Study of Galactic Cosmic Rays at high cut- off rigidity during solar cycle 23 Partha Chowdhury 1 and B.N. Dwivedi 2 1 Department of Physics, University.

Study of Galactic Cosmic Rays at high cut-off rigidity during solar cycle 23

Partha Chowdhury 1 and B.N. Dwivedi 2

1 Department of Physics, University of Calcutta & 2DEPARTMENT of Applied Physics, Institute of

Technology, Banaras Hindu University

• Layout of the Talk : Study of the time– lag between galactic cosmic

rays and some solar and geomagnetic parameters during solar cycle 23 ( May, 1996 - December, 2008)

We have divided solar cycle 23 into two phases viz.1] May, 1996 – december,2000 ( A > 0 epoch ) and 2] Jan. 2001 - December, 2008( A < 0 epoch).Then studied the time-lag between GCRs and solar parameters in both the phases

Then studied the time evolution of short and Then studied the time evolution of short and intermediate –term periodicities of Galactic intermediate –term periodicities of Galactic cosmic ray time series for complete cycle 23cosmic ray time series for complete cycle 23

Data and Methods :

GCRs data : Monthly and 27-day averaged values of CRs intensity obtained from Beijing neutron monitor station (cut-off rigidity 10 GV; altitude 48 m; 39.08 N; 116.26 E)

The solar and geomagnetic parameters used in this study are : 1] sunspot numbers (SSN), 2] 10.7 cm solar flux & 3] Geomagnetic Ap index.

We have divided cycle 23 in two parts viz. May 1996 – December 2000 (A > 0 state: solar magnetic field polarity is outward in northern hemisphere) and January 2001 – December 2008 (A < 0; solar magnetic field is outward in Polar Regions).

We have calculated the cross- correlation coefficients (CC) between solar/ geomagnetic parameters and CRs data with various time-lags (0, + 1, + 2…,. + 70 months) and determined the time-lag (L) corresponding to optimum correlation in both epochs, namely A > 0 and A < 0 .

The time-evolution of the main, short and intermediate term quasi-periodicities (16 – 500 days) using Morlet wavelet technique setting ω0 = 12 and considering a red-noise background

Figure 1. Solar modulation of galactic cosmic rays, monthly sunspot number and tilt angle α of the heliospheric current sheet. Marked by A+ (A−) are times when the solar magnetic field is directed inward (outward) from the Sun in the northern polar and outward (inward) in the southern polar region, as sketched on top (Scherer et al., 2004).

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 20091800

1850

1900

1950

2000

2050

2100

2150

Time (Years ) ------->

Beij

ing

G

CR

s -

----

----

-->

Beijing Cosmic Ray Counts ( 1996 - 2008 )

A gradual increase of GCRs after 2004, when the Sun was its descending phase

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 200820080

50

100

150

200

250

300

Time (Years) ------->

Mo

nth

ly s

un

sp

ot

nu

mb

er -

----

----

>

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 20082008600

800

1000

1200

1400

1600

1800

2000

2200

2400

Time (Years) ------->

10.7

cm

. so

lar f

lux -

----

----

--->

Sunspot no. gradually falls down after 2004 and abnormal low value during 2007-08

-40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40400

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.70.7

lags ( months) ----------->

No

rma

lize

d C

CR

--

--->

SSN vs Beijing CRs for ( 1.5.96 - 31.12.2000 ) A > 0

Cross-correlation and time –lag between monthly Beijing GCRs and sunspot numbers for A > 0 phase of cycle 23.Lag varies from 0 to 10 months.GCRs are lagging behind the SSN.

Solar polarity A < 0 ; CC :0.604 – 0.595 ; Lag = -1 – ( -6) months

GCRs are leading in A < 0 phase.

-40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40400.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

Lag ( months) ----------->

No

rma

lize

d C

CR

-

----

----

----

->SSN vs Beijing CRs for ( 1.1.2001- 31.12.2008) A< 0

Complete solar cycle 23 , CC : 0.501 Lag = -1 month; CC : 0.499 L= 4 months

Time –lag is small .

-70 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 7070

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

lag ( months) ------------>

No

rma

lize

d C

CR

--

----

----

--->

SSN vs Beijing for cycle 23 ( 1.5.96 - 31.12.2008)

Solar polarity A > 0 ; CC : 0.60 – 0.587 ; Lag = 0 - 2 months

GCRs are lagging behind 10.7 cm solar flux during A >0 phase.

-40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40400.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.70.7

lag ( months ) ---------------->

No

rmali

zed

CC

R

----

----

----

-->

10.7 cm solar flux vs Beijing ( 1.5.96 - 31.12.2000 ) A > 0

Solar polarity A < 0 ; CC : 0.604 – 0.595 ; Lag = -1 to (- 4) months . GCRs are leading than 10.7 cm solar flux during A <0 phase.

-40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40400.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

lag ( months ) ---------->

No

rmalized

CC

R -

----

----

----

>

10.7 cm solar flux vs Beijing GCRs ( 1.1.2001 - 31.12.2008 ) A < 0

Complete solar cycle 23 , CC : 0.5933 – 0.5926; Time Lag = 2- 4 months; Time –lag is small .

-70 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70700.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

lag ( months ) ------------->

No

rma

lize

d C

CR

--

----

----

>10.7 cm solar flux vs Beijing GCRs for cycle 23 ( 1.5.96 - 31.12.2008)

Solar polarity A > 0 ; CC : 0.934 ; ; Lag = 0 months

GCRs and Ap are in same phase during A >0 epoch.

-40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 400.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

lag ( months ) ----------->

No

rma

lis

ed

CC

R -

----

----

----

>

Ap vs Beijing GCRs for 1.5. 96 - 31.12. 2000 ( A > 0 )

Solar polarity A < 0 ; CC : 0.882 ; Time Lag = 0 months

GCRs and Ap are in same phase during A < 0 epoch

-40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40400.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

lag ( months ) ------------->

No

rma

lize

d C

CR

--

----

----

--->

Ap vs Beijing GCRs for 1.1.2001 - 31.12.2008 ; A < 0

Complete solar cycle 23 , CC : 0.899; Time Lag = 0 month;

Time –lag is zero .

-70 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70700.5

0.53

0.56

0.59

0.62

0.65

0.68

0.71

0.74

0.77

0.8

0.83

0.86

0.89

0.92Ap Index( nT) vs Beijing GCRs ( 27 day average) for cycle 23 ( 1.5.96 - 31.12.2008)

Lag ( in 27 days ) --------------->

No

rmali

zed

CC

R

----

----

----

----

>

Results and Discussion We have detected that during solar cycle 23 , the time lag between

Beijing GCRs data and solar and geomagnetic indices is small. This result is contradiction to the result of Mavromichalaki et al. ( 2007 ) and Kane ( 2011) who detected a high time-lag between GCRs of low cut-off rigidity ( Moscow NM station) during this cycle. However , our result support the earlier findings of zero time –lag between Ap and GCRs ( Mavromichalaki et al. 2007)

We have detected for SSN and 10.7 cm. solar flux , the lag is –ve for A < 0 state of the heliosphere, which implies that GCRs are leading .

The current sunspot minimum, which we have seen at the end of Cycle 23, has been one of the deepest minima that we have experienced in recent times with roughly 71–73% of the days in 2008 and 2009, respectively, being entirely spotless. Apart from this, Cycle 23 has shown several other peculiarities, such as a second maximum during the declining phase that is unusual for odd-numbered cycles, a slower rise to maximum than other odd numbered cycles, and a slower than average polar reversal. During cycle 23, polar fields have been at their lowest compared to cycles 21 and 22.Perhaps the weak solar magnetic has created low barrier to the propagation of GCRs and due to these reason GCRs was leading. More observational analysis is required to explain this time –lag.

Wavelet spectrum of Beijing NM data for complete solar cycle 23 ( May, 1996 - december,2008 ). Considering W0= 6 and lag-1 correlation coefficient is ~ 0.95.

Wavelet spectrum of Beijing NM data for complete solar cycle 23 ( May, 1996 - december,2008 ). Considering W0= 12 and lag-1 correlation coefficient is ~ 0.95.

Observed Quasi –periodicities in Beijing NM data for cycle 23

• A number of short & mid-term periodicities were detected through wavelet analysis and most of the periods are time variable. The significant periods are :

~27 –day period is detected during different phases of cycle 23 which is similar to the solar rotational periodicities.

The periods ~9 days are also detected which is considered as the 3rd harmonic of 27 –day period ( Sabbah & Kudela, 2011)

Other periods ~40 days, ~60 days and ~100 days were also detected. These are also detected in the data of daily sunspot number and coronal index data during cycle 23 ( Chowdhury & Dwivedi, 2011)

Rieger and near Rieger period of 120- 160 days.

1.3 -1.4 year period. This period was found in the data of solar wind speed , sunspot area as well as in the rotation rate of tachocline .

A detailed answer about these periodicities would be available when the complete modulation mechanism of GCRs within the heliosphere would be known.