H.-G. Yang

Synoptic observations of auroras along the postnoon oval: a survey with all-sky TV observations at Zhongshan, Antarctica

Abstract All-sky TV data obtained at Zhongshan, Antarctica, have been used to survey auroral displays along the postnoon auroral oval. The auroral occurrence peak around 15 MLT, which was previously shown by satellite observations, is confirmed to exist in ground observations as well. The so called ‘midday gap’ of discrete aurora, however, is not confirmed by ground observations. This survey reveals that the noon region appears to involve another auroral occurrence peak. The noon auroral peak observed from the ground is dominated by an aurora termed as dayside corona in the present study. A dayside corona is usually weak and changing rapidly in its appearance, luminosity and locale. The electron precipitation causing the dayside corona might be too soft, have reduced flux and/or have too rapid a motion of its rayed structures which would result in less luminosity, all of which could account for the ‘midday gap’ in satellite observations. We thus argue for a new synoptic picture of auroral displays along the postnoon oval, in which beside the 15 MLT peak, the noon region is filled with the dayside corona rather than a ‘gap’ in discrete aurora.

Observational properties of dayside throat aurora and implications on the possible generation mechanisms

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) By 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(|Bx|/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.

Coordinated observations of two types of diffuse auroras near magnetic local noon by Magnetospheric Multiscale mission and ground all-sky camera

Structured diffuse auroras are often observed near magnetic local noon (MLN), but their generation mechanisms are poorly understood. We have found that two types of structured diffuse auroras with ...