The sun had been making headlines recently. In September, NASA announced it had released its biggest solar flare for 12 years. This was pretty unusual considering it is supposedly heading into a period of quiet, where activity on its surface becomes muted—also known as the solar minimum.
Scientists' understanding of the sun is relatively limited. Sitting 93 million miles from Earth, NASA is currently planning a mission to “Touch the Sun,” in which a probe will be sent closer to the surface than any spacecraft before. The mission will explore the sun’s outer atmosphere, making observations that should help researchers better understand the workings of our star.
What we do know is that the sun operates on 11-year cycles, where activity peaks and wanes. The solar minimum is the period when fewer sunspots (active regions on the surface) appear, while the solar maximum is when most sunspots are recorded. The last solar minimum was in 2008.
NASA’s Solar Dynamics Observatory (SDO) tracks activity at the sun over these cycles and plots to get a longer term understanding of the sun’s overall output. This is called the Total Solar Irradiance. Dean Pesnell, project scientist at the SDO, recently wrote a blog post about whether the sun was getting brighter.
He explained that the TSI has been measured since 1996, with further data going back to 1978 allowing them to look at the sun’s output since this time. Results show that the TSI changes over time and that it goes up and down according to the solar cycle. However, there is also a baseline: “We would expect TSI to be the same at every solar minimum,” he wrote. “There is much discussion over whether the value of TSI at solar minimum is getting smaller with time, but it is not getting larger.
“These data show us that the Sun is not getting brighter with time. The brightness does follow the sunspot cycle, but the level of solar activity has been decreasing the last 35 years. The value at minimum may be decreasing as well, although that is far more difficult to prove. Perhaps the upcoming solar minimum in 2020 will help answer that question.”
In an email interview with Newsweek, Pesnell explains why we monitor the sun and what the solar cycles and the TSI mean.
What is the solar minimum and what will this tell us about the sun?
Solar minimum is the part of the solar cycle where few sunspots are seen for several years. The solar cycle takes about 11 years to go from peak to peak sunspot number. Minimum happens about seven years after one peak and four years before the next (the cycle is a little skewed.) Sunspots are places on the sun where strong magnetic fields poke through the surface. If there are no sunspots there are no strong magnetic fields poking through the surface. Understanding why strong magnetic fields develop is a big part of our research. Understanding why they have an 11-year cycle is another. To me, solar minimum is a time to look at the sun and try to predict what the next maximum will look like.
What is the SDO doing to prepare for the solar minimum?
SDO is ready to observe the sun as solar activity fades to minimum. We study the magnetic field, the waves at the surface, and the very hot material that solar activity produces high above the surface in the solar corona. Scientists around the world use SDO data to learn about the sun, solar activity and how that activity affects us.
What will be the key observations?
The most important observation for me is how the magnetic field in the polar regions of the sun forms. We believe that this field is a leading indicator of the next solar cycle. Right now that field indicates the sun will be as active in 2025 as it was in 2014. It's tough to wait eight years to see if you are correct!
What would happen to Earth if the sun did get significantly brighter or dimmer?
What happens to the Earth's temperature when the sun changes is a combination of what the sun does and the reaction of our atmosphere. The base temperature of the Earth increases as the sun gets brighter and decreases when the sun gets dimmer. The atmosphere of the Earth then increases that starting temperature. Without an atmosphere the Earth's surface would have a temperature around freezing (with enormous day to night changes). Our atmosphere retains some of the warmth and increases the average surface temperature to about 60 Farenheit (15.5 degrees Celsius). The seasonal temperature changes are still large, but the day to night changes are much smaller. So a dimmer sun should mean lower surface temperatures, but the calculation is difficult.
What does it mean if you find the TSI at solar minimum is getting smaller?
We know that the sun is currently going through a phase where its brightness should be slowly increasing. Very slowly. An increase by 25 percent takes a billion years. We can't measure the properties of the sun accurately enough to see that change over 50 years. So a measurable decrease in TSI at solar minimum is unexpected and therefore very interesting to solar physicists. It means our understanding of the sun has a gap that needs to be filled. This small drift could be related to solar activity. The sunspot number was above average in the middle of the 20th century and is now a bit below average. TSI might simply be returning to the value of the truly unblemished sun.