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On the Origin of the Strong Intermittent Nature of Interplanetary Magnetic Field
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We briefly discuss the strong intermittent nature of the interplanetary magnetic field, observing that similarity between
its statistical properties and the passive scalar ones exists, and may arise from different dynamical mechanisms.
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... The SW has a complex magnetic structure from large to small scales that includes dynamically interacting spaghetti-like magnetic flux tubes separated by current sheets (CSs) [14,62,15], magnetic islands or plasmoids near the heliospheric current sheet (HCS) [72,73,71,41], compressive and Alfvénic coherent structures [102,101], plasma waves, and shocks (see Sec. 2.3, 2.7). These structures show random, intermittent features with power-law spectral distributions [119,120,19]. ...
Turbulence is ubiquitous in space plasmas. It is one of the most important subjects in heliospheric physics, as it plays a fundamental role in the solar wind - local interstellar medium interaction and in controlling energetic particle transport and acceleration processes. Understanding the properties of turbulence in various regions of the heliosphere with vastly different conditions can lead to answers to many unsolved questions opened up by observations of the magnetic field, plasma, pickup ions, energetic particles, radio and UV emissions, and so on. Several space missions have helped us gain preliminary knowledge on turbulence in the outer heliosphere and the very local interstellar medium. Among the past few missions, the Voyagers have paved the way for such investigations. This paper summarizes the open challenges and voices our support for the development of future missions dedicated to the study of turbulence throughout the heliosphere and beyond.
... The SW has a complex magnetic structure from large to small scales that includes dynamically interacting spaghetti-like magnetic flux tubes separated by current sheets (CSs) (Borovsky, 2008;Greco et al., 2009;Borovsky, 2020), magnetic islands or plasmoids near the heliospheric current sheet (HCS) (Khabarova et al., 2015;Khabarova et al., 2016;Khabarova et al., 2021;Eriksson et al., 2022), compressive and Alfvénic coherent structures (Perrone et al., 2016;Perrone et al., 2017), plasma waves, and shocks (see Sections 2.3, 2.7). These structures show random, intermittent features with power-law spectral distributions (Sorriso-Valvo et al., 2005;Sorriso-Valvo et al., 2017;Bruno, 2019). The statistical nature has led to an apparent dichotomy between "random" turbulence and more "ordered" features possibly not manifestations of turbulence, thus posing challenging questions about their origin, interplay and roles in the SW heating and energetic particle transport and acceleration. ...
Turbulence is ubiquitous in space plasmas. It is one of the most important subjects in heliospheric physics, as it plays a fundamental role in the solar wind—local interstellar medium interaction and in controlling energetic particle transport and acceleration processes. Understanding the properties of turbulence in various regions of the heliosphere with vastly different conditions can lead to answers to many unsolved questions opened up by observations of the magnetic field, plasma, pickup ions, energetic particles, radio and UV emissions, and so on. Several space missions have helped us gain preliminary knowledge on turbulence in the outer heliosphere and the very local interstellar medium. Among the past few missions, the Voyagers have paved the way for such investigations. This paper summarizes the open challenges and voices our support for the development of future missions dedicated to the study of turbulence throughout the heliosphere and beyond.
... Turbulence in the solar wind describes the fluctuation of solar wind parameters over different spatial and temporal scales [33]. Intermittency, manifesting inhomogeneity in the energy transfer between scales [34][35][36][37], is a typical feature of turbulence [38]. If the probability distribution function (PDF) of the fluctuations for a given solar wind parameter is not Gaussian at different scales and increasingly departs from a normalized distribution when the timescale gets smaller, it reveals the presence of intermittency [36]. ...
Solar wind dynamic pressure pulses (DPPs) are small-scale plasma structures with abrupt and large-amplitude plasma dynamic pressure changes on timescales of seconds to several minutes. Overwhelming majority of DPP events (around 79.13%) reside in large-scale solar wind transients, i.e., coronal mass ejections, stream interaction regions, and complex ejecta. In this study, the intermittency, which is a typical feature of solar wind turbulence, is determined and compared during the time intervals in the undisturbed solar wind and in large-scale solar wind transients with clustered DPP events, respectively, as well as in the undisturbed solar wind without DPPs. The probability distribution functions (PDFs) of the fluctuations of proton density increments normalized to the standard deviation at different time lags in the three types of distinct regions are calculated. The PDFs in the undisturbed solar wind without DPPs are near-Gaussian distributions. However, the PDFs in the solar wind with clustered DPPs are obviously non-Gaussian distributions, and the intermittency is much stronger in the large-scale solar wind transients than that in the undisturbed solar wind. The major components of the DPPs are tangential discontinuities (TDs) and rotational discontinuities (RDs), which are suggested to be formed by compressive magnetohydrodynamic (MHD) turbulence. There are far more TD-type DPPs than RD-type DPPs both in the undisturbed solar wind and large-scale solar wind transients. The results imply that the formation of solar wind DPPs could be associated with solar wind turbulence, and much stronger intermittency may be responsible for the high occurrence rate of DPPs in the large-scale solar wind transients.
... Meanwhile, the solar wind fluctuations mix with non-propagating coherent structures, which are often observed and are an important ingredient of the dynamics and dissipation of the solar wind fluctuations (Tu, Marsch & Thieme 1989;Bruno & Bavassano 1991;Tu & Marsch 1993;Bruno et al. 2001;Sorriso-Valvo, Carbone & Bruno 2005;Greco et al. 2009;Wang et al. 2013;Chen et al. 2014;Yang et al. 2015;Perrone et al. 2016Perrone et al. , 2017Roberts et al. 2017;Yang et al. 2017bYang et al. ,c, 2018aWang et al. 2018;Roberts, Narita & Escoubet 2018). In addition, non-linearly interacting fluctuations are expected to generate coherent structures (Matthaeus et al. 2015), which are interpreted as current sheets, discontinuities, shocks, magnetic solitons, magnetic holes, Alfvén vortex, and so on. ...
Structures and propagating waves are often observed in solar wind turbulence. Their origins and features remain to be uncovered. In this work, we use 3D driven, compressible MHD turbulence simulations to investigate the global signatures of the driven fluctuations in whole spatial and temporal domain. With four-dimensional spatial-temporal (x, y, z, t) Fourier transformations implemented, we have identified two distinct main populations: waves, which satisfy the $\omega -\boldsymbol {k}$ dispersion relations and are propagating; and structures, which satisfy the polarization relations but non-propagating (ω = 0). Whereas the overall turbulent energy spectrum is still consistent with k−5/3, the contributions from waves and structures show very different behaviour in $\boldsymbol {k}$ space, with structures dominating at small k but waves becomes comparable to structures at large k. Overall, the fluctuations in the directions perpendicular to the large-scale mean field $\boldsymbol {B_0}$ are a manifestation of structures, while along the parallel direction, the fluctuations are dominated by waves. Also, a significant portion of the incompressible structures are the Alfvénic nature, and with imbalanced increased, the waves predominantly propagate in one direction and nearly perpendicular to $\boldsymbol {B_0}$. Differentiating the relative contributions from waves and structures could have important implications for understanding the non-linear cascade processes in the inertial range as well as particle-fluctuation interactions at small scales.
... Burlaga, 1991;Feynman and Ruzmaikin, 1994;Tu, 1994, 1997;Pagel and Balogh, 2001;Yordanova et al., 2009). Intermittency describes the inhomogeneity in the energy transfer between scales and is manifested as a lack of self-similarity in fluctuation distributions between scales (see, e.g., reviews by Horbury et al., 2005;Sorriso-Valvo et al., 2005;Bruno, 2019;Verscharen et al., 2019). In the solar wind, it can arise from coherent structures, such as current sheets and discontinuities. ...
In this work, we investigate magnetic field fluctuations in three coronal mass ejection (CME)-driven sheath regions at 1 AU, with their speeds ranging from slow to fast. The data set we use consists primarily of high-resolution (0.092 s) magnetic field measurements from the Wind spacecraft. We analyse magnetic field fluctuation amplitudes, com-pressibility, and spectral properties of fluctuations. We also analyse intermittency using various approaches; we apply the partial variance of increments (PVIs) method, investigate probability distribution functions of fluctuations, including their skewness and kurtosis, and perform a structure function analysis. Our analysis is conducted separately for three different subregions within the sheath and one in the solar wind ahead of it, each 1 h in duration. We find that, for all cases, the transition from the solar wind ahead to the sheath generates new fluctuations, and the intermittency and com-pressibility increase, while the region closest to the ejecta leading edge resembled the solar wind ahead. The spectral indices exhibit large variability in different parts of the sheath but are typically steeper than Kolmogorov's in the inertial range. The structure function analysis produced generally the best fit with the extended p model, suggesting that turbulence is not fully developed in CME sheaths near Earth's orbit. Both Kraichnan-Iroshinikov and Kolmogorov's forms yielded high intermittency but different spectral slopes, thus questioning how well these models can describe turbulence in sheaths. At the smallest timescales investigated, the spectral indices indicate shallower than expected slopes in the dissipation range (between −2 and −2.5), suggesting that, in CME-driven sheaths at 1 AU, the energy cascade from larger to smaller scales could still be ongoing through the ion scale. Many turbulent properties of sheaths (e.g. spectral indices and compressibility) resemble those of the slow wind rather than the fast. They are also partly similar to properties reported in the terrestrial magnetosheath, in particular regarding their intermittency, compressibility, and absence of Kolmogorov's type turbulence. Our study also reveals that turbulent properties can vary considerably within the sheath. This was particularly the case for the fast sheath behind the strong and quasi-parallel shock, including a small, coherent structure embedded close to its midpoint. Our results support the view of the complex formation of the sheath and different physical mechanisms playing a role in generating fluctuations in them.
... Several studies have shown that fluctuations in the solar wind are typically strongly intermittent (e.g., Burlaga, 1991;Feynman and Ruzmaikin, 1994;Tu, 1994, 1997;Pagel and Balogh, 2001;Yordanova et al., 2009). Intermittency describes inhomogenity in the energy transfer between scales, and is manifested as a lack of self-similarity in fluctuation distributions between scales (see, e.g., reviews by Horbury et al., 2005;Sorriso-Valvo et al., 2005;Bruno, 2019;Verscharen et al., 2019). 15 In the solar wind, it can arise from coherent structures such as current sheets and discontinuities. ...
Abstract. In this work, we investigate the magnetic field fluctuations in three coronal mass ejection (CME)-driven sheath regions at 1 AU with their speeds ranging from slow to fast. The data set we use consists primarily of high resolution (0.092 s) magnetic field measurements from the Wind spacecraft. We analyse magnetic field fluctuation amplitudes and fluctuation amplitudes normalised to the mean magnetic field, compressibility, and spectral properties of fluctuations. We also analyse intermittency using various approaches: we apply the partial variance of increments (PVI) method, investigate probability distribution functions of fluctuations, including their skewness and kurtosis, and perform a structure function analysis. Our analysis is conducted separately for three different subregions in the sheath and in the solar wind ahead of it, each 1 hr in duration. We find that, for all cases, the transition from the solar wind ahead to the sheath generates new fluctuations and the intermittency and compressibility increase, while the region closest to the ejecta leading edge resembled the solar wind ahead. The spectral indices exhibit large variability in different parts of the sheath, but are typically steeper than Kolmogorov's in the inertial range. The structure function analysis produced generally much better fit with the extended p -model (Kraichnan's form) than with the standard version, implying that turbulence is not fully developed in CME sheaths near Earth's orbit. The p -values obtained ( p ~0.8–0.9) also suggest relatively high intermittency. At the smallest timescales investigated, the spectral indices indicate relatively shallow slopes (between −2 and −2.5), suggesting that in CME-driven sheaths at 1 AU the energy cascade from larger to smaller scales could still be ongoing through the ion scale. Regarding many properties (e.g., spectral indices and compressibility) turbulent properties in sheaths, regardless their speed, resemble that of the slow wind, rather fast wind. They are also partly similar to properties reported in terrestrial magnetosheath, in particular regarding their intermittency, compressibility and absence of Kolmogorov's type turbulence. Our study also reveals that turbulent properties can vary considerably within the sheath. This was in particular the case for the fast sheath behind the strong and quasi-parallel shock, including a small, coherent structure embedded close to its midpoint. Our results support the view of the complex formation of the sheath and different physical mechanisms playing a role in generating fluctuations in them.
... Intermittency of velocity, magnetic field, and density has been widely reported in the solar wind turbulence since the 1990s (e.g., Burlaga 1991Burlaga , 1992Marsch & Liu 1993;Marsch & Tu 1994Tu et al. 1996;Horbury et al. 1997;Bruno et al. 1999Bruno et al. , 2001Bruno et al. , 2003Sorriso-Valvo et al. 1999, 2005Salem et al. 2009;Osman et al. 2012;Wu et al. 2013;Chen et al. 2014;Wang et al. 2014;Pei et al. 2016). To characterize the intermittency, the multifractal scaling in the structure functions are suggested. ...
Multi-order structure functions in the solar wind are reported to display a monofractal scaling when sampled parallel to the local magnetic field and a multifractal scaling when measured perpendicularly. Whether and to what extent will the scaling anisotropy be weakened by the enhancement of turbulence amplitude relative to the background magnetic strength? In this study, based on two runs of the magnetohydrodynamic (MHD) turbulence simulation with different relative levels of turbulence amplitude, we investigate and compare the scaling of multi-order magnetic structure functions and magnetic probability distribution functions (PDFs) as well as their dependence on the direction of the local field. The numerical results show that for the case of large-amplitude MHD turbulence, the multi-order structure functions display a multifractal scaling at all angles to the local magnetic field, with PDFs deviating significantly from the Gaussian distribution and a flatness larger than 3 at all angles. In contrast, for the case of small-amplitude MHD turbulence, the multi-order structure functions and PDFs have different features in the quasi-parallel and quasi-perpendicular directions: a monofractal scaling and Gaussian-like distribution in the former, and a conversion of a monofractal scaling and Gaussian-like distribution into a multifractal scaling and non-Gaussian tail distribution in the latter. These results hint that when intermittencies are abundant and intense, the multifractal scaling in the structure functions can appear even if it is in the quasi-parallel direction; otherwise, the monofractal scaling in the structure functions remains even if it is in the quasi-perpendicular direction.
... Turbulence is a potentially major factor in magnetospheric dynamics and in solar wind-magnetospheric couplings (Bruno et al., 2004). Recent theoretical results have shown that parametric decay of Alfven waves could be a source of coherent struc-tures, like shocks and current sheets, which are created when the instability is active (Valvo et al., 2005). Some of these fluctuations in the solar wind are remnants of those in the Sun's corona while others get affected through an active turbulent cascade of energy between scales . ...
The Aditya-L1 is first Indian solar mission scheduled to be placed in a halo orbit around the first Lagrangian point (L1) of Sun-Earth system in the year 2018-19. The approved scientific payloads onboard Aditya-L1 spacecraft includes a Fluxgate Digital Magnetometer (FGM) to measure the local magnetic field which is necessary to supplement the outcome of other scientific experiments onboard. The in-situ vector magnetic field data at L1 is essential for better understanding of the data provided by the particle and plasma analysis experiments, onboard Aditya-L1 mission. Also, the dynamics of Coronal Mass Ejections (CMEs) can be better understood with the help of in-situ magnetic field data at the L1 point region. This data will also serve as crucial input for the short lead-time space weather forecasting models.
The proposed FGM is a dual range magnetic sensor on a 6 m long boom mounted on the Sun viewing panel deck and configured to deploy along the negative roll direction of the spacecraft. Two sets of sensors (tri-axial each) are proposed to be mounted, one at the tip of boom (6 m from the spacecraft) and other, midway (3 m from the spacecraft). The main science objective of this experiment is to measure the magnitude and nature of the interplanetary magnetic field (IMF) locally and to study the disturbed magnetic conditions and extreme solar events by detecting the CME from Sun as a transient event. The proposed secondary science objectives are to study the impact of interplanetary structures and shock solar wind interaction on geo-space environment and to detect low frequency plasma waves emanating from the solar corona at L1 point. This will provide a better understanding on how the Sun affects interplanetary space.
In this paper, we shall give the main scientific objectives of the magnetic field experiment and brief technical details of the FGM onboard Aditya-1 spacecraft.
... Fluctuations in the solar wind are not scale-invariant, and their probability distribution functions (pdfs) change significantly from large scales to small scales. With decreasing scale, the pdf is observed to deviate from a Gaussian distribution, and to grow an extended tail on both wings, thus forming a non-Gaussian distribution, which is attributed to intermittency (Burlaga 1991;Marsch & Liu 1993;Marsch & Tu 1994;Tu et al. 1996;Horbury et al. 1997;Bruno et al. 1999Bruno et al. , 2001Bruno et al. , 2003Sorriso-Valvo et al. 2005;Salem et al. 2009;Osman et al. 2012;Wu et al. 2013;Wang et al. 2014;Pei et al. 2016). To identify and study intermittency, Bruno et al. (2001) proposed a method called the local intermittency measure, which is based on the normalized squared modulus of wavelet coefficients. ...
Solar wind fluctuations reveal the ubiquity of intermittency, which is believed to affect the spectral signatures of turbulence. In this work, based on simulation of driven compressible MHD turbulence, we apply the wavelet technique to the magnetic field and velocity to identify intermittency, and we analyze the influence of the intermittency on the quasi-perpendicular scaling in the inertial range. The numerical results show that the original magnetic and velocity fluctuations are anisotropic, and have a power anisotropy with a spectral index approaching the Iroshnikov–Kraichnan scaling in the direction quasi-perpendicular to the local mean magnetic field. As in observations of the solar wind fluctuations, as the scale decreases in the simulation, the calculated probability distribution functions (pdfs) of the wavelet coefficients become extended on both tails of the non-Gaussian distribution, with a rapid increase in flatness. After intermittency has been removed from the driven turbulence, at each scale, the pdfs approach a Gaussian distribution, with the flatness being ~3. Meanwhile, the quasi-perpendicular scaling for both fluctuations becomes steeper and close to a Kolmogorov scaling, which may be a result of the stronger intermittency in the quasi-perpendicular direction and at the smaller scales. These results suggest that there is intermittency superposed on the "background" turbulence that seems to have the Kolmogorov scaling, whereby the overall slope is getting flatter with the involvement of intermittency.
In this review we will focus on a topic of fundamental importance for
both plasma physics and astrophysics, namely the occurrence of
large-amplitude low-frequency fluctuations of the fields that describe
the plasma state. This subject will be treated within the context of the
expanding solar wind and the most meaningful advances in this research
field will be reported emphasizing the results obtained in the past
decade or so. As a matter of fact, Ulysses' high latitude observations
and new numerical approaches to the problem, based on the dynamics of
complex systems, brought new important insights which helped to better
understand how turbulent fluctuations behave in the solar wind. In
particular, numerical simulations within the realm of
magnetohydrodynamic (MHD) turbulence theory unraveled what kind of
physical mechanisms are at the basis of turbulence generation and energy
transfer across the spectral domain of the fluctuations. In other words,
the advances reached in these past years in the investigation of solar
wind turbulence now offer a rather complete picture of the
phenomenological aspect of the problem to be tentatively presented in a
rather organic way.
This textbook presents a modern account of turbulence. The
state-of-the-art is put into historical perspective five centuries after
the first studies of Leonardo and half a century after the first attempt
by A. N. Kolmogorov to predict the properties of flow at very high
Reynolds numbers. Such "fully developed turbulence" is ubiquitous in
both cosmical and natural environments, in engineering applications and
in everyday life.
Magnetic field and plasma data of Helios 1 and 2 between 0.3 and 1 AU have been used to investigate the Alfvenic character of the solar wind fluctuations with periods above 1 hour. Clear evidence for the existence of very long period Alfvenic waves, up to about 15 hours (in the spacecraft frame) close to the sun, has been obtained. The observations at different heliocentric distances suggest that the longest wavelengths are removed from the Alfvenic regime leaving the sun. 19 references.
Plasma and magnetic field measurements by Ulysses are used to investigate the radial evolution of hourly-scale Alfvénic fluctuations in the polar wind. The data span from 1.4 to 4.3 AU in heliocentric distance. Different radial regimes at different distances emerge. Inside about 2.5 AU the large outward traveling fluctuations decrease faster, in terms of energy per unit mass, than the small inward ones. This is in agreement with previous low-latitude observations inside 1 AU within the trailing edge of fast streams. As a result of this different gradient the ratio of inward to outward fluctuation energy rises to about 0.5 near 2.5 AU. Beyond this distance the radial gradient of the inward fluctuations becomes increasingly steeper, while that of the outward ones does not vary appreciably. A state is quickly reached where both populations decline at almost the same rate. These results on the behavior of outward and inward Alfvénic fluctuations are new and represent a constraint for models of turbulence evolution in steadily expanding flows like the polar wind. Finally, an extrapolation to regions near the Sun would suggest that Alfvénic fluctuations at hourly scale should not play a relevant role in solar wind heating and acceleration. Obviously, this last conclusion may be invalidated by non-WKB effects and by compressive and dissipative processess close to the Sun.
MHD fluctuations in the solar wind are reanalyzed here using Elsanic domain are more transverse in the perihelion, they radially evolve toward an equipartition of the energy in the three spatial dimensions at larger heliocentric distances. The e- spectrum does not change a lot at low frequencies but strongly steepens beyond about 0.0002 Hz. The spectra results on e- suggest that isotropic turbulence with an index of -5/3 is probably the ultimate state toward which solar wind MHD fluctuations evolve.
An assessment of local isotropy and universality in high-Reynolds-number
turbulent flows is presented. The emphasis is on the behavior of passive
scalar fields advected by turbulence, but a brief review of the relevant
facts is given for the turbulent motion itself. Experiments suggest that
local isotropy is not a natural concept for scalars in shear flows,
except, perhaps, at such extreme Reynolds numbers that are of no
practical relevance on earth. Yet some type of scaling exists even at
moderate Reynolds numbers. The relation between these two observations
is a theme of this paper.
This book provides a self-contained introduction to magnetohydrodynamics (MHD), with emphasis on nonlinear processes. The book outlines the conventional aspects of MHD theory, magnetostatic equilibrium and linear stability theory. It concentrates on nonlinear theory, starting with the evolution and saturation of individual ideal and resistive instabilities, continuing with a detailed analysis of magnetic reconnection and concluding with a study of the most complex nonlinear behavior, that of MHD turbulence. The last chapters describe three important applications of the theory: disruptive processes in tokomaks, MHD effects in the reversed field pinch, and solar flares.