Koh-Ichiro Oyama

Temperature enhancements and vertical winds in the lower thermosphere associated with auroral heating during the DELTA campaign

[1] A coordinated observation of the atmospheric response to auroral energy input in the polar lower thermosphere was conducted during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign. N2 rotational temperature was measured with a rocket-borne instrument launched from the Andoya Rocket Range, neutral winds were measured from auroral emissions at 557.7 nm with a Fabry-Perot Interferometer (FPI) at Skibotn and the KEOPS, and ionospheric parameters were measured with the European Incoherent Scatter (EISCAT) UHF radar at Tromso. Altitude profiles of the passive energy deposition rate and the particle heating rate were estimated using data taken with the EISCAT radar. The local temperature enhancement derived from the difference between the observed N2 rotational temperature and the MSISE-90 model neutral temperature were 70–140 K at 110–140 km altitude. The temperature increase rate derived from the estimated heating rates, however, cannot account for the temperature enhancement below 120 km, even considering the contribution of the neutral density to the estimated heating rate. The observed upward winds up to 40 m s �1 seem to respond nearly instantaneously to changes in the heating rates. Although the wind speeds cannot be explained by the estimated heating rate and the thermal expansion hypothesis, the present study suggests that the generation mechanism of the large vertical winds must be responsible for the fast response of the vertical wind to the heating event.

Ionospheric electron content and NmF2 from nighttime OI 135.6 nm intensity

[1]xa0This paper derives the theoretical relationship between vertical integrated intensity of OI 135.6 nm oxygen emission with integrated electron content (IEC) from 150 to 800 km altitude as well as F layer peak electron density (NmF2). Local time, seasonal, and solar cycle dependence of the relationship is investigated, and it is proposed as a conversion factor to retrieve IEC and NmF2 values from airglow measurements. The errors associated with the IEC and NmF2 estimation using the derived conversion factor are demonstrated for different local times and solar activity. The theoretical conversion factor is compared with that calculated using airglow measurements by the Global Ultraviolet Imager onboard the Theremosphere, Ionosphere, Mesosphere Energetics and Dynamics mission and the Global Ionospheric Map total electron content as well as the nadir integrated OI 135.6 nm intensity by the Tiny Ionospheric Photometer and NmF2 determined from the Global Positioning System Occultation Experiment, both onboard FORMOSAT-3/COSMIC.

Detection of long‐living neutral hydrated clusters in laboratory simulation of ionospheric D region plasma

[1]xa0The existence of hydrated cluster ions is known through in situ measurements in the D region of the ionosphere and laboratory simulation experiments. A series of experiments were conducted at Sagamihara, Japan with the intention of detecting some of the ions which, although predicted, had eluded detection in laboratory simulation. The other motivation was to look for heavier ions in laboratory simulations in conditions close to those in the D region. With the availability of better ion mass spectrometers, these could supposedly be detected by rocket measurements. Results of these experiments point to a new aspect, namely, the production of a neutral hydrated cluster molecule, which (a) has ionization potential of less than 10.2 eV, (b) has lifetimes in excess of 90 min, and (c) is formed within a limited pressure range. As this neutral cluster molecule has a mass number of 102, most probably it is NO⋅(H2O)4. A number of other important ions, which were detected earlier in laboratory experiments, were also seen in our data. These include NO+(H2O)n, NO+(H2O)nX, NO2+(H2O)n, H3O+(H2O)n, H3O+(H2O)nX, and O2+(H2O)n series. A few clusters {36+(H3O+OH), 60+(NO+NO) and 63+(NO+HO2)} and molecular ions {29+(N2H+), 33+(HO2+) and 43+(N3H+)} were also detected in these experiments. It was also found that, like the earlier experiments, the concentration of most of the hydrated ions showed an oscillatory behavior. The ion formation was observed only within a limited pressure range, which corresponds to the 50 to 100 km altitude range of the ionosphere.

In situ observations of instabilities in the mesopause region using foil chaff technique during the Waves in Airglow Structures Experiment (WAVE) campaigns

[1]xa0In order to investigate the dynamics of the mesopause region, foil chaff experiments were carried out successfully during the WAVE2000 and WAVE2004 campaigns at Kagoshima, Japan. In the WAVE2004 campaign, the height profiles of the horizontal and vertical wind speeds were obtained. The wind shear layers of >40 m/s/km were located around the altitudes of 89 and 95 km, and the profile of the Richardson number reveals the existence of dynamically unstable layers at about the same height region. Ripple observations using the all-sky imagers also support the possibility of dynamical instabilities. In the WAVE2000 campaign, a prominent small-scale feature around an altitude of 90.5 km appeared in both the horizontal and vertical chaff motions. These results demonstrate that the foil chaff technique is a valuable tool for in situ observation of small-scale turbulence features around the mesopause.

Foil chaff ejection systems for sounding rocket measurements of neutral winds in the mesopause region

The foil chaff technique is a simple in-situ technique to measure neutral winds in the mesopause region. In order to conduct foil chaff experiments by sounding rockets, two types of foil chaff ejection systems have been developed in Japan. The high resolution in altitude of the obtained neutral wind data by the foil chaff technique provides useful information for various studies on the dynamics in the mesopause region, when combined with simultaneous measurements of other geophysical parameters.