Ionospheric Tilts Research Articles

Assessment of Polar Ionospheric Observations by VIPIR/Dynasonde at Jang Bogo Station, Antarctica: Part 1—Ionospheric Densities

Vertical incidence pulsed ionospheric radar (VIPIR) has been operated to observe the polar ionosphere with Dynasonde analysis software at Jang Bogo Station (JBS), Antarctica, since 2017. The JBS-VIPIR-Dynasonde (JVD) provides ionospheric parameters such as the height profile of electron density with NmF2 and hmF2, the ion drift, and the ionospheric tilt in the bottomside ionosphere. The JBS (74.6°S, 164.2°E) is located in the polar cap, cusp, or auroral region depending on the geomagnetic activity and local time. In the present study, an initial assessment of JVD ionospheric densities is attempted by the comparison with GPS TEC measurements which are simultaneously obtained from the GPS receiver at JBS during the solar minimum period from 2017 to 2019. It is found that the JVD NmF2 and bottomside TEC (bTEC) show a generally good correlation with GPS TEC for geomagnetically quiet conditions. However, the bTEC seems to be less correlated with the GPS TEC with slightly larger spreads especially during the daytime and in summer, which seems to be associated with the characteristics of the polar ionosphere such as energetic particle precipitations and large density irregularities. It is also found that the Dynasonde analysis seems to show some limitations to handle these characteristics of the polar ionosphere and needs to be improved to produce more accurate ionospheric density profiles especially during disturbed conditions.

Observations of the Polar Ionosphere by the Vertical Incidence Pulsed Ionospheric Radar at Jang Bogo Station, Antarctica

Korea Polar Research Institute (KOPRI) installed an ionospheric sounding radar system called Vertical Incidence Pulsed Ionospheric Radar (VIPIR) at Jang Bogo Station (JBS) in 2015 in order to routinely monitor the state of the ionosphere in the auroral oval and polar cap regions. Since 2017, after two-year test operation, it has been continuously operated to produce various ionospheric parameters. In this article, we will introduce the characteristics of the JBS-VIPIR observations and possible applications of the data for the study on the polar ionosphere. The JBS-VIPIR utilizes a log periodic transmit antenna that transmits 0.5–25 MHz radio waves, and a receiving array of 8 dipole antennas. It is operated in the Dynasonde B-mode pulse scheme and utilizes the 3-D inversion program, called NeXtYZ, for the data acquisition and processing, instead of the conventional 1-D inversion procedure as used in the most of digisonde observations. The JBS-VIPIR outputs include the height profiles of the electron density, ionospheric tilts, and ion drifts with a 2-minute temporal resolution in the bottomside ionosphere. With these observations, possible research applications will be briefly described in combination with other observations for the aurora, the neutral atmosphere and the magnetosphere simultaneously conducted at JBS.

Ionospheric Tilt Measurements: Application to Traveling Ionospheric Disturbances Climatology Study

AbstractIonospheric tilt is a concept used to characterize local horizontal electron density gradients in the ionosphere using a mirror reflection model. Experimental results are presented that illustrate the validity and accuracy of such approach. Analysis of the tilt measurements collected in 2012–2014 with a digisonde at Ebro observatory (40.8°N, 0.5°E) is presented. Digisonde systems allow measuring angles of arrival of ionospherically reflected radio signals, from which the ionospheric tilts are derived. The tilts are represented in terms of North‐South and East‐West components. Using several years of observations, a climatological distribution of wavelike variations in the tilt records presumed to be associated with traveling ionospheric disturbances (TIDs) is established. Most of the observed TIDs have their main periods of 30 min to 1.5 hr which is rather typical for medium‐scale TIDs. Summertime appears to have the most frequent occurrence of TIDs. There is a good agreement between the presence of TIDs and sporadic E layer occurrence, suggesting that some TIDs can be driven by instability in the electric field which is initiated via an interhemispheric link between the E and F regions of the ionosphere. Direction of disturbance propagation is also analyzed and compared to the modeled neutral wind. There are indications that during the daytime TIDs tend to propagate in the direction opposite to the background neutral wind. This suggests that daytime TIDs are produced by atmospheric gravity waves originating in the lower atmosphere and experiencing background wind filtering effect on their upward propagation.

Seasonal Variability of the Midlatitude Traveling Ionospheric Disturbances From Wallops Island, VA, Dynasonde Data: Evidence of a Semiannual Variation

AbstractWe investigate the ionospheric variability due to traveling ionospheric disturbances (TIDs) over Wallops Island, VA, using Dynasonde‐derived measurements of ionospheric tilts and vertical Doppler speed between May 2013 and August 2016, covering the bottomside ionospheric F layer. The mean power spectral density is determined separately for each month for both ionospheric tilts and for the Doppler speed, with a 2 km resolution in a broad altitude range. This is accomplished using a spectral analysis technique based on the Lomb‐Scargle and Welch methods, with an added filtering criterion. The analysis is performed separately for the daytime and nighttime data. By investigating the seasonal variability of the power spectral density integral, a semiannual variation is highlighted in the overall level of daytime TID activity, with increased activity during summer and winter seasons in the northern hemisphere. This result provides a more complete picture than other recent publications that only highlighted a winter peak in the same geographical sector. Finally, the relative amplitude of the two peaks in TID activity is shown to vary significantly during the three years investigated and also as a function of the altitude.

Characteristics of acoustic gravity waves obtained from Dynasonde data

AbstractTraveling ionospheric disturbances (TIDs) are ubiquitous in the thermosphere‐ionosphere and are often assumed to be caused by acoustic gravity waves (AGWs). This study performs an analysis of the TID and AGW activity above Wallops Island, VA, during October 2013. The variations in electron density and ionospheric tilts obtained with the Dynasonde technique are used as primary indicators of wave activity. The temporal and spectral characteristics of the data are discussed in detail, using also results of the Whole Atmosphere Model (WAM) and the Global Ionosphere Plasmasphere Model (GIP). The full set of propagation parameters (frequency, and the vertical, zonal and meridional wave vector components) of the TIDs is determined over the 160–220 km height range. A test of the self‐consistency of these results within the confines of the theoretical AGW dispersion relation is devised. This is applied to a sample data set of 24 October 2013. A remarkable agreement has been achieved for wave periods between 52 and 21 min, for which we can rigorously claim the TIDs are caused by underlying acoustic gravity waves. The Wallops Island Dynasonde can operate for extended periods at a 2 min cadence, allowing determination of the statistical distributions of propagation parameters. A dominant population of TIDs is identified in the frequency band below 1 mHz, and for it, the distributions of the horizontal wavelengths, vertical wavelengths, and horizontal phase speeds are obtained.

High-resolution ionospheric observations and modeling over Belgium during the solar eclipse of 20 March 2015 including first results of ionospheric tilt and plasma drift measurements

The ionospheric behavior over Belgium during the partial solar eclipse of 20 March 2015 is analyzed based on high-resolution solar radio flux, vertical incidence sounding, and GPS TEC measurements. First results of ionosonde-based ionospheric plasma drift and tilt observations are presented and analyzed, including some traveling ionospheric disturbances caused by the eclipse. Also, collocated ionosonde and GPS measurements are used to reconstruct the time evolution of the vertical electron density distribution using the Royal Meteorological Institute (RMI) ionospheric specification system, called Local Ionospheric Electron Density profile Reconstruction (Liedr).

Real‐time HF ray tracing through a tilted ionosphere

High‐frequency (HF) direction‐finding (DF) systems measure the angles of arrival of signals at selected frequencies. With this information, ray tracing can accurately determine the location of the HF transmitters if the three‐dimensional (3‐D) electron density (Ne) distribution between the DF site and the transmitters is known. The usual approach is to use an ionospheric model like the International Reference Ionosphere (IRI) as a proxy for the density distribution. We describe a more realistic approach developed in cooperation with Codem Systems in Merrimack, New Hampshire. A collocated digisonde at the DF site measures the vertical electron density profile and the local ionospheric tilt, providing, in real time, the inputs for the construction of the 3‐D Ne distribution. The vertical profile is automatically obtained from the Automated Real Time Ionogram Scaler with True Height (ARTIST)‐scaled ionogram and the local tilt from the sky maps recorded after each ionogram. The characteristics of each layer, for example, critical frequencies and peak heights, are expressed as a function of latitude λ and longitude Ψ. In the neighborhood of the DF site each characteristic, for example, foF2, is given as foF2(λ, Ψ) = foF2m (1 + C7Δλ + C8ΔΨ) (1 + CλΔλ + CΨΔΨ). The coefficients C7 and C8 for any given azimuth direction are determined with the use of the Union Radio Scientifique Internationale/CCIR coefficients (which are also used in IRI), and the calculation of Cλ and CΨ makes use of the measured ionospheric tilt data; foF2m is the local, measured foF2 value. When the measured density profile and tilt data are available, the derived 3‐D density distribution represents the instantaneous ionosphere structure near the site. The numerical ray tracing includes the effects of the magnetic field and properly treats the spitze effect, making the ray‐tracing program especially useful for small distances. Ray tracing through simulated tilts shows that the differences in ground distances for one‐hop high‐frequency (HF) propagation vary from about 1 to 100 km depending on the assumed tilts and distances. Operational tests for distances up to approximately 100 km have demonstrated good results in determining the transmitter location in real time and have illustrated the importance of using the actual ionospheric profiles and tilts in the ray tracing.

Ionospheric electron heating, optical emissions, and striations induced by powerful HF radio waves at high latitudes: Aspect angle dependence

In recent years, large electron temperature increases of 300% (3000 K above background) caused by powerful HF‐radio wave injection have been observed during nighttime using the EISCAT incoherent scatter radar near Tromsø in northern Norway. In a case study we examine the spatial structure of the modified region. The electron heating is accompanied by ion heating of about 100 degrees and magnetic field‐aligned measurements show ion outflows increasing with height up to 300 m s−1 at 582 km. The electron density decreases by up to 20%. When the radar antenna was scanned between three elevations from near field‐aligned to vertical, the strongest heating effects were always obtained in the field‐aligned position. When the HF‐pump beam was scanned between the same three positions, the heating was still almost always strongest in the field‐aligned direction. Simultaneous images of the 630 nm O(1D) line in the radio‐induced aurora showed that the enhancement caused by the HF radio waves also remained localized near the field‐aligned position. Coherent HF radar backscatter also appeared strongest when the pump beam was pointed field‐aligned. These results are similar to some Langmuir turbulence phenomena which also show a strong preference for excitation by HF rays launched in the field‐aligned direction. The correlation of the position of largest temperature enhancement with the position of the radio‐induced aurora suggests that a common mechanism, upper‐hybrid wave turbulence, is responsible for both effects. Why the strongest heating effects occur for HF rays directed along the magnetic field is still unclear, but self‐focusing on field‐aligned striations is a candidate mechanism, and possibly ionospheric tilts may be important.

Design and realization of a goniopolarimeter with application to the study of ionospheric tilts

The purpose of the paper is to describe a high frequency goniopolarimeter operating in the range of 3–30 MHz. The system uses four sensors, each of them is composed of two cross polarization active loop antennas oriented east-west and north-south. By “goniopolarimeter” we mean a system which determines both arrival angles (goniometer function), the type of the mode (O or X), the magnitude of this mode and the corresponding Doppler shift. It works with known broadcast transmitters and permits measurements over a large geographic area. The signal process deals with narrowband signals, polarization filtering, nonlinear frequency analysis and interferometry technique. We present some experimental results and discuss the application of this system to the study of ionospheric tilts.

Systematic ionospheric electron density tilts (SITs) at mid-latitudes and their associated HF bearing errors

The bearings of a number of HF transmissions recorded continuously over many days are employed to investigate the large-scale horizontal gradients of electron density in the mid-latitude ionosphere (systematic ionospheric tilts or SITs). Although previous authors have explained these SITs and their associated bearing errors solely in terms of a dependence of ionospheric tilt on the variation of solar zenith angle, our observations indicate that plasma transport processes must also be taken into account. A 3D ray tracing technique is employed in conjunction with an ionospheric model based on predicted vertical-sounder data to simulate the experimental observations. Good agreement is obtained between predicted and observed bearing errors for south-to-north (S-N) transmission paths, although poor agreement is achieved for east-to-west (E-W) paths. The latter is due to the inability of our ionospheric model to yield accurate N-S ionospheric gradient information. Better agreement between computed and observed bearing errors for E-W paths is achieved by ray tracing through an ionospheric model based on measured median vertical-sounder data. A major limiting factor in the accurate simulation of SIT-induced bearing error is the day-to-day variation of ionospheric tilts.

Electron density profiles over the Southern Ocean from oblique incidence ionograms

This paper presents a first attempt to use oblique incidence ionograms over the 4500 km path from Sanae, Antarctica, to Grahamstown, South Africa, to deduce information about the ionosphere in the intervening regions. It is shown that existing methods for the reduction of oblique incidence ionograms to N( h) profiles give reasonable results even over the two-hop path involved. By comparison with vertical incidence ionograms made from a research ship below the reflection regions it is shown that the maximum observed frequency is normally limited by conditions at the southernmost reflection point, though this may be modified by ionospheric tilts, sunrise and sunset.

The influence of chordal paths on signals propagating to the near antipode of an HF radio transmitter

The results of an HF radio propagation experiment at 15.270 MHz over an antipodal path are presented. The observations show a large and sudden presunrise increase in signal strength that is not predicted by a computer propagation model (multihop ray tracing with some random scattering) previously used successfully for the same geometrical path on a frequency of 11.790 MHz. It is shown that the anomaly can be explained when chordal propagation supported by the longitudinal ionospheric tilts (determined from critical frequency predictions) occurring along the most probable propagation paths are included in the computer model.

Ray tracing studies for the topside ionosphere

The effects of nonvertical propagation on high-altitude topside sounder data reduction are investigated. Although the extraordinary, ordinary, and Z rays exhibit significant lateral deviation in a spherically stratified ionosphere, the resultant effects on analysis accuracy are generally tolerable; however ionospheric tilts produce very large effects. The joint use of the three modes for future analyses is suggested.

Elevation Angles of Ground Backscatter at 16 MHz and Large-Scale Ionospheric Tilts

A technique of recording the elevation angle by a phase comparison method is described and applied to ground backscatter echoes at 16 MHz. It is shown that the results may be explained in terms of ionospheric tilts of 1 � or 2� near sunset.

Seasonal variation of the [O I] 6300 a airglow at Townsville from isophote sky maps

Sky maps of red airglow at Townsville show a progressive development of an after mid-night anomaly from quiescent winter conditions to early summer. The enhancement is most prominent in the northern sky. Comparison is made with similar experiments carried out by other workers at an approximately conjugate station at Maui. The observations are discussed in terms of ionospheric parameters h′ƒ and ƒoF2 indicating the influence of ionospheric tilts and electron density on the results. Electron density appears to have a definite influence.

Chart for determining the effects of ionospheric tilts using an idealized model

Chart for determining the effects of ionospheric tilts using an idealized model

Fading and attenuation of high frequency radio waves propagated over long paths crossing the auroral, temperate and equatorial zones

This investigation is primarily concerned with the fading and attenuation of highfrequency radio signals propagated over a long path crossing the auroral zone. The fading of high-frequency signals propagated over non auroral paths of comparable length has also been studied, and some new results are obtained. The principal fading and attenuation measurements on which these conclusions are based were carried out in August 1957. For the auroral paths, there is no diurnal variation in fading speed except for a distinct minimum in time interval 1330–1900 PST, during which time the fading speed has little apparent dependence on magnetic activity along the path. In other time periods a positive correlation between magnetic activity and fading speed is found. It is suggested that the period of minimum fading speed is a consequence of the existence at that time of the kind of propagation mode made possible by ionospheric tilts. Attenuation over the long auroral zone path is found to be associated with polar blackouts as indicated by the absence of returned echo in vertical sounders located along the path. The percentage association varies with the location of the station relative to the path. This variation is consistent with the inferred propagation modes. It is found that during the hours 1330–1900 PST the attenuation cannot be attributed to the absorption that gives rise to blackouts as it can in the other hours. This is also explainable on the basis of the postulated tilt-mode propagation. Similar observations for temperate-latitude and transequatorial paths of comparable length indicate that there is a strong diurnal variation in fading speed. Some plausible explanations are offered.

Fading and attenuation of high-frequency radio waves propagated over long paths crossing the auroral, temperate and equatorial zones

This investigation is primarily concerned with the fading and attenuation of high-frequency radio signals propagated over a long path crossing the auroral zone. The fading of high-frequency signals propagated over non-auroral paths of comparable length has also been studied, and some new results are obtained. The principal fading and attenuation measurements on which these conclusions are based were carried out in August 1957. For the auroral paths, there is no diurnal variation in fading speed except for a distinct minimum in time interval 1330–1900 PST, during which time the fading speed has little apparent dependence on magnetic activity along the path. In other time periods a positive correlation between magnetic activity and fading speed is found. It is suggested that the period of minimum fading speed is a consequence of the existence at that time of the kind of propagation mode made possible by ionospheric tilts. Attenuation over the long auroral-zone path is found to be associated with “polar blackouts” as indicated by the absence of returned echo in vertical sounders located along the path. The percentage association varies with the location of the station relative to the path. This variation is consistent with the inferred propagation modes. It is found that during the hours 1330–1900 PST the attenuation cannot be attributed to the absorption that gives rise to blackouts as it can in the other hours. This is also explainable on the basis of the postulated tilt-mode propagation. Similar observations for temperate-latitude and transequatorial paths of comparable length indicate that there is strong diurnal variation in fading speed. Some plausible explanations arc offered.

The IGY Three-Frequency Backscatter Sounder

Design criteria and operating characteristics of the IGY Fixed-Frequency Backscatter Sounder are outlined. This three-frequency (12, 18, and 30 mc) rotating-antenna pulse sounder was designed at Stanford University for oblique-incidence ionosphere studies. During the IGY a network of these sounders has been operated at thirteen stations in polar, temperate, and equatorial regions. Data reduction and publication of data summaries have been carried out at Stanford using semiautomatic punch card methods. Preliminary analysis of data has yielded new information on sporadic-E, magnetic field aligned irregularities, large-scale traveling disturbances in the F region, and the effects of ionospheric tilts on long-distance propagation.

Effective Tilts of the Ionosphere at Places about 1000 km apart

A comparison of the bearing and elevation angles of radio waves from two stations situated about 2000 km to the North and to the West of the receiving site yields results which are sometimes consistent with the existence during the daytime of effective ionospheric tilts which are similar at places more than 1000 km apart. These tilts are of the order of half a degree and are in opposite directions during the morning and the afternoon. They may correspond to actual tilts of a uniform ionosphere or be due to horizontal gradients of electron density. The factor relating the tilt angle to the observed bearing deviations was measured experimentally and the effective tilt angle found to be approximately twice the bearing deviation.