The 154d periodicity was found in the cosmic ray intensity (RE) vs Flares, and some other peaks of coherency in the RC vs aa sub I, that when interpreted as aliased values, might correspond to recurring interplanetary magnetic field structures and solar wind streams. It cannot be excluded, however, that some of the correspondence with aa are of terrestrial origin. This study cannot be considered exhaustive due to the fact that other solar variables, such as polar hole size, are possibly correlated to cosmic ray intensities. However, the number of observations is small so that the interpretation of the results is very difficult

Attolini, M. R.

Cecchini, S.

Galli, M.

English

NASA Technical Reports Server

67 SH 4.4-14
A STUDY OF PERIODICITIES AND RECURRENCES IN SOLAR ACTIVITY
AND COSMIC RAY MODULATION
Attolini M. R., Cecchini S.
Istituto TE.S.R.E -C.N.R., Bologna, Italy
Galli M.
Dipartimento di Fisica dell'Universit_,Bologna, Italy
i. Introduction. Time variability on a wide range of time scales is the
main characteristic of cosmic rays observed inside the solar cavity.
As the intensity of galactic cosmic rays is assumed to be essentially
constant outside the heliosphere, the temporal changes observed must be
due to the interaction of CR particles with the IMF which is carried by
the solar wind.
The question is then to find out the pattern of the IMF and its flow,
to determine the time and spatial evolution of their configurations and
to relate them to CR variations.
Unfortunately the direct measurements of the IMF and of the solar
wind plasma are insufficient because they are limited to a region close
to the ecliptic plane as compared to the vast 3-dimensional extension of
the heliosphere. On the other hand CR particles provide an indirect
measurement of the global structure of IMF since they sample a large
volume of the solar cavity in their travel from its boundary to the Earth.
Yet one can tentatively try to derive important parameters of the
interplanetary medium by means of the observations of the source itself,
the Sun.
Solar output during the llyr activity cycle has many different
aspects such as the mnspot area and number, the development of polar
coronal holes, the solar flares and the solar wind.
Furthermore among the cyclic variations of the various kinds of
solar activity one has to distinguish the ones having same period but
differing in phase, amplitude and pattern.
It is common practice to use the sunspot number variation as the
standard of the solar cycle and to compare to it all the other relevant
parameters, even though this approach has not a good physical ground.
The same is true for CR intensity since Forbush (1954) discovered an
inverse correlation between this latter and the sunspot number. On the
other hand the changes in the solar wind characteristics are related to
the temperature variations of the corona, which in turn depend on different
solar features such as sunspots, flares, polar coronal holes, plages etc.
" It is then extremely interesting to investigate the possible relationship
between these phenomena and the CR intensity.
The spectral and cross-spectral power analysis is the tool that can
be used to consider over a wide range of frequencies in a global way
correlations on the same time scale between two time series.
We have already analyzed CR variation of ergodic nature (Attolini
et al., 1984) but we cannot exclude variations corresponding to spectral
lines.
From the operational point of view the best way to study line spectrum
is to obtain estimates of the power trough the periodograms and average
https://ntrs.nasa.gov/search.jsp?R=19850026697 2020-03-20T16:52:29+00:00Z
6 8 SH 4.4-14
them on a band as narrow as possible.
This has been done by averaging each periodogram to its consecutive
and then repeating this operation six time. in that way we obtain a binomial
smoothing which preserve the frequency information over 7 periodograms
corresponding to the average of 4.43 independent data equivalent. Since
before performing the FFT operation the number of data has been increased
to 1024 via zero padding, each coherency value corresponds to 2.13 independent
data equivalent. The coherency figure (YI2) is the one defined by Jenkins
and Watts (1968). The 95% significance level has been found using simulations
on random data series with the same number of data.
In our analysis we 'have used the monthly data series of the following
relevant parameters: sunspot number (Rz) from 1944 to 1984 (Solar Geophys.
Data); CR intensity (RC) from 1944 to 1984 (Huancayo-Climax pressure crrected
counting rates); aa indexes (aa) from 1944 to 1984 (Mayaud,1973,1975);
major flares from 1955 to 1979 (Dodson et al.,report UAG N.14,19,52,80).
2. Computations and Results.
a) RC vs Rz
The first thing to note in the comparison of the coherency spectra
in Figure la-c computed for three different time intervals, is that we can
identify two regions,the first one with frequencies higher than Ic/yr, the
other one with frequencies lower than Ic/yr. The first one contains few
peaks that exceed the 95% confidence level, which do not shift in frequency
when increasing the length of the series and whose amplitudes remain the
or at least increase with the increase of the number of data. In the other
range the peaks slightly wander in frequency and in amplitude; the long
wave of 22yr appears only in the longer data set and with low significant
level. All the peaks in this region correspond to nearly all possible
harmonics up to iOth order of the llyr cycle in all the series as it can be
better seen in the computation with zero padding of the data up to 2048.
The other significant peaks of the second region are at periodicities
lO.4m, 156.5d, 70.9d.
The correlation is negative (8) in the first two and positive (_) in the
last case. The 156.5 days may be related to the flare occurrence.
b) RC vs aa
Since solar wind plasma drives geomagnetic activity the aa index has
been used to investigate CR intensity variations caused by solar wind
changes. Following the study by Feynman(1982) we have considered the aa and
the aaT (i.e. aa corrected for Rz) monthly indexes. The results are shown
in Figbre 2. One can see that the coherencies between CR and either aa or
aa I do not differ very much. The only difference between the two is the llyr
cycle periodicity. The significant peaks are, however, different from the
RC vs Rz and the correlation is always negative. More meaningful is to
compare the CR intensity deduced after the subtraction of the sunspot
correlation computed according to Nagashima and Morishita (1980). In that
case the coherency (Figure 3) is significant in a greater number of frequencies.
These periodicities are linked to the polarity of the IMF and possibly some
of them represent the aliased peaks of more fundamental periodicities such
as 28.2,27.15 and 26.85 days typical either of sector structure as described
by Svalgaard and Wilcox (1975) or of the recurrent decreases in CR intensity
in correspondence to aaI increases. This last fact can be consistent with
the description of high speed stream effects on CR (lucci et ai.,1979). In
fact the analysis by Feynman (1982) has put in evidence that the property
69 SH 4.4-14
,_.
• • J •
"i 4
e
2 I , • ' "Io_
Fig.1
I
RC,_oo: 1_41I_jS_
:U6d
Fig. 2 , ,
0
r
'r,,.'I ' _e " ' '
ig 'F .3 o _ _ ,., _
70 SH 4.4-14
of the aa_ is the high recurrence probability.
Itlshould be remarked that a plasma stream that do not engulf the
Earth can nevertheless affect CR that reach us by inducing changes in the
3-dimensional IMF configuration.
c) RC vs Flares
Here the most outstanding periodicity is the one at 66.2 d and the
most interesting is the one at 153.4d which is similar to the one found in
hard solar flares ( Rieger at ai.,1984) and near to the 156.4d of RC vs Rz
(Figure 4). However the Rz vs Flare coherency does not show such a
correlation.
3. Conclusion. We want to stress that our work is aimed to find the
correlation of two data series in narrow freue__bands. The fact that
does not exist a correlation on a 2 periodogram band does not imply that a
very high correlation does not exist on a much wider frequency band (e.g.
CR vs Flares,Rzjaa as a whole). The existence of a narrow band correlation,
on the other hand, means only that a particular type of variation fn _'
series corresponds, with a very high probability, to a variations on t_e
same time scale in the other series.
Among the results here reported we have found the 154d periodicity
in the RC vs Flares, and some other peaks of coherency in the RC vs aa
that when interpreted as aliased values might correspond to recurring I_F
structures and solar wind streams. We cannot exclude however that some of
the correspondence with aa are of terrestrial origin.
This study cannot be considered exhaustive due to the fact that other
solar variables, such as polar hole size, are possibly correlated to CR
intensities,however the number of observations is small so that the
interpretation of the results is very difficult.
4. References. Attolini,M.R.,et al.,Nuovo Cim.,7C,413,1984// Dodson,H.W.
et al.,Report UAG Nos.14,19,52,80// Feynman,J.,J.Geophys.Res.,87,6153,1982//
Forbush,S.E.,J.Geophys.Res.,_9,525,1954// lucci,N, et al.,Nuovo Cim.,26,421,
1979.// Jenkins,G.M. and Watts,D.G.,Spectral Analysis and Its Applications,
Holden-Day,1968// Mayaud,P.N., IAGA Bulletin Nos.33,39// Nagashima,K.,and
Morishita,L.,Planet.Space Sci.,28,177,1980//Rieger,E.,et al.,Nature,312,
623,1984.
e ee
• _3&
g_


A study of periodicities and recurrences in solar activity and cosmic ray modulation

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server