Equatorial plasma bubbles observed by DMSP satellites during a full solar cycle: Toward a global climatology

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A12, 1434, doi: /2002ja009452, 2002 Equatorial plasma bubbles observed by DMSP satellites during a full solar cycle: Toward a global climatology C. Y. Huang, 1 W. J. Burke, 2 J. S. Machuzak, 2 L. C. Gentile, 1 and P. J. Sultan 2 Received 19 April 2002; revised 4 June 2002; accepted 18 June 2002; published 11 December [1] We have examined more than 75,000 latitudinal profiles of plasma densities measured by ion detectors on five Defense Meteorological Satellite Program (DMSP) satellites in the evening local time (LT) sector between 1989 and This survey established detection frequencies of equatorial bubbles (EPBs) at 840 km over the recent solar cycle. The annual rate of EPB detections decreased by more than an order of magnitude from >1000 during solar maximum to <100 during solar minimum years. EPB data were divided into 24 longitude sectors to determine seasonal and solar cycle variability in rates of encounter by DMSP. During the ascending and descending portions of the solar cycle, each longitude sector showed repeatable seasonal variations. The envelope of seasonally averaged rates of EPB encounters resembles the solar cycle variability for similar averages of the F 10.7 index. On both global and longitude sector scale sizes, annual rates of EPB encounters correlate with the yearly averages of F 10.7.We also find that throughout the solar cycle the EPB detections were overrepresented during times of high geomagnetic activity signified by Kp 5. During solar minimum years, about one third of the EPBs occurred when traces of the Dst index had significant negative slopes (ddst/dt 5 nt/hr). This suggests that electric field penetration of the inner magnetosphere is responsible for driving many EPBs. Comparisons of plasma and neutral density profiles in the evening sector, calculated using the Parameterized Ionospheric Model (PIM) and MSIS-86 Model, indicate that the height of the bottomside of the F layer is >100 km lower during solar minimum than solar maximum. However, the overall effect is to increase the growth rate of the Rayleigh Taylor instability at solar maximum in the bottomside F layer only by about a factor of 2. We suggest that the variability of electric fields in the postsunset equatorial ionosphere is the source of the observed discrepancy between EPB detections under solar maximum/minimum conditions. INDEX TERMS: 2439 Ionosphere: Ionospheric irregularities; 2162 Interplanetary Physics: Solar cycle variations (7536); 2415 Ionosphere: Equatorial ionosphere; 2411 Ionosphere: Electric fields (2712); KEYWORDS: plasma bubbles, solar cycle Citation: Huang, C. Y., W. J. Burke, J. S. Machuzak, L. C. Gentile, and P. J. Sultan, Equatorial plasma bubbles observed by DMSP satellites during a full solar cycle: Toward a global climatology, J. Geophys. Res., 107(A12), 1434, doi: /2002ja009452, Introduction [2] Three recent programmatic developments have spurred renewed interest in equatorial spread F and attendant phenomena. These are (1) the launch of the Republic of China Satellite (ROCSAT 1) in January 1999, (2) the upcoming launch of the Communication/Navigation Outage Forecasting System (C/NOFS) satellite in the fall of 2003, and (3) the initiation of the National Aeronautics and Space Administration (NASA) Living with a Star (LWS) program. 1 Institute for Scientific Research, Boston College, Chestnut Hill, Massachusetts, USA. 2 Space Vehicles Directorate, Air Force Research Laboratory, Hanscom AFB, Massachusetts, USA. Copyright 2002 by the American Geophysical Union /02/2002JA Flying in a low-inclination orbit at an altitude of 600 km, ROCSAT 1 regularly encounters equatorial bubbles (EPBs) in the evening local time (LT) sector. Chen et al. [2001] describe high-resolution measurements of plasma densities and velocities within bubbles crossed by ROCSAT 1. Measured scale sizes, extending from tens of kilometers to meters, span the longitudinal dimensions of bubbles down to those of plasma irregularities responsible for radio wave scintillation and radar backscatter. C/NOFS will also fly in a low-inclination orbit with apogee and perigee at 700 and 450 km, respectively. Sensors on C/NOFS will monitor such physical parameters as plasma density/velocity, electric fields, and neutral winds believed to trigger EPBs. By initiating the LWS program, NASA s Office of Space Science shifted from its traditional emphasis on geospace exploration to considering practical consequences of space plasmas, including the disruptive effects of EPBs on communications systems. SIA 7-1

2 SIA 7-2 HUANG ET AL.: EQUATORIAL PLASMA BUBBLES DURING A FULL SOLAR CYCLE [3] Huang et al. [2001] examined distributions of EPBs observed by polar-orbiting Defense Meteorological Satellite Program (DMSP) satellites in the evening sector during the solar maximum years 1989 and The study s two main results concerned EPB dependencies on season/longitude and on levels of geomagnetic activity. At topside altitudes the preponderance of EPB activity occurs during both equinoxes over a wide range of longitudes. In the Atlantic African sector, where the magnetic declination is westward, the maximum rate of EPB detection occurred near the December solstice. A significantly lesser peak in the rate of EPB activity occurred in the Pacific sector (eastward declination) near the June solstice. Seasonal differences have been largely explained by Tsunoda [1985] who suggested that bubble formation is favored when magnetic field lines are closely aligned with the terminator. At these times the flux tube integrated conductivity diminishes most quickly after sunset at low magnetic latitudes. The seasonal distribution and rate of occurrence observed by DMSP satellites qualitatively agreed with those of range spread F observed using ground-based sensors as reviewed by Aarons [1993]. [4] McClure et al. [1998] have proposed an alternative explanation of observed season versus longitude variations based on the seasonal location of the inter tropical convergence zone of tropospheric winds. Their contention is based on probabilities for the Atmosphere Explorer E (AE- E) satellite encountering equatorial F region irregularities (P EFI ). The AE-E database was accumulated during the solar maximum period. They suggest that P EFI = P seeds P inst where P seeds is the probability of having gravity wave induced seeds for spread F irregularities in the postsunset ionosphere and P inst represents the probability that the bottomside F layer is Rayleigh Taylor unstable. Observations of unusually frequent irregularity encounters by OGO 6 in the sector during November December 1969 and 1970 [Basu et al., 1976] are cited as supporting the seeding hypothesis. Gravity wave activity would be driven by El Niño heating of the central Pacific Ocean. Measurements by DMSP satellites during two El Niño years can be used to test this conjecture. [5] Huang et al. [2001] further showed that, contrary to previous reports [Aarons, 1991], EPB occurrences were overrepresented during periods of high (Kp 5) geomagnetic activity. They also noted phase-dependent characteristics of EPB occurrence during geomagnetic storms. Large plasma depletions were frequently observed during the growth and main phases of storms but seldom during recovery phases. This behavior is consistent with predictions of a model proposed by Scherliess and Fejer [1997] and Fejer and Scherliess [1997]. During the early phases, stormtime electric fields penetrate the inner magnetosphere. In the dusk sector, the mapped fields have eastward components [Wolf, 1970; Nopper and Carovillano, 1978] that increase the linear growth rate of EPBs [Ott, 1978; Anderson and Haerendel, 1979]. After several hours, injected ring current ions shield the inner magnetosphere [Harel et al., 1981]. On longer timescales, Joule heating at auroral latitudes stirs neutral winds that drive a counter dynamo generating westward (eastward) electric fields in the evening (postmidnight) sector [Scherliess and Fejer, 1997; Fejer and Scherliess, 1997]. [6] It is not our contention that the DMSP satellites flying in circular polar orbits in a fixed LT meridian sample all, or perhaps even most, EPBs. Scintillation [Groves et al., 1997] and radar [Hysell and Burcham, 1998] measurements indicate that relatively few bubbles reach the altitude of DMSP satellites. Plasma data from the Atmospheric Explorer E (AE-E) satellite in a low-inclination orbit show that EPBs can be spread out over a large range of LTs in the evening sector not sampled by DMSP [Hanson and Bamgboye, 1984]. Tsunoda et al. [1982] analyzed measurements by the ARPA long-range tracking and instrumentation radar (ALTAIR) acquired during three AE-E overflights of Kwajalein to show that the structure of bubbles, perpendicular to the local magnetic field are wedge shaped, extending upward from their bottomside source regions. [7] For understanding possible sampling biases, we note that DMSP F9 crossed the magnetic equator in the evening sector moving toward the southwest. All other satellites used in this study crossed the equator moving toward the northwest. Since EPBs align with the Earth s magnetic field, the highest probability of intercepting bubbles should occur in longitude sectors where DMSP satellites cross the largest range of magnetic longitudes during evening equatorial passes. If bubbles were evenly distributed in longitude, the F9 satellite, moving to the southwest, should have detected more EPBs in the Atlantic sector where the magnetic declination is westward. The other DMSP satellites used in this study, moving to the northwest, should have detected more bubbles in the eastern Pacific where the magnetic declination is eastward. The fact that all the DMSP satellites detected significantly more EPBs in the Atlantic sector and agreed with the characteristics of range spread F at a number of ground stations [Aarons, 1993] suggests that the statistical distributions reported here are representative of global distributions. [8] This report includes DMSP observations of EPBs from 1989 through the first quarter of The database thus covers a full solar cycle from solar maximum in 1991 through minimum in 1996 and back to maximum again in This paper has three objectives: (1) to verify that results from the earlier solar maximum conditions are representative, (2) to quantify solar cycle effects on EPB occurrence frequency, and (3) to compare EPB climatologies from DMSP with those derived from ground-based studies at various longitudes. Huang et al. [2001] conducted a similar comparison using ground measurements reviewed by Aarons [1993] as a benchmark. In recent years a network of ground-based detectors called the Scintillation Detection Aid (SCINDA) [Groves et al., 1997] has been installed at multiple locations around the world to support the C/NOFS mission. A companion paper by Burke et al. [2002] compares a subset of EPB detections reported here with seasonal variations of the S 4 index [Briggs and Parkin, 1963] for VHF signals measured near the magnetic equator at Ancon, Peru (11.79 S, E, dip latitude 0.9 ) between 1994 and Eventually we plan to compare DMSP observations of EPB occurrence rates with measurements from all of the SCINDA stations as their databases grow. 2. Satellite Data Selection and Observations [9] We have examined plasma density measurements from multiple DMSP satellites to establish a solar cycle climatology of EPB occurrence in the evening sector at

3 HUANG ET AL.: EQUATORIAL PLASMA BUBBLES DURING A FULL SOLAR CYCLE SIA 7-3 Table 1. EPB Encounters Year/satellite Orbits EPBs M-0 M-1 M-2 M /F /F /F /F /F /F /F /F /F /F /F /F /F /F /F /F /F topside altitudes. Nominally, DMSP satellites orbit the Earth in 98.7 inclined polar orbits at an altitude of 840 km. With a period of 104 min, each satellite completes 14 orbits per day or >5000 orbits per year. All DMSP satellites fly in Sun-synchronous orbits near either the or the LT meridians. In this study we consider plasma densities sampled at low magnetic latitudes in the evening sector from 1930 to 2130 LT. This corresponds to the prime LT sector for the triggering of ionospheric irregularities [Valladares et al., 1996]. [10] Table 1 summarizes the relevant DMSP flights by year, number of evening sector overpasses, and total number of EPBs observed, and also includes a breakdown of EPBs by the depth of plasma depletion. The database begins in 1989 as the solar cycle approached maximum, continues through solar maximum in 1991, solar minimum in 1996, and a second, as yet incomplete, maximum in In some years observations from more than one DMSP satellite were available and were included in the database to provide maximum coverage. In 1994 and 1997 new launches replaced older satellites so two DMSP flights are listed. In 1991, 1998, and 2000, complete coverage was obtained from two satellites for the entire year. Notation in Table 1 follows the EPB classification defined by Huang et al. [2001] to characterize ratios of ambient plasma density outside of a bubble to the minimum density inside it. M-0 corresponds to a plasma density ratio n 2; M-1 corresponds to 2 < n 10; M-2 to 10 < n 100; and M-3 to n > 100. Examples of the four categories are given by Huang et al. [2001, Figure 1]. [11] In general DMSP satellites make 5000 equatorial crossings each year in the evening LT sector. For satellites with nearly full-year coverage the total number of detected EPBs ranged from 73 near solar minimum (1996/F12) to 1112 near solar maximum (1989/F9), more than an order of magnitude increase. Since the rate of occurrence of range spread F detected in the 21 LT sector varies by a factor of 2 3 from solar maximum to solar minimum [Abdu et al., 1985], this result is curious. Data in Table 1 also show that deep (M-2 and M-3) bubbles are almost exclusively solar maximum phenomena. The deepest EPBs tend to occur in the early evening sector and move upward with high (>800 m/s) velocities [Hanson et al., 1997]. We also note that in 1991 DMSP F10 was launched into a slowly precessing orbit. Prior to the time when the orbital plane reached 1930 LT, F10 detected EPBs at a substantially less frequent rate than F9 during the early months of 1991 [Huang et al., 2001]. [12] We have sorted the DMSP data acquired over the full solar cycle into LT bins of 1 hour longitudinal extent and quarterly intervals centered on the equinox (March and September) and solstice (June and December) months. Figures 1a and 1b show the rates of EPB encounters by the various DMSP satellites over the solar cycle. The data were normalized by the number of evening sector equatorial crossings in a given period and are presented as percent of encounters. Data are plotted on a common scale so that occurrence frequencies at the different longitudes can be easily seen. We have averaged our longitudinal database into 24 hour-long sectors and plotted each sector for the entire solar cycle. In each panel can be seen variation in the distribution of bubbles during the solar cycle from a maximum during solar maximum ( ; 2000) to a minimum at solar minimum ( ). [13] In addition the variation with longitude can also be seen from the difference between the maximum percent occurrence in each plot. A common scale with a maximum of 100% has been used in the set of panels. The first panel centered at 7.5 E shows a maximum occurrence of 80%. Scanning down the panels which advance eastward in longitude it can be seen that the occurrence is high across the Atlantic and Africa ( E), falling to a low across the Pacific and Asia ( E). This trend repeats during the entire solar cycle. Further, upon closer examination it is also clear that within each year there are seasonal variations which agree with our earlier results [Huang et al., 2001]. As before maxima generally occur during equinox quarters. Exceptions to this occur in the longitudinal sector from E to E where maxima occur during the December solstice. [14] The effect of the solar cycle is apparent within each panel when the solar cycle average is compared with the quarterly average. There is a broad minimum from 1994 to Maximum occurrence frequencies generally coincide with the F 10.7 (solar radiation flux at 10.7 cm wavelength) index maxima during and again in In some sectors the maximum occurrence peaked in 1992 (see panels E). [15] To illustrate the season/longitude effect we show in Figure 2 the results from a sector in the Pacific, at E and contrast that with a sector in the Atlantic, at E, close to Ascension Island (345 E). First the difference in percent occurrence frequency is obvious over the entire solar cycle. Second the seasonal difference can be seen by examining the first two points in each panel. The first point in all the panels of Figure 1 corresponds to the August September October, or September equinox average in The second point corresponds to the November December January or December solstice average in 1988/ In the Pacific the maxima occur at the equinoxes so the solstice average is a minimum. In the Atlantic the maxima occur during the December solstice so the second point is a maximum. [16] In Figure 3 we show the annual average value of the F 10.7 index in standard flux units, compared with the annual

4 SIA 7-4 HUANG ET AL.: EQUATORIAL PLASMA BUBBLES DURING A FULL SOLAR CYCLE Figure 1. Percent of EPB encounters by DMSP satellites during the recent solar cycle, plotted as quarterly averages in 24 longitudinal sectors. The satellite data were acquired within 7.5 longitude of the listed longitude sector.

5 HUANG ET AL.: EQUATORIAL PLASMA BUBBLES DURING A FULL SOLAR CYCLE SIA 7-5 Figure 2. Percent of total EPB encounters by DMSP satellites plotted for 2 hours of LT during the recent solar cycle. Not only is there a large longitudinal difference but there are also repeated seasonal differences throughout the solar cycle. average percentage occurrence frequency of EPBs for the period (see Table 1). The different flights on which the bubbles were observed are indicated by different symbols. The correlation over the solar cycle is striking. Note that the maximum EPB occurrence was higher during the peak than during the peak, in agreement with the decrease in F 10.7 flux during the same times. [17] To quantify the solar cycle effect on bubble activity, Figure 4 contains a scatterplot of the rate of EPB encounters as a function of annual averages of hf 10.7 i. To avoid distortions caused by precession of the DMSP F10 orbit in 1991, we have removed its 669 EPB encounters in 4751 orbits from the data plotted in Figure 4. A linear regression analysis indicates that P B the probability of detecting a bubble for a given level of hf 10.7 i is given by P B ¼ 10:9 þ 0:16 hf 10:7 i with a regression coefficient of We emphasize that Figure 4 compares yearly averaged quantities and says nothing about EPB responses to daily variations of F This high correlation between annual variations in the global rate of EPB encounters with hf 10.7 i led us to make similar comparisons in each of the 15 longitude sectors. For consistency we again eliminated F10 measurements in The results of correlation analyses for all longitude bins are presented in numerical form in Table 2. From left to Figure 3. Scatterplot of annual percent of total EPB encounters by DMSP satellites plotted as a function of yearly averaged value of the hf 10.7 i index during the recent solar cycle.

6 SIA 7-6 HUANG ET AL.: EQUATORIAL PLASMA BUBBLES DURING A FULL SOLAR CYCLE Figure 4. Correlation between percent of EPB encounters by DMSP satellites and yearly averaged value of the hf 10.7 i index during the past solar cycle. right the columns of Table 2 give the central longitude of the bin, followed by the intercept, the slope, and the regression coefficient. Not surprisingly, the highest regression coefficients were found in the Atlantic African sector and the lowest in the Pacific sector. The regression coefficients exceeded 0.8 except in the Pacific sector, which was characterized by low EPB encounter rates. [18] It should be remembered that our EPB classification is based solely on ambient plasma densities inside and outside depletions. As discussed below, DMSP measured significantly higher ambient densities during solar maximum than during solar minimum years. A comparison with UHF scintillation data also shows a similar solar cycle effect [Burke et al., 2002]. However, the difference is more dramatic for EPBs detected by DMSP satellites. The maximum to minimum occurrence ratio for EPBs is about 40, while that for scintillations is about 3 4. Radar observations rely on the ratio dn/n, and both quantities experience analogous variations over the solar cycle [Fejer et al., 1993]. [19] McClure et al. [1998] suggested that El Niño heating of the central Pacific Ocean could act a source of gravity waves to seed the unusually high frequency of equatorial irregularities detected by the OGO-6 satellite during November December 1969 and 1970 [Basu et al.,1976]. Data presented in Figure 5 can be used to test this conjecture. The bottom panel of the figure essentially reproduces data given by McClure et al. [1998, Figure 3B]. The dash and solid lines represent cubic spline fits to measurements read off the figure. Respectively, they show probabilities for OGO-6 and AE encountering equatorial F region irregularities near December solstices, plotted as functions of geographic longitude. Both data sets were accumulated near solar maximum. Attention is directed to the local maximum in the OGO-6 measurements centered near 180. Corresponding AE-E data show a broad valley of low activity across the Asia Pacific sector. The upper panel of Figure 5 plots the percent occurrence of EPBs detected by DMSP in November December 1991 (solid line) and January 1992, as well a November December 1997 (dash line) and January The 1991 (1997) data were acquired near solar maximum (minimum). Both were accumulated during periods of high El Niño levels. We note that (1) in the Pacific sector DMSP measurements closely resemble those from the AE-E satellite and (2) under solar maximum conditions DMSP and AE-E measurements agree in the South American and Atlantic sectors. Plasma irregularity occurrences observed with OGO-6 are significantly larger than those reported from AE-E and DMSP. We have examined all DMSP measurements from the Pacific sector Table 2. Percent of EPB Encounters Versus hf 10.7 i Longitude Intercept Slope Correlation

7 HUANG ET AL.: EQUATORIAL PLASMA BUBBLES DURING A FULL SOLAR CYCLE SIA 7-7 Figure 5. Percent of EPB encounters by DMSP satellites during the December solstices of 1991 and 1997 (top), high El Niño intervals, compared with results of McClure et al. [1998] and Basu et al. [1976]. but failed to identify plausible signatures of El Niño effects on the rate of EPB detection. Results of our investigation appear to be inconsistent with the hypothesis that El Niño heating supports gravity wave seeding of EPBs [McClure et al., 1998]. 3. Discussion [20] Data presented in the previous section represent our second in a multistep plan for exploiting DMSP plasma density measurements acquired over the recent solar cycle. The first step, summarized in the report of Huang et al. [2001], established that polar-orbiting DMSP satellites provide suitable platforms for making meaningful measurements of EPB phenomenology. Agreement between the climatologies of EPB detections and range spread F observations from ground stations around the world [Aarons, 1993] established the utility of the DMSP satellites. To the best of our knowledge, measurements presented here provide the first global database for expanding the EPB climatology over a full solar cycle. The next step is to establish statistical and case by case relationships between scintillation activity observed on the ground and EPB detections by plasma monitors on DMSP satellites. The companion paper by Burke et al. [2002] establishes criteria for comparing variations of the S 4 index measured at Ancon, Peru during the last half of the recent solar cycle with data from the JULIA radar and DMSP F14. High correlations between Ancon and DMSP measurements suggest that quantitative relationships between scintillations and EPB activities can be obtained. As data from other SCINDA stations become available we plan to extend satellite and ground comparisons as the foundation for a more general global climatology for EPBs and radio scintillations. [21] Huang et al. [1987] compared occurrences of frequency and range spread F detected by ionosondes in the east Asia (25 N, 121 E) longitude sector over two solar cycles ( ). Range spread F corresponds most closely with plasma bubble activity [Aarons, 1993]. Although the data were binned by season and the level of F 10.7 their results are difficult to compare with those from DMSP. Whereas ionosonde measurements covered the entire night sector, DMSP observations were limited to the vicinity of the 2100 LT meridian. At this LT Huang et al. [1987] seldom encountered range spread F, which tended to occur later in the night. They even report finding an inverse correlation between solar activity and occurrence of spread F. Since EPBs observed near the magnetic equator at 840 km altitude, should map to the F layer near 14 Mlat, above the ionosonde stations, we are at a loss to explain this discrepancy. [22] Data presented in Figure 1 show that at any given location the rate of EPB occurrence depends on season and epoch in the solar cycle. The seasonal dependence has been discussed elsewhere [Tsunoda, 1985; Aarons, 1993; Huang et al., 2001]. The remainder of this discussion considers reasons why EPB activity shows more than an order of magnitude difference between solar maximum and minimum. It should be remembered that all our observations are obtained from a limited LT sector centered on 21 LT. During solar minimum the vertical speeds in the F layer are systematically lower [Fejer et al., 1979]. If the EPBs that occur during solar minimum years reach our altitude at later times, these are not recorded which would bias our statistical results. [23] We first state our assumption that EPBs begin in the bottomside of the F layer and, after passing through several linear growth periods, evolve into the nonlinear bubbles that percolate upward to DMSP altitudes. The dependence of the growth rate on the flux tube integrated conductivity [Sultan, 1996] is most evident in seasonal variability at different locations around the world [Tsunoda, 1985]. However, for simplicity in considering solar cycle effects, we take as our point of departure the linear growth rate g for the generalized Rayleigh Taylor instability using the local approximation [Kelley, 1989] g g0 rn R; n in n ð1þ where n is the ambient plasma density, R is the recombination rate and v in is the ion neutral collision frequency. The effective gravity [Ott, 1978] is given by g 0 ¼ g n in ðe 0 BÞ B 2 ; ð2þ where g is the acceleration due to gravity, E 0 represents the background electric field, and B is the Earth s magnetic field. The effects of neutral winds appear as part of E 0.

8 SIA 7-8 HUANG ET AL.: EQUATORIAL PLASMA BUBBLES DURING A FULL SOLAR CYCLE We have ignored the damping effects of plasma loss due to recombination. Since g and B are constant at a given location, EPB growth rate is mainly controlled by the variability of rn n, R, n in and E 0. We consider each of these in turn. [24] It is not possible to determine bottomside plasma density gradients from DMSP satellite measurements. However, the solar cycle variability of plasma densities at 840 km can be retrieved. We tabulated densities measured by DMSP satellites during 200 magnetic equatorial crossings in the evening sector during 1991 and Fifty crossings were taken from days near the two equinoxes and solstices of the 2 years. The only constraint imposed in the selection process was that no EPB activity occurred during a selected orbit. We found that the plasma density decreased by about a factor of thirty from (3.6 ± 2.2) 10 5 to (1.08 ± 0.41) 10 4 cm 3 between solar maximum and minimum. The topside ionospheres encountered by DMSP were clearly quite different during the 2 years. [25] We have estimated ionospheric density profiles near the magnetic equator at the longitudes of five longitudinally distributed locations for the equinox and solstice periods in 1991 and 1996 using the Parameterized Ionospheric Model (PIM) [Daniell et al., 1995]. Consistent with DMSP measurements, the PIM calculations indicate that the density at the peak of the F layer decreased from (1.3 ± 0.3) 10 6 to (5.5 ± 2.1) 10 5 cm 3 between solar maximum and minimum. The altitude of maximum plasma density decreased from 501 ± 73 to 332 ± 30 km during the interval. We note, however, that the scale size of the bottomside plasma density gradient l ¼ rn 1 n also decreased from 22 ± 3 to 14.7 ± 1.9 km. On the basis of the variation in l we might expect that, contrary to fact, the linear growth rate of bubbles should be greater by 50% under solar minimum conditions. [26] Equation (1) suggests that the value of n in and R in the bottomside ionosphere must play critical roles in the growth rate. The PIM calculations indicate that the bottomside of the F layer is more than 100 km lower in altitude near solar minimum than it is at solar maximum. The ion neutral collision frequency is n in n n sv i. Here n n, s, and v i represent the density of neutral atoms and molecules, the total ion-neutral cross section, and the mean thermal speed of ions, respectively. In the ionosphere s cm 2 and v i 10 5 cm/s for an O + dominated plasma. Since at a given time n n approximately decreases exponentially with altitude, we might expect that the damping effect of increased ion-neutral collisions strongly dominates over the contribution of l. However, the appropriate comparison is between neutral atmospheres under very different conditions. We have estimated values of n n near the peak of the F layer during the solar maximum/minimum years 1991/ 1996 using the MSIS-86 model [Hedin, 1987]. On average we found that n n near 330 km under solar minimum conditions was about three times larger than near 500 km at solar maximum. The overall effect is that in the post sunset, bottomside F layer l/n in is about twice as large in solar maximum than solar minimum. This is close to the observed difference in range spread F and nighttime airglow emissions [Abdu et al., 1985; Sahai et al., 2000] and suggests that growth rates for bottomside irregularities remains fairly constant over the solar cycle. [27] The second damping effect in equation (1), which may become increasingly important as the height of the bottomside F layer decreases, is the recombination rate. This should be enhanced as the molecular content of the local ionosphere increases. Determining the relative contributions of collisions and recombination is beyond the scope of this observational report. [28] The role of seeding of EPBs is also affected by the solar cycle. One candidate for initial generation of EPBs is tropospheric convection, in which atmospheric winds that flow mainly in the horizontal plane can produce uplift in regions of convergence. McClure et al. [1998, Figure 5] illustrate the location of the Intertropical Convergence Zone (ITCZ) relative to the magnetic dip equator and longitude. Regions in which the dip equator parallels the boundary of the ITCZ favor seeding of EPBs. However, during solar minimum years, the altitude of the ITCZ is lower than during solar maximum. The effect of the atmospheric uplift occurs at lower altitudes than is required for fast growth of the Rayleigh Taylor instability and thus fewer EPBs are triggered. [29] We conclude that the only term in equation (1) that may explain the large difference in EPB detection by DMSP between solar maximum and minimum is the electric field. Background electric fields that contribute to the growth of EPBs have three potential sources: (1) extension of the dayside dynamo effects into the postsunset ionosphere [Eccles, 1998], (2) uplift of the F layer in the evening sector by gravity waves [Kelley et al., 1981], and (3) expansion of the DP 2 current system to low magnetic latitudes [Wilson et al., 2001]. The atmospheric dynamo derives from vertical neutral winds driven by solar ultraviolet fluxes. Associated eastward electric fields cause plasma in the equatorial ionosphere to rise in the early evening LT sector. When the polarity of this electric field reverses several hours later, the ionospheric plasma falls. A commonly used proxy for ultraviolet radiation impacting the dayside ionosphere is the F 10.7 index. Since daily averaged values of F 10.7 fell from >200 units in the early 1990s to 70 units in 1996, we expect that dynamo contributions to evening sector electric fields diminish near solar minimum. The relation between vertical uplift and the F 10.7 index is given by Fejer et al. [1991, Figure 3]. [30] At first glance the contributions of gravity waves may appear to have a random influence on EPB activity. McClure et al. [1998] have argued that ITCZ contributions to the season longitude effects are systematic. Note however that the DMSP database failed to validate the suggestion that El Niño affects EPB occurrence in the Pacific sector. We also point out that the highest rates for DMSP detecting EPB activity occurred in the Atlantic sector near the December solstice. It is unclear how this observation can be reconciled with the ITCZ hypothesis. McClure et al. [1998, Figure 5] suggest that the probability of gravity wave seeding and seasonal variations should be small in this sector. [31] Huang et al. [2001, Figures 5 and 6] provide two pieces of empirical evidence that electric fields associated with geomagnetic activity influence the rate of EPB generation. First, the rate of EPB detections was overrepresented for Kp values >5. Second, intense deep, fast-moving bubbles were observed during the main phases of magnetic

9 HUANG ET AL.: EQUATORIAL PLASMA BUBBLES DURING A FULL SOLAR CYCLE SIA 7-9 storms. During the magnetic storms of June and July 1991, measurements by the Combined Release and Radiation Effects Satellite (CRRES) indicate that electric fields penetrated the inner magnetosphere at these times [Burke et al., 1998, 2000; Wilson et al., 2001]. Within the magnetosphere, penetration electric fields energize ring current ions and transport them earthward. The Dst index is the standard measure of ring current activity. During periods of ring current injection, the slope of Dst is negative. Mapped to the ionosphere, penetration electric fields drive low-latitude extensions of the global DP 2 current system [Nishida, 1968]. In the evening sector, associated eastward electric fields [Nopper and Carovillano, 1978] help destabilize the equatorial ionosphere and drive deep plasma bubbles [Huang et al., 2001]. [32] Auxiliary data collected for analysis of the DMSP database are simultaneous values of the Kp and Dst indices. We also obtained values of the interplanetary electric field and the dynamic pressure of the solar wind measured by the Wind and Advanced Composition Explorer (ACE) satellites in halo orbits around the L 1 Lagrangian point. A rough indicator of the solar cycle change in electric fields to drive ionospheric currents is reflected in the average values of Kp, which decreased from 3.3 ± 2.35 in 1991 to 1.68 ± 1.14 in Correspondingly, orbits with Kp > 5 decreased from 493 in 1991 to 87 in Of the 73 EPBs encountered by DMSP F12 in 1996, 15 occurred during intervals when Kp 5. Thus, while Kp 5 during <2% of the DMSP orbits in 1996, 20% of the EPBs were detected during those orbits. As we have observed near solar maximum [Huang et al., 2001], EPB occurrence is also overrepresented in periods of high geomagnetic activity during solar minimum. [33] A more sensitive proxy for monitoring the presence of penetration electric fields is the trace of Dst versus time. Negative slopes in this trace indicate periods of ring current intensification. We have created plots of Dst for the full solar cycle and marked them at times when a DMSP satellite encountered EPBs. Examples of these plots are shown by Huang et al. [2001, Figure 6]. During the solar maximum study, our focus was on the main phase of magnetic storms. However, in examining plots for solar minimum years when magnetic storms are mild and infrequent, we were struck by the relatively frequent occurrence of EPBs when Dst went through brief episodes of negative slope. Somewhat arbitrarily we assigned significance to periods when Dst decreased at rates 5 nt/hr for 2 or more hours. We found that in 1996, 29 of the 73 EPB encounters (40%) occurred during such periods. Similar observations were noted throughout the solar minimum years: 39 out of 120 EPBs (33%) in 1994, 40 out of 130 EPBs (31%) in 1995, 31 out of 96 EPBs (32%) in The cumulative evidence thus indicates that electric fields in the magnetosphere and ionosphere were depressed during solar minimum years. However, penetration electric fields contributed significantly to the growth of a large fraction of the bubbles observed at DMSP altitude. 4. Summary and Conclusions [34] This paper extends DMSP satellite observations of EPBs to cover a full solar cycle. The data were segmented into 24 bins each of 15 longitude. We have compared seasonal and annual occurrences of EPBs with similarly averaged values of the F 10.7 index. Our four main conclusions are: 1. The frequency of EPB occurrence at the DMSP altitude in any given longitude sector depended on both season and epoch in the solar cycle (Figures 1 and 2). 2. The rate of EPB detection in a given year was highly correlated with the yearly averaged value of the F 10.7 index (Figure 3). High correlations were found in all individual longitude bins except for the Pacific sector where the rate of EPB detection is low (Table 2). 3. As in the solar maximum phase, we found that the frequency of EPB detection was overrepresented during high geomagnetic activity. About one third of the EPB detections in a 4 year period around solar minimum occurred during intervals when electric fields penetrated the inner magnetosphere as signified by steep negative slopes in the Dst index. 4. A comparison of ionospheric plasma and neutral density profiles near the magnetic equator calculated using PIM and MSIS-86 for solar maximum and minimum conditions suggests that the dearth of EPBs detected by DMSP near solar minimum is mostly related to low levels of electric fields in the equatorial ionosphere at these times. 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