Modeling transport of energetic particles in corotating interaction regions: A case study

Abstract

We investigate energetic particle transport in corotating interaction regions (CIRs) through a case study. The CIR event we study occurred on 8 February 2008 and was observed by both the Advanced Composition Explorer (ACE) and the twin Solar TErrestrial RElations Observatory (STEREO) B spacecraft. An in situ reverse shock was observed by STEREO B (1.0 AU) but not ACE (0.98 AU). Using STEREO B observations and assuming the CIR structure does not vary significantly in the corotating frame, we estimate the shock location at later times for both the STEREO B and ACE observations. Further assuming the accelerated particle spectral shape at the shock does not vary with shock location, we calculate the particle differential intensities as observed by ACE and STEREO B at two different times by solving the focused transport equation using a Monte Carlo simulation. We assume that particles move along Parker's field and experience no cross‐field diffusion. We find that the modulation of sub‐MeV/nucleon particles is significant. To obtain reasonable comparisons between the simulations and the observations by both ACE and STEREO B, one has to assume that the CIR shock can accelerate more particles at a larger heliocentric distance than at a smaller heliocentric distance.

Properties of Suprathermal-through-Energetic He Ions Associated with Stream Interaction Regions Observed over Parker Solar Probe’s First Two Orb¬¬its

“…Our new results provide evidence that suprathermal He ions observed in six CIR/SIR events by ISeIS originate from sources or shocks beyond the location of PSP rather than from acceleration processes occurring at the locally observed compression regions (e.g., Giacalone et al 2002). Our results also suggest that rarefaction regions that typically follow the SIRs may facilitate easier particle transport throughout the inner heliosphere, particularly between ∼0.35 and 0.85 au in the ecliptic plane, where lowenergy ions do not undergo as significant energy losses due to adiabatic deceleration as has been predicted by existing models (Fisk & Lee 1980;Zhao et al 2016). Finally, we remark that although we have only observed one SIRassociated suprathermal-through-energetic He ion event inside ∼0.5 au, the PSP observations reported here pose a serious challenge for current particle transport models that predict Figure 10.…”

“…They concluded that local acceleration occurs at STEREO-A, where a reverse shock bounded the trailing edge. Based on the relative shapes of the He spectra, which either flattened or turned over below ∼0.5 MeV nucleon −1 , Zhao et al (2016) estimated that the particles seen at ACE and STEREO-B most likely originated from the same CIR shock that had subsequently moved out to ∼1.73 and ∼3.63 au, respectively, from the Sun. Putting the PSP observations of SIR-associated ions at ∼0.35 au in context of the results of Zhao et al (2016), we point out two new and surprising results, namely, (1) the He intensity-time profiles .…”

Pickup ion acceleration in the successive appearance of corotating interaction regions

“…The validity of the intershock acceleration shown in this study depends on how the particle energy is conserved during its propagation in the rarefaction region. As Zhao et al [] demonstrated, the magnetic focusing effect and the pitch angle diffusion process might alter the particle intensity profile, especially for low‐energy particles. In the present simulation, at least, the accelerated PUI moves in the rarefaction region without losing its energy (e.g., Figure ), where the weakened magnetic field makes the magnetic focusing effect ineffective.…”

“…Giacalone et al [] presented a theoretical model that shows that gradual solar wind compression can play a role in particle acceleration similar to the diffusive shock acceleration mechanism. Zhao et al [] numerically solved the focused transport equation including the magnetic focusing effect and the pitch angle diffusion process. They succeeded to obtain the CIR‐associated energetic particle intensity spectrum, which was more similar to the observational results than that of Fisk and Lee [].…”

Large solar energetic particle event that occurred on 2012 March 7 and its VDA analysis

“…For the energetic protons and electrons, the intensities in STB and SOHO present a nominal decay phase after the passage of the shock, while those in STA show a second enhancement in this period (after ∼05:00UT on March 9). It's very interesting to find that there is a corotating interaction region (CIR) passing through the STA coincidentally at that time, which may be considered to contribute to the second enhancement, as the CIR is also an efficient particle accelerator (Fisk & Lee , 1981;Reames et al, 1997;Zhao et al, 2016). To compare the solar energetic particle releases with those of other associated solar activities, it is necessary to compensate proton travel time from the Sun corresponding to their energies.…”