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Scientific Report 2007-2009
Astronomy & Astrophysics
A5. Search for periodicities in the solar energetic proton fluxes
Past studies revealed that many solar activity phenomena
undergo both periodic and quasi-periodic variations
on different time scales. Nevertheless, only a few
attempts were made so far to detect corresponding variations
in the occurence frequency of solar energetic particle
events. We tried to fill this gap searching for periodicities
in the proton fluxes, measured in the interplanetary
space, on time scales ranging from a few Bartels rotations
(27 days) up the the Schwabe period (≈11 years).
Figure 1: The wavelet power (normalized to the 95% significant
level) corresponding to the most relevant periods (as
reported in the legend) are plotted as a function of time for
channel P2 (top) and channel P11 (bottom). An upper cutoff
was applied to the data in order to smooth the discontinuities.
Our study was based on the data collected by the
Charged Particle Measurement Experiment (CPME),
aboard the satellite IMP 8, orbiting at ≈35 Earth radii in
the period from 1974 to 2001. Measurements were performed
in ten differential energy channels, but we used
only those taken in channels P2 (0.50 - 0.96 MeV) and
P11 (190 - 440 MeV), which were not affected by an
experiment malfunction occurred in 1989. Data were
analyzed using the wavelet transform (WT), a technique
which offers an important advantage with respect to the
Fourier transform, because it allows localization in time
of possible periodicities which are not present continuously
in the data set. It is known, however, that the
WT may lead to erroneus results, when applied to discontinuous
data, such as the proton fluxes considered in
our study. We investigated this issue by applying the
wavelet analysis to suitable test functions. It turned out
that eventual spurious frequencies can be discarded by
introducing an upper cutoff to reduce the amplitude of
the stronger discontinuities present in the data set.
Discarded the spurious periods, our analysis revealed
variations of the proton fluxes on the following time
scales (see Figure 1):
T = 3.8 years. This period has been singled out in
both the energy channels from 1977 to 1985 (the active
phase of solar cycle 21). It closely resembles the 3.7 year
period exhibited by the protospheric magnetic field in
the same time interval.
T = 1.7 - 2.2 years. This modulation, also present in
both the energy channels, is better observed from 1988
to 1993 (i.e., around the sunspot maximum of the cycle
22). It corresponds to the ”quasi biennial oscillations”
(QBO) which are known to characterize several features
of the solar activity: e.g., the number of H flares, the
total sunspot area, the 10.7 cm radio emission, and the
flux of the energetic electrons. The common origin of
all these phenomena is supported by the simultaneous
disappearance of their modulation during quiet periods.
T = 0.8 - 0.9 years. This modulation is observed in
shorter time intervals: its linkage with the variations of
other solar parameters deserves other studies.
We also note the lack, in our data set, of significant
modulations on time scales of 150 days and 5 - 6
years. However, the 150 day period (the so called ”Rieger
period”), revealed in several solar activity parameters,
could have been hardly singled out in our analysis, as
the proton fluxes were averaged over Bartels rotations
(27 days). On the other hand, the absence of the 5.5
year modulation appear to be more significant. In fact
it supports the hypothesis, advanced by some author,
that this periodicity, observed, e.g., in the sunspot number,
is an artifact produced by the asymmetric shape of
the solar cycle.
We finally stress that the procedure we successfully
introduced here to discard spurious periodicities may
be useful when the WT is applied to other sets of data
with strong discontinuities.
References
1. M. Lorenza, et al., Journ. Geophys. Res. 114, A01103,
(2009)
2. M. Storini, et al., Adv. Space Res. 41, 70 (2008).
Authors
G. Moreno
Sapienza Università di Roma 152 Dipartimento di Fisica