The soft X-ray Neupert effect as a proxy for solar energetic particle injection

• Main Authors: Steyn Ruhann, Strauss Du Toit, Effenberger Frederic, Pacheco Daniel
• Format: Article
• Language: English
• Published: EDP Sciences 2020-01-01
• Series: Journal of Space Weather and Space Climate
• Subjects: solar energetic particles; injection function; neupert effect; forecasting; nowcasting
• Online Access: https://www.swsc-journal.org/articles/swsc/full_html/2020/01/swsc200079/swsc200079.html

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.