If you have ever stood out in the cold for hours waiting for the Northern Lights, only to see a faint, boring grey arc that refuses to move, you are not alone. You might have checked your apps, seen a low "Kp index," and decided to go to bed. But if you had stayed just twenty minutes longer, you might have seen the sky explode with color. That explosion is called a substorm.
Timelapse of an substorm in Fairbanks, Alaska in October 2022.
At the Aurora Summit 2025, I gave a presentation on this exact topic because I believe understanding substorms is one of the best ways to improve your aurora chasing success. Below, I’ve broken down that talk into a practical guide to help you catch the best part of the show:
How a Substorm Works
To understand a substorm, you don't need a degree in physics. You just need to imagine a giant rubber band slingshot.
The Earth has a magnetic shield protecting us from the Sun’s energy, called the magnetosphere. This shield isn't a hard shell; it’s flexible. It stretches out behind the Earth like a comet's tail. When energy from the solar wind hits us, some of that energy gets stored in the Earth’s magnetic tail (called the magnetotail in scientific discussions), stretching it out further and further.
Think of this like pulling back a rubber band on a slingshot. You are loading it with potential energy. Eventually, that rubber band stretches too far. It can't hold the tension anymore, and it snaps.
That "snap" is the substorm or more specifically, the expansion phase of the substorm. The energy stored in the tail releases in a massive burst, shooting charged particles back toward Earth and lighting up the atmosphere.
Why Substorms Are the Best Part of the Chase
Many beginners obsess over the "Kp index," which is a global average of geomagnetic activity over a three-hour period (learn more about the Kp index and why it is not useful in my "Beginner's Guide to Aurora Chasing" e-book). The problem is that this delay causes substorms to be missed as they’re happening, with the Kp index reacting after the fact. The global nature of the Kp also means magnetic fluctuations caused by localized auroral disturbances are smoothed over and not represented.
A substorm is a specific event that happens within the auroral oval. It matters because it creates the most photogenic displays. If you are seeing aurora all night, the substorm will almost certainly be the highlight. This is when you get the bright, dancing ribbons and vibrant colors that are visible to the naked eye.
Substorms are also powerful enough to overpower a full moon. Even more importantly for chasers at mid-latitudes (like the northern tier of the U.S.), a strong substorm can push the aurora higher in the sky, making it visible further equatorward in places where the Kp index suggests it shouldn't be. I’ve seen bright, dancing aurora in Ely, Minnesota on webcams with a Kp of only 2, purely because a powerful substorm pushed the aurora high in the sky.
Since the aurora occurs hundreds of miles in altitude, it can be seen from very far away. The curvature of the Earth may block the green emission on the bottom of the aurora, but high-altitude reds can be seen from far away.
The Phases of a Substorm
Substorms operate on a cycle that usually lasts about 3 to 4 hours. If you can recognize which phase you are in, you will know when to get ready for a big show and when to take a break.
A "clean" substorm example which occured over southern Canada as seen from this webcam in Ely, Minnesota looking north. All three phases are obvious.
1. The Growth Phase (The "Loading" Bar)
Duration: 1–2 hours
During this phase, the "rubber band" is stretching. Energy is building up in the system. Visually, this can be a bit slow. You will usually see auroral arcs aligned east-to-west. They might look quiet, but watch them carefully. These arcs will slowly drift equatorward.
If you see a quiet band of aurora slowly pushing higher into the sky or further south, don't pack up. The system is charging like a battery. Sometimes, the aurora even gets dimmer during the growth phase which seems counterintuitive but actually means all the energy is being held in Earth’s magnetic fields away from reaching the aurora. The battery is just being charged very efficiently.
2. The Expansion Phase (The Main Event)
Duration: 15–30 minutes
This is the "snap" of the rubber band and the energy release. The quiet arc you were watching will suddenly brighten and twist, often forming distinct knots of brightness along the curve known as "auroral beads.” The aurora will surge from its equatorward position back toward the north very quickly in a process known as "poleward expansion.” While the earlier phases featured calm bands stretching East-to-West, this phase twists them into chaotic, dynamic arcs that stretch North-to-South, dancing wildly overhead. This is often called the "breakup."
Example of auroral beads during the onset of a substorm. Watch how quickly the beads grow and evolve into the poleward expansion. If the video does not automatically adjust, skip to around 2:26:00 in the video (around 22:32:02 on the timestamp in the top left) to see the auroral beads.
This is the holy grail of aurora chasing because it is fast, chaotic, and incredibly beautiful. Because there is more energy and particles coming into the atmosphere, the aurora appears physically taller and brighter. As high-energy particles rain down deeper into the atmosphere, they excite nitrogen molecules at the bottom of the curtains, creating a vibrant pink or purple lower edge.
Check out this compilation of real-time footage of intense corona auroras during substorms. Many of these sequences contain bright pink emissions on the bottomside of the classic green aurora.
Low-energy particles activate high-altitude atomic oxygen that glows a deep red. This vertical growth is why substorms are the secret weapon for chasers at lower latitudes. Because the aurora literally gets taller—sometimes reaching 300-500 km high (about 3-5x higher than typical green emissions)—it becomes visible from much further away, peeking over the horizon even if you are hundreds of miles south of the main show.
Timelapse of red aurora from Arizona during a substorm. The only color visible is red since the curvature of the Earth blocks the lower-altitude green emission.
3. The Recovery Phase (The Cooldown)
Duration: 1–3 hours
After the explosion, the magnetosphere needs to cool down and reset. The bright, defined ribbons break apart into patchy, pulsating lights. It’s a great time to take a breather and decide whether to head in for the night. While the recovery phase can sometimes last a long time with a new growth phase taking several hours to start again, the system can also recharge quickly. There may only be one substorm per night, but usually, you can expect two to three full cycles if you stay out long enough.
An example of a very intense display of pulsating aurora after a substorm. Real-time video filmed outside Calgary on November 4, 2021.
For me, if the first substorm happens at 1-2 am and I have class the next day, I usually head to bed since while there could be another substorm at 4-5 am, I don’t have the stamina to wait for that next pop.
How to Forecast a Substorm
Using the Solar Wind
It’s hard to predict when a substorm will take place, exactly, but the solar wind can give you a clue on how energy is flowing from the solar wind into Earth’s magnetic field. The charging process associated with the substorm growth phase is controlled by the direction of the solar wind's magnetic field, predominantly the north-south direction, or Bz.
When the Bz is pointing South, it connects with Earth's magnetic field and stretches the "rubber band" very efficiently. Think of a South Bz like plugging your phone into a high-wattage fast charger. The battery fills up rapidly, and the tension builds quickly.
However, substorms can still happen even when the Bz is zero or pointing North. In those cases, the charging is just much slower. It’s like trickle-charging your phone with an old, weak cable. It might take a long time to get to 100%, but the system is still building up energy and can eventually snap.
Typically, around 30 min of sustained, southward-pointing Bz is enough to stretch out Earth’s magnetic field to a point of criticality when it snaps, but this is just a general rule that seems to be violated more than followed. How the solar wind and Earth’s magnetosphere interact to produce substorms is a tricky subject, but regardless of the nuances, south Bz is still the most important ingredient to cooking up a substorm.
Predicting the exact onset of the expansion phase is notoriously hard. Even after sustained southward IMF that “should” be loading the system. The magnetosphere can sit in a primed growth phase and then unload at a time that isn’t obvious from the usual real-time plots.
Sometimes there is a clear nudge: a sharp IMF Bz turning or an interplanetary shock / pressure pulse can compress the system and help kick it out of growth phase into expansion. It’s not reliable enough to use like a stopwatch, but those moments are common triggers when things are already loaded.
And occasionally, instead of building toward a clean onset, the system slips into steady magnetospheric convection (SMC), where energy in ≈ energy out — like using your phone hard while it’s plugged in, so the battery percentage doesn’t rise even though it’s charging. I have touched on the concept of SMC in my blog post: "The Severe Geomagnetic Storm of October 2024."
Using the GOES Magnetometers
You don't need to guess when the "snap" is going to happen. You can watch the rubber band stretching in real-time using magnetometer data from the GOES satellites.
Locations of the GOES satellites.
The GOES satellites sit in space and measure the magnetic field. I have a blog article on my website that explains this platform in greater detail. The GOES satellites measure how stretched out the rubber band in Earth’s magnetic field is with lower values representing higher/greater amounts of stretching. Here is how to read the chart in two seconds:
- Line going DOWN: The rubber band is stretching. The battery is charging. The growth phase is happening.
- Line SPIKING UP: The rubber band has snapped. The explosion is happening. Look up!
If you see the line dipping low, you know something is building. If you see it shoot up vertically, you know the expansion phase has likely started.
GOES mag data annotated with the three substorm phases.
Using Mobile Apps
If you don’t want to stare at magnetometer plots all night, a few web-based “apps” can help you track substorm timing in a more user-friendly way, especially when you pair them with the GOES signature I just explained.
- Glendale is the most “substorm-aware” option I’ve found: it explicitly follows the substorm phases and can send alerts when the expansion phase begins (the onset or the moment you want to be outside). It also works globally, whereas GOES Mag is a North America-centric tool. The tradeoff is that it has a steeper learning curve, but the built-in guides make it manageable.
- Norlys is great for situational awareness: it shows an auroral oval/nowcast that reacts to substorm activity and includes a simple activity index. I use it to sanity-check how “lit up” the oval is and how things are evolving, but it can respond to substorms with a small delay compared to the true onset.
Norlys App homepage captured on Jan 5, 2026 at around 17:57 UTC.
- AuroraNotice is a tool currently underdevelopment by my friend and workshop partner Marc Rassel. It uses an AI model to calculate a percent change of a substorm occuring.
AuroraNotice homepage captured on Jan 5, 2026 at around 17:57 UTC.
My practical workflow is simple: use Glendale for “go time,” use Norlys for context, and use AuroraNotice (and webcams) to verify the sky is actually active and clear—then head outside immediately.
Using Webcams to Verify a Substorm Has Started
While satellite data is powerful, the best way to know exactly what the aurora is doing is to see a picture of it in real-time. I recommend using live aurora webcams—especially "all-sky" cameras that show the entire dome of the sky—as a reality check to verify the data.
You can actually use these cameras to identify which phase of the substorm is occurring. If you see the auroral arc rising "higher" in the webcam's frame (moving up from the northern horizon), the aurora is actually moving South (equatorward) towards you. This southward drift is the signature of the growth phase. It indicates the system is loading with energy.
Tracking substorm phases using an aurora webcam in Michigan.
The expansion phase is violent and incredibly fast. While the entire substorm cycle lasts hours, the intense, colorful "flare-up" might last minutes to tens of minutes. By keeping an eye on a webcam, you can spot the onset of the substorm by the classic auroral beads signature and run outside immediately to catch the aurora dancing!
Practical Tips for the Field
Where to point your camera
Because the auroral oval is fixed and we rotate underneath it, the time of the night can dictate what direction the aurora brightens from during a substorm. You can almost think of a rock being dropped into a pool of water. Depending on if you’re east or west of the initial splash (beginning of the substorm), you may see waves (aurora) approaching you from the west or east, respectively. Knowing the science helps you aim your camera in the right direction so you’re ready when the aurora starts to dance.
Example of a westward-traveling surge during a substorm (east is left of frame) I recorded using the AurorEye camera system in Fairbanks, Alaska.
- Evening: Close to sunset, you are not rotated yet underneath the auroral oval which is off to the northeast. Therefore, if a substorm occurs, you will likely see a brightening from east to west. Try pointing your camera towards the point where the auroral arc dips towards the horizon to the East.
- Morning: Close to sunrise, you have already rotated underneath the auroral oval, and substorms will start appearing from the west traveling east.
Substorm starting from the southern horizon with subsequent poleward expansion.
- Before the explosion: The expansion phase usually starts with the southernmost (growth-phase) arc brightening and then moving North. If you are watching a quiet arc, keep your eyes on the one furthest south—that is usually the fuse that lights the fire. I usually position my camera facing slightly northeast or northwest for this reason. If a substorm happens, I want to capture the poleward expansion since the aurora expands north, not south after beads form and the expansion phase is underway.
Auroral beads propagating from the west along a growth-phase auroral arc, propagating east with subsequent substorm expansion. Aurora webcam is located in Ely, MN. If the video does not automatically adjust, skip to around 4:59:11 in the video (around 00:56:20 on the timestamp in the top left) to see the auroral beads.
Perspective is everything
Why substorms are crucial for mid-latitude aurora chasers
For those chasing in the mid-latitudes (like the northern United States) or even lower, a substorm isn't just the highlight of the night, it might be the only thing you see. During quiet intervals, the auroral oval usually sits far to the north above locations like Churchill, Manitoba or Fairbanks, Alaska, completely hidden below the curvature of the Earth. You might stare at a dark horizon for hours seeing absolutely nothing.
However, when a substorm hits, the vertical growth of the aurora changes the game. As the display explodes upward, the high-altitude red emissions can peek over the horizon. This often appears as a brief, faint red glow or "pillars" on the northern horizon. Regardless of your latitude, substorms are always the best part of the show, but for chasers further away from the auroral oval, they are often the only part of the show.
Picture of the auroral oval from space overlaid with auroral visibility during quiet times (green line) vs. during a substorm (pink line). During a substorm, the aurora typically becomes taller with red emissions. These emissions can be seen from further away, extending the line of auroral visibility further equatorward.
What kind of aurora chaser are you?
When it comes down to it, learning about substorms is really about being an efficient aurora chaser. How much you need to obsess over this data depends entirely on your style.
The "All-Nighter”: If you are the type of person who is happy to stand outside in the freezing cold from dusk until dawn, staring at the sky the entire time, you honestly don't need to worry too much about forecasting. If you are out there all night, you are going to catch the substorm simply by default. You won't miss anything because you are always watching.
The "Guided" Traveler: If you are visiting a location like Alaska or Iceland and hire a professional aurora guide, you can take a backseat on the science. A good guide knows exactly what they are doing. It is their job to find clear skies and track these substorm cycles to make sure you are looking up at the right moment. They won't drive you back to the hotel five minutes before the sky explodes; they ensure you are in position for the peak of the show. Of course, this is all in theory. There are plenty of guides who don’t know about substorms, but the guides who work for me do. Make sure to check out tours ran by Maia and Levi, two expert aurora guides in Fairbanks who I am partnered with during aurora season.
The "Big Event" Hunter: Maybe you are a veteran who has seen the lights plenty of times, or you live far south and only care about the massive, once-a-year displays. In that case, you might be fine just monitoring the general NOAA Space Weather Prediction Center (SWPC) forecasts. You can wait for a G2 or G3 storm watch and only start paying attention to the real-time data when a major event is imminent. Make sure you are signed up for my aurora alerts, then to get the heads-up about these big shows!
The Efficient Chaser: However, if you want to see every possible aurora, or if you simply value your sleep and warmth, substorm knowledge is your best friend. Understanding why the aurora explodes allows you to go outside only when the probability is highest. It helps you catch displays that the general apps miss and ensures you are looking in the right direction when the magic happens. While you don't need to be obsessive to see the lights, understanding the rhythm of a substorm will make you a better, smarter, and more successful chaser.
Substorms are spontaneous, and we can't forecast them easily. My strategy is to “watch and react” - watch the GOES Magnetometer data for a spiking signature and the aurora for beads, then immediately head outside and look up!
Stay patient, watch the signs, and enjoy the show!
