Alfven waves contribute significantly to the solar coronal heating, the solar wind acceleration, as well as Alfv\'enic turbulence formation. As a universal process, magnetic reconnection has long been credited as a potentially crucial source of Alfven waves, but how magnetic reconnection trigger Alfven waves remains elusive. Here, with simulations of three-dimensional bursty interchange magnetic reconnection in the solar corona, for the first time, we find that Alfven waves are spontaneously excited in the reconnection sheet and propagate bi-directionally even along the unreconnected magnetic fields. The enhanced total pressure inherently carried by flux ropes gives kicks to the magnetic fields, and the nearly same propagation speed of the flux ropes and the kicks makes the kicks growing into the observed Alfven waves, which have large amplitudes and high frequencies, carrying substantial energy for heating the quiet corona and accelerating the solar wind. Our findings demonstrate that Alfven waves are natural products of three-dimensional intermittent magnetic reconnection, bringing its fundamental significance for energy release, transport, and conversion occurring in the plasma system.

How to cite: Yang, L., He, J., Feng, X., Li, H., Guo, F., Tian, H., and Shen, F.: Spontaneous Generation of Alfven Waves by Bursty Interchange Magnetic Reconnection in the Solar Corona, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4321, https://doi.org/10.5194/egusphere-egu24-4321, 2024.

Recent observations of the interplanetary magnetic field have enlarged the magnetic field topology family in the inner heliosphere, and raises interest in the relationship between those topologies and the evolution of solar wind. For example, switchbacks that are kinks of magnetic field lines accompanied by velocity spikes and enhancement of the proton parallel temperature, have free energy both of the field and of the particles, and thus have potential for a series of plasma instabilities. It is favorable to extract elementary topologies from complicated structures like switchbacks before we look further in. Based on Particle-in-Cell simulations, we discuss from elementary topologies on the effect they have on kinetic processes of the solar wind plasma.

How to cite: Lin, R., Lapenta, G., and He, J.: How will interplanetary magnetic field topology modify solar wind kinetics?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11114, https://doi.org/10.5194/egusphere-egu24-11114, 2024.

    Solar and interplanetary radio bursts are important phenomena to reflect the electron acceleration and kinetic process in the corona and space plasmas. The near-Sun radio observation by Parker Solar Probe (PSP) provides an good chance to make the approach and in situ measurements for the emission source of radio bursts. According to the radio dynamic spectrum, we found that a large number of weak radio bursts with higher cutoff frequency f_lo can only be detected by PSP. These bursts have several obvious characteristics: (1) short duration (1-5 min); (2) narrow frequency band (0.5-15 MHz); (3) weak peak intensity (~ 10^-15 V²/Hz). Their relative frequency drift rate decreases from > 0.01 s^-1 to < 0.01 s^-1 implies that they are not the typical type III and II radio bursts. Based on the plasma empirical models and the data from in situ detection by PSP, the fitted models indicate that the radiation of these bursts might be generated by the electron cyclotron maser emission in the acceleration region of solar wind (1.1-10 R_S). The spectral characteristics of these bursts manifest that these bursts come from small-scale emission source, which have experienced strong kinetic evolution. We propose that these weak bursts are possibly the solitary wave radiation (SWR) generated from electron cyclotron maser instability with the energetic electrons trapped and accelerated by solitary kinetic Alfvén wave (SKAW) in the close magnetic structure. The decay of SKAW can well explain the deceleration of the emitting sources and the short duration.

How to cite: Ma, B., Chen, L., and Wu, D.-J.: Emission Mechanism of radio bursts from the Solar Wind Acceleration Region observed by Parker Solar Probe in near-Sun plasmas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18356, https://doi.org/10.5194/egusphere-egu24-18356, 2024.

A nonlinear force-free field is solely determined by the normal components of magnetic field and current density on the entire boundary of the domain. Methods using three components of magnetic field suffer from either overspecification of boundary conditions and/or a nonzero divergence B problem. A vector potential formulation eliminates the latter issue, yet poses difficulties in imposing normal components of both magnetic field and current density on the boundary. This challenge arises due to the inability to fix all three components of the vector potential at the boundary while the vector potential within the interior domain undergoes iterative changes.

This paper explores four distinct boundary treatment approaches within the vector potential formulation. In two methods, the normal component of the vector potential is adjusted to satisfy the prescribed boundary-normal component of current density at each iteration, while the tangential components remain fixed throughout. In the other two methods, the tangential components of the vector potential are modified at each iteration, while the normal component remains fixed. Each group comprises both first-order and second-order boundary treatments. We conduct a comparative analysis of these methods against our established poloidal-toroidal formulation code and the optimization code in Solar Software.

While none of the four methods outperform our poloidal-toroidal formulation code, they demonstrate comparable or superior performance compared to the latter. This research is regarded to provide insights into optimizing boundary conditions for data-driven simulations using vector potentials. 

How to cite: Lee, M., Choe, G., and Yi, S.: Implementation of Boundary Conditions for Nonlinear Force-Free Field Computations Using Vector Potentials, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18387, https://doi.org/10.5194/egusphere-egu24-18387, 2024.

It is believed that the suprathermal populations of electrons and protons in the solar wind and terrestrial magnetosphere (with energies exceeding those of the bulk population up to several keV) can offer key answers to many major problems, such as the origin of energetic solar particles from interplanetary shocks, particle acceleration by the energy dissipation of the small-scale wave fluctuations, but also a certain level of kinetic turbulence which can explain the non-equilibrium quasi-stable states of space plasmas. Due to their low density and relatively high energy, these populations are collisionless, which means that their dynamics is governed by wave-particle interactions, especially resonant Landau or cyclotron interactions. We present here a series of recent results that demonstrate that the suprathermal populations, whether in the form of a less-drifting halo or the field-aligned beams/strahls, are mainly responsible for the resonant excitation of kinetic wave instabilities. The in-situ observations confirm both the electromagnetic fluctuations triggered by temperature anisotropy and the predominantly electrostatic excitations induced for instance by electron beams.

How to cite: Hamd, S. M. S., López, R. A., Lazar, M., and Poedts, S.: Resonant Wave Excitations of Suprathermal Populations in the Solar Wind and Terrestrial Magnetosphere, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7217, https://doi.org/10.5194/egusphere-egu24-7217, 2024.

The electrons in the solar wind often exhibit non-equilibrium velocity distribution functions. Observed non-equilibrium electron features in the inner heliosphere include a field-aligned beam (called "strahl"), a suprathermal halo population, a sunward deficit in the distribution, and temperature anisotropy. These features are the result of a complex interplay between global expansion effects, collisions, and local interactions between the particles and the electromagnetic fields. Global effects create, for example, the strahl via the mirror force in the decreasing magnetic field and the sunward deficit via reflection effects in the interplanetary electrostatic potential. Local wave-particle interactions such as instabilities and wave damping change the shape of these signatures and thus the overall properties and moments of the electron distribution.

We discuss the formation of the relevant features in the electron distribution and analyse their impact on the linear stability of whistler waves in the inner heliosphere. We then present results from our numerical ALPS code that is capable of evaluating the linear stability of plasma with arbitrary background distributions. With results from our ALPS code, we show that the strahl-core-deficit configuration near the Sun drives oblique whistler waves unstable. However, it leads to enhanced damping of parallel whistler waves compared to a Maxwellian configuration. As the distribution evolves, the sunward deficit fills with electrons, at which point the plasma becomes unstable and drives parallel whistler waves. Our results highlight the need to treat electrons statistically as a globally inhomogeneous plasma component and to account for the detailed shape of their distribution in the evaluation of the plasma's linear stability.

How to cite: Verscharen, D., Micera, A., Innocenti, M. E., Coburn, J., Boella, E., Pierrard, V., Liu, J., Owen, C. J., Nicolaou, G., and Klein, K. G.: Statistical mechanics of the electrons in the solar wind: stability and instability of whistler waves in the inner heliosphere, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8120, https://doi.org/10.5194/egusphere-egu24-8120, 2024.

The kinetic Alfvén wave (KAW) is an extension of the Alfvén mode to the kinetic scales that propagates at quasi-perpendicular angles with respect to the background magnetic field in a magnetized plasma. The KAW is characterized by a right-handed polarization in the plane orthogonal to the background magnetic field. This allows the KAW to resonate with electrons, as contrary to the electromagnetic ion-cyclotron (EMIC) wave, which resonates with positive ions due to it’s its left-handed polarization. The EMIC mode is also a kinetic extension of the classical Alfvén wave, and propagates at quasi-parallel wavenormal angles. Due to the relevance and overall presence of both of these modes in space plasmas, understanding the nature of the transition from the EMIC mode to the KAW is a matter of great interest for the study of the micro-scale physics of these systems.

The transition from left-hand to right-hand polarized Alfvén waves depends on the wavenumber, plasma beta, temperature anisotropy, and ion composition of the plasma. Along with the temperature anisotropy, the electron-to-proton temperature ratio T_e/T_p is of great relevance for the characterization of the thermal properties of a plasma. This ratio varies significantly between different space plasma environments. Thus, studying how variations on this ratio affect the polarization properties of electromagnetic waves becomes of high relevance highly relevant
for our understanding of the dynamics of space plasmas.

In this work, we present an extensive study on the effect of the thermal properties of electrons on the behaviour and characteristics of Alfvénic waves in fully kinetic linear theory, as well as on the transition from EMIC to KAW. We show that the temperature ratio T_e/T_p has strong and non-trivial effects on the polarization of the Alfvénic modes, especially at kinetic scales (k_┴ρ_L>1, where k_┴=k sinθ and ρ_L= cs/Ω_p, with c_s the plasma sound speed and Ωp the proton’s gyrofrequency) and β_e+β_p>0.5, where β_s=8πnkT_s/B² is the ratio between thermal and magnetic pressure. We conclude that electron inertia plays an important role in the kinetic scale physics of the KAW in the warm plasma regime, and thus cannot be excluded in hybrid models for computer simulations.

How to cite: Sepúlveda, N. F., Moya, P. S., López, R. A., and Verscharen, D.: The effect of the thermal properties of electrons on the dispersion of kinetic Alfvén waves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8536, https://doi.org/10.5194/egusphere-egu24-8536, 2024.

Magnetic helicity is an important concept in solar physics, with a number of theoretical statements pointing out the important role of magnetic helicity in solar flares and coronal mass ejections (CMEs). Here we construct a sample of 47 solar flares, which contains 18 no-CME-associated confined flares and 29 CME-associated eruptive flares. We calculate the change ratios of magnetic helicity and magnetic free energy before and after these 47 flares. Our calculations show that the change ratios of magnetic helicity and magnetic free energy show distinct different distributions in confined flares and eruptive flares. The median value of the change ratios of magnetic helicity in confined flares is −0.8%, while this number is −14.5% for eruptive flares. For the magnetic free energy, the median value of the change ratios is −4.3% for confined flares, whereas this number is −14.6% for eruptive flares. This statistical result, using observational data, is well consistent with the theoretical understandings that magnetic helicity is approximately conserved in the magnetic reconnection, as shown by confined flares, and the CMEs take away magnetic helicity from the corona, as shown by eruptive flares.

How to cite: Wang, Q., Yang, S., Zhang, M., Yang, X., and Zhu, X.:  Change Ratios of Magnetic Helicity and Magnetic Free Energy During MajorSolar Flares , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13542, https://doi.org/10.5194/egusphere-egu24-13542, 2024.

The exact coronal heating mechanism remains a riddle, but magnetically active regions are known to host coronal loops with extreme-UV emission. But also at much smaller sizes, up to 10 Mm, there are bipolar regions that can be associated with UV emission in coronal bright points (CBPs). We study the statistical properties of CBPs with continuous data from the SDO spacecraft to track the lifetime of CBPs. We use their tracking data to verify that the lower corona co-rotates with the photosphere. In a next step, we aim to reproduce an isolated CBP in a 3D magneto-hydrodynamic (MHD) simulation. To this end, we need observational data to drive the simulation from both, SDO/HMI and Hinode/NFI. As these instruments feature different resolutions and field-of-views, they also detect different levels of small-scale and large-scale magnetic structures in the photosphere. As we know, these magnetic patches are advected from photospheric horizontal motions and create the necessary Poynting flux at the base of the corona. Combining these data from two very different instruments is a task that needs careful overlaying, so that not artificial effects would appear in the MHD simulation. We use a multi-scale overlaying method to enlarge the field-of-view of Hinode with SDO data, to drive the simulation with consistent photospheric magnetic fields. The bottom and top boundaries are fully closed for any mass and heat flows. The output of the simulation will allow us to compute synthetic UV emission maps that we may compare directly to SDO and Hinode observations. With our model we test if the field-line braiding mechanism is sufficient to heat a CPB to the required temperature. We find that loop-like CBPs usually originate from bipolar regions. Weaker magnetic polarities produce fainter and hence cooler CBPs. And the same time, we find that lifetimes of typical CPBs are easily more than 6 hours. This supports the theory that the heating of CBP is mainly based on magnetic energy dissipation through a relatively steady and slow magnetic reconnection process.

How to cite: Kraus, I. and Bourdin, P.: Coronal Bright Points observed in the corona and photosphere with SDO and Hinode, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15727, https://doi.org/10.5194/egusphere-egu24-15727, 2024.

We simulate the propagation of relativistic test particles within the field of a 3D MHD simulation of the solar wind. The adiabatic ideal MHD equations are integrated numerically using the MPI-AMRVAC code. Test particles are initialized within the MHD simulation grid and advanced in time according to the guiding center equations, we employ a third-order accurate time prediction-correction method from Mignone et al. 2023 for integration. We also include possibility of diffusion in velocity space based on a particle-turbulence mean free path λ∥ along the magnetic field line. One of the first results where we consider 81 keV electrons injected at 0.139 AU heliocentric distance and mean free path λ∥ = 0.5 AU is in good qualitative agreement with measurements at 1 AU.

How to cite: Houeibib, A., Pantellini, F., and Griton, L.: Exploring Solar Energetic Particles Transport in the Inner Heliosphere, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17085, https://doi.org/10.5194/egusphere-egu24-17085, 2024.

Langmuir waves are frequently detected in the solar wind and affect the energetics of the plasma electrons. Previous observational studies have found Langmuir waves associated with magnetic holes in the solar wind. In our work, we aim to understand the connection between magnetic holes and these waves.

The Langmuir instability is a well-known consequence of electron beams in plasmas. We use particle-in-cell (PIC) simulations of a collisionless electron-ion plasma in a magnetic hole structure. We study the triggering of Langmuir waves by inhomogeneous beam instabilities in this configuration.

Our simulations reveal patterns that suggest that injecting a beam into the magnetic hole has the potential to create spatially inhomogeneous Langmuir wave packets by instability. We support our PIC simulations with linear stability calculations and discuss the conditions required for magnetic holes to emit Langmuir waves through our proposed mechanism. Our simulation results will be used in comparative studies with observational data of Langmuir-wave emitting magnetic holes.

How to cite: Liu, J., Verscharen, D., Coburn, J., Agudelo, J., Germaschewski, K., Reid, H., Nicolaou, G., and Owen, C.: Simulations of Langmuir-wave emission by magnetic holes in the solar wind, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18011, https://doi.org/10.5194/egusphere-egu24-18011, 2024.

A special observational signature in ICMEs that has puzzled researchers for a long time are so-called ”back regions” or ”magnetic-cloud-like (MCL)” parts of ICMEs which follow after the main flux rope rotation has passed the observer. These regions, occurring after the main flux rope rotation, display a peculiar behaviour where the magnetic field remains elevated with minimal rotation. Two proposed explanations involve magnetic reconnection during CME propagation or a purely geometric effect as the observer traverses the flux rope . 

We investigate in detail whether MCLs can be explained by an effect of the trajectory of an observer through a 3D expanding magnetic flux rope, without invoking magnetic reconnection as an explanation for those signatures. To this end, we employ the 3D coronal rope ejection (3DCORE) model, which has proven its ability to fit in situ magnetic fields of CME flux ropes. 

The model assumes an empirically motivated torus-like flux rope structure that expands self-similarly within the heliosphere, is influenced by a simplified interaction with the solar wind environment, and carries along an embedded analytical magnetic field. Developed for fitting, generating synthetic signatures, comparing models to observations, and analysing results, 3DCOREweb accelerates the determination of physical parameters, fostering research on the global magnetic structure of CMEs. Additionally, the interface aids in advancing our understanding of magnetic flux rope signatures arising from diverse 3D trajectories through CMEs.

How to cite: Rüdisser, H. T., Weiss, A. J., Möstl, C., Amerstorfer, U. V., Davies, E. E., and Weiler, E.: Understanding the effects of spacecraft trajectories through solar coronal mass ejection flux ropes using 3DCOREweb, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8372, https://doi.org/10.5194/egusphere-egu24-8372, 2024.

The temperature of the solar atmosphere increases from thousands to millions of degrees moving from the lower layer (chromosphere) to the outermost one (corona), while the density drops accordingly. The mechanism behind this phenomenon, known as a temperature inversion, is still unknown. In this work, we model a coronal loop as a collisionless plasma confined in a semicircular tube that is subject to the Sun's gravity and in thermal contact with a fully collisional chromosphere behaving as a thermostat at the loop's feet. By using kinetic N-particle simulations and analytical calculations, we show that rapid, intermittent, and short-lived heating events in the chromosphere drive the coronal plasma towards a non-equilibrium stationary state. The latter is characterized by suprathermal tails in the particles velocity distribution functions, exhibiting temperature and density profiles strikingly similar to those observed in the atmosphere of the Sun. These results suggest that a million-Kelvin solar corona can be produced without the local deposition of heat in the upper layer of the atmosphere that is typically assumed by standard approaches. We find that suprathermal distribution functions in the corona are self-consistently produced instead of postulated a priori, in contrast to classical kinetic models based on a velocity filtration mechanism.

How to cite: Barbieri, L., Casetti, L., Verdini, A., Landi, S., Papini, E., and Di Cintio, P.: Temperature inversion in a gravitationally bound plasma: Case of the solar corona, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20270, https://doi.org/10.5194/egusphere-egu24-20270, 2024.

Our research delves into solar wind's mesoscale features known as microstreams—periods of heightened speed and temperature lasting hours. Initially observed in Helios and Ulysses data, they're now prevalent in the 'young' solar wind by Parker Solar Probe and Solar Orbiter. Recent findings unveil microstreams' carriage of abundant Alfvénic perturbations—velocity spikes and magnetic switchbacks.

Employing a high-resolution 2.5 MHD model, we scrutinize the genesis of microstreams from emerging bipoles interacting with the ambient corona. Our simulations reveal the tearing-mode instability, forming plasmoids released into the solar wind. Our domain spans the lower corona to 20 Rs, enabling observation of plasmoid formation and their evolution into Alfvénic spikes (Gannouni et al. 2023). The magnetic emergence rate modulates microstream characteristics.

Additionally, 3D MHD simulations explore intermittent interchange reconnection in a 24x24x30Mm domain with a flux emergence rate of 1.38G/s. Reconnection spawns plasma jets and twisted magnetic field bundles, releasing hot plasma into the solar wind, inducing propagating waves and twists along magnetic field lines.

How to cite: Gannouni, B., Réville, V., and Rouillard, A.: Modelling the formation and  evolution of solar wind microstreams:from coronal plumes to propagating Alfvénic velocity spikes., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18200, https://doi.org/10.5194/egusphere-egu24-18200, 2024.