Solar energetic particle event onset as analyzed from simulated data

Abstract

Abstract. Solar energetic particle (SEP) event onset is analyzed using simulated data. A large number of simulations using different coronal and interplanetary (IP) scattering conditions are performed. Protons in the energy range of 0.13-57 MeV are considered. The simulated data are analyzed employing a velocity dispersion analysis (VDA) to the proton-flux onset times in 16 energy channels. As a result of the analysis, the apparent coronal release time, t 0 , and the apparent IP path length, s, of the first-observed particles are obtained. It is shown that typical IP scattering conditions, i.e., 1-GV radial mean free path of Λ rr = 0.1-1 AU with a ∝P 1/3 rigidity dependence, lead to apparent path lengths of s ∼ 1-2 AU, consistent with observations. Thus, SEP events with s ∼ 2 AU can simply be explained as a result of IP scattering. Models with coronal mean free paths small enough to enable proton acceleration to high energies in quasi-parallel shocks close to the Sun are shown to yield VDA results in agreement with observations, at least with Λ rr > ∼ 0.3. As a result of IP scattering, the inaccuracy of the release times is tens of minutes in events with s > ∼ 2 AU, which may prevent accurate timing of particle release using the VDA method in such events. Low contrast between the time-of-maximum intensity of the studied SEP event and the pre-event background intensity and/or a large difference between the forms of the respective energy spectra may also lead to errors in derived onset times. Best timing results (with errors typically less than 10 min) are obtained for large IP mean free paths, Λ rr > ∼ 0.3 AU (typically resulting in small values of s < ∼ 1.5 AU), and large intensity contrast between the SEP event and the pre-event background.

The firstSEPServerevent catalogue ~68-MeV solar proton events observed at 1 AU in 1996–2010

“…Figure 5 presents the distribution of l eff ¼ L=s obtained from all the 111 events with a VDA result. Excluding the unphysical values (the integral bin), the mean value and standard deviation of l eff are 0.54 and 0.21, respectively, which seem reasonable in light of simulation studies (Lintunen & Vainio 2004;Sáiz et al 2005). …”

“…Thus, probably the apparent path length values outside 1 < s [ 3 AU are artefacts of the analysis and the related release times should not be trusted. A similar result was obtained by Lintunen & Vainio (2004) analysing simulated data.…”

“…Rather than using the apparent path length obtained for protons, which is often severely affected by interplanetary scattering (Lintunen & Vainio 2004;Sáiz et al 2005), we used the nominal length of the Parker spiral L computed for the solar-wind speed u sw observed during the event, i.e.,…”

The injection of ten electron/3He-rich SEP events

“…1c for example). Numerical simulations of ion propagation (Lintunen & Vainio 2004;Sáiz et al 2005;Laitinen et al 2010) suggest that the inferred ion path lengths may have significant errors, but for events with weak scattering in the IPM, the errors in injection times are, typically, only several minutes, much smaller than the uncertainties in and the difference between the injection times estimated in this study. On the other hand, these injection times are inferred by fitting to the observations from the event onset through the peak (that are likely dominated by particles with little/weak scattering), not to the observations in the decay tail (that are probably dominated by scattered/reflected particles).…”

Solar flares, coronal mass ejections and solar energetic particle event characteristics

“…Thus, by linear fitting of the onset times as a function of the corresponding inverse velocity, estimates for both the solar release time (SRT) and the apparent path length of the particles can be obtained. The validity of the VDA method has been an item of active research by several authors (Lintunen & Vainio 2004;Sáiz et al 2005;Rouillard et al 2012;Vainio et al 2013), with the VDA method typically used to estimate the SRT of SEP events (Lario et al 2014(Lario et al , 2016. For the first time, in our analysis, we have incorporated in the VDA the new and improved E effective for each of the P2-P7 GOES energy channels that have been obtained by the cross-calibration of GOES/SEM with IMP-8/GME data (Sandberg et al 2014, their Tabs.…”