Earlier studies of the periodic changes in cosmic ray intensity by power spectral analysis provided some understanding of the daily variation in terms of ambient power. The solar rotation periodicities are investigated, using daily means of calgary neutron monitor data during 1965 to 1976. Significant peaks with periods of 27 and 13.5 days with varying magnitudes are observed

Agrawal, S. P.

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7 2 SH4.4-16
COSMIC RAY POWER SPECTRAL VARIATIONS:
3. SOLAR ROTATION PERIODICITIES
S. P. Agrawal
Department of Physics (Vlkram Space Physics Centre)
A.P.S. University, Rewa (M.P.) 486003, India
ABSTRACT
Earlier studies of the periodic changes in cosmic ray
Intensity by power spectral analysis provided some
understanding of the daily variation in terms of ambient
power. The solar rotation periodicities are investigated
here using daily means of Calgary neutron monitor data
during 1965-76. Significant peaks with periods of 27 and
13.5 days with varying magnitudes are observed.
i. Introduction. It is generally accepted that the galactic cosmic
radiation is modulated by the continuously varying solar plasma and
fields of the modulating re_ion within the hellosphere. Most cosmic ray
variational studies (I-I000 Gev), have used 4 decades of ground-based
measurements. These include _ eleven-year solar cycle varlation,
Forbush decreases, 27-day variation and the various harmonics of the
daily variation.
The 27-day variation has direct bearing on the solar features, on
or above the solar surface and remain stable for more than one solar
rotation of average period 27 days. The solar modulation is a
continuous process and any variations in solar output will affect the
interplanetary plasma and field, and in turn upon the flow of cosmic
rays. Thus a study of the 27-day recurrence in cosmic ray intensity can
provide us with information on solar output and its propagation in the
interplanetary medium. Here we derive the 27-day rotation periodicities
by power spectral analysis during the interval 1965-76, using the
procedure adopted by Lanzerotti et el., (1981).
2. Spectral analysis of cosmic ray data. Daily mean values of Calgary
neutron monitor data have been used over the interval 1965-76, for power
spectral analysis, to investigate the 27-day periodicies and its
harmonics. The Calgary neutron monitor is an 18-NM-64 type detector
with statistical fluctuations of 0.02% in daily means. Initially the
computations were carried out using one year's data at a time; however
the peaks under investigation, were not very clear. Hence we combined 2
3 years of data for deriving the power spectra. The choice of 2 years
of data at a time was found optimum to look for individual peaks and to
compare their variations during the solar cycle.
3. Results and Discussion. The results of the power spectral analysis
are shown in Figure i. Over a sunspot cycle, the cosmic ray intensity
changes (at median energies _ 10-15 Gev) by about 15-20%, and any two
successive years combined here generally correspond to similar solar
conditions, with very small changes in cosmic ray intensity. However,
large changes in cosmic ray intensity, which occur over periods of days
(Forbush type decreases), do affect to some extent the overall spectral
https://ntrs.nasa.gov/search.jsp?R=19850026699 2020-03-20T16:52:26+00:00Z
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SH4.4-16
shape, but are not effective in modulating the peaks observed. The
vertical dashed linen indicate frequencies corresponding to peaks at 27
and 13.5 days. Note that the main peaks occur for the 27-days period
and is quite significant for all the years, except for 1973-74.
However, the 13-14 day peak is not very significant for a number of
years, such as 1965-66, 1967-68 and 1975-76. These results were
essentially confirmed by Sulphur Mountain Cosmic ray data. The power
associated with the 27-day peak in 1973-74 is quite low and indicates
the possibility that even the stable periodic features of the solar
surface were not effective in producing any significant changes in the
cosmic ray intensity. Solar coronal holes are quite abundant during
1973-74 and also in 1975-76, and thus an absence of any solar rotation
periodicity in 1973-74 perhaps indicated that some other solar feature
may be responsible. Venkatesan et al., (1982) have shown that only
small changes occur in cosmic rays due to coronal holes. The Chree type
of analysis involving some specific solar features, in future, may
provide the answer for the observations reported here. It is likely
that the solar flares/solar active centres could be the source for the
solar rotati0n periodicity observed. The reason for this is not obvious
at present. /
4. Acknowledgement. The assistance of Messrs. Arun Tiwari and
Vishwarnath Khare in data reduction, etc. is acknowledged. I also wish
to acknowledge the Calgary data kindly provided by D. Venkatesan and T.
Mathews.
5. References.
Lanzerotti, L. J., Maclennan, C. J., _grawal, S. P. and Venkatesan, D.,
(1981), J. Geophys. Res., 86, 6951.
Venkatesan, D., Shukla, A. K., and Agrawal, S. P., (1982), Solar
Physics, 81, 375.
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SH4.4-16
_'I 7,-7zi
- 69-70
Z
o Io*
,'K
!
CALGARY
:l n i
°sl 65-66 jL
I I
i0-7 5 10-6
FREQ, cps.
Figure I. Power spectra for the Calgary neutron monitor for the
interval 1965-76. Vertical dashed lines indicate the 27 and 13.5
day period.


Cosmic ray power spectral variations. 3: Solar rotation periodicities

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