Hall current effects in dynamic magnetic reconnection solutions

The impact of Hall current contributions on flow driven planar magnetic merging solutions is discussed. The Hall current is important if the dimensionless Hall parameter (or normalized ion skin depth) satisfies cH>η where η is the inverse Lundquist number for the plasma. A dynamic analysis of the problem shows, however, that the Hall current initially manifests itself, not by modifying the planar reconnection field, but by inducing a non-reconnecting perpendicular "separator" component in the magnetic field. Only if the stronger condition c2/H > η is satisfied can Hall currents be expected to affect the planar merging. These analytic predictions are then tested by performing a series of numerical experiments in periodic geometry, using the full system of planar magnetohydrodynamic (MHD) equations. The numerical results confirm that the nature of the merging changes dramatically when the Hall coupling satisfies c2/H > η. In line with the analytic treatment of sheared reconnection, the coupling provided by the Hall term leads to the emergence of multiple current layers that can enhance the global Ohmic dissipation at the expense of the reconnection rate. However, the details of the dissipation depend critically on the symmetries of the simulation, and when the merging is "head-on" (i.e., comprises fourfold symmetry) the reconnection rate can be enhanced

Coronal magnetic energy release by current sheet reconnection

In this thesis we investigate the rapid release of energy in the solar corona, with a particular view to understanding the solar flare in which magnetic reconnection is thought to play a key role. A review of existing reconnection solutions is given in Chapters 2 and 3, with new analytic and numeric results are presented in subsequent chapters. Although much of the work in this thesis is computational, numerical investigations are always motivated theoretically. In Chapters 4 and 5 several aspects of two dimensional reconnection are investigated using a periodic time-dependent incompressible code. One of the main points is to check the veracity of the analytic solution of Craig and Henton (1995) by running the code from general initial conditions. Other aspects of 2-D merging covered include the tearing mode instability, osculation and the effects of finite compressibility. We employ a 3-D time-dependent code, in Chapter 4, to check that the analytically predicted spine and fan forms develop from general initial conditions. Scalings with resistivity of the associated current structures are also investigated. Most of the analytic work so far has revolved around single null magnetic configurations. Chapter 6 focuses on reconnection solutions in the presence of multiple nulls. Finally, we look at an application of the analytic theory in the context of particle acceleration. In Chapter 7 we trace proton orbits using a physically plausible analytic current sheet solution

Comparing Various Multi-Component Global Heliosphere Models

Modeling of the global heliosphere seeks to investigate the interaction of the solar wind with the partially ionized local interstellar medium. Models that treat neutr al hydrogen self-consistently and in great detail, together with the plasma, but that neglect magnetic fields, constitute a sub-category within global heliospheric models. There are several different modeling strategies used for this sub-category in the literature. Differences and commonalities in the modeling results from different strategies are pointed out

Comparing various multi-component global heliosphere models

Modeling of the global heliosphere seeks to investigate the interaction of the solar wind with the partially ionized local interstellar medium. Models that treat neutral hydrogen self-consistently and in great detail, together with the plasma, but that neglect magnetic fields, constitute a sub-category within global heliospheric models. There are several different modeling strategies used for this sub-category in the literature. Differences and commonalities in the modeling results from different strategies are pointed out. Plasma-only models and fully self-consistent models from four research groups, for which the neutral species is modeled with either one, three, or four fluids, or else kinetically, are run with the same boundary parameters and equations. They are compared to each other with respect to the locations of key heliospheric boundary locations and with respect to the neutral hydrogen content throughout the heliosphere. In many respects, the models' predictions are similar. In particular, the locations of the termination shock agree to within 7% in the nose direction and to within 14% in the downwind direction. The nose locations of the heliopause agree to within 5%. The filtration of neutral hydrogen from the interstellar medium into the inner heliosphere, however, is model dependent, as are other neutral results including the hydrogen wall. These differences are closely linked to the strength of the interstellar bow shock. The comparison also underlines that it is critical to include neutral hydrogen into global heliospheric models.Comment: 10 pages, 4 figures, submitted to a special section at A&A of an ISSI team "Determination of the physical Hydrogen parameters of the LIC from within the Heliosphere

Exclusion of Tiny Interstellar Dust Grains from the Heliosphere

The distribution of interstellar dust grains (ISDG) observed in the Solar System depends on the nature of the interstellar medium-solar wind interaction. The charge of the grains couples them to the interstellar magnetic field (ISMF) resulting in some fraction of grains being excluded from the heliosphere while grains on the larger end of the size distribution, with gyroradii comparable to the size of the heliosphere, penetrate the termination shock. This results in a skewing the size distribution detected in the Solar System. We present new calculations of grain trajectories and the resultant grain density distribution for small ISDGs propagating through the heliosphere. We make use of detailed heliosphere model results, using three-dimensional (3-D) magnetohydrodynamic/kinetic models designed to match data on the shape of the termination shock and the relative deflection of interstellar neutral H and He flowing into the heliosphere. We find that the necessary inclination of the ISMF relative to the inflow direction results in an asymmetry in the distribution of the larger grains (0.1 micron) that penetrate the heliopause. Smaller grains (0.01 micron) are completely excluded from the Solar System at the heliopause.Comment: 5 pages, 5 figures, accepted for publication in the Solar Wind 12 conference proceeding

Is the magnetic field in the heliosheath laminar or a turbulent bath of bubbles?

All the current global models of the heliosphere are based on the assumption that the magnetic field in the heliosheath, in the region close to the heliopause is laminar. We argue that in that region the heliospheric magnetic field is not laminar but instead consists of magnetic bubbles. Recently, we proposed that the annihilation of the "sectored" magnetic field within the heliosheath as it is compressed on its approach to the heliopause produces the anomalous cosmic rays and also energetic electrons. As a product of the annihilation of the sectored magnetic field, densely-packed magnetic islands/bubbles are produced. These magnetic islands/bubbles will be convected with the ambient flows as the sector region is carried to higher latitudes filling the heliosheath. We further argue that the magnetic islands/bubbles will develop upstream within the heliosheath. As a result, the magnetic field in the heliosheath sector region will be disordered well upstream of the heliopause. We present a 3D MHD simulation with very high numerical resolution that captures the north-south boundaries of the sector region. We show that due to the high pressure of the interstellar magnetic field a north-south asymmetry develops such that the disordered sectored region fills a large portion of the northern part of the heliosphere with a smaller extension in the southern hemisphere. We suggest that this scenario is supported by the following changes that occur around 2008 and from 2009.16 onward: a) the sudden decrease in the intensity of low energy electrons detected by Voyager 2; b) a sharp reduction in the intensity of fluctuations of the radial flow; and c) the dramatic differences in intensity trends between GCRs at V1 and 2. We argue that these observations are a consequence of V2 leaving the sector region of disordered field during these periods and crossing into a region of unipolar laminar field.Comment: 36 pages, 15 figures, submitted to Ap

Proton acceleration in analytic reconnecting current sheets

Particle acceleration provides an important signature for the magnetic collapse that accompanies a solar flare. Most particle acceleration studies, however, invoke magnetic and electric field models that are analytically convenient rather than solutions of the governing magnetohydrodynamic equations. In this paper a self-consistent magnetic reconnection solution is employed to investigate proton orbits, energy gains, and acceleration timescales for proton acceleration in solar flares. The magnetic field configuration is derived from the analytic reconnection solution of Craig and Henton. For the physically realistic case in which magnetic pressure of the current sheet is limited at small resistivities, the model contains a single free parameter that specifies the shear of the velocity field. It is shown that in the absence of losses, the field produces particle acceleration spectra characteristic of magnetic X-points. Specifically, the energy distribution approximates a power law ~ξ-3/2 nonrelativistically, but steepens slightly at the higher energies. Using realistic values of the “effective” resistivity, we obtain energies and acceleration times that fall within the range of observational data for proton acceleration in the solar corona