How the Auroral Activity Patterns are Created

The Total Energy Detector (TED) is an instrument in the Space Environment Monitor (SEM) that has been routinely flown on the NOAA/POES (formerly TIROS) series of polar orbiting meteorological satellites since TIROS-N was launched in November of 1978. The instruments in the SEM, now the second-generation SEM-2, were significantly upgraded beginning with NOAA-15. The upgraded TED, which is designed to monitor the power flux carried into the Earths's atmosphere by precipitating auroral charged particles, now covers particle energies from 50 to 20,000 electron volts (eV) as compared to the earlier TED that extended in energy to only 300 eV.

These measurements are made continually as the satellite passes over the polar aurora regions twice each orbit. Since 1978, observations from almost 300,000 transits over the auroral regions have been gathered under a variety of auroral activity conditions ranging from very quiet to extremely active. Power flux observations accumulated during a single transit over the polar region (which requires about 25 minutes as the satellite moves along its orbit) are used to estimate the total power input by auroral particles to a single polar region. This estimate, which is corrected to take into account how the satellite passes over a statistical auroral oval, is a measure of the level of auroral activity, much as Kp or Ap are measures of magnetic activity. A particle power input of less than 10 gigawatts (10,000,000,000 watts) to a single polar region, either in the North or the South, represents a very low level of auroral activity. A power input of more than 100 gigawatts represents a very high level of Auroral activity. Estimated power inputs as high as 500 gigawatts have been recorded into a single auroral region.

In order to create statistical patterns of auroral power flux, estimated power inputs were computed using observations obtained from more than 100,000 passes over both the northern and southern polar regions. These passes encompassed a wide range of local times and a variety of auroral activity conditions. These polar passes were then sorted into ten auroral activity levels, depending upon the power input estimate. The upper bounds of the first nine levels were defined by a geometric progression of power levels beginning at 2.5 gigawatts up to 96 gigawatts; the tenth level contained estimated power inputs greater than 96 gigawatts. Power flux observations--averaged over one degree of magnetic latitude--from all polar passes with a given activity level were then merged to produce a statistical pattern (a map) of auroral particle power deposition for an entire polar region. Because data were gathered from several satellites, in differing orbits, data were available for almost all local times at latitudes above 45 degrees geomagnetic.

In this fashion ten statistical patterns of auroral particle power input were created, one for each level of auroral activity. The statistical patterns show particle power flux to the atmosphere as a function of magnetic latitude and magnetic local time; coordinates that best order auroral phenomena.

Estimated hemispheric power estimates are computed for each pass over the polar regions as data arrive at the Space Weather Prediction Center from the satellite tracking stations. Once the power input is estimated, the corresponding statistical pattern of auroral power input is selected. Using the Universal Time of the satellite pass, the magnetic latitude and magnetic local time coordinates of the statistical pattern are converted to geographic coordinates; the pattern is then superimposed upon a geographic polar map of either the northern or southern hemisphere.

Normalization factor (n)
A normalization factor of less than 2.0 indicates a reasonable level of confidence in the estimate of power. The more the value of n exceeds 2.0, the less confidence should be placed in the estimate of hemispheric power and the activity level.

The process to estimate the hemispheric power, and the level of auroral activity, involves using this normalization factor which takes into account how effective the satellite was in sampling the aurora during its transit over the polar region. A large (> 2.0) normalization factor indicates that the transit through the aurora was not very effective and the resulting estimate of auroral activity has a lower confidence. In order for users to assess the confidence in a given estimate of auroral power, we now report the numerical value of the normalization factor in our web pages.