Concurrent study of bottomside spread F and plasma bubble events in the equatorial ionosphere during solar maximum using digisonde and ROCSAT-1

Abstract

Abstract. Data from the Jicamarca digisonde and the ROCSAT-1 satellite are employed to study the equatorial ionosphere on the west side of South America during April 1999-March 2000 for the concurrent bottomside spread F (BSSF) and plasma bubble events. This study, using digisonde and ROCSAT-1 concurrently, is the first attempt to investigate the equatorial spread F. Results show that BSSF and plasma bubble observations appear frequently respectively in the summer (January, February, November, and December) and in the equinoctial (March, April, September, and October) months, respectively, but are both rarely observed in the winter (May-August) months. The upward drift velocity during the concurrent BSSF and bubble observations has been determined to study the driving mechanism. This analysis shows that large vertical drift velocities favor BSSF and bubble formations in the equinoctial and summer months. Conversely, the smaller upward velocities during the winter months cause fewer BSSF and bubble occurrences. For the geomagnetic effect, the BSSF/bubble occurrence decreases with an increasing K p value in the equinoctial months, but no such correlation is found for the summer and winter months. Moreover, the anti-correlations between K p and dh F /dt are apparent in the equinoctial months, but not in the summer and winter months. These results indicate that in the equinoctial months the BSSF/bubble generations and the pre-reversal drift velocity can be suppressed by geomagnetic activity, because the disturbance dynamo effects could have decreased the eastward electric field near sunset. However, BSSF and bubble occurrences may not be suppressed by the geomagnetic activity in the summer and winter months.

Digisonde spread F and GPS phase fluctuations in the equatorial ionosphere during solar maximum

“…(b) The seasonal variations of relative occurrence for 200 < Fp (thick line with solid circles) and bubbles (dashed line with asterisks). Notice that the variations of spread F and bubble were reproduced from Lee et al [2005a, 2005b].…”

“…This similarity indicates that the occurrence of topside plasma bubbles is associated with the strong level of GPS phase fluctuations. On the basis of Lee et al [2005b], the larger upward E × B drift velocity would lift the bottomside F region irregularities to higher altitudes (topside plasma bubbles). Therefore the appearance of topside plasma bubbles means that irregularities distribute from bottomside to topside (600 km) in F region.…”

“…Recall that the upward E × B drift velocity in the D months is smaller than in the E months. Thus the irregularities in the D months may not rise into the altitudes as high as E months [ Lee et al , 2005b]. Therefore the duration of strong level GPS phase fluctuation will be shorter than that in the E months.…”

“…Let us now return to the postmidnight irregularities; it might be associated with the magnetic disturbance [Fejer et al, 1999] or the spread F fossil [Argo and Kelley, 1986] which drift from the west to the east. Among these two mechanisms, the fossil would be the major source of the Lee et al [2005aLee et al [ , 2005b.…”

Observation of F region irregularities near a northern equatorial anomaly crest during solar minimum using ionosonde, GPS receiver, and satellites

“…Because the upward velocities produced by the pre‐reversal enhancement (PRE) during solar minimum are smaller than during solar maximum [ Fejer et al ., ], the number of EPBs might be limited to reach DMSP altitudes at the appropriate local time. Furthermore, the smaller PRE upward velocity would be also responsible for the less occurrences of equatorial spread F (ESF) during solar minimum, since the PRE upward velocity is an important factor for the ESF formation [ Fejer et al ., ; Lee et al ., , ]. For another year of low solar activity, Chu et al .…”