osilasi solar magnetic cycle dependence in corotating modulation

7/25/2019 Osilasi Solar Magnetic Cycle Dependence in Corotating Modulation

1/11


2/11

186 Astrophys Space Sci (2009) 321: 185195

firmed in model calculations by Jokipii (1981), a natural

consequence of the drift effects. As the Sun rotates with a

periodicity of27-days, the Earth crosses the HCS once,

twice, thrice etc. during a rotation period. Consequently, the

separation between HCS and the Earth (and hence the helio-

magnetic latitude) changes during the course of solar rota-

tion.

GCR intensities were found to peak near HCS and de-

crease with helio-magnetic latitude irrespective of the na-

ture (outward to inward or vice-versa) of HCS crossings.

Further, the amplitude of decrease was found to be larger

during A> 0 than A < 0 epochs (Badruddin et al. 1985;

Newkirk and Fisk1985). These observations are consistent

with the stronger recurrent (27-day) GCR modulation ob-

served during A > 0, since sector crossings (HCS crossings)

typically occur near high speed stream leading edges near

solar minimum periods (Richardson et al. 1999). The ob-

servations of Badruddin et al. (1985) and Newkirk and Fisk

(1985) suggest that recurrent GCR modulation near ecliptic

arise because of latitudinal cosmic ray density gradient that

are arranged about the tilted HCS.

The HCS separates the two oppositely directed magnetic

polarity hemispheres of the heliosphere. The angle between

the plane of the current sheet and a plane that is an exten-

sion of the Suns equator is referred to as the tilt angle of

HCS. Stone (1987) and Cummings and Stone (1988), from

analysis based on Voyager cosmic ray measurements in a

solar cycle with A< 0, suggested that cosmic ray flux is

roughly organized by heliomagnetic latitude. On the other

hand, Reames and Ng (2001) observed peak intensities near

north-south crossing of HCS and valley near south-north

crossing of HCS, inconsistent with simple particle gradi-

ent organized around the current sheet. Zhang et al. (1995)demonstrated that recurrent cosmic ray modulations are not

intimately associated with the HCS and that 27-day recur-

rent variations in GCR intensity are not organized by helio-

magnetic latitude. Thus the whole area seems to be complex

and needs further investigation.

Kota and Jokipii (1991) presented the first global simula-

tion of the modulation of galactic cosmic rays by the three

dimensional solar wind with corotating interaction regions.

Their result shows both the small-scale response to CIRs and

global, drift-dominated effects. To account for the observed

polarity dependent recurrent modulation (i.e. larger modula-

tion in A > 0 epoch) in GCR intensity (as observed e.g., byBadruddin et al. 1985; Richardson et al. 1999), Kota and

Jokipii (2001) extended 3-D simulations including drifts,

and considered a southward displacement of HCS, rather

than a symmetric tilted dipole. Inclusion of asymmetrically

placed HCS in drift models provide results that are in quali-

tative agreement with the finding of Badruddin et al. (1985)

and Richardson et al. (1999). However, Richardson et al.

(1999) suggested that epoch dependence of the particle dif-

fusion coefficient (Chen and Bieber1993) may increase the

effect of solar wind convection on local cosmic ray inten-

sity during A > 0 epoch and that the observed dependence

in response of GCR to solar wind variations is sufficient to

explain the difference.

Reames and Ng (2001), whose findings regarding recur-

rent modulations were not consistent with the predictions

of then prevailing drift models of GCR modulations, sug-

gested that Fisk model of solar magnetic field (Fisk1996)

offers great potential for explaining the recurrent variations

in GCR intensity. However, a recent analysis of Ulysses data

by Roberts et al. (2007), found that the prediction of Fisk

model for latitudinal component of the heliospheric mag-

netic field are not seen in the observation in Ulysses data.

Burger and Hitge (2004) developed a divergence free Fisk-

Parker hybrid heliospheric magnetic field and studied the ef-

fect of hybrid field on GCR modulation by solving the 3-D

steady state Parker transport equation. They investigated the

27-day recurrent variations for both protons and electrons.

They have shown that hybrid field reduces intensities com-

pared to Parker field when A > 0. When A < 0, the global

effects of hybrid field are almost negligible. Their model

predictions are consistent with the observed results (Zhang

1997; Paizis et al. 1999) only when drift effects are included,

indicating that drifts are important for corotating modula-

tion.

The origin of recurrent modulation of GCR must be in

the solar wind and the interplanetary magnetic field as all

the basic processes of modulation (particle diffusion, con-

vection, drift and adiabatic deceleration) are controlled by

properties of magnetic field fluctuations, large-scale inter-

planetary magnetic field structures and solar wind velocity.

Corotating depressions in cosmic rays have been studied

in relation to solar wind plasma and field conditions (e.g.Barouch and Burlaga 1975; Badruddin and Yadav 1985;

Duggal and Pomerantz1977; Iucci et al.1979; Scholar et al.

1979; Duggal et al.1981; Venkatesan et al.1982; Burlaga et

al.1984; Mishra et al.1990; Badruddin1993,1997; Yadav

et al.1994; Richardson et al.1996,1999; Sabbah2000b; Gil

et al.2005; Gupta and Badruddin2005; Singh and Badrud-

din2007; see reviews by Venkatesan and Badruddin 1990;

Simpson 1998; Mckibben et al. 1999; Richardson 2004).

However, consensus eludes the conclusion as regards the

solar wind parameters playing important role in these de-

pressions; probably due to near simultaneous variations in

a number of parameters (e.g., solar wind velocity, magneticfield magnitude, magnetic turbulence etc.) observed during

cosmic ray depression. As a consequence of variations in

these parameters, several processes could contribute in mod-

ulation (Richardson et al.1996; Badruddin1997). Changes

in so1ar wind speed could cause variation in convection and

adiabatic cooling, diffusion coefficient may change due to

variations in turbulence level, and variation in field strength

may be responsible for causing variation in diffusion coeffi-

cients and particle drifts (Richardson2004).


3/11

Astrophys Space Sci (2009) 321: 185195 187

Further investigation is, therefore, required to ascertain

the status of recently proposed models, e.g. the convection-

diffusion model with epoch dependence of particle diffusion

coefficient (Richardson et al. 1999) and 3-D drift models

including drifts HCS (Kota and Jokipii 1991,2001) vis--

vis the experimental results including the solar polarity de-

pendent effects in recurrent modulation. Moreover, whether

the 27-day recurrent modulation contributes to the global

modulation and if so by which mechanism, is still not solved

completely (Simnett et al. 1998). Further studies of recurrent

modulation can, therefore, provide new insight into global

modulation phenomenon.

2 Method of analysis

The heliospheric current sheet evolves during the course of

Carrington rotation (C-rotation). For the study of GCR vari-

ations during C-rotation, we have adopted the method of

superposed epoch (SPE) analysis. Epoch (zero day) corre-

sponds to the C-rotation start time. Analysis has been per-

formed during low solar activity periods; to avoid large For-

bush decrease due to transient solar disturbances as they oc-

cur more frequently during high solar activity periods. If

Forbush-type decrease occurs during a Carrington period,

that rotation has been omitted from the analysis. Further,

during low solar activity conditions, the evolution of cur-

rent sheet during each C-rotation is relatively smooth. Due

to these reasons, low/minimum solar activity periods have

been selected for the analysis.

The GCR intensity data of two neutron monitors, one lo-

cated at Oulu (Latitude = 65.02 N, Longitude= 25.50 E,

Cut off rigidity= 0.81 GV) and other at Climax (Latitude=

37.37 N, Longitude = 106.18 W, Cut off rigidity =

3.03 GV) together with solar/heliospheric plasma and field

data have been utilized for the analysis. GCR intensity

and solar wind data have been analyzed during; (i) mini-

mum solar activity periods, 19761977 (A > 0), 19851986

(A < 0) and 19951996 (A > 0); (ii) decreasing and low

activity period in which high speed solar wind streams are

more frequently observed, 19741975 (A > 0), 19831984

(A < 0) and 19931994 (A > 0); (iii) low activity periods,

decreasing and minimum solar activity combined periods,

separately for each epoch 19741977 (A > 0), 19831986

(A < 0) and 19931996 (A > 0) in order to increase thestatistics.

3 Results

3.1 Temporal evolution during C-rotation

Earth based detectors (e.g. neutron monitors) used for the

measurement of GCR intensity show a variation of a period

of one day (diurnal variation) whose amplitude is 0.5%

due to the rotation of Earth. Since corotating decreases

observed by neutron monitors are usually not very large

(23%), use of daily averaged data for the study of coro-

tating decrease averages out the effect of daily variations.

In Figs. 1(a), 1(b), 1(c) we have plotted the SPE analysis

results of daily average GCR intensity data of Oulu and Cli-

max neutron monitors for the solar activity minimum peri-

ods of 19761977 (A > 0), 19851986 (A < 0) and 1995

1996 (A > 0) respectively. Upper panels of these figures

show the simultaneous SPE plots of solar wind velocity(V )

and heliospheric magnetic field (B ).

From the time profile of the SPE plots during the course

of C-rotations, shown in Figs. 1(a),1(b) and1(c), we infer

that the intensity oscillates during the course of C-rotation;

however, the 27-day variation during 19851986 (A < 0)

is different from that during 19761977 and 19951996

(A > 0). Further, intensity variations appear to follow (in

anti-phase) the variations in solar wind velocity during the

course of C-rotation in 19761977 and 19951996.

Since high-speed corotating streams are prominently ob-

served during a period of 23 years before the activity min-

imum, we have studied the recurrent modulation during C-

rotation periods, separately during three periods that lie be-

fore the minimum activity, i.e. 19741975 (A> 0), 1983

1984 (A< 0) and 19931994 (A > 0). Superposed epochs

Fig. 1a Superposed epoch analysis results showing the variations in

galactic cosmic ray intensity, solar wind velocity and interplanetary

magnetic field during the course of Carrington rotations in minimum

solar activity period (19761977) when the heliosphere is in a posi-

tive polarity state (A> 0); zero day corresponds to the beginning of

C-rotations in this period


4/11


Fig. 1b Same as for the Fig. 1(a) during 19851986 when the he-

liosphere is in a negative polarity state (A < 0)

Fig. 1c Same as for the Fig.1(a) during 19951996 when (A> 0)

Fig. 2a Superposed epoch analysis results showing the variations in

GCR intensity, solar wind velocity and IMF during the course of

C-rotations when the solar activity is low in the decreasing phase of

a solar cycle(19741975) and (A> 0): zero day correspond to the be-

ginning of C-rotations

results of GCR intensity, solar wind velocity, and IMF

strength with respect to C-rotations for three periods are

plotted in Figs.2(a),2(b) and2(c). From SPE time profiles

in these three plots, we observe different pattern in 27-dayrecurrent modulation during A > 0 (19831984) and A > 0

(19741975, 19931994) epoch. There are observable de-

pressions in intensity and corresponding enhancements in

solar wind parameters during 19741975 and 19931994

but the same is not the case in 19831984. In fact there is

a systematic rise in GCR intensity during 19831984 before

it becomes constant. Further, during 19741975 and 1993

1994 the 27-day variations appear to follow the variations

both inV andB , more closely variation inV.

Although the analyses exclusively during solar minimum

periods, and during the periods when high-speed solar wind

streams are prominently observed, have certain advantages,the number of C-rotations in each group (in which GCR in-

tensity was free from transient effects) was not large enough,

as we have omitted C-rotations in which transient (Forbush)

decreases are observed. In order to increase the statistics we

combined the minimum and decreasing low activity peri-

ods and performed SPE analysis of GCR intensity and so-

lar wind data during C-rotations for the periods 19741977,

19831986 and 19931996. Figures 3(a), 3(b) and 3(c) show

these SPE plots of GCR intensity and solar wind parameters


5/11


Fig. 2b Same as for the Fig.2(a) when A < 0 (19831984)

Fig. 2c Same as for the Fig.2(a) during 19931994 when A> 0

Fig. 3a Superposed epoch analysis results showing variations in GCR

intensity and solar plasma/field parameters (V and B), with respect

to beginning of C-rotations when the heliosphere was in positive state

(A > 0) and solar activity was low (19741977)

during these periods of two different solar polarities (A < 0

and A > 0). From an examination of the profiles of these

three SPE plots, we again observe that the temporal evolu-

tion during 19831986 (A < 0), is different from 19741977and 19931996 (A > 0). Moreover, the variations during

A> 0 at both the cosmic ray stations appear to follow the

variations in solar wind parameters, particularly that ofV.

From SPE plots, during (a) solar minimum periods,

(b) periods when high speed streams are predominantly ob-

served and (c) during low solar activity periods we con-

clude that the temporal evolution of GCR intensity during

C-rotation in A > 0 is different from that in A < 0.

3.2 Temporal variations in GCR intensity: correlation with

solar wind parameters

In order to provide quantitative basis to the observation in

preceding section (that the variations in GCR intensity dur-

ing C-rotations follow the solar wind velocity), and to assess

its significance we did linear regression analysis between

day-to-day variability in GCR intensity (in percent) and so-

lar wind parameters, and obtained correlation between GCR

intensity and solar wind parameters during different solar

and magnetic conditions. We have also tested the signifi-

cance of the correlation coefficients at 95% confidence level.


6/11


Fig. 3b Same as for the Fig.3(a) during periods 19831986 when the

polarity state of the heliosphere was negative (A< 0)

Fig. 3c Same as for the Fig.3(a) during another period (19931996)

of low solar activity in A > 0 state

Table 1 Correlation coefficients between GCR intensity(I )and solar

wind parameters (V, B) during the course of C-rotation in minimum

solar activity periods

Periods Ivs. V Ivs. B

19761977 0.66 0.18

19851986 0.20 0.44

19951996 0.89 0.40

We found that (negative) correlation coefficient between

GCR intensity (I )and solar wind velocity (V )is significant

only during 19761977 and 19951996 solar minimum pe-

riods when A > 0 but I is poorly correlated with V during

19851986 minimum when A < 0. The correlation between

B and I is relatively low during 19851986 (A < 0) and

19951996 (A > 0) and there is almost no correlation in

19761977 (A> 0). The values of correlation coefficients

are given in Table1.

It is found that the correlation coefficient between GCR

intensity(I )and solar wind velocity(V )is significant onlyin solar minimum periods 19761977 and 19951996, when

A > 0, but not in 19851986 when polarity state is opposite

(A < 0). It will be interesting to see whether this result sug-

gest that (a) the cosmic ray response to solar wind speed

variations is reduced in A < 0 minimum (Richardson et al.

1999), or (b) some other process/effect obscures the re-

sponse of solar wind velocity in A < 0 epochs. To represent

this result graphically, scatter plots along with best-fit line

(represented by I=C+mV linear equation), between aver-

aged GCR intensity and solar wind speed during the course

of C-rotation have been shown in Fig. 4for the minimum

periods 19761977, 19851986 and 19951996. The rate ofintensity change with velocity (m), intercept (C ), obtained

from the linear fit of the data, and correlation coefficient (R)

in three different minimum periods are given in respective

figures.

Correlation coefficients between GCR intensity and var-

ious interplanetary parameters during 19741975, 1983

1984 and 19931994 have been calculated; they are given in

Table2. It is interesting to note that the GCR intensity is sig-

nificantly correlated with day-to-day variations in solar wind

velocity(V )during 19741975 and it is reasonably good in

19931994 (A > 0) while correlation of I is comparatively

better with field magnitude (B )during 19831984 (A < 0).Scatter plots between GCR intensity and solar wind velocity

during these periods together with the best-fit linear curves

are shown in Fig. 5; the values of intercept (C), slope (m)

and correlation coefficient (R)are also given.

Correlation analysis between GCR intensity and solar

wind parameters V andB , during the low solar activity pe-

riods 19741977, 19831986 and 19931996 yielded the

correlation coefficients given in Table 3. Solar wind ve-

locity shows a better (anti-) correlation with GCR inten-


7/11


Fig. 4 Scatter plots and best-fit linear curve showing relationship be-

tween change in cosmic ray intensity and solar wind velocity, dur-

ing the course of C-rotation in solar minimum periods, 19761977

(A>

0), 19851986 (A

0)

sity during A > 0 epochs. The scatter plots between solar

wind velocity and GCR intensity together with the best-

fit curve, intercept (C), intensity gradient with solar wind

velocity (m), are shown in Fig. 6. It is more clearly seen

from these scatter plots that a good correlation exists be-

tween the two parameters (I and V) only during A > 0

epoch.

Table 2 Correlation coefficients between GCR intensity and solar

wind parameters, during the course of C-rotation, in low solar activ-

ity periods and declining phase of different solar cycles

Periods Ivs. V Ivs. B

19741975 0.65 0.36

19831984 0.06 0.42

19931994 0.42 0.17

Fig. 5 Scatter plots and best-fit linear curve showing relationship be-

tween cosmic ray intensity and solar wind velocity during two dif-

ferent polarity states of the heliosphere (A< 0 and A> 0) in three

(19741975, 19831984 and 19931994) low solar activity conditions

and during declining phases of three different solar activity cycles


8/11


Table 3 Correlation coefficients between GCR intensity and solar

plasma/field parameters during the course of C-rotations in extended

periods of low solar activity, 19741977 (A> 0), 19831984 (A< 0)

and 19931996 (A> 0)

Periods I vs.V I vs.B

19741977 0.35 0.05

19831986 0.08 0.44

19931996 0.68 0.29

Fig. 6 Scatter plots and best-fit curve showing relation between so-

lar wind velocity and cosmic ray intensity changes during the course

of C-rotation in low solar activity periods, (a) 19741977 (A> 0),

(b) 19831986 (A< 0) and (c) 19931996 (A> 0)

Thus from correlation analysis during solar minimum

periods, in periods with high speed streams and low solar

activity periods, we conclude that during the course of C-

rotation, correlation of GCR intensity with solar wind speed

is stronger for A > 0 than A < 0.

3.3 Maximum-minimum difference during C-rotations and

relation with tilt angle of HCS

Tilt angle of the HCS is taken as one-half of the sum of

the maximum latitudinal excursion (north and south) of

the solar neutral line during each C-rotation. Since tilt an-

gle of the HCS is an important parameter in drift mod-

els of cosmic ray modulation, we have studied the rela-

tionship between tilt angle and maximum-minimum differ-

ence during C-rotations. For this purpose, we have calcu-

lated the maximum-minimum difference in GCR intensity

during each C-rotation and plotted the maximum-minimum

difference (I) versus the tilt angle in individual C-rotations.

Figure7 shows the scatter plot, best fit linear curve for theminimum periods 19761977, 19851986 and 19951996;

correlation coefficient (R ), GCR intensity gradient with re-

spect to tilt angle of HCS (m) and intercept (C) are also

given. As correlation coefficient is small and insignificant,

the maximum-minimum difference in GCR intensity dur-

ing C-rotations does not seem to depend on tilt angle, at

least during solar minimum periods. We did similar corre-

lation analysis for the periods 19831984 and 19931994

(Fig.8); again we did not find a significant correlation be-

tween the two during these periods. Thus, separate stud-

ies during both solar minimum and near- minimum when

high-speed solar wind streams are predominantly observed,GCR intensity oscillation during C-rotation does not show a

clear dependence on tilt angle of HCS. However, whether

these results, shown in Figs. 7 and 8, are real or due to

limited range and/or small tilt-angles in each period is not

clear.

Similar analysis between I and tilt angle for extended

periods, 19831986 (A < 0) and 19931996 (A > 0) shows

somewhat better relationship between the two; although

the scatter in data point is large (see Fig. 9). However,

the gradient shows no dependence on polarity states, being

nearly same in both the epochs, 0.03% per degree tilt an-

gle.

4 Discussion

In the inner heliosphere, two basic kinds of recurrent phe-

nomenon in the solar wind and interplanetary magnetic

fields have been found. One is the HCS that is usually tilted

from the solar equator. As the Sun rotates, the heliomagnetic

latitude at the Earth/spacecraft, which is proportional to its


9/11


Fig. 7 Scatter plots, between maximum-minimum difference (ampli-

tudes of oscillations) in GCR intensity during different C-rotations

and tilt angle of HCS, in three solar minima periods (19761977,19851986 and 19951996)

distance to the current sheet, changes periodically. The other

recurrent phenomenon is the corotating interaction region

(CIR), which is the result of compression between slow and

fast solar wind streams. Since the slow wind mainly orig-

inates from the equatorial zone and the fast wind streams

come from polar coronal holes, a persistent equatorial exten-

sion of a polar coronal hole will produce a series of CIRs as

Fig. 8 Scatter plots, between GCR intensity oscillations and tilt an-

gles, during C-rotations in declining and low solar activity periods of

two solar cycles: 19831984 (A< 0) and 19931994 (A> 0)

observed by an interplanetary spacecraft. Both the HCS and

the CIRs are expected to modulate cosmic rays up to same

degree of latitude; thus both may produce 27-day recur-

rent variations in the cosmic ray flux (Simnett et al. 1998).

However, it is interesting to know whether the primary cause

of recurrent cosmic ray modulation is tilted HCS or CIRs.

Although the effects of HCS and CIR can not be easily dis-

tinguished, there are evidence supporting one or the other

and more studies are needed to resolve this question.

Although epoch dependent effects in recurrent modula-

tion were indirectly observed in the neutron monitor data

much earlier (e.g. Badruddin et al. 1985; Newkirk and Fisk1985), a detailed systematic study of this effect by Richard-

son et al. (1999) and Ulysses observations (e.g. Heber and

Burger 1999) have created renewed interest in this phe-

nomenon (e.g. Reames and Ng 2001; Kota and Jokipii

2001; Burger and Hitge2004; Richardson2004; Singh and

Badruddin2005; Gupta and Badruddin2005).

Richardson et al. (1999), and more recently, Gil et al.

(2005) and Singh and Badruddin (2005, 2007), have re-

ported that amplitudes of recurrent variations in GCR in-


10/11


Fig. 9 Relationship between amplitude of GCR oscillations and tilt

angle of HCS during two different polarity states of the heliosphere in

low solar activity periods, (a) 19831986 (A< 0) and (b) 19931996

(A > 0)

tensity are larger for A > 0 than for A < 0 periods of so-

lar magnetic cycle. In this study, we find the GCR intensity

variation during C-rotation in A > 0 is different from that in

A < 0 epoch. The SPE plots during C-rotations in all three

Figs.1, 2 and 3, depict a systematic rise in GCR intensity

during 1980s (A < 0) while it shows systematic decrease

and increase during the course of C-rotation in 1970s and

1990s (A> 0). This difference can be ascribed to the so-

lar field dependence of the transport parameters (Chen and

Bieber 1993) such as to enhance the effect of cosmic rayconvection during A > 0 (Richardson et al.1999). However,

it is also possible that the systematic rise seen in temporal

evolution during C-rotations in 1980s may arise due to the

increase of GCR level in 1980s which is sharper and more

systematic than in 1970s and 1990s; this difference is as-

cribed as to drift effects causing different types of GCR max-

ima in A > 0 and A < 0 periods (Jokipii and Thomas1981).

Thus, the systematic rise seen in SPE plots during 1980s is

probably masking the underlying 27-day variation.

The role of solar wind speed in corotating decreases has

been the focus of special attention since long. Iucci et al.

(1979), for example, observed that maximum depression in

cosmic ray density was correlated with the maximum speed

inside the solar wind stream and with the magnitude of the

increase in solar wind speed. Richardson et al. (1996) also

reported similar relationship with a weak positive correla-

tion. Richardson (2004) examined the relationship between

maximum depression in GCR intensity and maximum solar

wind velocity for the high speed streams separately during

A > 0 and A < 0 epoch. He found a weak correlation dur-

ing both the epochs; however correlation between the two

is better in A> 0 epochs compared to A< 0 epoch. From

our analysis we found that there is no significant correlation

between 27-day recurrent variations and solar wind speed

during A < 0 periods. However, during A > 0 epochs, there

is consistent and generally significant correlation with solar

wind speed, in agreement with Singh and Badruddin (2007).

For modulation models in which the variation in helio-

magnetic latitude is the primary source of the 27-day recur-rent variation of GCR, one would expect that the maximum-

minimum difference during C-rotations be correlated with

the tilt angle of the HCS. On the other hand, if CIRs are

the primary cause of recurrent cosmic ray modulation, one

would expect (a) a close anti-correlation with solar wind ve-

locity; (b) in every transition from slow solar wind to fast so-

lar wind, where CIR and magnetic field compression occur,

the GCR intensity decreases rapidly and then it is followed

by gradual recovery in the solar wind rarefaction region.

We found some, although a weak, dependence between

maximum-minimum differences in GCR intensity during C-

rotations and tilt angle of the HCS during minimum and lowsolar activity periods, consistent with those of Gil and Ala-

nia (2001). Moreover, the dependence, if any, is similar in

both A > 0 and A < 0.

5 Conclusions

Study of galactic cosmic ray (GCR) during C-rotation in low

solar activity conditions and in different polarity states (A 0) leads to the following conclusions:

1. GCR intensity oscillates during the course of Carringtonrotation.

2. The average behavior of GCR-oscillations during C- ro-

tation is different in A > 0 from that in A < 0 epoch.

3. Correlation of solar wind speed with GCR intensity dur-

ing the course of C-rotation is stronger for A > 0 than

A < 0.

4. The amplitudes of GCR-oscillations during C-rotations

show somewhat weak dependence on the tilt angle of the

heliospheric current sheet.


11/11


5. The results emphasize the solar magnetic cycle depen-

dence in corotating modulation of galactic cosmic rays

reinforcing the importance of drift-effects.

Acknowledgements We appreciate the generosity of Cliff Lopate

and llya Usoskin for Climax and Oulu neutron monitor data, Todd

Hoeksema for making available the Carrington rotation and tilt angle

data. We thank the reviewer of the paper for pointing out a recent result

relevant to this work and for other constructive comments.

References

Badruddin: Proc. 23rd Int. Cosmic Ray Conf.3, 727 (1993)

Badruddin: Astrophys. Space Sci.246, 171 (1997)

Badruddin, Yadav, R.S.: Proc. 19th Int. Cosmic Ray Conf. 4, 461

(1985)

Badruddin, Yadav, R.S., Yadav, N.R.: Planet. Space Sci. 33, 191

(1985)

Barouch, E., Burlaga, L.F.: J. Geophys. Res.80, 449 (1975)

Burger, R.A., Hitge, M.: Astrophys. J.617, L76 (2004)

Burlaga, L.F., McKibben, F.B., Ness, N.F., Schwenn, R., Lazarus, A.J.,

Mariani, F.: J. Geophys. Res.89, 6579 (1984)Chen, J., Bieber, J.W.: Astrophys. J.405, 375 (1993)

Cummings, A.C., Stone, E.C.: Proc. 6th Int. Solar Wind Conf.2, 599

(1988)

Duggal, S.P., Pomerantz, M.A.: Proc. 15th Int. Cosmic Ray Conf.3,

370 (1977)

Duggal, S.P., Tsurutani, B.T., Pomerantz, M.A., Tsao, C.H., Smith,

E.J.: J. Geophys. Res.86, 7473 (1981)

Fisk, L.A.: J. Geophys. Res.101, 15547 (1996)

Forbush, S.E.: Terr. Mag.43, 135 (1938)

Gil, A., Alania, M.V.: Proc 27st Int. Cosmic Ray Conf.9, 3725 (2001)

Gil, A., Iskra, K., Modzelewska, R., Alania, M.V.: Adv. Space Res.

35(4), 687 (2005)

Gupta, V., Badruddin: Proc. 29th Int. Cosmic Ray Conf.2, 61 (2005)

Heber, B., Burger, R.A.: Space Sci. Rev.89, 125 (1999)

Iucci, N., Parisi, M., Storini, M., Villoresi, G.: Nuovo Cimento2, 421(1979)

Jokipii, J.R.: Geophys. Res. Lett.8, 837 (1981)

Jokipii, J.R., Thomas, B.T.: Astrophys. J.243, 1115 (1981)

Kane, R.P.: Solar Phys.209, 207 (2002)

Kota, J., Jokipii, J.R.: Astrophys. J.265, 573 (1983)

Kota, J., Jokipii, J.R.: Geophys. Res. Lett.18, 1797 (1991)

Kota, J., Jokipii, J.R.: Proc. 27st Int. Cosmic Ray Conf.9, 3577 (2001)

Kudela, K., Storini, M., Hofer, M.Y., Belov, A.V.: Space Sci. Rev.93,

153 (2000)

Mavromichalaki, H., Vassilaki, A., Marmatsouri, E.: Solar Phys.115,

345 (1988)

Mckibben, R.B., Jokipii, J.R., Burger, R.A., Heber, B., Kota, J., Mc-

Donald, F.B., Paizis, C., Potgieter, M.S., Richardson, I.G.: Space

Sci. Rev.89, 307 (1999)

Mishra, B.L., Shrivastava, P.K., Agrawal, S.P.: Proc. 21st Int. Cosmic

Ray Conf.6, 299 (1990)Newkirk, G., Fisk, L.A.: J. Geophys. Res.90, 3391 (1985)

Ozguc, A., Atac, T.: New Astron.8, 745 (2003)

Paizis, C., Heber, B., Farrando, P., Raviart, A., Faleoni, B., Marzolla,

S., Potgieter, M.S., Bothmer, V., Kunow, H., Muller-Mellin, R.,

Posner, A.: J. Geophys. Res.104, 28241 (1999)

Parker, E.N.: Planet Space Sci.13, 9 (1965)

Potgieter, M.S., Burger, R.A., Ferreira, S.E.S.: Space Sci. Rev.97, 295

(2001)

Rao, U.R.: Space Sci. Rev.12, 719 (1972)

Reames, D.V., Ng, C.K.: Astrophys. J.563, L179 (2001)

Richardson, I.G.: Space Sci. Rev.121, 267 (2004)

Richardson, I.G., Wibbrenz, G., Cane, H.V.: J. Geophys. Res. 101,

13,483 (1996)

Richardson, I.G., Cane, H.V., Wibbrenz, G.: J. Geophys. Res. 104,

12,549 (1999)Roberts, D.A., Giacalone, J., Jokipii, J.R., Goldstein, M.L., Zepp, T.D.:

J. Geophys. Res.112, A08103 (2007)

Sabbah, I.: Geophys. Res. Lett.27, 1823 (2000a)

Sabbah, I.: Can. J. Phys.78, 293 (2000b)

Scholar, M., Hovestadt, D., Klecker, B., Gloecker, G.: Astrophys. J.

227, 323 (1979)

Shea, M.A., Smart, D.F.: Adv. Space Res.1, 147 (1981)

Simpson, J.A.: Space Sci. Rev.83, 7 (1998)

Singh, Y.P., Badruddin: Proc. 29th Int. Cosmic Ray Conf.2, 73 (2005)

Singh, Y.P., Badruddin: J. Geophys. Res.112, A05109 (2007)

Simnett, G.M., Kunow, H., Fluckiger, E., Heber, B., Horbury, T., Kota,

J., Lazarus, A., Roelof, E.C., Simpson, J.A., Zhang, M., Decker,

R.B.: Space Sci. Rev.83, 215 (1998)

Stone, E.C.: Proc. 20th Int. Cosmic Ray Conf.7, 105 (1987)

Venkatesan, D., Badruddin: Space Sci. Rev.52, 121 (1990)Venkatesan, D., Shukla, A.K., Agrawal, S.P.: Solar Phys. 81, 375

(1982)

Yadav, R.S., Sharma, N.K., Badruddin: Solar Phys.151, 393 (1994)

Zhang, M.: Astrophys. J.484, 841 (1997)

Zhang, M., Simpson, J.A., Mckibben, R.B., Johns, T.S., Smith, E.J.,

Phillips, J.L.: Proc. 24th Int. Cosmic Ray Conf.4, 956 (1995)

osilasi solar magnetic cycle dependence in corotating modulation

Documents