• V Sarabhai

Articles written in Proceedings – Section A

• Meteorological and extra-terrestrial causes of the daily variation of cosmic ray intensity

The daily variations of total cosmic ray intensity and the intensities of meson and electron components have been studied at Alimedabad with vertical geiger counter telescopes. The influence of meteorological factors on these variations has been examined, and it has been found that appropriate barometric coefficients for correcting the cosmic ray intensities can be obtained from a consideration of the semidiurnal components of the variations. The barometric coefficients for the three intensities are$$\begin{array}{l} \beta _r = - 4 \cdot 2\% per cm. Hg, \beta _M = - 2 \cdot 4\% per cm. Hg \\ \beta _K = - 14 \cdot 0\% per cm. Hg \\ \end{array}$$ The cosmic ray intensity variations are corrected with the appropriate coefficients for the daily variation of barometric pressure. No significant variation is then left in the electron intensity, implying that variations of this component are mostly caused by the mass absorption effect with a variation of barometric pressure. In total intensity and in meson intensity, on the other hand, there is a significant residual variation of about ·3% in amplitude. This is mainly diurnal in character with a maximum at 0900 hours I.S.T.

Reasons are given for concluding that the meson residual variation is not primarily caused by either the diurnal variation of temperature in the atmosphere or of geomagnetic elements. It is finally concluded that the bulk of the meson residual diurnal variation is extra-terrestrial in origin and is caused by continuous solar emission of cosmic ray particles. This conclusion is discussed in terms of the interpretation of omnidirectional and unidirectional measurements of the diurnal variation by other workers. A connection between changes in the amplitude and the hour of maximum of the diurnal variation has been suggested.

• Study of the anisotropy of cosmic rays with narrow angle telescopes

• A solar flare type increase in cosmic rays at low latitudes

During the hour following the big solar flare on 23-2-1956, an average increase of +5·7±0·8% has been observed in meson intensity measured with standard telescopes at Ahmedabad, Kodaikanal and Trivandrum. This is the first report of a significant solar flare type increase in cosmic rays near the geomagnetic equator. If the increase is due to solar protons travelling in approximately direct paths, the energy of the protous must extend from about 35–67·5 Bev. It is estimated that the average flux of such protons is approximately equal to 1·5 times the flux of general cosmic ray intensity in the same energy range. During the hour, the sun is estimated to have emitted more than 1028 protons of about 50 Bev energy.

• Variations of intensity and anisotropy of cosmic rays measured at the geomagnetic equator

A study of cosmic ray intensity variations has been conducted during 1956–57 at the equatorial mountain station of Kodaikanal, using a standard neutron monitor. The data have been examined to look for the relationship between the day-to-day changes of intensity, the variance of bi-hourly deviations, the occurrence of large bi-hourly deviations at different hours of the day and the associated parameters of the daily variation. The results are related to the electromagnetic state of interplanetary space as determined by streams of solar matter in the neighbourhood of the earth, carrying with them frozen magnetic fields. Comparison is made with the model elaborated by Dorman.

The principal conclusions are as follows:

Day-to-day changes of intensity involve increases as well as decreases with respect to a base intensity for the period in question.

The daily variation of intensity of local neutrons, at an equatorial mountain station during 1956–57, has often a large diurnal as well as a semi-diurnal component.

On days of high geomagnetic disturbance, the daily variation exhibits abrupt changes indicative of the source being situated at a distance shorter than the range of the geomagnetic field. On geomagnetically quiet days, the daily variation has a form consistent with its being related to an anisotropy in interplanetary space. On days of moderate geomagnetic disturbance, the daily variation has changeable characteristics.

Correlated day-to-day changes of mean intensity and daily variation have been confirmed. For geomagnetically disturbed days, the semidiurnal component is greater than the diurnal component for increases of intensity, and conversely for decreases of intensity.

An examination of the time series of Cp for high intensity and for low intensity indicates the presence of a component of the frozen magnetic field in the direction of the solar dipole field. However, during the period of observation the solar dipole field and the sunspot field were in the same direction. Therefore, the results obtained cannot be considered either to confirm or refute the possibility of beams carrying sunspot fields with them.

• Geomagnetic plasma probe for solar wind

Magnetograms from Alibag reveal that the range Δ H of the daily variation of the horizontal component is negatively correlated with the minimum value ΔHmin. during a day. This relationship is largely unaffected by the degree of geomagnetic disturbance and holds good during all phases of the 11-year cycle of solar activity. From the nature of the relationship between ΔH and ΔHmin. it is concluded that the daily variation of the geomagnetic field at a low latitude station outside the influence of the equatorial electroject must be regarded as largely due to a weakening of the ambient field on the night side rather than an enhancement of the field on the day side due to ionospheric currents. There exists a good correlation between (ΔH)2 and the kinetic energy density of the solar wind in interplanetary space measured by IMP-1 satellite. It is suggested that ΔH is largely the result of the partial ring currents related to the convective drift of the plasma from the tail of the magnetosphere. Moreover, using the relationships established during the IMP-1 period, the annual mean kinetic energy density of solar wind for geomagnetically quiet days for the past 11-year cycle is estimated, treating the earth as a plasma probe.

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