Electromagnetic Ion Cyclotron Research Articles

Multi‐Event Study of Simultaneous Observations of Isolated Proton Auroras at Subauroral Latitudes Using Ground All‐Sky Imagers and the Van Allen Probes

AbstractIsolated proton auroras (IPAs) appearing at subauroral latitudes are generated by energetic protons precipitating from the magnetosphere through interaction with electromagnetic ion cyclotron (EMIC) waves. Thus, an IPA is the ionospheric projection of the spatial and temporal variation of wave‐particle interaction regions in the magnetosphere. In this study, we conducted unique multi‐event analysis of simultaneous observations of IPAs and their source regions on 22 April, 7 September, and 22 March 2018, using all‐sky imagers at subauroral latitudes and the Van Allen Probes. When the satellite footprint passed over the IPAs associated with ground Pc1 geomagnetic pulsations, locally generated He+‐band EMIC waves with the same frequencies as the ground Pc1 pulsations were observed in all events. The IPAs and EMIC waves had comparable narrow widths in the latitudinal direction. The EMIC waves appeared during the rapid enhancement of the ring current proton flux at energy range of ∼10–50 keV, while they disappeared at the rapid decrease of the electron density. From these results, we conclude that the boundaries of the localized IPAs and EMIC waves were determined by the overlap region of energetic proton enhancement and the plasmasphere. This overlap of ring‐current protons and plasmasphere is a favorable condition for the pitch‐angle scattering of protons by the EMIC waves. Characteristic magnetic and electric field variations with the IPAs were not observed by the satellite, indicating that the IPAs were not accompanied by field‐aligned currents comparable to that of oval auroral arcs.

The Importance of Ion Composition for Radiation Belt Modeling

AbstractThe banded structure of electromagnetic ion cyclotron (EMIC) wave spectra and their resonant interactions with radiation belt electrons depend on the cold ion composition. However, there is a great deal of uncertainty in the composition in the inner magnetosphere due to difficulties in direct flux measurements. Here, we show that the hydrogen and helium band wave spectra are most consistent with a helium and oxygen composition of a few percent. Less than 10% of hydrogen band wave intensity is consistent with a high helium fraction of ∼20%. Similarly, only ∼20% helium band wave intensity is consistent with an oxygen torus ion composition. Furthermore, we find that the decay of the ultra‐relativistic electrons in the radiation belts by EMIC waves depends on the ion composition. The decay is most sensitive to the helium fraction, and the strongest agreement with Van Allen Probes data is found when the helium fraction is a few percent. We suggest that more observations of the cold ion composition would significantly help understand and set constraints on the decay of ultrarelativistic electrons in the radiation belts.

Spacecraft Potential Changes Associated With EMIC Waves in the Inner Magnetosphere

AbstractAlthough electromagnetic ion cyclotron (EMIC) waves are commonly observed in the magnetosphere and are believed to energize background cold ions, it is not clear whether EMIC waves play a significant role in determining spacecraft potential change. In this paper, we present two strong He‐band EMIC wave events observed by the Van Allen Probe‐B spacecraft inside the plasmasphere. One event occurred on 11 March 2016 when the spacecraft was on the dayside, and the other occurred on 9 October 2016 when the spacecraft was in the postmidnight sector. When a strong He‐band EMIC wave activity was detected, low‐energy ion flux enhancements occurred nearly simultaneously with the EMIC wave power enhancements. Both events presented in this study are clearly unique in that He‐band wave power and enhanced proton flux are extremely high. During the wave activity interval, we found that the spacecraft charged more positively without a significant change in the ambient electron density. We discuss whether low‐energy ions energized by EMIC waves can contribute to the spacecraft potential change.

Proton aurora and relativistic electron microbursts scattered by electromagnetic ion cyclotron waves

Charged particle precipitation from Earth’s magnetosphere results in stunning displays of the aurora and energy transfer into the atmosphere. Some of this precipitation is caused by wave-particle interactions. In this study, we present an example of a wave-particle interaction between Electromagnetic Ion Cyclotron waves, and magnetospheric protons and electrons. This interaction resulted in a co-located isolated proton aurora and relativistic electron microbursts. While isolated proton aurora is widely believed to be caused by Electromagnetic Ion Cyclotron waves, this unique observation suggests that these waves can also scatter relativistic electron microbursts. Theoretically, nonlinear interactions between Electromagnetic Ion Cyclotron waves and electrons are necessary to produce the intense sub-second microburst precipitation. Lastly, detailed analysis of the auroral emissions suggests that no chorus waves were present during the event. This is in contrast to the most commonly associated driver of microbursts, whistler mode chorus waves, and supports other less commonly considered driving mechanisms.

Long-lasting Electromagnetic Ion Cyclotron wave signatures at Indian Antarctic Station, Maitri

The long-lasting, persistence electromagnetic ion cyclotron (EMIC) waves can have a profound effect on the dynamics of the relativistic electrons in the Earth’s radiation belt. EMIC waves observed on the ground are the manifestation of their generation in the Earth’s inner magnetosphere. With the spacecraft measurements, it is difficult to determine their spatio-temporal extent and sustainability. In this context, in addition to the spacecraft observations the ground observations can give better insights into these features. We have identified the long-duration EMIC wave events observed at the subauroal station Maitri during 2011–2017. We have found a total of 12 EMIC wave events that are steadily observed for >10 h. The start times of these events are identified between 1.4–9.3 LT hours, and they lasted for 10–18 h. All these events are dominantly seen during the recovery phase of the weak-moderate geomagnetic storm. Comparison with spacecraft observations indicate the simultaneous EMIC waves in the magnetosphere within L–shell 4–6 with wider longitudinal coverage, and sufficiently long time presence in the magnetosphere. Simultaneous differential electron flux observations from Van Allen Probes indicate that approximately 90% of these EMIC waves were accompanied by a decrease in relativistic (2.6–4.2 MeV) electrons in the outer radiation belts.

Combined Scattering of Suprathermal Electrons by Whistler‐Mode Chorus and Electromagnetic Ion Cyclotron Waves in the Low‐Density Plasmatrough

AbstractBased on the observation from Van Allen Probes, we report an event of intense and simultaneous electromagnetic ion cyclotron (EMIC) and chorus waves inside the low‐density plasmatrough. By calculating the scattering rates and performing the diffusion simulations, we for the first time evaluate the combined scattering of suprathermal electrons by EMIC and chorus waves in the low‐density plasmatrough. Landau acceleration of suprathermal electrons by EMIC waves is mainly restricted at large pitch angles while their scattering loss by chorus waves occurs at small pitch angles well below 90°. When EMIC and chorus waves are simultaneously present, the scattering of suprathermal electrons by the two waves is very different from that by each type of waves alone. EMIC waves play a complementary role in the scattering of suprathermal electrons by chorus waves, which facilitate the scattering of suprathermal electrons over all pitch angles. The near‐equatorially minoring suprathermal electrons are firstly scattered toward intermediate pitch angles by EMIC waves and further toward the loss cone by chorus waves. Our results highlight the complementary role of different types of magnetospheric waves in the suprathermal electron scattering, which potentially have vital implications for the dynamics of suprathermal electrons in the planetary magnetosphere.

Analysis of Ionospheric Compressional Waves and Electron Density Oscillation During Storm Periods Using Swarm Observations

AbstractUsing Swarm satellite observations, we aimed to examine compressional electromagnetic ion cyclotron (EMIC) waves and concurrent plasma density oscillations during storm periods (2014–2018). Compressional waves were accompanied by precipitated energetic proton fluxes, which can lead to increased electron density in the F‐layer. Approximately one‐third of compressional waves were accompanied by electron density (Ne) oscillations. The Ne oscillation occurred more frequently during stronger magnetic storms and with EMIC wave events of larger power density. The morning sector and South Atlantic Anomaly region revealed high occurrence rates of Ne oscillations. Its peak occurrence region showed a local time drift during the storm evolution. These variations with magnetic local time and storm phases are consistent with those of EMIC waves. Phases of the EMIC compressional waves and Ne oscillations exhibited a strong correlation. The phases of EMIC waves and Ne oscillations were linearly related, apart from a bias of ∼17.5° or 200.5°. The relative amplitude ratio of most Ne oscillations to the compressional EMIC wave showed a strong correlation with the local Alfven velocity. When the Alfven velocity increased, the relative amplitude ratio also increased, and there appeared a saturation of the relative amplitude ratio at large Alfven velocity.

Resonance broadening effect for relativistic electron interaction with electromagnetic ion cyclotron waves

Relativistic electron scattering by electromagnetic ion cyclotron (EMIC) waves is one of the most effective mechanisms for >1 MeV electron flux depletion in the Earth's radiation belts. Resonant electron interaction with EMIC waves is traditionally described by quasi-linear diffusion equations, although spacecraft observations often report EMIC waves with intensities sufficiently large to trigger nonlinear resonant interaction with electrons. An important consequence of such nonlinear interaction is the resonance broadening effect due to high wave amplitudes. In this study, we quantify this resonance broadening effect in electron pitch-angle diffusion rates. We show that resonance broadening can significantly increase the pitch-angle range of EMIC-scattered electrons. This increase is especially important for ∼1 MeV electrons, where, without the resonance broadening, only those near the loss cone (with low fluxes) can resonate with EMIC waves.

Mass Density Inferred From Toroidal Wave Frequencies and Energization of Low‐Energy Helium Ions During H‐Band EMIC Wave Interval

AbstractVan Allen Probe A observed strongly enhanced hydrogen (H+) band electromagnetic ion cyclotron (EMIC) waves lasting >6 hr without helium (He+) band EMIC wave activity outside the plasmasphere on 23 February 2014. During the H‐band EMIC wave interval, multiharmonic toroidal waves were detected by the spacecraft. We estimated the local mass density using the observed toroidal wave frequencies and background electron number density. It was found that the average ion mass is close to 1. This indicates that the protons are the dominant ion species for the H‐band wave interval. From these observations, we suggest that the cold ion composition plays a major role in determining the spectral properties of EMIC waves. We also observed low‐energy He+ ion flux enhancements associated with the H‐band EMIC waves in the direction perpendicular to the background magnetic field. We discuss how the H‐band waves energize the low‐energy He+ ions in the perpendicular direction.

A Statistical Study of Modulation of Electromagnetic Ion Cyclotron Waves Observed on Ground

AbstractThe modulation of electromagnetic ion cyclotron (EMIC) waves by different geomagnetic pulsations is known to us from both ground and satellite observations. However, their dependence on the EMIC wave characteristics is not well explored. We report a statistical analysis of modulation of EMIC waves by short and long periodicities at the Indian Antarctic station, Maitri (L ≈ 5). We have analyzed the induction coil magnetometer data for the period of 2011–2017 and identified 6,845 EMIC wave events, out of which 5,502 events (80%) clearly showed the presence of short period modulation. These short period modulations are associated with repetitive rising tone EMIC wave emissions. Among these 5,502 EMIC wave events only 2,413 events showed presence of long period modulation, in addition to the short period modulation. Next we have examined the characteristics like start time, end time, peak frequency, frequency extent, maximum power, and dominant short and long periodicities present in each EMIC wave event. Based on the statistical analysis, we found that the dominant short and long periodicities in the EMIC waves are in the range of 1.5–3 min and 10–60 min, respectively. These short period decreases with an increase in the peak frequency of the EMIC wave. It is attributed to the decrease in magnetic field line oscillation period at lower L‐shells. Additionally, we noticed that the stronger EMIC wave events are likely to have a higher peak frequency. All these observed tendencies are examined in light of nonlinear theory, and they are found to be in good agreement.

Statistical Study of EMIC Wave Propagation Using Space‐Ground Conjugate Observations

AbstractIn the present study, we explore the observational characteristics of Electromagnetic Ion Cyclotron (EMIC) wave propagation from the source region to the ground. We use magnetometers aboard Geostationary Operational Environment Satellite (GOES) 13, the geosynchronous orbit satellite at 75°W, and at Sanikiluaq ground station (SNK, 79.14°W and 56.32°N in geographic coordinates, and L ∼ 6.0 in a dipole magnetic field) which is located in northern Canada. Using these magnetically conjugate observatories, simultaneous EMIC wave observations are carried out. We found a total of 295 coincident and 248 non‐coincident EMIC wave events between GOES 13 and the SNK station. Our statistical analysis reveals that the coincident events are predominantly observed on the dayside. The wave normal angles are slightly higher for the non‐coincident events than for coincident events. However, the coincidence of the waves is mostly governed by the intensity and duration of the wave. This is confirmed by the geomagnetic environment which shows higher auroral electrojet (AE) and Kp indices for the coincident events. We also found that some events show high‐frequency (f > 0.4 Hz) wave filtering. The statistics of the high‐frequency filtered and non‐filtered wave events show that there are clear magnetic local time (MLT) and F10.7 index differences between the two groups, as well as in ionospheric electron density measurements. In addition, we also found differences in the wave properties which possibly indicate that the propagation in the magnetosphere also plays an important role in the wave filtering.

Testing the Linearity of Combined Electron Scattering Effects Driven by Simultaneous H+ and He+ Band EMIC Waves

AbstractThe main aim of this study is to examine whether the combined scattering effects simultaneously induced by different plasma waves can be treated as the linear superposition (LS) of independent scattering effect caused by each single wave mode or not. Accordingly, this study investigates the individual and combined scattering effects of simultaneous H+ and He+ band electromagnetic ion cyclotron (EMIC) waves as an example. Both quasi‐linear theory and test particle simulations have been applied to evaluate the scattering effects in the form of diffusion rates and single particle tracing. The narrowband EMIC waves with amplitudes (1, 5, and 10 nT) and monochromatic EMIC waves with amplitudes ranging from 0.1 to 10 nT have been adopted in this study to involve both linear and nonlinear resonant scattering effects. The analysis on resonant condition indicates that H+ and He+ band EMIC waves can simultaneously scatter electrons with the same energies and pitch angles. For both narrowband and monochromatic EMIC waves, the combined diffusion rates induced by simultaneous two band waves are consistent with the LS of independent scattering rates caused by single band waves. The following single particle tracing presents two different wave‐particle interaction conditions at different magnetic latitudes. In conclusion, our results suggest that the linear superposed scattering results can effectively evaluate the combined resonant scattering effects driven by two‐band EMIC waves, which can easily be generalized to estimate the combined scattering effects simultaneously induced by different plasma waves for radiation belt modeling.

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.