Search

Flash aurora driven by an isolated chorus element can be a useful ionospheric indicator for identifying the source wave properties via wave‐particle interactions. Using ground observation and modeling approaches, here we report the temporal characteristics of flash aurora that depend on the chorus frequency width and the sweep rate. We found that the contraction time increases more than the expansion time in patchy auroral variations, due to the difference in the minimum electron energies resonated with the chorus wave packet away from the equatorial source to higher latitudes. Especially, the contraction time strongly depends on the higher‐frequency chorus waves due to cyclotron resonance with lower‐energy electrons. The model calculations support that the chorus element ranges from lower‐band to upper‐band frequencies with respect to half the gyrofrequency at the exact generation region. Our study provides the prompt (milliseconds) chorus‐driven electron dynamics through the spatiotemporal characteristics of flash aurora in the ionosphere. The wave frequency of a chorus wave, which is one of the electromagnetic wave emissions in magnetized plasmas, is an important parameter for characterizing energetic particles in the Earth's magnetosphere. Even though chorus waves are classified into two bands—lower‐ and upper‐band frequencies separated at half the electron gyrofrequency—it is not well established that both lower‐ and upper‐band chorus are essentially the same at the generation region. In this study, we investigate temporal characteristics of specific aurora caused by discrete chorus elements to identify the whole frequency band of chorus waves at the magnetospheric generation region. The aurora reflects the physical properties of the magnetospheric processes through the geomagnetic field lines from the generation region; thus, the aurora in the ionosphere becomes an ionospheric display on which the magnetospheric chorus waves are generated. Using auroral images along with model calculations, we find that the contraction time is longer than the expansion time as auroras vary their shape, depending on the frequency width of chorus wave packets. This study provides a new insight into the role of rapid (milliseconds) resonant interaction processes between discrete chorus elements and electrons over a wider energy range. High‐speed auroral observations allow us to probe chorus wave properties in detail through the cyclotron resonance as flash auroraThe temporal properties of flash aurora cannot be reproduced by the cyclotron resonance with an isolated lower‐band chorusAuroral observations and computations suggest an isolated chorus element distributing up to the upper‐band range in the source region High‐speed auroral observations allow us to probe chorus wave properties in detail through the cyclotron resonance as flash aurora The temporal properties of flash aurora cannot be reproduced by the cyclotron resonance with an isolated lower‐band chorus Auroral observations and computations suggest an isolated chorus element distributing up to the upper‐band range in the source region

The Solar Energetic Particle and imaging ultraviolet spectrograph (IUVS) instruments onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft discovered diffuse aurora that span across the nightside of Mars due to the interaction of solar energetic particles (SEPs) with the Martian atmosphere. However, it is unclear whether the diffuse aurora originates from energetic electrons or protons. We have developed a Monte Carlo model to calculate the limb intensity profile of the CO2+ultraviolet doublet (UVD) due to precipitation of energetic electrons and protons with energy ranges from 100 eV to 100 keV and from 50 keV to 5 MeV, respectively. We used electron and proton fluxes observed by MAVEN during the December 2014 SEP event and the September 2017 SEP event. Our results showed that proton‐induced CO2+UVD emission has a lower peak altitude than electron‐induced CO2+UVD emission. The calculated peak altitudes of the CO2+UVD limb profiles are 76 and 68 km in the December 2014 event and the September 2017 event, respectively. Extending the energy to 500 keV for electrons and 20 MeV for protons further improved our comparison to the IUVS observations. We have succeeded in reproducing peak altitudes and shapes of the observed CO2+UVD limb profiles using the SEP flux observed by MAVEN. This was possible by taking into account the contribution of energetic protons, indicating that both energetic electrons and protons contribute to producing the observed diffuse aurora. A Monte Carlo model was developed to investigate the contributions of precipitating electrons and protons to the diffuse auroral emissionProton‐induced CO2+ultraviolet doublet (UVD) emissions have lower peak altitudes than electron‐induced emissionsThe Mars Atmosphere and Volatile EvolutioN/imaging ultraviolet spectrograph (IUVS) limb emission profiles of CO2+UVD during two solar energetic particle (SEP) events were reproduced by considering the contribution of SEP protons A Monte Carlo model was developed to investigate the contributions of precipitating electrons and protons to the diffuse auroral emission Proton‐induced CO2+ultraviolet doublet (UVD) emissions have lower peak altitudes than electron‐induced emissions The Mars Atmosphere and Volatile EvolutioN/imaging ultraviolet spectrograph (IUVS) limb emission profiles of CO2+UVD during two solar energetic particle (SEP) events were reproduced by considering the contribution of SEP protons

The lack of data on solar wind have prevented a detailed understanding of extreme magnetic storms. To address this issue, we apply a machine learning technique in the form of an Echo State Network (ESN) to reconstruct solar wind data for several extreme magnetic storms for which little or no solar wind data were previously available. Multiple geomagnetic activity indices are used as the input data for the ESN, which produces a continuous time series of solar wind parameters as output. As a result, the solar wind parameters for the largest storm event in March 1989 are obtained, and the minimum Bz is estimated to be −95 nT ±10 nT. Two different types of solar wind profiles are discussed for the extreme magnetic storms, a sheath‐driven profile and a magnetic cloud‐driven profile. The results reported here will be highly useful as input data for future simulation studies modeling extreme magnetic storms. The detailed cause of extreme magnetic storms are unknown because of the very limited solar wind data. A machine learning technique can help to reconstruct the lost solar wind data from the geomagnetic response. New findings from the reconstructed solar wind data includes the unexpectedly large magnetic field strength of the solar wind that caused the largest magnetic storms in space age. A statistical analysis using the reconstructed solar wind data also tells the estimation of one in 100 years amplitude of the solar wind magnetic field for the first time. The method proposed in this study is also useful to obtain the long‐term continuous solar wind data since 1957, for the period even before the solar wind observation started. We use a machine learning technique called Echo State Network to reconstruct the solar wind parameters from geomagnetic activity indicesTwo different types of solar wind profiles are discussed for extreme magnetic storms, that is, sheath‐driven and cloud‐driven typesContinuous hourly data of the reconstructed solar wind parameters are obtained since 1957 We use a machine learning technique called Echo State Network to reconstruct the solar wind parameters from geomagnetic activity indices Two different types of solar wind profiles are discussed for extreme magnetic storms, that is, sheath‐driven and cloud‐driven types Continuous hourly data of the reconstructed solar wind parameters are obtained since 1957

Graphic Abstract:

Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar–terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar–terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.

We investigated the spatial characteristics of pulsating auroras (PsA) using digital camera measurements from the International Space Station. These measurements covered PsAs in ∼5‐h wide regions in the magnetic local time (MLT) direction in short time intervals (∼10 min). Analyses of two events suggested that the periodicity of the main pulsation of a PsA does not exhibit any clear dependence on the magnetic latitude (63–66°) and MLT (00–05). These results suggest that the periods are not controlled by the bounce time of trapped electrons but by local conditions such as gradients of temperature and/or density of electrons near the magnetospheric source region. We also analyzed the colors of PsAs as proxies for the energies of precipitating electrons. The blue and green channels of the digital camera are sensitive to the band emission of molecular nitrogen ions and the green line of oxygen atoms, respectively. Because the energy bands of precipitating electrons producing those two emissions are different, ratios of blue to green (B/G ratios) can be used as proxies for the energies of PsA electrons. The B/G ratio tends to be higher in the morning sector than in the midnight sector, which is consistent with the results of previous studies showing the MLT dependence of the energies of PsA electrons. This capability of estimating the energies of auroral electrons from digital camera images is expected to provide more opportunities for citizen scientists to contribute more deeply to auroral science. DSLR camera observations from the ISS revealed periodicities and colors of pulsating auroras over several thousands of kilometersPeriodicities of pulsating auroras analyzed for two events did not exhibit any clear dependence on magnetic latitude and local timeB/G ratios of DSLR camera images could be used as proxies for the energies of pulsating aurora electrons DSLR camera observations from the ISS revealed periodicities and colors of pulsating auroras over several thousands of kilometers Periodicities of pulsating auroras analyzed for two events did not exhibit any clear dependence on magnetic latitude and local time B/G ratios of DSLR camera images could be used as proxies for the energies of pulsating aurora electrons

In‐situ observations of spatial variations of the wave‐particle interaction region require a large number of satellite probes. As an alternative, flash‐type auroras, a kind of pulsating aurora, driven by discrete chorus elements, can be used to investigate the interaction region with a high spatial resolution. We estimated the spatial extent of wave‐particle interaction region from ground‐based observations of flash aurora at Gakona (62.39°N, 214.78°E), Alaska at subauroral latitudes, and found that the auroral expansion was predominantly to the low‐latitude side. The spatial displacement is thought to be caused by the propagation effects of chorus waves in the magnetosphere. Using ray tracing analysis to take into account chorus wave propagation, we reconstructed the spatiotemporal evolution of the volume emission rate and confirmed that the predominant expansion is toward the lower‐latitude side in the ionosphere. This study shows that chorus wave propagation in the magnetosphere gives new insight for characterizing the transverse size (across the geomagnetic field line) of wave‐particle interaction regions. The calculated spatial scale of the column auroral emission shows a correlation with the magnetic latitude of the resonance region at magnetic latitudes within 10° of the equatorial plane of the magnetosphere. Our results suggest that the spatial scale of a flash aurora is indirectly related to the chorus amplitude because the latitudinal range of the wave‐particle interaction is important for the growth of wave amplitude. Spatial evolution of the wave‐particle interaction region is extracted from flash auroral events and chorus‐ray tracingSpatial expansion of the wave‐particle interaction region depends on chorus wave propagation characterized by the refractive indexTransverse scale of the interaction region increases with the wider latitude range of cyclotron resonance owing to spreading chorus rays Spatial evolution of the wave‐particle interaction region is extracted from flash auroral events and chorus‐ray tracing Spatial expansion of the wave‐particle interaction region depends on chorus wave propagation characterized by the refractive index Transverse scale of the interaction region increases with the wider latitude range of cyclotron resonance owing to spreading chorus rays

We report three different types of relativistic electron precipitation (REP) events observed at International Space Station (ISS), associated with electromagnetic ion cyclotron (EMIC) waves or whistler mode waves as observed by the Arase satellite at conjugate locations near the magnetic equator. Three different detectors installed on the ISS were complementarily used; CALET/CHD as the detector of precipitating MeV electrons, MAXI/RBM as the detector of sub‐MeV electrons from horizontal and vertical directions, and SEDA‐AP/SDOM to quantitatively measure the energy spectrum. The REP event on 21 August 2017 shows a quasiperiodic intensity variation at ~1 Hz which corresponds to variations of the EMIC waves at the Arase altitudes. The REP event on 24 April 2017 shows rapid and irregular intensity variation which corresponds to the amplitude variation of chorus waves, while the REP events on 26 October 2017 shows a smooth quasiperiodic time variation at ~0.2 Hz which corresponds to the amplitude variation of “electrostatic” whistler mode waves. This study clearly demonstrates that the time variation of REP events at ISS are caused by various types of plasma waves near the magnetic equator. Three different types of relativistic electron precipitation (REP) events are observed at International Space Station (ISS)Electromagnetic ion cyclotron waves were observed during a REP event at conjugate locations near the magnetic equatorWhistler mode waves were observed during other REP events at conjugate locations near the magnetic equator Several different kinds of plasma waves were identified in the magnetosphere as the possible cause of relativistic electron precipitation (REP) events at International Space Station (ISS).

The full‐color photographs of aurora have been taken with digital single‐lens reflex cameras mounted on the International Space Station (ISS). Since these photographs do not have accurate time and geographical information, in order to use them as scientific data, it is necessary to calibrate the imaging parameters (such as looking direction and angle of view of the camera) of the photographs. For this purpose, we calibrated the imaging parameters using a city light image taken from the Defense Meteorological Satellite Program satellite following the method of Hozumi et al. (2016, https://doi.org/10.1186/s40623-016-0532-z). We mapped the photographs onto the geographic coordinate system using the calibrated imaging parameters. To evaluate the accuracy of the mapping, we compared the aurora taken simultaneously from ISS and ground. Comparing the spatial structure of discrete aurora and the temporal variation of pulsating aurora, the accuracy of the data set is less than 0.3 s in time and less than 5 km in space in the direction perpendicular to the looking direction of the camera. The generated data set has a wide field of view ( ∼1,100  ×900 km), and their temporal resolution is less than 1 s. Not only that, the field of view can sweep a wide area ( ∼3,000 km in longitude) in a short time ( ∼10 min). Thus, this new imaging capability will enable us to capture the evolution of fine‐scale spatial structure of aurora in a wide area. Accurate mapping method of the photographs taken from ISS was establishedMapped ISS images can be used for the study of fast moving auroraUnique wide FOV auroral data will reveal some global spatiotemporal variations of pulsating aurora