Radiation field formation and monitoring beyond LEO

Abstract

This brief review comprises the main features of radiation field formation due to galactic (GCR) and solar cosmic rays (SCR) beyond low Earth orbit (LEO) in the inner Solar system. We also consider the similarities and differences of radiation environment models applicable to the Moon and Mars missions, the main requirements for radiation monitoring, warning, and forecasting techniques for deep space missions. Existing modulation models for GCR provide a possibility to estimate easily the GCR contribution to the radiation field in the interplanetary space, at the Lunar or Martian surfaces. The situation with SCR contribution is much more complicated. For space weather purposes, time profiles of the 10–30 MeV protons in the most of solar proton events (SPE), unfortunately, are often complicated by several factors, especially by interplanetary shocks driven by Coronal Mass Ejections (CME). In particular, the particle-trapping region around the shock may or may not be the region with the highest intensity of solar energetic particles (SEP), depending on whether shock acceleration continues or diminishes with distance. At distances beyond 1 AU there can be interaction or merging of different transient shocks, and the corotating shocks begin to play a role, possibly by re-accelerating some of the SEPs from transient shocks. Also, the source locations, angular distribution and radial gradient of SEP intensity are very important. In a mission to Mars, for example, the radial distance will vary according to the spacecraft trajectory chosen. The flux of SEPs is expected to vary as a power law with radial distance from the Sun, and a power-law exponent of −3 would be expected from magnetic flux tube geometry. Since the radial distance to Mars is ∼1.5 AU, then the flux at the orbit of Mars would be expected to be about 1/3 of the flux at 1.0 AU along the same spiral path in the interplanetary magnetic field (IMF). A consideration of these expected variations suggests that the proton prediction problem for Mars is not dramatically different from the Earth. Autonomous sensors on board the spacecraft viewing in the optical, radio and soft X-ray wavelengths should be able to provide useful prediction information. Also, the radiation monitoring systems (RMS) are necessary to control the situation inside and outside the spacecraft, as well as on landing at the Mars’ surface. As follow from the long-term discussion of relative role of solar flares and CMEs in SEP production, existing paradigm of particle acceleration at near the Sun should be considerably improved. Also, it should be undertaken a partial revision of some existing models for SPE occurrence rate, proton energy spectrum, etc. In fact, we need a new concept of SPE in precise terms, taking into account a set of physical constraints for SEP acceleration and release processes.