Mathematics – Probability
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsh21a0726f&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #SH21A-0726
Mathematics
Probability
2159 Plasma Waves And Turbulence
Scientific paper
To study intermittent turbulence in the solar wind and how it scales, we analyzed moments and probability density functions (pdf) of fluctuations in the radial solar wind speed V(t+L)-V(t) reported by the SWEPAM instrument on the ACE spacecraft during 24 days in the middle of last year. After filling a few short data gaps by linear interpolation, we had a continuous set of 31,561 values of the solar wind speed at 64-second intervals. We used 15 lags from 64 to 1 million seconds. The pdfs at all L are fitted by Castaing distributions, as Sorriso-Valvo, et al. (GRL 26, p1801, 1999) noticed for other solar wind data sets. The Castaing parameters varied smoothly with lag, but a change occurred at L = about 1 hour: the parameters varied more rapidly at lags above 1 hour, than at shorter lags. However, since the structure function of order 3 is linear in lag at lags from 64 seconds to several days, it appears that the energy cascade was continuous. This also makes the fitted Castaing model identical to the K-O 1962 log-normal model, and consistent with a P-model. Thus we appear to have a clear case of a continuous turbulent cascade, in which the intermittency suddenly changes for some reason. Such a change is quite tractable in the Castaing scaling model. In terms of the K-O model, the mu parameter falls from about 2 at several hours lag, to 0.2 below one hour. In terms of the P-model (a discrete version of Castaing scaling), the corresponding P-value falls from about 0.9 to 0.7. In both models, behavior at L less than 1 hour is similar to that seen in the laboratory, but the large-lag behavior is not. The interesting physics question is, Why does the intermittency change at 1hour lag, and why is it so large on scales from 1 to 100 hours? What causes intermittency? We are very grateful to the ACE project and SWEPAM experimenters for the beautiful data of the SWEPAM instrument available on the ACE website. MAF is supported by NASA grant NAG 510995.
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