Formation of Alfvén Wave Ducts by Magnetotail Flow Bursts

Abstract

Geomagnetic activity in Earth's outer magnetosphere stimulates intense electromagnetic power flows that propagate to the high-latitude ionosphere as low-loss shear Alfvén waves-a type of magnetically guided, incompressible magnetohydrodynamic (MHD) wave (Alfvén, 1942;Hasegawa & Uberoi, 1982). Injection of Alfvénic power into Earth's Polar Regions has several important consequences. Alfvén waves signal the coming arrival in the ionosphere of disturbances in flows and magnetic fields in the outer reaches of the magnetosphere (Ferdousi & Raeder, 2016). Alfvén wave-induced electron precipitation produces spectacular dynamic aurora (Kataoka et al., 2021;Keiling et al., 2003). The waves contribute to the energization and exodus of heavy ions (principally O + ) from the ionosphere into space (Chaston et al., 2004;Hatch et al., 2016;Hull et al., 2019). And their collisional absorption in the cusp-region ionosphere-thermosphere locally heats and upwells the upper atmosphere (Dessler, 1959;Hogan et al., 2021;Tu et al., 2011), contributing to enhanced satellite drag in low-earth orbit. The distribution, intensity, and causality of low-altitude Alfvénic energy deposition thus underlies many space weather phenomena.The pattern of Alfvénic Poynting flux flowing to low altitude at frequencies exceeding 5.5 mHz has been termed the "Alfvénic oval" (Keiling, 2021). Its statistical distribution inferred from satellite measurements depends on the level of geomagnetic activity and has the form of an irregular, ≈10° wide band in magnetic latitude (MLAT) with nonuniform intensity in magnetic local time (MLT). The average Poynting flux is most persistent on the

Alfvén-fast wave coupling in a 3D non-uniform medium

“…These results are consistent with observations of Alfvén waves which show they are able to maintain their identity well when propagating through nonuniform plasmas, for example, in the Jovian magnetosphere, there is strong Alfvén wave activity at the foot point of the Io-Jupiter flux tube which is present with decreasing intensity throughout the footprint tail (Gershman et al, 2019). Other simulations have shown Alfvén waves propagating with high efficiency in the magnetosphere through the formation of Alfvén ducts which guide the Alfvén wave along a flux tube (Yang et al, 2022). Section 3.3 shows that the results of the earlier Sections 3.1 and 3.2, are not an isolated special case but are also observed where the extent of the nonuniform region differs.…”

Alfvén‐Fast Wave Coupling in a 2D Nonuniform Medium

“…These results are consistent with observations of Alfvén waves which show they are able to maintain their identity well when propagating through nonuniform plasmas, for example, in the Jovian magnetosphere, there is strong Alfvén wave activity at the foot point of the Io‐Jupiter flux tube which is present with decreasing intensity throughout the footprint tail (Gershman et al., 2019). Other simulations have shown Alfvén waves propagating with high efficiency in the magnetosphere through the formation of Alfvén ducts which guide the Alfvén wave along a flux tube (Yang et al., 2022).…”

“…As these waves propagate from the magneto-tail to the ionosphere, through several pronounced density gradients, it is likely that Alfvén-fast wave coupling will occur. This process may also be relevant to the formation of Alfvén wave ducts, which has been shown using global geospace simulations (Yang et al, 2022). Alfvén ducts are field-aligned channels that allow efficient transmission of Alfvénic energy due to their relatively uniform and low Alfvén wave conductance.…”