SH51B-1599
Observing and quantifying the solar wind signature of the magnetically complex corona.
The solar wind exhibits fluctuations over a broad range of timescales characteristic of magnetohydrodynamic (MHD) turbulence evolving in the presence of structures of coronal origin. In- situ spacecraft observations of plasma parameters are at minute (or below) resolution for intervals spanning the solar cycle and provide a large number of samples for statistical studies. The magnetic field power spectrum typically has two characteristic components, an inertial range of turbulence over several orders of magnitude with approximately Kolmogorov power law and at lower frequencies, an approximately '1/f' energy containing range believed to be of direct coronal origin. We focus on the behaviour of in- situ observations of fluctuations in the inner heliosphere as a function of solar cycle and solar wind speed; that is, with respect to coronal structure and dynamics. We employ a recently developed technique that sensitively distinguishes between fractal and multifractal scaling in the timeseries. Our working hypothesis is that since the latter can be characteristic of local MHD turbulence, the former maps more directly to features of coronal origin. We find a strong correlation between the scaling properties of magnetic energy density fluctuations and the magnetic complexity of the coronal magnetic fields. At solar maximum in the ecliptic, where the in- situ observations can be dominated by slow solar wind, the magnetic energy density as seen by WIND and ACE shows a fractal signature, whereas at minimum it is multifractal. This is corroborated by ULLYSES polar observations at solar minimum in quiet, fast solar wind where again, multifractal scaling is found. This high magnetic complexity in the corona corresponds to fractal, rather than multifractal scaling in magnetic energy density; remarkably, this fractal signature dominates the full dynamic range of observations, extending across timescales typically identified with both the '1/f' and 'inertial range'. The correlation of behaviour of other bulk plasma parameters observed in- situ with the magnetic complexity of the coronal will also be discussed. Since we are able to quantify scaling exponents, our results provide constraints on models for the solar wind. In particular, the fractal signature which we discuss here can be captured by a nonlinear Fokker Planck model, with the prospect of a quantitative mapping back to the corona.
SH51B-1600
Surface Alfven Wave Damping in a 3D Simulation of the Solar Wind
It is known that a source of additional momentum is needed to drive the solar wind. Here we investigate the effect of surface Alfvén wave damping in solar minima and solar maxima conditions. The surface Alfvén wave damping length L depends on the superradial expansion factor S of magnetic field lines. We calculate S for Carrington Rotation 1912 with a steady state solar background generated with the Space Weather Modeling Framework, and compare with estimates by Dobrzycka et al. 1999 using SOHO observations. We estimate the surface Alfvén wave damping for active regions, quiet sun, and the border between open and closed magnetic field lines. We address how our results can be incorporated in a MHD thermally driven-wind model.
SH51B-1601
Magnetic Reconnection in the Solar Streamer Belt as a Source of the Slow Solar Wind
The slow component of the solar wind is conjectured to originate in and around the solar streamer belt. The region beyond the cusp of an helmet streamer is characterized by the presence of a current sheet embedded in a plasma flow, and plasma density enhancements accelerating radially outward have been observed by the Large-Angle Spectrometric Coronagraph (LASCO) instrument on board the Solar and Heliospheric Observatory (SOHO). In the present work we investigate the stability of such configuration. Due to the coupling with the Kelvin-Helmholtz instability, magnetic reconnection can in fact give rise to the formation of density enhanced magnetic islands that accelerate outward. We have previously investigated this scenario in 2D cartesian simulations. In the present work we explore the behavior of such a system in spherical geometry. Global 3D MHD simulations have found that the velocity, at a fixed radius, grows from a slow value at the current sheet towards higher values towards the polar regions. The steady-state which was reached showed the bimodal characteristic of the solar wind, but the slow component did not show its characteristic variability. We present 2D numerical simulations, performed with the SAIC MHD spherical code (MAS), of the region beyond the cusp of an helmet streamer from 1 R☉ up to 20 R☉.
SH51B-1602
Small Bipoles Interacting with a Coronal Hole: MHD Simulations
Changes in the photospheric magnetic flux due to emergence, submergence, and surface flows drive the evolution of the coronal and heliospheric magnetic field. We have used our 3D MHD algorithm in spherical coordinates to study the interaction of the magnetic field of two bipoles with a coronal hole. We have prescribed as magnetic flux distribution at the lower boundary a smoothed Kitt Peak magnetogram for Carrington Rotation 1913 (late August 1996), to which we have added two small bipoles. After reaching a relaxed state, we have introduced surface flows, which evolve the magnetic flux distribution at the boundary. We have investigated the reconfiguration of coronal fields in response to these motions; in particular we show what happens to the open flux associated with the bipole when it is moved into a previously closed region.
SH51B-1603
Intrinsic Instability of Coronal Streamers
Plasma blobs are observed to be weak density enhancements as radially-stretched structures emerging from the cusps of coronal streamers. In this paper, it is suggested that the formation of blobs is a consequence of an intrinsic instability of coronal streamers occurring at a very localized region around the cusp. The evolutionary process of the instability, as revealed in our calculations, can be described as follows, (1) through the localized cusp region where the field is too weak to sustain the confinement, plasmas expand and stretch the closed field lines radially outwards as a result of the freezing-in effect of plasma-magnetic field coupling; the expansion brings a strong velocity gradient up to 150 km s-1 into the slow wind regime providing free energy necessary for the onset of the subsequent magnetohydrodynamic instability; (2) the instability manifests itself mainly as mixed streaming sausage-kink modes, the former results in pinches of elongated magnetic loops to provoke reconnections at one or multi locations to form blobs. Then, the streamer system returns to the configuration with a lower cusp point, subject to another cycle of the streamer instability. Although the instability is intrinsic, it does not lead to the loss of the closed magnetic flux, neither does it affect the overall feature of a streamer. The main properties of the modelled blobs, including their size, velocity profiles, density contrasts, and even their daily occurrence rate are in line with available observations.
SH51B-1604
Oxygen Observations by STEREO/PLASTIC in the Slow Solar Wind
We have analyzed solar wind oxygen in the data from the STEREO Plasma and Supra-thermal Ion Composition Experiment (PLASTIC). For this initial study we concentrate on the slow solar wind where the ion composition is stable, different ion species have nearly the same bulk speed, and the kinetic temperature is usually low. The mass of the detected ions is determined when the ions have both a valid time-of-flight and a residual energy measured by a Solid State Detector (SSD). The bulk speed, thermal speed and flow angles of O6+ are then calculated using the electrostatic analyzer and position data. The STEREO data are compared to similar measurement on ACE/SWICS.
SH51B-1605
Dynamic Instability Leading to Increased Interchange Reconnection Rates
Interchange reconnection is widely believed to play an important role in coronal magnetic field dynamics. In this investigation we investigate the 3D dynamics of interchange reconnection by extending the concept of a magnetic null-point to a null-volume, the so-called "acute-cusp field" configuration. The acute-cusp field geometry is characterized by high-beta plasma confined with favorable curvature, surrounded by a low-beta environment. First, we construct an initial translationally-symmetric potential field configuration. This configuration contains the required topological characteristics of four separate flux systems in the perpendicular plane. We then drive the system by a slow, incompressible, uniform flow at the boundary. The resulting evolution is calculated by solving numerically the MHD equations in full 3D Cartesian coordinates using the Adaptively Refined MHD Solver developed at the U.S. Naval Research Laboratory. Field shearing along the topological boundaries changes the shape of the acute-cusp field surface separating the high and low plasma beta regions. An extended, 2D current sheet is generated by the photospheric driving. We discuss the effect of 3D perturbations on the current sheet dynamics and on the rate of the resulting interchange reconnection. Finally, we discuss the implications of our simulations for coronal observations. This work has been supported, in part, by the NASA HTP and SR&T programs.