V. E. Kunitsyn

An investigation of motions of the equatorial anomaly crest

Using more than 350 ionospheric images reconstructed tomographically, studies on the motion of the anomaly crest and its geophysical implications are carried out. On an average day, the crest forms at 09:00 LT and, in the next two hours, moves poleward with a speed of about 1° per hour as it intensifies. This poleward motion is slowed as the crest reaches its highest latitude where it stays for several hours until early afternoon. Thereafter, the crest starts to weaken as it recedes with a speed of about 0.5° per hour equatorward. During 12:00 - 14:00 LT, the crest latitude is found to correlate with the fountain strength and the total number of electrons in a cross sectional plane at the observational longitude of the whole equatorial ionosphere.

Possibilities of the near‐space environment radio tomography

A number of radio tomography schemes of the ionosphere and near-space environment (Earths magnetosphere, plasmasphere, and protonosphere) are analyzed by utilizing multisatellite systems. A new approach to space-time tomography is proposed. It is shown that application of high orbital systems of Global Positioning System and Global Navigation Satellite System type with ground-based receivers is promising in the case of space-time tomography. Computer simulation results showed the possibility of realizing radio tomography of the near-space environment with the help of some multisatellite systems.

On generation of an assembly of images in ionospheric tomography

Radio tomography experiments have demonstrated the promising potential of applying tomographic methods in imaging various ionospheric structures. In actual implementation of image reconstructions one is faced with many choices, which include the following: whether to use the total phase, relative phase, or Doppler as the projection data, how to approximate the projection operator, what inversion algorithm to employ, and the choice of how to include the ancillary data and constraints on the constructed image. Each choice results in an image compatible with the given or measured projection data, yet each choice results in an image different from that of the others, with its own attendant artifacts and distortions. Collectively, the images produced by all the possible choices comprise an assembly of images. In this simulation study of one ionospheric model, 113 members of such an assembly are generated. All images look similar in gross features with a root-mean-square deviation not more than 29% from the mean. As expected, the largest deviation occurs near the region of highest gradients. By averaging all of the images in the assembly we show that the mean image is superior because of its smallest root-mean-square deviation from the true image. This conclusion, drawn on the simulation study of one model, may in fact have a general applicability, and we discuss why this may be so.

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.

Japan megathrust earthquake on March 11, 2011: GPS-TEC evidence for ionospheric disturbances

Two-dimensional distributions of the vertical total electron content in the ionosphere above the Japan under-sea mega-earthquake on March 11, 2011 are reconstructed using high-temporal-resolution (2-min) data from the Japan GPS network. A diverging ionospheric perturbation with multicomponent spectral composition is identified emerging after the main shock. The disturbances in the ionospheric electron concentration caused by acoustic gravity waves generated by the earthquake-related processes. The initial phase of this disturbance can be used as a marker in the tsunami early warning systems. The surface energy of the earthquake estimated from the amplitude of the ionospheric disturbance is close to the estimate based on the seismic data. Other disturbances (ionospheric responses to the Rayleigh and tsunami waves, a solitary ionospheric pulse) are also analyzed. Physical interpretation of the identified ionospheric disturbances is presented.

Radiotomographical Detection of Ionosphere Disturbances Caused by Ground Explosions

Long-lived local disturbances of the ionospheric density over the site of ground industrial explosions were detected by the ionosphere radiotomography method. It is assumed that the density anomalies arise because of the initiation of vortex motion in a neutral component after acoustic impulse passage.

Investigation of SBAS L1/L5 Signals and Their Application to the Ionospheric TEC Studies

With the development of satellite-based augmentation systems (SBAS), the dual-frequency L1/L5 observations from a number of geostationary satellites are now available. It provides the possibility of retrieving ionospheric total electron content (TEC) from these observations using the same approach as for dual-frequency GPS/GLONASS observations. In this letter, we study L1/L5 signals of American Wide Area Augmentation System (WAAS) and Indian GPS and geo-augmented navigation system (GAGAN) geostationary satellites observed with geodetic Global Navigation Satellite System (GNSS) receivers located at equatorial and midlatitudes and estimate corresponding geostationary TEC and errors of such estimations. One-hundred-second TEC RMS was found to reach up to 1.5 TEC unit (TECU) with typical values of 0.25-0.5 TECU, which is several times greater than for GPS/GLONASS observations. TEC RMS also manifests UT dynamics, which is specific for the satellite and not relevant to the signal paths. SBAS TEC was found to be in good agreement with the data of nearest ionosondes. We also conduct the wavelet analysis of geostationary TEC, providing typical periods of observed variations at different timescales and discuss the capabilities of SBAS TEC observations in connection with ionospheric effects of X1.7 solar flare on October 25, 2013.

Radiotomography and HF ray tracing of the artificially disturbed ionosphere above the Sura heating facility

We present the results of the radiotomographic imaging of the artificial ionospheric disturbances obtained in the recent experiments on the modification of the midlatitude ionosphere by powerful HF radiowaves carried out at the Sura heater. Radio transmissions from low orbital PARUS beacon satellites recorded at the specially installed network of three receiving sites were used for the remote sensing of the heated ionosphere. We discuss the possibility to generate acoustic-gravity waves (AGWs) with special regimes of ionospheric heating (with the square wave modulation of the effective radiated power at the frequency lower than or of the order of the Brunt-Vaisala frequency of the neutral atmosphere at ionospheric heights during several hours) and present radiotomographic images of the spatial structure of the disturbed volume of the ionosphere corresponding to the directivity pattern of the heater, as well as the spatial structure of the wave-like disturbances, which are possibly heating-induced AGWs, diverging from the heated area of the ionosphere. We also studied the HF propagation of the pumping wave through the reconstructed disturbed ionosphere above the Sura heater, showing the presence of heater-created, field-aligned irregularities that effectively serve as “artificial radio windows.”

Ionospheric TEC estimation with the signals of various geostationary navigational satellites

With the development of receiver equipment and GNSS and SBAS constellations, the coherent dual-frequency L-band transmissions are now available from a number of geostationary satellites. These signals can be used for ionospheric total electron content (TEC) estimation. The quality of these data, i.e., the level of noise in such TEC estimation is of great interest and importance. We present results of comparisons of noise patterns in TEC estimation using signals of geostationary satellites of augmentation systems such as the Indian GAGAN, the European EGNOS and the American WAAS, as well as the signals from the Chinese Beidou navigation system. We used data from two receiving sites in the European part of Russia and the USA, which are equipped with JAVAD Delta receivers. We found out that the noise level in TEC estimation based on geostationary satellites of the Beidou system is one order smaller than that for SBAS and corresponds to those of GPS/GLONASS at the same elevation angles. Typically, the TEC RMS was about 0.05 TECU for GPS/GLONASS satellites at elevation range 5–15°, 0.06 TECU for Beidou geostationary satellites at elevation range 15–25°, 0.6 TECU for GAGAN at elevation range 15–25°, 0.7 TECU for WAAS at elevation 45°, and 5 TECU for EGNOS at elevation 20°. We also discuss the capabilities of geostationary TEC observations in connection with the recent G4 geomagnetic storm of March 2015 using six IGS MGEX stations in the American, Southeast Asian and Australian sectors. We demonstrate the hemispheric asymmetry in the ionospheric TEC response during this storm.

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.