Comparative analysis of low‐altitude ENA emissions in two substorms

Abstract

We report on the dynamics of low‐altitude energetic neutral atom (ENA) emissions during two substorms that occurred during the main phases of two storms: (1) a CIR‐driven storm on 11 October 2008 and (2) a coronal mass ejection (CME)‐driven storm on 5 April 2010. For both of these storms, we have complementary spacecraft and ground‐based observations. The dual‐spacecraft Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) mission obtained ENA images containing low‐altitude emissions (LAEs). Substorm dynamics is inferred from THEMIS all‐sky imagers. TWINS‐observed LAEs are compared with trapped/loss cone proton fluxes from the low‐altitude NOAA/MetOp spacecraft constellation. We find that the timing and intensity profiles of LAEs are different for the two selected events. For the 11 October 2008 event, the LAEs rise during substorm recovery phase and storm main phase. On 5 April 2010, the LAEs tend to peak near substorm onset. We argue that the different LAE behavior results from different pitch‐angle distributions (PADs) of the ion source population. Ion PADs are isotropic during substorm recovery phase for the 11 October 2008 event and have empty loss cone for the 5 April 2010 event. For both cases, LAE intensification marks the onset of activity in the magnetotail and precedes the large substorm onset. We conclude that the LAE production starts in the transition region between the magnetotail and ring current and may expand/move into the inner magnetosphere together with ring current formation.

Statistical correlation of low‐altitude ENA emissions with geomagnetic activity from IMAGE/MENA observations

“…Results from the initial processing of the data indicate reasonable distributions that agree with prior studies [ Søraas and Aarsnes , ; Pollock et al , ; Buzulukova et al , ; Vorobjev and Yagodkina , ]. The combined pitch angles appear centered slightly above 90°, which for nearly mirroring neutrals escaping the atmosphere is expected.…”

Magnetosphere dynamics during the 14 November 2012 storm inferred from TWINS, AMPERE, Van Allen Probes, and BATS-R-US–CRCM

“…A ring current model can be considered as a bounce-averaged kinetic code and normally operates with a large number (from hundreds to thousands) of "species", i.e., distributed in energy, pitch angle, or first and second adiabatic invariants. Currently, there are several major ring current models used by the community: the standard Rice Convection Model (RCM) (Harel et al, 1981;Toffoletto et al, 2003) or the RCM-E, a version that includes frictional dissipation (Lemon et al, 2003;Chen et al, 2015); the Magnetospheric Specification Model (MSM) (Wang et al, 2003(Wang et al, , 2004; the Comprehensive Ring Current Model (CRCM) (Fok et al, 1993(Fok et al, , 1995(Fok et al, , 2001(Fok et al, , 2014 and its successor, the Comprehensive Inner Magnetosphere-Ionosphere model (CIMI) (Fok et al, 2014); the Ring current-Atmosphere interactions Model (RAM) (Jordanova et al, 1994(Jordanova et al, , 2001 and its extended version with magnetic equilibrium solver Zaharia, 2008); the Hot Electron and Ion Drift Integrator (HEIDI) code (Liemohn et al, 1999); and the Inner Magnetosphere Particle Transport and Acceleration model (IMPTAM) (Ganushkina et al, 2012).…”

“…Valek et al (2010) showed that LAEs respond quickly to geomagnetic activity and are more dynamic than the ring current emissions (RCEs). In a follow-up study, Buzulukova et al (2013) showed that LAEs first increase during the substorm growth phase when RCEs may be on the edge of detectability and concluded that the parent ions responsible for these LAEs originated in the near-Earth tail region. It follows from these studies that the global ion precipitation in the near-Earth region will be reflected in the LAE index.…”

Empirical Model of Precipitating Ion Oval

“…For ions, this condition may be satisfied under several sets of circumstances. During storms and substorms, injections can transport ions with isotropic pitch angle distributions into the nightside inner magnetosphere, filling the loss cone (Buzulukova et al, ; Fok et al, ; Moore & Arnoldy, ; Valek et al, ). Trapped ions can also be pitch angle scattered into the loss cone by Coulomb collisions or wave‐particle interactions (Cornwall et al, ; Fok et al, , ; Frey et al, ; Gary et al, ; Williams & Lyons, ; Yuan et al, ).…”