Unusual Polar Conditions in Solar Cycle 24 and Their Implications for Cycle 25

Abstract

We report on the prolonged solar-maximum conditions until late 2015 at the north-polar region of the Sun indicated by the occurrence of high-latitude prominence eruptions (PEs) and microwave brightness temperature close to the quiet-Sun level. These two aspects of solar activity indicate that the polarity reversal was completed by mid-2014 in the south and late 2015 in the north. The microwave brightness in the south-polar region has increased to a level exceeding the level of the Cycle 23/24 minimum, but just started to increase in the north. The northsouth asymmetry in the polarity reversal has switched from that in Cycle 23. These observations lead us to the hypothesis that the onset of Cycle 25 in the northern hemisphere is likely to be delayed with respect to that in the southern hemisphere. We find that the unusual condition in the north is a direct consequence of the arrival of poleward surges of opposite polarity from the active region belt. We also find that multiple rush-to-the-pole episodes were indicated by the PE locations that lined up at the boundary between opposite-polarity surges. The high-latitude PEs occurred in the boundary between the incumbent polar flux and the insurgent flux of opposite polarity.

Long-term solar activity studies using microwave imaging observations and prediction for cycle 25

“…It must be appreciated that it is difficult to observe the weak polar magnetic field especially because the field is generally perpendicular to the line of sight. During the well observed reversal in cycle 24, Gopalswamy et al (2016) found that the reversal was complete within 6 months to a year from the time the zero-field condition was attained at the solar poles. The end of high-latitude prominence activity was much closer (within six months) to the time of polarity reversal.…”

“…9) as in cycle 24. The prolonged zero-field condition before reversal has been shown to be due to surges that violate Joy's law (Sun et al 2015;Mordvinov et al 2016;Gopalswamy et al 2016). Such surges cause the reversal to be episodic instead of being sharp.…”

The Sun’s Fast Dynamo Action

“…The Sun's polar magnetic fields are much weaker than active region fields; nevertheless, they have far-reaching importance because of their unipolarity over large spatial scales and because of their role in the solar activity cycle [29]. Sporadically a north-south asymmetry in the polarity reversal is observed; that is, the one polar reversal preceded the other for more than a year (e.g., [30,31]).…”

Beyond the Minisolar Maximum of Solar Cycle 24: Declining Solar Magnetic Fields and the Response of the Terrestrial Magnetosphere

“…The other study that we found for such long-term trends for the MP stand-off distances is by McComas et al (2013), wherein the authors reported the canonical stand-off distance of the MP to be about 11 Earth radii (R E ), for the period 2009 to 2013, covering the minimum of cycle 23 to the early rise phase of cycle 24, compared to about 10 R E for the period 1974 to 1994, covering cycles 21-22, while the normal value of the MP is usually ∼10 R E . It must however, be kept in mind that different researchers have used different latitude ranges to estimate polar fields, namely, poleward of 45 • (Bisoi, Janardhan, Chakrabarty, et al, 2014;Janardhan et al, 2011), 55 • (Wilcox Solar Observatory polar fields, http://wso.stanford.edu/Polar.html, Janardhan et al (2018)), 60 • (Gopalswamy et al, 2012(Gopalswamy et al, , 2016Sun et al, 2015;de Toma, 2011), and70 • (Muñoz-Jaramillo et al, 2012). Recently, Samsonov et al (2019), using solar wind observations and empirical magnetopause models, also reported an increase in average annual magnetopause stand-off distance by nearly 2 R E between 1991 (9.7 R E ) and 2009 (11.6 R E ).…”

“…In earlier studies, we have also shown the steady decline in solar wind microturbulence levels in the inner heliosphere, spanning heliocentric distances from 0.2 to 0.8 AU (Bisoi, Janardhan, Ingale, et al, 2014;Janardhan et al, 2011Janardhan et al, , 2015, in sync with the decline in photospheric magnetic fields. The long-term declining trends seen in both photospheric magnetic fields and solar wind microturbulence levels over the entire inner heliosphere, coupled with the unusually deep solar minimum in cycle 23 and the very unusual solar polar field conditions seen in cycle 24 (Gopalswamy et al, 2016;Janardhan et al, 2018), implies that these changes could directly affect the terrestrial magnetosphere. IPS essentially provides one with an idea of the large-scale structure of the solar wind (Ananthakrishnan et al, 1995(Ananthakrishnan et al, , 1980.…”