Showing papers by "Zoran Mikic published in 1999"

Magnetohydrodynamic modeling of the global solar corona

Abstract: A three-dimensional magnetohydrodynamic model of the global solar corona is described. The model uses observed photospheric magnetic fields as a boundary condition. A version of the model with a polytropic energy equation is used to interpret solar observations, including eclipse images of the corona, Ulysses spacecraft measurements of the interplanetary magnetic field, and coronal hole boundaries from Kitt Peak He 10 830 A maps and extreme ultraviolet images from the Solar Heliospheric Observatory. Observed magnetic fields are used as a boundary condition to model the evolution of the solar corona during the period February 1997–March 1998. A model with an improved energy equation and Alfven waves that is better able to model the solar wind is also presented.

Magnetohydrodynamic modeling of the solar corona during Whole Sun Month

Abstract: The Whole Sun Month campaign (August 10 to September 8, 1996) brought together a wide range of space-based and ground-based observations of the Sun and the interplanetary medium during solar minimum. The wealth of data collected provides a unique opportunity for testing coronal models. We develop a three-dimensional magnetohydrodynamic (MHD) model of the solar corona (from 1 to 30 solar radii) applicable to the WSM time period, using measurements of the photospheric magnetic field as boundary conditions for the calculation. We compare results from the computation with daily and synoptic white-light and emission images obtained from ground-based observations and the SOHO spacecraft and with solar wind measurements from the Ulysses and WIND spacecraft. The results from the MHD computation show good overall agreement with coronal and interplanetary structures, including the position and shape of the streamer belt, coronal hole boundaries, and the heliospheric current sheet. From the model, we can infer the source locations of solar wind properties measured in interplanetary space. We find that the slow solar wind typically maps back to near the coronal hole boundary, while the fast solar wind maps to regions deeper within the coronal holes. Quantitative disagreements between the MHD model and observations for individual features observed during Whole Sun Month give insights into possible improvements to the model.

Three-dimensional Solutions of Magnetohydrodynamic Equationsfor Prominence Magnetic Support: Twisted Magnetic Flux Rope

Abstract: The search for a background magnetic configuration favorable for prominence support has been given a great deal of attention for several decades. The most recent theoretical studies seem to agree that a promising candidate for the support of the dense and cooler prominence material, which fulfills several of the theoretical and observational requirements such as twist, shear along the neutral line, and dips, is a magnetic flux rope. The most convincing models take an infinitely long periodic configuration that consists of a linear constant-α force-free magnetic field. These models, however, assume values of α that are close to its maximum possible value. In this Letter, we report our recent results, which show that it is indeed possible to produce a configuration that consists of a twisted magnetic flux tube embedded in an overlaying, almost potential, arcade such that high electric currents (and therefore values of α) are confined to the inner twisted magnetic flux rope. We present two MHD processes—corresponding to two different types of boundary conditions—that produce such a configuration. Our results show that the process associated variations of Bz at the photospheric level by applying an electric field involving diffusion is much more efficient for creating a structure with more twist and dips.

The Solar Origin of Corotating Interaction Regions and Their Formation in the Inner Heliosphere

Abstract: Corotating Interaction Regions (CIRs) form as a consequence of the compression of the solar wind at the interface between fast speed streams and slow streams. Dynamic interaction of solar wind streams is a general feature of the heliospheric medium; when the sources of the solar wind streams are relatively stable, the interaction regions form a pattern which corotates with the Sun. The regions of origin of the high speed solar wind streams have been clearly identified as the coronal holes with their open magnetic field structures. The origin of the slow speed solar wind is less clear; slow streams may well originate from a range of coronal configurations adjacent to, or above magnetically closed structures. This article addresses the coronal origin of the stable pattern of solar wind streams which leads to the formation of CIRs. In particular, coronal models based on photospheric measurements are reviewed; we also examine the observations of kinematic and compositional solar wind features at 1 AU, their appearance in the stream interfaces (SIs) of CIRs, and their relationship to the structure of the solar surface and the inner corona; finally we summarise the Helios observations in the inner heliosphere of CIRs and their precursors to give a link between the optical observations on their solar origin and the in-situ plasma observations at 1 AU after their formation. The most important question that remains to be answered concerning the solar origin of CIRs is related to the origin and morphology of the slow solar wind.

An iterative method for the reconstructionbreak of the solar coronal magnetic field. I. Method for regular solutions

Abstract: We present a method for reconstructing the coronal magnetic field, assumed to be in a non-linear force-free state, from its values given in the photosphere by vector magnetograph measurements. In this paper, that is the first of a series, we propose a method that solves the boundary value problem set in the functional space of regular solutions (i.e., that do not contain current sheets). This is an iterative method introduced by Grad and Rubin. It is associated with a well-posed boundary-value problem. We present some results obtained with this method on two exact solutions of the magnetostatic equations, used as theoretical magnetograms. Unlike some other extrapolations methods, that are associated with ill-posed boundary value problems, our method allows extrapolation to arbitrarily large heights, with no blowing up due to the presence in these methods of an intrinsic instability that makes errors growing up exponentially.

A Test for Coronal Magnetic Field Extrapolations

Abstract: As models for the physical properties of the corona above solar active regions grow more sophisticated, we will require better means for testing them. In this paper we discuss and apply such a test to a magnetic field model for an active region. This test is based on the expectation that the temperatures at different points on a given magnetic field line should be well correlated because of the rapid transport of heat along field lines in the corona. We use radio observations of an active region to measure the temperatures on field lines as they cross two isogauss surfaces (at 430 and 750 G) in the corona. The field lines and isogauss surfaces are derived from a coronal magnetic field model obtained via a nonlinear force-free field extrapolation of a photospheric vector magnetogram; for comparison, we also investigate a potential-field extrapolation of the same magnetogram. In a region in which strongly sheared fields are present, the nonlinear force-free field model does indeed show a good correlation between the temperatures in the two surfaces at points on the same field line, while the potential-field model does not. This diagnostic acts both as a test of the magnetic field model as well as of the interpretation of the radio data, and we show how this test can also aid in understanding the radio data.

The Three-dimensional Coronal Magnetic Field during Whole Sun Month

Abstract: Combining models and observations, we study the three-dimensional coronal magnetic field during a period of extensive coordinated solar observations and analysis known as the Whole Sun Month (WSM) campaign (1996 August 10-September 8). The two main goals of the WSM campaign are addressed in this paper, namely, (1) to use the field configuration to link coronal features observed by coronagraphs and imaging telescopes to solar wind speed variations observed in situ and (2) to study the role of the three-dimensional coronal magnetic field in coronal force balance. Specifically, we consider how the magnetic field connects the two fastest wind streams to the two regions that have been the main foci of the WSM analysis: the equatorial extension of the north coronal hole (known as the Elephant's Trunk) and the axisymmetric streamer belt region on the opposite side of the Sun. We then quantitatively compare the different model predictions of coronal plasma and solar wind properties with observations and consider the implications for coronal force balance and solar wind acceleration.

Interplanetary scintillation measurements of the solar wind during Whole Sun Month: Comparisons with coronal and in situ observations

Abstract: Two-site observations of interplanetary scintillation using the EISCAT facility can provide measurements of solar wind velocity at any point in the heliosphere between 15 and 120 solar radii (R). In this paper we discuss a series of observations made as part of the Whole Sun Month campaign (August 10 to September 8, 1996) and compare the results with coronal data and in-situ measurements made during the campaign. The results of the comparison revealed extremely good agreement between solar wind speeds measured by IPS at 16-73 R and in situ measurements at 213 R and beyond, both in the general morphology of the solar wind and in the absolute velocities observed. These results confirm that structures in the solar wind, originating in the corona, preserve their form out to 910 R or more. Observations of fast solar wind were always associated with coronal holes and slow wind with the bright corona. Velocities intermediate between normal fast and slow flow speeds are associated with interaction regions between fast and slow flow and are also found above the boundaries of coronal holes.

In‐ecliptic CIR‐associated energetic particle events and polar coronal hole structures: SOHO/COSTEP observations for the Whole Sun Month Campaign

Abstract: The Solar and Heliospheric Observatory (SOHO), in halo orbit around the L1 Lagrangian point of the Sun-Earth system, combines a unique set of instruments for studies of the Sun and the heliosphere. SOHO's Comprehensive Suprathermal and Energetic Particle Analyser measures in situ particles in the energy range 44 keV/particle to above 53 MeV/nucleon. For the time period of the Whole Sun Month Campaign in mid 1996 we have identified recurrent energetic particle intensity increases in association with corotating interaction regions (CIRs) in the energy range <10 MeV. Solar wind measurements of the Wind spacecraft were used to estimate the corresponding magnetic source location in Carrington longitude for comparison of energetic particles with synoptic maps of the lower corona, derived from images of SOHO's Extreme-ultraviolet Imaging Telescope. The comparison reveals a close relationship of latitudinal extensions of polar coronal holes, situated in regions up to 40° away from the ecliptic, with CIR-associated in-ecliptic particle events.

Magnetohydrodynamic models of solar coronal magnetic fields

Abstract: We present some results concerning the possibility of determining the structure of solar active regions using measurements of the vector magnetic field on the Sun surface as boundary conditions for the new numerical extrapolation codes. From these computations the main features of these configurations, as shear and twist (which are particular forms of magnetic helicity), are then used as ingredients to define model problems and solved for the magnetohydrodynamic (MHD) analysis of solar eruptive phenomena, in which ejection (or redistribution) of helicity occurs.