A climatology of equatorial plasma bubbles from DMSP 1989–2004



[1] After examining evening sector plasma density measurements from polar-orbiting Defense Meteorological Satellite Program (DMSP) spacecraft for 1989–2004, we have established a statistical database of more than 14,400 equatorial plasma bubble (EPB) observations. EPBs are irregular plasma density depletions in the postsunset ionosphere that degrade communication and navigation signals. In general, the DMSP observations support Tsunoda's (1985) hypothesis that EPB rates peak when the terminator is aligned with the Earth's magnetic field, but unpredicted offsets are also evident in many longitude sectors. Plots of EPB rates for solar cycle phases: maximum 1989–1992 and 1999–2002, minimum 1994–1997, and transition years 1993, 1998, and 2003 reveal significant differences in the climatologies for solar maximum and minimum, between the two solar maxima, and in the transition years. To assess local time effects on EPB rates, we also compare observations from F12, F14, F15, and F16 at slightly different postsunset local times for 2000–2004. This study was undertaken to facilitate improvements in ionospheric models in preparation for the Communication/Navigation Outage Forecasting System (C/NOFS) mission.

1. Introduction

[2] Equatorial plasma bubbles (EPBs) are the result of the nonlinear evolution of the generalized Rayleigh-Taylor (R-T) instability in the postsunset ionosphere in which bottomside plasma interchanges with plasma near and above the peak of the F layer. Irregularities with scale sizes of meters to kilometers develop within these density depletions. As these large-scale irregularities propagate into the topside ionosphere, smaller-scale irregularities within them diffract radio waves, severely degrading communication and navigation signals at low magnetic latitudes. The objective of the Communication/Navigation Outage Forecasting System (C/NOFS) mission is to forecast low-latitude ionospheric disturbances that cause these disruptions. To prepare for the launch of the C/NOFS satellite, we examined in situ plasma density measurements from the Special Sensor-Ions, Electrons, and Scintillation (SSIES) instruments on Defense Meteorological Satellite Program (DMSP) spacecraft and developed a global climatology of EPB occurrence to enhance the predictive capability of current ionospheric models.

[3] DMSP spacecraft fly in circular, Sun-synchronous polar orbits at an altitude of ∼840 km and an inclination of 98.7°. The trajectories of the spacecraft chosen for this study, F9, F10, F12, F14, F15, and F16, all cross the magnetic equator in the postsunset local time (LT) sector (1900–2200 LT). “Sun-synchronous” orbits are fixed with respect to the mean LT, but atmospheric drag lowers the orbit approximately a kilometer per year. Since the initial orbits of F10 and F12 were not quite Sun-synchronous, their orbital planes drifted in LT at a rate of ∼30 min or less per year.

[4] Although using polar-orbiting spacecraft at this altitude to study equatorial phenomena presents certain challenges, the preliminary global climatology developed by Burke et al. [2003, 2004] and Huang et al. [2001, 2002] demonstrated the utility of DMSP for EPB observations. Huang et al. [2001] first surveyed EPB activity for solar maximum years 1989 and 1991 and found that seasonal versus longitudinal distributions were generally consistent with ground-based measurements [Aarons, 1993] and the Scherliess and Fejer [1997] storm time model. Huang et al. [2001] also noted that the number of EPBs increased in the main phase of geomagnetic storms but was suppressed during the recovery phase. Huang et al. [2002] then extended their study to include multiple DMSP spacecraft over a full solar cycle 1989–2000 and suggested that penetration electric fields drive many storm time EPBs. Burke et al. [2003] compared the DMSP EPB observations with coordinated ground measurements from the Jicamarca unattended long-term studies of the ionosphere and atmosphere (JULIA) radar and a scintillation monitor in Ancon, Peru (MLat 0.9°N) and determined that overall trends in seasonal averages of EPB occurrence correlated closely despite differences in the actual rates since many ionospheric disturbances that cause measurable S4 scintillations do not reach DMSP altitudes.

[5] Burke et al. [2004] found that DMSP EPB rates were well correlated with plasma density measurements for March–April 2000 and 2002 from the Republic of China Satellite (ROCSAT-1) in a 35° inclination orbit at 650 km when the spacecraft crossed the same longitude sector within ±15 min. Furthermore, ROCSAT-1 EPB rates peaked around the LT of the DMSP orbits, confirming that the spacecraft were well placed to observe EPBs. Burke et al. [2004] also examined scintillation measurements from a network of GPS receivers in South America. On the basis of unexpected minima in EPB rates near the west coast of South America and significant differences in scintillations observed to the east and to the west of the GPS receivers along the 287° meridian [Valladares et al., 2004], they proposed that energetic electron precipitation from the inner radiation belt affects the global distribution of EPBs and the climatology of radio wave scintillations. Enhanced drift loss cone precipitation of inner belt electrons to the west of the South Atlantic Anomaly [Luhmann and Vampola, 1977] would increase E layer conductance and thereby inhibit nonlinear EPB growth. The present study updates the EPB climatology of Burke et al. [2004] with data from F12 for 1999–2000, F14 and F15 for 2003, and F14, F15, and F16 for 2004 and examines variations in EPB rates during phases of the solar cycle.

[6] To guide our EPB studies, we use a formula for the linear growth rate γ of the generalized R-T instability derived by Zalesak and Ossakow [1982] and later adapted by Sultan [1996]:

equation image

where gL represents the downward acceleration due to gravity, Vp = E × B/B2 the vertical component of plasma drift due to the zonal component of the electric field Ez at the magnetic equator, UnP the vertical component of neutral wind velocity perpendicular to B, and νineff the effective, flux tube integrated, F region, ion-neutral collision frequency weighted by number density in the flux tube. Ln is the scale length of the vertical gradient of the flux tube integrated plasma density measured at the equator. RT is the flux tube integrated recombination rate [Basu, 1997]; ΣPE and ΣPF are the E and F region contributions to ΣP, the field line integrated Pedersen conductivity. Ln is positive in the bottomside of the F layer in the postsunset ionosphere. Eastward components of E and downward components of UnP contribute to a positive growth rate γ. Growth periods, 1/γ, are ∼10 min, and large-amplitude irregularities develop over several growth periods. Since g and B are constants at a given location, R-T growth rates are controlled by the variability of Ez, UnP, ΣPE, ΣPF, νineff, RT, and through the flux tube integrated quantities by the height of the F layer. Tsunoda [1985] predicted that EPB rates should peak near times when α, the angle between equatorial declination and the dusk terminator, is equal to zero. This places the conjugate E regions in darkness at the same time, simultaneously lowering their conductivities while the equatorial ionosphere is still raised in altitude, creating a favorable environment for the rapid growth of instabilities.

2. Instrumentation

[7] DMSP spacecraft average 14 orbits of ∼104 min each per day, ∼5100 orbits per year. Satellites twice traverse all magnetic latitudes between ±20° MLat within a narrow LT range during every orbit and regress ∼25° in longitude between ascending nodes. SSIES includes a spherical Langmuir probe on an 0.8 m boom to measure the densities and temperatures of ambient electrons and three ion sensors mounted on a conducting plate facing ram [Rich and Hairston, 1994]: (1) an ion trap to measure total ion density, (2) an ion drift meter to measure horizontal (VH) and vertical (VV) cross-track components of plasma drifts, and (3) a retarding potential analyzer to measure ion temperatures and in-track components of plasma drift V.

3. Observations

[8] Table 1 summarizes the DMSP EPB observations by year, spacecraft, average LT of the spacecraft orbit over the course of the year, number of orbits, number of orbits with EPBs, and number of orbits with EPBs classified by the depth of the deepest depletion with respect to the nearby undisturbed plasma. Irregularities within depletions made it difficult to count individual EPBs; each cluster was classified as one occurrence according to the depth of the deepest depletion in that equatorial pass. Data were sorted by the longitude of the spacecraft's magnetic equatorial crossing rather than the EPB longitude. DMSP and ROCSAT-1 observed most EPBs within ±20° of the equator [Burke et al., 2004].

Table 1. DMSP EPBs by Year, Spacecraft, LT of Spacecraft Orbit, Number of Orbits, Number of Orbits With EPBs, and EPBs Classified by the Depth of the Deepest Depletion δN With Respect to the Nearby Undisturbed Plasma Densitya
  • a

    M-0 if δN ≤ 2; M-1 if 2 < δN ≤ 10; M-2 if 10 < δN ≤ 100; M-3 if δN > 100 [cf. Huang et al., 2001, Figure 1].

Totals  134364144124236939169293

[9] Solar cycle, seasonal, and longitudinal effects are evident in the data. The number of orbits with EPBs ranged from ∼1100 per year during solar maximum to <100 during solar minimum. Overall, more EPBs were encountered in the America-Atlantic-Africa sector around the equinoxes in March–April and September–October than near the solstices in June–July and December–January [Huang et al., 2002]. EPB rates were generally lower in the Pacific sector. During solar minimum, ∼1/3 of the EPBs occurred during times when plots of Dst had significant and sustained negative slopes suggesting the presence of penetration electric fields [Huang et al., 2002]. Distributions of M-2 and M-3 EPBs show strong solar cycle dependences, and most M-3s occurred during the main phase of magnetic storms [Huang et al., 2001].

[10] EPB rates were calculated as the ratio of the number of orbits in which at least one EPB was detected divided by the total number of orbits for each month of the year in 24 longitude sectors of 15°. On average, there were ∼18 orbits per month for each satellite in each longitude bin. To provide some perspective on the size of the statistical samples, Table 2 is an array of the total number of EPBs and orbits per month in each longitude sector for all the spacecraft listed in Table 1.

Table 2. Number of DMSP EPBs/Orbits Per Month in Each Longitude Sector for the Full DMSP EPB Database 1989–2004a
  • a

    Lon is longitude, which marks the western edge of each 15° sector.


[11] EPB rates were then grouped according to the F10.7 index as solar maximum (1989–1992 and 1999–2002), minimum (1994–1997), and transition years (1993, 1998, and 2003) and displayed in color contour plots similar to equatorial spread F occurrence patterns devised by P. J. Sultan (private communication, 2000). In all solar cycle phases, DMSP encountered more EPBs in the Atlantic-Africa sector than in the Pacific. Figure 1 plots EPB rates for 1989–1992 on a month-versus-longitude grid with longitude ranging from −180° to 180° in 24 bins of 15°. Colors represent the rates as indicated. Superimposed black lines mark the days when α = 0°.

Figure 1.

Contour plot of EPB rates for solar maximum 1989–1992 on a month versus longitude grid. Black lines mark the two times per year when α = 0° within each 15° longitude bin. EPBs occurred throughout the year in the Atlantic-Africa sector; rates were highest from September to December.

[12] Both seasonal and longitudinal effects are evident with the highest EPB rates during March–April (46%–62%) and September–December (60%–68%) in the America-Atlantic-Africa region. Note that EPB rates actually remained high throughout the year in the Atlantic-Africa sector and that the maximum rates were significantly higher than those during the 1999–2002 solar maximum (40%–51%) shown in Figure 2. This is consistent with Huang et al. [2002], who reported a strong linear relationship between EPB rates and the yearly averaged F10.7 index both globally and in individual longitude sectors [cf. Huang et al., 2002, Figure 4 and Table 2]. The distribution for 1999–2002 is also remarkably symmetric; EPB rates were quite high in the Atlantic-Africa sector around both spring and fall equinoxes with distinct minima (0%–5%) near the summer solstice in the America-Atlantic and India sectors.

Figure 2.

Contour plot of EPB rates for solar maximum 1999–2002 in the same format as Figure 1. EPB rates are fairly symmetric, high in the America-Atlantic-Africa sector both early and late in the year.

[13] The solar minimum climatology plot in Figure 3 presents a striking contrast to those for the solar maxima. DMSP EPB rates were generally less than 5% except in the America-Atlantic-Africa sector and a few isolated areas. Most EPBs were observed early in the year; the highest rate (21%) occurred in the Africa sector in March. Since the height distribution of EPBs is still in question, it is important to note that ionospheric conditions during solar minimum may limit the number of EPBs that reach DMSP altitudes at the appropriate local times. C/NOFS may provide critical information since the satellite's 13° inclination orbit with an apogee of 710 km and perigee of 375 km should maximize the probability of observing EPBs even under solar minimum conditions.

Figure 3.

Contour plot of EPB rates for solar minimum 1994–1997 in the same format as Figure 1. EPB rates were generally ≤5%. Highest rate (21%) occurred in the Africa sector in March.

[14] Climatologies for the transition years presented elsewhere by Gentile et al. [2006] exhibit similar trends that vary with the onset or decline of the cycle. As the solar cycle declined in 1993, EPB rates were higher (50%–60%) during March–April in the Atlantic-Africa sector and lower (25%–30%) in September–October. A significant number of EPBs were encountered in the India sector in February, but by fall, very few EPBs were observed in this area. Only localized hot spots (∼30%) appear in the Atlantic-Africa sector early in 1998, but as the solar cycle approached maximum, EPB rates rose significantly (40%–48%). Maximum EPB rates for 2003 (29%–31%) were lower than those in 1993 or 1998, which is reasonable for the declining phase of solar cycle 24; however, contrary to expectations, the highest rates occurred late in the year because of the number of EPBs encountered during the October–November solar storms.

[15] Note that observations from different LTs were combined in the climatology plots. In comparing DMSP and ROCSAT-1 plasma densities, Burke et al. [2004] found that ROCSAT-1 EPB rates at 650 km varied with magnetic local time (MLT) from ∼0 near 18.7 MLT to a fairly consistent level between 20.3 MLT and midnight, the LT range of most of the DMSP orbits. For DMSP satellites at 848 km, the average LT varies from year to year (see Table 1). To determine the variation in EPB rate as a function of LT for DMSP, we compared data from F12, F14, F15, and F16 for 2000–2004. This time interval was advantageous because several spacecraft were available simultaneously during the maximum phase of the solar cycle. Of the four satellites, F15 was at the latest LT; thus we assumed that F15 was in the ∼20–24 MLT range when occurrence rates were relatively constant. Yearly EPB rates for F15 from 2000 to 2004 were 20%, 20%, 19%, 7%, and 5%, respectively. To normalize the data and remove the influence of the solar cycle variation, we multiplied the rate for each spacecraft in a given year by a factor which normalized the F15 rate for that year at 20%. Figure 4 shows that DMSP EPB rates rose from ∼0% near 19.3 MLT to a maximum near 21.0 MLT, with the half-maximum rate at ∼20.0 MLT. For ROCSAT-1, the half-maximum rate occurred at ∼19.5 MLT. Attributing this difference to the average speed of the rising bubbles, we estimate an upward drift speed of ∼110 m/s, well within the expected range for EPBs [Ott, 1978].

Figure 4.

LT distribution of EPBs observed from DMSP F12, F14, F15, and F16 at 848 km altitude for 2000–2004 as a function of LT. Nonsolar maximum years 2002–2004 have been scaled to solar maximum years 2000–2001.

4. Discussion and Conclusions

[16] The primary objective of the DMSP EPB studies has been to exploit global and continuous observations of plasma densities at low magnetic latitudes in the evening LT sector in preparation for the C/NOFS mission. The solar cycle climatology plots provide a more detailed picture of seasonal and longitudinal trends in EPB occurrence rates at DMSP altitudes for a range of solar activity conditions. As with any statistical study, our methodology offers advantages and disadvantages relative to other techniques. Each DMSP spacecraft measures plasma densities at 848 km near a specific LT; equatorial crossings regress in longitude by ∼25° from one orbit to the next. Therefore a DMSP satellite misses many EPBs, especially during solar minimum. However, it does observe internal irregularities within depletions as it follows the magnetic flux tube. While satellites such as Atmosphere Explorer E and ROCSAT-1 flying in low-inclination orbits sample plasma density irregularities at all longitudes and LTs, their operational lifetimes were limited to a few years. Burke et al. [2004] demonstrated that if ROCSAT-1 crossed an EPB at a given longitude and a DMSP satellite crossed the magnetic equator at the same longitude within ±15 min the probability of detecting the same depletion was nearly 100%. In other words, EPBs that reached the ROCSAT-1 altitude (650 km) usually ascended to altitudes >848 km. Also, ROCSAT-1 saw the highest EPB rates at LTs that coincided with those of the DMSP orbits. Thus the combined databases contribute a great deal to our understanding of the nonlinear growth and evolution of EPBs. However, it is important to note that the available ROCSAT-1 data were for March–April of 2000 and 2002, months around the spring equinox during solar maximum years when EPB rates are relatively high [Burke et al., 2004]. No data were available for a similar solar minimum comparison.

[17] Although there have been some apparent conflicts when we compared our DMSP climatology with others derived from ground-based radars and monitors of radio wave scintillations at fixed geographic locations, we also have several cases which confirmed the DMSP measurements. The initial solar maximum study by Huang et al. [2001] found good agreement with ground-based measurements reported by Aarons [1993]. Burke et al. [2003] compared DMSP rates of EPBs detected near the west coast of South America with the intensities of UHF scintillations observed at Ancon. They showed that when S4 > 0.8, EPB detections at 848 km and UHF scintillations followed the same pattern. More recently, Wiens et al. [2006] found excellent agreement between GPS scintillation data for September 2001 to June 2003 from Asmara (MLat 7°N) and DMSP F15 EPB rates for the longitude sector of East Africa at 2130 MLT. Although the GPS rates were slightly higher, which is reasonable for plasma irregularities at low altitudes, seasonal patterns were well correlated. The authors will make the DMSP EPB data available upon request for comparison studies with other satellite or ground-based instrumentation. Our hope is that in the C/NOFS era we will understand this complex manifestation of space weather well enough to predict its occurrence.


[18] This work was supported under Air Force Office of Scientific Research task 2311SDA1, Air Force contract F19628-02-C-0012 with Boston College, and the National Polar-orbiting Operational Environmental Satellite System Internal Government Studies Program.