Frequency and duration of disturbances in the mid‐latitude F region of the ionosphere

Abstract

Disturbances of the ionospheric F region at two well‐separated mid‐latitude stations were identified on rapid run ionograms by spread echoes, blackouts, and anomalies in structure or critical frequency. The diurnal variation in the probability of disturbance consistently shows minima around sunrise and sunset. Details such as a brief postsunset enhancement in winter are revealed by the fine time resolution of the data. Disturbance probabilities were lowest in autumn and spring (8% and 9%), while that for winter (33%) exceeded the summer level (20%). Spread F constituted 32% of the events, and disturbances producing sharply defined ionogram signatures 77%, with some overlapping of types. The majority of disturbances were not correlated with geomagnetic phenomena. Disturbances at the two stations were essentially independent (spatial correlation of 0.15). Spread F events were of longer duration (25 min) than sharp disturbances (20 min). TIDs were the exception, with a most probable duration of 80 min, but accounted for only 8.3% of the sharp disturbances. The absence of disturbances in the F region at dusk and especially at dawn coincides with periods of instability in the E region.

Morphology of quantified ionospheric range spread-F over a wide range of midlatitudes in the Australian longitudinal sector

“…A similar increase in the June solstitial spread-F in East Asia (Japan and Taiwan) was reported by Sinno andKan (1980), andHuang (1978). The June solstice maximum in spread-F was also reported for the South African longitudinal sector (Lambert, 1988). Bowman (1992) found that "midlatitude spread-F is caused primarily by off-vertical reflections of radio waves from tilted isoionic surfaces produced by the passage of medium-scale travelling ionospheric disturbances (MSTIDs)".…”

A statistical study of midlatitude spread F at Wallops Island, Virginia

“…[5] The present study significantly extends the work of Lambert [1988] by utilizing data from a single midlatitude site over an entire solar cycle (1996 -2006). In order to process such a large data set an automated detection algorithm for identifying both range and frequency spread F has been developed; it is described in detail in section 3.…”

“…The study includes data from a single solar minimum year, and covers all four seasons. Thus Lambert [1988] was the first to examine the seasonal variations of midlatitude spread F (MSF) in a statistically significant way.…”

Ionospheric frequency spread and its relationship with range spread in mid‐latitude regions

“…Experimental results indicate that there is not a close relationship between the recording of medium-scale TIDs and geomagnetic activity. For example, Lambert [1988], commenting on both nighttime spread F disturbances and daytime medium-scale TIDs, states, "The majority of disturbances were not correlated with geomagnetic phenomena." Also, statistical studies at latitudes similar to that of Bribie Island [Bowman, 1979[Bowman, , 1982 indicate a tendency for spread F to occur several days after increased geomagnetic activity.…”