Studies Of Storm-time And Pre-earthquake Ionospheric Disturbances

The ionosphere is the closest ionization area to the earth surface,which is closely related to human activities.The ionosphere could cause reflection,refraction,scatter and absorption of the radio wave propagating through it,and then influence the communication,navigation,broadcasting,radar positioning and so on.With the development of the space exploration technologies,more and more ionospheric data are obtained by various observation methods and greatly promote the research progress of the ionosphere.The solar wind-magnetosphere-ionosphere coupling and the lithosphere-atmosphere-ionosphere coupling have always been the hot issues.It is significant and important for the modeling of the space environment and the forecasting of the disastrous space weather to understand the characteristics of the storm-time and pre-earthquake ionospheric disturbances.In this dissertation,both case studies and statistical analysis are conducted to study the characteristics of the storm-time ionospheric disturbances,as well as the spatial and temporal characteristics of the pre-earthquake ionospheric anomalies.The main studies performed and results obtained in this dissertation can be summarized as follows:(1)We present both observations and numerical simulations of the low and equatorial ionospheric response to a super magnetic storm during 9-10 Nov.2004.The ionophseric observations include GPS-TEC,ExB drift obtained by Jicamarca Radio Observatory Incoherent Scatter Radar(JRO ISR),and hmF2 and foF2 obtained by the ionosonde at JRO.Both observations and simulations show super plasma fountains during the daytime and nighttime respectively,due to the vertical ExB drift caused by the strong eastward prompt penetrate electric field.And then the super plasma fountains are suppressed by the westward disturbed dynamo electric filed occurring subsequently.During the daytime,the F;appears as a result of the super plasma fountain.Different with the results of Balan et al.(2008),SAMI2 could reproduce and explain the generation,development,and suppression of the equatorial F3,which is well consistent with the ionogram.During the nighttime,because of the lack of photochemical effect,there appears huge depletion at the low and equatorial ionosphere after the lifting of the ionosphere.The numerical simulations are consistent with the observations without consideration of the neutral wind effect,which indicate that the electrodynamic effects play a major role in controlling the ionosphere for this storm event.(2)To further understand the dependence of the ionospheric storm on longitude and latitude,we have made a statistical analysis of the ionospheric TEC response during 217 geomagnetic storms with main phase undergoing only one step growth from the year 2001 to 2015,including observations at 6 different latitude zones along one meridian in the American sector and 4 different longitudes at the middle latitude zone.It shows that the ionospheric storm has prominent latitudinal dependence,with negative storm prevailing at high latitudes and positive storm at lower latitudes.The maximum ratio of positive to negative storms is observed at around 30°N magnetic latitude.The ionospheric response depends also on the phases of the storm.The occurrence of positive storm decreases during recovery phase when compared to that during main phase,while the negative storm presents opposite pattern.However,the occurrence of positive storm increases at equatorial and low latitudes during recovery phase.Additionally,during main phase,the occurrence of negative storm is higher at equatorial and low latitudes than that at middle latitudes.The local time dependences of the ionospheric storm onsets are quite different for different latitudes.At high-latitudes,the negative storm mainly occurs during daytime while the positive storm mainly occurs during nighttime.At middle latitudes,the negative storm mainly occurs from post-middle night to morning hours and the positive storm mainly occurs during daytime.At equatorial latitudes,both the positive and negative storms mainly occur during nighttime.For all latitudes,the typical time delay between the main phase onset and the onset time of ionospheric negative storm is longer than 10 hours,while it is shorter than 10 hours for positive storm(expect at low and equatorial latitudes).We further check the longitudinal dependence of the ionospheric storm at middle latitude,and find that the occurrence of positive storm is higher in American and Asian sectors than that in European sector,and the non-significant storm is mostly observed in European sector.The‘forbidden time interval'of negative storm in Asian sector is different with that in American and European sectors.(3)Based on CODE GIM data,a statistical analysis of local TEC anomalies before 121 low-depth(D?100km)strong(Mw>7.0)earthquakes has been made using the sliding median differential calculation method combined with a new approach of image feature extraction.The results show that significant local TEC anomalies could be observed 0-6 days before 80 earthquakes,about 66.1%out of the total.The positive anomalies occur more often than negative ones.For 26 cases,both positive and negative anomalies are observed before the shock.The pre-earthquake TEC anomalies show local time recurrence for 38 earthquakes,which occur around the same local time on different days.The local time distribution of the pre-earthquake TEC anomalies mainly concentrates between 19 and 06 LT,roughly from the sunset to sunrise.Most of the pre-earthquake TEC anomalies do not locate above the epicenter,but shift to the south.The pre-earthquake TEC anomalies could be extracted near the magnetic conjugate point of the epicenter for 40 events,which is 50%out of the total 80 cases with significant local TEC anomalies.In general,the signs of the anomalies around epicenter and its conjugate point are the same,but the abnormal magnitude and lasting time are not.The hypothesis of the anomalous electric field in seismo-ionoshpere coupling are used to explain most of the observed phenomena;however,to further understand the interaction between earthquake and ionosphere,more earthquake cases and experimental data including both ground-based and space-borne observations should be investigated in the future.