On the cause and extent of outer radiation belt losses during the 30 September 2012 dropout event

Turner, D. L.; Angelopoulos, V.; Morley, S.; Henderson, M. G.; Reeves, G. D.; Li, W.; Baker, D. N.; Huang, C. L.; Boyd, A. J.; Spence, Harlan E.; Claudepierre, S. G.; Blake, J. B.; Rodriguez, J. V.

doi:10.1002/2013ja019446

Cited by 114 publications

(154 citation statements)

References 52 publications

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“…In fact the increasing loss rates as particle energy increases is suggestive of EMIC waves, but conversely, as particle energy decreases the observed rates decrease too slowly (if current ideas about the minimum resonant energy are correct). Similar conclusions, based on different events, were recently reached by , Turner et al (2014), and Hudson et al (2014).…”

Section: J Albert: Diffusion Simulations 5 Conclusionsupporting

confidence: 86%

Radial diffusion simulations of the 20 September 2007 radiation belt dropout

Albert

2014

Ann. Geophys.

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Abstract. This is a study of a dropout of radiation belt electrons, associated with an isolated solar wind density pulse on 20 September 2007, as seen by the solid-state telescopes (SST) detectors on THEMIS (Time History of Events and Macroscale Interactions during Substorms). Omnidirectional fluxes were converted to phase space density at constant invariants M = 700 MeV G −1 and K = 0.014 R E G 1/2 , with the assumption of local pitch angle α ≈ 80 • and using the T04 magnetic field model. The last closed drift shell, which was calculated throughout the time interval, never came within the simulation outer boundary of L * = 6. It is found, using several different models for diffusion rates, that radial diffusion alone only allows the data-driven, time-dependent boundary values at L max = 6 and L min = 3.7 to propagate a few tenths of an R E during the simulation; far too slow to account for the dropout observed over the broad range of L * = 4-5.5. Pitch angle diffusion via resonant interactions with several types of waves (chorus, electromagnetic ion cyclotron waves, and plasmaspheric and plume hiss) also seems problematic, for several reasons which are discussed.

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Section: J Albert: Diffusion Simulations 5 Conclusionsupporting

confidence: 86%

Radial diffusion simulations of the 20 September 2007 radiation belt dropout

Albert

2014

Ann. Geophys.

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“…The major similarities include that (1) EMIC wave scattering is highly energy selective, only occurring for >~1 MeV electrons, depending on the L shell and wave band; (2) while EMIC waves can cause efficient scattering loss of >~1 MeV relativistic electrons near the loss cone, the electron population at large pitch angles cannot undergo resonant interactions with EMIC waves, and therefore the latter seems unlikely to fully explain the storm time outer zone flux dropouts [e.g., Reeves et al, 2003;Turner et al, 2014aTurner et al, , 2014b. Complementary to the above points, our investigation comprehensively assesses the effectiveness of EMIC waves in affecting the outer radiation belt electron dynamics by computing EMIC wave-driven relativistic electron scattering rates and loss cone filling index and evaluating the resultant electron pitch angle evolution and loss time scales in the broad spatial coverage of L = 3-7, corresponding to different EMIC wave bands and different wave normal angle models.…”

Section: 1002/2015ja021466mentioning

confidence: 99%

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

et al. 2015

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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 < ~40° for electrons > ~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.

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“…Pressure-driven currents determine the distribution of magnetic field across the inner magnetosphere. Time evolution of the field determines the motion of radiation belt particles and can be responsible for rapid electron losses during storm main phase [e.g., Ukhorskiy et al, 2006;Bortnik et al, 2006;Kim et al, 2008;Turner et al, 2014;Ukhorskiy et al, 2015]. Hot plasma also provides the energy source for multiple instabilities generating wave modes that produce local acceleration and loss of radiation belt electrons (see reviews for Friedel et al [2002], Hudson et al [2008], and Thorne [2010]), drive their radial transport [e.g., Lanzerotti et al, 1969;Ukhorskiy et al, 2009], and cause precipitation of energetic particles into the ionosphere (see review for Millan and Thorne [2007]).…”

Section: Introductionmentioning

confidence: 99%

Ion acceleration at dipolarization fronts in the inner magnetosphere

et al. 2017

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During geomagnetic storms plasma pressure in the inner magnetosphere is controlled by energetic ions of tens to hundreds of keV. Plasma pressure is the source of global storm time currents, which control the distribution of magnetic field and couple the inner magnetosphere and the ionosphere. Recent analysis showed that the buildup of hot ion population in the inner magnetosphere largely occurs in the form of localized discrete injections associated with sharp dipolarizations of magnetic field, similar to dipolarization fronts in the magnetotail. Because of significant differences between the ambient magnetic field and the dipolarization front properties in the magnetotail and the inner magnetosphere, the physical mechanisms of ion acceleration at dipolarization fronts in these two regions may also be different. In this paper we discuss a new acceleration mechanism enabled by stable trapping of ions at the azimuthally localized dipolarization fronts. It is shown that trapping can provide a robust mechanism of ion energization in the inner magnetosphere even in the absence of large electric fields.

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On the cause and extent of outer radiation belt losses during the 30 September 2012 dropout event

Cited by 114 publications

References 52 publications

Radial diffusion simulations of the 20 September 2007 radiation belt dropout

Radial diffusion simulations of the 20 September 2007 radiation belt dropout

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

Ion acceleration at dipolarization fronts in the inner magnetosphere

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