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Dangerous solar weather seems to be a fixture in the news. Every few days, another potentially disastrous sunspot crosses the face of the Sun. As we approach solar maximum, the solar flares are coming faster and faster. Mostly, it means Earthbound viewers get more than our normal share of nights illuminated by the aurora. More intense solar storms just drive the polar lights closer to the Equator.
Still, solar flares are complex phenomena with real hazards for Earth, even this far away from the Sun.
What Is a Solar Flare?
A solar flare is a burst of radiation: photons of all wavelengths, traveling at the speed of light. Solar flares are closely related to another solar phenomenon called a coronal mass ejection, or CME. Where there’s a flare, there’s likely to be a CME, just like where there’s smoke there’s fire. CMEs physically throw solar material out into space, in the form of huge jets or splashes of charged particles (plasma, sometimes protons or electrons) “threaded” with magnetic field lines. A solar flare is visible with just the normal eight-minute light delay between Earth and the Sun, while a CME takes two to three days to get here.
Captured during a solar flare in October 2022
Credit: NASA Solar Dynamics Observatory
Solar flares occur in active regions, often around sunspots. Such active regions can be many times the diameter of Earth, with convective cells called granules that build up strained, twisted magnetic fields. These intense magnetic fields can reach right through the cool, dense photosphere, magnetically linking the corona to deeper strata.
Eventually, there’s too much magnetic tension, and the field snaps into a lower-energy configuration. That sudden release of magnetic energy, which takes place on the scale of minutes to tens of minutes, powers solar flares. The same energy releases are believed to produce CMEs, although the precise relationship between CMEs and flares is poorly understood.
Credit: NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-Keith
According to NASA, “One can think of the explosions using the physics of a cannon. The flare is like the muzzle flash, which can be seen anywhere in the vicinity. The CME is like the cannonball, propelled forward in a single, preferential direction, this mass ejected from the barrel only affecting a targeted area.”
Sidebar: What Is a Halo CME?
Sometimes, a sunspot lets go when it’s pointed precisely at the Earth (or another observer, such as one of our sun-dipping solar satellites). Instead of firing off in a direction, it makes what looks like a glowing halo around the Sun. This happened during a period of intense solar flare activity in May of 2024. In a halo CME, the observer sees something like what you’d see if you had a garden hose pointed directly at a video camera lens.
(Credit: NASA/SDO/SOHO)
Because they’re pointed dead-on at Earth, halo CMEs can be the most dangerous of all. To appear as a halo, CMEs need to originate close to the central meridian on either the near or far side of the Sun as viewed from the coronagraph, although about 10% of halo CMEs originate from the ‘limb’ (the leading or trailing edge of the solar disc).
Classes of Solar Flares
Scientists divide solar flares into five classes, according to their peak brightness in X-ray wavelengths. In order from smallest / least intense to most intense and troublesome:
A-class flares are the smallest, with no detectable consequences on Earth.
B-class flares are ten times more powerful than A-class flares, but ten times smaller or weaker than C-class flares. Solar observatories can see these flares, but they don’t interfere with our satellites.
C-class flares are small, with few noticeable consequences here on Earth. In fact, class C used to be the bottom of the scale; classes A and B were added retroactively as our equipment became sensitive enough to detect them.
M-class flares are medium-sized, capable of causing minor radiation storms. Flares of this magnitude can also interfere with GPS and cause brief radio blackouts around the North and South Poles.
X-class flares are the biggest and most problematic; they are major events that can blanket the planet in radio blackouts, physically damage satellites and infrastructure, and cause long-lasting radiation storms in the upper atmosphere. (Who knew Earth could be on the list of Extreme hazard planets from No Man’s Sky?)
An X-class solar flare from August 7, 2023
Credit: NASA Solar Dynamics Observatory
Numbers after the class designation give the relative strength of flares within each class. Each class is a tenfold increase (an M1 is ten times as strong as a C1) except for X-class flares, which have no ceiling of intensity. X-class flares have been estimated to reach up to an X45; the largest on record, a 2003 Carrington-class near miss first believed to be an X28 (because it totally saturated the sensors on NASA’s GOES satellites), was later modeled at a revised X45.
Space Weather
Every 11 years, the Sun enters a new solar cycle, during which it slowly shifts from quiescence to a roar and back again. When the Sun is dormant, there’s usually less than one solar flare a week, but close to solar maximum, there can be more than one flare per day. For the current solar cycle, solar maximum is expected in 2025.
When a solar flare or CME reaches Earth, it does so on the sun-facing, “day” side of the planet. There, the atmosphere—mostly the ionosphere—absorbs the brunt of the extreme X-rays and UV radiation. Absorbing all that energy can further ionize the ionosphere, which causes broad-spectrum electromagnetic noise that can interfere with radio communication. On the dark side of Earth, colorful lights may appear to dance in the night sky: the aurora borealis and aurora australis (Northern and Southern polar lights).
Taken during the solar storms of May 2024.
Credit: NASA Aurorasaurus / Kashmir Wilkinson
On NOAA’s radio disturbance severity scale, the threshold for detectable interference doesn’t even start until a flare is at least M-class in magnitude. M-class and X-class flares can both cause widespread radio jamming on the day side of the planet, but the most dangerous X-class flares can even disrupt radio communications and GPS on the night side.
Space weather also presents a very different threat to satellites: increased physical resistance from Earth’s atmosphere. When the atmosphere absorbs the energy of a CME, it heats up and expands like a marshmallow over a campfire. This makes it harder for satellites to avoid falling back to Earth. During a 2024 solar storm, thousands of SpaceX satellites had to make abrupt course corrections, ascending to avoid the sudden increase in atmospheric drag after a solar flare and its associated CME.
Rage Against the Magnetosphere
No discussion of solar weather would be complete without nodding to extreme solar storms like the Carrington Event of 1859. As Solar Cycle 10 approached its peak, a massive sunspot complex released a solar flare and a fast-moving CME that struck the Earth head-on. The wave of charged particles caused a powerful geomagnetic storm, the likes of which has never been seen again.
Perhaps the most famous bout of inclement solar weather in history, the resulting storm is now known as the Carrington Event. Auroras were visible in Florida and Puerto Rico. On the East Coast, the aurora was widely reported to be bright enough to read the newspaper at midnight. Telegraph operators corresponded for hours without power, relying solely on the “auroral current.” Telluric currents permeated the Earth itself, dampened only by igneous bedrock. Currents induced in conductive infrastructure like long-distance power lines and train tracks led to arcing and sparks that started wildfires.
Sunspots on July 18, 2024, with the sunspots of the Carrington Event superimposed.
Credit: spaceweather.com
Based on how fast the CME hit Earth after the solar flare and drawings of the solar disc at the time, it’s now thought that there was a separate Earth-directed CME just before the one that caused the Carrington event itself. There was another big aurora, visible at near-tropical latitudes, two days before the Carrington event. Such a wave of charged particles would have cleared a path between Earth and the Sun, a bit like curling, leaving a track with little magnetic debris to slow the CME that came immediately afterward. Normally, a CME takes two or three days to reach Earth, but the Carrington CME struck us just under 18 hours after its release.
Much more is known about a series of violent solar storms in August 1972. The same region of instability produced 67 solar flares while it was directly facing Earth on just one of the five solar rotations through which it persisted. One flare was so bright that our nuclear defense satellites mistakenly registered it as a nuclear bomb. During the peak of the geomagnetic storm, “numerous” magnetically-sensitive Destructor sea mines spontaneously detonated within about thirty seconds near the Vietnamese island of Hon La (magnetic latitude ≈9 degrees).
Outside the Earth’s magnetosphere, had the Apollo astronauts been caught on an EVA or a moonwalk, the flare was enough to cause radiation poisoning on the spot. The event burned through half the ozone layer on the day side of the planet, and put two years’ worth of wear on orbiting satellites in half an hour. Only four of those flares were X-class.
According to NOAA, the Sun releases an X-class flare about eight times in a typical Earth year. Thankfully, flares can be pointed in any direction, so the odds of any given CME striking Earth are quite low.
Wear sunscreen, kids. You never know. 🌞