Outflow of low‐energy ions and the solar cycle

Abstract

Magnetospheric ions with energies less than tens of eV originate from the ionosphere. Positive low-energy ions are complicated to detect onboard sunlit spacecraft at higher altitudes, which often become positively charged to several tens of volts. We use two Cluster spacecraft and study low-energy ions with a technique based on the detection of the wake behind a charged spacecraft in a supersonic ion flow. We find that low-energy ions usually dominate the density and the outward flux in the geomagnetic tail lobes during all parts of the solar cycle. The global outflow is of the order of 10 26 ions/s and often dominates over the outflow at higher energies. The outflow increases by a factor of 2 with increasing solar EUV flux during a solar cycle. This increase is mainly due to the increased density of the outflowing population, while the outflow velocity does not vary much. Thus, the outflow is limited by the available density in the ionospheric source rather than by the energy available in the magnetosphere to increase the velocity.

Seasonal variations and north–south asymmetries in polar wind outflow due to solar illumination

“…Moreover, integrated over the polar cap area, these H + flux densities lead to fluxes similar to those found by other studies of the polar wind (see e.g. Nagai et al, 1984;Cully et al, 2003;Huddleston et al, 2005;Engwall et al, 2009;André et al, 2015).…”

“…Cully et al (2003) using Akebono data, however, observed an increase for both H + and O + , of ∼ 300 and ∼ 2000 %, respectively. Furthermore, Engwall et al (2009) and André et al (2015) found an increase of the ion total flux of ∼ 200 and ∼ 100 %, respectively. It is worth mentioning that Yau et al (1988) and Cully et al (2003) were restricted to an energy range of ∼ 10 eV-17 keV and ∼ 1-70 eV, respectively, and Engwall et al (2009) and André et al (2015), using an indirect method involving the spacecraft potential, should have theoretically observed all ions.…”

“…Furthermore, Engwall et al (2009) and André et al (2015) found an increase of the ion total flux of ∼ 200 and ∼ 100 %, respectively. It is worth mentioning that Yau et al (1988) and Cully et al (2003) were restricted to an energy range of ∼ 10 eV-17 keV and ∼ 1-70 eV, respectively, and Engwall et al (2009) and André et al (2015), using an indirect method involving the spacecraft potential, should have theoretically observed all ions. However, the last of these methods can only be used when the thermal energy of the ions is lower than both their bulk kinetic energy and the energy needed to overcome the spacecraft potential.…”

The Earth: Plasma Sources, Losses, and Transport Processes

“…A supersonic flow of positive low-energy ions can create an enhanced wake behind a positively charged spacecraft. Here the ions are diverted by the potential structure and not by the much smaller spacecraft (Engwall et al 2006(Engwall et al , 2009b(Engwall et al , 2009aAndré and Cully 2012;André et al 2015). The conditions for the enhanced wake formation sketched in Fig.…”

“…Using the Cluster dataset and this unique method to obtain low-energy ion fluxes in the night-side (X GSM < 0) André et al (2015) showed the occurrence rate of low-energy ions was 60-70 % in the lobes to out to X GSM of about 15 R E during all parts of the solar cycle, as indicated in Fig. 13.…”