Early observations indicated that the Earth's Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. ...Subsequent studies showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep 'slot' region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location. Recent observations have revealed unexpected radiation belt morphology, especially at ultrarelativistic kinetic energies (more than five megaelectronvolts). Here we analyse an extended data set that reveals an exceedingly sharp inner boundary for the ultrarelativistic electrons. Additional, concurrently measured data reveal that this barrier to inward electron radial transport does not arise because of a physical boundary within the Earth's intrinsic magnetic field, and that inward radial diffusion is unlikely to be inhibited by scattering by electromagnetic transmitter wave fields. Rather, we suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave-particle pitch angle scattering deep inside the Earth's plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate.
Two wave packets of second harmonic poloidal Pc 4 waves with a wave frequency of ~7 mHz were detected by Van Allen Probe A at a radial distance of ~5.8 RE and magnetic local time of 13 hr near the ...magnetic equator, where plasmaspheric refilling was in progress. Proton butterfly distributions with energy dispersions were also measured at the same time; the proton fluxes at 10–30 keV oscillated with the same frequency as the Pc 4 waves. Using the ion sounding technique, we find that the Pc 4 waves propagated eastward with an azimuthal wave number (m number) of ~220 and ~260 for each wave packet, respectively. Such eastward propagating high‐m (m > 100) waves were seldom reported in previous studies. The condition of drift‐bounce resonance is well satisfied for the estimated m numbers in both events. Proton phase space density was also examined to understand the wave excitation mechanism. We obtained temporal variations of the energy and radial gradient of the proton phase space density and find that temporal intensification of the radial gradient can generate the two wave packets. The cold electron density around the spacecraft apogee was >100 cm−3 in the present events, and hence the eigenfrequency of the Pc 4 waves became lower. This causes the increase of the m number which satisfies the resonance condition of drift‐bounce resonance for 10–30 keV protons and meets the condition for destabilization due to gyrokinetic effect.
Key Points
The first direct observation of drift‐bounce resonance that excites eastward propagating second harmonic poloidal waves with high m number
These waves are excited by intensification of radial gradient of proton phase space density due to substorm injection
Cold electrons also contribute to the wave excitation by increasing the m number to satisfy gyro‐kinetic destabilization condition
At solar minimum, the solar wind is observed at high solar latitudes as a predominantly fast (> 500 km/s), highly Alfvenic, rarefied stream of plasma originating deep within coronal holes, while near ...the ecliptic plane it is interspersed with a more variable slow (< 500 kms) wind. The precise origins of the slow wind streams are less certain, with theories and observations supporting sources from the tips of helmet streamers, interchange reconnection near coronal hole boundaries, and origins within coronal holes with highly diverging magnetic fields. The heating mechanism required to drive the solar wind is also an open question and candidate mechanisms include Alfven wave turbulence, heating by reconnection in nanoflares, ion cyclotron wave heating and acceleration by thermal gradients1. At 1 au, the wind is mixed and evolved and much of the diagnostic structure of these sources and processes has been lost. Here we present new measurements from Parker Solar Probe at 36 to 54 solar radii that show clear evidence of slow, Alfvenic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals associated with jets of plasma and enhanced Poynting flux and interspersed in a smoother and less turbulent flow with near-radial magnetic field. Furthermore, plasma wave measurements suggest electron and ion velocity-space micro-instabilities that have been identified with plasma heating and thermalization processes. Our measurements suggest an impulsive mechanism associated with solar wind energization and a heating role for micro-instabilities and provide strong evidence for low latitude coronal holes as a significant contribution to the source of the slow solar wind.
We present Van Allen Probe observations of electromagnetic ion cyclotron (EMIC) waves triggered solely due to individual substorm-injected ions in the absence of storms or compressions of the ...magnetosphere during 9 August 2015. The time at which the injected ions are observed directly corresponds to the onset of EMIC waves at the location of Van Allen Probe A (L = 5.5 and 18:06 magnetic local time). The injection was also seen at geosynchronous orbit by the Geostationary Operational Environmental Satellite and Los Alamos National Laboratory spacecraft, and the westward(eastward) drift of ions(electrons) was monitored by Los Alamos National Laboratory spacecraft at different local times. The azimuthal location of the injection was determined by tracing the injection signatures backward intime to their origin assuming a dipolar magnetic field of Earth. The center of this injection location wasdetermined to be close to 20:00 magnetic local time. Geostationary Operational Environmental Satelliteand ground magnetometer responses confirm substorm onset at approximately the same local time.The observed EMIC wave onsets at Van Allen Probe were also associated with a magnetic field decrease.The arrival of anisotropic ions along with the decrease in the magnetic field favors the growth of the EMICwave instability based on linear theory analysis.
Using measurements from the Van Allen Probes, we show that field‐aligned fluxes of electrons energized by dispersive Alfvén waves (DAWs) are prominent in the inner magnetosphere during active ...conditions. These electrons have preferentially field‐aligned anisotropies from 1.2 to >2 at energies ranging from tens of electron volts to several kiloelectron volts (keV), with largest values being coincident with magnetic field dipolarizations. Comparisons reveal that DAW energy densities and Poynting fluxes are strongly correlated with precipitating electron energies and energy fluxes and also O+ ion outflow energies. These observations yield empirical inner magnetosphere relations between the DAW and electron inputs and the O+ ion outflow response, providing important constraints for models. They also suggest that DAWs play an important role in enhancing field‐aligned electron input into the ionosphere that facilitates the outflow and subsequent energization of O+ ions in the wave fields into the inner magnetosphere.
Plain Language Summary
We use satellite observations in the inner magnetosphere to study field‐aligned electrons at kiloelectron volt energies and below. These electrons are invariably coincident with intense low‐frequency electromagnetic waves called dispersive Alfvén waves (DAWs), which are prevalent during substorms and geomagnetic storms. Given that DAWs have parallel electric fields known to accelerate electrons, their simultaneous occurrences indicate that these waves are accelerating the electrons in the equatorial inner magnetosphere. Indicative of energization and heating in DAWs, the energies, number fluxes and energy fluxes of earthward‐moving electrons are observed to increase with increasing DAW Poynting fluxes and energy densities. Given the ability of electron precipitation and DAWs to drive and produce energized ion outflow, we also tested for and demonstrated that oxygen ion outflow energies are strongly correlated with the electron and wave energy fluxes. Least squares fits yielded empirical relationships between the DAW, electron, and oxygen ion outflow characteristics, providing important constraints for models of plasma transport in the magnetospheric‐ionospheric system. Owing to their persistent occurrence during active conditions and field‐aligned sense, these wave energized electrons are expected to affect the growth and spatial distribution of other waves known to impact inner magnetosphere source and loss processes.
Key Points
Inner magnetosphere field‐aligned electron fluxes and dispersive Alfvén waves are strongly enhanced during storms and substorms
Field‐aligned electron energy and energy fluxes are strongly correlated with dispersive Alfvén wave energy densities and Poynting fluxes
Oxygen ion outflows show strong correlations with field‐aligned electron precipitation and dispersive Alfvén waves
Spherical double probe electric field sensors become electrically coupled to magnetospheric plasma during operation, leading to an instrument response that varies with the local plasma environment. ...Here, a method is developed for determining this variable coupling impedance for each measurement direction by using periods of favorable boom, wave, and magnetic field geometry. Comparing electric field complex amplitudes between 30 Hz and 10 kHz observed along each boom direction to those predicted from simultaneous magnetic field measurements and cold plasma theory allows for the amplitude and phase response of the instrument to be quantified over the full range of plasma densities encountered on‐orbit. A sheath model is developed to describe how the sheath resistance, sheath capacitance, and relative effective length vary as a function of plasma density. An additional empirical correction is also included to describe the phase response along the spin‐axis. The modeled sheath correction is subsequently tested for case studies of burst‐mode data and statistical analyses of survey‐mode data. It is demonstrated that the levels of agreement between observations and theoretical predictions based on Faraday's Law are substantially greater for the sheath corrected data than for uncorrected observations. Comparisons between observations with oppositely directed Poynting vector directions reveals that the sheath correction reconciles a bifurcated distribution in the uncorrected data to a single peak centered on agreement with Faraday's Law. A full sheath corrected EMFISIS L4 survey mode data set has been produced for final archive. Full details of the sheath correction are also provided for manual data correction.
Key Points
Variable coupling impedance between the instrument and plasma has been quantified for the electric field measurements on Van Allen Probes
Sheath correction is demonstrated to substantially improve agreement between data and theoretical predictions from Faraday's Law
Sheath‐corrected EMFISIS L4 data set produced for archive. Full details are also provided to facilitate manual data corrections
The relationship between dispersive Alfvén waves (DAWs), magnetospheric activity, and O+ ion outflow/energy density is examined using measurements from the Van Allen Probes. We show that correlated ...DAW activity and O+ outflow/energization is a characteristic feature of the inner magnetosphere during active conditions and during storms persists for several hours over large L‐shell and azimuthal ranges of the plasma sheet. Though enhanced during substorm and storm active periods, these correlated features are most intense during geomagnetic storms. Comparisons show a linear relationship between DAW electric (and magnetic) field energy density and outflowing O+ energy. Statistical measurements from a large number of storms also reveal a linear relationship between DAW energy density and gross enhancements in energetic O+ energy densities. These observations support the notion that DAWs play an important role in the energization of O+ ions into and within the inner magnetosphere.
Plain Language Summary
Geomagnetic storms are major disturbances in the Earth's magnetosphere, during which the particle content and pressure in the magnetosphere increase considerably. Much of the pressure increase is due to singly charged oxygen ions that come from the ionosphere. How this happens is not clear. Analyzing satellite observations, we found evidence suggesting that a particular type of low‐frequency electromagnetic wave called a dispersive Alfvén wave may be playing a key role. These waves are found to be more prevalent and intense in the magnetosphere during storms. Oxygen ion energies are shown to increase with increasing intensities of these waves. The oxygen ion contribution to pressure also increases in association with intensified wave activity. These observations support the notion that the waves energize and heat oxygen ions into and within the magnetosphere over extended periods of time, which leads to significant magnetospheric pressure increases.
Key Points
Dispersive Alfven wave energy density is enhanced during substorms and geomagnetic storms
Oxygen ion outflow energy and energy density are correlated with dispersive Alfven wave energy density
Observations support modeling that dispersive Alfven waves enhance oxygen ion energy density in the inner magnetosphere
We deduce the electron plasma density from the NASA Van Allen Probes Electric Field and Waves and Electric and Magnetic Field Instrument Suite and Integrated Science measurements and extract the ...plasmasphere boundaries throughout 2012–2019. We use the gradient method for locating the plasmapause at Lpp and the 100 cm−3 density threshold for the plasmasphere outer edge at L100. We show how, where, and when both Lpp and L100 coincide when the plasmapause gradient exists. L100 is demonstrated to bound the plasmasphere at large L‐shell in the dusk. The plasmasphere expands farther out than predicted from the Carpenter and Anderson (1992, https://doi.org/10.1029/91JA01548) model. We generate statistics of the plasmasphere boundaries binned by L‐shell, magnetic local time (MLT), and geomagnetic indices, leading to new models for radiation belt codes. The L100 boundary commonly varies by ∼±0.5 L, increasing with activity up to ∼±1 L, becomes MLT‐dependent for Kp > ∼2, and is preferentially steep on the night side for non‐quiet times and a wider region in the afternoon sector.
Plain Language Summary
The plasmasphere is a region of plasma extending out from the ionized upper part of the atmosphere to distances of 2–6 Earth Radii. The plasmasphere plasma is the coldest plasma (1/100–1/1,000,000 of the energy of other plasma) in the space around Earth where the particle motions are regulated by Earth's magnetic field (the magnetosphere). It is also high density, 100–10,000 times higher than elsewhere in the magnetosphere. The outer edge of the plasmasphere, called the plasmapause, typically drops from >100 to <10 cm−3 over a relatively short distance. Waves that energize radiation belt particles (chorus) are found outside the plasmasphere. Inside the plasmasphere are different waves (hiss) that cause radiation belt particles to precipitate into Earth's atmosphere. Therefore, models predicting the radiation belt's behavior need to know the plasmapause location. To predict the plasmapause position, we analyze 7 years of Van Allen Probes data to find the plasma density in two different ways, using both the 100 cm−3 density and the density gradient. We look at how their locations change with the level of geomagnetic storm activity and deduce new plasmasphere boundaries models for space weather codes.
Key Points
We deduce the electron plasma density from Electric Field and Waves and Electric and Magnetic Field Instrument Suite and Integrated Science measurements (2012–2019) and extract the plasmasphere boundaries
New plasmasphere boundary statistics and laws, binned by L, magnetic local time, and geomagnetic indices are generated to be used in space weather codes
A density‐based boundary is more frequently defined than is a gradient‐based boundary, and yields a more frequently applicable model
The dual‐spacecraft Van Allen Probes mission has provided a new window into mega electron volt (MeV) particle dynamics in the Earth's radiation belts. Observations (up to E ~10 MeV) show clearly the ...behavior of the outer electron radiation belt at different timescales: months‐long periods of gradual inward radial diffusive transport and weak loss being punctuated by dramatic flux changes driven by strong solar wind transient events. We present analysis of multi‐MeV electron flux and phase space density (PSD) changes during March 2013 in the context of the first year of Van Allen Probes operation. This March period demonstrates the classic signatures both of inward radial diffusive energization and abrupt localized acceleration deep within the outer Van Allen zone (L ~4.0 ± 0.5). This reveals graphically that both “competing” mechanisms of multi‐MeV electron energization are at play in the radiation belts, often acting almost concurrently or at least in rapid succession.
Key Points
Clear observations to higher energy than ever before
Precise detection of where and how acceleration takes place
Provides “new eyes” on megaelectron Volt
The source of O+ in the storm time ring current Kistler, L. M.; Mouikis, C. G.; Spence, H. E. ...
Journal of geophysical research. Space physics,
June 2016, 2016-06-00, 20160601, Letnik:
121, Številka:
6
Journal Article
Recenzirano
Odprti dostop
A stretched and compressed geomagnetic field occurred during the main phase of a geomagnetic storm on 1 June 2013. During the storm the Van Allen Probes spacecraft made measurements of the plasma ...sheet boundary layer and observed large fluxes of O+ ions streaming up the field line from the nightside auroral region. Prior to the storm main phase there was an increase in the hot (>1 keV) and more isotropic O+ ions in the plasma sheet. In the spacecraft inbound pass through the ring current region during the storm main phase, the H+ and O+ ions were significantly enhanced. We show that this enhanced inner magnetosphere ring current population is due to the inward adiabatic convection of the plasma sheet ion population. The energy range of the O+ ion plasma sheet that impacts the ring current most is found to be from ~5 to 60 keV. This is in the energy range of the hot population that increased prior to the start of the storm main phase, and the ion fluxes in this energy range only increase slightly during the extended outflow time interval. Thus, the auroral outflow does not have a significant impact on the ring current during the main phase. The auroral outflow is transported to the inner magnetosphere but does not reach high enough energies to affect the energy density. We conclude that the more energetic O+ that entered the plasma sheet prior to the main phase and that dominates the ring current is likely from the cusp.
Key Points
Auroral outflow during the storm main phase mainly impacts <1 keV plasma sheet population
The >1 keV (hot) more isotropic plasma sheet O+ population increases prior to the main phase
Inward transport of the hot O+ dominates the ring current; this O+ is likely from the cusp
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