Large Electron Densities in the Early Morning Equatorial Ionosphere
Observed with UV Instruments from Space
Abstract
The equatorial ionosopheric anomalies (EIA) at night are the slowly
recombining remnants of the dayside ionosphere, and charged particle
densities slowly decay during the course of the night. Thus the electron
density in ionosphere in the early morning (0300-0400 Local time) is
usually very low and the ionospheric UV 135.6 nm O+ recombination
emission is rarely detectable from current UV remote sensing
instruments. However, there are times when the EIA have unusually high
density even during these morning times and are observable by the
DMSP/SSUSI and TIMED/GUVI instruments. By using other UV ‘colors’ -
130.4 nm (from monatomic Oxygen) and N2 Lyman Birge Hopfield bands - we
can establish that this emission is definitely from the ionosphere
recombination emission. We will show examples of this phenomenon, and
correlate these occurrences to geomagnetic storm events. We estimate the
electron density in the early morning EIA and compare with other
ionosphere observations and climatological models. In the figure below,
we show the 135.6 nm radiance seen by DMSP F16 SSUSI as it crosses the
equator around 210 degrees longitude (over the Pacific Ocean) at 03:45
local time. The equatorial anomaly peaks are clearly visible in the
SSUSI data. These radiances are background subtracted, which is not
perfect and introduces a small (-1 Rayleigh) bias to the resulting
radiances. DMSP = Defense Meteorological Satellite Program, SSUSI =
Special Sensor Ultraviolet Spectrographic Imager; TIMED = Thermosphere,
Ionosphere, Mesosphere Energetics and Dynamics, GUVI = Global
UltraViolet Imager