Glossary

Space weather has a lot of jargon. Here's what all the terms in Lumina actually mean.

Space weather terms

IMF — Interplanetary Magnetic Field: The Sun's magnetic field, carried through space by the solar wind. When it reaches Earth, its orientation relative to Earth's magnetic field determines whether energy can enter the magnetosphere.
B_z — the north-south component of the IMF: The single most important number for aurora. B_z is measured in nanoteslas (nT). Negative (southward) B_z means the IMF opposes Earth's field — this opens the gate and lets energy pour into the magnetosphere, driving aurora. Positive (northward) B_z means the fields are aligned and the gate stays mostly closed. When aurora watchers talk about "B_z going south," this is what they mean.
B_y — the east-west component of the IMF: How much the IMF is leaning sideways. A strong B _y tilts the field lines and can affect exactly where on Earth the energy enters. Combined with B _z , B _y determines the clock angle — the overall direction of the IMF in the plane perpendicular to the Sun-Earth line.
B_t — total IMF magnitude: The overall strength of the interplanetary magnetic field, regardless of direction: \(B_t = \sqrt{B_x^2 + B_y^2 + B_z^2}\) . Higher B_t means a stronger field, which can drive stronger coupling when B_z is southward.
Solar wind speed: How fast the solar wind is moving, typically 300–800 km/s. Faster wind carries more kinetic energy and drives stronger magnetospheric coupling. During CME passages, speeds can exceed 1000 km/s.
Solar wind density: Number of particles per cubic centimetre in the solar wind. Higher density means more particles available to interact with the magnetosphere. Typical values: 1–20 particles/cm³. Very high density combined with high speed produces high dynamic pressure.
Dynamic pressure: The ram pressure of the solar wind against Earth's magnetosphere, calculated from speed and density. Higher pressure compresses the magnetosphere, which can intensify coupling. Measured in nanopascals (nPa).
Newell coupling: A formula that estimates how efficiently solar wind energy couples into the magnetosphere, based on the IMF clock angle and solar wind speed. Named after Pat Newell, who derived it from satellite observations. The formula involves \(\sin^{8/3}(\theta/2) \times v^{4/3}\) , making it especially sensitive to southward B_z and high solar wind speed.
Clock angle: The direction of the IMF in the plane perpendicular to the Sun-Earth line, measured in degrees: 0° = purely northward, 90° = purely eastward (toward dusk), 180° = purely southward, 270° = purely westward (toward dawn). A clock angle near 180° (southward) is optimal for aurora.

Geomagnetic terms

Kp — planetary K-index: A 3-hour global measure of geomagnetic activity on a scale of 0–9. Derived from a network of ground magnetometers. Kp ≥ 5 indicates a geomagnetic storm (G1). Kp is a lagging indicator — it tells you what happened over the last 3 hours, not what's happening right now.
G-scale: The NOAA geomagnetic storm scale: G1 (Minor, Kp=5) through G5 (Extreme, Kp=9). Each level corresponds to specific aurora visibility expectations at different latitudes.
HP — Hemispheric Power: The estimated total auroral particle energy deposition in gigawatts (GW), derived from NOAA's OVATION model. It's a real-time measure of how much energy is actually hitting the upper atmosphere, separately for the northern and southern hemispheres. Lumina primarily uses the southern hemisphere value.
Auroral oval: The ring-shaped region around each magnetic pole where aurora typically occurs. The oval expands equatorward as geomagnetic activity increases. Its position is estimated from hemispheric power and OVATION model data.
Magnetosphere: The region of space dominated by Earth's magnetic field. It acts as a shield against the solar wind, but when the IMF is southward, energy can enter through magnetic reconnection at the magnetopause.
Magnetotail: The night-side extension of the magnetosphere, stretched far behind Earth by the solar wind. Energy from southward IMF builds up here before being released in substorms.
Substorm: A sudden release of energy stored in the magnetotail, typically lasting 30–60 minutes. Substorms produce bright, dynamic, fast-moving aurora. They can happen multiple times in one night and are the main event for aurora watchers at AU/NZ latitudes.
Geomagnetic storm: A global disturbance of Earth's magnetic field, usually triggered by a CME impact or a high-speed solar wind stream. Storms last hours to days and can drive repeated substorms. Classified on the G-scale.
MLT — Magnetic Local Time: Time measured relative to the magnetic pole rather than the geographic pole. Magnetic midnight is when you're on the night side of the magnetic pole — typically the best time for aurora viewing.
L1 — Lagrange Point 1: A gravitationally stable point about 1.5 million km sunward of Earth where NOAA's SOLAR1, DSCOVR & ACE satellites sit. Solar wind measured at L1 takes 30–90 minutes to reach Earth (the propagation delay).

Solar terms

CME — Coronal Mass Ejection: A massive eruption of plasma and magnetic field from the Sun's corona. CMEs travelling toward Earth can trigger geomagnetic storms 1–4 days after eruption. Halo CMEs (appearing as a ring around the Sun in coronagraph images) are the most likely to be Earth-directed.
Solar flare: A sudden brightening on the Sun caused by magnetic reconnection in the solar atmosphere. Flares are classified C, M, or X (increasing intensity). Flares often (but not always) accompany CMEs.
Coronal hole: A region of the Sun's corona that's cooler and less dense than surrounding areas, allowing high-speed solar wind to escape. Coronal hole high-speed streams can cause recurrent geomagnetic activity on a 27-day cycle (the Sun's rotation period).

Lumina-specific & miscellaneous terms

Field Mode: Lumina's short-term (~10–15 min) probability mode. Uses arrival-shifted IMF data to estimate current aurora odds. Best for immediate go/no-go decisions.
Plan Mode: Lumina's planning-horizon (~30–90 min) probability mode. Uses raw L1 data plus storm memory and visibility factor. Best for deciding whether to head out later tonight.
Storm memory: Lumina's model of magnetotail charge state (0–1). Rises during sustained southward IMF, decays slowly afterward. Carries forward built-up energy so the probability doesn't drop to zero during brief lulls.
SQM — Sky Quality Meter: A measure of sky darkness in magnitudes per square arcsecond (mag/arcsec²). Higher numbers = darker skies. Used in Lumina for location-based viewing quality assessment.
Visibility factor: A multiplier (0.15–1.0) that scales Plan mode probability based on whether the auroral oval is above your horizon. Accounts for atmospheric extinction at low elevation angles.
ΔH (delta-H): The deviation of the horizontal component of the ground magnetic field from its quiet-day baseline, measured in nanoteslas. A negative ΔH at sub-auroral stations indicates an active substorm electrojet overhead.