Christian Jacquey

Location and propagation of the magnetotail current disruption during substorm expansion : analysis and simulation of an ISEE multi-onset event

Magnetic field variations measured onboard the ISEE-1 spacecraft in the tail northern lobe (∼22 RE geocentric distance) during a substorm with multiple onsets are analyzed and simulated using a model current sheet. This remote sensing study of the cross-tail current sheet shows that substorm onsets are associated with a partial disruption of the cross-tail current that begins in the near-earth plasma sheet (∼7 RE) and that expands tailward during substorm expansion with a velocity of the order of 300 km/s. The two consecutive substorm onsets, which occur during a very active period (AE > 500 nT), correspond to a similar partial disruption of the near-earth current sheet that expands tailwards over a distance of the order of 80 RE.

Tailward propagating cross‐tail current disruption and dynamics of near‐Earth Tail: A multi‐point measurement analysis

Plasma and magnetic field data taken simultaneously in the near-Earth plasma sheet (6.6–13 RE) aboard three satellites, GEOS-2, ISEE-1, and ISEE-2, are used to infer directly the location and tailward propagation velocity of the partial cross-tail current disruption associated with a well-defined substorm. This study shows that the disruption starts at 6–9 RE and propagates down the tail with a velocity of the order of 150 to 250 km/s over tens of earth radii during the substorm expansion phase. Moreover, magnetic variations measured aboard GEOS-2 suggest that the partial current disruption also propagates longitudinally. After a time delay of about 2 minutes after the disruption onset, an injection front of accelerated particles is measured at geostationary orbit. This time delay is interpreted as indicating that the front propagates earthward with a velocity of the order of 20 to 160 km/s and also eastward. The overall post-onset magnetic signatures measured at geostationary orbit are shown to be mainly due to an expansion (thickening) of the current sheet in the close vicinity of GEOS-2 during the expansion phase.

Radiation belt electron precipitation due to VLF transmitters: Satellite observations

In the Earth’s inner magnetosphere, the distribution of energetic electrons is controlled by pitch-angle scattering by waves. A category of Whistler waves originates from powerful ground-based VLF transmitter signals in the frequency range 10–25 kHz. These transmissions are observed in space as waves of very narrow bandwidth. Here we examine the significance of the VLF transmitter NWC on the inner radiation belt using DEMETER satellite global observations at low altitudes. We find that enhancements in the 100–600 keV drift-loss cone electron fluxes at L values between 1.4 and 1.7 are linked to NWC operation and to ionospheric absorption. Waves and particles interact in the vicinity of the magnetic equatorial plane. Using Demeter passes across the drifting cloud of electrons caused by the transmitter; we find that 300 times more 200 keV electrons are driven into the drift-loss cone during NWC transmission periods than during non-transmission periods. The correlation between the flux of resonant electrons and the Dst index shows that the electron source intensity is controlled by magnetic storm activity.

Multispacecraft observation of magnetic cloud erosion by magnetic reconnection during propagation

During propagation, Magnetic Clouds (MC) interact with their environment and, in particular, may reconnect with the solar wind around it, eroding away part of its initial magnetic flux. Here we quantitatively analyze such an interaction using combined, multipoint observations of the same MC flux rope by STEREO A, B, ACE, WIND and THEMIS on November 19-20, 2007. Observation of azimuthal magnetic flux imbalance inside a MC flux rope has been argued to stem from erosion due to magnetic reconnection at its front boundary. The present study adds to such analysis a large set of signatures expected from this erosion process. (1) Comparison of azimuthal flux imbalance for the same MC at widely separated points precludes the crossing of the MC leg as a source of bias in flux imbalance estimates. (2) The use of different methods, associated errors and parametric analyses show that only an unexpectedly large error in MC axis orientation could explain the azimuthal flux imbalance. (3) Reconnection signatures are observed at the MC front at all spacecraft, consistent with an ongoing erosion process. (4) Signatures in suprathermal electrons suggest that the trailing part of the MC has a different large-scale magnetic topology, as expected. The azimuthal magnetic flux erosion estimated at ACE and STEREO A corresponds respectively to 44% and 49% of the inferred initial azimuthal magnetic flux before MC erosion upon propagation. The corresponding average reconnection rate during transit is estimated to be in the range 0.12-0.22 mV/m, suggesting most of the erosion occurs in the inner parts of the heliosphere. Future studies ought to quantify the influence of such an erosion process on geo-effectiveness. ©2012. American Geophysical Union. All Rights Reserved.

Currents and associated electron scattering and bouncing near the diffusion region at Earth's magnetopause

Based on high-resolution measurements from NASAs Magnetospheric Multiscale mission, we present the dynamics of electrons associated with current systems observed near the diffusion region of magnetic reconnection at Earths magnetopause. Using pitch angle distributions (PAD) and magnetic curvature analysis, we demonstrate the occurrence of electron scattering in the curved magnetic field of the diffusion region down to energies of 20 eV. We show that scattering occurs closer to the current sheet as the electron energy decreases. The scattering of inflowing electrons, associated with field-aligned electrostatic potentials and Hall currents, produces a new population of scattered electrons with broader PAD which bounce back and forth in the exhaust. Except at the center of the diffusion region the two populations are collocated and appear to behave adiabatically: the inflowing electron PAD focuses inward (toward lower magnetic field), while the bouncing population PAD gradually peaks at 90° away from the center (where it mirrors owing to higher magnetic field and probable field-aligned potentials).

Tracing solar wind plasma entry into the magnetosphere using ion-to-electron temperature ratio

When the solar wind Mach number is low, typically such as in magnetic clouds, the physics of the bow shock leads to a downstream ion-to-electron temperature ratio that can be notably lower than usual. We utilize this property to trace solar wind plasma entry into the magnetosphere by use of Cluster measurements in the vicinity of the dusk magnetopause during the passage of a magnetic cloud at Earth on November 25, 2001. The ion-to-electron temperature ratio was indeed low in the magnetosheath (Ti/Te ∼ 3). In total, three magnetopause boundary layer intervals are encountered on that day. They all show that the low ion-to-electron temperature ratio can be preserved as the plasma enters the magnetosphere, and both with and without the observation of Kelvin-Helmholtz activity. This suggests that the ion-to-electron temperature ratio in the magnetopause boundary layer, which is usually high, is not prescribed by the heating characteristics of the plasma entry mechanism that formed these boundary layers. In the future, this property may be used to (1) further trace plasma entry into inner regions and (2) determine the preferred entry mechanisms if other theoretical, observational and simulation works can give indications on which mechanisms may alter this ratio.

Three‐dimensional current systems and ionospheric effects associated with small dipolarization fronts

We present a case study of eight successive plasma sheet (PS) activations (usually referred to as bursty bulk flows or dipolarization fronts), associated with small individual B-ZGSM increases on 31 March 2009 (0200-0900 UT), observed by the Time History of Events and Macroscale Interactions During Substorms mission. This series of events happens during very quiet solar wind conditions, over a period of 7 h preceding a substorm onset at 1230 UT. The amplitude of the dipolarizations increases with time. The low-amplitude dipolarization fronts are associated with few (1 or 2) rapid flux transport events (RFT, E-h > 2 mV/m), whereas the large-amplitude ones encompass many more RFT events. All PS activations are associated with small and localized substorm current wedge (SCW)-like current system signatures, which seems to be the consequence of RFT arrival in the near tail. The associated ground magnetic perturbations affect a larger part of the contracted auroral oval when, in the magnetotail, more RFT are embedded in PS activations (> 5). Dipolarization fronts with very low amplitude, a type usually not included in statistical studies, are of particular interest because we found even those to be associated with clear small SCW-like current system and particle injections at geosynchronous orbit. This exceptional data set highlights the role of flow bursts in the magnetotail and leads to the conclusion that we may be observing the smallest form of a substorm or rather its smallest element. This study also highlights the gradual evolution of the ionospheric current disturbance as the plasma sheet is observed to heat up.

A study of the changes of the near‐Earth plasma sheet and lobe driven by multiple substorms: Comparison with a full particle simulation of reconnection

Comparisons of multispacecraft observations and full-particle simulations are used to understand magnetotail changes during substorms and the related cross-tail current disruptions/reductions. We first show that the electric field accompanying current disruptions can be measured in the tail lobe from the drift velocity of oxygen beams. A stormy period is studied here with a fleet of spacecraft including the four Cluster spacecraft and the Double Star spacecraft TC-1 in the tail, ACE and Geotail respectively in the solar wind and magnetosheath, and five LANL geostationary satellites, thus allowing the determination of the direction of propagation of the substorm disturbances. Each substorm here corresponds to an energy-loading period followed by a dipolarization of the magnetic field seen from 11 to 18 R-E. Plasma sheet thinning inside 12 R-E occurs during energy loading and is enhanced at the onset of strong dissipations of magnetic energy, which precede by several minutes particle injections at 6.6 R-E. Dipolarizations coincide with an increase of the lobe electric field, up to several mV/m. This study shows that the onset of the magnetic energy conversion occurs at about similar to 10-11 R-E and that once initiated, the perturbation propagates both toward the Earth and toward the distant tail. Comparisons of the measurements with recently published 2D full particle simulations of the reconnection process by Oka et al. (2008) indicate a good agreement between data and simulated magnetic lobe signatures. This suggests that the lobe magnetic changes are the signature of a tailward retreating neutral line, with its associated current disruption/reduction.

Solar wind control of the terrestrial magnetotail as seen by STEREO

At the beginning of 2007 the twin STEREO spacecraft provided a unique opportunity to study the global solar wind control of the terrestrial magnetotail under typical solar activity minimum conditions. The STEREO-B (STB) spacecraft flew in the vicinity of the far terrestrial magnetotail, while the STEREO-A (STA) spacecraft was located in front of the Earth performing measurements in the undisturbed solar wind. In February, the STB spacecraft was located in the magnetosheath most of the time but experienced several incursions into the distant magnetotail. Comparison of STA and STB observations determines unambiguously whether solar wind events such as energetic particle enhancements observed by STB are of pure solar origin or due to the influence of the terrestrial magnetosphere. During this time period in 2007, there were solar minimum conditions with alternating fast and slow solar wind streams that formed corotating interaction regions, which were the dominating source of magnetospheric disturbances encountering the Earth almost every week. Under these conditions, STB experienced multiple bow shock and magnetopause crossings due to the induced highly dynamic behavior of the terrestrial magnetotail and detected bursts of tailward directed energetic ions in the range of 110–2200 keV accompanied by suprathermal electrons of ~700–1500 eV, which were not seen in the undisturbed solar wind by STA. The corotating interaction regions triggered these energetic particle enhancements, and we demonstrate their magnetosphere-related origin. Even after leaving the magnetosheath in March 2007, STB continued to observe antisunward directed energetic ion bursts until May up to a distance of ~ 800 RE behind Earth, the largest distance to which solar wind and magnetospheric interaction has been observed. These results show that Earth is a very significant source of energetic particles in its local interplanetary environment.

Magnetosphere-ionosphere response to an enhanced energetic coupling with the solar wind

A remote sensing study of the dynamics of the cross-tail current has been performed using ISEE magnetic field data during a 12-hour period when these spacecraft were in the tail lobe. IMP 8 data taken simultaneously in the interplanetary medium showed that the solar wind was continuously supplying power to the magnetosphere. On a global scale the magnetosphere responded directly, as evidenced by the good general correlation between the auroral electrojets and the interplanetary magnetic field (IMF). A detailed study of the ISEE and ground magnetic fields reveals the occurrence of four main successive cross-tail current disruptions associated with substorm onsets. As a general trend, each disruption was followed by a cross-tail current increase while in the meantime the intensity of the auroral electrojet decreased. A new substorm started when the cross-tail current again reached a high value and while the polar ionosphere was relaxing to a quieter state. These observations also apply to the last substorm of the series, which coincided with a northward turning of the IMF. These results strongly support a cross-tail current threshold instability as the main cause of auroral substorms.