Effects of Ion Slippage in Earth's Ionosphere and the Plasma Sheet

Abstract

Although triggering mechanisms for substorm onsets remain highly controversial, consensus has reached that violation of frozen‐in flux condition in the central plasma sheet is required. In this study, we carry out a numerical gedanken experiment to investigate the effects of the violation by assuming ions slip with respect to the magnetic field lines in the late substorm growth phase while electrons remain magnetized, without specifying the microphysics. The simulation results predict (1) a thin arc and a strong westward electrojet associated with downward‐upward‐downward field‐aligned currents and westward‐eastward‐westward horizontal flows in the ionosphere, which are found to be consistent with a preonset arc observed by the Swarm and the all‐sky imager; (2) a rapid creation of a bubble‐blob pair in the plasma sheet with a tailward hump of Bz that may lead to tearing instability.

Current Sheet Thinning in the Wake of a Bubble Injection

“…It also has the advantage to investigate the transient fast flow with designated injection interval by changing the boundary conditions, which remains a challenge for standalone MHD codes. Similar to our previous work (e.g., Wang et al., 2021; Yang et al., 2011), we set up the initial and boundary conditions using the empirical magnetic field (Tsyganenko, 1989) and plasma (Tsyganenko & Mukai, 2003) models driven by typical solar wind and geomagnetic conditions, that is, IMF B Z = –3.0 nT, solar wind number density N = 5.0 cm −3 , velocity V = 400 km/s, Kp = 1.0, cross‐polar‐cap potential drops = 50 kV. As most bursty bulk flows (BBFs) are found between the mid‐tail region and the near‐Earth region (−30 < X < −10 Re) and with the velocity greater than 300 km/s (e.g., Angelopoulos et al., 1994; Ohtani et al., 2004), we inject the bubble from the tail at −30 Re following 30 min of adiabatic convection.…”

“…It is also notable that although the stretching behind the bubble has been formulated earlier by the ion slippage scenario (Wang et al, 2021), it differs from this study in many aspects. The frozen-in flux condition is violated in the ion slippage for the generation of the bubble-blob pair, resulting in a Hall electric current in the plasma sheet closing the downward FAC associated with the blob and the upward FAC associated with the bubble.…”

“…Interchange instability, which is critically dependent on the gradient of the flux‐tube entropy PV 5/3 (here P is the plasma pressure and V is the flux tube volume per unit magnetic flux), is found to play an important role in the injection process. The bubble‐blob scenario and its refined form, the ion slippage scenario, were used to explore the stretching and thinning in the wake (Hu et al., 2011; Sitnov & Swisdak, 2011; Wang et al., 2021; Yang et al., 2011). Although these ideas reproduced current sheet thinning and the pre‐onset auroral arc in the ionosphere, they require a pair of low PV 5/3 bubble and high PV 5/3 blob moving in opposite directions, which has not been observationally confirmed so far.…”