Variations Of Dayside Aurora Revealed From Polar Observations

The auroras are one of the luminosity phenomena in the Earth's space that can be visible to the naked eyes.They are the result of collisions between atoms or molecules in the upper atmosphere and energetic particles that have their origin in the magnetosphere.With transportation of energy from solar wind/magnetosphere to the Earth's ionosphere,the particles reach the atmosphere and produce the aurora after having undergone various degrees of acceleration and redistribution within the terrestrial magnetosphere.The energy and momentum of solar wind/magnetosphere are injected into the Earth's ionosphere through the polar region,and the aurora is a display of the solar wind-magnetosphere coupling and magnetospheric dynamic process in the polar region.The auroras not only provide an important energy source for ionosphere in the polar region but also increases ionospheric ionization and thus ionospheric conductivity,Joule heating at high latitude.They play an important role in ionosphere-thermosphere coupling process.Hence,the investigation of aurora offers insight into the complex solar wind-magnetosphere-ionosphere-thermosphere coupling process,and is also of great significance to the prediction of space weather.Due to the limited local time coverage of the ground and in situ particle observations,previous studies did not well track some variations of precipitating particle energy and fully understand the mechanism involved,especially for quantitative statistical analysis of dayside aurora.The dayside aurora,for example afternoon aurora,generally occurs under sunlit conditions,and it will be contaminated by the strong day glow in the imaging observations for equinoctial seasons and summer.How to determine and remove the dayglow contamination from auroral images is a challenge in studying aurora.Recently,with the development of aerospace technology and the instruments of auroral detection,satellite equipped with ultraviolet imagers(Polar/UVI and TIMED/GUVI etc.)have been new tools for auroral observations,which provide a great opportunity to investigate the global auroral phenomena.N2 LBH(Lyman-Birge-Hopfield band)bands are the major components of auroral spectra in the far ultraviolet.They can be monitored on the global scale by the ultraviolet imagers/cameras aboard satellites.The LBHL band(160-180 nm)emissions,which lie outside the wavelength of substantial O2 absorption,experience neither O2 absorption nor undergo multiple scattering.Due to these particular features,the column emission intensity of LBH-long band is estimated to be proportional to the auroral energy flux.The intensity ratio of LBHL to LBHS(140-160 nm)can be used to calculate the average energy of precipitating electrons.In this thesis,using N2 LBH band ultraviolet imager(UVI)data on board Polar satellite,we focus on the investigation of dayglow in auroral images and developing dayglow model.Using the developed dayglow model,we obtain the more accurate and reliable aurora to construct auroral model.We also investigate the universal time(UT)variations of afternoon aurora and explore the mechanism involved.The main results of this thesis are listed as follows:1.Based on the Polar/UVI auroral observations,we investigated the variations of dayglow in the auroral images and developed an empirical dayglow model for auroral images.The N2 Lyman-Birge-Hopfield band dayglow emissions related to solar radiation can be characterized as a cosine-like function of the solar zenith angle(Dayglow=Amp×cos2(Phi × SZA)).The amplitude(Amp)and phase(Phi)factors are used to quantify the cosine-like function.We first obtained the hourly Amp and Phi factors in summers from 1996-2000 using least squares fitting method,then investigated the solar cycle and universal time(UT)variations of these two shape factors.It was found that Amp factors were non-linearly dependent on the solar activity for all years,and they also showed clear UT variations under both low and high solar activities.The Phi factors decreases slightly with the increase of solar activity with no UT variations.A dayglow model for summer auroral images was constructed to consider the variations of Amp and Phi factors.To more accurately capture the dependences of dayglow emissions on solar zenith angle,we consider the dayglow emissions both in poleward and equatorward boundaries of the auroral oval in the new method.It is demonstrated that,besides the strong solar cycle and universal time dependencies,the amplitude factors of the cosine-like function show prominent seasonal variations,and they are the largest in summer and smallest in winter.The annual variation of amplitude factors is related to the annual variation of solar zenith angle;however,their semiannual variation is associated with neither geomagnetic activity nor the neutral densities.The dayglow phase factors show nearly constant values within each season,throughout the year,and they are higher in summer and equinoctial seasons and lower in winter.A seasonal dependent dayglow model is constructed by considering the solar cycle,UT and seasonal variations of the two factors,which is beneficial to deriving the dayside aurora and investigating the seasonal variations of aurora.2.Applying the method of dayglow removal introduced above,we quantitatively investigated the universal time variation of afternoon aurora in the equinoxes for geomagnetically quiet conditions(Kp=1)using auroral images from ultraviolet imager(UVI)aboard the Polar satellite.The variation of afternoon aurora are then compared with solar illumination effects(the solar zenith angle and the consequent ionospheric conductivity)and the dipole tilt angle,as well as the observational region 1 upward field-aligned currents(FACs)from AMPERE data.The averaged afternoon auroral emissions have pronounced universal time(UT)variations with valley at around 01:00-03:00 UT and peak at around 17:00-19:00 UT.They generally vary with variations of the solar illumination,the dipole tilt angle and the observed region 1 upward FACs as a function of UT.The afternoon auroral intensity is anticorrelated with the solar zenith angle(SZA)and positively proportional to the solar EUV-produced Pedersen conductivity,region 1 upward FACs and dipole tilt angle.Additionally,they depend weakly on solar activity.These results suggest that in the afternoon auroral region,the peak auroral emissions are closely associated with the peak conductivity and the maximum upward FACs.Other mechanisms,such as the dipole tilt angle,may also contribute.Further comparison between the northern afternoon aurora and the FACs in the two conjugate hemispheres suggests little contributions on the auroral UT variations from the interhemispheric FACs in the equinoxes.3.Based on the developed dayglow model,a seasonal and Kp dependent auroral model is constructed using Polar/UVI auroral images from 1997-1999.The previous auroral model rarely considered the seasonal features of aurora,especially for afternoon aurora.After the dayglow was removed automatically from the original UVI images via our model,the remaining auroral precipitation energy flux is fitted by the Epstein function.We obtain the fitted parameters from 1997-1999 applying the least squares fitting method,and investigate their Kp and seasonal dependences.Based on the Kp and seasonal variations of these fitted parameters,we construct a seasonal and Kp dependent model.As compared with other auroral models,our developed model exhibits seasonal dependences of dayside aurora,and it would provide an important input for modeling effort in the ionosphere and thermosphere,since the aurora becomes an important energy source for the upper atmosphere.The research results of this thesis reveal the characteristics of dayglow variations in the polar region,and are beneficial to obtaining more accurate and reliable aurora from UVI image data aboard the satellite.They provided an effective method for the statistical study of the global auroral phenomena.In addition,the investigation of the UT variations of afternoon aurora and involved mechanism and developing auroral model are of great importance for the study of the response of ionosphere-thermosphere caused by the aurora,the improvement of ionospheric model,space weather prediction and understanding solar wind-magnetosphere-ionosphere-thermosphere coupling processes.