Ground-based Hα Research Articles

Generating Space-based SDO/HMI-like Solar Magnetograms from Ground-based Hα Images by Deep Learning

Recently, the method of estimating magnetic field through monochromatic images by deep learning has been proposed, demonstrating good morphological similarity but somewhat poor magnetic polarity consistency relative to real observation. In this paper, we propose to estimate magnetic field from Hα images by using a conditional generative adversarial network (cGAN) as the basic framework. The Hα images from the Global Oscillation Network Group are used as the inputs and the line-of-sight magnetograms of the Helioseismic Magnetic Imager (HMI) are used as the targets. First, we train a cGAN model (Model A) with shuffling training data. However, the estimated magnetic polarities are not very consistent with real observations. Second, to improve the accuracy of estimated magnetic polarities, we train a cGAN model (Model B) with the chronological Hα and HMI images, which can implicitly exploit the magnetic polarity constraint of time-series observation to generate more accurate magnetic polarities. We compare the generated magnetograms with the target HMI magnetograms to evaluate the two models. It can be observed that Model B has better magnetic polarity consistency than Model A. To quantitatively measure this consistency, we propose a new metric called pixel-to-pixel polarity accuracy (PPA). With respect to PPA, Model B is superior to Model A. This work gives us an insight that the time-series constraint can be implicitly exploited through organizing training data chronologically, and this conclusion also can be applied to other similar tasks related to time-series data.

Obtaining space-based SDO/AIA solar UV and EUV images from ground-based Hα observations by deep learning

In this work, we explore the mappings from solar images taken in Hα (6563 Å) by the Global Oscillation Network Group (GONG) on the ground to those observed in eight different wavelengths (94, 131, 171, 193, 211, 304, 335 and 1600 Å) by SDO/AIA in space. Eight mappings are built by training the conditional Generative Adversarial Networks (cGANs) on datasets with 500 paired images, which are [Hα, AIA94], [Hα, AIA131], [Hα, AIA171], [Hα, AIA193], [Hα, AIA211], [Hα, AIA304], [Hα, AIA335] and [Hα, AIA1600]. We evaluate the eight trained cGANs models on validation and test datasets with 154-pair images and 327-pair images, respectively. The model generated fake AIA images match the corresponding observed AIA images well on large-scale structures such as large active regions and prominences. But the small-scale flare loops and filament threads are difficult to reconstruct. Four quantitative comparisons are carried out on the validation and test datasets to score the mappings. We find that the model-generated images in 304 and 1600 Å match the corresponding observed images best. This exploration suggests that the cGANs are promising methods for mappings between ground-based Hα and space-based EUV/UV images, while some improvements are necessary.

First Characterization of the Neutral ISM in Two Local Volume Dwarf Galaxies

Abstract We present the first H i spectral-line images of the nearby, star-forming dwarf galaxies UGC 11411 and UGC 8245, acquired as part of the “Observing for University Classes” program with the Karl G. Jansky Very Large Array (VLA). These low-resolution images localize the H i gas and reveal the bulk kinematics of each system. Comparing with Hubble Space Telescope (HST) broadband and ground-based Hα imaging, we find that the ongoing star formation in each galaxy is associated with the highest H i mass surface density regions. UGC 8245 has a much lower current star formation rate than UGC 11411, which harbors very high surface brightness Hα emission in the inner disk and diffuse, lower surface brightness nebular gas that extends well beyond the stellar disk as traced by HST. We measure the dynamical masses of each galaxy and find that the halo of UGC 11411 is more than an order of magnitude more massive than the halo of UGC 8245, even though the H i and stellar masses of the sources are similar. We show that UGC 8245 shares similar physical properties with other well-studied low-mass galaxies, while UGC 11411 is more highly dark matter dominated. Both systems have negative peculiar velocities that are associated with a coherent flow of nearby galaxies at high supergalactic latitude.

Large-amplitude Longitudinal Oscillations in a Solar Filament

Abstract In this paper, we report our multiwavelength observations of the large-amplitude longitudinal oscillations of a filament observed on 2015 May 3. Located next to active region 12335, the sigmoidal filament was observed by the ground-based Hα telescopes from the Global Oscillation Network Group and by the Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory. The filament oscillations were most probably triggered by the magnetic reconnection in the filament channel, which is characterized by the bidirectional flows, brightenings in EUV and soft X-ray, and magnetic cancellation in the photosphere. The directions of oscillations have angles of 4°–36° with respect to the filament axis. The whole filament did not oscillate in phase as a rigid body. Meanwhile, the oscillation periods (3100–4400 s) have a spatial dependence, implying that the curvature radii (R) of the magnetic dips are different at different positions. The values of R are estimated to be 69.4–133.9 Mm, and the minimum transverse magnetic field of the dips is estimated to be 15 G. The amplitudes of S5-S8 grew with time, while the amplitudes of S9-S14 damped with time. The oscillation amplitudes range from a few to ten Mm, and the maximum velocity can reach 30 km s−1. Interestingly, the filament experienced mass drainage southward at a speed of ∼27 km s−1. The oscillations continued after the mass drainage and lasted for more than 11 hr. After the mass drainage, the oscillation phases did not change much. The periods of S5-S8 decreased, while the periods of S9-S14 increased. The amplitudes of S5-S8 damped with time, while the amplitudes of S9-S14 grew. Most of the damping (growing) ratios are between −9 and 14. We offer a schematic cartoon to explain the complex behaviors of oscillations by introducing thread-thread interaction.

THE HOT COMPANION AND CIRCUMBINARY DISK OF THE Be STAR HR 2142

ABSTRACT We present a spectroscopic investigation of the Be+sdO binary system HR 2142 that is based upon large sets of ultraviolet observations from the International Ultraviolet Explorer (IUE) and ground-based Hα observations. We measured radial velocities for the Be star component from these spectra and computed a revised orbit. In order to search for the spectral signature of the hot subdwarf, we cross-correlated the short wavelength end of each IUE spectrum with a model hot star spectrum, and then we used the predicted Doppler shifts of the subdwarf to shift-and-add all the cross-correlation functions to the frame of the subdwarf. This merged function shows the weak signal from the spectral lines of the hot star, and a best fit is obtained with a mass ratio , companion temperature kK, projected rotational velocity km s−1, and a monochromatic flux ratio near 1170 Å of . This hot subdwarf creates a one-armed spiral, tidal wake in the disk of the Be star, and we present a circumbinary disk model that can explain the occurrence of shell absorption lines by gas enhancements that occur where gas crossing the gap created by the subdwarf strikes the disk boundaries. The faint companion of HR 2142 may be representative of a significant fraction of Be stars with undetected former mass donor companion stars.

Real-time Flare Detection in Ground-Based Hα Imaging at Kanzelhöhe Observatory

Kanzelh\"ohe Observatory (KSO) regularly performs high-cadence full-disk imaging of the solar chromosphere in the H$\alpha$ and CaIIK spectrallines as well as the solar photosphere in white-light. In the frame of ESA's Space Situational Awareness (SSA) programme, a new system for real-time H$\alpha$ data provision and automatic flare detection was developed at KSO. The data and events detected are published in near real-time at ESA's SSA Space Weather portal (http://swe.ssa.esa.int/web/guest/kso-federated). In this paper, we describe the H$\alpha$ instrument, the image recognition algorithms developed, the implementation into the KSO H$\alpha$ observing system and present the evaluation results of the real-time data provision and flare detection for a period of five months. The H$\alpha$ data provision worked in $99.96$% of the images, with a mean time lag between image recording and online provision of 4s. Within the given criteria for the automatic image recognition system (at least three H$\alpha$ images are needed for a positive detection), all flares with an area $\ge$50 micro-hemispheres and located within $60^\circ$ of the Sun's center that occurred during the KSO observing times were detected, in total a number of 87 events. The automatically determined flare importance and brightness classes were correct in $\sim$85%. The mean flare positions in heliographic longitude and latitude were correct within $\sim$1$^\circ$. The median of the absolute differences for the flare start times and peak times from the automatic detections in comparison to the official NOAA (and KSO) visual flare reports were 3 min (1 min).

Computer Vision for the Solar Dynamics Observatory (SDO)

In Fall 2008 NASA selected a large international consortium to produce a comprehensive automated feature-recognition system for the Solar Dynamics Observatory (SDO). The SDO data that we consider are all of the Atmospheric Imaging Assembly (AIA) images plus surface magnetic-field images from the Helioseismic and Magnetic Imager (HMI). We produce robust, very efficient, professionally coded software modules that can keep up with the SDO data stream and detect, trace, and analyze numerous phenomena, including flares, sigmoids, filaments, coronal dimmings, polarity inversion lines, sunspots, X-ray bright points, active regions, coronal holes, EIT waves, coronal mass ejections (CMEs), coronal oscillations, and jets. We also track the emergence and evolution of magnetic elements down to the smallest detectable features and will provide at least four full-disk, nonlinear, force-free magnetic field extrapolations per day. The detection of CMEs and filaments is accomplished with Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph (LASCO) and ground-based Hα data, respectively. A completely new software element is a trainable feature-detection module based on a generalized image-classification algorithm. Such a trainable module can be used to find features that have not yet been discovered (as, for example, sigmoids were in the pre-Yohkoh era). Our codes will produce entries in the Heliophysics Events Knowledgebase (HEK) as well as produce complete catalogs for results that are too numerous for inclusion in the HEK, such as the X-ray bright-point metadata. This will permit users to locate data on individual events as well as carry out statistical studies on large numbers of events, using the interface provided by the Virtual Solar Observatory. The operations concept for our computer vision system is that the data will be analyzed in near real time as soon as they arrive at the SDO Joint Science Operations Center and have undergone basic processing. This will allow the system to produce timely space-weather alerts and to guide the selection and production of quicklook images and movies, in addition to its prime mission of enabling solar science. We briefly describe the complex and unique data-processing pipeline, consisting of the hardware and control software required to handle the SDO data stream and accommodate the computer-vision modules, which has been set up at the Lockheed-Martin Space Astrophysics Laboratory (LMSAL), with an identical copy at the Smithsonian Astrophysical Observatory (SAO).

AN X-RAY VIEW OF STAR FORMATION IN THE CENTRAL 3 kpc OF NGC 2403

Archival Chandra observations are used to study the X-ray emission associated with star formation in the central region of the nearby SAB(s)cd galaxy NGC 2403. The distribution of X-ray emission is compared to the morphology visible at other wavelengths using complementary Spitzer, Galaxy Evolution Explorer, and ground-based Hα imagery. In general, the brightest extended X-ray emission is associated with H ii regions and to other star-forming structures but is more pervasive, existing also in regions devoid of strong Hα and UV emission. This X-ray emission has the spectral properties of diffuse hot gas (kT ∼ 0.2 keV) whose likely origin is in gas shock heated by stellar winds and supernovae with ≲20% coming from faint unresolved X-ray point sources. This hot gas may be slowly cooling extra-planar remnants of past outflow events, or a disk component that either lingers after local star formation activity has ended or that has vented from active star-forming regions into a porous interstellar medium.

The relative timing of supra-arcade downflows in solar flares

Context. Supra-arcade downflows (generally dark, sunward-propagating features located above the bright arcade of loops in some solar flares) have been reported mostly during the decay phase, although some have also been reported during the rise phase of solar flares. Aims. We investigate, from a statistical point of view, the timing of supra-arcade downflows during the solar flare process, and thus determine the possible relation of supra-arcade downflows to the primary or secondary energy release in a flare. Methods. Yohkoh Soft X-ray Telescope (SXT) imaging data are examined to produce a list of supra-arcade downflow candidates. In many of our events supra-arcade downflows are not directly observed. However, the events do show laterally moving (or “waving”) bright rays in the supra-arcade fan of coronal rays which we interpret as due to dark supra-arcade downflows. The events are analysed in detail to determine whether the supra-arcade downflows (or the proxy waving coronal rays) occur during a) the rise and/or decay phases of the soft X-ray flare and b) the flare hard X-ray bursts. It is also investigated whether the supra-arcade downflows events show prior eruptive signatures as seen in SXT, other space-based coronal data, or reported in ground-based Hα images. Results. A substantial majority of supra-arcade downflow events show downflows which start during the soft X-ray flare rise phase (73%), occur during hard X-ray bursts (90%), and have prior eruptive signatures associated with them (73%). However, we find a single event (2% of the total) which clearly and unambiguously showed supra-arcade downflows starting during the soft X-ray flare decay phase. Conclusions. Since the majority of supra-arcade downflows occur during the rise phase of the soft X-ray flare and the time of hard X-ray bursts, and have prior eruptive signatures this suggests that they are related to the main flare energy release process. Furthermore, the suggested association of supra-arcade downflows with recently reconnected magnetic field lines means they may indeed be considered as evidence for a magnetic reconnection process. The single supra-arcade downflow event which unambiguously started during the decay phase of the flare occurred during hard X-ray bursts and thus appears to be related to late energy release.

A new superwind galaxy: XMM–Newton observations of NGC 6810

We present the first imaging X-ray observation of the highly inclined (i= 78°) Sab Seyfert 2 galaxy NGC 6810 using XMM–Newton, which reveals soft X-ray emission that extends out to a projected height of ∼7 kpc away from the plane of the galaxy. The soft X-ray emission beyond the optical disc of the galaxy is most plausibly extraplanar, although it could instead come from large galactic radius. This extended X-ray emission is spatially associated with diffuse Hα emission, in particular with a prominent 5-kpc-long Hα filament on the north-west of the disc. A fraction ≲35 per cent of the total soft X-ray emission of the galaxy arises from projected heights |z| ≥ 2 kpc. Within the optical disc of the galaxy the soft X-ray emission is associated with the star-forming regions visible in ground-based Hα and XMM–Newton optical monitor near-UV imaging. The temperature, supersolar α-element-to-iron abundance ratio, soft X-ray/Hα correlation, and X-ray to far-infrared (FIR) flux ratio of NGC 6810 are all consistent with local starbursts with winds, although the large base radius of the outflow would make NGC 6810 one of the few ‘disc-wide’ superwinds currently known. Hard X-ray emission from NGC 6810 is weak, and the total E= 2–10 keV luminosity and spectral shape are consistent with the expected level of X-ray binary emission from the old and young stellar populations. The X-ray observations provide no evidence of any active galactic nucleus activity. We find that the optical, IR and radio properties of NGC 6810 are all consistent with a starburst galaxy, and that the old classification of this galaxy as a Seyfert 2 galaxy is probably incorrect.

Ultraviolet–to–Far‐Infrared Properties of Local Star‐forming Galaxies

We present the results of a multiwavelength study of nearby galaxies aimed at understanding the relation between the ultraviolet and far-infrared emission in star-forming galaxies. The data set comprises new ultraviolet (from HST STIS), ground-based Hα, and radio continuum observations, together with archival infrared data (from IRAS and ISO). The local galaxies are used as benchmarks for comparison of the infrared-to-ultraviolet properties with two populations of high-redshift galaxies: the submillimeter star-forming galaxies detected by SCUBA and the ultraviolet-selected Lyman break galaxies (LBGs). In addition, the long wavelength baseline covered by the present data enables us to compare the star formation rates (SFRs) derived from the observed ultraviolet, Hα, infrared, and radio luminosities and to gauge the impact of dust opacity in the local galaxies. We also derive a new calibration for the nonthermal part of the radio SFR estimator, based on the comparison of 1.4 GHz measurements with a new estimator of the bolometric luminosity of the star-forming regions. We find that more actively star-forming galaxies show higher dust opacities, which is in line with previous results. We find that the local star-forming galaxies have a lower Fλ(205 μm)/Fλ(UV) ratio by 2-3 orders of magnitude than the submillimeter-selected galaxies and may have a similar or somewhat higher Fλ(205 μm)/Fλ(UV) ratio than LBGs. The Fλ(205 μm)/Fλ(UV) ratio of the local galaxy population may be influenced by the cool dust emission in the far-infrared heated by nonionizing stellar populations, which may be reduced or absent in the LBGs.

Multiwavelength Star Formation Indicators: Observations

We present a compilation of multiwavelength data on different star formation indicators for a sample of nearby star forming galaxies. Here we discuss the observations, reductions and measurements of ultraviolet images obtained with STIS on board the Hubble Space Telescope (HST), ground-based Hα, and VLA 8.46 GHz radio images. These observations are complemented with infrared fluxes, as well as large-aperture optical, radio, and ultraviolet data from the literature. This database will be used in a forthcoming paper to compare star formation rates at different wave bands. We also present spectral energy distributions (SEDs) for those galaxies with at least one far-infrared measurements from ISO, longward of 100 μm. These SEDs are divided in two groups, those that are dominated by the far-infrared emission, and those for which the contribution from the far-infrared and optical emission is comparable. These SEDs are useful tools to study the properties of high-redshift galaxies.

Interplanetary shocks unconnected with earthbound coronal mass ejections

[1] An associated study by Howard and Tappin (2005) identified 7 Earthbound forward shocks (of which 3 were geoeffective) which were not connected with any detectable coronal mass ejection activity along the Sun-Earth line. This largely unexplored result lends evidence to the fact that some large interplanetary transients are not detected by coronagraphs. This letter explores two possibilities for the formation of the interplanetary foreward shock, namely Corotating Interaction Regions (CIR) or Erupting Magnetic Structures (EMS). Data from EPAM, SWEPAM and MAG on board ACE provided details of the interplanetary shock and associated energetic ions along the Sun-Earth line, while evidence of magnetic field reorientation at the Sun was investigated using EIT on board SOHO, the GOES network and ground-based Hα and radio telescopes. No evidence was found to associate 6 of the shocks with CIRs, although we were uncertain about one event, and in each case evidence of chromospheric activity at the Sun was detected between the estimated onset time of the transient and the arrival of the shock at ACE. The nature of this surface activity included X-Ray (≥C5.0) and Hα flares, associated Type III and Type II radio bursts and disappearing filaments. These results lead to the proposal that EMS are the likely source of some interplanetary transients.

Properties of structured coronal mass ejections in solar cycle 23

The Solar and Heliospheric Observatory has provided to date doubtlessly the best dataset of coronal observations ever taken over the course of a solar cycle. Structured coronal mass ejections, i.e. events with clear white-light fine structures, have been selected from the dataset of the large angle spectroscopic coronagraph during the period 1996–2002. Their source regions in the low corona and photosphere have been deduced by analyzing data from the Extreme-ultraviolet Imaging Telescope and Michelson Doppler Imager onboard the Solar and Heliospheric Observatory and from ground-based Hα measurements. Based on this subset of coronal mass ejections, originally compiled to study their three-dimensional configuration, we have analyzed the variation of their properties during the present solar cycle. These properties include latitude and tilt angle of source regions of coronal mass ejections, position angle of coronal mass ejections, and deviation of coronal mass ejections with respect to their corresponding source regions. The results show a pronounced and systematic confinement of position angles at equatorial latitudes and equatorward deviations during low solar activity. In contrast, fluctuating position angles and deviations in all directions were found at times of high solar activity. A method to explain the observed deflections is presented.

On the three-dimensional configuration of coronal mass ejections

Coronal mass ejections (CMEs) are a direct consequence of the dynamic nature of the solar atmosphere. They represent fundamental processes in which energy is transferred from the Sun into interplanetary space, including geospace. Their origin, 3D structure and internal magnetic field configuration are to date not well understood. The SOHO spacecraft, launched by the end of 1995, has provided unprecedented data on CMEs since instruments switched on in 1996. From a detailed investigation of the full set of LASCO (Large Angle Spectroscopic Coronagraph) observations from 1996 to the end of 2002, a set of structured CME events has been identified, which exhibits white-light fine structures likely indicative of their internal magnetic field configuration and possible 3D structure. Their source regions in the low corona and photosphere have been inferred by means of complementary analyses of data from the Extreme-Ultraviolet Imaging Telescope (EIT) and Michelson Doppler Imager (MDI) on board SOHO, and ground-based Hα measurements. According to the results of this study, structured CMEs arise in a self-similar manner from pre-existing small scale loop systems, overlying regions of opposite magnetic polarities. From the characteristic pattern of the CMEs' source regions in both solar hemispheres, a generic scheme is presented in which the projected white-light topology of a CME depends primarily on the orientation and position of the source region's neutral line on the solar disk. The paper also provides information about the white-light characteristics of the analysed CMEs, such as angular width and position angle, with respect to their source region properties, such as heliographic location, inclination and length, including the frequency and variation of these parameters over the investigated time period.

A High Spatial Resolution X‐Ray and Hα Study of Hot Gas in the Halos of Star‐forming Disk Galaxies. I. Spatial and Spectral Properties of the Diffuse X‐Ray Emission

We present arcsecond resolution Chandra X-ray and ground-based optical Hα imaging of a sample of 10 edge-on star-forming disk galaxies (seven starburst and three spiral galaxies), a sample that covers the full range of star formation intensity found in disk galaxies. The X-ray observations make use of the unprecedented spatial resolution of the Chandra X-ray observatory to more robustly than before remove X-ray emission from point sources and hence obtain the X-ray properties of the diffuse thermal emission alone. We have combined the X-ray observations with existing, comparable-resolution, ground-based Hα and R-band imaging and present a mini-atlas of images on a common spatial and surface brightness scale to aid cross-comparison. In general, the morphology of the extraplanar diffuse X-ray emission is very similar to the extraplanar Hα filaments and arcs, on both small and large scales (scales of tens of parsecs and kiloparsecs, respectively). The most spectacular cases of this are found in NGC 1482 (for which we provide the first published X-ray observation) and NGC 3079. We provide a variety of quantitative measures of how the spectral hardness and surface brightness of the diffuse X-ray emission varies with increasing height z above the plane of each galaxy. Of the eight galaxies in which diffuse X-ray emitting halos are found (the starbursts and the normal spiral NGC 891), significant spatial variation in the spectral properties of the extraplanar emission (|z| ≥ 2 kpc) is only found in two cases: NGC 3628 and NGC 4631. In general, the vertical distribution of the halo-region X-ray surface brightness is best described as an exponential, with the observed scale heights of the sample galaxies lying in the range Heff ~ 2-4 kpc. The presence of extraplanar X-ray emission is always associated with the presence of extraplanar optical line emission of similar vertical extent. No X-ray emission was detected from the halos of the two low-mass normal spiral galaxies NGC 6503 and NGC 4244. Active galactic nuclei, where present, appear to play no role in powering or shaping the outflows from the starburst galaxies in this sample. The Chandra ACIS X-ray spectra of extraplanar emission from all these galaxies can be fitted with a common two-temperature spectral model with an enhanced α-to-iron element ratio. This is consistent with the origin of the X-ray emitting gas being either metal-enriched merged SN ejecta or shock-heated ambient halo or disk material with moderate levels of metal depletion onto dust. Our favored model is that SN feedback in the disks of star-forming galaxies create, via blow-out and venting of hot gas from the disk, tenuous exponential atmospheres of density scale height Hg ~ 4-8 kpc. The soft thermal X-ray emission observed in the halos of the starburst galaxies is either this preexisting halo medium, which has been swept up and shock-heated by the starburst-driven wind, or wind material compressed near the walls of the outflow by reverse shocks within the wind. In either case, the X-ray emission provides us with a powerful probe of the properties of gaseous halos around star-forming disk galaxies.

The Complex Neutral Gas Dynamics of the Dwarf Starburst Galaxy NGC 625

We present new multiconfiguration H I aperture synthesis imaging of the nearby dwarf starburst galaxy NGC 625 obtained with the Australia Telescope Compact Array. Total H I column density images show gas well aligned with the optical major axis and low column density H I extending to greater than 6 optical scale lengths. The H I velocity field, on the other hand, is highly disturbed, with neutral gas at nearly all detected velocities within the central region. After considering various interpretations, we find that a blowout scenario most accurately describes the data. Since at our resolution we do not detect any large evacuated holes in the H I disk, we interpret this blowout to be the result of the extended (both spatially and temporally) star formation event that NGC 625 has undergone in the last 100 Myr. This is one of the clearest examples of H I outflow detected in a dwarf galaxy. We find no obvious external trigger for this extended star formation event. We detect strong radio continuum emission from the largest H II regions; comparing to our Hubble Space Telescope and ground-based Hα fluxes suggests either appreciable amounts of extinction toward the star formation regions or the contribution of nonthermal sources to the radio continuum luminosity.

Discovery of a Double Ring in the Dwarf Galaxy Markarian 409

We report the discovery of a double ring of emission-line regions around the nucleus of the blue compact dwarf galaxy Mrk 409 as seen by deep, ground-based Hα images. Echelle spectroscopy obtained at Magellan I with MIKE shows the presence of ionized gas flowing out of the galaxy from a very massive (≥7 × 10^6 M_☉) nuclear starburst with projected expansion velocities of ~50 km s-^1. Different scenarios for the formation of these rings are discussed. While the innermost, nuclear ring is most probably formed by the interaction of a starburst-driven shock with the surrounding interstellar medium, the origin of the outer ring is less clear.

Massive Star Clusters in Ongoing Galaxy Interactions: Clues to Cluster Formation

We present HST WFPC2 observations, supplemented by ground-based Hα data, of the star-cluster populations in two pairs of interacting galaxies selected for being in very different kinds of encounters seen at different stages. Dynamical information and n-body simulations provide the details of encounter geometry, mass ratio, and timing. In NGC 5752/4 we are seeing a weak encounter, well past closest approach, after about 2.5 × 108 yr. The large spiral NGC 5754 has a normal population of disk clusters, while the fainter companion NGC 5752 exhibits a rich population of luminous clusters with a flatter luminosity function. The strong, ongoing encounter in NGC 6621/2, seen about 1.0 × 108 yr past closest approach between roughly equal-mass galaxies, has produced an extensive population of luminous clusters, particularly young and luminous in a small region between the two nuclei. This region is dynamically interesting, with such a strong perturbation in the velocity field that the rotation curve reverses sign. From these results, in comparison with other strongly interacting systems discussed in the literature, cluster formation requires a threshold level of perturbation, with stage of the interaction a less important factor. The location of the most active star formation in NGC 6621/2 draws attention to a possible role for the Toomre stability threshold in shaping star formation in interacting galaxies. The rich cluster populations in NGC 5752 and NGC 6621 show that direct contact between gas-rich galaxy disks is not a requirement to form luminous clusters and that they can be triggered by processes happening within a single galaxy disk (albeit triggered by external perturbations).

Solar Magnetic Field Studies Using the 12 Micron Emission Lines. IV. Observations of a Delta Region Solar Flare

We have recently developed the capability to make solar vector (Stokes IQUV) magnetograms using the infrared line of Mg I at 12.32 μm. On 2001 April 24, we obtained a vector magnetic map of solar active region NOAA 9433, fortuitously just prior to the occurrence of an M2 flare. Examination of a sequence of SOHO/Michaelson Doppler Imager magnetograms and comparison with ground-based Hα images shows that the flare was produced by the cancellation of newly emergent magnetic flux outside of the main sunspot. The very high Zeeman sensitivity of the 12 μm data allowed us to measure field strengths on a spatial scale which was not directly resolvable. At the flare trigger site, opposite polarity fields of 2700 and 1000 G occurred within a single 2'' resolution element, as revealed by two resolved Zeeman splittings in a single spectrum. Our results imply an extremely high horizontal field strength gradient (5 G km-1) prior to the flare, significantly greater than seen in previous studies. We also find that the magnetic energy of the cancelling fields was more than sufficient to account for the flare's X-ray luminosity.