Stars as Laboratories for Fundamental Physics - MPP Theory Group

372 Chapter 10
10.3.5 Summary
The main features and results of the solar neutrino experiments discussed
in this section, and of near-future experiments to be discussed
in Sect. 10.9, are summarized in Tab. 10.9. More technical aspects can
be found in the original papers quoted in this section and in previous
papers by the referenced authors. Overviews of many experimental aspects
can be found in Bahcall (1989), Davis, Mann, and Wolfenstein
(1989), and Koshiba (1992). The two theoretical predictions shown
are somewhat extreme in that TL93 yields lowish neutrino fluxes when
compared with BP92 (no diffusion), while BP95 with the inclusion of
helium and metal diffusion is presently at the upper end of what is
being predicted on the basis of standard solar models. It is clear, of
course, that including diffusion would also increase the TL93 fluxes.
10.4 Time Variations
10.4.1 Day-Night Effect
It is commonly assumed that the Sun is in a stationary state so that
the solar neutrino flux should be constant in time. One may still expect
certain temporal variations of the measured flux. Between day
and night the line of sight between the Kamiokande detector and the
Sun intersects with different parts of the Earth. If neutrinos oscillate,
for certain masses and mixing angles the Earth’s matter would
alter the oscillation pattern such that the counting rate at Kamiokande
would be expected to vary between day and night. The day rate is
found to be 0.90 ± 0.10 stat ± 0.12 syst times the average, the night rate
1.04 ± 0.10 stat ± 0.12 syst , i.e. there is no significant difference (Suzuki
1995). The radiochemical detectors with their long exposure times cannot
resolve a possible day-night difference.
10.4.2 Seasonal Variation
Because of the ellipticity of the Earth’s orbit the distance to the Sun
varies during the year from a minimum of 1.471×10 13 cm in January
to a maximum of 1.521×10 13 cm in July, i.e. it varies by ±1.67% from
its average during the year. Therefore, the solar neutrino flux varies
by ±3.3% from average between January and July. This effect is too
small to be observed by any of the present-day experiments.
This variation can be amplified if neutrinos oscillate. Notably, if
the vacuum oscillation length of the monochromatic 7 Be neutrinos is