40 research outputs found
Detecting and Characterising Small-Scale Brightenings in Solar Imaging Data
Observations of small-scale brightenings in the low solar atmosphere can
provide valuable constraints on possible heating/heat-transport mechanisms. We
present a method for the detection and analysis of brightenings and demonstrate
its application to IRIS EUV time-series imagery. The method uses band-pass
filtering, adaptive thresholding and centroid tracking, and records an event's
position, duration, and total/maximum brightness. Area, brightness, and
position are also recorded as functions of time throughout their lifetime.
Detected brightenings can fragment or merge over time, thus the number of
distinct regions constituting a brightening event is recorded over time and the
maximum number of regions are recorded as a simple measure of an event's
coherence/complexity. A test is made on a synthetic datacube composed of a
static background based on IRIS data, Poisson noise and
randomly-distributed, moving, small-scale Gaussian brightenings. Maximum
brightness, total brightness, area, and duration follow power-law distributions
and the results show the range over which the method can extract information.
The recorded maximum brightness is a reliable measure for the brightest and
most accurately detected events with an error of 6%. Area, duration, and speed
are generally underestimated by 15% with an uncertainty of 20-30%. Total
brightness is underestimated by 30% with an uncertainty of 30%. Applying this
method to real IRIS QS data spanning 19 minutes over a 54.40"55.23" FOV
yields 2997 detections. 1340 of these either remain un-fragmented or fragment
to two distinct regions at least once during their lifetime equating to an
event density of arcsecs. The method will be
used for a future large-scale statistical analysis of several QS data sets from
IRIS, other EUV imagers, as well as H- and visible photospheric
imagery.Comment: 19 pages, 12 figure
A Solar-cycle Study of Coronal Rotation:Large Variations, Rapid Changes, and Implications for Solar-wind Models
Information on the rotation rate of the corona, and its variation over
latitude and solar cycle, is valuable for making global connections between the
corona and the Sun, for global estimates of reconnection rates, and as a basic
parameter for solar wind modelling. Here, we use a time series of tomographical
maps gained from coronagraph observations between 2007 - 2020 to directly
measure the longitudinal drift of high-density streamers over time. The method
reveals abrupt changes in rotation rates, revealing a complex relationship
between the coronal rotation and the underlying photosphere. The majority of
rates are between -1.0 to +0.5/day relative to the standard Carrington
rate of 14.18/day, although rates are measured as low as
-2.2/day and as high as 1.6/day. Equatorial rotation rates
during the 2008 solar minimum are slightly faster than the Carrington rate,
with an abrupt switch to slow rotation in 2009, then a return to faster rates
in 2017. Abrupt changes and large variations in rates are seen at all
latitudes. Comparison with a magnetic model suggests that periods of equatorial
fast rotation are associated with times when a large proportion of the magnetic
footpoints of equatorial streamers are near the equator, and we interpret the
abrupt changes in terms of the latitudinal distribution of the streamer
photospheric footpoints. The coronal rotation rate is a key parameter for solar
wind models, and variations of up to a degree per day or more can lead to large
systematic errors over forecasting periods of longer than a few days. The
approach described in this paper gives corrected values that can form a part of
future forecasting efforts
Dark Off-limb Gap:Manifestation of a Temperature Minimum and the Dynamic Nature of the Chromosphere
We study off-limb emission of the lower solar atmosphere using
high-resolution imaging spectroscopy in the H and Ca II 8542 \r{A} lines
obtained with the CHROMospheric Imaging Spectrometer (CHROMIS) and the CRisp
Imaging SpectroPolarimeter (CRISP) on the Swedish 1-m Solar Telescope. The
H line wing images show the dark intensity gap between the photospheric
limb and chromosphere which is absent in the Ca II images. We calculate
synthetic spectra of the off-limb emissions with the RH code in the
one-dimension spherical geometry and find good agreement with the observations.
The analysis of synthetic line profiles shows that the gap in the H line
wing images maps the temperature minimum region between the photosphere and
chromosphere due to the well known opacity and emissivity gap of Balmer lines
in this layer. However, observed gap is detected farther from the line core in
the outer line wing positions than in the synthetic profiles. We found that an
increased microturbulence in the model chromosphere is needed to reproduce the
dark gap in the outer line wing, suggesting that observed H gap is the
manifestation of the temperature minimum and the dynamic nature of the solar
chromosphere. The temperature minimum produces a small enhancement in synthetic
Ca II line-wing intensities. Observed off-limb Ca II line-wing emissions show
similar enhancement below temperature minimum layer near the edge of the
photospheric limb.Comment: 14 pages, 8 figures, accepted in Ap
Multi-wavelength observations of the 2014 June 11 M3.9 flare:Temporal and spatial characteristics
We present multi-wavelength observations of an M-class flare (M3.9) that
occurred on 2014 June 11. Our observations were conducted with the Dunn Solar
Telescope (DST), adaptive optics, the multi-camera system ROSA (Rapid
Oscillations in Solar Atmosphere) and new HARDcam (Hydrogen-Alpha Rapid
Dynamics) camera in various wavelengths, such as Ca~II~K, Mg~I~b (at 5172.7
Ang), and H narrow-band, and G-band continuum filters. Images were
re-constructed using the Kiepencheuer-Institut Speckle Interferometry Package
(KISIP) code, to improve our image resolution. We observed intensity increases
of 120-150% in the Mg, Ca~K and H narrow band filters during
the flare. Intensity increases for the flare observed in the SDO EUV channels
were several times larger, and the GOES X-rays increased over a factor of 30
for the harder band. Only a modest delay is found between the onset of flare
ribbons of a nearby sympathetic flare and the main flare ribbons observed in
these narrow-band filters. The peak flare emission occurs within a few seconds
for the Ca~K, Mg, and H bands. Time-distance techniques find
propagation velocities of 60 km/s for the main flare ribbon and as
high as 300 km/s for smaller regions we attribute to filament eruptions. This
result and delays and velocities observed with SDO (100 km/s) for
different coronal heights agree well with the simple model of energy
propagation versus height, although a more detailed model for the flaring solar
atmosphere is needed. And finally, we detected marginal quasi-periodic
pulsations (QPPs) in the 40--60 second range for the Ca~K, Mg and H
bands, and such measurements are important for disentangling the detailed
flare-physics.Comment: 16 Pages, 7 Figures, 1 Table (1 video in on-line journal); Accepted
in Research in Astronomy and Astrophysic
Multi-wavelength imaging and spectral analysis of jet-like phenomena in a solar active region using IRIS and AIA
High-resolution observations of dynamic phenomena give insights into the properties and processes that govern the low solar atmosphere. We present an analysis of jet-like phenomena emanating from a penumbral footpoint in active region (AR) 12192 using imaging and spectral observations from the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. These jets are associated with line-of-sight Doppler speeds of ±10–22 km s−1 and bright fronts that seem to move across the plane-of-sky at speeds of 23–130 km s−1. Such speeds are considerably higher than the expected sound speed in the chromosphere. The jets have signatures that are visible both in the cool and hot channels of IRIS and AIA. Each jet lasts on average 15 minutes and occurs 5–7 times over a period of 2 hr. Possible mechanisms to explain this phenomenon are suggested, the most likely of which involve p-mode or Alfvén wave shock trains impinging on the transition region and corona as a result of steepening photospheric wavefronts or gravity waves
Association between Tornadoes and Instability of Hosting Prominences
We studied the dynamics of all prominence tornadoes detected by the Solar
Dynamics Observatory/Atmospheric Imaging Assembly from 2011 January 01 to
December 31. In total, 361 events were identified during the whole year, but
only 166 tornadoes were traced until the end of their lifetime. Out of 166
tornadoes, 80 (48%) triggered CMEs in hosting prominences, 83 (50%) caused
failed coronal mass ejections (CMEs) or strong internal motion in the
prominences, and only 3 (2%) finished their lifetimes without any observed
activity. Therefore, almost all prominence tornadoes lead to the
destabilization of their hosting prominences and half of them trigger CMEs.
Consequently, prominence tornadoes may be used as precursors for CMEs and hence
for space weather predictions.Comment: 16 pages, 5 figures, Accepted in Ap
Spectral Characteristics and Formation Height of Off-limb Flare Ribbons
Flare ribbons are bright manifestations of flare energy dissipation in the
lower solar atmosphere. For the first time, we report on high-resolution
imaging spectroscopy observations of flare ribbons situated off-limb in the
H and Ca II 8542 {\AA} lines and make a detailed comparison with
radiative hydrodynamic simulations. Observations of the X8.2-class solar flare
SOL2017-09-10T16:06 UT obtained with the Swedish Solar Telescope reveal bright
horizontal emission layers in H line wing images located near the
footpoints of the flare loops. The apparent separation between the ribbon
observed in the H wing and the nominal photospheric limb is about 300 -
500 km. The Ca II 8542 {\AA} line wing images show much fainter ribbon
emissions located right on the edge of the limb, without clear separation from
the limb. RADYN models are used to investigate synthetic spectral line profiles
for the flaring atmosphere, and good agreement is found with the observations.
The simulations show that, towards the limb, where the line of sight is
substantially oblique with respect to the vertical direction, the flaring
atmosphere model reproduces the high contrast of the off-limb H ribbons
and their significant elevation above the photosphere. The ribbons in the Ca II
8542 {\AA} line wing images are located deeper in the lower solar atmosphere
with a lower contrast. A comparison of the height deposition of electron beam
energy and the intensity contribution function shows that the H line
wing intensities can be an useful tracer of flare energy deposition in the
lower solar atmosphereComment: 18 pages, 10 figures, accepted in Ap