Auroral research group, Boston University
Selected research topics
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Conjunction event list of Heliophysics and Geospace System Observatory
The list includes recent (since 2017) conjunction events between the satellites in the magnetosphere and between the satellites and major ground-based sites. It is free to use, modify and redistribute them.
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Fine-Scale Structures of STEVE Revealed by 4K Imaging
Strong Thermal Emission Velocity Enhancement (STEVE) is often perceived as a homogeneous arc with a purple or mauve color. However, this notion is based on photographs with a long exposure time. We conducted a 4K video observation of STEVE with unprecedentedly high spatial and temporal resolution. The video revealed that STEVE is not a homogeneous arc but consists of rich fine-scale structures that could not be seen in regular photographs. The fine-scale structures had wavelengths of 1–10 s of km with multiple spectral peaks. The fine-scale structures moved westward at 8.9 km/s. We suggest that the fine-scale structures of STEVE correspond to fine-scale plasma structures in the fast plasma streams in the upper atmosphere. However, it is challenging to explain how the upper atmospheric glow can form the fine-scale structure because of the long chemical reaction time. This paper was highlighted by the Science News article.
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Unsolved problems in Strong Thermal Emission Velocity Enhancement (STEVE) and the picket fence
A review article has summarized the current understanding and open questions of the STEVE phenomenon. We first introduce the basic characteristics of STEVE and historical observations of STEVE-like emissions, particularly the case on 11 September 1891. Then, we discuss major open questions about STEVE: 1) Why does STEVE preferentially occur in equinoxes? 2) How do the solar wind and storm/substorm conditions control STEVE? 3) Why is STEVE rare, despite that STEVE does not seem to require extreme driving conditions? 4) What are the multi-scale structures of STEVE? 5) What mechanisms determine the properties of the picket fence? 6) What are the chemistry and emission mechanisms of STEVE? 7) What are the impacts of STEVE on the ionosphere−thermosphere system? Also, 8) what is the relation between STEVE, stable auroral red (SAR) arcs, and the subauroral proton aurora? This paper was highlighted by the Washington Post article.
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Proton aurora and Subauroral Red (SAR) Arc
Photographs that were taken by citizen scientists at Strathmore (Canada), Orimattila (Finland) and Ikaalinen (Finland) revealed peculiar green bands and blobs and red arcs in the night sky equatorward of the auroral oval. Using concurrent scientific instruments, we identified that those are a type of proton aurora that is illuminated by electrons induced by proton precipitation from space. The proton aurora transitions to a stable auroral red (SAR) arc. A series of photographs suggests that the SAR arc is initiated by proton precipitation, which has been suggested but never been demonstrated previously. The citizen scientist photographs played a critical role in unveiling the interaction between the proton aurora and SAR arc. This work has been featured by science news media.
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Space-Ground Observations of Dynamics of Substorm Onset Beads
Our research on plasma instability that initiates auroral breakup has been featured as the cover image of the journal.
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Book: Cross-Scale Coupling and Energy Transfer in the Magnetosphere-Ionosphere-Thermosphere System
The book provides a systematic understanding of Magnetosphere-Ionosphere-Thermosphere dynamics. Cross-scale coupling has become increasingly important in the Space Physics community. Although large-scale processes can specify the averaged state of the system reasonably well, they cannot accurately describe localized and rapidly varying structures in space in actual events. Such localized and variable structures can be as intense as the large-scale features. This book covers observations on quantifying coupling and energetics and simulation on evaluating impacts of cross-scale processes. It includes an in-depth review and summary of the current status of multi-scale coupling processes, fundamental physics, and concise illustrations and plots that are usable in tutorial presentations and classrooms.
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STEVE (strong thermal emission velocity enhancement)
STEVE is a newly discovered optical emission in the upper atmosphere, and has become one of the hottest topics in auroral researches. We investigate its origin in space through spacecraft and remote sensing techniques. Our recent research on STEVE emission has been covered by American Geophysical Union's Press Release.
Precursors of auroral substorms
Substorm is the most dynamic and spectacular phenomenon in auroral phenomena, which can be seen with naked eyes. While the phenonenology has been well documented, the time sequence leading to substorm onset has been a long-standing debate for more than 40 years in the space physics community. We found precursor of auroral substorms using the THEMIS all-sky imager network. The pre-onset sequence starts from the poleward boundary of the auroral oval leading to faint auroral streamers toward the equatorward boundary, suggesting enhanced earthward plasma flows in the magnetosphere. This result was published in NASA press release and Journal of Geophysical Research with more than 100 citations.
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Driver of pulsating aurora
Pulsating auroras are beautiful emissions that blink in the atmosphere with a periodicity of 5 to 40 seconds. Earlier work in October, based on observations both from NASA's THEMIS spacecraft and from the ground, we solved the mystery of how the pulsating auroras were formed -- these spectacular light shows were driven by chorus waves observed deep in space. The lessons we are learning from this novel experimental technique is links aurora in the ionosphere to a location which is separated by over 40,000 km in the magnetosphere. This work was published in Science and covered by News media.
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Imaging day-night plasma transport
Our interview article has been published in Science Diffusion.
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Searching the nature of substorm onset triggering
Another big question regarding substorms is to understand the nature of waves that occur at the very beginning of substorm onset. Graduate student, Bea Gallardo-Lacourt, used a high time resolution mode of SuperDARN radar data together with THEMIS all sky imager data and discovered large-amplitude oscillatory flows. Those flows are found to highly correlate with auroral waves buth with a finite phase lag. Through a careful examination of four events with different beam configurations, she determined the spatial relationship between ionospheric flows and upward field-aligned currents during the initial stage of substorm onset. This work was published in Journal of Geophysical Research and was selected as a Featured Article. She received a Student Poster Award in the 2014 CEDAR Workshop and 2014 AGU Outstanding Student Paper Award.
