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

[1] This review surveys the basic theories, observational methods, satellite algorithms, and land surface models for terrestrial evapotranspiration, E (or λE, i.e., latent heat flux), including a long-term variability and trends perspective. The basic theories used to estimate E are the Monin-Obukhov similarity theory (MOST), the Bowen ratio method, and the Penman-Monteith equation. The latter two theoretical expressions combine MOST with surface energy balance. Estimates of E can differ substantially between these three approaches because of their use of different input data. Surface and satellite-based measurement systems can provide accurate estimates of diurnal, daily, and annual variability of E. But their estimation of longer time variability is largely not established. A reasonable estimate of E as a global mean can be obtained from a surface water budget method, but its regional distribution is still rather uncertain. Current land surface models provide widely different ratios of the transpiration by vegetation to total E. This source of uncertainty therefore limits the capability of models to provide the sensitivities of E to precipitation deficits and land cover change.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2011RG000373

A review of global terrestrial evapotranspiration: observation, modeling, climatology, and climatic variability

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

/pdf/the-solar-probe-plus-mission-humanity-s-first-visit-to-our-e2fb5pur3z.pdf

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

A Review of Global Terrestrial Evapotranspiration: Observation, Modeling, Climatology, and Climatic Variability

MHD Structures, Waves and Turbulence in the Solar Wind: Observations and Theories

The radial evolution of MHD turbulence in the inner heliosphere, as observed by the Helios probes between 0.3 and 1 AU, is reanalyzed and newly evaluated by means of Elsasser variables. It is found that the spectra of the energy of the outward (e+) and inward (e−) oriented fluctuations, of the cross helicity and cross correlation and of related dimensionless ratios, like the ones named after Alfven or Elsasser, all undergo considerable radial evolution with an overall trend for the spectra to steepen. These evolutionary tendencies are established both by individual typical case studies for a recurrent high-speed stream and neighboring low-speed flows as well as by a statistical analysis of many sample spectra pertaining to various heliocentric distances. In particular, the e+ spectrum considerably steepens in the low-frequency part. Whereas the fluctuations in the Alfvenic 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 2×10−4Hz. The spectral results on e− suggest that isotropic turbulence with an index of −5/3 is probably the ultimate state toward which solar wind MHD fluctuations seem to evolve. The radial evolution of the cross correlation eR is also analyzed.

On the radial evolution of MHD turbulence in the inner heliosphere

Magnetic field and plasma data collected by the Helios spacecraft between 0.3 and 1 AU near the activity minimum of solar cycle 21 have been analyzed to establish spectral characteristics of compressive fluctuations in the inner solar wind and their relation to the morphology of the plasma flow and magnetic field. The compressive turbulence level is found to be closely related to the stream structure or latitudinal location with respect to the heliospheric current sheet. Compressive turbulence in low-speed flows is more fully developed and intense. The spectra are radially invariant and come close to a −5/3 spectral law. In contrast, fast stream turbulence becomes increasingly compressive, in terms of radially growing amplitudes of δn/n and δB/B, with increasing heliocentric distance. The spectra reveal a flatter high-frequency part, which gradually seems to get straightened out at large solar distances. Single case studies, as well as averaged spectra and their spectral features, are presented and discussed in the context of the theoretical turbulence literature.

Spectral and spatial evolution of compressible turbulence in the inner solar wind

The probability distributions of field differences x( )=x(t+ )-x(t), where the variable x(t) may denote any solar wind scalar field or vector field component at time t, have been calculated from time series of Helios data obtained in 1976 at heliocentric distances near 0.3 AU. It is found that for comparatively long time lag , ranging from a few hours to 1 day, the differences are normally distributed according to a Gaussian. For shorter time lags, of less than ten minutes, significant changes in shape are observed. The distributions are often spikier and narrower than the equivalent Gaussian distribution with the same standard deviation, and they are enhanced for large, reduced for intermediate and enhanced for very small values of x. This result is in accordance with fluid observations and numerical simulations. Hence statistical properties are dominated at small scale by large fluctuation amplitudes that are sparsely distributed, which is direct evidence for spatial intermittency of the fluctuations. This is in agreement with results from earlier analyses of the structure functions of x. The non-Gaussian features are differently developed for the various types of fluctuations. The relevance of these observations to the interpretation and understanding of the nature of solar wind magnetohydrodynamic (MHD) turbulence is pointed out, and contact is made with existing theoretical concepts of intermittency in fluid turbulence.

/pdf/non-gaussian-probability-distributions-of-solar-wind-3bn4npapds.pdf

Chia-Ying Tu

Papers

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

MHD Structures, Waves and Turbulence in the Solar Wind: Observations and Theories

On the radial evolution of MHD turbulence in the inner heliosphere

Spectral and spatial evolution of compressible turbulence in the inner solar wind

Non-Gaussian probability distributions of solar wind fluctuations