Ionospheric Scatter Research Articles

Effects of solar and geomagnetic activities on the sub-ionospheric very low frequency transmitter signals received by the DEMETER micro-satellite

In the framework of seismic precursor electromagnetic investigations, we analyzed the very low frequency (VLF) amplitude signals recorded by the Instrument Champ Electrique (ICE) experiment on board the DEMETER micro-satellite. The sun-synchronous orbits of the micro-satellite allowed us to cover an invariant latitude of between -65° and +65° in a time interval of about 40 min. We considered four transmitter signals emitted by stations in Europe (France, FTU, 18.3 kHz; Germany, DFY, 16.58 kHz),Asia (Japan, JP, 17.8 kHz) and Australia (Australia, NWC, 19.8 kHz). We studied the variations of these VLF signals, taking into consideration: the signal-to-noise ratio, sunspots, and the geomagnetic activity. We show that the degree of correlation in periods of high geomagnetic and solar activities is, on average, about 40%. Such effects can be fully neglected in the period of weak activity. We also find that the solar activity can have a more important effect on the VLF transmitter signal than the geomagnetic activity. Our data are combined with models where the coupling between the lithosphere, atmosphere and ionosphere is essential to explain how ionospheric disturbances scatter the VLF transmitter signal.

A survey of plasma irregularities as seen by the midlatitude Blackstone SuperDARN radar

The Super Dual Auroral Radar Network (SuperDARN) is a chain of HF radars that monitor plasma dynamics in the ionosphere. In recent years, SuperDARN has expanded to midlatitudes in order to provide enhanced coverage during geomagnetically active periods. A new type of backscatter from F region plasma irregularities with low Doppler velocity has been frequently observed on the nightside during quiescent conditions. Using three years of data from the Blackstone, VA radar, we have implemented a method for extracting this new type of backscatter from routine observations. We have statistically characterized the occurrence properties of the Sub Auroral Ionospheric Scatter (SAIS) events, including the latitudinal relationships to the equatorward edge of the auroral oval and the ionospheric projection of the plasmapause. We find that the backscatter is confined to local night, occurs on ≈70% of nights, is fixed in geomagnetic latitude, and is equatorward of both the auroral region and the plasmapause boundary. We conclude that SAIS irregularities are observed within a range of latitudes that is conjugate to the inner magnetosphere (plasmasphere).

First observations of ionospheric irregularities and flows over the south geomagnetic pole from the Super Dual Auroral Radar Network (SuperDARN) HF radar at McMurdo Station, Antarctica

[1] Observations from the first 100 days of operation of the Super Dual Auroral Radar Network (SuperDARN) radar at McMurdo Station, Antarctica are presented. The observations are examined for incidence of ionospheric scatter, which is higher than has been found at the auroral zone radars of the SuperDARN network. During some hours of each day, the probability of observing ionospheric scatter exceeded 90%. In the later portion of the period examined, there was a period of time each day during which the scatter incidence dropped significantly. Incidence was as examined versus solar illumination of the ionosphere, and it was found that the decreases coincided with periods when the ionosphere was in darkness. From this, it is concluded that propagation effects were the reason for the decreases. Plasma velocity observations from the period are also presented and compared with the prevailing interplanetary magnetic field (IMF). The observations were averaged over the central polar cap (Λ > 85°) and were selected for intervals when the radar line of sight was nearly parallel to the Earth-Sun (E-S) line or perpendicular (E-S-perp) to it. A linear relationship was found between the IMF z component and the E-S velocity and similar relationship between the E-S-perp velocity and the IMF y component. There was a significant spread of velocities about the best fit lines. The IMF and average velocity time series were cross-correlated for a number of intervals, and it was found that the average correlation was about 50%, though during some intervals, the correlation exceeds 80%. No explanation is given for the variance of the correlation.

A new approach for identifying ionospheric backscatter in midlatitude SuperDARN HF radar observations

The Super Dual Auroral Radar Network (SuperDARN) is a network of HF radars that are traditionally used for monitoring phenomena in the Earth's ionosphere at high latitudes. The radar backscatter is due primarily to reflections from plasma irregularities in the ionosphere, known as ionospheric scatter, and to signal reflected from the ground, known as ground scatter. In recent years, SuperDARN has expanded to midlatitudes to provide improved coverage of the auroral region during times of enhanced geomagnetic activity. In addition to high‐speed auroral flows, the radars commonly see a variety of low‐velocity plasma drift associated with the quiet time midlatitude ionosphere. The traditional method of distinguishing between scatter types in SuperDARN data was developed for high latitudes and depends solely on the Doppler velocity and Doppler spectral width of each data point. This method has proven inadequate for identifying quiet time midlatitude ionospheric scatter. In this paper, we present a new technique for the classification of SuperDARN data, which operates on a distributed range time basis and involves procedures similar to “depth first search.” Using the new method for classification of ground and ionospheric scatter, we show a dramatic improvement in the determination of ionospheric scatter within extended (>1 h) events. Compared to the traditional method, the number of ionospheric measurements resolved increases by more than 50%. The new classification algorithm identifies discrete events of ionospheric scatter and can be applied to statistical analysis of event occurrence and characteristics.

Comparison between some methods for estimating thermospheric parameters using ionospheric incoherent scatter measurements

Three methods for estimating the atomic oxygen density and exospheric temperature using the ionospheric incoherent scatter measurements have been compared. These methods are based on the usage of the relationship obtained from the energy conservation equation. They are applicable under the conditions of the atmosphere composed of oxygen atoms and ions. The first method is based on the regression analysis of finding minimal deviations of the calculated electron densities (N) from the values measured at altitudes of 300–450 km. The second method is based on solving the set of equations of the analytical expression relating N to ionospheric characteristics. These equations are given for several altitudes. The third method is related to the selection of such atomic oxygen densities at which the neutral gas temperature remains constant at an altitude above 350 km. All methods give similar results. The calculations based on the measurements performed at the Irkutsk radar on September 23, 1998, showed that the atomic oxygen densities at 400 km, estimated using the first and second methods, account for 0.48 of the value predicted by the MSIS-86 model, and such densities estimated using the third method account for 0.59 of the model value. In this case the estimated exospheric temperatures are larger than the values obtained using the thermospheric model by 11% for the first and second methods and by 14% for the third method.

Probabilistic identification of high‐frequency radar backscatter from the ground and ionosphere based on spectral characteristics

We present an analysis of the spectral characteristics of 1‐hop HF radar ground scatter and ‐ and 1‐hop ionospheric scatter as measured by the Super Dual Auroral Radar Network. Our objective is to determine criteria that separate signals scattered from the ground and the ionosphere. We find that for both ground scatter and ionospheric scatter that the probability density function of backscatter Doppler velocity decreases exponentially with velocity, but with significantly different e‐folding velocities for the two types of backscatter. We use this observation to separate the total probability density of Doppler velocity and spectral width into two component distributions. This process yields the posterior probability that a signal of given Doppler velocity and spectral width is ground scatter. The resulting criterion for classification of a particular signal as ground scatter, v < 33.1 m/s + 0.139w − (0.00133 s/m)w2, significantly reduces the probability that a signal will be erroneously classified as ionospheric scatter, while only moderately increasing the probability that an ionospheric scatter signal will be erroneously classified as ground scatter. Finally, we validate the ground scatter probability function by demonstrating that the backscatter virtual height increases as expected with increasing probability of ground scatter.

Effects of mixed scatter on SuperDARN convection maps

Abstract. The SuperDARN radars map high-latitude ionospheric plasma drift velocities by measuring the Doppler frequency shift of HF signals scattered by decameter electron density irregularities. In many cases the ionospheric returns are contaminated by strong scatter from the ground or sea surface. In this paper we develop and test a two-component fitting algorithm to separate ionospheric and surface scatter components. Application of the technique to sample data reveals that mixed scatter may considerably distort ionospheric convection patterns derived from the radar data and can cause underestimation of the plasma drift velocity.

Observations of Plasma Line Splitting in the Ionospheric Incoherent Scatter Spectrum

Wide-bandwidth ionospheric incoherent scatter (IS) spectra obtained using the Arecibo IS radar show the occurrence of a split in the plasma line (i.e., two plasma lines) when the plasma frequency is close to the second harmonic of the electron gyrofrequency. This split is predicted in the IS theory for a magnetized plasma, but observations have never been reported. Here we present the experimental results and theoretical calculations supporting the observations. These results may assist in understanding the behavior of Langmuir waves in the magnetized plasma and are a validation of what historically was a somewhat controversial aspect of the IS theory.

Ted Harrison 1930-2007

Associate of the RAS, pioneer in ionospheric incoherent scatter theory and observation, practical manager and leader, not least of EISCAT.

An auroral westward flow channel (AWFC) and its relationship to field-aligned current, ring current, and plasmapause location determined using multiple spacecraft observations

Abstract. An auroral westward flow channel (AWFC) is a latitudinally narrow channel of unstable F-region plasma with intense westward drift in the dusk-to-midnight sector ionosphere. AWFCs tend to overlap the equatorward edge of the auroral oval, and their life cycle is often synchronised to that of substorms: they commence close to substorm expansion phase onset, intensify during the expansion phase, and then decay during the recovery phase. Here we define for the first time the relationship between an AWFC, large-scale field-aligned current (FAC), the ring current, and plasmapause location. The Tasman International Geospace Environment Radar (TIGER), a Southern Hemisphere HF SuperDARN radar, observed a jet-like AWFC during ~08:35 to 13:28 UT on 7 April 2001. The initiation of the AWFC was preceded by a band of equatorward expanding ionospheric scatter (BEES) which conveyed an intense poleward electric field through the inner plasma sheet. Unlike previous AWFCs, this event was not associated with a distinct substorm surge; rather it occurred during an interval of persistent, moderate magnetic activity characterised by AL~−200 nT. The four Cluster spacecraft had perigees within the dusk sector plasmasphere, and their trajectories were magnetically conjugate to the radar observations. The Waves of High frequency and Sounder for Probing Electron density by Relaxation (WHISPER) instruments on board Cluster were used to identify the plasmapause location. The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) EUV experiment also provided global-scale observations of the plasmapause. The Cluster fluxgate magnetometers (FGM) provided successive measurements specifying the relative location of the ring current and filamentary plasma sheet current. An analysis of Iridium spacecraft magnetometer measurements provided estimates of large-scale ionospheric FAC in relation to the AWFC evolution. Peak flows in the AWFC were located close to the peak of a Region 2 downward FAC, located just poleward of the plasmapause. DMSP satellite observations confirmed the AWFC was located equatorward of the nightside plasmasheet, sometimes associated with ~10 keV ion precipitation.

The Survey of Ionospheric Scattering Function

Because of its advantages, such as broad sounding range and real-time sounding, etc, ionospheric oblique back sounding has been regarded as one of the most important ionospheric sounding means. Ionospheric oblique back sounding is capable to get features of HF back scatting channel and the sounding data will be shown by the Frequency-Time delay-Amplitude 3D graphics in the form of scattering function. The Doppler frequency shift fd, the multidiameter spread according to time and channel fading can be obtained directly by surveying the scattering function of the HF channel. The new type ionospheric oblique backsounding system (IOBSS) developed by Wuhan University has been put in use and got many sounding data.

Magnetic local time, substorm, and particle precipitation-related variations in the behaviour of SuperDARN Doppler spectral widths

Abstract. Super Dual Auroral Radar Network (DARN) radars often detect a distinct transition in line-of-sight Doppler velocity spread, or spectral width, from <50ms–1 at lower latitude to >200ms–1 at higher latitude. They also detect a similar boundary, namely the range at which ionospheric scatter with large spectral width suddenly commences (i.e. without preceding scatter with low spectral width). The location and behaviour of the spectral width boundary (SWB) (and scatter boundary) and the open-closed magnetic field line boundary (OCB) are thought to be closely related. The location of the nightside OCB can be inferred from the poleward edge of the auroral oval determined using energy spectra of precipitating particles measured on board Defence Meteorology Satellite Program (DMSP) satellites. Observations made with the Halley SuperDARN radar (75.5° S, 26.6° W, geographic; –62.0°Λ) and the Tasman International Geospace Environment Radar (TIGER) (43.4° S, 147.2° E; –54.5°Λ) are used to compare the location of the SWB with the DMSP-inferred OCB during 08:00 to 22:00 UT on 1 April 2000. This study interval was chosen because it includes several moderate substorms, whilst the Halley radar provided almost continuous high-time resolution measurements of the dayside SWB location and shape, and TIGER provided the same in the nightside ionosphere. The behaviour of the day- and nightside SWB can be understood in terms of the expanding/contracting polar cap model of high-latitude convection change, and the behaviour of the nightside SWB can also be organised according to substorm phase. Previous comparisons with DMSP OCBs have proven that the radar SWB is often a reasonable proxy for the OCB from dusk to just past midnight (Chisham et al., 2004). However, the present case study actually suggests that the nightside SWB is often a better proxy for the poleward edge of Pedersen conductance enhanced by hot particle precipitation in the auroral zone. Simple modeling implies that the large spectral widths must be caused by ~10-km scale velocity fluctuations. Key words. Ionosphere (auroral ionosphere; ionospheremagnetosphere interactions) – Magnetospheric physics (storms and substorms)

Effects of Ionospheric Radio Wave Scattering in the Decameter Interferometry

We consider some theoretical issues concerning diffraction of radio waves in a randomly irregular ionosphere with application to the problems of long-wavelength interferometry of cosmic objects. The statistical characteristics of intensity fluctuations of the decameter radio emission from discrete sources in the case of ground-based observations by two-element interferometers with very long and small baselines are analyzed. Analytical expressions are obtained for the autocorrelation function of the radiation intensity and for the scintillation spectrum of a point source in the limiting cases of large and small phase increments in an irregular ionospheric plasma. We find that in the case of radio interferometric reception, the scintillation spectrum corresponding to observations of a source by a single antenna is transferred from the zero-frequency region to the region of the Doppler frequency of the interferometer. It is shown that decameter ground-based and space-borne radio interferometers can be used to study the angular distribution of the radio brightness of cosmic sources under conditions of both quiet and disturbed ionosphere.

Passive VHF radar for ionospheric physics

Recent technological advances enable a new class of passive radar instruments. These radars have no dedicated transmitter, observing serendipitous scatter of existing sources. Such radars may have very high performance and cost far less than conventional radars. The resulting equipment is essentially reduced to simple antennas, desktop computers, and Global Positioning System equipment. The safety hazards, interference problems, licensing issues, and financial costs associated with high-power transmitters are conspicuously absent. We will offer general design considerations and describe our own instrument, which observes the scatter of commercial FM broadcasts. Our system provides far better range and Doppler resolution than any conventional radar used in ionospheric coherent scatter studies, and is completely free of any range or Doppler aliasing problems. There are two principal drawbacks to passive radars: the “front end” signal processing cost is very large, and there is a significant data transport problem. However, spectacular advances in low-cost computing and internet bandwidth have rendered these problems quite easy to solve.

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.

High‐latitude ionospheric perturbations and gravity waves: 1. Observational results

The Saskatoon HF radar has been used to observe ionospheric perturbations and gravity waves at high latitudes. The observations on December 29, 1993, taken during an extended period of northward interplanetary magnetic field, are analyzed in detail. Two distinct intervals of backscatter are received, a near‐range interval of ground scatter echoes from 900 to 1500 km and a far‐range interval of ionospheric scatter from 2000 to 2600 km. The ground scatter intensity shows strong quasi‐periodic enhancements caused by Earth‐reflected gravity waves. The ionospheric echoes, which form a circular band of radius about 600 km, occur in the auroral zone below 80° magnetic latitude, just equatorward of the convection reversal. They cover a magnetic longitude interval from −60° to 80°, corresponding to a magnetic local time period from about 1100 to about 1800 LT. From the shape of the Earth‐reflected gravity wave intensity bands on the latitude‐time plots, those gravity waves can be traced back to the ionospheric perturbation region, which lies in the high‐latitude echo zone, namely the auroral zone. This is confirmed by a two‐dimensional cross‐correlation between the time series for the intensity of the gravity‐wave modulated ground scatter and the auroral zone ionospheric scatter. That correlation shows evidence for a time delay corresponding to the Earth‐reflected gravity wave propagation time between the regions. Also, the correlation peaks for a range separation about 1100 km, which implies that the normal one‐and‐a‐half‐hop propagation mode usually used to explain the presence of the far‐range auroral echoes cannot be operative and that the Pedersen ray “trapped” ionospheric mode is responsible for the observations.

CUTLASS observations of convection in the cusp region

The Finnish radar of CUTLASS has been operational since February 1995. Between 0620 and 0800 UT on August 25 1995, a region of ionospheric scatter, which was limited to only a few degrees of magnetic latitude, moved poleward at a rate of some 7 degrees per hour. The line of sight velocity measurements on a special high time resolution beam indicate a flow reversal boundary which moved poleward throughout the interval. Flow was towards (away from) the radar on the poleward (equatorward) side of the boundary. The flow towards the radar along this beam was at a high speed, >800 m s −1, while the speed away from the radar was less than 200 m s −1. Spatial plots indicate high speed flow away from the radar in the west of the field of view. A tentative interpretation is that the high speed flow is caused by lobe cell reconnection while the low speed flow is associated with either the contraction of the polar cap or due to viscous processes at the magnetopause.

F-region seasonal behavior as measured by the MU radar

We present MU radar ionospheric incoherent scatter results from the period December 1986 to August 1988. We extend our previous 3-month model of the ion vector velocity to a full year and determine seasonal variations. We also present solstitial and equinoctial patterns of electron-density and plasma-temperature behavior. Compared with our previous 3-month model, the full-year velocity model is largely diurnal in nature, the higher harmonic being very variable and tending to average out as noise over a long period. The seasonal behavior of the electron density may be explained largely in terms of the behaviors of the neutral composition and of the timing of the diurnal change of direction of the neutral wind with respect to the time of sunset. According to the neutral composition a high-altitude O + layer and a low-altitude molecular-ion layer compete for density predominance. The molecular-ion layer often becomes stronger during summer, and also during times of strong magnetic disturbance. The mean equinox daytime F-region electron and ion temperatures ( T e and T i ), and particularly their ratio ( T r = T e T i ), is controlled largely by the electron density, with other specific time-of-day effects. At night T r = 1, and at sunrise T r > 1 at all F-region altitudes measured. During midday we have two cases. For a peak F-layer density of about 10 6 cm −3 or greater, T r > 1 below 300 km altitude and T r = 1 above 300 km altitude; for lower electron densities T r > 1 at all F-region altitudes. Near sunset T r generally increases as the electron density decreases while photoionization is continuing.

A MU radar‐based study of mid‐latitude F region response to a geomagnetic disturbance

During the night of January 20–21, 1989, ionospheric incoherent scatter power measurements were made with the MU (middle and upper atmosphere) radar at Shigaraki (geographic latitude 34.85°N, longitude 136.10°E; geomagnetic latitude 24.9°, longitude 204.3°), Japan, and the electron density profiles in the 180‐ to 1000‐km height range were derived at 8‐min intervals. The observations showed the presence of three F region disturbances during the night. During the very large first disturbance, which lasted from 2300 to 0240 LT approximately, the height of maximum electron density Nm increased by 220 km in 2 hours to reach an altitude of 600 km. The other two, smaller disturbances occurred during 0300–0500 LT and 0530–0700 LT approximately. A detailed interpretation of the above F region disturbances is given. Examination of some high‐ and middle‐latitude magnetograms showed the beginning of an intense geomagnetic substorm at auroral latitudes at the start of the first F region disturbance, and a less intense substorm around the starting time of the second F region disturbance. On the basis of this evidence, the first two F region disturbances are interpreted as the result of large vertical drifts of F region ionization due to the substorm‐generated east‐west electric fields appearing at mid‐latitudes. The patterns of h′F variations during this night at five ionospheric stations in Japan support the above interpretation. Additionally, during the second disturbance the possible presence of a wind perturbation due to the equatorward propagation of a wave disturbance, generated probably by the first major substorm, is indicated by the MU radar data and the ionosonde data. The third F region disturbance is attributed to a neutral wind perturbation associated with a gravity wave traveling equatorward, the wave being generated most plausibly by the Joule heating during the first major substorm. The brief, unusual distortions of the F layer shape, as revealed by MU radar observations, are also interpreted as the consequence of an unusually large convergence of ionization in the bottomside of the F layer due to the unusually large electric field and wind perturbations. Theoretical calculations are made using the MU radar data and F region continuity equation, and the east‐west electric field values during the disturbances are quantified. During the first major rise of the F layer during 2300–0115 LT the inferred eastward electric field in the ionosphere at Shigaraki is found to be as large as 6±1 mV/m; and after this disturbance a westward electric field and/or a poleward meridional wind is found to occur for some time. An eastward electric field of about 3.0 mV/m is found to occur during the second F region disturbance, along with a possible meridional wind perturbation. The large eastward electric field around local midnight is most unusual in both magnitude and direction, because the theoretical models of disturbance electric fields predict a strong westward electric field in this LT sector; this result has important implications for understanding the physics of disturbance electric fields at middle and low latitudes during substorms.

Neutral and ion composition changes in the F region over Millstone Hill during the Equinox Transition Study

We use overhead ionospheric incoherent scatter radar data from Millstone Hill to deduce the stormtime variations of the thermospheric temperature, O and N2 densities, and F1 region ion composition during the Equinox Transition Study of September 17–24, 1984. Using the measured neutral temperature profile and O density at 400 km altitude, we extrapolate to determine O density changes in the lower thermosphere. Using these O densities and the measured electron densities, we deduce the variation of the N2 density. Our initial attempt leads to the deduction of large depletions in both O and N2 in the lower thermosphere. On using the MSIS model neutral temperatures for the extrapolation, however, we compute O depletions having magnitudes more comparable with theoretical predictions and N2 variations consistent with expected thermal expansion effects. We speculate that the radar‐measured neutral temperatures were contaminated by ion‐neutral frictional heating effects during the disturbed periods of the ETS and that this served to invalidate neutral density extrapolations made with these temperatures. Further, we compute the height of 50% O+ ion composition in the F1 region and the height of the peak electron density in the ionosphere, and, upon comparing these, find that the height of the peak on the disturbed days was located in the lower F1 region molecular‐ion layer rather than in the F2 region O+ layer. This occurrence has serious consequences for methods of deducing neutral winds from the height of the F layer, for these methods rely on the peak containing primarily O+ ions.