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Register a new userHelical motions of fine-structure prominence threads observed by Hinode and IRIS
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Takenori Okamoto Joten
Publication date
2016
fields
Physics
and research's language is
English
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Fine-structure dynamics in solar prominences holds critical clues to understanding their physical nature of significant space-weather implications. We report evidence of rotational motions of horizontal helical threads in two active-region prominences observed by the emph{Hinode} and/or emph{IRIS} satellites at high resolution. In the first event, we found transverse motions of brightening threads at speeds up to 55~km~s$^{-1}$ seen in the plane of the sky. Such motions appeared as sinusoidal space--time trajectories with a typical period of $sim$390~s, which is consistent with plane-of-sky projections of rotational motions. Phase delays at different locations suggest propagation of twists along the threads at phase speeds of 90--270~km~s$^{-1}$. At least 15 episodes of such motions occurred in two days, none associated with any eruption. For these episodes, the plane-of-sky speed is linearly correlated with the vertical travel distance, suggestive of a constant angular speed. In the second event, we found Doppler velocities of 30--40~km~s$^{-1}$ in opposite directions in the top and bottom portions of the prominence, comparable to the plane-of-sky speed. The moving threads have about twice broader line widths than stationary threads. These observations, when taken together, provide strong evidence for rotations of helical prominence threads, which were likely driven by unwinding twists triggered by magnetic reconnection between twisted prominence magnetic fields and ambient coronal fields.
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We report on observations of recurrent jets by instruments onboard the Interface Region Imaging Spectrograph (IRIS), Solar Dynamics Observatory (SDO) and Hinode spacecrafts. Over a 4-hour period on July 21st 2013, recurrent coronal jets were observed to emanate from NOAA Active Region 11793. FUV spectra probing plasma at transition region temperatures show evidence of oppositely directed flows with components reaching Doppler velocities of +/- 100 km/s. Raster Doppler maps using a Si IV transition region line show all four jets to have helical motion of the same sense. Simultaneous observations of the region by SDO and Hinode show that the jets emanate from a source region comprising a pore embedded in the interior of a supergranule. The parasitic pore has opposite polarity flux compared to the surrounding network field. This leads to a spine-fan magnetic topology in the coronal field that is amenable to jet formation. Time-dependent data-driven simulations are used to investigate the underlying drivers for the jets. These numerical experiments show that the emergence of current-carrying magnetic field in the vicinity of the pore supplies the magnetic twist needed for recurrent helical jet formation.
We report on observations of a solar prominence obtained on 26 April 2007 using the Extreme Ultraviolet Imaging Spectrometer on Hinode. Several regions within the prominence are identified for further analysis. Selected profiles for lines with formation temperatures between log(T)=4.7-6.3, as well as their integrated intensities, are given. The line profiles are discussed. We pay special attention to the He II line which is blended with coronal lines. Our analysis confirms that depression in EUV lines can be interpreted by two mechanisms: absorption of coronal radiation by the hydrogen and neutral helium resonance continua, and emissivity blocking. We present estimates of the He II line integrated intensity in different parts of the prominence according to different scenarios for the relative contribution of absorption and emissivity blocking on the coronal lines blended with the He II line. We estimate the contribution of the He II 256.32 line in the He II raster image to vary between ~44% and 70% of the rasters total intensity in the prominence according to the different models used to take into account the blending coronal lines. The inferred integrated intensities of the He II line are consistent with theoretical intensities obtained with previous 1D non-LTE radiative transfer calculations, yielding a preliminary estimate for the central temperature of 8700 K, central pressure of 0.33 dyn/cm^2, and column mass of 2.5 10^{-4} g/cm^2. The corresponding theoretical hydrogen column density (10^{20} cm^{-2}) is about two orders of magnitude higher than those inferred from the opacity estimates at 195 {AA}. The non-LTE calculations indicate that the He II 256.32 {AA} line is essentially formed in the prominence-to-corona transition region by resonant scattering of the incident radiation.
Threads are the building blocks of solar prominences and very often show longitudinal oscillatory motions that are strongly attenuated with time. The damping mechanism responsible for the reported oscillations is not fully understood yet. To understand the oscillations and damping of prominence threads it is mandatory to investigate first the nature of the equilibrium solutions that arise under static conditions and under the presence of radiative losses, thermal conduction and background heating. This provides the basis to calculate the eigenmodes of the thread models. The nonlinear ordinary differential equations for hydrostatic and thermal equilibrium under the presence of gravity are solved using standard numerical techniques and simple analytical expressions are derived under certain approximations. The solutions to the equations represent a prominence thread, i.e., a dense and cold plasma region of a certain length that connects with the corona through a prominence corona transition region (PCTR). The solutions can also match with a chromospheric-like layer if a spatially dependent heating function localised around the footpoints is considered. We have obtained static solutions representing prominence threads and have investigated in detail the dependence of these solutions on the different parameters of the model. Among other results, we have shown that multiple condensations along a magnetic field line are possible, and that the effect of partial ionisation in the model can significantly modify the thermal balance in the thread and therefore their length. This last parameter is also shown to be comparable to that reported in the observations when the radiative losses are reduced for typical thread temperatures.
A comprehensive study of the physical parameters of active region fan loops is presented using the observations recorded with the Interface Region Imaging Spectrometer (IRIS), the EUV Imaging Spectrometer (EIS) on-board Hinode and the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) on-board the Solar Dynamics Observatory (SDO). The fan loops emerging from non-flaring AR~11899 (near the disk-center) on 19th November, 2013 are clearly discernible in AIA 171~{AA} images and those obtained in ion{Fe}{8} and ion{Si}{7} images using EIS. Our measurements of electron densities reveal that the footpoints of these loops are approximately at constant pressure with electron densities of $log,N_{e}=$ 10.1 cm$^{-3}$ at $log,[T/K]=5.15$ (ion{O}{4}), and $log,N_{e}=$ 8.9 cm$^{-3}$ at $log,[T/K]=6.15$ (ion{Si}{10}). The electron temperature diagnosed across the fan loops by means of EM-Loci suggest that at the footpoints, there are two temperature components at $log,[T/K]=4.95$ and 5.95, which are picked-up by IRIS lines and EIS lines respectively. At higher heights, the loops are nearly isothermal at $log,[T/K]=5.95$, that remained constant along the loop. The measurement of Doppler shift using IRIS lines suggests that the plasma at the footpoints of these loops is predominantly redshifted by 2-3~km~s$^{-1}$ in ion{C}{2}, 10-15~km~s$^{-1}$ in ion{Si}{4} and $~$15{--}20~km~s$^{-1}$ in ion{O}{4}, reflecting the increase in the speed of downflows with increasing temperature from $log,[T/K]=4.40$ to 5.15. These observations can be explained by low frequency nanoflares or impulsive heating, and provide further important constraints on the modeling of the dynamics of fan loops.
In this paper we study the soft X-ray (SXR) signatures of one particular prominence. The X-ray observations used here were made by the Hinode/XRT instrument using two different filters. Both of them have a pronounced peak of the response function around 10 A. One of them has a secondary smaller peak around 170 A, which leads to a contamination of SXR images. The observed darkening in both of these filters has a very large vertical extension. The position and shape of the darkening corresponds nicely with the prominence structure seen in SDO/AIA images. First we have investigated the possibility that the darkening is caused by X-ray absorption. But detailed calculations of the optical thickness in this spectral range show clearly that this effect is completely negligible. Therefore the alternative is the presence of an extended region with a large emissivity deficit which can be caused by the presence of cool prominence plasmas within otherwise hot corona. To reproduce the observed darkening one needs a very large extension along the line-of-sight of the region amounting to around 10$^5$ km. We interpret this region as the prominence spine, which is also consistent with SDO/AIA observations in EUV.
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