The prediction of auroral oval location and activity level is made using ground magnetometer data from the line of CANOPUS instruments stretching along a common meridian (approximately) from Taloyoak in the north to Pinawa in the south. The magnetometers are triaxial, measuring the north -south component of the magnetic field (X), the east-west component (Y) and the vertical component (Z). The plots in the top left corner of the web page show latitude profiles of those three components at each time a prediction is made. If you think of the auroral region as carrying a large electric current, that current either flows eastward (in the evening hours) or westward in the morning hours. An eastward electrojet makes a positive X-component perturbation and the westward electrojet makes a negative X-component perturbation. The Z-component tells us about the location of the edges of these east-west currents, and we use the peaks in the Z-component to identify the poleward and equatorward edges of the electrojet every 5 minutes. On the bottom panel , you can see the time evolution of the electrojet width and position since the start of the UT day, at the longitude of the Churchill line. While the Churchill line gives a position of the borders at only one local time, we use a statistical data base of prior information to construct the oval which requires you know the edges of the current system at all local times.
Risk is computed from an algorithm that involves how far equatorward the poleward edge of the oval has been pushed and how long it has been there. The dotted white line on the bottom panel shows the position of the poleward edge of the oval under average activity conditions. We are only trying to predict big events, so we are interested in how far below the average position of the poleward edge of the oval the instantaneous position has moved. If the instantaneous position of the poleward edge of the electrojet is at or poleward of the average position, we say there is no risk of a really big event.
But....what do we mean by "event"? For us an "event" is a substorm, the largest of the auroral disturbances which usually occurs in the hours around midnight. The energy for these disturbances is stored in the earth's magnetic tail and how much energy is stored can be estimated from the size of the polar cap - the "black" area poleward of the auroral oval. The bigger the polar cap...the more energy is stored which can be released suddenly to make a big disturbance. Our Risk algorithm is an estimator of the size of the polar cap and how long it has been expanded also has some impact on how big the disturbance will be when it comes.
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