A geomagnetic storm has strengthened to G3 — "strong" on NOAA's five-level scale — and forecasters say tonight could bring the best northern lights show of the week to parts of the United States. The storm reached Kp 7 on Saturday afternoon, at 1:58 p.m. EDT (10:58 a.m. PDT), though North America was in daylight at the time, so Europe got the first show.
The Kp index runs from 0 to 9 and maps onto NOAA's G1 (minor) through G5 (extreme) storm scale. Saturday's first Kp 7 reading put the storm firmly in G3 territory. NOAA data put hemispheric power — a measure of the energy raining into the upper atmosphere — at 160.7 gigawatts, roughly five times the 33 GW measured that morning.
That surge means the coronal mass ejection is still sweeping past Earth and the magnetic field remains disturbed, leaving the door open for more auroral activity after dark.
The storm is the product of a busy week on the Sun. On Tuesday, October 6, an M1.4-class solar flare — mid-strength on the A-through-X flare scale — erupted above Active Region 4549, a cluster of sunspots near the center of the Sun's Earth-facing side. NASA's Solar Dynamics Observatory and the SOHO spacecraft watched two coronal mass ejections leave the Sun about four hours apart.
The second one mattered most: NOAA forecasters call it a "partial halo CME." In coronagraph images it appears as a partial ring around the instrument's field of view — the signature of a cloud expanding more or less directly toward Earth.
A third eruption on October 8 is expected to sideswipe Earth around October 10–11. The UK Met Office has warned of another moderate storm this weekend, with a slim chance of further G3-strength intervals.
The stronger the storm, the farther south the aurora can be seen. On NOAA's scale, a G2 storm can push the lights down to New York and Idaho; a G3 storm extends the range to Illinois and Oregon. If the storm peaks, some forecasts say the aurora could dip as far south as Kansas, Missouri, Kentucky, West Virginia, Virginia and Maryland.
That remains conditional. Whether the lights actually appear depends on the speed, density and magnetic direction of the solar wind — things forecasters can only confirm once the material arrives. NOAA advises checking its 30-minute aurora forecast before heading out.
Timing is on skywatchers' side in one respect: October 10 is a new moon. Moonlight is the aurora-hunter's enemy — a full moon can wash out faint displays. This weekend there is only a thin waning crescent that rises just before sunrise, leaving the prime viewing window, typically 10 p.m. to 2 a.m. local time, completely dark.
The other two requirements: clear skies and distance from city lights. A dark spot with a wide-open view of the northern horizon gives the best odds.
If the aurora is faint, the naked eye may see only a grayish, fog-like glow on the northern horizon. Don't give up: a smartphone's night mode can often pull out green or pink hues the eye misses. Point it north and let a long exposure do the work.
Auroras happen when charged particles from the Sun collide with oxygen and nitrogen in Earth's atmosphere, producing curtains of green, red, purple or blue light. The Sun is currently in solar maximum — the most active phase of its 11-year cycle — which is why auroras have been appearing night after night this week.
The show may not end tonight: forecasts point to more geomagnetic activity on Sunday, October 11. If clouds or timing spoil Saturday, the northern sky is worth another look tomorrow.