By Simon Thomas
Friday 24 October 2014 – updated 30 October 2014
Sunspots are areas on the Sun which appear dark in contrast to the solar disk. They are associated with complex magnetic fields which inhibit convection and are therefore not as hot as the surroundings (sunspots are typically 3000-4500 K compared to the surrounding at, on average, 5780K). The magnetic fields in sunspots are arranged vertically and are either pointing inwards or outwards. Therefore, when we discuss “sunspots” we are typically referring to sunspot-pairs, where the magnetic field points out of one, then loops and connects inwards to other sunspot.
Active regions on the Sun, including sunspots, are of interest to us as they are effectively source regions for solar flares and coronal mass ejections (CMEs). Solar flares are bright flashes interpreted as a huge release of energy. The emitted x-ray and ultraviolet (UV) radiation from solar flares can affect satellite communication systems at Earth. A flare is often, but not always followed by a CME, a subsequent eruption of plasma and magnetic field out from the Sun and into space. If a large CME erupts and impacts Earth, then it can trigger “geomagnetic storms” which can result in low-latitude aurora, disruption to power grids and enhanced radiation doses to humans at high altitude. Full space weather hazards and risks are discussed by Hapgood (2010).
The large sunspot AR2192, which has made the news this week, was not too unusual when it was last visible before rotating around the opposite side of the Earth. As our only spacecraft which view the Sun on the other side of the Sun are now turned off while the Sun is between them and Earth, we have not been able to follow the sunspot through its progression until recently. However, since it has rotated around and become visible to ground-based and observations from the Solar Dynamics Observatory, we have been able to observe and analyse its progression and development. See this link for a video clip of the progression of this sunspot from youtube.
So, why has sunspot AR2192 caused excitement within the media? Well, firstly it has just been confirmed to be the largest to be observed in almost 25 years. It is by far the largest sunspot in this current Solar Cycle and has been visible to the naked eye (with the help of filters – don’t look directly at the Sun!). This interest has been enhanced as it coincided with a recent partial eclipse viewable from the USA. To be able to comprehend the size of such a sunspot, as the pictures on the solar disk do not give it justice, we can compare with the sizes of the planets. This is shown in Figure 1 where the sunspot on the 22 October is shown to scale to be approximately the size of Jupiter, and 14 times the size of Earth.
Figure 1 – Sunspot AR2192 on 22 October 2014 compared to the sizes of Jupiter and Earth. Image courtesy of www.thesuntoday.org.
Since this image was taken, the sunspot has grown larger still.
The second point relating to our interest in this sunspot is in the history of such sizeable sunspots. The last time we saw a sunspot of comparable size, it was associated with consecutive Earth-directed CMEs known as the “Halloween Storms” which took place in 2003. These caused aurora to be observed as far south as Texas and the Mediterranean, and caused a power outage in Sweden. There have, however, been larger sunspots since records began. Figure 2 shows a graph of the largest yearly sunspots of the 20th Century. The largest peak on here is the so called “Great Spot of 1947”. This grandly-named sunspot was the largest recorded, but in contrast to the 2003 event that produced the Halloween Storms, this, it appears, did not produce a large Earth-directed CME. This does not, however, mean that it did not release a huge CME in a different direction as it rotated around, but it is unfortunate that we have only had a means of observing remote coronal mass ejections and flares in recent years.
It is worth discussing now that just because a sunspot is large, this does not necessarily mean it is capable of producing hazards to us on Earth. Firstly, the active region (including the sunspot) must have suitably complex and compressed magnetic fields to release a CME in the first place. Secondly, even if the active region is capable of producing CMEs, the eruption of this CME would have to be at the correct angle to intercept Earth. CMEs in the solar system are fairly large structures, but Earth is a small target compared to the number of possible trajectory angles of the CME.
Figure 2 – Largest yearly sunspots observed from 1900-2000 in millionths of the total solar disk. Courtesy of David Hathaway, NASA.
So, with that said, what are the prospects of getting some significant space weather from AR2192? Currently, the new Met Office space weather prediction centre have been keeping a close eye on the evolution of this sunspot and this is a sizeable, early challenge for their forecasters. The sunspot group has been producing a high frequency of powerful solar flares, but as of yet, no significant CMEs in our direction. This led the Met Office to release the following forecast on 23 October 2014, for the weekend: “Solar activity is likely to remain at moderate to high levels with a chance (30%) of X-class flares. Geomagnetic activity is expected to remain elevated as a further coronal hole high speed stream becomes geo-effective but with only a slight chance (15%) of minor storms”. The coronal hole part relates to a large, slightly darker region of the Sun ahead of AR2192 which has been releasing faster solar wind than the ambient wind speed.
The predictions from the Met Office and a similar forecast from NOAA show that it is very likely that there will be further solar flares, perhaps up to the most intense, X-class of flare. However, it does appear that there is a low chance of a CME impacting in the next ~3 days. However, this does not mean that this would not happen in the slightly longer term; CMEs generally take a matter of days to reach Earth and so as the sunspot is now Earth-directed, a CME associated with an anticipated flare may not reach us until early next week. Such a flare with an associated CME has not occurred, as of 1500 UTC on Friday 24 October.
Finally, let’s think about the longer term. Are we likely to see any more sunspots of this size in the coming years and what is the chance of a large Earth-directed CME? The sunspot cycle has just peaked at solar maximum and solar activity is starting to decline. As the maximum sunspot size roughly follows the sunspot cycle, it is unlikely that we shall see another of this size for a good few years, but not impossible! Secondly, solar activity in general appears to be in decline from the very large cycles we saw in the late 20th Century. The latest solar cycle has been very weak compared to previous cycles in both sunspot numbers as well as solar wind parameters such as the magnetic field in near-Earth space. This is shown in the top two panels of Figure 3 (from Lockwood et al., 2012). These show the sunspot number (R) in the top panel and the near-Earth magnetic field in the second. The black lines in these are the data and it is apparent that the number of sunspots has began to decline here and this has coincided in the magnetic field drop.
Figure 3 – Data and predictions from Lockwood et al. (2012) of sunspot number (R; top panel), the near-Earth magnetic field strength (B; second panel), cosmic ray number from a station in Finland (Onm; third panel), and the aa index, showing changes in the Earth’s magnetic field (aa; final panel). Black lines are raw data, purple lines are reconstructions and red to blue are likely patterns that the observations will follow.
With the number of sunspots reducing, it is unlikely that there will be as frequent “super-sunspots” as were seen during the last century. Moreover, the predictions from Figure 3 (based on predicted variations from previous scenarios in ice-core data) show that sunspot number is likely to reduce even further with weaker magnetic fields. Thus, although solar activity is very difficult to predict, sunspot AR2192 has given us a rare opportunity to study the evolution and activity of such a sizeable sunspot, which will hopefully give a useful and significant contribution to future space weather forecasts.
UPDATE (30 October): As AR2192 is disappearing around the east (right-hand) limb of the Sun, it is still increasing with size. It has produced a large number of M- and X- class solar flares but no sizeable coronal mass ejections, which was as anticipated by the Met Office prediction centre. Sunspots can persist for several solar rotations (which take approximately 27-days to complete), and so it is possible that we shall see the active region again in a few weeks.
REFERENCES
Hapgood, M.A., 2010. Towards a Scientific Understanding of the Risk from Extreme Space Weather. Adv. Space Res., 47, 2059–2072
Lockwood, M., M. J. Owens, L. Barnard, C. J. Davis, and S. R. Thomas, 2012. What is the Sun up to? Astron. and Geophys., 53, 3.9–3.15