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 us to resolve turbulent fluctuations from ion scales down to a fraction of electron scales, i.e., from ~10 super(2) km to ~300 m. We show that for k sub(perpendicular)rhoe isin 0.03, 3 (which corresponds approximately to the frequency in the spacecraft frame f isin 3, 300 Hz), all the observed spectra can be described by a general law E(k sub(perpendicular)) is proportional to k super(-8/3) sub(perpendicular) exp(-k sub(perpendicular)rhoe), where k sub(perpendicular) is the wavevector component normal to the background magnetic field and rhoe the electron Larmor radius. This exponential tail found in the solar wind seems compatible with the Landau damping of magnetic fluctuations onto electrons.
The prediction of a supersonic solar wind
was first confirmed by spacecraft near Earth
and later by spacecraft at heliocentric distances as small as 62 solar radii
. These missions showed that plasma ...accelerates as it emerges from the corona, aided by unidentified processes that transport energy outwards from the Sun before depositing it in the wind. Alfvénic fluctuations are a promising candidate for such a process because they are seen in the corona and solar wind and contain considerable energy
. Magnetic tension forces the corona to co-rotate with the Sun, but any residual rotation far from the Sun reported until now has been much smaller than the amplitude of waves and deflections from interacting wind streams
. Here we report observations of solar-wind plasma at heliocentric distances of about 35 solar radii
, well within the distance at which stream interactions become important. We find that Alfvén waves organize into structured velocity spikes with duration of up to minutes, which are associated with propagating S-like bends in the magnetic-field lines. We detect an increasing rotational component to the flow velocity of the solar wind around the Sun, peaking at 35 to 50 kilometres per second-considerably above the amplitude of the waves. These flows exceed classical velocity predictions of a few kilometres per second, challenging models of circulation in the corona and calling into question our understanding of how stars lose angular momentum and spin down as they age
.
ABSTRACT We present a study of magnetic field fluctuations in a slow solar wind stream, close to ion scales, where an increase of the level of magnetic compressibility is observed. Here, the nature ...of these compressive fluctuations is found to be characterized by coherent structures. Although previous studies have shown that current sheets can be considered the principal cause of intermittency at ion scales, here we show for the first time that, in the case of the slow solar wind, a large variety of coherent structures contributes to intermittency at proton scales, and current sheets are not the most common. Specifically, we find compressive ( ), linearly polarized structures in the form of magnetic holes, solitons, and shock waves. Examples of Alfvénic structures ( ) are identified as current sheets and vortex-like structures. Some of these vortices have , as in the case of Alfvén vortices, but the majority of them are characterized by . Thanks to multi-point measurements by the Cluster spacecraft, for about 100 structures we could determine the normal, the propagation velocity, and the spatial scale along this normal. Independently of the nature of the structures, the normal is always perpendicular to the local magnetic field, meaning that k > k . The spatial scales of the studied structures are found to be between two and eight times the proton gyroradius. Most of them are simply convected by the wind, but 25% propagate in the plasma frame. Possible interpretations of the observed structures and the connection with plasma heating are discussed.
Estimating the temperature of solar wind particles and their anisotropies is particularly important for understanding the origin of their deviations from thermal equilibrium and the effects this has. ...In the absence of energetic events, the velocity distribution of electrons reveals a dual structure with a thermal (Maxwellian) core and a suprathermal (kappa) halo. This article presents a detailed observational analysis of these two components, providing estimations of their temperatures and temperature anisotropies, and decoding any potential interdependence that their properties may indicate. The dataset used in this study includes more than 120 000 of the distributions measured by three missions in the ecliptic within an extended range of heliocentric distances from 0.3 to over 4 AU. The core temperature is found to decrease with the radial distance, while the halo temperature slightly increases, clarifying an apparent contradiction in previous observational analyses and providing valuable clues about the temperature of the kappa-distributed populations. For low values of the power-index kappa, these two components manifest a clear tendency to deviate from isotropy in the same direction, which seems to confirm the existence of mechanisms with similar effects on both components,
e.g.
, the solar wind expansion, or the particle heating by the fluctuations. However, the existence of plasma states with anticorrelated anisotropies of the core and halo populations and the increase in their number for high values of the power-index kappa suggest a dynamic interplay of these components, mediated, most probably, by the anisotropy-driven instabilities.
ABSTRACT We perform the first statistical study of the effects of the interaction of suprathermal electrons with narrow-band whistler mode waves in the solar wind (SW). We show that this interaction ...does occur and that it is associated with enhanced widths of the so-called strahl component. The latter is directed along the interplanetary magnetic field away from the Sun. We do the study by comparing the strahl pitch angle widths in the SW at 1 AU in the absence of large scale discontinuities and transient structures, such as interplanetary shocks, interplanetary coronal mass ejections, stream interaction regions, etc. during times when the whistler mode waves were present and when they were absent. This is done by using the data from two Cluster instruments: Spatio Temporal Analysis of Field Fluctuations experiment (STAFF) data in the frequency range between ∼0.1 and ∼200 Hz were used for determining the wave properties and Plasma Electron And Current Experiment (PEACE) data sets at 12 central energies between ∼57 eV (equivalent to ∼10 typical electron thermal energies in the SW, ET) and ∼676 eV (∼113 ET) for pitch angle measurements. Statistical analysis shows that, during the intervals with the whistler waves, the strahl component on average exhibits pitch angle widths between 2° and 12° larger than during the intervals when these waves are not present. The largest difference is obtained for the electron central energy of ∼344 eV (∼57 ET).
We investigate the nature of magnetic turbulent fluctuations, around ion characteristic scales, in a fast solar wind stream, by using Cluster data. Contrarily to slow solar wind, where both Alfvénic ...( δ b > δ b ) and compressive ( δ b > δ b ) coherent structures are observed, the turbulent cascade of fast solar wind is dominated by Alfvénic structures, namely, Alfvén vortices, with a small and/or finite compressive part, with the presence also of several current sheets aligned with the local magnetic field. Several examples of vortex chains are also recognized. Although an increase of magnetic compressibility around ion scales is observed also for fast solar wind, no strongly compressive structures are found, meaning that the nature of the slow and fast winds is intrinsically different. Multispacecraft analysis applied to this interval of fast wind indicates that the coherent structures are almost convected by the flow and aligned with the local magnetic field, i.e., their normal is perpendicular to B , which is consistent with a two-dimensional turbulence picture. Understanding intermittency and the related generation of coherent structures could provide a key insight into the nonlinear energy transfer and dissipation processes in magnetized and collisionless plasmas.
The solar wind protons are accelerated to supersonic velocities within the distance of 10 solar radii from the Sun, as a consequence of a complex physical mechanism including particle kinetic effects ...as well as the field‐particle energy and momentum exchange. We use a numerical kinetic model of the solar wind, accounting for Coulomb collisions (BiCoP), and model a solar wind accelerated only by the ambipolar electrostatic filed (E) arising due to the difference in mass between electron and proton, and assuring quasi‐neutrality and zero current. We study the effect E, which was found to be on the order of Dreicer electric field (ED) (Dreicer, 1959), has on the resulting electron velocity distribution functions. The strahl electron radial evolution is represented by means of its pitch‐angle width (PAW), and the strahl parallel temperature (Ts,‖). A continuous transition between collisional and weakly collisional regime results in broader PAW, compared to the single‐exobase prediction imposed by the exospheric models. Collisions were found to scatter strahl electrons below 250 eV, which in turn has an effect on the measured Ts,‖. A slight increase was found in Ts,‖ with radial distance, and was stronger for the more collisional run. We estimate that the coronal electron temperature inferred from the observations of Ts,‖ in the solar wind, would be overestimated for between 8% and 15%.
Key Points
We use a kinetic model of expanding solar wind accounting for Coulomb collisions. This model produces a slow, supersonic solar wind proton population accelerated only through the ambipolar electric field, which arises due to the difference of mass between electron and proton
The self‐consistently calculated ambipolar electric field in the model is on the order of Dreicer electric field
We present the radial evolution of the strahl electron component under the influence of Coulomb collisions
The Solar Orbiter mission Müller, D.; St. Cyr, O. C.; Zouganelis, I. ...
Astronomy and astrophysics (Berlin),
10/2020, Letnik:
642
Journal Article
Recenzirano
Odprti dostop
Aims.
Solar Orbiter, the first mission of ESA’s Cosmic Vision 2015–2025 programme and a mission of international collaboration between ESA and NASA, will explore the Sun and heliosphere from close up ...and out of the ecliptic plane. It was launched on 10 February 2020 04:03 UTC from Cape Canaveral and aims to address key questions of solar and heliospheric physics pertaining to how the Sun creates and controls the Heliosphere, and why solar activity changes with time. To answer these, the mission carries six remote-sensing instruments to observe the Sun and the solar corona, and four in-situ instruments to measure the solar wind, energetic particles, and electromagnetic fields. In this paper, we describe the science objectives of the mission, and how these will be addressed by the joint observations of the instruments onboard.
Methods.
The paper first summarises the mission-level science objectives, followed by an overview of the spacecraft and payload. We report the observables and performance figures of each instrument, as well as the trajectory design. This is followed by a summary of the science operations concept. The paper concludes with a more detailed description of the science objectives.
Results.
Solar Orbiter will combine in-situ measurements in the heliosphere with high-resolution remote-sensing observations of the Sun to address fundamental questions of solar and heliospheric physics. The performance of the Solar Orbiter payload meets the requirements derived from the mission’s science objectives. Its science return will be augmented further by coordinated observations with other space missions and ground-based observatories.
We analyze the radial variation of the power spectra of the magnetic field from 0.3 to about 0.9 AU, using Helios 2 spacecraft measurements in the fast solar wind. The time resolution of the magnetic ...field data allows us to study the power spectra up to 2 Hz. Generally, the corresponding spectral break frequency and the Doppler-shifted frequencies, which are related to the proton gyroradius and inertial scales, are close to a frequency functionof of about 0.5 Hz at a distance of 1 AU from the Sun. However, studying the radial evolution of the power spectra offers us the possibility to distinguish between those scales. Recent Ulysses observations show that, while the proton scales vary, stays nearly constant with the heliocentric distance R. In our study we confirm that varies within a small interval of 0.2, 0.4 Hz only, as R varies from 0.3 to 0.9 AU. Moreover, if we assume parallel propagating fluctuations (with respect to the solar wind flow or background magnetic field), we can show that none of the proton scales are coincident with the break scale. If, however, we take into account the two-dimensional nature of the turbulent fluctuations, then we can show that the spatial scale corresponding to does follow the proton inertial scale, lambdap(R), but not the proton gyroradius scale, rhop(R), as a function of heliocentric distance. These observations indicate that the spectral break at the proton inertial scale might be related to the Hall effect, or be controlled by the ion-cyclotron damping of obliquely propagating fluctuations or the formation of current sheets scaling like lambdap, which could be responsible for ion heating through magnetic reconnection.
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