The electron density at the midpoint of the plasmapause Provisionally Accepted

Richard E. Denton^1* Phoebe M. Tengdin^{1, 2} David P. Hartley³ Jerry Goldstein⁴ Jinmyoung Lee¹ Kazue Takahashi⁵

• ¹Dartmouth College, United States
• ²Miraex, Switzerland
• ³The University of Iowa, United States
• ⁴Southwest Research Institute (SwRI), United States
• ⁵Applied Physics Laboratory, Johns Hopkins University, United States

The high density plasmasphere in the magnetosphere is often separated from a lower density region outside of the plasmasphere, called the plasmatrough, by a sharp gradient in electron density called the plasmapause. Here we use plasmapause events identified from electron density data from the ISEE, CRRES, Polar, and IMAGE missions, to find models for the electron density at the midpoint of the plasmapause, n e,pp . A good model for n e,pp should include dependence on L, which is the strongest dependence. But models can be improved by including weaker dependencies on the magnetic local time, MLT, the solar EUV index F10.7, and geomagnetic activity as indicated by averages of Kp and AE. The most complicated model that we present, found using the nonlinear genetic algorithm TuringBot, predicts n e,pp within a factor of 1.64, and is within the range of observed plasmapause densities for about 96% of our events. These models can be useful for separating plasma populations into plasmasphere-like and plasmatrough-like populations. We also make available our database of electron density measurements categorized into various populations.