Small-scale Irregularities Research Articles

Spread-F Radar Observations in the Midlatitude Ionosphere Using an Ionosonde–Radiodirection Finder

We present the results of experimental studies of the spread-F phenomenon in the midlatitude ionosphere, which were obtained using a broadband chirp HF radar of bistatic configuration on the Cyprus—Rostov-on-Don oblique sounding path. In the evening, night, and early morning hours of the winter and spring months, in a quiet geomagnetic environment, scattered signals were detected at frequencies exceeding the maximum observable frequency (MOF) of the directsignal 1F mode with the following parameters: delays of 6‐7.5 to 8.5‐11 ms, frequencies of 10‐14 to 13‐19 MHz, vertical angles of arrival of 20 ◦ to 45 ◦ , azimuthal angles of arrival of −30 ◦ to 50 ◦ , and amplitude of scattered signal by 70‐80 dB less than the direct-signal amplitude. Based on modeling of HF propagation and scattering and on comparison with the experimental data, it is established that the observable anormalous signal is due to the backscattering of radio waves by small-scale irregularities with sizes about 8‐10 m, which occupy an extended area of the midlatitude ionosphere (50 ◦ ‐55 ◦ N, 35 ◦ ‐48 ◦ E) at altitudes both below and above the F-layer maximum (250‐450 km). Relative fluctuations of the electron number density of the scattering irregularities are estimated as δN ≈ 7.8 · 10 −3 , which is about a factor of 3 to 5 greater than δN for natural fluctuations of the electron number density under typical conditions in the midlatitude ionospheric F region. Possible relation between spread F and the traveling ionospheric disturbances is discussed.

Dependence of the Pc4 magnetic pulsation parameters on the radiated power of the EISCAT HF heating facility

The interaction between the Earth’s ionosphere and magnetosphere in a situation when artificial disturbances are generated in the F region of the auroral ionosphere with the EISCAT/Heating facility is studied. An experiment was performed in the daytime when the facility effective radiated power changed in a stepwise manner. Wavelike disturbances with periods of (130–140) s corresponding to Pc4 pulsations were simultaneously registered by the method of bi-static backscatter and with ground magnetometers. The variations in the Doppler frequency shift were correlated with the changes in the facility power. Incoherent scatter radar measurements at a frequency of 930 MHz (Tromso) and numerical calculations were used in an analysis. It has been indicated that the ionospheric drift of small-scale artificial ionospheric irregularities was modulated by magnetospheric Alfven waves. The possible effect of powerful HF radioemission on the Alfven wave amplitude owing to the modification of the magnetospheric resonator ionospheric edge reflectivity and the generation of an outgoing Alfven wave above the region where the ionospheric conductivity is locally intensified has been considered.

Anisotropy of small-scale stratospheric irregularities retrieved from scintillations of a double star α-Cru observed by GOMOS/ENVISAT

Abstract. In this paper, we discuss estimating anisotropy of air density irregularities (ratio of characteristic horizontal and vertical scales) from satellite observations of bi-chromatic scintillations of a double star whose components are not resolved by the detector. The analysis is based on fitting experimental auto- and cross-spectra of scintillations by those computed using the 3-D spectral model of atmospheric irregularities consisting of anisotropic and isotropic components. Application of the developed method to the scintillation measurements of the double star α-Cru by GOMOS (Global Ozone Monitoring by Occultation of Stars) fast photometers results in estimates of anisotropy coefficient of ~15–20 at altitudes 30–38 km, as well as other parameters of atmospheric irregularities. The obtained estimates of the anisotropy coefficient correspond to small-scale irregularities, close to the buoyancy scale.

Influence of an inhomogeneous structure of the high-latitude ionosphere on the long-distance propagation of high-frequency radio waves

We present the results of experimental studies of the features of long-distance propagation of high-frequency radio waves on the large-extent subauroral Magadan–Rostov-on-Don and midlatitude Khabarovsk–Rostov-on-Don and Irkutsk–Rostov-on-Don paths, which were obtained using the ionosonde-finder with a chirp output signal. Anomalous (lateral) signals with delays of about 1–2 ms with respect to a direct signal, which arrive from the azimuths 10°–20°, are observed on the Magadan–Rostov-on-Don path. The lateral signals were observed in the morning and antemeridian hours in the time interval 08:00–10:40 MSK. In the evening and night hours, the lateral signals were not observed. During magnetic activity, the amplitude of the lateral signals was greater than that observed prior to a magnetic storm by 5–10 dB. Location of the ionospheric-perturbation regions responsible for the appearance of the lateral signals was determined as φgeogr ≈ 69°–71°N (φmagn ≈ 65°–66°N), and λ ≈ 51°–58°E. The mechanisms of the lateral-signal propagation due to lateral refraction of radio waves on patches with enhanced electron number density and due to scattering of radio waves from small-scale irregularities are considered.

Investigation of ionospheric scintillation at UKM station, Malaysia during low solar activity

In this paper the investigation of the occurrence of ionospheric scintillation with S4≥0.2 was conducted by using a dual-frequency GISTM (GPS Ionospheric Scintillation and TEC monitor) at Universiti Kebangsaan Malaysia station, Malaysia (2.55°N, 101.46°E; geomagnetic: 7.39°S, 173.63°E) between September 2009 and December 2010. The study shows that significant nighttime amplitude scintillation event with 0.4≤S4<0.6 mainly occurred in the months of March, September and October, while significant daytime amplitude scintillation activity took place in November and December with 0.3≤S4<0.5. Moreover, nighttime amplitude scintillation observed at UKM station always occurred with phase scintillations, total electron content (TEC) depletions, rate of change of TEC (ROT) fluctuations and the enhancement of rate of TEC index (ROTI). Nevertheless, during daytime amplitude scintillation, TEC depletions and ROT fluctuations were much weaker than those that occurred during nighttime and this may be caused by small scale irregularities in the E region, called sporadic-E (Es), while the occurrences of nighttime amplitude scintillation maybe caused by the ionospheric irregularities in the F region.

Ionospheric and hardware fluctuations of the optical path during radio occultation sounding of the earth’s atmosphere in the COSMIC experiment

Ionospheric and hardware fluctuations of optical paths are the most significant sources of errors in determination of the parameters of the neutral atmosphere using the radio occultation method at altitudes above 8 km. The paper presents a spectral analysis of the fluctuations in the optical path during radio occultation measurements in two channels of the GPS system for the perigee altitudes of probing beams in the range from 80 to 100 km, where the contribution of the neutral atmosphere is negligibly small. For analysis, a data set of 300 measurements made during the COSMIC experiment was used. The spectral analysis revealed the maximum cross-correlation of the fluctuations in two channels at a spatial frequency of 2.5 rad/km. Weaker correlation for lower spatial frequencies is determined by hardware noise. Weaker correlation for higher spatial frequencies is determined by the effects of diffraction by small-scale ionospheric irregularities.

Cluster structure of artificial ionospheric turbulence according to the data of the radar measurements by the radio-direction finder ionosonde

We present the results of experimental studies of an artificial ionospheric turbulence, which were obtained using the radio-direction finder ionosonde with linear frequency chirp of output signal. Several scattered signals with close distance-frequency and angular-frequency characteristics were observed, which is related to radio-wave scattering by various clustered areas. The features of the aspect scattering of radio waves on the IZMIRAN–“Sura”–Rostov-on-Don path were simulated. The spatial dimensions and the structure of the region filled by small-scale artificial ionospheric irregularities are estimated. The displacement of some clusters from the main region is of the order of 10–40 km with the smaller and larger displacements observed in the daytime and evening ionospheres, respectively.

Effects of modification of the polar ionosphere with high-power short-wave extraordinary-mode HF waves produced by the spear heating facility

We present the results of modifying the F2 layer of the polar ionosphere experimentally with highpower HF extraordinary-mode waves. The experiments were performed in October 2010 using the short-wave SPEAR heating facility (Longyearbyen, Spitsbergen). To diagnose the effects of high-power HF waves by the aspect-scattering method in a network of diagnostic paths, we used the short-wave Doppler radar CUTLASS (Hankasalmi, Finland) and the incoherent scatter radar ESR (Longyearbyen, Spitsbergen). Excitation of small-scale artificial ionospheric irregularities was revealed, which were responsible for the aspect and backward scattering of the diagnostic signals. The measurements performed by the ESR incoherent scatter radar simultaneously with the heating demonstrated changes in the parameters of the ionospheric plasma, specifically, an increase in the electron density by 10–25 % and an increase in the electron temperature by 10–30 % at the altitudes of the F2 layer, as well as formation of sporadic ionization at altitudes of 140–180 km (below the F2 layer maximum). To explain the effects of ionosphere heating with HF extraordinary-mode waves, we propose a hypothesis of transformation of extraordinary electromagnetic waves to ordinary in the anisotropic, smoothly nonuniform ionosphere.

A New Theoretical Approach of Wavelet-Based Multifractal Characterization of HF Channel Scattering Function: Theoretical and Physical Interpretations

In high-frequency (HF) ionospheric communications, the dynamic nature of the high-latitude ionosphere creates small-scale ionospheric irregularities capable of scattering HF radio waves. In addition, there are large-scale irregularities which may be identified using some specific methods. However, small-scale irregularities cannot be easily identified and characterized. The aim of this paper is to introduce a new wavelet-based multifractal dimension mapping approach for the identification and the quantification of the small-scale irregularities. Numerical results show the improvement of the proposed methods compared to those obtained by the classical Legendre multifractal analysis. In our analysis, we have used the scattering function as a tool for the HF channel characterization. Our analysis deals with experimental scattering functions measured during the solar eclipse on August 11, 1999.

Detection of a large-scale irregularity in the region of the main ionospheric trough according to the data of topside sounding on board the Intercosmos-19 satellite

Complicated ionograms of topside sounding on board the Intercosmos-19 satellite, which were registered on November 26, 1980, in the dusk sector (1800 LT) at the latitudes of the equatorward wall (55°–62° ILAT) of the main ionospheric trough (MIT), are analyzed. They are characterized by the presence of two extra traces at distances larger than the main traces. Approaching the MIT minimum, all traces become more scattered, converge, and join into one strongly diffusive trace. An attempt of interpretation of the complicated ionograms on the basis of trajectory calculations performed by the method of characteristics in the “complex” two-dimensional version (in two mutually intersecting planes) is undertaken. The modeling shows that the extra traces could be related to the presence of a large-scale irregularity stretched along the geomagnetic meridian at the equatorward wall of the MIT. The calculations make it possible to estimate the parameters of the irregularity: the intensity is δfoF2 ∼ 30%, the length is several hundred kilometers, the semi-thickness is 50–60 km, and the height is 350 km. The possible formation causes of the irregularity are discussed. The intensification of the diffuseness of all traces is related to the increase in the intensity of small-scale irregularities, which is usually observed when approaching the MIT minimum.

Images of bottomside irregularities observed at topside altitudes

We analyzed plasma and field measurements acquired by the Communication/Navigation Outage Forecasting System (C/NOFS) satellite during an eight‐hour period on 13–14 January 2010 when strong to moderate 250 MHz scintillation activity was observed at nearby Scintillation Network Decision Aid (SCINDA) ground stations. C/NOFS consistently detected relatively small‐scale density and electric field irregularities embedded within large‐scale (∼100 km) structures at topside altitudes. Significant spectral power measured at the Fresnel (∼1 km) scale size suggests that C/NOFS was magnetically conjugate to bottomside irregularities similar to those directly responsible for the observed scintillations. Simultaneous ion drift and plasma density measurements indicate three distinct types of large‐scale irregularities: (1) upward moving depletions, (2) downward moving depletions, and (3) upward moving density enhancements. The first type has the characteristics of equatorial plasma bubbles; the second and third do not. The data suggest that both downward moving depletions and upward moving density enhancements and the embedded small‐scale irregularities may be regarded as Alfvénic images of bottomside irregularities. This interpretation is consistent with predictions of previously reported theoretical modeling and with satellite observations of upward‐directed Poynting flux in the low‐latitude ionosphere.

Using GPS-SCINDA observations to study the correlation between scintillation, total electron content enhancement and depletions over the Kenyan region

This paper presents the first results of total electron content (TEC) depletions and enhancement associated with ionospheric irregularities in the low latitude region over Kenya. At the low latitude ionosphere the diurnal behavior of scintillation is driven by the formation of large scale equatorial depletions which are formed by post-sunset plasma instabilities via the Rayleigh–Taylor instability near the magnetic equator. Data from the GPS scintillation receiver (GPS-SCINDA) located at the University of Nairobi (36.8°E, 1.27°S) for March 2011 was used in this study. The TEC depletions have been detected from satellite passes along the line of sight of the signal and the detected depletions have good correspondence with the occurrence of scintillation patches. TEC enhancement has been observed and is not correlated with increases in S4 index and consecutive enhancements and depletions in TEC have also been observed which results into scintillation patches related to TEC depletions. The TEC depletions have been interpreted as plasma irregularities and inhomogeneities in the F region caused by plasma instabilities, while TEC enhancement have been interpreted as the manifestation of plasma density enhancements mainly associated with the equatorial ionization anomaly crest over this region. Occurrence of scintillation does happen at and around the ionization anomaly crest over Kenyan region. The presence of high ambient electron densities and large electron density gradients associated with small scale irregularities in the ionization anomaly regions have been linked to the occurrence of scintillation.

Monitoring, tracking and forecasting ionospheric perturbations using GNSS techniques

The paper reviews the current state of GNSS-based detection, monitoring and forecasting of ionospheric perturbations in Europe in relation to the COST action ES0803 “Developing Space Weather Products and Services in Europe”. Space weather research and related ionospheric studies require broad international collaboration in sharing databases, developing analysis software and models and providing services. Reviewed is the European GNSS data basis including ionospheric services providing derived data products such as the Total Electron Content (TEC) and radio scintillation indices. Fundamental ionospheric perturbation phenomena covering quite different scales in time and space are discussed in the light of recent achievements in GNSS-based ionospheric monitoring. Thus, large-scale perturbation processes characterized by moving ionization fronts, wave-like travelling ionospheric disturbances and finally small-scale irregularities causing radio scintillations are considered. Whereas ground and space-based GNSS monitoring techniques are well developed, forecasting of ionospheric perturbations needs much more work to become attractive for users who might be interested in condensed information on the perturbation degree of the ionosphere by robust indices. Finally, we have briefly presented a few samples illustrating the space weather impact on GNSS applications thus encouraging the scientific community to enhance space weather research in upcoming years.

Results of Russian experiments dealing with the impact of powerful HF radiowaves on the high-latitude ionosphere using the EISCAT facilities

We present the results of complex experiments dealing with the impact of powerful HF radiowaves on the high-latitude ionosphere using the European Incoherent Scatter Scientific Association (EISCAT) facilities. During the ionospheric F-region heating by powerful extraordinary (X-mode) polarized HF radiowaves under the conditions of heating near the critical f H frequency f H ≈ f x F2 of the extraordinary wave of the F2-layer, we were first to detect the excitation of intense artificial small-scale ionospheric irregularities (ASIs), accompanied by electron temperature increases by approximately 50%. The results of coordinated satellite and ground-based observations of the powerful HF radiowave impact on the high-latitude ionosphere are considered. During ionospheric F-region heating by powerful HF radiowaves of ordinary polarization (O-mode) during evening hours, the phenomenon of ion outflow accompanied by electron temperature increases and thermal plasma expansion was revealed. Concurrent DMSP-F15 satellite measurements at a height of about 850 km indicate an O+ ion density increase. The CHAMP satellite observations identified ULF emissions at the modulation frequency (3 Hz) of the powerful HF radiowave, generated during modulated emissions of the powerful HF radiowave of O-polarization and accompanied by a substantial increase in the electron temperature and ASI generation.

Evolutionary phases of equatorial spreadFincluding L band scintillations and plumes in the context of GPS total electron content variability: A case study

[1] The evolution of large-scale (few kilometers), medium-scale (few hundreds of meters), and small-scale (meters) size plasma density irregularities in the postsunset equatorial F region, in the context of characteristic GPS total electron content (GTEC) variations, are reported from Indian longitudes. The ionograms and GTEC from a GPS receiver installed as a part of the GPS Aided Geo Augmentation Network (GAGAN) project for satellite-based navigation are obtained from an equatorial station at Trivandrum (8.5°N, 76.91°E, dip latitude 0.5°N). The variations in the GTEC with respect to TEC are considered to represent the seed perturbations for the plasma instability that results in the equatorial spread F (ESF) irregularities and are treated as a perturbation factor (P). The VHF radar at Gadanki (13.5°N, 79.17°E, dip latitude 6.4°N) provided the small-scale structures of ESF. The background thermospheric conditions that affect the growth of the plasma instability through ion-neutral collision frequency (νin) are estimated using the F region base height (h′F)and the representative scale height of the neutral atmosphere and are represented by a growth factor (G). The present case study reveals a close coupling between the background ionospheric conditions and the baseline perturbations in deciding the evolutionary phases of ESF. It has been shown that although large-scale (kilometer scale) irregularities are formed without any constraints when the background ionospheric-thermospheric conditions are favorable in the presence of fluctuations in GTEC, consistently, the medium-scale and small-scale irregularities show remarkable similarity with the variations in the product of the perturbation and growth factors.

Modeling ionospheric &amp;lt;I&amp;gt;fo&amp;lt;/I&amp;gt;F2 by using empirical orthogonal function analysis

Abstract. A similar-parameters interpolation method and an empirical orthogonal function analysis are used to construct empirical models for the ionospheric foF2 by using the observational data from three ground-based ionosonde stations in Japan which are Wakkanai (Geographic 45.4° N, 141.7° E), Kokubunji (Geographic 35.7° N, 140.1° E) and Yamagawa (Geographic 31.2° N, 130.6° E) during the years of 1971–1987. The impact of different drivers towards ionospheric foF2 can be well indicated by choosing appropriate proxies. It is shown that the missing data of original foF2 can be optimal refilled using similar-parameters method. The characteristics of base functions and associated coefficients of EOF model are analyzed. The diurnal variation of base functions can reflect the essential nature of ionospheric foF2 while the coefficients represent the long-term alteration tendency. The 1st order EOF coefficient A1 can reflect the feature of the components with solar cycle variation. A1 also contains an evident semi-annual variation component as well as a relatively weak annual fluctuation component. Both of which are not so obvious as the solar cycle variation. The 2nd order coefficient A2 contains mainly annual variation components. The 3rd order coefficient A3 and 4th order coefficient A4 contain both annual and semi-annual variation components. The seasonal variation, solar rotation oscillation and the small-scale irregularities are also included in the 4th order coefficient A4. The amplitude range and developing tendency of all these coefficients depend on the level of solar activity and geomagnetic activity. The reliability and validity of EOF model are verified by comparison with observational data and with International Reference Ionosphere (IRI). The agreement between observations and EOF model is quite well, indicating that the EOF model can reflect the major changes and the temporal distribution characteristics of the mid-latitude ionosphere of the Sea of Japan region. The error analysis processes imply that there are seasonal anomaly and semi-annual asymmetry phenomena which are consistent with pre-existing ionosphere theory.

Modification of the high-latitude ionosphere by high-power hf radio waves. 2. Results of coordinated satellite and ground-based observations

We present the results of coordinated satellite and ground-based observations of the high-latitude ionospheric phenomena induced by high-power high-frequency (HF) radio waves. The ion outflow phenomenon accompanied by a strong increase in the electron temperature and thermal expansion of plasma was observed in the evening hours, when the high-latitude ionospheric F region was heated by high-power O-mode HF radio waves. The DMSP F15 satellite recorded an increase in the ion number density O+ at an altitide of about 850 km in that period. Ultralow-frequency (ULF) radiation at the modulation frequency 3 Hz of the high-power HF radio waves, which was generated in the ionosphere irradiated by high-power O-mode HF radio waves and accompanied by a strong increase in the electron temperature and the generation of artificial small-scale ionospheric irregularities, was recorded by the CHAMP satellite during the heating experiment in Tromso in November 5, 2009. The results of the DEMETER satellite observations of extremely low frequency (ELF) radiation at the modulation frequency 1178 Hz of the high-power radio waves in the heating experiments were analyzed using the event of March 3, 2009 as an example.

Artificial small-scale field-aligned irregularities in the high latitude F region of the ionosphere induced by an X-mode HF heater wave

[1] The effects on the high-latitude F region of the ionosphere by X-mode powerful HF radio waves injected towards the magnetic zenith (MZ) are analysed. The experiments were conducted using the EISCAT/Heating facility and UHF radar at Tromso, Norway, the CUTLASS (SuperDARN) radar and the EISCAT ionosonde (dynasonde). The results show that the X-mode HF pump wave, radiated into the magnetic zenith from the HF heater, can generate very strong small-scale artificial field aligned irregularities (AFAIs) in the F-region of the high-latitude ionosphere. These irregularities, with spatial scales across the geomagnetic field of the order of 8–15 m, are generated when the heater frequency is above the ordinary-mode critical frequency but comparable with the extraordinary-mode critical frequency. The generation of the X-mode AFAIs was accompanied by electron temperature (Te) enhancements up to 50% above the background level and an increase in the electron density (Ne) by up to 30%.

Climatology of GPS phase scintillation and HF radar backscatter for the high-latitude ionosphere under solar minimum conditions

Abstract. Maps of GPS phase scintillation at high latitudes have been constructed after the first two years of operation of the Canadian High Arctic Ionospheric Network (CHAIN) during the 2008–2009 solar minimum. CHAIN consists of ten dual-frequency receivers, configured to measure amplitude and phase scintillation from L1 GPS signals and ionospheric total electron content (TEC) from L1 and L2 GPS signals. Those ionospheric data have been mapped as a function of magnetic local time and geomagnetic latitude assuming ionospheric pierce points (IPPs) at 350 km. The mean TEC depletions are identified with the statistical high-latitude and mid-latitude troughs. Phase scintillation occurs predominantly in the nightside auroral oval and the ionospheric footprint of the cusp. The strongest phase scintillation is associated with auroral arc brightening and substorms or with perturbed cusp ionosphere. Auroral phase scintillation tends to be intermittent, localized and of short duration, while the dayside scintillation observed for individual satellites can stay continuously above a given threshold for several minutes and such scintillation patches persist over a large area of the cusp/cleft region sampled by different satellites for several hours. The seasonal variation of the phase scintillation occurrence also differs between the nightside auroral oval and the cusp. The auroral phase scintillation shows an expected semiannual oscillation with equinoctial maxima known to be associated with aurorae, while the cusp scintillation is dominated by an annual cycle maximizing in autumn-winter. These differences point to different irregularity production mechanisms: energetic electron precipitation into dynamic auroral arcs versus cusp ionospheric convection dynamics. Observations suggest anisotropy of scintillation-causing irregularities with stronger L-shell alignment of irregularities in the cusp while a significant component of field-aligned irregularities is found in the nightside auroral oval. Scintillation-causing irregularities can coexist with small-scale field-aligned irregularities resulting in HF radar backscatter. The statistical cusp and auroral oval are characterized by the occurrence of HF radar ionospheric backscatter and mean ground magnetic perturbations due to ionospheric currents.

Inclusion of Surface Topography into the Vector Vorticity Equation Model (VVM)

[1] Surface topography is implemented into the vector vorticity equation model with a block representation of mountains in the height coordinate. The kinematic boundary conditions at the surface are satisfied with a proper computational boundary condition for vorticity. This approach has been tested in various ways, including idealized 2D mountain waves, the much-studied “Boulder Downslope Windstorm” case, and 3D orographic precipitation over a ridge with and without small-scale irregularities. The model performs reasonably well in all of these cases, with no obvious computational difficulties due to the use of coordinate surfaces intersecting the surface and is ready to be used for arbitrarily prescribed surface topography without any artificial smoothing.