Incoherent Scatter Theory Research Articles

Radar observations of thermal plasma oscillations in the ionosphere

AbstractIncoherent scatter radar observations of ionospheric plasmas rely on echoes from electron density fluctuations with properties governed by the dispersion relations for ion acoustic and Langmuir waves. Radar observations of echoes associated with Langmuir waves (plasma lines) from thermal plasma are weak, and only a few near‐thermal level measurements have been reported. Plasma line echoes are typically only observed with existing radars only when the Langmuir waves are enhanced by suprathermal electrons. A new observation technique has been developed which is sensitive enough to allow observations of these echoes without the presence of suprathermal electrons up to at least 1000 km. This paper presents recent observations from the Arecibo Observatory 430 MHz incoherent scatter radar which show plasma line echoes during the night when no suprathermal enhancement is expected to be present. The observations are compared with theory, and the results are found to be in agreement with classical incoherent scatter theory for thermal plasmas. The theoretical ratio of the ion line and plasma line power spectral density is within approximately 3 dB of the predicted value. The finding adds a new observational capability, allowing electron density to also be observed at night using the plasma line well into the top side of the ionosphere, increasing the accuracy of the electron density measurement.

Study on incoherent scatter theory of high density dusty plasma

Incoherent scatter radar is one of the most important detection instruments of the space plasma. But because of the low dust density in natural space plasma, the contribution of charged dust to incoherent scatter spectrum can be completely ignored, therefore the incoherent scattering theory has not appeared in dusty plasma. In the solid rocket plume, the propellant combustion can form a large number of nanometer- and micronmeter-sized dusty particles, and produce a high electron density from high temperature ionization, which makes considerable contributionto charged dusty particles with the high density. Therefore, we develop the incoherent scattering theory of dusty plasma in order to calculate the scattering characteristics of high density dusty plasma produced by rocket plume, for example. The theoretical model including electrons, ions and dusty particles is established by combining effects of charged dusty particles. The incoherent scatter spectral lines of ion resonance region and dust resonance regionare calculated. The effects of dusty particle radius, temperature and density on spectral line structure are discussed. With the increases of dusty particle radius and density, the amplitude of power spectrum increases. With the increase of dust temperature, the amplitude of power spectrum decreases. In the dust resonance region, the control mechanism of dust in spectrum is similar to that of the ions. With the increase of particle size (mass) and decrease of the temperature, the spectrum width narrows, and amplitude and area increase with the increase of density. But in the ion resonance region, the dust control mechanism is completely different, and the influence of the dust on ion line is in the way of attracting ions. So with the increase of dust density, ion line characteristics do not show that the area increases, and dust controls ions by adjusting the Debye radius or electrostatic shielding ball size. By comparing the ion lines with and without dust under the same parameters conditions, the amplitude of the ion line with dust is much larger than that without dust, and the resonance frequency of the ion line is greatly changed. With the dust particles of a relatively high density, one can enhance the ion line, hence the incoherent scattering phenomenon can be more easily observed in rocket plume. On the other hand, due to significant changes of frequency and amplitude in the ion line spectrum, the incoherent scattering inversion method based on the traditional theory will cause a large error in the inversion parameter, even a failure of parameter retrieval. The incoherent scattering theory and relevant physical laws of dusty plasma are presented, which are of great significance for establishing the incoherent scattering theory system and studying the rocket plume parameters.

Ted Harrison 1930-2007

Associate of the RAS, pioneer in ionospheric incoherent scatter theory and observation, practical manager and leader, not least of EISCAT.

Diurnal variability of the gyro resonance line observed with the Arecibo incoherent scatter radar at E‐ and F1‐region altitudes

In this paper, we report on observations of the diurnal variation of a resonance line that occurs near the gyro frequency as seen with the incoherent scatter radar (ISR) at the Arecibo Observatory. This gyro resonance line has rarely been observed in ISR data, but recent results, exemplified by the observations of Bhatt et al. (2006), have implied that the gyro line is a common ionospheric feature. The observations presented here were made with much better altitude resolution in comparison with previous results and represent full 24 hour coverage for several days. The data indicate that an increase in the gyro line intensity near sunrise and sunset is an everyday occurrence. As the increase in intensity is occurring at sunset, the gyro line frequency drops from the expected frequency to one near zero frequency. As the increase in intensity is occurring at sunrise, the gyro line frequency increases from near zero frequency to one near the expected gyro line frequency. This behavior of the gyro line frequency and intensity has not been observed before. We explain these results using normal Maxwellian incoherent scatter theory together with the diurnal variation of the density in the E and F1 regions. These results may explain previous observations of the gyro line made at Arecibo and suggest avenues of future research for studying this exciting feature of the incoherent scatter spectrum.

A new approach in incoherent scatter F region E × B drift measurements at Jicamarca

Since 1996 incoherent scatter F region plasma drift measurements at Jicamarca have been implemented using a new signal processing approach replacing the traditional pulse‐to‐pulse correlation method. The new method, based on Doppler spectrum estimation and nonlinear least squares fitting to model spectra obtained from incoherent scatter theory, improves the instrumental sensitivity remarkably under low signal‐to‐noise conditions. With the new method it has become possible to obtain very high quality drifts data at nearly all hours of the day throughout most F region heights. Altitudinal smoothing of the drifts data to reduce measurement noise is no longer necessary, and studies of the height variations of drifts can be performed with much greater certainty than before. Small‐amplitude gravity wave oscillations have been detected at F region heights and a vortical circulation of the F region plasma has been observed in the post sunset period.

Turbulence scattering layers in the middle‐mesosphere observed by the EISCAT 224‐MHz radar

Spectral measurements of signals scattered from altitudes well below 70 km have for the first time been obtained with the EISCAT 224‐MHz radar. Observations with the antenna pointing away from the vertical at times show spectral power distributions departing markedly from those expected from collision‐dominated incoherent scatter theory. Separated peaks in the spectra occur in regions of abrupt shear in the velocity of the neutral wind within the scattering volume. Anomalously large backscattered power levels are sometimes observed in the vicinity of such shears at altitudes near 68 km. It is unlikely that these echoes are due to incoherent (Thomson) scatter. Instead, it is proposed that the abrupt velocity shears develop neutral turbulence in narrow layers and that these layers cause the enhanced backscatter. Calculations of the expected backscatter power levels at 224 MHz for incoherent scatter and for turbulence scatter show that the turbulence component can be the dominant one at heights below about 68 km if the potential refractive index gradient and the turbulence energy dissipation rates are large enough.

Recent D-region research using incoherent scatter radar

An attempt has been made to review some of the more recent developments in incoherent scatter research relating to the D-region of the ionosphere, and most accent has been placed on the most recent research. As a preamble, the basic relevant incoherent scatter theory is introduced, but without so much mathematics as to confuse the reader. Research has tended to concentrate on three distinct areas: (i) electron density and energy distribution, (ii) ion chemistry and (iii) neutral dynamics, corresponding to the three parameters directly measured by the radars: power, spectral width and bulk Doppler shift respectively. This review is thus divided into corresponding sections.

First observations of summer polar mesospheric backscatter with a 224 MHz radar

Exceptionally strong echoes from the mesopause region have been observed with the EISCAT 224 MHz radar during the summer of 1987. The echoes are so strong that they cannot be explained by incoherent scatter theory but may instead be associated with neutral air turbulence. We discuss the variability of these echoes in terms of time and height distribution, and outline some characteristics of vertical velocity oscillations observed at the echo height.