engleski

The September 2017 events and their imprints at Earth, Mars and STEREO-A

During the declining phase of the current solar cycle, heliospheric activity has suddenly and drastically increased starting from a simple sunspot in Active Region (AR) 2673, which transformed into a complex region with three X-class flares accompanied by several Earth-directed Coronal Mass Ejections (CME) from 4th to 6th of September. Only a few days later, on 10th September, the same AR 2673 produced solar energetic particles (SEPs) which were registered as a ground level enhancement (GLE) at Earth and the biggest GLE on the surface of Mars as observed by the Radiation Assessment Detector (RAD) since the landing of the Curiosity rover in August 2012. Both Earth and Mars saw an impulsive and intense enhancement of the accelerated protons with energies larger than hundreds of MeV whereas STEREO-A, despite being at the back side of the event, detected gradually increasing fluxes of particles transported there across the heliospheric magnetic field. These high energy particles were mainly accelerated by the flares and shocks which were associated with three consecutive CMEs launched on 9th and 10th of September. Based on STEREO-A and SOHO coronograph images, we identified the initial three-dimensional kinematics of the three CMEs using the Graduated Cylindrical Shell (GCS) model. The first two CMEs had moderate launch speeds while the last one was extremely fast (larger than 2500 km/s at 20 solar radii). These three CMEs interacted as they propagated outwards into the heliosphere and the resulting complex interplanetary CME (ICME) together with its associated shock was highly likely related to the effective acceleration of particles at such high energies causing GLE at both Earth and Mars. The arrival of the ICME at Mars caused a very significant Forbush decrease seen by the Radiation Assessment Detector (RAD) on the surface of Mars and the arrival time is only a few hours later than that at Earth which is about 0.5 AU closer to the Sun than Mars. We investigated the interaction of three CMEs and propagation of the consequent ICME using the Drag Based Model (DBM) as well as the WSA-ENLIL plus cone model and the simulated results are compared with in-situ measurements at both Earth and Mars. The comparison shows that in order to better predict the ICME arrival and its potential space weather impact at Earth and other heliospheric locations, it is essential to 1) analyze the evolution of the ICME kinematics, especially during interactions of different CMEs and 2) better understand the spatially and temporally varying interplanetary conditions of the heliosphere.

coronal mass ejections, solar energetic particles, space weather

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