11 research outputs found
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
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
Eruptions from quiet Sun coronal bright points : II. Non-potential modelling
Support for VAPOR is provided by the U.S. National Science Foundation (grants # 03-25934 and 09-06379, ACI-14-40412), and by the Korea Institute of Science and Technology Information. C. M. thanks the National Natural Science Foundation of China (41474150). The HMI data are provided courtesy of NASA/SDO and corresponding science teams. The HMI data have been retrieved using the Stanford University’s Joint Science Operations Centre/Science Data Processing Facility. M.M. and K.G. thank the ISSI Bern for the support to the team “Observation-Driven Modelling of Solar Phenomena”.Context. Our recent observational study shows that the majority of coronal bright points (CBPs) in the quiet Sun are sources of one or more eruptions during their lifetime. Aims. Here, we investigate the non-potential time-dependent structure of the magnetic field of the CBP regions with special emphasison the time-evolving magnetic structure at the spatial locations where the eruptions are initiated. Methods. The magnetic structure is evolved in time using a non-linear force-free field (NLFFF) relaxation approach based on a timeseries of helioseismic and magnetic imager (HMI) longitudinal magnetograms. This results in a continuous time series of NLFFFs.The time series is initiated with a potential field extrapolation based on a magnetogram taken well before the time of the eruptions. This initial field is then evolved in time in response to the observed changes in the magnetic field distribution at the photosphere. The local and global magnetic field structures from the time series of NLFFF field solutions are analysed in the vicinity of the eruption sites at the approximate times of the eruptions. Results. The analysis shows that many of the CBP eruptions reported in a recent publication contain twisted flux tube located atthe sites of eruptions. The presence of flux ropes at these locations provides in many cases a direct link between the magnetic field structure, their eruption, and the observation of mini coronal mass ejections (mini-CMEs). It is found that all repetitive eruptions are homologous. Conclusions. The NLFFF simulations show that twisted magnetic field structures are created at the locations hosting eruptions inCBPs. These twisted structures are produced by footpoint motions imposed by changes in the photospheric magnetic field observations.The true nature of the micro-flares remains unknown. Further 3D data-driven magnetohydrodynamic modelling is required to show how these twisted regions become unstable and erupt.Publisher PDFPeer reviewe
Coronal Sources and In Situ Properties of the Solar Winds Sampled by ACE During 1999 - 2008
We identify the coronal sources of the solar winds sampled by the ACE
spacecraft during 1999-2008, and examine the in situ solar wind properties as a
function of wind sources. The standard two-step mapping technique is adopted to
establish the photospheric footpoints of the magnetic flux tubes along which
the ACE winds flow. The footpoints are then placed in the context of EIT
284~\AA\ images and photospheric magnetograms, allowing us to categorize the
sources into four groups: coronal holes (CHs), active regions (ARs), the quiet
Sun (QS), and "Undefined". This practice also enables us to establish the
response to solar activity of the fractions occupied by each kind of solar
winds, and of their speeds and O/O ratios measured in situ. We
find that during the maximum phase, the majority of ACE winds originate from
ARs. During the declining phase, CHs and ARs are equally important contributors
to the ACE solar winds. The QS contribution increases with decreasing solar
activity, and maximizes in the minimum phase when QS appear to be the primary
supplier of the ACE winds. With decreasing activity, the winds from all sources
tend to become cooler, as represented by the increasingly low O/O
ratios. On the other hand, during each activity phase, the AR winds tend to be
the slowest and associated with the highest O/O ratios, and the
CH winds correspond to the other extreme, with the QS winds lying in between.
Applying the same analysis method to the slow winds only, here defined as the
winds with speeds lower than 500 km s, we find basically the same
overall behavior, as far as the contributions of individual groups of sources
are concerned. This statistical study indicates that QS regions are an
important source of the solar wind during the minimum phase.Comment: 24 pages, 7 figures, accepted for publication in Solar Physic
Sources of quasi-periodic propagating disturbances above a solar polar coronal hole
Quasi-periodic propagating disturbances (PDs) are ubiquitous in polar coronal
holes on the Sun. It remains unclear as to what generates PDs. In this work, we
investigate how the PDs are generated in the solar atmosphere by analyzing a
fourhour dataset taken by the Atmospheric Imaging Assembly (AIA) on board the
Solar Dynamics Observatory (SDO). We find convincing evidence that spicular
activities in the solar transition region as seen in the AIA 304 {\AA} passband
are responsible for PDs in the corona as revealed in the AIA 171 {\AA} images.
We conclude that spicules are an important source that triggers coronal PDs.Comment: Accepted for publication in ApJ
Anisotropic nonthermal motions in the transition region of solar active regions
Context. We study the nonthermal motions in the transition region of active regions (ARs) using center-to-limb observations of Si I