Estimation of SEP and CME kinetic energies

According to Mewaldt et al. (2005a) several steps are involved in estimating the SEP kinetic energies (Emslie et al., 2004; Mewaldt et al., 2005b):

(i) Fitting spectra. The energy spectra were obtained by combining data from the ULEIS, EPAM and SIS instruments on ACE, the PET instrument on SAMPEX, and the GOES EPS sensor on GOES-8 and GOES-11. The spectra, extending from <0.1 to >100 MeV/nuc, were fit with one of two spectral forms: the double-power-law form of Band et al. (1993) or the model of Ellison and Ramaty (1985). The spectral fits were integrated from 0.01 to 1000 MeV/nuc to obtain the integrated fluencies at 1 AU.

(ii) Correcting for particles that cross 1 AU more than once. To obtain the energy/cm2 escaping from 1 AU necessary to correct for the number of times that the average particle crosses 1 AU due to scattering on interplanetary turbulence. It was used the simulation by Giacalone (2005) shown in Fig. 4.31.1, which gives a logarithmic dependence on energy. On average, this reduces the estimated energy content of accelerated particles by a factor of ~3 to 4.

(iii) Correcting for longitude and latitude profiles. Studies of heavy ions >10 MeV/nuc show that the largest SEP events originate near central meridian. This is also seen in the longitude distribution of large proton events observed by GOES (see Fig. 4.31.1). From these data sets it was derived longitudinal e-folding longitudes of 45° for western events and 25° for eastern events. The e-folding latitude was chosen to be the average of these (about 35°). Using these dependences, it is possible to integrate the total particle energy escaping through 1 AU. In order to test whether the longitudinal profiles assumed here are reasonable, Helios 1 and 2 and IMP-8 data (Reames et al., 1996) were used to compare the estimated event fluencies from three separate vantage points, as shown in Fig. 4.31.2. The locations of the three spacecraft were spread over 158° in one event and 66° in the second event. The radial differences in the spacecraft locations were also

corrected for by assuming that SEP fluencies scale as ^ r , where r is the distance from the Sun (Reames and Ng, 1998). The uncertainties on the fluency estimates were taken to be the square-root of the sum of the correction factors for longitude, latitude, and multiple crossings (Mewaldt et al., 2005b). The agreement of the three independent estimates suggests that there are not significantly underestimated the uncertainties in this model.

Fig. 4.31.1. The left panel shows the average number of times solar protons pass outward across 1 AU as a function of energy; based on a simulation by Giacalone (2005) assuming a mean free path of 0.2 AU. A logarithmic dependence was fit to these results and extrapolated to higher energy. The right panel shows the longitude distribution of large SEP events observed by the NOAA GOES satellites from 1976-2003. The e-folding longitudes used in this study are indicated. From Mewaldt et al. (2005a).

Fig. 4.31.1. The left panel shows the average number of times solar protons pass outward across 1 AU as a function of energy; based on a simulation by Giacalone (2005) assuming a mean free path of 0.2 AU. A logarithmic dependence was fit to these results and extrapolated to higher energy. The right panel shows the longitude distribution of large SEP events observed by the NOAA GOES satellites from 1976-2003. The e-folding longitudes used in this study are indicated. From Mewaldt et al. (2005a).

Septe

' I

Corrected values *

Was this article helpful?

0 0

Post a comment