Incoherent Scatter Radar Data Research Articles

Medium-scale 4-D ionospheric tomography using a dense GPS network

Abstract. The ionosphere above Scandinavia in December 2006 is successfully imaged by 4-dimensional tomography using the software package MIDAS from the University of Bath. The method concentrates on medium-scale structures: between 100 km and 2000 km in horizontal size. The input consists of TEC measurements from the dense GPS network Geotrim in Finland. In order to ensure sufficient vertical resolution of the result, EISCAT incoherent scatter radar data from Tromsø are used as additional input to provide the vertical profile information. The TEC offset of the measurements is unknown, but the inversion procedure is able to determine this automatically. This auto-calibration is shown to work well. Comparisons with EISCAT radar results and with occultation results show that the inversion using EISCAT data for profile information is much better able to resolve vertical profiles of irregular structures than the inversion using built-in profiles. Still, with either method the intensities of irregular structures of sizes near the resolution (about 100 km horizontal size) can be underestimated. Also, the accuracy of the inversion worsens above areas where no receivers are available. The ionosphere over Scandinavia in December 2006 often showed a dense E-layer in early morning hours, which generally disappeared during midday when a dense F-layer was present. On 14 December, a strong coronal mass ejection occurred, and many intense irregularities appeared in the ionosphere, which extended to high altitudes.

Spectral enhancements associated with Pi1B events observed at high latitude

Spectral enhancements observed in conjunction with Pi1B by a ground‐based search‐coil magnetometer located at Longyearbyen in Svalbard, Norway, are examined as evidence of resonant structures in the ionosphere excited by broadband ultra low frequency geomagnetic micropulsations. We examine the morphology of spectral enhancements in dynamic Fourier spectra using a series of 102 Pi1B events recorded from February through October, 2008, followed by a quantitative analysis of a 49‐event subset that allows us to derive an estimate of the range of quality factors that can be expected for Pi1B‐associated spectral enhancements. Finally, we use European Incoherent Scatter (EISCAT) radar electron density data recorded during two Pi1B events from the original data set‐one during a sunlit ionosphere and one during a dark ionosphere‐in conjunction with Mass‐Spectrometer Incoherent Scatter (MSIS) and International Model Ionosphere (IRI) model data as input to a model of eigenmodes of the ionospheric Alfvén resonator. We compare the results of this model to the observed spectra and show that the observations can be partially explained as (instantaneous) resonant frequencies of the ionospheric Alfvén resonator.

Plasma parameter analysis of the Langmuir decay process via Particle-in-Cell simulations

Abstract. The beam-plasma mechanism, based on the Langmuir decay process, has been proposed to explain naturally enhanced ion-acoustic lines (NEIALs), which are spectral distortions in incoherent scatter radar (ISR) data frequently observed in the vicinity of auroral arcs. In this work the effect of the Langmuir decay process on the ISR spectrum is studied and compared with an analytical model for different plasma parameters by using an electrostatic parallel particle-in-cell (EPPIC) code. Simulations show that the code is working in accordance with theory for a wide range of beam and plasma values and that the features of the spectrum are sensitive to changes of those values. These results suggest that the EPPIC code might be used to build a spectrum-plasma parameter model which will allow estimation of beam and plasma parameters from observed spectra. Simulations also confirm that background electron density (ne) plays an important role in determining the maximum detectable wavenumber of the enhancement. Specifically, results demonstrate that an increase in ne makes the enhancements of the ion acoustic more likely line at large wavenumbers, a finding consistent with statistical studies showing more frequent NEIAL occurrence near solar maximum. Finally, the simulations expose some inaccuracies of the current theoretical model in quantifying the energy passed from the beam to the Langmuir waves as well as with the range of enhanced wavenumbers. These differences may be attributable to the weak Langmuir turbulent regime assumption used in the theory.

A nighttime temperature maximum in the thermosphere above Saint Santin in winter

The Saint Santin (45°N) incoherent scatter radar data base shows a local enhancement in nighttime ion temperature that occurs in winter only. Its amplitude can exceed 100 K and averages 40 K on the basis of a conservative computation. The enhancement peaks near 4 LT and has a duration of about 4 hours. We discuss two possible explanations for this feature: (1) it is caused by the convergence of global winds at the global pressure minimum, under which Saint Santin rotates at winter solstice, and (2) it is an extension or propagation of the well‐known low‐latitude midnight temperature maximum to midlatitudes, perhaps to that pressure minimum.

Time constants in the ionosphere from neural network models

Neural network (NN) models for the low latitude and the polar ionosphere from the D- to the F-region were developed which are based on incoherent scatter radar data from Arecibo and EISCAT Svalbard, respectively. The various geophysical input parameters defining the NN are not only the ones that represent the time one wants to predict, but also the geophysical conditions prior to the time of the prediction. The optimum length of these preceding periods are derived for the two models are different, but a period of 60days is a compromise acceptable for both latitudes. Furthermore from the Arecibo data time constants of electron density decay after sundown are derived which – arguably – are also relevant elsewhere, including the polar latitudes. Whereas at all altitudes the electron densities decay exponentially after sundown, below 300km there is an additional variation with solar zenith angle.

Statistical characterization of the American sector subauroral polarization stream using incoherent scatter radar

[1] We examine Millstone Hill incoherent scatter radar data collected over two solar cycles between 1979 and 2001 to determine average characteristics and features of storm time ion velocity and flux transport in the American sector midlatitude ionosphere. We use over 1100 radar azimuth scans identified as containing enhanced westward ion velocities associated with the subauroral polarization stream (SAPS), covering the 12–06 magnetic local time (MLT) sector and 50°–68° invariant latitude for weak to moderate disturbance levels with Dst from 50 to −200 nT. We find the magnetic latitude peak location of the SAPS flow channels decreases linearly with both Dst and MLT with a very good degree of correlation. We also examine for the first time SAPS peak westward ion fluxes, which transport material westward with magnitude between 3 × 1013 and 3 × 1014 m−2 s−1 in a manner nearly invariant to activity level. This invariance is maintained by an inverse relationship between electron density and ion velocity magnitudes with increasing Dst. Westward log ion flux and ion velocity are maximum in the dusk sector and decrease linearly with increasing MLT, smoothly varying across the dusk terminator. Finally, velocity distributions show that material in the afternoon SAPS flow is swept westward to earlier MLT values, delivering O+ flux to the cusp region. In contrast, SAPS streams in the post terminator sectors are fixed east-west in the Sun-Earth inertial frame, effectively maintaining entrained ion fluxes at the same MLT.

A modeling study offoF2andhmF2parameters measured by the Arecibo incoherent scatter radar and comparison with IRI model predictions for solar cycles 21, 22, and 23

[1] This work presents the results of a local empirical model that describes the behavior of the ionospheric F2 region peak. The model was developed using nearly 25 years of incoherent scatter radar (ISR) measurements made at the Arecibo Observatory (AO) between 1985 and 2009. The model describes the variability of the F2 peak frequency (foF2) and F2 peak height (hmF2) as a function of local time, season, and solar activity for quiet-to-moderate geomagnetic activity conditions (Kp < 4+). Our results show that the solar activity control of hmF2 and foF2 over Arecibo can be better described by a new proxy of the solar flux (F107P), which is presented here. The variation of hmF2 parameter with F107P is virtually linear, and only a small saturation of the foF2 parameter is observed at the highest levels of solar flux. The winter anomaly and asymmetries in the variation of the modeled parameters between equinoxes were detected during the analyses and have been taken into account by the AO model. Comparisons of ISR data with international reference ionosphere (IRI) model predictions indicate that both CCIR and URSI modes overestimate foF2 during the daytime and underestimate it at night. As expected, this underestimation is not observed in the AO model. Our analyses also show that the hmF2 parameter predicted by the IRI modes shows a saturation point, which causes hmF2 to be underestimated at high solar activity. The underestimation increases with higher levels of solar activity. Finally, we also found that IRI predictions of the seasonal variability of foF2 and hmF2 over Arecibo can be improved by using a small correction that varies with solar activity and local time.

Using incoherent scatter radar to investigate the neutral wind long-term trend over Arecibo

[1] Thermospheric neutral winds can be the most important driver when modeling ionospheric densities and temperatures. Several papers in this special edition show interesting features of the neutral winds behavior during the last 30 years at the Arecibo Observatory (18.3°N, 66.75°W; ∼28.25° dip latitude) using Fabry-Perot Interferometer (FPI) data. A neutral wind vector that changes its direction, becoming more dominantly eastward over the years and a meridional neutral wind component that decreases in magnitude, were found. The main goal of this work is to look for similar evidence of long-term trends in the radar derived winds that might support these recent discoveries and explore the associated ionospheric parameter measurements to look for the effects of these changing winds on the ionosphere. With this purpose in mind, Incoherent Scatter Radar (ISR) measurements of the F region vector drifts are used to derive the corresponding meridional thermospheric neutral wind along the magnetic field over Arecibo during 20 years. Major findings include a decreasing long-term trend (lowering) of the height where the F2- layer peak density occurs (hmF2), which could be related with a more increased downward flux of the ionosphere. A slight decrease in the peak density of the F2- layer (NmF2) after local midnight also was found during the period studied. The meridional wind along the magnetic field derived from ISR data also revealed a long-term trend, becoming more northward during the period studied, with a maximum variation between 02:30UT and 05:30UT.

Validation of CHAMP electron temperature measurements by incoherent scatter radar data

[1] The CHAMP performed electron temperature, Te, measurements during its mission period from 2000 to 2010. For the validation of these Te data comparisons with incoherent scatter radar observations at Arecibo and Tromso (EISCAT) have been performed. Data from 94 (143) close encounters of the satellite with the Arecibo (Tromso) radar are available for the validation. Results at Tromso were reasonable, but at Arecibo significant differences, in particular for low temperature, were observed. Investigations showed that CHAMP Te measurements have a bias which switches sign between northbound and southbound orbit arcs. The global distribution of the bias shows systematic latitudinal structures antisymmetric to the magnetic equator. After correction of this effect, CHAMP Te data show a good agreement with the radar observations at both sites. From the mean relative deviation we deduce that CHAMP Te data are low by 3% with a standard deviation of 8%. Validated CHAMP Te data are available for the period 20 February 2002 through 20 February 2010.

Three‐dimensional numerical simulation of equatorial F region plasma irregularities with bottomside shear flow

A three‐dimensional numerical simulation of plasma density irregularities in the postsunset equatorial F region ionosphere leading to equatorial spread F (ESF) is described. The simulation advances the plasma number density and electrostatic potential forward in time by enforcing the constraints of quasi‐neutrality and momentum conservation. The magnetic field lines are not modeled as equipotentials. Simulations are performed for cases with no background winds, with no background electric field or gravity, and with winds, a background electric field, and gravity all working in concert. The first run produced generalized Rayleigh Taylor (GRT) instability, and the second produced collisional shear instability (CSI). The combined run produced an instability which developed into an intense ESF event more quickly and with more realistic characteristics than the other two. Simulation results are compared with incoherent and coherent scatter radar data from the magnetic equator. A number of signature ESF characteristics are shown to be reproduced by the simulation.

An analysis of the topside ionosphere parameters based on the long-duration Irkutsk incoherent scatter radar measurements

The topside ionosphere parameters are studied based on the long-duration Irkutsk incoherent scatter radar (52.9N, 103.3E) measurements conducted in September 2005, June and December 2007. As a topside ionosphere parameter we chose the vertical scale height (VSH) related to the gradient of the electron density logarithm above the peak height. For morphological studies we used median electron density profiles. Besides the median behavior we also studied VSH disturbances (deviations from median values) during the magnetic storm of September 11th 2005. We compared the Irkutsk incoherent scatter radar data with the Millstone Hill and Arecibo incoherent scatter radar observations, the IRI-2007 prediction (using the two topside options) and VSH derived from the Irkutsk DPS-4 Digisonde bottomside measurements.

Combining incoherent scatter radar data and IRI‐2007 to monitor the open‐closed field line boundary during substorms

The size of the polar cap is very important for understanding the substorm process as well as reconnection rates in general. In this work we build on previous studies which use a combination of European Incoherent Scatter radar (EISCAT) electron temperature (Te) measurements from two radars running simultaneously to track the motion of the open‐closed field line boundary (OCB). The second radar gives an estimate of the background variation of Te with altitude, which can then be subtracted from the radar beam being used to estimate the OCB location. We demonstrate that using the international reference ionosphere 2007 (IRI‐2007) model can remove the second radar requirement and therefore increase the number of cases which could benefit from background Te subtraction. In this paper we focus our analysis on substorm intervals. We find that the IRI‐2007 method produces an OCB proxy location which on average is 0.25° altitude adjusted corrected geomagnetic coordinate latitude equatorward of the two‐radar method. On comparing both the two‐radar and IRI‐2007 Te OCB finding methods with the OCB identified in the DMSP particle data and IMAGE satellite data we find that both EISCAT methods perform quite well, and neither method is particularly favored over the other. We find that the magnitude of the mean offset to the IMAGE OCB varies between 0.1° and 2.7° latitude, dependent on the event and the IMAGE camera used.

Dynamic variability in F-region ionospheric composition at auroral arc boundaries

Abstract. The work presents a data-model synthesis examining the response of the auroral F-region ion temperature, composition, and density to short time scale (&lt;1 min) electric field disturbances associated with auroral arcs. Ion temperature profiles recorded by the Sondrestrom incoherent scatter radar (ISR) are critically analyzed with the aid of theoretical calculations to infer ion composition variability. The analyses presented include a partial accounting for the effects of neutral winds on frictional heating and show promise as the groundwork for future attempts to address ion temperature-mass ambiguities in short-integration ISR data sets. Results indicate that large NO+ enchancements in the F-region can occur in as little as 20 s in response to impulsive changes in ion frictional heating. Enhancements in molecular ion density result in recombination and a depletion in plasma, which is shown to occur on time scales of several minutes. This depletion process, thus, appears to be of comparable importance to electrodynamic evacuation processes in producing auroral arc-related plasma depletions. Furthermore, the potential of ionospheric composition in regulating the amounts and types of ions supplied to the magnetosphere is outlined.

Determining the parameters of the layer of scattering irregularities, forming coherent echo, according to the Irkutsk IS Radar data

The method for determining the parameters of the layer of scattering irregularities, form coherent echo, based on the Irkutsk Incoherent Scatter (IS) Radar data is presented. It has been indicted that the method has the necessary accuracy. The processing of experimental data for the periods of September 25–26, 1998, and July 15–16, 2000, indicated that our results are in good agreement with the data obtained by other researchers. The analysis of the experiments with a high time resolution made it possible to reveal the time variation in the determined parameters. The time variations in the layer thickness and height do not contradict the data obtained by other researchers, and the time variations in the angular sensitivity of irregularities has been observed for the first time and requires further studies.

Correlation method for determining the ionospheric plasma drift velocity at the Irkutsk Incoherent Scatter Radar

The correlation method for processing the Irkutsk Incoherent Scatter Radar (IISR) data and the method for calculating the ionospheric plasma drift velocity are described. The properties of the complex autocorrelation function of an incoherent scatter signal are considered. The IISR data for September 2005 are analyzed.

Comparison of COSMIC occultation‐based electron density profiles and TIP observations with Arecibo incoherent scatter radar data

In June 2006 an early opportunity arose to compare occultation‐based electron density profiles with incoherent scatter radar data. The former were made available by the constellation of satellites called COSMIC. We find that the value of the peak plasma density in the F region is reasonably well characterized but that the altitude of the peak is overestimated above about 300 km and underestimated below that height. A simple Abel transform is not suitable for determining the E region profiles, even in daytime. We also compared the emission strength recorded by the Tiny Ionospheric Photometer (TIP) on board COSMIC to the Arecibo measurement of the TEC as part of an ongoing effort to calibrate the former. The TIP and Arecibo data both show the development of an ionospheric storm of the type referred to as high‐intensity, long‐duration continuous AE activity. It is surprising that such a modest magnetic storm (Dst ∼ −2.5 nT) resulted in the anomaly moving to nearly 30° magnetic latitude.

Thermospheric density model blending techniques: Bridging the gap between satellites and sounding rockets

Uncertainty in the atmospheric density is a crucial error source in calculating orbits of satellites in low Earth orbit. As a result, establishing accurate thermospheric neutral density models is important for predicting the motion of these satellites. Unfortunately, since density data in the altitude range between 140 and 200 km are sparse, predicting the neutral density to estimate atmospheric drag effects on the motion of satellites operating in this altitude region is subject to relatively large errors. A previous study found that the Jacchia‐Bowman model (JB2006) is the most reliable thermospheric empirical neutral density model above 200 km and the Naval Research Laboratory's Mass Spectrometer Incoherent Scatter (NRLMSISE‐00) model, whose core formulation is based on incoherent scatter radar data, can be considered a more reliable neutral density model below approximately 140 km. We have developed a simple bridging technique to blend the two models between these two regions. A two‐body model with atmospheric drag was used to compare effects of various atmospheric density models. These preliminary tests are conducted by propagating the positions of satellites orbiting between 140 and 200 km, with various ballistic coefficients, using the JB2006, the NRLMSISE‐00, and the bridging technique.

PFISR and ROPA observations of pulsating aurora

Previous observations have shown that pulsating aurora sometimes occurs with patches of a vertical extent that is thinner than would be expected for aurora caused by collisional processes, implying that local ionospheric processes are important in causing the narrow luminosity enhancement. However, Poker Flat Incoherent Scatter Radar (PFISR) data from four pulsating aurora events, during the Rocket Observations of Pulsating Aurora (ROPA) mission in January and February 2007, show that the electron density profile associated with the pulsating patches had a thickness of ∼ 15 – 25 km in all four cases and that, therefore, these are not examples of such thin enhancements. A numerical model of the associated volume emission rates for the night of the ROPA launch supports this conclusion. In the process of modeling the volume emission rates, the PFISR data are inverted to calculate the associated electron energy distribution for comparison with in situ electron measurements from ROPA and the REIMEI satellite. The modeled distribution shows a diffuse plasma sheet population which gradually decreases in energy over the course of the event, resulting in ∼ 6 – 8 keV precipitation by the end of the PFISR data interval, in agreement with the ROPA/REIMEI measurements.

Optical estimation of auroral ion upflow: 2. A case study

This work presents a detailed multi‐instrument and modeling case study of an ion upflow event occurring over the Sondrestrom incoherent scatter radar on 17 February 2001. This case study is used to demonstrate the effectiveness of a previously developed optical estimator of ion upflow. The optical estimator of ion upflow is applied in a two step process: (1) near‐infrared (NIR) optical observations in the 700–850 nm wavelength range are inverted to reconstruct the energy distribution of precipitating electrons then (2) the reconstructed precipitation is applied to a combined fluid‐kinetic model of the ionosphere (TRANSCAR) to estimate the plasma response to the auroral particles. Plasma parameters estimated by this procedure are compared with plasma parameters derived from incoherent scatter radar data recorded during the event and are shown to be in reasonably good agreement. However, this study highlights an important limitation of the estimation technique, the lack of a direct measure of the ambient ionospheric state (nighttime plasma density). Means for addressing this limitation are discussed and may eventually allow for estimation of ionospheric state parameters under certain conditions by using optical sensors.

Global model comparison with Millstone Hill during September 2005

A direct comparison between simulation results from the Global Ionosphere Thermosphere Model (GITM) and measurements from the Millstone Hill incoherent scatter radar (ISR) during the month of September 2005 is presented. Electron density, electron temperature, and ion temperature results are compared at two altitudes where ISR data is the most abundant. The model results are produced, first using GITM running in one dimension, which allows comparison at the Millstone Hill location throughout the entire month. The model results have errors ranging from 20% to 50% over the course of the month. In addition, the F2 peak electron density (NmF2) and height of the peak (HmF2) are compared for the month. On average the model indicates higher peak electron densities as well as a higher HmF2. During the time period from 9 September through 13 September, the trends in the data are different than the trends in the model results. These differences are due to active solar and geomagnetic conditions during this time period. Three‐dimensional (3‐D) GITM results are presented during these active conditions, and it is found that the 3‐D model results replicate the trends in the data more closely. GITM is able to capture the positive storm phase that occurred late on 10 September but has the most difficulty capturing the density depletion on 11 and 12 September that is seen in the data. This is probably a result of the use of statistical high‐latitude and solar drivers that are not as accurate during storm time.