**2. Solar proton events and data**

The energetic proton events are detected by several detectors onboard the GOES spacecraft at Geosynchronous orbit [13] and the list of proton events is available at NOAA SWPC (https://ngdc.noaa.gov/stp/space-weather/interplanetary-data/solarproton-events/SEP%20page%20code.html). The integrated energies for 5-min averages >10 MeV, measured in particle flux units (pfu) defined the proton flux. The start of the SPE was considered when the first three consecutive fluxes were ≥10 pfu and the end was the last time when the flux was greater than or equal to 10 pfu. The last three events in 2017 were picked from the list of Solar Proton Event Archive by European Space Agency (ESA) (https://space-env.esa.int/noaa-solar-proton-event-a rchive/). Each proton event was verified in the GLE list available at Neutron Monitor Database Event Search Tool (NEST) (https://www.nmdb.eu/nest/gle\_list.php) to confirm if it's associated with the GLE.

#### **2.1 SPE relationship with solar and GCR activity**

SPEs are accompanied by shock and flare components and their observation depends on the connectivity of the observer to the flare site. The X-ray flares are used to classify the size of the events with X and M-class being the most powerful and occurring more frequently during the active phase of the solar cycle. SPEs occurrence peaks during the solar maxima with frequent spikes of proton flux occurring due to super active regions associated with large successive eruptions. Out of the total 42 recorded SPEs, only 6 were associated with weak C-class flare eruption while the rest were associated with X and M-class flares. The C-class flares associated SPEs occurred mostly during the ascending phase of the solar cycle. Most energetic events with pfu exceeding

### *Solar Proton Activity over the Solar Cycle 24 and Associated Space Radiation Doses DOI: http://dx.doi.org/10.5772/intechopen.103832*

100 pfu occurred during the solar maxima/active periods (86% of the events with >100 pfu occurred from 2012 to 2015). **Figure 1** shows the bar graph of the yearly number of SPEs from 2010 to 2017 with the yearly averaged sunspot number. The number of SPEs for the four consecutive solar cycles were 59, 72, 79, and 42 for solar cycles 21, 22, 23 and 24 respectively (https://ngdc.noaa.gov/stp/space-weather/ interplanetary-data/solar-proton-events/SEP%20page%20code.html) while the maximum smoothed sunspot number is 232.9, 212.5, 180.3, and 81.8 for the four solar cycles respectively (https://wwwbis.sidc.be/silso/datafiles) [14]. The sharp decline of sunspot number from solar cycle 23–24 corresponds with the least number of SPEs in solar cycle 24. The occurrence rate of GLE events follows a similar pattern with 18%, 20%, 22%, and 2% of the total GLE events (72) for the 4 solar cycles respectively. The least numbers of SPEs and GLEs were recorded in the solar cycle 24 with the least maximum smoothed sunspot number. Solar cycle 24 has been ranked as the fourth weakest of the 24 solar cycles since 1755 and the weakest in 100 years (https://www.weather.gov/ news/201509-solar-cycle) [15]. This was caused by the impaired growth of the polar field. The simulation of the solar surface field by Jiang and Schüssler [16] showed emerging low latitude bipolar regions with an opposite orientation of the magnetic polarities in the north–south direction. This was the cause of growth impairment of the polar field and hence resulted in a weak magnetic field throughout the solar cycle 24. **Figure 2** shows the variation of sunspot, proton flux at energy levels of >10, >30 and > 60, X-ray intensity, and GCRs intensity recorded by Oulu neutron monitor from 2010 to 2017. The intensity of GCRs recorded by neutron monitors shows an anticorrelation relationship with the rate of particle flux entering the earth's atmosphere and sunspot number [12, 17].
