Astrophysical magnetic fields and nonlinear dynamo theory

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.

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The Solar Wind as a Turbulence Laboratory

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

The question of what heats the solar corona remains one of the most important problems in astrophysics. Finding a definitive solution involves a number of challenging steps, beginning with an identification of the energy source and ending with a prediction of observable quantities that can be compared directly with actual observations. Critical intermediate steps include realistic modeling of both the energy release process (the conversion of magnetic stress energy or wave energy into heat) and the response of the plasma to the heating. A variety of difficult issues must be addressed: highly disparate spatial scales, physical connections between the corona and lower atmosphere, complex microphysics, and variability and dynamics. Nearly all of the coronal heating mechanisms that have been proposed produce heating that is impulsive from the perspective of elemental magnetic flux strands. It is this perspective that must be adopted to understand how the plasma responds and radiates. In our opinion, the most promising explanation offered so far is Parker's idea of nanoflares occurring in magnetic fields that become tangled by turbulent convection. Exciting new developments include the identification of the “secondary instability” as the likely mechanism of energy release and the demonstration that impulsive heating in sub-resolution strands can explain certain observed properties of coronal loops that are otherwise very difficult to understand. Whatever the detailed mechanism of energy release, it is clear that some form of magnetic reconnection must be occurring at significant altitudes in the corona (above the magnetic carpet), so that the tangling does not increase indefinitely. This article outlines the key elements of a comprehensive strategy for solving the coronal heating problem and warns of obstacles that must be overcome along the way.

On Solving the Coronal Heating Problem

Topside ionospheric instabilities of electrostatic ion acoustic and ion cyclotron waves to field aligned currents in single and multiion plasmas

Topside current instabilities

Polar wind, describing upward plasma expansion of topside polar ionosphere and acceleration of positive H and He ions

The polar wind

The hydrodynamic properties of a steadily expanding corona are explored for situations in which departures from spherically symmetric outflow are large, in the sense that the geometrical cross section of a given flow tube increases outward from the Sun faster than r
2 in some regions. Assuming polytropic flow, it is shown that in certain cases the flow may contain more than one critical point. We derive the criterion for determining which of these critical points is actually crossed by the transonic solution which begins at the Sun and extends continuously outward. Next, we apply the theory to geometries which exhibit rapid spreading of the flow tubes in the inner corona, followed by more-or-less radial divergence at large distances. This is believed to be the type of geometry found in coronal hole regions. The results show that, if this initial divergence is sufficiently large, the outflow becomes supersonic at a critical point encountered low in the corona in the region of high divergence, and it remains supersonic at all greater heights in the corona. This feature strongly suggests that coronal hole regions differ from other open-field regions of the corona in that they are in a ‘fast’, low density expansion state over much of their extent. Such a dynamical configuration makes it possible to reconcile the low values of electron density observed in coronal holes with the large particle fluxes in the associated high speed streams seen in the solar wind.

Dynamics of coronal hole regions

Plasma transport models of polar ionosphere, discussing physical processes involved and effects on electron concentration, ion composition and speeds

High-latitude plasma transport: The polar wind

A model is constructed to represent the interaction between the solar wind and the neutral component of the interstellar gas. It is found that the neutral gas has several important effects on the solar-wind expansion beyond the orbit of the earth and that it should be possible to infer the presence of the neutral gas from observations of the solar wind made by a space probe traveling into the outer solar system. The effects include a deceleration and heating of the supersonic solar wind, a cooling of and pressure reduction in the subsonic solar wind, and a tightening of the spiral magnetic field in the supersonic solar wind.

Interaction of the solar wind with the neutral component of the interstellar gas.

In order to understand better the dynamical processes in the solar atmosphere that are associated with coronal mass ejections (CMEs), we have carried out a study of prominence activity using Hα observations obtained at the Mauna Loa Solar Observatory (MLSO). After developing clear definitions of active prominences (APs) and eruptive prominences (EPs), we examined 54 Hα events to identify distinguishing characteristics of APs and EPs and to study the relationship between prominence activity and CMEs. The principal characteristics we found to distinguish clearly between APs and EPs are maximum projected radial height, projected radial velocity, and projected radial acceleration. We determined CME associations with Hα events by using white-light data from the Mk III K-Coronameter at MLSO and the LASCO C2 Coronagraph on SOHO. We found that EPs are more strongly associated with CMEs than are APs and that the CMEs associated with EPs generally have cores, while those associated with APs do not. A majority of the EPs in the study exhibit separation of escaping material from the bulk of the prominence—the latter initially lifting away from and then returning toward the solar surface. This separation tends to occur in the height range from 1.20 to 1.35 R0, and we infer that it involves the formation of an X-type neutral line in this region, which allows disconnection of part of the prominence material. This disconnection view of prominence eruption seems most consistent with flux rope models of prominence support.

Thomas E. Holzer

Papers

The polar wind

Dynamics of coronal hole regions

High-latitude plasma transport: The polar wind

Interaction of the solar wind with the neutral component of the interstellar gas.

Active and Eruptive Prominences and Their Relationship to Coronal Mass Ejections