A Survey of 25 Years' Transpolar Voltage Data From the SuperDARN Radar Network and the Expanding‐Contracting Polar Cap Model. Issue 9 (27th August 2021)

• Record Type: Journal Article
• Authors: Lockwood, Mike; McWilliams, Kathryn A.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 9(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-08-27
• Subjects: magnetosphere -- ionosphere -- convection -- non‐steady state -- magnetic reconnection -- viscous‐like interaction; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2021JA029554
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: We use 214, 410 hourly observations of transpolar voltage, ΦPC, from 25 years' observations by the Super Dual Auroral Radar Network radars to confirm the central tenet of the expanding‐contracting polar cap model of ionospheric convection that ΦPC responds to both dayside and nightside reconnection voltages (ΦD and ΦN ). We show that ΦPC not only increases at a fixed level of the nightside auroral electrojet AL index with increasingly southward interplanetary magnetic field (IMF) (identifying the well‐known effect of ΦD on ΦPC ) but also with increasingly negative AL at a fixed southward IMF (identifying a distinct effect of ΦN on ΦPC ). We also study the variation of ΦPC with time elapsed since the IMF last pointed southward, Δt, and show that low/large values occur when (− AL ) is small/large. Lower numbers of radar echoes, n e, mean that the "map‐potential" reanalysis technique used to derive ΦPC is influenced by the model used: we present a sensitivity study of the effect of the threshold of n e required to avoid this. We show that for any threshold n e, ΦPC falls to about 15 kV for Δ t greater than about 15 h, indicating any viscous‐like voltage ΦV is considerably smaller than this. It is shown that both ΦPC and (− AL ) increase with increased solar wind dynamic pressure p SW, but not as much as the midlatitude geomagnetic index am . We conclude p SW increases both ΦD and ΦN through increasing the magnetic shear across the relevant current sheet but has a larger effect on midlatitude geomagnetic indices because of the effect of additional energy stored in the tail lobes. Plain Language Summary: Large‐scale circulation of the ionized upper atmosphere over the poles is driven by the solar wind flow by the process of magnetic reconnection that interconnects the magnetic field of Earth with that embedded in the solar wind flow. In long‐term averages, the rate at which this "open" magnetic flux is transferred away from the Sun and in the polar ionospheres is the same, but on shorter timescales the field can stretch between the two locations and this decouples the transfer rates. The expanding‐contracting polar cap model has been very useful in predicting the faster changes that can occur in the ionosphere, changes that can have an influence on a wide range of modern operational systems from satellites to power grids. Two key concepts that the model is based on were obtained from only very small data sets and here we use 214, 410 hourly observations from 25 years of observations by the Super Dual Auroral Radar Network to show that the concepts do apply all the time. Key Points: Transpolar voltage depends on both dayside and nightside reconnection voltages and both are enhanced by larger solar wind dynamic pressure Large voltages seen when the interplanetary magnetic field (IMF) points northward decay away with elapsed time since the IMF last pointed southward Ongoing nightside reconnection mimics the effect of proposed viscous‐like interactions which are shown to cause considerably less than 15 kV of voltage