Linear Polarization in H-alpha Flares
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|1st Author:||Tomoko KAWATE|
|2nd Author:||Yoichiro HANAOKA|
|Published:||4 February 2019|
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The solar chromosphere plays an essential role in the development of solar flares, and the Hα line historically led the way to much of what we now know about flare physics. Such an emission line can in principle have polarization, which could greatly extend its diagnostic potential.
The origin of the linear polarization in chromospheric lines during solar flares is not fully understood yet. Linear polarization found in flare ribbons, especially in Hα, has been investigated and reported by many authors as event studies.. A large-scale survey (Ref) analyzed 30 flares observed with imaging and spectropolarimetry techniques in Hα using high-frequency modulation, and concluded that no flares show such significant linear polarization signals above 0.07%. This conflicted with earlier positive observational results, suggesting spurious polarization signals caused by seeing or by flaring structures evolving in time. To investigate the polarization during flares further, a high-speed polarization modulation is required to suppress spurious signals to as low a level as possible. Here we report new results (Ref. ), both statistical and event-based, of Hα linear polarization in flaring regions taken with a reasonably high-frequency modulation as well as with high temporal and spatial resolutions.
A statistical approach
Our polarization measurements used ferroelectric liquid crystal polarimeter, which was installed in one of the tubes of the Solar Flare Telescope of the National Astronomical Observatory of Japan. The frequency of the polarization modulation in our observations was about 120 Hz, which greatly reduces the effects of seeing. The cadence of the polarization data was 1 s, at spatial sampling 2"/pixel. This instrument operated regularly between 2004 and 2005, with flares automatically detected on the basis of intensity increase at the Hα line center. As a result, 71 Hα flares, including 64 GOES events, were recorded between 2004 July and 2005 December. The GOES events include 4 X-class flares and 13 M-class flares.
Examining the spatially- and temporally-binned pixels (macropixels) that showed a flare enhancement, we investigated linear polarization signals in flare ribbons in the 71 events. In Figure 1, 4 macropixels show polarization exceeding the maximum polarimetric error (0.65%). All of the four macropixels above the error are observed during an identical flare on December 2, 2005, and they show net polarization. Therefore, the detection of the polarization in these macropixels is considered to be reliable, and that the polarization signal has a solar origin.
Significant linear polarization in an Hα flare ribbon
We investigate more details of the event in which the strong polarization signals appear. The event was an M6.5 class flare occurring in NOAA 10826, located at S03E19. Just after the flare, strong coronal dimming appeared near the active region. Figure 2 shows Stokes IQUV images in Hα as well as images taken with other instruments at the hard X-ray peak time. Two bright sources appeared in the hard X-ray (50-100 keV) image at solar helioprojective coordinates (-300, -90) and (-290, -80), and they were connected by soft X-ray (12-25 keV) loop-like structures. The two hard X-ray sources are located in the opposite magnetic polarities seen in the photospheric line-of-sight magnetogram, and this fact suggests that the soft X-ray loops correspond to magnetic field lines connecting the two hard X-ray sources. The Hα intensity image shows the brightest source corresponding to the X-ray sources, and in addition, two other bright sources located at (-340, -87) and (-265, -70). They are in opposite magnetic polarities, and connected by a bright EUV loop seen in the 195Â image. Thus, we suggest that small arch structures connect two hard X-ray footpoints and that a loop over the arch structures lies approximately along the magnetic neutral line as shown in the cartoon in Figure 2 (lower right). Among the four Hα footpoints, strong Q/I and U/I polarization signals of ~1% appeared at (-340, -87), which we interpret as one of the footpoints of the EUV loop. The averaged degree of the linear polarization at the hard X-ray sources is less than 0.2% which is below the polarimetric error level. These facts suggest that neither hard X-ray, soft X-ray, nor Hα fluxes correlate with the Hα linear polarization.
We found only one event that showed significant linear polarization signals among our 71 Hα flares including 64 events with GOES classifications. Many preceding papers have reported the characteristics of the linear polarization in Hα for the events with apparently significant polarization signals. In the cases where the polarization was not detected, it has been considered that the sensitivity of the polarimetry was insufficient. The high-frequency modulation technique in our datasets now establish that linear polarization does not appear in most of the flares, but there are a few flares which actually show such an effect.
From our statistical results, we conclude that linear polarization in Hα with a few percent is rarely observed. A mechanism for producing Hα linear polarization is therefore not a commonly found phenomenon in in ordinary flares. Furthermore, the result that only one minor kernel in our detected flare showed significant polarization means that not all the kernels in a flare necessarily have a common condition relating to the production of the linear polarization. Here we focus on CMEs and eruptions. From some γ-ray observations of behind-the-limb flares, protons can be accelerated by CME shocks and propagate toward solar surface along the magnetic field, i.e., toward the footpoints of coronal loops (Ref.,). The strong linear polarization appeared at one of the footpoints of a coronal loop in the flare studied here, and the direction of polarization was aligned with the coronal loop just above the footpoint. Thus, it is possible that the observed polarization was produced through impact polarization caused by accelerated protons, not electrons. This indirect evidence meets the scenario that linear polarization is produced by impact polarization of accelerated protons, but we cannot explain the degree of the polarization of 1%, as suggested in Ref. . More investigations with highly accurate polarization measurements of solar flares and with numerical simulations of radiative magneto-hydrodynamics at flare kernels in the chromosphere are needed to clarify the origin of linear polarization at H-alpha in solar flares.