Interplanetary Propagation of Solar Energetic Particle Heavy Ions Observed at 1 Au and the Role of Energy Scaling

Abstract

We have studied ∼0.3 to >100 MeV nucleon −1 H, He, O, and Fe in 17 large western hemisphere solar energetic particle events (SEP) to examine whether the often observed decrease of Fe/O during the rise phase is due to mixing of separate SEP particle populations, or is an interplanetary transport effect. Our earlier study showed that the decrease in Fe/O nearly disappeared if Fe and O were compared at energies where the two species interplanetary diffusion coefficient were equal, and therefore their kinetic energy nucleon −1 was different by typically a factor ∼2 ("energy scaling"). Using an interplanetary transport model that includes effects of focusing, convection, adiabatic deceleration, and pitch angle scattering we have fit the particle spectral forms and intensity profiles over a broad range of conditions where the 1 AU intensities were reasonably well connected to the source and not obviously dominated by local shock effects. The transport parameters we derive are similar to earlier studies. Our model follows individual particles with a Monte Carlo calculation, making it possible to determine many properties and effects of the transport. We find that the energy scaling feature is preserved, and that the model is reasonably successful at fitting the magnitude and duration of the Fe/O ratio decrease. This along with successfully fitting the observed decrease of the O/He ratio leads us to conclude that this feature is best understood as a transport effect. Although the effects of transport, in particular adiabatic deceleration, are very significant below a few MeV nucleon −1 , the spectral break observed in these events at 1 AU is only somewhat modified by transport, and so the commonly observed spectral breaks must be present at injection. For scattering mean free paths of the order of 0.1 AU adiabatic deceleration is so large below ∼200 keV nucleon −1 that ions starting with such energies at injection are cooled sufficiently as to be unobservable at 1 AU. Because of the complicating factors of different spectral break energies for different elements, it appears that SEP abundances determined below the break are least susceptible to systematic distortions.

Longitudinal Properties of a Widespread Solar Energetic Particle Event on 2014 February 25: Evolution of the Associated Cme Shock

“…The more gradual increase observed at L1 for low-energy protons (bottom panel of Figure 1(c)) and low-energy ions (Figure 8(c)) with respect to the faster increase observed for electrons (Figure 8(a)) and high-energy ions and protons (Figures 8(a) and (c)) may be due to a rigidity-dependent transport for the particles to escape from the shock front. Similar differences in low and high energy ion intensities were observed for the event on 2005 January 20 (e.g., Figure 2 in Mason et al 2012). …”

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

“…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). Future investigations of detailed particle propagation will require a focused transport modeling (including the effects of adiabatic cooling, magnetic focusing, and pitch angle scattering) (e.g., Zhang 1999; Qin et al 2006;Li 2008;Mason et al 2012), which exceeds the scope of this paper.…”

Spectral Properties of Large Gradual Solar Energetic Particle Events. Ii. Systematic Q/M Dependence of Heavy Ion Spectral Breaks

“…Next, we point out that the increases in the Fe/O ratio observed during the early phases of large SEP events has been successfully and quantitatively modeled in many studies as a transport effect (e.g., Tylka et al 1999Tylka et al , 2005Tylka et al , 2012Mason et al 2012Mason et al , 2014Reames et al 2013), and that in particular, the following three observational results in conjunction with the new results presented in this paper provide strong support for the counterargument that ∼0.1-500 MeV nucleon −1 H-Fe nuclei observed during large gradual SEPs are indeed accelerated by near-Sun CME-driven shocks. Thus, the bulk of the observations show that the suggestion that hypothesized "flare" contributions dominate the early phases of SEP event composition or the event-integrated SEP Fe/O abundances is not supported.…”

“…2. Second, many studies (e.g., Tylka et al 1999;Mason et al 2006Mason et al , 2012Reames 2015) have shown that temporal variations in the <100 MeV nucleon −1 Fe/O abundances in large SEP events are diminished or even eliminated if the Fe and O time-intensity profiles are compared at a scaled energy for the different species, indicating that these variations are due to differences in the Q/M ratios of the different species, and therefore better explained by interplanetary transport models that include the effects of focusing, diffusion, convection, adiabatic deceleration, and pitch-angle scattering. 3.…”

“…In the present study, we eliminated all events with possible contributions from local IP shock-associated populations (see Paper 1) in order to highlight SEP acceleration processes. Further, we use event-integrated fluences, rather than time-intensity profiles (see Mason et al 2012), to study the SEP spectral properties. In particular, Mason et al (2012) used a detailed model of interplanetary propagation and showed that transport from the inner solar system can lower the break energy systematically for all species as well as lower the slopes by ∼10%-20% but that the basic spectral form remained intact (their Figure 14).…”