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In classical thermodynamics the entropy is an extensive quantity, i.e.\ the sum of the entropies of two subsystems in equilibrium with each other is equal to the entropy of the full system consisting of the two subsystems. The extensitivity of entropy has been questioned in the context of a theoretical foundation for the so-called $\kappa$-distributions, which describe plasma constituents with power-law velocity distributions. We demonstrate here, by employing the recently introduced {\it regularized $\kappa$-distributions}, that entropy can be defined as an extensive quantity even for such power-law-like distributions that truncate exponentially., Comment: Preprint accepted for publication in Phys. Rev. E

In this paper, we study where mirror mode structures are generated near unmagnetized solar system bodies (Venus and comet P/Halley measured in situ by Venus Express and Giotto). To estimate the location of the mirror mode source region at Venus, we apply a turbulent diffusion model of mirror mode structures, which has already been successfully tested in planetary magnetosheaths (Earth, Jupiter, Saturn). It enables us to estimate the distance between the measured location of the mirror mode and the origin of the mirror mode structure through the mirror mode size. We find that the scenario of mirror mode excitation at the bow shock with subsequent convection and diffusion downstream to the magnetopause is valid for Venus. In the cometary case, however, we find that the size of the mirror mode structure is comparable to the gyroradius of water group ions. This suggests local production of mirror mode structures in the cometary magnetosheath, most likely through fresh ion pickup, as opposed to the convection and diffusion mechanism at Venus.

International audience; We present observations of multiple reconnection in the Earth's magnetotail. In particular, we observe an ion temperature anisotropy characterized by large temperature along the magnetic field, between the two active X-lines. The anisotropy is associated with right-hand polarized waves at frequencies lower than the ion cyclotron frequency and propagating obliquely to the background magnetic field. We show that the observed anisotropy and the wave properties are consistent with linear kinetic theory of fire hose instability. The observations are in agreement with the particle-in-cell simulations of multiple reconnection. The results suggest that the fire hose instability can develop during multiple reconnection as a consequence of the ion parallel anisotropy that is produced by counter-streaming ions trapped between the X-lines.

During a magnetic storm on 23 June 2015, several very intense substorms took place, with signatures observed by multiple spacecraft including DMSP and Magnetospheric Multiscale (MMS). At the time of interest, DMSP F18 crossed inbound through a poleward expanding auroral bulge boundary at 23.5 h magnetic local time (MLT), while MMS was located duskward of 22 h MLT during an inward crossing of the expanding plasma sheet boundary. The two spacecraft observed a consistent set of signatures as they simultaneously crossed the reconnection separatrix layer during this very intense reconnection event. These include (1) energy dispersion of the energetic ions and electrons traveling earthward, accompanied with high electron energies in the vicinity of the separatrix; (2) energy dispersion of polar rain electrons, with a high‐energy cutoff; and (3) intense inward convection of the magnetic field lines at the MMS location. The high temporal resolution measurements by MMS provide unprecedented observations of the outermost electron boundary layer. We discuss the relevance of the energy dispersion of the electrons, and their pitch angle distribution, to the spatial and temporal evolution of the boundary layer. The results indicate that the underlying magnetotail magnetic reconnection process was an intrinsically impulsive and the active X‐line was located relatively close to the Earth, approximately at 16–18 RE., Key Points Simultaneous observations of the PSBL during a strong substorm suggest a near‐tail reconnection site being located at 16–18 RE For the first time, we resolve both energy and pitch angle dispersion of the electron evidence that the boundary layer has temporal originFirst observation of thin energetic electron layers in PSBL with thicknesses of the electron gyroscale

International audience; In this paper we use the full armament of the MMS (Magnetospheric Multiscale) spacecraft to study magnetic reconnection in the turbulent magnetosheath downstream of a quasi-parallel bow shock. Contrarily to the magnetopause and magnetotail cases, only a few observations of reconnection in the magnetosheath have been reported. The case study in this paper presents, for the first time, both fluid-scale and kinetic-scale signatures of an ongoing reconnection in the turbulent magnetosheath. The spacecraft are crossing the reconnection inflow and outflow regions and the ion diffusion region (IDR). Inside the reconnection outflows D shape ion distributions are observed. Inside the IDR mixing of ion populations, crescent-like velocity distributions and ion accelerations are observed. One of the spacecraft skims the outer region of the electron diffusion region, where parallel electric fields, energy dissipation/conversion, electron pressure tensor agyrotropy, electron temperature anisotropy, and electron accelerations are observed. Some of the difficulties of the observations of magnetic reconnection in turbulent plasma are also outlined.

International audience; Magnetic reconnection may proceed via multiple reconnection sites which implies the collision between the counterstreaming reconnection jets. We present the magnetotail observations of two reconnection X lines and the region of the reconnection jets collision in between. Using favorable location of two Cluster spacecraft probes, we analyzed the evolution of collision process. Under strengthened collision, the ion-scale current sheet-like structure formed between the colliding jets, becomes compressed. The strong emission of whistler waves is observed in the center of the compressed structure. Such observations might be helpful in understanding the multiple reconnection dynamics at the magnetopause where the colliding reconnection jets in the core of the flux rope have been recently observed by MMS.

International audience; Recent global hybrid and fully kinetic simulations of the terrestrial magnetosphere have shown that the turbulent quasi-parallel (QP) magnetosheath contains multi-scale interacting structures such as jets, flux ropes, waves, vortices, reconnecting current sheets, over the fluid, ion and electron scales. Multi-scale flux-rope interactions in magnetosheath plasma turbulence downstream of a QP shock are investigated on the basis of particle, plasma and field measurements from the Magnetospheric Multiscale (MMS) mission satellites. It is demonstrated that magnetic flux rope interactions are associated with magnetic flux pileup, density depletion and with formation of proton scale current sheets, electron scale spiky structures and particle heating. This study also demonstrates how patchy dissipation and plasma heating can occur in collisionless turbulent plasmas.

International audience; Collisionless space plasma turbulence can generate reconnecting thin current sheets as suggested by recent results of numerical magnetohydrodynamic simulations. The Magnetospheric Multiscale (MMS) mission provides the first serious opportunity to verify whether small ion-electron-scale reconnection, generated by turbulence, resembles the reconnection events frequently observed in the magnetotail or at the magnetopause. Here we investigate field and particle observations obtained by the MMS fleet in the turbulent terrestrial magnetosheath behind quasi-parallel bow shock geometry. We observe multiple small-scale current sheets during the event and present a detailed look of one of the detected structures. The emergence of thin current sheets can lead to electron scale structures. Within these structures, we see signatures of ion demagnetization, electron jets, electron heating, and agyrotropy suggesting that MMS spacecraft observe reconnection at these scales.

In this paper, we introduce two simple and inexpensive versions of the well-known Foucault method for measuring the speed of light. In a footprint of just 20 cm by 270 cm with readily available laboratory items and a webcam, we obtained $c=296720\pm3000$ km/s, and $c=302295\pm3000$ km/s, respectively, both within less than a per cent of the defined value. The experiment also prepares students to work with large amounts of data., Comment: 9 pages, 12 figures

We experimentally analyze Rayleigh scattering in coupled planar microcavities. We show that the correlations of the disorder in the two cavities lead to inter-branch scattering of polaritons, that would otherwise be forbidden by symmetry. These longitudinal correlations can be inferred from the strength of the inter-branch scattering., Comment: 6 pages, 10 figures

Entanglement generation in microcavity exciton-polaritons is an interesting application of the peculiar properties of these half-light/half-matter quasiparticles. In this paper we theoretically investigate their luminescence dynamics and entanglement formation in single, double, and triple cavities. We derive general expressions and selection rules for polariton-polariton scattering. We evaluate a number of possible parametric scattering schemes in terms of entanglement, and identify the ones that are experimentally most promising., Comment: 25 pages, 8 figures

[1] Long-term observations of proton cyclotron waves in the upstream region of Venus raise the question of under which general solar wind conditions these waves are generated and maintained. The waves are characterized by their occurrence at the local proton cyclotron frequency and left-hand polarization, both in the spacecraft frame. Magnetometer data of the Venus Express spacecraft for two Venus years of observations are analyzed before, during, and after the occurrence of these waves. The configuration of the upstream magnetic field and the solar wind velocity is investigated, to study if the waves are generated from a ring distribution of pickup ions in velocity space or from a parallel pickup ion beam, i.e., for quasi-parallel conditions of solar wind velocity and magnetic field when the solar wind motional electric field is weak. It is found that stable and mainly quasi-parallel magnetic field conditions for up to ∼20 min prior to wave observation are present, enabling sufficient ion pickup and wave growth to obtain observable waves in the magnetometer data. Persistent waves occur mainly under quasi-parallel conditions. This is in agreement with linear theory, which predicts efficient wave growth for instabilities driven by field-aligned planetary ion beams, already for low pickup ion density. The occurrence of highly coherent waves at 4 RV upstream toward the Sun implies that planetary neutral hydrogen is initially picked up at least 5 RV toward the Sun from a sufficiently dense Venus hydrogen exosphere. Fil: Delva, M.. Austrian Academy of Sciences; Austria Fil: Mazelle, C.. Universitá Paul Sabatier; Francia Fil: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Volwerk, M.. Austrian Academy of Sciences; Austria Fil: Vörös, Z.. University of Innsbruck; Austria Fil: Zhang, T. L.. Austrian Academy of Sciences; Austria