UV Brightenings Research Articles

Transition region response to quiet-Sun Ellerman bombs

Context. Quiet-Sun Ellerman bombs (QSEBs) are key indicators of small-scale photospheric magnetic reconnection events. Recent high-resolution observations have shown that they are ubiquitous and that large numbers of QSEBs can be found in the quiet Sun. Aims. We aim to understand the impact of QSEBs on the upper solar atmosphere by analyzing their spatial and temporal relationship with the UV brightenings observed in transition region diagnostics. Methods. We analyzed high-resolution Hβ observations from the Swedish 1-m Solar Telescope and utilized k-means clustering to detect 1423 QSEBs in a 51 min time series. We used coordinated and co-aligned observations from the Interface Region Imaging Spectrograph (IRIS) to search for corresponding signatures in the 1400 Å slit-jaw image (SJI) channel and in the Si IV 1394 Å and Mg II 2798.8 Å triplet spectral lines. We identified UV brightenings from SJI 1400 using a threshold of 5σ above the median background. Results. We focused on 453 long-lived QSEBs (> 1 min) and found 67 cases of UV brightenings from SJI 1400 occurring near the QSEBs, both temporally and spatially. Temporal analysis of these events indicates that QSEBs start before UV brightenings in 57% of cases, while UV brightenings lead in 36% of instances. The majority of the UV brightenings occur within 1000 km of the QSEBs in the direction of the solar limb. We also identify 21 QSEBs covered by the IRIS slit, four of which show emissions in the Si IV 1394 Å and/or Mg II 2798.8 Å triplet lines, at distances within 500 km of the QSEBs in the limb direction. Conclusions. We conclude that a small fraction (15%) of the long-lived QSEBs contribute to the localized heating observable in transition region diagnostics, indicating they play a minimal role in the global heating of the upper solar atmosphere.

The relation between X-ray and ultraviolet variability of quasars

ABSTRACT The relation between X-ray and UV/optical variability in AGNs has been explored in many individual sources, however a large sample study is yet absent. Through matching the XMM-Newton serendipitous X-ray and UV source catalogs with SDSS quasars, we build a sample of 802 epoch-pairs of 525 quasars showing clear variability in logFX − logFUV space. After correcting for the effect of photometric noise, we find 35.6 ± 2.1 per cent of the epoch-pairs show asynchronous variability between X-ray and UV (brightening in one band but dimming in the other). This indicates only in 28.8 ± 4.2 per cent of the epoch-pairs the X-ray and UV variability are intrinsically coordinated. The variability synchronicity exhibits no dependence on physical parameters of quasars or the time lag of the epoch-pairs, except for stronger variability tends to have stronger synchronicity. Switches between synchronous and asynchronous variability are also seen in individual sources. The poor coordination clearly contradicts both the X-ray reprocessing model and the accretion rate variation model for AGN variability. The ratios of the observed X-ray variability amplitude to that in UV span a broad range and peak at ∼2. The dominant fraction of the ratios appear too small to be attributed to X-ray reprocessing, and too large for accretion rate variation. The inhomogeneous disc model which incorporates both X-ray and UV/optical variability in AGNs is favoured by the observed stochastic relation between X-ray and UV variations.

Thermal Instability–Induced Fundamental Magnetic Field Strands in the Solar Corona

Abstract Thermal instability is a fundamental process of astrophysical plasmas. It is expected to occur whenever the cooling is dominated by radiation and cannot be compensated for by heating. In this work, we conduct 2.5D radiation MHD simulations with the Bifrost code of an enhanced activity network in the solar atmosphere. Coronal loops are produced self-consistently, mainly through Joule heating, which is sufficiently stratified and symmetric to produce thermal nonequilibrium. During the cooling and driven by thermal instability, coronal rain is produced along the loops. Due to flux freezing, the catastrophic cooling process leading to a rain clump produces a local enhancement of the magnetic field, thereby generating a distinct magnetic strand within the loop up to a few Gauss stronger than the surrounding coronal field. These strands, which can be considered fundamental, are a few hundred kilometers in width, span most of the loop leg, and emit strongly in the UV and extreme UV (EUV), thereby establishing a link between the commonly seen rain strands in the visible spectrum with the observed EUV coronal strands at high resolution. The compression downstream leads to an increase in temperature that generates a plume-like structure, a strongly emitting spicule-like feature, and short-lived brightening in the UV during the rain impact, providing an explanation for similar phenomena seen with IRIS. Thermal instability and nonequilibrium can therefore be associated with localized and intermittent UV brightening in the transition region and chromosphere, as well as contribute to the characteristic filamentary morphology of the solar corona in the EUV.

Coronal Mini-jets in an Activated Solar Tornado-like Prominence

Abstract High-resolution observations from the Interface Region Imaging Spectrometer reveal the existence of a particular type of small solar jet, which arose singly or in clusters from a tornado-like prominence suspended in the corona. In this study, we perform a detailed statistical analysis of 43 selected mini-jets in the tornado event. Our results show that the mini-jets typically have (1) a projected length of 1.0–6.0 Mm, (2) a width of 0.2–1.0 Mm, (3) a lifetime of 10–50 s, (4) a velocity of 100–350 km s−1, and (5) an acceleration of 3–20 km s−2. Based on spectral diagnostics and EM-Loci analysis, these jets seem to be multithermal small-scale plasma ejections with an estimated average electron density of ∼2.4 × 1010 cm−3 and an approximate mean temperature of ∼2.6 × 105 K. Their mean kinetic energy density, thermal energy density, and dissipated magnetic field strength are roughly estimated to be ∼9 erg cm−3, 3 erg cm−3, and 16 G, respectively. The accelerations of the mini-jets, the UV and EUV brightenings at the footpoints of some mini-jets, and the activation of the host prominence suggest that the tornado mini-jets are probably created by fine-scale external or internal magnetic reconnections (a) between the prominence field and the enveloping or background field or (b) between twisted or braided flux tubes within the prominence. The observations provide insight into the geometry of such reconnection events in the corona and have implications for the structure of the prominence magnetic field and the instability that is responsible for the eruption of prominences and coronal mass ejections.

Mid-infrared evolution of η Carinae from 1968 to 2018

η Car is one of the most luminous and massive stars in our Galaxy and is the brightest mid-IR source in the sky outside our solar system. Since the late 1990s, the central source has dramatically brightened at UV and optical wavelengths. This might be explained by a decrease in circumstellar dust extinction. We aim to establish the mid-IR flux evolution and further our understanding of the star’s UV and optical brightening. Mid-IR images from 8−20 μm were obtained in 2018 with VISIR at the Very Large Telescope. Archival data from 2003 and 2005 were retrieved from the ESO Science Archive Facility, and historical records were collected from publications. We present mid-IR images of η Car with the highest angular resolution to date at the corresponding wavelengths (≥0.22″). We reconstruct the mid-IR evolution of the spectral energy distribution of the spatially integrated Homunculus nebula from 1968 to 2018 and find no long-term changes. The bolometric luminosity of η Car has been stable over the past five decades. We do not observe a long-term decrease in the mid-IR flux densities that could be associated with the brightening at UV and optical wavelengths, but circumstellar dust must be declining in our line of sight alone. Short-term flux variations within about 25% of the mean levels could be present.

DETERMINING HEATING RATES IN RECONNECTION FORMED FLARE LOOPS OF THE M8.0 FLARE ON 2005 MAY 13

We analyze and model an M8.0 flare on 2005 May 13 observed by TRACE and RHESSI to determine the energy release rate from magnetic reconnection that forms and heats numerous flare loops. The flare exhibits two ribbons in UV 1600 {\AA} emission. Analysis shows that the UV light curve at each flaring pixel rises impulsively within a few minutes, and decays slowly with a timescale >10 min. Since the lower atmosphere (transition region and chromosphere) responds to energy deposit nearly instantaneously, the rapid UV brightening is thought to reflect the energy release process in the newly formed flare loop rooted at the footpoint. We utilize spatially resolved (down to 1 arcsec) UV light curves and thick-target hard X-ray emission to construct heating functions of a few thousand flare loops anchored at UV foot points, and compute plasma evolution in these loops using the EBTEL model. The modeled coronal temperatures and densities of these flare loops are used to calculate synthetic soft X-ray spectra and light curves, which compare favorably with those observed by RHESSI and GOES/XRS. The time-dependent transition region DEM for each loop during its decay phase is also computed with a simplified model and used to calculate the optically-thin C IV line emission, which dominates the UV 1600 {\AA} bandpass during the flare. The computed C IV line emission decays at the same rate as observed. This study presents a method to constrain heating of reconnection-formed flare loops using all available observables independently, and provides insight into the physics of energy release and plasma heating during the flare. With this method, the lower limit of the total energy used to heat the flare loops in this event is estimated to be 1.22e31 ergs, of which only 1.9e30 ergs is carried by beam-driven upflows during the impulsive phase, suggesting that the coronal plasmas are predominantly heated in situ.

INVESTIGATING TWO SUCCESSIVE FLUX ROPE ERUPTIONS IN A SOLAR ACTIVE REGION

We investigate two successive flux rope (FR1 and FR2) eruptions resulting in two coronal mass ejections (CMEs) on 2012 January 23. Both FRs appeared as an EUV channel structure in the images of high temperature passbands of the Atmospheric Imaging Assembly prior to the CME eruption. Through fitting their height evolution with a function consisting of linear and exponential components, we determine the onset time of the FR impulsive acceleration with high temporal accuracy for the first time. Using this onset time, we divide the evolution of the FRs in the low corona into two phases: a slow rise phase and an impulsive acceleration phase. In the slow rise phase of the FR1, the appearance of sporadic EUV and UV brightening and the strong shearing along the polarity inverse line indicates that the quasi-separatrix-layer reconnection likely initiates the slow rise. On the other hand for the FR2, we mainly contribute its slow rise to the FR1 eruption, which partially opened the overlying field and thus decreased the magnetic restriction. At the onset of the impulsive acceleration phase, the FR1 (FR2) reaches the critical height of 84.4$\pm$11.2 Mm (86.2$\pm$13.0 Mm) where the decline of the overlying field with height is fast enough to trigger the torus instability. After a very short interval ($\sim$2 minutes), the flare emission began to enhance. These results reveal the compound activity involving multiple magnetic FRs and further suggest that the ideal torus instability probably plays the essential role of initiating the impulsive acceleration of CMEs.

HARD X-RAY AND ULTRAVIOLET OBSERVATIONS OF THE 2005 JANUARY 15 TWO-RIBBON FLARE

In this paper, we present comprehensive analysis of a two-ribbon flare observed in UV 1600{\AA} by Transition Region and Coronal Explorer and in HXRs by Reuven Ramaty High Energy Solar Spectroscopic Imager. HXR (25-100 keV) imaging observations show two kernels of size (FWHM) 15?? moving along the two UV ribbons. We find the following results. (1) UV brightening is substantially enhanced wherever and whenever the compact HXR kernel is passing, and during the HXR transit across a certain region, the UV count light curve in that region is temporally correlated with the HXR total flux light curve. After the passage of the HXR kernel, the UV light curve exhibits smooth monotonical decay. (2)We measure the apparent motion speed of the HXR sources and UV ribbon fronts, and decompose the motion into parallel and perpendicular motions with respect to the magnetic polarity inversion line (PIL). It is found that HXR kernels and UV fronts exhibit similar apparent motion patterns and speeds. The parallel motion dominates during the rise of the HXR emission, and the perpendicular motion starts and dominates at the HXR peak, the apparent motion speed being 10-40 km s-1. (3) We also find that UV emission is characterized by a rapid rise correlated with HXRs, followed by a long decay on timescales of 15-30 minutes. The above analysis provides evidence that UV brightening is primarily caused by beam heating, which also produces thick-target HXR emission. The thermal origin of UV emission cannot be excluded, but would produce weaker heating by one order of magnitude. The extended UV ribbons in this event are most likely a result of sequential reconnection along the PIL, which produces individual flux tubes (post-flare loops), subsequent non-thermal energy release and heating in these flux tubes, and then the very long cooling time of the transition region at the feet of these flux tubes.

RECONNECTION AND ENERGETICS IN TWO-RIBBON FLARES: A REVISIT OF THE BASTILLE-DAY FLARE

We conduct a semi-quantitative analysis of two-ribbon flares to investigate the observational relationship between magnetic reconnection and energetics by revisiting the Bastille-day flare, particularly the UV and hard X-ray (HXR) observations. The analysis establishes that prominent UV emission is primarily produced by precipitating electrons that also produce HXRs. In addition, reconnection and subsequent energy release along adjacent field lines along the polarity inversion line (PIL) combined with elongated decay of UV emission may account for the observed extended UV ribbons whereas HXR sources with rapid decay appear mostly as compact kernels. Observations also show that HXR sources and UV brightenings exhibit an organized parallel motion along the magnetic PIL during the rise of the flare, and then the perpendicular expansion of UV ribbons dominate during the peak. With a 2.5 dimensional approximation with the assumed translational dimension along the PIL, we derive geometric properties of UV ribbons and infer the pattern of reconnection as with a varying magnetic guide field during reconnection. It is shown that HXR and UV emissions evolve in a similar way to reconnection rates determined by the perpendicular “motion.” The analysis suggests that a relatively strong guide field may be present during the rise of the flare, whereas particle acceleration and non-thermal energy release are probably more efficient with an enhanced reconnection rate with a relatively weak guide field. We discuss the role of the guide field in reconnection and particle energization, as well as novel observational experiments that may be conducted to shed new light on these issues.

OBSERVATIONS OF CHROMOSPHERIC FLARE RE-BRIGHTENINGS

We investigate an active region that produced three C-class flares and one M-class flare within 2.5 hr. The morphology and location of the C-flares indicate that these events constitute a set of homologous flares. Radio observations indicate the occurrence of a downward-moving plasmoid during the impulsive phase of the M flare. We use TRACE 1700 A filtergrams and SOHO Michelson Doppler Imager magnetograms to examine the character of the UV brightenings; i.e., we search for re-brightenings of former flare areas both across the series of events and within one and the same event. We find that essentially the same footpoints re-brighten in each C flare. Based on the progression of both the derived magnetic flux change rate and the observed Radio Solar Telescope Network microwave emission, we speculate about a further re-brightening during the decay phase of the M flare as a further member of the series of homologous flares. We conclude that the postflare field is driven to repeated eruption by continuous, shear-increasing, horizontal, photospheric flows, as one end of the involved magnetic arcade is anchored in the penumbra of a large sunspot. The observed motion pattern of the UV kernels indicates that the arcade evolves during the series of events from a both highly sheared and heavily entangled state to a still sheared but more organized state.

UV TRANSIENT BRIGHTENINGS ASSOCIATED WITH A CORONAL MASS EJECTION

In this paper, we analyze transient UV brightenings in spectra acquired by SOHO/UltraViolet Coronagraph Spectrometer (UVCS) on 2003 June 2 in association with a coronal mass ejection (CME) that occurred at the West limb of the Sun at 08:54 UT. Brightenings have been observed in lines from cool (C III, O VI), intermediate (Si VIII, Si XII), and high ([Fe XVIII]) temperature ions over about 7 hr from the CME. Brightenings in cool lines are interpreted in terms of mini-ejections that appear at the time of, and after, the passage of the CME front through the UVCS slit. We give here their temperature and density and we point out that, assuming a spherical shape, a few of these mini-CMEs can provide a mass comparable to that quoted for typical CMEs. Hot lines, like the [Fe XVIII] line at 974.9 A which shows up in the CME associated current sheet (CS), undergo transient brightness as well, but hot lines brightenings are more difficult to interpret. We propose here a scenario where they are signatures of the passage through the UVCS slit of plasmoids similar to those observed in the filamentary CS of the magnetotail that form as a consequence of the tearing-mode instability or of a time-dependent Petschek-type reconnection.

Preflare Eruption Triggered by a Tether‐cutting Process

We have examined the preflare activity of an M1.2 flare that occurred in NOAA active region 8440 on 1999 January 16, using images from the Soft X-Ray Telescope (SXT) on board Yohkoh, 1600 A UV images from the Transition Region and Coronal Explorer (TRACE), X-ray flux data from the GOES satellite, and magnetograms from Big Bear Solar Observatory (BBSO). During the preflare phase, we note a weak GOES X-ray flux enhancement just 4 minutes before the main flare begins. The SXT images show that this enhancement occurs at one footpoint of a soft X-ray loop bundle, which exactly coincides with the kernel of the major flare. The series of TRACE images provides the following pieces of evidence for small-scale magnetic reconnections associated with the preflare activity. (1) A small-scale UV sigmoid is seen at the X-ray loop footpoint before the preflare activity, and it is located along the polarity inversion line. (2) The brightest among the UV brightenings is exactly coincident and cospatial with the soft X-ray brightening observed by the Yohkoh SXT and GOES. (3) There were several interactions and brightenings among small UV loops. After these brightenings, the connectivity of the UV loops was apparently changed. As a result, a large rising loop structure was formed, with a maximum rising speed of about 40 km s−1. (4) The main flare occurred in this structure. In the aspects of the overall configuration and morphological change of UV loops, the preflare activity is quite consistent with the tether-cutting model with a single-bipole magnetic explosion. We suggest that the preflare activity and the main flare in this event not only have similar physical mechanisms, but also have a causal relation.

Solar impulsive EUV and UV brightenings in flare footpoints and their connection with X-ray emission

Context. The thick-target model of the impulsive phase of solar flares describes the propagation of non-thermal electron beams into the chromosphere. The stopping depth of the electron is related to its energy. Thus, the thermal reaction at different levels should be connected to the parameters of the electron spectrum. Aims. We investigate the impulsive thermal reaction produced by non-thermal electron beams at different heights of the solar atmosphere observed in solar flare footpoints. Methods. We used methods developed for the investigation of the impulsive SXR brightenings. The EUV and UV images obtained by TRACE in the 171 A and 1600 A filter were used as the basic data set for this analysis. For the first time, EUV images have been desaturated using diffraction pattern properties. Results. We find that the thermal impulsive reaction observed in solar flare footpoints is different for UV and EUV brightenings. Results are compared to those obtained for the impulsive SXR brightenings. We find that observed dissimilarities are connected to the properties of the non-thermal electron spectrum and to the depth of their penetration into the chromosphere. Moreover, we show that the lower levels of the chromosphere are excited by more energetic electrons.

LOW ATMOSPHERE RECONNECTIONS ASSOCIATED WITH AN ERUPTIVE SOLAR FLARE

It has been a big mystery what drives filament eruptions and flares. We have studied in detail an X1.8 flare and its associated filament eruption that occurred in NOAA Active Region 9236 on November 24,2000. For this work we have analyzed high temporal (about 1 minute) and spatial (about 1 arcsec) resolution images taken by Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory, Hoc centerline and blue wing () images from Big Bear Solar Observatory, and 1600 UV images by the Transition Region and Corona Explorer (TRACE). We have found that there were several transient brightenings seen in H and, more noticeably in TRACE 1600 images around the preflare phase. A closer look at the UV brightenings in 1600 images reveals that they took place near one end of the erupting filament, and are a kind of jets supplying mass into the transient loops seen in 1600 . These brightenings were also associated with canceling magnetic features (CMFs) as seen in the MDI magnetograms. The flux variations of these CMFs suggest that the flux cancellation may have been driven by the emergence of the new flux. For this event, we have estimated the ejection speeds of the filament ranging from 10 to 160 km for the first twenty minutes. It is noted that the initiation of the filament eruption (as defined by the rise speed less than 20 km ) coincided with the preflare activity characterized by UV brightenings and CMFs. The speed of the associated LASCO CME can be well extrapolated from the observed filament speed and its direction is consistent with those of the disturbed UV loops associated with the preflare activity. Supposing the H/UV transient brightenings and the canceling magnetic features are due to magnetic reconnect ion in the low atmosphere, our results may be strong observational evidence supporting that the initiation of the filament eruption and the preflare phase of the associated flare may be physically related to low-atmosphere magnetic reconnection.

Close Correlation among Hα Surges, Magnetic Flux Cancellations, and UV Brightenings Found at the Edge of an Emerging Flux Region

Abstract Surge activities were observed at the edge of an emerging flux region. We studied the relations between the features around the surges in various data sets: magnetogram, $\mathrm{H}\alpha$, G-band, UV, EUV, and soft X-rays. We showed that the surge activities in $\mathrm{H}\alpha$ and the brightenings in TRACE 1600 Å images correlate well in both time and space with the cancellation of magnetic fluxes around an emerging flux region. In particular, at the onset of surge activity, a close correlation among them was clearly found. These facts are consistent with the magnetic reconnection model. The released energy through magnetic reconnection, which is estimated to be $10^{28} \,\mathrm{erg}$, is sufficiently large to produce surge activities. No prominent brightenings were observed in soft X-ray and EUV images during the surge activities. This may suggest that the energy releases occurred at a layer of high densities.

CDS UV Brightenings Explained by Quasi-separatrices and Bald Patches in an S-shape Active Region

We present multi-instrument observations of AR 8048, made between June 3 and June 5 1997 as part of SoHO JOP033. This active region (AR) has a sigmoid-like global shape and undergoes transient brightenings through which the stored energy is released.Using a magneto-hydrostatic model, we compute coronal magnetic field. The large-scale magnetic lines confirm the sigmoidal characteristics of the AR. The field lines most closely matching the hotter SoHO/CDS loops extend along the quasi-separatrix-Iayers (QSLs) of the coronal field. Transition region (TR) brightenings observed with SoHO/CDS can be associated with both QSL intersections with the photosphere, and places where separatrices corresponding to bald patches (BPs, sites where field lines are tangent to the photosphere) lie at the photospheric plane. There are suggestions that the element abundances measured in the TR may depend on the type of topological structure present. TR brightenings associated with QSLs have coronal abundances, while those associated with BP separatrices have abundances closer to photospheric values.

Physical Parameters of Erupting Luminous Blue Variables: NGC 2363‐V1 Caught in the Act

A quantitative study of the luminous blue variable NGC 2363-V1 in the Magellanic galaxy NGC 2366 (D = 3.44 Mpc) is presented, based on ultraviolet and optical Hubble Space Telescope STIS spectroscopy. Contemporary WFPC2 and William Herschel Telescope imaging reveals a modest V-band brightness increase of ~0.2 mag per year between 1996 January-1997 November, reaching V = 17.4 mag, corresponding to MV = -10.4 mag. Subsequently, V1 underwent a similar decrease in V-band brightness, together with a UV brightening of 0.35 mag from 1997 November to 1999 November. The optical spectrum of V1 is dominated by H emission lines, with Fe II, He I and Na I also detected. In the ultraviolet, a forest of Fe absorption features and numerous absorption lines typical of mid-B supergiants (such as Si II, Si III, Si IV, C III, C IV) are observed. From a spectral analysis with the non-LTE, line-blanketed code of Hillier & Miller, we derive stellar parameters of T* = 11 kK, R* = 420 R☉, log (L/L☉) = 6.35 during 1997 November, and T* = 13 kK, R* = 315 R☉, log (L/L☉) = 6.4 for 1999 July. The wind properties of V1 are also exceptional, with ≃ 4.4 × 10-4M☉ yr-1 and v∞ ≃ 300 km s-1, allowing for a clumped wind (filling factor = 0.3) and assuming H/He ~ 4 by number. The presence of Fe lines in the UV and optical spectrum of V1 permits an estimate of the heavy elemental abundance of NGC 2363 from our spectral synthesis. Although some deficiencies remain, allowance for charge exchange reactions in our calculations supports a SMC-like metallicity, that has previously been determined for NGC 2363 from nebular oxygen diagnostics. Considering a variety of possible progenitor stars, V1 has definitely undergone a giant eruption, with a substantial increase in stellar luminosity, radius, and almost certainly mass-loss rate, such that its stellar radius increased at an average rate of ~4 km s-1 during 1992 October-1995 February. The stellar properties of V1 are compared to other LBVs, including η Car and HD 5980 during its brief eruption in 1994 September, the latter newly analyzed here. The mass-loss rate of the HD 5980 eruptor compares closely with V1, but its bolometric luminosity was a factor ~6 times larger.

3-D magnetic configurations for filaments and flares: The role of “magnetic dips” and “bald patches”

The 3-D magnetic configuration of a filament and of a low energy flare is reconstructed, using linear magnetohydrostatic ( lmhs) extrapolations. In both cases, we find observational signatures of energy release at the locations of computed “bald patches” separatrices, characterised by field lines which are tangent to the photosphere. The filament was observed on Sept. 25, 1996, in Hα with the MSDP on the German VTT, Tenerife, as well as in Si iv with SOHO/SUMER. It is modeled as a twisted flux-tube deformed by the magnetic polarities observed with SOHO/MDI. The shape and location of the computed dipped field lines are in good agreement with the shape of the filament and its feet observed in Hα. Some “bald patches” (BPs) are present where the distribution of dips reaches the photosphere. We show that some of the large scale field lines rooted in BPs can be related to bright fine structures in Si iv. We propose that the plasma there is heated by ohmic dissipation from the currents expected to be present along the BP separatrices. The flare was observed on May 18, 1994, in soft X-rays with Yohkoh/SXT, and in Hα at Mitaka (Japan). The magnetic field is directly extrapolated from a photospheric magnetogram from Kitt Peak Observatory. The intersections with the photosphere of the computed separatrices match well the bright Hα ribbons. The later are associated to three BPs, with overlaying dipped field lines. We show that enhanced densities are present in these dips, which can be correlated with dark Hα fibrils. Both cases show the importance of dipped field lines and BPs in the solar atmosphere. Energy release via ohmic dissipation as well as reconnection along BP separatrices is proposed to provide heating observed as UV brightenings in filament channels and even as small flares.

Auroral activity associated with pressure pulses and substorms: A comparison between global fluid modeling and Viking UV imaging

Global three‐dimensional fluid simulations in conjunction with Viking UV observations are used to investigate the response of the auroral region to solar wind pressure pulses. The fluid simulations differ from ideal MHD in that the steady state electron momentum equation which incorporates the Hall and gradient pressure terms is used instead of the idealized Ohm's law. This treatment produces important corrections to the currents that map into the noon and midnight sectors of the auroral regions and can be important in determining magnetospheric activity during periods of changing interplanetary magnetic field (IMF). Two pressure pulses observed by IMP 8 in the solar wind are shown to produce transient (a few minute) variations in the auroral oval, starting with brightenings near noon which then quickly propagate into the nightside. Of the two pulses, the second pulse which is accompanied by southward IMF produces the larger nightside activity. The model shows that the observed UV brightenings are accompanied by large increases in the auroral field‐aligned currents (of the order of 1 MA) at approximately the same MLT as the UV brightenings. However, the current enhancements produced by each pulse are shown to develop along slightly different latitudes of the existing region 1 currents so that the current system can become filamented. The noon currents are also shown to be highly dependent on both By and Bz components of the IMF.

Correlated observations of impulsive UV and hard X-ray bursts in solar flares from the solar maximum mission

An investigation is conducted of the temporal and spatial structures of UV and hard X-ray bursts in a disk and a limb flare observed with instruments on the Solar Maximum Mission satellite. Attention is given to the transient UV brightening before the flare, the impulsive enhancement of UV continuum emission, the relationship between emission source region and particle acceleration region, and large scale excitations. The most active part of the active region appears to be the most flare-productive region. These regions exhibit high UV activities with numerous UV transient bursts occurring in many small kernels.