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Center for Mathematical Science and Advanced Technology (MAT)

Seminar Schedule

[MAT Seminar]

Date:
2024/05/24(Friday) 13:00 - 15:00
Language:
Japanese
Speaker:
Takashi Minoshima
Title:
BepiColombo and STELEO observations and numerical modelling of solar energetic particle events in March 2022.
Abstract:
Solar Energetic Particles (SEPs) are high-energy charged particles spanning from a few keV to several GeV, which are generated in association with energetic phenomena on the Sun (flares and CMEs), and then are ejected into interplanetary space. Given that SEPs offer insights into acceleration and transport mechanisms in collisionless plasmas, understanding their origin and dynamics is of great interest to space plasma physics and relevant fields (e.g., high-energy astrophysics and laser plasma physics). Furthermore, accurate prediction of the SEP profile is required for space weather operations, as SEPs exceeding 10 MeV pose primary threats to the space environment, including radio communication failure, malfunction and degradation of equipment onboard aircraft and satellites, and radiation exposure of astronauts during extravehicular activities. This is expected to become more and more important in the future as humanity expands its activity beyond the Earth’s magnetosphere.

Unraveling the SEP physics presents a formidable challenge. On the theoretical front, the rigorous description of SEPs requires a first-principle kinetic treatment while the system scale of the Sun-Earth environment is far beyond the magnetohydrodynamic scale. On the observational front, SEPs have been conventionally observed by in-situ measurement of an individual satellite, resulting in information about physical mechanisms of acceleration and transport of SEPs being convolved during their journey from the Sun to the observation point. Presently, many satellites are operating to measure SEPs at different radii and longitudes, giving a valuable opportunity to integrate theoretical modeling with multi-satellite observations to extract their physical knowledge.

M and X-class flares, along with subsequent CMEs, occurred on 28 and 30 March, 2022. The propagations of CMEs and background solar wind were reconstructed by the global MHD simulation of the heliosphere based on the ground-based interplanetary scintillation (IPS) observation, IPS-SUSANOO. Two associated SEP events were observed simultaneously by BepiColombo at 0.6 AU and STEREO A at 1 AU. Fortunately, both satellites were positioned approximately along the same Parker spiral. In both events, the Solar Particle Monitor onboard BepiColombo/MMO detected 40-200 MeV ions approximately within one hour after the flare, with their peak exhibiting velocity dispersion indicative of ballistic motion. Time-of-flight analysis suggests the expected starting point to be 0.5-0.7 AU from the BepiColombo, close to the Sun. This is corroborated by the same analysis of 1.8-10 MeV ions detected by the Low Energy Telescope (LET) onboard STEREO A. The BepiColombo Environment Radiation Monitor (BERM) detected 1.5 MeV ions, providing a comparative study with 1.8 MeV ions detected by LET. While the difference in their peak times suggests ballistic motion along the magnetic field, the decay profile observed by LET is more prolonged than that observed by BERM. This observation may imply an increase in scattering over time, potentially indicating a reduction in mean free path to below 0.1 AU, as inferred from focused transport simulations. In this paper, we will present detailed observations and numerical modelling results, and discuss potential scenarios for the transport of SEPs between BepiColombo and STEREO locations.

[MAT Seminar]

Date:
2024/05/22(Wednesday) 13:00 - 15:00
Language:
English
Speaker:
Ayumi Ozawa
Title:
Development and application of the phase-reduction theory for understanding interacting oscillatory systems
Abstract:
When oscillatory units interact, they often self-organize and exhibit nontrivial behavior. For example, they adjust their rhythms to each other and yield coherent oscillations. This phenomenon, called synchronization, has been observed in various systems, including the atmosphere-ocean system. In this talk, I will present my past and current research projects on interacting oscillatory systems and my research plan at JAMSTEC. The talk will be organized as follows. First, I will give an overview of my work. I will then focus on my research on feedback control of interacting oscillators. This study is motivated by a wide range of examples of desirable and undesirable synchronized oscillations. Previous studies indicated that a simple global feedback loop can suppress synchronized oscillations by desynchronization, i.e. making the oscillations asynchronous. Later, another study reported that a similar feedback loop may also induce oscillation quenching, where the oscillations of individual oscillators cease. However, it was unclear under what conditions which of the two, desynchronization or oscillation quenching, occurs. We therefore constructed and analyzed a simple mathematical model of oscillators under feedback by utilizing the phase-reduction theory, which allows us to describe the dynamics of one oscillator with a single variable, the phase. The model is simple enough to allow detailed analyses, but its dynamics is rich enough to exhibit both desynchronization and oscillation quenching. We obtained the phase diagrams of the collective state and proposed a methodology to tune the feedback. Next, I will briefly introduce my ongoing project on the collective dynamics of small aquatic animals that exhibit circular trajectories. Lastly, I will explain my main research project at JAMSTEC. In this project, I will develop a phase-reduction theory for delay partial differential equations and apply it to a model of synchronized sea-surface temperatures over two ocean currents to elucidate their interaction.

[MAT Seminar]

Date:
2024/05/15(Wednesday) 13:00 - 15:00
Language:
English
Speaker:
Go Eguchi
Title:
Fish schooling and drafting behaviors: Fish can adjust angle of attack and curvature of body to minimize the total force acting on its body
Abstract:
Fish schooling has various advantages, one of which is the improvement in hydrodynamic propulsive efficiency. It is said that wake flow induced by tail beat makes reverse Kármán vortices, follower fish could get the advantage of thrust because of the reduction of experienced flow velocity and local pressure. We verified the velocity field of wake flow when fish swim in parallel by using visualization method, Particle Image Velocimetry (PIV). The results demonstrated that the existence of a region of reduction flow velocity, but not enough to explain all the energy-saving in fish schooling. Therefore, we focused on the behaviors, “drafting”.
Drafting is the act of swimming with force transmission among individuals without any physical contact. For example, it has been reported that some fish can follow other individual while swimming with minimal tail beat. These drafting occur in front or side of other individual. We investigated this mechanism by stably reproducing drafting using actual fish and a flat plate or hydrofoil model. The velocity and pressure field around the fish and forces acting on fish body were verified three experimental approach, PIV, Computational Fluid Dynamics (CFD) and model experiments using two-axis load cell. These results showed that fish can balance the anterior-posterior and lateral direction forces acting on its body by having an angle of attack or adjusting curvature of body in response to changes in pressure and flow fields. It is suggested that this mechanism may applied to energy saving in fish schooling.
In this seminar, I introduce the above in detail. In the latter, I explain “Observation of fish schooling dynamics in fish preserve nets” which I have started working on as a postdoctoral researcher, and explain the research plan, and outline of the business trip scheduled for May 16-23.