O⁺Escape During the Extreme Space Weather Event of 4–10 September 2017

Abstract

We have investigated the consequences of extreme space weather on ion outflow from the polar ionosphere by analyzing the solar storm that occurred early September 2017, causing a severe geomagnetic storm. Several X‐flares and coronal mass ejections were observed between 4 and 10 September. The first shock—likely associated with a coronal mass ejection—hit the Earth late on 6 September, produced a storm sudden commencement, and began the initial phase of the storm. It was followed by a second shock, approximately 24 hr later, that initiated the main phase and simultaneously the Dst index dropped to Dst = −142 nT and Kp index reached Kp = 8. Using COmposition DIstribution Function data on board Cluster satellite 4, we estimated the ionospheric O+ outflow before and after the second shock. We found an enhancement in the polar cap by a factor of 3 for an unusually high ionospheric O+ outflow (mapped to an ionospheric reference altitude) of 1013 m−2 s−1. We suggest that this high ionospheric O+ outflow is due to a preheating of the ionosphere by the multiple X‐flares. Finally, we briefly discuss the space weather consequences on the magnetosphere as a whole and the enhanced O+ outflow in connection with enhanced satellite drag.

The Oxygen Ion Circulation in The Outer Terrestrial Magnetosphere and Its Dependence on Geomagnetic Activity

“…The average total O + velocity v and magnetic field strength B pm in the plasma mantle (−5 < X G S M <−2R E ) is 120 km/s and 50 nT during the strongest geomagnetic conditions. Schillings et al () observed a convection speed v ⊥pc in the polar cap of 20.6 km/s (at an altitude of 6.5 R E and magnetic field strength B pc of 145 nT) during a strong geomagnetic storm associated with the extreme space weather event in September 2017. This convection speed can be scaled to the considered plasma mantle as km/s.…”

Ionospheric Response Observed by EISCAT During the 6–8 September 2017 Space Weather Event: Overview

“…The timing for individual events discussed in subsections 3.1–3.5 are marked between panels (d) and (e). The Cluster traversal of the cusp and polar cap (Schillings et al, ) is also marked. SEM = Space Environmental Monitor; IMF = interplanetary magnetic field.…”

“…For this examination, we took a 30‐min running average of N e ·V i at 400 km altitude from the Svalbard 42 m antenna between 08:30 and 11:00 UT (about 10–12 LT), because the outflow is maximized at this local time for this latitude according to the past statistics (e.g., Moore et al, , and references therein). The simultaneous Cluster observation of the same event at high altitudes in the prenoon region shows that the ICME arrivals caused an increase of ionospheric ion outflow (Schillings et al, ). Therefore, the increased solar irradiation most likely contributed to preconditions for enhancement of the ion outflow.…”

“…This unique observation in terms of timing and the location of EISCAT gives us the chance to monitor how the ionosphere, particularly the ion flow, responded to the sequence of these events. This paper provides an overview of the EISCAT common programme data with a focus on ion heating and escape, and complementary information to the Cluster observations of the ion escape presented in Schillings et al ().…”

“…Finally, EISCAT observed the local noon sector when a large southward turning of IMF to purely and stable southward (Bz ~−17 nT) arrived at the Earth on 8 September. For the second ICME, Cluster traversed the cusp and polar cap (the main route of the ion escape) before and after the ICME arrival (Schillings et al, ).…”