Centrifugal acceleration at high altitudes above the polar cap: A Monte Carlo simulation

Abstract

A Monte Carlo simulation was used to study the outflow of O+ and H+ ions along three flight trajectories above the polar cap up to altitudes of about 15 RE. Barghouthi (2008) developed a model on the basis of altitude and velocity‐dependent wave‐particle interactions and a radial geomagnetic field which includes the effects of ambipolar electric field and gravitational and mirror forces. In the present work we improve this model to include the effect of the centrifugal force, with the use of relevant boundary conditions. In addition, the magnetic field and flight trajectories, namely, the central polar cap (CPC), nightside polar cap (NPC), and cusp, were calculated using the Tsyganenko T96 model. To simulate wave‐particle interactions, the perpendicular velocity diffusion coefficients for O+ ions in each region were determined such that the simulation results fit the observations. For H+ ions, a constant perpendicular velocity diffusion coefficient was assumed for all altitudes in all regions as recommended by Nilsson et al. (2013). The effect of centrifugal acceleration was simulated by considering three values for the ionospheric electric field: 0 (no centrifugal acceleration), 50, and 100 mV/m. It was found that the centrifugal acceleration increases the parallel bulk velocity and decreases the parallel and perpendicular temperatures of both ion species at altitudes above about 4 RE. Centrifugal acceleration also increases the temperature anisotropy at high altitudes. At a given altitude, centrifugal acceleration decreases the density of H+ ions while it increases the density of O+ ions. This implies that with higher centrifugal acceleration more O+ ions overcome the potential barrier. It was also found that aside from two exceptions centrifugal acceleration has the same effect on the velocities of both ions. This implies that the centrifugal acceleration is universal for all particles. The parallel bulk velocities at a given value of ionospheric electric field were highest in the cusp followed by the CPC followed by the NPC. In this study a region of no wave‐particle interaction was assumed in the CPC and NPC between 3.7 and 7.5 RE. In this region the perpendicular temperature was found to decrease with altitude due to perpendicular adiabatic cooling.

The fate of O<sup>+</sup> ions observed in the plasma mantle and cusp: particle tracing modelling and Cluster observations

“…They found that centrifugal forces increase the outflowing O + flux with 2 orders of magnitude when the convection electric field is enhanced from 0 mV/m to 100 mV/m. A similar result has been shown by Abudayyeh et al (2015), who used a Monte Carlo simulation based on the Tsyganenko T96 model and included the effects of the ambipolar electric field as well as gravitational and mirror forces. Additionally, Abudayyeh et al…”

“…At an altitude range of 1.2 Re to 15.2 Re, Barghouthi et al (2016) employed the same 1-D Monte Carlo model used by Abudayyeh et al (2015) to investigate energetic H + and O + outflows along two trajectories (from the polar cap to the cusp) and compared them with Cluster data. Considering the centrifugal acceleration, the ambipolar electric field and the waveparticle interaction, they concluded that the latter was the most important mechanism especially at higher altitudes (cusp).…”

O+ and H+ above the polar cap: Observations and semikinetic simulations

“…In the latter case the divergence of the geomagnetic field lines converts the perpendicular energy into the parallel direction which increases the parallel velocity. Several acceleration mechanisms were studied such as parallel potential drops [Lundin et al, 1995;Maggiolo et al, 2006], centrifugal acceleration [Cladis, 1986;Horwitz et al, 1994;Demars et al, 1996;Nilsson et al, 2008Nilsson et al, , 2010Abudayyeh et al, 2015a] and wave-particle interaction [Chang and Coppi, 1981;Chang et al, 1986;Retterer et al, 1987Retterer et al, , 1994; Barakat and Barghouthi, 1994;Barghouthi, 1997;Barghouthi et al, 1998;Bouhram et al, 2004;Barghouthi and Atout, 2006;Barghouthi, 2008;Barghouthi et al, 2011;Slapak et al, 2011;Waara et al, 2011Waara et al, , 2012Abudayyeh et al, 2015b].…”

“…where s is a vector along the magnetic field; V E = E × B∕B 2 is the convection velocity; andb is a unit vector in the direction of the magnetic field . For a detailed description of the effect of centrifugal acceleration on the outflow of ions along various flight trajectories above the polar cap refer to Nilsson et al [2008Nilsson et al [ , 2010 and Abudayyeh et al [2015a].…”

“…The model used in this study is a continuation to the model used in Abudayyeh et al [2015aAbudayyeh et al [ , 2015b. Abudayyeh et al [2015a] upgraded the Barghouthi model [Barghouthi, 1997;Barghouthi et al, 1998;Barghouthi, 2008;Barghouthi et al, 2011] to include the effect of centrifugal acceleration.…”