Natsuo Sato’s research while affiliated with National Institute of Polar Research and other places

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Publications (104)


Morphologies of omega band auroras
  • Article
  • Full-text available

August 2017

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48 Reads

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6 Citations

Earth Planets and Space

Natsuo Sato

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Akira Sessai Yukimatu

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Yoshimasa Tanaka

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Tomoaki Hori

We examined the morphological signatures of 315 omega band aurora events observed using the Time History of Events and Macroscale Interactions during Substorm ground-based all-sky imager network over a period of 8 years. We find that omega bands can be classified into the following three subtypes: (1) classical (O-type) omega bands, (2) torch or tongue (T-type) omega bands, and (3) combinations of classical and torch or tongue (O/T-type) omega bands. The statistical results show that T-type bands occur the most frequently (45%), followed by O/T-type bands (35%) and O-type bands (18%). We also examined the morphologies of the omega bands during their formation, from the growth period to the declining period through the maximum period. Interestingly, the omega bands are not stable, but rather exhibit dynamic changes in shape, intensity, and motion. They grow from small-scale bumps (seeds) at the poleward boundary of preexisting east–west-aligned auroras, rather than via the rotation or shear motion of preexisting east–west-aligned auroras, and do not exhibit any shear motion during the periods of auroral activity growth. Furthermore, the auroral luminosity is observed to increase during the declining period, and the total time from the start of the growth period to the end of the declining period is found to be about 20 min. Such dynamical signatures may be important in determining the mechanism responsible for omega band formation.

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Omega band pulsating auroras observed onboard THEMIS spacecraft and on the ground

June 2015

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32 Reads

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12 Citations

Natsuo Sato

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Yoshimasa Tanaka

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[...]

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Akira Sessai Yukimatu

We examined a fortuitous case of an omega band pulsating aurora observed simultaneously on the ground at Sanikiluaq in Canada and onboard the THEMIS spacecraft on March 1, 2011. We observed almost the entire process of the generation of the omega band aurora from the initial growth to the declining through expansion period. The omega band aurora grew from a faint seed, not via distortion of a pre-existing east–west band aurora. The size scale of the omega band aurora during the maximum period was ~500 km in the north–south direction and ~200 km in the east–west direction. The meso-scale omega band aurora consisted of more than 15 patches of complex-shaped small-scale auroras. Each patch contained an intense pulsating aurora with a recurrent period of ~9-12 s and a poleward-moving form. The footprints of the THEMIS-D and THEMIS-E spacecraft crossed the poleward part of the omega band aurora. THEMIS-D observed significant signatures in the electromagnetic fields and particles associated with the time at which the spacecraft crossed the omega band aurora. In particular, it was found that the Y and Z components of the DC electric field intensity, especially the Z component, modulated with almost the same period as that of the optical pulsating auroras. The electrostatic low-frequency waves of less than 30 Hz showed quasi-periodic intensity variations similar to those of the DC electric field. These observations suggest that DC electric field variation and low-frequency electrostatic waves may play important roles in the driving mechanism of omega band pulsating auroras.


Figure 1: Data flow and capacity-building/outreach of the IUGONET. The observational data are archived in each institutional database, and the metadata database and analysis software (UDAS) are developed. Some educational activities are performed in this development part of the IUGONET. These developed products are used for the international STP research community, including those in developing countries.
Table 1 Countries that are associated with IUGONET activities
Figure 2: Global network of ground-based observations that are registered to the IUGONET system.
Figure 3: Observatories and instruments registered in IUGONET MDDB as viewed through Google Earth around Kyoto and Nagoya, Japan. (a) Observatories. Yellow indicates observatories operated by Kyoto University, and orange indicates those operated by Nagoya University. (b) Display of the metadata of the all-sky camera of OMTI at Shigaraki, offered by clicking ‘Instrument’ metadata viewed through Google Earth. (c) Result of search MDDB followed by clicking IUGONET Metadata (description) in (b). (d) Result of clicking ‘Related URL’ shown in (b).
Figure 3 (See legend on next page.)

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The capacity-building and science-enabling activities of the IUGONET for the solar-terrestrial research community , Satoru UeNo 5 and IUGONET project team

January 2015

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172 Reads

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4 Citations

Earth Planets and Space

Background This paper presents an overview of the capacity-building activities and science-enabling services of the Inter-university Upper atmosphere Global Observation NETwork (IUGONET) project. This Japanese program, which started in 2009, is building a metadata database (MDDB) of ground-based observations and is developing an analysis software to handle the data linked to the MDDB system for use by the solar-terrestrial physics community. Because the institutional members of the IUGONET are mainly universities in Japan, we explore tools that can contribute to advanced education as well as promote research activities. Findings In this paper, we describe the utilities of the IUGONET for education, including our capacity-building activities in developing countries. We have regularly facilitated training seminars for Japanese students on the use of our tools (IUGONET MDDB and the software), and we have held capacity-building seminars for young scientists in developing countries. In addition to the MDDB, we have prepared various ‘gateway’ tools for users who are unfamiliar with ‘keywords’ to search for data. One of these is a geographical display tool that uses Google Earth (KML file), which is included as supplemental material to this paper. The usefulness of the IUGONET has been proven over its first 5 years of operation by the increasing number of its users, which has led to the production of approximately 500 scientific papers, including 42 thesis papers. Conclusions The IUGONET community collaborates with the Scientific Committee on Solar-Terrestrial Physics program, not only in its scientific activities, but also in the establishment of E-infrastructure and capacity building.


Observed correlation between pulsating aurora and chorus waves at Syowa Station in Antarctica: A case study

August 2012

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41 Reads

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15 Citations

Journal of Geophysical Research Atmospheres

A high correlation between a pulsating auroral patch and grouped chorus waves was observed on 17 April 2006 at Syowa Station in Antarctica. The spatial distribution of aurora-chorus correlation coefficients is evaluated in order to determine the source region. A pulsating patch at the highest-correlation pixel shows a one-to-one correspondence with the intensity variation of the grouped chorus waves, consisting of successive rising-tone elements with a duration and spacing of 2-3 s and 20-30 s, respectively. The generation region of the chorus waves is estimated from the latitude and longitude dependence of the equatorial electron gyrofrequencies using the IGRF geomagnetic field model. The extent of the estimated latitude and longitude is consistent with the spatial distribution of the high-correlation aurora-chorus region. The time difference between the chorus waves and the scattered electrons is also evaluated to discuss the validity of the source region. It shows that electrons reached the ionosphere sooner than the associated chorus waves by ˜1 s, consistent with the theoretical value for conjugate pulsating aurora generated at the equator. These results support the hypothesis that pulsating aurora is caused by pitch angle scattering of high-energy electrons by whistler mode chorus waves, via a cyclotron resonance at the equator. These results are the first ground-based observations of high correlations between a spatially extended aurora and chorus waves.


Ground-Based Aurora Conjugacy and Dynamic Tracing of Geomagnetic Conjugate Points

January 2012

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23 Reads

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4 Citations

Geophysical Monograph Series

We report on highly similar auroras that were simultaneously recorded with allsky TV cameras situated at two geomagnetically conjugate points at Tjornes in Iceland and at Syowa in Antarctica. During this event, various types of aurora were observed including auroral breakup, curl-type aurora, north-south-directed bandtype aurora, and pulsating aurora. We examined their characteristics for their similarity and dissimilarity in terms of shapes, movements, and luminosity variations at both observatories. We were also able to trace the movements and displacement of conjugate auroras in the Northern and Southern Hemispheres with a high spatial-temporal resolution. We discuss their conjugate characteristics with reference to solar wind-magnetosphere-ionosphere coupling processes.


Displacement of conjugate points during a substorm in a global magnetohydrodynamic simulation

June 2011

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26 Reads

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9 Citations

Journal of Geophysical Research Atmospheres

We reproduce a magnetospheric reconfiguration under southward interplanetary magnetic field (IMF) conditions using numerical magnetohydrodynamic simulations. To investigate the relative displacements of the geomagnetic conjugate points, we trace both footprints of the geomagnetic field lines during the magnetospheric reconfiguration under positive and negative IMF B-y conditions. Several substormlike features, namely, the formation of a near-Earth neutral line, a fast earthward flow, and tailward releases of the plasmoid, occur about 1 h after a southward turning of the IMF. The surveyed field line traced from the near-Earth magnetotail was strongly distorted duskward in the north and south after the substorm onset for positive and negative IMF B-y, respectively. The maximum of the relative longitudinal displacement of both footprints is 4.5 and 5.5 h in magnetic local time for positive and negative IMF B-y, respectively. While observational studies have indicated that the IMF orientation is the main factor controlling the relative displacement of the conjugate points, the present simulation-based study with a constant IMF orientation shows for the first time that the combined effects of plasma pressure, magnetic field intensity, and the field-aligned current density distribution along the field line are likely to be major factors controlling the relative displacement of conjugate points.


Modulation of ionospheric conductance and electric field associated with pulsating aurora

March 2010

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79 Reads

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66 Citations

Journal of Geophysical Research Atmospheres

We present, for the first time, a quasi-periodic modulation of ionospheric parameters, associated with the occurrence of pulsating auroras, such as electron density, conductance, and electric field. In March 2008, simultaneous campaign-based measurements of pulsating auroras were conducted over Tromsoe (69.60N, 19.20E), Norway, using an all-sky TV camera (ATV) and the EISCAT UHF system. During an interval within this campaign period, pulsating auroras, with periods of 8-17 s, were observed by the ATV in the morning local time sector (approximatelly 05 MLT). In this interval, quasi-periodic oscillations were identified in the raw electron density obtained by EISCAT. The electron density at lower altitudes in the E region (95-115 km) was enhanced by a factor of 3-4 immediately after the optical pulsation became "on". The height-integrated Hall conductance was also elevated by a factor of 1.5-2 almost in harmony with the electron density variation. The response of the electron density and Hall conductance to the appearance of the pulsating aurora was almost immediate. However, both did not decrease to the background level promptly after optical pulsation ceased. This was primarily because it took a few seconds for the electron density to decrease through recombination with ambient ions at these altitudes. Interestingly, electric field measurements performed by the remote antenna at Kiruna showed that redirection of the electric field occurred when the pulsating aurora was "on". We propose a model in which the enhancement of Hall conductance within patches of the pulsating aurora caused charge accumulation at the edges of the patches, and the electric field was then modified by the resulting polarization electric field. An estimation of the electric field modulation with this model well reproduced the actual electric field observations carried out by EISCAT, which confirmed the validity of the model. These results imply that the ionization caused by high-energy electron precipitation associated with a pulsating aurora has a significant effect on the ionospheric conductivity and current system. This modification of the ionosphere may facilitate characterization of the morphological features of pulsating auroras. In particular, modification of the electric field would affect the spatial structure of pulsating aurora patches, such as their motion and shapes.


Conjugate Aurora: A Case Study of Westward Traveling Surge

December 2009

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26 Reads

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1 Citation

We report on spatial and temporal conjugacy of meso-scale (~10-200 km) westward traveling surge auroras that were simultaneously acquired with all-sky TV cameras situated at two geomagnetically conjugate points, at Raufarhofn in Iceland and at Syowa Station in Antarctica on 30 September 2000. During this event, dynamic features of WTS for westward traveling, meso-scale vortex and curl structure and small-scale ray structure, latitudinal location of surge front of WTS showed good conjugacy. On the other hand, longitudinal displacement of surge front between Syowa and Raufarhofn was 570 km, and westward traveling speed is about 1.4 km/sec in the southern hemisphere but it is about 0.8 km/sec in the northern hemisphere.


Fig. 1. The ground-based UKM GPS measurements at Husafell, Iceland. 
Fig. 2. Solar-geomagnetic conditions during the period of March-May 2009. The sunspot number (SSN), the 3-hour Kp and the daily Ap indices were obtained from NOAA/SEC () and hourly provisional Dst (nT units) for this work was from the World Data Center-C2 for geomagnetism, Kyoto (). 
Fig. 3. Diurnal variations of VTEC and PWV at bipolar conjugate stations
Fig. 5. The TEC changes between conjugate stations on 17 th May of 2009
Fig. 6. Association between TEC and PWV measurements between bipolar conjugate stations V. CONCLUSSION AND FURTHER WORK
Solar-Terrestrial Observations at Bipolar Conjugate Points Using Global Positioning System

November 2009

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189 Reads

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1 Citation

GPS technique is an essential tool to explore of the near-Earth space environment towards improved space weather prediction and such methods provide suitable platforms for the studies solar-climate relationships. Using the atmospheric precipitable water vapour (PWV) as a terrestrial response and the ionospheric total electron content (TEC) as a measure solar activity which both derived from ground-based GPS observations, some physical mechanisms of how solar activity exerts their influences on weather/climate changes can be explained. This paper presents an characterization of both TEC and PWV using ground-based GPS receivers to insight Sun-Earth coupling mechanisms through bipolar conjugate points.


Estimation of enhanced electron density in the lower ionosphere using correlation between natural VLF emission intensity and CNA

July 2009

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10 Reads

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7 Citations

Antarctic Record

In This Stud., We Suggest A New Remule Sensing Tech. for Enhanced Electron Density in the Lower Ionosphere by Using the Correlation between VLF Whistler Mode Waves and CNA, Both Observed on the Ground. the Ionospheric Attenuations for VLF Whistler Mode and HF Waves Are Calculated by Using Full-wave Anal. to Evaluate Their Correlations for Various Ionospheric Electron Density Profiles Enhanced by Precipitating Electrons. the Calculation Results Show Negative Correlations between CNA and VLF Whistler Mode Waves in Accordance with the Observ. Results. Then, the Gradient of the Negative Correlation Becomes Larger with Decreasing Altitude of Maximum Electron Density. Thus, We Found That the Correlation Provides Info. on the Vertical Profile of the Enhanced Electron Density in the Lower Ionosphere Caused by Electron Precipitation. This Allows the Stud. of Electron Precipitation in the Daytime, in Addition to Trimpi Events at Nighttime. 2009 Natl. Inst. of Polar Res..


Citations (60)


... Using interhemispheric conjugate observations, Fujii et al. [1987] showed that the patchy-type pulsating aurora appears nearly simultaneously in both hemispheres. According to recent studies, however, the conjugacy of pulsating auroras is generally poor in shape and phase [e.g., Minatoya et al., 1995; Sato et al., 2004; Watanabe et al., 2007], a result that is inconsistent with the above traditional picture for the pulsating auroras. Sato et al. [2004] estimated the altitude of the electron modulation for a pulsating aurora using the time-of-flight (TOF) method of electron energy-dispersion observed by the FAST satellite. ...

Reference:

Electron and wave characteristics observed by the THEMIS satellites near the magnetic equator during a pulsating aurora
Development of image data processing system for the conjugate auroral TV data
  • Citing Article
  • July 1994

Antarctic Record

... Journal of Geophysical Research: Space Physics coupling processes and resulting plasma instabilities may play a significant role in generating postmidnight MPEs. We note that Sato et al. (2017) similarly concluded their study of omega bands by strongly suggesting that magnetosphere-ionosphere coupling should play an important role in the formation of omega band auroras. ...

Morphologies of omega band auroras

Earth Planets and Space

... Nonconjugate auroral spirals reported by Sato et al. [1998b] also suggest the existence of asymmetrical FACs. Minatoya et al. [1995] found a distinct lack of correlation between pulsating auroras in the conjugate areas. Sato et al. [1998a] showed that pulsating patches appear in both hemispheres but the shapes are not necessarily the same. ...

Absence of Correlation between Periodic Pulsating Auroras in Geomagnetically Conjugate Areas.
  • Citing Article
  • July 1995

Journal of geomagnetism and geoelectricity

... Observations at high latitudes suggest that conjugate point displacement can reach over 100 km in solar quiet (Sq) periods. Minatoya et al. (1996) found that when geomagnetic conditions changed from a disturbed state to the recovery phase, a large longitudinal displacement occurred, reaching 500 -800 km from the point indicated by the geomagnetic field model. However, Belon et al. (1969) found evidence of displacement even greater than 1000 km at high latitudes (66° and 71°). ...

Large Displacements of Conjugate Auroras in the Midnight Sector.
  • Citing Article
  • January 1996

Earth Planets and Space

... The increase in CNA is inferred using ground-based Relative Ionospheric Opacity METER (riometers) (Little & Leinbach, 1959). The mechanism that causes the high-energy electron precipitation resulting in CNA is the electron-cyclotron resonance and subsequent pitch angle diffusion (Kennel & Petschek, 1966;Ozaki et al., 2009Ozaki et al., , 2019Tsurutani & Lakhina, 1997) and low-energy electromagnetic ion cyclotron-driven electron precipitation (Hendry et al., 2017;Ozaki et al., 2018). ...

Estimation of enhanced electron density in the lower ionosphere using correlation between natural VLF emission intensity and CNA
  • Citing Article
  • July 2009

Antarctic Record

... Omega bands are thought to be associated with substorm onset (Wild et al., 2000) and substorm recovery phase (Vanhamäki et al., 2009), but we have shown that dawn spikes can occur independently of substorms. Typically, the omega band Ps6 magnetic signature has peak-to-peak values of a few 10 s nT (Sato et al., 2015) and average dB/dt ≈ 50 nT min 1 (Vokhmyanin et al., 2021), though extreme examples have been reported with dB/dt of several 100 s nT min 1 (Apatenkov et al., 2020). Most reported omega bands have a latitudinal extent of a few 100 s km (e.g., Vokhmyanin et al., 2021), consistent with the auroral emissions presented in Figure 6. ...

Omega band pulsating auroras observed onboard THEMIS spacecraft and on the ground
  • Citing Article
  • June 2015

... Regarding item 3, many papers (Palmer et al., 1996;Hoffmann et al., 1999 and references therein) have noticed a semidiurnal behavior of VHF-PMSE occurrences. This behavior is also true for four PMSE events detected in the austral summer of 1997/1998 with the Syowa HF radars (Ogawa et al., 2003b). Using HF radar data from Syowa Station under quiet geomagnetic conditions during 46 months from March 1997until December 2000, Hosokawa et al. (2004 have investigated seasonal and yearly variations of near range echoes, which were recognized as PMSEs by Ogawa et al. (2002). ...

Characteristics of polar mesosphere summer echoes observed with the Syowa Station HF radars

... S. Chakraborty (1) , N. Nishitani (2) , J. B. H. Baker* (1) , P. Ponomarenko (3) , and J. M. Ruohoniemi (1) (1) Center for Space Science and Engineering Research, Virginia Tech, USA, e-mail: shibaji7@vt.edu (2) Institute for Space-Earth Environmental Research, Nagoya University, Aichi, Japan (2) University of Saskatchewan, Canada A solar flare effect (SFE) is a transient disturbance in sub-auroral, middle, and low latitude dayside ionospheric currents due to changes in the ionospheric electric field and conductivity caused by flare-enhanced photoionization [1]. ...

Characteristics of solar flare effect in the high-latitude ionosphere as observed by the SuperDARN radars

... For archiving purposes, the metadata database system for cross-searching various data distributed across many institutions was developed based on the existing repository software called DSpace as the core component and the Space Physics Archive Search and Extract (SPASE) data model as the metadata format. Yatagai et al. (2015) gave an overview of the capacity-building activities and science-enabling services related to IUGONET. They regularly facilitated training seminars for Japanese students and also in developing countries. ...

The capacity-building and science-enabling activities of the IUGONET for the solar-terrestrial research community , Satoru UeNo 5 and IUGONET project team

Earth Planets and Space

... At least four types occur, and while all of them occur during substorms, two of them, medium frequency burst (MFB) and impulsive auroral hiss, stand out because of their specific association with the expansion phase (Makita 1979;LaBelle et al. 1994; reviews by Sazhin et al. 1993;LaBelle and Treumann 2002). MFB is a broadband left-hand polarized emission of typically a few minutes duration at substorm onset LaBelle et al. 1997LaBelle et al. , 2005Sato et al. 2008). It is best observed from a station poleward of the onset location. ...

Auroral radio emission and absorption of medium frequency radio waves observed in Iceland

Earth Planets and Space