Aurora Alert July 4-5 – Sun Sends Sparks Earth’s Way

4th of July fireworks at Duluth, Minnesota's Bayfront Park. Credit: Bob King
4th of July fireworks at Duluth, Minnesota’s Bayfront Park. Credit: Bob King

Expecting a little fireworks this 4th of July? When you’re done with tonight’s Independence blast, be sure to look north on the way home. NOAA space weather forecasters are predicting a G1 geomagnetic storm overnight. This one looks like it will start late – around midnight – and last through tomorrow morning. Dribs and drabs may linger into tomorrow evening July 5. G1 or minor storms generally mean a modest display of northern lights across the northern U.S. states and southern Canada.

Two photos of the Sun in far ultraviolet light show how little changed a coronal hole seen in early June appears next to its photo taken 4 weeks later on July 2. Credit: NASA/SDO
The Sun photographed in far UV light by NASA’s Solar Dynamics Observatory showing the same coronal hole in the Sun’s southern hemisphere four weeks apart.  At left, an image taken on July 2; at right, the same hole on June 5 one solar rotation earlier. Credit: NASA/SDO

High-speed streams of particles flowing from a coronal hole will brush by Earth tonight and perhaps set off auroras. Coronal holes appear as dark gaps in the glowing canopy of gases in the Sun’s atmosphere in photos taken in ultraviolet light.  Like a firework fountain that sprays color sparks, a hole marks a place where the solar wind of electrons and protons flows freely away from our star at high speeds, unconstrained by magnetic fields.

Normal solar winds blow around 250 miles per second (400 km/sec), but holes release gusts at twice that speed and can reach Earth in 2-4 days.

Some coronal holes can linger for months and reappear rotation after rotation. They’re called recurrent holes. This is one of them. Since the Sun spins on its axis approximately once every 27 days, aurora predictions often recur at 27-day intervals once a significant coronal hole sets up shop.

Solar flares and other storm activity can occur anytime, but coronal holes’ longevity can make a space weather forecaster’s job at least a little easier. What do you bet we’ll be looking at a similar forecast around July 31?