S. P. Christon

Ion composition of the near-Earth plasma sheet in storm and quiet intervals: Geotail/EPIC measurements

We investigate the ion composition of the near-Earth plasma sheet in storm and quiet intervals, using energetic (9-210 keV) particle flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft. In 1998 four magnetic storms (minimum Dst -20 nT. The energy density of the H + , He + , and O + ions was computed from the EPIC/STICS data for these storm and quiet-time events. We obtained the following results: (1) The energy density is higher during storms than during quiet times for all ion species (H + , He + , and O + ); (2) the He + /H + energy density ratio during storms is 0.01-0.02, while that during quiet times is ∼0.01; and (3) the O + /H + energy density ratio is significantly larger during storms (0.2-0.6) than during quiet times (0.05-0.1). To explain these results we suggested a current sheet acceleration mechanism in which ions are energized by the dawn-to-dusk convection electric field in a mass-dependent way in the course of interaction with the current sheet.

Geotail observations of plasma sheet ion composition over 16 years: On variations of average plasma ion mass and O+ triggering substorm model

[1] We examined long-term variations of ion composition in the plasma sheet, using energetic (9.4-212.1 keV/e) ion flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on board the Geotail spacecraft. EPIC/STICS observations are available from 17 October 1992 for more than 16 years, covering the declining phase of solar cycle 22, all of solar cycle 23, and the early phase of solar cycle 24. This unprecedented long-term data set revealed that (1) the He + /H + and O + /H + flux ratios in the plasma sheet were dependent on the F10.7 index; (2) the F10.7 index dependence is stronger for O + /H + than He + /H + ; (3) the 0 + /H + flux ratio is also weakly correlated with the ∑Kp index; and (4) the He 2+ /H + flux ratio in the plasma sheet appeared to show no long-term trend. From these results, we derived empirical equations related to plasma sheet ion composition and the F10.7 index and estimated that the average plasma ion mass changes from ∼1.1 amu during solar minimum to ∼2.8 amu during solar maximum. In such a case, the Alfven velocity during solar maximum decreases to ∼60% of the solar minimum value. Thus, physical processes in the plasma sheet are considered to be much different between solar minimum and solar maximum. We also compared long-term variation of the plasma sheet ion composition with that of the substorm occurrence rate, which is evaluated by the number of Pi2 pulsations. No correlation or negative correlation was found between them. This result contradicts the O + triggering substorm model, in which heavy ions in the plasma sheet increase the growth rate of the linear ion tearing mode and play an important role in localization and initiation of substorms. In contrast, O + ions in the plasma sheet may prevent occurrence of substorms.

Plasma sheet and (nonstorm) ring current formation from solar and polar wind sources

We consider the formation of the plasma sheet and geosynchronous region (nonstorm) ring current in the framework of collisionless test particle motions in three-dimensional magnetospheric fields obtained from self-consistent MHD simulations. Simulation results are compared with observations of the near-Earth plasma sheet from the Polar spacecraft during 2001 and 2002. Many particles were initiated in two regions representative of the solar wind source upstream of the bow shock and the polar wind source outside the plasmasphere, both of which are dominated by protons (H+). Proton trajectories are run until they precipitate into the atmosphere, escape from the simulation space, or become stably trapped. These calculations produce a database of proton characteristics in each 1 RE3 volume element of the magnetosphere and yield velocity distributions as well as bulk plasma properties. We report results reflecting steady growth phase conditions after 45 min of southward interplanetary field, BZ = −5 nT (BY = 0), and for conditions resulting after 2 hours of northward BZ = +5 nT. The results for simulated velocity distributions are consistent with the Polar soundings of the current sheet from lobe to lobe and with AMPTE/CCE observations of (nonstorm) ring current region protons. The simulations help us identify the differentiation between solar and polar wind H+ ions in observations. The weak NBZ ring current-like pressure is primarily polar wind protons, while the moderately active SBZ ring current-like pressure is primarily solar wind protons. The solar and polar wind contributions to the SBZ ring current are comparable in density, but the solar protons have a higher average energy. For SBZ, solar wind protons enter the nonstorm ring current region primarily via the dawn flank and to a lesser degree via the midnight plasma sheet. For NBZ, solar wind protons enter the ring current-like region via the cusp and flanks. Polar wind protons enter the nonstorm ring current through the midnight plasma sheet in both cases. Solar and ionospheric plasmas thus take different transport paths to the geosynchronous (nonstorm) ring current region and may thus be expected to respond differently to substorm dynamics of the magnetotail.

Change of energetic ion composition in the plasma sheet during substorms

It has been reported by previous studies that the energetic particle flux of ions of ionospheric origin like O+ ions is more enhanced than that of H+ ions in the near-Earth tail (X ∼ −6 to −16 RE) during substorms. To explain this strong O+ flux enhancement, some studies have surmised that thermal O+ ions in the plasma sheet boundary layer or the lobe are strongly accelerated at the magnetic reconnection region (X ∼ −20 to −30 RE), and are subsequently transported into the near-Earth plasma sheet with earthward plasma flows. However, other studies have supposed that the strong O+ flux enhancement is caused by local magnetic field reconfiguration (local dipolarization). In the present study, we used Geotail/EPIC measurements of energetic (60 keV to 3.6 MeV) ion flux to test the above two scenarios. We investigated ion composition in the plasma sheet while earthward plasma flows and/or dipolarization signatures were observed. In terms of energy density ratio of oxygen ions to protons, the observational results can be summarized as follows: (1) earthward plasma flows without dipolarization signatures did not accompany large increases of the ratio in most cases; (2) when earthward plasma flows appeared with dipolarization signatures, they accompanied increases of the ratio; and (3) most of dipolarization events were associated with large increases of the ratio. These results suggest that the strong increase in the energetic oxygen constituent in the near-Earth plasma sheet is due to acceleration of ions during dipolarization, consistent with the latter scenario.

Low-charge-state heavy ions upstream of Earth's bow shock and sunward flux of ionospheric O+1, N+1, and O+2 ions: Geotail observations

Energetic ∼10–210 keV/e low-charge-state heavy ions (LCSHI) were measured sunward of Earths bow shock (to X ∼ 30 RE) during numerous intervals lasting from minutes to hours using Geotail/STICS in 1995–1998. LCSHI fluxes are strong and continuous in a few tens of intervals, during which a strong component of ionospheric origin O+1, N+1, and O+2 streams sunward on nearly radial IMF. Most often though only ‘trace’ levels are present. LCSHI flux is typically, but not exclusively, observed during diffuse upstream ion events. LCSHI are accompanied by sunward energetic electron bursts in three of the four cases shown and twice by enhanced IMF fluctuations. As O+1 streams sunward in the spacecraft frame during the strongest case, H+, He+2, He+1, and O+6 fluxes have weak anisotropies and the H+ energy spectrum is kappa-like. The strongest LCSHI fluxes tend to occur duskward of local noon during disturbed geomagnetic conditions. On average, LCSHI flux is more uniformly distributed across the dayside.

Dynamics of ions of ionospheric origin during magnetic storms : Their acceleration mechanism and transport path to ring current

We investigated the spatial and temporal properties of the 9-210 keV/e ion composition in the near-Earth plasma sheet (at geocentric distance, r, of 8-15 R E ), using energetic ion flux data acquired by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument onboard the Geotail spacecraft. We analyzed data covering 8.3 years to find the MLT-r distribution of the H + , He + , and O + energy densities (as well as the He + /H + and O + /H + energy density ratios) as a function of the SYM-H index. We obtained the following results: (1) The energy density increases as SYM-H decreases; (2) The energy density change depends on ion mass: the largest change occurs in O + and the smallest change occurs in H + ; (3) The energy density shows a dawn-dusk asymmetry when SYM-H < -50 nT, being larger on the duskside than on the dawnside; and (4) The He + /H + and O + /H + energy density ratios in the plasma sheet are similar to those in the ring current. From these results we conclude that ionospheric He + and O + ions are transported to the plasma sheet, accelerated there by the dawn-to-dusk electric field in a mass-dependent manner (heavier ions gain more energy than lighter ions), and injected into the ring current region. Both the ion flux from the ionosphere and the energy gain in the plasma sheet become large when the geomagnetic disturbance becomes intense.

Solar and Ionospheric Plasmas in the Ring Current Region

We consider formation of ring current-like plasmas in the inner magnetosphere and explore the degree to which they derive from heating and outflow of ionospheric plasmas. Recent observations show ring current proton injection into the ring current is relatively smooth and continuous, while O + injection is episodic in close association with substorms. We use collisionless test particle motions in magnetospheric fields from a magnetohydrodynamic simulation. The simulation is used to generate bulk properties and detailed velocity distributions at key locations, for comparison with observations. Particles are initiated in regions representative of the solar wind proton source upstream of the bow shock, the polar wind proton source, and the auroral zone enhanced outflows of O + , which we term auroral wind. Results reflect steady growth phase conditions after 45 minutes of southward interplanetary field. Solar wind protons enter the ring current principally via the dawn flank bypassing the midnight plasma sheet, while polar wind protons and auroral wind O + enter the ring current through the midnight plasma sheet. Thus, solar wind and ionospheric plasmas take very different transport paths to the ring current region. Accordingly, they are expected to respond differently to substorm dynamics of the magnetotail, as observed recently by remote neutral atom imaging from the IMAGE mission. Polar wind protons make a minor contribution to ring current pressure under steady conditions, but auroral wind O + has the potential to dominate the ring current, when outflow is strongly enhanced during periods of enhanced solar wind dynamic pressure fluctuations.

Ion composition and charge state of energetic particles in flux ropes/plasmoids

Measurements from the energetic particles and ion composition (EPIC) instrument on Geotail are used to investigate the relative abundance of different ion species at energies ∼10 keV to 3 MeV in flux ropes or plasmoids detected at various downstream distances from ∼23 RE to ∼190 RE in the tail. In terms of charge state, we find that during encounters with these structures most of the oxygen ions are singly charged, i.e., O+ ions of ionospheric origin, while most of the helium ions are doubly charged, i.e., He++ ions of solar wind origin. Therefore the ion species from the solar wind and the ionosphere appear to be thoroughly mixed inside flux ropes or plasmoids, just like the ion composition in the tail outside these structures. This study also reveals a variety of changes in the abundance of ionospheric oxygen ions within flux ropes or plasmoids. These range from simultaneous enhancements of ionospheric oxygen ions within the structure to delayed enhancements or no significant change of ionospheric oxygen ions throughout the entire structure. The amount of ionospheric oxygen ions present in flux ropes or plasmoids increases with higher geomagnetic activity as gauged by the Kp index, but it shows no apparent trend with the downstream distance. The lack of consistent enhancement of ionospheric oxygen ions inside these structures suggests that oxygen ions may not play a significant role in their formation.

Correction to “Pressure changes associated with substorm depolarization in the near-Earth plasma sheet”

] In the paper “Pressure changes associated with sub-storm depolarization in the near‐Earth plasma sheet” byY.Miyashitaetal.(J.Geophys.Res.,115,A12239,doi:10.1029/2010JA015608) the title was incorrect. The correct title isPressure changes associated with substorm dipolarization inthe near‐Earth plasma sheet.

Geomagnetic Activity Dependence of Occurrence Probability and Spatial Distribution of Upstream Events

Abstract We investigated upstream events observed by the ion composition system (ICS) sensor of the energetic particles and ion composition (EPIC) instrument on board the Geotail spacecraft. We examined how occurrence probability and spatial distribution of upstream events depend on the geomagnetic activity. The results showed that the upstream events were observed more frequently in the dawn side during intense geomagnetic activity in particular. We also analyzed carbon-nitrogen-oxygen ions during the upstream events. From the above results we discuss origin of the upstream energetic ions.