1. Conclusions In summary, this analysis of the topside sounder data from ISS-b leads to the following preliminary conclusions:  There is no apparent preference for midlatitude spread echoes to occur over continental land masses.  There are very large seasonal variations in the occurrence probability of midlatitude spreading over distinct geographic domains. These seasonal variations are largest over the oceanic regions.  The highest occurrence probability for midlatitude spread echoes is over the north Atlantic in the November-January period. The smallest occurrence probability is over the north Pacific, in the same interval.  Occurrence probabilities up to about 30% are quite common at all locales. Acknowledgments We thank Dr. T. Maruyama for the ISS-b data. The first author thanks Patrick Roddy for assistance. This work was supported by NASA grant NNG04WC19G Introduction Ionosonde signatures of spread echo conditions are not strictly limited to regions near the magnetic equator. A number of radar and satellite studies have shown that radio scintillation and large scale density irregularities in the F region plasma also occur at midlatitudes, although less frequently. Fukao et al. [1991] observed spread F type ionograms quite far from the magnetic equator, and Hanson and Johnson [1992] observed mid-latitude density perturbations at dip latitudes as high as 40 degrees using the AE-E satellite. Our focus in this work is to determine whether midlatitude spread echoes have any statistically significant seasonal or geographical variability. Future Work It may be interesting to compare the statistics we have derived here to global weather patterns. For example, the existence of monsoon zones in the equatorial zone in southeast Asia can be expected to launch copious quantities of gravity waves, which might in turn be expected to trigger outbreaks of spreading events. It may be fruitful to compare satellite observations of midlatitude gravity waves at F region heights to the occurrence probability plots shown here. We have begun a study of this nature using DE-2 data, but the results are not yet ready for such a detailed comparison. Seasonal and Longitudinal Variations of Midlatitude Topside Spread Echoes Based on ISS-b Observations A. M. Mwene, G. D. Earle, J. P. McClure William. B. Hanson Center for Space Sciences, University of Texas at Dallas References [1]Fukao, S., et al., Turbulent upwelling of the mid-latitude ionosphere: 1.Observational results by the MU radar, J. Geophys.Res., 96, 3725, 1991. [2]Hanson, W. B. and F. S. Johnson, Lower midlatitude ionospheric disturbances and the Perkins instability, Planet. Space Sci., 40,1615, 1992. [3]Maruyama, T., and N. Matuura, Global distribution of occurrence probability of spread echoes based on ISS-b observation, J. Radio Res. Lab., 27, 201, 1980. [4]McClure, J.P. S. Singh, D.K. Bamgboye, F.S. Johnson, and H. Kil,Occurrence of equatorial F region irregularities: Evidence for tropospheric seeding, J. Geophys. Res., 103, 29,119, 1998. Instrumentation and Coverage The topside sounder instrument from the ISS-b satellite is used as our diagnostic tool. The satellite provided useful data from August 1978 through December 1980, with intermittent tape recorder outages and data dump intervals resulting in roughly a 30% duty cycle. The satellite was inserted into a 70 degree inclination orbit, with apogee and perigee at 1220 km and 972 km, respectively. The 150 W topside sounder instrument used for this study covered the frequency range from 0.5-14.8 MHz in 0.1 MHz steps, with a receiver bandwidth of 6 kHz. Figure 1 shows the satellite coverage over the course of one season. The points on the map correspond to the locations at which topside ionograms were obtained. Midlatitude coverage is relatively good for all seasons except for the May-July solstice period. We have therefore omitted this interval from our analysis. Data Presentation Figures 2-4 show logarithmically scaled histogram plots of the Maruyama index values for each of the geographic regions defined in Table 1. Each of the figures corresponds to a different season; logarithmic axes have been used in order to highlight the regions on each graph for which the index value is greater than four. It is important to remember that the regions defined in Table 1 correspond to very different geographic areas (in km 2 ). However, it is valid to compare the seasonal variations for a given geographic area. In Figures 2-4 the left column of histograms corresponds to oceanic regions, and the right column corresponds to land masses. The seasonal variations become more apparent when the data from Figures 2-4 are presented as occurrence probabilities. These have been calculated as follows for each region: The occurrence probabilities as a function of season and geographic domain are presented in Figure 5. Discussion With reference to Figure 5, there are very large seasonal differences in occurrence probabilities for midlatitude spread echoes in the north Atlantic, south Atlantic, and north Pacific regions. Somewhat less striking seasonal variations are evident in Asia and Europe. The other geographic domains have much less pronounced seasonal variations. The occurrence of spread echoes over the north Atlantic region is particularly variable. This region shows the highest (November-January) and second lowest (August-September) occurrence probabilities. The overall occurrence probabilities for MSF are quite large when classified using the Maruyama and Matuura [1980] index. This may be caused by incursion of high and/or low latitude irregularities into the midlatitude domain. In general there are no differences between the number of spreading events occurring over land masses and over oceans. Table. 1.Definitions of the regions of interest. Fig. 1.Satellite coverage map showing regions of interest. Fig. 5. Topside spread echo occurrence probabilities as a function of season and location. Fig. 2. Maruyama and Matuura’s [1980] spread echo index variations for each region in Feb-Apr. Procedure Maruyama and Matuura [1980] describe the process of inferring a simple index corresponding to spread echo conditions from the ISS-b topside sounder data. Index values greater than four correspond to widespread regions of spread echoes. McClure et al. [1998] offer a good overview of this classification method, particularly as it applies to equatorial spread F. We use the Maruyama index in our analysis to identify regions at magnetic latitudes between ±20 and ± 50 degrees that have significant spreading. Table 1 shows the breakdown of the various geographic regions, and Figure 1 shows these regions on a world map. Fig. 4. Same format as Figure 2 for Nov-Jan. Fig. 3. Same format as Figure 2 for Aug-Oct. This is surprising, since it might be expected that more thunderstorms and subsequently more gravity wave seeding for spreading would be expected over land masses, where orographic features exist. The lack of such a correlation may be due to the fact that gravity waves can be ducted over very large horizontal distances, so that waves generated over land masses may propagate for thousands of kilometers before generating perturbations that lead to midlatitude spread echoes. Abstract A preliminary study of the seasonal and longitudinal variations of spread echoes from the Ionosphere Sounding Satellite (ISS) using the topside sounding data has been undertaken. Significant longitudinal and seasonal variations in midlatitude spread echoes are observed. The north Atlantic region has the highest occurrence probability in the winter solstice. The smallest occurrence is in the north Pacific in the same interval. Occurrence probabilities of up to about 30% are quite common.