M. Malingre

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.

On the origin of sporadic keV ion injections observed by Interball‐Auroral during the expansion phase of a substorm

During the expansion phase of a substorm on October 10, 1997, the Interball-Auroral probe traveling across the auroral zone in the midnight sector detected sporadic injections of H+ and O+ ions with energies of a few keVs. We show that these injections are of ionospheric origin and are related to the large fluxes of low-energy (a few tens of eV) upflowing ions near the poleward boundary of the auroral oval. Using test particle trajectory simulations, we demonstrate that these low-energy ionospheric ions may be subjected to a rapid (on the timescale of expansion phase) circulation inside the plasma sheet. After their ejection from the poleward propagating auroral bulge, these ions travel into the midtail where they behave nonadiabatically and experience energization up to the keV range. As a result of magnetic moment damping in the current sheet, some of these ionospheric ions escape from the equatorial magnetosphere and travel back toward low altitudes. Numerical calculations reveal that upon reaching Interball-Auroral near ∼3 RE altitude, these particles give rise to the observed keV injections, the sporadic character of which is due to the chaotic nature of ion motion in the magnetotail.

Low‐energy upflowing ion events at the poleward boundary of the nightside auroral oval: High‐altitude Interball‐Auroral probe observations

Ion data acquired by the Interball-Auroral satellite during crossings of the poleward boundary of the auroral oval in the 2200-0300 MLT sector at altitudes of ∼2.5–3 Earths radii reveal the frequent occurrence of thermal and superthermal H+ ion outflows. These events are strongly correlated with suprathermal electron fluxes and broadband electromagnetic ULF waves. The pitch angle distributions give evidence of transverse heating occurring in a latitudinally narrow layer at the boundary between the polar cap and the plasma sheet boundary layer, over a broad altitude range extending up to the satellite altitude. The distributions evolve with latitude, exhibiting fluxes maximizing at pitch angles close to 90° at the poleward edge of the outflow structure and at pitch angles closer to the upward field-aligned direction at lower latitudes. The data analysis suggests that ion cyclotron resonance interaction with ULF electromagnetic turbulence can account for the observed heating, even if we cannot totally exclude that transverse velocity shears and nonresonant stochastic transverse acceleration sometimes contribute to the ion energization in view of the dc electric field fluctuations commonly observed at the same times. During the expansion phase of substorms the region of transverse heating at the poleward boundary of the discrete auroral oval exhibits a latitudinal structure characterized by an alternate occurrence of latitudinally narrow regions of intense and weak ion fluxes. These latitudinal variations are associated with magnetic fluctuations at a frequency of ∼2×10−2 Hz, interpreted in terms of hydromagnetic Alfven waves. Equatorward of the heating region, the energy spectrograms recorded during the same events exhibit an energy-latitude dispersion signature with energy decreasing as latitude decreases. This dispersion is the result of the velocity filter effect due to the large-scale convection and of the poleward motion of the ion heating source associated with the poleward motion of the high-latitude edge of the active auroral region. The poleward edge of the low-energy ion structure marked by a sharp latitudinal gradient of the ion flux appears as a reliable midaltitude criterion for identifying the poleward boundary of the soft electron layer lying at the high-latitude edge of the plasma sheet boundary layer.