K. Igarashi

A large-scale traveling ionospheric disturbance during the magnetic storm of 15 September 1999

enhancement of GPS total electron content (� 1.0 � 10 16 m � 2 ). Multipoint and imaging observations of these parameters show that the LSTID moved equatorward over Japan with a velocity of � 400–450 m/s. From a comparison with the Sheffield University Plasmasphere-Ionosphere Model (SUPIM) we conclude that an enhancement (250–300 m/s) of poleward neutral wind (that is propagating equatorward) caused these observational features of the LSTID at midlatitudes. To investigate generation of the LSTID by auroral energy input, we have used auroral images obtained by the Polar UVI instrument, magnetic field variations obtained at multipoint ground stations, and the empirical Joule heating rate calculated by the assimilative mapping of ionospheric electrodynamics (AMIE) technique. Intense auroral energy input was observed at 0800–1100 UT (4–6 hours before the LSTID), probably causing equatorward neutral wind at lower latitudes. It is likely that the poleward wind pulse that caused the observed LSTID was generated associated with the cessation of this equatorward wind. The effect of Lorentz force is also discussed. INDEX TERMS: 0310 Atmospheric Composition and Structure: Airglow and aurora; 2427 Ionosphere: Ionosphere/atmosphere interactions (0335); 2435 Ionosphere: Ionospheric disturbances; 2437 Ionosphere: Ionospheric dynamics; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: large-scale traveling ionospheric disturbance, thermosphere–ionosphere coupling, magnetic storm, airglow imaging, GPS network, ionosonde

Statistical study of short‐period gravity waves in OH and OI nightglow images at two separated sites

[1]xa0Using airglow images of the near-infrared OH band (720–910 nm) and OI (557.7 nm) line, we investigated seasonal, latitudinal, and local time variations of short-period gravity waves. The images were obtained at two locations in Japan that are ∼1200 km apart, Rikubetsu (43.5°N, 143.8°E) and Shigaraki (34.9°N, 136.1°E), between October 1998 and October 1999. Our analysis has focused on small-scale gravity waves with wavelengths less than 40 km and dominant phase speeds of ∼20–50 m/s. Wave occurrences for both OH and OI at Rikubetsu and Shigaraki are significantly higher than 60%, with a slightly larger value in summer. The occurrences increase from evening to midnight. There are no obvious local time dependencies in horizontal wavelength, propagation direction, and phase speed. The propagation directions in summer are either northward or northeastward at both locations. However, in winter the propagation directions at Rikubetsu are generally westward (NW, W, and SW), whereas those at Shigaraki are only southwestward. From simultaneous wind observation by the MF radars at Wakkanai (45.4°N, 141.7°E) and Yamagawa (31.2°N, 130.6°E), we discuss possible influences of Doppler and thermal ducting, wind filtering, and source distribution of gravity waves propagating from the lower atmosphere to the airglow heights in the mesopause region.

Imaging observations of the equatorward limit of midlatitude traveling ionospheric disturbances

This paper reports the first attempt to observe the equatorward limit of medium-scale traveling ionospheric disturbances (TIDs) in the middle latitudes. The TIDs usually propagate southwestward in the northern hemisphere. An all-sky cooled-CCD imager measured 630-nm airglow at a southern island of Japan, Okinawa (26.9°N, 128.3°E, geomagnetic latitude (MLAT) = 17.0°), during the FRONT-2 campaign of August 4–15, 1999. The TIDs were detected at the mainland of Japan (∼21°–36° MLAT) by the total electron content (TEC) observations of more than 1000 GPS receivers. In the August 4 event, the TIDs moving southwestward was seen only in the northern sky of Okinawa as a depletion band in the 630-nm airglow images. In the August 6 event, the TIDs were not seen in the 630-nm images at Okinawa, although weak TID activity was observed by the GPS network at the mainland of Japan. The TEC data also showed weakening of the TID activity below 18° MLAT. Based on these observations, we suggest that there is a possible limit of medium-scale TID propagation around ∼18° MLAT.

Thermospheric wind during a storm‐time large‐scale traveling ionospheric disturbance

[1]xa0A prominent large-scale traveling ionospheric disturbance (LSTID) was observed in Japan during the major magnetic storm (Dst ∼ −358 nT) of 31 March 2001. It was detected as enhancements of the 630-nm airglow and foF2, GPS-TEC variations, and a decrease in F-layer virtual height at 1700–1900 UT (0200–0400 LT). It moved equatorward with a speed of ∼600 m/s. The decrease in the F-layer height was also detected by the MU radar at Shigaraki. Thermospheric wind variations were observed by the MU radar through ion drift measurement and by a Fabry-Perot interferometer (FPI) through a Doppler shift of the 630-nm airglow line at Shigaraki. The wind data show a turn of the meridional wind from −94 m/s (equatorward) to +44 m/s (poleward) during the LSTID, indicating that an intense poleward wind in the thermosphere passed over Shigaraki as an atmospheric gravity wave and caused the observed ionospheric features of the LSTID. Intense poleward wind was also detected at mesospheric altitudes (95–100 km) by the MU radar (through meteor echoes) and by the FPI (through the 558-nm airglow) with a delay of ∼2 hours from the thermospheric wind, indicating downward phase progression of the wave. Generation of the observed poleward wind in the auroral zone was investigated using magnetic field data and auroral energy input estimated by the assimilative mapping of ionospheric electrodynamics (AMIE) technique. We suggest that simple atmospheric heating and/or the Lorentz force in the auroral zone do not explain the observed poleward wind enhancement.

Multi‐point observation of short‐period mesospheric gravity waves over Japan during the FRONT Campaign

Simultaneous observations of short-period gravity waves were carried out using three all-sky cooled-CCD imagers of OH airglow at Moshiri (northern edge of Japan) and at Shigaraki and Bisei (middle part of Japan) during the FRONT campaign of May 1998. These stations were separated by horizontal distances of 250–1300 km, and they offered a unique opportunity to investigate the spatial extent of gravity waves in the mesopause region. The images from Shigaraki and Bisei showed short-period gravity waves (horizontal wavelength: 20–40 km, phase front: ∼east-west aligned) moving northward (phase velocity: 20–40 m/s) on May 19, 21 and 22. At Moshiri, similar east-west waves moving northward were observed on May 19 and 21, indicating a broad horizontal extent of the gravity waves. Waves at Moshiri were not evident on May 22, however. The horizontal wind measured by the MF radar at Wakkanai (near Moshiri) showed height profiles that were similar to those at Yamagawa (southern edge of Japan) on May 19 and 21 but quite dissimilar on May 22. On the basis of these data, we suggest that short-period gravity waves in the mesopause region can have a broad spatial extent of more than 1000 km. The generation and propagation of internal gravity waves are discussed as a cause of the spatial extent of the waves.

Ionospheric and atmospheric disturbances around Japan caused by the eruption of Mount Pinatubo on 15 June 1991

Abstract The disturbances observed by the Japanese ionospheric observation network following the explosions of Mount Pinatubo on 15 June 1991, are presented. Remarkable ionospheric fluctuations with periods of about 20 min appeared in the records of HF Doppler and total electron content (TEC) data. Traveling wave fronts of ionospheric disturbances scaled from these data give a northward horizontal velocity of 290 m/s. The surface pressure fluctuations due to the passage of atmospheric waves were confirmed by the microbarograph chain data in Japan. There existed three kinds of northward traveling pressure fluctuations; short-period (2–5 min) fluctuations with a horizontal velocity of 290 m/s, long-period (8–10 min) fluctuations with 300 m/s, and 17-min fluctuations with 281 m/s. It is concluded that the ionospheric and surface pressure waves were caused by the eruption of Mount Pinatubo.

Simultaneous mesosphere/lower thermosphere and thermospheric F region observations during geomagnetic storms

[1]xa0Simultaneous mesosphere/lower thermosphere (MLT) and thermospheric F region observations made during geomagnetic storms are presented using the MU radar (35°N, 136°E) in alternate meteor and incoherent scatter (IS) modes, MF radar (45°N, 136°E), optical mesosphere thermosphere imagers (OMTI) (35°N, 136°E) and over 1000 GPS receivers in Japan. The observations were made during one of the MTEC-S (mesosphere-thermosphere experiments for coupling studies) campaigns conducted during 23 March to 2 April 2001 when a major storm with Dst reaching −358 nT and a moderate storm with Dst reaching −112 nT occurred and solar activity varied from high to very high (F10.7 = 220 to 270). The campaign has provided zonal and meridional wind velocities at MLT altitudes (80–95 km) over the MU and MF radars; meridional wind velocity in the upper thermosphere (220–450 km) and ionospheric electron density (150–600 km), peak height and plasma drift velocity over the MU radar; all-sky airglow intensities at 557.7 nm, 630.0 nm, and 777.4 nm wavelengths over the MU radar location; and vertical GPS-TEC all over Japan. Study of the data indicates that following the major storm, the meridional wind in the upper thermosphere over the MU radar (35°N) becomes weak (less poleward) during daytime and its diurnal amplitude reduces. The winds and tides at MLT altitudes show changes during both nonstorm times and storm times, but this study is not able to confirm any statistically significant changes related to the storms. In the ionosphere, following the onset of the major morning storm, the electron density increases at altitudes near and above the ionospheric peak, which after about 5 hours is followed by large increase in density at all altitudes above about 200 km. On the following night, though the density is generally low, there is a large increase in density in the bottomside for about 3 hours centered at 0230 LT. During this time, a rare intense optical activity has occurred at all wavelengths (557.7 nm, 630 nm, and 777.4 nm) over the low midlatitude location (35°N), and an associated large-scale traveling ionospheric disturbance (LSTID) has propagated from north to south at F region altitudes.

MF radar observations of mean winds and tides over Poker Flat, Alaska (65.1° N, 147.5° W)

Abstract. MF radar wind measurements in the mesosphere and lower thermosphere over Poker Flat, Alaska (65.1° N, 147.5° W) are used to study the features of mean winds and solar tides. Continuous observation with the newly installed radar is in progress and in the present study we have analyzed a database of the first 27 months (October 1998–December 2000) of observation. The observed mean wind climatology has been compared with previous measurements and the latest empirical model values (HWM93 model). Similarly, the tidal characteristics are described and compared with the Global Scale Wave Model (GSWM00). The mean wind characteristics observed are fairly consistent with previous wind measurements by the Poker Flat MST radar. The main feature of the zonal circulation is the annual variation with summer westward flow and winter eastward flow. The annual mean zonal wind has a west-ward motion at altitudes below 90 km. The annual mean meridional circulation has mainly southward motion at 70–100 km. There is very good agreement between the radar zonal winds and the HWM93 model winds. Comparison of the meridional winds shows some discrepancy. Analysis of two years of data indicated that the year-to-year consistency is preserved in the mean circulation in the mesosphere. Tidal characteristics observed are also consistent with previous measurements. Semidiurnal tides have the largest amplitudes in summer while the weakest amplitude is observed during the winter months. The vertical wavelength is longer during the summer season compared to the winter season. Comparison with the GSWM00 produces mixed results. There is reasonable agreement between the observed and modeled phases. Diurnal tide amplitudes are comparable in magnitude with that of the semidiurnal tide. Seasonal variation is less evident in the amplitudes. Comparison of the observed tidal parameters with the GSWM00 reveals some agreement and discrepancies. Key words. Meteorology and atmospheric dynamics (climatology; middle atmosphere dynamics; waves and tides)

Imaging observations of midlatitude ionospheric disturbances during the geomagnetic storm of February 12, 2000

Regular all-sky imaging observations of the F region OI 630 nm nightglow emission are carried out at Rikubetsu (43.5°N) and Shigaraki (34.8°N), Japan since 1998. In this paper, we present observations of the OI 630 nm emission images from these two sites with a cluster of other observations during the geomagnetic storm of February 12, 2000. The airglow observations were obtained between 1500 UT and 2000 UT (0000-0500 LT) on this night. Images from both Rikubetsu and Shigaraki show the presence of mesoscale-enhanced airglow bands moving slowly (∼20-30 m s -1 ) to the southwest direction. The mesoscale band structures were also detected by an extensive network of GPS signal observations over Japan. The airglow observations at Shigaraki also show intense OI 630 nm emission in the south with several small-scale intensity depletion structures. We suggest that these small-scale intensity depletion structures (scale size 30-50 km) are caused by possible nonlinear interaction between the mesoscale TIDs and enhanced regions of the equatorial ionospheric anomaly. The ionosonde observations from Wakkanai (45.4°N) and Kokubunji (35.7°N) show spread F signatures between 1130 and 2100 UT and 1215 and 1715 UT, respectively, whereas no spread F was observed at Yamagawa (31.2°N) and Okinawa (26.3°N). The phase fluctuations of GPS signal were observed at latitudes higher than 30°N for 1400-1900 UT. These observations suggest enhanced storm time ionospheric disturbances were confined in the midlatitudes.

Tidal structure and variability in the mesosphere and lower thermosphere over Yamagawa and Wakkanai

Abstract Observations of winds in the mesosphere and lower thermosphere (MLT) measured at Yamagawa (31.2°N, 130.6°E) and Wakkanai (45.4°N, 141.7°E) are used to study the structure/variability of solar semidiurnal and diurnal tides. MF radar observations over these sites were recently started by the Communications Research Laboratory, Tokyo, Japan. A database of 3–4 years has been used in the present analysis. The tidal features are discussed, and these are compared with the results from other similar mid-latitude sites. The semidiurnal and diurnal tides are also compared with a recent tidal model. The analysis on semidiurnal tides indicates that the variations over these sites seem to have reasonable similarities. Generally, at both locations the semidiurnal tidal amplitudes have the values in the range 5– 15 m / s . Although seasonal variations in the semidiurnal tide amplitude are not clearly seen, the multi-year average indicates that the maximum amplitude is observed in August/September at both sites. The summer season is characterized by a large vertical wavelength, while the winter season has a comparatively smaller vertical wavelength. Comparison of the observed semidiurnal tide amplitude and phase parameters, and the Global Scale Wave Model (GSWM-98) outputs is generally not satisfactory and confirms the need for further effort in the development of a more realistic model. Diurnal tide amplitude at Yamagawa and Wakkanai shows significant differences in their strengths. The amplitude is consistently larger in Yamagawa. Phase structure shows a better agreement between the sites. Again, comparison of the observed tidal parameters and the GSWM-98 values is generally not good. However, the overall assessment is that the comparison is better in the case of diurnal tides than that of semidiurnal tides. The best comparison result is observed for the diurnal tidal phase. The interannual variability observed in both the semidiurnal and diurnal tides is larger in the Yamagawa data compared to the Wakkanai data. The semidiurnal and diurnal tidal phases around 90 km generally exhibit a bimodal state with the phase being almost constant near the summer (June) and winter (December) solstices, and making rapid transitions in the equinoxes. The duration of the constant phase is longer in the summer solstice than in the winter solstice.