Observational Study Of Polar Ionospheric Convection Under Radial Interplanetary Magnetic Fields

The IMF Bz and By components play an important role in the solar wind-magnetosphere coupling system.These two parameters almost control all of the energy,mass,and momentum transportation from the solar wind to the magnetosphere.In the past,the effects of IMF Bz and By have been received more attention from researchers.The understanding of Bx's effect was not sufficient.In the last decade,it has been found that Bx plays an important role in magnetopause configuration,reconnection rate,and ionospheric potential difference.The radial interplanetary magnetic field is a special condition of the Interplanetary Magnetic Field(IMF,||/|| 0.9)in which the Bx component is dominant and the By and Bz components are only minuscule.Based on Super DARN radar observations,the effects of Bx on ionospheric convection are investigated by excluding the effects of By and Bz under the condition of radial interplanetary magnetic field.The specific results are as follows:(1)In this study,we present a statistical study on the radial IMFs for all the time scales of20,30,40,50,60,and 90 minutes from the year 2002 to 2017.The latitude of the equator boundary of ionospheric convection(which is also called the Heppner-Maynard Boundary)is a proxy of the solar wind-magnetosphere-ionosphere(S-M-I)coupling level.We find that the latitude of HMB at midnight is about 64°-65° during the sunward and anti-sunward directions of radial IMFs.Moreover,the AE indices are small in both directions of events,which means that only very weak night-side geomagnetic activity is likely to occur during the radial intervals.Most solar wind-driven electric fields are less than 0.8m V/m in both directions of events,and the difference in the mean values of the electric fields is very small.During the radial IMF periods,the IMF Bx component has the most obvious effect on the midnight latitudes of HMB,while the By and Bz components have no significant effect on HMB midnight latitudes.With the increase of the Bx magnitude of the radial IMFs,the HMB midnight latitude moves towards the lower latitude.It suggested that the driving effect of solar wind increases with the increase of the Bx magnitude.During the radial IMFs,the contribution of the Bz component may be covered by the contribution of the Bx component due to the draping effect.There is a decreasing trend between the solar wind velocity(Vx)and HMB latitude,but no trend could be found observed between the By or Bz components and the HMB midnight latitude.Our results suggest that the coupling function without considering the Bx term may not be applicable to the condition of radial IMFs.This can also lead to a mismatch between the coupling level and the electric field driven by the solar wind.The radial interplanetary magnetic field causes a north-south asymmetry in the magnetopause boundary layer,which is responsible for the difference in the large-scale coupling between the sunward and anti-sunward radial IMFs.(2)The radial interplanetary magnetic field with negative Bx is favorable for lobe reconnection in the northern hemisphere due to the draping effect.We present an interval of radial IMF with four sudden increases of Bz component from the OMNI IMF database between07:30 to 09:00 UT on 19 February 2017.During this time,the other solar wind parameters remained basically stable.Several minutes after each sudden and weak increase of the IMF Bz the floe burst was observed.After the enhancement of the IMF Bz component,the flow bursts disappeared rapidly.At 08:38 UT,we also observed an auroral spot in the cusp region.The aurora spot produced in this region is in the same position as the cusp spot caused by the lobe reconnection.Meanwhile,the horizontal ion drift velocity is in the same direction as the flow burst observed by Super DARN han radar in the polar cap.A significant reverse ion dispersion signal was observed in DMSP in-situ data.The above observations are all evidence of the occurrence of lobe reconnection.(3)From the traditional theory,the geomagnetic activity is relatively quiet under the radial interplanetary magnetic field.We examined the occurrence of geomagnetic disturbances during all radial IMF events carefully from the year 2002 to 2020.In 19 years of long-term observations,we found only one special case of auroral substorms under radial IMF conditions.During the substorm,the geomagnetic X component of Zhongshan station decreased rapidly,and the Pi2 pulsation was also observed.In the all-sky aurora image,the aurora suddenly brightens from the lower latitudes.After the aurora onset,the all-sky camera at Zhongshan station observed vortex aurora of the westward traveling surge,and DMSP/SUSSI also observed a large scale westward traveling surge during the substorm peak period.It is found that the sudden decrease of dayside ionospheric convections and the previous steady magnetospheric convection are the triggering of the substorm under radial IMF conditions.The paper mainly studies the ionospheric large-scale and small-scale convection under the condition of radial interplanetary magnetic field based on Super DARN radar and other observations,in order to study the influence of radial interplanetary magnetic field on the magnetospheric state.Which can improve our understanding of the solar wind magnetosphere ionospheric coupling system,and provides an important theoretical basis for establishing a more perfect prediction model in the future.