DOUBLE-HUMP H+ VELOCITY DISTRIBUTION IN THE POLAR WIND

The polar wind is an ambipolar plasma outflow from the terrestrial ion osphere at high latitudes. As the ions drift upward along geomagnetic flux tubes, they move from collision-dominated (ion barosphere) to col lisionless (ion exosphere) regions. A transition layer is embedded bet ween these two regions where the ion characteristics change rapidly. A Monte Carlo simulation was used to study the steady-state flow of Hions through a background of O+ ions. The simulation domain covered th e collision-dominated, transition, and collisionless regions. The mode l properly accounted for the divergence of magnetic field lines, the g ravitational force, the electrostatic field, and H+-O+ collisions. The H+ velocity distribution, f(H+), was found to be very close to Maxwel lian at low altitudes (deep in the barosphere). As the ions drifted to higher altitudes, f(H+) formed an upward tail. In the transition laye r, the upward tail evolved into a second peak with a kidney bean shape , and hence, f(H+) developed a double-humped shape. The second peak gr ew with altitude and eventually became dominant as the ions reached th e exosphere. This behavior is due to the interplay between the electro static force and the velocity-dependent Coulomb collisions. Moreover, the H+ heat flux, q(H+), was found to change rapidly with altitude in the transition layer from a positive maximum to a negative minimum. Th is remarkable feature of q(H+) is closely related to the coincident fo rmation of the double-humped structure of f(H+). The double-hump distr ibution might destabilize the plasma or, at least, cause enhanced ther mal fluctuations. The double-hump f(H+), and the associated wave turbu lence, have several consequences with regard to our understanding of t he polar wind and similar space physics problems. The plasma turbulenc e can significantly alter the behavior of the plasma in and above the transition region and, therefore, should be considered in future polar wind models. The wave turbulence can serve as a signature for the for mation of the double-hump f(H+). Also, more sophisticated (than the ex isting bi-Maxwellian 16-moment) generalized transport equations might be needed to properly handle problems such as the one considered here.