Auroral substorms, dynamic phenomena that occur in the upper atmosphere at night, are caused by global reconfiguration of the magnetosphere, which releases stored solar wind energy. These storms are ...characterized by auroral brightening from dusk to midnight, followed by violent motions of distinct auroral arcs that suddenly break up, and the subsequent emergence of diffuse, pulsating auroral patches at dawn. Pulsating aurorae, which are quasiperiodic, blinking patches of light tens to hundreds of kilometres across, appear at altitudes of about 100 kilometres in the high-latitude regions of both hemispheres, and multiple patches often cover the entire sky. This auroral pulsation, with periods of several to tens of seconds, is generated by the intermittent precipitation of energetic electrons (several to tens of kiloelectronvolts) arriving from the magnetosphere and colliding with the atoms and molecules of the upper atmosphere. A possible cause of this precipitation is the interaction between magnetospheric electrons and electromagnetic waves called whistler-mode chorus waves. However, no direct observational evidence of this interaction has been obtained so far. Here we report that energetic electrons are scattered by chorus waves, resulting in their precipitation. Our observations were made in March 2017 with a magnetospheric spacecraft equipped with a high-angular-resolution electron sensor and electromagnetic field instruments. The measured quasiperiodic precipitating electron flux was sufficiently intense to generate a pulsating aurora, which was indeed simultaneously observed by a ground auroral imager.
We investigate plasma transport to and plasma heating in the plasma sheet in the noon‐midnight meridian, characterizing protons with temperature colder than the core plasma sheet protons (<700 eV). ...We extract the density and temperature of the cold protons from velocity distribution functions measured by the Hot Plasma Composition Analyzer instrument on board the Magnetospheric Multiscale spacecraft in the radial distance (r) of 6–25 Re, performing two‐component Maxwellian fits. We selected time intervals with no fast flow observed, to examine the characteristics of magnetotail plasma not directly affected by magnetic reconnection and associated phenomena. In the region of r > ∼10 Re, the two‐component populations are identified more frequently near the plasma sheet boundary than the central plasma sheet. The cold component density peaks near the boundary, in contrast to the hot components which display high density near the central plasma sheet. These characteristics suggest that cold protons are convected from the lobe by the open field lines and then heated and mixed with the plasma sheet hot plasma near the lobe‐plasma sheet boundary. The statistical features of the extracted cold components indicate that, in the tailward regions (r > ∼20 Re), temperature increases with decreasing vertical distance from the plasma sheet (represented by plasma β) in a similar trend to the hot components. In the near‐Earth plasma sheet (r < ∼15 Re), cold proton temperature is lower at higher‐β regions; the density decreases as increasing r. These features suggest that cold protons in the near‐Earth plasma sheet are of ionospheric origin, transported to the plasma sheet in the closed magnetic field configuration.
Plain Language Summary
The Earth's magnetosphere is filled by charged particles (mostly electrons and protons) originating from the Sun and Earth's upper atmosphere. Most of those plasma is transported anti‐sunward and then stored on the tailward side of the magnetosphere (called the magnetotail). The plasma is heated up to >1 kilo‐electron volts and thus forms a sheet‐like hot region called the plasma sheet nearly on the equatorial plane. This study is successful in observationally determining global characteristics of pre‐heated cold protons in the plasma sheet, by utilizing a spectral fitting method that enables us to extract density and temperature of cold protons from in‐situ plasma measurements. The characteristics suggest that cold protons are convected toward the plasma sheet by the open field lines and then efficiently mixed with the plasma sheet hot plasma. In the near‐Earth magnetotail, cold protons are likely of atmospheric origin, supplied to the plasma sheet in the closed field line configuration.
Key Points
We characterize cold protons in the magnetotail around midnight, applying two‐component Maxwellian fits to velocity distribution functions
Cold protons are efficiently heated and mixed with hot plasma near the plasma sheet boundary rather than the central plasma sheet
Cold protons in the near‐Earth plasma sheet, r < ∼10 Re, are likely of ionospheric origin, transported in the closed field lines
We investigated how velocity and flux of ionospheric ion upflows vary during magnetic storms driven by corotating interaction regions (CIRs) and coronal mass ejections (CMEs), using data from the ...European Incoherent Scatter (EISCAT) Tromsø UHF and Svalbard radars between 1996 and 2015. The characteristics of ion upflows were compared with ion and electron temperature variations measured with the EISCAT radars and also joule heating rate, electric field, and field‐aligned current distribution derived from the Weimer model. Upward ion velocity increases in the nighttime at Tromsø (66.2 °N geomagnetic latitude) just after the CIR‐ and CME‐driven storms, corresponding to electron temperature enhancements due to soft particle precipitation and also ion temperature enhancements in the strong westward electric field region. The CME‐driven storms have larger upward ion flux (~1.7 × 1013 m2/s) than those under the CIR‐driven storms (~0.3 × 1013 m2/s). In the daytime, ion upflows are seen at Longyearbyen, Svalbard (75.2 °N geomagnetic latitude), with an upward flux of typically 1013 m2/s for small CIR and CME storm cases. Substantial ion upflows last for a few days after the storm onsets under small CIR storms, whereas they last for only a day under small CME storms. Under both the cases, the substantial ion upflows are associated with an enhancement of the Region 1 field‐aligned current, eastward electric field and Joule heating rate. For large CME storms, substantial ion upflows are absent in the daytime probably due to equatorward expansion of the auroral oval.
Key Points
Flux and velocity of ionospheric ion upflows during magnetic storms are quantitatively examined
CME‐driven storms have about 4 times larger upward ion flux in the nighttime than those under CIR‐driven storms
Dayside ion upflows under small CIR‐driven storms continue a few days longer than those under small CME‐driven storms
The Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) on the two Van Allen Probes spacecraft is the magnetosphere ring current instrument that will provide data for answering the three ...over-arching questions for the Van Allen Probes Program: RBSPICE will determine “how space weather creates the storm-time ring current around Earth, how that ring current supplies and supports the creation of the radiation belt populations,” and how the ring current is involved in radiation belt losses. RBSPICE is a time-of-flight versus total energy instrument that measures ions over the energy range from ∼20 keV to ∼1 MeV. RBSPICE will also measure electrons over the energy range ∼25 keV to ∼1 MeV in order to provide instrument background information in the radiation belts. A description of the instrument and its data products are provided in this chapter.
Equatorial noise (EN) emissions are observed inside and outside the plasmapause. EN emissions are referred to as magnetosonic mode waves. Using data from Van Allen Probes and Arase, we found ...conversion from EN emissions to electromagnetic ion cyclotron (EMIC) waves in the plasmasphere and in the topside ionosphere. A low‐frequency part of EN emissions becomes EMIC waves through branch splitting of EN emissions, and the mode conversion from EN to EMIC waves occurs around the frequency of M/Q = 2 (deuteron and/or alpha particles) cyclotron frequency. These processes result in plasmaspheric EMIC waves. We investigated the ion composition ratio by characteristic frequencies of EN emissions and EMIC waves and obtained ion composition ratios. We found that the maximum composition ratio of M/Q = 2 ions is ~10% below 3,000 km. The quantitative estimation of the ion composition will contribute to improving the plasma model of the deep plasmasphere and the topside ionosphere.
Plain Language Summary
Equatorial noise (EN) emissions are whistler mode waves. Using Van Allen Probe and Arase (ERG) plasma wave data, we found that EN emissions propagate toward the Earth and are converted to electromagnetic ion cyclotron (EMIC) waves in the deep plasmasphere and the topside ionosphere. We suggest that minor ions with a mass per charge (M/Q) = 2, that is, deuteron or alpha particles, play an important role in this process. The processes reported here are a new generation process of plasmaspheric EMIC waves. Moreover, we determined the ion composition ratio using characteristics of wave dispersion. We derived the altitude profile of the ion composition ratio and identified the maximum ratio of M/Q = 2 ions of about 10% in the deep plasmasphere.
Key Points
The first measurements of the conversion from equatorial noise to EMIC waves are presented
Existence of M/Q = 2 ions (deuteron or alpha particle) in the deep plasmasphere is essential to cause the conversion
The ion composition ratio is quantitatively estimated in the deep plasmasphere using characteristics of the wave dispersion
Resonant interactions of energetic electrons with electromagnetic whistler‐mode waves (whistlers) contribute significantly to the dynamics of electron fluxes in Earth's outer radiation belt. At low ...geomagnetic latitudes, these waves are very effective in pitch angle scattering and precipitation into the ionosphere of low equatorial pitch angle, tens of keV electrons and acceleration of high equatorial pitch angle electrons to relativistic energies. Relativistic (hundreds of keV), electrons may also be precipitated by resonant interaction with whistlers, but this requires waves propagating quasi‐parallel without significant intensity decrease to high latitudes where they can resonate with higher energy low equatorial pitch angle electrons than at the equator. Wave propagation away from the equatorial source region in a non‐uniform magnetic field leads to ray divergence from the originally field‐aligned direction and efficient wave damping by Landau resonance with suprathermal electrons, reducing the wave ability to scatter electrons at high latitudes. However, wave propagation can become ducted along field‐aligned density peaks (ducts), preventing ray divergence and wave damping. Such ducting may therefore result in significant relativistic electron precipitation. We present evidence that ducted whistlers efficiently precipitate relativistic electrons. We employ simultaneous near‐equatorial and ground‐based measurements of whistlers and low‐altitude electron precipitation measurements by ELFIN CubeSat. We show that ducted waves (appearing on the ground) efficiently scatter relativistic electrons into the loss cone, contrary to non‐ducted waves (absent on the ground) precipitating only <150 keV electrons. Our results indicate that ducted whistlers may be quite significant for relativistic electron losses; they should be further studied statistically and possibly incorporated in radiation belt models.
Key Points
Near‐equatorial and ground‐based measurements of whistler‐mode waves are accompanied by relativistic electron precipitation
In the presence (absence) of ducted wave propagation, as monitored by propagation to the ground, the precipitating electron energies are above (below) 150 keV
Ducted whistler‐mode waves may play a key role in relativistic electron loss in the inner magnetosphere
Abstract
Both solar wind and ionospheric sources contribute to the magnetotail plasma sheet, but how their contribution changes during a geomagnetic storm is an open question. The source is critical ...because the plasma sheet properties control the enhancement and decay rate of the ring current, the main cause of the geomagnetic field perturbations that define a geomagnetic storm. Here we use the solar wind composition to track the source and show that the plasma sheet source changes from predominantly solar wind to predominantly ionospheric as a storm develops. Additionally, we find that the ionospheric plasma during the storm main phase is initially dominated by singly ionized hydrogen (H
+
), likely from the polar wind, a low energy outflow from the polar cap, and then transitions to the accelerated outflow from the dayside and nightside auroral regions, identified by singly ionized oxygen (O
+
). These results reveal how the access to the magnetotail of the different sources can change quickly, impacting the storm development.
We present the results of a multi‐point and multi‐instrument study of electromagnetic ion cyclotron (EMIC) waves and related energetic proton precipitation during a substorm. We analyze the data from ...Arase (ERG) and Van Allen Probes (VAPs) A and B spacecraft for an event of 16 and 17 UT on December 1, 2018. VAP‐A detected an almost dispersionless injection of energetic protons related to the substorm onset in the night sector. Then the proton injection was detected by VAP‐B and further by Arase, as a dispersive enhancement of energetic proton flux. The proton flux enhancement at every spacecraft coincided with the EMIC wave enhancement or appearance. This data show the excitation of EMIC waves first inside an expanding substorm wedge and then by a drifting cloud of injected protons. Low‐orbiting NOAA/POES and MetOp satellites observed precipitation of energetic protons nearly conjugate with the EMIC wave observations in the magnetosphere. The proton pitch‐angle diffusion coefficient and the strong diffusion regime index were calculated based on the observed wave, plasma, and magnetic field parameters. The diffusion coefficient reaches a maximum at energies corresponding well to the energy range of the observed proton precipitation. The diffusion coefficient values indicated the strong diffusion regime, in agreement with the equality of the trapped and precipitating proton flux at the low‐Earth orbit. The growth rate calculations based on the plasma and magnetic field data from both VAP and Arase spacecraft indicated that the detected EMIC waves could be generated in the region of their observation or in its close vicinity.
Plain Language Summary
Electromagnetic ion cyclotron (EMIC) waves are believed to play a significant role in the dynamics of energetic protons and relativistic electrons in the Earth's magnetosphere. The properties of these waves are being intensively studied. We consider the conditions of the EMIC wave generation and the dynamics of the wave source during a substorm event using a unique configuration of three spacecraft (Arase and two Van Allen Probes). All spacecraft were at approximately the same distance from the Earth, forming a chain across the evening local time sector. Analyzing parameters of the wave generation obtained from in situ measured proton distribution function, we came to the conclusion that the waves could be generated within the substorm area, sometimes close to, but not necessary at the spacecraft location. As the substorm expands in longitude, the EMIC wave source exhibits a longitudinal drift. When substorm expansion stops, the wave generation region expands due to the magnetic drift of protons injected during the substorm. The observed wave properties show that the waves are able to precipitate energetic protons into the atmosphere. This is confirmed by observations of low orbiting satellites measuring proton precipitating fluxes.
Key Points
Westward propagation of the EMIC wave generation region is due to both the substorm expansion and azimuthal drift of injected protons
Strong pitch‐angle diffusion regime is confirmed by observations of proton fluxes at low altitude and the diffusion coefficient calculation
The diffusion coefficient maximum corresponds well to the energy range of the observed proton precipitation
We present the first direct evidence of an in situ excitation of drift‐compressional waves driven by drift resonance with ring current protons in the magnetosphere. Compressional Pc4–5 waves with ...frequencies of 4–12 mHz were observed by the Arase satellite near the magnetic equator at L ∼ 6 in the evening sector on 19 November 2018. Estimated azimuthal wave numbers (m) ranged from −100 to −130. The observed frequency was consistent with that calculated using the drift‐compressional mode theory, whereas the plasma anisotropy was too small to excite the drift‐mirror mode. We discovered that the energy source of the wave was a drift resonance instability, which was generated by the negative radial gradient in a proton phase space density at 20–25 keV. This proton distribution is attributed to a temporal variation of the electric field, which formed the observed multiple‐nose structures of ring current protons.
Plain Language Summary
During magnetic storms and substorms, energetic ions are sporadically injected into the geospace, which distorts the stable population and velocity distributions of ions in space. At these moments, various plasma instabilities lead to ultra‐low frequency (ULF) wave excitations. The lowest‐frequency waves in the ULF range have a wavelength comparable to the size of the Earth and are typically analyzed using magnetohydrodynamic principles. This approach considers the plasma environment using macroscale parameters such as pressure and density. In this paper, we report a spacecraft observation of a broadband compressional ULF wave that cannot be interpreted using magnetohydrodynamics. Such waves have rarely been reported and analyzed; however, their interaction with energetic ions is important to understand magnetospheric energy dynamics. The plasma conditions were described using the kinetic theory, which involves particle velocity distributions. We observed that a drift resonance occurred between the energetic protons and waves, while the gradient instability condition was satisfied for a part of time. Therefore, we concluded that the wave was in a drift‐compressional mode excited through drift resonance and gradient instability. The interpretation of compressional waves via satellite observations of energetic ions has been receiving increasing attention to understand their excitation mechanism.
Key Points
Pc4–5 compressional ultra‐low frequency waves with an azimuthal wave number of −130 were observed in the nose structure on duskside
Theoretically predicted values of drift‐compressional mode frequency match the observed wave frequency
Both radial ion temperature gradient and drift resonance of 20–25 keV protons serve as energy sources of the wave
During the magnetic storm starting on September 7, 2017, the MEP‐i instrument onboard the Arase (ERG) satellite observed molecular ions (O2+/NO+/N2+) in the ring current. The molecular ions were ...observed by Arase in four orbits during this magnetic storm. This indicates that there was a continuous molecular ion supply from the ionosphere. During the storm main phase around the second Dst minimum (∼−100 nT) on September 8, 2017, the European Incoherent Scatter (EISCAT) radar observed the ion upflow (∼50–150 m s−1) in the low‐altitude (250–350 km) ionosphere together with strong ion heating up to >2,000 K. The convective electric field derived from the electron heating observed by EISCAT at an altitude of approximately 110 km was also enhanced by a factor of 2. The observations suggest that the additional ion heating at low altitudes helps to cause the fast upflow and transport molecular ions upward. The flux decreases from 280 to 350 km altitudes due to the dissociative recombination was estimated to be approximately two orders of magnitude. This resulted in significant molecular ion flux remaining at 350 km altitude. These results suggest that the low‐altitude ion upflow caused by the ion frictional heating enables molecular ions to escape to space against rapid loss by the dissociative recombination.
Plain Language Summary
Molecular ions (O2+/NO+/N2+) in the ring current are sometimes observed during magnetic storms. These molecular ions come from the deep ionosphere and considered good tracers of escape mechanisms from the Earth's ionosphere to the magnetosphere. However, it has not been revealed how these molecular ions are transported upward, especially by ion upflows in the low‐altitude ionosphere (∼250–350 km). It is difficult to transport sufficient flux of molecular ions due to deceleration by the strong gravitational force and rapid decrease by dissociative recombination even during magnetic storms. In this paper, we report the analysis results of the fast ion upflow (∼100 m s−1) event in the low‐altitude ionosphere observed by the European Incoherent Scatter (EISCAT) radar, while the Arase (ERG) satellite observed molecular ions in the inner magnetosphere during the magnetic storm starting on September 7, 2017. The results suggest that ion frictional heating created the ion upflow and could be a source of outflows at higher altitudes in the ionosphere to supply the molecular ions into the magnetosphere.
Key Points
Arase satellite observed molecular ions in the ring current during the magnetic storm starting on September 7, 2017
An ion upflow from the deep ionosphere was observed with enhancements of electric field and ion temperature by EISCAT
The fast upflow caused by the ion frictional heating enables molecular ions to escape to space against dissociative recombination
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