[1] We statistically analyze the ionospheric scale heights in the lower topside ionosphere based on the electron density (N e ) and temperature profiles observed from the incoherent scatter radar (ISR) at Arecibo (293.2°E, 18.3°N), Puerto Rico. In this study, a database containing the Arecibo ISR observations from 1966 to 2002 has been used in order to investigate the diurnal and seasonal variations and solar activity dependences of the vertical scale height (VSH), which is deduced from the electron concentration profiles defined as the value of Àdh/d(ln(N e )), and the effective scale height (H m ), which is defined as the scale height in the Chapman-a function to approximate the N e profiles. As a measure of the slope of the height profiles of the topside electron density, the derived VSH and H m show marked diurnal and seasonal variations and solar activity dependences. Their features are discussed in terms of thermal structures in the lower topside ionosphere. We also investigate the quantitative relationships between H m , VSH, and plasma scale height (H p ) over Arecibo. The similarities and differences in these scale heights are discussed. Results suggest that both the contributions from topside temperature structure and diffusion processes can also greatly control VSH and H m through changing the profile shape.

[1] We present observations of the F-region ionosphere over Arecibo, Puerto Rico (18.34°N, 66.75°W), during the January-February 2008 and January-February 2009 sudden stratospheric warming (SSW) events. For the first period (2008), we have used incoherent scatter radar (ISR) electron density and temperature measurements from the Arecibo Observatory (AO), as well as relative total electron content (TEC) derived from a dual-frequency GPS receiver. For the second event (2009), during which we observed the largest recorded stratospheric warming, we have used the relative GPS TEC. Our analysis indicates that the ionosphere over Arecibo exhibits perturbations after the SSW, the effects are most visible during the daytime. The strongest signatures are observed in the TEC measurements, represented by large enhancements (with respect to non SSW days), particularly during daytime hours. However, the local time dependence of these enhancements is not the same in the two events. In addition, the data show that our results are consistent with the larger than normal daytime vertical drift differences observed at the magnetic equator over Jicamarca. The electron temperature is also affected during the daytime due to changes in electron density, indicating that the electron temperatures is influenced, indirectly, by changes in planetary wave activity in the lower altitudes.Citation: Chau, J. L., N. A. Aponte, E. Cabassa, M. P. Sulzer, L. P. Goncharenko, and S. A. González (2010), Quiet time ionospheric variability over Arecibo during sudden stratospheric warming events,

[1] We present a new algorithm to infer information on the properties of charged meteoric smoke particles (MSPs) from the shape of incoherent scatter radar spectra. We show that in the presence of charged MSPs the spectrum can be approximated as the sum of two Lorentzians. These two distinct spectral lines correspond to two diffusion modes in the D-region plasma, i.e., one due the presence of positive ions and one because of heavy charged MSPs. The widths and amplitudes of these two spectral lines yield information on the radius and number density (the latter only for positively charged particles) of the charged MSPs. We apply this new algorithm to measurements obtained with the 430 MHz ISR at Arecibo and demonstrate that the observed spectra indeed bear the features anticipated in the presence of charged MSPs. Resulting values of retrieved MSP number densities and radii fall well within the range of values expected from models and independent in situ observations. Citation: Strelnikova, I., M. Rapp, S. Raizada, and M. Sulzer (2007), Meteor smoke particle properties derived from Arecibo incoherent scatter radar observations, Geophys. Res. Lett., 34, L15815,

Abstract. Very accurate measurements of electron density can be made at Arecibo Observatory, Puerto Rico, by applying the coded long-pulse (CLP) radar tectmique [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 highfrequency cutoff consistent with calculated values of the Bmnt-V/iis/il/i 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 wave's horizontal wave vector relative to the geomagnetic field. Limited observations with extended altitude coverage indicate that wave imprints can be detected at thennospheric heights as high as 500 kan.