The processes responsible for the acceleration of solar energetic particles (SEPs) are still not well understood, including whether SEP electrons and protons are accelerated by common or separate processes. Using a numerical particle transport model that includes both pitch-angle and perpendicular spatial diffusion, we simulate, among other quantities, the onset delay for MeV electrons and protons and compare the results to observations of SEPs from widely separated spacecraft. Such observations have previously been interpreted, in a simple scenario assuming no perpendicular diffusion, as evidence for different electron and proton sources. We show that, by assuming a common particle source together with perpendicular diffusion, we are able to simultaneously reproduce the onset delays for both electrons and protons. We argue that this points toward a common accelerator for these particles. Moreover, a relatively broad particle source is required in the model to correctly describe the observations. This is suggestive of diffusive shock acceleration occurring at large shock structures playing a significant role in the acceleration of these SEPs.
The acceleration and injection of solar energetic particles (SEPs) near the Sun is one of the major unsolved problems in contemporary SEP transport modeling efforts. Here, we establish a new approach to the injection problem by utilizing a correlation between the soft X-ray thermal emission in solar flares, and their hard X-ray counterpart, the so-called Neupert effect, which is indicative of the presence of non-thermal particles. We show that the resulting injection function, in the initial phase of the flare, is similar to those inferred from inverting the transport problem based on in-situ observations. For few cases, we find early injections with no in-situ correspondence, that can be caused by particles accelerated before there is a magnetic connection between the source and the spacecraft. The method has limitations for long-duration injections, since it is not applicable to the decay phase of the flare where particle trapping might play a role. For a sample of SEP events in 1980, observed with the Helios-1 and IMP8 spacecraft, we show the results of a 2D SEP transport model based on this approach. We discuss that, with this method, a physics-based, real-time operational SEP now-cast model for the heliosphere is feasible.
Multi-spacecraft observations of widespread solar energetic particle (SEP) events indicate that perpendicular (to the mean field) diffusion is an important SEP transport mechanism. However, this is in direct contrast to so-called spike and drop-out events, which indicate very little lateral transport. To better understand these seemingly incongruous observations, we discuss the recent progress made towards understanding and implementing perpendicular diffusion in transport models of SEP electrons. This includes a re-derivation of the relevant focused transport equation, a discussion surrounding the correct form of the pitch-angle dependent perpendicular diffusion coefficient and what turbulence quantities are needed as input, and how models lead to degenerate solutions of the particle intensity. Lastly, we evaluate the validity of a diffusion approach to SEP transport and conclude that it is valid when examining a large number of (an ensemble of) events, but that individual SEP events may exhibit coherent structures related to the magnetic field turbulence at short timescales that cannot be accounted for in this modelling approach.
The existence of a Fisk-type heliospheric magnetic field (HMF) has been debated ever since Fisk proposed an alternative to the traditional view of the HMF first proposed by Parker. Several modifications of the original Fisk field model have been published in the past, for example, the Schwadron field and the Fisk–Parker hybrid HMF model. This study presents a new generalized Fisk HMF model that implements for the first time a spatially dependent differential rotation rate of the photosphere, by mapping magnetic field lines from the solar wind source surface to the photosphere. Data analysis methods of Forsyth et al. are used to search for a signature of a Fisk-type field during solar minimum conditions, using the magnetic field data from the first solar orbit of the Ulysses spacecraft. The new generalized Fisk field agrees better with the observed magnetic field winding angle than a standard Parker field during the majority of intervals scanned by Ulysses.
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