Motion Of Flux Tubes Research Articles

Impact of the Dipole Tilt Angle on the Ionospheric Plasma as Modeled with IPIM

AbstractWe ran the IRAP Plasmasphere Ionosphere Model (IPIM) for several flux tubes at different distances from Earth for very quiet conditions, in order to model the convection and corotation transport of closed flux tubes in the plasmasphere for tilted/eccentric dipolar magnetic field configuration and for solstice and equinox conditions. For each simulation, the model was run for 30 days in order to study the long‐term evolution of the plasmasphere‐ionosphere system. The goal is to study the combined effect of corotation and convection, which causes eventually a lag of the flux tube motion with respect to the Earth. This combination of nonuniform rotation and subcorotation in average becomes important especially in the outer plasmasphere and causes nonuniform drift of the dipole tilt projection in the plane of the flux tube with time, changing drastically the plasma distribution along this flux tube. In the classical representation of the plasmasphere, the ionosphere only depends on angular Magnetic Local Time (MLT) sector. We show that due to the tilt effect, solar illumination along the flux tube and field‐aligned transport varies from one day to the other and causes also a day‐to‐day variation in the plasma distribution along the tube. This effect implies that a stable representation of the plasmasphere in MLT during quiet conditions is an erroneous view, and no real dynamic equilibrium can be reached, in particular close to the stagnation point.

Dipolarization Fronts: Tangential Discontinuities? On the Spatial Range of Validity of the MHD Jump Conditions

AbstractMagnetotail dipolarization fronts (DFs) are referred as the sharp increase of the northward magnetic field component, embedded in bursts of fast earthward moving plasma flows, so called bursty bulk flows. Earlier studies often considered DFs as tangential discontinuities (TDs), which can be understood as thin vertical current layers of earthward moving flux tubes, so called dipolarzing flux bundles (DFBs), which separate the ambient plasma sheet plasma from the low entropy plasma within the DFB. Here we present a statistical study of 23 DFs observed by the Magnetospheric Multiscale mission during 2017 and 2018 when the apogee was at 25 RE in the magnetotail. We perform a test of the Walén relation to distinguish whether the observed DFs have rather a TD or rotational discontinuity character and evaluate the plasma flow across the DFs in detail. The results show that on MHD large scales, all 23 DFs can be considered as TD like, but sometimes may have a significant normal plasma flow across it: for 16 events (∼70%), the plasma flows mainly tangential to the DFs, while for seven events (∼30%), the plasma flows mainly across the DFs. Based on the findings present in this study, we further hypothesize that the DF structure becomes more distorted and unstable in a (locally) more dipolarized background magnetic field region, which may additionally facilitate the plasma flow across the front.

Multiscale Currents Observed by MMS in the Flow Braking Region.

We present characteristics of current layers in the off‐equatorial near‐Earth plasma sheet boundary observed with high time‐resolution measurements from the Magnetospheric Multiscale mission during an intense substorm associated with multiple dipolarizations. The four Magnetospheric Multiscale spacecraft, separated by distances of about 50 km, were located in the southern hemisphere in the dusk portion of a substorm current wedge. They observed fast flow disturbances (up to about 500 km/s), most intense in the dawn‐dusk direction. Field‐aligned currents were observed initially within the expanding plasma sheet, where the flow and field disturbances showed the distinct pattern expected in the braking region of localized flows. Subsequently, intense thin field‐aligned current layers were detected at the inner boundary of equatorward moving flux tubes together with Earthward streaming hot ions. Intense Hall current layers were found adjacent to the field‐aligned currents. In particular, we found a Hall current structure in the vicinity of the Earthward streaming ion jet that consisted of mixed ion components, that is, hot unmagnetized ions, cold E × B drifting ions, and magnetized electrons. Our observations show that both the near‐Earth plasma jet diversion and the thin Hall current layers formed around the reconnection jet boundary are the sites where diversion of the perpendicular currents take place that contribute to the observed field‐aligned current pattern as predicted by simulations of reconnection jets. Hence, multiscale structure of flow braking is preserved in the field‐aligned currents in the off‐equatorial plasma sheet and is also translated to ionosphere to become a part of the substorm field‐aligned current system.

Stagnation of Saturn's auroral emission at noon

AbstractAuroral emissions serve as a powerful tool to investigate the magnetospheric processes at Saturn. Solar wind and internally driven processes largely control Saturn's auroral morphology. The main auroral emission at Saturn is suggested to be connected with the magnetosphere‐solar wind interaction, through the flow shear related to rotational dynamics. Dawn auroral enhancements are associated with intense field‐aligned currents generated by hot tenuous plasma carried toward the planet in fast moving flux tubes as they return from tail reconnection site to the dayside. In this work we demonstrate, based on Cassini auroral observations, that the main auroral emission at Saturn, as it rotates from midnight to dusk via noon, occasionally stagnates near noon over a couple of hours. In half of the sequences examined, the auroral emission is blocked close to noon, while in three out of four cases, the blockage of the auroral emission is accompanied with signatures of dayside reconnection. We discuss some possible interpretations of the auroral “blockage” near noon. According to the first one, it could be related to local time variations of the flow shear close to noon. Auroral local time variations are also suggested to be initiated by radial transport process. Alternatively, the auroral blockage at noon could be associated with a plasma circulation theory, according to which tenuously populated closed flux tubes as they return from the nightside to the morning sector experience a blockage in the equatorial plane and they cannot rotate beyond noon.

Modeling Kelvin–Helmholtz Instability in Soft X-Ray Solar Jets

Development of Kelvin–Helmholtz (KH) instability in solar coronal jets can trigger the wave turbulence considered as one of the main mechanisms of coronal heating. In this review, we have investigated the propagation of normal MHD modes running on three X-ray jets modeling them as untwisted and slightly twisted moving cylindrical flux tubes. The basic physical parameters of the jets are temperatures in the range of 5.2–8.2 MK, particle number densities of the order of109 cm−3, and speeds of 385, 437, and 532 km s−1, respectively. For small density contrast between the environment and a given jet, as well as at ambient coronal temperature of 2.0 MK and magnetic field around 7 G, we have obtained that the kink (m=1) mode propagating on moving untwisted flux tubes can become unstable in the first and second jets at flow speeds of≅348 and 429 km s−1, respectively. The KH instability onset in the third jet requires a speed of≅826 km s−1, higher than the observed one. The same mode, propagating in weakly twisted flux tubes, becomes unstable at flow speeds of≅361 km s−1for the first and of 443 km s−1for the second jet. Except the kink mode, the twisted moving flux tube supports the propagation of higher (m>1) MHD modes that can become unstable at accessible jets’ speeds.

Plasmapause formation at Saturn

AbstractCassini observations during a rapid, high‐latitude, dawnside pass from Saturn's lobe to inner magnetosphere on 25 June 2009 provide strong evidence for the formation of a “plasmapause” at Saturn by Vasyliunas‐type nightside reconnection of previously mass‐loaded flux tubes. A population of hot, tenuous plasma that lies between the lobe and the dense inner magnetospheric plasma is consistent with a region formed by very recent injection from a reconnection region in the tail, including low density, high temperature, supercorotational flow, a significant O+ content, and the near‐simultaneous observation of enhanced Saturn kilometric radiation emissions. The sharp boundary between that region and the cool dense inner magnetospheric plasma thus separates flux tubes that were involved in the reconnection from those that successfully traversed the nightside without mass loss. This event demonstrates that tail reconnection can strip off inner magnetospheric plasma in to at least dipole L = 8.6. Clear evidence of flux tube interchange driven by the sharp boundary is found, both inward moving flux tubes of hotter plasma and, for the first time, the outward moving cool population. The outward moving cool regions have azimuthal sizes less than 1 RS, were probably created within the past 1.2 h, and have outflow speeds greater than about 5 km/s. At the outer edge of the reconnected region, there is also a possible signature of Dungey‐type lobe reconnection following the initial Vasyliunas‐type reconnection. Observations from this event are entirely consistent with previously described global MHD simulations of tail reconnection, plasmoid departure, and Saturnward injection of reconnected flux.

A multi-scale magnetotail reconnection event at Saturn and associated flows: Cassini/UVIS observations

We present high-resolution Cassini/UVIS (Ultraviolet Imaging Spectrograph) observations of Saturn’s aurora during May 2013 (DOY 140–141). The observations reveal an enhanced auroral activity in the midnight-dawn quadrant in an extended local time sector (∼02 to 05LT), which rotates with an average velocity of ∼45% of rigid corotation. The auroral dawn enhancement reported here, given its observed location and brightness, is most probably due to hot tenuous plasma carried inward in fast moving flux tubes returning from a tail reconnection site to the dayside. These flux tubes could generate intense field-aligned currents that would cause aurora to brighten. However, the origin of tail reconnection (solar wind or internally driven) is uncertain. Based mainly on the flux variations, which do not demonstrate flux closure, we suggest that the most plausible scenario is that of internally driven tail reconnection which operates on closed field lines. The observations also reveal multiple intensifications within the enhanced region suggesting an x-line in the tail, which extends from 02 to 05LT. The localised enhancements evolve in arc and spot-like small scale features, which resemble vortices mainly in the beginning of the sequence. These auroral features could be related to plasma flows enhanced from reconnection which diverge into multiple narrow channels then spread azimuthally and radially. We suggest that the evolution of tail reconnection at Saturn may be pictured by an ensemble of numerous narrow current wedges or that inward transport initiated in the reconnection region could be explained by multiple localised flow burst events. The formation of vortical-like structures could then be related to field-aligned currents, building up in vortical flows in the tail. An alternative, but less plausible, scenario could be that the small scale auroral structures are related to viscous interactions involving small-scale reconnection.

PHOTOSPHERIC PROPERTIES OF WARM EUV LOOPS AND HOT X-RAY LOOPS

We investigate the photospheric properties (vector magnetic fields and horizontal velocity) of a well-developed active region, NOAA AR 10978, using the Hinode Solar Optical Telescope specifically to determine what gives rise to the temperature difference between ''warm loops'' (1-2 MK), which are coronal loops observed in EUV wavelengths, and ''hot loops'' (>3 MK), coronal loops observed in X-rays. We found that outside sunspots, the magnetic filling factor in the solar network varies with location and is anti-correlated with the horizontal random velocity. If we accept that the observed magnetic features consist of unresolved magnetic flux tubes, this anti-correlation can be explained by the ensemble average of flux-tube motion driven by small-scale random flows. The observed data are consistent with a flux tube width of ∼77 km and horizontal flow at ∼2.6 km s{sup –1} with a spatial scale of ∼120 km. We also found that outside sunspots, there is no significant difference between warm and hot loops either in the magnetic properties (except for the inclination) or in the horizontal random velocity at their footpoints, which are identified with the Hinode X-Ray Telescope and the Transition Region and Coronal Explorer. The energy flux injected into the coronal loops by the observed photospheric motion ofmore » the magnetic fields is estimated to be 2 × 10{sup 6} erg s{sup –1} cm{sup –2}, which is the same for both warm and hot loops. This suggests that coronal properties (e.g., loop length) play a more important role in giving rise to temperature differences of active-region coronal loops than photospheric parameters.« less

Color-magnetic flux tubes in quark matter cores of neutron stars

We argue that if color-superconducting quark matter exists in the core of a neutron star, it may contain a high density of flux tubes, carrying flux that is mostly color magnetic, with a small admixture of ordinary magnetic flux. We focus on the two-flavor color-superconducting (‘2SC’) phase, and assume that the flux tubes are energetically stable, although this has not yet been demonstrated. The density of flux tubes depends on the nature of the transition to the color-superconducting phase, and could be within an order of magnitude of the density of magnetic flux tubes that would be found if the core were superconducting nuclear matter. We calculate the cross-section for Aharonov–Bohm scattering of gapless fermions off the flux tubes, and the associated collision time and frictional force on a moving flux tube. We discuss the other forces on the flux tube, and find that if we take into account only the forces that arise within the 2SC core region, then the timescale for expulsion of the color flux tubes from the 2SC core is of order 1010 years.

Electron density measurement in a pulsed-power plasma by FIR laser beam deflection and/or interferometry

A pulsed-power experiment has been designed to produce arc-shaped magnetic flux tubes similar to ascending solar flares. The tubes are filled with hydrogen plasma (electron temperature ≤ 10 eV, electron density 2...3×1020 m−3) and expand with a velocity of ~2.5 cm/μs, while keeping their cross section constant at a radius of about 1.5 cm. For measuring the spatial electron density distribution within the moving flux tube, a single cw laser beam can be used. The information taken from the laser beam, which traverses the vacuum vessel perpendicular to the plane of the plasma arch, can be either the phase shift or the beam deflection due to the density gradient. Assuming a parabolic distribution with a central electron density of 2 × 1020 m−3, the maximum deflection angle occurring at an impact parameter of 0.7 amounts to γmax/deg ≊ 10−5 × (λ/μm)2. Hence, a FIR laser operating at λ = 433 μm would be deflected by γmax = 1.9° only. Alternatively, a beam passing through the plasma centre would experience a plasma-induced phase shift of Δϕmax/rad ≊ 10−2 × (λ/μm), yielding 4.3 rad for a FIR laser (λ = 433 μm) and 0.1 rad for a CO2 laser (λ = 10.6 μm). While the former is readily detectable in a standard interferometer, the latter requires a more advanced technique of measurement to achieve the necessary resolution. On the other hand, the short wavelength compared to FIR radiation allows for a very narrow beam and hence for a high spatial resolution. For these reasons a so-called coupled-cavity scheme for a CO2 laser interferometer is presently under development.

High‐resolution observations of extremely bright penumbral grains

AbstractWe observed a cluster of extremely bright penumbral grains located at the inner limb‐side penumbra of the leading sunspot in active region NOAA 10892. The penumbral grains in the cluster showed a typical peak intensity of 1.58 times the intensity I0 of the granulation surrounding the sunspot. The brightest specimen even reached values of 1.8–2.0 I0, thus, exceeding the temperatures of the brightest granules in the immediate surroundings of the sunspot. We find that the observed sample of extremely bright penumbral grains is an intermittent phenomenon, that disappears on time scales of hours. Horizontal flow maps indicating proper motions reveal that the cluster leaves a distinct imprint on the penumbral flow field. We find that the divergence line co‐located with the cluster is displaced from the middle penumbra closer towards the umbra and that the radial outflow velocities are significantly increased to speeds in excess of 2 km s–1. The extremely bright penumbral grains, which are located at the inner limb‐side penumbra, are also discernible in offband Hα images down to Hα ± 0.045 nm. We interpret the observations in the context of the moving flux tube model arguing that hotter than normal material is rapidly ascending along the inner footpoint of the embedded flux tube, i.e., the ascending hot material is the cause of the extremely bright penumbral grains. This study is based on speckle‐reconstructed broad‐band images taken at 600 nm and chromospheric Hα observations obtained with two‐dimensional spectroscopy. All data were taken with adaptive optics under very good seeing conditions at the Dunn Solar Telescope, National Solar Observatory/Sacramento Peak, New Mexico on 2006 June 10. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Conjugate observations of ENA signals in the high‐altitude cusp and proton auroral spot in the low‐altitude cusp with IMAGE spacecraft

On 28 April 2001, significant enhancements of neutral atom signals were detected in the direction of a high‐altitude cusp by the Low‐Energy Neutral Atom (LENA) imager onboard the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE). Simultaneously, proton auroral emission was observed in the low‐altitude cusp by the Far‐Ultraviolet Instrument (FUV) on the IMAGE spacecraft. The temporal variations of their intensities showed a good correlation, suggesting they had a common source. During a brief period, the proton auroral spot moved antisunward and dawnward in conjunction with the motion of the ionospheric footprint of the LENA cusp signal. The Tsyganenko‐96 model shows that the possible source location of the LENA cusp signals maps to the FUV spot. Considering the solar wind variations, we have attributed this “moving proton auroral spot” to a moving flux tube that was created by transient reconnection on the dayside magnetopause and contained relatively high proton densities.

Global MHD simulation of flux transfer events at the high‐latitude magnetopause observed by the Cluster spacecraft and the SuperDARN radar system

A global magnetohydrodynamic numerical simulation is used to study the large‐scale structure and formation location of flux transfer events (FTEs) in synergy with in situ spacecraft and ground‐based observations. During the main period of interest on the 14 February 2001 from 0930 to 1100 UT the Cluster spacecraft were approaching the Northern Hemisphere high‐latitude magnetopause in the postnoon sector on an outbound trajectory. Throughout this period the magnetic field, electron, and ion sensors on board Cluster observed characteristic signatures of FTEs. A few minutes delayed to these observations the Super Dual Auroral Radar Network (SuperDARN) system indicated flow disturbances in the conjugate ionospheres. These “two‐point” observations on the ground and in space were closely correlated and were caused by ongoing unsteady reconnection in the vicinity of the spacecraft. The three‐dimensional structures and dynamics of the observed FTEs and the associated reconnection sites are studied by using the Block‐Adaptive‐Tree‐Solarwind‐Roe‐Upwind‐Scheme (BATS‐R‐US) MHD code in combination with a simple open flux tube motion model (Cooling). Using these two models the spatial and temporal evolution of the FTEs is estimated. The models fill the gaps left by measurements and allow a “point‐to‐point” mapping between the instruments in order to investigate the global structure of the phenomenon. The modeled results presented are in good correlation with previous theoretical and observational studies addressing individual features of FTEs.

Simultaneous tracking of reconnected flux tubes: Cluster and conjugate SuperDARN observations on 1 April 2004

Abstract. While the Cluster spacecraft were located near the high-latitude magnetopause, between 11:30–13:00 UT on 1 April 2004, a series of medium to large scale (40 nT, 0.6–1.2 Re) FTEs were observed. During this pass, simultaneous and conjugated SuperDARN measurements are available that show a global flow pattern which is consistent with the expected (mapped) north-west motion of (predominantly sub-solar) reconnected, magnetic flux at the magnetopause. We focus on analysing the local response of three FTEs, tracking their magnetopause motion via the four-spacecraft measurements together with their corresponding ground mapped motions. For two of these FTEs, where the tracking is strongly coordinated with the ionospheric flow at each footprint of the implied flux tubes in the Northern Hemisphere, conditions corresponded to stable, increasing (>100°) clock angle, while the third event, where the correspondence is less strong, coincided with low (<100°) clock angle. Flux tube motion, both measured and modeled from the inferred X-line, qualitatively matches the clear velocity enhancements in ionospheric convections with northward and westward flow at each location in the Northern Hemisphere, measured simultaneously by SuperDARN, and also roughly matches the observed, south-eastward ionospheric flow in the Southern Hemisphere at the time of these events. The time periods of these velocity enhancements infer that the evolution time of the FTEs is about 4–6 min from its origin on magnetopause to its addition to the polar cap. However, the ionospheric response time in the Southern Hemisphere might be 2 min longer for the 12:31 UT FTE (and 6 min longer for the 12:51 UT FTE) than the response time in the Northern Hemisphere.

Electron circulation in Saturn's magnetosphere

We present a model wherein electrons produced in Saturn's inner magnetosphere circulate through a combination of outward and inward motions driven by the centrifugal interchange instability and azimuthal motion through gradient and curvature drifts. Cool (<100 eV) electrons produced inside L ∼ 12 move slowly outward. To balance outflowing flux, inward transport occurs in small scale injection events. Electrons in these inwardly moving flux tubes are heated adiabatically to energies greater than 100 eV and their pitch angle distributions evolve from isotropic to “pancake” (peaked at 90°). We show that this evolution is observed, and that the pitch angle distributions observed inside a plasma injection are consistent with loss free inward adiabatic transport from L ∼ 11. As the flux tube moves inward the warm electrons undergo energy dependent gradient and curvature drifts out of the inwardly moving flux tube and find themselves superposed on cold, locally produced, plasma. At this point they turn around and are transported along with the cold plasma back toward the outer magnetosphere. With reasonable assumptions about scattering and loss this motion can naturally lead to the “butterfly” electron pitch angle distributions (with local minima at both 90° and 0/180°) that are observed in the warm electron plasma component. We note that we cannot reproduce the butterfly distributions using loss free outward adiabatic transport alone. This is to be expected because there exist pitch angle dependent losses in the form of Saturn's neutral gas cloud and E‐ring.

Flux transfer events simultaneously observed by Polar and Cluster: Flux rope in the subsolar region and flux tube addition to the polar cusp

In this paper we present observational evidence of a flux transfer event observed simultaneously at low‐latitude by Polar and at high‐latitude by Cluster. This event occurs on 21 March 2002, when both Cluster and Polar are located near local noon but with a large latitudinal separation. During the event, Cluster is moving outbound from the polar cusp to the magnetosheath and Polar is in the magnetosheath near the equatorial magnetopause. The observations show that a flux transfer event occurs between the equator and the northern cusp. Polar and Cluster observe the FTE's two open flux tubes: Polar encounters the southward moving flux tube near the equator and Cluster encounters the northward moving flux tube at high latitude. The low‐latitude FTE appears to be a flux rope with helical magnetic field lines as it has a strong core field and the magnetic field component in the boundary normal direction exhibits a strong bipolar variation. Unlike the low‐latitude FTE, the high‐latitude FTE observed by Cluster does not exhibit the characteristic bipolar perturbation in the magnetic field. However, the plasma data clearly reveal its open flux tube configuration. It shows that the magnetic field lines have straightened inside the FTE and become more aligned to the neighboring flux tubes as it moves to the cusp. Enhanced electrostatic fluctuations have been observed within the FTE core, both at low and high latitudes. This event provides a unique opportunity to understand high‐latitude FTE signatures and the nature of time‐varying reconnection. It shows that existing FTE models cannot accommodate all the features in global observations, and coordinated measurements from largely spaced multiple spacecraft place important constraints which are crucial to the development and refinement of FTE models.

On the location of dayside magnetic reconnection during an interval of duskward oriented IMF

Abstract. We present space- and ground-based observations of the signatures of magnetic reconnection during an interval of duskward-oriented interplanetary magnetic field on 25 March 2004. In situ field and plasma measurements are drawn from the Double Star and Cluster satellites during traversals of the pre-noon sector dayside magnetopause at low and high latitudes, respectively. These reveal the typical signatures of flux transfer events (FTEs), namely bipolar perturbations in the magnetic field component normal to the local magnetopause, enhancements in the local magnetic field strength and mixing of magnetospheric and magnetosheath plasmas. Further evidence of magnetic reconnection is inferred from the ground-based signatures of pulsed ionospheric flow observed over an extended interval. In order to ascertain the location of the reconnection site responsible for the FTEs, a simple model of open flux tube motion over the surface of the magnetopause is employed. A comparison of the modelled and observed motion of open flux tubes (i.e. FTEs) and plasma flow in the magnetopause boundary layer indicates that the FTEs observed at both low and high latitudes were consistence with the existence of a tilted X-line passing through the sub-solar region, as suggested by the component reconnection paradigm. While a high latitude X-line (as predicted by the anti-parallel description of reconnection) may have been present, we find it unlikely that it could have been responsible for the FTEs observed in the pre-noon sector under the observed IMF conditions. Finally, we note that throughout the interval, the magnetosphere was bathed in ULF oscillations within the solar wind electric field. While no one-to-one correspondence with the pulsed reconnection rate suggested by the ground-based observation of pulsed ionospheric flow has been demonstrated, we note that similar periodicity oscillations were observed throughout the solar wind-magnetosphere-ionosphere system. These findings are consistent with previously proposed mechanisms of solar wind modulation of the dayside reconnection rate.

Magnetospheric convection during prolonged intervals with southward interplanetary magnetic field

We have used a global magnetohydrodynamic simulation to investigate magnetospheric convection during a prolonged interval with modest southward driving. The interval simulated occurred on 13 and 14 February 2001 during a period when the interplanetary magnetic field (IMF) remained weakly southward for over 12 hours. Early in the simulation a near‐Earth neutral line located between 20 RE and 40 RE in the tail drove flows earthward and then around the near‐Earth obstacle to return magnetic flux to the dayside. The earthward flow led to increased ionospheric conductance and developed a modest earthward pressure gradient in the inner plasma sheet. Ionospheric line tying slowed the earthward flow. The pressure gradient caused the inner tail to become interchange unstable and lead to the tailward flow. When the tailward moving flux tube reached the near‐Earth neutral line additional reconnection occurred between the tailward moving flux tube and IMF field lines created by lobe reconnection thereby returning flux to the nightside tail lobes. Late in the event the new lobe field lines reconnected at the duskside magnetopause creating additional rapid tailward motion. The tailward flow led to a reduction in both the ionospheric conductance and near‐Earth tail pressure gradient and earthward flow was again established. This sequence repeated at least three times during the interval simulated.

The multi-component field topology of sunspot penumbrae

Context: Sunspot penumbrae harbor highly structured magnetic fields and flows. The moving flux tube model offers an explanation for several observed phenomena, e.g. the Evershed effect and bright penumbral grains. Aims: A wealth of information can be extracted from spectropolarimetric observations. In order to deduce the structure of the magnetic field in sunspot penumbrae, detailed forward modeling is necessary. On the one hand, it gives insight into the sensitivity of various spectral lines to different physical scenarios. On the other hand, it is a very useful tool to guide inversion techniques. In this work, we present a generalized 3D geometrical model that embeds an arbitrarily shaped flux tube in a stratified magnetized atmosphere. Methods: The new semi-analytical geometric model serves as a frontend for a polarized radiative transfer code. The advantage of this model is that it preserves the discontinuities of the physical parameters across the flux tube boundaries. This is important for the detailed shape of the emerging Stokes Profiles and the resulting net circular polarization (NCP). Results: (a) The inclination of downflows in the outer penumbra must be shallower than approximately 15 degrees. (b) Observing the limb-side NCP of sunspots in the Fe I 1564.8 nm line offers a promising way to identify a reduced magnetic field strength in flow channels. (c) The choice of the background atmosphere can significantly influence the shape of the Stokes profiles, but does not change the global characteristics of the resulting NCP curves for the tested atmospheric models.

Dynamics of an Isolated Blob in the Presence of the X-Point

The interplay of X-point shearing and axial plasma redistribution along a moving flux tube is discussed. Blobs limited to the main scrape-off-layer and blobs entirely confined in the divertor region are identified. A strong effect of the radial tilt of the divertor plate on “divertor” blobs is found. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)