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Properties and associations of F- and D-region ionospheric structures during weak electron precipitation at very high latitude are investigated using the Longyearbyen incoherent scatter radar. The radio absorption deduced from the electron density observations revealed evidence of a persistent low-altitude absorption layer peaking at 80-85 km. Inversion of the electron density profile to give an estimate of the incoming energetic electron spectrum suggests that its source may be the solar wind. Strong similarities are seen between variations in the F and D regions which suggest that the electron flux reaching the D-region is being modulated in energy by the variations of electron density in the F-region. [ABSTRACT FROM AUTHOR]

Using previously developed methods for determining the three-dimensional spatial–temporal structure of traveling ionospheric disturbances and the automatic detection of wave disturbances, we analyzed data obtained simultaneously with the Irkutsk Incoherent Scatter Radar and Irkutsk ionosonde. The analysis relies on long continuous series of observations acquired during winter seasons in 2010–2014. We obtained representative statistics of traveling ionospheric disturbances characteristics including the full velocity vector. We analyzed the characteristics of traveling ionospheric disturbances with 1–6 h periods comparing them against the dispersion relations for internal gravity waves in the Boussinesq and Hines approximations. It was shown that, with due consideration for the horizontal neutral wind, most of the observed ionospheric disturbances agrees with the laws of internal gravity waves propagation in the upper atmosphere. It was found that azimuthal anisotropy of internal gravity waves characteristics allows us to obtain the diurnal variations of zonal and meridional neutral winds in the upper atmosphere. [ABSTRACT FROM AUTHOR]

This research investigates anomalous nighttime ion density behaviour over the Kharkiv, Ukraine incoherent scatter radar (49.6° N, 36.3° E, 45.3° inv) during the equinoxes of 2006–2010. The observations show that the altitude of the transition from O + to lighter ions was much lower than empirical and physical models predict. The standard physical model produces very good agreement for the O + densities but underestimates the H + densities by a factor of 2 in March 2006 and a factor of 3 in March 2009. The anomalously low transition height is a result of similar lowering of the ionospheric peak height and also of significantly increased H + density. The lower ionospheric peak height may be caused by weaker nighttime neutral winds. The calculations indicate that the higher measured topside ionosphere H + densities are most likely due to higher neutral hydrogen densities. Both factors could be the result of weaker than usual magnetic activity, which would reduce the energy input to high latitudes. Prolonged low activity periods could cause a global redistribution of hydrogen and also allow more neutral hydrogen to settle down from the exosphere into the mid-latitude ionosphere. The finding of the need for higher H densities agrees well with recent H-alpha airglow measurements and it is important for accurate modelling of plasmasphere refilling rates and night-time N m F 2 values. [ABSTRACT FROM AUTHOR]

We present two cases of incoherent-scatter ion line enhancements in conjunction with auroral arcs drifting through the radar beam. The up- and downshifted ion line shoulders as well as the spectral region between them are enhanced equally and simultaneously. The power enhancements are one order of magnitude above the thermal level and are concentrated in less than 15 km wide altitude ranges at the ionospheric F region peak. The auroral arc passages are preceded by significantly enhanced ion temperatures in the E region, assumed to be caused by transient electric fields associated with velocity shears. We use a Hall MHD model of velocity shears perpendicular to the geomagnetic field and show that a Kelvin–Helmholtz instability will grow for the two presented cases. [ABSTRACT FROM AUTHOR]

Variations in the evening/nighttime ionosonde vertical plasma drift velocities inferred from the time rate of change of both the base of the F-layer height (Vz (h′F) ) and height of the peak electron density (Vz (hmF2) ) from an equatorial station were compared for better description of the E×B drifts. For better interpretation, both results were compared with the Incoherent Scatter (IS) radar observations (Vz (ISR) ) which is taken to be the most accurate method of measuring drift, and therefore the data of reference level. An equinoctial maximum and June solstice minimum in post-sunset pre-reversal enhancement (PRE) was observed for Vz (hmF2) , Vz (ISR) , and Vz (h′F) . The percentage correlation between Vz hmF2 and Vz h′F ranges within 55–70%. While PRE for Vz (hmF2) peaked at 19 LT for all seasons, Vz (h′F) peaked at 18 LT for September equinox and December solstice, and start earlier. The nighttime downward reversal peak magnitudes for Vz (hmF2) and Vz (h′F) are respectively within the range of −4 to −14 and −2 to −14 m/s; whereas Vz (ISR) ranges within −12 and −34 m/s; and the peak time was reached earlier with the ionosonde observations than for the ISR. The PRE peak magnitude for Vz (hmF2) , Vz (h′F) and Vz (ISR) varies between 3–14, 2–14, and 4–14 m/s for the entire seasons. Our results revealed higher drift correlation coefficients in both Vz (hmF2) vs. Vz (ISR) (0.983) and Vz (h′F) vs. Vz (ISR) (0.833) relationships during the equinoxes between 16–20 LT, at which time the F-layer altitude is higher than the 300 km threshold value; and lower for solstice period (0.326 and 0.410 in similar order). A better linear relationship between Vz (hmF2) and Vz (h′F2) was observed during the reversal (19–21 LT) phase period. PRE velocity was shown to be seasonal and solar activity dependent. Both Vz hmF2 and Vz h′F compares almost equally with the ISR measurement. However, the PRE peak magnitude for the drift inferred using h′F2 is closer to the corresponding ISR magnitude during the equinoxes; whereas the drift inferred from hmF2 best represent the ISR magnitude for solstices. We established that both Vz hmF2 and Vz h′F are governed by the same mechanism at nighttime, and as such any of them can be used to infer vertical drift as long as the 300 km threshold value condition is considered, otherwise chemical correction may be required for the F-layer uplift. [ABSTRACT FROM AUTHOR]

Arctic ionospheric variations during the main phase of a magnetic storm on 14–15 December, 2006 were investigated to characterize the high energy particle precipitation caused effects, based on multi-instrument observations. These include electron density observations provided by the Global Positioning System (GPS) total electron content (TEC) measurements, European Incoherent Scatter (EISCAT) radar, the radio occultation (RO) from both the CHAMP satellite and the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellite, as well as the ionospheric absorption of cosmic radio noise measured by the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjärvi in the northern Finland (69.05°N, 20.79°E). Significant increases in the electron density for these different observations were found in the Arctic ionosphere during the main phase of the magnetic storm. These increase occurred in Scandinavian, Northwest part of Russia and Svalbard (SNRS) region, primarily at an altitude of about 110km. These results are first reported for the SNRS region, and our study contributes to a more complete description of this space weather event during 14–15 December, 2006. Our observations also provide direct evidence that the stormtime E-layer electron density enhancement (e.g., the sporadic E) can form a nearly dominant portion in the observed TEC increase. These increases were accompanied by the ionospheric absorption enhancement at the altitude of about 90km. The Y-component of magnetic field to the south of SNRS decreased, indicating strong upward field aligned electric current in the Arctic ionosphere. These features are interpreted as the effect of the high energy electron precipitation during the magnetic storm, which is caused by the sub-storm reflected on AL index and the measurements of IMAGE (International Monitor for Auroral Geomagnetic Effects) chain. The average energy of the precipitation electrons reached to about 10keV and the boundary of the high energy electron precipitation was also found to move poleward with a speed of about 800m/s. [ABSTRACT FROM AUTHOR]

Abstract: A multi-beam incoherent scatter radar technique has been developed at the Jicamarca Radio Observatory in order to maximize the number of ionospheric parameters that can be estimated simultaneously. The technique interleaves radar observations with antenna beams pointing perpendicular and oblique to the Earth's magnetic field. For the estimation of the ionospheric parameters, we have modeled the magnetic aspect angle variations of the signal power and cross-correlation data measured in multiple directions. The data model, formulated in terms of soft-target radar equations, was built based on the theories of incoherent scattering and magneto-ionic propagation. Applying a nonlinear least-squares inversion algorithm, we have succeeded in measuring simultaneously ionospheric electron densities N e , electron-to-ion temperature ratios as well as vertical and zonal plasma drifts. In the past, radar users had to choose between either perpendicular-to- or oblique modes, thus, the application of this technique extends the current capabilities of the Jicamarca radar. [Copyright &y& Elsevier]

We present ground-based auroral observations from Resolute Bay, Nunavut, Canada (74.73°N, 94.9°W) during January 2011. Two electron-multiplying CCD (EMCCD) imagers were operated at 31 frames per second. One was equipped with an all-sky field of view (FOV) lens and the other with a narrow (19°) FOV lens, centered on the geographic zenith (0° Az., 90° El.), a few degrees away from magnetic zenith (315° Az., 88° El.). The Resolute Incoherent Scatter Radar (RISR) was operating in a mode that enabled common-volume observations with the imagers. Being well inside the polar cap, the magnetic field at Resolute Bay is considered ‘open’ and connects to the lobes of the magnetotail. However, there is no clear consensus on whether polar cap aurorae occur on open or closed field lines. The electron acceleration is likely driven by direct solar wind processes, distant tail lobe processes or plasma sheet processes. One possible mechanism for accelerating the precipitating electrons is the parallel electric field of inertial Alfvén waves. The dynamic nature of the small-scale auroral features, observed on several nights, and the altitude extent of the ionization observed with RISR provide support for this hypothesis. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the spatio-temporal structure of traveling ionospheric disturbances characteristics is studied on the base of the electron density profiles measured by two beams of the Irkutsk incoherent scatter radar and the Irkutsk Digisonde. The technique for determination of spatial–temporal structure of wavelike ionospheric disturbances was developed using cross-correlation and spectrum analysis of electron density. The automated method for extracting ionospheric disturbances including both long-period day-to-day variability and short-period variations has been developed. Full analyses of January 15–February 17, 2011 data, including total velocity vector, was carry out for 1–6h ionospheric disturbances, corresponding to internal gravitational waves. The propagation characteristics agree with those obtained from the known studies of the wavelike ionospheric disturbances. An automated method of ionospheric disturbances analysis was created on the basis of regular continuous measurements with the Irkutsk Digisonde. The statistical analysis of electron density disturbances was carried out for 2004–2009 period. [Copyright &y& Elsevier]

Abstract: We report on EISCAT ionospheric heating experiments carried out on 13 September 2010 at Tromsø, Norway. This paper focuses on the long-lasting enhanced ion- and plasma-lines observed by the VHF incoherent scatter radar. In previous experiments, the enhanced ion-lines observed by VHF radar lasted ~200ms. Observations of long-lasting enhanced ion- and plasma-lines in the VHF radar have not been reported before. The enhanced lines may be caused by Parametric Decay Instability (PDI). In addition, the VHF data show that the enhanced ion-lines are descending in altitude with time during the heater-on period. Likewise, the temporal and spatial evolution of the enhanced plasma-lines also show a similar morphology and generally occur at the same height. At last a possible mechanism for the altitude evolution is given in this paper. [Copyright &y& Elsevier]

Abstract: The investigation concerns the size and motion of spatial fine structure in auroral radio absorption as observed by imaging riometer. Electron density profiles in the D and E regions determined by simultaneous incoherent-scatter radar were inverted to provide an estimate of the energy spectrum of the incoming electrons, and it is shown that the gradient-curvature drift at these energies is not consistent with the observed motion of the absorption fine structure, being too large by a factor of 5 or 6. However, a comparison with F-region drifts determined by the SuperDARN radar network supports a hypothesis that the cause lies in moving structures within the magnetosphere. [Copyright &y& Elsevier]

On 1 August 2008, a total solar eclipse occurred and the footprint of its umbra passed North–East of the Svalbard archipelago, where the EISCAT Svalbard Radars (ESR) are installed. This eclipse is peculiar because it was seen at high-latitude, thus with large solar zenith angles and it was the first time that incoherent scatter radars were able to sound the ionospheric response at such a high latitude. The ESR data, after correction, show the typical signatures of a solar eclipse: depletion in electron density in the E- and F-regions, a decrease in electron temperature in the F-region, and a downflow of ions. Also, the UHF dish of the mainland EISCAT system measured a decrease in the same parameters, showing the influence of the penumbra (partial eclipse) on the ionosphere. We have modelled the response of the high-latitude ionosphere to this solar eclipse with the TRANSCAR code. Our simulations reproduce rather well the observations above the UHF radar in Tromsø and the ESR on Svalbard. In particular, we show the influence of particle precipitation above the ESR and we suspect an effect above Tromsø of the photoelectrons coming from the opposite hemisphere. Another interesting and quite unusual feature is the observation of the summer ionospheric cusp undergoing the eclipse. Radar data show structures of enhanced electron temperature, due to soft particle precipitation through the polar cusp. [ABSTRACT FROM AUTHOR]

Abstract: We describe a sounding technique that allows us to improve spatial resolution at the Irkutsk Incoherent Scatter Radar (IISR) without losing spectral resolution. The technique also allows us to decrease temperature estimation errors caused by the Faraday effect. The technique is based on transmitting various duration pulses without any modulation and on subtracting correlation matrices of the received signal grouped by sounding pulse duration. We show theoretically and experimentally that the technique allows us to solve the problem of improving spatial resolution. Accumulation time for the technique is approximately four times longer than that for the alternating codes technique with the same spatial resolution. The number of lags in the correlation function with high spatial resolution does not depend on necessary spatial resolution. In the proposed technique, all the lags are obtained with the same spatial resolution and with the same signal-to-noise ratio. The technique is valid within the quasi-static ionospheric parameter approximation. [Copyright &y& Elsevier]

A number of studies have addressed the principal assumption used by the SuperDARN network of HF radars that the scatter from F region field-aligned irregularities has a Doppler shift given by the cosine component of the EXB plasma drift. However, the slopes of the best-fit line to the measured points have consistently been low, perhaps implying contamination from irregularities with smaller amplitude drifts. This work is motivated by testing the same assumption on a more optimal experimental setting: using the north face of the Resolute Bay incoherent scatter radar (RISR-N), which provides high resolution magnitude and direction measurements of EXB drift in a region where the ionospheric flow is mostly uniform relative to auroral latitudes, thereby reducing echo mixing due to spatio-temporal structuring. We compared the EXB drift measured by RISR-N to the line-of-sight Doppler velocities measured PolarDARN which is composed of Rankin Inlet and Inuvik HF radars, both having a field of view over Resolute Bay. An aggregate scatter plot of all the echoes observed during a 5-day period in early May 2011 contains two distinct groups of echoes. The first group is hypothesized to be from the E region because the echoes appear above a threshold EXB drift and at small flow angles, which are characteristics of primary Farley–Buneman waves. The Doppler velocity of the second group of echoes increase linearly with the EXB drift and is identified here as F region echoes. A special joint fit showed a slope of 0.85 for the F region echoes. During the observation period, the F region electron density has large variations between 1 and 10×1011, in particular due to the polar cap patches. Considering a representative F region electron density of 5×1011 m−3, which has a refraction index of 0.86 at 12.5MHz (for PolarDARN frequencies) and the fact that the measured velocity is the product of the actual velocity and the refractive index, the expected slope is 0.86, in agreement with our measurement. The fit for E region Doppler velocities shows a saturation speed at 170m/s, while the data were spread between 100 and 300m/s. Although the saturation speed is somewhat lower than the ion acoustic speed, it is acceptable considering earlier work attributing similarly low velocities to large aspect angles at the scattering altitudes. [ABSTRACT FROM AUTHOR]

Abstract: During March–April 2011 a campaign of coordinated observations was undertaken between the Poker Flat Advanced Modular Incoherent Scatter Radar and the Poker Flat Scanning Doppler Imager. These instruments provide horizontally resolved maps of plasma and neutral parameters in Earth's thermosphere. We report on data collected during the campaign, and use these data to investigate two key aspects of ion–neutral coupling, namely Joule heating and the ion–neutral collision frequency. Volumetric Joule heating rates were often well correlated with measured ion temperature enhancements. The contribution of the neutral wind dynamo to the observed heating rates was positive when the absolute horizontal magnetic field perturbation ( ) was less than approximately 40nT, and negative above that level. The total momentum–transfer ion–neutral collision frequency was estimated to be at an altitude of 260km, which, for a neutral composition of 75% atomic oxygen, yielded an estimate of the O+–O collision frequency of . [Copyright &y& Elsevier]

Abstract: We present coordinated optical and radar observations using the Poker Flat Incoherent Scatter Radar (PFISR) in Alaska and the Sondrestrom radar in Greenland. Several cases were examined where intense, similar-looking dynamic auroral structures were observed in the magnetic zenith. The presence or absence of Naturally Enhanced Ion Acoustic Lines (NEIALs) was investigated in both sets of radar data. In all cases, the aurora exhibited small-scale ( –1km), dynamic features that were observed with both all-sky and narrow-field of view (19°) imagers. In all of the cases examined, the PFISR radar observed NEIALs while the Sondrestrom radar did not, despite similar auroral morphology. The main difference between the radars, namely the probing wave number, provides strong evidence for a limiting spatial scale of between 10 and 15cm for the enhanced wave activity responsible for NEIALs. This has implications for constraining the models of NEIAL generation mechanisms. [Copyright &y& Elsevier]

Abstract: While it may be a general belief that the optimal baud length for radar measurements of range extended targets should be close to the desired resolution, this is only an approximate truth for weak targets and not true at all for strong targets. We use full measurement error estimates with proper correlations and find numerically the baud length which optimises the posteriori variance of an extended target. While the pulse is assumed to be a simple boxcar with a given fixed energy and the baud length is the only design parameter, the results extend to many traditional ways of pulse compression coding through arguments derived from recent results on rigorous experiment comparison and perfect coding. [Copyright &y& Elsevier]

Abstract: It is now well understood that the occurrence of PMSE is closely connected to the presence of ice particles. These ice particles modify the ambient electron density by electron attachment which occasionally leads to large electron density depletions which have also been called ‘biteouts’. There has been some debate in the literature regarding the relative depth of such depletions which is usually expressed by the parameter . Here, is the charge number density of ice particles and n e is the electron density. In this paper, we present, for the first time, the statistical distribution of using measurements with the EISCAT VHF- and UHF-radars. Based on 25h of simultaneous observations, we derived a total of 757 values based on 15min of data each. In each of these cases, PMSE were observed with the EISCAT VHF-radar but not with the UHF-radar and the UHF-measurement were hence used to determine the electron density profile. From these 757 cases, there are 699 cases with , and only 33 cases with (21 cases with ). A correlation analysis of versus PMSE volume reflectivities further reveals that there is no strong dependence between the two parameters. This is in accordance with current PMSE-theory based on turbulence in combination with a large Schmidt-number. The maxima of from each profile show a negative relationship with the undisturbed electron densities deduced at the same altitudes. This reveals that the variability of mainly depends on the variability of the electron densities. In addition, variations of aerosol number densities may also play a role. Although part of the observations were conducted during the HF heating experiments, the so-called overshoot effects did not significantly bias our statistical results. In order to avoid missing biteouts because of a superposition of coherent and incoherent scatter in the UHF-data, we finally calculated spectral parameters n by applying a simple fit to auto-correlation functions as introduced by Strelnikova and Rapp (2010). Corresponding statistical results of the parameter n indicate that charged ice particles do exist in the vicinity of PMSE (i.e., ) but they did not efficiently modify ambient electron densities so that clear ‘biteouts’ are observed. [Copyright &y& Elsevier]

The midnight temperature maximum (MTM) is studied using ion temperature data from the incoherent scatter radar at the Arecibo Observatory (18.3°N, 66.2°W). The MTM is characterized by fitting the radar data with a function that takes into account diurnal, semidiurnal and terdiurnal components. Under the hypothesis that the MTM is related to the amplification of the terdiurnal wave, a Gaussian amplification window is included in the fitting to automatically determine the time of occurrence, temporal duration, and amplitude of the MTM observed. This study focused initially on altitudes near 300km, the typical height of MTM observations from Fabry Perot Interferometers (FPIs). Out of the 491 days available between 1967 and 2010 only 82 showed reliable temperature determination throughout the night at this typically bottomside altitude of often low density and sharp density gradient. The analysis was expanded to include 229 usable nights at heights close to 330km and 367km, where better conditions for temperature determination exist. Most of these nights showed an MTM with amplitudes between 20 and 150K and peak occurrence times during local summer months. The seasonal dependence of MTM parameters is also investigated and compared with previous experimental and modeling studies. [ABSTRACT FROM AUTHOR]

Abstract: Jicamarca measurements of electron temperatures at high altitudes (500–1500km) from the last solar minimum routinely show variations of hundreds of Kelvin from day-to-day. Possible sources of these variations are explored using the SAMI2-PE is another model of the ionosphere including photoelectron transport (SAMI2-PE) model, which includes a multistream photoelectron transport model. Changes to the electric fields, meridional winds, and thermospheric densities can all change the electron densities and temperatures at high altitudes. The high altitude electron temperatures are primarily determined by a balance between heating from photoelectrons which travel up the field lines and thermal diffusion which carries heat back down the field lines. The winds and electric fields will change the altitude and densities of the off-equatorial F-region peaks, especially on the field lines connected to the equatorial arcs. The densities and temperatures in the plasmasphere will self consistently adjust themselves to achieve diffusive equilibrium with the off-equatorial F-regions. Furthermore, decreases in the density and/or altitude of the F-region makes it easier for photoelectrons to escape to high altitudes. These connections between the equatorial plasmasphere, the off-equatorial F-regions, and the neutral thermosphere suggest that high altitude measurements at Jicamarca could be used to study thermospheric variability. [Copyright &y& Elsevier]

Abstract: Motivated by a recent controversy in the literature about the agreement or disagreement of spectral widths (after conversion to Doppler-velocities) of PMSE observed simultaneously with the EISCAT VHF and UHF radars in Tromsø, Norway, this issue was reconsidered taking into account a much larger data set than in any of the previous studies. While these previous studies only considered case studies of a few minutes in length, we considered a total of 380min of simultaneous and common volume observations with these two radars at a time resolution of 30s, i.e., a total of 760 samples. This study is important to discern whether PMSE at these two frequencies originate from the same coherent scattering mechanism or if different physical mechanisms are at work. We considered both shape and width of the considered spectra and found that – within the available spectral resolution – the VHF spectra are on average well described by a Gauss shape (with spectral parameter n=2—which is derived from a generalized formulation of the autocorrelation function), whereas the UHF-spectra show a small deviation from this shape with an average n of 1.6. Spectral widths do largely agree but show a small systematic difference, i.e., the UHF spectra are on average 0.1m/s narrower than the VHF spectra at an average spectral width of 3.5m/s. This small systematic effect is largely explained considering the overall effect of beam-, shear-, and wave-broadening. This means that the slightly different beam widths of the VHF and UHF-radars and the hence slightly different observing volumes account for the small systematic difference in mean spectral width. Finally, we demonstrated that the small deviation of UHF-spectra from a perfect Gauss shape can be either due to relatively small signal-to-noise ratios of the UHF-data and/or a superposition of incoherent and coherent scatter in cases where both contributions add equally to the total observed power. Since VHF PMSE are much more stronger than the incoherent scatter background this effect cannot be recognized in VHF observations. This superposition effect may occasionally also lead to larger deviations between VHF and UHF spectra in that it can potentially narrow the UHF-spectra significantly. However, excluding these rather rare and exotic cases, we conclude that the majority of our observations are compatible with a single coherent scattering at both VHF and UHF. [Copyright &y& Elsevier]

Abstract: Many of the expressions for the reflectivity of polar mesospheric summer echoes (PMSE) predict that the echo strength is controlled by the electron density. We present several observations made using the Poker Flat Incoherent Scatter Radar which are difficult to explain based on those expressions, including observations at night with no detectable incoherent scatter below 90km and during aurora where the reflectivity shows no response to abrupt changes in ionization. We derive a new expression for the reflectivity and are still able to explain all of these observations as scatter from free electrons. When the electron density is much smaller than the ice density the reflectivity is a strong function of electron density. In the opposite limit the reflectivity is solely controlled by the ice density because only a fraction of the electron density variance generated at the largest scales can get convected to sufficiently small scales. [Copyright &y& Elsevier]

Abstract: We analyze the data obtained using the Arecibo incoherent scatter radar to examine the response of the topside ionosphere to a moderate geomagnetic storm that occurred during the period March 7–11, 2008. During this time period a magnetic storm with a non-monotonic main phase decrease in the Dst index occurred. The recovery phase also exhibited a secondary Dst decrease. During the initial phase of the storm, Te and Ti increased coincident with the arrival of the solar wind. The main phase registered an increase in proton concentration proportional to Ne while temperatures reached the lowest values. Variations in O+ concentration were not significant but a reduction in helium fraction was observed. Soon after the peak of the storm, the transition height between the topside ionosphere and the protonosphere, where H+ ions dominate composition, was lower than would be expected during quiet conditions and this behavior lasted for approximately 12h. [Copyright &y& Elsevier]

Abstract: A nighttime sporadic E (Es) layer responsible for range spread Es trace in ionogram and quasi-periodic echoes scattered from 5-m field-aligned electron density irregularities (FAIs) is investigated by using a ground-based 30MHz coherent scatter radar, ionosonde and FORMOSAT-3/COSMIC satellite. A comparison shows a positive correlation between the 30MHz radar backscatter and critical frequency foEs of the Es trace, which are consistent with earlier observational results. Interferometry measurement indicates that the spatial patterns of the radar echoes from the FAIs were in patchy form with vertical extent of about a few kilometers and horizontal dimensions of about 10–20km. The echo structures experienced quasi-periodic oscillations not only in vertical but also in horizontal directions with periods of approximately 5–6min. Observed Doppler velocities of the FAIs also exhibited quasi-periodic oscillations with the same periods. From their downward phase movement, these oscillations were very likely attributed to the modulations of upward propagating gravity waves with vertical wavelengths of about 10–20km. In addition, vertical shears of the FAI Doppler velocity were observed, which are believed to result from the change in the phase of gravity wave with height. A physical process is proposed in this article to account for the relation between gravity wave-perturbed neutral wind and height variation of the FAI Doppler velocity. It is believed that the oscillations in the Doppler velocity profiles are associated with the wave-induced polarization electric fields in a finite plasma structure in the azimuth direction. [Copyright &y& Elsevier]

Abstract: Measurements of turbulent energy dissipation rates by radar spectral-width procedures require differencing two numbers to produce a difference that is small relative to the two initial numbers. In particular, the square of the so-called “beam-broadening” component must be subtracted from the square of a measured spectral width. Because the difference is relatively small, it is very sensitive to statistical fluctuations in each of the initial measured parameters. The method by which the measured spectral width is determined can impact the accuracy of the measurements, and, in addition, the beam-broadened component is affected by errors in the measured wind, variability in the mean wind, wind-shear, and aspect-sensitivity of the scatterers. All these effects can impact the measurements of turbulence, and in some cases can even produce physically unrealistic “negative” values of turbulence strength. In this paper, we investigate the relative importance of (i) variability of the mean wind within the averaging period, (ii) digitization errors and the accuracy of determination of the spectral width, (iii) the particular beam-broadening model employed, and (iv) anisotropy of the scatterers. Although these terms are often discussed in the literature, we have quantified their relative importance. The accuracy of determination of the spectral width is the most important source of error, followed by variability of the mean wind in the averaging period. In addition to these studies, we also develop a new formula for accurate determination of the beam-broadened spectral width. This includes a new term that was missing from earlier formulations. [Copyright &y& Elsevier]

Abstract: A statistical analysis has been made of 26 Polar Mesospheric Summer Echoes (PMSE) active modulation experiments between 2002 and 2007. Observed with the EISCAT VHF radar the PMSE signature can be reduced by means of heating the ionosphere with powerful high frequency (HF) radio waves. However, PMSE modulation experiments sometimes fail. We use a computational model to estimate the enhanced electron temperatures due to ionospheric heating from HF radio waves in the D-region. We show that the statistical PMSE modulation for a fixed HF heater-induced electron temperature enhancement appears to be independent of altitude. In addition, for experiments where the PMSE modulation experiment failed, we show that the atmospheric attenuation of the HF heater wave was too great for the HF wave to have any effect on the PMSE layer. [Copyright &y& Elsevier]

Abstract: EISCAT field-aligned observations for winter and equinoctial seasons under quiet geomagnetic conditions were used to study electron concentration Ne(h) distribution in the evening sector of the main ionospheric trough. Both morphological results and physical interpretation are considered. A typical variation of the F2-layer maximum height is hmF2 increase in the trough minimum followed by hmF2 sharp decrease in the polar wall of the trough and in the auroral oval. The effective scale height H eff of plasma distribution in the topside F2-region increases in the trough minimum. Specific Ne(h) profiles with H eff decreasing with height observed in the trough minimum are due to upward flow of plasma from the F2-region. Such Ne(h) distributions are associated with narrow deep troughs, but only if the trough is formed in non-sunlit conditions. [Copyright &y& Elsevier]

Abstract: In this paper we discussed the penetration effects of common interplanetary magnetic cloud (MC) structures like sheath region, both sheath and magnetic cloud boundary layer (MCBL), MC body, and shock-running into a preceding MC on the equatorial ionosphere during intense (SYM-H ≤−100nT) geomagnetic storms. Using solar wind data obtained from the ACE and WIND spacecraft, we have identified these four types of MC structures responsible for the electric field penetration events detected by Jicamarca incoherent scatter radar. After elimination of the propagation delay, the observations show that the equatorial electric field (EEF) was changed immediately following the arrival of solar wind disturbance. Moreover, the duration of EEF corresponded well with that of the corresponding MC structure interval. We suggest that identifying the solar wind structures associated with penetration electric field may shed light on the understanding of the penetration processes and further help exploring their effects on the ionospheric plasma. [ABSTRACT FROM AUTHOR]

Abstract: Coordinated Poker Flat Incoherent Scatter Radar (PFISR) and imaging observations are examined in order to study the radar scattering signatures of optically visible meteors. Preliminary observations in 2006 revealed that optical meteors were detected by PFISR. A recent campaign in the winter of 2009, employed a radar mode optimized for meteor Doppler shifts. This paper presents a case study from 21 January 2009. Seven out of the 338 meteors observed with PFISR were also detected optically. Six out of those seven were detected by the side lobes of the radar and not the main beam. A positive correlation was found between the corrected backscattered radar power and the optical brightness of the meteors, as well as between optical brightness and absolute speed. Meteors originating in the east had higher speeds and more glancing incidence angles, while meteors originating in the north had lower speeds and more direct incidence angles. [Copyright &y& Elsevier]

Abstract: We present a seasonal study of the presence and characteristics of meteoric smoke particles (MSPs) in the D-region plasma derived from observations using the Gregorian and line feeds of the 430MHz dual-beam Arecibo Observatory (AO) incoherent scatter radar (ISR) in Puerto Rico . MSPs are the product of re-condensation of ablated meteoric material and are believed to be the condensation nuclei for the formation of ice particles in the polar mesopause region. These CNs can then be responsible for the formation of polar mesospheric clouds (PMCs), noctilucent clouds (NLCs) and polar mesospheric summer echoes (PMSEs). For this work, we simultaneously employed both AO antenna feeds to define two radar beams inclined east and west of zenith (). Because of the non-vertical pointing, the sampled spectra are Doppler shifted due to the rapidly changing neutral dynamics of the MLT region. We correct this effect by removing the Doppler shift using the radial velocities estimated every min and then integrate the corrected spectra for longer periods to enhance signal-to-noise ratio (SNR) and better investigate the variability of MSP properties. We determine MSP radii and number densities utilizing a method similar to the one developed by in which the autocorrelation function (ACF) is approximated as the sum of two exponential decays, i.e., the power spectrum is approximated as the sum of two Lorentzians. This method, which assumes mono-disperse particles, allows us to determine mean particle properties in the 80–95km altitude range during the hours of 10–14 AO LT when the detected SNR from the D-region is highest. Results from this work show MSP radii approximately 0.6–1.5nm is size depending on altitude and season. Also, MSP densities as a function of altitude and season are determined with the aid of the IRI model resulting in values on the order of per cubic centimeter. Our error analysis shows that spectral broadening from atmospheric sources such as neutral turbulence induced by gravity wave vertical variance, and non-zero vertical winds results in change in derived MSP radii. Also, since our dataset covers different seasons, we investigate a potential correlation between the seasonal variations of the derived MSPs properties with that of the meteoric input function (MIF) in the MLT above Arecibo. [Copyright &y& Elsevier]

Abstract: We report first results from a unique experiment performed at the HIPAS ionospheric modification facility in conjunction with the Poker Flat incoherent scatter radar in Alaska. High-power radio waves at 2.85MHz, which corresponds to the second electron gyro-harmonic at ~245km altitude, were transmitted into the nighttime ionosphere. Clear evidence of F-region ionospheric electron temperature enhancements were found, for the first time at this pump frequency, maximizing when the pump frequency is close to the second gyro-harmonic and double resonance. This is consistent with previous pump-enhanced artificial optical observations. We estimate the plasma heating efficiency to be approximately double that for higher pump frequencies. [Copyright &y& Elsevier]

Abstract: The Poker Flat Incoherent Scatter Radar (PFISR) is the first dedicated ISR built with an electronically steerable array. This paper demonstrates the capabilities of PFISR for producing three-dimensional volumetric images of -region ionization patterns produced by the aurora. The phase table was configured to cycle through 121 beam positions arranged in an grid. A 13-baud Barker coded pulse was used, which provided range resolution out to a maximum range of 250km. Backscattered power was converted to electron density by correcting for path loss and applying the Buneman approximation assuming equal electron and ion temperatures. The results were then interpolated onto a three-dimensional cartesian grid. Volumetric images are presented at 5-min, 1-min, and 14.6-s integration times (corresponding to 960, 192, and 48 pulses-per-position, respectively) to illustrate the tradeoff between spatio-temporal resolution and data quality. At 14.6s cadence, variability in plasma density within the volume appears to be fully resolved in space and time, a result that is supported by both observational evidence and theoretical predictions of ionospheric response times. Some potential applications of this mode for studying magnetosphere–ionosphere interactions in the auroral zone are discussed. [Copyright &y& Elsevier]

Abstract: In this work, we use a semi-empirical model of the micrometeor input function (MIF) together with meteor head-echo observations obtained with two high power and large aperture (HPLA) radars, the 430MHz Arecibo Observatory (AO) radar in Puerto Rico (18°N, 67°W) and the 450MHz Poker flat incoherent scatter radar (PFISR) in Alaska (65°N, 147°W), to study the seasonal and geographical dependence of the meteoric flux in the upper atmosphere. The model, recently developed by Janches et al. [2006a. Modeling the global micrometeor input function in the upper atmosphere observed by high power and large aperture radars. Journal of Geophysical Research 111] and Fentzke and Janches [2008. A semi-empirical model of the contribution from sporadic meteoroid sources on the meteor input function observed at arecibo. Journal of Geophysical Research (Space Physics) 113 (A03304)], includes an initial mass flux that is provided by the six known meteor sources (i.e. orbital families of dust) as well as detailed modeling of meteoroid atmospheric entry and ablation physics. In addition, we use a simple ionization model to treat radar sensitivity issues by defining minimum electron volume density production thresholds required in the meteor head-echo plasma for detection. This simplified approach works well because we use observations from two radars with similar frequencies, but different sensitivities and locations. This methodology allows us to explore the initial input of particles and how it manifests in different parts of the MLT as observed by these instruments without the need to invoke more sophisticated plasma models, which are under current development. The comparisons between model predictions and radar observations show excellent agreement between diurnal, seasonal, and latitudinal variability of the detected meteor rate and radial velocity distributions, allowing us to understand how individual meteoroid populations contribute to the overall flux at a particular location and season. [Copyright &y& Elsevier]

Abstract: In this paper, we analyze the temporal variability of the propagation and dissipation of two southeast (SE)ward-propagating gravity waves (GWs) observed by the Poker Flat Incoherent Scatter Radar (PFISR) on 13 December 2006. We determine the GW vertical wavelengths as a function of altitude along each constant wave phase line, then extract the neutral, horizontal winds every (one-half of a wave period) along the direction of GW propagation as a function of altitude using an accurate, dissipative GW dispersion relation and MSIS temperatures. We find that the neutral wind in the northwest (NW) direction above PFISR was composed of a slowly varying “mean” of plus a slowly moving, large-scale wave with a period of 3–5h. These winds added at , creating a large NW wind of . This wind caused these two GWs to become evanescent or nearly evanescent some of the time, although their amplitudes increased up to . We find that the winds accelerated in the SEward direction by in 30–40min at . We hypothesize that these accelerations are thermospheric body forces caused by the dissipation of SEward-propagating GWs excited by mountain wave breaking near the mesopause NW of PFISR. This hypothesis is supported by tropospheric winds and the consistency of the observed GW periods. Finally, we ray trace these GWs through the extracted winds, and using a simple single-ion plasma model, compare the theoretical and measured plasma responses. We find that theory agrees reasonably well with observations. [Copyright &y& Elsevier]

Abstract: The recently commissioned Poker Flat Incoherent Scatter Radar (PFISR) began a continuous operation measurement program for the duration of the International Polar Year (IPY). The IPY began on 1 March 2007 and is an 18-month period of intense polar study. PFISR began its IPY campaign on 1 March 2007 and this paper describes the first 10 months of observations. The PFISR IPY science goals revolve around distinguishing between ionospheric climate and weather variability, and to determine the relative role of geomagnetic weather from the magnetosphere versus that driven from the atmosphere below. This latter goal may well be aided by the fact that the IPY period is at solar minimum, a time when major geomagnetic activity occurrence should be minimized. However, as nature would have it once the IPY observations began it was found that geomagnetic activity was a recurrent feature lasting the entire 10 months being discussed here. The PFISR IPY database will also be used as a long-term fiducial data set against which ionospheric models are to be compared. Hence, this paper provides a documentation of the contents of the database. Case studies as well as statistical studies of how the ionospheric climate and weather can be separated are presented. A particular emphasis is placed upon the F-region ion temperature observations. These appear to provide a very direct measure of geomagnetic energy input to the ionosphere–thermosphere system. Examples are shown in which 150K F-region ion temperature increases are associated with very moderate geomagnetic disturbances in which the daily average 3-h Kp is only 2.5. [Copyright &y& Elsevier]

Abstract: Flow bursts within the ionosphere are the ionospheric signatures of flow bursts in the plasma sheet and have been associated with poleward boundary intensifications (PBIs). Some PBIs extend equatorward from the polar cap boundary, where they can be roughly divided into north–south-aligned and east–west-aligned structures. In this paper, we present two flow burst events observed by the new Poker Flat Advanced Modular Incoherent Scatter Radar (PFISR) in the pre-midnight auroral zone on 28 April 2007, one towards the west and the other towards the east. In both cases, enhanced flows lasted for about 8–10min with peak velocities exceeding 1500m/s. The concurrently measured electron density showed that the flow bursts occurred in low conductivity regions. However, near the poleward (equatorward) edge of the westward (eastward) flow burst, strong electron density enhancements were observed in the E region, indicating the presence of discrete auroral arcs. Auroral images from the Polar spacecraft were available at the time of the eastward flow burst and they indicate that this burst was associated with an east–west-aligned auroral structure that connected at later MLT to a north–south structure. In addition, simultaneous precipitating particle energy spectrum measured by the the Defense Meteorological Satellites Program (DMSP) F13 satellite reveals that this auroral structure resulted from mono-energetic electron precipitation associated with a significant field-aligned potential drop. These observations show direct evidence of the relationship between flow bursts, field-aligned currents and auroral intensifications, and suggest that eastward/westward flow bursts are associated with east–west-oriented PBI structures that have extended well within the plasma sheet. This is in contrast to the equatorward-directed flow that has been previously inferred for PBIs near the polar cap boundary and for north–south auroral structures. This paper illustrates the use of the PFISR radar for studying the magnetosphere–ionosphere coupling of flow bursts. [Copyright &y& Elsevier]

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

Abstract: The plasma sheet moves earthward (equatorward in the ionosphere) after enhancements in convection, and the electrodynamics of this response is strongly influenced by Region 2 magnetosphere–ionosphere coupling. We have used Poker Flat Advanced Modular Incoherent Scatter Radar (PFISR) observations associated with two relatively abrupt southward turnings of the IMF to provide an initial evaluation of aspects of this response. The observations show that strong westward sub-auroral polarization streams (SAPS) flow regions moved equatorward as the plasma sheet electron precipitation (the diffuse aurora) penetrated equatorward following the IMF southward turnings. Consistent with our identification of these flows as SAPS, concurrent DMSP particle precipitation measurements show the equatorial boundary of ion precipitation equatorward of the electron precipitation boundary and that westward flows lie within the low-conductivity region between the two boundaries where the plasma sheet ion pressure gradient is expected to drive downward R2 currents. Evidence for these downward currents is seen in the DMSP magnetometer observations. Preliminary examination indicates that the SAPS response seen in the examples presented here may be common. However, detailed analysis will be required for many more events to reliably determine if this is the case. If so, it would imply that SAPS are frequently an important aspect of the inner magnetospheric electric field distribution, and that they are critical for understanding the response of the magnetosphere–ionosphere system to enhancements in convection, including understanding the earthward penetration of the plasma sheet. This earthward penetration is critical to geomagnetic disturbance phenomena such as the substorm growth phase and the formation of the stormtime ring current. Additionally, for one example, a prompt electric field response to the IMF southward turnings is seen within the inner plasma sheet. [Copyright &y& Elsevier]

Abstract: Line-of-sight F region plasma drifts measured with the Poker Flat Incoherent Scatter Radar (PFISR) during the NASA JOULE II experiment period are compared with estimates derived from E region coherent scatter spectra observed with a 30MHz imaging radar on common magnetic field lines. The estimates are based on empirical formulas determined through earlier comparisons between in situ and ground-based measurements. Good agreement is found in the current study when the radar measurements are sufficiently well collocated and the convection is sufficiently homogeneous for meaningful comparison. The mean and variance of the background electric field are also estimated from the coherent scatter data and used to assess the contribution of fine structure in the convection pattern to Joule heating. Fine structure in the scale size regime between and 200km is found to increase the second moment of the electric field by up to 50% for intervals of a few minutes during a substorm when the flow was highly irregular. The average contribution to the net Joule heating rate is generally likely to be modest, however. [Copyright &y& Elsevier]

Abstract: Novel coincident 3-D radar, lidar and optical image measurements of dynamical structures in polar mesosphere summer echoes (PMSE) and noctilucent clouds (NLC) are presented. Common volume mesospheric measurements were made over central Alaska using the new Poker Flat Incoherent Scatter Radar (PFISR), a co-located Rayleigh lidar and remote, two-station digital image observations, enabling the first detailed investigation of the horizontal and vertical structures of NLC and PMSE. Coincident measurements were made of an unusual NLC display recorded on 10–11 August 2007, characterized by a broad luminous band that contained several prominent wave forms. Concurrent lidar and image measurements established the presence of NLC within the radar volume from ∼09:00UT (01:00LT), when the solar depression angle was 10.4°, until dawn. Strong but intermittent PMSE were detected by PFISR, with distinct patchy structures that exhibited a similar southward motion as the NLC. Detailed comparison of the 3-D PMSE structures and the NLC lidar and image data have revealed striking similarities when account was taken of the NLC layer altitude, suggesting a direct link between their small-scale spatial signatures (within the current resolution of the radar measurements). At the same time, the lidar detected a sustained increase in the backscatter signal, while the imagers revealed the development of copious short horizontal wavelength (4.9km) billow waves. We conclude that strong wind shears associated with the Kelvin–Helmholtz billow instabilities played a key role in the development of a neutral turbulence layer in close proximity to the NLC layer resulting in the strong but intermittent PMSE detected at 450MHz on this occasion. [Copyright &y& Elsevier]

Abstract: We report on combined spectral measurements of the D-region ionosphere and the polar summer mesosphere and lower thermosphere performed on 9 June 2007 with the 450MHz Poker Flat Incoherent Scatter Radar (PFISR) under active auroral conditions. Observations during the first 7h occurred at nighttime and revealed strong temporal correlations between enhanced auroral precipitation, the occurrence of polar mesosphere summer echoes (PMSE), and enhanced electron densities () extending to much lower altitudes than the previously reported at these latitudes without the presence of a polar cap absorption (PCA) event. PMSE and lower D-region echoes most often occurred together and PMSE were more often visible to PFISR before the of the D-region was sufficiently high to be detected. These measurements also enabled definition of high precision horizontal winds (errors of ) in the altitude range, which has previously been possible only with MF and HF radars, with the Arecibo 430MHz IS radar (ISR), or with other ISRs during PCA events. This observing technique allowed for of almost continuous measurements of the meridional and zonal wind fields and thus permitted the measurements of atmospheric tides over this large altitude range. We determine amplitude and phase for both the diurnal and semi-diurnal tides showing the latter to be stronger as expected at these latitudes. We also present observations of a gravity wave during a quiet period indicating that PFISRs sensitivity is sufficiently high to obtain reliable spectral information even when electron densities are not enhanced by aurora or a PCA event and are as low as . These results show the capabilities of PFISR to study the neutral dynamics in the polar D-region and mesosphere and lower thermosphere (MLT) over an unprecedented altitude range. [Copyright &y& Elsevier]

Abstract: In this paper, we report on multi-beam spectral observations of polar mesospheric summer echoes (PMSE) at 450MHz (Bragg scattering wavelength of ) carried out with the Poker Flat Incoherent Scatter Radar (PFISR) located near Fairbanks, Alaska. The observations presented in this paper occurred with auroral particle precipitation, which enhanced the otherwise low nighttime D-region ionization. The observations indicate two classes of spectra associated with PMSE at this frequency: a relatively rare, “broad” class of spectra that seems to be particularly turbulent with spectral widths (root mean square velocity fluctuations) of 6–7m/s, and the more common “narrow” spectra, with spectral widths close to 1m/s. The results are discussed in terms of the turbulence scattering theory of PMSE. Using the theories of Rapp and Lübken [2003. On the nature of PMSE: electron diffusion in the vicinity of charged particles revisited. Journal of Geophysical Research 108, 8437, doi:10.1029/2002JD002857], we find that neutral turbulence together with enhancement of the Schmidt number by the presence of charged ice can indeed explain the observations, even at these small scales. The echoes are likely associated with large charged ice particles (a few 10s of nanometers in radius). The narrowest echoes, while seemingly resulting from relatively modest neutral turbulence, thus acquire long diffusion times which allow them to drift with the background winds for tens of seconds, possibly explaining the predominance of these narrow-width echoes. [Copyright &y& Elsevier]

Abstract: In this work we present a novel use of the Poker Flat Incoherent Scatter Radar (PFISR) to study meteor-head echoes with wide (W) beams. Until now, most of the meteor-head echo studies have been performed with High-Power Large-Aperture Radars (HPLARs) using very narrow (N) beams. At PFISR we have implemented an antenna compression approach using a defocusing scheme, similar to Chirp (linear frequency modulation) in pulse compression. The resulting effective beam is times wider than the narrowest PFISR beam. Using the signal-to-noise ratio (SNR) as a proxy measurement of cross-section, from the combined W and N beam experiments, our main results are: (1) observed meteors in the W beam are approximately half the number of meteors observed in the N beam, (2) we detected 10 times more large cross-section (strong) meteors ( if they were measured by the N mainlobe) than using only the N beam, and (3) more than 15% of the total N meteors were observed in the N sidelobes, therefore being at least 20dB stronger if they were observed in the N mainlobe. Our results are summarized in a corrected distribution of relative meteor cross-sections as if all of them were observed with the N mainlobe, namely correcting their SNR values depending on where in the beam they were detected (sidelobes or mainlobe). In addition, we show a qualitative meteor cross-section distribution that one can obtain combining W and N beams. The resulting distribution is incomplete, since the W beam is not sensitive enough to detect the very small (weak) meteors, but could provide new information about the large cross-section events. [Copyright &y& Elsevier]

Abstract: We report in this and a companion paper [Fentzke, J.T., Janches, D., Sparks, J.J., 2008. Latitudinal and seasonal variability of the micrometeor input function: A study using model predictions and observations from Arecibo and PFISR. Journal of Atmospheric and Solar-Terrestrial Physics, this issue, doi:10.1016/j.jastp.2008.07.015] a complete seasonal study of the micrometeor input function (MIF) at high latitudes using meteor head-echo radar observations performed with the Poker Flat Incoherent Scatter Radar (PFISR). This flux is responsible for a number of atmospheric phenomena; for example, it could be the source of meteoric smoke that is thought to act as condensation nuclei in the formation of ice particles in the polar mesosphere. The observations presented here were performed for full 24-h periods near the summer and winter solstices and spring and autumn equinoxes, times at which the seasonal variability of the MIF is predicted to be large at high latitudes [Janches, D., Heinselman, C.J., Chau, J.L., Chandran, A., Woodman, R., 2006. Modeling of the micrometeor input function in the upper atmosphere observed by High Power and Large Aperture Radars, JGR, 11, A07317, doi:10.1029/2006JA011628]. Precise altitude and radar instantaneous line-of-sight (radial) Doppler velocity information are obtained for each of the hundreds of events detected every day. We show that meteor rates, altitude, and radial velocity distributions have a large seasonal dependence. This seasonal variability can be explained by a change in the relative location of the meteoroid sources with respect to the observer. Our results show that the meteor flux into the upper atmosphere is strongly anisotropic and its characteristics must be accounted for when including this flux into models attempting to explain related aeronomical phenomena. In addition, the measured acceleration and received signal strength distribution do not seem to depend on season; which may suggest that these observed quantities do not have a strong dependence on entry angle. [Copyright &y& Elsevier]

Abstract: High latitude ionospheric convection patterns and models are obtained based on a composite analysis of incoherent scatter radar (ISR) observations in the North America sector at Millstone Hill and Sondrestrom (MHS). These observations span a combined latitude range of over time periods starting from 1978 for Millstone Hill and from 1995 for Sondrestrom. These models, regional to the America sector, represent average and steady patterns of the convection as driven by the interplanetary magnetic field (IMF) By and Bz or by Kp and the By direction, and update the existing MHS models. This paper describes the ISR and IMF data processing and modeling technique and discusses convection changes under a variety of conditions. In particular, we have examined By and Bz effects on the electrostatic potential at dawn and dusk, the location of the cells, and the shape of the cells; we also present the convection change in response to magnetic activity. This long-term data set has enabled us to study seasonal variations of the convection. The cross polar cap potential is on average higher in equinox than in solstice. With the advance of season from winter to summer, the convection cells appear to move antisunward, and the axis across the two cells rotates toward earlier local times; meanwhile, for By negative, the crescent-shaped dawn cell and circular dusk cell become well defined with a reduced cross polar cap potential, while for By positive, the cross polar cap potential tends to be higher. The dusk–dawn cell similarity in summer is found to be high for By negative and low for By positive, supportive of the theory of lobe cell generation in summer. Because of the asymmetry of the convection response to By negative and positive, and of seasonal differences in the convection, it is speculated that there exist perceptible hemispheric differences in the cross polar cap potential drop. [Copyright &y& Elsevier]

Abstract: We introduce a simple but effective order statistics filter for the pulse-level interference and meteor processing of Arecibo ionosphere observation data. Using this filter and the techniques introduced by Wen et al. [2005. Adaptive filtering for the separation of incoherent scatter and meteor signals for Arecibo Observation Data. Journal of Atmospheric and Solar-Terrestrial Physics 67, 1190–1195] we effectively remove/separate the unwanted signals, such as impulsive interference and meteor returns, encountered during incoherent scatter radar (ISR) observations of the ionosphere. We further analyze the separated signals to obtain uniquely “cleaned” incoherent scatter power data and scientifically valuable meteor parameters. We present the processed incoherent scatter results from 22/23 March 2004 observations and the altitude and the speed distributions of the separated meteor signals. The nighttime photochemical E-region is clearly revealed for the first time as a result of meteor and interference removal. Additionally, these results reveal the first major bias in large aperture radar meteor headecho results—the meteor speed distribution is flattened by the absence of at least 50% of the events with duration less than 10ms revealed by meteor-specific observations. Meteor data derived from standard incoherent scatter data always displays this bias. [Copyright &y& Elsevier]

Abstract: Using data acquired with a Digisonde DPS-4 and an Incoherent Scatter Radar both from Jicamarca Radio Observatory, Peru, comparisons of ionospheric vertical drift velocities derived from these two instruments are carried out in this work. They show good agreement during sunset and evening hours (around sunset, up to approximately 22LT), and also, during the post-midnight (between 02 and 03LT) and sunrise hours (until 08LT), when the F-layer bottom side height is around and above 300km, as well as during events of magnetically disturbed periods when the ionospheric plasma drift may be influenced by the perturbation electric fields arising from (1) direct penetration from high to low magnetic latitudes, and (2) disturbance winds dynamo action driven by Joule heating due to increased energy and momentum deposition into the high-latitude thermosphere–ionosphere system. The results of this study lend confidence to using Digisonde drift data to study phenomena related to night-time magnetically disturbed periods. [Copyright &y& Elsevier]

Abstract: The Arecibo 430MHz incoherent scatter radar (ISR) has been used to observe the ionosphere and meteors over 40 years. The meteor signals have traditionally been treated as the interference in ionosphere observation data, while more recently the incoherent scatter signals have been described as background noise in the meteor observation data. In this paper we present signal processing techniques that separate these two different signals into different geophysically interesting data sets. We investigate two techniques for two different data sets. For meteor observation data an adaptive filter is used to locate and separate meteor returns from the total data so that we improve meteor detection rate. For ionosphere observation data we use a filterbank followed by the short time Fourier transform (STFT) analysis to remove the meteor signals from the incoherent scatter results. By doing this the incoherent scatter data will not be contaminated and we can also analyze meteor characteristics from the separated signal. [Copyright &y& Elsevier]

A theoretical ionospheric model is employed to investigate the ionospheric behavior as observed by the incoherent-scatter radar (ISR) at Millstone Hill during the September 21–27, 1998 storm. The observed NmF2 presented a significant negative phase on September 25, and a G condition (hmF2<200 km) was also observed. The model results based on the standard input parameters (climatological model values) are in good agreement with the observed electron densities under quiet conditions, but there are large discrepancies during disturbed periods. The exospheric temperature Tex, neutral winds, atomic oxygen density [O] and molecular nitrogen density [N2], and solar flux are inferred from the ISR ion temperature profiles and from the electron density profiles. Our calculated results show that the maximum Tex is higher than 1700 K, and an averaged decrease in [O] is a factor of 2.2 and an increase in [N2] at 300 km is about 1.8 times for the disturbed day, September 25, relative to the quiet day level. Therefore, the large change of [N2]/[O] ratio gives a good explanation for the negative phase at Millstone Hill during this storm. Furthermore, at the disturbed nighttime the observations show a strong NmF2 decrease, accompanied by a significant hmF2 increase after the sudden storm commencement (SSC). Simulations are carried out based on the inferred Tex. It is found that the uplift of F2 layer during the period from sunset to post-midnight is mainly associated with the large equatorward winds, and a second rise in hmF2 after midnight results from the depleted Ne in the bottom-side of F2 layer due to the increased recombination, while the “midnight collapse” of hmF2 is attributed to the large-scale traveling atmospheric disturbances. [Copyright &y& Elsevier]

A long-term statistical analysis of thermospheric tides in an altitude range from 150 to 400 km is presented. The analysis is based on a dataset with 31 multi-day experiments conducted from an incoherent scatter radar at Arecibo Observatory between 1984 and 2015. This is the first time that the climatological mean and seasonal variations of the thermospheric tides and their response to solar activities are reported using an extensive dataset. The climatological mean amplitude of diurnal tide (DT) is dominant while the amplitudes of the semidiurnal tide (SDT) and terdiurnal tide (TDT) are comparable. Below 250 km, the SDT and TDT phases show downward propagation with vertical wavelengths of 300 and 240 km, respectively. Above 250 km, the DT is the most prominent component except in autumn while the SDT dominates below 250 km except in winter. Above 250 km, the DT is the strongest in winter and its amplitude varies around 28 m/s. Below 250 km, the SDT is prominent in autumn and fluctuates around 35 m/s. The TDT is the most important in winter compared to the other three seasons and its amplitude is slightly less than 20 m/s. The DT and SDT amplitudes show the opposite response to solar activity. Above 250 km, the former increases with increasing solar activity while the latter is the opposite. The enhanced DT amplitude and its phase structure under the high solar activity indicates that the in-situ EUV radiation plays a major role in generating the thermospheric DT over Arecibo. • A long-term observational study on the thermospheric tides is presented over Arecibo for the first time. • Climatology and seasonal variation of the thermospheric tides are revealed. • The thermospheric diurnal tide responds most strongly to solar activity among the three tidal components. [ABSTRACT FROM AUTHOR]