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.

/pdf/the-solar-wind-as-a-turbulence-laboratory-4mad4q4xdr.pdf

The Solar Wind as a Turbulence Laboratory

The EUV Imaging Spectrometer (EIS) on Hinode will observe solar corona and upper transition region emission lines in the wavelength ranges 170 – 210 A and 250 – 290 A. The line centroid positions and profile widths will allow plasma velocities and turbulent or non-thermal line broadenings to be measured. We will derive local plasma temperatures and densities from the line intensities. The spectra will allow accurate determination of differential emission measure and element abundances within a variety of corona and transition region structures. These powerful spectroscopic diagnostics will allow identification and characterization of magnetic reconnection and wave propagation processes in the upper solar atmosphere. We will also directly study the detailed evolution and heating of coronal loops. The EIS instrument incorporates a unique two element, normal incidence design. The optics are coated with optimized multilayer coatings. We have selected highly efficient, backside-illuminated, thinned CCDs. These design features result in an instrument that has significantly greater effective area than previous orbiting EUV spectrographs with typical active region 2 – 5 s exposure times in the brightest lines. EIS can scan a field of 6×8.5 arc min with spatial and velocity scales of 1 arc sec and 25 km s−1 per pixel. The instrument design, its absolute calibration, and performance are described in detail in this paper. EIS will be used along with the Solar Optical Telescope (SOT) and the X-ray Telescope (XRT) for a wide range of studies of the solar atmosphere.

http://www.rxollc.com/windt/papers/2007_SolPhys_243_19.pdf

The EUV Imaging Spectrometer for Hinode

Alfven waves have been invoked as a possible mechanism for the heating of the Sun's outer atmosphere, or corona, to millions of degrees and for the acceleration of the solar wind to hundreds of kilometers per second. However, Alfven waves of sufficient strength have not been unambiguously observed in the solar atmosphere. We used images of high temporal and spatial resolution obtained with the Solar Optical Telescope onboard the Japanese Hinode satellite to reveal that the chromosphere, the region sandwiched between the solar surface and the corona, is permeated by Alfven waves with strong amplitudes on the order of 10 to 25 kilometers per second and periods of 100 to 500 seconds. Estimates of the energy flux carried by these waves and comparisons with advanced radiative magnetohydrodynamic simulations indicate that such Alfven waves are energetic enough to accelerate the solar wind and possibly to heat the quiet corona.

https://science.sciencemag.org/content/sci/318/5856/1574.full.pdf

Chromospheric alfvenic waves strong enough to power the solar wind.

Solar Probe Plus (SPP) will be the first spacecraft to fly into the low solar corona. SPP’s main science goal is to determine the structure and dynamics of the Sun’s coronal magnetic field, understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate energetic particles. Understanding these fundamental phenomena has been a top-priority science goal for over five decades, dating back to the 1958 Simpson Committee Report. The scale and concept of such a mission has been revised at intervals since that time, yet the core has always been a close encounter with the Sun. The mission design and the technology and engineering developments enable SPP to meet its science objectives to: (1) Trace the flow of energy that heats and accelerates the solar corona and solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles. The SPP mission was confirmed in March 2014 and is under development as a part of NASA’s Living with a Star (LWS) Program. SPP is scheduled for launch in mid-2018, and will perform 24 orbits over a 7-year nominal mission duration. Seven Venus gravity assists gradually reduce SPP’s perihelion from 35 solar radii (
 $R_{S}$
 ) for the first orbit to ${<}10~R_{S}$
 for the final three orbits. In this paper we present the science, mission concept and the baseline vehicle for SPP, and examine how the mission will address the key science questions

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The Solar Probe Plus Mission: Humanity’s First Visit to Our Star

A comprehensive overview is presented of recent observational and theoretical results on solar wind structures and fluctuations and magnetohydrodynamic waves and turbulence, with preference given to phenomena in the inner heliosphere. Emphasis is placed on the progress made in the past decade in the understanding of the nature and origin of especially small-scale, compressible and incompressible fluctuations. Turbulence models to describe the spatial transport and spectral transfer of the fluctuations in the inner heliosphere are discussed, and results from direct numerical simulations are dealt with. Intermittency of solar wind fluctuations and their statistical distributions are briefly investigated. Studies of the heating and acceleration effects of the turbulence on the background wind are critically surveyed. Finally, open questions concerning the origin, nature and evolution of the fluctuations are listed, and possible avenues and perspectives for future research are outlined.

MHD structures, waves and turbulence in the solar wind : observations and theories

We report the first detection of postflare loop oscillations seen in both Doppler shift and intensity. The observations were recorded in an Fe xix line by the SUMER spectrometer on SOHO in the corona about 70 min after anM-class flare on the solar limb. The oscillation has a period of about 17 min in both the Doppler velocity and the intensity, but their decay times are different (i.e., 37 min for the velocity and 21 min for the intensity). The fact that the velocity and the intensity oscillations have exactly a 1/4-period phase difference points to the existence of slow-mode standing waves in the oscillating loop. This interpretation is also supported by two other pieces of evidence: (1) the wave period and (2) the amplitude relationship between the intensity and velocity are as expected for a slow-mode standing wave.

/pdf/slow-mode-standing-waves-observed-by-sumer-in-hot-coronal-19b9sp54v3.pdf

Slow-mode standing waves observed by SUMER in hot coronal loops

[1] In this paper we analyze the temperature anisotropy of velocity distribution functions (VDFs) of protons measured by the Helios spacecraft in fast solar wind. We concentrate on data obtained during the primary mission, including the first perihelion passage, of Helios 2 in a distance range between 0.98 and 0.29 AU for the days 23 through 114 of the year 1976. The main goal is to provide solid statistical evidence on the relation between the anisotropy and the proton plasma beta, parameters that play a key role in the regulation of the shape of the core. It is believed to be formed by resonant interactions between ion cyclotron waves and protons, as described by the quasi-linear theory of pitch angle diffusion. In the analysis of the VDF, particular attention is paid to the symmetry axis, which can be determined by the directions of either the magnetic field, the proton heat flux, or the alpha-proton relative drift. We analyze in detail the core part of the proton VDF, carefully avoiding a possible influence of the proton beam component.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003JA010330

On the temperature anisotropy of the core part of the proton velocity distribution function in the solar wind

We study the fluctuations of density, magnetic and velocity fields in the frequency range from (1 day)−1 ≈ 1.2×10−5 Hz to (2.8 min)−1≈ 6×10−3 Hz, as measured by the primary Helios mission (118 days), at heliocentric distances ranging from 0.3 to 1 AU. We address the question of the existence of nonlinear cascades in the observed turbulence, possibly separate for the two “inward” and “outward” components, corresponding to opposite directions of propagation along the large-scale magnetic field. We consider energies per unit mass, not per unit volume, in order to work with variables which are not very sensitive to the heliocentric distance variations. We find that while the whole spectrum of total (kinetic plus magnetic) turbulent energy undergoes very large daily variations both in its amplitude and spectral shape the instantaneous spectrum follows a power law in the frequency range 10−4 to 6×10−3 Hz. We show that both the amplitude and the spectral index m depend on the proton temperature, in a monotonic way, so that a large temperature (thermal speed about 60 km/s) leads to a low level of turbulence with a steep, Kolmogorov-like spectrum (m ≈ −1.8), while a low temperature (thermal speed about 16 km/s) leads to a flatter spectrum (m ≈ −1.2) with a high level of turbulence. This relation is independent from heliocentric distance, at least between 0.3 and 1 AU. Decomposing the turbulent energy into two components, “outward” and “inward,” we find that the spectrum of the outward component also follows very closely the daily proton temperature variations, while the inward component's spectrum is less sensitive to the temperature but also varies with the relative level of rms proton density fluctuations. As a consequence, Alfvenic periods (in which energy is dominated by the outgoing component) occur mainly when density fluctuations are low and temperature is high, which does not contradict the classical view that they are found in the “trailing edges of high-speed streams” (Belcher and Davis, 1971). The existence of inertial ranges controlled by the level of density fluctuations is not completely new (see the numerical simulations of purely hydrodynamic turbulence by Pouquet and Passot (1987)), but the strong dependence of both turbulent energy level and spectral slope on temperature is a new, unexpected property of solar wind turbulence which remains to be explained.

On the origin of solar wind MHD turbulence: Helios data revisited

Magnetic field and plasma data, obtained by the Helios 1 and 2 spacecraft in the solar wind near 0.3 AU during the years 1975 to 1976, have been analyzed by calculating 12 kinds of spectra related to the Elsasser variables, δZ+ = δV + δVA, and δZ− = δV − δVA, where δV and δVA are the bulk velocity and Alfven velocity fluctuations, respectively. For small amplitude Alfven waves the fluctuation variable δZ+ simply relates to outward propagation and δZ− to an inward sense of propagation, if the ambient magnetic field B0 is directed inward. The frequency range analysed in this paper is 6×10−6 Hz to 6×10−3 Hz. It is found that (1) the autocorrelation length for δZ− is much larger than for δZ+ in both the high-speed and low-speed wind. (2) The power spectra of δZ−, especially in high-wind speed, are steeper in the low-frequency range and flatten in the high-frequency range. (3) In the low-frequency range, the power spectra for the components of δZ+ tend to be isotropic with respect to the three polarization directions, while the spectra of δZ− are dominated by the radial component. In the high-frequency domain, the spectra of both δZ+ and δZ− are dominated by the transverse component in high-speed wind and are more isotropic in low-speed wind. (4) The spectra related to the residual energy or the cross-correlation in low-speed flows have a power law with the slope near to −5/3. However, in high-speed flows the corresponding data are widely distributed in a cloud of points with an upper envelope near to the spectrum of δZ−. The origin of all these spectra and their importance for the solar wind physics have also been discussed. Several generation mechanisms are suggested as candidates. In the flat part of e− spectrum, the fluctuations may be generated by non-local (in wave number space) interactions with the low-frequency part of the e+ spectrum, or just by parametric decay of the high-frequency part of the e+ spectrum. The steep part of e− (f) may be related to small-scale stream tubes, or be influenced by pressure waves, nonlinear cascading, and the interaction with the outgoing Alfven waves.

Basic properties of solar wind MHD turbulence near 0.3 AU analyzed by means of Elsässer variables

UNLIKE plasma instruments used on previous space missions to Mars, the TAUS instrument on Phobos 2 was designed so that the energy per charge and angular spectra of three species of ions could be measured separately. These species were H+ and He2+ characteristic of the solar wind, and 'heavy ions' collected in one integral channel covering the mass per charge (M/q) range 3 to infinite, which we anticipated to find predominantly in the near-martian regime. In all spacecraft orbits around Mars we found a sharp boundary, separating the shocked solar wind from the martian magnetosphere which was characterized by the absence of solar-wind-like plasma. As the plasma inside the magnetosphere, and particularly in the tail, was dominated by heavy ions with number densities orders of magnitude higher than found in the solar wind, we assumed it was mainly of martian origin. Typically, heavy ions of low tailward flow velocity were seen near the boundary of the magnetotail, whereas high-speed tailward plasma flows of such ions were detected deeper inside the tail, a region not investigated before. Near the centre of the martian magnetotail a plasma regime, comparable to the terrestrial as well as the venusian1 plasma sheet, was detected, characterized by highly supersonic tailward streams of heavy ions. The flux of planetary ions leaving Mars through its magnetotail is tentatively estimated to be of the order of a few times 1025 s-1. Such loss rates would be significant for the dissipation of the martian atmosphere on cosmological timescales.

E. Marsch

Papers

Slow-mode standing waves observed by SUMER in hot coronal loops

On the temperature anisotropy of the core part of the proton velocity distribution function in the solar wind

On the origin of solar wind MHD turbulence: Helios data revisited

Basic properties of solar wind MHD turbulence near 0.3 AU analyzed by means of Elsässer variables

Ions of martian origin and plasma sheet in the martian magnetosphere: initial results of the TAUS experiment