Yu. V. Yasyukevich

Influence of GPS/GLONASS differential code biases on the determination accuracy of the absolute total electron content in the ionosphere

Systematic error arises when the total electron content (TEC) is estimated with the simultaneous use of phase and code GPS/GLONASS measurements. This is related to the different signal propagation times at L1 and L2 frequencies in the radio frequency path of the transmitting and receiving equipment, the so-called differential code biases. A differential code bias of 1 ns results in an error of ~2.9 TECU when TEC is determined. Differential code bias variations on a long time interval, which were obtained at the CODE laboratory, were analyzed. It has been found that the systematic variation in these biases and considerable seasonal variations apparently caused by the environmental state (temperature and humidity), which sometimes reach 20 TECU (in TEC units), are observed for several stations. The algorithm for determining differential code biases at an individual station and the results of correction for absolute slant TEC are also presented. Presented results show algorithm effectiveness for various geographical regions and solar activity.

Effect of magnetic storms and substorms on GPS slips at high latitudes

The dynamics of slips in navigation signal parameters of GPS from 2010 to 2014 is considered for the stations of the IGS and CHAIN networks located in the Arctic region. On the basis of almost continuous (more than 8 million hours) observations at around 200 receiving stations, we investigate the probability of “instrumental” loss of phase and pseudo-range as well as short-term variations in the high rate of change of the total electron content (TEC) in different geomagnetic conditions. Quantitative estimates for the impact of geomagnetic disturbances on the slips of these parameters are given. The slip probabilities for TEC are significantly (100–200 times) higher than those of purely instrumental slips and grow during geomagnetic storms and substorms. The growth of instrumental slips may be caused by the increased absorption that occurs during geomagnetic storms, among other reasons, and is an indicator of auroral intrusions of highenergy particles.

Ionospheric response to solar flares of C and M classes in January–February 2010

The ionospheric response to solar flares is analyzed for the case of the beginning of solar activity growth, when the background ionization of the ionosphere is still low enough. It is shown that the algorithms and methods of averaging variations and derivative of the total electron content (TEC) over the entire sunlit ionosphere almost always make it possible to identify the ionospheric response even to close in time weak solar flares of the C class. It is found that the response to a solar flare rather intense in the X-ray range can have almost no manifestations, which is caused by the fact that the flare does not reveal itself in the ultraviolet part of the spectrum. A map of the TEC derivative over the Japan territory with an average resolution of ∼18 km is drawn for the M6.4 flare (February 7, 2010). Before the flare maximum, the TEC derivatives are synchronously increasing over the entire Japan, while after the flare maximum the values of the TEC derivative vary not so synchronously, and local differences are seen.

Global electron content during solar cycle 23

Global electron content (GEC) as a new ionospheric parameter was first proposed by Afraimovich et al. [2006]. GEC is equal to the total number of electrons in the near-Earth space. GEC better than local parameters reflects the global response to a change in solar activity. It has been indicated that, during solar cycle 23, the GEC dynamics followed similar variations in the solar UV irradiance and F10.7 index, including the 11-year cycle and 27-day variations. The dynamics of the regional electron content (REC) has been considered for three belts: the equatorial belt and two midlatitude belts in the Northern and Southern hemispheres (±30° and 30°–65° geomagnetic latitudes, respectively). In contrast to GEC, the annual REC component is clearly defined for the northern and southern midlatitude belts; the REC amplitude is comparable with the amplitude of the seasonal variations in the Northern Hemisphere and exceeds this amplitude in the Southern Hemisphere by a factor of ∼1.7. The dayside to nightside REC ratio, R(t), at the equator is a factor of 1.5 as low as such a GEC ratio, which indicates that the degree of nighttime ionization is higher, especially during the solar activity maximum. The pronounced annual cycle with the maximal R(t) value near 8.0 for the winter Southern Hemisphere and summer Northern Hemisphere is typical of midlatitudes.

The response of the ionosphere to the earthquake in Japan on March 11, 2011 as estimated by different GPS-based methods

The results of detecting ionospheric disturbances by different methods based on GPS observations during the mega-earthquake in Japan (March 11, 2011) are analyzed. It is shown that different methods of analysis and data processing technologies provide generally similar results and suggest quite a complex morphology of the ionospheric response to this seismic event. We distinguish three types of wave disturbances that appeared in the ionosphere in response to the earthquake: slow gravity waves, acoustic-gravity oscillations, and fast disturbances corresponding to the Rayleigh waves. Such an analysis of the wave phenomena and the comparison between the results provided by different methods has been carried out for the first time.

Diurnal and longitudinal variations in the earth’s ionosphere in the period of solstice in conditions of a deep minimum of solar activity

The results of studies of longitudinal and LT variations in parameters of the ionosphere–plasmasphere system, obtained using the Global Self-Consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP), assimilation ionospheric model IRI Real-Time Assimilation Mapping (IRTAM), and satellite and ground-based observational data are presented in the paper. The study of the main morphological features of longitudinal and LT variations in the critical frequency of the ionospheric F2 layer (foF2) and total electron content (TEC) depending on latitude in the winter solstice during a solar-activity minimum (December 22, 2009) is carried out. It is shown that the variations in foF2 and TEC, on the whole, are identical, and so mutually substitutable, while creating empirical models of these parameters in quiet geomagnetic conditions. The longitudinal and LT variations in both foF2 and TEC are within an order of magnitude everywhere except for the equator anomaly region, where LT variation is larger by an order of magnitude than longitudinal variation. According to the results of the study, in the American longitudinal sector at all latitudes of the Southern (summer) Hemisphere, maxima of foF2 and TEC are formed. The near-equatorial and high-latitudinal maxima are separated out from these. The estimate of the contribution into the longitudinal variation in foF2 and TEC for various local time sectors and at various latitudes has been obtained for the first time. In the Southern (summer) Hemisphere, longitudinal variation in foF2 and TEC is formed in the nighttime.

A statistical study of medium-scale ionospheric disturbances generated by solar terminator registered over Japan in 2008

A statistical study of wave packets of terminator origin using GEONET GPS data for 87 days in 2008 was done. There were some seasonal and latitudinal features in occurrence of the packets. In spring packets are registered 1.7 times more often in the south region than in the north one and in summer −1.3 times more often in the north than south region. Autumn packets registration rate have two maxima: 4 hour after and at the same time as terminator appears over the registration point. Similar distribution maxima are registered in summer in the south of Japan.

Determination of the Level of Diagnostic Slips of the Total Electron Content from GPS Observations in Different Latitudinal Regions

This paper presents the results of the statistical analysis of the total electron content (TEC) variation rate distributions for different sets of approximately 400 IGS network stations located at high (∼200 stations), middle (> 100 stations), and equatorial latitudes (> 100 stations).. The analysis is carried out for 2010 and 2014 under different geo- and heliophysical conditions, as well as phases of the minimum and practical maximum of the 24th solar cycle. The aim of the analysis is the experimental determination of the threshold value of the magnitude of sudden TEC changes associated with the tails of the investigated distributions for a given physically adequate level of significance. It is found that the threshold value from which the measured values can be considered statistical noise depends on the latitude of the observation region. The maximum threshold occurs for the Arctic region and the minimum occurs for the equatorial group of stations.

Experimental observations of carrier phase acceleration in conditions of polar ionosphere

The paper presents a comparative analysis of the quality of measuring the phase carrier of signals from GLONASS and GPS navigation satellites and spectrum of small-scale irregularities of electron density in conditions of a nonstationary and inhomogeneous polar ionosphere. It is shown that small-scale irregularities of the electron density in the ionosphere can lead to a substantial nonstationary increase in the acceleration of the phase carrier of both GPS and GLONASS by a factor of 1.3–2.5 from the background level.

Estimating the absolute total electron content, spatial gradients and time derivative from the GNSS data

Global navigation satellite systems have enabled to study the ionosphere in different regions of the world. The total electron content (TEC) of the Earth ionosphere can be determined from code and phase dual-frequency pseudorange measurements performed by receivers of GNSS signals. This technique is widely described in the literature (B. Hofmann-Wellenhof, H. Lichtenegger, J. Collins. New York: Springer-Verlag Wien, 389 p. 1998). To obtain the absolute TEC values, phase measurements are usually used, because they are weakly noised, and the ambiguity of the initial phase definition is eliminated with code ones. Thus, there occurs a systematic error termed differential code biases (DCBs). To determine absolute TEC accounting for DCBs from the data of a single GPS/GLONASS station as well as spatial gradients and time derivative, we have developed an algorithm. The algorithm includes estimating DCBs by using a simple model of measurements: equation where IV is the absolute vertical TEC value; Δ□(Δl) is the latitude (longitude) difference between the ionospheric point coordinate □ (l) and that of the station □ 0 (l 0 ); Δt is the difference between the measurement time t and the time t 0 , for which the calculation is performed; G □ =∂I V /∂□, G l =∂I V /∂l, G q_□ =∂2I V /∂□2, G q_l =∂2I V /∂l2 are linear and quadratic spatial TEC gradients; G t =∂I V /∂t and G q_t =∂2I V /∂t2 are the first and second time derivatives.