Research Items (22)
- May 2018
- 2018 2nd URSI Atlantic Radio Science Meeting (AT-RASC)
It has been reported that the dynamics of energetic (tens to hundreds of keV) electrons and ions is inconsistent with the theoretical picture in which the large-scale electric field is a superposition of corotation and convection electric fields. Combining one year of measurements by the Super Dual Auroral Radar Network, DMSP F-18, and the Van Allen Probes, we show that subauroral polarization streams (SAPSs) are observed when energetic electrons have penetrated below L = 4. Outside the plasmasphere in the premidnight region, potential energy is subtracted from the total energy of ions and added to the total energy of electrons during SAPS onset. This potential energy is converted into radial motion as the energetic particles drift around Earth and leave the SAPS azimuthal sector. As a result, energetic electrons are injected deeper than energetic ions when SAPSs are included in the large-scale electric field picture, in line with observations.
More than 2 years of magnetic and electric field measurements by the Van Allen Probes are analyzed with the objective of determining the average effects of magnetic activity on the electric drift below L = 3. The study finds that an increase in magnetospheric convection leads to a decrease in the magnitude of the azimuthal component of the electric drift, especially in the nightside. The amplitude of the slowdown is a function of L, magnetic local time, and Kp, in a pattern consistent with the storm time dynamics of the ionosphere and thermosphere. To a lesser extent, magnetic activity also alters the average radial component of the electric drift below L = 3. A global picture for the average variations of the electric drift with Kp is provided as a function of L and magnetic local time. It is the first time that the signature of the ionospheric disturbance dynamo is observed in near-equatorial electric drift measurements.
- Mar 2018
Nishimura et al [2010, 2011, 2013 and in their comment, hereafter called N18] have suggested that chorus waves interact with equatorial electrons to produce pulsating auroras. We agree that chorus can scatter electrons >10 keV, as do Time Domain Structures (TDS). Lower energy electrons occurring in pulsating auroras cannot be produced by chorus but such electrons are scattered and accelerated by TDS. TDS often occur with chorus and have power in their spectra at chorus frequencies. Thus, the absence of power at low frequencies is not evidence that TDS are absent, as an example shows. Through examination of equatorial electric field waveforms and electron pitch angle distributions measured on the THEMIS satellites (in place of examining field and particle spectra, as done by Nishimura et al), we show that chorus cannot produce the field-aligned electrons associated with pulsating auroras in the Nishimura et al  events, but Time Domain Structures can. Equatorial field-aligned electron distributions associated with pulsating auroras and created by TDS in the absence of chorus or any other wave at the equator are also shown.
- Feb 2018
L-star” was introduced in geomagnetically trapped particle dynamics. It is thus timely to review the use of adiabatic theory in present-day studies of the radiation belts, with the intention of helping to prevent common misinterpretations and the frequent confusion between concepts like “distance to the equatorial point of a field line”, McIlwain's L-value and the trapped particle's adiabatic L* parameter. And too often do we miss in the recent literature a proper discussion of the extent to which some observed time- and space-signatures of particle flux could simply be due to changes in magnetospheric field, especially insofar as off-equatorial particles are concerned. We present a brief review on the history of radiation belt parametrization, some “recipes” on how to compute adiabatic parameters, and we illustrate our points with a real event in which magnetospheric disturbance is shown to adiabatically affect the particle fluxes measured onboard the Van Allen Probes.
We examine a characteristic feature of the magnetosphere-ionosphere coupling, namely, the persistent and latitudinally narrow bands of rapid westward ion drifts called the subauroral 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 2 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 9 h 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. Fifty percent of the events fit within the time interval [0;18] hours.
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.
- Aug 2017
- 2017 XXXIInd General Assembly and Scientific Symposium of the International Union of Radio Science (URSI GASS)
- Jul 2017
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 2 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. Accounting for the thermospheric corotation lag at ionospheric E region altitudes results in significantly better agreement between the model and the observations.
For 200 days in 2016 while Time History of Events and Macroscale Interactions during Substorms D (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° and 160–180° divided by the flux at 80–100°) greater than 100. Evidence is presented that they resulted from cusp electrons moving from open to closed magnetospheric field lines due to their E × B/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 3 RE 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 toward the Sun during the summer.
The electric drift E × B/B2 plays a fundamental role for the description of plasma flow and particle acceleration. Yet it is not well-known in the inner belt and slot region because of a lack of reliable in situ measurements. In this article, we present an analysis of the electric drifts measured below L ~ 3 by both Van Allen Probes A and B from September 2012 to December 2014. The objective is to determine the typical components of the equatorial electric drift in both radial and azimuthal directions. The dependences of the components on radial distance, magnetic local time, and geographic longitude are examined. The results from Van Allen Probe A agree with Van Allen Probe B. They show, among other things, a typical corotation lag of the order of 5 to 10% below L ~ 2.6, as well as a slight radial transport of the order of 20 m s−1. The magnetic local time dependence of the electric drift is consistent with that of the ionosphere wind dynamo below L ~ 2 and with that of a solar wind-driven convection electric field above L ~ 2. A secondary longitudinal dependence of the electric field is also found. Therefore, this work also demonstrates that the instruments on board Van Allen Probes are able to perform accurate measurements of the electric drift below L ~ 3.
- Jul 2016
We have used electric and magnetic measurements by Van Allen Probe B from 2013 to 2014 to examine the equatorial electric drift E × B/B2 at one field line coordinate set to Arecibo's 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.
- Jan 2016
We examine a characteristic effect, namely the ubiquitous appearance of structured peaks and valleys called zebra stripes in the spectrograms of energetic electrons and ions trapped in the inner belt below L~3. We propose an explanation of this phenomenon as a purely kinematic consequence of particle drift velocity modulation caused by F-region zonal plasma drifts in the ionosphere. In other words, we amend the traditional assumption that the electric field associated with ionospheric plasma drives trapped particle distributions into rigid corotation with the Earth. An equation based on a simple first-order model is set up to determine quantitatively the appearance of zebra stripes as a function of magnetic time. Our numerical predictions are in agreement with measurements by the RBSPICE detector onboard Van Allen Probes, namely: (1) the central energy of any peak identified in the spectrum on the dayside is the central energy of a spectral valley on the night side, and vice versa; (2) there is also an approximate peak-to-valley inversion when comparing the spectrum of trapped electrons with that of trapped ions in the same place; (3) the actual energy separation between two consecutive peaks (or number of stripes) in the spectrogram of a trapped population is an indicator of the time spent by the particles drifting under quiet conditions.
- Apr 2015
Time domain structures (TDS) (electrostatic or electromagnetic electron holes, solitary waves, double layers, etc.) are ≥1 ms 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.
An accurate forecast of flare and coronal mass ejection (CME) initiation requires precise measurements of the magnetic energy buildup and release in the active regions of the solar atmosphere. We designed a new space weather mission that performs such measurements using new optical instruments based on the Hanle and Zeeman effects. The mission consists of two satellites, one orbiting the L1 Lagrangian point (Spacecraft Earth, SCE) and the second in heliocentric orbit at 1AU trailing the Earth by 80° (Spacecraft 80, SC80). Optical instruments measure the vector magnetic field in multiple layers of the solar atmosphere. The orbits of the spacecraft allow for a continuous imaging of nearly 73% of the total solar surface. In-situ plasma instruments detect solar wind conditions at 1AU and ahead of our planet. Earth-directed CMEs can be tracked using the stereoscopic view of the spacecraft and the strategic placement of the SC80 satellite. Forecasting of geoeffective space weather events is possible thanks to an accurate surveillance of the magnetic energy buildup in the Sun, an optical tracking through the interplanetary space, and in-situ measurements of the near-Earth environment.
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 Earth's 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.
- Jul 2014
The mechanisms for accelerating electrons from thermal to relativistic energies in the terrestrial magnetosphere, on the sun, and in many astrophysical environments have never been verified. We present the first direct observation of two processes that, in a chain, cause this acceleration in Earth's outer radiation belt. The two processes are parallel acceleration from electron-volt to kilovolt energies by parallel electric fields in time-domain structures (TDS), after which the parallel electron velocity becomes sufficiently large for Doppler-shifted upper band whistler frequencies to be in resonance with the electron gyration frequency, even though the electron energies are kilovolts and not hundreds of kilovolts. The electrons are then accelerated by the whistler perpendicular electric field to relativistic energies in several resonant interactions. TDS are packets of electric field spikes, each spike having duration of a few hundred microseconds and containing a local parallel electric field. The TDS of interest resulted from nonlinearity of the parallel electric field component in oblique whistlers and consisted of ∼0.1 msec pulses superposed on the whistler waveform with each such spike containing a net parallel potential the order of 50 V. Local magnetic field compression from remote activity provided the free energy to drive the two processes. The expected temporal correlations between the compressed magnetic field, the nonlinear whistlers with their parallel electric field spikes, the electron flux and the electron pitch angle distributions were all observed.
This study falls within the field of the Earth’s radiation belt dynamics. It consists of modeling the radial diffusion process based on a spatiotemporal resolution higher than the resolution at which radiation belt dynamics are described in terms of a diffusion equation. The approach has been organized in three parts. First, we described radial diffusion theoretically, highlighting the main drivers of the phenomenon and giving a ready-made formula of the radial diffusion coefficients. Then, based on this formula, we aimed to quantify the radial diffusion coefficients. In order to reach this goal, we developed analytical and numerical procedures, and then, observational procedures. Finally, we discussed the results and the pros and cons of each method. This study highlights the central role of asymmetric variations of the electromagnetic fields and induced electric fields in the driving of the intensity of the radial diffusion process. It provides tracks for numerical and experimental quantification of these two drivers. It also provides tools for a critical review of the literature. It paves the way for a more accurate determination of radial diffusion coefficients based on a more precise description of the electromagnetic environment and its variations.
In this paper, we show that the correlation that exists between magnetic variations and induced electric fields through Faraday's law is of prime importance for adequately characterizing electromagnetic radial diffusion. Accordingly, we present an approach to derive electromagnetic radial diffusion coefficients based on magnetic field measurements at geostationary orbit. It consists of setting a very simple theoretical electromagnetic field model, considering the magnetic field as a background dipolar field on which two small time disturbances are superimposed: a symmetric disturbance and an asymmetric disturbance. Within this framework, electromagnetic radial diffusion is quantified analytically, taking into account both induced electric and magnetic contributions. The role played by the time variations of the field asymmetry is highlighted. From this, we deduce instantaneous field asymmetries from measurements of the magnetic field at the same time in two different places of the geostationary orbit. Then, we perform a statistical analysis of the time variations of this signal based on more than 7 years of data from the NOAA-GOES 8, NOAA-GOES 10, and NOAA-GOES 12 spacecraft, working with time resolutions of 1 and 5 min. We show that the asymmetry signal is not stationary, having time-dependent statistical properties, and we question accordingly the standard formulation of the electromagnetic radial diffusion coefficient and the role of drift-resonant interactions. Finally, we provide new electromagnetic radial diffusion coefficients at geostationary orbit as a function of electron kinetic energy and Kp index from 0 to 4.
- Apr 2013
It has been shown that the magnetic field perturbations induce two different effects on the particles drifting around the Earth. With a symmetric perturbation, because of the induced electric field, particles are displaced in location, but as their third adiabatic invariant is not violated at all with this motion, the particles come back to their initial drift shell when the perturbation is finished. Only the asymmetric part of the perturbation does induce radial diffusion. Therefore we investigated the possibility to obtain instantaneous asymmetries using magnetic field measurements on board the NOAA GOES satellites. Since 1999, there is always 2 GOES satellites located at longitudes 75°W and 135°W, separated by around 4h in local time. This separation is sufficient to obtain a good determination of the asymmetry, when both satellites are located in the dawn or in the dusk sectors. Using the period between September 1998 and May 2003 and the satellites GOES 8 and 10, it was possible to estimate the asymmetries of the magnetic field for a large range of magnetic activity, and deduce the radial diffusion coefficients. The results for dawn and dusk sectors were compared each other, and compared to pre existing models of radial diffusion. The average coefficients obtained can vary by several orders of magnitude from very quiet (Kp=0) to very active (Kp=9). These results were also compared with the ones obtained using GOES 10 and 12 during the period 2003-2006.
In this paper, a new approach for the derivation of the instantaneous rate of change of the third adiabatic invariant is introduced. It is based on the tracking of the bounce-averaged motion of guiding centers with assumptions that are only kept to the necessary conditions for definition and conservation of the first two adiabatic invariants. The derivation is first given in the case of trapped equatorial particles drifting in a time varying magnetic field in the absence of electrostatic potential. It is then extended to more general cases including time varying electric potentials and non-equatorial particles. Finally, the general formulation of the third adiabatic invariant time derivative is related to the description of the radial diffusion process occurring in the radiation belts. It highlights the links that exist between previous theoretical works with the objective of a better understanding of the radial diffusion process. A theoretical validation in the specific case of equatorial particles drifting in a magnetic field model whose disturbed part is limited to the first terms of a spherical expansion is also presented.
- Apr 2012
Radiation belts can be considered as an open-system whose population is fed by external sources. For trapped electrons and protons of energies below a few MeV, plasmasheet injections constitute the main seed population. Acceleration processes inside radiation belts will diffuse plasmasheet population to inner regions and energize them. A statistical survey has been conducted using NPOES and THEMIS data in order to characterize near Earth plasmasheet according to geomagnetic activity parameters, radial distance and magnetic local time. Interesting features have been highlighted, in particular the unexpected good correlation between low-orbit and equatorial measurements. Such works constitute advances for a better radiation belts modelling. In particular, even if THEMIS spacecrafts are currently measuring really quiet geomagnetic activity since their launches, these statistical results highlight their great potentiality in improving our knowledge on radiation belts trapping processes.