The Feature of Ionospheric Mid-Latitude Trough during Geomagnetic Storms Derived from GPS Total Electron Content (TEC) Data

This study aims to investigate the features of the ionospheric mid-latitude trough over North America by using the MIT total electron content data obtained during three geomagnetic storms that occurred in August 2018, September 2017, and March 2015. The mid-latitude trough position sharply moves equatorward from the quiet-time subauroral latitude to mid-latitude with the decrease in SYM-H during geomagnetic storms. We find that the ionospheric behavior of TEC around the mid-latitude trough position displays three kinds of ionospheric storm effect: negative ionospheric storm effect, unchanged ionospheric behavior, and positive ionospheric storm effect. These ionospheric storm effects around the mid-latitude trough position are not always produced by the mid-latitude trough. The ionospheric storm effects produced by the mid-latitude trough are limited in the narrow mid-latitude trough regions, and are transmitted to other regions with the movement of the mid-latitude trough.

Dynamic processes in the magnetic field and in the ionosphere during the 30 August–2 September 2019 geospace storm: influence on high frequency radio wave characteristics

The concept that geospace storms are comprised of synergistically coupled magnetic storms, ionospheric storms, atmospheric storms, and storms in the electric field originating in the magnetosphere, the ionosphere, and the atmosphere (i.e., electrical storms) was validated a few decades ago. Geospace storm studies require the employment of multiple-method approaches to the Sun–interplanetary medium–magnetosphere–ionosphere–atmosphere–Earth system. This study provides general analysis of the 30 August–2 September 2019 geospace storm, the analysis of disturbances in the geomagnetic field and in the ionosphere, as well as the influence of the ionospheric storm on the characteristics of high frequency (HF) radio waves over the People's Republic of China. The main results of the study are as follows. The energy and power of the geospace storm have been estimated to be 1.5×1015 J and 1.5×1010 W, and thus, this storm is weak. The energy and power of the magnetic storm have been estimated to be 1.5×1015 J and 9×109 W, i.e., this storm is moderate, and a characteristic feature of this storm is the duration of the main phase of up to 2 d. The recovery phase also was lengthy and was no less than 2 d. On 31 August and 1 September 2019, the variations in the H and D components attained 60–70 nT, while the Z-component variations did not exceed 20 nT. On 31 August and 1 September 2019, the level of fluctuations in the geomagnetic field in the 100–1000 s period range increased from 0.2–0.3 to 2–4 nT, while the energy of the oscillations showed a maximum in the 300–400 to 700–900 s period range. During the geospace storm, a moderately to strongly negative ionospheric storm manifested itself by the reduction in the ionospheric F-region electron density by a factor of 1.4 to 2.4 times on 31 August and 1 September 2019, compared to the its values on the reference day. Appreciable disturbances were also observed to occur in the ionospheric E region and possibly in the Es layer. In the course of the ionospheric storm, the altitude of reflection of radio waves could sharply increase from ∼150 to ∼300–310 km. The atmospheric gravity waves generated within the geospace storm modulated the ionospheric electron density; for the ∼30 min period oscillation, the amplitude of the electron density disturbances could attain ∼40 %, while it did not exceed 6 % for the ∼15 min period. At the same time, the height of reflection of the radio waves varied quasi-periodically with a 20–30 km amplitude. The results obtained have made a contribution to the understanding of the geospace storm physics, to developing theoretical and empirical models of geospace storms, to the acquisition of detailed understanding of the adverse effects that geospace storms have on radio wave propagation, and to applying that knowledge to effective forecasting of these adverse influences.

Geospace storm effects on August 5-6, 2019

Geospace storms are the synergistically interacting magnetic storms, ionospheric storms, atmospheric storms, and the storms in an electric field of magnetospheric, ionospheric, and atmospheric origins. Geospace storms are very diverse, and no two of them behave exactly the same. Therefore, studying the effects of each new storm becomes an urgent task for us. Such research will reveal both the general laws and individual characteristics of storm processes. The purpose of this paper is to present general information about the geospace storm, the results of the analysis of features of magnetic and ionospheric storms. To analyze the magnetic environment, we used the measurement results of magnetic field fluctuations in the range from 1 s to 1000 s, performed at the Magnetometric Observatory of V. N. Karazin Kharkiv National University, and variations of three components of the geomagnetic field, performed at the Low-frequency observatory of the IRA NASU. We analyzed the ionospheric environment using multi-frequency multi-path measurements performed at Harbin Engineering University (China) and also the data of ionosonde. The main results of the work are as follows. An increase in the main parameters of the solar wind on August 5, 2019, led to a geospace storm, which was mainly observed on August 5 and 6, 2019. The main phase of the magnetic storm took place on August 5, 2019, from 06:00 a.m. to 08:30 a.m. The recovery phase lasted at no less than 4 days. The magnetic storm shows significant variations of all components of the geomagnetic field, and there is an increase by order of magnitude of the oscillations’ level of the geomagnetic field in the range from 400 s to 950 s. During the ionospheric storm, significant disturbances occurred in the F region of the ionosphere. The E-region of the ionosphere remained weakly perturbed. The ionospheric storm has severely affected the Doppler spectra of radio waves in the 5 – 10 MHz frequency range. The Doppler spectra are significantly broadened, and the Doppler frequency shift and its quasi-periodic change with a period of 20–40 minutes and a duration of 120–240 minutes have taken place. The quasi-periodic variations of the Doppler frequency shift are due to quasi-periodic variations in the electron concentration, and the amplitude of their relative perturbations varied from 3% to 16%. On one of these paths, the amplitude of the Doppler frequency shift reached 0.7 Hz. And in this case, the amplitude of the relative perturbations of the electron concentration could reach 80 - 90%. In addition, the ionospheric storm little affected the signal amplitude on most radio paths.

The development of an ionospheric storm-time index for the South African region

This thesis presents the development of a regional ionospheric storm-time model which forms the foundation of an index to provide a quick view of the ionospheric storm effects over South African mid-latitude region. The model is based on the foF2 measurements from four South African ionosonde stations. The data coverage for the model development over Grahamstown (33.3◦S, 26.5◦E), Hermanus (34.42◦S, 19.22◦E), Louisvale (28.50◦S, 21.20◦E), and Madimbo (22.39◦S, 30.88◦E) is 1996-2016, 2009-2016, 2000-2016, and 2000-2016 respectively. Data from the Global Positioning System (GPS) and radio occultation (RO) technique were used during validation. As the measure of either positive or negative storm effect, the variation of the critical frequency of the F2 layer (foF2) from the monthly median values (denoted as _foF2) is modeled. The modeling of _foF2 is based on only storm time data with the criteria of Dst 6 -50 nT and Kp > 4. The modeling methods used in the study were artificial neural network (ANN), linear regression (LR) and polynomial functions. The approach taken was to first test the modeling techniques on a single station before expanding the study to cover the regional aspect. The single station modeling was developed based on ionosonde data over Grahamstown. The inputs for the model which related to seasonal variation, diurnal variation, geomagnetic activity and solar activity were considered. For the geomagnetic activity, three indices namely; the symmetric disturbance in the horizontal component of the Earth’s magnetic field (SYM − H), the Auroral Electrojet (AE) index and local geomagnetic index A, were included as inputs. The performance of a single station model revealed that, of the three geomagnetic indices, SYM − H index has the largest contribution of 41% and 54% based on ANN and LR techniques respectively. The average correlation coefficients (R) for both ANN and LR models was 0.8, when validated during the selected storms falling within the period of model development. When validated using storms that fall outside the period of model development, the model gave R values of 0.6 and 0.5 for ANN and LR respectively. In addition, the GPS total electron content (TEC) derived measurements were used to estimate foF2 data. This is because there are more GPS receivers than ionosonde locations and the utilisation of this data increases the spatial coverage of the regional model. The estimation of foF2 from GPS TEC was done at GPS-ionosonde co-locations using polynomial functions. The average R values of 0.69 and 0.65 were obtained between actual and derived _foF2 over the co-locations and other GPS stations respectively. Validation of GPS TEC derived foF2 with RO data over regions out of ionospheric pierce points coverage with respect to ionosonde locations gave R greater than 0.9 for the selected storm period of 4-8 August 2011. The regional storm-time model was then developed based on the ANN technique using the four South African ionosonde stations. The maximum and minimum R values of 0.6 and 0.5 were obtained over ionosonde and GPS locations respectively. This model forms the basis towards the regional ionospheric storm-time index.

Physics of geospace storms

A review of our knowledge about the coupling of solar-terrestrial processes, manifestations of geospace storms, and variations in space weather is presented. Space weather effects are analyzed within the system paradigm concept. The system where geospace storms occur is a Sun–interplanetary medium–magnetosphere–ionosphere–atmosphere–Earth (interior spheres) aggregate (SIMMIAE). An early twenty-first- century geospace superstorm that occurred on November 7 – 10, 2004, is examined in detail. Clustered instrument observations of this storm effects are presented. The investigation of the physical effects of geospace storms is noted to be the most important field of study in space geophysics. The problem of subsystem coupling in the SIMMIAE system during a geospace storm is interdisciplinary in nature. Its solution requires an application of the system approach. The problem has a multifactor character. The subsystem response is determined by the simultaneous (synergetic) impact of a few disturbing factors. It is important to note that the SIMMIAE is an open, nonlinear, and nonstationary system. Within it, direct coupling and feedback processes, positive and negative linkages operate. Due to the myriads of manifestations of geospace storms, because of the unique nature of each storm, the investigation of occurring physical effects is far from complete. In addition to a thorough investigation of the storm’s physical effects, there is an urgent need to model and forecast the storms adequately and in detail. The solution to these problems will facilitate the survival and steady progress of our civilization, relying more and more on new state-of-the-art technology. The more technologically reliant our society is, the more vulnerable the civilization's infrastructure to solar and geospace storm impacts becomes. A classification of geostorms based on Akasofu's epsilon parameter has been advanced. Six types of geostorm have been introduced, and a geostorm index has been suggested. A classification of ionospheric storms and disturbances based on the magnitude of variations in the peak density of the F2 layer has been suggested. Five types of the ionospheric storm have been introduced. An ionospheric index characterizing the intensity of negative and positive ionospheric storms has been suggested. A classification of ionospheric storms and disturbances based on the magnitude of variations in the lower-ionosphere electron density has been proposed. Six types of the positive ionospheric storm have been introduced. The appropriate ionospheric index has been suggested. The physics-based model of the evolution of each group of ionospheric storms and disturbances has been determined. The linkages among magnetic, ionospheric, and atmospheric storms, as well as electric field disturbances, have been shown.