Space weather comes with its own vocabulary — Kp index, CME, OVATION, Bz — that can feel impenetrable when you first encounter it. This glossary cuts through the jargon and explains each concept in straightforward language. Whether you are a first-time aurora chaser or someone who has been watching the skies for years, understanding these terms will help you read forecasts more confidently and make better decisions about when to go outside.
Terms are arranged alphabetically. Each entry links to the broader science described on our Science page when relevant.
Auroral Oval
A ring-shaped zone centered on each magnetic pole where aurora activity is most concentrated. The oval is not a fixed circle — it expands toward the equator during geomagnetic storms and contracts toward the poles during quiet periods. Solar Ruler's globe displays the current state of the auroral oval using real-time NOAA data. At Kp 5 and above, the oval can stretch dramatically southward, bringing aurora to latitudes that rarely see it.
Auroral Substorm
A sudden, short-lived intensification of aurora activity that can occur even during overall quiet geomagnetic conditions. During a substorm, the night-side of Earth's magnetosphere releases stored energy in a burst, causing aurora to rapidly brighten, develop rays and folds, and spread across the sky. Substorms typically last 30 minutes to two hours and are one of the most dramatic things to witness under a dark sky. Multiple substorms can occur in a single night.
Coronal Mass Ejection (CME)
A massive eruption from the Sun's corona that launches billions of tons of magnetized plasma into space. When a CME is aimed at Earth and arrives one to three days later, it compresses and disturbs our planet's magnetic field, often producing a geomagnetic storm. The intensity of the storm — and the aurora it produces — depends on the speed and magnetic orientation of the incoming plasma cloud. A southward-pointing magnetic field in the CME (called negative Bz) is the condition most favorable for triggering strong aurora.
Coronal Hole
A region on the Sun's surface where the magnetic field opens outward into space rather than looping back, allowing a faster, denser stream of solar wind to escape. Coronal holes are a steady, recurring source of elevated solar wind that can produce moderate geomagnetic activity and aurora, even without a solar flare or CME. High-speed streams from coronal holes often arrive at Earth with 24–72 hours of notice and can produce aurora-worthy conditions for one to three days.
Geomagnetic Storm
A worldwide disturbance of Earth's magnetic field caused by an influx of solar wind energy. Geomagnetic storms are rated on the G-scale from G1 (minor) to G5 (extreme). A G1 storm (Kp 5) can produce aurora visible from the northern US and central Europe. A G5 storm (Kp 9) — the type seen during the Carrington Event of 1859 — can push aurora toward the tropics and cause widespread disruption to power grids and satellite systems. Modern forecasters track incoming CMEs using satellites positioned between Earth and the Sun.
G-Scale
A five-level scale used by NOAA's Space Weather Prediction Center to classify the severity of geomagnetic storms. G1 is a minor storm (Kp 5); G5 is an extreme storm (Kp 9). Each step up the scale represents aurora visibility extending to progressively lower latitudes. A G3 storm (Kp 7) can bring visible aurora as far south as Oregon, Illinois, and Pennsylvania in the United States. The G-scale is used alongside the Kp index, though they are directly related — each G level corresponds to a specific range of Kp values.
Interplanetary Magnetic Field (IMF)
The magnetic field carried through space by the solar wind. When the IMF is oriented southward — opposite to Earth's northward magnetic field — it connects with Earth's field through a process called magnetic reconnection, allowing solar wind energy to efficiently enter the magnetosphere and power geomagnetic storms and aurora. The north-south component of the IMF is called Bz. Negative Bz (southward) is the most important indicator of incoming aurora-producing conditions. Solar Ruler's users often watch the Bz value in real-time to gauge whether a geomagnetic storm will intensify.
Kp Index
The planetary K-index, a 0–9 scale that measures the average global geomagnetic disturbance level over a three-hour period. Kp 0–1 represents very quiet conditions; Kp 4 is active but below storm threshold; Kp 5–6 is a minor-to-moderate geomagnetic storm; Kp 7–9 is a major-to-extreme storm. The Kp index is the most widely used number in aurora forecasting because it translates directly into a rough estimate of where on Earth the aurora will be visible. Solar Ruler's live OVATION model incorporates current geomagnetic conditions to show the auroral oval in real time.
Magnetosphere
The region of space surrounding Earth that is dominated by Earth's magnetic field, extending from roughly 60,000 kilometers on the dayside (facing the Sun) to hundreds of thousands of kilometers on the nightside, where it is stretched into a long magnetotail. The magnetosphere acts as a shield against the solar wind, deflecting most of the incoming charged particles around the planet. However, during geomagnetic storms, the magnetosphere is compressed and energy enters the system, ultimately driving aurora at the poles. Without the magnetosphere, Earth's atmosphere would be slowly eroded by the solar wind over geologic time.
Magnetic Reconnection
A process in which magnetic field lines from two different sources break and reconnect in a new configuration, releasing large amounts of energy in the process. In the context of aurora, reconnection occurs when the southward-pointing interplanetary magnetic field connects with Earth's northward-pointing magnetic field at the dayside boundary of the magnetosphere. This allows solar wind energy to enter the magnetosphere and drive electrical currents that ultimately produce the aurora. Reconnection can also occur on the nightside of the magnetosphere, releasing energy stored in the magnetotail and triggering auroral substorms.
NOAA Space Weather Prediction Center (SWPC)
The United States government agency responsible for monitoring and forecasting space weather. NOAA SWPC operates a network of ground-based and space-based sensors that track solar activity, solar wind conditions, and the state of Earth's magnetosphere in real time. The center issues alerts, watches, and warnings for geomagnetic storms, solar radiation events, and other space weather hazards. Solar Ruler relies on NOAA's OVATION aurora model — one of SWPC's real-time products — to power the live aurora oval displayed on its globe.
OVATION Model
A real-time scientific model developed by NOAA that predicts the intensity and location of aurora activity based on incoming solar wind measurements. OVATION — which stands for Oval Variation, Assessment, Tracking, Intensity, and Online Nowcasting — uses data from satellites positioned upstream of Earth to estimate where aurora will appear and how bright it will be, approximately 30–60 minutes in advance. Solar Ruler fetches this data and maps it onto its 3D globe as the glowing green band you see over the polar regions.
Solar Cycle
The approximately 11-year cycle of solar activity during which the Sun's magnetic field gradually becomes more complex, reaches a period of peak activity called solar maximum, then calms to a solar minimum before repeating. During solar maximum, the Sun produces more sunspots, solar flares, and coronal mass ejections, leading to more frequent and intense aurora displays at lower latitudes. During solar minimum, activity is suppressed and aurora tends to be confined to higher latitudes. The current solar cycle (Solar Cycle 25) began in December 2019 and is expected to peak around 2025.
Solar Flare
A sudden, intense burst of radiation from the Sun's surface caused by the release of magnetic energy around sunspot regions. Solar flares are classified by their X-ray intensity: C-class (minor), M-class (moderate), and X-class (major). The radiation from a flare travels at the speed of light and reaches Earth in about eight minutes, where it can disrupt radio communications and GPS signals on the sunlit side of the planet. Flares often, but not always, accompany coronal mass ejections. The flare itself does not directly cause aurora — the CME that may follow it, arriving one to three days later, is what drives the geomagnetic storm and aurora display.
Solar Wind
A continuous stream of charged particles — mostly electrons and protons — that flows outward from the Sun's corona in all directions at speeds of 250–800 kilometers per second. The solar wind carries the Sun's magnetic field into space, creating what is called the heliosphere — a vast bubble of solar influence that extends far beyond the orbit of Pluto. At Earth's orbit, the solar wind is monitored by spacecraft positioned at the L1 Lagrange point, about 1.5 million kilometers toward the Sun. Data from these spacecraft provides roughly 30–60 minutes of warning before solar wind conditions reach Earth and begin affecting the magnetosphere.
Substorm (see Auroral Substorm)
See Auroral Substorm above. Substorms are one of the most exciting and photogenic aspects of aurora watching — a strong substorm can transform a quiet green glow into a full-sky light show within seconds. They are not always correlated with high Kp values, meaning aurora can be spectacular even when the overall geomagnetic conditions appear moderate.
Terminator
The boundary between the sunlit day side and the dark night side of Earth. In aurora forecasting, the terminator is important because aurora is only visible on the night side of the planet — the atmosphere must be dark for the aurora's light to be seen against the sky. Solar Ruler's aurora model masks out the daytime hemisphere to show only where aurora would be visible to observers on the ground. Near the terminator, aurora that is active may be washed out by twilight, especially at high altitudes.
Zenith Aurora
Aurora that appears directly overhead, filling the entire sky above the observer, rather than on the horizon. Zenith aurora typically means you are standing beneath one of the most active parts of the auroral oval. The display often appears as a corona — a circular, symmetrical burst of rays radiating outward from a central point directly above, like light shining down through a circular skylight. Experiencing zenith aurora is considered by many observers to be the ultimate aurora encounter, and it is rare enough that even experienced chasers describe it as one of the most moving natural experiences of their lives.