ORIGINAL RESEARCH article
Thermospheric Neutral Winds as the Cause of Drift Shell Distortion in Earth’s Inner Radiation Belt
- 1Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA, United States
- 2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, United States
- 3NOAA Space Weather Prediction Center, Boulder, CO, United States
- 4Space Vehicles Directorate, Air Force Research Laboratory, Kirtland AFB, Albuquerque, NM, United States
Recent analysis of energetic electron measurements from the Magnetic Electron Ion Spectrometer instruments onboard the Van Allen Probes showed a local time variation of the equatorial electron intensity in the Earth’s inner radiation belt. The local time asymmetry was interpreted as evidence of drift shell distortion by a large-scale electric field. It was also demonstrated that the inclusion of a simple dawn-to-dusk electric field model improved the agreement between observations and theoretical expectations. Yet, exactly what drives this electric field was left unexplained. We combine in-situ field and particle observations, together with a physics-based coupled model, the Rice Convection Model (RCM) Coupled Thermosphere-Ionosphere-Plasmasphere-electrodynamics (CTIPe), to revisit the local time asymmetry of the equatorial electron intensity observed in the innermost radiation belt. The study is based on the dawn-dusk difference in equatorial electron intensity measured at L = 1.30 during the first 60 days of the year 2014. Analysis of measured equatorial electron intensity in the 150–400 keV energy range, in-situ DC electric field measurements and wind dynamo modeling outputs provide consistent estimates of the order of 6–8 kV for the average dawn-to-dusk electric potential variation. This suggests that the dynamo electric fields produced by tidal motion of upper atmospheric winds flowing across Earth’s magnetic field lines - the quiet time ionospheric wind dynamo - are the main drivers of the drift shell distortion in the Earth’s inner radiation belt.
The Van Allen Probes (Mauk et al., 2013) have provided unprecedented amounts of high quality energetic (10–100 keV) electron flux measurements near the magnetic equator in the inner belt and slot region (Reeves et al., 2016) below an equatorial altitude of about 3 Earth Radii (
Radiation belt particles are magnetically trapped high energy particles whose motion exhibits three quasi-periodic types of motion occurring on three distinct timescales (e.g., Northrop and Teller, 1960). The slowest periodicity corresponds to a drift motion around the planet that is the combination of an energy-independent electric drift and a gradient-curvature magnetic drift proportional to the particle’s momentum. Radiation belt particles’ drift motion defines closed surfaces known as drift shells. At high enough energies (>100 keV), the role played by large-scale electric fields is usually omitted: Radiation belt particles’ momentum is expected to be conserved along a drift shell and the flux is expected to be constant at a fixed
Even during geomagnetically quiet times, large-scale electric fields of the inner magnetosphere are more complex than the simple superposition of 1) a corotation electric field due to the rigid corotation of a perfectly conducting ionosphere and 2) a convection electric field set up by the coupling between the solar wind and the magnetosphere (e.g. Wolf et al., 2006). Dynamo electric fields produced by tidal motion of upper atmospheric winds flowing across the Earth’s magnetic field lines—the ionospheric wind dynamo (Richmond, 1989)—are usually larger than subauroral convection electric fields below L ∼ 2 (e.g., Figure 1 in the review by Mozer (1973). Longitudinal, seasonal, and solar cycle variations of the mid- and low-latitude ionospheric electric fields have been reported, together with a large day-to-day variability present even during geomagnetically quiet times (e.g., Fejer, 1993; Chau et al., 2010; Pfaff et al., 2010).
The objective of this study is to re-examine the time interval analyzed by Selesnick et al. (2016) in order to determine the origin of the large-scale electric field causing the observed radiation belt distortion. Observational and modeling resources are introduced in Section 2 to describe the electric field properties associated with the drift shell distortion observed. In particular, we present a method to infer electric field properties directly from an analysis of asymmetries measured in differential directional fluxes. Experimental, numerical and analytical experiments provide consistent electric potential variation estimates, of the order of
2. Materials and Methods
Leveraging Liouville’s theorem and adiabatic invariant theory, we conduct an analysis of energetic electron directional differential fluxes measured by the Van Allen Probes to infer electric field properties in the inner belt. The resulting electric field properties are compared with outputs from Van Allen Probes field measurements and from a physics-based coupled model, RCM-CTIPe.
2.1. Field and Particle Measurements Onboard the Van Allen Probes
The time interval of the study corresponds to the first 60 days of the year 2014. This time interval was also selected for the study by Selesnick et al. (2016) because 1) the two orbital legs are near dawn and dusk at
The Van Allen Probes (Fox and Burch, 2014) were twin spacecraft with similar highly elliptical orbits (perigee at
In this study, we rely primarily on the directional differential fluxes provided by the MagEIS instrument (processed to level 3). Electric field measurements are also analyzed to compare and contrast with proposed electric field models. Experimental electric field information comes from measurements by the Electric Field and Waves (EFW) instrument (Wygant et al., 2013) and by the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) (Kletzing et al., 2013). Field and particle measurements immediately following spacecraft maneuvers are omitted.
2.1.1. Differential Fluxes for Equatorial Electrons
The analysis focuses on equatorial electron intensity in the 150–400 keV energy range. MagEIS data below 400 keV is normally well above background. We follow the approach developed by Selesnick et al. (2016) to determine fluxes of equatorially mirroring electrons. MagEIS pitch-angle resolved measurements are extrapolated when the spacecraft is close to the magnetic equator. Specifically, directional differential fluxes are extrapolated to determine the equatorial electron intensity,
We define the magnetic shell parameter,
When experimental information is required at a fixed
2.1.2. Electric Drift Measurements
Electric drift measurements are pre-processed following the approach developed over the years by Lejosne and Mozer (2016a, 2016b, 2019) and briefly described below. After a slight correction applied to the orientation of the magnetometer axes (Lejosne and Mozer, 2016a), the spin-averaged (
2.2. Numerical Model RCM-CTIPe
Electric potential and electric field values at
2.3. Theoretical Framework for the Inference of Electric Field Properties From Directional Differential Flux Analysis
The objective of this section is to show how to infer information on electric field properties from measured variations of differential fluxes of equatorially trapped particles. This theoretical framework is similar to the one developed by Lejosne and Mozer (2020) for the analysis of zebra stripe patterns. It is adapted below to the case of drift shell distortion by quasi-static electric fields.
Leveraging Liouville’s theorem and adiabatic invariant theory, the variation of equatorial electron fluxes measured at different local times along the same
2.3.1. Link Between Fluctuations in Directional Differential Fluxes of Equatorially Trapped Particles and Kinetic Energy Variations
According to Liouville’s theorem, the phase space density,
This equation can also be rewritten in terms of kinetic energy,
The variations discussed above occur along the same equatorial drift shell, i.e.:
Assuming small distortions of the field, the fluctuation measured,
Combining Eqs 3, 4 and 8, and noting that
2.3.2. Link Between Kinetic Energy Variation and Electric Potential Variation
The time rate of change of the average kinetic energy of a guiding center of charge
where Ω is the angular velocity vector of the Earth’s rotation, and
2.3.3. Summary of the Theoretical Framework
A change of variables from
Theoretically, the electric potential variation,
The assumptions underlying this equation are that there is no significant source or loss mechanism on the time scale of the equatorially trapped population drift period, no external non-electromagnetic force, no parallel electric field, no other significant source of magnetic field time variation besides Earth’s rotation, and no significant time variation of the electric potential on the timescale of the electron drift period (this includes the timescale of spacecraft motion from the inbound to the outbound crossings of
Section 3.1. presents experimental information on the electric potential variation between dawn and dusk at
3.1. Experimental Results
3.1.1. Experimental Information on the Electric Potential Variation Between Dawn and Dusk Based on Measured Equatorial Electron Intensity Asymmetry
Figure 1 is an introduction to the approach. During the first 60 days of 2014, the Van Allen Probes crossings of
FIGURE 1. The variation of electric potential between the inbound and outbound locations of the same
The time variation of the electric potential variation,
The approach is extended to both Van Allen Probes, and to all four MagEIS energy channels between 150 and 400 keV. The results are presented Figure 2. While the times series for the electric potential variations seem to depend on the measuring spacecraft (Figures 2A,B), the averages, and standard deviations of the electric potential variation,
FIGURE 2. (A) Time series of electric potential variations according to four MagEIS channels comprised between 150 and 400 keV onboard the Van Allen Probes A (RBA). The time series inferred from each individual energy channel are represented by thin lines while the average over all four energy channels is represented by a thick solid green line. (B) Time series of electric potential variations according to four MagEIS channels comprised between 150 and 400 keV onboard the Van Allen Probes B (RBB). The time series for each energy channel are represented by thin lines while the average over all four energy channels is represented by a thick solid purple line. (C) Medians and standard deviations for the electric potential variation between dawn and dusk at L = 1.3 during the first 60 days of the year 2014 according to four MagEIS channels measuring differential direction fluxes for kinetic energies,
The analysis suggests that the average value for the electric potential variation between dawn and dusk is
3.1.2. Experimental Information on the Electric Potential Between Dawn and Dusk Based on Electric Field Measurements
While Van Allen Probes only provide local electric field samples at
FIGURE 3. Characteristics of the (A) radial and (B) azimuthal components of the electric field measured by both Van Allen Probes and projected to the magnetic equator during crossings of
3.2. Model-Observation Comparison for the Electric Potential and Electric Field Components
In this section, we provide electric potential variation between dawn and dusk predicted by analytic and numerical models.
3.2.1. Comparison With an Analytical Expression for the Electric Potential Variation
The simple electrostatic potential proposed by Selesnick et al. (2016),
3.2.2. Comparison With Numerical Values During a Quiet Time Run With RCM-CTipe
24 h worth of electric field values during a quiet-time run with the RCM-CTIPe are provided in Figure 4. The quiet time run corresponds to a day in spring (set to March 17, 2013) during which the solar wind-magnetosphere dynamo is artificially forced to 0. In that context, the electric fields are driven solely by the quiet time wind dynamo. Because the magnetic local time (MLT) is a combination of universal time (UT) and longitude, the time variations observed at a given MLT are also representative of the longitudinal dependence of the electric field. Ionospheric values have been projected to the magnetic equator along dipolar equipotential field lines. This projection means that the radial (poleward) ionospheric component of the electric field has been multiplied by a factor
FIGURE 4. Characteristics of the (A) radial and (B) azimuthal components of the electric field projected to the magnetic equator of
3.3. Conclusion of the Experimental Analysis
The different estimates for the dawn-dusk electric potential variation at
TABLE 1. Estimates of the electric potential variation between dawn and dusk, and average azimuthal electric field component between the dusk and dawn regions of the magnetic equator of
Table 1 shows that different methods provide consistent estimates for the electric potential variation between dawn and dusk, with a magnitude of the order of 6–8 kV. The similarity is remarkable given that: 1) the neutral winds and associated wind dynamo vary from day to day and 2) every method was applied under a unique set of conditions.
An electric potential variation of 6–8 kV between dawn and dusk is ∼15 times greater than would be predicted by the usual Maynard and Chen (1975) parametrization of the Volland (1973) - Stern (1975) convection electric field model during quiet geomagnetic activity (
Van Allen probes electric field measurements and simulations by RCM-CTIPe are in qualitative agreement, despite differences in their time intervals. These similarities confirm that the electric potential variation results mainly from the quiet time wind dynamo. Comparing Figures 3, 4 also demonstrates that the Van Allen Probes can resolve the effects of neutral winds on global electric fields.
4. Discussion: Time Variations of the Electric Potential Variation as Determined by the Measured Asymmetry in the Equatorial Electron Intensity
While the modeling and experimental results provide similar estimates for the average magnitude of the dawn-dusk electric potential variation, they do not explain the high variability revealed by the particle data analysis. The time series of dawn-dusk electric potential variation,
Data analyses similar to the one presented Section 3.1.1. were performed 1) using measurements by the Radiation Belt Storm Probes Ion Composition Experiment (Mitchell et al., 2013) in place of MagEIS, and/or 2) at higher
We searched for possible dependencies of the time series,
FIGURE 5. Various indicators [(A) F10.7, (B) Kp, (C) Dst and (D) Polar Cap Potential PCP] for the electromagnetic conditions associated with (E) the time dependence of the dawn-dusk electric potential variation, dU. The SuperMag substorm lists from Forsyth et al. (2015), Newell and Gjerloev (2011) and Ohtani and Gjerloev (2020) are indicated Panel (D) in black, blue and red, respectively.
Realistic modeling of the subauroral electric field evolution is required to further investigate the origin of the variability reported here.
Data Availability Statement
Publicly available datasets were analyzed in this study. This data can be found here: The Van Allen Probes electric field data are in public access in the RBSP/EFW database: www.space.umn.edu/rbspefw-data. The MagEIS and magnetic ephemeris data are available from the ECT Science Operations and Data Center, https://rbsp-ect.newmexicoconsortium.org/rbsp_ect.php Simulation outputs can be obtained by contacting NM.
The first author is the lead and corresponding author. All other authors are listed in alphabetical order. We describe contributions to the paper using the CRediT (Contributor Roles Taxonomy) categories (Brand et al., 2015). Conceptualization and Funding Acquisition: SL and NM. Resources: SL, MF, NM, and RS. Methodology, Software, Validation, Formal Analysis and Investigation: SL and RS. Writing-Original Draft, Visualization and Project Administration: SL. Writing-Review and Editing: SL, RS, and MF.
SL work was performed under JHU/APL Contract No. 922613 (RBSP-EFW), NASA Grant Awards 80NSSC18K1223 and 80NSSC20K1351. RS work is supported by the Space Vehicles Directorate of the Air Force Research Laboratory.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
The authors are grateful to Seth Claudepierre, the MAGEIS team, and the RBSPICE team for insightful discussions.
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Keywords: Earth’s inner radiation belt, thermospheric neutral winds, ionospheric wind dynamo, electric fields, radial transport
Citation: Lejosne S, Fedrizzi M, Maruyama N and Selesnick RS (2021) Thermospheric Neutral Winds as the Cause of Drift Shell Distortion in Earth’s Inner Radiation Belt. Front. Astron. Space Sci. 8:725800. doi: 10.3389/fspas.2021.725800
Received: 15 June 2021; Accepted: 13 August 2021;
Published: 01 September 2021.
Edited by:Jean-Francois Ripoll, CEA DAM Île-de-France, France
Reviewed by:Paul O’Brien, The Aerospace Corporation, United States
Margaret Chen, The Aerospace Corporation, United States
Copyright © 2021 Lejosne, Fedrizzi, Maruyama and Selesnick. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Solène Lejosne, firstname.lastname@example.org