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Io has an atmosphere produced by volcanism and sublimation of frosts deposited around active volcanoes. However, the time variation of atomic oxygen escaping Io's atmosphere is not well known. In this paper, we show a significant increase in atomic oxygen around Io during a volcanic event. Brightening of Io's extended sodium nebula was observed in the spring of 2015. We used the Hisaki satellite to investigate the time variation of atomic oxygen emission around Io during the same period. This investigation reveals that the duration of atomic oxygen brightness increases from a volcanically quiet level to a maximum level during the same approximate time period of 30 days as the observed sodium brightness. On the other hand, the recovery of the atomic oxygen brightness from the maximum to the quiet level (60 days) was longer than that of the sodium nebula decreasing (40 days). Additionally, a dawn-dusk asymmetry of the atomic oxygen emission is observed. [ABSTRACT FROM AUTHOR]

Abstract: The radial transport of plasma in the Io torus has been considered to be controlled by interchange instability, whereas the time scale (–80 days) estimated from matching physical chemistry models to spectral observations was much larger than the scale ( day) expected from the process. Previous studies using a magnetospheric convection model suggested that a corotation lag associated with mass ejection from Io can suppress the plasma transport. However, essential processes of fluid dynamics, vortex formation and nonlinear mode interactions were not taken into account but have a critical role in transport and structure development due to plasma instabilities. We performed nonlinear simulations of interchange instability in the Io torus with a reduced magnetohydrodynamic model in two-dimensional dipole magnetic field coordinates. If a single azimuthal perturbation in density is initially given, pure finger-like patterns develop over day but are stabilized due to a strong gradient of magnetic fields. If the initial torus density increases, vortex patterns are spontaneously formed through a nonlinear mode coupling, resulting in a collapse of the fingers. In the case of a multiple perturbation, a merging of vortices appears and the torus plasma expands radially to . The radial transport time scale at the saturation stage of convection energies is estimated to be days in this case, while it is days in the single perturbation case. By including effects of the corotation lag, a radially striated structure is produced within , but the radial transport is still fast, days. We conclude that the radial transport could be controlled by the nonlinear effects of fluids and discuss implications for the ribbon-like structures inside the torus as well as a contribution of the corotation lag. [Copyright &y& Elsevier]

Abstract: Ulysses had a “distant encounter” with Jupiter in February 2004. The spacecraft passed from north to south, and it observed Jovian radio waves from high to low latitudes (from +80° to +10°) for few months during its encounter. In this study, we present a statistical investigation of the occurrence characteristics of Jovian quasi-periodic bursts, using spectral data from the unified radio and plasma wave experiment (URAP) onboard Ulysses. The latitudinal distribution of quasi-periodic bursts is derived for the first time. The analysis suggested that the bursts can be roughly categorized into two types: one having periods shorter than 30min and one with periods longer than 30min, which is consistent with the results of the previous analysis of data from Ulysses’ first Jovian flyby [MacDowall, R.J., Kaiser, M.L., Desch, M.D., Farrell, W.M., Hess, R.A., Stone, R.G., 1993. Quasi-periodic Jovian radio bursts: observations from the Ulysses radio and plasma wave. Experiment. Planet. Space Sci. 41, 1059–1072]. It is also suggested that the groups of quasi-periodic bursts showed a dependence on the Jovian longitude of the sub-solar point, which means that these burst groups are triggered during a particular rotational phase of the planet. Maps of the occurrence probability of these quasi-periodic bursts also showed a unique CML/MLAT dependence. We performed a 3D ray tracing analysis of the quasi-periodic burst emission to learn more about the source distribution. The results suggest that the longitudinal distribution of the occurrence probability depends on the rotational phase. The source region of quasi-periodic bursts seems to be located at an altitude between 0.4 and 1.4 Rj above the polar cap region (L>30). [Copyright &y& Elsevier]

Abstract: Ulysses had a “distant encounter” with Jupiter when it was within 0.8AU of the planet during February, 2004. The passage of the spacecraft was from north to south, and observations of the Jovian radio waves were carried out for a few months from high to low latitudes (+80° to +10°) of Jupiter. The statistical study performed during this “distant encounter” event provided the occurrence characteristics of the Jovian broadband kilometric radiation (bKOM), including the high-latitude component as follows: (1) the emission intensity of bKOM was found to have a sinusoidal dependence with respect to the central meridian longitude (CML), showing a broad peak at ∼180°, (2) bKOM was preferably observed in the magnetic latitudinal range from ∼+30° to +90°, and the emission intensities at the high latitudes were found to be two times larger than that at the equatorial region, and (3) the emission intensity was controlled possibly by the sub solar longitude (SSL) of Jupiter. The intensity had a sharp peak around SSL ∼210°. A 3D ray tracing approach was applied to the bKOM in order to examine the source distribution. It was suggested that: (1) the R-X mode waves generated through the Cyclotron Maser Instability process would be unable to reproduce the intense high-latitude component of the bKOM, (2) the L-O mode, which was assumed to be generated at frequencies near the local plasma frequency, was considered to be the dominant mode for past and present observations at mid- and high-latitudinal regions, and (3) the high-latitude component of bKOM was found to have a source altitude of 0.9–1.5Rj (Rj: Jovian radii), and to be distributed along magnetic field lines having L>10. [Copyright &y& Elsevier]

Previous studies have shown that auroral kilometric radiation (AKR) can play an important role in the magnetosphere‐atmosphere coupling and has the right‐handed extraordinary (R‐X), left‐handed ordinary (L‐O) and left‐handed extraordinary (L‐X) modes. However, the L‐X mode has not been directly observed in the lower latitude magnetosphere yet, probably because of its very limited frequency range. Here, using observations of the Arase satellite on 6 September 2018, we present an AKR event with two distinct bands (8–20 and 300–1000 kHz) around the location: L = 8 and latitude = −37°. The low (high) band is identified as the L‐X (R‐X) mode based on the polarization and frequency ranges. Simulations of 3‐D ray tracing show that most of ray paths with 14 (11 and 18) kHz pass (miss) the location of Arase, basically consistent with observations. Our study provides direct evidence that the L‐X mode can propagate from high latitudes downward to lower latitudes. Plain Language Summary: Auroral kilometric radiation (AKR) is a widely existing radio emission with kilometric wavelength at the Earth, contributing to the magnetosphere‐atmosphere coupling. Similar emissions have been observed on all magnetic planets of the solar system. Previous studies have shown that AKR primarily occurs in the R‐X mode, with a small contribution in the L‐O and L‐X modes. The L‐X mode at lower latitudes has not been directly observed so far, most likely due to its extremely limited frequency range. Here, we present an L‐X mode (peak frequency ∼14 kHz) in the lower latitude magnetosphere observed by the Arase satellite. Using the 3‐D ray tracing method, we simulate ray paths with different initial wave parameters and source locations. Simulations show that ray paths with 14 (11 and 18) kHz pass (miss) the location of the Arase satellite and are highly dependent on initial wave parameters and the location of source. Our results provide a direct evidence that the L‐X mode from high latitude source regions can propagate downward to lower latitudes under suitable conditions. This study enriches the understanding of AKR propagation characteristics in the magnetosphere. Key Points: An auroral kilometric radiation (AKR) event with two distinct bands (8–20 kHz and 300–1000 kHz) is observed around the location: L = 8 and latitude = −37°Based on the polarization and frequency ranges, the low (high) band AKR is identified as the L‐X (R‐X) mode3‐D ray tracing simulations show that L‐X mode can propagate downward to lower latitudes, basically consistent with observations [ABSTRACT FROM AUTHOR]

Plasmaspheric hiss waves are important to shape the Earth's electron radiation belt. These waves are commonly envisioned to have a long lifetime which allows them to permeate the global plasmasphere from a spatially restricted source. However, this hypothesis has not been experimentally confirmed yet, because of the challenging observational requirements in terms of location and timing. With wave and particle measurements from five magnetospheric satellites and detailed modeling, we present the first report of long lifetime (∼42 s) hiss rays in the substorm‐disturbed plasmasphere. The low‐frequency hiss waves are found to originate from the middle piece of the plasmaspheric plume, bounce between two hemispheres, and eventually drift into the plasmaspheric core. These hiss rays can travel through ∼3 hr magnetic local time and ∼4 magnetic shell. Such a long‐time and large‐scale permeation of hiss rays could benefit from the ducting process by plasmaspheric field‐aligned density irregularities. Plain Language Summary: Earth's plasmasphere is populated by a type of whistler‐mode wave named plasmaspheric hiss which is able to shape the electron radiation belt. Hiss waves were commonly envisioned to have a long lifetime which allows them to permeate the global plasmasphere from a spatially restricted source. Although there have been numerous studies on the source of plasmaspheric hiss waves, the hypothesis regarding their long lifetime remains not experimentally confirmed yet because of the challenging observational requirements in terms of location and timing. On the basis of wave and particle measurements from five magnetospheric satellites covering the entire plasmasphere and detailed modeling, we show that the hiss rays can survive at least 42 s in the plasmasphere disturbed by substorms. Within the survival period, these hiss rays migrated from the middle piece of the plasmaspheric plume to the plasmaspheric core, whose path lengths reached 25 Earth radii. Such a long‐time and large‐scale permeation of hiss rays from the plasmaspheric plume to the plasmaspheric core could benefit from the ducting process by plasmaspheric field‐aligned density irregularities. Key Points: Low‐frequency hiss waves were excited by energetic electrons inside the dayside plasmaspheric plume following substormsLow‐frequency hiss rays survived at least 42 s, allowing themselves to migrate from the plasmaspheric plume to the plasmaspheric corePlasmaspheric density ducts facilitated the permeation of hiss rays from the plasmaspheric plume to the plasmaspheric core [ABSTRACT FROM AUTHOR]

This study presents a first simultaneous observation of N 2 + Meinel (0,0) band (hereafter, N 2 + (M)) aurora by cutting-edge short-wavelength infrared imaging spectrograph (NIRAS-2) and monochromatic camera (NIRAC) installed at the Kjell Henriksen Observatory (78 ∘ N, 16 ∘ E). On January 21 2023, N 2 + (M) intensification that is associated with a band-shape aurora structure was observed by the NIRAS-2 and the NIRAC having temporal resolutions of 30 s and 20 s, respectively. In addition, the European incoherent scatter Svalbard Radar also observed electron density variations at the same time. Electron density measured at altitude range from 100 km 120 km shows similar variations as of N 2 + (M) intensity, which implies that a primary source of N 2 + (M) emissions is direct collisions of N 2 by precipitating electrons penetrating down to around 100 km altitude (up to 10 keV). However, the observation also demonstrated moderate correlations between N 2 + (M) intensity and electron density above 140 km, which implies that different N 2 + (M) generation process, N 2 charge exchange with O + , may work up to near 160 km and make a non-negligible contribution to N 2 + (M) emissions. This hypothesis would be verified with further radar observations or stereo imaging observations useful to estimate the vertical distribution of the emission layers. The N 2 + (M) is a very promising target wavelength for aurora observation because the quality of sensors is highly expected to improve further and further. Continuous observations with our new instruments will undoubtedly provide an important information of N 2 + (M) characteristics, for future missions of both balloon-borne and satellite-borne imaging. [ABSTRACT FROM AUTHOR]

This study presents a first simultaneous observation of N 2 + Meinel (0,0) band (hereafter, N 2 + (M)) aurora by cutting-edge short-wavelength infrared imaging spectrograph (NIRAS-2) and monochromatic camera (NIRAC) installed at the Kjell Henriksen Observatory (78 ∘ N, 16 ∘ E). On January 21 2023, N 2 + (M) intensification that is associated with a band-shape aurora structure was observed by the NIRAS-2 and the NIRAC having temporal resolutions of 30 s and 20 s, respectively. In addition, the European incoherent scatter Svalbard Radar also observed electron density variations at the same time. Electron density measured at altitude range from 100 km 120 km shows similar variations as of N 2 + (M) intensity, which implies that a primary source of N 2 + (M) emissions is direct collisions of N 2 by precipitating electrons penetrating down to around 100 km altitude (up to 10 keV). However, the observation also demonstrated moderate correlations between N 2 + (M) intensity and electron density above 140 km, which implies that different N 2 + (M) generation process, N 2 charge exchange with O + , may work up to near 160 km and make a non-negligible contribution to N 2 + (M) emissions. This hypothesis would be verified with further radar observations or stereo imaging observations useful to estimate the vertical distribution of the emission layers. The N 2 + (M) is a very promising target wavelength for aurora observation because the quality of sensors is highly expected to improve further and further. Continuous observations with our new instruments will undoubtedly provide an important information of N 2 + (M) characteristics, for future missions of both balloon-borne and satellite-borne imaging. [ABSTRACT FROM AUTHOR]

The Hunga Tonga–Hunga Ha'apai (HTHH) undersea volcanic eruption that occurred at 04:15 UT on 15 January 2022 is one of the most explosive events in the modern era, and a vertical plume reached approximately 55 km, corresponding to a height of the lower mesosphere. The intense explosion and subsequent plume generated acoustic and atmospheric gravity waves detected by ground-based instruments worldwide. Because a global-scale atmospheric and ionospheric response to the large volcanic eruption has not yet been observed, it provides a unique opportunity to promote interdisciplinary studies of coupling processes in lithosphere–atmosphere–ionosphere with ground-based and satellite observations and modeling. Further, this event allows us to elucidate the propagation and occurrence features of traveling ionospheric disturbances, the generation of equatorial plasma bubbles, the cause of electron density holes around the volcano, and the magnetic conjugacy of magnetic field perturbations. The most notable point among these studies is that the medium-scale travelling traveling ionospheric disturbances (MSTIDs) have magnetic conjugacy even in the daytime ionosphere and are generated by an external electric field, such as an E-region dynamo field, due to the motions of neutrals in the thermosphere. This advocates a new generation mechanism of MSTIDs other than the neutral oscillation associated with atmospheric gravity waves and electrified MSTIDs, which are frequently observed during daytime and nighttime, respectively. This paper reviews the recent studies of atmospheric and ionospheric disturbances after the HTHH volcanic eruption and summarizes what we know from this extreme event analysis. Further, we analyzed new datasets not shown in previous studies to give some new insights to understanding of some related phenomena. As a result, we also found that 4-min plasma flow oscillations caused by the acoustic resonance appeared with the amplitude of approximately 30 m/s in the northern hemisphere a few hours before the initial arrival of the air pressure waves. The propagation direction was westward, which is the same as that of the daytime MSTIDs with a magnetic conjugate feature. This result suggests that the 4-min oscillations are generated by an external electric field transmitted to the northern hemisphere along magnetic field lines. [ABSTRACT FROM AUTHOR]

The High Frequency Analyzer (HFA) is a subsystem of the Plasma Wave Experiment onboard the Arase (ERG) spacecraft. The main purposes of the HFA include (1) determining the electron number density around the spacecraft from observations of upper hybrid resonance (UHR) waves, (2) measuring the electromagnetic field component of whistler-mode chorus in a frequency range above 20 kHz, and (3) observing radio and plasma waves excited in the storm-time magnetosphere. Two components of AC electric fields detected by Wire Probe Antenna and one component of AC magnetic fields detected by Magnetic Search Coils are fed to the HFA. By applying analog and digital signal processing in the HFA, the spectrograms of two electric fields (EE mode) or one electric field and one magnetic field (EB mode) in a frequency range from 10 kHz to 10 MHz are obtained at an interval of 8 s. For the observation of plasmapause, the HFA can also be operated in PP (plasmapause) mode, in which spectrograms of one electric field component below 1 MHz are obtained at an interval of 1 s. In the initial HFA operations from January to July, 2017, the following results are obtained: (1) UHR waves, auroral kilometric radiation (AKR), whistler-mode chorus, electrostatic electron cyclotron harmonic waves, and nonthermal terrestrial continuum radiation were observed by the HFA in geomagnetically quiet and disturbed conditions. (2) In the test operations of the polarization observations on June 10, 2017, the fundamental R-X and L-O mode AKR and the second-harmonic R-X mode AKR from different sources in the northern polar region were observed. (3) The semiautomatic UHR frequency identification by the computer and a human operator was applied to the HFA spectrograms. In the identification by the computer, we used an algorithm for narrowing down the candidates of UHR frequency by checking intensity and bandwidth. Then, the identified UHR frequency by the computer was checked and corrected if needed by the human operator. Electron number density derived from the determined UHR frequency will be useful for the investigation of the storm-time evolution of the plasmasphere and topside ionosphere. [ABSTRACT FROM AUTHOR]

In the tenuous atmospheric bodies of our solar system, space weathering on the celestial surface is an important process for its chemical and physical evolution and ambient environment on timescales of celestial evolution. Space plasma is a dominant energy and material source for space weathering. Plasma irradiation experiment in the laboratory is an effective method for modeling space weathering driven by space plasma. However, comprehensive modeling of plasma space weathering has not yet been conducted because the capabilities of the earlier facilities were not optimized for realistic space weathering; for example, the incident electron and ion were not irradiated in the same condition. Here, we developed a plasma irradiation system, Plasma Irradiation Emulator for Celestial Environments (PIECE) of the solar system bodies, which reproduces plasma space weathering in tenuous atmospheric bodies by the electron and ion irradiations in the same condition. We successfully developed a system with high electron and ion number fluxes of ∼ 10 13 - 10 16 particles cm - 2 s - 1 at any acceleration energy in the range of 1–30 keV, which leads to a fluence of e.g., ∼ 10 18 - 10 21 particles cm - 2 s - 1 , with a 1-day irradiation time. This fluence corresponds to a plasma irradiation time of ~ 103–106 years on Europa. [ABSTRACT FROM AUTHOR]

Equatorial plasma bubbles are a phenomenon of plasma density depletion with small-scale density irregularities, normally observed in the equatorial ionosphere. This phenomenon, which impacts satellite-based communications, was observed in the Asia-Pacific region after the largest-on-record January 15, 2022 eruption of the Tonga volcano. We used satellite and ground-based ionospheric observations to demonstrate that an air pressure wave triggered by the Tonga volcanic eruption could cause the emergence of an equatorial plasma bubble. The most prominent observation result shows a sudden increase of electron density and height of the ionosphere several ten minutes to hours before the initial arrival of the air pressure wave in the lower atmosphere. The propagation speed of ionospheric electron density variations was ~ 480–540 m/s, whose speed was higher than that of a Lamb wave (~315 m/s) in the troposphere. The electron density variations started larger in the Northern Hemisphere than in the Southern Hemisphere. The fast response of the ionosphere could be caused by an instantaneous transmission of the electric field to the magnetic conjugate ionosphere along the magnetic field lines. After the ionospheric perturbations, electron density depletion appeared in the equatorial and low-latitude ionosphere and extended at least up to ±25° in geomagnetic latitude. [ABSTRACT FROM AUTHOR]

Abstract: Temporal variation of Jupiter's northern aurora during enhanced Io volcanic activity was detected using the EXCEED spectrometer on board the Hisaki Earth‐orbiting planetary space telescope. It was found that in association with reported Io volcanic events in early 2015, auroral power and estimated field‐aligned currents were enhanced during day of year 40–120. Furthermore, the far ultraviolet color ratio decreased during the event, indicating a decrease of auroral electron mean energy and total acceleration by <30%. During the episode of enhanced Io volcanic activity, Jupiter's magnetosphere contains more source current via increased suprathermal plasma density by up to 42%; therefore, it would have required correspondingly less electron acceleration to maintain the enhanced field‐aligned current and corotation enforcement current. Sporadic large enhancements in auroral emission detected more frequently during the active period could have been contributed by nonadiabatic magnetospheric energization. [ABSTRACT FROM AUTHOR]

Reconstruction of reliable plasmaspheric electron density profiles is important for understanding physical processes in the plasmasphere. This paper proposes a technique that can be applied to correct the plasmaspheric electron density profiles using ray tracing by scrutinizing dispersion analyses of lightning whistlers. The Global Core Plasma Model and the International Reference Ionosphere were introduced as a reference electron density profile. Modifications of this electron density profile were then proposed to satisfy the dispersion characteristics of lightning whistlers measured by satellites in the plasmasphere. We first introduce two kinds of functions to modify the electron density: constant and linear, the linear function is more adequate. We applied our method to two lightning whistler events on 14 August 2017, measured by the Plasma Wave Experiment/Waveform Capture aboard the Arase satellite, and analyzed the dispersion of the observed lightning whistlers. We show how the density modification affects the delay time of the ray path and satisfies the dispersion characteristics under the appropriate adjustments. [ABSTRACT FROM AUTHOR]

Bodies such as planets, moons, and asteroids in our solar system are the brightest objects in the low-frequency radio astronomy at ≲10 GHz. The low-frequency radio emissions from our solar system bodies exhibit various observed characteristics in the spectrum, polarization, periodicity, and flux. The observed characteristics are essential probes for explorations of the bodies' magnetosphere, atmosphere, surface, and even their interior. Generation and propagation theories of the radio emissions associate the characteristics with fundamental physics embedded in the environments: e.g., auroral electron acceleration, betatron acceleration, and atmospheric momentum transfer. Here we review previous studies on the low-frequency radio emissions from our solar system bodies to unveil some outstanding key questions on the dynamics and evolution of the bodies. To address the key questions by the future observations with the Square Kilometre Array (SKA), we made feasibility studies for detection and imaging of the radio emissions. Possible extensions of the solar system observations with SKA to the exoplanets are also proposed in the summary. [ABSTRACT FROM AUTHOR]

Recent studies have shown that auroral kilometric radiation (AKR) can propagate from the polar source cavity into the equatorial radiation belt but there is no direct evidence yet. By examining 19‐month data from Arase satellite and Van Allen Probes, we report a unique event that distinct AKR emissions are observed simultaneously at high and low latitudes, with the peak wave spectra in the frequency 300–400 kHz. Using the three‐dimensional ray tracing method, we simulate the AKR ray paths at three typical frequencies (220, 350, and 550 kHz), and show that more ray paths for 350 kHz can propagate downward through the locations of Arase satellite and Van Allen Probe B than those for other two frequencies. This result can successfully explain the observation and provides a direct evidence that AKR can propagate to low latitude radiation belt under suitable conditions. Plain Language Summary: Auroral kilometric radiation (AKR) is a widely existing electromagnetic wave in the magnetosphere and has potential for acceleration or precipitation loss of energetic electrons, yielding serious damage to orbiting space systems or ozone destruction in the atmosphere. Previous observation and numerical simulation have shown that AKR originated from the polar cavity region can be presented in the low latitude even in the equator. However, there is no direct observational evidence yet due to lack of the correlated data at higher and lower latitudes. Here, we examine observations of Arase satellite and Van Allen Probes from 25 March 2017 to 30 October 2018, and find a unique AKR event detected simultaneously by the Arase satellite (at high latitude) and Probe B (at low latitude). Using 3‐D ray tracing method, we simulate AKR paths for different frequencies (220, 350, and 550 kHz) with suitable wave normal angles and azimuthal angles. We show that more ray paths for 350 kHz can pass through the locations of Arase satellite and Probe B than those for other two frequencies, consistent with the observations. The current result provides a direct evidence that AKR can propagate to low latitude radiation belt under suitable conditions. Key Points: A unique auroral kilometric radiation (AKR) event is reported simultaneously by Arase satellite and Van Allen Probes during 19‐month observationsSimulations show that more AKR for 350 kHz pass the locations of two satellites, consistent with observationsAKR raypaths are highly dependent on wave frequency, initial wave normal angles and azimuthal angles [ABSTRACT FROM AUTHOR]

Auroral kilometric radiation (AKR) is generated at high latitudes and can propagate down to low latitudes. Due to the lack of direct observations, the characteristics of AKR in the middle and low latitudes of two hemispheres have not been studied so far. Here, using observations of the Arase satellite from 23 March 2017 to 31 July 2019, we present the first statistical study of AKR distribution in the northern (Magnetic latitude Mlat = 0°–40°) and southern (Mlat = −40°–0°) hemispheres. Results (totally 30,353 samples) show that relatively high occurrence rates (>30%) of AKR in the northern (southern) hemisphere primarily stay in the region of magnetic local time MLT = 17–24 (MLT = 21–05). About 60% of wave samples in the northern (southern) hemisphere are observed in the frequency range of ≤300 kHz (>300 kHz). The asymmetric distribution in two hemispheres can further enrich our understanding of AKR. Plain Language Summary: Auroral kilometric radiation (AKR) is a strong radio emission with kilometric wavelength at the Earth. They have a potential for accelerating electrons to relativistic energies or scattering electrons into the atmosphere, leading to serious damage to spacecrafts or ozone destruction. Because the parallel electric field contributing to AKR generation should be different in the northern and southern hemispheres, it is necessary to study the distribution characteristics of AKR in two hemispheres. Here, we examine the data of Arase satellite from 23 March 2017 to 31 July 2019, and find that the distributions of AKR samples in two hemispheres are asymmetric. The occurrence rate in the southern hemisphere is greater than that in the northern hemisphere. More AKR samples in the northern (southern) hemisphere occur from dusk to midnight (pre‐midnight to dawn). More AKR samples in the northern (southern) hemisphere are observed in the frequency range of ≤300 kHz (>300 kHz). This study provides more information about AKR in the magnetosphere. Key Points: The distribution characteristics of Auroral kilometric radiation (AKR) in two hemispheres are presented for the first time based on observations from Arase satelliteAKR samples in the northern (southern) hemisphere are observed relatively frequently in the region of magnetic local time = 17–24 (MLT = 21–05)More AKR samples in the northern (southern) hemisphere stay in the low (high) frequency range [ABSTRACT FROM AUTHOR]

We report on the multi-frequency multi-epoch radio observations of the magnetar XTE J1810−197, which exhibited a radio outburst from 2018 December after its 10-year quiescent period. We performed quasi-simultaneous observations with VERA (22 GHz), Hitachi (6.9 GHz and 8.4 GHz), Kashima (2.3 GHz), and Iitate (0.3 GHz) radio telescopes located in Japan to trace the variability of the magnetar radio pulsations during the observing period from 2018 December 13 to 2019 June 12. The pulse width becomes narrower as the observing frequency goes higher, analogous to the general profile narrowing behavior of ordinary pulsars. When assuming a simple power law in the range of 2.3 GHz and 8.7 GHz, the radio spectrum of the magnetar goes steeper with the average spectral index 〈α〉 ≈ −0.85 for the first four months. The wide-band radio spectra gathered from our observations and the literature suggest that XTE J1810−197 would have a double-peaked spectrum with a valley point in 22–150 GHz, where the first spectral peak implies a gigahertz-peaked spectrum (GPS) feature with a peak at a few GHz. The GPS and the high-frequency peak have been identified in the spectra of other radio-loud magnetars, thus they may be intrinsic features that can give a new insight to the understanding of various emission mechanisms and the surrounding environments of radio magnetars. Our study emphasizes the importance of simultaneous long-term broad-band observations toward radio-loud magnetars to capture the puzzling spectral features and establish a link to other types of neutron stars. [ABSTRACT FROM AUTHOR]

We report on the relationship between a pulsating aurora and a relativistic electron microburst using simultaneous observations of ground‐based fast auroral imagers with the FIREBIRD‐Ⅱ CubeSat for the first time. We conducted a detailed analysis of an event on October 8, 2018 and found that the occurrence of the pulsating aurora with internal modulations corresponds to the flux enhancement of electrons with energy ranging from ∼220 keV to >1 MeV detected with Flight Unit 4, one of FIREBIRD's CubeSat, with a time delay of ∼585 ms. Combining of this time delay result and time of flight model, we suggest that the pulsating aurora and the microburst occur simultaneously due to the chorus waves at different latitudes along the same field‐line as predicted by Miyoshi et al. (2020, https://doi.org/10.1029/2020gl090360). Plain Language Summary: A pulsating aurora is known as the aurora that blinks in seconds to tens of seconds and a microburst is the periodic and short durational precipitation of ∼MeV energy electrons. These two phenomena have similar spatiotemporal features such as their hierarchy structure in time and occurrence time from midnight to morning. Otherwise, the energy of the electrons is different between them. The pulsating aurora is generated by precipitation of ∼5–∼30 keV electrons. Recent theoretical study predicts that chorus waves propagating along the field line, which are the low frequency waves, generate ∼10 keV electron precipitation that causes the pulsating aurora and the microburst at the different latitudes on the same field line, that is, both pulsating aurora and microbursts are same product of the chorus wave particle interactions. However, there has been a lack of observational evidence. In this study, we detected the microbursts during the pulsating aurora from coordinated ground‐based and satellite observations for the first time. The observed timing of microburst electrons and the pulsating aurora are consistent with the theoretical model, that is, chorus waves propagating along the field line generate simultaneously the microbursts and low‐energy electron precipitation that cause the pulsating aurora along the same field‐line. Key Points: We identified relativistic electron microburst during a pulsating aurora for the first timeWe theoretically explain the detected time delay between the relativistic electron microburst and optical pulsationRelativistic to low‐energy electron precipitations are commonly caused by chorus waves propagating along the same field line [ABSTRACT FROM AUTHOR]

Auroral kilometric radiations (AKR) are strong radio emission phenomena, and can produce significant acceleration or scattering of radiation belt electrons. The variation of AKR wave amplitude with the latitude (λ) has not been reported so far owing to lack of measurements. Here, using observations of the Arase satellite and Van Allen Probes from March 23, 2017 to July 31, 2019, we present the first statistical study on the AKR electric field amplitude (Et) in the radiation belts for |λ| = 0° − 40° and L‐shell L = 3.0–6.2. Results (totally 14,770 samples) show that Et can be described by a concise formula: Et(λ) = E0 exp(ξ sin |λ|), decreasing with decreasing latitude. Fitting parameters E0 and ξ are limited in the ranges: E0 = 0.054–0.340 mV/m and ξ = 3.0–4.2. Wave amplitudes are greater (smaller) under intense (weak) geomagnetic conditions. This study helps to better quantify the gyroresonance between AKR and radiation belt electrons. Plain Language Summary: Auroral kilometric radiation (AKR) is a strong radio emission phenomenon which is generated in the low density polar cavity. They can propagate downward into radiation belts and produce significant acceleration or pitch angle scattering of energetic electrons. Those electrons can yield serious damage to spacecraft or ozone destruction. Because wave‐particle interaction generally occurs at each point along the field line and diffusion coefficients are directly proportional to the square of wave amplitude, it is necessary to specify the field‐aligned wave amplitude. However, previous calculations of AKR‐electron interaction were based on the assumption of the constant field‐aligned wave amplitude. Here, using observations of the Arase satellite and Van Allen Probes from March 23, 2017 to July 31, 2019, we report a field‐aligned distribution of AKR wave amplitude for the first time. Gaussian fitting for the electric field power spectral density is performed to obtain the wave amplitude of each AKR sample. We calculate averaged wave amplitudes of total 14,770 samples on the dayside and nightside. Results show that wave amplitude can be described by a concise formula, decreasing obviously with decreasing latitude. Wave amplitudes are greater (smaller) under intense (weak) geomagnetic condition. This paper provides the further understanding of AKR‐electron interaction. Key Points: The field‐aligned distribution of AKR wave amplitude is presented for the first time based on Arase satellite and Van Allen ProbesWave amplitudes can be approximately described by a concise exponential formula and decrease obviously with decreasing latitudeThe AKR sample and amplitude are roughly symmetric around the equator, wave amplitudes increase with increasing geomagnetic activities [ABSTRACT FROM AUTHOR]

Plasmaspheric hiss is an important whistler‐mode emission shaping the Van Allen radiation belt environment. How the plasmaspheric hiss waves are generated, propagate, and dissipate remains under intense debate. With the five spacecraft of Van Allen Probes, Exploration of energization and Radiation in Geospace (Arase), and Geostationary Operational Environmental Satellites missions at widely spaced locations, we present here the first comprehensive observations of hiss waves growing from the substorm‐injected electron instability, spreading within the plasmasphere, and dissipating over a large spatial scale. During substorms, hot electrons were injected energy‐dispersively into the plasmasphere near the dawnside and, probably through a combination of linear and nonlinear cyclotron resonances, generated whistler‐mode waves with globally drifting frequencies. These waves were able to propagate from the dawnside to the noonside, with the frequency‐drifting feature retained. Approximately 5 hr of magnetic local time away from the source region in the dayside sector, the wave power was dissipated to e−4 of its original level. Plain Language Summary: A noisy band of electromagnetic waves with frequencies ranging from tens of hertz to several kilohertz in the Earth's plasmasphere is termed plasmaspheric hiss. These waves are recognized to shape the Van Allen radiation belt environment and then affect the spacecraft survivability and lifetime. How the plasmaspheric hiss waves are generated, propagate, and dissipate has been a fundamental unanswered question since their discovery. With three space missions scattered in the inner magnetosphere, we present here the first comprehensive observations of hiss waves growing from the substorm‐injected electron instability, spreading within the plasmasphere, and dissipating over a large spatial scale. These findings have significant implications for the modeling of the plasmaspheric hiss waves and the Van Allen radiation belt dynamics. Key Points: Near the dawnside, energy‐dispersively injected electrons generated plasmaspheric hiss with globally drifting frequenciesThe frequency drift feature allowed tracing the plasmaspheric hiss propagation from the dawnside to the noonsideThe dayside plasmaspheric hiss dissipated away roughly within a magnetic local time span of 5 hr [ABSTRACT FROM AUTHOR]

Plasmaspheric hiss is an important whistler‐mode emission shaping the Van Allen radiation belt environment. How the plasmaspheric hiss waves are generated, propagate, and dissipate remains under intense debate. With the five spacecraft of Van Allen Probes, Exploration of energization and Radiation in Geospace (Arase), and Geostationary Operational Environmental Satellites missions at widely spaced locations, we present here the first comprehensive observations of hiss waves growing from the substorm‐injected electron instability, spreading within the plasmasphere, and dissipating over a large spatial scale. During substorms, hot electrons were injected energy‐dispersively into the plasmasphere near the dawnside and, probably through a combination of linear and nonlinear cyclotron resonances, generated whistler‐mode waves with globally drifting frequencies. These waves were able to propagate from the dawnside to the noonside, with the frequency‐drifting feature retained. Approximately 5 hr of magnetic local time away from the source region in the dayside sector, the wave power was dissipated to e−4 of its original level. Plain Language Summary: A noisy band of electromagnetic waves with frequencies ranging from tens of hertz to several kilohertz in the Earth's plasmasphere is termed plasmaspheric hiss. These waves are recognized to shape the Van Allen radiation belt environment and then affect the spacecraft survivability and lifetime. How the plasmaspheric hiss waves are generated, propagate, and dissipate has been a fundamental unanswered question since their discovery. With three space missions scattered in the inner magnetosphere, we present here the first comprehensive observations of hiss waves growing from the substorm‐injected electron instability, spreading within the plasmasphere, and dissipating over a large spatial scale. These findings have significant implications for the modeling of the plasmaspheric hiss waves and the Van Allen radiation belt dynamics. Key Points: Near the dawnside, energy‐dispersively injected electrons generated plasmaspheric hiss with globally drifting frequenciesThe frequency drift feature allowed tracing the plasmaspheric hiss propagation from the dawnside to the noonsideThe dayside plasmaspheric hiss dissipated away roughly within a magnetic local time span of 5 hr [ABSTRACT FROM AUTHOR]

A new type of terrestrial line spectra has been detected by the Arase satellite. These spectra are called hectometric line spectra. They consist primarily of constant frequency narrowband components at frequencies between 525 and 1,700 kHz, which originate and are sometimes amplified from AM broadcasting waves and other generated emissions. These line spectra can be amplified or generated by the odd‐harmonic electrostatic cyclotron instability while in the Z mode. Hectometric line spectra need to be mode‐converted to the L‐O mode to be received at higher altitudes because the Z mode can only exist when its frequency is lower than the local upper hybrid resonance frequency. Wave penetration into the ionosphere and mode conversion are enabled by low‐density regions and steep density gradients in ionospheric irregularities. Note that the broadcasting waves passed through the ionosphere with frequencies lower than the maximum plasma frequency along their paths. Plain Language Summary: A new type of terrestrial line spectra that consists primarily of constant frequency narrowband emissions at frequencies of 525 to 1,700 kHz and additional emissions with changing frequencies has been detected by the Arase (ERG) satellite. The former originate and are amplified from AM broadcasting waves in a similar frequency band. The latter are naturally generated emissions or emissions stimulated by broadcasting waves. These spectra are called hectometric line spectra because the wavelengths of the spectra are between 176 and 571 m. Broadcasting waves whose frequencies are lower than the maximum plasma frequency along their paths can penetrate into the ionosphere in the Z mode. Then, they can be amplified or generated by the odd‐harmonic electrostatic cyclotron instability while in the Z mode. Hectometric line spectra need to be mode‐converted to the L‐O mode (the free‐space mode) to be received at higher altitudes because waves in the Z mode are trapped in regions where their frequency is lower than the local upper hybrid resonance frequency. Key Points: New spectra with constant and some changing frequency lines with hectometric wavelengths have been detected by the Arase satelliteTheir frequency range is similar to the AM broadcast band, and most lines originate from broadcasting while others are naturalThese waves cannot normally pass through the ionosphere but require density depletion, mode conversion, and electrostatic instability [ABSTRACT FROM AUTHOR]

Abstract: The Arase spacecraft is capable of observing ultralow frequency waves in the inner magnetosphere at intermediate magnetic latitudes, a region sparsely covered by previous spacecraft missions. We report a series of impulsively excited fundamental toroidal mode standing Alfvén waves in the midnight sector observed by Arase outside the plasmasphere at magnetic latitudes 13–24°. The wave onsets are concurrent with Pi2 onsets detected by the Van Allen Probe B spacecraft at the magnetic equator in the duskside plasmasphere and by ground magnetometers at low latitudes. The duration of each toroidal wave packet is ∼20 min, which is much longer than that of the corresponding Pi2 wave packet. The toroidal waves cannot be the source of high‐latitude Pi2 waves because they were not detected on the ground near the magnetic field footprint of Arase. Overall, the toroidal wave event lasted more than 2 hr and allowed us to use the wave frequency to estimate the plasma mass density at L = 6.1–8.3. The mass density (in amu/cm3) is higher than the electron density (cm−3) by a factor of ∼6, which implies that 17–33% of the ions were O+. [ABSTRACT FROM AUTHOR]

Abstract: Temporal and spatial variations of the midlatitude ionospheric trough during a geomagnetic storm on 4 April 2017 have been investigated using Global Navigation Satellite System total electron content data together with Arase satellite observations. After the geomagnetic storm commencement, the trough minimum location moves equatorward from 60 to 48° in geomagnetic latitude within 4 hr. The trough minimum location identified from the Global Navigation Satellite System total electron content data is located near the footprint of an abrupt drop of electron density detected by the Arase High‐Frequency Analyzer instrument. The longitudinal variation of the trough minimum location shows a significant variation with a scale of 1,000–2,500 km during both storm and quiet times. This phenomenon has not yet been reported by previous studies. After the onset of the storm recovery phase, the trough minimum location rapidly moves poleward back to the quiet time location within 4 hr. [ABSTRACT FROM AUTHOR]

Extreme ultraviolet (EUV) spectra of Venus in the wavelength range 520 − 1480 Å with 3 − 4 Å resolutions were obtained in March 2014 by an EUV imaging spectrometer EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics) on the HISAKI spacecraft. Due to its high sensitivity and long exposure time, many new emission lines and bands were identified. Already known emissions such as the O II 834 Å, O I 989 Å, H I Ly − β 1026 Å, and the C I 1277 Å lines (Broadfoot et al. , 1974; Bertaux et al. , 1980; Feldman et al., 2000) are also detected in the EXCEED spectrum. In addition, N 2 band systems such as the Lyman-Birge-Hopfield ( a 1 Π g − X 1 Σ g + ) (2, 0), (2, 1), (3, 1), (3, 2) and (5, 3) bands, the Birge-Hopfield ( b 1 Π u − X 1 Σ g + ) (1, 3) band, and the Carroll-Yoshino ( c 4 ′ 1 Σ u + − X 1 Σ g + ) (0, 0) and (0, 1) bands together are identified for the first time in the Venusian airglow. We also identified the CO Hopfield-Birge ( B 1 Σ + − X 1 Σ + ) (1, 0) band in addition to the already known (0, 0) band, and the CO Hopfield-Birge ( C 1 Σ + − X 1 Σ + ) (0, 1), (0, 2) bands in addition to the already known (0, 0) band (Feldman et al., 2000; Gérard et al. , 2011). [ABSTRACT FROM AUTHOR]

This paper summarizes the specifications and initial evaluation results of Wire Probe Antenna (WPT) and Electric Field Detector (EFD), the key components for the electric field measurement of the Plasma Wave Experiment (PWE) aboard the Arase (ERG) satellite. WPT consists of two pairs of dipole antennas with ~ 31-m tip-to-tip length. Each antenna element has a spherical probe (60 mm diameter) at each end of the wire (15 m length). They are extended orthogonally in the spin plane of the spacecraft, which is roughly perpendicular to the Sun and enables to measure the electric field in the frequency range of DC to 10 MHz. This system is almost identical to the WPT of Plasma Wave Investigation aboard the BepiColombo Mercury Magnetospheric Orbiter, except for the material of the spherical probe (ERG: Al alloy, MMO: Ti alloy). EFD is a part of the EWO (EFD/WFC/OFA) receiver and measures the 2-ch electric field at a sampling rate of 512 Hz (dynamic range: ± 200 mV/m) and the 4-ch spacecraft potential at a sampling rate of 128 Hz (dynamic range: ± 100 V and ± 3 V/m), with the bias control capability of WPT. The electric field waveform provides (1) fundamental information about the plasma dynamics and accelerations and (2) the characteristics of MHD and ion waves in various magnetospheric statuses with the magnetic field measured by MGF and PWE-MSC. The spacecraft potential provides information on thermal electron plasma variations and structure combined with the electron density obtained from the upper hybrid resonance frequency provided by PWE-HFA. EFD has two data modes. The continuous (medium-mode) data are provided as (1) 2-ch waveforms at 64 Hz (in apoapsis mode, L > 4) or 256 Hz (in periapsis mode, L < 4), (2) 1-ch spectrum within 1-232 Hz with 1-s resolution, and (3) 4-ch spacecraft potential at 8 Hz. The burst (high-mode) data are intermittently obtained as (4) 2-ch waveforms at 512 Hz and (5) 4-ch spacecraft potential at 128 Hz and downloaded with the WFC-E/B datasets after the selection. This paper also shows the initial evaluation results in the initial observation phase. [ABSTRACT FROM AUTHOR]

Jupiter's moon Io, which orbits deep inside the magnetosphere, is the most geologically active object in the solar system. Kurdalagon Patera, a volcano on Io, erupted in 2015 and became a substantial source of Jovian magnetospheric plasma. Based on Earth-orbiting spacecraft observations, Io plasma torus (IPT) exhibited the peak intensity (nearly double) of ionic sulfur emissions roughly 2 month later, followed by a decay phase. This environmental change provides a unique opportunity to determine how the more heavily loaded magnetosphere behaves. Indeed, the extreme ultraviolet spectroscope for exospheric dynamics onboard the Earth-orbiting spacecraft Hisaki witnessed the whole interval via aurora and IPT observations. A simple-minded idea would be that the centrifugal force acting on fast co-rotating magnetic flux tubes loaded with heavier contents intensifies their outward transport. At the same time, there must be increased inward convection to conserve the magnetic flux. The latter could be accompanied by (1) increased inward velocity of field lines, (2) increased frequency of inward transport events, (3) increased inward flux carried per event, or (4) combinations of them. The Hisaki observations showed that the densities of major ions in the IPT increased and roughly doubled compared with pre-eruption values. The hot electron fraction, which sustains the EUV radiation from the IPT, gradually increased on a timescale of days. Pairs of intensified aurora and IPT brightening due to the enhanced supply of hot electrons from the mid-magnetosphere to the IPT upon aurora explosions observed during both quiet and active times, enabled the study of the mid-magnetosphere/IPT relationship. Hisaki observations under active Io conditions showed that: (1) the hot electron fraction in the torus gradually increased; (2) brightening pairs were more intense; (3) the energy supplied by the largest event maintained enhanced torus emission for less than a day; (4) the time delay of a torus brightening from a corresponding aurora intensification was roughly 11 h, that is, the same as during quiet times, suggesting that the inward convection speed of high-energy electrons does not change significantly. [ABSTRACT FROM AUTHOR]

We report a dawn-dusk difference of periodic variations of oxygen EUV dayglow (OII 83.4 nm, OI 130.4 nm and OI 135.6 nm) in the upper atmosphere of Venus observed by the Hisaki spacecraft in 2015. Observations show that the periodic dayglow variations are mainly controlled by the solar EUV flux. Additionally, we observed characteristic ∼1 day and ∼4 day periodicities in the OI 135.6 nm brightness. The ∼1 day periodicity was dominant on the duskside while the ∼4 day periodicity was dominant on the dawnside. Although the driver of the ∼1 day periodicity is still uncertain, we suggest that the ∼4 day periodicity is caused by gravity waves that propagate from the middle atmosphere. The thermospheric subsolar-antisolar flow and the gravity waves dominantly enhance eddy diffusion on the dawnside, and the eddy diffusion coefficient changes every ∼4 days due to large periodic modulations of wind velocity of the super-rotating atmosphere. Since the ∼4 day modulations on the dawnside are not continuously observed, it is possible that there is an intermittent coupling between the thermosphere and middle atmosphere due to variations of wave source altitudes. Moreover, if there are variations of the wind velocity in the mesosphere or lower thermosphere, it is possible that gravity waves occasionally propagate to the thermosphere even on the duskside due to periodic disappearance of the critical level and the ∼4 day periodic O atomic modulations occur. Thus, our observations imply that the ∼4 day periodicity of the EUV dayglow may reflect the dynamics of the middle atmosphere of Venus. We also examined the effects of the solar wind on the dayglow variations by shifting the solar wind measurements from earth to Venus. We did not find clear correlations between them. However, since there are no local measurements of the solar wind at Venus, the effect of the solar wind on the dayglow is still uncertain. [ABSTRACT FROM AUTHOR]

Chryse and Acidalia Planitiae (CAP) are known as one of the areas which have abundant sites of recurring slope lineae (RSL) on Mars. We present a radar survey of shallow subsurface structures across the CAP regions using the Mars SHAllow RADar sounder (SHARAD) onboard the Mars Reconnaissance Orbiter (MRO). A total of 25 subsurface reflectors were identified. The detected reflectors do not constitute apparent subsurface structures larger than 30 km. Because those have no counterparts in nearby tracks, we could not suggest that those are real subsurface structures. This study suggests that the CAP region does not have special wide-spread subsurface features which could be linked to the RSL sites. • We found subsurface reflectors at 25 locations across Chryse and Acidalia Planitiae, without structures larger than 30 km. • Because those have no counterparts in nearby tracks, we could not conclude that those are real subsurface structures. • Chryse and Acidalia Planitiae have abundant Recurring Slope Lineae, but the detected reflectors were not linked to them. [ABSTRACT FROM AUTHOR]

The Sprint-A satellite with the EUV spectrometer (Extreme Ultraviolet Spectroscope for Exospheric Dynamics: EXCEED) was launched in September 2013 by the Epsilon rocket. Now it is orbiting around the Earth (954.05 km×1156.87 km orbit; the period is 104 minutes) and one has started a broad and varied observation program. With an effective area of more than 1 cm and well-calibrated sensitivity in space, the EUV spectrometer will produce spectral images (520-1480 Å) of the atmospheres/magnetospheres of several planets (Mercury, Venus, Mars, Jupiter, and Saturn) from the Earth's orbit. At the first day of the observation, EUV emissions from the Io plasma torus (mainly sulfur ions) and aurora (H Lyman and Werner bands) of Jupiter have been identified. Continuous 3-month measurement for Io's plasma torus and aurora is planned to witness the sporadic and sudden brightening events occurring on one or both regions. For Venus, the Fourth Positive (A Π-X Σ) system of CO and some yet known emissions of the atmosphere were identified even though the exposure was short (8-min). Long-term exposure from April to June (for approximately 2 months) will visualize the Venusian ionosphere and tail in the EUV spectral range. Saturn and Mars are the next targets. [ABSTRACT FROM AUTHOR]

Io's atmosphere is predominately SO 2 that is sustained by a combination of volcanic outgassing and sublimation. The loss from the atmosphere is the main mass source for Jupiter's large magnetosphere. Numerous previous studies attributed various transient phenomena in Io's environment and Jupiter's magnetosphere to a sudden change in the mass loss from the atmosphere supposedly triggered by a change in volcanic activity. Since the gas in volcanic plumes does not escape directly, such causal correlation would require a transient volcano-induced change in atmospheric abundance, which has never been observed so far. Here we report four observations of atmospheric SO 2 and NaCl from the same hemisphere of Io, obtained with the IRAM NOEMA interferometer on 11 December 2016, 14 March, 6 and 29 April 2017. These observations are compared to measurements of volcanic hot spots and Io's neutral and plasma environment. We find a stable NaCl column density in Io's atmosphere on the four dates. The SO 2 column density derived for December 2016 is about 30% lower compared to the SO 2 column density found in the period of March to April 2017. This increase in SO 2 from December 2016 to March 2017 might be related to increasing volcanic activity observed at several sites in spring 2017, but the stability of the volcanic trace gas NaCl and resulting decrease in NaCl/SO 2 ratio do not support this interpretation. Observed dimmings in both the sulfur ion torus and Na neutral cloud suggest rather a decrease in mass loading in the period of increasing SO 2 abundance. The dimming Na brightness and stable atmospheric NaCl furthermore dispute an earlier suggested positive correlation of the sodium cloud and the hot spot activity at Loki Patara, which considerably increased in this period. The environment of Io overall appears to be in a rather quiescent state, preventing further conclusions. Only Jupiter's aurora morphology underwent several short-term changes, which are apparently unrelated to Io's quiescent environment or the relatively stable atmosphere. • First sub-mm study of Io's atmospheric SO2 and NaCl over a period of several months. • Comparison of the atmosphere results to data from volcanic hot spots and environment. • The NaCl mixing ratio appears to be not correlated with Loki's thermal brightness. [ABSTRACT FROM AUTHOR]

This study investigates the long-term variation of Saturn Kilometric Radiation (SKR) intensity observed by the Radio and Plasma Wave Science (RPWS) instrument on board the Cassini spacecraft from 2004 (southern summer) to 2017 (northern summer). The results show that the SKR intensity was brighter in the summer hemisphere than in the winter hemisphere, which was clearly seen in the south-to-north SKR intensity ratio. Over the long-term, the southern SKR intensity became 100 times smaller during northern summer, while the northern SKR intensity remained fairly constant. It means that the reversal in the intensity ratio was mainly caused by the long-term reduction of southern SKR intensity as Saturn's southern hemisphere moved from summer to winter, not the enhancement of northern SKR from winter to summer. We also investigated the possible contributions from the long-term solar EUV flux and solar wind dynamic pressure during the solar cycles 23 and 24, but we found that their impact on the SKR long-term variations was less than Saturn's seasonal changes associated with the variation of the tilt of its rotational axis with respect to the Sun. We further compared the long-term variation of the SKR intensity and the SKR period over half a Kronian year. The former showed more systematic variations which do not compare to those seen in the SKR periods. [ABSTRACT FROM AUTHOR]

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 AraseInstantaneous frequency of a rising tone emission is analyzed and compared with the nonlinear growth theoryEMIC waves have short‐time frequency modulations caused by phase‐bunched ion motion in the phase space [ABSTRACT FROM AUTHOR]

The Exploration of energization and Radiation in Geospace (ERG) project aims to study acceleration and loss mechanisms of relativistic electrons around the Earth. The Arase (ERG) satellite was launched on December 20, 2016, to explore in the heart of the Earth’s radiation belt. In the present paper, we introduce the specifications of the Plasma Wave Experiment (PWE) on board the Arase satellite. In the inner magnetosphere, plasma waves, such as the whistler-mode chorus, electromagnetic ion cyclotron wave, and magnetosonic wave, are expected to interact with particles over a wide energy range and contribute to high-energy particle loss and/or acceleration processes. Thermal plasma density is another key parameter because it controls the dispersion relation of plasma waves, which affects wave-particle interaction conditions and wave propagation characteristics. The DC electric field also plays an important role in controlling the global dynamics of the inner magnetosphere. The PWE, which consists of an orthogonal electric field sensor (WPT; wire probe antenna), a triaxial magnetic sensor (MSC; magnetic search coil), and receivers named electric field detector (EFD), waveform capture and onboard frequency analyzer (WFC/OFA), and high-frequency analyzer (HFA), was developed to measure the DC electric field and plasma waves in the inner magnetosphere. Using these sensors and receivers, the PWE covers a wide frequency range from DC to 10 MHz for electric fields and from a few Hz to 100 kHz for magnetic fields. We produce continuous ELF/VLF/HF range wave spectra and ELF range waveforms for 24 h each day. We also produce spectral matrices as continuous data for wave direction finding. In addition, we intermittently produce two types of waveform burst data, “chorus burst” and “EMIC burst.” We also input raw waveform data into the software-type wave-particle interaction analyzer (S-WPIA), which derives direct correlation between waves and particles. Finally, we introduce our PWE observation strategy and provide some initial results. [ABSTRACT FROM AUTHOR]

We developed the onboard processing software for the Plasma Wave Experiment (PWE) onboard the Exploration of energization and Radiation in Geospace, Arase satellite. The PWE instrument has three receivers: Electric Field Detector, Waveform Capture/Onboard Frequency Analyzer (WFC/OFA), and the High-Frequency Analyzer. We designed a pseudo-parallel processing scheme with a time-sharing system and achieved simultaneous signal processing for each receiver. Since electric and magnetic field signals are processed by the different CPUs, we developed a synchronized observation system by using shared packets on the mission network. The OFA continuously measures the power spectra, spectral matrices, and complex spectra. The OFA obtains not only the entire ELF/VLF plasma waves’ activity but also the detailed properties (e.g., propagation direction and polarization) of the observed plasma waves. We performed simultaneous observation of electric and magnetic field data and successfully obtained clear wave properties of whistler-mode chorus waves using these data. In order to measure raw waveforms, we developed two modes for the WFC, ‘chorus burst mode’ (65,536 samples/s) and ‘EMIC burst mode’ (1024 samples/s), for the purpose of the measurement of the whistler-mode chorus waves (typically in a frequency range from several hundred Hz to several kHz) and the EMIC waves (typically in a frequency range from a few Hz to several hundred Hz), respectively. We successfully obtained the waveforms of electric and magnetic fields of whistler-mode chorus waves and ion cyclotron mode waves along the Arase’s orbit. We also designed the software-type wave-particle interaction analyzer mode. In this mode, we measure electric and magnetic field waveforms continuously and transfer them to the mission data recorder onboard the Arase satellite. We also installed an onboard signal calibration function (onboard SoftWare CALibration; SWCAL). We performed onboard electric circuit diagnostics and antenna impedance measurement of the wire-probe antennas along the orbit. We utilize the results obtained using the SWCAL function when we calibrate the spectra and waveforms obtained by the PWE. [ABSTRACT FROM AUTHOR]

This paper presents detailed performance values of the Magnetic Search Coil (MSC) that is part of the Plasma Wave Experiment on board the Arase (ERG) satellite. The MSC consists of a three-axis search coil magnetometer with a 200-mm-long magnetic core. The MSC plays a central role in the magnetic field observations, particularly for whistler mode chorus and hiss waves in a few kHz frequency range, which may cause local acceleration and/or rapid loss of radiation belt electrons. Accordingly, the MSC was carefully designed and developed to operate well in harsh radiation environments. To ascertain the wave-normal vectors, polarizations, and refractive indices of the plasma waves in a wide frequency band, the output signals detected by the MSC are fed into the two different wave receivers: one is the WaveForm Capture/Onboard Frequency Analyzer for waveform and spectrum observations in the frequency range from a few Hz up to 20 kHz, and the other is the High Frequency Analyzer for spectrum observations in the frequency range from 10 to 100 kHz. The noise equivalent magnetic induction of the MSC is 20fT/Hz1/2 at a frequency of 2 kHz, and the null depth of directionality is − 40 dB, which is equivalent to an angular error less than 1∘. The MSC on board the Arase satellite is the first experiment using a current-sensitive preamplifier for probing the plasma waves in the radiation belts. [ABSTRACT FROM AUTHOR]