We have known for more than a century that solar activity shows a pronounced cyclic variation with an 11-year period. About every 11 years there is a maximum in activity and sunspots become very prominent and very common. Back in my younger days, I was a keen solar observer during one particular holiday around the 1980 sunspot maximum, in which I had a very nice telescope available on loan and was observing the Sun two or three times a day for a couple of months, drawing the projected solar disk and its constant changes. My memory of that maximum of sunspot activity was of the incredible number of large sunspots and sunspot groups that were visible almost every day. Wind forward thirty years to the recent solar maximum and my abiding memory is of how few large spots were visible. Occasionally there were sunspots visible to the naked-eye, which I logged and reported, but they were few and infrequent and most of the activity visible on the solar disk was in the form of small sunspots and groups of sunspots: it gave sunspot numbers that looked impressive at times, but the actual activity behind the numbers was minimal.
In fact, even with the plethora of small sunspots, the 2014 maximum was an unusually weak one and unusually late. Of the twenty-four sunspot maxima observed in the telescopic era, according to the data of the Royal Observatory of Belgium, only the first two – Solar Cycle # 1 and Solar Cycle #2, peaking in 1761 and 1769 respectively – were clearly weaker than the recent one, which is the about the fifth weakest ever observed and the lowest solar maximum for over a century. In fact, for the first time in over a century, since the very weak maximum of 1906, there was even a spotless day at maximum, while the sunspot minimum in 2009 was the deepest in several centuries.
Records are very patchy before 1750. We know that sunspot activity was normal in the early 17th Century – Galileo saw plenty of sunspots – but, from about 1650 to 1700, solar activity all but disappeared. This was the Maunder Minimum and we know that it coincided with the Little Ice Age in Europe. There was also a second, briefer period of much lower solar activity in the early 19th Century, named the Dalton Minimum. We also know that of the five highest sunspot maxima observed, four have been since the start if the Space Age and there is a strong suggestion that there is a cycle of about a century in the level of sunspot activity, on top of the regular 11-year sunspot cycle.
Obviously, solar activity shows major variations of level. Why should this matter?
The first reason is that there is the, as yet, little understood apparent correlation between climate and the solar cycle. Periods of sustained, very low solar activity seem to coincide with periods of unusually cold weather, at least in the northern hemisphere, for which we have the best records. Given the importance of climate change to civilisation, understanding the different components involved, both man-made and natural, is of fundamental importance to humanity. It is anti-intuitive, but we know that the sun is actually a little brighter when there are many sunspots, than when solar activity is low, but the effect is a tiny one and in no way large enough, apparently, to drive the changes in climate that are seen.
The second reason is the link between solar activity and radiation in space, which affects both human and unmanned spaceflight.
The radiation danger is of two kinds:
- At solar maximum, solar flares can cause bursts of x-rays that can last for a few minutes up to an hour or more followed, some hours later, by storms of high-energy protons that can increase the high-energy proton flux by a factor of as much as one hundred thousand for a period of time from a few hours to a few days.
- At solar minimum, the cosmic ray flux increases, as more Galactic cosmic rays penetrate into the inner solar system.
We can see both these effects in this fabulous plot from the SREM – Space Radiation Environment Monitor – mounted on the Herschel Space Observatory:
Herschel launched during the very deep solar minimum in 2009, with the mission ending just about at the moment when sunspot activity peaked. You can see that the baseline level of activity was quite high and then started to decline as solar activity picked up, with brief peaks due to solar radiation storms.
We have a good record of solar x-ray flares and protons storms from the mid-1970s, thanks to the series of GOES satellites that have studied the space environment since then. The recent solar maximum has shown fewer large x-ray flares and proton storms than any of three previous solar maxima. While the effects of a solar storm are limited in Low-Earth orbit, where astronauts are protected within the Earth’s magnetic bubble, for astronauts in deep space, a solar proton storm could be a killer. It is a risk that NASA took by sending astronauts to the Moon at the time of solar maximum. Fortunately, the 1969 maximum was much lower than that of 1958, or 1980, so the actual danger from solar storms was correspondingly lower, but no one knew that at the time. Had a major solar storm happened during an Apollo mission, the astronauts would have turned the Service Module towards the Sun and would have used the combination of the Service Module and the heat shield to protect them from radiation but, for astronauts on the lunar surface, death from radiation poisoning was a very real threat. Their only recourse would have been to abort any lunar activity and make an emergency lift-off, in the hope of getting to the relative safety of the Command Module in time. It is a well-known fact that a massive storm happened in August 1972, between the Apollo 16 and 17 missions, that would have given an astronaut on the Moon an estimated dose of 400 rem: not necessarily fatal, but very serious.
Fortunately, solar proton storms are relatively short-lived and there is some warning of their arrival. While the x-rays arrive in 8 minutes, the energetic protons following the x-rays take a minimum of about 6 hours to arrive and can take much longer. This allows astronauts time to get into a radiation shelter which, it is assumed, any long-duration mission into deep space will have to carry.
Less can be done about cosmic rays. These give a constant, low-intensity radiation dose. The relationship between cosmic rays and sunspot activity is well-known and summarised in this plot:
In the plot, the panels are, from top to bottom: the sunspot number, the Moscow cosmic ray flux (this is part of a decades-long series of measures taken from Moscow that have long been regarded as the standard for measuring cosmic ray activity), the average solar x-ray flux, the average flux of high-energy protons and, in the final panel, the interplanetary magnetic field.
We can see how the x-ray and proton fluxes track the sunspot number. In contrast, cosmic rays rise at solar minimum and drop at solar maximum. Coronal Mass Expulsions (CMEs), produced by solar activity sweep aside the cosmic rays so, when solar activity is high, there are more and larger CMEs and the inner solar system is shielded more efficiently from cosmic ray bombardment. As solar activity drops to historically low levels since the start of the space age, the cosmic rays are increasing. We can see this in the very high energy cosmic rays. During the 2009 solar minimum, the high-energy cosmic ray flux was about 20% higher than at any previous solar minimum.
NASA has strict limits for accumulated radiation dosage for its astronauts. It is anticipated that around the next solar minimum, expected around 2021, that an astronaut may reach the maximum permissible career dosage in as little as 250 days in space, unless their capsules are more heavily shielded (more mass in shielding means less payload capacity). As 250 days is about the length of a flight to Mars, the matter becomes important for potential flights to Mars or to the asteroids: long-duration manned missions into deep space may have to be limited to times of solar maximum.
A second effect is that of cosmic rays on unmanned satellites. Multiple studies on long-duration satellites have shown the correlation between what are called Single Event Upsets (SEUs) and radiation levels in orbit. An SEU is a bit-flip in on-board memory caused by a hit from particle radiation. Sometimes the memory area hit is inoffensive, or the impact affects an area that is not even in use but, on occasion, a bit-flip can occur in a critical area of memory. The effects of an SEU can range from loss of data, to corruption of software, damage to an instrument and even, in extreme cases, loss of mission. One of the tasks of ground controllers is to carry out checks for and patch SEUs, when they happen. Further effects are that high-energy radiation bombardment slowly degrades the transparency of glass, damaging optical instruments on missions and slowly degrades solar panels, affecting the energy-generating capabilities of satellites.
One speculation is that solar activity may be dropping to a Maunder Minimum. More likely on present evidence, is that we are seeing the onset of a Dalton minimum, as in the early 19th Century. Apart from the inconvenience to long-duration manned spaceflight, that would offer scientists some amazing opportunities to study the long-term effects on low levels of solar activity on the space environment. A important potential consequence of much higher cosmic ray fluxes in the future may be the need to pay more attention to radiation hardening of satellites.