4,985 research outputs found
Thermal and non-thermal emission from reconnecting twisted coronal loops
Twisted magnetic fields should be ubiquitous in flare-producing active
regions where the magnetic fields are strongly non-potential. It has been shown
that reconnection in helical magnetic coronal loops results in plasma heating
and particle acceleration distributed within a large volume, including the
lower coronal and chromospheric sections of the loops. This scenario can be an
alternative to the standard flare model, where particles are accelerated only
in a small volume located in the upper corona. We use a combination of MHD
simulations and test-particle methods, which describe the development of kink
instability and magnetic reconnection in twisted coronal loops using resistive
compressible MHD, and incorporate atmospheric stratification and large-scale
loop curvature. The resulting distributions of hot plasma let us estimate
thermal X-ray emission intensities. The electric and magnetic fields obtained
are used to calculate electron trajectories using the guiding-centre
approximation. These trajectories combined with the MHD plasma density
distributions let us deduce synthetic HXR bremsstrahlung intensities. Our
simulations emphasise that the geometry of the emission patterns produced by
hot plasma in flaring twisted coronal loops can differ from the actual geometry
of the underlying magnetic fields. The twist angles revealed by the emission
threads (SXR) are consistently lower than the field-line twist present at the
onset of the kink-instability. HXR emission due to the interaction of energetic
electrons with the stratified background are concentrated at the loop
foot-points in these simulations, even though the electrons are accelerated
everywhere within the coronal volume of the loop. The maximum of HXR emission
consistently precedes that of SXR emission, with the HXR light-curve being
approximately proportional to the temporal derivative of the SXR light-curve.Comment: (accepted for publication on A&A
Observation of multiple sausage oscillations in cool postflare loop
Using simultaneous high spatial (1.3 arc sec) and temporal (5 and 10 s)
resolution H-alpha observations from the 15 cm Solar Tower Telescope at ARIES,
we study the oscillations in the relative intensity to explore the possibility
of sausage oscillations in the chromospheric cool postflare loop. We use
standard wavelet tool, and find the oscillation period of ~ 587 s near the loop
apex, and ~ 349 s near the footpoint. We suggest that the oscillations
represent the fundamental and the first harmonics of fast sausage waves in the
cool postflare loop. Based on the period ratio P1/P2 ~ 1.68, we estimate the
density scale height in the loop as ~ 17 Mm. This value is much higher than the
equilibrium scale height corresponding to H-alpha temperature, which probably
indicates that the cool postflare loop is not in hydrostatic equilibrium.
Seismologically estimated Alfv\'en speed outside the loop is ~ 300-330 km/s.
The observation of multiple oscillations may play a crucial role in
understanding the dynamics of lower solar atmosphere, complementing such
oscillations already reported in the upper solar atmosphere (e.g., hot flaring
loops).Comment: 13 pages, 4 figures, accepted in MNRA
Prominence Activation by Coronal Fast Mode Shock
An X5.4 class flare occurred in active region (AR) NOAA11429 on 2012 March 7.
The flare was associated with very fast coronal mass ejection (CME) with its
velocity of over 2500 km/s. In the images taken with STEREO-B/COR1, a dome-like
disturbance was seen to detach from expanding CME bubble and propagated
further. A Type-II radio burst was also observed at the same time. On the other
hand, in EUV images obtained by SDO/AIA, expanding dome-like structure and its
foot print propagating to the north were observed. The foot print propagated
with its average speed of about 670 km/s and hit a prominence located at the
north pole and activated it. While the activation, the prominence was strongly
brightened. On the basis of some observational evidence, we concluded that the
foot print in AIA images and the ones in COR1 images are the same, that is MHD
fast mode shock front. With the help of a linear theory, the fast mode mach
number of the coronal shock is estimated to be between 1.11 and 1.29 using the
initial velocity of the activated prominence. Also, the plasma compression
ratio of the shock is enhanced to be between 1.18 and 2.11 in the prominence
material, which we consider to be the reason of the strong brightening of the
activated prominence. The applicability of linear theory to the shock problem
is tested with nonlinear MHD simulation
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