Roughly every 11 years, headlines announce the peak of a natural variation known as the solar cycle, pointing to a proliferation of sunspots, potentially dangerous solar flares and beautiful aurora.
Image: Guhathakurta, M. et al, “The Solar Cycle Turned Sideways,” Space Weather, Wiley, doi: 10.1002/swe.20039
This period of increased activity is known as solar maximum, the latest of which is predicted to arrive in August. But scientists who study the sun say that we should be paying just as much attention to the sun’s calmer periods.
“Space weather doesn’t go away during solar minimum, it just changes form and approaches a different extreme,” said astrophysicist Madhulika Guhathakurta, who lead’s NASA’s “Living With a Star” project and co-authored an opinion piece on this topic that appeared in Space Weather March 19.
The solar cycle, say scientists like Guhthakurta, is an oscillation between two extreme states, each with their own distinct conditions and hazards, rather than a simple change between periods of high and low activity.
The sun constantly spews out radiation and charged particles, which rush outward through interplanetary space in our solar system. The levels of ambient plasma, magnetic fields and particles around the Earth make up our local space weather.
During solar max, the sun’s surface erupts with solar flares and coronal mass ejections — huge prominences that carry charged particles and radiation out into space. If these explosions hit the Earth, they can damage satellites in space, create radio blackouts, pose a danger to astronauts and, in the event of a solar megastorm, disrupt power lines and other infrastructure on the ground.
Increased extreme ultraviolet radiation at solar max also heats up the Earth’s atmosphere, causing it to puff out at the very top where it touches space. This creates more drag forces on satellites and outposts like the International Space Station, causing them to fall back toward Earth, which vexes satellite operators and requires reboosting to bring these objects back to their proper place. But the effect isn’t all bad: Orbital debris known as space junk tends to fall back too. Since these bits are small and get no extra boost, they burn up in the atmosphere, helping clean up the space environment.
Most of these events go away during solar minimum, only to be replaced with another set of space weather hazards. A decrease in the solar wind allows more galactic cosmic rays to enter the solar system.
These fast-moving high-energy ions can shatter DNA strands, increasing the risk of cancer for astronauts. This is one of the main obstacles facing the recently announced Inspiration Mars mission, which plans to send two people to fly by the Red Planet in 2018, during solar min. Thinking too simplistically about solar min as a calm time “can actually do damage if space travelers and mission planners start to take it seriously,” said Guhathakurta.
The decrease in ultraviolet radiation causes the Earth’s atmosphere to cool and contract, meaning satellites have less trouble staying in orbit. On the other hand, space junk tends to accumulate at this time.
Because of these complex interactions, Guhathakurta and her co-author liken the solar cycle to the terrestrial climate pattern known as El Nino and La Nina. This oscillation in the Pacific Ocean occurs every two to seven years. Like solar max and min, El Nino and La Nina have their own set of distinct properties, some of which are good and some of which are not. El Nino can bring heavy rainfall and floods to the West Coast but it keeps New England relatively warm and dry. In Ecuador and Peru, El Nino means good farming weather. La Nina, on the other hand, can cause droughts in the western Pacific, flooding in South America and mild summers in northern North America.
The inspiration for thinking about the solar cycle in this way came to Guhathakurta during the most recent minimum during 2008 and 2009. Sunspot counts were very low but cosmic rays surged to the highest intensity seen during the Space Age. Earth’s upper atmosphere collapsed and space junk boomed.
“That doesn’t sound very quiet, does it?” said Guhathakurta.
Thinking about space weather in this way is great, said Robert Rutledge, lead of the forecast office at the NOAA/National Weather Service Space Weather Prediction Center. “It’s a point that needs to be made and one that hasn’t been made a lot.”
Most people tend to think about solar storms, which happen more frequently at solar max, as having the biggest impact on human activity. But many space activities, especially satellites, can be affected by solar minimum in subtle ways.
Because the most recent solar min was very prolonged and low, “some people’s atmospheric drag models started to break down,” said Rutledge. “It was outside the realm of what they had accounted for.”