Space Weather Effect Of Shock-Interplanetary Coronal Mass Ejection Structures

As a kind of large scale complex structure in the interplanetary space,Shock-Interplanetary Coronal Mass Ejection(S-ICME)plays an important role in space weather effect.On the one hand,S-ICME is a vital interplanetary origin of intense geomagnetic storms.On the other hand,S-ICME may lead to enhanced Solar Energetic Particle events.In this paper,the influence of S-ICMEs on geomagnetic storms and solar ener-getic particles is analyzed based on the long-term observation of Wind satellite.According to Wind observation from 1995 to 2017,we establish a shock list that has 568 fast forward shocks.The shock parameters are obtained by the R-H iteration method.Generally speaking,shocks near the Earth are relatively slow and weak,the mean speed is 442 km/s,mean density compression ratio and Alfven Mach number is 2.0 and 2.7.In addition,the shocks are more likely to be quasi-perpendicular.In the solar maximum,shocks occure more and tend to be faster,while the shock density compression ratio and Alfven Mach number do not follow the solar cycle variations.According to the structures appeared within 24 hours after the shock,these shocks can be divided into three categories,which are ICME-driven shocks,SIR-driven shocks and'driverless' shocks.Comparing these three kinds of shocks,we find that ICME-driven shocks are the fastest and the strongest.In the solar maximum,ICME-driven shocks account the most,while in solar minimum,more shocks are SIR-driven shocks.Of the 568 shocks,58 propagates inside ICMEs.Most of these shocks inside ICMEs appeared in solar maximum.Compared to other interplanetary shocks which propagate in background solar wind,shocks inside ICMEs are relatively fast but weak.Their mean speed is about 100 km/s higher,but their Alfven Mach number is about 0.6 lower.The shocks are not evenly distributed in ICMEs,with up to 73%shocks appear-ing in the last 1/3 of the ICME intervals.This demonstrates that 45%shocks can not survive in ICMEs due to the low plasma ? inside.The geomagnetic storm on September 8,2017 is a strong multi-step magnetic storm with a peak intensity of-142nT.Based on in-situ observation,we determine that the in-terplanetary origin of the geomagnetic storm is the S-ICME event that began at 16:50 UT on September 7,2017.By recovering the shocked part of the S-ICME to an un-compressed state,and substituting both the observational data and recovered state into various Dst prediction models,we find that the compression of the shock enhanced the intensity of this geomagnetic storm by roughly a factor of two.For the first time,this work quantitatively analyze the effects of shock compression on enhancing the geoef-fectiveness of an ICME.In addition,in this case,the interaction between the shock and the host ICME increased the intensity of solar energetic particles by a factor of five.Using the same approach,we analyze 11 geomagnetic storms caused by S-ICMEs since 1995.The results show that due to the compression of the shock,the peak values of the southward magnetic field and the dawn-dusk electric field in ICMEs are increased by 2.0 times and 2.4 times,respectively,and the intensity of the geomagnetic storm is increased by 1.4 times.Without shock compression,the geomagnetic effects of these ICMEs will be significantly reduced,with five(63%)strong geomagnetic storms re-duced to moderate geomagnetic storms.In addition,from the theoretical and statistical point of view,we prove that there is a significant positive correlation between the shock density compression ratio and its ability to enhance the geomagnetic storm.Based on the Wind observations,we find that 12 ICMEs are accompanied with ex-traordinary energetic proton intensity enhancements inside.9 of them are S-ICME com-plex structures,which account for 75%.This shows that compared with other ICMEs,the S-ICMEs are more prone to have increased energetic particle intensities.By com-paring energetic proton enhanced I-ICMEs and S-ICMEs,we find that in all energy channels,especially in the high-energy channels,the flux enhancements in S-ICME are more pronounced than the I-ICMEs.In addition,shocks in these energetic particle enhanced S-ICMEs have greater speed,density compression ratio,and Alfven Mach number compared to other shocks propagating inside ICMEs.This indicates that in these events,the acceleration of energetic protons is related to the shocks inside.