Downflow Velocities Research Articles

Photospheric signatures of retraction and reconnection in realistic magnetohydrodynamic simulations

Context. Magnetic flux emergence and cancelling in the quiet Sun is a frequently observed phenomenon. The two possible physical flux-removal mechanisms involved in this cancelling process are retraction and reconnection. Aims. We seek to find distinct observational signatures characterising retraction and reconnection. Methods. We carried out three-dimensional non-grey radiative magnetohydrodynamic (MHD) simulations of convection near the solar surface and the solar photosphere using the STAGGER code, and employing different initial conditions: (1) mixed-polarity simulations with alternating horizontal stripes of opposite vertical magnetic field and separated by a zero field stripe, and (2) flux emergence simulations with continuous injection of magnetic flux from the lower boundary. These initial conditions are meant to represent two different situations in the solar photosphere, namely magnetic flux cancelling in the absence or presence of magnetic flux emergence, respectively. Results. We analyse the observational signatures of magnetic flux-removal processes for flux emergence as well as for mixed-polarity MHD simulations. In the flux emergence simulation, we are able to identify ubiquitous reconnection events anywhere from the solar surface to the upper photosphere. For a few of those reconnection events, we can identify supersonic upflow velocities in the upper photosphere as well as strong temperature enhancements. We also see strong electric currents very close to the locations where reconnection takes place, as well as supersonic horizontal velocities leading to sideways plasma compression. In the mixed-polarity simulations, we only detect observational signatures of magnetic field retraction related to large downflow velocities that appear in between regions where opposing horizontal velocities converge. These horizontal velocities are often supersonic, leading to heating due to shock dissipation. We do not see clear signatures of magnetic reconnection in these mixed-polarity simulations. Conclusions. We suggest that, in the emerging flux regions of the quiet Sun, the main flux-removal process is reconnection, while in regions without flux emergence, retraction is the dominant flux-removal process.

Evolution of downflows in the transition region above a sunspot over short time-scales

Context. Downflows with potentially super-sonic velocities have been reported to occur in the transition region above many sunspots; however, how these signatures evolve over short time-scales in both spatial and spectral terms is still unknown and requires further research. Aims. In this article, we investigate the evolution of downflows detected within spectral lines sampling the transition region on time-scales of the order of minutes and we search for clues as to the formation mechanisms of these features in co-temporal imaging data. Methods. For the purposes of this article, we used high-resolution spectral and imaging data sampled by the Interface Region Imaging Spectrograph on the 20 and 21 May 2015 to identify and analyse downflows. Additionally, photospheric and coronal imaging data from the Hinode and Solar Dynamics Observatory satellites were studied to provide context about the wider solar atmosphere. Results. Four downflows were identified and analysed through time. The potential super-sonic components of these downflows had widths of around 2″ and were observed to evolve over time-scales of the order of minutes. The measured apparent downflow velocities were structured both in time and space, with the highest apparent velocities occurring above a bright region detected in Si IV 1400 Å images. Downflows with apparent velocities below the super-sonic threshold that was assumed here were observed to extend a few arcseconds away from the foot-points, suggesting that the potential super-sonic components are linked to larger-scale flows. The electron density and mass flux for these events were found to be within the ranges of 109.6–1010.2 cm−3 and 10−6.81–10−7.48 g cm−2 s−1, respectively. Finally, each downflow formed at the foot-point of thin “fingers”, extending out around 3–5″ in Si IV 1400 Å data with smaller widths (< 1″) than the super-sonic downflow components. Conclusions. Downflows can appear, disappear, and recur within time-scales of less than one hour in sunspots. As the potential super-sonic downflow signatures were detected at the foot-points of both extended fingers in Si IV 1400 SJI data and sub-sonic downflows in Si IV 1394 Å spectra, it is likely that these events are linked to larger-scale flows within structures such as coronal loops.

Dynamics and connectivity of an extended arch filament system

Aims.In this study, we analyzed a filament system, which expanded between moving magnetic features (MMFs) of a decaying sunspot and opposite flux outside of the active region from the nearby quiet-Sun network. This configuration deviated from a classical arch filament system (AFS), which typically connects two pores in an emerging flux region. Thus, we called this system an extended AFS. We contrasted classical and extended AFSs with an emphasis on the complex magnetic structure of the latter. Furthermore, we examined the physical properties of the extended AFS and described its dynamics and connectivity.Methods.The extended AFS was observed with two instruments at the Dunn Solar Telescope (DST). The Rapid Oscillations in the Solar Atmosphere (ROSA) imager provided images in three different wavelength regions, which covered the dynamics of the extended AFS at different atmospheric heights. The Interferometric Bidimensional Spectropolarimeter (IBIS) provided spectroscopic Hαdata and spectropolarimetric data that was obtained in the near-infrared (NIR) Ca IIλ8542 Å line. We derived the corresponding line-of-sight (LOS) velocities and used He IIλ304 Å extreme ultraviolet (EUV) images of the Atmospheric Imaging Assembly (AIA) and LOS magnetograms of the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) as context data.Results.The NIR Ca IIStokes-Vmaps are not suitable to definitively define a clear polarity inversion line and to classify this chromospheric structure. Nevertheless, this unusual AFS connects the MMFs of a decaying sunspot with the network field. At the southern footpoint, we measured that the flux decreases over time. We find strong downflow velocities at the footpoints of the extended AFS, which increase in a time period of 30 min. The velocities are asymmetric at both footpoints with higher velocities at the southern footpoint. An EUV brigthening appears in one of the arch filaments, which migrates from the northern footpoint toward the southern one. This activation likely influences the increasing redshift at the southern footpoint.Conclusions.The extended AFS exhibits a similar morphology as classical AFSs, for example, threaded filaments of comparable length and width. Major differences concern the connection from MMFs around the sunspot with the flux of the neighboring quiet-Sun network, converging footpoint motions, and longer lifetimes of individual arch filaments of about one hour, while the extended AFS is still very dynamic.

Gravitational wave signals from 3D neutrino hydrodynamics simulations of core-collapse supernovae

We present gravitational wave (GW) signal predictions from four 3D multi-group neutrino hydrodynamics simulations of core-collapse supernovae of progenitors with 11.2 Msun, 20 Msun, and 27 Msun. GW emission in the pre-explosion phase strongly depends on whether the post-shock flow is dominated by the standing accretion shock instability (SASI) or convection and differs considerably from 2D models. SASI activity produces a strong signal component below 250 Hz through asymmetric mass motions in the gain layer and a non-resonant coupling to the proto-neutron star (PNS). Both convection- and SASI-dominated models show GW emission above 250 Hz, but with considerably lower amplitudes than in 2D. This is due to a different excitation mechanism for high-frequency l=2 motions in the PNS surface, which are predominantly excited by PNS convection in 3D. Resonant excitation of high-frequency surface g-modes in 3D by mass motions in the gain layer is suppressed compared to 2D because of smaller downflow velocities and a lack of high-frequency variability in the downflows. In the exploding 20 Msun model, shock revival results in enhanced low-frequency emission due to a change of the preferred scale of the convective eddies in the PNS convection zone. Estimates of the expected excess power in two frequency bands suggests that second-generation detectors will only be able to detect very nearby events, but that third-generation detectors could distinguish SASI- and convection-dominated models at distances of ~10 kpc.

Dynamics of small-scale convective motions

Previous studies have discovered a population of small granules with diameters less than 800 km showing differing physical properties. High resolution simulations and observations of the solar granulation, in combination with automated segmentation and tracking algorithms, allow us to study the evolution of the structural and physical properties of these granules and surrounding vortex motions with high temporal and spatial accuracy. We focus on the dynamics of granules (lifetime, fragmentation, size, position, intensity, vertical velocity) over time and the influence of strong vortex motions. Of special interest are the dynamics of small granules compared to regular-sized granules. We developed a temporal tracking algorithm based on our developed segmentation algorithm for solar granulation. This was applied to radiation hydrodynamics simulations and high resolution observations of the quiet Sun by SUNRISE/IMaX. The dynamics of small granules differ in regard to their diameter, intensity and depth evolution compared to regular granules. The tracked granules in the simulation and observations reveal similar dynamics (lifetime, evolution of size, vertical velocity and intensity). The fragmentation analysis shows that the majority of granules in simulations do not fragment, while the opposite was found in observations. Strong vortex motions were detected at the location of small granules. Regions of strong vertical vorticity show high intensities and downflow velocities, and live up to several minutes. The analysis of granules separated according to their diameter in different groups reveals strongly differing behaviors. The largest discrepancies can be found within the groups of small, medium-sized and large granules and have to be analyzed independently. The predominant location of vortex motions on and close to small granules indicates a strong influence on the dynamics of granules.

Fitting peculiar spectral profiles in He I 10830 Å absorption features

AbstractThe new generation of solar instruments provides better spectral, spatial, and temporal resolution for a better understanding of the physical processes that take place on the Sun. Multiple‐component profiles are more commonly observed with these instruments. Particularly, the Hei 10830 Å triplet presents such peculiar spectral profiles, which give information on the velocity and magnetic fine structure of the upper chromosphere. The purpose of this investigation is to describe a technique to efficiently fit the two blended components of the Hei 10830 Å triplet, which are commonly observed when two atmospheric components are located within the same resolution element. The observations used in this study were taken on 2015 April 17 with the very fast spectroscopic mode of the GREGOR Infrared Spectrograph (GRIS) attached to the 1.5‐m GREGOR solar telescope, located at the Observatorio del Teide, Tenerife, Spain. We apply a double‐Lorentzian fitting technique using Levenberg‐Marquardt least‐squares minimization. This technique is very simple and much faster than inversion codes. Line‐of‐sight Doppler velocities can be inferred for a whole map of pixels within just a few minutes. Our results show sub‐ and supersonic downflow velocities of up to 32km s–1 for the fast component in the vicinity of footpoints of filamentary structures. The slow component presents velocities close to rest. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamics of the Fe II 396.94 nm emission line observed at solar disk center

Aims. We have studied the structure and dynamics of the upper photosphere and lower chromosphere of the quiet Sun. Methods. We analyzed a sequence of scans of slit spectra of the Ca II H line at 396.8 nm, with a spectral resolution of 850 000, and a spatial resolution of about one arcsec, taken in the quiet Sun at disk center, with a duration of about 55 min and an area coverage of 185 �� × 3.5 �� . We used statistical methods to characterize size, lifetime, and dynamics of emission features of the Fe II line at 396.94 nm. Results. We have identified about 780 distinct features where the Fe II line shows emission stronger than 3% above the local continuum. Their lifetime is on the order of one minute, and the spatial extent is about or less than 2 �� . On average, Fe II emission features show a significant redshift of 1.3 km s −1 , with respect to the position of the absorption line. The redshift of the Fe II emission is absent close to the solar limb. The Fe II emission is coupled with a strongly enhanced intensity level of the Ca II wings. A time line analysis shows that other photospheric lines show significant redshift one minute before, and a blueshift after, the occurrence of an emission event. A redshift of the Ca II H line core precedes the redshift of the Fe II line by about 30 s. The occurrence of the H2v emissions is strongly reduced before and significantly increased after the Fe II emission events. Conclusions. The temporal behavior of the Fe II emission line and both the Ca II H line core and H2v emission suggest a connection between Fe II emission and chromospheric activity. The presence of a significant redshift during the Fe II emissions and the absence of this redshift near the limb suggest that the observed redshift is indeed caused by a downflow in the line-forming region. From our result concerning the intensity fluctuations in the core of the Fe II line, and from the magnitude of the downflow velocities we conclude that the Fe II emissions at 396.94 nm occur in the photosphere.

Peripheral downflows in sunspot penumbrae

Sunspot penumbrae show high-velocity patches along the periphery. The high-velocity downflow patches are believed to be the return channels of the Evershed flow. We aim to investigate their structure in detail using Hinode SOT/SP observations. We employ Fourier interpolation in combination with spatially coupled height dependent LTE inversions of Stokes profiles to produce high-resolution, height-dependent maps of atmospheric parameters of these downflows and investigate their properties. High-speed downflows are observed over a wide range of viewing angles. They have supersonic line-of-sight velocities, some in excess of 20km/s, and very high magnetic field strengths, reaching values of over 7 kG. A relation between the downflow velocities and the magnetic field strength is found, in good agreement with MHD simulations. The coupled inversion at high resolution allows for the accurate determination of small-scale structures. The recovered atmospheric structure indicates that regions with very high downflow velocities contain some of the strongest magnetic fields that have ever been measured on the Sun.

Changes in CSF flow after one-stage posterior vertebral column resection in scoliosis patients with syringomyelia and Chiari malformation Type I

Phase contrast-cine MRI (PC-cine MRI) studies in patients with syringomyelia and Chiari malformation Type I (CM-I) have demonstrated abnormal CSF flow across the foramen magnum, which can revert to normal after craniocervical decompression with syrinx shrinkage. In order to investigate the mechanisms leading to postoperative syringomyelia shrinkage, the authors studied the hydrodynamic changes of CSF flow in the craniocervical junction and spinal canal in patients with scoliosis associated with syringomyelia after one-stage deformity correction by posterior vertebral column resection. Preoperative and postoperative CSF flow dynamics at the levels of the foramen magnum, C-7, T-7 (or apex), and L-1 were assessed by electrocardiogram-synchronized cardiac-gated PC-cine MRI in 8 adolescent patients suffering from severe scoliosis with syringomyelia and CM-I (scoliosis group) and undergoing posterior vertebral column resection. An additional 8 patients with syringomyelia and CM-I without spinal deformity (syrinx group) and 8 healthy volunteers (control group) were also enrolled. Mean values were obtained for the following parameters: the duration of a CSF cycle, the duration of caudad CSF flow (CSF downflow [DF]) and cephalad CSF flow (CSF upflow [UF]), the ratio of DF duration to CSF cycle duration (DF%), and the ratio of UF duration to CSF cycle duration (UF%). The ratio of the stationary phase (SP) duration to CSF cycle duration was calculated (SP%). The maximum downflow velocities (VD max) and maximum upflow velocities (VU max) were measured. SPSS (version 14.0) was used for all statistical analysis. Patients in the scoliosis group underwent one-stage posterior vertebral column resection for deformity correction without suboccipital decompression. The mean preoperative coronal Cobb angle was 102.4° (range 76°-138°). The mean postoperative Cobb angle was 41.7° (range 12°-75°), with an average correction rate of 59.3%. During the follow-up, 1 patient with hypermyotonia experienced a significant decrease of muscle tension and 1 patient with reduced anal sphincter tone manifested recovery. A total of 5 patients demonstrated a significant decrease (> 30%) in syrinx size. With respect to changes in CSF flow dynamics, the syrinx group was characterized by slower and shorter downflow than the control group, and the difference was more significant at the foramen magnum and C-7 levels. In patients with scoliosis, CSF downflow at the foramen magnum level was significantly restricted, and a prolonged stationary phase indicated increased obstruction of CSF flow. After posterior vertebral column resection, the peak velocity of CSF flow at the foramen magnum increased, and the downflow phase duration was markedly prolonged. The parameters showed a return to almost normal CSF dynamics at the craniocervical region, and this improvement was maintained for 6-12 months of follow-up. There were distinct abnormalities of CSF flow at the craniocervical junction in patients with syringomyelia. Abnormal dynamics of downflow could be aggravated by associated severe spinal deformity and improved by correction via posterior vertebral column resection.

Fundamental parameters and granulation properties of Alpha Centauri A and B obtained from inversions of their spectra

Properties of stellar granulation are obtained by inverting spectra of the late-type stars α Centauri A and B. Our inversions are based on a multi-component model of the stellar photosphere and take into account the center-to-limb variation and rotational broadening. The different atmospheric components describe the areas harboring up-, down- and horizontal flows. The inversions are constrained by fitting not only the flux profiles, but also their line bisectors, and by using a simple mass conservation scheme. The inversions return the properties of convection at the stellar surface, including the stratification of the thermodynamic parameters, as well as fundamental parameters such as the gravitational acceleration, v sini and the element abundances. For α Cen A (G2V) the derived stratifications of the temperature and convective velocity are very similar to the Sun, while for α Cen B (K1V) we find similar up- and downflow velocities, but lower horizontal speeds and a reduced overshoot. The latter is consistent with the smaller scale height of the atmosphere, while mass conservation arguments taken with the lower horizontal speed imply that the granules on α Cen B are smaller than on the Sun. Both these properties are in good agreement with the hydrodynamic simulation of Nordlund & Dravins (1990, A&A, 228, 155). The inversions also return the fundamental parameters (Teff ,l ogg, abundances, v sini, etc.) of the two stars. These values are on the whole in good agreement with literature values. Also, most of them do not strongly depend on the details of the inversion. However, importantly, the element abundances are 0.1 to 0.15 dex lower when a 2- or 3-component inversion is carried out than with a 1-component inversion.

ADAS analysis of the differential emission measure structure of the inner solar corona

We present a study of the differential emission measure (DEM) of a “quiet Sun” area observed in the extreme ultraviolet at normal incidence by the Coronal Diagnostic Spectrometer (CDS) on the SOHO spacecraft. The data used for this work were taken using the NISAT_S observing sequence. This takes the full wavelength ranges from both the NIS channels (308–381 Å and 513–633 Å) with the 2 arcsec by 240 arcsec slit, which is the narrowest slit available, yielding the best spectral resolution. In this work we contrast the DEM from subregions of 2 80 arcsec2 with that obtained from the mean spectrum of the whole raster (20 240 arcsec2). We find that the DEM maintains essentially the same shape in the subregions, differing by a constant factor between 0.5 and 2 from the mean DEM, except in areas were the electron density is below cm-3 and downflow velocities of 50 km s-1 are found in the transition region. Such areas are likely to contain plasma departing from ionisation equilibrium, violating the basic assumptions underlying the DEM method. The comparison between lines of Li-like and Be-like ions may provide further evidence of departure from ionisation equilibrium. We find also that line intensities tend to be lower where velocities of the order of 30 km s-1 or higher are measured in transition region lines. The DEM analysis is also exploited to improve the line identification performed by [CITE] and to investigate possible elemental abundance variations from region to region. We find that the plasma has composition close to photospheric in all the subregions examined.

On the consequences of a non-equilibrium ionisation balance for compact flare emission and dynamics

We carry out a hydrodynamic simulation of a compact flare and find significant non-equilibrium distributions for the ionisation balance during the impulsive and gradual phases, which can strongly alter the radiative emission. This has major implications for attempts to derive the theoretical intensities of emission lines used for spectroscopic diagnostic analyses of the plasma properties. During the impulsive phase we find that the emissivities of He I, He II and C IV in the transition region can be strongly enhanced above their expected equilibrium values, followed by a significant reduction which increases the amount of chromo- spheric plasma ablated into the corona. Furthermore, during the flare heating the overall charge state of the coronal ions can be significantly lower than is suggested by an equilibrium ionisation balance and, therefore, line ratio measurements will yield plasma temperatures that are much greater than the formation temperature of the emitting ion. During the gradual phase the emissivity at transition region temperatures remains suppressed, compared with its equilibrium value, with correspondingly reduced downflow velocities and increased radiative cooling time-scales. Finally, we synthesise the emission as it would be detected by TRACE in its 171 A and 195 A wavelength bands, and find that the filter ratio technique can give reasonably good estimates of the plasma temperature in quiescence, though when the populations of Fe VIII, Fe IX, Fe X and Fe XII exhibit departures from equilibrium the temperatures derived from filter ratio measurements become unreliable.

Spectral Diagnostics of Chromospheres—A Signature of Emergent Magnetic Flux

The chromospheres of the Sun and solar-like stars respond to the underlying magnetic activity. The spectral lines of singly ionized calcium (e.g., Ca II K at 3933 Å) exhibit single or double reversals and serve as diagnostic tools for the magnetic activity. We present observations showing that the Ca II K spectral line may exhibit reversals in addition to the well-known K1, K2, and K3 components, when observed with high angular and spectral resolution. These Ca II K spectral line anomalies occur when small-scale, subarcsecond magnetic fields emerge through the chromosphere. The anomalous profiles originate at locations that are cospatial with Hα brightening at footpoints of dark active region fibrils, or active region filaments. The photospheric magnetic field is bipolar (small opposite polarity magnetic nodules) or has neutral lines at cospatial locations where the spectral lines are anomalous. Small-scale reconnective processes can cause these anomalous profiles. From a simultaneous time series of magnetic field data, we find that the emerging magnetic flux is associated with these profiles. The additional reversals primarily occur redward of the familiar K3 absorption trough, implying chromospheric downflow velocities that lead to the formation of shocks. The magnitude of the Doppler shift is of the order 40-50 km s-1. We present evidence to suggest that a combination of subresolution elements, each of which has regular profiles with large relative Doppler shifts could explain the formation of some of these profiles. A heuristic model that combines buoyant magnetic fields, convective collapse, gas evacuation, shock formation, heating, and a multicomponent model atmosphere can reproduce these profiles.

Reciprocity-type relations for waves in a moving inhomogeneous fluid

The reciprocity principle is known to be invalid for acoustic waves in a moving fluid due to differences in up- and down-flow wave propagation velocities. A variation of the reciprocity principle, a flow reversal theorem (FRT), which states symmetry of some field quantity with respect to interchange of the source and receiver positions and the simultaneous reversal of flow, was considered by many authors during the last four decades. It was proven under that or other specific assumptions about fluid parameters and/or flow velocity space dependence or in a framework of some asymptotic representations of the acoustic field. A simple but rather general proof of the FRT will be presented which is valid for sound as well as acoustic-gravity waves in an arbitrary three-dimensional inhomogeneous moving fluid with time-independent parameters provided divergence of the flow velocity is zero. The fluid may be unbounded or have pressure-release, rigid, or impedance boundaries. The key point in the approach is the choice of a set of dependent variables in hydrodynamic equations for describing the wave field. An implication of the FRT on wave action conservation for cw sound in moving fluid is considered. Some possible FRT applications are indicated. [Work supported by NRC.]

Relation between cool and hot post-flare loops of 26 June 1992 derived from optical and X-ray (SXT-Yohkoh) observations

We have analyzed the physical conditions of the plasma in post-flare loops with special emphasis on dynamics and energy transport using SXT-data (hot plasma) and optical ground-based data from Pic du Midi, Wroclaw, and Ondřejov (cool plasma). By combining the Hα observations with the SXT images we can understand the relationship between cool and hot plasmas, the process of cooling post-flare loops and the mechanism which maintains the long duration of these loops. Using recent results of NLTE modeling of prominence-like plasmas, we derive the emission measure of cool Hα loops and this gives us a realistic estimate of the electron density (2.2 × 1010 cm−3). Then, by comparing this emission measure with that of hot loops derived from SXT data, we are able to estimate the ratio between electron densities in hot and cool loops taking into account the effect of geometrical filling factors. This leads to the electron density in hot loops 7 × 109 cm−3. We also derive the temperature of hot X-ray loops (≃ 5.5 × 106 K), which, together with the electron density, provides the initial values for solving the time-dependent energy balance equation. We obtain the cooling times which are compared to a typical growth-time of the whole loop system (∼ 2000 s). In the legs of cool Hα loops, we observe an excess of the emission measure which we attribute to the effect of Doppler brightening (due to large downflow velocities).

Electron precipitation and mass motion in the 1991 June 9 white-light flare

We use Hα line profiles as a diagnostic of mass motion and nonthermal electron precipitation in the white-light flare (WLF) of 1991 June 9 01:34 UT. We find only weak downflow velocities (≈10 km s−1) at the site of white-light emission, and comparable velocities elsewhere.We also find that electron precipitation is strongest at the WLF site. We conclude that continuum emission in this flare was probably caused by nonthermal electrons and not by dynamical energy transport via a chromospheric condensation.

Solar Doppler-shift measurements in the NE VII 465 A emission line

Using a Dopplergram obtained with the Naval Research Laboratory extreme ultraviolet spectrograph on Skylab, we have searched for Doppler shifts in the Ne VII 465 A emission line, which is formed at a temperature of about 500,000 K. In the quiet sun we find no measurable average Doppler shift to a measurement accuracy of +/- 18 km/s. Small regions of the quiet sun do, however, display measurable Doppler shifts. In active regions, we measure Doppler shifts indicating downflow velocities of up to 70 km/s.

Model study for subsurface trajectory of 75FeSi and SiMn ferroalloys in steel

The present study, using a low temperature model, is aimed at examining the feasibility of subsurface melting of 75FeSi and SiMn alloys added to steel baths. Wooden objects, simulating 75FeSi and SiMn of various sizes and shapes, were prepared so as to have same density ratio of 75FeSi and SiMn to steel, and their subsurface trajectories in water or zinc chloride solution following three meters of free fall through air were analyzed using video images. In general, subsurface retention times were observed to gradually increase with size of object up to a steady value. For those objects simulating 75FeSi, it was found to be dependent on shapes, spherical objects having longer retention times. The effect of shape was less pronounced for objects simulating SiMn. In a gas stirred bath, smaller cuboids for SiMn were entrained. Comparisons with estimated melting times of 75FeSi suggest that regular sized 75FeSi (ex. 20 to 50 mm) will float up and melt at the surface, when dropped to the steel bath in a ladle. A similar situation is expected with SiMn alloys. Subsurface melting may be feasible by reducing the size of 75FeSi addition from 1 to 5 mm dia. and of SiMn down to about 10 mm while simultaneously applying sufficient turbulent mixing of steel bath or providing entraining downflow velocities in the range of 10 to 30 cm/s.

H-alpha redshifts as a diagnostic of solar flare heating

The dynamics of chomospheric condensations formed during chromospheric evaporation are studied using coordinated X-ray and H-alpha observations of five solar flares. It is shown that the peak downflow velocity of condensations predicted by simple hydrodynamic compression of the chromosphere is in good agreement with empirical downflow velocities implied by impulsive phase H-alpha red wing Doppler shifts. It is found that the H-alpha wing redshift provides a diagnostic of the pressure excess in the evaporating region and the coronal energy flux driving chromospheric evaporation. Because preflare coronal pressures derived for each of the flares are high, it is suggested that the chromosphere succeeds in radiating excess flare heat energy without undergoing explosive chromospheric evaporation.

Chromospheric Downflows as a Diagnostic of Solar Flare Heating

AbstractUsing coordinated X-ray and Hα observations of five solar flares, we investigate the dynamics of chromospheric condensations formed during chromospheric evaporation. We show that the peak downflow velocity of condensations predicted by simple hydrodynamic compression of the chromosphere is in good agreement with empirical downflow velocities implied by impulsive phase Hα redwing Doppler shifts. This agreement indicates that the Hα wing redshift provides a useful diagnostic of the pressure excess in the evaporating region and, hence, the energy flux driving chromospheric evaporation.