SHOCKS IN THE POLAR WIND

The occurrence of shock waves in the terrestrial polar wind was predic ted many years ago by a time-dependent three-dimensional model based o n hydrodynamic equations. These shocks were seen to occur for counters treaming ion populations and for cases when a convecting flux tube ent ered a region of sharply increasing electron temperature, such as the dayside cusp. Other studies conducted at about the same time showed th at the shocks induced by counterstreaming ion populations may simply b e artifacts of the adopted hydrodynamic model. The validity of shocks induced by electron temperature enhancements has remained an open ques tion. Using a macroscopic particle-in-cell (PIG) code, we have verifie d the hydrodynamic prediction that sudden electron temperature enhance ments can launch shock waves in a convecting flux tube of plasma. Our simulation follows a flux tube as it convects antisunward across the d ayside auroral oval, the polar cap, and the nightside auroral oval. Th e electron temperature at 2000 km altitude is assumed to be relatively low (3000 K) in the subauroral ionosphere and in the polar cap but mu ch higher (7000 K) in the dayside and nightside auroral oval. As the f lux tube enters the auroral oval, either on the dayside or the nightsi de, forward and reverse shock pairs in the Hf component of the plasma are created at the bottom of the flux tube and propagate upward until they exit the simulation region at the top. The forward and reverse sh ock fronts propagate at speeds greater than and less than the drift sp eed of the Hf gas, respectively.