| The optical auroral emissions in the upper atmosphere are evidence of the capture of energetic particles from the solar wind. The energetic particles are precipitated along magnetic field line from dayside magnetopause boundary or magnetotail to ionosphere in the polar region. Night aurora is related to substorm, but there is a intrinsic relation between the dayside aurora and the magnetospheric boundary layers. It is one of the important methods to understand magnetospheric dynamic processes by studying the auroral characteristics in dayside oval.Dayside auroral arc is the basic form of discrete aurora, the brightest and most obvious kind of aurora, which occurs from 06 to 18 magnetic local time (MLT). The precipitating electron energy of dayside auroral arc is between several hundred eV and 10 keV. The stable arcs are elongated in longitude (>100 km) and narrowed (-18 km) in latitude. Dayside auroral arcs observed at Yellow River Station (YRS) occur at near 76° MLAT in the prenoon and postnoon sectors. They are rare near magnetic noon. Based on the auroral observation data obtained from the multiple-wavelength all-sky images (ASIs) at YRS in the Arctic, together with the DMSP data and the SuperDARN radars’observations, we have studied on dayside auroral arcs and focused on the widths, alignment, motions and emission intensities.We analyze 17571 dayside auroral arcs obtained at YRS to measure the arc widths. The widths of the stable arcs is 18.5° 13 km, which is associated with the scale of precipitating electron acceleration mechanism. The precipitating electron of the narrowed arc maybe accelerated by Alfven wave. It is also found the widths of dayside auroral arcs are narrower as it is close to magnetic noon. That is because that the boundary layer (e.g. boundary plasma sheet, BPS) is narrower as it is close to magnetic noon.Based on ASIs, we quantify the alignment of dayside auroral arcs. Arc tilt is defined as the angle the arc alignment makes with the local east-west direction. It is found arc tilt is decreased linearly with the increasing MLT. There is a reversal point of arc tilt located at near 10.S MLT. That is because the reversal position is related to the throat of ionospheric convection, which is prior to magnetic noon under average interplanetary magnetic field (IMF). It is also found there is a shift of the reversal point of arc tilt to magnetic noon in the lower latitude. We further studied the effects of interplanetary magnetic field (IMF) on the location of the arc tilt reversal point. We found the throat of ionospheric convection would be shifted with IMF By and the reversal position shifts toward magnetic noon (dawn side) for negative (positive) By.There seem to be three classes of dayside arc motions:motions affected by the earth’s rotation, slow motions (<200 m/s), and much faster ones (>200 mis). Motions related to the earth’s rotation are not real motions of auroal arcs, which are mainly determined by arc tilt. They are poleward at dawn and turn to equatorward in the postnoon sector, with an average speed of 51 m/s. Slow motions are associated with ionospheric convection. The occurrence rate of PMAAs increases as it is close to magnetic noon, with an average speed of 132 m/s. The effects of IMF are further studied. Fast motion is affacted by IMF Bz. Arc motion turns to poleward or equatorward after IMF Bz turns to north to south, which is associated with inflation or deflation of polar cap.The 557.7 nm emission intensity exists two maximum value in hot spot and warm spot region, with the average values of 2.2 and 2.9 kR respectively. But, there is a maximum near magnetic noon for 630.0 nm emission intensity, with an average value of 1.5 kR. Together with particle spectra detected from DMSP, it is found average energy of the electrons is proportional to the I557.7/I630.0 ratio and there is a positive correlation between the total energy flux and 7557.7. The typical region of electron precipitation is BPS in the prenoon and postnoon sectors. It is also found some precipitating electrons with low average energy from the region of mantle. The source region of the precipitating electrons was identified as the low latitude boundary layer adjacent to magnetic noon. Arcs in this region are located in lower latitude, with the low average energy of precipitating electron less than 600 eV.In this thesis, the width, the alignment, the motion and source region of dayside auroral arcs have been systematically studied. However, it is needed to do some case studies to determin the acceleration mechanism of dayside auroral arc. The features of dayside auroral arcs in different source regions and fine structure (such as curls, spirals) of dayside auroral arcs are needed to investigated in the future study. |
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