Author name code: nelson-chris
ADS astronomy entries on 2022-09-14
author:"Nelson, Christopher J." OR "Nelson, Chris J."
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Title: IRIS burst properties in active regions
Authors: Nelson, C. J.; Kleint, L.
Bibcode: 2022arXiv220811013N
Altcode:
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.
Title: The European Solar Telescope
Authors: Quintero Noda, C.; Schlichenmaier, R.; Bellot Rubio, L. R.;
Löfdahl, M. G.; Khomenko, E.; Jurcak, J.; Leenaarts, J.; Kuckein,
C.; González Manrique, S. J.; Gunar, S.; Nelson, C. J.; de la Cruz
Rodríguez, J.; Tziotziou, K.; Tsiropoula, G.; Aulanier, G.; Collados,
M.; the EST team
Bibcode: 2022arXiv220710905Q
Altcode:
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.
Title: Polymeric jets throw light on the origin and nature of the
forest of solar spicules
Authors: Dey, Sahel; Chatterjee, Piyali; Murthy, O. V. S. N.; Korsós,
Marianna B.; Liu, Jiajia; Nelson, Christopher J.; Erdélyi, Robertus
Bibcode: 2022NatPh..18..595D
Altcode:
Spicules are plasma jets that are observed in the dynamic interface
region between the visible solar surface and the hot corona. At any
given time, it is estimated that about 3 million spicules are present
on the Sun. We find an intriguing parallel between the simulated
spicular forest in a solar-like atmosphere and the numerous jets of
polymeric fluids when both are subjected to harmonic forcing. In a
radiative magnetohydrodynamic numerical simulation with sub-surface
convection, solar global surface oscillations are excited similarly to
those harmonic vibrations. The jets thus produced match remarkably well
with the forests of spicules detected in observations of the Sun. Taken
together, the numerical simulations of the Sun and the laboratory fluid
dynamics experiments provide insights into the mechanism underlying
the ubiquity of jets. The non-linear focusing of quasi-periodic waves
in anisotropic media of magnetized plasma as well as polymeric fluids
under gravity is sufficient to generate a forest of jets.
Title: Oscillations in the line-of-sight magnetic field strength in
a pore observed by the GREGOR Infrared Spectrograph (GRIS)
Authors: Nelson, C. J.; Campbell, R. J.; Mathioudakis, M.
Bibcode: 2021A&A...654A..50N
Altcode: 2021arXiv210710183N
Context. Numerous magnetohydrodynamic oscillations have been reported
within solar pores over the past few decades, including in line-of-sight
(LOS) velocities, intensities, and magnetic field strengths.
Aims: 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.
Methods: A solar pore was observed on
1 September 2019 using the GREGOR Infrared Spectrograph instrument for
around one hour. Full-Stokes vectors were sampled in a 37 Å window
containing the Fe I 15 648.52 Å line (effective Landé g factor of
3). The LOS magnetic field strength was inferred using the strong-field
approximation. Additionally, the Stokes Inversion based on Response
functions code was used to gain a more complete understanding of the
physical properties of the solar atmosphere at the locations of these
oscillations.
Results: Oscillations of more than 100 G are
observed in the LOS magnetic field in the period window between 600
and 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 they
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 τ5000 = −0.5 (potentially caused by
compression) and other effects (e.g. changes in the optical depth or the
inclination of the magnetic field) may account for these changes.
Conclusions: The oscillations in the separations of the Stokes-V lobes
of the 15 648.52 Å 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.
Title: Flare-induced Sunquake Signatures in the Ultraviolet as
Observed by the Atmospheric Imaging Assembly
Authors: Quinn, Sean; Mathioudakis, Mihalis; Nelson, Christopher J.;
Milligan, Ryan O.; Reid, Aaron; Jess, David B.
Bibcode: 2021ApJ...920...25Q
Altcode: 2021arXiv210505704Q
Sunquakes (SQs) have been routinely observed in the solar photosphere,
but it is only recently that signatures of these events have been
detected in the chromosphere. We investigate whether signatures of
SQs are common in ultraviolet (UV) continua that sample the solar
plasma several hundred km above where SQs are typically detected. We
analyze observations from the Solar Dynamics Observatory's Atmospheric
Imaging Assembly (SDO/AIA) 1600 and 1700 Å passbands, for SQ signatures
induced by the flares of Solar Cycle 24. We base our analysis on the
62 SQs detected in the recent statistical study presented by Sharykin
& Kosovichev. We find that 9 out of 62 SQ candidates produced a
response that is clearly detected in running-difference images from
the AIA 1600 and 1700 Å channels. A binary frequency filter with a
width of 2 mHz, centered on 6 mHz, was applied to the data. The first
signature of each SQ was detected at distances between 5.2 and 25.7
Mm from the associated flare ribbon. Time-distance and regression
analysis allowed us to calculate the apparent transverse velocities
of the SQs in the UV data sets and found maximum velocities as high
as 41 km s-1, 87 Mm away from the SQ source. Our analysis
shows that flare-induced SQ signatures can be detected in the SDO/AIA
1600 and 1700 Å passbands, hinting at their presence in the lower
chromosphere. There was no apparent correlation between GOES flare
classification and the appearance of the SQ at these heights.
Title: Temporal evolution of small-scale internetwork magnetic fields
in the solar photosphere (Corrigendum)
Authors: Campbell, R. J.; Mathioudakis, M.; Collados, M.; Keys, P. H.;
Asensio Ramos, A.; Nelson, C. J.; Kuridze, D.; Reid, A.
Bibcode: 2021A&A...652C...2C
Altcode:
No abstract at ADS
Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
(DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
Instrument Scientists; DKIST Science Working Group; DKIST Critical
Science Plan Community
Bibcode: 2021SoPh..296...70R
Altcode: 2020arXiv200808203R
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) will revolutionize our ability to measure, understand,
and model the basic physical processes that control the structure
and dynamics of the Sun and its atmosphere. The first-light DKIST
images, released publicly on 29 January 2020, only hint at the
extraordinary capabilities that will accompany full commissioning of
the five facility instruments. With this Critical Science Plan (CSP)
we attempt to anticipate some of what those capabilities will enable,
providing a snapshot of some of the scientific pursuits that the DKIST
hopes to engage as start-of-operations nears. The work builds on the
combined contributions of the DKIST Science Working Group (SWG) and
CSP Community members, who generously shared their experiences, plans,
knowledge, and dreams. Discussion is primarily focused on those issues
to which DKIST will uniquely contribute.
Title: Temporal evolution of small-scale internetwork magnetic fields
in the solar photosphere
Authors: Campbell, R. J.; Mathioudakis, M.; Collados, M.; Keys, P. H.;
Asensio Ramos, A.; Nelson, C. J.; Kuridze, D.; Reid, A.
Bibcode: 2021A&A...647A.182C
Altcode: 2021arXiv210200942C
Context. While the longitudinal field that dominates in photospheric
network regions has been studied extensively, small-scale transverse
fields have recently been found to be ubiquitous in the quiet
internetwork photosphere and this merits further study. Furthermore,
few observations have been able to capture how this field evolves.
Aims: We aim to statistically characterize the magnetic vector in
a quiet Sun internetwork region and observe the temporal evolution of
specific small-scale magnetic features.
Methods: We present
two high spatio-temporal resolution observations that reveal the
dynamics of two disk-centre internetwork regions taken by the new GREGOR
Infrared Spectrograph Integral Field Unit with the highly magnetically
sensitive photospheric Fe I line pair at 15648.52 Å and 15652.87
Å. We record the full Stokes vector and apply inversions with the
Stokes inversions based on response functions code to retrieve the
parameters characterizing the atmosphere. We consider two inversion
schemes: scheme 1 (S1), where a magnetic atmosphere is embedded in
a field free medium, and scheme 2 (S2), with two magnetic models
and a fixed 30% stray light component.
Results: The magnetic
properties produced from S1 inversions returned a median magnetic
field strength of 200 and 240 G for the two datasets, respectively. We
consider the median transverse (horizontal) component, among pixels
with Stokes Q or U, and the median unsigned longitudinal (vertical)
component, among pixels with Stokes V, above a noise threshold. We
determined the former to be 263 G and 267 G, and the latter to be 131
G and 145 G, for the two datasets, respectively. Finally, we present
three regions of interest, tracking the dynamics of small-scale magnetic
features. We apply S1 and S2 inversions to specific profiles of interest
and find that the latter produces better approximations when there is
evidence of mixed polarities. We find patches of linear polarization
with magnetic flux density of the order of 130−150 G and find that
linear polarization appears preferentially at granule-intergranular
lane boundaries. The weak magnetic field appears to be organized in
terms of complex `loop-like' structures, with transverse fields often
flanked by opposite polarity longitudinal fields.
Title: Significance of Cooling Effect On Comprehension of Kink
Oscillations of Coronal Loops
Authors: Shukhobodskaia, Daria; Shukhobodskiy, Alexander A.; Nelson,
Chris J.; Ruderman, Michael S.; Erdélyi, Robert
Bibcode: 2021FrASS...7..106S
Altcode:
Kink oscillations of coronal loops have been widely studied, both
observationally and theoretically, over the past few decades. It
has been shown that the majority of observed driven coronal loop
oscillations appear to damp with either exponential or Gaussian
profiles and a range of mechanisms have been proposed to account for
this. However, some driven oscillations seem to evolve in manners which
cannot be modelled with purely Gaussian or exponential profiles, with
amplification of oscillations even being observed on occasions. Recent
research has shown that incorporating the combined effects of coronal
loop expansion, resonant absorption, and cooling can cause significant
deviations from Gaussian and exponential profiles in damping profiles,
potentially explaining increases in oscillation amplitude through time
in some cases. In this article, we analyse 10 driven kink oscillations
in coronal loops to further investigate the ability of expansion and
cooling to explain complex damping profiles. Our results do not rely
on fitting a periodicity to the oscillations meaning complexities in
both temporal (period changes) and spatial (amplitude changes) can be
accounted for in an elegant and simple way. Furthermore, this approach
could also allow us to infer some important diagnostic information
(such as, for example, the density ratio at the loop foot-points)
from the oscillation profile alone, without detailed measurements
of the loop and without complex numerical methods. Our results imply
the existence of correlations between the density ratio at the loop
foot-points and the amplitudes and periods of the oscillations. Finally,
we compare our results to previous models, namely purely Gaussian and
purely exponential damping profiles, through the calculation of chi^2
values, finding the inclusion of cooling can produce better fits in
some cases. The current study indicates that thermal evolution should
be included in kink-mode oscillation models in the future to help
us to better understand oscillations that are not purely Gaussian
or exponential.
Title: Torsional oscillations within a magnetic pore in the solar
photosphere
Authors: Stangalini, Marco; Erdélyi, Robertus; Boocock, Callum;
Tsiklauri, David; Nelson, Christopher J.; Del Moro, Dario; Berrilli,
Francesco; Korsós, Marianna B.
Bibcode: 2021NatAs...5..691S
Altcode: 2021NatAs.tmp...82S
Alfvén waves have proven to be important in a range of physical
systems due to their ability to transport non-thermal energy over long
distances in a magnetized plasma. This property is of specific interest
in solar physics, where the extreme heating of the atmosphere of the
Sun remains unexplained. In an inhomogeneous plasma such as a flux
tube in the solar atmosphere, they manifest as incompressible torsional
perturbations. However, despite evidence in the upper atmosphere, they
have not been directly observed in the photosphere. Here, we report the
detection of antiphase incompressible torsional oscillations observed in
a magnetic pore in the photosphere by the Interferometric Bidimensional
Spectropolarimeter. State-of-the-art numerical simulations suggest that
a kink mode is a possible excitation mechanism of these waves. The
excitation of torsional waves in photospheric magnetic structures
can substantially contribute to the energy transport in the solar
atmosphere and the acceleration of the solar wind, especially if such
signatures will be ubiquitously detected in even smaller structures
with the forthcoming next generation of solar telescopes.
Title: Power-law energy distributions of small-scale impulsive events
on the active Sun: results from IRIS
Authors: Vilangot Nhalil, Nived; Nelson, Chris J.; Mathioudakis,
Mihalis; Doyle, J. Gerry; Ramsay, Gavin
Bibcode: 2020MNRAS.499.1385V
Altcode: 2020MNRAS.tmp.2719N; 2020arXiv200903123V
Numerous studies have analysed inferred power-law distributions
between frequency and energy of impulsive events in the outer solar
atmosphere in an attempt to understand the predominant energy supply
mechanism in the corona. Here, we apply a burst detection algorithm to
high-resolution imaging data obtained by the Interface Region Imaging
Spectrograph to further investigate the derived power-law index, γ,
of bright impulsive events in the transition region. Applying the
algorithm with a constant minimum event lifetime (of either 60 s or
110 s) indicated that the target under investigation, such as Plage
and Sunspot, has an influence on the observed power-law index. For
regions dominated by sunspots, we always find γ < 2; however,
for data sets where the target is a plage region, we often find that
γ > 2 in the energy range (~1023, ~1026)
erg. Applying the algorithm with a minimum event lifetime of three
time-steps indicated that cadence was another important factor, with
the highest cadence data sets returning γ > 2 values. The estimated
total radiative power obtained for the observed energy distributions
is typically 10-25 per cent of what would be required to sustain the
corona indicating that impulsive events in this energy range are not
sufficient to solve coronal heating. If we were to extend the power-law
distribution down to an energy of 1021 erg, and assume parity
between radiative energy release and the deposition of thermal energy,
then such bursts could provide 25-50 per cent of the required energy
to account for the coronal heating problem.
Title: Umbral chromospheric fine structure and umbral flashes modelled
as one: The corrugated umbra
Authors: Henriques, Vasco M. J.; Nelson, Chris J.; Rouppe van der
Voort, Luc H. M.; Mathioudakis, Mihalis
Bibcode: 2020A&A...642A.215H
Altcode: 2020arXiv200805482H
Context. The chromosphere of the umbra of sunspots features an
assortment of dynamic fine structures that are poorly understood and
often studied separately. Small-scale umbral brightenings (SSUBs),
umbral microjets, spikes or short dynamic fibrils (SDFs), and umbral
dark fibrils are found in any observation of the chromosphere with
sufficient spatial resolution performed at the correct umbral flash
stage and passband. Understanding these features means understanding
the dynamics of the umbral chromosphere.
Aims: We aim to fully
understand the dynamics of umbral chromosphere through analysis of the
relationships between distinct observed fine features and to produce
complete models that explain both spectral profiles and the temporal
evolution of the features. We seek to relate such understanding to
umbral flashes.
Methods: We studied the spatial and spectral
co-evolution of SDFs, SSUBs, and umbral flashes in Ca II 8542 Å
spectral profiles. We produced models that generate the spectral
profiles for all classes of features using non-local thermodynamic
equilibrium radiative transfer with a recent version of the NICOLE
inversion code.
Results: We find that both bright SSUBs and dark
SDF structures are described with a continuous feature in the parameter
space that is distinct from the surroundings even in pixel-by-pixel
inversions. We find a phase difference between such features and
umbral flashes in both inverted line-of-sight velocities and timing
of the brightenings. For umbral flashes themselves we resolve,
for the first time in inversion-based semi-empirical modelling,
the pre-flash downflows, post-flash upflows, and the counter-flows
present during the umbral flash phase. We further present a simple
time-dependent cartoon model that explains the dynamics and spectral
profiles of both fine structure, dark and bright, and umbral flashes
in umbral chromospheres.
Conclusions: The similarity of the
profiles between the brightenings and umbral flashes, the pattern of
velocities obtained from the inversions, and the phase relationships
between the structures all lead us to put forward that all dynamic
umbral chromospheric structures observed to this date are a locally
delayed or locally early portion of the oscillatory flow pattern that
generates flashes, secondary to the steepening large-scale acoustic
waves at its source. Essentially, SSUBs are part of the same shock or
merely compression front responsible for the spatially larger umbral
flash phenomenon, but out of phase with the broader oscillation.
Title: Evolution of downflows in the transition region above a
sunspot over short time-scales
Authors: Nelson, C. J.; Krishna Prasad, S.; Mathioudakis, M.
Bibcode: 2020A&A...640A.120N
Altcode: 2020arXiv200613617N
Context. Downflows with potentially super-sonic velocities have been
reported to occur in the transition region above many sunspots; however,
how these signatures evolve over short time-scales in both spatial and
spectral terms is still unknown and requires further research.
Aims: In this article, we investigate the evolution of downflows
detected within spectral lines sampling the transition region on
time-scales of the order of minutes and we search for clues as to
the formation mechanisms of these features in co-temporal imaging
data.
Methods: For the purposes of this article, we used
high-resolution spectral and imaging data sampled by the Interface
Region Imaging Spectrograph on the 20 and 21 May 2015 to identify and
analyse downflows. Additionally, photospheric and coronal imaging data
from the Hinode and Solar Dynamics Observatory satellites were studied
to provide context about the wider solar atmosphere.
Results:
Four downflows were identified and analysed through time. The potential
super-sonic components of these downflows had widths of around 2″ and
were observed to evolve over time-scales of the order of minutes. The
measured apparent downflow velocities were structured both in time and
space, with the highest apparent velocities occurring above a bright
region detected in Si IV 1400 Å images. Downflows with apparent
velocities below the super-sonic threshold that was assumed here
were observed to extend a few arcseconds away from the foot-points,
suggesting that the potential super-sonic components are linked to
larger-scale flows. The electron density and mass flux for these events
were found to be within the ranges of 109.6-1010.2
cm-3 and 10-6.81-10-7.48 g
cm-2 s-1, respectively. Finally, each downflow
formed at the foot-point of thin "fingers", extending out around
3-5″ in Si IV 1400 Å data with smaller widths (< 1″) than
the super-sonic downflow components.
Conclusions: Downflows
can appear, disappear, and recur within time-scales of less than one
hour in sunspots. As the potential super-sonic downflow signatures
were detected at the foot-points of both extended fingers in Si IV
1400 SJI data and sub-sonic downflows in Si IV 1394 Å spectra, it
is likely that these events are linked to larger-scale flows within
structures such as coronal loops.
Title: Solar Flare Prediction Using Magnetic Field Diagnostics above
the Photosphere
Authors: Korsós, M. B.; Georgoulis, M. K.; Gyenge, N.; Bisoi, S. K.;
Yu, S.; Poedts, S.; Nelson, C. J.; Liu, J.; Yan, Y.; Erdélyi, R.
Bibcode: 2020ApJ...896..119K
Altcode: 2020arXiv200512180K
In this article, we present the application of the weighted horizontal
gradient of magnetic field (WGM) flare prediction method
to three-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 nonlinear 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 hr. Certain caveats
and an outlook for future work along these lines are also discussed.
Title: Evolution of supersonic downflows in a sunspot
Authors: Nelson, C. J.; Krishna Prasad, S.; Mathioudakis, M.
Bibcode: 2020A&A...636A..35N
Altcode: 2020arXiv200302489N
Context. Supersonic 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 timescales longer than a few hours.
Aims: We aim to analyse the lead sunspot of AR 12526 to infer the
properties and evolution of supersonic downflows occurring within
it using high-spatial and spectral resolution data.
Methods:
We analysed 16 large, dense raster scans sampled by the Interface
Region Imaging Spectrograph. These rasters tracked the lead sunspot
of AR 12526 across the solar disc at discrete times between 27 March
2016 and 2 April 2016, providing spectral profiles from the Si IV, O
IV, Mg II, and C II lines. Additionally, we studied one sit-and-stare
observation acquired on 1 April 2016 centred on the sunspot to analyse
the evolution of supersonic downflows on shorter timescales.
Results: Supersonic downflows are variable within this sunspot both
in terms of spatial structuring and velocities. Thirteen of the 16
raster scans display some evidence of supersonic downflows in the Si
IV 1394 Å line co-spatial to a sustained bright structure detected
in the 1400 Å slit-jaw imaging channel; a peak velocity of 112 km
s-1 is recorded on 29 March 2016. Evidence for supersonic
downflows in the O IV 1401 Å line is found in 14 of these rasters; the
spatial structuring in this line often differs from that inferred from
the Si IV 1394 Å line. Only one example of a supersonic downflow is
detected in the C II 1335 Å line and no downflows are found in the Mg
II 2796 Å lines at these locations. In the sit-and-stare observations,
no dual flow is initially detected, however, a supersonic downflow
develops after approximately 60 min. 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 min before the
end of the observation.
Conclusions: Supersonic downflows are
found in the Si IV 1394 Å line in 13 of the 16 rasters studied in this
work. 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
footpoints of coronal fan loops. Our results indicate that one raster
is not enough to conclusively draw inferences about the properties of
supersonic downflows within a sunspot during its lifetime.
Title: Multipoint Observations of Solar Wind Conditions and
Magnetopause Motion
Authors: Bagheri, F.; Lopez, R. E.; Dredger, P. M.; Bonde, R. E. F.;
Bui, M.; Chapagain, N.; Nelson, C.; Xing, C.
Bibcode: 2019AGUFMSM51C3198B
Altcode:
We study the magnetopause motion and structure based on observation
from multiple crossings of the magnetopause of THEMIS A, D and E with
simultaneous solar wind observations from Wind, ACE, THEMIS B, and
THEMIS C. The solar wind data shows the IMF was radial and the velocity
of solar wind was almost constant. The THEMIS A, D and E observations
are consistent with the classic picture of pressure balance across the
magnetopause. The thickness of the Chapman-Ferraro current layer is
about one gyro diameter. The average magnetopause speeds were in the low
10's of km/s, however, speeds up to 100km/s were observed. During this
period there is no evidence of hot flow anomalies in the magnetosheath
near the magnetopause. Although the orientation of the IMF during the
observations would have been predicted to cause wavy motions of the
magnetopause boundary, in our study there is no evidence of such kind
of motion.
Title: Co-spatial velocity and magnetic swirls in the simulated
solar photosphere
Authors: Liu, Jiajia; Carlsson, Mats; Nelson, Chris J.; Erdélyi,
Robert
Bibcode: 2019A&A...632A..97L
Altcode: 2019arXiv191110923L
Context. Velocity or intensity swirls have now been shown to be
widely present throughout the photosphere and chromosphere. It has
been suggested that these events could contribute to the heating
of the upper solar atmosphere, via exciting Alfvén pulses, which
could carry significant amounts of energy. However, the conjectured
necessary physical conditions for their excitation, that the magnetic
field rotates co-spatially and co-temporally with the velocity field,
has not been verified.
Aims: We aim to understand whether
photospheric velocity swirls exist co-spatially and co-temporally with
photospheric magnetic swirls, in order to demonstrate the link between
swirls and pulses.
Methods: The automated swirl detection
algorithm (ASDA) is applied to the photospheric horizontal velocity
and vertical magnetic fields obtained from a series of realistic
numerical simulations using the radiative magnetohydrodynamics (RMHD)
code Bifrost. The spatial relationship between the detected velocity
and magnetic swirls is further investigated via a well-defined
correlation index (CI) study.
Results: On average, there are
∼63 short-lived photospheric velocity swirls (with lifetimes mostly
less than 20 s, and average radius of ∼37 km and rotating speeds of
∼2.5 km s-1) detected in a field of view (FOV) of 6 ×
6 Mm-2, implying a total population of velocity swirls of
∼1.06 × 107 in the solar photosphere. More than 80% of the
detected velocity swirls are found to be accompanied by local magnetic
concentrations in intergranular lanes. On average, ∼71% of the
detected velocity swirls have been found to co-exist with photospheric
magnetic swirls with the same rotating direction.
Conclusions:
The co-temporal and co-spatial rotation in the photospheric velocity
and magnetic fields provide evidence that the conjectured condition for
the excitation of Alfvén pulses by photospheric swirls is fulfilled.
Title: Investigation of light ion fusion reactions with plasma
discharges
Authors: Schenkel, T.; Persaud, A.; Wang, H.; Seidl, P. A.; MacFadyen,
R.; Nelson, C.; Waldron, W. L.; Vay, J. -L.; Deblonde, G.; Wen, B.;
Chiang, Y. -M.; MacLeod, B. P.; Ji, Q.
Bibcode: 2019JAP...126t3302S
Altcode: 2019arXiv190503400S
The scaling of reaction yields in light ion fusion to low reaction
energies is important for our understanding of stellar fuel chains
and the development of future energy technologies. Experiments become
progressively more challenging at lower reaction energies due to the
exponential drop of fusion cross sections below the Coulomb barrier. We
report on experiments where deuterium-deuterium (D-D) fusion reactions
are studied in a pulsed plasma in the glow discharge regime using a
benchtop apparatus. We model plasma conditions using particle-in-cell
codes. Advantages of this approach are relatively high peak ion currents
and current densities (0.1 to several A/cm2) that can be
applied to metal wire cathodes for several days. We detect neutrons from
D-D reactions with scintillator-based detectors. For palladium targets,
we find neutron yields as a function of cathode voltage that are over
100 times higher than yields expected for bare nuclei fusion at ion
energies below 2 keV (center of mass frame). A possible explanation is a
correction to the ion energy due to an electron screening potential of
1000 ± 250 eV, which increases the probability for tunneling through
the repulsive Coulomb barrier. Our compact, robust setup enables
parametric studies of this effect at relatively low reaction energies.
Title: Spatially Resolved Signatures of Bidirectional Flows Observed
in Inverted-Y Shaped Jets
Authors: Nelson, C. J.; Freij, N.; Bennett, S.; Erdélyi, R.;
Mathioudakis, M.
Bibcode: 2019ApJ...883..115N
Altcode: 2019arXiv190805132N
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 localized bidirectional
flows and extended shock waves; however, these signatures are rarely
observed as extremely high spatial-resolution data are required. Here,
we use Hα imaging data sampled by the Swedish Solar Telescope’s CRisp
Imaging SpectroPolarimeter to investigate whether bidirectional flows
can be detected within inverted-Y shaped jets near the solar limb. These
jets are apparent in the Hα line wings, while no signature of either
jet is observed in the Hα line core, implying reconnection took place
below the chromospheric canopy. Asymmetries in the Hα line profiles
along the legs of the jets indicate the presence of bidirectional 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 ±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 suggests 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 bidirectional flows within
inverted-Y shaped jets and could be the driver of surges.
Title: Evidence of ubiquitous Alfvén pulses transporting energy
from the photosphere to the upper chromosphere
Authors: Liu, Jiajia; Nelson, Chris J.; Snow, Ben; Wang, Yuming;
Erdélyi, Robert
Bibcode: 2019NatCo..10.3504L
Altcode:
The multi-million degree temperature increase from the middle to
the upper solar atmosphere is one of the most fascinating puzzles
in plasma-astrophysics. Although magnetic waves might transport
enough energy from the photosphere to heat up the local chromosphere
and corona, observationally validating their ubiquity has proved
challenging. Here, we show observational evidence that ubiquitous
Alfvén pulses are excited by prevalent intensity swirls in the
solar photosphere. Correlation analysis between swirls detected at
different heights in the solar atmosphere, together with realistic
numerical simulations, show that these Alfvén pulses propagate upwards
and reach chromospheric layers. We found that Alfvén pulses carry
sufficient energy flux (1.9 to 7.7 kW m-2) to balance the
local upper chromospheric energy losses ( 0.1 kW m-2) in
quiet regions. Whether this wave energy flux is actually dissipated
in the chromosphere and can lead to heating that balances the losses
is still an open question.
Title: The Effect Of Cooling On Driven Kink Oscillations Of Coronal
Loops
Authors: Nelson, Chris J.; Shukhobodskiy, Alexander A.; Erdélyi,
Robertus; Mathioudakis, Mihalis
Bibcode: 2019FrASS...6...45N
Altcode: 2019arXiv190513137N
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).
Title: Automated Swirl Detection Algorithm (ASDA) and Its Application
to Simulation and Observational Data
Authors: Liu, Jiajia; Nelson, Chris J.; Erdélyi, Robertus
Bibcode: 2019ApJ...872...22L
Altcode: 2018arXiv180402931L
Swirling motions in the solar atmosphere have been widely observed in
recent years and suggested to play a key role in channeling energy
from the photosphere into the corona. Here, we present a newly
developed Automated Swirl Detection Algorithm (ASDA) and discuss
its applications. ASDA is found to be very proficient at detecting
swirls in a variety of synthetic data with various levels of noise,
implying our subsequent scientific results are astute. Applying ASDA
to photospheric observations with a pixel size of 39.2 km sampled by
the Solar Optical Telescope on board Hinode suggests a total number
of 1.62 × 105 swirls in the photosphere, with an average
radius and rotating speed of ∼290 km and <1.0 km s-1,
respectively. Comparisons between swirls detected in Bifrost numerical
MHD simulations and both ground-based and space-borne observations
suggest that (1) the spatial resolution of data plays a vital role in
the total number and radii of swirls detected, and (2) noise introduced
by seeing effects could decrease the detection rate of swirls, but has
no significant influences in determining their inferred properties. All
results have shown that there is no significant difference in the
analyzed properties between counterclockwise or clockwise rotating
swirls. About 70% of swirls are located in intergranular lanes. Most
of the swirls have lifetimes of less than twice the cadences, meaning
future research should aim to use data with much higher cadences than
6 s. In the conclusions, we propose some promising future research
applications where ASDA may provide useful insight.
Title: Solar Ultraviolet Bursts
Authors: Young, Peter R.; Tian, Hui; Peter, Hardi; Rutten, Robert J.;
Nelson, Chris J.; Huang, Zhenghua; Schmieder, Brigitte; Vissers, Gregal
J. M.; Toriumi, Shin; Rouppe van der Voort, Luc H. M.; Madjarska, Maria
S.; Danilovic, Sanja; Berlicki, Arkadiusz; Chitta, L. P.; Cheung, Mark
C. M.; Madsen, Chad; Reardon, Kevin P.; Katsukawa, Yukio; Heinzel, Petr
Bibcode: 2018SSRv..214..120Y
Altcode: 2018arXiv180505850Y
The term "ultraviolet (UV) burst" is introduced to describe small,
intense, transient brightenings in ultraviolet images of solar active
regions. We inventorize their properties and provide a definition
based on image sequences in transition-region lines. Coronal signatures
are rare, and most bursts are associated with small-scale, canceling
opposite-polarity fields in the photosphere that occur in emerging flux
regions, moving magnetic features in sunspot moats, and sunspot light
bridges. We also compare UV bursts with similar transition-region
phenomena found previously in solar ultraviolet spectrometry and
with similar phenomena at optical wavelengths, in particular Ellerman
bombs. Akin to the latter, UV bursts are probably small-scale magnetic
reconnection events occurring in the low atmosphere, at photospheric
and/or chromospheric heights. Their intense emission in lines with
optically thin formation gives unique diagnostic opportunities
for studying the physics of magnetic reconnection in the low solar
atmosphere. This paper is a review report from an International Space
Science Institute team that met in 2016-2017.
Title: Magnetic Braids in Eruptions of a Spiral Structure in the
Solar Atmosphere
Authors: Huang, Zhenghua; Xia, Lidong; Nelson, Chris J.; Liu, Jiajia;
Wiegelmann, Thomas; Tian, Hui; Klimchuk, James A.; Chen, Yao; Li, Bo
Bibcode: 2018ApJ...854...80H
Altcode: 2018arXiv180105967H
We report on high-resolution imaging and spectral observations of
eruptions of a spiral structure in the transition region, which
were taken with the Interface Region Imaging Spectrograph, and the
Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic
Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The eruption
coincided with the appearance of two series of jets, with velocities
comparable to the Alfvén speeds in their footpoints. Several pieces of
evidence of magnetic braiding in the eruption are revealed, including
localized bright knots, multiple well-separated jet threads, transition
region explosive events, and the fact that all three of these are
falling into the same locations within the eruptive structures. Through
analysis of the extrapolated 3D magnetic field in the region, we found
that the eruptive spiral structure corresponded well to locations
of twisted magnetic flux tubes with varying curl values along their
lengths. The eruption occurred where strong parallel currents,
high squashing factors, and large twist numbers were obtained. The
electron number density of the eruptive structure is found to be ∼3 ×
1012 cm-3, indicating that a significant amount
of mass could be pumped into the corona by the jets. Following the
eruption, the extrapolations revealed a set of seemingly relaxed loops,
which were visible in the AIA 94 Å channel, indicating temperatures
of around 6.3 MK. With these observations, we suggest that magnetic
braiding could be part of the mechanisms explaining the formation of
solar eruption and the mass and energy supplement to the corona.
Title: The formation of small-scale umbral brightenings in sunspot
atmospheres
Authors: Nelson, C. J.; Henriques, V. M. J.; Mathioudakis, M.; Keenan,
F. P.
Bibcode: 2017A&A...605A..14N
Altcode: 2017arXiv170505617N
Context. Sunspot atmospheres have been shown to be highly inhomogeneous
hosting both quasi-stable and transient features, such as small-scale
umbral brightenings (previously named "umbral micro-jets") and dark
fibril-like events.
Aims: We seek to understand the morphological
properties and formation mechanisms of small-scale umbral brightenings
(analogous to umbral micro-jets). In addition, we aim to understand
whether links between these events and short dynamic fibrils,
umbral flashes, and umbral dots can be established.
Methods:
A Swedish 1 m Solar Telescope (SST) filtergram time-series sampling
the Ca II H line and a CRisp Imaging Spectro-Polarimeter (SST/CRISP)
full-Stokes 15-point Ca II 8542 Å line scan dataset were used. The
spatial resolutions of these datasets are close to 0.1'' and 0.18''
with cadences of 1.4 s and 29 s, respectively. These data allowed
us to construct light-curves, plot line profiles, and to perform
a weak-field approximation in order to infer the magnetic field
strength.
Results: The average lifetime and lengths of the 54
small-scale brightenings identified in the sunspot umbra are found
to be 44.2 s (σ = 20 s) and 0.56'' (σ = 0.14''), respectively. The
spatial positioning and morphological evolution of these events in
Ca II H filtergrams was investigated finding no evidence of parabolic
or ballistic profiles nor a preference for co-spatial formation with
umbral flashes. Line scans in Ca II 8542 Å and the presence of Stokes
V profile reversals provided evidence that these events could form in a
similar manner to umbral flashes in the chromosphere (I.e. through the
formation of shocks either due to the steepening of localised wavefronts
or due to the impact of returning material from short dynamic fibrils,
a scenario we find evidence for). 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, in fact, the actual
cause of the increased emission. Finally, a sub-set of small-scale
brightenings were observed to form at the foot-points of short dynamic
fibrils.
Conclusions: The small-scale umbral brightenings studied
here do not appear to be jet-like in nature. Instead they appear to be
evidence of shock formation in the lower solar atmosphere. We found
no correlation between the spatial locations where these events were
observed and the occurrence of umbral dots and umbral flashes. These
events 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 that
these features form due to the shocking of fibrilar material in the
lower atmosphere upon its return under gravity.
Title: IRIS Burst Spectra Co-spatial to a Quiet-Sun Ellerman-like
Brightening
Authors: Nelson, C. J.; Freij, N.; Reid, A.; Oliver, R.; Mathioudakis,
M.; Erdélyi, R.
Bibcode: 2017ApJ...845...16N
Altcode: 2017arXiv170705080N
Ellerman bombs (EBs) have been widely studied over the past two
decades; however, only recently have the 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 Hα 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 behavior
of EBs in active regions. Two QSEBs in this sample occurred co-spatial
to increased emission in SDO/AIA 1600 Å and IRIS slit-jaw imager 1400
Å data; however, these intensity increases were smaller than those
reported co-spatially with 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 Hα 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.
Title: On the relationship between magnetic cancellation and UV
burst formation
Authors: Nelson, C. J.; Doyle, J. G.; Erdélyi, R.
Bibcode: 2016MNRAS.463.2190N
Altcode: 2016arXiv160806505N; 2016MNRAS.tmp.1156N
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 Interface Region Imaging Spectrograph 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 1014-1015 Mx s-1. Hα 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 upflow 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.
Title: Magnetic Flux Cancellation in Ellerman Bombs
Authors: Reid, A.; Mathioudakis, M.; Doyle, J. G.; Scullion, E.;
Nelson, C. J.; Henriques, V.; Ray, T.
Bibcode: 2016ApJ...823..110R
Altcode: 2016arXiv160307100R
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 Å 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) × 1024 erg of stored magnetic energy
disappears from the bipolar region during EB burning. The inversions
also show flux cancellation rates of 1014-1015
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.
Title: High-cadence observations of spicular-type events on the Sun
Authors: Shetye, J.; Doyle, J. G.; Scullion, E.; Nelson, C. J.;
Kuridze, D.; Henriques, V.; Woeger, F.; Ray, T.
Bibcode: 2016A&A...589A...3S
Altcode: 2016arXiv160108087S
Context. 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 rapid blue-shifted excursions
(RBEs) and rapid red-shifted excursions (RREs) which are thought to
be on-disk counterparts of Type-II spicules) 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-1.
Aims:
This article seeks to quantify and study rapidly appearing spicular-type
events. We also compare the multi-object multi-frame blind deconvolution
(MOMFBD) and speckle reconstruction techniques to understand if
these spicules are more favourably observed using a particular
technique.
Methods: We use spectral imaging observations taken
with the CRisp Imaging SpectroPolarimeter (CRISP) on the Swedish 1-m
Solar Telescope. Data was sampled at multiple positions within the Hα
line profile for both an on-disk and limb location.
Results: The
data is host to numerous rapidly appearing features which are observed
at different locations within the Hα line profile. The feature's
durations vary between 10-20 s and lengths around 3500 km. Sometimes,
a time delay in their appearance between the blue and red wings of
3-5 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.
Conclusions:
We provide the first statistical analysis of these spicules and suggest
that these observations can be interpreted as the line-of-sight (LOS)
movement of highly dynamic spicules moving in and out of the narrow 60
mÅ transmission filter that is used to observe in different parts of
the Hα 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 Hα line. Additional
work involving data at other wavelengths is required to investigate
the nature of their possible wave-like activity.
Title: High Cadence Observations and Analysis of Spicular-type Events
Using CRISP Onboard SST
Authors: Shetye, J.; Doyle, J. G.; Scullion, E.; Nelson, C. J.;
Kuridze, D.
Bibcode: 2016ASPC..504..115S
Altcode:
We present spectroscopic and imaging observations of apparent ultra-fast
spicule-like features observed with CRisp Imaging SpectroPolarimeter
(CRISP) at the Swedish 1-m Solar Telescope (SST). The data shows
spicules with an apparent velocity above 500 km s-1,
very short lifetimes of up to 20 s and length/height around 3500
km. The spicules are seen as dark absorption structures in the Hα
wings ±516 mÅ, ±774 mÅ and ±1032 mÅ which suddenly appear and
disappear from the FOV. These features show a time delay in their
appearance in the blue and red wings by 3-5 s. We suggest that their
appearance/disappearance is due to their Doppler motion in and out of
the 60 mÅ filter. See Fig. 1 for the evolution of the event at two
line positions.
Title: On The Role of MHD Waves in Heating Localised Magnetic
Structures
Authors: Erdélyi, R.; Nelson, C. J.
Bibcode: 2016ASPC..504..153E
Altcode:
Satellite and ground-based observations from e.g. SOHO, TRACE, STEREO,
Hinode, SDO and IRIS to DST/ROSA, IBIS, CoMP, STT/CRISP have provided
a wealth of evidence of waves and oscillations present in a wide range
of spatial scales of the magnetised solar atmosphere. Our understanding
about localised solar structures has been considerably changed in light
of these high spatial and time resolution observations. However, MHD
waves not only enable us to perform sub-resolution magneto-seismology of
magnetic waveguides but are also potential candidates to carry and damp
the necessary non-thermal energy in these localised waveguides. First,
we will briefly outline the basic recent developments in MHD wave
theory focussing on linear waves. Next, we discuss the role of
the most frequently studied wave classes, including the Alfven, and
magneto-acoustic kink and sausage waves. The current theoretical (and
often difficult) interpretations of the detected solar atmospheric
wave and oscillatory phenomena within the framework of MHD will be
shown. Last, the latest reported observational findings of potential MHD
wave flux, in terms of localised plasma heating, in the solar atmosphere
is discussed, bringing us closer to solve the coronal heating problem.
Title: Small-scale Structuring of Ellerman Bombs at the Solar Limb
Authors: Nelson, C. J.; Scullion, E. M.; Doyle, J. G.; Freij, N.;
Erdélyi, R.
Bibcode: 2015ApJ...798...19N
Altcode: 2014arXiv1410.5715N
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 disk. In
this article, we make use of high-resolution, high-cadence observations
of an Active Region at the solar limb, collected by the CRisp Imaging
SpectroPolarimeter (CRISP) instrument, to identify EBs and infer their
physical properties. The upper atmosphere is also probed using the
Solar Dynamic Observatory's Atmospheric Imaging Assembly (SDO/AIA). We
analyze 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-disk events. Two in-depth case studies are also presented that
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 Hα 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 Hα line core,
and are not observed by the SDO/AIA EUV filters.
Title: The Detection of Upwardly Propagating Waves Channeling Energy
from the Chromosphere to the Low Corona
Authors: Freij, N.; Scullion, E. M.; Nelson, C. J.; Mumford, S.;
Wedemeyer, S.; Erdélyi, R.
Bibcode: 2014ApJ...791...61F
Altcode: 2014arXiv1408.4621F
There have been ubiquitous observations of wave-like motions in
the solar atmosphere for decades. Recent improvements to space- and
ground-based observatories have allowed the focus to shift to smaller
magnetic structures on the solar surface. In this paper, high-resolution
ground-based data taken using the Swedish 1 m Solar Telescope is
combined with co-spatial and co-temporal data from the Atmospheric
Imaging Assembly (AIA) on board the Solar Dynamics Observatory
(SDO) satellite to analyze running penumbral waves (RPWs). RPWs
have always been thought to be radial wave propagation that occurs
within sunspots. Recent research has suggested that they are in fact
upwardly propagating field-aligned waves (UPWs). Here, RPWs within a
solar pore are observed for the first time and are interpreted as UPWs
due to the lack of a penumbra that is required to support RPWs. These
UPWs are also observed co-spatially and co-temporally within several
SDO/AIA elemental lines that sample the transition region and low
corona. The observed UPWs are traveling at a horizontal velocity of
around 17 ± 0.5 km s-1 and a minimum vertical velocity
of 42 ± 21 km s-1. The estimated energy of the waves is
around 150 W m-2, which is on the lower bound required to
heat the quiet-Sun corona. This is a new, yet unconsidered source of
wave energy within the solar chromosphere and low corona.
Title: Effects of Stratification and Flows on P 1/ P
2 Ratios and Anti-node Shifts Within Closed Loop Structures
Authors: Erdélyi, R.; Hague, A.; Nelson, C. J.
Bibcode: 2014SoPh..289..167E
Altcode: 2013arXiv1306.1051E
The solar atmosphere is a dynamic environment, constantly evolving
to form a wide range of magnetically dominated structures (coronal
loops, spicules, prominences, etc.) which cover a significant
percentage of the surface at any one time. Oscillations and waves
in many of these structures are now widely observed and have led
to the new analytic technique of solar magneto-seismology, where
inferences of the background conditions of the plasma can be deduced
by studying magneto-hydrodynamic (MHD) waves. Here, we generalise
a novel magneto-seismological method designed to infer the density
distribution of a bounded plasma structure from the relationship of
its fundamental and subsequent harmonics. Observations of the solar
atmosphere have emphatically shown that stratification, leading to
complex density profiles within plasma structures, is common thereby
rendering this work instantly accessible to solar physics. We show,
in a dynamic waveguide, how the period ratio differs from the idealised
harmonic ratios prevalent in homogeneous structures. These ratios show
strong agreement with recent observational work. Next, anti-node shifts
are also analysed. Using typical scaling parameters for bulk flows
within atmospheric waveguides, e.g., coronal loops, it is found that
significant anti-node shifts can be predicted, even to the order of 10
Mm. It would be highly encouraged to design specific observations to
confirm the predicted anti-node shifts and apply the developed theory
of solar magneto-seismology to gain more accurate waveguide diagnostics
of the solar atmosphere.
Title: Excitation of an outflow from the lower solar atmosphere and
a co-temporal EUV transient brightening
Authors: Nelson, C. J.; Doyle, J. G.
Bibcode: 2013A&A...560A..31N
Altcode: 2013arXiv1310.8490N
Aims: We analyse an absorption event within the Hα line wings,
which has been identified as a surge, and the co-spatial evolution
of an EUV brightening, with spatial and temporal scales analogous
to a small blinker.
Methods: We conduct a multi-wavelength,
multi-instrument analysis using high-cadence, high-resolution data,
collected by the Interferometric BIdimensional Spectrometer on the
Dunn Solar Telescope, as well as the space-borne Atmospheric Imaging
Assembly and Helioseismic and Magnetic Imager instruments on board
the Solar Dynamics Observatory.
Results: One large absorption
event situated within the plage region trailing the lead sunspot of
AR 11579 is identified within the Hα line wings. This event is found
to be co-spatially linked to a medium-scale (around 4'' in diameter)
brightening within the transition region and corona. This ejection
appears to have a parabolic evolution, first forming in the Hα blue
wing before fading and reappearing in the Hα red wing, and comprises a
number of smaller fibril events. The line-of-sight photospheric magnetic
field shows no evidence of cancellation leading to this event.
Conclusions: Our research has identified clear evidence that at least
a subset of transient brightening events in the transition region is
linked to the influx of cooler plasma from the lower solar atmosphere
during large eruptive events, such as surges. These observations agree
with previous numerical researches on the nature of blinkers and,
therefore, suggest that magnetic reconnection is the driver of the
analysed surge events; however, further research is required to confirm
this.