Study of anisotropic variation of cosmic rays intensity with solar activity

doi:10.4236/ns.2011.32014

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Vol.3, No.2, 101-103 (2011) Natural Science

http://dx.doi.org/10.4236/ns.2011.32014

Study of anisotropic variation of cosmic rays intensity

with solar activity

Chandra Mani Tiwari*, Devendra Sharma, Lalji Tiwari, Ajay Kumar Saxena, Dadan Prasad Tiwari

Department of Physics, A. P. S. University, Rewa (M.P.), India; *Corresponding Author: [email protected]

Received 27 September 2010; revised 28 October 2010; accepted 20 November 2010.

ABSTRACT

The annual average values of amplitudes and

phases of first two harmonics of cosmic ray

anisotropy have been derived by using the

harmonic analysis technique for the period 1989

to 2004, which covers mostly the major period

of solar cycles 22 and 23. In this paper we have

taken the pressure corrected hourly data for Kiel

neutron monitor station (cut off rigidity ≈ 2.29

GV) to derive the harmonic component of cos-

mic ray daily variation and compared with the

data of Halekala neutron monitor (cut off rigidity

≈ 13.2 GV) for the period 1991 to 2004. From the

analysis it has been concluded that the diurnal

amplitude and phase of daily variation of cos-

mic rays have been found to be correlated with

solar activity. However, the semi-diurnal ampli-

tude and phase are inversely correlated with

solar activity for both the stations.

Keywords: Cosmic Ray, Anisotropic Variation,

Solar Parameter

1. INTRODUCTION

The anisotropic variations in cosmic ray intensity

which are observed only in the heliosphere can be easily

detected by the ground based detectors [1-6]. Among the

various cosmic ray intensity variations, 27-day varia-

tions, Forbush decreases and solar daily variations have

been widely investigated by number of researchers [6-8].

The large differences in the diurnal and semi-diurnal

variation of cosmic ray intensity indicate that large

changes occur in interplanetary space for continuous

periods, which are associated with the spatial distribu-

tion of cosmic ray intensity as well as geomagnetic dis-

turbances. The amplitudes and phases of first two har-

monics of cosmic ray daily variation and their average

characteristics have been particularly emphasized in a

series of papers [9-10]. Since the realization of “in situ”

observations; the convection-diffusion and the inter-

planetary magnetic field (IMF) gradient as well as cur-

vature drift phenomena in galactic cosmic ray particles;

all together manifest itself as a time variation in the

count rate of the monitor, a phenomena called solar daily

variation or cosmic ray anisotropies [11-15].

In this paper we have collected the data of diurnal and

semi-diurnal amplitudes and phases of cosmic ray ani-

sotropies for the period 1989-2004 of Kiel neutron mon-

itor (a high-latitude station) and for the period 1991-

2004 of Haleakala (a low-latitude) neutron monitor sta-

tion and correlated with sunspot number (Rz) covering

the previous solar cycle 22 and present solar cycle 23.

2. METHOD OF ANALYSIS

Generally, cosmic ray intensity shows significant ani-

sotropic variation on a day-to-day basis with most prob-

able amplitude of 0.4% to 0.5% at high and low latitude

neutron monitor stations. During the period 1989 to

2004, covering the major portion of solar cycles 22 and

23, the amplitudes and phases of the first two harmonics

of the daily variation of high energy cosmic rays have

been obtained on a day-to-day basis by using the pres-

sure corrected hourly data of neutron monitors, well dis-

tributed particularly in latitudes, to cover different cut-

off rigidities. Such data enable us to study the rigidity-

dependent variations. These observational results for

first and second (diurnal and semi-diurnal) harmonics

have been compared with the solar and geomagnetic

parameters. The hourly pressure corrected cosmic ray

neutron monitor data of Kiel (a high-latitude station with

low cut-off rigidity) and Haleakala (a low-latitude sta-

tion with high cut-off rigidity) neutron monitor stations

have been obtained from the website www.cosmic ray

neutron monitor data NGDC/WDC STP, Boulder-Cosmic

Rays. The amplitudes and Phases (time of maximum) of

the anisotropic variation of cosmic rays have been de-

rived from these data by simple harmonic analysis. The

annual average is calculated from individual daily vec-

tors after rejecting the days with universal time (UT)

associated cosmic ray variations. The daily values of

C. M. Tiwari et al. / Natural Science 3 (2011) 101-103

102

solar and geomagnetic parameters have been taken from

Solar Geophysical Data Books.

3. DISCUSSION AND CONCLUSIONS

The solar activities play a significant role in modulat-

ing the cosmic ray intensity. It modifies interplanetary

and geomagnetic parameters. The cosmic ray daily vari-

ations which are due to spinning motion of the earth, are

particularly described in this analysis. In fact, the largest

amplitudes are observed during the declining phase of

solar activity (Figures 1 and 2). In other words, we infer

that the semi-diurnal amplitude for Kiel and Haleakala

neutron monitor stations are negatively correlated with

sunspot numbers, (Figures 3 and 4) which is opposite to

that found for the diurnal amplitudes.

Nevertheless, the semi-diurnal phase i.e. the time of

maximum for Kiel and Haleakala is positively correlated

(Kiel r = 0.84, Haleakala r = 0.53) with sunspot number,

as was also the case for the diurnal phase. The results are

presented here for the recent periods.

Figure 1. The crossplot between the first harmonic (diurnal

variation) annual average amplitude (for Kiel as well as for

Haleakala in %) with sunspot numbers, for the interval

1989-2004 for Kiel and 1991-2004 for Haleakala. The best fit

lines are also shown.

Figure 2. The crossplot between the first harmonic (diurnal

variation) annual average phase values (in hours) for (Kiel/

Haleakala) neutron monitor with sunspot numbers for the in-

terval 1989-2004 for Kiel and 1991-2004 for Haleakala. The

best fit lines are also shown.

Figure 3. The crossplot between the second harmonic (semi-

diurnal variation) annual average amplitudes in (%) for Kiel/

Haleakala neutron monitor with sunspot numbers, for the in-

terval 1989-2004 for Kiel and 1991-2004 for Haleakala. The

best fit lines are also shown.

Figure 4. The crossplot between the second harmonic (semi-

diurnal variation) annual average phase values (in hours) for

Kiel as well as for Haleakala neutron monitor station with

sunspot numbers for the interval 1989-2004 for Kiel and

1991-2004 for Haleakala. The best fit lines are also shown.

1) Significant positive correlations of amplitudes

and phase for both the stations as well as for the

solar parameter (Rz) have been found for diurnal

variation. From the analysis, it is observed that

the diurnal amplitude and phase show a signifi-

cant correlation with sunspot activity.

2) The amplitude as well as the time of maximum

of the diurnal phase has been found to increase

with the increase of sunspot numbers, i.e. diurnal

amplitude as well as phase is generally high

during high solar activity period. The negative

correlations of the semi-diurnal amplitudes with

sunspot number signify that during maximum

sunspot activity periods, the semi-diurnal ampli-

tudes have least magnitudes.

3) The semi-diurnal amplitude for Kiel and Halea-

kala neutron monitor stations are negatively cor-

related with sunspot number, which is opposite

to that found for the diurnal amplitudes.

4) Nevertheless, the semi-diurnal phase i.e. the time

of maximum for Kiel and Haleakala is positively

correlated (Kiel r = 0.84, Haleakala r = 0.53)

C. M. Tiwari et al. / Natural Science 3 (2011) 101-103

103

with sunspot number, as was also observed in

case of the diurnal phase.

4. ACKNOWLEDGEMENT

The authors are thankful to world Data Centers (ngdc and Omni web

centers) whose data have been used by the present investigators for the

analysis.

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http://www.lip.pt/events/2006/ecrs/proc/ecrs06-so123.pdf