Electromagnetic ion cyclotron (EMIC) waves are transverse plasma waves generated by anisotropic proton distributions with Tperp > Tpara. They are believed to play an important role in the dynamics of ...the ring current and potentially, of the radiation belts. Therefore it is important to know their localization in the magnetosphere and the magnetospheric and solar wind conditions which lead to their generation. Our earlier observations from three Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes demonstrated that strong magnetospheric compressions associated with high solar wind dynamic pressure (Pdyn) may drive EMIC waves in the inner dayside magnetosphere, just inside the plasmapause. Previously, magnetospheric compressions were found to generate EMIC waves mainly close to the magnetopause. In this work we use an automated detection algorithm of EMIC Pc1 waves observed by THEMIS between May 2007 to December 2011 and present the occurrence rate of those waves as a function of L‐shell, magnetic local time (MLT), Pdyn, AE, and SYMH. Consistent with earlier studies we find that the dayside (sunward of the terminator) outer magnetosphere is a preferential location for EMIC activity, with the occurrence rate in this region being strongly controlled by solar wind dynamic pressure. High EMIC occurrence, preferentially at 12–15 MLT, is also associated with high AE. Our analysis of 26 magnetic storms with Dst < −50 nT showed that the storm‐time EMIC occurrence rate in the inner magnetosphere remains low (<10%). This brings into question the importance of EMIC waves in influencing energetic particle dynamics in the inner magnetosphere during disturbed geomagnetic conditions.
Key Points
Dayside outer magnetosphere is a preferential location for EMIC waves
Dayside EMIC occurrence rate is controlled by solar wind pressure
Storm‐time EMIC occurrence in the inner magnetosphere remains low
We study the effect of electromagnetic ion cyclotron (EMIC) waves on the loss and pitch angle scattering of relativistic and ultrarelativistic electrons during the recovery phase of a moderate ...geomagnetic storm on 11 October 2012. The EMIC wave activity was observed in situ on the Van Allen Probes and conjugately on the ground across the Canadian Array for Real‐time Investigations of Magnetic Activity throughout an extended 18 h interval. However, neither enhanced precipitation of >0.7 MeV electrons nor reductions in Van Allen Probe 90° pitch angle ultrarelativistic electron flux were observed. Computed radiation belt electron pitch angle diffusion rates demonstrate that rapid pitch angle diffusion is confined to low pitch angles and cannot reach 90°. For the first time, from both observational and modeling perspectives, we show evidence of EMIC waves triggering ultrarelativistic (~2–8 MeV) electron loss but which is confined to pitch angles below around 45° and not affecting the core distribution.
Key Points
EMIC wave activity is not associated with precipitation of MeV electrons
EMIC waves do not deplete the ultra‐relativistic belt down to 90°
EMIC waves cause loss of low pitch angle electrons with energies ~2–8 MeV
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.
Plain Language Summary
In this study, we explore the distribution of extremely high energy electrons that surround near‐Earth space (from ∼20,000 km up to ∼35,000 km). Such electrons are trapped by the Earth's magnetic field, forming the radiation belts. There are several mechanisms of how such electrons can vanish. So‐called phase space density profiles help us to distinguish between different causes of electron loss. Our statistical analysis of the minima in the phase space density profiles and additional comparison with the depletions of measured electron fluxes showed that a fast localized loss process is frequently acting in the heart of the outer radiation belt. We associate this loss process with wave–particle interactions between electrons and electromagnetic ion cyclotron (EMIC) waves. This conclusion is confirmed by global modeling and demonstrates the importance of EMIC waves in the dynamics of the radiation belts.
Key Points
PSD minima are commonly observed throughout a year of observations, indicating a major role of fast‐localized losses of multi‐MeV electrons
A modeled year reproduces the distribution of PSD minima over the same range of μ and K as observations only when EMIC waves are included
On average, 53% of multi‐MeV flux depletions below L* = 5 (αeq < 75 °) are associated with PSD minima
Recently, electromagnetic ion cyclotron (EMIC) wave generation in plasmaspheric plumes has been the subject of extensive discussion. Theory predicts that regions of detached cold, dense plasma ...immersed in relatively low background magnetic field should aid EMIC wave growth and may provide conditions for interaction between the EMIC waves and relativistic (MeV) electrons, leading to energetic particle loss into the atmosphere. Since plasmaspheric plumes are specific to disturbed geomagnetic conditions, the link between EMIC waves and plumes may be especially important for radiation belt dynamics during magnetic storms. In this work, we present an in situ survey of EMIC waves in plasmaspheric plumes using data from the Cluster satellites and will address the question of whether plumes are important for EMIC wave generation from a statistical perspective. We used a survey of plasmaspheric plumes between 2001 and 2006 identified from the Waves of High frequency and Sounder for Probing of Electron density by Relaxation (WHISPER) sounder measurements. We further identified EMIC waves from simultaneous (with WHISPER) magnetic field measurements by the fluxgate magnetometer instruments and investigated the relationship between these two data sets. Only 10% of the time when Cluster‐observed plumes along its orbit did we also observe EMIC waves. The wave occurrence outside plumes is further significantly reduced and is ~20 times lower in immediately adjacent regions than inside plumes. We found that cold plasma density was not a good predictor of EMIC occurrence inside the plumes and that the absolute density does not affect the EMIC probability. On the other hand, enhanced solar wind dynamic pressure significantly increases EMIC wave occurrence rate inside the plumes.
Key Points
We analyzed 6 years of the Cluster satellite data
EMIC waves were seen during 10% of the time when Cluster observed plumes
Enhanced solar wind pressure controls EMIC occurrence in plumes
In this study, we performed a series of long‐term and individual storm simulations with and without hiss, chorus, and electromagnetic ion cyclotron (EMIC) waves. We compared simulation results ...incorporating different wave modes with Van Allen Probes flux observations to illustrate how hiss and chorus waves aid EMIC waves in depleting multi‐MeV electrons. We found that EMIC, hiss, and chorus waves are required to reproduce satellite measurements in our simulations. Our results indicate that hiss waves play a dominant role in scattering near‐equatorial mirroring electrons, and they assist EMIC waves, which scatter only small pitch angle electrons. The best agreement between the observations and the simulations (long‐term and 17 January 2013 storm) is achieved when hiss, chorus, and EMIC waves are included.
Key Points
Hiss and chorus waves aid EMIC waves in depleting near equatorial mirroring electrons
In the presence of EMIC waves, chorus wave‐particle interactions can result in a net loss instead of acceleration of multi‐MeV electrons
In long‐term simulations, hiss waves play a dominant role in aiding multi‐MeV electron depletion by EMIC waves
We present coordinated ground satellite observations of solar wind compression‐related dayside electromagnetic ion cyclotron (EMIC) waves from 25 September 2005. On the ground, dayside structured ...EMIC wave activity was observed by the CARISMA and STEP magnetometer arrays for several hours during the period of maximum compression. The EMIC waves were also registered by the Cluster satellites for half an hour, as they consecutively crossed the conjugate equatorial plasmasphere on their perigee passes at L ∼ 5. Simultaneously, conjugate to Cluster, NOAA 17 passed through field lines supporting EMIC wave activity and registered a localized enhancement of precipitating protons with energies >30 keV. Our observations suggest that generation of the EMIC waves and consequent loss of energetic protons may last for several hours while the magnetosphere remains compressed. The EMIC waves were confined to the outer plasmasphere region, just inside the plasmapause. Analysis of lower‐frequency Pc5 waves observed both by the Cluster electron drift instrument (EDI) and fluxgate magnetometer (FGM) instruments and by the ground magnetometers show that the repetitive structure of EMIC wave packets observed on the ground cannot be explained by the ultra low frequency (ULF) wave modulation theory. However, the EMIC wave repetition period on the ground was close to the estimated field‐aligned Alfvénic travel time. For a short interval of time, there was some evidence that EMIC wave packet repetition period in the source region was half of that on the ground, which further suggests bidirectional propagation of wave packets.
Electromagnetic ion cyclotron (EMIC) waves play an important role in the dynamics of ultrarelativistic electron population in the radiation belts. However, as EMIC waves are very sporadic, developing ...a parameterization of such wave properties is a challenging task. Currently, there are no dynamic, activity‐dependent models of EMIC waves that can be used in the long‐term (several months) simulations, which makes the quantitative modeling of the radiation belt dynamics incomplete. In this study, we investigate Kp, Dst, and AE indices, solar wind speed, and dynamic pressure as possible parameters of EMIC wave presence. The EMIC waves are included in the long‐term simulations (1 year, including different geomagnetic activity) performed with the Versatile Electron Radiation Belt code, and we compare results of the simulation with the Van Allen Probes observations. The comparison shows that modeling with EMIC waves, parameterized by solar wind dynamic pressure, provides a better agreement with the observations among considered parameterizations. The simulation with EMIC waves improves the dynamics of ultrarelativistic fluxes and reproduces the formation of the local minimum in the phase space density profiles.
Key Points
Addition of EMIC waves improves the long‐term simulation of multi‐MeV electron dynamics
Solar wind dynamic pressure provides better parameterization of EMIC wave presence than Kp, Dst, and AE indices and solar wind velocity
Simulation with parameterized EMIC waves reproduces a phase space density deepening minimum
The turbulent energy cascade that is characteristic of bursty bulk flow (BBF) braking regions in the Earth's magnetotail has been shown to be the energy source of large‐amplitude electric fields ...(>50 mV/m) which can, in turn, result in local energetic electron acceleration. These pre‐energized electrons can move inward to stronger magnetic fields being further accelerated and can eventually supply an energetic tail to electron distributions in the outer radiation belt. Using wave and plasma measurements from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites during four tail seasons from 2015 to 2019, we study the process of BBF magnetic and kinetic energy being transferred to electrons by turbulent electric fields from a statistical perspective. We identify turbulent BBF regions by the presence of high‐amplitude electric fields. We show that the high‐amplitude electric fields are associated with an increase in electron temperature by three times compared to quiet times and with a ten‐fold increase in temperature fluctuations. They are also associated with strong variations of energetic electron fluxes, indicative of local acceleration. We further discuss the implications of these findings and the role of this pre‐energized electron population in supplying the outer radiation belt.
Plain Language Summary
Bursty bulk flows are high‐speed plasma flows, observed in Earth's nightside magnetosphere. They move toward Earth from distances 20–30 Earth radii in the magnetotail where they are generated by magnetic reconnection. Closer to Earth (6–12 Re tailward), they slow down and deflect, as their energy dissipates in a turbulent cascade. Using data from NASA THEMIS satellites, we show that high‐amplitude electric fields generated in this cascade, strongly energize electrons. If further pushed inward, these pre‐energized electrons will be accelerated by Earth's magnetic field, and can eventually supply the outer radiation belt.
Key Points
Large‐amplitude electric fields can lead to ten‐fold electron temperature fluctuations and three‐fold energetic electron flux variations
Temperature and flux variations rather than absolute temperature and flux values are indicative of local acceleration
The accelerated electrons may serve as a seed population for the high‐energy tail of the outer radiation belt
Large‐amplitude electric fields (>50 mV/m) typical to bursty bulk flow (BBF) braking regions of the Earth's magnetotail can accelerate energetic electrons and ions to many times their initial thermal ...energies. We follow up on the Usanova and Ergun (2022), https://doi.org/10.1029/2022JA030336, study of electron energization and examine wave and plasma observations from the THEMIS satellites over four tail seasons to investigate the transfer of BBF energy to ions by turbulent electric fields. The results show that the large‐amplitude electric fields are accompanied by an ion temperature increase of ∼50% when compared to times when the turbulence is not observed. Electric field turbulence is also associated with a roughly ten‐fold increase in temperature fluctuations and a five‐fold increase in variations of energetic ion fluxes. We discuss the contribution of this turbulent energy transfer process to the dynamics of energetic ions in the magnetosphere.
Plain Language Summary
Bursty bulk flows are high‐speed ion flows propagating toward Earth from the reconnection sites. On their approach to Earth, they decelerate and divert, while generating a turbulent cascade through which their energy dissipates. We use data from NASA's THEMIS satellites to show that high‐amplitude turbulent electric fields are produced through this energy dissipation process, which, in turn, transfer energy to ions. Further, we discuss the contribution of this turbulent energy transfer to the energetic ion dynamics in the inner magnetosphere.
Key Points
Large‐amplitude electric fields are linked to a 1,000% increase in ion temperature fluctuations and a 500% increase in ion flux variations
The effect on ion temperature is smaller than on electron temperature, being 50% versus 300%
The accelerated energetic ions may contribute to ring current and plasmasheet energization
The cold plasmaspheric plasma, the ring current and the radiation belts constitute three important populations of the inner magnetosphere. The overlap region between these populations gives rise to ...wave-particle interactions between different plasma species and wave modes observed in the magnetosphere, in particular, electromagnetic ion cyclotron (EMIC) waves. These waves can resonantly interact with multiple particle species, being an important loss process for both ring current ions and radiation belt electrons, as well as a cold plasma heating mechanism. This mini-review will focus on the interaction between EMIC waves and cold and thermal plasma, specifically the role of EMIC waves in cold and thermal electron and ion heating. It will discuss early theoretical results in conjunction with numerical modelling and recent satellite observations, and address outstanding problems and controversies in this field.
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