Explanation | July 2000 Proton Event

Solar Protons Detected by POES Omnidirectional Sensors

Northern Hemisphere Solar Protons in Northern Hemisphere
Southern Hemisphere Solar Protons in Southern Hemisphere

Solar Proton Events (SPE) and Their Impact on HF Communications

When very energetic protons (>10 MeV) produced by processes at the sun and interplanetary space arrive at Earth and enter the atmosphere over the polar regions, much enhanced ionization is produced at altitudes below 100 km. Ionization at these low altitudes is particularly effective in absorbing HF radio signals and can render HF communications impossible throughout the polar regions.

SWPC monitors the energetic proton detectors on NOAA's GOES satellites to determine when an SPE is in progress. (An SPE is defined as a flux of >10 MeV protons greater than 10 particles cm-2 sec-1 ster-1.) While the GOES satellite instrument can determine the presence and temporal profile of an SPE, those observations provide no information about the extent of the area over which those protons enter the atmosphere in the polar region or about which radio propagation paths are affected by the increased ionization.

NOAA's Polar Orbiting Environmental Satellites (POES) carry proton detectors similar to GOES. However, each POES satellite transits a polar region twice each orbit and can provide a direct measure of the boundaries and extent of the solar proton fluxes entering the atmosphere during an SPE.

The plots displayed above are designed to provide an up-to-date picture of the extent of the Northern and Southern polar area being affected during an SPE. This also provides an indication of those HF radio propagation paths that will be badly degraded because of the signal absorption.

In this display, all available satellite transits over the polar region during the most recent 8-hours are plotted. Currently there are four operating POES providing about a dozen transits over both the Northern and Southern polar regions within that time interval. The 16-second averaged count rates from the >15 MeV (nominally 15 -70 MeV) proton detector on each satellite are color coded and plotted along that satellite's transit. The geographic location of each data point is not the location of the sub-satellite point but rather the location mapped along the geomagnetic field from the satellite to 120 km altitude where the solar protons enter the atmosphere and produce the excess ionization. The beginning of the most recent satellite transit is marked with a red square and its end with a red triangle. Data plotted in white indicates insignificant count rates. The universal time at the start of the pass is given in the caption along with the name of the satellite providing the information.

Typically the sensor count rates are at cosmic ray background and the color plotted for each satellite transit is white. In the Southern Hemisphere display there is a geographic region over South America and the South Atlantic where the sensor count rates are normally high and are plotted yellow. This region is the southern extension of the South Atlantic Anomaly where the energetic particle intensities are normally large and does not indicate the presence of solar protons. In addition to the enhanced intensities associated with the South Atlantic Anomaly, there are regions of enhanced sensor responses (appearing as swathes of yellow and red) that appear within regions closer to the poles in both the northern and southern hemisphere. These elevated sensor responses are confined to data gathered by the SEM-1 instrument on NOAA POES satellites up to NOAA-14, and do not appear in data from the new SEM-2 instrument on NOAA POES satellites from NOAA-15 onward. This apparent high sensor response is, in fact, elevated background response due to high energy electrons from Earth's radiation belts and is not to be confused with regions of solar proton influxes. Improvements in the solid state detector electronics design has eliminated this problem in the SEM-2 instruments although it remains in the earlier SEM-1 design.

L-value Contour Lines

The outermost circle on these plots marks 30 degrees geographic latitude. The area over which solar protons enter the atmosphere, however, does not follow geographic coordinates but geomagnetic. For this reason we have included three geomagnetic L-value contours on the solar proton displays. The outermost contour is L=3, the center is L=4, and the innermost is L=6. The equator boundary of solar proton influx generally follows an L value contour and these contours will assist the viewer in extrapolating the boundary of solar proton influx to all longitudes.

The Solar Proton Event of July 2000

During SPEs the sensor response over a large portion of each satellite transit will be much increased and the corresponding color code will be yellow to deep red. An example of this display using data acquired during the SPE of July 2000 is shown below. This sample display clearly shows the bounds of the solar proton influx for individual satellite transits. With modest extrapolation, the boundaries can be defined on a global basis.

Northern Hemisphere Solar Protons in Northern Hemisphere
Southern Hemisphere Solar Protons in Southern Hemisphere