N.M. Polekh

Mid-latitude effects of the May 15, 1997 magnetic storm

We investigate the May 15, 1997 magnetic storm effects on the mid- and low-latitude ionosphere. The study is based on using the data from three chains of ionospheric stations located approximately along the meridians 20°, 140° and 280°E in the geomagnetic latitude range 13–65°N. Variations in f0F2 are considered. Estimates of the zonal electric fields are made. Results of our analysis show that the main ionospheric effects of the storm under consideration are: (1) long-lasting intense negative disturbances during the storm main and recovery phases at subauroral and mid-latitudes; (2) positive disturbances at stations of the European and American chains observed prior to the storm, regardless of the local time; (3) a positive peak of Δf0F2 at stations of the Asian chain during the storm main phase in the evening hours; (4) a similarity of the form of the Δf0F2-variations at different latitudes and (5) the largest effect on the F region is observed at the Asian chain. The resulting differences of the Δf0F2-variations can be driven both by the local time of the sudden storm commencement and by magnetic dip. It is not mere chance that the largest differences are observed along the meridian 140°E where the difference between the geographic and magnetic poles is the largest.

Longitudinal variation of critical frequencies in polar F-region

Abstract Based on data from a network of ionospheric stations located in the range of geographic longitudes 19°–285° and invariant latitudes 53°–70°N we have investigated diurnal behaviour variations in F2-layer critical frequencies for different seasons and different levels of solar activity. The study revealed that the longitudinal effect in diurnal f o F2 variations is most conspicuous in the summertime at the invariant latitude about 55–57°N and manifests itself in the shift of the f o F2 maximum into the evening and night-time hours on the Yakutsk (129.6°) and Ottawa (284°) meridians in the region of westward declination. It is likely that such a behaviour of f o F2 is conditioned by a change in the dynamic regime of the high-latitude ionosphere associated with the magnetic anomaly.

Ionospheric effects of magnetospheric and thermospheric disturbances on March 17–19, 2015

Using vertical and oblique radio-sounding data, we analyze the ionospheric and thermospheric disturbances during the magnetic storm that occurred in northeastern Russia on March 17–19, 2015. We consider the heliospheric sources that induced the magnetic storm. During the main and early recovery phases, the midlatitude stations are characterized by extremely low values of electron density at the F2 layer maximum. Using oblique sounding data, we recorded signals that propagated outside the great circle arc. In evening and night hours, no radio signals were found to pass along the Norilsk–Irkutsk and Magadan–Irkutsk paths. The observed ionospheric effects are shown to be caused by a sharp shift of the boundaries of the main ionospheric trough to the invariant latitude 46° N during the main phase of the magnetic storm. The negative ionospheric disturbance during the recovery phase of the storm, which was associated with significant variations in the composition of the neutral atmosphere, led to a change in the mode composition of received radio signals and a decline in observed maximal frequencies in daytime hours of March 18, 2015 by more than 2 times.

Variability of parameters of the F2-layer maximum in the quiet midlatitude ionosphere under low solar activity: 1. Statistical properties

Results of statistical analysis of the properties of variability of F2-layer maximum parameters (critical frequency foF2 and the height hmF2) in quiet midlatitude ionosphere under low solar activity in the daytime (1000–1500 LT) and nighttime (2200–0300 LT) hours are presented on the basis of Irkutsk station data for 2007–2008. It is found that the distribution density of δfoF2 could be presented as consisting of two distinctly different normal laws of this distribution, one of which corresponds to weak (|δfoF2| < 10%) fluctuations in foF2 and the other corresponds to strong (30% > |δfoF2| > 10%) fluctuations. Weak fluctuations in foF2 to a substantial degree are related to ionospheric variability at times less of than 1–3 h and determine the δfoF2 variability in the daytime hours. Strong fluctuations in foF2 are mainly related to day-to-day variability of the ionosphere at a fixed local time, the variability increasing by approximately a factor of 3 during the transition from day to night and determining the δfoF2 variability in the nighttime hours. The distribution density of ΔhmF2 is close to the normal distribution law. An interpretation of the different character of the distribution densities of δfoF2 and ΔhmF2 is given.

Geospheric effects of the solar flare of December 13, 2006

Disturbances in the magnetic field and magnetospheric and ionospheric plasma registered on December 14–16, 2006, during a strong magnetic storm caused by a solar flare of 4B/X3.4 class are studied. It is shown that in the north of Yakutia, interactions between the Earth’s magnetosphere and the region of high dynamic pressure of the solar wind led to the formation of sporadic layers in the ionospheric E and F regions, depletion of the critical frequency of the F2 layer, and total absorption. At the end of the magnetic storm’s main phase, anomalously high values of foF2 exceeding the quiet level by a factor of 1.5–1.7 were detected. It was found that the disturbances detected by ground-based observatories had developed on the background of changes in the temperature, density, and the pitch-angle distribution of particles at the geostationary orbit manifesting radial shifts of magnetospheric structures (magnetopause, cusp/cleft, and plasma sheet) relative to the observation points. A conclusion is drawn that in this case, changes in the near-Earth plasma and magnetic field manifest the dynamics of the physical conditions at the magnetospheric boundary and diurnal rotation of the Earth.

Using IRI-95 in FMCW signal simulation

Abstract A ray tracing code for long distance propagation via the ionosphere has been established and applied with electron density profiles taken from IRI-90 and -95. The theoretical results for different distances were compared with experimental ones obtained by Chirp ionosonde techniques. When comparing maximum observable frequencies good agreement was found even with a transequatorial path.

Variability of parameters of the F2-layer maximum in the quiet midlatitude ionosphere under low solar activity: 2. Strong fluctuations of critical frequency

This paper presents a qualitative analysis of the properties and particular examples of strong (10% < |δfoF2| < 30%) and very strong (|δfoF2| > 30%) fluctuations in the critical frequency of the F2 layer (foF2) of the quiet ionosphere at midlatitudes under low solar activity according to the Irkutsk station data for 2007–2008. It is found that strong day-to-day fluctuations in foF2 are mainly related to changes in thermospheric parameters, which have a nature of planetary waves and tides. Evidently, very strong day-to-day fluctuations in foF2 are caused by superposition of the effects in the ionosphere caused by changes in the thermospheric parameters and those related to a complex of processes of solar wind interaction with the magnetosphere, including the effects caused by the reversal of the vertical component of the solar wind magnetic field southwards. The increase in foF2 during nighttime hours in winter up to values typical for the daytime maximum in foF2 is the brightest example of very strong changes in foF2 in the quiet ionosphere.

Peculiarities of the ionospheric response to geomagnetic storm on 25 September, 1998, in the East-Asian region

We present the results derived from investigating the ionospheric effects of a large geomagnetic storm on September 25, 1998, based on analyzing the data of ionospheric stations located in the region of East Siberia and the Far East. For the sake of comparison, we used some European stations. In addition to vertical-incidence sounding data, measurements showed that even with a relatively small difference of longitudes, the recovery phase has substantial differences of ionospheric distrubances. The study revealed the critical frequency variations from stations of the Norilsk chain and at Salekhard during the recovery phase of the storm when negative disturbances recurred after positive disturbances. The sign reversal of disturbance in the displacement from the west to the east is difficult to explain, and further investigation is required.

THE SOLAR WIND MAGNETIC CLOUD OF OCTOBER 18-20, 1995: EFFECT ON IONOSPHERE OF THE RUSSIAN ASIAN REGION

Based on data from a chain of ionospheric stations we investigate the response of ionospheric parameters to the magnetic storm of October, 18-23, 1995. During this storm an abrupt decrease in critical frequencies to 35-45% in the F-region in the daytime hours and intense sporadic E-layers of the auroral type at night were recorded by the entire chain of stations. The analysis of experimental data suggests that the trough boundaries are displaced as far as 41 ° magnetic latitude.

Modeling of the electron density distribution in the polar ionosphere

Abstract This paper presents the precepts to the development of a high-latitude semi-empirical model and the results obtained by modeling electron density in the height range 100 to 500 km. When constructing the model, advantage was taken of an earlier approach used to model the midlatitude ionosphere, in which, based on empirical data a map of critical frequencies foF2 is deduced. The electron density profile is calculated by solving the set of aeronomic equations controlling the electron and ion distribution.