Atsushi Yamazaki

We characterize ultraviolet (UV) contrasts of Venus at the cloud top level, ∼70 km altitude, at low latitudes. We define the contrast as the albedo difference between bright and dark points at low latitudes of a single image. Our target is small‐scale contrast features, spanning 60–80 km, as seen in the reflected UV images acquired at 283 and 365 nm in 2016–2021 by the UV Imager (UVI) aboard JAXA's Akatsuki Venus orbiter. The number of analyzed images is 4,288 at 283 nm and 4,286 at 365 nm. The mean solar phase angle dependence of UV contrasts shows peak values of 45.6 (12.5)% at 283 nm and 53.9 (14.1)% at 365 nm near 40° solar phase angle, while the data span 0°–80°. In general, 283‐nm contrasts are slightly smaller than those at 365 nm, by ∼7% on average. For the first time, we compare the phase angle dependence of contrasts at 283 and 365 nm, and show that the 365‐nm contrast decreases more rapidly with increasing phase angle from 40°: −0.44 (0.06)%/degree at 283 nm and −0.60 (0.07)%/degree at 365 nm. This slope difference may be attributed to differences in the vertical distribution of the UV absorbers; for example, the unknown absorber has a smaller scale height than that of the gas above 70 km (3 km). We confirm the dominant temporal variations in contrasts, with the well‐known 4–5‐day periodicity followed by month‐scale fluctuations. Because of strong temporal variations, we are unable to separate a possible local‐time dependence of contrasts in this study.

Ultraviolet (UV) emissions from molecular hydrogen in Saturn's atmosphere consist of bright auroral emission over the poles and disk‐wide airglow. The dayside disk emits substantial intensities (dayglow) previously detected by various instruments. Exploration of dayside disk emissions provides insight into the interaction between the solar flux and planetary atmosphere. The respective contributions from the dayglow and the aurorae has not yet been evaluated, and the daily variability of the dayglow over timescales of weeks remains somewhat unexplored. For three weeks in 2014, the low‐Earth orbiting telescope, Hisaki, monitored Saturn while Cassini concurrently observed Saturn's northern hemisphere from within the magnetosphere. The total emitted power varies by a factor of 2.26 over 23 days as observed by Hisaki, and 1.29 over 17 days by Cassini. Cassini data allow separation of the auroral power; the total power is dominated by that from the dayglow emission, with the northern aurora contributing ∼10%–26%. The disk‐wide dayglow component displays strong correlation with solar activity, with Pearson correlation coefficients >0.8. The Cassini data further show that the global power at longer wavelengths (155–162 nm) depends more strongly on the solar spectrum than shorter wavelengths (70–148 nm). The auroral contribution to the total emission (70–170 nm) is ∼2%–14%. It is concluded that the total power is dominated by the dayglow from the disk, with solar photons controlling the dayglow emission.

Fluorescence line diagnostics of the Fe K α line at ∼6.4 keV observed in both solar and stellar flares can constrain the latitude and size of the flare loop, even in the absence of imaging observations. However, they are hampered by the unresolved origin of stellar Fe K α lines; i.e., it is unclear which of two mechanisms—photoionization by hard X-ray photons or collisional ionization by nonthermal electrons—is the dominant process. We present clear evidence for the photoionization origin based on simultaneous far-ultraviolet (FUV) and soft X-ray observations of a superflare on the RS Canum Venaticorum–type star UX Arietis with the Extreme Ultraviolet Spectroscope for Exospheric Dynamics instrument (900−1480 Å) on board the Hisaki and Neutron Star Interior Composition Explorer (0.2−12 keV). The flare started at 22:50 UT on 2018 November 15 and released 2 × 10 ³⁶ erg in the 900−1480 Å band and 3 × 10 ³⁶ erg in the 0.3−4 keV band. The FUV emission, a proxy for nonthermal activity, peaked approximately 1.4 hr before the soft X-rays. In contrast, the Fe K α line, detected at a statistical significance of 5.3 σ with an equivalent width of 6 7 − 20 + 28 eV, peaked simultaneously with the thermal X-ray maximum rather than the nonthermal FUV peak—strongly supporting the photoionization hypothesis. Radiative transfer calculations, combined with the observed Fe K α line intensity, further support the photoionization scenario and demonstrate the potential of this line to provide the flare geometry.

Despite decades of exploration, Venus's climate system governing the atmosphere beneath the cloud top (70 km altitude) remains poorly understood due to the persistent thick cloud cover. Here, we present the first detailed characterization of temporal variability in the thermal structure of Venus's low‐latitude atmosphere from the sub‐cloud region to the mesosphere (45–80 km), using 257 vertical temperature profiles obtained from Venus Express and Akatsuki radio occultation measurements in 2006–2024. Remarkable decadal temperature variations of 10 K were observed below 55 km, far exceeding those of 2 K seen at comparable pressure levels in Earth's tropics. In terms of the relative variation, the change on Venus over a decade is 1.4%, which is larger than the corresponding value of 0.6% in Earth's tropical region, indicating that the variability on Venus is more pronounced. The variation in temperature partly correlates with that in the ultraviolet albedo of Venus's cloud, suggesting potential links to the solar heating, cloud physics, and photochemistry. Furthermore, the variations in temperature and geopotential height are nearly in phase with those in zonal wind speed at the cloud top, indicating a strong coupling between the atmospheric dynamics in the upper cloud and the underlying thermal structure. While the mechanisms behind the observed variations remain elusive, our findings reveal that Venus has a more pronounced intrinsic climate variability than Earth. To elucidate Venus's climate system, we need long‐term and continuous observations by successive future missions.

Nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) were investigated for wave parameters, occurrence, and seasonal–longitudinal climatology using 630 nm OI airglow from the Visible and Near‐Infrared Spectral Imager (VISI) aboard the International Space Station. Although MSTIDs are widely studied from the ground, their global behavior remains less understood due to limited geographic coverage. We present new insights into MSTID climatology derived from spaceborne measurements. VISI's two fields of view, pointing forward and backward from nadir, enable discrimination of ionospheric signals from ground‐based light contamination. Case studies show that the two views capture subtle structural differences consistent with an equatorward‐tilted vertical geometry of MSTIDs. Comparisons with simultaneous total electron content from Japanese nationwide global navigation satellite system receiver network and with all‐sky airglow imagers of the Optical Mesosphere Thermosphere Imagers network reveal that measured wave parameters—wavelength, wavefront orientation, phase speed, and amplitude—agree with established MSTID characteristics. Analysis of 3 years (2013–2015) of VISI data yielded 676 MSTID events across both hemispheres, exhibiting strong seasonal and longitudinal variations. Local summer maxima and activity enhancements appear in regions with high sporadic‐E (Es) occurrence. In the Northern Hemisphere, the primary peak occurs around the June solstice, with particularly high rates in the Asia–west Pacific sector. In the Southern Hemisphere, a semiannual pattern emerges, featuring a primary peak during the December solstice and a secondary peak during the June solstice in the same Asia–west Pacific sector. These findings suggest Es‐related E–F coupling plays a key role in MSTID generation, with conjugate‐hemisphere coupling further modulating global distribution.

And the unidentified UV absorber are major absorbers in the near‐ultraviolet region in the Venusian atmosphere and influence the climate system. We developed a method to retrieve and the imaginary part of the refractive index , a proxy for the unidentified absorber, from 15,002 pairs of 283 and images taken by the Ultraviolet imager (UVI) onboard Akatsuki. We obtained the distributions of and the unidentified absorber with high spatial resolution every 2 hours for about 6.5 years. We analyzed the local time and latitudinal distributions of and the unidentified absorber. The mixing ratio at the cloud top around altitude depends on latitude and local time, ranging from 80 to , within the range of previous observations. and the unidentified absorber are abundant at low latitudes and decrease toward mid‐latitudes, which is attributed to vertical transport by the Hadley circulation. The abundance is maximized around 14:30 at low latitudes; the distribution is explained by the vertical transport by the semi‐diurnal tide and consistent with the cloud‐top velocity distribution. The local time and latitude dependence of obtained is consistent with a three‐dimensional general circulation model involving photochemical reactions. We also derived the long‐term variations in and the unidentified absorber. increased from 2016 to 2019, showed no clear trend until early 2021, and decreased through 2022. No clear correlation was seen between the unidentified absorber and the mean super rotation speed, although it contributes to the solar heating rate.

Plasma originating from the satellite Io forms a dense plasma region known as the Io plasma torus (IPT) in the Jovian inner magnetosphere. Slightly inside the Io‐orbit is a distinct structure called the “ribbon,” where the plasma spreads along the magnetic field lines. The ribbon moved dawnward owing to the dawn‐to‐dusk electric field. Extreme ultraviolet (EUV) observations of the IPT showed that the electric field was enhanced under compressed conditions of the magnetosphere caused by solar wind. However, no reports have been published on the influence of solar wind on the radial position of the ribbon. Here, we show the correlation between the temporal variation in the ribbon's position and solar wind. We analyzed the visible ([SII] 6716, 6731 Å) IPT images observed by the Tohoku 60‐cm telescope (T60) and the EUV emissions observed by the Hisaki satellite. We found that the position of the ribbon shifted dawnward when the solar wind dynamic pressure was enhanced. The dawnward shift was more significant on the dawn side than on the dusk side, indicating that the change in the electric field was inhomogeneous. The simultaneous observations of T60 and the Hisaki satellite on 19–23 February 2016 indicated that the averaged intensity of the electric field derived from T60 was 3.9 ± 0.9 mV/m, consistent with that of 2.8 ± 1.2 mV/m derived from the Hisaki satellite. Our results demonstrate how solar wind affects the nonuniformity of the electric field in the inner magnetosphere.

Plasma originating from the satellite Io forms a dense plasma region known as the Io plasma torus (IPT) in the Jovian inner magnetosphere. Slightly inside the Io-orbit is a distinct structure called the “ribbon,” where the plasma spreads along the magnetic field lines. The ribbon moved dawnward owing to the dawn-to-dusk electric field. Extreme ultraviolet (EUV) observations of the IPT showed that the electric field was enhanced under compressed conditions of the magnetosphere caused by solar wind. However, no reports have been published on the influence of solar wind on the radial position of the ribbon. Here, we show the correlation between the temporal variation in the ribbon’s position and solar wind. We analyzed the visible ([SII] 6716, 6731 Å) IPT images observed by the Tohoku 60-cm telescope (T60) and the EUV emissions observed by the Hisaki satellite. We found that the position of the ribbon shifted dawnward when the solar wind dynamic pressure was enhanced. The dawnward shift was more significant on the dawn side than on the dusk side, indicating that the change in the electric field was inhomogeneous. The simultaneous observations of T60 and the Hisaki satellite on February 19–23, 2016 indicated that the averaged intensity of the electric field derived from T60 was 3.9±0.9 mV/m, consistent with that of 2.8±1.2 mV/m derived from the Hisaki satellite. Our results demonstrate how solar wind affects the nonuniformity of the electric field in the inner magnetosphere.

We present the simultaneous and conjugated auroral emission and particle data obtained by a low‐altitude polar‐orbiting micro‐satellite, Reimei, for elucidating their latitudinal distributions and variations in the nightside auroral oval. Here are reported a few notable examples of the Reimei observations with high time and spatial resolutions, namely ∼120 msec. and ∼1.2 km × 1.2 km for multispectral auroral images and 40 msec. for energy‐pitch angle distributions of electrons and ions with energies of 10 eV–12 keV, respectively. The auroral images show various fine‐scale auroral activities characterized by the following types of auroral forms and variations: faint bands, streaming multiple arcs, shearing arcs, and vortices/curls, which are typical of the latitudinal properties of auroras. The particle analyzer simultaneously observed various properties of electron energy‐pitch angle and latitudinal distributions, and their temporal variations, each of which corresponds to a type of the auroral activities. Their features are summarized below. Reimei repetitively observed inverted‐V signatures of low‐energy (<1 keV) field‐aligned electrons in addition to the higher‐energy (several keV) diffuse electrons in low‐latitude auroral oval. In more active regions at higher latitudes, the dominant energy flux responsible for the multiple‐arc emissions was carried by the well‐known inverted‐V electron precipitation. The rapidly rotating vortices or so‐called curls of fine‐scale discrete auroras near the poleward boundary of the auroral oval were closely associated with the significant energy fluxes of spiky field‐aligned electron bursts with energy‐time dispersions produced by dispersive Alfvén waves.

The 100% cloud coverage of Venus leads to its high reflectance of the solar irradiance. However, in the ultra-violet (UV) range Venus’ brightness is reduced due to the absorptions of an unknown absorber and SO2 gas. The unknown absorber is a mystery not only for its chemical composition, but also for its vertical location. This latter factor is an important constraint to estimate a solar heating rate. In this study, we use disk-integrated phase curves and estimate a possible vertical location of the unknown absorber, relative to the cloud top level. We also expand the same method to estimate the SO2 gaseous abundance at the cloud top level.We used 283 and 365-nm narrow filter images taken by the UV camera onboard Akatsuki (UVI, [1]). We have selected global Venus images with a sufficient signal over 3 years (2016-2018) to calculate the disk-integrated reflectance. Total ~6,000 images per filter have been collected in a broad range of solar phase angle, 0-155°, and we calculated mean phase curves at the two UV wavelengths as shown in Fig.1. We used a pre-conditioned backward Monte Carlos (PBMC) algorithm that solves the vector radiative transfer equation [2] to fit the observed mean phase curves. This PBMC was successfully used to analyze Venus’ phase curves in the visible range [3]. In this study, we assumed a global Venus cloud top structure with a 70-km cloud top level (unity tau) and a 4-km aerosol scale height, and changed a relative location of the unknown absorber layer from the assumed cloud top level. Cloud aerosols’ microphysical properties were taken from previous observation analysis [4,5]. We find that the 365-nm phase curve is sufficiently sensitive on the vertical location of the unknown absorber, as this is the only one absorption agent. Our best fits suggest that the unknown absorber layer should be centered at 3 km below the cloud top level, or lower, regardless of its thickness (2, 6, 10 km thickness, Fig. 2 shows that with the 6-km thickness). The 283-nm phase curve has been analyzed for the first time. However, we find a difficulty on a distinction of absorbing agents between the SO2 gas and the unknown absorber (Fig. 3), using only the phase curve.We have analyzed the 3-year mean solar phase curves at 283 and 365 nm acquired by UVI/Akatsuki. Our results at 365 nm suggest that the unknown absorber layer should be located near and below the cloud top level, but not above. This supports the analysis of Pioneer Venus observations [6], suggesting that the unknown absorber is a layer below the cloud top level. We rule out a well-mixed layer above the cloud top level, which was suggested as a possible solution [7]. Our analysis shows its limit at 283 nm to understand abundance of the SO2 gas due to the overlapped absorption of the unknown absorber. This limitation perhaps can be able to overcome with spectral information, such as planned observation by the UV spectrometer onboard BepiColombo during its cruise phase (August 2020).Figure 1 Observed mean disk-integrated reflectance at 365 nm (a) and 283 nm (b) as a function of solar phase angle (degree). Grey dots are individual data points, and the red curve is an average over 3 years (2016-2018).Figure 2 Analysis at 365 nm with an assumed 6-km thick unknown absorber layer. A2 refers to the unknown absorber. (a) Reduced Chi-square contour compared to the observed phase curve. Free variables are an absorption ratio of the unknown absorber (A2_R) at 365 nm and a vertical location of the unknown absorber layer. Dashed line indicates the cloud top level, 70 km, so ‘-’ means below the cloud top level and ‘+’ means above. (b) A comparison between the observed mean phase curve (red) and a simulated best-fit curve (black, the ‘x’ mark on (a)).Figure 3 Analysis at 283 nm. This used the best-fit condition of the unknown absorber layer as retrieved in Fig.2: the 6-km thick layer of unknown absorber is centered at 3 km below the cloud top level. (a) Reduced Chi-square contour, compared to the observed phase curve. Free variables are an absorption of the unknown absorber (A2_R) at 283 nm and an abundance of SO2 gas at the cloud top level (70 km). The yellow line indicates the mean SO2 gas abundance retrieved from ground-based mid-IR observations cloud top level in 2016-2018 (22.4 ppbv at 70 km. This is a converted value from the 450 ppbv at 61 km [8] using the same manner in [9]). (b) Comparison between the observed mean phase curve (red) and simulated curves (blue and oranges), as indicated in (a). References[1] Yamazaki et al. 2018, EP&S, 70, 23, https://doi.org/10.1186/s40623-017-0772-6[2] García Muñoz and Mills 2015, A&A 573, A72, https://doi.org/10.1051/0004-6361/201424042[3] García Muñoz et al. 2014, A&A, 566, L1, https://doi.org/10.1051/0004-6361/201423531[4] Luginin et al. 2016, Icarus, 277, 154, https://doi.org/10.1016/j.icarus.2016.05.008[5] Lee et al. 2017, AJ, 154, 44, https://doi.org/10.3847/1538-3881/aa78a5[6] Esposito, 1980, JGR, 85, 8151, https://doi.org/10.1029/JA085iA13p08151[7] Molaverdikhani et al. 2012, Icarus 217, 648–660, http://dx.doi.org/10.1016/j.icarus.2011.08.008[8] Encrenaz et al. 2019, A&A, 623, A70, https://doi.org/10.1051/0004-6361/201833511[9] Marcq et al. 2020, Icarus, 252, 335:113368, https://doi.org/10.1016/j.icarus.2019.07.002