Study On The Ionospheric Disturbances During The Solar Eclipse And The Sudden Stratospheric Warming

The ionosphere is overlapped with the lower thermosphere.The main source of the ionospheric disturbances is the Sun,including the seasonal and daily variations caused by the movements of the Sun,as well as the coronal mass ejection(CME)and solar flares coming from the Sun.The waves from the lower atmosphere,such as planetary waves,tides,gravity waves and so on,play a significant role in the ionosphere.Therefore,the atmosphere-ionosphere system is determined by the complicatedly non-linear chemical,dynamical,electrodynamical and radiation processes and affected by the external and internal processes.The solar eclipse is a common celestial event.When it happens,the Moon obscures the disk of the Sun leading the rapid decrease of the solar irradiation in a limited area on the Earth,which even can be comparable to that during nighttime.However,compared to the normally day-to-night variations,the rapid decrease and then increase of the solar irradiation results in more perturbations and non-linear processes in the atmosphere-ionosphere system.For each solar eclipse,the ionospheric response is unique and complex depending on latitude,local time,geomagnetic conditions,solar cycle,etc.,which provides us a good opportunity to study the atmosphere-ionospheric response to the impulse variations of solar EUV over a limited region of the Earth.The sudden stratospheric warming(SSW)is a large-scale meteorological process that mainly occurs in the polar stratosphere on the northern hemisphere during winter.For a typical event,the stratospheric temperature rises by ten to several tens of kelvins at?10 hPa pressure level(?32 km),while the zonal mean zonal winds decrease and even reverse to westward.It is now widely accepted that SSWs are triggered by the growth of upward propagating planetary waves that can interact with the mean flow in the polar stratosphere.The warming events can significantly affect the mesosphere and lower thermosphere(MLT)region,including the planetary waves,gravity waves,and tides,ect.Recently,studies on SSWs have been extended upward even to the ionosphere.Therefore,SSWs are helpful to study the coupling between the ionosphere and the lower atmosphere.We studied the ionospheric disturbances during the solar eclipse and the SSW.The main work and conclusions are as follows.1.We used(a)foF2 and hmF2 obtained from a meridional chain of eight observation locations at 120°E,ranging from 30.5°N to 42.8°N,and(b)zonal winds and meridional winds of the Wuhan meteor radar to observe the ionospheric response to the 2013 SSW at midlatitudes(>30°N)under the high solar activity.To our knowledge,this was perhaps the first observation of the ionospheric response to SSW at midlatitudes under high solar activity with the well-set and organized semidiurnal pattern(the increased/elevated foF2/hmF2)during daytime and the frequent enhancement after sunset.The time-period spectra of the average foF2 and zonal winds and meridional winds all presented the quasi-16 day planerary wave-like oscillations,indicating a crucial role of the quasi-16 day planetary waves in the coupling between the ionosphere and the lower atmosphere during the warming event.The harmonic analysis revealed that the diurnal,semidiurnal,and terdiurnal tides in foF2 were enhanced to different degrees.And the amplitudes of the semidiurnal tides were modulated by a?16-day periodic component.This indicates that the quasi-16 day planetary waves interacted with the semidiurnal tides that drove the E-region dynamo,perturbed the F region in the ionosphere.2.Based on the five ionosondes in South Korea and Japan,we studied the mid-latitude ionospheric variations during the solar eclipse on May 20,2012.It was observed that the ionospheric response was delayed to the solar eclipse.The foF2 responsed slower at higher latitudes.The ionosphere at different latiudes perturbed much differently from each other,though with similar obscuration.This may be caused by the downward diffusive plasma from the plasmasphere resulting from the decrase of the temperature in the lunar shadow,which made up the ionization loss and delayed the ionospheric depression.At higher latitudes,the dip angles were larger,which means there were more plasma flux diffusing downward along the magnetic field lines delaying the ionospheric reponse later.In addition,a gravity wave induced by the solar eclipse propagated upward at 100-150 km,with periods of?43-51 min,indicating that the source of the solar eclipse was under 100 km,outside the F region.3.We used the Global Ionosphere-Thermosphere Model(GITM)simulated the solar eclipse on August 21,2017 and compared the results with the total electron content(TEC)measurements of GPS and F2 layer peak electron density(NmF2)from six ionosondes.Model-data comparisons showed a relatively consistent depletion and enhancement in the ionosphere during and after the eclipse.It was found that the TEC decreased by?54.3%in GITM and?57.6%in GPS;and the NmF2 decreased by?20-50%in GITM and?40-60%in the measurements.Importantly,both the model and the measurements revealed an enhancement in the ionosphere after the solar eclipse.The TEC(NmF2)was enhanced by?10%(10%)in GITM and?10-25%(10-40%)in meausrements.The analysis of the GITM results indicated that the enhanced[O]/[N2]are mainly responsible for the post-eclipse enhancement.The divergence of the horizontal winds drove the increase in[O]after the eclipse allowing an increase in the ionization rate.The slower charge exchange due to both the decreased ion temperature and[N2]allowed an increase of O+ density in the F region also.