Figure 7.7: The maximum particle intensities at1 AU for different injection locations. All the injections have been shifted to correspond to an injection atφ0 = 90◦(indicated by the vertical red dashed line).
118 7.3. DISCUSSION sented in this chapter, when a realistic HMF is included, show similar rippled distributions at Earth. Here, this is caused by SEPs following large scale meandering field lines while also experiencing different transport conditions (different solar wind speeds and different levels of focusing/mirroring). Figure 7.7 shows that each SEP injection has a different maximum par- ticle intensity distribution at1AU, depending on the location of the injection at all azimuthal angles, and hence represents a different realization of the HMF, as discussed inStrauss et al.
[2020]. Each modelled HMF injection has a rippled peak distribution and would be impossi- ble to predict without knowing the exact HMF realization throughout the heliosphere which is dominated by the large-scale meandering of field lines. However, on average, the different realizations seem to follow the Gaussian-like result when a Parker HMF is adopted.
Summary and Conclusions
Space weather (SWx) events have the potential to cripple technology, both in orbit around the Earth and terrestrial infrastructure. Since society has become increasingly dependent on tech- nology (that can be destroyed by a large SWx event), another Carrington event would have disastrous consequences. Therefore, SWx prediction has become a crucial part of the space sci- ences. To that end, this study attempted to provide a roadmap of the origin of SEPs by investi- gating indirect solar energetic particle (SEP) observations across the electromagnetic spectrum.
A SWx prediction model based on the Neupert effect was presented, and a numerical, turbu- lent magnetic field was implemented in a SEP transport model to showcase the importance of using a more realistic heliospheric magnetic field (HMF).
The necessary background related to the solar structure, from the solar core out to the chro- mosphere, was discussed. The association between active regions (ARs) and the11year solar cycle was reviewed as well as the origin of sunspots, prominences, filaments, and coronal holes (CHs). The HMF was introduced and its connection with the solar wind (SW) explained. The different sources of SEPs were discussed and the differences between so-called impulsive and gradual events were examined. These two categories of events are assumed to be the two ex- treme scenarios and that the vast majority of events are a mixture of impulsive and gradual events. The transport of SEPs in the inner heliosphere was also briefly explained. Indirect SEP observations across the multi-wavelength electromagnetic spectrum were introduced, starting at the lower energy radio emission and progressed through visible light, up to the higher en- ergy emissions such as solar X-rays and gamma-rays. Coronal type II and III radio bursts were investigated and a distinction was made between soft X-rays (SXR) and hard X-rays (HXR). A brief description of several spacecraft and their instruments was given, including the recently launched Parker Solar Probe (PSP) and Solar Orbiter (SolO) missions.
The two-dimensional (2D) SEP transport model ofStrauss and Fichtner[2015] was introduced, together with a small scale parameter study. The focussed transport equation (FTE) (in spher- ical coordinates) was used to simulate SEP transport between the Sun and the Earth using the pitch-angle diffusion coefficient ofDr¨oge et al.[2010]. An isotropic impulsive SEP injection was simulated at the Sun and the results, such as the longitudinal extent of the injection and the escape times, were discussed. The results from the 2D model were compared to the results of a 3D model, and it was found that the 2D model was sufficient for studying SEP transport.
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Gaussian curves were fitted to the modelling results and showed that if appropriate observa- tions are available much closer to the Sun, such as the observations of PSP and SolO, it should be possible to distinguish between the effects of perpendicular diffusion and the initial injection broadness. The transport model was also used to simulate the transport of protons, although it remains only a proof-of-concept. The results showed the onset delay between electrons and protons, and this could be used in future to test certain SWx forecasting models.
A SWx forecasting model based on the Neupert effect was introduced. The SEP transport model ofStrauss and Fichtner[2015] was used, but instead of using an impulsive injection func- tion to simulate the SEP injections, SXR data from the GOES spacecraft was used as a proxy for the injection function. The Neupert effect exploits the causal relationship between SXRs and HXRs and therefore is able to indicate when HXRs can be expected without directly observing HXRs. The results were compared to the inversion method ofPacheco et al.[2019]. A reasonably good comparison was found, with the results based on the Neupert effect seemingly capturing the initial particle release effectively, although it did not capture the extended release of SEPs.
The differences between the two approaches were also discussed. A real-time now-casting ver- sion of the this model was set up athttps://fskbhe1.puk.ac.za/spaceweather/sep_
predictor.html to illustrate the efficiency and convenience of using the Neupert effect as an injection proxy.
The structured SW model ofLi et al.[2016] was implemented in the SEP transport model of Strauss and Fichtner [2015]. Several properties of this data-driven, numerical magnetic field were discussed, including its divergence, its focusing length, and winding angles. The simu- lated particle intensity distributions at1AU showed a rippled peak distribution in contrast to the approximately uniform Gaussian-like distribution when the standard Parker HMF geom- etry was used. The different scenarios that could be responsible for the rippled intensity dis- tributions were discussed; amongst these the fact that large scale meandering magnetic field lines can lead to the mixing of empty and energetic particle filled magnetic flux tubes since some flux tubes may or may not be well connected to the injection region. Each modelled HMF injection had a rippled peak distribution, but on average, the different realisations followed the Gaussian-like result when a Parker HMF was adopted.
This study was ended with a brief overview of the North-West University (NWU) solar tele- scope project. Observations of the past five years were shown, after which the future plans for this project were discussed.
Avenues of future research related to this study are:
• further investigation into simulating proton transport in the inner heliosphere,
• including more solar emission mechanisms, such as radio emission, to add to the SWx prediction model based on the Neupert effect,
• using the modelled magnetic field results of more Carrington rotations fromLi et al.[2016]
and implementing it in the SEP transport model ofStrauss and Fichtner[2015],
• observing the Sun with the dedicated solar telescopes and the eCallisto radio spectrom- eter to monitor the Sun and provide in-house SWx prediction data to develop more in- volved SWx prediction models.
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Appendix
8.1 The North-West University solar telescope project
The Centre for Space Research (CSR) at the North-West University (NWU), South Africa, started a solar telescope project in2017. The weather in Potchefstroom is ideal for solar observations for the biggest part of the year, especially during the dry winter months, and is located1340m above sea level. Mr Ruhann Steyn and Prof Du Toit Strauss are the principal scientists respon- sible for the commissioning of this project. The goal of the solar telescope project is three-fold:
• to train undergraduate and postgraduate students in solar observations;
• to provide community engagement opportunities for local school learners and
• to add observational solar physics to the CSR’s research repertoire.
The project has gained traction over the past five years and this Appendix is a brief review of where the project started, what has already been observed, what is the current project status, and what future work can be expected from this project.