FISS Observations of Vertical Motion of Plasma in Tiny Pores
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Pores can be exploited for the understanding of the interaction between small-scale vertical magnetic field and the surrounding convective motions as well as the transport of mechanical energy into the chromosphere along the magnetic field. For better understanding of the physics of pores, we investigate tiny pores in a new emerging active region (AR11117) that were observed on 26 October 2010 by the Solar Optical Telescope (SOT) on board Hinode and the Fast Imaging Solar Spectrograph (FISS) of the 1.6 meter New Solar Telescope (NST). The pores are compared with nearby small magnetic concentrations (SMCs), which have similar magnetic flux as the pores but do not appear dark. Magnetic flux density and Doppler velocities in the photosphere are estimated by applying the center-of-gravity method to the Hinode/Spectro-Polarimeter data. The line-of-sight motions in the lower chromosphere are determined by applying the bisector method to the wings of the Hα and the Ca ii 8542 Å line simultaneously taken by the FISS. The coordinated observation reveals that the pores are filled with plasma which moves down slowly and are surrounded by stronger downflow in the photosphere. In the lower chromosphere, we found that the plasma flows upwards inside the pores while the plasma in the SMCs is always moving down. Our inspection of the Ca ii 8542 Å line from the wing to the core shows that the upflow in the pores slows down with height and turns into downflow in the upper chromosphere while the downflow in the SMCs gains its speed. Our results are in agreement with the numerical studies which suggest that rapid cooling of the interior of the pores drives a strong downflow, which collides with the dense lower layer below and rebounds into an upflow.
KeywordsChromosphere Imaging spectroscopy Photosphere Pore Surface magnetic field
While sunspots can grow to tens of thousands of kilometers, pores are patches of strong magnetic field only about a few Mm wide at the photosphere without penumbral structures. Their simple magnetic field configuration allows investigation of the dynamic interaction between solar magnetic elements and granular convection, and of the transport of energy by flows or waves along strong magnetic field to the chromosphere and corona.
are magnetized areas whose (magnetic) radius is larger than its continuum radius (Keppens and Martinez Pillet 1996),
are structured with fine structures (e.g. an umbral dot and a light bridge),
are surrounded by downflow channels (Keil et al. 1999).
Recently, Hinode/SOT observation by Cho et al. (2010) confirmed that the plasma in tiny pores (R<2″) is always moving down, with speeds between 100 and 500 m s−1. They found that the darkness increases as the surrounding plasma flows toward them and decreases as the plasma moves away from them. The plasma in the tiny pores in the photosphere is always moving down and the pores are surrounded by a strong neighboring downward flow (highly red-shifted). There are several numerical simulations of magnetic flux tubes in the solar photosphere. For example, Cameron et al. (2007) and Knölker and Schüssler (1988) predicted downflows at the pores. Steiner et al. (1998) and Kato et al. (2011) carried out numerical simulations of dynamic interaction between magnetic flux tubes and the surrounding gas motion. Especially, Kato et al. (2011) reported that the convective downdrafts in the surroundings of a magnetic element in the photosphere leads to a slow and upwardly propagating magnetoacoustic wave, which develops into a shock wave in the chromospheric layers of the magnetic flux tube. Meanwhile, very few chromospheric observations above the pores and small magnetic concentrations (SMCs) have been made, since observations of chromospheric manifestations of pores/SMCs are difficult. This is a major impediment to the understanding of the physics of pores.
The Fast Imaging Solar Spectrograph (FISS; Ahn et al. 2008; Chae et al. 2012) in the Coudé room of the 1.6 m New Solar Telescope (NST; Goode et al. 2003, 2010) at Big Bear Solar Observatory is a unique system that can perform fast-scan spectroscopy of extended areas. Since the NST/FISS is suited to improve the current understanding of the dynamics above the pores/SMCs, we have undertaken a coordinated observation with the Solar Optical Telescope (SOT; Tsuneta et al. 2008), on board the Hinode satellite (Kosugi et al. 2007). Our observation target was a new emerging active region (AR11117) on 26 October 2010.
2 Observation and Analysis
We have carried out a joint observation of AR11117 with NST/FISS and Hinode/SOT from 25 October to 29 October 2010. The photospheric observation of the pores was mainly done by Hinode/SOT. G-band and Ca ii H-line (3968 Å) images were obtained with the Broad-band Filter Imager (BFI) of Hinode/SOT with a pixel size of 0″.16 and 30 s time cadence for a field of view (FOV) of 75″×75″. Basic data processing was performed by using the SolarSoft package ( http://www.lmsal.com/solarsoft ). G-band images were aligned to track the pores and SMCs by using the local correlation tracking technique, and the Ca ii images were shifted by the same amount to align them with simultaneous G-band images by assuming the same pointing of the G-band and Ca ii images by the BFI. The Spectro-Polarimeter (SP) of Hinode/SOT recorded the magnetically sensitive Fe i 6301.5/6302.5 lines with a spectral resolution of ≈ 21 mÅ. The SP mapped the region with a normal mode raster scan of 512 steps and time cadence of ≈ 50 min. The effective pixel size was 0″.16 along the slit and 0″.15 across it. The field of view was about 75″×81″. Dark, flat, and wavelength corrections on the SP data were performed by using the SolarSoft package. We have derived line-of-sight magnetic field strengths and line-of-sight velocities from the SP data by using the center-of-gravity (COG) method of Rees and Semel (1979). The SP scans were aligned with BFI images by over-plotting SP continuum data on G-band images.
For the observation of the lower chromosphere, we ran NST/FISS during 17 – 19 UT from 25 October to 29 October. Our observation target was the pores near AR11117. The seeing at the Big Bear Observatory was fairly good on 26 October so that we selected FISS data taken on that day for the present study. FISS scanned the region including the pores located in the small box of Figure 1(a). The FISS obtained raster-scanned spectral images in two spectral lines, Hα and Ca ii 8542 Å, simultaneously. The FISS mapped the region with the default mode in which the scanner moves one step, waits for each camera to take one frame, and then moves to the next step. Each step took 0.15 s and the slit was moved over 400 steps. The raster cadence was about 63 s. The effective pixel size was 0″.16 along and across the slit. The number of pixels along the slit was about 256 so that the field of view was about 40″ in the slit direction. The scan width was about 64″. The spectral resolution of FISS is approximately 19 mÅ and 21 mÅ in Hα and Ca ii 8542 Å, respectively. Details of basic processing including data acquisition, flat fielding, correction, and compression are described by Chae et al. (2012). For wavelength calibration of the spectral lines, we used the two telluric lines of water vapor in the vicinity of each line (6561.097/6565.545 Å for Hα, 8540.817/8546.222 Å for Ca ii).
3.1 General Characteristics of the Pore and SMC
Why do not the SMCs appear dark in the G-band image even though they are similar to the pores in magnetic flux density? A standard explanation by Spruit and Zwaan (1981) is that pores as well as sunspots are dark by inhibition of convection, while smaller flux tubes such as SMCs may not necessarily be dark because of radiative energy input from their hot walls into the evacuated interior. For a similar reason, the filling fraction of magnetic field in a single pixel was proposed as an alternative explanation of the difference between pores and SMCs (Knölker and Schüssler 1988). Morinaga et al. (2008) reported that the filling factor of pores (0.8 – 0.9) is bigger than that of G-band bright points (0.6). We have estimated the mean filling factor values inside our pores by applying the Milne–Eddington (ME) inversion method (Skumanichi and Lites 1987) to our SP data. As a result, we found that the filling factor in pores (0.85 for P1 and 0.84 for P2) is a little bigger than that of SMCs (0.81 for SMC1 and 0.77 for SMC2), being in agreement with these studies.
The line-of-sight velocity in the photosphere deduced from SP data shown in Figure 3 suggests that the tiny pores are surrounded by strong downflows, and are filled with weak downflows inside them. This is consistent with the results of Cho et al. (2010), who suggested that slow downflows inside pores and fast downflows outside may be associated with the formation and maintenance of pores. The downflow channels around the pores and SMCs were well known from previous observations (e.g. Keil et al. 1999; Nagata et al. 2008; Morinaga et al. 2008) and from numerical simulations (Cameron et al. 2007; Spruit 1979).
The pores considered in this study are fairly small (diameter < 2″). The time evolution of tiny pores may be different from ordinary pores so that it might be interesting to investigate how these intense magnetic structures evolve during our observation. Unfortunately, there were no SDO/HMI data and Hinode/SOT Narrow-band Filter Imager data during our observation period, so we could not directly inspect the evolution of the magnetic field. However, we examined the evolution of the pores by using Hinode/SOT-BFI G-band observation. The G-band movie (electronic supplementary material) shows that the pores underwent temporal change in intensity and size, but they did not significantly grow or decay during the observation.
3.2 Vertical Motion in the Lower Chromosphere
Since the intensity level in the line spectra reflects the formation height as shown in Figure 2 (Cauzzi et al. 2008, 2009), the bisectors measured from the wings of the line represent the Doppler shift at a lower altitude of the atmosphere. In this study, we considered the wavelength ranges of Δλ=1.0 – 2.5 Å in Ca ii 8542 Å, and Δλ=1.9 Å and Δλ=0.5 – 1.2 Å in Hα, respectively.
As shown in Figure 5, the penumbra shows the blueshift (upflows), while most of the quiet region has redshift (downflows). Pores and SMCs are located at the boundary of the redshift cells. The white contours in Figure 5 showing the level of 1500 gauss (G) delineate the boundaries of strong magnetic patches obtained by Hinode/SP observation, of which spatial resolution is about 0″.2. The effective spatial resolution of the FISS map is about 1″ (Chae et al. 2012), which is similar to the seeing limit at BBSO; therefore the accuracy of the alignment between the NST/FISS and Hinode/SOT is similar to the resolution of the FISS map. As compared with the Hinode/SP Doppler map in Figure 3(d), Figure 5 shows structures larger in scale and therefore the characteristic scales of the velocity field look different. This may be because granule-size photospheric features seen by Hinode/SP disappear in the lower chromosphere, and/or the spatial resolution or sensitivity of NST/FISS is lower than that of Hinode/SP. In both images, we found upflows on the pores and downflows at the SMCs. The upflow areas at the location of the pores in Figure 5 look larger than the photospheric size of pores in the G-band image (Figure 3(a)) and they are slightly shifted in position relative to the pores. It may be related to the expansion of the flux tube, or swaying of it by the ambient horizontal flows in the upper atmosphere.
We inspected the line-of-sight velocity change across the wavelength of the line. As a result, we found that the velocity changes with wavelength in Ca ii 8542 Å but this is not the case in Hα. This is because the Hα line is formed either in chromospheric fibrils or in the deep photosphere whereas the Ca ii 8542 Å line samples the layer between them (Cauzzi et al. 2008, 2009). We speculate that the velocity changes with wavelength seen in Ca ii 8542 Å may reflect an intrinsic property of the vertical flows above the pores/SMCs in the lower chromosphere.
3.3 Evolution of the Vertical Motion
The time evolution of pores and SMCs and their interaction with the surrounding gas motions can be thoroughly studied when the structures are stably imaged with high spatial resolution and sensitivity. Unfortunately, the performance of currently working low-order adaptive optics of NST/FISS is not satisfactory in this regard.
4 Discussion and Conclusion
We have derived the physical characteristics of small pores based on the simultaneous observation of Hinode/SOT and NST/FISS. To our knowledge, this is the first detailed report on the connection of LOS plasma flows in tiny pores between the photosphere and the lower chromosphere through the determination of the chromospheric LOS velocities in pores and SMCs simultaneously, as well as their photospheric LOS velocities.
Our observations on the dynamic property of flux tubes could be understood by considering a dynamic interaction between solar magnetic elements and granular convection simulated by Steiner et al. (1998) and Kato et al. (2011). According to their two-dimensional MHD simulation, the interior of a pore cools rapidly and induces strong downflow around the pore owing to radiative losses and suppression of energy transport by convection in the pore. Their simulation predicts that the downflow turns into upflow after interacting with a dense and deep layer under the pore. This theoretical description is well consistent with our observations of LOS velocities in the photosphere and low chromosphere. It is likely that only strong downflow can excite an upward propagating shock wave as predicted by Grossmann-Doerth, Schüssler, and Steiner (1998), and it is expected that the upward shock is continuously observed in the upper photosphere and lower chromosphere.
In SMCs, we detected downflows, but no upflows at all. This might be explained by the absence of localized rapid cooling in SMCs because of the heat coming from the surroundings. In other words, the existence of downflows only suggests that a more or less continuous process may be more important in SMCs than a localized, impulsive cooling process unlike pores. If the downflow at the photosphere is caused by the convective collapse, the time scale of the event is several minutes (e.g. Narayan 2011). Persistent downflows over 20 min might represent the spatially averaged property of multiple flux tubes which are subject to convective instability.
In summary, we observed upward-directed dynamic motions at the tiny pores in the lower chromosphere by inspecting the wing spectra of chromospheric lines, which might be related to upwardly propagating magnetoacoustic waves predicted by Steiner et al. (1998) and Kato et al. (2011).
We are expecting that the high-order adaptive optics system for NST will provide stable observations with high spatial/spectral/temporal (10 s) resolutions, which will contribute to understanding the nature of the upward motion in the pores and SMCs, as well as their interaction with ambient plasma.
We are grateful to the referee for helpful and constructive comments. This work was supported by the “Development of Korea Space Weather Center” of KASI and the KASI basic research funds and by the Korea Research Foundation Grant funded by the Korean Government (KRF-2008-220-C00022, 2011-0028102). One of the authors (K.-S.C.) is very thankful to R.J. Rutten for his valuable comments and to S. Tsuneta for supporting of collaborative visiting in NAOJ, where the part of present work has been carried out. Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in cooperation with ESA and NSC (Norway).
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