Corona IV
Presiding: J Karpen, Naval Research Laboratory; J Brosius, The Catholic University of America
SP22A-01 10:30h
Dissertation Talk: Brightness and Magnetic Evolution of Solar Coronal Bright Points
Dissertation: A study of the brightness and magnetic evolution of several Extreme ultraviolet (EUV) coronal bright points (hereafter BPs) using CDS, EIT, MDI (onboard SOHO) and TRACE is presented. The results confirm that, down to 1" resolution, BPs are made of small loops with lengths of ~6 Mm and cross-sections of ~2 Mm. The loops are very dynamic, evolving in time scales as short as 1 -- 2 minutes. This is reflected in a highly variable EUV response with fluctuations highly correlated in spectral lines at transition region temperatures (in the range 3.2×104 -- 3.5×105 K). A wavelet analysis of the intensity variations reveals, for the first time, the existence of quasi-periodic oscillations with periods ranging 400--1000 s, in the range of periods characteristic of the chromospheric network. The link between BPs and network bright points is discussed. Co-spatial and co-temporal TRACE and MDI images also reveal the signature of heating events that produce sudden EUV brightenings simultaneous to magnetic flux cancellations. This is interpreted in terms of magnetic reconnection events.
SP22A-02 11:00h
Coronal Bright Points at the Minimum of Sunspot Activity
We will present the results of our study of coronal bright points (XBPs) using SoHO/EIT data and an automatic identification procedure. The latitude-longitude distribution of XBPs obtained during the period of very low sunspot activity (March-April 1996) is not uniform; it exhibits structures reminiscent of active longitudes and large-scale circular patterns. We also find XBPs associated with emergence of ephemeral active regions. We interpret these results as a possible indication of two types of XBPs that are characteristic of quiet and active Sun.
SP22A-03 11:15h
Nine Years of EIT Bright Points
We discuss early results derived from an algorithm that automates the detection, cataloging, and analysis of Extreme-ultraviolet (EUV) "Bright Points" (BP) from nine years of data acquired by the Extreme-ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO). In particular, we describe the temporal and spatial variations of the 1.3x108 EUV BPs observed by SOHO to date.
SP22A-04 11:30h
A Hierarchical Application of the Minimum Current Corona
We study the energy and helicity injected into the corona of a photospheric quadrupole by quasi-static motion. The Minimum Current Corona (MCC) method provides a lower bound on coronal free energy by quantifying the coronal linkages between discrete photospheric source regions. A quadrupolar distribution can be modeled naturally with 4 photospheric sources. The coronal field connects the sources in 4 different ways (4 domains) and includes one separator. The energy and helicity injection into this system were explored by by Longcope and Magara (2004). The 4-source configuration can be considered the most significant element of an MCC hierarchy whereby the continuous photospheric field is represented by an increasing number of discrete sources. The numbers of linkages and separators increase with the number of sources. This hierarchy asymptotically approaches a quasi-static MHD evolution line-tied to a continuous photospheric field. We demonstrate this hierarchy using a quadrupolar example which can be compared to the results of numerical simulations of quasi-static evolution. This work is supported by NSF.
SP22A-05 11:45h
Self Consistent Modeling of Current Sheet Structure and Transport Processes
A simple magnetohydrodynamic (MHD) model of a fully ionized, collision dominated plasma that includes the electrical conductivity and thermoelectric tensors in Ohm's law is used to generate current sheet solutions that, where the assumption of full ionization is valid, are characterized by ranges of temperature, density, magnetic field strength, and flow speed that correspond to those of the transition region and corona. The electrical conductivity and thermoelectric tensors are functions of temperature, number density, and magnetic field strength. The model contains a prescribed sheared magnetic field with a characteristic length scale L. The characteristic sheet width is 2L, but the temperature has transition region or coronal values only within a central plasma sheet (CPS) that has a width one or more orders of magnitude smaller than 2L. The CPS is essentially the diffusion region. The heating rate per unit mass, and the flow speed in the CPS are orders of magnitude larger, and the density is orders of magnitude smaller than in the surrounding plasma. The heating rate per unit volume is a maximum in the CPS, and falls off steadily outside the CPS. The heating is driven almost entirely by the convection electric field. The current density and heating rate each consist of a thermoelectric component driven by the temperature gradient, and a conduction component driven by the center of mass electric field. These components largely cancel one another, yielding a total current density and heating rate that are orders of magnitude smaller than either of their components. This suggests that thermoelectric effects are important in determing current sheet structure. This work was supported by NSF grant ATM-0242820 to the Institute for Scientific Research.