4.1. Irregularities in the Equatorial Region
 In recent years, global distributions of equatorial plasma bubbles or density fluctuation irregularity structures from in situ measurements of ion or electron density variations have been reported by Oya et al.  and Watanabe and Oya  with the Hinotori data; by Kil and Heelis , and McClure et al.  with the AE-E data; and by Huang et al. [2001, 2002], and Burke et al. [2004a, 2004b] from data in the DMSP spacecraft series. There are other global distribution surveys from topside sounders such as by Alouette 1 and 2 [Calvert and Schmid, 1964; Muldrew, 1980] and by ISS-b [Maruyama and Matuura, 1980, 1984]. Among these, the AE-E results by Kil and Heelis , and by McClure et al.  have the complete global s/l distribution of the occurrence rate.
 The 5 years of ROCSAT-1 observations have resulted in an unprecedented high spatial resolution in a two-dimensional global distribution of the seasonal, longitudinal and latitudinal variations for the topside density irregularity occurrence rate shown in Figure 3. The seasonal changes in the occurrence rate due to solar variability and magnetic effects are also presented (Figures 6 and 8). Comparison of the ROCSAT result with the AE-E result of McClure et al.  is shown in Figure 4 for the reason that McClure et al. have shown that the AE-E result can reproduce all other published results from ISS-b observations [Maruyama and Matuura, 1980, 1984], Hinotori observations [Watanabe and Oya, 1986], and OGO 6 observations [Basu et al., 1976].
 Because the selection criterion adopted by McClure et al.  to study large irregularity patch in a scale size of ∼500 km is different from the selection criterion in the current report for tracing the exact spatial extent of the density irregularity structure for sizes from 7.5 to 75 km, a different occurrence rate on the density irregularity is noted in Figure 4. However, Figure 4 clearly indicates that the two global s/l distribution patterns in the occurrence rate have all the same major features. Many similar longitudinal variation of increase or decrease in occurrence rate in the two results can be identified for every season. However, there are also differences. Some noticeable differences are listed in the following. The first one is in the Pacific region during the June solstice. The peak occurrence rate from the AE-E result is located west of longitude 180°, while the peak of the ROCSAT result is located slightly east of longitude 180°. The second difference is the location of the maximum occurrence rate during the December solstice. The ROCSAT data indicate a single maximum of occurrence rate at longitude −35°, while the AE-E data indicate a dip at this location but is surrounded by two maxima at longitude −10° and at −50°. A final point of difference is the existence of finite occurrence rates in the Indian sector in comparison with other longitude locations observed by ROCSAT during the two equinox seasons. All these differences can be resolved from the fact that ROCSAT observation is composed of 5-year data set so that better statistics has been resulted. There are many small peaks and valleys in the s/l distribution of the irregularity occurrence rate in the ROCSAT yearly result. However, when the 5-year data are averaged into Figure 4, many small peaks and valleys are smoothed out. Though not shown here, we have noticed that the s/l distribution of equatorial density irregularity for the year 2000 indicates that the second peak of high occurrence in the Pacific region during the June solstice is located west of longitude of 180° similar to the AE-E result. For other years, the peak shifts to east of 180° during the June solstice. As for the case of a single peak versus double peaks in the maximum occurrence rate in the South American-Atlantic region during the December solstice, the ROCSAT data in 2003 indicate double peaks as in the AE-E result. The rest of ROCSAT data indicate a single maximum peak in the December solstice. The case of a single maximum peak at longitude –35° has also been shown by McClure et al.  in their replot of ISS-b data of Maruyama and Matuura  and in the Hinotori data of Watanabe and Oya . Thus many small differences between the ROCSAT result and the AE-E result can be resolved by the argument of statistics. The other possibility is that there are in deed some subtle differences between the existence of mesoscale density irregularity and the intermediate-scale density irregularity in the topside ionosphere, for which the cause and effect in manifesting the density irregularity should be studied in detail. However, this falls outside the scope of current report in which the emphasis is the contrasting properties of the equatorial density irregularity versus the midlatitude irregularity.
 Therefore we can conclude that the ROCSAT observation of the global s/l distribution of equatorial irregularity structure is indeed the same as what has been observed by AE-E two decades ago. On the basis of different approaches of counting the irregularity structures in two different scale sizes, one can also state that irregularity structures of different scale sizes will manifest the same global s/l distribution from the end result of multistage cascading process of the gravitational Rayleigh-Taylor instability. The height difference sampled by AE-E at 375 to 400 km and by ROCSAT-1 at 600 km as well as the choice of the σ value is irrelevant in the statistical study of the irregularity occurrence distribution. It further implies that the global variation in the seeding and growth conditions for the instability process that results in major features in global irregularity pattern seems to persist for past 25 years. There is still no consensus answer to what background condition should be.
 Now return to the result of Figure 3. With the impressive visual presentation for the global distribution of density irregularity pattern, one can immediately conclude that the equatorial density irregularity is evidently confined within a band between the two Appleton crests around ±15° in dip latitude. We have been using the term “equatorial density irregularities” to describe the density irregularities that are confined within ±15° in dip latitude. However, in Figure 5, we realize that the latitudinal extent of the equatorial irregularities can reach to ±30°. This is quite farther outside the Appleton crest as was initially thought.
 The s/l variations of the occurrence rate for equatorial density irregularities as well as the shifting of high occurrence rate in longitude are clearly noticed in Figure 3. The high occurrence rate in longitude is seen to move from around 0° during the March equinox to around 30° in the June solstice. It then moves back to 20° in the September equinox and settles at −30° during the December solstice. The result of high occurrence rate in longitude regions between −60° and 0° (South America and Atlantic region where the magnetic declination is highly negative) during the December solstice supports the theory that alignment of the sunset terminator with the magnetic meridian as one major cause for the irregularity occurrence [Tsunoda, 1985; Abdu et al., 1981, 1982]. Moreover, this region happens to be near the South Atlantic Anomaly region so that the enhanced vertical drift due to low magnetic field has also been thought as the cause [Huang et al., 2001]. However, neither theory can explain why the magnetic declination and strength do not play any important role in the high occurrence rates of irregularities in longitude regions between 0° and 60° in the June solstice as well as during the two equinoxes. Since the ROCSAT observation reproduces the AE-E result, control of the seasonal variation in global equatorial irregularity occurrence pattern may still be laid in the seeding perturbations of atmospheric source such as from the Intertropical Convection effect in atmosphere as was proposed by McClure et al. . We do not think we can add any further information in this report regarding to the cause of global s/l distribution of equatorial irregularities.
 The current report, however, provides new additional statistical results of the magnetic and solar variability effects on the global distribution of equatorial irregularity occurrences. In Figure 6, we noticed that the irregularities occur more frequently during the quiet time than during the disturbed time. Such outcome can be realized through the understanding of suppressing the postsunset prereversal enhancement during the disturbed period [Fejer, 1991; Fejer and Scherliess, 1997] as the growth rate of an equatorial density irregularity is related to the enhanced zonal electric field which is related to the prereversal enhancement. However, it is noted that during the disturbed periods the occurrence locations of density irregularities will spread outside the Appleton crests. In addition, more longitudinal spread in the occurrence pattern is also noted during the disturbed periods in the two solstice seasons. Thus in some local region, the irregularity occurrence does indeed seem to be increased in comparison with the quiet time observation. However, the overall global distribution of the occurrence rate is still lower during the disturbed period than in the quiet time.
 On the other hand, the probability of occurrence increases with solar activity. This can also be understood from the fact that the atmospheric driver for the zonal electric field is stronger during high solar activity period to enhance the growth in the instability process. However, there are data from the AE-E observations indicated that more cases of equatorial irregularities around the F-peak are observed during low solar activity period than during high solar activity period (B. Fejer, personal communication, 2005). Such opposite observations should be noted from the fact that the AE-E data used in the study comprise only finite numbers of observations. The long-term statistics would prove otherwise as indicated in the current report. In fact, the morphological study of gigahertz equatorial scintillation experiments carried out by Feng and Liu  has indicated that the occurrence of gigahertz scintillation decreases with magnetic activity but increases with solar activity. A high occurrence of deep scintillation within the Appleton crests during a year of high solar activity than during a year of low solar activity has also been reported by Aarons . Thus our result of solar variability effect on the occurrence of equatorial irregularities agrees with past results of scintillation experiments.
 The latitudinal variation shown in Figure 5 clearly indicates the existence of a latitudinal demarcation that separates the irregularities in the equatorial region from that at midlatitudes. Although such demarcation is inferred from the observations of midlatitude irregularities in two longitude regions only, between −30° and 190° in the Southern Hemisphere and between 185° and 340° in the Northern Hemisphere, there is no reason to believe that density irregularities at midlatitudes in other longitude regions outside the ROCSAT coverage will behave differently because the global distribution of equatorial irregularities has already tapered off significantly at dip latitudes of ±20° and terminated at about ±30°. Even with limited data in midlatitude coverage from the ROCSAT observation, we still can conclude that there exist two different distributions of intermediate-scale density irregularities, the equatorial region versus the midlatitude. The two distributions behave similarly in the seasonal variation as seen in Figure 5, but oppositely under magnetic effect as seen in Figures 6 and 7, in solar variability effect as seen in Figures 8 and 9, and in local time distribution in Figure 13. Table 1 summarizes the different characteristics of the density irregularities in the equatorial region versus at midlatitude in the current report. The result implies that the density irregularities in the equatorial region and at midlatitude belong to two different populations of intermediate-scale density irregularities at topside ionosphere.
Table 1. Summary of Characteristics for Density Irregularities in the Equatorial Region and at Midlatitudes
| ||Density Irregularities in Equatorial Region||Density Irregularities at Midlatitudes|
|Local time distribution||peaks before midnight at ∼2100 LT||broad peak after midnight at ∼0300 LT|
|Seasonal effect||slightly more during equinoxes and different longitudinal distribution||more in winter hemisphere|
|Geomagnetic effect||more occurrences during low-Kp period||no effect|
|Solar activity effect||high solar activity period has more||low solar activity period has more|
 The cause for terminating an equatorial density irregularity at about ±30° in dip latitude is explained in the following. As the Rayleigh-Taylor instability develops in the equatorial bottomside ionosphere, the flux tube that contains low-density plasma from bottomside ionosphere begins to rise. To maintain adequate growth of the instability, the growth rate γ in equation (1) should be positive. In the study by Sultan  with modeled ionospheric and atmospheric parameters, he has shown that the height integrated density N begins to decrease above 900 km in altitude that is about ±20° in dip latitude. As the growth rate γ becomes negative above 900 km, one will see the decaying bubble only. Thus, according to the model, one should not observe a rising bubble above ±20° in dip latitude. The height below 900 km has also been quoted by Singh et al.  as the most likely regions to observe plasma bubbles. The statistical result of the ROCSAT observation indicates that the occurrence of equatorial irregularity structure decreases significantly at ±20° and terminates at about ±30° as indicated in Figure 5. This put the maximum height for the existence of an equatorial irregularity structure (that is related to an equatorial bubble) at about 2000 km. Such conclusion confirms the topside ionogram results reported from old Alouette 1 and 2 observations. Though limited with only 15-month of Alouette-1 data, Calvert and Schmid  reported that the occurrence rate of aspect-sensitive scattering events from thin field-aligned irregularity in the equatorial region terminates at about ±30° in the 2000–2100 LT sector. The Alouette 2 results reported by Muldrew  indicated that from a statistical study of the propagation of ducted echoes in the topside ionogram, plasma bubble irregularities related to the ducted echoes (due to thick field-aligned irregularities) are limited to altitudes lower than L ≲1.2 (dip latitude ≲24°). Although the current conclusion for the maximum apex height of an equatorial plasma bubble reaching to 2000 km is somewhat higher than past observations of 1100 km [Maruyama and Matuura, 1984], or 1400 km [Mendillo and Tyler, 1983], it is still lower than some other observations of 3500 km [Burke et al., 1979] or 2500 km [Sahai et al., 1994]. Nonetheless, the statistical ceiling height of an equatorial bubble seems to be set at 2000 km altitude.
 Following this argument, we draw a picture depicted in Figure 14 to show a morphological development of an equatorial plasma bubble observed by the traversing ROCSAT-1 at a constant height of 600 km from equatorial region to midlatitude. Observation of midlatitude density irregularity structures by ROCSAT is also illustrated in Figure 14. A similar graphic illustration with multiple flux tubes of filled plasma bubbles has been presented by Whalen  to indicate consecutive observations of equatorial spread F events from ground stations distributed in dip latitude. An immediate implication of Figure 14 is that the equatorial plasma bubble structures belong to a population of density irregularity that is different from the midlatitude irregularity.
Figure 14. Schematic presentation of the evolution of the equatorial density depletion irregularities versus midlatitude irregularities as detected by ROCSAT at 600 km altitude.
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 The property of equatorial density irregularity from the triggering mechanism shown in equation (1) has been well documented in the literature. For example, Sultan  has used this equation to demonstrate how the irregularities should be distributed in s/l and local time occurrence patterns. The new additional properties of the occurrence dependences on magnetic and solar variability effects could also be examined with the aid of equation (1). However, this will not be carried out here. Instead, we shall focus on the newly observed properties of midlatitude irregularities obtained by ROCSAT.
4.2. Irregularities in the Midlatitude Regions
 Midlatitude ionospheric irregularities have long been studied with echoes in ionosonde [Bowman, 1960, 1985], by propagation properties in scintillation experiments [Yeh et al., 1968; Rodger, 1976; Rodger and Aarons, 1988], or with echoes in coherent radar observation [Behnke, 1979; Fukao et al., 1991]. The first direct in situ measurement in space was made by AE-E in a report by Hanson and Johnson . The observed mesoscale (50 to 1000 km) density depletions varied in phase with the radial flow motion when AE-E was located at ∼260 km altitude in bottomside ionosphere. This observation was interpreted to be related to the Perkins instability [Perkins, 1973]. Similarly, observations of ionospheric band-like motion by Arecibo radar [Behnke, 1979] and F region field-aligned irregularities (FAI) by MU radar [Fukao et al., 1991; Kelley and Fukao, 1991] have also been interpreted with the Perkins instability. On the other hand, observations of midlatitude electric field fluctuations (MEFs) made with Dynamic Explorer 2 (DE 2) were explained with the field-aligned current flowing between the conjugate ionospheres without resorting to any instability process [Saito et al., 1995]. No clear relationship between the MEFs and the midlatitude spread F has been established in that report. The local time distribution of MEFs' occurrence pattern that is peaked after midnight is very similar to the distribution pattern of high occurrence rate from the ROCSAT observation. However, it needs to be emphasized again that the current ROCSAT observation of the midlatitude irregularities starts from the dip latitude of ±30°. This is a different distribution of the intermediate-scale density irregularity after the termination of the equatorial irregularity at ±30°. It is also different from the conventional understanding of midlatitude irregularities in which the occurrence rate peaks at ∼±30°, as in the case of MEF observation by DE 2. Therefore the difference between the property of midlatitude irregularity in this report and in some past observations should be expected.
 The statistical property of midlatitude irregularities has not been established because not many cases of midlatitude irregularities have been studied with large amount of data. From what is available in the literature (as to our best knowledge), we construct Table 2 to compare properties of midlatitude irregularities observed by different experiments. It is noted that the scale size of the observed midlatitude irregularity in the table is an important factor when comparison is made. Different triggering mechanisms could be related to different midlatitude irregularities of various scale sizes. Furthermore, we have excluded the observations of traveling ionospheric disturbances (TIDs) in Table 2 because we are only concerned about the phenomena of scale lengths in ∼50 km or less. However, it should be kept in mind that the TIDs might be the root cause of many observations in Table 2. Another important feature noted in Table 2 is the lower limit in dip latitude for observing midlatitude irregularities in different experiments. It seems that many experiments have observed the midlatitude irregularities around the dip latitude of ≳30°. This dip latitude is higher than the limiting dip latitude of ∼18° for propagating the medium-scale (50 to 1000 km) TIDs in a report by Shiokawa et al. . This lower limit in dip latitude for the MTIDs does not contradict the results in Table 2 because phenomena of different scale sizes are addressed. In fact, ROCSAT has observed many cases of mesoscale density and flow undulations as well as the plasma blob events at about 20° in dip latitude. These ROCSAT observations are not included in the current report from the autosearch result of intermediate-scale irregularities. They are presented in a separate report (S.-Y. Su et al., preprint, 2006) for the study of the mesoscale density and flow undulations in conjunction with the intermediate-scale density irregularities.
Table 2. Occurrence Characteristics of Nighttime Midlatitude Irregularities Observed by Different Experiments
| ||FAI||MEFc||Density Irregularitiesd||Sporadic Ee (Spread Es)||Scintillationf|
|Topside Soundera||Ground Radarb|
|Scale length||∼12 to 30 m||3 m||∼4 to 40 km||7.5 to 75 km|| || |
|Lowest dip latitude or observation latitude||<30°||at 29.3°||broad peak between 25° and 40°||<30°||∼30°|| |
|Seasonal variation||dominant when sunset terminator is aligned with magnetic meridian||more in summer season||no dependence||more in winter hemisphere||more in summer hemisphere||more in summer season|
|Solar variability effect|| ||more in low solar activity period||no correlation||more in low solar activity period||more in low solar activity period|| |
|Magnetic conditions||no correlation|| || ||no correlation||more during Kp ∼2 period||no correlation|
|Local time dependence||high occurrence after midnight||high occurrence after midnight||high occurrence after midnight||high occurrence after midnight||no definitive conclusion|| |
 Even though many midlatitude irregularities have been explained with the model of the Perkins instability, not all observations fit the theoretical prediction such as the motion of field-aligned irregularities observed by MU radar [Kelley and Fukao, 1991]. Furthermore, it has been noted by many investigators [see, e.g., Kelley and Fukao, 1991; Tsunoda et al., 2004] that the growth rate of the Perkins instability is too low to produce any significant irregularity structure to be observed in F region. On the other hand, many recent papers on the instabilities of the sporadic E layers [Cosgrove and Tsunoda, 2001, 2002, 2003; Tsunoda and Cosgrove, 2001; Haldoupis et al., 2003; Tsunoda et al., 2004] indicate that the electrodynamically coupled E and F regions will produce F region structure more rapidly than by the Perkins instability acting alone. If the current ROCSAT observation of midlatitude irregularities is related to the sporadic E layer instability, then the occurrence condition of the sporadic E layer instability can be inferred from the current report. One important feature for the occurrence of the sporadic E layer instability lies in its relationship with the wind shear condition of the neutral winds. The wind shear condition might be related to the solar variability as indicated in the report of Bowman  in which more sporadic E instability (spread Es event) occurrence is observed during the solar minimum year of 1953 than during the solar maximum year of 1949 and 1957. This would imply that the midlatitude irregularities are more likely to occur during a low solar activity period than during a high solar activity period as indicated from the ROCSAT observations.
 Finally, as the roughness of density irregularity structure at midlatitudes is concerned, we noticed with reference to Figure 2 that the spatial extent in tracing the midlatitude density irregularity decreases rapidly when the σ value increases as compared with the case of equatorial density irregularity. Thus it seems that the midlatitude irregularity is not as rough in density structure as the one in the equatorial region. From the scintillation experiments reviewed by Aarons , it is also learned that the midlatitude scintillation activity is not as intense as that encountered in the equatorial region.