**Abstract**

The least number of proton events and ground-level enhancements was recorded in the solar cycle 24 which corresponds with the least smoothed sunspot number compared to the last three previous solar cycles. This was attributed to the weak sun's polar field and decreasing strength of the interplanetary magnetic field at the start of the solar cycle. The majority contribution to background radiation dose within our earth's atmosphere is galactic cosmic rays and trapped particles in the Van Allen Belts. However, solar proton events cause sudden spikes in radiation doses, and this depends on the fluence and energy spectra of the events. While these doses are least detected in the lower atmosphere, they have significant radiation damage to spacecraft electronic components and astronauts on long space missions and at higher atmospheric altitudes. Therefore, the prediction of such events and estimation of their effective radiation damage is an important consideration for planning long space missions and spacecraft design materials.

**Keywords:** energetic particles, galactic cosmic rays, fluence, energy spectra, radiation dose

### **1. Introduction**

The sun's activity varies over the 11 years of a solar cycle with active and quiet phases. Over this period, several energetic particles are released into the interplanetary medium and accelerated towards the earth along the interplanetary magnetic field lines by several mechanisms discussed by Reames [1]. The sudden increase of energetic particle flux is detected by particle detectors on several satellites as solar proton events (SPEs). The proton events can also be detected by neutron monitors on ground stations as suppression of galactic cosmic ray (GCR) intensity (Furbish decrease), mostly observed on geomagnetic storm days [2]. For such proton events, they are defined as ground level enhancements (GLEs) and are associated with a 'hard'spectrum of energies >500 MeV [3]. Gradual SPEs are driven by CMEs and interplanetary shocks with a high proton to electron ratio while impulsive events are X-ray flashes associated with solar flares lasting for a few hours to about a day and with a high electron to proton ratio [1, 4, 5]. A few energetic protons from SPEs can escape the geomagnetic shielding and penetrate through the open magnetic field lines at the polar regions of the earth's atmosphere. These deposit their energy on spacecraft and satellite components at higher orbital altitudes causing space hazardous effects like spacecraft charging, single event upsets, and high radiation levels [6, 7]. The high radiation doses can also be detected by dosimeters onboard aircraft flying at high aviation altitudes mostly during intense proton and GLE events despite the atmospheric mass shielding [8–10]. GCRs consist of protons, alpha particles, and heavier nuclei which interact with the atmospheric constituents to produce secondary particles that can be observed at the ground level station by neutron and muon detectors. They have high energies ranging between 1 and 20 GeV, most of these energetic particles are deflected away by the magnetosphere and earth's magnetic field. The few penetrating particles contribute to background radiation levels within the earth's atmosphere. The important property of GCRs is the magnetic rigidity which defines their ability to resist bending in a magnetic field; this value varies from 0 at the poles to about 17 at the equatorial latitudes [11]. Therefore, the radiation doses due to GCRs are more enhanced at the poles than at the equator. Solar activity is a modulation factor for the propagation of GCRs and this is defined by an anti-correlation relationship between GCR intensity and sunspot number [12].

In this paper, we study the proton event, solar and cosmic ray activity during the solar cycle 24 and relate them to space radiation doses. We study and discuss the effect of particle fluence and energy spectra on dose contribution and some of the effects of high space radiation doses. This is an important consideration for planning long space missions and designing appropriate shielding materials for spacecraft manufacturers.
