Electromagnetic Ion Cyclotron Waves Research Articles

Combined Scattering of Concurrent Unusual High‐Frequency EMIC Waves and MS Waves on Radiation Belt Electrons

AbstractWe report a concurrent event of unusual high‐frequency Electromagnetic ion cyclotron (EMIC) and Magnetosonic (MS) waves in correspondence with butterfly distributions for sub‐MeV electrons and flux decay of MeV electrons, observed by Van Allen Probe A on 16 October 2017. The scattering effects are quantitatively investigated by test particle simulations. The results indicate that the unusual EMIC waves mainly scatter electrons from hundreds of keV to several MeV at pitch angles <60° while the MS waves mainly influence sub‐MeV and MeV electrons at pitch angles >60°. Their combined scattering effects are suitably analyzed to be justified as the linear superposition of the effect induced by each wave mode. The Fokker‐Planck simulations exhibit the formation of butterfly pitch angle distributions for sub‐MeV electrons and the obvious flux decay for electrons ∼1.8 MeV at all pitch angles, showing similar variation trends to the observations. Our study suggests that the occurrence of the two waves can significantly influence the electrons over the whole pitch angle range, which can help us better understand the radiation belt dynamics.

Amplitude Dependence of Nonlinear Precipitation Blocking of Relativistic Electrons by Large Amplitude EMIC Waves.

Recent work has shown that ElectroMagnetic Ion Cyclotron (EMIC) waves tend to occur in four distinct regions, each having their own characteristics and morphology. Here, we use nonlinear test‐particle simulations to examine the range of energetic electron scattering responses to two EMIC wave groups that occur at low L‐shells and overlap the outer radiation belt electrons. The first group consists of low‐density, H‐band region b waves, and the second group consists of high‐density, He‐band region c waves. Results show that while low‐density EMIC waves cannot precipitate electrons below ∼16 MeV, the high density EMIC waves drive a range of linear and nonlinear behaviors including phase bunching and trapping. In particular, a nonlinear force bunching effect can rapidly advect electrons at low pitch‐angles near the minimum resonant energy to larger pitch angles, effectively blocking precipitation and loss. This effect contradicts conventional expectations and may have profound implication for observational campaigns.

Nonlinear Interaction Between H+ Band and He+ Band EMIC Waves: Van Allen Probe Observations and Hybrid Simulations

AbstractNonlinear theory has predicted that wave‐wave coupling from different band electromagnetic ion cyclotron (EMIC) waves can occur in multiple‐ion plasmas. However, no direct observational evidence has been reported yet. Here we present multiple substructures on the frequency spectrum of EMIC waves with observations of Van Allen Probes. Some substructures are demonstrated to be excited by nonlinear interactions between preexisting H+ band and He+ band EMIC waves through the bicoherence analysis. Additionally, hybrid simulation displays that a new wave is motivated at sum frequencies after two independent pump waves of different band EMIC waves are injected, implying that the nonlinear wave‐wave interactions could drive EMIC waves with higher wavenumbers. Therefore, this letter provides the first observational evidence of wave‐wave coupling between H+ band and He+ band EMIC waves.

Localized Excitation of Electromagnetic Ion Cyclotron Waves From Anisotropic Protons Filtered by Magnetic Dips

AbstractThe excitation of electrostatic and/or electromagnetic waves in the plasma universe is often associated with anisotropic velocity distributions of charged particles. In Earth's inner magnetosphere, this anisotropy can gradually develop as particles injected from the magnetotail drift around the Earth at different speeds depending on their energy and pitch angle. Here, we show that the perpendicular‐moving and bouncing ions can be separated more abruptly near the injection front. These pitch‐angle filters are localized magnetic dip structures formed by the diamagnetic behavior of the injected particles, which can trap perpendicular‐moving ions and allow bouncing ions to overtake. The resulting ion anisotropy facilitates the rapid generation of electromagnetic ion cyclotron (EMIC) waves, which in turn can largely reshape the Van Allen radiation belts. This scenario is examined by case and statistical observations, together with numerical simulations that reproduce most of the observational signatures, to support the causal relationship between magnetic dips, anisotropic ion distributions, and localized excitation of EMIC waves. Our study highlights the important roles of magnetic dips in the inner magnetospheric dynamics, as pitch‐angle filters of the injected ions and traveling hotspots of EMIC wave activities.

Hot Plasma Effects on Electron Resonant Scattering by Electromagnetic Ion Cyclotron Waves

AbstractResonant scattering by electromagnetic ion cyclotron (EMIC) waves is one of the most effective mechanisms of relativistic electron losses in Earth’s inner magnetosphere. Low‐altitude spacecraft measurements, however, often show that the energy range of precipitating electrons is wider than theoretical predictions based on the cold plasma dispersion of EMIC waves. To explain this discrepancy, we examine the diffusion rates of EMIC waves by including hot plasma effects in their dispersion relation. Using the observed ion distribution functions, we investigate the hot plasma effects on the EMIC wave dispersion for a wide frequency range. We develop analytical equations for hot plasma effects on EMIC dispersion, and apply this model to diffusion rate evaluations. We show that hot ion effects tend to increase the minimum resonant energy for the frequency range around wave intensity maxima, but can decrease the minimum resonant energy for the higher‐frequency part of wave spectra.

Simultaneous Evolutions of Inner Magnetospheric Plasmaspheric Hiss and EMIC Waves Under the Influence of a Heliospheric Plasma Sheet

Plasmaspheric hiss and electromagnetic ion cyclotron (EMIC) waves are two of the most important magnetospheric plasma waves inside the Earth's high-density plasmasphere. In this work, utilizing observations from the WIND and Van Allen Probes missions, we investigate the simultaneous evolutions of inner magnetospheric plasmaspheric hiss and EMIC waves under the influence of a heliospheric plasma sheet (HPS). The long duration impingement of high-density solar wind HPS onto the magnetosphere produced hot anisotropy electrons and protons, and then caused excitations of plasmaspheric hiss and EMIC waves in the duskside plasmasphere (L ∼ 6.75, MLT ∼ 16.85). In contrast, the solar wind decompression associated with the subsequent cessation of the HPS led to decreases in hot electron and proton fluxes, triggering prompt disappearances of the compression-related plasmaspheric hiss and EMIC waves. The presented results highlight the importance of solar wind conditions for the Earth's space weather and provide new insights into the solar wind-magnetosphere coupling.

Second‐Harmonic Generation of EMIC Waves in the Inner Magnetosphere: Theoretical Analyses and Hybrid Simulations

AbstractObservations of non‐linear second harmonic (SH) in various waves have recently been reported by several studies. However, the quantitative relation of the amplitudes between the fundamental wave (FW) and the SH has not been derived and verified yet. In this letter, the relation equations for the FW and the SH are simplified under the cold plasma assumption, which is reasonable in the inner magnetosphere. The quantitative relation of the amplitudes between the FW and the SH of the electromagnetic ion cyclotron wave is theoretically derived and verified through 1‐D hybrid simulations. The quantitative influences of the FW frequency and wave normal angle on the SH excitation mechanism are obtained.

Novel EMIC Wave Propagation Pathway Through Buchsbaum Resonance and Inter‐Hemispheric Wave Interference: Swarm Observations and Modeling

AbstractIn situ conjugate electromagnetic ion cyclotron (EMIC) waves observed by the Swarm mission in both hemispheres are presented. A complex and unusual pattern of Alfvénic EMIC wave energy is observed, with a mid‐latitude peak close to the source at L =3.3, as well as a secondary lower L‐peak. A wave propagation model reveals that the secondary peak at L =1.7 may be explained by wave power being redirected equatorward due to the Buchsbaum resonance, crossing and interfering with the same EMIC wave power propagating equatorwards from the opposite hemisphere. This interference creates a coherent equatorial driver for a low‐L field line resonance at the secondary peak, and which is associated with strong shear‐to‐fast mode coupling in the ionosphere. This behavior complicates the interpretation of low‐Earth orbit EMIC data for applications assessing radiation belt loss. Combined low Earth orbit observations and modeling enable these novel and localized magnetosphere‐ionosphere EMIC wave propagation pathways to be identified.

A New Population of Ultra‐Relativistic Electrons in the Outer Radiation Zone

AbstractVan Allen Probes measurements revealed the presence of the most unusual structures in the ultra‐relativistic radiation belts. Detailed modeling, analysis of pitch angle distributions, analysis of the difference between relativistic and ultra‐realistic electron evolution, along with theoretical studies of the scattering and wave growth, all indicate that electromagnetic ion cyclotron (EMIC) waves can produce a very efficient loss of the ultra‐relativistic electrons in the heart of the radiation belts. Moreover, a detailed analysis of the profiles of phase space densities provides direct evidence for localized loss by EMIC waves. The evolution of multi‐MeV fluxes shows dramatic and very sudden enhancements of electrons for selected storms. Analysis of phase space density profiles reveals that growing peaks at different values of the first invariant are formed at approximately the same radial distance from the Earth and show the sequential formation of the peaks from lower to higher energies, indicating that local energy diffusion is the dominant source of the acceleration from MeV to multi‐MeV energies. Further simultaneous analysis of the background density and ultra‐relativistic electron fluxes shows that the acceleration to multi‐MeV energies only occurs when plasma density is significantly depleted outside of the plasmasphere, which is consistent with the modeling of acceleration due to chorus waves.

Effect of Solar Wind Pressure and Substorm Linked Particle Injection on Local Time Distribution of Electromagnetic Ion Cyclotron Waves

Several observations of electromagnetic ion cyclotron (EMIC) waves have been reported from the Earth’s magnetosphere. It is an important component of space weather research due to its efficient role in precipitating highly energetic electrons from the Earth’s radiation belt. In general, the interplanetary solar wind conditions and geomagnetic activity control the generation of EMIC waves by modifying the ambient plasma conditions in the generation region. Therefore, EMIC wave occurrence patterns observed both in space and on the ground are influenced by the variations in the parameters like auroral electrojet (AE index), solar wind dynamic pressure, and Dst, which represent the magnetospheric conditions. We present a detailed local time distribution of EMIC waves observed at the Antarctic station, Maitri (geographic 70.7oS, 11.8oE, L = 5), for 2011–2017. We have cataloged the EMIC wave occurrence in four frequency ranges (between 0.12–2 Hz) and examined their occurrence pattern for different levels of AE index and solar wind pressure. The present analysis reveals that at the ground station, Maitri, the occurrence of EMIC waves is dominated in the lower frequency range (0.12–1 Hz) with lower occurrence in the higher frequency range (>1Hz). It is found that the EMIC waves having frequencies > 1 Hz occur dominantly in the early morning hours with a peak close to 5.7 LT hours, and they are linked to the magnetic activity that occurred in the preceding days, whereas the effect of solar wind dynamic pressure and AE index on the local time occurrence of EMIC waves is unambiguously seen for the waves with <1 Hz. The study revealed the larger AE index shifts occurrence of EMIC waves in the dusk sector.

Depletions of Multi-MeV Electrons and Their Association to Minima in Phase Space Density.

Fast‐localized electron loss, resulting from interactions with electromagnetic ion cyclotron (EMIC) waves, can produce deepening minima in phase space density (PSD) radial profiles. Here, we perform a statistical analysis of local PSD minima to quantify how readily these are associated with radiation belt depletions. The statistics of PSD minima observed over a year are compared to the Versatile Electron Radiation Belts (VERB) simulations, both including and excluding EMIC waves. The observed minima distribution can only be achieved in the simulation including EMIC waves, indicating their importance in the dynamics of the radiation belts. By analyzing electron flux depletions in conjunction with the observed PSD minima, we show that, in the heart of the outer radiation belt (L* < 5), on average, 53% of multi‐MeV electron depletions are associated with PSD minima, demonstrating that fast localized loss by interactions with EMIC waves are a common and crucial process for ultra‐relativistic electron populations.

Trifurcate structure of oxygen band EMIC waves excited in a warm magnetospheric plasma

Applying linear dispersion theory to a warm collisionless plasma, we investigate the effect of hot ion composition on electromagnetic ion cyclotron (EMIC) wave generation. The growth rate of oxygen band waves can divide into three components if hot anisotropic heavy ions are included. Their wave frequencies (0.05 ΩH+, 0.03 ΩH+, and 0.01 ΩH+) are sorted in relation to the cyclotron frequencies of H+, He+, and O+ ions (ΩH+, ΩH+/4, and ΩH+/16). The three sub-bands within the oxygen band form an unusual trifurcate structure of growth rate. A link between the three sub-bands and three hot species has been definitively established. That is to say, hot H+, He+, and O+ ions are responsible for the generation of sub-bands. The unstable frequencies of oxygen band waves are directly modulated by the actual composition of magnetospheric ions. The largest growth can arise between the extremely low frequency and the gyrofrequency of O+ (ΩO+). When O+ ions become the dominant hot component during the storm phase, oxygen waves can be generated at much lower frequencies than ΩO+. We perform a parametric study of oxygen sub-band generation in the magnetosphere by using a statistical survey of the plasma composition measured by the Van Allen Probes. The trifurcate structure of growth rate appears at wide L shells from the outer plasmasphere to the geostationary orbit. The new findings demonstrate that wave structures in the oxygen band are more complex than the hydrogen and helium bands. This may provide insight into the nature of the EMIC instability.

EMIC‐Wave Driven Electron Precipitation Observed by CALET on the International Space Station

AbstractWe present an analysis of the relativistic electron precipitation (REP) event measured by the CALorimetric Electron Telescope (CALET) experiment on board the International Space Station during a relatively weak geomagnetic storm on 31 December 2016. CALET observations were compared with the measurements of the Van Allen Probes in the near‐equatorial plane to investigate the global radiation belt dynamics and the REP drivers. The magnetically conjugate observations from these two missions demonstrate that the significant MeV precipitation directly detected by CALET in low‐Earth orbit during a period of radiation belt depletion following the passage of a high‐speed stream, was associated with dusk‐side electromagnetic ion cyclotron (EMIC) waves. In addition, the combined wave, REP and trapped electron data suggest that the reported radiation belt depletion can be likely ascribed to the concomitant loss effects of EMIC wave scattering driving the atmospheric precipitation, as well as outward radial diffusion associated with magnetopause shadowing.

Simultaneous Observations of EMIC‐Induced Drifting Electron Holes (EDEHs) in the Earth's Radiation Belt by the Arase Satellite, Van Allen Probes, and THEMIS

AbstractWe present an observation of rapid flux depressions in relativistic electrons, which is referred to as “EMIC‐induced drifting electron holes (EDEHs).” The Arase, Van Allen Probes, and THEMIS detected simultaneously electron flux fluctuations. The time variation of flux shows depressions of 1‐min scale with energy dispersion, which appear only in the relativistic energy range and small pitch angles. These characteristics of the flux depression indicate that electromagnetic ion cyclotron waves caused pitch angle scattering on a short time scale in a longitudinally limited region. The Arase satellite detected the local depression of the phase space density of 1,000 MeV/G electron, indicating that EMIC waves cause the true loss of electrons. Tracing the energy dispersion profile of EDEHs, we show that EDEHs are formed at localized region in the dusk side. Multisatellite observations demonstrate that a series of EDEHs eventually cause a substantial depression of the radiation belt on 1‐hr time scale.

Effects of Hot Protons on the Pitch Angle Scattering of Ring Current Protons by EMIC Waves

AbstractVia cyclotron resonant interactions, electromagnetic ion cyclotron (EMIC) waves play an important role in the loss of ring current protons. In this study, by calculating the proton bounce‐averaged pitch angle diffusion coefficients using both the cold and hot plasma dispersion relations, we investigate the effects of hot protons on the EMIC wave‐induced scattering loss of ring current protons. Our results show that, for H+ band (He+ band) EMIC waves, inclusion of hot protons results in significant decrease of pitch angle diffusion coefficients of ∼10–60 keV (4–30 keV) protons, while the scattering efficiency of higher energy protons increases at low pitch angles and decreases at relatively high pitch angles. We also find that the cold plasma approximation seriously underestimates the loss timescales of protons at energies from a few keV to tens of keV but overestimate that of higher energy protons. The differences in proton loss timescales caused by hot protons are generally less than a factor of ∼5 for H+ band but can exceed an order of magnitude for He+ band, showing a strong dependence on the hot proton concentration and temperature anisotropy and L‐shell. This study confirms that effects of hot protons play a crucial role in the EMIC wave driven loss of ring current protons and should be included in future modeling of ring current dynamics.

Magnetic Local Time and Latitude Distribution of Ionospheric Large‐Spatial‐Scale EMIC Wave Events: Swarm Observations

AbstractIn the present work, the magnetic local time and latitude (MLT and MLat) distributions of ionospheric large‐spatial‐scale (&gt;20° MLat) electromagnetic ion cyclotron waves were investigated using high‐resolution 50‐Hz geomagnetic field data from Swarm A and B satellites. Both parallel and transverse waves were studied in a comparative manner for different geomagnetic activities and seasons. Frequent occurrences of large‐spatial‐scale waves in the South Atlantic Anomaly and North America, where the parallel waves propagate over the longest distance, were observed. Waves appear mostly in the 02–10 MLT sector wherein the pre‐noon parallel waves propagate farthest in latitudinal range. With the enhancement of geomagnetic activity, both transverse and parallel waves increase in occurrence. The dayside occurrence rate is higher during weak geomagnetic activity, whereas the situation is reversed on the nightside and duskside. The dayside waves are located outside the mid‐latitude trough, and the nightside waves are located near (inside) the equatorward boundary of the mid‐latitude trough. Large‐spatial‐scale waves tend to occur at the equinoxes when the absolute value of the dipole tilt angle is less than 10°, while the long‐distance transmission in the waveguide occurs in the pre‐noon in summer. Parallel waves propagate in the region where the electron density is higher than that of the transverse waves. There is a close relationship between EMIC wave and electron density oscillation.

Modeling the Dynamics of Energetic Protons in Earth's Inner Magnetosphere

AbstractRecently, Van Allen Probes data have revealed the interesting energy‐dependent dynamics of energetic protons in Earth's inner magnetosphere. To quantify the role of radial diffusion and charge exchange to the observed energy‐dependent dynamics of ring current protons, a 1D radial diffusion model with charge exchange loss is implemented. The observed proton flux over the long‐term period of November 2012 to September 2013 is first converted to phase space densities, which are then simulated by our model driven by an outer boundary condition at L* = 5.5 derived from the data. The simulation results show that our model generally captures the transport and acceleration of energetic protons at μ = 30, 50, and 80 MeV/G and K = 0.11 G1/2 RE, suggesting that radial diffusion is the dominant source mechanism for &gt;75 keV protons at L* = 3.5–5.5. In addition, the observed fast decay of protons at lower μ and slow decay at higher μ are well captured by the model, demonstrating the dominant role of charge exchange in explaining the observed energy‐dependent proton decay. For higher μ protons, prompt losses of protons on the time scale of hours are observed over a wide range of L*, which is too fast to be explained by charge exchange. Some of the prompt losses at high L* regions are reproduced by the model with outward radial diffusion to the outer boundary. However, many prompt losses observed at lower L* regions are not captured by the model, which could be due to other loss mechanisms including electromagnetic ion cyclotron wave scattering and field line curvature scattering.

Electromagnetic Ion Cyclotron Harmonic Waves Generated via Nonlinear Wave‐Wave Couplings

AbstractIn this letter, we report an electromagnetic ion cyclotron (EMIC) harmonic event observed by the Van Allen Probe B, whose generation is demonstrated to result from nonlinear wave‐wave resonances through the bicoherence analysis. Hybrid simulation shows that the second to sixth harmonics could be excited in sequence after a pump EMIC wave is initially injected, which is in consistent with the observations of EMIC harmonic. It indicates the important role of the second harmonic in the generation of higher harmonics. Finally, the statistical distribution for the amplitude of EMIC second harmonic waves indicates that the energy transfer rate from the fundamental wave to the second harmonic is mainly less than 2%, which might be too low to result in third and other higher harmonics above the observational level. Our results will give some new insights into the excitation of EMIC harmonic waves in the inner magnetosphere.

Magnetotail Ion Structuring by Kinetic Ballooning-Interchange Instability.

By combining three‐probe THEMIS observations and 3‐D Particle‐in‐Cell simulations, we identify key structures on the ion gyroradius scale that occur in connection with ballooning‐interchange instability heads in the Earth's magnetotail. The mesoscale structures occur at sites of strong ion velocity shear and vorticity where the thermal ion Larmor radius is about half of the width of the head. Finer structures occur at the smaller scales characterizing the wavelength of the electromagnetic ion cyclotron waves generated at the heads. These two processes act to erode and thin the current sheet, thereby forming a local magnetotail configuration that is favorable for reconnection.

Effects of EMIC Wave‐Driven Proton Precipitation on the Ionosphere

AbstractWe investigate the proton precipitation caused by electromagnetic ion cyclotron (EMIC) waves and its impact on the ionosphere with the self‐consistent ring current‐atmosphere interactions model (RAM‐SCBE) coupled with an ionospheric particle transport model Global Airglow (GLOW). The EMIC wave diffusion process causes significant precipitation of tens of keV protons, particularly in the afternoon to midnight sector. These precipitating energetic protons further impact the ionosphere‐thermosphere and contribute to the ionization in the E/F regions. The integrated auroral conductance is significantly enhanced in the dusk‐to‐midnight sector. Although the EMIC waves do not directly interact with the ring current electrons, after the EMIC wave scattering included in the model, remarkable changes are found in the global distribution of precipitating electron flux. This means that the addition of proton precipitation driven by EMIC waves results in feedback effects on the ring current electron dynamics through the circulation system. Validation is also conducted by comparing the simulated precipitation flux and ionospheric electron density with observations.