Christopher T. Fallen

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.

First artificial periodic inhomogeneity experiments at HAARP

Experiments involving the generation and detection of artificial periodic inhomogeneities have been performed at the High Frequency Active Auroral Research Program (HAARP) facility. Irregularities were created using powerful X-mode HF emissions and then probed using short (10 μs) X- and O-mode pulses. Reception was performed using a portable software-defined receiver together with the crossed rhombic antenna from the local ionosonde. Echoes were observed reliably between about 85 and 140 km altitude with signal-to-noise ratios as high as about 30 dB. The Doppler shift of the echoes can be associated with the vertical neutral wind in this altitude range. Small but persistent Doppler shifts were observed. The decay time constant of the echoes is meanwhile indicative of the ambipolar diffusion coefficient which depends on the plasma temperature, composition, and neutral gas density. The measured time constants appear to be consistent with theoretical expectations and imply a methodology for measuring neutral density profiles. The significance of thermospheric vertical neutral wind and density measurements which are difficult to obtain using ground-based instruments by other means is discussed.

Calculations of 4278 Å artificial auroral airglow emissions resulting from powerful HF radio transmissions

Powerful HF electromagnetic waves transmitted into the ionosphere from the High-frequency Active Auroral Research Program (HAARP) facility in Alaska and the European Incoherent Scatter Scientific Association (EISCAT) facility in Norway have induced artificial aurora with ground-measurable 4278 Å wavelength emissions (B. Gustavsson et al., Ann. Geophys., 23, 1747-1754, 2005; T. B. Pedersen et al., Geophys. Res. Lett. 37, 2, L02106, 2010). This artificial “blue-line” emission can result from the electronic transition of an N2+(1N) ion to its ground state. The two main sources of N2+(1N) ions in the F region are photoionization and electron-impact ionization of N2 molecules. Experimental and theoretical evidence suggests that impact ionization of N2 molecules by electrons accelerated by wave-plasma interactions to energies exceeding 18 eV is responsible at least in part for artificial 4278 Å wavelength emissions observed during ionosphere radio modification experiments at HAARP and EISCAT (H. C. Carlson et al., J. Atmos. Solar Terr. Phys., 44, 12, 1089-1100, 1982; A. V. Gurevich et al., J. Atmos. Terr. Phys., 47, 11, 1057-1070, 1985).