The anomalously high concentrations of 10Be in Antarctic ice cores1,2, uncorrelated with δ18O, are consistent with an increase in the atmospheric cosmic ray (CR) flux from CR acceleration in propagating interstellar shock waves, which envelop the heliosphere and whose source may be ancient supernovas. This mechanism is an alternative to the model where decreases in geomagnetic field intensity, associated with geomagnetic reversals, periodically enhance the CR flux at the top of the atmosphere. That CR variations attributable to interstellar events such as supernova shock waves have so far not been observed in the CR record is a long-standing issue 3-5. If our interpretation of the 10Be spikes is correct, it marks the first such observation. The possibility that 'direct' observation of the interstellar medium can be made using the 10Be record would be an important adjunct to the study of cosmic rays in the interstellar medium.
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