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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.


Equatorial profiles of total magnetic field and field lines of model dipolarization front at two locations in the magnetotail where the ambient magnetic field is (a) weaker and (b) stronger than the negative depletion of the field associated with the front. The reconnecting line in Figure 3a is shown in red.
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fig03: Equatorial profiles of total magnetic field and field lines of model dipolarization front at two locations in the magnetotail where the ambient magnetic field is (a) weaker and (b) stronger than the negative depletion of the field associated with the front. The reconnecting line in Figure 3a is shown in red.

Mentions: [21] Depending on the value of the ambient magnetic field, the Bz depletion of a dipolarization front may or may not cause reconnection ahead of the front. Both cases are illustrated in Figure 3 showing the superposition of the front described in previous section with the T96 magnetic field at different locations in the tail. Figure 3a corresponds to the front at x=−16RE. The ambient magnetic field there is B0=3.35 nT and the front is therefore preceded by a region of negative Bz bounded by two neutral points. The bottom part of Figure 3a shows magnetic field lines in the meridional plane. The reconnecting field line is marked in red. Similarly, Figure 3b shows the front at x=−12RE where the ambient magnetic field of B0=6.28 nT exceeds the magnitude of the negative dip ahead of the front. Consequently, the front in this case depletes the field but does not cause reconnection. Before conducting full three-dimensional simulations, we analyze proton dynamics in these two cases in two dimensions restricting our consideration to the equatorially mirroring particles.


Rapid acceleration of protons upstream of earthward propagating dipolarization fronts.

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

Equatorial profiles of total magnetic field and field lines of model dipolarization front at two locations in the magnetotail where the ambient magnetic field is (a) weaker and (b) stronger than the negative depletion of the field associated with the front. The reconnecting line in Figure 3a is shown in red.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig03: Equatorial profiles of total magnetic field and field lines of model dipolarization front at two locations in the magnetotail where the ambient magnetic field is (a) weaker and (b) stronger than the negative depletion of the field associated with the front. The reconnecting line in Figure 3a is shown in red.
Mentions: [21] Depending on the value of the ambient magnetic field, the Bz depletion of a dipolarization front may or may not cause reconnection ahead of the front. Both cases are illustrated in Figure 3 showing the superposition of the front described in previous section with the T96 magnetic field at different locations in the tail. Figure 3a corresponds to the front at x=−16RE. The ambient magnetic field there is B0=3.35 nT and the front is therefore preceded by a region of negative Bz bounded by two neutral points. The bottom part of Figure 3a shows magnetic field lines in the meridional plane. The reconnecting field line is marked in red. Similarly, Figure 3b shows the front at x=−12RE where the ambient magnetic field of B0=6.28 nT exceeds the magnitude of the negative dip ahead of the front. Consequently, the front in this case depletes the field but does not cause reconnection. Before conducting full three-dimensional simulations, we analyze proton dynamics in these two cases in two dimensions restricting our consideration to the equatorially mirroring particles.

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.