Arcade Structures Research Articles

Can Substorm Particle Acceleration Be Applied to Solar Flares?

Abstract Using test particle studies in the electromagnetic fields of three-dimensional magnetohydrodynamic (MHD) simulations of magnetic reconnection, we study the energization of charged particles in the context of the standard two-ribbon flare picture in analogy to the standard magnetospheric substorm paradigm. In particular, we investigate the effects of the collapsing field (“collapsing magnetic trap”) below a reconnection site, which has been demonstrated to be the major acceleration mechanism that causes energetic particle acceleration and injections observed in Earth’s magnetotail associated with substorms and other impulsive events. We contrast an initially force-free, high-shear field (low beta) with low and moderate shear, finite-pressure (high-beta) arcade structures, where beta represents the ratio between gas (plasma) and magnetic pressure. We demonstrate that the energization affects large numbers of particles, but the acceleration is modest in the presence of a significant shear field. Without incorporating loss mechanisms, the effect on particles at different energies is similar, akin to adiabatic heating, and thus is not a likely mechanism to generate a power-law tail onto a (heated or not heated) Maxwellian velocity distribution.

Enhancement of city breathability with half open spaces in ideal urban street canyons

The design guidelines for assessing the wind environment at the pedestrian level have become increasingly important in urban planning over the past few decades. The purpose of this study is therefore to conduct computational fluid dynamics simulations to examine the flow behavior of the aeration around the buildings, with and without an arcade design in an ideal situation in urban street canyons. The results suggest the effectiveness of adopting an arcade design to sustain a stable wind environment and improve the flow aeration over areas with urban street canyons. Computations are also carried out to examine the ventilation performance of the pedestrian pathway layer by varying the parameters of building height, canyon width, and height and width of the arcade. The simulated results are then further processed by using a multivariable regression analysis on a group of dimensionless parameters as well as visualization with the scaling law on arcade structures to facilitate a design for arcade buildings that would enhance city breathability. This correlation which uses multiple dimensionless parameters will act as a reference for future arcade designs.

Impacts of torus model on studies of geometrical relationships between interplanetary magnetic clouds and their solar origins

Abstract Our recent analysis of interplanetary magnetic clouds (MCs) showed that the orientations of MC axes determined by a model fitting with curvature of MCs taken into account (referred to as a torus model, hereafter) can be significantly different from those obtained from fittings with a straight cylinder model. Motivated by this finding, we re-examined geometrical relationships between magnetic field structures of MCs and their solar origins. This paper describes the results of the re-examination with special attention paid to two MC events, for which different orientations of MC axes were obtained from a torus model and a cylinder model. For both cases, it is shown that the torus models give the MC geometries of magnetic field structures in good agreement with those of coronal arcade structures which were formed in association with the launch of MCs along the magnetic field inversion lines. Summarizing the analysis results for 12 MCs investigated here, we conclude that: (1) the formation of coronal arcade structure is a good indication of MC formation; (2) MC geometries can be obtained that are consistent with the coronal arcades with respect to the axis orientation and the magnetic field structure including chirality, indicating that no significant direction changes occurred during the propagation of MCs through the interplanetary medium.

Sandstone-conglomerate rocky forms – more than a tourist attraction

Natural, thrown open to the sightseeing rockies are undoubtedly a tourist attraction, but furthermore they constitute a precious source of geological and geomorphological information. A special scientific-cognitive and teaching value of the rocky forms is based on the possibility of carrying out direct observations on the character of deposits and on shaping these forms by exogenic processes. A good example of the rocky forms with such values is represented by sandstone-conglomerate rockies in the Outer Carpathians. One of the regions where such rockies occur is the Wiśnicz Foothil, where, among others, the Brodziński Stones (after a poet), the Mushroom Stone (after a mushroom-like shape) and the Chronowskie Tors (after the Chronów town) are localized. The Mushroom Stone serves to demonstrate general, common features of the origin of the rocky forms from the Istebna Beds (the Istebna Formation) observed by the author also in the Moravian Beskid Mts. and Silesian Beskid Mts. and the Small Beskid Mts. Epigenetic arcade structures, cellular structures (honey comb structures) and bowl structures form characteristic relief of rockies. The rocky forms have a complex origin, resulting from many interconnected factors and processes that develop in time with various intensity.

Solar Arcades as Possible Minimum Dissipative Relaxed States

In this work, arcade structures are modeled to be in minimum dissipative relaxed states using a two-fluid description of the plasma. The obtained relaxed state is non-force-free in nature and appropriate to an open system with external drives. The Euler – Lagrange equations are solved in Cartesian coordinates subject to the appropriate photospheric boundary conditions. The solutions are seen to support flow-containing arcade-like magnetic field configurations with inherent dissipative properties that may play an important role in coronal heating. An interesting feature observed is the generation of different types of arcades with the variation of a single parameter characterizing the relaxed state. Also, observations with the LASCO coronagraph on board the SOHO spacecraft suggest that helmet streamers originating from the Sun may have an internal triple-arcade structure. The two-fluid relaxed state obtained here is also seen to support such structures.

Energy and Mass Supply in the Decay Phase of Long-Duration Solar Flare Events

A new mechanism is proposed to supply energy and mass in the decay phase of long-duration solar flare events. The long-duration event (LDE) flares are known to be caused by filament eruptions, and bright arcade structures continue for several hours or even a day. To overcome the short cooling time, continuous energy and mass supplies are required. Continuous magnetic reconnection in the current sheet extended above the arcade has been proposed to supply energy and mass. However, no direct observations of the current sheet have been reported. The new mechanism that we are proposing is based on the recent observations of downflows onto the top of arcades along the vertical spiky features observed by the Yohkoh soft X-ray telescope. We interpret these downflows as free-falling plasma. Plasma falling from a high altitude can convert its potential energy into thermal energy and heat itself with an average temperature of several million kelvins. Observations show that the downflows continue for several hours. Coronagraph observations by the Solar and Heliospheric Observatory Large Angle and Spectrometric Coronagraph also show many falling features after coronal mass ejections. Instead of magnetic reconnection, we propose that these falling plasmas are the source of the energy and mass supply for LDEs in the long-lasting decay phase.

SHARP DECREASES OF SOLAR METRIC RADIO STORM EMISSION

We discuss a little-known variety of sharp decreases of long-duration meter-wavelength noise storms and type IV bursts. A survey of the IZMIRAN and AIP radio observations shows that a decrease or nearly complete disappearance of the continuum and bursts developing over tens of minutes without a subsequent recovery of the radio flux occasionally occurs. The decrease is usually preceded by a short-duration (several tens of minutes) enhancement of the radio emission. In these events, the onset of the flux decrease drifts from high to low frequencies with a rate of −(0.05–0.35) MHz s−1, comparable to the drift rates of noise-storm onsets and of chains of type I bursts. White-light coronagraph observations, as well as the characteristics of the accompanying microwave and soft X-ray emissions, provide evidence that such radio decreases appear to be associated with coronal mass ejections (CMEs) and post-CME phenomena. Yohkoh/SXT images show radio flux decrease events which are accompanied by significant rearrangements of coronal structures. We suggest that the radio flux variations are caused by CME interactions with pre-existing coronal arcade structures which are sources of noise storms and energetic electron acceleration. The fact that the noise-storm decreases develop with delays of several tens of minutes relative to the associated microwave burst peak, when the corresponding CME front is located at heights of several R ⊙, however, is not explained.

Simulations of Three‐Dimensional Reconnection in the Solar Corona

Using ideal and resistive MHD, we investigate the stability and dynamic evolution of three- dimensional magnetic —eld con—gurations, representing stretched arcade structures above a dipolar ii photospheric ˇˇ magnetic —eld. Two types of con—gurations are studied that diUer by the amount of divergence (ii fanning ˇˇ) of the initial —eld lines as viewed in the horizontal direction perpendicular to the photospheric magnetic neutral line and, correspondingly, by the radial decrease of magnetic —eld strength and current density. The two sets of con—gurations are found to diUer in their stability behavior. The strongly fanning —elds, associated with a rapid radial decrease of the —eld strength, current density, and plasma pressure, are more stable. A stability diUerence is found also when the con—gurations are —rst subjected to a converging motion of photospheric footpoints toward the neutral line, which leads to the buildup of thin current sheets in the region above. This current sheet formation is more pronounced for the weakly fanning —elds. For similar current density enhancements, the occurrence of anomalous dissipation (resistivity) initiates magnetic reconnection in either con—guration. However, the eUects are much more drastic in magnitude and spread in the weakly diverging —eld structure. In the unstable cases, a strongly localized electric —eld parallel to the magnetic —eld develops, which results in integrated voltages with maximum values of the order of a few hundred MeV, both on open and closed —eld lines. For comparison, we studied both low-beta, force-free, and high-beta initial states. The weakly fanning high-beta con—gurations tend to show more drastic instability eUects than the corresponding low-beta —elds, but the stabilization of the strongly fanning —elds pertains to both low-beta and high-beta —elds. The three-dimensional reconnection in the unstable cases generates a region of intertwined magnetic —ux tubes with diUerent topologies that lie below a region of closed —ux ropes not aUected by reconnection. The topological changes could be the source of open —ux tubes that are occasionally observed within coronal mass ejections, as recently discussed by Gosling et al. The fast outward —ow generated in these simulations aUects only the regions of changing topology but does not cause the above-lying closed —ux ropes to move (within the times considered). This may be seen as an indication that reconnection may be associated with the onset of a —are, initiated after the eruption of a coronal mass ejection, but is not the driver of the coronal mass ejection itself. Subject headings: MHDSun: coronaSun: —aresSun: magnetic —elds

Heating and activity of the solar corona: 3. Dynamics of a low beta plasma with three‐dimensional null points

We investigate the self‐consistent nonlinear evolution of an initially force‐free three‐dimensional magnetic field subjected to stress on two boundaries. The results illustrate how complicated magnetic field structures, such as those found in the solar corona, evolve dynamically when forced by stress from boundaries and how the energy which is temporarily stored in the magnetic field may be converted into other forms of energy such as heat, flow energy, and fast particles. The initial model state is triple periodic and contains eight magnetic null points. During the time evolution, the current density concentrates near particular locations in space that can be identified with the singular field lines connecting pairs of null points of the initial state. Current sheets are found to grow out of the singular lines formed by the intersection of surfaces across which the magnetic connectivity is discontinuous. Jets of plasma shoot out from the edges of the currents sheets, driven by the “sling‐shot” Lorentz force created by reconnecting magnetic field lines. As a result of the reconnection, most of the magnetic connectivity between the two boundaries is lost, and the remaining magnetic field develops arcade‐like structures along the boundaries. These arcade structures are long‐lived, and the system enters a quasi‐stationary state, where small‐scale current sheets are continually appearing and disappearing. The distribution of size of these current sheets is limited only by the numerical resolution. The current sheets dissipate the energy supplied by the boundary driving and also slowly deplete the magnetic energy from the initial constant alpha magnetic field. The dissipation occurs in an increasing number of current sheets of decreasing size at higher numerical resolution, which keeps the overall reconnection rate nearly independent of the numerical resolution. This suggests that “fast reconnection” may occur through the collaborative effort of a large number of many small‐scale current sheets, rather than in the very large magnetic Reynolds number limit of single current sheets that has been so extensively discussed in the literature. This has important applications to both the problem of understanding coronal heating and the search for efficient flare energy release mechanisms.

Quasi-static evolution in multiple arcades

The slow dynamical evolution of solar atmospheric magnetic field structures via the equilibrium equation has been the subject of a number of investigations. In many of these studies the quasi-static evolution of the field and the associated plasma has been investigated for a single arcade structure. In this paper we present results for multiple arcade structures. For multiple arcades we do not find bifurcations and the consequent multiple solutions as the field is evolved through a sequence of equilibrium states, in contrast to the findings for single arcade. Further we show that particular polarity arrangements within a pair of arcade structures lead to quite different topologies of the field as it is evolved. When new flux emerges from the photospheric boundary under a pre-existing magnetic arcade the results suggest that such a mechanism will initiate coronal mass ejection.

Magnetic arcade evolution and instability

The evolution of two-dimensional coronal magnetic arcades driven by photospheric shear flows is studied by numerical solution of the resistive MHD equations neglecting pressure and gravitational forces. By varying the distribution of the frozen-in photospheric magnetic flux, the shear flow profile and the magnetic Reynolds number, a fairly general picture is obtained. Isolated arcades develop in a quasi-selfsimilar stable way, invalidating previous studies of equilibrium sequences ▽2ψ = αF(ψ) with monotonically increasing parameter α. Groups of several interacting arcades show a more complex behavior. When of sufficiently large height arcade structures tend to bifurcate, leading to plasmoid (or filament) formation. Usually this is a slow resistive process and the plasmoid is confined in the arcade interior. Configurations containing at least three arcades may give rise to fast plasmoid ejection.

The effect of gravity on the stability of a line-tied coronal magnetohydrostatic equilibrium

Abstract The magnetohydrodynamic stability of a class of magnetohydrostatic equilibria is investigated. The effect of gravity is included as well as the stabilising influence of the dense photospheric line-tying. Although the two-dimensional equilibria exhibit a catastrophe point, when the ratio of plasma pressure to magnetic pressure exceeds a critical value, arcade structures, with both footpoints connected to the photosphere, become unstable to three-dimensional disturbances before the catastrophe point is reached. Numerical results for field lines that are open into the solar corona suggest that they are completely stable. Although there is no definite proof of stability, this would allow the point of non-equilibrium to be reached.

Theoretical study of onset conditions for solar eruptive processes

We apply the model of quasistatic equilibrium sequences to describe the time development of magnetic field structures in the plasma of the solar corona, and to determine onset points of a dynamical evolution. The representation of the magnetic field by Euler potentials provides a realistic modeling of the photospheric boundary conditions. We present a numerical method suited for the computation of magnetohydrodynamic equilibrium states and for analysing their stability against perturbations within ideal MHD. Pressure and magnetic footpoint displacement can be prescribed separately as boundary conditions. We consider magnetic arcade structures typical for large two-ribbon flares. Our results indicate that a finite pressure gradient seems to be essential for the existence of onset points. Furthermore, it is shown that magnetic shear destabilizes for intermediate values, but can have a stabilizing effect for a large amount of shear.

Hydromagnetic stability of coronal arcade structures: The effects of photospheric line tying

A model is given of the magnetic-field equilibrium and possible dynamic excitations of a solar coronal arcade. Such structures are well observed in the spectral range from Hα to X-rays and often give rise to two-ribbon flares. However, the preflare state must be stable to ideal magnetohydrodynamic disturbances, and we treat this problem with particular attention to the necessary foot-point boundary conditions. Having devised a reasonably general perturbation set, we use an energy-principle analysis to show the strong stabilizing influence of inertial field-line tying at the photosphere.