A trifecta is within our grasp. The same coronal hole that’s been around since August is facing Earth again and blasting us with a wind of high-speed particles. This persistent feature sparked both the late August and September auroras. Why the repeat? The sun rotates on its axis once about every 4 weeks, making this the third time in a row we’ve come face to face with the hole.
Space weather forecasters predict a minor geomagnetic storm with Kp index reaching 5 starting this afternoon (Oct. 24) and continuing through the night. Peak “winds” are expected to reach 375 miles per second (600 km/sec) or 1,350,000 miles an hour. While the individual particles number in the trillions, it’s thin stuff. Nothing like the pressure one feels when pushed around by powerful Earth winds. Yet the particles do have energy, and once ensnared in Earth’s magnetic field they can be accelerated into the upper atmosphere at 10 percent the speed of light.
Even the tiniest of things pack a punch at those speeds, the reason that oxygen and nitrogen atoms respond by glowing pink and green when solar electrons crash into them. The process is straightforward. A crazy-fast electron strikes an atom. The energy it imparts raises an electron in that atom to a higher orbit around the nucleus. But only temporarily. The electron wants to return to its former orbit like a person who gets up to answer the phone and then returns to their recliner.
When it does fall back, the energy gained in the collision is released as light, typically green or red light. Zillions of particles doing this simultaneously are responsible for the aurora’s light and colors.
Skywatchers in the northern states and should keep an eye out tonight for a subtle glowing arc and perhaps a few dancing rays low in the northern sky. You can start watching at nightfall — around 8 p.m. local time — but the aurora is usually best later, typically after 10 o’clock local time. The forecast calls for a one-night only show.
Aurora or not anyone with fair weather will be able to enjoy a splendidly thin lunar crescent early Saturday morning (Oct. 26) and welcome Mars back to the morning sky. Depending on when you look Mars may or may not be visible with the naked eye since it’s still relatively faint at magnitude 1.8. Bring binoculars to spot it and use them to better enjoy the eerie glow of the earth-lit moon, too.
Late October brings some of the latest sunrises of the year, making it relatively easy to observing dawn celestial antics without losing sleep. Take advantage before we lose Daylight Saving Time on Nov. 3. If you’re really into crescents we’ll see a record-breaking knife-edged one on Sunday morning (Oct. 27) about a half-hour before sunrise very low in the eastern sky. For details on the opportunity to see this rare sight, check out my how-to guide at Sky & Telescope.