IONOSPHERE

Ionosphere is the charged or the ionized region of the atmosphere above about 60 Km. This region is divided into different layers or regions; D, E, F1 and F2. These regions correspond to different altitude and electron density levels. The ionosphere structure is variable with day and night, seasons and different latitudes. In the night region, the D and F1 layers disappear. The density profiles of day and night time of midlatitude region is shown in figure1.


density profiles

          FIGURE 1
(Day and night time electron density profiles for sunspot maximum (solid lines) and sunspot minimum (dashed lines).


Figure from - Tascione, T.F., Introduction to the Space Environment, 2nd Ed.

The F2 layer does not disappear at sunset, instead its density slowly decreases throughout the night. Occasionally the nighttime F2 region becomes unstable at midlatitudes. At these times the density of the plasma is not smoothly varying with height; instead it contains sharp gradients and density structures.

Ionospheric Irregularities

The ionospheric irregularities mentioned above are temporal and spatial variations of the electron density lasting from a couple minutes to a few hours. In the lower latitudes of the F region of the ionosphere, these irregularities are called Equatorial Spread F. Occurrence of similar irregularities in the midlatitude F region is also called spread F, but at the midlatitudes the mechanism of formation of spread F is not understood. Density perturbations form the basis of spread F. Gravity waves are also widely recognized as a major cause for these density perturbations.

The following figure shows an ionogram for a normal event.

non spread

FIGURE 2
(Midlatitude Non-Spread F event for a 15 minute sweep on November 20, 2004 at 1 UT)

spread

FIGURE 3
(Midlatitude Spread F event for a 15 minute sweep on November 21, 2004 at 1 UT)


Ionograms obtained for spread F show thickness or spread in the F region which is more than that obtained for a normal ionosphere (figure 3). Instead of a relatively thin band of reflections, the ionograms made during a spread F event seem to show multiple echoes at different heights for the same frequency. This is called range spreading. It is also possible for the ionograms to show bands of echoes from the same height for multiple frequencies. This is called frequency spreading.

Figures 2 and 3 also show a second hop or echo in the ionogram. This is due to the double reflection of the pulse from the ionosphere to ground to ionosphere and back at the receiver. Thus the wave is reflected twice before being received and hence it is almost twice the height of the first echo. 

Ionospheric sounding is one of the oldest and most accurate ways to study the ionosphere. Ionosondes are used to send signals in the ionosphere. These signals when reflected back indicate presence of ionization. The frequency at which the reflection occurs is related to the plasma density at the altitude from which the echo occurs.

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