The Van Allen radiation belts were first discovered in 1958 by the Explorer series of spacecraft1. The dynamic outer belt consists primarily of relativistic electrons trapped by the Earth's magnetic ...field. Magnetospheric processes driven by the solar wind2 cause the electron flux in this belt to fluctuate substantially over timescales ranging from minutes to years3. The most dramatic of these events are known as flux 'dropouts' and often occur during geomagnetic storms. During such an event the electron flux can drop by several orders of magnitude in just a few hours4, 5 and remain low even after a storm has abated. Various solar wind phenomena, including coronal mass ejections and co-rotating interaction regions6, can drive storm activity, but several outstanding questions remain concerning dropouts and the precise channels to which outer belt electrons are lost during these events. By analysing data collected at multiple altitudes by the THEMIS, GOES, and NOAA-POES spacecraft, we show that the sudden electron depletion observed during a recent storm's main phase is primarily a result of outward transport rather than loss to the atmosphere. PUBLICATION ABSTRACT
Abstract
Thermalization and heating of plasma flows at shocks result in unstable charged-particle distributions that generate a wide range of electromagnetic waves. These waves, in turn, can further ...accelerate and scatter energetic particles. Thus, the properties of the waves and their implication for wave−particle interactions are critically important for modeling energetic particle dynamics in shock environments. Whistler-mode waves, excited by the electron heat flux or a temperature anisotropy, arise naturally near shocks and foreshock transients. As a result, they can often interact with suprathermal electrons. The low background magnetic field typical at the core of such transients and the large wave amplitudes may cause such interactions to enter the nonlinear regime. In this study, we present a statistical characterization of whistler-mode waves at foreshock transients around Earth’s bow shock, as they are observed under a wide range of upstream conditions. We find that a significant portion of them are sufficiently intense and coherent (narrowband) to warrant nonlinear treatment. Copious observations of background magnetic field gradients and intense whistler wave amplitudes suggest that phase trapping, a very effective mechanism for electron acceleration in inhomogeneous plasmas, may be the cause. We discuss the implications of our findings for electron acceleration in planetary and astrophysical shock environments.
Winter air pollution in Ulaanbaatar, Mongolia is among the worst in the world. The health impacts of policy decisions affecting air pollution exposures in Ulaanbaatar were modeled and evaluated under ...business as usual and two more-strict alternative emissions pathways through 2024. Previous studies have relied on either outdoor or indoor concentrations to assesses the health risks of air pollution, but the burden is really a function of total exposure. This study combined projections of indoor and outdoor concentrations of PM2.5 with population time-activity estimates to develop trajectories of total age-specific PM2.5 exposure for the Ulaanbaatar population. Indoor PM2.5 contributions from secondhand tobacco smoke (SHS) were estimated in order to fill out total exposures, and changes in population and background disease were modeled. The health impacts were derived using integrated exposure-response curves from the Global Burden of Disease Study.
Annual average population-weighted PM2.5 exposures at baseline (2014) were estimated at 59 μg/m3. These were dominated by exposures occurring indoors, influenced considerably by infiltrated outdoor pollution. Under current control policies, exposures increased slightly to 60 μg/m3 by 2024; under moderate emissions reductions and under a switch to clean technologies, exposures were reduced from baseline levels by 45% and 80%, respectively. The moderate improvement pathway decreased per capita annual disability-adjusted life year (DALY) and death burdens by approximately 40%. A switch to clean fuels decreased per capita annual DALY and death burdens by about 85% by 2024 with the relative SHS contribution increasing substantially.
This study demonstrates a way to combine estimated changes in total exposure, background disease and population levels, and exposure-response functions to project the health impacts of alternative policy pathways. The resulting burden analysis highlights the need for aggressive action, including the elimination of residential coal burning and the reduction of current smoking rates.
Radiation in space was the first discovery of the space age. Earth's radiation belts consist of energetic particles that are trapped by the geomagnetic field and encircle the planet1. The electron ...radiation belts usually form a two-zone structure with a stable inner zone and a highly variable outer zone, which forms and disappears owing to wave-particle interactions on the timescale of a day, and is strongly influenced by the very-low-frequency plasma waves. Recent observations revealed a third radiation zone at ultrarelativistic energies2, with the additional medium narrow belt (long-lived ring) persisting for approximately 4 weeks. This new ring resulted from a combination of electron losses to the interplanetary medium and scattering by electromagnetic ion cyclotron waves to the Earth's atmosphere. Here we show that ultrarelativistic electrons can stay trapped in the outer zone and remain unaffected by the very-low-frequency plasma waves for a very long time owing to a lack of scattering into the atmosphere. The absence of scattering is explained as a result of ultrarelativistic particles being too energetic to resonantly interact with waves at low latitudes. This study shows that a different set of physical processes determines the evolution of ultrarelativistic electrons. PUBLICATION ABSTRACT
Foreshock bubbles (FBs), transient ion foreshock phenomena formed from highly concentrated suprathermal foreshock ions upstream of solar wind discontinuities, produce intense disturbances in the ...magnetosphere‐ionosphere system and can accelerate particles to even higher energies. Rotational discontinuities are known to drive FBs. Tangential discontinuities (TDs), however, have not previously been considered as drivers of FBs because they have no normal magnetic field component, preventing access of field‐aligned particles upstream. However, given that suprathermal foreshock ions have gyroradii larger than the width of TDs, they may pass upstream of TDs and generate FBs. Using multipoint observations from Advanced Composition Explorer, WIND, and Time History of Events and Macroscale Interactions during Substorms (THEMIS), we report on two cases of TD‐driven FBs. The FBs were identified using classical FB selection criteria, and the driving TDs were identified using ideal MHD criteria, to within the limits of observational error. Our results add another potential solar wind driver of FBs and imply that FBs may be even more common than previously thought.
Key Points
A hypothesis about TD‐driven FB formation is presented
TD‐driven FBs are observed and identified
We present observations of the radiation belts from the Helium Oxygen Proton Electron and Magnetic Electron Ion Spectrometer particle detectors on the Van Allen Probes satellites that illustrate the ...energy dependence and L shell dependence of radiation belt enhancements and decays. We survey events in 2013 and analyze an event on 1 March in more detail. The observations show the following: (a) at all L shells, lower energy electrons are enhanced more often than higher energies; (b) events that fill the slot region are more common at lower energies; (c) enhancements of electrons in the inner zone are more common at lower energies; and (d) even when events do not fully fill the slot region, enhancements at lower energies tend to extend to lower L shells than higher energies. During enhancement events the outer zone extends to lower L shells at lower energies while being confined to higher L shells at higher energies. The inner zone shows the opposite with an outer boundary at higher L shells for lower energies. Both boundaries are nearly straight in log(energy) versus L shell space. At energies below a few 100 keV, radiation belt electron penetration through the slot region into the inner zone is commonplace, but the number and frequency of “slot filling” events decreases with increasing energy. The inner zone is enhanced only at energies that penetrate through the slot. Energy‐ and L shell‐dependent losses (that are consistent with whistler hiss interactions) return the belts to more quiescent conditions.
Key Points
Radiation belt dynamics are a strong function of energy and L shell
Events that fill the slot region are common at lower energies and rare at higher energies
During enhancement events different energies are enhanced in different spatial regions
We present a unique 2 h ground‐based observation of concurrent magnetospheric hiss, chorus, VLF triggered emissions as well as ELF/VLF signals generated locally by the High Frequency Active Auroral ...Research Program (HAARP) facility. Eccentricity of observed wave polarization is used as a criteria to identify magnetospheric emissions and estimate their ionospheric exit points. The observations of hiss and chorus in the unique background of coherent HAARP ELF/VLF waves and triggered emissions allow for more accurate characterization of hiss and chorus properties than in typical ground‐based observations. Eccentricity and azimuth results suggest a moving ionospheric exit point associated with a single ducted path at L ~ 5. The emissions exhibit dynamics in time suggesting an evolution of a magnetospheric source from hiss generation to chorus generation or a moving plasmapause location. We introduce a frequency band‐limited autocorrelation method to quantify the relative coherency of the emissions. A range of coherency was observed from high order of coherency in local HAARP transmissions and their echoes to lower coherency in natural chorus and hiss emissions.
Key Points
Unique simultaneous observation of hiss, chorus, and HAARP‐induced triggered emissions
Eccentricity and autocorrelation are valuable tools for analysis of hiss and chorus emissions
Observations suggest evolution of magnetosphere whistler mode source
The Magnetospheric Multiscale (MMS) mission has probed Earth's magnetosphere, magnetosheath, and near‐Earth solar wind for over 8 years. We utilize an unsupervised learning algorithm, Gaussian ...mixture model clustering, along with feature generation and simple post‐cleaning methods to automatically classify 8 years of MMS dayside observations into four plasma regions (magnetosphere, magnetosheath, solar wind, and ion foreshock) at 1‐min resolution. With these plasma regions distinguished, we have also identified boundary surfaces (e.g., magnetopause, bow shock). We validate our results on manually generated and rule based region labels described in the literature. We report overlap rates in our cluster determined magnetopauses and bow shocks against Scientist‐in‐the Loop (SITL) identified transitions and published databases. Our features are general and our model is extensible, potentially making it applicable to observational data from multiple other missions.
Key Points
We use an extensible unsupervised Gaussian mixture model (GMM) algorithm to automatically classify 8 years of dayside magnetospheric multiscale (MMS) plasma data
Our model distinguishes between solar wind, ion foreshock, magnetosheath, and magnetosphere with 97.8% accuracy compared to manual labels
We provide classified labels and transitions at 1‐min resolution and stable region specific lists for all 8 years of dayside MMS data
When backstreaming foreshock ions pass through approaching solar wind discontinuities, they may get concentrated upstream of the discontinuities and form foreshock bubbles (FBs). Because FBs result ...in very intense global pressure variations upstream of and inside Earth's magnetosphere, they are important for solar wind‐magnetosphere coupling. Information about these recently discovered phenomena is limited, however. To elucidate FB spatial structure, evolution, expansion, and formation conditions, we use multipoint Time History of Events and Macroscale Interactions during Substorms observations in which three or more spacecraft observed the same events. From single‐case studies of two foreshock bubbles and one hot flow anomaly (HFA), we demonstrate how we determine FB spatial structure, evolution, and expansion and propose a model to explain these properties. We discuss the different conditions leading to formation of FBs and HFAs. Multiple case studies of six FBs, five HFAs, and one spontaneous HFA show that FBs typically expand faster than HFAs and their expansion speed is likely determined by the solar wind speed.
Key Points
Structure, evolution, and expansion of FBs are examined
Formation condition differences between FBs and HFAs are discussed
FBs expand faster than HFAs, and their expansion speed is proportional to the solar wind speed
Earth's foreshock is a region within the solar wind upstream of Earth's bow shock filled with backstreaming solar wind particles reflected at the shock. Within this region, when the interplanetary ...field is approximately radial, foreshock bubbles (FBs) can be formed when the backstreaming particles interact with approaching discontinuities embedded in the solar wind. Foreshock bubbles can grow to 5–10 RE in scale, well upstream of the bow shock. Having a high concentration of thermalized upstream ions and slow, or even sunward, speeds within them, these transient phenomena deflect the solar wind by forming a new shock ahead of them. Although FBs eventually succumb to solar wind dynamic pressure and crash onto Earth's bow shock and magnetopause, they may last long enough to allow solar wind reflection at their own shocks, which forms a new FB foreshock region upstream of them. The FB shock may be of different obliquity than the parent bow shock providing new and diverse opportunities for particle acceleration. Using a case study from Time History of Events and Macroscale Interactions during Substorms, we demonstrate that ions and electrons are reflected at the FB shock, where they acquire energies consistent with shock acceleration theory. These are the first definitive observations of a new ion and electron foreshock region upstream of the FB shock with implications for shock acceleration in general.
Key Points
First definitive observation of a new ion and electron foreshock region upstream of an FB shock
Ions were accelerated through adiabatic reflection (shock drift acceleration) at the FB shock
FB shock could be an additional accelerator and a source of preaccelerated particles at the parent shock
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