On the Acceleration and Anisotropy of Ions Within Magnetotail Dipolarizing Flux Bundles

Abstract

Dipolarizing flux bundles (DFBs), earthward propagating structures with enhanced northward magnetic field Bz, are usually believed to carry a distinctly different plasma population from that in the ambient magnetotail plasma sheet. The ion distribution functions within the DFB, however, have been recently found to be largely controlled by the ion adiabaticity parameter κ in the ambient plasma sheet outside the DFB. According to these observations, the ambient κ values of 2–3 usually correspond to a strong perpendicular anisotropy of suprathermal ions within the DFB, whereas for lower κ values the DFB ions become more isotropic. Here we utilize a simple, test particle model to explore the nature of the anisotropy and its dependence on the ambient κ values. We find that the anisotropy originates from successive ion reflections and reentries to the DFB, during which the ions are consecutively accelerated in the perpendicular direction by the DFB‐associated electric field. This consecutive acceleration may be interrupted, however, when magnetic field lines are highly curved in the ambient plasma sheet. In this case, the ion trajectories become stochastic outside the DFB, which makes the reflected ions less likely to return to the DFB for another cycle of acceleration; as a consequence, the perpendicular ion anisotropy does not appear. Given that the DFB ions are a free energy source for instabilities when they are injected toward Earth, our simple model (that reproduces most observational features on the anisotropic DFB ion distributions) may shed new lights on the coupling process between magnetotail and inner magnetosphere.

Ion Anisotropy in Earth's Magnetotail Current Sheet: Multicomponent Ion Population

“…Observations of strong plasma flows, v xy > 100 km/s, are characterized by enhanced transverse anisotropy: for the entire energy range of E i / T i ∈ [1,100] or E i > 1 keV (and even E i > 100 eV for v xy > 500 km/s). This anisotropic ion population is likely generated within dipolarized flows (see, e.g., Runov et al, ), due to ion acceleration by transient electric fields, ∼ v x B z (Birn et al, ; Pritchett & Runov, ; Zhou et al, ). Indeed, Figures b and e show that the quiet magnetotail distribution (i.e., three‐population anisotropy) is observed for B z < 10 nT ( B z > 10 nT is the typical range of magnetic field intensity during dipolarizations in the magnetotail; see Fu et al, ; Liu et al, ; Runov et al, ).…”

“…In such a high‐ β plasma, the ion anisotropy required to balance the strong tension force j y B z (i.e., strong j y current) is only a few percent of the averaged pressure (stronger anisotropy would result in firehose and mirror instabilities anyway; see discussions in Noetzel et al, ; Vörös, ). However, even such weak anisotropy has only been observed in very disturbed CSs: around reconnection regions (Aunai et al,; Hietala et al, , ) or within dipolarized plasma flows (Birn et al, ; Cheng et al, ; Runov et al, ; Zhou et al, ). The quiet time magnetotail CS is generally characterized by almost (within the accuracy of measurements) isotropic ions (Stiles et al, ; Walsh et al, ).…”

Ion‐Beam‐Driven Intense Electrostatic Solitary Waves in Reconnection Jet

“…During reconnection processes, the initially separated plasmas become magnetically connected, and the connected plasmas get ejected from reconnection region, producing high‐speed plasma flows, commonly referred to as reconnection jets or burst bulk flows (Angelopoulos et al, ; Cao et al, , ). Reconnection jets have been suggested to be responsible for energy dissipation, particle heating, and acceleration in space plasmas (e.g., Khotyaintsev et al, ; Fu et al, ; Lapenta et al, ; Lu, Angelopoulos, et al, ; Zhou et al, ; Sitnov et al, ; Chen, Fu, Liu, et al, ; Zhao et al, ).…”

Electron‐Scale Measurements of Dipolarization Front

“…During their earthward propagation, strong particle and wave activity can occur in the vicinity of the DFs. For example, ambient ions can be efficiently accelerated due to reflection ahead of the DFs (e.g., Drake et al, 2014;Zhou et al, 2010Zhou et al, , 2011Zhou et al, , 2018; electrons can also be accelerated up to suprathermal energies inside the flux pileup regions (e.g., Fu et al, 2011;Khotyaintsev et al, 2011) or the dipolarizing flux bundles (e.g., behind the DFs, due to adiabatic processes (e.g., Birn et al, 2013;Fu et al, 2011;Gabrielse et al, 2012;Liu et al, 2009;Lu et al, 2016;Pan et al, 2012;Wu et al, 2013) or nonadiabatic processes (e.g., Huang et al, 2012;Hwang et al, 2014;Liu et al, 2016;Panov et al, 2013); several types of waves, such as lower hybrid drift waves (e.g., Khotyaintsev et al, 2011;Zhou et al, 2009), broadband high-frequency electrostatic waves (Yang et al, 2017) and whistler waves (e.g., Breuillard et al, 2016;Fu et al, 2014), have been widely reported near the DFs; pitch angle distribution (PAD) of suprathermal electrons can evolve dramatically around DFs (e.g., Fu, Khotyaintsev, Vaivads, André, Sergeev, et al, 2012;Liu, Fu, Xu, Wang, et al, 2017;Runov et al, 2013); Strong energy conversion can happen at the DF (e.g., Angelopoulos et al, 2013;Huang et al, 2015;Khotyaintsev et al, 2017;Lapenta et al, 2014;Liu et al, 2018;Yao et al, 2017), due to intense currents and electric fields.…”