Longitude Sector Research Articles

Ionospheric signatures of a plasmaspheric plume over Europe

Previously, ionospheric signatures of plasmaspheric plumes were reported only at the North American longitude sector. This led to the hypothesis that the geomagnetic field configuration at those longitudes played a vital role in the observation of plasmaspheric plume signatures only over the American continent. Combining ground‐based GPS total electron content (TEC), EISCAT incoherent scattering radar (ISR), and DMSP F15 ion drift meter observations we have observed greatly elevated density over the European continent during storm recovery phase on 12 September 2005. The TEC seen over Europe has a tongue of enhanced ionization extending to higher latitudes, which is identical to the plasmaspheric plume signatures that have been often observed over North America. Therefore, our observations clearly demonstrate that ionospheric signatures of plasmaspheric plumes are not limited to the North American sector and suggest that they may be observed at any longitude as long as a dense array of instruments are available to identify these signatures.

Large-scale patterns, complexes of solar activity and ‘active longitudes’

AbstractTypical latitude zones and longitude sectors with a dominant occurrence of newly emerging magnetic flux were systematically detected during three last activity cycles. Two long time persistent longitude sectors with the preferred occurrence of a new strong magnetic flux are characteristic for magnetic flux distribution and their rotation rate is latitude dependent in the relationship with differential rotation rate. Recent new information about the large-scale flows in convection zone relates to a concept of the expected giant cells and jets and show a new relationship with different scales of the large-scale circulation. Non-axially symmetric horizontal flow in upper part of convection zone gives a good motivation for an extension of the existing axially symmetric 2D models into new 3D concept.

Global, low‐latitude, vertical E × B drift velocities inferred from daytime magnetometer observations

Navigation and communication, Department of Defense and civilian, customers rely on accurate, low‐latitude specification of ionospheric parameters, globally, that are not currently realistic on a day‐to‐day basis. This paper describes, demonstrates, and speculates about the data sets that are required inputs to the operational ionospheric models that will correct these deficiencies. In order to investigate quiet time, vertical E × B drift velocities at two different longitude sectors, magnetometer observations were obtained for the period between January 2001 and December 2004 from the magnetometers at Jicamarca (0.8°N dip latitude) and Piura (6.8°N dip latitude) in Peru and from Davao (1.4°S dip latitude) and Muntinlupa (6.3°N dip latitude) in the Philippine sector. We choose only geomagnetically “quiet” days, when the 3‐hourly Kp value never exceeds a value of 3 over the entire day, and when the daily Ap value is less than 10. These are “binned” into three seasons, December solstice, equinox, and June solstice periods. A neural network trained for the Peruvian sector was applied to each of the days in both the Peruvian and Philippine sectors, providing ΔH‐inferred vertical E × B drift velocities between 0700 and 1700 local time. For each season, the average E × B drift velocity curves are compared with the Fejer‐Scherliess, climatological E × B drift velocity curves in both the Peruvian and Philippine sectors. In the Peruvian sector, the comparisons are excellent, and in the Philippine sector they are very good. We demonstrate that realistic magnetometer‐inferred E × B drifts can be obtained in the Peruvian sector on a day‐to‐day basis and speculate that on the basis of the average, quiet day comparisons, realistic E × B drifts can be obtained on quiet days in the Philippine sector.

Theoretically modeling the low‐latitude, ionospheric response to large geomagnetic storms

In the low‐latitude, ionospheric F region, the primary transport mechanism that determines the electron and ion density distributions is the magnitude of the daytime, upward E × B drift velocity. During large geomagnetic storms, penetration of high‐latitude electric fields to low latitudes can often produce daytime, vertical E × B drift velocities in excess of 50 m/s. Employing a recently developed technique, we can infer these daytime, upward E × B drift velocities from ground‐based magnetometer observations at Jicamarca and Piura, Peru, as a function of local time (0700–1700 LT). We study the ionospheric response in the Peruvian longitude sector to these large upward drifts by theoretically calculating electron and ion densities as a function of altitude, latitude, and local time using the time‐dependent Low‐Latitude Ionospheric Sector (LLIONS) model. This is a single‐sector ionosphere model capable of incorporating data‐determined drivers, such as E × B drift velocities. For this study, we choose three large storms in 2003 (29 and 30 October and 20 November) when daytime E × B drift velocities approached or exceeded 50 m/s. Initial results indicate that the large, upward E × B drift velocities on 29 October produced equatorial anomaly crests in ionization at ±20° dip latitude rather than the usual ±16° dip latitude. We compare the theoretically calculated results with a variety of ground‐based and satellite observations for these three periods and discuss the implications of these comparisons as they relate to the capabilities of current theoretical models and our ability to infer ionospheric drivers such as E × B drifts (Anderson et al., 2002).

Statistical analysis of the ionization ledge in the equatorial ionosphere observed from topside sounder satellites

For the purpose of clarifying the occurrence character of the ionization ledge and its relation to the diurnal variation of the eastward electric field, plasma density profiles in the equatorial ionosphere of the American longitude sector obtained from topside sounder on-board the Ohzora (EXOS-C) and the ISIS-2 satellites were analyzed. The ionization ledge appeared in a limited dip latitude region from −13.5° to 19.3°, near the dip equator. The structure of the ionization ledge tends to align along the geomagnetic field line, and to appear within local time sectors from morning (09 LT) to slightly after midnight (02 LT). Comparing with the eastward electric field deduced by analyzing the variations of the geomagnetic field on the ground, the ionization ledge tends to occur with the enhancement of the eastward electric field. The occurrence probability shows a seasonal variation giving maxima in equinox seasons and minima in solstice seasons. Comparing with the seasonal dependence of the upward drift velocity based on the variation of the electric field, the seasonal variation of the occurrence probability is well interpreted as the character of the ionospheric electric field.

Imaging science at El Leoncito, Argentina

Abstract. Thermospheric and mesospheric structures are studied using an all-sky imager located at El Leoncito, Argentina (31.8° S, 69.3° W, –18° mag lat). This site has relatively high geographic latitude for a location under the crest of the equatorial ionization anomaly (EIA), and thus observations can be used to study the intrusion of several equatorial processes into the midlatitude domain. In addition, it has a conjugate point close to the field of view of our companion imager at Arecibo, PR, allowing for the study of inter-hemispheric effects. Four types of phenomena were studied using 630.0 nm and 777.4 nm observations: (1) highly-structured airglow depletions associated with the Rayleigh-Taylor instability/equatorial spread-F (RTI/ESF) process, (2) brightness waves (BW) associated with the midnight temperature maximum (MTM), (3) strong airglow enhancements associated with the positive phase of ionospheric storms, and (4) simple (non-structured) bands of airglow depletions with characteristics matching a Perkins-like instability. Using 557.7 nm mesospheric observations, a fifth category of study deals with gravity waves probably generated by lower atmospheric disturbances, and mesospheric bores related to strong vertical temperature gradients. While ESF depletions and BW events are detected fairly frequently, the mid-latitude bands are not, and thus their successful imaging at El Leoncito offers the first example of the coupling from mid-latitudes to low-latitudes in the South American longitude sector. Preliminary results on these features are presented in this paper. Taken together, these five types of optical structures offer the opportunity to investigate coupling, both in altitude and latitude, of aeronomic processes at low latitudes in an under-sampled longitude sector in the Southern Hemisphere.

Equatorial scintillations in relation to the development of ionization anomaly

Abstract. The irregularities in the electron density distribution of the ionosphere over the equatorial region frequently disrupt space-based communication and navigation links by causing severe amplitude and phase scintillations of signals. Development of a specification and forecast system for scintillations is needed in view of the increased reliance on space-based communication and navigation systems, which are vulnerable to ionospheric scintillations. It has been suggested in recent years that a developed equatorial anomaly in the afternoon hours, with a steep gradient of the F-region ionization or Total Electron Content (TEC) in the region between the crest and the trough, may be taken as a precursor to scintillations on transionospheric links. Latitudinal gradient of TEC measured using Faraday Rotation technique from LEO NOAA 12/14 transmissions during the afternoon hours at Calcutta shows a highly significant association with L-band scintillations recorded on the INMARSAT link, also from Calcutta, during the equinoxes, August through October 2000, and February through April 2001. The daytime equatorial electrojet is believed to control the development of the equatorial anomaly and plays a crucial role in the subsequent development of F-region irregularities in the post-sunset hours. The diurnal maximum and integrated value (integrated from the time of onset of plasma influx to off-equatorial latitudes till local sunset) of the strength of the electrojet in the Indian longitude sector shows a significant association with post-sunset L-band scintillations recorded at Calcutta during the two equinoxes mentioned earlier. Generation of equatorial irregularities over the magnetic equator in the post-sunset hours is intimately related to the variation of the height of the F-layer around sunset. Ionosonde data from Kodaikanal, a station situated close to the magnetic equator, has been utilized to calculate the vertical drift of the F-layer over the magnetic equator for the period August through October 2000. The post-sunset F-region height rise over the magnetic equator shows a remarkable correspondence with the occurrence of scintillations at Calcutta located near the northern crest of the equatorial anomaly. Existence of a flat-topped ionization distribution over the magnetic equator around sunset has been suggested as a possible indication of occurrence of post-sunset scintillations. Width of the latitudinal distribution of ionization obtained from DMSP satellite shows some correspondence with post-sunset L-band scintillations. During the period of observation of the present study (August through October 2000, and February through April 2001), it has been observed that although the probability of occurrence of scintillations is high on days with flat-topped ion density variation over the equator, there are cases when no scintillations were observed even when a pronounced flat top variation was recorded.

A climatology of equatorial plasma bubbles from DMSP 1989–2004

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.

Morphology of the ultraviolet Io footprint emission and its control by Io's location

A total of 74 images of the ultraviolet footprint of the Io flux tube (IFT) on Jupiter's upper atmosphere made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope have been analyzed to characterize their location, morphology, and brightness distribution. The observations cover a wide range of central meridian Jovian longitudes and Io orbital positions and include north and south footprint emissions. Comparing the location of the IFT with that expected from the VIP4 model of the Jovian magnetic field, we find that the lead angle is generally not significantly different from zero in the System III longitude sector 125°–195°. Instead, the lead angles reach about 8° in the 50° sector, coinciding with a region of possible magnetic anomaly. We observe that the brightness of the main footprint shows intrinsic intensity changes that appear to be controlled by the system III longitude of Io and its position above or below the center of the torus. The size of the primary spot magnetically maps into a region varying from 1 to over 10 Io diameters in Io's orbital plane. Multiple footprints are observed with varying brightness relative to the mean spot. The number of spots is found to increase as Io gets closer to the torus outer edge facing the spots. The separation between the first and second spots is typically 1°–3° of longitude and increases when Io is displaced from the torus center in the direction of the IFT signature. These features confirm that Alfvén waves play an important role and generate energization of precipitated electrons. However, the observed variation of the FUV spot structure with Io's position appears inconsistent with models where reflections of Alfvén wings occur between the torus boundary and Jupiter's ionosphere. Instead, the multiple spots apparently correspond to electron precipitation generated by Alfvén waves reflected inside the plasma torus.

A global climatology for equatorial plasma bubbles in the topside ionosphere

Abstract. We have developed a global climatology of equatorial plasma bubble (EPB) occurrence based on evening sector plasma density measurements from polar-orbiting Defense Meteorological Satellite Program (DMSP) spacecraft during 1989-2004. EPBs are irregular plasma density depletions in the post-sunset ionosphere that degrade communication and navigation signals. More than 14400 EPBs were identified in ~134000 DMSP orbits. DMSP observations basically agree with Tsunoda's (1985) hypothesis that EPB rates peak when the terminator is aligned with the Earth's magnetic field, but there are also unpredicted offsets in many longitude sectors. We present an updated climatology for the full database from 1989-2004 along with new plots for specific phases of the solar cycle: maximum 1989-1992 and 1999-2002, minimum 1994-1997, and transition years 1993, 1998, and 2003. As expected, there are significant differences between the climatologies for solar maximum and minimum and between the two solar maximum phases as well. We also compare DMSP F12, F14, F15, and F16 observations at slightly different local times during 2000-2004 to examine local time effects on EPB rates. The global climatologies developed using the DMSP EPB database provide an environmental context for the long-range prediction tools under development for the Communication/Navigation Outage Forecasting System (C/NOFS) mission.

Discrepancy between the nighttime molecular ion composition given by the International Reference Ionosphere model and airglow measurements at low latitudes

The 630‐nm nighttime airglow is radiated by O(1D) atoms, which are produced by the dissociative recombination of O2+ ions. The typical approach used to calculate the red line emission rate at night is based on the assumption that O2+ is mainly produced by the reaction of O+ with molecular oxygen. In the case that the O2+ density is much smaller than the O+ density, [O+] = ne in the F2 region. Good agreement between measured nighttime integrated emission rates and the emission rates calculated by this typical approach, using both electron densities measured by incoherent scatter radars and given by the International Reference Ionosphere (IRI) model, has been shown. However, the O2+ densities given by the IRI model are much higher than the densities produced by the reaction of O+ with O2, and these densities do not correspond to the condition [O+] = ne. In this case, the typical approach cannot be applied and molecular ions must be included in the emission rate calculations. The integrated emission rates calculated including the molecular ion density given by the IRI model have been found to be much higher than the measured 630.0‐nm emission rates. This discrepancy takes place at latitudes below about ±30° in the western longitude sector, mainly for the period from March to November, and the disagreement is higher than 1 order of magnitude at the equator. In addition, we model the F2 region green line O(1S) emission at 557.7 nm resulting from the dissociative recombination of O2+. Using measurements of this volume emission rate made by the Wind Imaging Interferometer (WINDII) satellite, we are able to show that IRI overestimates the O2+ density (and ion fraction) on the bottomside of the F2 region. A revision of the ion composition in the IRI model on the bottomside seems to be needed on the basis of these results. Airglow measurements may be useful in constraining such a revision. A revision could utilize the formulae for the relationship between the molecular ion densities and neutral densities derived here, using the Mass Spectrometer Incoherent Scatter (MSIS) neutral densities and the IRI electron density. These calculations are based on the assumption that O2+ and NO+ are only produced through ion‐molecular reactions. Such a revision would correct the magnitude and altitudinal dependence of the molecular ion fraction in the IRI model.

Highly resolved observations of trace gases in the lowermost stratosphere and upper troposphere from the Spurt project: an overview

Abstract. During SPURT (Spurenstofftransport in der Tropopausenregion, trace gas transport in the tropopause region) we performed measurements of a wide range of trace gases with different lifetimes and sink/source characteristics in the northern hemispheric upper troposphere (UT) and lowermost stratosphere (LMS). A large number of in-situ instruments were deployed on board a Learjet 35A, flying at altitudes up to 13.7 km, at times reaching to nearly 380 K potential temperature. Eight measurement campaigns (consisting of a total of 36 flights), distributed over all seasons and typically covering latitudes between 35° N and 75° N in the European longitude sector (10° W–20° E), were performed. Here we present an overview of the project, describing the instrumentation, the encountered meteorological situations during the campaigns and the data set available from SPURT. Measurements were obtained for N2O, CH4, CO, CO2, CFC12, H2, SF6, NO, NOy, O3 and H2O. We illustrate the strength of this new data set by showing mean distributions of the mixing ratios of selected trace gases, using a potential temperature-equivalent latitude coordinate system. The observations reveal that the LMS is most stratospheric in character during spring, with the highest mixing ratios of O3 and NOy and the lowest mixing ratios of N2O and SF6. The lowest mixing ratios of NOy and O3 are observed during autumn, together with the highest mixing ratios of N2O and SF6 indicating a strong tropospheric influence. For H2O, however, the maximum concentrations in the LMS are found during summer, suggesting unique (temperature- and convection-controlled) conditions for this molecule during transport across the tropopause. The SPURT data set is presently the most accurate and complete data set for many trace species in the LMS, and its main value is the simultaneous measurement of a suite of trace gases having different lifetimes and physical-chemical histories. It is thus very well suited for studies of atmospheric transport, for model validation, and for investigations of seasonal changes in the UT/LMS, as demonstrated in accompanying and elsewhere published studies.

Equatorial F region evening vertical drift, and peak height, during southern winter months: A comparison of observational data with the IRI descriptions

The equatorial F region evening vertical drift, due to pre-reversal electric field enhancement, is an important condition for the spread F/plasma bubble irregularity generation, that is more frequent during summer-equinoctial months over South America. A comparative study of these vertical drifts with their IRI representations was presented at the Grahamstown IRI 2003 workshop. During southern winter months the post-sunset ESF development is relatively infrequent over South America due to the generally weaker intensity of the sunset zonal electric field, which, however, is critical in determining the equatorial spread F (ESF) development under magnetospherically disturbed conditions. Therefore a detailed understanding of the characteristics of the evening F layer vertical drift, hmF2 and foF2 during southern winter months is important for developing/improving their representations in the IRI scheme. In this paper we have undertaken a study of these parameters over the Brazilian equatorial sites, Sao Luis (2.33S, 44.2W, dip angle: −0.5°, declination angle: 21W°) and the low latitude site, Cachoeira Paulista (22.6°S, 315°E; dip angle: −32°) in comparison with their existing representations in the IRI. The study is made as a function of the solar flux varying from the solar activity minimum to maximum conditions. Some of the results in the Brazilian longitude sector are compared with results from Jicamarca (12°S, 76.9°W; dip latitude: 1°N, declination angle: ∼3°E) in Peru, separated by a large difference in magnetic declination angle. The magnetic equatorial and the low latitude stations analyzed here are all located in the southern geographic hemisphere. Systematic patterns of difference between the observed characteristics of these parameters and their IRI representations are identified for eventual corrections to their existing representations in the IRI model. The study has yielded further important clues towards a better understanding of the possible mechanism for the infrequent ESF occurrence in winter over South America, and especially over Brazil.

Diurnal, seasonal and latitudinal variation of ion concentrations as obtained from the SROSS C2 satellite over Indian low and equatorial latitudes during solar minimum

In situ measurements carried out by the Retarding Potential Analyzer on board the SROSS C2 satellite at equatorial and low latitudes during the low solar activity period of 1995 and 1996 are used to study the diurnal, seasonal and latitudinal variations of the concentrations of O +, H +, He + and O 2 + ions at the average altitude of ∼500 km in the Indian longitude sector. The concentration of the major ion O + is minimum before local sunrise and reaches its peak near local noon or in the afternoon hours depending on season and latitude of observation. The minimum and maximum densities vary with season and latitude. The H + concentration is found to be one order of magnitude less than that of O + during the daytime at all latitudes. At night, hydrogen ion concentration is comparable to the concentration of atomic oxygen ion, which indicates a lowering of the O + to H + transition altitude in this period. The ratio of night to day H + concentration is higher in summer than in winter and equinox. The measured concentration of He + is one order of magnitude less than that of H +. Helium ion concentration is higher in the December solstice compared to that in the June solstice in the forenoon hours while in the afternoon the relative densities in the solstices reverse in magnitude. The He + density is low at night and remains at a constant higher level during the daytime. The low nighttime He + densities indicate a higher O + to He + transition altitude compared to O + to H + transition altitudes. O 2 + concentration shows higher daytime values with peak in the midday or afternoon hours but it is a minor constituent at ∼500 km during solar minimum over the Indian longitude sector compared to H + or even He +.

Theoretical study of the low‐ and midlatitude ionospheric electron density enhancement during the October 2003 superstorm: Relative importance of the neutral wind and the electric field

During magnetic storms the ionospheric total electron content (TEC) at low‐ and midlatitudes often shows great enhancements, which may be associated with mechanisms producing midlatitude storm‐enhanced density (SED). The TEC enhancements may result from different ionospheric drivers such as electric fields, neutral winds, and neutral composition effects. To study the importance of the ionospheric drivers in producing the TEC enhancement, we perform numerical simulations for the 29–30 October 2003 superstorm period in the American longitude sector (∼ −70°W) using the Sheffield University Plasmasphere Ionosphere Model (SUPIM) with values for the neutral wind, temperature, and composition provided by the National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere General Circulation Model (TIEGCM). Various numerical experiments were run to identify the relative importance of the storm‐time ionospheric drivers. For carrying out the storm‐time SUPIM simulation, the storm‐time upward/poleward E × B drifts are derived from ROCSAT‐1 satellite measurements at low and equatorial latitudes and input to SUPIM, while the storm‐time neutral wind and composition disturbances are obtained from TIEGCM run. The simulation results presented in this paper, mainly during the evening period, show that the enhanced upward E × B drifts due to storm‐time eastward penetration electric field can expand the low‐latitude equatorial ionization anomaly (EIA) to higher latitudes and produce the TEC enhancement. However, by the effect of penetration electric fields alone, the TEC enhancement is less than by combining the storm‐generated equatorward neutral winds and the penetration electric fields. Disturbance neutral composition effects decrease the plasma density at higher latitudes and increase it at low and equatorial latitudes. However, the composition effects do not produce a density increase as large as that produced by the neutral‐wind and electric‐field effects. Our simulations suggest that the storm‐generated equatorward neutral winds play an important role in producing the TEC enhancement at low‐ and midlatitudes, in addition to the eastward penetration electric field.

Magnetic signature of the ionospheric disturbance dynamo at equatorial latitudes: “Ddyn”

During magnetic storms, wind disturbances produced by auroral phenomena can affect the whole thermospheric circulation and associated ionospheric dynamo currents for many hours after the end of the storms. In this paper we define criteria to select a new simple type of ionospheric disturbance dynamo events that allow a simple interpretation over all longitude sectors. These events exhibit a weak auroral activity during at least 24 UT hours, on the day after the storm. We analyze the magnetic disturbances “Ddyn” observed at equatorial latitudes in the three longitude sectors of such selected events. It is found for all the cases that the amplitude of the H component of the Earth's magnetic field is reduced, on the day after storm at equatorial latitudes, in agreement with the ionospheric disturbance dynamo model (Blanc and Richmond, 1980). The observation of H component decrease on the day after storm is longitudinally asymmetric. The observed signature of the ionospheric disturbance dynamo process in a specific longitude sector is strongly dependent on the magnitude, the start time, and the duration of the storm.

On the onset and meridional propagation of the ionospheric F2-region response to geomagnetic storms

The meridional propagation velocities of the ionospheric F2-region response to 268 geomagnetic storms are calculated. Ionospheric vertical sounding data of 1 h time resolution from several stations located in a longitude sector approximately centred along the great circle that contains both the geomagnetic poles and the geographic poles are used. Most meridional propagation velocities from high to low latitudes are less than 600 m/s. The smaller velocities are typical of global neutral meridional wind circulation and the larger are representative of traveling atmospheric disturbances. Simultaneous disturbances at several locations are more frequent during positive phases than during negative phases. Negative phase meridional propagation velocities associated with meridional neutral winds are less frequent in the southern hemisphere when compared with corresponding velocities observed in the northern hemisphere. This may be related to the fact that the distance between the geomagnetic pole and the equator is smaller in the northern hemisphere. Most negative phase onsets are within the 06–10 LT interval. For middle geomagnetic latitudes a “forbidden time interval” between 11 and 14 LT is present. The positive phase onsets show the “dusk effect”.

Global distribution of equatorial plasma bubbles in the premidnight sector during solar maximum as observed by KOMPSAT‐1 and Defense Meteorological Satellite Program F15

We investigated the global distribution of equatorial plasma bubbles (EPBs) using in situ plasma density measurements from Korea Multipurpose Satellite‐1 (KOMPSAT‐1) and Defense Meteorological Satellite Program (DMSP) F15 during the solar maximum period from June 2000 to August 2001. The results were generally consistent with those of previous studies. EPBs were observed at all longitudes around the magnetic dip equator in the equinoctial seasons with the peak occurrence in the American‐Atlantic‐African regions. During the June solstice, EPBs occurred predominantly in the African sector, with enhancements in the magnetic north in the Indian and west Pacific regions, but were totally suppressed in the American‐Atlantic sector. During the December solstice, EPBs occurred frequently in the American‐Atlantic sector but were suppressed in the other longitude sectors, especially in the Pacific sector. The EPB occurrence probability was seen to be correlated with the observed topside plasma density and the model prereversal upward drift speed of ambient plasmas (Fejer et al., 1999), with their respective dominance dependent on the seasons. However, the peak EPB occurrence in the American‐Atlantic sector during the December solstice was displaced somewhat from the region of peak density and upward drift, probably due to a strong solar terminator influence on the flux tube–integrated E region Pedersen conductivity and due to anomaly morphology. The peak EPB occurrence in the African sector during the June solstice is consistent only with the high ambient density in that region, for there was no coincidence with the maximum vertical drift or the minimum E region Pedersen conductivity.

Evidence for the tongue of ionization under northward interplanetary magnetic field conditions

The activities of the International Ionospheric Tomography Community open up new possibilities of simultaneously imaging the large‐scale spatial structure of the ionosphere in different longitude sectors. In the study, tomography receiver chains in Scandinavia and Greenland were used to provide a wide view of the plasma density structure in the winter, magnetic postnoon sector under conditions of stable, positive interplanetary magnetic field Bz component. The spatial distributions of the plasma are discussed in light of a high‐latitude plasma convection pattern pertinent to the conditions, which is supported by DMSP flow measurements. The observations are consistent with a tongue of dayside photoionization being drawn antisunward by the convection pattern to form an arc of enhanced plasma density around the periphery of the polar cap.

Toward a synthesis of equatorial spread F onset and suppression during geomagnetic storms

We present initial steps toward unifying our understanding of storm time equatorial spread F (ESF) by searching for the common elements in past case studies and statistical occurrence patterns. We show that the development (or inhibition) of equatorial irregularities during magnetically active periods can be understood using the AE‐parameterized Fejer‐Scherliess model for disturbance vertical drifts versus storm time and local time. This model takes into account the different sources of perturbation electric fields (magnetospheric and ionospheric dynamos) that ultimately drive the equatorial vertical drifts, showing prompt and delayed effects in the premidnight sector (where both generation and suppression can occur), as well as in the postmidnight period where generation dominates. The postsunset period exhibits the greatest variability for storm time ESF versus longitude, and thus we demonstrate the Fejer‐Scherliess model's applicability in a test case (6 April 2000) that had an AE pattern compatible with their parameterization scheme. The model successfully accounts for the pronounced longitude confinement in the observed postsunset ESF patterns. Finally, we move beyond the empirically derived relationships between geomagnetic indices and the occurrence of ESF (Aarons, 1991) into a framework of true solar‐terrestrial parameters that drive such effects. Additional case studies taken from the published literature are then used to show a consistent linkage between postsunset ESF onset and the interplanetary electric field (IEF) Esw. While AE, Dst, Kp and Dst/dt indices were used in earlier studies to determine the dusk‐longitude sector of disturbance electric fields, here we attribute to the IEF the main role in the determination of this longitude sector.