ST1.9

We report analysis of sub-Alfvénic magnetohydrodynamic (MHD) perturbations in the low-ß radial-field solar wind employing the Parker Solar Probe spacecraft data from 31 October to 12 November 2018. We calculate wave vectors using the singular value decomposition method and separate MHD perturbations into three eigenmodes (Alfvén, fast, and slow modes) to explore the properties of sub-Alfvénic perturbations and the role of compressible perturbations in solar wind heating. The MHD perturbations show a high degree of Alfvénicity in the radial-field solar wind, with the energy fraction of Alfvén modes dominating (~45%-83%) over those of fast modes (~16%-43%) and slow modes (~1%-19%). We present a detailed analysis of a representative event on 10 November 2018. Observations show that fast modes dominate magnetic compressibility, whereas slow modes dominate density compressibility. The energy damping rate of compressible modes is comparable to the heating rate, suggesting the collisionless damping of compressible modes could be significant for solar wind heating. These results are valuable for further studies of the imbalanced turbulence near the Sun and possible heating effects of compressible modes at MHD scales in low-ß plasma.

How to cite: Zhao, S., Yan, H., Liu, T., Liu, M., and Shi, M.: Analysis of Magnetohydrodynamic Perturbations in the Radial-field Solar Wind from Parker Solar Probe Observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8501, https://doi.org/10.5194/egusphere-egu22-8501, 2022.

This study investigates long-lasting radial interplanetary magnetic field (IMF) intervals in which IMF points along the solar wind flow direction for several hours. We use 419 such events identified in Wind observations during 1995-2019, and we focus on the behavior of magnetic field fluctuations. Using the power spectral density (PSD) calculated over 1-hour radial IMF intervals and PSDs in adjacent regions prior to and after the radial IMF interval, we address: (i) the power of IMF fluctuations, (ii) median slopes of PSDs in both inertial and kinetic ranges, (iii) the proton temperature and its anisotropy, and (vi) the occurrence rate of wavy structures and their polarization. Comparison of PSDs in radial IMF intervals with those in prior and after them revealed that the fluctuation magnitude is low in the radial IMF intervals in both MHD and kinetic ranges and the spectral power increases with the cone angle in the MHD range. It may be related to the observation limitations because the dominant 2D component of the magnetic fluctuation is hard to observe if the sampling direction is aligned with the mean magnetic field. Moreover, the proton temperature is more isotropic, and the occurrence rate of wave structures is higher for radial IMF events. The waves have no preferred polarization in the frequency range from 0.1 to 1 Hz. It suggests that the radial IMF structure leads to a different development of turbulence than the typical Parker-spiral orientation.

How to cite: Pi, G., Pitna, A., Nemecek, Z., and Safrankova, J.: An examination of the magnetic fluctuations in long-lasting radial IMF events, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1369, https://doi.org/10.5194/egusphere-egu22-1369, 2022.

First perihelion Parker Solar Probe magnetic field measurements (MAG and SCM merged data) allow to resolve the fluctuations on a wide range of scales: from MHD to ion plasma scales and smaller. We trace the cascade of the fluctuations and investigate the structures formed. Using the total energy of magnetic fluctuations in time and scales, we show that coherent structures cover all the observed scales. The filling factor of the structures is a few percents. We analyze the magnetic fluctuations at different frequency ranges. We observe the coexistence of events at MHD, ion and sub-ion scales in the form of sharp discontinuities and/or vortex-like events. The approach of selecting structures by total energy alone is not complete, as it can miss structures with change in magnetic field modulus. For completeness, we perform the same analysis on longitudinal magnetic fluctuations.

How to cite: Vinogradov, A., Alexandrova, O., Maksimovich, M., Artemyev, A., Mangeney, A., Vasiliev, A., Issautier, K., Moncuquet, M., and Petrukovich, A.: PSP observations of the solar wind coherent structures from MHD to sub-ion scales at 0.17 AU, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9796, https://doi.org/10.5194/egusphere-egu22-9796, 2022.

We investigate properties of solar-wind like plasma turbulence using direct numerical simulations. We analyze the transition from large (magnetohydrodynamic) scales to ion ones using two-dimensional hybrid (fluid electrons, kinetic ions) simulations of decaying turbulence. To quantify turbulence properties we apply spectral transfer and Karman-Howarth-Monin equations for extended compressible Hall MHD to the simulated results. The simulation results indicate that the transition from MHD to ion scales (the so called ion break) results from a combination of an onset of Hall physics and of an effective dissipation owing to the pressure-strain energy-exchange channel and resistivity. We discuss the simulation results in the context of the solar wind.

How to cite: Hellinger, P., Montagud-Camps, V., Franci, L., Matteini, L., Papini, E., Verdini, A., and Landi, S.: Ion-scale transition of plasma turbulence: Pressure-strain effect, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8357, https://doi.org/10.5194/egusphere-egu22-8357, 2022.

Turbulence near an interplanetary shock is of practical interest because turbulent magnetic fluctuations are key to the diffusive shock acceleration and transport of energetic particles, which can lead to significant space weather effects.  In this work, we examine burst-mode observations by the Magnetospheric Multiscale Mission (MMS) for an interplanetary shock passage at a distance of 25 R_e­ on 8 January 2018.  The instrumental resolution offers an opportunity to examine the energy transfer rate of solar wind turbulence in both the upstream and downstream regions. We implement a Hampel filtering-based technique to mitigate the instrumental noise in plasma moment data. We use a Kolmogorov-Yaglom Law for the third-order structure function and a von Kármán-decay law to calculate the energy dissipation rates at the inertial scale and energy-containing scale, respectively. The results show that the region downstream of the shock has stronger and better developed turbulence and a higher energy transfer rate than the upstream region. N.N. has been supported by STFC studentship and UCL Doctoral School. This research has also been supported by grant RTA6280002 from Thailand Science Research and Innovation, by the MMS Theory and Modeling team grant 80NSSC19K0565, and the NASA LWS program grant 80NSSC20K0377 under NMC subcontract 655-001.

How to cite: Ngampoopun, N., Ruffolo, D., Bandyopadhyay, R., and Matthaeus, W.: Analysis of Turbulence Energy Transfer at an Interplanetary Shock Observed by MMS, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6596, https://doi.org/10.5194/egusphere-egu22-6596, 2022.

Spectral transfer equations allow to  quantify the value of the energy flux of a turbulent flow across concentric shells in Fourier space. Karman-Howarth-Monin equations serve as a complement to the Spectral Transfer analysis, since they  quantify  as well the energy transfer rate of turbulence across scales via third-order structure functions, but also provide information on the directionality of the flux. We have extended the use of these methods to study the cascade of cross-helicity and compare it to the energy cascade  in 3D compressible MHD simulations. Our results show that the cross-helicity cascade reaches stationarity after the energy cascade, thus indicating a slower turbulence development for this invariant. Once fully developed, the cross-helicity cascade matches the main features of the energy one.

How to cite: Montagud-Camps, V., Hellinger, P., Verdini, A., Papini, E., Franci, L., Matteini, L., and Landi, S.: Quantification of the cross-helicity cascade with Karman-Howarth-Monin and Spectral transfer equations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5115, https://doi.org/10.5194/egusphere-egu22-5115, 2022.

In recent years, the Kolmogorov's statistical formalism of exact law that describes incompressible hydrodynamic turbulence, has been extended to compressible magnetized fluid described by isothermal or polytropic closure. Such exact laws permit an evaluation of the energy cascade rate, assumed within this formalism to be equivalent to the dissipation rate. Its estimation in the solar wind can help to better understand particle heating in such collisionless media. But previous exact laws are insufficient in a system led by pressure anisotropy. We propose a general exact law of Hall-MHD turbulence based on models with a pressure tensor that allows us to study various known equations of state as particular limits, derive a new one corresponding to the CGL (i.e., gyrotropic pressure tensor), and correlate the cascade rate to instable plasma conditions. In the incompressible MHD limit we provide a generalization of the Politano & Pouquet law to pressure-anisotropic plasmas.

How to cite: Simon, P. and Sahraoui, F.: A link between turbulent cascade and gyrotropic pressure instabilities in compressible and magnetized fluids., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4524, https://doi.org/10.5194/egusphere-egu22-4524, 2022.

How to cite: Teissier, J.-M. and Müller, W.-C.: Inverse transfer of magnetic helicity in isothermal supersonic turbulence, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11566, https://doi.org/10.5194/egusphere-egu22-11566, 2022.

Like the solar wind in general, interplanetary coronal mass ejections (ICMEs) display magnetic field and velocity fluctuations across a wide range of scales. These fluctuations may be interpreted as Alfvénic wave packets propagating parallel or anti-parallel to the local magnetic field direction, with cross helicity, σ_c, quantifying the difference in power between the counter-propagating fluxes. We have determined σ_c at inertial range frequencies in a large sample of ICME flux ropes and sheaths observed by the Wind spacecraft at 1 au. The mean σ_c value was low for both the flux ropes and sheaths, with the balance tipped towards the positive, anti-sunward direction. The low values indicate that Alfvénic fluxes are more balanced in ICMEs than in the solar wind at 1 au, where σ_c tends to be larger and anti-sunward fluctuations show a greater predominance. Superposed epoch profiles show σ_c falling sharply in the upstream sheath and being typically close to balance inside the flux rope near the leading edge. More imbalanced, solar wind-like σ_c values are found towards the trailing edge and further from the rope axis. The presence or absence of an upstream shock also has a significant effect on σ_c. Coronal and interplanetary origins of low σ_c in ICMEs are discussed.

How to cite: Good, S., Hatakka, L., Ala-Lahti, M., Soljento, J., Osmane, A., and Kilpua, E.: Cross helicity of interplanetary coronal mass ejections, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11274, https://doi.org/10.5194/egusphere-egu22-11274, 2022.

In the solar wind, the differential flow between the alpha particles and the protons is an important source of free energy for driving A/IC waves and FM/W waves unstable. Large-scale slow-mode waves can modulate the differential flow, leading to non-negligible locally time-dependent changes in the drift velocity.

We investigate the behaviour of the maximum differential flow with multi-fluid wave theory in the parameter range 0<U_α/V_A,p<1.5 and 0.1<β_p<10 assuming quasi-perpendicular propagation of the slow mode wave, where U_α is the background alpha particle beam speed, V_A,p is the proton Alfvén velocity, and β_p is the ratio of the thermal proton energy to the magnetic field energy. We derive an analytical expression for the fluctuation in differential flow, the result of which we confirm through numerical evaluation of the multi-fluid wave equation. The thresholds in terms of U_α/V_A,p for the instability of the A/IC and FM/W instabilities in the presence of slow mode waves decrease with increasing slow-mode amplitude and decreasing β_p.

We statistically investigate the differential flow between alpha particles and protons based on spacecraft measurements with Solar Orbiter for intervals with clearly identified slow-mode waves as an observational test of our theoretical predictions. We find that slow mode fluctuations play an important role in the driving of A/IC and FM/W instabilities which are important for the energy transfer in the solar wind.

How to cite: Zhu, X., Verscharen, D., He, J., and Owen, C. J.: Slow-Mode-Driven Alfvén/Ion-Cyclotron (A/IC) and Fast-Magnetosonic/Whistler (FM/W) Instabilities in the Presence of an Alpha-Particle Beam in the Solar Wind, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11941, https://doi.org/10.5194/egusphere-egu22-11941, 2022.

Magnetic holes are plasma structures that trap a large number of particles in a magnetic field that is weaker than its surroundings. The unprecedented high time-resolution in-situ observations by NASA's Magnetospheric Multi-Scale (MMS) mission enable us to study the particle dynamics in the Earth's magnetosheath plasma in great detail. For the first time, we reveal the local generation of whistler waves by the Landau-resonant instability of electron beams as a response to the large-scale evolution of a magnetic hole. As the magnetic hole converges, we find a pair of counter-streaming electron beams are formed near the hole's center as a consequence of the combined action of betatron cooling and Fermi acceleration. The beams trigger the generation of slightly oblique whistler waves near the hole center, which is supported by a remarkable agreement between observations and our ALPS model predictions. Our findings show that kinetic effects and wave-particle interactions are fundamental to the dynamics and the evolution of magnetic holes as an important type of coherent structures in collisionless plasmas.

How to cite: Jiang, W., Verscharen, D., Li, H., Wang, C., and Klein, K.: Local Emission of Whistler Waves by Landau Resonance As a Signature of a Converging Magnetic Hole, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12969, https://doi.org/10.5194/egusphere-egu22-12969, 2022.