Electromagnetic Ion Cyclotron Wave Growth Research Articles

Potential impacts of hydrogen band EMIC waves on the ion velocity distributions: MMS observations

In this paper, the influence of the hydrogen (H+) band electromagnetic ion cyclotron (EMIC) waves on the hydrogen and helium velocity distributions has been studied. The hydrogen band EMIC waves have been investigated in the inner magnetosphere using the magnetospheric multiscale mission. The EMIC waves for the frequency range typical frequency have been frequently observed in the Earth's magnetosphere and have received considerable attention for energy transport across the magnetosphere. In this manuscript, we studied the velocity distribution of cold/hot proton and helium ions at different times of the event under consideration. For cold (1–600 eV) hydrogen ions, the velocity distribution is directly proportional to the growth rate of the EMIC wave, whereas the hot (1–40 keV) hydrogen ions have a ring distribution, which are not strongly influenced by the growth of EMIC waves like cold hydrogen ions, but the helium (1 eV–40 keV) ions are rarely influenced by EMIC waves.

First Observations of O2+ Band EMIC Waves in the Terrestrial Magnetosphere

AbstractAlthough electromagnetic ion cyclotron (EMIC) waves are often known to exhibit three (H+, He+, and O+) usual bands, other peculiar minor ions (excluding He+ and O+) might also play an important role in the spectra of EMIC waves. This letter reports an interesting case that He+ band EMIC waves appear to split into O2+ and He+ band emissions. Wave frequency spectra suggesting that the organization of the wave emissions by the O2+ gyrofrequency motivates an investigation of the effects of O2+ presence on EMIC wave growth using linear theory. The calculated growth rates show that the original He+ band EMIC waves might be locally excited when O2+ is absent, while significant growth occurs for O2+ band and He+ band EMIC waves when O2+ is present. Our results provide insight into the generation of O2+ band EMIC waves.

Oxygen torus and its coincidence with EMIC wave in the deep inner magnetosphere: Van Allen Probe B and Arase observations

We investigate the longitudinal structure of the oxygen torus in the inner magnetosphere for a specific event found on 12 September 2017, using simultaneous observations from the Van Allen Probe B and Arase satellites. It is found that Probe B observed a clear enhancement in the average plasma mass (M) up to 3–4 amu at L = 3.3–3.6 and magnetic local time (MLT) = 9.0 h. In the afternoon sector at MLT ~ 16.0 h, both Probe B and Arase found no clear enhancements in M. This result suggests that the oxygen torus does not extend over all MLT but is skewed toward the dawn. Since a similar result has been reported for another event of the oxygen torus in a previous study, a crescent-shaped torus or a pinched torus centered around dawn may be a general feature of the O+ density enhancement in the inner magnetosphere. We newly find that an electromagnetic ion cyclotron (EMIC) wave in the H+ band appeared coincidently with the oxygen torus. From the lower cutoff frequency of the EMIC wave, the ion composition of the oxygen torus is estimated to be 80.6% H+, 3.4% He+, and 16.0% O+. According to the linearized dispersion relation for EMIC waves, both He+ and O+ ions inhibit EMIC wave growth and the stabilizing effect is stronger for He+ than O+. Therefore, when the H+ fraction or M is constant, the denser O+ ions are naturally accompanied by the more tenuous He+ ions, resulting in a weaker stabilizing effect (i.e., larger growth rate). From the Probe B observations, we find that the growth rate becomes larger in the oxygen torus than in the adjacent regions in the plasma trough and the plasmasphere.

EMIC Waves in the Outer Magnetosphere: Observations of an Off-Equator Source Region.

Electromagnetic ion cyclotron (EMIC) waves at large L shells were observed away from the magnetic equator by the Magnetospheric MultiScale (MMS) mission nearly continuously for over four hours on 28 October 2015. During this event, the wave Poynting vector direction systematically changed from parallel to the magnetic field (toward the equator), to bidirectional, to antiparallel (away from the equator). These changes coincide with the shift in the location of the minimum in the magnetic field in the southern hemisphere from poleward to equatorward of MMS. The local plasma conditions measured with the EMIC waves also suggest that the outer magnetospheric region sampled during this event was generally unstable to EMIC wave growth. Together, these observations indicate that the bidirectionally propagating wave packets were not a result of reflection at high latitudes but that MMS passed through an off‐equator EMIC wave source region associated with the local minimum in the magnetic field.

The Detached Auroras Induced by the Solar Wind Pressure Enhancement in Both Hemispheres From Imaging and In Situ Particle Observations

AbstractThis paper presents simultaneous detached proton auroras that appeared in both hemispheres at 11:06 UT, 08 March 2012, just 2 min after a sudden solar wind pressure enhancement (~11:04 UT) hit the Earth. They were observed under northward interplanetary magnetic field Bz condition and during the recovery phase of a moderate geomagnetic storm. In the Northern Hemisphere, Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager observed that the detached arc occurred within 60°–65° magnetic latitude and covered a few magnetic local time (MLT) hours ranging from 0530 to 0830 MLT with a possible extension toward noon. At the same time (11:06 UT), Polar Orbiting Environment Satellites 19 detected a detached proton aurora around 1300 MLT in the Southern Hemisphere, centering ~62° magnetic latitude, which was at the same latitudes as the northern detached arc. This southern aurora was most probably a part of a dayside detached arc that was conjugate to the northern one. In situ particle observations indicated that the detached auroras were dominated by protons/ions with energies ranging from around 20 keV to several hundreds of keV, without obvious electron precipitations. These detached arcs persisted for less than 6 min, consistent with the impact from pressure enhancement and the observed electromagnetic ion cyclotron (EMIC) waves. It is suggested that the increasing solar wind pressure pushed the hot ions in the ring current closer to Earth where the steep gradient of cold plasma favored EMIC wave growth. By losing energy to EMIC waves the energetic protons (>20 keV) were scattered into the loss cone and produced the observed detached proton auroras.

EMIC waves covering wide L shells: MMS and Van Allen Probes observations

AbstractDuring 04:45:00–08:15:00 UT on 13 September in 2015, a case of Electromagnetic ion cyclotron (EMIC) waves covering wide L shells (L = 3.6–9.4), observed by the Magnotospheric Multiscale 1 (MMS1) are reported. During the same time interval, EMIC waves observed by Van Allen Probes A (VAP‐A) only occurred just outside the plasmapause. As the Van Allen Probes moved outside into a more tenuous plasma region, no intense waves were observed. Combined observations of MMS1 and VAP‐A suggest that in the terrestrial magnetosphere, an appropriately dense background plasma would make contributions to the growth of EMIC waves in lower L shells, while the ion anisotropy, driven by magnetospheric compression, might play an important role in the excitation of EMIC waves in higher L shells. These EMIC waves are observed over wide L shells after three continuous magnetic storms, which suggests that these waves might obtain their free energy from those energetic ions injected during storm times. These EMIC waves should be included in radiation belt modeling, especially during continuous magnetic storms. Moreover, two‐band structures separated in frequencies by local He2+ gyrofrequencies were observed in large L shells (L > ~6), implying sufficiently rich solar wind origin He2+ likely in the outer ring current. It is suggested that multiband‐structured EMIC waves can be used to trace the coupling between solar wind and the magnetosphere.

Effect of hot anisotropic He+ ions on the growth and damping of electromagnetic ion cyclotron waves in the inner magnetosphere

AbstractPhysics of electromagnetic ion cyclotron (EMIC) waves is complicated by inclusion of heavy ions. In particular, He+ ions in the magnetosphere have long been considered to play important roles. Motivated by recent observations, we examine the effect of the inclusion of hot anisotropic He+ ions in addition to the usual hot anisotropic protons. We solve the kinetic dispersion relation for this examination and find the following results. First, inclusion of hot anisotropic He+ ions leads to the growth of EMIC waves at frequencies below the He+ gyrofrequency (He band) and a reduction of the EMIC wave growth rates (or damping of the waves) at frequencies between the proton and He+ gyrofrequencies (H band). Second, this effect is more dramatic for higher temperatures of He+ that would play a role in damping EMIC waves for both frequency bands and especially for cases without a He+ temperature anisotropy. Lastly, the effect is more prominent for cold plasma dominant conditions such as the region inside the plasmasphere or plume than for hot proton dominant conditions such as the region outside the plasmasphere. We propose that this last effect can at least partially explain the satellite observations indicating the preferred (though not exclusive) occurrence of He band waves inside the plasmasphere for the times when hot anisotropic He+ ions are supplied from the plasma sheet and ring current.

A parametric study of the linear growth of magnetospheric EMIC waves in a hot plasma

Since electromagnetic ion cyclotron (EMIC) waves in the terrestrial magnetosphere play a crucial role in the dynamic losses of relativistic electrons and energetic protons and in the ion heating, it is important to pursue a comprehensive understanding of the EMIC wave dispersion relation under realistic circumstances, which can shed significant light on the generation, amplification, and propagation of magnetospheric EMIC waves. The full kinetic linear dispersion relation is implemented in the present study to evaluate the linear growth of EMIC waves in a multi-ion (H+, He+, and O+) magnetospheric plasma that also consists of hot ring current protons. Introduction of anisotropic hot protons strongly modifies the EMIC wave dispersion surface and can result in the simultaneous growth of H+-, He+-, and O+-band EMIC emissions. Our parametric analysis demonstrates that an increase in the hot proton concentration can produce the generation of H+- and He+-band EMIC waves with higher possibility. While the excitation of H+-band emissions requires relatively larger temperature anisotropy of hot protons, He+-band emissions are more likely to be triggered in the plasmasphere or plasmaspheric plume where the background plasma is denser. In addition, the generation of He+-band waves is more sensitive to the variation of proton temperature than H+-band waves. Increase of cold heavy ion (He+ and O+) density increases the H+ cutoff frequency and therefore widens the frequency coverage of the stop band above the He+ gyrofrequency, leading to a significant damping of H+-band EMIC waves. In contrast, O+-band EMIC waves characteristically exhibit the temporal growth much weaker than the other two bands, regardless of all considered variables, suggesting that O+-band emissions occur at a rate much lower than H+- and He+-band emissions, which is consistent with the observations.

Source of seed fluctuations for electromagnetic ion cyclotron waves in Earth’s magnetosphere

We consider a nonlinear wave energy cascade from the low frequency range into the higher frequency domain of electromagnetic ion cyclotron (EMIC) wave generation as a possible source of seed fluctuations for EMIC wave growth due to the ion cyclotron instability in Earth’s magnetosphere. The presented theoretical analysis shows that energy cascade from the Pc 4–5 frequency range (2–22mHz) into the range of Pc 1–2 pulsations (0.1–5Hz), i.e. into the frequency range of EMIC waves, is able to supply the needed level of seed fluctuations that guarantees growth of EMIC waves up to the observable level during one pass through the near equatorial region where the ion cyclotron instability takes place. We also analyze the magnetic field data from the Polar and Van Allen Probes spacecraft to test the suggested nonlinear mechanism. In this initial study we restrict our analysis to magnetic fluctuation spectra only. We do not analyze the third-order structure function, but judge whether a nonlinear energy cascade is present or whether it is not by only analyzing the appearance of power-law distributions in the low-frequency part of the magnetic field spectra. While the power-law spectrum alone does not guarantee that a nonlinear cascade is present, the power-law distribution is a strong indication of the possible development of a nonlinear cascade. Our analysis shows that a nonlinear energy cascade is indeed observed in both the outer and inner magnetosphere data, and EMIC waves are growing from this nonthermal background. All the analyzed data are in good agreement with the theoretical model presented in this study. Overall, the results of this study support a nonlinear energy cascade in Earth’s magnetosphere as a mechanism which is responsible for supplying seed fluctuating energy in the higher frequency domain where EMIC waves grow due to the ion cyclotron instability.

Effect of spatial density variation and O+ concentration on the growth and evolution of electromagnetic ion cyclotron waves

AbstractWe simulate electromagnetic ion cyclotron (EMIC) wave growth and evolution within three regions, the plasmasphere (or plasmaspheric plume), the plasmapause, and the low‐density plasmatrough outside the plasmapause. First, we use a ring current simulation with a plasmasphere model to model the particle populations that give rise to the instability for conditions observed on 9 June 2001. Then, using two different models for the cold ion composition, we do a full‐scale hybrid code simulation in dipole coordinates of the EMIC waves on a meridional plane at magnetic local time = 18 and at 1900 UT within a range of L shell from L=4.9 to 6.7. EMIC waves were observed during 9 June 2001 by Geostationary Operational Environmental Satellite (GOES) spacecraft. While an exact comparison between observed and simulated spectra is not possible here, we do find significant similarities between the two, at least at one location within the region of largest wave growth. We find that the plasmapause is not a preferred region for EMIC wave growth, though waves can grow in that region. The density gradient within the plasmapause does, however, affect the orientation of wavefronts and wave vector both within the plasmapause and in adjacent regions. There is a preference for EMIC waves to be driven in the He+ band (frequencies between the O+ and He+ gyrofrequencies) within the plasmasphere, although they can also grow in the plasmatrough. If present, H+ band waves are more likely to grow in the plasmatrough. This fact, plus L dependence of the frequency and possible time evolution toward lower frequency waves, can be explained by a simple model. Large O+ concentration limits the frequency range of or even totally quenches EMIC waves. This is more likely to occur in the plasmatrough at solar maximum. Such large O+ concentration significantly affects the H+ cutoff frequency and hence the width in frequency of the stop band above the He+ gyrofrequency. EMIC wave surfaces predicted by cold plasma theory are altered by the finite temperature of the ring current H+.

Model of electromagnetic ion cyclotron waves in the inner magnetosphere

AbstractThe evolution of He+‐mode electromagnetic ion cyclotron (EMIC) waves is studied inside the geostationary orbit using our global model of ring current (RC) ions, electric field, plasmasphere, and EMIC waves. In contrast to the approach previously used by Gamayunov et al. (2009), however, we do not use the bounce‐averaged wave kinetic equation but instead use a complete, nonbounce‐averaged, equation to model the evolution of EMIC wave power spectral density, including off‐equatorial wave dynamics. The major results of our study can be summarized as follows. (1) The thermal background level for EMIC waves is too low to allow waves to grow up to the observable level during one pass between the “bi‐ion latitudes” (the latitudes where the given wave frequency is equal to the O+–He+ bi‐ion frequency) in conjugate hemispheres. As a consequence, quasi‐field‐aligned EMIC waves are not typically produced in the model if the thermal background level is used, but routinely observed in the Earth's magnetosphere. To overcome this model‐observation discrepancy we suggest a nonlinear energy cascade from the lower frequency range of ultralow frequency waves into the frequency range of EMIC wave generation as a possible mechanism supplying the needed level of seed fluctuations that guarantees growth of EMIC waves during one pass through the near equatorial region. The EMIC wave development from a suprathermal background level shows that EMIC waves are quasi field aligned near the equator, while they are oblique at high latitudes, and the Poynting flux is predominantly directed away from the near equatorial source region in agreement with observations. (2) An abundance of O+ strongly controls the energy of oblique He+‐mode EMIC waves that propagate to the equator after their reflection at bi‐ion latitudes, and so it controls a fraction of wave energy in the oblique normals. (3) The RC O+ not only causes damping of the He+‐mode EMIC waves but also causes wave generation in the region of highly oblique wave normal angles, typically for θ > 82°, where a growth rate γ > 10−2rad/s is frequently observed. The instability is driven by the loss cone feature in the RC O+ distribution function, where ∂F/∂v⟂>0 for the resonating O+. (4) The oblique and intense He+‐mode EMIC waves generated by RC O+ in the region L≈2–3 may have an implication to the energetic particle loss in the inner radiation belt.

Excitation of EMIC waves detected by the Van Allen Probes on 28 April 2013

AbstractWe report the wave observations, associated plasma measurements, and linear theory testing of electromagnetic ion cyclotron (EMIC) wave events observed by the Van Allen Probes on 28 April 2013. The wave events are detected in their generation regions as three individual events in two consecutive orbits of Van Allen Probe‐A, while the other spacecraft, B, does not detect any significant EMIC wave activity during this period. Three overlapping H+ populations are observed around the plasmapause when the waves are excited. The difference between the observational EMIC wave growth parameter (Σh) and the theoretical EMIC instability parameter (Sh) is significantly raised, on average, to 0.10 ± 0.01, 0.15 ± 0.02, and 0.07 ± 0.02 during the three wave events, respectively. On Van Allen Probe‐B, this difference never exceeds 0. Compared to linear theory (Σh > Sh), the waves are only excited for elevated thresholds.

Effects of ion abundances on electromagnetic ion cyclotron wave growth rate in the vicinity of the plasmapause

Electromagnetic ion cyclotron (EMIC) waves in multi-ion species plasmas propagate in branches. Except for the branch corresponding to the heaviest ion species, which has only a resonance at its gyrofrequency, these branches are bounded below by a cutoff frequency and above by a resonant gyrofrequency. The condition for wave growth is determined by the thermal anisotropies of each ion species, j, which sets an upper bound, ωj∗, on the wave frequency below which that ion species contributes positively to the growth rate. It follows that the relative positions of the cutoffs and the critical frequencies ωj∗ play a crucial role in determining whether a particular wave branch will be unstable. The effect of the magnetospheric ion abundances on the growth rate of each branch of the EMIC instability in a model where all the ion species have kappa velocity distributions is investigated by appealing to the above ideas. Using the variation of the cutoff frequencies predicted by cold plasma theory as a guide, optimal ion abundances that maximise the EMIC instability growth rate are sought. When the ring current is comprised predominantly of H+ ions, all branches of the EMIC wave are destabilised, with the proton branch having the maximum growth rate. When the O+ ion abundance in the ring current is increased, a decrease in the growth rate of the proton branch and cyclotron damping of the helium branch are observed. The oxygen branch, on the other hand, experiences an increase in the maximum growth rate with an increase in the O+ ion abundance. When the ring current is comprised predominantly of He+ ions, only the helium and oxygen branches of the EMIC wave are destabilised, with the helium branch having the maximum growth rate.

Statistical analysis of EMIC waves in plasmaspheric plumes from Cluster observations

AbstractRecently, 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.

Impact of cold O+ ions on the generation and evolution of EMIC waves

We explore the effect of cool O+ ions (~11 eV) on the generation and nonlinear evolution of electromagnetic ion cyclotron (EMIC) waves in the magnetosphere, using hybrid (kinetic ions, fluid electrons) simulations with a dipolar magnetic field. The instability is driven by the temperature anisotropy of the hot (keV), ring current protons whose density is a few percent of the background plasma consisting of cold (eV) protons and O+ ions with concentrations varying between 0 and 30% of the cold protons. The results show that for O+ ion concentration of 7% or less, the properties of the EMIC waves are similar to those in the absence of O+ ions where waves are generated at low latitudes and propagate all the way to the ionospheric boundary. Further increases in O+ density (~15%) result in waves being confined in latitude due to processes explored in this paper. At O+ densities of 30% and higher, the growth of EMIC waves is weak or non‐existent due to cyclotron resonant damping by the O+ ions. Comparing the results of the runs with no and 15% O+ ions shows that the propagation properties of the EMIC waves change dramatically in the presence of O+ ions. Specifically, they show that waves are propagating parallel and anti‐parallel to the magnetic field in both hemispheres due to reflection at points that move to higher latitudes with time. This in turn results in the nonlinear generation of field aligned electrostatic waves with large perturbations in the density of the cold protons and O+ ions and also local heating of these ions. Examination of the wave properties also shows that EMIC waves are only present in regions of space where the density of the hot protons is larger than 1% of the background level. In other words, the propagation properties of the EMIC waves are controlled by the density of the hot protons. This finding is further confirmed by performing a test hybrid simulation in which hot protons reaching latitude of 22° are removed from the run resulting in the confinement of the waves to this and lower latitudes.

Energetic particle injection, acceleration, and loss during the geomagnetic disturbances which upset Galaxy 15

On 5 April 2010 a series of energetic electron injections, acceleration, and loss events appeared to induce an operational anomaly in the Galaxy 15 geosynchronous communications satellite. We describe the energetic electron precipitation conditions leading to the anomaly. A few hours prior to the anomaly electron acceleration at >0.6 MeV, and loss at >30 keV, were observed simultaneously. The acceleration took place in the region of the Galaxy 15 satellite on the nightside and the precipitation of electrons primarily on the dayside. The precipitation was confined to L‐shells outside of the plasmapause and appeared to be driven by chorus waves via a weak diffusion process. An hour prior to the anomaly, a solar wind shock event generated a few minutes of 30–150 keV electron precipitation but only on the dayside, over a largeL‐shell range (4.8 <L < 13). The timing of the precipitation burst was consistent with electromagnetic ion cyclotron (EMIC) waves seen on the dayside, but the high geomagnetic latitude of the precipitation suggests that EMIC wave growth associated with high cold density regions in the plasmasphere is unlikely to have played a role. A substorm injection event shortly after the shock appears to have ultimately triggered the upset on Galaxy 15. However, the peak >30 keV electron precipitation fluxes of 1.35 × 107 el cm−2 s−1 sr−1 were roughly the same level as other large substorm events previously analyzed, indicating either a sensitivity to the energetic electron environment prior to the event or that the satellite was in a vulnerable situation.

A comparison of magnetic field measurements and a plasma‐based proxy to infer EMIC wave distributions at geosynchronous orbit

Wave‐particle interactions are fundamental to the dynamics of the outer radiation belt. Electromagnetic ion cyclotron (EMIC) waves can resonate with energetic electrons, causing pitch angle diffusion and scattering of the electrons into Earth's atmosphere. These waves act locally; thus, accurately measuring their spatial and temporal distributions is critical to understanding their contribution to radiation belt electron losses. Using Los Alamos National Laboratory Magnetospheric Plasma Analyzer data from geosynchronous orbit, we examine a plasma‐based proxy for enhanced EMIC wave growth during a set of 52 relativistic electron flux dropout events. This proxy is compared to in situ wave measurements from the GOES satellites, also at geosynchronous orbit, for single‐wave events as well as a superposed epoch statistical analysis. The proxy is extended to calculate an amplitude for the inferred waves, to enable a more quantitative comparison to the in situ GOES EMIC measurements. Signatures of EMIC waves are present in both the proxy and the direct wave observations at similar local times as well as epoch times. The waves are most prevalent in the afternoon sector, with enhanced occurrences beginning half a day before the onset of the dropouts and peaking in the day following. We see agreement in occurrence between the proxy and waves both statistically and in individual instances. This study demonstrates the powerful applications of plasma data to infer wave distributions in space.

The role of Shabansky orbits in compression‐related electromagnetic ion cyclotron wave growth

Electromagnetic ion cyclotron waves at high L values near local noon are often found to be related to magnetospheric compression events. These waves arise from temperature anisotropies in trapped warm plasma populations. There are several possible mechanisms that can generate these temperature anisotropies, including both energizing and nonenergizing processes. In this work we investigate a nonenergizing process arising from dayside bifurcated magnetic field minima. There are two kinds of behavior particles undergo in the presence of bifurcated minima: particles with high initial equatorial pitch angles (EPAs) are forced to execute so‐called Shabansky orbits and mirror at high latitudes without passing through the equator, while those with lower initial EPAs will pass through the equator with higher EPAs than before; as a result, perpendicular energies near the equator increase at the cost of parallel energies. By using a 3‐D particle tracing code in a tunable analytic compressed‐dipole field, we explore the effects of Shabansky orbits on the anisotropy of the warm plasma and contrast with the anisotropy resulting from drift shell splitting. We show that Shabansky orbits are an independent source of temperature anisotropy for warm dayside ions.

EMIC waves observed at geosynchronous orbit during solar minimum: Statistics and excitation

[1] We identified 1875 wave events in magnetic field data from geosynchronous orbit. Most of these events were transverse with respect to the background magnetic field, left-hand polarized, and were observed in the post-noon magnetic local time sector at frequencies just below the helium gyrofrequency. Combined, these observations strongly suggest that most of these events are Electromagnetic Ion Cyclotron (EMIC) waves. Average wave amplitudes are presented, binned by frequency, geomagnetic activity and magnetic local time. The amplitude increases with increasing geomagnetic activity; increased activity also narrows the local time sector in which the waves are observed. A superposed epoch analysis of solar wind parameters and geomagnetic activity indices shows that 12 hours before wave onset the AE and Kp index increased, indicating storm and substorm activity that injects hot ion populations needed to drive the EMIC instability and providing ample time for those populations to drift into the post-noon local time sector. Just before wave onset a sudden enhancement in the AE index and the solar wind dynamic pressure are observed, indicating that a final perturbation of the magnetosphere is needed to excite EMIC wave growth. EMIC waves are thought to cause loss of relativistic particles in the radiation belt. Large solar wind densities have been associated with low flux of relativistic particles during the recovery phase of geomagnetic storms. We show that EMIC waves are preferentially generated during intervals of large solar wind density, indicating that such conditions drive EMIC waves which in turn cause enhanced loss of relativistic particles.

Physical mechanisms of compressional EMIC wave growth

On 29 June 2007, electromagnetic ion cyclotron (EMIC) waves were observed on the ground by the Canadian Array for Realtime Investigations of Magnetic Activity (CARISMA) network of magnetometers between L = 4 and L = 6 in response to a significant magnetospheric compression. Here a new MHD/particle method for studying EMIC wave growth in the magnetosphere is used to provide a detailed study of the compression event. We compare equatorial field line crossings of NASA's Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and CARISMA observation sites to frequency‐integrated wave growth rates from the MHD/particle method. Simulated temperatures were constant in time, suggesting an absence of energizing processes during this event. Many particles experienced so‐called Shabansky orbits during this event, in which their drift motion was confined to high latitudes in the dayside magnetosphere. We propose a new nonenergizing process, driven by ions undergoing Shabansky orbits, for generating ion temperature anisotropies. In addition, the fundamental role of the plasmasphere in generating EMIC waves from the free energy of warm ion temperature anisotropies is discussed.