31 research outputs found
Cool transition region loops observed by the Interface Region Imaging Spectrograph
We report on the first Interface Region Imaging Spectrograph (IRIS) study of
cool transition region loops. This class of loops has received little attention
in the literature. A cluster of such loops was observed on the solar disk in
active region NOAA11934, in the Si IV 1402.8 \AA\ spectral raster and 1400 \AA\
slit-jaw (SJ) images. We divide the loops into three groups and study their
dynamics and interaction. The first group comprises relatively stable loops,
with 382--626\,km cross-sections. Observed Doppler velocities are suggestive of
siphon flows, gradually changing from -10 km/s at one end to 20 km/s at the
other end of the loops. Nonthermal velocities from 15 to 25 km/s were
determined. These physical properties suggest that these loops are impulsively
heated by magnetic reconnection occurring at the blue-shifted footpoints where
magnetic cancellation with a rate of Mx/s is found. The released
magnetic energy is redistributed by the siphon flows. The second group
corresponds to two footpoints rooted in mixed-magnetic-polarity regions, where
magnetic cancellation occurred at a rate of Mx/s and line profiles
with enhanced wings of up to 200 km/s were observed. These are suggestive of
explosive-like events. The Doppler velocities combined with the SJ images
suggest possible anti-parallel flows in finer loop strands. In the third group,
interaction between two cool loop systems is observed. Evidence for magnetic
reconnection between the two loop systems is reflected in the line profiles of
explosive events, and a magnetic cancellation rate of Mx/s
observed in the corresponding area. The IRIS observations have thus opened a
new window of opportunity for in-depth investigations of cool transition region
loops. Further numerical experiments are crucial for understanding their
physics and their role in the coronal heating processes.Comment: Accepted for publication in Ap
Explosive events on sub-arcsecond scale in IRIS observations: a case study
We present study of a typical explosive event (EE) at sub-arcsecond scale
witnessed by strong non-Gaussian profiles with blue- and red-shifted emission
of up to 150 km/s seen in the transition-region Si IV 1402.8 \AA, and the
chromospheric Mg II k 2796.4 \AA\ and C II 1334.5 \AA\ observed by the
Interface Region Imaging Spectrograph at unprecedented spatial and spectral
resolution. For the first time a EE is found to be associated with very
small-scale (120 km wide) plasma ejection followed by retraction in the
chromosphere. These small-scale jets originate from a compact bright-point-like
structure of 1.5" size as seen in the IRIS 1330 \AA\ images. SDO/AIA and
SDO/HMI co-observations show that the EE lies in the footpoint of a complex
loop-like brightening system. The EE is detected in the higher temperature
channels of AIA 171 \AA, 193 \AA\ and 131 \AA\ suggesting that it reaches a
higher temperature of log T (K). Brightenings observed in the AIA
channels with durations 90--120 seconds are probably caused by the plasma
ejections seen in the chromosphere. The wings of the C II line behave in a
similar manner as the Si IV's indicating close formation temperatures, while
the Mg II k wings show additional Doppler-shifted emission. Magnetic
convergence or emergence followed by cancellation at a rate of
Mx s is associated with the EE region. The combined changes of the
locations and the flux of different magnetic patches suggest that magnetic
reconnection must have taken place. Our results challenge several theories put
forward in the past to explain non-Gaussian line profiles, i.e. EEs. Our case
study on its own, however, cannot reject these theories, thus further in-depth
studies on the phenomena producing EEs are required.Comment: 16 figures, accepted for publication in Ap
Plasma parameters and geometry of cool and warm active region loops
How the solar corona is heated to high temperatures remains an unsolved
mystery in solar physics. In the present study we analyse observations of 50
whole active-region loops taken with the Extreme-ultraviolet Imaging
Spectrometer (EIS) on board the Hinode satellite. Eleven loops were classified
as cool (<1 MK) and 39 as warm (1-2 MK) loops. We study their plasma parameters
such as densities, temperatures, filling factors, non-thermal velocities and
Doppler velocities. We combine spectroscopic analysis with linear force-free
magnetic-field extrapolation to derive the three-dimensional structure and
positioning of the loops, their lengths and heights as well as the magnetic
field strength along the loops. We use density-sensitive line pairs from Fe
XII, Fe XIII, Si X and Mg VII ions to obtain electron densities by taking
special care of intensity background-subtraction. The emission-measure loci
method is used to obtain the loop temperatures. We find that the loops are
nearly isothermal along the line-of-sight. Their filling factors are between 8%
and 89%. We also compare the observed parameters with the theoretical RTV
scaling law. We find that most of the loops are in an overpressure state
relative to the RTV predictions. In a followup study, we will report a heating
model of a parallel-cascade-based mechanism and will compare the model
parameters with the loop plasma and structural parameters derived here.Comment: ApJ, accepted for publicatio
Narrow-line-width UV bursts in the transition region above Sunspots observed by IRIS
Various small-scale structures abound in the solar atmosphere above active
regions, playing an important role in the dynamics and evolution therein. We
report on a new class of small-scale transition region structures in active
regions, characterized by strong emissions but extremely narrow Si IV line
profiles as found in observations taken with the Interface Region Imaging
Spectrograph (IRIS). Tentatively named as Narrow-line-width UV bursts (NUBs),
these structures are located above sunspots and comprise of one or multiple
compact bright cores at sub-arcsecond scales. We found six NUBs in two datasets
(a raster and a sit-and-stare dataset). Among these, four events are
short-living with a duration of 10 mins while two last for more than 36
mins. All NUBs have Doppler shifts of 15--18 km/s, while the NUB found in
sit-and-stare data possesses an additional component at 50 km/s found
only in the C II and Mg II lines. Given that these events are found to play a
role in the local dynamics, it is important to further investigate the physical
mechanisms that generate these phenomena and their role in the mass transport
in sunspots.Comment: 8 pages, 4 figures and 1 table, accepted for publication in ApJ
Energetics of Hi-C EUV Brightenings
We study the thermal structure and energetics of the point-like EUV
brightenings within a system of fan loops observed in the active region
\textsl{AR~11520}. These brightenings were simultaneously observed on 2012 July
11 by the HIgh-resolution Coronal (Hi-C) imager and the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We identified 27
brightenings by automatically determining intensity enhancements in both Hi-C
and AIA ~193~{\AA} light curves. The energetics of these brightenings were
studied by using the Differential Emission Measure (DEM) diagnostics. The DEM
weighted temperatures of these transients are in the range with radiative energies ~ergs and densities
a few times ~cm. To the best of our knowledge, these
are the smallest brightenings in EUV ever detected. We used these results to
determine the mechanism of energy loss in these brightenings. Our analysis
reveals that the dominant mechanism of energy loss for all the identified
brightenings is conduction rather than radiation.Comment: 17 pages, 8 Figures, 3 tables, accepted for publication in Astronomy
and Astrophysic
Detection of Opposite Magnetic Polarity in a Light Bridge: Its Emergence and Cancellation in association with LB Fan-shaped Jets
Light bridges (LBs) are relatively bright structures that divide sunspot
umbrae into two or more parts. Chromospheric LBs are known to be associated
with various activities including fan-shaped jet-like ejections and
brightenings. Although magnetic reconnection is frequently suggested to be
responsible for such activities, not many studies present firm evidence to
support the scenario. We carry out magnetic field measurements and imaging
spectroscopy of a LB where fan-shaped jet-like ejections occur with co-spatial
brightenings at their footpoints. We study LB fine structure and magnetic field
changes using TiO images, Near-InfraRed Imaging Spectropolarimeter, and Halpha
data taken by the 1.6~m Goode Solar Telescope. We detect magnetic flux
emergence in the LB that is of opposite polarity to that of the sunspot. The
new magnetic flux cancels with the pre-existing flux at a rate of 5.6x10^18
Mx/hr. Both the recurrent jet-like ejections and their base brightenings are
initiated at the vicinity of the magnetic flux cancellation, and show apparent
horizontal extension along the LB at a projected speed of up to 18.4km/s to
form a fan-shaped appearance. Based on these observations, we suggest that the
fan-shaped ejections may have resulted from slipping reconnection between the
new flux emerging in the LB and the ambient sunspot field.Comment: 24pages, 11figures, accepted by the Ap