Impulsive Solar Energetic Particle Events: Extreme-Ultraviolet Waves and Jets

Impulsive solar energetic particle (ISEP) events show peculiar elemental composition, with enhanced 3He and heavy-ion abundances, markedly different from our Solar System composition. Furthermore, the events are characterized by a wide variety of energy spectral shapes from power laws to rounded spectra toward the low energies. Solar sources of the events have been firmly associated with coronal jets. Surprisingly, new observations have shown that events are often accompanied by so-called extreme-ultraviolet (EUV) coronal waves–a large-scale phenomenon compared to jets. This paper outlines the current understanding of the linkage of EUV waves with jets and energetic ions in ISEP events.

Evaluation of the UMASEP-10 Version 2 Tool for Predicting All >10 MeV SEP Events of Solar Cycles 22, 23 and 24

The prediction of solar energetic particle (SEP) events may help to improve the mitigation of adverse effects on humans and technology in space. UMASEP (University of Málaga Solar particle Event Predictor) is an empirical model scheme that predicts SEP events. This scheme is based on a dual-model approach. The first model predicts well-connected events by using an improved lag-correlation algorithm for analyzing soft X-ray (SXR) and differential proton fluxes to estimate empirically the Sun–Earth magnetic connectivity. The second model predicts poorly connected events by analyzing the evolution of differential proton fluxes. This study presents the evaluation of UMASEP-10 version 2, a tool based on the aforementioned scheme for predicting all >10 MeV SEP events, including those without associated flare. The evaluation of this tool is presented in terms of the probability of detection (POD), false alarm ratio (FAR) and average warning time (AWT). The best performance was achieved for the solar cycle 24 (i.e., 2008–2019), obtaining a POD of 91.1% (41/45), a FAR of 12.8% (6/47) and an AWT of 2 h 46 min. These results show that UMASEP-10 version 2 obtains a high POD and low FAR mainly because it is able to detect true Sun–Earth magnetic connections.

Variable Ion Compositions of Solar Energetic Particle Events in the Inner Heliosphere: A Field Line Braiding Model with Compound Injections

We propose a model for interpreting highly variable ion composition ratios in solar energetic particle (SEP) events recently observed by the Parker Solar Probe (PSP) at 0.3–0.45 au. We use numerical simulations to calculate SEP propagation in a turbulent interplanetary magnetic field with a Kolmogorov power spectrum from large scales down to the gyration scale of energetic particles. We show that when the source regions of different species are offset by a distance comparable to the size of the source regions, the observed energetic particle composition He/H can be strongly variable over more than two orders of magnitude, even if the source ratio is at the nominal value. Assuming a 3He/4He source ratio of 10% in impulsive 3He-rich events and the same spatial offset of the source regions, the 3He/4He ratio at observation sites also vary considerably. The variability of the ion composition ratios depends on the radial distance, which can be tested by observations made at different radial locations. We discuss the implications of these results on the variability of ion composition of impulsive events and on further PSP and Solar Orbiter observations close to the Sun.

Evidence for Energetic Neutral Hydrogen Emission from Solar Particle Events

We report the probable detection of energetic neutral hydrogen atoms (ENAs) at >0.8 MeV in several large solar energetic particle events observed between 1997 and 2004. The low Earth orbiting SAMPEX satellite detected transient increases of quasi-trapped equatorial protons beginning typically ∼3 hr after the X-ray flare and lasting for up to several hours. Since the magnetic cutoff rigidity is >10 GV at the magnetic latitude where the particles were observed, we interpret the signal as due to ENAs that penetrate Earth’s magnetic field and charge exchange in the upper atmosphere, whereupon the charged particles may become trapped. One event outside our survey period (2006 December 5) had previously reported solar flare ENAs, the only example of this phenomenon of which we are aware. Although the statistics are limited, the events we report suggest that the ENAs are produced as the flare-associated coronal mass wjection moves through the corona, as concluded previously for the 2006 December 5 event. The finding of ENAs emitted in conjunction with large solar flares opens a new avenue to understanding these events.

The study of the strongest solar event on a minimum of the 24th solar cycle

The strongest solar flares of the 24th solar cycle erupted on September 6, 2017, and it was the 8th strongest solar flare recorded since 1996. This extreme solar flare occurred at the minimum of the 24th solar cycle. The active region is located in the Western Hemisphere and produced the violent explosion of class X9.3 and X2.2 on September 6, X1.3 on September 7, and X8.2 on September 10, 2017. The injection duration of the solar energetic particles of the solar event was 17 minutes. All data for this solar event was collected from the Advanced Composition Explorer and simulated for particles’ motion using the transport equation and solved by the numerical technique. We obtained the injection time of the solar energetic particle propagation by comparing fitting between the simulation results and the spacecraft data. Injection time taken by high-energy particles to travel from the Sun to the Earth was found to be in the range of 39 to 743 minutes. At the peak of this solar flare, the coronal mass ejection was detected, which increased the injection time. The Kp-index of this solar flare was 4; thus, there was no effect on the Earth. The Kp-index value increased to 8 on September 7-8, 2017, due to another solar event from the same sunspot region, indicating the effect of solar flare and CME, which resulted in the appearance of aurora.

Acceleration of Solar Energetic Particles by the Shock of Interplanetary Coronal Mass Ejection

Interplanetary coronal mass ejection (ICME) shocks are known to accelerate particles and contribute significantly to solar energetic particle events. We have performed magnetohydrodynamic-particle in cell simulations of ICME shocks to understand the acceleration mechanism. These shocks vary in Alfvénic Mach numbers as well as in magnetic field orientations (parallel and quasi-perpendicular). We find that diffusive shock acceleration plays a significant role in accelerating particles in a parallel ICME shock. In contrast, shock drift acceleration (SDA) plays a pivotal role in a quasi-perpendicular shock. High-Mach shocks are seen to accelerate particles more efficiently. Our simulations suggest that background turbulence and local particle velocity distribution around the shock can indirectly hint at the acceleration mechanism. Our results also point toward a few possible in situ observations that could validate our understanding of the topic.

Tracking the Source of Solar Type II Bursts through Comparisons of Simulations and Radio Data

The most intense solar energetic particle events are produced by coronal mass ejections (CMEs) accompanied by intense type II radio bursts below 15 MHz. Understanding where these type II bursts are generated relative to an erupting CME would reveal important details of particle acceleration near the Sun, but the emission cannot be imaged on Earth due to distortion from its ionosphere. Here, a technique is introduced to identify the likely source location of the emission by comparing the dynamic spectrum observed from a single spacecraft against synthetic spectra made from hypothesized emitting regions within a magnetohydrodynamic (MHD) numerical simulation of the recreated CME. The radio-loud 2005 May 13 CME was chosen as a test case, with Wind/WAVES radio data being used to frame the inverse problem of finding the most likely progression of burst locations. An MHD recreation is used to create synthetic spectra for various hypothesized burst locations. A framework is developed to score these synthetic spectra by their similarity to the type II frequency profile derived from the Wind/WAVES data. Simulated areas with 4× enhanced entropy and elevated de Hoffmann–Teller velocities are found to produce synthetic spectra similar to spacecraft observations. A geometrical analysis suggests the eastern edge of the entropy-derived shock around (−30°, 0°) was emitting in the first hour of the event before falling off, and the western/southwestern edge of the shock centered around (6°, −12°) was a dominant area of radio emission for the 2 hr of simulation data out to 20 solar radii.

Ensemble Numerical Simulations of Realistic SEP Events and the Inspiration for Space Weather Awareness

The solar energetic particle (SEP) event is a kind of hazardous space weather phenomenon, so its quantitative forecast is of great importance from the aspect of space environmental situation awareness. We present here a set of SEP forecast tools, which consists of three components: 1) a simple polytropic solar wind model to estimate the background solar wind conditions at the inner boundary of 0.1AU (about 20 R⊙); 2) an ice-cream-cone model to estimate the erupted CME parameters; and 3) the improved Particle Acceleration and Transport in the Heliosphere (iPATH) model to calculate particle fluxes and energy spectra. By utilizing the above models, we have simulated six realistic SEP events from August 14, 2010 to September 10, 2014, and compared the simulated results to the GOES spacecraft observations. The results show that the simulated fluxes of > 10MeV particles agree with the observations while the simulated fluxes of > 100MeV particles are higher than the observed data. One of the possible reasons is that we have adopted a simple method in the model to calculate the injection rate of energetic particles. Furthermore, we have conducted the ensemble numerical simulations over these events and investigated the effects of different background solar wind conditions at the inner boundary on SEP events. The results imply that the initial CME density plays an important role in determining the power spectrum, while the effect of varying background solar wind temperature is not significant. Naturally, we have examined the influence of CME initial density on the numerical prediction results for virtual SEP cases with different CME ejection speeds. The result shows that the effect of initial CME density variation is inversely associated with CME speed.

Solar Energetic Particle Event occurrence prediction using Solar Flare Soft X-ray measurements and Machine Learning

The prediction of the occurrence of Solar Energetic Particle (SEP) events has been investigated over many years and multiple works have presented significant advances in this problem. The accurate and timely prediction of SEPs is of interest to the scientific community as well as mission designers, operators, and industrial partners due to the threat SEPs pose to satellites, spacecrafts and crewed missions. In this work we present a methodology for the prediction of SEPs from the soft X-rays of solar flares associated with SEPs that were measured in 1 AU. We use an expansive dataset covering 25 years of solar activity, 1988-2013, which includes thousands of flares and more than two hundred identified and catalogued SEPs. Neural networks are employed as the predictors in the model providing probabilities for the occurrence or not of an SEP which are converted to yes/no predictions. The neural networks are designed using current and state-of the-art tools integrating recent advances in the machine learning field. The results of the methodology are extensively evaluated and validated using all the available data and it is shown that we achieve very good levels of accuracy with correct SEP occurrence prediction higher than 85% and correct no-SEP predictions higher than 92%. Finally we discuss further work towards potential improvements and the applicability of our model in real life conditions.