Magnetic reconnection is a fundamental process in astrophysical plasma, as it enables the dissipation of energy at kinetic scales as well as large-scale reconfiguration of the magnetic topology. In the solar wind, its quantitative role in plasma dynamics and particle energization remains an open question that is starting to come into focus as more missions now probe the inner heliosphere. In particular, the first encounters of the Parker Solar Probe (PSP) mission with the Sun have revealed that the Heliospheric Current Sheet (HCS) was often reconnecting close to the Sun, opening question about the impact of HCS reconnection on the nearby solar wind.
In this work, we first make a thorough catalog of all HCS crossings measured PSP (encounter 5 to the latest available) and find that 88\% of crossings present magnetic reconnection signatures. This statistically confirms that magnetic reconnection is prevalent in the near Sun HCS. We then quantify the level of turbulence within the HCS and find enhanced energy at kinetic scales compared to the nearby solar wind, usually devoid of magnetic switchbacks. We furthermore highlight the frequent observation of mirror mode instabilities within the structure of the HCS, hinting that this process plays a particular role in the energy dissipation. These mirror mode instabilities are also observed within HCS crossings observed by Solar Orbiter further in the heliosphere. We finally plan to study the evolution of the HCS structure through multi-spacecraft observation.
Collectively, these results show that the HCS may play an important role in the energization of the near Sun solar wind. We discuss the impact of these observations on our current understanding of HCS reconnection and solar wind turbulence.
How to cite: Fargette, N., Eastwood, J., Matteini, L., Waters, C. L., Génot, V., and Réville, V.: On the role of mirror mode instabilities in the reconnecting Heliospheric Current Sheet dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18278, https://doi.org/10.5194/egusphere-egu25-18278, 2025.
Space plasma turbulence incorporates multi-scale coexisting occurrences of many physical phenomena such as waves, large amplitude field and plasma fluctuations, formation of coherent structures and the large variety of associated energy transfer, transport and conversion processes. For example, magnetic reconnection converts magnetic energy to kinetic and thermal energies and accelerates particles. Contrarily, dynamo action refers to energy conversion processes through which magnetic fields are generated or/and amplified at the expense of kinetic energy. Magnetic reconnection has been extensively studied on the basis of in-situ measurements at large-scale magnetospheric boundaries, in the turbulent magnetosheath and in the solar wind. Dynamo processes have been investigated mainly through numerical studies and in laboratory liquid metal and laser experiments. In-situ observations of dynamo processes require certain physical assumptions to calculate gradients from single-point data in the solar wind. Here we study for the first time the kinematic small-scale dynamo in the turbulent magnetosheath. In the kinematic approach the back reaction of the amplified magnetic field to plasma flows is neglected. Small-scale dynamos can generate or amplify magnetic fields at scales comparable to, or smaller than, the characteristic scales of flow gradients in 3D plasma turbulence. The flow gradients are estimated on the basis of in-situ multi-point MMS measurements. Theoretical predictions and numerical simulation results for the turbulent kinematic dynamo are tested. Specifically, the expected stretching of the magnetic field by velocity gradients, the effect of compressions and the concurrent occurrence of pressure anisotropy instabilities are investigated. The observations show that the magnetosheath data exhibit the expected turbulent dynamo signatures. Since the increase of magnetic field is associated with the loss of kinetic energy, the small-scale dynamo represents an inherent ingredient of plasma turbulence.
How to cite: Vörös, Z., Roberts, O. W., Narita, Y., Emiliya, Y., Nakamura, R., Schmid, D., Settino, A., Wolwerk, M., Wedlund, C. S., Varsani, A., Sorriso-Valvo, L., Bourdin, P. A., and Kis, Á.: Turbulent small-scale kinematic dynamo in the terrestrial magnetosheath, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3759, https://doi.org/10.5194/egusphere-egu25-3759, 2025.
The very first observations by Mariner 5 highlighted the presence of Alfvénic fluctuations in the solar wind identified as nearly incompressible fluctuations accompanied by large correlations between velocity and magnetic field components as predicted by the magnetohydrodynamics (MHD) theory. Since then, Alfvénic fluctuations have been observed to be ubiquitous especially in high-speed solar wind streams, but are also in some cases in slow wind streams, which may in turn exhibit a strong Alfvénic character. The so-called Alfvénic slow wind resembles the fast wind in many aspects, but may also differ from it. Indeed, recent observations performed by Solar Orbiter have shown that the fast wind may display a strong Alfvénic content of the fluctuations than the one observed in the Alfvénic slow wind, especially closer to the Sun.
In this context, Solar Orbiter offers a unique opportunity to study the origin and radial evolution of the Alfvénic solar wind. In this particular study, we present a comparative study between different Alfvénic streams, both fast and slow, at different heliocentric distances, focusing on the characterization of Alfvénicity of different streams with particular reference to the energy balance of the fluctuations.
The aim of this work is to deepen our understanding of what are the mechanisms responsible for the evolution of Alfvénicity in solar wind fluctuations and to understand better to what extent the two solar wind regimes show different Alfvénic content of the fluctuations and eventually evolve in a different way.
How to cite: D Amicis, R., Benella, S., Bruno, R., De Marco, R., Velli, M., Perrone, D., Sorriso Valvo, L., Alterman, B. L., Sioulas, N., Franci, L., Verdini, A., Matteini, L., Telloni, D., Owen, C. J., Louarn, P., and Livi, S.: What determines the departure from equipartition of energy in Alfvénic fluctuations in solar wind streams? Insights from Solar Orbiter observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4040, https://doi.org/10.5194/egusphere-egu25-4040, 2025.
We have studied the decay of turbulence in the solar wind. Fluctuations carried by the expanding wind are naturally damped because of flux conservation, slowing down the development of a turbulent cascade. The latter also damps fluctuations but results in plasma heating. We analyzed time series of the velocity and magnetic field (v and B, respectively) obtained by the WIND spacecraft at 1 au. Fluctuations were recast in terms of the Elsasser variables, z± = v ± B/√4πρ, with ρ being the average density, and their second- and third-order structure functions were used to evaluate the Politano-Pouquet relation, modified to account for the effect of expansion.
We find that expansion plays a major role in the Alfvénic stream, those for which z+ ≫ z‑. In such a stream, expansion damping and turbulence damping act, respectively, on large and small scales for z+, and also balance each other. Instead, z‑ is only subject to a weak turbulent damping because expansion is a negligible loss at large scales and a weak source at inertial range scales.
These properties are in qualitative agreement with the observed evolution of energy spectra that is described by a double power law separated by a break that sweeps toward lower frequencies for increasing heliocentric distances. However, the data at 1 au indicate that injection by sweeping is not enough to sustain the turbulent cascade. We derived approximate decay laws of energy with distance that suggest possible solutions for the inconsistency: in our analysis, we either overestimated the cascade of z± or missed an additional injection mechanism; for example, velocity shear among streams.
How to cite: Verdini, A., Hellinger, P., Landi, S., Grappin, R., Montagud-Camps, V., and Papini, E.: Decay of magnetohydrodynamic turbulence in the expanding solar wind: WIND observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9622, https://doi.org/10.5194/egusphere-egu25-9622, 2025.
Exploration of space plasmas is entering a new era of multi-satellite constellation measurements that will determine fundamental properties of turbulence, with unprecedented precision. Familiar but imprecise approximations must be abandoned and replaced with more advanced approaches. We present the novel multispacecraft technique LPDE (Lag-Polyhedra Derivative Ensemble) for evaluating third-order statistics, using simultaneous measurements at many points. The method differs from existing approaches in that (i) it is inherently three-dimensional; (ii) it provides a statistically significant number of estimates from a single data stream; and (iii) it allows for a direct visualization of energy flux in turbulent plasma. Implications for HelioSwarm and Plasma Observatory and comparison with single-spacecraft approaches are discussed.
How to cite: Pecora, F., Servidio, S., Greco, A., Yang, Y., Matthaeus, W. H., Chasapis, A., Primavera, L., Hellinger, P., Pucci, F., Oughton, S., Gershman, D. J., Giles, B. L., and Burch, J. L.: Measuring the turbulent energy cascade rate with multiple spacecraft, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4534, https://doi.org/10.5194/egusphere-egu25-4534, 2025.
The evolving subset of turbulent structures facilitates the energy transfer from large to small spatial scales, on average. Currently, it is not known how the discontinuities that develop between these structures alter the energy transfer in the solar wind. Quantifying the energy transfer to small scales is essential to explain the apparent plasma heating during its advection through the heliosphere. We analyse the energy transfer rate conditioned on the magnetic field line topology of the associated structures in the solar wind. Magnetic field line topology is classified using invariants of the magnetic field gradient tensor constructed from the Cluster spacecraft configuration on scale of approximately 40 proton gyro-radii. Third order structure functions are estimated for five solar wind intervals and conditioned on the contemporaneous values of the topological invariants. We determine how the global mean energy transfer rates correlate with the topology of the turbulence.
How to cite: Hnat, B., Chapman, S., and Watkins, N.: Statistics of the turbulent energy transfer rate conditioned on magnetic field line topology in the solar wind, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6877, https://doi.org/10.5194/egusphere-egu25-6877, 2025.
Solar wind provides a natural laboratory for the plasma turbulence. The core problem is the energy cascade process in the inertial range, which has been a long-standing fundamental question. Many efforts are put into the theoretical modellings to explain the observational features in the solar wind. However, there are always questions remained. Here we report a new scenario that the inertial regime of the solar wind turbulence consists of two subranges based on the observation. We perform multi-order structure function analyses for one high-latitude fast solar wind interval at 1.48 au measured by Ulysses and one slow solar wind at 0.17 au measured by Parker Solar Probe (PSP). We identify the existence of two subranges in the inertial range according to their distinct scaling features. Based on the observational features, we propose that the possible mechanisms that subrange 1 is Iroshnikov-Kraichnan-like turbulence and subrange 2 is the intermittency-dominated region. The scenario of two subranges and their scaling laws not only shed new lights for the plasma turbulence, but also unify previous results that cause debates, making the observed scaling laws prepared for further theoretical modeling.
How to cite: Wu, H., Huang, S., He, J., Yang, L., Sorriso-Valvo, L., Wang, X., and Yuan, Z.: A new scenario with two subranges in the inertial regime of solar wind turbulence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14782, https://doi.org/10.5194/egusphere-egu25-14782, 2025.
In space plasma, due to the absence of collisions, the phase space present a complex structuring and strong deviations from thermal equilibrium. Previous works have highlighted this aspect in both magnetosheath data and numerical simulation through an hermite decomposition of the ion velocity distribution function. The hermite spectrum of the vdf is expected to to have a precise spectral slope and to present anisotropy in a magnetic field dominated environment. Such a tool is particularly suited for the vdf representation since each order of the hermite decomposition corresponds to a moment of the vdf.
In this work we study, by using the Parker Solar Probe ion vdfs, the evolution of the hermite spectrum and the vdf fine features with respect to radial distance and solar wind conditions.
These results are useful to understand how the phase space evolve in the inner heliosphere and how this effect the heating in collissionless plasma.
How to cite: Larosa, A., Pezzi, O., Bowen, T., Chasapis, A., Trotta, D., Sorriso-Valvo, L., Chen, C., Livi, R., and Verniero, J.: On the velocity phase space cascade in the inner heliosphere, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17532, https://doi.org/10.5194/egusphere-egu25-17532, 2025.
Posters on site: Wed, 30 Apr, 08:30–10:15 | Hall X4
The solar wind is highly turbulent, which results in power-law spectra and intermittency for magnetic and velocity fluctuations within the inertial range.
Using fast solar wind intervals measured during solar minima between 0.3 au and 3.16 au, a clear break emerges within the traditional inertial range, with signatures of two inertial sub-ranges with f-3/2 and f-5/3 power laws in the magnetic power spectra. The intermittency, measured through the scaling law of the kurtosis of magnetic field fluctuations, further confirms the existence of two different power laws separated by a clear break. A systematic study on the evolution of the said sub-ranges as a function of heliospheric distance shows correlation of the break scale with both the turbulence outer scale and the typical ion scales. Finally, using Parker Solar Probe data measured closer to the Sun, we highlight the role of switchbacks and switchback patches in generating such scale breaks.
How to cite: Sorriso-Valvo, L., Mondal, S., Banerjee, S., Larosa, A., Wu, H., Sioulas, N., Telloni, D., D'Amicis, R., and Yordanova, E.: Emergence of a characteristic scale in the Alfvénic solar wind turbulence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11551, https://doi.org/10.5194/egusphere-egu25-11551, 2025.
The solar wind plasma is observed to fluctuate over a broad range of space and time scales, extending from scales above the magnetic field correlation scale to below those associated with the particle gyration. At scales larger than the gyroscale, the fluctuations are typically categorised as 1) non-compressive fluctuations that have Alfvénic correlation, 2) compressive fluctuations that perturb the plasma density and pressure. While the amplitude of the compressive fluctuations are subdominant to the Alfvénic component, they have unique dynamics that drastically alter the plasma. For example, compressive fluctuations perturb the pressure anisotropy and beam drift speeds. This may drive the perturbed plasma unstable, generating microscale waves that scatter particles and alter the effective mean free path. In addition, compressive fluctuations perturb the magnetic field strength, leading to stochastic heating and transit time damping. Therefore, an understanding of compressive fluctuations is vital to a complete picture of the plasma thermodynamics. To build on our understanding of the solar wind in the inner heliosphere, we combine observations from Solar Orbiter, Parker Solar Probe, and the Wind spacecraft to study compressive fluctuations. We compare amplitude ratios and polarisations to numerical models to understand the efficiency of various generation mechanisms of compressive fluctuations and how they heat and modify the thermodynamics of the solar wind plasma.
How to cite: Coburn, J., Verscharen, D., Tenerani, A., and Owen, C.: The Thermodynamic Impact of Compressive Fluctuations on the Solar Wind in the Inner Heliosphere, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3330, https://doi.org/10.5194/egusphere-egu25-3330, 2025.
Magnetic reconnection is an energy dissipating process, in which magnetic field topology is modified, eroding the magnetic field, and turning the magnetic energy into thermal and kinetic energy of the plasma. Magnetic reconnection has been observed through a wide range of scales in the solar system, from thousands of ion inertial lengths in the heliospheric current sheet to few electron inertial lengths in Earth’s magnetosheath. However, the smaller scales were not accessible in the solar wind until the launch of Solar Orbiter and Parker Solar Probe, and therefore ion-scale magnetic reconnection had not been studied in the solar wind.
Non-linear interactions drive turbulence in the solar wind, transferring energy across scales at a constant rate, seen as a constant slope in the energy spectrum of magnetic fluctuations. However, a spectral break is observed at scales close to and below the ion inertial length. It has been proposed that the magnetic energy dissipated through magnetic reconnection at scales of the ion inertial length or smaller can account in part for this break in the magnetic fluctuation energy spectrum.
In the present work, we have harnessed the high cadence of the Solar Orbiter in-situ instrumentation (Solar Wind Analyzer and Magnetometer) to search for magnetic reconnection at scales in the order of few to tens ion inertial lengths. We compiled a catalog of 979 thin current sheets, 5% of which undergo reconnection. Statistics of CS properties and Solar Wind conditions around these has been performed, with a double aim: assessing the relation between turbulence and reconnection; and evaluate the influence of different Solar Wind parameters on ion-scale reconnection.
How to cite: F. Albert, I., Toledo-Redondo, S., Montagud-Camps, V., Castilla, A., Lavraud, B., Fargette, N., Louarn, P., Owen, C., and Zouganelis, Y.: Small-scale Current Sheets and Magnetic Reconnection in the Turbulent Solar Wind, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17439, https://doi.org/10.5194/egusphere-egu25-17439, 2025.
The power spectral densities (PSDs) of ion moments and magnetic field turbulence in the solar wind can be fitted by a power law with the power index of -5/3 in the MHD range of frequencies and with the power index ranging from 2 to 4 at frequencies exceeding the proton gyroscale. However, the density PSD often exhibits a significant flattening at the high-frequency part of the MHD range but a similar effect was not observed for any other quantity. For this reason, the paper analyzes the power spectra of solar wind and magnetic field fluctuations computed in the frequency range around the break between MHD and kinetic scales. We use Spektr-R proton moments and Wind magnetic field at 1 AU and concentrate on the overall PSD profiles of the density, thermal speed, parallel and perpendicular components of magnetic field and velocity fluctuations and investigate statistically the role of parameters like the fluctuation amplitude, collisional age, temperature anisotropy or ion beta. The statistics based on more than 10 thousand of 20-minute intervals shows that the compressive component of magnetic field fluctuations behaves like the density fluctuation in the old, low-beta solar wind. On the other hand, a similar profile was not observed for either bulk or thermal speeds. The dependence on the collisional age initiated the comparison with Solar Orbiter and PSP observations in the inner heliosphere that would shed light on the processes leading to a formation of these spectral features.
How to cite: Safrankova, J., Nemecek, Z., and Nemec, F.: Flattening of the magnetic field power spectral density profile, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4798, https://doi.org/10.5194/egusphere-egu25-4798, 2025.
We investigate properties of ideal second-order magneto-hydrodynamic (MHD) and Hall MHD invariants (kinetic+magnetic energy and different helicities) in a two-dimensional hybrid simulation of decaying plasma turbulence. The combined (kinetic+magnetic) energy decays at large scales, cascades (from large to small scales) via the MHD non-linearity at intermediate scales. This cascade partly continues via the Hall coupling to sub-ion scales. The cascading energy is transferred (dissipated) to the internal energy at small scales via the resistive dissipation and the pressure-strain effect. The mixed (X) helicity, an ideal invariant of Hall MHD, exhibits a strange behaviour whereas the cross helicity (the ideal invariant in MHD but not in Hall MHD), in analogy to the energy, decays at large scales, cascades from large to small scales via the MHD+Hall non-linearities, and is dissipated at small scales via the resistive dissipation and an equivalent of the pressure-strain effect. In contrast, the magnetic helicity is very weakly generated through the resistive term and does not exhibit any cascade; furthermore, the magnetic and cross helicities are not coupled in the hybrid approximation, so that the corresponding helicity barrier does not exist.
How to cite: Hellinger, P. and Montagud Camps, V.: Rugged magnetohydrodynamic invariants in weakly collisional plasma turbulence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5936, https://doi.org/10.5194/egusphere-egu25-5936, 2025.
Turbulent processes play a key role in the dynamics of solar wind plasma fluctuations, governing energy transfer within the heliosphere and driving particle acceleration. In this study, we aim to investigate the nature of large- and small-scale fluctuations in the upstream and downstream regions of interplanetary shocks. By analyzing magnetic field fluctuations using both traditional and recently developed methods, we examine changes in correlation length, Taylor scale, and Reynolds number from upstream to downstream regions. Plasma and magnetic field measurements from the ACE, WIND, and DSCOVR missions are utilized in this analysis. Correlation lengths are determined using autocorrelation and cross-correlation functions applied across data from the three spacecraft. When analyzing the Reynolds number, we observe a decrease in values when transitioning from upstream to downstream regions, suggesting turbulence resetting in the case under consideration. Building on the findings of a case study, we extend our investigation by performing a statistical analysis of these parameters across multiple shocks.
How to cite: Abushzada, I., Pitna, A., Nemecek, Z., and Safrankova, J.: Autocorrelation and Cross-Correlation of MHD Turbulence across IP Shock: Multispacecraft Analysis, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6875, https://doi.org/10.5194/egusphere-egu25-6875, 2025.
How to cite: Jiang, W., Li, H., Verscharen, D., Zheng, J., Klein, K., Riquelme, M., Liu, J., and Wang, C.: Multi-scale Dynamics of Coherent Electron Trapping and Diffusion in Earth's Magnetosheath, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7618, https://doi.org/10.5194/egusphere-egu25-7618, 2025.
We present a comprehensive analysis of the evolution of the turbulent energy dissipation at interplanetary (IP) shocks observed by Parker Solar Probe (≈0.4 AU), Solar Orbiter (≈0.8 AU), and Wind (1 AU). Our previous study reveals the conservation of the energy dissipating mechanisms across different types of IP shocks except fast reverse. Motivated to investigate the thickness of the shock transition region in terms of the dissipation of magnetic field turbulent energy, we adopt pairs of quasi-perpendicular fast forward (FF) and reverse (FR) shocks observed at Parker Solar Probe, Solar Orbiter, and Wind. By comparing these pairs of shock, we anticipate examining (1) whether FF and FR shocks are systematically different, (2) the dependence of the shock transition thickness on critical Mach number, and (3) on heliocentric distance. We present several parameters, i.e., cross- and magnetic helicity, and the amplitude of magnetic field fluctuations for the estimation of their correlation with the spectral index evolving through shock. The abrupt changes of the plasma parameters along with the spectral index shorter than the temporal resolution of the plasma measurement are overall observed showing their minimal correlations. This suggests a role of IP shock as a thin boundary simply distinguishing two different plasmas. We will extend this hypothesis toward a statistical study including near-shock processes such as particle acceleration and wave activities.
How to cite: Park, B., Pitna, A., Safrankova, J., and Nemecek, Z.: Evolution of Turbulent Energy Dissipation at Quasi-perpendicular Fast Interplanetary Shocks: The thickness of shock transition region, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10420, https://doi.org/10.5194/egusphere-egu25-10420, 2025.
Electromagnetic fluctuations in the solar wind cover a wide range of scales, from sun-rotation period to sub-electron scales. We study Cluster Guest Investigator data when 2 satellites were at 7 km distance, that corresponds to few electron Larmor radius. We find a typical spectral shape within the kinetic range and signatures of intermittency up to electron scales. Local analysis of magnetic fluctuations at electron scales indicates presence of vortex-like coherent structures, which can be interpreted in terms of electron scale Alfven vortices. We discuss a possible connection of these small-scale vortices with coherent structures at ion scales. The results at 1 au will be compared with spectral properties and coherent structures at kinetic scales observed by Parker Solar Probe closer to the Sun.
How to cite: Alexandrova, O., Jovanovic, D., Hellinger, P., Demoulin, P., Maksimovic, M., Bale, S., and Mangeney, A.: Solar wind turbulent fluctuations within the kinetic range of scales, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11571, https://doi.org/10.5194/egusphere-egu25-11571, 2025.
Properties of the solar wind in different types of plasma (e.g., heliospheric current sheet, coronal hole, ejecta, sub-Alfvénic) are known to exhibit distinct features. Based on Parker Solar Probe measurements of the solar wind in the inner heliosphere, we compare the similarities and differences between two streams originating from different sources at the same radial distance. Despite sharing similar properties, including cross helicity, residual energy, Elsasser ratio, and magnetic compressibility, notable differences are observed. For the solar wind associated with active regions, the turbulence exhibits lower magnetic field fluctuation amplitudes, shallower magnetic field spectrum, and stronger intermittency, whereas the turbulence associated with coronal holes displays opposite characteristics. The switchback properties of these two streams are also discussed. Our results further explore the variabilities of solar wind turbulence, which may have implications for solar wind heating and acceleration.
How to cite: Wang, T., Chen, W., Yang, L., He, J., and Fu, H.: Turbulence features of the solar wind from different source regions based on Parker Solar Probe observation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15511, https://doi.org/10.5194/egusphere-egu25-15511, 2025.
Magnetohydrodynamic (MHD) shocks are one of the key nonlinear phenomena which occur in plasmas and can influence a dynamical evolution of a system at wide range of spatial scales. In the vicinity of the shock fronts, a majority of the dissipation of the incident bulk energy takes place. Furthermore, the incident fluctuations have profound effect on the shock front itself and also on the respective evolution of the transmitted/generated modes. Recently, several approaches have been developed focusing on the evolution of various plasma wave modes across MHD shocks. In this work, we investigate the transmission of quasi-2D turbulent fluctuations across fast forward shocks in the framework of the Zank et al. (2021) model. We take advantage of concurrent measurements of upstream and downstream plasma of a terrestrial bow shock, employing observations of the Wind spacecraft and Magnetophere Multiscale Mission (MMS). This partially mitigates two main limitations of single spacecraft studies, (a) the variability of incident plasma and magnetic field fluctuations and (b) the effects that stem from the evolution of fluctuations as they propagate away from the shock front. Our results suggest that the Zank et al. (2021) model predicts the downstream levels of fluctuations excellently for the quasi-perpendicular regime of the bow shock. We discuss the deviations between the predicted and observed levels of downstream fluctuations, highlighting the influence of bow shock nonplanarity and variable obliquity.
How to cite: Pitna, A., Zank, G., Zhao, L., Nakanotani, M., Gautam, S. P., Silwal, A., Abushzada, I., Park, B., Safrankova, J., and Nemecek, Z.: Evolution of Turbulent Fluctuations across Terrestrial Bow Shock, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9809, https://doi.org/10.5194/egusphere-egu25-9809, 2025.
The phenomenon of energy cascade in Alfvénic solar wind turbulence has traditionally been studied assuming ideal plasmas, where viscosity (ν) and resistivity (η) are equal and very small. However, recent observations suggest that in the solar wind, viscous-like effects related to velocity act on much larger scales compared to magnetic dissipation. The main novelty of this study lies in assuming phenomenological distinctions among dissipation mechanisms and hence assuming different values for ν and η.
In this work, we investigate the third-order Yaglom law for magnetohydrodynamic (MHD) turbulence through a combination of theoretical analysis and simulations. Specifically, we study the energy budget law for visco-resistive MHD and explore how differing viscosities and resistivities affect the energy cascade. The Yaglom relation, rewritten in terms of Elsässer variables, deviates from the ideal case due to the assumption ν ≠ η. This relation, which involves a third-order moment calculated from velocity and magnetic fields, provides a direct measure of the energy transfer rate across scales.
Our preliminary results, supported by direct numerical simulations, indicate that these findings could enhance the interpretation of solar wind and magnetosheath observations. The third-order moment is indeed particularly relevant as it enables a detailed comparison of energy transfer mechanisms, highlighting the differences that arise when the dissipation processes in the velocity and the magnetic field are different.
How to cite: Fortugno, E. M., Scarivaglione, L., Servidio, S., and Carbone, V.: Third-Order Law for MHD Turbulence Varying the Dissipation Mechanisms, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16301, https://doi.org/10.5194/egusphere-egu25-16301, 2025.
Solar Orbiter observations provide an unprecedented opportunity to study plasma turbulence in the solar wind. On magnetohydrodynamic scales intermittent structures mediate the cascade, due to non-linear wave-wave interactions and coherent structures. Those coherent structures are often quantified and identified by the Partial Variance Increment (PVI).
We obtain magnetic field fluctuations from observations of homogeneous turbulence by wavelet decompositions which preferentially resolve either signatures of coherent structures or wave-packets. Comparing the PVI obtained from both wavelet decompositions, this provides a new, physics based method to determine the PVI threshold above which fluctuations may be coherent structures.
We find a single PVI threshold in each of the kinetic and inertial ranges above which coherent structures typically dominate. This threshold is insensitive to the plasma conditions or heliocentric distance. Therefore, it suggests a ubiquitous constraint on the turbulent phenomenology. This can inform estimates of the heating rates of the solar wind due to the turbulence.
How to cite: Bendt, A. and Chapman, S.: Ubiquitous threshold for coherent structures in the kinetic and inertial ranges of solar wind turbulence from Solar Orbiter observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3592, https://doi.org/10.5194/egusphere-egu25-3592, 2025.
In the transition range of the solar wind turbulence, the magnetic spectrum has been observed to be strongly anisotropic with respect to local mean field. However, the generation mechanism of the anisotropy remains not well understood. There are two typical types of waves existing in the transition range, including ion cyclotron waves (ICWs) and kinetic Alfven waves (KAWs) propagating in the directions parallel and perpendicular to magnetic field, respectively. In this work, we perform a statistical study on the effects of the waves on the spectral anisotropy of the transition range. We select 31 intervals from the measurements of Parker Solar Probe between 2018 and 2021. The magnetic helicity (sigma_m) diagnosis is applied on the magnetic field data at the frequency domain [0.1 Hz, 10 Hz], and the wavelet coefficients with sigma_m < -0.5 and sigma_m > 0.4 are considered as signals of ICWs and KAWs, respectively. We then remove them and find that the spectral anisotropy in the transition range becomes significantly weaker. Specifically, the spectra in the quasi-parallel direction statistically get shallower, and the average spectral index changes from -5.68±0.74 to -4.72±0.56. By contrast, the spectra in the perpendicular direction get slightly steeper, and the index changes from -3.63±0.34 to -3.95±0.41. Moreover, the anisotropic scaling in the transition range is found to be k⊥ ~ k∥1.55±0.33. The new results about the magnetic field spectra after the removal of ICW and KAW will help to further understand the possible mechanisms that cause the spectral anisotropy in the transition range.
How to cite: Wang, X. and Zhang, H.: Effects of Waves on the Spectral Anisotropy of Transition Range in the Solar Wind Turbulence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14164, https://doi.org/10.5194/egusphere-egu25-14164, 2025.
