On the origin of pressure and magnetic perturbations ahead of dipolarization fronts

Abstract

Dipolarization fronts (DFs), earthward-propagating structures in the Earth's magnetotail current sheet with sharp enhancements of the northward magnetic field B z , are typically preceded by minor decreases in B z . Other characteristic DF precursor signatures, including earthward flows and plasma density/pressure enhancements, have been explained in the context of ion acceleration and reflection at dipolarization fronts. In the same context here we simulate the spatial distribution of plasma pressure earthward of a convex DF. The resultant pressure distribution, which shows clear dawn-dusk asymmetries with greater enhancements at the DF duskside, agrees with statistical observations. The simulation further reveals that the reflected ions can carry a secondary current earthward of the advancing DF, which explains the characteristic signature of the B z dip immediately ahead of the DF.

Reconnection flow jets in 3D as a source of structured dipolarization fronts

“…In contrast, the pressure enhancement is much larger (up to five to six times the original value) and is confined to the central portion (30 y/d i 38) and extends much further downstream than does the There are also considerably weaker enhancements in both P ixx and the density immediately ahead of the secondary front. Some of these pressure and density features are consistent with the calculations of Zhou et al 2014aZhou et al , 2014b regarding the role of ion reflection in the asymmetric breaking and dawnward deflection of dipolarization fronts. However, as shown in Fig.…”

Energetic ions in dipolarization events

“…Figure 4 also shows the shift of precursor ion fluxes at energies above ∼ 10 keV, which tend to start at ≈ −60 ∘ and shift to larger values of at later times. This shift can be easily understood from the mechanism of ion reflection and acceleration in the dipolarization front [Zhou et al, , 2014, which in its basics we also confirm. If an ion is reflected into the low electric field region ahead of the front, the first signal of accelerated ions should be seen at −90 ∘ , followed by increasing values of up to 90 ∘ near the front, if the front were perpendicular to its earthward propagation direction.…”

“…If an ion is reflected into the low electric field region ahead of the front, the first signal of accelerated ions should be seen at −90 ∘ , followed by increasing values of up to 90 ∘ near the front, if the front were perpendicular to its earthward propagation direction. This result can be modified by an inclination or curvature of the front and a different propagation direction [Zhou et al, 2014]. Another important factor is the fact that the electric field ahead of the front does not vanish but is finite, consistent with the finite flow speed.…”

“…In a number of papers, Zhou et al [2010Zhou et al [ , 2011Zhou et al [ , 2012aZhou et al [ , 2014 have emphasized observations of energetic ions of a few tens of keV preceding the arrival of dipolarization fronts, coincident with an increase of the plasma density and flow velocity. They attributed these observations to ion reflection and acceleration at the propagating front and supported this view with test particle investigations in a simplified model.…”

“…(The latter was modified by Zhou et al [2014], who assumed a circular shape of the DF in the equatorial plane.) The former precludes the transport of particles from the rear to the front, while the latter precludes entry into the acceleration region from the side, both of which were found to play a role in our simulations [e.g., Birn et al, 2013].…”