Xing Cao

Resonant scattering of outer zone relativistic electrons by multiband EMIC waves and resultant electron loss time scales

To improve our understanding of the role of electromagnetic ion cyclotron (EMIC) waves in radiation belt electron dynamics, we perform a comprehensive analysis of EMIC wave-induced resonant scattering of outer zone relativistic (>0.5 MeV) electrons and resultant electron loss time scales with respect to EMIC wave band, L shell, and wave normal angle model. The results demonstrate that while H+-band EMIC waves dominate the scattering losses of ~1–4 MeV outer zone relativistic electrons, it is He+-band and O+-band waves that prevail over the pitch angle diffusion of ultrarelativistic electrons at higher energies. Given the wave amplitude, EMIC waves at higher L shells tend to resonantly interact with a larger population of outer zone relativistic electrons and drive their pitch angle scattering more efficiently. Obliquity of EMIC waves can reduce the efficiency of wave-induced relativistic electron pitch angle scattering. Compared to the frequently adopted parallel or quasi-parallel model, use of the latitudinally varying wave normal angle model produces the largest decrease in H+-band EMIC wave scattering rates at pitch angles  ~5 MeV. At a representative nominal amplitude of 1 nT, EMIC wave scattering produces the equilibrium state (i.e., the lowest normal mode under which electrons at the same energy but different pitch angles decay exponentially on the same time scale) of outer belt relativistic electrons within several to tens of minutes and the following exponential decay extending to higher pitch angles on time scales from <1 min to ~1 h. The electron loss cone can be either empty as a result of the weak diffusion or heavily/fully filled due to approaching the strong diffusion limit, while the trapped electron population at high pitch angles close to 90° remains intact because of no resonant scattering. In this manner, EMIC wave scattering has the potential to deepen the anisotropic distribution of outer zone relativistic electrons by reshaping their pitch angle profiles to “top-hat.” Overall, H+-band and He+-band EMIC waves are most efficient in producing the pitch angle scattering loss of relativistic electrons at ~1–2 MeV. In contrast, the presence of O+-band EMIC waves, while at a smaller occurrence rate, can dominate the scattering loss of 5–10 MeV electrons in the entire region of the outer zone, which should be considered in future modeling of the outer zone relativistic electron dynamics.

Resonant scattering of central plasma sheet protons by multiband EMIC waves and resultant proton loss timescales

This is a companion study to Liang et al. (2014) which reported a “reversed” energy-latitude dispersion pattern of ion precipitation in that the lower energy ion precipitation extends to lower latitudes than the higher-energy ion precipitation. Electromagnetic ion cyclotron (EMIC) waves in the central plasma sheet (CPS) have been suggested to account for this reversed-type ion precipitation. To further investigate the association, we perform a comprehensive study of pitch angle diffusion rates induced by EMIC wave and the resultant proton loss timescales at L = 8–12 around the midnight. Comparing the proton scattering rates in the Earths dipole field and a more realistic quiet time geomagnetic field constructed from the Tsyganenko 2001 (T01) model, we find that use of a realistic, nondipolar magnetic field model not only decreases the minimum resonant energies of CPS protons but also considerably decreases the limit of strong diffusion and changes the proton pitch angle diffusion rates. Adoption of the T01 model increases EMIC wave diffusion rates at > ~ 60° equatorial pitch angles but decreases them at small equatorial pitch angles. Pitch angle scattering coefficients of 1–10 keV protons due to H+ band EMIC waves can exceed the strong diffusion rate for both geomagnetic field models. While He+ and O+ band EMIC waves can only scatter tens of keV protons efficiently to cause a fully filled loss cone at L > 10, in the T01 magnetic field they can also cause efficient scattering of ~ keV protons in the strong diffusion limit at L > 10. The resultant proton loss timescales by EMIC waves with a nominal amplitude of 0.2 nT vary from a few hours to several days, depending on the wave band and L shell. Overall, the results demonstrate that H+ band EMIC waves, once present, can act as a major contributor to the scattering loss of a few keV protons at lower L shells in the CPS, accounting for the reversed energy-latitude dispersion pattern of proton precipitation at low energies (~ keV) on the nightside. The pitch angle coverage for H+ band EMIC wave resonant scattering strongly depends on proton energy, L shell, and field model. He+ and O+ band EMIC waves tend to cause efficient scattering loss of protons at higher energies, thereby importantly contributing to the isotropic distribution of higher energy (> ~ 10 keV) protons at higher L shells on the nightside where the geomagnetic field line is highly stretched. Our results also suggest that scattering by H+ band EMIC waves may significantly contribute to the formation of the reversed-type CPS proton precipitation on the dawnside where both the wave activity and occurrence probability is statistically high.

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.

Scattering of Ultra-relativistic Electrons in the Van Allen Radiation Belts Accounting for Hot Plasma Effects

Electron flux in the Earth’s outer radiation belt is highly variable due to a delicate balance between competing acceleration and loss processes. It has been long recognized that Electromagnetic Ion Cyclotron (EMIC) waves may play a crucial role in the loss of radiation belt electrons. Previous theoretical studies proposed that EMIC waves may account for the loss of the relativistic electron population. However, recent observations showed that while EMIC waves are responsible for the significant loss of ultra-relativistic electrons, the relativistic electron population is almost unaffected. In this study, we provide a theoretical explanation for this discrepancy between previous theoretical studies and recent observations. We demonstrate that EMIC waves mainly contribute to the loss of ultra-relativistic electrons. This study significantly improves the current understanding of the electron dynamics in the Earth’s radiation belt and also can help us understand the radiation environments of the exoplanets and outer planets.

Bounce resonance scattering of radiation belt electrons by low-frequency hiss: Comparison with cyclotron and Landau resonances

Bounce-resonant interactions with magnetospheric waves have been proposed as important contributing mechanisms for scattering near-equatorially mirroring electrons by violating the second adiabatic invariant associated with the electron bounce motion along a geomagnetic field line. This study demonstrates that low-frequency plasmaspheric hiss with significant wave power below 100 Hz can bounce-resonate efficiently with radiation belt electrons. By performing quantitative calculations of pitch-angle scattering rates, we show that low-frequency hiss induced bounce-resonant scattering of electrons has a strong dependence on equatorial pitch-angle αeq. For electrons with αeq close to 90°, the timescale associated with bounce resonance scattering can be comparable to or even less than 1 hour. Cyclotron- and Landau-resonant interactions between low-frequency hiss and electrons are also investigated for comparisons. It is found that while the bounce and Landau resonances are responsible for the diffusive transport of near-equatorially mirroring electrons to lower αeq, pitch-angle scattering by cyclotron resonance could take over to further diffuse electrons into the atmosphere. Bounce resonance provides a more efficient pitch-angle scattering mechanism of relativistic (≥ 1 MeV) electrons than Landau resonance due to the stronger scattering rates and broader resonance coverage of αeq, thereby demonstrating that bounce resonance scattering by low-frequency hiss can contribute importantly to the evolution of the electron pitch-angle distribution and the loss of radiation belt electrons.

Bounce resonance scattering of ring current electrons by H+ band EMIC waves

We present a detailed investigation of bounce-resonant pitch angle scattering of ring current electrons caused by electromagnetic ion cyclotron (EMIC) waves. It is found that H+ band EMIC waves can resonate with near-equatorially mirroring electrons over a wide range of L shells (i.e., 3 ≤ L ≤ 6) and energies and lead to the efficient transport of ring current electrons (i.e., ∼10 keV to 100 keV) from near 90° pitch angles to lower pitch angles. Computations of the bounce-resonant pitch angle scattering rates show a strong dependence on the L shell, electron energy, and resonance harmonics. When the L-shell increases, the orders of bounce resonance contributing to the whole scattering coefficient decrease, and meanwhile, it becomes difficult for the bounce resonance of higher orders to occur. Furthermore, when the electron energy increases, the bounce resonance orders decrease. Our results demonstrate that bounce-resonant scattering by H+ band EMIC waves can be an important loss mechanism for ∼10–100 keV electrons because of the absence of cyclotron resonance for ring current electrons interacting with EMIC waves. We conclude that bounce resonant scattering by H+ band EMIC waves should be incorporated into future modeling efforts of the ring current electron dynamics.We present a detailed investigation of bounce-resonant pitch angle scattering of ring current electrons caused by electromagnetic ion cyclotron (EMIC) waves. It is found that H+ band EMIC waves can resonate with near-equatorially mirroring electrons over a wide range of L shells (i.e., 3 ≤ L ≤ 6) and energies and lead to the efficient transport of ring current electrons (i.e., ∼10 keV to 100 keV) from near 90° pitch angles to lower pitch angles. Computations of the bounce-resonant pitch angle scattering rates show a strong dependence on the L shell, electron energy, and resonance harmonics. When the L-shell increases, the orders of bounce resonance contributing to the whole scattering coefficient decrease, and meanwhile, it becomes difficult for the bounce resonance of higher orders to occur. Furthermore, when the electron energy increases, the bounce resonance orders decrease. Our results demonstrate that bounce-resonant scattering by H+ band EMIC waves can be an important loss mechanism for ∼10–100 k...

A Statistical Survey of Radiation Belt Dropouts Observed by Van Allen Probes

Relativistic electron flux in the Earth’s radiation belt are observed to drop by orders of magnitude on timescales of a few hours. Where do the electrons go? This is one of the most important outstanding questions in radiation belt studies. Here we perform statistical analysis on the radiation belt dropouts based on four years of electron phase space density data from Van Allen Probes. Our results show that the dropouts at larger ∗ regions have higher occurrence, and cover a wider range in and (the first the second adiabatic invariants) compared to those at low ∗ regions. By comparing the statistical distribution of the dropout occurrence and ratio with the minimum resonant energy curve by EMIC waves, we find that EMIC wave scattering is the dominant loss mechanism at low ∗ regions, while the dropouts at high ∗ are due to a combination of EMIC wave scattering and outward radial diffusion associated with magnetopause shadowing, with outward radial diffusion leading to larger loss than EMIC wave scattering. The radiation belt dropouts at high ∗ regions also have strong , dependence. The electron variation at low and low is dominated by fast injection or convection followed by fast decay, leading to very low occurrence of electron dropouts. However, at high , low and other high regimes, electrons suffer from abrupt dropouts and show high occurrence of dropouts at the high regimes due to a combination of the two loss mechanisms.

Bounce resonance scattering of radiation belt electrons by H+band EMIC waves: Bounce Resonance by EMIC Waves

We perform a detailed analysis of bounce-resonant pitch angle scattering of radiation belt electrons due to electromagnetic ion cyclotron (EMIC) waves. It is found that EMIC waves can resonate with near-equatorially mirroring electrons over a wide range of L shells and energies. H band EMIC waves efficiently scatter radiation belt electrons of energy>100 keV from near 90° pitch angles to lower pitch angles where the cyclotron resonance mechanism can take over to further diffuse electrons into the loss cone. Bounce-resonant electron pitch angle scattering rates show a strong dependence on L shell, wave normal angle distribution, and wave spectral properties. We find distinct quantitative differences between EMIC wave-induced bounce-resonant and cyclotron-resonant diffusion coefficients. Cyclotron-resonant electron scattering by EMIC waves has been well studied and found to be a potentially crucial electron scattering mechanism. The new investigation here demonstrates that bounce-resonant electron scattering may also be very important. We conclude that bounce resonance scattering by EMIC waves should be incorporated into future modeling efforts of radiation belt electron dynamics.