Christopher Mouikis

Ion composition and pressure changes in storm time and nonstorm substorms in the vicinity of the near-Earth neutral line

[i] Using CLUSTER/CODIF data from close to ∼ 19 Re in the magnetotail, we have performed a superposed epoch analysis of storm time and nonstorm substorms to determine how the ion composition changes during a substorm. We find that the median O + density and pressure in the plasma sheet are a factor of 5 higher during storm times than during nonstorm times. However, we do not observe significant changes in the composition during a substorm that would indicate that ionospheric outflow is playing a dynamic role in loading the plasma sheet or triggering the substorm at this location. There are differences between the storm time and nonstorm substorms, and it is intriguing to consider whether the composition differences play a role. The storm time substorms exhibit more loading and faster unloading than the nonstorm substorms. In addition, we observe differences in the H + and O + behavior at onset in the storm time substorms that we attribute to the different dynamics of the two ion species at the reconnection site and during the field reconfiguration due to their different gyroradii. The H + density and pressure decrease over the whole energy range at substorm onset, while the O + density and pressure decrease less, and the O + temperature increases. That more O + is left after substorm onset indicates that either the O + is more quickly replenished from O + in the lobes and/or that the more energetic O + , due to its larger gyroradius, is not depleted when the field reconfigures and is accelerated in the thin current sheet.

Ballooning instability in the presence of a plasma flow: A synthesis of tail reconnection and current disruption models for the initiation of substorms

The drift ballooning mode (DBM) instability near the inner edge of the plasma sheet (IEPS) is studied further by including a nonstationary earthward flow and flow shear in the analysis. Both equatorial and off-equatorial regions are considered. It is found that the presence of a decelerated earthward flow destabilizes both the M− and M+ branches of the DBM in a large portion of the current sheet near the IEPS and substantially increases the growth rate of the instability. The flow shear in the premidnight sector causes the conventional ballooning mode to weakly subside, while it slightly enhances the growth rate for the Alfvenic ballooning mode. The combination of the earthward flow and flow shear makes both the Alfvenic ballooning mode and conventional ballooning mode grow much faster than they would without the flow, giving rise to coupled Alfvenic slow magnetosonic waves, field-aligned currents, and the formation of a current wedge. A synthesis of tail reconnection and cross-tail current disruption scenarios is proposed for the substorm global initiation process: When the fast flow produced by magnetic reconnection in the midtail abruptly decelerates at the IEPS, it compresses the plasma populations earthward of the front, transports momentum to them, and pushes them farther earthward. This creates the configuration instability in a large portion of the inner tail magnetic field lines on both the tailward side and earthward side of the braking point. As soon as the ionospheric conductance increases over a threshold level, the auroral electrojet is greatly intensified, which leads to the formation of the substorm current wedge and dipolarization of the magnetic field. This substorm paradigm combines the near-Earth neutral line and near-Earth current disruption scenarios for the initiation of substorms and may also synthesize dynamical processes in the rnagnetosphere-ionosphere coupling and field line resonance during the substorm onset. We intend to use this global model to explain substorm expansion onsets occurring under the southward interplanetary magnetic field condition.

Comprehensive particle and field observations of magnetic storms at different local times from the CRRES spacecraft

The response of ring current intensification to three magnetic storms sampled at dawn, midnight, and dusk is investigated. We use a comprehensive set of data from the CRRES satellite, using plasma, energetic particle (ion composition), electric field, and magnetic field data, which is ideal for investigating the interrelationship between the ring current strength as measured by D st , the particle (current carriers) in the outer radiation belt, their effects on the global magnetic field, and the convection effects caused by large dawn-dusk electric fields. This yields a comprehensive and self-consistent picture of storm time radiation belt formation based entirely on data. At all local times investigated strong, stretching (flattening) of the magnetic field down to L < 4 is observed during the storm main phase, showing that this field line stretching is not limited to midnight. Ring current ions above 100 keV are shown to form a partial ring current during the main phase as they are only sampled at dawn during the recovery phase when the electric field vanishes. Comparing this feature to Kp-dependent models of the proton Alfven layer shows that dawn is only accessible to these ions after the main phase. Ionospheric origin ions (O + ) follow dynamics very similar to those of H + , indicating a source in the plasma sheet. Solar wind ions (He ++ ) are controlled by their solar wind source and have immediate access during the main phase. He + , which is generated in the ionosphere as well as by charge exchange, has behavior similar to that of O + and H + . In contrast to the current view, plasma sheet ions in the energy range from 5 to 28 keV contribute significantly in the energy density of the ring current during the storm main phase.

Excitation of EMIC waves detected by the Van Allen Probes on 28 April 2013

We report the wave observations, associated plasma measurements, and linear theory testing of electromagnetic ion cyclotron (EMIC) wave events observed by the Van Allen Probes on 28 April 2013. The wave events are detected in their generation regions as three individual events in two consecutive orbits of Van Allen Probe-A, while the other spacecraft, B, does not detect any significant EMIC wave activity during this period. Three overlapping H+ populations are observed around the plasmapause when the waves are excited. The difference between the observational EMIC wave growth parameter (Σh) and the theoretical EMIC instability parameter (Sh) is significantly raised, on average, to 0.10 ± 0.01, 0.15 ± 0.02, and 0.07 ± 0.02 during the three wave events, respectively. On Van Allen Probe-B, this difference never exceeds 0. Compared to linear theory (Σh > Sh), the waves are only excited for elevated thresholds.

Flattened current sheet and its evolution in substorms

In this research, the properties of a tail current sheet, which has a flattened geometry, and its evolution during substorm activity have been investigated. The geometrical configuration of the magnetic field and the spatial distribution of the current density in a flattened current sheet have been revealed with certainty for the first time. It is found that such a flattened current sheet has sufficiently strong B-y (GSM) within its neutral sheet that the magnetic field lines (MFLs) in the neutral sheet are lie almost in the GSM equatorial plane and that the normal directions are generally northward. Detailed analyses show that, the magnetic field lines are spiral-like, not plane curves, which are left-handed or right-handed spirals for B-y > 0 or B-y < 0. This magnetic rotation occurs predominantly in the neutral sheet. The flattened current sheet may be very thin, and the thickness of the neutral sheet is much less than the minimum radius of the curvature of the MFLs in the current sheet. The analysis also suggests that the neutral sheet current is field-aligned and lies mainly duskward. The curvature current makes little contribution to the total current in the flattened current sheet. The main current carriers in the neutral sheet of the flattened current sheet are electrons. A statistical survey shows that there is one positive correlation between B-y in the flattened current sheet and IMF B-y and penetration efficiency is 0.67. Flattened current sheets may occur in both quiet and disturbed periods and may appear at all phases of the substorms. During the growth phase of a substorm event, the neutral sheet of the flattened current sheet is shown to become progressively thinner, while the associated current density is increasing gradually. It is found that the northern turning of the IMF has triggered the explosive growth phase at the end of the growth phase, which lasts several minutes. At the explosive growth phase, the flattened current sheet becomes much thinner and the current density in the neutral sheet then increases considerably and reaches a value larger than 0.017 mu Am-2. Just after the onset of the substorm, the current density in the neutral sheet drops abruptly and varies turbulently.

Flow shear near the boundary of the plasma sheet observed by Cluster and Geotail

[1] We report on the transient strong flow shear (large northward/southward electric field) events accompanied by energetic ion beams and localized field-aligned currents observed at the boundary of the premidnight plasma sheet by Cluster in the Northern Hemisphere and Geotail in the Southern Hemisphere. The events took place associated with plasma sheet expansion during a substorm interval, with the main positive bay onset at 1155 UT on 10 October 2001. Typical timescales of these events were 1-5 minutes. Cluster multipoint analysis showed that the field-aligned currents consist of upward and downward current layers, the latter located at the outermost edge of the plasma sheet and concentrated in a region with a thickness of 1600 km. Low-energy proton flow suggested that the electric field was southward at the outer part and northward at the inner part, with a magnitude exceeding 10 mV/m. The electric field reversal region also corresponds to the boundary between beam-like electrons and more isotropic electron distributions. Geotail observed corresponding plasma and field disturbances simultaneously inside the plasma sheet. We suggest that the strong bipolar electric fields could be related to the Hall effect of the transient reconnection process tailward of Cluster and Geotail and to the leading edge of the plasma flow jetting Earthward from the reconnection region.

Cluster and DMSP observations of SAID electric fields

[1] We report on magnetically conjugate Cluster and the Defense Meteorological Satellite Program (DMSP) satellite observations of subauroral ion drifts (SAID) during moderate geomagnetic activity levels on 8 April 2004. To our knowledge, the field-aligned separation of DMSP and Cluster (%28,000 km) is the largest separation ever analyzed with respect to the SAID phenomenon. Nonetheless, we show coherent, subauroral magnetosphere-ionosphere (MI) coupling along an entire field line in the post-dusk sector. The four Cluster satellites crossed SAID electric field channels with meridional magnitude E M of 25 mV/m in situ and latitudinal extent DL % 0.5° in the southern and northern hemispheres near 07:00 and 07:30 UT, respectively. Cluster was near perigee (R % 4 R E) and within 5° (15°) of the magnetic equator for the southern (northern) crossing. The SAID were located near the plasmapause—within the ring current-plasmasphere overlap region. Downward field-aligned current signatures were observed across both SAID crossings. The most magnetically and temporally conjugate SAID field from DMSP F16A at 07:12 UT was practically identical in latitudinal size to that mapped from Cluster. Since the DMSP ion drift meter saturated at 3000 m/s (or

Walen and slow-mode shock analyses in the near-Earth magnetotail in connection with a substorm onset on 27 August 2001

114 mV/m) and the electrostatically mapped value for E M from Cluster exceeded 300 mV/m, a magnitude comparison of E M was not possible. Although the conjugate measurements show similar large-scale SAID features, the differences in substructure highlight the physical and chemical diversity of the conjugate regions.

Near-Earth substorm features from multiple satellite observations

plasma density N � 0.3 cm � 3 and the plasma ion b � 1.5. The Walen analysis applied to the tailward flow interval from 0400:36 UT to 0403:14 UT is consistent with the ions being accelerated to � 73% of the Alfven speed across a slow-mode shock connected to a near-Earth neutral line located on the earthward side of the spacecraft. The occurrence of a small plasmoid-type magnetic flux rope during the leading edge of the tailward flows provides further support in favor of an active region of magnetic reconnection earthward of Cluster. The more field-aligned earthward flows between 0406 UT and 0408 UT, however, failed to satisfy the Walen test. Rankine-Hugoniot analyses of upstream and downstream plasma and magnetic field parameters confirm the presence of a slow-mode shock in connection with the passage of the tailward flow region but not with the 0406 UT to 0408 UT earthward flow interval. The confirmed shock satisfies the critical slow-mode requirements: MI * � 1.0 and MSM * > 1.0 on the upstream side and MSM * � 19 RE of the near-Earth magnetotail. INDEX TERMS: 7835 Space Plasma Physics: Magnetic reconnection; 7851 Space Plasma Physics: Shock waves; 2744 Magnetospheric Physics: Magnetotail; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: magnetic reconnection, shock waves, magnetotail boundary layers

Ion composition of substorms during storm-time and non-storm-time periods

We investigate a substorm on 3 October 2004 during which 11 satellites were located in near-Earth magnetotail (X-GSM > -10 R-E). Double Star 1 (TC-1), Cluster, and LANL-97 satellites were closely aligned in the dawn-dusk direction (<1 R-E apart) for this conjunction. After substorm expansion onset, TC-1 observed plasma sheet thinning at X approximate to -5.5 RE and later detected signature of plasma flow shear that may be associated with an auroral arc. Analysis of the dawn-dusk magnetic perturbations from GOES-10 and Polar suggests that these could be caused by a substorm current system consisting of not only the azimuthal closure of field-aligned currents (the substorm current wedge) but also the meridional closure of field-aligned currents. The temporal sequence of substorm activity (particle injection, current disruption, and dipolarization) revealed by these satellites indicates that the substorm expansion activity was initiated close to the Earth and spread later to further downstream distances. Furthermore, TC-1 and Cluster data show that there is no close relationship between some dipolarizations and Earthward plasma flows in the near-Earth region. The overall development of substorm activity is in agreement with the near-Earth initiation model for substorms. A temporal evolution of the magnetic field reconfiguration and plasma boundary motion during this substorm is constructed from these observations.