Diffuse aurora at the Earth's high latitude regions is mainly caused by the low-energy (0.1-30 keV) electron precipitation which carries the major energy flux into the nightside upper atmosphere. ...Previous studies have demonstrated that combined scattering by the upper- and lower- band chorus waves acts as the dominant cause of diffuse auroral precipitation, but that is not necessarily the case as these two types of waves do not always occur simultaneously, with the lower-band more often. Here we report that the lower-band chorus satisfying the preferred condition can generate their second harmonics so as to trigger the diffuse auroral electron precipitation. We find that the lower-band chorus alone can only cause the precipitation of electrons greater than 4 keV, while the self-consistently generated second harmonic is weak but still able to result in the electron precipitation below 4 keV. The combined effect of those modes results in the observed pancake electron distributions and the diffuse aurora. Our results clearly demonstrate an alternative but universal mechanism of chorus-driven diffuse aurora in the Earth, which may also apply to the auroral formation in other planetary magnetospheres.
The Van Allen radiation belts contain ultrarelativistic electrons trapped in Earth's magnetic field. Since their discovery in 1958, a fundamental unanswered question has been how electrons can be ...accelerated to such high energies. Two classes of processes have been proposed: transport and acceleration of electrons from a source population located outside the radiation belts (radial acceleration) or acceleration of lower-energy electrons to relativistic energies in situ in the heart of the radiation belts (local acceleration). We report measurements from NASA's Van Allen Radiation Belt Storm Probes that clearly distinguish between the two types of acceleration. The observed radial profiles of phase space density are characteristic of local acceleration in the heart of the radiation belts and are inconsistent with a predominantly radial acceleration process.
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Excitation mechanisms of highly oblique, quasi‐electrostatic lower band chorus waves are investigated using Van Allen Probes observations near the equator of the Earth's magnetosphere. Linear growth ...rates are evaluated based on in situ, measured electron velocity distributions and plasma conditions and compared with simultaneously observed wave frequency spectra and wave normal angles. Accordingly, two distinct excitation mechanisms of highly oblique lower band chorus have been clearly identified for the first time. The first mechanism relies on cyclotron resonance with electrons possessing both a realistic temperature anisotropy at keV energies and a plateau at 100–500 eV in the parallel velocity distribution. The second mechanism corresponds to Landau resonance with a 100–500 eV beam. In both cases, a small low‐energy beam‐like component is necessary for suppressing an otherwise dominating Landau damping. Our new findings suggest that small variations in the electron distribution could have important impacts on energetic electron dynamics.
Key Points
Observed temperature anisotropy alone was too weak to excite highly oblique waves damped by dense 100‐500 eV Maxwellian electrons
A 100‐300 eV plateau in the parallel electron PSD can reduce Landau damping to allow highly oblique wave excitation via cyclotron resonance
Small 100‐500 eV electron beams observed sporadically can generate highly oblique waves directly through Landau resonance
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We present NASA Van Allen Probes observations of wave‐particle interactions between magnetospheric ultra‐low frequency (ULF) waves and energetic electrons (20–500 keV) on 31 October 2012. The ULF ...waves are identified as the fundamental poloidal mode oscillation and are excited following an interplanetary shock impact on the magnetosphere. Large amplitude modulations in energetic electron flux are observed at the same period (≈ 3 min) as the ULF waves and are consistent with a drift‐resonant interaction. The azimuthal mode number of the interacting wave is estimated from the electron measurements to be ~40, based on an assumed symmetric drift resonance. The drift‐resonant interaction is observed to be localized and occur over 5–6 wave cycles, demonstrating peak electron flux modulations at energies ~60 keV. Our observation clearly shows electron drift resonance with the fundamental poloidal mode, the energy dependence of the amplitude and phase of the electron flux modulations providing strong evidence for such an interaction. Significantly, the observation highlights the importance of localized wave‐particle interactions for understanding energetic particle dynamics in the inner magnetosphere, through the intermediary of ULF waves.
Key Points
First conclusive evidence of electron drift‐resonance with poloidal ULF waves.
First to show the energy dependence to the amplitude/phase expected from theory.
Observation shows the drift‐resonant interaction occurs over a localized region.
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The Interstellar Boundary Explorer (IBEX) has obtained all-sky images of energetic neutral atoms emitted from the heliosheath, located between the solar wind termination shock and the local ...interstellar medium (LISM). These flux maps reveal distinct nonthermal (0.2 to 6 kilo-electron volts) heliosheath proton populations with spectral signatures ordered predominantly by ecliptic latitude. The maps show a globally distributed population of termination-shock-heated protons and a superimposed ribbonlike feature that forms a circular arc in the sky centered on ecliptic coordinate (longitude λ, latitude β) = (221°, 39°), probably near the direction of the LISM magnetic field. Over the IBEX energy range, the ribbon's nonthermal ion pressure multiplied by its radial thickness is in the range of 70 to 100 picodynes per square centimeter AU (AU, astronomical unit), which is significantly larger than the 30 to 60 picodynes per square centimeter AU of the globally distributed population.
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Dipolarizing flux bundles (DFBs) are small flux tubes (typically <3 RE in XGSM and YGSM) in the nightside magnetosphere that have magnetic field more dipolar than the background. Although DFBs are ...known to accelerate particles, creating energetic particle injections outside geosynchronous orbit (trans‐GEO), the nature of the acceleration mechanism and the importance of DFBs in generating injections inside geosynchronous orbit (cis‐GEO) are unclear. Our statistical study of cis‐GEO DFBs using data from the Van Allen Probes reveals that just like trans‐GEO DFBs, cis‐GEO DFBs occur most often in the premidnight sector, but their occurrence rate is ~1/3 that of trans‐GEO DFBs. Half the cis‐GEO DFBs are accompanied by an energetic particle injection and have an electric field 3 times stronger than that of the injectionless half. All DFB injections are dispersionless within the temporal resolution considered (11 s). Our findings suggest that these injections are ushered or produced locally by the DFB, and the DFB's strong electric field is an important aspect of the injection generation mechanism.
Key Points
Dipolarizing flux bundles (DFBs) have significant occurrence rates inside the geosynchronous orbit
The average electric field of DFBs with injections is 3 times that of those without
All energetic particle injections accompanying DFBs are dispersionless
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Van Allen radiation belts consist of relativistic electrons trapped by Earth's magnetic field. Trapped electrons often drift azimuthally around Earth and display a butterfly pitch angle distribution ...of a minimum at 90° further out than geostationary orbit. This is usually attributed to drift shell splitting resulting from day-night asymmetry in Earth's magnetic field. However, direct observation of a butterfly distribution well inside of geostationary orbit and the origin of this phenomenon have not been provided so far. Here we report high-resolution observation that a unusual butterfly pitch angle distribution of relativistic electrons occurred within 5 Earth radii during the 28 June 2013 geomagnetic storm. Simulation results show that combined acceleration by chorus and magnetosonic waves can successfully explain the electron flux evolution both in the energy and butterfly pitch angle distribution. The current provides a great support for the mechanism of wave-driven butterfly distribution of relativistic electrons.
Simulations of energetic neutral atom (ENA) maps predict flux magnitudes that are, in some cases, similar to those observed by the Interstellar Boundary Explorer (IBEX) spacecraft, but they miss the ...ribbon. Our model of the heliosphere indicates that the local interstellar medium (LISM) magnetic field (BLISM) is transverse to the line of sight (LOS) along the ribbon, suggesting that the ribbon may carry its imprint. The force-per-unit area on the heliopause from field line draping and the LISM ram pressure is comparable with the ribbon pressure if the LOS approximately 30 to 60 astronomical units and BLISM approximately 2.5 microgauss. Although various models have advantages in accounting for some of the observations, no model can explain all the dominant features, which probably requires a substantial change in our understanding of the processes that shape our heliosphere.
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The dominant feature in Interstellar Boundary Explorer (IBEX) sky maps of heliospheric energetic neutral atom (ENA) flux is a ribbon of enhanced flux that extends over a broad range of ecliptic ...latitudes and longitudes. It is narrow (approximately 20° average width) but long (extending over 300° in the sky) and is observed at energies from 0.2 to 6 kilo-electron volts. We demonstrate that the flux in the ribbon is a factor of 2 to 3 times higher than that of the more diffuse, globally distributed heliospheric ENA flux. The ribbon is most pronounced at approximately 1 kilo-electron volt. The average width of the ribbon is nearly constant, independent of energy. The ribbon is likely the result of an enhancement in the combined solar wind and pickup ion populations in the heliosheath.
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Radiation belt electron flux dropouts during the main phase of geomagnetic storms have received increasing attention in recent years. Here we focus on a rarely reported nonstorm time dropout event ...observed by Van Allen Probes on 24 September 2013. Within several hours, the radiation belt electron fluxes exhibited a significant (up to 2 orders of magnitude) depletion over a wide range of radial distances (L > 4.5), energies (∼500 keV to several MeV) and equatorial pitch angles (0°≤αe≤180°). STEERB simulations show that the relativistic electron loss in the region L = 4.5–6.0 was primarily caused by the pitch angle scattering of observed plasmaspheric hiss and electromagnetic ion cyclotron waves. Our results emphasize the complexity of radiation belt dynamics and the importance of wave‐driven precipitation loss even during nonstorm times.
Key Points
Nonstorm time dropout of radiation belt electron fluxes
Quasi‐linear simulation of electron radiation belt evolution
Importance of wave‐driven precipitation loss for radiation belt dynamics
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