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ACE News #42 - March 17, 2000

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Forecasting Shock-Accelerated Energetic Particles with ACE

During large solar energetic particle (SEP) events associated with coronal mass ejections (CMEs) the highest particle intensities are often observed when the interplanetary shock driven by the CME passes the spacecraft. At 1 AU these locally-accelerated particles occasionally extend to very high energies, creating a risk of increased radiation dose to astronauts and space hardware. For example, during the large SEP events of October 1989 the (already very intense) flux of >100 MeV protons suddenly increased by ~ x10 for several hours as the shock passed 1 AU. The effect of these shock-associated particle increases is often magnified because the shock may also cause a geomagnetic storm that suddenly lowers the geomagnetic cutoff at Space Station latitudes.

In order to provide the capability to forecast such radiation risks to astronauts, two additional count-rates were added to the Real Time Solar Wind (RTSW) data stream that ACE provides to NOAA. These count rates, obtained from the Solar Isotope Spectrometer (SIS), record protons with energies >10 MeV and >30 MeV. Increases in these rates coincident with the passage of an interplanetary shock at ACE can indicate a local population of shock-accelerated high-energy particles. An example of such an event is shown above, where the count rate of > 10 MeV protons measured by ACE at L1 is compared with the > 10 MeV proton flux measured in geosynchronous orbit by GOES-8. A shock at 2315 UT on day 267 (indicated by the triangle) was identified by the MAG and SWEPAM instruments on ACE. Note that the time profiles at ACE and GOES are nearly identical with a time delay of 43 ± 5 minutes. This delay indicates that the shock (with its associated particle fluxes) traveled the 1.5 million-km from L1 to Earth at an average speed of 590 ± 90 km/s. This and other examples demonstrate that real-time data from ACE can provide up to 1 hour advanced warning of high-energy protons associated with large interplanetary shocks.

Contributed by Christina Cohen and Richard Mewaldt of Caltech.

For real-time data from ACE see http://www.srl.caltech.edu/ACE/ASC/rtsw.html.
See http://www.srl.caltech.edu/ACE/CRIS_SIS/sis.html for more on SIS.

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Last modified 17 March 2000 Steve Sears