Incoherent Scatter Observations Research Articles

Charged dust in the D-region incoherent scatter spectrum

We investigate the influence of charged dust on the incoherent scatter from the D-region ionosphere. Incoherent scatter is observed with high-power, large aperture radars and results from electromagnetic waves scattering at electrons that are coupled to other charged components through plasma oscillations. The influence of charged dust can hence be considered an effect of dusty plasma. The D-region contains meteoric smoke particles that are of nanometre size and form from incoming ablating meteors. Detection of such charged dust in the incoherent scatter spectrum from the D-region has previously been proposed and studied to some degree. We here present model calculations to investigate the influence of the charged dust component with a size distribution, instead of the one size dust components assumed in other works. The developed code to calculate the incoherent scatter spectrum from the D-region including dust particles with different sizes and different positive and negative charge states is made available (https://doi.org/10.18710/GHZIIY). We investigate how sizes, number density and charge state of the dust influence the spectrum during different ionospheric conditions. We consider the ionospheric parameters for the location of the EISCAT VHF radar during a year and find that conditions are most suitable for dust detection in winter below 80 km at times with increased electron densities. The prospects to derive dust parameters increase, when the incoherent scatter observations are combined with those of other instruments to provide independent information on electron density, neutral density and temperature.

Radar Investigation of Postsunset Equatorial Ionospheric Instability Over Kwajalein During Project WINDY

AbstractAdvanced Research Projects Agency Long‐Range Tracking and Instrumentation Radar (ALTAIR) data acquired during the NASA Waves and INstabilities from a Neutral DYnamo (WINDY) sounding rocket campaign on Kwajalein Atoll in late August and September 2017 are analyzed to study the development of postsunset F‐region ionospheric plasma density irregularities associated with equatorial spread F (ESF) conditions. Unstable conditions existed on nine of ten observing nights during the campaign. The first indication of instability onset each night was a patchy layer of coherent scatter in the bottomside/valley region. Patchy bottom‐type layers are telltales of plasma convective instability driven by vertical current in the F region associated with bottomside shear flow. The resulting flow evolves rapidly into fully developed instability through a bootstrapping process. Sometimes, the resulting topside depletions exhibited large‐scale structuring with a spatial scale too large to be associated with the preferred scale for plasma instability. We examine incoherent scatter observations of the background ionospheric morphology prior to the appearance of bottom‐type layers and find that a small degree of large‐scale bottomside structuring is sometimes present. Numerical simulations suggest that this structuring can act as a seed and provide the initial conditions necessary to account for the clustering of the topside plumes that emerge later.

Radio Beacon and Radar Assessment and Forecasting of Equatorial F Region Ionospheric Stability

AbstractIonospheric conditions on two adjacent nights in March 2019 were observed at the Jicamarca Radio Observatory using a combination of incoherent scatter, coherent scatter, and high‐frequency (HF) radio modes. The HF data came from a network of beacons consisting of three transmitters and six receivers operating at two frequencies and deployed regionally. The HF beacons employ pseudorandom noise coding and can be used to measure group delay (pseudorange) and Doppler shift, and the time derivative of optical path length. A method for inferring volumetric estimates of electron density regionally from the HF data is described. The radar and HF data are interpreted in light of a direct numerical simulation of the ionospheric interchange instability to elucidate why convective plumes and equatorial spread F conditions occurred on one night but not the other. The numerical simulation accurately predicted whether convective plumes would develop on a given night, utilizing initial conditions and forcings derived from the incoherent scatter data. The HF data were consistent with the incoherent scatter observations and remained intelligible throughout the equatorial spread F event. Crests in the bottomside electron density associated with convective plumes at higher altitudes could be seen propagating through the region in the HF data. It should be possible to incorporate HF data in assimilative simulations of interchange instabilities in order to predict where and when individual convective plumes emerge.

Artificial ionospheric modification: The Metal Oxide Space Cloud experiment

Clouds of vaporized samarium (Sm) were released during sounding rocket flights from the Reagan Test Site, Kwajalein Atoll in May 2013 as part of the Metal Oxide Space Cloud (MOSC) experiment. A network of ground-based sensors observed the resulting clouds from five locations in the Republic of the Marshall Islands. Of primary interest was an examination of the extent to which a tailored radio frequency (RF) propagation environment could be generated through artificial ionospheric modification. The MOSC experiment consisted of launches near dusk on two separate evenings each releasing ~6 kg of Sm vapor at altitudes near 170 km and 180 km. Localized plasma clouds were generated through a combination of photoionization and chemi-ionization (Sm + O → SmO+ + e–) processes producing signatures visible in optical sensors, incoherent scatter radar, and in high-frequency (HF) diagnostics. Here we present an overview of the experiment payloads, document the flight characteristics, and describe the experimental measurements conducted throughout the 2 week launch window. Multi-instrument analysis including incoherent scatter observations, HF soundings, RF beacon measurements, and optical data provided the opportunity for a comprehensive characterization of the physical, spectral, and plasma density composition of the artificial plasma clouds as a function of space and time. A series of companion papers submitted along with this experimental overview provide more detail on the individual elements for interested readers.

ЭФФЕКТЫ СОЛНЕЧНОГО ЗАТМЕНИЯ 20 МАРТА 2015 Г. В ИОНОСФЕРЕ НАД ЕВРОПОЙ: РЕЗУЛЬТАТЫ ИОНОЗОНДОВЫХ НАБЛЮДЕНИЙ

ЭФФЕКТЫ СОЛНЕЧНОГО ЗАТМЕНИЯ 20 МАРТА 2015 Г. В ИОНОСФЕРЕ НАД ЕВРОПОЙ: РЕЗУЛЬТАТЫ ИОНОЗОНДОВЫХ НАБЛЮДЕНИЙ

ТЕПЛОВЫЕ И ДИНАМИЧЕСКИЕ ПРОЦЕССЫ В ИОНОСФЕРЕ ВО ВРЕМЯ ЧАСТНОГО ЗАТМЕНИЯ СОЛНЦА 20 МАРТА 2015 г. НАД ХАРЬКОВОМ. РЕЗУЛЬТАТЫ РАСЧЕТОВ

ТЕПЛОВЫЕ И ДИНАМИЧЕСКИЕ ПРОЦЕССЫ В ИОНОСФЕРЕ ВО ВРЕМЯ ЧАСТНОГО ЗАТМЕНИЯ СОЛНЦА 20 МАРТА 2015 г. НАД ХАРЬКОВОМ. РЕЗУЛЬТАТЫ РАСЧЕТОВ

Fine structure in midlatitude sporadic E layers

Fine structure in midlatitude sporadic E layer patches or “clouds” is apparent in incoherent scatter observations from the Arecibo Radio Telescope. The fine structure is wavelike with predominant horizontal wavelengths as large as about 2–3km. We attribute the structure to a drift wave instability operating in the collisional regime. A linear, local dispersion relation for the waves is described which predicts growth driven by polarization electric fields in the cloud. A numerical simulation produces wave growth and other features consistent with the dispersion relation, including finite parallel wavenumbers. The kilometric irregularities are thought to be the primary waves from which secondary, meter-scale waves in the layers can form.

Solar eclipse of August 1, 2008, above Kharkov: 1. Results of incoherent scatter observations

The observation results of the effects in the geospace plasma during a partially (magnitude ∼0.42) solar eclipse are presented. The experimental data were obtained with an incoherent scatter radar of the Institute of the Ionosphere (near Kharkov). During the eclipse, the density at the F2 layer maximum decreased by 32%, the foF2 critical frequency decreased by 17.5%, and the altitude of the F2 layer maximum increased insignificantly. At altitudes of 290–680 km, the electron density decreased by ∼25%. During the eclipse, the electron and ion temperature decreased by 70–180 and 0–140 K, respectively, at altitudes of 190–490 km. Near the eclipse main phase, the plasma velocity vertical component decreased by 10–45 m/s at altitudes of 200–470 km, respectively. At the time of the eclipse main phase, the hydrogen ion fractional density increased by 50% as compared to the reference day at altitudes of 450–650 km.

Soliton-induced spectrally uniform ion line power enhancements at the ionospheric F region peak

We present European Incoherent Scatter (EISCAT) observations of spectrally uniform ion line power enhancements (SUIPE), where the up- and downshifted shoulder and the spectral valley between them are enhanced simultaneously and equally. We have identified 48 cases of this type of ion line enhancement in data from the EISCAT Svalbard radar taken during the International Polar Year (extending from March 2007 to the end of February 2008). The SUIPEs are observed at altitudes between 210 km and 280 km with a standard deviation of 9% of the average occurrence height 230 km. The power enhancements are one order of magnitude above the thermal level. The SUIPEs occur at the ionospheric F region density peak with 85% of the cases located within 10 km of the peak. These characteristics are in good agreement with the predictions of a recently published model for soliton-induced ion-line enhancements at the F region peak. The SUIPE occurrence shows a clear preference for magnetically disturbed conditions, with the likelihood of occurrence increasing with increasing K index. A majority of the events occur in the magnetic evening to pre-midnight sector.

A new analysis method for determining polar ionosphere and upper atmosphere characteristics from ESR data: Illustration with IPY period

The one‐year incoherent scatter (IS) data set recorded at the EISCAT Svalbard Radar (ESR) during the International Polar Year (IPY) is an opportunity for studies on long‐term variations in the ionosphere and neutral atmosphere. Such studies place severe requirements on the analysis of any IS observations which result in two major difficulties. First, concerning the ionosphere, the challenge is to make accurate measurements of the temperature profiles in the F1‐region, where the ion composition is changing, highly variable, and unknown. Changes in ion mass and temperature affect the received spectra in almost identical ways, and ambiguities in one present insurmountable difficulties in the determination of the other. The second difficulty, concerning the neutrals, is that it is necessary to separate the contributions from the neutral atmosphere and from frictional heating. In the F1‐region, differential drift induces differential heating of the atomic and molecular ions which can be used to distinguish between frictional and neutral atmosphere coupling. In this paper, we present in detail a method to solve these two difficulties: 13‐moment, parallel, two‐ion (atomic and molecular) energy equations are used to fit the full IS profile along the magnetic field. The molecular‐atomic ion composition transition is assumed to follow an analytical hyperbolic tangent function, and the full altitude profile of the measured ion temperature is then used to fit the desired parameters. The results are validated against simulated data from a numerical model, and then against real measurements. A 36‐hour data set was compared with the numerical model TRANSCAR, making some unknowns available from the numerical model (ion composition, frictional heating, neutral temperature), providing a convincing demonstration of the validity of the method. In particular, this “full profile” method is sufficiently robust for routine use, even for periods with relatively high convection electric fields. Finally, the application of the method to the IPY ESR data set is presented and some initial trends are identified.

PMSE-observations with the EISCAT VHF and UHF-radars: Statistical properties

Observations of polar mesosphere summer echoes (PMSE) have been carried out with the EISCAT VHF radar during five summer seasons between 2003 and 2007 as well as with the EISCAT UHF radar in July 2004 and 2005. From a total of 523 h of observations with the VHF radar, PMSE have been detected for more than 300 h overall. In July 2004 and 2005 when PMSE were detected by both radars, the occurrence rate of the VHF PMSE is as large as 85.0% whereas the UHF PMSE are much rarer with an occurrence rate of 5.3%. Comparison of the year-to-year variation between the occurrence rate of PMSE at 224 MHz and noctilucent clouds (NLC) observed with a nearby lidar shows similar properties. The year-to-year variation of the occurrence rate of PMSE at 53.5 MHz, however, is quite different. This might imply that PMSE at 224 MHz require the presence of larger ice particles than at 53.5 MHz, and hence show similar behavior as NLC. In contrast, the correlations of the occurrence rate of PMSE at 224 MHz with the solar and geomagnetic activities both show positive correlations which are, however less significant compared with the PMSE at 53.5 MHz. In addition, the occurrence rate and volume reflectivities of PMSE derived from the observations at 224 MHz both show remarkable characteristics of seasonal variation and diurnal variation. As expected from previous PMSE observations at 53.5 MHz, the seasonal variation is determined by the condition of supersaturation with respect to ice. The diurnal variations, however, are much more complicated, as they can be significantly influenced by the temperature changes due to tidal activity as well as the ionization level due to the solar radiation and high-energy particle precipitation. Taking into account the ‘turbulence with large Schmidt number’-theory of PMSE, a comparison between the volume reflectivities of simultaneous VHF PMSE and UHF PMSE is carried out and shows 99% of the observations with reflectivity ratios larger than a minimum value predicted by theory. Last but not least, we considered electron densities derived from UHF incoherent scatter observations at altitudes above the PMSE-layers (i.e., 88 and 90 km) during cases with VHF PMSE with and without UHF PMSE as well as a comparison of spectral widths from observations with both radars. This reveals that the occurrence of UHF-PMSE most likely requires larger electron densities and larger spectral widths (corresponding to enhanced neutral turbulence) at altitudes above 84 km.

New incoherent scatter diagnostic methods for the heated D-region ionosphere

Abstract. A new incoherent scatter (IS) diagnostic technique for the actively heated D-region ionosphere is presented. In this approach, an exponential autocorrelation function (ACF) was fitted to the data by using a Markov Chain Monte Carlo (MCMC) inversion and the Sodankylä Ion Chemistry model (SIC). The method was applied for a set of combined EISCAT heating and VHF radar experiments carried out in November 2006. A newly designed radar experiment, sippi, based on optimised phase codes and direct sampling of the transmitted and the received IS signal, was used to produce ACF estimates of the target plasma. Systematic features associated with the heating were found by comparing the mean ACFs corresponding to heated and unheated periods. However, the data analysis revealed reasonable electron temperature estimates at the altitudes of the expected maximum effect (70–75 km) only in three cases out of six, corresponding to the highest signal-to-noise ratio (SNR) conditions. In two of these cases, the electron temperature was increased by a factor of 5 to 7, which is in a good agreement with the theoretical heating modelling. In the case of the first presented data example, the model overestimates the effect. These are the first successful IS observations of the HF-induced maximum electron temperature enhancements in the D-region ionosphere.

Comparison and validation studies related to the modeling ionospheric convection and the European incoherent scatter observations in the polar cap

[1] We use the ionospheric velocity measurements obtained by the European incoherent scatter (EISCAT) Svalbard radar (ESR) for comparison with the electric field distribution predicted by the new ionospheric convection model based on realistic maps of field-aligned currents. The comparative analysis of model output and observations is based on the data contained both the north–south and east–west components of the ionospheric electric field. Such a comparison provides an independent check of the model. It is shown that the model represents accurately the large-scale features of statistical electric field inferred from the ion velocity measured by the ESR. From observations and modeling we quantitatively determine the dependence of the ionospheric electric field strength on the IMF conditions and how this varies with magnetic local time. For specification of the relationship we derive average convection patterns for sorting by the magnitude and direction of the IMF. The dependence is clearly seen by contrasting the results for two magnitudes of the IMF strength (BT = 1 and 5 nT), for two opposite directions of the IMF Bz component, and for By+, By−. The magnitude of the IMF has the main impact on the average flows under southward IMF conditions. The sign of IMF By is the most important factor influenced the convection system under the IMF northward. For By− the northward component is negative during a major part of the day indicating counterclockwise plasma flow around the pole. For By+, the situation is opposite, the electric field is directed poleward and the plasma rotate clockwise. These are confirmed nicely by the ESR observations.

Optical and radio measurements of a 630‐nm airglow enhancement over Japan on 9 September 1999

Using a comprehensive data set from optical and radio instruments, we investigate a midnight brightness wave that appeared in 630‐nm airglow images over Japan on the night of 9 September 1999. This may be the first such observation of the brightness wave with an all‐sky imager in the East‐Asian longitudinal sector. The imager at Shigaraki (35.6°N, 136.1°E) tracked a north‐northeastward propagation of the wave with an apparent velocity of 500 m s−1 after midnight. Ionosonde observations at five stations in Japan showed that rapid descent of the F2 layer propagated northward beyond 35°N with decreasing amplitude. Incoherent scatter observations with the MU radar at Shigaraki also revealed that the F2 peak altitude decreased from 360 to 280 km during the event. During the F2 layer descent the altitude profile of the electron density became sharp, enhancing the F2 peak electron density. After the F2 layer altitude reached 280 km, electron density in the F2 layer rapidly decreased because of increased neutral density at low altitude. A Fabry‐Perot interferometer (FPI) at Shigaraki observed northward neutral winds of 10–70 m s−1 during the event. A model calculation demonstrates that the meridional winds estimated from the MU radar electron density profiles are fairly consistent with those observed with the FPI. From these results we conclude that the observed northward wind enhancements, probably caused by the midnight temperature maximum, pushed down the plasma in the F2 layer to lower altitudes along the geomagnetic field to cause the 630‐nm airglow intensity enhancement.

Geomagnetic storm effects at F1-layer heights from incoherent scatter observations

Abstract. Storm effects at F1-layer heights (160–200 km) were analyzed for the first time using Millstone Hill (mid-latitudes) and EISCAT (auroral zone) incoherent scatter (IS) observations. The morphological study has shown both increases (positive effect) and decreases (negative effect) in electron concentration. Negative storm effects prevail for all seasons and show a larger magnitude than positive ones, the magnitude of the effect normally increasing with height. At Millstone Hill the summer storm effects are small compared to other seasons, but they are well detectable. At EISCAT this summer decrease takes place only with respect to the autumnal period and the autumn/spring asymmetry in the storm effects is well pronounced. Direct and significant correlation exists between deviations in electron concentration at the F1-layer heights and in the F2-layer maximum. Unlike the F2-layer the F1-region demonstrates a relatively small reaction to geomagnetic disturbances despite large perturbations in thermospheric parameters. Aeronomic parameters extracted from IS observations are used to explain the revealed morphology. A competition between atomic and molecular ion contributions to Ne variations was found to be the main physical mechanism controlling the F1-layer storm effect. The revealed morphology is shown to be related with neutral composition (O, O2, N2) seasonal and storm-time variations. The present day understanding of the F1-region formation mechanisms is sufficient to explain the observed storm effects.Key words. Atmospheric composition and structure (thermosphere-composition and chemistry); ionosphere (ion chemistry and composition; ionospheric disturbances)

Annual and semiannual variations of the midlatitude ionosphere under low solar activity

The annual and semiannual variations of the midlatitude ionosphere under low solar activity are studied using middle and upper (MU) radar (135°E, 35°N) incoherent scatter observations and Sheffield University plasmasphere‐ionosphere model (SUPIM). The variations of the daytime electron density (Ne) and electron and ion temperatures (Te and Ti) at 200–600 km altitudes measured by the radar under low solar activity (F10.7 ≤ 120) are satisfactorily reproduced for the first time by incorporating the radar measured values of the magnetic meridional neutral wind velocity (Uθ) and northward perpendicular plasma drift velocity (V⟂) into SUPIM that uses mass spectrometer incoherent scatter 1986 (MSIS‐86) for neutral densities and neutral temperatures. The study shows that the annual and semiannual variations of the midlatitude ionosphere during daytime at altitudes near and above the ionospheric peak under low solar activity depend more on the direct effect of the neutral wind arising through the changes in ionospheric height than on the indirect effect arising through the changes in thermospheric composition (or atomic to molecular concentration ratio); the indirect effect, however, predominates on the variations at altitudes below the ionospheric peak. The electron temperature (Te) undergoes similar but almost opposite seasonal and semiannual variations as the electron density. The ion temperature (Ti) is closer to neutral temperature than to electron temperature at altitudes up to ∼400 km and shows comparatively weak annual and semiannual variations.

Equinoctial transitions in the ionosphere and thermosphere

Abstract. Equinoctial summer/winter transitions in the parameters of the F2-region are analyzed using ground-based ionosonde and incoherent scatter observations. Average transition from one type of diurnal NmF2 variation to another takes 20–25 days, but cases of very fast (6–10 days) transitions are observed as well. Strong day-time NmF2 deviations of both signs from the monthly median, not related to geomagnetic activity, are revealed for the transition periods. Both longitudinal and latitudinal variations take place for the amplitude of such quiet time NmF2 deviations. The summer-type diurnal NmF2 variation during the transition period is characterized by decreased atomic oxygen concentration [O] and a small equatorward thermospheric wind compared to winter-type days with strong poleward wind and increased [O]. Molecular N2 and O2 concentrations remain practically unchanged in such day-to-day transitions. The main cause of the F2-layer variations during the transition periods is the change of atomic oxygen abundance in the thermosphere related to changes of global thermospheric circulation. A possible relationship with an equinoctial transition of atomic oxygen at the E-region heights is discussed.Key words. Atmospheric composition and structure (thermosphere – composition and chemistry) – Ionosphere (ionosphere- atmosphere interactions; ionospheric disturbances)

<i>Letter to the Editor</i>Effects of hot oxygen in the ionosphere: <i>TRANSCAR</i> simulations

Abstract. Recent studies of the ion energy balance in the mid-latitude ionosphere have led to the suggestion that hot neutral atomic oxygen may play a significant role; the presence of a population of hot O could explain some of the problems met in balancing the ion energy budget for Incoherent Scatter (IS) observations. The aim of the present study is to look at such effects by using numerical simulation. The TRANSCAR model is a time-dependent, 13-moment ionosphere model developed for high latitude studies. It was first adapted for mid-latitude conditions. In a first step the model was calibrated and cross-checked with St. Santin IS measurements for the winter case of 27 January 1972 around noon using, in particular, the MSIS neutral atmosphere model. This provides a reference diurnal variation of the ionosphere. The second step investigated the influence of a maxwellian population of hot neutral atomic oxygen introduced in addition to the standard neutral atmosphere. The paper describes the initial comparison between the model and St. Santin IS data, and then the effects induced by a hot atomic oxygen population.Key words. Ionosphere (ionosphere-atmosphere interactions; ion chemistry and composition; mid-latitude ionosphere)

Comparative study of electron density from incoherent scatter measurements at Arecibo with the IRI-95 model during solar maximum

Abstract. Arecibo (18.4 N, 66.7 W) incoherent scatter (IS) observations of electron density N(h) are compared with the International Reference Ionosphere (IRI-95) during midday (10–14 h), for summer, winter and equinox, at solar maximum (1981). The N(h) profiles below the F2 peak, are normalized to the peak density NmF2 of the F region and are then compared with the IRI-95 model using both the standard B0 (old option) and the Gulyaeva-B0 thickness (new option). The thickness parameter B0 is obtained from the observed electron density profiles and compared with those obtained from the IRI-95 using both the options. Our studies indicate that during summer and equinox, in general, the values of electron densities at all the heights given by the IRI model (new option), are generally larger than those obtained from IS measurements. However, during winter, the agreement between the IRI and the observed values is reasonably good in the bottom part of the F2 layer but IRI underestimates electron density at F1 layer heights. The IRI profiles obtained with the old option gives much better results than those generated with the new option. Compared to the observations, the IRI profiles are found to be much thicker using Gulyaeva-B0 option than using standard B0.Key words: Ionosphere (modelling and forecasting)

Upper atmospheric observations at the Arecibo Observatory: Examples obtained using new capabilities

The Arecibo Observatory will soon complete a major instrumental upgrade which will provide improved capabilities for observations of the upper atmosphere. As in the past, Arecibo capabilities center on 430‐MHz incoherent scatter radar (ISR) measurements of the ionosphere made locally or in conjunction with other national and international incoherent scatter facilities; the upgrade will add the capability for simultaneous two beam incoherent scatter observations and will extend plasma line measurements to ±15 MHz. Aeronomical studies may also be performed using a 46.8‐MHz coherent scatter radar, the feed of which is coaxial with the 430‐MHz line feed, the newly refurbished medium‐frequency (MF) radar facility, and a new digital ionosonde. The original airglow laboratory continues to house the Fabry‐Perot interferometers, Ebert‐Fastie spectrometer, and tilting filter photometers used for observations of mesospheric, thermospheric, and exospheric airglow, while a new laboratory provides a permanent home for the lidars used for resonance fluorescence observations of atomic metal layers in the mesopause region and for Doppler Rayleigh measurements of the upper stratosphere and lower mesosphere. Finally, the recently upgraded high‐frequency, high‐power transmitter facility has performed active aeronomical and plasma physical studies alone and in conjunction with other Arecibo and visitor‐supplied instruments. Observations performed as the upgrade nears completion have already provided a first look at what lies ahead in areas as diverse and fundamental as the structure and dynamics of the turbopause, the properties of upper atmospheric tides, the electrodynamics of the E and F regions, the dynamics of light ions in the topside, and the physics of plasma turbulence. Examples of these and other observations are presented, and the opportunities for future investigations are discussed.