You can often count on coronal holes to provide auroras even when solar activity is low. Today for instance there’s only one small sunspot group, and it’s very “quiet.” But invisible to the eye, a coronal hole has been blasting us like a garden hose with a stream of subatomic particles that raises the possibility for auroras tonight. A coronal hole is an opening in the sun’s magnetic field that allows the strong gusts of charged particles — electrons and protons — to stream freely from the sun at speeds around a million miles an hour (500 km/sec).
Earth is surrounded by a magnetic field the points north, the same that directs as the compass needle. The material leaving the sun also has a magnetic field that can point either north or south. If the field is north, it aligns with Earth’s field like two north poles of a magnet that repel each other. The burst of “wind” slides safely by, leaving the Earth unaffected. But if the solar field points south, it can connect with Earth’s field like the north and south poles of two magnets snapping together.
During the reconnection, particles from the sun swizzle down Earth’s field lines at high speed straight down into the atmosphere over the polar regions to spark auroras. That’s what may happen tonight. NOAA’s space weather forecast calls for minor G1 storming this evening through about 1 a.m. Central Time. With the waning moon rising around 11:30 p.m., we’ll have an hour or so of dark sky before any lunar interference.
A minor storm means it’s likely to be visible from the northern regions of border states such as New York, Michigan, Wisconsin, Minnesota, North Dakota and so on. Keep watch on the northern sky: most auroras start with a low arc of pale green light about a fist above the horizon. If the arc brightens, thickens or doubles, that’s a good sign there’s more in the offing. Stick around or check every half-hour to see how it’s coming. You can also periodically check the extent of the aurora by clicking on the Aurora 30-minute forecast. This will show how far south the auroral oval reaches.
I’ll be in northern Minnesota tonight with both eyes open!