The Minimum of Solar Cycle 23: As Deep as It Could Be?

Abstract

In this work we introduce a new way of binning sunspot group data with the purpose of better understanding the impact of the solar cycle on sunspot properties and how this defined the characteristics of the extended minimum of cycle 23. Our approach assumes that the statistical properties of sunspots are completely determined by the strength of the underlying large-scale field and have no additional time dependencies. We use the amplitude of the cycle at any given moment (something we refer to as activity level) as a proxy for the strength of this deep-seated magnetic field.We find that the sunspot size distribution is composed of two populations: one population of groups and active regions and a second population of pores and ephemeral regions. When fits are performed at periods of different activity level, only the statistical properties of the former population, the active regions, is found to vary.Finally, we study the relative contribution of each component (small-scale versus large-scale) to solar magnetism. We find that when hemispheres are treated separately, almost every one of the past 12 solar minima reaches a point where the main contribution to magnetism comes from the small-scale component. However, due to asymmetries in cycle phase, this state is very rarely reached by both hemispheres at the same time. From this we infer that even though each hemisphere did reach the magnetic baseline, from a heliospheric point of view the minimum of cycle 23 was not as deep as it could have been.

The reversal of the Sun’s magnetic field in cycle 24

“…Despite a relatively low amplitude in cycle 23, the asynchronicity in sunspot activity between two hemispheres helped in maintaining a relatively high level of magnetic activity during minimum of cycle 23. Muñoz-Jaramillo et al [2015] have shown that although the minimum of cycle 23 was one of the lowest in recent history, it did not reach the lowest possible limit because the activity in each hemisphere reached minimum at different times. At the end of cycle 23, sunspot activity in the southern hemisphere lasted longer, as compared to the northern hemisphere.…”

A Remarkable Recent Transition in the Solar Dynamo

“…The maximum umbral field strength stayed virtually constant, at least till 2016. An important piece of additional of information is that the minimum of cycle 23 is not the deepest that is possible (Muñoz-Jaramillo et al 2015). The lowest value of the umbral magnetic field as measured with the traditional Fe 1564.8 nm line is 1500 Gauss.…”

Time-dependent properties of sunspot groups

“…In order to better visualise the small changes in the distribution of the area of sunspot groups, we plot the logarithmic variation of the area, as suggested by earlier studies (e.g. Petrovay & van Driel-Gesztelyi 1997;Muñoz-Jaramillo et al 2015;Namekata et al 2019). As we specified earlier, in our analysis we neglected all small groups where the total (umbral and penumbral) area at its maximum did not reach 50 MSH.…”