What causes the northern lights?

SWPC's aurora page walks through the chain. Electrons gain energy in the downwind tail of the magnetosphere, the night side, and at lower altitudes along auroral field lines. They then follow Earth's magnetic field down toward the polar regions. There they collide with oxygen and nitrogen atoms and molecules, pass energy to them, and lift them to higher energy states.

When those atoms and molecules drop back to lower states, they release the energy as light. SWPC compares it to a neon sign. Protons also cause aurora, but SWPC notes it is faint and diffuse and usually not easy to see with the eye.

Because the electrons follow field lines, the aurora lands where those lines meet the atmosphere at high magnetic latitude. SWPC says the result is two ovals, approximately centered on the magnetic poles: the aurora borealis in the north and the aurora australis in the south.

SWPC names the best place to watch as an oval-shaped region between about 60 and 75 degrees, where aurora can be seen on more than half of the nights of a year. The page does not say whether it means geographic or geomagnetic latitude, so the table shows both for the 10 Glow places.

The five towns in Alaska, Canada, Iceland and Norway fall inside the zone on both measures. The five lower 48 towns fall outside on both, which is why they depend on storms that push the oval south.

Distance from the ovals is measured in geomagnetic latitude, not map latitude. Minneapolis sits at 45.0 north on the map but 53.5 on the dipole. The geomagnetic latitude guide explains the shift.

The ovals are always there. They grow brighter and wider when more energy flows from the solar wind into the magnetosphere. SWPC's storm page says the conditions that do this best are hours of fast solar wind with a magnetic field pointing south, opposite Earth's own field, at the front of the magnetosphere.

The largest storms come from coronal mass ejections, clouds of solar plasma carrying their own magnetic field. SWPC says they usually take several days to reach Earth, and as little as 18 hours for some of the most intense storms. High-speed solar wind streams cause storms too, usually weaker but sometimes longer.

When activity rises, SWPC says the aurora extends toward the equator. During large events it can be seen as far south as the US, Europe and Asia. Kp and the NOAA G-scale measure that growth: see what the Kp index means and the storm levels hub.

SWPC describes a typical night. Evening aurora often forms tall rays like the folds of a curtain, which can line up into arcs that cross the sky from horizon to horizon. Near midnight the arcs can twist and sway, then expand across the whole sky, fast and bright, at the peak of a substorm. In the early morning the forms turn diffuse and cloud-like, blinking on and off for hours until sunrise.

That rhythm is why SWPC recommends the hours around midnight. The best time of night guide turns it into clock times for 10 towns, and the oval map shows where the ovals sit right now.