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Proba-1/SREM - 10 year Anniversary

Proba-1 was launched 22 October 2001 and since 29 October 2001 has been returning data on it's radiation environment taken by the ESA standard radiation environment monitor SREM. These data show how the Earth's radiation belts change with time and location and show the effects of solar "particle events" that inject more radiation into the near-Earth environment. Proba is in a low altitude polar orbit (with altitude ranging between 560 and 680 km) and passes continuously in and out of the radiation belts. At high latitudes it crosses geomagnetic field lines that are connected to higher altitudes so that it is able to monitor the state of the "outer" radiation belt.

The figure above shows a complete summary of the measurements from some of the Proba energy channels (TC3: electrons with energy > 0.8 MeV; S12: protons with energy > 26 MeV). It represents the evolution of a cross-section through the radiation belts during the mission. The upper panel shows the dynamic outer radiation belt where radiation levels can rise and fall as a consequence of "storms" and vertical streaks appear where solar particle events occur. The vertical axis represents approximately where the field line crosses the equator in units of Earth-radii, and so is also closely related to geomagnetic latitude. (A field line with L = 6.6.Re crosses the equator up at about the geostationary orbit and crosses the Proba orbit at a geomagnetic latitude of around 65 degrees.) 

The lower panel shows the inner radiation belt encountered by Proba in the "South Atlantic anomaly". This is much more stable and the regular short term variations are due to orbital effects. The long term trend is due to the slow effect of the solar cycle on the Earth's atmosphere - radiation belt particles are lost in collisions with atmospheric neutrals and as solar activity declines the atmosphere shrinks, allowing radiation levels to rise. The 109 day oscillation is an effect of the eccentric orbit as the line of apsides rotates around the orbital plane.

Proba's lifetime has far exceeded expectations and being a small satellite it inevitably has limited power. The available power has decreased over time as the solar arrays are degraded by radiation. As a result, power was not available to operate for a long period in 2009 and for most of 2010. But smart operations have allowed data collection to resume. A contract was recently signed with RUAG (CH) to develop a replacement for the highly successful SREM series - the next generation radiation monitor (NGRM) - that should fly on many future European spacecraft and be available commercially.

Plots of the SREM data are also available.