Aurora glossary: the space-weather terms defined

This glossary defines the terms that appear in aurora forecasts, space-weather bulletins and the rest of this site. Each entry gives one definition and then what the term changes for someone standing outside hoping to see something, because a definition that does not connect to a decision is not much use.

Terms are grouped by what they describe rather than alphabetically: the Sun and the solar wind first, then Earth’s magnetic environment, then the indices, then what appears in the sky. Where a term has a full article behind it, the entry links there.

The grouping is not cosmetic. Each group answers a different question, and knowing which group a term belongs to tells you what kind of decision it can support.

The Sun and the solar wind

Solar wind

The solar wind is the continuous outflow of charged particles and embedded magnetic field from the Sun’s corona. It reaches Earth at speeds typically between about 300 and 800 kilometres per second, and it never stops — there is no such thing as a night with no solar wind, only nights with wind that couples poorly to Earth’s magnetic field.

Speed matters less than most forecasts imply. A fast stream with an unfavourable magnetic orientation produces little, while moderate wind with a strongly southward field can produce a good display. How to read solar wind speed, density and Bz covers the interaction in detail.

Interplanetary magnetic field (IMF)

The interplanetary magnetic field is the Sun’s magnetic field carried outward by the solar wind. Its orientation on arrival at Earth determines how efficiently energy enters the magnetosphere, and it cannot be predicted from solar observations — it has to be measured as the wind arrives.

That single fact sets the ceiling on aurora forecasting. Everything beyond about an hour ahead is a forecast of the wind, not of the field embedded in it.

Bz

Bz is the north-south component of the interplanetary magnetic field. It is reported in nanotesla, and the sign is what matters: a negative, or southward, Bz points opposite to Earth’s field where the two meet on the dayside, which allows magnetic reconnection to open a route for energy into the magnetosphere.

A sustained Bz near −10 nT is a genuinely promising sign. A brief dip is not, because the magnetosphere needs a period of loading before it has energy to release. Bt, the total field strength, sets the maximum size Bz can reach, so a strong Bt with a northward field means the potential is there but not yet turned on.

Coronal mass ejection (CME)

A coronal mass ejection is a large expulsion of plasma and magnetic field from the Sun’s corona. Transit to Earth takes roughly 15 to 90 hours depending on speed, and CMEs drive the strongest geomagnetic storms.

A CME’s arrival time can be modelled from coronagraph images with useful but limited precision, and its internal magnetic orientation cannot be measured remotely at all. This is why a three-day forecast can correctly identify the night and still be wrong about whether anything happens.

Coronal hole and corotating interaction region

A coronal hole is a region of open magnetic field in the corona from which fast solar wind escapes. Because the Sun rotates in about 27 days as seen from Earth, a long-lived coronal hole can produce recurring disturbances at roughly that interval — which is the entire basis of the 27-day outlook.

A corotating interaction region is the compressed boundary formed where that fast wind overtakes slower wind ahead of it. It can produce a respectable geomagnetic disturbance with no eruption on the Sun at all, which surprises people who are watching only for flares.

Solar flare

A solar flare is a sudden release of energy in the solar atmosphere. Its radiation arrives in about eight minutes and disrupts high-frequency radio on the sunlit side of Earth.

A flare on its own does not produce aurora. Flares are frequently reported in general news as aurora warnings, but what matters for the sky is whether an associated CME was launched and whether it is headed toward Earth.

L1 point

L1 is the Sun-Earth Lagrange point about 1.5 million kilometres sunward of Earth, where the two bodies’ gravity allows a spacecraft to hold station upstream in the solar wind. Instruments there sample the wind roughly 20 to 60 minutes before it reaches Earth, the range depending on how fast the wind is travelling.

That interval is the physical basis of every reliable short-range aurora forecast, and its length is why the nowcast is measured in tens of minutes rather than hours. Which satellites and observatories produce aurora data names the specific spacecraft.

Earth’s magnetic environment

Magnetosphere and magnetotail

The magnetosphere is the region around Earth where the planet’s own magnetic field dominates over the interplanetary field. Solar wind pressure compresses it on the dayside and draws it into a long tail on the nightside, the magnetotail, which extends far beyond the Moon’s orbit.

The magnetotail is where energy is stored before an auroral display. Understanding that storage step is what separates a useful mental model from the common one in which particles travel from the Sun and arrive in the sky an hour later.

Magnetic reconnection

Magnetic reconnection is the process in which oppositely directed magnetic field lines break and reconnect into a new configuration, converting stored magnetic energy into particle energy and heat. It happens twice in the aurora sequence: on the dayside, where it loads the tail, and later in the tail itself, where it releases that energy.

Ring current and auroral electrojet

The ring current is a westward current carried by energetic ions drifting around Earth at a few Earth radii. Its strengthening during a storm depresses the surface magnetic field, and that depression is what the Dst index measures.

The auroral electrojet is a separate, much closer current flowing horizontally in the ionosphere at about 100 kilometres altitude. Its rapid changes are what induce currents in long grounded conductors on the surface — see what a geomagnetic storm actually does.

Geomagnetic latitude and corrected geomagnetic latitude

Geomagnetic latitude measures position relative to Earth’s magnetic pole rather than the geographic pole. Corrected geomagnetic latitude, usually abbreviated CGM, is obtained by tracing the magnetic field line through a location in a field model and relating it to an equivalent latitude, which represents the field’s real offset and distortion far better than a single correction.

This is the coordinate that decides which Kp value a location needs, and it can differ from map latitude by several degrees. Why your neighbour sees more aurora than you works through the comparison; the geomagnetic latitude calculator computes it for a specific place.

IGRF

The International Geomagnetic Reference Field is the standard mathematical model of Earth’s main magnetic field, revised every five years under IAGA. Every geomagnetic coordinate calculation is computed against some version of it, which is why two calculators can return slightly different values for the same address and both be correct.

Magnetic local time and magnetic midnight

Magnetic local time measures the hour in the magnetic coordinate system rather than by the Sun. Magnetic midnight is the moment a location passes through 00 MLT, placing it most directly on the nightside of the magnetosphere.

Substorm onsets cluster in the hours around magnetic midnight, and magnetic midnight does not coincide with clock midnight — the offset depends on longitude and can exceed an hour. For deciding when to stand outside, it is the more useful clock.

The indices

Kp, ap and Ap

Kp is a planetary index of geomagnetic disturbance reported for each three-hour UT interval, on a quasi-logarithmic scale from 0 to 9 in thirds, derived from 13 subauroral observatories and published by GFZ in Potsdam. The ap index is the linear equivalent for the same interval in nanotesla, and Ap is the daily average of that day’s eight ap values.

The distinction matters because Kp values must not be averaged arithmetically — that is what ap exists for. The Kp index explained, and what it cannot tell you covers the limits of the index as a viewing guide.

Hp30 and Hp60

Hp30 and Hp60 are high-cadence geomagnetic indices on the same scale as Kp but computed over 30-minute and 60-minute intervals. Unlike Kp they are not capped at 9, so an extreme storm is not flattened at the top of the scale.

They exist because a three-hour average hides exactly the timescale on which aurora actually varies. A sharp substorm can produce an excellent 20 minutes inside a three-hour block that reports a modest Kp.

Dst and AE

Dst is an hourly index in nanotesla measuring the depression in Earth’s horizontal field caused by the ring current, computed from four near-equatorial observatories. A deeply negative Dst is the standard definition of a strong magnetic storm.

AE, the auroral electrojet index, is computed at one-minute cadence from a chain of auroral-zone observatories and tracks the electrojet directly. Of the common indices it is the one that responds on the timescale of a substorm.

The NOAA G-scale

The NOAA G-scale rates geomagnetic storms from G1 to G5 and pairs each level with documented effects and an average frequency. The mapping to Kp is fixed: G1 is Kp 5, G2 is Kp 6, G3 is Kp 7, G4 is Kp 8 including a 9−, and G5 is Kp 9.

The scale is more useful than a bare Kp number because each level carries an explicit statement of how far south aurora has historically been reported. How the geomagnetic indices differ sets them side by side.

What appears in the sky

Auroral oval

The auroral oval is the ring-shaped region around each geomagnetic pole where auroral emission is most likely at a given moment. It is offset toward the nightside, and it expands toward lower geomagnetic latitudes as activity rises.

It is a statistical region rather than a painted band, and because the emission sits about 100 kilometres up, it is visible from well outside its edge by looking toward the pole. Why the lights form a ring explains the geometry.

OVATION

OVATION is the empirical auroral precipitation model behind NOAA’s 30-minute aurora forecast map. It shows modelled energy deposited into the atmosphere at each point, not the probability that a person will see something from the ground.

Reading the coloured map as a viewing probability for your own garden is one of the most common mistakes in this subject. How to read the OVATION map covers what the colours mean.

Substorm

An auroral substorm is a cycle of roughly one to three hours in which energy stored in the magnetotail is released abruptly. It has three phases — growth, expansion and recovery — and the expansion phase is when a quiet arc suddenly brightens and races poleward.

Most memorable displays are substorm expansions. What a substorm is is the article behind this entry.

Diffuse and discrete aurora

Diffuse aurora is a broad, structureless glow produced by electrons scattered into the atmosphere over a wide region. It is often below the threshold of unaided human vision while showing clearly in a photograph.

Discrete aurora is the structured aurora of arcs, bands and rays, produced by electrons accelerated along magnetic field lines. It is brighter, sharper and what people mean by the northern lights.

Arc, band, ray and corona

An arc is a smooth band of light stretching roughly east to west with a sharp lower edge and a diffuse upper edge. A band is an arc that has developed folds or kinks. Rays are vertical striations aligned with the magnetic field.

A corona is rays appearing to converge on a point overhead. It is a perspective effect produced by looking straight up a set of parallel rays rather than a separate kind of aurora. The names for what you are looking at sets out the full classification.

International Brightness Coefficient (IBC)

The IBC rates visual auroral brightness on four steps: IBC I is comparable to the Milky Way, IBC II to moonlit cirrus, IBC III to moonlit cumulus, and IBC IV to full moonlight.

It describes what an eye sees rather than energy deposited, which is why a camera can produce a vivid image of an IBC I display that looked like a grey smudge in person.

STEVE

STEVE, short for Strong Thermal Emission Velocity Enhancement, is a narrow mauve arc seen at lower geomagnetic latitudes than typical aurora and aligned roughly east to west. It is produced by a fast-flowing, heated stream of plasma rather than by particle precipitation, so it is not aurora in the strict sense.

It was named by aurora photographers and characterised by researchers working with citizen-science reports alongside satellite measurements — an unusually direct case of amateur observation feeding a scientific result.

Airglow

Airglow is faint continuous emission from the upper atmosphere, present on every clear night regardless of geomagnetic activity. It appears in long exposures as a mottled green or reddish wash.

It is the most common cause of a false aurora report from a photograph. If the glow does not move, does not have vertical structure, and appears in every direction rather than toward the pole, it is very likely airglow.

Dark adaptation

Dark adaptation is the process by which the eye becomes sensitive to low light over roughly 20 to 30 minutes. Colour vision is largely lost in the process, which is why a display that photographs as vivid green looks grey to the eye.

Checking a phone screen resets much of that adaptation. That single habit ruins more aurora nights than a poor forecast does. Camera settings for the aurora covers the exposure side.

Bottom line

The terms in this glossary split into three groups that answer different questions. Solar wind terms — Bz, Bt, CME, coronal hole, L1 — answer whether energy is arriving and how soon. Magnetic terms — corrected geomagnetic latitude, magnetic local time, auroral oval — answer whether you are in the right place at the right hour. Index terms — Kp, ap, Dst, AE, Hp30, the G-scale — are summaries of what already happened, useful for context but always lagging.

The most consequential single term for a short-range decision is Bz: sustained and southward is the signal, and it can only be known about 20 to 60 minutes ahead because it must be measured at L1. The most consequential term for a long-range decision is corrected geomagnetic latitude, because it does not change and it sets the threshold everything else is judged against. And the most commonly misread term is OVATION, because a modelled emission map is not a viewing probability for a specific address.