Solar storms to get worse
as activity drops?
BY KEITH COOPER
Posted: 30 March 2012
A decline in solar activity over the coming decades may result in choppier space weather with ferocious solar storms becoming more frequent, according to new research presented at the RAS’ National Astronomy Meeting at the University of Manchester this week.
The Sun goes through cycles – not merely the familiar eleven-year cycle of activity and sunspots, but also a greater cycle that sees the eleven-year cycles grow more intense, rising to a ‘grand maximum’, before dropping off again. Our Sun has just passed through one of these grand maxima and now its decline begins. PhD student Luke Barnard of the University of Reading has hence asked what this decline will mean for solar storms that have the potential to wreak havoc with electrical systems on Earth and in orbit while at the same time generating beautiful aurorae.
An enormous coronal mass ejection erupts from the Sun, spewing charged particles into space that are accelerated out towards the planets, including Earth. Image: NASA/SDO.
“Solar activity is controlled by the evolution of the solar dynamo,” Barnard tells Astronomy Now. “Currently we are unable to produce a predictive model for the solar dynamo and it’s for this reason we look to our past experience of solar activity to produce an analogue forecast of future activity.”
Barnard investigated records of 24 past grand cycles, but using the past as a template of the future is full of uncertainty – how do we know what is shown in our records is typical, or that the future will follow the trends of the past? Consequently, Barnard assigns probabilities to possible outcomes over the next forty years. He cites an eight percent chance that solar activity will plummet to levels below the quiet Sun of the Maunder Minimum – a period between 1645 and 1715 when nary a sunspot was seen on the Sun’s disc, coinciding with the so-called ‘Little Ice Age’ in Europe that saw the River Thames freeze over. More likely, however, is that solar activity will drop by half over the next four decades, says Barnard. Somewhat paradoxically, however, a 50 percent drop in activity doesn’t mean we’re going to have an easier time of it in regards to powerful solar storms.
Such storms are generated when explosive events that we call coronal mass ejections (CMEs) generate shock waves that can accelerate particles in the solar wind up to higher energies, sometimes with Earth in their sights. The magnitude of activity on the Sun is obviously a factor in their strength, but competing against that is the strength of the interplanetary magnetic field, carried away from the Sun on the solar wind. A weaker magnetic field, caused when activity is lower on the Sun, results in stronger shock waves that can accelerate particles more efficiently. Hence the strength of a solar storm depends on the balance between the two and, if solar activity drops to fifty percent its current value, activity will still be great enough to produce high energy events, while the interplanetary magnetic field will be sufficiently weak to allow particles to be accelerated. Hence, this could see the rate of extremely powerful solar storms rise from the current average of five per century to eight, creating a greater hazard for electronic systems, spacecraft and astronauts. On the off-chance that we should experience a recurrence of the Maunder Minimum, says Barnard, then the number of solar storms per century will drop to as low as two since solar activity will be too low to instigate many powerful events in the first place.
The weakening of the interplanetary magnetic field will also allow more galactic cosmic rays coming from beyond the Solar System to reach Earth. A fifty percent drop in solar activity, says Barnard, will allow the cosmic ray rate to rise by 1.5 times, whereas another Maunder Minimum would see that increase to 2.5 times. Either way, it is not good news, and aviation and space agencies are already planning extra shielding on future craft with this in mind. Unfortunately, should another Maunder Minimum occur it probably would not be sufficient to counteract manmade global warming on Earth based on atmospheric carbon dioxide levels and feedback mechanisms (find out more here).