The sensitivity of quasi‐linear scattering rates to the wave normal distribution of chorus waves is studied using the Full Diffusion Code newly developed at the University of California, Los Angeles. ...Scattering rates are computed for field‐aligned, oblique (∼20° wave normal angles), and highly oblique (∼40° wave normal angles) cases. For radiation belt electrons, scattering rates are relatively insensitive to the assumed distribution of wave normal angles at high energies; while when the energy is smaller, in the range of tens of keV, knowledge of the wave normal distribution becomes important. It is shown that Landau resonance becomes very important for the scattering of electrons with energies of tens of keV as waves become more oblique. Scattering rates for various order resonances and energies are presented. Our results show that, for a fixed ratio of plasma to gyrofrequency and fixed spectral properties of waves, scattering rates scale as an inverse of magnitude of the magnetic field. We also show that resonant scattering of 10 keV and 100 keV occurs within 10° and 20° latitude of the geomagnetic equator, respectively. At 1 MeV, dominant scattering occurs above 20° latitude. We also present local scattering rates as a function of energy and latitude. Implications of the presented results for the upcoming satellite mission's planning, future measurements, and radiation belt modeling are discussed.
We perform a comprehensive analysis to evaluate hiss‐induced scattering effect on the pitch angle evolution and associated decay processes of relativistic electrons. The results show that scattering ...by the equatorial, highly oblique hiss component is negligible. Quasi‐parallel approximation is good for evaluation of hiss‐driven electron scattering rates ≤ 2 MeV. However, realistic wave propagation angles as a function of latitude must be considered to accurately quantify hiss scattering rates above 2 MeV, and ambient plasma density is also a critical parameter. While the first‐order cyclotron and the Landau resonances are dominant for hiss scattering < 2 MeV electrons, higher‐order resonances become important and even dominant at intermediate pitch angles for ultrarelativistic (≥ 3 MeV) electrons. Hiss‐induced electron pitch angle evolution shows an initially rapid transport from high to lower pitch angles, with a gradual approach toward equilibrium, and a final exponential decay as a whole. Although hiss scattering rates near the loss cone control the pitch angle evolution and the ultimate loss of ultrarelativistic electrons, the scattering bottleneck significantly affects the loss rate and leads to characteristic top hat‐shaped pitch angle distributions at energies < 1 MeV. Decay timescales are on the order of a few days, tens of days, and > 100 days for 500 keV, 2 MeV, and 5 MeV electrons, respectively, consistent with recent observations from the Van Allen Probes and indicating that scattering by hiss can realistically account for the long‐term loss process and the pitch angle evolution of relativistic electrons in the plasmasphere following storm time injections.
Key Points
Latitudinal wave normal model and plasma density is critical to hiss scattering
Electrons transport to low pitch angles, reach equilibrium, and decay wholly
Decay timescales vary from a few days to > 100 days for 0.5–5 MeV electrons
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Individuals' unverified information sharing on social media, namely, sharing information without verification, is a major cause of the widespread misinformation amid the COVID-19 pandemic. The ...association between perceived information overload and unverified information sharing has been well documented in the cognitive overload approach. However, little is known about the underlying mechanism of this process. This study aims to explore the mediating role of anxiety and the moderating role of perceived herd between perceived information overload and unverified information sharing on WeChat. Anxiety demonstrates people's emotional response to the pandemic, whereas perceived herd describes a willingness to share certain information if it has been shared by many. The results of an online survey in China (
= 525) showed that perceived information overload was positively associated with unverified information sharing. In addition, this relationship was partially mediated by anxiety. Moreover, perceived herd positively moderated the link between anxiety and unverified information sharing, such that the indirect effect of perceived information overload on unverified information sharing
anxiety was significant in conditions where the level of perceived herd was high, whereas the indirect effect was not significant in conditions where the level of perceived herd was low. The moderated mediation model extends the cognitive overload approach and indicates that unverified information sharing is not only an individual strategy to cope with information overload but also a herding behavior to manage anxiety. Practical implications for curbing people's tendencies toward unverified information sharing on social media are discussed.
Outer zone radiation belt electrons can undergo gyroresonant interaction with various magnetospheric wave modes including whistler‐mode chorus outside the plasmasphere and both whistler‐mode hiss and ...electromagnetic ion cyclotron (EMIC) waves inside the plasmasphere. To evaluate timescales for electron momentum diffusion and pitch angle diffusion, we utilize bounce‐averaged quasi‐linear diffusion coefficients for field‐aligned waves with a Gaussian frequency spectrum in a dipole magnetic field. Timescales for momentum diffusion of MeV electrons due to VLF chorus can be less than a day in the outer radiation belt. Equatorial chorus waves (∣λW∣ < 15 deg) can effectively accelerate MeV electrons. Efficiency of the chorus acceleration mechanism is increased if high‐latitude waves (∣λW∣ > 15 deg) are also present. Our calculations confirm that chorus diffusion is a viable mechanism for generating relativistic (MeV) electrons in the outer zone during the recovery phase of a storm or during periods of prolonged substorm activity when chorus amplitudes are enhanced. Radiation belt electrons are subject to precipitation loss to the atmosphere due to resonant pitch angle scattering by plasma waves. The electron precipitation loss timescale due to scattering by each of the wave modes, chorus, hiss, and EMIC waves, can be 1 day or less. These wave modes can separately, or in combination, contribute significantly to the depletion of relativistic (MeV) electrons from the outer zone over the course of a magnetic storm. Efficient pitch angle scattering by whistler‐mode chorus or hiss typically requires high latitude waves (∣λW∣ > 30 deg). Timescales for electron acceleration and loss generally depend on the spectral properties of the waves, as well as the background electron number density and magnetic field. Loss timescales due to EMIC wave scattering also depend on the ion (H+, He+, O+) composition of the plasma. Complete models of radiation belt electron transport, acceleration and loss should include, in addition to radial (cross‐L) diffusion, resonant diffusion due to gyroresonance with VLF chorus, plasmaspheric hiss, and EMIC waves. Comprehensive observational data on the spectral properties of these waves are required as a function of spatial location (L, MLT, MLAT) and magnetic activity.
A statistical analysis on the radiation belt dropouts is performed based on 4 years of electron phase space density data from the Van Allen Probes. The μ, K, and L* dependence of dropouts and their ...driving mechanisms and geomagnetic and solar wind conditions are investigated using electron phase space density data sets for the first time. Our results suggest that electronmagnetic ion cyclotron (EMIC) wave scattering is the dominant dropout mechanism at low L* region, which requires the most active geomagnetic and solar wind conditions. In contrast, dropouts at high L* have a higher occurrence and are due to a combination of EMIC wave scattering and outward radial diffusion associated with magnetopause shadowing. In addition, outward radial diffusion at high L* is found to cause larger dropouts than EMIC wave scattering and is accompanied with active geomagnetic and solar wind drivers.
Plain Language Summary
Radiation belt dropout is one of the most dramatic variations in Earth's magnetosphere, which means the relativistic electron fluxes can decrease a few orders in just several hours. Two mechanisms have been proposed to explain the quick depletion of radiation belt electrons: loss through magnetopause and the precipitate into the atmosphere. However, their relative contribution is still not clear now. In this paper, we use the 4‐year measurement of Van Allen Probes to investigate the statistical features and underlying physical mechanisms of radiation belt dropouts. Through using electron phase space density rather than electron flux, we reveal the real loss of electrons in the radiation belt. The results show that electrons at low L* are mainly precipitated to atmosphere while high L* electrons both loss to magnetopause and atmosphere.
Key Points
EMIC wave scattering is the dominant dropout mechanism at low L*, accompanied with the most active geomagnetic and solar wind conditions
Dropouts at high L* have a higher occurrence and are due to a combination of EMIC wave scattering and outward radial diffusion
At high L*, outward radial diffusion causes larger dropouts than EMIC wave scattering, and it favors more active conditions
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
To achieve a better understanding of the dominant loss mechanisms for the rapid dropouts of radiation belt electrons, three distinct radiation belt dropout events observed by Van Allen Probes are ...comprehensively investigated. For each event, observations of the pitch angle distribution of electron fluxes and electromagnetic ion cyclotron (EMIC) waves are analyzed to determine the effects of atmospheric precipitation loss due to pitch angle scattering induced by EMIC waves. Last closed drift shells (LCDS) and magnetopause standoff position are obtained to evaluate the effects of magnetopause shadowing loss. Evolution of electron phase space density (PSD) versus L* profiles and the μ and K (first and second adiabatic invariants) dependence of the electron PSD drops are calculated to further analyze the dominant loss mechanisms at different L*. Our findings suggest that these radiation belt dropouts can be classified into distinct classes in terms of dominant loss mechanisms: magnetopause shadowing dominant, EMIC wave scattering dominant, and combination of both mechanisms. Different from previous understanding, our results show that magnetopause shadowing can deplete electrons at L* < 4, while EMIC waves can efficiently scatter electrons at L* > 4. Compared to the magnetopause standoff position, it is more reliable to use LCDS to evaluate the impact of magnetopause shadowing. The evolution of electron PSD versus L* profile and the μ, K dependence of electron PSD drops can provide critical and credible clues regarding the mechanisms responsible for electron losses at different L* over the outer radiation belt.
Key Points
Radiation belt dropouts can be classified into three distinct classes in terms of dominant loss mechanisms
Magnetopause shadowing can deplete electrons at L* < 4, while EMIC waves can efficiently scatter electrons at L* > 4
The μ, K dependence of electron PSD drops can provide critical and credible clues regarding the electron loss mechanisms at different L*
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Earth's diffuse aurora occurs over a broad latitude range and is primarily caused by the precipitation of low-energy (0.1-30-keV) electrons originating in the central plasma sheet, which is the ...source region for hot electrons in the nightside outer magnetosphere. Although generally not visible, the diffuse auroral precipitation provides the main source of energy for the high-latitude nightside upper atmosphere, leading to enhanced ionization and chemical changes. Previous theoretical studies have indicated that two distinct classes of magnetospheric plasma wave, electrostatic electron cyclotron harmonic waves and whistler-mode chorus waves, could be responsible for the electron scattering that leads to diffuse auroral precipitation, but it has hitherto not been possible to determine which is the more important. Here we report an analysis of satellite wave data and Fokker-Planck diffusion calculations which reveals that scattering by chorus is the dominant cause of the most intense diffuse auroral precipitation. This resolves a long-standing controversy. Furthermore, scattering by chorus can remove most electrons as they drift around Earth's magnetosphere, leading to the development of observed pancake distributions, and can account for the global morphology of the diffuse aurora.
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DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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.
Key Points
Obliquity of EMIC waves can reduce the scattering efficiency
EMIC wave scattering can cause the top‐hot electron distribution
Different EMIC bands are important for RB electrons at different energies
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
To investigate the hot plasma effects on the cyclotron‐resonant interactions between electromagnetic ion cyclotron (EMIC) waves and radiation belt electrons in a realistic magnetospheric environment, ...calculations of the wave‐induced bounce‐averaged pitch angle diffusion coefficients are performed using both the cold and hot plasma dispersion relations. The results demonstrate that the hot plasma effects have a pronounced influence on the electron pitch angle scattering rates due to all three EMIC emission bands (H+, He+, and O+) when the hot plasma dispersion relation deviates significantly from the cold plasma approximation. For a given wave spectrum, the modification of the dispersion relation by hot anisotropic protons can strongly increase the minimum resonant energy for electrons interacting with O+ band EMIC waves, while the minimum resonant energies for H+ and He+ bands are not greatly affected. For H+ band EMIC waves, inclusion of hot protons tends to weaken the pitch angle scattering efficiency of >5 MeV electrons. The most crucial differences introduced by the hot plasma effects occur for >3 MeV electron scattering rates by He+ band EMIC waves. Mainly due to the changes of resonant frequency and wave group velocity when the hot protons are included, the difference in scattering rates can be up to an order of magnitude, showing a strong dependence on both electron energy and equatorial pitch angle. Our study confirms the importance of including hot plasma effects in modeling the scattering of ultra‐relativistic radiation belt electrons by EMIC waves.
Key Points
Both cold and hot dispersion relations are used in the calculations of EMIC wave‐induced bounce‐averaged pitch angle diffusion coefficients
Hot plasma effects can significantly influence the scattering loss of radiation belt electrons induced by all the three EMIC wave bands
The differences of diffusion coefficients are caused by the changes of resonant frequency and wave group velocity
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In this paper, we report the proof of the existence of density ducts in the Earth's magnetosphere by studying in situ observations of whistler‐mode chorus waves using NASA's Van Allen Probe‐A data. ...Chorus waves, originally excited inside the density ducts with wave normal angles (WNAs) smaller than the Gendrin angle at the near‐equatorial region, are efficiently confined to a limited area inside density ducts (i.e., ducted regions), and remain with small WNAs as they propagate toward higher latitudes. The ducted region becomes narrower for the higher‐frequency waves. Chorus waves with WNAs larger than the Gendrin angle are not guided by density ducts. Our study reveals that density ducts can effectively control the property and distribution of chorus waves, and may ultimately regulate electron dynamics in the Earth's or other planetary radiation belts.
Key Points
Chorus waves with WNAs smaller than Gendrin angle are confined to a limited area inside ducts, and keep small WNAs up to high latitudes
Chorus waves with WNAs larger than the Gendrin angle are not guided by density ducts
The ducted region becomes narrower for the higher‐frequency chorus waves
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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