arXiv subjects

Olga Alexandrova

Publications and source records attributed to Olga Alexandrova.

29 records · Page 2Linked to original sources

arXiv2019

Magnetospheric Multiscale Observation of Kinetic Signatures in the Alfvén Vortex

Alfvén vortex is a multi-scale nonlinear structure which contributes to intermittency of turbulence. Despite previous explorations mostly on the spatial properties of the Alfvén vortex (i.e., scale, orientation, and motion), the plasma characteristics within the Alfvén vortex are unknown. Moreover, the connection between the plasma energization and the Alfvén vortex still remains unclear. Based on high resolution in-situ measurement from the Magnetospheric Multiscale (MMS) mission, we report for the first time, distinctive plasma features within an Alfvén vortex. This Alfvén vortex is identified to be two-dimensional ($k_{\bot} \gg k_{\|}$) quasi-monopole with a radius of ~10 proton gyroscales. Its magnetic fluctuations $δB_{\bot}$ are anti correlated with velocity fluctuations $δV_{\bot}$, thus the parallel current density $j_{\|}$ and flow vorticity $ω_{\|}$ are anti-aligned. In different part of the vortex (i.e., edge, middle, center), the ion and electron temperatures are found to be quite different and they behave in the reverse trend: the ion temperature variations are correlated with $j_{\|}$, while the electron temperature variations are correlated with $ω_{\|}$. Furthermore, the temperature anisotropies, together with the non-Maxwellian kinetic effects, exhibit strong enhancement at peaks of $|ω_{\|}| (|j_{\|}|)$ within the vortex. Comparison between observations and numerical/theoretical results are made. In addition, the energy-conversion channels and the compressibility associated with the Alfvén vortex are discussed. These results may help to understand the link between coherent vortex structures and the kinetic processes, which determines how turbulence energy dissipate in the weakly-collisional space plasmas.

physics.space-ph↗

arXiv2017

Variability Of The Magnetic Field Power Spectrum In The Solar Wind At Electron Scales

At the electron scales the power spectrum of solar-wind magnetic fluctuations can be highly variable and the dissipation mechanisms of the magnetic energy into the various particle species is under debate. In this paper we investigate data from the Cluster mission's STAFF Search Coil magnetometer when the level of turbulence is sufficiently high that the morphology of the power spectrum at electron scales can be investigated. The Cluster spacecraft sample a disturbed interval of plasma where two streams of solar wind interact. Meanwhile, several discontinuities (coherent structures) are seen in the large scale magnetic field, while at small scales several intermittent bursts of wave activity (whistler waves) are present. Several different morphologies of the power spectrum can be identified: (1) two power laws separated by a break (2) an exponential cutoff near the Taylor shifted electron scales and (3) strong spectral knees at the Taylor shifted electron scales. These different morphologies are investigated by using wavelet coherence, showing that in this interval a clear break and strong spectral knees are features which are associated with sporadic quasi parallel propagating whistler waves, even for short times. On the other hand, when no signatures of whistler waves at 0.1 - 0.2fce are present, a clear break is difficult to find and the spectrum is often more characteristic of a power law with an exponential cutoff.

physics.plasm-ph↗

arXiv2017

Anisotropies of the magnetic field fluctuations at kinetic scales in the solar wind : Cluster observations

We present the first statistical study of the anisotropy of the magnetic field turbulence in the solar wind between 1 and 200Hz, i.e. from proton to sub-electron scales. We consider 93 10-minute intervals of Cluster/STAFF measurements. We find that the fluctuations $δB_{\perp}^2$ are not gyrotropic at a given frequency $f$, a property already observed at larger scales ($\parallel$/$\perp$ mean parallel/perpendicular to the average magnetic ${\bf B_0}$). This non-gyrotropy gives indications on the angular distribution of the wave vectors ${\bf k}$: at $f <$ 10Hz, we find that $k_{\perp}\gg k_{\parallel}$, mainly in the fast wind; at $f >$ 10Hz, fluctuations with a non-negligible $k_{\parallel}$ are also present. We then consider the anisotropy ratio $δB_{\parallel}^2/δB_{\perp}^2$, which is a measure of the magnetic compressibility of the fluctuations. This ratio, always smaller than 1, increases with $f.$ It reaches a value showing that the fluctuations are more or less isotropic at electron scales, for $f \geq 50$Hz. From 1 to 15-20Hz, there is a strong correlation between the observed compressibility and the one expected for the kinetic Alfvén waves (KAW), which only depends on the total plasma $β$. For $f > $ 15-20Hz, the observed compressibility is larger than expected for KAW; and it is stronger in the slow wind: this could be an indication of the presence of a slow-ion acoustic mode of fluctuations, which is more compressive and is favored by the larger values of the electron to proton temperature ratio generally observed in the slow wind.

physics.space-ph↗

arXiv2017

Gyrokinetic theory of magnetic structures in high-beta plasmas of the Earths magnetopause and of the slow solar wind

Nonlinear effects of the trapping of resonant particles by the combined action of the electric field and the magnetic mirror force is studied using a gyrokinetic description that includes the finite Larmor radius effects. A general nonlinear solution is found that is supported by the nonlinearity arising from the resonant particles, trapped by the combined action of the parallel electric field and the magnetic mirror force. Applying these results to the space plasma conditions, we demonstrate that in the magnetosheath plasma, coherent nonlinear magnetic depression may be created associated with the nonlinear mirror mode and supported by the population of trapped ions forming a hump in the distribution function. These objects may appear either isolated or as the train of weakly correlated structures (the cnoidal wave). In the Solar wind and in the Earths magnetopause, characterized with anisotropic electron and ion temperatures that are of the same order of magnitude, we find coherent magnetic holes of the same form that are attributed to the two branches of the nonlinear magnetosonic mode, the electron mirror and the field swelling mode, including also the kinetic Alfven mode, and supported by the population of trapped electrons. The localized magnetic holes may have the form of a moving oblique slab or of an ellipsoid parallel to the magnetic field and strongly elongated along it, that propagates along the magnetic field and may be convected in the perpendicular direction by a plasma flow. While the ion mirror structures are purely compressional magnetic, featuring negligible magnetic torsion and electric field, the magnetosonic and kinetic Alfven structures possess a finite electrostatic potential, magnetic compression, and magnetic torsion, but the ratio of the perpendicular and parallel magnetic fields remains small.

physics.plasm-ph↗

arXiv2016

Observation of an MHD Alfvén vortex in the slow solar wind

In the solar wind, magnetic field power spectra usually show several power-laws. In this paper, magnetic field data from the Cluster mission during an undisturbed interval of slow solar wind is analyzed at 0.28Hz, near the spectral break point between the ion inertial and dissipation/dispersion ranges. Assuming Taylor's frozen-in condition, it corresponds to a proton kinetic scale of $kv_A/Ω_p \sim 0.38$, where $v_A$ and $Ω_p$ are the Alfvén speed and proton angular gyrofrequency, respectively. Data show that the Cluster spacecraft passed through a series of wavepackets. A strong isolated wavepacket is found to be in accordance with the four Cluster satellites crossing an Alfvén vortex, a nonlinear solution to the incompressible MHD equations. A strong agreement is seen between the data from four satellites and a model vortex with a radius of the order of $40$ times the local proton gyro-radii. The polarization at different spacecraft is compared and is found to agree with the vortex model, whereas it cannot be explained solely by the linear plane wave approach.

astro-ph.SR↗

arXiv2016

Coherent events and spectral shape at ion kinetic scales in the fast solar wind turbulence

In this paper we investigate spectral and phase coherence properties of magnetic fluctuations in the vicinity of the spectral transition from large, magnetohydrodynamic (MHD) to sub-ion scales using in-situ measurements of the Wind spacecraft in a fast stream. For the time interval investigated by Leamon et al. (1998) the phase-coherence analysis shows the presence of sporadic quasi-parallel Alfvén Ion Cyclotron (AIC) waves as well as coherent structures in the form of large-amplitude, quasi-perpendicular Alfvén vortex-like structures and current sheets. These waves and structures importantly contribute to the observed power spectrum of magnetic fluctuations around ion scales; AIC waves contribute to the spectrum in a narrow frequency range whereas the coherent structures contribute to the spectrum over a wide frequency band from the inertial range to the sub-ion frequency range. We conclude that a particular combination of waves and coherent structures determines the spectral shape of the magnetic field spectrum around ion scales. This phenomenon provides a possible explanation for a high variability of the magnetic power spectra around ion scales observed in the solar wind.

astro-ph.SR↗

arXiv2014

Whistler mode waves and the electron heat flux in the solar wind: Cluster observations

The nature of the magnetic field fluctuations in the solar wind between the ion and electron scales is still under debate. Using the Cluster/STAFF instrument, we make a survey of the power spectral density and of the polarization of these fluctuations at frequencies $f\in[1,400]$ Hz, during five years (2001-2005), when Cluster was in the free solar wind. In $\sim 10\%$ of the selected data, we observe narrow-band, right-handed, circularly polarized fluctuations, with wave vectors quasi-parallel to the mean magnetic field, superimposed on the spectrum of the permanent background turbulence. We interpret these coherent fluctuations as whistler mode waves. The life time of these waves varies between a few seconds and several hours. Here we present, for the first time, an analysis of long-lived whistler waves, i.e. lasting more than five minutes. We find several necessary (but not sufficient) conditions for the observation of whistler waves, mainly a low level of the background turbulence, a slow wind, a relatively large electron heat flux and a low electron collision frequency. When the electron parallel beta factor $β_{e\parallel}$ is larger than 3, the whistler waves are seen along the heat flux threshold of the whistler heat flux instability. The presence of such whistler waves confirms that the whistler heat flux instability contributes to the regulation of the solar wind heat flux, at least for $β_{e\parallel} \ge$ 3, in the slow wind, at 1 AU.

astro-ph.SR↗

arXiv2013

Solar Wind Turbulence and the Role of Ion Instabilities

Solar wind is probably the best laboratory to study turbulence in astrophysical plasmas. In addition to the presence of magnetic field, the differences with neutral fluid isotropic turbulence are: weakness of collisional dissipation and presence of several characteristic space and time scales. In this paper we discuss observational properties of solar wind turbulence in a large range from the MHD to the electron scales. At MHD scales, within the inertial range, turbulence cascade of magnetic fluctuations develops mostly in the plane perpendicular to the mean field. Solar wind turbulence is compressible in nature. The spectrum of velocity fluctuations do not follow magnetic field one. Probability distribution functions of different plasma parameters are not Gaussian, indicating presence of intermittency. At the moment there is no global model taking into account all these observed properties of the inertial range. At ion scales, turbulent spectra have a break, compressibility increases and the density fluctuation spectrum has a local flattening. Around ion scales, magnetic spectra are variable and ion instabilities occur as a function of the local plasma parameters. Between ion and electron scales, a small scale turbulent cascade seems to be established. Approaching electron scales, the fluctuations are no more self-similar: an exponential cut-off is usually observed indicating an onset of dissipation. The nature of the small scale cascade and a possible dissipation mechanism are still under debate.

astro-ph.SR↗

arXiv2012

Solar wind turbulent spectrum at plasma kinetic scales

The description of the turbulent spectrum of magnetic fluctuations in the solar wind in the kinetic range of scales is not yet completely established. Here, we perform a statistical study of 100 spectra measured by the STAFF instrument on the Cluster mission, which allows to resolve turbulent fluctuations from ion scales down to a fraction of electron scales, i.e. from $\sim 10^2$ km to $\sim 300$ m. We show that for $k_{\perp}ρ_e \in[0.03,3]$ (that corresponds approximately to the frequency in the spacecraft frame $f\in [3,300]$ Hz), all the observed spectra can be described by a general law $E(k_\perp)\propto k_\perp^{-8/3}\exp{(-k_\perp ρ_e)}$, where $k_{\perp}$ is the wave-vector component normal to the background magnetic field and $ρ_e$ the electron Larmor radius. This exponential tail found in the solar wind seems compatible with the Landau damping of magnetic fluctuations onto electrons.

astro-ph.SR↗

arXiv2009

Universality of solar wind turbulent spectrum from MHD to electron scales

In order to investigate the universality of magnetic turbulence in space plasmas we analyze seven time periods in the free solar wind of different origin, slow or fast, and under different plasma conditions. The orientation of magnetic field to the flow velocity was always quasi-perpendicular. Unique combination of three instruments on Cluster spacecraft which operate in different frequency ranges give us the possibility to resolve spectra up to 300 Hz. We show that spectra measured under different plasma conditions have a similar shape. Such a quasi-universal spectrum consists of three parts: two power laws and an exponential domain. At MHD scales, Kolmogorov's law $\sim k^{-5/3}$ is found. At scales smaller than the ion characteristic scales, a $k^{-2.8}$ law is observed. At scales $kρ_e\sim (0.1-1)$, where $ρ_e$ is the electron gyroradius, the magnetic spectrum follows an exponential law $\exp(-k^{1/2})$, indicating the onset of dissipation. This is the first observation of an exponential magnetic spectrum in space plasmas. We show that among several spatial kinetic plasma scales, the electron Larmor radius plays the role of a dissipation scale in space plasma turbulence.

physics.plasm-ph↗

arXiv2008

Spectra and anisotropy of magnetic fluctuations in the Earth's magnetosheath: Cluster observations

We investigate the spectral shape, the anisotropy of the wave vector distributions and the anisotropy of the amplitudes of the magnetic fluctuations in the Earth's magnetosheath within a broad range of frequencies. We present the first observations of a Kolmogorov-like inertial range of Alfvenic fluctuations in the magnetosheath flanks, below fci. In the vicinity of fci, a spectral break is observed, like in solar wind turbulence. Above the break, the energy of compressive and Alfvenic fluctuations generally follow a power law with a spectral index between -3 and -2. Concerning the anisotropy of the wave vector distribution, we observe a change in its nature in the vicinity of ion characteristic scales: if at MHD scales there is no evidence for a dominance of a slab (k||>kperp) or 2D (kperp>k||) turbulence, above the spectral break, (f>fci, kc/wpi>1) the 2D turbulence dominates. This 2D turbulence is observed in six selected one-hour intervals among which the average proton beta varies from 0.8 to 9. It is observed for both the transverse and compressive magnetic fluctuations, independently on the presence of linearly unstable modes at low frequencies or Alfven vortices at the spectral break. We then analyse the anisotropy of the magnetic fluctuations in a time dependent reference frame based on the field B and the flow velocity V directions. Within the range of the 2D turbulence, at scales [1,30]kc/wpi, and for any beta we find that the magnetic fluctuations at a given frequency in the plane perpendicular to B have more energy along the BxV direction. This non-gyrotropy of the fluctuations is consistent with gyrotropic fluctuations at a given wave vector, with kperp>k||, which suffer a different Doppler shift along and perpendicular to V in the plane perpendicular to B.

astro-ph↗