Following a news article from Stanford, slashdot has followed up with a report on observed changes in decay rates of radioactive elements caused by solar flares. This is not really new and follows other claims that nuclear decay rates change by about 0.1% on an annual cycle (or other longer cycles corresponding to various forms of solar activity) of course Motl has reported it two years ago.
The explanation for the effect offered by the scientists involved is that it is caused by changes in neutrino fluxes from the Sun. There are small variations in beutrino flux of this sort but the effect on deacy rates seems unlikely because neutrino interactions as we know them are too weak for this to happen. It would have to be something far outside the standard model that has somehow avoided detection in very sensitive experiments such as Super-Kamiokande. If not neutrinos then the next suggestion is some as-yet-unknown particle. It is an appealing idea but it is still unlikely that something like that could affect nuclear decay rates without being observed in different ways in other experiments.
Could the effect be caused by something more basic such as changes in electric or magnetic fields or even temperature in the environment of the experiment? If such dramatic effects on decay rates could be caused in these ways it would have been observed in controlled experiments a long time ago, so this can be ruled out. Environmental effects on the measuring apparatus are another matter, so suspicions immediately arise.
Putting aside the solar flare result for the moment, it turns out that someone has already done some more careful experiments to look for annual variations of decay rates and found nothing (See Evidence against correlations between nuclear decay rates and Earth–Sun distance) here is one of their plots. I have chosen the one that looks like it comes closest to showing some effect to my eye but it is nowhere near the claimed effect shown by the sine curve and is not statistically significant.
Could there be some effect seen better with solar flares? The trouble is that solar flares are relatively brief. One claim was for an effect that lasted 43 minutes. The studies of annual variations were performed over a period of two years and still the statistical errors are just a little below the expected effect. If solar flares caused an effect observable over 43 minutes it would surely have to be much larger to stand out from the noise.
Sadly then, the warning signs are not aligning in favour of these results, but ruling out an effect conclusively might require more experiments.