Solar Energetic Particle Events of July 2017: Multi-spacecraft
Observations near 1 and 1.5 AU
Abstract
We investigate the solar events of late solar cycle 24 in July 2017
observed by a number of spacecraft in the inner heliosphere widely
separated in heliolongitude and radial distance. These include
spacecraft at L1 point, STEREO-A, near Earth satellites, and MAVEN (near
Mars). The GRASP payload onboard Indian GSAT-19 satellite provides a new
vantage point for Solar Energetic Particle (SEP) observations near
Earth. There were two major Coronal Mass Ejections (CMEs) and a Stream
Interaction Region (SIR) event in July 2017, which is a period during
the deep descending phase of the historically weak solar cycle 24. The
16 July CME was Earth directed and the 24 July CME was STEREO-A and Mars
directed. Earth and Mars were on the opposite sides of the solar disk,
while Mars and STEREO-A were aligned with respect to the nominal Parker
spiral field. The 24 July event was stronger and wider in
heliolongitude. This CME-driven shock had magnetic connectivity to
Earth, which produced an SEP event at Earth ~two days
later. The spectral indices of the event observed directly at STEREO-A
and at the remote location of ACE was found to be similar. The 16 July
SIR event was observed by both MAVEN and STEREO-A. Higher particle
intensities (a factor of 6 enhancement for 1 MeV protons) are observed
by MAVEN (at 1.58 AU) compared to STEREO-A (at 0.96 AU). Also a spectral
hardening is observed while comparing the spectral indices at these two
locations, indicating proton acceleration at the SIR forward shock
during the radial propagation of 0.62 AU in the interplanetary space.