Proton acceleration at two‐dimensional dipolarization fronts in the magnetotail

Abstract

Transient reconnection events in the planetary magnetotail give rise to fast plasma jets, whose leading edges are called dipolarization fronts. We perform a test particle simulation of ion acceleration in a 2-D model of dipolarization fronts. We study the dependence of acceleration on several parameters of the model. We obtain that the interaction between the dipolarization front and particles depends on their initial energy h 0 , resulting in a moderate increase of the gained energy with h 0 . The front amplitude B max has been varied from 10 nT to 40 nT in order to study its influence on the particle gyro-radii and, in turn, on interaction with the front. As expected, the energy gained by particles increases with B max . We find that the acceleration strongly depends on the front velocity v , which we chose to vary from 200 km/s to 750 km/s. The simulations pointed out that higher energies have been reached with fast fronts, ranging from 50 to 100 keV. We also study how the average maximum energy attained by ions, ⟨h max ⟩, depends on the y extent of the front: at variance with expectations, the energy gain decreases with the increase of the y size of the front for small values of that size; increasing further, the scaling of ⟨h max ⟩ with the y size seems to indicate a slow growth. We also study the energy and velocity distributions functions of ions accelerated by dipolarization fronts and compare these distributions with spacecraft observations.

Ion acceleration at dipolarization fronts in the inner magnetosphere

“…Second, as was previously mentioned, observations of ion injections suggest that He ++ ions are accelerated to the energies approximately twice larger than H + and He + ions, whereas in our simulation the difference in acceleration is not as large. Third, similar to previous studies [e.g., Artemyev et al, 2012;Greco et al, 2014], the magnetic perturbation of the front was approximated with a soliton-like quasi-stationary structure propagating earthward relative to global magnetospheric convection. A fully self-consistent model of dipolarization fronts in the inner magnetosphere is required to validate this assumption.…”

“…Second, similar to recent studies [e.g., Artemyev et al, 2012;Greco et al, 2014], we assume that the front propagates radially inward:…”

Heavy ion acceleration at dipolarization fronts in planetary magnetotails

“…We consider a two‐dimensional (2‐D) configuration of the DF containing the main magnetic field component B z and two components of the electric field E x , E y which only depend on the coordinates ( x , y ). Furthermore, the DF propagates along the x axis with a constant velocity v ϕ (see schematic view of DF configuration and system geometry in Zhou et al [] and Greco et al []). We introduce dimensionless coordinates ϕ = ( x − v ϕ t )/ L , ψ = y / R , where L is the DF magnetic ramp thickness and R is the DF half width.…”

“…To determine the components of the DF electromagnetic fields, we separate variables ϕ and ψ and solve Maxwell's equations for given b ( ϕ ) function describing the profile of the front magnetic field B z along x axis. In this case, DF magnetic field component takes the form , where B max is the amplitude of the DF magnetic field, parameter k = π L /2 R defines the relation between DF scales along x and y axes, and ψ ∈[−1,1] (see Appendix in Greco et al []). The relevant components of DF electric field E = E x e x + E y e y are found from Maxwell equations: [∇× E ] = − ( ∂ B z / ∂ t ) e z and (∇· E ) = 0 (absence of free charges at DF; see discussion in Greco et al []).…”

Proton and heavy ion acceleration by stochastic fluctuations in the Earth's magnetotail

“…With regard to the geospace environment once more, ion acceleration at dipolarization fronts in the geomagnetic tail has been studied, among others, by Ashour-Abdalla et al (2011), Ukhorskiy et al (2013), Birn and Hesse (2014), Greco et al (2014) and Greco et al (2015). Perri et al (2009) and Greco et al (2010) considered the combined effect of a steady-state dawn-dusk electric field and of stochastic Fermi acceleration due to the presence of transient magnetic structures: by performing a two-dimensional (2-D) test particle simulation, they showed that proton energies of up to 80-100 keV can be reached, in agreement with the above observations.…”