2. LONGITUDINAL DIFFERENCES IN THE EQUATORIAL SPREAD F CHARACTERISTICS BETWEEN VIETNAM AND BRAZIL Hoang Thai Lan, Abdu M. A, MacDougall J. W, Batista I. S. 1. HO CHI MINH CITY INSTITUTE OF PHYSICS, VAST, VIETNAM 2. INSTITUTO NACIONAL DE PESQUISAS ESPACIAIS – INPE, BRAZIL 3. UNIVERSITY OF WESTERN ONTARIO, LONDON, CANADA.

3. LONGITUDINAL DIFFERENCES IN THE EQUATORIAL SPREAD F CHARACTERISTICS BETWEEN VIETNAM AND BRAZIL Predicting Equatorial Spread F (ESF) is one of the goals of the International Space Weather effort. Our study is along this line. We present here the results of a comparative study of the Equatorial Spread F and the F layer critical parameter, that is the base height of the F layer bottomside over the two equatorial sites, Ho Chi Minh City in Vietnam and Sao Luis - in Brazil. This study focuses on the quiet time seasonal behavior of the F layer parameters in the two widely separated longitude sectors, and addresses the question as to what can we learn from such comparative studies with respect to the ambient ionospheric and thermospheric parameters that are believed to control the ESF generation and hence its longitudinal occurrence pattern.

4. Basic Principles in the Ionosphere at Low Latitudes  In a magnetic field, the charged particle motion is anisotropic. It is determined by the distribution of charged and neutral particles.  Thus, forces may driven ions and electrons at different speeds producing a current, that may have a divergence.  But, polarization electric fields are produced to make the total current divergence free everywhere.  Then, modified electric fields redistribute the ionization and change the anisotropic motions.

5. The Equatorial spread F phenomenon is a manifestation of the plasma bubble irregularities that occur under the unique electrodynamical conditions of the post sunset ionosphere. The post sunset equatorial F layer undergoes a rapid uplift due to an enhancement in the eastward electric field, widely known as the pre-reversal electric field enhancement that arises from the dominating role of an F layer dynamo. The positive electron density gradient region of the rapidly rising F layer bottomside becomes unstable to density perturbations leading to the growth of plasma bubble irregularities by the well-known Rayleigh -Taylor instability mechanism. The plasma bubbles develop as flux tube aligned vertically rising plasma depletions of large scale sizes with secondary irregularities developing by cascading processes at the steepening density gradients of the bubbles.

6. A scheme of the related coupling processes The following interdependent factors, of the ambient ionosphere–thermosphere system, control the spread F development: 1) The evening F layer height and the vertical plasma drift due to the pre-reversal electric field enhancement (PRE) that are controlled by the thermospheric zonal wind (eastward in the evening) and the longitudinal/local time gradient in the integrated E layer conductivity near sunset (SS.ΔΣ). 2) Development of a steep density gradient at the F layer bottomside where instability is initiated by seed perturbations (that widely believed to be from gravity waves). 3) The integrated Pedersen conductivity of the unstable flux tube which is controlled by thermospheric meridional/transequatorial winds.

7. The present analysis of the ionosonde spread F data is limited to examining any possible relative/complementary roles of the pre-reversal electric field enhancement, the F layer heights (h’F) and meridional winds in causing the seasonal variation in the spread F occurrences and intensity that are found to be markedly different in the two widely separated longitudes. Ho Chi Minh City - HCM (10.51 N, 106.33 E, dip lat: +2.9º) Sao Luis – SL (2.33 S, 44.2W, dip lat: -2°)

8. DATA PRESENTATION  The monthly occurrence rate of spread F.  The corresponding monthly average F layer base height (h’F) variations.  For the period: July 2002 - June 2003, which corresponded to a period of medium to high solar flux values.

9. RESULTS: The monthly percentage occurrence of spread F over HCM as a function of local time  The spread F starts mostly between 18:30 to 19:00 LT and attains a peak occurrence before midnight in almost all months.  The month of July with peak occurrence of ~ 25% at ~ 22 LT appears to present a yearly maximum with comparable occurrence in August and September and again in April and May. Strangely the month of June shows only post-midnight occurrence, as it does in October.  The general seasonal trend appears to be characterized by a tendency for a minimum in spread F occurrence during the months around December and relatively larger occurrence during the months around July.

10. The monthly average h’F local time variation over HCM We note that in general the months of larger spread F occurrence (July, August and September) have relatively higher post-sunset peak of F layer height as might be expected. Correspondingly the h’F peaks are smaller in the months of smaller spread F occurrence.

11. The monthly percentage occurrence of spread F for Sao Luis  The post-sunset spread F occurrence over Sao Luis is significantly more frequent than over Ho Chi Minh.  The season of highest occurrence rate extends from September through December solstice to March when the occurrence rate is often nearly 100 %.  Seasonally the minimum incidence of spread F is during the months centered on July.  However, the post midnight spread F shows relatively higher occurrence rates around June and December.  It may be noted that in J-months (May, June and July) the post-sunset spread F occurrence rate increases with local time at a much slower rate up till around midnight and presents larger values in the post-midnight hours. In contrast, during the D-months the post-sunset occurrence rate increases rather rapidly reaching peak values near 20 LT with the post-midnight occurrence rates presenting generally relatively smaller values.

12. The monthly average h’F local time variation over Sao Luis The post-sunset h’F attains peak values that are lower and at later local time in J-months as compared to their larger values peaking at earlier local times in D-months. This shows that a larger evening vertical drift is mainly responsible for the rapid increase to larger values of the post-sunset spread F in D-months in contrast to the smaller values of these parameters in J-months.

13. DISCUSSION An inter comparison of the spread F statistics presented for the two widely separated longitude sectors brings out two main points: 1.Clearly the spread F occurrence in general is significantly less frequent over HCM than over Sao Luis throughout the year. 2.Over HCM the spread F occurs more frequently around July with peak occurrence of only ~ 25%, whereas the spread F occurs at a significantly higher rate over Sao Luis during all months, with a seasonal maximum during a few months centered on December (and especially in January when it could occur on almost 100% of the nights).  In this context, it is to be pointed out that the larger occurrence rate corresponding the more intense spread F event.  Part of the reason for a generally lower rate of spread F occurrence (and therefore relatively weaker events) over Ho Chi Minh would appear to be the generally lower height of F layer over this station.

14. Comparison of the monthly average height of the F layer over HCM and SL  It shows a systematically lower F layer post-sunset height indicating also lower pre-reversal electric field enhancement over Ho Chi Minh as compared to Sao Luis. It suggests that this indeed is an important cause for such a difference.  As an interesting point we note that the time rate of change of the h’F near 18 LT during the July, August and Sep. months (of the seasonal peak in spread F) over Ho Chi Minh appears as large as that in the same months over Sao Luis. Though the height rise was smaller the magnitude of the pre-reversal electric field enhancement should have caused significantly larger rate of spread F occurrence in these months over Ho Chi Minh contrary to that is actually observed.  The difficulty to account for this observed seasonal behavior would call for the role of other factors in influencing the spread F occurrence.

15. An interesting point concerns the preference for the spread F to occur during the post-midnight/pre-sunrise hours in some months. Over Ho Chi Minh such a trend is seen in June and October whereas over Sao Luis it is seen in June and December solstice months.

16.  The F layer height preceding the spread F is an important indicator of these geophysical conditions.  Over Ho Chi Minh the spread F occurred without any conspicuous increase in the F layer height as seen in over Sao Luis, that F layer height rise appears to have preceded in most cases during December and not in July. The post-midnight/pre-sunrise spread F could occur under different geophysical conditions.

17. Discussion  A type of post-midnight spread F is known to occur under the effect of disturbance electric field with an associated F layer height increase.  Another type of pre-sunrise spread F occurs without any conspicuous F layer height increase. This class has been attributed to wind induced instability presumably under the action of zonal wind in the presence of large scale horizontal gradients in bottom-side electron density.  These different types need to be investigated in more detail, however.

18. CONCLUSIONS: The main conclusions of the study may be summarized as follows: 1. The range spread F occurrence over Vietnam is significantly less frequent (the monthly percentage occurrence being normally less than 25%) in all seasons of its occurrence than over Brazil where the percentage occurrence could attain close to 100% during its seasonal maximum. 2.In the Vietnamese longitude, during the analyzed period, the post-sunset spread F occurs more frequently around July months extending to September (with a strange exception of June) and less frequently around December, whereas in the Brazilian longitude sector the seasonal maximum is around December and minimum around July.

19. CONCLUSIONS 3.The F layer heights (in terms of the h’F) over Vietnam is in general systematically lower than those over Brazil which seems to be a main cause of the reduced spread F over Vietnam. 4.Post-midnight spread F is very frequent over Sao Luis showing higher occurrence rate than post- sunset events, during December and July months, whereas it is less frequent over Vietnam.  Further data analysis is to be pursued for a better clarification of some of these points.

20. Thank you for your attention! Special thanks are going to UNOOSA, NASA And Korea organization!