Three days, not fourteen

Where the oval is

Glacier National Park, St. Mary, MT

Badlands National Park Wind Cave National Park Devils Tower National Monument Glacier National Park

The oval stops 5 degrees north

Peak forecast Kp 3, Fri, Sep 18

Right now, an hour upstream

The field is pointing south

The arriving field points south at -1.4 nT. Earth's field points north out there, so the two reconnect and energy gets in. This is the condition the aurora actually needs.

Bz, the north south field	-1.4 nT
Held southward for	8 minutes
Solar wind speed	446 km/s
Reaches Earth in	about 56 minutes
Clock angle	246 degrees, near due south
Coupling	5,573

Measured by DSCOVR at the L1 point, about 930,000 miles sunward, and last updated 4:08am UTC. ⚠️ This says the driver is switched on, not that anything is visible from the ground. Coupling is the Newell function in its own units, useful for comparing one hour with another and nothing else.

The next three days

When	Kp	Where the oval sits
Thu, Sep 17 9:00pm	2.0	The oval stops 7 degrees north of here.
Fri, Sep 18 12:00am	2.0	The oval stops 7 degrees north of here.
Fri, Sep 18 3:00am	2.7	The oval stops 5.6 degrees north of here.
Fri, Sep 18 6:00am	2.7	The oval stops 5.6 degrees north of here.
Fri, Sep 18 9:00am	2.7	The oval stops 5.6 degrees north of here.
Fri, Sep 18 12:00pm	2.7	The oval stops 5.6 degrees north of here.
Fri, Sep 18 3:00pm	3.0	The oval stops 5 degrees north of here.
Fri, Sep 18 6:00pm	2.7	The oval stops 5.6 degrees north of here.
Fri, Sep 18 9:00pm	2.0	The oval stops 7 degrees north of here.
Sat, Sep 19 12:00am	2.3	The oval stops 6.3 degrees north of here.
Sat, Sep 19 3:00am	2.3	The oval stops 6.3 degrees north of here.
Sat, Sep 19 6:00am	2.3	The oval stops 6.3 degrees north of here.
Sat, Sep 19 9:00am	2.3	The oval stops 6.3 degrees north of here.
Sat, Sep 19 12:00pm	2.3	The oval stops 6.3 degrees north of here.
Sat, Sep 19 3:00pm	2.7	The oval stops 5.6 degrees north of here.
Sat, Sep 19 6:00pm	2.3	The oval stops 6.3 degrees north of here.
Sat, Sep 19 9:00pm	2.0	The oval stops 7 degrees north of here.
Sun, Sep 20 12:00am	2.0	The oval stops 7 degrees north of here.
Sun, Sep 20 3:00am	2.0	The oval stops 7 degrees north of here.
Sun, Sep 20 6:00am	2.0	The oval stops 7 degrees north of here.
Sun, Sep 20 9:00am	2.0	The oval stops 7 degrees north of here.
Sun, Sep 20 12:00pm	2.0	The oval stops 7 degrees north of here.
Sun, Sep 20 3:00pm	2.3	The oval stops 6.3 degrees north of here.
Sun, Sep 20 6:00pm	1.7	The oval stops 7.7 degrees north of here.

What this page will not tell you

It will not tell you that you are going to see the aurora. Whether a faint glow low in the north is visible to a particular pair of eyes on a particular night depends on how bright the display is, how much air it is shining through, whether the moon is up and what is on the horizon behind it. The published thresholds for that disagree with each other, so this page does not pick one.

What it can say exactly is where the oval's southern edge sits and where you sit, and that is what the numbers above are.

Why this site is further north than it looks

The aurora follows the magnetic pole, not the geographic one, and the magnetic pole sits over northern Canada. That pushes American latitudes up. Glacier National Park is at 48.7 degrees on a map and 55.4 degrees magnetically, a gap of about 7 degrees. It is the reason the northern plains see aurora at all, and the reason the same latitude in France mostly does not.

The oval reaches overhead here at about Kp 5.4. Below that it is a northern horizon object, if it is anything.

The northern horizon here

The land to the north rises 7.7 degrees at worst and 3.5 on average, and due north it is 1.5 degrees. That is enough to matter. The aurora at this latitude sits low in the north, so a ridge that would be beneath notice for anything overhead can take it away entirely.

⚠️ Computed from elevation data, not from a visit. Same arithmetic as the terrain figures on the forecast pages, and the same limits: the nearest sample sits 820 feet out, so nothing closer than that is in it.

Where the numbers come from

Kp is NOAA's planetary geomagnetic index, published by the Space Weather Prediction Center, which is a National Weather Service office. They run the three day forecast, the thirty minute aurora nowcast and an alert service that sends a message when a storm is on the way.

NOAA SWPC, 30 minute aurora forecast, for tonight rather than this week
NOAA SWPC, planetary K index, the number in the table above
NOAA SWPC alerts, watches and warnings, which will email you when a storm is coming

The oval's edge is placed using the Feldstein and Starkov relation, which puts it at 66.5 magnetic degrees when Kp is 0 and moves it about two degrees south per Kp step.