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Register a new userPolar Coronal Plumes as Tornado-Like Jets
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We examine the dynamical behavior of white light polar plume structures in the inner corona that are observed from the ground during total solar eclipses, based on their EUV hot and cool emission line counterparts observed from space. EUV observations from SDO/AIA of a sequence of rapidly varying coronal hole structures are analyzed. Evidence of events showing acceleration in the 1.25 Mk line of Fe XII at 193 A is given. The structures along the plume show an outward velocity of about 140 kms-1 that can be interpreted as an upwards propagating wave in the 304 A and 171 A lines; higher speeds are seen in 193 A (up to 1000 km/s). The ejection of the cold He II plasma is delayed by about 4 min in the lowest layer and more than 12 min in the highest level compared to the hot 193 A behavior. A study of the dynamics using time-slice diagrams reveals that a large amount of fast ejected material originates from below the plume, at the footpoints. The release of plasma material appears to come from a cylinder with quasi-parallel edge-enhanced walls. After the initial phase of a longitudinal acceleration, the speed substantially reduces and the ejecta disperse into the environment. Finally, the detailed temporal and spatial relationships between the cool and hot components were studied with simultaneous multi-wavelength observations, using more AIA data. The outward-propagating perturbation of the presumably magnetic walls of polar plumes supports the suggestion that Alfven waves propagate outwardly along these radially extended walls.
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Both coronal plumes and network jets are rooted in network lanes. The relationship between the two, however, has yet to be addressed. For this purpose, we perform an observational analysis using images acquired with the Atmospheric Imaging Assembly (AIA) 171{AA} passband to follow the evolution of coronal plumes, the observations taken by the Interface Region Imaging Spectrograph (IRIS) slit-jaw 1330{AA} to study the network jets, and the line-of-sight magnetograms taken by the Helioseismic and Magnetic Imager (HMI) to overview the the photospheric magnetic features in the regions. Four regions in the network lanes are identified, and labeled ``R1--R4. We find that coronal plumes are clearly seen only in ``R1&R2 but not in ``R3&``R4, even though network jets abound in all these regions. Furthermore, while magnetic features in all these regions are dominated by positive polarity, they are more compact (suggesting stronger convergence) in ``R1&``R2 than that in ``R3&``R4. We develop an automated method to identify and track the network jets in the regions. We find that the network jets rooted in ``R1&``R2 are higher and faster than that in ``R3&``R4,indicating that network regions producing stronger coronal plumes also tend to produce more dynamic network jets. We suggest that the stronger convergence in ``R1&``R2 might provide a condition for faster shocks and/or more small-scale magnetic reconnection events that power more dynamic network jets and coronal plumes.
Solar coronal plumes long seemed to possess a simple geometry supporting spatially coherent, stable outflow without significant fine structure. Recent high-resolution observations have challenged this picture by revealing numerous transient, small-scale, collimated outflows (jetlets) at the base of plumes. The dynamic filamentary structure of solar plumes above these outflows, and its relationship with the overall plume structure, have remained largely unexplored. We analyzed the statistics of continuously observed fine structure inside a single representative bright plume within a mid-latitude coronal hole during 2016 July 2-3. By applying advanced edge-enhancement and spatiotemporal analysis techniques to extended series of high-resolution images from the Solar Dynamics Observatorys Atmospheric Imaging Assembly, we determined that the plume was composed of numerous time-evolving filamentary substructures, referred to as plumelets in this paper, that accounted for most of the plume emission. The number of simultaneously identifiable plumelets was positively correlated with plume brightness, peaked in the fully formed plume, and remained saturated thereafter. The plumelets had transverse widths of 10 Mm and intermittently supported upwardly propagating periodic disturbances with phase speeds of 190-260 km/s and longitudinal wavelengths of 55-65 Mm. The characteristic frequency (3.5 mHz) is commensurate with that of solar p-modes. Oscillations in neighboring plumelets are uncorrelated, indicating that the waves could be driven by p-mode flows at spatial scales smaller than the plumelet separation. Multiple independent sources of outflow within a single coronal plume should impart significant fine structure to the solar wind that may be detectable by Parker Solar Probe and Solar Orbiter.
Coronal plumes are bright magnetic funnels found in quiet regions (QRs) and coronal holes (CHs). They extend high into the solar corona and last from hours to days. The heating processes of plumes involve dynamics of the magnetic field at their base, but the processes themselves remain mysterious. Recent observations suggest that plume heating is a consequence of magnetic flux cancellation and/or convergence at the plume base. These studies suggest that the base flux in plumes is of mixed polarity, either obvious or hidden in SDO HMI data, but do not quantify it. To investigate the magnetic origins of plume heating, we select ten unipolar network flux concentrations, four in CHs, four in QRs, and two that do not form a plume, and track plume luminosity in SDO AIA 171 A images along with the base flux in SDO HMI magnetograms, over each flux concentrations lifetime. We find that plume heating is triggered when convergence of the base flux surpasses a field strength of 200 to 600 G. The luminosity of both QR and CH plumes respond similarly to the field in the plume base, suggesting that the two have a common formation mechanism. Our examples of non-plume-forming flux concentrations, reaching field strengths of 200 G for a similar number of pixels as for a couple of our plumes, suggest that a critical field might be necessary to form a plume but is not sufficient for it, thus, advocating for other mechanisms, e.g. flux cancellation due to hidden opposite-polarity field, at play.
To study the dynamics of coronal holes and the role of waves in the acceleration of the solar wind, spectral observations were performed over polar coronal hole regions with the SUMER spectrometer on SoHO and the EIS spectrometer on Hinode. Using these observations, we aim to detect the presence of propagating waves in the corona and to study their properties. The observations analysed here consist of SUMER spectra of the Ne VIII 770 A line (T = 0.6 MK) and EIS slot images in the Fe XII 195 A line (T = 1.3 MK). Using the wavelet technique, we study line radiance oscillations at different heights from the limb in the polar coronal hole regions. We detect the presence of long period oscillations with periods of 10 to 30 min in polar coronal holes. The oscillations have an amplitude of a few percent in radiance and are not detectable in line-of-sight velocity. From the time distance maps we find evidence for propagating velocities from 75 km/s (Ne VIII) to 125 km/s (Fe XII). These velocities are subsonic and roughly in the same ratio as the respective sound speeds. We interpret the observed propagating oscillations in terms of slow magneto-acoustic waves. These waves can be important for the acceleration of the fast solar wind.
The nature of flows in tornado-prominences is an open issue. While the AIA imager aboard the Solar Dynamics Observatory (SDO) allowed us to follow the global structure of a tornado-like prominence during five hours, the Interface Region Imaging Spectrograph (IRIS), and the Multi subtractive Double pass spectrograph (MSDP) permitted to obtain plasma diagnostics of its fine structures. We aim to address two questions. Is the observed plasma rotation conceptually acceptable in a flux rope magnetic support configuration with dips? How is the plasma density distributed in the tornado-like prominence? We calculated line-of-sight velocities and non-thermal line widths using Gaussian fitting for Mg II lines and bisector method for H-alpha line. We determined the electron density from Mg II line integrated intensities and profile fitting methods using 1D NLTE radiative transfer theory models. The global structure of the prominence observed in H-alpha, and Mg II h and k lines fits with a magnetic field structure configuration with dips. Coherent Dopplershifts in red- and blue-shifted areas observed in both lines were detected along rapidly-changing vertical and horizontal structures. However, the tornado at the top of the prominence consists of multiple-fine threads with opposite flows suggesting counter streaming flows rather than rotation. Surprisingly we found that the electron density at the top of the prominence could be larger (10^11 cm^{-3}) than in the inner part of the prominence. We suggest that the tornado is in a formation state with cooling of hot plasma in a first phase, and following that, a phase of leakage of the formed blobs with large transverse flows of material along long loops extended away of the UV prominence top. The existence of such long magnetic field lines on both sides of the prominence would avoid the tornado-like prominence to really turn around its axis.
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