Occurrence Rates of Electromagnetic Ion Cyclotron (EMIC) Waves With Rising Tones in the Van Allen Probes Data Set

In Fourier time-frequency power spectrograms of satellite magnetic field data, electromagnetic ion cyclotron (EMIC) waves may feature discrete, rising tone structures that rapidly increase in frequency. Using data from the Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) fluxgate magnetometer, we conducted a statistical study of EMIC waves from September 2012 through June 2016. We compared the occurrence rates and spatial distributions for all EMIC waves with those for rising tone EMIC waves as a function of magnetic local time (MLT) and L shell, as well as a function of R XY and Z in solar-magnetic (SM) coordinates. Overall, EMIC waves occurred during 2.4% of the time period considered, but rising tone EMIC waves were only found during 0.2% of the time period considered. About 7%-8% of the minutes of orbital coverage with H+ or He+ band EMIC waves had rising tones. The regions of peak occurrence rates for H+ and He+ band waves, as well as waves with rising tones, were found in the noon and dusk sectors for 4 < L < 6. The preferred regions for H+ waves as a function of R XY and Z SM suggest an association with magnetospheric compressions near noon and interactions between plumes and the ring current near dusk. Peak occurrence rates for O+ band waves were found between 2 < L < 4 at all MLT, and over a wide range of L shells near dusk. No rising tones were found in the O+ band.

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.

Determining EMIC Wave Vector Properties Through Multi-Point Measurements: The Wave Curl Analysis

Electromagnetic ion cyclotron (EMIC) waves play important roles in particle loss processes in the magnetosphere. Determining the evolution of EMIC waves as they propagate and how this evolution affects wave-particle interactions requires accurate knowledge of the wave vector, k. We present a technique using the curl of the wave magnetic field to determine k observationally, enabled by the unique configuration and instrumentation of the Magnetospheric MultiScale (MMS) spacecraft. The wave curl analysis is demonstrated for synthetic arbitrary electromagnetic waves with varying properties typical of observed EMIC waves. The method is also applied to an EMIC wave interval observed by MMS on October 28, 2015. The derived wave properties and k from the wave curl analysis for the observed EMIC wave are compared with the Waves in Homogenous, Anisotropic, Multi-component Plasma (WHAMP) wave dispersion solution and with results from other single- and multi-spacecraft techniques. We find good agreement between k from the wave curl analysis, k determined from other observational techniques, and k determined from WHAMP. Additionally, the variation of k due to the time and frequency intervals used in the wave curl analysis is explored. This exploration demonstrates that the method is robust when applied to a wave containing at least 3-4 wave periods and over a rather wide frequency range encompassing the peak wave emission. These results provide confidence that we are able to directly determine the wave vector properties using this multi-spacecraft method implementation, enabling systematic studies of EMIC wave k properties with MMS.

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.

The plasma environment inside geostationary orbit: A Van Allen Probes HOPE survey

The two full precessions in local time completed by the Van Allen Probes enable global specification of the near-equatorial inner magnetosphere plasma environment. Observations by the Helium-Oxygen-Proton-Electron (HOPE) mass spectrometers provide detailed insight into the global spatial distribution of electrons, H+, He+, and O+. Near-equatorial omnidirectional fluxes and abundance ratios at energies 0.1-30 keV are presented for 2 ≤ L ≤ 6 as a function of L shell, magnetic local time (MLT), and geomagnetic activity. We present a new tool built on the UBK modeling technique for classifying plasma sheet particle access to the inner magnetosphere. This new tool generates access maps for particles of constant energy for more direct comparison with in situ measurements, rather than the traditional constant μ presentation typically associated with UBK. We present for the first time inner magnetosphere abundances of O+ flux relative to H+ flux as a function of Kp, L, MLT, and energy. At L = 6, the O+/H+ ratio increases with increasing Kp, consistent with previous results. However, at L < 5 the O+/H+ ratio generally decreases with increasing Kp. We identify a new "afternoon bulge" plasma population enriched in 10 keV O+ and superenriched in 10 keV He+ that is present during quiet/moderate geomagnetic activity (Kp < 5) at ~1100-2000 MLT and L shell 2-4. Drift path modeling results are consistent with the narrow energy and approximate MLT location of this enhancement, but the underlying physics describing its formation, structure, and depletion during higher geomagnetic activity are currently not understood.