Auroral ion outflow: low altitude energization

The SIERRA nightside auroral sounding rocket made observations of the origins of ion upflow, at topside F-region altitudes (below 700 km), comparatively large topside plasma densities (above 20 000/cc), and low energies (10 eV). Upflowing ions with bulk velocities up to 2 km/s are seen in conjunction with the poleward edge of a nightside substorm arc. The upflow is limited within the poleward edge to a region (a) of northward convection, (b) where Alfvénic and Pedersen conductivities are well-matched, leading to good ionospheric transmission of Alfvénic power, and (c) of soft electron precipitation (below 100 eV). Models of the effect of the soft precipitation show strong increases in electron temperature, increasing the scale height and initiating ion upflow. Throughout the entire poleward edge, precipitation of moderate-energy (100s of eV) protons and oxygen is also observed. This ion precipitation is interpreted as reflection from a higher-altitude, time-varying field-aligned potential of upgoing transversely heated ion conics seeded by the low altitude upflow. 1 Science background and outline Low altitude ion energization is one of the boundary conditions of magnetosphere/ionosphere coupling. The transfer of auroral energy through precipitation, Poynting flux, convection electric fields, and Joule heating to the cold lower ionosphere provides the initiation of a process that moves atmospheric oxygen out into the far magnetosphere. Sounding rockets are ideally situated for observing the low altitude Correspondence to: K. A. Lynch ([email protected]) signatures of auroral ion energization, providing in situ examples of both kinetic and fluid processes that can be used to constrain models of ion outflow. Their altitude range connects the low altitude fluid observations provided by radar studies to the high altitude in situ observations of spacecraft. The SCIFER (Arnoldy et al., 1996) and AMICIST (Lynch et al., 1996; Bonnell et al., 1996) experiments observed the low altitude signatures of broadband ELF (BBELF) transverse ion heating subsequently quantified by higher-altitude Freja (Andre et al., 1998; Knudsen et al., 1998a, b; Wahlund et al., 1998) and FAST observations (Lynch et al., 2002; Strangeway et al., 2005). This paper reporting SIERRA rocket observations on the nightside and Frederick-Frost et al. (2007) reporting SERSIO rocket observations on the dayside both describe even lower altitude signatures: the initiation of ion heating and upflow that may seed wave particle interactions at higher altitudes. Data sets from radar observations (Semeter et al., 2005; Doe et al., 1993; Wahlund et al., 1993) provide lower altitude fluid observations, but are unable to follow the kinetics of wave particle interactions. Fluid moment calculations are able to model and quantify ion outflow rates up to altitudes of about 250 km; above that, ion heating and outflow are often observed that are inconsistent with fluid calculations (Stromme et al., 2004). Sounding rocket observations, while limited to the specific case studies of their launch events, provide an important linkage between radar and satellite studies. Empirical classification (Wahlund et al., 1993) of ion upflow events as seen in radar databases into Type 1 (convection driven) and Type 2 (precipitation driven) is supported by the literature of sounding rocket observations. Both Type 1 (Moore et al., 1996; St. Maurice et al., 1976) and Type 2 (Frederick-Frost et al., 2007; Lynch et al., 1996) cases have Published by Copernicus Publications on behalf of the European Geosciences Union. 1968 K. A. Lynch et al.: Auroral ion outflow Table 1. SIERRA particle instrumentation specifics.