Timescales of Dayside and Nightside Field‐Aligned Current Response to Changes in Solar Wind‐Magnetosphere Coupling

Abstract

Principal component analysis is performed on Birkeland or field‐aligned current (FAC) measurements from the Active Magnetosphere and Planetary Electrodynamics Response Experiment, to determine the response of dayside and nightside FACs to reversals in the orientation of the interplanetary magnetic field (IMF) and the occurrence of substorms. Dayside FACs respond promptly to changes in IMF BY, but the nightside response is delayed by up to an hour and can take up to 4 hr to develop fully, especially during northward IMF. Nightside FAC asymmetries grow during substorm growth phase when the IMF has a significant BY component, and also promptly at substorm onset. Our findings suggest that magnetotail twisting and/or BY penetration into the magnetotail, due to subsolar reconnection with east‐west orientated IMF, are the main cause of these nightside FAC asymmetries and that asymmetries also arise due to magnetotail reconnection of these twisted field lines.

A Deep Learning‐Based Approach for Modeling the Dynamics of AMPERE Birkeland Currents

“…These time scales are in agreement with Milan et al. (2018), who used principal component analysis and reported that NBZ currents typically take between 45 and 90 min to fully develop. In comparison, McPherron et al.…”

“…We can note two main features from our model's response to the “simulated” substorm interval. First, we find that the nightside currents intensify and expand, in immediate response to a drop in SML index and later subside during the “simulated” recovery phase, beginning at T Pred = 40 min, as expected during a substorm interval due to an increase in nightside reconnection rate (Clausen, Milan, et al., 2013; Milan et al., 2018). Second, we find an increase in the area of the polar cap/R1‐oval indicating an increase (Clausen, Baker, et al., 2013) after the onset of a substorm, which later recedes back to its original state during the recovery phase.…”

“…(1966). Since then, data from magnetometers on several satellite missions such as Dynamics Explorer 2 (Weimer, 2001), Orsted/Magsat (Papitashvili et al., 2002), DMSP (Ohtani et al., 2005), and Challenging Minisatellite Payload (CHAMP) (Laundal, Finlay, et al., 2018; Laundal, Reistad, et al., 2018) have provided crucial insights into the nature of solar wind‐magnetosphere‐ionosphere coupling (e.g., Iijima & Potemra, 1976a; Lysak, 1990; Milan et al., 2018). For example, the large‐scale structure and magnitude of FACs are strongly controlled by the interplanetary magnetic field (IMF) and season (e.g., Fujii et al., 1981; Iijima & Potemra, 1976a, 1978; Potemra, 1985).…”

“…They further reported that the initial response is strongest near noon, whereas the response on the nightside grew stronger after an initial delay of ∼10 min. Clearly, the response of high‐latitude ionosphere to changes in IMF is still an active area of research, and several key aspects such as the reconfiguration times (Milan et al., 2018; Moretto et al., 2018) associated with the Birkeland currents are yet to be determined.…”

Interplanetary Magnetic Field Control of Polar Ionospheric Equivalent Current System Modes

“…This is the well‐known directly driven response of the electrojets to the IMF (Kamide & Kokubun, ). At longer lags, we find that the DP2 and DP1 responses are more strongly correlated to the IMF variations than the initial response, despite being presumably less directly driven (e.g., Milan et al, ; Weimer, ). This seems counterintuitive but can be explained via an ionospheric conductivity feedback.…”