arXiv subjects

C. J. Nelson

Publications and source records attributed to C. J. Nelson.

At least 19 recordsLinked to original sources

arXiv2024

Spatial distributions of EUV brightenings in the quiet-Sun

The identification of large numbers of localised transient EUV brightenings, with small spatial scales, in the quiet-Sun corona has been one of the key early results from Solar Orbiter. However, much is still unknown about these events. Here, we aim to better understand EUV brightenings by investigating their spatial distributions, specifically whether they occur co-spatial with specific line-of-sight magnetic field topologies in the photospheric network. EUV brightenings are detected using an automated algorithm applied to a high-cadence (3 s) dataset sampled over ~30 min on 8 March 2022 by the Extreme Ultraviolet Imager's 17.4 nm EUV High Resolution Imager. Data from the Solar Dynamics Observatory's Helioseismic and Magnetic Imager and Atmospheric Imaging Assembly are used to provide context about the line-of-sight magnetic field and for alignment purposes. We found a total of 5064 EUV brightenings within this dataset that are directly comparable to events reported previously in the literature. These events occurred within around 0.015-0.020 % of pixels for any given frame. We compared eight different thresholds to split the EUV brightenings into four different categories related to the line-of-sight magnetic field. Using our preferred threshold, we found that 627 EUV brightenings (12.4 %) occurred co-spatial with Strong Bipolar configurations and 967 EUV brightenings (19.1 %) occurred in Weak Field regions. Fewer than 10 % of EUV brightenings occurred co-spatial with Unipolar line-of-sight magnetic field no matter what threshold was used. Of the 627 Strong Bipolar EUV Brightenings, 54 were found to occur co-spatial with cancellation whilst 57 occurred co-spatial with emergence. EUV brightenings preferentially occur co-spatial with the strong line-of-sight magnetic field in the photospheric network. They do not, though, predominantly occur co-spatial with (cancelling) bi-poles.

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arXiv2024

A multi-instrument study of ultraviolet bursts and associated surges in AR 12957

The relationship between UV Bursts and solar surges is complex, with these events sometimes being observed together and sometimes being observed independently. Why this sporadic association exists is unknown, however, it likely relates to the physical conditions at the site of the energy release that drives these events. Here, we aim to better understand the relationship between UV Bursts and solar surges through a multi-instrument analysis of several associated events that occurred around the trailing sunspot in AR 12957. We use data from Solar Orbiter, the Solar Dynamics Observatory (SDO), and the Interface Region Imaging Spectrograph (IRIS) to achieve our aims. These data were sampled on 3rd March 2022 between 09:30:30 UT and 11:00:00 UT, during which time a coordinated observing campaign associated with the Slow Solar Wind Connection Solar Orbiter Observing Plan took place. Numerous small-scale negative polarity magnetic magnetic features (MMFs) are observed to move quickly (potentially up to 3.3 km/s) away from a sunspot until they collide with a more stable positive polarity plage region around 7 Mm away. Several UV Bursts are identified in IRIS slit-jaw imager (SJI) 1400 Å data co-spatial to where these opposite polarity fields interact, with spatial scales (2 Mm<) and lifetimes (20< min) larger than typical values for such events. Two surges are also observed to occur at these locations, with one being short (5 Mm) and hot (bright in IRIS SJI images), whilst the other is a cooler (dark in coronal imaging channels), longer surge that appears to fill an active region loop. Magnetic reconnection between the negative polarity MMFs around the sunspot and the positive polarity plage region appears to be the driver of these events. Both the speed of the MMFs and the locally open magnetic topology of the plage region could possibly be important for forming the surges.

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arXiv2023

EUV brightenings in the quiet-Sun: Signatures in spectral and imaging data from the Interface Region Imaging Spectrograph

Localised transient EUV brightenings, sometimes named `campfires', occur throughout the quiet-Sun. However, there are still many open questions about such events, in particular regarding their temperature range and dynamics. In this article, we aim to determine whether any transition region response can be detected for small-scale EUV brightenings and, if so, to identify whether the measured spectra correspond to any previously reported bursts in the transition region, such as Explosive Events (EEs). EUV brightenings were detected in a ~29.4 minute dataset sampled by Solar Orbiter's Extreme Ultraviolet Imager on 8 March 2022 using an automated detection algorithm. Any potential transition region response was inferred through analysis of imaging and spectral data sampled through coordinated observations conducted by the Interface Region Imaging Spectrograph (IRIS). EUV brightenings display a range of responses in IRIS slit-jaw imager (SJI) data. Some events have clear signatures in the Mg II and Si IV SJI filters, whilst others have no discernible counterpart. Both extended and more complex EUV brightenings are found to, sometimes, have responses in IRIS SJI data. Examples of EUI intensities peaking before, during, and after their IRIS counterparts were found in lightcurves constructed co-spatial to EUV brightenings. Importantly, therefore, it is likely that not all EUV brightenings are driven in the same way, with some seemingly being magnetic reconnection driven and others not. A single EUV brightening occurred co-spatial to the IRIS slit, with its spectra matching the properties of EEs. EUV brightenings is a term used to describe a range of small-scale event in the solar corona. The physics responsible for all EUV brightenings is likely not the same and, therefore, more research is required to assess their importance towards global questions in the field, such as coronal heating.

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arXiv2022

Investigating datasets with high IRIS burst prevalence

Approximately 0.01 % of all Si IV 1394 A spectra sampled in 2013 and 2014 by the Interface Region Imaging Spectrograph (IRIS) have IRIS burst profiles. However, these events are not evenly distributed across datasets with 19.31 % of these spectra being identified in only six rasters. Here, we investigate five of these six datasets, to understand why they contain so many IRIS burst profiles. This research will help guide future targeted analyses of IRIS bursts. We analyse five datasets sampled by the IRIS satellite, studying both Si IV 1394 A spectra and 1400 A filter slit-jaw imager (SJI) data. IRIS burst profiles are identified through the use of an automated algorithm. Additionally, we study co-spatial line-of-sight photospheric magnetic field maps sampled by the Solar Dynamics Observatory's Helioseismic and Magnetic Imager (SDO/HMI) instrument. The majority of identified IRIS burst profiles (12401 out of 13904) found in the five datasets analysed here were localised to seven small regions in the time-distance domain (temporal durations of <4 hours and spatial lengths of <12" along the slit). The SJI data co-spatial to these regions contained long-lived or repetitive compact brightenings, matching the defined properties of UV bursts, which remained close to the IRIS slit throughout their evolutions. The IRIS burst profiles were not limited to the brightest pixels in the fields of view (FOVs) nor did they comprise the majority of bright (>500 DN/s) pixels. These IRIS burst profiles occurred co-spatial to evolving (e.g. cancelling) opposite polarity magnetic fields where magnetic reconnection is thought to be possible. More than 10 % of the IRIS burst profiles identified during the entirety of 2013 and 2014 are contained in just seven small regions in the time-distance domain where long-lived (lifetimes >10 minutes) or repetitive UV bursts occurred along the axis of the IRIS slit.

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arXiv2022

Pre-emergence Signatures Of Horizontal Divergent Flows In Solar Active Regions

Solar active regions (ARs) play a fundamental role in driving many of the geo-effective eruptions which propagate into the Solar System. However, we are still unable to consistently predict where and when ARs will occur across the solar disk by identifying pre-emergence signatures in observables such as the Doppler velocity (without using Helioseismic methods). Here we aim to determine the earliest time at which pre-emergence signatures, specifically the Horizontal Divergent Flow (HDF), can be confidently detected using data from the Solar Dynamics Observatory's Helioseismic and Magnetic Imager (SDO/HMI). Initially, we follow previous studies using the thresholding method, which searches for significant increases in the number of pixels that display a specific line-of-sight velocity. We expand this method to more velocity windows and conduct a basic parameter study investigating the effect of cadence on the inferred results. Our findings agree with previous studies with $37.5$% of ARs displaying a HDF, with average lead times between the HDF and flux emergence of $58$ minutes. We present a new potential signature of flux emergence which manifests as cadence-independent transient disruptions to the amplitudes of multiple velocity windows and recover potential pre-emergence signatures for 10 of the 16 ARs studied, with lead times of 60-156 minutes. Several effects can influence both the estimated times of both HDF and flux emergence suggesting that one may need to combine Doppler and magnetic field data to get a reliable indicator of continued flux emergence.

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arXiv2022

IRIS burst properties in active regions

Interface Region Imaging Spectrograph (IRIS) bursts are localised features thought to be driven by magnetic reconnection. Although these events are well-studied, it remains unknown whether their properties vary as their host active regions (ARs) evolve. Here, we aim to understand whether the measurable properties of IRIS bursts are consistent during the evolution of their host ARs. We study 42 dense 400-step rasters sampled by IRIS. These rasters each covered one of seven ARs, with each AR being sampled at least four times over a minimum of 48 hours. An automated detection algorithm is used to identify IRIS burst profiles. Data from the Solar Dynamics Observatory's Helioseismic and Magnetic Imager are also used to provide context about the co-spatial line-of-sight magnetic field. Of the rasters studied, 36 were found to contain IRIS burst profiles. Five ARs (11850, 11909, 11916, 12104, and 12139) contained IRIS burst profiles in each raster that sampled them whilst one AR (11871) was found to contain no such spectra at any time. A total of 4019 IRIS burst profiles belonging to 752 connected objects, which we define as parent IRIS bursts, were identified. IRIS burst profiles were only detected within compact regions in each raster, with these regions appearing to increase in size as the host ARs aged. No systematic changes in the frequency of IRIS burst profiles or the spectral characteristic of IRIS burst profiles through time were found for these ARs. Finally, 93 % of parent IRIS bursts with areas between 1 arcsec^2 and 4 arcsec^2 occurred co-spatial to bi-poles in the photosphere. Overall, IRIS bursts have remarkably consistent spectral and spatial properties throughout the evolution of ARs. These events predominantly form within the cores of larger and more complex ARs, with the regions containing these events appearing to increase in size as the host region itself evolves.

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arXiv2022

The European Solar Telescope

The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope (SST), the German Vacuum Tower Telescope (VTT) and GREGOR, the French Télescope Héliographique pour l'Étude du Magnétisme et des Instabilités Solaires (THÉMIS), and the Dutch Open Telescope (DOT). With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems.

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arXiv2021

Oscillations In The Line-of-Sight Magnetic Field Strength In A Pore Observed By The GREGOR Infrared Spectrograph (GRIS)

Numerous magnetohydrodynamic oscillations have been reported within solar pores over the past decades, including in line-of-sight (LOS) velocities, intensities, and magnetic field strengths. Our aim is to identify whether high-amplitude oscillations in the LOS magnetic field strength can be detected within a pore located in Active Region 12748 and to investigate which physical mechanisms could be responsible for them. A solar pore was observed on the 1st September 2019 using the GREGOR Infrared Spectrograph for around one hour. Full-Stokes vectors were sampled in a 37 A window containing the Fe I 15648.52 A line (effective Lande g-factor of 3). The LOS magnetic field strength is inferred using the strong-field approximation. The Stokes Inversion based on Response functions code is used to gain a more complete understanding the properties of the solar atmosphere at the locations of these oscillations. Oscillations of more than 100 G are observed in the LOS magnetic field in the period window 600-1272 s at three localised (>1"^2) regions. These oscillations have coherence across individual regions indicating that jitter cannot account for their occurrence. Longer-period amplitude variations, amplitudes over 200 G, are also detected but these have periods outside of the cone-of-influence. Numerical inversions confirm both oscillations in the LOS magnetic field strength at optical depths of around log-tau_5000=-0.5 (potentially caused by compression) and other effects (e.g., changes in the optical depth or the inclination of the field) may account for these changes. The oscillations in the separations of the Stokes-V lobes of the 15648.52 A line appear to be solar in nature. Future work will be required to understand whether these are truly oscillations in the magnetic field strength at a specific depth in the solar atmosphere or whether other effects are responsible for these signatures.

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arXiv2020

Evolution Of Downflows In The Transition Region Above A Sunspot Over Short Time-Scales

Downflows with high velocities 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. In this article, we investigate the evolution of downflows detected within the transition region on time-scales of the order minutes and search for clues as to their formation mechanisms in co-temporal imaging data. The high-resolution spectral and imaging data used to identify downflows here were sampled by IRIS on the 20th and 21st May 2015. Imaging data from the Hinode and SDO satellites provided context about the wider solar atmosphere. Four downflows were identified and analysed. The potential super-sonic components of these downflows had widths of around 2" and evolved over time-scales of the order 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 50 km s^{-1} were observed to extend a few arcsecs away from the foot-points suggesting the potential super-sonic components are linked to larger-scale flows. The electron density and mass flux for these events were within the ranges 10^{9.6}-10^{10.2} cm^{-3} and 10^{-6.81}-10^{-7.48} g cm^{-2} s^{-1}, respectively. 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 downflows. Downflows can appear, disappear, and recur within time-scales of less than one hour in sunspots. As potential super-sonic downflow signatures were detected at the foot-points of both extended fingers in Si IV 1400 Å SJI data and sub-sonic downflows in Si IV 1394 Å spectra, it is likely that these events are linked to flows within structures such as coronal loops.

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arXiv2020

Solar Flare Prediction Using Magnetic Field Diagnostics Above the Photosphere

In this article, we present the application of the weighted horizontal gradient of magnetic field (WGM) flare prediction method to 3-dimensional (3D) extrapolated magnetic configurations of 13 flaring solar active regions (ARs). The main aim is to identify an optimal height range, if any, in the interface region between the photosphere and lower corona, where the flare onset time prediction capability of WGM is best exploited. The optimal height is where flare prediction, by means of the WGM method, is achieved earlier than at the photospheric level. 3D magnetic structures, based on potential and non-linear force-free field extrapolations, are constructed to study a vertical range from the photosphere up to the low corona with a 45 km step size. The WGM method is applied as a function of height to all 13 flaring AR cases that are subject to certain selection criteria. We found that applying the WGM method between 1000 and 1800 km above the solar surface would improve the prediction of the flare onset time by around 2-8 hours.Certain caveats and an outlook for future work along these lines are also discussed.

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arXiv2020

Evolution Of Super-Sonic Downflows In A Sunspot

Super-sonic downflows have been observed in transition region spectra above numerous sunspots; however, little research has been conducted to date into how persistent these signatures are within sunspots on time-scales longer than a few hours. Here, we analyse the lead sunspot of AR 12526 to infer the properties and evolution of super-sonic downflows occurring within it. Sixteen large, dense raster scans sampled by IRIS are analysed. These rasters tracked the lead sunspot of AR 12526 at discrete times between the 27th March 2016 and the 2nd April 2016. One sit-and-stare observation acquired on the 1st April 2016 is also studied in order to analyse the evolution of super-sonic downflows on shorter time-scales. Super-sonic downflows are variable within this sunspot both in terms of spatial structuring and velocities. 13 of the 16 raster scans display some evidence of super-sonic downflows in the Si IV 1394 Å line co-spatial to a sustained bright structure detected in the 1400 Å slit-jaw imaging channel, with a peak velocity of 112 km s^{-1} being recorded on the 29th March 2016. Evidence for super-sonic downflows in the O IV 1401 Å line was found in 14 of these rasters, with the spatial structuring differing from that inferred from the Si IV 1394 Å line. In the sit-and-stare data, no dual flow is initially detected, however, a super-sonic downflow does develop after 60 minutes. This downflow accelerates from 73 km s^{-1} to close to 80 km s^{-1} in both the Si IV 1394 Å and O IV 1401 Å lines over the course of 20 minutes before the end of the observation. The morphology of these downflows evolved over the course of both hours and days and was often different in the Si IV 1394 Å and O IV 1401 Å lines. These events were found co-spatial to a bright region in the core of the Si IV 1394 Å line which appeared to form at the foot-points of coronal fan loops.

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arXiv2019

Spatially resolved signatures of bi-directional flows observed in inverted-Y shaped jets

Numerous apparent signatures of magnetic reconnection have been reported in the solar photosphere, including inverted-Y shaped jets. The reconnection at these sites is expected to cause localised bi-directional flows and extended shock waves; however, these signatures are rarely observed as extremely high spatial-resolution data are required. Here, we use H-alpha imaging data sampled by the Swedish Solar Telescope's CRisp Imaging SpectroPolarimeter to investigate whether bi-directional flows can be detected within inverted-Y shaped jets near the solar limb. These jets are apparent in the H-alpha line wings, while no signature of either jet is observed in the H-alpha line core, implying reconnection took place below the chromospheric canopy. Asymmetries in the H-alpha line profiles along the legs of the jets indicate the presence of bi-directional flows, consistent with cartoon models of reconnection in chromospheric anemone jets. These asymmetries are present for over two minutes, longer than the lifetimes of Rapid Blue Excursions, and beyond \pm 1 Å into the wings of the line indicating that flows within the inverted-Y shaped jets are responsible for the imbalance in the profiles, rather than motions in the foreground. Additionally, surges form following the occurrence of the inverted-Y shaped jets. This surge formation is consistent with models which suggest such events could be caused by the propagation of shock waves from reconnection sites in the photosphere to the upper atmosphere. Overall, our results provide evidence that magnetic reconnection in the photosphere can cause bi-directional flows within inverted-Y shaped jets and could be the driver of surges.

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arXiv2019

The Effect Of Cooling On Driven Kink Oscillations Of Coronal Loops

Ever since their detection two decades ago, standing kink oscillations in coronal loops have been extensively studied both observationally and theoretically. Almost all driven coronal loop oscillations (e.g., by flares) are observed to damp through time often with Gaussian or exponential profiles. Intriguingly, however, it has been shown theoretically that the amplitudes of some oscillations could be modified from Gaussian or exponential profiles if cooling is present in the coronal loop systems. Indeed, in some cases the oscillation amplitude can even increase through time. In this article, we analyse a flare-driven coronal loop oscillation observed by the Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA) in order to investigate whether models of cooling can explain the amplitude profile of the oscillation and whether hints of cooling can be found in the intensity evolution of several SDO/AIA filters. During the oscillation of this loop system, the kink mode amplitude appears to differ from a typical Gaussian or exponential profile with some hints being present that the amplitude increases. The application of cooling coronal loop modelling allowed us to estimate the density ratio between the loop and the background plasma, with a ratio of between 2.05-2.35 being returned. Overall, our results indicate that consideration of the thermal evolution of coronal loop systems can allow us to better describe oscillations in these structures and return more accurate estimates of the physical properties of the loops (e.g., density, scale height, magnetic field strength).

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arXiv2017

IRIS Burst Spectra Co-Spatial To A Quiet-Sun Ellerman-Like Brightening

Ellerman bombs (EBs) have been widely studied over the past two decades; however, only recently have counterparts of these events been observed in the quiet-Sun. The aim of this article is to further understand small-scale quiet-Sun Ellerman-like brightenings (QSEBs) through research into their spectral signatures, including investigating whether the hot signatures associated with some EBs are also visible co-spatial to any QSEBs. We combine H$α$ and Ca II $8542$ Å line scans at the solar limb with spectral and imaging data sampled by the Interface Region Imaging Spectrograph (IRIS). Twenty one QSEBs were identified with average lifetimes, lengths, and widths measured to be around $120$ s, $0.63$", and $0.35$", respectively. Three of these QSEBs displayed clear repetitive flaring through their lifetimes, comparable to the behaviour of EBs in Active Regions (ARs). Two QSEBs in this sample occurred co-spatial with increased emission in SDO/AIA $1600$ Å and IRIS slit-jaw imager $1400$ Å data, however, these intensity increases were smaller than reported co-spatial to EBs. One QSEB was also sampled by the IRIS slit during its lifetime, displaying increases in intensity in the Si IV $1393$ Å and Si IV $1403$ Å cores as well as the C II and Mg II line wings, analogous to IRIS bursts (IBs). Using RADYN simulations, we are unable to reproduce the observed QSEB H$α$ and Ca II $8542$ Å line profiles leaving the question of the temperature stratification of QSEBs open. Our results imply that some QSEBs could be heated to Transition Region temperatures, suggesting that IB profiles should be observed throughout the quiet-Sun.

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arXiv2017

The formation of small-scale umbral brightenings in sunspot atmospheres

Sunspot atmospheres are highly inhomogeneous hosting both quasi-stable and transient features, such as `umbral micro-jets' and dark fibril-like events. We seek to understand the morphological properties and formation mechanisms of small-scale umbral brightenings (SSUBs; analogous to umbral micro-jets) and to understand whether links between these events and short dynamic fibrils, umbral flashes, and umbral dots can be established. An SST filtergram time-series sampling the Ca II H line and a CRISP full-Stokes 15-point Ca II 8542 A line scan dataset were used. The average lifetime and lengths of 54 SSUBs identified in the sunspot umbra are found to be 44.2 seconds (sigma=20 seconds) and 0.56" (sigma=0.14"). The spatial positioning and morphological evolution of these events was investigated finding no evidence of parabolic or ballistic profiles nor a preference for co-spatial formation with umbral flashes. The presence of Stokes V profile reversals provided evidence that these events could form through the development of shocks in the chromosphere. The application of the weak-field approximation indicated that changes in the line-of-sight magnetic field were not responsible for the modifications to the line profile and suggested that thermodynamic effects are the actual cause of the increased emission. Finally, a sub-set of SSUBs were observed to form at the foot-points of short dynamic fibrils. Overall, we found no correlation between the spatial locations where these events were observed and the occurrence of umbral dots and umbral flashes. SSUBs, however, have lifetimes and spectral signatures comparable to umbral flashes and are located at the footpoints of short dynamic fibrils, during or at the end of the red-shifted stage. It is possible, therefore, that these features form due to the shocking of fibrilar material in the lower atmosphere upon its return under gravity.

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arXiv2016

On The Relationship Between Magnetic Cancellation and UV Burst Formation

Burst-like events with signatures in the UV are often observed co-spatial to strong line-of-sight photospheric magnetic fields. Several authors, for example, have noted the spatial relationship between Ellerman bombs (EBs) and Moving Magnetic Features (MMFs), regions of flux which disconnect from a sunspot or pore before propagating away in the moat flow and often displaying evidence of cancellation. In this article, data collected by the Solar Dynamics Observatory's Helioseismic and Magnetic Imager and Atmospheric Imaging Assembly are analysed in an attempt to understand the potential links between such cancellation and UV burst formation. Two MMFs from AR 11579, three bi-poles from AR 11765, and six bi-poles (four of which were co-spatial to IRIS bursts) in AR 11850 were identified for analysis. All of these cancellation features were found to have lifetimes of the order hours and cancellation rates of the order 10^14-10^15 Mx s^-1. H-alpha line wing data from the Dunn Solar Telescope's Interferometric BIdimensional Spectrometer were also available for AR 11579 facilitating a discussion of links between MMFs and EBs. Using an algebraic model of photospheric magnetic reconnection, the measured cancellation rates are then used to ascertain estimates of certain quantities (such as up-flow speeds, jet extents, and potential energy releases) which compared reasonably to the properties of EBs reported within the literature. Our results suggest that cancellation rates of the order measured here are capable of supplying enough energy to drive certain UV bursts (including EBs), however, they are not a guaranteeing condition for burst formation.

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arXiv2016

High--cadence observations of spicular-type events on the Sun

Chromospheric observations taken at high cadence and high spatial resolution show a range of spicule like features, including Type I, Type II (as well as RBEs and RREs) and those which seem to appear within a few seconds, which if interpreted as flows would imply mass flow velocities in excess of 1000 km/s. This article seeks to quantify and study rapidly appearing spicular type events. We also compare the MOMFBD and speckle reconstruction techniques in order to understand if such spicules are more favourably observed using a particular technique. We use spectral imaging observations taken with the CRISP on the Swedish 1 m Solar Telescope. Data was sampled at multiple positions within the Halpha line profile for both an ondisk and limb location. The data is host to numerous rapidly appearing features which are observed at different locations within the Halpha line profile. The feature's durations vary between 10 and 20 s and lengths around 3500 km. Sometimes, a time delay in their appearance between the blue and red wings of 3 and 5 s is evident, whereas sometimes they are near simultaneous. In some instances features are observed to fade and then re emerge at the same location several tens of seconds later. We provide the first statistical analysis of these spicules and suggest that these observations can be interpreted as the LOS movement of highly dynamic spicules moving in and out of the narrow 60 mA transmission filter used to observe in different parts of the Halpha line profile. The LOS velocity component of the observed fast chromospheric features, manifested as Doppler shifts, are responsible for their appearance in the red and blue wings of Halpha line. Additional work involving data at other wavelengths is required to investigate the nature of their possible wavelike activity.

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arXiv2014

Small-Scale Structuring Of Ellerman Bombs at Solar Limb

Ellerman bombs (EBs) have been widely studied in recent years due to their dynamic, explosive nature and apparent links to the underlying photospheric magnetic field implying that they may be formed by magnetic reconnection in the photosphere. Despite a plethora of researches discussing the morphologies of EBs, there has been a limited investigation of how these events appear at the limb, specifically, whether they manifest as vertical extensions away from the disc. In this article, we make use of high-resolution, high-cadence observations of an AR at the solar limb, collected by the CRISP instrument, to identify EBs and infer their physical properties. The upper atmosphere is also probed using the SDO/AIA. We analyse 22 EB events evident within these data, finding that 20 appear to follow a parabolic path away from the solar surface at an average speed of 9 km s^(-1), extending away from their source by 580 km, before retreating back at a similar speed. These results show strong evidence of vertical motions associated with EBs, possibly explaining the dynamical `flaring' (changing in area and intensity) observed in on-disc events. Two in-depth case studies are also presented which highlight the unique dynamical nature of EBs within the lower solar atmosphere. The viewing angle of these observations allows for a direct linkage between these EBs and other small-scale events in the H-alpha line wings, including a potential flux emergence scenario. The findings presented here suggest that EBs could have a wider-reaching influence on the solar atmosphere than previously thought, as we reveal a direct linkage between EBs and an emerging small-scale loop, and other near-by small-scale explosive events. However, as previous research found, these extensions do not appear to impact upon the H-alpha line core, and are not observed by the SDO/AIA EUV filters.

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