Ionogram height–time–intensity observations of descending sporadic E layers at mid-latitude
A new methodology of ionosonde height–time–intensity (HTI) analysis is introduced which allows the investigation of sporadic E layer ( E s) vertical motion and variability. This technique, which is useful in measuring descent rates and tidal periodicities of E s, is applied on ionogram recordings ma...
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| Published in: | Journal of atmospheric and solar-terrestrial physics 2006-01, Vol.68 (3), p.539-557 |
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| Main Authors: | , , , , |
| Format: | Article |
| Language: | eng |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | A new methodology of ionosonde height–time–intensity (HTI) analysis is introduced which allows the investigation of sporadic
E layer (
E
s) vertical motion and variability. This technique, which is useful in measuring descent rates and tidal periodicities of
E
s, is applied on ionogram recordings made during a summer period from solstice to equinox on the island of Milos (36.7°N; 24.5°E). On the average, the ionogram HTI analysis revealed a pronounced semidiurnal periodicity in layer descent and occurrence. It is characterized by a daytime layer starting at 120
km near 06
h local time (LT) and moving downward to altitudes below 100
km by about 18
h LT when a nighttime layer appears above at ∼125
km. The latter moves also downward but at higher descent rates (1.6–2.2
km/h) than the daytime layer (0.8–1.5
km/h). The nighttime
E
s is weaker in terms of critical sporadic
E frequencies (
foEs), has a shorter duration, and tends to occur less during times close to solstice. Here, a diurnal periodicity in
E
s becomes dominant. The HTI plots often show the daytime and nighttime
E
s connecting with weak traces in the upper
E region which occur with a semidiurnal, and at times terdiurnal, periodicity. These, which are identified as upper
E region descending intermediate layers (DIL), play an important role in initiating and reinforcing the sporadic
E layers below 120–125
km. The observations are interpreted by considering the downward propagation of wind shear convergent nodes that associate with the
S
2,3 semidiurnal tide in the upper
E region and the
S
1,1 diurnal tide in the lower
E region. The daytime sporadic
E layer is attributed to the confluence of semidiurnal and diurnal convergent nodes, which may explain the well-known pre-noon daily maximum observed in
foEs. The nighttime layer is not well understood, although most likely it is associated with the intrusion of the daytime DIL into the lower
E region due to vertical wind shear convergence nodes descending with the semidiurnal tide. It was also found that the descent rates of sporadic
E may not always represent the vertical phase velocities of the tides, especially in the nighttime layers. Finally, the ionosonde HTI analysis is a promising new tool for exploring long-duration data sets from ionosondes around the globe to obtain preliminary climatological studies of neutral wind dynamics at
E region heights in the lower thermosphere. |
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| ISSN: | 1364-6826 1879-1824 |