BepiColombo is a joint mission between the European Space Agency, ESA, and the Japanese Aerospace Exploration Agency, JAXA, to perform a comprehensive exploration of Mercury. Launched on
20
th
...October 2018 from the European spaceport in Kourou, French Guiana, the spacecraft is now en route to Mercury.
Two orbiters have been sent to Mercury and will be put into dedicated, polar orbits around the planet to study the planet and its environment. One orbiter, Mio, is provided by JAXA, and one orbiter, MPO, is provided by ESA. The scientific payload of both spacecraft will provide detailed information necessary to understand the origin and evolution of the planet itself and its surrounding environment. Mercury is the planet closest to the Sun, the only terrestrial planet besides Earth with a self-sustained magnetic field, and the smallest planet in our Solar System. It is a key planet for understanding the evolutionary history of our Solar System and therefore also for the question of how the Earth and our Planetary System were formed.
The scientific objectives focus on a global characterization of Mercury through the investigation of its interior, surface, exosphere, and magnetosphere. In addition, instrumentation onboard BepiColombo will be used to test Einstein’s theory of general relativity. Major effort was put into optimizing the scientific return of the mission by defining a payload such that individual measurements can be interrelated and complement each other.
The brightness of aurorae in Earth's polar region often beats with periods ranging from sub-second to a few tens of a second. Past observations showed that the beat of the aurora is composed of a ...superposition of two independent periodicities that co-exist hierarchically. However, the origin of such multiple time-scale beats in aurora remains poorly understood due to a lack of measurements with sufficiently high temporal resolution. By coordinating experiments using ultrafast auroral imagers deployed in the Arctic with the newly-launched magnetospheric satellite Arase, we succeeded in identifying an excellent agreement between the beats in aurorae and intensity modulations of natural electromagnetic waves in space called "chorus". In particular, sub-second scintillations of aurorae are precisely controlled by fine-scale chirping rhythms in chorus. The observation of this striking correlation demonstrates that resonant interaction between energetic electrons and chorus waves in magnetospheres orchestrates the complex behavior of aurora on Earth and other magnetized planets.
Over the past few decades, numerous geochemical and geological observations of early Mars have suggested the presence of surface river systems during the late Noachian and early Hesperian. A large ...volume of liquid water combined with stable and long-term fluvial activity is required to explain these river systems, suggesting that early Mars must have experienced a long period during which the climate was sufficiently warm and wet for stable surface liquid water and precipitation to occur. We performed 3-dimensional climate simulations for the late Noachian and early Hesperian using a paleo-Mars global climate model (PMGCM), assuming a CO2/H2O/H2 atmosphere under the “Faint Young Sun” condition and using the assumed topography before the formation of the Tharsis load. Cases of surface pressures of up to 2 bar and obliquities of 20°, 40°, and 60° were investigated. We also developed a river transport model, CRIS (Catchment-based RIver Simulator), which enabled us to estimate global river discharge and the transport of bedload and suspended load sediments from the PMGCM output for a variety of sediment particle sizes.
Our simulation results revealed that CO2 atmospheres with surface pressures of above 2 bar and a few percent of H2 composition produced a hospitable surface environment that could support the stable liquid water on the surface for long periods. Annual precipitation increased with surface pressure, and intense precipitation occurred around the low to mid-latitudes, where the majority of the valley networks are currently observed. Our river model CRIS produced river runoff, which agrees with some parts of the observed valley distribution, and revealed that these valleys could be formed in ~104 years, which agrees with previous geological studies. However, our simulation results also suggested that Martian valleys were not all created as a result of precipitation-fed river activity, and other mechanisms such as seasonal or transient snow melting should be simultaneously considered to explain Martian valley networks.
•A GCM simulated the paleoclimate of Mars before Tharsis for multiple inclinations.•River transport processes on surface were simulated coupled with GCM precipitation.•Surface runoff for 40° obliquity agreed with the locations of valley networks.•Snow accumulation for 60° obliquity agreed with the locations of valley networks.•Future coupling between atmosphere, hydrosphere and cryosphere would be needed.
We report observations of a stellar occultation by Pluto on 2019 July 17. A single-chord high-speed (time resolution = 2 s) photometry dataset was obtained with a CMOS camera mounted on the Tohoku ...University 60 cm telescope (Haleakala, Hawaii). The occultation light curve is satisfactorily fitted to an existing atmospheric model of Pluto. We find the lowest pressure value at a reference radius of
r
= 1215 km among those reported after 2012. These reports indicate a possible rapid (approximately 21
−5
+4
% of the previous value) pressure drop between 2016, which is the latest reported estimate, and 2019. However, this drop is detected at a 2.4
σ
level only and still requires confirmation from future observations. If real, this trend is opposite from the monotonic increase of Pluto’s atmospheric pressure reported by previous studies. The observed decrease trend is possibly caused by ongoing N
2
condensation processes in the Sputnik Planitia glacier associated with an orbitally driven decline of solar insolation, as predicted by previous theoretical models. However, the observed amplitude of the pressure decrease is larger than the model predictions.
We present the results of a multi‐point and multi‐instrument study of electromagnetic ion cyclotron (EMIC) waves and related energetic proton precipitation during a substorm. We analyze the data from ...Arase (ERG) and Van Allen Probes (VAPs) A and B spacecraft for an event of 16 and 17 UT on December 1, 2018. VAP‐A detected an almost dispersionless injection of energetic protons related to the substorm onset in the night sector. Then the proton injection was detected by VAP‐B and further by Arase, as a dispersive enhancement of energetic proton flux. The proton flux enhancement at every spacecraft coincided with the EMIC wave enhancement or appearance. This data show the excitation of EMIC waves first inside an expanding substorm wedge and then by a drifting cloud of injected protons. Low‐orbiting NOAA/POES and MetOp satellites observed precipitation of energetic protons nearly conjugate with the EMIC wave observations in the magnetosphere. The proton pitch‐angle diffusion coefficient and the strong diffusion regime index were calculated based on the observed wave, plasma, and magnetic field parameters. The diffusion coefficient reaches a maximum at energies corresponding well to the energy range of the observed proton precipitation. The diffusion coefficient values indicated the strong diffusion regime, in agreement with the equality of the trapped and precipitating proton flux at the low‐Earth orbit. The growth rate calculations based on the plasma and magnetic field data from both VAP and Arase spacecraft indicated that the detected EMIC waves could be generated in the region of their observation or in its close vicinity.
Plain Language Summary
Electromagnetic ion cyclotron (EMIC) waves are believed to play a significant role in the dynamics of energetic protons and relativistic electrons in the Earth's magnetosphere. The properties of these waves are being intensively studied. We consider the conditions of the EMIC wave generation and the dynamics of the wave source during a substorm event using a unique configuration of three spacecraft (Arase and two Van Allen Probes). All spacecraft were at approximately the same distance from the Earth, forming a chain across the evening local time sector. Analyzing parameters of the wave generation obtained from in situ measured proton distribution function, we came to the conclusion that the waves could be generated within the substorm area, sometimes close to, but not necessary at the spacecraft location. As the substorm expands in longitude, the EMIC wave source exhibits a longitudinal drift. When substorm expansion stops, the wave generation region expands due to the magnetic drift of protons injected during the substorm. The observed wave properties show that the waves are able to precipitate energetic protons into the atmosphere. This is confirmed by observations of low orbiting satellites measuring proton precipitating fluxes.
Key Points
Westward propagation of the EMIC wave generation region is due to both the substorm expansion and azimuthal drift of injected protons
Strong pitch‐angle diffusion regime is confirmed by observations of proton fluxes at low altitude and the diffusion coefficient calculation
The diffusion coefficient maximum corresponds well to the energy range of the observed proton precipitation
Kawai et al. (2021) reported the first ground‐satellite conjugate observation of nighttime medium‐scale traveling ionospheric disturbances (MSTIDs), by analyzing measurements from an airglow imager ...at Gakona (geographic latitude: 62.39°N, geographic longitude: 214.78°E, magnetic latitude: 63.60°N) and the Arase satellite in the magnetosphere on 3 November 2018. The Arase satellite observed variations in both the polarization electric field and the electron density as the Arase footprint passed through the MSTID structures in the ionosphere. In this study, we investigated whether these electric field and density variations associated with MSTIDs at subauroral latitudes are always observed by Arase in the magnetosphere. We used three airglow imagers installed at Gakona, Athabasca (geographic latitude: 54.60°N, geographic longitude: 246.36°E, magnetic latitude: 61.10°N), and Kapuskasing (geographic latitude: 49.39°N, geographic longitude: 277.81°E, magnetic latitude: 58.70°N) and the Arase satellite. We found eight observations of MSTIDs conjugate with Arase. They indicate that electric field and density variations associated with MSTIDs are not always observed in the magnetosphere. These variations tend to be observed in the magnetosphere during geomagnetically quiet times and when the amplitude of the MSTID is large. We categorized the MSTIDs into those caused by plasma instabilities and gravity waves and found that the electric field and density variations can be observed in the magnetosphere for both types of MSTIDs.
Plain Language Summary
Medium‐scale traveling ionospheric disturbances (MSTIDs) involve the propagation of electron density perturbations in the ionosphere. Kawai et al. reported an event in which MSTIDs were generated in the magnetosphere‐ionosphere coupled system using an airglow imager at Gakona (62.39°N, 214.78°E, 63.60°MLAT) and the Arase satellite. This was the first conjugate observation of nighttime MSTIDs with a magnetospheric satellite. In this study, we investigate whether those electric field and density variations associated with MSTIDs are always observed by Arase in the inner magnetosphere. We found eight conjugate observations of nighttime MSTIDs using airglow imagers at Gakona, Athabasca (54.60°N, 246.36°E, 61.10°MLAT), and Kapuskasing (49.39°N, 277.81°E, 58.70°MLAT) and the Arase satellite. They indicate that electric field and density variations associated with MSTIDs are not always observed in the magnetosphere. These variations tend to be observed in the magnetosphere during geomagnetically quiet times and when the amplitude of the MSTID is large. We categorized the MSTIDs into those caused by plasma instabilities and gravity waves and found that the electric field and density variations can be observed in the magnetosphere for both types of MSTIDs.
Key Points
Electric field variations by medium‐scale traveling ionospheric disturbances (MSTIDs) are not always observed in the magnetosphere
Magnetosphere‐ionosphere coupling of MSTIDs depends on the Kp index and the amplitude of the MSTIDs
Arase observed electric field and electron density variations associated with MSTIDs by both plasma instabilities and gravity waves
We have developed the automatic detection scheme for upper hybrid resonance (UHR) frequency using a convolutional neural network (CNN) from the electric field spectra obtained by the plasma wave ...experiment (PWE) aboard Arase. In this paper, we investigate the practical capability of this scheme in terms of actual scientific use case. We find that the average error rate is below 7.8% when the wave frequency is above 30 kHz and the wave spectral intensity is less than 10−5 mV 2/m2/Hz. About 91% of the data obtained by the high‐frequency analyzer (HFA) aboard the Arase satellite satisfies these conditions. To improve the accuracy of the determined UHR frequencies in a wide frequency range, we used another receiver, the onboard frequency analyzer (OFA), which enables us to detect low‐frequency UHR emissions. We confirmed that the averaged error rate derived by the OFA spectra becomes better than that derived from the HFA spectra in a frequency range below 20 kHz. We report the performance of the UHR frequency determination under the different geomagnetic conditions. We find that the UHR frequency can be determined with good accuracy using the CNN from the frequency‐time diagram both during geomagnetically quiet and disturbed conditions. We conclude that the CNN‐based UHR frequency determination is a reliable method to derive the electron density along the satellite orbit through observations of UHR frequencies, and this method contributes to studies on dynamics of the plasmasphere.
Plain Language Summary
Determining the upper hybrid resonance (UHR) frequency is the most popular way to determine the quantitative electron density in space. The high‐frequency analyzer (HFA) and onboard frequency analyzer (OFA) aboard the Arase satellite measure electric field spectra at high‐ and low‐frequency ranges. The nominal time resolutions of the HFA and OFA spectra are 8 and 1 s, respectively. Determining the UHR frequency by conventional visual inspection requires huge resources for researchers; therefore, some automatic determination methods have been proposed in recent years. In this study, we evaluate the accuracy of UHR frequency determination by convolutional neural networks (CNN). We find that the averaged error rate of the automatically determined UHR frequency is less than 7.8% for the most events (91% of the data set), and we conclude that the CNN‐based UHR frequency determination is the reliable method to estimate the electron density along the satellite orbit.
Key Points
We investigated the practical capability of the CNN‐based UHR frequency detection and found that the error is below 7.8% for most events
We successfully improved the accuracy of UHR frequency determination in a low‐frequency range by combining the OFA spectra
We confirmed that the UHR emissions in plasmasphere can be determined with good accuracy both during quiet and disturbance periods
Auroral brightening is one of the most common phenomena that occur during substorm onset and is usually recognized as a projection of the substorm‐associated magnetospheric plasma dynamics to the ...ionosphere. However, electromagnetic fields and plasma features associated with the substorm brightening arc have not been well understood. In this study, we present a comprehensive observation of the source plasma and field variations of a substorm brightening aurora in the inner magnetosphere. We performed a unique conjugate observation of a substorm brightening auroral arc observed by a ground‐based camera and by the Arase satellite in the magnetospheric source region at L ∼ 6. The event was observed at Tromsø (69.6°N, 19.2°E), Norway, on 12 October 2017. The brightening arc indicates east‐west structures with longitudinal scales of ∼0.5°–2.0°. Field‐aligned bi‐directional electrons with an energy range between 66 and 1,800 eV were detected by the satellite, simultaneously with the appearance of the brightening arc in the camera. These electrons were probably supplied from the auroral brightening region in the ionosphere, indicating that the satellite was on the same field line of the brightening aurora. The magnetic and electric field data show characteristic fluctuations and earthward Poynting flux around the time that the satellite crossed the aurora. Anti‐phase oscillations between the thermal pressure and the magnetic pressure are also reported. Based on these observations, we suggest the possibility that a ballooning instability occurred in the source region of the substorm brightening arc in the inner magnetosphere at L ∼ 6.
Plain Language Summary
A frequently occurring source of variations in the magnetosphere is the substorm, a process that causes energy dissipation into the atmosphere. Substorm is presented as the development of aurorae at high latitudes in the ionosphere. The study of substorm processes helps in understanding the near‐Earth space environment and the space weather. Along Earth's magnetic field lines, the aurora at a latitude of ∼65°N can be traced to ∼4–7 Earth radii away from the Earth at the equatorial plane in space. Using a ground‐based auroral camera, we can construct the correspondence between auroral motion and field and plasma variation at the satellite. This study reports such a unique event of substorm brightening arc observed at Tromsø, Norway, on 12 October 2017. Satellite observed bi‐directional electrons prove the connection between aurora break‐up at ∼100 km altitude and its source region in the magnetosphere at ∼30,000 km away from Earth. Based on the magnetic wave spectrograms, auroral bead‐like structures and other observational results, we suggest the possibility that a ballooning plasma instability occurred in the source region of the substorm brightening arc in the inner magnetosphere.
Key Points
Observation of plasma and field features in the source region of a sudden brightening auroral arc during a minor substorm onset at L ∼ 6
Energization of particles, field‐aligned electrons, and electromagnetic field fluctuations were observed during the arc crossing by Arase
Several observational facts indicate the possibility of ballooning instability occurring at this substorm onset
The dark colors of Jupiter's North Equatorial Belt (NEB, 7-17degN) appeared to expand northward into the neighboring one in 2015, consistent with a 35 year cycle. Inversions of thermal-IR imaging ...from the Very Large Telescope revealed a moderate warming and reduction of aerosol opacity at the cloud tops at 17-20degN, suggesting subsidence and drying in the expanded sector. Two new thermal waves were identified during this period: (i) an upper tropospheric thermal wave (wave number 16-17, amplitude 2.5 K at 170 mbar) in the mid-NEB that was anticorrelated with haze reflectivity; and (ii) a stratospheric wave (wave number 13-14, amplitude 7.3 K at 5 mbar) at 20-30degN. Both were quasi-stationary, confined to regions of eastward zonal flow, and are morphologically similar to waves observed during previous expansion events.
Over‐Darkening of Pulsating Aurora Hosokawa, K.; Miyoshi, Y.; Oyama, S.‐I. ...
Journal of geophysical research. Space physics,
April 2021, Letnik:
126, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Recent analyses of high‐time resolution ground‐based optical observations of pulsating aurora (PsA) have reported that the brightness of PsA sometimes decreases below the diffuse background level ...immediately after the ON phase of the main pulsation finishes. To date, however, the generation mechanism of such an “over‐darkening PsA” is still unclarified. In this study, we investigated the characteristics of the over‐darkening PsA by using simultaneous observations of PsA with an electron multiplying charge coupled device all‐sky camera in Sodankylä, Finland and the Arase satellite. During one of the conjunction events in Scandinavia on March 29, 2017, almost all the PsA pulses showed clear over‐darkening characteristics. By analyzing the 2D all‐sky images at the times of over‐darkening we discovered that over‐darkening areas appeared in the trailing edge of PsA patches and moved in tandem with the poleward propagating patches. It was also found that similar over‐decreasing characteristics were not seen in the chorus data from the wave instruments onboard Arase located at the magnetospheric counterpart of PsA. These results indicate that the over‐darkening PsA is not caused by a temporal variation of chorus at a fixed point, but is produced by a propagation of over‐darkening area with PsA patches. That is, the over‐darkening PsA is a result of compounding effects of spatial structure and recurrent propagation of PsA. The mechanism creating the dark area is still unknown, but the existence of over‐darkening PsA suggests that the temporal variation of PsA is not always a perfect copy of the modulation of lower‐band chorus waves in the magnetosphere.
Plain Language Summary
Pulsating auroras (PsAs) are characterized by quasi‐periodic variations in the brightness whose period typically ranges from a few to a few tens of second. Coordinated ground/satellite observations in the last decade demonstrated that the main optical pulsation well correlates with the intensity modulation of electromagnetic wave called “chorus” in the magnetosphere. Recent optical observations of PsA using high‐speed cameras have reported that the brightness of PsA often decreases below the diffuse background level immediately after the ON phase of the optical pulsation. In this study, we investigate the characteristics of such “over‐darkening PsAs” by using simultaneous observations of PsA with an all‐sky camera in Finland and the magnetospheric satellite Arase. By analyzing the 2D all‐sky images of over‐darkening PsA on March 29, 2017, we discovered that over‐darkening areas appeared in the trailing edge of PsA patches and moved in tandem with the poleward propagating patches. Similar over‐decreasing characteristics were not identified in the chorus data from Arase located at the magnetospheric counterpart of PsA. These results indicate that the over‐darkening PsA is not a pure temporal variation of chorus at a fixed point, but a result of compounding effects of spatial structure and dynamical motion of PsA.
Key Points
Over‐darkening of pulsating aurora (PsA) was observed during an interval of conjugate observation with the Arase satellite
Corresponding over‐decreasing of chorus wave intensity was not seen in the wave data from Arase
Over‐darkening is caused by a passage of dark region on the trailing edge of the PsA patch across the sensing area
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