The ionosphere consists of several layers, generally referred to as D (60 km - 90 km), E (90 km - 150 km) and F ( >150 km) regions. Horizontal ionospheric currents, such as the polar electrojets, the equatorial electrojet and the Sq (solar quiet) current systems are largely confined to the E region. At night time, however, when E region conductivity is low, currents in the higher F region can play an important role. This is particularly true for the tropical ionosphere. Near the equator the magnetic field lines have long sections within the F Region. Above an altitude of 400 km these field lines cross regions of enhanced plasma density, referred to as equatorial ionisation anomaly or Appleton anomaly. The Appleton anomaly is caused by a fountain effect of uprising plasma, occurring in the evening and pre-midnight hours, giving rise to enhanced plasma density about 15° north and south of the magnetic dip equator. Interesting magnetic signatures in the F-region are produced by pressure and gravity driven currents and by plasma instabilities and cavities ("plasma bubbles").