Shock Connectivity and the Late Cycle 24 Solar Energetic Particle Events in July and September 2017

Abstract

As solar activity steadily declined toward the cycle 24 minimum in the early months of 2017, the expectation for major solar energetic particle (SEP) events diminished with the sunspot number. It was thus surprising (though not unprecedented) when a new, potentially significant active region rotated around the East limb in early July that by midmonth was producing a series of coronal eruptions, reaching a crescendo around 23 July. This series, apparently associated with the birth of a growing pseudostreamer, produced the largest SEP event(s) seen since the solar maximum years. Activity abated with the decay of the active region, but a second episode of magnetic flux emergence in the same area in early September initiated a new round of eruptions. The western longitude of the erupting region, together with its similar coronal setting in both cases, resulted in a set of nearly homologous multipoint SEP event periods at Earth, Solar TErrestrial RElations Observatory‐A and Mars (Mars Atmosphere and Volatile EvolutioN) for July and September 2017. We use a combination of WSA‐ENLIL‐cone heliospheric simulation results, together with SEPMOD SEP event modeling, to illustrate how the event similarities at the three observer sites can be understood from their relative positions and their connectivities to the generated interplanetary shocks.

Observations and Impacts of the 10 September 2017 Solar Events at Mars: An Overview and Synthesis of the Initial Results

“…For this article, we use the WSA‐Enlil simulation results as a tool toward interpreting the solar wind and CME observations at Mars. The simulation details are described in the Space Weather paper by Luhmann et al () that uses the Enlil results to model the associated SEP observations for the July and September 2017 event periods. In general, the limitations of WSA‐Enlil to accurately model the CME event include the uncertainties in the input solar photospheric field maps that are used to drive the background solar wind simulations, the use of a geometrical cone shape to approximate the shape of a CME, the accuracy of the CME parameterization (injection direction, time, and size), and the lack of the ejecta field description in Enlil, which affects the CME expansion during the propagation (Lee et al, , ; Mays et al, ).…”

“…These September events began with two large M‐class flares and one X‐class flare observed at Mars (Chamberlin et al, ) together with CME activity seen in the solar coronagraphs at Earth and STA. High fluxes of solar energetic particles (SEPs) associated with the flare and CME eruption were also detected (e.g., see Luhmann et al, ). In section 2 we provide an overview of the solar and heliospheric observations of the 10 September 2017 solar activity to provide context for sections 3‐6 where we present and synthesize the initial results presented in this GRL volume on the space weather impacts at Mars.…”

September 2017's Geoeffective Space Weather and Impacts to Caribbean Radio Communications During Hurricane Response

“…The alert threshold was exceeded semicontinuously as far back as mid‐July, driven by several coronal hole high speed streams resulting in stream interaction regions (SIRs), which are common during the declining phase of a solar cycle. For further context and study, see Luhmann et al (, their Figure 3) and review OMNIWeb's solar wind parameters and SWPC's alert timeline (our Table ).…”

“…The SUVI images are taken at the time nearest to the X‐ray peaks for the given event. For model estimates of the propagation of these interplanetary CMEs (ICMEs) through the heliosphere, see Luhmann et al (, their Figure 4). In particular, the distinctly different trajectory and longitudinal extent near 1 AU for the 6 and 10 September eruptions, respectively, correlate well with the G4 “severe” and G3 “strong” geomagnetic disturbances observed at Earth.…”

Simulation of Prompt Acceleration of Radiation Belt Electrons During the 16 July 2017 Storm

“…A moderately strong forward shock and its CME driver from a western solar disc active region impacted Earth on 16 July 2017 (Luhmann et al, ) around 05:50 UT followed by a subsequent pressure impulse characterized by a southward IMF B z rotation at 09:40 UT. Solar wind parameters are plotted in Figure a along with magnetopause location from the Shue () model and SYM‐H .…”