53 research outputs found
Amplification of compressional MHD waves in systems with forced entropy oscillations
The propagation of compressional MHD waves is studied for an externally
driven system. It is assumed that the combined action of the external sources
and sinks of the entropy results in the harmonic oscillation of the entropy
(and temperature) in the system. It is found that with the appropriate resonant
conditions fast and slow waves get amplified due to the phenomenon of
parametric resonance. Besides, it is shown that the considered waves are
mutually coupled as a consequence of the nonequilibrium state of the background
medium. The coupling is strongest when the plasma . The
proposed formalism is sufficiently general and can be applied for many
dynamical systems, both under terrestrial and astrophysical conditions.Comment: 14 pages, 4 figures, Accepted to Physical Review
Long-period oscillations of active region patterns: least-squares mapping on second-order curves
Active regions (ARs) are the main sources of variety in solar dynamic events.
Automated detection and identification tools need to be developed for solar
features for a deeper understanding of the solar cycle. Of particular interest
here are the dynamical properties of the ARs, regardless of their internal
structure and sunspot distribution. We studied the oscillatory dynamics of two
ARs: NOAA 11327 and NOAA 11726 using two different methods of pattern
recognition. We developed a novel method of automated AR border detection and
compared it to an existing method for the proof-of-concept. The first method
uses least-squares fitting on the smallest ellipse enclosing the AR, while the
second method applies regression on the convex hull.} After processing the
data, we found that the axes and the inclination angle of the ellipse and the
convex hull oscillate in time. These oscillations are interpreted as the second
harmonic of the standing long-period kink oscillations (with the node at the
apex) of the magnetic flux tube connecting the two main sunspots of the ARs. In
both ARs we have estimated the distribution of the phase speed magnitude along
the magnetic tubes (along the two main spots) by interpreting the obtained
oscillation of the inclination angle as the standing second harmonic kink mode.
After comparing the obtained results for fast and slow kink modes, we conclude
that both of these modes are good candidates to explain the observed
oscillations of the AR inclination angles, as in the high plasma regime
the phase speeds of these modes are comparable and on the order of the
Alfv\'{e}n speed. Based on the properties of the observed oscillations, we
detected the appropriate depth of the sunspot patterns, which coincides with
estimations made by helioseismic methods. The latter analysis can be used as a
basis for developing a magneto-seismological tool for ARs.Comment: 10 pages, 6 figures, Accepted for publication in A&
Quasi-oscillatory dynamics observed in ascending phase of the flare on March 6, 2012
Context. The dynamics of the flaring loops in active region (AR) 11429 are
studied. The observed dynamics consist of several evolution stages of the
flaring loop system during both the ascending and descending phases of the
registered M-class flare. The dynamical properties can also be classified by
different types of magnetic reconnection, related plasma ejection and aperiodic
flows, quasi-periodic oscillatory motions, and rapid temperature and density
changes, among others. The focus of the present paper is on a specific time
interval during the ascending (pre-flare) phase. Aims. The goal is to
understand the quasi-periodic behavior in both space and time of the magnetic
loop structures during the considered time interval. Methods.We have studied
the characteristic location, motion, and periodicity properties of the flaring
loops by examining space-time diagrams and intensity variation analysis along
the coronal magnetic loops using AIA intensity and HMI magnetogram images (from
the Solar Dynamics Observatory(SDO)). Results. We detected bright plasma blobs
along the coronal loop during the ascending phase of the solar flare, the
intensity variations of which clearly show quasi-periodic behavior. We also
determined the periods of these oscillations. Conclusions. Two different
interpretations are presented for the observed dynamics. Firstly, the
oscillations are interpreted as the manifestation of non-fundamental harmonics
of longitudinal standing acoustic oscillations driven by the thermodynamically
nonequilibrium background (with time variable density and temperature). The
second possible interpretation we provide is that the observed bright blobs
could be a signature of a strongly twisted coronal loop that is kink unstable.Comment: 12 pages, 10 figures, A&A, in pres
Case study on the identification and classification of small-scale flow patterns in flaring active region
We propose a novel methodology to identity flows in the solar atmosphere and
classify their velocities as either supersonic, subsonic, or sonic. The
proposed methodology consists of three parts. First, an algorithm is applied to
the Solar Dynamics Observatory (SDO) image data to locate and track flows,
resulting in the trajectory of each flow over time. Thereafter, the
differential emission measure inversion method is applied to six AIA channels
along the trajectory of each flow in order to estimate its background
temperature and sound speed. Finally, we classify each flow as supersonic,
subsonic, or sonic by performing simultaneous hypothesis tests on whether the
velocity bounds of the flow are larger, smaller, or equal to the background
sound speed. The proposed methodology was applied to the SDO image data from
the 171 {\AA} spectral line for the date 6 March 2012 from 12:22:00 to 12:35:00
and again for the date 9 March 2012 from 03:00:00 to 03:24:00. Eighteen plasma
flows were detected, 11 of which were classified as supersonic, 3 as subsonic,
and 3 as sonic at a level of significance. Out of all these cases, 2
flows cannot be strictly ascribed to one of the respective categories as they
change from the subsonic state to supersonic and vice versa. We labelled them
as a subclass of transonic flows. The proposed methodology provides an
automatic and scalable solution to identify small-scale flows and to classify
their velocities as either supersonic, subsonic, or sonic. We identified and
classified small-scale flow patterns in flaring loops. The results show that
the flows can be classified into four classes: sub-, super-, trans-sonic, and
sonic. The detected flows from AIA images can be analyzed in combination with
the other high-resolution observational data, such as Hi-C 2.1 data, and be
used for the development of theories of the formation of flow patterns.Comment: 13 pages, 7 figures, Accepted for publication in A&
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
Statistical properties of coronal hole rotation rates: Are they linked to the solar interior?
The present paper discusses results of a statistical study of the
characteristics of coronal hole (CH) rotation in order to find connections to
the internal rotation of the Sun. The goal is to measure CH rotation rates and
study their distribution over latitude and their area sizes. In addition, the
CH rotation rates are compared with the solar photospheric and inner layer
rotational profiles. We study coronal holes observed within latitude
and longitude degrees from the solar disc centre during the time span from the
1 January 2013 to 20 April 2015, which includes the extended peak of solar
cycle 24.We used data created by the Spatial Possibilistic Clustering Algorithm
(SPoCA), which provides the exact location and characterisation of solar
coronal holes using SDO=AIA 193 {\AA} channel images. The CH rotation rates are
measured with four-hour cadence data to track variable positions of the CH
geometric centre. North-south asymmetry was found in the distribution of
coronal holes: about 60 percent were observed in the northern hemisphere and 40
percent were observed in the southern hemisphere. The smallest and largest CHs
were present only at high latitudes. The average sidereal rotation rate for 540
examined CHs is degrees/d. Conclusions. The latitudinal
characteristics of CH rotation do not match any known photospheric rotation
profile. The CH angular velocities exceed the photospheric angular velocities
at latitudes higher than 35-40 degrees. According to our results, the CH
rotation profile perfectly coincides with tachocline and the lower layers of
convection zone at around 0.71 ; this indicates that CHs may be
linked to the solar global magnetic field, which originates in the tachocline
region.Comment: 8 pages, 8 figures, Accepted for publication in A&
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