Eruptive Flux Research Articles

Numerical modeling of solar magnetostatic structures bounded by current sheets

A numerical method for efficiently determining the magnetostatic equilibrium configuration of erupted solar flux concentrations, such as sunspots and flux tubes, is presented. The magnetic structures are taken to be approximately vertically oriented and axisymmetric in the surface layers and are assumed to be isolated from the surrounding photosphere by a vanishingly thin current sheet. Since the location of the current sheet is initially unknown, the final structure is generated iteratively as a free-surface problem, with the magnetic configuration for each iterate being obtained from the horizontal force balance equation, subject to the appropriate boundary conditions. Multigrid methods are used at each stage to solve the equilibrium equation, which is mapped algebraically into a body-fitted coordinate system via transfinite interpolation techniques. Several model flux tubes and sunspots are computed to illustrate the procedure, and the accuracy of the numerical method is assessed against exact analytic solutions. 32 refs.

Sulfur dioxide from Nevado del Ruiz volcano, Colombia: total flux and isotopic constraints on its origin

Nevado del Ruiz volcano has been releasing extraordinarily large volumes of sulfur dioxide gas from its Arenas crater since September, 1985 and probably since the renewal of volcanic activity began in late 1984. The combined release from eruptive and passive flux has been approximately (3.4 ± 1.0) × 10 6 metric tons in the four years after November, 1984. This value combined with data of others on pre-eruption sulfur content of the magma, allows the calculation of a minimum required volume of magma of 0.92 km 3. Consideration of the continued high flux and typical ratios of erupted to degassed magma indicate that the true minimum volume of magma involved is probably at least 4.6–9.2 km 3. A systematic study of the various species of sulfur in the volcanic plume and the hydrothermal system has provided a characterization of the geochemistry and sulfur-isotopic variation. Consideration has been given to five potential sources of the sulfur dioxide: dissolution of subvolcanic evaporites; remobilization and oxidation of native sulfur within summit crater fumaroles; large-scale release from the hydrothermal system by reduction of sulfate or water-rock interaction; assimilation and oxidation of a pre-existing sulfide deposit; and magmatic volatiles. The data are most realistically explained by the passage of magmatic gas through the hydrothermal system, with some disproportionation of sulfur dioxide to produce the high sulfate content and low pH of the waters. This model is also most consistent with the distribution of acid-sulfate-chloride hot springs, high chloride and fluoride content of the hydrothermal waters, sulfide deposition within the volcanic pile, and the transient increase in sulfate content of the acid-sulfate-chloride hot springs that reached a maximum approximately one year after the November, 1985 eruption. The magmatic model is in apparent conflict with the absence of general deformation from the time that data began to be collected, about one week before the eruption, until the time this manuscript was finalized (September, 1989). Although this conflict cannot be resolved with certainty, it is noted that the absence of deformation is only an indication of the lack of magma movement, not of the absence of magma. An alternative magmatic model considers the available geological data and proposes that the system is related to a large-scale caldera. In this model, the degassing is related to a long-term process of exsolution of magmatic volatiles from a large volume of crystallizing magma left from the construction of modern Ruiz. This model can better explain the combined geological and geophysical data, as well as the geochemical and isotopic data. The nature of the degassing data at Ruiz is interesting because it is an example of an explosive eruption cycle in which the gas flux is actually dominated by passive release. The nature of the degassing allows us to hypothesize that some major change in the system occurred in April–May, 1986, and to predict that the present high flux can be expected to last for another decade.

The effect of newly erupting flux on the polar coronal holes

He i 10830 A images show that early in sunspot cycles 21 and 22, large bipolar magnetic regions strongly affected the boundaries of the nearby polar coronal holes. East of each eruption, the hole boundary immediately contracted poleward, leaving a band of enhanced helium network. West of the eruption, the boundary remained diffuse and gradually expanded equatorward into the leading, like-polarity part of the bipolar magnetic region. Comparisons between these observations and simulations based on a current-free coronal model suggest that: (1) The Sun's polar magnetic fields are confined to relatively small caps of high average field strength, apparently by a poleward meridional flow. (2) The enhanced helium network at high latitude marks the location of relatively strong polar fields that have become linked to the newly erupted bipolar region in that hemisphere. (3) The distortion of the polar-hole boundary is accompanied by a corresponding distortion of the equatorial neutral sheet in the outer corona, in which the amount of warping depends on the magnitude of the erupted flux relative to the strength of the Sun's polar magnetic fields.