ALBERT

Description: Storm sudden commencements (SSC) often precede geomagnetic storms. Commonly, it takes some hours from the step-like change that marks the SSC to the start of the magnetic storm activity. In a subset of cases, however, auroral activity starts almost instantaneously after the SSC. To the authors knowledge, the conditions that enable this rapid activation have not been investigated in detail before. Here we consider all the sudden commencements (SC) during the years 2000–2020. Our focus is on the initial response of the auroral currents on the nightside. For that purpose, we make use of the IMAGE Magnetometer Network in Fennoscandia. In about 30% of SC events an initial activation of the westward electrojet is observed. Magnetic deflections of the northward component, surpassing frequently 1,000 nT, are observed only 4 min after the SC. These intense westward currents, flowing typically in narrow channels of 1°–2° latitudinal width, last some 10 min. The electrojets are conjugate to regions in the magnetosphere near geostationary orbits. In several cases geomagnetic substorm onsets are observed about 30 min after the SC. These start typically at fairly high latitude, around 71° magnetic latitude. This is an indication for rather quiet conditions preceding the onset. The magnetic pulse of the SC seems to play an important role in initiating the strong electrojets and the substorms. These initial activities are of relevance for space weather effects because of their strong and rapid variations. This paper provides detailed observations of the initial auroral activity following some SCs.

Description: Ionospheric currents have widely been investigated by using magnetic measurements from low-Earth orbiting satellites. However, the assumptions of deriving currents from the magnetic measurements have not always been well considered. In this study we performed a detailed analysis of the ionospheric radial current (IRC) and inter-hemispheric field-aligned current (IHFAC) estimates at equatorial and low latitudes derived from the single-satellite and dual-spacecraft (dual-SC) approaches of European Space Agency (ESA's) Swarm constellation. Data considered cover a 5-year period, from 17 April 2014 to 16 April 2019. We found for most of the cases, the IRCs and IHFACs derived from both approaches show consistent latitudinal profiles. However, there are several cases with discrepancy exceeding 5 nA/m2 between two approaches. On average, the diurnal variations of IHFACs from both approaches agree well with each other for all seasons. But the amplitudes of single-satellite results reach only about 70% of those from the dual-SC. This difference is attributed to the fact that only the magnetic field By component is utilized in the single-satellite approach, while both Bx and By components are considered in the dual-SC approach. Above the magnetic equator, the IRCs derived from single-satellite approach show clear tidal signatures, while such signature cannot be found in the IRCs from dual-SC approach. We interpret these tidal-signature of IRCs as spurious results, caused by equatorial electrojet contributions to the ∆By component. The dual-SC derived IRCs show notable differences between ascending and descending orbits. Such differences might be due to a violation of the assumed perfect calibration of Swarm A and C. We suggest a systematic spacecraft-fixed bias in the along-track magnetic field component (Bx) between Swarm A and C. By interpreting the IRC differences, we obtain bias values of ∆Bx reaching 1 nT. Our results reveal that ionospheric currents are better characterized by the dual-SC approach. But comparison with single-satellite current estimates can help to identify weakness.

Description: In this study, we perform the first comprehensive comparison of ion density (Ni) in the topside ionosphere measured by the Langmuir probe (LP) and faceplate (FP) of the thermal ion imager on board Swarm satellites. Our results show a systematic difference between the LP and FP derived Ni values, and the systematic difference shows prominent dependences on solar flux, local time, and season. Although both Ni datasets show generally good linear regression with electron density (Ne) measurements from the incoherent scatter radar (ISR) located at Jicamarca, the Ni derived from LP shows an additional dependence on the solar flux, while such a dependence cannot be seen in the FP-derived Ni. We suggest that the solar flux dependence of LP-derived Ni is related to the ion compositions change at Swarm altitude, which has not been properly accounted for in the LP processing algorithm. More light ions (e.g., H+), diffusing down from the plasmasphere to the Swarm altitude, seem to cause the overestimation of Ni from LP during low solar activity. A linear relation between the Swarm LP-derived Ni and ISR Ne is derived, and such a function is recommended to be implemented into further updates of the Swarm LP plasma density data.

Description: In the present work, the magnetic local time and latitude (MLT and MLat) distributions of ionospheric large-spatial-scale (〉20° MLat) electromagnetic ion cyclotron waves were investigated using high-resolution 50-Hz geomagnetic field data from Swarm A and B satellites. Both parallel and transverse waves were studied in a comparative manner for different geomagnetic activities and seasons. Frequent occurrences of large-spatial-scale waves in the South Atlantic Anomaly and North America, where the parallel waves propagate over the longest distance, were observed. Waves appear mostly in the 02–10 MLT sector wherein the pre-noon parallel waves propagate farthest in latitudinal range. With the enhancement of geomagnetic activity, both transverse and parallel waves increase in occurrence. The dayside occurrence rate is higher during weak geomagnetic activity, whereas the situation is reversed on the nightside and duskside. The dayside waves are located outside the mid-latitude trough, and the nightside waves are located near (inside) the equatorward boundary of the mid-latitude trough. Large-spatial-scale waves tend to occur at the equinoxes when the absolute value of the dipole tilt angle is less than 10°, while the long-distance transmission in the waveguide occurs in the pre-noon in summer. Parallel waves propagate in the region where the electron density is higher than that of the transverse waves. There is a close relationship between EMIC wave and electron density oscillation.

Description: In this study we performed a detailed analysis of the ionospheric currents at different latitudes derived from the single- and dual-spacecraft (dual-SC) approaches of ESA’s Swarm constellation. We found at low and middle latitudes, the inter-hemispheric field-aligned current (IHFAC) derived from both approaches agree qualitatively with each other for all seasons, but the amplitudes of single-spacecraft results reach only about 70% of those from the dual-SC. This difference between IHFACs is attributed to the fact that only the magnetic field By component is utilized in the single-satellite approach, while both Bx and By components are considered in the dual-SC approach. Above the magnetic equator, the ionospheric radial current from single-spacecraft approach shows spurious tidal signatures, caused by equatorial electrojet (EEJ) contributions to the component, while the EEJ does not contaminate dual-satellite results. At auroral latitudes, similar underestimation of FACs from the single-spacecraft is also found at both dawn and dusk sides, with most prominent deviations at latitudes between the Region 1 and Region 2 currents. Our results confirm that the advanced current estimates from the Swarm dual-SC approach provide a good opportunity for better understanding the ionospheric current system.

Description: The Earth’s ionosphere affects the propagation of signals from the Global Navigation Satellite Systems (GNSS). Due to the non-uniform coverage of available observations and complicated dynamics of the region, developing accurate models of the ionosphere has been a long-standing challenge. Here, we present a Neural network-based model of Electron density in the Topside ionosphere (NET), which is constructed using 19 years of GNSS radio occultation data. The NET model is tested against in situ measurements from several missions and shows excellent agreement with the observations, outperforming the state-of-the-art International Reference Ionosphere (IRI) model by up to an order of magnitude, especially at 100-200 km above the F2-layer peak. This study provides a paradigm shift in ionospheric research, by demonstrating that ionospheric densities can be reconstructed with very high fidelity. The NET model depicts the effects of numerous physical processes governing the topside dynamics and can have wide applications in ionospheric research.

Description: The ionosphere is an ionized part of the upper atmosphere, where the number of free electrons is large enough to affect the propagation of electromagnetic signals, including those of the Global Navigation Satellite Systems (GNSS) systems. Knowing electron density values in the ionosphere is crucial for both industrial and scientific applications. Here, we present a novel Neural network model of Electron density in Earth's Topside ionosphere (NET). The model is trained on 19 years of radio occultation data collected by the CHAMP, GRACE, and COSMIC missions. We assume a linear decay of scale height with altitude and create a model consisting of 4 parameters, namely the F2-peak density and height (NmF2 and hmF2) and the slope and intercept of scale height decay in the topside (dHs/dh and H0). The resulting NET model, based on feedforward neural networks, takes as input the geographic and magnetic position, magnetic local time, day of year, and solar and geomagnetic indices. The model has been extensively validated on more than a hundred million in-situ measurements from CHAMP, CNOFS and Swarm satellites, as well as on the GRACE/KBR data. The NET model yields highly accurate and unbiased reconstructions of electron density in the topside ionosphere for all seasonal and solar activity conditions and can have wide applications in ionospheric research.