THE SOUTHWORTH PLANETARIUM
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Founded January 1970
Julian Date: 2459193.16
2020-2021: LVI
THE DAILY ASTRONOMER
Wednesday, December 9, 2020
Aurora Alert!
*We're taking yet another day-long break from the Remote Planetarium to
bring you a four point bulletin about the aurora borealis, known to English
speakers as the "Northern Lights."*
While you're seated (or standing) at your desk at this very moment,
super charged material recently expelled by the Sun is roaring through
interplanetary space en route to Earth and other points beyond. While we
complacent Earthlings can be confident that no harm will befall us
-since this is 2020, our goblet is overflowing with harm already, thank
you- we might well experience a particularly dramatic aurora show starting,
perhaps, tonight.
Let's explain what's happening:
Charged particles emitted from coronal holes along the sun's outer layer
have been expelled through coronal holes. The image below, courtesy of the
Solar Dynamics Observatory, shows these holes as dark splotches toward the
image's upper right. Material propelled out in this *coronal mass
ejection* travels
through space at 2 -3 million miles per hour. At this rate, the particles
reach Earth about 3 days after expulsion. Because the Sun is 8.3
light minutes from our planet, we can observe this ejection well before the
associated material reaches us.
[image: coronalhole_sdo_blank.jpg]
At the upper right, one can see dark regions called coronal holes from
which charged particles were expelled in an event called a *coronal mass
ejection. *
This ejection occurred earlier this week and soon the CME particles will
arrive, resulting in intense geomagnetic storm activity. We should
also experience auroral activity associated with these storms. Forecasters
from NOAA (National Oceanic and Atmospheric Administration) expect that the
most intense activity will occur on December 9 - 10th. On those nights,
the aurora borealis might be visible as far south as Illinois and Oregon.
Scientists measure geomagnetic storm intensity on the Kp scale which ranges
from 0 - 9. 0 indicates calm conditions. 9 indicates intense
geomagnetic storm. Any rating 5 or higher indicates storm-level activity.
The higher the Kp index, the greater the region of potential auroral
activity, as seen in the image below, courtesy of the NOAA Space Weather
Prediction Center.
[image: HHDMKGNSIREONBQNWL3OEF3A6A.png]
The Kp index value will equal 7 during the peak activity. Consequently,
aurora might be visible throughout the northern tier of the United States.
Auroral activity is limited to regions surrounding the magnetic
poles. According to the World Magnetic Model 2020, the northern
hemisphere's magnetic pole is located at 86.50°N and 164.04°E. When charged
solar particles reach Earth. the planet's magnetic field directs the
particles toward both poles. These charged particles "excite" atoms and
molecules within the upper atmosphere (90- 150 km above Earth's surface.)
Excitation occurs when the atoms absorb a specific amount of energy that
elevates them into higher energy levels. When an excited atom settles
back to its original state, it will emit a visible light photon with an
energy equal to the energy it originally absorbed. These emitted photons
comprise the aurora light we see.
Different gases produce different colors.
- Excited *oxygen* atoms generally produce a greenish-yellow or red
light,
- *Nitrogen* emits a bluish light.
[image: unnamed.jpg]
While aurora events are not as easy to predict as lunar phases, planet
positions or even galaxy collisions, one might well expect to see the
aurora in our sky tonight and tomorrow night in New England. Weather
permitting, of course...and the less said about that, the better.
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