ToolComputedCited
Which storm level typically reaches my geomagnetic latitude?
Answer
| Level | Kp | Aurora | Per cycle |
|---|---|---|---|
| G1Minor | Kp = 5 | "aurora is commonly visible at high latitudes (northern Michigan and Maine)" | 1,700900 days |
| G2Moderate | Kp = 6 | "aurora has been seen as low as New York and Idaho (typically 55° geomagnetic lat.)" | 600360 days |
| G3Strong | Kp = 7 | "aurora has been seen as low as Illinois and Oregon (typically 50° geomagnetic lat.)"Typical latitude reaches this location | 200130 days |
| G4Severe | Kp = 8, including a 9- | "aurora has been seen as low as Alabama and northern California (typically 45° geomagnetic lat.)"Typical latitude reaches this location | 10060 days |
| G5Extreme | Kp = 9 | "aurora has been seen as low as Florida and southern Texas (typically 40° geomagnetic lat.)"Typical latitude reaches this location | 44 days |
Worked example
Buffalo, New York sits at 42.89N, which the IGRF-14 dipole places at 52.0 degrees geomagnetic latitude. SWPC's G2 row gives a typical latitude of 55 degrees, north of Buffalo. The G3 row gives 50 degrees, south of it. So the lowest level whose typical latitude reaches Buffalo is G3, listed at Kp = 7.
SWPC's G2 row also names New York as a place aurora has been seen. The dipole match and the named states are separate statements, and Glow quotes both rather than merging them.
| Fairbanks | 65.7° | No storm (Kp 1) |
| Anchorage | 61.9° | No storm (Kp 3) |
| Seattle | 53.0° | G3 |
| Minneapolis | 53.5° | G3 |
| Duluth | 55.4° | G1 or less |
| Portland | 52.9° | G3 |
| Buffalo | 52.0° | G3 |
| Reykjavik | 68.8° | No storm (Kp 0) |
| Tromsø | 67.5° | No storm (Kp 0) |
| Yellowknife | 68.5° | No storm (Kp 0) |
What Kp is and is not
Kp is a planetary index of geomagnetic activity on a scale of 0 to 9. It describes the whole planet, not your sky. SWPC's viewing tips describe what each band means for the aurora:
- "For Kp in the range 0 to 2, the aurora will be far north, quite dim in intensity, and not very active."
- "For Kp in the range of 3 to 5, the aurora will move further from the poles, it will become brighter and there will be more auroral activity"
- "For Kp in the range 6 to 7, the aurora will move even further from the poles and will become quite bright and active."
- "For Kp in the range 8 to 9, the aurora will move even further towards the equator and it will become very bright and very active."
Glow never prints a planetary Kp as a local forecast. When you want to know about tonight, the OVATION cell for your coordinates is the local number.
How the G-scale maps to latitude
NOAA's storm scale runs from G1 (Kp 5) to G5 (Kp 9). For G2 through G5 it names states where aurora has been seen and a typical geomagnetic latitude: 55, 50, 45 and 40 degrees. G1 names northern Michigan and Maine without a latitude.
Glow computes your centered dipole latitude first. At 60 degrees or more you sit in SWPC's 60 to 75 degree band, where aurora can be seen on more than half of all nights, so no storm is needed. From 55 to 60 degrees, G1 (Kp 5) or less usually brings aurora into view. Below 55 Glow picks the lowest level whose typical latitude is at or below yours: at 53.0 degrees, 55 is too far north and 50 is the first row that reaches you, so the answer is G3.
Separately, SWPC's tips page puts the average oval edge at 66 degrees at Kp 0, moving 2 degrees per step. That rule answers a stricter question, when the oval sits overhead, so it gives a higher Kp than the viewing latitudes on the storm scale.
The frequency column is quoted as SWPC gives it: the number of storms at that level per solar cycle, and the number of days they span.
Why this is not a forecast
The match tells you what level of storm has typically been needed at your geomagnetic latitude. It says nothing about whether a storm is coming. For tonight, read your OVATION cell in the chance tool.
SWPC does not state which coordinate system its typical latitudes use. A centered dipole can differ from corrected geomagnetic coordinates by a few degrees, so treat a result near a boundary as either level.
Questions
- What Kp do I need to see the northern lights?
- Find your dipole latitude with the tool. At 60 degrees or more no storm is needed; from 55 to 60, Kp 5 or less usually does it. Below that, SWPC's scale lists typical latitudes of 50 degrees for Kp 7 (G3), 45 for Kp 8 (G4) and 40 for Kp 9 (G5).
- How often do G3 storms happen?
- NOAA's scale counts 200 G3 storms per 11-year cycle, spread over about 130 days.
- Is a Kp 5 night enough for Seattle?
- Kp 5 is G1, which SWPC describes as commonly visible at high latitudes such as northern Michigan and Maine, with no typical latitude. Seattle reads 53.0 degrees on the dipole, which the scale first reaches at G3.
- Why does Glow not show the current Kp?
- Kp is a planetary index, not a local forecast. Glow shows the OVATION cell for your coordinates instead, with its timestamps.
- What does typical mean here?
- It is SWPC's word. The scale says aurora has been seen as low as the listed states, typically at the listed geomagnetic latitude. It is not a guarantee for any storm.