J.-A. Sauvaud

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.

A two satellite study of nightside flux transfer events in the plasma sheet

Abstract High time resolution measurements of magnetic and electric fields and of the kiloelectronvolt electron flux at X ∼ −21 Re in the central magnetotail were used in the study of short-duration (time scale 1 min) magnetic field events during two consecutive substorms on 23 March 1979. These events were observed at ISEE-1 and 2 (having 0.3 Re separation along the Z-coordinate), both inside and outside of the thinned plasma sheet, close (in time) to the polarity reversal of the plasma flow from tailward to earthward direction in association with the transient plasma sheet expansions. The enhanced magnetic flux closure (southward Bz during tailward streaming events, northward during earthward streaming events), plus enhanced plasma flow during these events, suggest that they are nightside magnetic flux transfer events (NFTEs). The typical behaviour of the magnetic field during a NFTE (compression, rotation then field decrease), the signatures of overpressure in such structures and their close correlation with ground-based observed auroral activations and high energy particle bursts signify that NFTE structures may result from the impulsive reconnection in the plasmasheet. Systematic differences in the magnetic variations found at ISEE-1 and 2 indicate current concentration at the outer plasmasheet boundary during the passage of a NFTE. Two NFTEs appeared to have structures similar to magnetic flux ropes. The NFTE structure and associated current system, as well as the interpretation of NFTE signatures in terms of the impulsive reconnection, are discussed. During the “poleward leap” stage of one substorm, the spacecraft within the central part of the plasma sheet observed the intense earthward magnetic flux transport associated with superimposed NFTE structures. The estimated amount of this flux transfer explained well the observed amplitude of the poleward expansion of the westward electrojet in the conjugate auroral zone.

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.

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.

Measurements of the ion escape rates from Venus for solar minimum

[1]xa0We report the first direct measurements of the Venusian atmospheric erosion rate due to the interaction with the solar wind. The erosion through the ion escape is determined during the period of the minimum solar activity from 24 May 2006 to 12 December 2007. The ion fluxes are measured in the energy range 10 eV to 25 keV by an ion mass spectrometer on board the Venus Express spacecraft and sampled statistically dense in the volume in the Venusian wake. The rates are Q(H+) = 7.1 · 1024 s−1, Q(He+) = 7.9 · 1022 s−1, and Q(O+) = 2.7 · 1024 s−1. The reported escape rates measured for the solar minimum are close to the rates estimated for the solar maximum from the Pioneer Venus Orbiter observations. We may thus propose that the atmospheric loss due to solar wind interaction depends weakly on the solar conditions. The paper also presents in detail how the global escape rates are deduced from the in situ measurements.

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 20u2009eV. 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).

Statistical study of magnetic cloud erosion by magnetic reconnection

Several recent studies suggest that magnetic reconnection is able to erode substantial amounts of the outer magnetic flux of interplanetary magnetic clouds (MCs) as they propagate in the heliosphere. We quantify and provide a broader context to this process, starting from 263 tabulated interplanetary coronal mass ejections, including MCs, observed over a time period covering 17u2009years and at a distance of 1u2009AU from the Sun with Wind (1995–2008) and the two STEREO (2009–2012) spacecraft. Based on several quality factors, including careful determination of the MC boundaries and main magnetic flux rope axes, an analysis of the azimuthal flux imbalance expected from erosion by magnetic reconnection was performed on a subset of 50 MCs. The results suggest that MCs may be eroded at the front or at rear and in similar proportions, with a significant average erosion of about 40% of the total azimuthal magnetic flux. We also searched for in situ signatures of magnetic reconnection causing erosion at the front and rear boundaries of these MCs. Nearly ~30% of the selected MC boundaries show reconnection signatures. Given that observations were acquired only at 1u2009AU and that MCs are large-scale structures, this finding is also consistent with the idea that erosion is a common process. Finally, we studied potential correlations between the amount of eroded azimuthal magnetic flux and various parameters such as local magnetic shear, Alfven speed, and leading and trailing ambient solar wind speeds. However, no significant correlations were found, suggesting that the locally observed parameters at 1u2009AU are not likely to be representative of the conditions that prevailed during the erosion which occurred during propagation from the Sun to 1u2009AU. Future heliospheric missions, and in particular Solar Orbiter or Solar Probe Plus, will be fully geared to answer such questions.

Observations of auroral electron inverted-V structures by the AUREOL-3 satellite

Abstract We present a detailed statistical study of the distribution of the inverted-V electron precipitation commonly detected in the 500–2000 km altitude range aboard the AUREOL-3 satellite. These structured precipitations are statistically observed inside the auroral oval with a maximum occurrence in the evening-midnight sector. They correspond to primary electron fluxes peaked at energies generally below 10 keV and precipitating in large-scale structures (50–100 km in latitudinal width). It is shown that, as predicted by collisionless kinetic theories, most inverted-V structures present a clear relationship between the field-aligned current density carried by the 1–20 keV primary electrons and the potential drop inferred from particle distribution functions. Furthermore the study demonstrates the existence of strong electron heating, inside the acceleration region.

A statistical analysis of properties of small transients in the solar wind 2007–2009: STEREO and Wind observations

We present a comprehensive statistical analysis of small solar wind transients (STs) in 2007–2009. Extending work on STs by Kilpua et al. (2009) to a 3 year period, we arrive at the following identification criteria: (i) a duration < 12 h, (ii) a low proton temperature and/or a low proton beta, and (iii) enhanced field strength relative to the 3 year average. In addition, it must have at least one of the following: (a) decreased magnetic field variability, (b) large, coherent rotation of the field vector, (c) low Alfven Mach number, and (d) Te/Tp higher than the 3 year average. These criteria include magnetic flux ropes. We searched for STs using Wind and STEREO data. We exclude Alfvenic fluctuations. Case studies illustrate features of these configurations. In total, we find 126 examples, ∼81% of which lie in the slow solar wind (≤ 450 km s−1). Many start or end with sharp field and flow gradients/discontinuities. Year 2009 had the largest number of STs. The average ST duration is ∼4.3 h, 75%<6 h. Comparing with interplanetary coronal mass ejections (ICMEs) in the same solar minimum, we find the major difference to be that Tp in STs is not significantly less than the expected Tp. Thus, whereas a low Tp is generally considered a very reliable signature of ICMEs, it is not a robust signature of STs. Finally, since plasma β∼1, force-free modeling of STs having a magnetic flux rope geometry may be inappropriate.

Altitude dependence of nightside Martian suprathermal electron depletions as revealed by MAVEN observations

The MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft is providing new detailed observations of the Martian ionosphere thanks to its unique orbital coverage and instrument suite. During most periapsis passages on the nightside ionosphere suprathermal electron depletions were detected. A simple criterion was implemented to identify the 1742 depletions observed from 16 November 2014 to 28 February 2015. A statistical analysis reveals that the main ion and electron populations within the depletions are surprisingly constant in time and altitude. Absorption by CO2 is the main loss process for suprathermal electrons, and electrons that strongly peaked around 6 eV are resulting from this interaction. The observation of depletions appears however highly dependent on altitude. Depletions are mainly located above strong crustal magnetic sources above 170 km, whereas the depletions observed for the first time below 170 km are globally scattered onto the Martian surface with no particular dependence on crustal fields.