Stars as Laboratories for Fundamental Physics - MPP Theory Group

Solar Neutrinos 373
of order the annual distance variation, the 7 Be flux measured in the
chlorine and gallium detectors could vary between zero and its predicted
full rate. Moreover, at different times of the year the neutrinos have
to traverse on average a different amount of terrestrial matter which
could affect neutrino oscillations and thus lead to an annual variation.
There is no evidence for such an effect in any of the detectors.
GALLEX reports a counting rate of (82 ± 15 stat ) SNU for October−
March and (78 ± 14 stat ) SNU for April−September, i.e. there is no significant
difference (GALLEX collaboration 1994).
A semiannual variation could be caused by nonstandard neutrino interactions
with the solar magnetic field (Sect. 10.7). The solar equatorial
plane is at an angle of 7 ◦ 15 ′ relative to the Earth’s orbital plane (the
ecliptic). Around 7 June and 8 December, the solar core is viewed from
Earth through the solar equator where the magnetic field is thought
to be weaker than at higher latitudes. The Kamiokande II data (1040
live detector days in 1987−1990) were subdivided into three-months
periods which include (rate Γ I ) or exclude (Γ II ) the intersection points,
respectively. The relative difference in counting rate was found to be
(Γ I − Γ II )/(Γ I + Γ II ) = −0.06 ± 0.11 stat ± 0.02 syst , i.e. there was no indication
for a time variation (Hirata et al. 1991). The Homestake data
also do not show any evidence for a semiannual variation.
10.4.3 Correlation with Solar Cycle at Homestake
The Sun shows a prominent magnetic activity cycle which is thought
to be due to dynamo action within the convective surface layers, driven
by the nonuniform rotation of the Sun (Stix 1989). One of the bestknown
manifestations of this activity is the cycle of sunspots, measured
by their total number appearing on the solar disk. 54 A given solar cycle
lasts for about 11 years from one minimum of sunspot number to
the following; the first recorded cycle begins with the minimum around
A.D. 1755. Sunspots are caused by magnetic flux tubes which break
through the surface; they are thought to be manifestations of a subsurface
toroidal magnetic field with opposite directions between the
southern and northern hemisphere. In addition, the Sun has a poloidal
(dipole) field. The fields reverse polarity after 11 years so that the full
magnetic cycle lasts 22 years.
54 In the following, the “number of sunspots” refers to the international sunspot
index according to the Zürich system where both spots and spot groups are counted
and averaged from the reports of many solar observatories. The sunspot index is
regularly published in Solar Geophysical Data.