In this paper, our current understanding and recent advances in the study of ionospheric storms is reviewed, with emphasis on the F2-region. Ionospheric storms represent an extreme form of space weather with important effects on ground- and space-based technological systems. These phenomena are driven by highly variable solar and magnetospheric energy inputs to the Earth's upper atmosphere, which continue to provide a major difficulty for attempts now being made to simulate the detailed storm response of the coupled neutral and ionized upper atmospheric constituents using increasingly sophisticated global first principle physical models. Several major programs for coordinated theoretical and experimental study of these storms are now underway. These are beginning to bear fruit in the form of improved physical understanding and prediction of ionospheric storm effects at high, middle, and low latitude.

Ionospheric Storms — A Review

The International GNSS Service (IGS) Working Group on Ionosphere was created in 1998. Since then, the Scientific community behind IGS, in particular CODE, ESA, JPL and UPC, have been continuosly contributing to reliable IGS combined vertical total electron content (VTEC) maps in both rapid and final schedules. The details on how these products are being generated, performance numbers, proposed improvement as far as VTEC evolution trends during near one Solar Cycle, are summarized in this paper. The confirmation of (1) the good performance of the IGS combined VTEC maps, and (2) the characteristic VTEC variability periods, are two main results of this work.

/pdf/the-igs-vtec-maps-a-reliable-source-of-ionospheric-28b28fg9g1.pdf

The IGS VTEC maps: a reliable source of ionospheric information since 1998

[1] With the current data availability from both ground- and space-based sources, the network of ground-based Global Positioning System (GPS) receivers, GPS occultation receivers, in situ electron density sensors, and dual-frequency beacon transmitters, the time is right for a comprehensive review of the history, current state, and future directions of ionospheric imaging. A brief introduction and history of ionospheric imaging is presented, beginning with computerized ionospheric tomography. Then, a comprehensive review of the current state of ionospheric imaging is presented. The ability of imaging algorithms to ingest multiple types of data and use advanced inverse techniques borrowed from meteorological data assimilation to produce four-dimensional images of electron density is discussed. Particular emphasis is given to the mathematical basis for the different methods. The science that ionospheric imaging addresses is discussed, and the scientific contributions that ionospheric imaging has made are described. Finally, future directions for this research area are outlined.

/pdf/history-current-state-and-future-directions-of-ionospheric-2aok12z48z.pdf

History, current state, and future directions of ionospheric imaging

The main source of error in the estimation of TEC (total electron content) from dual Global Positioning System (GPS) data is the effect of the differential satellite and receiver instrumental delay biases. These biases are normally estimated simultaneously with the TEC. However, the additional estimation of the instrumental biases may constitute an insurmountable burden in some practical applications like real-time estimation of TEC, or the estimation may be difficult or correlated to the ionospheric parameters, particularly in situations where the TEC behavior may be harder to model (equatorial or auroral zone, ionospheric storms, etc.). A priori values of the instrumental biases, estimated under good conditions or with global networks, could solve those problems if we could determine how stable those instrumental biases are in time and how often we need to check or reestimate their values. In this paper we will present our estimation of the GPS satellite and receiver instrumental biases from 19 months of data and the study of their variation during that time. We will also show some situations of changes in the instrumental biases and the possible influence of antispoofmg (AS). The main conclusion of this work is that the variation of the estimated differential GPS satellite biases during the 19 months is smaller than 1 ns (1 ns = 2.86 × 1016 e/m2) in most of the cases, with a mean RMS of 0.15 ns. For the GPS receivers used, that variation is greater than for the satellites, with the larger variations corresponding to physical changes in the receivers. The difference of the estimated differential instrumental biases between two consecutive days is in practically all cases smaller than 0.5 ns for the GPS satellites and smaller than 1 ns for the GPS receivers. Regarding the influence of AS, we have detected some significant changes in the instrumental biases of some satellites and some stations whether AS is activated or not. Our main conclusion is that due to the stability of the GPS instrumental biases, only an estimation or calibration of them (under optimal conditions) from time to time is required.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97RS01457

Estimation of total electron content using GPS data: How stable are the differential satellite and receiver instrumental biases?

Modeling the Neutral-Atmosphere Propagation Delay in Radiometric Space Techniques

Results from the June 1998 combined ionospheric campaign (CIC) are presented. The CIC represents an attempt to focus a large number of different instruments on one interesting geophysical region. The Center for Ionospheric Research (CIR) at Applied Research Laboratories, the University of Texas at Austin (ARL:UT), has had several computerized ionospheric tomography (CIT) receivers deployed in the Caribbean region since July 1997. In this paper we compare CIT data, GPS TEC data and data from the incoherent scatter radar at Arecibo to try to obtain an understanding of the temporal and spatial distribution of ionospheric structure observed during the campaign. We use the three data sets as inputs to the 3DVAR tomography algorithm developed at CIR and present results of the 3DVAR “objectively analyzed” electron density field. An ionization wall was found near 40° latitude in agreement with previous Millstone Hill and DMSP observations in high Kp. Several elongated density depletions were also detected.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000GL000053

Combined Ionospheric Campaign 1: Ionospheric tomography and GPS total electron count (TEC) depletions

Complementary measurements from three different electron density measurement techniques are presented for the time period of the November 4, 1993, storm. Computerized ionospheric tomography (CIT) data from an array of nine ground stations operating as part of the Mid-America CIT Experiment (MACE 93) is presented along with data from a digital ionosonde operating near the midpoint of the CIT array. Corroborating data from the DMSP satellites are also presented. Taken together the data provide evidence of a strongly disturbed ionosphere with rapid variations of electron density structures in space and time. The CIT data show a deep equatorward surge of the the midlatitude trough to nearly 50° geomagnetic latitude, while the ionosonde shows dramatic variations in the virtual height of the ionosphere. DMSP data confirm the equatorward surge of the trough and also display a number of sharp latitudinal variations in vertical drift velocities. An interesting occurrence of spread F also occurred during this time period.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JA03994

Ionospheric observations of the November 1993 storm

IRI data ingestion and ionospheric tomography

Results from the June 1998 Combined Ionospheric Campaign (CIC) will be presented. The CIC represents an attempt to focus a large number of different instruments on one interesting geophysical region. One of the objectives of these campaigns is to develop suitable data sets for ingestion into data assimilative models and also to serve as validation sets for these models. The Center for Ionospheric Research (CIR) at Applied Research Laboratories, University of Texas at Austin, has had several computerized ionospheric tomography (CIT) receivers deployed in the Caribbean region since July 1997. Analysis of the CIT data from the first CIC will be presented, and comparison with other data sets will be made. Analysis will initially focus on examining the total electron content (TEC) data from the CIT receivers and ground-based GPS TEC data and correlating it with other data sets. Subsequently, the analysis will shift to performing four-dimensional electron density estimations using the Ionospheric Data Assimilation 3D (IDA3D) algorithm developed at CIR. The resulting electron density estimates will be compared with other data sources both for accuracy of the technique and scientific investigations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999RS002424

Computerized ionospheric tomography analysis of the Combined Ionospheric Campaign

Ionospheric products from sensors and models were compared to investigate strengths and limitations of each. Total electron content data from computerized ionospheric tomography (CIT) and TOPEX sensors in the Caribbean region in 1997 were compared to estimates produced by models Parameterized Ionospheric Model (PIM) and Raytrace/ICED-Bent-Gallagher (RIBG) and global maps from GPS. A 5 total electron content unit (TECU) bias was observed in TOPEX. CIT and TOPEX confirmed the location and structure of the equatorial anomaly. A GPS map confirmed the location of the anomaly but did not reproduce structure less than 1000 km in latitude and 1500 km in longitude and underestimated TEC by at least 11 TECU or 25%. PIM positioned the anomaly 13° equatorward of its observed location and greatly underestimated (∼50%) the rise in content over 5°-25°N range. RIBG overestimated the latitudinal extent of the anomaly and underestimated TEC at the peak by 40%. Additional comparisons were made using CIT and ionosonde sensors at midlatitude during the summer of 1998. Fourteen days of TEC, hmF2, NmF2, and half-thickness comparisons showed reasonable agreement between CIT and ionosonde for TEC and NmF2. The hmF2 and half-thickness comparisons were contaminated by noise, which accounted for a significant portion of the ionospheric variation. Daytime cases where CIT overestimated maximum density were attributed to underestimating layer thickness. Finally, TOPEX and multiple GPS sensors were compared to verify regional ionospheric conditions associated with occurrence of nighttime ionospheric depletions in the Caribbean during Combined Ionospheric Campaigns in June of 1998. From 0300 to 0800 UT on June 26, GPS and TOPEX showed elevated nighttime content over the entire Caribbean region. Vertical TEC approached 25 TECU in some places with interspersed depletions, which in some cases evacuated nearly the entire ionospheric content.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999RS002418

David S. Coco

Papers

Combined Ionospheric Campaign 1: Ionospheric tomography and GPS total electron count (TEC) depletions

Ionospheric observations of the November 1993 storm

IRI data ingestion and ionospheric tomography

Computerized ionospheric tomography analysis of the Combined Ionospheric Campaign

Verification of ionospheric sensors