Incoherent Scatter Radar Technique Research Articles

Refraction and Faraday rotation in the incoherent scatter radar technique

AbstractIn this paper we consider the radiolocation equation for the incoherent scatter radar (ISR) method; the equation accounts for refraction effect and Faraday rotation in the VHF frequency domain. Our research shows that we cannot ignore refraction in processing the incoherent scatter data when lag‐product is used to obtain the ionospheric characteristics. Effect of refraction can explain some uncertainties that arise during processing and interpretation of the ISR data. This effect shows that together with the “ion line” of incoherent scatter spectrum, a relatively small “electron line” also plays a significant role in forming the ionosphere response. The “electron line” behaves as an additional “radar induced” wideband radiolocation signal affected by refraction and Faraday rotation; it depends on the ionosphere temperature and ion composition. The presence of a wideband “electron line” produces specific distortions of the ISR spectrum during 3D‐forming of the ionosphere response inside the ISR antenna diagram pattern. During testing, we found that the signal from the ISR “electron line” can be measured at much higher altitudes comparing to the “ion line.” Hence, to improve the incoherent scatter technique, we can take the “electron line” and refraction into account.

A multi-beam incoherent scatter radar technique for the estimation of ionospheric electron density and [formula omitted] profiles at Jicamarca

A multi-beam incoherent scatter radar technique has been developed at the Jicamarca Radio Observatory in order to maximize the number of ionospheric parameters that can be estimated simultaneously. The technique interleaves radar observations with antenna beams pointing perpendicular and oblique to the Earth's magnetic field. For the estimation of the ionospheric parameters, we have modeled the magnetic aspect angle variations of the signal power and cross-correlation data measured in multiple directions. The data model, formulated in terms of soft-target radar equations, was built based on the theories of incoherent scattering and magneto-ionic propagation. Applying a nonlinear least-squares inversion algorithm, we have succeeded in measuring simultaneously ionospheric electron densities Ne, electron-to-ion temperature ratios Te/Ti as well as vertical and zonal plasma drifts. In the past, radar users had to choose between either perpendicular-to-B or oblique modes, thus, the application of this technique extends the current capabilities of the Jicamarca radar.

Comparison of dayside magnetic separatrix signatures in HF and incoherent scatter radar data

Both high‐frequency (HF) and incoherent scatter (IS) radar have previously been used to identify the open‐closed magnetic field separatrix and to measure the magnetospheric reconnection rate. The HF radar signature of open magnetic field on the dayside is a high spectral width in the backscatter Doppler spectrum. The IS radar signature of closed field lines is the presence of ionization indicating boundary plasma sheet precipitation. We investigate the consistency of these different signatures of the separatrix and show that the consistency depends on the local reconnection electric field. The two signatures yield a consistent identification of the separatrix at local times where the reconnection electric field is low. In the merging gap, however, where the reconnection electric field is high, the signature in the HF radar signature moves several degrees equatorward of the signature in the IS radar. These observations are backed up by DMSP precipitating particle observations, which indicate that the IS radar technique is properly identifying the poleward edge of the dayside BPS precipitation but also that the trapping boundary of magnetospheric ions and the electron edge occur at a latitude consistent with the HF radar signature of the separatrix. These observations are consistent with the theory that a portion of the dayside BPS is on open magnetic field lines. The latitudinal width of the dayside BPS on open field lines is given by ΔΛ = 0.14°Erec[mV/m]. We conclude that the HF radar signature is the more accurate identifier of the magnetic separatrix in the region of active reconnection.

Incoherent scatter radar measurement of vertical winds in the mesosphere

We report a significant improvement in the measurement of mesospheric winds using the incoherent scatter radar technique. With a height resolution of 600 m and a time resolution of 1 min, we show that the accuracy of the vertical wind measured by the Arecibo incoherent scatter radar is about 1 m/s in the altitude range of 68–88 km. The observed vertical wind amplitude was of the order of a few meters per second. The wind variation was dominated by gravity waves having an apparent period of 34 min below 80 km and 13.8 min above 80 km. The vertical wavelength of the latter gravity wave was 36 km, which was more than twice that of the former. The much improved accuracy was achieved by changing the radar inter‐pulse‐period from 1 ms to 4 ms. This simple change shortens the integration time by about two orders of magnitude comparing with the scheme used in the past. The improved efficiency ought to provide new research opportunities in the D‐region using the incoherent scatter radar technique.

Dynamical effects of geomagnetic storms and substorms in the middle‐latitude ionosphere: An observational campaign

An observational campaign was conducted in October 1992 for ∼36 hours, at three high‐ to low‐latitude sites near 75°W longitude (Sondre Stromfjord, Millstone Hill, and Arecibo). Vector plasma drift velocities are obtained using the incoherent scatter radar technique at each site. Neutral winds were measured using a Fabry‐Perot interferometer, and 6300 Å airglow structures were imaged at the midlatitude site. Electric fields and meridional winds for the period were perturbed when magnetic storms and substorms occurred on the day and night of the campaign. The penetration of magnetospheric electric field and the following interplays between ionospheric electrodynamics and thermospheric wind perturbations in the midlatitude ionosphere are assessed using the multidiagnostic measurements. Evidence for traveling atmospheric disturbances (TADs) and large‐scale gravity waves induced by auroral heating effects upon the thermosphere is identified. Diffuse aurora and a stable aurora red (SAR) arc were observed from Millstone Hill during the night of the campaign. The SAR arc moved southward when there were westward electric field perturbations, indicating plasmasphere compression in the postmidnight sector under substorm conditions. The SAR arc location was used to infer the motion of the magnetospheric shielding layer past the Millstone Hill site. Ionospheric F region disturbances in hmF2, NmF2, and total electron content were driven by the observed dynamics, exhibiting a complex mix of wind and electric field perturbations. While standard model episodes of penetration and shielding/overshielding occurred during the daytime event, such unambiguous clarifications were far less obvious during the nighttime event. This is perhaps due to the prolonged period of moderate geomagnetic activity that served as the background conditions for the substorms that occurred during the campaign.

Outstanding student paper awards

Ten students received Outstanding Student Paper Awards for Solar Physics and Aeronomy Section papers they presented last December at AGU's Fall Meeting in San Francisco, Calif.Peter Sultan, who was honored for his presentation 'Zonal Drift Velocities of Equatorial Spread F Irregularity Plumes,' is a doctoral student at Boston University. He received an Sc.B. in physics from Brown University in 1986 and an M.A. in astronomy and physics from Boston University in 1989. His research interests are equatorial ionospheric physics, active chemical release experiments, and incoherent scatter radar techniques. His dissertation work is a study of artificially triggered equatorial spread‐F, centering on the 1990 CRRES‐at‐Kwajalein sounding rocket campaign.

Electrodynamic properties of auroral surges

The incoherent scatter radar technique provides an excellent means to study the ionization and electric fields associated with auroral precipitation events. One of the most intense and dynamic auroral events is the so‐called surge or breakup aurora that accompanies auroral substorms. For our purposes we define a surge as a transient intensification of auroral precipitation that occurs simultaneously with a pronounced negative bay in the ground magnetometer data. We present data obtained during five such events in 1980 and 1981. Measurements made by the Chatanika radar are combined with simultaneous data obtained from ground‐based magnetometers and all‐sky cameras and space‐based instruments on the Dynamics Explorer satellites. The combined data sets yield information about the auroral forms, electric fields, and currents that are associated with surges. Prior to the surge, auroral forms move equatorward, develop ray structure, and intensify. The surge is identified by an apparent poleward motion of the aurora producing aurorally associated ionization that extends over several hundred kilometers in latitude. The presurge auroral forms are embedded in a region of northward electric field. The auroral forms that comprise the surge span a region within which the meridional electric field is small and at times southward. A westward electric field is often but not always present within the surge. The behavior of the westward electric field is significantly different from the north‐south field, in that sharp spatial gradients are absent even in very disturbed conditions. Although the westward Hall currents are mostly responsible for the negative bays that accompany the surge, at times the westward Pedersen current sustained by the westward electric field can be important. Sudden variations in the H component of the ground magnetogram can be caused by motions of the aurora or by temporal variations in the fields or conductivities. We present a model that simulates the observed changes in electric field and precipitation that accompany surges. The perturbation in the electric field produced by the surge is simulated by adding negative potential in regions of intense precipitation.

The contributions of the incoherent scatter radar technique to the development of thermospheric models

The contributions of the incoherent scatter radar technique to the development of thermospheric models is traced from the development of the radar technique in 1958. Emphasis is given to the qualitative information regarding thermospheric processes and behavior provided by the radar studies and the role that these results have played in the structural development of thermospheric models. The complementary nature of radar and satellite data in the development of recent models is stressed. In addition, a summary of pertinent radar developments over the past two years is presented. Primary among these are measurements made by high-latitude radars, multi-radar coordinated observations, the establishment of an incoherent scatter radar data base, and publication of a number of studies concerning high-latitude measurements, solar-cyle maximum results, long-term data sets, and lower thermosphere tides.

Incoherent Scatter Radar User Workshop

The incoherent scatter radar technique has developed over the years into one of the most powerful tools for investigating physical processes in the upper atmosphere. The National Science Foundation (NSF) now supports a chain of four incoherent scatter facilities at Sondrestromfjord (Greenland), Millstone Hill (Massachusetts), Arecibo (Puerto Rico), and Jicamarca (PERU). Six European nations support the EISCAT facility in northern Scandinavia, and France also has a radar at St. Santin. Recently, the organizations reponsible for each of the six radars agreed to participate in a centralized data base being established at the National Center for Atmospheric Research (NCAR) to make their data more readily accessible to the scientific community at large.

Incoherent scatter observations of an artificially modified ionosphere

The launch of NASA's HEAO‐C satellite by an Atlas/Centaur rocket on September 20, 1979, provided the first “experiment of opportunity” to test incoherent scatter radar techniques for the diagnostic study of a chemically‐induced modification of the ionosphere. The cause of the disturbance was the rocket's exhaust cloud of H2 and H2O molecules that cause a rapid recombination of the F region plasma at heights above 250 km. The launch from the Kennedy Space Center (28°N, 81°W) was monitored by the incoherent scatter radar at Millstone Hill (42.6°N, 71.5°W) using low elevation angle observations over ranges exceeding 2000 km. The experiment employed various pulse lengths, integration times and scanning modes to explore optimization methods for an upcoming series of Space Shuttle induced ionospheric hole experiments. The results from the HEAO‐hole campaign include the first unambiguous observations of a gas expansion “snowplow effect,” the derivation of local and height‐integrated plasma recombination rates, and the full spatial, temporal, and dynamical morphologies of a large‐scale ionospheric hole. The campaign succeeded in demonstrating that incoherent scatter radars can play a primary role in space plasma physics “active experiments” during the next decade.

Measurement of electric fields in the ionosphere by incoherent scatter radar techniques

Various methods for the extraction of electric fields from incoherent scatter radar observations are discussed. The random errors in electric field determinations are derived and it is shown that under typical conditions observatories such as EISCAT should measure electric fields with an r.m.s. error of about 1 mv/m with one minute integration time. Some preliminary EISCAT results are shown and compared with other observations.

Observations of the Ca+ twilight airglow from intermediate layers of ionization

Application of complementary optical and incoherent scatter radar techniques has detected the presence of Ca+ in lower thermospheric intermediate layers over Arecibo. The altitude distribution and density of the calcium ion is inferred from the variation of twilight resonant scattering with solar depression angle while the Arecibo 430Mhz radar measured electron densities. Comparison between the Ca+ and electron column densities indicates that the composition of low‐altitude intermediate layers was 2% Ca+. This value is consistent with both rocket mass spectrometer measurements and with the relative concentration of calcium expected for meteoric debris. These results have implications regarding both possible nighttime tropical E region ionization sources and metal ion circulation in the tropical thermosphere.

Incoherent scatter contributions to studies of the dynamics of the lower thermosphere

This paper reviews what has recently been learned by means of the incoherent scatter radar technique concerning the dynamics of the lower thermosphere (100–150 km). In this region, thermal tides generated in situ and propagating upward from below appear to be the principal source of motions at low and middle latitudes. The winds that they establish can drive ions across magnetic field lines, thereby establishing electric currents and polarization electric fields (which in turn modify the motions of the ions). Efforts to understand this region have traditionally been made through calculations of the tidal amplitudes, from studies of magnetometer records, and from vertical wind profiles observed via chemical releases from rockets. The direct measurements of the temperatures, winds, and electric fields made possible by means of incoherent scatter radar probing have confirmed some early ideas and also have revealed considerably more complexity than had been anticipated. Presently available evidence indicates that the interval 100– 125 km is dominated by upward propagating solar semidiurnal tidal modes (e.g., (2, 4) and (2, 5)) generated in the mesosphere where background winds couple energy out of the (2, 2) mode, which is the mode most strongly excited in the stratosphere‐mesosphere. These higher‐order modes appear to be damped above 120 km by viscous dissipation. In the region 125–150 km the (1, −2) trapped diurnal mode generated by direct absorption of solar EUV seems to be the predominant one at midlatitudes, although some of the energy remaining in the semidiurnal (2, 2) mode appears to leak up to these heights. Nearer the equator it appears that the (2, 2) tide dominates the region below 200–300 km, particularly in summer. These tides create winds that appear capable of explaining a variety of related phenomena including the Sq (solar quiet) dynamo current system, the generation of sporadic E layers and layers intermediate between the E and F layers, and the drift of ions in directions normal to the magnetic field at F region heights (i.e., electric field induced drifts) seen within the plasmasphere during the day.

Modulation langmuir probe and incoherent scatter radar measurements of ionospheric electron temperature

Results are presented from a comparative study of ionospheric electron temperatures measured by the modulation Langmuir probes on the satellites Explorer 31 and ESRO-IA, and various incoherent scatter radar stations. Agreement is found between the two techniques to within the quoted errors. The discrepancy between slow-sweep Langmuir probe and incoherent scatter radar techniques is discussed in the light of these new results.

The Study of E‐Region Ion Concentration and Composition by Incoherent Scatter Radar

This paper reviews the use of the incoherent scatter radar technique for studies of the E‐region ion concentration and composition. A number of existing incoherent scatter radars are capable of securing profiles of ion concentration in the E region with a height resolution of 1 or 2 km by integrating the signals for several minutes. The advantage of these measurements is the ease with which a continuous time history may be secured. The height resolution is probably open to improvement by using coded pulses, but this remains to be attempted. The technique is of less value in studying the ion composition; and to date, only the relative abundance of atomic (O+) to molecular (principally NO+ and O2+) ions has been deduced from incoherent‐scatter measurements. Even here it has usually been necessary to adopt a model for the ion temperature as a function of altitude. Nevertheless, these composition measurements clearly show a seasonal variation of the O/N2 ratio in the thermosphere which is believed to be responsible for the seasonal anomaly in F‐region density. Recently, the accuracy of the measurements and the data reduction procedures have been improved to the point where the relative ion abundance can be determined over the altitude interval 120–200 km, given only that this be a continuous smooth function.