This week has real aurora potential. Tonight through Wednesday night, the NOAA space weather forecast calls for minor to moderate geomagnetic storms during overnight hours. Sometimes solar storms called flares are responsible for aurora; they blast bits and pieces of the sun (electrons and protons) toward Earth at extremely high speed. During the biggest solar flare ever recorded on April 2, 2001, material shot out from the sun at 4.3 million miles an hour (7 million km/hr). Incredibly, it took less than a day for the first squirming electrons to cross the 93 million mile gap between sun and Earth.
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A large opening in the sun's magnetic field called a coronal hole is the source of this week's excitement. Normally, the sun's magnetic field loops back down to its "surface." Particles still stream from the sun into space but at slower speeds and in smaller number. We call it the solar wind. But a coronal hole is a like an enormous open window, a place where the sun's magnetic field opens up and allows the particles to fly away into space at much higher speeds and in greater number.
Holes can pack a punch! Normal solar wind speeds are around 900,000 mph (400 km/sec), but wind leaving through a coronal hole can race away at up to 1.8 million mph (800 km/sec).
This hole is unusually large, and the gaseous stream headed our way is pointing in the right magnetic direction (south pole first) to link up with Earth's north-pointing magnetic field. Once connected, the material has a path into our planet's upper atmosphere, where it can collide with air molecules to fire up an aurora display.
Assuming the forecast holds, we should have a minor G1 storm this evening (March 26) from about 10 p.m. to 1 a.m Central Daylight Time and a G2 or moderate storm during that same time period Monday evening. Moderate G2 storms are again predicted for Tuesday night from nightfall through about 1 a.m. CDT.
So for tonight, the aurora might be seen across the tops of the northern tier of states, southern Canada and northern Europe. If a moderate storm occurs, the aurora will be seen further south.
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By the way, this coronal hole has been around before. The sun rotates once about every 27 days. Coronal holes, especially big ones, can last for several rotations. Back at the beginning of March, winds from this very same gap spawned powerful auroras at arctic latitudes. Let's see if it can go one better and make an appearance in realms where people outnumber moose.
** To learn more about the northern lights and how to predict when and where they might occur, pick up a copy of my book Night Sky with the Naked Eye at Amazon or Barnes & Noble . I devote a chapter to the topic and discuss how to get up-to-the-minute aurora forecasts.
