Solar Wind Fluctuations Research Articles

INTERPRETING MAGNETIC VARIANCE ANISOTROPY MEASUREMENTS IN THE SOLAR WIND

The magnetic variance anisotropy ($\mathcal{A}_m$) of the solar wind has been used widely as a method to identify the nature of solar wind turbulent fluctuations; however, a thorough discussion of the meaning and interpretation of the $\mathcal{A}_m$ has not appeared in the literature. This paper explores the implications and limitations of using the $\mathcal{A}_m$ as a method for constraining the solar wind fluctuation mode composition and presents a more informative method for interpreting spacecraft data. The paper also compares predictions of the $\mathcal{A}_m$ from linear theory to nonlinear turbulence simulations and solar wind measurements. In both cases, linear theory compares well and suggests the solar wind for the interval studied is dominantly Alfv\'{e}nic in the inertial and dissipation ranges to scales $k \rho_i \simeq 5$.

POWER ANISOTROPY IN THE MAGNETIC FIELD POWER SPECTRAL TENSOR OF SOLAR WIND TURBULENCE

We observe the anisotropy of the power spectral tensor of magnetic field fluctuations in the fast solar wind for the first time. In heliocentric RTN coordinates the power in each element of the tensor has a unique dependence on the angle between the magnetic field and velocity of the solar wind (\theta) and the angle of the vector in the plane perpendicular to the velocity (\phi). We derive the geometrical effect of the high speed flow of the solar wind past the spacecraft on the power spectrum in the frame of the plasma P(k) to arrive at the observed power spectrum P(f,\theta,\phi) based on a scalar field description of turbulence theory. This allows us to predict the variation in the \phi direction and compare it to the data. We then transform the observations from RTN coordinates to magnetic-field-aligned coordinates. The observed reduced power spectral tensor matches the theoretical predictions we derive in both RTN and field-aligned coordinates which means that the local magnetic field we calculate with wavelet envelope functions is an accurate representation of the physical axis of symmetry for the turbulence and implies that on average the turbulence is axi-symmetric. We also show that we can separate the dominant toroidal component of the turbulence from the smaller but significant poloidal component and that these have different power anisotropy. We also conclude that the magnetic helicity is anisotropic and mostly two-dimensional, arising from wavevectors largely confined to the plane perpendicular to B.

Scale-free texture of the fast solar wind

The higher-order statistics of magnetic field magnitude fluctuations in the fast quiet solar wind are quantified systematically, scale by scale. We find a single global non-Gaussian scale-free behavior from minutes to over 5 h. This spans the signature of an inertial range of magnetohydrodynamic turbulence and a ~1/f range in magnetic field components. This global scaling in field magnitude fluctuations is an intrinsic component of the underlying texture of the solar wind and puts a strong constraint on any theory of solar corona and the heliosphere. Intriguingly, the magnetic field and velocity components show scale-dependent dynamic alignment outside of the inertial range.

Statistical investigation of hourly OMNI solar wind data

[1] Hourly OMNI solar wind data are sorted into categories reflecting membership of each data point to either slow or fast solar wind streams, or to either coronal mass ejection or corotating interaction region environments. The categorization is inspired by Yermolaev et al. (2009) and modified from there. Durations and coverage fractions of each category are investigated, together with their dependence on the solar activity cycle. The results are in line with physical expectations for the solar wind at 1 AU. A further analysis, treating hourly solar wind fluctuations as a constrained random walk process, is carried out independently for each solar wind category and discussed. The resulting step size distributions are found to be largely symmetric across zero, resembling a random walk deviation from a long-term average. This constrained random walk can in principle be used to fill gaps in the OMNI data and perform other OMNI data extrapolations.

Nonaxisymmetric Anisotropy of Solar Wind Turbulence

A key prediction of turbulence theories is frame-invariance, and in magnetohydrodynamic (MHD) turbulence, axisymmetry of fluctuations with respect to the background magnetic field. Paradoxically the power in fluctuations in the turbulent solar wind are observed to be ordered with respect to the bulk macroscopic flow as well as the background magnetic field. Here, nonaxisymmetry across the inertial and dissipation ranges is quantified using in situ observations from Cluster. The observed inertial range nonaxisymmetry is reproduced by a "fly through" sampling of a direct numerical simulation of MHD turbulence. Furthermore, fly through sampling of a linear superposition of transverse waves with axisymmetric fluctuations generates the trend in nonaxisymmetry with power spectral exponent. The observed nonaxisymmetric anisotropy may thus simply arise as a sampling effect related to Taylor's hypothesis and is not related to the plasma dynamics itself.

Transport of solar wind fluctuations: A two-component model

We present a new model for the transport of solar wind fluctuations which treats them as two interacting incompressible components: quasi-two-dimensional turbulence and a wave-like piece. Quantities solved for include the energy, cross helicity, and characteristic transverse length scale of each component, plus the proton temperature. The development of the model is outlined and numerical solutions are compared with spacecraft observations. Compared to previous single-component models, this new model incorporates a more physically realistic treatment of fluctuations induced by pickup ions and yields improved agreement with observed values of the correlation length, while maintaining good observational accord with the energy, cross helicity, and temperature.

On the energy cascade rate of solar wind turbulence in high cross helicity flows

[1] Improved observations and analysis of solar wind fluctuations at 1 AU show that the total energy spectrum, kinetic plus magnetic, is in better agreement with the Kraichnan 3/2 scaling than with the Kolmogorov 5/3 scaling, consistent with simulations of incompressible MHD turbulence with a strong ambient magnetic field which exhibit a perpendicular energy spectrum proportional to k⊥−3/2. The Kraichnan scaling is especially clear in solar wind flows having large values of the normalized cross helicity σc. In this study, a generalization of Boldyrev's theory that applies to turbulence with nonvanishing cross helicity is used to estimate the energy cascade rate for high cross helicity flows in the solar wind at 1 AU. The analysis of 85 intervals of solar wind data from the Wind spacecraft characterized by normalized cross helicities in the range ∣σc∣ > 0.88 yields typical values of the energy dissipation rate ɛ ranging from 100 to 2400 J/kg/s. This is roughly a factor of 4 or 5 smaller than estimates of the proton heating rate at 1 AU needed to account for the nonadiabatic solar wind expansion. The results suggest that the reduction in turbulent heating in high cross helicity flows creates a tangible deficit in the heating rate required by empirical models based on average solar wind properties.

First lunar wake passage of ARTEMIS: Discrimination of wake effects and solar wind fluctuations by 3D hybrid simulations

The spacecraft P1 of the new ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) mission passed the lunar wake for the first time on February 13, 2010. We present magnetic field and plasma data of this event and results of 3D hybrid simulations. As the solar wind magnetic field was highly dynamic during the passage, a simulation with stationary solar wind input cannot distinguish whether distortions were caused by these solar wind variations or by the lunar wake; therefore, a dynamic real-time simulation of the flyby has been performed. The input values of this simulation are taken from NASA OMNI data and adapted to the P1 data, resulting in a good agreement between simulation and measurements. Combined with the stationary simulation showing non-transient lunar wake structures, a separation of solar wind and wake effects is achieved. An anisotropy in the magnitude of the plasma bulk flow velocity caused by a non-vanishing magnetic field component parallel to the solar wind flow and perturbations created by counterstreaming ions in the lunar wake are observed in data and simulations. The simulations help to interpret the data granting us the opportunity to examine the entire lunar plasma environment and, thus, extending the possibilities of measurements alone: A comparison of a simulation cross section to theoretical predictions of MHD wave propagation shows that all three basic MHD modes are present in the lunar wake and that their expansion governs the lunar wake refilling process.

Unique observations of a geomagnetic SI+ − SI− pair: Solar sources and associated solar wind fluctuations

This paper describes the occurrence of a pair of oppositely directed sudden impulses (SI) in the geomagnetic field (ΔX) at ground stations, called SI+ − SI− pairs, that occurred between 1835 UT and 2300 UT on 23 April 1998. The SI+ − SI− pair was closely correlated with corresponding variations in the solar wind density, while solar wind velocity and the southward component of the interplanetary magnetic field (Bz) did not show any correspondence. Further, this event had no source on the visible solar disk. However, a rear‐side partial halo coronal mass ejection (CME) and an M1.4 class solar flare behind the west limb took place on 20 April 1998, the date corresponding to the traceback location of the solar wind flows. This event presents empirical evidence, which to our knowledge is the most convincing evidence for the association of specific solar events to the observations of an SI+ − SI− pair. In addition, it shows that it is possible for a rear‐side solar flare to propagate a shock toward the Earth.

LINEAR THEORY OF WEAKLY AMPLIFIED, PARALLEL PROPAGATING, TRANSVERSE TEMPERATURE-ANISOTROPY INSTABILITIES IN MAGNETIZED THERMAL PLASMAS

A rigorous analytical study of the dispersion relations of weakly amplified transverse fluctuations with wave vectors ($\vec{k}\parallel \vec{B}$) parallel to the uniform background magnetic field $\vec{B}$ in an anisotropic bi-Maxwellian magnetized electron-proton plasma is presented. We determine the conditions for which the weakly amplified LH-handed polarized Alfven-proton-cyclotron and RH-handed polarized Alfven-Whistler-electron-cyclotron branches can be excited. The results of our instability study are applied to the observed solar wind magnetic turbulence. The Alfvenic instability diagram explains well the main characteristic properties of the observed solar wind fluctuations. Especially, the observed confinement limits at small parallel plasma beta values are explained.

Why are halo coronal mass ejections faster?

Halo coronal mass ejections (CMEs) have been to be significantly faster than normal CMEs, which is a long-standing puzzle. In order to solve the puzzle, we first investigate the observed properties of 31 limb CMEs that clearly display loop-shaped frontal loops. The observational results show a strong tendency that slower CMEs are weaker in white-light intensity. Then, we perform a Monte Carlo simulation of 20000 artificial limb CMEs that have an average velocity of ∼523 km s−1. The Thomson scattering of these events is calculated when they are assumed to be observed as limb and halo events, respectively. It is found that the white-light intensity of many slow CMEs becomes remarkably reduced when they turn from being viewed as a limb event to being viewed as a halo event. When the intensity is below the background solar wind fluctuation, it is assumed that they would be missed by coronagraphs. The average velocity of “detectable" halo CMEs is ∼922 km s−1 very close to the observed value. This also indicates that wider events are more likely to be recorded. The results soundly suggest that the higher average velocity of halo CMEs is due to that a majority of slow events and some of narrow fast events carrying less material are so faint that they are blended with the solar wind fluctuations, and therefore are not observed.

Minimum variance analysis‐based propagation of the solar wind observations: Application to real‐time global magnetohydrodynamic simulations

The performance of the bulk of the current real‐time space weather applications is dependent on the accuracy of the driver solar wind data. One of the central aspects associated with the accuracy of the driver data is the determination of the propagation delay time of single‐spacecraft solar wind observations from Lagrangian point L1 to the Earth. In this work, the value of the minimum variance analysis‐based solar wind propagation technique as applied to real‐time global magnetohydrodynamic (MHD) simulations is investigated. Both the method that uses the newly introduced technique for minimizing the effect of the poorly determined phase planes and the minimum variance analysis‐based setup by Weimer and King (2008) along with the standard simple convection delay‐based (no phase planes used) solar wind propagation technique are applied to global MHD‐based modeling of the ground magnetic field and geomagnetically induced current variations. All computations are carried out in a real‐time setting. It is shown by means of comparisons to ground‐based observations that while the minimum variance analysis‐based propagation techniques can be used to optimize the timing associated with the propagated solar wind fluctuations, the improvement is so modest that from the statistical viewpoint, the benefit over using the simpler propagation technique vanishes for the studied storm period when the information is passed through real‐time global MHD modeling process.

Evidence that solar wind fluctuations substantially affect global convection and substorm occurrence

We have used examples of Poker Flat and Sondrestrom incoherent‐scatter radar observations of flows within the ionospheric mapping of the nightside plasma sheet and of Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft observations within the nightside plasma sheet to investigate whether the features found in the companion paper by Kim et al. (2009) within the dayside polar cap are also seen within the nightside plasma sheet. We find evidence that this is indeed the case: intensified interplanetary ULF fluctuations substantially enhance nightside convection flows and the fluctuations are reflected in the fluctuation power of the nightside flows. Additionally, our observations show evidence for an enhancement of earthward convection within the inner plasma sheet and an increase in plasma pressure within the plasma sheet in association with enhanced interplanetary ULF fluctuations. We have also found evidence that the enhancement in convection and plasma sheet pressure associated with strong interplanetary fluctuations may lead to a dramatic increase in substorm occurrence under northward interplanetary magnetic field conditions. More detailed testing of the above results is needed. However, if corroborated, it would indicate that interplanetary ULF fluctuations have a substantial effect on global convection and are an important contributor to the large‐scale transfer of solar wind energy to the magnetosphere‐ionosphere system, to plasma sheet structure and dynamics, and to the occurrence of disturbances such as substorms.

Evidence that solar wind fluctuations substantially affect the strength of dayside ionospheric convection

Ionospheric convection is occasionally observed to be substantially enhanced even when the interplanetary magnetic field (IMF) is not strongly southward and the IMF By is not large. Such enhanced convection flows tend to exhibit large oscillations with ∼10–30 min periodicity. We have considered the solar wind characteristics that lead to these oscillatory convection enhancements. We have used an extensive set of Sondrestrom radar observations of ionospheric convection within the dayside polar cap. We find that IMF ULF power is closely associated with the strength of dayside convection. Convection flows during periods of large north–south IMF fluctuations are observed to be as strong as for steady and large southward IMF periods. Enhanced convection is also observed during northward IMF intervals when the interplanetary magnetic field exhibits high ULF power. We find that ULF power enhances the convection strength, independent of an observed direct effect from the solar wind speed. These observations thus suggest that IMF ULF fluctuations can significantly influence ionospheric convection. Therefore, in addition to the well‐established contributions from the direction and magnitude of the IMF and the solar wind dynamic pressure, ULF fluctuations may also be an important contributor to coupling of the solar wind to the magnetosphere‐ionosphere system. We speculate that resonance between IMF fluctuations and natural magnetospheric oscillation frequencies or magnetopause boundary oscillations might be responsible for the connection between ionospheric convection and IMF ULF power. We have also found evidence for a connection between the ULF power in the solar wind dynamic pressure and the strength of convection.

Multi-spacecraft measurement of anisotropic power levels and scaling in solar wind turbulence

Abstract. Measurements by the four Cluster spacecraft in the solar wind are used to determine quantitatively the field-aligned anisotropy of magnetohydrodynamic inertial range turbulence power levels and spectral indexes. We find, using time-lagged second order structure functions, that the spectral index is near 2 around the field-parallel direction, which is consistent with a "critical balance" turbulent cascade. Solar wind fluctuations are found to be anisotropic with power mainly in wavevectors perpendicular to the mean field, where the spectral index is around 5/3.

Quantitative estimates of the slab and 2‐D power in solar wind turbulence using multispacecraft data

A quantitative estimate of the field‐aligned anisotropy of magnetohydrodynamic inertial range turbulence is obtained by comparing multispacecraft data with a fully three‐dimensional turbulent magnetic field numerical model. The simulated turbulence is a superposition of slab fluctuations parallel to the mean field and 2‐D fluctuations perpendicular to it. This model is fitted to spatial autocorrelation functions, computed from measurements made by the four Cluster spacecraft in the solar wind, by varying the slab‐2‐D fraction. The mean power in the 2‐D fluctuations, 81 ± 3%, is consistent with solar wind fluctuations being anisotropic with energy mainly in wave vectors perpendicular to the large‐scale mean magnetic field. Eight intervals of multipoint magnetic field data are analyzed, and a large degree of variation in the anisotropy estimates about the mean is observed. The origin of this variability is unclear. However, variations in the anisotropy estimates are correlated between different field components.

DEPENDENCE OF SOLAR-WIND POWER SPECTRA ON THE DIRECTION OF THE LOCAL MEAN MAGNETIC FIELD

(Abridged) Wavelet analysis can be used to measure the power spectrum of solar wind fluctuations along a line in any direction with respect to the local mean magnetic field. This technique is applied to study solar wind turbulence in high-speed streams in the ecliptic plane near solar minimum using magnetic field measurements with a cadence of eight vectors per second. The analysis of nine high-speed streams shows that the reduced spectrum of magnetic field fluctuations (trace power) is approximately azimuthally symmetric about B_0 in both the inertial range and dissipation range; in the inertial range the spectra are characterized by a power-law exponent that changes continuously from 1.6 \pm 0.1 in the direction perpendicular to the mean field to 2.0 \pm 0.1 in the direction parallel to the mean field. The large uncertainties suggest that the perpendicular power-law indices 3/2 and 5/3 are both consistent with the data. The results are similar to those found by Horbury et al. (2008) at high heliographic latitudes.

Velocity fluctuations in polar solar wind: a comparison between different solar cycles

Abstract. The polar solar wind is a fast, tenuous and steady flow that, with the exception of a relatively short phase around the Sun's activity maximum, fills the high-latitude heliosphere. The polar wind properties have been extensively investigated by Ulysses, the first spacecraft able to perform in-situ measurements in the high-latitude heliosphere. The out-of-ecliptic phases of Ulysses cover about seventeen years. This makes possible to study heliospheric properties at high latitudes in different solar cycles. In the present investigation we focus on hourly- to daily-scale fluctuations of the polar wind velocity. Though the polar wind is a quite uniform flow, fluctuations in its velocity do not appear negligible. A simple way to characterize wind velocity variations is that of performing a multi-scale statistical analysis of the wind velocity differences. Our analysis is based on the computation of velocity differences at different time lags and the evaluation of statistical quantities (mean, standard deviation, skewness, and kurtosis) for the different ensembles. The results clearly show that, though differences exist in the three-dimensional structure of the heliosphere between the investigated solar cycles, the velocity fluctuations in the core of polar coronal holes exhibit essentially unchanged statistical properties.

Correlation length of large‐scale solar wind velocity fluctuations measured tangent to the Earth's orbit: First results from Stereo

Proton velocity data from identical plasma instruments on board NASA's two Stereo spacecraft are used to investigate the correlation length of large‐scale radial velocity fluctuations in the solar wind in the direction tangent to the Earth's orbit. The data cover the period from February 2007 through August 2007, near solar minimum in 2007. Estimates of the spatial correlation function are based on simultaneous 10 min averages of the solar wind speed at the two spacecraft that are binned according to the distance between the spacecraft. The results for the correlation function are noisy due, in part, to the modest number of data points in each bin. Linear least squares fits are used to extract the correlation length from noisy data. It is found that the transverse correlation length (along the Y‐direction) is 0.25 ± 0.02 AU which is approximately equal to the longitudinal correlation length (along the X‐direction) 0.28 ± 0.05 AU measured using the same data. Assuming that the autocorrelation time of the velocity signal at one spacecraft is determined by the rate of rigid rotation of solar wind source structures past the Earth‐Sun line, the observed equivalence of longitudinal and transverse integral scales can be understood as a rotation effect. It must be emphasized that the correlation length studied here pertains to large‐scale solar wind fluctuations associated with the large‐scale flow, not to the smaller‐scale fluctuations associated with solar wind turbulence.

The effect of upstream turbulence and its anisotropy on the efficiency of solar wind – magnetosphere coupling

Abstract. The importance of space weather and its forecasting is growing as interest in studying geoeffective processes in the Sun – solar wind – magnetosphere – ionosphere coupled system is increasing. This paper introduces the proper selection criteria for solar wind magnetic turbulence events during duskward electric field and southward Bz driven geomagnetic storms. Two measures for the strength of solar wind fluctuations were investigated: the standard deviations of magnetic field components and a proxy for the so-called Shebalin anisotropy angles. These measures were compared to the strength of geomagnetic storms obtained from a SYM-H index time series. We found a weak correlation between standard deviation of interplanetary magnetic field GSM component Bz and SYM-H index, and a strong correlation between Shebalin anisotropy angle and the SYM-H index, which can be the result of an increase of probability of magnetic reconnection in fluctuating magnetic fields.