Yu Morton

Observations of ion layer motions during the AIDA campaign

Abstract The AIDA-89 campaign has yielded the most comprehensive set of low-latitude incoherent scatter radar power profiles and derived electron concentration results ever made. These results have been used to study the time-height trajectories of 80–150 km ion layers and serve to gauge both the periodicity and variability of ion layer structure throughout the campaign. Features of the AIDA ion layer trajectories point to a dynamics ‘zoo’ of processes ranging from multiday-period waves, tides and acoustic-gravity waves (AGWs) to geomagnetic storm effects and evidence of coupled neutral sodium and ion layer/plasma processes. The semidiurnal and diurnal tides are evidenced in the almost always present layers, the Tidal Ion Layers (TILs), which are identified by their regular and periodic trajectories that also display regions of variable mixing or confluence of the various tides. The TILs are contrasted with the truly sporadic layers that include sporadic E and sporadic intermediate layers. The sporadic layers may be formed due to interaction of the tidal wind system with AGWs. The formation process may involve horizontal as well as vertical ion convergence mechanisms and/or various non-linear effects. Limits to the study derive from volume undersampling due to use of the single radar beam.

Feasibility study of a multi-carrier dual-use imaging radar and communication system

The paper describes a modelling scenario aimed at exploring high-resolution imaging, communication and in-band coexistence with other signals potential of a multi-carrier radar/communication system based on ultra-wideband waveforms obtained via orthogonal frequency division multiplexing (OFDM). Imaging performance is simulated for a well-studied scenario of stripmap SAR, for which backprojection algorithm is used in image reconstruction. The case of two airborne platforms is considered and image data communication between the platforms is modelled. This feasibility study lays foundation for subsequent analysis of a multi-platform sensor network scenario based on novel ultra-wideband radar architectures.

GLOBAL MESOSPHERIC TIDAL WINDS OBSERVED BY THE HIGH RESOLUTION DOPPLER IMAGER ON BOARD THE UPPER ATMOSPHERE RESEARCH SATELLITE

This paper presents preliminary results of a study of mesospheric and lower thermosphedc diurnal tidal winds obtained with the High Resolution Doppler Imager (HI) on the Upper Atmosphere Research Satellite (UARS). Zonal averages of the meddional and zonal wind fields are performed at fixed local times. These analyses reveal tidal structures in the mean meridional wind field during the months of February and March 1992. Results are presented for a typical day in February 1992. The observed maximum amplitude of meridional wind is approximately 75 m s -1. The observed vertical wavelength is about 20 km, with amplitudes increasing linearly with height. In addition to the tides, the zonal wind field reveals many features of the mesospheric

Further evidence for a 6-h tide above Arecibo

Abstract Since the publication of results suggesting the existence of a 6-h tide in the E region above Arecibo [( Tong et al ., 1988) J. geophys. Res. 93 , 10047–10051], much more data has been collected and analyzed. In particular, the time-height trajectories of middle and upper E region Tidal Ion Layers (TILs) for early January 1989 closely resemble those from early January 1981, which first revealed the presence of a 6-h quasi-periodic intermediate layer structure. Further, the January 1989 observations form an ‘overture’ to the March–May 1989 AIDA (Arecibo Initiative in Dynamics of the Atmosphere) campaign, which yielded a total of 28 days of additional data regarding TIL motion. Interestingly, the AIDA data set is dominated, above about 120 km altitude, by sporadic intermediate layers [( mathews et al ., 1993) J. atmos. terr. Phys. 55 , 447–457] and certainly does not show the consistent 6-h period TIL feature seen in the two January data sets. In reviewing all data collected over the past 10 yr and the extensive 1989 observations in particular, we conclude that the basic TIL structure is controlled by two separate tidal wind patterns. We refer to these as the normal pattern and the ‘deep-winter’ pattern. The normal pattern includes the combination of diurnal and semidiurnal tides, while the deep-winter pattern has an additional 6-h tidal component. The deep winter pattern remains unexplained, but we suggest that the 6-h periodicity, which appears to be phase locked with the semidiurnal tide, is generated via in situ non-linear frequency doubling of the semidiurnal tide. The January 1989 results also manifest a TIL structure, below 100 km altitude, which has not been previously reported.

Observations of ion and sodium layer coupled processes during AIDA

Abstract Extended measurements of the neutral sodium layer using lidar and of the lower ionosphere using Incoherent Scatter Radar were conducted at Arecibo during the spring 1989 AIDA campaign. We present results from four nights of these rather extensive, almost common volume, simultaneous observations of the 80–110 km altitude region. Although in each set of results there are unique features that appear to be associated with the diverse dynamics of the atmosphere, the general nocturnal motion of both the sodium and ion layers is controlled by the tidal wind. Additionally, there is often a very strong correlation between the presence of narrow ionization and narrow sodium layers below 100 km altitude. These sporadic E and sporadic sodium layers usually occur at the same heights and with a sometimes striking correspondence in the details of vertical and temporal structure. These details are seen most clearly in a series of time-height plots of sodium concentration over which are overlaid the corresponding time-height trajectories of the ionization layers. Included in this series of plots are the two most spectacular of the layering events observed during AIDA. The most intense of the events occurred on the night of 30–31 March and was characterized by ion and sodium layers, with about 6 km vertical extent, that formed near 95 km altitude. During this event the sodium column content was observed to increase by a factor of about 10, from 1.5–2.0 × 10 9 to 20 × 10 9 cm 2 , in 90 min. The second of the major events occurred on the night of 5–6 April and was characterized by a strong vertical oscillation of the sodium layer with a peak-to-peak amplitude of 2–3 km and a period of 10.4 min. For contrast with the two major events and because of the completeness of the data sets, results from the nights of 4–5 April and 7–8 April are also given.

Comparison of HRDI wind measurements with radar and rocket observations

Wind fields in the mesosphere and lower thermosphere are obtained with the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) by observing the Doppler shifts of emission lines in the O2 Atmospheric band. The validity of the measured winds depends on an accurate knowledge of the positions on the detector of the observed lines in the absence of a wind induced Doppler shift. These positions have been determined to an accuracy of approximately 5 ms−1 from the comparison of winds measured by HRDI with those obtained by MF radars. Excellent agreement is found between HRDI measured winds and winds observed with radars and rockets. In addition, the sensitivity of HRDI to migrating tides and other large scale waves is demonstrated.

Comparison of the effect of high‐latitude and equatorial ionospheric scintillation on GPS signals during the maximum of solar cycle 24

Radio signal scintillation caused by electron density irregularities in the ionosphere affects the accuracy and integrity of Global Navigation Satellite Systems, especially in the equatorial and high-latitude regions during solar maxima. Scintillation in these two regions, nevertheless, is usually influenced by different factors and thus has different characteristics that cause different effects on GNSS signals. This paper compares the characteristics of high-latitude and equatorial scintillation using multifrequency GPS scintillation data collected at Gakona, Alaska, Jicamarca, Peru, and Ascension Island during the 24th solar maximum. Several statistical distributions are established based on the data to characterize the intensity, duration, and occurrence frequency of scintillation. Results show that scintillation in the equatorial region is generally more severe and longer lasting, while high-latitude scintillation is, in general, more moderate and usually dominated by phase fluctuations. Results also reveal the different impacts of solar activity, geomagnetic activity, and seasons on scintillation in different geographic locations.

Gravity Wave Propagation in a Nonisothermal Atmosphere with height Varying Background Wind

We derive a gravity wave propagation equation for a compressible and non-isothermal atmosphere with a variable background wind profile. Impact of all the gradient terms on the vertical wavenumber depends only on the intrinsic horizontal phase velocity and the background atmosphere. For the background wind variation, any one of the linear first order derivative, second order derivative and the square of the first order derivative terms can be the dominant term under different conditions. For temperature variation, only the linear first order derivative is important for waves having a slow intrinsic horizontal phase velocity. Our equation indicates that the effect of wind shear on the vertical wavenumber is opposite to that predicted by the Taylor-Goldstein equation, which assumes an incompressible fluid. We also derive an expression for the amplitude of the vertical wind perturbation. Citation: Zhou, Q., and Y. T. Morton (2007), Gravity wave propagation in a nonisothermal atmosphere with height varying background wind, Geophys. Res. Lett., 34, L23803, doi:10.1029/2007GL031061.

GPS Civil Signal Self-Interference Mitigation During Weak Signal Acquisition

Current global positioning system receivers can acquire weak satellite signals with if there is C/NO = 15 dB/Hz if there is no self-interference from other strong satellite signals. This correspondence presents a computational efficient partitioned subspace projection method to mitigate the self-interference. The method is evaluated using simulated signals and a block-based weak signal acquisition algorithm.

Comparative Studies of GPS Multipath Mitigation Methods Performance

Coupled amplitude delay lock loops (CADLL) is a recently proposed multipath estimation and mitigation technique based on joint estimation of line-of-sight (LOS) and multipath signal amplitude, code phase, and carrier phase. The CADLL performance is evaluated against two widely known multipath mitigation methods: the high-resolution correlator (HRC), representative of the correlators combination methods, and the multipath mitigation technique (MMT), representative of multipath estimation methods. Multiple tests emulating various scenarios are performed to demonstrate that CADLL always generates better results than the other two methods. Additionally, CADLL has better noise performance, can estimate multipath signals using shorter integration time, and is capable of tracking dynamic multipath signals. Simulation tests using a statistical urban multipath signal model prove that CADLL is effective in estimating and mitigating multipath in severe multipath environments. These simulation results are further validated using satellite signals generated by Spirent Global Navigation Satellite System (GNSS) 6700.