Apparent Flows above an Active Region Observed with the [ITAL]Transition Region and Coronal Explorer[/ITAL] (original) (raw)
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Characterization of intensity and spatial variations along coronal loops
Astronomy and Astrophysics, 2005
We describe dynamical features and evolutionary characteristics of brightening coronal loops. We describe intensity variations, both in space and time, along a coarse grain loop structure, confirming high speed velocity scenarios. We apply the method to TRACE space-born images that show a compound of several magnetic threads. MICA ground-based images display a unique non-resolved loop structure. We confirm that a coherent behavior of the intensity along neighboring magnetic tubes occurs, i.e. we obtain a similar pattern from both telescopes: each has two branches, suggesting the sliding down of plasma in both directions from a given position on the loop structure. The apparent sliding down occurs in approximately 12 min. After the first appearance, TRACE registers two reiterations of the phenomenon suggesting a wave-based explanation. The feasibility of wave-based and flow-based models is analyzed. In either case, in order to explain the coherent coronal behavior the scenario of apparently non-interacting coronal threads requires theoretical explanations that consider uniform chromospheric conditions covering the footpoints of all the related magnetic tubes. We suggest a characteristic longitude of coherence.
Coronal upflows from edges of an active region observed with EUV Imaging Spectrometer onboard Hinode
In order to better understand the plasma supply and leakage at active regions, we investigated physical properties of the upflows from edges of active region NOAA AR10978 observed with the EUV Imaging Spectrometer (EIS) onboard Hinode. Our observational aim is to measure two quantities of the outflows: Doppler velocity and electron density. These upflows in the corona, referred to as active region outflows (hereafter the outflows), were discovered for the first time by EIS due to its unprecedented high sensitivity and spectral resolution. Those outflows are emanated at the outer edge of a bright active region core, where the intensity is low (i.e., dark region). It is well known by a number of EIS observations that the coronal emission lines at the outflow regions are composed of an enhanced component at the blue wing (EBW) corresponding to a speed of ∼ 100 km s −1 , added to by the stronger major component almost at rest. This EBW can be seen in line profiles of Fe xii-xv whose formation temperatures are around log T [K] = 6.2-6.3. It has been suggested that the outflows are (1) an indication of upflows from the footpoints of coronal loops induced by impulsive heating in the corona, (2) induced by the sudden pressure change after the reconnection between closed active region loops and open extended loops located at the edge of an active region, (3) driven by the contraction occurring at the edge of an active region which is caused by horizontal expansion of the active region, and (4) the tips of chromospheric spicules heated up to coronal temperature. While a number of observations have been revealed such aspects of the outflows, however, their electron density has not been known until present, which is one of the important physical quantities to consider the nature of the outflows. In addition, the Doppler velocity at the transition region temperature (log T [K] ≤ 6.0) has not been measured accurately in the outflow regions because of the difficulties in EIS spectroscopic analysis (e.g., the lack of onboard calibration lamp for absolute wavelength, and the temperature drift of line centroids according to the orbital motion of the satellite). In this thesis, we analyzed the outflow regions in NOAA AR10978 in order to measure Doppler velocity within wide temperature range (log T [K] = 5.5-6.5) and electron density by using an emission line pair Fe xiv 264.78Å/274.20Å. Since EIS does not have an absolute wavelength reference onboard, we need another reference for the precise measurement of the Doppler velocity. In this thesis, we exploited Doppler velocity of the quiet region as the reference, which was studied in Chapter 3. EUV emission lines observed in the quiet region are known to indicate redshift corresponding to v ≃ 10 km s −1 at log T [K] ≤ 5.8, while those above that temperature have not been established where a number of emission lines observed with EIS exist. Since the corona is optically thin, spectra outside the limb are superposed symmetrically along the line of sight, which leads to the reasonable idea that the limb spectra take a Doppler velocity of v = 0. We derived the Doppler velocity of the quiet region at the disk center by studying the center-to-limb variation of line centroid shifts for eleven emission lines from the transition region and the corona. By analyzing the spectroscopic data which cover the meridional line of the Sun from the south pole to the north pole, we determined the Doppler velocity of the quiet region with 5.7 ≤ log T [K] ≤ 6.3 in the accuracy of ≃ 3 km s −1 for the first time. It is shown that emission lines below log T [K] = 6.0 have Doppler velocity of almost zero with an error of 1-3 km s −1 , while those above that temperature are blueshifted with gradually increasing magnitude: v = −6.3 ± 2.1 km s −1 at log T [K] = 6.25 1. 1 Positive (Negative) velocity indicates a motion away from (toward) us.
A Brightening Coronal Loop Observed byTRACE. I. Morphology and Evolution
The Astrophysical Journal, 2000
We analyze the transient brightening of a solar coronal loop observed, at high time cadence (30 s) and spatial resolution pixel size), with the T ransition Region and Coronal Explorer (T RACE) in the 171 (0A .5 band on 1998 June 26. The loop, located in AR 8253, is B1010 cm long and inclined with respect to A the vertical to the solar surface. Its geometry and shape do not change signiÐcantly during the brightening, which lasts for D2 hr and is preceded by highly dynamic events in nearby and perhaps interacting loops. The loop footpoints brighten Ðrst ; after D10 minutes, moving brightness fronts rise initially from the northern footpoint, and after another D7 minutes from the southern one, at an apparent speed D100 km s~1, the whole loop becoming clearly visible afterward. During the rising phase the loop evolves coherently as a single magnetic tube. The brightness proÐle is asymmetric with respect to the loop apex at all times ; the brightness contrast between the footpoints and the apex decreases with time from a ratio of D10 to D3. After the loop has become all visible, the several parallel Ðlaments which form it follow an independent evolution. Assuming a plasma temperature of B1 MK, we infer a plasma density of D6 ] 108 cm~3 and a pressure of D0.2 dyn cm~2 close to the loop apex at the luminosity maximum. A companion paper is devoted to modeling the rising phase of this event.
Observational Aspects of the Three-Dimensional Coronal Structure Over a Solar Activity Cycle
The Astrophysical Journal, 2010
Solar rotational tomography is applied to almost eleven years of Large Angle Spectrometric Coronagraph C2/Solar and Heliospheric Observatory data, revealing for the first time the behavior of the large-scale coronal density structures, also known as streamers, over almost a full solar activity cycle. This study gives an overview of the main results of this project. (1) Streamers are most often shaped as extended, narrow plasma sheets. The sheets can be extremely narrow at times ( 0.14 × 10 6 km at 4 R ). This is over twice their heliocentric angular thickness at 1 AU. (2) At most times outside the height of solar maximum, there are two separate stable large helmet streamer belts extending from mid-latitudes (in both north and south). At solar minimum, the streamers converge and join near the equator, giving the impression of a single large helmet streamer. Outside of solar minimum, the two streamers do not join, forming separate high-density sheets in the extended corona (one in the north, another in the south). At solar maximum, streamers rise radially from their source regions, while during the ascending and descending activity phases, streamers are skewed toward the equator. (3) For most of the activity cycle, streamers share the same latitudinal extent as filaments on the disk, showing that large-scale stable streamers are closely linked to the same large-scale photospheric magnetic configuration, which give rise to large filaments. (4) The poleward footpoints of the streamers are often above crown polar filaments and the equatorial footpoints are above filaments or active regions (or above the photospheric neutral lines which underlie these structures). The high-density structures arising from the equatorial active regions either rise and form the equatorial footpoints of mid-latitude quiescent streamers, or form unstable streamers at the equator, not connected to the quiescent streamer structure at higher latitude (so there are often three streamer sheets sharing the same extended longitudinal region). (5) Comparison between the tomography results and a potential field source surface model shows that streamers are not necessarily associated with a magnetic polarity reversal, but rather are regions containing field lines arising from widely separated sources at the Sun. We call these convergence sheets. (6) There is considerable differential rotation of streamers at high latitudes, which makes comparison between disk and coronal structure complicated. The presence of differential rotation has implications for many areas of coronal and heliospheric research.
Measurements of Flow Speeds in the Corona Between 2 and 30 R ☉
The Astrophysical Journal, 1997
Time-lapse sequences of white-light images, obtained during sunspot minimum conditions in 1996 by the Large Angle Spectrometric Coronagraph on the Solar and Heliospheric Observatory, give the impression of a continuous outÑow of material in the streamer belt, as if we were observing Thomson scattering from inhomogeneities in the solar wind. Pursuing this idea, we have tracked the birth and outÑow of 50È100 of the most prominent moving coronal features and Ðnd that :
Spectroscopic Diagnosis of Propagating disturbances in coronal loops: Waves or flows?
arXiv (Cornell University), 2012
The analysis of multiwavelength properties of propagating disturbances (PDs) using Hinode/EIS observations is presented. Quasi-periodic PDs were mostly interpreted as slow magnetoacoustic waves in early studies, but recently suggested to be intermittent upflows of the order of 50−150 km s −1 based on the Red-Blue (RB) asymmetry analysis of spectral line profiles. Using the forward models, velocities of the secondary component derived from the RB analysis are found significantly overestimated due to the saturation effect when its offset velocities are smaller than the Gaussian width. We developed a different method to examine spectral features of the PDs. This method is assuming that the excessive emission of the PD profile against the background (taken as that prior to the PD) is caused by a hypothetic upflow. The derived LOS velocities of the flow are on the order of 10−30 km s −1 from the warm (1−1.5 MK) coronal lines, much smaller than those inferred from the RB analysis. This result does not support the flow interpretation but favors of the early wave interpretation.
Geophysical Research Letters, 1995
The solar corona was observed with an externally occulted White Light Coronagraph carried on the SPAR-TAN 201-01 spacecraft for a 47 hour period beginning on April 11, 1993. At this phase of the descending solar magnetic activity cycle there were well developed coronal helmet streamers located over both the east and west limbs of the sun. Of additional interest in the SPARTAN data are the finer scale streamer structures observed in the low latitude corona which are partially resolved by the SP201-01 instrument. The purpose of this investigation was to determine the physical and morphological characteristics of the streamers and the fine scale ray structures observed in the region between streamers. A comparison of these low latitude rays with the polar rays observed in the north and south polar holes during the same flight suggest that they have similar morphology and physical characteristics.
Propagating intensity disturbances in fan-like coronal loops: flows or waves?
Quasi-periodic intensity disturbances propagating upward along the coronal structure have been extensively studied using EUV imaging observations from SOHO/EIT and TRACE. They were interpreted as either slow mode magnetoacoustic waves or intermittent upflows. In this study we aim at demonstrating that time series of spectroscopic observations are critical to solve this puzzle. Propagating intensity and Doppler shift disturbances in fanlike coronal loops are analyzed in multiple wavelengths using sit-and-stare observations from Hinode/EIS. We find that the disturbances did not cause the blue-wing asymmetry of spectral profiles in the warm (∼1.5 MK) coronal lines. The estimated small line-of-sight velocities also did not support the intermittent upflow interpretation. In the hot (∼2 MK) coronal lines the disturbances did cause the blue-wing asymmetry, but the double fits revealed that a high-velocity minor component is steady and persistent, while the propagating intensity and Doppler shift disturbances are mainly due to variations of the core component, therefore, supporting the slow wave interpretation. However, the cause for blueward line asymmetries remains unclear.
2016
Quasi-periodic intensity disturbances propagating upward along the coronal structure have been extensively studied using EUV imaging observations from SOHO/EIT and TRACE. They were interpreted as either slow mode magnetoacoustic waves or intermittent upflows. In this study we aim at demonstrating that time series of spectroscopic observations are critical to solve this puzzle. Propagating intensity and Doppler shift disturbances in fanlike coronal loops are analyzed in multiple wavelengths using sit-and-stare observations from Hinode/EIS. We find that the disturbances did not cause the blue-wing asymmetry of spectral profiles in the warm (∼1.5 MK) coronal lines. The estimated small line-of-sight velocities also did not support the intermittent upflow interpretation. In the hot (∼2 MK) coronal lines the disturbances did cause the blue-wing asymmetry, but the double fits revealed that a high-velocity minor component is steady and persistent, while the propagating intensity and Doppler shift disturbances are mainly due to variations of the core component, therefore, supporting the slow wave interpretation. However, the cause for blueward line asymmetries remains unclear.