Electromagnetic ion cyclotron (EMIC) waves potentially cause precipitation loss of relativistic electrons from the outer radiation belt to the atmosphere through pitch angle scattering. However, the ...direct evidence of each EMIC wave element and burst of precipitation has not yet been reported. Here we show the temporal and spatial correspondence of the EMIC waves with relativistic electron precipitation (REP) during the geomagnetic storm of 27 March 2017. EMIC waves were observed at several stations in North America. REP was detected as a decrease of subionospheric radio amplitudes observed at Athabasca, Canada. When isolated proton aurora, observed at Athabasca, appeared on the radio propagation path, we found a good correspondence between the temporal variations of REP and EMIC waves, and REP preceded EMIC waves by 24 s. This time lag is consistent with the travel time difference between relativistic electrons and EMIC waves from the magnetospheric equatorial plane to the ionosphere.
Plain Language Summary
The flux of relativistic electrons in the outer radiation belt can vary in a time scale of hours to days during magnetically disturbed condition. One of the loss mechanisms of relativistic electrons is precipitation into the atmosphere due to interaction between electrons and plasma waves in the magnetosphere. Electromagnetic ion cyclotron waves, which are excited in the magnetosphere, are effective for causing precipitation loss of relativistic electrons. These waves can have rising spectral structure with an increase in frequency quasiperiodically. In this study, we made use of multiple ground‐based observations and confirmed where and when relativistic electrons precipitated due to scattering by these waves. For the first time, we found direct evidence of correspondence between each wave element with rising tone and each burst of electron precipitation. We suggest that the precipitation of relativistic electrons occurred when the intensity of waves increased in the period of a few tens of seconds to minutes.
Key Points
Relativistic electron precipitation and electromagnetic ion cyclotron waves were observed during the main phase of a geomagnetic storm
Isolated proton auroras appeared on the radio propagation paths on which relativistic electron precipitation was detected
We found a good correspondence between the time variations of relativistic electron precipitation and electromagnetic ion cyclotron waves
HISAKI (SPRINT-A) satellite is an earth-orbiting Extreme UltraViolet (EUV) spectroscopic mission and launched on 14 Sep. 2013 by the launch vehicle Epsilon-1. Extreme ultraviolet spectroscope ...(EXCEED) onboard the satellite will investigate plasma dynamics in Jupiter’s inner magnetosphere and atmospheric escape from Venus and Mars. EUV spectroscopy is useful to measure electron density and temperature and ion composition in plasma environment. EXCEED also has an advantage to measure spatial distribution of plasmas around the planets. To measure radial plasma distribution in the Jovian inner magnetosphere and plasma emissions from ionosphere, exosphere and tail separately (for Venus and Mars), the pointing accuracy of the spectroscope should be smaller than spatial structures of interest (20 arc-seconds). For satellites in the low earth orbit (LEO), the pointing displacement is generally caused by change of alignment between the satellite bus module and the telescope due to the changing thermal inputs from the Sun and Earth. The HISAKI satellite is designed to compensate the displacement by tracking the target with using a Field-Of-View (FOV) guiding camera. Initial checkout of the attitude control for the EXCEED observation shows that pointing accuracy kept within 2 arc-seconds in a case of “track mode” which is used for Jupiter observation. For observations of Mercury, Venus, Mars, and Saturn, the entire disk will be guided inside slit to observe plasma around the planets. Since the FOV camera does not capture the disk in this case, the satellite uses a star tracker (STT) to hold the attitude (“hold mode”). Pointing accuracy during this mode has been 20–25 arc-seconds. It has been confirmed that the attitude control works well as designed.
The reception properties of the Radio Wave Instrument (RWI) onboard JUICE (Jupiter Icy Moons Explorer) have been determined using numerical methods applied to a mesh‐grid model of the spacecraft. The ...RWI is part of the RPWI (Radio and Plasma Wave Investigation) and consists of three perpendicular dipoles mounted on a long boom. We determined their effective lengths vectors and capacitive impedances of 8–9 pF. We also investigated the change in effective antenna angles as a function of solar panel rotation and calculated the directivity of the antennas at higher frequencies up to the maximum frequency of 45 MHz of the receiver. We found that the RWI dipoles can be used for direction‐finding with an accuracy of 2° up to a frequency of 1.5 MHz. Additionally we calculated the influence of strong pulses from the JUICE active radar on RPWI and found that they should do no harm to its sensors and receivers.
Plain Language Summary
In this paper we calculate the reception properties of the antennas of the Radio Wave Instrument (RWI) onboard the JUICE (Jupiter Icy Moons Explorer) spacecraft using numerical computer simulations. The RWI is part of the RPWI (Radio and Plasma Wave Investigation) and consists of three perpendicular dipoles mounted on a long boom. With this antenna system the scientists want to determine the intensity, the polarization, and the incoming radio wave direction of Jovian radio emissions. This can only be properly done when the reception properties of the antennas are well known, and for this necessary calibration we calculate the so‐called effective length vector. It can describe the reception properties of an antenna, being constant (direction‐independent) when the wavelength is large compared to the dimensions of the spacecraft. The antenna pattern of an RWI dipole has a toroidal shape like a donut at lower frequencies, but gets multiple lobes at higher frequencies. We also calculated the influence of the strong pulses from the JUICE active radar on the RPWI sensors and receivers and found that no harm should be done to them.
Key Points
The antennas of the Radio Wave Instrument (RWI) onboard Jupiter Icy Moons Explorer were calibrated by numerical computer simulations
Results include the calculation of effective lengths vectors, antenna impedances, and high‐frequency characteristics
Strong active radar pulses should not harm the RWI and Langmuir probe receivers
Plagioclase feldspars are among the most prevalent minerals in the solar system, and are present in many chondritic and achondritic meteorite families. Nevertheless, spectral features of plagioclases ...have never been unambiguously and directly observed in remote observations of asteroids. We report here the detection of an absorption band at 12.2 m on Vesta spectra provided by ground-based spectral observations at the Subaru Telescope. This signature represents the first direct evidence of a widespread presence of crystalline Ca-rich plagioclase on Vesta and reveals that its regolith is comminuted to a very fine grain size, smaller than a few tens of microns, indicating that the mechanical brecciation process has been very effective. The crystalline nature of plagioclase strongly suggests that impacts alone cannot be the sole mechanism for regolith formation on Vesta and a milder process, such as thermal fatigue, should be invoked as an important and concomitant process Thermal fatigue should be considered a very effective process in regolith production and rejuvenation not only for near-Earth asteroids but even for large asteroids located in the main belt.
Recent studies suggest that electrons with energies up to several hundred keV precipitate into the atmosphere associated with pulsating aurora (PsA). It is debated the highest energy of precipitating ...electrons associated with PsA. Here we report for the first time that the energy extends to relativistic energies. PsA was observed by THEMIS all‐sky imagers during a substorm that occurred on 27 March 2017. Energetic electron precipitation was detected by very low frequency subionospheric propagation. We found similar time variations between the auroral intensity and perturbation of the received radio signal intensity when the PsA occurred on the radio path. The perturbation showed a short recovery time of ~2 s. The recovery time indicates relaxation from ionospheric modification due to energetic electron precipitation and depends on the stopping altitude of the electrons. The recovery time required a stopping altitude of 50–60 km and indicates that the PsA is accompanied by relativistic electron precipitation.
Plain Language Summary
Auroral patches frequently show periodic variations in brightness with periods from a few to a few tens of seconds. They are called pulsating auroras. Recent studies showed that pulsating auroras are caused by the scattering of auroral electrons by electric and magnetic waves in near‐Earth space. Theoretical studies predicted that not only the auroral electrons but very high‐energy electrons in radiation belts are scattered by the waves and precipitate into the atmosphere. To visualize the high‐energy electron precipitation, we used the ground‐based observation of man‐made very low‐frequency radio signals. We found, for the first time, that pulsations of the high‐energy electron precipitation are associated with the pulsating aurora. Because the very low frequency radio signal can propagate at long distances reflected between the ground and the lower edge of the ionosphere, radio broadcasts were used to adjust our watch, submarine communication, and so on. When the electron precipitations occur above a radio propagation path, they cause ionization in the lower ionosphere and modulation in the received radio signal. They can also contribute to a loss of the outer radiation belt during magnetic storms and have an impact on the chemistry of the middle atmosphere.
Key Points
Energetic electron precipitation was observed with subionospheric VLF propagation during a substorm on 27 March 2017
Similar time variations between the pulsating aurora and energetic electron precipitation were found
The short recovery time of the subionospheric perturbation indicates that pulsating aurora accompanied relativistic electron precipitation
► We developed a new model for the venusian mesosphere and thermosphere. ► In the venusian thermosphere, the subsolar-to-antisolar flow is dominant. ► Below about 90
km, a weak return flow of the ...subsolar-to-antisolar flow is driven. ► Kelvin wave propagates up to around 130
km in the venusian upper atmosphere. ► Kelvin wave causes the temporal variation of the O
2-1.27
μm nightglow emission.
We have developed a new general circulation model (GCM) for the venusian mesosphere and thermosphere (80-about 180
km). Our GCM simulations show that winds in the subsolar-to-antisolar direction (SS–AS) are predominant above about 90
km. A weak return flow of the SS–AS is seen below about 90
km. We performed GCM simulations imposing the planetary-scale waves (thermal tides, Rossby wave, and Kelvin wave) at the lower boundary. Although the diurnal and semidiurnal tides are damped below 95
km, the Rossby wave propagates up to around 130
km. However, the amplitude of the Rossby wave is too small (<1
m/s) to affect the general circulation. On the other hand, the Kelvin wave propagates up to about 130
km with a maximum zonal wind fluctuation of approximately 5.9
m/s on average. The amplitude of the Kelvin wave sometimes exceeds 10
m/s around the terminator. The Kelvin wave causes a temporal variation in the wind velocity at the altitude of the O
2-1.27
μm nightglow emission (about 95
km). Using a newly developed 1-D nightglow model and the composition distribution calculated from our GCM, we investigated the impact of the Kelvin wave on the nightglow distribution. Our results suggest that the Kelvin wave would cause temporal variations in the nightglow emission in the 23:50–00:20
LT region with an intensity of 1.1–1.3
MR and a period of approximately 4
days.
Jupiter’s magnetosphere is a strong particle accelerator that contains ultrarelativistic electrons in its inner part. They are thought to be accelerated by whistler-mode waves excited by anisotropic ...hot electrons (>10 kiloelectron volts) injected from the outer magnetosphere. However, electron transportation in the inner magnetosphere is not well understood. By analyzing the extreme ultraviolet line emission from the inner magnetosphere, we show evidence for global inward transport of flux tubes containing hot plasma. High-spectral-resolution scanning observations of the Io plasma torus in the inner magnetosphere enable us to generate radial profiles of the hot electron fraction. It gradually decreases with decreasing radial distance, despite the short collisional time scale that should thermalize them rapidly. This indicates a fast and continuous resupply of hot electrons responsible for exciting the whistler-mode waves.
•Cloud top structure of Venus was investigated from the Subaru/COMICS mid-IR images.•The rotations of the polar features in the two hemispheres are apparently in phase.•Varying non-solar-fixed ...features appear in the images after high-pass filtering.•The center-to-limb brightness temperatures have a systematic day–night asymmetry.•Cloud top height and cloud scale height were obtained from center-to-limb analysis.
We have investigated the cloud top structure of Venus by analyzing ground-based images taken at the mid-infrared wavelengths of 8.66μm and 11.34μm. Venus at a solar phase angle of ∼90°, with the morning terminator in view, was observed by the Cooled Mid-Infrared Camera and Spectrometer (COMICS), mounted on the 8.2-m Subaru Telescope, during the period October 25–29, 2007. The disk-averaged brightness temperatures for the observation period are ∼230K and ∼238K at 8.66μm and 11.34μm, respectively. The obtained images with good signal-to-noise ratio and with high spatial resolution (∼200km at the sub-observer point) provide several important findings. First, we present observational evidence, for the first time, of the possibility that the westward rotation of the polar features (the hot polar spots and the surrounding cold collars) is synchronized between the northern and southern hemispheres. Second, after high-pass filtering, the images reveal that streaks and mottled and patchy patterns are distributed over the entire disk, with typical amplitudes of ∼0.5K, and vary from day to day. The detected features, some of which are similar to those seen in past UV images, result from inhomogeneities of both the temperature and the cloud top altitude. Third, the equatorial center-to-limb variations of brightness temperatures have a systematic day–night asymmetry, except those on October 25, that the dayside brightness temperatures are higher than the nightside brightness temperatures by 0–4K under the same viewing geometry. Such asymmetry would be caused by the propagation of the migrating semidiurnal tide. Finally, by applying the lapse rates deduced from previous studies, we demonstrate that the equatorial center-to-limb curves in the two spectral channels give access to two parameters: the cloud scale height H and the cloud top altitude zc. The acceptable models for data on October 25 are obtained at H=2.4–4.3km and zc=66–69km; this supports previous results determined from spacecraft observations.
ABSTRACT Solar micro-type III radio bursts are elements of the so-called type III storms and are characterized by short-lived, continuous, and weak emissions. Their frequency of occurrence with ...respect to radiation power is quite different from that of ordinary type III bursts, suggesting that the generation process is not flare-related, but due to some recurrent acceleration processes around the active region. We examine the relationship of micro-type III radio bursts with coronal streamers. We also explore the propagation channel of bursts in the outer corona, the acceleration process, and the escape route of electron beams. It is observationally confirmed that micro-type III bursts occur near the edge of coronal streamers. The magnetic field line of the escaping electron beams is tracked on the basis of the frequency drift rate of micro-type III bursts and the electron density distribution model. The results demonstrate that electron beams are trapped along closed dipolar field lines in the outer coronal region, which arise from the interface region between the active region and the coronal hole. A 22 year statistical study reveals that the apex altitude of the magnetic loop ranges from 15 to 50 RS. The distribution of the apex altitude has a sharp upper limit around 50 RS suggesting that an unknown but universal condition regulates the upper boundary of the streamer dipolar field.
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