Mitsumu K. Ejiri

Modeling of solar wind control of the ring current buildup: A case study of the magnetic storms in April 1997

The ring current buildup depending on the solar wind density and the time-dependent convection field is examined by using a particle tracing scheme. To obtain the current density, energetic protons injected from the night side boundary at L=9 are traced under the dipolar magnetic field and the Kp dependent model of the Volland-Stern type convection field. The source distribution function at the boundary is assumed to be isotropic Maxwellian with a temperature of 5 keV and a number density depending on the solar wind density. We derive the corrected Dst (Dst*) from the Biot-Savart integral over the whole three-dimensional distribution of the calculated current density. The results are compared with the observed Dst* in the three successive storms in April 1997; the major variation of the Dst* is mainly due to the convection electric field and the solar wind density as well.

Development of the multi-spectral auroral camera onboard the index satellite

Abstract To investigate the fine-scale auroral structures, high time and spatial resolution imaging observations of optical auroras will be made by a multi-spectral auroral camera (MAC) onboard the INDEX satellite which will be launched by an H2A rocket as a piggyback satellite into a polar orbit at an altitude of ∼700 km. Monochromatic auroral image data at emissions of N2+ first negative band (427.8 nm), OI (557.7 nm), and N2 first positive band (670 nm) are obtained by MAC with the field-of-view (FOV) of 7.6° using three independent CCD cameras in combination with interference filters. MAC will operate in the nightside auroral region by two operation modes in the following. (1) Simultaneous measurement with particle sensors (ESA/ISA). In this mode, MAC observes an imaging area of ∼80×80 km (at a 100 km altitude) around a magnetic footprint with spatial and time resolutions of ∼1.2 km and 120 msec, respectively. (2) Auroral height distribution measurement. The attitude of INDEX satellite is changed to direct the FOV of MAC on the limb of the Earth. In this mode, MAC observes an imaging area of ∼270×270 km (at a 2000 km distance from the satellite) with spatial and time resolutions of ∼4 km and 1 sec, respectively. In this paper, the science mission, the instrumentation, and observation modes concerning on MAC will be presented.

Wedge‐like dispersion of sub‐keV ions in the dayside magnetosphere: Particle simulation and Viking observation

Particle drift simulation of low-energy ions (less than a few keV) with a simple geomagnetic field (dipole) and the Volland-Stern type convection electric field is found to be capable of investigating dispersed sub-keV ion events deep inside the dayside ring current region observed by Viking. Here we show three types of such events and simulation results. From the shape of the dispersion in the energy-latitude diagrams, they are called wedge-like dispersions: type 1 (characteristic energy monotonically increasing with latitude), type 2 (energy increasing with latitude and subsequently decreasing with latitude), and type 3 (energy monotonically decreasing with latitude). All these cases are understood as results of energy-dependent drift motion of ions coming from the nightside near-Earth tail if the distribution function in the source region varies in time and space. The results indicate that the wedge-like ion dispersions even may be associated with a substorm injection and/or a localized plasma flow channel in the near-Earth tail.

Ozone profiles in the high-latitude stratosphere and lower mesosphere measured by the Improved Limb Atmospheric Spectrometer (ILAS)-II: comparison with other satellite sensors and ozonesondes

A solar occultation sensor, the Improved Limb Atmospheric Spectrometer (ILAS)-II, measured 5890 vertical profiles of ozone concentrations in the stratosphere and lower mesosphere and of other species from January to October 2003. The measurement latitude coverage was 54–71°N and 64–88°S, which is similar to the coverage of ILAS (November 1996 to June 1997). One purpose of the ILAS-II measurements was to continue such high-latitude measurements of ozone and its related chemical species in order to help accurately determine their trends. The present paper assesses the quality of ozone data in the version 1.4 retrieval algorithm, through comparisons with results obtained from comprehensive ozonesonde measurements and four satellite-borne solar occultation sensors. In the Northern Hemisphere (NH), the ILAS-II ozone data agree with the other data within ±10% (in terms of the absolute difference divided by its mean value) at altitudes between 11 and 40 km, with the median coincident ILAS-II profiles being systematically up to 10% higher below 20 km and up to 10% lower between 21 and 40 km after screening possible suspicious retrievals. Above 41 km, the negative bias between the NH ILAS-II ozone data and the other data increases with increasing altitude and reaches 30% at 61–65 km. In the Southern Hemisphere, the ILAS-II ozone data agree with the other data within ±10% in the altitude range of 11–60 km, with the median coincident profiles being on average up to 10% higher below 20 km and up to 10% lower above 20 km. Considering the accuracy of the other data used for this comparative study, the version 1.4 ozone data are suitably used for quantitative analyses in the high-latitude stratosphere in both the Northern and Southern Hemisphere and in the lower mesosphere in the Southern Hemisphere.

Simultaneous airglow, lidar, and radar measurements of mesospheric gravity waves over Japan

[1] To investigate gravity wave dynamics in the mesosphere and lower thermosphere (MLT) region, we conducted coordinated observations of mesospheric gravity waves over Japan during the Aeronomy and Dynamics Observation campaign. Two all-sky airglow imagers were used in this campaign to derive a two-dimensional structure of the gravity waves; these imagers were installed at the middle and upper atmosphere (MU) observatory in Shigaraki (34.9°N, 136.1°E) and at the Dynic Astropark Observatory in Taga (35.2°N, 136.3°E). Simultaneous measurements of the horizontal winds and the temperature in the MLT region were provided by the meteor-mode observations of the MU radar at Shigaraki and by a sodium temperature lidar at Uji (34.9°N, 135.8°E), respectively. On 2 October 2008, gravity waves having a horizontal wavelength of ∼170 km, wave period of ∼1 h, and propagating northeastward at ∼50 m s -1 were observed in the airglow keograms. Similar wave structures were observed in the time series of the meteor wind and lidar temperature data; the polarity of these waves varied consistently with the airglow intensity variations according to the linear theory of gravity waves. The phase speeds and momentum fluxes of the gravity waves, estimated from the wind and temperature observations, were also in good agreement with those obtained from the airglow measurements. These results demonstrate, both qualitatively and quantitatively, that an identical gravity wave structure was detected in all the airglow intensities, radar winds, and lidar temperature.

Validation of the Improved Limb Atmospheric Spectrometer‐II (ILAS‐II) Version 1.4 nitrous oxide and methane profiles

This study assesses polar stratospheric nitrous oxide (N(2)O) and methane (CH(4)) data from the Improved Limb Atmospheric Spectrometer-II (ILAS-II) on board the Advanced Earth Observing Satellite-II (ADEOS-II) retrieved by the Version 1.4 retrieval algorithm. The data were measured between January and October 2003. Vertical profiles of ILAS-II volume mixing ratio (VMR) data are compared with data from two balloon-borne instruments, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) and the MkIV instrument, as well as with two satellite sensors, the Odin Sub-Millimetre Radiometer (SMR) for N(2)O and the Halogen Occultation Experiment (HALOE) for CH(4). Relative percentage differences between the ILAS-II and balloon/satellite data and their median values are calculated in 10-ppbv-wide bins for N(2)O (from 0 to 400 ppbv) and in 0.05-ppmv-wide bins for CH(4) (from 0 to 2 ppmv) in order to assess systematic differences between the ILAS-II and balloon/satellite data. According to this study, the characteristics of the ILAS-II Version 1.4 N(2)O and CH(4) data differ between hemispheres. For ILAS-II N(2)O VMR larger than 250 ppbv, the ILAS-II N(2)O agrees with the balloon/SMR N(2)O within +/- 20% in both hemispheres. The ILAS-II N(2)O in the VMR range from 30-50 to 250 ppbv (corresponding to altitudes of similar to 17-30 km in the Northern Hemisphere (NH, mainly outside the polar vortex) and similar to 13-21 km in the Southern Hemisphere (SH, mainly inside the polar vortex) is smaller by similar to 10-30% than the balloon/SMR N(2)O. For ILAS-II N(2)O VMR smaller than 30 ppbv (>similar to 21 km) in the SH, the differences between the ILAS-II and SMR N(2)O are within +/- 10 ppbv. For ILAS-II CH(4) VMR larger than 1 ppmv ( similar to 30 km) and the ILAS-II CH(4) for its VMR smaller than 1 ppmv (>similar to 25 km) only in the NH, are abnormally small compared to the balloon/satellite data.

Variations of nitric oxide in the mesosphere and lower thermosphere over Antarctica associated with a magnetic storm in April 2012

We report extreme enhancements of the nitric oxide (NO) column density observed with the ground-based millimeter-wave spectroscopic radiometer installed at Syowa Station, Antarctica, during a large geomagnetic storm in April 2012. From the NO spectrum line shape and NO column density relationship with solar radiation, we concluded that the NO was emitted in the altitude range between 75 km and 100 km. The column density of NO gradually increased during the recovery phase. In addition to variations on a time frame of several days, we found diurnal variations. The increase of NO was related to precipitated electrons in the energy range of 30–300 keV observed by Polar-orbiting Operational Environmental Satellite (POES)/The Meteorological Operational (METOP). We found a rapid response (within 1 h) and a one-to-one correspondence between them. For the first time, we show that a remarkable increase of the column density of NO is caused by dawn-dusk asymmetry of the plasma sheet electrons.

Small‐scale gravity waves near the mesopause observed by four all‐sky airglow imagers

Nocturnal airglow images obtained at the Shigaraki middle and upper atmosphere (MU) observatory (34.9°N, 136.1°E), Japan, were analyzed to study the three-dimensional structure of small-scale gravity waves in the mesopause region. Airglow images from the near-infrared OH (layer height of ∼86 km) and O2(0, 1) (∼94 km) bands and the visible O I (∼96 km) and Na (∼90 km) lines were obtained simultaneously by using four all-sky cooled charge-coupled device imagers with a high time resolution of 0.5–2.0 min. A clear wave packet (horizontal wavelength of 60–70 km) propagating downward was observed for 2000–2030 LT on January 28, 1998. The downward motion of the wave packet is identified from a comparison of the edge location of the packet at the four airglow layers. The observed waves became faint first in O I (the highest altitude) and last in OH (the lower altitude), suggesting also that the wave packet passed through each of the airglow layers downward. From the simultaneous wind observation by the MU radar, we conclude that the observed downward propagating wave packet was generated by wave reflection at higher altitudes in the background wind field that was opposite to the horizontal k vector of the waves. For the same event, we estimate the vertical wavelength of the waves to be ∼5 km or 20–30 km by comparing the wave phases observed in the four airglow layers. The vertical wavelengths estimated from the dispersion relation are consistent with the latter value.

New statistical analysis of the horizontal phase velocity distribution of gravity waves observed by airglow imaging

We have developed a new analysis method for obtaining the power spectrum in the horizontal phase velocity domain from airglow intensity image data to study atmospheric gravity waves. This method can deal with extensive amounts of imaging data obtained on different years and at various observation sites without bias caused by different event extraction criteria for the person processing the data. The new method was applied to sodium airglow data obtained in 2011 at Syowa Station (69°S, 40°E), Antarctica. The results were compared with those obtained from a conventional event analysis in which the phase fronts were traced manually in order to estimate horizontal characteristics, such as wavelengths, phase velocities, and wave periods. The horizontal phase velocity of each wave event in the airglow images corresponded closely to a peak in the spectrum. The statistical results of spectral analysis showed an eastward offset of the horizontal phase velocity distribution. This could be interpreted as the existence of wave sources around the stratospheric eastward jet. Similar zonal anisotropy was also seen in the horizontal phase velocity distribution of the gravity waves by the event analysis. Both methods produce similar statistical results about directionality of atmospheric gravity waves. Galactic contamination of the spectrum was examined by calculating the apparent velocity of the stars and found to be limited for phase speeds lower than 30 m/s. In conclusion, our new method is suitable for deriving the horizontal phase velocity characteristics of atmospheric gravity waves from an extensive amount of imaging data.

A thermospheric Na layer event observed up to 140 km over Syowa Station (69.0°S, 39.6°E) in Antarctica

We report a thermospheric Na layer event (up to 140 km) observed by lidar in the night of 23–24 September 2000 at Syowa (69.0°S, 39.6°E), Antarctica. The thermospheric Na number densities were 2–9 cm−3 at 110–140 km, 3 orders of magnitude smaller than the peak density of the normal layer at 80–110 km. The thermospheric Na layers exhibited a wave-like structure with a period of 1–2 h. The colocated ionospheric/auroral observations showed sporadic E layers over Syowa through the night and an enhancement of the ionospheric/auroral activity around south side of Syowa at the event beginning. Adopting the theory by Chu et al. (2011), we hypothesize that the thermospheric Na layers are neutralized from converged Na+ layers. An envelope calculation shows good consistency with the observations.