Solar illumination control of ionospheric outflow above polar cap arcs

TL;DR

This study investigates how solar illumination influences ion outflows above polar cap arcs, revealing a dependence on solar zenith angle and local ionospheric conditions that affect ion fluxes and potential drops.

Contribution

It provides new insights into the relationship between solar illumination, ionospheric outflows, and electric fields above polar cap arcs using Cluster data.

Findings

O$^{+}$ flux higher above sunlit ionosphere

Dependence of ion flux on solar zenith angle

Low-magnitude potential drops occur above sunlit regions

Abstract

We measure the flux density, composition, and energy of outflowing ions above the polar cap, accelerated by quasi-static electric fields parallel to the magnetic field and associated with polar cap arcs, using Cluster. Mapping the spacecraft position to its ionospheric foot point, we analyze the dependence of these parameters on the solar zenith angle (SZA). We find a clear transition at SZA between We find a clear transition at SZA between $\sim$94$^{\circ}$ and $\sim$107$^{\circ}$, with the O$^{+}$ flux higher above the sunlit ionosphere. This dependence on the illumination of the local ionosphere indicates that significant O$^{+}$ upflow occurs locally above the polar ionosphere. The same is found for H$^{+}$, but to a lesser extent. This effect can result in a seasonal variation of the total ion upflow from the polar ionosphere. Furthermore, we show that low-magnitude field-aligned potential drops are preferentially observed above the sunlit ionosphere, suggesting a feedback effect of ionospheric conductivity.