Publications (10)5.55 Total impact
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Article: High Resolution Observations using Adaptive Optics: Achievements and Future Needs
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ABSTRACT: Over the last few years, several interesting observations were obtained with the help of solar Adaptive Optics (AO). In this paper, few observations made using the solar AO are enlightened and briefly discussed. A list of disadvantages with the current AO system are presented. With telescopes larger than 1.5m are expected during the next decade, there is a need to develop the existing AO technologies for large aperture telescopes. Some aspects of this development are highlighted. Finally, the recent AO developments in India are also presented.01/2008; -
Article: Euv Spectroscopy of the Sunspot Region Noaa 7981 Using Soho – II. Velocities and Line Profiles
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ABSTRACT: We have studied the dynamics in the sunspot transition region between the chromosphere and the corona and investigated the extension of the flow field into the corona. Based on EUV spectra of a medium size sunspot and its surroundings, NOAA 7981, observed with CDS and SUMER on SOHO, we derive line-of-sight velocities and study the line profiles for a series of emission lines.The flow field in the low corona is found to differ markedly from that in the transition region. In the transition region the relative line-of-sight velocity shows an upflow in the umbra and relatively large areas with downflow that cover part of the penumbra. The spatial extent of these areas with upflow and downflow increases with increasing temperature in the transition region, but the whole flow field changes character as the temperature increases from the upper transition region to the low corona. Based on a calibration of the SUMER wavelength scale we find that the entire sunspot transition zone appears to be moving downwards towards the chromosphere. The relation between this finding and the general tendency for transition-region lines to show a net red shift is discussed.Several of the transition-region spectral line profiles are observed to show two line components with Gaussian shape and line-of-sight velocities that differ markedly. Several of the line profiles that are composed of two spectral line components occur close to the dividing line between up- and downflow. A discussion of this observation is presented. In small regions with spatial extent of a few arc sec we detect enhanced continuum emission underlying explosive events. The similarities between explosive events with continuum emission and the moustaches observed in H close to sunspots are so striking that we are tempted to introduce the notation transition-region moustaches.Solar Physics 04/1998; 179(2):279-312. · 2.78 Impact Factor -
Article: Euv Spectroscopy of the Sunspot Region Noaa 7981 Using Soho – I. Line Emission and Time Dependence
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ABSTRACT: EUV spectra of a medium-size sunspot and its surroundings, NOAA 7981, were obtained on 2 August 1996 with the Coronal Diagnostic Spectrometer (CDS) and the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) on the Solar and Heliospheric Observatory (SOHO). The spectral lines formed in the transition region and corona show considerable structure and large deviations from a uniform spatial distribution over the active region. Enhanced EUV emissions in transition region lines are concentrated in small regions outside the umbra of the sunspot throughout most of the observing sequence. Only during a short, active period do we find an enhanced line emission that reaches into the umbra. Preliminary values for the umbral intensity are given.Marked changes are detected between the spatial distribution of line emission in the transition region and the low corona. The difference is not limited to cool and hot non-flaring loops not being cospatial, but includes differences both regarding the time variability and the orientation and size of the emission features. Whereas both rapid ( 4 and 2 min) and slow ( 10 and 12 h) temporal variations are found in the chromosphere (Hei 584 ) and transition region (Ov 629 ), the response in the low corona (Mgix 368 ) is slow ( 5 h). Furthermore, marked differences between the spatial distributions in the Mgviii 315 , Mgix 368 lines formed in the low corona and the coronal Fexiv 334 , Fexvi 360 lines are detected.Solar Physics 03/1998; 179(1):43-74. · 2.78 Impact Factor -
Article: EUV Sunspot Plumes Observed with SOHO
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ABSTRACT: Bright EUV sunspot plumes have been observed in five out of nine sunspot regions with the Coronal Diagnostic Spectrometer -- CDS on SOHO. In the other four regions the brightest line emissions may appear inside the sunspot but are mainly concentrated in small regions outside the sunspot areas. These results are in contrast to those obtained during the Solar Maximum Mission, but are compatible with the Skylab mission results. The present observations show that sunspot plumes are formed in the upper part of the transition region, occur both in magnetic unipolar-- and bipolar regions, and may extend from the umbra into the penumbra. Comment: 8 pages, 3 figures, to be published in ApJ Letters01/1998; -
Article: Statistical Properties of Asymmetries in and around Sunspots
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Article: The Non-Uniformity in the Sunspot Transition Region
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Article: Three Dimensional EUV Imaging of Sunspot Regions Observed with SOHO
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Article: EUV Line Emission and Time Dependence in the Sunspot Region NOAA 7981
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Article: Velocity and Magnetic Fields in and around Pores
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Article: EUV spectroscopy of the sunspot region NOAA 7981 using SOHO I. Line emission and tune dependence
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- Solar Physics (2)
Institutions
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1998
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University of Oslo
- Institute of Theoretical Astrophysics
Oslo, Oslo, Norway
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