Differential Doppler measurements of the ionosphere during a solar eclipse

The effects of the solar eclipse of 26 February 1979 on the ionosphere were measured using differential Doppler techniques. Nayy navigation satellite passes were monitored at 12 sites located across the North American continent. These data yield a measurement of the vertical columnar electron content along a north-south line. Different sites monitoring the same pass provide simultaneous observations of ionospheric variations along different longitude lines. Two satellite passes occurred during or just after the eclipse. These data show a shoulderjust northward of the umbra region and a trough just behind the umbra containing large horizontal gradients. This sharp trough recovered quickly with a half-life of about 10 min.     

Shadow bands during the total solar eclipse of 3 November 1994

We recorded shadow bands just before and just after the total phase of the solar eclipse of 3 November 1994. The recordings were made using two broad-band silicon photodiodes separated by 100 mm. They were mounted on a plate that faced the eclipsed Sun, which was at an altitude of 32.4° as seen from our observing site 4500 m above sea level between Putre and Lake Chungara in northern Chile. The irradiance fluctuations associated with the shadow bands were around 0.008 W m⁻² rms on a background of about 2–8 W m⁻². The cross-correlation function indicates that the shadow bands were moving at a speed of about 1.8 m s⁻¹ perpendicular to their extent. The power spectral density functions are in accord with the shadow band theory of Codona (1986). We carried out a similar experiment in Baja California during the eclipse of 11 July 1991. In spite of teh considerable differences between the two circumstances, our results on the two occasions are broadly similar.     

Shadow bands during the total solar eclipse of 11 July 1991

We recorded shadow bands just before and just after the total phase of the solar eclipse of 11 July 1991. The recordings were made using two broad band silicon photodiodes separated horizontally by 100 mm. They faced the zenith, near to where the eclipsed Sun lay as seen from our observing site close to San José del Cabo in Baja California. The irradiance fluctuations associated with the shadow bands were around 0.04 W m⁻² peak to peak on a background of 1–3 W m⁻². The cross-correlation function indicates that the shadow bands were moving at about 1.8 m s⁻¹ perpendicular to their extent. The power spectral density functions are in accord with the shadow band theory of Codona [(1986), Astron. Astrophys. 164, 415–427].     

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.     

A model of the lower ionosphere above Red Lake,Canada, during the 26 February 1979 solar eclipse

Measurements of ionization sources, ionization profiles and minor atmospheric constituents were conducted during the 26 February 1979 solar eclipse above Red Lake, Canada. A model of the lower thermosphere has developed to describe the D- and E-regions of the ionosphere for this case with the model being guided by the measurements. During the eclipse a rather intense particle precipitation event was in progress. For this reason, an auroral deposition code was coupled to a chemical-kinetics code to calculate degraded primary and secondary electron fluxes, ionization rates, positive ion and electron densities. The model was calibrated with the experimental measurements of electron flux below 100 km and electron density between 70 and 150 km. This calculation not only satisfactorily described the ionization in the E-region but also the gross electron density characteristics of the D-region. Bursts in the observed electron flux were also simulated with the model to give electron density profiles that were remarkably consistent with small perturbations seen in the electron density measurements.     

Upper atmosphere wind observations over Southern Australia during the total solar eclipse of 23 October 1976

Observations of winds in the 80–100 km height region were made at three locations in South Australia during the total solar eclipse of 23 October 1976. One station (Tantanoola) was located in the eclipse path while the others (Adelaide and Woomera) were situated several hundred kilometers north of the path of totality. Wind variations caused either directly by cooling of the 90 km region or by the propagation of a bow wave generated in the lower atmosphere were searched for but no events were found that could be ascribed unambiguously to the eclipse.     

Atmospheric electricity measurements at Pune during the solar eclipse of 18 March 1988

Measurements of atmospheric electric potential gradient, conductivity of both polarities and space charge at the ground surface at Pune during the partial solar eclipse of 18 March 1988 have been made. In spite of no appreciable change in atmospheric temperature at the ground surface, all the atmospheric electric parameters showed remarkable changes during the period of eclipse. Results do not support any vertical transport of charge, either by conduction or by convection, near to the ground surface.     

HF Doppler observations of gravity waves during the 16 February 1980 solar eclipse

Observations by the HF Doppier technique of the ionospheric effects of the 16 February 1980 solar eclipse in Africa are presented. Some oscillations which are detected at two stations can be attributed to a travelling coherent structure. Its characteristics are consistent with a gravity wave generated by the eclipse.     

HF Doppler observations of 23 October, 1976 total solar eclipse over south-eastern Australia

Observations of the Doppler shift of HF transmissions on 2.5 MHz and 4.5 MHz propagated via the ionosphere during the 23 October, 1976 eclipse over south-eastern Australia are reported. The 4.5 MHz signal, reflected from the F-region, showed the characteristic frequency shift attributable to recombination and/or motion of the reflection height after first contact. Spreading of the Doppler signal occurred after totality. Some wave activity was present in the record which is normal for this time of day. The 2.5 MHz observations showed little activity up to totality when sporadic E conditions set in, masking the detection of the response of the normal E-region to the eclipse.     

26 February 1979 total solar eclipse induced LF (60 kHz) phase retardation

The LF phase retardation induced by the total solar eclipse of Monday, 26 February 1979 and observed by monitoring the 12.5 km path length 60 kHz WWVB transmission from Fort Collins, U.S.A., to Calgary, Canada, as the path of totality at the 80 km height of the D-layer swept over Billings, Montana, U.S.A., near the midpoint of this transmission path is reported.     

Atmospheric electrical potential gradient measurements during the annular solar eclipse of 29 April 1976

This paper gives the results of measurements of the potential gradient of the electric field of the atmosphere during the annular eclipse of the sun, on 29 April 1976, on the island of Santorini. The potential gradient during the annular eclipse showed a diminution. The results of measurements of the intensity of the total solar radiation and the temperatures of the air and the earth's surface are also given.     

Positive and negative ion composition measurements in the D- and E-regions during the 26 February 1979 solar eclipse

Two rockets bearing quadrupole mass spectrometers capable of measuring both positive and negative ion composition were launched from Red Lake, Canada, during the solar eclipse. Both instruments had liquid helium cryopumps and shock-attaching conical samplers. The payloads also contained two Gerdien condensers to measure total positive and negative ion concentrations and ion mobilities. Attitude control systems aligned the payloads with the velocity vector throughout ascent and descent. The first rocket was launched so that the D-region was in darkness 35 ± 8 s on the upleg and about 150 ± 15 s on the downleg for the study of ionospheric decay processes. The second rocket was fired after totality into 75% solar illumination for the study of ionospheric recovery. The positive ion composition above 105 km exhibited a strongly increasing NO⁺/O₂⁺ ratio with time after second contact due to O₂⁺ charge transfer with NO and a sharply diminished ionization rate. However, in both nights, the ionization below 105 km was created mainly by energetic particle deposition as exemplified by the increased ion concentrations and the composition signatures of a particle event: asignificant enhancement of O₂⁺ below 105 km and large amounts of H₅O₂⁺ ions in the D-region which result from the O₂⁺ clustering scheme. H₅O₂ was the major ion in the upper D-region while H₇O⁺₃, H₉O₄⁺ and H₅O₂⁺ were dominant ions at lower altitudes. Numerous minor species were also detected. The negative ion distributions in both flights exhibited a distinct shelf at 83 ± 2 km, decreasing by more than an order of magnitude by 90 km and with minima near 75 km. In the 75–90 km range, a significant percentage of the negative ions had masses exceeding 160 a.m.u. Comparisons are made with prior negative ion measurements during similar daytime auroral zone absorption (AZA) events. Two striking characteristics of the precipitating particles were apparent from these and past observations in daytime AZA events: there is a near absence of low energy electrons capable of ionizing above about 105 km and there is'a significant spatial and/or temporal variability in the electron flux. This paper is devoted principally to a presentation of the ion composition measurements and associated uncertainties.     

VLF transmissions observed at anomalously high latitude above the ionosphere in the conjugate hemisphere

Ariel 3 and 4 satellite observations of the GBR 16 kHz and NAA 17.8 kHz transmissions above the ionosphere in the conjugate hemisphere show that their wave-fields generally show a rapid reduction in signal strength for geomagnetic latitudes greater than 55°–60°. Sometimes, however, the signal strength has been observed to be high in the invariant latitude range > 60°. At certain times during these observations, the signal showed clear evidence of amplification, whilst at other times the pattern of signal strength was displaced to higher latitude with the signal strength integrated over latitude being unchanged from that normally observed.It is shown that the plasmapause can guide both the NAA and GBR signals but that the efficiency of this guiding depends on the plasmapause position. The important condition is found that the plasmapause must be situated sufficiently equatorwards that half the equatorial electron gyrofrequency at the plasmapause position is greater than (or approximately equal to) the transmitter signal frequency. Ray-tracing calculations in a realistic magnetosphere model indicate that for the 16 kHz GBR signal, the efficiency of guiding falls off for L_pp, (the L-value of the plasmapause) > 3.0 and guiding effectively ceases for L_pp > 3.5.Guidance by the plasmapause results in a wave-field at higher latitude than for non-guided propagation. This will only occur when, following geomagnetic storms, the plasmapause position is at a sufficiently low L-value. This is in agreement with the experimental observations of anomalously high latitude signal reception following strong magnetic storms (K_p ≥ 4+).     

Magnetic observations at the time of the 23 October 1976 solar eclipse in Australia

The solar eclipse of 23 October 1976 passed across south-east Australia in the local solar afternoon. This paper records the magnetic observations of a line of ten temporary stations stretching from a region of fifty percent obscuration to the path of totality. Magnetic conditions at the time were mildly disturbed. Analyses of the data have sought an eclipse effect in the basic quiet daily variation, and also in the disturbance variations. However, no eclipse effect in the primary ionospheric currents has been clearly distinguished above spatial uneveness, due to local differential induction, in the induced secondary currents flowing in the earth.     

HF scattering induced by field-aligned irregularities in the equatorial E-region during total solar eclipses

In this paper, results and analyses of solar eclipse effects on the lower ionosphere are presented. After the first contact of the total eclipse on 16 February 1980, an absorption increment of 12 dB was observed. At the same time, the frequency of amplitude fading increased largely and Doppler frequency shift disturbances appeared. The calculation of signal strength is carried out by means of Booker's scattering theory, supposing an outer scale T_o = 1000 m and an inner scale T_i = 5 m, of space scale spectrum of field-aligned irregularities in the equatorial E-region. The calculated results agree fairly well with observations. Results showed that, because of the formation of lower ionospheric field-aligned irregularities in the course of the obscuration of solar local ionization source, radio wave scattering was strengthened.     

Nitric oxide and lower ionosphere quantities during solar particle events of October 1989 after rocket and ground-based measurements

The most dramatic demonstrations of solar activity are solar proton flares. One such very strong flare, accompanied by a solar proton event (SPE) and a large ground level enhancement of cosmic rays on Earth, was observed in October 1989. During this SPE, ion density and nitric oxide concentration profiles were measured by rockets launched from the Soviet research vessel ‘Akademik Shirshov’ in the southern part of the Indian Ocean. The rocket experiment yielded the first in-situ measurement of NO concentration increased by SPE. The NO concentrations estimated from ion-pair production rates due to measured fluxes of high energy particles agree fairly well with the observed NO concentrations in the stratopause region. The results of rocket measurements are compared with measurements of the radio wave absorption in the lower ionosphere performed at similar latitudes in central Europe. Model calculations of absorption show that while the night-time enhancement of absorption can be explained by increased electron density related to the measured increase of ion density as a consequence of enhanced penetration of high energy particles, the daytime increase of absorption needs to be explained mainly in terms of the observed increase of nitric oxide concentration.