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.     

Mesospheric wind studies during AIDA Act '89: morphology and comparison of various techniques

The Arecibo Initiative in Dynamics of the Atmosphere (AIDA) '89 was a multi-instrument campaign designed to compare various mesospheric wind measurement techniques. Our emphasis here is the comparison of the incoherent scatter radar (ISR) measurements with those of a 3.175 MHz radar operating a s an imaging Doppler interferometer (1131). We have performed further analyses in order to justify the interpretation of the long term IDI measurements in terms of prevailing winds and tides. Initial comparison of 14 profiles by Hines et al., 1993, J. atmos. terr. Phys. 55, 241–288, showed good agreement between the ISR and IDI measurements up to about 80 km, with fair to poor agreement above that altitude. We have compiled statistics from 208 profiles which show that the prevailing wind and diurnal and semidiurnal tides deduced from the IDI data provide a background wind about which both the IDI and ISR winds are normally distributed over the height range from 70 to 97 km. The 3.175 MHz radar data have also been processed using an interferometry (INT) technique [Van Baelen and Richmond 1991, Radio Sts. 26, 1209–1218] and two spaced antenna (SA) techniques [Meek, 1980, J. atmos. terr. Phys. 42, 837–839; Briggs. 1984, MAP Handbook, Vol. 13, pp. 166–186] to determine the three dimensional wind vector. These are then compared with the IDI results. Tidal amplitudes and phases were calculated using the generalized analysis of Groves, 1959, S. atmos. terr. Phys. 16, 344–356, historically used on meteor wind radar data. Results show a predominance of the diurnal S₁¹ tidal mode in the altitude range 70–110 km, reaching a maximum amplitude 45 ms⁻¹ at 95 km, with semidiurnal amplitudes being about 10–15 ms⁻¹ throughout the height range considered. There is evidence of the two day wave in data from 86–120 km, with amplitudes on the order of 20 ms⁻¹.     

Estimates of gravity wave momentum fluxes in the winter and summer high mesosphere over northern Scandinavia

As part of the MAP/WINE campaign (winter 1983–1984) and the MAC/SINE campaign (summer 1987) high resolution wind profiles were obtained in the upper mesosphere using the foil cloud technique. Vertical winds were derived from the fall rate of the foil clouds and are used for estimating the momentum fluxes associated with vertical wavelengths shorter than about 10 km. From the ensemble average of 15 observations over an altitude range of 74–89 km we calculate a zonal net momentum flux of +12.6 ± 4.5 m²s⁻² in summer. The average of 14 measurements in winter between 73 and 85 km indicates a zonal net momentum flux of −3.7 ± 2.4 m²2 s⁻².     

Electric fields and electron density irregularities in the equatorial electrojet

Two Centaure rockets were launched from Thumba (0 47′S dip). India, with a new arrangement of double probe sensors for the simultaneous measurements of the irregularities in the electron density and the electric field along and perpendicular to the spin axis of the rocket. These experiments were carried out during the period when type I irregularities were observed with the VHF backscatter radar at Thumba. Irregularities with scale sizes ranging from a few meters to a few kilometers in the electron density and in the electric field components both in the east-west and the vertical direction could be studied with these experiments. Irregularities in the electric field in the medium scale size range (30–300 m) were observed with peak to peak amplitudes up to 20 mV m⁻¹ and in the small scale (⩽ 15 m) with peak to peak amplitudes up to 5 mV m⁻¹. Horizontally propagating waves with horizontal scale sizes up to 2.5 km were observed in the region below 105.5 km. Using linear theory for the electrojet irregularities, it was found that for 5 % perturbations in the electron density, the amplitude of the electric field can be as large as 20–30 mV m⁻¹. The spectrum of the irregularities in the vertical electric field in the rocket frame of reference was calculated and it was found that for the range of scale sizes between 10 and 70 m, the mean spectral index was −2.7 and −2.6. while in the scale size range 2–10 m it was −4.0 and −5.1 for the flights C-77 and C-73, respectively.     

Field-perpendicular and field-aligned plasma flows observed by EISCAT during a prolonged period of northward IMF

The effect of a prolonged period of strongly northward Interplanetary Magnetic Field (IMF) on the high-latitude F-region is studied using data from the EISCAT Common Programme Zero mode of operation on 11–12 August 1982. The analysis of the raw autocorrelation functions is kept to the directly derived parameters N_e, T_e, T_i and velocity, and limits are defined for the errors introduced by assumptions about ion composition and by changes in the transmitted power and system constant. Simple data-cleaning criteria are employed to eliminate problems due to coherent signals and large background noise levels. The observed variations in plasma densities, temperatures and velocities are interpreted in terms of supporting data from ISEE-3 and local riometers and magnetometers. Both field-aligned and field-perpendicular plasma flows at Tromsø showed effects of the northward IMF: convection was slow and irregular and field-aligned flow profiles were characteristic of steady-state polar wind outflow with flux of order 10¹² m⁻² s⁻¹. This period followed a strongly southward IMF which had triggered a substorm. The substorm gave enhanced convection, with a swing to equatorward flow and large (5 × 10¹² m⁻² s⁻¹), steady-state field-aligned fluxes, leading to the possibility of O⁺ escape into the magnetosphere. The apparent influence of the IMF over both field-perpendicular and field-aligned flows is explained in terms of the cross-cap potential difference and the location of the auroral oval.     

The nocturnal intermediate layer over South Georgia : solar and magnetic influence on occurrence

Ionograms from South Georgia (54°S, 37°W; L = 1.9) are used to investigate the influence of magnetic and solar activity upon the occurrence of the nocturnal intermediate layer. The diurnal variation in occurrence of this layer exhibits a peak at about 2000 LT and a subsidiary peak after midnight. It is shown that magnetic activity has no significant influence on the behaviour before midnight, but is positively correlated with the size of the post-midnight peak. The effect of varying solar activity is to introduce a local time shift (42 min for a change in mean solar activity from 80 to 130× 10⁻²² Wm⁻²Hz⁻¹)inthediurnal variation without otherwise changing the overall morphology of the layer significantly. Most of the features of the intermediate layer before midnight can be explained by a wind shear mechanism associated with the solar semi-diurnal tide. Some possible causes for the post-midnight observations are considered, but no firm conclusions can be made.     

Schumann resonance effects of electrical conductivity perturbations in an exponential atmospheric/ionospheric profile

Eigenmode solutions are computed for the n = 1 … 3 Schumann resonances in a perturbed, unmagnetized vertical atmospheric conductivity profile σ = 10⁻¹⁶ exp (z/3.1) mho m⁻¹ for z ⩽ 100 km and σ = 10⁻² mho m⁻¹ for z > 100 km. For the unperturbed exponential profile the radial electric field E_r is nearly constant z ≲ 40 km, and decreases rapidly above 50 km. The tangential field E_ϑ > E_r for z ≳ 65 km. The Joule dissipation profile in this case has an absolute maximum at about 50 km and a smaller relative maximum at 90 km with a deep relative minimum at 65 km. The maximum dissipation thus occurs in the middle atmosphere, making the Schumann resonances particularly susceptible to conductivity perturbations in this region. The perturbations of this study comprise Gaussian-shaped enhancements or depressions of FWHM ≈ 10 km impressed on the unperturbed profile. Eigenfrequencies and Q-values are computed for the full range of perturbation amplitudes 10⁻³−10³ and altitudes 30–90 km. The perturbations induce overall eigenfrequency variations of ± 1.0, ±1.5, and ±2.5 Hz in the n = 1, 2, and 3 modes, respectively, and Q-values spanning the range 3.5–11.0. The results of this calculation extend those of previous works investigating the Schumann resonance response to atmospheric conductivity perturbations, and may be useful for interpreting experimental observations in terms of external ionization source intensities of GCR, Lyman-α, or solar cosmic or X-rays, or variations in middle atmospheric chemical constituents.     

Observed and calculated F2 peak heights and derived meridional winds at mid-latitudes over a full solar cycle

The peak height of the F2 layer, hmF2, has been calculated using the ‘servo’ model of Rishbeth et al. [(1978), J. atmos. terr. Phys. 40, 767], combined with the hedin et al. [(1988), J. geophys. Res. 93, 9959] neutral wind model. The results are compared with observed values at noon and midnight derived from ionosonde measurements at two mid-latitude stations, Boulder and Wallops Island, over a full solar cycle. The reduced height of the F2 layer, zmF2, is also computed for the same period using the observed hmF2 values and the MSIS-86 model. Day-night, seasonal, and solar cycle variations in zmF2 are attributed to neutral composition changes and winds. Anomalously low values of hmF2 and zmF2 during summer both at solar minimum and during the solar cycle maximum in magnetic activity may be associated with increases in the molecular to atomic ion concentration ratio. Under these circumstances the F2 peak may lie significantly below the O⁺ peak height calculated by the servo model. Neutral meridional winds at Wallops Island are derived from the servo model using the observed hmF2 values and the calculated O⁺ ‘balance height’. It is shown that if the anomalously low hmF2 values are used, unrealistically large poleward winds are derived, which are inconsistent with both theory and observations made using other techniques. For most conditions the F2 peak is clearly an O⁺ peak, and daily mean winds at hmF2 derived from the servo model are consistent with the hedin et al. (1988) wind model. Unexpectedly, the results do not show an abrupt transition in the thermospheric circulation at the equinoxes. Diurnal curves of the servo model winds reveal a larger day-night difference at solar minimum than at solar maximum.     

VELOX: a new VLF/ELF receiver in Antarctica for the Global Geospace Science mission

VELOX (VLF/ELF Logger Experiment), a new facility for systematically studying the characteristics of magnetospherically generated ELF/VLF radio noise received at a high-latitude ground station (Halley, Antarctica, 76°S, 26°W, L = 4.3), measures continuously at 1 s resolution the absolute power (peak, mean, and minimum), arrival azimuth, and polarisation ellipticity in 8 logarithmically spaced frequency bands ranging from 500 Hz to 9.3 kHz. All filtering etc. is done in real time using Digital Signal Processing (DSP) techniques. Key parameters (1 kHz and 3 kHz power channels only, at 1-minute intervals) for each day are extracted and regularly transferred to the Global Geospace Study Central Data Handling Facility. Data from the first year of operation (1992) show that, whilst the upper channels (6 kHz and 9.3 kHz) are dominated by thunderstorm (spheric) noise, which is strongest at night and repeatable from day to day, magnetospheric chorus and hiss emissions are more important in the 1–4 kHz range of high attenuation in the Earth-ionosphere waveguide. They are highly variable in intensity from below system noise level (15–20 dB above the reference level 10⁻³³ T² Hz⁻¹) up to a maximum of 60–70 dB. Three classes of event are usually observed during specific local time sectors: substorm-related chorus events in the midnight-dawn sector, dawn chorus, and hiss-like events in the afternoon; all may occasionally be completely absent on quiet days. The substorm chorus events are shorter (typically 10–20 minutes) and more narrow-band than dawn chorus. Both upper and lower cut-off frequencies rise rapidly (∼ 100 Hz/min), consistent with the energy dispersion of resonant electrons as they drift eastward from injection near midnight, and with the inward drift, driven by substorm-enhanced electric fields, of whistler ducts which support propagation to the ground. Afternoon emission events are often punctuated by sudden deep fading, to noise level within 1–2 minutes, usually followed by complete recovery after a few minutes. All frequencies in the emission band are affected simultaneously. The explanation for this effect is unknown, though it could be a cut-off of propagation through the ionosphere to the ground by irregularities or gradients tilting the wave-normals out of the transmission cone. A similar system to VELOX will be deployed on a network of Automatic Geophysical Observatories extending to higher latitudes, south of Halley.     

A narrow auroral arc observed with EISCAT

On the evening of 13 January 1983 we made simultaneous observations of optical and radar aurora using low light television cameras together with the EISCAT radar system. At 19 h 16 m 06 s UT an extremely bright auroral arc moved rapidly (about 2 km s⁻¹) through the EISCAT radar beam. The associated rapid rise and fall in the E-region electron density indicates that there was an intense narrow electron beam associated with the optical arc. We estimate that the ionisation rate in the E-region increased at least 20-fold (from 1 × 10¹⁰ m⁻³ s⁻¹ to >2 x 10¹¹ m⁻³ s⁻¹) for 1 or 2 s as the arc passed by. In addition, there was a brief (<4 s) increase of 130% in the signal returned from 250 km altitude which coincided with the arc crossing the radar beam at that height. In view of this coincidence, we find that a possible explanation is that the increase arose from short-lived molecular ions, for example vibrationally excited N⁺₂ ions, produced in the F-region by soft precipitation associated with the arc.     

Latitudinal and vertical parameters of the equatorial electrojet from an autonomous data set

The magnetic field expressions from the current ribbon and thick current versions of the continuous distribution of current density model and their merits have been presented. For the first time both the latitudinal and vertical parameters of the equatorial electrojet (EEJ) have been derived from the same set of data. The local noon and daytime means of certain key parameters of the EEJ are shown to be in good agreement with those from other sources. Selected local noon means include: peak current density j_o, 10.58 ± 0.34 A/km²; peak current intensity j_o, 224 ± 9 A/km; total eastward current I₊, 74 ± 5 kA ; EEJ current focal distance w, 300 ± 5 km ; half thickness at half of peak current density p, 7.0 ± 0.1 km; peak westward current location x_m, 5.13 ± 0.08° dip latitude; and EEJ latitudinal extent L₁, 12 ± 1° dip latitude. The problem of model calculated landmark distances of EEJ being consistently shorter than observations, encountered by Onwumechiliet al. [J. geomagn. Geoelecl. 41, 443 (1989)] has been solved.     

Electron temperature and electron density in the F-region of the ionosphere. I. Observed relationship

Ionospheric data from three incoherent scatter stations over the height range 225–450 km were studied for all daylight hours over a wide range of solar conditions. The relationship between electron temperature T_e, electron density Nand solar flux at 10.7 cm wavelength S_10.7 was expressed as T_e = A−B·(N−5 × 10¹¹) + C·(S_10.7−750), where N is in units of m⁻³ and S_10.7 in kJy.This provided a very satisfactory expression for all data taken at Malvern and St. Santin between 0800 and 1600 LT. For data taken at Arecibo, however, the linearity broke down at low electron densities. The data from all three stations were therefore divided into two sets according to electron density and reexamined.ForN < 5 × 10¹¹ m⁻³ B increased steadily with height and decreased steadily with latitude.For N > 5 × 10¹¹ m⁻³ B did not appear to vary with height, with season or with latitude. C was approximately constant for all sets of data.The different mechanisms involved in the heat balance of the electron population are discussed and a qualitative explanation for the relationship is proposed.     

Recent work on mid-latitude and equatorial sporadic-E

Theoretical and experimental work since 1970 is summarized. Mid-latitude sporadic-E is most likely due to a vertical shear in the horizontal east-west wind and this theory accounts for the detailed observations of the wind and electron density profiles. Preferred heights of sporadic-E are separated by about 6km and descending layers are often seen moving down with velocities in the range 0.6–4 ms⁻. Sometimes sporadic-E layers are very flat and uniform, and at other times form clouds of electrons 2–100km in size moving horizontally at 20–130 ms⁻¹. Sporadic-E is probably not correlated with meteor showers; this is a rather surprising result since the ions are meteor debris.The major problems with windshear theory are to account for the dramatic seasonal variation and, to a lesser extent, for the geographical and diurnal distributions.The Q-type equatorial sporadic-E appears to be due to the gradient instability. There is a very much smaller amount of new experimental data available in this area.     

Fractal analysis of gravity wave spectra in the middle atmosphere

Wind fluctuations in the middle atmosphere behave like colored noise processes. They have a continuum of scales without dominant features and a power spectrum density (PSD) that often decays with frequency ƒ as ƒ^−β. Spectral index β is generally obtained through least-square fit to PSD estimated by Fourier methods. Graphs of colored noise have fractal plane-filling properties depending on β. An efficient method for finding β using the fractal dimension (D), based on analysis of 1/ƒ noise in galactic X-ray luminosities by McHardy I. and Czerny B., (1987, Nature325, 696), is described. An empirical relation is found between D and β and its validity is confirmed in limiting cases. Then D is obtained from power-law dependence of a length metric L(μ) on scale μ. The method is applied to middle-atmospheric velocity data from the Poker Flat radar in Alaska. Variations of D follow those in β, from an earlier analysis by Bemraet al., (1986, Handbook for MAP20, 216), but show an offset of 0.1–0.2 even after corrections for outliers, gaps, and additive noise. Usefulness of this method for screening data as an aid to spectral analysis is examined.     

EISCAT observations of ion composition and temperature anisotropy in the high-latitude F-region

The papers by Winseret al. [(1990) J. atmos. terr. Phys.52, 501] and Häggström and Collis [(1990) J. atmos. terr. Phys.52, 519] used plasma flows and ion temperatures, as measured by the EISCAT tristatic incoherent scatter radar, to investigate changes in the ion composition of the ionospheric F-layer at high latitudes, in response to increases in the speed of plasma convection. These studies reported that the ion composition rapidly changed from mainly O⁺ to almost completely (>90%) molecular ions, following rapid increases in ion drift speed by >1 km s⁻¹. These changes appeared inconsisent with theoretical considerations of the ion chemistry, which could not account for the large fractions of molecular ions inferred from the obsevations. In this paper, we discuss two causes of this discrepancy. First, we reevaluate the theoretical calculations for chemical equilibrium and show that, if we correct the derived temperatures for the effect of the molecular ions, and if we employ more realistic dependences of the reaction rates on the ion temperature, the composition changes derived for the faster convection speeds can be explained. For the Winser et al. observations with the radar beam at an aspect angle of ϕ = 54.7° to the geomagnetic field, we now compute a change to 89% molecular ions in < 2 min, in response to the 3 km s⁻¹ drift. This is broadly consistent with the observations. But for the two cases considered by Häggström and Collis, looking along the field line (ϕ = 0°), we compute the proportion of molecular ions to be only 4 and 16% for the observed plasma drifts of 1.2 and 1.6 km s⁻¹, respectively. These computed proportions are much smaller than those derived experimentally (70 and 90%). We attribute the differences to the effects of non-Maxwellian, anisotropic ion velocity distribution functions. We also discuss the effect of ion composition changes on the various radar observations that report anisotropies of ion temperature.     

Do beats affect sun-weather correlations?

Kinget al. (1977) have presented evidence to suggest that variations in the height of the 500 mbar level may be solar induced. Using a superposed epoch analysis they show a high correlation between the 27.5 day variation (frequency 0.0364 day⁻¹) in the sunspot number and the tropospheric pressure using a ten year run of data. It is suggested here that using such an analysis, side bands in the isobaric height variations spaced equally either side of the 0.0364 day⁻¹ component may add to enhance the variation at 0.0364 day⁻¹. That such a situation may occur is confirmed by an analysis of isobaric height data at three Australian locations for the winter of 1973 which shows that the isobaric surfaces do not respond to the solar component, but that frequency components spaced equally either side of the solar components do occur.     

Effective electron collision frequency measurements in the E- and F-regions

The collision frequency v in the ionosphere has often been determined by measuring differences in the amplitude and group path of two closely spaced signals reflected in the region of high group retardation. In this paper we describe a method of measuring v using a CW double-side-band modulated signal reflected obliquely in the ionosphere. This allows v to be determined on a continuous basis and it is found that the value of v obtained is 1–5 × 10⁴ s⁻¹ for the E-region and ~ 10³ s⁻¹ for the F-region. It is shown that measurements made just after sunset, when the E-region is still present, are more representative of E-region values than F-region.     

Internal structure of the equatorial ionospheric dynamo

Using the Sq current profiles measured with rocket born magnetometer, off the coast of Peru, by Daviset al. (J. geophys. Res. 72, 1845) comparison is made between measured and calculated profiles near noon on geomagnetic dip equator, and a mismatch is pointed out in the height pattern of Sq current. Theory is worked out to determine the eastward electric field (E_y) with which computed j_y (eastward current density) coincides with the observed one. It is found that the neutral wind plays very important rôle in keeping E_y height-independent. E_y is found to be about 0.6 mVm⁻¹, ranging from 0.5 to 0.7 mVm⁻¹ in some cases. Flow of meridional current is obtained and its effect on jet current construction is discussed.     

Comparison of stratospheric ozone profiles retrieved from microwave-radiometer and Dobson-spectrometer data

Passive Microwave Remote Sensors (MRS) can provide information on the composition of the atmosphere by measuring the thermal radiation emitted from rotational transitions of atmospheric molecules Ozone profiles simultaneously obtained from an MRS and a Dobson instrument (‘Umkehr’ method) are compared over a time period of approximately four months. The microwave measurements yield ozone concentrations which are 20–30% higher than the ‘Umkehr’ values. A critical, though not well known, parameter for the microwave inversion procedure is the foreign gas pressure broadening parameter (C) for the observed 142GHz ozone resonance. Throughout the intercomparison we used a value of 3 MHz mb⁻¹. There is recent theoretical and experimental indication that C is more likel y to be as low as 2.5 MHz mb⁻¹. If we use this new value for C all microwave retrieved profiles decrease by 20–25%, thus leading to a far better agreement with the ‘Umkehr’ results. Our measurements therefore strongly support the proposed value of 2.5 MHz mb⁻¹. A final answer on MRS ‘Umkehr’ correlation accuracy cannot be given. We feel that comparison on a day-to-day basis may be rather meaningless and monthly mean values should be used. On the other hand, there was relatively little change in these mean values during the intercomparison period The MRS showed its potential to retrieve ozone profiles also under adverse meteorological conditions, such as cloud cover or fog.