Dangerous Solar Energetic Particles Dissipate From the NearEarth

Dangerous Solar Energetic Particles Dissipate From the Near-Earth Space Mainly Through Adiabatic Cooling Mechanism Fits to the time-intensity profiles of 15 -44 Mev protons observed at Earth by the GOES satellite. The red and blue lines are simulation results with and without adiabatic energy loss, respectively.

When a solar flare or CME shock event occurs on the sun, energetic particles up to 10 Ge. V are injected into the interplanetary space. Within a few minutes to a few hours, the space surrounding the Earth is filled with these dangerous high-energy radiations from the sun. The high radiation level can persist for many days to over a month depending on how fast the particles dissipate through the interplanetary medium. For most of us, we used to think solar energetic particles dissipate by a diffusive transport process out of the heliosphere, which we commonly call leakage. Recent results from this project found that the leakage is not the main dissipation mechanism for large graduate events that last more than one day. The main dissipation mechanism is the prevailing adiabatic energy loss experienced by particles in the expanding solar wind plasma. By inclusion of the effect of adiabatic energy loss into the traditional focused transport equation of energetic particle, the team is able to fit the time profile of long-duration solar energetic particle event (Figure). The huge difference of particle intensity after one day into the event between the two models suggests that the adiabatic energy loss is the far most important dissipation mechanism of solar energetic particles. Contributors: M. Zhang and G. Qin