| 25-Apr-2025 | Mr. Takato Otsu (Kyoto Univ.) | |
| How Are Stellar Postflare Loops Observed? ~Implications from Solar Data~ | ||
| Stellar flares are known as the sudden brightening on stars. Recent Hα spectroscopic observations suggest that stellar flares are not just brightening phenomena but related to various active phenomena. Correctly understanding the dynamics of stellar flares are essential not only for the unified understanding of solar and stellar flares but also for the accurate estimation of effect of stellar flares on surrounding exoplanets. However, since the surfaces of distant stars cannot be spatially resolved, it is challenging to observationally characterize their dynamics. The Hα spectra of stellar flares are thought to have a complex combination of contributions from flare ribbons, filament eruptions, and postflare loops, based on analogy with solar flares. Therefore, in order to understand Hα observations of stellar flares, it is necessary to clarify how each component of a solar flare appears in spatially integrated Hα data. In this talk, we will especially introduce how postflare loops are observed in spatially integrated (Sun-as-a-star) data. To clarify behaviors of postflare loops in spatially integrated data, we performed the Sun-as-a-star analysis of the X1.6 flare that occurred near the northwest limb on 2023 August 5 using GOES (soft X-rays, ~ 10^7 K), SDO/AIA (EUV, ≥10^5 K) and SMART/SDDI at Hida Observatory, Kyoto University (Hα, ∼10^4 K), focusing on postflare loops. As a result, this flare showed signatures corresponding to the important dynamics of the postflare loops even in the spatially integrated data: (1) Cooling of postflare loops was confirmed as peak time difference in soft X-rays, EUV, and Hα. Notably, the Hα light curve showed two distinct peaks corresponding to the flare ribbons and the postflare loops. (2) Downflows were confirmed as simultaneous red/blue-shifted absorptions in Sun-as-a-star Hα spectra. (3) Apparent rise of postflare loops was confirmed as the stop of the decay for the Hα light curve. These results are keys to detect stellar postflare loops in multiwavelength observations and grasp their dynamics with spatially integrated data. We also discuss the dependence of our results on flare locations and its possible applications to stellar observations. Moreover, we mention the statistical study of postflare loops for further understanding of stellar flares. | ||
| 04-Apr-2025 | Dr. Tom Van Doorsselaere (KU Leuven) | |
| Global solar coronal models driven with Alfven and kink waves | ||
| The solar corona has a mysterious heating source, which may be magnetohydrodynamic (MHD) waves. In recent years, the so-called AWSOM models are a new generation of solar atmospheric models, which incorporate the heating and forces of Alfven waves on top of more classical effects. They are outperforming older models capturing most structures of the solar corona. However, they are still lacking in the polar open field regions, where artificial heating is still necessary to match the observations. In this talk, I will highlight our description of wave heating by kink waves. We develop a new formalism that allows to describe the kink wave in a similar way as Alfven waves. In this new development, we generalise the Elsasser variables to Q-variables in order to follow waves that are not Alfven waves. In the talk, I will explain the governing equations, and highlight early outcomes of the proof-of-concept in 1D, 2D and 3D configurations. I will show that kink waves may play the role of heating the polar open field regions, replacing the artificial heating terms required in the earlier models. | ||
| 18-Dec-2024 | Mr. Shun Ishigami (SOKENDAI/NAOJ) | |
| Spectroscopic Study of Heating Distributions and Mechanisms Using Hinode/EIS | ||
| 01-May-2024 | Ms. Jargalmaa Batmunkh (Niigata Univ.) | |
| Deep learning compression of solar polarization spectra from Hinode SOT/SP | ||
| Solar polarization spectra contain crucial physical information about the Sun, contributing to the exploration of diverse solar events. The characteristics of solar spectral data, encompassing spatial information, wavelength, and polarimetry, introduces a multi-dimensional complexity, posing challenges in data processing. Furthermore, the variations in noise levels among polarization parameters further complicates the task. To address this, our objective is to develop a specialized compression model using deep learning methods, with the potential to broaden its applications, such as solar flare prediction. We constructed deep autoencoder (DAE) and 1D-convolutional autoencoder (CAE) models for compressing Stokes I and V parameters. These models operate by taking polarimetric spectra as input, encoding them into a compact representation, and then decoding to produce an output closely resembling the initial input. The CAE model showed greater promise in reconstructing Stokes signals, achieving a 14% reduction in deviation compared to the outcomes produced by the DAE model. Additionally, we have obtained preliminary results in solar flare prediction using the CAE model. It resulted in higher error rate for pre-flare instances compared to non-flare instances, suggesting that our method holds the potential to predict flares before their actual occurrence. | ||
| 18-Oct-2023 | Ms. Mai Yamashita (Univ. of Hyogo) | |
| Starspots, Chromospheric Emission Lines, and Flares of Pre-Main-Sequence Stars | ||
| Observational studies of chromospheric activity have been extensively carried out for main-sequence stars. It is revealed that young solar-type stars have strong surface magnetic field, enormous starspots, and strong chromospheric emission lines. We investigated chromospheric activities of pre-main-sequence (PMS) stars by spectroscopic and photometric observations. First, we studied the Ca II infrared emission lines at 8542 A (Yamashita et al. 2020). We conducted mediun or high-resolution spectroscopy with Nayuta/MALLS and Subaru/HDS in addition to use of archival spectra obtained by Keck/HIRES, VLT/UVES, and VLT/X-Shooter. Most PMS stars have narrow Ca II IRT emission lines whose intensities are as large as the maximum of the zero-age main-sequence (ZAMS) stars. The chromosphere of most of PMSs is suggested to be completely filled by the Ca II emitting region. We also searched the periodic light variation caused by a starspot. The light curves of 26 PMS stars were obtained from TESS photometric data (Yamashita et al. 2022). The amplitudes of the light curves are 0.001 - 1.155 mag. We found that the light variations and Ca II emission line strengths of PMS stars are as large as those of the most active superflare stars and two orders of magnitudes larger than those of the Sun, and are located on the extensions of the superflare stars. In summary, PMS stars have very active chromosphere driven by strong dynamo process due to the fast rotation and the long convection timescale. We also detected flares in the TESS light curves. The energies of the flares are estimated to be 10^33 - 10^35 erg, which is comparable with the energy of a superflare. | ||
| 15-Sep-2023 | Ms. Teia Mihailescu (UCL) | |
| Intriguing Plasma Composition Pattern in a Solar Active Region: a Result of Non-Resonant Alfvén Waves? | ||
| The plasma composition of the solar corona is different from that of the solar photosphere. Elements that have a low first ionisation potential (FIP) are preferentially transported to the corona and, therefore, show enhanced abundances in the corona compared to the photosphere. The level of enhancement is measured using the FIP bias parameter. The highest FIP bias values are typically observed in active regions, but they also vary at sub-active region level. In this work, we use data from the EUV Imaging Spectrometer (EIS) on Hinode to study the plasma composition in an active region following an episode of significant new flux emergence into the pre-existing magnetic environment of the active region. We use two FIP bias diagnostics: Si X 258.375 Å/S X 264.233 Å (formation temperature of 1.5 MK) and Ca XIV 193.874 Å /Ar XIV 194.396 Å (formation temperature of 4 MK). We observe different plasma composition patterns in the newly emerging loops and the preexisting loops (those that had been formed before the flux emergence). This result can be interpreted in the context of the ponderomotive force model, which proposes that the enhancement of low-FIP elements in the corona is generally driven by Alfvén waves. We suggest that the low-FIP elements enhancement pattern observed in the emerging loops could be driven by resonant waves originating in the active region core loops. Conversely, we suggest that the pattern observed in preexisting loops could be driven by non-resonant waves and we discuss potential sources of these waves. This different type of wave activity is what could explain the different patterns of low-FIP elements enhancement in these two sets of loops. | ||