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The disturbances of ionospheric radio channel during 17–24 March 2015 magnetic storm are investigated. The heliospheric sources which caused the storm are considered. Based on space-distributed multipurpose chirp ionosonde data effects of geomagnetic disturbances influence on conditions of HF signal propagation are studied.

The results for modeling of HF radio waves propagation characteristics are given for the periods of solar flares 25.02.2014, 25.10.2013, 13-14.05.2013. The distance–frequency and amplitude-frequency propagation characteristics are calculated on the base of the complex algorithm which includes modules of ionosphere and plasmasphere global models and radio waves propagation model. The results of calculations were compared with experimental data of oblique ionosphere sounding obtained by chirp ionosonde on paths Magadan – Irkutsk, Khabarovsk – Irkutsk and Norilsk – Irkutsk.

The dependence of the origination of G conditions in the ionospheric F region on solar and geomagnetic activity has been determined based on numerical simulation of the ionosphere over points 50° N, 105° E and 70° N, 105° E for summer conditions at noon. It has been found that the threshold value of the Kp geomagnetic activity index (KpS), beginning from which a G condition can originate, is minimal for a low solar activity level at relatively high latitudes during the recovery phase of a geomagnetic storm. On average, KpS increases with increasing solar activity, but G conditions can originate at high solar activity levels and be absent at moderate ones for certain Kp values, which was apparently predicted for the first time. These properties of the origination of G conditions do not contradict the known results of a G-condition statistical analysis performed based on the data from the global network of ionospheric stations.

This paper studies the role of magnetospheric factors, such as convection and energetic electron precipitation during the formation of positive disturbances in the total electron content under the conditions of the summer evening ionosphere. A numerical model of the ionosphere and plasmasphere, where time variations in the magnetospheric convection velocity and electron precipitation parameters correspond to the main phase of a magnetic storm, has been used for this purpose. It has been indicated that the total electron content sharply increases (the “dusk effect”) in the eastern and western sectors at approximately the same geomagnetic latitudes corresponding to the subauroral zone provided that a sudden storm commencement is registered in the morning hours. local time. This peak of the total electron content is formed as a result of joint reconstruction of the magnetospheric convection pattern and energetic electron precipitation during the main phase of a storm. In this case, magnetospheric convection plays the main role, raising the F2 layer by 40–80 km into the region with a lower recombination rate.

We study the possibilities of the Theoretical Ionosphere Model (TIM) developed at the Institute of Solar-Terrestrial Physics, Siberian Branch, Russian Academy of Sciences, for calculating the HF-radiowave propagation characteristics. The results of simulation based on the TIM are compared with calculations based on the IRI model and data from experimental observations. Analysis of the results of calculations for the maximum usable frequency (MUF) have shown that with the same input data (coordinates of the receipt and transmission points, the route length, date, and time), the differences in the calculated MUFs (using two different models supplying radio routes with ionospheric information) amount to ∼1% in the daytime and reach 10% at night.

This paper proposes a new method for estimating the contribution from different ionospheric regions to the response of total electron content variations to the solar flare which uses the effect of partial shadowing of the atmosphere by the terrestrial globe. The study uses GPS stations located near the boundary of the shadow on the ground in the nightside hemisphere. The beams between the satellite-borne transmitter and the receiver on the ground for these stations pass partially through the atmosphere lying in the region of total shadow and partially through the illuminated atmosphere. The analysis of the ionospheric effect of a powerful solar flare of class X5.7/3B that was recorded on July 14, 2000 (10:24 UT, N22W07) in quiet geomagnetic conditions (Dst=-10 nT) has shown that about 20% of the TEC increase correspond to the ionospheric region lying below 100 km, about 5% refer to the ionospheric E-region (100-140 km), about 30% correspond to the F1-region (140-200 km), and about 30% to regions lying above 300 km., LaTeX, 6 pages, 4 figures, 1 table

The ionospheric response to a geomagnetic disturbance is a complex set of events caused by both the upper atmosphere and ionosphere parameters and characteristics of the magnetosphere and solar wind. A situation is particularly complicated during the large geomagnetic storm. We present the results derived from investigating of the ionospheric response to large geomagnetic storms with the values of index Dst < (-200 ÷ -300nT) observed during two last cycles of solar activity. Our analysis of the behavior of the ionosphere is based on using the measurements from a network of ionospheric stations located at different latitudes in the longitudinal sector of 60-150°E. Also there are presented the results of numerical modeling of ionospheric parameters during the geomagnetic storm on April, 2000δ which show a good agreement of calculations and measurements. As the results modeling illustrates prolonged negative ionospheric disturbances observed during geomagnetic storms may be produced by the change of thermosphere composition.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

in the ionospheric response to a geomagnetic storm are obtained depending on latitude and season. At middle latitudes the most interesting is that the positive and negative phases of ionospheric disturbance prevail in winter and summer, respectively. To interpret the observed variations in the ionospheric structure, modeling is performed, using a theoretical ionospheric model. The analysis of the processes governing the response of the midlatitude ionosphere to a geomagnetic storm showed a good agreement between the results of modeling and measurements and made it possible to detect the determining role of the neutral composition in the observed variations in ionospheric parameters. At auroral and subauroral stations the variability of the electron concentration during a storm is much better pronounced. According to the results of the analysis of the trajectories of the ionospheric plasma convection, this variability is caused by the joint action of the convection and energetic electron precipitations. The disturbances in the neutral composition in this latitudinal region influence the background electron concentration level. INDEX TERMS: 2441 Ionosphere: Ionospheric storms; 2447 Ionosphere: Modeling and forecasting; 2407 Ionosphere: Auroral ionosphere; KEYWORDS: Ionospheric storms; Ionosphere-magnetosphere interactions; Numerical simulations.