Several international space agencies plan to send astronauts beyond low-Earth orbit in the coming decades to explore the Moon or other nearby planetary objects. Humans leaving the Earth's magnetosphere enter the solar wind, potentially exposing themselves to prompt solar energetic particle (SEP) events, which are sudden outbursts of energetic particle radiation of solar origin. Accurate warning of SEP radiation hazards through an operational forecasting system, even if only an hour in advance, allows contingency plans to be set in motion rapidly.
The potential for expanding mission operations capabilities with such warnings has been acknowledged by the NASA Space Radiation Analysis Group at Johnson Space Center. As NASA gears up to send astronauts to the Moon and Mars, projected radiation doses on such long-term missions approach current career limits, so avoiding sudden exposure from SEP events becomes crucial.
SEP ions are most harmful to humans when they arrive in bulk at energies of tens of megaelectron volts, potentially delivering a large radiation dose to astronauts within a short period of time. These ions take between 20 minutes and several hours longer to travel from the Sun to Earth or Mars than specific signals near light speed that are emitted or released at the same time as the ions.
The research community now realizes that such fast moving signals hold the key to predicting bulk ion arrivals at Earth. Multiple research groups, in what appears to be a new trend, now attempt rapid forecasting of prompt SEP events by exploiting precursor signals that indicate particle release from the Sun. Among the signals that can be monitored by space- or ground-based instruments are relativistic electrons (RE), relativistic ions, and plasma radiation excited by lower-energy electrons near the Sun [Dorman et al., 2003; Posner and Hassler, 2004; Kuwabara et al., 2006a, 2006b; Looper et al., 2006; Posner, 2007; Laurenza et al., 2009]. Scientists are investigating these precursors for future operational use in forecasts.
For several reasons, REs in particular show great promise in forecasting efforts. Always present in SEP events, they also have the advantage of probing the passageways along the interplanetary magnetic field lines ahead of the ions. In addition, the number of electrons arriving (the intensity profile) is linked with the intensity of the later-arriving ions. Electron detection in the vicinity of the Earth thus not only provides evidence that the Sun has released charged particles into the heliosphere but also provides information that scientists can use to forecast the intensity of the harmful ions soon to arrive.
The method of using REs to forecast incoming ions currently relies on data from the Comprehensive Suprathermal and Energetic Particle Analyzer (COSTEP) instrument on board the Solar and Heliospheric Observatory (SOHO). The upcoming transition of most of the live science data stream from SOHO to the new Solar Dynamics Observatory mission leaves a narrow window of opportunity for testing the real-time performance of the RE method.
Taking advantage of this opportunity, our collaboration between Southwest Research Institute, the University of Kiel, and NASA Goddard Space Flight Center (GSFC) has created a real-time forecast system called Relativistic Electron Alert System for Exploration (REleASE), which went online on 7 February 2008 (see Figure 1). About 1 year into real-time forecasting, our new system has proven two aspects: a low false alarm rate, about one per year, and a sensitivity to even weak particle events, such as one that occurred on 4 November 2008. REleASE is now distributed worldwide from Web servers of the University of Kiel, COSTEP's principal investigator institution, at http://www-etph.physik.uni-kiel.de/missions/soho/costep/realtime/forecast/, and of GSFC's Community Coordinated Modeling Center at http://swan.gsfc.nasa.gov/helio.html.