Aurora Glossary
Aurora hunting comes with its own vocabulary. Whether you’re reading a forecast, joining an online community or chatting with a guide, understanding these terms will help you make sense of conditions and improve your chances of seeing a great display.
This glossary covers the most common terms you’ll encounter — from solar weather and geomagnetic activity to the visual characteristics of the aurora itself.
Solar Weather
- Solar Wind
- A continuous stream of charged particles (mostly electrons and protons) flowing outward from the Sun at speeds of 300–800 km/s. When the solar wind is fast and dense, it increases the likelihood of auroral activity. Speeds above 400 km/s are generally considered favourable for aurora.
- CME (Coronal Mass Ejection)
- A massive burst of magnetised plasma erupted from the Sun’s corona. CMEs travel through space and can strike Earth’s magnetic field 1–3 days after leaving the Sun, often triggering geomagnetic storms and strong auroral displays. They are responsible for most major aurora events visible at lower latitudes.
- Solar Flare
- A sudden, intense burst of electromagnetic radiation from the Sun’s surface. Solar flares are classified by strength: A, B, C, M and X (weakest to strongest). While flares themselves don’t directly cause aurora, they often accompany CMEs that do.
- Coronal Hole
- An area of the Sun’s corona where the magnetic field opens outward, allowing solar wind to escape at higher speeds. Coronal holes produce recurring high-speed solar wind streams that can enhance auroral activity for several days. Because the Sun rotates roughly every 27 days, the same coronal hole can affect Earth on a recurring cycle.
- Solar Cycle
- An approximately 11-year cycle of solar activity, measured by the number of sunspots. During solar maximum, CMEs and flares are more frequent, increasing the chances of strong aurora. During solar minimum, activity is lower but aurora still occurs — often driven by coronal holes.
- Sunspot
- A temporarily cooler, darker region on the Sun’s surface caused by intense magnetic activity. Sunspots are often the source of solar flares and CMEs. More sunspots generally means more potential for geomagnetic storms.
- Solar Energetic Particles (SEPs)
- High-energy particles accelerated by solar flares or CME-driven shocks. These can arrive at Earth within minutes to hours and occasionally contribute to auroral activity, though their primary impact is on satellites and polar radio communications.
Geomagnetic Indicators
- Bz (Interplanetary Magnetic Field — North/South Component)
- The north-south orientation of the interplanetary magnetic field (IMF) carried by the solar wind. When Bz turns southward (negative), it can connect with Earth’s magnetic field and allow solar wind energy to enter the magnetosphere — fuelling aurora. A Bz of -10 nT or lower is considered strong and favourable. When Bz is northward (positive), Earth’s magnetic shield remains largely closed and aurora is less likely.
- Bt (Total Interplanetary Magnetic Field Strength)
- The total strength of the interplanetary magnetic field, regardless of direction. A high Bt combined with a strongly negative Bz indicates potent conditions for geomagnetic activity.
- Kp-index
- A global geomagnetic activity index scaled from 0 to 9. It’s updated every 3 hours and indicates how disturbed Earth’s magnetic field is. Higher values mean stronger geomagnetic storms and aurora visible at lower latitudes:
- Kp 0–1 — Quiet. Aurora confined to high latitudes.
- Kp 2–3 — Unsettled. Aurora visible from typical aurora zone locations.
- Kp 4–5 — Active to minor storm. Aurora expands southward; visible from northern Scotland, southern Scandinavia, northern US states.
- Kp 6–7 — Moderate to strong storm. Aurora visible from central UK, central Europe, central US.
- Kp 8–9 — Severe storm. Aurora potentially visible from southern Europe and southern US. Rare events.
- G-scale (Geomagnetic Storm Scale)
- NOAA’s 5-level scale for geomagnetic storms: G1 (minor) to G5 (extreme). G1 corresponds to Kp5, G5 to Kp9. The scale helps communicate the severity and potential impacts of a storm.
- Dst Index (Disturbance Storm Time)
- A measure of the ring current around Earth during geomagnetic storms. A strongly negative Dst (e.g. -100 nT or lower) indicates a significant storm is in progress and the aurora oval is likely expanded.
- Magnetosphere
- The region of space around Earth controlled by its magnetic field. The magnetosphere deflects most solar wind but can be disturbed by CMEs and high-speed streams, leading to geomagnetic storms and aurora.
- Substorm
- A sudden, localised burst of auroral activity caused by energy release in Earth’s magnetotail. Substorms can produce vivid, fast-moving aurora even when overall geomagnetic activity (Kp) is relatively low. They typically last 1–3 hours and are often the cause of sudden "aurora explosions" that catch forecasters off guard.
- Magnetotail
- The elongated tail of Earth’s magnetosphere, stretched away from the Sun by the solar wind. Energy stored in the magnetotail is periodically released during substorms, accelerating particles towards Earth’s poles and causing aurora.
Aurora Characteristics
- Aurora Oval
- A ring-shaped zone centred on Earth’s magnetic poles where aurora is most likely to occur. The oval expands equatorward during geomagnetic storms and contracts poleward during quiet times.
- Arc
- The most common auroral form — a long, east-west aligned band of light stretching across the sky. Arcs can be homogeneous (uniform brightness) or rayed (with vertical striations). They often appear first as activity begins.
- Band
- Similar to an arc but with folds, curves or kinks. Bands indicate more dynamic conditions and often precede more active displays.
- Corona
- A dramatic auroral form where rays appear to converge at a single point directly overhead (the magnetic zenith). A corona indicates you are directly beneath the aurora and often occurs during strong activity. It’s one of the most spectacular sights for aurora watchers.
- Curtains / Drapes
- Rapidly moving, folded sheets of light that resemble hanging fabric blowing in the wind. These occur during active phases and can move quickly across the sky.
- Pulsating Aurora
- Patches of aurora that rhythmically brighten and dim, typically on a timescale of a few seconds. Usually seen during the recovery phase of a substorm and often lower in the sky.
- Breakup
- The explosive onset of a substorm, where a quiet arc suddenly brightens, develops rapid movement and expands poleward. Breakups produce some of the most dynamic and memorable aurora displays.
- STEVE (Strong Thermal Emission Velocity Enhancement)
- A recently identified atmospheric phenomenon that appears as a narrow, mauve or white-green ribbon of light, often accompanied by a green "picket fence" structure. STEVE is not technically aurora (it’s caused by a different mechanism) but often appears alongside auroral displays at sub-auroral latitudes.
- Proton Aurora
- A diffuse glow caused by energetic protons rather than electrons. Proton aurora is typically faint and spread over a broad area, often appearing as a dim, uniform brightening of the sky. It’s usually not visible to the naked eye but can be detected by cameras.
- Discrete vs Diffuse Aurora
- Discrete aurora has clearly defined structures — arcs, rays, curtains — and is what most people think of as "the Northern Lights." Diffuse aurora is a faint, structureless glow spread across the sky, often visible in photographs but hard to see with the naked eye.
Forecasting Terms
- ACE / DSCOVR
- Spacecraft positioned at the L1 Lagrange point (approximately 1.5 million km from Earth towards the Sun). They measure real-time solar wind conditions — speed, density, and magnetic field — giving roughly 15–60 minutes advance warning before those conditions reach Earth. DSCOVR is the primary operational satellite; ACE serves as backup.
- L1 Lagrange Point
- A gravitationally stable point between Earth and the Sun where monitoring spacecraft are positioned. Solar wind measured here will arrive at Earth shortly after, making it the ideal early-warning location.
- Ovation Map
- A model produced by NOAA that predicts the location and intensity of the aurora oval in near real-time. It uses live solar wind data to estimate where the aurora is most likely visible. More useful than the Kp-index for determining whether the aurora will be visible from your specific location.
- Solar Wind Density
- The number of particles per cubic centimetre in the solar wind. Higher density (above ~10 particles/cm³) combined with high speed and southward Bz can amplify geomagnetic activity and aurora brightness.
- Shock / Sudden Impulse
- A sharp increase in solar wind speed and density — often the leading edge of a CME hitting Earth’s magnetosphere. Shocks can trigger immediate geomagnetic activity and are often followed by sustained storming.
- Interplanetary Shock
- A discontinuity in the solar wind where speed, density and magnetic field change abruptly. These are often associated with the arrival of CMEs and can cause a sudden compression of Earth’s magnetosphere.
Viewing Conditions
- Magnetic Midnight
- The time when your location is pointed most directly away from the Sun in magnetic coordinates — roughly when the aurora oval passes closest overhead. This is the statistically best time for aurora viewing. It doesn’t correspond exactly to clock midnight and varies by location, but typically will be around an hour earlier than the conventional solar midnight.
- Magnetic Zenith
- The point in the sky directly along your local magnetic field line — where aurora appears to converge overhead during a corona display. At auroral latitudes, this is typically slightly poleward of straight up.
- Light Pollution
- Artificial light from towns, cities and industry that brightens the night sky and reduces contrast, making faint aurora difficult or impossible to see. Getting away from light pollution — even 10–20 km from a town — dramatically improves visibility.
- Nautical Twilight
- When the Sun is 6–12° below the horizon. Aurora can become visible during nautical twilight, though full darkness (astronomical twilight or darker) is preferred. Relevant at high latitudes where true darkness is limited in autumn and spring.
- Moonlight
- A full or bright gibbous moon illuminates the landscape beautifully but washes out faint aurora. A new moon or thin crescent provides the darkest skies. However, moderate moonlight can actually enhance aurora photography by lighting foreground elements.
- Dark Adaptation
- The process by which your eyes adjust to darkness, taking 20–30 minutes to reach full sensitivity. Checking your phone screen or using white light resets this process. Use red light to preserve your night vision.
- Bortle Scale
- A nine-level scale measuring night sky darkness, from Class 1 (excellent dark sky) to Class 9 (inner-city sky). For aurora viewing, anywhere Class 4 or darker gives good results. The scale helps you assess how much light pollution affects your location.
Community Shorthand
- KP / Kp
- Shorthand for the Kp-index (see above). You’ll often see people say "Kp is 5 tonight" in aurora groups.
- IMF
- Interplanetary Magnetic Field — the magnetic field embedded in the solar wind. Its Bz component is the most important factor for aurora.
- CH HSS
- Coronal Hole High-Speed Stream — fast solar wind flowing from a coronal hole. Often written in shorthand in space weather discussions.
- CIR (Co-rotating Interaction Region)
- The boundary where fast solar wind from a coronal hole catches up with slower wind ahead of it, creating a compression region that can trigger moderate geomagnetic activity.
- Earth-directed / Geoeffective
- Describes a CME or solar wind feature that is heading towards Earth and likely to interact with our magnetosphere. Not all solar eruptions are Earth-directed — only those aimed at our planet will affect aurora.
- Glancing Blow
- When a CME only clips Earth’s magnetosphere rather than hitting it directly. This produces weaker effects than a direct hit but can still trigger moderate aurora.
- Solar Noon / Solar Midnight
- The times when the Sun is at its highest and lowest points relative to your location. Aurora is statistically most likely around magnetic midnight but can occur at any time during dark hours when conditions are strong enough.
Understanding these terms will help you interpret forecasts, follow space weather updates, and know when to head outside. For current conditions, check our live aurora forecast.