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December 16 (Mon)
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Dr. T. Sakao (ISAS)
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Development Studies of Precision Wolter Mirrors for Solar Observations
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We present overview and status of our development studies on precision Wolter (grazing-incidence) mirrors for future solar X-ray observations. Imaging observations of the Sun in soft X-rays provide us with rich information on dynamic behavior of high-temperature plasmas in the corona, including those created during flares. However, fabrication of conventional precision Wolter mirrors for X-ray astronomical observations (such as those for Yohkoh/SXT, Hinode/XRT, and Chandra X-ray observatory) is no longer possible, and new ways of mirror fabrication must be seeked for to realize precision imagery of the X-ray Sun. The Wolter mirrors we have been developing aim to achieve angular resolution and scattering performance equivalent to, or even better than, any other previous X-ray telescopes for solar observations.
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December 4 (Wed)
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Dr. T. Kaneko (ISEE/Nagoya Univ.)
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Numerical study on internal velocity field variation of eruptive solar prominence
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Solar prominences are cool dense plasma clouds in the hot tenuous corona. They sometimes erupt and evolve into coronal mass ejections. Revealing the origin of cool dense plasmas and their impact on the
eruptive mechanisms has been a long term issue in solar physics. Moreover, there is an increasing interest in the predictability of solar eruptions for modern space weather forecast. It is known that the interiors of prominences are filled with turbulent flows. Recent observational studies using SMART/SDDI (Hida Obs.) reported that the standard deviation of the Doppler velocity in the eruptive prominences started to increase before eruptions even when the mean velocity was unchanged. This finding is useful to predict prominence eruptions; however, the origin of velocity field variation and the relationship with the magnetically-driven eruptive mechanisms are unclear. In this study, we reproduced eruption of a turbulent prominence using three-dimensional MHD simulation including gravity, optically thin radiative cooling, and nonlinear anisotropic thermal conduction. In our simulation, the cool dense prominence was formed by radiative condensation (thermal instability). The prominence erupted after exceeding the critical height of torus instability. Before eruption, the velocity field inside the prominence had complicated distribution containing both upflows and downflows associated with the Kelvin-Helmholtz instability (KHI). We confirmed that the vertical velocity created increasing standard deviation, which was quantitatively consistent with the observation. We discuss how the nonlinear growth of the KHI contributes to slow-rise of prominence-hosting flux rope until reaching the critical height of torus instability.
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November 18 (Mon)
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Dr. S. Gunar (Astronomical Institute/CAS)
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Modelling of entire prominences with their multiple fine structures: the 3D Whole-Prominence Fine Structure models
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We will present the 3D Whole-Prominence Fine Structure (WPFS) models which combine 3D magnetic field configurations of entire prominences
with a detailed description of the prominence plasma. The plasma is located in magnetic dips in hydrostatic equilibrium and is distributed
along hundreds of fine structures within the 3D magnetic field configurations. The prominence plasma has realistic density and temperature distributions including the prominence-corona transition region. Thanks to this, the 3D WPFS models provide us with a representation of prominences (and filaments) with a complexity that begins to approach real prominences. We have developed two WPFS models - the first by Gunar & Mackay (2015) and the second by Gunar et al. (2018). These models allow us to investigate links between the distribution of the prominence plasma, configuration of its magnetic field and observations of the prominence/filament fine structures obtained in UV/EUV, optical and radio domains from various vantage points. The models thus serve as a virtual laboratory that helps us to test different physical properties of prominences.
We will present the basic building blocks of our models and highlight some of the results obtained by the 3D prominence modelling. We will, for example, investigate the evolution of prominences/filaments in response to changes in the underlying photospheric flux distribution. We will also explore the role of projection effects and visibility of prominence fine structures in our understanding of prominences. In addition, we will look at simulations of prominences and filaments in ALMA wavelengths.
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November 8 (Fri)
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Dr. S. Krucker (SSL, UCB/FHNW)
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Solar X-ray Observations with NuSTAR
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Designed with true focusing optics for extremely high sensitivity, NuSTAR is an astrophysical observatory, but unlike virtually every other high-energy astrophysics mission to date, it is capable of being pointed at the Sun. NuSTAR observes the Sun above 2.5 keV with a sensitivity over 200 times better than that of RHESSI, the previously most capable solar mission in this energy range. This talk will review NuSTAR solar observations focusing on the smallest detectable flares and their importance to coronal heating.
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August 26 (Mon)
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Dr. V. Polito (LMSAL/BAERI)
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UV spectroscopy of solar flares: recent insights from IRIS and EIS
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Finding a definitive model for flares still remains one of the most challenging problems in astrophysics. Spectroscopic instruments such as
Hinode/EIS and IRIS observe many individual spectral lines simultaneously and allow a diagnosis of the fundamental parameters of the plasma (including flows, density, temperature and emission measure), which can then be matched against the results of competing theoretical models. This talk will focus on recent results gained from IRIS and EIS in the field of UV spectroscopy of solar flares, in particular addressing long-standing issues regarding the chromospheric evaporation process during the impulsive phase of flares.
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August 5 (Mon)
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Mr. M. Yoshida (NAOJ/Sokendai)
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Lyman-Alpha Scattering Polarization in Off-Limb Spicules and Its Constraint on Their Magnetic Field
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The magnetic field of spicules is one of the critical physical quantity that needs to be observationally determined to quantitatively evaluate the energy transported to the corona. The CLASP (Chromospheric Lyman-Alpha Spectro-Polarimeter) is a sounding rocket experiment launched in 2015 to obtain spectro-polarimetric data in the hydrogen Lyman-alpha line. CLASP performed the sit-and-stare observations in the quiet Sun near the limb for 5 minutes with a slit perpendicular to the limb and successfully captured off-limb spicules. The Lyman-alpha line is well suited to investigate how spicules affect corona because it is sensitive to higher temperatures than other chromospheric lines due to large optical thickness. Another advantage of this spectral line is the Hanle effect, namely the modification of the scattering polarization by the magnetic field, operates at the field strength approximately between 10 and 100 G, which is comparable to the typical field strength of spicules suggested by previous studies using other chromospheric lines in visible and infrared ranges. In this talk, we report the first detection of the scattering polarization of spicules in the Lyman-alpha line, which can be exploited to constrain the magnetic field information via the Hanle effect. The linear polarization Q/I signal (positive Q/I indicates polarization parallel to the limb) during the observing time is 0% to 2% in off-limb locations. The linear polarization U/I signal changes in time from +1% to -1%. The accuracy of the polarization signal is about 0.1--0.3%. The positive Q/I values during the observation period indicate that 90deg scattering by the anisotropic incident radiation from the solar disk is dominant. The temporal variation observed in U/I can be considered to be due to the temporal variation of magnetic field vector (i.e., operation of the Hanle effect) and/or to the symmetry properties of the radiation field. We discuss possible constraints on the magnetic field in the spicules observed by CLASP.
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July 25 (Thu)
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Dr. S. Toriumi (ISAS)
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Comparative Study of Data-driven Coronal Field Models
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To investigate the magnetic field and estimate the accumulated magnetic energy in the solar corona, data-driven magnetic field models have
recently been developed. In contrast to widely used NLFFF methods, these schemes calculate the temporally evolving coronal field in response to the sequentially updated photospheric boundary vector field. In this study, we systematically compare different types of data-driven models by applying our MHD flux emergence simulation as a ground-truth data set. As a result, all models succeed in, at least, reproducing the twisted magnetic flux rope structure in the atmosphere. In the quantitative aspects, however, we observe higher degree of model dependence. Several models overestimate the magnetic energy and helicity, which indicates excessive accumulations of magnetic twist
throughout the computation. We discuss origins of the discrepancies from the ground truth and suggest possible future implementations.
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June 17 (Mon)
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Mr. T. Hasegawa (ISAS)
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Formation Properties of the O I 1027 and 1028A lines near the Lyman-beta for the Diagnostics with Solar-C_EUVST
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The future Solar-C_EUVST mission (Imada et al. 2017) will bring the possibility of inferring the physical information of the solar atmosphere observing the solar spectrum at extreme ultra-violet (EUV) wavelengths and aims to have access to different atmospheric layers from the chromosphere to the outer corona. In this sense, we consider that the neutral oxygen doublet lines O I 1027 and 1028 A are good candidates for understanding the properties of the chromosphere and transition region when these lines are combined with Lyman-beta. The latter is a strong candidate of the upcoming solar mission Solar-C_EUVST while the former lines are located in its wing. In this work, we examine general spectroscopic properties of those lines in the quiet sun by synthesizing them assuming non-local thermal equilibrium conditions and partial redistribution effects. We estimate the heights where the spectral lines are sensitive to the physical parameters with a 1D semi-empirical atmospheric model. Furthermore, we synthesize the intensity spectrum using the 3D "enhanced network" simulation computed with the Bifrost RMHD code. The average formation height of Lyman-beta is about 2400 km, where temperature is ~2x10^4 K. The O I 1027 and 1028 A lines form at about 1650 and 1500 km in average, respectively, in the upper chromosphere where its temperature is ~8x10^3 K. The spatial distribution of signals confirms that the lines are sensitive to the thermodynamics of the solar atmosphere at lower layers than those covered by the Lyman-beta. These results indicate that the O I lines of interest are good candidates for EUV spectroscopy and complement the Lyman-beta line bringing the possibility of simultaneous multi-layer observations.
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May 23 (Thu)
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Dr. A. C. Sterling (NASA)
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Update on Coronal Jets, and Their Connections to Other Solar Features
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We review recent ideas about solar coronal jets and their causes. Many --- if not most or all --- such jets result from the eruption of small-scale filaments, or "minifilaments." Moreover, more recent work has confirmed earlier hints that magnetic flux cancelation is often the trigger that induces eruption of the minifilaments. While typical coronal jets have base size sizes of ~10,000 km. We present evidence for smaller-scale jet features, called "jetlets, which have base sizes ~4000, are seen in EUV and UV, and which occur in the network; we consider whether jets extend down to the size of chromospheric spicules (widths ~300 km). We then consider the opposite size scale, of the size of small active regions, and show how coronal jets are starting to provide clues to the mechanism that drives larger-scale CME-producing eruptions.
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May 20 (Mon)
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Dr. K. Iwai (Nagoya Univ.)
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Heliospheric Physics and Space Weather with Interplanetary Scintillation Observations
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Interplanetary scintillation (IPS) is a radio scattering phenomenon caused by the disturbances in the solar wind. The ground-based IPS observation has been an important technique to investigate the heliosphere. Now, we are going to have many opportunities to investigate the solar corona and its connection to the inner heliosphere such as PSP, MMO, Solar orbiter, and Solar-C_EUVST. The synergy between these instruments and ground-based IPS observations should be a key issue for us. We have developed an IPS estimation system based on a global magnetohydrodynamic (MHD) simulation of the inner heliosphere. In this system, the background solar wind and propagation of the CMEs are calculated by a global MHD simulation. The IPS response is estimated by the three-dimensional density distribution of the inner heliosphere derived from the MHD simulation. The simulated IPS response is compared with the actual IPS observations made by ISEE, Nagoya University, to achieve more realistic simulation result. Now, this system has been installed in the space weather forecasting system in NICT to predict the arrival time of CMEs. This system is also useful to compare the IPS data with other space-based observational data.
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May 8 (Wed)
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Mr. Y. Kawabata (ISAS)
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Observational Studies on Non-potential Magnetic Field in Solar Active Regions
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Solar active regions sometimes produce explosive events, such as solar flares and coronal mass ejections causing several influences to the geomagnetic environment. The main interest of our study is how the non-potential magnetic field is distributed in active regions. The measurements of the magnetic field have been mainly performed at the photospheric height. The coronal magnetic field through polarimetric observations are difficult even with the state-of-art instruments. To overcome the difficulty, the nonlinear force-free field (NLFFF) modeling has been extensively used to infer the three-dimensional (3D) magnetic field in the solar corona. We attempt to investigate the non-potential magnetic field and its 3D structure in active regions, while we tackle the technical problem in the NLFFF modeling.
We focus on two different viewpoints with the NLFFF modeling and observations.
(1) We investigate the robustness and dependency of the NLFFF calculation with respect to the initial guess of the 3D magnetic field.
While previous studies often use potential field as the initial guess in the NLFFF modeling, we adopt the linear force-free fields with different constant force-free alpha as the initial guesses.
(2) We examine the direct measurements of the chromospheric magnetic field in the whole active regions through the spectropolarimetric observations at He I10830 A.
The results of NLFFF extrapolation from the photosphere are compared with the direct measurements.
We obtained following findings.
(1) The dependency of the initial condition in the NLFFF extrapolation is smaller in the strong magnetic field region. Therefore, the magnetic field at the lower height tends to be less affected by the initial condition.
(2) Chromospheric magnetic field may have larger non-potentiality compared to the photospheric magnetic field.
The large non-potentiality in the chromospheric height may not be reproduced by the NLFFF extrapolation from the photospheric magnetic field. The magnitude of the underestimation of the non-potentiality at the chromospheric height may reach 30-40 degree in signed shear angle. We conclude that the magnetic field in the upper atmosphere may have higher non-potentiality than previously thought based on the NLFFF. Because the non-potentialty is crucial in the MHD instability, this finding will improve understand onset mechanism for solar flares and CMEs, which affect the environment in the solar system.
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Apr 22 (Mon)
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Dr. T. Shimizu (ISAS)
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PDF
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Recent progress and future prospects in solar observations
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With a brief summary of recent progress in solar observations, I will
discuss future prospects of solar researches for 2020s and beyond.
The discussion will cover new projects and what I want to do in 2019 and coming years.
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