Author name code: hasan ADS astronomy entries on 2022-09-14 =author:"Hasan, S.S." OR =author:"Hasan, S. Sirajul" ------------------------------------------------------------------------ Title: National Large Solar Telescope Authors: Hasan, S. S.; Banerjee, D.; Ravindra, B.; Sankarasubramanian, K.; Rangarajan, K. E. Bibcode: 2017CSci..113..696H Altcode: No abstract at ADS 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 Bibcode: 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. Title: Solar Astronomy at High Altitude Authors: Hasan, S. S.; Bagare, S. P.; Rangarajan, K. E. Bibcode: 2014PINSA..80..815H Altcode: No abstract at ADS Title: Scientific Programmes with India's National Large Solar Telescope and their contribution to Prominence Research Authors: Hasan, S. S. Bibcode: 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.

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. Title: Acoustic emission from magnetic flux tubes in the solar network Authors: Vigeesh, G.; Hasan, S. S. Bibcode: 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. 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. Bibcode: 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 < 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. Title: India's National Large Solar Telescope Authors: Hasan, S. S. Bibcode: 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. Title: The National Large Solar Telescope (NLST) of India Authors: Hasan, S. S. Bibcode: 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. Title: Three-dimensional Simulations of Magnetohydrodynamic Waves in Magnetized Solar Atmosphere Authors: Vigeesh, G.; Fedun, V.; Hasan, S. S.; Erdélyi, R. Bibcode: 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. 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 Bibcode: 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. Title: Stokes Diagnostics of Magneto-Acoustic Wave Propagation in the Magnetic Network on the Sun Authors: Vigeesh, G.; Steiner, O.; Hasan, S. S. Bibcode: 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. Title: Magnetohydrostatic equilibrium in starspots: dependences on color (Teff) and surface gravity (g) Authors: Rajaguru, S. P.; Hasan, S. S. Bibcode: 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 (Teff), 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 Teff due to a combination of pressure and opacity variations against Teff. 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. Bibcode: 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. Title: National Large Solar Telescope of India Authors: Hasan, S. S. Bibcode: 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.

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. Title: Solar physics in India: developments from the nineteenth century to the present era Authors: Hasan, S. S.; Choudhuri, Arnab Rai; Rajaguru, S. P. Bibcode: 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. Title: NLST: the Indian National Large Solar Telescope Authors: Hasan, S. S.; Soltau, D.; Kärcher, H.; Süss, M.; Berkefeld, T. Bibcode: 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 Title: NLST: India's National Large Solar Telescope Authors: Hasan, S. S.; Soltau, D.; Kärcher, H.; Süß, M.; Berkefeld, T. Bibcode: 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. Title: The Indian National Large Solar Telescope (NLST) Authors: Hasan, S. S. Bibcode: 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.

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. Title: Republication of: Radial Movement in Sunspots Authors: Hasan, S. S. Bibcode: 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. Title: Magnetic Coupling between the Interior and Atmosphere of the Sun Authors: Hasan, S. S.; Rutten, R. J. Bibcode: 2010ASSP...19.....H Altcode: 2010mcia.conf.....H No abstract at ADS Title: Solar Physics at the Kodaikanal Observatory: A Historical Perspective Authors: Hasan, S. S.; Mallik, D. C. V.; Bagare, S. P.; Rajaguru, S. P. Bibcode: 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. Title: Spectropolarimetry with the NLST Authors: Sankarasubramanian, K.; Hasan, S. S.; Rangarajan, K. E. Bibcode: 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. Title: Wave propagation and energy transport in the magnetic network of the Sun Authors: Vigeesh, G.; Hasan, S. S.; Steiner, O. Bibcode: 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.
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.
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 106 erg cm-2 s-1 at chromospheric height levels.
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. 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2007A&A...473..931S Altcode: Aims:We study the spatial damping of linear compressional MHD waves in a homogeneous, isothermal, and unbounded prominence.
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.
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 × 102 km for the Kolmogorov turbulence. Correspondingly, the damping length of slow modes is 1.3 × 101 km and for fast modes 1.9× 102 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 10-7 to 102 s, and the Kolmogorov turbulence dominates for longer wave periods between 103 to 105 s. From the damping length of fast modes, it is found that the Kraichnan turbulence dominates from very short to long wave periods.
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× 103 km for Kolmogorov turbulence and 3.5 × 103-3.1 × 104 km for Kraichnan turbulence. Correspondingly, for fast modes, the damping length is in the range 2.6× 105-2.3× 106 km for Kolmogorov turbulence and 1.7 × 107-1.5× 108 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. Bibcode: 2007AIPC..919.....H Altcode: No abstract at ADS Title: Kodai School on Solar Physics Authors: Hasan, S. S.; Banerjee, D. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2005A&A...444L..29H Altcode: 2005astro.ph.11472H Context: .
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.
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.
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.
Conclusions: . Title: Dynamics of the Solar Magnetic Network: Two-dimensional MHD Simulations Authors: Hasan, S. S.; van Ballegooijen, A. A.; Kalkofen, W.; Steiner, O. Bibcode: 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. Bibcode: 2005BASI...33..362R Altcode: No abstract at ADS Title: Dynamics of the Magnetized Solar Atmosphere Authors: Hasan, S. S. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 K2V 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. Bibcode: 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 Teff>=5500 K and logg>=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. Bibcode: 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. Bibcode: 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. Bibcode: 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 K2V bright points Authors: Sivaraman, K. R.; Gupta, S. S.; Livingston, W. C.; Damé, L.; Kalkofen, W.; Keller, C. U.; Smartt, R.; Hasan, S. S. Bibcode: 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 K2V bright points in the interior of the network and corresponding magnetic elements. We find that about 60% of the K2V bright points spatially coincide with magnetic elements of flux density > 4 Mx cm-2. About 25% of the K2V bright points with equally enhanced emission lie over bipole elements where the fields are > 4 Mx cm-2 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 < 4 Mx cm-2 which is the noise level set by us for the magnetic scans. When magnetic elements of opposite polarity merge and form bipoles, the associated K2V 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 > 4 Mx cm-2, 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 K2V bright points with magnetic elements. Title: Dynamical Processes in Flux Tubes and their Role in Chromospheric Heating Authors: Hasan, S. S. Bibcode: 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. Bibcode: 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-1. 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. Bibcode: 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 & 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 & 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. Bibcode: 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, & 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-1 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 1998IJMPD...7...49D Altcode: Eigenfrequencies of radial pulsations of "slowly" rotating neutron stars are calculated in a general relativistic formalism given by Chandrasekhar & 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 1998BASI...26..181H Altcode: No abstract at ADS Title: The Influence of Magnetic Flux Tubes on Their Environment Authors: Hasan, S. S.; Kalkofen, W. Bibcode: 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 & 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. Bibcode: 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 \& Christensen-Dalsgaard (1992) and Banerjee, Hasan \& 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. Bibcode: 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. Bibcode: 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 Kx, 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 Kx until reaching a maximum, and thereafter it drops very sharply. As n increases, the maximum shifts to lower values of Kx. For fixed values of Kx 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 Kx 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. Bibcode: 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. Bibcode: 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, lmax ~ 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. Bibcode: 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. Bibcode: 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. Bibcode: 1996BASI...24..341G Altcode: No abstract at ADS Title: Dynamical Effects in Solar Photospheric Flux Tubes Authors: Hasan, S. S. Bibcode: 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 -2, 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1992iiaa.book.....P Altcode: No abstract at ADS Title: Heating in Intense Flux Tubes (With 4 Figures) Authors: Hasan, S. S. Bibcode: 1991mcch.conf..408H Altcode: No abstract at ADS Title: Magnetoatmospheric Oscillations in Sunspot Umbrae Authors: Hasan, S. S. Bibcode: 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. Bibcode: 1991iiaa.book.....P Altcode: No abstract at ADS Title: Dynamical effects and energy transport in intense flux tubes Authors: Hasan, S. S. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Betac, where Betac denotes a (tube) radius-dependent critical value. A bifurcation at Beta = Betac 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. Bibcode: 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. Bibcode: 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. Bibcode: 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-1, 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 r2 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 r2, 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1981BASI....9...74H Altcode: No abstract at ADS Title: Magnetohydrodynamic Equilibrium and Stability of Pre-Flare Loops - Constant Pitch Field Authors: Hasan, S. S. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.