Why Are There Massive Solar Flares Every 11 Years?
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Every 11 years, our sun’s magnetic activity rises and falls, generating more dark sunspots at the peak of the cycle and fewer at the dip. The biggest flares are caused by magnetic waves crashing into each other along the sun's equator.
The most powerful solar flare since 2006's happened September 6, 2017, and was an X9.3! September 4-10, 2017 had a single complex sunspot called Active Region 2673 emit six other radiation flares. Four were X-class solar flares. This scrambled shortwave radio signals in Africa for an hour and created huge auroras that were visible down to Arkansas on September 7, 2017.
It seems bizarre to have this massive solar activity is during an unusually weak cycle.
In the 1960s, solar researchers discovered the peak flare rate happens a few years after the sunspot's maximum. Even stranger, the strongest flares tend to occur on the cycle’s downslope. The quietest cycles may even produce the biggest flares. The biggest solar storm in recorded history, called the Carrington event, occurred at the end of another especially weak cycle in early September 1859. Modern simulations estimate that flare may have been an X45.
“When you’re descending to a quiescent phase of the cycle and things are getting more organized and simplified, how is it we are getting things this complex?” asks solar physicist Madhulika Guhathakurta, spokesperson for NASA’s Heliophysics Division. “It still remains an interesting question.”
McIntosh and his colleagues believe they've answered this riddle. The powerful solar flares occur when opposing bands of magnetism "vie for supremacy." Similar to jet streams on Earth, which usually staying at certain latitudes, except that these solar magnetic bands slide all around during the cycle. They start near the sun’s poles, about 55° N and 55° S, and move toward the equator, pulling on each other with tremendous magnetic force twisting in opposite directions. When they finally meet at the equator, BOOM!!! KAPOW!!!
These complicated sunspots are called deltas, look like dark and light spots, and represent various magnetic poles.
After 1.5 years, the magnetic bands neutralize each other, ending the solar cycle. During weak cycles, the magnetic wrestling takes longer and builds complex "deltas" that eventually explode.
Scientists will continue observing the sun's activity and running simulations to further understand how our sun works in order to predict and prepare for dangerous events.
Sources:
https://www.sciencenews.org/article/sun-solar-flares-paradox
S. McIntosh et al. The solar magnetic activity band interaction and instabilities that shape quasi-periodic variability. Nature Communications. Published online April 7, 2015. doi: 10.1038.ncomms7491.
S. McIntosh and R. Leamon. On magnetic activity band overlap, interaction, and the formation of complex solar active regions. Astrophysical Journal Letters. Vol. 796, November 10, 2014. doi: 10.1088/2041-8205/796/1/L19.
E. Cliver and W. Dietrich. The 1859 space weather event revisited: limits of extreme activity. Journal of Space Weather and Space Climate. Published online October 21, 2013. doi: 10.1051/swsc/2013053.
