Severe Ionospheric Storms Research Articles

Global View of Ionospheric Disturbance Impacts on Kinematic GPS Positioning Solutions During the 2015 St. Patrick's Day Storm

AbstractThe 2015 St. Patrick's Day geomagnetic storm caused numerous disturbances of the ionosphere, particularly, plasma irregularities, large‐scale traveling ionospheric disturbances, and equatorial ionization anomaly enhancement. This study for the first time quantifies the global‐scale impacts of the ionospheric disturbances on Global Positioning System (GPS) precise point positioning (PPP) solutions during this extreme space weather event by taking advantage of 5,500 + GNSS stations installed worldwide. The overall impact was more severe at high latitudes, while PPP degradation at low latitudes was associated with different types of ionospheric disturbances. Specifically, our results show that kinematic PPP solutions degraded following an intensified auroral particle precipitation during the storm's main phase (06–23 UT) when up to ~70% of the high‐latitude stations experienced degraded position solutions in the multimeter range at 16–18 UT. Around magnetic noon and midnight, the storm‐induced plasma irregularities caused notable PPP errors (>10 m) at high latitudes. Interhemispheric differences were observed with a more severe impact seen in the Southern Hemisphere, where PPP outage lasted for ~12 hr during the second main phase (12–23 UT). At low latitudes, post sunset equatorial plasma irregularities were suppressed across most longitudes, but large PPP errors (>2 m) associated with storm‐induced plasma bubbles were registered at the Indian sector at 14–18 UT. The storm‐induced equatorial ionization anomaly enhancement and large‐scale traveling ionospheric disturbances were responsible for the low‐latitude PPP degradation at dayside sectors. This study fills the research gap between physical and practical aspects of severe ionospheric storm effects.

A New Algorithm for High-Integrity Detection and Compensation of Dual-Frequency Cycle Slip under Severe Ionospheric Storm Conditions.

Many strategies for treating dual-frequency cycle slip, which can seriously affect the performance of a carrier-phase-based positioning system, have been studied over the years. However, the legacy method using the Melbourne-Wübbena (MW) combination and ionosphere combination is vulnerable to pseudorange multipath effects and high ionospheric storms. In this paper, we propose a robust algorithm to detect and repair dual-frequency cycle slip for the network-based real-time kinematic (RTK) system which generates high-precision corrections for users. Two independent and complementary carrier-phase combinations, called the ionospheric negative and positive combinations in this paper, are employed for avoiding insensitive pairs. In addition, they are treated as second-order time differences to reduce the impact of ionospheric delay even under severe ionospheric storm. We verified that the actual error distributions of these monitoring values can be sufficiently bounded by the normal Gaussian distribution. Consequently, we demonstrated that the proposed method ensures high-integrity performance with a maximum probability of missed detection of 7.5 × 10−9 under a desired false-alarm probability of 10−5. Furthermore, we introduce a LAMBDA-based cycle slip compensation method, which has a failure rate of 1.4 × 10−8. Through an algorithm verification test using data collected under a severe ionospheric storm, we confirmed that artificially inserted cycle slips are successfully detected and compensated for. Thus, the proposed method is confirmed to be effective for handling dual-frequency cycle slips of the network RTK system.

Methodology of automated ionosphere front velocity estimation for ground‐based augmentation of GNSS

AbstractExtreme ionospheric anomalies occurring during severe ionospheric storms can pose integrity threats to Global Navigation Satellite System (GNSS) Ground‐Based Augmentation Systems (GBAS). Ionospheric anomaly threat models for each region of operation need to be developed to analyze the potential impact of these anomalies on GBAS users and develop mitigation strategies. Along with the magnitude of ionospheric gradients, the speed of the ionosphere “fronts” in which these gradients are embedded is an important parameter for simulation‐based GBAS integrity analysis. This paper presents a methodology for automated ionosphere front velocity estimation which will be used to analyze a vast amount of ionospheric data, build ionospheric anomaly threat models for different regions, and monitor ionospheric anomalies continuously going forward. This procedure automatically selects stations that show a similar trend of ionospheric delays, computes the orientation of detected fronts using a three‐station‐based trigonometric method, and estimates speeds for the front using a two‐station‐based method. It also includes fine‐tuning methods to improve the estimation to be robust against faulty measurements and modeling errors. It demonstrates the performance of the algorithm by comparing the results of automated speed estimation to those manually computed previously. All speed estimates from the automated algorithm fall within error bars of ± 30% of the manually computed speeds. In addition, this algorithm is used to populate the current threat space with newly generated threat points. A larger number of velocity estimates helps us to better understand the behavior of ionospheric gradients under geomagnetic storm conditions.

Are ionospheric storms the same during different solar cycles?

AbstractThe ionosphere's response to geomagnetic storms has been studied since the earliest days of terrestrial space physics. In terms of temporal coverage, the largest data sets used extensively have been from the global network of ionosondes. Many previous investigations examined the behavior of the F layer's maximum electron density (Nmax)—often contrasting the difference seen between storms that occur during solar maximum years versus those during solar minimum years. We report on the first attempt to study systematically the patterns of ionospheric disturbance seen during different solar cycles. We select two midlatitude sites with long‐term consistency in data: Wallops Island (Virginia) and Hobart (Tasmania)—stations with comparable geographic and geomagnetic coordinates—but in different hemispheres and widely separated longitude sectors. We compare average ionospheric storm patterns using over 200 moderate to severe geomagnetic storms within each of solar cycle #20 (October 1964 to June 1976) and cycle #23 (May 1996 to December 2008). We compute average patterns of ΔNmax(%), measured with respect to monthly mean conditions, following storm and local time. The overall results show remarkable consistency in characteristic patterns of an ionospheric storm: a short positive phase that occurs during the daytime hours on the first day of a storm, with a prolonged negative phase on subsequent days. Statistical differences occur in the overall magnitudes and longevities of these patterns, consistently showing that cycle #23 had less severe ionospheric storms. An analysis of geomagnetic indices shows that degrees of disturbance were, in fact, lower during solar cycle #23 than cycle #20.

Nonlinear Filter for Ionosphere Divergence Error Reduction in LAAS

The Hatch filter, the industry-standard method for smoothing GPS code measurements with carrier data in the local area augmented system (LAAS), is susceptible to divergence errors during severe ionospheric storms. The work presented here develops an alternate single-frequency filter called a nonlinear divergence elimination (NLDE) filter that mitigates divergence errors without increasing measurement noise. Application of the filter to GPS data indicates that the nonlinear filter not only zeroes steady-state divergence but also reduces maximum transient errors by 0-24% relative to the Hatch filter.

Ionospheric Threat Mitigation by Geometry Screening in Ground-Based Augmentation Systems

Large spatial variations in ionospheric delay of Global Navigation Satellite System signals observed during severe ionospheric storms pose potential threats to the integrity of the Ground-Based Augmentation System, which supports aircraft precision approaches and landing. Range-domain monitoring within the Ground-Based Augmentation System ground facility cannot completely eliminate all possible ionospheric threats, because ionospheric gradients are not observable to the ground monitor if they impact the satellite-to-ground lines of sight with the worst-possible geometry and velocity. This paper proposes an algorithm called position-domain geometry screening to remove potentially hazardous satellite geometries under worst-case ionospheric conditions. This is done by inflating one ormore integrity parameters broadcast by the ground facility. Hence, the integrity of the system can be guaranteedwithout anymodification of existing avionics. This paper develops an algorithm that allows the ground station to conservatively estimate the worst-case ionospheric errors for Ground-Based Augmentation System users. The results of this algorithm determine which potential aircraft satellite geometries are safe and which are unsafe, and inflation of the broadcast vig parameter is used to make all unsafe geometries unusable for the Ground-Based Augmentation System. Although the elimination of unsafe geometries reduces system availability, this paper shows that acceptable availability for category I precision approaches is attainable at Memphis International Airport and Newark Liberty International Airport while guaranteeing system integrity under anomalous ionospheric gradients.

Behaviour of large scale structures of the electron content as a key parameterfor range errors in GNSS applications

The Total Electron Content (TEC) of the ionosphere is a key parameter for describing the ionospheric state. This paper deals with the large scale behaviour of TEC under low and high solar activity conditions. Large scale structures of the plasma density are formed by fundamental ionospheric processes mainly driven by solar radiation input, neutral winds and electric fields. The monitoring of large scale structures contributes to a comprehensive understanding of these coupling mechanisms which are rather complex particularly under perturbed geomagnetic conditions. The paper addresses techniques to monitor TEC with sufficient accuracy of a few TEC units (1016m-2) to measure large scale structures over Europe and over the polar areas. The availability of GPS data from global GPS receiver networks as e.g., those from the International GPS Service (IGS) is dense enough to generate TEC maps on a continuous base. A model assisted technique is briefly described for mapping TEC over the European and polar areas. A statistical estimation of horizontal TEC gradients reveals large scale gradients of up to about 6 TECU/1000 km under high solar activity conditions at an occurrence probability level of about 1%. Occasionally, during severe ionospheric storms this value may increase by a factor of 10 or even more. A close correlation of large scale gradients and the geomagnetic activity has been found giving the chance to forecast TEC gradient amplitudes by using predicted geomagnetic indices. Since TEC is proportional to first-order range errors in Global Satellite Navigation Systems (GNSS) such as the US GPS and the Russian GLONASS the study of the behaviour of this parameter has a practical meaning in GNSS based navigation and positioning. The paper addresses the close relationship between TEC and ranging errors in GNSS. Having in view Galileo, the planned Europe?s own global satellite navigation system, some aspects related to the mitigation of ionospheric propagation errors within the European Geostationary Navigation Overlay System (EGNOS) are discussed. Since EGNOS will augment the two above mentioned satellite navigation systems and make them suitable for safety critical applications such as flying aircraft or navigating ships through narrow channels the ionospheric propagation errors have to be mitigated as much as possible.

The influence of geomagnetic storms on the estimation of GPS instrumental biases

Abstract. An algorithm has been developed to derive the ionospheric total electron content (TEC) and to estimate the resulting instrumental biases in Global Positioning System (GPS) data from measurements made with a single receiver. The algorithm assumes that the TEC is identical at any point within a mesh and that the GPS instrumental biases do not vary within a day. We present some results obtained using the algorithm and a study of the characteristics of the instrumental biases during active geomagnetic periods. The deviations of the TEC during an ionospheric storm (induced by a geomagnetic storm), compared to the quiet ionosphere, typically result in severe fluctuations in the derived GPS instrumental biases. Based on the analysis of three ionospheric storm events, we conclude that different kinds of ionospheric storms have differing influences on the measured biases of GPS satellites and receivers. We find that the duration of severe ionospheric storms is the critical factor that adversely impacts the estimation of GPS instrumental biases. Large deviations in the TEC can produce inaccuracies in the estimation of GPS instrumental biases for the satellites that pass over the receiver during that period. We also present a semi quantitative analysis of the duration of the influence of the storm.

Impact and mitigation of ionospheric anomalies on ground‐based augmentation of GNSS

This paper describes the impact that extreme ionospheric spatial gradients occurring during severe ionospheric storms have on Global Navigation Satellite System Ground Based Augmentation Systems and how the U.S. Local Area Augmentation System (LAAS) mitigates the integrity risk due to these events. Gradients in slant ionospheric delay of as large as 425 mm/km over baselines of 40–100 km have been observed in the United States during ionospheric storms since April 2000 by both the U.S. Wide Area Augmentation System and the network of Continuously Operating Reference Stations receivers. Because ionospheric gradients affecting a LAAS site may not be observed by the LAAS Ground Facility (LGF) before users are affected, a simulation‐based method has been developed to determine, in near real time, which potential LAAS user geometries would be unacceptably threatened by a hypothetical worst‐case ionosphere gradient. Geometries of this type are made unavailable to LAAS users by having the LGF inflate the broadcast ionospheric‐gradient sigma, which increases the Vertical Protection Level of the unacceptable geometries so that they exceed the allowed Vertical Alert Limit; thus ensuring user safety.

Analysis of long-range network RTK during a severe ionospheric storm

The network-based GPS technique provides a broad spectrum of corrections to support RTK (real-time kinematic) surveying and geodetic applications. The most important among them are the ionospheric corrections generated in the reference network. The accuracy of these corrections depends upon the ionospheric conditions and may not always be sufficient to support ambiguity resolution (AR), and hence accurate GPS positioning. This paper presents the analyses of the network-derived ionospheric correction accuracy under extremely varying – quiet and stormy – geomagnetic and ionospheric conditions. In addition, the influence of the correction accuracy on the instantaneous (single-epoch) and on-the-fly (OTF) AR in long-range RTK GPS positioning is investigated, and the results, based on post-processed GPS data, are provided. The network used here to generate the ionospheric corrections consists of three permanent stations selected from the Ohio Continuously Operating Reference Stations (CORS) network. The average separation between the reference stations was ∼200 km and the test baseline was 121 km long. The results show that, during the severe ionospheric storm, the correction accuracy deteriorates to the point when the instantaneous AR is no longer possible, and the OTF AR requires much more time to fix the integers. The analyses presented here also outline the importance of the correct selection of the stochastic constraints in the rover solution applied to the network-derived ionospheric corrections.

GPS TEC and scintillation measurements from the polar ionosphere during the October 2003 storm

Severe ionospheric storms occurred at the end of October 2003. During the evening of 30 October a narrow stream of high electron concentration plasma crossed the polar cap in the antisunward ionospheric convection. A GPS scintillation receiver in the European high arctic, operating at 1.575 GHz, experienced both phase and amplitude scintillation on several satellite‐to‐ground links during this period. Close examination of the GPS signals revealed the scintillation to be co‐located with strong gradients in Total Electron Content (TEC) at the edge of the plasma stream. The gradient‐drift instability is a likely mechanism for the generation of the irregularities causing some of the scintillation at L band frequencies during this storm. The origin of the high TEC is explored and the possible implications of the work for scintillation forecasting are noted. The results indicate that the GPS scintillation over Svalbard can originate from traceable ionospheric plasma structures convecting from the American sector.

Магнитная, ионосферная и атмосферная бури большой интенсивности в сентябре 1998 года: сравнение результатов измерений и моделирования

Магнитная, ионосферная и атмосферная бури большой интенсивности в сентябре 1998 года: сравнение результатов измерений и моделирования

The use of Millstone Hill and EISCAT incoherent scatter observations for the F2-layer storm effects analysis

A self-consistent method for day-time F-region modeling was applied to EISCAT and Millstone Hill radar observations during severe ionospheric storms. One observed by EISCAT on Apr 03, 1992 shows a strong N mF2 decrease which is attributed to a strong imposed electric field, in another case observed at Millstone Hill on Apr 10, 1990 such a decrease is caused by neutral composition and temperature changes. Calculations indicate that the ionosphere was molecular-ion dominated even at F2-layer heights, requiring a correction to the routinely-evaluated T e(h), T i(h) and N e(h) profiles.

Ionospheric effects of the March 1990 Magnetic Storm: Comparison of theory and measurement

This paper presents a comparison of the measured and modeled ionospheric response to magnetic storms at Millstone Hill and Arecibo during March 16‐23, 1990. Magnetic activity was low until midday UT on day 18 when Kp reached 6, days 19 and 20 were quiet, but a large storm occurred around midnight UT on day 20 (Kp=7) and it was moderately disturbed (Kp=4) for the remainder of the study period. At Millstone Hill, the daytime peak electron density (NmF2) showed only a modest 30% decrease in response to the first storm and recovered to prestorm values before the onset of the second storm. The model reproduces the daytime peak electron density well for this period. However, the severe storm on March 20 caused a factor of 4 depletion in electron density, while the model densities were not greatly affected. The inclusion of vibrationally excited nitrogen (N2*) in the model was unable to account for the observed large electron density depletions afterward March 20. The storm did not appear to affect the overall magnitude of the electron density at Arecibo very much, but did cause unusual wavelike structure in the peak density and peak height following the storm. The model reproduces the daytime NmF2 very well for Arecibo, but after sunset the model densities decay too rapidly. This study indicates that successful modeling of severe ionospheric storms will require better definition of the storm time inputs, especially of the neutral atmosphere.

A global description of the F-region during the ionospheric storm of 17 December 1971

Total electron content data—observed continuously by means of geostationary radio beacons—provide a good means to study the ionosphere during severe ionospheric storms. Data of 20 sites were collected in order to study the ionospheric storm of 17 December 1971. A remarkable increase in the total electron content occurred at midlatitude stations throughout the northern hemisphere. Stations at auroral and equatorial latitudes also showed sharp increases which were both LT and latitude dependent whereas both sites in the southern hemisphere showed substantial depletions in the total electron content.

The middle-latitude F region during some severe ionospheric storms

Topside ionograms taken during some severe ionospheric storms show F2-layer critical frequencies that are less than F1-layer critical frequencies. A comparison of the electron density profiles derived from Alouette I ionograms obtained during these disturbed periods with those obtained during undisturbed periods show that the height of the F2 layer moves up, the plasma scale height above the F2 peak increases, and the column electron content up to 1000 km decreases. Data taken during the April 18, 1965, storm are treated in detail and it is found that an increase in the electron loss rate by a factor of 16 times the undisturbed rate can account for the decrease in electron density.

The absorption effect in the arctic during a severe ionospheric storm

A complete set of ionospheric data for the northern hemisphere has been analyzed for September 11–14, 1957 during a period when a severe ionospheric and magnetic storm occurred. Synoptic charts were constructed at three hour intervals for f min, f° Es and f° F2. The analytical method used allowed the transformation of blackout data into f min data, so that reliable charts of equal f min could be obtained. About 20 hours after the occurrence of a solar flare, f min started to increase over the polar cap. With time, these values increased steadily to about 7 mc/s with highest values near the pole. This increase—leading to blackout conditions at several inner Arctic stations—lasted for somewhat over 24 hr (Polar Cap Absorption—PCA). At the time of the Sudden Commencement, a steep gradient of f min occurs as a ring along the auroral zone. This gradient of 3–4 mc/s is confined to a width of a few hundred kilometers. After the Sudden Commencement, i.e. during the main phase of the storm, f min decreased over the inner Arctic so that normal values of about 2 mc/s were found 8 hours after the SC. However, a ring of high values prevailed during the main phase, first along the auroral belt, then moving southward towards a dip angle of about 71°.