The Structure Characteristics of Simple Strong Magnetic Cloud

MHD characteristics of interplanetary magnetic clouds has been studied for the simple case without large corotating stream interactions.The present discussions involve two strong clouds,the Dcember 19,1980,and March 19,1980 events moving in a high and a low speed solar wind streams respectively.The simple strong clouds are characterized by a main structure of low density and temperature but high magnetic field,the field vector of which rotates with a monotonically changing inclination 6 to the ecliptic plane.The main structure is proceeded by a turbulent high density and temperature structure,and followed by another less dense density enhancement.Large gradients of magnetic pressure P_m are observed at both the front and rear boundaries of the cloud.Inside the main structure of cloud,the Alfvenic speed C_a and ratios of the magnetic pressure to both the kinetic energy density and thermal pressure,μ_k and μ_p respectively are at least one order of magnitude higher than that in the background stream.Such a structure would help the MHD disturbances behind the cloud to propagate quickly across the cloud,and concntrate at the front boundary.It is suggestive that the magnetic gradient force due to expanding strong magnetic cloud as well as the dissipation of MHD waves in the high density structure could accelerate and heat the solar wind ahead of the cloud,build up the observed steep front in the speed profile and generate a forward shock.Also,there are indications that such expanding strong magnetic structure would be able to decelerate the solar wind from behind,and result in a double saw-teeth like speed profile similar to that observed in the outer,heliosphere.The structures of simple strong magnetic cloud could be compared with the structures observed successively in a typical coronal mass ejection event,i.e.a bright high density outer loop driven by a dark cavity with low density,presumably high magnetic field,inside which is an inner prominence core.In addition,the geomagnetic disturbances and cosmic ray decreases caused by the clouds" are briefly discussed in association with the Kp category of geomagnetic storms.