Just in time before the moon thickens and brightens the sky, there’s a chance for a G1 (minor) geomagnetic storm tonight. The last solar blast — a G2 or moderate storm — occurred on August 2-3 and sparked a very attractive display in the lower half of the northern sky seen from the northern U.S. and Canada. Look toward the northern horizon at nightfall tonight for any potential activity. The most common early evening auroral form is the arc which looks like a “rainbow” of pale green light about one fist (sometimes less!) high in the northern sky.
NOAA space weather experts expect the modest storm to happen between about 10 p.m. and 1 a.m. CDT (3:00-6:00 UT). Its cause is another in a series of “holes” in the sun’s outer atmosphere or corona called a coronal hole. Invisible in ordinary light, they stand out as dark voids in the glowing corona in photos taken in ultraviolet light. Unrestrained by solar magnetic fields, material streams directly from the holes into outer space and occasionally past the Earth. When its subatomic contents make a connection with our own magnetic field, the contents can stream down into the planet’s upper atmosphere, bump into air molecules and excite them to emit light. Voila — northern lights! Or southern lights a.k.a. the aurora australis if you’re watching from New Zealand.
IRIS Spots Plasma Rain on Sun’s Surface
While we’re on the topic of solar disturbances, NASA’s Interface Region Imaging Spectrograph, or IRIS, captured a mid-level solar flare directly along the sun’s limb. Often associated with sunspots, solar flares are powerful explosions of radiation from the sun’s surface with energies as high as a billion hydrogen bombs.
During flares, huge amounts of pent-up magnetic energy is released, heating the sun’s atmosphere and releasing energized particles out into space where they can potentially impact Earth and lead to auroras. Astronomers study flares to see how solar material and energy move throughout the sun’s lower atmosphere, so we can better understand how flares and other short-lived events affect the surface and corona.
In the video, we see solar material cascading down to the sun’s surface in great loops called post-flare loops or coronal rain immediately following the flare. This material is plasma, a gas in which positively and negatively charged particles have separated, forming a super-hot mix that follows paths guided by complex magnetic forces in the sun’s atmosphere. As the plasma falls down, it rapidly cools – from millions down to around 50,000°F. The corona is much hotter than the sun’s surface, a mystery astronomers are still working to puzzle out.
The spray of bright pixels that appears at the end of the video isn’t caused by the solar flare, but occurs when high-energy particles bombard IRIS’s camera sensor.