GRAIL USOs; Another in-flight quartz radiation experiment

The Gravity Recovery and Interior Laboratory (GRAIL) mission successfully ended in December, 2012 after an extended science phase of over 280 days mapping the gravitation field gradient of the Moon with a precision of better than 50 E-6 g´s over the entire lunar surface. The mission was performed by two tandem flying spacecraft, both of which carried a Microwave Dual One-way Ranging (DOWR) instrument that together formed a highly precise relative measure of the distance between the two spacecraft as they orbited the Moon. The precision of the radio link maintained by the DOWR instruments was derived from the frequency stability of the ultra-stable oscillators (USOs) on-board each spacecraft. The opportunity to observe the USOs frequency throughout the GRAIL mission provides a record of not only the intrinsic performance of the oscillators, but their behavior during exposure to space conditions. We describe the effect to the frequency of each GRAIL USO, A and B, resulting from the March 7th, 2012 X5.4 level solar flare, just several days into GRAIL´s science collection phase. We discuss the impact of this radiation exposure, and the asymmetric behavior of two USOs coincidentally perturbed by the same space-weather event.