Radial and latitudinal gradients of anomalous cosmic ray oxygen in the inner heliosphere

We use data from the Ulysses, Advanced Composition Explorer, and Solar Terrestrial Relations Observatory spacecraft to determine the radial and latitudinal gradients of anomalous cosmic ray oxygen in two energy ranges from 4.5-15.6 MeV/nuc in the inner heliosphere for the first time during an A<0 part of the solar cycle. Using measurements from early 2007 to mid-2008, we find that the radial gradient is similar to 48 +/- 13%/AU, consistent with previous inferences for A<0 and larger than that determined in the inner heliosphere for A>0. The latitudinal gradient is consistent with zero and in the range -0.3 to 0.4%/degree, suggesting that during A<0 these particles are not able to propagate into the inner heliosphere by rapid drift along the heliospheric current sheet if the tilt of the sheet is as large as 30 degrees. Citation: Cummings, A. C., C. Tranquille, R. G. Marsden, R. A. Mewaldt, and E. C. Stone (2009), Radial and latitudinal gradients of anomalous cosmic ray oxygen in the inner heliosphere, Geophys. Res. Lett., 36, L18103, doi:10.1029/2009GL039851.