Plasma Physics at West Virginia University
12 Jun

Predicting Aurora Sightings

Amy | June 12th, 2013

By Dr. Amy Keesee, WVU Plasma Physicist.

On April 13, 2013, social media was abuzz with predictions that there would be good viewing conditions for aurora in the northern United States. Many people went outside that night to look up, but unfortunately, there wasn’t much to see. While many may have been disappointed, this was an excellent opportunity for the general public to learn about the Sun, geomagnetic storms, and space weather. In particular, while such activity from the Sun and the resulting geomagnetic storms can have (potentially negative) effects on humans and our technology (see previous posts by Paul and myself), it is important to note that our ability to predict such space weather is in its infancy.

The Sun is monitored by numerous instruments on satellites such as the Solar Dynamics Observatory (SDO), the Solar and Heliospheric Observatory (SOHO), and the dual-satellite mission STEREO. These instruments observed a coronal mass ejection launched from the Sun on April 11th. These images can be seen using the NASA Integrated Space Weather Analysis System.

Using calculations of the speed and direction of the coronal mass ejection (CME) from these images, scientists predict whether the CME will hit Earth, including whether it will be a direct or glancing blow, as well as an approximate time. Scientists also use models to predict these characteristics. One such model is the WSA-ENLIL-CONE model. The model of the Earth-directed CME that launched from the Sun on March 15, 2013 can be seen here. In this image, the Sun is the white circle and Earth is the yellow circle.

AccuWeather created a map of auroral viewing conditions for April 13th.

What may have caused confusion with this map is that it was not a prediction of the aurora itself, but an assessment of the cloud cover in the region of possible auroral viewing zones. However, the cutoff used to indicate the “not visible” region is based on a storm with a Kp index of 9. It would require a very intense geomagnetic storm to reach such levels. The Kp index is a scale, similar to the Richter scale for earthquakes, that is based on measurements of the changes in Earth’s magnetic field caused by currents driven in space by the storm. Stronger storms drive stronger currents, and thus larger changes in magnetic field measurements. Aurora are often seen at high latitudes, even during weak storms. As the storms grow stronger, the aurora can be seen at lower latitudes.

It turned out that the CME that hit Earth on April 13th drove a relatively weak event, with a Kp index of 4. One reason for the weak storm was that the CME contained a primarily northward pointing magnetic field. One of the drivers of large storms is magnetic reconnection of the magnetic field in the CME with Earth’s magnetic field. (Learn more about reconnection in Luke’s post.) The magnetic fields must be pointing in opposite directions to occur. Earth’s magnetic field is like a bar magnet, with the magnetic field lines coming out of the South geographic pole and into the North geomagnetic pole, so that they point northward. For reconnection to occur, the magnetic field in the CME must, therefore, point southward. We currently do not have the ability to predict which direction the magnetic field in the CME is pointing. There are satellites with instruments that can measure the magnetic field in the CME, but their location gives about a one-hour notice prior to the CME hitting Earth’s magnetic field.

Plasma physicists at WVU are working to understand many elements of the Sun-Earth system, including coronal mass ejections, magnetic reconnection, and the dynamics of geomagnetic storms. Hopefully in the future, scientists will be able to predict space weather with similar accuracy to the regular weather forecasts.

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