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\begin{document}

\title{A survey of 25 years' transpolar voltage data from the SuperDARN radar
network and the Expanding-Contracting Polar Cap model}



\author[1,1]{Michael Lockwood}%
\author[2,2]{Kathryn A McWilliams}%
\affil[1]{University of Reading}%
\affil[2]{University of Saskatchewan}%


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  \date{May 27, 2024}


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\selectlanguage{english}
\begin{abstract}
We use 214410 hourly observations of the transpolar voltage,
\selectlanguage{greek}Φ\selectlanguage{english}\textsubscript{PC}, from 25 years of observations by the SuperDARN
radars, to confirm the central tenet of the Expanding-Contracting Polar
Cap (ECPC) model of ionospheric convection that \selectlanguage{greek}Φ\selectlanguage{english}\textsubscript{PC}
responds to both dayside and nightside reconnection voltages
(\selectlanguage{greek}Φ\selectlanguage{english}\textsubscript{D} and \selectlanguage{greek}Φ\selectlanguage{english}\textsubscript{N}). We show \selectlanguage{greek}Φ\selectlanguage{english}\textsubscript{PC}
increases at a fixed level of nightside auroral electrojet AL index with
increasingly southward IMF (identifying the well-known effect of
\selectlanguage{greek}Φ\selectlanguage{english}\textsubscript{D} on \selectlanguage{greek}Φ\selectlanguage{english}\textsubscript{PC}) but also with increasingly
negative AL at a fixed southward IMF (identifying a distinct effect of
\selectlanguage{greek}Φ\selectlanguage{english}\textsubscript{N} on \selectlanguage{greek}Φ\selectlanguage{english}\textsubscript{PC} ). We study the variation of
\selectlanguage{greek}Φ\selectlanguage{english}\textsubscript{PC} with time elapsed \selectlanguage{greek}Δ\selectlanguage{english}t since the IMF last pointed
southward and show that low/large values occur when -AL is small/large.
We have to allow for the fact that at lower numbers of radar echoes,
n\textsubscript{e} , the matched potential re-analysis technique used to
derive is influenced by the model used: this is done by a sensitivity
study of the threshold of n\textsubscript{e} required. We show that for
any threshold \selectlanguage{greek}Φ\selectlanguage{english}\textsubscript{PC} falls to about 15kV for \& \selectlanguage{greek}Δ\selectlanguage{english}t greater
than about 15 hours giving an upper limit to the viscous-like voltage.
It is shown that both \selectlanguage{greek}Φ\selectlanguage{english}\textsubscript{PC} and -AL
\selectlanguage{greek}\textsubscript{increase with increased solar wind dynamic pressure
p\textsubscript{sw} , but not as much as the mid-latitude geomagnetic
range index am. We conclude p\textsubscript{sw} increases both
Φ\textsubscript{D} and Φ\textsubscript{N} through increasing the
magnetic shear across the relevant current sheet but has a bigger effect
on mid-latitude geomagnetic activity indices via the additional energy
stored in the tail lobes.}\selectlanguage{english}%
\end{abstract}%



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