Abstract:
The galactic cosmic rays (GCR) are charged particles, primarily protons and
helium nuclei. In the present work, by applying data analysis techniques, we have
investigated the proposal that GCR can in
uence precipitation. Wavelet transforms
have been applied for the rst time to study the problem. This is because the
wavelet is the best tool available today to analyse non-stationary and non-linear
time series. The method suggests a common 9:8 year cycle in the cosmic ray
ux
(CRF) and precipitation time series during the considered period [1979-2008] at
Thule(76.5 N). Finally, the point process method on wavelet maps has been applied
at Thule. The method depicts that CRF leads the precipitation by about 2 and 9
months at Thule.
For the present analysis, six geographically diverse locations across the globe
have been selected. The six stations are Thule (76.5 N), Climax (39.4 N), Huancayo
(12 S), Namibia (19.12 S), Potchefstroom (26.4 S) and Hermanus (34.25 S); all
of them are neutron monitor stations. For the present study, precipitation data
have been retrieved from Global Precipitation Climatology Project (GPCP). GPCP
blends estimates based on various data sources to produce a global gridded data
set, taking advantage of the strengths of each data type. A reliable set of cosmic
ray
ux data at the six neutron monitor stations mentioned above is available
from National Geophysical Data Centre (NGDC). For cloud cover, the data has
been extracted from the International Satellite Cloud Climatology Project (ISCCP)
archives.
In the present analysis, the correlation between the GCR
ux and precipitation
has been found to be signi cantly high at higher latitudes. However, no signi cant
correlation between low cloud cover(LCC) and GCR was detected at any of the
six stations. The LCC data was taken from ISCCP. The absence of correlation
between the GCR and LCC has been attributed by Svensmark(2003) to calibration
problems. Thus, we are not in a position to come to a de nite conclusion about
the absence or presence of correlation between LCC and GCR.
Using the Fourier power spectrum, a signi cant 10 year period has been detected
in both cosmic ray
ux and precipitation at Thule, Potchefstroom and Hermanus.
Such a cycle is absent in the precipitation data at Huancayo and Namibia, in the
tropics.
One of the ways to explain the physical mechanism underlying a GCR-precipitation
connection can be stated as follows. GCR being charged particles a ects the earth's
global electrical circuit, and may thus stimulate the formation of charged cloud con-
densation nuclei (CCN) in the atmosphere. Therefore, higher GCR
uxes would
lead to more charged CCN in the atmosphere. It has been noticed1 that charged
CCN are more capable of attracting neutral or oppositely charged ambient parti-
cles in the atmosphere. The higher charged-CCN concentrations at times of high
GCR
uxes would lead to increased ice nucleation which eventually enhances the
precipitation release in cold clouds2. This mechanism is expected to be stronger at
the higher latitudes due to low geomagnetic cuto value. Thus, the concentration
of GCR is higher at higher latitudes, as in case of Thule, where cold clouds are
common and ice nucleation in cold clouds helps in precipitation.
Thus the higher correlation coe cient and the presence of a common 9.8 year
cycle in both CRF and precipitation time series at Thule, using power spectrum
and wavelet power spectrum method, supports the above mechanism.
Kniveton and Todd in 2001 proposed a relation between the CRF, precipitation
and precipitation e ciency. Using data from 1979 to 1999 they found evidence of
statistically strong relationships between the three variables over ocean surfaces
at mid to high latitudes. Our work con rms their conclusion. In addition, we
found non-stationrity and non-linearity in the precipitation time series, and used
the wavelet transform in our analysis. Moreover, the 9:8 year cycle in precipitation
and CRF time series has been found. Here, we nd that CRF leads precipitation
by 2 months at Thule.