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This paper describes the plasma distribution solver (PDS), a theoretical model that determines plasma number density and pressure profiles along a magnetic field line of magnetized planets. The PDS computes velocity distribution functions of each particle species at an arbitrary point along a field line to meet the force balance and the accessibility. The number density, mean flow velocity, and pressure calculated by taking the zero‐th, first‐, second‐order moments of the velocity distribution functions enable us to obtain field‐aligned profiles of the quantities strictly consistent with the assumed ionospheric/magnetospheric boundary conditions. The spatial distribution of physical quantities, such as Alfvén speed and plasma β, was also obtained from the PDS results. We applied the PDS to the Jupiter‐Io system. When the temperature anisotropy is presented for the ion species originated from Io, the ion species other than protons, especially O+, can hardly reach mid‐to‐high latitudes. The number density of electrons which satisfy the charge neutrality with ions is also reduced compared to the case when the temperature is assumed to be isotropic. From these differences in the number density profile, we found the change of the boundary position, which characterizes dispersive Alfvén waves from the relation between the plasma β and the mass ratio between electrons and ions. Key Points: We developed a solver to calculate the number density and the plasma pressure profiles along the magnetic field line of planetsThese profiles satisfy the ionospheric/magnetospheric boundary conditions strictlyThe profiles are derived from the velocity distribution of electrons and multi‐species ions, considering the accessibility of particles [ABSTRACT FROM AUTHOR]

Inner magnetospheric electrons are precipitated in the ionosphere via pitch‐angle (PA) scattering by lower band chorus (LBC), upper band chorus (UBC), and electrostatic electron cyclotron harmonic (ECH) waves. However, the PA scattering efficiency of low‐energy electrons (0.1–10 keV) has not been investigated via in situ observations because of difficulties in flux measurements within loss cones at the magnetosphere. In this study, we demonstrate that LBC, UBC, and ECH waves contribute to PA scattering of electrons at different energy ranges using the Arase (ERG) satellite observation data and successively detected the moderate loss cone filling, that is, approaching strong diffusion. Approaching strong diffusion by LBC, UBC, and ECH waves occurred at ∼2–20 keV, ∼1–10 keV, and ∼0.1–2 keV, respectively. The occurrence rate of the approaching strong diffusion by high‐amplitude LBC (>50 pT), UBC (>20 pT), and ECH (>10 mV/m) waves, respectively, reached ∼70%, ∼40%, and ∼30% higher than that without simultaneous wave activity. The energy range in which the occurrence rate was high agreed with the range where the PA diffusion rate of each wave exceeded the strong diffusion level based on the quasilinear theory. Key Points: The pitch‐angle scattering efficiencies by plasma waves are statistically investigated using in situ observationsLower band chorus waves caused approaching strong diffusion with the highest occurrence rate in the energy range of ∼2–20 keVElectrostatic electron cyclotron harmonic waves could contribute approaching strong diffusion in the ∼0.1–1 keV energy range [ABSTRACT FROM AUTHOR]

Intense lower band chorus waves are ubiquitous in the inner magnetosphere. Their properties have been modeled by various codes and investigated using measurements of many spacecraft missions. This study aims to compare simulated and observed properties of chorus waves. We present detailed comparisons between results from four different codes of nonlinear chorus wave generation and statistical observations from satellites, focusing on the fine structure of such chorus waves. We show that simulations performed with these different codes well reproduce the observed wave packet characteristics, although in somewhat complementary parameter domains as concerns wave packets sizes, amplitudes, and frequency sweep rates. In particular, simulations generate both the frequently observed short wave packets with high positive and negative frequency sweep rates, and the more rare long and intense packets with mainly rising tones. Moreover, simulations reproduce quantitatively both the increase of the size of the observed chorus wave packets with their peak amplitude, and the fast decrease of their frequency sweep rate as their size increases. This confirms the reliability of the main existing codes for accurately modeling chorus wave generation, although we find that initial conditions should be carefully selected to reproduce a given parameter range. Key Points: Four different codes of chorus wave nonlinear generation well reproduce the observed wave packet characteristicsThe simulations recover complementary domains of observed wave packets sizes, amplitudes, and frequency sweep ratesThe simulations reproduce the observed short wave packets with high positive and negative frequency sweep rates [ABSTRACT FROM AUTHOR]

The Magnetospheric Multiscale (MMS) spacecraft observed many enhancements of electromagnetic ion cyclotron (EMIC) waves in an event in the late afternoon outer magnetosphere. These enhancements occurred mainly in the troughs of magnetic field intensity associated with a compressional ultralow frequency (ULF) wave. The ULF wave had a period of ∼2–5 min (Pc5 frequency range) and was almost static in the plasma rest frame. The magnetic and ion pressures were in antiphase. They are consistent with mirror‐mode type structures. We apply the Wave‐Particle Interaction Analyzer method, which can quantitatively investigate the energy transfer between hot anisotropic protons and EMIC waves, to burst‐mode data obtained by the four MMS spacecraft. The energy transfer near the cyclotron resonance velocity was identified in the vicinity of the center of troughs of magnetic field intensity, which corresponds to the maxima of ion pressure in the compressional ULF wave. This result is consistent with the idea that the EMIC wave generation is modulated by ULF waves, and preferential locations for the cyclotron resonant energy transfer are the troughs of magnetic field intensity. In these troughs, relatively low resonance velocity due to the lower magnetic field intensity and the enhanced hot proton flux likely contribute to the enhanced energy transfer from hot protons to the EMIC waves by cyclotron resonance. Due to the compressional ULF wave, regions of the cyclotron resonant energy transfer can be narrow (only a few times of the gyroradii of hot resonant protons) in magnetic local time. Key Points: Electromagnetic ion cyclotron wave enhancements were detected in a compressional ultralow frequency (ULF) waveTroughs of magnetic field intensity of the ULF wave are preferential locations for the cyclotron resonant energy transferDue to compressional ULF wave, regions of the cyclotron resonant energy transfer can be narrow in magnetic local time [ABSTRACT FROM AUTHOR]

We report the properties of the ionospheric exit point and characteristics of several types of very low frequency waves, including chorus and quasiperiodic emissions, based on a comprehensive dataset of simultaneous observations between ground and space. Whistler-mode waves were observed at Kannuslehto (L = 5.5, KAN), Finland, and in the inner magnetosphere by the Japanese Arase satellite. During the 2017--2018 winter campaign, we found 13 cases showing one-to-one correspondence of wave spectra between KAN and Arase. This is the first time that such a large number of conjugated events have been reported at once. The duration of the events ranged from a few minutes up to 3 h, with 90% of events detected in the afternoon sector. While the occurrence rate is higher during daytime, this can also be related to a majority of the detected waves being quasiperiodic emissions, a known dayside phenomenon. Arase was usually located within 30° of the equator, at L *4--5, and detected mostly waves propagating at oblique angles (≥20°). Frequently, the ionospheric magnetic footprint of Arase was located equatorwards (south) from KAN, often in the same geographical area. We investigated the probable location of the ionospheric exit point of the waves from the location of the footprint of Arase and the angle of arrival of waves detected at KAN. Using density measurements at Arase we discuss magnetospheric wave propagation; we find that, in most cases, waves were unducted in their propagation from the satellite to the ground. [ABSTRACT FROM AUTHOR]

We report the properties of the ionospheric exit point and characteristics of several types of very low frequency waves, including chorus and quasiperiodic emissions, based on a comprehensive dataset of simultaneous observations between ground and space. Whistler‐mode waves were observed at Kannuslehto (L = 5.5, KAN), Finland, and in the inner magnetosphere by the Japanese Arase satellite. During the 2017–2018 winter campaign, we found 13 cases showing one‐to‐one correspondence of wave spectra between KAN and Arase. This is the first time that such a large number of conjugated events have been reported at once. The duration of the events ranged from a few minutes up to 3 h, with 90% of events detected in the afternoon sector. While the occurrence rate is higher during daytime, this can also be related to a majority of the detected waves being quasiperiodic emissions, a known dayside phenomenon. Arase was usually located within 30° of the equator, at L ∼4–5, and detected mostly waves propagating at oblique angles (≥20°). Frequently, the ionospheric magnetic footprint of Arase was located equatorwards (south) from KAN, often in the same geographical area. We investigated the probable location of the ionospheric exit point of the waves from the location of the footprint of Arase and the angle of arrival of waves detected at KAN. Using density measurements at Arase we discuss magnetospheric wave propagation; we find that, in most cases, waves were unducted in their propagation from the satellite to the ground.Key Points: First time such a large number (13) of space‐ground conjugated events of magnetospheric whistler‐mode ELF/VLF waves is reportedIonospheric exit point of the waves is usually located at least ∼250 km equatorwards (south) of ground station in Finland at L ∼5.5All conjugated events but one were observed on the afternoon side of the magnetosphere [ABSTRACT FROM AUTHOR]

Recent availability of a considerable amount of satellite and ground-based data has allowed us to analyze rare conjugated events where extremely low and very low frequency waves from the same source region are observed in different locations. Here, we report a quasiperiodic (QP) emission, showing one-to-one correspondence, observed by three satellites in space (Arase and the Van Allen Probes) and a ground station. The main event was on 29 November 2018 from 12:06 to 13:08 UT during geomagnetically quiet times. Using the position of the satellites we estimated the spatial extent of the area where the one-to-one correspondence is observed. We found this to be up to 1.21 Earth's radii by 2.26 hr MLT, in radial and longitudinal directions, respectively. Using simple ray tracing calculations, we discuss the probable source location of these waves. At ~12:20 UT, changes in the frequency sweep rate of the QP elements are observed at all locations associated with magnetic disturbances. We also discuss temporal changes of the spectral shape of QP observed simultaneously in space and on the ground, suggesting the changes are related to properties of the source mechanisms of the waves. This could be linked to two separate sources or a larger source region with different source intensities (i.e., electron flux). At frequencies below the low hybrid resonance, waves can experience attenuation and/or reflection in the magnetosphere. This could explain the sudden end of the observations at the spacecraft, which are moving away from the area where waves can propagate. [ABSTRACT FROM AUTHOR]

Chorus emissions composed of coherent whistler mode waves are responsible for pitch angle scattering of energetic electrons. This scattering is closely related to energetic electron precipitation into the atmosphere, contributing to pulsating auroras. Conventionally, energetic electrons are considered to satisfy the cyclotron resonance condition over the energy range of a few to tens of kiloelectron volts and are scattered toward the loss cone by waves. However, previous simulation studies indicate that low pitch angle electrons tend to be scattered away from the loss cone by coherent whistler mode waves. We examine the mechanism of anomalous trapping at low pitch angles, deriving a particle equation with low pitch angle assumptions. An additional term that is conventionally neglected represents the Lorentz force caused by the wave magnetic field and the parallel particle velocity. Therefore, due to the large v‖×Bw Lorentz force, low pitch angle electrons satisfying the cyclotron resonant condition are scattered away from the loss cone and effectively trapped by waves. We perform test particle simulations in a one‐dimensional dipole magnetic field with a whistler mode wave model and reproduce the anomalous trapping of electrons. The simulation results show that the majority of electrons at high and moderate pitch angles are scattered toward low pitch angle regions while low pitch angle electrons are strongly scattered toward high pitch angle regions. Consequently, a coherent chorus element produces a bump in the electron pitch angle distribution. Key Points: Coherent whistler mode waves cause anomalous trapping of electrons at low pitch anglesTest particle simulations demonstrate the theoretically predicted motion of electronsThe relation between wave intensity and electron flux near the loss cone is not proportional [ABSTRACT FROM AUTHOR]

Abstract: We carry out a series of self‐consistent electron hybrid code simulations for the dependence of chorus generation process on the temperature anisotropy and density of energetic electrons in the Earth's inner magnetosphere. We use the same magnetic field gradient in the simulation system and different temperature anisotropy AT for the initial distribution of energetic electrons at the magnetic equator. We conduct 6 sets of simulations for different AT from 4 to 9, changing the initial number density Nh of energetic electrons at the equator in each set of simulations. By analyzing the spectra obtained in the simulation results, we identify chorus elements with rising tones in the results for higher Nh but no distinct chorus in smaller Nh. We compare the simulation results with estimations of the threshold and optimum amplitude proposed by the nonlinear wave growth theory. We find that the chorus generation processes reproduced in the simulation results are consistently explained by the theoretical estimates. We also compare the simulation results with linear growth rates for all simulation runs. We find clear disagreement between the spectral characteristics of reproduced chorus and the predictions by the linear theory. The present study clarifies that the spectra of chorus are essentially different from those predicted by the linear theory and are determined fully by nonlinear processes of wave‐particle interactions in the chorus generation region. [ABSTRACT FROM AUTHOR]