The Statistical Study Of Oxygen Ions Characteristics In The Dayside Magnetopause

Magnetic reconnection is an important physical process of energy transfer and ma-terial exchange between the solar wind and the magnetosphere.Oxygen ions(O+)can not only participate in magnetic reconnection,but also reduce the reconnection rate,thus affecting the solar wind energy and material to enter the Earth space.The study of the variations of O+ on solar wind conditions,geomagnetic activity and solar UV/EUV flux as well as its spatial distributions under different conditions mentioned above is not only benefit to understand the O+ transport and leakage in the magnetosphere,but also provides hints for investigation of the reconnection and K-H instability processes in the dayside magnetopause.It also helps to analyze the effects of oxygen ions on solar wind energy and material entering the Earth space.In this study,using in-situ data measured by Magnetospheric Multiscale(MMS)satellites at the dayside magnetopause,the re-sponses of O+(1-40 keV)density on solar wind conditions,geomagnetic activity and solar UV/EUV flux and its spatial distributions for the different ranges of above pa-rameters are analyzed.The dependencies of O+ fluxes spatial distributions on different energy ranges are also investigated.First of all,the variations of the H+,O+ density(1-40 keV)and corresponding ratio on solar wind parameters(Psw,IMF By,and IMF Bz),geomagnetic activity(SYM-H)and solar UV/EUV flux(F10.7)are investigated.It is found that O+ density shows expo-nentially decrease with SYM-H index and increase with solar wind dynamic pressure,but the dependencies of O+ density is more stronger than H+ density.The H+ den-sity changes little with IMF By and IMF Bz.But O+ density and O+/H+ density ratio increase with the absolute values of IMF By and IMF Bz.Otherwise,the H+ density has little correlation with F10.7 index,but O+ density increases with F10.7 index at the beginning and then becomes stable.Secondly,the spatial distributions of H+,O+ density(1-40 keV)and therefore den-sity ratio for different ranges of parameters(including SYM-H index,IMF By,IMF Bz,solar wind dynamic pressure,and F10.7 index)are investigated.These distributions are projected into the XY plane in Geocentric Solar Magneto spheric(GSM)coordinates according to different parameter scales.And the asymmetry index is defined for quanti-tative description of dawn-dusk asymmetry.The O+ density shows duskside asymmetry under southward IMF when IMF By is positive.Meanwhile it shows dawnside asym-metry under northward IMF when IMF By is negative.However,H+ density doesn't show significant dawn-dusk asymmetry under the different IMF conditions.The in-fluence of the dawn-dusk IMF direction on dawn-dusk asymmetry is significant.The positive IMF By can strengthen the O+ density duskside asymmetry when the IMF is southward,and the negative IMF By can strengthen the O+ density dawnside asymme-try when the IMF is northward.The H+density asymmetry index increases with solar wind dynamic pressure,but solar wind dynamic pressure seems to make a minor effect on O+ density dawn-dusk asymmetry.Similarly,the H+ density asymmetry index also increases with the F10.7 index,but the F10.7 index seems to play a minor role in the O+density dawn-dusk asymmetry as well.Then,the distributions of the H+,O+ fluxes and their ratios in XYGSM depending on different energy ranges are studied.The O+ fluxes dawn-dusk asymmetry changes little as particle energy enhances from?1 keV to?35 keV.While the H+ fluxes show more prominent duskside asymmetry as energy enhances from?10 keV to?35 keV.The O+/H+ fluxes ratios increase with energy more distinctly at the dawnside magnetopause boundary layer than that at the duskside,therefore it shows more significant dawnside asymmetry.These results suggest that the H+and O+ at energies from 1 keV to 40 keV experience different processes when they are transported to the dayside magnetopause boundary layer.At last,the variations of energetic O+(1-40 keV)at the duskside magnetopause boundary layer on the solar wind and geomagnetic activity conditions(IMF By,IMF Bz,solar wind dynamic pressure,and AE index)during intense substorm are investi-gated specifically.It is found that the maximum values of the O+ density and O+/H+density ratio occurred during the intense substorm recovery phase and corresponding to southward IMF.The O+ density presents an exponential growth with solar wind dy-namic pressure.When the IMF is southward,the O+ density shows an exponential in-crease with IMF Bz absolute value.Meanwhile,the O+/H+ density ratio exponentially increases with IMF By.This result agrees with previous studies of the relationship between O+ abundance and IMF By in the near-Earth magnetosphere during intense substorm.It is suggested that O+ abundance(O+/H+)in the duskside magnetopause boundary layer has a close relation to O+ variations in the near-Earth magnetosphere during intense substorms.