Outflow of low-energy O⁺ ion beams observed during periods without substorms

Abstract. Numerous observations have shown that ions flow out of the ionosphere during substorms with more fluxes leaving as the substorm intensity increases (Wilson et al., 2004). In this article we show observations of low-energy (few tens of electron volts) ionospheric ions flowing out periods without substorms, determined using the Wideband Imaging Camera (WIC) and Auroral Electrojet (AE) indices. We use Cluster ion composition data and show the outflowing ions are field-aligned H⁺, He⁺ and O⁺ beams accelerated to energies of ~40–80 eV, after correcting for spacecraft potential. The estimated fluxes of the low-energy O⁺ ions measured at ~20 000 km altitude are >10³–10⁵ cm⁻² s. Assuming the auroral oval is the source of the escaping ions, the measured fluxes correspond to a flow rate of ~10¹⁹–10²¹ ions s⁻¹ leaving the ionosphere. However, periods without substorms can persist for hours suggesting the low-energy ions flowing out during these times could be a major source of the heavy ion population in the plasma sheet and lobe.