• B. N. Dwivedi

      Articles written in Journal of Astrophysics and Astronomy

    • Density and temperature diagnostics of solar emission lines from NeV/MgV and SiVII/MgVII Ions

      Anita Mohan B. N. Dwivedi P. K. Raju

      More Details Abstract Fulltext PDF

      We present NeV/MgV and SiVII/MgVII theoretical line intensity ratios as a function of electron densityNe and temperatureTe. These are shown in the form of ratio-ratio diagrams, which should in principle allow bothNe andTe to be deduced for the emitting region of the solar plasma. We apply these diagnostics in the solar atmosphere, and discuss the available observations made from space. In most cases, however, we deduceNe andTe from the computed absolute line intensities in a spherically symmetric model atmosphere of the Sun. Possible future applications of this investigation to spectral data from the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) are briefly discussed.

    • The EUV spectrum of sunspot plumes observed by SUMER on SOHO

      W. Curdt B. N. Dwivedi U. Feldman

      More Details Abstract Fulltext PDF

      We present results from sunspot observations obtained by SUMER on SOHO. In sunspot plumes the EUV spectrum differs from the quiet Sun; continua are observed with different slopes and intensities; emission lines from molecular hydrogen and many unidentified species indicate unique plasma conditions above sunspots. Sunspot plumes are sites of systematic downflow. We also discuss the properties of sunspot oscillations

    • Physics of the sun’s hot atmosphere

      B. N. Dwivedi

      More Details Abstract Fulltext PDF

      This article briefly overviews the physics of the Sun’s hot atmosphere, using observations from recent solar spacecrafts: Yohkoh, SOHO, TRACE and RHESSI.

    • Magnetosonic waveguide model of solar wind flow tubes

      A. K. Srivastava B. N. Dwivedi

      More Details Abstract Fulltext PDF

      We consider solar wind flow tubes as a magnetosonic wave-guide. Assuming a symmetric expansion in edges of slab-modelled wave-guide, we study the propagation characteristics of magnetosonic wave in the solar wind flow tubes. We present the preliminary results and discuss their implications.

    • Oblique Propagation and Dissipation of Alfv´en Waves in Coronal Holes

      A. K. Srivastava B. N. Dwivedi

      More Details Abstract Fulltext PDF

      We investigate the effect of viscosity and magnetic diffusivity on the oblique propagation and dissipation of Alfvén waves with respect to the normal outward direction, making use of MHD equations, density, temperature and magnetic field structure in coronal holes and underlying magnetic funnels. We find reduction in the damping length scale, group velocity and energy flux density as the propagation angle of Alfvén waves increases inside the coronal holes. For any propagation angle, the energy flux density and damping length scale also show a decrement in the source region of the solar wind (< 1.05 R) where these may be one of the primary energy sources, which can convert the inflow of the solar wind into the outflow. In the outer region (> 1.21 R), for any propagation angle, the energy flux density peaks match with the peaks of MgX 609.78 Å and 624.78 Å linewidths observed from the Coronal Diagnostic Spectrometer (CDS) on SOHO and the non-thermal velocity derived from these observations, justify the observed spectroscopic signature of the Alfvén wave dissipation.

    • Magnetic Field in the Gravitationally Stratified Coronal Loops

      B. N. Dwivedi A. K. Srivastava

      More Details Abstract Fulltext PDF

      We study the effect of gravitational stratification on the estimation of magnetic fields in the coronal loops. By using the method of MHD seismology of kink waves for the estimation of magnetic field of coronal loops, we derive a new formula for the magnetic field considering the effect of gravitational stratification. The fast-kink wave is a potential diagnostic tool for the estimation of magnetic field in fluxtubes. We consider the eleven kink oscillation cases observed by TRACE between July 1998 and June 2001. We calculate magnetic field in the stratified loops (𝐵str) and compare them with the previously calculated absolute magnetic field (𝐵abs). The gravitational stratification efficiently affects the magnetic field estimation in the coronal loops as it affects also the properties of kink waves. We find ≈22% increment in the magnetic field for the smallest (𝐿 = 72 Mm) while ≈42% increment in the absolute magnetic field for the longest (𝐿 = 406 Mm) coronal loops. The magnetic fields 𝐵str and 𝐵abs also increase with the number density, if the loop length does not vary much. The increment in the magnetic field due to gravitational stratification is small at the lower number densities, however, it is large at the higher number densities. We find that damping time of kink waves due to phase-mixing is less in the case of gravitationally stratified loops compared to nonstratified ones. This indicates the more rapid damping of kink waves in the stratified loops. In conclusion, we find that the gravitational stratification efficiently affects the estimation of magnetic field and damping time estimation especially in the longer coronal loops.

    • Evidence of Magnetoacoustic Oscillations above the Brightened and Magnetically Active Quiet-Sun

      A. K. Srivastava B. N. Dwivedi

      More Details Abstract Fulltext PDF

      We study the near photospheric Fe I 6302 Å (V-component) and chromospheric Ca II H 3968 Å radiance oscillations in the lower solar atmosphere above the four brightened and magnetically active regions of the quiet-Sun as observed by the solar optical telescope onboard Hinode. At the chosen locations (L1, L3, L4), we obtain the statistically significant periods of 5.0 min both in Ca II H and Fe I fluxes, which is interpreted as the likely signature of the presence of 5.0 min magnetoacoustic oscillations. It is also found that the higher period (7.0 min) of magnetoacoustic waves, as observed at another location (L2), may be generated locally in the quiet-Sun tube and leak to higher heights. We suggest that 5.0 min global acoustic oscillations are converted into magnetoacoustic one in the strongly magnetized quiet-Sun fluxtubes. Noting that the magnetoacoustic waves in strongly magnetized regions can transfer energy twice compared to the acoustic waves in the non-magnetized regions, the directly observed these 5.0 min magnetoacoustic oscillations at photospheric and chromospheric heights above the strongly magnetized regions reveal the transfer of significant amount of energy to partially balance the localized energy losses.

  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

© 2017-2019 Indian Academy of Sciences, Bengaluru.