| ACE News Archives | ACE News #57 - December 6, 2001 |
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ACE Science Nuggets |
Four years after the November 1997 solar energetic particle (SEP) event inaugurated the current solar maximum period, solar activity continues at high levels, as evidenced by two very large SEP events that occurred in November 2001. The upper panel of the Figure compares the intensity profiles of the four largest events of Solar Cycle 23, including > 10 MeV proton data from GOES-8 and >10 MeV/nucleon oxygen and iron intensities from the SIS instrument on ACE. During the recent Nov. 4-9, 2001 event the proton intensity reached 32,000 particles per cm2sr-sec, making it the most intense event (so far) of this solar cycle. The other three events are close behind.
The histograms below show measured proton peak intensities and the H, O, and Fe fluences summed over each event. Note that the largest fluence does not necessarily occur in the event with the largest peak intensity. Indeed, it appears that the largest peak intensities have occurred in events closer to central meridian in which there is a very strong, high-speed (>1000 km/sec) shock that reaches Earth within 1-2 days (dotted lines), accompanied by locally-accelerated particles that extend up to high energies. In these cases, the intensity drops once the shock has passed Earth. It is also interesting that the locally-accelerated particles have been enriched in heavier species such as O and Fe relative to the composition of particles accelerated closer to the Sun. In the November 2000 event that was directed much further from the Earth-Sun line the shock speed was less, there was little evidence for local acceleration >10 MeV/nucleon, and the peak intensity was not as great.
How do the events of Solar Cycle 23 compare to the >10 MeV fluences of the largest SEP events over the last 45 years? The fluences of the two largest events exceed 109 per cm2sr, which puts them in the top 20 but not in the top 10, according to summaries by Feynman, Shea and Smart, and NOAA. The event with the largest >10 MeV fluence occurred 41 years ago in November 1960. Although the Solar Cycle 23 events have not been the largest, with the array of very capable spacecraft now in space it is probably fair to say that they will be among the best studied SEP events of all time.
Contributed by Christina Cohen and Richard Mewaldt of Caltech and Ron Zwickl of NOAAs Space Environment Lab.
Last modified 6 December 2001, by
Andrew Davis