Mizuki Yoneda

Weakening of Jupiter's main auroral emission during January 2014

In January 2014 Jupiters FUV main auroral oval decreased its emitted power by 70% and shifted equatorward by ∼1°. Intense, low-latitude features were also detected. The decrease in emitted power is attributed to a decrease in auroral current density rather than electron energy. This could be caused by a decrease in the source electron density, an order of magnitude increase in the source electron thermal energy, or a combination of these. Both can be explained either by expansion of the magnetosphere or by an increase in the inward transport of hot plasma through the middle magnetosphere and its interchange with cold flux tubes moving outward. In the latter case the hot plasma could have increased the electron temperature in the source region and produced the intense, low-latitude features, while the increased cold plasma transport rate produced the shift of the main oval.

Inter-University upper Atmosphere Global Observation Network (IUGONET)

An overview of the Inter-university Upper atmosphere Global Observation NETwork (IUGONET) project is presented with a brief description of the products to be developed. This is a Japanese inter-university research program to build the metadata database for ground-based observations of the upper atmosphere. The project also develops the software to analyze the observational data provided by various universities/institutes. These products will be of great help to researchers in efficiently finding, obtaining, and utilizing various data dispersed across the universities/institutes. This is expected to contribute significantly to the promotion of interdisciplinary research, leading to more a comprehensive understanding of the upper atmosphere.

MiniTAO/MAX38 first light: 30-micron band observations from the ground-based telescope

We successfully carried out 30-micron observations from the ground-based telescope for the first time with our newly developed mid-infrared instrument, MAX38, which is mounted on the University of Tokyo Atacama 1.0-m telescope (miniTAO telescope). Thanks to the high altitude of the miniTAO (5,640m) and dry weather condition of the Atacama site, we can access the 30-micron wavelength region from ground-based telescopes. To achieve the observation at 30- micron wavelength, remarkable devices are employed in MAX38. First, a Si:Sb 128x128 array detector is installed which can detect long mid-infrared light up to 38-micron. Second, we developed metal mesh filters for 30-micron region band-pass filter, which are composed of several gold thin-films with cross-shaped holes. Third, a cold chopper, a 6-cm square plane mirror controlled by a piezoelectric actuator, is built into the MAX38 optics for canceling out the atmospheric turbulence noise. It enables square-wave chopping with a 50-arcsecound throw at a frequency more than 5- Hz. Finally, a low-dispersion grism spectrometer (R~50) will provide information on the transmission spectrum of the terrestrial atmosphere in 20 to 40 micron. In this observation, we clearly demonstrated that the atmospheric windows around 30-micron can be used for the astronomical observations at the miniTAO site.

Perfomance verification of the ground-based mid-infrared camera MAX38 on the MiniTAO Telescope

We have evaluated on-sky performances of a mid-infrared camera MAX38 (Mid-infrared Astronomical eXploerer) on the miniTAO 1-meter telescope. A Strehl ratio at the N-band is estimated to be 0.7-0.8, and it reaches to 0.9 at the 37.7 micron, indicating that diffraction limited angular resolution is almost achieved at the wavelength range from 8 to 38 micron. System efficiencies at the N and the Q-band are estimated with photometry of standard stars. The sensitivity at the 30 micron cannot be exactly estimated because there are no standard stars bright enough. We use the sky brightness instead. The estimated efficiencies at the 8.9, 18.7, and 31.7 micron are 4%, 3%, 15% , respectively. One-sigma sensitivity in 1 sec integration of each filter is also evaluated. These give good agreements with the designed values. Preliminary scientific results are briefly reported.

Evaluations of new atmospheric windows at thirty micron wavelengths for astronomy

Thirty micron has remained one of unexplored frontiers of ground-based astronomical observations. Recent developments of extreme high sites including the Chajnantor TAO site (5,640m) enable us to access the this wavelengths from the ground. The expected transmittance seems clear enough for astronomical observations, but practical evaluations based on astronomical data has not been carried out yet. We have analyzed images obtained at the 31.7 micron with a mid-infrared camera MAX38 attached on a mini-TAO 1.0-meter telescope. 109 images of a star IRC+10420 and 11,114 images of the sky have been reduced. Clear relationship between the measured photocurrents and the perceptible water vapor has been found. Simple estimation of the photocurrents with of the ATRAN model gives good agreements with the measurements, indicating that the ATRAN model reproduce the atmospheric transmittance reasonably well. This also supports our assumption that the scaling factor 0.85 of the PVW at the Chajnantor TAO site to the PWV at the APEX. The average transmittance in the 31.7 micron is achieved to be over 20% when the PWV below 0.6 mm. In some cases clear degradation up to 10% in the transmittance is found. It may be caused by droplets of liquid or iced water with a size over 10 micron although the causes are not exactly specified. Diurnal time variations of the sky photocurrents are also investigated. The sky is sometimes bright and usually unstable in the twilight time. On the other hand the sky around the noontime does not show clear difference from the night sky. It may suggest that the observing condition at the thirty micron windows remain good even in the daytime.

Optical and IR observations of planetary and exoplanetary atmospheres

The spatial and temporal variations of planetary atmospheric phenomena (e.g., auroras on Jupiter and Io, as well as the polarization of expolanetary atmospheres) are extremely complicated. Indeed, the wide range of these phenomena, and the cross-scale coupling processes that exist between them, means that is difficult to understand their underlying mechanisms. To study and understand such planetary atmospheric phenomena it is therefore essential to perform continuous and flexible monitoring with a suitable telescope. From previous studies it is known that Jupiter’s auroras are generated by two separate mechanisms: a rapid 10-hour rotation of the magnetic field (known as the ‘internal source’) and the variation of the solar wind (known as the ‘external source’).1 Although much effort has been made to determine which of these sources plays the major role in causing the spatial and temporal variations of Jupiter’s auroras, the mechanism has still not been resolved.2 In addition, it has been shown that active volcanoes on Io (a moon of Jupiter) intermittently inject large amounts of gases into the magnetosphere and can modulate the auroral activity.3 Past studies have also revealed that the light from exoplanet host stars is polarized with the variation period of the planet’s orbital motion. This result suggests that the polarization is caused by Rayleigh scattering in the exoplanetary atmosphere.4 The examples of exoplanetary atmospheres that have so far been studied, however, have been very limited. Further observations are thus required to fully characterize the atmospheres of different types of exoplanets. Figure 1. The 60cm Cassegrain and Coude telescope (T60) installed at the summit of Mount Haleakala, Hawaii.