Can Enhanced Flux Loading by High‐Speed Jets Lead to a Substorm? Multipoint Detection of the Christmas Day Substorm Onset at 08:17 UT, 2015

Abstract

This paper describes highly interesting observations of the 25 December 2015 substorm onset at 08:17 UT during northward interplanetary magnetic field (IMF) while magnetosheath contained several intervals of negative B z. A unique alignment of several spacecraft near the Earth-Sun line together with magnetohydrodynamic simulations, ground-based magnetometer, and auroral observations allow a comprehensive timing analysis of the events leading to substorm onset. Perplexingly, prior to substorm onset Geotail measured for 26 min positive IMF B z just upstream of the bow shock, while simultaneously MMS spacecraft measured several intervals of strong negative B z in the dayside magnetosheath. These strong pulses of negative B z in the magnetosheath were associated with high dynamic pressure magnetosheath jets, likely created by foreshock transients during strongly radial IMF interval. Multipoint plasma and magnetic field measurements from ARTEMIS and THEMIS spacecraft were used to determine tail reconnection time at 8:14 and location at x = −33R E. Ground-based observations of Pi2 pulsations and auroral brightening, with observations of a dipolarization front by THEMIS spacecraft, allowed determination of substorm onset to be at ≈08:17. All MMS spacecraft detected the same magnetosheath jet structure with B z = −25 nT at ∼08:00 while IMF was northward. Based on DMSP observations and timing analysis we propose that these jets produced magnetopause reconnection leading to final, critical flux enhancement in the midtail region, which may have triggered reconnection 12-14 min later after jet observations.

Electron Acceleration by Magnetosheath Jet‐Driven Bow Waves

“…Journal of Geophysical Research: Space Physics enhancement (Figure 1.3 h), bow waves could intensify or trigger the magnetopause reconnection resulting in magnetospheric and ionospheric perturbation, such as substorms (e.g., Hietala et al, 2018;Nykyri et al, 2019). As for electrons below 2 keV, they were likely solar wind electrons heated/accelerated by the bow shock.…”

On the alignment of velocity and magnetic fields within magnetosheath jets

“…A second, smaller group of jets appears to be associated with the passage of IMF discontinuities, in particular when the character of the shock changes from quasi-perpendicular to quasi-parallel (Dmitriev and Suvorova, 2012;Savin et al, 2012;Archer et al, 2012;Plaschke et al, 2017). In this context, jets have also been associated with hot flow anomalies (HFAs) that can occur when an IMF discontinuity interacts with the bow shock (Schwartz et al, 2000;Omidi and Sibeck, 2007).…”

“…The angle between B and V is φ B,V . (Hietala et al, 2018), as well as on the associated triggering of substorms (Nykyri et al, 2019).…”

Long‐Term Variations of the Geomagnetic Activity: A Comparison Between the Strong and Weak Solar Activity Cycles and Implications for the Space Climate

“…This may cause global-scale geomagnetic disturbances, commonly known as geomagnetic storms (Chapman & Bartels, 1940;Gonzalez et al, 1994). On the other hand, a low rate magnetic reconnection for an intermittent interval may lead to the precipitation of ∼10-100 keV electrons and ions in the auroral atmosphere resulting in auroral substorms (Akasofu, 1964;Nykyri et al, 2019;Ohtani, 2001;Tsurutani & Meng, 1972). Intense auroral substorms continuing for a few days to a week without occurrence of any major geomagnetic storms have been called high-intensity long-duration continuous auroral electrojet (AE) activities (HILDCAAs: Hajra et al, 2013Hajra et al, , 2014Tsurutani & Gonzalez, 1987).…”