Abstract
Abstract. Two-dimensional (2D) test particle simulations based on shock profiles issued from 2D full particle-in-cell (PIC) simulations are used in order to analyze the formation processes of ions back streaming within the upstream region after interacting with a quasi-perpendicular curved shock front. Two different types of simulations have been performed based on (i) a fully consistent expansion (FCE) model, which includes all self-consistent shock profiles at different times, and (ii) a homothetic expansion (HE) model in which shock profiles are fixed at certain times and artificially expanded in space. The comparison of both configurations allows one to analyze the impact of the front nonstationarity on the back-streaming population. Moreover, the role of the space charge electric field El is analyzed by either including or canceling the El component in the simulations. A detailed comparison of these last two different configurations allows one to show that this El component plays a key role in the ion reflection process within the whole quasi-perpendicular propagation range. Simulations provide evidence that the different field-aligned beam (FAB) and gyro-phase bunched (GPB) populations observed in situ are essentially formed by a Et×B drift in the velocity space involving the convective electric field Et. Simultaneously, the study emphasizes (i) the essential action of the magnetic field component on the GPB population (i.e., mirror reflection) and (ii) the leading role of the convective field Et in the FAB energy gain. In addition, the electrostatic field component El is essential for reflecting ions at high θBn angles and, in particular, at the edge of the ion foreshock around 70∘. Moreover, the HE model shows that the rate BI% of back-streaming ions is strongly dependent on the shock front profile, which varies because of the shock front nonstationarity. In particular, reflected ions appear to escape periodically from the shock front as bursts with an occurrence time period associated to the self-reformation of the shock front.
Highlights
While upstream ions of the incoming solar wind interact with the curved terrestrial bow shock, a certain percentage is reinjected back into the solar wind and propagates along the interplanetary magnetic field (IMF); they form the so-called ion foreshock
Even if we restrict ourselves to the quasi-perpendicular region, different types of back-streaming ions are identified, and (a) some are characterized by a gyrotropic velocity distribution and form the field-aligned ion beam population; (b) others exhibit a nongyrotropic velocity distribution and form the gyro-phase bunched ion population
It evidences approximately a distribution with a maximum slightly noncentered at v⊥ = 0. This distribution can be viewed as a mixing of gyrophase bunched (GPB) and field-aligned beam (FAB) populations, even if the GPB population with a pitch angle different zero is the largest one
Summary
While upstream ions of the incoming solar wind interact with the curved terrestrial bow shock, a certain percentage is reinjected back into the solar wind and propagates along the interplanetary magnetic field (IMF); they form the so-called ion foreshock. Even if we restrict ourselves to the quasi-perpendicular region (i.e., for 45◦ ≤ θBn ≤ 90◦, where θBn is the angle between the local shock normal and the IMF), different types of back-streaming ions are identified, and (a) some are characterized by a gyrotropic velocity distribution and form the field-aligned ion beam population (hereafter FAB); (b) others exhibit a nongyrotropic velocity distribution and form the gyro-phase bunched ion population (hereafter GPB) None of these populations yet have a wellestablished origin, and different mechanisms have been proposed for years (Möbius et al, 2001; Kucharek et al, 2004), including (i) scenarios based on the specular reflection The FAB population loses their initial phase coherency by suffering several bounces along the front, which is in contrast with the GPB population which suffers mainly one bounce (i.e., mirror reflection process) This important result was not expected and greatly simplifies the question on each population origin (Savoini and Lembège, 2015). What is the impact of the space charge electric field localized within the shock ramp on the reflection process?
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
