Ionosphere and plasmasphere electron density profiles

The ionosphere-plasmasphere region of Earth's environment extends from ∼60 km altitude above the surface to a geocentric distance of roughly 4 R E . Radio wave propagation in this region is predominantly controlled by the electron density distribution and the geomagnetic field. Measuring and modeling the electron density profiles Ne in this near Earth plasma regime has been a challenge ever since this space plasma regime was discovered, almost one hundred years ago. Great progress has been made in specifying the Ne profiles, especially in the ionosphere, as a function of time, location, season, and solar and magnetic activity, and yet, many open questions remain. Early observations with groundbased ionosondes and partial reflection radars discovered the non-monotonic structure of the ionosphere with densities peaking at ∼90 km (D layer), ∼110 km (E layer), ∼150 km (F1 layer or ledge), and ∼300 km (F2 layer). The global network of ionosondes routinely monitors the bottomside ionosphere up to hmF2, the height of the F2 peak (http://spidr.ngdc.noaa.gov/spidr/; http://giro.uml.edu/). Rocket measurements provided the most reliable profile information on the D layer profiles, but of course only at a few places and at a few times. The most thoroughly tested ionospheric profiles are arguably given by the IRI, an empirical model that uses readily measurable characteristics like foF2, hmF2, etc. for its specification as shown in Figure 1. Beginning in the middle of the last century, satellite and incoherent scatter radar (ISR) observations extended the density measurements above the height of the F2 layer peak and into the plasmasphere. The ISRs operate on frequencies well above foF2, the maximum plasma frequency in the ionosphere, and can measure Ne up to ∼1000 km, and also electron and ion temperatures, which are important for the development of physics-based ionospheric models. From the mid-1960s to the late 1970s, the US/Canadian Alouette and ISIS satellites carried topside sounders that measured the topside Ne profiles from the satellite altitude down to hmF2 (http://nssdc.gsfc.nasa.gov/space/isis/isis-status.html). The currently used topside profile models are largely based on these data. The Japanese ISS-b and Ohzora satellites also carried topside sounders. Topside Ne profile data form the Russian Cosmos-1809 and Intercosmos-19 satellites are available on http://antares.izmiran.rssi.ru/ projects/IK19/. Sounders (ionosondes) provide the most reliable Ne profile information, however there are currently no topside sounders measuring the topside ionosphere. Instead, indirect techniques have been developed in recent decades that invert transionospheric radio signals from satellites into electron density profiles using tomography and radio-occultation techniques. Earth's plasmasphere is the upward extension of the low- and mid-latitude ionosphere. Also filled with cold plasma, it extends to the plasmapause located along the L≈4 shell during magnetically quiet conditions, but this boundary is highly dynamic varying from ∼2–7 R E . In situ Ne measurements in the plasmasphere were made by instruments on a range of satellites, e.g., on LUNIK, OGOxx, IMP2, GEOS 1 and 2, ELECTRON 2 and 4, INTERCOSMOS, ISEE1, CLUSTER, and others. These in situ density data have been statistically analyzed to derive “average” plasmaspheric electron density distributions for different conditions and regions. In contrast to these in situ measurements, a VLF sounder on IMAGE made instantaneous measurements of entire profiles along the geomagnetic field line passing through the satellite. 2-D density distributions from L=1.5 to L=4 are obtained within ∼20 min of observation (Figure 2).