The background covariance matrix establishes the transition from the data-to model-driven regions in the ionospheric data assimilation algorithms. To construct the background covariance matrix, the information about the spatial ionospheric correlations of model errors is required. This paper focuses on the horizontal component of the covariance matrix. It is the first study that presents global maps of zonal and meridional ionospheric correlation lengths derived for IRI-2016 model errors. The model errors were calculated using 20 years of GPS total electron content (TEC) values from the Jet Propulsion Laboratory Global Ionospheric Maps (GIMs) for different seasons, geomagnetic conditions, and universal times. The correlation lengths derived from IRI model errors were analyzed and compared to correlation lengths derived from day-today ionospheric variability calculated from GIM. It was found that the global distributions of the zonal and meridional correlation lengths between the two approaches are very different and that the correlation lengths derived from day-today TEC variability cannot be used as a proxy for the construction of covariance matrix for ionospheric data assimilation. A new method is proposed for the modeling of azimuthal distribution of the correlation distances that considers the nonisotropic nature of the distribution of correlations around the reference point.

The construction of the background covariance matrix is an important component of ionospheric data assimilation algorithms, such as Ionospheric Data Assimilation Four‐Dimensional (IDA4D). It is a matrix that describes the correlations between all the grid points in the model domain and determines the transition from the data‐driven to model‐driven regions. The vertical component of this matrix also controls the shape of the assimilated electron density profile. To construct the background covariance matrix, the information about the spatial ionospheric correlations is required. This paper focuses on the vertical component of the model covariance matrix. Data from five different incoherent scatter radars (ISR) are analyzed to derive the vertical correlation lengths for the International Reference Ionosphere (IRI) 2016 model errors, because it is the background model for IDA4D. The vertical distribution of the correlations is found to be asymmetric about the reference altitude around which the correlations are calculated, with significant differences between the correlation lengths above and below the reference altitude. It is found that the correlation distances not only increase exponentially with height but also have an additional bump‐on‐tail feature. The location and the magnitude of this bump are different for different radars. Solar flux binning introduces more pronounced changes in the correlation distances in comparison to magnetic local time (MLT) and seasonal binning of the data. The latitudinal distribution of vertical correlation lengths is presented and can be applied to the construction of the vertical component of the background model covariance matrix in data assimilation models that use IRI or similar empirical models as the background.

This paper presents the evaluation of the recently developed covariance model for the Ionospheric Data Assimilation Four-Dimensional (IDA4D) technique. The ionospheric data are generated using the Observation System Simulation Experiment Tool from the known ionospheric state produced by the physics-based Thermosphere-Ionosphere-Mesosphere Electrodynamics General Circulation Model. Several experiments are conducted to assess performance of IDA4D with data-driven vertical and horizontal covariance matrices. We show that the vertical part of the covariance model plays the most important role because it preserves the vertical structure of the F-region density layer and helps to correct a tomographic issue that arises when the slant total electron content is assimilated along the intersecting rays. The results show that the new covariance model improves the fidelity of IDA4D algorithm, making it more suitable for the regional assimilation with dense ground-based Global Positioning System data coverage.FORSYTHE ET AL.

Occurrence of the E region plasma irregularities is investigated using two Super Dual Auroral Radar Network (SuperDARN) South Pole (SPS) and Zhongshan (ZHO) radars that sample the same magnetic latitude deep within the high‐latitude plasma convection pattern but from two opposite directions. It is shown that the SPS and ZHO velocity distributions and their variations with the magnetic local time are different, with each distribution being asymmetric; i.e., a particular velocity polarity is predominant. This asymmetry in the E region velocity distribution is associated with the bump‐on‐tail of the distribution near the nominal ion acoustic speed Cs that is most likely due to the Farley‐Buneman instability (FBI) echoes or an inflection point of the distribution below nominal Cs that is most likely due to the gradient drift instability echoes. In contrast, the distribution of the convection velocity component was found to be symmetric, i.e., with no bump‐on‐tail or an inflection point, but with a bias (i.e., uniform shift) toward a particular polarity. It is demonstrated that the asymmetry in the convection pattern between the eastward and westward zonal components is unexpectedly strong, with the westward zonal component being predominant, especially at lower latitudes, while also exhibiting a strong interplanetary magnetic field By dependence. The observations are consistent with the notion that the asymmetry in the E region velocity distribution is highly sensitive to the bias in the convection component caused by the zonal convection component asymmetry and that the bump‐on‐tail or inflection point features may also depend on the irregularity height and the presence of strong density gradients modifying the FBI threshold value.

Characteristics and formation mechanisms of E region plasma irregularities at high latitudes are investigated using observations with the newly deployed Super Dual Auroral Radar Network (SuperDARN) radar at the South Pole Antarctic station (SPS) near a magnetic latitude (MLAT) of 75°S. It is shown that E region echo occurrence at SPS exhibits a diurnal variation that is significantly different from those at auroral and polar cap latitudes. Moreover, analysis of major spectral populations also showed a distinct and previously unreported diurnal pattern. The plasma drift velocity estimates are derived at E region ranges of SPS, leveraging the SPS radar's position well within the MLAT region where SuperDARN convection estimates are well constrained by the data. It is shown that E region irregularity occurrence increases when the convection direction is within the SPS field of view and/or when the plasma drift component is comparable with the nominal ion‐acoustic speed Cs of 350 m/s. This is the expected behavior for irregularities generated directly by the modified two‐stream plasma instability (MTSI). On the other hand, irregularity velocity dependence on convection velocity showed an unexpected saturation at velocity values smaller than nominal Cs. It is demonstrated that the convection velocity at which irregularity velocity starts to differ from the convection component and to approach a maximum value is dependent on the magnetic aspect angle. Moreover, the maximum velocity value itself also depends on the aspect angle. The observed behavior is discussed in context of recent models that involve evolving aspect angles as a key characteristic of MTSI saturation.

Commercial Global Navigation Satellite System radio occultation ionospheric measurements collected by the Spire CubeSat constellation could significantly improve the determination of the global ionospheric state. However, the systematic validation work is needed before using the Spire data for scientific and assimilative purposes. In this work, the electron density profiles retrieved from Spire total electron content measurements using Abel inversion corrected by horizontal asymmetry are compared to digisonde measurements and Arecibo incoherent scatter radar data. Four events when the radio occultation measurements occurred close to three digisondes showed a strong agreement between these independent measurements. The comparison of F2 layer peak and height obtained from Spire data to digisonde measurements at 34 stations around the globe from Global Ionospheric Radio Observatory network also showed a strong agreement. Additionally, a good agreement was obtained in the comparison of Spire profiles with Arecibo electron density measurements. Spire Measurements and Inversion TechniqueAs of August 2019, the Spire constellation consists of 84 nanosatellites in LEO and around 20-30 satellites devoted to collecting RO measurements. When complete, Spire plans a full constellation of over 100 satellites simultaneously collecting RO measurements. Each satellite is equipped with a tiny, low power, Spire-built GNSS-RO receiver and an upward facing precise orbit determination antenna that produce dual-frequency observations, with 1-Hz raw observation frequency, allowing the derivation of ionospheric sTEC measurements. During the preprocessing of the raw phase observations, outliers are eliminated and cycle slips are

Origins and characteristics of small‐scale plasma irregularities in the polar ionosphere are investigated using a dual radar setup in which the E region is probed from opposite directions by two Super Dual Auroral Radar Network facilities at the McMurdo and Dome Concordia Antarctic stations. In certain time intervals, velocity agreement is observed when velocities are compared at the same physical location in the horizontal plane. Such an agreement is widely expected if velocity at a given location is largely controlled by the convection electric field. In other cases, however, velocity agreement is unexpectedly observed when measurements are considered at the same slant range (distance along the radar beam) for both radars. This implies that it is not the electric field at a given location that is a controlling factor. Raytracing results show that the same range agreement may be explained for certain E region density conditions when echo altitude increases with radar range. Backscatter observations under generally unfavorable conditions for irregularity generation and the critical role of propagation conditions in the polar cap are discussed. The observed E region velocity in the polar cap is demonstrated to depend indirectly on the plasma density distribution, which is important for establishing the fundamental dependence on the convection electric field.

Ionospheric correlation time is an important parameter that contains information about the temporal variability, structures, and dynamics of the ionosphere. This parameter is also important in forecasting of the ionospheric state. Ionospheric data assimilation algorithms employing empirical background models, such as Ionospheric Data Assimilation Four‐Dimensional (IDA4D), apply Gauss‐Markov approximation for the propagation of temporal updates from one time stamp to the next. In this process, the relaxation parameter, or the correlation time, determines to what degree the projected state depends on the background model and analysis density. An ad hoc approach is usually applied to choose this user‐defined parameter. This paper focuses on the estimation of the ionospheric correlation time using high temporal resolution global ionospheric maps (GIMs). It is found that the correlation time changes significantly with latitude. The longest correlation time is observed in the equatorial region, whereas the shortest correlation time is observed at high latitudes and in the polar cap regions. The global distribution of the correlation time exhibits seasonal variation and depends on the solar flux conditions. The correlation time at the equatorial and mid‐latitude regions increases with increasing solar and geomagnetic activity, whereas the correlation time at high‐latitude regions decreases. The results of this study can be directly applied to improve ionospheric data assimilation models.