Is diffuse aurora driven from above or below?

Abstract

In the diffuse aurora, magnetospheric electrons, initially precipitated from the inner plasma sheet via wave‐particle interaction processes, degrade in the atmosphere toward lower energies, and produce secondary electrons via impact ionization of the neutral atmosphere. These initially precipitating electrons of magnetospheric origin can also be additionally reflected back into the magnetosphere, leading to a series of multiple reflections by the two magnetically conjugate atmospheres that can greatly impact the initially precipitating flux at the upper ionospheric boundary (700–800 km). The resultant population of secondary and primary electrons cascades toward lower energies and escape back to the magnetosphere. Escaping upward electrons traveling from the ionosphere can be trapped in the magnetosphere, as they travel inside the loss cone, via Coulomb collisions with the cold plasma, or by interactions with various plasma waves. Even though this scenario is intuitively transparent, this magnetosphere‐ionosphere coupling element is not considered in any of the existing diffuse aurora research. Nevertheless, as we demonstrate in this letter, this process has the potential to dramatically affect the formation of electron precipitated fluxes in the regions of diffuse auroras.

The Magnetosphere‐Ionosphere Electron Precipitation Dynamics and Their Geospace Consequences During the 17 March 2013 Storm

“…As has been shown by Khazanov et al (2014) about 15-40% of the total aurora energy returns back to the magnetosphere and the conjugate ionosphere. As discussed by Khazanov et al (2015), , 2017a, the first step in such simulations is initiation of electron precipitation from the Earth's plasma sheet via wave-particle interactions (WPIs) into both magnetically conjugate points (e.g., as is simulated by Chen, Lemon, Guild, et al, 2015). As discussed by Khazanov et al (2015), , 2017a, the first step in such simulations is initiation of electron precipitation from the Earth's plasma sheet via wave-particle interactions (WPIs) into both magnetically conjugate points (e.g., as is simulated by Chen, Lemon, Guild, et al, 2015).…”

“…Collisionless processes dominate in the inner magnetosphere, whereas collisional interactions are important in the ionosphere, and Khazanov et al (2018) have emphasized that first-principle simulations of precipitating electron fluxes in both regimes are required to understand spatial and temporal variations of ionospheric conductance and related electric fields. The second step is to account for multiple atmospheric reflections of electrons between the ionosphere and magnetosphere at the two magnetically conjugate points, as treated in the SuperThermal Electron Transport (STET) model (Khazanov et al, 2015(Khazanov et al, , 2017a(Khazanov et al, , 2017b(Khazanov et al, , 2018Khazanov, Himwich, et al, 2016). The second step is to account for multiple atmospheric reflections of electrons between the ionosphere and magnetosphere at the two magnetically conjugate points, as treated in the SuperThermal Electron Transport (STET) model (Khazanov et al, 2015(Khazanov et al, , 2017a(Khazanov et al, , 2017b(Khazanov et al, , 2018Khazanov, Himwich, et al, 2016).…”

Low Energy Precipitating Electrons in the Diffuse Aurorae

“…The secondary electrons are assumed to have isotropic distribution and their productions are described by the ionization collisional term in equation (1) above (see also equation 5 in Khazanov et al, 2014). Cross sections for elastic collisions, state specific excitation, and ionization are taken from Solomon et al (1988), Khazanov et al (2014), , Khazanov et al (2017), and provide extensive details on the STET code. For the atmospheric model, STET uses the Mass-Spectrometer-Incoherent-Scatter model, which describes the neutral temperature and densities in the upper atmosphere (above~100 km) (Hedin et al, 1991).…”

“…Blue arrows show the secondary electrons (adapted fromKhazanov et al, 2017). The primary electrons are shown by red and yellow arrows.…”

Solar Illumination Dependence of the Auroral Electrojet Intensity: Interplay Between the Solar Zenith Angle and Dipole Tilt

“…The generation of secondary electrons, which can be an important component of the diffuse precipitation (Khazanov et al, 2017), may also contribute to its interhemispheric asymmetry. If more secondary electrons are produced in the sunlit nightside hemisphere, possibly because of a largerscale height, they would contribute to the enhancement of the diffuse precipitation in the dark hemisphere.…”