We consider VHF amplitude scintillations, GPS phase fluctuations, ionosonde measurements, maps of GPS total electron content (TEC), observations of daytime aurora and TIMED GUVI images during the large magnetic storms of October 29–31, 2003, and find two distinct classes of plasma processes that produce midlatitude ionospheric irregularities. One is associated with auroral plasma processes; the other, with storm enhanced density (SED) gradients, a part of which occur in close proximity to sub‐auroral polarization stream (SAPS) electric fields as discussed by J. C. Foster et al. (2002). We analyze in detail the storm event of October 30, 2003. The SAPS‐associated plasma structures may occur by an ion temperature gradient convective instability (M. J. Keskinen et al., 2004), but structuring by auroral processes requires elucidation.

We examine the correspondence between high latitude ionospheric scintillation measurements made at 250MHz with the occurrence of 10MHz HF coherent radar backscatter, on 13 and 14 December 2002. We demonstrate that when the ionospheric intersection point of the scintillation measurements is co-located with significant HF radar backscatter, the observed scintillation, quantified by the S4 index, is elevated. Conversely, when the radar indicates that backscatter is observed away from the intersection point due to movements of the auroral zone, the observed scintillation is low. This suggests that scintillation is highly location-dependent, being enhanced in the auroral zone and being lower at sub-auroral latitudes. The coexistence of scintillation and HF radar backscatter, produced by ionospheric density perturbations with scale sizes of 100s of metres and ~15 m, respectively, suggests that a broad spectrum of density fluctuations is found in the auroral zone.

UHF scintillation measurements of zonal ionospheric drifts have been conducted at Ancon, Peru since 1994 using antennas spaced in the magnetic east-west direction to cross-correlate geo-synchronous satellite signals. An empirical model of average drift over a wide range of Kp and solar flux conditions was constructed from successive two-dimensional fits of drift vs. the parameters and day of year. The model exhibits the typical local time trend of maximum eastward velocity in the early evening with a gradual decrease and reversal in the early morning hours. As expected, velocities at all hours increase with the solar flux and decrease with Kp activity. It was also found that vertical drifts could contribute to the variability of drift measurements to the east of Ancon at a low elevation angle. The vertical drift at the ionospheric intersection to the east can be estimated when combined with nearly overhead observations at Ancon or a similar spaced-antenna site at Antofagasta, Chile. Comparisons on five days with nearly simultaneous measurements of vertical drift by the Julia radar at Jicamarca, Peru show varying agreement with the spaced-antenna estimates. Statistical results from 1997 to 2001 generally agree with radar and satellite studies.

We have constructed latitudinal profiles of the total electron content (TEC) using measurements from six GPS receivers conducted during 1998. The TEC profiles have been divided into two groups: One corresponds to days when plumes or equatorial spread F (ESF) develops, and the second group portrays days of no‐ESF condition. The presence/absence of ESF is based on the signature of the coherent echoes measured by the Jicamarca Unattended Long‐Term Investigation (JULIA) radar and records of scintillations from two sites spaced in latitude. One scintillation station is located near the magnetic equator (Ancon) and the other 12° southward (Antofagasta). The TEC profiles display the typical day‐to‐day and seasonal variability seen at low latitudes. During the equinoxes, we observed quite often the crests of the anomaly located between 12° and 20° away from the magnetic equator and a trough in‐between. The monthly distribution of the appearance of the anomaly and the local time of their appearance are in very good agreement with the reported variability of the upward vertical drifts and the current theory of the equatorial fountain effect. During the equinoxes and the December solstice, the TEC anomaly is observed almost every day, sometimes when there is no ESF activity. Nevertheless, fine inspection of the TEC latitudinal profiles suggests the existence of a close relationship between the temporal evolution of the TEC profiles near sunset and the onset of ESF. We have examined the TEC latitudinal distributions in two different ways. First, we calculated time difference profiles using the distributions corresponding to 1800 and 2000 LT. Second, we used a parameterization of the TEC distributions obtained at 2000 LT. The first method indicates quite drastic increases of the crest values and sharp decreases near the trough during ESF days. In contrast, during days of no ESF there exist almost uniform TEC decreases at all latitudes. The second method displays a preferred high crest/trough ratio (>2), small TEC values at the trough, and large latitudinal integrated values during ESF events.

Coordinated radio and optical measurements of the structure and dynamics of the postsunset equatorial ionosphere were conducted on October 1, 1994, from Agua Verde, Chile (11.3°S magnetic latitude (MLat)). The measurements clearly show a north‐south aligned undulation or ripple on the bottomside of the F layer at 2000 LT, appearing as an eastward propagating decrease in the 630.0‐nm airglow, resembling a traveling ionospheric disturbance in the digital portable ionosonde measurements and causing a total electron content decrease in the Global Positioning System (GPS) satellite measurements. The initial development of this feature, toward the east and away from the magnetic equator, took place in an otherwise smooth, unstructured ionosphere. Spread F began to develop in the ionograms at 2020 LT, and, at this same time, local onset of satellite signal scintillation was detected using the multiple ray paths throughout the sky available from the GPS satellite constellation transmitting at L band frequencies. UHF scintillation measurements from Ancon, Peru, along the same magnetic field line, show that intense scintillation and ionospheric irregularities had developed over the magnetic equator almost 60 min prior to their development at 11°S MLat. The observations suggest that the east‐west electric field expected to be present within the earlier developed depletion and scintillation region at the magnetic equator mapped along magnetic field lines to lower altitudes and higher latitudes, resulting in an undulation or dome‐shaped structure, before evolving into a fully developed depletion (with associated ionospheric irregularities) all along the magnetic flux tube.

Data collection for the first ground-based ionospheric tomography campaign in North America was conducted over a 48-hour period in mid-November 1991. The data consist of records of ionospheric total electron content (TEC) from a number of passes of the U. S. Navy Navigation Satellite System spacecraft over a chain of ground-based receiving stations. Data collection and reduction techniques are discussed; these include the determination of absolute TEC from the different phase advances induced by the ionosphere in each component of the dual-frequency spacecraft signal. The use of tomographic methods to reconstruct ionospheric electron densities over a two-dimensional (2-D) region of the Earth's ionosphere at a number of different times is demonstrated. Specifically, two distinct tomographic methods, the algebraic reconstruction technique and a maximum entropy method, are used to mathematically invert the records of TEC. The resulting 2-D contour ''maps'' of ionospheric electron density are then compared to similar maps produced by the Millstone Hill incoherent backscatter radar facility located at Westford, Massachusetts. Both qualitative and quantitative measures of agreement among the different reconstructions and the radar maps are presented. The behavior of the ionosphere over the course of the experiment is discussed.

An instrumented rocket payload was launched into a polar cap F layer aurora to investigate the energetic particle, plasma, and electric circuit parameters of a Sun‐aligned arc. The rocket was launched from Sondrestrom, Greenland, on March 15, 1985, at 0205:52 UT (approximately midnight corrected geomagnetic local time). On‐board instruments measured energetic electron flux, ion composition and density fluctuations, electron density and temperature, electron density fluctuations, and ac and dc electric fields. The payload traversed a rapidly moving Sun‐aligned, F layer arc near apogee (429 km altitude). Real‐time all‐sky imaging photometer measurements of the location and motion of the aurora, conducted from an aircraft in the vicinity of the trajectory, were used to determine the proper geophysical situation for launch. Comparison of the in situ measurements with remote optical measurements shows that the arc was produced by fluxes of low‐energy (<1 keV) electrons. Field‐aligned potentials in the arc inferred from the electron spectra had a maximum value of approximately 300 V, and from the spectral shape a parent population of preaccelerated electrons characteristic of the boundary plasma sheet or magnetosheath was inferred. Electric field components along and across the arc show sunward flow within the arc and duskward drift of the arc consistent with the drift direction and speed determined from optical imaging. Thus this arc is drifting duskward under the influence of the convection electric field. Within the arc the thermal plasma density shows a moderate decrease, even in the presence of precipitating electron fluxes. Three possible explanations for this (field‐aligned currents, chemistry, and transport) are considered. The field‐aligned currents associated with the arc are calculated from measured electric fields and computed conductivities. The calculated upward field‐aligned current agrees very well in magnitude and location with that carried by the energetic electron flux. The ac electric field and ion mass spectrometer measurements allowed the first direct comparison of the low‐frequency cutoff in the wave spectra with the local lower hybrid frequency. There is good agreement at high altitudes, and the disagreement at low altitudes can be explained by propagation effects. Finally, ionospheric irregularity and electric field fluctuations indicate two different generation mechanisms on the dawnside and duskside of the arc. On the duskside, parameters are suggestive of an interchange process, while on the dawnside, fluctuation parameters are consistent with a velocity shear instability.

The dominant coupling between the ionosphere and magnetosphere is now understood to be Birkeland currents. During substorms the Birkeland current system in the midnight sector is generally interpreted as a cross‐tail current interruption and diversion along magnetic field lines to the conducting ionosphere, which carries the intensified auroral electrojet. In the present study, the equatorial current diversion, the Birkeland current intensification, and the auroral electrojet increase in activity were observed to be initiated simultaneously at the onset of substorms. Our principal finding is that the field‐aligned currents of the components of the current wedge have been observed simultaneously on the ground, at low‐altitude satellite orbits, and at geosynchronous satellite orbits. Recent dynamic modeling work by Chen et al. (1982) appears to support our findings. These observations contribute significantly toward the confirmation of the substorm current wedge model and complement and extend the results obtained by Nagai (1982).

The Air Force Geophysics Laboratory's Airborne Ionospheric Observatory (AIO) performed radio, optical, and scintillation measurements on a series of north‐south flight legs along the Chatanika radar magnetic meridian. The incoherent scatter radar was operated in a north‐south magnetic meridian scan mode to measure ionospheric parameters (electron density, temperature, and ion drift) from 600 km north to 600 km south of the radar and from ∼80 to 700 km altitude. Ionospheric structure measured by the radar is compared with remote optical and ionosonde measurements from the AIO and with precipitating electron characteristics measured by a Defence Meteorological Satellite Program satellite. Long‐lived F region plasma enhancements (plasma blobs) were observed during this experiment. From the simultaneous measurements it is shown that these enhancements were not locally produced by precipitating particle fluxes. F region electron concentrations, calculated from simultaneously observed precipitating electron fluxes, are significantly less than those observed in the plasma enhancements. These premidnight features (plasma blobs) were in fact observed to be convecting sunward (to the west) at a few hundred meters per second and are thus presumed to have been produced well upstream in the convection flow within a region of significantly greater production rate. Intense scintillation of satellite signals due to ionospheric irregularities is generally confined to these regions of enhanced F region density. Simultaneous measurements of independent parameters identifying the auroral E layer (1–20 keV precipitating electrons, 4278‐Å N2+ emission, f0E, and radar electron densities) all agree well as to location and the latitude profile of the auroral E layer and associated diffuse aurora.

We rated the auroral activity in 749 southern hemisphere DMSP images taken during 1972 and 1973 on a qualitative scale with active (A), moderate (M), quiet (Q), and no aurora (N) categories. The overall occurrence rates of images with A, M, Q, and N ratings are 0.17, 0.55, 0.16, and 0.11, respectively. After one DMSP orbit (102 min), both A and Q conditions recur in about one half of the cases. Thus successive images showing A conditions and successive images showing Q conditions occur more often than expected from the overall rates of A and Q images. The difference between the recurrence rate and overall occurrence rate of M images is much smaller (0.63 versus 0.55), suggesting that moderate auroral activity, as seen in a series of DMSP images, occurs essentially randomly. Nearly random, short‐period (15–30 min) bursts of auroral activity have been observed by Krukonis and Whalen (1980) in montages of all‐sky photographs taken 1 min apart. Our results, which cover a much longer time span, are consistent with this behavior and confirm that auroral activity occurs on time scales that differ from the full duration of the classic substorm pattern.

Low‐ and high‐altitude spacecraft carrying scientific magnetometers have probed the magnetic fields of the terrestrial magnetosphere. These measurements reveal that field‐aligned currents, possibly powered by dynamo action, flow over large regions of the earth's neighborhood. The purpose of this paper is to report on highlights of the U.S. contributions to the IMS effort in the area of understanding the global nature of the field‐aligned currents and their relation to auroral and ionospheric phenomena. It has been established now that field‐aligned currents flow on the same field lines connecting distant regions to low ionospheric altitudes. Similarly, currents flow on L shells extending over several hours of local time. However, it is becoming increasingly apparent that the sources of the currents in the various latitudes and longitudes are not the same. For this reason we discuss separately the following current‐laden regions: (1) the noon sector, (2) the dawn and dusk sectors, (3) the midnight sector, and (4) the polar cap. Special attention is paid to polar cap ionospheric irregularities and the relation of auroral kilometric radiation to field‐aligned currents and optical auroral emissions.

The latitude of the equatorward auroral boundary near local midnight has been determined for 162 Defense Meteorological Satellite Program (DMSP) images in November‐December 1972. When grouped according to Kp and AE, these observations show approximate linear decreases in the average boundary latitude with increasing values of these magnetic indices. There appears also to be a slight diurnal variation in the boundary location. Mapping of the appropriate McIlwain injection boundaries to auroral latitudes shows good agreement with the average DMSP equatorward auroral boundary latitude. Similar analyses at 2000 and 2200 CGLT (corrected geomagnetic local time) using a different set of DMSP images yield similar results, with somewhat poorer agreement under quiet conditions.