DOI： 10.1029/2023JA031402

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DOI： 10.1186/s40623-023-01829-0

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DOI： 10.3389/fspas.2023.1147531

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DOI： 10.1029/2022JA031157

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DOI： 10.1186/s40623-023-01774-y

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PubMed

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DOI： 10.1186/s40623-022-01665-8

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DOI： 10.1029/2021GL097107

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PubMed

DOI： 10.1029/2021JA029965

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DOI： 10.1029/2021JA030230

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DOI： 10.1029/2021JA029502

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DOI： 10.1029/2021JA029925

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DOI： 10.1029/2021JA029300

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DOI： 10.1016/j.polar.2021.100669

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DOI： 10.1016/j.polar.2021.100690

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DOI： 10.1029/2021JA029192

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DOI： 10.1016/j.polar.2021.100671

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DOI： 10.1029/2020JA029090

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DOI： 10.1029/2020GL089451

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DOI： 10.1029/2020JA028101

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DOI： 10.1029/2020JA028126

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DOI： 10.1029/2020JA028067

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DOI： 10.1029/2020JA027782

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DOI： 10.1029/2019JA027473

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DOI： 10.1186/s40623-020-01168-4

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PubMed

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DOI： 10.1029/2019JA027261

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DOI： 10.1029/2019EA000911

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DOI： 10.23919/ursigass49373.2020.9232189

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DOI： 10.23919/ursigass49373.2020.9232206

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DOI： 10.1186/s40623-019-1092-9

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DOI： 10.1186/s40623-019-1070-2

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DOI： 10.1029/2019SW002179

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DOI： 10.1186/s40645-019-0300-3

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DOI： 10.1029/2018SW002130

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DOI： 10.1029/2019SW002168

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DOI： 10.5194/angeo-37-215-2019

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DOI： 10.1186/s40645-019-0270-5

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DOI： 10.1038/s41467-018-07996-z

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PubMed

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DOI： 10.23919/ursiap-rasc.2019.8738444

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DOI： 10.12737/stp-52201906

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DOI： 10.1109/piers-spring46901.2019.9017884

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DOI： 10.1029/2018GL079812

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DOI： 10.1016/j.jastp.2017.09.013

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DOI： 10.1029/2018GL079777

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DOI： 10.1029/2018JA025690

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DOI： 10.1029/2018GL078723

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DOI： 10.1016/j.jastp.2018.03.022

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DOI： 10.1186/s40623-018-0867-8

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DOI： 10.1002/2017RS006488

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DOI： 10.1002/2017GL075765

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DOI： 10.1002/2017JA024198

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DOI： 10.1186/s40623-017-0745-9

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DOI： 10.1186/s40623-017-0742-z

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DOI： 10.1002/2017JA024611

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DOI： 10.1002/2017JA023990

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Simultaneous imaging observations of a noctilucent cloud (NLC) from five sites in Hokkaido, Japan (43.17-45.36 N), were successfully carried out using digital cameras in the early hours of the morning (around 02:00 LST) on June 21, 2015. This is the first NLC event that has been captured from multiple sites in Japan. The simultaneous images obtained from multiple sites made it possible to calculate the exact altitude (=83.9 +- 0.1 km) and spatial distribution (47.5-50.0 N and 143.0-147.5 E) of the NLC by triangulation and image correlation methods. Based on a comparison of atmospheric parameters of the upper mesosphere provided by satellites and a middle-frequency (MF) radar in northern Hokkaido (Wakkanai) with the cloud distribution obtained from the Aeronomy of Ice in the Mesosphere satellite, this particular event is considered to be the result of southward advection of the NLC from a higher-latitude (i.e., colder) region. Anomalies in the upper mesospheric temperature of the northern hemispheric summer in 2015 were examined using AURA satellite data, because this is the first NLC event that has been identified in Japan. However, no remarkable temperature variations relative to other years were found in upper mesosphere. Based on a comparison between the NLC period and the record of sky conditions archived by the Japan Meteorological Agency, a high percentage of cloud (especially low-level) cover during the summer in Hokkaido cannot be ruled out as a possible reason why the NLC had not previously been sighted in Hokkaido.

DOI： 10.1186/s40623-016-0562-6

Language：English   Publishing type：Research paper (scientific journal)  

Information on the vertical angle of arrival (elevation) is crucial in determining propagation modes of high-frequency (HF, 3-30 MHz) radio waves travelling through the ionosphere. The most advanced network of ionospheric HF radars, SuperDARN (Super Dual Auroral Radar Network), relies on interferometry to measure elevation, but this information is rarely used due to intrinsic difficulties with phase calibration as well as with the physical interpretation of the measured elevation patterns. In this work, we propose an empirical method of calibration for SuperDARN interferometry. The method utilises a well-defined dependence of elevation on range of ground scatter returns. "Fine tuning" of the phase is achieved based on a detailed analysis of phase fluctuation effects at very low elevation angles. The proposed technique has been successfully applied to data from the mid-latitude Hokkaido East SuperDARN radar. It can also be used at any other installation that utilises HF interferometry.

DOI： doi: 10.1186/s40623-015-0310-3

Language：English   Publishing type：Research paper (scientific journal)  

We investigate the characteristics of the subauroral polarization stream (SAPS), with focus on the relationship between geomagnetic parameters and occurrence characteristics of SAPS. This study's observations were made using the Super Dual Auroral Radar Network (SuperDARN) Hokkaido East radar, which can observe the Far East region of Russia and has been in operation since 2006. In particular, we identify the lowest limit of SAPS speed, which has not been discussed in previous literature, in order to examine the lowest threshold of electric field able to generate SAPS as a result of magnetosphere-ionosphere (M-I) coupling. In order to conduct a comprehensive investigation of SAPS occurrence characteristics, we analyzed events with wider ranges of velocity and magnetic latitude (MLAT) than those in previous studies. As a result of quantitative estimation, we found two categories of westward flows that were reasonably separated using a speed threshold of 150-200 m/s. For the faster flows above the speed threshold, there is a clear correlation between MLAT and SYM-H geomagnetic index, whereas for the slower flows, there is no such correlation. The faster flows are considered to be SAPS, whereas the slower flows are probably associated with mid-latitude F-region ionospheric irregularities not directly related to storms or substorms. This slowest limit of SAPS gives us a minimum electric field of 7.5-10 mV/m that generates SAPS. However, this field strength is not strong enough to cause frictional heating, which is generally considered to be a crucial mechanism for generating SAPS. This result suggests that frictional heating is not always necessary to generate SAPS.

DOI： 10.1186/s40623-015-0299-7

Language：English   Publishing type：Research paper (scientific journal)  

We describe an automated technique to determine parameters of traveling ionospheric disturbances (TIDs) using the Super Dual Auroral Radar Network (SuperDARN) high frequency (HF) radar data. The technique is based on the analysis of minimum ground backscatter range variations corresponding to different radar beams. Using this technique, we processed the SuperDARN Hokkaido radar data for 2011 and revealed statistical distributions of medium-scale TID (MSTID) azimuth and apparent horizontal velocity. We found four peaks with a distinct diurnal and seasonal dependence in the MSTID azimuth occurrence rate distributions. Northeast MSTID azimuths (20° to 50°) are typical of the summer and equinox morning hours; southeast azimuths (100° to 140°) prevail in the winter daytime; southwest azimuths (190° to 220°) are typical mostly in the summer and equinox nighttime and in the equinox evening; northwest azimuths (280° to 320°) are typical of the summer daytime and evening. The apparent horizontal velocities are generally within the 100 to 160 m/s range. The obtained results agree well with earlier studies by other researchers. However, there are also certain differences. The summer daytime northwestward MSTIDs are not indicated in the earlier studies. The nighttime horizontal velocities are 1.5 to 2 times higher than those in the daytime. Furthermore, winter velocity values are about 1.5 times higher than those in other seasons. These differences might be associated with the peculiarities of the data recorded by different facilities, or the features of the processing techniques, and require further investigation for their interpretation.

DOI： 10.1186/s40623-015-0192-4

Language：English   Publishing type：Research paper (scientific journal)  

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 ~8.0mV/m
in the F region and ~2.0mV/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 from6 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 × 105m/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 Alfvén waves must be taken in account.

DOI： 10.1002/2012JA018585

Language：English   Publishing type：Research paper (scientific journal)  

The Super Dual Auroral Radar Network Hokkaido HF radar often detects periodically reoccurring E region echoing regions propagating toward or away from it. In this work, we consider 117 of such events identified for 2008-2012. These are shown to occur at nighttime, preferentially during summer, although significant number for events was found for winter. Statistics for the local time of occurrence, magnitude of the speed and polarity of progressions, and temporal and spatial periodicities are presented. We show that the power of echoes is linearly related to their Doppler velocity which makes it possible to identify the events on both power and Doppler velocity plots. Other peculiar characteristics of echoes are discussed. The onset of
events is associated with gravity waves propagation through the radar field of view.

DOI： 10.1002/2013JA019422

Language：English   Publishing type：Research paper (scientific journal)  

DOI： http://dx.doi.org/10.1016/j.asr.2014.01.011

Language：English   Publishing type：Research paper (scientific journal)  

DOI： in press

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 12-18

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Ionospheric responses to the 2011 off the Pacific coast of Tohoku Earthquake are studied using the SuperDARN Hokkaido radar, which is located at (43.5^o N, 143.6^o E) and which monitors the ionosphere over a wide horizontal area. The radar observed an oscillation of the vertical motion of the ionosphere with a period of about 1 to 2 min. The disturbance propagated northward, away from the epicenter with the velocity of about 6.2, 4.5, 3.9 and 3.5 km/s. The latter three values are basically consistent with the propagation of the Earth's surface waves reported in several previous studies. The propagation velocities decreased with time, which has not been reported in previous studies for this propagation velocity range. The peak-to-peak amplitudes of Doppler velocities of ground/sea scatter echoes observed by the radar were up to 200 m/s, which is considerably larger than previously-reported values using HF Doppler measurements, although they are not extremely large for this historical earthquake (M = 9.0). This is the first time that ionospheric data have been obtained with high temporal (8 s) and spatial (22.5 km) resolutions following a giant earthquake, which enables us to discuss the detailed characteristics of the propagation of coseismic ionospheric disturbances.

DOI： doi:10.5047/eps.2011.07.003

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

We present midlatitude medium-scale traveling ionospheric disturbances (MSTIDs)
observed with a Super Dual Auroral Radar Network (SuperDARN) HF radar at around
10 MHz in Hokkaido, Japan, in combination with a 630-nm all-sky imager and a GPS
network (GEONET) that provides total electron content (TEC) data. MSTIDs propagating
southward from high latitudes are detected at first with the HF radar and then with
the imager and GEONET. We analyze two MSTID events, one in winter (event 1) and
the other in summer (event 2), to find that MSTIDs appear simultaneously, at least, at
55–25N. It is shown that nighttime MSTIDs propagate toward the southwest over a
horizontal distance of about 4000 km, and daytime MSTIDs do so toward the southeast.
Daytime radar echoes are due to ground/sea surface (GS) scatter, while nighttime
echoes in event 1 return from 15-m-scale F region field-aligned irregularities (FAIs) and
those in event 2 are due to GS scatter. Doppler velocities of the nighttime F region
FAI echoes in event 1 are negative (motion away from the radar) within strong echo
regions and are positive (motion toward the radar) within weak echo regions. This fact
suggests that the strong (weak) echoes return from suppressed (enhanced) airglow/TEC
areas, in line with previous observations over central Japan. The nighttime MSTIDs in
events 1 and 2 are often accompanied by concurrent coherent echoes from FAIs in
sporadic E (Es) layers. The Es echo areas in event 2 rather coincide with suppressed
airglow/TEC areas in the F region that are connected with the echo areas along the
geomagnetic field, indicating the existence of E and F region coupling at night.

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Two-dimensional observations of ionospheric plasma flows possibly caused by overshielding are reported for the first time. The observations were made by the mid-latitude SuperDARN Hokkaido radar in Japan during a major magnetic storm on December 15, 2006. The magnetosphere was exposed continuously to a southward interplanetary magnetic field (IMF) for several hours during the main phase of the storm. Immediately following the subsequent northward turning of the IMF, an anti-sunward plasma flow was observed for about 14 min in the pre-dusk sector at magnetic latitudes of 50-60°, reaching a maximum line-of-sight speed of 70-80 m/s. These features are consistent with a simulation of coupling between the ring current and the ionosphere associated with an overshielding condition. Within 1 h of the first observation, a similar anti-sunward flow was observed during a period of southward-oriented IMF. However, the simulation cannot account for the anti-sunward flow in this case. It is suggested that the shielding/overshielding condition is not simply caused by the northward turning of IMF. This second overshielding-like condition is attributable to a sudden contraction of the polar cap associated with the substorm, or to a sudden strengthening of the inertial current converted from the abrupt injection of magnetospheric ions. However, none of both fully accounts for the observations.

Language：English   Publishing type：Research paper (scientific journal)  

During the intense magnetic storm of 15 December 2006, the midlatitude Super Dual Auroral Radar Network (SuperDARN) Hokkaido radar observed a dynamical character of rapid, westward flows at 50–56 magnetic latitude. The simulation that couples the inner magnetosphere and the subauroral ionosphere was performed using a realistic boundary condition of the hot ion distribution determined from four Los Alamos National Laboratory satellites at 6.6 R E . The following results are obtained using the simulation: (1) In general, morphology of the azimuthal component of the simulated ionospheric plasma flow is consistent with that known as the subauroral polarization stream (SAPS), (2) an increase in the hot ion density in the plasma sheet results in the temporal reduction and subsequent intensification of the rapid flow at certain subauroral latitudes with a delay of ∼40 min, and (3) influence of the plasma sheet temperature on the rapid flow is not evident. The simulated line-of-sight velocity is compared with that obtained by the SuperDARN Hokkaido radar. Agreement between them is found in terms of the temporal and spatial variations of the rapid flows as well as the flow velocity. It is suggested that the dynamical character of the subauroral plasma flow is a direct manifestation of the plasma pressure distribution in the inner magnetosphere (the ring current) especially during the magnetic storm.

Language：English   Publishing type：Research paper (scientific journal)  

SuperDARN Hokkaido HF radar, capable of measuring the subauroral ionospheric plasma convection especially during storms, has been in continuous operation since the beginning of December 2006. We report the first two-dimensional observation of a dynamic variation of convection flow reversal in subauroral postmidnight sector during the storm main phase on 29 January 2007. The flow reversal region is extended over 20° in longitude and 5° in latitude, lasting for about 10-15 min, and the maximum flow speed is about 0.5-1.0 km/s. The flow reversal structure is reasonably reproduced by the ring current simulation coupled with the ionosphere, suggesting that it is produced by the region 2 field-aligned current associated with the ring current enhancement during the storm main phase. The dynamic variation of the flow reversal structure is interpreted as a transient eastward extension of the elongated dusk convection cell to the postmidnight and equatorward of the dawn cell, associated with the variation of the ring current whose structure is controlled by the interplanetary magnetic field and solar wind dynamic pressure. It is suggested that the ring current variation is highly coupled with the interplanetary parameters and is much more complicated than ever thought.

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

HF ray path calculation is performed in order to identify possible
ionospheric backscatter echo area for an HF radar at mid-latitude. The calculation is
made on the basis of the R.M. Jones and J. J. Stephenson (U.S. Dept. of Commerce,
OT Rep. 75-76, 1975) HF ray path tracing algorithm plus the IRI-2001 ionosphere
model. It is shown that depending on the local time and geomagnetic activity, the
possible ionospheric backscatter regions have different distributions. In any case the
backscatter region is large enough, indicating the capability of a planned HF radar in
Hokkaido (43.5 N, 143.6 E), Japan.

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

The spectral width of the ionospheric backscatter echoes obtained with the SuperDARN radars has been regarded as a useful tool for locating specific ionospheric regions such as the cusp. In this paper we report the presence of ionospheric echoes with high (> 450 m/s) Doppler velocity and very low (< 60 m/s) spectral width, observed by the CUTLASS and Syowa East and South SuperDARN radars. These echoes have the following characteristics. (1) They have a close correlation with geomagnetic activity such that as the Dst index decreases, the radars tend to observe ionospheric echoes with high Doppler velocity and very low spectral width more frequently. (2) Their existence does not depend on magnetic local time. (3) They are located preferably in the polar cap region, where anti-sunward convection prevails. (4) They sometimes exist over a wide range, so they are more likely to be F-region rather than E-region echoes. The occurrence of these echoes during active periods is associated with the suppression of the electric field turbulence. The present result appears consistent with the previous paper by Golovchanskaya et al. [2002], who showed the negative correlation between the electric field turbulence level and geomagnetic activity.

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Ionospheric convection over the southern polar cap on May 11-12, 1999 has been studied by using the Syowa East and South HF radar data and the DMSP ion driftmeter data, when the solar wind density was very low and geomagnetic activity was low. The overall convection pattern is consistent with the previous results by Ohtani et al. [2000]. However, the Syowa radars and the DMSP satellites observed very high (> 1500 m/s) westward plasma flows at dusk directed from the nightside toward the dayside only in the southern (dark) hemisphere. The high-speed flow was observed continuously across the fields of view of both radars from 1530 UT on May 11 to 0200 UT on May 12, when the solar wind density was close to minimum. Comparison with the DMSP particle and auroral image data shows that the westward flow regions were located in the middle of the auroral precipitation area. The strong asymmetry of the convection between the two hemispheres indicates the importance of the presence (absence) of solar illumination for the absence (presence) of the strong and localized ionospheric flows.

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

One example of the response of ionospheric convection and the polar cap boundary to a sudden change in the IMF orientation has been studied when the IMF suddenly changed from northward (+6 nT) to strongly southward (-19 nT), at 1716 UT on September 5, 1995, The Bz component was fairly constant for about 2 hours before and about 25 min after the sudden IMF change. The convection flow changed almost simultaneously over a global extent. This initial change of the convection pattern can be characterized by a sudden formation of a large flow vortex in the afternoon sector. On the other hand, the response of the polar cap boundary (or its proxy) is more complicated. The radar and particle data indicate the immediate equatorward expansion of the precipitation regions in the noon and premidnight sectors. About 10 to 20 minutes after the initial change, there were changes observed in the dusk region, namely an equatorward expansion of the current reversal boundary in the dusk sector between 1740 and 1750 UT, and an equatorward expansion of the convection reversal boundary. The delayed responses were observed 18 to 8 minutes before a substorm onset was recorded at mid-latitude stations at 1756 UT. These observations indicate that there were two kinds of ionospheric responses to the southward turning of the IMF; the first response is the formation of the convection vortex and the equatorward shift of the polar cap boundary at noon and at 21 MLT, and the second response is the equatorward expansion of the convection reversal boundary in the dusk sector. We make the case that the first response is associated with the propagation of magnetosonic waves, and that the second response is consistent with the Cowley and Lockwood [1992] picture of the redistribution of the newly created open flux in the polar cap region.

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

We examined the dynamics of the ionospheric plasma in the dayside sector by using the HF radar data at Iceland West and at Finland from 1100 to 1230 UT on September 5, 1995. During that period, the solar wind density was high and the IMF was strongly southward. The dayside magnetopause was highly compressed nearly to the geosynchronous orbit. The two radars simultaneously detected a poleward flow burst in the noon sector which, assuming uniformity of flow in the region of the data gap (1.5 h of MLT) between the two radars, showed a magnetic local time extent of 5 hours. This local time extent is 2 to 3 hours wider than previous results. The maximum poleward plasma velocity of the flow burst is about 750 m/s, and the latitudinal size of the flow burst region is about 100 to 200 km. This flow burst region initially expanded in longitude up to 5 hours, and then shifted poleward with a phase speed of 400 to 670 m/s. The flow burst has a duration of about 20 minutes. This large-scale poleward flow burst is likely to be due to large-scale reconnection occurring at the dayside magnetopause and subsequent convection as the magnetic field lines are transported across the polar cap.

Language：English  

Language：English  

Language：English