Frank T. Djuth

High‐resolution studies of atmosphere‐ionosphere coupling at Arecibo Observatory, Puerto Rico

Very accurate measurements of electron density can be made at Arecibo Observatory, Puerto Rico, by applying the coded long-pulse (CLP) radar technique [Sulzer, 1986a] to plasma line echoes from daytime photoelectrons [Djuth et al., 1994]. In the lower thermosphere above Arecibo, background neutral waves couple to the ionospheric plasma, typically yielding ∼1–3% electron density “imprints” of the waves. These imprints are present in all observations made to date; they are decisively detected at 30–60 standard deviations above the “noise level” imposed by the measurement technique. Complementary analysis and modeling efforts provide strong evidence that these fluctuations are caused by internal gravity waves. Properties of the neutral waves such as their period and vertical wavelength are closely mirrored by the electron density fluctuations. Frequency spectra of the fluctuations exhibit a high-frequency cutoff consistent with calculated values of the Brunt-Vaisala frequency. Vertical half wavelengths are typically in the range 2–25 km between 115- and 160-km altitude, and the corresponding phase velocities are always directed downward. Some waves have vertical wavelengths short enough to be quenched by kinematic viscosity. In general, the observed electron density imprints are relatively “clean” in that their vertical wavelength spectrum is characteristically narrow-banded. It is estimated that perturbations in the horizontal wind field as small as 2–4 m/s can give rise to the observed electron density fluctuations. However, the required wind speed can be significantly greater depending on the orientation of the neutral waves horizontal wave vector relative to the geomagnetic field. Limited observations with extended altitude coverage indicate that wave imprints can be detected at thermospheric heights as high as 500 km.

Height dependence of the observed spectrum of radar backscatter from HF-induced ionospheric Langmuir turbulence

Observations of the spectrum of 430-MHz radar backscatter from HF-induced Langmuir turbulence with height discrimination are described. During very stable ionospheric conditions under which the height of the below-threshold backscatter spectrum had changed by less than 300 m during a 7-min period, a 20-s-long temporary increase in the HF power from 3 MW ERP to 38 MW equivalent radiated HF power resulted in subsequent strong above-threshold spectra extending to heights up to 1200 m greater than the height of the below-threshold spectrum for more than a minute. The generation of irregularities in the plasma density during the 20 s of enhanced HF power is suggested as a possible cause of this persistence of strong above-threshold spectra at greater heights. The initial temporal evolution of the backscatter spectrum from Langmuir turbulence after the start of HF transmissions was observed for different heights. The observational results are compared with the predictions of existing theories of Langmuir turbulence.

Altitude characteristics of plasma turbulence excited with the Tromsø Superheater

Langmuir/ion turbulence excited with the upgraded high-power (1.2-GW effective radiated power) HF heating facility at Tromso, Norway, has been recently studied with the European Incoherent Scatter VHF and UHF incoherent scatter radars. In this report we focus on the altitudinal development of the turbulence observed at the highest HF power levels available. Quite remarkably, the observed plasma turbulence plunges downward in altitude over timescales of tens of seconds following HF beam turn-on; the bottom altitude is generally reached after ∼30 s. This phenomenon has a well-defined HF power threshold. It is most likely caused by changes in the electron density profile brought about by HF heating of the electron gas. If this is the case, then the heat source must be nonlinearly dependent on HF power. Overall, the characteristics of the Tromso turbulence are quite distinctive when compared to similar high-resolution measurements made at Arecibo Observatory, Puerto Rico. After HF transmissions have been made for tens of seconds at Tromso, billowing altitude structures are often seen, in sharp contrast to layers of turbulence observed at Arecibo.

Response of the Arecibo ionosphere to large HF-induced electron temperature enhancements

Abstract During recent experiments at Arecibo, Puerto Rico, large (1000 - >2500 K) enhancements in electron temperature have been observed when high-power radio waves reflect near the nighttime F region peak. The electron temperature enhancements are accompanied by significant (50 – 200 K) increases in ion temperature, large (10 – 25%) reductions in electron density, and strong HF-induced spread F . When large electron temperature enhancements develop, the ionosphere appears to become dynamically unstable resulting in the production of geomagnetic field-aligned striations having elevated electron temperatures and depleted electron densities. In the present work, a theoretical description of the F region thermal response is provided along with several new experimental results. An interesting aspect of this type of radio wave heating experiment is that it offers the possibility of directly measuring aeronomic parameters that are difficult to determine with other techniques.

Low‐latitude 10 eV electrons: Nighttime plasma line as a new research capability

The incoherent scatter radar (ISR) plasma line (PL) in daylight is excited by photoelectrons. Measurement of its intensity (κTp) has long been used for their study. At night, despite the absence of any other excitation mechanism, the PL intensity should have a thermal amplitude level κTe, determined by the electron gas temperature Te. To the contrary Carlson et al. (1982) found nighttime PLs over Arecibo enhanced >3 times above thermal intensities despite the absence of any known causative mechanism. Here we present discovery that nighttime PLs frequently recur, with quite variable enhancement. In the absence of direct solar EUV, these enhanced PLs must be produced by particle precipitation, manifested by the presence of variable recurring F region ~10 eV electron fluxes. We see this as offering a new tool for space environment studies, opening a new era of particle precipitation research and ISR calibration.

MF/HF/VHF Radar Observations of Polar Mesosphere Summer Echoes (PMSE)

PMSE, or polar mesosphere summer echoes, refers to a uniquely strong radar backscatter target that occurs during the summer season near the high latitude mesopause. The radar echoes are thought to be associated with noctilucent clouds (NLC), the highest clouds over the Earth, due to the many similarities between both phenomena; e.g., altitude near the temperature minimum (mesopause region/85-km), seasonality, geographical location (northern and southern hemispheres), etc. An increased number of NLC sightings over the last century led to the suggestion that they are an indicator of climate change. Since the association of NLC with charged ice particles is now accepted as the source of PMSE, radar is the ideal observational technique to monitor long-term variations in the mesopause region and, possibly, the earths atmospheric temperature. Lower temperatures in the upper mesosphere could be manifested in a larger number of NLC and PMSE events. Although PMSE have been well known since their discovery in the early eighties, the radar mechanism producing the echoes is not yet fully understood. Comparisons of VHF radar and rocket measurements during PMSE showed evidence of both turbulent and non-turbulent scattering mechanisms acting simultaneously or separately in the medium. The majority of the radar observations have been conducted at VHF/50 MHz, the reference sensors employed traditionally for PMSE studies; very few observations have been reported using radars operating at multiple frequencies. In an effort to extract new clues on this intriguing phenomenon, we conducted radar observations of PMSE at six different frequencies: 2.43, 4.53, 4.9, 28, 50, and 139 MHz, using radar facilities over the central Alaskan region. The echo morphology at the different frequencies is described in case studies wherein PMSE events were observed concurrently using at least two radar systems. The identity of MF and HF radar echoes as PMSE is resolved for the first time by means of simultaneous measurements made with VHF radars. On the basis of echo duration and signal strength, we suggest that HF radars would be optimal for PMSE monitoring. MF radars show highly organized PMSE layers quite often but are more susceptible to ionospheric absorption and higher altitude returns associated with geomagnetic activity. However, since a number of MF stations are located at polar or near polar latitudes, including Antarctica, it may be possible to use the PMSE signature studied here to investigate its long term variability as well as its low latitude boundary. The latter could be an indicator of global change and/or of lower temperatures in the Earths mesosphere, a necessary ingredient for NLC formation.

Using radio-induced fluorescence to determine the horizontal structure of ion layers in the mesosphere and lower thermosphere

Two-dimensional images of Sporadic-E layers have been produced using a new technique called radio induced fluorescence (RIF). This technique makes the ion layers glow when being stimulated by high power radio waves. Normally the ion-layers do not radiate visible emissions. Experiments on January 1998 at Arecibo Observatory in Puerto Rico have shown that the layers can be made to glow at 557.7 nm and other wavelengths by illuminating them with radio waves at 3.175 MHz with effective radiated powers of 80 megawatts. The regions of the sporadic-E layers that have electron densities greater than the critical density for reflection of the radio waves emit electrons that collide with and excite atmospheric atomic oxygen and molecular nitrogen. A charge-coupled-device (CCD) imager located on the ground is used to capture images of the glowing E-region structures. The camera exposure times were in the range of 15 to 45 seconds. The images obtained using this technique show a wide variety of structures in the sporadic-E layers. Some layers cover the 15 x 30 km region illuminated by the radio wave beam. Other layers show strong modulation of the E-region by neutral wind instabilities. Two-dimensional computer simulations of the coupling between neutral wind turbulence and the ion layers simulate the structure in the images.