Outline
Many believe that turbulence is ubiquitous in solar
physics, although just how global turbulence is generated and evolves in the
solar environment remains to be investigated. For example, although the solar
wind appears to be a turbulent magnetofluid, the input spectrum from the solar
corona is almost certainly not Kolmogoroff, and, in fact appears to be significantly
flatter, perhaps indicating that log-normal processes dominate, producing an f–1 input spectrum. On the
other hand, convection in solar interior and shear flows at the base of the
solar convection zone known as tachocline do appear to be turbulent. Solar
variability, which may be a consequence of turbulent magnetic phenomena, is an
important issue from the solar physics, as well as terrestrial climate and
space weather point of view. Most phenomena observed at and above the solar surface, such as sunspots, the magnetic
network, coronal mass ejections, flares, and
fluctuations in the amount and the spectral distribution of the emitted
radiation may be connected
in various ways to turbulent processes. The
observed phenomena are essentially the response of the outer solar layers to
thermally or magnetically driven excitations near the bottom of the convection
zone.
Processes in
the solar interior, which cannot be directly observed, control solar magnetic
activity, which causes fluctuations in the Sun’s radiative output as well as in
the solar wind. It is thought, that a turbulent
dynamo mechanism, originating at the interface between the convection zone and radiative core, produces the
magnetic field of the Sun. The heliospheric magnetic field originates in the
solar corona and affects solar wind dynamics. The stream boundaries and the velocity and magnetic field gradients between
the fast and slow solar wind appear to be a source of interplanetary
turbulence. There are many meso-scale structures in the slow solar wind, which
is intrinsically variable in time and space and convects structures such as
tangential discontinuities of coronal origin.
Many magnetic structures
exist in the corona on different length scales, e.g. the plumes in the
interstreamer regions, coronal spikes at smaller spatial scales, elongated and
thin, thread-like structures that extend outward in the innermost corona.
Turbulence and the associated turbulent pressure in the upper convective layer
are potentially important for the physics of solar oscillations.
Within the last two decades or
so, major advances have been made in quantifying the physics of turbulence in
different solar and heliospheric layers (e.g. convective zone, solar corona and
solar wind) These studies are broadly based, involving observational analysis,
numerical modeling and theoretical approaches. The importance of turbulent
processes in maintaining the larger-scale phenomena has become widely
recognized in solar physics. Nevertheless, our knowledge of the interaction
between turbulence and large-scale solar processes remains rudimentary. This
aspect has not received the attention it deserves, in part because current
understanding and models have failed to represent adequately the intellectual
challenge of a problem that hinges on formulating,
in a closed fashion, the inherently nonlinear interactions between large scales
and small scales of motions. This is especially true in studies of the energy flow and links between physical processes
in the solar interior and the
magnetic-field-dominated regime in the solar atmosphere, and the
particle-dominated regime in the heliosphere. The advanced understanding of the
basic question, “why and how does the Sun vary?» will be achieved by
considering fundamental processes related to turbulence at all relevant scales.
It is now accepted that studying turbulence in solar physics involves more
diverse processes then do traditional studies that focus on mechanistic rather
than scale-interaction aspects. For example, the
future Solar Obiter mission will provide major
advances in identifying the links between activity on the Sun’s surface and the
resulting evolution of its atmosphere and the inner heliosphere, using
close-up, high-resolution remote sensing in combination with new in-situ
measurements. In this context, and facing the interdisciplinary issues involved, one requires
advanced observational strategies, novel physical and mathematical formulations
and, most importantly, individuals who are prepared to tackle solar science issues
of this scope, breadth and challenge.
Apparently,
there is a pressing need for new initiatives in the development of the
foundations of our current understanding of multi-scale phenomena in a tenuous
magnetofluid. We need to assess the basics of solar turbulence and to implement
the major advances in our studies of the global dynamics of the Sun and Inner
Heliosphere. It is clear that the way forward in studying turbulence in the Sun
and solar environment is to summarize first the current status in this research
field, and then to define clearly and concretely the hot problems, using the
existing experience on relevant topics and exchanging ideas from the different
fields.
The proposed list of the main issues that need
to be examined includes:
·
The structure and role of the tachocline in the solar
convection and dynamics. Interplay of convection, rotation and magnetic fields
in tachocline dynamics, possible models. The role of turbulence at the
tachocline and in the transport of angular momentum.
·
The physical mechanisms for entropy transport near the
top of the magnetized convection zone. A realistic physical picture of heat
transport, consistent with convection simulations, which will make clear how
the large-scale thermal structure seen at the photosphere comes about.
·
The physics of the interaction between convection and
magnetism. Interplay between the small-scale turbulent convection at the
photosphere and the weak convective overshoot in the convection zone. Models of
emergence of magnetic flux from the base of the convection zone. Physics of
highly anisotropic advective transport of magnetic flux.
·
Relevance of current turbulence models to solar
convection, estimates of the convective fluxes, evaluation of the assumptions,
which are typically made in the turbulence models and needs for development of
advanced theoretical models, including influence of the overshoot layer and
developing a non-local convective picture.
Possible ways to link the local simulations of the near-surface layers
to the global simulations of the convection-zone dynamics.
·
Relevance of solar convection zone properties to
issues of helioseismology, namely, thermodynamic and wave properties of the
transition from the convection zone to the visible surface; the influence of
the upper turning points of the p-modes and the role of surface magnetic
fields; the modification of hydrostatic balance by turbulent pressure on the
mode frequencies and the dependence from the averaging over large 3D
thermodynamic fluctuations and over large 3D thermodynamic fluctuations and
over large amplitude 3D velocity fields; non-adiabatic effects at or near the
optical surface, and stochastic excitation and linear damping of oscillations.
·
The problem of the origin of supergranulation in the
solar photosphere, and the effect of stratification and surface effects. Role
of convective motions in the photosphere in the generation of coronal MHD
waves; the emergence and cancellation of photospheric magnetic flux and the
resulting consequences for the coronal magnetic loops, the chromosphere and the
transition region magnetic network.
·
The fact that corona is highly inhomogeneous and
non-stationary, revealed by observations from Yohkoh and SOHO and TRACE,
complicates the theoretical description of MHD waves, giving rise to a variety
of phenomena including phase mixing, resonant absorption and dispersive
ducting. Our current theoretical understanding is limited to simple models
describing MHD waves in coronal structures modeled as magnetic flux tubes. More
refined theoretical models are needed to extend our understanding of corona
waves and oscillations to more realistic configurations with an appropriate
incorporation of various physical effects, including field-line curvature,
non-adiabaticity, plasma inhomogeneity, and the presence of flows.
·
The role of turbulence in the flow of energy through
different layers of the solar atmosphere to the inner most heliosphere, and in
the relationship between corona and solar wind, through the heating of the
corona, the acceleration of the solar wind, and the origin of the
interplanetary magnetic field. The role
of turbulence in the boundaries between the fast and slow solar wind. The role
of flares and coronal bursts as probes for intermittent dissipative events
within turbulence, relationship between turbulence in space and in laboratory
plasmas.
·
The transport of angular momentum from Sun base to the
solar wind is not well understood. Physical mechanisms for kinetic and magnetic
stresses originating in the solar wind, and carrying angular momentum from the
Sun, is relevant to the understanding of turbulence in the inner heliosphere,
its radial evolution and dissipation.
We propose
to form a team that critically reviews the current state of the global
variability of the Sun and Inner Heliosphere, in particular concerning
turbulence and its relation with solar variables and processes, with the
purpose to suggest novel ways in making substantial progress in the field. In
this way new theoretical and modeling efforts and space-observations will be
initiated. The proposed team consists of experts in magnetohydrodynamics,
turbulence theory, physics of the Sun and Inner Heliosphere, and in solar
observations. It is to be expected that the complementary expertise of the team
members having very different views will lead to a better understanding of the
challenging problems of solar physics, and open new prospects and avenues in
Sun and Inner Heliosphere research.
A critical number
of scientists covering all the required expertise have been invited. The team will meet twice to discuss the
issues outlined above. Understanding that the proposed issues are wide-ranging
and difficult to resolve, we plan a careful preparation prior to the first
meeting in order to identify the immediate problems. The current state of
research on solar turbulence will be examined in the first meeting, and key
problems to be tackled and the way forward for all identified problems will be
defined. Smaller sub teams led by one of the study team members will attempt to
would attempt to write up the way forward in all relevant topics in the period
between the first and the second meeting.
We will evaluate the made progress during the second meeting and will
continue joint work on brainstorming papers for publication in form of a joint
article and will plan a special book. We
will keep contact with a wide science community not involved in our team to get
feedbacks and contributions to our work. The possible organization and program
of an
Two
meetings of the team members are planned. We
suggest having our first meeting in summer 2003. The second meeting will be scheduled
6-8 months later. The meetings would last 6 days including probably either a
Saturday or a Sunday to minimize return fees.
ISSI facilities
We will
need the larger meeting room on the first floor and expect to use the computers
on both 1st and 3rd floors for electronic mail
connections. No special computational facilities are required.
Output
We are confident
that this project will be of great interest worldwide. Therefore
we attach considerable importance to dissemination and to consequent feedback. The main expected output will be articles
submitted to leading journals, co-authored by team members. We would anticipate specialized review
papers in the form of joint articles. A special issue of Space Science Series of ISSI will be
discussed at the first meeting. Lectures will be presented at relevant
conferences and workshops. ISSI making our interaction possible will be
acknowledged in all relevant publications. However, we specially stress the
importance of spending as much time as possible for
free discussions between team members. A project ”Home Page” will be maintained and updated
after the formal termination of the project.
Additionally, but necessarily more vaguely, the team members will use the results
in teaching, further research, and collaboration with others on future
projects. New proposals to ISSI for
smaller study teams will be initiated and submitted.