Ten years of geomagnetic field observations by the CHAMP satellite are used for a systematic investigation of the counter equatorial electrojet (CEJ). For the first time a comprehensive characterization of CEJ is presented. CEJs occur preferably during early morning, and their occurrence rate is down to 4% at noon. The CEJ occurrence rate shows a clear annual variation with a peak around July-August and a secondary peak in January. The late summer peak is related to the effect of meteor dust ablation. The CEJ amplitude is closely controlled by magnetic activity, showing a good correlation with the a P index. Nonmigrating solar tides are the main reason for longitudinal patterns of occurrence rate. The most prominent wavenumber 1 longitudinal structures during all seasons can be attributed to the tidal components SW3 and SPW1. The wavenumber 4 becomes largest during late summer-autumn season, which is related to the DE3 component. Also, the influence of lunar tides is evident in the CEJ occurrence rate.

The intense magnetic storm on 17–18 March 2015 caused large disturbances of the ionosphere. Based on the plasma density (Ni) observations performed by the Swarm fleet of satellites, the Gravity Recovery and Climate Experiment mission, and the Communications/Navigation Outage Forecasting System satellite, we characterize the storm‐related perturbations at low latitudes. All these satellites sampled the ionosphere in morning and evening time sectors where large modifications occurred. Modifications of plasma density are closely related to changes of the solar wind merging electric field (Em). We consider two mechanisms, prompt penetration electric field (PPEF) and disturbance dynamo electric field (DDEF), as the main cause for the Ni redistribution, but effects of meridional wind are also taken into account. At the start of the storm main phase, the PPEF is enhancing plasma density on the dayside and reducing it on the nightside. Later, DDEF takes over and causes the opposite reaction. Unexpectedly, there appears during the recovery phase a strong density enhancement in the morning/prenoon sector and a severe Ni reduction in the afternoon/evening sector, and we suggest a combined effect of vertical plasma drift, and meridional wind is responsible for these ionospheric storm effects. Different from earlier studies about this storm, we also investigate the influence of storm dynamics on the initiation of equatorial plasma irregularities (EPIs). Shortly after the start of the storm main phase, EPIs appear in the postsunset sector. As a response to a short‐lived decline of Em, EPI activity appears in the early morning sector. Following the second start of the main phase, EPIs are generated for a few hours in the late evening sector. However, for the rest of the storm main phase, no more EPIs are initiated for more than 12 h. Only after the onset of recovery phase does EPI activity start again in the postmidnight sector, lasting more than 7 h. This comprehensive view of ionospheric storm effects and plasma irregularities adds to our understanding of conditions that lead to ionospheric instabilities.

By utilizing the high‐resolution and precise vector magnetic field measurements from CHAMP during 2001–2005, the characteristics of the net auroral currents calculated by Ampère's integral law are comprehensively investigated. It is found that the net currents deduced from noon‐midnight (dawn‐dusk) orbits are directed duskward (antisunward). The intensities of the net currents increase linearly when the merging electric field (Em) is growing, exhibiting maximum values of about 2 (1) MA for the net duskward (antisunward) currents when Em exceeds 4 mV/m. For the first time the seasonal variations of the different net currents are shown. The net currents deduced from full orbits show only little seasonal dependence due to a compensation of the effects between the hemispheres. Conversely, the net currents deduced separately for the two hemispheres exhibit prominent seasonal dependences. For the net duskward currents the amplitudes and slopes of Em dependence are both larger by a factor of about 2 in summer than in winter. The related cross‐polar cap Pedersen currents are higher in the sunlit hemisphere due to enhanced conductivity. The summer‐time duskward currents are larger in the Northern Hemisphere than in the Southern Hemisphere by a factor of 1.5. Conversely, the net antisunward currents show an opposite seasonal dependence. The ratio of summer to winter intensity amounts to about 0.7. In this case the currents are stronger in the Southern Hemisphere.

Abstract. Similar to the Dst index, the SYM-H index may also serve as an indicator of magnetic storm intensity, but having distinct advantage of higher time-resolution. In this study the NARX neural network has been used for the first time to predict SYM-H index from solar wind (SW) and IMF parameters. In total 73 time intervals of great storm events with IMF/SW data available from ACE satellite during 1998 to 2006 are used to establish the ANN model. Out of them, 67 are used to train the network and the other 6 samples for test. Additionally, the NARX prediction model is also validated using IMF/SW data from WIND satellite for 7 great storms during 1995-1997 and 2005, as well as for the July 2000 Bastille day storm and November 2001 superstorm using Geotail and OMNI data at 1 AU, respectively. Five interplanetary parameters of IMF B z , B y and total B components along with proton density and velocity of solar wind are used as the original external inputs of the neural network to predict the SYM-H index about one hour ahead. For the 6 test storms registered by ACE including two super-storms of min. SYM-H< −200 nT, the correlation coefficient between observed and NARX network predicted SYM-H is 0.95 as a whole, even as high as 0.95 and 0.98 with average relative variance of 13.2% and 7.4%, respectively, for the two super-storms. The prediction for the 7 storms with WIND data is also satisfactory, showing averaged correlation coefficient about 0.91 and RMSE of 14.2 nT. The newly developed NARX model shows much better capability than Elman network for SYM-H prediction, which can partly be attributed to a key feedback to the input layer from the output neuron with a suitable length (about 120 min). This feedback means that nearly real information of the ring current status is effectively directed to take part in the prediction of SYM-H index by ANN. The proper history length of the output-feedback may Correspondence to: S. Y. Ma (syma@whu.edu.cn) mainly reflect on average the characteristic time of ring current decay which involves various decay mechanisms with ion lifetimes from tens of minutes to tens of hours. The Elman network makes feedback from hidden layer to input only one step, which is of 5 min for SYM-H index in this work and thus insufficient to catch the characteristic time length.

Abstract. By using the accelerometer measurements from CHAMP and GRACE satellites, the tidal signatures of the thermospheric mass density and zonal wind at midlatitudes have been analyzed in this study. The results show that the mass density and zonal wind at southern midlatitudes are dominated by a longitudinal wave-1 pattern. The most prominent tidal components in mass density and zonal wind are the diurnal tides D0 and DW2 and the semidiurnal tides SW1 and SW3. This is consistent with the tidal signatures in the F region electron density at midlatitudes as reported by Xiong and Lühr (2014). These same tidal components are observed both in the thermospheric and ionospheric quantities, supporting a mechanism that the non-migrating tides in the upper atmosphere are excited in situ by ion-neutral interactions at midlatitudes, consistent with the modeling results of Jones Jr. et al. (2013). We regard the thermospheric dynamics as the main driver for the electron density tidal structures. An example is the in-phase variation of D0 between electron density and mass density in both hemispheres. Further research including coupled atmospheric models is probably needed for explaining the similarities and differences between thermospheric and ionospheric tidal signals at midlatitudes.

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.

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Using 2 years of magnetic field measurements from the Swarm constellation, we present a detailed study of the equatorial electrojet (EEJ) and its longitudinal gradient (ΔEEJ). This study represents for the first time the tidal characteristics derived from the longitudinal gradient of EEJ. Our analysis mainly focuses on the months around August (133 days centered on 15 August, day of year: 161–293) of 2014 and 2015 when the longitudinal wave number 4 (WN4) pattern is known to be most prominent. The EEJ intensity, derived from the average of the Swarm A and C current estimates, peaks around 11:30 LT and exhibits a clear WN4 pattern. These features are compatible with earlier CHAMP observations. The ΔEEJ, which can be considered as a high‐pass filtered result of EEJ, although having much smaller values than the EEJ, exhibits clearly the local time gradient of the EEJ diurnal variation. This kind of high‐pass filtering makes the tidal signatures in ΔEEJ more prominent. The crests of longitudinal WN4 patterns in ΔEEJ have locations different from those of EEJ. Prominent tidal components in ΔEEJ during August months are DE3, DW5, SW3, SW4, SW6, SPW1, SPW2, and SPW4. For a given wave number pattern the westward propagating components are more amplified in ΔEEJ than the eastward ones, which explains their numerous appearance. Using spectral analysis, we can confirm that the observed amplitude ratios of different tidal components between ΔEEJ and EEJ are as expected. Also, the phase differences between ΔEEJ and EEJ fit reasonably the theoretical values. The preferred amplification of westward propagating tides in ΔEEJ allows for a more detailed investigation of these components, which are assumed to be closer related to local electrodynamics processes.