Feedback Interactions Between the Ionosphere and Magnetosphere at Middle Latitude

Abstract

Ultra-low frequency (ULF) electromagnetic waves carry significant fluxes of the electromagnetic power, mass, and momentum in the highly coupled solar wind-magnetosphere-ionosphere system. Their role in the global energy exchange increases many times during magnetically active times like the geomagnetic storms and substorms. This is one of the main reasons why these waves and the associated field-aligned currents are extensively studied at high latitudes in connection with the discrete auroral arcs and other non-luminous phenomena.One of the most frequently considered mechanisms responsible for the generation of the large-amplitude ULF waves is the global magnetospheric field-line resonator (FLR) formed by the magnetic field line bounded by the conjugate locations in the ionosphere. Shear Alfvén waves can form a standing pattern between these locations. If there is an external or internal driver that provides power with a frequency matching one of the eigenfrequencies of the system, then the resonator can produce large-amplitude ULF waves. This idea had been proposed in classical papers by Cummings et al. (1969);Southwood (1974) and extensively studied after that, for example, Chen and Hasegawa (1974); Streltsov and Lotko (1995).The existence of the global magnetospheric resonator is strongly supported by the frequent observations obtained with ground magnetometers and radars, particularly, by these electromagnetic oscillations with discrete frequencies of 0.8, 1.3, 1.9, and 3.5 mHz in the night-side auroral zone (Fenrich et al., 1995;Samson, Harrold, et al., 1992). The same frequencies also have been detected in the field-aligned particle fluxes and luminosity of the discrete auroral arcs (Xu et al., 1993). The fact that at high altitudes these frequencies match the eigenfrequencies of the magnetic field lines stretched to the magnetotail (Lui & Cheng, 2001) provides a rationale to interpret them as a manifestation of the global field line resonator (Samson, Wallis, et al., 1992;Walker et al., 1992).FLR can be driven by several physical mechanisms. It can be a reconnection in the magnetotail (Angelopoulos et al., 2002), or the Kelvin-Helmholtz instabilities on the magnetopause or on the flanks of the magnetosphere in the so-called low-latitude boundary layers (Galinsky & Sonnerup, 1994;Marin et al., 2014). It can be energetic electron injections (Pilipenko et al., 2002) or feedback-active ionosphere-magnetosphere interactions driven by the electric field in the ionosphere (