The dayside midlatitude plasma trough

The electrostatic probe experiment aboard the ISIS-1 satellite detects a quasi-stable dayside plasma trough in the protonosphere with a characteristic time of at least 2 weeks. This behavior contrasts with measurements at 1000 km from Explorer-22, which indicate a trough of a characteristic time less than a satellite orbit period. Associated with the protonospheric trough is an electron temperature maximum approximately 6000 K that is sharp at midnight and broad at noon. In spring and summer, a second noontime temperature maximum often appears poleward of 70 invariant latitude, accompanied by an enhancement in the ionization. The ISIS-1 data indicates that local processes such as dayside F-region photoionization and ionization by energetic cusp particles produce enhancements in plasma density and electron temperature. These enhancements modify the plasmaspheric boundary to varying degrees along a magnetic field tube, thus leading to a plasmas pheric boundary with distinctive features at various positions along a field tube.