E. Scullion

Unresolved fine-scale structure in solar coronal loop-tops

New and advanced space-based observing facilities continue to lower the resolution limit and detect solar coronal loops in greater detail. We continue to discover even finer substructures within coronal loop cross-sections, in order to understand the nature of the solar corona. Here, we push this lower limit further to search for the finest coronal loop substructures, through taking advantage of the resolving power of the Swedish 1 m Solar Telescope/CRisp Imaging Spectro-Polarimeter (CRISP), together with co-observations from the Solar Dynamics Observatory/Atmospheric Image Assembly (AIA). High-resolution imaging of the chromospheric Hα 656.28 nm spectral line core and wings can, under certain circumstances, allow one to deduce the topology of the local magnetic environment of the solar atmosphere where its observed. Here, we study post-flare coronal loops, which become filled with evaporated chromosphere that rapidly condenses into chromospheric clumps of plasma (detectable in Hα) known as a coronal rain, to investigate their fine-scale structure. We identify, through analysis of three data sets, large-scale catastrophic cooling in coronal loop-tops and the existence of multi-thermal, multi-stranded substructures. Many cool strands even extend fully intact from loop-top to footpoint. We discover that coronal loop fine-scale strands can appear bunched with as many as eight parallel strands within an AIA coronal loop cross-section. The strand number density versus cross-sectional width distribution, as detected by CRISP within AIA-defined coronal loops, most likely peaks at well below 100 km, and currently, 69% of the substructure strands are statistically unresolved in AIA coronal loops.

Solar Science with the Atacama Large Millimeter/Submillimeter Array—A New View of Our Sun

The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere—a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA’s scientific potential for studying the Sun for a large range of science cases.

Magnetic Flux Cancellation in Ellerman Bombs

Ellerman Bombs (EBs) are often found to be co-spatial with bipolar photospheric magnetic fields. We use Hα imaging spectroscopy along with Fe i 6302.5 A spectropolarimetry from the Swedish 1 m Solar Telescope (SST), combined with data from the Solar Dynamic Observatory, to study EBs and the evolution of the local magnetic fields at EB locations. EBs are found via an EB detection and tracking algorithm. Using NICOLE inversions of the spectropolarimetric data, we find that, on average, (3.43 ± 0.49) × 10^24 erg of stored magnetic energy disappears from the bipolar region during EB burning. The inversions also show flux cancellation rates of 10^14–10^15 Mx s−1 and temperature enhancements of 200 K at the detection footpoints. We investigate the near-simultaneous flaring of EBs due to co-temporal flux emergence from a sunspot, which shows a decrease in transverse velocity when interacting with an existing, stationary area of opposite polarity magnetic flux, resulting in the formation of the EBs. We also show that these EBs can be fueled further by additional, faster moving, negative magnetic flux regions.

Ellerman bombs with jets: cause and effect

Ellerman Bombs (EBs) are thought to arise as a result of photospheric magnetic reconnection. We use data from the Swedish 1 m Solar Telescope to study EB events on the solar disk and at the limb. Both data sets show that EBs are connected to the foot points of forming chromospheric jets. The limb observations show that a bright structure in the Hα blue wing connects to the EB initially fueling it, leading to the ejection of material upwards. The material moves along a loop structure where a newly formed jet is subsequently observed in the red wing of Hα. In the disk data set, an EB initiates a jet which propagates away from the apparent reconnection site within the EB flame. The EB then splits into two, with associated brightenings in the inter-granular lanes. Micro-jets are then observed, extending to 500 km with a lifetime of a few minutes. Observed velocities of the micro-jets are approximately 5–10 km s−1, while their chromospheric counterparts range from 50 to 80 km s−1. MURaM simulations of quiet Sun reconnection show that micro-jets with properties similar to those of the observations follow the line of reconnection in the photosphere, with associated Hα brightening at the location of increased temperature.

High-frequency torsional Alfvén waves as an energy source for coronal heating

The existence of the Sun’s hot atmosphere and the solar wind acceleration continues to be an outstanding problem in solar-astrophysics. Although magnetohydrodynamic (MHD) modes and dissipation of magnetic energy contribute to heating and the mass cycle of the solar atmosphere, yet direct evidence of such processes often generates debate. Ground-based 1-m Swedish Solar Telescope (SST)/CRISP, Hα 6562.8 Å observations reveal, for the first time, the ubiquitous presence of high frequency (~12–42 mHz) torsional motions in thin spicular-type structures in the chromosphere. We detect numerous oscillating flux tubes on 10 June 2014 between 07:17 UT to 08:08 UT in a quiet-Sun field-of-view of 60” × 60” (1” = 725 km). Stringent numerical model shows that these observations resemble torsional Alfvén waves associated with high frequency drivers which contain a huge amount of energy (~105 W m−2) in the chromosphere. Even after partial reflection from the transition region, a significant amount of energy (~103 W m−2) is transferred onto the overlying corona. We find that oscillating tubes serve as substantial sources of Alfvén wave generation that provide sufficient Poynting flux not only to heat the corona but also to originate the supersonic solar wind.

Stable Umbral Chromospheric Structures

Aims. To understand the morphology of the chromosphere in sunspot umbra. We investigate if the horizontal structures observed in the spectral core of the Caii H line are ephemeral visuals caused by the shock dynamics of more stable structures, and examine their relationship with observables in the H-alpha line. Methods. Filtergrams in the core of the Caii H and H-alpha lines as observed with the Swedish 1-m Solar Telescope are employed. We utilise a technique that creates composite images and tracks the flash propagation horizontally. Results. We find 0. ′′ 15 wide horizontal structures, in all of the three target sun spots, for every flash where the seeing was moderate to good. Discrete dark structures are identified that are sta ble for at least two umbral flashes, as well as systems of struc tures that live for up to 24 minutes. We find cases of extremely extended s tructures with similar stability, with one such structure s howing an extent of 5 ′′ . Some of these structures have a correspondence in H-alpha but we were unable to find a one to one correspondence for every occurrence. If the dark streaks are formed at the same heights as umbral flashes then there are systems of structures with strong departures from the vertical for all three analysed sunspot s. Conclusions. Long-lived Caii H filamentary horizontal structures are a common and likely e ver-present feature in the umbra of sunspots. If the magnetic field in the chromosphere of the umb ra is indeed aligned with the structures, then the present th eoretical understanding of the typical umbra needs to be revisited.

First simultaneous SST/CRISP and IRIS observations of a small-scale quiet Sun vortex

Ubiquitous small-scale vortices have recently been found in the lower atmosphere of the quiet Sun in state-of-the-art solar observations and in numerical simulations. We investigate the characteristics and temporal evolution of a granular-scale vortex and its associated upflows through the photosphere and chromosphere of a quiet Sun internetwork region. We analyzed high spatial and temporal resolution ground- and spaced-based observations of a quiet Sun region. The observations consist of high-cadence time series of wideband and narrowband images of both H-alpha 6563 A and Ca II 8542 A lines obtained with the CRisp Imaging SpectroPolarimeter (CRISP) instrument at the Swedish 1-m Solar Telescope (SST), as well as ultraviolet imaging and spectral data simultaneously obtained by the Interface Region Imaging Spectrograph (IRIS). A small-scale vortex is observed for the first time simultaneously in H-alpha, Ca II 8542 A, and Mg II k lines. During the evolution of the vortex, H-alpha narrowband images at -0.77 A and Ca II 8542 A narrowband images at -0.5 A, and their corresponding Doppler Signal maps, clearly show consecutive high-speed upflow events in the vortex region. These high-speed upflows with a size of 0.5-1 Mm appear in the shape of spiral arms and exhibit two distinctive apparent motions in the plane of sky for a few minutes: (1) a swirling motion with an average speed of 13 km/s and (2) an expanding motion at a rate of 4-6 km/s. Furthermore, the spectral analysis of Mg II k and Mg II subordinate lines in the vortex region indicates an upward velocity of up to about 8 km/s along with a higher temperature compared to the nearby quiet Sun chromosphere. The consecutive small-scale vortex events can heat the upper chromosphere by driving continuous high-speed upflows through the lower atmosphere.

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 features 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.

OBSERVING THE FORMATION OF FLARE-DRIVEN CORONAL RAIN

Flare-driven coronal rain can manifest from rapidly cooled plasma condensations near coronal loop-tops in thermally unstable post-flare arcades. We detect 5 phases that characterise the post-flare decay: heating, evaporation, conductive cooling dominance for ~120 s, radiative / enthalpy cooling dominance for ~4700 s and finally catastrophic cooling occurring within 35-124 s leading to rain strands with s periodicity of 55-70 s. We find an excellent agreement between the observations and model predictions of the dominant cooling timescales and the onset of catastrophic cooling. At the rain formation site we detect co-moving, multi-thermal rain clumps that undergo catastrophic cooling from ~1 MK to ~22000 K. During catastrophic cooling the plasma cools at a maximum rate of 22700 K s-1 in multiple loop-top sources. We calculated the density of the EUV plasma from the DEM of the multi-thermal source employing regularised inversion. Assuming a pressure balance, we estimate the density of the chromospheric component of rain to be 9.21x10^11 +-1.76x10^11 cm-3 which is comparable with quiescent coronal rain densities. With up to 8 parallel strands in the EUV loop cross section, we calculate the mass loss rate from the post-flare arcade to be as much as 1.98x10^12 +/-4.95x10^11 g s-1. Finally, we reveal a close proximity between the model predictions of 10^5.8 K and the observed properties between 10^5.9 K and 10^6.2 K, that defines the temperature onset of catastrophic cooling. The close correspondence between the observations and numerical models suggests that indeed acoustic waves (with a sound travel time of 68 s) could play an important role in redistributing energy and sustaining the enthalpy-based radiative cooling.

OBSERVING CASCADES OF SOLAR BULLETS AT HIGH RESOLUTION. II.

High resolution observations from the Swedish 1-m Solar Telescope revealed bright, discrete, blob-like structures (which we refer to as solar bullets) in the Hα 656.28 nm line core that appear to propagate laterally across the solar atmosphere as clusters in active regions (ARs). These small-scale structures appear to be field aligned and many bullets become triggered simultaneously and traverse collectively as a cluster. Here, we conduct a follow-up study on these rapidly evolving structures with coincident observations from the Solar Dynamics Observatory/Atmospheric Imaging Assembly. With the co-aligned data sets, we reveal (a) an evolving multithermal structure in the bullet cluster ranging from chromospheric to at least transition region temperatures, (b) evidence for cascade-like behavior and corresponding bidirectional motions in bullets within the cluster, which indicate that there is a common source of the initial instability leading to bullet formation, and (c) a direct relationship between co-incident bullet velocities observed in Hα and He ii 30.4 nm and an inverse relationship with respect to bullet intensity in these channels. We find evidence supporting that bullets are typically composed of a cooler, higher density core detectable in Hα with a less dense, hotter, and fainter co-moving outer sheath. Bullets unequivocally demonstrate the finely structured nature of the AR corona. We have no clear evidence for bullets being associated with locally heated (or cooled), fast flowing plasma. Fast MHD pulses (such as solitons) could best describe the dynamic properties of bullets whereas the presence of a multithermal structure is new.