GPS satellite measurements: ionospheric slab thickness and total electron content

A preliminary analysis was made of ionospheric slab thickness, τ, and total electron content, TEC, for southern Australia using GPS satellite measurements. It was found that at mid-latitudes τ has similar overall diurnal, seasonal and latitudinal variations in the southern hemisphere as in the northern hemisphere. However, there are appreciable differences between τ in the two hemispheres which would justify appropriate modifications to ionospheric models based on northern hemisphere data before being applied confidently to the southern hemisphere. The usefulness of GPS satellites together with ionosondes over a spread of latitudes was demonstrated in determining long-term variations of TEC and τ over a large area. It was concluded that as few as four GPS receivers could provide TEC for the whole of Australia in real-time, though approximately six receivers in convenient locations would be required in practice.     

A case study of a high latitude ionospheric electron density depletion

Incoherent scatter measurements inside and outside an ionospheric electron density depletion are described. The density depletion is probably caused by an enhancement of NO⁺-ions and subsequent dissociative recombination. The NO⁺-ions are increased because high electric fields present at the geographical location of the density depletion speed up the reaction O⁺ + N₂ → NO⁺ + N. The electron as well as the ion temperature within the density depletion are strongly enhanced, the latter due to Joule heating, also caused by the electric field.     

Short term variabilities of ionospheric electron content (IEC) and peak electron density (NP) during solar cycles 20 and 21 for a low latitude station

Hourly values of IEC and of f₀F₂ (critical frequency) for a low latitude station, Hawaii (21.2°N, 157.7°W), during the solar maxima (1969 and 1981) and minima (1965 and 1985) years of two consecutive solar cycles, 20 and 21, are used to study the day to day variabilities of the ionospheric parameters IEC and NP. It is found that there is good correspondence in the day to day variations of IEC and NP from one solar cycle to the other for both solar maximum and minimum years in the two solar cycles. Depending on solar phase and season, while the mean daytime IEC and NP variations range from about 20% to 35%, the mean night time values vary from about 25% to 60%. The mean daytime variations in NP for the solar minimum phase are remarkably higher in all the three seasons compared to the solar maximum phase. However, no such increase is observed in the mean daytime IEC variations, indicating the highly variable nature of the daytime ionospheric F region compared to the topside during solar minimum for this low latitude station. The winter night time IEC also seems to be a relatively stable parameter during the solar minimum. The short term day to day variabilities of the day time peak values of IEC and NP (ie IEC_max and NP_max) are not closely associated with the variations in F_10.7 solar flux. Contrary to the common expectation, the variabilities in both the parameters, particularly in NP_max, are somewhat reduced during the solar maximum (when the variability in F_10.7 solar flux is much higher compared to the solar minimum) which is more evident in the stronger 21 solar cycle. A larger number of significant components are seen in the spectra of the percentage variation of both IEC_max and NP_max during both solar phases of the two solar cycles compared to the corresponding F_10.7 solar flux spectra. The number of additional components for both the parameters with periods less than 15 days are more for the low solar activity years than for the solar maximum years.     

On medium scale travelling ionospheric disturbances as seen through total electron content power spectra

Modulation phase of 140 MHz with respect to 360 MHz signals from ATS 6 satellite recorded at Slough (51.5°N, 0.6°W geographic latitude and longitude; 54.3° geomagnetic latitude) at one-minute interval are power spectral analysed to derive dominant periodicities corresponding to Travelling Ionospheric Disturbances. From the significant peaks in the spectra, an occurrence peak at periods between 10 and 15 min and a secondary peak at 60–65 min are seen. From cross-spectral analysis of the same records from three stations separated by a few hundred kilometers for a short period, the speed and azimuth of the propagating disturbances are determined. During the day-time, most of the waves in the period range 30–100 min are seen to propagate at azimuths of 90–160°. At night-time they propagate poleward. Theoretical computations of the azimuth response of TEC to typical gravity waves, including the effect of neutral winds, show that the observed azimuths of propagation are in reasonably good agreement with theoretical predictions.     

Observations of high latitude ionospheric conductances

A brief historical review of the development of models of the ionospheric conductivities with special emphasis on high latitude regions and the auroral zone is presented. It is with great admiration that we must conclude that the physical understanding of the importance of the ionospheric conductances was well perceived by pioneers like Schuster and Birkeland a hundred years ago. Progress in the basic theoretical fundamentals was achieved in the late 1920s and 1930s. Realistic estimates were not derived until the first rocket probes measured the electron and ion content at different altitudes in the 1950s.Today we have a superior technique in resolving electron density profiles of high time and height resolution by incoherent scatter radars on the ground. The challenge that we are facing is to obtain global conductivity maps, especially at high latitudes, with a time and spatial resolution which match the details in auroral substorm phenomena. If that can be achieved, great progress in the understanding of detailed dynamical coupling in the ionosphere, magnetosphere, and thermosphere systems is expected. The imaging technique as demonstrated by the DE-satellite can be the tool which eventually materializes our desires for increased knowledge.     

A study of gravity waves in ionospheric electron content at L = 4

Using satellite radio beacon transmissions, travelling ionospheric disturbances have been observed in the electron content at L = 4. Waves are a common feature at this latitude, present for at least 98% of all daylight hours. The amplitude is usually 1–4% of the mean electron content and periods range between 15 and 90 minutes. Simultaneous observation of two satellite beacons, giving an effective east-west separation of 350 km, indicated apparent east-to-west velocities of 200–700 m/s.A search was made for a likely source of the waves, using data from magnetometers and riometers, from incoherent scatter radar measurements of Joule heating, and from orbiting satellite measurements of electron influx, but no definite source could be established.It is also shown that travelling disturbances are closely related to occurrences of spread-F on ionograms at high latitudes.     

A quantitative study of the high latitude ionospheric trough using EISCAT's common programmes

Eighteen days of EISCAT data were used in a systematic study of the high latitude trough. Apart from a few days at midwinter, the pattern was the same in all cases. Near midnight the reversal of plasma flow from westward to eastward caused significant frictional heating of the ion population. At the same time a strong plasma velocity was observed upwards along the magnetic field line. This was the result of

• 1.(i) a southward neutral wind
• 2.(ii) a vertical wind driven by Joule heating
• 3.(iii) diffusion. Both enhanced recombination—associated with the increase in ion temperature—and the escape of plasma along the field line contribute to the drop in electron density.

     

Modeling equatorial ionospheric electric fields

This paper gives a brief overview of the processes responsible for the equatorial electric field, and reviews relevant modeling work of these processes, with emphases on basic aspects and recent progress. Modeling studies have been able to explain most of the observed features of equatorial electric fields, although some uncertainties remain. The strong anisotropy of the conductivity and the presence of an east-west electric field lead to a strong vertical polarization electric field in the lower ionosphere at the magnetic equator, whose magnitude can be limited by plasma irregularities. Local winds influence the structure of the equatorial polarization field in both the E and F regions. The evening pre-reversal enhancement of the eastward electric field has been modeled by considering a combination of effects due to the presence of a strong eastward wind in the F region and to east-west gradients of the conductivity, current, and wind. Models of coupled thermosphere-ionosphere dynamics and electrodynamics have demonstrated the importance of mutual-coupling effects. The low-latitude east-west electric field arises mainly from the global ionospheric wind dynamo and from the magnetospheric dynamo, but models of these dynamos and of their coupling have not yet attained accurate predictive capability.     

Solar cycle variation in the total electron content at Sagamore Hill

The TEC data obtained at Sagamore Hill observatory by using ATS-3 beacon signal during the period from November 1967 to December 1976 have been used to analyze the solar cycle variations of total electron content at invariant latitude 54°. The correlation coefficients between TEC and sunspot number were found to be largest if 12 month running mean values were used, and to be smallest if monthly mean values were used. By the method of linear regression analysis, the contour charts for real diurnal and seasonal variations of TEC at given sunspot number were constructed and described. The diurnal variation of TEC was represented by the sum of its diurnal mean and first three harmonic components. The solar cycle variations of these components were also given.     

Mechanisms for observed total electron content pulsations at mid latitudes

Total electron content variations in the Pc3–Pc4 range of frequencies of the order of 4 parts in 10⁴ have been reported in apparent correlation with simultaneous ground based magnetic pulsation observations. By means of a term-by-term analysis of the continuity equation for electrons, the plausibility of various mechanisms is investigated. The most likely explanation is in terms of localized increases in the electron density at F-region heights caused by the field-aligned (compressional) component of the pulsation magnetic field. The analysis predicts a tendency for the amplitude of the TEC pulsations to vary in antiphase with ground-based measurements of the north-south component of the pulsation field.     

Antarctic polar cap total electron content observations from Casey Station

In early 1990 a modified JMR-1 satellite receiver system was installed at Casey Station, Antarctica (g.g. 66.28°S, 110.54° E, -80.4°A, magnetic midnight 1816UT, L = 37.8), in order to monitor the differential phase between the 150 and 400 MHz signals from polar orbiting NNSS satellites. Total electron content (TEC) was calculated using the differential phase and Casey ionosonde foF2 data, and is presented here for near sunspot maximum in August 1990 and exactly one year later. The data are used to investigate long-lived ionization enhancements at invariant latitudes polewards of − 80° A, and the ‘polar hole’, a region from −70 to − 80° A on the nightside of the polar cap where reduced electron densitiy exists because of the long transport time of plasma from the dayside across the polar cap. A comparison is made between the Casey TEC data and the Utah State University Time Dependent Ionospheric Model (TDIM) which uses as variables the solar index (F 10.7), season (summer, winter or equinox), global magnetic index (K_p), IMF B_y direction, and universal time (UT) [sojkaet al. (1991) Adv. Space Res.11(10), 39].     

TID,electron density and ionospheric sounding

For the approximation of a parabolic ionospheric layer, the relation between the fluctuation amplitude of radio phase path in HF vertical sounding and sinusoidal disturbance parameters has been found. Horizontal disturbance propagation has not been considered. The determining factors are the relation of the vertical wavelength of the disturbance to the half-thickness of the layer and between operating and critical frequency.     

Comparison of total electron content calculated using the IRI with observations in China

Values of total electron content (TEC) calculated using the International Reference Ionosphere (IRI-86 and IRI-90) are compared with the observations at Xinxiang based on the Faraday rotation measurement. It is found that the IRI gives acceptable values with respect to the observations during low solar activity. Generally the IRI-90 is better than the IRI-86 and the URSI coefficients are better than the CCIR coefficients in the calculation of TEC. Making use of the foF2 and M(3000)F₂ calculated using the Asia Oceania Region F₂-layer mapping (AOR) instead of using the CCIR or the URSI coefficients, the IRI gives more accurate TEC values. In October-April during high solar activity, however, the IRI obviously underestimates TEC in the daytime, which could be due to an improper topside electron density profile.     

Ray-tracing the paths of very low latitude whistler-mode signals

VLF whistler-mode signals with very low group delays (75–160 ms) received at night in Dunedin, N.Z., from the 23.4 kHz MSK transmissions of NPM, Hawaii (21.5°N, 158°W), are explained by ray-tracing along unducted paths. The typical vertical and horizontal electron density gradients of the night equatorial ionosphere are found to be sufficient to explain not only the typical group delays but also their decrease during the night and the typical frequency shifts observed on these signals. An important feature appears to be the decreasing starting and finishing latitudes (and the decreasing maximum height of the path) during the course of the night. The amplitude of the signals in relation to the expected collisional absorption in the ionosphere is discussed. A simple but quite accurate analytical expression suitable for ray-tracing is derived for the night electron density in the height range 170–1400 km, based on non-isothermal diffusive equilibrium and O⁺/O friction.     

Ionospheric electron content observations during the total solar eclipse of February 16, 1980

Observations on the Faraday rotation of a transionospheric VHF signal obtained from a network of four stations near the path of totality during the total solar eclipse of 16 February 1980 are reported. A small decrease of 3–4% in the total ionization has been obtained around the time of totality. Absence of any periodic structure following the eclipse indicates that the TIDs are not of significant amplitude in the present case to be detected by the Faraday rotation technique.     

Experimental observations of very low latitude man-made whistler-mode signals

VLF signals at 23.4 kHz from NPM in Hawaii (lat 21.5°N) are commonly received at night in Dunedin, New Zealand with very low group delays of between about 75 ms and 160 ms and frequency shifts of several tens of milliHertz or more. The ratio of the frequency shift to the rate of change of group delay generally agrees with the ratio which would be expected from signals which have travelled through the equatorial ionosphere. Normal whistler-mode signals with delays of 0.3–0.6 s are quite frequently observed at the same time.     

Storm-time variation of foE at a low latitude station

foE changes during geomagnetic storms are studied for Ahmedabad. Individual storms show erratic effects. The average curves show a possible 5–10% decrease in the post-Main Phase Onset period and a somewhat larger decrease after about 40 storm hours.