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Rapid acceleration of protons upstream of earthward propagating dipolarization fronts.

Ukhorskiy AY, Sitnov MI, Merkin VG, Artemyev AV - J Geophys Res Space Phys (2013)

Bottom Line: According to our numerical simulations, both trapping and quasi-trapping can produce rapid acceleration of protons by more than an order of magnitude.Quasi-trapping does not cause particle scattering out of the equatorial plane.Energization levels in this case are limited by the number of encounters particles have with the front before they get magnetized behind it.

View Article: PubMed Central - PubMed

Affiliation: The Johns Hopkins University Applied Physics Laboratory, Laurel Maryland, USA.

ABSTRACT

[1] Transport and acceleration of ions in the magnetotail largely occurs in the form of discrete impulsive events associated with a steep increase of the tail magnetic field normal to the neutral plane (B z ), which are referred to as dipolarization fronts. The goal of this paper is to investigate how protons initially located upstream of earthward moving fronts are accelerated at their encounter. According to our analytical analysis and simplified two-dimensional test-particle simulations of equatorially mirroring particles, there are two regimes of proton acceleration: trapping and quasi-trapping, which are realized depending on whether the front is preceded by a negative depletion in B z . We then use three-dimensional test-particle simulations to investigate how these acceleration processes operate in a realistic magnetotail geometry. For this purpose we construct an analytical model of the front which is superimposed onto the ambient field of the magnetotail. According to our numerical simulations, both trapping and quasi-trapping can produce rapid acceleration of protons by more than an order of magnitude. In the case of trapping, the acceleration levels depend on the amount of time particles stay in phase with the front which is controlled by the magnetic field curvature ahead of the front and the front width. Quasi-trapping does not cause particle scattering out of the equatorial plane. Energization levels in this case are limited by the number of encounters particles have with the front before they get magnetized behind it.

No MeSH data available.


Examples of two different types of proton trajectories in the case when the front is preceded by a negative Bz depletion. Particle energy is indicated with color.
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fig08: Examples of two different types of proton trajectories in the case when the front is preceded by a negative Bz depletion. Particle energy is indicated with color.

Mentions: [33] In the first simulation, protons were initiated at x0=−16RE and the front was launched from x=−16.61RE, such that particles were initially located at the reconnection point ahead of the front, where according to our two-dimensional analysis they can be stably trapped. Figure 8a shows distribution of final proton energy computed as a function of the initial gyrophase (ψ0). There are two groups of particles in the distribution. Particles with the initial gyrophase around π (i.e., those initially moving in the direction opposite to the front) penetrate into the region of the increased magnetic field behind the front deeper than their gyroradius. These particles get magnetized behind the front and E×B drift toward Earth. They fall behind the front due to the difference between the front velocity u and the velocity of their E×B motion, uE≃uB1/(B0+B1), until the electric field at their location diminishes such that the gradient-curvature drift becomes prevalent at which point they separate from the front. These particles exhibit no or weak acceleration.


Rapid acceleration of protons upstream of earthward propagating dipolarization fronts.

Ukhorskiy AY, Sitnov MI, Merkin VG, Artemyev AV - J Geophys Res Space Phys (2013)

Examples of two different types of proton trajectories in the case when the front is preceded by a negative Bz depletion. Particle energy is indicated with color.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4497486&req=5

fig08: Examples of two different types of proton trajectories in the case when the front is preceded by a negative Bz depletion. Particle energy is indicated with color.
Mentions: [33] In the first simulation, protons were initiated at x0=−16RE and the front was launched from x=−16.61RE, such that particles were initially located at the reconnection point ahead of the front, where according to our two-dimensional analysis they can be stably trapped. Figure 8a shows distribution of final proton energy computed as a function of the initial gyrophase (ψ0). There are two groups of particles in the distribution. Particles with the initial gyrophase around π (i.e., those initially moving in the direction opposite to the front) penetrate into the region of the increased magnetic field behind the front deeper than their gyroradius. These particles get magnetized behind the front and E×B drift toward Earth. They fall behind the front due to the difference between the front velocity u and the velocity of their E×B motion, uE≃uB1/(B0+B1), until the electric field at their location diminishes such that the gradient-curvature drift becomes prevalent at which point they separate from the front. These particles exhibit no or weak acceleration.

Bottom Line: According to our numerical simulations, both trapping and quasi-trapping can produce rapid acceleration of protons by more than an order of magnitude.Quasi-trapping does not cause particle scattering out of the equatorial plane.Energization levels in this case are limited by the number of encounters particles have with the front before they get magnetized behind it.

View Article: PubMed Central - PubMed

Affiliation: The Johns Hopkins University Applied Physics Laboratory, Laurel Maryland, USA.

ABSTRACT

[1] Transport and acceleration of ions in the magnetotail largely occurs in the form of discrete impulsive events associated with a steep increase of the tail magnetic field normal to the neutral plane (B z ), which are referred to as dipolarization fronts. The goal of this paper is to investigate how protons initially located upstream of earthward moving fronts are accelerated at their encounter. According to our analytical analysis and simplified two-dimensional test-particle simulations of equatorially mirroring particles, there are two regimes of proton acceleration: trapping and quasi-trapping, which are realized depending on whether the front is preceded by a negative depletion in B z . We then use three-dimensional test-particle simulations to investigate how these acceleration processes operate in a realistic magnetotail geometry. For this purpose we construct an analytical model of the front which is superimposed onto the ambient field of the magnetotail. According to our numerical simulations, both trapping and quasi-trapping can produce rapid acceleration of protons by more than an order of magnitude. In the case of trapping, the acceleration levels depend on the amount of time particles stay in phase with the front which is controlled by the magnetic field curvature ahead of the front and the front width. Quasi-trapping does not cause particle scattering out of the equatorial plane. Energization levels in this case are limited by the number of encounters particles have with the front before they get magnetized behind it.

No MeSH data available.