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.