L.G. Smith

Global zones of energetic particle precipitation

Abstract Global zones of energetic particle precipitation in the nighttime atmosphere during disturbed magnetic conditions are identified in rocket, satellite and ground-based measurements, and are supported by theoretical arguments. The previously-known equatorial zone of precipitated energetic ions and neutrals (10 A mid-latitude precipitation zone consisting mainly of energetic protons is observed between L ≈ 2.1 and the plasmapause (L ≈ 3.5); it has a broad maximum near L ≈ 2.6. This flux (10

Seasonal variation of the lower ionosphere at Wallops Island during the IQSY.

Abstract Electron density profiles for the lower ionosphere between 55 and 105 km are presented for each of five Nike-Apache rockets launched from Wallops Island, Virginia near times of solstice or equinox of the International Quiet Sun Years of 1964 and 1965, at the same solar zenith angle of 60°. Summer solstice and equinox electron densities are larger than those of winter solstice by a factor of about four between 65 and 80 km. Above and below this region systematic seasonal differences are not apparent. The profiles are compared with profiles of Aikin, Belrose, Deeks and Hall.

Growth of the D-region at sunrise

Abstract D-region electron density profiles of high accuracy (±10 per cent), and high resolution (0·1 km) are presented for each of 5 Nike Apache rockets launched at sunrise from Wallops Island, Virginia at solar zenith angles of 108, 96, 94, 94, and 85°. Orderly increases of electron density between 70 and 80 km are observed as the solar zenith angle decreases from 108 to 94 degrees. At a zenith angle of 94°, the observed electron density between 87 and 90 km is larger by a factor of 6 on a morning of high VLF absorption which commenced at 98° compared with mornings of normal VLF absorption which commenced at 94°. At a zenich angle of 85°, a “C-layer” or local maximum electron density of about 90 cm−3 at 68 km, and a decreased electron density at 75 km are observed.

Sporadic-E layers and unstable wind shears

Abstract Electron density profiles of sporadic-E layers have been observed with good height resolution using rocket-borne probes. These generally show a simple shape consistent with the effect of a linear wind shear acting on metallic ions. Occasionally more complex shapes have been recorded, including double peaks and, on one occasion, a nearly rectangular profile. A direct method of obtaining the wind profile from the concentration profile of metallic ions has been developed. The metallic ion concentration profile itself is obtained from the electron density profile. Both procedures derive from the steady-state continuity equation. For linear wind shears it is found that the maximum value of the shear is about 50 m s−1 km−1 which corresponds to a Richardson number of 1 4 . Layers of complex shape are associated with non-linear wind shears in which the maximum shear considerably exceeds this value. It is concluded that the complex profiles of sporadic-E layers can be interpreted as an effect of unstable wind shears.

Comparative in situ studies of the unstable day-time equatorial E-region

Abstract Plasma waves measured by probes on sounding rockets are used to characterize the unstable equatorial E-region and to reflect the changing irregularity morphology with respect to altitude within the layer. We present measurements from three sounding rockets launched at the geomagnetic equator from Punta Lobos. Peru, including both a strong and a mild electrojet experiment conducted at midday and a weak electrojet experiment conducted in the late afternoon. The observed irregularities are analyzed in relation to simultaneous measurements of the electron number density and to either the measured or inferred profiles of the electron current density. The linear growth rate for the combined two-stream and gradient drift instabilities is computed using these profiles and the changing unstable wavenumber regimes are then compared to the power spectra of the wave observations as a function of altitude. We have allowed for long wavelength waves in the growth rate and have included the effect of recombination. In each case, the waves are observed only in the altitude regions, which, on the basis of the growth rate, are predicted to be unstable for horizontally propagating waves. Further, although the conversion of observed frequency to wavenumber is not definitive, the theoretical range of wavenumbers that will be unstable agrees at least qualitatively with the corresponding frequencies which have associated fluctuations displayed in spectrograms of the in situ time series measurements. In the case of the strong electrojet experiment, both the growth rate calculations and the wave observations show a region of high frequency (short wavelength) oscillations in the upper portion of the layer, where the medium was unstable to the primary two-stream instability. In the mild electrojet experiment, current measurements show that the two-stream threshold was not met, which is supported by the absence of observed high frequency oscillations for this flight. The medium may still have been unstable to this process as a secondary mechanism, as suggested in the 3 m backscatter radar data, implying that the large scale wave electric fields were on the order of or greater than, the polarization electric field. Where the payloads encountered a positive (upwards) gradient in electron density, all three rockets show a strong low frequency component which we attribute, in general, to the gradient drift instability.

Vertical incidence absorption calculated using electron density profiles from rocket experiments and comparison with observations during the winter anomaly

Abstract In the first part of this paper ground-based measurements of vertical incidence absorption are compared with values calculated from electron density profiles from simultaneous rocket experiments. These data were obtained at Wallops Island during an investigation of the winter anomaly in radiowave absorption. The measurements were made at 3.03 MHz during five of the rocket flights and at 1.8 MHz on four others. Satisfactory agreement between observed and calculated values of absorption is obtained for an electron collision frequency model vm = 6.3 × 105p where p is the pressure (in N m−2) from CIRA 1972. In the second part of the paper this electron collision frequency model is used in the calculation of vertical incidence absorption as a function of frequency between 1.7 and 3.0 MHz. The nine electron density profiles from the winter season are used together with five additional profiles from rocket flights in other seasons, all profiles being for a solar zenith angle of about 60°. Examination of the calculated values shows that the absorption is more sensitive to frequency changes than expected particularly for winter days, even at frequencies much less than ƒoE . The occurrence and number of multiple echoes in the ionograms is consistent with the calculated absorption values.

Reflection of radio waves by sporadic-E layers

A full-wave analysis of the reflection coefficient is developed and applied to electron-density profiles of midlatitude sporadic-E layers observed by rocket-borne probes. It is shown that partial reflection from the large electron-density gradients at the upper and lower boundaries of sporadic-E layers does not account for the partial transparency observed by ionosondes.

Positive ion composition and derived particle heating in the lower auroral ionosphere

Two positive ion mass spectrometers of the University of Bern were launched from Kiruna on two Taurus-Orion payloads on 16 November (salvo B) and 30 November (salvo A2) as part of the European Energy Budget Campaign 1980. The payloads also included experiments of the University of Illinois for measurements of electron density and energetic particles. The ionization sources were predominantly precipitating electrons in salvo B and precipitating protons in salvo A2. Positive ion composition is evaluated from the ascent measurements in the altitude range 60–170 km. Altitude profiles of total and partial ion densities are calculated from the mass spectrometer ion currents normalized to electron density measurements at 90 km. Typical characteristics of the positive ion composition under winter-time auroral conditions are large NO+O+2 density ratios, with maximum values of 20 at 118 km in salvo B and 100 at 100 km in salvo A2, respectively. The transition height from dominant proton hydrates H+(H2O)3 and H+(H2O)4 to dominant molecular ions NO+ and O+2 occurred at 79 km in salvo B and 76 km in salvo A2. The measured N + and O+ number densities are generally in good agreement with model calculations. From these calculations the measured 28+ is interpreted mainly as N+2 above 110 km in salvo B and above 105 km in salvo A2, and is Si+ below these altitudes. Electron density and ion composition data are used for the calculation of ion-electron pair production rates in both flights. The energy flux of precipitating particles, derived from the altitude integrated electron-ion pair production, is 0.85 mW m−2 in salvo B and 1.00 mW m−2 in salvo A2.

Energetic electrons in the midlatitude nighttime E-region

Observations of the electron density profile and flux of energetic electrons obtained in two rocket flights at Wallops Island near midnight are presented. The ionization rates of the upper E region deduced from the electron density profiles are found to support the dependence on Kp established in previous observation. Calculations of the ionization rates using the observed electron fluxes show agreement with the values derived from the electron density profiles.

Electron-density irregularities in the day-time equatorial ionosphere

Abstract Electron-density irregularities have been observed in the day-time equatorial ionosphere using probe experiments carried in the payloads of two rockets launched near Lima, Peru, during Project Condor, 1983. Simultaneous observations of the mesosphere and upper E -region were obtained using the radar at Jicamarca. A layer of mesospheric irregularities is identified as originating in neutral atmosphere turbulence. In the electrojet the rocket data show the regions of Type 2 irregularities (between 90 and 105 km) and Type 1 irregularities (between 103 and 108 km), differentiated by the slopes of their respective spectra. The irregularities of the upper E -region seen in the rocket data are reconciled with the radar data by postulating a strong aspect sensitivity.