explanation      blue bibcodes open ADS page with paths to full text
Author name code: hasan
ADS astronomy entries on 2022-09-14
=author:"Hasan, S.S." OR =author:"Hasan, S. Sirajul" 

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Title: National Large Solar Telescope
Authors: Hasan, S. S.; Banerjee, D.; Ravindra, B.; Sankarasubramanian,
   K.; Rangarajan, K. E.
2017CSci..113..696H    Altcode:
  No abstract at ADS

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Title: Division II: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Scrijver, Karel J.; Klimchuk,
   James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
   Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
   Vršnak, Bojan; Yan, Yihua
2015IAUTB..28..106V    Altcode:
  The Business Meeting of Commission 10 was held as part of the Business
  Meeting of Division II (Sun and Heliosphere), chaired by Valentin
  Martínez-Pillet, the President of the Division. The President of
  Commission 10 (C10; Solar activity), Lidia van Driel-Gesztelyi, took
  the chair for the business meeting of C10. She summarised the activities
  of C10 over the triennium and the election of the incoming OC.

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Title: Solar Astronomy at High Altitude
Authors: Hasan, S. S.; Bagare, S. P.; Rangarajan, K. E.
2014PINSA..80..815H    Altcode:
  No abstract at ADS

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Title: Scientific Programmes with India's National Large Solar
    Telescope and their contribution to Prominence Research
Authors: Hasan, S. S.
2014IAUS..300..355H    Altcode:
  The primary objective of the 2-m National Large Solar Telescope (NLST)
  is to study the solar atmosphere with high spatial and spectral
  resolution. With an innovative optical design, NLST is an on-axis
  Gregorian telescope with a low number of optical elements and a high
  throughput. In addition, it is equipped with a high order adaptive
  optics system to produce close to diffraction limited performance. <P
  />NLST will address a large number of scientific questions with a focus
  on high resolution observations. With NLST, high spatial resolution
  observations of prominences will be possible in multiple spectral
  lines. Studies of magnetic fields, filament eruptions as a whole,
  and the dynamics of filaments on fine scales using high resolution
  observations will be some of the major areas of focus.

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Title: Acoustic emission from magnetic flux tubes in the solar network
Authors: Vigeesh, G.; Hasan, S. S.
2013JPhCS.440a2045V    Altcode: 2013arXiv1304.5193V
  We present the results of three-dimensional numerical simulations to
  investigate the excitation of waves in the magnetic network of the Sun
  due to footpoint motions of a magnetic flux tube. We consider motions
  that typically mimic granular buffeting and vortex flows and implement
  them as driving motions at the base of the flux tube. The driving
  motions generates various MHD modes within the flux tube and acoustic
  waves in the ambient medium. The response of the upper atmosphere to
  the underlying photospheric motion and the role of the flux tube in
  channeling the waves is investigated. We compute the acoustic energy
  flux in the various wave modes across different boundary layers
  defined by the plasma and magnetic field parameters and examine the
  observational implications for chromospheric and coronal heating.

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Title: Observations and Modeling of the Emerging Extreme-ultraviolet
    Loops in the Quiet Sun as Seen with the Solar Dynamics Observatory
Authors: Chitta, L. P.; Kariyappa, R.; van Ballegooijen, A. A.;
   DeLuca, E. E.; Hasan, S. S.; Hanslmeier, A.
2013ApJ...768...32C    Altcode: 2013arXiv1303.3426C
  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 line-of-sight magnetograms, we derive the integrated
  and unsigned photospheric magnetic flux at the loop footpoints in the
  photosphere. These loops are bright in the EUV channels of AIA. Using
  the six AIA EUV filters, we construct the differential emission measure
  (DEM) in the temperature range 5.7-6.5 in log T (K) for several hours
  of observations. The observed DEMs have a peak distribution around
  log T ≈ 6.3, falling rapidly at higher temperatures. For log T &lt;
  6.3, DEMs are comparable to their peak values within an order of
  magnitude. The emission-weighted temperature is calculated, and its
  time variations are compared with those of magnetic flux. We present
  two possibilities for explaining the observed DEMs and temperatures
  variations. (1) Assuming that the observed loops are composed of
  a hundred thin strands with certain radius and length, we tested
  three time-dependent heating models and compared the resulting DEMs
  and temperatures with the observed quantities. This modeling used
  enthalpy-based thermal evolution of loops (EBTEL), a zero-dimensional
  (0D) hydrodynamic code. The comparisons suggest that a medium-frequency
  heating model with a population of different heating amplitudes can
  roughly reproduce the observations. (2) We also consider a loop model
  with steady heating and non-uniform cross-section of the loop along
  its length, and find that this model can also reproduce the observed
  DEMs, provided the loop expansion factor γ ~ 5-10. More observational
  constraints are required to better understand the nature of coronal
  heating in the short emerging loops on the quiet Sun.

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Title: India's National Large Solar Telescope
Authors: Hasan, S. S.
2012ASPC..463..395H    Altcode:
  India's 2-m National Large Solar Telescope (NLST) is aimed primarily
  at carrying out observations of the solar atmosphere with high spatial
  and spectral resolution. A comprehensive site characterization program,
  that commenced in 2007, has identified two superb sites in the Himalayan
  region at altitudes greater than 4000-m that have extremely low water
  vapor content and are unaffected by monsoons. With an innovative optical
  design, the NLST is an on-axis Gregorian telescope with a low number of
  optical elements to reduce the number of reflections and yield a high
  throughput with low polarization. In addition, it is equipped with
  a high-order adaptive optics to produce close to diffraction limited
  performance. To control atmospheric and thermal perturbations of the
  observations, the telescope will function with a fully open dome, to
  achieve its full potential atop a 25 m tower. Given its design, NLST can
  also operate at night, without compromising its solar performance. The
  post-focus instruments include broad-band and tunable Fabry-Pérot
  narrow-band imaging instruments; a high resolution spectropolarimeter
  and an Echelle spectrograph for night time astronomy. This project
  is led by the Indian Institute of Astrophysics and has national
  and international partners. Its geographical location will fill the
  longitudinal gap between Japan and Europe and is expected to be the
  largest solar telescope with an aperture larger than 1.5 m till the
  ATST and EST come into operation. An international consortium has
  been identified to build the NLST. The facility is expected to be
  commissioned by 2016.

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Title: The National Large Solar Telescope (NLST) of India
Authors: Hasan, S. S.
2012IAUSS...6E.211H    Altcode:
  The Indian National Large Solar Telescope (NLST) will be a
  state-of-the-art 2-m class telescope for carrying out high-resolution
  studies in the solar atmosphere. Recent numerical simulations suggest
  that crucial physical processes like vortex flow, dissipation of
  magnetic fields and the generation of MHD waves can occur efficiently
  over length scales of tens of kilometers. Current telescopes are unable
  to resolve solar feature to this level at visible wavelengths. NLST
  will not only achieve good spatial resolution, but will also have
  a high photon throughput in order to carry out spectropolarimetric
  observations to accurately measure vector magnetic fields in the
  solar atmosphere with a good signal to noise ratio. The main science
  goals of NLST include: a) Magnetic field generation and the solar
  cycle; b) Dynamics of magnetized regions; c) Helioseismology; d)
  Long term variability; e) Energetic phenomena and Activity; and f)
  Night time astronomy. The optical design of the telescope is optimized
  for high optical throughput and uses a minimum number of optical
  elements. A high order adaptive optics system is integrated as part
  of the design that works with a modest Fried's parameter of 7-cm to
  give diffraction limited performance. The telescope will be equipped
  with a suite of post-focus instruments including a high resolution
  spectrograph and a polarimeter. NLST will also be used for carrying
  out stellar observations during the night. The mechanical design of
  the telescope, building, and the innovative dome takes advantage of
  the natural air flush which will help to keep the open telescope in
  temperature equilibrium. Critical to the successful implementation of
  NLST is the selection of a site with optimum atmospheric properties,
  such as the number of sunshine hours and good "seeing" over long
  periods. A site characterization programme carried over several
  years has established the existence of suitable sites in the Ladakh
  region. After its completion, currently planned for 2016, NLST will
  fill a gap in longitude between the major solar facilities in the world
  and will be for some years the largest solar telescope in the world.

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Title: Three-dimensional Simulations of Magnetohydrodynamic Waves
    in Magnetized Solar Atmosphere
Authors: Vigeesh, G.; Fedun, V.; Hasan, S. S.; Erdélyi, R.
2012ApJ...755...18V    Altcode: 2011arXiv1109.6471V
  We present results of three-dimensional numerical simulations of
  magnetohydrodynamic (MHD) wave propagation in a solar magnetic flux
  tube. Our study aims at understanding the properties of a range of MHD
  wave modes generated by different photospheric motions. We consider two
  scenarios observed in the lower solar photosphere, namely, granular
  buffeting and vortex-like motion, among the simplest mechanism for
  the generation of waves within a strong, localized magnetic flux
  concentration. We show that granular buffeting is likely to generate
  stronger slow and fast magnetoacoustic waves as compared to swirly
  motions. Correspondingly, the energy flux transported differs as a
  result of the driving motions. We also demonstrate that the waves
  generated by granular buffeting are likely to manifest in stronger
  emission in the chromospheric network. We argue that different
  mechanisms of wave generation are active during the evolution of a
  magnetic element in the intergranular lane, resulting in temporally
  varying emission at chromospheric heights.

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Title: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Klimchuk,
   James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
   Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
   Vršnak, Bojan; Yan, Yihua
2012IAUTA..28...69V    Altcode:
  Commission 10 of the International Astronomical Union has more than
  650 members who study a wide range of activity phenomena produced by
  our nearest star, the Sun. Solar activity is intrinsically related
  to solar magnetic fields and encompasses events from the smallest
  energy releases (nano- or even picoflares) to the largest eruptions
  in the Solar System, coronal mass ejections (CMEs), which propagate
  into the Heliosphere reaching the Earth and beyond. Solar activity is
  manifested in the appearance of sunspot groups or active regions, which
  are the principal sources of activity phenomena from the emergence of
  their magnetic flux through their dispersion and decay. The period
  2008-2009 saw an unanticipated extended solar cycle minimum and
  unprecedentedly weak polar-cap and heliospheric field. Associated with
  that was the 2009 historical maximum in galactic cosmic rays flux since
  measurements begun in the middle of the 20th Century. Since then Cycle
  24 has re-started solar activity producing some spectacular eruptions
  observed with a fleet of spacecraft and ground-based facilities. In
  the last triennium major advances in our knowledge and understanding
  of solar activity were due to continuing success of space missions as
  SOHO, Hinode, RHESSI and the twin STEREO spacecraft, further enriched
  by the breathtaking images of the solar atmosphere produced by the
  Solar Dynamic Observatory (SDO) launched on 11 February 2010 in the
  framework of NASA's Living with a Star program. In August 2012, at the
  time of the IAU General Assembly in Beijing when the mandate of this
  Commission ends, we will be in the unique position to have for the
  first time a full 3-D view of the Sun and solar activity phenomena
  provided by the twin STEREO missions about 120 degrees behind and
  ahead of Earth and other spacecraft around the Earth and ground-based
  observatories. These new observational insights are continuously
  posing new questions, inspiring and advancing theoretical analysis
  and modelling, improving our understanding of the physics underlying
  magnetic activity phenomena. Commission 10 reports on a vigorously
  evolving field of research produced by a large community. The number
  of refereed publications containing `Sun', `heliosphere', or a synonym
  in their abstracts continued the steady growth seen over the preceding
  decades, reaching about 2000 in the years 2008-2010, with a total of
  close to 4000 unique authors. This report, however, has its limitations
  and it is inherently incomplete, as it was prepared jointly by the
  members of the Organising Committee of Commission 10 (see the names
  of the primary contributors to the sections indicated in parentheses)
  reflecting their fields of expertise and interest. Nevertheless, we
  believe that it is a representative sample of significant new results
  obtained during the last triennium in the field of solar activity.

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Title: Stokes Diagnostics of Magneto-Acoustic Wave Propagation in
    the Magnetic Network on the Sun
Authors: Vigeesh, G.; Steiner, O.; Hasan, S. S.
2011SoPh..273...15V    Altcode: 2011SoPh..tmp..349V; 2011arXiv1104.4069V
  The solar atmosphere is magnetically structured and highly
  dynamic. Owing to the dynamic nature of the regions in which the
  magnetic structures exist, waves can be excited in them. Numerical
  investigations of wave propagation in small-scale magnetic flux
  concentrations in the magnetic network on the Sun have shown that
  the nature of the excited modes depends on the value of plasma β
  (the ratio of gas to magnetic pressure) where the driving motion
  occurs. Considering that these waves should give rise to observable
  characteristic signatures, we have attempted a study of synthesised
  emergent spectra from numerical simulations of magneto-acoustic
  wave propagation. We find that the signatures of wave propagation
  in a magnetic element can be detected when the spatial resolution
  is sufficiently high to clearly resolve it, enabling observations in
  different regions within the flux concentration. The possibility to
  probe various lines of sight around the flux concentration bears the
  potential to reveal different modes of the magnetohydrodynamic waves
  and mode conversion. We highlight the feasibility of using the Stokes-V
  asymmetries as a diagnostic tool to study the wave propagation within
  magnetic flux concentrations. These quantities can possibly be compared
  with existing and new observations in order to place constraints on
  different wave excitation mechanisms.

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Title: Magnetohydrostatic equilibrium in starspots: dependences on
    color (T<SUB>eff</SUB>) and surface gravity (g)
Authors: Rajaguru, S. P.; Hasan, S. S.
2011IAUS..273..276R    Altcode: 2010arXiv1010.0126R
  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 molecular bands, of temperature contrasts
  of starspots in cool stars, and a grid of Kurucz stellar model
  atmospheres constructed to cover layers of sub-surface convection zone,
  we examine how the above relationship scales with effective temperature
  (T<SUB>eff</SUB>), surface gravity g and the associated changes in
  opacity of stellar photospheric gas. We calculate expected field
  strengths in starpots and find that a given relative reduction in
  temperatures (or the same darkness contrasts) yield increasing field
  strengths against decreasing T<SUB>eff</SUB> due to a combination of
  pressure and opacity variations against T<SUB>eff</SUB>.

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Title: Spectroscopic Observation of Oscillations in the Corona During
    the Total Solar Eclipse of 22 July 2009
Authors: Singh, Jagdev; Hasan, S. S.; Gupta, G. R.; Nagaraju, K.;
   Banerjee, D.
2011SoPh..270..213S    Altcode: 2011SoPh..tmp...36S
  We performed high resolution spectroscopy of the solar corona during
  the total solar eclipse of 22 July 2009 in two emission lines: the green
  line at 5303 Å due to Fe XIV and the red line at 6374 Å due to Fe X,
  simultaneously from Anji (latitude 30°28.1' N; longitude 119°35.4'
  E; elevation 890 m), China. A two-mirror coelostat with 100 cm focal
  length lens produced a 9.2 mm image of the Sun. The spectrograph using
  140 cm focal length lens in Littrow mode and a grating with 600 lines
  per millimeter blazed at 2 μm provided a dispersion of 30 mÅ and
  43 mÅ per pixel in the fourth order around the green line and third
  order around the red line, respectively. Two Peltier cooled 1k × 1k
  CCD cameras, with a pixel size of 13 μm square and 14-bit readout at
  10 MHz operated in frame transfer mode, were used to obtain the time
  sequence spectra in two emission lines simultaneously. The duration of
  totality was 341 s, but we could get spectra for 270 s after a trial
  exposure at an interval of 5 s. We report here on the detection of
  intensity, velocity, and line width oscillations with periodicity in
  the range of 25 - 50 s. These oscillations can be interpreted in terms
  of the presence of fast magnetoacoustic waves or torsional Alfvén
  waves. The intensity ratios of green to red emission lines indicate
  the temperature of the corona to be 1.65 MK in the equatorial region
  and 1.40 MK in the polar region, relatively higher than the expected
  temperature during the low activity period. The width variation of
  the emission lines in different coronal structures suggests different
  physical conditions in different structures.

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Title: National Large Solar Telescope of India
Authors: Hasan, S. S.
2011ASInC...2...37H    Altcode:
  The National Large Solar Telescope NLST will be a state-of-the-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 that have extremely low water vapor content and
  are unaffected by monsoons are under evaluation. This project is
  led by the Indian Institute of Astrophysics and has national and
  international partners. Its geographical location will fill the
  longitudinal gap between Japan and Europe and is expected to be the
  largest solar telescope with an aperture larger than 1.5 m till ATST and
  EST come into operation. <P />NLST is an on-axis alt-azimuth Gregorian
  multi-purpose open telescope with the provision of carrying out night
  time stellar observations using a spectrograph at the final focus. The
  telescope utilizes an innovative design with low number of reflections
  to achieve a high throughput and low polarization. High order adaptive
  optics is integrated into the design that works with a modest Fried
  parameter of 7-cm to give diffraction limited performance. The telescope
  will be equipped with a suite of post-focus instruments including a
  high-resolution spectrograph and a polarimeter.

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Title: Solar physics in India: developments from the nineteenth
    century to the present era
Authors: Hasan, S. S.; Choudhuri, Arnab Rai; Rajaguru, S. P.
2011ASInC...2..367H    Altcode:
  Modern solar astronomy took root in India with the discovery of helium
  during the total solar eclipse of 1868 and followed by the setting up
  of the Kodaikanal Observatory in 1899. We provide a brief overview of
  the growth of this field, describe the various Indian solar observing
  facilities and summarize the highlights of solar research in India
  during the last few decades.

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Title: NLST: the Indian National Large Solar Telescope
Authors: Hasan, S. S.; Soltau, D.; Kärcher, H.; Süss, M.;
   Berkefeld, T.
2010SPIE.7733E..0IH    Altcode: 2010SPIE.7733E..16H
  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 design for NLST (National Large Solar Telescope)
  has been completed. The optical design of the telescope is optimized
  for high optical throughput and uses a minimum number of optical
  elements. A high order AO system is integrated part of the design that
  works with a modest Fried's parameter of 7-cm to give diffraction
  limited performance. The telescope will be equipped with a suite
  of post-focus instruments including a high resolution spectrograph
  and a polarimeter. NLST will also be used for carrying out stellar
  observations during the night. The mechanical design of the telescope,
  building, and the innovative dome is optimized to take advantage of
  the natural air flush which will help to keep the open telescope in
  temperature equilibrium. After its completion (planned for 2014),
  NLST will fill a gap in longitude between the major solar facilities
  in USA and Europe, and it will be for years the largest solar telescope
  in the world

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Title: NLST: India's National Large Solar Telescope
Authors: Hasan, S. S.; Soltau, D.; Kärcher, H.; Süß, M.;
   Berkefeld, T.
2010AN....331..628H    Altcode:
  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 project and the science goals which shall be
  addressed with this telescope. NLST is a solar telescope with high
  optical throughput and will be equipped with an integrated Adaptive
  Optics system. It is optimized for a site with the kind of seeing and
  wind conditions as they are expected at a lake site in the Himalayan
  mountains. The telescope can also be used for certain night time
  applications. We also give the scientific rationale for this class
  of telescope.

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Title: The Indian National Large Solar Telescope (NLST)
Authors: Hasan, S. S.
2010IAUS..264..499H    Altcode:
  The Indian National Large Solar Telescope (NLST) will be a
  state-of-the-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 that have extremely low water vapor
  content and are unaffected by monsoons are under evaluation. This
  project is led by the Indian Institute of Astrophysics and has national
  and international partners. <P />NLST is an on-axis alt-azimuth
  Gregorian multi-purpose open telescope with the provision of carrying
  out night time stellar observations using a spectrograph. The telescope
  utilizes an innovative design with low number of reflections to achieve
  a high throughput and low instrumental polarization. High order adaptive
  optics is integrated into the design that works with a modest Fried's
  parameter of 7-cm to give diffraction limited performance. The telescope
  will be equipped with a suite of post-focus instruments including a
  high resolution spectrograph and a polarimeter. A detailed concept
  design of the telescope is presently being finalized and fabrication
  is expected to begin in 2010 with first light in 2014.

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Title: Republication of: Radial Movement in Sunspots
Authors: Hasan, S. S.
2010ASSP...19....2H    Altcode: 2010mcia.conf....2H
  Displacements of the lines of hydrogen and calcium in the neighbourhood
  of sunspots, indicating violent motions in the line of sight, is
  a common characteristic of spot disturbances. Such phenomena are
  frequently observed during periods of active change in spot development,
  or during the genesis of a spot. These line-shifts rarely affect the
  spectra of other elements than those of the higher chromospheres. In
  very violent outbursts, in addition to the hydrogen and calcium lines,
  those of He, Mg, Na, and some of the enhanced displacements may be
  either an increase or a decrease of wave-length, and may amount to
  several Ångström units, indicating movements of approach or recession
  of several hundred kilometers per second. These movements are seldom
  maintained for more than a few minutes at a time, and are usually to
  be found in the immediate neighbourhood of sports, rarely within the
  umbral area.

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Title: Magnetic Coupling between the Interior and Atmosphere of
    the Sun
Authors: Hasan, S. S.; Rutten, R. J.
2010ASSP...19.....H    Altcode: 2010mcia.conf.....H
  No abstract at ADS

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Title: Solar Physics at the Kodaikanal Observatory: A Historical
    Perspective
Authors: Hasan, S. S.; Mallik, D. C. V.; Bagare, S. P.; Rajaguru, S. P.
2010ASSP...19...12H    Altcode: 2009arXiv0906.0144H; 2010mcia.conf...12H
  The Kodaikanal Observatory traces its origins to the East India Company,
  which started an observatory in Madras "for promoting the knowledge of
  astronomy, geography, and navigation in India." Observations began in
  1787 at the initiative of William Petrie, an officer of the Company,
  with the use of two 3-in achromatic telescopes, two astronomical
  clocks with compound pendulums, and a transit instrument. By the early
  nineteenth century, the Madras Observatory had already established a
  reputation as a leading astronomical center devoted to work on the
  fundamental positions of stars, and a principal source of stellar
  positions for most of the southern hemisphere stars. John Goldingham
  (1796-1805, 1812-1830), T.G. Taylor (1830-1848),W.S. Jacob (1849-1858),
  and Norman R. Pogson (1861-1891) were successive Government Astronomers
  who led the activities in Madras. Scientific highlights of the work
  included a catalogue of 11,000 southern stars produced by theMadras
  Observatory in 1844 under Taylor's direction using the new 5-ft
  transit instrument.

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Title: Spectropolarimetry with the NLST
Authors: Sankarasubramanian, K.; Hasan, S. S.; Rangarajan, K. E.
2010ASSP...19..156S    Altcode: 2010mcia.conf..156S
  India's National Large Solar Telescope (NLST) will provide
  opportunities to observe the Sun with high spatial, spectral, and
  polarimetric resolution. The large aperture also enables high-cadence
  spectropolarimetry with moderate spatial resolution. A multi-slit
  spectropolarimeter is planned as one of the back-end instruments for
  this powerful telescope, primarily to measure vector magnetic fields
  in both active and quiet regions. An integral-field unit added with the
  multi-slit spectropolarimeter will enable fast-cadence observation. Here
  we discuss the scientific requirements for such an instrument, along
  with advantages and limitations of the concept and preliminary design
  details.

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Title: Wave propagation and energy transport in the magnetic network
    of the Sun
Authors: Vigeesh, G.; Hasan, S. S.; Steiner, O.
2009A&A...508..951V    Altcode: 2009arXiv0909.2325V
  Aims. We investigate wave propagation and energy transport in
  magnetic elements, which are representatives of small scale magnetic
  flux concentrations in the magnetic network on the Sun. This is
  a continuation of earlier work by Hasan et al. (2005, ApJ, 631,
  1270). The new features in the present investigation include
  a quantitative evaluation of the energy transport in the various
  modes and for different field strengths, as well as the effect of the
  boundary-layer thickness on wave propagation.<BR /> Methods: We carry
  out 2D MHD numerical simulations of magnetic flux concentrations for
  strong and moderate magnetic fields for which β (the ratio of gas to
  magnetic pressure) on the tube axis at the photospheric base is 0.4 and
  1.7, respectively. Waves are excited in the tube and ambient medium by
  a transverse impulsive motion of the lower boundary.<BR /> Results: The
  nature of the modes excited depends on the value of β. Mode conversion
  occurs in the moderate field case when the fast mode crosses the β =
  1 contour. In the strong field case the fast mode undergoes conversion
  from predominantly magnetic to predominantly acoustic when waves are
  leaking from the interior of the flux concentration to the ambient
  medium. We also estimate the energy fluxes in the acoustic and magnetic
  modes and find that in the strong field case, the vertically directed
  acoustic wave fluxes reach spatially averaged, temporal maximum values
  of a few times 10<SUP>6</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP> at
  chromospheric height levels.<BR /> Conclusions: The main conclusions
  of our work are twofold: firstly, for transverse, impulsive excitation,
  flux tubes/sheets with strong fields are more efficient than those with
  weak fields in providing acoustic flux to the chromosphere. However,
  there is insufficient energy in the acoustic flux to balance the
  chromospheric radiative losses in the network, even for the strong
  field case. Secondly, the acoustic emission from the interface between
  the flux concentration and the ambient medium decreases with the width
  of the boundary layer.

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Title: Intensity Oscillation in the Corona as Observed during the
    Total Solar Eclipse of 29 March 2006
Authors: Singh, Jagdev; Hasan, S. S.; Gupta, G. R.; Banerjee, D.;
   Muneer, S.; Raju, K. P.; Bagare, S. P.; Srinivasan, R.
2009SoPh..260..125S    Altcode:
  We obtained the images of the eastern part of the solar corona in the
  Fe XIV 530.3 nm (green) and Fe X 637.4 nm (red) coronal emission lines
  during the total solar eclipse of 29 March 2006 at Manavgat, Antalya,
  Turkey. The images were obtained using a 35 cm Meade telescope equipped
  with a Peltier-cooled 2k × 2k CCD and 0.3 nm pass-band interference
  filters at the rates of 2.95 s (exposure times of 100 ms) and 2.0 s
  (exposure times of 300 ms) in the Fe XIV and Fe X emission lines,
  respectively. The analysis of the data indicates intensity variations at
  some locations with period of strongest power around 27 s for the green
  line and 20 s for the red line. These results confirm earlier findings
  of variations in the continuum intensity with periods in the range
  of 5 to 56 s by Singh et al. (Solar Phys.170, 235, 1997). The wavelet
  analysis has been used to identify significant intensity oscillations
  at all pixels within our field of view. Significant oscillations with
  high probability estimates were detected for some locations only. These
  locations seem to follow the boundary of an active region and in the
  neighborhood, rather than within the loops themselves. These intensity
  oscillations may be caused by fast magneto-sonic waves in the solar
  corona and partly account for heating of the plasma in the corona.

---------------------------------------------------------
Title: Processes in the magnetized chromosphere of the Sun
Authors: Hasan, S. S.
2009IAUS..257..121H    Altcode:
  We review physical processes in magnetized chromospheres on the Sun. In
  the quiet chromosphere, it is useful to distinguish between the magnetic
  network on the boundaries of supergranules, where strong magnetic
  fields are organized in mainly vertical flux tubes and internetwork
  regions in the cell interiors, which have traditionally been associated
  with weak magnetic fields. Recent observations from Hinode, however,
  suggest that there is a significant amount of horizontal magnetic flux
  in the cell interior with large field strength. Furthermore, processes
  that heat the magnetic network have not been fully identified. Is the
  network heated by wave dissipation and if so, what is the nature of
  these waves? These and other aspects related to the role of spicules
  will also be highlighted. A critical assessment will be made on the
  challenges facing theory and observations, particularly in light of
  the new space experiments and the planned ground facilities.

---------------------------------------------------------
Title: Numerical simulation of wave propagation in magnetic network
Authors: Vigeesh, G.; Hasan, S. S.; Steiner, O.
2009IAUS..257..185V    Altcode:
  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 of about 150 km at the base of the photosphere and
  expand upward and become uniform. We consider flux sheets of different
  field strengths. Waves are excited by means of transverse motions at
  the lower boundary, to simulate the effect of granular buffeting. We
  look at the magneto-acoustic waves generated within the flux sheet
  and the acoustic waves generated in the ambient medium due to the
  excitation. We calculate the wave energy fluxes separating them into
  contributions from the acoustic and the Poynting part and study the
  effect of the different field strengths.

---------------------------------------------------------
Title: Chapter 2: Overview of the Sun
Authors: Hasan, S. S.
2008psa..book....9H    Altcode:
  The Sun plays a central role in two important respects: firstly, it
  provides a cosmic laboratory for investigating processes that cannot
  be simulated in the terrestrial environment and secondly, because
  of its relative closeness it serves as a proxy for understanding
  conditions in other stars. Formed about 4.6 billion years from a cloud
  of gas dust and frozen ice, the Sun at the current epoch in its life
  is a normal main-sequence star of spectral classification G2 with an
  average surface temperature of around 5700 K. It displays an astounding
  range of phenomena on myriad spatial and temporal scales that have
  traditionally defied comprehension. The Sun's magnetic field, that
  varies on a 22-year cycle, triggers activity and powerful eruptions
  that affect regions extending from the Earth's atmosphere to the
  distant edges of the solar system. Despite the inherent complexities
  of these processes, some progress has been achieved in understanding
  them through recent spectacular advances in observational techniques
  coupled with theoretical modelling...

---------------------------------------------------------
Title: Numerical simulation of wave propagation in the presence of
    a magnetic flux sheet
Authors: Vigeesh, G.; Steiner, O.; Hasan, S. S.
2008ESPM...12.3.24V    Altcode:
  We model network magnetic fields as consisting of individual magnetic
  flux sheets located in intergranular lanes. With a typical horizontal
  size of about 150 km at the base of the photosphere, they expand upward
  and merge with their neighbors at a height of about 600 km. Above
  a height of approximately 1000 km the magnetic field starts to
  become uniform. Granular buffeting is thought to excite waves in this
  medium, which is modeled by means of transversal motions at the lower
  boundary. The transverse driving, generates both fast and slow waves
  within the flux sheet and acoustic waves in the ambient medium. We
  consider flux sheets of different field strengths and different
  boundary-layer widths. Separating the energy flux of the waves into
  contributions due to the acoustic flux and the Poynting flux, we show
  the longitudinal and transversal components of both and study their
  temporal evolution.

---------------------------------------------------------
Title: Chromospheric dynamics
Authors: Hasan, S. S.
2008AdSpR..42...86H    Altcode:
  This review focuses on dynamics of the solar chromosphere, which serves
  as a good proxy for understanding processes in stellar chromospheres
  as well. In the quiet chromosphere it is useful to distinguish between
  the magnetic network on the boundaries of supergranulation cells,
  where strong magnetic fields are organized in mainly vertical magnetic
  flux tubes, and internetwork regions in the cell interior, where
  magnetic fields are weaker and dynamically less important. Recently,
  some progress has been made in understanding the physics of the
  non-magnetic chromosphere. On the other hand, the physical processes
  that heat the magnetic network have not been fully identified. Is the
  network heated by wave dissipation and if so, what are their physical
  characteristics? These and other aspects relating to the dynamics and
  energy transport mechanisms will be discussed in detail. In addition,
  some of the outstanding problems in the field such the driving mechanism
  for spicules and the nature of internetwork magnetic fields will also
  be highlighted. Furthermore, a critical assessment will be made on the
  challenges facing theory and the direction for future investigations,
  particularly in the light of the new space experiments, will be
  highlighted.

---------------------------------------------------------
Title: Dynamics of the Solar Magnetic Network. II. Heating the
    Magnetized Chromosphere
Authors: Hasan, S. S.; van Ballegooijen, A. A.
2008ApJ...680.1542H    Altcode: 2008arXiv0802.3509H
  We consider recent observations of the chromospheric network and
  argue that the bright network grains observed in the Ca II H and K
  lines are heated by an as-yet-unidentified quasi-steady process. We
  propose that the heating is caused by dissipation of short-period
  magnetoacoustic waves in magnetic flux tubes (periods less than 100
  s). Magnetohydrodynamic (MHD) models of such waves are presented. We
  consider wave generation in the network due to two separate processes:
  (1) transverse motions at the base of the flux tube and (2) the
  absorption of acoustic waves generated in the ambient medium. We
  find that the former mechanism leads to efficient heating of the
  chromosphere by slow magnetoacoustic waves propagating along magnetic
  field lines. This heating is produced by shock waves with a horizontal
  size of a few hundred kilometers. In contrast, acoustic waves excited
  in the ambient medium are converted into transverse fast modes that
  travel rapidly through the flux tube and do not form shocks, unless
  the acoustic sources are located within 100 km from the tube axis. We
  conclude that the magnetic network may be heated by magnetoacoustic
  waves that are generated in or near the flux tubes.

---------------------------------------------------------
Title: Wave propagation in multiple flux tubes and chromospheric
    heating
Authors: Hasan, S. S.; van Ballegooijen, A.; Steiner, O.
2008IAUS..247...82H    Altcode: 2007IAUS..247...82H
  This investigation is a continuation of earlier work on the dynamics of
  the magnetic network. In a previous calculation (Hasan et al. 2005),
  we examined the response of a single flux tube to transverse motions
  of its footpoints. We now extend this analysis to a more realistic
  model of the network consisting of multiple flux tubes. We apply a
  transverse velocity perturbation uniformly along the lower boundary
  located at the base of the photosphere. Our 2-D MHD simulations enable
  us to study the complex wave pattern due to waves generated in the
  individual tubes as well as their interaction with those emanating
  from adjacent tubes. Our results show that the dominant heating of the
  chromosphere occurs due to slow magnetoacoustic waves in a region that
  is close to the central region of the flux tube.

---------------------------------------------------------
Title: Inferring the chromospheric magnetic topology through waves
Authors: Hasan, S. S.; Steiner, O.; van Ballegooijen, A.
2008IAUS..247...78H    Altcode: 2007IAUS..247...78H
  The aim of this work is to examine the hypothesis that the wave
  propagation time in the solar atmosphere can be used to infer the
  magnetic topography in the chromosphere as suggested by Finsterle et
  al. (2004). We do this by using an extension of our earlier 2-D MHD
  work on the interaction of acoustic waves with a flux sheet. It is well
  known that these waves undergo mode transformation due to the presence
  of a magnetic field which is particularly effective at the surface
  of equipartition between the magnetic and thermal energy density, the
  β = 1 surface. This transformation depends sensitively on the angle
  between the wave vector and the local field direction. At the β =
  1 interface, the wave that enters the flux sheet, (essentially the
  fast mode) has a higher phase speed than the incident acoustic wave. A
  time correlation between wave motions in the non-magnetic and magnetic
  regions could therefore provide a powerful diagnostic for mapping the
  magnetic field in the chromospheric network.

---------------------------------------------------------
Title: Spatial damping of compressional MHD waves in prominences
Authors: Singh, K. A. P.; Dwivedi, B. N.; Hasan, S. S.
2007A&A...473..931S    Altcode:
  Aims:We study the spatial damping of linear compressional MHD waves in
  a homogeneous, isothermal, and unbounded prominence. <BR />Methods: We
  derive a general dispersion relation invoking the Newtonian radiation
  and turbulent viscosity. The turbulent viscosity is derived from SUMER
  and CDS observations for Kraichnan and Kolmogorov turbulences. Since
  we are interested in the spatial damping, the dispersion relation is
  solved numerically considering ω as real and k as complex corresponding
  to slow, fast, and thermal modes. <BR />Results: Both the slow and
  fast modes show strong damping, but the thermal mode is absent. The
  turbulent viscosity derived from observations can be a viable mechanism
  for the spatial damping of slow and fast modes. For a wave period of
  1 s, the damping length for slow and fast modes is found to be 1.1
  × 10<SUP>2</SUP> km for the Kolmogorov turbulence. Correspondingly,
  the damping length of slow modes is 1.3 × 10<SUP>1</SUP> km and for
  fast modes 1.9× 10<SUP>2</SUP> km for the Kraichnan turbulence. From
  the damping length study of slow modes, it is found that Kraichnan
  turbulence dominates for short wave periods between 10<SUP>-7</SUP> to
  10<SUP>2</SUP> s, and the Kolmogorov turbulence dominates for longer
  wave periods between 10<SUP>3</SUP> to 10<SUP>5</SUP> s. From the
  damping length of fast modes, it is found that the Kraichnan turbulence
  dominates from very short to long wave periods. <BR />Conclusions: The
  Kraichnan and Kolmogorov turbulence can be a viable damping mechanism
  for the spatial damping of short-period oscillations. In particular,
  the short-period oscillations (5-15 min) observed in quiescent limb
  prominences, which seem to be due to internal fundamental slow modes,
  have damping lengths in the range 1.9-3.7× 10<SUP>3</SUP> km for
  Kolmogorov turbulence and 3.5 × 10<SUP>3</SUP>-3.1 × 10<SUP>4</SUP> km
  for Kraichnan turbulence. Correspondingly, for fast modes, the damping
  length is in the range 2.6× 10<SUP>5</SUP>-2.3× 10<SUP>6</SUP> km for
  Kolmogorov turbulence and 1.7 × 10<SUP>7</SUP>-1.5× 10<SUP>8</SUP>
  km for Kraichnan turbulence. This study underlines the importance of
  turbulent viscosity for explaining the damping of both slow and fast
  modes, which, hitherto, has not been explored.

---------------------------------------------------------
Title: Kodai School on Solar Physics
Authors: Hasan, S. S.; Banerjee, D.
2007AIPC..919.....H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Kodai School on Solar Physics
Authors: Hasan, S. S.; Banerjee, D.
2007kssp.conf.....H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Damping of MHD Waves in Quiescent Prominences (P50)
Authors: Singh, K. A. P.; Hasan, S. S.; Dwivedi, B. N.
2006ihy..workE.142S    Altcode:
  The effects of radiative losses due to Newtonian cooling and MHD
  turbulence have been considered to examine the damping of linear MHD
  waves in unbounded quiescent prominences. Taking account of isotropic
  viscosity in the momentum equation and viscous as well as radiation
  terms in energy equation, we derive a general fifth-order dispersion
  relation. The analytical solutions of the general dispersion relation
  have been obtained. It is shown that the damping of magnetoacoustic
  waves depends on the equilibrium density, magnetic field, temperature,
  frequency and wave number. The fifth-order general dispersion relation
  has been solved numerically. We have compared our results with the
  observations taken from the VTT telescope at Sac Peak. We find that the
  slow mode waves are mainly affected by radiation but fast mode waves
  remain unaffected, while noting that both of them are damped due to
  MHD turbulence. We also find that classical viscosity hardly plays a
  role in damping the magnetoacoustic waves. The radiative losses give
  acceptable damping lengths for the slow mode waves for the radiative
  relaxation times in the range 10 - 103 s. It has been found that for a
  given value of radiative relaxation time, the high frequency slow mode
  waves are highly damped. We have also investigated the possible role
  of MHD turbulence in damping of MHD waves and found that a turbulent
  viscosity can re-produce the observed damping time and damping length
  in prominences, especially in PCTR. We find that MHD turbulence alone
  can explain the damping of magnetoacoustic waves in prominences. From
  prominence seismology, the values of opacity and turbulent kinematic
  viscosity have been inferred.

---------------------------------------------------------
Title: Dynamics of the Magnetized Solar Chromosphere
Authors: Hasan, S. S.
2006ihy..workE..29H    Altcode:
  This review focuses on dynamics of the magnetized solar chromosphere. In
  the quiet chromosphere we distinguish between the magnetic network on
  the boundary of super-granulation cells, where strong magnetic fields
  are organized in mainly vertical magnetic flux tubes, and inter-network
  regions in the cell interior, where magnetic fields are weak and
  dynamically unimportant. Observations have firmly established the
  presence of oscillations in the solar chromosphere. Both the network
  and inter-network media show bright points (BPs), which are prominent
  in the emission peaks in the cores of the Ca II H and K lines. However,
  the dynamical and spectral properties of network and inter-network BPs
  are quite different. In the latter the chromospheric velocity power
  spectrum is dominated by oscillations having power in the 5-7 mHz
  range, which can essentially be regarded as acoustic waves, whereas
  the network exhibits low-frequency oscillations with periods 7-20
  min. The qualitative properties of inter-network BPs are reasonably well
  understood, including their formation in upward propagating acoustic
  shocks that encounter downward-flowing gas. On the other hand, the
  physical processes that heat the magnetic network have not been fully
  identified. Are network BPs heated by wave dissipation and if so,
  what is the nature of these waves? These and other aspects relating
  to the dynamics and energy transport mechanisms will be discussed
  in detail. Furthermore, a critical assessment will be made on the
  challenges facing theory and the direction for future investigations,
  particularly in the light of the new space experiments, will be
  highlighted.

---------------------------------------------------------
Title: Wave Propagation in the Magnetic Network on the Sun
Authors: Hasan, S. S.; Vigeesh, G.; van Ballegooijen, A. A.
2006IAUS..233..116H    Altcode:
  Hasan et al. (2005) have recently presented 2-D dynamical calculations
  on wave propagation in in the magnetic network of the Sun. The latter
  is idealized as consisting of non-potential flux tubes in the quiet
  solar chromosphere. It is of interest to understand how the nature of
  wave propagation is influenced by the choice of initial equilibrium
  configuration of the magnetic field. We examine this by comparing
  the earlier calculations with those when the network is modelled as
  a potential structure. Our calculations demonstrate that the nature
  of the wave propagation is significantly different, particularly
  the transport of energy which for the potential case, occurs more
  isotropically than for the non-potential configuration.

---------------------------------------------------------
Title: Chromospheric Dynamics
Authors: Hasan, S. S.
2006cosp...36.3643H    Altcode: 2006cosp.meet.3643H
  This review focuses on dynamics of the solar chromosphere which serves
  as a good proxy for understanding processes in stellar chromospheres
  In the quiet chromosphere we distinguish between the magnetic network
  on the boundary of supergranulation cells where strong magnetic fields
  are organized in mainly vertical magnetic flux tubes and internetwork
  regions in the cell interior where magnetic fields are weak and
  dynamically unimportant Observations have firmly established the
  presence of oscillations in the solar chromosphere Both the network
  and internetwork media show bright points BPs which are prominent in
  the emission peaks in the cores of the Ca II H and K lines However the
  dynamical and spectral properties of network and internetwork BPs are
  quite different In the latter the chromospheric velocity power spectrum
  is dominated by oscillations having power in the 5-7 mHz range which can
  essentially be regarded as acoustic waves whereas the network exhibits
  low-frequency oscillations with periods 7-20 min The qualitative
  properties of internetwork BPs are reasonably well understood including
  their formation in upward propagating acoustic shocks that encounter
  downward-flowing gas On the other hand the physical processes that
  heat the magnetic network have not been fully identified Are network
  BPs heated by wave dissipation and if so what is the nature of these
  waves These and other aspects relating to the dynamics and energy
  transport mechanisms will be discussed in detail Furthermore a critical

---------------------------------------------------------
Title: Influence of magnetic field on the Doppler measurements
    of velocity field in the solar photosphere and implications for
    helioseismology
Authors: Rajaguru, S. P.; Wachter, R.; Hasan, S. S.
2006ilws.conf...21R    Altcode:
  Shapes of spectral lines and their sensitivity to fluid motions are
  strongly altered in magnetised regions of the solar atmosphere. Sunspots
  and plages (bright network regions) are prime examples. Here we study
  the temporal behaviour of Ni I (6768 A, used by MDI onboard SOHO) in
  sunspots and plages using realistic models of sunspots and network flux
  tubes as input to a radiative transfer code SPINOR (Frutiger et al.,
  2000) that models spectral line formation in magnetic fields. We examine
  the sunspot case based on a simple model of oscillations superposed on
  the Maltby model (empirical) of sunspot atmosphere. The plage regions
  are studied using a 2-d MHD simulation of oscillatory motions (Hasan
  et al., 2005) in network flux tubes. We discuss the changes that the
  altered line profiles would cause in the Doppler measurements of the
  velocity field, especially those by helioseismic imaging instrument
  MDI onboard SOHO.

---------------------------------------------------------
Title: Probing the internal magnetic field of slowly pulsating
    B-stars through g modes
Authors: Hasan, S. S.; Zahn, J. -P.; Christensen-Dalsgaard, J.
2005A&A...444L..29H    Altcode: 2005astro.ph.11472H
  Context: .<BR /> Aims: .We suggest that high-order g modes can be used
  as a probe of the internal magnetic field of SPB (slowly pulsating
  B) stars. The idea is based on earlier work by the authors hich
  analytically investigated the effect of a vertical magnetic field on p
  and g modes in a plane-parallel isothermal stratified atmosphere. It
  was found that even a weak field can significantly shift the g-mode
  frequencies - the effect increases with mode order.<BR /> Methods:
  .In the present study we adopt the classical perturbative approach
  to estimate the internal field of a 4 solar mass SPB star by looking
  at its effect on a low-degree (l=1) and high-order (n=20) g mode with
  a period of about 1.5 d. <BR /> Results: .We find that a polar field
  strength of about 110 kG on the edge of the convective core is required
  to produce a frequency shift of 1%. Frequency splittings of that order
  have been observed in several SPB variables, in some cases clearly too
  small to be ascribed to rotation. We suggest that they may be due to
  a poloidal field with a strength of order 100 kG, buried in the deep
  interior of the star.<BR /> Conclusions: .

---------------------------------------------------------
Title: Dynamics of the Solar Magnetic Network: Two-dimensional
    MHD Simulations
Authors: Hasan, S. S.; van Ballegooijen, A. A.; Kalkofen, W.;
   Steiner, O.
2005ApJ...631.1270H    Altcode: 2005astro.ph..3525H
  The aim of this work is to identify the physical processes that occur
  in the network and contribute to its dynamics and heating. We model the
  network as consisting of individual flux tubes, each with a nonpotential
  field structure, that are located in intergranular lanes. With a typical
  horizontal size of about 150 km at the base of the photosphere, they
  expand upward and merge with their neighbors at a height of about 600
  km. Above a height of approximately 1000 km the magnetic field starts
  to become uniform. Waves are excited in this medium by means of motions
  at the lower boundary. We focus on transverse driving, which generates
  both fast and slow waves within a flux tube and acoustic waves at
  the interface of the tube and the ambient medium. The acoustic waves
  at the interface are due to compression of the gas on one side of
  the flux tube and expansion on the other. These longitudinal waves
  are guided upward along field lines at the two sides of the flux
  tube, and their amplitude increases with height due to the density
  stratification. Being acoustic in nature, they produce a compression
  and significant shock heating of the plasma in the chromospheric part of
  the flux tube. For impulsive excitation with a time constant of 120 s,
  we find that a dominant feature of our simulations is the creation of
  vortical motions that propagate upward. We have identified an efficient
  mechanism for the generation of acoustic waves at the tube edge, which
  is a consequence of the sharp interface of the flux concentration. We
  examine some broad implications of our results.

---------------------------------------------------------
Title: How Similar are Starspots to Sunspots?
Authors: Rajaguru, S. P.; Kurucz, R. L.; Hasan, S. S.
2005BASI...33..362R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Dynamics of the Magnetized Solar Atmosphere
Authors: Hasan, S. S.
2005BASI...33..338H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Erratum: Dynamics and heating of the magnetic network on the
    Sun. Efficiency of mode transformation
Authors: Hasan, S. S.; Ulmschneider, P.
2004A&A...428.1017H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Modulation in the solar irradiance due to surface magnetism
    during cycles 21, 22 and 23
Authors: Jain, K.; Hasan, S. S.
2004A&A...425..301J    Altcode:
  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 total solar irradiance variability during solar
  cycles 21, 22 and 23. We find that most of the solar cycle variation in
  the total solar irradiance can be accounted for by the absolute magnetic
  field strength on the solar disk, if fields associated with dark and
  bright regions are considered separately. However, there is a large
  scatter in the calculated and observed values of TSI during solar cycle
  21. On the other hand, the multiple correlation coefficients obtained
  for solar cycles 22 and 23 are 0.88 and 0.91 respectively. Furthermore,
  separate regression analyses for solar cycles 22 and 23 do not show
  any significant differences in the total solar irradiance during these
  cycles. Our study further strengthens the view that surface magnetism
  indeed plays a dominant role in modulating solar irradiance.

---------------------------------------------------------
Title: Dynamics and heating of the magnetic network on the
    Sun. Efficiency of mode transformation
Authors: Hasan, S. S.; Ulmschneider, P.
2004A&A...422.1085H    Altcode: 2004astro.ph..6626H
  We aim to identify the physical processes which occur in the magnetic
  network of the chromosphere and which contribute to its dynamics
  and heating. Specifically, we study the propagation of transverse
  (kink) MHD waves which are impulsively excited in flux tubes through
  footpoint motions. When these waves travel upwards, they get partially
  converted to longitudinal waves through nonlinear effects (mode
  coupling). By solving the nonlinear, time-dependent MHD equations
  we find that significant longitudinal wave generation occurs in the
  photosphere typically for Mach numbers as low as 0.2 and that the
  onset of shock formation occurs at heights of about 600 km above the
  photospheric base. We also investigate the compressional heating due
  to longitudinal waves and the efficiency of mode coupling for various
  values of the plasma β, that parameterises the magnetic field strength
  in the network. We find that this efficiency is maximum for field
  strengths corresponding to β≈ 0.2, when the kink and tube wave
  speeds are almost identical. This can have interesting observational
  implications. Furthermore, we find that even when the two speeds are
  different, once shock formation occurs, the longitudinal and transverse
  shocks exhibit strong mode coupling.

---------------------------------------------------------
Title: Reconstruction of the past total solar irradiance on short
    timescales
Authors: Jain, Kiran; Hasan, S. S.
2004JGRA..109.3105J    Altcode:
  The aim of this investigation is to present a new analysis of short-term
  variations in total solar irradiance by developing regression models
  and to extend these to epochs when irradiance measurements were not
  available. In our models the sunspot area is used to quantify sunspot
  darkening while facular brightening is calculated using facular area,
  10.7 cm radio flux and Mg II core-to-wing ratio. Models developed
  with various proxies are compared with a view to identify the role of
  key parameters in solar variability. We also study the relationship
  between different facular proxies and show that the facular area and
  10.7 cm radio flux do not vary linearly with the Mg II core-to-wing
  ratio. We emphasize that the facular term in current empirical models
  (using facular area or radio flux proxies) on short time scale needs to
  have a nonlinear component in order to obtain a better correlation with
  observed irradiance. Our analysis demonstrates that the correlation for
  daily variations in solar irradiance improves by 10% using a quadratic
  term in the model based on radio flux as a facular proxy, which is
  a significant improvement on earlier models. On the other hand, the
  correlation remains unchanged in the model using Mg II core-to-wing
  ratio. Thus we point out that various proxies for facular brightenings
  contribute differently to solar irradiance. We estimate the solar
  irradiance variations at epochs before irradiance observation began,
  in particular to the start of the radio flux measurements, and find that
  there is no drastic increase in radiative output during the most active
  solar cycle 19 while for cycle 20 we observe a much lower irradiance
  during maximum.

---------------------------------------------------------
Title: Reconstruction of Total Solar Irradiance on Multiple Time
    Scales
Authors: Jain, Kiran; Hasan, S. S.
2003BASI...31..315J    Altcode:
  We have developed regression models of total solar irradiance on
  different time scales by parameterizing the combined influence of
  sunspots and faculae. These models are useful in identifying the key
  parameters responsible for temporal variations.

---------------------------------------------------------
Title: Kink and Longitudinal Oscillations in the Magnetic Network
on the Sun: Nonlinear Effects and Mode Transformation
Authors: Hasan, S. S.; Kalkofen, W.; van Ballegooijen, A. A.;
   Ulmschneider, P.
2003ApJ...585.1138H    Altcode:
  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 found that footpoint motions of the tubes can generate
  sufficient wave energy for chromospheric heating. We assumed that
  the kink and longitudinal waves are decoupled and linear. We overcome
  these limitations by treating the nonlinear MHD equations for coupled
  kink and longitudinal waves in a thin flux tube. For the parameters
  we have chosen, the thin tube approximation is valid up to the layers
  of formation of the emission features in the H and K lines of Ca II,
  at a height of about 1 Mm. By solving the nonlinear, time-dependent MHD
  equations we are able to study the onset of wave coupling, which occurs
  when the Mach number of the kink waves is of the order of 0.3. We also
  investigate the transfer of energy from the kink to the longitudinal
  waves, which is important for the dissipation of the wave energy in
  shocks. We find that kink waves excited by footpoint motions of a
  flux tube generate longitudinal modes by mode coupling. For subsonic
  velocities, the amplitude of a longitudinal wave increases as the square
  of the amplitude of the transverse wave, and for amplitudes near Mach
  number unity, the coupling saturates and becomes linear when the energy
  is nearly evenly divided between the two modes.

---------------------------------------------------------
Title: Magnetic Flux Tubes and Activity on the Sun
Authors: Hasan, S. S.
2003LNP...619..173H    Altcode: 2003lsp..conf..173H
  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 and emergence of magnetic fields from the solar interior;
  (b) the nature of the surface magnetic fields, especially in the form
  of small-scale flux tubes; and (c) dynamical processes in flux tubes
  and heating of the magnetic chromosphere.

---------------------------------------------------------
Title: Physics of Photospheric Magnetic Field (Invited review)
Authors: van Ballegooijen, A. A.; Hasan, S. S.
2003ASPC..286..155V    Altcode: 2003ctmf.conf..155V
  No abstract at ADS

---------------------------------------------------------
Title: The dynamics of the quiet solar chromosphere
Authors: Kalkofen, W.; Hasan, S. S.; Ulmschneider, P.
2003dysu.book..165K    Altcode:
  Wave propagation in the nonmagnetic chromosphere is described for
  plane and spherical waves, and excitation by means of impulses in small
  source regions in the photosphere; excitation for flux tube waves in
  the magnetic network is described for large, single impulses and for a
  fluctuating velocity field. Observational signatures of the various wave
  types and their effect on chromospheric heating are considered. It is
  concluded that calcium bright points in the nonmagnetic chromosphere
  are due to spherical acoustic waves, and that for the oscillations
  in the magnetic network, transverse waves are more important than
  longitudinal waves; they may penetrate into the corona, giving rise
  to some coronal heating.

---------------------------------------------------------
Title: Dynamics of Chromospheres
Authors: Hasan, S. S.
2002AAS...200.5303H    Altcode: 2002BAAS...34..729H
  This review focuses on dynamics of the solar chromosphere, which serves
  as a good proxy for understanding processes in stellar chromospheres. In
  the quiet chromosphere we distinguish between the magnetic network on
  the boundary of supergranulation cells, where strong magnetic fields
  are organized in mainly vertical magnetic flux tubes, and internetwork
  regions in the cell interior, where magnetic fields are weak and
  dynamically unimportant. Observations have firmly established the
  presence of oscillations in the solar chromosphere. The internetwork
  medium is dominated with oscillations having power in the 5-7 mHz
  range, which can essentially be regarded as acoustic waves. Significant
  progress has been made recently in modeling wave propagation in the
  non-magnetic medium and applying these calculations to interpreting
  the properties of K<SUB>2V</SUB> grains. Nevertheless, there are still
  several open questions which need to be addressed, specifically the
  departure from 1-D geometry and the inclusion of oblique propagation:
  these can have important consequences. The dynamics of the magnetic
  network, on the other hand, is dominated by low frequency waves with
  periods in the 4-15 min. range, which can be interpreted as transverse
  MHD waves, generated in thin flux tubes by granular buffeting. Through
  nonlinear effects, these modes generate longitudinal MHD waves, that
  form shocks and dissipate in the low to middle chromosphere. Alternative
  theoretical scenarios for interpreting network oscillations will also
  be discussed as well as their observational consequences. Finally, we
  consider some implications of the above models to stellar chromospheres.

---------------------------------------------------------
Title: Convective Intensification of Magnetic Flux Tubes in Stellar
    Photospheres
Authors: Rajaguru, S. P.; Kurucz, R. L.; Hasan, S. S.
2002ApJ...565L.101R    Altcode: 2002astro.ph..1026R
  The convective collapse of thin magnetic flux tubes in the
  photospheres of Sun-like stars is investigated using realistic
  models of the superadiabatic upper convection zone layers of these
  stars. The strengths of convectively stable flux tubes are computed as
  a function of surface gravity and effective temperature. We find that
  while stars with T<SUB>eff</SUB>&gt;=5500 K and logg&gt;=4.0 show flux
  tubes highly evacuated of gas, and hence strong field strengths due to
  convective collapse, cooler stars exhibit flux tubes with lower field
  strengths. Observations reveal the existence of field strengths close
  to thermal equipartition limits even in cooler stars, implying highly
  evacuated tubes, for which we suggest possible reasons.

---------------------------------------------------------
Title: Coronal Heating by Kink Waves
Authors: Hasan, S. S.; Kalkofen, W.; Ulmschneider, P.
2001AGUSM..SH41B01H    Altcode:
  We examine the hypothesis that kink waves contribute to coronal
  heating. In earlier work we demonstrated that the excitation of
  kink oscillations flux tubes in the magnetic network of the Sun
  through their footpoint motions can provide sufficient energy for
  chromospheric heating. This calculation assumed that: (a) the waves
  could be treated using the linear approximation, and (b) the kink
  and longitudinal waves were decoupled. These approximations, although
  valid in the lower atmosphere, break down in the upper chromosphere,
  where the wave amplitude becomes comparable with the tube speed. We
  overcome the earlier limitations by numerically solving the nonlinear
  MHD equations for coupled kink and longitudinal waves. Using a specified
  form of the footpoint motions, which is compatible with observations,
  we solve the nonlinear time-dependent MHD equations for a thin flux
  tube extending vertically from the sub-photosphere to the base of the
  corona. Our code is able to resolve shocks and also self-consistently
  treats mode transformation. We calculate the energy fluxes in vertically
  propagating kink waves and show that there is in principle adequate
  energy in the waves to heat the corona.

---------------------------------------------------------
Title: Photospheric Flow Fields and Properties of Embedded Small-scale
    Magnetic Flux Concentrations
Authors: Rajaguru, S. P.; Srikanth, R.; Hasan, S. S.
2001IAUS..203..205R    Altcode:
  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 intensity, velocity (Doppler) images and magnetograms. The
  convective flow patterns are mapped using the Local Correlation Tracking
  (LCT) algorithm. The locations and strengths of the flux elements
  with respect to the flow cells are shown to reflect the depths of the
  associated downflows. This property is in turn, in combination with
  results of calculations on the convective collapse process that forms
  strong field elements, used to derive some properties of the different
  scales of convection.

---------------------------------------------------------
Title: Radiative Transfer Effects and the Dynamics of Small-Scale
    Magnetic Structures on the Sun
Authors: Rajaguru, S. P.; Hasan, S. S.
2000ApJ...544..522R    Altcode:
  The dynamical consequences of radiative energy transport on the
  evolution of gas confined to small-scale magnetic structures on the Sun
  are studied. Convective collapse, which transforms weak-field structures
  into intense structures of field strengths in the 1-2 kG range on
  the photosphere, is strongly influenced by radiative heating from the
  surroundings and cooling due to losses in the vertical direction. We
  first present analytic results in the quasi-adiabatic approximation
  to attempt a qualitative understanding of the influence of radiative
  effects on the convective stability of flux tubes. We demonstrate the
  destabilizing action of vertical radiative losses, that tend to enhance
  convective collapse and produce strong tubes at a relatively smaller
  horizontal scale than those expected from calculations based solely on
  horizontal radiative energy transport. Our calculations clearly point to
  an asymmetry between upflow and downflow perturbations-only the latter
  are amplified in the presence of vertical radiative transport. Using
  a realistic model of the solar atmospheric structure and treating
  radiative transfer in the diffusion and Eddington approximations,
  we next perform numerical stability analyses and produce size
  (flux)-strength relations for solar flux tubes. Our results provide
  a physical explanation for the observed flux-dependent (equivalently
  size-dependent) field strengths of the solar small-scale magnetic
  structures in the form of weak intranetwork and strong network
  components.

---------------------------------------------------------
Title: Results from a revisit to the K<SUB>2V</SUB> bright points
Authors: Sivaraman, K. R.; Gupta, S. S.; Livingston, W. C.; Damé,
   L.; Kalkofen, W.; Keller, C. U.; Smartt, R.; Hasan, S. S.
2000A&A...363..279S    Altcode:
  We have used pairs of temporally simultaneous CaII K-line
  spectroheliograms and magnetic area scans to search for spatial
  correlation between the CaII K<SUB>2V</SUB> bright points in the
  interior of the network and corresponding magnetic elements. We find
  that about 60% of the K<SUB>2V</SUB> bright points spatially coincide
  with magnetic elements of flux density &gt; 4 Mx cm<SUP>-2</SUP>. About
  25% of the K<SUB>2V</SUB> bright points with equally enhanced emission
  lie over bipole elements where the fields are &gt; 4 Mx cm<SUP>-2</SUP>
  for both polarity elements which merge and presumably cancel and
  result in low fields. The rest, 15%, of the bright points coincide
  with areas of fields &lt; 4 Mx cm<SUP>-2</SUP> which is the noise
  level set by us for the magnetic scans. When magnetic elements of
  opposite polarity merge and form bipoles, the associated K<SUB>2V</SUB>
  bright points show excess emission. Although such excess emission is a
  magnetic-field-driven phenomenon, the measured value of the field at the
  site of the bipole is typically low, and these cases would therefore be
  excluded in the count of coincidences of excess emission with excess
  magnetic fields. In our opinion, these cases of excess emission at
  the sites of the bipoles, as well as at the sites of fields &gt;
  4 Mx cm<SUP>-2</SUP>, are both instances of magnetic-field-related
  emissions. If the former are not taken into account as coincidences,
  the correlation will drop down and this might be interpreted as not an
  obvious correlation. Our present results, taking into account the low
  fields of merging bipoles, establish the association of K<SUB>2V</SUB>
  bright points with magnetic elements.

---------------------------------------------------------
Title: Dynamical Processes in Flux Tubes and their Role in
    Chromospheric Heating
Authors: Hasan, S. S.
2000JApA...21..283H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Excitation of Oscillations in the Magnetic Network on the Sun
Authors: Hasan, S. S.; Kalkofen, W.; van Ballegooijen, A. A.
2000ApJ...535L..67H    Altcode: 2000astro.ph..4246H
  We examine the excitation of oscillations in the magnetic network of
  the Sun through the footpoint motion of photospheric magnetic flux
  tubes located in intergranular lanes. The motion is derived from a
  time series of high-resolution G-band and continuum filtergrams using
  an object-tracking technique. We model the response of the flux tube
  to the footpoint motion in terms of the Klein-Gordon equation, which
  is solved analytically as an initial value problem for transverse
  (kink) waves. We compute the wave energy flux in upward-propagating
  transverse waves. In general we find that the injection of energy into
  the chromosphere occurs in short-duration pulses, which would lead
  to a time variability in chromospheric emission that is incompatible
  with observations. Therefore, we consider the effects of turbulent
  convective flows on flux tubes in intergranular lanes. The turbulent
  flows are simulated by adding high-frequency motions (periods 5-50 s)
  with an amplitude of 1 km s<SUP>-1</SUP>. The latter are simulated by
  adding random velocity fluctuations to the observationally determined
  velocities. In this case, we find that the energy flux is much
  less intermittent and can in principle carry adequate energy for
  chromospheric heating.

---------------------------------------------------------
Title: The Nature of Wave Excitation in the Magnetic Network
Authors: Hasan, S. S.; Kalkofen, W.
2000SPD....31.0129H    Altcode: 2000BAAS...32..806H
  We examine the nature of wave excitation in the magnetic network of
  the Sun through the footpoint motion of photospheric magnetic flux
  tubes located in intergranular lanes. This contribution builds on
  earlier work (Hasan &amp; Kalkofen 1999, ApJ 519, 899) where it was
  suggested that impulsively generated MHD kink, or transverse, waves can
  contribute efficiently to chromospheric heating. On the other hand, it
  was argued that this scenario for heating the magnetic network would
  produce "strongly intermittent chromospheric emission consisting of
  brief, intense flashes superimposed on a very low background" (Hasan,
  Kalkofen &amp; van Ballegooijen 2000, ApJL, in press). In this paper,
  we analyse in greater detail the consequences based on the above picture
  by adopting different forms for the footpoint point velocity. We compute
  the transverse wave energy flux injected into the chromosphere through
  (a) impulsive and (b) turbulent footpoint motions. Finally, we point
  out observational implication of our calculations.

---------------------------------------------------------
Title: Excitation of Oscillations in Photospheric Flux Tubes through
    Buffeting by External Granules
Authors: Hasan, S. S.; Kalkofen, W.
1999ApJ...519..899H    Altcode:
  We examine the excitation of transverse (kink) and longitudinal
  (sausage) waves in magnetic flux tubes by granules in the solar
  photosphere. The investigation is motivated by the interpretation
  of network oscillations in terms of flux tube waves. We model the
  interaction between a granule, with a specified transverse velocity,
  and a vertical flux tube in terms of the Klein-Gordon equation, which
  we solve analytically as an initial value problem for both wave modes,
  assuming the same external impulse. The calculations show that for
  magnetic field strengths typical of the network, the energy flux in
  transverse waves is higher than in longitudinal waves by an order
  of magnitude, in agreement with the chromospheric power spectrum of
  network oscillations observed by Lites, Rutten, &amp; Kalkofen. But
  for weaker fields, such as those that might be found in internetwork
  regions, the energy fluxes in the two modes are comparable. This result
  implies that if there are internetwork oscillations in magnetic flux
  tubes, they must show the cutoff periods of both longitudinal and
  transverse modes at 3 minutes and at 7 minutes or longer. We also find
  that granules with speeds of about 2 km s<SUP>-1</SUP> can efficiently
  excite transverse oscillations in frequent short-duration (typically
  1 minute) bursts that can heat the corona.

---------------------------------------------------------
Title: Excitation of oscillations in the magnetic network on the Sun
Authors: Hasan, S. S.; Kalkofen, W.
1999AAS...194.9309H    Altcode: 1999BAAS...31..990H
  We examine the excitation of oscillations in the magnetic network of
  the Sun through the buffeting action of external granules on vertical
  magnetic flux tubes extending through the photosphere. We assume that
  the granules motions are turbulent with an extended Kolmogorov energy
  spectrum. We model the interaction of the external flow field with
  the flux tube in terms of the Klein-Gordon equation, that is solved
  analytically as an initial value problem for transverse (kink) and
  longitudinal (waves). We compute the wave energy flux in both the modes
  for various magnetic field strengths and compare these results with our
  previous calculations in which the network oscillations are excited in
  short duration pulses by the occasional rapid motions of granules. We
  examine the consequences of our results for coronal heating.

---------------------------------------------------------
Title: 2D radiative equilibrium models of magnetic flux tubes
Authors: Hasan, S. S.; Kalkofen, W.; Steiner, O.
1999ASSL..243..409H    Altcode: 1999sopo.conf..409H
  No abstract at ADS

---------------------------------------------------------
Title: The excitation of oscillations in network bright points.
Authors: Kalkofen, W.; Hasan, S. S.
1999joso.proc..137K    Altcode:
  The authors study the excitation of flux tube waves in the photosphere
  by granular buffeting of magnetic flux tubes and show that for the
  strong magnetic fields of the network the emitted energy flux appears
  mainly in transverse (kink) waves, and only to a much smaller extent
  in longitudinal (sausage) waves.

---------------------------------------------------------
Title: Radiative Cooling, Convective Downflows and the Formation of
    Small-Scale Strong Magnetic Field Structures on the Sun
Authors: Rajaguru, S. Paul; Hasan, S. S.
1999soho....9E..16R    Altcode:
  Radiative energy transport has important dynamical consequences for
  the evolution of gas confined in small-scale magnetic structures on
  the Sun. The convective collapse process which transforms weak field
  structures into intense structures of strength in the range 1-2 kG,
  which eventually form sunspots in the emerging flux regions, is
  affected by the radiative heating from the surroundings and cooling
  due to vertical losses. Through a simple analysis of such radiative
  transfer effects in the diffusion approximation for a thin magnetic flux
  tube, we show the destabilizing action of vertical radiative losses in
  the form of an accelerated downflow, thereby verifying theoretically
  the thermal origin of the convective collapse process, as suggested
  by Parker (1978). We also perform numerical stability calculations
  involving the generalised Eddington approximation for radiative transfer
  and produce size(flux)-strength relations for the solar small-scale
  magnetic structures. Our results provide a physical explanation for
  the flux dependent (equivalently size dependent) field strength of
  the solar small-scale magnetic structures and the two-component (weak
  intra-network and the strong network) distribution is explained in terms
  of physical parameters that control the convective collapse process.

---------------------------------------------------------
Title: Spectral line radiation from solar small-scale flux tubes. II
Authors: Hasan, S. S.; Kneer, F.; Kalkofen, W.
1998A&A...332.1064H    Altcode:
  We examine spectral line radiation from small-scale magnetic flux tubes
  in the solar atmosphere. This is a continuation of work by Kneer et
  al. (1996). The main difference with the previous investigation is in
  the choice of the external atmosphere. Earlier we adopted an atmosphere
  resembling the empirical quiet Sun model for the ambient medium. In the
  present study, we iteratively adjust the temperature structure of the
  external atmosphere to fit the Stokes I and V profiles and the average
  continuum intensities with those obtained from observations. Our
  models are hotter in the uppermost photospheric layers and cooler
  in the deeper layers than the quiet Sun model and agree well with
  semi-empirical flux tube models.

---------------------------------------------------------
Title: Excitation of Longitudinal Modes in Solar Magnetic Flux Tubes
    by p-Modes
Authors: Hasan, S. S.; Kalkofen, W.
1998ASPC..154..767H    Altcode: 1998csss...10..767H
  This is a continuation of earlier work by Hasan (1997) on the
  interaction of longitudinal (sausage) waves in a slender flux tube
  with p-modes in the ambient medium. We use a realistic stratification
  for the flux tube and external atmospheres based upon the models of
  Hasan &amp; Kalkofen (1994). The MHD equations for a thin flux tube
  are solved as an initial value problem incorporating radiative and
  convective energy transport. Our calculations confirm the linear
  prediction that the interaction is non-resonant. We find that the
  response (for a fixed order) increases with mode degree l up to a
  maximum and then falls off sharply as l increases. For the f-mode,
  l_max ~650. The amplitude of the oscillations tend to become stationary
  implying a balance between energy input from p-modes and losses through
  radiative damping and leakage from boundaries. Low order p-modes with
  degrees of several hundred appear to be most efficient for exciting
  longitudinal oscillations in flux tubes. The energy flux in these
  oscillations appears to be insufficient for chromospheric heating,
  but may contribute partially to the required flux.

---------------------------------------------------------
Title: Why Does the Sun Have Kilogauss Magnetic Fields?
Authors: Hasan, S. S.; van Ballegooijen, A. A.
1998ASPC..154..630H    Altcode: 1998csss...10..630H
  Magnetic fields in the solar photosphere are concentrated in flux
  tubes with kilogauss field strength surrounded by nearly field-free
  plasma. Observations show that the flux tubes are located in convective
  downdrafts where the temperature is lower than average. We assume that
  the convective downdrafts extend to large depths in the convection
  zone, and that flux tubes follow the downdrafts to these depths. We
  develop a model for the magnetic field strength B(z) in the flux
  tubes as a function of depth z below the surface. Our calculations
  reveal that epsilon, the ratio of magnetic pressure to gas pressure,
  has a large depth variation: at the base of the convection zone where
  epsilon ~10^{-5} (B ~10^5 G), while at the top epsilon ~1, in broad
  agreement with solar observations. Thus the model can explain why the
  field strength at the photosphere is around 1 kG.

---------------------------------------------------------
Title: Radial Modes of Rotating Neutron Stars in the
    Chandrasekhar-Friedman Formalism
Authors: Datta, B.; Hasan, S. S.; Sahu, P. K.; Prasanna, A. R.
1998IJMPD...7...49D    Altcode:
  Eigenfrequencies of radial pulsations of "slowly" rotating neutron
  stars are calculated in a general relativistic formalism given by
  Chandrasekhar &amp; Friedman. It is found that the square of the
  frequencies are always a decreasing function of the central density of
  the neutron star. The decrease of the squared frequency is sensitive
  to the equation of state of neutron star matter, and is illustrated
  using realistic models.

---------------------------------------------------------
Title: The influence of magnetic fields on radiative damping of
    magnetoatmospheric oscillations
Authors: Banerjee, Dipankar; Hasan, S. S.; Christensen-Dalsgaard, J.
1998IAUS..185..423B    Altcode:
  We examine the non adiabatic effects on the modes of an isothermal
  stratified magnetic atmosphere. The present investigation
  is a continuation of earlier work by Banerjee, Hasan &amp;
  Christensen-Dalsgaard (1995, 1996, 1997), where the interaction of
  various elementary modes in a stratified magnetised atmosphere
  was studied in the purely adiabatic limit. The inclusion of
  radiative dissipation in magnetoatmospheric wave problem - even in
  the over simplified Newtonian Cooling approximation - is of great
  importance. Including Newtonian Cooling, in the weak field limit, an
  analytic expression for the dispersion relation is derived which allows
  the effect of weak magnetic field on the modes to be studied. We examine
  the nature of the eigen frequency curves in the diagnostic diagram and
  find that, similar to the previous analysis, the modes undergo avoided
  crossings. We study the full frequency spectrum and the interaction
  amongst various modes. Strong mode coupling in the avoided crossing
  regions permits energy leakage. Our results, find application in the
  analysis of waves in flux-tubes on the Sun. We study the influence of
  magnetoatmospheric waves on spectral line profiles.

---------------------------------------------------------
Title: The Excitation of Oscillations in Network Bright Points
Authors: Kalkofen, W.; Hasan, S. S.
1998joso.proc..137K    Altcode:
  We study the excitation of flux tube waves in the photosphere by
  granular buffeting of magnetic flux tubes and show that for the strong
  magnetic fields of the network the emitted energy flux appears mainly
  in transverse (kink) waves, and only to a much smaller extent in
  longitudinal (sausage) waves.

---------------------------------------------------------
Title: Structure of small-scale magnetic flux tubes and excitation
    of sausage mode oscillations
Authors: Hasan, S. S.
1998BASI...26..181H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Influence of Magnetic Flux Tubes on Their Environment
Authors: Hasan, S. S.; Kalkofen, W.
1998ASPC..154..838H    Altcode: 1998csss...10..838H
  We present new calculations for model atmospheres in magnetic flux
  tubes extending vertically through the photosphere and convection zone
  of the Sun. This study is a continuation of earlier work by Hasan &amp;
  Kalkofen (1994) on the equilibrium structure of intense magnetic flux
  tubes. We construct static models of flux tubes by solving the equations
  of energy transport and radiative transfer. The most significant aspect
  of our study is the inclusion of multidimensional radiative transfer in
  cylindrical geometry and the influence of the flux tube on the ambient
  medium. Our models satisfy the condition of radiative equilibrium;
  the lower boundary intensity includes the effect of convection. We
  determine the structure of the thermal boundary layer at the interface
  of the flux tube and the ambient medium. We find that the temperature
  does not change abruptly from its value on the flux tube axis to the
  ambient value far from the tube. Rather, there is a transition layer
  at the interface, where there is a significant horizontal temperature
  gradient. Detailed calculations have been carried out to examine the
  physical conditions in this layer as well as its horizontal extent.

---------------------------------------------------------
Title: The influence of radiative damping on the modes of a magnetized
    isothermal atmosphere
Authors: Banerjee, D.; Hasan, S. S.; Christensen-Dalsgaard, J.
1997ASSL..225..277B    Altcode: 1997scor.proc..277B
  We examine the influence of radiative damping on the modes of an
  isothermal magnetized plasma. This investigation is an extension of
  earlier work by Hasan \&amp; Christensen-Dalsgaard (1992) and Banerjee,
  Hasan \&amp; Christensen-Dalsgaard (1995, 1996), which treated the
  effects of a uniform vertical magnetic field on the adiabatic modes
  of a stratified atmosphere. We focus on the properties of the Lamb
  mode and examine its interaction with a magnetic mode. We show that
  radiative dissipation can significantly alter the Lamb-mode frequency
  and lead to a enhanced damping of the waves at the avoided crossings
  in the $k-\omega$ diagram. This mechanism could be important for wave
  damping in flux tubes on the Sun.

---------------------------------------------------------
Title: Dynamical phenomena in sunspots. I. Time dependent relaxation
    to equilibrium.
Authors: Gangadhara, R. T.; Hasan, S. S.
1997A&A...327..786G    Altcode:
  We have adapted a general purpose time-dependent 2-D code to study
  dynamical phenomena in sunspots. In the first part of our investigation,
  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 equations
  to examine the evolution of a sunspot towards equilibrium, starting
  from an arbitrary initial state. Initially, we choose a potential
  magnetic field configuration and assume hydrostatic equilibrium along
  field lines, which allows the pressure variation along the field to
  be determined, for a known temperature distribution. We also assume
  that all quantities in the tube have a smooth and continuous radial
  variation. In particular the pressure increases radially from the
  tube axis to the photospheric value. The absence of Lorentz forces
  to balance the radial pressure gradient, leads to an inflow of gas
  towards the axis accompanied by an increase in the magnetic field
  strength. A complex flow pattern develops in the tube, which eventually
  dies out due to escape of matter upwards along the field lines. In the
  quasi-equilibrium state it is found that the field lines near the center
  of a large spot assume a configuration which is almost potential while
  those at the periphery depart significantly from the initial state,
  due to being pushed inwards by the gas flow. Our method is applicable
  to both thin and thick flux tubes. Further it can be readily extended
  to any coordinate system with 2 or 3 coordinates, and to discontinuous
  configurations such as current sheets. Forthcoming studies will focus on
  an extension of the present study to an analysis of dynamical effects
  in sunspots associated with nonlinear waves and examine the transport
  of energy by these to the corona.

---------------------------------------------------------
Title: The Linear Response of a Magnetic Flux Tube to Buffeting by
    External p-Modes. I.
Authors: Hasan, S. S.
1997ApJ...480..803H    Altcode:
  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 case of an isothermal atmosphere is considered,
  which has the mathematical advantage that the differential equation
  for the vertical component of the Lagrangian displacement in the tube
  can be solved analytically. A lower boundary condition is employed
  that permits the tube wave to leak out through this boundary. This
  has the important consequence that the p-mode interaction with flux
  tubes does not exhibit a resonant behavior. The detailed behavior of
  the vertical displacement in the tube and its dependence on various
  parameters are examined. An equation for the wave energy in a thin flux
  tube is derived along with analytic expressions for the wave energy
  density and vertical energy flux. The variation of the tube response Ξ
  (defined as the ratio of the total wave energy, integrated over the
  length of the tube, to the p-mode energy in the external atmosphere)
  is investigated for different values of the dimensionless external
  horizontal wavenumber K<SUB>x</SUB>, mode order n, and β, where
  β is the ratio of the gas to magnetic pressure in the tube that,
  by assumption, is constant with depth. It is found that when n is
  small, the response of the tube increases gradually with K<SUB>x</SUB>
  until reaching a maximum, and thereafter it drops very sharply. As n
  increases, the maximum shifts to lower values of K<SUB>x</SUB>. For
  fixed values of K<SUB>x</SUB> and β, Ξ increases with n and then
  falls off after reaching a maximum. A similar dependence of Ξ on β is
  found. Line widths of p-modes are also calculated, and their dependence
  on K<SUB>x</SUB> and frequency is studied. Finally, an application of
  the results to the solar atmosphere is discussed, and the limitations
  of the model are pointed out.

---------------------------------------------------------
Title: Effect of Newtonian Cooling on Waves in a Magnetized Isothermal
    Atmosphere
Authors: Banerjee, Dipankar; Hasan, S. S.; Christensen-Dalsgaard, J.
1997SoPh..172...53B    Altcode: 1997ESPM....8...53B
  We examine the influence of nonadiabatic effects on the modes of an
  isothermal stratified magnetic atmosphere. The present investigation is
  a continuation of earlier work by Hasan and Christensen-Dalsgaard (1992)
  and Banerjee, Hasan, and Christensen-Dalsgaard (1995, 1996), where
  the interaction of various elementary modes in a stratified magnetized
  atmosphere was studied in the purely adiabatic limit. The inclusion of
  radiative dissipation based on Newton's law of cooling demonstrates
  the importance of this effect in the study of magnetoatmospheric
  waves. We analyze the physical nature of magnetoacoustic gravity (or
  MAG) oscillations in the presence of Newtonian cooling and find that
  the eigenfrequency curves in the diagnostic diagram, as in the previous
  analysis, undergo avoided crossings. However, the qualitative nature of
  the mode interaction is strongly influenced by radiative dissipation,
  which leads to strong mode damping in the avoided-crossing regions. We
  demonstrate this effect for the interaction between the Lamb mode and
  a magnetic mode. Our results could be important in the analysis of
  waves in flux tubes on the Sun.

---------------------------------------------------------
Title: Excitation of longitudinal modes in Magnetic Flux Tubes by
    external p-modes
Authors: Hasan, S. S.; Kalkofen, W.
1997SPD....28.0237H    Altcode: 1997BAAS...29R.899H
  This is a continuation of earlier work by Hasan (1997, ApJ 480,
  in press) on the interaction of sausage waves in a slender flux
  tube with p-modes in the ambient medium. In the above paper, the
  time-asymptotic response of a vertical tube in the solar photosphere
  due to the buffeting action by external p-modes was examined using
  linear theory. For mathematical tractability an isothermal atmosphere
  was assumed, which allowed the problem to be solved exactly. This
  study provided insight into the behavior of the tube response and
  its dependence on various parameters. The previous analysis has
  now been extended in three important respects; firstly, a realistic
  stratification for the flux tube and external atmospheres has been
  used based upon the models of Hasan and Kalkofen (1994, ApJ 436,
  355). Secondly, the interaction has been treated as an initial value
  problem, which enables the gradual buildup of energy in flux tube
  oscillations to be studied. Thirdly, non-adiabatic effects involving
  radiative transport have been taken into account. Our calculations
  confirm the linear prediction that the interaction is non-resonant. We
  find that the response does not exhibit a monotonic variation with the
  p-mode degree l (for a fixed order), but increases to a maximum and
  then falls off sharply as l increases. For the f- mode, l<SUB>max</SUB>
  ~ 700. The amplitude of the oscillations tend to become stationary
  (i.e. constant in time) implying a balance between energy input
  from p-modes and losses through radiative damping and leakage from
  boundaries. The dominant contribution to the energy flux in the upper
  photosphere comes from the enthalpy flux. This flux by itself appears to
  be insufficient for chromospheric heating, but may contribute partially
  to the required flux. P-modes of low order and degree appear to be
  most efficient for exciting longitudinal oscillations in flux tubes.

---------------------------------------------------------
Title: The effect of p-modes on thin magnetic flux tubes
Authors: Hasan, S. S.; Bogdan, T. J.
1996BASI...24..125H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Wave leakage in a magnetized isothermal atmosphere
Authors: Banerjee, Dipankar; Hasan, S. S.; Christensen-Dalsgaard, J.
1996BASI...24..325B    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Modelling sunspot equilibrium through a solution of the time
    dependent MHD equations
Authors: Gangadhara, R. T.; Hasan, S. S.
1996BASI...24..341G    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Dynamical Effects in Solar Photospheric Flux Tubes
Authors: Hasan, S. S.
1996Ap&SS.243..155H    Altcode: 1996IAUCo.154..155H
  The interaction of an intense flux tube, extending vertically through
  the photosphere, with p-modes in the ambient medium is modelled
  by solving the time dependent MHD equations in the thin flux tube
  approximation. It is found that a resonant interaction can occur,
  which leads to the excitation of flux tube oscillations with large
  amplitudes. The resonance is not as sharp as in the case of an
  unstratified atmosphere, but is broadened by a factor proportional
  toH <SUP>-2</SUP>, whereH is the local pressure scale height. In
  addition, the inclusion of radiative transport leads to a decrease in
  the amplitude of the oscillations, but does not qualitatively change
  the nature of the interaction.

---------------------------------------------------------
Title: Spectral line radiation from solar small-scale magnetic
    flux tubes.
Authors: Kneer, F.; Hasan, S. S.; Kalkofen, W.
1996A&A...305..660K    Altcode:
  We consider spectral line radiation from small-scale magnetic model flux
  tubes in the solar atmosphere. The structure of the tube is determined
  from the magnetostatic equations in the thin flux tube approximation. We
  assume that the tube is in energy equilibrium and pressure balance
  with the ambient medium. For the latter, we construct a quiet sun model
  with an artificial heating term in order to reproduce the VAL C model,
  treating the medium as a plane-parallel atmosphere. The flux tube models
  are parameterized by the plasma β_0_ (the ratio of gas the pressure
  to the magnetic pressure), the convective efficiency parameter α,
  and the radius R_0_ at height z=0 (τ_5000_=1) in the quiet sun. The
  Stokes I and V profiles emerging from the models and averaged over
  areas that include the neighbourhood of the flux tube are calculated
  for various spectral lines with different sensitivity for magnetic
  field strength and temperature. The profiles are compared with high
  spatial resolution observations of plages near disc centre that have
  been obtained with the Gregory Coude Telescope at the Observatorio del
  Teide/Tenerife. The information contained in both I and V profiles is
  found to be very useful in constraining the theoretical models. The
  best match of models with observations is achieved for values of β_0_
  between 0.3 and 0.5. For a sufficiently wide separation of the V extrema
  of the strongly split lines, a broadening mechanism is required. Pure
  velocity (microturbulent) broadening compatible with observations
  of strongly split lines gives too much broadening for weakly split
  lines. A broadening that is proportional to the Lande factor, i.e.,
  magnetic broadening, appears to be more appropriate. This suggests
  dynamic models with temporary enhancement of the magnetic field
  strength. The continuum intensity of our models is higher and the
  absorption and V amplitude in the FeII 6149A line are stronger than
  observed. An improvement in the match between model predictions and
  observations is likely to come from models in which the ambient gas
  has a lower temperature as well as a lower temperature gradient than
  are found in the quiet, field-free sun. Such models are currently
  under development for cylindrical flux tubes.

---------------------------------------------------------
Title: The Influence of a Vertical Magnetic Field on Oscillations
    in an Isothermal Stratified Atmosphere. II.
Authors: Banerjee, Dipankar; Hasan, S. S.; Christensen-Dalsgaard, J.
1995ApJ...451..825B    Altcode:
  We examine the effect of a uniform vertical magnetic field on the modes
  of an isothermal stratified atmosphere. The present investigation is
  a continuation of earlier work by Hasan &amp; Christensen-Dalsgaard in
  which this problem was studied for rigid boundary conditions. In this
  paper, the earlier results are extended to different sets of boundary
  conditions. We demonstrate explicitly how these boundary conditions
  affect the various elementary wave modes present in the atmosphere. In
  the weak-field limit, an analytic expression for the dispersion relation
  is derived, which allows the effect of a weak magnetic field on the
  modes to be studied. We show that, to lowest order in our perturbation
  expansion, the oscillation spectrum can be analyzed in terms of (a) p-
  and g-like modes; (b) a magnetic Lamb mode; (c) magnetic or slow modes;
  and (d) a gravity- Lamb mode. The first three of these were present
  in the previous analysis for rigid boundaries, whereas the last is
  a consequence of the vertical gradients of the displacements at the
  boundaries. We focus our attention on the properties of this mode and
  show that it is present even in the moderate to strong field case as a
  magnetogravity-Lamb mode. The recognition and physical interpretation
  of this mode is a new feature of the present analysis. We also examine
  the nature of the eigenfrequency curves in the diagnostic (or K-Ω)
  diagram and find that, similar to the previous analysis, the modes
  undergo avoided crossings. However, the nature of the solutions in
  the present case is more complicated, especially when triple-mode
  interactions occur. Furthermore, the connectivity of the curves in
  the K-Ω diagram can be strongly influenced by the choice of boundary
  conditions. Our results, though somewhat idealized, find application
  in the analysis of waves in sunspots. It is conjectured that conditions
  for the existence of the magnetogravity-Lamb mode may also be satisfied
  in the subphotospheric layers of the Sun.

---------------------------------------------------------
Title: Helioseismology - Impact and Implications
Authors: Hasan, S. S.
1995BASI...23..337H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Equilibrium Structure of Solar Magnetic Flux Tubes: Energy
    Transport with Multistream Radiative Transfer
Authors: Hasan, S. S.; Kalkofen, W.
1994ApJ...436..355H    Altcode:
  We examine the equilibrium structure of vertical intense magnetic
  flux tubes on the Sun. Assuming cylindrical geometry, we solve the
  magnetohydrostatic equations in the thin flux-tube approximation,
  allowing for energy transport by radiation and convection. The radiative
  transfer equation is solved in the six-stream approximation, assuming
  gray opacity and local thermodynamic equilibrium. This constitutes
  a significant improvement over a previous study, in which the
  transfer was solved using the multidimensional generalization of the
  Eddington approximation. Convection in the flux tube is treated using
  mixing-length theory, with an additional parameter alpha, characterizing
  the suppression of convective energy transport in the tube by the
  strong magnetic field. The equations are solved using the method of
  partial linearization. We present results for tubes with different
  values of the magnetic field strength and radius at a fixed depth in
  the atmosphere. In general, we find that, at equal geometric heights,
  the temperature on the tube axis, compared to the ambient medium,
  is higher in the photosphere and lower in the convection zone, with
  the difference becoming larger for thicker tubes. At equal optical
  depths the tubes are generally hotter than their surroundings. The
  results are comparatively insensitive to alpha but depend upon whether
  radiative and convective energy transport operate simultaneously or
  in separate layers. A comparison of our results with semiempirical
  models shows that the temperature and intensity contrast are in broad
  agreement. However, the field strengths of the flux-tube models are
  somewhat lower than the values inferred from observations.

---------------------------------------------------------
Title: Thin flux tube models with multistream radiative transfer
Authors: Hasan, S. S.; Kalkofen, W.
1994smf..conf..334H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Influence of a Vertical Magnetic Field on Oscillations
    in an Isothermal Stratified Atmosphere
Authors: Hasan, S. S.; Christensen-Dalsgaard, J.
1992ApJ...396..311H    Altcode:
  The effect of a uniform vertical magnetic field on the modes
  of an isothermal stratified atmosphere is examined. The general
  solutions of the wave equation for an isothermal magnetized medium
  are given. Asymptotic expansions of these solutions are presented in
  the strong- and weak-field limits. For a weak field, it is found that,
  to lowest order of the present perturbation expansion, the oscillation
  spectrum can be analyzed in terms of p- and g-like modes, magnetic
  Lamb modes, and magnetic or slow modes. The frequency corrections
  for each of the modes due to coupling with the remaining modes are
  calculated. It is demonstrated that when the frequencies of different
  modes are almost identical, strong mode coupling occurs and the waves
  acquire a mixed character. An analysis of the solutions in the vicinity
  of the degenerate frequencies is carried out. The solutions reveal the
  presence of avoided crossings, which occur at the otherwise degenerate
  frequencies. Examples of different types of avoided crossings are given.

---------------------------------------------------------
Title: Indian Institute of Astrophysics. Annual report 1990 - 91.
Authors: Parthasarathy, M.; Hasan, S. S.; Srinivasan, R.
1992iiaa.book.....P    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Heating in Intense Flux Tubes (With 4 Figures)
Authors: Hasan, S. S.
1991mcch.conf..408H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Magnetoatmospheric Oscillations in Sunspot Umbrae
Authors: Hasan, S. S.
1991ApJ...366..328H    Altcode:
  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 axisymmetric normal modes of magnetoatmospheric
  waves were determined, and a diagnostic diagram was generated
  for different field strengths. The diagram shows the existence of
  'avoided crossings' in the solution of magnetoatmospheric modes in a
  sunspot atmosphere. It was found that, for low values of the radial
  wavenumber k, corresponding to observed oscillations, a simple global
  classification is not possible. The oscillations in the low photosphere
  and below are of either fast or mixed type, but tend to acquire a slow
  or acoustic character above the temperature minimum.

---------------------------------------------------------
Title: Indian Institute of Astrophysics. Annual report 1989 - 90.
Authors: Parthasarathy, M.; Hasan, S. S.
1991iiaa.book.....P    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Dynamical effects and energy transport in intense flux tubes
Authors: Hasan, S. S.
1990GMS....58..157H    Altcode:
  The present effort to obtain a realistic model of conditions inside
  intense flux tubes employs calculations which treat radiative transport
  in the Eddington approximation, while allowing for convective energy
  transport within the flux tube. After constructing an equilibrium
  atmosphere within the tube, this equilibrium is perturbed through
  the introduction of a small downflow. Complex oscillatory behavior
  is observed in which upflow and downflow phases do not appear to be
  symmetric. Vertical energy transport through radiation emerges as
  very important, especially in the proximity of continuum optical-depth
  unity. Observational implications are noted.

---------------------------------------------------------
Title: Wave Propagation in Sunspots
Authors: Hasan, S. S.
1990IAUS..142..189H    Altcode:
  Wave propagation in sunspot umbrae is analyzed; the stratification
  in a typical umbra is approximated by a model atmosphere, extending
  vertically from a depth of a few thousand kilometers below the
  photosphere, to the transition region. A Rayleigh-Ritz variational
  technique is used to calculate the normal mode frequencies of the
  umbra for different values of the horizontal wave number. The nature
  of the wave modes is examined by decomposing the eigenvectors into
  longitudinal and transverse components.

---------------------------------------------------------
Title: Classification of Magnetoatmospheric Modes in Sunspot Umbrae
Authors: Hasan, S. S.; Sobouti, Y.
1990IAUS..138..255H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Wave modes in thick photospheric flux tubes - Classification
    and diagnostic diagram
Authors: Hasan, S. S.; Abdelatif, T.
1990GMS....58...93H    Altcode:
  The nature of wave motions in thick photospheric flux tubes is
  analyzed. The aim of this investigation is to determine the normal modes
  of a stratified atmosphere with a vertical magnetic field and to discuss
  their properties. The results are displayed in the form of a diagnostic
  diagram. An interesting feature of the solutions is the existence of
  'avoided crossings', which occur when adjacent order modes approach
  each other in the diagnostic diagram. In general, the character of
  a mode changes with height in the atmosphere. Results are applied to
  umbral oscillations, and it is found that the observed oscillations
  with periods in the range 2-3 min, correspond to low-order modes in
  these calculations. For low horizontal wave number K, the modes, in
  the photosphere, have almost equal contributions from longitudinal
  and transverse components. As K increases, the transverse component
  begins to dominate. In the chromosphere, the modes are essentially
  transverse and can be identified with slow modes.

---------------------------------------------------------
Title: Indian Institute of Astrophysics. Annual report 1987 - 88.
Authors: Hasan, S. S.
1989iiaa.book.....H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Energy Transport in Intense Flux Tubes on the
    Sun. I. Equilibrium Atmospheres
Authors: Hasan, S. Sirajul
1988ApJ...332..499H    Altcode:
  Model calculations are performed to determine self-consistently the
  equilibrium atmosphere in an intense flux tube. The generalization
  of the Eddington approximation to three dimensions is used to develop
  a zeroth-order transfer equation valid for an axisymmetric thin flux
  tube. Convection is included by using a mixing length formalism and
  a parameter to incorporate its inhibition by the magnetic field. The
  results suggest that the temperature on the axis of a flux tube is
  lower than the ambient medium at the same geometric level. At equal
  optical depths, however, the temperature in the tube is higher. At an
  optical depth of unity, the temperature difference is typically about
  500 K. In the optically thin layers, horizontal exchange of heat is
  efficient and the temperature inside the tube is insensitive to the
  magnetic field strength. The equilibrium stratification is almost
  independent of the degree of convective inhibition.

---------------------------------------------------------
Title: Mode classification and wave propagation in a magnetically
    structured medium
Authors: Hasan, S. S.; Sobouti, Y.
1987MNRAS.228..427H    Altcode:
  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 version of Helmholtz's theorem, the decompose
  the perturbations into an irrotational component and a solenoidal
  component, which they further split into the sum of poloidal and
  toroidal components. These components are identified with p, g and
  toroidal modes of a fluid. The normal modes of the tube are determined
  using a Rayleigh-Ritz variational technique. The authors' technique
  efficiently isolates all the modes to high orders. They first consider
  some special cases, in order to highlight some interesting properties of
  the modes. Then, they choose a parameter range to study the properties
  of oscillations in intense flux tubes on the Sun.

---------------------------------------------------------
Title: Stability of cool flux tubes in the solar chromosphere. Linear
    analysis.
Authors: Hasan, S. S.; Kneer, F.
1986A&A...158..288H    Altcode:
  The linear stability of cool flux tubes in the solar chromosphere
  which are initially in radiative equilibrium is examined. Owing to
  the presence of carbon monoxide, there exists a narrow region near the
  temperature minimum where the temperature gradient becomes steep enough
  to drive a convective instability. The thin flux tube equations are
  used and in a simple manner radiative heat exchange with the ambient
  medium are included. Initial states of constant beta (where beta is
  the ratio of gas to magnetic pressure) are considered. Results for
  various values of beta are presented. It is found that for beta less
  than 5.7 the tube is overstable with periods in the range 300-600 s. At
  beta = 5.7 a bifurcation occurs into two purely growing modes. Growth
  rates, eigenvectors of the fundamental modes are calculated and phase
  relationships are examined. It is suggested that overstable oscillations
  should invariably be associated with cool flux tubes. These oscillations
  transport energy and can thus change the thermodynamic structure of flux
  tubes. It is conjectured that the CO overstability may be responsible
  for spicules.

---------------------------------------------------------
Title: Oscillatory motions in intense flux tubes
Authors: Hasan, S. S.
1986MNRAS.219..357H    Altcode:
  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 atmospheres by means of linear analysis; in the
  latter case, a height-dependent radiation exchange time constant
  is used. For purposes of comparison with earlier studies, results
  for the adiabatic case are also given. These results indicate that
  states, characteristicaly stable in the adiabatic limit, can be
  driven overstable when heat exchange is included in the analysis. It
  was found that for solar stratification, oscillatory behavior occurs
  for Beta less than Beta<SUB>c,</SUB> where Beta<SUB>c</SUB> denotes
  a (tube) radius-dependent critical value. A bifurcation at Beta =
  Beta<SUB>c</SUB> was also found owing to overstability into two
  purely unstable modes. The height dependence of the eigenvectors
  is discussed along with the sensitivity of the results on boundary
  conditions. Observational consequences of the study are also noted.

---------------------------------------------------------
Title: One-Dimensional Model Calculations of Flux Tubes
Authors: Hasan, S. S.
1986ssmf.conf..121H    Altcode:
  The author presents 1-D model calculations of intense flux tubes on the
  Sun. Assuming an initial state in hydrostatic and thermal equilibrium
  (with the ambient medium), he models the collapse of such a tube due
  to a convective instability by numerically solving the time dependent
  MHD equations in the thin flux tube approximation. The results indicate
  that the collapse produces a final state with field strengths in the
  kG range. This state is not steady, but one exhibiting overstable
  oscillations (in time). Furthermore, flows are generated in the
  tube during the collapse with a peak amplitude 1 - 2 km/s, but with a
  time-averaged value around zero. Some of the observational implications
  are discussed.

---------------------------------------------------------
Title: Heating of solar magnetic elements by downflows
Authors: Hasan, S. S.; Schuessler, M.
1985A&A...151...69H    Altcode:
  The idea that magnetic elements in the photosphere and lower
  chromosphere of the sun are heated by downflowing gas is quantitatively
  examined. The time-dependent hydromagnetic equations are solved
  numerically in the slender flux tube approximation. Viscous terms are
  retained, and the radiative exchange of heat between the flux tube
  and the ambient medium are included. Hydrogen ionization and its
  thermodynamic consequences are treated self-consistently. Starting
  from a state of hydrostatic and thermal equilibrium, the temporal
  response due to the onset of a downflow in the tube is studied. After a
  transient phase lasting a few minutes, a stationary state results that
  is substantially hotter than the ambient medium over a fairly large
  height range. Chapman's facular model can be reproduced remarkably well
  by adjusting the mass flux entering the tube at the upper boundary. The
  results are comparatively insensitive to viscosity (nu less than or
  equal to 10 to the 12th sq cm/s), while radiative heat exchange is
  significant. Some observational implications are discussed, and it is
  suggested that the necessary mass flux could be provided by overstable
  oscillations during their downflow phase.

---------------------------------------------------------
Title: Convective instability in a solar flux tube. II - Nonlinear
    calculations with horizontal radiative heat transport and finite
    viscosity
Authors: Hasan, S. S.
1985A&A...143...39H    Altcode:
  Convective instability in a thin flux tube is examined in the presence
  of horizontal radiative heat transport and finite viscosity. The
  temporal behaviour of flux tubes initially in hydrostatic and thermal
  equilibrium is studied by solving the nonlinear time dependent equations
  for a thin flux tube. An important result of the investigation is the
  demonstration of overstable oscillations with periods typically about
  1000s in intense flux tubes on the Sun. Overstability is a direct
  consequence of radiative exchange between the flux tube and the
  surrounding medium. Detailed calculations are presented for a broad
  range of parameters which characterize the strength of the magnetic
  field and the tube radius in the intial state. An initial stratification
  based on a model atmosphere is used. The results indicate that that
  overstable oscillations are produced as an end state of the convective
  instability. It is found that the surface magnetic field that results
  is in the observed kG range. Furthermore, there is also an oscillating
  flow (unidirectional at any given instant) with an amplitude between
  1 and 2 km s<SUP>-1</SUP>, with an average value that is approximately
  zero. The observational implications of these results are discussed.

---------------------------------------------------------
Title: Convective collapse and overstable oscillations in solar
    flux tubes.
Authors: Hasan, S. S.
1984ESASP.220..227H    Altcode: 1984ESPM....4..227H
  The collapse of solar flux tubes by a convective instability is
  investigated. The final state consists of overstable oscillations with
  periods typically around 1000 s.

---------------------------------------------------------
Title: Time-dependent convective collapse of flux tubes
Authors: Hasan, S. S.
1983IAUS..102...73H    Altcode:
  The time-dependent collapse of a slender flux tube extending vertically
  in the convection zone of the sun is modelled. Starting from an
  initial state in which the flux tube is in hydrostatic equilibrium, the
  non-linear MHD equations are used to examine its temporal evolution. A
  detailed study of the flow variables and magnetic field within the tube
  is presented. It is seen that asymptotically in time a unique state of
  dynamic equilibrium is established, irrespective of the value of beta-O
  (the ratio of the thermal to magnetic pressure at the initial epoch).

---------------------------------------------------------
Title: Transient Response of the Solar Wind to Changes in Flow
    Geometry - Flows in Coronal Holes
Authors: Hasan, S. S.; Venkatakrishnan, P.
1982SoPh...80..385H    Altcode:
  The transient response of the solar wind to changes in geometry is
  examined. An initial stationary flow in a configuration that diverges
  as r<SUP>2</SUP> is assumed. This state corresponds to the usual solar
  wind solution. The effect on the flow through a tube whose area A(r,
  t) diverges faster than r<SUP>2</SUP>, with the degree of divergence
  increasing in time, is considered. The asymptotic form of A(r, t)
  is chosen to mimic the form inferred in coronal holes. A detailed
  parameter study relating the form of A(r, t) to the pattern of flow in
  the tube is presented. It is observed that in the limit of large time
  (large compared to τ, the time constant for change in geometry of a
  flow tube) the solutions obtained from a time-dependent analysis can
  depend upon τ. For sufficiently large τ, the asymptotic solution is
  the same as the steady state solution obeying the correct boundary
  conditions and possessing a smooth sonic transition. However, if
  the geometry changes rapidly enough, solutions exhibiting shock-like
  discontinuities can also exist. This is essentially a new feature that
  emerges from the present investigation. Finally, it is suggested that
  this study may be useful in describing flows in evolving coronal holes.

---------------------------------------------------------
Title: Comment on the paper `a new resonance in the solar atmosphere'
    by Joseph V. Hollweg
Authors: Venkatakrishnan, P.; Hasan, S. S.
1982SoPh...75...79V    Altcode:
  In the absence of genuine forcing terms, there is no resonance between
  linear fast mhd and gravito-acoustic waves.

---------------------------------------------------------
Title: A Time Dependent Model for Spicule Flow
Authors: Hasan, S. S.; Venkatakrishnan, P.
1981SoPh...73...45H    Altcode:
  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.

---------------------------------------------------------
Title: Time-dependent interaction of granules with magnetic flux tubes
Authors: Venkatakrishnan, P.; Hasan, S. S.
1981JApA....2..133V    Altcode:
  The time-dependent interaction of the granulation velocity field with
  a magnetic flux tube is investigated here. It is seen that when a
  magnetic field line is displaced normal to itself so as to simulate
  the buffeting action of granules, a flow of gas is initiated along the
  field. By choosing a lateral velocity field which is consistent with
  observations of granules, it is found that the resulting gas motion is
  a downward flow with a velocity compatible with the observed downflow
  in isolated photospheric flux tubes. It is therefore proposed that the
  observed photospheric downflow is a manifestation of the interaction
  of granules with flux tubes.

---------------------------------------------------------
Title: A time dependent model for spicule flow.
Authors: Hasan, S. S.; Venkatakrishnan, P.
1981BASI....9...74H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Magnetohydrodynamic Equilibrium and Stability of Pre-Flare
    Loops - Constant Pitch Field
Authors: Hasan, S. S.
1980SoPh...67..267H    Altcode:
  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 in the loop typical values of parameters which describe the
  equilibrium are found for a magnetic field with a constant twist. The
  stability of this configuration is examined next and it is shown that
  for the force-free case, the structure is always unstable to kinking for
  any degree of twist. However, a slight deviation from the force-free
  configuration, through the presence of a small positive transverse
  pressure gradient, can stabilize the loops for moderate degrees of
  twist. The range of wave-numbers for which instability occurs and the
  maximum growth rates are also presented. Lastly, it is shown that the
  pressure gradients required to stabilize a pre-flare loop do not lead
  to conflict with observations.

---------------------------------------------------------
Title: Flow of Gas Along a Magnetic Field with Time Dependent Geometry
Authors: Hasan, S. S.; Venkatakrishnan, P.
1980KodOB...3....6H    Altcode:
  The flow of gas along a magnetic field with time dependent geometry
  has been studied. It is seen that the velocity of the flow in the
  direction of the magnetic field depends both on the magnitude of the
  velocity of flow perpendicular to the field as well as on its spatial
  variation. Further, the nature of the flow is not very sensitive to
  the choice of base temperature and polytropic index. The application
  of this study to magnetofluid dynamic flow on the Sun is discussed.

---------------------------------------------------------
Title: A Magneto-Hydrodynamic Study of Pre-Flare Loops.
Authors: Hasan, S. S.
1979phsp.coll..233H    Altcode: 1979IAUCo..44..233H; 1979phsp.conf..233H
  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 their surroundings; (2) a few hours before the
  flare the configuration gradually acquires energy in the form of
  currents; (3) during the period of energy build-up, the loops are
  magnetohydrodynamically stable; and (4) the flare occurs only after
  there is adequate energy in the currents. Assuming a cylindrical
  geometry, equations are then presented and solved for the pressure
  and magnetic field equilibrium distribution within the loop using
  a typical energy value associated with a subflare and assuming the
  case of an approximately force-free field. An equation for the MHD
  stability of the system is then solved as an eigenvalue problem for
  the frequency of the normal modes. It is shown that the force-free
  configuration is unstable for all cases considered, with the wavenumber
  region for instability increasing inversely with pitch. It is concluded
  that stable configurations for loops possessing adequate energy for a
  flare are possible only if positive pressure gradients of sufficient
  magnitude exist.

---------------------------------------------------------
Title: The Alfvén-Carlquist Double-Layer Theory of Solar Flares
Authors: Hasan, S. S.; Ter Haar, D.
1978Ap&SS..56...89H    Altcode:
  We use the Vlasov equations for ions and electrons to develop a theory
  of a double layer in which there are both free and trapped electrons
  and ions. We find the equations which replace the Langmuir condition
  and the Bohm conditions and by numerically solving the resultant
  differential equation we find for particular choices of distribution
  functions the potential distribution in the layer. We discuss the
  applicability of this theory to solar flares, and show that conditions
  in solar flares may be such that double layers can exist for which the
  free particles have a power-law energy distribution. These particles
  will be accelerated in a double layer and may in this way account for
  the production of high-energy particles during the impulsive phase of
  solar flares.