The equatorial ionosphere is host to the most dramatic and enigmatic plasma instability mechanism in the geospace environment. Equatorial spread F (ESF) was discovered in early ionosonde measurements and interpreted theoretically using Rayleigh‐Taylor theory. Subsequent diagnostic and modeling advances have improved substantially our understanding of ESF onset and evolution and its associated effects on the ionosphere throughout the low‐latitude domain. The degree to which ESF mechanisms penetrate into the lower midlatitudes is a topic of current study, a reverse of the familiar concept of high‐to‐low latitude coupling for space weather phenomena. Optical diagnostic systems, first ground based and now space based, reveal the presence of ESF structures via images of airglow depletions that are aligned in the approximately north‐south direction spanning the geomagnetic equator. Ground‐based all‐sky camera systems used to capture the two‐dimensional horizontal patterns of airglow depletions are the main source of observations showing that ESF processes intrude to midlatitudes in the L ∼ 1.5 domain. In this paper we review the process of mapping airglow depletions along geomagnetic field lines to the equatorial plane, hence defining the maximum apex heights achieved. A case study comparison of simultaneous radar backscatter data from Kwajalein with optical data from Wake Island, sites that share common magnetic meridians in the Pacific section, confirms the utility of the approach and its applicability to sites at other longitudes. Modeling studies based on buoyancy arguments using flux tube–integrated mean density values versus L shell apex heights show that instability‐induced plasma depletions starting at F layer bottomside heights easily reach altitudes above 2000 km in the equatorial plane, implying that ESF intrusions to lower midlatitudes should be a relatively frequent occurrence.

Defense Meteorological Satellite Program (DMSP) spacecraft at 840 km observe a 27‐day variation in plasma density and temperature at all subauroral latitudes. At the peak of the solar cycle, evening‐sector variations are ∼40–50% in plasma density and ∼5–10% in electron temperature. The percent of variation decreases with decreasing solar activity to or below the threshold of detectability for the DMSP sensors. We compare in situ densities with simultaneous observations of total electron content but find that similar variations are not present in a consistent manner. Thus we conclude that the variations exist mostly as topside phenomena. However, comparisons with variations in the radio flux at 10.7 cm ( F 10.7 ), a standard proxy for solar EUV, indicate that the topside variations are driven by the solar EUV flux. When compared with the variations of several alternative proxies for the solar EUV flux, we find that only one of them correlates better than F 10.7 . Because the topside ionosphere couples with the plasmasphere, we suggest that similar 27‐day variations should appear in plasmaspheric parameters.

This paper compares evening sector measurements by the Jicamarca unattended long‐term studies of the ionosphere and atmosphere (JULIA) radar, the Ancon scintillation monitor, and plasma density sensors on Defense Meteorological Satellite Program (DMSP) satellites. During more than half of the 110 nights of JULIA operations in 1998 and 1999, backscatter was observed from plumes extending above the layer of bottomside spread F. On 98% of the nights with no plumes, the S4 index measured at Ancon was <0.8. On ∼90% nights with plumes, S4 > 0.8. DMSP F14 crossed the magnetic equator within 7.5° longitude of Ancon near the 2100 local time (LT) meridian on 61 nights. During 32 overpasses, DMSP detected no equatorial plasma bubbles (EPBs), and JULIA detected no plumes. DMSP encountered EPBs on only 9 of the remaining 29 nights when JULIA observed plumes. Two plumes detected by JULIA on 15 April 1999 did not coincide with nearby EPBs crossed by the two satellites on the same evening. We also compared the seasonally averaged percent of nights with S4 ≥ 0.8 at Ancon with the percent of orbits in which a DMSP satellite detected EPBs. Data were accumulated between May 1994 and the first quarter of 2001. On a global scale at solar minimum, DMSP encountered very few EPBs. In years near solar maximum the two data sets were well correlated. However, there were more nights with S4 ≥ 0.8 at Ancon than EPB encounters by DMSP satellites. This discrepancy reflects the effects of different sampling intervals and the fact that about a third of the plumes fail to reach the DMSP altitude. Still, a correlation coefficient of 0.88 indicates that EPB detection at 840 km is a good indicator that scintillation activity is occurring near the spacecraft's longitude at the Earth's surface. The data also suggest that bubbles are often generated in bursts rather than at nearly uniform intervals.

The ionosphere often becomes turbulent and develops electron density irregularities. These irregularities scatter radio waves to cause amplitude and phase scintillation and affect satellite communication and GPS navigation systems. The effects are most intense in the equatorial region, moderate at high latitudes and minimum at middle latitudes. The thermosphere and the ionosphere seem to internally control the generation of irregularities in the equatorial region and its forcing by solar transients is an additional modulating factor. On the other hand, the irregularity generation mechanisms in the high-latitude ionosphere seem to be driven by magnetospheric processes and, therefore, high-latitude scintillations can be tracked by following the trail of energy from the sun in the form of solar flares and coronal mass ejections. The development of a global specification and forecast system for scintillation is needed in view of our increased reliance on space-based communication and navigation systems, which are vulnerable to ionospheric scintillation. Such scintillation specification systems are being developed for the equatorial region. An equatorial satellite equipped with an appropriate suite of sensors, capable of detecting ionospheric irregularities and tracking the drivers that control the formation of ionospheric irregularities, has also been planned for the purpose of specifying and forecasting equatorial scintillations. In the polar region, scintillation specification and forecast systems are yet to emerge although modeling and observations of polar cap plasma structures, their convection and associated irregularities have advanced greatly in recent years. Global scintillation observations made during the S-RAMP Space Weather Month in September 1999 are currently being analyzed to study the effects of magnetic storms on communication and navigation systems.

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 2001. 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 F10.7 index. On both global and longitude sector scale sizes, annual rates of EPB encounters correlate with the yearly averages of F10.7. 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.

The Defense Meteorological Satellite Program (DMSP) flights F9 and F10 crossed postsunset local time sectors approximately 14 times per day in Sun‐synchronous orbits at an altitude of ∼840 km. We have examined a large database of postsunset plasma density measurements acquired during ∼ 15,000 equatorial crossings made by DMSP F9 in 1989 and 1991 and DMSP F10 in 1991. On 2086 of these crossings equatorial plasma bubbles (EPBs) were observed as intervals of depleted and irregular plasma densities. We have analyzed these EPB events to determine their distributions with season, longitude (S/L), and levels of geomagnetic activity. The global S/L distributions of EPBs observed by the DMSP satellites are shown to be in general agreement with results from discrete ground‐based measurements. That is, the seasonal variations detected at 840 km in longitude bins hosting radar/scintillation observatories appear similar to those reported from the ground. Over the Atlantic sector where EPBs occur frequently, we found good agreement with predictions of a simple model proposed by Tsunoda [1985]. In the Pacific sector the frequency of EPB occurrence is considerably lower, and poor counting statistics preclude confident predictions regarding the absolute value of seasonal variations. We suggest that relatively large equatorial magnetic fields at Flayer altitudes in the Pacific (∼0.34 G) sector more strongly inhibit the growth of the Rayleigh‐Taylor instability than at Atlantic (∼0.25 G) longitudes. Contrary to general belief, we found that EPBs occurred regularly during geomagnetic storms, especially in the initial and main phases. EPB activity appears to have been suppressed from many hours to clays during storm recovery phases.

A simple parameter has been developed to describe the measured latitudinal profiles of the ion density at 840 km near the magnetic equator. This parameter, which we call the “asymmetry index,” is found to be well correlated with empirical neutral wind model output. Such a correlation makes it possible to use the index as a proxy representing the relative strength and direction of the transequatorial neutral wind. Topside ion densities are measured routinely by in situ sensors aboard Defense Meteorological Satellite Program (DMSP) spacecraft near 0600, 0900, 1800 and 2100 hours local time. DMSP data over the full range of solar activity levels and available local times are considered and compared to model neutral winds. While the shape of the observed profile (ni versus latitude) is controlled both by dynamics and by chemistry and varies as a function of season, longitude, local time, solar cycle, and geomagnetic activity, the neutral wind is clearly a significant factor. The profile asymmetry index is not a direct measurement of the instantaneous wind but rather represents the integrated effects of vertical and interhemispheric plasma transport due to the neutral wind over several hours. The index is shown to duplicate well (data/model correlation coefficient R ∼ 0.8) the longitudinal and seasonal morphology of the neutral wind blowing along the geomagnetic meridian for all available local times during solar maximum conditions, but only near the 0900 and 1800 LT meridians during solar minimum. These results suggest that during these times the meridional neutral wind is the dominant influence on the ion density profile shape. Thus the asymmetry index is a useful parameter for the study of the dynamics of the low‐latitude ionosphere. In particular, such a parameter would be important for use in operational ionospheric models that require a real‐time neutral wind input and in the verification and testing of future thermospheric wind models.

In this paper we present a study of the ionospheric effects of a halo coronal mass ejection (CME) initiated on the Sun on September 20, 1999, and causing the largest magnetic storm during this month on September 22–23, 1999, with the hourly Dst index being −167 nT at ∼2400 UT on September 22. The recurrent CME on October 18 caused an even larger magnetic storm on October 22, 1999, with Dst of −231 nT at ∼0700 UT. The ionospheric effects of these two major magnetic storms are studied through their effects on a prototype of a Global Positioning System (GPS)‐based navigation system called Wide Area Augmentation System (WAAS) being developed by the Federal Aviation Administration for use in the continental United States and their impact on global VHF/UHF communication systems. It is shown that the penetration of transient magnetospheric electric fields equatorward of the shielding region at midlatitudes, which have been well‐correlated in the past with rapid changes in the well‐known Dst index (or through its recently available high resolution 1‐min counterpart the SYM‐H index), can cause large increases of total electron content (TEC), TEC fluctuations, and saturated 250‐MHz scintillation, and these, in turn, may have significant impacts on WAAS. The local time of Dst changes (and not just Dst magnitude) was found to be very important for WAAS, since the largest effects on TEC are seen near dusk. The prompt penetration of these magnetospheric electric fields all the way to the magnetic equator causes augmentation or inhibition of equatorial spread F. The global ionospheric response to these storms has been obtained from ground‐based TEC observations with a GPS network and space‐based in situ density and electric field measurements using the Republic of China Satellite‐1 (ROCSAT‐I) and several Defense Meteorological Satellite Program satellites. These prompt penetration electric fields cause VHF/UHF scintillations and GPS TEC variations at low latitudes in the specific longitude sector for which the early evening period corresponds to the time of rapid Dst variations and maximum Dst phase. The effects of the delayed ionospheric disturbance dynamo and those of decreased magnetospheric convection on postmidnight irregularity generation are shown to be confined to a part of the same longitude range that actively responded to the prompt penetration of electric fields in the early evening sector.

The effects of the great magnetic storm of July 15, 2000 on the equatorial ionosphere have been studied by ground‐based and satellite in‐situ measurements. A large westward plasma drift in the evening equatorial ionosphere was observed as a result of the ionospheric disturbance dynamo. In that environment, the IMF Bz turned southward and presumably caused penetration of E‐fields to low latitudes. This E‐field initiated the onset of 250 MHz and L‐band scintillations at Ascension Island (15°W) and precipitous TEC decrease at Fortaleza, Brazil (38°W), bounding the narrow longitude region in the South Atlantic. These impulsive ionospheric effects were extremely well correlated with abrupt decreases of SYM‐H (1‐min resolution Dst). The DMSP in‐situ measurements showed the presence of severe ion density bite‐outs extending over 30° latitude in the South Atlantic Magnetic Anomaly region. The ROCSAT‐1 satellite measured upward and large southward ion drifts in the same sector.

Measurements of total ion density at 840 km altitude at all latitudes in four local time sectors are available from 1987 to the present from spacecraft of the Defense Meteorological Satellite Program (DMSP). Thus all phases of the past solar cycle are represented in the data set. We present comparisons of the measurements with values obtained from climatology models of the ionosphere. The models examined are the International Reference Ionosphere (IRI) model, the Parameterized Ionospheric Model (PIM) and the Reilley‐ICED‐Bent‐ Gallagher (RIBG) model. We show that all of the models reproduce some of the features of the observed topside ionosphere, but none of the models match all of the observations.