|Session:||Session 6: Magnetic Field Transport from the Sun to the Heliosphere (06)|
|Date:||Thursday, June 16, 2005|
|Time:||08:30 - 15:00|
The Magnetic Connection between the Sun and the Heliosphere
Lockheed Martin, UNITED STATES
The magnetic field at the surface of the Sun is shaped by several transport processes which act on a broad, continuous spectrum of magnetic bipoles from small ephemeral regions to the largest active region complexes. The coronal field above the Sun's surface somehow manages to relax to a near-potential state despite the variety of motions of the field's photospheric footpoints. On the largest scales, the fields are forced open into the heliosphere. In this tutorial, I review some of the uncertainties which researchers face in modelling the Sun's environment, explore the dynamic geometry of the connection from Sun to heliosphere, discuss the role of the various surface transport processes, and look at several of the mental images that have been reshaped over the past few years, including the magnetic canopy and the role of active regions in the divide between fast and slow winds. The state of the Sun-heliosphere connection in the Sun's distant past and future are explored.
Transport of Open Magnetic Flux on the Sun and its Consequences
University of Michigan, UNITED STATES
The component of the magnetic field of the Sun that opens into the heliosphere, the so-called open magnetic flux, appears to be relatively constant during the solar cycle, and thus its distribution and evolution can usefully be considered a transport problem, with the transport governed by various diffusive processes: It diffuses in the random convective motions of the photosphere, as does all magnetic flux; it reconnects with coronal loops on the quiet Sun at their base and is displaced randomly; it reconnects in the canopy of loops and is displaced; it becomes braided and entangled in the overlying corona, and executes diffusive motions. Theories that have been developed to describe this diffusion will be discussed and some solutions to the governing equations for the distribution and evolution of the open flux will be presented. As a result of the various diffusive processes, the coronal plasma will behave as if it has a finite conductivity, with a resulting large-scale electric field. The diffusion approach can thus be used to provide a convenient description of the heating and acceleration of the solar wind and the acceleration of energetic particles.
Magnetic Flux Transport On the Sun
Naval Research Laboratory, UNITED STATES
Magnetic flux erupts in bipolar concentrations and spreads out on the surface via the non-stationary convection process called supergranular diffusion. The diffusing patterns are sheared by differential rotation , carried poleward by meridional flow, and weakened by annihilative encounters with regions of opposite polarity. Although several thousand eruptions occur in the sunspot belts during a typical sunspot cycle, only a few hundred very large eruptions contribute appreciably to the large-scale field. The alternating polarities of these sources from one cycle to the next (Hale's Law) lead to the cyclic behavior of the polar fields. These observations have helped us to understand and predict several aspects of the large-scale field and its heliospheric extenstion, and promise to do so in the future.
Evolution of Twisted Magnetic Flux Tubes Emerging into the Solar Corona
National Center for Atmospheric Research, UNITED STATES
We present MHD simulations in both 2D axisymmetric and 3D spherical geometries of the evolution of a twisted magnetic flux rope emerging into the low-β corona previously occupied by a potential arcade field. We describe both the initial quasi-static evolution whereby stable equilibrium structures form with stored free magnetic energy, and the eventual loss of equilibrium (or confinement) of the twisted magnetic flux rope as sufficient twist is being transported into the corona, resulting in the onset of a CME. We discuss the observational properties of a line-tied twisted flux rope in the corona as CME precursors, which can produce features such as soft-X ray sigmoids, filaments, and cavities. For a 3D line-tied flux tube confined by an external arcade, we find that with the buildup of a moderate amount of twist (< 2 full winds of field-line twist about the axis), the flux tube can erupt through the arcade at a localized area, with most of the arcade field remaining closed. The non-linear evolution of the kink instability facilitates the loss of confinement of the flux rope by changing its orientation at the apex such that it becomes easier for the flux rope to part and erupt through the arcade.
Magnetic Reconnection in the Solar Wind
Los Alamos National Laboratory, UNITED STATES
We have obtained direct evidence for local, quasi-stationary magnetic reconnection in the solar wind near 1 AU using solar wind plasma and magnetic field data obtained by the Advanced Composition Explorer, ACE. The prime evidence consists of intervals of accelerated or decelerated plasma flow observed within magnetic field reversal regions that we interpret as encounters with (generic) Petschek-type reconnection exhausts that are bounded by Alfven waves. A total of 42 such events have now been identified in approximately 7 years of data. This paper summarizes observational aspects of the physical properties of solar wind reconnection exhausts, including observed accelerations and decelerations, the plasma and magnetic field conditions for which reconnection occurs, the degree of plasma interpenetration within the exhausts, the nature of the transitions at the edges of the exhausts, changes in magnetic field topology associated with the exhausts, and initial evidence for and against slow mode shocks and reconnection-associated particle acceleration.
The magnetic configuration of the solar corona and inner heliosphere: A self-consistent MHD model incorporating the effects of differential rotation
Riley, Pete; Lionello, R; Linker, J.; Mikic, Z.
Science Applications International Corporation, UNITED STATES
Previously, we developed a global, empirically based model of the solar corona and inner heliosphere, driven by observed line-of-sight photospheric magnetic field. As such it was capable of modeling specific time periods of interest and provided a realistic global context with which to interpret in situ observations. While the model successfully reproduced many features of the observations, it was limited in several important ways: First, a number of ad hoc assumptions were implemented to derive the boundary conditions for the heliospheric portion of the model; and second, differential rotation was not included. In this study, we address these limitations. Specifically, by including the effects of Alfven waves, we describe seamless, self-consistent solutions all the way from the solar surface to 1 AU. In addition, we incorporate the effects of differential rotation at the photosphere to assess the effects of various magnetic reconfiguration processes. In particular, interchange reconnection, which opens previously closed field lines, releases the trapped plasma and may provide the dominant source of the slow solar wind.
Voyager 1 Observations of the Heliospheric Magnetic Field beyond 83 AU
Burlaga, Leonard1; Ness, N. F.2; Acuna, M.3; Lepping, R. P.3; Connerney, J.E.P.3
1NASA/GSFC, UNITED STATES; 2Bartol Research Institute, University of Delaware, Newark, Delaware, 19716, UNITED STATES; 3NASA-Goddard Space Flight Center, Greenbelt, Maryland, 20771, UNITED STATES
Voyager 1 is currently near 95 AU at ~34° North latitude. This presentation surveys the observations of the magnetic field made beyond 83 AU from 2002 to 2005. We find no evidence that V1 crossed the termination shock through mid-2004. The properties of the magnetic field observed by Voyager 1 during 2002 and 2003 are consistent with extrapolations of previous observations of the heliospheric magnetic field. A merged interaction region (MIR) with strong magnetic fields was observed beginning DOY 36, 2003. A second MIR was observed following a quasi-perpendicular shock that was detected on DOY 213, 2004. Near DOY 352, 2004, there was a transition to moderately strong magnetic fields that persisted with occasional large fluctuations to at least DOY 60, 2005. The possibilities that these 2004/2005 magnetic field observations are related to 1) a MIR, 2) a crossing of the heliospheric current sheet, and 3) a crossing of the termination shock will be discussed.
Did the heliospheric magnetic field really double during the last 100 years?
Mursula, Kalevi; Martini, D.
University of Oulu, FINLAND
It is known since several decennia that geomagnetic activity, as measured by the aa index, has considerably increased during the last 100 years. This increase, together with the in situ measurements of the heliospheric parameters, has been used to suggest that the heliospheric magnetic field has more than doubled during the 20th century. Here we reanalyze the long-term measurements of the Earth’s magnetic field at several stations since the beginning of the 20th century. We calculate a new measure of local geomagnetic activity at these stations, and develop a new centennial index Cp of global geomagnetic activity. The Cp index correlates extremely well (e.g., better than aa index) with the well known Ap index that exists since 1932, and can therefore be used as a reliable continuation of the Ap index by 30 years. According to the Cp index, the overall increase of global geomagnetic activity was only half of the increase depicted by the aa index. Obviously, this essentially reduces the earlier estimate on the centennial increase in the heliospheric magnetic field.
Cluster Observations of the Field and Plasma Structure of Current Sheets in the Solar Wind.
University College London, UNITED KINGDOM
When the apogee of the Cluster spacecraft is on the dayside between mid-January and mid-April each year, the 4 spacecraft spend several hours per orbit upstream of the bow shock, and thus sample the fields and plasmas of the solar wind. The multi-point nature of the Cluster mission allows an unambiguous determination of many properties of structures convecting with the solar wind. For example, 4 spacecraft timing analysis can be used to determine the orientation and thickness of 2-dimensional structures embedded in the solar wind, while the 'curlometer' technique can be used to determine the associated electric current density in the manner described, for example, by Eastwood et al. . In this paper we present analyses of Cluster data from a number of crossings of current sheets in the solar wind. We describe the current profile within these sheets. The current often occurs within discrete filaments, may also show significant asymmetry between upstream and downstream edges. We also examine the ion and electron distributions within and around these sheets in an effort to determine how the current and the pressure gradients are supported.
Large Scale Solar Wind Modeling with Subgridscale Turbulence Methods
Matthaeus, William H1; Dmitruk, P2; Goldstein, M L3; Usmanov, A4
1University of Delaware, UNITED STATES; 2Bartol, University of Delaware, UNITED STATES; 3NASA-GSFC, UNITED STATES; 4GSFC-NASA, UNITED STATES
Application of subgridscale modeling methods to the MHD equations describing large scale solar wind flow might extend the effective range of dynamical scales, enabling better treatment of the influence of turbulence on large-scale dynamics, along with the effect of dynamic large-scale fields on the cascade process. We describe and discuss an integrated system of large-scale MHD modeling equations, including and meso-scale turbulence modeling. The turbulence model couples in a consistent and physically acceptable way to a large-scale MHD model through five special terms appearing in the (Reynolds averaged) large-scale MHD equations. Meanwhile the large-scale fields (velocity, magnetic field, density) influence the small-scale turbulence in precise ways that include an accurate accounting for inhomogeneity of the system. The five terms that require subgridscale closure are (1) MHD Reynolds stress, (2) Fluctuation magnetic pressure, (3) mean turbulent induced electric field, (4) Flux of mean turbulent ram pressure, and (5) energy deposition due to turbulent dissipation. Simple closure strategies are suggested, as well as more elaborate approaches based on turbulence transport equations. The new system, once developed and tested, will amount to a subgridscale model of MHD turbulence. We suggest that this will extend the effective range of dynamically included scales by several orders of magnitude.
Three dimensional structure of magnetohydrodynamic turbulence in the solar wind
Horbury, Timothy1; Forman, M. A.2; Oughton, S.3
1Imperial College London, UNITED KINGDOM; 2State University of New York, Stony Brook, UNITED STATES; 3University of Waikato, Hamilton, NEW ZEALAND
Knowledge of the three dimensional power spectrum of solar wind MHD turbulence is important for predicting the propagation of energetic particles, as well as understanding the nature of the turbulence itself. However, spacecraft measurements are taken only along the solar wind flow direction, resulting in a so-called ``reduced’’ spectrum, and making the determination of the full spectrum very difficult. We present a new analysis of Ulysses magnetic field data taken in high speed polar solar wind, using a novel wavelet method, which results in a detailed estimate of the reduced spectrum. We demonstrate for the first time that fluctuations with wave vectors parallel (``slab’’) and perpendicular (``2D’’) to the magnetic field have different spectral indices, in agreement with some recent theories of MHD turbulence; and that the fraction of power in slab fluctuations is only a few percent, with important consequences for energetic particle propagation. Finally, we discuss deviations of the data from the slab/2D model and the consequences for our understanding of the turbulent cascade in collisionless plasmas.
Measurement of the electric fluctuation spectrum of solar wind magnetohydrodynamic
Bale, Stuart1; Kellogg, P. J.2; Mozer, F. S.1; Horbury, T. S.3; Reme, H.4
1University of California, UNITED STATES; 2University of Minnesota, UNITED STATES; 3Imperial College, London, UNITED KINGDOM; 4CESR, Toulouse, FRANCE
Magnetohydrodynamic (MHD) turbulence in the solar wind is observed to show the spectral behavior of classical Kolmogorov fluid turbulence over an inertial subrange and departures from this at short wavelengths, where energy should be dissipated. Here we present the first measurements of the electric field fluctuation spectrum over the inertial and dissipative wavenumber ranges in a beta~1 plasma. The k^-5/3 inertial subrange is observed and agrees strikingly with the magnetic fluctuation spectrum; the wave phase speed in this regime is shown to be consistent with the Alfven speed. At smaller wavelengths k rho_i 1 the electric spectrum is softer and is consistent with the expected dispersion relation of short-wavelength kinetic Alfven waves. Kinetic Alfven waves damp on the solar wind ions and electrons and may act to isotropize them. This effect may explain the fluid-like nature of the solar wind.