Xiangji Huang

Deducing topside profiles and total electron content from bottomside ionograms

Abstract A new technique of estimating the ionospheric topside profile from the information contained in the groundbased ionograms is introduced. The electron density profile above the F2 layer peak is approximated by an α-Chapman function with a constant scale height that is derived from the bottomside profile shape near the F2 peak. The scale height is obtained from the bottomside profile by representing the latter in terms of α-Chapman functions with scale heights H(h) that vary as a function of height. The scale height at the layer peak is then used for the topside profile. The total electron content (TEC) is obtained by integrating the electron density from h = 0 to ∞. The ionogram derived TEC values, ITEC, are compared with incoherent scatter radar, Faraday, and TOPEX TEC measurements showing very good agreement at middle latitudes and the magnetic equator.

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.

First results from the Radio Plasma Imager on IMAGE

The Radio Plasma Imager (RPI) is a 3 kHz to 3 MHz radio sounder, incorporating modern digital processing techniques and long electronically-tuned antennas, that is flown to large radial distances into the high-latitude magnetosphere on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Clear echoes, similar to those observed by ionospheric topside sounders, are routinely observed from the polar-cap ionosphere by RPI even when IMAGE is located at geocentric distances up to approximately 5 Earth radii. Using an inversion technique, these echoes have been used to determine electron-density distributions from the polar-cap ionosphere to the location of the IMAGE satellite. Typical echoes from the plasmapause boundary, observed from outside the plasmasphere, are of a diffuse nature indicating persistently irregular structure. Echoes attributed to the cusp and the magnetopause have also been identified, those from the cusp have been identified more often and with greater confidence.

The accuracy of ionogram-derived N(h) profiles

Abstract At the Millstone Hill station the Incoherent Scatter Radar (ISR) and a Digisonde 256 are simultaneously operating. Some characteristic true heights determined by both instruments are compared with each other, possible reasons for observed difference are indicated.

Online data base of satellite sounder and insitu measurements covering two solar cycles

Abstract Accurate descriptions of the solar cycle variations of ionospheric parameters are an important goal of ionospheric modeling. Reliable predictions of these variations are of essential importance for almost all applications of ionospheric models. Unfortunately there are very few global data sources that cover a solar cycle or more. In an effort to expand the solar cycle coverage of data readily available for ionospheric modeling, we have processed a large number of satellite data sets from the sixties, seventies, and early eighties and have made them online accessible as part of NSSDCs ftp archive (http://nssdcftp.gsfc.nasa.gov/spacecraft data/) and its ATMOWeb retrieval and plotting system (http://nssdc.gsfc.nasa.gov/atmoweb/). We report about two data restoration efforts supported through NASAs Applied Information Systems Research Program (AISRP). The first project deals with insitu data from a large number of US, Canadian, Japanese and German satellites that measured ionospheric densities and temperatures from 1964 to 1983. The accumulated data base includes data from the BE-B, DME-A, AE-B, Alouette 2, ISIS 1, 2, OGO-6, AEROS A, AE-C, -D, -E, Hinotori, ISS-b and DE-2 satellite missions. The second project involves the production of digital topside sounder ionograms from the ISIS 1 and 2 satellites and their subsequent inversion to produce electron-density profiles. Approximately 340,000 ionograms are available from NSSDC as of July 2002. An automatic topside ionogram scaler with true height algorithm (TOPIST) was developed as part of this project and is now being used to obtain electron density profiles from these ionograms. Providing global coverage over more than two solar cycles the database established by this two projects is a valuable asset for improvements of the International Reference Ionosphere model and for ionospheric research.

Magnetospheric Active Wave Measurements

The magnetospheric community has conducted active wave measurements, which are made by radiating known signals to space and measuring the dispersion of the signals when propagating through magnetospheric plasma, for more than three decades. Because of the advances in space electronics and signal processing technology, active wave experiments can now cover a large dynamic range in frequency and be made in a large spatial range for versatile applications. They are becoming a promising new technique to probe the space plasma conditions. In this review we demonstrate the capability of magnetospheric sounding technique employed by the radio plasma imager (RPI) on board the IMAGE satellite. This new technique, combined with the mathematical density inversion algorithm, measures the plasma density in situat the satellite location and remotely and instantaneously along the magnetic field line from one hemisphere to the other down to as low as half an Earth radius (Re) in altitude. The technique has been formally validated. The database from the RPI active measurements covers all local times from 1.5 Re to 5 Re under different geomagnetic activities. Empirical models that are being developed specify the density as functions of radial distance, latitude, local time, distance along the field line from the earths surface, solar wind conditions, geomagnetic indices, and other possible variables that affect the density distribution. The models can describe the statistical behaviors of the plasma distribution and can also provide snapshots of the plasma conditions on occasions. Dynamical processes that cause variations from the average models, such as depletion/refilling processes and plasma convection tail formation, can be studied. When applied to multiple satellites, or a constellation of satellites, the active wave measurements also make it possible for magnetospheric tomography, in which transmissions and reception of various waves within the constellation are used to derive the plasma density and magnetic field component in the constellation plane.

Validating ionospheric models with measured electron density profiles

Abstract On behalf of an URSI Working Group 3 initiated study (VIM), three ionospheric models, IRI, PL/PRISM and FLIP, are compared with electron density profiles derived from ionograms Millstone Hill. Four months of data in 1989/90 were analyzed. For most of the time, N(h) profiles were available every 15 minutes providing a good statistical database for the evaluation of the ionospheric models in terms of diurnal and seasonal variations.

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.

IRI in Windows environment

Abstract The International Reference Ionosphere (IRI) is a widely used model recommended for international use by URSI and COSPAR. An interface to Windows has been developed and the stand-alone IRI program is executable on PC systems using Windows 95/98 or Windows NT as the platform. This IRI-Windows version allows computation and plotting of any of the IRI parameters as function of any one or two variables: height, latitude, longitude, UT or LT time, and solar activity. The user can conveniently specify the variables and various options of the model. This paper gives a brief description of the program features and the operating procedure.