Gary S. Sales

Plasma density distribution along the magnetospheric field: RPI observations from IMAGE

A new technique is introduced that remotely measures the plasma density profile in the plasmasphere. Radio plasma imager (RPI) echo observations provide echo delay time as function of frequency, from which the plasma density as function of position along the magnetic field line can be calculated. An example from the nightside plasmasphere (L=3) shows the density having its minimum value near the equator and rapidly increasing densities along the field line above 40° magnetic latitude. The density increases at a faster rate toward the ionosphere than the field strength. The index of the power law of the density as a function of field strength increases from a few tenths near the equator to close to unity near 40° and greater than 2 near the ionosphere.

Ionospheric sounding in support of over‐the‐horizon radar

Precise coordinate registration for HF over-the-horizon (OTH) radar applications requires accurate knowledge of the ionospheric structure. In the mid-1980s Digisonde 256 systems were deployed in the American sector to provide this information from strategically located sites via telephone lines to the user. The mid-1990s saw the development of a new advanced system, the Digisonde portable sounder, or DPS, now being deployed in Australia in support of the Australian OTH radar system. A summary of the new features provided by the DPS is as follows: low radio frequency power (300 W); narrow transmission bandwidth; advanced automatic scaling; and control and data access via the Internet. The availability of real-time electron density profiles as function of time from a network of stations makes it possible to calculate the three-dimensional electron density distribution in the region of interest using Fourier transform techniques. The resulting density maps are the basis for the OTH radar coordinate registration. The DPS uses Doppler interferometry to determine the development of ionospheric irregularities.

Spread F and the structure of equatorial ionization depletions in the southern anomaly region

Combined optical and radio sensors provide a unique characterization of the structure of equatorial emission depletion regions connected to rising bubbles over the magnetic equator. In Chile, as part of the MISETA campaign in fall 1994, a CCD-enhanced all-sky imaging photometer provided optical images of the postsunset appearance and motions of the depletion bands at a magnetic dip latitude of 11°S. Concurrently, a Digisonde collocated with the photometer monitored the appearance of spread F. In between the ionograms, the sounder operated as a Doppler interferometer identifying the locations of F layer irregularities associated with the spread F. They were found to lie inside the emission depletion regions. The HF sounder, requiring orthogonality with the field-aligned F layer irregularities to generate the spread structure, tracked these irregularities inside the emission depletion bands as they drifted eastward. Ray tracing simulations show that the radio waves become trapped within the depletion regions when the depletions are within 300 km of the sounder site. Model calculations indicate that the sounder rays encounter orthogonality with the Earths magnetic field within the depletion bubble southward from the site, consistent with the local dip angle. The combination of optical images with HF radio sounding demonstrated that radio imaging in the equatorial ionosphere can be done with a digital ionosonde that operates as a Doppler interferometer. The Digisonde measurements and ray tracing show for the first time that the spread F signatures on ionograms are the result of coherent scatter from irregularities primarily within the walls of the depletion.

Radio sounding in space: magnetosphere and topside ionosphere

Abstract Modern sounding techniques have been developed for the space-borne exploration of Earths magnetosphere and topside ionosphere. Two new satellite instruments will use the advanced techniques of the ground-based Digisondes. The Radio Plasma Imager (RPI), a low-frequency sounder with 500-m dipole antennas designed to sweep from 3 kHz to 3 MHz, will be part of NASAs IMAGE mission to be launched in February 2000 into an elliptical orbit with an altitude at apogee of 7 R e . While in the magnetospheric cavity, RPI will receive echoes from the magnetopause and the plasmasphere and will measure the direct response of the magnetospheres configuration to changes in the solar wind. With three orthogonal dipole antennas (two 500-m tip-to-tip antennas in the spin plane used for transmission and reception, one 20-m antenna along the spin axis for reception only) the arrival angle of returning echoes can be determined with high accuracy. The other instrument is the TOPside Automated Sounder (TOPAS), which was originally conceived for the Ukrainian WARNING mission with a launch date in 2001. Using one antenna for transmission and three orthogonal 10-m antennas for reception, TOPAS will be able to determine the arrival angle of ionospheric echoes and their wave polarization. It will then be possible to automatically scale the topside ionograms and calculate the electron density profiles in real time. Operating as a high-frequency radar, TOPAS will for the first time measure topside plasma velocities by tracking the motions of plasma irregularities.

Equatorial F region irregularity morphology during an equinoctial month at solar minimum

A large number of instruments was used in October 1996 to record activities in the equatorial ionosphere above South America. In a month at solar minimum, data were obtained at various levels of magnetic activity and various levels of ionospheric irregularity development. With this multi-instrumented study, it was possible to utilize optical data, radar, GPS transmissions, and ionosondes at various sites in the equatorial region. The concept of this paper is to review the plethora of events which occurred during this month with a view to describing the interrelationship of the wide variety of irregularity developments. Data were obtained on nights when no irregularities were observed at any location in the equatorial region across South America. There were nights when only localized irregularity structures with relatively narrow latitudinal and longitudinal effects were noted close to the magnetic equator. We noted the occasional presence in the 02–06 local time period of plume structures with data available from optical observations as well as from phase and amplitude scintillation. During a major magnetic storm on one night, October 22–23, a long lasting high altitude plume was detected by the Jicamarca radar. On this night, irregularities were noted all across South America and even beyond the western and eastern coasts. This plume produced ionospheric effects which could be traced to turbulence at over 2000 km above the magnetic equator. With additional data from high latitude stations and from Guam and Kwajelein, it was possible to link and compare irregularity development in the same time period over a large portion of the globe. The aim of this paper is to give a day-to-day picture of the occurrence and intensity of equatorial irregularity development over a month-long period rather than a short case study or the converse, long term statistics over several seasons. Using this database and the modeling of total electron content as a function of solar flux, we outline the possibilities and limitations for forecasting irregularity activity in this region for a period of low solar flux. Forecasting is limited and calls for experimental data for necessary and sufficient gradients and wind conditions for plumes to fully develop.

Radio wave active Doppler imaging of space plasma structures: Arrival angle, wave polarization, and Faraday rotation measurements with the radio plasma imager

Radio sounding in the magnetosphere by the radio plasma imager on the IMAGE spacecraft will determine the dimensions and shape of the cavity between the magnetopause and the plasmapause. Omnidirectional transmission of pulsed radio signals results in echoes arriving from many directions. Quadrature sampling and Doppler analysis of the signals received on three orthogonal antennas will make it possible to determine the angles of arrival of the echoes, their polarization ellipses, and the Faraday rotation. Decomposition of the echo signals into the two characteristic waves is used to identify the O- and X-wave components.

Comparing TID simulations using 3‐D ray tracing and mirror reflection

Measurements of Doppler frequencies and angles of arrival (AoA) of ionospherically reflected HF waves are a means of detecting the occurrence of traveling ionospheric disturbances (TIDs), using the time variations of these measurements. Simulations are made using the Huang and Reinisch [2006] ray tracing technique and the IRI electron density model in an effort to reproduce measured (or simulated) signatures. The TID is represented by a wavelike perturbation of the 3D electron density with an amplitude that varies sinusoidally with time and travels horizontally in the ionosphere in a given direction. By judiciously selecting the TID parameters the raytracing simulation can reproduce observed Doppler frequencies and AoA. Raytracing in a 3D realistic ionosphere is, however, excessively time consuming considering the involved homing procedures. To simplify the procedure we simulated the results for a mirror reflection model [Paznukhov et al., 2012]. The height and tilt of the undisturbed reflection surface are adjusted to agree with assumed or measured AoAs. This undisturbed reflection surface is then deformed into a wavelike moving surface. The rays reflected from the corrugated surface vary with time, and the Doppler frequencies and AoAs are determined. The simulation results from the ray tracing through the IRI-model ionosphere and the mirror model are compared to assess the applicability of the mirror model.

Study of equatorial clutter using observed and simulated long‐range backscatter ionograms

This synoptic study of equatorial clutter used over 6200 long-range (8000 nautical miles, or 14,816 km) backscatter ionograms from the east coast over-the-horizon backscatter (OTH-B) radar system at Bangor, Maine, obtained from 1100 to 0500 LT during the 22-month period from October 1991 to July 1993. Data were collected in five 7.5°-wide azimuthal sectors (radar beams 1-6,1-8, 2-4, 2-8, and 3-5) from 58°T to 170°T. The backscatter clutter signatures can be identified with an equatorial clutter region covering a dip latitude range of ±40°. Equatorial clutter typically begins after sunset and continues through midnight, with a minimum of activity between 1700 and 2200 LT. The clutter range shows some dependence on solar activity, with a reduction in range from the radar from high to moderate solar activity periods. These data did not show any dependence of clutter on operating frequency or on magnetic activity. Synthetic backscatter ionograms were generated using the parameterized ionospheric model (PIM) for the ray trace technique. The analysis shows that the equatorial dome of the ionosphere favors the chordal mode of reflection from the south side (the farther side from the radar) of the dome, which produces horizontal traces as clutter signatures on the backscatter ionograms. The reduction in range at moderate solar activity is associated with the lowering of the mean height of the ionospheric dome. In beams 2-8 and 3-5 the ionospheric dome structure is associated with the equatorial (Appleton) anomaly. This dome structure geometry favors the chordal mode of reflection.

Regional ionospheric mapping

Abstract A new multidimensional mapping technique determines the three dimensional electron density distribution of the ionosphere over an area of several thousand kilometers from the time sequence of sounder data recorded at several locations within the area. Electron density profiles from five Digisonde stations in the Northeastern region of the North American continent are used to construct the electron distribution in a 30° longitude by 30° latitude region. Fourier transforms of the profile time histories over a period of 32 hours from each station determine the spectrum components. For each spectrum component a damped plane wave is fitted to the amplitudes and phases at the five stations.

The effects of powerful oblique radio transmission on the ionosphere on vertical sounding data

Abstract Results of experiments measuring the effects of a powerful oblique HF radio transmission on the ionosphere are described. In one experiment carried out in 1991 on a longitudinal path in the south-west part of the U.S.A., additional traces were found on vertical sounding (VS) ionograms generated by an ionosonde located near the mid-point of the path. Numerical synthesis of VS ionograms using reasonable ionospheric models produce additional traces showing that they can be formed by a large scale inhomogeneity of electron density that is produced by a powerful HF transmitter. This inhomogeniety appears in a region of the caustic wedge. These extra traces can be caused either by enhanced or depleted electron densities, although the characteristics associated with each is different. The negative (depleted) disturbance, stretched along the interior branch of the caustic, gives a more adequate description of the observed effects.