Storm-enhanced Density Research Articles

Ionospheric Threat Parameterization for Local Area Global-Positioning-System-Based Aircraft Landing Systems

Observations of extreme spatial rates of change of ionospheric electron content and the characterization strategy for mitigation applied by the U.S. local area augmentation system are shown. During extreme ionospheric activity, the gradient suffered by a global navigation satellite system user a few kilometers away from a ground reference station may reach as high as 425 mm of delay (at the GPS L1frequency) per km of user separation. The method of data analysis that produced these results is described, and a threat space that parameterizes these possible threats to user integrity is defined. Certain configurations of user, reference station, global navigation satellite system satellite, and ionospheric storm-enhanced density may inhibit detection of the anomalous ionosphere by the reference station.

Storm‐enhanced plasma density features, daytime polar cap plasma enhancements, and their underlying plasma flows investigated during superstorms

This study investigates the effects of November superstorms on the daytime‐evening northern topside ionosphere. We utilize multi‐instrument interplanetary and geomagnetic data to analyze the interplanetary shock/electric (E) field events. To assess their impact, field‐aligned DMSP (Defense Meteorological Satellite Program) passes are employed. We tracked evening storm‐enhanced density (SED) features and daytime polar plasma enhancements occurring regularly that allowed us to learn about the basic mechanisms responsible for their development. Results show that large vertical downward plasma flows supplied the plasma building up the SED features and polar plasma enhancements detected. These plasma formations occurred during the main phase with the forward superfountain and during a substorm with the reverse fountain. These provide observational evidence that primary downward plasma flows, triggered by detachment processes caused by the westward subauroral polarization stream (SAPS) E‐fields, existed and became transported sunward. At SED latitudes, these primary downward plasma flows were intensified by the forward superfountain when its upward plasma flow spilled over into the equatorial anomaly and cascaded down the magnetic field lines. When the reverse fountain was active, such intensification was not available to the primary downward plasma flows at SED latitudes. As the downward streaming plasma found its way to the polar cap, it resulted in the development of daytime polar plasma enhancements. Statistically, daytime polar plasma enhancements occurred at all longitudes contradicting the myth that the North American region is better suited for the development of large plasma enhancements in the polar cap.

F region behavior in the SED plume during a geomagnetic superstorm: A case study

We report observations from the GPS total electron content (TEC), ionosonde, and Millstone Hill incoherent scatter radar showing some interesting features of the height profile of the ionospheric electron density that occurred during a passage of a storm‐enhanced density (SED) plume during the geomagnetic superstorm on 20 November 2003. First, the TEC increases to about 6.58 times the quiet day value, while the F peak plasma density (NmF2) only enhances by a factor of 1.37. Second, the F2 layer peak height (hmF2) is dramatically lifted from the quiet time altitude of 250 km to an unusual altitude of 577 km. Furthermore, the topside Chapman scale height HT is larger by a factor of 6.25 compared to the quiet time value. As a result, the electron density content with an α‐Chapman function above 700 km contributes about 59% to the total content during the SED event. Our analysis shows that during a passage of the SED plume, the enhancement of TEC is mainly attributed to the great increase of HT with a very high hmF2.

Unusually elongated, bright airglow plume in the polar cap F region: Is it a tongue of ionization?

We report an event of unusually elongated, bright airglow plume, which is considered as an optical manifestation of tongue of ionization (TOI) in the central polar cap. This optical structure was detected with an all‐sky airglow imager at Resolute Bay (74.73°N, 265.07°E) during a large magnetic storm on December 15, 2006. The absolute optical intensity of the plume was ≈1 kR, which is much brighter than that of non‐stormtime polar cap patches. Two‐dimensional imaging capability of the all‐sky imager demonstrates that some meso‐scale structures (≈250–600 km) were embedded within the plume. Simultaneous ion density and drift measurements with the DMSP spacecraft strongly suggest that the plume was extending from the dayside as a narrow stream of dense plasma and thus is an optical manifestation of polar cap TOI. The DMSP data also implies that the possible source of the plume is a narrow stream of storm enhanced density (SED) transported from lower latitudes. The DMSP auroral particle observation demonstrates that the polar cap extremely expanded equatorward during this interval. This extreme expansion allowed the anti‐sunward convection to capture plasmas within the SED and deliver them deep into the polar cap as a luminous airglow plume. This observation claims that the plasma transport from the dayside lower latitudes plays an important role in controlling the plasma environment in the polar cap ionosphere during magnetic storms.

Monitoring storm-enhanced density using IGS reference station data

Storm-enhanced density (SED) is a geomagnetic storm phenomenon, characterized by a plume of enhanced total electron content (TEC) that initially moves poleward and sunward extending out from a larger region of enhanced TEC in the mid-latitudes. SED is associated with extreme mid-latitude space weather effects. Sharp gradients in the TEC are found along the borders of SED plumes and at the boundaries of the larger TEC region (the base of the plume). These large TEC gradients can cause significant errors in DGPS and WADGPS positioning and can result in serious consequences for applications such as railway control, highway traffic management, emergency response, commercial aviation and marine navigation, all of which require high precision, real-time positioning. Data from the global IGS network of GPS receivers have enabled the spatial and temporal visualization of these SED plumes, allowing ionospheric researchers to study this phenomenon and investigate the potential for developing prediction techniques and real-time warning systems. GPS TEC maps provided by analysis of the data from the IGS network have now been widely disseminated throughout the atmospheric research community and have become one of the standard means of studying the effects of geomagnetic storms on the ionosphere. These maps have enabled researchers to identify that the SED phenomenon occurs globally, is associated with large TEC gradients (at times greater than 100 TEC units per degree latitude), and is a magnetically conjugate phenomenon. This paper reports on the recent advances in our understanding of the SED phenomenon enabled by GPS observations.

Studies of storm-enhanced density impact on DGPS using IGS reference station data

DGPS services are provided in support of land and marine applications by many government agencies worldwide. Horizontal positioning accuracies in the order of several metres are typically achieved for these systems. Under high levels of ionospheric activity, however, significant degradations in DGPS positioning accuracies can occur. In particular, gradients of up to 50 ppm are associated with a feature known as storm-enhanced density (SED). This feature is a localized enhancement of total electron content (TEC) extending north through the mid-latitudes into the polar region. DGPS positioning errors of 20 m or more can persist for hours during such events. In this paper, archived IGS data from GPS reference stations are used to derive high-resolution TEC maps for two SED events. The impact of SED effects on DGPS horizontal positioning accuracies is then quantified using data from select IGS reference stations in North America and Europe. Results indicate that positioning accuracies may be degraded by factors as large as 10–20 during such events.

Study of Ionospheric Irregularities in the Vicinity of SED Region During a Super Geomagnetic Storm

基于IGS提供的TEC数据, 研究了2003年10月大磁暴期间的暴时密度增强(Storm Enhanced Density, SED)现象; 利用GPS观测数据, 计算出ROTI (Standard deviations of ROT)指数, 分析了SED边界附近电离层小尺度不均匀体结构的时间和空间演变. 研究表明, 在磁暴主相期间SED边界附近不均匀体随着磁暴的发展逐渐增多; 在主相的中后期不均匀体的分布密集度达到最大; 在恢复相期间, 不均匀体分布很少; 随着磁暴的发展, 不均匀体开始主要集中在40~45ºN范围内, 随后向高纬漂移, 主要集中在45~55ºN范围内.

DMSP/GPS observations of intense ion upflow in the midnight polar ionosphere associated with the SED plume during a super geomagnetic storm

We report observations from the GPS TEC and DMSP F‐15 satellite showing that a very strong upward field‐aligned (FA) plasma velocity and flux at F‐region heights in the auroral zone/polar cap boundary during a passage of the polar tongue of ionization (TOI) full of storm‐enhanced density (SED) materials occurred in the event of the super geomagnetic storm on Nov. 20, 2003. The upward FA ion velocities in excess of 460 m/s are obtained from observations of the DMSP F‐15 satellite. With enhancements of the plasma density caused by the TOI, FA ion fluxes are estimated to about 1.2 × 1014 ions m−2s−1, which are comparable to those observed by the ground‐based radar in the polar cusp. Therefore, during the super geomagnetic storm a s SED can cause a TOI plume at F region altitudes, which leads to a strong upward ion flux reaching 1–2 orders of magnitude larger than that of the typical flux at midnight auroral zone/polar cap boundary. Through estimations of the influence of those upflow ions associated with SED plumes on the development of the super storm, our result suggests that the midnight auroral zone/polar cap boundary becomes an important source to directly provide rich O+ to the ring current during super geomagnetic storms.

GPS observations at quasi-conjugate points under disturbed conditions

The conjugacy effects of the GPS scintillation activities during the geomagnetic storms of October 2003, November 2003 and July 2004 have been investigated at the approximately geomagnetically conjugate stations: Scott Base, Antarctica (SBA) and Resolute Cornwallis Island (RESO) in the high Arctic region. The measurements aim at investigation of the similarities and differences of the scintillation activities occurring at the conjugate points in the polar regions under storm conditions and examine the relationship between the Storm Enhanced Density (SED) and scintillation activity.

The occurrence of ionospheric signatures of plasmaspheric plumes over different longitudinal sectors

Plasmaspheric plumes have ionospheric signatures and are observed as storm‐enhanced density (SED) in global positioning system (GPS) total electron content (TEC). These ionospheric signatures have been primarily observed over the American sector and in a few limited examples over the European sector. This study examines the longitudinal occurrence frequency of plasmaspheric plumes. We analyzed all images from the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) Extreme Ultraviolet Imager (EUV) databases for the first half of 2001 and identified a total of 31 distinct plume intervals observed during different storm events. Out of the total IMAGE EUV plumes that we identified, 12 were projected over North America, 10 over Asia, and the remaining 9 were over Europe and the Atlantic Ocean. Using ground‐based GPS TEC from MIT's Madrigal database, we searched for corresponding SED/TEC plumes at different longitudinal sector and found 12 ionospheric SED plume signatures over North America, 4 over Europe, and 2 over Asia. This indicates that the observation probability of an ionospheric SED plume when a plasmaspheric plume is seen is 100% in the American sector, 50% in the European sector, and 20% in the Asian sector. This could be due to the fact that the plumes may be either positioned beyond the limit of the ground‐based GPS field of view, which happens mainly when there is less plasmaspheric erosion, or are too weak to be detected by the sparse number of GPS receivers over Asia. The in situ plasma densities from the available coincident defense metrological satellite program (DMSP) satellites were also used to study the characteristics of SED/TEC plume at DMSP orbiting altitude (i.e., ∼870 km). The TOPographic EXplorer (TOPEX) altimeter TEC also is used to identify the conjugate SED/plume signature over the Southern Hemisphere.

Storm-Time Strong Field-Aligned Ion Upflow in Region of the SED Plume

We report the observations from the GPS TEC and DMSP F-13 satellites showing that very strong upward field-aligned (FA) ion velocity and flux in the outer region of the storm-enhanced density (SED) occurred in the event of the geomagnetic storm on 29–31 May 2003. By a method of coordinate transformation, upward FA ion velocities in excess of 250 m/s are obtained from the observations of the DMSP F-13 satellite. Further, an FA ion flux is estimated to be about 4.5 × 1013 ions/m2s in the dusk sector. The estimated FA ion velocity and flux provide a powerful direct proof to support the scenario that there is a strong coupling of particles between the ionosphere and plasmasphere in the region of the SED plume. In the process, FA ion flux transports from the ionosphere to the plasmasphere in the region of the SED plume. Therefore, the plume of SED in the ionosphere provides an important source to the enhanced density of O+ in the storm-time plasmasphere.

Space Weather and the Global Positioning System

Space Weather and the Global Positioning System

Potential for issuing ionospheric warnings to Canadian users of marine DGPS

Under normal operating conditions marine DGPS horizontal positioning accuracies on the order of several meters are achieved in North America. Degradations in positioning accuracy can occur during enhanced ionospheric activity. An ionospheric phenomenon known as storm enhanced density (SED) is observed to develop in the middle to high latitudes during ionospheric storm events. Very large gradients in total electron content are observed in the vicinity of this feature with DGPS positioning errors increased by a factor of 10–30 versus quiet conditions. The specific evolution of a given SED event and the magnitude of expected impact are not generally predictable. A method to monitor development of SED is to compute ionospheric maps in real time. Local gradients can then be computed for various geographic regions from North American maps of ionospheric delay. Sources of real‐time ionospheric information include the Wide Area Augmentation System (WAAS) and the Canadian GPS•C service. These are wide area differential GPS systems. In this paper, a real‐time ionospheric warning system is investigated for North American (primarily Canadian) DGPS users based on available real‐time data. The WAAS and GPS•C ionospheric models are inadequate to resolve ionospheric gradients for 100–200 km scale sizes. Raw GPS data from GPS•C reference sites can be used, however, to observe large ionospheric gradients and interpret the expected impact on DGPS users. Potential exists to issue marine user warnings based on this method. Results of this work can readily be extended to land DGPS applications, such as the NDGPS service in the United States.

Storm enhanced densities (SED) as possible sources for Cleft Ion Fountain dayside ionospheric outflows

Foster et al. (2002) and others have reported on elevated ionospheric density regions being convected from the subauroral plasmaspheric region toward noon, in association with convection of plasmaspheric tails in the dayside magnetosphere. It has been suggested that these so‐called Storm Enhanced Density (SED) regions could serve as ionospheric plasma source populations for cleft ion fountain outflows. To investigate this scenario, the University of Texas at Arlington Dynamic Fluid Kinetic (DyFK) model is employed to simulate the entry of a high‐density plasmasphere‐like flux tube entering the cleft region and subjected to an episode of wave‐driven transverse ion heating. In these transport simulations, the O+ ion density at higher altitudes is found to increase while the densities at lower altitudes decrease, following this heating episode. Substantial O+ outfluxes are found indicating that the SED‐associated O+ ions acquire sufficient energy to reach higher altitudes after the effects of transverse wave heating. O+ outfluxes of 2–3.5 × 108 cm−2 s−1 persist for more than 90 minutes during the simulation episode considered. The simulation results indicate that SED can be a significant source for the cleft ion fountain, and thus in turn, O+ ions to the magnetosphere.

Longitude sector comparisons of storm enhanced density

Examples of storm enhanced density (SED) formation over Russia and Northern Europe are presented. These events, which persisted 15–20 hours, were fixed in local time near noon over Europe and then later observed over the American sector. The amount of total electron content (TEC) at the base of the SED erosion plume is found to be greatest in the American sector. A persistent, repeatable pattern is apparent in the time evolution of the latitude location of the SED plume base, although the latitudinal rate of change differs between the two sectors. In the European sector the invariant latitude (Λ) of the SED plume base is observed to be between 61°–63° Λ and at a time close to local noon. In the American sector, the position of the base of the plume shifts from local noon towards dusk, and moves to a lower latitude at a nearly fixed longitude.

Storm enhanced density: magnetic conjugacy effects

Abstract. In the early phases of a geomagnetic storm, the low and mid-latitude ionosphere are greatly perturbed. Large SAPS electric fields map earthward from the perturbed ring current overlapping and eroding the outer plasmasphere and mid-latitude ionosphere, drawing out extended plumes of storm enhanced density (SED). We use combined satellite and ground-based observations to investigate the degree of magnetic conjugacy associated with specific features of the stormtime ionospheric perturbation. We find that many ionospheric disturbance features exhibit degrees of magnetic conjugacy and simultaneity which implicate the workings of electric fields. TEC enhancements on inner-magnetospheric field lines at the base of the SED plumes exhibit localized and longitude-dependent features which are not strictly magnetic conjugate. The SED plumes streaming away from these source regions closely follow magnetic conjugate paths. SED plumes can be used as a tracer of the location and strength of disturbance electric fields. The SED streams of cold plasma from lower latitudes enter the polar caps near noon, forming conjugate tongues of ionization over the polar regions. SED plumes exhibit close magnetic conjugacy, confirming that SED is a convection electric field dominated effect. Several conclusions are reached: 1) The SED plume occurs in magnetically-conjugate regions in both hemispheres. 2) The position of the sharp poleward edge of the SED plume is closely conjugate. 3) The SAPS electric field is observed in magnetically conjugate regions (SAPS channel). 4) The strong TEC enhancement at the base of the SED plume in the north American sector is more extensive than in its magnetic conjugate region. 5) The entry of the SED plume into the polar cap near noon, forming the polar tongue of ionization (TOI), is seen in both hemispheres in magnetically-conjugate regions.

Conjugate localized enhancement of total electron content at low latitudes in the American sector

We report evidence for a discrete localized enhancement of total electron content (TEC) in magnetic-conjugate regions at dusk in the American sector during large magnetic storms. After their formation, the conjugate enhancements persist at these longitudes and corotate into the night sector. We use ground-based GPS TEC and Millstone Hill radar observations, and DMSP overflights to investigate this phenomenon. Both poleward and eastward components of the disturbance electric field are needed to account for the observed redistribution of cold plasma at low and mid latitudes inside the plasmapause. We conclude that horizontal plasma transport associated with disturbance electric fields is the dominant mechanism for producing the conjugate TEC enhancements. The repeatability and geographic localization of this feature indicate a longitudinal specificity for the process associated with its formation. This long-lasting conjugate enhancement in TEC serves as a corotating source for the erosion plumes of storm-enhanced density seen during such events.

On a possible relationship between density depletions in the SAA region and storm-enhanced densities in the conjugate hemisphere

During the great magnetic storm of July 15, 2000, the ROCSAT-1 satellite with an orbital altitude of 640 km detected a large density depletion region at low latitudes (<35°) in the southern hemisphere while storm-enhanced density (SED) was observed in the northern hemisphere. Convective electric fields deduced from measurements of the ROCSAT-1 drift meters and retarding potential analyzer (RPA) indicate negligible electric fields in the eastern portion of the SED region where density enhancement was evident, suggesting a lack of ion upward motion at the equator of the SED longitudes. However, a localized electric field enhancement was observed in association with South Atlantic magnetic anomaly density structures (MADS) during the event. The zonal electric field component was eastward corresponding to radial convection, and the radial electric field component on the magnetic meridional plane was outward corresponding to westward convection. In general, both components of the observed convective electric fields were enhanced inside the density depletion region and reduced inside the density enhancement. The electric field patterns are consistent with those arising from a circular region of flux tubes with enhanced conductance in the presence of a background electric field. The mapping of the MADS region and convective electric fields from the southern hemisphere to the northern hemisphere suggests that these localized electric fields could drive ionospheric plasma from the conjugate MADS region to the SED at higher latitudes. We thus propose a new idea that the conjugate MADS region could be the plasma source for producing the SED.

On the relationship of SAPS to storm-enhanced density

We use magnetic field-aligned mapping between the ionosphere and the magnetosphere to intercompare ground-based observations of storm enhanced density (SED), and plasmasphere drainage plumes imaged from space by the IMAGE EUV imager, with the enhanced inner-magnetosphere/ionosphere SAPS electric field which develops during large storms. We find that the inner edge of the SAPS electric field overlaps the erosion plume and that plume material is carried sunward in the SAPS overlap region. The two phenomena, SED in the ionosphere and the erosion plume at magnetospheric heights, define a common trajectory for sunward-propagating cold plasma fluxes in the midnight—dusk–postnoon sector. The SAPS channel at ionospheric heights and its projection into the equatorial plane serve to define the sharp outer boundary of the erosion plume. The SAPS electric field abuts and overlaps both the plasmasphere boundary layer and the plasmasphere erosion plume from pre-midnight through post-noon local times.

Variations in the midlatitude and equatorial ionosphere during the October 2003 magnetic storm

The October 2003 geomagnetic storm (often called the Halloween storm) was one of the largest storms (as measured by Dst) yet recorded. The storm‐induced synoptic‐scale changes in the ionosphere's plasma content and density can be viewed through space weather maps created by objective analysis algorithms. For this study, these maps, which specify the electron density in altitude, latitude, and longitude, are created by the ionospheric data assimilation three dimensional (IDA3D), a three‐dimensional variation algorithm of the ionospheric electron density. These maps, representing the average conditions in the ionosphere over a 15 min sampling time, show how dramatically the ionosphere changed during the Halloween storm. Following the southward turning of the interplanetary magnetic field, the dayside electron content is significantly reduced in the equatorial ionosphere between ±18° magnetic latitude and is enhanced poleward of this latitude. This is the expected behavior when the equatorial fountain is enhanced by a strong penetration electric field. In addition, the electron content is significantly increased in the dayside midlatitude ionosphere, which corresponds to a storm‐enhanced density (SED) plume. Above 40° magnetic latitude, the dayside plasma content is significantly reduced in the regions adjacent to the SED structure, which enhances the electron content gradient. Electron density maps in the altitude–magnetic latitude plane show an increase in the topside electron densities within an SED plume.