Measurement of stratospheric and mesospheric winds with a submillimeter wave limb sounder: results from JEM/SMILES and simulation study for SMILES-2

Satellite missions for measuring winds in the troposphere and thermosphere will be launched in a near future. There is no plan to observe winds in the altitude range between 30-90 km, though middle atmospheric winds are recognized as an essential parameter in various atmospheric research areas. Sub-millimetre limb sounders have the capability to fill this altitude gap. In this paper, we summarize the wind retrievals obtained from the Japanese Superconducting Submillimeter Wave Limb Emission Sounder (SMILES) which operated from the International Space Station between September 2009 and April 2010. The results illustrate the potential of such instruments to measure winds. They also show the need of improving the wind representation in the models in the Tropics, and globally in the mesosphere. A wind measurement sensitivity study has been conducted for its successor, SMILES-2, which is being studied in Japan. If it is realized, sub-millimeter and terahertz molecular lines suitable to determine line-of-sight winds will be measured. It is shown that with the current instrument definition, line-of-sight winds can be observed from 20 km up to more than 160 km. Winds can be retrieved with a precision better than 5 m s(-1) and a vertical resolution of 2-3 km between 35-90 km. Above 90 km, the precision is better than 10 m s(-1) with a vertical resolution of 3-5 km. Measurements can be performed day and night with a similar sensitivity. Requirements on observation parameters such as the antenna size, the satellite altitude are discussed. An alternative setting for the spectral bands is examined. The new setting is compatible with the general scientific objectives of the mission and the instrument design. It allows to improve the wind measurement sensitivity between 35 to 90 km by a factor 2. It is also shown that retrievals can be performed with a vertical resolution of 1 km and a precision of 5-10 m s(-1) between 50 and 90 km. RAGAM A, 1953, PHYSICAL REVIEW, V92, P144

MgB2 hot-electron bolometer mixers for sub-mm wave astronomy

Spectroscopy and photometry in the terahertz (THz) range of remote space objects allows for a study of their chemical composition, because this range covers rotational lines from simple molecules and electron transition lines from atoms and ions. Due to high spectral resolution, THz heterodyne receivers al- low for studying dynamical properties of space objects manifested in doppler-shifted emission lines. Niobium nitride (NbN) hot-electron bolometer (HEB) mixers currently used at frequencies >1 THz, provide a typical gain bandwidth (GBW) of 3 GHz, and consequently, a noise bandwidth (NBW) of 4 GHz. This property severely limits the functionality of astronomical instruments. Moreover, the low critical temperature (Tc = 8–11 K) of NbN ultrathin films necessitates usage of liquid helium (LHe) for device cooling, which reduces lifetime of spaceborne missions.In this thesis, a study of HEB mixers dedicated for sub-mm wave astronomy applications made from magnesium diboride (MgB2) ultrathin films is presented. It is shown that MgB2 HEB mixers reach a unique combination of low noise, wide noise bandwidth, and high operation temperature when 8 nm thick MgB2 films (Tc = 30 K) are used. The hybrid physical chemical vapour deposition (HPCVD) technique allows for reproducible deposition of such thin films. The high Tc of MgB2 (39 K), and consequently, short (3 ps) electron- phonon interaction time result in a GBW of up to 10 GHz and possibility of operation at temperatures >20 K, where compact cryocoolers are available. The GBW was observed to be almost independent on both bias voltage and bath temperature. A NBW of 11 GHz with a minimum double sideband (DSB) receiver noise temperature of 930 K is achieved at a 1.63 THz local oscillator (LO) and a 5 K bath temperature. At 15 K and 20 K, noise temperatures are 1100 K and 1600 K, respectively. From 0.69 THz to 1.63 THz noise increases by only 12%, and hence, low noise performance is expected even at higher frequencies. The minimum receiver noise temperature is achieved in a quite large range of both bias voltages (5–10 mV) and LO power. Compared to initial results, higher sensitivity and larger NBW are due to a larger HEB width (lower contact resistance), applied in-situ contact cleaning, and a smaller film thickness. The increase of noise temperature when operation temperature rises from 5 K to 20 K is due to a reduction of conversion gain by 2–4 dB caused be the reduced LO power absorbed in the HEB. The output noise of the HEB remains the same (120–220 K depending on the bias point)