The radiation-management strategy includes three components:
(1) Autonomous safing. Chandra's onboard computer constantly receives data from the onboard radiation monitor EPHIN (Electron, Proton, Helium Instrument) and autonomously safes the science instruments if certain EPHIN thresholds are exceeded. Unfortunately, the lowest EPHIN proton channel measures energies (5-8 MeV) significantly higher than the lowest energy protons (0.1 MeV) that can damage the front-illuminated CCDs. Nonetheless, the EPHIN provides autonomous protection against large solar-radiation storms and inadvertent unprotected entry into the radiation belts.
(2) Intervention. The SOT has built a suite of tools to monitor real-time data from NOAA's Space Environment Center (SEC) -- primarily, from NASA's Advanced Composition Explorer (ACE, at L1, 0.01-AU sunward of the earth) and NOAA's Geostationary Operational Environmental Satellites (GOES) spacecraft. The ACE Electron, Proton, and Alpha Monitor (EPAM) conveniently provides real-time data on protons in the energy range most damaging to the ACIS CCDs. Thus, the EPAM directly measures the radiation environment to which Chandra is exposed while in the solar wind. A CXC computer automatically monitors the appropriate EPAM channels and issues alerts to the anomaly-response team when certain thresholds are exceeded. The team then meets via telecon to access the radiation risk and, if deemed necessary, to intervene to protect the science instruments and stop the load.
(3) Modeling. Unfortunately, Chandra's orbit passes through the magnetosphere, magnetosheath, and the solar wind. While the science instruments are always protected when passing through the inner magnetosphere (including the radiation belts), they routinely operate in the outer magnetosphere, where solar-wind measurements do not directly apply. It is in the outer magnetosphere that most of the remaining radiation degradation is occurring. Thus, scientists from Sverdrup Technology, working with the MSFC Space Environments Group and Project Science, have developed the Chandra Radiation Model (CRM) to describe the radiation environment throughout Chandra's orbit. The CRM serves as a tool for mission planning and for real-time estimation of the radiation environment, as described below.
The Chandra Radiation Model (CRM) is a database derived from data of NASA spacecraft which have sampled the proton-environment regions through which Chandra passes. The team at MSFC is now updating the database to add data from Polar to the existing data from Geotail, in order to sample more fully the proton environment away from the ecliptic. In addition, the Sverdrup scientists are implementing streamline mapping into the model, in order to populate more accurately the database in regions not directly sampled by Geotail and Polar orbits. Because, magnetospheric proton fluxes are correlated with geomagnetic activity, the CRM stratifies the global 3-D data in a fourth dimension -- the planetary K index (Kp), a measure of this activity. This Kp-dependent model then serves as a basis for performing long-term, probablistic, predictions of the environment (determined by the frequency distribution of Kp) and for real-time estimation of the proton flux along the orbit (driven by real-time estimates of Kp). The SOT has incorporated the Kp-driven CRM estimates of magnetospheric protons into the real-time radiation-monitoring system, along with direct ACE-EPAM measurements of solar-wind protons. This serves as a powerful tool in real-time estimation of the 0.1-0.2-MeV proton environment throughout Chandra's orbit. During the next year, we expect that the CRM will also become a useful tool in long-term radiation management, by identifying specific periods during which Chandra is particularly vulnerable to magnetospheric protons.
- Martin Weisskopf and Steve O'Dell
See also ``Shelter from the Storm: Protecting the Chandra X-ray Observatory from Radiation"