We have explored the long standing question about effects of IMF By orientation on the morphology of the dayside aurora for the Northern and Southern Hemispheres. It is shown, under similar solar wind driving, the postnoon auroral hot spot, which is centered at 1500 MLT in the Northern Hemisphere (Newell et al. 1996; Liou et al. 1997), also exist in the Southern Hemisphere and at the same local time. It is also found that in the postnoon 1400–1600 MLT sector of the Northern Hemisphere, the peak auroral energy flux is larger (~ 11%) for negative IMF By than for positive IMF By. The postnoon aurora can reach higher latitudes for positive IMF By compared to negative IMF By, resulting in larger energy flux there for positive than negative IMF By. This IMF By polarity dependence for the postnoon aurora is reversed and more pronounced in the Southern Hemisphere—the peak auroral energy flux is larger (~ 23%) for positive than negative IMF which are also present asymmetricy. In the prenoon sector, the auroral response to the IMF By orientation is in general opposite to that of the postnoon aurora but less pronounced. This result suggests an asymmetrical response of the aurora in the dawn and dusk sectors to the orientation of IMF By.
The present result that a clear IMF By effect on the postnoon aurora and a weaker IMF By effect on the prenoon aurora is consistent with the recent finding that upward field-aligned currents are well correlated with auroral precipitation in the dusk sector, but not in the dawn sector (Korth et al. 2014). The present finding is, to a certain extent, also consistent with the reports that negative IMF By favors the occurrence of auroras in the afternoon sector (e.g., Murphree et al. 1981; Vo and Murphree 1995; Trondsen et al. 1999). One must note that these studies are qualitative and therefore cannot address the absolute intensity of the aurora. These authors have associated their findings to the field-aligned currents and ionospheric convection, which also appear to response asymmetrically to the orientation of the IMF By component (e.g., Burch et al. 1985). The present finding of the lack of dependence of the total auroral energy flux deposited in the postnoon sector (1400–1600) on the IMF By orientation is consistent with the finding reported by Liou et al. (1998), who found the total auroral power in the afternoon (1300–1800 MLT) is linearly proportional to the magnitude of IMF By. This can also explain why Yang et al. (2013) did not find a dependency of the afternoon auroral intensity on the orientation of IMF By component. Based on the present and these previous results, the orientation of IMF By does not lead to more energy precipitation in the postnoon sector but affects the precipitation pattern in the ionosphere. In the Northern (Southern) Hemisphere, a negative (positive) By component changes the precipitation pattern such that the postnoon is more focused and brighter. This is the reason why previous studies often found enhanced auroral intensity in the Northern Hemisphere during IMF By < 0.
The topology change of the magnetosphere is an important manifestation of solar wind–magnetosphere coupling, and magnetic field merging/reconnection plays the key role in the coupling. The anti-parallel merging model (Reiff and Burch 1985) predicts two merging sites in opposite side of the noon in each hemisphere for a finite IMF By component. It is generally believed that this north–south asymmetry in the merging sites, when coupled with the solar wind drag, is responsible for the dawn–dusk asymmetry in the ionospheric plasma convection (e.g., Ruohoniemi and Greenwald 1996), field-aligned currents (e.g., Burch et al. 1985; Weimer 2001), and perhaps dayside auroras (e.g., Murphree et al. 1981).
Postnoon auroras are collocated with the upward region-1 field-aligned currents (Iijima and Potemra 1976). Enhanced dayside auroral arcs are associated with enhanced upward field-aligned currents and a strong convection reversal in the ionosphere. Plasma convection in the ionosphere can generate field-aligned currents (e.g., Sato and Iijima 1979) proportional to the flow vorticity (ω = ∆ × v), which is largest at the flow reversal (ω ~ v/r) for a solid body rotation. In the Northern Hemisphere, the duskside convection cell reversal is found to be crescent in shape for IMF By < 0 and rounded for IMF By > 0 (Ruohoniemi and Greenwald 1996), and the reverse is true for the convection cell in the dawn sector. The convection flow pattern is reversed in response to the reversal of IMF By in the Southern Hemisphere (e.g., Förster and Haaland 2015). A schematic drawing of the ionospheric plasma convection and field-aligned currents in the Northern Hemisphere associated with IMF By > 0 and IMF By < 0 is given in Fig. 6. In the dusk sector, a larger flow shear for IMF By < 0 than for IMF By > 0 can produce a larger upward field-aligned current and more intense auroral precipitation there. In the dawn sector, a larger field-aligned current associated with a larger flow shear for IMF By > 0 than for IMF By < 0 is expected. However, the field-aligned current is downward and is not associated with electron precipitation. The present result supports the above description of the convection flow and field-aligned currents in response to the orientation of IMF By.
In the dawn sector, however, the present result does not support the convection model, which predicts little influence in the dawn sector because the region-1 field-aligned current is dawnward and is not associated with particle precipitation. While diffuse auroras dominate the dawn sector particle precipitation because nightside electrons drift dawnward, auroral arcs can also appear, thought less frequent and intense, in the dawn sector (e.g., Newell et al. 1996). It is possible that field-aligned currents associated with auroral arcs in the dawn sector also enhanced in response to a stronger convection for IMF By > 0. However, we are not aware of any report about field-aligned current enhancements associated with the orientation of IMF By.
The convection model may explain the expansion of the postnoon aurora to higher latitudes for positive IMF By. When IMF By is positive, the larger dusk cell can expand to high latitudes, as well as its associated field-aligned currents, as shown in Fig. 6a. Electron precipitation associated with the field-aligned currents can produce auroras, probably weak, at higher latitudes. A little change in the prenoon aurora is expected because the field-aligned currents are dawnward. This is not consistent with our observations, which show an opposite response comparing to the postnoon aurora. On the other hand, it has been reported that the center of the Southern polar cap, when fitted with a circle, shifts duskward when IMF By > 0 and dawnward when IMF By < 0 (Holzworth and Meng 1984). This is consistent with our result shown in Fig. 5b, d. Dayside anti-parallel magnetic field merging can occur in the high-latitude afternoon magnetopause for IMF By > 0 and high-latitude prenoon for IMF By > 0. Open magnetic fluxes are pulled into the lobe from dayside, through the noon-midnight meridian, to the nightside of opposite quadrant due to the magnetic tension force. This effect is opposite between the two hemispheres. Therefore, it is expected that the Northern oval moves dawnward for IMF By > 0 and duskward for IMF By < 0, as shown in Fig. 5a, b.
The present result of a hemispheric asymmetry in the dayside aurora may also suggest existence of an interhemispheric current associated with a finite IMF By component, in addition to the large-scale region-1 and region-2 currents systems responsible for steady-state convection. A field-aligned current system associated with the IMF By component has been theoretically considered (e.g., Leontyev and Lyatsky 1974). According to this model, the solar wind motional electric field, after reconnection, polarizes the polar cap (open flux) region with opposite charges, depending on the orientation of IMF By, setting up field-aligned currents. The field-aligned currents can flow in the ionosphere into low latitudes and form a thin layer of interhemispheric field-aligned currents in the close region (Kozlovsky et al. 2003). A schematic drawing of this model is shown in the bottom panels of Fig. 6. As shown in Fig. 6c, a positive IMF By component will induce an interhemispheric field-aligned current out of the Southern Hemisphere, whereas a negative IMF By component will induce an interhemispheric field-aligned current out of the Northern Hemisphere. The IMF By-induced interhemispheric field-aligned current implies a larger (weaker) auroral intensity in the hemisphere where currents are flowing out. Therefore, this model predicts that dayside auroras are more intense in the Northern Hemisphere for IMF By < 0 and in the Southern Hemisphere for IMF By > 0. The present analysis result clearly indicates that the condition is more completed. Since this model applies perhaps only to regions close to the open-closed boundary, Fig. 5a, b indicates that the model correctly predicts the dawn sector but not the dusk sector for both hemispheres.
Finally, the present finding provides an impact in space weather forecasting. With simplicity in mind, current auroral models are parameterized by the Kp index (Hardy et al. 1991) or the solar wind coupling function (Newell et al. 2014). These models are not capable of simulating the IMF By effect as presented here because both parameters are not taking the IMF By orientation into consideration. Second, these models assumed symmetric auroral ovals and were constructed using combined data from both hemispheres. This assumption is clearly not valid as it is clearly shown in the present result. Future work on improvements of these models based on the present result is crucial for accurate aurora forecasting.