Survey of 0.1‐ to 16‐keV/e plasma sheet ion composition

Abstract

A large statistical survey of the 0.1‐ to 16‐keV/e plasma sheet ion composition has been carried out using data obtained by the Plasma Composition Experiment on ISEE 1 between 10 and 23 RE during 1978 and 1979. This survey includes more than 10 times the quantity of data used in earlier studies of the same topic and makes it possible to investigate in finer detail the relationship between the ion composition and the substorm activity. The larger data base also makes it possible for the first time to study the spatial distribution of the principal ion species. As found in previous studies, the ion composition has a large variance at any given value of the AE index, but a number of distinct trends emerge when the data are averaged at each activity level. During quiet conditions the plasma sheet is dominated by ions of solar origin (H+ and He++), as found in earlier studies, and these ions are most numerous during extended periods of very low activity (AE ≲ 30 γ). The quiet time density of these ions is particularly large in the flanks of the plasma sheet (GSM Y ∼ ± 10 RE), where it is about twice as large as it is near the central axis of the plasma sheet (Y = Z = 0). In contrast, the energy of these ions peaks near the central axis. When the AE index approaches zero for extended periods (several hours), the energy of the solar ions approach values that are similar to solar wind kinetic energies (∼1 keV/nucleon). Conversely, as the AE index increases, the solar ion energy increases. When a correction is made for the finite instrumental energy window, the data indicate that the solar H+ and He++, on the average, retain more nearly equal energy/nucleon than equal energy/charge. With increasing AE index the solar ion density decreases at all GSM Z, on the average, and the solar ions are partially replaced by ions of terrestrial origin. The most conspicuous of the terrestrial ions is the O+, which has an average energy of about 3–4 keV/ion at all activity levels. The increase in the O+ density is strongest around local midnight (GSM |Y| ≲ 5 RE), where the O+ often becomes the most numerous ion during strongly disturbed conditions (AE ∼ 1000 γ). At each level of substorm activity the average O+ density has a long‐term variability, increasing by a factor of 3 between early 1978 and early 1979, possibly in response to changing solar EUV radiation.

Polar/Toroidal Imaging Mass‐Angle Spectrograph survey of earthward field‐aligned proton flows from the near‐midnight tail

“…This phenomenon may well be a fundamental physical aspect of the proton "blasts," but it appears somewhat separate from the energy and pitch angle dispersions in the data, suggesting that the instrument often encounters sharp spatial flux gradients in addition to the temporal effects. For a brief analysis of this aspect, consider again the last couple of dispersion traces in Plate 3, in particular, the one dissected in Extending that line of reasoning to globally active conditions, one may speculate that the central plasma sheet then shows the cumulative effect of widespread "blasting," especially near local midnight, since its temperature peaks there [Lennartsson and Shelley, 1986]. Indeed, the top panel of Plate 3, representing strongly disturbed conditions, has many dispersions spanning some 7 ø invariant latitude, and the slope of the dispersions varies somewhat randomly, as though a large radial range of the near-midnight (---0010 MLT) tail is active concurrently.…”

“…The number density in the center of the H + trace, at 0933:06 UT, is 0.42 cm -3, whereas the peak O + density, at 0937:13 UT, excluding the upflowing component at lower energy, is 0.07 cm -3, suggesting an O+/H + ratio of •17% at the source. That is fairly typical of the central plasma sheet during geomagnetically active conditions, considering that solar activity at this time is still in its early rising phase [Lennartsson and Shelley, 1986]. …”

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

“…Magnetotail ion composition measurements in that energy range were first made more than 20 years ago on the ISEE 1 satellite, within 23 R E [Lennartsson and Shelley, 1986, and references therein], and are presently being made with the Cluster satellites within 19 R E [e.g., Rème et al, 2001]. Near-Earth ion composition measurements ($2 R E ) were begun even earlier with the polar-orbiting S3-3 satellite and were indeed the first to both detect the energetic ion outflow and provide substantial statistical material [e.g., Ghielmetti et al, 1978;Collin et al, 1981].…”

“…[4] According to the several years of ISEE 1 measurements the virtually omnipresent O + ions, arguably the ''quintessential'' ionosphere origin ions, have a broad energy distribution in the tail plasma sheet beyond 10 R E , typically averaging $3-4 keV [Lennartsson and Shelley, 1986]. This may be contrasted with the (indirect) DE 1 results just quoted and with the finding in this present study that the mean O + energy at R $ 2 R E , near the Polar perigee, is only $0.1-0.4 keV.…”

Kelvin-Helmholtz vortices and secondary instabilities in super-magnetosonic regimes

“…In this case we would not expect a large transport of plasma and energy or, in general, a strong interaction between the solar wind and the magnetosphere. However, during the period in which the IMF points northwards, a strong mixing layer is observed (Lennartsson and Shelley, 1986;Mitchell et al, 1987;Fujimoto et al, 1998) at low latitudes. In order to account for this observed exchange of mass and energy it was proposed (Belmont and Chanteur, 1989;Otto and Fairfield, 2000;Nakamura and Fujimoto, 2005) that the velocity shear between the magnetosphere and the magnetosheath leads to the onset of the Kelvin-Helmholtz (KH) instability.…”