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1.
A time dependent model for the flow of gas in a spicule is studied. In this model, the flow occurs in a magnetic flux sheath. Starting from hydrostatic equilibrium, the flux sheath is allowed to collapse normal to itself. The collapse induces a flow of gas along the magnetic field and this flow is identified as a spicule. A variety of sheath geometries and velocity patterns for the normal flow have been studied. It is observed that a large curvature in the field geometry and a large initial value for the normal flow are necessary to achieve spicule-like velocities. The duration for which a large velocity of normal flow is required is much shorter than the average lifetime of a spicule. It is proposed that the initial rapid collapse occurs during an impulsive spicule phase and it is the subsequent gradual relaxation of the flow which is observed as a spicule. 相似文献
2.
Alok Ranjan Tiwary Shibu K. Mathew A. Raja Bayanna P. Venkatakrishnan Rahul Yadav 《Solar physics》2017,292(4):49
The Multi-Application Solar Telescope (MAST) is a 50 cm off-axis Gregorian telescope that has recently become operational at the Udaipur Solar Observatory (USO). An imaging spectropolarimeter is being developed as one of the back-end instruments of MAST to gain a better understanding of the evolution and dynamics of solar magnetic and velocity fields. This system consists of a narrow-band filter and a polarimeter. The polarimeter includes a linear polarizer and two sets of liquid crystal variable retarders (LCVRs). The instrument is intended for simultaneous observations in the spectral lines 6173 Å and 8542 Å, which are formed in the photosphere and chromosphere, respectively. In this article, we present results from the characterization of the LCVRs for the spectral lines of interest and the response matrix of the polarimeter. We also present preliminary observations of an active region obtained using the spectropolarimeter. For verification purposes, we compare the Stokes observations of the active region obtained from the Helioseismic Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) with that of MAST observations in the spectral line 6173 Å. We find good agreement between the two observations, considering the fact that MAST observations are limited by seeing. 相似文献
3.
Motivated by the various examples of spatial variability in the power of the acoustic spectrum, we attempted to look for spatial variability in the peak frequency of the spectrum. However, the determination of this peak frequency on a spatial scale of a single pixel (8 arc sec for the GONG data) is limited by the stochastic variations in the power spectrum presumably caused by the stochastic nature of the excitation process. Averaging over a large number of spectra (100 spectra from a 10 × 10 pixel area) produced stabler spectra. The peak frequencies of 130 such locations were found to be distributed with a FWHM of about 130 Hz. A map of the spatial variation of this peak frequency did not show any strong feature with statistically significant deviation from the mean of the distribution. Likewise, the scatter in the peak frequencies masked the detection of magnetic-field-induced changes in the peak frequency. On a much larger scale, the N latitudes showed a slightly lower value of the peak frequency as compared to the S latitudes, although the difference (25 Hz) is barely larger than the r.m.s. spread (20 Hz). 相似文献
4.
A few methods of estimating Fried's parameter (r
0) from specklegrams of solar features are described. Some of these methods were used to estimate r
0 for the speckle data obtained from Kodaikanal Observatory (KO), Uttar Pradesh State Observatory (UPSO) and Udaipur Solar Observatory (USO). The average value of r
0 was found to be 3 cm at USO and UPSO during our observations. At KO, values of r
0 ranging from 6 to 10 cm were estimated. 相似文献
5.
P. Venkatakrishnan 《Astrophysics and Space Science》1996,243(1):43-47
The mechanisms that could possibly heat the corona are briefly reviewed with emphasis on their observability. Observing enhanced wave flux at footpoints of active regions would confirm wave heating. Observation of nonthermal electrons in tiny coronal events (nanoflares) would confirm dissipation of current sheets. Presence of large scale flows in coronal arcades would underline the importance of turbulent resistivity for coronal heating. A comparison of HeI absorption in quiet and active regions demonstrates the difficulty of interpreting data that connect chromospheric dynamics with coronal heating. Finally, the implications of the search for observations of coronal heating processes are mentioned. 相似文献
6.
We investigate a potential field calculation for off disk-center vector magnetograms that uses all the three components of the measured field. There is neither any need for interpolation of grid points between the image plane and the heliographic plane nor for an extension or a truncation to a heliographic rectangle. Hence, the method provides the maximum information content from the photospheric field as well as the most consistent potential field independent of the viewing angle. The introduction of polarimetric noise produces a less tolerant extrapolation procedure than using the line-of-sight extrapolation, but the resultant standard deviation is still small enough for the practical utility of this method.National Research Council Resident Research Associate on leave from Indian Institute of Astrophysics, Bangalore 560034, India. 相似文献
7.
A filament disappearance event was observed on 22 May 2008 during our recent campaign JOP 178. The filament, situated in the Southern Hemisphere, showed sinistral chirality consistent with the hemispheric rule. The event was well observed by several observatories, in particular by THEMIS. One day, before the disappearance, Hα observations showed up- and down-flows in adjacent locations along the filament, which suggest plasma motions along twisted flux rope. THEMIS and GONG observations show shearing photospheric motions leading to magnetic flux canceling around barbs. STEREO A, B spacecraft with separation angle 52.4°, showed quite different views of this untwisting flux rope in He ii 304 Å images. Here, we reconstruct the three-dimensional geometry of the filament during its eruption phase using STEREO EUV He ii 304 Å images and find that the filament was highly inclined to the solar normal. The He ii 304 Å movies show individual threads, which oscillate and rise to an altitude of about 120 Mm with apparent velocities of about 100 km?s?1 during the rapid evolution phase. Finally, as the flux rope expands into the corona, the filament disappears by becoming optically thin to undetectable levels. No CME was detected by STEREO, only a faint CME was recorded by LASCO at the beginning of the disappearance phase at 02:00 UT, which could be due to partial filament eruption. Further, STEREO Fe xii 195 Å images showed bright loops beneath the filament prior to the disappearance phase, suggesting magnetic reconnection below the flux rope. 相似文献
8.
Sanjay Gosain Sanjiv Tiwari Jayant Joshi P. Venkatakrishnan 《Journal of Astrophysics and Astronomy》2008,29(1-2):107-111
The Solar Vector Magnetograph (SVM) at Udaipur Solar Observatory saw its first light in April 2005. The retrieval of vector fields from the imaging spectro-polarimetric observational data requires a substantial amount of computer post-processing. The GUI-based data reduction and analysis software have been developed to make the data processing pipeline user-friendly and less time-consuming. In this paper we describe these software packages. 相似文献
9.
K. Sankarasubramanian G. Srinivasulu A. V. Ananth P. Venkatakrishnan 《Journal of Astrophysics and Astronomy》2000,21(3-4):241-244
A Stokes Polarimeter has been developed using a masked CCD arrangement for the measurement of the vector magnetic field of
sunspots. Charge shifting within the CCD is used to record near simultaneous orthogonal polarisation. The testing of the Stokes
Polarimeter and the behavior of the integrated system combined with the Kodaikanal tower tunnel telescope will be discussed. 相似文献
10.
The observations of Stokes line profiles require an accurate knowledge of the instrumental polarisation caused by optical components in the path of the light beam. In this context we present a theoretical approach to the instrumental polarisation caused by the 3-mirror coelostat system of the Kodaikanal Solar Tower.Collaborative Program in Astronomy and Astrophysics, Department of Physics, Indian Institute of Science, Bangalore 560 012 India. 相似文献