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K-Index (Kp 0 – Kp 9), Disturbance levels (nT) & Geomagnetic Storms explained
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Kp-index (Kp 0 – Kp 9)
Kp-index (Kp 0 – Kp 9)
- The Kp-index (Kp 0 – Kp 9) is a scale used to characterise the magnitude of geomagnetic disturbances. A geomagnetic storm starts at Kp5 after which the G-scale (G1 – G5) is also used.
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| Kp | Meaning | Aurora reach |
| Kp 0 – Kp 2 | Quiet | Arctic only |
| Kp 3 | Unsettled/Active | High Arctic only |
| Kp 4 | Active | Far north of Scotland (rare) |
| Kp 5 | Minor storm | Scotland |
| Kp 6 | Moderate storm | Northern England |
| Kp 7 | Strong storm | Southern UK possible |
| Kp 8 – Kp 9 | Severe/extreme | Visible across all UK |
| . |
Magnetometers (Kp-index)
The colour-coded system has four levels magnetometers to automatically detect
when it is likely that aurora can be seen from the UK.
| O Green alert: no significant activity (small influx of particles due to some reconnections mostly at the magnetotail.) O | Geomagnetic Latitudes Kp 0 – quiet (weak & slow) Kp 1 – quiet (weak & slow) Kp 2 – Unsettled (moderate) Kp 3 – Unsettled (substorms possible) | O Aurora is unlikely to be visible by eye or camera from anywhere in the UK. O |
| O Yellow alert: minor geomagnetic activity (Coronal hole sending fast winds after days / enhanced solar wind.) | Geomagnetic Latitudes Kp 4 – active (multiple substorms) (High latitudes – active auroral display) (Low latitudes – weak aurora display) | O It is likely that aurora will be visible by eye and camera from anywhere in the UK. O |
| O Amber alert: possible aurora (Coronal hole sending fast winds or Coronal Mass Ejections (CME) / enhanced solar wind.) O | Geomagnetic Latitudes Kp 5 (G1) – minor storm (High latitudes – more active auroral) (Low latitudes – auroral display likely) Kp 6 (G2) – minor storm (High latitudes – strong auroral display) (Low latitudes – auroral display very likely) | O Aurora is likely to be visible by eye from Scotland, northern England and Northern Ireland; possibly visible from elsewhere in the UK. Photographs of aurora are likely from anywhere in the UK. O |
| O Red alert: aurora likely (Large CME caused by solar storms or flares / very enhanced solar wind with strong shock wave.) O | Geomagnetic Latitudes Kp 7 (G3) – strong storm Kp 8 (G4) – severe storm Kp 9 (G5) – extreme storm (High latitudes – exteremly strong auroral) (Low latitudes – strong auroral exteremly) | O It is likely that aurora will be visible by eye and camera from anywhere in the UK. O |
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Disturbance levels (nT)
AuroraWatch UK Disturbance levels (nT)
- AuroraWatch UK uses four alert levels based on geomagnetic disturbance measured in nT (nanotesla).
| Level 1 – Green | Less than 50 nT (Kp0 – Kp3) | No chance |
| Level 2 – Yellow | Between 50 – 100 nT (Kp4) | Northern Scotland only |
| Level 3 – Amber | Between 100 – 200 nT (Kp5) | Northern England possible / Southern UK almost never |
| Level 4 – Red | Between 200 – 300 nT (Kp6) | That’s the level where: – the auroral oval is pushed far enough south. – the glow can rise above the northern light-pollution dome. – the brightness is strong enough for the naked-eye. |
| Level 4 – Red | Between 350-500+ nT (Kp6 – Kp7) | On very rare, extreme storms: Southern England can see – bright arcs – Rays – Colour (pink/green) – Aurora reaching overhead. |
| Level 4 – Red | over 500+ nT (Kp7 – Kp9) | Major storm, visible across all of the UK. |
Aurora visible for southern England / London
Typical disturbance (nT) needed for naked‑eye aurora in southern England
- For southern England places like Milton Keynes, Oxford, London, Bristol, Reading, the aurora usually becomes visible only when the geomagnetic disturbance reaches roughly:
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| nT (nanotesla) | Aurora visible for southern England |
| Less than 150 nT | Almost never visible |
| 150 – 200 nT | Only faint photography aurora, and only from very dark northern horizons. |
| 200 – 300 nT | Real naked‑eye potential |
| 300 – 400 nT | Strong naked‑eye aurora likely |
| 400 – 500 nT | Likely visible across the whole of the UK |
| 500+ nT | Major storm, visible across all of the UK |
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Disturbance Levels Needed for London (nT)
- With high light pollution and London’s low latitude, the auroral oval must be pushed far south before anything becomes visible:
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| nT (nanotesla) | Aurora visible for London |
| 300 – 350 nT | A good chance is likely for London. Arcs and pillars may be visible, especially from dark sites. (Epping Forest, North Downs, Chilterns). |
| 350 – 500+ nT | Strong, obvious aurora visible across all of southern England. Colours and structure likely. |
| 500 – 700+ nT | Major storm. Aurora can reach overhead even in London. |
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Five categories measure Geomagnetic Storms (G1 – G5)
Five categories measure Geomagnetic Storms (G1 – G5)
- While the storms create a beautiful aurora, they also can disrupt navigation systems such as the Global Navigation Satellite System (GNSS) and create harmful geomagnetic induced currents (GICs) in the power grid and pipelines. An increase in the geomagnetic disturbance index called Kp is observed.
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| Geomagnetic storm: G1 (minor) | G1 storms may trigger weak power grid fluctuations and have some minor impacts on satellite operations. |
| O Geomagnetic storm: G2 (moderate) | To suggest high-latitude power systems may experience voltage alarms while long-duration G2 storms may cause transformer damage. HF radio propagation may also fade at higher latitudes. |
| O Geomagnetic storm: G3 (strong) | Once storm levels reach G3, intermittent satellite navigation and low-frequency radio navigation problems may occur, and HF radio may be intermittent. Voltage corrections may be needed on power systems, and false alarms may be triggered on some protection devices. |
| O Geomagnetic storm: G4 (severe) | Power system administrators may be feeling like their hands are getting full as there could be widespread voltage control issues. Satellite and space operations may see surface charging and tracking issues. |
| O Geomagnetic storm: G5 (extreme) | Luckily, storm levels of this magnitude are relatively rare. Still, when they occur, power systems may see widespread voltage control problems and some grid systems may experience complete collapse or blackouts, and transformers may suffer damage. |
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