Electron Fluence Model Help


The tool collects the latest eight days of GOES-8 >2 MeV electron flux and calculates the 24 hour running fluence. The GOES-8 is a geosynchronous satellite at about 75o West and the fluence levels and thresholds are valid for geosynchronous satellites.

There are two threshold levels defined, one for medium risk of anomaly (yellow) and one for high risk of anomaly (red). The default threshold levels are 5*107 cm-2 sr-1 for yellow 5*108 cm-2 sr-1 for red. When the daily fluence reach above the yellow or red levels the risk for internal charging increases, which in turn may cause satellite anomalies. The user can also change the fluence thresholds to levels that might be more appropriate for a specific spacecraft. When the tool is started the latest GOES-8 data is loaded. To study other days the year, month, and date fields may be changed. Then push the "Plot" button to load the data and update the plot.

Background

Energetic electrons can penetrate the shielding of a spacecraft and deposit the charge in dielectric materials. If the electron flux stays at high levels for an extended period charging can take place within the spacecraft. When the charge reaches the electric breakdown level of the dielectric a discharge occurs which may trigger an anomaly.

In a study by Wrenn [1] it was found that geosynchronous communications spacecraft regularly experienced anomalies that were caused by internal dielectric charging. By calculating the daily fluence of the >2 MeV electron flux, measured by the GOES satellites, threshold levels could be determined when there was an increased risk of internal charging anomalies.

The figure to the right shows the daily fluence of >2 MeV electrons over a complete solar cycle (left panel) and the sunspot number (right panel) [2]. The years are given on the left side from top to bottom and one row covers 27 days (one solar rotation). The green areas correspond to when the daily fluence is less than 5*107 cm-2 sr-1, the yellow when the fluence lies in the range 5*107 cm-2 sr-1 to 5*108 cm-2 sr-1, and the red when the fluence exceeds 5*108 cm-2 sr-1. The yellow and red areas are thus periods of increased risk if internal charging anomalies.

It is also clear from the figure that the high fluence levels are concentrated around the years 1993-1996. This period coincides with the declining phase of the solar cycle (see the right panel where the sunspot number is decreasing). During the declining phase the solar wind is dominated by long lasting high speed plasma streams that elevate the MeV electron flux to high levels. As we expect that the sunspot maximum for the current cycle will occur around 2000/2001 the internal charging will again peak around 2005.

References

  1. Wrenn, G.L., Conclusive evidence for internal dielectric charging anomalies on geosynchronous communications spacecraft, J. Spacecraft and Rockets, 32, 514-520, 1995.
  2. Wrenn, G.L., Private communications, 2000.

Peter Wintoft, Wednesday, August 29, 2001