By Dr. Kristen Miller
Faculty Member, Space Studies
Movies such as Knowing (2009) and Solar Flare (2008) depict the danger and damage solar storms can inflict. But are these solar events really as destructive as Hollywood would have you believe? To understand solar storms, however, it is necessary to first understand sunspots.
An Explanation of Sunspots
If you’ve ever looked at the sun through a telescope, then you know that its surface is not uniform. The sun has dark spots on its surface called sunspots, and they change over time.
Sunspots appear dark because they are cooler than the surrounding material of the sun. Don’t let that fool you, though – sunspots are still plenty hot!
The typical sunspot is about 3,800 degrees Kelvin (6,380 degrees Fahrenheit). It only seems cool in comparison to the rest of the solar surface, which is at 5,800 degrees Kelvin (9,980 degrees Fahrenheit — this is the temperature of the solar photosphere).
Material that is hot naturally emits light, and the hotter the material, the brighter the light it emits. Sunspots, which are cooler than the surrounding material, emit less light. This difference in temperature is what makes them appear dark against the background of the sun’s surface.
So why are sunspots cooler? In the sun, energy transport occurs by two methods: radiation and convection. Radiation transport is energy transported by photons of light. The energy produced by fusion in the core of the sun is transported outward by this method.
Inside the sun, in a large layer called the convective layer that surrounds the sun’s core, the density of material is too high for photons to effectively move energy. Instead, heat transfer in this region occurs via a process known as convection, which is how this layer gets its name.
For instance, if you’ve ever watched boiling water on a stove, then you know how convection works. When water boils, pockets of heated water near the bottom of the pan rise to the top, releasing their energy before sinking back down. The roiling movement associated with the rise and fall of these water pockets is what we recognize as the process of boiling.
Convection in the sun occurs in the same way. Energy is transported by the rising of hot, buoyant pockets of solar material that originate near the core. These heated materials rise up and release their energy when they encounter cooler layers. They sink back down later, and the process continues.
Strong magnetic fields can inhibit this process, however. Solar material is ionized, so it is tied to the sun’s magnetic field. When convection occurs, the movement of the material tangles the magnetic field lines. Strong magnetic fields resist this movement and can actually prevent convection from occurring in localized spots. The lack of upward convective energy movement in these regions results in lower temperatures, creating a dark “sunspot” on the solar surface.
Strong magnetic fields not only make sunspots cooler, but they also make them the epicenter of violent solar activity. Strong magnetic fields become buoyant, and as a result, magnetic field loops can rise above the surface of the sun.
Sunspots mark the “footpoints” of these loops – the places where the magnetic field breaks free of the surface. These loops are not stable; they twist and reconnect, allowing a loop of magnetic flux plus ionized solar material to become disconnected and fly outward into space. This type of event is known as a solar flare, and it is a violent, energetic occurrence.
The Immense Power of Solar Flares and Coronal Mass Ejections
Solar flares typically release about the equivalent of 24 million megatons of TNT per second. That means that each second, a solar flare produces 10 million times more energy than a volcanic explosion.
Coronal mass ejections (CMEs) are formed in a similar way to solar flares, but they are ejections of actual solar material rather than purely solar energy. CMEs are much more powerful and more destructive than flares. They can eject billions of tons of charged solar material at speeds ranging from hundreds to thousands of kilometers per second, which is millions of miles per hour.
Assessing the Threat of Solar Storms to Earth
It’s no wonder that these solar storms seem like the perfect culprit for a Hollywood disaster movie! However, even though both solar flares and CMEs are very powerful and violent events, neither Earth nor life on Earth is in imminent danger of being destroyed for a few reasons.
First, studying sunspots can give us early warning of solar storms. Scientists study sunspots as part of an effort to monitor and predict solar storms that can impact Earth.
We have found that sunspots follow a regular pattern of intensity over an 11-year period. Within this cycle, sunspots increase in frequency and also change in location.
At the start of the cycle, known as solar minimum, there are only a few sunspots at mid- latitudes. By the end of the cycle, known as solar maximum, the sun’s surface may be covered with hundreds of sunspots concentrated around the equator. The frequency of solar storms rises and falls in accordance with this cycle and is fairly predictable.
Second, solar storms like sunspots and CMEs are directional. This means that most solar storms do not intersect with the Earth, which is lucky for us.
Scientists estimate that the most powerful solar storms only hit Earth approximately once every 25 years; smaller storms have frequencies of roughly once every three to four years.
Third, NASA and other space agencies around the world closely monitor the Sun in an effort to better understand solar storms and give early warning. One of the newest solar missions is the Solar Orbiter mission, which is a joint effort between the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA).
Solar Orbiter will study the sun’s magnetic field as well as the solar wind and storms from a unique vantage point well inside the orbit of Mercury. This close-up view gives us an unprecedented look at conditions in the solar environment and the early evolution of solar storms.
In February 2021, for instance, instruments on board Solar Orbiter captured a video of a CME as it emerged from the solar surface. This observation is very exciting because it is the closest look at a CME to date and demonstrates the potential of the Solar Orbiter mission.
How Solar Storms Affect the Earth
However, occasionally a solar flare or CME does hit the Earth. What happens then?
The most recent storm that hit the Earth was the Halloween storm of 2003; this unusual storm was the product of a series of dramatic magnetic reconnection events on the sun that resulted in 17 solar flares between October 19 and November 7 of that year. On Earth, the impact of these storms caused some minor power outages and a temporary loss of satellite service.
The largest solar storm to hit the Earth in recent history was the Carrington Event of 1859. This event disrupted telegraph communications worldwide and resulted in minor injuries due to shocks.
Both of the Halloween storm and the Carrington Event did damage to our technology and power grids, but neither of them posed a serious threat to life on Earth. The biggest danger from solar storms today is to our orbiting satellites that provide communications and global positioning system (GPS) services, as well as to astronauts on the International Space Station (ISS), who are exposed to the brunt of the storms. In both cases, monitoring of space storms allows us to prepare for such events and be sure that both our technology and people are safe.
Solar Storms Also Affect Space Exploration and Space Missions
Solar storms and the solar cycle also affect space exploration and the timing of space missions. You might think that it would be safer to plan missions to the moon or Mars, for example, during a period of solar minimum when the likelihood of a solar flare or CME event is very low.
Surprisingly, though, solar maximum is the safest time for space travel. While solar storms are more frequent during solar maximum, the sun’s magnetic field is also strongest at this time. The sun’s magnetic field spreads through the solar system during solar maximum; this expansion provides protection from galactic radiation that is much more dangerous than the radiation produced by solar storms.
Solar storms do happen from time to time and they can cause damage, but they will not destroy the Earth or life on Earth. They might make you think twice about going to space at the wrong time, though.
Space studies majors can learn more about solar storms in the online B.S. in space studies in the course “SCIN 134: Introduction to Astronomy with Lab” and also in “SPST 465: Space Weather.” In both classes, students explore the sun-Earth connection and the current state of space weather study and prediction.
About the Author
Dr. Kristen Miller is an associate professor of Space Studies. She holds a B.S. in physics from Brigham Young University, a M.S. in astronomy from the University of Maryland, College Park, and a Ph.D. in astronomy from the University of Maryland, College Park. Her thesis work studied turbulence in magnetic fields in the protostellar disks surrounding young stars using supercomputer simulations, investigating both the ways in which turbulence allows angular momentum transport within the disks and how coupling of the gas to the field influences the direction of the accretion flow onto the protostar.
Currently, Dr. Miller leads the Supernova Search Program, a program dedicated to detecting supernovae and other transient objects in nearby galaxies. She also leads the Analog Research Group, which is working to send a team of students on a Mars/Lunar analog experience at the Mars Desert Research Station.
Dr. Miller is the co-advisor of the student chapter of the American Institute of Aeronautics and Astronautics (AIAA). She is the section chair for the Women in Space session of the SESA 2021 conference and also serves on a variety of committees at the university.