An Encounter With the Ion and Electron Diffusion Regions at a Flapping and Twisted Tail Current Sheet

Abstract

Soon after the discovery of the geomagnetic tail at the start of the space age (Ness, 1965), it was found that the tail can sometimes move rapidly in a north-south direction (Speiser & Ness, 1967). This flapping motion was deduced from a reversal in the polarity of the Earth-Sun component of geomagnetic tail field, B x , concomitant with a decrease in the total magnetic field strength. The typical duration of this up-down motion is a couple of minutes, with an amplitude of a few R E (Runov et al., 2009; Sergeev et al., 2003; Toichi & Miyazaki, 1976). Tail flapping is now a well-known phenomenon that has been repeatedly reported by spacecraft making observations close to the tail current sheet (CS, or neutral sheet (NS)) in the near-tail region (R ∼−15 to −30 R E). Not well established is what gives rise to it. Over the years both internal (see e.g.,

Electron‐Only Reconnection as a Transition From Quiet Current Sheet to Standard Reconnection in Earth's Magnetotail: Particle‐In‐Cell Simulation and Application to MMS Data

“…Therefore, Farrugia et al. (2021), using an initial configuration of the Harris current sheet, should have also seen the ion heating in their simulations (although they do not show). Following the above rationale, we plot j ⋅ E ′ and T i in the MMS 17 June 2017 event in Figures 5g and 5h, respectively.…”

“…However, Farrugia et al (2021) argue that this event is a standard reconnection event, and the reason why the spacecraft do not observe any ion flow increase in this event is that the spacecraft traverse close to the reconnection site where the ion outflow reverses. Farrugia et al (2021) also obtain a fast reconnection rate of ∼0.077 for the MMS 17 June 2017 event based on the reconnection electric field E M , which is close to the reconnection rate of standard reconnection. However, the reconnection electric field E M is usually tens of times smaller than the normal electric field E N (e.g., Torbert et al, 2018), therefore, even a small inaccuracy in the choice of the (L, M, N) coordinate system yields a large error of E M .…”

“…In this event, the MMS spacecraft observe fast electron flows in the L and M directions, whereas the ion flows do not increase. However, Farrugia et al (2021) argue that this event is a standard reconnection event, and the reason why the spacecraft do not observe any ion flow increase in this event is that the spacecraft traverse close to the reconnection site where the ion outflow reverses. Farrugia et al (2021) also obtain a fast reconnection rate of ∼0.077 for the MMS 17 June 2017 event based on the reconnection electric field E M , which is close to the reconnection rate of standard reconnection.…”

“…However, Farrugia et al. (2021) argue that this event is a standard reconnection event, and the reason why the spacecraft do not observe any ion flow increase in this event is that the spacecraft traverse close to the reconnection site where the ion outflow reverses. Farrugia et al.…”

“…Such a transition is the onset of magnetotail reconnection caused by the electron tearing mode instability (Pritchett, 2005a(Pritchett, , 2005b(Pritchett, , 2010Hesse & Schindler, 2001;Liu et al, 2014;Pritchett & Lu, 2018). However, Farrugia et al (2021) argue that the MMS 17 June 2017 event is not an electron-only reconnection event but a standard reconnection event. Given the above controversy, here we study the transition from quiet current sheet to electron-only reconnection and then to standard reconnection using a 2-D PIC simulation, and then we use the simulation results to show that the MMS 17 June 2017 event is an electron-only reconnection event.…”