Solar Wind Observations Research Articles

Short‐wavelength turbulence in the solar wind: Linear theory of whistler and kinetic Alfvén fluctuations

There is a debate as to the identity of the fluctuations which constitute the relatively high‐frequency plasma turbulence observed in the solar wind. One school holds that these modes are kinetic Alfvén waves, whereas another opinion is that they are whistler modes. Here linear kinetic theory for electromagnetic fluctuations in homogeneous, collisionless, magnetized plasmas is used to compute two dimensionless transport ratios, the electron compressibility Ce and the magnetic compressibility C for these two modes. The former is a measure of the amplitude of density fluctuations, and the latter indicates the relative energy in magnetic fluctuations in the component parallel to the background magnetic field Bo. For βe ≪ 1, [C]Alfven ≪ [C]whistler, and the latter quantity is of order 0.5 at whistler propagation strongly oblique to Bo. Such values of C are sometimes measured at relatively high frequencies and βe ≪ 1 in the solar wind; thus, it is concluded that such observations correspond to whistler mode turbulence. But other solar wind observations indicate that kinetic Alfvén fluctuations also contribute to relatively high‐frequency solar wind turbulence.

Are periodic solar wind number density structures formed in the solar corona?

We present an analysis of the alpha to proton solar wind abundance ratio (AHe) during a period characterized by significant large size scale density fluctuations, focusing on an event in which the proton and alpha enhancements are anti‐correlated. In a recent study using 11 years (1995–2005) of solar wind observations from the Wind spacecraft, N. M. Viall et al. [2008] showed that periodic proton density structures occurred at particular radial length‐scales more often than others. The source of these periodic density structures is a significant and outstanding question. Are they generated in the interplanetary medium, or are they a relic of coronal activity as the solar wind was formed? We use AHe to answer this question, as solar wind elemental abundance ratios are not expected to change during transit. For this event, the anti‐phase nature of the AHe variations strongly suggests that periodic solar wind density structures originate in the solar corona.

The response of the polar cusp to a high-speed solar wind stream studied by a multispacecraft wavelet analysis

We investigate the coupling between the solar wind and the magnetospheric cusp during the passage of a Corotating Interaction Region (CIR) high-speed stream. A CIR event on 11 January 2002 is selected for this study. We use ACE (Advanced Composition Explorer) and Geotail plasma and magnetic field data for interplanetary solar wind observations and Cluster plasma and magnetic field data for the cusp observations. The response of the solar wind on the polar cusp was studied and made visible with the wavelet and cross-wavelet analysis. During the CIR event we observe significantly enhanced wave power for waves with periods ranging from 15 to 25 min (1.1–0.7 mHz). Significant cross-correlation power of the ACE and Geotail interplanetary magnetic field (IMF) Bz- and Ey-component and the solar wind Vx-component versus the Cluster cusp H + density, the thermal temperature and energetic electron flux (50–244 keV) were found at the corresponding frequencies. We conclude that the solar wind Alfvén waves affect the magnetospheric dayside cusp density and heat the cusp. We interpret these correlation results as evidence of high-speed solar wind—magnetosphere coupling driven by waves in the solar wind.

EXACT VECTORIAL LAW FOR AXISYMMETRIC MAGNETOHYDRODYNAMICS TURBULENCE

Three-dimensional incompressible magnetohydrodynamics turbulence is investigated under the assumptions of homogeneity and axisymmetry. We demonstrate that previous works of Chandrasekhar may be improved significantly by using a different formalism for the representation of two-point correlation tensors. From this axisymmetric kinematics, the equations ` al a von K´´ an–Howarth are derived from which an exact relation is found in terms of measurable correlations. The relation is then analyzed in the particular case of a medium permeated by an imposed magnetic field B0. We make the ansatz that the development of anisotropy implies an algebraic relation between the axial and the radial components of the separation vector r and we derive an exact vectorial law which is parameterized by the intensity of anisotropy. The critical balance proposed by Goldreich & Sridhar is used to fix this parameter and to obtain a unique exact expression; the particular limits of correlations transverse and parallel to B0 are given for which simple expressions are found. Predictions for the energy spectra are also proposed by a straightforward dimensional analysis of the exact law; it gives a stronger theoretical background to the heuristic spectra previously proposed in the context of the critical balance. We also discuss the wave turbulence limit of an asymptotically large external magnetic field which appears as a natural limit of the vectorial relation. A new interpretation of the anisotropic solar wind observations is eventually discussed.

Finite dissipation and intermittency in magnetohydrodynamics

We present an analysis of data stemming from numerical simulations of decaying magnetohydrodynamic (MHD) turbulence up to grid resolution of 1536(3) points and up to Taylor Reynolds number of approximately 1200 . The initial conditions are such that the initial velocity and magnetic fields are helical and in equipartition, while their correlation is negligible. Analyzing the data at the peak of dissipation, we show that the dissipation in MHD seems to asymptote to a constant as the Reynolds number increases, thereby strengthening the possibility of fast reconnection events in the solar environment for very large Reynolds numbers. Furthermore, intermittency of MHD flows, as determined by the spectrum of anomalous exponents of structure functions of the velocity and the magnetic field, is stronger than that of fluids, confirming earlier results; however, we also find that there is a measurable difference between the exponents of the velocity and those of the magnetic field, reminiscent of recent solar wind observations. Finally, we discuss the spectral scaling laws that arise in this flow.

Substorm evolution as revealed by THEMIS satellites and a global MHD simulation

The major substorm that occurred on 1 March 2008 had excellent spacecraft coverage by the THEMIS spacecraft in the magnetotail, GOES 11, and GOES 12 at geosynchronous orbit and Geotail in the dayside magnetosheath. A global magnetohydrodynamic simulation of this substorm, driven by Wind solar wind observations, accurately reproduced the magnetospheric observations. The simulation revealed the complexity of magnetotail dynamics during the substorm, in particular, in the near‐Earth plasma sheet. Reconnection began prior to the substorm on closed field lines and a flux rope formed there. Around substorm onset, the simulation exhibited flow vortices near the locations of THEMIS P3 and P4, in agreement with observations at P3 and P4. These vortices were associated with a duskside neutral line that formed early in the substorm. Six minutes later, another neutral line formed on the dawnside of the tail. These neutral lines then merged to form a single large reconnection region that extended across the tail and greatly expanded the flux rope. The least active part of the tail was the region around midnight. Strong flows were seen in the observations and in the simulation during the two intensifications of this substorm; in particular, tailward flows were seen at THEMIS P1 and P2. Reconnection on closed field lines, vortices in the near‐Earth region, a channel of strong tailward flow, and enhanced precipitation into the ionosphere all contributed to substorm development.

Electrostatic Short-Scale Termination of Solar-Wind Turbulence

Recent hybrid Vlasov simulations [F. Valentini, Phys. Rev. Lett. 101, 025006 (2008)10.1103/PhysRevLett.101.025006] have shown that the short-scale termination of solar-wind turbulence is characterized by the occurrence of longitudinal electrostatic fluctuations. Beside the ion-acoustic branch, in agreement with solar-wind observations, a novel branch of acoustic-like waves, with phase velocity close to the ion thermal speed, has been recovered in the simulations. In this Letter, we show that these waves turn out to be Bernstein-Greene-Kruskal-like solutions of the hybrid Vlasov-Maxwell equations, driven by kinetic effects of resonant particle trapping. We also discuss the development of the solar-wind turbulent spectra across the ion inertial length and especially stress the fact that turbulence privileges acoustic paths to develop towards short wavelengths.

Alfvén wave evolution within corotating interaction regions associated with the formation of magnetic holes/decreases

One‐dimensional magnetohydrodynamics (MHD) simulations were performed to validate the evolution of large‐amplitude Alfvénic fluctuations embedded in the high‐speed solar wind as it interacts with the low‐speed stream flowing ahead. It is well known that the interaction of the high‐ and low‐speed streams results in the formation of a corotating interaction region (CIR) bounded by forward and reverse shocks. After passing through the reverse shock, the initial Alfvénic fluctuation disintegrates into two Alfvén modes traveling in opposite directions in a plasma rest frame. These modes bound a region where the magnetic field intensity is largely below the background level. The appearance of this magnetic depression, called a “magnetic hole” (MH) or “magnetic decrease” (MD), is highly consistent with previous solar wind observations in that the structure is often identified close to the reverse shock within CIRs and holds pressure balance. The process by which such evolution of Alfvénic fluctuations results in MH/MD formation is elucidated: A local maximum in the field components of the fluctuation, which have originally monotonic gradients (e.g., dBy/dx < 0), is formed due to the amplification when the field passes through the reverse shock. This causes local current reversal (dBy/dx > 0) within the fluctuation, where the initial force balance (∇p ∼ J × B) is violated. The resultant force sweeps the plasma backward to form a pressure increase and simultaneous magnetic decrease within the CIR, which can be associated with a MH/MD. This model naturally describes the formation process of a MH/MD.

Neutral solar wind generated by lunar exospheric dust at the terminator

We calculate the flux of neutral solar wind observed on the lunar surface at the terminator due to solar wind protons penetrating exospheric dust grains with (1) radii greater than 0.1 μm and (2) radii greater than 0.01 μm. For grains with radii larger than 0.1 μm, the ratio of the neutral solar wind flux produced by exospheric dust to the incident ionized solar wind flux is estimated to be ∼10−4–10−3 for solar wind speeds in excess of 800 km s−1 but much lower (<10−5) at average to slow solar wind speeds. However, when the smaller grain sizes are considered, this ratio is estimated to be ≥10−5 at all speeds and at speeds in excess of 700 km s−1 reaches ∼10−3. These neutral solar wind fluxes are easily measurable with current low‐energy neutral atom instrumentation. Observations of neutral solar wind from the surface of the Moon would provide independent information on the distribution of very small dust grains in the lunar exosphere that would complement and constrain optical measurements at ultraviolet and visible wavelengths.

Solar wind control of plasma number density in the near-Earth plasma sheet: three-dimensional structure

Abstract. The plasma number density in the near-Earth plasma sheet depends on the solar wind number density and the north-south component of interplanetary magnetic field (IMF Bz) with time lag and duration of several hours. We examined the three-dimensional structure of such dependences by fitting observations of plasma sheet and solar wind to an empirical model equation. Analyses were conducted separately for northward and southward IMF conditions. Effects of solar wind speed and IMF orientation were also examined by further subdivision of the dataset. Based on obtained results, we discuss (i) the relative contribution of the ionosphere and solar wind to plasma sheet mass supply, (ii) the entry mechanisms for magnetosheath particles, and (iii) the plasma transport in the plasma sheet. We found that solar wind number density dependence is weaker and IMF Bz dependence is stronger for faster solar wind with southward IMF, which suggests the contribution of ionospheric particles. Further from the Earth, different interplanetary conditions result in different structures of solar wind dependence, which indicate different solar wind entry mechanisms: (1) southward IMF results in a strong dependence on solar wind number density in the flank high-latitude region, (2) slow solar wind with northward IMF leads to lower-latitude peaks of solar wind number density dependence in the flank region, (3) fast solar wind with northward IMF results in a strong dependence on solar wind number density at the down-tail dusk flank equator, and (4) solar wind number density dependence is stronger in the downstream of quasi-parallel bow shock. These features are attributable to (1) low-latitude dayside reconnection entry, (2) high-latitude dayside reconnection entry, (3) entry due to decay of Kelvin-Helmholtz vortices, and (4) diffusive entry mediated by kinetic Alfven waves, respectively. Effect of IMF Bz and its time lags show plasma sheet reconfiguration associated with enhanced convective transport under southward IMF. Duration of IMF Bz effect under northward IMF is interpreted in terms of turbulent diffusive transport.

Nonlinear Guiding Center Theory of Perpendicular Diffusion: Derivation from the Newton‐Lorentz Equation

We revisit the problem of charged-particle scattering in the direction perpendicular to a mean magnetic field. By combining the well-established nonlinear guiding center theory with the Newton-Lorentz equation, we derive an integral equation for perpendicular scattering without the assumption that charged particles follow magnetic field lines. A comparison with solar wind observations is also presented.

Weaker solar wind from the polar coronal holes and the whole Sun

Observations of solar wind from both large polar coronal holes (PCHs) during Ulysses' third orbit showed that the fast solar wind was slightly slower, significantly less dense, cooler, and had less mass and momentum flux than during the previous solar minimum (first) orbit. In addition, while much more variable, measurements in the slower, in‐ecliptic wind match quantitatively with Ulysses and show essentially identical trends. Thus, these combined observations indicate significant, long‐term variations in solar wind output from the entire Sun. The significant, long‐term trend to lower dynamic pressures means that the heliosphere has been shrinking and the heliopause must be moving inward toward the Voyager spacecraft. In addition, our observations suggest a significant and global reduction in the mass and energy fed in below the sonic point in the corona. The lower supply of mass and energy may result naturally from a reduction of open magnetic flux during this period.

On the importance of accurate solar wind measurements for studying magnetospheric dynamics

We have examined the validity of assuming that the solar wind observed at the L1 Lagrange point is a good predictor of the environment at the Earth by using solar wind observations during a magnetic storm from four spacecraft (ACE, Wind, IMP‐8, and Geotail). The storm on 23 and 24 May 2000 occurred when a magnetic cloud reached the Earth. We carried out a running cross‐correlation analysis between the solar wind parameters observed on pairs of the solar wind spacecraft and found good agreement early in the storm. The peak cross‐correlations between magnetic field components exceeded 90% throughout this interval, while the peak cross‐correlations between the dynamic pressure observations was about 70% or better. The main differences were at higher frequencies. However, later after the main cloud had passed the Earth the peak cross‐correlations decreased, especially between ACE at L1 and spacecraft nearer the Earth. Late in the storm the monitors were not observing the same solar wind. We examined the effect of these differences by using our global magnetohydrodynamic simulation to model the magnetosphere using observations from three monitors. Early in the event we found very good agreement between simulated magnetospheres based on input from different solar wind monitors but later the agreement became very bad. If we assume that the monitors near the Earth provide the best measurement of the solar wind that interacts with the magnetosphere, then later in the magnetic storm the solar wind observations at ACE were not a good indication of the solar wind reaching the Earth.

Stream Interactions and Interplanetary Coronal Mass Ejections at 5.3 AU near the Solar Ecliptic Plane

We have performed a survey of the characteristics of two types of large spatial-scale solar-wind structures, stream interaction regions (SIRs), and interplanetary coronal mass ejections (ICMEs), near 5.3 AU, using solar-wind observations from Ulysses. Our study is confined to the three aphelion passes of Ulysses, and also within ± 10° of the solar ecliptic plane, covering a part of 1992, 1997 – 1998, and 2003 – 2005, representing three slices of different phases of the solar activity cycle. Overall, there are 54 SIRs and 60 ICMEs in the survey. Many are merged in hybrid events, suggesting that they have undergone multiple interactions prior to reaching Jovian orbit. About 91% of SIRs occur with shocks, with 47% of such shocks being forward – reverse shock pairs. The solar-wind velocity sometimes stays constant or even decreases within the interaction region near 5.3 AU, in contrast with the gradual velocity increase during SIRs at 1 AU. Shocks are driven by 58% of ICMEs, with 94% of them being forward shocks. Some ICMEs seem to have multiple small flux ropes with different scales and properties. We quantitatively compare various properties of SIRs and ICMEs at 5.3 AU, and study their statistical distributions and variations with solar activity. The width, maximum dynamic pressure, and peak perpendicular pressure of SIRs all become larger than ICMEs. Dynamic pressure (Pdyn) is expected to be important for Jovian magnetospheric activity. We have examined the distributions of Pdyn of SIRs, ICMEs, and general solar wind, but these cannot explain the observed bimodal distribution of the location of the Jovian magnetopause. By comparing the properties of SIRs and ICMEs at 0.72, 1, and 5.3 AU, we find that the ICME expansion slows down significantly between 1 and 5.3 AU. Some transient and small streams in the inner heliosphere have merged into a single interaction region.

Stream Interactions and Interplanetary Coronal Mass Ejections at 0.72 AU

We present comprehensive surveys of 203 stream interaction regions (SIRs) and 124 interplanetary CMEs (ICMEs) during 1979 – 1988 using Pioneer Venus Orbiter (PVO) in situ solar-wind observations at 0.72 AU and examine the solar-cycle variations of the occurrence rate, shock association rate, duration, width, maximum total perpendicular pressure (P t), maximum dynamic pressure, maximum magnetic field intensity, and maximum velocity change of these two large-scale solar-wind structures. The medians, averages, and histogram distributions of these parameters are also reported. Furthermore, we sort ICMEs into three groups based on the temporal profiles of P t, and we investigate the variations of the fractional occurrence rate of three groups of ICMEs with solar activity. We find that the fractional occurrence rate of magnetic-cloud-like ICMEs declined with solar activity, consistent with our former 1-AU results. This study at 0.72 AU provides a point of comparison in the inner heliosphere for examining the radial evolution of SIRs and ICMEs. The width of SIRs and ICMEs increases by 0.04 and 0.1 AU, respectively, and the maximum P t decreases to about 1/3 from Venus to Earth orbit. In addition, our work establishes the statistical properties of the solar-wind conditions at 0.72 AU that control the solar-wind interaction with Venus and its atmosphere loss by related processes.

Time-dependent test-particle scattering perpendicular to a mean magnetic field: the four transport regimes and validity of the FLRW limit

By employing a compound transport model the problem of charged test-particle scattering perpendicular to a mean magnetic field is investigated. In comparison with previous approaches a full time-dependent description is provided. According to these new results there are at least four different transport regimes, namely, the ballistic regime, the strong diffusive regime, the FLRW limit and the subdiffusive regime. It is also shown that the FLRW limit, and therefore quasilinear theory, can only be correct for intermediate time-scales and for high energetic particles. For low energetic particles there is a strong diffusive regime with κ⊥/κ∥ ∼ 1. These results are very important for understanding solar wind observations and recent test-particle simulations.

Intermittency of solar wind density fluctuations and its relation to sharp density changes

The paper presents the study of turbulent properties of the solar wind plasma, namely, the intermittency of fluctuations of the solar wind ion flux in the earlier unexplored region of comparatively high frequencies (0.01–1 Hz). Special attention is given to a comparison of intermittency for solar wind observation intervals containing sharp (shorter than 10 min) and high-amplitude (greater than 20%) changes of the ion flux to intervals without such changes. The solar wind observation intervals containing sharp changes of the flux are found to be essentially more intermittent than the intervals of quiet solar wind. Such a comparison allows one to reveal the fundamental difference in turbulent properties of the solar wind depending on the presence or absence of sharp boundaries in plasma structures.

Formation of suprathermal electron distributions in the quiet solar corona

Context. Solar wind electron velocity distribution functions (VDFs) show enhanced levels of suprathermal electrons as compared to a Maxwellian distribution. Previous studies show that the suprathermal tails of solar wind VDFs can be fitted by kappa distributions, and that a coronal origin of the suprathermal electrons is possible. Aims. The generation of suprathermal electrons by resonant interaction with whistler waves in the corona is investigated under quiet solar conditions without any flare activity. The magnetic field geometry is that of a closed magnetic loop. The electron-whistler interaction is described within the framework of quasilinear theory, that leads to pitch-angle diffusion of the electrons in the reference frame of the waves. Methods. A study of electron VDFs requires a kinetic description of the electrons. The model used in this paper is based on a numerical solution of the Boltzmann-Vlasov equation for the electrons, considering Coulomb collisions and wave-electron interaction. The waves are assumed to enter the simulation box with a given power-law spectrum, which evolves inside the box due to wave propagation and absorption by the electrons. Starting from a nearly Maxwellian initial electron VDF, the temporal evolution of the VDF is calculated until a final steady state has been reached. Results. The results show that a population of suprathermal electrons develops in a closed coronal loop. The electron VDF can be approximated by a power-law in the energy range of 4-10 keV. The power-law index is in agreement with the solar wind observations. For lower energy, the electrons are thermalized in the dense model coronal loop, and the efficiency of the acceleration mechanism decreases for higher energies. The energy range of the simulation box has to be chosen sufficiently large, and the influence of the loop geometry on the results is also studied. Conclusions. These numerical studies show that the quiet solar corona is capable of producing suprathermal electron VDFs with similar characteristics to those observed in the solar wind. This study is focused on a closed region in the solar corona, but if such an electron population is present in the corona, it should also appear in the solar wind.

A statistical study of hot flow anomalies using Cluster data

Hot flow anomalies (HFAs) are studied using observations of the RAPID suprathermal charged particle detector, the FGM magnetometer, and the CIS plasma detector aboard the four Cluster spacecraft. Previously, we studied several specific features of tangential discontinuities on the basis of Cluster measurements in February–April 2003. In this paper, we confirm the following results: the angle between the Sun direction and the tangentional discontinuity (TD) normal is larger than 45° during HFAs, the magnetic field directional change is large. We then present evidence for a new necessary condition for the formation of HFAs, that is, the solar wind speed is significantly ( about 200 km / s or Δ M f = 2.3 ) higher than the long-term average. The existence of this condition is also confirmed by simultaneous ACE MAG and SWEPAM solar wind observations at the L1 point 1.4 million km upstream of the Earth. The results are compared with recent hybrid simulations.

Non-linear damping of slab modes and cosmic ray transport

By applying recent results for the slab correlation time-scale on to cosmic ray scattering theory, we compute cosmic ray parallel mean free paths within the quasi-linear limit. By employing these results on to charged particle transport in the Solar system, we demonstrate that much larger parallel mean free paths can be obtained in comparison to previous results. A comparison with solar wind observations is also presented to show that the new theoretical results are much closer to the observations than the previous results.