Dominique C. Delcourt

Impulsive enhancements of oxygen ions during substorms.

It has been observed that H+ is the dominant ion species in the plasma sheet and the ring current during quiet times. However, the O+/H+ density ratio increases with increasing geomagnetic storm and substorm activity. Energetic neutral atom (ENA) images from Imager for Magnetopause-to-Aurora Global Exploration/High Energy Neutral Atom (IMAGE/HENA) reveal the rapid increase of O+ ring current at substorm expansion. Finding the cause of this substorm-associated O+ enhancement is the main focus of this paper. Two possible sources are suggested: direct injection from the ionosphere and energization of the preexisting oxygen ions in the magnetosphere. We perform numerical simulations to examine these two mechanisms. Millions of O+ are released from the auroral region during a simulated substorm by the Lyon-Fedder-Mobarry MHD model. The subsequent trajectories of these outflowing ions are calculated by solving the full equation of particle motion. A few minutes into the substorm expansion phase, an enhancement in O+ pressure is found on the nightside at ∼12 RE. After careful analysis, we conclude that this pressure peak is coming from energization of the preexisting O+ in the plasma sheet. The direct injection mechanism will introduce a significant time lag between strong ionospheric outflow and magnetospheric enhancement, so that it cannot explain the observed O+ bursts. Using the temperature and density established by the test-particle calculations as boundary conditions to a ring current model, we calculate the O+ fluxes and the corresponding ENA emissions during the model substorm. We are able to reproduce observable features of oxygen ENA enhancements as seen by IMAGE/HENA.

Particle acceleration by inductive electric fields in the inner magnetosphere

Abstract We review some features of charged particle dynamics during substorm dipolarization of the magnetic field lines in the inner magnetosphere. In the parallel direction, particles traveling at low latitudes are subjected to prominent equatorward-oriented acceleration due to the rapid direction-changing E × B drift. This centrifugal effect leads to focusing of ions with small parallel speeds into the equatorial region, which in turn yields the development of substantial (a few kV) parallel potential drops and the production of magnetic field-aligned electron beams. In the perpendicular direction, when the gyroperiod is comparable to the field-variation time scale, particles can experience abrupt nonadiabatic energization. As an example, O + ions originating from the ionosphere are accelerated up to hundreds of keV while being injected earthward, hence providing a seed population for the outer ring current. These results suggest that the electric field induced by relaxation of the magnetic field lines can play an active role in the dynamics of the inner magnetosphere during the main phase of geomagnetic storms.

Electron pitch angle distributions following the dipolarization phase of a substorm: Interball‐Tail observations and modeling

We investigate Interball-Tail observations of electron pitch angle distributions after the dipolarization phase of a substorm. For 10 keV electrons we observe beamlike, coniclike, and perpendicularly peaked distributions at L ∼ 11, L ∼ 9, and L ∼ 7, respectively. We examine the efficiency of betatron heating and Fermi acceleration associated with adiabatic transport of the electrons during the substorm dipolarization phase. This dipolarization phase was modeled using transition between different K p levels within a realistic magnetic field model. The calculations reproduce well the evolution of the high-energy electron flux in the parallel and perpendicular directions. They also reproduce well the pitch angle distribution observed by Interball-Tail at 10 keV, after the dipolarization phase. It is shown that Fermi acceleration is the leading process, compared to betatron heating. The production of the coniclike distributions is narrowly linked to the existence of a transition region between dipolelike and taillike magnetic fields, at about L ∼ 9.

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.

A simple model of magnetic moment scattering in a field reversal

We examine the nonadiabatic motion of charged particles in a field reversal using a simple centrifugal impulse model. It is shown that, though this model cannot account for long term behaviors, it accurately describes the abrupt magnetic moment change experienced by the particles between two consecutive adiabatic (magnetic moment conserving) sequences. Good agreement is obtained between analytical estimates and trajectory computations. Our analysis also suggests a new pitch angle dependent parameter to characterize the jumps in magnetic moment. Application to particle motion in a cusped field geometry demonstrates that magnetic moment damping and subsequent particle escape can be adequately modeled by means of such centrifugal impulses.

Application of the centrifugal impulse model to particle motion in the near-Earth magnetotail

Particles traveling in the geomagnetic tail do not conserve their magnetic moment (first adiabatic invariant) due to significant field variations on the length scale of their Larmor radius. We examine the possibility of describing these magnetic moment changes by the action of an impulsive centrifugal force on the timescale of the particle cyclotron turn. Trajectory calculations demonstrate that such a centrifugal impulse model adequately describes the nonadiabatic particle behavior for situations where the κ parameter (defined as the square root of the minimum curvature radius-to-maximum Larmor radius ratio) is of the order of 1 to 2. In particular, it is shown that this behavior can be organized by another parameter (referred to as κα), which is proportional to κ but depends upon the particle pitch angle, namely, one obtains: systematic magnetic moment enhancements for κα ≪ 1, large gyrophase effects with possibly prominent damping of the magnetic moment for κα ∼ 1, and nearly constant magnetic moment for κα ≫ 1. More generally, we show that the centrifugal impulse approximation applies to an intermediate orbit regime at the transition between the fully adiabatic (magnetic moment conserving) regime and that regime where particles experience meandering motions about the field minimum. It applies to ion transport in the near-Earth magnetotail where the magnetic field lines evolve from dipolar to taillike configurations and where the κ parameter nears unity. In this region of space the model predictions are in agreement with numerical results, revealing both enhanced trapping (due to magnetic moment enhancement) and possible precipitation (due to magnetic moment damping) of plasma sheet ions depending upon their pitch and phase angles.

Remote analysis of cleft ion acceleration using thermal plasma measurements from Interball Auroral Probe

Three dimensional distributions of low energy (0–80 eV) ions have been obtained in the high-latitude dayside sector between 10,000 and 20,000 km by the Hyperboloid experiment onboard Interball-Auroral Probe. H+, He+ and O+ ions exhibit a latitude-energy dispersion characteristic of the cleft fountain. Test particle simulations are used to investigate the properties of the outflowing ion source region. Regardless of ion mass, it is shown that the bulk of the outflowing population originates from a narrow (< 2°) latitudinal interval inside the dayside cleft. Ion acceleration in the direction perpendicular to the magnetic field is shown to occur at all altitudes at least up to 10,000 km, that is, higher than previously reported in cleft fountain studies. The simulations clearly display a gradual decrease of the heating efficiency with increasing altitude and suggest a weaker gradient for O+ than for H+. Parallel acceleration at low altitudes also appears to contribute to the net ion energization within the cleft.

Spatial structure of the cusp/cleft ion fountain : A case study using a magnetic conjugacy between Interball AP and a pair of SuperDARN radars

The spatial structure of low-energy ion outflows associated with heating processes in the dayside cusp/cleft region is investigated by using a magnetic conjugacy between Interball Auroral Probe (AP) and the Saskatoon-Kapuskasing pair of the Super Dual Auroral Radar Network (SuperDARN). The interplanetary magnetic field during this event is characterized by By < Bz < 0 components, which breaks the symmetry of morning and afternoon convection cells relative to the noon meridian. As a result, plasma convection over the afternoon polar cap is not antisunward, but almost azimuthally oriented. The three-dimensional thermal ion distributions measured by the Hyperboloid experiment on board Interball AP are used as input of numerical simulations in order to investigate the spatial structure of the ion heating processes. The simulations include the complete guiding center motion of ions under the effect of gravity, geomagnetic field, and convection field measured by SuperDARN radars. In contrast to the classical cleft ion fountain picture, we demonstrate that the observed ions originate from a wide latitudinal interval and that the heating region likely coincides with the polar cusp. The numerical simulations allow us to reconstruct the downstream (relative to convection) boundary of the heating region, as well as ion distributions along this boundary. Ions are found to cross this boundary and to exit the heating region over a broad range of altitudes (up to at least 15,000 km) with an average altitude increasing with ion mass, their average pitch angle increasing with altitude in agreement with altitude cumulative heating scenarios.

Ion and electron distribution functions in the distant magnetotail: modeling of Geotail observations

Ion and electron distribution functions measured by the Geotail spacecraft in the distant magnetotail are investigated considering single-particle dynamics in sharp field reversals. The observed H+ distribution functions exhibit two components, namely, a low-energy core and a high-energy population propagating tailward. In agreement with previous studies, we show that ion dynamics in a two-dimensional (2-D) reconnection field geometry naturally leads to such distributions and that the neutral line location can be analytically inferred from the velocity space structure. As for electron distribution functions, Geotail observations reveal characteristic “flattop” profiles with enhanced flux in the parallel direction at low energies and in the perpendicular direction at high energies. We interpret these distributions as the result of the electron chaotic behavior in the reconnection region. This chaotic regime, which occurs when the particle Larmor radius is comparable to the magnetic field line curvature radius, can lead to prominent pitch angle scattering. Near the neutral line, we show that at a given energy, this regime yields flux depletion in the parallel direction and enhancement in the perpendicular one. The flattop profile arises from this behavior affecting only the (relatively) high-energy part of the electron distribution. Conversely, the threshold energy above which electrons exhibit such a behavior contains information on the field reversal characteristics. The electron distribution functions obtained numerically are in agreement with the observations, which support this interpretation framework and allow us to estimate the thickness of the neutral sheet.

RING CURRENTS AND INTERNAL PLASMA SOURCES

The discovery of terrestrial O+ and other heavy ions in magnetospheric hot plasmas, combined with the association of energetic ionospheric outflows with geomagnetic activity, led to the conclusion that increasing geomagnetic activity is responsible for filling the magnetosphere with ionospheric plasma. Recently it has been discovered that a major source of ionospheric heavy ion plasma outflow is responsive to the earliest impact of coronal mass ejecta upon the dayside ionosphere. Thus a large increase in ionospheric outflows begins promptly during the initial phase of geomagnetic storms, and is already present during the main phase development of such storms. We hypothesize that enhancement of the internal source of plasma actually supports the transition from substorm enhancements of aurora to storm-time ring current development in the inner magnetosphere. Other planets known to have ring current-like plasmas also have substantial internal sources of plasma, notably Jupiter and Saturn. One planet having a small magnetosphere, but very little internal source of plasma, is Mercury. Observations suggest that Mercury has substorms, but are ambiguous with regard to the possibility of magnetic storms of the planet. The Messenger mission to Mercury should provide an interesting test of our hypothesis. Mercury should support at most a modest ring current if its internal plasma source is as small as is currently believed. If substantiated, this hypothesis would support a general conclusion that the magnetospheric inflationary response is a characteristic of magnetospheres with substantial internal plasma sources. We quantitatively define this hypothesis and pose it as a problem in comparative magnetospheres.