Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userTemporal and Spatial Scales in Coronal Rain Revealed by UV Imaging and Spectroscopic Observations
98
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Coronal rain corresponds to cool and dense clumps in the corona accreting towards the solar surface, and is often observed above solar active regions. They are generally thought to be produced by thermal instability in the corona and their lifetime is limited by the time they take to reach the chromosphere. Although the rain usually fragments into smaller clumps while falling down, their specific spatial and temporal scales remain unclear. In addition, the observational signatures of the impact of the rain with the chromosphere have not been clarified yet. In this study, we investigate the time evolution of velocity and intensity of coronal rain above a sunspot by analyzing coronal images obtained by the AIA onboard the SDO satellite as well as the Slit-Jaw Images (SJIs) and spectral data taken by the IRIS satellite. We identify dark and bright threads moving towards the umbra in AIA images and in SJIs, respectively, and co-spatial chromospheric intensity enhancements and redshifts in three IRIS spectra, Mg II k 2796 Angstrom, Si IV 1394 Angstrom, and C II 1336 Angstrom. The intensity enhancements and coronal rain redshifts occur almost concurrently in all the three lines, which clearly demonstrates the causal relationship with coronal rain. Furthermore, we detect bursty intensity variation with a timescale shorter than 1 minute in Mg II k, Si IV and C II spectra, indicating that a length scale of rain clumps is about 2.7 Mm if we multiply the typical time scale of the busty intensity variation at 30 sec by the rain velocity at 90 $mathrm{km s}^{-1}$. Such rapid enhancements in the IRIS lines are excited within a time lag of 5.6 sec limited by the temporal resolution. These temporal and spatial scales may reflect the physical processes responsible for the rain morphology, and are suggestive of instabilities such as Kelvin-Helmholtz.
rate research
Read More
We report on the discovery of periodic coronal rain in an off-limb sequence of {it Solar Dynamics Observatory}/Atmospheric Imaging Assembly images. The showers are co-spatial and in phase with periodic (6.6~hr) intensity pulsations of coronal loops of the sort described by Auchere et al. (2014) and Froment et al. (2015, 2017). These new observations make possible a unified description of both phenomena. Coronal rain and periodic intensity pulsations of loops are two manifestations of the same physical process: evaporation / condensation cycles resulting from a state of thermal nonequilibrium (TNE). The fluctuations around coronal temperatures produce the intensity pulsations of loops, and rain falls along their legs if thermal runaway cools the periodic condensations down and below transition-region (TR) temperatures. This scenario is in line with the predictions of numerical models of quasi-steadily and footpoint heated loops. The presence of coronal rain -- albeit non-periodic -- in several other structures within the studied field of view implies that this type of heating is at play on a large scale.
Coronal rain is the well-known phenomenon in which hot plasma high in the Suns corona undergoes rapid cooling (from > 10^6 K to < 10^4 K), condenses, and falls to the surface. Coronal rain appears frequently in active region coronal loops and is very common in post-flare loops. This Letter presents discovery observations, which show that coronal rain is ubiquitous in the embedded bipole very near a coronal hole boundary. Our observed structures formed when the photospheric decay of active region leading sunspots resulted in a large parasitic polarity embedded in a background unipolar region. We observe coronal rain to appear within the legs of closed loops well under the fan surface, as well as preferentially near separatrices of the resulting coronal topology: the spine lines, null point, and fan surface. We analyze 3 events using SDO Atmospheric Imaging Assembly (AIA) observations in the 304, 171, and 211 {/AA} channels, as well as SDO Helioseismic and Magnetic Imager (HMI) magnetograms. The frequency of rain formation and the ease with which it is observed strongly suggests that this phenomenon is generally present in null-point topologies of this size scale. We argue that these rain events could be explained by the classic process of thermal nonequilibrium or via interchange reconnection at the null; it is also possible that both mechanisms are present. Further studies with higher spatial resolution data and MHD simulations will be required to determine the exact mechanism(s).
The solar atmosphere may be heated by Alfven waves that propagate up from the convection zone and dissipate their energy in the chromosphere and corona. To further test this theory, we consider wave heating in an active region observed on 2012 March 7. A potential field model of the region is constructed, and 22 field lines representing observed coronal loops are traced through the model. Using a three-dimensional (3D) reduced magneto-hydrodynamics (MHD) code, we simulate the dynamics of Alfven waves in and near the observed loops. The results for different loops are combined into a single formula describing the average heating rate Q as function of position within the observed active region. We suggest this expression may be approximately valid also for other active regions, and therefore may be used to construct 3D, time-dependent models of the coronal plasma. Such models are needed to understand the role of thermal non-equilibrium in the structuring and dynamics of the Suns corona.
Small and elongated, cool and dense blob-like structures are being reported with high resolution telescopes in physically different regions throughout the solar atmosphere. Their detection and the understanding of their formation, morphology and thermodynamical characteristics can provide important information on their hosting environment, especially concerning the magnetic field, whose understanding constitutes a major problem in solar physics. An example of such blobs is coronal rain, a phenomenon of thermal non- equilibrium observed in active region loops, which consists of cool and dense chromospheric blobs falling along loop-like paths from coronal heights. So far, only off-limb coronal rain has been observed and few reports on the phenomenon exist. In the present work, several datasets of on-disk H{alpha} observations with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar Telescope (SST) are analyzed. A special family of on-disk blobs is selected for each dataset and a statistical analysis is carried out on their dynamics, morphology and temperatures. All characteristics present distributions which are very similar to reported coronal rain statistics. We discuss possible interpretations considering other similar blob-like structures reported so far and show that a coronal rain interpretation is the most likely one. Their chromospheric nature and the projection effects (which eliminate all direct possibility of height estimation) on one side, and their small sizes, fast dynamics, and especially, their faint character (offering low contrast with the background intensity) on the other side, are found as the main causes for the absence until now of the detection of this on-disk coronal rain counterpart.
The spectrum of gyrosynchrotron emission from solar flares generally peaks in the microwave range. Its optically-thin, high-frequency component, above the spectral peak, is often used for diagnostics of the nonthermal electrons and the magnetic field in the radio source. Under favorable conditions, its low-frequency counterpart brings additional, complementary information about these parameters as well as thermal plasma diagnostics, either through gyrosynchrotron self-absorption, free-free absorption by the thermal plasma, or the suppression of emission through the so-called Razin effect. However, their effects on the low-frequency spectrum are often masked by spatial nonuniformity. To disentangle the various contributions to low-frequency gyrosynchrotron emission, a combination of spectral and imaging data is needed. To this end, we have investigated Owens Valley Solar Array (OVSA) multi-frequency images for 26 solar bursts observed jointly with Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during the first half of 2002. For each, we examined dynamic spectra, time- and frequency-synthesis maps, RHESSI images with overlaid OVSA contours, and a few representative single-frequency snapshot OVSA images. We focus on the frequency dependence of microwave source sizes derived from the OVSA images and their effect on the low-frequency microwave spectral slope. We succeed in categorizing 18 analyzed events into several groups. Four events demonstrate clear evidence of being dominated by gyrosynchrotron self-absorption, with an inferred brightness temperature of $geq10^8$~K. The low-frequency spectra in the remaining events are affected to varying degree by Razin suppression. We find that many radio sources are rather large at low frequencies, which can have important implications for solar energetic particle production and escape.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
