Naturally occurring chorus emissions are a class of electromagnetic waves found in the space environments of the Earth and other magnetized planets. They play an essential role in accelerating ...high-energy electrons forming the hazardous radiation belt environment. Chorus typically occurs in two distinct frequency bands separated by a gap. The origin of this two-band structure remains a 50-year old question. Here we report, using NASA's Van Allen Probe measurements, that banded chorus waves are commonly accompanied by two separate anisotropic electron components. Using numerical simulations, we show that the initially excited single-band chorus waves alter the electron distribution immediately via Landau resonance, and suppress the electron anisotropy at medium energies. This naturally divides the electron anisotropy into a low and a high energy components which excite the upper-band and lower-band chorus waves, respectively. This mechanism may also apply to the generation of chorus waves in other magnetized planetary magnetospheres.
A Three-dimensional Map of the Heliosphere from IBEX Reisenfeld, Daniel B.; Bzowski, Maciej; Funsten, Herbert O. ...
The Astrophysical journal. Supplement series,
06/2021, Letnik:
254, Številka:
2
Journal Article
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Abstract
The Interstellar Boundary Explorer (IBEX) mission has shown that variations in the energetic neutral atom (ENA) flux from the outer heliosphere are associated with the solar cycle and ...longer-term variations in the solar wind (SW). In particular, there is a good correlation between the dynamic pressure of the outbound SW and variations in the later-observed IBEX ENA flux. The time difference between observations of the outbound SW and the heliospheric ENAs with which they correlate ranges from approximately 2 to 6 yr or more, depending on ENA energy and look direction. This time difference can be used as a means of “sounding” the heliosheath, that is, finding the average distance to the ENA source region in a particular direction. We apply this method to build a 3D map of the heliosphere. We use IBEX ENA data collected over a complete solar cycle, from 2009 through 2019, corrected for survival probability to the inner heliosphere. Here we divide the data into 56 “macropixels” covering the entire sky. As each point in the sky is sampled once every 6 months, this gives us a time series of 22 points macropixel
–1
on which to time-correlate. Consistent with prior studies and heliospheric models, we find that the shortest distance to the heliopause,
d
HP
, is slightly south of the nose direction (
d
HP
∼ 110–120 au), with a flaring toward the flanks and poles (
d
HP
∼ 160–180 au). The heliosphere extends at least ∼350 au tailward, which is the distance limit of the technique.
We report in situ observations by the Van Allen Probe mission that magnetosonic (MS) waves are clearly relevant to the background plasma number density. As the satellite moved across dense and ...tenuous plasma alternatively, MS waves occurred only in lower density region. As the observed protons with “ring” distributions provide free energy, local linear growth rates are calculated and show that magnetosonic waves can be locally excited in tenuous plasma. With variations of the background plasma density, the temporal variations of local wave growth rates calculated with the observed proton ring distributions show a remarkable agreement with those of the observed wave amplitude. Therefore, the paper provides a direct proof that background plasma densities can modulate the amplitudes of magnetosonic waves through controlling the wave growth rates.
Key Points
Background plasma densities can modulate the amplitudes of MS waves through controlling the wave growth rates
MS waves can been locally excited in tenuous plasma due to the free energy from “ring” distribution protons
Background plasma densities can play an important role in the excitation of magnetosonic waves
Plain Language Summary
For generation of magnetosonic waves, background plasma was proposed to be a key factor in theory, but clear observations have not been provided, so far. In this letter, we report on in situ satellite observations that background plasma densities can modulate the amplitudes of magnetosonic waves through controlling the wave growth rates. This modulation mechanism is expected to reveal the spacial distribution of MS waves and its role in evolution of the radiation belts.
Fast magnetosonic (MS) waves play an important role in the dynamics of the inner magnetosphere. Theoretical prediction and simulation have demonstrated that MS waves can heat cold ions. However, ...direct observational evidence of cold ion heating by MS waves has so far remained elusive. In this paper, we show a typical event of cold ion heating by magnetosonic waves in a density cavity of the plasmasphere with observations of the Van Allen Probe mission on 22 August 2013. During enhancements of the MS wave intensity in the density cavity, the fluxes of trapped H+ and He+ ions with energies of 10–100 eV were observed to increase, implying that cold plasmaspheric ions were heated through high‐order resonances with the MS waves. Based on simultaneous observations of ring current protons, we have calculated local linear growth rates, which demonstrate that magnetosonic waves can be locally generated in the density cavity. Our results provide a direct observational proof of the energy coupling process between the ring current and plasmasphere; that is, through exciting MS waves, the free energy stored in the ring current protons with ring distributions is released. In the density cavity of the plasmasphere, both cold H+ and He+ ions are heated by MS waves. As a result, the energy of the ring current can be transferred into the plasmasphere.
Plain Language Summary
Fast magnetosonic (MS) waves play an important role in the dynamics of the inner magnetosphere. Theoretical prediction and simulation have demonstrated that MS waves can heat cold ions. However, direct observational evidence of cold ion heating by MS waves has so far remained elusive. In this paper, we provide a direct observational evidence to cold ion heating by magnetosonic waves in a density cavity of the plasmasphere. Finally, we demonstrate the energy coupling process between the ring current and plasmasphere, that is, through exciting MS waves; the free energy stored in the ring current protons with ring distributions is released. In the density cavity of the plasmasphere, both cold H+ and He+ ions are heated by MS waves. As a result, the energy of ring current could be transferred into the plasmasphere.
Key Points
This paper has shown in situ observations of cold ion heating by MS waves in the plasmaspheric cavity
Magnetosonic waves can be locally generated in the plasmaspheric density cavity
This paper provides a direct observational evidence to the energy coupling process between ring current and plasmasphere through MS waves
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
Isolated proton auroras (IPAs) appearing at subauroral latitudes are generated by energetic protons precipitating from the magnetosphere through interaction with electromagnetic ion cyclotron (EMIC) ...waves. Thus, an IPA is the ionospheric projection of the spatial and temporal variation of wave‐particle interaction regions in the magnetosphere. In this study, we conducted unique multi‐event analysis of simultaneous observations of IPAs and their source regions on 22 April, 7 September, and 22 March 2018, using all‐sky imagers at subauroral latitudes and the Van Allen Probes. When the satellite footprint passed over the IPAs associated with ground Pc1 geomagnetic pulsations, locally generated He+‐band EMIC waves with the same frequencies as the ground Pc1 pulsations were observed in all events. The IPAs and EMIC waves had comparable narrow widths in the latitudinal direction. The EMIC waves appeared during the rapid enhancement of the ring current proton flux at energy range of ∼10–50 keV, while they disappeared at the rapid decrease of the electron density. From these results, we conclude that the boundaries of the localized IPAs and EMIC waves were determined by the overlap region of energetic proton enhancement and the plasmasphere. This overlap of ring‐current protons and plasmasphere is a favorable condition for the pitch‐angle scattering of protons by the EMIC waves. Characteristic magnetic and electric field variations with the IPAs were not observed by the satellite, indicating that the IPAs were not accompanied by field‐aligned currents comparable to that of oval auroral arcs.
Plain Language Summary
Isolated proton auroras (IPAs) appearing at subauroral latitudes (∼55–65°) are generated by energetic protons precipitating from the Earth's magnetosphere, possibly through interaction with plasma waves, that is, electromagnetic ion cyclotron (EMIC) waves. An IPA indicates the spatial and temporal variation of wave‐particle interaction regions in the magnetosphere. EMIC waves are expected to contribute to the rapid loss of radiation‐belt particles into the atmosphere and affect human activities in space. In this study, we report unique multi‐event analysis of simultaneous observations of IPAs and their source regions, using all‐sky imagers at subauroral latitudes and the Van Allen Probes on 22 April, 7 September, and 22 March 2018. When the satellite footprint passed over the IPAs, locally generated EMIC waves were observed in all events. The IPAs and EMIC waves had comparable narrow widths in the latitudinal direction. These results indicate that the IPAs were caused by the EMIC waves. The appearance and disappearance of the EMIC waves were correlated with the rapid enhancement of energetic proton flux and the rapid decrease of the local electron density, respectively. We conclude that the boundaries of the localized IPAs and EMIC waves were determined by the overlap region of energetic proton enhancement and the plasmasphere.
Key Points
Three ground‐satellite conjugate observations of isolated proton auroras (IPAs) and their magnetospheric source regions are reported
Van Allen Probes observed electromagnetic ion cyclotron (EMIC) waves which had radial ranges corresponding to latitudinal widths of IPAs in the ionosphere in all events
We conclude that the IPAs and EMIC waves are excited in the region where enhanced ring current flux overlapped with stable plasmasphere
Drift‐resonance wave‐particle interaction is a fundamental collisionless plasma process studied extensively in theory. Using cross‐spectral analysis of electric field, magnetic field, and ion flux ...data from the Van Allen Probe (Radiation Belt Storm Probes) spacecraft, we present direct evidence identifying the generation of a fundamental mode standing poloidal wave through drift‐resonance interactions in the inner magnetosphere. Intense azimuthal electric field (Eφ) oscillations as large as 10mV/m are observed, associated with radial magnetic field (Br) oscillations in the dawn‐noon sector near but south of the magnetic equator at L∼5. The observed wave period, Eφ/Br ratio and the 90° phase lag between Br and Eφ are all consistent with fundamental mode standing Poloidal waves. Phase shifts between particle fluxes and wave electric fields clearly demonstrate a drift resonance with ∼90 keV ring current ions. The estimated earthward gradient of ion phase space density provides a free energy source for wave generation through the drift‐resonance instability. A similar drift‐resonance process should occur ubiquitously in collisionless plasma systems. One specific example is the “fishbone” instability in fusion plasma devices. In addition, our observations have important implications for the long‐standing mysterious origin of Giant Pulsations.
Key Points
Unambiguous identification of drift‐resonance in magnetosphere
Broad implications for ring current and ground observations
Drift‐resonance similar to fishbone instability in Tokamak
Magnetospheric banded chorus is enhanced whistler waves with frequencies ωr<Ωe, where Ωe is the electron cyclotron frequency, and a characteristic spectral gap at ωr≃Ωe/2. This paper uses spacecraft ...observations and two‐dimensional particle‐in‐cell simulations in a magnetized, homogeneous, collisionless plasma to test the hypothesis that banded chorus is due to local linear growth of two branches of the whistler anisotropy instability excited by two distinct, anisotropic electron components of significantly different temperatures. The electron densities and temperatures are derived from Helium, Oxygen, Proton, and Electron instrument measurements on the Van Allen Probes A satellite during a banded chorus event on 1 November 2012. The observations are consistent with a three‐component electron model consisting of a cold (a few tens of eV) population, a warm (a few hundred eV) anisotropic population, and a hot (a few keV) anisotropic population. The simulations use plasma and field parameters as measured from the satellite during this event except for two numbers: the anisotropies of the warm and the hot electron components are enhanced over the measured values in order to obtain relatively rapid instability growth. The simulations show that the warm component drives the quasi‐electrostatic upper band chorus and that the hot component drives the electromagnetic lower band chorus; the gap at ∼Ωe/2 is a natural consequence of the growth of two whistler modes with different properties.
Key PointsThe frequency gas of banded chorus is explained by linear dispersion theoryBanded chorus is excited by two distinct anisotropic electron componentsTheory, simulations, and observations agree
Isolated proton auroras (IPAs) appearing at subauroral latitudes are generated by energetic protons precipitating from the magnetosphere through interaction with electromagnetic ion cyclotron (EMIC) ...waves. An IPA thus indicates the spatial scale and temporal variation of wave‐particle interactions in the magnetosphere. In this study, a unique event of simultaneous ground and magnetospheric satellite observations of two IPAs were conducted on March 16, 2015, using an all‐sky imager at Athabasca, Canada and Van Allen Probes. The Van Allen Probes observed two isolated EMIC waves with frequencies of ∼1 and 0.4 Hz at L ≈ 5.0 when the satellite footprint crossed over the two IPAs. This suggests that the IPAs were caused by localized EMIC waves. Proton flux at 5–20 keV increased locally when the EMIC waves appeared. Electron flux at energies below ∼500 eV also increased. Temperature anisotropy of the energetic protons was estimated at 1.5–2.5 over a wide L‐value range of 3.0–5.2. Electron density gradually decreased from L = 3.5 to 5.4, suggesting that the EMIC wave at L ≈ 5.0 was located in the gradual plasmapause. From these observations, we conclude that the localized IPAs and associated EMIC waves took place because of localized enhancement of energetic proton flux and plasma density structure near the plasmapause. The magnetic field observed by the satellite showed small variation during the wave observation, indicating that the IPAs were accompanied by the weak field‐aligned current.
Plain Language Summary
Isolated proton aurora (IPA) appearing at subauroral latitudes (∼55°–65°) is generated by energetic protons precipitating from Earth's magnetosphere, possibly through interaction with plasma waves called electromagnetic ion cyclotron (EMIC) waves. The IPA indicates the spatial scale and temporal variation of wave‐particle interactions in the magnetosphere. EMIC waves are expected to contribute to the rapid loss of radiation‐belt particles into the atmosphere. Thus, IPAs present essential information related to EMIC waves. In this study, we report unique simultaneous ground and magnetospheric satellite observations of two IPAs using an all‐sky imager at Athabasca, Canada and Van Allen Probes acquired on March 16, 2015. Van Allen Probes observed two isolated EMIC waves with frequencies of ∼1 and 0.4 Hz when the satellite footprint crossed over the two IPAs. This indicates that the IPAs were caused by localized EMIC waves. We conclude that the localized IPAs and associated EMIC waves took place due to localized enhancement of energetic proton number flux and local plasma density structure near the plasmapause.
Key Points
We report unique simultaneous ground and magnetospheric satellite observations of two isolated proton auroras at subauroral latitudes
Van Allen Probes observed two EMIC waves at ∼1 and 0.4 Hz during crossings of the isolated proton auroras
When the EMIC waves were observed, 5–20 keV proton flux was locally enhanced near a steep density decrease in the plasmapause region
Suprathermal electrons (~0.1–10 keV) in the inner magnetosphere are usually observed in a 90°‐peaked pitch angle distribution, formed due to the conservation of the first and second adiabatic ...invariants as they are transported from the plasma sheet. We report a peculiar field‐aligned suprathermal electron (FASE) distribution measured by Van Allen Probes, where parallel fluxes are 1 order of magnitude higher than perpendicular fluxes. Those FASEs are found to be closely correlated with large‐amplitude hiss waves and are observed around the Landau resonant energies. We demonstrate, using quasilinear diffusion simulations, that hiss waves can rapidly accelerate suprathermal electrons through Landau resonance and create the observed FASE population. The proposed mechanism potentially has broad implications for suprathermal electron dynamics as well as whistler mode waves in the Earth's magnetosphere and has been demonstrated in the Jovian magnetosphere.
Plain Language Summary
Hiss waves are structureless and incoherent “hissy” emissions found in the magnetized near‐Earth space, typically in a frequency range from 0.1 to 2 kHz. Hiss waves have traditionally been treated as an energetic electron removal mechanism, because they can precipitate energetic electrons through resonant interactions and cause electron loss into the atmosphere. Here we show, by presenting observations from NASA's Van Allen Probes, that intense hiss waves are accompanied by enhancements of field‐aligned suprathermal electrons. We propose that hiss waves can accelerate suprathermal electrons as they travel at the same speed in the direction along the magnetic field. Computer simulations successfully reproduce the rapid enhancement of field‐aligned suprathermal electrons under the impact of hiss waves, with detailed features similar to observations.
Key Points
Pronounced field‐aligned suprathermal electron enhancements are observed in correlation with intense hiss waves
Numerical simulations reproduce field‐aligned electron distributions similar to observations
Intense hiss waves can accelerate field‐aligned suprathermal electrons via Landau resonance on a timescale of several minutes
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