Space weather forecasters and aurora chasers often talk about the "planetary K index" (Kp). It's like a Richter Scale for geomagnetic storms, ranking global geomagnetic activity with numbers ranging from 0 (low) to 9 (extreme). When Kp shoots above 5, a storm is in progress and auroras ring the Arctic Circle. When Kp is near 0, you wouldn't expect to see anything. Yet on Nov. 24th, Kp was 0 when aurora tour guide Oliver Wright witnessed a magnificent display over Abisko, Sweden:
"The sky went completely crazy with vivid greens and pinks," says Wright. "Totally not what we expecting to see with the Kp score as low as it was. "
What happened? Wright himself solved the mystery: "We have just been looking at magnetometer data from the nearby town of Kiruna. Locally, there was a magnetic disturbance corresponding to a K index of 7--a strong geomagnetic storm. It just goes to show how geomagnetic activity can be much more localized than expected."
Indeed, this episode and others like it highlight a shortcoming of space weather forecasting: For decades, analysts have used a global index (Kp) to express forecasts of regional activity. It would be like a terrestrial weather forecaster announcing "there is a chance of rain today around the Arctic Circle." Okay. But where around the Arctic Circle?
Regional forecasting of geomagnetic effects is a relatively new frontier in space weather research. NOAA's Space Weather Prediction Center and its university partners are starting to make strides in this area. For instance, experimental Geospace Ground Magnetic Perturbation Maps show realtime estimates of geomagnetic activity in a five-by-five degree polar grid, providing more local information than ever before. Try watching the movies the next time a "planetary" geomagnetic storm is in progress; the regional information may surprise you.