R. Sheehan

Scintillations, plasma drifts, and neutral winds in the equatorial ionosphere after sunset

An equatorial campaign was conducted during September 25 to October 7, 1994, to investigate the neutral and plasma dynamics in the equatorial ionosphere after sunset in relation to the day-to-day variability of the occurrence of equatorial spread F (ESF). The campaign was organized under the auspices of National Science Foundations Multi-Instrumented Studies of the Equatorial Thermosphere Aeronomy program (MISETA), which included the Jicamarca radar, spaced-antenna satellite scintillation, digisonde, all-sky imager, and Fabry-Perot interferometer (FPI) measurements near the magnetic equator in Peru. During a part of the period September 27 to October 3, the Geophysics Directorate of Phillips Laboratory performed measurements away from the magnetic equator at Aguaverde, Chile (magnetic latitude: 11°S) located 800 km to the east of the Jicamarca meridian using geostationary and GPS satellite scintillation, digisonde and all-sky imager systems. The incoherent scatter radar results indicate that the postsunset enhancement of upward plasma drift, even though of the order of only 20 m s−1 during the solar minimum period, is a necessary condition for the generation of ESF. In view of the extreme difficulty of determining the neutral wind speed during the early evening hours by the FPI due to low airglow intensity, it was not possible to unequivocally associate the observed postsunset enhancements with strong eastward neutral winds. However, considering a few observations contiguous to the campaign period, it appears that such a causal relationship may exist. The scintillation drift measurements in Peru and Chile indicated that the zonal irregularity drift was smaller away from the magnetic equator, implying a variation of neutral wind with latitude. This is reproduced in the altitude variation of zonal drift observed by the Jicamarca radar. During a magnetic storm, scintillation measurements indicated that eastward drifts near the magnetic equator are accompanied by westward drifts near the anomaly peak, which is consistent with the effects of a disturbance dynamo. The campaign results indicate that in order to resolve the variability of ESF, a careful probing of neutral dynamics as a function of latitude needs to be undertaken during the postsunset period.

Equatorial scintillation and systems support

The need to nowcast and forecast scintillation for the support of operational systems has been recently identified by the interagency National Space Weather Program. This issue is addressed in the present paper in the context of nighttime irregularities in the equatorial ionosphere that cause intense amplitude and phase scintillations of satellite signals in the VHF/UHF range of frequencies and impact satellite communication, Global Positioning System navigation, and radar systems. Multistation and multifrequency satellite scintillation observations have been used to show that even though equatorial scintillations vary in accordance with the solar cycle, the extreme day-to-day variability of unknown origin modulates the scintillation occurrence during all phases of the solar cycle. It is shown that although equatorial scintillation events often show correlation with magnetic activity, the major component of scintillation is observed during magnetically quiet periods. In view of the day-to-day variability of the occurrence and intensity of scintillating regions, their latitude extent, and their zonal motion, a regional specification and short-term forecast system based on real-time measurements has been developed. This system, named the Scintillation Network Decision Aid, consists of two latitudinally dispersed stations, each of which uses spaced antenna scintillation receiving systems to monitor 250-MHz transmissions from two longitudinally separated geostationary satellites. The scintillation index and zonal irregularity drift are processed on-line and are retrieved by a remote operator on the Internet. At the operator terminal the data are combined with an empirical plasma bubble model to generate three-dimensional maps of irregularity structures and two-dimensional outage maps for the region.

The multi-instrumented studies of equatorial thermosphere aeronomy scintillation system : Climatology of zonal drifts

A spaced-antenna scintillation system was installed at Ancon, Peru, in May 1994 to measure scintillation of 250-MHz signals from a geostationary satellite by three antennas spaced in the magnetic east-west direction. These measurements were used to establish the climatology of the zonal drift of the irregularities which cause equatorial scintillations. The major objective of this study is to compare this drift climatology to the climatology of zonal neutral wind which is the driver of the equatorial electrodynamics. A comparison of these two climatologies in conjunction with scintillation statistics may provide some clues regarding factors which help or hinder the formation of equatorial spread-F (ESF). With these objectives in mind, the first years drift and scintillation statistics have been presented as a function of local time, season and magnetic activity and compared with the statistics of ion drift published earlier from incoherent scatter radar observations. The scintillation drift is in good agreement with the Jicamarca radar observations except for the fact that the local time dependence of our drift observations exhibit a broader maximum. The broad maximum may be attributed to lower ion drag experienced in the presence of ESF due to sustained uplifting of the ionosphere. During magnetically active periods, the scintillation drift often exhibits east to west reversals presumably because of the disturbance dynamo effects. The westward drifts during such reversals may be as large as 100 m/s. We have also modeled the zonal drifts as a seasonal basis by using Hedins neutral wind model and Andersons fully analytical ionospheric model. The modeled zonal drifts present good quantitative agreement with the drifts obtained with the scintillation technique.

Measurement of the latitudinal distributions of total electron content during equatorial spread F events

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.

Characteristics of plasma structuring in the cusp/cleft region at Svalbard

Satellite scintillation, all-sky optical imager, and digisonde observations were coordinated during a cusp campaign conducted at Ny Alesund, Svalbard (78.9°N, 11.8°E 75.7°N corrected geomagnetic latitude, over the period January 4–15, 1997. This paper is focused on a study of the distribution and dynamics of mesoscale (tens of kilometers to tens of meters) electron density irregularities in the dayside auroral region. This study has been performed at Ny Alesund, Svalbard, by measuring the effects of these irregularities on the amplitude scintillation of 250-MHz transmissions from a quasi-stationary polar satellite as well as the amplitude and phase scintillation of 1.6-GHz signals from Global Positioning System (GPS) satellites. These GPS scintillation measurements were augmented by the use of dual-frequency (1.2 and 1.6 GHz) GPS phase data acquired at the same station by the Jet Propulsion Laboratory for the International GPS Geodynamic Service (IGS). The continuous 250-MHz scintillation observations explored the daytime auroral ionosphere 2° poleward of Ny Alesund and showed that the scintillation spectra are often broad, as may be expected for irregularities in a turbulent flow region. Such irregularity dynamics were detected poleward of the nominal cusp region over the interval of 0600–1500 magnetic local time. The period of observations included the magnetic storm of January 10–11, 1997, when GPS observations of the IGS detected polar cap patches with total electron contents of 3×1016 m−2 and large-scale (tens of kilometers) phase variations at the GPS frequency of 1.6 GHz that corresponded to temporal gradients of 2×1016 m−2 min−1. However, amplitude scintillations at the GPS frequency of 1.6 GHz could not be detected in association with these large-scale phase variations, indicating that the irregularities with wavelengths less than the Fresnel dimension of 400 m were below the detectable limit. This is shown to be consistent in the context of enhanced ionospheric convection determined by digisonde and scintillation spectra.

Modeling daytime F layer patches over Sondrestrom

A comprehensive, time-dependent, high-latitude, one-species F region model has been developed to study the various physical processes which are believed to affect the polar cap plasma density distributions as a function of altitude, latitude, longitude, and local time. These processes include production of ionization by solar extreme ultraviolet radiation and particle precipitation; loss through charge exchange with N 2 and O 2 ; and transport by diffusion, neutral winds, and convection E×B drifts. In our initial calculations we have modeled highly structured plasma densities characterized by digisonde observations at Sondrestrom using both a time-dependent global convection pattern and spatially localized regions of transient high-speed flow

Modeling the formation of polar cap patches using large plasma flows

Recent measurements made with the Sondrestrom incoherent scatter radar have indicated that the formation of polar cap patches can be closely associated with the flow of a large plasma jet. In this paper, we report the results of a numerical study to investigate the role of plasma jets on patch formation, to determine the temporal evolution of the density structure, and to assess the importance of O+ loss rate and transport mechanisms. We have used a time-dependent model of the high-latitude F region ionosphere and model inputs guided by data collected by radar and ground-based magnetometers. We have studied several different scenarios of patch formation. Rather than mix the effects of a complex of variations that could occur during a transient event, we limit ourselves here to simulations of three types to focus on a few key elements. The first attempt employed a Heelis-type pattern to represent the global convection and two stationary vortices to characterize the localized velocity structure. No discrete isolated patches were evident in this simulation. The second modeling study allowed the vortices to travel according to the background convection. Discrete density patches were seen in the polar cap for this case. The third case involved the use of a Heppner and Maynard pattern of polar cap potential. Like the second case, patches were seen only when traveling vortices were used in the simulation. The shapes of the patches in the two cases of moving vortices were defined by the geometrical aspect of the vortices, i.e. elliptical vortices generated elongated patches. When we “artificially” removed the Joule frictional heating, and hence any enhanced O+ loss rate, it was found that transport of low density plasma from earlier local times can contribute to ∼60% of the depletion. We also found that patches can be created only when the vortices are located in a narrow local time sector, between 1000 and 1200 LT and at latitudes close to the tongue of ionization.

Equatorial plasma depletion precursor signatures and onset observed at 11° south of the magnetic equator

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.

Formation of polar cap patches associated with north-to-south transitions of the interplanetary magnetic field

On January 15, 1991, the Sondrestrom incoherent scatter radar probed the midday high-latitude ionosphere to gather evidence for the formation and entry of polar cap patches. During the experiment the interplanetary magnetic field (IMF) BZ was positive and steady for few hours until 1548 UT when a short negative excursion of BZ occurred. Prior to the BZ excursion, and when this parameter was directed northward, the Sondrestrom radar detected a quasi-stationary density feature situated near the poleward boundary of the oval. Convergent electric fields and slightly elevated Te values were seen accompanying the F region density feature. A numerical model of the high-latitude ionosphere that uses a steady north BZ Heppner and Maynard convection pattern suggests that under these IMF conditions a tongue of ionization (TOI) can be formed near the midday sector, but it is confined to the poleward boundary of the auroral oval. It does not traverse into the polar cap. This simulated BZ northward TOI resembles the density feature seen in the radar data prior to the BZ negative excursion. When the BZ value was oriented southward, the radar detected the density feature moving poleward and then disappearing to the north of the radar field of view. At this time of BZ negative the radar data also displayed elevated Ti values and a new pattern of line-of-sight velocities. Nearly 34 min after the density feature departed from the field of view of the Sondrestrom radar, the Qaanaaq digisonde measured a factor of 2 increase in the ƒoF2 values. Similar enhancements are typically attributed to the passage of a patch. We also conducted a numerical simulation of the transit of the density feature from its initial location near the polar cap boundary up to its passage through the Qaanaaq station. The time that the density feature reaches Qaanaaq in our simulations is in good agreement with the actual time that the enhanced patch-like number density was observed at Qaanaaq. The BZ switching mechanism does not dispute the validity of other patch formation mechanisms; it merely suggests that a BZ northward TOI can end up as a polar cap patch if a timely reversal of BZ occurs.

Simultaneous observations of polar cap patches and Sun‐aligned arcs during transitions of the IMF

This paper presents the first observations of simultaneous polar cap patches and polar cap arcs in a single common 1000-km field of view, and identifies a model that explains the interplanetary magnetic field (IMF) dependencies of the observed phenomenology. To study the characteristics of the polar cap optical emissions in the 630.0 nm line during transitions of the IMF Bz, we have scanned images taken at Qaanaaq, Greeland, between 1989 and 1994. We found that on a few occasions, when Bz changed from a south to a north orientation, a particular pattern of polar cap patches and Sun-aligned arcs coexisted. No similar pattern of coexisting arcs and patches was found during north-to-south IMF transitions. The detailed analyses of three of these events are presented here in which patches and polar cap arcs are clearly identified to reside simultaneously within the Qaanaaq imager field of view. The digisonde located also at Qaanaaq is used to confirm that the optical patches correspond to enhancements in the number density and a simultaneous decrease of the hmF2 value. These two factors increase the capability of the imager to differentiate between patches and the background airglow. Data collected by the DMSP F8 satellite during one of the events reaffirm the appearance of polar cap precipitation during the Bz positive period. The J4 sensor on board DMSP F8 detected typical electron fluxes commonly associated with polar cap arcs. The coexistence of patches and arcs is due to a slower response of the patches in exiting the polar cap, and then the relatively sudden appearance of polar cap arcs presumably driven by dayside reconnection between the IMF and open flux drawn initially equatorward toward the cusp. This model, of dayside reconnection switching from equatorward of the cusp for Bz south to poleward of the cusp for Bz north, likewise explains why arcs and patches are seen by the imager to coexist for rapid Bz reversals only from south to north and not from north to south.