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Reverse-Polarity Sunspot Group Does Not Belong to Cycle 25, Observatory Says


The Royal Observatory of Belgium’s Solar-Terrestrial Centre of Excellence (STCE) has asserted that the reverse polarity sunspot group 2720 observed in late August belongs to the current solar cycle — Cycle 24 — and does not represent the start of Cycle 25.

“Because of its reversed polarity, some websites claimed sunspot group 2720 was possibly one of the first groups of new solar Cycle 25,” the Centre said. “This is simply not true, in view of its very low 8° latitude. The next solar Cycle 25 sunspot group should have both reversed magnetic polarity and much higher heliographic latitude, typically 20° to 40° from the equator. Only two tiny, short-lived numbered sunspot groups are currently assigned to new solar Cycle 25, sunspot group 2620 in December 2016 and 2694 in January 2018.”

STCE said that while both of those small sunspots have been assigned to Cycle 25, some uncertainty exists as to just which sunspot cycle they actually belong to. STCE said some additional sunspot groups that belong to Cycle 25 were so tiny and short lived that they were not assigned a sunspot number. “During each solar cycle, about 3% of all active regions have reversed polarity but do not belong to the previous or next solar cycle,” the Centre said. “With 2,000 to 3,000 sunspot groups per solar cycle, this means that every solar cycle has a few dozen reverse-polarity sunspots that belong to the ongoing sunspot cycle despite their reverse polarity.”

After examining magnetograms of the sun’s surface, well-known Amateur Radio solar observer and propagation authority Carl Luetzelschwab, K9LA, agreed that AR2720 is reversed in polarity from other sunspots in the northern solar hemisphere. What confuses the issue, he said, it its low latitude, as a Cycle 25 sunspot area should be at a much higher latitude.

The same weekend of sunspot group 2720, a radio blackout lasting about a day took place, affecting the HF amateur bands as well as GPS systems. Solar watcher Tamitha Skov, in her YouTube report, called the G3-level geomagnetic storm “one of the top five storms of the solar cycle.”

As ARRL Propagation Reporter Tad Cook, K7RA, noted in his August 31 edition of The K7RA Solar Update, “For HF operations, we want to see high solar flux and sunspot numbers and low A index, a measure of geomagnetic instability. On Sunday, August 26, we saw high A index numbers from an unexpected crack, opening in Earth’s magnetic field. Solar wind spewed forth, and the planetary A index rose to 76. During this period the planetary K index — a component of the A index — rose to 7 over a 6-hour period. Seven is a big K index number.”

Cook went on to note that Alaska felt the full force of the geomagnetic storm, with an A index in Fairbanks of 90 — a very high number.    



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