Substorm Injection Research Articles

A multi-satellite survey scheme for addressing open questions on the Earth’s outer radiation belt dynamics

The Earth’s outer radiation belt is highly dynamic, containing relativistic electron fluxes that can increase by several orders of magnitude during magnetospheric disturbances. This greatly increases the likelihood of spacecraft malfunction or failure and significantly influences the solar-terrestrial system’s energy and mass coupling, highlighting the importance of fully understanding the mechanisms governing these dynamics from both theoretical and practical perspectives. Although many theories have been proposed, further research is essential to quantify the specific contributions of different dynamic mechanisms for improving space weather forecasting. To address this, observations of the outer radiation belt with high spatial–temporal resolution to distinguish the spatial and temporal variations are essential. We introduce a 10-CubeSat constellation survey scheme in the geosynchronous transfer orbit (GTO) to achieve this required observation. Three baseline instruments are proposed to be employed: the high energy electron detector (HEED), the search coil wave detector (SCWD), and the magnetometer (MAG). Two groups of physical processes will be investigated: wave-particle interactions involving charged particles interacting with whistler-mode waves, electromagnetic ion cyclotron (EMIC) waves, and ultra-low frequency (ULF) waves; and radial transport encompassing shock-induced injections, substorm injections, storm convection and magnetopause shadowing. The performance parameters of instruments and platform of the constellation are presented. Additionally, aligned with the concept of constellation survey, we outline the COSPAR-coordinated space program, COnstellation of Radiation BElt Survey (CORBES), which will provide a crucial scientific contribution in the absence of the Van Allen Probes. The program’s excellent observational capability enables a comprehensive understanding of the underlying physical mechanisms governing the outer radiation belt dynamics and improved space weather forecasting.

A Sub‐relativistic Electron Three‐Belt Event in the Earth's Radiation Belts: Observation and Explanation

AbstractThe Van Allen Probes mission contributed to the discovery of the relativistic (∼500 keV–2 MeV) and ultra‐relativistic (∼>2 MeV) electron three‐belt structure in Earth's radiation belts. This structure results from the partial depletion of the preexisting outer belt and the replenishment of a new outer belt. Ultra‐low frequency and very‐low frequency waves are believed to play important roles in these processes, and substorm injections are usually not responsible for the formation of the external outer belt. In this study, based on observations from the Arase and NOAA‐18 (National Oceanic and Atmospheric Administration‐18) satellites, we report an electron three‐belt event in the energy range of ∼100–200 keV (i.e., sub‐relativistic electrons). According to the evolution of the phase space density and the dawn‐dusk asymmetric flux enhancement during this event, we conclude that the depletion of the upper part of the original outer belt was due to outward radial diffusion accompanied by magnetopause shadowing effect and convection, and the formation of the external outer belt was due to increased electron flux from convection as well as substorm injections. This discovery and its preliminary explanation may help understand the electron three‐belt structure in radiation belts more comprehensively.

The impact of drifting substorm-injected electrons on pulsating aurora initiation and intensification

Pulsating aurora, which consists of diffuse patches blinking on and off, is caused by pitch angle scattering of radiation belt electrons into the loss cone by lower-band chorus waves. Understanding the drivers of pulsating aurora is important as it is a long-lasting and widespread phenomenon, accounting for significant energy transfer from the solar wind into the ionosphere. Substorm injections, which transport electrons from the magnetotail into the inner magnetosphere, are one source of electrons in this region. Injections have been observed simultaneously with pulsating aurora during conjunctions between ground cameras and satellites. In addition, previous work has also shown that substorms can enhance chorus activity (the fundamental process that produces pulsating aurora), providing a mechanism linking substorms to pulsating aurora. To further study this connection, we used the Van Allen Probes and all-sky cameras to look at events where pulsating aurora and substorm injections were observed at different locations in Magnetic Local Time (MLT), rather than focusing only on conjunctions. To make this comparison, we calculated the drift rate of electrons originating from observed injections and projected their motion forward in time until their Magnetic Local Time was the same as the ground camera. When the electrons are located at the same MLT as the ground camera, the pulsating aurora they cause would most likely occur in the field of view of the camera. We compared the time drifting substorm-injected particles arrived at the MLT of the camera to when pulsating aurora was observed. We found several instances where the initiation or intensification of pulsating aurora was accompanied by the arrival of substorm-injected electrons. This observation gives further evidence that pulsating aurora can be enhanced by or occur after substorm injections.

Longitudinal Development of Cosmic Noise Absorption Based on Multipoint Observations at Subauroral Latitudes During Storm‐Time Substorms on 25–28 August 2018

AbstractEnhancements in electron density in the D‐region ionosphere attributed to the precipitation of high‐energy electrons, have previously been inferred from increases in cosmic radio noise absorption (CNA) using ground‐based riometers. However, there have been few studies of CNA observations at multi‐point stations distributed in longitudes. Thus, the spatio‐temporal development of the global distribution of CNA is not well understood. In this study, we investigated the longitudinal extent of CNA using simultaneous riometer observations at six stations at subauroral latitudes in Canada, Alaska, Russia, and Iceland. These stations are located encircling the earth at ∼60° north magnetic latitudes. We have conducted simultaneous observations of CNA at these stations since October 2017. Here we focus on seven substorms during a geomagnetic storm 25–28 August 2018 and study the spatio‐temporal development of the global distribution of CNA during these substorms. For all seven substorms, some stations observed CNA enhancements after the substorm onsets. In five cases, the CNA enhancements started around midnight and expanded eastward. The other two cases show westward and anti‐sunward development of CNA. The eastward expansion of CNA indicates the eastward drift of high‐energy electrons, which is the source of the CNA, due to gradient and curvature drift in the geomagnetic field. The westward expansion of CNA may correspond to westward expansion of the substorm injection region due to dawn‐to‐dusk electric fields. These results indicate that spatio‐temporal development of CNA at subauroral latitudes corresponds to high energy electron drift in the inner magnetosphere.

Different Behaviors of Outer Radiation Belt keV and MeV Electrons During Two Sequential Geomagnetic Storms

AbstractElectron flux dropout is an extraordinary phenomenon in the outer radiation belt, which is characterized by a drastic depletion in trapped electron populations over a broad range of energies, pitch angles, and L‐shells. Using data from multiple satellites, we study a 5‐day dropout of MeV electrons and a several‐hour dropout of keV electrons in geostationary Earth orbit (GEO) during two sequential geomagnetic storms. Outward radial diffusion caused real loss of MeV electrons, while asymmetric and depressed magnetic fields led to adiabatic variations in keV electron fluxes. In addition to time scale differences, low‐energy keV, medium‐energy keV and high‐energy MeV electron fluxes behaved differently, involving an increasing trend, an initial decreasing trend followed by an increasing trend, and a decreasing trend, respectively, as observed by Radiation Belt Storm Probes (RBSPs) and low‐altitude satellites during the first storm. Substorm injections accompanied by inward radial diffusion initially produced an enhanced flux of electrons with energies up to ∼100 keV. Chorus waves generated in morning side of magnetosphere were responsible for scattering part of this seed population into the atmosphere. In turn, these enhanced but short‐lived chorus waves accelerated lower‐energy keV electrons to achieve a moderate energy level of hundreds of keV within one day. Additionally, the initial slow outward radial diffusion was associated with MeV electron loss, and there was insufficient time for acceleration by chorus waves, so only MeV electrons could maintain long‐term dropout. Competition between energy‐ and L‐shell‐dependent acceleration and loss mechanisms caused different keV and MeV electron behaviors.

Distribution and Evolution of Chorus Waves Modeled by a Neural Network: The Importance of Imbalanced Regression

AbstractWhistler‐mode chorus waves play an essential role in the acceleration and loss of energetic electrons in the Earth’s inner magnetosphere, with the more intense waves producing the most dramatic effects. However, it is challenging to predict the amplitude of strong chorus waves due to the imbalanced nature of the data set, that is, there are many more non‐chorus data points than strong chorus waves. Thus, traditional models usually underestimate chorus wave amplitudes significantly during active times. Using an imbalanced regressive (IR) method, we develop a neural network model of lower‐band (LB) chorus waves using 7‐year observations from the EMFISIS instrument onboard Van Allen Probes. The feature selection process suggests that the auroral electrojet index alone captures most of the variations of chorus waves. The large amplitude of strong chorus waves can be predicted for the first time. Furthermore, our model shows that the equatorial LB chorus’s spatiotemporal evolution is similar to the drift path of substorm‐injected electrons. We also show that the chorus waves have a peak amplitude at the equator in the source MLT near midnight, but toward noon, there is a local minimum in amplitude at the equator with two off‐equator amplitude peaks in both hemispheres, likely caused by the bifurcated drift paths of substorm injections on the dayside. The IR‐based chorus model will improve radiation belt prediction by providing chorus wave distributions, especially storm‐time strong chorus. Since data imbalance is ubiquitous and inherent in space physics and other physical systems, imbalanced regressive methods deserve more attention in space physics.

External and Internal Causes of the Stormtime Intensification of the Dawnside Westward Auroral Electrojet

AbstractThe dawn‐dusk asymmetry of magnetic depression is a characteristic feature of the storm main phase. Recently Ohtani (2021, https://doi.org/10.1029/2021JA029643) reported that its magnitude is correlated with the dawnside westward auroral electrojet (AEJ) intensity, and suggested that the dawnside AEJ intensification is a fundamental process of the stormtime magnetosphere‐ionosphere coupling. In this study we observationally address the cause of the dawnside AEJ intensification in terms of four scenarios. That is, the dawnside AEJ intensifies because (a) the external driving of global convection strengthens, (b) solar wind compression enhances energetic electron precipitation, and therefore, ionospheric conductance, through wave‐particle interaction, (c) the substorm current wedge forms in the dawn sector, and (d) energetic electrons injected by nightside substorms drift dawnward, and the subsequent precipitation enhances ionospheric conductance. We find an event that fits each scenario, and therefore, none of these scenarios can be precluded. However, the result of a superposed epoch analysis shows that some causes are more prevalent than others. More specifically, (a) although the enhancement of external driving may precondition the dawnside AEJ intensification, it is rarely the direct cause; (b) external compression probably explains only a small fraction of the events; (c) prior to the dawnside AEJ intensification, the westward AEJ tends to intensify in the midnight sector along with mid‐latitude positive bays, which suggests that the substorm injection of energetic electrons is the most prevalent cause. This last result may also be explained by the dawnside expansion of the substorm current wedge, which, however, is arguably far less common.

Simulation of radiation belt wave-particle interactions in an MHD-particle framework

In this paper we describe K2, a comprehensive simulation model of Earth’s radiation belts that includes a wide range of relevant physical processes. Global MHD simulations are combined with guiding-center test-particle methods to model interactions with ultra low-frequency (ULF) waves, substorm injections, convective transport, drift-shell splitting, drift-orbit bifurcations, and magnetopause shadowing, all in self-consistent MHD fields. Simulation of local acceleration and pitch-angle scattering due to cyclotron-scale interactions is incorporated by including stochastic differential equation (SDE) methods in the MHD-particle framework. The SDEs are driven by event-specific bounce-averaged energy and pitch-angle diffusion coefficients. We present simulations of electron phase-space densities during a simplified particle acceleration event based on the 17 March 2013 event observed by the Van Allen Probes, with a focus on demonstrating the capabilities of the K2 model. The relative wave-particle effects of global scale ULF waves and very-low frequency (VLF) whistler-mode chorus waves are compared, and we show that the primary acceleration appears to be from the latter. We also show that the enhancement with both ULF and VLF processes included exceeds that of VLF waves alone, indicating a synergistic combination of energization and transport processes may be important.

Amplitude‐Dependent Properties and Excitation Mechanisms of EMIC Waves in the Earth's Inner Magnetosphere

AbstractLarge‐amplitude (Bw > 1 nT) electromagnetic ion cyclotron (EMIC) waves can cause the rapid loss of >1 MeV electrons, greatly impacting radiation belt dynamics. With long‐term Van Allen Probe B observations from 2013 to 2018, we conducted a statistical survey to reveal the amplitude‐dependent EMIC wave properties and excitation mechanisms in the Earth's inner magnetosphere. Statistical results show that large‐amplitude EMIC waves prefer to occur in the afternoon‐dusk sector in the northern hemisphere and tend to be more left‐hand polarized with smaller wave normal angles. In addition, the high proton beta parallel conditions also favor the generation of larger‐amplitude EMIC waves. From the variations of EMIC wave occurrence rate as a function of SuperMAG electrojet (SME) index and solar wind dynamic pressure, we find that the small‐amplitude EMIC waves are generally triggered by high solar wind dynamic pressure, while large‐amplitude EMIC wave generation is both affected by substorm activity and solar wind dynamic pressure. The normalized magnetic field perturbations during EMIC wave appearance, which enable us to distinguish the relative roles of magnetospheric compression and substorm injection in the excitation of different‐amplitude EMIC waves, provide further evidence that as wave amplitude increases, substorm injection plays a more important role in EMIC wave excitation, and magnetospheric compression is also an indispensable trigger.

Contribution of Kinetic Alfvén Waves to Energetic Electron Precipitation From the Plasma Sheet During a Substorm

AbstractEnergetic (≳50 keV) electron precipitation from the magnetosphere to the ionosphere during substorms can be important for magnetosphere‐ionosphere coupling. Using conjugate observations between the THEMIS, ELFIN, and DMSP spacecraft during a substorm, we have analyzed the energetic electron precipitation, the magnetospheric injection, and the associated plasma waves to examine the role of waves in pitch‐angle scattering plasma sheet electrons into the loss cone. During the substorm expansion phase, ELFIN‐A observed 50–300 keV electron precipitation from the plasma sheet that was likely driven by wave‐particle interactions. The identification of the low‐altitude extent of the plasma sheet from ELFIN is aided by DMSP global auroral images. Combining quasi‐linear theory, numerical test particle simulations, and equatorial THEMIS measurements of particles and fields, we have evaluated the relative importance of kinetic Alfvén waves (KAWs) and whistler‐mode waves in driving the observed precipitation. We find that the KAW‐driven bounce‐averaged pitch‐angle diffusion coefficients near the edge of the loss cone are ∼10−6–10−5 s−1 for these energetic electrons. The due to parallel whistler‐mode waves, observed at THEMIS ∼10‐min after the ELFIN observations, are ∼10−8–10−6 s−1. Thus, at least in this case, the observed KAWs dominate over the observed whistler‐mode waves in the scattering and precipitation of energetic plasma sheet electrons during the substorm injection.

Examination of Radiation Belt Dynamics During Substorm Clusters: Magnetic Local Time Variation and Intensity of Precipitating Fluxes

AbstractSubstorms are short‐lived but significant reconfigurations of the geomagnetic field during which energetic particles are injected into the inner magnetosphere close to magnetic midnight. There is currently a need to quantify substorm‐driven energetic electron precipitation (EEP) to better understand its role in radiation belt dynamics and to quantify its impact on the atmosphere. As substorm injections trigger chorus waves, which have strong MLT, AE, and L‐shell dependence, we investigate the dependence of EEP in terms of these variables. We utilize many decades of low Earth orbit satellite observations to examine the typical statistical variability around substorm events identified by the Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE) algorithm. In contrast to trapped flux enhancements, enhanced EEP is found to occur even for the quietest AE range of those considered (AE ≤ 100 nT, 100 nT < AE ≤ 300 nT, AE ≥ 300 nT). The MLT‐dependent analysis for all AE‐ranges shows a well‐defined variation in >30 keV EEP magnitude, with a distinct and deep minimum in the late afternoon sector (15–18 MLT), and maxima in the mid to late morning sector (6–12 MLT). The patterns show similarities to previously published whistler‐mode lower band chorus distributions with MLT. Clusters of substorms reliably produce enhancements in electron precipitation for >30 keV and >300 keV, with steadily increasing peak precipitation magnitudes with increasing AE. The peak precipitation flux L‐shell also moves inwards with increasing AE, in a similar way for the two energy ranges.

The Effects of Substorm Injection of Energetic Electrons and Enhanced Solar Wind Ram Pressure on Whistler‐Mode Chorus Waves: A Statistical Study

AbstractWhistler‐mode chorus waves in the inner magnetosphere are typically excited by an electron temperature anisotropy. The anisotropy can be driven by two sources: particle injections from the tail (such as during substorms) and the solar wind ram pressure on the dayside magnetosphere. Based on 5 years of data from Van Allen Probe A, we have separately studied the effects of substorm injection of energetic ∼10–100 keV electrons and their gradient and curvature drifts () and enhanced solar wind ram pressure () on the generation of whistler‐mode chorus waves. We use time‐modified and indices to take into account time delays. We find that during the period of large but small , chorus waves are mainly observed in the midnight through dawn to noon sectors ( MLT 13) near the magnetic equator (|MLAT| < 10°) at L = 4.5–6.5. With an increase in both the chorus occurrence rates and the wave amplitudes increase. While under the condition of enhanced but small , chorus waves are preferentially detected on the dayside ( MLT 14) in a wide range of latitudes (|MLAT| < 20°) at outer L‐shells (L = 5.5–6.5). With the increase of , chorus occurrence rates also increase, while the amplitude remains relatively constant. Our study supports the two mechanisms for chorus excitation in the Earth’s magnetosphere and provides a better understanding of the global distribution and properties of chorus waves.

Non‐Adiabatic Acceleration of Injected Electrons in the Inner Magnetosphere: Joint Observations by the Van Allen Probe and the BeiDa Imaging Electron Spectrometer

AbstractWe present joint observations of a substorm injection event on 5 February 2016. The enhancements of energetic electron (∼30 to ∼300 keV) fluxes, the drift echoes of the injected electrons, and the quasi‐monochromatic ultralow frequency oscillations embedded in the flux variations were simultaneously observed by the BeiDa Imaging Electron Spectrometer and the Van Allen Probe B. These vivid features of substorm injection were concretely captured by multiple spacecraft within the geosynchronous distance for the first time. The radial separation of the spacecraft (∼0.3 RE) allowed for a quantitative comparison of the electron flux and phase space density (PSD) across different drift shells. It is found that a more intense increase of equatorial PSD occurred at the lower L‐shell (L* ∼5.5), which indicated a non‐adiabatic acceleration of the injected electrons in the inner magnetosphere.

Broadband Whistler Waves and Differential Electron Fluxes in the Equatorial Region of the Magnetosphere behind the Plasmapause during Substorm Injections

This paper considers the whistler waves in the frequency band from 3 to 30 kHz observed on the Van Allen Probe-B satellite on March 17, 2019, when the satellite was on L-shells from 2.8 to 5.4. The upper frequency in the emission spectrum followed the course of the electron gyrofrequency {{f}_{{ce}}} and was lower than it by 1–5 kHz. The emission spectrum often had two spectral maxima (above and below fce/2); the maximum at frequencies above fce/2 could be either more or less intense. High-frequency whistler waves at frequencies > fce/2 were observed simultaneously with an increase in low-energy electron fluxes with energies more than 102 eV, which had transverse anisotropy. To explain the observed spectrum, we used simultaneous satellite measurements of the cold plasma density and differential fluxes of energetic electrons in the energy range from 0.015 to 250 keV in a wide range of pitch angles to determine the electron distribution function and calculate local linear growth rate as a function of frequency f and wave normal angle theta . The calculations were performed for three cyclotron resonances (n = 1, 0, –1) that make the largest contributions to the wave growth rate. The calculations showed the presence of a pronounced maximum at frequencies (0.8–0.9) fce. The energy range and pitch angles of electrons with a maximum contribution to wave excitation at these frequencies were estimated.

Nonlinear Wave Growth Analysis of Chorus Emissions Modulated by ULF Waves

AbstractWe present Van Allen Probes observations of periodic chorus wave emissions in the troughs of compressional ultralow frequency (ULF) waves. During this event, the spectral gap of chorus waves gradually widens as the spacecraft moves from the equatorial source region towards higher latitudes. Moreover, chorus wave intensity increases and frequency range widens after a substorm injection. We show that the periodic occurrence of chorus waves is attributed to the modulation of threshold amplitude for nonlinear growth of chorus waves by the second spatial derivative of ULF compressional magnetic field. The widening gap can be interpreted in terms of the nonlinear damping mechanism. A good agreement is also found between the nonlinear wave growth theory and the observations regarding the influence of substorm injection on the chorus. These findings support the applicability of the nonlinear theory in describing the chorus wave generation and damping, together with their modulations by ULF waves.

Relationship of the SAR Arc Dynamics to Substorm Injection Based on the Aurorae Observation. Magnetospheric Phenomena in the Plasmapause Vicinity

Relationship of the SAR Arc Dynamics to Substorm Injection Based on the Aurorae Observation. Magnetospheric Phenomena in the Plasmapause Vicinity

Multispacecraft Observation of the Presubstorm Long‐Lasting Poloidal ULF Wave

AbstractWe report global dayside observation of Pc4–5 wave at the late recovery phase of a magnetic storm from seven satellites. Oscillations lasted for 15 hr and revealed the wave appearance before the substorm onset. The continuous observation showed that substorm injections did not affect the wave behavior, and all the wave parameters change was caused by magnetospheric plasma recovery after a magnetic storm. The wave was identified as a second harmonic poloidal Alfvén wave propagating westward with a high azimuthal wave number (|m| ∼ 210) and coupled with 100 keV protons through the drift‐bounce resonance. A large radial gradient of phase space density was an energy source for the wave.

A comparative study of body potential transient profile of geosynchronous orbit spacecraft in single and double Maxwellian plasma models

AbstractEstimation of the transient profile of body potential of spacecraft in recommended cases of plasma models is crucial because of its importance to estimate electrostatic discharge (ESD) events. The spacecraft structure consisting of a cuboid with two coplanar rectangular plates is modeled as a lumped equivalent capacitance circuit. Method of moments is used to compute absolute and coupled capacitances, which directly affect the transient profile of body potential. Additionally, closer to the actual plasma model representing substorm injection events is vital for reliable estimation of absolute charging. Accordingly, in this work, we attempt a comprehensive analysis of the charging of a spacecraft in Maxwellian plasma models with the inclusion of induced electron yields for the probable particle incidences. Subsequently, the critical range of particle energy susceptible to failures related to ESD is investigated for a spacecraft structure of Aluminum. The comparison of transient variations of body potential in single and double Maxwellian plasma environments identifies the critical aspects of normal and worst cases concerning the prediction of ESD events, which can be useful in the early stage of a spacecraft design.

The Link Between Wedge‐Like and Nose‐Like Ion Spectral Structures in the Inner Magnetosphere

AbstractThe wedge‐like and nose‐like ion spectral structures, named after their characteristic shapes in the energy‐time spectrograms, appear to be distinctively different structures in the Earth's inner magnetosphere. Here. we present a case study with conjugate observations from the Arase spacecraft and the twin Van Allen Probes on July 1 and 2, 2017, which displayed the characteristic signatures of the wedge‐like and nose‐like ion structures, respectively. When the spacecraft nearly intersected at L = 2.8, the two structures overlapped with enhanced ion fluxes in the energy range of 1–10 keV. These observations suggest that the wedge‐like and nose‐like spectral signatures are merely the manifestations of one single structure along different spacecraft trajectories. This finding is further validated by the reproduction of both structures from a particle‐tracing model, which also indicates their formation processes associated with the intermittent substorm injections in the nightside magnetosphere.

Concurrent Empirical Magnetic Reconstruction of Storm and Substorm Spatial Scales Using Data Mining and Virtual Spacecraft

Data mining (DM) has ushered in a new era of empirical magnetic reconstructions of the magnetosphere via application of the k-nearest neighbors (kNN) method. In this approach, the combined magnetosphere storm-substorm state is characterized by the Sym-H and AL indices, their time derivatives, and the solar wind electric field vBzIMF. However, using the DM reconstructions to account for the substorm contributions to the ring current as well as describing storm-time substorms remains a problem. The inner region r ≤ 12RE, where the ring current develops, has a much higher density of data than the tail region 12RE ≤ r ≤ 22RE, where substorms operate. This results in two models inconsistent in their scales dictated by the corresponding data densities. The inner model reconstructs storm time dynamics, including the formation of the westward and eastward ring current and pressure distributions. The outer model captures substorm features, including the thinning and rapid dipolarization of the tail sheet during the growth and expansion phases, respectively. However, the substorm model is insufficient to reconstruct the eastward ring current while the storm model cannot fully reproduce substorm effects because it overfits in the tail region. This issue is addressed by constructing a hybrid model which is fit using virtual magnetic field observations generated by sampling the other two models. The resulting merged resolution model concurrently captures the spatial scales associated with both storms in the inner region and substorms in the near-tail region. Hence it is particularly useful for investigation of the storm-substorm relationship, including storm-time substorms and the impact of individual substorm injections to the buildup of the storm-time ring current.