If you’re a North American who bought a pair of eclipse glasses early for the April 8 event, or just someone who has old ones lying around, now might be a good time to get them out. The giant group of sunspots, collectively known as AR3576, is approaching the center of the side of the Sun facing us. It’s so big that Perseverance was able to see it from Mars, and it’s gotten bigger since then, so if you have safe solar viewing equipment, you should see it without magnification. That could be especially memorable if the AR3576 also spits out X-class flares, the impact of which will go down in history.
Large sunspots do not always cause large solar flares, but there is a connection. It is therefore very likely that we will soon witness X-flares associated with AR3576. If we don’t, we can still expect to encounter them before this solar cycle is over – after all, we already had one in December. So what are they?
What are solar flares?
As we know, the sun is a constant source of light, a form of electromagnetic radiation, which also radiates at other wavelengths. However, every now and then a small (or sometimes not so small) part of the sun emits more electromagnetic radiation than normal. The extra brightness isn’t so great that we notice the sun as a whole getting brighter, but if we have telescopes pointed at the area we can see the brightening spot.
Flashes are a result of irregularities in the sun’s magnetic field. Initially, the field will block some of the heat rising from the center of the sun, creating a sunspot. However, when the field becomes entangled or reorganizes, the energy released can accelerate charged particles through the Sun’s atmosphere, creating a rapid additional burst of energy.
It’s not hard to see AR3576 in this recent image of the Sun: not just one sunspot, but an immense cluster.
Image credit: SDO/NASA
The larger the sunspot, the greater the potential for solar flares, but the relationship between them is far from perfect. Both rise and fall with the 11-year solar cycle, the peak of which we have just seen or will soon see.
A large spot doesn’t guarantee major outbreaks, but it certainly increases the chances.
How are solar flares categorized?
Torches are divided into five classes based on the peak power in watts per square meter (W/m2), counting only the energy released between 1 and 8 Angstroms (known as soft X-rays). For decades, successive geostationary operational environmental satellites (GOES) have been tasked with measuring the energy released by flares so that they can be classified.
The smallest flares, A class, peak at less than 10-7 W/m2, which is almost too small to notice at our distance. B and C classes (10-7-10-6 and 10-6–10-5 W/m2) are of interest to solar astronomers, but have little effect on most people.
M-class flares (10-5–10-4 W/m2) can be associated with aurora and sometimes radio interference and other uncomfortable phenomena.
The largest flares, those with more than 10-4 W/m2, are called X class. There is no theoretical limit to how large the X-class can be. Within the other classes, the outbursts are divided into 1-10, but the largest outburst encountered by GOES was so powerful that its detectors became saturated, forcing astronomers to estimate its size at X40-X45.
The number of an X flare indicates how many times more energetic it is than an X1 flare, so an X9 is nine times as powerful as an-4 W/m2.
The largest solar flare ever recorded consisted of saturated instruments that could measure up to X28, allowing us to estimate how much higher it was.
Image credit: SOHO/EIT (ESA & NASA)
Are solar flares a threat?
Absolutely, but not directly.
The first thing to note is that the problem is rarely the flares themselves. The threat comes from coronal mass ejections (CMEs) that hurl charged particles into space. When large CMEs encounter Earth’s magnetosphere, they can create geomagnetic storms, producing everything from beautiful lights near the poles to generating induced currents that can disrupt power grids and plunge regions into darkness.
Most flares do not produce CMEs, but the larger a flare is, the more powerful a CME it can activate.
As you’ve probably noticed, the world hasn’t suffered any serious consequences from recent X-flares, but just because not every X-flare is harmful doesn’t mean that none are.
Unsurprisingly, we have more to fear from an X50 flare than an X5. However, direction is at least as important as size. Half of the X-flares are on the far side of the Sun, and we are only aware of them if a spacecraft is in the right position to notice them. Even flares that are on the side we can see won’t affect us much if they are pointed away from Earth.
The eruptions we should worry about are the ones that are both powerful and cause direct hits to Earth’s magnetosphere.
The standard against which flares are measured is the Carrington event. Although the damage was limited to a few shocked telegraph operators and a few fires, the damage occurred in a world where telegraph lines were the only long stretches of wire in which induced currents could grow to dangerous levels. Today, a similar event could take out satellites, crash trains, or blow up electrical transformers, rendering our communications networks and energy sources useless and potentially taking weeks or months to restore.
Furthermore, the Carrington Event may be far from the limit. Tree rings reveal evidence of so-called Miyake events which, as far as we know, are flares that make the Carrington event seem small. We don’t know what a Miyake event would do to a technological civilization like ours, but it’s unlikely we would enjoy it.