Response Of Geosynchronous Magnetic Field To Solar Wind Variations

Since the geosynchronous orbit is important to space activities,the response of geosynchronous magnetic filed to the interplanetary disturbance is one of the hot topics nowadays in space physics and space weather research.The knowledge of the physics process and properties is vital for us to understand the coupling of solar wind and magnetosphere.This paper carries out the data analysis of the multiple spacecraft observations and MHD simulation to investigate the response of the geosynchronous magnetic field to interplanetary disturbances(including solar wind dynamical pulses and interplanetary shocks).We do statistical survey of the response of geosynchronous magnetic field to solar wind dynamic pressure pulses,which is easier to identified in solar wind and check out the assured response at geosynchronous orbit,leading to a more believable one to one responses.In the time range from 1998 to 2005 we find 111 solar wind dynamic pressure pulses which produce geosynchronous magnetic field responses for very quiet magnetosphere condition(Dst＞-50).These responses are often observed by two or three GOES spacecraft at different local times in geosynchronous orbit.The magnitudes of the geosynchronous magnetic field changes(dB_z) have a peak near the noon meridian,similar to the results obtained in the study of the response of the geosynchronous field to the large and sharp solar wind dynamic pressure variations.However,the relative change of the geosynchronous magnetic field dB_z/AV-B_z(where AV-B_z is the average of the geosynchronous magnetic field B_z observed during the response to the pressure pulse) depends weakly on the local time,thus the change of B_z(dB_z) is proportional to the average field(AV-B_z).As the magnitude of the relative change of solar wind dynamic pressure(dP_d/P_d) increases(or decreases),the rate of geosynchronous magnetic field variation increases(or decreases) correspondingly. These results imply that the magnitude of the geosynchronous magnetic field response could be determined by AV-B_z.In addition,the interplanetary field orientation does not affect the response significantly.Using an MHD code which models the global behavior of the solar wind-magnetosphere-ionosphere system,we reproduce the main characteristics of the observations.We have also studied the geosynchronous magnetic filed responses to interplanetary (IP) shocks.We have identified 250 interplanetary fast forward shocks in the solar wind,of which 216 IP shocks can produce the obvious geosynchronous magnetic field responses.Under very quite magnetospheric condition (Dst＞-30) we find the similar result as the responses to Pd pulses.The amplitude of the geosynchronous magnetic flied responses has a peak near the local noon,but the relative amplitude of the response depends weakly on local time.When the magnetosphere is appropriately compressed,the amplitude of the geosynchronous magnetic field(d/B_z) is proportional to the time-average value of geosynchronous magnetic field(Av-B_z).As the upstream solar wind dynamic pressure increases the slope of the linear fit line increases.At the local noon,the response amplitude(dB_z) is proportional to the changes of the square root of solar wind dynamic pressure.Using the shock-geosynchronous response-SYM-H response relationship,we investigate the relationship between the geosynchronous magnetic field responses and SYM-H responses.We find that at the noon meridian of geosynchronous orbit,the magnetic response amplitude is highly correlated with the amplitude of SYM-H responses,which means both geosynchronous magnetic field responses at noon and SYM-H indices response are caused by the chance of magnetopause currents.Through the comparison between the amplitude of the SYM-H response and the change of square root of solar wind dynamic pressure,we conclude that the SYM-H could be used to roughly represent the amplitude of SSC or SI+.