Van Allen Probes Spacecraft Research Articles

Energetic electron detection packages on board Chinese navigation satellites in MEO

Energetic electron measurements and spacecraft charging are of great significance for theoretical research in space physics and space weather applications. In this paper, the energetic electron detection package (EEDP) deployed on three Chinese navigation satellites in medium Earth orbit (MEO) is reviewed. The instrument was developed by the space science payload team led by Peking University. The EEDP includes a pinhole medium-energy electron spectrometer (MES), a high-energy electron detector (HED) based on ΔE-E telescope technology, and a deep dielectric charging monitor (DDCM). The MES measures the energy spectra of 50−600 keV electrons from nine directions with a 180°×30° field of view (FOV). The HED measures the energy spectrum of 0.5−3.0 MeV electrons from one direction with a 30° cone-angle FOV. The ground test and calibration results indicate that these three sensors exhibit excellent performance. Preliminary observations show that the electron spectra measured by the MES and HED are in good agreement with the results from the magnetic electron-ion spectrometer (MagEIS) of the Van Allen Probes spacecraft, with an average relative deviation of 27.3% for the energy spectra. The charging currents and voltages measured by the DDCM during storms are consistent with the high-energy electron observations of the HED, demonstrating the effectiveness of the DDCM. The observations of the EEDP on board the three MEO satellites can provide important support for theoretical research on the radiation belts and the applications related to space weather.

A Model of the Subpacket Structure of Rising Tone Chorus Emissions

AbstractThe nonlinear growth theory of chorus emissions is used to develop a simple model of the subpacket formation. The model assumes that the resonant current, which is released from the source to the upstream region, radiates a new whistler mode wave with a slightly increased frequency, which triggers a new subpacket. Saturation of the growth in amplitude is controlled by the optimum amplitude. Numerical solution of advection equations for each subpacket, with the chorus equations acting as the boundary conditions, produces a chorus element with a subpacket structure. This element features an upstream shift of the source region with time and an irregular growth of frequency, showing small decreases between adjacent subpackets. The influence of input parameters on the number of subpackets, the shift of the source, the frequency sweep rate, and the maximum amplitude is analyzed. The model well captures basic features of instantaneous frequency measurements provided by the Van Allen Probes spacecraft. The modeled wave field can be used in future particle acceleration studies.

Conjugate Observations of Quasiperiodic Emissions by the Van Allen Probes Spacecraft and Ground‐Based Station Kannuslehto

AbstractWhistler mode waves observed in the Earth's inner magnetosphere at frequencies between about 0.5 and 4 kHz which exhibit a nearly periodic time modulation of the wave intensity are called quasiperiodic (QP) emissions. Conjugate measurements of QP events at several different locations can be used to estimate their spatial extent and spatiotemporal variability. Results obtained using conjugate QP measurements provided by the ground‐based station Kannuslehto (L≈5.5) and the Van Allen Probes spacecraft (L shells between about 1.1 and 6.5) between September 2012 and November 2017 are presented. Altogether, 26 simultaneously detected events were analyzed. The event modulation periods and frequency‐time structures were generally the same at all observation points. Spatial separations of the spacecraft and the ground‐based station during conjugate observations are typically within about 40° in azimuth and from about 1 to 3 in L shell. RBSP consistently observes events at lower L shells than Kannuslehto, with the event occurrence primarily inside of the plasmasphere. Ratios of Poynting fluxes observed by the spacecraft and on the ground are used to evaluate event intensity variations related to the spacecraft position. It is found that the intensity decreases considerably both at low L shells and outside of the plasmasphere. Finally, an event containing a gap in its frequency‐time structure related to a sudden change of its properties is analyzed in detail.

Localization of the Source of Quasiperiodic VLF Emissions in the Magnetosphere by Using Simultaneous Ground and Space Observations: A Case Study

AbstractWe study quasiperiodic very low frequency (VLF) emissions observed simultaneously by Van Allen Probes spacecraft and Kannuslehto and Lovozero ground‐based stations on 25 December 2015. Both Van Allen Probes A and B detected quasiperiodic emissions, probably originated from a common source, and observed on the ground. In order to locate possible regions of wave generation, we analyze wave‐normal angles with respect to the geomagnetic field, Poynting flux direction, and cyclotron instability growth rate calculated by using the measured phase space density of energetic electrons. We demonstrate that even parallel wave propagation and proper (downward) Poynting flux direction are not sufficient for claiming observations to be in the source region. Agreement between the growth rate and emission bands was obtained for a restricted part of Van Allen Probe A trajectory corresponding to localized enhancement of plasma density with scale of 700 km. We employ spacecraft density data to build a model plasma profile and to calculate ray trajectories from the point of wave detection in space to the ionosphere and examine the possibility of their propagation toward the ground. For the considered event, the wave could propagate toward the ground in the geomagnetic flux tube with enhanced plasma density, which ensured ducted propagation. The region of wave exit was confirmed by the analysis of wave propagation direction at the ground detection point.

Fine Harmonic Structure of Equatorial Noise with a Quasiperiodic Modulation

AbstractEquatorial noise emissions (fast magnetosonic waves) are electromagnetic waves observed routinely in the equatorial region of the inner magnetosphere. They propagate with wave vectors nearly perpendicular to the ambient magnetic field; that is, they are limited to frequencies below the lower hybrid frequency. The waves are generated by instabilities of ring‐like proton distribution functions, which result in their fine harmonic structure with intensity maxima close to harmonics of the proton cyclotron frequency in the source region. Although most equatorial noise emissions are continuous in time, some events exhibit a clear quasiperiodic time modulation of the wave intensity, with typical modulation periods on the order of minutes. We analyze 72 such events (17 observed by the Cluster spacecraft, 55 observed by the Van Allen Probes spacecraft) for which high‐resolution data were available. The analysis of the observed harmonic structure allows us to determine source radial distances of the events. It is found that the calculated source radial distances are generally close to the radial distances where the events were observed. The harmonic numbers where the events are generated range between about 12 and 30. Two events for which the spacecraft passed through the generation region were identified and analyzed. No simultaneous ultra‐low‐frequency magnetic field pulsations and no periodic plasma number density variations were observed. Although the in situ measured proton distribution functions were shown to be responsible for the wave growth, an insufficient resolution of the particle instruments prevented us from detecting a quasiperiodic modulation possibly present in the particle data.

Dynamical Coupling of Energetic Electrons and Upper‐Hybrid Thermal Fluctuations in the Earth's Radiation Belt

AbstractThe inner magnetosphere including the radiation belt environment is replete with quasi‐electrostatic fluctuations with peak frequency in the upper‐hybrid frequency range. Some examples are demonstrated with the Van Allen Probe spacecraft data. These features have recently been explained in the framework of spontaneously emitted thermal noise theory. Such an environment is also characterized by quasi‐isotropic population of energized electrons, which naturally leads one to ask whether these electrons and the upper‐hybrid fluctuations influence each other. The present paper explores the potential causal relationship between the two features via kinetic theory. It is shown that indeed, isotropic energetic electrons and upper‐hybrid frequency thermal fluctuations can be dynamically coupled and that they could coexist in a quasi‐steady state manner.

Specifying High‐Altitude Electrons Using Low‐Altitude LEO Systems: The SHELLS Model

AbstractWe describe an artificial neural network model of the near‐Earth space radiation environment. The geomagnetic activity index and low‐earth‐orbit (LEO) electron flux measurements from the National Oceanic and Atmospheric Administration Polar‐orbiting Operational Environmental Satellite (POES) operational spacecraft are used as model training inputs. Electron fluxes from National Aeronautics and Space Administration's Van Allen Probe spacecraft form the training outputs. We demonstrate that the model can accurately specify outer radiation belt ( –7) electron fluxes at two energies, 350 keV and 1 MeV. Various performance metrics are calculated using out‐of‐sample data, and we find high correlations and low errors between the model specification and the observed flux. We emphasize that once the model is trained, the Van Allen Probes data are no longer needed at model run time; only the POES fluxes and the index are required to specify the outer electron belt using the model.

Electron acoustic envelope solitons in non-Maxwellian plasmas

The NASA Van Allen Probes spacecraft has recently observed broadband electrostatic turbulence in the inner magnetosphere which has been conjectured to be produced by large amplitude electrostatic solitary waves of generally two types [D.M. Malaspina, J.R. Wygant, R.E. Ergun, G.D. Reeves, R.M. Skoug, B.A. Larsen, J. Geophys. Res.: Space Phys. 120, 4246 (2015); C.S. Dillard, I.Y. Vasko, F.S. Mozer, O.V. Agapitov, J.W. Bonnell, Phys. Plasmas 25, 022905, (2018)]. The solitary waves with highly asymmetric bipolar parallel electric field have been recently shown to correspond to the electron-acoustic plasma mode (existing due to two-temperature electron population) [C.S. Dillard, I.Y. Vasko, F.S. Mozer, O.V. Agapitov, J.W. Bonnell, Phys. Plasmas 25, 022905, (2018)]. These findings along with the observations of non-Maxwellian electrons in terrestrial magnetosheath and planetary magnetospheres [W. Masood, S.J. Schwartz, M. Maksimovic, A.N. Fazakerley, Ann. Geophys. 24, 1725 (2006); W. Masood, S.J. Schwartz, J. Geophys. Res. 113, A01216 (2008); M.N.S. Qureshi, W. Nasir, W. Masood, P.H. Yoon, H.A. Shah, S.J. Schwartz, J. Geophys. Res.: Space Phys. 119, 10059 (2014); M.N.S. Qureshi, W. Nasir, R. Bruno, W. Masood, MNRAS 488, 954 (2019)] have prompted us to theoretically investigate the problem of amplitude modulation of the electron acoustic waves (EAWs) in plasmas whose ingredients are stationary ions, inertial cold electrons and warm (r,q) distributed inertialess electrons. The nonlinear Schrodinger equation (NLSE) that governs the modulational instability (MI) of the EAWs has been derived using the standard reductive perturbation technique (RPT). The presence of the warm (r,q) distributed electrons has been shown to influence the existence conditions of MI of the EAWs and both the indices have been found to cause a decrease in the growth rate of MI. A detailed comparison has also been drawn between double spectral index (r,q), kappa and Maxwellian distribution functions and the differences are highlighted. Additionally, the nondimensional parameter α=nc0/ne0 which is the equilibrium density ratio of the cold to hot electron component, has been shown to play an important role in the formation of envelope solitary excitations.

The Parameterization Method of Discrete VLF Chorus Emissions

We present a method for automatic search and parameterization of discrete elements of very low-frequency (VLF) chorus emissions. The method is based on the processing of dynamic spectrograms using a special scanning algorithm, which is intended for seeking the discrete elements of VLF chorus emissions and calculation of their parameters. We propose to create the optimal dynamic spectrograms for the scanning algorithm by using short-time Fourier transform. The paper gives general recommendations for calculation of spectrograms and their preprocessing. The scanning algorithm is based on processing of the dynamic-spectrogram images by using the methods of mathematical morphology. The developed method was tested for several cases of chorus emissions observed by the Van Allen Probes spacecraft. It is shown that the total errors related to “false positive” detection and missing the target are about 10% of the elements visible to the human eye when the optimal parameters of the scanning algorithm are used and intense discrete elements are processed. The method can be applied to both spacecraft and ground-based wave data. The results of using the method can be employed for verification of physical theories of the formation of chorus emissions and determining their statistical properties.

Conjugate Ionosphere‐Magnetosphere Observations of a Sub‐Alfvénic Compressional Intermediate‐m Wave: A Case Study Using EKB Radar and Van Allen Probes

AbstractA Pc5 wave was simultaneously observed in the ionosphere by EKB radar and in the magnetosphere by both Van Allen Probe spacecraft within a substorm activity. The wave was located in the nightside, in 1.5‐ to 3‐hr magnetic local time sector, and in the region corresponding to the magnetic shells with maximal distances 4.6–7.8 Earth's radii. As it was found using both the radar and spacecraft data, the wave had frequency of about 1.8 mHz and azimuthal wave number m≈−10; that is, the wave was westward propagating. The EKB radar data revealed the equatorward wave propagating in the ionosphere, which corresponded to the earthward propagation in the magnetosphere. Furthermore, the field‐aligned magnetic component was approximately 2 times larger than both transverse components and accompanied by antiphase pressure oscillations; that is, the wave is compressional and diamagnetic. According to both radar and spacecraft measurements, among two transverse magnetic components, the dominant one was the poloidal. The wave was possibly driven by substorm‐injected energetic protons registered by the spacecraft: the proton fluxes were modulated with the wave frequency at energies of about 90 keV, which corresponded to the energy of the drift wave‐particle resonance. The wave frequency was much lower than the minimal frequency of the field line resonance calculated using the spacecraft data. We conclude that the wave is not the Alfvén mode, but some kind of compressional wave, for example, the drift‐compressional mode.

Multiyear Measurements of Radiation Belt Electrons: Acceleration, Transport, and Loss.

In addition to clarifying morphological structures of the Earth's radiation belts, it has also been a major achievement of the Van Allen Probes mission to understand more thoroughly how highly relativistic and ultrarelativistic electrons are accelerated deep inside the radiation belts. Prior studies have demonstrated that electrons up to energies of 10 megaelectron volts (MeV) can be produced over broad regions of the outer Van Allen zone on timescales of minutes to a few hours. It often is seen that geomagnetic activity driven by strong solar storms (i.e., coronal mass ejections, or CMEs) almost inexorably leads to relativistic electron production through the intermediary step of intense magnetospheric substorms. In this study, we report observations over the 6‐year period 1 September 2012 to 1 September 2018. We focus on data about the relativistic and ultrarelativistic electrons (5 MeV) measured by the Relativistic Electron‐Proton Telescope sensors on board the Van Allen Probes spacecraft. This work portrays the radiation belt acceleration, transport, and loss characteristics over a wide range of geomagnetic events. We emphasize features seen repeatedly in the data (three‐belt structures, “impenetrable” barrier properties, and radial diffusion signatures) in the context of acceleration and loss mechanisms. We especially highlight solar wind forcing of the ultrarelativistic electron populations and extended periods when such electrons were absent. The analysis includes new display tools showing spatial features of the mission‐long time variability of the outer Van Allen belt emphasizing the remarkable dynamics of the system.

Quasiperiodic Whistler Mode Emissions Observed by the Van Allen Probes Spacecraft

AbstractQuasiperiodic (QP) emissions are whistler mode electromagnetic waves observed in the inner magnetosphere that exhibit a QP time modulation of the wave intensity. We analyze 768 QP events observed during the first 5 years of the operation of the Van Allen Probes spacecraft (September 2012 to October 2017). Multicomponent wave measurements performed in the equatorial region, where the emissions are likely generated, are used to reveal new experimental information about their properties. We show that the events are observed nearly exclusively inside the plasmasphere. Wave frequencies are mostly between about 0.5 and 4 kHz. The events observed at larger radial distances and on the duskside tend to have slightly lower frequencies than the emissions observed elsewhere. The maximum event frequencies are limited by half of the equatorial electron gyrofrequency, suggesting the importance of wave ducting. Modulation periods are typically between about 0.5 and 5 min, and they increase with the in situ measured plasma number density. This increase is consistent with the main mechanisms suggested to explain the origin of the QP modulation. Two‐point measurements performed by the Van Allen Probes are used to estimate a typical spatial extent of the emissions to about 1 RE in radial distance and 1.5 hr in magnetic local time. Detailed wave analysis shows that the emissions are right‐hand circularly polarized, and they usually come from several different directions simultaneously. They, however, predominantly propagate at rather low wave normal angles and away from the geomagnetic equator.

Fast Diffusion of Ultrarelativistic Electrons in the Outer Radiation Belt: 17 March 2015 Storm Event.

Inward radial diffusion driven by ULF waves has long been known to be capable of accelerating radiation belt electrons to very high energies within the heart of the belts, but more recent work has shown that radial diffusion values can be highly event‐specific, and mean values or empirical models may not capture the full significance of radial diffusion to acceleration events. Here we present an event of fast inward radial diffusion, occurring during a period following the geomagnetic storm of 17 March 2015. Ultrarelativistic electrons up to ∼8 MeV are accelerated in the absence of intense higher‐frequency plasma waves, indicating an acceleration event in the core of the outer belt driven primarily or entirely by ULF wave‐driven diffusion. We examine this fast diffusion rate along with derived radial diffusion coefficients using particle and fields instruments on the Van Allen Probes spacecraft mission.

Pitch Angle Scattering and Loss of Radiation Belt Electrons in Broadband Electromagnetic Waves

AbstractA magnetic conjunction between Van Allen Probes spacecraft and the Balloon Array for Radiation‐belt Relativistic Electron Losses (BARREL) reveals the simultaneous occurrence of broadband Alfvénic fluctuations and multi‐timescale modulation of enhanced atmospheric X‐ray bremsstrahlung emission. The properties of the Alfvénic fluctuations are used to build a model for pitch angle scattering in the outer radiation belt on electron gyro‐radii scale field structures. It is shown that this scattering may lead to the transport of electrons into the loss cone over an energy range from hundreds of keV to multi‐MeV on diffusive timescales on the order of hours. This process may account for modulation of atmospheric X‐ray fluxes observed from balloons and constitute a significant loss process for the radiation belts.

Longitudinal Dependence of Whistler Mode Electromagnetic Waves in the Earth's Inner Magnetosphere

AbstractWe use the measurements performed by the DEMETER (2004–010) and the Van Allen Probes (2012–016, still operating) spacecraft to investigate the longitudinal dependence of the intensity of whistler mode waves in the Earth's inner magnetosphere. We show that a significant longitudinal dependence is observed inside the plasmasphere on the nightside, primarily in the frequency range 400 Hz to 2 kHz. On the other hand, almost no longitudinal dependence is observed on the dayside. The obtained results are compared to the lightning occurrence rate provided by the Optical Transient Detector/Lightning Imaging Sensor mission normalized by a factor accounting for the ionospheric attenuation. The agreement between the two dependencies indicates that lightning‐generated electromagnetic waves may be responsible for of the observed effect, thus substantially affecting the overall wave intensity in the given frequency range. Finally, we show that the longitudinal dependence is most pronounced for waves with oblique wave of normal angles.

Equatorial Noise With Quasiperiodic Modulation: Multipoint Observations by the Van Allen Probes Spacecraft

AbstractElectromagnetic wave measurements performed by the two Van Allen Probes spacecraft are used to analyze equatorial noise emissions with a quasiperiodic modulation of the wave intensity. These waves are confined to the vicinity of the geomagnetic equator, and they occur primarily on the dayside. In situ plasma number density measurements are used to evaluate density variations related to the wave occurrence. It is shown that the events are sometimes effectively confined to low‐density regions, being observed at successive satellite passes over a time duration as long as 1 hr. The events typically occur outside the plasmasphere, and they often cease to exist just at the plasmapause. The analysis of the spatial separations of the spacecraft at the times when the events were observed simultaneously by both of them allows us to estimate the event spatial dimensions. It is found that the event spatial extent is typically lower than about 0.25RE in radial distance and within about 1 hr in magnetic local time. Modulation periods of the events decrease with increasing plasma number density up to about 100 cm−3. Principally no dependence is observed at larger densities, possibly indicating a propagation from other locations.

Poloidal Mode Wave‐Particle Interactions Inferred From Van Allen Probes and CARISMA Ground‐Based Observations

AbstractUltralow‐frequency wave and test particle models are used to investigate the pitch angle and energy dependence of ion differential fluxes measured by the Van Allen Probes spacecraft on 6 October 2012. Analysis of the satellite data reveals modulations in differential flux resulting from drift resonance between H+ ions and fundamental mode poloidal Alfvén waves detected near the magnetic equator at L ∼ 5.7. Results obtained from simulations reproduce important features of the observations, including a substantial enhancement of the differential flux between ∼20 and 40° pitch angle for ion energies between ∼90 and 220 keV and an absence of flux modulations at 90°. The numerical results confirm predictions of drift‐bounce resonance theory and show good quantitative agreement with observations of modulations in differential flux produced by ultralow‐frequency waves.

Radial Transport of Higher‐Energy Oxygen Ions Into the Deep Inner Magnetosphere Observed by Van Allen Probes

AbstractThe transport mechanism of the ring current ions differs among ion energies. Lower‐energy (≲150 keV) ions are well known to be transported convectively. Higher‐energy (≳150 keV) protons are reported to be transported diffusively, while there are few reports about transport of higher‐energy oxygen ions. We report the radial transport of higher‐energy oxygen ions into the deep inner magnetosphere during the late main phase of the magnetic storm on 23–25 April 2013 observed by the Van Allen Probes spacecraft. An enhancement of 1–100 mHz magnetic fluctuations is simultaneously observed. Observations of 3 and 30 mHz geomagnetic pulsations indicate the azimuthal mode number is ≤10. The fluctuations can resonate with the drift and bounce motions of the oxygen ions. The results suggest that the combination of the drift and drift‐bounce resonances is responsible for the radial transport of higher‐energy oxygen ions.

Electrostatic Steepening of Whistler Waves.

We present surprising observations by the NASA Van Allen Probes spacecraft of whistler waves with substantial electric field power at harmonics of the whistler wave fundamental frequency. The wave power at harmonics is due to a nonlinearly steepened whistler electrostatic field that becomes possible in the two-temperature electron plasma due to the whistler wave coupling to the electron-acoustic mode. The simulation and analytical estimates show that the steepening takes a few tens of milliseconds. The hydrodynamic energy cascade to higher frequencies facilitates efficient energy transfer from cyclotron resonant electrons, driving the whistler waves, to lower energy electrons.

Van Allen Probes Observations of Drift‐Bounce Resonance and Energy Transfer Between Energetic Ring Current Protons and Poloidal Pc4 Wave

AbstractA poloidal Pc4 wave and proton flux oscillations are observed in the inner magnetosphere on the dayside near the magnetic equator by the Van Allen Probes spacecraft on 2 March 2014. The flux oscillations are observed in the energy range of 67.0 to 268.8 keV with the same frequency of the poloidal Pc4 wave. We find pitch angle and energy dispersion in the phase difference between the poloidal magnetic field and the proton flux oscillations, which are features of drift‐bounce resonance. We estimate the resonance energy to be ~120 keV for pitch angle (α) of 30° or 150° and 170–180 keV for α = 50° or 130°. To examine the direction of energy flow between protons and the wave, we calculate the sign of the gradient of proton phase space density (df/dW) on both the inbound and outbound legs of the orbit. We find the gradient to be outward on both legs, which means that energy is transferred from the protons to the wave. During the poloidal Pc4 wave event, the Dst* index shows a measurable increase of ~6.7 nT. We estimate the total energy loss of the ring current from the recovery of the Dst* index and from the variation of proton flux by the drift‐bounce resonance. We suggest that energy transfer from the ring current protons to the poloidal Pc4 wave via the drift‐bounce resonance contributes to up to ~85% of the increase of the Dst* index.