AuroracastUKLearn
Solar Activity
Understanding Hp30, Bz, solar wind and CMEs — the forces that drive UK aurora displays.
The Hp30 Index
The Hp30 index measures global geomagnetic disturbance on a 0–9 scale — the same as the older Kp index, but recalculated every 30 minutes instead of every 3 hours. This extra resolution catches the sharp onset of a substorm hours before Kp reflects it. Hp30 is published by GFZ Potsdam and is the index Auroracast uses for all live readings, forecasts, and alerts.
Bz — The Key Number
The Bz component of the interplanetary magnetic field (IMF) is arguably the single most important number for aurora watchers. It describes the north-south orientation of the magnetic field carried in the solar wind.
Bz negative (southward)
When Bz points south, it aligns anti-parallel with Earth's northward magnetic field. This allows magnetic reconnection — solar wind energy pours into the magnetosphere, accelerating particles toward the poles and energising aurora. The more negative, the stronger the activity.
Bz positive (northward)
When Bz points north, it aligns parallel with Earth's field. The magnetosphere stays closed. Even with high solar wind speed or elevated Hp30, aurora will be weak or absent. Always check Bz before heading out.
A Bz of −10 nT sustained for 30+ minutes typically produces Hp30 5–6. Values below −20 nT during a CME can drive Hp30 7–9 storms.
Solar Wind
The solar wind is a continuous stream of charged particles (mostly protons and electrons) flowing outward from the Sun at 300–800 km/s. It carries the Sun's magnetic field into space as the interplanetary magnetic field (IMF).
Speed
Faster wind (>500 km/s) amplifies geomagnetic effects. Wind from coronal holes is typically faster than background wind.
Density
Higher particle density means more energy delivered to the magnetosphere per second. A dense, fast stream is a powerful driver.
Pressure
Dynamic pressure = ½ρv². Sudden pressure increases can cause Sudden Storm Commencements (SSCs) — rapid brightening of aurora before the main phase.
Coronal Mass Ejections (CMEs)
A CME is a large expulsion of plasma and magnetic field from the Sun's corona. They are far more energetic than the background solar wind and are responsible for the strongest geomagnetic storms.
- Eruption — A solar flare or filament eruption launches a CME from the solar surface, often associated with an X-ray flare detected by GOES satellites.
- Transit — CMEs travel at 500–3000 km/s. A typical Earth-directed CME takes 1–3 days to arrive. NOAA models provide arrival estimates within ±6 hours.
- Sheath — A turbulent compressed region arrives first, often with variable Bz. Aurora may begin here but is unpredictable.
- Main body — The magnetic cloud follows, often with strong sustained southward Bz — this is when the best aurora typically occurs.
- Recovery — Bz returns northward, Hp30 falls, aurora fades over 6–24 hours.
Solar Cycle
The Sun follows an approximately 11-year activity cycle between solar minimum (few sunspots, quiet) and solar maximum (many sunspots, frequent flares and CMEs). We are currently in Solar Cycle 25, which peaked around 2024–2025 and is one of the strongest cycles in decades — explaining the spectacular aurora seen across the UK in 2024.
Even as cycle 25 passes its peak, strong CME events can still occur for several years on the descending phase. UK aurora watchers remain in a favourable window through 2026–2028.
Coronal Holes
A coronal hole is a region of the Sun's corona where the magnetic field opens outward into space, allowing solar wind to escape at high speed (500–800 km/s). Unlike CMEs, coronal hole streams arrive with ~27-day recurrence as the Sun rotates. They typically produce moderate, sustained activity (Hp30 4–6) rather than the sudden peaks of CMEs.
If there was aurora 27 days ago from a recurring coronal hole, watch for a repeat. Auroracast's forecast incorporates NOAA coronal hole stream predictions.