Dayside and nightside magnetic field responses at 780 km altitude to dayside reconnection

Abstract

During southward interplanetary magnetic field, dayside reconnection will drive the Dungey cycle in the magnetosphere, which is manifested as a two‐cell convection pattern in the ionosphere. We address the response of the ionospheric convection to changes in the dayside reconnection rate by examining magnetic field perturbations at 780 km altitude. The Active Magnetosphere and Planetary Electrodynamics Response Experiment data products derived from the Iridium constellation provide global maps of the magnetic field perturbations. Cluster data just upstream of the Earth's bow shock have been used to estimate the dayside reconnection rate. By using a statistical model where the magnetic field can respond on several time scales, we confirm previous reports of an almost immediate response both near noon and near midnight combined with a 10–20 min reconfiguration time of the two‐cell convection pattern. The response of the ionospheric convection has been associated with the expansion of the polar cap boundary in the Cowley‐Lockwood paradigm. In the original formulation of this paradigm the expansion spreads from noon to midnight in 15–20 min. However, also an immediate global response has been shown to be consistent with the paradigm when the previous dayside reconnection history is considered. In this paper we present a new explanation for how the immediate response can be accommodated in the Cowley‐Lockwood paradigm. The new explanation is based on how MHD waves propagate in the magnetospheric lobes when newly reconnected open flux tubes are added to the lobes, and the magnetopause flaring angle increases.

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

“…The nightside FAC magnitude, 1 , also falls prior to t 0 and for the following 15 min, before rising over a period of 90 min before gradually diminishing; this variation closely mirrors the behavior of AL. The onset of the nightside response is almost as prompt as the dayside (the short delay is interpreted by Snekvik et al (2017) as the time taken for pressure perturbations to propagate through the lobes) but takes the onset of substorm activity and associated nightside reconnection for the response to maximize. As indicted in Figures 2j-2l, 4 > 0 is indicative of NBZ FACs associated with reverse lobe cells, and this situation develops in the lead up to t 0 ; it then takes until t 0 + 30 min for this signature to reverse.…”

“…These expansion and contraction phases should be accompanied by enhanced FACs on the dayside followed by enhanced FACs on the nightside (Milan, ), behavior that has been confirmed by Anderson et al (), Coxon et al (), and Milan et al (). Prompt responses of nightside convection and FACs to changes in dayside reconnection have also been reported, interpreted as the propagation time of pressure perturbations from the magnetopause to the neutral sheet through the lobes (e.g., Snekvik et al, , and references therein).…”

Solar Wind and Seasonal Influence on Ionospheric Currents From Swarm and CHAMP Measurements

“…Naively, this suggests using as high time resolution input as possible. However, we believe it is better to apply some time averaging because (1) even for the direct solar wind influence, it takes about 10–20 min before the global current patterns adapt to changes (Snekvik et al, ), (2) in situ high‐resolution measurements of the solar wind may not be a good representation of its large‐scale structure due to spatial variations/turbulence, and (3) the time shift from the solar wind monitor to the magnetopause will be imperfect.…”

On the origin of plasma sheet reconfiguration during the substorm growth phase

“…It then saturates for the rest of the growth phase (Figure c), indicating that the near‐Earth equatorial convection entered the new quasi‐stationary and significantly intensified regime (see Figure g for illustration). It is worth noting that both detected time scales of the simulated near‐Earth convection development (nearly instant reaction to the dayside reconnection and the 10–15 min reconfiguration) are consistent with time scales observed for ionospheric convection evolution as well as for variation of magnetic field perturbations measured at low‐altitude spacecraft at the nightside during the north‐to‐south IMF B z changes [see, e.g., Snekvik et al , , and references therein].…”