Magnetic reconnection driven by recombination during collision of two current loops

It is shown by using a 3-D resistive MHD simulation code, taking into account the recombination effect, that magnetic reconnection during collision of two current loops can be enhanced by recombination. It is also shown that the temperature in the thin current sheet formed between two loops increases from few to about thirty times larger than a case of no recombination, depending on both the plasma beta and the strength of recombination. The simulation results obtained here may be applicable for a mechanism of chromospheric heating and as an explanation of X-ray bright points as well as solar flares observed in the chromosphere.Dedicated to Cornelis de Jager     

Nonlinear magnetosonic waves propagating perpendicular to a magnetic neutral sheet

Nonlinear magnetosonic waves propagating in a magnetic neutral sheet are investigated within the framework of a fluid model. It is shown that the behavior of the magnetosonic waves is governed by a ‘modified Burgers equation’ with an additional termc(η)? due to the relevant slowly varying background plasma parameter (density or magnetic field), $$\frac{{\partial \phi }}{{\partial \eta }}$$ where ?(ξ, η) is the amplitude of the wave, \(\xi = \int {k_x } {\text{d}}x + k_y y - \omega t\) , and η=εx is the coordinate stretched by a smallness parameter ε. When we consider fast magnetosonic waves propagating toward the neutral region across the magnetic field, they grow and undergo rapid steepening after passing through the neutral region; i.e., shock formation is promoted by the background inhomogeneity. By the numerical computation of the above equation, the time evolution is examined for two initial disturbances, the pulse type (gaussian) and the wave train type (sinusoidal wave). The relevance of the interactions between the magnetosonic shock waves and the neutral sheet plasma to a triggering mechanism of sympathetic flares is also suggested.     

Formation of tonalite from basaltic magma at the Komahashi-Daini Seamount,northern Kyushu-Palau Ridge in the Philippine Sea,and growth of Izu-Ogasawara (Bonin)-Mariana arc crust

Tonalitic rocks dredged from the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge are classified as biotite-hornblende tonalites and hornblende tonalites. These rocks have radiometric ages of 37-38 Ma, indicating that felsic plutonic activity occurred during the early stages of Izu-Ogasawara (Bonin)-Mariana (IBM) arc volcanism. Therefore, this tonalite complex has great importance for understanding the initial processes of island arc and continental crust formation. These tonalitic rocks exhibit the following petrological and geochemical characteristics: (1) common lamellar twins and oscillatory zoning patterns in plagioclase phenocrysts throughout the compositional range; (2) hornblende tonalite shows parallel REE patterns and increasing total REE content with increasing SiO₂, except for an increasingly strong negative Eu anomaly at higher SiO₂ levels; and (3) isotopic composition remains constant over a wide silica variation. We compare this tonalite with younger tonalities of the same arc from the Tanzawa Complex (10-5 Ma), central Japan, considered to represent the lower-middle crust of the IBM arc, and find the following differences: (1) cumulate textures found in Tanzawa tonalites are not observed in samples from the Komahashi-Daini Seamount; and (2) Komahashi-Daini Seamount tonalites, unlike those from Tanzawa, exhibit linear variations of Zr and REEs vs. SiO₂ plots. These data and other observations support the interpretation that tonalite in the Komahashi-Daini Seamount was produced by crystal fractionation from basaltic magma. We suggest that fractional crystallization operated during the early stage of oceanic island arc formation to produce tonalite, whereas tonalities in later stages formed largely by partial melting of basaltic lower crust, as represented by the tonalites in the Tanzawa Complex.     

Measurement of the solar wind velocity with cometary tail rays

Cometary tail rays are traces of the magnetic fields caught in the cometary magnetosphere. Time variations of these rays give us a way to measure the local solar wind velocity at the location of a comet. We introduce a simple method for determining the radial velocity of the solar wind by observing the ray folding motion, and show an example of its application to comet P/Brorsen-Metcalf 1989o, which resulted in 340 ± 35 km s^–1.     

Modulational instability of fast magnetosonic waves in a solar plasma

Transverse amplitude modulations of fast magnetosonic waves propagating perpendicular to the background magnetic field are shown to be unstable on a time scale τ ～- λ_∥/V _aφ, if the wave amplitude φ exceeds a critical value, φ _c = C _s/V _a. The slow modes generated by the modulational instability under gravity can propagate along the magnetic field with the characteristic velocity, V _ph = g/2k _∥ V _aφ. The applications of this modulational instability and slow-mode generation mechanism to a solar plasma are discussed.     

A model of solar flares triggered by interactions between force-free current loops and plasmoids

We present a model of solar flares triggered by collisions between current loops and plasmoids. We investigate a collision process between a force-free current loop and a plasmoid, by using 3-D resistive MHD code. It is shown that a current system can be induced in the front of a plasmoid, when it approaches a force-free current loop. This secondary current produced in the front of the plasmoid separates from the plasmoid and coalesces to the force-free current loop associated with the magnetic reconnection. The core of the plasmoid stays outside the reconnection region, maintaining high density. The core can be confined by the current system produced around the plasmoid. This collison process between a current loop and a plasmoid may explain the triggering of solar flares observed byYohkoh.     

Variations of magnetic susceptibility and fine quartz accumulation rate in Daisen loam over the past 200 000 years: Interaction between winter and summer monsoons in south-west Japan

Abstract A loam section near Daisen volcano, South-west Japan, has been examined for low-field magnetic susceptibility (MS) and fine quartz accumulation rate. Fission track dating of tephra layers interbedded in the deposit shows that the loam age ranges from about 200 ka to the Present. The MS was measured for both bulk sample and the < 63 μm fine fraction. Fine quartz contents in the < 63 μm fraction were also determined using acid-alkali digestions and recalculated to derive fine quartz accumulation rate (Rqz). Grain size analysis was then carried out on the separated fine quartz. Low-field MS varies from low frequency magnetic suspectibility (χ(LF)) 5 to 100 (× 10⁻⁶ m³/kg) for bulk samples and from 1 to 30 for fine fractions. The fine fraction χ(LF) variation correlated with Chinese loess MS stratigraphy, which indicated changes in pedogenic enhancement of the MS and is reflected by summer monsoon intensity. The Rqz are high in cool climate stages, with volumes between 0.2 and 0.4 (× 10⁻² kg/m² per yr), whereas in warm stages the rate falls to about 0.1. These values compare well with those reported from the Hokkaido and Kanto areas, suggesting the fine quartz originates from tropospheric dust. The strong winter monsoons during glacial stages alternated with weak summer monsoons as a result of a southward shift of the jet stream. In interglacials, summer monsoons were stronger. Seasonal alternating monsoons appear to have operated in South-west Japan through the past 200 000 years.     

Early tholeiitic and calc-alkaline arc magmatism of middle to Late Eocene Age in the southern Ogasawara (Bonin) forearc

Analysis of middle and upper Eocene rocks from the IBM forearc, including the Ogasawara and Mariana Islands, help illuminate early arc volcanism of the proto-IBM arc. Dredged volcanic rocks from the forearc are two-pyroxene basalt to andesite, and may be divided into two groups, tholeiite and calc-alkaline, on the basis of mineralogy, petrography, and bulk chemistry. Tholeiites are characterized by high HFSE contents, high crystallization temperatures, and low water contents. In contrast, the calc-alkaline rocks are characterized by low HFSE contents, low crystallization temperatures, and higher water contents. These characteristics indicate that magma genesis for the two series differed. The tholeiites resulted from high degrees of partial melting of slightly depleted mantle under anhydrous conditions, whereas the calc-alkaline rocks were generated by low degrees of melting of depleted mantle under hydrous conditions. We believe that differences in mantle depletion arose from compositional layering and fluid zonations caused by MORB volcanism and slab dehydration, respectively. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Acceleration through X-type coalescence in a small energetic flare

We studied the acceleration conditions in the small but fairly energetic flare of May 21, 1984 at 1326 UT. The most pronounced aspect of this flare was a series of 13 microwave/X-ray spikes, each lasting for about 0.1 s. A previous study has shown that each of these was due to a series of successive sudden formations of small plasma knots of high-energy particles. Each of these knots lost its energy in about 50 ms. In the present study we show that these knots can originate by the process of X-type (3-D) flux tube coalescence. The predicted rise time (30 to 50 ms) and energy are in good agreement with the observationally derived parameters.     

Plasma jet and shock formation during current loop coalescence in solar flares

We report on the results of plasma jet and shock formation during the current loop coalescence in solar flares. It is shown by a theoretical model based on the ideal MHD equation that the spiral, two-sided plasma jet can be explosively driven by the plasma rotational motion induced during the two current loop coalescence process. The maximum velocity of the jet can exceed the Alfvén velocity, depending on the plasma (= c _s ² v _A ² ) ratio. The acceleration time getting to the maximum jet velocity is quite short and le than 1 s. The rebound following the plasma collapse driven by magnetic pinch effect can strongly induce super-Alfvénic flow. We present the condition of the shock formation. We briefly discuss the high-energy particle acceleration during the plasma collapse as well as by the shocks.