The Sun is Earth's nearest star--a giant orb of hydrogen and helium about 93 million miles away. To many people, it looks like the same constant ball of light day after day as it moves across the sky. However, our Sun actually goes through a cycle of increasing and decreasing activity that lasts for about 11 years.
A Magnetic Personality
The Sun is a turbulent place where energy and light are generated at its core via a process called nuclear fusion. At its surface, the Sun seethes, boils, and churns at a temperature of roughly 10,000 degrees Fahrenheit while constantly spewing a stream of gases and electrically charged particles into space. Called solar wind, these particles can travel over one million miles per hour. Luckily though, the Earth has a magnetic field that surrounds us like a protective bubble, deflecting most of this harmful radiation. When this happens at the poles, the radiation sometimes appears as bright auroras (aka, the northern and southern lights) from the ground.
Like the Earth, the Sun, which is made of electrified gases called plasma, also generates its own magnetic field via a process called a dynamo, which is driven by internal fluid motion. However, the Sun's magnetic field is colossally more powerful and complex than Earth's. It is so large in fact, that it reaches far past the edge of our solar system, and works similarly in that it helps to protect the planets within it from harmful cosmic particles from farther out in space.
All solar activity is driven by the Sun's magnetic fields, so the more complex they are, the more solar activity there is. Plus, since the Sun's gases are constantly moving, they stretch and twist the magnetic fields.
We can see manifestations of the Sun's magnetic field in the form of sunspots, which are cooler, dark areas on the sun's surface that mark regions where magnetism is the strongest. These areas are cooler (a frigid 6,500 degrees Fahrenheit) because the magnetic fields are so strong that they keep some of the heat within the Sun from reaching the surface. Sunspots are associated with various types of solar activity and the number and location of sunspots are a key indicator of the Sun's overall activity.
What is the Solar Cycle?
The solar cycle is a nearly periodic change in the Sun's activity between the time where we can observe the most and least number of sunspots, and generally lasts around 11 years. Sometimes the surface of the Sun is very active with lots of sunspots, while other times it is quieter with only a few or even none.
Also, at the peak of each solar cycle, the Sun's magnetic field changes polarity as its inner magnetic dynamo reorganizes itself. This can stir up stormy space weather around our planet. The cosmic particles from deep space that the field protects us from may also be affected, since when a magnetic field reversal occurs, it becomes more wavy, and can act as a better shield against them.
What are some examples of solar activity?
Solar activity associated with space weather that can affect the Earth includes phenomena such as:
- solar flares
- coronal mass ejections (CMEs)
- high-speed solar wind
- solar energetic particles
Solar flares and CMEs are types of large solar eruptions that spew forth from the violent surface of the Sun. However, their sizes are massively different, they look and travel differently, and their effects on surrounding planets vary. Solar flares are localized intense bursts of radiation, and some of the energy they release can reach the Earth relatively quickly (in less than 10 minutes) if our planet is in its path. Additionally, high-energy solar energetic particles are believed to be released just ahead of solar flares and CMEs.
CMEs are much larger eruptions that hurl massive clouds of magnetized plasma far into space, plowing right through the continuous flow of charged particles that normally stream from the Sun, called solar wind, and can reach Earth in up to three days. Although flares do not cause or launch CMEs, they are often--but not always--associated with a given event.
High-speed solar wind is more powerful than regular solar wind, and it streams from areas of the sun known as coronal holes, or large regions in the corona that are less dense than their surroundings. Think of high-speed solar wind as a strong gust as opposed to the slower breeze of normal solar wind.
These different forms of solar activity happen regularly and can erupt out in any direction from the Sun. These events can even lead to geomagnetic storms, which are brief disturbances in Earth's magnetic field and atmosphere caused by these bursts of radiation and charged particles. Earth is only affected if we end up being in the line of fire.
What Are the Solar Minimum and Maximum?
Solar maximum is the peak of the solar cycle--when the number of sunspots and solar activity is at its highest. When they decrease to a minimum, that is considered the solar minimum. Both solar maximum and minimum are wider periods of time rather than specific points in time.
Where Are We Right Now?
Extensive recording of sunspots began in 1755. Since then, we have officially gone through 24 complete solar cycles. Solar cycle 24 began in December of 2008, where sunspots and overall activity were at a minimum. Its maximum activity was reached in April 2014.
For the last two years (2019 and 2020), the sun has been pretty quiet with a record-setting low number of sunspots. In 2019, there were 281 days without any sunspots (77%) and in 2020, there have been 181 days without any sunspot activity (70%) thus far.
Based on the solar cycle, scientists have known we are due for another solar minimum, which will mark the start of solar cycle 25. Thus, they have been watching the Sun carefully , keeping track of sunspots to find out exactly when that will be (or if it already occurred). Since scientists need to see the sunspot number rise before determining when they were at their lowest number, it is only possible to recognize solar minimum after it has occurred. On top of that, the Sun is extremely variable, so scientists use long-term data--typically, 13 months' worth to describe any one given month--to build a picture of the Sun's progress through the solar cycle. Now, based on all their observations, they have determined that the solar minimum occurred this past December, 2019.
How Can Solar Activity Affect Us on Earth?
Despite the Sun being 93 million miles away, space weather has a big impact on Earth as well as the entire solar system. Earlier we mentioned how the normal constant stream of charged particles (solar wind) from the Sun reaches us on Earth, and that our planet's magnetic field helps protect us from most of it. However, when solar activity ramps up, there is a higher chance that high-energy solar energetic particles or a large amount of charged particles from flares or CMEs can bombard the Earth all at once.
This radiation and associated geomagnetic storms can potentially affect power grids on Earth as well as radio signals and communications systems used by airlines and government agencies like the Department of Defense and the Federal Emergency Management Agency. They can also affect our satellite operations and GPS navigation capabilities. Fortunately, the FAA routinely receives alerts of solar flares, and can divert flights away from the poles, where radiation levels may increase, during these events. Planes also have backup systems available for pilots in case solar events cause problems with the instruments.
Additionally, astronauts on the International Space Station have to be extra careful , particularly if they are doing a spacewalk. Outside of the Earth's protective atmosphere, the extra radiation they are exposed to may cause radiation poisoning or other harmful health effects.
Note: The International Space Station has a special section called the Zvezda module for this purpose that is lined with special shielding meant to help protect astronauts during a large solar storm.
Because these events can happen unpredictably and some can reach Earth within minutes, NOAA's Space Weather Prediction Center monitors the activity on the Sun and makes forecasts, predictions, and alerts.