Distribution of upflowing ionospheric ions in the high‐altitude polar cap and auroral ionosphere

Abstract

A statistical study is presented of the occurrence frequency distribution of upflowing ionospheric ions (UFI) in the high‐altitude (8000–23,000 km) auroral and polar cap ionosphere, using measurements from the energetic ion composition spectrometer aboard Dynamics Explorer 1. In this survey, distinction is made between UFI of different masses (H+ and O+), energies (0.01–1, 1–4, and 4–17 keV/e), and pitch angle characteristics (within 20° from , between 20° and 80° from , and transverse to ). The increased statistical accuracy of the data and the added dimensions in this study relative to earlier work at lower altitudes provide a more detailed picture of UFI events. Occurrence frequencies are determined for both H+ and O+ UFI at the different energies as a function of altitude, invariant latitude, magnetic local time, and magnetic activity (Kp index). It is shown that the occurrence frequency and intensity distribution of O+ UFI have a marked magnetic dependence, whereas the H+ UFI occurrence displays little variation with magnetic activity. The occurrence frequency of O+ conics decreases with altitude, in relation to that of H+ conics. Also, ≈90° H+ conics occur 2–3 times more frequently than O+. The invariant latitude‐local time distribution of the occurrence frequency for both H+ and O+ is auroral‐oval‐like in that the frequency peaks at a more equatorward latitude and has a broader latitudinal extent in the nightside than in the dayside. However, this association with the auroral oval is energy‐dependent, the oval for low‐energy UFI being distinctly more poleward than that for energetic UFI. The low‐energy O+ UFI occurrence is relatively uniform in local time; the H+ UFI occurrence frequency distribution shows a broad distribution peaking in the noon sector. Both the H+ and O+ conics distributions display a dawn‐dusk asymmetry in favor of the dusk sector. In the polar cap (poleward of the statistical auroral oval), the H+ and O+ ion outflows (≈2×1024 s−1 and 3×1024 s−1, respectively, at quiet times, and ≈3×1024 s−1 and 1×1025 s−1 at disturbed times) represent a significant ionospheric source to the magnetotail. The results of this study are shown to be consistent with previous, less detailed surveys of UFI at lower altitudes and provide a basis with which to examine possible acceleration mechanisms.

Polar/Toroidal Imaging Mass‐Angle Spectrograph observations of suprathermal ion outflow during solar minimum conditions

“…Reports of the occurrence probability of ion beams and conics based on daia from the DE 1/EICS instrument show an increased probability of detecting an ion beam or conic above 16,000 km [Yau et al, 1984[Yau et al, , 1985a[Yau et al, , 1985bKondo et al, 1990;Peterson et al, 1992Peterson et al, , 1995. Kondo et al [1990] showed that, at low levels of magnetic activity (defined as Kp < 3*), and for energies less than 1 keV, the probability of encountering an H* or O* ion beam at 20,000 km was more than twice that at 8000 km.…”

The structure of high altitude O&lt;sup&gt;+&lt;/sup&gt; energization and outflow: a case study

“…Gorney et al, 1981;Yau et al, 1984;Miyake et al, 1993). Energization is typically transverse to the magnetic field and may start just above the ionosphere.…”

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

“…In particular, the much faster TIMAS measurement cycle allows us to retain full energy resolution in the statistical records, as opposed to the three broad energy bands employed with the EICS statistical database, namely 0.01 -1, 1 -4, and 4 -17 keV/e [e.g., Yau et al, 1984]. The DE 1 statistics, while very large in terms of net ion number flows, provide rather limited information about average ion energies and energy flow rates.…”