Incoherent Scatter Observations Research Articles

A study of the forenoon ionospheric F2 layer behavior over the middle and upper atmospheric radar

The middle and upper atmosphere (MU) radar incoherent scatter observations of electron density Ne profiles for 1986–1995 at Shigaraki (34.85°N, 136.10°E) show a forenoon density depression (bite‐out) in summer both on individual days and in statistical averages. The depression takes place generally between 0700 and 1200 LT, above the F2 peak for low solar activity and around the F2 peak for high solar activity. The maximum electron density NmF2 shows, in both percentage and time duration, a greater depression for low solar activity than for high solar activity. With increasing altitude, the forenoon Ne depression develops earlier for low solar activity. In combination with the MU radar observations of ion drifts and plasma temperatures, our modeling indicates the following: (1) Transport processes such as the enhancement of electron temperature Te, the poleward neutral wind, and the reduced upper boundary ionization density help to reduce Ne and result in a negative ∂Ne/∂t in the morning hours in summer. Among those processes the morning enhancement of Te is found to be most important for the presence of the forenoon bite‐out, since it greatly increases the O+ diffusion such that ionization outflow is significantly intensified. (2) The forenoon bite‐out occurs only in summer, not in other seasons, because the smaller atmospheric composition ratio [O]/[N2] in summer leads to a smaller daytime background Ne and thus to an enhanced Te, and the after‐sunrise photochemical contributions (mainly the O+ production) are so small that ∂Ne/∂t also tends to be smaller. These two conditions give prominence to the transport contributions. (3) The solar activity dependences of the forenoon bite‐out can be explained basically in terms of the low‐electron‐density background. (4) The height of formation of the forenoon bite‐out is also related to the sunrise effect on photoionization.

Some F2-layer effects during the January06-11,1997 cedar storm period as observed with theMillstoneHill incoherent scatter facility

An observed daytime negative storm effect on 10 January and strongpost-midnight N m F 2 increases on 7 and 8 January followed by a steep T e decrease down to T n are analyzed. Daytime neutral composition and T n(h) were inferred from incoherent scatter (IS) radar observations using aself-consistent approach to F 2 -layer modelling. Observed daytime T e , T i , N e profiles were corrected for the disturbed day using the calculated ioncomposition. The main reason for the observed N m F 2 negative storm effectis the O/(N 2 O 2 ) ratio decrease in the thermosphere. The calculated O/N 2 ratio at hm F 2 is less by a factor of 3.7 than MSIS prediction for 10January and is close to MSIS for quiet days. Strong Nm F 2 night-time increase by afactor of 3 is due to a plasma compression mechanism under the action of observed EB drift(westward E-field) moving plasma from higher L shells to lower ones and squeezing it into the F 2 -region. Moderate (1-2)10 8 cm -2 s -1 O fluxes in agreement with IS observations were shown to be sufficient to account for the night-timeN m F 2 increase. This elevation in the F 2 -region electron densityis shown to be the main reason for the observed steep Te post-midnight decrease. 1999 ElsevierScience Ltd. All rights reserved.

The use of Millstone Hill and EISCAT incoherent scatter observations for the F2-layer storm effects analysis

A self-consistent method for day-time F-region modeling was applied to EISCAT and Millstone Hill radar observations during severe ionospheric storms. One observed by EISCAT on Apr 03, 1992 shows a strong N mF2 decrease which is attributed to a strong imposed electric field, in another case observed at Millstone Hill on Apr 10, 1990 such a decrease is caused by neutral composition and temperature changes. Calculations indicate that the ionosphere was molecular-ion dominated even at F2-layer heights, requiring a correction to the routinely-evaluated T e(h), T i(h) and N e(h) profiles.

Day-to-day thermosphere parameter variation as deduced from Millstone Hill incoherent scatter radar observations during March 16-22, 1990 magnetic storm period

Abstract. A self-consistent method for day-time F2-region modelling was applied to the analysis of Millstone Hill incoherent scatter observations during the storm period of March 16-22, 1990. The method allows us to calculate in a self-consistent way neutral composition, temperature and meridional wind as well as the ionized species height distribution. Theoretically calculated Ne(h) profiles fit the observed daytime ones with great accuracy in the whole range of heights above 150 km for both quiet and disturbed days. The overall increase in Tex by 270 K from March 16 to March 22 reflects the increase of solar activity level during the period in question. A 30% decrease in [O] and a two-fold increase in [N2] are calculated for the disturbed day of March 22 relative to quiet time prestorm conditions. Only a small reaction to the first geomagnetic disturbance on March 18 and the initial phase of the second storm on March 20 was found in [O] and [N2] variations. The meridional neutral wind inferred from plasma vertical drift clearly demonstrates the dependence on the geomagnetic activity level being more equatorward on disturbed days. Small positive F2-layer storm effects on March 18 and 20 are totally attributed to the decrease in the northward neutral wind but not to changes in neutral composition. A moderate (by a factor of 1.5) O/N2 ratio decrease relative to the MSIS-83 model prediction is required to describe the observed NmF2 decrease on the most disturbed day of March 22, but virtually no change of this ratio is needed for March 21.

Electrodynamics of midlatitude spread F: 1. Observations of unstable, gravity wave‐induced ionospheric electric fields at tropical latitudes

In part 1 of our series exploring the role of electrical forces in midlatitude spread F, we present observations of an electrodynamically driven traveling ionospheric disturbance which passed over Arecibo Observatory between 22 and 24 AST on January 26, 1993. The total electric potential differences driving the wave were of the order of 1 kV. Our analysis indicates that this disturbance is the result of a midlatitude F region plasma instability seeded by a thermospheric gravity wave. Two novel measurements, in addition to typical incoherent scatter observations, were crucial to this determination: the use of 6300 Å airglow images from the coupling, energetics, and dynamics of atmospheric regions (CEDAR) allsky imager to track the two‐dimensional, mesoseale dynamics of the disturbance and the use of a portable ionosonde to simultaneously measure the fieldline integrated ionospheric conductivity in the conjugate hemisphere. We have also determined that this disturbance, like several previously observed midlatitude disturbances, is consistent with our theoretical knowledge of the basic instability of the midlatitude ionosphere described originally by Perkins [1973].

Self-consistent modelling of the daytime electron density profile in the ionospheric F region

Abstract. A theoretical self-consistent method for the description of daytime Ne(h) profiles in the ionospheric F region measured by EISCAT is proposed. It is based on the use of a theoretical F-region model and measured electron density, Ne(h), electron, Te(h), and ion temperature, Ti(h), and field-aligned plasma drift Vl(h) profiles. The method describes the observed Ne(h) profile with high accuracy for quiet and disturbed conditions. Two versions of the method are considered: in the first the exospheric temperature Tex is derived from a procedure minimizing [log(Ne(h)obs / Ne(h)cal]2, in the second Tex is deduced from the ion energy conservation in the F region. The method allows us to infer from the incoherent-scatter observations: concentrations of atomic oxygen, [O], molecular oxygen, [O2], molecular nitrogen, [N2] the vertical plasma drift, W, the exospheric temperature. Tex, and the shape parameter S in the neutral temperature profile. The ratio ([O+]/Ne) calculated by the theoretical model is used to correct Te(h), Ti(h) and Ne(h) profiles routinely measured with EISCAT which are known to depend strongly on the actual applied ion-composition model. Such a correction is especially important for geomagnetically disturbed periods when the F region is strongly enriched with molecular ions. We conclude that four of the six thermospheric parameters, namely [O], [N2], W and Tex can be confidently inferred from the EISCAT observations, while the other two derived parameters, [O2] ans S are less reliable. The method can be used for the analysis of long-term (seasonal, solar cycle) as well as for day-to-day variations of the thermospheric parameters and the F-region ion composition using daytime incoherent-scatter observations.

Fractional lags in alternating codes: Improving incoherent scatter measurements by using lag estimates at noninteger multiples of baud length

Alternating codes are nowadays a standard measurement technique in the incoherent scatter observations. In most conditions they give the best accuracy for the plasma autocorrelation function estimates in the E and F region measurements. We explain here how oversampling can be used to further increase the speed of the experiments. In an oversampled alternating code the sampling interval and the length of the postdetection filter are a fraction of the baud length, and autocorrelation function estimates are calculated both for integer lag values, which are multiples of the baud length, and for the intermediate values, which are called fractional lags. We show that alternating codes with fractional lags give data for multiple range resolutions and that the experiments are always more efficient than the standard alternating code experiments. Finally, we study how the oversampling affects the signal processing.

Vertical electron density profiles from the Digisonde network

The global Digisonde network provides a vast volume of ionogram-derived electron density profile data from some 50 stations. The current true height inversion program NHPC differs significantly from the original algorithms developed for real time calculation on PC 286 machines. NHPC successfully inverts about 90% of the autoscaled ionograms. A stand-alone version of NHPC can invert h′(f) traces from any digital or analog ionogram. For each layer, the N(h) profile is expressed in terms of shifted Chebyshev polynomials with a logarithmic argument containing the starting plasma frequency and the critical frequency of the layer. The peak height and a set of coefficients specify the electron density distribution of each layer. A valley model derived from incoherent scatter observations at Arecibo describes the E-F trasition. The frequent problem of missing h′(f) data points at the beginning of a trace is addressed by analytical expansion in the true height (profile) domain rather than by direct extrapolation of the h′(f) trace. The validity of the Digisonde NHPC profiles has been verified by comparison with incoherent scatter radar profiles.

Ionospheric plasma response to HF radio waves operating at frequencies close to the third harmonic of the electron gyrofrequency

Experimental results concerning European incoherent scatter observations of heater‐induced electron temperature enhancements, anomalous absorption of low‐power HF probe waves, and the spectrum of stimulated electromagnetic emission (SEE) in the sidebands of a high‐power HF electromagnetic wave are presented. For the experiments reported in this paper, an O mode pump wave was transmitted vertically into the F region above Tromsø, Norway, while the injected frequency was varied in small steps around the third harmonic of the electron gyrofrequency. Systematic variations with pump frequency were observed in the data obtained from all three diagnostics. Measurements of anomalous absorption, the downshifted maximum (DM) spectral feature, and heater‐induced electron temperature enhancements all exhibited broad minima as the heater frequency approached the third harmonic of the electron gyrofrequency. In addition, the signal strength of the HF probe wave measured during heater off periods is also reduced at these and higher heater frequencies. The experimental findings suggest that at heater frequencies in the vicinity of the third gyroharmonic, small‐scale field‐aligned irregularities are not excited, whereas very small scale irregularities, of the order of a few electron cyclotron radii, which are responsible for the production of fast electrons, may be generated. The observed reduction in the diagnostic signal strength is then attributed to the ionized patches produced by these energetic electrons.

Two different types of enhanced ion acoustic fluctuations observed in the upper ionosphere

We present European incoherent scatter observations of several events that show enhanced ion acoustic fluctuations in the altitude region 400–1500 km. The incoherently scattered spectra have one or both the ion acoustic lines enhanced, and sometimes a peak close to the zero frequency is observed. It is shown that these observations can be separated into two types depending on the surrounding physical conditions. The first type is related to strongly enhanced electron temperatures, ion outflows with large vertical velocities, an altitudinal extend of 300–700 km, and auroral arcs and precipitating particles of 100 eV to 10 keV. The spectra are in that case strongly asymmetric. The second type corresponds to slightly enhanced electron temperatures, no ion outflows, spatial extends of 100–200 km, and an apparent lack of precipitating particles of less than 1 keV. The corresponding spectra show mostly both the ion lines enhanced. These observations are compared with the existing interpretations.

Correction to “Midlatitude incoherent scatter observations of helium and hydrogen ions”

Correction to “Midlatitude incoherent scatter observations of helium and hydrogen ions”

Mid‐latitude incoherent scatter observations of helium and hydrogen ions

We have successfully employed a constrained non‐linear least squares analysis of incoherent scatter data from the topside ionosphere over Arecibo, Puerto Rico to simultaneously deduce concentrations of helium ions, protons, and oxygen ions as well as ion and electron temperatures. The constraint encourages smooth temperature profiles, and its use avoids unrealistic parameter values in regions where light ions are more than 80% of the total. Here we report proton concentrations that remain higher than helium ion concentrations, even though the latter reach 30% of the total and exceed the oxygen ion densities at altitudes as low as 700 km at night and 1500 km in the day‐time. Before local sunrise the effects of conjugate‐point photoelectrons are clearly seen, while after sunrise the transition height between oxygen and helium rises much more rapidly than does the transition from oxygen to hydrogen.

Effects of magnetospheric electric fields and neutral winds on the low‐middle latitude ionosphere during the March 20‐21, 1990, storm

During the geomagnetic storm of March 20‐21, 1990, substorm activity is clearly evident in magnetometer data collected during the night at the middle‐ and low‐latitude stations Fredericksburg (38.2°N, 282.6°E) and San Juan (18.1°N, 293.8°E). At the same time, incoherent scatter radars at Millstone Hill (42.6°N, 288.5°E) and Arecibo (18.3°N, 293.25°E) observed ionospheric storm effects, which included the penetration of magnetospheric electric fields and disturbance neutral winds to the latitude of Arecibo. The eastward electric fields associated with the substorm disturbances result in increases in the F2 peak height (hmF2) at Arecibo. Decreases in hmF2 follow as a result of increased downward diffusion and/or the effects of an ion drag induced poleward wind. During the intervals between the electric field penetration events, equatorward surges in the neutral wind result in westward electric fields by the disturbance dynamo mechanism. At these times the horizontal ionization drifts are not as strong as the neutral winds, apparently because of a partial shorting out of the dynamo electric fields as a result of some E region conductivity. The anticorrelation between the components of the ion drift parallel (V∥) and perpendicular to the magnetic field in the northward direction (V⊥N) results in approximately horizontal (constant altitude) ion drift motion throughout the interval. Calculations of spatial gradients in the electron density and in the components of the ion velocity are carried out using the multi‐directional incoherent scatter observations at Arecibo. The results show that the variations in electron density during this disturbed interval follow closely the motion term in the F2 region continuity equation, with both advection of spatial gradients and divergence of the ion flow important at times.

Incoherent scatter observations of thin ionization layers at Sondrestrom

High resolution incoherent-scatter observations of E-region thin (1–3 km) metallic ion layers are presented. Data were collected during three different periods from August 1990 to August 1991, in three different experimental modes. First, the antenna was directed vertically and the entire duty cycle was devoted to Barker coded multi-pulse [ Zamlutti (1980) J. atmos. terr. Phys. 42, 975–982] measurements to determine the densities and temperatures in the E-region with 300 m resolution. The second experiment measured the F-region electric field as well as the high resolution E-region densities. For the third experiment the antenna was scanned magnetic north-south while only the E-region densities were measured. The experiments were carried out on 16 different nights for a period of 4 h each night at a time near magnetic midnight. Thin ionization layers were observed on 12 of the 16 nights. The first experiment demonstrated that the thin layers are composed of a significant fraction of heavy metallic ions; assuming the layers are composed of a mixture of Fe + and Mg + a composition estimate of 63% Fe + was obtained in one example. The second experiment investigated the relationship between the direction of the electric field and the presence of the thin layers. In these observations thin layers were only present when the electric field was pointed in the magnetic north-west or south-west quadrants, most frequently when the field was near magnetic west. Correlation between layer altitude and field direction was also observed, layers occurring at higher altitudes for fields directed in the north-west, and lower altitudes for fields directed to the south-west. The observations are compatible with the electric field mechanism for thin ionization layer formation. The scanning experiment showed that the layers were of a limited latitudinal extent, typically about 100 km up to a maximum of about 200 km.

Incoherent scatter observations of tides in the lower thermosphere at high latitudes

Incoherent scatter measurements at high latitudes show that in the height range between 100 and 130 km the velocity of the neutral atmosphere shows a prominent sinusoidal component with a 12-hour period. The variation of phase with height for this component is approximately constant from day to day, and corresponds to the predicted variation for the (2,4) mode of the global tide; at greater heights, the variation of phase with height is much slower and corresponds to the (2,2) mode. The amplitude of this semi-diurnal mode as measured at EISCAT shows a deep modulation with a period of about 53 hours, and this is interpreted as the result of a non-linear interaction in the mesosphere between the semi-diurnal tide and the most prominent planetary wave at these latitudes. A diurnal mode is also observed which shows even greater day-to-day variability. There is good qualitative agreement between the measurements made at EISCAT, Chatanika and Søndrestrøm, especially for the semidiurnal component. There is also reasonable agreement with the predictions made using the various models of the lower thermosphere, but the effect of ion on-drag driven by magnetospheric convection is difficult to model in detail, especially at Søndrestrøm. Tidal modes are also observed in the semi-diurnal variation of ion temperature.

Incoherent scatter observations of the E and F regions

This paper outlines the principles of the incoherent scatter technique and briefly reviews the ways in which it has been used to study the ionospheric E and F regions. Examples are given of the important scientific advances that have been made over the last thirty years.

Early incoherent scatter observations at Jicamarca

We describe some of the highlights of the incoherent scatter observations made at Jicamarca in the 1960s. Some of the observations were then and are still now unique and worthy of further study. Of particular note are (1) a long series of electron density measurements extending to altitudes as high as 10,000 km on a few occasions, (2) density and temperature observations during a total solar eclipse, (3) temperature and composition measurements extending past the O +-H + transition region, and (4) observations of the ion gyro resonance effect for protons.

Comparison of incoherent scatter observations of electron density, and electron and ion temperature at Millstone Hill with the International Reference Ionosphere

Millstone Hill incoherent scatter (IS) observations of electron density ( N e , electron temperature ( T e ) and ion temperature ( T i ) are compared with the International Reference Ionosphere (IRI-86) for both noon and midnight, for summer, equinox and winter, at both solar maximum (1979–1980) and solar minimum (1985–1986). The largest difference in N e is found in the topside, where values of N e given by IRI-86 are generally larger than those obtained from IS measurements, by a factor which increases with increasing height, and which has a mean value near two at 600 km. Apart from the bottom of the profile, which is tied to the CIRA neutral temperature, the IRI-86 T e model has no solar cycle variation. However, the IS measurements during the summer reveal larger T e at solar maximum than at solar minimum. At other seasons higher T e at solar maximum occurs only during the daytime at the greater heights. Nighttime T e is shown by the IS radar to be generally larger in winter than in summer, an effect not included in the IRI. This is apparently due to photoelectron heating during winter from the sunlit ionosphere conjugate to Millstone Hill. The day-night difference in T i given by IRI-86 above 600km is not as large in the IS measurements.

Radar and optical measurements of ionospheric processes associated with intense subauroral electric fields

Observations of very large poleward directed electric fields were obtained with a clustered set of instrumentation that included the Millstone Hill incoherent scatter radar, the Boston University Mobile Ionospheric Observatory, and the HILAT and Defense Meteorological Satellite Program (DMSP) F6 and F7 satellites. In this paper we concentrate on data from the Millstone Hill incoherent scatter radar which was operated on selected evenings in a rapid azimuthal scan, centered on magnetic west. The mode was designed with the express purpose of measuring line‐of‐sight drift velocity and electron density as a function of latitude during events with large localized electric fields. During this same period, the Defense Nuclear Agency HILAT satellite made northern hemisphere measurements every 100 min of ion drift, density, and field‐aligned currents across the equatorial boundary of the auroral oval. A detailed study of optical data in this region is provided in a companion paper. On the evenings of April 20 and 21, 1985, during an intense magnetic storm (Kp > 8+), large ionospheric electric fields (E > 80 mV/m) were detected along the edge of the auroral oval with the Millstone Hill incoherent scatter radar. These constitute the first definitive incoherent scatter observations of this phenomenon. An L shell‐aligned (zero order) deep trough in electron density was colocated with these large electric fields at L shells as low as L = 2.8. These data indicate that the trough develops much more quickly than present theories predict, at least near the F peak. We also report elevated ion and electron temperatures in the trough and conjecture that these may contribute to the rapid decay. We also show that the associated field‐aligned currents are very weak, as predicted by Banks and Yasuhara (1978) but that it is the F region structure which dominates the conductivity gradient rather than the E region emphasized by the earlier work. We also discuss the data set in light of competing theories for the production of large electric fields and for undulations of the edge of the diffuse aurora. In particular we discuss the importance of large radial ion temperature gradients indicated by the DMSP data we present.

Observations of atmospheric gravity waves with incoherent scatter radar

Incoherent scatter observations of the ionosphere have been used to monitor the passage of atmospheric gravity waves. The period of a wave and its vertical wavelength can be derived from profiles of electron concentration or field-aligned plasma velocity, and measurements of this kind have been used to confirm the Hines' dispersion equation. On occasions waves have been observed which maintain a regular periodicity over many cycles. Incoherent scatter can also be used to study the source region of large-scale atmospheric gravity waves in the auroral zone. Such observations have identified Joule heating and Lorentz forcing in the auroral electrojet as the main mechanisms for generating large-scale waves which are later observed at mid-latitude propagating equatorwards. The observations often reveal a quasi-periodicity in the substorm electric field which largely determines the periodicity of the wave launched by the electrojet, although this may be modified in the far-field by filtering and dispersion.