We study the effect of electromagnetic ion cyclotron (EMIC) waves on the loss and pitch angle scattering of relativistic and ultrarelativistic electrons during the recovery phase of a moderate ...geomagnetic storm on 11 October 2012. The EMIC wave activity was observed in situ on the Van Allen Probes and conjugately on the ground across the Canadian Array for Real‐time Investigations of Magnetic Activity throughout an extended 18 h interval. However, neither enhanced precipitation of >0.7 MeV electrons nor reductions in Van Allen Probe 90° pitch angle ultrarelativistic electron flux were observed. Computed radiation belt electron pitch angle diffusion rates demonstrate that rapid pitch angle diffusion is confined to low pitch angles and cannot reach 90°. For the first time, from both observational and modeling perspectives, we show evidence of EMIC waves triggering ultrarelativistic (~2–8 MeV) electron loss but which is confined to pitch angles below around 45° and not affecting the core distribution.
Key Points
EMIC wave activity is not associated with precipitation of MeV electrons
EMIC waves do not deplete the ultra‐relativistic belt down to 90°
EMIC waves cause loss of low pitch angle electrons with energies ~2–8 MeV
Self‐consistent kinetic solutions from test‐particle simulations are used to improve the derivation of electron temperature (Te) and density (ne) from Langmuir probes in space. While Langmuir probes ...are well understood, the non‐ideal characteristics of space‐flight instruments can influence the accuracy of Te and ne and how they are derived. In particular, when in an ionosphere, spacecraft motion often causes a sensor wake and exposes its surface to oxidation. This work has two primary goals. We present kinetic solutions of general interest then apply our findings to the Langmuir Probe and Waves (LPW) instrument on the MAVEN satellite. Of general interest, (1) kinetic solutions show that mechanical mounting of a Langmuir probe sensor and controlling the potential of nearby surfaces is critical for accuracy. (2) An ion wake generated by the sensor can greatly modify how ne must be derived. (3) Interestingly, small voltage variations on the surface of the sensor do not significantly diminish the accuracy of Te and ne. (4) On the other hand, surface resistance on the sensor can appreciably disturb the derivation of Te. The LPW instrument is recalibrated with the aid of kinetic solutions and published results from laboratory experiments. The systematic uncertainty (as opposed to random variations) in Te is improved to as low as ±0.005 eV (±60 K) when ne > ∼3 × 104 cm−3. This recalibration leads to some of the most accurate measurements of Te made in space and can result in improved modeling of Mars' ionosphere.
Key Points
Self‐consistent kinetic solutions reveal that a sensor wake can alter the I‐V characteristic of a Langmuir probe
The measurement accuracy of Te by the MAVEN Langmuir Probe and Waves instrument is dramatically improved from kinetic modeling
Self‐consistent kinetic solutions from a test‐particle simulation improve the accuracy of the electron density measurement
Effects of whistler mode hiss waves in March 2013 Ripoll, J.‐F.; Santolík, O.; Reeves, G. D. ...
Journal of geophysical research. Space physics,
July 2017, 2017-07-00, 20170701, Letnik:
122, Številka:
7
Journal Article
Recenzirano
We present simulations of the loss of radiation belt electrons by resonant pitch angle diffusion caused by whistler mode hiss waves for March 2013. Pitch angle diffusion coefficients are computed ...from the wave properties and the ambient plasma data obtained by the Van Allen Probes with a resolution of 8 h and 0.1 L shell. Loss rates follow a complex dynamic structure, imposed by the wave and plasma properties. Hiss effects can be strong, with minimum lifetimes (of ~1 day) moving from energies of ~100 keV at L ~ 5 up to ~2 MeV at L ~ 2 and stop abruptly, similarly to the observed energy‐dependent inner belt edge. Periods when the plasmasphere extends beyond L ~ 5 favor long‐lasting hiss losses from the outer belt. Such loss rates are embedded in a reduced Fokker‐Planck code and validated against Magnetic Electron and Ion Spectrometer observations of the belts at all energy. Results are complemented with a sensitivity study involving different radial diffusion and lifetime models. Validation is carried out globally at all L shells and energies. The good agreement between simulations and observations demonstrates that hiss waves drive the slot formation during quiet times. Combined with transport, they sculpt the energy structure of the outer belt into an “S shape.” Low energy electrons (<0.3 MeV) are less subject to hiss scattering below L = 4. In contrast, 0.3–1.5 MeV electrons evolve in an environment that depopulates them as they migrate from L ~ 5 to L ~ 2.5. Ultrarelativistic electrons are not affected by hiss losses until L ~ 2–3.
Key Points
Computations of daily pitch angle diffusion coefficients and electron lifetimes from properties of hiss waves observed in March 2013
Good agreement found between MagEIS flux observations and 1‐D Fokker‐Planck simulations based on our hiss loss term for quiet times
Combined with transport, hiss waves loss drives the daily energy structure of the radiation belts, with a typical S‐shaped outer belt
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
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.
Purely compressional electromagnetic waves (fast magnetosonic waves), generated at multiple harmonics of the local proton gyrofrequency, have been observed by various types of satellite instruments ...(fluxgate and search coil magnetometers and electric field sensors), but most recent studies have used data from search coil sensors, and many have been restricted to high harmonics. We report here on a survey of low‐harmonic waves, based on electric and magnetic field data from the Electric Fields and Waves double probe and Electric and Magnetic Field Instrument Suite and Integrated Science fluxgate magnetometer instruments, respectively, on the Van Allen Probes spacecraft during its first full precession through all local times, from 1 October 2012 to 13 July 2014. These waves were observed both inside and outside the plasmapause (PP), at L shells from 2.4 to ~6 (the spacecraft apogee), and in regions with plasma number densities ranging from 10 to >1000 cm−3. Consistent with earlier studies, wave occurrence was sharply peaked near the magnetic equator. Waves appeared at all local times but were more common from noon to dusk, and often occurred within 3 h after substorm injections. Outside the PP occurrence maximized broadly across noon, and inside the PP occurrence maximized in the dusk sector, in an extended plasmasphere. We confirm recent ray‐tracing studies showing wave refraction and/or reflection at PP‐like boundaries. Comparison with waveform receiver data indicates that in some cases these low‐harmonic magnetosonic wave events occurred independently of higher‐harmonic waves; this indicates the importance of including this population in future studies of radiation belt dynamics.
Key Points
Low‐harmonic magnetosonic waves can occur independently of higher harmonics
Waves often occurred within 3 h after substorm injections
Observations confirm ray‐tracing studies of wave refraction and/or reflection
The evolution of the radiation belts in L‐shell (L), energy (E), and equatorial pitch angle (α0) is analyzed during the calm 11‐day interval (4–15 March) following the 1 March 2013 storm. Magnetic ...Electron and Ion Spectrometer (MagEIS) observations from Van Allen Probes are interpreted alongside 1D and 3D Fokker‐Planck simulations combined with consistent event‐driven scattering modeling from whistler mode hiss waves. Three (L, E, α0) regions persist through 11 days of hiss wave scattering; the pitch angle‐dependent inner belt core (L ~ <2.2 and E < 700 keV), pitch angle homogeneous outer belt low‐energy core (L > ~5 and E~ < 100 keV), and a distinct pocket of electrons (L ~ 4.5, 5.5 and E ~ 0.7, 2 MeV). The pitch angle homogeneous outer belt is explained by the diffusion coefficients that are roughly constant for α0 ~ <60°, E > 100 keV, 3.5 < L < Lpp ~ 6. Thus, observed unidirectional flux decays can be used to estimate local pitch angle diffusion rates in that region. Top‐hat distributions are computed and observed at L ~ 3–3.5 and E = 100–300 keV.
Plain Language Summary
We study the evolution of the radiation belts during quiet geomagnetic times from satellite observations and numerical codes. We reach a global understanding of the trapped electrons variation with time, space, energy, and pitch angle (the angle of the velocity vector with the magnetic field). We exhibit three stable regions, which are less sensitive to scattering from hiss waves, while, on the other hand, hiss causes flux decay over 12 days that forms the slot region between the inner and outer belt. The existing theory explains why the outer belt electron decay is independent of pitch angle but dependent upon energy. This implies that satellite observations can reveal local pitch angle diffusion rates, themselves intimately connected with the wave properties. Thus, a connection is made between observed wave properties and observed/computed scattered electron flux, consistent with theory. Regions where the flux is pitch angle dependent are isolated in the low‐energy slot region where we show that the real shape is a smoothed version of the ideal top‐hat distribution computed from theory. The impact of this work is improved understanding of the belt evolution for space weather prediction, with a proposed event‐driven method that accurately (within ×2) predicts the electron flux decay after storms.
Key Points
Global computations of the (L, E, α0) structure of the evolving radiation belt during quiet times agree well with observations
The inner belt decay is pitch angle dependent, while the outer belt is much more homogeneous with two distinct (L, E) regions
The homogeneity of the pitch angle diffusion coefficient due to hiss waves explains the uniform outer belt decay and why 1D and 3D simulations agree
On 17 March 2013, a large magnetic storm significantly depleted the multi‐MeV radiation belt. We present multi‐instrument observations from the Van Allen Probes spacecraft Radiation Belt Storm Probe ...A and Radiation Belt Storm Probe B at ~6 Re in the midnight sector magnetosphere and from ground‐based ionospheric sensors during a substorm dipolarization followed by rapid reenergization of multi‐MeV electrons. A 50% increase in magnetic field magnitude occurred simultaneously with dramatic increases in 100 keV electron fluxes and a 100 times increase in VLF wave intensity. The 100 keV electrons and intense VLF waves provide a seed population and energy source for subsequent radiation belt enhancements. Highly relativistic (>2 MeV) electron fluxes increased immediately at L* ~ 4.5 and 4.5 MeV flux increased >90 times at L* = 4 over 5 h. Although plasmasphere expansion brings the enhanced radiation belt multi‐MeV fluxes inside the plasmasphere several hours postsubstorm, we localize their prompt reenergization during the event to regions outside the plasmasphere.
Key Points
Substorm dynamics are important for highly relativistic electron energization
Cold plasma preconditioning is significant for rapid relativistic energization
Relativistic / highly relativistic electron energization can occur in < 5 hrs
We have studied the spatial location relative to the plasmapause (PP) of the most intense electromagnetic ion cyclotron (EMIC) waves observed on Van Allen Probes A and B during their first full ...precession in local time. Most of these waves occurred over an L range of from −1 to +2 RE relative to the PP. Very few events occurred only within 0.1 RE of the PP, and events with a width in L of < 0.2 RE occurred both inside and outside the PP. Wave occurrence was always associated with high densities of ring current ions; plasma density gradients or enhancements were associated with some events but were not dominant factors in determining the sites of wave generation. Storm main and recovery phase events in the dusk sector were often inside the PP, and dayside events during quiet times and compressions of the magnetosphere were more evenly distributed both inside and outside the PP. Superposed epoch analyses of the dependence of wave onset on solar wind dynamic pressure (Psw), the SME (SuperMAG auroral electrojet) index, and the SYM‐H index showed that substorm injections and solar wind compressions were temporally closely associated with EMIC wave onset but to an extent that varied with frequency band, magnetic local time, and storm phase, and location relative to the PP. The fact that increases in SME and Psw were less strongly correlated with events at the PP than with other events might suggest that the occurrence of those events was affected by the density gradient.
Key Points
A statistical and superposed epoch study of the 158 most intense EMIC wave events in the Van Allen Probes data set has been performed
EMIC wave onsets were often temporally linked to recent particle injections and/or increases in solar wind pressure
Plasma density gradients or enhancements were not dominant factors in determining the sites of wave generation
We present cross‐scale magnetospheric observations of the 17 March 2015 (St. Patrick's Day) storm, by Time History of Events and Macroscale Interactions during Substorms (THEMIS), Van Allen Probes ...(Radiation Belt Storm Probes), and Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS), plus upstream ACE/Wind solar wind data. THEMIS crossed the bow shock or magnetopause 22 times and observed the magnetospheric compression that initiated the storm. Empirical models reproduce these boundary locations within 0.7 RE. Van Allen Probes crossed the plasmapause 13 times; test particle simulations reproduce these encounters within 0.5 RE. Before the storm, Van Allen Probes measured quiet double‐nose proton spectra in the region of corotating cold plasma. About 15 min after a 0605 UT dayside southward turning, Van Allen Probes captured the onset of inner magnetospheric convection, as a density decrease at the moving corotation‐convection boundary (CCB) and a steep increase in ring current (RC) proton flux. During the first several hours of the storm, Van Allen Probes measured highly dynamic ion signatures (numerous injections and multiple spectral peaks). Sustained convection after ∼1200 UT initiated a major buildup of the midnight‐sector ring current (measured by RBSP A), with much weaker duskside fluxes (measured by RBSP B, THEMIS a and THEMIS d). A close conjunction of THEMIS d, RBSP A, and TWINS 1 at 1631 UT shows good three‐way agreement in the shapes of two‐peak spectra from the center of the partial RC. A midstorm injection, observed by Van Allen Probes and TWINS at 1740 UT, brought in fresh ions with lower average energies (leading to globally less energetic spectra in precipitating ions) but increased the total pressure. The cross‐scale measurements of 17 March 2015 contain significant spatial, spectral, and temporal structure.
Key Points
Observations by THEMIS, Van Allen Probes, and TWINS contain much spatial, spectral, and temporal variation
During main phase, all three missions measured two‐peak ion spectrum in center of partial ring current
Encounters with bow shock, magnetopause (by THEMIS) and plasmapause (RBSP) reproduced by models
PDF
