日時： 金曜 13:00 - 14:30
場所： ZOOM （参加ご希望の方は nakamura.satoko[at]isee.nagoya-u.ac.jp まで事前にご連絡ください。
|Verification of the inversion method for electric field and velocity on the photosphere
|Abstract : To understand the mechanism of solar flares and coronal activity, it is necessary to investigate the three-dimensional structure of the solar coronal magnetic field. However, since the coronal magnetic field cannot be observed directly, a data-driven MHD simulation that numerically reproduces the coronal magnetic field from the observation data of the photospheric magnetic field has recently been developed. To conduct a data-driven simulation, we must determine the electric and velocity fields on the photosphere as boundary conditions. Several methods have recently been developed to do this. For instance, Fisher et al. (2010) developed a method for this purpose, exploiting the vector magnetic field's poloidal-toroidal decomposition (PTD). Kaneko et al. (2021) used this method to simulate observed mesoscale flares successfully. However, it has not been quantitatively investigated how accurately the PTD reproduces the electric and velocity fields and the dependencies of the PTD on the observation parameters. The objective of this study is to quantitatively evaluate the reproducibility of the electric field and velocity field by PTD. Therefore, we produced two potential magnetograms to make the analysis simple. The two magnetograms correspond to sequential data in which the magnetic field is uniformly advected for a constant time at a certain velocity. Therefore, the reproducibility can be evaluated quantitatively by comparing the velocities and electric fields reproduced from the two magnetograms with the advective velocities and the corresponding electric fields. As a result, we found that the reproduction of the electric field is relatively good in the strong magnetic field region, while the electric field is hardly reproduced in the region where the magnetic field is weaker . It was also found that the reproduced electric field tends to be smaller than the true value, even in the strong magnetic field region. In addition, we will investigate the dependency of the reproducibility on the time interval and spatial structure of the magnetic field simulated by R2D2 (RSST and Radiation for Deep Dynamics)
|Pressure distribution of ions and electrons in the inner magnetosphere during CIR and CME driven storms observed by Arase satellite
|Abstract : Geomagnetic storms are the main component of space weather, and the main phase of the geomagnetic storms are driven by Coronal Mass Ejections (CMEs) or Corotating Interaction Regions (CIRs). Enhancement of the ring current is a typical feature of the geomagnetic storm and a global decrease in the H component of the geomagnetic field is observed during the main phase of the geomagnetic storm. The ring current represents a diamagnetic current driven by the plasma pressure in the inner magnetosphere. The plasma pressure is mainly contributed by protons in an energy range of a few to a few hundreds of keVs. The O + contribution is also important, and sometimes dominates H + during the geomagnetically active period. However, the contribution of electron to the ring current is not well understood. Recently, we showed that the electron pressure also contributes to the depression of ground magnetic field during the November 2017 CIR-driven storm by comparing Ring current Atmosphere interactions Model with Self Consistent magnetic field (RAM-SCB) simulation, Arase in-situ plasma/particle data, and ground-based magnetometer data [Kumar et al., 2021]. In this seminar, statistical results of the spatial and temporal distribution of electrons and ions pressure with different energies and their contribution to the depression of the magnetic field during main phase, early recovery and late recovery phase for selected CIR and CME storms using in situ plasma/particle data obtained by Arase will be presented.
|Investigating differences of magnetic fields in solar active regions that produce eruptive and confine flares
|Abstract : Solar flares and coronal mass ejections (CMEs) are massive eruptions in the solar corona. Because they may impact geospace and social infrastructures, their prediction is important. Although larger flares are more likely to be accompanied by CMEs, some flares, even X-class flares, are not accompanied by CMEs. Hence, the relationship between flares and CMEs is not fully understood, and it is challenging to clearly predict what kinds of flares produce CMEs or not. Recently, some studies tried to clarify the difference between eruptive and confined flares, which associate CME and no CME, respectively. Toriumi et al. (2017) statistically analyzed several structural parameters of the active regions (ARs) and flares for eruptive and confined flare events. Lin et al. (2020, 2021) introduced the new parameters to discriminate the eruptive and confined flares based on the nonlinear force-free field model. Liu et al. (2022) calculated the force-free parameter a and analyzed the time history of the averaged αin the source regions of eruptive event and confined events. We focus the three magnetic parameters a, the horizontal magnetic field intensity Bh, and the intensity of the non-potential magnetic field Bnp, and analyzed how the statistics (average and standard deviation) of these parameters in the high free-energy region (HiFER) change during the eruptive and confined flares. We think that changing of high twist energy region’s distribution and Taylar Relaxation are related to flare has CME or not.
|PORUNAKATU Shreedevi Radhakrishna
|EMIC wave induced proton precipitation during the 27-28 May 2017 storm: Modelling and Observations
EMIC wave induced proton precipitation during the 27-28 May 2017
storm: Modelling and Observations
Electro Magnetic Ion Cyclotron (EMIC) waves are known to cause the ion precipitation into the mid-latitude ionosphere during geomagnetic storms. Recent studies have shown that the ion precipitation induced by EMIC waves can contribute significantly to the total energy flux deposited
into the ionosphere and severely affect the magnetosphere-ionosphere coupling. In this study, the temporal and spatial evolution of the proton precipitation into the ionosphere and its correspondence to the EMIC wave activity in the inner magnetosphere is examined using simulations of the BATSRUS+RAM-SCB model. During the geomagnetic storm of 27-28 May 2017, the Van Allen Probes and Arase satellite observed typical signatures of EMIC waves in the inner magnetosphere in the dusk-midnight sector. Ground magnetometers at high latitude stations also showed the presence of PC1/EMIC waves. During the main phase of the storm, the DMSP satellites observed enhanced proton precipitation at locations where the ground/space-based magnetic field measurements detected enhanced EMIC wave activity. The source and distribution of proton temperature anisotropy in the equatorial plane associated with EMIC waves are investigated to understand the excitation of the waves. A comparison of the precipitating proton fluxes obtained from the simulations with the particle measurements from the DMSP satellites show that EMIC wave scattering can account for the 30 keV proton precipitation at subauroral latitudes. The results are also compared with the proton fluxes/EMIC wave activity measured by the Arase satellite.
|The Study of the Spatial Structure of the Precipitating Electron Energy and Field Aligned Currents in the Omega Band Aurora
|Abstract : The omega band aurora is named for its shape, which resembles the Greek letter Ω. Omega band auroras often appear from the late expansion phase to the recovery phase of substorms, and have a tendency to drift eastward. The detailed mechanism of the omega band aurora is not clear yet, but the prevailing theory is that auroral streamers associated with BBFs (Bursty Bulk FLow) grow into omega band auroras. In addition, a variety of auroras exist inside the omega band aurora, including discrete auroras, diffuse auroras, and pulsating auroras. However, no energetic structure within the Omega Band structure has ever been estimated. In this study, we observed the omega band aurora at two wavelengths (427.8 nm and 844.6 nm) using two EMCCD cameras installed in Tromsø, Norway, and estimated the two-dimensional distribution of precipitating electron characteristic energy and downward energy flux of the omega band aurora from its emission intensity ratio. We analyze the two events on 2017/3/2/2 1:30-2:30 (MLT: around 4:00~5:00) and 2018/2/2/24 1:00-3:00 (MLT: around 3:30~5:30). In common with the two events, no characteristics are observed in the east-west direction of the omega band aurora for the precipitating electron characteristic energy. However, in the north-south direction, We find that the pulsating auroral region at low latitudes have a particularly high precipitation of a few keV. In the east-west direction of the omega band aurora, we find the downward energy flux is higher in the west side of the aurora. In both events, the satellite was flying over the magnetospheric conjugate of the omega band aurora. Therefore, we estimate the direction of the field aligned current at the magnetosphere from the variation of the satellite's magnetic field data. In this study, we compare the spatial structure of the energy and the spatial distribution of the magnetic field line currents obtained in this way with previous studies, and confirm the correspondence with related magnetospheric side phenomena such as BBFs.
|Reconstructions of solar cycles for the last 4 centuries based on historical records: recent developments
|Abstract : The solar activity has multiple measurement indices. Among them, there are two longest-term indices of sunspot group number and relative sunspot number based on historical sunspot observations. Their analyses have increased importance, as the WDC SILSO is working on production of International Sunspot Number (ver 3). This presentation briefly overview recent progresses in analyses of historical sunspot records, including developments of the butterfly diagrams covering almost four centuries of sunspot positions.
|Variations of molecular ions in the inner magnetosphere observed by the Arase satellite
|Abstract : In the Earth’s magnetosphere, several kinds of ions originate from both the solar wind and the ionosphere. Molecular ions in the magnetosphere originate in the Earth’s ionosphere. The dynamics leading to the ionospheric outflow of O+ ions have been subjects of numerous studies, but molecular ions are not well understood due to insufficient observations. The Arase satellite has observed various kinds of ions since 2017, using two ion analyzers, LEPi and MEPi, which cover the energy range from 10 eV/q to 180 keV/q. We conducted an analysis of the time-of-flight (TOF) data from the Arase (ERG) satellite to investigate the temporal variations of molecular ions, including O2+, NO+, and N2+, in the inner magnetosphere for 6 years from the solar declining phase to the rising phase. We estimate the molecular ion counts by subtracting the background contamination of the oxygen counts. While the number of clear molecular events is small, the derived molecular ion counts show a good correlation with the solar wind dynamic pressure and the Sym-H index. The detected molecular ions events are related to the arrivals of CIRs and CMEs, and it should be noted that SC or SSC caused by the dynamic pressure enhancements of the solar wind are almost always observed before the enhancement of the molecular ions in the inner magnetosphere. We found that the molecular ions exist in very low geomagnetic activity, suggesting that the molecular ions constantly exist in the inner magnetosphere. Different variations of the molecular ions associated with the solar cycle from the oxygen ions suggest that the escaping mechanisms of the molecular ions at the ionospheric altitudes are different from the oxygen ions.
|Anomalous UV emission from Li- and Na-like ions in the solar transition region
|Abstract : The solar ultraviolet intensities of spectral lines originating from Li- and Na-like ions have been observed to surpass the expectations derived from plasmas with coronal approximation. The violation of the coronal approximation can be partially attributed to non-equilibrium ionization (NEI) due to dynamic processes occurring in the vicinity of the transition region. However, the quantitative analysis of these dynamic effects has not yet been conducted. To investigate the impact of these dynamics, a set of equations governing NEI for multiple ion species was solved numerically in conjunction with 1.5-dimensional magnetohydrodynamic equations describing an Alfven-wave-heated coronal loop. Following the injection of Alfven waves from the photosphere, the system undergoes a time evolution characterized by phases of evaporation, condensation, and quasi-steady states. During the evaporation phase, the ionization fractions of Li- and Na-like ions were observed to increase, with a maximum enhancement of 1.6 when compared to the fractions in ionization equilibrium. This over-fractionation of Li- and Na-like ions was found to be induced by the evaporation process, while collisions between shocks and the transition region did not exhibit deviations from ionization equilibrium. Consequently, the intensities calculated using the coronal approximation underestimated the intensities of Li- and Na-like ions by up to 60\%. Conversely, under-fractions of at least 0.9 was observed during the condensation phase and the quasi-steady state. Given that the degree of over/under-fraction exhibits a dependency on mass motions, our study has a possibility to impose limitations on both the mass circulation in coronal heating and mass loss processes.
|Property of numerical Cerenkov radiation in plasma particle-in-cell simulations
|Abstract : Numerical methods of particle-in-cell (PIC) simulations for plasma were studied since 1960s and were established in early 1990s. There are several recent advances in numerical methods for plasma PIC simulations in terms of numerical accuracy. A performance measurement of a latest plasma PIC code is conducted on recent scalar processors for studying impact of the higher-degree/higher-order numerical methods on the computing cost. As a benchmark problem, the numerical Cerenkov instability is examined. The numerical tests show that the growth rate of the numerical Cerenkov instability is reduced and the property of the energy conservation law in plasma PIC simulations is improved significantly with higher-degree/higher-order methods. However, the computing time with higher-degree/higher-order methods does not increase substantially.
|Wide energy electron precipitation associated with pulsating aurorae: Arase and EISCAT conjugate observations
|Abstract : A model has been proposed in previous studies that lower-band whistler-mode chorus (LBC) waves propagating towards higher latitudes can produce electron precipitation over a wide energy range from a few keV to a few MeV (Miyoshi+, (2015, 2020, 2021)). However, there have been very few successful cases of conjugate observations by scientific satellites and incoherent scatter radars, so it is unclear whether this model can be applied to various cases. To validate this model, we investigated a conjugate observation event on March 12, 2022 in Tromsoe, Norway conducted by the Arase satellite (Miyoshi+, 2018) and the European Incoherent Scatter (EISCAT) radar. First, we use the inversion method to estimate an energy spectrum of precipitating electrons during the time when the LBC waves were excited from the height profile of electron density obtained by the EISCAT radar. The result shows that the maximum energy of the precipitating electrons is found to be about 200 to 300 keV, and the maximum propagation latitude of the LBC waves is estimated to be around 20 to 40 degrees. Next, we calculate the energy spectrum of precipitating electrons based on quasi-linear theory from wave and electron data observed by the Arase satellite and compared it with the one estimated from the EISCAT radar. The result is that the precipitating electron spectrum up to 20 to 300 keV is well reproduced and can be explained by the model of the previous study when the maximum propagation latitude of the LBC waves is assumed to be about 25 degrees. We also confirm that the 20 to 30 keV electrons scattered by the LBC waves reach strong diffusion limit.
|Do electromagnetic ion cyclotron waves really energize thermal ions in the magnetosphere?
|Abstract : Electromagnetic Ion Cyclotron (EMIC) waves are generated through cyclotron wave-particle interactions and affect the plasma environment in the magnetosphere. Ion energization by EMIC waves in the inner magnetosphere is investigated using spacecraft observations by comparing variations in ion distributions and waves. The energy transfer between the plasma waves and ions can be quantitatively evaluated by calculating the inner product of the wave electric field vector and the ion velocity vector using wave-particle interaction analysis (WPIA). WPIA was used with 4.5 years of data from the Arase spacecraft, and the statistical results were investigated alongside the qV・E spatial distribution of the positive region in the inner magnetosphere. By comparing the possible theoretical mechanisms of ion flux enhancement, we concluded that there is not a significant contribution of energization by EMIC waves to warm plasma in the magnetosphere.
|Denis P. Cabezas
|Acceleration mechanism of filament eruptions and its relation with coronal mass ejections
|Abstract : Solar filament eruptions usually appear to occur in association with the sudden explosive release of magnetic energy accumulated in long-lived arched magnetic structures. Different scenarios have been postulated to explain the onset and triggering mechanism of the eruptions but only some of them address the question of the upward acceleration. Understanding the physical conditions under which a filament can be ejected with high-speed and large acceleration are important issues for the context of modeling and predicting solar eruptions, and also to understand the initiation mechanism of coronal mass ejections (CME). In this talk, I will be presenting a case study (as an example) of a fast-filament eruption using imaging-spectroscopy observations and applying numerical models. I will focus on different aspects of the eruption with particular emphasis on the detection of highly Doppler shifted plasma and the time development of the velocity and acceleration. I will also discuss the importance of deriving the “true velocity” of filament/prominence eruptions that can be useful as initial inputs in CME models, which in turn can help to improve the predictions of the CMEs time-arrival at Earth. In the final part I will introduce briefly another example of a fast-filament eruption, event that we just started to analyze.
|Cross-field energy transport by magnetic reconnection in solar wind formation
|Abstract : The Alfvén wave and magnetic reconnection have been two important candidates in the scenarios of solar wind formation. The complexity of the three-dimensional solar atmosphere has prohibited us from understanding a quantitative picture of the formation process of the solar atmosphere. We here report a new method to quantify their importance in the three-dimensional, dynamic atmosphere in numerical simulations, focusing on the scalar transport process along and across magnetic field lines. The developed method is applied to the comprehensive MHD simulation of solar wind. As the numerical model accounts for magnetic structure generation and plasma wave excitation by thermal convection, we can measure the quantitative contributions of the Alfvén wave and magnetic reconnection in the energy transport to the solar wind.
|Bayesian rotation inversion of solar-like stars (progress report)
|Abstract : Helioseismology has revealed the rotational profile of the solar convective envelope in great detail, providing us with crucial constraints on the dynamo theory inside the Sun. Then, why not solar-like stars, which are defined as stars similar to the Sun in terms of, e.g., the mass, age, and cyclic magnetic activity? In this context, there have been several attempts of asteroseismic inferences on internal rotation of solar-like stars. One of the primary results is that there have not been strong rotational velocity shears in the radial direction throughout the interiors of solar-like stars. However, it is possible that the result originates from the fairly low resolution of asteroseismic rotation inversion. In this talk, I am going to show you how we can quantitatively assess that possibility based on the Bayesian statistics. I will also talk about future prospects to revisit internal rotational profiles of solar-like stars.
|Impact of different types of electric fields on Magnetosphere-Ionosphere coupling
|Abstract : It is well known that various types of electric field perturbations affect the global Magnetosphere-Ionosphere-Thermosphere system during geomagnetically disturbed conditions. These include Prompt penetration of the interplanetary electric field, Disturbance dynamo-generated electric field, substorm-induced electric field, and solar wind pressure-induced electric field. In this talk, the generation of these electric fields and their impacts on global ionospheric plasma distributions will be discussed based on recent observations. These results will help us to understand a few critical aspects of the effects of space weather events on magnetosphere-ionosphere coupling processes.
|Study on the time evolution of the electron acceleration site in a solar flare by using Time-of-Flight analysis method
|Abstract : It is well known that a large number of particles are accelerated during a solar flare. However, the particle acceleration process has not been clearly revealed yet. As for the acceleration site, there are few observational studies since it is difficult to identify directly it from imaging observations. The most outstanding study was done by Aschwanden et al. (1996) using the so-called Time-of-Flight (ToF) analysis technique. They concluded that the electron acceleration site is located slightly above the flare loop. Although the time evolution of the acceleration site during a flare is important for understanding the acceleration process, there are no studies on this topic. In this situation, we try to obtain new information on the evolution of the acceleration site using high-time resolution X-ray data derived from the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope. To investigate the time evolution of the acceleration site, ToF analysis was applied for each of the time-windows including an outstanding spike that appeared in the hard X-ray light curve for an M-class flare occurring on 25 June 2015. Then, a time series of time-lags between two different energy ranges were derived. Considering the mean-energy of electrons contributing to the X-rays in each energy band, we converted the time-lag to the ToF distance which corresponds to the distance between the acceleration site and the energy-loss site (chromosphere). It was found that the ToF distance became large in the later phase during the flare. This indicates that the acceleration site moved to a different magnetic loop system during the flare. To confirm this interpretation, we analyzed 1600A images observed with Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO). Then, we found that a new brightening region appeared in the later phase. The magnetic field line from this point is longer than the first one. So that we concluded that increasing ToF distance was caused by changing particles orbit.
|The inner edge locations of SAPS electric field and the ring current in the inner magnetosphere: why they often do not match?
|Abstract : Relative displacement of subauroral polarization streams (SAPS) and the ring current boundary in the inner magnetosphere is extensively investigated by analyzing particle and field data obtained by the Arase satellite and ionospheric convection data obtained by Super Dual Auroral Radar Network (SuperDARN) data. Upon SAPS emergence in the ionosphere, the enhancement of electric field is observed also on the magnetospheric side, particularly in the intermediate region between the electron plasma sheet and the ion ring current (IRC) as a satellite passes the equatorial magnetosphere in the dusk to early morning sector. Our statistical study shows that the inner edge locations of SAPS and IRC match well in some cases, while they do not in some other cases. Their correlation with substorm occurrence further indicates that the latter cases are always associated with preceding substorm activities. The result suggests that part of the IRC located inward of SAPS has nothing to do with an ongoing SAPS: it has been injected by preceding substorms and then no longer flows downward field-aligned current to the ionosphere.
|Time variation of the pitch angle distribution of the accelerated electrons in the solar flares
|Abstract : In solar flares, a large amount of electrons are accelerated and produce strong non-thermal microwave radiation (gyrosynchrotron radiation) and hard-X rays. The information on the pitch angle of the accelerated electrons is one of the important keys to understanding the acceleration mechanism. The non-thermal microwave radiation is essential because it is related to the pitch angle distribution of the accelerated electrons. We analyzed an M-class flare that occurred on 2012 July 19 using the 17GHz microwave data in Nobeyama radioheliograph (NoRH). In this flare, we found slow upward propagation of microwave sources from both of the footpoint regions. The propagation speed is about 1000 km/s. The speed is not realistic in terms of the speed of the accelerated electrons (Ve ~ speed of light). What makes this slow microwave propagation? After we considered various possibilities, it is suggested that the time variation of the pitch angle distribution of the accelerated electrons is needed in order to explain this phenomenon.
|Data-driven MHD Simulation Study of a Deflected Solar Eruption in NOAA Active Region 11283
|Abstract : Solar flares are commonly known to be caused by the release of magnetic energy accumulated in the solar corona. To understand the mechanism of solar flares, it is crucial to study the dynamics of coronal magnetic fields. However, due to the challenges associated with directly measuring three-dimensional coronal magnetic fields, the complete understanding of this mechanism remains elusive. To overcome these observational limitations, numerical modeling of coronal magnetic fields based on magnetohydrodynamics (MHD) theory has been developed. These numerical approaches enable exploration of the mechanisms involved in flare initiation. More recently, data-driven MHD modeling, which continuously introduces time-series of observational data in the bottom boundary, has been developed. This modeling is expected to provide a more realistic reproduction of coronal magnetic fields by updating the simulated magnetic field with observational data. In this study, we conduct a long-term data-driven simulation of the M5.3 flare observed on Sep., 6, 01:59 UT, 2011 in NOAA Active Region 11283 (AR 11283), known for its flare productivity, to explore the mechanism of the flare production in this AR. We use a model developed by Kaneko et al. (2021) in which the electric field is inversely derived from the time-series of photospheric magnetic field data, and the inverted velocity field, computed by the inverted electric field and the magnetic field, is introduced in the bottom boundary. We use 36 snapshots of the photospheric magnetic field (SHARP) obtained from SDO/HMI, and the simulation covers the period from Sep., 4, 19:48 UT, 2011 to Sep., 6, 06:48 UT, 2011, with each snapshot taken at hourly intervals. In our modeling, we have successfully reproduced both the formation of magnetic flux ropes (MFRs), bundles of helically-sheared magnetic field lines, and the eruption of the reproduced MFRs, which exhibits an inclination with respect to the photosphere. To explain the mechanism of the inclined eruption, we introduce a new methodology for calculating the decay index, which takes into account all eruptive directions of a torus plasma. This methodology allows us to evaluate the possibility for the torus instability growth and to determine the potential eruptive direction of the torus plasma.
|A test-particle simulation study of particles dynamics in the Earth’s magnetosphere
|Abstract : The Near-Earth environment hosts various processes like energetic particle dynamics, wave- particle interaction, particle acceleration/deceleration/loss, etc. These activities make this a subject of scientific interest and practical importance like space exploration or satellite communication etc. In this context, the Earth’s magnetosphere is an excellent natural laboratory for studying particle dynamics and their interaction with plasma waves. To probe the various magnetospheric phenomena, tools like theoretical analysis, simulations, and satellite or ground-based observations can be used based on the requirement. In this talk, I will be discussing about the test-particle simulation approach to understand the fundamental motion of Earth’s radiation belt phenomena.
|Plasmoids or flux ropes in the planetary magnetic tail
Plasmoids or flux ropes are traveling plasmas with helical magnetic
field lines. They are created by magnetic reconnection in the solar
corona and the planetary magnetic tail, and is associated with solar
flare and auroral breakup. In this talk, I will (1) review plasmoids
in the tail, (2) test their force-free nature using
four-satellite MMS observations, and (3) discuss the dawn-dusk location
Below are details of (3). Magnetotail reconnection had previously been recognized near the midnight meridian, as is the case in MHD simulations. The Geotail spacecraft mission discovered that the reconnection location is displaced toward dusk, or pre-midnight, in the Earth’s magnetotail. This dusk preference may be caused by the Hall electric field, which transports the magnetic flux dawnward and thins the current sheet on the duskside, as is the case in simulations with the Hall effect. In contrast, the Messenger spacecraft observed that the reconnection location was displaced toward dawn in Mercury’s magnetotail. This study reviews observational evidence of reconnection in the Earth’s magnetotail to address the controversy surrounding the dawn-dusk displacement of the reconnection location. The dusk preference is evident for tailward-moving plasmoids, but depends on studies for earthward-moving structures. Thus, the reconnection may not occur on the meridian where planetward-moving structures were observed. I predict that the BepiColombo/MMO spacecraft will observe tailward-moving plasmoids on the duskside of Mercury’s magnetotail.
|Spectral structures of electromagnetic ion cyclotron (EMIC) waves in the inner magnetosphere using the Van Allen Probes and the Arase satellites.
|Abstract : Electromagnetic ion cyclotron (EMIC) waves play an important role in controlling magnetospheric plasma dynamics. Especially, EMIC wave- particle interactions cause the loss of energetic protons and relativistic electrons in the Earth’s magnetosphere by pitch-angle scattering. Temperature anisotropy of energetic protons acts as the free energy to excite EMIC waves by ion cyclotron instability. The proton hole or proton hill in the phase space can also trigger nonlinear wave growth, causing EMIC rising (or falling)-tone emissions. EMIC waves caused by different generation processes exhibit unique spectral characteristics in the magnetic and electric fields. Here, we present several examples of EMIC waves with different spectral types in the inner magnetosphere during the Van Allen Probes and Arase eras (2013-2021). We investigate the characteristics of EMIC waves showing various spectral structures and discuss the possible free energy sources and generation processes for each type of wave.
|Undersea volcanic eruption off the coast of Tonga impacted on space beyond the Earth's atmosphere (トンガ沖海底火山噴火は大気圏を超えて宇宙空間にまで影響を与えていた)
|Abstract : The explosive eruption of the undersea volcano "Hunga Tonga-Hunga Haʻapai" (hereafter "Tonga volcano") off the coast of Tonga in the South Pacific on 15 January 2022, was a cataclysmic event, with ash plumes reaching an altitude of 57 km, the highest in recorded history. Recent ground-based and satellite observations have revealed that the eruption affected the upper atmosphere, including the thermosphere and ionosphere. To elucidate the generation mechanism of traveling ionospheric disturbances (TIDs) and equatorial plasma bubbles (EPBs) triggered by the explosive eruption of the Tonga volcano, we analyzed various kinds of observation data obtained from the Arase satellite, the Himawari-8 satellite, the SuperDARN HOP (Hokkaido Pair of) radars, GNSS (Global Navigation Satellite System), and ionosondes. As a result, we could follow the development process of TIDs and EPBs caused by large-scale atmospheric disturbances associated with the Tonga volcanic eruption. We also found that EPBs can reach space at an altitude of at least 2,000 km beyond the top of the Earth's atmosphere. This finding is surprising because EPBs were previously thought to be a phenomenon confined to the ionosphere. EPBs are one of the most important phenomena in space weather research because they severely affect satellite positioning and communications. Therefore, our results suggest that it is necessary to include the volcanic eruptions which cause large-scale atmospheric disturbances in the targets of space weather research.
|Solar near-surface shear layer and poleward meridional flow
|Abstract : The solar near-surface shear layer (NSSL) is where the angular velocity increases with depth. While the layer has been predicted with sunspot tracking long ago, the physical maintenance mechanism of the layer has yet to be understood. Since the spatial and temporal scales of the thermal convection drastically change in the NSSL, it is difficult to reproduce them in numerical simulations. Several attempts have been to this end (Hotta+2015, Matilsky+2019), and only part of the NSSL has been reproduced. Here, we carry out an unprecedentedly large-scale numerical simulation (12.8 billion grid points and 12 million time steps) to understand the maintenance mechanism of the NSSL. We nicely reproduce the NSSL in the calculation. Our analyses indicate that the turbulence transports the angular momentum radially inward to construct the poleward meridional flow. The magnetic tension on the sheared poleward meridional flow is balanced with the Coriolis force in the NSSL.
|Study on characteristics of global ionospheric disturbances during geomagnetic storms using worldwide Global Navigation Satellite System (GNSS) observation data
|Abstract : The solar extreme ultraviolet ionizes the Earth’s atmosphere above the altitude of 80 km, creating the ionosphere. It is known that the ionosphere affects radio propagations used in positioning, broadcast, and so on. The effect of ionosphere to the radio propagations depends on the radio frequency and the ionospheric electron density. Based on this feature, we can obtain the sum of electron density along the signal path between global navigation satellite system (GNSS) satellites and receivers, called total electron content (TEC). Ionospheric electron density variations are caused by various reasons (earthquake, volcanic eruption, solar eclipse, solar flare, geomagnetic storm, etc..). Geomagnetic storm, which are one of them, vary global ionospheric electron density severely. Therefore, we need to study characteristics of ionospheric response during geomagnetic storms. In our study, we show the example clarifying spatiotemporal variations of global ionospheric electron density during geomagnetic storms statistically using the long-term global GNSS-TEC data.
|Ryohtaroh T. Ishikawa
|Small-scale dynamics in the solar photosphere challenging with spectral line analyses
|Abstract : In the quiet region in the solar photosphere, turbulent convective motions of the granular flows naturally drive subgranular-scale flows. However, it is difficult to evaluate such small-scale velocities because of the limited instrumental resolution. We found spectral line broadening events during granules' fading process, while their physical mechanism is still unclear. To understand the line-broadening mechanism, we observe fading granules with the Hinode-SOT/SP and perform spectral line inversions. The microturbulence term or complicated velocity gradients are required to explain the wide line profiles in fading granules. Moreover, we investigate broadening events of synthesized spectra in fading granules reproduced by the MURaM simulation. Our results indicate that the small-scale turbulent motions are excited in the fading process and such turbulent flows contribute to the line broadening. The spectral line widths have the potential to be a tracer of the photospheric turbulent flows. I will also discuss an observational strategy to challenge these small-scale dynamics in the solar atmosphere with new instruments such as DKIST and SUNRISE-III.
|Observational evidence for the fast modulation of pulsating aurora
|Abstract : Pulsating auroras (PsAs) are characterized by a series of on-off transitions with a period of ~10 s, known as the main pulsation (the original name is 5-15 s quasiperiodic pulsations). The main pulsation is caused by the chorus bursts with a quasi-periodic modulation of wave intensity. A faster modulation (~3 Hz) called internal modulation (same as the microbursts in the X-ray data) is embedded in the PsA, which has been directly demonstrated to be related to repetitive rising-tone chorus elements. Much faster modulation (called fast modulation later) with frequencies up to 54 Hz have been reported by Kataoka et al., but the authors indicated they only detect the fast modulation in that one event. Due to the lack of high temporal resolution measurements and joint observations with magnetospheric satellites, the observational evidence of fast modulation is quite rare, and its formation mechanism is rarely studied. In this study, we present the observational evidence of the fast modulation (>10 Hz) and potentially reveal that the fast modulation is related to the subelement of chorus waves based on the joint observations from 100-Hz all-sky imagers and Arase satellite. Frequency spectrum of the aurora intensity shows the ~0.3 Hz, 4 Hz, and >10 Hz modulations, corresponding to the main pulsation, internal modulation, and fast modulation. The internal modulation (~4 Hz) and fast modulation (>10 Hz) are well-structured within the aurora patch. In addition, the joint observation event between hiss-like waves and PsA shows the ~0.07 Hz, and >10 Hz modulations, corresponding to the main pulsation and fast modulation. Both the rising-tone chorus and hiss-like waves are composed of a series of subelements, and the PsAs have the fast modulation (>10 Hz).
|Atmosphere-ionosphere-magnetosphere (AIM) coupling through energetic electron precipitation: Highlighting the results of the LAMP sounding rocket experiment
|Abstract : Energetic particle precipitation (EPP) is a fundamental driver of space weather in the coupled atmosphere-ionosphere-magnetosphere (AIM) system. Electrons with energies ranging from hundreds of eV to MeV are precipitated from the magnetosphere into the thermosphere/middle atmosphere. This leads to energy deposition at various altitudes, which in turn enhances ionization and changes ion compositions. The wave-particle interaction (WPI) plays a vital role in EPP, where nonlinear chorus waves serve as a mediating agent, causing electron acceleration as well as precipitation. We have proposed a model where relativistic electron microbursts, which are high-energy tails of pulsating aurora electrons, are produced by chorus wave-particle interactions. Coordinated observations between Arase/RBSP satellites and a ground-based EISCAT radar showed consistent results with this model, providing evidence that EPP significantly impacts the middle atmosphere, including ozone destruction. To confirm this model through in-situ observations, we conducted the LAMP (Loss through Auroral Microburst Pulsations) sounding rocket experiment at the Poker Flat Research Range in the US in March 2022. The experiment provides definitive evidence that microburst precipitation occurs concurrently with pulsating aurora. In this presentation, we will present the findings of the LAMP sounding rocket experiment. Additionally, we will show the new sounding rocket project, LAMP-2, which will collaborate with EISCAT_3D to understand ion upflow by WPI as well as EPP.
|Introduction to Integrated Studies Seminar and Comments toward Advanced Prediction of Solar Flares and CMEs
|Abstract : The solar-terrestrial environment is a complex system that consists of nonlinear, non-equilibrium, and multi-scale interacting processes. The Integrated Studies Division aims to improve our understanding of the dynamics of various phenomena in the solar-terrestrial environment through data analyses, numerical simulations, and modelings. Integrated Studies Seminar plays a vital role in discussing different research topics of solar-terrestrial environmental research with all division members. In the first part of this seminar, I will briefly introduce the integrated studies seminar and explain how important the holistic aspect is to understand and predict the dynamics of the solar-terrestrial environment. In the second part of my talk, we will discuss the current status of predictive research on solar flares and CMEs, and the issues to improve them. Although many institutes have developed the predictive scheme of solar flares and CMEs for accurate space weather forecasting, the predictive power is still insufficient. Even our recently developed physics-based prediction scheme for large flares (kappa-scheme: Kusano et al. 2020) and the discrimination scheme for CME formation regions by Lin et al. (2021) cannot accurately predict the occurrence of all flares and CMEs. Therefore, it is crucial to scientifically examine why they cannot predict some events to improve our understanding of flares and CMEs and advance our predictive capabilities. We will discuss several issues for this purpose.