Solène Lejosne

Time domain structures: What and where they are, what they do, and how they are made

Time domain structures (TDS) (electrostatic or electromagnetic electron holes, solitary waves, double layers, etc.) are ≥1ms pulses having significant parallel (to the background magnetic field) electric fields. They are abundant through space and occur in packets of hundreds in the outer Van Allen radiation belts where they produce magnetic-field-aligned electron pitch angle distributions at energies up to a hundred keV. TDS can provide the seed electrons that are later accelerated to relativistic energies by whistlers and they also produce field-aligned electrons that may be responsible for some types of auroras. These field-aligned electron distributions result from at least three processes. The first process is parallel acceleration by Landau trapping in the TDS parallel electric field. The second process is Fermi acceleration due to reflection of electrons by the TDS. The third process is an effective and rapid pitch angle scattering resulting from electron interactions with the perpendicular and parallel electric and magnetic fields of many TDS. TDS are created by current-driven and beam-related instabilities and by whistler-related processes such as parametric decay of whistlers and nonlinear evolution from oblique whistlers. New results on the temporal relationship of TDS and particle injections, types of field-aligned electron pitch angle distributions produced by TDS, the mechanisms for generation of field-aligned distributions by TDS, the maximum energies of field-aligned electrons created by TDS in the absence of whistler mode waves, TDS generation by oblique whistlers and three-wave-parametric decay, and the correlation between TDS and auroral particle precipitation, are presented.

Van Allen Probe measurements of the electric drift E × B/B2 at Arecibo's L = 1.4 field-line coordinate†

We have used electric and magnetic measurements by Van Allen Probe B from 2013 to 2014 to examine the equatorial electric drift ExB/B2 at one field-line coordinate set to Arecibos incoherent scatter radar location (L = 1.43). We report on departures from the traditional picture of corotational motion with the Earth in two ways: (1) the rotational angular speed is found to be 10% smaller than the rotational angular speed of the Earth, in agreement with previous works on plasmaspheric notches, and (2) the equatorial electric drift displays a dependence in magnetic local time, with a pattern consistent with the mapping of the Arecibo ionosphere dynamo electric fields along equipotential magnetic field lines. The electric fields due to the ionosphere dynamo are therefore expected to play a significant role when discussing for instance the structure and dynamics of the plasmasphere or the transport of trapped particles in the inner belt.

Pulsating auroras produced by interactions of electrons and time domain structures

Previous evidence has suggested that either lower band chorus waves or kinetic Alfven waves scatter equatorial kilovolt electrons that propagate to lower altitudes where they precipitate or undergo further low altitude scattering to make pulsating auroras. Recently, time domain structures (TDS) were shown, both theoretically and experimentally, to efficiently scatter equatorial electrons. To assess the relative importance of these three mechanisms for production of pulsating auroras, eleven intervals of equatorial THEMIS data and a four-hour interval of Van Allen Probe measurements have been analyzed. During these events, lower band chorus waves produced only negligible modifications of the equatorial electron distributions. During the several TDS events, the equatorial 0.1-3 keV electrons became magnetic-field-aligned. Kinetic Alfven waves may also have had a small electron scattering effect. The conclusion of these studies is that time domain structures caused the most important equatorial scattering of ~1 keV electrons toward the loss cone to provide the main electron contribution to pulsating auroras. Chorus wave scattering may have provided part of the highest energy (>10 keV) electrons in such auroras.

Subauroral Polarization Streams (SAPS) Duration as Determined From Van Allen Probe Successive Electric Drift Measurements

We examine a characteristic feature of the magnetosphere-ionosphere coupling, namely, the persistent and latitudinally narrow bands of rapid westward ion drifts called the Sub-Auroral Polarization Streams (SAPS). Despite countless works on SAPS, information relative to their durations is lacking. Here, we report on the first statistical analysis of more than 200 near-equatorial SAPS observations based on more than two years of Van Allen Probe electric drift measurements. First, we present results relative to SAPS radial locations and amplitudes. Then, we introduce two different ways to estimate SAPS durations. In both cases, SAPS activity is estimated to last for about nine hours on average. However, our estimates for SAPS duration are limited either by the relatively long orbital periods of the spacecraft or by the relatively small number of observations involved. 50 % of the events fit within the time interval [0;18] hours.

Model‐observation comparison for the geographic variability of the plasma electric drift in the Earth's innermost magnetosphere

Plasmaspheric rotation is known to lag behind Earth rotation. The causes for this corotation lag are not yet fully understood. We have used more than two years of Van Allen Probe observations to compare the electric drift measured below L~2 with the predictions of a general model. In the first step, a rigid corotation of the ionosphere with the solid Earth was assumed in the model. The results of the model-observation comparison are twofold: (1) radially, the model explains the average observed geographic variability of the electric drift; (2) azimuthally, the model fails to explain the full amplitude of the observed corotation lag. In the second step, ionospheric corotation was modulated in the model by thermospheric winds, as given by the latest version of the Horizontal Wind Model (HWM14). Accounting for the thermospheric corotation lag at ionospheric E-region altitudes results in significantly better agreement between the model and the observations.

Extremely Field‐Aligned Cool Electrons in the Dayside Outer Magnetosphere

For 200 days in 2016 while THEMIS_D was in the dayside, equatorial, magnetosphere, its electron energy coverage was modified such that the first 15 energy steps covered the range of 1-30 eV and 16 steps covered energies to 30 keV. These measurements were free of backgrounds from photoelectrons, secondaries, or ionospheric plasma plumes. Three energy bands of electrons were observed: cold electrons having energies below 1 eV (plasmaspheric plumes measured by the spacecraft potential); cool electrons, defined as electrons having energies of 1-25 eV; and hot electrons having energies of 25 eV to 30 keV. The cool electron fluxes at fixed radial distances varied by an order-of-magnitude from one orbit to the next. These fluxes often increased with increasing radial distance, suggesting an external source. They were extremely field-aligned, having pitch angle ratios (flux at 0-20 degrees and 160-180 degrees divided by the flux at 80-100 degrees) greater than 100. Evidence is presented that they resulted from cusp electrons moving from open to closed magnetospheric field lines due to their ExB/B2 drift. They constituted the majority of the electron energy density at such times and places. They were not associated with magnetopause reconnection because they were not observed at the magnetopause but they were observed as far as three Earth radii inside of it. Their occurrence probability in the outer magnetosphere was ~50% in June and ~10% in September, suggesting a dayside source attributed to the tilt of the northern cusp towards the sun during the summer equinox.

An algorithm for approximating the L * invariant coordinate from the real‐time tracing of one magnetic field line between mirror points

The L * invariant coordinate depends on the global electromagnetic field topology at a given instance and the standard method for its determination requires a computationally expensive drift contour tracing. This fact makes L * a cumbersome parameter to handle. In this paper, we provide new insights on the L * parameter and we introduce an algorithm for an L * approximation that only requires the real-time tracing of one magnetic field line between mirrors points. This approximation is based on the description of the variation of the magnetic field mirror intensity after an adiabatic dipolarization, i.e., after the non-dipolar components of a magnetic field have been turned off with a characteristic time very long in comparison with the particles’ drift periods. The corresponding magnetic field topological variations are deduced assuming that the field line foot points remain rooted in the Earths surface and the drift average operator is replaced with a computationally cheaper circular average operator. The algorithm results in a relative difference of a maximum of 12 % between the approximate L * and the output obtained using the International Radiation Belt Environment Modeling Library (IRBEM-LIB), in the case of the Tsyganenko 89 model for the external magnetic field (T89). This margin of error is similar to the margin of error due to small deviations between different magnetic field models at geostationary orbit. This approximate L * algorithm represents therefore a reasonable compromise between computational speed and accuracy of particular interest for real-time space weather forecast purposes