• S A Stephens

      Articles written in Proceedings – Section A

    • Directional variation of geomagnetic cut-off rigidity around Hyderabad, India

      R R Daniel S A Stephens

      More Details Abstract Fulltext PDF

      The geomagnetic cut-off rigidities for cosmic ray particles arriving at the top of the atmosphere over Hyderabad, India (geographic latitude 17.6°N. and longitude 78.5° E.), as a function of zenith and azimuthal angles and the vertical cut-off rigidities for a few neighbouring locations, have been made using the sixth degree simulation of the geomagnetic field by the C.D.C. 3600 Computer.

    • Recent observations on cosmic electrons and their consequences on different cosmic ray models and related astrophysical quantities

      R R Daniel S A Stephens

      More Details Abstract Fulltext PDF

      An experiment has been carried out to study the electron component of the primary cosmic radiation at energies >12 GeV using a hypersensitised nuclear emulsion stack, flown oriented in the east-west plane over Hyderabad, India. The results of this experiment, on the basis of 28 identified electrons of energy above 12 GeV are: (i) the integral flux of electrons above an effective energy of 16 GeV is 0·51±·10 per m.2 sec. sr.; (ii) the differential energy spectrum between 12 and 300 GeV can be represented as N(E)dE=12·7 E−2.1±·2dE m.−2 sec.−1 sr.−1; and (iii) the fraction of positrons among the total electrons in the energy region 12 to 30 GeV has been discussed.

      A critical study has been made on the applicability of different cosmic ray models by making use of the observed differential energy spectrum of electrons and the relevant astrophysical parameters associated with the confinement regions. The confinement regions considered are: (i) the universe as a whole, (ii) the super cluster to which our galaxy belongs, (iii) the galactic halo and (iv) the galactic disc. The consequences of the recently postulated universal black body radiation at 3° K. on the cosmic ray models have also been considered. Some of the crucial experiments needed to set more stringent constrains on the models which would then permit meaningful interpretation, are enumerated.

    • Rigidity spectrum of cosmic ray helium nuclei

      K C Anand R R Daniel S A Stephens B Bhowmik C S Krishna P K Aditya R K Puri

      More Details Abstract Fulltext PDF

      An experiment has been carried out using an oriented stack of nuclear emulsions to determine the rigidity spectrum of cosmic ray helium nuclei between 12 and 40 GV, by taking advantage of the variation of the geomagnetic cut-off rigidity in the east-west plane over Hyderabad, India. Altotal of 2433 identified helium nuclei recorded in the stack, has been divided into 8 angular intervals in the east-west plane corresponding to 8 different cut-off rigidities. From this the integral fluxes of helium nuclei at the top of the atmosphere have been obtained for all the 8 rigidity intervals. The vertical flux above an effective threshold rigidity of 16·73 GV has been determined with high statistical accuracy and has a value of 15·0±0·5 helium nuclei (m2.sr.sec.)−1. The rigidity spectrum of these nuclei between 12 and 40 GV can be well represented by a power law of the type N (>R) =1990 R−1.74±0.11 (m2.sr.sec.)−1 and is the first direct determination so far made in this rigidity region.

      The differential rigidity spectra of protons, helium nuclei and S-nuclei of the cosmic radiation in the vicinity of the earth at solar minimum (1965) have been constructed with the existing world data and it is found that for rigidities ≳ 10 GV, the three spectra have, within experimental errors, the same slope of 2·6. The ratio P/He and He/S of the differential fluxes have been studied as a function of rigidity. It is found that for R>2 GV, the ratio P/He has, within experimental errors, a constant value of 6·3; as for the ratio He/S, it seems that the experimental data above a GV is not inconsistent with a constant value of 14 over the entire rigidity interval considered here.

    • Electrons magnetic fields and background radio emission in the galaxy

      K C Anand R R Daniel S A Stephens

      More Details Abstract Fulltext PDF

      Extensive data now available on the non-thermal background radio emission from different celestial directions, and recent measurements on the energy spectrum of cosmic ray electrons in the vicinity of the earth permit one to deduce information on the mean magnetic fields and cosmic electron spectra needed to exist in different regions of the Galaxy. It is found that in order to explain quantitatively the background radio brightness distributions from the Galaxy one needs (i) the same or nearly same electron spectrum that exists in the near interstellar space, to exist in almost all regions of Galactic space, (ii) a mean magnetic field close to 6×10−6 Gauss in the Disc in the direction of the Anti-centre, (iii) a mean magnetic field close to 2·5×10−6 Gauss in the radio Halo and (iv) a mean magnetic field probably close to 9·5×10−6 Gauss towards the Galactic Ridge in the direction of the Centre. Some inferences are also drawn on the confinement of cosmic rays in the Galaxy.

    • On the existence of the Galactic Radio Halo

      K C Anand R R Daniel S A Stephens

      More Details Abstract Fulltext PDF

      Radio astronomical data available on the galactic background radiation has been carefully examined to deduce information on the existence or otherwise of a Galactic Radio Halo. It is shown that, though considerable doubt has been recently expressed by some regarding its existence, no convincing and quantitative reasoning has been so far advanced towards this; on the other hand, there are substantial arguments in its favour.

    • Atmospheric gamma-rays of 1–200 GeV over Hyderabad

      S A Stephens

      More Details Abstract Fulltext PDF

      Using a stack of hypersensitized nuclear emulsions, exposed under 10.2 g.cm−2 of residual atmosphere over Hyderabad, India, gamma-rays of energy > 1 GeV have been studied. It is found that the observed differential energy spectrum can be best represented by two power law spectra, one for energies less than 30 GeV and the other for higher energies but with the same spectral index of 2.6; however the intensities at energies greater than 30 GeV are about three times higher than what would be expected from the extrapolation of the spectrum at lower enerjies. The implication of this observation has been examined. The neutral pion production spectrum over Hyderabad has been derived from the gamma-ray spectrum. Using this information the energy spectrum of atmospheric electrons has been deduced and a comparison made with the observed spectrum below the geomagnetic threshold energy over Hyderabad; therefrom an upper limit of 0.5 particles/(m2.sr.sec.) has been deduced for the flux of re-entrant albedo electrons above 2 GeV.

  •  

© 2021-2022 Indian Academy of Sciences, Bengaluru.