B. J. Watkins

Coordinated optical and radar observations of ionospheric pumping for a frequency pass through the second electron gyroharmonic at HAARP

On 4 February 2005, the High-frequency Active Auroral Research Program (HAARP) facility was operated in O and X mode while pointing into the magnetic zenith to produce artificial optical emissions in the ionospheric F layer. The pump frequency was set to 2.85 MHz to ensure passing through the second electron gyroharmonic of the decaying ionosphere. Optical recordings at 557.7 and 630 nm were performed simultaneously with the side-viewing high frequency (HF) and colocated ultra high frequency (UHF) ionospheric radars. No X-mode effects were found. For O-mode pumping, when passing from below to above the second gyroharmonic frequency, the optical intensity shows a distinct increase when the plasma frequency passes through the second electron gyroharmonic, while the UHF backscatter changes from persistent to overshoot in character. The optical intensity decreases when pump wave reflection ceases, dropping to zero when upper-hybrid resonance ceases. The HF radar backscatter increases when the upper-hybrid resonance frequency passes from below to above the second gyroharmonic frequency. These observations are consistent with the coexistence of the parametric decay and thermal parametric instabilities above the second gyroharmonic. The combined optical and radar data provide evidence that up to three electron-acceleration mechanisms are acting, sometimes simultaneously, depending on the pump frequency relative to the second gyroharmonic. In addition, we provide the first evidence of lower-hybrid waves in HF radar centerline data and show that the parametric decay instability producing Langmuir waves can be stimulated in the magnetic zenith at high latitudes despite the pump wave not reaching the nominal frequency-matching height.

A high‐latitude observation of sporadic sodium and sporadic E‐layer formation

High-resolution radar and lidar measurements of sporadic sodium (Na) and sporadic E (E) layers were made at the Sondrestrom incoherent-scatter radar facility on 11 December 1997. These measurements suggest a causal link between Es and Nas, supporting the proposed mechanism in which Na+ ions in the Es are neutralized to form the Nas. This Nas, by contrast, does not appear to have been formed by the presence of auroral precipitation or ionization, and, in fact, the sodium density is seen to decrease during an auroral event.

Numerical simulation of the formation of thin ionization layers at high latitudes

A one-dimensional simulation of high latitude sporadic-E layers has been developed to investigate the role of electric field in layer formation. The simulation model computes the ion densities (O+, O2+, N+, N2+, NO+, Fe+) and temperatures as a function of altitude. The stationary state momentum equation and continuity equation is solved for each ion species. Then the energy equation is solved for electrons, neutrals, and a generic ion having the mean ion mass and velocity. The model does not include any photoionization or particle precipitation. Chemical production and loss are included for the five standard ion species, but not for the Fe+ ions. An initial altitude profile is assumed for the ions, and the model computes the altitude distribution as a function of time. The horizontal direction of the electric field is important for determining the layer structure and its altitude. An electric field directed between magnetic west and north results in convergent plasma flow and thus layer formation at about 120–130 km altitude. For fields directed to the west and south layers form at lower altitudes(100–115km). With an assumed dip angle of 75° the layers formed by the N-W quadrant fields form quickly (<10min), are quite thin, and remain stationary in altitude. Layers formed by S-W quadrant fields are initially thick but compress in time (15–20 min). Additionally they move down in altitude as they compress. The large perpendicular electric field (50 mV/m) used in the simulation leads to significant heating of the ions and to a smaller extent the electrons and neutrals. The layer formed by the field directed in the N-W quadrant occurs in the region of significant heating, and the temperature profile shows some enhancement at the layer altitude.

Artificial ionospheric layers driven by high‐frequency radiowaves: An assessment

High-power ordinary mode radio waves produce artificial ionization in the F-region ionosphere at the European Incoherent Scatter (EISCAT at Tromso, Norway) and High-frequency Active Auroral Research Program (HAARP at Gakona, Alaska, USA) facilities. We have summarized the features of the excited plasma turbulence and descending layers of freshly-ionized (“artificial”) plasma. The concept of an ionizing wavefront created by accelerated suprathermal electrons appears to be in accordance with the data. The strong Langmuir turbulence (SLT) regime is revealed by the specific spectral features of incoherent radar backscatter and stimulated electromagnetic emissions. Theory predicts that the SLT acceleration is facilitated in the presence of photoelectrons. This agrees with the intensified artificial plasma production and the greater speeds of descent but weaker incoherent radar backscatter in the sunlit ionosphere. Numerical investigation of propagation of O-mode waves and the development of SLT and descending layers have been performed. The greater extent of the SLT region at the magnetic zenith than at vertical appears to make magnetic zenith injections more efficient for electron acceleration and descending layers. At high powers, anomalous absorption is suppressed, leading to the Langmuir and upper hybrid processes during the whole heater-on period. The data suggest that parametric UH interactions mitigate anomalous absorption at heating frequencies far from electron gyroharmonics and also generate SLT in the upper hybrid layer. The persistence of artificial plasma at the terminal altitude depends on how close the heating frequency is to the local gyroharmonic.

Height‐dependent ionospheric variations in the vicinity of nightside poleward expanding aurora after substorm onset

High-latitude ionospheric variations at times near auroral substorms exhibit large temporal variations in both vertical and horizontal extents. Statistical analysis was made of data from the European Incoherent Scatter UHF radar at Tromso, Norway, and International Monitor for Auroral Geomagnetic Effects magnetometer for finding common features in electron density, ion and electron temperatures and relating these to currents and associated heating. This paper particularly focused on the height dependencies. Results show clear evidences of large electric field with corresponding frictional heating and Pedersen currents located just outside the front of the poleward expanding aurora, which typically appeared at the eastside of westward traveling surge. At the beginning of the substorm recovery phase, the ionospheric density had a large peak in the E region and a smaller peak in the F region. This structure was named as C form in this paper based on its shape in the altitude-time plot. The lower altitude density maximum is associated with hard auroral electron precipitation probably during pulsating aurora. We attribute the upper F region density maximum to local ionization by lower energy particle precipitation and/or long-lived plasma that is convected horizontally into the overhead measurement volume from the dayside hemisphere.

MF radar observations of mean winds and tides over Poker Flat, Alaska (65.1° N, 147.5° W)

Abstract. MF radar wind measurements in the mesosphere and lower thermosphere over Poker Flat, Alaska (65.1° N, 147.5° W) are used to study the features of mean winds and solar tides. Continuous observation with the newly installed radar is in progress and in the present study we have analyzed a database of the first 27 months (October 1998–December 2000) of observation. The observed mean wind climatology has been compared with previous measurements and the latest empirical model values (HWM93 model). Similarly, the tidal characteristics are described and compared with the Global Scale Wave Model (GSWM00). The mean wind characteristics observed are fairly consistent with previous wind measurements by the Poker Flat MST radar. The main feature of the zonal circulation is the annual variation with summer westward flow and winter eastward flow. The annual mean zonal wind has a west-ward motion at altitudes below 90 km. The annual mean meridional circulation has mainly southward motion at 70–100 km. There is very good agreement between the radar zonal winds and the HWM93 model winds. Comparison of the meridional winds shows some discrepancy. Analysis of two years of data indicated that the year-to-year consistency is preserved in the mean circulation in the mesosphere. Tidal characteristics observed are also consistent with previous measurements. Semidiurnal tides have the largest amplitudes in summer while the weakest amplitude is observed during the winter months. The vertical wavelength is longer during the summer season compared to the winter season. Comparison with the GSWM00 produces mixed results. There is reasonable agreement between the observed and modeled phases. Diurnal tide amplitudes are comparable in magnitude with that of the semidiurnal tide. Seasonal variation is less evident in the amplitudes. Comparison of the observed tidal parameters with the GSWM00 reveals some agreement and discrepancies. Key words. Meteorology and atmospheric dynamics (climatology; middle atmosphere dynamics; waves and tides)

Seasonal occurrence of thin metallic ion layers at high latitudes

A year-long series of incoherent-scatter radar observations of the high-latitude ionosphere conducted at Sondrestrom in Greenland has revealed an apparent increase in the rate of occurrence of thin metallic ion layers within the E-region during the summer months, consistent with the known seasonal effect existing at middle latitudes. Unlike the situation at temperate latitudes, where shears in the neutral wind drive layer formation, layers at high latitudes are often formed by the action of large-scale electric fields of magnetospheric origin. It is suggested here that a summertime increase in layer occurrence at high latitudes is attributable to a seasonal variation in the structure of the convective electric field, which determines the regions in which the necessary conditions for layer formation are present.

Artificial ionosphere layers for pumping-wave frequencies near the fourth electron gyroharmonic in experiments at the HAARP facility

In this paper we consider the action (in the magnetic-zenith direction) of powerful high frequency (HF) radiation of ordinary polarization on the ionosphere F region. We deal with frequencies f0 > 4fce (fce is the electron cyclotron frequency) of 1.7 GW equivalent radiated power. This action results in the appearance in the ionosphere of an artificial ionization layer. The layer descends with respect to the basic (unperturbed) layer at a rate of ∼500 m s−1 down to the altitude, where f0 ≈ 4fce.

Determination of auroral heat fluxes and thermal ion outflows using a numerical ionospheric model and incoherent-scatter radar data

A comprehensive one-dimensional model of the polar ionosphere has been used in conjunction with incoherent-scatter radar data from Sondrestrom, Greenland, to determine downward heat fluxes and thermal ion outflows at very high latitudes. For periods of very quiet geomagnetic activity the model closely simulates the observed time-dependent behavior of the electron density, ion and electron temperatures. To obtain this similarity between model and data, the upper boundary conditions of the model, namely downward heat flux, and magnetic field-aligned ion flows, are continually adjusted with time to provide a best fit with data. The heat fluxes and ion flows are determined indirectly from this fitting procedure. The technique has been applied to a 10-hour daytime data set for February 12, 1990, to search for enhanced downward heat fluxes and outward thermal ion fluxes associated with dayside auroral oval. Variations of heat flux ranged from about 2×109 to 2×1010 eV cm−2 s−1, and vertical outward fluxes of ionization ranged from about zero to 8×108 cm−2 s−1. For both quantities the peak values occurred when the radar site was located under the dayside auroral oval. It is suggested that these marked upward thermal ion flows in the dayside auroral ionosphere may be associated with energetic O+ ion outflows that have been observed at high altitudes with spacecraft.

Lower thermospheric wind variations in auroral patches during the substorm recovery phase

Measurements of the lower thermospheric wind with a Fabry-Perot interferometer (FPI) at Tromso, Norway, found the largest wind variations in a night during the appearance of auroral patches at the substorm recovery phase. Taking into account magnetospheric substorm evolution of plasma energy accumulation and release, the largest wind amplitude at the recovery phase is a fascinating result. The results are the first detailed investigation of the magnetosphere-ionosphere-thermosphere coupled system at the substorm recovery phase using comprehensive data sets of solar wind, geomagnetic field, auroral pattern, and FPI-derived wind. This study used three events in November 2010 and January 2012, particularly focusing on the wind signatures associated with the auroral morphology, and found three specific features: (1) wind fluctuations that were isolated at the edge and/or in the darker area of an auroral patch with the largest vertical amplitude up to about 20 m/s and with the longest oscillation period about 10 min, (2) when the convection electric field was smaller than 15 mV/m, and (3) wind fluctuations that were accompanied by pulsating aurora. This approach suggests that the energy dissipation to produce the wind fluctuations is localized in the auroral pattern. Effects of the altitudinal variation in the volume emission rate were investigated to evaluate the instrumental artifact due to vertical wind shear. The small electric field values suggest weak contributions of the Joule heating and Lorentz force processes in wind fluctuations. Other unknown mechanisms may play a principal role at the recovery phase.