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Sun to skip solar cycle 25?
Posted: 15 June 2011

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According to three independent studies of the Sun's interior, visible surface and corona, solar cycle 25 will have significantly reduced activity, or may not even appear at all.

“This is highly unusual and unexpected,” says Frank Hill, associated director of the National Solar Observatory's (NSO) Solar Synoptic Network. “But the fact that three completely different views of the Sun point in the same direction is a powerful indicator that the sunspot cycle may be going into hibernation.”

The Sun viewed in visible light, at minimum phase (2006) and maximum phase (2001).

Solar activity waxes and wanes every 11 years, which is half of the 22-year magnetic interval which sees the Sun’s magnetic poles reverse. Cycle 24 is currently heading towards maximum, but it was late to get started. Now, the observation of a quiescent jet stream, fading sunspots and slower activity near the poles is suggesting that the Sun could be headed for hibernation. Scientists are asking if this quiet period could be a repeat of the famous Maunder Minimum, a period between 1645 and 1715 where near to no sunspots were seen.

“We expected to see the start of the zonal flow for Cycle 25 by now but we see no sign of it,” says Hill. This zonal flow inside the Sun travels east-west and starts at mid-latitudes, migrating towards the equator by solar maximum, and is tracked by looking at surface pulsations caused by sound reverberating through the Sun. “This indicates that the start of Cycle 25 may be delayed to 2021 or 2022, or may not happen at all.”

Mobile "jet streams" in the Sun migrate from the poles toward the equator as the solar cycle progresses. At left (solar minimum) the red jet streams are located near the poles. At right (solar maximum) they have migrated close to the equator. The jet streams are associated with the locations where sunspots emerge during the solar cycle, and are thought to play an important role in generating the Sun's magnetic field.

Meanwhile, Matt Penn and William Livingston see a long-term weakening in the strength of sunspots, reflected in the decline in the Sun's magnetic field strength over the last two solar cycles. Typical sunspots have magnetic field strengths of 2,500-3,500 gauss, and must reach at least 1,500 gauss to form a dark spot. What Penn and Livingston see from analysing 13 years of data from the McMath-Pierce Telescope at Kitt Peak in Arizona, is a decline in average field strength of about 50 gauss per year during Cycle 23 and now in Cycle 24. If the trend continues, the magnetic strength will sink below the threshold required to produce sunspots.

Finally, using four decades of observations with NSO’s 40-cm coronagraphic telescope, Richard Altrock, manager of the Air Force’s coronal research program at NSO’s Sunspot, NM, facilities has observed a slowing of the typical rapid poleward march of magnetic activity in the corona.

Latitude-time plots of jet streams under the Sun's surface show the surprising shutdown of the solar cycle mechanism. New jet streams typically form at about 50 degrees latitude (as in 1999 on this plot) and are associated with the following solar cycle 11 years later. New jet streams associated with a future 2018-2020 solar maximum were expected to form by 2008 but are not present even now, indicating a delayed or missing Cycle 25.

“In cycles 21 through 23, solar maximum occurred when this rush appeared at an average latitude of 76 degrees,” says Altrock. Typically, new solar activity emerges first at about 70 degrees latitude, trending towards the equator as the cycle ages. At the same time, the new magnetic fields push remnants of the older cycle as far as 85 degrees poleward. “Cycle 24 started out late and slow and may not be strong enough to create a rush to the poles, indicating we’ll see a very weak solar maximum in 2013, if at all,” continues Altrock. “If the rush to the poles fails to complete, this creates a tremendous dilemma for the theorists, as it would mean that Cycle 23’s magnetic field will not completely disappear from the polar regions (the rush to the poles accomplishes this feat). No one knows what the Sun will do in that case.”