N. M. Pavlova

Anomalous variations in the structure of the ionospheric F 2 region at geomagnetic midlatitudes of the Southern and Northern hemispheres in going from summer to winter conditions at high solar activity

The structure and dynamics of the ionosphere and plasmasphere at high solar activity under quiet geomagnetic conditions of June 2–3, 1979, and January 5–6, 1980, over Millstone Hill station and Argentine Islands ionosonde, the locations of which are approximately magnetically conjugate, have been theoretically calculated. The plasma drift velocity, determined by comparing the calculated and measured heights of the F2 layer maximum (hmF2), and the correction of [N2] and [O2], found in the NRLMSISE-00 model, make it possible to coordinate the electron densities (NmF2) calculated at the hmF2 height and the measured anomalous variations in NmF2 over the Argentine Islands ionosonde as well as the calculated and measured NmF2 and electron temperature at the hmF2 height over Millstone Hill station. It has been shown that, if the interference of the diffusion velocities of O+(4S) and H+ ions is taken into account, the additional heating of plasmaspheric electrons leads to an increase in the flux of O+(4S) ions from the topside ionosphere to lower F2 layer altitudes, as a result of which an anomalous nighttime increase in NmF2 6, observed on January 6, 1980, over Millstone Hill station, is mainly produced. The second component of the formation of anomalous night-time NmF2 is the plasma drift along the magnetic field caused by the neutral wind, which shifts O+(4S) ions to higher altitudes where the recombination rate of O+(4S) with N2 and O2 is lower and slows down a decrease in NmF2 in the course of time. It has been shown that the influence of electronically excited O+ ions and vibrationally excited N2 and O2 molecules on electron density (Ne) considerably differs under winter and summer conditions. This difference forms significant part of the winter anomaly in Ne at heights of the F2 region and topside ionosphere over Millstone Hill station.

Effect of solar radiation refraction on the zenith angle and times of the sunrise and sunset in the atmosphere

The analytical formulas for the solar radiation refraction angle and the times of the sunrise and sunset at altitudes of the atmosphere, which make it possible to take into account the dependence of these parameters on the temperature, density, and temperature vertical gradient in the atmosphere specified at the ground level, have been obtained. It has been indicated that solar radiation refraction pronouncedly affects the times of the sunrise and sunset at altitudes of the atmosphere.

Variations in statistical parameters of the NmF2 winter anomaly with latitude and solar activity

The maximal R ratios of the winter-to-summer NmF2 values of each ionosonde are calculated for a specified UT under daytime quiet geomagnetic conditions and at approximately equal levels of solar activity, based on foF2 measurement data of 98 ionosondes at mid- and low geomagnetic latitudes of the Northern and Southern hemispheres for 1957–2009. The P(R > 1) conditional probability of NmF2 winter anomaly observations, as well as the most probable RMP and average of R values are calculated for low, moderate, and high solar activity on the base of foF2 measurements during the periods December 22 ± 30 days and June 21 ± 30 days. Variations in P(R > 1), RMP, and 〈R〉 with latitude and solar activity are analyzed.

Statistical study of anomalous nighttime maximums in the NmF 2 diurnal variations in the region of appearance of the equatorial anomaly northern crest

The occurrence probabilities of the first and second anomalous nighttime local maximums in the diurnal variations in the electron density at a maximum of the ionospheric F2 layer (NmF2) in the region where the crest (hump) of the equatorial anomaly originates in the northern geographic hemisphere have been studied using the data of the stations for vertical sounding of the ionosphere (Paramaribo, Dakar, Quagadougou, Ahmedabad, Delhi, Calcutta, Chongoing, Guangzhou, Taipei, Chung-Li, Okinawa, Yamagawa, Panama, and Bogota) from 1957 to 2004. It has been demonstrated that the anomalous nighttime NmF2 maximums are least frequently formed at ∼53° geomagnetic longitude. The calculations have indicated that the studied probabilities are independent of solar activity. Geomagnetic activity weakly affects the rate of occurrence of the first nighttime NmF2 maximum at geomagnetic longitudes of approximately 140° to 358°. At geomagnetic longitudes of approximately 16° to 70° (i.e., in the longitudinal zone of a decreased occurrence frequency of anomalous nighttime maximums), the occurrence probability of the first anomalous nighttime NmF2 maximum under geomagnetically quiet conditions is pronouncedly lower than under geomagnetically disturbed conditions. The dependence of the occurrence probabilities of the first and second anomalous nighttime NmF2 maximums on the month number in a year has been studied.

Anomalous variations in the ionospheric F 2-layer structure at geomagnetic midlatitudes of the Southern and Northern hemispheres at the transition from summer to winter conditions under low solar activity

The structure and dynamics of the ionosphere and plasmasphere at low solar activity under quiet geomagnetic conditions on January 15–17, 1985, and July 10–13, 1986, over Millstone Hill station and Argentine Islands ionosonde, the locations of which are approximately magnetically conjugate, have been theoretically calculated. The detected correction of the model input parameters makes it possible to coordinate the measured and calculated anomalous variations in the electron density NmF2 at the height hmF2 of the ionospheric F2 layer over Argentine Islands ionosonde as well as the calculated and measured values of NmF2 and electron temperature at the hmF2 height over Millstone Hill station. It has been shown that vibrationally excited N2 and O2 molecules almost do not influence the formation of the winter anomaly under the conditions of low solar activity. A difference between the influence of electronically excited O+ on Ne ions under winter and summer conditions forms not more than 11% of the Ne winter anomaly event in the F2 layer and topside ionosphere. The model without electronically excited O+ ions reduces the duration of the Ne winter anomaly event. It has been shown that the seasonal variations in the composition of the neutral atmosphere form mainly the NmF2 winter anomaly event over the Millstone Hill radar at low solar activity.

Influence of the plasma zonal E × B drift on the electron concentration in the low-latitude ionospheric F region at the minimum of solar activity near the spring equinox

The three-dimensional nonstationary theoretical model of the concentrations and temperatures of electrons and ions in the ionospheric F region and plasmasphere at low and middle latitudes is used to study variations in the concentration NmF2 and height hmF2 of the ionospheric F2 layer under the action of the plasma zonal drift in the direction geomagnetic west-geomagnetic east perpendicularly to the electric E and geomagnetic B fields. The calculated and measured values of NmF2 and hmF2 for 16 ionospheric sounding stations during the quiet geomagnetic period on March 28–29, 1964 at low solar activity are compared. This comparison made it possible to correct the input parameters of the model: [O] from the NRLMSISE-00 model and the meridional component of the neutral wind velocity from the HWW90 model. It is shown that the nighttime NmF2 values decrease up to twice at low solar activity in the low-latitude ionosphere, and the hmF2 values change by up to 16 km, if the plasma zonal E×B drift is not taken into account. Under the daytime conditions, the influence of the plasma zonal E×B drift on NmF2 can be neglected.

Comparison of electron concentrations in the ionospheric E-layer maximum in spring conditions obtained by calculations and Moscow ionosonde measurements

The electron concentrations in the ionospheric E-layer maximum NmE, as measured by the Moscow ionosonde, are compared with the results of theoretical calculations of NmE for geomagnetically quiet conditions at low solar activity on April 1, 1986, and April 6, 1996, moderate solar activity on April 9, 1978, and April 6, 1998, and high solar activity on April 20, 1980, and April 15, 1991. On the basis of this comparison, a correction of the model flux of solar X-ray radiation is proposed. The discovered variability of the correction factors manifests the influence of solar X-ray radiation flux variations on NmE variability. The dependence of the influence of the neutral constituents ionization by photoelectrons on NmE on the solar activity level is studied.

Effectiveness of predicting radiowave propagation in the ionosphere based on the IRI-2001 ionospheric model

The standard deviation of predicting radiowave propagation using the IRI-2001 model is estimated based on the data of oblique-incidence sounding on the England-IZMIRAN (∼2500 km) and Cyprus-IZMIRAN (∼2300 km) practically perpendicular paths and vertical radiosounding of the ionosphere at the common point (IZMIRAN) during the period March 2002–2007. The observational period covers a wide range of solar activity values: the sunspot number R varied from 112 at a maximum (2002) to 4.8 at a minimum (2007). The results of an analysis of the complex experimental data, which agree with their description by the IRI-2001 model, completed with the calculations of the electron collision frequency, are presented.

Formation mechanisms of the midlatitudinal NmF2 semiannual anomaly under daytime quiet geomagnetic conditions at low solar activity

The F-region peak electron densities NmF2 measured during daytime quiet geomagnetic conditions at low solar activity on January 22, 2008, April 8, 1997, July 12, 1986, and October 26, 1995, are compared. Ionospheric parameters are measured by the ionosonde and incoherent scatter radar at Millstone Hill and calculated with the use of a 1D nonstationary ionosphere–plasmasphere model of number densities and temperatures of electrons and ions at middle geomagnetic latitudes. The formation of the semiannual anomaly of the midlatitudinal NmF2 under daytime quiet geomagnetic conditions at low solar activity is studied. The study shows that the semiannual NmF2 anomaly occurs due to the total impact of three main causes: seasonal variations in the velocity of plasma drift along the geomagnetic field due to the corresponding variations in the components of the neutral wind velocity; seasonal variations in the composition and temperature of the neutral atmosphere; and the dependence of the solar zenith angle on a number of the day in the year at the same solar local time.

Dependences of the NmF2 midlatitude statistical characteristics on the month of a year under geomagnetically quiet conditions near noon at low solar activity

The month-to-month variations in the statistical characteristics of the electron density at the mid-latitude ionospheric F2 layer maximum (NmF2) were studied for geomagnetically quiet conditions near noon. The ionospheric F2 layer critical frequencies, measured by ionosondes in Ashkhabad, Tashkent, Rostov, Irkutsk, Moscow, Sverdlovsk, Tomsk, and Magadan at low solar activity from 1957 to 2013, were used in the performed statistical analysis. The mathematical expectation and the arithmetical mean value of NmF2, the arithmetical mean value of the NmF2 median, and the most probable NmF2 value were calculated for each month. The months with local extremums of these NmF2 statistical characteristics were indicated. It was demonstrated that the semiannual symmetry in the implementation time of local maximums of the NmF2 mathematical expectation, arithmetical mean values, and mean median is observed in the month-to-month variations over Ashkhabad, Tashkent, and Sverdlovsk. The semiannual symmetry of the most probable NmF2 value was found over Ashkhabad. The NmF2 statistical characteristics were compared with one another for each ionosonde at a fixed month. This comparison shows that the values of the most probable NmF2 and the NmF2 mathematical expectation and arithmetically mean median can pronouncedly differ from one another, and the maximal difference of the NmF2 mathematical expectation from the NmF2 arithmetical mean is 0.9%.