C. E. Valladares

Ionospheric effects of major magnetic storms during the International Space Weather Period of September and October 1999: GPS observations, VHF/UHF scintillations, and in situ density structures at middle and equatorial latitudes

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

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.

Longitudinal variability of equatorial plasma bubbles observed by DMSP and ROCSAT‐1

Abstract : We compare observations of equatorial plasma bubbles (EPBs) by polar-orbiting satellites of the Defense Meteorological Satellite Program (DMSP) with plasma density measurements from the Republic of China Satellite (ROCSAT-1) in a low-inclination orbit. DMSP data were acquired in the evening sector at low magnetic latitudes between 1989 and 2002. ROCSAT-1 plasma densities were measured in March and April of 2000 and 2002. Observations of individual EPBs detected by both ROCSAT-1 and DMSP were well correlated when satellite orbital paths crossed the same longitude within approximately plus or minus 15 min. We compiled a statistical database of ROCSAT-1 occurrence rates sorted by magnetic local time (MLT), magnetic latitude, and geographic longitude. The rate of ROCSAT-1 EPB encounters at topside altitudes rose rapidly after 1930 MLT and peaked between 2000 and 2200 MLT, close to the orbital planes of DMSP F12, F14, and F15. EPB encounter rates have Gaussian distributions centered on the magnetic equator with half widths of ^8. Longitudinal distributions observed by ROCSAT-1 and DMSP are qualitatively similar, with both showing significantly fewer occurrences than expected near the west coast of South America. A chain of GPS receivers extending from Colombia to Chile measured a west-to-east gradient in S4 indices that independently confirms the existence of a steep longitudinal gradient in EPB occurrence rates. We suggest that precipitation of energetic particles from the inner radiation belt causes the dearth of EPBs. Enhancements in the post sunset ionospheric conductance near the South Atlantic Anomaly cause a decrease in growth rate for the generalized Rayleigh-Taylor instability. Results indicate substantial agreement between ROCSAT-1 and DMSP observations and provide new insights on EPB phenomenology.

Modeling Polar Cap F-Region Patches Using Time Varying Convection

Here the authors present the results of computerized simulations of the polar cap regions which were able to model the formation of polar cap patches. They used the Utah State University Time-Dependent Ionospheric Model (TDIM) and the Phillips Laboratory (PL) F-region models in this work. By allowing a time varying magnetospheric electric field in the models, they were able to generate the patches. This time varying field generates a convection in the ionosphere. This convection is similar to convective changes observed in the ionosphere at times of southward pointing interplanetary magnetic field, due to changes in the B[sub y] component of the IMF.

Interplanetary magnetic field dependency of stable sun-aligned polar cap arcs

This is the first analysis, using a statistically significant data set, of the morphological dependence of the presence, orientation, and motion of stable sun-aligned polar cap arcs upon the vector interplanetary magnetic field (IMF). For the one winter season analyzed we had 1392 all-sky 630.0-nm images of 2-min resolution containing a total of 150 polar cap arcs, all with corresponding values of the IMF as measured by IMP 8 or ISEE 2. After demonstrating an unbiased data set with smooth normal distributions of events versus the dimensions of time, space, and IMF component, we examine IMF dependencies of the properties of the optical arcs. A well-defined dependence for Bz is found for the presence/absence of stable Sun-aligned polar cap arcs. Consistent with previous statistical studies, the probability of observing polar cap aurora steadily increases for larger positive values of Bz, and linearly decreases when Bz becomes more negative. The probability of observing Sun-aligned arcs within the polar cap is determined to vary sharply as a function of the arc location; arcs were observed 40% of the time on the dawnside and only 10% on the duskside. This implies an overall probability of at least 40% for the whole polar cap. 20% of the arcs were observed during “southward IMF conditions”, but in fact under closer inspection were found to have been formed under northward IMF conditions; these “residual” positive Bz arcs had a delayed residence time in the polar cap of about what would be expected after a north to south transition of Bz. A firm dependence on By is also found for both the orientation and the dawn-dusk direction of motion of the arcs. All the arcs are Sun-aligned to a first approximation, but present deviations from this orientation, depending primarily upon the location of the arc in corrected geomagnetic (CG) coordinates. The arcs populating the 06-12 and the 12-18 quadrants of the CG coordinate system point toward the cusp. The By dependency of the arc alignment is consistent with a cusp displacement in local time according to the sign of By. We found that the arc direction of motion depended both on By and the arc location within the polar cap. For a given value of By, two well-defined regions (or cells) exist. Within each cell the arcs move in the same direction toward the boundary between the cells. The arcs located in the duskside move dawnward; those in the dawnside move duskward. The relative size of these dusk and dawn regions (or cells) are controlled by the magnitude of By. This persistent dusk-dawn motion of the polar cap arcs is interpreted in terms of newly open flux tubes entering the polar cap and exerting a displacement of the convective cells and the polar cap arcs that are embedded within them.

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.

Dynamics of equatorial F region irregularities from spaced receiver scintillation observations

Spaced receiver observations of amplitude scintillations on a 244 MHz signal, at an equatorial station, have been used to study random temporal changes associated with the scintillation-producing irregularities and the variability of their motion. The computed drift of the scintillation pattern shows the presence of velocity structures associated with equatorial bubbles in the early phase of their development. On magnetically quiet days, after 22:00 LT, the estimated drifts fall into a pattern which is close to that of the ambient plasma drift. There is considerable decorrelation between the two signals until 22:00 LT. The power spectra of the most highly correlated scintillations recorded by spaced receivers indicate that the associated irregularities are confined to a thin layer on the bottomside of the equatorial F region. This suggests that the convection pattern associated with bottomside irregularities is stable due to the dominance of ion-neutral collisions over ion inertia.

Understanding space weather to shield society : A global road map for 2015-2025 commissioned by COSPAR and ILWS

There is a growing appreciation that the environmental conditions that we call space weather impact the technological infrastructure that powers the coupled economies around the world. With that co ...