Study On Energy Spectra Of Ground Level Enhancement Events During Solar Cycle 23

The space weather caused by the Sun has great influence both on the space enviornment and on the Earth,which poses a serious threat to space activities and human life.It is important for us to study and prevent space weather effects.Solar energetic particle(SEP)event is belonging to space weather,and the SEPs are from solar flare eruption and from the coronal materials or solar wind.In some extreme large SEPs,Ge V particles can reach the Earth's atmosphere and cause a cascade of secondary particles with intensities above the background level that is produced by the galactic cosmic rays so that the counts of ground-based neutron monitors are enhanced,which is called ground level enhancement(GLE)of SEPs.The energy spectrum of SEPs can be used to calculate its radiation dose.Therefore,studying the energy spectrum of GLEs is of great significance in preventing the disaster caused by space weather and can promote the understanding of the origin,acceleration,and propagation mechanisms of energetic particles.There were a total of 16 GLEs during solar cycle 23,and many spacecraft provided abundant observations for these events,which promoted the study of GLEs.Based on the extended measurements of energetic protons from spacecratf,Mewaldt et al.tested three spectral forms for fitting proton energy spectra of GLEs during solar cycle 23 in energies ranges from ?0.1 MeV to 500-700 MeV.They found that the GLE spectra are best fitted by the double power law shape and obatained the values of the four spectral parameters,which are the normalization constant C,low-energy power law slope ?1,high-energy power law slope ?2,and break energy E0.In this work,we analyze the four spectral parameters with other observations for the 16 GLEs.The energy spectra of 3 GLEs are influenced by some effects,so that we exclude these events for simplicity purpose.According to the level that interplanetary shock accelerates particles,we divide these 13 GLEs into two categories,and we find 3 methods to dinstinguish between these two categories.By statistically analyzing the spectral parameters,we obtain the empirical models of the spectral paramters for the two categories of GLEs separately,and thus we establish the energy spectrum prediction model of GLEs by combining the empirical models with the double power law equation.Finally,we obtain prediction results for seven selected GLEs from cycle 22 and cycle 24 to check the validity of the prediction model,and we find that the model can give a relatively good results in energy ranging from ?6 to 100-200 MeV for the GLEs with longitude greater than W40.