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Midlatitude mesosphere summer echoes (MSE) at the VHF band (VHF-MSE) were observed for 13 years (2000-2002 and 2009-2018) with a 46.5 MHz radar at Wakkanai, Japan (45.4ºN, 141.8ºE). VHF-MSE are active during June-July and appear only in the daytime mainly at altitudes of 80-88 km with a maximum occurrence at 85 km and altitude extents of 1-4 km for a duration of about half an hour or more. The VHF-MSE occurrences are positively correlated with solar activity, but not with geomagnetic activity except for very high activity. Such long-term characteristics are mostly consistent with past VHF-MSE observations at higher midlatitudes in Europe. No VHF-MSE were observed in 2002, 2014 and 2018, possible reasons for which are discussed. It is shown that cold ice particles in the upper mesosphere inducing MSE are advected from high latitudes to midlatitudes with equatorward wind. Thus, the MSE occurrences over Wakkanai are fundamentally controlled by both the solar activity and equatorward ice particle advection. One example of MSE at the HF band (HF-MSE) is presented to discuss spatial and temporal relationship between VHF-MSE and HF-MSE.

We report the history and progress of the Japanese contribution to the Super Dual Auroral Radar Network (SuperDARN) project; especially focusing on the initial stage of SENSU ( S yowa South and E ast HF Radars of N IPR for SU perDARN) project. The SENSU Syowa South HF radar operated by NIPR was one of the starting members of the initial SuperDARN radar operating at Syowa Station in Antarctica in 1995. It provided bi-directional common-volume observations with the Halley radar. In 1997 the SENSU Syowa East radar was installed. The King Salmon radar organized by NICT was installed in Alaska, USA in 2001. The Hokkaido East radar organized by Nagoya University was installed in Hokkaido, Japan in 2006, and the Hokkaido West radar began operation in 2014. The Japanese group has also contributed to SuperDARN by hosting three SuperDARN workshops in Japan namely: 1) in Tokyo in 1998 hosted by NIPR, 2) in Hokkaido in 2007 hosted by Nagoya University and 3) at Yamanashi in 2019 hosted by NICT.

We report simultaneous observations of atmospheric gravity waves (AGW) in OI (557.7nm) and OH airglow images and VHF radar backscatter from field-aligned irregularities (FAI) in the E-region during the SEEK-2 (Sporadic-E Experiment over Kyushu 2) campaign period from 29 July to 9 August 2002. An all-sky imager was operated at Nishino-Omote (30.5 N, 130.1 E), Japan. On 14 nights, 17 AGW events were detected in OI and OH airglow images. AGW propagated mostly toward the northeast or southeast. From comparison with the E-region FAI occurrence, which is detected by a nearby VHF radar (31.57MHz), we found that AGW tended to propagate southeastward during FAI events. This result suggests that the interaction between AGW and E-region plasma plays an important role in generating FAI. Furthermore, polarization electric fields generated directly by AGW may contribute to the FAI generation. Keywords. Atmospheric composition and structure (Airglow and aurora), Ionosphere (Ionospheric irregularities, Mid-latitude ionosphere)

Peculiar near range echoes observed in summer with the SuperDARN HF radar in Finland are presented. The echoes were detected at four frequencies of 9, 11, 13 and 15 MHz at slant ranges of 105–250 km for about 100 min. Interferometer measurements indicate that the echoes are returned from 80–100 km altitudes with elevation angles of 20°–60°. Echo power (< 16 dB), Doppler velocity (between –30 and + 30 ms-1) and spectral width (< 60 ms-1) fluctuate with periods of several to 20 min, perhaps due to short–period atmospheric gravity waves. When the HF radar detected the echoes, a vertical incidence MST radar, located at Esrange in Sweden (650 km north of the HF radar site), observed polar mesosphere summer echoes (PMSE) at altitudes of 80–90 km. This fact suggests that the near range HF echoes are PMSE at HF band, although both radars did not probe a common volume. With increasing radar frequency, HF echo ranges are closer to the radar site and echo power becomes weaker. Possible mechanisms to explain these features are discussed.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; thermospheric dynamics; waves and tides; instruments and techniques)

A Rayleigh–Raman lidar system was installed in January 2011 at Syowa Station, Antarctica (69.0° S, 39.6° E). Polar mesospheric clouds (PMCs) were detected by lidar at around 22:30 UTC (LT −3 h) on 4 February 2011, which was the first day of observation. This was the first detection of PMCs over Syowa Station by lidar. On the same day, a Super Dual Auroral Radar Network (SuperDARN) HF radar with oblique-incidence beams detected polar mesospheric summer echoes (PMSE) between 21:30 and 23:00 UTC. This event is regarded as the last PMC activity around Syowa Station during the austral summer season (2010–2011), since no other PMC signals were detected by lidar in February 2011. This is consistent with results of PMC and mesopause temperature observations by satellite-born instruments of AIM (Aeronomy of Ice in the Mesosphere)/CIPS (Cloud Imaging and Particle Size) and AURA/MLS (Microwave Limb Sounder) and horizontal wind measurements taken by a separate MF radar. Doppler velocity of PMSE observed by the HF radar showed motion toward Syowa Station (westward). This westward motion is consistent with the wind velocities obtained by the MF radar. However, the PMSE region showed horizontal motion from a north-to-south direction during the PMC event. This event indicates that the apparent horizontal motion of the PMSE region can deviate from neutral wind directions and observed Doppler velocities.

We investigated Pi2 pulsations in the nightside ionosphere that began at 14:15 UT (2315 LT) on 11 July 2010, and they were observed with high-temporal (8 s) resolution by beam 4 of the Super Dual Auroral Radar Network (SuperDARN) Hokkaido radar. These pulsations were simultaneously observed in both the ground/sea scatter echoes reflected from the F region height and in ionospheric echoes from field-aligned irregularities in the sporadic Es region. They had the same period of 110 s and approximately no phase lag. From the radar observations and the International Geomagnetic Reference Field model, the amplitude of the eastward (EEW) component of the electric field of the Pi2 pulsations in the ionosphere was estimated ~8.0 mV/m in the F region and ~2.0 mV/m in the E region. Corresponding Pi2 pulsations appeared dominantly in the horizontal northward magnetic field component (H) at nearby ground stations, Moshiri (MSR), St. Paratunka (PTK), and Stecolny (STC), with amplitudes ranging from 6 nT (MSR) to 10 nT (STC). At the dominant frequency of 8.8 mHz, the coherences between H and EEW were high (>0.9), the cross phases of EEW relative to H were −56° and −45°, and the amplitude ratios were 2.7 × 105 m/s and 8.4 × 105 m/s, in the E and F regions, respectively. Based on a comparison of these results with theoretical predictions, we suggest that the concept of a pure cavity mode is not sufficient to explain the combined observations for midlatitude Pi2 waves and that the contribution of an Alfven waves must be taken in account.

After publication of this article (Nishitani et al. 2019), it was brought to our attention that the figure 5 is incorrect, where the positions of DCE and SPS were misplaced. The correct figure 5 is as below, the original publication has been corrected.

昭和基地における夜光雲(NLC)と極域夏季中間圏レーダーエコー(PMSE)の同時観測について報告する.2009 年2 月11 日の2000 UT から2130 UTの間,昭和基地の南側において夜光雲が目視観測された.同時に,昭和基地SuperDARN 短波レーダーが近距離レンジにおいて特徴的なエコーを観測した.このエコーは小さいドップラー速度とスペクトル幅を持ち,短波レーダーによって観測されるPMSEの特徴と調和的であるが,同じ時間帯にNLCが観測されたことから,この特徴的な近距離エコーがPMSE であることが確認された.これに加え,NLCとPMSE の空間分布は良い一致を示し,これは短波レーダーによる観測がPMSEやNLC の2 次元分布を知る上で有用なツールになることを示唆するものである., This paper reports simultaneous observations of visible noctilucent clouds (NLC) and polar mesosphere summer echoes (PMSE) at Syowa Station (69°01′S, 38°61′E) in Antarctica. During a 1.5 h interval from 2000 to 2130 UT (2300 to 0030 LT) on Feb. 11, 2009, visible NLC were observed south of Syowa Station. The oblique sounding HF radar of SuperDARN at Syowa Station simultaneously observed peculiar echoes in the closest two range gates. The echoes had a small Doppler velocity and a narrow spectral width, which are consistent with the characteristics of PMSE in the SuperDARN data. The simultaneous appearance of the visible NLC and peculiar near-range echoes observed by the HF radar suggests that the echoes were actually a signature of PMSE in the HF band. In addition, the data from the simultaneous measurements show that the spatial distributions of NLC and PMSE in the HF band were collocated with each other, which implies that oblique sounding HF radar is a useful tool for estimating the two-dimensional horizontal distribution of PMSE.

Giant ionospheric disturbances induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) on 11 March 2011 are studied by using data from the SuperDARN Hokkaido HF radar and GPS receiver network (GEONET) in Japan. The HF radar observations revealed strong disturbances to the north of Hokkaido that propagated northward at velocities of 6.7–1.8 km/s triggered by northward-propagating seismic surface waves. An induction magnetometer in Hokkaido recorded part of the seismic wave propagation from the epicenter. After the passage of the 6.7–1.8 km/s waves the radar observed northward-propagating disturbances (343–136 m/s) due to atmospheric gravity waves (AGW) perhaps excited near the epicenter. Interestingly, the radar first detected peculiar disturbances with periods of about 2–4 min caused by the acoustic resonance. GEONET, which covers the area on the south of the radar field of view, provided total electron content (TEC) data. Comparisons between radar and TEC observations indicate the following: (1) 6.7–1.8 km/s waves observed with the radar do not always have counterparts in TEC. (2) Acoustic waves of 1.3–0.7 km/s identified in TEC are not observed with the radar. (3) Disturbances caused by both AGW and acoustic resonance are simultaneously discernible in both TEC and radar data.

There is mounting evidence that plasma structure in nighttime equatorial F layer evolves from large-scale wave structure (LSWS) in the bottomside F layer. This process cannot be ignored because equatorial plasma bubbles (EPBs), arise from large-amplitude LSWS; and, because intense radiowave scintillations are associated with EPBs, understanding the LSWS-to-EPB process is a crucial step toward reliable space-weather forecasting. In this regard, the transequatorial propagation (TEP) experiment appears to be most useful among available research instruments. After a lapse of 30 years, the TEP experiment has been resurrected [e.g., Maruyama and Kawamura, 2006]; a goal of this research is to understand TEP measurements well enough so that they can be used to diagnose the LSWS-to-EPB process. Toward this end, new results are presented in two companion papers. Herein (P1), off-great-circle (OGC) propagation paths are shown to consist of two types, discrete and diffuse. The new findings include: (1) a generalized multi-reflection model that can explain most of the observed properties; (2) the discrete type is supported by multi-reflections from an unstructured upwelling, (3) the diffuse type is supported by reflections from plasma structure in EPBs; and (4) the observed east-west (EW) asymmetry can be explained in terms of a distorted upwelling or plasma structure along the west wall of an upwelling. In Paper 2 (P2), a second form of observed EW asymmetry is explained in terms of plasma structure, which is not aligned with the geomagnetic field. The findings strongly confirm a close relationship between upwellings, ESF patches, and OGC paths.

Mesosphere summer echoes (MSE) were observed during 2000–2001 and in 2009 with a 46.5-MHz VHF radar at Wakkanai, Japan (45.4°N). MSE at VHF (VHF-MSE) are active at 80–90 km altitudes in the daytime mainly during June–July. Echo layer thickness ranges from a few to about 5 km, and layer altitude and echo intensity are often modulated by short-period atmospheric gravity waves while the layer descends gradually at 0.2–0.8 m/s with time, maybe due to tides. Detailed analyses of three VHF-MSE events show that meridional winds at the MSE altitudes over Wakkanai observed with a collocated 1.96-MHz MF radar are equatorward, indicating that cold ice particles and/or irregularities are advected southward from higher latitudes. It is found that VHF-MSE are often accompanied by simultaneous MSE at MF (MF-MSE) from a common volume, suggesting that plasma turbulence and/or short-scale electron density structures are responsible for the generation of VHF- and MF-MSE.

The plasma instability process during internal gravity wave propagation through the ionospheric E region is considered. The growth rate of the instability has been found and it has been shown that it depends on perturbation wavelength, gravity wave parameters and direction of propagation. The conditions for the instability are favorable when the vorticity of the associated neutral motion becomes antiparallel to the geomagnetic field. In the proposed instability mechanism plasma irregularities could seed the large-scale sporadic E layer structuring because they are generated in situ as a part of the same neutral wind structure that serves to initiate the formation of the layer.

We present the characteristics of small-scale (

We review measurements of nighttime atmospheric/ionospheric waves in the upper atmosphere in Japan, Indonesia, and Australia, using all-sky airglow imagers of optical mesosphere thermosphere imagers (OMTIs). The imagers observe two-dimensional patterns of airglow emissions from oxygen (wavelength: 557.7 nm) and hydroxyl (OH) (near-infrared band) in the mesopause region (80–100 km) and from oxygen (630.0 nm) in the thermosphere/ionosphere (200–300 km). Several statistical studies were done to investigate propagation characteristics of small-scale (less than 100 km) gravity waves in the mesopause region and medium-scale traveling ionospheric disturbances (MSTIDs, ∼100–1,000 km) in the thermosphere/ionosphere. Clear seasonal variations of occurrence and propagation directions were reported for these waves. The propagation directions in the mesopause region are controlled by wind filtering, ducting processes and relative location to the wave sources in the troposphere. Poleward-propagating waves tend to be observed in the summer in the mesopause region at several stations, suggesting that mesospheric gravity waves are generated by intense convective activity in the equatorial troposphere. On the other hand, systematic equatorward and westward motions were observed for all seasons for nighttime MSTIDs in the midlatitude ionosphere with geomagnetic conjugacy between the northern and southern hemispheres. Ionospheric instabilities may play important role for the generation and propagation of these MSTIDs. We also give an example of simultaneous observation of quasi-periodic southward-moving waves in the mesopause region and in the thermosphere at the geographic equator. From these results, we discuss mean wind acceleration by mesospheric gravity waves and penetration of gravity waves from the mesosphere to the thermosphere.

Using Global Positioning System (GPS) satellites, we have been conducting equatorial ionospheric scintillation observations at Kototabang, Indonesia since January 2003. Scintillations caused by equatorial plasma bubbles appear between 2000 and 0100 LT in equinoctial months with a seasonal asymmetry, and their activity decreases with decreasing solar activity. A comparison between scintillation index (S 4) and Earth’s brightness temperature (T bb) variations suggests that the scintillation activity can be related to tropospheric disturbances over the Indian Ocean to the west of Kototabang. To understand better the reasons of day-to-day variability of S 4, we analyze S 4, T bband lower thermospheric neutral wind ("Equation missing") data. The results show that S 4 fluctuates with periods of about 2.5, 5, 8, 14 and 25 days, possibly due to atmospheric waves from below and that similar periods are also found in the T bband "Equation missing" variations. Using a general circulation model, we made numerical simulations to determine the behavior of neutral wind in the equatorial thermosphere. The results indicate the following: (1) 2- to 20-day waves dissipate rapidly above about an altitude of 125 km, and 0.5- to 3-hour waves become predominant above 100 km, (2) zonal winds above 200 km altitude are, on the whole, eastward during sunset-sunrise, (3) zonal wind patterns due to short-period (1–4 h) atmospheric gravity waves (AGWs) above 120 km altitude change day by day, exhibit wavy structures with scale lengths of about 30–1000 km and, as a whole, move eastward at about 100−1 while changing patterns over time. These simulations suggest that the Rayleigh-Taylor instability responsible for plasma bubble generation can be seeded by AGWs with short periods of about 0.5–3 h, and that background conditions necessary for this instability are modulated by planetary-scale atmospheric waves propagating up to an altitude of about 120 km from below.

We investigate the relationship between medium-scale traveling ionospheric disturbance (MSTID) and sporadic E ( E s ) layer activities in summer nights by analyzing total electron content (TEC) data obtained from a global positioning system (GPS) network in Japan and ionosonde data obtained at Kokubunji, Japan during May–August in 2001–2005. MSTID activity is defined as δ I / I ¯ , where δ I is standard deviation of the TEC perturbations over Kokubunji within 1 h, and I ¯ is the background TEC. By analyzing nighttime-averaged (19-02 LT) values of MSTID activity and E s layer parameters, we find that the MSTID activity is closely correlated with f 0 E s and f 0 E s - f b E s . This result suggests that MSTID and the spatial structures of E s layer could be generated by an electro-dynamical coupling process between the E s layer and F region through polarization electric fields. Furthermore, we suggest that the appearance of the E s layer in the summer hemisphere could play an important role in generating MSTIDs in both hemispheres.

On 15 February 2007, several duskward moving sun-aligned (SA) auroral forms have been observed by the all-sky camera at Resolute Bay, Nunavut (Canada). Concurrent observations with the Rankin Inlet (RANK) PolarDARN HF radar within the field-of-view of the camera showed signatures of moving auroral forms in all signal parameters with the most remarkable effects being the echo power drop and velocity reversal as the arc reached a specific radar beam/gate. Spatial and temporal variations of the velocity in the vicinity of the SA form are investigated. It is shown that the form-associated convection reversal was located poleward (duskward) of the global-scale convection reversal associated with the dawn cell of the large-scale convection pattern. Thus, the RANK radar was monitoring the polar cap portion of the global-scale convection pattern and its transition from the IMF ByBy>0 situation. Magnetic perturbations associated with the SA form passing the zenith of several magnetometers are investigated. It is shown that although magnetometer signatures of the moving form were clear, the convection pattern derivation from magnetometer records alone is not straightforward.

As part of the CPEA (Coupling Processes in the Equatorial Atmosphere) project, we have been conducting ground-based optical and radio observations of the ionosphere and thermosphere at Kototabang in Indonesia, Japan, and Australia. First, this paper gives a brief overview of some results, paying special attention to 100-1000 km scale plasma disturbances in the F region ionosphere, i.e., medium-scale traveling ionospheric disturbances (MSTIDs) and plasma bubbles. MSTIDs over the equator are observed within and in the south of the F region equatorial anomaly crest and have, on the average, a phase velocity of 300 m s−1 toward the south, a period of 40 min, and a wavelength of 700 km. Plasma bubbles move to the east at ∼ 100 m s−1, have a scale of about 100 km with spacings of 200-250 km, and are embedded within plasma structures with a scale of about 1000 km. Interestingly, giant plasma bubbles can be simultaneously detected at lower midlatitudes in southern and northern hemispheres that are connected by the geomagnetic field line, and are very identical in appearance in the both hemispheres. Then, we present newly-obtained characteristics of equatorial ionospheric scintillations of 1.6 GHz GPS radio waves associated with plasma bubbles. Continuous scintillation observations for two years at Kototabang indicate that the scintillations appear predominantly from sunset to midnight in equinoctial months. Such a seasonal variation is also recognized from a statistical study of bubble occurrences over the Philippines, Singapore, and Indonesia. To investigate possible dynamical coupling between the ionosphere/thermosphere and troposphere over the equator, we compare the scintillation (bubble) activity and Earth’s brightness temperature variation over the Indian Ocean measured by meteorological satellites. The results indicate that there can exist meaningful correlations between the scintillation occurrence and tropospheric disturbance at 80°-95°E longitudes, i.e. 5°-20° west of Kototabang. Possible processes to seed plasma bubbles are discussed.

To investigate drift velocities of a few hundred meter-scale irregularities associated with equatorial plasma bubbles, we used three single-frequency GPS receivers at the Equatorial Atmosphere Radar (EAR) site at Kototabang, Indonesia (0.20°S, 100.32°E; geomagnetic latitude 10.6°S), since January 2003. The GPS receivers sampled GPS signal intensity at a rate of 20 Hz. Distances between the receivers were 116, 127, and 152 m. An analysis of scintillation index (S 4 ) in two years (2003-2004) revealed that the scintillations often occurred between 2000-0100 LT at equinoxes and that their occurrence rate was higher during March-April than during September-October. Drift velocities of irregularities were measured using cross-correlation analysis with the time series of the GPS signal intensity obtained from the three receivers. From a statistical analysis of the drift velocities, the eastward component of drift velocity just after sunset is found to be greater during March-April than during September-October. Based on these results, for the first time, we suggest that the east-west component of plasma drift velocity (or vertical electric field) may be related to the evolution of plasma irregularities causing scintillations throughout the mechanism causing the prereversal enhancement of the eastward electric fields. The equinoctial asymmetry of the drift velocity could be attributed to the equinoctial asymmetry of neutral winds in the thermosphere.

We have made a comprehensive measurement of a front-like structure in the mesosphere at the equator at Kototabang, Indonesia (0.2°S, 100.3°E), using an airglow imager, an airglow temperature photometer, a meteor radar, and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. The event was detected in airglow images of both OH-band (peak emission altitude: 87 km) and 557.7-nm (96-km) emissions, as an east-west front-like structure propagating northward with a velocity of 52-58 m/s. Wave trains with a horizontal wavelength of 30-70 km were observed after the passage of the front. The airglow intensity decreased for all the mesospheric emissions of OI (557.7 nm), OH-band, O2-band (altitude: 94 km), and Na (589.3 nm) (90 km) after the front passage. The rotational temperatures of both the OH-band and the O2-band also decreased ∼10 K. An intense shear in northward wind velocity of 80 m/s/6 km was observed at altitudes of 84-90 km by the meteor radar. The kinetic temperature profile at altitudes of 20-120 km was observed near Kototabang by TIMED/SABER. The front-like structure and trailing waves were similar to those of the mesospheric tidal bore. However, we found that the ducting condition, which is necessary to form a bore, was not satisfied for the observed wave parameters. We speculate that the intense wind shear may play some role for the generation of the front-like structure.

The physics of the unstable E-region plasma is based on the modified two stream, or Farley-Buneman, and the gradient drift instabilities. The theory combines both mechanisms into a single dispersion relation which applies for the directly generated short-scale plasma waves, known as type1 irregularities. In the absence of a plasma gradient it is only the two stream mechanism acting which favors wave excitation if E×B electron drifts relative to the ions exceed a threshold slightly above the ion acoustic speed. On the other hand, the theory also predicts that a destabilizing (stabilizing) electron density gradient acts to decrease (increase) the ion acoustic threshold, and hence the wave phase velocities at threshold, depending on the gradient strength and the wavelength. Given a destabilizing plasma gradient, the threshold reduction is larger at longer than shorter wavelengths and thus the best way to test the gradient role is by simultaneous observations of type1 waves at two or more radio backscatter frequencies. The present paper relies on dual frequency backscatter observations of 1.1 m and 3.1 m type1 irregularities made simultaneously at 144 MHz and 50 MHz, respectively, in mid-latitude sporadic E-layers. Using as typical plasma gradient scale lengths for destabilized sporadic E-layers those that are obtained from rocket electron density profiles, the radar observations are compared with the predictions of kinetic theory. The results suggest that the plasma density gradient effect on meter scale Farley-Buneman waves is not important. This is reinforced further by the analysis of backscatter from destabilized meteor trail plasma when very steep gradients are expected in electron density. The present findings, and more from past studies, question the electron density gradient role in the generation of short-scale plasma waves as predicted by the linear instability theory. This deserves attention and more study.

We present the first coordinated observations of quasi-periodic (QP) radar echoes from sporadic-E (Es) field-aligned irregularities (FAIs), OI 557.7-nm airglow, and neutral winds in a common volume over Shigaraki, Japan (34.9° N, 136.1° E) on the night of 5 August 2002 during the SEEK-2 campaign. QP echo altitudes of 90-110 km were lower than usual by 10 km, enabling us to make a detailed comparison among QP echoes, airglow intensity, and neutral wind at around 96 km altitude. Eastward movement of the QP echo regions is consistent with the motions of neutral winds, airglow structures, and FAIs, suggesting that the electrodynamics of Es-layers is fundamentally controlled by the neutral atmospheric dynamics. During the QP echo event, the echo altitudes clearly went up (down) in harmony with an airglow enhancement (subsidence) that also moved to the east. This fact suggests that the eastward-moving enhanced airglow region included an upward (downward) component of neutral winds to raise (lower) the altitude of the wind-shear node responsible for the Es formation. The airglow intensity, echo intensity, and Doppler velocity of FAIs at around 96 km altitude fluctuated with periods from 10 min to 1h, indicating that these parameters were modulated with short-period atmospheric disturbances. Some QP echo regions below 100km altitude contained small-scale QP structures in which very strong neutral winds exceeding 100 m/s existed. The results are compared with recent observations, theories, and simulations of QP echoes. Keywords. Ionosphere (Ionosphere-atmosphere interactions; Ionospheric irregularities; Mid-latitude ionosphere)

Polar mesosphere summer echoes (PMSEs) have been well studied using vertical incidence VHF radars at northern high-latitudes. In this paper, two PMSE events detected with the oblique incidence SuperDARN HF radars at Hankasalmi, Finland (62.3° N) and Syowa Station, Antarctica (69.0° S), are analyzed, together with simultaneous VHF and medium-frequency (MF) radar data. Altitude resolutions of the HF radars in the mesosphere and the lower thermosphere are too poor to know exact PMSE altitudes. However, a comparison of Doppler velocity from the HF radar and neutral wind velocity from the MF radar shows that PMSEs at the HF band appeared at altitudes within 80-90km, which are consistent with those from previous vertical incidence HF-VHF radar results. The HF-VHF PMSE occurrences exhibit a semidiurnal behavior, as observed by other researchers. It is found that in one event, PMSEs occurred when westward semidiurnal winds with large amplitude at 85-88km altitudes attained a maximum. When the HF-VHF PMSEs were observed at distances beyond 180km from MF radar sites, the MF radars detected no appreciable signatures of echo enhancement. Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; thermospheric dynamics; waves and tides)

Measurements of OH rotational temperatures have been carried out at Shigaraki, Japan (34.8°N, 136.1°E), for 8 nights in September–December 2000, using the Spectral Airglow Temperature Imager (SATI) and a tilting-filter zenith photometer (MC4), simultaneously. Clear positive correlation was obtained between the two temperatures. However, the absolute temperatures obtained by SATI were unusually low (∼130– 180 K ) with differences of ∼60 K from the MC4 temperatures. We discuss possible causes of the temperature differences, particularly for SATI, which newly uses OH(6-2) Q-branch lines (MC4 uses OH(6-2) P-branch lines) and imaging optics for the temperature measurement.

Polar mesosphere summer echoes(PMSE) are strong VHF-UHF radar echoes from the high-latitude cold mesopause at around 80-90km altitudes in summer. Although a number of in situ and radar observations of PMSE have been made until now, generation mechanisms of PMSE and scattering processes of radar waves due to PMSE-associated irregularities are still controversial. In this paper, PMSE detected for the first time in December 1997 and January 1998 with the oblique incidence SuperDARN HF radars at Syowa Station, Antarctica(69.0°S , 39.6°E ), are summarized to reveal the characteristics of PMSE at HF band. They appear at slant ranges of 180-315km with elevation angles of 15°-30° between 1030 and 1230UT or between 2100 and 0140UT, and are characterized by durations of 65-110min with intermittent subsidence and quasi-periodic oscillations of echo power with periods of 5-20min, due to short-period atmospheric gravity waves. Detailed analysis of the December 15, 1997 event reveals the followings: 1) echo power is less than 30dB, Doppler velocity between -40 and +40m/s, and spectral width less than 50m/s, respectively, 2) there exists no particular correlation among power, velocity and width, and 3) PMSE occurrence can be related to eastward neutral wind due to semi-diurnal tide that may induce the decrease in the mesospheric temperature.

E region radar echo parameters (echo power, Doppler velocity and spectral width) obtained with the Syowa Station HF radar under disturbed geomagnetic conditions are qualitatively analyzed to study how echoing region changes due to HF wave refraction caused by ionospheric disturbance. It is found that with increasing disturbance level, echo ranges become shorter because of wave refraction during propagation due to more enhanced D and E region electron density. When geomagnetic H component variation (ΔH) is less than about -900 nT, echoes are returned from the central E region where geomagnetic aspect angle is close to zero. When ΔH is very high (= - 1500nT), the echoes are backscattered from the D and lower E regions and their power, Doppler velocity and spectral width are largely suppressed. The results suggest that we must always consider, more or less, wave refraction effect in analyzing near-range E region HF radar echoes.

This paper reports the performance and capability of a newly developed zenith spectrometer (for measurements of airglow and aurora) that uses an acousto-optic tunable filter (AOTF). The AOTF can scan the pass-band of the spectrometer between 450 and 700 nm with a bandwidth of 2-3 nm by changing the RF driver frequency from 180 to 100 MHz. The absolute sensitivity of the spectrometer is ～0.1-1.5 counts/Rayleigh/s per spectral step. The spectrometer is fully automated. The O I (557.7 nm) airglow line can be clearly identified in test observations of midlatitude airglow performed at Shigaraki Observatory, Japan. Based on an estimate of the signal-to-noise ratio, we conclude that the full auroral spectrum (450-700) nm can be measured by the AOTF spectrometer with a time resolution of ～100 s and a signal-to-noise ratio of ～100 for an auroral emission intensity of 10 kR. An example of the auroral spectra is shown for observations made at Syowa Station in Antarctica.

Vertical total electron content (GPS-TEC) data obtained from the dual-frequency GPS receiver network (GEONET) in Japan are compared with those calculated using the Sheffield University plasmasphere-ionosphere model (SUPIM). The model is also used to estimate the electron content in the plasmaspheric sections of GPS ray paths for the three seasons of high solar activity (F10.7 = 165) under magnetically quiet conditions. According to the estimates, the plasmaspheric sections of vertical GPS ray paths over Japan at altitudes above the O+ to H+ transition height and above the upper altitude (2500 km) of Faraday rotation contain up to 11 and 9 TEC units (1 TEC unit = 1016 electrons m−2) of free electrons, respectively. The free electrons present above the Faraday rotation altitude can cause propagation errors of up to 4.9 ns in time delay and 1.6 m in range at the GPS L1 (1.57542 GHz) frequency. The plasmaspheric electron content, PEC, changes appreciably with season and latitude and very little with the time of the day. However, the percentage contribution of PEC to GPS-TEC changes most significantly with the time of the day; the contribution varies from a minimum of about 12% during daytime at equinox to a maximum of about 60% at night in winter.

The dual frequency radio signals of the Global Positioning System (GPS) allow measurements of the total number of electrons, called total electron content (TEC), along a ray path from GPS satellite to receiver. We have developed a new technique to construct two-dimensional maps of absolute TEC over Japan by using GPS data from more than 1000 GPS receivers. A least squares fitting procedure is used to remove instrumental biases inherent in the GPS satellite and receiver. Two-dimensional maps of absolute vertical TEC are derived with time resolution of 30 seconds and spatial resolution of 0.15° × 0.15° in latitude and longitude. Our method is validated in two ways. First, TECs along ray paths from the GPS satellites are simulated using a model for electron contents based on the IRI-95 model. It is found that TEC from our method is underestimated by less than 3 TECU. Then, estimated vertical GPS TEC is compared with ionospheric TEC that is calculated from simultaneous electron density profile obtained with the MU radar. Diurnal and day-to-day variation of the GPS TEC follows the TEC behavior derived from MU radar observation but the GPS TEC is 2 TECU larger than the MU radar TEC on average. This difference can be attributed to the plasmaspheric electron content along the GPS ray path. This method is also applied to GPS data during a magnetic storm of September 25, 1998. An intense TEC enhancement, probably caused by a northward expansion of the equatorial anomaly, was observed in the southern part of Japan in the evening during the main phase of the storm.

Southwestward-propagating medium-scale traveling ionospheric disturbances (MSTIDs) observed over Shigaraki (34.85°N, 136.10°E) in Japan on the night of May 22, 1998 are analyzed in detail. The MSTIDs were detected with a 630.0 nm (OI) all-sky imager at Shigaraki and a large number of GPS (Global Positioning System) receivers distributed around Shigaraki. Each GPS receiver provided total electron content (TEC) between the GPS altitude (20,200 km) and the ground. MSTID amplitudes varied in space and time, and showed decay and enhancement during the southwestward propagation, suggesting that amplitudes of atmospheric gravity waves and the interaction process between gravity waves and F region plasma were highly variable. It is found that spatial and temporal fluctuations of the 630 nm intensity are well correlated with those of GPS-TEC except for a certain period of time. The Scheffield University Plasmasphere Ionosphere Model (SUPIM) is used to obtain theoretical relationships between the 630 nm airglow intensity and GPS-TEC and between their fluctuation amplitudes. The results indicate that the fluctuation amplitudes observed in weak airglow regions are caused by an electron density fluctuation of about ±20% occurring around an altitude of 250 km, where the 630 nm emission rate reaches a maximum, below the F layer peak altitude. Highly enhanced 630 nm intensity and GPS-TEC within a bright airglow region are due to an electron density enhancement of about 150% occurring at altitudes below 300 km.

May 10–11, 1999, is known as an interval of very weak solar wind, with an interplanetary plasma density of

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.

A large data-set of line-of-sight Doppler velocity obtained with the Antarctic Syowa East HF radar from February to December 1997 is analyzed to discuss the statistical characteristics of Doppler velocity (V_D) at ranges of 180-1200 km and their implications. Syowa Station K-indices during the observation period were between 0 and 7 with a maximum occurrence at K = 1. On average V_D has a minimum of about 100 m/s at 180-225 km ranges. With increasing range it increases monotonically to attain a maximum of 300-350 m/s at 400-500 km, decreases gradually to reach 250-300 m/s at about 700km, and again increases slowly at farther ranges. These values of V_D and the range vary depending on both local time and radar beam direction. In the light of recent knowledge of plasma instabilities in the ionosphere we suggest that such range profile of V_D is mainly caused by the combined effects of altitude-dependent phase velocities of ionospheric plasma waves, HF wave refraction due to enhanced E region electron density, and latitude-dependent electric field. We infer that the low V_D (～ 100 m/s) at ranges of 180-225 km may originate in part from neutral winds and/or turbulence of the neutral atmosphere.

Nearly simultaneous observations of 12- and 50-MHz coherent echoes at the Antarctic Syowa station are considered for studying the power-Doppler velocity relationship at these significantly different radar frequencies. We concentrate on postmidnight measurements when both 12- and 50-MHz echoes were seen as narrow bands (∼150–200 km) of strong backscatter at short distances of

Simultaneous CUTLASS and EISCAT Svalbard radar (ESR) observations on February 1, 1998, are used to study the generation of plasma density suppression, which may ultimately result in polar cap patch formation, occurring around the cusp region under IMF Bz negative and By positive conditions. The CUTLASS HF radars in Iceland and Finland observed F region plasma drifts in a wide area including the ESR field of view while the ESR monitored electron density, electron temperature, ion temperature, and ion motion along the geomagnetic field. We focus on two events for which the density suppressions (30–60%) formed in harmony with strong plasma drifts (> 1500 m s−1) lasting for 5–12 min. In one event, the suppression is mainly caused by enhanced chemical reactions in the F region due to the intensified convection flows which also raise ion temperature by 1500–2000 K through frictional heating. This process can chop preexisting high-density region produced by energetic particle precipitation, maybe giving rise to polar patches. In the other event, the density suppression is related to an appearance of eastward directed high-speed plasma jets in a limited region with a latitudinal width of > 100 km and a longitudinal extension of > 500 km. The ESR data, however, show no ion temperature increase, suggesting that the suppression may not be caused by enhanced chemical reactions. We tentatively propose that the eastward plasma jets transported less dense plasma from earlier local times over the ESR. Rapid change of IMF By polarity is another candidate for producing the density suppression. A role of HF radar wave refraction in explaining this event is discussed.

The F-region Radio and Optical measurement of Nighttime TID (FRONT) campaign was conducted to clarify the non-classical features of traveling ionospheric disturbances (TIDs) at mid-latitudes in May, 1998 and August, 1999. A cluster of all-sky CCD cameras and a GPS receiver network observed a wide area of the ionosphere over Japan to detect the spatial structure and temporal evolution of TIDs. The propagation direction of the nighttime TID detected during the FRONT campaign periods is restricted to the southwest. The time evolution of their amplitude indicates that the TID structure is intensified as it travels from high-latitudes to low-latitudes. The significant coincidence between the structures of 630 nm band airglow and total electron content indicates that the perturbations take place in the bottomside of the ionospheric F region. Coherent echoes from the field-aligned irregularities were observed by the MU radar in the nights when the TID activity was high.

Seasonal variations of activity and horizontal propagation direction of gravity waves in the MLT region at mid-latitude over Shigaraki, Japan (35 N) are investigated for various time and spatial scales using the MU radar observations and recent airglow imaging observations. Semiannual variation with solsticial maxima and equinoctial minima are generally observed for short period (