Dayside Dip Equator Research Articles

A Small Peak in the Swarm‐LP Plasma Density Data at the Dayside Dip Equator

AbstractIn this paper, we statistically investigate an artifact in Langmuir Probe (LP) observations of Swarm satellites. A small peak of electron density (Ne) is frequently found in the Swarm data around the dayside dip equator. On the contrary, they appear in neither the Total Electron Content data of the Swarm/Global Positioning System Receivers nor COSMIC‐2 in‐situ measurements at similar altitudes but with low orbit inclination. Arguably, this peak does not represent natural ionospheric irregularities but is likely to result from artifacts. The phenomena are found regardless of the season, solar activity, and the velocity direction of the satellite (ascending and descending). They predominantly occur when the magnetic declination is close to zero, that is, when the Swarm ram direction and the Earth's magnetic field are aligned under sunlight. Hence, we attribute the phenomenon to intensified secondary electrons escape when the geomagnetic field lines are normal to conducting surfaces that emit secondary electrons. Since the magnitude of the artifact is only a few percent of the large‐scale background, it does not have a serious impact on the value of the Swarm/LP data in scientific research. Nevertheless, future efforts to determine the exact cause of the artifacts will contribute to improving the reliability and quality of plasma density and temperature measured by Swarm/LP.

Day‐to‐day variability of equatorial electrojet and its role on the day‐to‐day characteristics of the equatorial ionization anomaly over the Indian and Brazilian sectors

AbstractThe equatorial electrojet (EEJ) is a narrow band of current flowing eastward at the ionospheric E region altitudes along the dayside dip equator. Mutually perpendicular electric and magnetic fields over the equator results in the formation of equatorial ionization anomaly (EIA), which in turn generates large electron density variabilities. Simultaneous study on the characteristics of EEJ and EIA is necessary to understand the role of EEJ on the EIA variabilities. This is helpful for the improved estimation of total electron content (TEC) and range delays required for satellite‐based communication and navigation applications. Present study reports simultaneous variations of EEJ and GPS‐TEC over Indian and Brazilian sectors to understand the role of EEJ on the day‐to‐day characteristics of the EIA. Magnetometer measurements during the low solar activity year 2004 are used to derive the EEJ values over the two different sectors. The characteristics of EIA are studied using two different chains of GPS receivers along the common meridian of 77°E (India) and 45°W (Brazil). The diurnal, seasonal, and day‐to‐day variations of EEJ and TEC are described simultaneously. Variations of EIA during different seasons are presented along with the variations of the EEJ in the two hemispheres. The role of EEJ variations on the characteristic features of the EIA such as the strength and temporal extent of the EIA crest has also been reported. Further, the time delay between the occurrences of the day maximum EEJ and the well‐developed EIA is studied and corresponding results are presented in this paper.

Transmission line model for the near‐instantaneous transmission of the ionospheric electric field and currents to the equator

AbstractThe simultaneous onset of the preliminary impulse (PI) of the geomagnetic sudden commencement at high latitude and dayside dip equator is explained by means of the TM0 mode waves propagating at the speed of light in the Earth‐ionosphere waveguide (EIW) [Kikuchi et al., 1978]. A couple of issues remain to be addressed in the EIW model: (1) How is the TM0 mode wave excited by the field‐aligned currents (FACs) in the polar region? (2) How are the quasi‐steady ionospheric currents achieved by the TM0 mode waves? (3) How simultaneous or delayed are the onset and peak of the equatorial PI with respect to the high‐latitude PI? To address these issues, we examine the TEM (TM0) mode wave propagation in the finite‐length transmission lines replacing the pair of FACs (magnetosphere‐ionosphere (MI) transmission line) and the Earth‐ionosphere waveguide (ionosphere‐ground (IG) transmission line). The issue (1) is addressed by showing that a fraction of the TEM mode wave is transmitted from the MI to IG transmission lines through the polar ionosphere. To address the issues (2) and (3), we examine the properties of the finite‐length IG transmission line with finite ionospheric conductivity. It is shown that the ionospheric currents start to grow instantaneously and continue to grow gradually with time constants of 1–10 s depending on the ionospheric conductivity. The MIG transmission line enables us to explain the instantaneous onset and delayed peak time of the equatorial PI and quick electric field response of the low‐latitude ionosphere and inner magnetosphere.

Polar-equatorial ionospheric currents driven by the region 2 field-aligned currents at the onset of substorms

[1] The dawn-to-dusk convection electric field increases during the growth phase of substorms, driving DP2 currents composed of two-cell current vortices in high latitude and leading to an increase in the eastward electrojet at the dayside dip equator (EEJ). During the expansion phase of substorms, electric field and currents are often reversed in direction to the normal DP2 currents at subauroral to equatorial latitudes when the convection electric field reduces abruptly. The reversed current at the dayside dip equator appears as a counterelectrojet (CEJ) and causes an equatorial enhancement of the negative bay in the afternoon sector. Conversely, the convection electric field increases during substorms as deduced from sawtooth events, causing an increase in the EEJ. In this study, by analyzing isolated substorms with magnetometer array data and SuperDARN (Super Dual Auroral Radar Network) convection maps, we deduce that the reversed electric field and currents develop at the subauroral-to-equatorial latitudes at the onset of substorms, while the convection electric field increases at auroral latitudes. These observations imply that both the region 1 and region 2 field aligned currents (R1 and R2 FACs) develop on the dayside concurrently with the current wedge responsible for the midlatitude positive bay at midnight. The substorm-associated R2 FACs are strong enough to cause the reversed current at the subauroral latitude and the CEJ at the equator. We also deduce that the dayside equatorial CEJ begins simultaneously with or even earlier than the midlatitude positive bay at midnight while the midlatitude positive bay onset is delayed by several minutes as the station departs from the midnight meridian. These observations suggest that the substorm begins with the intensification of the R2 FACs responsible for the equatorial CEJ.

High‐latitude reconnection effect observed at the dayside dip equator as a precursor of a sudden impulse

Geomagnetic sudden impulses (SI) observed at the dayside dip equator normally show a decrease and then an increase in the H magnetic field component, i.e., a preliminary reverse impulse (PRI) followed by a main impulse. Using global geomagnetic field measurements, we examine an unusual SI event observed at the dayside dip equator, which shows a clear precursor ∼1 min before the PRI onset. The precursor was observed simultaneously at both the dayside dip equator and in the southern polar region but was not observed at all in the northern polar region. The global ground variations after the PRI onset were, however, consistent with a conventional SI model of the magnetospheric response to a sudden enhancement of the solar wind dynamic pressure. Considering that the interplanetary magnetic field Bx, By, and Bz components were positive with the By component dominant for this event, we suggest that the SI precursor was caused by high‐latitude magnetic reconnection that occurred only (or first) in the dawn quadrant of the southern hemisphere, the effect of which has for the first time been clearly identified at the dayside dip equator. This implies that electric field effects occurring in the polar ionosphere due to magnetopause reconnection may be rapidly monitored at the dayside dip equator. In addition, we argue that the quasi‐simultaneous (time difference is less than 10 s) appearance of the disturbance fields both in the polar region and at the dip equator confirmed in this study is extremely important for theoretically explaining the transmission of the polar electric field to the dayside dip equator, because different theories give different travel times for a disturbance field propagating from polar region to the dip equator. Our observations lend strong evidence for validity of the waveguide model.

Global features of Pi 2 pulsations obtained by independent component analysis

Ground Pi 2 pulsations are mixtures of several components reflecting (1) propagations of fast and shear Alfvén wave, (2) resonances of plasmaspheric/magnetospheric cavity and magnetic field lines, and (3) transformations to ionospheric current systems. However, it has been unclear how they coupled with each other and how their signals are distributed at different latitudes. The present work is intended to pilot the future possibilities whether we can identify the global system of Pi 2 pulsations by Independent Component Analysis (ICA). We have successfully decomposed an isolated Pi 2 event on a quiet day observed at the CPMN stations into two components. One was the global oscillation that occurs from nightside high to equatorial latitudes with the common waveform and has an amplitude maximum at nightside high latitude. Another component was localized at nightside high latitudes. Its amplitudes were quite weak at low latitudes, but were enhanced near dayside dip equator.

Comparative study of Geomagnetic Sudden Commencement (SC) between Oersted and ground observations at different local times

Oersted is a low‐altitude (638–849 km) polar‐orbiting satellite. Using vector magnetic field measurements obtained from Oersted, we identified more than 20 geomagnetic sudden commencement (SC) events on both dayside (0600–1800 MLT) and nightside (1800–0600 MLT). The unique properties reflected by these events have never been reported before. The SCs observed by Oersted in the B// (compressional) component on the nightside had the nearly same waveforms as those observed on the ground in the H (northward) component. We suggest that the SCs observed by Oersted on the nightside were dominantly caused by the enhanced magnetopause currents, which were transmitted by the compressional hydromagnetic waves, and the effects of the ionospheric current (IC) were negligible on the nightside. The SC waveforms observed by Oersted on the dayside were apparently different from those observed on the ground. Near the dayside dip equator (DDE), corresponding to preliminary reverse impulses (PRIs) observed in the ground H component, Oersted always observed positive impulses in the B// component, which suggest that the PRIs at the DDE are generated by westward ICs. On the dayside, corresponding to positive main impulses (MIs) of SCs observed in the ground H component, the Oersted B// component always presented clear decreases, which implies that an eastward IC was excited after the PRI. The generation mechanism for the westward and eastward ICs are discussed according to previously proposed models. On the dayside, we suggest that the waveforms observed both on the ground and at Oersted during the time period of PRI and MI were superposition of the incident compressional waves and the disturbance fields caused by the ICs. The features observed by Oersted just above the ionosphere are significant complementary to our empirical knowledge for SCs.

On the nature of response of dayside equatorial geomagnetic H-field to sudden magnetospheric compressions

A step-like increase in the solar wind ram pressure causes a sudden compression of the magnetosphere which manifests as a sudden commencement (SC) in the geomagnetic field. SC manifests in two basic forms in the geomagnetic H-field of the dayside dip equatorial region, namely, as a positive impulse termed SC(+), and as a positive impulse preceded by a sharp negative impulse termed, SC(−, +) or SC *. Unlike at high latitudes, the factor(s) that determine this bi-modal response of the equatorial geomagnetic H-field to sudden magnetospheric compressions are not known. As an exploratory step in identifying the causative factor(s), we have performed a statistical study of the characteristics of the daytime (06-18 IST) SCs recorded at the equatorial station, Kodaikanal (10.25N, 77.5E, dipole latitude 0.6N) and the low latitude station, Alibag (18.6N, 72.9E, dipole latitude 9.5N) over the period 1957–2002. A total of 304 SCs have been analyzed out of which 99 (32.6 per cent) were of SC * type. We find that the average value of the amplitude of the positive main impulse (MI) in H-component of SC * is higher than that of the conventional SC(+). This difference in the MI amplitude is statistically significant and persists even when the analysis is restricted to the interval of SC * occurrence, namely, 0730–1730 IST. Such a behavior is not seen in the amplitude of the MI at Alibag, away from the influence of the equatorial electrojet. It is found for the first time that a linear relationship exists between the amplitudes of the preliminary reverse impulse (PRI) and MI in SC * events. Our study of the H-field response at dayside dip equatorial stations of the Circum-pan Pacific Magnetometer Network (CPMN) to sudden magnetospheric compressions under different IMF orientation (northward and southward), showed that the impact of an impulse in solar wind dynamic pressure under southward IMF is associated with SC * in both the events considered. On the other hand, under northward IMF the dynamic pressure increase consistently led to SC(+) in the two events studied. The statistical and case study results imply that, when compared to SC(+), the contribution of ionospheric currents of polar origin gain prominence when SC * is excited at dayside dip equator, and the orientation of IMF Bz may be one of the (if not the only one) deterministic factors underlying the bi-modal response of the dayside equatorial H-field to a sudden increase in solar wind dynamic pressure.

Global Pc5 caused by a DP 2–type ionospheric current system

A global Pc5 geomagnetic pulsation (period of ∼6 min) was observed coherently from the auroral to equatorial latitudes in the local time sector of 0700–2100 LT during 1834–1900 UT on 21 April 1993. The Pc5 at the dayside dip equator (Sao Luiz, Brazil, and Ancon, Peru) was characterized by an equatorial enhancement with an enhancement ratio of ∼4. In the afternoon sector (1200–1600 magnetic local time) the Pc5 at the auroral latitude was coherent with that at the dip equator within a time resolution of 3 s. The Pc5 amplitudes decreased sharply away from the auroral zone but were enhanced in the dayside dip equator, in a manner that resembled the latitudinal profile of a DP 2 magnetic fluctuation (period of ∼40 min) studied by Kikuchi et al. [1996] except for the enhanced amplitude in the auroral zone. In the dayside auroral oval the Pc5 fluctuations in the magnetic X/H component were reversed in phase between the morning and afternoon sectors. Additionally, the magnetic Y/D component fluctuations around noon were correlated with the dayside equatorial Pc5. At most dayside middle‐/low‐latitude stations the Pc5 fluctuations in the H component, which are regarded as evidence for magnetospheric compressions, were strong. However, near the dawn terminator the D component was dominant rather than the H component, suggesting the influences of ionospheric currents originating in the polar region. There was no significant azimuthal phase propagation away from noon for the auroral to equatorial Pc5 signals. The global observational results suggest that the dawn‐dusk electric field in the polar ionosphere accompanying a pair of field‐aligned currents extends instantaneously to the equatorial ionosphere and completes a DP 2–type ionospheric current system responsible for the global coherent Pc5.

A concurrent modulation of the auroral luminosity and ground Pc3 activities at dip-equator

A diffuse and faint aurora was recorded in the all-sky image of Syowa station near the southern boarder (most tailward part) of the Syowa field-of-view during the period from 2200-0000 UT, 1 August 1987. The Syowa station was at the midnight sector, 2200 MLT-0000 MLT. Meanwhile, geomagnetic pulsations in Pc3 band were observed in the morning sector (0700-0900 LT) by ground magnetometer at the dip-equator (MEL: Melekeok in Western Pacific region, −1.12°, 205.06° in geomagnetic coordinates). We demonstrate a possible correlation of the occurrences between these two events at midnight auroral zone and at dayside dip-equator.

Letter to the editor: Electric field fluctuations (25-35 min) in the midnight dip equatorial ionosphere

Abstract. Measurements with a HF Doppler sounder at Kodaikanal (10.2°N, 77.5°E, geomagnetic latitude 0.8°N) showed conspicuous quasi-periodic fluctuations (period 25-35 min) in F region vertical plasma drift, Vz in the interval 0047-0210 IST on the night of 23/24 December, 1991 (Ap = 14, Kp < 4-). The fluctuations in F region vertical drift are found to be coherent with variations in Bz (north-south) component of interplanetary magnetic field (IMF), in geomagnetic H/X components at high-mid latitude locations both in the sunlit and dark hemispheres and near the dayside dip equator, suggestive of DP2 origin. But the polarity of the electric field fluctuations at the midnight dip equator (eastward) is the same as the dayside equator inferred from magnetic variations, contrary to what is expected of equatorial DP2. The origin of the coherent occurrence of equatorial electric field fluctuations in the DP2 range of the same sign in the day and night hemispheres is unclear and merits further investigations.Key words: Ionosphere (electric fields and currents; equatorial ionosphere; ionosphere-magnetosphere interactions)

Electric field fluctuations (25–35 min) in the midnight dip equatorial ionosphere

Measurements with a HF Doppler sounder at Kodaikanal (10.2∞N, 77.5∞E, geomagnetic latitude 0.8∞N) showed conspicuous quasi-periodic fluctuations (period 25-35 min) in F region vertical plasma drift, Vz in the interval 0047-0210 IST on the night of 23/24 December, 1991 (Ap = 14, Kp <4 ) ). The fluctuations in F region vertical drift are found to be coherent with variations in Bz (north-south) component of interplan- etary magnetic field (IMF), in geomagnetic H/X com- ponents at high-mid latitude locations both in the sunlit and dark hemispheres and near the dayside dip equator, suggestive of DP2 origin. But the polarity of the electric field fluctuations at the midnight dip equator (eastward) is the same as the dayside equator inferred from magnetic variations, contrary to what is expected of equatorial DP2. The origin of the coherent occurrence of equatorial electric field fluctuations in the DP2 range of the same sign in the day and night hemispheres is unclear and merits further investigations.

Amplitude modulation of the equatorial electrojet (EEJ) during a magnetospheric storm

Abstract An encounter of the Earth with a high velocity solar wind stream triggered the magnetic storm of 29 January 1995. The storm lasted for the following seven days, keeping the level of the Dst index in the range of −25–−50 nT. At the dip-equator, a regular enhancement of the magnetic H component during daytime, referred to as the Equatorial Electrojet (EEJ), was seen to be suppressed and modified during this storm interval. We attempted to classify the type of the EEJ modification by analyzing magnetometer data from two stations at the dip-equator but located in the opposite hemisphere, and energetic particle data from two geosynchronous satellites located close to the ground magnetometer meridian. As a result, three different types of modulation of EEJ amplitudes, with time scales of 15 min to 1 day, were found to appear during the periods when the flux level of low energy charged particles (>30 keV) in the midnight magnetosphere was increased above the quiet level. These modulations were characterized by referring to the nightside particle signatures. We argue that, although the EEJ is a local enhancement of the ionospheric currents at the dayside dip-equator, the EEJ is definitely affected by changes of the magnetosphere, probably in various ways arising from the complexities of the storm effects.

Equatorial electrojet as part of the global circuit: a case-study from the IEEY

Abstract. Geomagnetic storm-time variations often occur coherently at high latitude and the day-side dip equator where they affect the normal eastward Sq field. This paper presents an analysis of ground magnetic field and ionospheric electrodynamic data related to the geomagnetic storm which occured on 27 May 1993 during the International Equatorial Electrojet Year (IEEY) experiment. This storm-signature analysis on the auroral, mid-latitude and equatorial ground field and ionospheric electrodynamic data leads to the identification of a sensitive response of the equatorial electrojet (EEJ) to large-scale auroral return current: this response consists in a change of the eastward electric field during the pre-sunrise hours (0400-0600 UT) coherently to the high-, mid-, and equatorial-latitude H decrease and the disappearance of the EEJ irregularities between the time-interval 0800-0950 UT. Subsequent to the change in h'F during pre-sunrise hours, the observed foF2 increase revealed an enhancement of the equatorial ionization anomaly (EIA) caused by the high-latitude penetrating electric field. The strengthening of these irregularities attested by the Doppler frequency increase tracks the H component at the equator which undergoes a rapid increase around 0800 UT. The ∆H variations observed at the equator are the sum of the following components: SR, DP, DR, DCF and DT.Keywords. Equatorial electrojet · Magnetosphere-ionosphere interactions · Electric fields and currents · Auroral ionosphere · Ionospheric disturbances

Wave characteristics of geomagnetic pulsations across the dip equator

In order to clarify the wave characteristics of Pi2 and Pc 4–5 magnetic pulsations around the dip equator, we analyzed magnetic data from the latitudinally dense magnetometer array in Brazil. We found that the phase difference between Pi2 pulsations observed at globally separated low‐latitude stations is small, whereas Pi2 pulsations observed within the dayside dip equator region of ±2° latitude show phase lags of about 30° ∼ 50° behind those in the off‐dip equator region. Pc 4–5 magnetic pulsations at the dip equator also show the same phase character. Pi2 amplitudes are enhanced in the equatorial region, where the phase lags of pulsations must be associated with the enhancement of ionospheric conductivity. The equatorial phase lags can be explained by invoking the induction effect of the equatorial enhanced ionospheric current above the good conductor Earth.

DP 2 electric field fluctuations in the dusk‐time dip equatorial ionosphere

We have studied the geomagnetic and ionospheric manifestations of the DP 2 activity that occurred on April 7, 1995 using high time resolution magnetometer data of IMAGE network in Scandinavia and of Alcantara (dip 1.2° N), Brazil, and F layer vertical plasma drift, Vz measured by an HF Doppler radar at Kodaikanal (dip 4° N), India. Quasi‐periodic fluctuations in dusk‐time (1730–1900 LT) F layer vertical plasma drift (eastward electric field) occurred over Kodaikanal coherent with DP 2 type magnetic fluctuations (period≈25 min) at the day‐side dip equator and auroral/subauroral latitudes. This first‐ever observation at the dusk‐side equator is in agreement with the two‐cell equivalent current system previously proposed for DP 2 fluctuations. The results strongly suggest that the magnetospheric electric field responsible for DP 2 fluctuations penetrates to equatorial ionosphere on the dusk‐side as on the day‐side. An additional observation is that the electric field fluctuation amplitude increases towards the nightside suggesting the influence of dusk sector electrodynamics in the observed signature of the DP 2 electric field.

Equatorial electrojet as part of the global circuit: a case-study from the IEEY

Geomagnetic storm-time variations often occur coherently at high latitude and the day-side dip equator where they affect the normal eastward Sq field. This paper presents an analysis of ground magnetic field and ionospheric electrodynamic data related to the geomagnetic storm which occured on 27 May 1993 during the International Equatorial Electrojet Year (IEEY) experiment. This storm-signature analysis on the auroral, mid-latitude and equatorial ground field and ionospheric electrodynamic data leads to the identification of a sensitive response of the equatorial electrojet (EEJ) to large-scale auroral return current: this response consists in a change of the eastward electric field during the pre-sunrise hours (0400-0600 UT) coherently to the high-, mid-, and equatorial-latitude H decrease and the disappearance of the EEJ irregularities between the time-interval 0800-0950 UT. Subsequent to the change in h'F during pre-sunrise hours, the observed foF2 increase revealed an enhancement of the equatorial ionization anomaly (EIA) caused by the high-latitude penetrating electric field. The strengthening of these irregularities attested by the Doppler frequency increase tracks the H component at the equator which undergoes a rapid increase around 0800 UT. The ∆H variations observed at the equator are the sum of the following components: SR, DP, DR, DCF and DT.Keywords. Equatorial electrojet · Magnetosphere-ionosphere interactions · Electric fields and currents · Auroral ionosphere · Ionospheric disturbances

Direct penetration of the polar electric field to the equator during a DP 2 event as detected by the auroral and equatorial magnetometer chains and the EISCAT radar

The quasi‐periodic DP 2 magnetic fluctuations (period of 30–40 min) appearing coherently at the auroral and equatorial latitudes during the day are analyzed based on the high time resolution magnetometer data recorded at the International Monitor for Auroral Geomagnetic Effects (IMAGE) stations in Scandinavia and at the Brazilian and African equatorial stations. It is shown that the correlation between the DP 2 magnetic fluctuations at both latitudes is excellent (correlation coefficient of 0.9). No discernible time shift has been found within the resolution of 25 s. The European incoherent scatter (EISCAT) radar observations in Scandinavia show that the DP 2 fluctuations at auroral latitudes are caused by an ionospheric Hall current which is controled by the convection electric field. The DP 2 fluctuations exhibit a strong decrease in magnitude with decreasing latitude, however, it is enhanced considerably at the dip equator with an amplitude comparable to that at the subauroral latitude. The considerable equatorial enhancement of the magnitude of the DP 2 fluctuations with an enhancement ratio of 4 is due to the concentration of the electric current along the highly conductive dayside equatorial ionosphere. These observational facts can be explained in terms of an ionospheric current which is generated by the magnetospheric electric field at the high latitude and extends to the equatorial ionosphere almost instantaneously. From the viewpoint of the electric field penetration, we conclude that the magnetospheric electric field penetrates to the equatorial ionosphere through the polar ionosphere almost instantaneously within the time resolution of 25 s. The nearly instantaneous propagation of the electric field to the equator can be explained primarily by a parallel plane transmission line model composed of the conductive Earth and ionosphere. In addition to our finding of the fast propagation of the DP 2 electric field, it is found that an impulsive magnetic change with a timescale of 100 s appears at the dayside dip equator with a time delay of about 10 s, which requires to include the effect of the high conductivity of the dayside equatorial ionosphere in future studies of the propagation model.

Another look at equatorial metallic ions in the F region

An extensive survey is made of the equatorial occurrences of the metallic ion Fe+ as detected by the ion mass spectrometer on the equatorial orbiting satellite Atmosphere Explorer E. It considers the longest time period (4 years) data base available for the study of the equatorial metallic ion distributions. The distributions of the Fe+ ions are explored through the use of percentages of occurrences within spatial and temporal windows of the independent variables: dip latitude, local time, altitude, longitude, and season. Concentrations of the metallic ions exceeding 10, 30, and 100 cm−3 were considered. The number of occurrences in the F region were most frequent at the dayside dip equator. Diurnally, the events were not appreciable in the F region until a few hours after dawn, reaching a maximum near noon followed by a secondary maximum in the afternoon. Near and after dusk the Fe+ ions extended on the average to higher altitudes than during the day and became less and less frequent from midnight to dawn. Seasonally, the distributions between 200 and 300 km were skewed away from the dip equator during the day with the maximum frequency of occurrence north (south) of the dip equator during a period centered on the December (June) solstice; the average nighttime distributions did not have the opposite seasonal relationship. No correlation was evident at all between any features of the frequency of the F region events and meteoritic input variations. There was evidence for a spreading of the events in latitude with increasing altitude. Theoretical studies of the dynamical evolution of the equatorial metallic ion distributions thus far published do not account for some of the observed features.