Observing the Quiet Sun: A Weak Cycle Offers Opportunity for Expanding Solar Understanding

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Monday, November 18, 2013

Since the dawn of the Space Age in the 1950s, solar maxima have coincided with strong upticks in sunspot activity but the current cycle, Cycle 24, which was expected to peak in Spring of 2013, has been unusually quiet. Solar activity remained at a moderate level, making it one of the weakest cycles in the past 100 years, as evidenced by the low number of sunspots appearing on the Sun's surface. With fewer and smaller sunspots, weaker magnetic fields that are associated with the sunspots, and less outgoing (radiative) energy, many wonder what might be causing the Sun's quiescence.

Observing the Quiet Sun
Top: Image of the Sun taken on October 15 2013 by the Atmospheric Imaging Assembly (AIA) telescope that flies on the NASA Solar Dynamics Observatory (SDO) satellite. Most of the sunspot activity in this image can be seen in the Sun's Southern Hemisphere, while the Northern Hemisphere seems to have already gone through its peak in activity.
Middle: Shows a record of sunspot area measures going back to solar cycle 12 in 1898. Areas are measured in millionth of solar hemisphere and averaged over three solar rotations.
Bottom: A drawing by Johannes Hevelius created in 1644, one year before the start of the Maunder Minimum, shows the passage of a large spot across the solar disk, which occurred over eight consecutive days.

Some have suggested that the decline in solar activity may be a result of the 100-year Gleissberg Cycle. Others have speculated that sunspots and solar activity may be changing, with these changes potentially marking the beginning of a new Maunder Minimum. Because of the close connection between Earth and Sun, scientists are interested in better understanding the Sun's variable output, particularly when the star is not acting in a manner that people have come to expect.

"Cycle 24 is different from recent cycles. You need to go back almost a century, to 1928, to find a cycle as weak as this," says Giuliana de Toma, a scientist at the National Center for Atmospheric Research's High Altitude Observatory (HAO). "Weak cycles have happened before, but this is the first time we have had an opportunity to observe a weak cycle with modern instrumentation, which makes Cycle 24 very interesting."

Since the 1950s, the Sun's activity has been high, with the largest cycle in the historical record (Cycle 19) occurring during this time frame. But beginning with Cycle 23 in 1996, things started to change. While still an above-average cycle, Cycle 23 was noticeably weaker than the preceding ones and had fewer very large sunspots. Then, with Cycle 24, solar activity declined even more. Sunspots of all sizes have decreased in number, with large sunspots particularly scarce.

"This does not make Cycle 24 anomalous" says de Toma. "Weak cycles have fewer sunspots, and especially fewer of the largest sunspots – this is why weak cycles are weak." 

A possible explanation for the recent decline in solar activity is that the Sun is near the minimum of a "Gleissberg Cycle." While regular sunspot observations began in the 18th century, scientists turn to cosmic rays as a proxy to understanding solar activity that occurred prior to having a direct observational record. Isotopes (Berrylium-10 and Carbon-14), which provide insight on the variation in cosmic radiation incident on Earth, indicate that the amount of radiative energy emitted by the Sun seems to change over a period of about 80-100 years. Known as the Gleissberg cycle, this pattern is believed to be of solar origin. While this theory is consistent with the fact that sunspot cycle patterns were similarly weak at the beginning of the 1800 and 1900, given the brevity of a direct solar observational record, scientists are unsure if this is a real phenomenon and, if real, what might cause such modulation in solar activity every 100 years. If valid, a Gleissberg Cycle might explain why the Sun is currently quieter.

Matt Penn and Bill Livingston at the Solar National Observatory have put forward the suggestion that the Sun may be entering a period of inactivity. They reported a decline in magnetic field and darkness of sunspots, independent of the current weak solar cycle. According to their analysis, observations taken with the McMath-Pierce telescope located at the Sonoran Desert-based Kitt Peak Solar Observatory show that sunspots are becoming less dark at a worrisome rate of about 2% a year since the early '90s. If true, in extrapolating this trend forward in time, the Sun will experience a new Maunder Minimum by the year 2022.

The Maunder Minimum, which lasted from 1645 to 1715, was a period when sunspots rarely appeared. While little is known about the causes of the Maunder Minimum, one interesting aspect of this period is that it coincided with lower than average temperatures in Europe and North America. However, this was also a period of high volcanic activity that likely contributed to the observed cooling. Despite happening only once in the hundreds of years that humans have been counting sunspots, by using cosmic rays as a proxy of pre-historic solar activity, the Maunder Minimum is not a unique occurrence.

So is the Sun on course for a long period of low activity? The decline found in the McMath-Pierce data occurs mostly in the early years, when only a very few measurements were collected. This decline has not been confirmed by other observations, in spite of the fact that other groups have looked for similar trends in different data sets. To better understand the recent slowdown in solar activity, de Toma and her colleagues at the California State University, Northridge examined the solar observations collected at the San Fernando Observatory (SFO) with the Cartesian Full Disk Telescope (CFDT1) that goes back to 1986. This data set includes more than 20,000 sunspot observations and is the longest record of accurate photometric sunspot measures, that is, accurate measurements of sunspot brightness.

"While the total area and number of sunspots have decreased in Cycles 23 and 24, the relationship between the area and brightness of the sunspots has not changed," de Toma says. "We do not find that sunspots are getting less dark during the 27-year period spanned by the SFO observations. Sunspots are not different now than they were in Cycle 22, when activity was high."

This evidence suggests that while the Sun's quiet has not been seen within recent history, it does not imply that the Earth will experience a Maunder Minimum, or that sunspots are disappearing.

The quiet solar maximum provides astrophysicists with a unique opportunity to hone their understanding of the solar dynamo and their ability to predict the strength of upcoming solar cycles. As for predicting a Maunder Minimum, this is still out of our reach, according to de Toma.

"We still do not know how and why the Maunder Minimum started, so we cannot predict when another one will occur," she says. "We know from drawings made by Johannes Hevelius in the 17th century that there were large sunspots on the Sun shortly before the Maunder Minimum stated, so the lack of large spots is not an omen of an incoming Maunder Minimum."

In addition to the intellectual challenge, another benefit of this weaker solar activity is that fewer coronal mass ejections occur during these quieter cycles. This results in quieter space weather, which can affect communications and electrical grids on Earth, as well as space missions.

“That said, space weather does not go away when the Sun is quiet. Powerful solar eruptions can still occur. The 1859 Carrington flare, which caused the strongest geomagnetic storm on record, occurred during a normal cycle, not a very active one" cautions de Toma. “Indeed, in July 2012 we observed the fastest coronal mass ejection ever recorded, which blasted away from the Sun at a record speed of about 3,000 kilometer-per-second – luckily, it was not directed toward the Earth.”