Solar wind control of Earth's H+ and O+ outflow rates in the 15‐eV to 33‐keV energy range

Abstract

Earth's high‐latitude outflow of H+ and O+ ions has been examined with the Toroidal Imaging Mass‐Angle Spectrograph instrument on the Polar satellite in the 15‐eV to 33‐keV energy range over an almost 3‐year period near solar minimum (1996–1998). This outflow is compared with solar wind plasma and interplanetary magnetic field (IMF) data from the Wind spacecraft, the latter having been time shifted to the subsolar magnetopause and averaged for 15 min prior to each sampling of Earth's magnetic field‐aligned ion flow densities. When the flow data are arranged according to the polarity of the IMF Bz (in GSM coordinates) and limited to times with Bz > 3 nT or Bz < −3 nT, the total rate of ion outflow is seen to be significantly enhanced with negative Bz, typically by factors of 2.5–3 for the O+ and 1.5–2 for the H+, more than previously reported from similar but less extensive comparisons. With either IMF Bz polarity the rate of ion outflow is well correlated with the solar wind energy flow density, especially well with the density of kinetic energy flow. The rate of ion outflow within the instrument's energy range is a strong function of the Polar satellite altitude, increasing almost threefold from perigee (R ∼ 2 RE) toward apogee (R ∼ 4–9 RE) for O+ ions, i.e., up to 1026 ions s−1 or more per hemisphere. The apogee enhancement may be still larger for the H+, but it is obscured by mantle flow of cusp origin solar H+. Ion mean energy also increases with altitude, leading to about a twentyfold increase in the O+ energy flow rate from Polar perigee to apogee altitude, reaching values of 20 GW or more per hemisphere. While the perigee outflow of H+ has little or no seasonal modulation, in terms of ions s−1 the O+ outflow rates at both altitudes do increase during local summer and so does the rate of cusp origin H+ flow near apogee. The latter rate, in fact, has very similar seasonal modulation as the O+ rates, suggesting that it has a significant influence on the O+ outflow.