Research On The Decay Phase Of Solar Energetic Proton Events Based On The Multi-spacecraft Observations

The solar energetic particle(SEP)event has a great impact on human space activities.The study of SEP events helps to understand the physical mechanism of SEP accelerating and propagating in the heliosphere.Multi-spacecraft observations often reveal a interesting “reservoir” phenomenon: observers located at different longitudes,latitudes,and heliocentric distances among the interplanetary space see large differences in particle fluxes during the ascending phase of solar energetic particle events.But during the decay phase,the fluxes of the SEP are almost equal and evolve similarly over time.During different solar cycles,we combine spacecraft with different radial distances and longitudes at the ecliptic to observe the proton reservoir phenomenon of SEP events.Data comes from two Helios and IMP-8 spacecraft during the 21 st solar cycle,and STEREO A&B with SOHO spacecraft during the 23-24 th solar cycle.We found 24 proton reservoir events from more than 200 SEP events,and analyze the data.Base on the analysis of the observation results,we find that:(1)the larger peak value and the longer duration of SEP event,the more likely the reservoir phenomenon would occur.(2)The higher energy of SEP particles,the more likely the reservoir would occur.(3)In solar energetic proton event,if the two Helios spacecraft observe the reservoir phenomenon,basically this phenomenon would also appears in the IMP-8 observation;and the joint observations of STEREO A&B and SOHO also verifies this finding.Based on the above observations,we conclude that:(1)The forming of the ecliptic proton reservoir is dominated by shock,while the interplanetary perpendicular diffusion and magnetic mirror effects do not play a leading role.(2)Compare to lower energy particles,the injection process of higher energy particles mainly occurs near the Sun.In the process of particle injection,the intensity and the spatial changing of SEP source fuction determine whether the reservoir can occur.