Although most studies of the effects of electromagnetic ion cyclotron (EMIC) waves on Earth's outer radiation belt have focused on events in the afternoon sector in the outer plasmasphere or plume ...region, strong magnetospheric compressions provide an additional stimulus for EMIC wave generation across a large range of local times and L shells. We present here observations of the effects of a wave event on 23 February 2014 that extended over 8 h in UT and over 12 h in local time, stimulated by a gradual 4 h rise and subsequent sharp increases in solar wind pressure. Large‐amplitude linearly polarized hydrogen band EMIC waves (up to 25 nT p‐p) appeared for over 4 h at both Van Allen Probes, from late morning through local noon, when these spacecraft were outside the plasmapause, with densities ~5–20 cm−3. Waves were also observed by ground‐based induction magnetometers in Antarctica (near dawn), Finland (near local noon), Russia (in the afternoon), and in Canada (from dusk to midnight). Ten passes of NOAA‐POES and METOP satellites near the northern foot point of the Van Allen Probes observed 30–80 keV subauroral proton precipitation, often over extended L shell ranges; other passes identified a narrow L shell region of precipitation over Canada. Observations of relativistic electrons by the Van Allen Probes showed that the fluxes of more field‐aligned and more energetic radiation belt electrons were reduced in response to both the emission over Canada and the more spatially extended emission associated with the compression, confirming the effectiveness of EMIC‐induced loss processes for this event.
Key Points
Compression‐induced EMIC waves were observed across 12 h of local time
EMIC‐triggered emissions appeared during the strongest compression
Intense EMIC waves outside the plasmasphere depleted the radiation belts
We simulate the radiation belt electron flux enhancements during selected Geospace Environment Modeling (GEM) challenge events to quantitatively compare the major processes involved in relativistic ...electron acceleration under different conditions. Van Allen Probes observed significant electron flux enhancement during both the storm time of 17–18 March 2013 and non–storm time of 19–20 September 2013, but the distributions of plasma waves and energetic electrons for the two events were dramatically different. During 17–18 March 2013, the SYM‐H minimum reached −130 nT, intense chorus waves (peak Bw ~140 pT) occurred at 3.5 < L < 5.5, and several hundred keV to several MeV electron fluxes increased by ~2 orders of magnitude mostly at 3.5 < L < 5.5. During 19–20 September 2013, the SYM‐H remained higher than −30 nT, modestly intense chorus waves (peak Bw ~80 pT) occurred at L > 5.5, and electron fluxes at energies up to 3 MeV increased by a factor of ~5 at L > 5.5. The two electron flux enhancement events were simulated using the available wave distribution and diffusion coefficients from the GEM focus group Quantitative Assessment of Radiation Belt Modeling. By comparing the individual roles of local electron heating and radial transport, our simulation indicates that resonant interaction with chorus waves is the dominant process that accounts for the electron flux enhancement during the storm time event particularly near the flux peak locations, while radial diffusion by ultralow‐frequency waves plays a dominant role in the enhancement during the non–storm time event. Incorporation of both processes reasonably reproduces the observed location and magnitude of electron flux enhancement.
Key Points
Energetic electron fluxes are enhanced during storm and non–storm time events, but wave and electron structures are dramatically different
Local heating by whistler mode chorus wave is the major contributor to the flux enhancement near the peak during 17–18 March 2013
Radial diffusion by ultralow‐frequency wave is the major contributor to the observed flux enhancement during 19–20 September 2013
Electromagnetic ion cyclotron (EMIC) waves can drive precipitation of tens of keV protons and relativistic electrons, and are a potential candidate for causing radiation belt flux dropouts. In this ...study, we quantitatively analyze three cases of EMIC‐driven precipitation, which occurred near the dusk sector observed by multiple Low‐Earth‐Orbiting (LEO) Polar Operational Environmental Satellites/Meteorological Operational satellite programme (POES/MetOp) satellites. During EMIC wave activity, the proton precipitation occurred from few tens of keV up to hundreds of keV, while the electron precipitation was mainly at relativistic energies. We compare observations of electron precipitation with calculations using quasi‐linear theory. For all cases, we consider the effects of other magnetospheric waves observed simultaneously with EMIC waves, namely, plasmaspheric hiss and magnetosonic waves, and find that the electron precipitation at MeV energies was predominantly caused by EMIC‐driven pitch angle scattering. Interestingly, each precipitation event observed by a LEO satellite extended over a limited L shell region (ΔL ~ 0.3 on average), suggesting that the pitch angle scattering caused by EMIC waves occurs only when favorable conditions are met, likely in a localized region. Furthermore, we take advantage of the LEO constellation to explore the occurrence of precipitation at different L shells and magnetic local time sectors, simultaneously with EMIC wave observations near the equator (detected by Van Allen Probes) or at the ground (measured by magnetometers). Our analysis shows that although EMIC waves drove precipitation only in a narrow ΔL, electron precipitation was triggered at various locations as identified by POES/MetOp over a rather broad region (up to ~4.4 hr MLT and ~1.4 L shells) with similar patterns between satellites.
Key Points
We show three cases of proton and relativistic electron precipitation observed simultaneously with EMIC waves
EMIC‐driven precipitation was observed by POES/MetOp satellites at different locations over a broad L‐MLT region
Each precipitation event extended over ΔL ~ 0.3 on average, showing that wave‐driven pitch angle scattering is localized
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 stimulation of electromagnetic ion cyclotron (EMIC) waves by a magnetospheric compression is perhaps the closest thing to a controlled experiment that is currently possible in magnetospheric ...physics, in that one prominent factor that can increase wave growth acts at a well‐defined time. We present a detailed analysis of EMIC waves observed in the outer dayside magnetosphere by the four Magnetosphere Multiscale (MMS) spacecraft, Van Allen Probe A, and GOES 13 and by four very high latitude ground magnetometer stations in the western hemisphere before, during, and after a modest interplanetary shock on 14 December 2015. Analysis shows several features consistent with current theory, as well as some unexpected features. During the most intense MMS wave burst, which began ~ 1 min after the end of a brief magnetosheath incursion, independent transverse EMIC waves with orthogonal linear polarizations appeared simultaneously at all four spacecraft. He++ band EMIC waves were observed by MMS inside the magnetosphere, whereas almost all previous studies of He++ band EMIC waves observed them only in the magnetosheath and magnetopause boundary layers. Transverse EMIC waves also appeared at Van Allen Probe A and GOES 13 very near the times when the magnetic field compression reached their locations, indicating that the compression lowered the instability threshold to allow for EMIC wave generation throughout the outer dayside magnetosphere. The timing of the EMIC waves at both MMS and Van Allen Probe A was consistent with theoretical expectations for EMIC instabilities based on characteristics of the proton distributions observed by instruments on these spacecraft.
Key Points
MMS observed a burst of independent transverse EMIC waves with orthogonal linear polarizations following the shock
Wave onsets at both MMS and Van Allen Probe A were consistent with theoretical expectations based on particle observations
A rarely observed minimum in wave power at the gyrofrequency of He++ ions was present both before and after the shock
Well‐defined ULF Pc 1 geomagnetic pulsations have been observed simultaneously from a ground array of five search‐coil magnetometers in the morning sector of Antarctica on Mar. 23, 2007. Distributed ...over a very extensive range of geomagnetic latitudes (−62° to −87°, spanning ∼2920 km geographically) approximately along a magnetic meridian, the array showed poleward propagation of the Pc 1 waves in the ionospheric waveguide. It is observed that attenuation factors are between ∼8 and 20 dB/1000 km and the polarization sense changes from left‐hand to right‐hand as the waves are ducted poleward. However, a complex polarization pattern (i.e., change in ellipticity and major axis angle) was seen on the ground, which might be attributed to the array being close to the wave injection region where the superposed effect of incident waves and ducted waves is dominant. A CHAMP satellite conjunction showed a transverse and nearly linearly polarized Pc 1 ULF wave at the altitude of the ducting layer (∼350 km) over a limited latitudinal extent (−53° to −61° ILAT). The polarization analysis performed using the ground data supports the idea that CHAMP detected the wave activity near the wave injection region. The observations are unique in that the ducted waves, seen over an array with unprecedented geomagnetic latitudinal range and positioning along a magnetic meridian (a condition that provides the most efficient ducting), have rarely been measured before.
We present a statistical analysis on the plasmaspheric mass density derived from the field line resonance (FLR) observations by the Mid‐continent MAgnetoseismic Chain (McMAC). McMAC consists of nine ...stations in the United States and Mexico along the 330° magnetic longitude, spanning L‐values between 1.5 and 3.4. Using the gradient method and an automated procedure for FLR detection, we studied a full year of McMAC observations between July 2006 and June 2007. We find that the rate of FLR detection can reach as high as 56% around local noon at L = 2.7, and the detection rates at higher and lower L‐values decline due to the occasional presence of the plasmapause and weaker FLR signals, respectively. At L‐values between 1.8 and 3.1, the inferred equatorial plasma mass density follows the L‐dependence of L−4. By comparing the mass density with the electron density, we found that the ion mass gradually decreased from 1.7 amu at L = 1.8 to 1 amu at L = 3.1. The plasma mass density exhibits an annual variation that maximizes in January, and at L = 2.4 the ratio between January and July densities is 1.6. Our observations also show a local time dependence of plasmaspheric mass density that stays steady in the morning and rises postnoon, a phenomenon that may be attributed to the equatorial ionization anomaly as a part of the plasma neutral coupling at low latitude.
Key Points
Ground observations reveal statistical properties of plasmaspheric density.
Plasmaspheric mass density exhibits an annual variation that peaks in winter.
Plasmaspheric mass density stays steady in the morning and rises post‐noon.
Electromagnetic ion cyclotron (EMIC) waves at large L shells were observed away from the magnetic equator by the Magnetospheric MultiScale (MMS) mission nearly continuously for over four hours on 28 ...October 2015. During this event, the wave Poynting vector direction systematically changed from parallel to the magnetic field (toward the equator), to bidirectional, to antiparallel (away from the equator). These changes coincide with the shift in the location of the minimum in the magnetic field in the southern hemisphere from poleward to equatorward of MMS. The local plasma conditions measured with the EMIC waves also suggest that the outer magnetospheric region sampled during this event was generally unstable to EMIC wave growth. Together, these observations indicate that the bidirectionally propagating wave packets were not a result of reflection at high latitudes but that MMS passed through an off‐equator EMIC wave source region associated with the local minimum in the magnetic field.
Plain Language Summary
Electromagnetic ion cyclotron (EMIC) waves are a fundamental plasma instability in space environments. In near‐Earth space, these waves act as one mechanism for energetic electrons in the radiation belts to be lost to the atmosphere. Because EMIC waves are important for the transport of energy throughout the magnetosphere, understanding where and how these waves are generated, as well as how the waves move along a magnetic field line, is necessary for understanding the full cycle of energization and loss of plasma. The two previous case studies of EMIC waves at high latitudes in the outer magnetosphere were not able to determine if the waves were generated at those high latitudes or if the wave signatures were due to reflection of the waves back toward the magnetic equator, which has important implications for waves seen from the ground. The observations presented here show EMIC waves in the outer magnetosphere away from the equator nearly continuously over several hours. Using the wave Poynting flux direction (which indicates how the waves are moving along the magnetic field), we show unambiguously for the first time that these EMIC waves are from a local source region at higher latitudes.
Key Points
Several hours of EMIC wave activity were observed off‐equator in the outer magnetosphere with plasma conditions favorable for local growth
Changes in direction of the wave Poynting vector indicate transition of source region from poleward, to local, to equatorward of spacecraft
Observations confirm association of EMIC wave source region with local minimum‐B of the field line, possibly related to Shabansky orbits
Magnetospheric plasma waves play a significant role in ring current and radiation belt dynamics, leading to pitch angle scattering loss and/or stochastic acceleration of the particles. During a ...non‐storm time dropout event on 24 September 2013, intense electromagnetic ion cyclotron (EMIC) waves were detected by Van Allen Probe A (Radiation Belt Storm Probes‐A). We quantitatively analyze a conjunction event when Van Allen Probe A was located approximately along the same magnetic field line as MetOp‐01, which detected simultaneous precipitation of >30 keV protons and energetic electrons over an unexpectedly broad energy range (>~30 keV). Multipoint observations together with quasi‐linear theory provide direct evidence that the observed electron precipitation at higher energy (>~700 keV) is primarily driven by EMIC waves. However, the newly observed feature of the simultaneous electron precipitation extending down to ~30 keV is not supported by existing theories and raises an interesting question on whether EMIC waves can scatter such low‐energy electrons.
Plain Language Summary
Energetic electrons can move from the magnetosphere into the Earth's upper atmosphere and cause chemical changes in the atmosphere leading to ozone reduction. The present paper studies the physical process that causes such electron precipitation. When a charged particle interacts with a plasma wave, its trajectory can be altered such that the particle falls into the upper atmosphere, but this process occurs only for a specific range of particle energy. In this study, we use one satellite (MetOp‐01) orbiting in the upper atmosphere (altitude ~800 km) that can detect particle precipitation, and a Van Allen Probes satellite, which provides wave measurements in the equatorial magnetosphere. During the electron precipitation detected by MetOp‐01, a Van Allen Probes satellite observed strong electromagnetic ion cyclotron (EMIC) waves. Multipoint satellite observations together with quasi‐linear theory provide a direct evidence that the observed electron precipitation is primarily driven by EMIC waves. Another new interesting finding is that the precipitation occurs not only for electrons at high energies (>~1 MeV) but also at low energies (down to ~30 keV). This newly observed feature is not supported by existing theories and raises an interesting question whether EMIC waves can interact with such low‐energy electrons as well.
Key Points
Strong electromagnetic ion cyclotron (EMIC) waves were observed during a non‐storm time electron dropout event
Simultaneous particle precipitation was observed for >30 keV protons and energetic electrons in a broad energy range (>~30 keV)
Quasi‐linear theory shows that EMIC waves dominate precipitation of high‐energy electrons but underestimates low‐energy electron precipitation
Propagation of ULF Pc1–2 geomagnetic pulsations in the ionospheric waveguide (duct), centered around the F2 region altitude of maximum ionospheric electron density, is studied using the data obtained ...from a ground array of five search coil magnetometers in Antarctica distributed over a very extensive range of geomagnetic latitudes from subauroral to polar cap (−62° to −87°, spanning 2920 km geographically). A statistical study including a total of 138 Pc1–2 wave ducting events in 2007 presents wave power attenuation factors that vary between ∼10 and 14 dB/1000 km and the change of polarization during the wave propagation under different ionospheric sunlight conditions. It is also shown that the frequency cutoff of the wave events in the waveguide is dependent on the ionospheric conductivity, and the wave power attenuation increases with increasing frequency. In addition, it appears that initial propagation direction is related to attenuation, which supports the idea that meridional propagation is most efficient as predicted in previous studies. The results show the observations of ducted waves over such an unprecedented latitudinal extent along a magnetic meridian, which have rarely been measured before, and thus provide very important information in regard to wave ducting under various ionospheric conditions.
Key Points
ULF wave propagation in the ionospheric waveguide
Antarctic ground magnetometer array observed poleward wave power attenuation
Frequency cutoff in the waveguide is dependent on the ionospheric conductivity
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