Storm-enhanced Density Plume Research Articles

The High Latitude Ionospheric Response to the Major May 2024 Geomagnetic Storm: A Synoptic View

AbstractThe high latitude ionospheric evolution of the May 10‐11, 2024, geomagnetic storm is investigated in terms of Total Electron Content and contextualized with Incoherent Scatter Radar and ionosonde observations. Substantial plasma lifting is observed within the initial Storm Enhanced Density plume with ionospheric peak heights increasing by 150–300 km, reaching levels of up to 630 km. Scintillation is observed within the cusp during the initial expansion phase of the storm, spreading across the auroral oval thereafter. Patch transport into the polar cap produces broad regions of scintillation that are rapidly cleared from the region after a strong Interplanetary Magnetic Field reversal at 2230UT. Strong heating and composition changes result in the complete absence of the F2‐layer on the eleventh, suffocating high latitude convection from dense plasma necessary for Tongue of Ionization and patch formation, ultimately resulting in a suppression of polar cap scintillation on the eleventh.

A Statistical Analysis of the Morphology of Storm‐Enhanced Density Plumes Over the North American Sector

AbstractThe storm‐enhanced density (SED) is a large‐scale midlatitude ionospheric electron density enhancement in the local afternoon sector, which exhibits substantial spatial gradients and thus can impose detrimental effects on modern navigation and communication systems, causing potential space weather hazards. This study has identified a comprehensive list of 49 SED events over the continental US and adjacent regions, by examining strong geomagnetic storms occurring between 2000 and 2023. The ground‐based Global Navigation Satellite System (GNSS) total electron content and data from a new TEC‐based ionospheric data assimilation system were used to analyze the characteristics of SED. For each derived SED events, we have quantified its morphology by employing a Gaussian function to parameterize key characteristics of the SED, such as the plume intensity, central longitude, and half‐width. A statistical analysis of SEDs was conducted for the first time to characterize their climatological features. We found that the SED distribution exhibits a higher peak intensity and a narrower width as geomagnetic activity strengthens. The peak intensity of SED has maximum values around the equinoxes in their seasonal distribution. Additionally, we observed a solar cycle dependence in the SED distribution, with more events occurring during the solar maximum and declining phases compared to the solar minimum. SED plumes exhibit a sub‐corotation feature with respect to the Earth, characterized by a westward drift speed between 50 and 400 m/s and a duration of 3–10 hr. These information advanced the current understanding of the spatial‐temporal variation of SED characteristics.

Impacts of Subauroral Polarization Streams on Storm‐Enhanced Density Plume and Consequently on Polar Tongue of Ionization

AbstractThe influences of subauroral polarization streams (SAPS) on storm‐enhanced density (SED) plume and consequently on polar tongue of ionization (TOI), an important topic in the field of magnetosphere‐ionosphere‐thermosphere coupling, however, remain undetermined. The Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) with/without an empirical SAPS model has been used to investigate the impacts of SAPS on local ionosphere‐thermosphere system, which in turn affect the SED plume and consequently the TOI. The modeled total electron content and ion drift velocities agree reasonably well with the observations of Global Navigation Satellite System and Defense Meteorological Satellite Program satellites on 17 March 2013. The TIEGCM simulations show that SAPS can significantly affect the electron density of SED plume and consequently of TOI. The ionospheric‐thermospheric effect of SAPS can reduce the electron density of the SED plume in the afternoon sector and enhance the SED plume in the noon sector, which moves the SED plume dawnward. SAPS‐induced electron depletions in the throat region undergo anti‐sunward convection into the polar cap to weaken the TOI. A term‐by‐term analysis of the O+ ion continuity equation in the F‐region shows that the electron density depletions on the duskward edge of the SED plume are mainly due to the increased local plasma loss rates because of SAPS elevated plasma‐neutral temperatures and O/N2 reduction because of thermosphere upwelling. The electron density enhancements on the edge of the SED plume near noon are mainly due to the SAPS‐induced westward plasma E × B transports and the O/N2 increment because of thermospheric downwelling.

Sub‐Auroral Flows and Associated Magnetospheric and Ionospheric Phenomena Developed During 7–8 September 2017

AbstractThis study investigates the Sub‐Auroral Polarization Streams (SAPS) and Sub‐Auroral Ion Drifts (SAID) along with the associated magnetospheric and ionospheric phenomena developed during 7–8 September 2017. Then, a series of substorms occurred before and during the geomagnetic storm (SYM‐Hmin = −146 nT). Results are obtained by utilizing multi‐point observations. These show that during the pre‐storm period, when the ElectroMagnetic Ion‐Cyclotron waves resulted in drop‐out events, a series of sunward (westward) SAID‐in‐SAPS flows developed in the dusk‐midnight sector due to the small‐scale flow bursts reaching the SAPS channel and the westward SAPS flows were streaming antisunward after midnight. During the storm period, dawnside sunward (eastward) SAPS also developed, Kelvin‐Helmholtz (K‐H) vortices rolled along the magnetopause and the hot zone became structured by reflected K‐H waves near the plasmapause leading to auroral undulations (AUs). New findings include the correlation of AUs and K‐H waves in the structured hot zone implying that the Near‐Earth Plasma Sheet acted as a resonator after activation by the K‐H vortices on the magnetopause. Further evidence is provided by the dayside Equatorward Boundary Intensification and Poleward Moving Auroral Forms of which development was supported by the K‐H vortices on the magnetopause by carrying the magnetosheath plasma into the magnetosphere. Adding to previous studies, new results also include the correlated observations of storm‐enhanced density (SED) plume and poleward drifts underlying both the equatorward plasma bubbles and the higher latitude plasma density depletions, and thus verifying their poleward movement within the SED base and SED plume.

Simulation of Cold Ion Transport Originating From the SED Plume Into Dayside Magnetosphere

AbstractIn this study, based on the Dynamic‐Kinetic Fluid (DyFK) model, we conduct simulations on the field‐aligned transport of cold ions (<1 eV) within three independent closed flux tubes from different starting points (L = 3.0 Re, L = 3.122 Re and L = 3.5 Re, respectively) during the 2015 St. Patrick's Day storm. These flux tubes magnetically connect the plasmaspheric plume with the ionospheric Storm Enhanced Density (SED) plume but drift toward the dayside magnetopause under E×B effects. Meanwhile, we can obtain the density variations of three ions species (H+, O+, and He+, respectively) around the flux tubes' apex (at the magnetic equator) as a function of the L shell. The simulations present the temporal evolution of cold plasma along these drifting flux tubes. We found that with increasing initial L shell location of the flux tube, there are more ionospheric cold light ions (H+ and He+) flowing into the equatorial region while the inflows of cold heavy ions (O+) toward the equator decrease. As the simulated flux tube drifts toward dayside magnetopause from the main plasmasphere (L = 3.0 Re), the SED sourced heavy ions pile up near the magnetic equator at low L shells (L < 4.0 Re) due to gravitational effects, while the light ions rapidly reach density equilibrium along the flux tube. With the flux tube drifting beyond L > 4.0 Re, the piled‐up cold heavy ions will later transport into the magnetic equatorial region and furnish the dayside magnetosphere, demonstrating that the SED is another supplier of magnetospheric cold heavy ions (O+).

3‐D Regional Ionosphere Imaging and SED Reconstruction With a New TEC‐Based Ionospheric Data Assimilation System (TIDAS)

AbstractA new TEC‐based ionospheric data assimilation system (TIDAS) over the continental US and adjacent area (20°–60°N, 60°–130°W, and 100–600 km) has been developed through assimilating heterogeneous ionospheric data, including dense ground‐based Global Navigation Satellite System (GNSS) Total Electron Content (TEC) from 2,000+ receivers, Constellation Observing System for Meteorology, Ionosphere, and Climate radio occultation data, JASON satellite altimeter TEC, and Millstone Hill incoherent scatter radar measurements. A hybrid Ensemble‐Variational scheme is utilized to reconstruct the regional 3‐D electron density distribution: a more realistic and location‐dependent background error covariance matrix is calculated from an ensemble of corrected NeQuick outputs, and a three‐dimensional variational (3DVAR) method is adopted for measurement updates to obtain an optimal state estimation. The spatial‐temporal resolution of the reanalyzed 3‐D electron density product is as high as 1° × 1° in latitude and longitude, 20 km in altitude, and 5 min in universal time, which is sufficient to reproduce ionospheric fine structure and storm‐time disturbances. The accuracy and reliability of data assimilation results are validated using ionosonde and other measurements. TIDAS reanalyzed electron density is able to successfully reconstruct the 3‐D morphology and dynamic evolution of the storm‐enhanced density (SED) plume observed during the St. Patrick's day geomagnetic storm on 17 March 2013 with high fidelity. Using TIDAS, we found that the 3‐D SED plume manifests as a ridge‐like high‐density channel that predominantly occurred between 300 and 500 km during 19:00–21:00 UT for this event, with the F2 region peak height being raised by 40–60 km and peak density enhancement of 30%–50%.

3‐D Tomographic Reconstruction of SED Plume During 17 March 2013 Storm

AbstractThe three‐dimensional computerized ionospheric tomography (3DCIT) technique is used to reconstruct the spatial distribution of storm‐enhanced density (SED) based on the global positioning system total electron content measurements over the North American area during the 17 March 2013 storm. The reconstruction results are carefully validated with observations from three ionosonde stations, the constellation observing system for meteorology, ionosphere, and climate (COSMIC) radio occultations, and the Millstone Hill incoherent scatter radar. The electron density profiles from the 3DCIT reconstruction show a good agreement with the ionosonde and COSMIC electron density profiles. The 3DCIT‐derived electron density difference between the storm day of 17 March and the quiet day of 16 March also captures the similar SED plume signature that was observed by the Millstone Hill incoherent scatter radar. The 3DCIT reconstruction allows us for the first time to unveil the 3‐D configuration of the SED plume and its spatiotemporal evolution. It was found that the SED plume first appeared around 400 km and then expanded downward to ~300 km as well as upward to ~500 km over the course of a 3‐hr period from 19 to 22 UT on 17 March. Our study also showed that the density enhancement within the SED plume occurred mostly above the storm time F layer peak height.

Statistical Study of Ion Upflow and Downflow Observed by PFISR

AbstractIon upflow in the F region and topside ionosphere can greatly influence the ion density and fluxes at higher altitudes and thus has significant impact on ion outflow. We investigated the statistical characteristics of ion upflow and downflow using a 3‐year (2011–2013) data set from the Poker Flat Incoherent Scatter Radar (PFISR). Ion upflow is twice more likely to occur on the nightside than on the dayside in PFISR observations, while downflow events occur more often in the afternoon sector. Upflow and downflow on the dayside tend to occur at altitudes ~500 km, higher than those on the nightside. Both upflow and downflow occur more frequently as ion convection speed increases. Upflow observed from 16 to 6 magnetic local time through midnight is associated with temperature and density enhancements. Occurrence rates of upflow on the nightside and downflow on the dayside increase with geomagnetic activity level. On the nightside, occurrence rate of ion upflow increases with enhanced solar wind and interplanetary magnetic field (IMF) drivers as well as southwestward local magnetic perturbations. The lack of correlation of upflow on the dayside with the solar wind and IMF parameters is because PFISR is usually equatorward of the dayside auroral zone. Occurrence rate of downflow does not show strong dependence on the solar wind and IMF conditions. However, it occurs much more frequently on the dayside when the IMF By > 10 nT and the IMF Bz < −10 nT, which we suggest is associated with the decaying of the dayside storm‐enhanced density (SED) and the SED plume.

Large‐Scale Ionospheric Disturbances During the 17 March 2015 Storm: A Model‐Data Comparative Study

AbstractThis paper presents a detailed model‐data comparative study of the 17 March 2015 geomagnetic storm using the high‐resolution version of the thermosphere‐ionosphere‐electrodynamic general circulation model and the total electron content observations from a dense global navigation satellite system network. Driven by time‐dependent high‐latitude ionospheric convection and auroral precipitation inputs, together with an empirically defined subauroral plasma stream (SAPS) field, our simulation reproduce many observed storm‐related ionospheric phenomena, including large‐scale traveling ionospheric disturbances over Europe, the effects of prompt penetration electric field over South and Central America, and the formation of a storm‐enhanced density (SED) plume across the continental United States. Our simulation results reaffirm a number of important characteristics concerning the SED plume: (1) enhanced background ionospheric density is a necessary but not sufficient condition, and enhanced ion drift is required to form the SED plume; (2) the SAPS flow channel does not directly transport the plasma from midnight to postnoon via dusk to form the SED plume, instead, the SED plume is formed at the equatorward and westward edge of the SAPS channel; and (3) the SED plume appears to subcorotate with respect to the Earth.

Direct Observations of a Polar Cap Patch Formation Associated With Dayside Reconnection Driven Fast Flow

AbstractDayside solar‐produced concentrated F region plasma can be transported from the midlatitude region into the polar cap during geomagnetically disturbed period, creating plasma density irregularities like polar cap patches, which can cause scintillation and degrade performance of satellite communication and navigation at polar latitudes. In this paper, we observed and investigated a dynamic formation process of a polar cap patch during the 13 October 2016 intense geomagnetic storm. During the storm main phase, storm‐enhanced density (SED) was formed within an extended period of strong southward interplanetary magnetic field (IMF) Bz condition. Total electron content (TEC) map shows that a polar cap patch was segmented from the SED plume. The Sondrestrom Incoherent Scatter Radar (ISR) was right underneath the segmentation region and captured the dynamic process. It shows that the patch segmentation was related with a sudden northeastward flow enhancement reaching ~2 km/s near the dayside cusp inflow region. The flow surge was observed along with abrupt E region electron temperature increase, F region ion temperature increase, and density decrease. The upstream solar wind and IMF observations suggest that the flow enhancement was associated with dayside magnetic reconnection triggered by a sudden and short period of IMF By negative excursion. Quantitative estimation suggests that plasma density loss due to enhanced frictional heating was insufficient for the patch segmentation because the elevated F region density peaking at ~500 km made dissociative recombination inefficient. Instead, the patch was segmented from the SED by low‐density plasma transported by the fast flow channel from earlier local time.

Ionospheric F-region response to the 26 September 2011 geomagnetic storm in the Antarctica American and Australian sectors

Abstract. The ionospheric response at middle and high latitudes in the Antarctica American and Australian sectors to the 26–27 September 2011 moderately intense geomagnetic storm was investigated using instruments including an ionosonde, riometer, and GNSS receivers. The multi-instrument observations permitted us to characterize the ionospheric storm-enhanced density (SED) and tongues of ionization (TOIs) as a function of storm time and location, considering the effect of prompt penetration electric fields (PPEFs). During the main phase of the geomagnetic storm, dayside SEDs were observed at middle latitudes, and in the nightside only density depletions were observed from middle to high latitudes. Both the increase and decrease in ionospheric density at middle latitudes can be attributed to a combination of processes, including the PPEF effect just after the storm onset, dominated by disturbance dynamo processes during the evolution of the main phase. Two SEDs–TOIs were identified in the Southern Hemisphere, but only the first episode had a counterpart in the Northern Hemisphere. This difference can be explained by the interhemispheric asymmetry caused by the high-latitude coupling between solar wind and the magnetosphere, which drives the dawn-to-dusk component of the interplanetary magnetic field. The formation of polar TOI is a function of the SED plume location that might be near the dayside cusp from which it can enter the polar cap, which was the case in the Southern Hemisphere. Strong GNSS scintillations were observed at stations collocated with SED plumes at middle latitudes and cusp on the dayside and at polar cap TOIs on the nightside.

Small‐scale structure of the midlatitude storm enhanced density plume during the 17 March 2015 St. Patrick's Day storm

AbstractKilometer‐scale density irregularities in the ionosphere can cause ionospheric scintillation—a phenomenon that degrades space‐based navigation and communication signals. During strong geomagnetic storms, the midlatitude ionosphere is primed to produce these ∼1–10 km small‐scale irregularities along the steep gradients between midlatitude storm enhanced density (SED) plumes and the adjacent low‐density trough. The length scales of irregularities on the order of 1–10 km are determined from a combination of spatial, temporal, and frequency analyses using single‐station ground‐based Global Positioning System total electron content (TEC) combined with radar plasma velocity measurements. Kilometer‐scale irregularities are detected along the boundaries of the SED plume and depleted density trough during the 17 March 2015 geomagnetic storm, but not equatorward of the plume or within the plume itself. Analysis using the fast Fourier transform of high‐pass filtered slant TEC suggests that the kilometer‐scale irregularities formed near the poleward gradients of SED plumes can have similar intensity and length scales to those typically found in the aurora but are shown to be distinct phenomena in spacecraft electron precipitation measurements.

PFISR observation of intense ion upflow fluxes associated with an SED during the 1 June 2013 geomagnetic storm

AbstractThe Earth's ionosphere plays an important role in supplying plasma into the magnetosphere through ion upflow/outflow, particularly during periods of strong solar wind driving. An intense ion upflow flux event during the 1 June 2013 storm has been studied using observations from multiple instruments. When the open‐closed field line boundary (OCB) moved into the Poker Flat incoherent scatter radar (PFISR) field of view, divergent ion fluxes were observed by PFISR with intense upflow fluxes reaching ~1.9 × 1014 m−2 s−1 at ~600 km altitude. Both ion and electron temperatures increased significantly within the ion upflow, and thus, this event has been classified as a type 2 upflow. We discuss factors contributing to the high electron density and intense ion upflow fluxes, including plasma temperature effect and preconditioning by storm‐enhanced density (SED). Our analysis shows that the significantly enhanced electron temperature due to soft electron precipitation in the cusp can reduce the dissociative recombination rate of molecular ions above ~400 km and contributed to the density increase. In addition, this intense ion upflow flux event is preconditioned by the lifted F region ionosphere due to northwestward convection flows in the SED plume. During this event, the OCB and cusp were detected by DMSP between 15 and 16 magnetic local times, unusually duskward. Results from a global magnetohydrodynamics simulation using the Space Weather Modeling Framework have been used to provide a global context for this event. This case study provides a more comprehensive mechanism for the generation of intense ion upflow fluxes observed in association with SEDs.

Observations of ion‐neutral coupling associated with strong electrodynamic disturbances during the 2015 St. Patrick's Day storm

AbstractWe use incoherent scatter radar observations at Millstone Hill (MHO) and Arecibo (AO) and topside ionosphere in situ Defense Meteorological Satellite Program (DMSP) observations during the great geomagnetic storm on 17–18 March 2015 to conduct a focused study on ion‐neutral coupling and storm time ionosphere and thermosphere dynamics. Some of these observations were made around the time of large ionospheric drifts within a subauroral polarization stream (SAPS). During the storm main phase, we identify multiple disturbance characteristics in the North American late afternoon and dusk sector. (1) Strong subauroral westward drifts occurred between 20 and 24 UT near MHO, accompanied by a storm enhanced density plume passage over MHO in the afternoon with a poleward/upward ion drift. The strongly westward flow reached 2000 m/s speed near the poleward plume edge. (2) Prompt penetration electric field signatures, appearing as poleward/upward ion drifts on the dayside over both MHO and AO, were consistent with DMSP vertical drift data and contributed to plume development. (3) Meridional wind equatorward surges occurred during daytime hours at MHO, followed by 2–3 h period oscillations at both MHO and AO. The zonal electric field at AO was strongly correlated with the wind oscillation. (4) Large ion temperature enhancements as well as 50+ m/s upward ion drifts throughout the E and F regions were observed during the SAPS period. These were presumably caused by strong frictional heating due to large plasma drifts. The heating effects appeared to drive significant atmospheric upwelling, and corresponding ion upflow was also observed briefly. This study highlights some of the important effects of fast plasma transport as well as other disturbance dynamics on ion‐neutral coupling during a single intensification period within a great geomagnetic storm.

High-latitude ionospheric irregularities: differences between ground- and space-based GPS measurements during the 2015 St. Patrick’s Day storm

We present an analysis of ionospheric irregularities at high latitudes during the 2015 St. Patrick’s Day storm. Our study used measurements from ~2700 ground-based GPS stations and GPS receivers onboard five low earth orbit (LEO) satellites—Swarm A, B and C, GRACE and TerraSAR-X—that had close orbit altitudes of ~500 km, and the Swarm in situ plasma densities. An analysis of the rate of TEC index (ROTI) derived from LEO–GPS data, together with Swarm in situ plasma probe data, allowed us to examine the topside ionospheric irregularities and to compare them to the main ionospheric storm effects observed in ground-based GPS data. We observed strong ionospheric irregularities in the topside ionosphere during the storm’s main phase that were associated with storm-enhanced density (SED) formation at mid-latitudes and further evolution of the SED plume to the polar tongue of ionization (TOI). Daily ROTI maps derived from ground-based and LEO–GPS measurements show the pattern of irregularities oriented in the local noon–midnight direction, which is a signature of SED/TOI development across the polar cap region. Analysis of the Swarm in situ plasma measurements revealed that, during the storm’s main phase, all events with extremely enhanced plasma densities (>106 el/cm3) in the polar cap were observed in the Southern Hemisphere. When Swarm satellites crossed these enhancements, degradation of GPS performance was observed, with a sudden decrease in the number of GPS satellites tracked. Our findings indicate that polar patches and TOI structures in the topside ionosphere were predominantly observed in the Southern Hemisphere, which had much higher plasma densities than the Northern Hemisphere, where SED/TOI structures have already been reported earlier. LEO–GPS data (ROTI and topside TEC) were consistent with these results.

Polar tongue of ionization (TOI) and associated Joule heating intensification investigated during the magnetically disturbed period of 1–2 October 2001

AbstractWe investigate storm‐enhanced density (SED) and polar tongue of ionization (TOI) over North America under southward Interplanetary Magnetic Field conditions. We focus on the 30 September to 1 October 2001 medium magnetic storm's recovery phase (Period 1) and on the last substorm (Period 2) of the following 2 October substorm series. We aim to study the SED‐TOI structure in the time frame of solar wind energy input to the magnetosphere‐ionosphere system and in terms of Joule heating. We utilize GPS total electron content maps tracking SED plume and polar TOI, and spectrogram images detecting polar rain and precipitation void and thus evidencing dayside merging. The variations of merging electric (E) field (EM) and its mapped‐down polar equivalent (EP), energy input efficiency (EIeff), and modeled Joule heating rate (QJoule) are monitored. Results show multiple Joule heating intensification points implying multiple energy deposition points at high latitudes where the magnetic pole was one of the preferred locations. During the higher EIeff (~1.5%) Period 2, the polar TOI was associated with a well‐defined strong QJoule intensification and with polar rain (or void) on the dayside (or nightside). During the lower EIeff (~0.5%) Period 1, only weak QJoule intensification occurred in the absence of both polar TOI and polar rain. We highlight the polar TOI's potential impact on the thermosphere. We conclude that (i) strong (EM ≈ 5 mV/m during Period 2) or weak (EM ≈ 0.5–2 mV/m during Period 1) EM facilitated energy deposition close to the magnetic pole and (ii) EIeff could be used as a diagnostic of the polar TOI's intensity.

Positive and negative ionospheric storms occurring during the 15 May 2005 geomagnetic superstorm

AbstractThis study focuses on the 15 May 2005 geomagnetic superstorm and aims to investigate the global variation of positive and negative storm phases and their development. Observations are provided by a series of global total electron content maps and multi‐instrument line plots. Coupled Thermosphere‐Ionosphere‐Plasmasphere electrodynamics (CTIPe) simulations are also employed. Results reveal some sunward streaming plumes of storm‐enhanced density (SED) over Asia and a well‐developed midlatitude trough over North America forming isolated positive and negative storms, respectively. The simultaneous development of positive and negative storms over North America is also shown. Then, some enhanced auroral ionizations maintained by strong equatorward neutral winds appeared in the depleted nighttime ionosphere. Meanwhile, the northern nighttime polar region became significantly depleted as the SED plume plasma could not progress further than the dayside cusp. Oppositely, a polar tongue of ionization (TOI) developed in the daytime southern polar region. According to CTIP simulations, solar heating locally maximized (minimized) over the southern (northern) magnetic pole. Furthermore, strong upward surges of molecular‐rich air created O/N2 decreases both in the auroral zone and in the trough region, while some SED‐related downward surges produced O/N2 increases. From these results we conclude for the time period studied that (1) composition changes contributed to the formation of positive and negative storms, (2) strengthening polar convection and increasing solar heating of the polar cap supported polar TOI development, and (3) a weaker polar convection and minimized solar heating of the polar cap aided the depletion of polar plasma.

Direct observations of the full Dungey convection cycle in the polar ionosphere for southward interplanetary magnetic field conditions

AbstractTracking the formation and full evolution of polar cap ionization patches in the polar ionosphere, we directly observe the full Dungey convection cycle for southward interplanetary magnetic field (IMF) conditions. This enables us to study how the Dungey cycle influences the patches' evolution. The patches were initially segmented from the dayside storm enhanced density plume at the equatorward edge of the cusp, by the expansion and contraction of the polar cap boundary due to pulsed dayside magnetopause reconnection, as indicated by in situ Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. Convection led to the patches entering the polar cap and being transported antisunward, while being continuously monitored by the globally distributed arrays of GPS receivers and Super Dual Auroral Radar Network radars. Changes in convection over time resulted in the patches following a range of trajectories, each of which differed somewhat from the classical twin‐cell convection streamlines. Pulsed nightside reconnection, occurring as part of the magnetospheric substorm cycle, modulated the exit of the patches from the polar cap, as confirmed by coordinated observations of the magnetometer at Tromsø and European Incoherent Scatter Tromsø UHF radar. After exiting the polar cap, the patches broke up into a number of plasma blobs and returned sunward in the auroral return flow of the dawn and/or dusk convection cell. The full circulation time was about 3 h.

Investigating storm‐enhanced density and polar tongue of ionization development during the 22 October 1999 great storm

AbstractWe investigate large‐scale plasma density increases occurring during the 22 October 1999 great storm and focus on storm‐enhanced density (SED) and polar tongue of ionization (TOI) development. Observations include two‐hourly Global Ionosphere Map series coupled with multi‐instrument in situ, space‐based, and ground‐based data plots, with Super Dual Auroral Radar Network two‐cell convection maps and with model‐generated neutral wind vector maps. Results demonstrate the equatorial electrojet events occurring in the Australian and American sectors, the high‐density plasma features, and their underlying plasma transportation processes. During the main phase, a series of four prompt penetration electric field (PPEF) events occurred with subauroral polarization stream E field development forming a plasmaspheric drainage plume. These E field events caused the repeated development of equatorial ionization anomaly (EIA), SED bulge, and SED plume during the local dusk‐midnight hours in those sectors that covered these local times. Showing a westward movement in accordance with their local time dependence, the EIA‐SED structure developed first in the American sector, later on over the Pacific, and finally in the Australian sector. The SED plume plasma found its way into the polar cap through the dayside cusp region, created in the north some large polar cap enhancements reaching up to seven times of the quiet time levels, and evolved in each hemisphere into a polar TOI. We speculate that the enhanced growth of EIA, transporting high‐density solar‐produced plasma to the SED bulge via strong net eastward E field effects, and the mechanical effects of equatorward neutral winds contributed to the polar TOI's increasingly better development.

Large plasma density enhancements occurring in the northern polar region during the 6 April 2000 superstorm

AbstractWe focus on the ionospheric response of northern high‐latitude region to the 6 April 2000 superstorm and aim to investigate how the storm‐enhanced density (SED) plume plasma became distributed in the regions of auroral zone and polar cap plus to study the resultant ionospheric features and their development. Multi‐instrument observational results combined with model‐generated, two‐cell convection maps permitted identifying the high‐density plasma's origin and the underlying plasma transportation processes. Results show the plasma density feature of polar cap enhancement (PCE; ~600 × 103 i+/cm3) appearing for 7 h during the main phase and characterized by increases reaching up to 6 times of the quiet time values. Meanwhile, strong westward convections (~17,500 m/s) created low plasma densities in a wider region of the dusk cell. Oppositely, small (~750 m/s) but rigorous westward drifts drove the SED plume plasma through the auroral zone, wherein plasma densities doubled. As the SED plume plasma traveled along the convection streamlines and entered the polar cap, a continuous enhancement of the tongue of ionization (TOI) developed under steady convection conditions. However, convection changes caused slow convections and flow stagnations and thus segmented the TOI feature by locally depleting the plasma in the affected regions of the auroral zone and polar cap. From the strong correspondence of polar cap potential drop and subauroral polarization stream (SAPS), we conclude that the SAPS E‐field strength remained strong, and under its prolonged influence, the SED plume provided a continuous supply of downward flowing high‐density plasma for the development and maintenance of PCEs.