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ACE News #51 - January 29, 2001

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The Disappearance of ACR Oxygen with Solar Maximum

Anomalous Cosmic Rays (ACRs) are accelerated at the solar wind termination shock far out in the heliosphere. Those ACRs that are observed at 1 AU have diffused back through the heliosphere, moving upstream against the steady flow of turbulent magnetic fields that are carried with the solar wind. This difficult upstream motion reduces both the number density and energy of the inward-bound ACRs in a process called solar modulation. As solar activity has been increasing over the duration of the ACE mission, the intensity of ACR oxygen has been steadily decreasing.

The left panel above shows oxygen spectra as observed with SIS at 1 AU during solar quiet days. The spectrum for the period A, 9/97-3/98, is taken to be the spectrum at solar minimum, and it shows the two major components of the quiet-time oxygen population at these energies: at the lower energies, the high intensity population is the ACR component, and at the higher energies the spectrum is dominated by the galactic cosmic ray (GCR) component. Subsequent spectra show lower levels of both ACR and GCR oxygen.

By the latter half of 2000, the intensity of oxygen at about 8 MeV/nuc has been reduced by a factor of over 80. In contrast, GCR oxygen at about 80 MeV/nuc has decreased by a factor of less than 6. The right panel above shows the changing ratio of oxygen to carbon at energies between 7.1 and 10 MeV/nuc. For oxygen, these energies are the lowest that can be detected with SIS. Each point on this panel represents one of the periods from the left panel. The O/C ratio experiences large changes over the course of the mission, from a value of over ~30 in the solar minimum period, which is consistent with ACRs, to a value of about 2.2. This latter ratio is more similar to that observed in solar energetic particle events or in events associated with corotating interaction regions. Thus, it is possible that by the latter half of 2000, ACR oxygen below ~10 MeV/nucleon may have disappeared below the background of particles of solar origin.

Contributed by Luke Sollitt, Alan Cummings, and Richard Mewaldt of Caltech.

For more information on the SIS instrument, see The SIS Home Page.

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Last modified 29 January 2001, by Andrew Davis