On the Origin of Gamma-Ray Emission from the Behind-the-Limb Flare on 29 September 1989

We were all very surprised when SOL1989-09-29, a flare known to be well-occulted, actually produced detectable gamma radiation even in the 2.223 MeV neutron-capture line, which really implies the deep atmosphere. This region should be invisible from the perspective of the Earth because of being occulted, but this simple cartoon provides a straightforward explanation: the flare and its CME created a large-scale shock wave which, according to conventional wisdom, then accelerated the solar cosmic rays (SEPs). Some of these then, confounding the mirror force and the likely-to-be-negligibly-small loss cone, somehow swim back upstream to impact the solar photosphere quite far from the flare. Conceptually the short (upper) curly line segment would be an SEP "auguring in" to an open-field region, such as might be found in a coronal hole. The longer curly line is an outward-bound SEP also coming from the shock front.

      In a sense this is the most plausible mechanism, but it still seems unlikely because of the well-known limb darkening of the 2.223 MeV line. If this event could violate that so strongly, why wouldn't any event do so, statistically speaking?

      As a footnote on the cartoon, one should note that a bow shock normally adheres to the shape of its source disturbance. Often people draw wide-ranging shocks - possibly reflecting the shapes of their French curves - that may or may not be realistic, nor may it guide us to the most favorable regions of the shock. In this case perhaps the effective part of the disturbance was down near the chromosphere. A later cartoon by the same author envsions more flexibility in terms of the possible acceleration regions (and conditions).