linkages between the atmosphere on the Earth and sun Phần 9

434 G.R. Gupta et al. have measured oscillations in coronal holes in the polar off-limbregions of the Sun. All these studies point to the presence of compressional waves thought to be slow magnetoacoustic waves as found by Deforest and Gurman (1998); O’Shea et al. (2006, 2007). Recently, Gupta et al. (2009) have reported the detection of these waves in the disk part of the polar coronal hole (hereafter PCH). They also find a difference in nature of the compressional waves between bright (network) and dark (internetwork) regions in the PCH. In this contribution, we extend such analysis to another dataset. More detail is given in Gupta et al. (2009). 2 Observations and Data Analysis The data used in this analysis were taken on 25 February 1997, during 00:00– 13:59UT with the 1 30000 slit of SUMER and an exposure time of 60s in the NIV 765A and NeVIII 770A lines in a southern PCH. Details of the data reduc-tion are given in Gupta et al. (2009). The chromosphere and transition region show enhanced-intensity network boundaries and darker internetwork cells. Presumably, the magnetic field is pre-dominantly concentrated at the network boundaries and, within coronal holes, the footpoints of coronal funnels emanate from these network boundaries. As the ob-serving duration of this dataset is very long, the locations of bright and dark pixels along the slit change with time. For this reason, we have analyzed the whole dataset pixel by pixel and timeframe by timeframe. For example, for one given moment, we first determined the average intensity along the slit. All pixels having an intensity higher than 1.25 times this average intensity were chosenas brightpixels.If such pixels are brightforat least 60min (or 60 timeframes), then these are considered to be a bright network location over that time interval. The bright pixel identification is done only for the low-temperature NIV line; the network pixels obtained from it are assumed to be the same in the higher-temperatureNeVIII line. 3 Results and Discussion Figure 1 shows a representative example of the oscillations measured in a bright region of the PCH. We use wavelet analysis to provide information on the temporal signal variation (Torrence and Compo 1998). Further details on this wavelet anal-ysis are found in Gupta et al. (2009); O’Shea et al. (2001) and references therein. Figure 1 shows oscillations of about 18min periodicity in both lines at the same lo-cation. This suggests that these two layers are linked by a propagatingwave passing from one layer to the other. To test this hypothesis and to ascertain the nature of the propagating waves, we measured phase delays in intensity and in Dopplershift be-tweenthetwolinesateachofthemeasurablepixelsalongtheslit forafullfrequency Network Loop Oscillations with EIS/Hinode A.K. Srivastava, D. Kuridze, T.V. Zaqarashvili, B.N. Dwivedi, and B. Rani Abstract We analyze a time sequence of HeII256.32A images obtained with EIS/Hinode, sampling a small magnetic loop in magnetic network. Wavelet anal-ysis indicates 11-minperiodicityclose to the loop apex. We interpret this oscillation as forcing through upward leakage by the fundamental acoustic eigenmode of the underlying field-free cavity. The observed loop length corresponds to the value pre-dicted from this mechanism. 1 Introduction Field-free cavities under bipolar magnetic canopies (Centeno et al. 2007) in the vicinityof magneticnetworkare likelyto serve as resonatorsforfast magnetoacous-tic waves (Kuridze et al. 2007). Srivastava et al. (2008) have studied the properties of the fundamental fast magnetoacoustic mode in brightened magnetic network. It leaks through the magnetic network into the upper solar atmosphere. Recently, Martine´z Gonza´lez et al. (2007) found evidence for low-lying loops in magnetic internetwork. In EIS/Hinode observations of bright magnetic network, we found a small loop located near the south pole. We search for magnetoacoustic oscillations in this loop through wavelet analysis. 2 Observations The observations were acquired on 11 March 2007 during 19:04–19:54UT in the study HPW005 QS Slot 60m. The slot-center position was .X D 11800; Y D 97300/, with a 4000 51200 field of view (Fig.1). The data were binned A.K. Srivastava () and B. Rani Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital, India D. Kuridze and T.V. Zaqarashvili Abastumani Astrophysical Observatory, Tbilisi, Georgia B.N. Dwivedi Department of Applied Physics, Institute of Technology, Banaras Hindu University, Varanasi, India S.S. Hasan and R.J. Rutten (eds.), Magnetic Coupling between the Interior 437 and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3-642-02859-5 50, Springer-Verlag Berlin Heidelberg 2010 Dynamical Evolution of X-Ray Bright Points with Hinode/XRT R. Kariyappa, B.A. Varghese, E.E. DeLuca, and A.A. van Ballegooijen Abstract We analyzed a 7-h long time sequence of soft X-ray images obtained on 14 April 2007 from a quiet region using the X-Ray Telescope (XRT) onboard Hinode. The aim was to observe intensity oscillations in coronal XBPs of differ-ent brightness and to study differences, if any, in the periodicity of the intensity variations and the heating mechanism during their dynamical evolution. We have compared the XRT images with GONG magnetograms using Coronal Modeling Software (CMS), and found that some of the XBPs are located at magnetic bipoles. The coronal XBPs are highly dynamic and oscillatory in nature, showing a wide variety of time scales in their intensity variations. 1 Introduction Coronal X-ray bright points (XBPs) were discovered using a soft X-ray telescope on a sounding rocket in the late 1960s (Vaiana et al. 1978). Their nature remained enigmatic. Later, using Skylab and Yohkoh/SXT X-ray images, XBPs were studied in detail (Golub et al. 1974; Longcope et al. 2001; Hara and Nakakubo-Morimoto 2003). The number of XBPs that is daily present on the visible hemisphere of the Sun varies from several hundred to a few thousand (Golub et al. 1974), with 800 on the entire solar surface at anygiventime (Zhanget al. 2001).The numberof coronal bright points varies inverselywith the solar activity cycle (Sattarovet al. 2002; Hara and Nakakubo-Morimoto2003).The XBP diameters are about 10–2000 (Golub et al. 1974). Their lifetime ranges from a few hours to a few days (Zhang et al. 2001; Kariyappa and Varghese 2008). In this contribution, we report the analysis of XBPs on soft X-ray images ob-tained from Hinode/XRT and on magnetograms from GONG. We briefly discuss the dynamical evolution of the XBPs in relation to the magnetic field. R. Kariyappa () and B.A. Varghese Indian Institute of Astrophysics, Bangalore, India E.E. DeLuca and A.A. van Ballegooijen Harvard-Smithsonian Center for Astrophysics, Cambridge, USA S.S. Hasan and R.J. Rutten (eds.), Magnetic Coupling between the Interior 440 and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3-642-02859-5 51, Springer-Verlag Berlin Heidelberg 2010 Dynamical Evolution of X-Ray Bright Points with Hinode/XRT 441 2 Results and Discussion We use a 7-h (17:00–24:00UT) time sequence of soft X-ray images obtained on 14 April 2007 with the X-Ray Telescope (XRT) onboardthe Hinode mission, using the Ti polyfilterforaquietregionnearthecenterofthesolardisk.We selected14XBPs for analysis, marking them as XBP1, XBP2, ..., XBP14, and two background,very dark coronal comparison regions as XBP15 and XBP16. The XRT images have been calibrated using the SSW subroutine xrt prep.pro (Kariyappa and Varghese 2008). We also obtained the full-disk magnetograms obtained with GONG during the XRT observing period. These magnetograms have been co-registered with the XRT images using the Coronal Modeling Software (CMS) developed by the fourth author. The XBPs, defined as the sites where intense brightness enhancement is seen, are highly dynamic in nature. We derived light curves for the XBPs by summing their brightness over small square image cut-outs covering the selected XBPs. Fig. 1 GONG magnetogram overlayed on an XRT image using CMS modeling. The magnetic field lines are computed from a potential-field extrapolation of the magnetogram Helicity at Photospheric and Chromospheric Heights S.K. Tiwari, P. Venkatakrishnan, and K. Sankarasubramanian Abstract In the solar atmosphere, the twist parameter ˛ has the same sign as magnetic helicity. It has been observed using photospheric vector magnetograms that negative/positive helicity is dominant in the northern/southern hemisphere of the Sun. Chromospheric features show dextral/sinistral dominance in the north-ern/southern hemisphere and sigmoids observed in X-rays also have a dominant sense of reverse-S/forward-S in the northern/southern hemisphere. It is of interest whether individual features have one-to-one correspondence in terms of helicity at different atmospheric heights. We use UBF H˛ images from the Dunn Solar Tele-scope (DST) andother H˛ data fromUdaipurSolar ObservatoryandBig Bear Solar Observatory.Near-simultaneousvector magnetogramsfrom the DST are used to es-tablish one-to-one correspondence of helicity at photospheric and chromospheric heights. We plan to extend this investigation with more data including coronal intensities. 1 Introduction Helicity is a physical quantity that measures the degree of linkage and twistedness in the field (Berger and Field 1984). It is derived from a volume integral over the scalar product of the magnetic field B and its vector potential A. Direct calculation of helicity is not possible due to the nonuniqueness of the vector potential A and the limited availability of data sampling different layers of the solar atmosphere. The force-free parameter ˛ estimates one component of helicity, that is, twist, the other component being writhe, which cannot be derived from the available data. This ˛ is a measure of degree of twist per unit axial length. It has the same sign as magnetic helicity (Pevtsov et al. 2008, Pevtsov 2008). It is now well known that negative/positive helicity dominates in the northern/southern hemisphere. For S.K. Tiwari () and P. Venkatakrishnan Udaipur Solar Observatory, Physical Research Laboratory, Udaipur, India K. Sankarasubramanian Space Astron. & Instrument. Div., ISRO Satellite Center, Bangalore, India S.S. Hasan and R.J. Rutten (eds.), Magnetic Coupling between the Interior 443 and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3-642-02859-5 52, Springer-Verlag Berlin Heidelberg 2010 ... - tailieumienphi.vn