Olga M. Pirog

Solar activity variations of nighttime ionospheric peak electron density

[1] Monthly median NmF2 (maximum electron density of the F2-layer) data at Okinawa, Yamagawa, Kokubunji, and Wakkanai have been collected to investigate the solar activity dependence of the nighttime ionosphere. The result shows that there are seasonal and latitudinal differences of the solar activity variation of nighttime NmF2. The main seasonal effects are as follows: nighttime NmF2 increases with F107 linearly in equinoctial months (March and September), and it tends to saturate with F107 increasing in summer solstice month (June). What is peculiar is that there is an amplification trend of nighttime NmF2 with F107 in winter solstice month (December). The latitudinal difference is mainly displayed by the evolvement course of the variation trend between NmF2 and F107. Using hmF2 (peak height of the F2-layer) data and the NRLMSISE00 model, we estimated the recombination loss around the F2-peak at different solar activity levels. We found that the solar activity variation of the recombination processes around the F2-peak also shows seasonal dependence, which can explain the variation trends of nighttime NmF2 with F107 qualitatively, and field-aligned plasma influx plays an important role in the equatorial ionization anomaly (EIA) crest region. During the first several hours following sunset in December, there are faster recombination processes around the F2-peak at medium solar activity level in mid-latitude regions. This feature is suggested to be responsible for inducing the amplification trend in winter. In virtue of the calculation of neutral parameters at 300-km altitude and hmF2 data, the variation trend of the recombination processes around the F2-peak with F107 can be explained. It shows that both the solar activity variations of hmF2 and neutral parameters (neutral temperature, density, and vibrational excited N2) are important for the variation trend of nighttime NmF2 with F107. Furthermore, the obvious uplift of hmF2 at low solar activity level following sunset in December is important for the amplification trend.

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.

Regular changes in the critical frequency of the F2 layer of the quiet midlatitude ionosphere

A method for constructing the empirical model of the F2 layer critical frequency (foF2) under magnetically quiet conditions, aimed at analyzing disturbances of any nature, is proposed. This method has been analyzed, and typical features of regular changes in foF2 of the quiet ionosphere and day-to-day foF2 variability are analyzed using the data from Irkutsk and Slough stations as an example. In particular, it has been obtained that this model differs from the international IRI model, and this difference is mainly caused by the fact that the foF2 values in the IRI model do not correspond to quiet conditions. Therefore, this model gives a larger amplitude of the annual and semiannual variations in foF2 than the IRI model. In addition, this model more accurately reproduces the rate of foF2 annual variations at a fixed local time, especially in equinoxes, when foF2 variations can exceed 1 MHz within one month.

Variations of the ground-measured solar ultraviolet radiation during the solar eclipse on March 9, 1997

Abstract Results of investigations of the solar ultraviolet radiation measured on the ground during the solar eclipse observed over Irkutsk on March 9, 1997 are presented. The spectral distribution variations (radiation 296–326 nm) are revealed and discussed in connection with proposed ozone-eclipse effect. It is suggested that variations in spectra observed at the time close to the maximum phase of eclipse can be caused by the multiple scattering effect of the ultraviolet radiation.

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.

The upper mesosphere/lower thermosphere wind field nonzonality as possible sign of the external forcing from above and from below

Abstract The geographical nonzonality of the global wind field was investigated through a long-term study of upper mesopause/lower thermosphere horizontal winds (average altitude 95 km) near Irkutsk, East Siberia, Russia (52° N, 104° E) in comparison with similar wind measurements at the stations, which are well spaced along the 52° N latitude circle. The longitudinal/regional variations in the dynamics of the upper middle atmosphere are illustrated. This effect may significantly manifest itself in the different amplitudes of the prevailing zonal wind, in the different times of the winter-spring transition of the zonal circulation, in the different response to the sudden stratosphere warmings, and in the different response to the geomagnetic storms and Solar Proton Events. The reasons of the nonzonality could be planetary scale mesopause oscillations structure; the external forcing from below due to different conditions for the generation and propagation upward of the internal atmospheric waves; the real difference of the geomagnetic latitudes. The nonzonality of the mesopause wind field may be important when empirical and physical upper atmosphere models are being formed.

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.

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.