filament butterfly wings, as well as the poleward propagation of the highest latitude filament (panel C) across the 55 degree latitude marked by the horizontal dashed lines. Unlike hemispheric sunspot activity, this "rush to the poles" appears to be well-synchronized across the two hemispheres.

Using multi-scale tracers of solar activity we examine the timing of these events in relation to the excitation of new activity and find that the time taken for the solar plasma to communicate this transition is of the order of one solar rotation - but could be shorter. Utilizing uniquely comprehensive solar observations from the Solar Terrestrial Relations Observatory (STEREO) and Solar Dynamics Observatory (SDO) we see that this transitional event is strongly longitudinal in nature. Combined, these characteristics suggest that information is communicated through the solar interior rapidly. A range of possibilities exist to explain such behavior: for example gravity waves on the solar tachocline, or that the magnetic fields present in the Sun’s convection zone could be very large, with a poloidal field strengths reaching 50 kG - considerably larger than conventional explorations of solar and stellar dynamos estimate. Regardless of the mechanism responsible, the rapid timescales demonstrated by the Sun’s global magnetic field reconfiguration present strong constraints on first-principles numerical simulations of the solar interior and, by extension, other stars.

Publication Name: Solar Physics First HAO
Author's Name: Scott McIntosh
Author's Email: mscott@ucar.edu