Clearly the most prominent signal we have of a flare occurrence is the bursts of electromagnetic radiation that originate in the flare event and travel in a straight-line path to Earth, thus reaching Earth in about 8 minutes, compared with flare-generated solar energetic particles (SPEs), which can arrive as soon as 18 minutes after the flare.
Our ability to predict the occurrence of SPEs due to flares is imprecise at best, and the accuracy of the prediction varies inversely with the length of the advance warning desired. In addition, the probability of flare SPEs reaching Earth depends on the location of the flare on the sun's surface. The magnetic field line that connects Earth to the sun has its origin on the sun at location W55. Thus the closer the flare event is to this location, the shorter the time it takes the solar flare particles to reach Earth. With increasing distances from location W55 on the sun, the protons take longer to reach Earth or may not be connected by magnetic field lines to Earth; thus such SPEs would not be observed on Earth.
Coronal mass ejections are large eruptions of coronal material that produce interplanetary shocks when they move at high speeds from the sun through the solar wind. These shocks produce SPEs by accelerating a small fraction of the solar wind particles during their passage through interplanetary space. It is now appreciated that not all coronal ejections that produce SPEs have related flares, and the contribution of nonflare events to the SPE population has only recently been appreciated. Such events are detectable in soft x-ray images obtainable above Earth's absorbing atmosphere. However, it is not yet possible to predict from such images whether the disturbance launched toward Earth is fast enough to produce a shock and thus SPEs.
Assuming the goal of allowing time for crew members, in space or on another planetary body, to reach shielded locations, current monitoring systems using visual observations for predicting SPEs are not acceptable. The fact that a space vehicle during its travel may be connected to a flare on the sun by magnetic field lines that originate on the far side creates a blind spot for Earth-based monitoring stations. A series of satellite platforms for monitoring the sun's activity could keep the sun's surface and corona under continual surveillance, but such a system is complex and costly. Thus there is a need for new and innovative ideas in this area. The spacecraft itself may be equipped with the necessary instrumentation to allow crew members to participate in the monitoring process. However, this system would, like Earth-based stations, have blind spots and should be viewed as a complement to any series of space platform monitoring stations.
The required advance warning time will be dictated by the mission program. Under the assumption that crew members will return to a base camp on Mars for rest at the end of each day, an advance warning of 8 hours would be desirable. Shorter warning periods may curtail the scope of a mission if the level of safety is to be maintained.
The current state of the science is that Earth-based monitoring stations can only partially support a space vehicle in predicting the occurrence of SPEs at the vehicle. A system that monitors the global surface of the sun and corona should be developed and knowledge improved on how to interpret information acquired by monitoring the sun. Research is necessary to provide an understanding of the basic mechanisms that trigger solar events, the precursor signals that can be detected from terrestrial or satellite observation, and the ability to determine the location of flare occurrence.
1. Dicello, J.F., Schillaci, M.E., and Liu, L. 1990. Cross sections for pion, neutron, proton, and heavy-ion production from 800-MeV protons incident upon aluminum and silicon. Nucl. Instrum. Methods B45: 135–138.