In situ observations and modeling work have confirmed that singly charged oxygen ions, O+, which are of Earth's ionospheric origin, are heated/accelerated up to >100 keV in the magnetosphere. The ...energetic O+ population makes a significant contribution to the plasma pressure in the Earth's inner magnetosphere during magnetic storms, although under quiet conditions, H+ dominates the plasma pressure. The pressure enhancements, which we term energization, are caused by adiabatic heating through earthward transport of source population in the plasma sheet, local acceleration in the inner magnetosphere and near‐Earth plasma sheet, and enhanced ion supply from the topside ionosphere. The key issues regarding stronger O+ energization than H+ are nonadiabatic local acceleration, responsible for increase in O+ temperature, and more significant O+ supply than H+, responsible for the increase in O+ density. Although several acceleration mechanisms and O+ supply processes have been proposed, it remains an open question what mechanism(s)/process(es) play the dominant role in stronger O+ energization. This review paper summarizes important previous spacecraft observations, introduces the proposed mechanisms/processes that generate O+‐rich energetic plasma population, and outlines possible scenarios of O+ pressure abundance in the Earth's inner magnetosphere.
Key Points
Summarizes previous in‐situ and remote observations of O+ energization
Discusses possible mechanisms of O+ energization
Suggests the importance of O+ energization on the substorm time scale (<30min)
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We estimate the reconnection rates for eight dayside magnetopause reconnection events observed by the Cluster spacecraft and compare them with the predictions of the Cassak‐Shay Formula (Rcs) Cassak ...and Shay (2007). The measured reconnection rate is determined by calculating the product of the inflow velocity and magnetic field in the magnetosheath inflow region. The predicted reconnection rate is calculated using the plasma parameters on both sides of the current layer, including the contributions of magnetosheath H+, magnetospheric hot H+ and O+, and magnetospheric cold ions. The measured reconnection rates show clear correlations with Rcs with an aspect ratio of 0.07. The O+ and cold ions can contribute up to ~30% of the mass density, which may reduce the reconnection rate for individual events. However, the variation of the reconnection rate is dominated by the variation of the magnetosheath parameters. In addition, we calculated the predicted reconnection rate using only magnetosheath parameters (Rsh). The correlation of the measured rate with Rsh was better than the correlation with Rcs, with an aspect ratio of 0.09. This might indicate deviations from the Cassak‐Shay theory caused by the asymmetric reconnection structure and kinetic effects of different inflow populations. A better aspect ratio is expected to be between the ones determined using Rcs and Rsh. The aspect ratio does not show a clear dependence on the O+ concentration, likely because the O+ contribution is too small in these events. The aspect ratio also does not show a clear correlation with density asymmetry or guide field.
Key Points
Measured magnetopause reconnection rates show clear correlations with the Cassak‐Shay formula
Observed magnetospheric hot O+ and cold ions contribute up to ~30% of mass density in reconnection
The diffusion region aspect ratio exhibits no clear correlation with O+ abundance or guide field
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Knowledge of the ion composition in the near-Earth’s magnetosphere and plasma sheet is essential for the understanding of magnetospheric processes and instabilities. The presence of heavy ions of ...ionospheric origin in the magnetosphere, in particular oxygen (O
+
), influences the plasma sheet bulk properties, current sheet (CS) thickness and its structure. It affects reconnection rates and the formation of Kelvin-Helmholtz instabilities. This has profound consequences for the global magnetospheric dynamics, including geomagnetic storms and substorm-like events. The formation and demise of the ring current and the radiation belts are also dependent on the presence of heavy ions. In this review we cover recent advances in observations and models of the circulation of heavy ions in the magnetosphere, considering sources, transport, acceleration, bulk properties, and the influence on the magnetospheric dynamics. We identify important open questions and promising avenues for future research.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Abstract
A study using Arase data gives the first observational evidence that the frequency drift of electromagnetic ion cyclotron (EMIC) waves is caused by cyclotron trapping. EMIC emissions play an ...important role in planetary magnetospheres, causing scattering loss of radiation belt relativistic electrons and energetic protons. EMIC waves frequently show nonlinear signatures that include frequency drift and amplitude enhancements. While nonlinear growth theory has suggested that the frequency change is caused by nonlinear resonant currents owing to cyclotron trapping of the particles, observational evidence for this has been elusive. We survey the wave data observed by Arase from March, 2017 to September 2019, and find the best falling tone emission event, one detected on 11th November, 2017, for the wave particle interaction analysis. Here, we show for the first time direct evidence of the formation of a proton hill in phase space indicating cyclotron trapping. The associated resonance currents and the wave growth of a falling tone EMIC wave are observed coincident with the hill, as theoretically predicted.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The ionosphere is one of the important sources for magnetospheric plasma, particularly for heavy ions with low charge states. We investigate the effect of solar illumination on the number flux of ion ...outflow using data obtained by the Fast Auroral SnapshoT (FAST) satellite at 3000–4150 km altitude from 7 January 1998 to 5 February 1999. We derive empirical formulas between energy inputs and outflowing ion number fluxes for various solar zenith angle ranges. We found that the outflowing ion number flux under sunlit conditions increases more steeply with increasing electron density in the loss cone or with increasing precipitating electron density (> 50 eV), compared to the ion flux under dark conditions. Under ionospheric dark conditions, weak electron precipitation can drive ion outflow with small averaged fluxes (~ 10
7
cm
−2
s
−1
). The slopes of relations between the Poynting fluxes and outflowing ion number fluxes show no clear dependence on the solar zenith angle. Intense ion outflow events (> 10
8
cm
−2
s
−1
) occur mostly under sunlit conditions (solar zenith angle < 90°). Thus, it is presumably difficult to drive intense ion outflows under dark conditions, because of a lack of the solar illumination (low ionospheric density and/or small scale height owing to low plasma temperature).
Graphical abstract
Electron heating in the magnetic reconnection exhaust is investigated with particle‐in‐cell simulations, space observations, and theoretical analysis. Spatial variations of the electron temperature ...(Te) and associated velocity distribution functions (VDFs) are examined and understood in terms of particle energization and randomization processes that vary with exhaust locations. Inside the electron diffusion region (EDR), the electron temperature parallel to the magnetic field (Te∥) exhibits a local minimum and the perpendicular temperature (Te⊥) shows a maximum at the current sheet midplane. In the intermediate exhaust downstream from the EDR and far from the magnetic field pileup region, Te⊥/Te∥ is close to unity and Te is approximately uniform, but the VDFs are structured: close to the midplane, VDFs are quasi‐isotropic, whereas farther away from the midplane, VDFs exhibit field‐aligned beams directed toward the midplane. In the far exhaust, Te generally increases toward the midplane and the pileup region, and the corresponding VDFs show counter‐streaming beams. A distinct population with low v∥ and high v⊥ is prominent in the VDFs around the midplane. Test particle results show that the magnetic curvature near the midplane produces pitch angle scattering to generate quasi‐isotropic distributions in the intermediate exhaust. In the far exhaust, electrons with initial high v∥ (v⊥) are accelerated mainly through curvature (gradient‐B) drift opposite to the electric field, without significant pitch angle scattering. The VDF structures predicted by simulations are observed in magnetotail reconnection measurements, indicating that the energization mechanisms captured in the reported simulations are applicable to magnetotail reconnection with negligible guide field.
Key Points
Exhaust e‐heating results from acceleration, gyrotropization, pitch angle scattering, and mixing
Magnetotail exhaust electron distributions confirm our results from theory and simulation
Pitch angle scattering is elucidated with theory and test particles to explain isotropization
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) on both the Van Allen Probes spacecraft is a time-of-flight versus total energy instrument that provided ion composition data over ...the ring current energy (∼7 keV to ∼1 MeV), and electrons over the energy range ∼25 keV to ∼1 MeV throughout the duration of the mission (2012 – 2019). In this paper we present instrument calibrations, implemented after the Van Allen Probes mission was launched. In particular, we discuss updated rate dependent corrections, possible contamination by “accidentals” rates, and caveats concerning the use of certain products. We also provide a summary of the major advances in ring current science, obtained from RBSPICE observations, and their implications for the future of inner magnetosphere exploration.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
In the inner magnetosphere, the Arase spacecraft has observed electromagnetic ion cyclotron (EMIC) emissions with both rising and falling frequencies. The instantaneous frequency analyses on the ...electromagnetic fields of the EMIC rising tone emission have been performed by the Hilbert‐Huang transform. The time variation of the instantaneous frequency shows a good agreement with the nonlinear theory for the frequency evolutions. Rapid instantaneous frequency modulation is also found during the rising tone emission. We estimate the peak‐to‐peak time of the fluctuation in the frequency and find that the fluctuation is caused around a half of the particle trapping time. From the motion of the phase‐bunched particle around the resonant velocity, it is expected that the nonlinear resonant current, which induces the falling frequency is formed in half the trapping time.
Plain Language Summary
The Arase spacecraft observed nonlinear EMIC rising and falling tone emissions in the inner magnetosphere. Instantaneous frequency of nonlinear EMIC rising tone emission is analyzed by Hilbert‐Huang Transformation. Fast frequency modulation of the rising tone emission is found in the instantaneous frequency, which can be caused by the phase‐bunched particles in the phase space.
Key Points
Typical nonlinear EMIC rising and falling tone emissions have been observed by the electric and magnetic field detectors onboard Arase
Instantaneous frequency of a rising tone emission is analyzed and compared with the nonlinear growth theory
EMIC waves have short‐time frequency modulations caused by phase‐bunched ion motion in the phase space
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We have performed a statistical study investigating how substorm triggering and unloading is affected by the heavy ion content of the magnetotail plasma sheet. During the substorm growth phase, ...magnetic flux is accumulated in the tail lobes until the magnetotail reaches an unstable state. A near‐Earth neutral line then forms, and this excess flux is reconnected. The increased lobe magnetic flux during the substorm growth phase increases the magnetopause flaring angle. As a result, a greater fraction of the solar‐wind dynamic pressure is observed in the tail lobes and plasma sheet. Therefore, the increase and decrease of the lobe magnetic flux can be monitored by observing the increase and decrease in the magnetotail pressure. Using Cluster data from 2001 to 2004, we have determined how the maximum pressure (or flaring angle) and the rate of change of pressure (or flaring angle) during substorms depend on the O+ content of the plasma sheet. In addition, we have estimated the maximum magnetic flux, and rate of change of the magnetic flux. Our results show that both the maximum tail pressure and the rate of change in the pressure are positively correlated with the amount of O+ in the plasma sheet. When the measurements are normalized to account for the external solar‐wind pressure and the different Cluster locations in the tail, the maximum accumulated flux and the unloading rate still correlate positively with the O+ density and O+/H+ ratio. This suggests that the additional O+ makes it more difficult to trigger the substorm onset, but once it is triggered, the unloading is faster. This could either indicate that the presence of O+ increases the reconnection rate, or that it initiates reconnection over a broader width of the tail.
Key Points
The heavy ion effects on substorm triggering and unloading rate were tested.The maximum accumulated flux and the unloading rate increase with the O+.More O+ prevent substorm triggering but speed the unloading.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The validation of the magnetically self‐consistent inner magnetospheric model RAM‐SCB developed at Los Alamos National Laboratory is presented here. The model consists of two codes: a kinetic ring ...current–atmosphere interaction model (RAM) and a 3‐D equilibrium magnetic field code (SCB). The validation is conducted by simulating two magnetic storm events and then comparing the model results against a variety of satellite in situ observations, including the magnetic field from Cluster and Polar spacecraft, ion differential flux from the Cluster/CODIF (Composition and Distribution Function) analyzer, and the ground‐based SYM‐H index. The model prediction of the magnetic field is in good agreement with observations, which indicates the model's capability of representing well the inner magnetospheric field configuration. This provides confidence for the RAM‐SCB model to be utilized for field line and drift shell tracing, which are needed in radiation belt studies. While the SYM‐H index, which reflects the total ring current energy content, is generally reasonably reproduced by the model using the Weimer electric field model, the modeled ion differential flux clearly depends on the electric field strength, local time, and magnetic activity level. A self‐consistent electric field approach may be needed to improve the model performance in this regard.
Key Points
The inner magnetosphere model RAM‐SCB can well reproduce the B field
The modeled ion flux shows dependence on the driving electric field model
The ring current dynamics is generally captured by the model
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