Statistical Study Of Solar Wind Dynamic Pressure Pulses

In interplanetary space,small-scale solar wind plasma structures with abrupt increase or decrease in plasma dynamic pressure are frequently observed.Found from the high resolution data,they usually have a very sharp front with dynamic pressure changing abruptly in a pulse form on timescales of seconds to a few minutes.They are usually called solar wind dynamic pressure pulses(DPPs)when they could be isolated in the sense that there are only small variations in the preceding and succeeding regions.The abrupt changes in solar wind dynamic pressure can affect the energy and momentum from the solar wind to the magnetosphere-ionosphere coupling system and cause various disturbances particularly under the condition of southward interplanetary magnetic field(IMF).Based on the list of Solar Wind Dynamic Pressure Pulse(DPP)events observed by the WIND spacecraft during the period from 1995 to2014,we statistically analyze the distribution tendency about orientation and type distribution of 13042 non shock DPPs.The results are as follows:The occurrence distribution of the angle change between the magnetic field vector at preceding regions and that of succeeding regions is well fit by the double power-law distribution: the spectrum index of a population of smaller changes that is-1/28 and the spectrum index of a population of larger changes that is-1/49,and the angle occurrence distribution has a breakpoint at 30 degrees.Then we apply Minimum variance analysis(MVA)method to find the normals of DPP events.In order to ensure reliable normal estimates,the MVA eigenvalue ratio which is greater than 2 and the spreading angle that is greater than 30 degrees should be taken into consideration.It is found that the distribution of the normals of 5002 DPP events are mainly concentrated in a certain region that the range of polar angle is from-50 degrees to 50 degrees and the range of azimuthal angle is from 160 degrees to 250 degrees,the center of which is(-22.83 degrees,186.59 degrees).The orientation of DPP is key parameter that effects the tranist time between the position that DPPs areobserved by WIND spacecraft and subsolar point of magnetosphere.According to the direction and character about magnetic filed in preceding regions and succeeding regions of DPP events,DPPs can be classified to four types of discontinuities: tangential discontinuity(TD),rotational discontinuity(RD),either discontinuity(ED)and neither discontinuity(ND),of which the proportions are46.37%,19.57%,27.49%,and 6.57%.It shows that most DPPs are belonging to TDs.We also analyze the distribution of the angle between of IMF and the normal direction of DPP events,there are 70.30% of 5002 cases of DPP events that the angle is larger than 60 degrees and only 12.33% of 5002 cases of DPP events that the angle is smaller than 30 degrees.Beside,the ratio of TD and RD increases during the solar minimum.