Incoherent Scatter Radar Data Research Articles

Imaging the structure of a large‐scale TID using ISR and TEC data

Here we present North American observations of the 3‐dimensional structure of large‐scale traveling ionospheric disturbances (TIDs) that affected the low‐ and mid‐latitude ionosphere during the stormtime period of October 1–2, 2002. Using TEC data from the American network of dual‐frequency GPS receivers, we are able to examine the effect of the TIDs on the background TEC and thus image their horizontal scales. Incoherent scatter radar data from the Arecibo Observatory give information on the vertical structure of the disturbances. The meridional and zonal structure of the TIDs are explored by computing the TEC perturbation, a method which has a resolution of a fraction of a TEC unit. The technique is effective in mapping the global structure of large‐scale disturbances, as the lower latitude perturbations can be examined at the same time as the higher latitude sources. The method may prove valuable in studying the formation and propagation of TIDs.

First ground‐based OI 630 nm optical measurements of daytime cusplike and F‐region auroral precipitation

We have used a high‐resolution (0.012 nm at 630 nm) imaging spectrograph to measure auroral emissions in daytime along the magnetic meridian from Sondrestrom Incoherent Scatter Radar (ISR) facility, at Kangerlussuaq, Greenland (67°N, 51°W; 74.5°N Mag. Lat.). Measured red line (OI 630 nm) brightness during a magnetically disturbed day (January 21, 2001) shows emission enhancements starting at 1330 UT (1130 MLT) and reaching a four‐fold enhancement above the normal dayglow brightness about two hours later. The onset enhancement, characterized by a sharp equatorward boundary, was measured concurrently with ISR Ne and Te, whose morphologies are generally indicative of cusplike precipitation. The 630 nm emission enhancements after 1518 UT correspond to a series of post‐noon F‐region arcs as measured by the ISR. Modeled 630 nm airglow brightness using ISR data, agree well with both the brightness and time history of the daytime optical measurements.

Seasonal and solar activity changes of electron temperature in the F-region and topside ionosphere

Incoherent scatter radar data from Arecibo, for high solar activity (HSA) (1989–1990) as well as for low solar activity (LSA) (1974–1977) periods, are used to study the seasonal and solar activity variations in electron temperature ( T e) for noontime conditions. Inspite of large day-to-day variations, clear seasonal variations in average T e can be identified for both solar activity periods, with winter temperatures significantly higher in the topside (400–700 km) ionosphere. Further, comparison of average T e profiles for each season reveals distinct solar activity variations – a large increase in the F-region (200–350 km) T e, during summer and equinox as compared to winter, occurs as one moves from low to HSA. In the topside, however, electron temperature changes little with solar activity. Comparisons with IRI-95 and Truhlik et al. (2000) models show a reasonable agreement within one standard deviation of the measured values.

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.

The DTM-2000 empirical thermosphere model with new data assimilation and constraints at lower boundary: accuracy and properties

The drag temperature model (DTM) is a semi-empirical model describing the temperature, density and composition of the Earth's thermosphere. Its first version (Ann. Geophys. 34 (1978) 9) used direct measurements of exospheric temperature and atmospheric densities derived from satellite drag data. It has later been refined (J. Geodesy 72 (1998) 161). However, both models have their lower boundaries at 120 km , which are not constrained by observations. Consequently in the lower thermosphere, the modelled temperature and density structure is uncertain. For predicting satellite orbits in the lower thermosphere, more realistic density models are required. We present a new DTM model having the following improvements: (a) Temperature and its gradient at 120 km are represented in agreement with theory and observation, using incoherent scatter radar and satellite-borne interferometer data. (b) Atmosphere explorer (AE) data, which have not been assimilated in DTM-94, are used as they cover a complete solar activity cycle. (c) The Mg II index is used whenever possible to represent the solar UV and EUV emission intensity instead of the solar decimetre radio flux, since it is more representative of solar instantaneous chromospheric activity than F 10.7 is. The basic DTM mathematical representation of temperature and composition is used, with, however, some additions and modifications to take into account the variations at 120 km altitude. The temperature modelling accuracy has improved by 5–8%, and there is no model bias as a function of solar activity. The oxygen and helium modelling has improved as well, and this is demonstrated by the estimated drag scale coefficients issued from precise orbit computation. The scaling coefficients estimated using DTM-2000 are systematically closer to unity than those resulting from employing DTM-94 and MSIS-86 in the orbit computation. The minor constituents (O 2 and H) modelling is unchanged. The molecular nitrogen modelling is not improved, but this is, at least partly, caused by the poor data quality. Despite these improvements, semi-empirical thermosphere models still suffer of weaknesses. First, they assume a steady-state equilibrium which is not necessarily reached in any circumstances. Second, the basic process of atmospheric heating by EUV is assumed to be represented by some indices (Mg II, F 10.7), while particle precipitation is represented by an index associated to a latitude without a longitude effect ( K p), although winds that transport energy suggest this effect. Third, the data accuracy and their incomplete geographical and temporal coverage are significant sources of uncertainty. Assimilation of a long time series of data, with a complete geographical coverage, and using the Mg II index will probably increase model accuracy from the present-day RMS of 19% to 10–15%. The complete CHAMP accelerometer data set may allow the achievement of that goal after 5 years of operations in 2005.

Relative contribution of ionospheric conductivity and electric field to ionospheric current

Based on 10 years of European Incoherent Scatter (EISCAT) radar data, we have investigated the variation of ionospheric conductivities, electric fields, and currents over magnetic local time (MLT). Pedersen and Hall currents are generally enhanced especially on the nightside, and the MLT dependencies of these two types of ionospheric currents is similar, as previously found for field‐aligned currents and auroral electrojets, respectively. We readdress the question to what extent the conductivity or the electric field contributes to the ionospheric current in different periods of MLT. On average, higher Pedersen conductivities are seen at 2000–0800 MLT in comparison with other MLT intervals, and these conductivities crucially contribute to Pedersen currents over two MLT sectors, around midnight and in the late morning. However, when the Pedersen current is stronger, not only the Pedersen conductivity but also the electric field becomes higher statistically on the nightside. Hall conductivities are also higher at about 2000–0800 MLT, showing two maxima, around midnight and in the late morning, and they increase more strongly than Pedersen conductivities on a statistical basis. Thus the nightside electrojets are mainly due to high Hall conductivities.

Solar EUV flux, exospheric temperature and thermospheric wind inferred from incoherent scatter measurements of the electron density profile at Millstone Hill and Shigaraki

We explore a method for inferring solar EUV flux, atmospheric composition and wind using ionospheric electron density Ne profile measurements. Incoherent scatter radar data from Millstone Hill and Shigaraki measured on October 5, 1989 are assimilated into a theoretical model whose driving forces, solar EUV flux, exosphere temperature Tex, and meridional wind, are adjustable. Adjustments are made to give best match between the model Ne profile and the data. The derived Tex values, found to be low near noon at Millstone and high in the afternoon at Shigaraki, are essentially those required to give the [O]/[N2] ratio necessary to fit the data. Our inferred EUV fluxes for the two sites are similar. Our technique of using profile data may resolve the ambiguity in deriving EUV and [O]/[N2] from electron‐density measurements.

A case study of HF radar spectral width in the post midnight magnetic local time sector and its relationship to the polar cap boundary

Abstract. The aim of this paper is to advance the current understanding of the spectral width parameter observed by coherent high frequency (HF) radars. In particular, we address the relationship of a frequently observed gradient, between low ( < 200 m/s) and high ( > 200 m/s) spectral width, to magnetospheric boundaries. Previous work has linked this gradient in the spectral width, in the nightside sector of magnetic local time, to the Polar Cap Boundary (PCB), and also to the boundary between the Central Plasma Sheet (CPS) and the Plasma Sheet Boundary Layer (PSBL). The present case study investigates the former by comparison with the 630.0 nm optical emission. No suitable data were available to test the second of the two hypotheses. It is found that during the interval in question the spectral width gradient is within the region of the 630.0 nm optical emission. A comparison of coherent and incoherent scatter radar data is also conducted, which indicates that values of high spectral width are typically collocated with elevated F-region electron temperatures. We conclude that the high spectral width region in the interval under study is associated with particle precipitation and also that the spectral width gradient is not a reliable method for locating the PCB.Key words. Ionosphere (auroral ionosphere; ionospheric irregularities)

Regional and local ionospheric models based on Millstone Hill incoherent scatter radar data

Local and regional statistical models to describe Millstone Hill incoherent scatter radar observations of electron density, electron temperature and ion temperature since 1976 are developed using a bin‐fit technique. The local models generate ionospheric variations with local time, day number, and altitude from 150–1000 km. The prior day's F107 and the Ap index from the previous 3 hour period are keyed inputs to specify solar and geomagnetic activity. The regional models have a latitude coverage of 32–55° geodetic and an altitude coverage of 200–600 km. These climatology models are capable of reproducing primary ionospheric variation features seen in previous studies as well as several newly revealed features, such as the semiannual variation of electron density. They are accessible through the World Wide Web at the URL http://www.openmadrigal.org.

Plasma conditions in auroral roar source regions inferred from radio and radar observations

A medium frequency interferometer (MFI) has been established in Sondrestrom, Greenland, to measure the direction of arrival (DOA) of auroral roar emissions at 2.5–3.0 MHz. Combined with electron density data from the Sondrestrom incoherent scatter radar (ISR), these DOA data provide a powerful test of the most‐cited generation mechanism of auroral roar emissions. This mechanism requires that the relationship between the electron plasma frequency (fpe) and the electron gryofrequency (fce) in the source regions isfpe2 = (n2 − 1)fce2, where n = 2, 3 is the harmonic number of the emission. For each of five auroral roar events studied, to within experimental uncertainty this frequency‐matching condition is met throughout the source region of the emissions inferred from the MFI and ISR data. Furthermore, ray‐tracing analysis indicates that signals at the roar frequency are able to propagate from the source region to the location of the MFI receiver.

Observations of persistent dayside F region electron temperature enhancements associated with soft magnetosheathlike precipitation

A series of experiments with the Sondrestrom incoherent scatter (IS) radar (66.99°N latitude, 50.95°W longitude) were designed to examine F region structure in the dayside auroral oval in order to search for plasma signatures from magnetospheric regions such as the cusp, boundary plasma sheet, low‐latitude boundary layer, and mantle. This IS radar mode, optimized to search for ionospheric features which remain fixed in local time, was coordinated with overflights of the DMSP F‐10 satellite on 2 days in September 1992. For both study days, IS radar data show persistent (∼ 7 hour), enhanced Te regions at 300 to 500 km. These enhanced Te regions evolve during periods in which relatively unstructured, laminar Ne densities are observed and thus are not merely the result of a structured suppression of electron cooling. The cores of these Te enhancements were observed at latitudes and magnetic local times corresponding to DMSP satellite measurement of soft (< 100 eV) cusplike precipitation. These Te hotspots move systematically equatorward with increasing geomagnetic activity and display a sharp field‐aligned equatorward edge at the location of satellite cusp detection. Unlike prior IS radar/satellite cusp investigations, no significant Ne enhancements were measured coincident with Te hotspots. A simple ionospheric model is invoked to confirm that such soft cusplike precipitation does not significantly alter the magnitude of the ambient plasma density, and we argue that cusp detection based on collocated Ne and Te enhancements is seldom possible. The local time persistence of the Te enhancements, beyond the typical cusp widths, suggests an association with additional dayside magnetospheric regimes such as the low‐latitude boundary layer. Both latitudinal and vertical Te gradients maximize at the location of satellite cusp detection, suggesting that the heat source is a divergence of magnetospheric heat flux on freshly reconnected geomagnetic field lines.

Characteristics of the postnoon auroras inferred from EISCAT radar measurements

Svalbard European Incoherent Scatter (EISCAT) radar and Polar satellite ultraviolet imager data have been used to investigate structure and dynamics of the postnoon auroral oval (in the so‐called “14 MLT” region). Southward turnings of the interplanetary magnetic field were associated with bright afternoon and evening arcs which drifted equatorward. Increase in the equatorward arc velocity occurred about 27 ± 7 min after the IMF Bz turned southward at the subsolar point. The observed equatorward motion may occur independently of the convection: In one of the cases, the velocity of the drifting arc was observed to be about 100 – 200 m/s southward whereas the ionospheric plasma drifted northward at a velocity of 160 m/s. The arcs were located in the region of convection shear that was shown by the ground magnetometers and the EISCAT data on ion velocity and temperature. In the cases when electric field data were available the meridional component of the electric field varied from about −60 mV/m (southward) equatorward of the arc to +30 mV/m (northward) poleward of the arc. Ionospheric plasma parameters were measured during the passes of the arcs through the radar beam. Downward field‐aligned ion flow at the 300 – 550 km altitude was observed poleward of the arc, whereas upward flows were observed equatorward of the arc. This variation is explained by the difference in the vertical gradient of plasma density. Convection flows associated with arc, and differences in electron precipitation and sunlight conditions create the vertical gradients of plasma density that determine the ambipolar diffusion and observed ion motion.

MIDAS-W: a workstation-based incoherent scatter radar data acquisition system

Abstract. The Millstone Hill Incoherent Scatter Data Acquisition System (MIDAS) is based on an abstract model of an incoherent scatter radar. This model is implemented in a hierarchical software system, which serves to isolate hardware and low-level software implementation details from higher levels of the system. Inherent in this is the idea that implementation details can easily be changed in response to technological advances. MIDAS is an evolutionary system, and the MIDAS hardware has, in fact, evolved while the basic software model has remained unchanged. From the earliest days of MIDAS, it was realized that some functions implemented in specialized hardware might eventually be implemented by software in a general-purpose computer. MIDAS-W is the realization of this concept. The core component of MIDAS-W is a Sun Microsystems UltraSparc 10 workstation equipped with an Ultrarad 1280 PCI bus analog to digital (A/D) converter board. In the current implementation, a 2.25 MHz intermediate frequency (IF) is bandpass sampled at 1 µs intervals and these samples are multicast over a high-speed Ethernet which serves as a raw data bus. A second workstation receives the samples, converts them to filtered, decimated, complex baseband samples and computes the lag-profile matrix of the decimated samples. Overall performance is approximately ten times better than the previous MIDAS system, which utilizes a custom digital filtering module and array processor based correlator. A major advantage of MIDAS-W is its flexibility. A portable, single-workstation data acquisition system can be implemented by moving the software receiver and correlator programs to the workstation with the A/D converter. When the data samples are multicast, additional data processing systems, for example for raw data recording, can be implemented simply by adding another workstation with suitable software to the high-speed network. Testing of new data processing software is also greatly simplified, because a workstation with the new software can be added to the network without impacting the production system. MIDAS-W has been operated in parallel with the existing MIDAS-1 system to verify that incoherent scatter measurements by the two systems agree. MIDAS-W has also been used in a high-bandwidth mode to collect data on the November, 1999, Leonid meteor shower.Key words: Electromagnetics (instruments and techniques; signal processing and adaptive antennas) – Ionosphere (instruments and techniques)

MIDAS-W: a workstation-based incoherent scatter radar data acquisition system

The Millstone Hill Incoherent Scatter Data Acquisition System (MIDAS) is based on an abstract model of an incoherent scatter radar. This model is implemented in a hierarchical software system, which serves to isolate hardware and low-level software implementation details from higher levels of the system. Inherent in this is the idea that implementation details can easily be changed in response to technological advances. MIDAS is an evolutionary system, and the MIDAS hardware has, in fact, evolved while the basic software model has remained unchanged. From the earliest days of MIDAS, it was realized that some functions implemented in specialized hardware might eventually be implemented by software in a general-purpose computer. MIDAS-W is the realization of this concept. The core component of MIDAS-W is a Sun Microsystems UltraSparc 10 workstation equipped with an Ultrarad 1280 PCI bus analog to digital (A/D) converter board. In the current implementation, a 2.25 MHz intermediate frequency (IF) is bandpass sampled at 1 µs intervals and these samples are multicast over a high-speed Ethernet which serves as a raw data bus. A second workstation receives the samples, converts them to filtered, decimated, complex baseband samples and computes the lag-profile matrix of the decimated samples. Overall performance is approximately ten times better than the previous MIDAS system, which utilizes a custom digital filtering module and array processor based correlator. A major advantage of MIDAS-W is its flexibility. A portable, single-workstation data acquisition system can be implemented by moving the software receiver and correlator programs to the workstation with the A/D converter. When the data samples are multicast, additional data processing systems, for example for raw data recording, can be implemented simply by adding another workstation with suitable software to the high-speed network. Testing of new data processing software is also greatly simplified, because a workstation with the new software can be added to the network without impacting the production system. MIDAS-W has been operated in parallel with the existing MIDAS-1 system to verify that incoherent scatter measurements by the two systems agree. MIDAS-W has also been used in a high-bandwidth mode to collect data on the November, 1999, Leonid meteor shower.Key words: Electromagnetics (instruments and techniques; signal processing and adaptive antennas) – Ionosphere (instruments and techniques)

On the electrical structure of airglow depletion/Height layer bands over Arecibo

Using a combination of airglow images and incoherent scatter radar data, we have explored the electrical structure of the airglow depleted, height layer bands over a mid‐latitude site. We find a reproducible electrical signature in both components of the electric field in all events studied. The most pronounced feature is a large northward/upward electric field in the heart of the structure. The latter is identical to the radially outward field reported for mid‐latitude conjugate electric fields [Saito et al., 1995], found to trace the poleward edge of the equatorial anomaly. We favorably compare the Arecibo drift to a typical satellite event. These electric fields may reflect a nonlinear evolved state of the Perkins instability or some, as yet, unexplained coupling between atmospheric motion and the plasma embedded in it. We show here that the F‐region Pedersen conductivity is much lower in these structures than outside and suggest that this is related to a polarization electric field inside the structure.

Comparison between SUPIM simulations and measured TEC for the January, 1997 storm

A method of extrapolating electron densities (Ne) from Incoherent Scatter Radar (ISR) data to high altitudes is presented, and is used to estimate the vertical Total Electron Content (TEC) over Arecibo. Although primarily intended for use on the Ne profiles obtained using the new topside frequency hopping mode at Arecibo which provides measurements to over 2000 km altitude, we show that if the O+ ‐H+ transition altitude is low enough to be observed, then the method may also be applied to the data taken using the standard World Day multi‐radar mode, and thus to a great fraction of the World Day data from low solar flux conditions. This method is applied to Arecibo ISR data taken during the January 6–11 1997 CEDAR storm period, and compared with measured TEC from the Global Positioning System (GPS) satellite network. In addition, the TEC is compared to TEC calculated from the Sheffield University Plasmasphere Ionosphere Model (SUPIM) as part of an ongoing project of model validation.

The upper atmosphere during June 5–11, 1991

Millstone Hill ionospheric incoherent scatter radar data collected during June 5–11, 1991, are analyzed to determine F region neutral and ion composition, density, and temperature. The 10.7‐cm solar flux index was at the very high level of 250, and the magnetic activity index Kp ranged up to the highest possible level of 9 during this period, making it of particular interest for understanding upper atmosphere behavior for such a nearly limiting combination of conditions experienced by the thermosphere. We find that the Mass Spectrometer Incoherent Scatter 86 (MSIS‐86) model overestimates the daytime exospheric temperature and the neutral O concentration for this high level of solar activity. These results differ substantially from storm periods analyzed previously. The ionospheric F region converts from largely atomic ions to largely molecular ions during the periods of strongest magnetic activity. We show that this condition leads to erroneous determination of neutral atmosphere parameters, including temperature, density, and drift velocity, if not accounted for in incoherent scatter data analysis.

Non-stationary Alfvén resonator: vertical profiles of wave characteristics

A Pc1/IPDP event recorded by the Finnish search coil magnetometers on 15 December 1984 was analyzed in a companion paper (Mursula et al., 2000. Non-stationary Alfvén resonator: new results on Pc1 pearls and IPDP events. J. Atmos. Solar-Terr. Phys. 62(4), 299–309) using numerical simulations of the ionospheric Alfvén resonator (IAR). EISCAT incoherent scatter radar data were used to determine the vertical profiles of ionospheric plasma parameters. In this paper, the detailed altitude profiles of several wave characteristics at the IAR eigenfrequency are computed up to 1000 km height, including, e.g., the real normalized amplitude of the magnetic wave field component, ellipticity and orientation of the polarization ellipse in the horizontal plane. We also calculate the altitude profile of the energy flux density (Poynting vector). These features illustrate in detail the ionospheric effects on the wave spectral structure in a non-stationary IAR, and their significance in the formation of the Pc1/IPDP signal on the ground.

An updated climatology of thermospheric neutral winds and F region ion drifts above Millstone Hill

Incoherent scatter radar data from 63 experiments during the period 1984–1997 have been used to update earlier published climatologies of ion drifts and neutral thermospheric winds at Millstone Hill. Data are binned according to season, solar activity, geomagnetic activity, and local solar time. The derived ion drift patterns are similar to those previously published, but we now find that the main effect of the increased magnetic activity is an enhancement of the usual nighttime west‐ward ion drift. A tidal decomposition of the winds from geomagnetically quiet conditions is carried out for each season/solar activity level to extract diurnal means, diurnal amplitudes and phases, and semidiurnal components. These winds are compared with previous results at Millstone Hill and the HWM‐93 model. Using our expanded wind database, statistically significant semidiurnal components are now found in both summer and winter, at both solar maximum and solar minimum. Our bin‐averaged results confirm earlier published findings that at all seasons the diurnal mean winds are more strongly equatorward and the diurnal amplitudes are stronger at solar cycle minimum than at solar cycle maximum. Diurnal amplitudes derived in the present work are larger than those found previously because we now use a smaller value for the O+, O collision frequency. Differences in the diurnal mean and diurnal amplitude between recent solar minimum data (1993–1997) and data from the previous solar minimum (1984–1986) are attributed to a difference in the EUV flux as inferred from Millstone Hill electron density data which one would not expect from the F10.7 index. This affects the ion drag and apparently also the relative importance of the EUV and high latitude heat sources.

Cavitating Langmuir Turbulence Observed during High-Latitude Ionospheric Wave Interaction Experiments

Incoherent scatter radar data obtained during high power radio wave ionospheric interaction experiments performed in northern Scandinavia show an induced spectral line at the O-mode pump frequency simultaneous with strong enhancements at the natural Langmuir resonance frequency, both just below the reflection height. These characteristics provide an unmistakable signature of cavitating (a.k.a. strong) Langmuir turbulence, and show that this turbulence occurs not only in midlatitude but also during high latitude ionospheric interaction experiments. Cavitating turbulence is thus not dependent on specific geophysical conditions but is due to fundamental wave-plasma interaction processes in the ionospheric plasma.