Observational properties of throat aurora are investigated in detail by using 7 year continuous auroral observations obtained at Yellow River Station (magnetic latitude 76.24°N). From our inspection, throat aurora is often observed under the condition of stripy diffuse aurora contacting with the persistent discrete auroral oval, and the long‐period throat aurora observations generally consist of intermittent subsequences of throat aurora brightening followed by poleward moving auroral form and throat aurora dimming. We also noticed that the orientation of throat aurora is aligned along the ionospheric convection flow, and its local time distribution shows clear dependence on the interplanetary magnetic field (IMF) B_y component. These observational results indicate that factors inside the magnetosphere may play important role on occurrence of throat aurora. We thus suggest that throat aurora may present the ionospheric signature of redistribution of reconnection rate on the magnetopause by cold magnetospheric plasma flowing into the reconnection site. In addition, we also found that the occurrence rate of throat aurora clearly decreases with increase of the IMF cone angle (arccos(|B_x|/B)), which is very similar with the occurrence rate of high‐speed jet (HSJ) observed in magnetosheath depending on the IMF cone angle. This is suggested as that the HSJs occurred outside the magnetosphere may also play important role for generation of throat aurora by triggering magnetopause reconnection or by direct impacting. Although further studies are needed to clarify how the throat auroras are generated in detail, the relevant observations about throat aurora have presented important implications on a variety open questions, such as distribution and generation of cold plasma structures in the outer magnetosphere, magnetopause deformation, and possible relation between HSJ and reconnection.

By using 7 years optical auroral observations obtained at Yellow River Station (magnetic latitude 76.24°N) at Ny‐Alesund, Svalbard, we performed the first extensive survey for the dayside diffuse auroras (DDAs) and acquired observational results as follows. (1) The DDAs can be classified into two broad categories, i.e., unstructured and structured DDAs. The unstructured DDAs are mainly distributed in morning and afternoon, but the structured DDAs predominantly occurred around the magnetic local noon (MLN). (2) The unstructured DDAs observed in morning and afternoon present obviously different properties. The afternoon ones are much stable and seldom show pulsating property. (3) The DDAs are more easily observed under geomagnetically quiet times. (4) The structured DDAs mainly show patchy, stripy, and irregular forms and are often pulsating and drifting. The drifting directions are mostly westward (with speed ~5 km/s), but there are cases showing eastward or poleward drifting. (5) The stripy DDAs are exclusively observed near the MLN and, most importantly, their alignments are confirmed to be consistent with the direction of ionospheric convection near the MLN. (6) A new auroral form, called throat aurora, is found to be developed from the stripy DDAs. Based on the observational results and previous studies, we proposed our explanations to the DDAs. We suggest that the unstructured DDAs observed in the morning are extensions of the nightside diffuse aurora to the dayside, but that observed in the afternoon are predominantly caused by proton precipitations. The structured DDAs occurred near the MLN are caused by interactions of cold plasma structures, which are supposed to be originated from the ionospheric outflows or plasmaspheric drainage plumes, with hot electrons from the plasma sheet. We suppose that the cold plasma structures for producing the patchy DDAs are in lumpy and are more likely from the plasmaspheric drainage plumes. The cold plasma structure for producing the stripy DDAs should be in wedge like and is generated by conveying the cold plasmas from lower L‐shell toward higher L‐shell with magnetospheric convection, and that for producing the irregular DDAs is resulted from deforming the wedge‐like structure by disturbance. The throat aurora is supposed to be projection of a newly opened flux of reconnection. In addition, we also found that structured DDAs correspond to structured electron precipitations in the ionosphere, which implies that the cold plasma structures in the magnetosphere are magnetically mapped to the ionosphere and act as a duct for producing the structured DDAs. We argue that we have presented some new observational results about DDA in this paper, which will be useful for fully understanding the DDAs.

Magnetopause transients, observing as brief entries into the magnetosheath by satellites, are commonly observed in the vicinity of the magnetopause and have been explained by several possible mechanisms. However, satellite observations alone are insufficient to determine the dynamics and context of transients. Throat auroras are characterized as north‐south aligned discrete auroral forms extending from the equatorward edge of the discrete auroral oval that are only observed near dayside convection throat region and have been suggested as the ionospheric signature of localized magnetopause indentations. Using coordinated observations from the Magnetospheric Multiscale Mission (MMS) and ground‐based all‐sky imagers, we show apparent one‐to‐one correspondences between transients observed by MMS near the subsolar magnetopause and throat auroras observed on the ground. The correspondence is valid not only for typical throat aurora with larger spatial scale but also for these with tiny scales. We even notice that the transient durations observed by satellite are approximately proportional to the width (east‐west extension) of the throat aurora. These results provide direct evidence that throat auroras are ground signatures for the magnetopause transients. With the aid of auroral observations, we suggest that these transients reflect localized magnetopause indentations but are not produced by motion of the entire magnetopause. We also found that most transients observed here are associated with earthward flow enhancements, which indicates that high‐speed jets in the magnetosheath could be a driver for producing these transients.

Solar flares can severely disturb the Earth's ionosphere, leading to changes of atmospheric chemistry and degradation of Global Navigation Satellite System (GNSS) signals. The key to better assess these effects is to more accurately quantify the electron density enhancements induced by solar flares, which is still uncertain. Thanks to the good coverage and quiet electromagnetic environment provided by the Great Wall Station (GWS, 62.22°S, 58.96°W) in Antarctica, our Very‐Low‐Frequency (VLF) receiver has recorded clean VLF signatures produced by three X‐class solar flares from March to May in 2022. These measurements motivate us to revisit the uncertain question related to the ionospheric effects induced by solar flares. Different from previous studies, we model solar flare events from satellite measurements of X‐ray fluxes to ground‐based measurements of VLF transmitter signals by combining a flare irradiance model, an X‐ray propagation model, a D‐region chemistry model, and a VLF propagation model. For eight transmitting frequencies and propagation paths, modeling results can satisfactorily explain our VLF measurements at GWS during the three X‐class flares, especially so for the X1.3 event occurring on 30 March 2022. Moreover, the electron density results that reconcile space‐ and ground‐based measurements suggest that the VLF reflection height can be lowered to ∼66 km by an X1.1 flare, in line with previous studies. The present set of numerical models can be thus utilized in future studies to improve the nowcasting capability of VLF sensing technique for monitoring solar flares, as well as the accuracy of GNSS navigation/positioning during solar flares.

Solar flares have a severe impact on the near-earth space environment, during which the VLF signals observed by the ground-based instrument exhibit abnormal changes. However, the similarity and differences of VLF signal responses to solar flares over daytime and nighttime propagation paths are still unclear. Previous magnetograph measurements suggest that solar flares can also influence the dark hemisphere by the induction currents caused by the change in the ionospheric electrical conductivity of the sunlit hemisphere. To examine these effects, we have analyzed the solar flare effects on VLF propagation along two paths that are suited along the north–south direction, but with a time difference of 12 h. From late March to late May in the year of 2022, a total of 32 flare events with clear VLF responses are selected in order to analyze the similarity and differences between daytime and nighttime propagation paths. Different from the previous magnetograph measurements, it is found that the solar flare effects can only be observed from daytime VLF propagation paths. Moreover, present results show that the amplitude and phase variation of the VLF signal increases almost linearly with the magnitude of solar flares, and the stronger the solar flare, the more obvious the influence on VLF signals. However, the two paths exhibit notably different sensitivity in terms of amplitude and phase variation to solar flare class. Future studies that aim at nowcasting solar flare events using ground-based VLF receivers need to take these effects into account. The goal is to better understand the effects of solar flares on the lower ionosphere, with a view toward improving the nowcasting capability of the VLF technique for solar flares.