| ACE News Archives | ACE News #65 - Oct 15, 2002 |
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ACE Science Nuggets |
As galactic cosmic rays diffuse through the outward-flowing interplanetary medium (the solar wind) to an observer near the orbit of Earth, they are expected to suffer adiabatic energy loss. The magnitude of this energy loss is understood to vary over the solar cycle, with a substantially greater energy loss near solar maximum than near solar minimum. The Fisk model of solar modulation, which takes account of this energy loss, has been used effectively to fit the varying energy spectra of cosmic rays over the solar cycle (see ACE News #58.) Now, observations of specific cosmic-ray isotopes by the Cosmic-Ray Isotope Spectrometer (CRIS) on ACE, have directly demonstrated differences of energy loss between solar minimum and solar maximum.
Some secondary nuclei in the cosmic rays are unstable, but can decay only by electron capture. These nuclei are produced by fragmentation of heavier nuclei in collisions with nuclei of the interstellar gas. Since they are produced fully ionized, they are stable unless they attach an electron from the interstellar medium. The cross-section for electron attachment is strongly energy dependent; at high energies it is so small that attachment is very unlikely before the nuclei are lost to fragmentation or leakage from the galaxy. At lower energies the attachment cross-section is higher, and there is substantial probability of attachment and subsequent decay. 49V is stable when fully ionized, but with an atomic electron it decays to 49Ti with a half-life of the order of a year. The left-hand figure shows the energy dependence of the 49Ti/49V ratio observed by CRIS during two years near solar minimum. This ratio of daughter to parent increases with decreasing energy, as expected from the increased probability of electron attachment. (See also ACE News #47.)
The right-hand figure shows the same 49Ti/49V ratio observed by CRIS during a similar period near solar maximum. Here, over the entire energy range we observe that the ratio has essentially the same value as it had above 300 MeV/nucleon during solar minimum. Thus the nuclei observed as low as 120 MeV/nucleon during solar maximum must have come from the population that was observed above 300 MeV/nucleon during solar minimum. In other words the mean energy loss in the solar system was higher during solar maximum than during solar minimum by at least 170 MeV/nucleon. Similar results are observed by CRIS in the 51V/51Cr ratio, confirming the changing extent of energy loss with changing level of solar modulation.
Contributed by Martin Israel and Lauren Scott of Washington University in St. Louis.
Last modified 15 October 2002, by
Andrew Davis