Energetic auroral and polar ion outflow at DE 1 altitudes: Magnitude, composition, magnetic activity dependence, and long‐term variations

Abstract

Data from Dynamics Explorer 1 Energetic Ion Composition Spectrometer (DE 1 EICS) in the period from September 1981 to May 1984 were used to determine the mass composition, magnitude, magnetic activity dependence, long‐term variations, and topology (MLT‐invariant latitude distribution) of energetic (0.01–17 keV/el) terrestrial outflow. The September 1981 to May 1984 period coincided with the declining phase of the current solar cycle (cycle 21), when the monthly average of the solar radio flux at 10.7 cm, F10.7, decreased from a high of 222 ( × 10−22 W m−2 Hz−1) in September 1981 to a low of 93 in November 1983. At both magnetically quiet and active times, the O+ outflow rate exhibited long‐term variations which correlated with the declining solar radio flux. Overall, the O+ outflow rate in the 1981–1982 period was a factor of 2 larger than the 1983–1984 rate. Any corresponding variation in the H+ outflow rate, if present, was much smaller and not statistically significant. At solar maximum (in 1981–1982), the total ion outflow rate (H+ and O+) was 1.5×1026 ions s−1 at active times (3 ≤ Kp ≤ 5) and 5×1025 ions s−1 at quiet times (Kp ≤ 2). The active time flow was predominantly O+; the O+/H+ ratio was 3. At quiet times, H+ and O+ flows were comparable, and the ratio was 0.9. In 1983–1984, a period of reduced solar activity, the total ion outflow rate was 1.0×1026 ions s−1 at active times and 4×1025 ions s−1 at quiet times. The O+/H+ ratio was 1.1 and 0.6, respectively. In both H+ and O+, the outflow was dominated by < 1‐keV ions. Ions of 1–17 keV/el constituted less than 10% of the total ion outflow. The O+ ion outflow rate increased exponentially with the Kp index, the rate at very disturbed times (Kp ≥ 6) being a factor of 30 larger than the quiet time (Kp = 0) value. The increase in the H+ outflow rate with Kp was more modest, the disturbed time (Kp ≥ 6) rate being a factor of 5 larger than the quiet time value. At quiet times (Kp = 0–2), 10–20% of H+ and 20–25% of O+ ion outflow occurs above 80° invariant. At active times (Kp = 3 ‐ 5), 20–30% of H+ and 30–35% of O+ outflows occur above 76° invariant. Defining the polar cap as the latitude region above 80° invariant at low Kp and above 76° invariant at high Kp, the ratio of polar cap to auroral ion outflow is 0.2–0.3 at quiet times and 0.4–0.5 at active times. In other words, the polar cap ion outflow is smaller than, but nevertheless nonnegligible compared with, the auroral ion outflow, particularly at active times. For both H+ and O+, the distributions of upward ion fluxes peak near 78° invariant in the noon sector. The averaged quiet time H+ upward flux at the peak invariant latitude, normalized to 1000‐km altitude, is 2 ‐ 3×108 cm−2 s−1. The corresponding quiet time O+ upward flux is ∼2×108 cm−2 s−1 at solar maximum and increases to ∼5×108 cm−2 s−1 at active times (Kp = 3 ‐ 5). In the night side the upward ion flux peaks at lower latitude. At quiet times the peak flux in the midnight sector is a factor of 3–5 smaller than the peak flux in the noon sector. At active times it is a factor of 2 smaller. Over 95% of the quiet time (energetic) terrestrial ion outflow occurs above 60° invariant (L = 4), as does over 90% of the active time outflow. It is concluded (1) that energetic (0.01–17 keV/el) auroral upflowing ions are an adequate source of O+ in the plasma sheet at both quiet and active times and (2) that polar cap upflowing ions are the primary source for the observed ion streams in the magnetotail lobe.