# The Sun and Space Weather
## 6 Solar Eruptions and SEPs  
Solar energetic particles (SEPs) are nonthermal electrons and ions that have energies well above the thermal plasma particles. Energetic ions can be accelerated up to multi-GeV energies during solar eruptions. The energetic electrons and ions are detected directly by particle detectors in space. They are also inferred from the electromagnetic emissions they  
Figure 14: A low-latitude coronal hole observed by SDO/AIA at 193 Å (**a**), the underlying photospheric magnetic field strength from SDO/HMI (**b**), and solar wind parameters inaround the CIR with a leading shock (**c**) (from [PERSON] et al. 2015). The coronal hole was near the central meridian on 30 May 2013 and resulted in an intense storm (\(-\)119 nT) on 1 June. Magnetic field components Bx, By, and Bz are shown in panels (**d–f**), respectively. The three indicators of a stream interaction region are the increase in solar wind speed (**g**), increasing temperature (**h**), and density peak (**i**). The Dst index (**j**) shows an intense storm (Dst \(<-\)100 nT). produce while interacting with the ambient medium. SEPs were first observed in the early 1940s using ionization chambers at the ground level [147]. The SEP events detected at Earth's surface are called ground-level enhancement (GLE) events. GLE events are now detected using neutron monitors or muon telescopes that detect secondary neutrons and muons caused by primary SEPs. The events reported by [PERSON] [147] were associated with intense H-alpha flares on the Sun; thus, flares became known as the accelerator of SEPs. Type II solar radio bursts were first detected in 1947 [148; 149] and have been attributed to a fast-mode MHD shock [150]. Type II bursts are caused by nonthermal electrons accelerated at the shock via the plasma emission mechanism [72]. [PERSON] et al. [151] found that energetic proton events occur in major eruptions accompanied by type II and type IV radio bursts, intense X-ray and microwave emission, and relativistic electrons. [PERSON] et al. [152] suggested that energetic storm particle (ESP) events are accelerated by IP shocks when they are at Earth. [PERSON] et al. [153] attributed the shocks to CMEs, and hence the shock paradigm for SEPs became firmly established [154; 155; 156]. The importance of CMEs in the occurrence of SEP events is illustrated in Figure 15. These are CMEs associated with large SEP events, defined as those with proton intensity expressed in particle flux units (pfu = 1 particle cm\({}^{-2}\) s\({}^{-1}\) sr\({}^{-1}\)) exceeding 10 in the \(>\)10 MeV energy channel. Such events have been determined to have important space weather consequences by the NOAA. The CME speeds range from \(\sim\)600 km/s to \(>\)3000 km/s, with an average speed of \(\sim\)1500 km/s. The speed is clearly much larger than that of an average CME. A vast majority (\(>\)80%) of SEP-associated CMEs are halos, indicating that such CMEs are very wide, further confirmed by the average width (\(\sim\)180\({}^{\circ}\)) of the non-halo CMEs. The distribution of CME source locations on the Sun heavily favors the Western Hemisphere because the accelerated particles propagate along magnetic field lines that have a Parker spiral configuration. The nominal connection angle is \(\sim\)W58 for a background solar wind speed of \(\sim\) 400 km/s. While most of the high-intensity events are from the Western Hemisphere, there are some events originating from the Eastern Hemisphere, and even from the east limb. The east-limb events are of low intensity (\(\leq\)100 pfu), but the CME speeds are extraordinarily high. For example, the average CME speed of the seven east-limb SEP events in Table 2 is \(\sim\)1956 km/s, while the average SEP intensity (Ip) detected by GOES is only 44 pfu. The low intensity is a consequence of the poor connectivity--Earth is connected to the extreme west flank of the CME shock. It must be noted that a spacecraft behind the east limb is well-connected to such east-limb events and hence would observe an intense particle event.