Received December 28, 1999A. K. Svirzhevskaya and N. S. Svirzhevskiy
P. N. Lebedev Physical Institute, Moscow, Russia
Received December 28, 1999
The North-South asymmetry in cosmic ray fluxes in the lower atmosphere at the high-latitude stations Murmansk and Mirny (Antarctic) has been discussed by many authors [Charakhch'yan, 1986a, 1986b; Charakhch'yan and Charakhch'yan, 1979; Charakhch'yan and Stozhkov, 1983; Stozhkov et al., 1996; Svirzhevskaya et al., 1997]. Nevertheless, the physical mechanism responsible for different particle fluxes in the northern and southern hemispheres incident at large angles to the ecliptic is still unclear. The attempts to find a parameter in the solar wind or interplanetary magnetic field (IMF) that would correlate with the asymmetry in the particle fluxes for a long time period have also failed.
Probably, there are several causes for the NS asymmetry. Any North-South asymmetry in the solar wind and IMF parameters can lead to asymmetry in the particle fluxes. Various scenarios for development of asymmetry also are in favor of the fact that there are different sources of the asymmetry. Most often, the asymmetry arises when time variations in the particle fluxes in the North and South are in phase, but somewhat shifted in time and differ in magnitude. However, sometimes the particle fluxes at the two stations are observed to be in antiphase, as, for instance, in February-June, 1973.
Charakhch'yan [1986b] put forward the hypothesis that properties of the NS asymmetry are determined by the sign of the total magnetic field of the Sun and, hence, can experience 22-year variations. At first glance, this hypothesis was not confirmed because a large positive NS asymmetry (like in the 1970s) was expected to take place after the solar magnetic field polarity reversal in 1991, when the IMF direction became the same as in 1971-1979. In 1991-1998, the North-South asymmetry was indeed large, but it changed the sign. Probably, the 22-year periodicity is manifested only in the magnitude of the asymmetry. So it was nearly zero under negative IMF in the northern hemisphere of the heliosphere (N- S+ ) in 1963-1970 and 1980-1988 and large under the IMF of opposite sign (N+ S- ) in 1971-1979 and 1991-1998.
Below we discuss the data on the NS asymmetry averaged over a 11-year cycle and monthly averages for 1970-1998. Major specific features of this phenomenon and their relation to the solar wind velocity and IMF direction are considered. The dependence of the NS asymmetry on the threshold rigidity of radiation and the annual wave in its magnitude are presented.
To quantitatively characterize the asymmetry, the parameter
ANS = 100 
(NMurm
- NMirn)/(NMurm + NMirn)
is
typically used. Here,
NMurm and
NMirn are the fluxes of
charged particles at Murmansk and Mirny, respectively. The temporal
behavior of the NS asymmetry in 1970-1998 for full fluxes of
charged particles measured by Geiger counters is shown in Figure 1.
ANS is given for four intervals of atmospheric pressure:
200-300, 300-400, 400-500, and 500-600 g cm-2. Temporal
variations in
ANS are synchronous in all pressure intervals, and
the value of asymmetry depends on pressure
x so that the lowest
ANS magnitudes correspond to
x = 200-300 g cm-2 and the highest
ANS magnitudes correspond to
x = 500-600 g cm-2.
It is likely
that in 1980-1988 the asymmetry was suppressed, while in the 1970s
and 1990s it was sometimes as large as 10-12%. Beginning from
1989,
ANS became, on the average, negative,
and, in addition, a
pseudoannual wave with a minimum approximately in the middle of the
year appeared. The asymmetry for vertical particle fluxes in the
atmosphere measured by telescopes involving Geiger counters in the
pressure intervals 300-400, 400-500, and 500-600 g cm-2 confirms
that the picture is stable (Figure 2).
The NS asymmetry manifests itself also in the particle fluxes averaged over an 11-year cycle (Figure 3). In the 1970s and 1990s, the value of the asymmetry ANS was 2-4%. On the average, in the 1970s the particle fluxes from the northern direction of the heliosphere were larger than from the southern direction. In the 1990s, the opposite situation took place. It can be expected that some physical parameters on the Sun or in the heliosphere varied in a similar fashion. The value of the asymmetry ANS in the 1980s was by an order of magnitude lower than in the 1970s and 1990s. Note that there is no pronounced correlation between the solar wind velocity and the asymmetry averaged over an 11-year cycle. The average solar wind velocity Vsw near the Earth in the 1970s, 1980s, and 1990s was 443.6, 443.3, and 438.9 km s-1, respectively.
Earlier, Svirzhevskaya et al. [1997] showed, using a large positive asymmetry of 1973 as an example, that ANS does not change when the Earth passes from one IMF sector to another. The parameter which is related to the interplanetary plasma and IMF and behaves in a similar manner at that time, is the difference between the solar wind velocities in neighboring sectors, dVsw = V+ - V-. There was an evident anticorrelation between ANS and dVsw in the first half of 1973.
In this work the value ANS and sign of the NS asymmetry are compared with the difference between the solar wind velocities dVsw = V+ - V- above and below the heliospheric current sheet (HCS) in 1990-1998. The OMNI files were used as initial data on the IMF and solar wind [NASA NSSDC, 1998]. The field in the northern hemisphere of the heliosphere directed away from the Sun forms a "+" sector near the helioequator if the azimuthal IMF component Bj falls into the longitudinal interval 45o-225o with the average value of 135o in the ecliptic (right-handed) coordinate system with the Ox axis pointed from the Earth's center toward the Sun. The field in the southern hemisphere directed toward the Sun forms a "-" sector near the helioequator in the longitudinal interval 225o-45o with the average value of 315o. For the "+" and "-" sectors, monthly average velocities V+ and V- were derived, and then dVsw was calculated.
The North-South asymmetry in cosmic rays and the difference
between
the solar wind velocities
dVsw in 1990-1998 are shown in
Figure 4.
ANS and
dVsw vary in antiphase during two and a half years
in succession (1992-1994), in the early 1995, and in 1996. During
these periods, the relation between
ANS and
dVsw is the same
(anticorrelation) as in February-June, 1973. During some periods
(1991, the second half of 1994 and of 1995) the anticorrelation is
disturbed. The correlation coefficient between two data series for
the whole period 1990-1997 is low and equals
-0.33.
This leads to
the conclusion that the difference between the solar wind
velocities
dVsw above and below the heliospheric current sheet
can be one of the factors controlling the NS asymmetry. However, it
is not the only source of the NS asymmetry.
In 1991-1997 (with the exception of 1995), the annual wave was observed in the NS asymmetry. The oscillations in ANS were not symmetric with respect to zero, they were shifted toward negative values. During this period, the annual wave was also observed in the velocity difference dVsw. In 1994-1997, it was probably associated with the change in the Earth's heliolatitude. In March, the Earth occupies the extreme southern position in heliolatitude and the solar wind velocity Vsw in the Southern sector is considerably higher than in the Northern one. In half a year, in early September, the Earth's heliolatitude is ~7o N, and Vsw is higher in the Northern sector than in the Southern. In 1992-1993, the annual wave in dVsw had a different phase. Note also the asymmetry of dVsw with respect to its zero value. The solar wind velocity to the south of the heliospheric current sheet in 1994, 1995, and 1996 was higher than to the north of the sheet, as if the Earth moved to greater distances from the heliospheric current sheet in the negative (southern) sector than in the positive (northern) sector. This could happen if the HCS extended northward from the helioequator. The northward extension of the HCS was confirmed by the Ulysses data in 1996 [Forsyth et al., 1997], but there are reasons to believe that in 1994 the HCS extended southward from the helioequator by approximately 10o [Simpson, 1996].
Below we discuss some properties of the NS asymmetry in charged particle fluxes in the lower atmosphere. The source of secondary radiation under atmospheric pressure x > 300 g cm -2 are galactic cosmic rays (GCR), mainly protons with a rigidity greater than 4 GV. The anisotropic distribution of primary radiation with this rigidity beyond the magnetosphere is the cause of the observed NS asymmetry.
The NS asymmetry
is typically described by invoking the drift
currents
B (grad n)r,
where
B is the IMF strength and (grad
n)r is the radial component of gradient of the distribution
function of cosmic ray density
n [Pomerantz and Bieber, 1984].
These currents reverse the sign when the Earth
crosses the sector boundary. The fact that the NS asymmetry does
not change its sign when the Earth traverses the HCS means that the
drift currents of the
B (grad n)r
type do not give a
significant contribution to
ANS in our case.
Svirzhevskaya et al. [1997] discussed a possible scheme for formation of the NS asymmetry involving the diffusion current of particles which has the same direction in neighboring sectors. In case of different solar wind velocities at opposite sides of the HCS, an asymmetric convective particle outflow takes place. It leads to a lower density of particles in the sector with a higher wind velocity. In this case a density gradient of cosmic rays perpendicular to the HCS and the diffusion current associated with this gradient arise. The current flows from the sector with a lower Vsw to the higher-velocity sector and does not change its direction at the sector boundary. The diffusion current supports the asymmetry as long as the difference between the solar wind velocities dVsw in neighboring sectors remains sufficiently large. It is quite clear what sign the asymmetry has in this situation: the additional particle flux is directed from the sector with a low solar wind velocity toward the higher-velocity sector. Of importance for the asymmetry onset is a sufficiently large difference between the solar wind velocities in different sectors rather than the value of Vsw by itself. This scheme describes well the large asymmetry in February-June, 1973. When high-velocity solar wind fluxes were observed in the Southern hemisphere of the heliosphere, dVsw was negative and ANS was positive. One can see from Figure 4 that this scheme correctly predicts the asymmetry sign for the greater part of the 1990s, but during some periods (1991, the second half of 1994 and of 1995) the anticorrelation is violated. The condition dVsw > 0 (or dVsw < 0) is not sufficient for the onset of negative or positive asymmetry in these cases.
A large stable asymmetry during two years (1996-1997) should be noted. In the first half of 1996 the sector structure at the Earth's orbit was nearly absent - for the most part of the period the Earth was in the southern sector, that is, in the magnetic field of the same sign. The asymmetry in this case could be caused by the equatorward drift of particles from the South pole. The drift current in this case could be detected only at Mirny where asymptotic reception cones pointed southward.
As follows from the drift theories of modulation, in the 1970s and
1990s the drift currents were directed from the heliosphere poles
toward the equator and further, along the HCS, to the heliosphere
boundary
[Potgieter and Moraal, 1985].
In the 1980s, the
drift currents flew along the HCS toward the Sun and then to the
poles. The longitudes of the asymptotic directions for particles
with a rigidity of 10 GV
for Murmansk and Mirny stations during the
whole year were within the angle of
90o directed from the
Earth toward the Sun. Therefore, detectors in the lower atmosphere
at these stations can efficiently detect particles only in case of
the (N+ S- )
IMF orientation when particles move from the poles
toward the helioequator. For the opposite IMF, detectors were
pointed in the drift direction, and, hence, were not sensitive to
particles.
Thus the mutual orientation of asymptotic directions of Murmansk
and Mirny stations and drift currents depends on the IMF direction.
In the 1970s and 1990s, drift particle fluxes were efficiently
detected in the lower atmosphere. The conditions for development of
asymmetry were, therefore, favorable, and
ANS (by modulus) was
indeed large during those years. In the 1980s, detectors in the
lower atmosphere were not sensitive to the drift particle fluxes,
and
ANS 
0.
The major difference between
estimates of the NS asymmetry from
neutron data and atmospheric measurements
is that it is calculated
for different threshold GCR rigidities. The NS asymmetry in the
atmosphere is observed in a wide range of pressures,
200-700 g cm-2.
For each pressure
x in the atmosphere there is a definite
cutoff threshold
Ra of the primary GCR spectrum. It is such that
the secondary radiation from particles with rigidity
R < Ra does
not reach the level of observation of
x due to absorption in the
atmosphere. Therefore, the dependence of the asymmetry value
ANS on pressure
x can be recalculated into the dependence of
ANS on
Ra, and the NS asymmetry can be estimated for different
atmospheric thresholds, from 1.5 to 6 GV.
The dependence of
ANS on
Ra for 1973 is shown in Figure 5.
It is evident from Figure 5
that the NS asymmetry exhibits
a strong dependence on energy. For
instance, in the pressure interval 150-200 g cm-2 which
corresponds to atmospheric thresholds of 1.5-2.5 GV,
the asymmetry
is by 5-10 times smaller than for pressure 400-600 g cm-2 where
atmospheric thresholds are 4-6 GV.
The rigidity thresholds of
high-latitude neutron monitors determined by the atmospheric cutoff
are of the order of 1.5 GV. Therefore, in the
ANS calculations a
small asymmetry (by the number of particles) in the energy region
5 GeV
superimposes on large "symmetric"
fluxes of particles
with lower energy, and the resulting asymmetry is underestimated.
Thus it can be concluded that stratospheric measurements are better
suited for observations of the NS asymmetry than are neutron
monitors.
Figure 6 shows an annual wave in the North-South asymmetry of
cosmic rays in the troposphere. It is partly caused by
meteorological effects in the atmosphere. For instance, seasonal
variations in temperature and heights of barometric levels at
stratospheric stations in different hemispheres of the Earth are in
antiphase. The maximum
ANS is in August when the longitude of
the asymptotic direction for Mirny station is perpendicular to the
magnetic field B, that is, it lies in the
B grad n plane. In
this case Mirny has optimal conditions for detecting the drift
particle fluxes. Probably, in August the particle fluxes are
enhanced at Mirny due to contribution of drifts as well, and this
manifests itself as an annual wave in asymmetry. Numerically, the
annual variation is
ANS 2%.
As noted above, the attempts to find one characteristic in the solar wind or IMF that would correlate with the NS asymmetry in the particle fluxes in the atmosphere for a sufficiently long period of time have failed. Some properties of the NS asymmetry suggest that its source can be particle drifts. Any asymmetry in basic characteristics of the solar plasma - solar wind velocity responsible for convection and also the IMF strengths and disturbances causing particle drift and diffusion - must lead to asymmetry in particle fluxes. Therefore, to quantitatively describe the asymmetry, the transport equation for distributions of the solar wind velocities and IMF characteristics in the Northern and Southern hemispheres of the heliosphere should be solved for specific time periods.
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Charakhch'yan, T. N., Mysteries of zonal modulation of cosmic rays, Geomagn. Aeron., 26 (1), 10, 1986b (in Russian).
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Charakhch'yan, T. N., and Yu. I. Stozhkov, Zonal modulation and anomalous component of cosmic rays, Izv. Akad. Nauk SSSR Ser. Fiz., 47 (9), 1674, 1983 (in Russian).
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