A numerical study of the pre-ejection, magnetically-sheared corona as a free boundary problem

A class of magnetostatic equilibria with axial symmetry outside a unit sphere in the presence of plasma pressure and an r ^–2 gravitational field is constructed. The structure contains a localized current-carrying region confined by a background bipolar potential field, and the shape of the region changes subject to the variation of the electric current. The continuity requirement for the magnetic field and plasma pressures at the outer boundary of the cavity defines a free boundary problem, which is solved numerically using a spectral boundary scheme. The model is then used to study the expansion of the current-carrying region, caused by the buildup of magnetic shear, against the background confining field. The magnetic shear in our model is induced by the loading of an azimuthal field, accompanied by a depletion of plasma density.We show that due to the additional effect of confinement by the dense surrounding plasma, the energy of the magnetic field can exceed the energy of its associated open field, presumably a necessary condition for the onset of coronal mass ejections. (However, the plasma beta of the confining fluid is higher than that in the outer boundary of a realistic helmet-streamer structure.) Furthermore, under the assumption that coronal mass ejections are driven by magnetic buoyancy, the result from our model study lends further support to the notion of a suspended magnetic flux rope in the low-density cavity of a helmet-streamer as a promising pre-ejection configuration.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Avalanche models for solar flares (Invited Review)

This paper is a pedagogical introduction to avalanche models of solar flares, including a comprehensive review of recent modeling efforts and directions. This class of flare model is built on a recent paradigm in statistical physics, known as self-organized criticality. The basic idea is that flares are the result of an ‘avalanche’ of small-scale magnetic reconnection events cascading through a highly stressed coronal magnetic structure, driven to a critical state by random photospheric motions of its magnetic footpoints. Such models thus provide a natural and convenient computational framework to examine Parker's hypothesis of coronal heating by nanoflares. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1013301521745     

Solar Spectral Irradiance Variability in November/December 2012: Comparison of Observations by Instruments on the International Space Station and Models

Onboard the International Space Station (ISS), two instruments are observing the solar spectral irradiance (SSI) at wavelengths from 16 to 2900 nm. Although the ISS platform orientation generally precludes pointing at the Sun more than 10?–?14 days per month, in November/December 2012 a continuous period of measurements was obtained by implementing an ISS ‘bridging’ maneuver. This enabled observations to be made of the solar spectral irradiance (SSI) during a complete solar rotation. We present these measurements, which quantify the impact of active regions on SSI, and compare them with data simultaneously gathered from other platforms, and with models of spectral irradiance variability. Our analysis demonstrates that the instruments onboard the ISS have the capability to measure SSI variations consistent with other instruments in space. A comparison among all available SSI measurements during November–December 2012 in absolute units with reconstructions using solar proxies and observed solar activity features is presented and discussed in terms of accuracy.     

Numerical Simulation of a Solar Active Region. I: Bastille Day Flare

We present three-dimensional unsteady modeling and numerical simulations of a coronal active region, carried out within the compressible single-fluid MHD approximation. We focus on AR 9077 on 14 July 2000, and the triggering of the X5.7 GOES X-ray class “Bastille Day” flare. We simulate only the lower corona, although we include a virtual photosphere and chromosphere below. The boundary conditions at the base of this layer are set using temperature maps from line intensities and line-of-sight magnetograms (SOHO/MDI). From the latter, we generate vector magnetograms using the force-free approximation; these vector magnetograms are then used to produce the boundary condition on the velocity field using a minimum energy principle (Longcope, Astrophys. J. 612, 1181, 2004). The reconnection process is modeled through a dynamical hyper-resistivity which is activated when the current exceeds a critical value (Klimas et al., J. Geophys. Res. 109, 2218, 2004). Comparing the time series of X-ray fluxes recorded by GOES with modeled time series of various mean physical variables such as current density, Poynting energy flux, or radiative loss inside the active region, we can demonstrate that the model properly captures the evolution of an active region over a day and, in particular, is able to explain the initiation of the flare at the observed time.     

Predictive Capabilities of Avalanche Models for Solar Flares

We assess the predictive capabilities of various classes of avalanche models for solar flares. We demonstrate that avalanche models cannot generally be used to predict specific events because of their high sensitivity to the embedded stochastic process. We show that deterministically driven models can nevertheless alleviate this caveat and be efficiently used for predictions of large events. Our results suggest a new approach for predictions of large (typically X-class) solar flares based on simple and computationally inexpensive avalanche models.     

The solar corona as a minimum energy system?

This paper is an exploration of the possibility that the large-scale equilibrium of plasma and magnetic fields in the solar corona is a minimum energy state. Support for this conjecture is sought by considering the simplest form of that equilibrium in a dipole solar field, as suggested by the observed structure of the corona at times of minimum solar activity. Approximate, axisymmetric solutions to the MHD equations are constructed to include both a magnetically closed, hydrostatic region and a magnetically open region where plasma flows along field lines in the form of a transonic, thermally-driven wind. Sequences of such solutions are obtained for various degrees of magnetic field opening, and the total energy of each solution is computed, including contributions from both the plasma and magnetic field. It is shown that along a sequence of increasingly closed coronal magnetic field, the total energy curve is a non-monotonic function of the parameter measuring the degree of magnetic field opening, with a minimum occurring at moderate field opening.For reasonable choices of model parameters (coronal temperature, base density, base magnetic field strength, etc.), the morphology of the minimum energy solution resembles the observed quiet, solar minimum corona. The exact location energy minimum along a given sequence depends rather sensitively on some of the adopted parameter values. It is nevertheless argued that the existence of an energy minimum along the sequences of solutions should remain a robust property of more realistic coronal wind models that incorporate the basic characteristics of the equilibrium corona- the presence of both open and closed magnetic regions.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Fluorine-bearing phases in lamproites

Summary The distribution of F between phlogopite, amphibole, apatite, and glass has been determined for 261amproites from Leucite Hills, Wyoming; West Kimberley, Western Australia; Smoky Butte, Montana; Prairie Creek, Arkansas; and Gaussberg, Antarctica. The F contents of these phases are inconsistent in different lamproite localities, with distinct variations, for example, between the Leucite Hills and West Kimberley lamproites. Fluorine commonly occurs in greatest abundance in phlogopite and apatite. Evolved glass in some lamproites may have no detectable F. With increasing evolution of lamproites, F tends to decrease in whole rocks, amphibole and glass. Fluorine also shows a variety of relationships with BaO in phlogopite and apatite. Based on plots of F/(F + OH) in apatite and phlogopite, apatite and amphibole, and phlogopite and amphibole, the distribution of F is variable in different lamproites and hence determination of partition coefficients is not possible. The F distribution patterns are affected by lack of mutual equilibrium between F-bearing phases, possible removal of F in a fluid phase during magma degassing, variations in physical conditions between different phases during crystallization of the magma, and other causes. This study suggests that F preferentially enters solid rather than liquid phases. Hence, on partial melting of a F-rich mantle source, the F entering melts is unlikely to be sufficient to produce high F-bearing lamproites.

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Fluor führende Phasen in Lamproiten

Zusammenfassung Die Verteilung von Fluor zwischen Phlogopit, Amphibol, Apatit und Glas wurde für 26 Lamproite aus den Leucite Hills, Wyoming; West Kimberley, West-Australien, Smokie Butte, Montana; Prairie Creek, Arkansas; und Gaussberg, Antarktis, bestimmt. Der Fluorgehalt dieser Phasen schwankt in verschiedenen Lamproitvorkommen, und besonders Kimberley. Fluor kommt im allgemeinen besonders reichlich in Phlogopit und Apatit vor. Das Glas in manchen Lamproiten führt keine nachweisbaren Fluorgehalte. Mit zunehmender Evolution von Lamproiten tendiert F in den Gesamtgesteins-analysen, wie auch in Amphibol und im Glas abzunehmen. Fluor zeigt auch Beziehungen zu den BaO Gehalten in Phlogopit und Apatit. Auf der Basis von Plots von F/(F + OH) in Apatit und Phlogopit, Apatit und Amphibol, und Phlogopit und Amphibol, läßt sich erkennen, daß die Verteilung von F in verschiedenen Lamproiten variabel ist und daher eine Bestimmung von Verteilungskoefizienten nicht möglich ist. Die F-Verteilungsmuster werden von dem Mangel an gegenseitigem Euqilibrium zwischen F-führenden Phasen, möglichem Wegtransport von F in einer fluiden Phase während MagmaEntgasung, Variationen in den physikalischen Bedingungen zwischen verschiedenen Phasen bei der Kristallisation des Magmas, und durch andere Ursachen beeinflußt. Diese Untersuchung zeigt, daß F vorzugsweise in festen und nicht in flüssigen Phasen untergebracht wird. Daher wird das Fluor, das in Schmelzen bei der teilweisen Rufschmelzung von fluor-reichen Mantelgesteinen untergebracht wird, kaum ausreichen, um fluor-reiche Lamproite zu erzeugen.

     

Solar Magnetic Activity and Total Irradiance Since the Maunder Minimum

We develop a model for estimating solar total irradiance since 1600 AD using the sunspot number record as input, since this is the only intrinsic record of solar activity extending back far enough in time. Sunspot number is strongly correlated, albeit nonlinearly with the 10.7-cm radio flux (F _10.7), which forms a continuous record back to 1947. This enables the nonlinear relationship to be estimated with usable accuracy and shows that relationship to be consistent over multiple solar activity cycles. From the sunspot number record we estimate F _10.7 values back to 1600 AD. F _10.7 is linearly correlated with the total amount of magnetic flux in active regions, and we use it as input to a simple cascade model for the other magnetic flux components. The irradiance record is estimated by using these magnetic flux components plus a very rudimentary model for the modulation of energy flow to the photosphere by the subphotospheric magnetic flux reservoir feeding the photospheric magnetic structures. Including a Monte Carlo analysis of the consequences of measurement and fitting errors, the model indicates the mean irradiance during the Maunder Minimum was about 1 ± 0.4 W m⁻² lower than the mean irradiance over the last solar activity cycle.     

The Spectroscopic Orbit of the Planetary Companion Transiting HD 209458

Three new silicon-bearing radicals of astrophysical interest, SiCCH and the two nearly isoenergetic isomers SiCN and SiNC, were detected in a laboratory discharge in their X2Pi ground states by Fourier transform microwave and millimeter-wave absorption spectroscopy. Hyperfine structure was observed in the low rotational transitions of the (2)Pi(1/2) ladder, and well-resolved Lambda-doubling was observed in both fine-structure ladders. With the spectroscopic constants derived from the laboratory measurements, the spectra of all three can be calculated to an uncertainty of less than 0.1 km s(-1) in equivalent radial velocity over the entire range of interest to radio astronomers. SiCN, with a dipole moment of 2.9 D, is probably the most promising of the three for astronomical discovery.