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Browsing by Author "Hasan, S. S"

Browsing by Author "Hasan, S. S"

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  • Kink and Longitudinal Oscillations in the Magnetic Network on the Sun: Nonlinear Effects and Mode Transformation 
    Hasan, S. S; Kalkofen, W; van Ballegooijen, A. A; Ulmschneider, P (The American Astronomical Society, 2003-03)
    We examine the propagation of kink and longitudinal waves in the solar magnetic network. Previously, we investigated the excitation of network oscillations in vertical magnetic flux tubes through buffeting by granules and ...
  • The Linear Response of a Magnetic Flux Tube to Buffeting by External p-Modes. I 
    Hasan, S. S (The American Astronomical Society, 1997-05)
    The linear response of a thin vertical magnetic flux tube to buffeting by p-modes in the ambient atmosphere is examined with the aim of understanding the interaction of acoustic modes with sausage tube waves. The idealized ...
  • Magnetic flux tubes and activity on the sun 
    Hasan, S. S (Springer, 2003)
    Activity on the Sun is associated with magnetic fields, involving a complex interaction between the field and plasma. In this review I focus on three fundamental aspects of magnetic activity: (a) the generation, storage ...
  • A magneto-hydrodynamic study of pre-flare loops 
    Hasan, S. S (Institute of Theoretical Astrophysics, 1979)
    The magnetohydrodynamic equilibrium and stability of a configuration which may apply to solar pre-flare loops is analyzed based on the following scenario: (1) the loops exist much prior to the flare and in equilibrium with ...
  • Magnetoatmospheric oscillations in sunspot umbrae 
    Hasan, S. S (The American Astronomical Society, 1990-01)
    The nature of umbral oscillations is investigated using an empirical model for the sunspot umbra, based on the model of Maltby et al. (1986). Approximating the sunspot as a thick flux tube of circular cross section, the ...
  • Magnetohydrodynamic equilibrium and stability of pre-flare loops - Constant pitch field 
    Hasan, S. S (D. Reidel Publishing Co.,, 1980-08)
    The equilibrium and stability of a loop in which energy storage occurs prior to a solar flare is discussed. Working on the hypothesis, that the onset of the flare begins only after sufficient magnetic energy has been stored ...
  • Magnetohydrostatic equilibrium in starspots: dependences on color (Teff) and surface gravity (g) 
    Rajaguru, S. P; Hasan, S. S (Cambridge University Press, 2011-08)
    Temperature contrasts and magnetic field strengths of sunspot umbrae broadly follow the thermal-magnetic relationship obtained from magnetohydrostatic equilibrium. Using a compilation of recent observations, especially in ...
  • MHD waves on the Sun 
    Hasan, S. S (Indian Association for the Cultivation of Science, 1992)
    Theoretical aspects of wave propagation in an atmosphere with a vertical magnetic field are reviewed. Starting from the ideal magneto hydrodynamic equations. The differential equations governing linear oscillations presented ...
  • Mode classification and wave propagation in a magnetically structured medium 
    Hasan, S. S; Sobouti, Y (Blackwell, 1987-09)
    The authors examine the structure of motions that can occur in a vertical magnetic flux tube with a rectangular cross-section. A polytropic stratification is assumed in the vertical direction. The authors use a gauged ...
  • Modelling sunspot equilibrium through a solution of the time dependent MHD equations 
    Gangadhara, R. T; Hasan, S. S (Astronomical Society of India, 1996)
    We numerically simulate the dynamical relaxation to equilibrium of a sunspot. Treating the sunspot as a thick axisymmetric flux tube in cylindrical geometry, we solve the time dependent MHD equation to examine the evolution ...
  • Modulation in the solar irradiance due to surface magnetism during cycles 21, 22 and 23 
    Jain, K; Hasan, S. S (EDP Sciences, 2004-10)
    Magnetic field indices derived from synoptic magnetograms of the Mt. Wilson Observatory, i.e. Magnetic Plage Strength Index (MPSI) and Mt. Wilson Sunspot Index (MWSI), are used to study the effects of surface magnetism on ...
  • National Large Solar Telescope of India 
    Hasan, S. S (Astronomical Society of India, 2011)
    The National Large Solar Telescope NLST will be a state-ofthe- art 2-m class telescope for carrying out high-resolution studies of the solar atmosphere. Sites in the Himalayan region at altitudes greater than 4000-m ...
  • NLST: India's National Large Solar Telescope 
    Hasan, S. S; Soltau, D; Kärcher, H; Süß, M; Berkefeld, T (Wiley-Blackwell, 2010-06)
    This article introduces the new Indian 2 m telescope which has been designed by MT Mechatronics in a detailed conceptual design study for the Indian Institute of Astrophysics, Bangalore. We describe the background of the ...
  • NLST: the Indian National Large Solar Telescope 
    Hasan, S. S; Soltau, D; Kärcher, H; Süss, M; Berkefeld, T (SPIE-The International Society for Optical Engineering, 2010-07)
    India is planning a new solar telescope with an aperture of 2-m for carrying out high resolution studies of the Sun. Site characterization is underway at high altitude locations in the Himalayan mountains. A detailed concept ...
  • Numerical Simulation of Wave Propagation in Magnetic Network 
    Vigeesh, G; Hasan, S. S; Steiner, O (Cambridge University Press, 2009-03)
    We present 2-D numerical simulations of wave propagation in the magnetic network. The network is modelled as consisting of individual magnetic flux sheets located in intergranular lanes. They have a typical horizontal size ...
  • Observations and Modeling of the Emerging Extreme-Ultraviolet Loops in the Quiet Sun as Seen With the Solar Dynamics Observatory 
    Pradeep Chitta, L; Kariyappa, R; Van Ballegooijen, A. A; DeLuca, E. E; Hasan, S. S; Hanslmeier, A (IOP Publishing, 2013-05-01)
    We used data from the Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) to study coronal loops at small scales, emerging in the quiet Sun. With HMI ...
  • On the interchange instability of solar magnetic flux tubes. II. The influence of energy transport effects 
    Bunte, M; Hasan, S. S; Kalkofen, W (European Southern Observatory, 1993-06)
    We examine the interchange instability of thin photospheric magnetic flux tube models which satisfy both force and energy balance with their surroundings. The stability of the tubes is independent of the efficiency of ...
  • Oscillatory motions in intense flux tubes 
    Hasan, S. S (Royal Astronomical Society, 1986-03)
    The intricate nature of oscillatory motions in intene flux tubes is examined and states in which the ratio of gas to magnetic pressure (Beta) is constant are considered. Results are presented for both polytropic and real ...
  • Overview of the Sun 
    Hasan, S. S (World Scientific, 2008)
  • Photospheric Flow Fields and Properties of Embedded Small-scale Magnetic Flux Concentrations 
    Rajaguru, S. P; Srikanth, R; Hasan, S. S (Astronomical Society of the Pacific, 2001)
    The association between the different scales of convection on the solar photosphere and the field strengths/flux contents of discrete magnetic flux elements are analysed using simultaneously recorded SOHO MDI high resolution ...

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