ST1.5

Turbulent density fluctuations are investigated in the solar wind at sub-ion scales using calibrated spacecraft potential. The measurement technique using the spacecraft potential allows for a much higher time resolution and sensitivity when compared to direct measurements using plasma instruments. Using this novel method, density fluctuations can be measured with unprecedentedly high time resolutions for in situ measurements of solar wind plasma at 1 a.u. By investigating 1 h of high-time resolution data, the scale dependant kurtosis is calculated by varying the time lag τ to calculate increments between observations. The scale-dependent kurtosis is found to increase towards ion scales but then plateaus and remains fairly constant through the sub-ion range in a similar fashion to magnetic field measurements. The sub-ion range is also found to exhibit self-similar monofractal behavior contrasting sharply with the multi-fractal behavior at large scales. The scale-dependent kurtosis is also calculated using increments between two different spacecraft. When the time lags are converted using the ion bulk velocity to a comparable spatial lag, a discrepancy is observed between the two measurement techniques. Several different possibilities are discussed including a breakdown of Taylor’s hypothesis, high-frequency plasma waves, or intrinsic differences between sampling directions.

How to cite: Roberts, O., Thwaites, J., Sorriso-Valvo, L., Nakamura, R., and Voros, Z.: Higher-Order Statistics in Compressive Solar Wind Plasma Turbulence: High-Resolution Density Observations From the Magnetospheric MultiScale Mission, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2032, https://doi.org/10.5194/egusphere-egu21-2032, 2021.

The space between the Sun and our planet is not empty. It is filled with the expanding plasma of the solar corona called the Solar Wind, which is a tenuous weakly collisional plasma composed mainly by protons and electrons. Due to the lack of sufficient collisions, the electron velocity distribution function in the Solar Wind usually exhibits a variety of non-thermal characteristics that deviate from the thermodynamic equilibrium. These deviations from equilibrium provide a local source for electromagnetic fluctuations, intimately related to the shape of the distribution function, and associated with the commonly observed kinetic instabilities such as the whistler-cyclotron for T_⊥/ T_∥>1, and firehose for T_⊥/ T_∥<1 and large enough plasma beta. In this work we carry out systematic statistical study of correlations of various plasma moments and interplanetary magnetic fluctuations as a function of time, in order to describe the role and evolution of these parameters in the solar plasma through the solar cycle. We consider a large time interval during solar cycle 23, ranging from solar minimum (1995-1996) to solar maximum (2000-2001). Using NASA's Wind space mission and its SWE and High-Resolution MFI instruments with resolutions of 6-15 sec and 11 vectors/sec, respectively, we show that collisionless kinetic instabilities can regulate the electron distribution as the whistler-cyclotron and firehose instability thresholds bound the temperature and plasma beta electron distributions, and such regulation is more effective during solar minimum. Subsequently, the magnetic fluctuations level increases as the electron VDF acquires a configuration close to the thresholds. In addition, we note that there is a high difference between the fast and slow wind regimes given a greater tendency towards larger collisionallity and isotropization for low speeds streams, and magnetic fluctuations amplitude decreases as collisional age increases. In summary, our results indicate that collisionless plasma processes and Coulomb collisions effects coexist and both seem to play relevant roles in shaping the observed electron distributions.

How to cite: Silva, J., Moya, P., and Viñas, A.: Study of the relationship between the observations of electron distributions in the solar wind and interplanetary magnetic field fluctuations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3332, https://doi.org/10.5194/egusphere-egu21-3332, 2021.

Turbulence in space plasmas usually exhibits two regimes separated by a spectral break that divides the so called inertial and kinetic ranges. Large scale magnetic fluctuations are dominated by MHD non-linear wave-wave interactions following a -5/3 or -3/2 slope power-law spectrum. After the break, at scales in which kinetic effects take place, the magnetic spectrum follows a steeper power-law k^{- α} shape given by a spectral index α > 5/3. The location of the break and the particular value of α, depend on plasma conditions, and different space environments can exhibit different spectral indices. Despite its ubiquitousness, the possible effects of a turbulent background spectrum in the quasilinear relaxation of solar wind temperatures are usually not considered. In this work, a quasilinear kinetic theory is used to study the evolution of the proton temperatures in a solar wind-like plasma composed by cold electrons and bi-Maxwellian protons, in which electromagnetic waves propagate along a background magnetic field. Four wave spectrum shapes are compared with different levels of wave intensity. We show that a sufficient turbulent magnetic power can drive stable protons to transverse heating, resulting in an increase in the temperature anisotropy and the reduction of the parallel proton beta. Thus, stable proton velocity distribution can evolve in such a way as to develop kinetic instabilities. This may explain why the constituents of the solar wind can be observed far from thermodynamic equilibrium and near the instability thresholds.

How to cite: Moya, P. S. and Navarro, R. E.: Effects of the Background Turbulence on the Relaxation of Ion Temperature Anisotropy in Space Plasmas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3597, https://doi.org/10.5194/egusphere-egu21-3597, 2021.

Alfvénic turbulence denotes a regime of MHD turbulence in which Alfvén waves propagating in a given direction along the mean field are dominant, as commonly found in polar regions/coronal holes/fast solar wind. 

Generalization to Alfvénic turbulence of the Iroshnikov-Kraichnan (IK) weak theory concluded that one should observe a time increase of the imbalance between both Alfvén species and observe the so-called “spectral pinning”, i.e., steep spectra (with spectral index m₊>3/2) for the dominant energy E₊ and flat spectra (with index m-<3/2) for the sub-dominant energy E-.

Since then, observations in the inner heliosphere have shown on the contrary a decrease of imbalance with time, with both species showing the same flat spectra (m_± → -3/2) when imbalance is large.

We show here using direct MHD simulations that both behaviors may occur, the control parameters being the solar wind expansion rate as well as initial conditions of the plasma close to the Sun.

How to cite: Grappin, R., Verdini, A., and Müller, W.-C.: Spectral evolution of Alfvénic turbulence, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9861, https://doi.org/10.5194/egusphere-egu21-9861, 2021.

Turbulence in the solar wind is developed along a vast range of scales, generally under weakly compressible and strong magnetic field plasma conditions.
The effects of weakly and moderate compressibility (Mach ≤1) and turbulence anisotropy on the energy transfer rate are investigated at MHD and Hall MHD scales. For this purpose, the results of two and three-dimensional compressible Hall MHD simulations are analyzed using a new form of the Karman-Howarth-Monin (KHM) equations that accounts for compressible effects down to Hall MHD scales.
The KHM are dynamic equations directly derived from the basic fluid equations that describe the plasma, such as the Hall MHD equations. They provide a relation between the two-point cross-correlations in real space or II-order structure functions, the III-order structure functions and the energy cascade rate of turbulence. These relations depend upon turbulence anisotropy. The effects of compressibility and the Hall term on anisotropy and the estimation of the energy cascade rate via the KHM equations are discussed.

How to cite: Montagud-Camps, V., Hellinger, P., Verdini, A., Landi, S., Papini, E., Franci, L., and Matteini, L.: Karman-Howarth-Monin equation for compressible Hall MHD  turbulence: 2D and 3D Hall MHD simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11106, https://doi.org/10.5194/egusphere-egu21-11106, 2021.

We study intermittency of turbulence in the young solar wind at 0.17 au with NASA/Parker Solar Probe during the first perihelion. We use a merged FIELDS/Search Coil and Fluxgate Magnetometers data for magnetic field, SWEAP/SPC instrument for ions and RFS/FIELDS quasi thermal noise data for electrons to characterize the plasma environment. The merged magnetic waveforms have 3.4 ms time resolution, which allows us to resolve a wide range of scales, going from MHD inertial range to sub-ion range. We apply a wavelet transform to the magnetic waveforms and we observe localized enhancements in power density that form corresponding peaks in Local Intermittency Measure (LIM) going from MHD to kinetic scales. These LIM peaks are not present in the random-phase signal with the same Fourier amplitudes. This indicates the presence of coherent structures in the observed signal. To detect coherent structures at a given timescale, we use the maximum of the random-phase signal LIM at the same scale as a threshold. We observe a variety of coherent events from MHD to kinetic scales. We estimate the filling factor of the structures as well as their minimum variance properties and local topology. The physical connections between intermittency and solar wind heating are discussed.

How to cite: Vinogradov, A., Alexandrova, O., Artemyev, A., Maksimovic, M., Alexei, V., Petrukovich, A., Bale, S., Issautier, K., and Moncuquet, M.: Young solar wind coherent structures from inertial to sub-ion range, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11561, https://doi.org/10.5194/egusphere-egu21-11561, 2021.

How to cite: Kontar, E. and Reid, H.: Solar type III radio burst fine structure from Langmuir wave motion through turbulent plasma, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12013, https://doi.org/10.5194/egusphere-egu21-12013, 2021.

We investigate the spectral properties of the turbulence in the solar wind which is a weakly collisional astrophysical plasma, accessible by in-situ observations. Using the Helios search coil magnetometer measurements in the fast solar wind, in the inner heliosphere, we focus on properties of the turbulent magnetic fluctuations at scales smaller than the ion characteristic scales, the so-called kinetic plasma turbulence. At such small scales, we show that the magnetic power spectra between 0.3 and 0.9 AU from the Sun have a generic shape ~f^-8/3exp(-f/f_d) where the dissipation frequency f_d is correlated with the Doppler shifted frequency f_ρe of the electron Larmor radius. This behavior is statistically significant: all the observed kinetic spectra are well described by this model, with f_d=f_ρe/1.8. These results provide important constraints on the dissipation mechanism in nearly collisionless space plasmas.

How to cite: Alexandrova, O., Krishna Jagarlamudi, V., Hellinger, P., Maksimovic, M., Shprits, Y., and Mangeney, A.: Spectrum of kinetic plasma turbulence at 0.3-0.9 AU from the Sun, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12072, https://doi.org/10.5194/egusphere-egu21-12072, 2021.

Recently selected for phase A study for NASA’s Heliophysics MidEx Announcement of Opportunity, the HelioSwarm Observatory proposes to transform our understanding of the physics of turbulence in space and astrophysical plasmas by deploying nine spacecraft to measure the local plasma and magnetic field conditions at many points, with separations between the spacecraft spanning MHD and ion scales.  HelioSwarm resolves the transfer and dissipation of turbulent energy in weakly-collisional magnetized plasmas with a novel configuration of spacecraft in the solar wind. These simultaneous multi-point, multi-scale measurements of space plasmas allow us to reach closure on two science goals comprised of six science objectives: (1) reveal how turbulent energy is transferred in the most probable, undisturbed solar wind plasma and distributed as a function of scale and time; (2) reveal how this turbulent cascade of energy varies with the background magnetic field and plasma parameters in more extreme solar wind environments; (3) quantify the transfer of turbulent energy between fields, flows, and ion heat; (4) identify thermodynamic impacts of intermittent structures on ion distributions; (5) determine how solar wind turbulence affects and is affected by large-scale solar wind structures; and (6) determine how strongly driven turbulence differs from that in the undisturbed solar wind. 

How to cite: Spence, H., Klein, K., and Science Team, H.: HelioSwarm: The Nature of Turbulence in Space Plasmas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12092, https://doi.org/10.5194/egusphere-egu21-12092, 2021.

Appearance of measurements of the interplanetary medium parameters with high temporal resolution gave rise to a variety of investigations of turbulent cascade at ion kinetic scales at which processes of plasma heating was believed to operate. Our recent studies based on high frequency plasma measurements at Spektr-R spacecraft have shown that the turbulent cascade was not stable and dynamically changed depending on the plasma conditions in different large-scale solar wind structures. These changes was most significant at the kinetic scales of the turbulent cascade. Slow undisturbed solar wind was characterized by the consistency of the spectra to the predictions of the kinetic Alfven wave turbulence model. On the other hand, the discrepancy between the model predictions and registered spectra were found in stream interaction regions characterized by crucial steepening of spectra at the kinetic scales with slopes having values up to -(4-5). This discrepancy was clearly shown for plasma compression region Sheath in front of the magnetic clouds and CIR in front of high speed streams associated with coronal holes. Present study is focused on the break preceding the kinetic scales. Currently the characteristic plasma parameters associated with the formation of the break is still debated. Number of studies demonstrated that the break was consistent with distinct characteristic frequencies for different values ​​of the plasma proton parameter beta βp. Present study consider the ratio between the break frequency determined for ion flux fluctuation spectra according to Spektr-R data and several characteristic plasma frequencies used traditionally in such cases. The value of this ratio is statistically compared for different large-scale solar wind streams. We analyze both the classical spectrum view with two slopes and one break and the spectrum with flattening between magnetohydrodynamic and kinetic scales.  Our results show that for the Sheath and CIR regions characterized typically by βp ≤1 the break corresponds statistically to the frequency determined by the proton gyroradius. At the same time such correspondence are not observed either for the undisturbed slow solar wind with similar βp value or for disturbed flows associated with interplanetary manifestations of coronal mass ejections, where βp << 1. The results also shows that in slow undisturbed solar wind the break is closer to the frequency determined by the inertial proton length. Thus, apparently the transition between streams of different speeds may result in the change of dissipation regimes and plays role in plasma heating at these areas. This work was supported by the RFBR grant No. 19-02-00177a

How to cite: Riazantseva, M., Rakhmanova, L., Yermolaev, Y., Lodkina, I., Zastenker, G., Safrankova, J., Nemecek, Z., and Prech, L.: Ion-scale break of the plasma fluctuation spectra in different large-scale solar wind streams, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13569, https://doi.org/10.5194/egusphere-egu21-13569, 2021.

When point-like galactic and extragalactic radio sources are observed through the solar corona by ground-based radio telescopes, plasma density fluctuations in the turbulent solar wind scatter these photons, leading to an observed broadening and/or elongation of such sources. By observing this broadening for several sources, over several days, we can get information about e.g. the wavenumber and radial dependence of solar wind density fluctuations at very small scales (~30m - 8km) inside the Alfven radius, thereby capturing details of the turbulence dissipation range. Here, we present very initial results of such a study with the MeerKAT radio telescope in South Africa (being, of course, a precursor to the much larger Square Kilometer Array, SKA), discuss the preliminary results, and compare these with theoretical estimates and previous observations.

How to cite: Strauss, D. T., Botha, G., Chibueze, J., Kontar, E., Engelbrecht, E., Lotz, S., Wicks, R., Krupar, V., Bale, S., Maharaj, S., Jeffrey, N., Nel, A., Steyn, R., and van den Berg, J.: Observing solar wind turbulence in the corona with ground-based radio telescopes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14180, https://doi.org/10.5194/egusphere-egu21-14180, 2021.

We explore the multi-faceted important features of turbulence (e.g., anisotropy, dispersion, diffusion) in the three-dimensional (3D) wavenumber domain (k, k_perp1, k_perp2), by employing the k-filtering technique to the high-quality measurements of fields and plasmas from multi-spacecraft constellation (i.e., MMS). We compute the 3D power spectral densities (PSDs) of magnetic and electric fluctuations (marked as PSD(δB(k)) and PSD(δE′‹v_i›(k))), both of which show prominent spectral anisotropy in the sub-ion range. We calculate the ratio between PSD(δE′‹v_i›(k)) and PSD(δB(k)), the distribution of which is related with nonlinear dispersion relation. We also compute the ratio between electric spectra in different frames of ion flow, that is PSD(δE′local v_i)/PSD(δE′‹v_i›), to demonstrate the turbulence ion diffusion region (T- IDR) in the wavenumber space. The T-IDR has an anisotropy and a preferential direction of wavevectors, which is generally consistent with the plasma wave theory prediction based on the dominance of kinetic Alfvén wave (KAW). This work manifests the worth of the k-filtering technique in diagnosing turbulence comprehensively, especially when the electric field is involved.

How to cite: Lin, R., He, J., Zhu, X., Zhang, L., Duan, D., Sahraoui, F., and Verscharen, D.: Power Anisotropy, Dispersion Signature and Diffusion Region in the 3D Wavenumber Domain of Space Plasma Turbulence, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14329, https://doi.org/10.5194/egusphere-egu21-14329, 2021.

We present the results from a spacetime study of Hall-MHD and Hybrid-kinetic numerical simulations of decaying turbulence. By combining Fourier analysis and Multivariate Iterative Filtering (a new technique developed for the analysis of nonstationary nonlinear signals) we calculate the kω-power spectrum of magnetic, velocity, and density fluctuations at the maximum of turbulent activity. Results show that the magnetic power spectrum at sub-ion scales is formed by localized structures and/or perturbations with temporal frequencies much smaller than the ion-cyclotron frequency Ω_i. Going toward smaller ion-kinetic scales, the contribution of low-medium frequency perturbations (ω < 3Ω_i) to the magnetic spectrum becomes important. Our analysis clearly indicates that such low-frequency perturbations have no kinetic-Alfvén neither Ion-cyclotron origin. At higher frequencies, we clearly identify signatures of both whistler and kinetic-Alfvén wave activity. However, their energetic contribution to the turbulent cascade is negligible. We conclude that the dynamics of turbulence at sub-ion scales is mainly shaped by localized intermittent structures, with no contribution of wavelike perturbations.

How to cite: Papini, E., Cicone, A., Franci, L., Piersanti, M., Landi, S., Verdini, A., and Hellinger, P.: Sounding plasma turbulence at sub-ion scales with Fast Iterative Filtering in space and time., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15525, https://doi.org/10.5194/egusphere-egu21-15525, 2021.

Recent measurments of Parker Solar Probe show that alfvenic fluctuations in the solar wind often appear in the form of swithcback with constant total magnetic field. Our aim is to understand if and how such fluctuations can contribute to the heating or acceleration of the solar wind, via the Parametric Instability. The intability of one dimensional Alfvénic fluctuations has been extensively studied in both homogenoeus plasma and in the expanding solar wind, less so for the two-dimensional case which is closer to expected three-dimensional nature of switchbacks. In this work we study under which condition an Alfvén wave with a two dimensional spectrum (as introduced in Primavera et al ApJ 2019) can decay in the expanding solar wind and we will present preliminary results.

How to cite: Verdini, A., Grappin, R., Malara, F., Primavera, L., and Del Zanna, L.: Parametric instability in the expanding solar wind, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15590, https://doi.org/10.5194/egusphere-egu21-15590, 2021.

Ion cyclotron waves (ICWs) frequently occur in the solar wind and are detected by PSP within 0.3 AU (Bowen et al. 2020), by MESSENGER from 0.3 AU to 0.7 AU (Jian et al. 2010, Boardsen et al. 2015) and by STEREO at 1 AU (Jian et al. 2009; He et al. 2011). However, the relation between the wave properties and the kinetic features of different ion components (proton core, proton beam and helium) are not widely discussed in the existing literature. We statistically analyze the polarization and propagation properties of hundreds of ICW events using measurements from the Solar Orbiter spacecraft. We find three types of ICW events in terms of their occurrence and duration: clustering ICW events with long durations; sporadic ICW events immersed in a quiet background magnetic field; and ICW events alongside discontinuities. We perform an investigation of the ion velocity distribution functions (VDFs) and draw comparisons of the kinetic behavior of each ion component during intervals with and without ICWs. The plasma parameters of the different ion components are acquired by our newly developed fitting program.

How to cite: Zhu, X., He, J., Verscharen, D., Duan, D., Owen, C., and Horbury, T.: Statistical Study of ICW Events and Associated Ion Velocity Distributions in the Inner Heliosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16078, https://doi.org/10.5194/egusphere-egu21-16078, 2021.