ICTS

In continuation, I will cover quasi-linear theory and moment method leading to fluid equations.

In this session, we will see how in situ solar wind data are downloaded and how the bulk properties of the solar wind are derived from measured solar wind particle fluxes.

In this pedagogical lecture, I will introduce astrophysical turbulence, focusing on two key environments, the interstellar medium (ISM) of galaxies and the intracluster medium (ICM) of galaxy clusters.These turbulent environments also provide a setting for magnetic-field amplification through turbulent dynamo action.

High-fidelity plasma simulations are indispensable for understanding and designing future fusion reactors, but their high computational cost presents a major challenge for reactor optimization and integrated modeling predictions. This seminar will introduce the concept of surrogate models—fast computational approximations of expensive physics simulations—and discuss their growing role in fusion research. Focusing on energetic particle transport driven by Alfvén eigenmode instabilities in ITER, the talk will present machine-learning-based surrogate models developed using nonlinear FAR3d simulations. Two complementary approaches, Gaussian process regression and neural networks, will be discussed for rapid prediction of energetic beam and alpha-particle transport together with predictive uncertainty estimates. The resulting surrogate models reproduce nonlinear transport with high accuracy while reducing computational cost by five to six orders of magnitude, illustrating the potential of scientific machine learning to enable next-generation integrated modeling and reactor design.

It is an illustrious fact that the intact solar plasma system naturally serves as a complex plasma laboratory with a mystic plethora of collective modes. The complex plasma stability dynamics of the self-gravitationally bounded Sun and its circumambient unbounded radially evolving atmosphere are critically investigated in the framework of the gravito-electrostatic sheath (GES) model. The exceptionality, especially featuring the latter, is systematically applied to illuminate solar wind acceleration characteristics. The three categorically distinct collective modes are found to get excited in the linearly perturbed solar plasmas. Their modal characteristics are illustratively analyzed and observationally actualized. The vital applicability of this thematic study in resolving some major long-standing plasma solar problems is finally projected.

The inner heliosphere constitutes an integral component to understanding space weather, serving as the dynamic medium through which solar drivers propagate and ultimately impact the near-Earth space environment. A primary challenge in forecasting these impacts is the highly complex, multi-scalar nature of the coupled Heliosphere-Magnetosphere-Ionosphere (HMI) system, which exhibits an immense separation of spatial and temporal scales. Consequently, a single traditional numerical model is no longer sufficient to fully capture the intricate, cross-scale interplay of plasma processes operating within this tightly coupled system of systems.

In this talk, I shall present a coupled, end-to-end data-driven HMI model designed to understand macro-scale dynamics and bridge these disparate environments. This integrated framework leverages the Space Weather Adaptive SimulaTion (SWASTi) framework for 3D magnetohydrodynamic (MHD) modeling of background solar wind evolution and coronal mass ejections, which is seamlessly combined with MagPIE, a two-way coupled magnetosphere-ionosphere module that evaluates global field-aligned currents and ionospheric potentials. Crucially, the underlying solar boundary conditions are driven by a novel Physics-Informed Neural Networks (PINNs) based Surface Flux Transport (SFT) model, which ensures mesh-independent flux conservation of active magnetic regions.

Additionally, I shall discuss different case studies from such a model and its applicablity to confront in-situ observations from Aditya L1. Furthemore, I will also delve upon ensemble studies and means of going beyond MHD for striving a detailed understanding of sub-grid plasma processes in HMI system.

Even though the interstellar medium (ISM) of star-forming galaxies has been known to have a multiphase structure (broadly hot, warm, and cold phases) since the 1970s, how magnetic fields differ between the ISM phases is still unknown. Using a combination of theory, simulations, and observations, this talk explores how the multiphase nature of the ISM shapes magnetic fields. In particular, the relationship between the turbulent kinetic and magnetic energy densities will be established using Zeeman magnetic field measurements and pulsar observations. The findings from these studies will enhance our understanding of the role of magnetic fields in star formation and galaxy evolution, and also prepare us to harness the upcoming wealth of radio polarisation data from the SKA.

Cosmic rays are nonthermal charged particles with energies spanning more than ten orders of magnitude. In many environments, such as the interstellar medium and galaxies/galaxy clusters, cosmic ray energy density is comparable to that of the thermal, magnetic fields, and turbulent gas motions, making them a dynamically important component. Yet key questions remain: how do they reach such high energies, what governs their transport, and how do they affect the plasma they pass through?

In this talk, I will combine plasma theory and first-principles kinetic simulations to show how cosmic rays drive plasma instabilities and turbulence, and what limits the growth of the magnetic fluctuations. I will then discuss why such self-generated turbulence is crucial for cosmic ray acceleration, propagation, and confinement. I will conclude by highlighting key implications of cosmic ray-driven turbulence, from astrophysical shocks to the launching of galactic winds and the interpretation of multiwavelength observations.

By December 2025, the Parker Solar Probe (PSP) had completed 26 orbits aroundthe Sun. During this period, the spacecraft encountered numerous slow solar windintervals, the majority of which were Alfvenic. In this study, we systematicallyinvestigate the plasma properties of these intervals, focusing on the radialevolution of key parameters, including plasma density, magnetic field strength,magnetic field variance, temperature, and cross-helicity. By analyzing multiple slowAlfvenic and non-Alfvenic wind intervals spanning a wide range of heliocentricdistances, we provide a comprehensive characterization of the radial evolution ofthese two distinct slow solar wind populations. This work contributes to a deeperunderstanding of the origin and evolution of the slow solar wind in the innerheliosphere.

One of the main challenges of space physics simulations, including those of turbulence, is to reach simulation boxes/ simulation times large enough to be of relevance, while including a sufficient amount of kinetic-scale information to reproduce the complicated interaction of small and large scales in colllisionless environments. In this talk, we will describe a number of strategies pursued at the Institute of Theoretical Physics I of the Ruhr University Bochum to reach this fundamental aim.

I will discuss signatures of two stream and Weibel instabilities in the collisionless astrophysical plasmas. The primary data source that we have relied on in this study is the multi-spacecraft observational data.

The particle dynamics in plasmas has a highly multi-scale nature in terms of spatial scales, temporal scales, and energy range. In the solar wind, it spans about eight orders of magnitude in length (from fractions of the electron gyroradius to the Sun-Earth distance) and about five in energy. These ranges are way more extreme in astrophysical systems. Tackling the particle dynamics is thus a very challenging task, especially when the background plasma is highly turbulent.
No single numerical model can capture this full range of scales. I will describe and compare complementary models of plasma turbulence, such as Hall-MHD, hybrid and fully kinetic models, highlighting their similarities and differences across ranges of scales, from fluid scales through ion-kinetic scales down to electron scales. This comparison identifies where each model is reliable and where it breaks down, and justifies combining them into a single multi-scale framework.
Building on this, we have developed an innovative approach, the multi-scale Box-in-Box (BiB), which combines these models to describe particle energization in turbulent plasmas over a massive range of scales. First, we model the turbulent energy cascade from very large to very small scales coupling different models (MHD, hybrid, and fully kinetic), using a portion of a large simulation as initial condition for one with higher resolution multiple times in sequence. Then, we advance the trajectories of hundreds of millions of test particles on the evolving turbulent background from small to large scales while their gyroradius (and energy) increases. Our BiB approach allows for modelling the large-scale propagation of energetic particles in turbulent plasmas while retaining a realistic and self-consistent description of the microphysics responsible for particle energization. BiB simulations are key tools for developing new visualisation and analysis techniques for future multi-scale space missions such as NASA HelioSwarm and ESA Plasma Observatory.

Turbulence and magnetic fields are present in most astrophysical environments, and their significant dynamical roles are widely acknowledged. One transport effect with a direct impact on star formation processes is the diffusion of magnetic fields. This effect challenges the commonly employed assumption that magnetic flux is ``frozen-in'' on large scales, an assumption that holds only when ohmic resistivity is considered negligible. In this presentation, I will discuss several open questions regarding magnetic field diffusion during star formation, as well as our contributions toward quantifying, via numerical simulations, the transport rates driven by MHD turbulence. I will also discuss our recent efforts to characterize magnetic fields and turbulence in the interstellar medium by studying the optical polarization of starlight, which arises from the absorption of light by anisotropic dust grains aligned with the ambient field lines.