On the Asymmetry Between Upward and Downward Field‐Aligned Currents Interacting With the Ionosphere

Abstract

The paper presents results from the numerical study of the magnetosphere-ionosphere interactions driven by the large-scale electric field in the magnetically conjugate, high-latitude regions of northern and southern hemispheres. Simulations of the two-fluid MHD model demonstrate that these interactions can lead to a generation of a system of small-scale, intense field-aligned currents with a significant difference in size and amplitude between the upward and downward currents. In particular, in both hemispheres, the downward currents (where the electrons are flowing from the ionosphere) become more narrow and intense than the adjacent upward currents. At high latitudes, the field-aligned currents are closely related to the discrete auroral arcs. The fact that this mechanism produces very narrow and intense downward currents embedded into the broader upward current regions makes it relevant to the explanation of the "black" auroral arcs appearing as narrow, dark strips embedded in the broad luminous background.

Resonant Alfvén Waves in the Lower Auroral Ionosphere: Evidence for the Nonlinear Evolution of the Ionospheric Feedback Instability

“…Second, the spiky peaks of the electric field coincide with the regions of plasma density depletions. This is similar to the numerical results of Lysak and Song (2002), but in contrast to the simulation results presented in Figure 3 of Streltsov (2018). While in the steady‐state the downward j ∥ (corresponding to upward motion of ionospheric electrons) is expected to peak at the location of the maximum density gradient, this is not necessarily the case in a dynamic situation.…”

ULF Waves Generated Near the Plasmapause by the Magnetosphere‐Ionosphere Interactions

“…The important feature of the simulated electric field illustrated in Figures e and f is that it reproduce reasonably well the “spiky” character of the small‐scale electric fields observed at low altitudes by the DMSP satellite (Figure d). It has been shown by Streltsov () that these “spikes” are the characteristic feature of the small‐scale electric fields produced by the IFI on its latter, strongly nonlinear stage of the development. Because one of the main goal of this study was to reproduce DMSP observations using the background parameters from CRRES, the paper shows the results from the simulation when strong “spikes” appears in the electric field at low altitudes, at this time the “spiky” character of the field also becomes visible in the equatorial magnetosphere.…”

“…This field contains large‐amplitude positive spikes: If the field is detrended by removing the low‐frequency part, then the amplitude of the positive peaks in will be 3–5 times larger than the amplitude of the neighboring negative peaks. This is a characteristic feature of the small‐scale electric fields produced by the IFI simultaneously developing in two magnetically conjugate regions in the ionosphere (Streltsov, ). These fields are associated with the narrow, downward field‐aligned currents interacting with the ionosphere.…”

“…We use a two‐fluid MHD model recently used to simulate ULF waves detected near the plasmapause by Van Allen Probes satellites by Streltsov and Mishin (). The model has been discussed in detail in several papers about the IFI (e.g., Streltsov & Lotko, ; Streltsov, ), and it will be mentioned here briefly for completeness. It consists of the electron parallel momentum equation, the density continuity equation, and the current continuity equation describing dispersive Alfvén waves in the low‐ magnetospheric plasma.…”

“…These waves can be amplified by the feedback interactions with the ionosphere, if the necessary conditions for the development of the ionospheric feedback instability (IFI) are satisfied. IFI has been extensively studied for almost 50 years (e.g., Atkinson, ; Lysak, ; Lysak & Song, ; Miura & Sato, ; Pokhotelov et al, ; Russell et al, ; Streltsov & Lotko, ; Streltsov, ; Sato, ; Trakhtengertz & Feldstein, , ; Watanabe et al, ), and, currently, it is considered one of the most probable mechanisms for the generation of small‐scale, intense ULF waves at high latitudes.…”