Initial nighttime ionospheric observations with advanced ionospheric probe onboard FORMOSAT-5

FORMOSAT-5 satellite was launched into a sun-synchronous orbit at 720 km altitude with 98.28° inclination on 25 August 2017. The onboard scientific payload, Advanced Ionospheric Probe (AIP) is capable of measuring topside ionospheric ion density, cross-track flow velocities, ion composition and temperature, and electron temperature. Initial observations of nighttime midlatitude ionospheric density and vertical flow velocity variations at 2230 LT sector during a few quiet magnetic days in December 2017 are studied here. Longitudinal density variations in the equatorward edge of midlatitude ionospheric trough (MIT) region are noticed. Accompanied with this density variation, the vertical flow velocities also behave differently. Although the density difference has been stated due to zonal wind effect related to the declination of the geomagnetic field lines, the vertical flow velocity variation seems to play the opposite role. All these density and vertical flow observations in the northern winter hemisphere can only be explained by the longitudinal differences in the diffusion velocity coming down from the protonsphere (plasmasphere). In addition, the hemispheric asymmetry in the vertical flow velocity can also be explained by the interaction between the topside ionosphere and the protonsphere. The observed vertical flow variations near MIT at different longitudes should present a new potential tool for the study of MIT formation.     

Midlatitude ionospheric F2-layer response to eruptive solar events-caused geomagnetic disturbances over Hungary during the maximum of the solar cycle 24: A case study

In our study we analyze and compare the response and behavior of the ionospheric F2 and of the sporadic E-layer during three strong (i.e., Dst?<??100nT) individual geomagnetic storms from years 2012, 2013 and 2015, winter time period. The data was provided by the state-of the art digital ionosonde of the Széchenyi István Geophysical Observatory located at midlatitude, Nagycenk, Hungary (IAGA code: NCK, geomagnetic latitude: 46.17° geomagnetic longitude: 98.85°). The local time of the sudden commencement (SC) was used to characterize the type of the ionospheric storm (after Mendillo and Narvaez, 2010). This way two regular positive phase (RPP) ionospheric storms and one no-positive phase (NPP) storm have been analyzed. In all three cases a significant increase in electron density of the foF2 layer can be observed at dawn/early morning (around 6:00 UT, 07:00 LT). Also we can observe the fade-out of the ionospheric layers at night during the geomagnetically disturbed time periods. Our results suggest that the fade-out effect is not connected to the occurrence of the sporadic E-layers.     

Hemisphere-coupled modeling of nighttime medium-scale traveling ionospheric disturbances

Nighttime medium-scale traveling ionospheric disturbances (MSTIDs), which have tilted frontal structures in the midlatitude ionosphere, are investigated by the midlatitude ionosphere electrodynamics coupling (MIECO) model in this study. It has been proposed that the electrodynamic coupling between the E and F regions plays an important role in generating MSTIDs within a few hours. An intriguing aspect of MSTIDs is that they were simultaneously observed at magnetic conjugate locations in the Northern and Southern Hemispheres. In order to study the hemisphere-coupled electrodynamics, the MIECO model has been upgraded to consist of two simulation domains for both hemispheres in which the electrostatic potential is solved by considering electrodynamics in both hemispheres. The simultaneous occurrence of MSTIDs at the magnetic conjugate stations has clearly been reproduced when the F-region neutral wind satisfies the unstable condition in both hemispheres and a sporadic-E layer is given only at the Northern (summer) Hemisphere. Even if the unstable condition is satisfied in the summer hemisphere, an unfavorable F-region neutral wind in the winter hemisphere largely suppresses the growth of MSTIDs in both hemispheres.     

Collocated ionosonde and dense GPS/GLONASS network measurements of midlatitude MSTIDs

To analyze midlatitude medium-scale travelling ionospheric disturbances (MSTIDs) over Kazan (55.5°N, 49°E), Russia, the sufficiently dense network of GNSS receivers (more than 150 ground-based stations) were used. For the first time, daytime MSTIDs in the form of their main signature (band structure) on high-resolution two-dimensional maps of the total electron content perturbation (TEC maps) are compared with ionosonde data with a high temporal resolution. For a pair of events, a relationship between southwestward TEC perturbations and evolution of F2 layer traces was established. So F2 peak frequency varied in antiphase to TEC perturbations. The ionograms show that during the movement of plasma depletion band (overhead ionosonde) the F2 peak frequency is the highest, and vice versa, for the plasma enhancement band, the F2 peak frequency is the lowest. One possible explanation may be a greater inclination of the radio beam from the vertical during the placement of a plasma enhancement band above the ionosonde, as evidenced by the absence of multiple reflections and the increased occurrence rate of additional cusp trace. Another possible explanation may be the redistribution of the electron content in the topside ionosphere with a small decrease in the F peak concentration of the layer with a small increase in TEC along the line-of-sight. Analysis of F2 peak frequency variation shows that observed peak-to-peak values of TEC perturbation equal to 0.4 and 1 TECU correspond to the values of ΔN/N equal to 13% and 28%. The need for further research is evident.     

Study of mid-latitude ionospheric convection during quiet and disturbed periods using the SuperDARN Hokkaido radar

Westward ionospheric convective flows around midnight are frequently observed at mid-latitudes. They can be generated by so-called disturbance dynamo mechanisms working mainly in the mid-latitudes. To understand the influence of disturbance dynamo effects in the mid-latitudes, we studied the latitudinal distribution of westward flows in association with several kinds of geomagnetic disturbances using the SuperDARN Hokkaido radar. This radar creates high temporal resolution (1 s to 2 min), two-dimensional observations measuring the line-of-sight velocities of ionospheric plasma irregularities, which can be regarded as line-of-sight velocities of ionospheric convection in the mid-latitude region from 40° to 50°. This region could not be monitored using preexisting SuperDARN radars. In this study, we used ionospheric echo data obtained by the SuperDARN Hokkaido radar over 5 years (December 2006 to November 2011). We identified westward flows around midnight at about 40° to 55° geomagnetic latitude. Additionally, the data showed that the westward flow around midnight intensified under high geomagnetic activity (high Kp). This suggests that the disturbance dynamo could affect the mid-latitude ionospheric convection. We performed Superposed Epoch Analysis (SEA) to study the influences from the geomagnetic disturbances on mid-latitude ionospheric convection. We found no obvious influence during major storms (minimum Dst below −60 nT). SEA was also used to study the temporal and latitudinal dependence on the influences from substorms. From analysis of 36 events of AL-defined substorms, we saw that the influence of substorms lasted from 5 to 20 h after the onset between 44° and 53° geomagnetic latitude. The westward flow at mid-latitude grew to a maximum at 12 h after the geomagnetic substorm onset. This is consistent with the results of past numerical simulation studies of the disturbance dynamo effects.     

Improving the modeling of bottomside thickness parameters over midlatitudes and high latitudes

This paper investigates bottomside thickness parameters at Digisonde stations over midlatitude and high latitude regions, and compares the diurnal, seasonal, and solar activity variations in 2014 and 2009. The geographic latitudes of high latitude considered in this work are located beyond ±60° and those of midlatitude are located between ±40° and ±60°. The IRI-modeled B0 with ABT-2009 option (B0_IRI) are also examined and compared with four kinds of the B0 values, i.e., the observed B0 (B0_obs) from GIRO, the computed B0 following to Jamjareegulgarn et al. (2017a) (B0_old), the calculated B0 with a correction factor regarding to Jamjareegulgarn et al. (2017b) (B0_new), and the B0 with an average correction factor (B0_new_c_av). The average correction factors are proposed additionally in this work so as to assist occasionally the experimental B0 nonexistence of Digisonde which are equal to 0.2658 and 0.2058 for midlatitudes and high latitudes, respectively. Results show that the diurnal variations of B0_new and B0_new_c_av are in a good agreement with those of B0_obs evidently compared with those of B0_IRI and B0_old at every station during the three seasons over high and middle latitudes. During the three seasons, the diurnal variations of B0_new_c_av show similar trends and are close to one another with the B0_obs and the B0_new with small deviations. The differences between the B0obs and the B0_new_c_av also show similar trends and are close to one another with those between the B0obs and the B0_new. In contrast, the B0_IRI with ABT-2009 option seems to predict the B0 values poorly during the three seasons at high latitudes and some seasons at midlatitudes. The proposed B0_new is useful for computing approximately the observed B0 and the ionogram-based total electron content (ITEC) of Digisonde, and the plasma scale height over midlatitudes and high latitudes.     

Spatio-temporal analysis of ionospheric disturbances for ground based augmentation systems over a midlatitude region

Forcings from above and below the ionosphere can cause disturbances that need to be detected and corrected for navigation systems. Ground Based Augmentation Systems (GBAS) are used to give corrections to aircraft navigation systems while landing. These systems use regional ionosphere monitoring algorithms to detect the anomalies in the ionosphere. The aim of this study is to understand occurrence of ionosphere anomalies and their trends over Turkey. A comprehensive analysis of spatio-temporal variability of ionosphere is carried out for a midlatitude GPS network using Slant Total Electron Content (STEC). Differential Rate Of TEC (DROT), which is a measure of the amount of deviation of temporal derivative of TEC from its trend, is used to detect and classify the level of such disturbances. The GPS satellite tracks are grouped into north, east, west and over directions. The 24 h is divided into six time intervals. The percentage occurrence of each DROT category and the deviation from STEC trend in magnitude are calculated and grouped into satellite track directions and time intervals for 2010 (low solar activity), 2011 and 2012 (medium solar activity). The highest level of disturbances is observed in north and west directions, and during sunrise and sunset hours. The dominant periods of percentage occurrences are diurnal (22–25 h), semidiurnal (12–13 h) and terdiurnal (8–9 h) followed by quasi two-day and quasi 16-day periods. Disturbances corresponding to 50% < DROT < 70% are mostly visible during low solar activity years with magnitudes from 1 to 2 TECU. Geomagnetic storms can cause aperiodic larger scale disturbances that are mostly correlated with DROT > 70%. In 2012, the magnitude of such disturbances can reach 5 TECU. The anisotropic and dynamic nature of midlatitude ionosphere is reflected in the spatio-temporal and spectral distributions of DROT, and their percentage occurrences. This study serves a basis for future studies about development of a regional ionosphere monitoring for Turkey.     

Investigation of the planar midlatitude ionospheric trend under different levels of solar activity

A better understanding of the ionosphere through accurate mathematical models is no doubt a crucial element. This study focuses on the challenging problem of building a model representing the complex structure of the midlatitude ionosphere. Previous studies have shown that a regional planar model is suitable in representing the total electron content (TEC) trend in the midlatitude ionosphere in both hemispheres. In this study, the planar trend model for 12 non-overlapping northern hemisphere regions in three groups of geographically near 4 regions is further investigated under different levels of solar activity; low, moderate and high. To that end, the coefficients of the model are estimated in the least squares sense using total electron content values from global ionospheric maps (GIMs) for the years 2009, 2012 and 2014. Subsequently, these coefficients are used to reconstruct estimated TEC maps which are then compared with actual GIM-TEC by investigating their difference in normalized $L_2$ norm squared sense. The regional planar trend model provides a particularly successful representation in the years 2012 and 2014 for which the solar activity level is the dominant factor determining the TEC trend. Under low solar activity conditions of 2009, other factors such as ocean currents, temperature variations and meteorological phenomena are suspected to have a considerable effect in some regions depending on their geographic location and on seasonal trends in those regions. As an example, studies show that under the influence of the Pacific Decadal Oscillation (PDO) and Siberian High (SH), a significant cooling trend between 2004 and 2018 in autumn is observed in Eurasia, which, in conjunction with the low solar activity levels, may be related to the deviations from the actual GIM-TEC in 2009 in these regions. As solar radiation increases, however, such bottom-side forcings are masked in 2012 and 2014 and these deviations are no longer observed.