Effects of pre-reversal enhancement of E × B drift on the latitudinal extension of plasma bubble in Southeast Asia

We investigated the effects of the F region bottomside altitude (h’F), maximum upward E × B drift velocity, duration of pre-reversal enhancement and the integral of upward E × B drift on the latitudinal extension of equatorial plasma bubbles in the Southeast Asian sector using the observations recorded by three GPS receivers and two ionosondes. The GPS receivers are installed at Kototabang (0.2°S, 100.3°E; 9.9°S magnetic latitude), Pontianak (0.02°S, 109.3°E; 9.8°S magnetic latitude) and Bandung (6.9°S, 107.6°E; 16.7°S magnetic latitude) in Indonesia. The ionosondes are installed at magnetically equatorial stations, Chumphon (10.7°N, 99.4°E; 0.86°N magnetic latitude) in Thailand and Bac Lieu (9.3°N, 105.7°E; 0.62°N magnetic latitude) in Vietnam. We analysed those observations acquired in the equinoctial months (March, April, September and October) in 2010–2012, when the solar activity index F10.7 was in the range from 75 to 150. Assuming that plasma bubbles are the major source of scintillations, the latitudinal extension of the bubbles was determined according to the S4 index. We have found that the peak of h’F, maximum upward E × B drift and the integral of upward E × B drift during the pre-reversal enhancement period are positively correlated with the maximum latitude extension of plasma bubbles, but that duration of pre-reversal enhancement does not show correlation. The plasma bubbles reached magnetic latitudes of 10°–20° in the following conditions: (1) the peak value of h’F is greater than 250–450 km, (2) the maximum upward E × B drift is greater than 10–70 m/s and (3) the integral of upward E × B drift is greater than 50–250 m/s. These results suggest that the latitudinal extension of plasma bubbles is controlled mainly by the magnitude of pre-reversal enhancement and the peak value of h’F at the initial phase of development of plasma bubbles (or equatorial spread F) rather than by the duration of pre-reversal enhancement.


Role of Pre-reversal Enhancement (PRE) on Plasma Bubble Generation
Plasma bubble refers to depletion of plasma density at the nighttime equatorial and low-latitude region. Even though evolution of plasma bubble may need a seed perturbation to trigger the instability, one of the favourable ionospheric conditions for plasma bubble growth is the enhancement of the F region at the evening terminator; this condition is called pre-reversal enhancement (PRE). PRE associated with an enhanced upward E × B drift in the post-sunset sector significantly affects the equatorial plasma bubble, or equatorial spread F (ESF), generation through the Rayleigh-Taylor instability (RTI) mechanism. The enhancement of E (eastward electric field) is caused by F-region dynamo at evening terminator. The PRE affects the creation of the bubbles by lifting the ionosphere to high altitudes where the growth rate of the RTI is large because of the small ion-neutral collision.
Morphologically, a plasma bubble originates at the magnetic equator and extends poleward, reaching the equatorial ionisation anomaly (EIA) crest region and sometimes beyond, as shown in Figure 2. The plasma bubble contains irregularities (electron density fluctuations) of various scale sizes. Furthermore, total electron content (TEC) depletions can be associated with plasma bubble.

Purpose of Study
In this study, we aim to investigate the relations between PRE and the latitudinal extension of plasma bubbles in the Southeast Asian region. We focus on the strength of PRE to see its effect on latitudinal extension of plasma bubble. We use the peak value of h'F and the magnitude of vertical drift velocity, calculated from data obtained by two ionosondes in the equatorial region, to analyse the effect of the upward E × B drift. We assume that the scintillation region coincides with plasma bubble; therefore, the maximum latitude of scintillation can indicate the maximum latitude of plasma bubble. The latitudinal extension of scintillation is observed by GPS receivers installed at three sites in Indonesia. Figure 1 shows the geometry of the observations. As shown, two ionosondes are located near the magnetic equator at Chumphon (10.7°N, 99.4°E; 3.3°N magnetic latitude) in Thailand and at Bac Lieu (9.3°N, 105.7°E; 1.7°N magnetic latitude) in Vietnam as a part of the SEALION (Southeast Asia low-latitude ionospheric network) project. From the rate of increase in h'F, we estimated E × B drift during the PRE period. Three GPS receivers installed in the western part of Indonesia are used to observe ionospheric scintillation activity at night in the equinoctial months of 2010, 2011, and 2012. The field-of-view of all GPS receivers covers an area from near the magnetic equator up to 15°S in geographic latitude. The geometry of the observations therefore allows us to study the relation between the upward E × B drift during the PRE period and the latitudinal extension of scintillation at the low-latitude region in the Southeast Asian sector.

Observation Setup
In this study, we analyse the relations of the peak h'F, maximum eastward E, duration of eastward E, and the integral of eastward E versus the maximum latitudinal extension of scintillations. To investigate these relations, we used two datasets. The first (Group 1) is used for comparison of peak h'F, maximum eastward E, time duration of eastward E, and the integral of eastward E obtained from the Chumphon ionosonde with the maximum latitudinal extension of scintillation observed by the GPS receivers at Kototabang and Bandung. The other (Group 2) is the same comparison, but using the Bac Lieu ionosonde and the GPS receivers at Pontianak and Bandung.

Analysis Method
We analyse the peak h'F, maximum eastward E, duration of eastward E, and the integral of eastward E as the PRE strenght. Figure 2 shows local time variations of h'F at 3 MHz and vertical drift derived from the rate of change of h'F (i.e., dh'F/dt), as obtained by the ionograms at Chumphon on 26 October 2012. The value of h'F increased between 18:00 and 19:20 LT, reached a maximum at 19:20 LT, and decreased afterward. In this study, we take PRE period as the interval for h'F enhancement, that is, the time from when h'F starts to increase until it reaches a peak. As shown in Fig Fig. 2, the PRE period is between 18:00 and 19:20 LT. We choose and maximum eastward E (dh'F/dt) that occurs within PRE period. We also consider the duration of eastward E as a parameter affecting the latitudinal extension of scintillations. We define the duration of eastward E as the period from when eastward E begins to increase until the last positive value of vertical drift occurs, before eastward E becomes

reaches a peak. As shown in
The last parameter we also consider that could be affecting the latitudinal extension of ation is the integral of eastward E during the PRE period. We computed the integral of eastward E is as the integral of vertical drifts over time, with the time interval taken as the interval of eastward E. For example, as shown in Figure 2, we calculate the sum of vertical drift from 18:10 LT to 19:20 LT. Thus, the sum of vertical drift over time in that interval is the integral of eastward E in the PRE period. In our analysis, we avoid using h'F and dh'F/dt during the period of strong spread F occurrence.
Observation geometry of the study. n 18:00 and 19:20 LT. We choose that occurs within PRE period. We also consider the duration of eastward E as a parameter affecting the latitudinal extension of f eastward E as the period from when eastward E begins to increase until the last positive value of vertical drift occurs, before eastward E becomes The last parameter we also consider that could be affecting the latitudinal extension of ation is the integral of eastward E during the PRE period. We computed the integral of eastward E is as the integral of vertical drifts over time, with the time interval taken as the he sum of vertical drift from 18:10 LT to 19:20 LT. Thus, the sum of vertical drift over time in that interval is the In our analysis, we avoid using h'F and dh'F/dt during We can reasonably suppose that scintillations could be present when the plasma bubble Thus, in this study, we assume that the scintillation coincides with plasma bubble. And, we assume that maximum latitude of scintillation region can be present even beyond the EIA crest region. We have carefully examined the relations, finding that scintillation, the intensity is generally of scintillation intensity and maximum latitudinal extension of scintillation as obtained from the Kototabang-Bandung (Group 1) this figure, strong scintillation (S4 index > 0.5) occurs between 5°S and 10°S, which can be taken as the EIA crest region. Moreover, weak scintillation is distributed not only at the same latitude as the EIA latitude but also both inside and outside of this latitude. This analysis emphasises that weak scintillation could be present as far as the extent of plasma bubble. This fact also suggests that GPS scintillation data, particularly for weaker s observing the latitudinal extension of plasma bubble. We can reasonably suppose that scintillations could be present when the plasma bubble Thus, in this study, we assume that the scintillation coincides with plasma bubble. And, we assume that maximum latitude of scintillation region can be present even beyond the EIA crest region. We have carefully examined the relations, finding that at the highest latitude of scintillation, the intensity is generally weak (S4 index < 0.5). Figure 3 displays the distribution of scintillation intensity and maximum latitudinal extension of scintillation as obtained from the and Pontianak-Bandung (Group 2) GPS receivers. As shown in this figure, strong scintillation (S4 index > 0.5) occurs between 5°S and 10°S, which can be taken as the EIA crest region. Moreover, weak scintillation is distributed not only at the same e as the EIA latitude but also both inside and outside of this latitude. This analysis emphasises that weak scintillation could be present as far as the extent of plasma bubble. This fact also suggests that GPS scintillation data, particularly for weaker scintillation, can be used for observing the latitudinal extension of plasma bubble.

Local time variations of height of the F layer and vertical drift, as derived from Chumphon ionograms with 26 October 2012 data.
We can reasonably suppose that scintillations could be present when the plasma bubble occurs.
Thus, in this study, we assume that the scintillation coincides with plasma bubble. And, we assume that maximum latitude of scintillation region can be present even beyond the EIA crest at the highest latitude of displays the distribution of scintillation intensity and maximum latitudinal extension of scintillation as obtained from the Bandung (Group 2) GPS receivers. As shown in this figure, strong scintillation (S4 index > 0.5) occurs between 5°S and 10°S, which can be taken as the EIA crest region. Moreover, weak scintillation is distributed not only at the same e as the EIA latitude but also both inside and outside of this latitude. This analysis emphasises that weak scintillation could be present as far as the extent of plasma bubble. This cintillation, can be used for

Results and Discussion
As Figure  Our result also emphasise that maximum latitude to which plasma bubble extends is only weakly dependent on the duration of eastward E. Our result indicates that duration of eastward E may not play an important role in the altitudinal and latitudinal extension of plasma bubble, whereas the magnitude of eastward E at the initial phase of plasma bubble generation is a primary factor for plasma bubble extension. Hence, our findings indicate that the altitudinal and latitudinal extension of plasma bubbles may be linearly dependent on the magnitude of eastward E within the PRE period, rather than on the duration of eastward E.

Conclusion
We investigated the effects of the peak h'F and magnitude and time duration of eastward electric field within the PRE period on the latitudinal extension of plasma bubble in the Southeast Asian region. Our findings emphasise good correlations of the peak h'F, maximum vertical E × B drift, and the integral of vertical E × B drift versus the maximum latitudinal extension of plasma bubble and a weak correlation between the duration of vertical E × B drift and the maximum latitudinal extension of plasma bubble. Our finding indicates that the key factor of plasma bubble extension is the magnitude of eastward electric field, not the duration of eastward E, because this factor can lift the F layer reaching to higher altitudes, where the growth rate of plasma bubble is larger, so that plasma bubble can extend to higher altitudes and latitudes.