We’ve seen the Northern Lights on full display so far this year, and we can expect more of the same in the months and even years ahead. But why? Why has the Aurora Borealis been so unusually bright, and why has it been visible so much farther south than normal? Here’s the science behind what’s going on.
Aurora chasers across Canada and the northern United States have been having an amazing year so far. Especially across the Prairie provinces, night skies have been alive with arcs and ribbons of the Northern Lights.
On no less than four separate occasions so far in 2024 — on May 10, August 12, September 12, and October 10 — auroras were so bright and active that they could be seen across all of Canada, and in even down into the southern United States.
The Aurora Borealis shines over Collingwood, ON, on the night of May 10, 2024. (UGC/Angie Gibson)
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The May 10 event, specifically, has now been named the strongest geomagnetic storm in over 20 years. This geomagnetic storm was sparked by a massive cloud of charged particles — a ‘coronal mass ejection’ or ‘solar storm‘ — that erupted from the Sun a few days before. Along with the disturbance it caused to Earth’s magnetic field as it passed us, countless particles from the cloud were funnelled down into the atmosphere, where they produced auroras.
Based on the intensity of the CME impact, NOAA’s Space Weather Prediction Center classified it as G5 (extreme) — at the very top of their scale for geomagnetic storm strength. While displays of the Northern Lights are typically confined to regions of northern or central Canada, auroras that accompanied this geomagnetic storm were reportedly seen as far south as the Yucatan Peninsula.
A satellite composite map of North America shows the southerly extent of the auroral arc on the night of May 10-11, 2024. The Northern Lights would appear directly overhead across that arc, however, since the auroras occur at least 80 kilometres above the ground, it would be easily seen farther south, with observers around the Gulf of Mexico seeing them on the northern horizon. (CIMSS/Suomi NPP)
The last time an event of that magnitude was seen was during the Great Halloween Solar Storms of 2003.
There’s an excellent chance that the May 10 event won’t even be the strongest geomagnetic storm or aurora display that we see this solar cycle.
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The cause of all this Northern Lights activity, ultimately, is the Sun. More specifically, it is due to how solar activity interacts with Earth’s geomagnetic field and the upper atmosphere.
This graphic shows the Sun, filtered in extreme ultraviolet light to pick up the coronal loops that reveal heightened activity, between 2010 and 2020, from the start to the end of Solar Cycle 24. The graph reveals the number of sunspots counted each month of the cycle. (NASA)
Every 11 years, our Sun goes through a cycle of higher and lower activity. The number of sunspots gradually increases, as does the number and intensity of solar flares and coronal mass ejections, and the normally sedate flow of the solar wind becomes broken up by more ‘blustery’ streams. Roughly halfway through the cycle, all of this activity reaches a maximum. Then, afterward, it all decreases until it reaches a minimum and the cycle ends.
Sunspots are small, dark, cool regions of the Sun’s surface — “small” at least compared to the size of the Sun itself. They are cooler, and thus darker, than their surroundings due to the tangle of magnetic fields that exist along their border. These fields are the very reason the sunspot exists, as they ‘lock-in’ that portion of the Sun’s surface, preventing it from sinking and being replaced by fresh, hotter material rising from below.
These same tangled magnetic fields are also the source of solar flares and most coronal mass ejections (CMEs).
These two views of the Sun show the strongest Earth-directed solar flare so far in Cycle 25, the X9 flare from October 3, 2024. On the left, several dark sunspots dot the surface of the Sun, while on the right, the corresponding magnetic activity is shown, revealing coronal loops and dark filaments of solar matter, with the bright flash of the flare centered in the lower half of the disk. (NASA SDO)
During the roughly 12 days it takes for a sunspot to pass across the face of the Sun, it can grow and possibly group together with other sunspots, forming complex magnetic connections. The loops along its edge respond to this activity, but by the laws of physics, always seek out the least complicated connections possible. Sometimes, to achieve that, the tangles in the loops suddenly and violently unravel, releasing an intense blast of energy. We call this blast a solar flare.
Most flares (A-class, B-class, and C-class) are barely distinguishable from the rest of the solar activity going on around them. However, the strongest — M-class and X-class flares — show up to orbiting telescopes like NASA’s Solar Dynamics Observatory as bright bursts of light. They can also pose a risk to astronauts in orbiting spacecraft and they tend to saturate the ionosphere on the day side of Earth, disrupting long-range radio transmissions and satellite GPS signals.
The massive “prominence” shown on the left erupted from the Sun on August 31, 2012, forming the CME shown on the right as it expanded away from the Sun. (NASA SDO, NASA/NOAA SOHO)
A coronal mass ejection typically occurs during a solar flare, when the new connections made between the magnetic fields around a sunspot result in a section of one of the magnetic loops getting sheared off and hurled away. The resulting eruption, sometimes called a solar storm, becomes a massive cloud of charged solar particles, energized and accelerated away by the flare, and carrying a ‘piece’ of the Sun’s magnetic field along with it as it expands outward into space.
If a CME sweeps past Earth, it has the potential to cause a geomagnetic storm and auroras. Its impact, though, depends on the conditions within the cloud.
Auroras are caused by solar particles, usually electrons, plunging into Earth’s upper atmosphere. There, they smack into oxygen and nitrogen in the air and pass on their energy in the process. The oxygen and nitrogen then release that energy as flashes of coloured light, with oxygen producing green and red light (the two most common colours), and nitrogen usually emitting blue and purple. Different colours of light can blend into other hues like yellow, orange, and pink.
As mentioned above, a coronal mass ejection is a cloud of solar particles that erupts from the Sun, usually following a solar flare. The cloud has a certain density (the amount of particles it holds), the particles in it have a certain temperature (the energy they absorbed from the Sun and solar flare), and the cloud moves at a certain speed. Given that the cloud is made up of moving charged particles, it also carries with it electric and magnetic fields.
This diagram demonstrates the interaction between a CME’s magnetic field and Earth’s geomagnetic field, where a CME with a positive field direction is repelled (top example), and a CME with a negative field direction is attracted (bottom example). (Javalab/Scott Sutherland)
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Under normal conditions, particles picked up from the solar wind follow the outermost ‘layer’ of Earth’s magnetic field down into the atmosphere. Thus, they stream in only near the poles and any auroras that form are confined to those same regions.
When a coronal mass ejection passes by Earth, it represents a large influx of particles in a fairly short time. The more dense a CME is, the more particles enter the atmosphere, and the brighter the auroras get. The more energy the cloud absorbed from the flare, the greater amount of energy the particles have to pass on, which can make the auroras even brighter. Additionally, higher energy particles tend to make it deeper into the atmosphere, where they can interact with nitrogen to introduce even more colour. Also, the faster the CME is travelling, the stronger its overall effect, which drives the intensity of the auroras up even further.
The one thing all this depends on, though, is the magnetic field of the CME.
Solar wind real-time from November 8 through 10, 2024 shows the magnetic field going negative (top plot, red line), which caused an increase in aurora activity. The values read on this graph represent the solar wind and CME particles that are roughly an hour out from arriving at Earth, giving space weather forecasters a heads-up for what’s coming. (NOAA SWPC)
If its magnetic field points in a “positive” direction, similar to Earth’s, chances are we won’t see anything happen. It’s like pushing the north poles of two magnets together: Earth and the cloud repel each other.
However, the more “negative” the CME’s magnetic field direction is, the more it will be attracted by and connect with Earth’s magnetic field. This gives particles in the cloud a fast-track to stream into the atmosphere to produce auroras.
This colourful display of the Northern Lights appeared over Lambton, Quebec, on October 6, 2024. (UGC/Alex Dostie)
This can even help the auroras to spread farther from the poles, as well.
Stronger magnetic connections between the CME cloud and Earth essentially open up gaps in Earth’s magnetic field, allowing particles to stream into the upper atmosphere at lower latitudes. This results in Northern Lights displays that spread much farther south than normal (as well as the Southern Lights being visible much farther north than usual).
Sunspots, solar flares, coronal mass ejections, and auroras are all examples of space weather.
One of the challenging parts of space weather forecasting is telling the public when auroras will be visible and how bright and widespread they’ll be.
Satellites like NASA’s Solar Dynamics Observatory (SDO), the NASA/ESA Solar and Heliospheric Observatory (SOHO), and the new GOES-19 geostationary weather satellite, all record activity either on or around the Sun. SDO can record sunspots, solar flares and it can see the initial eruption of a CME. SOHO and GOES-19 can do this as well, plus they use special instruments known as coronagraphs to track CMEs as they expand away from the Sun. From these satellites, forecasters can get a reasonable estimate of a CME’s density, energy, and speed.
This forecast from May 10, 2024 showed a potential G4 (severe) geomagnetic storm upon the arrival of a coronal mass ejection that erupted from a large sunspot region a few days before. The resulting geomagnetic storm reached G4 and then G5 (extreme) as the night progressed. (NOAA SWPC)
Other satellites, like the Advanced Composition Explorer (ACE) and the Deep Space Climate Observatory (DSCOVR), read the real-time conditions of the solar wind and CME particles that pass them. Located directly between the Earth and Sun, roughly 1.5 million km away from us, they provide more precise information on the particles’ density, energy, and speed. Most importantly, they also read the magnetic field the particles carry.
So, while forecasters can give us a fairly reasonable idea of whether auroras are possible from an Earth-directed CME, they need to wait until the cloud is only about an hour from reaching Earth before knowing with more certainty.
The big aurora events we’ve seen so far have all occurred during the ‘ramp-up’ of solar activity towards the maximum of this cycle.
On October 15, 2024, experts from NASA and NOAA gathered to announce that we have now entered the period of maximum activity for Solar Cycle 25!
Solar activity in December 2019, at the start of Solar Cycle 25 (left) versus activity in October 2024, at the beginning of the period of solar maximum for this cycle (right). These views, captured by NASA’s Solar Dynamics Observatory, filter for extreme ultraviolet light emitted at temperatures of around 1 million Kelvin, showing off arcs of solar plasma known as ‘coronal loops’ and regions where the magnetic field near the surface is exceptionally strong. (NASA SDO)
This period of maximum activity will last through the rest of 2024, throughout all of 2025, and likely well into 2026. That means we have at least another 12 to 18 months with potential for seeing the Northern Lights.
So, if you are interested in seeing more keep an eye on the news for powerful solar flares, specifically high-end M-class and any X-class flares. Also, look for mention of a coronal mass ejection or CME accompanying it.
