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Indoor device-free object sensing can be utilized in many applications such as occupant-centered control of building utilities and assisted living. To satisfy the mathematical requirement of many observation points for arbitrary indoor layout and furnishing, radio-frequency identification (RFID) offers a low-cost solution with a plethora of maintenance-free passive tags. Both the received signal strength indicator and carrier phase from tag backscattering can be assembled to generate the voxel reflectivity distribution by the inverse method. We adopt the regularized truncated pseudo-inverse method, and devise the strategies for optimal selection of the critical parameters. Compared with conventional matched filtering, our method is more robust against random noises in the collected data. An experimental prototype was established to evaluate the system robustness and performance, and the dipole antennas were used to replace patch antennas to enhance the system signal-to-noise ratio (SNR). The regularized truncated pseudo-inverse method together with the improved system SNR has successfully shown higher locating accuracy and lower computational cost.

With the proliferation of the Internet of Things and radio-frequency identification (RFID), the ambient radio signals can be leveraged for indoor occupant monitoring. In this article, we have employed passive RFID tags in the ambient for occupant counting by a deep-learning solution. The reader collects both carrier phase and received signal strength from each tag, which are inputs to a convolutional neural network. A novel background calibration is proposed to reduce phase offsets and noises in the presence of heavy multipath, which further improves model accuracy. Our results show satisfactory performance, with 0.82 probability for detecting the correct number of occupants, and 1.0 if ±1 error is permitted. The model also exhibits occupant location and posture independent learning, allowing limited and faster training data collection. To demonstrate generalized learning without strong bias to indoor setup, we have also transferred this pre-trained model to another similar-sized room, achieving 0.85 – 1.0 accuracy for different tag-receiver placements and furnishing.

The Ionospheric Connection Explorer, or ICON, is a new NASA Explorer mission that will explore the boundary between Earth and space to understand the physical connection between our world and our space environment. This connection is made in the ionosphere, which has long been known to exhibit variability associated with the sun and solar wind. However, it has been recognized in the 21st century that equally significant changes in ionospheric conditions are apparently associated with energy and momentum propagating upward from our own atmosphere. ICON's goal is to weigh the competing impacts of these two drivers as they influence our space environment. Here we describe the specific science objectives that address this goal, as well as the means by which they will be achieved. The instruments selected, the overall performance requirements of the science payload and the operational requirements are also described. ICON's development began in 2013 and the mission is on track for launch in 2017. ICON is developed and managed by the Space Sciences Laboratory at the University of California, Berkeley, with key contributions from several partner institutions.

We present an analysis of in-situ thermal ion measurements from a cusp auroral sounding rocket. Using a forward modeling procedure, we find most-probable thermal ion temperature and parallel (field...

Incoherent scatter radar observations of ionospheric plasmas rely on echoes from electron density fluctuations with properties governed by the dispersion relations for ion-acoustic and Langmuir waves. Radar observations of echoes associated with Langmuir waves (plasma lines) from thermal plasma are weak and only a few near-thermal level measurements have been reported. Plasma line echoes are typically only observed with existing radars only when the Langmuir waves are enhanced by suprathermal electrons. A new observation technique has been developed which is sensitive enough to allow observations of these echoes without the presence of suprathermal electrons up to at least 1000 km. This paper presents recent observations from the Arecibo Observatory 430-MHz incoherent scatter radar which show plasma-line echoes during the night when no suprathermal enhancement is expected to be present. The observations are compared with theory, and the results are found to be in agreement with classical incoherent scatter theory for thermal plasmas. The theoretical ratio of the ion line and plasma line power spectral density is within approximately 3 dB of the predicted value. The finding adds a new observational capability, allowing electron density to also be observed at night using the plasma line well into the top side of the ionosphere, increasing the accuracy of the electron density measurement.

The Rocket Experiment for Neutral Upwelling 2 (RENU2) rocket was launched on 13 December 2015 at 07:34 UT. The payload transited the cusp region during a neutral upwelling event, supported by a comprehensive set of onboard and ground-based instrumentation. RENU2 data highlight two important processes. One is that a proper understanding of neutral upwelling by Poleward Moving Auroral Forms (PMAFs) requires a treatment that mimics the quasiperiodic passage of a sequence of PMAFs. As a PMAF reaches a flux tube, its physical consequences must be determined including the residual history of effects from previous passages, implying that understanding such a process requires an accounting of the system hysteresis. Second, RENU2 observations suggest that neutral density enhancements driven by precipitation and/or Joule heating can be highly structured in altitude and latitude. In addition, timescales involving neutral dynamics suggest that the structuring must be slowly changing, for example, over the course of 10 to tens of minutes. publishedVersion

Publisher's version available at https://doi.org/10.1029/2018RS006688 We argue that combining a high‐power, large‐aperture radar transmitter with several large‐aperture receiving arrays to make a geospace radar—a radar capable of probing near‐Earth space from the upper troposphere through to the solar corona—would transform geospace research. We review the emergence of incoherent scatter radar in the 1960s as an agent that unified early, pioneering research in geospace in a common theoretical, experimental, and instrumental framework, and we suggest that a geospace radar would have a similar effect on future developments in space weather research. We then discuss recent developments in radio‐array technology that could be exploited in the development of a geospace radar with new or substantially improved capabilities compared to the radars in use presently. A number of applications for a geospace radar with the new and improved capabilities are reviewed including studies of meteor echoes, mesospheric and stratospheric turbulence, ionospheric flows, plasmaspheric and ionospheric irregularities, and reflection from the solar corona and coronal mass ejections. We conclude with a summary of technical requirements.

Thermal ion retarding potential analyzers (RPAs) are used to measure in situ auroral ionospheric plasma parameters. This article analyzes data from a low-resource RPA in order to quantify the capability of the sensor. The RPA collects a sigmoidal current-voltage (I-V) curve, which depends on a non-linear combination of Maxwellian plasma parameters, so a forward-modeling procedure is used to match the best choice plasma parameters for each I-V curve. First, the procedure is used, given constraining information about the flow moment, to find scalar plasma parameters-ion temperature, ion density, and spacecraft sheath potential-for a single I-V curve interpreted in the context of a Maxwellian plasma distribution. Second, two azimuthally separated I-V curves from a single sensor on the spinning spacecraft are matched, given constraining information on density and sheath potential, to determine the bulk plasma flow components. These flows are compared to a high-fidelity, high-resource flow diagnostic. In both cases, the procedure's sensitivity to variations in constraining diagnostics is tested to ensure that the matching procedure is robust. Finally, a standalone analysis is shown, providing plasma scalar and flow parameters using known payload velocity and International Reference Ionosphere density as input information. The results show that the sensor can determine scalar plasma measurements as designed, as well as determine plasma DC flows to within hundreds of m/s error compared to a high-fidelity metric, thus showing their capability to replace higher-resource methods for determining DC plasma flows when coarse-resolution measurements at in situ spatial scales are suitable.

Equatorial plasma depletions have significant impact on radio wave propagation in the upper atmosphere, causing rapid fluctuations in the power of radio signals used in telecommunication and GPS navigation, thus playing a crucial role in space weather impacts. Complex structuring and self-organization of equatorial plasma depletions involving bifurcation, connection, disconnection and reconnection are the signatures of nonlinear evolution of interchange instability and secondary instabilities, responsible for the generation of coherent structures and turbulence in the ionosphere. The aims of this paper are three-fold: (1) to report the first optical imaging of reconnection of equatorial plasma depletions in the South Atlantic Magnetic Anomaly, (2) to investigate the optical imaging of equatorial ionospheric intermittent turbulence, and (3) to compare nonlinear characteristics of optical imaging of equatorial plasma depletions for two different altitudes at same times. We show that the degree of spatiotemporal complexity of ionospheric intermittent turbulence can be quantified by nonlinear studies of optical images, confirming the duality of amplitude-phase synchronization in multiscale interactions. By decomposing the analyses into North-South and East-West directions we show that the degree of non-Gaussianity, intermittency and multifractality is stronger in the North-South direction, confirming the anisotropic nature of the interchange instability. In particular, by using simultaneous observation of multi-spectral all-sky emissions from two different heights we show that the degree of non-Gaussianity and intermittency in the bottomside F-region ionosphere is stronger than the peak F-region ionosphere. Our results are confirmed by two sets of observations on the nights of 28 September 2002 and 9 November 2002.

Jicamarca measurements of electron temperatures at high altitudes (500–1500 km) 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.

Progress simulating equatorial spread F (ESF) in pursuit of a space-weather forecast capability is summarized. ESF is the main manifestation of space weather at low magnetic latitudes in the ionosphere and is responsible for disrupting communication, navigation, imaging, and surveillance systems important to the Air Force and other federal agencies. A 3D numerical simulation of the plasma instabilities responsible for ESF written at Cornell has been developed and upgraded under this award. Plasma number density, electric field, and neutral wind data necessary for driving the simulation have been collected in campaigns at the Jicamarca Radio Observatory conducted approximately semi-annually. The simulation code is initialized and forced using campaign data. Simulation results, specifically the plasma depletions and plumes characteristic of ESF, are compared with coherent scatter radar imagery of ESF irregularities made at Jicamarca. Such imagery is directly comparable to the 3D simulation products, offering a prediction-assessment strategy that is uniquely conducive to closure. In multiple campaign studies, the simulation was able to recover the ionospheric dynamics that unfolded in nature including the occurrence or non-occurrence of ESF

We present experimental evidence and modeling results which indicate that eastward thermospheric wind is the primary controlling factor of equatorial spread-F initiation in the post-sunset ionosphere. Eastward wind-driven Pedersen currents are able to polarize F-region density perturbations with westward tilting wavefronts into rapidly growing modes to trigger the formation of spread-F bubbles. The described process is so rapid that seeding requirements of spread-F initiation by external factors such as gravity waves are effectively eliminated.

Jicamarca radar observations of bottom-type coherent scattering layers in the post-sunset bottomside F-region ionosphere are presented and analyzed. The morphology of the primary waves seen in radar images of the layers supports the hypothesis of kudeki+bhattacharyya-1999 that wind-driven gradient drift instabilities are operating. In one layer event when topside spread F did not occur, irregularities were distributed uniformly in space throughout the layers. In another event when topside spread F did eventually occur, the irregularities within the pre-existing bottom-type layers were horizontally clustered, with clusters separated by about 30km. The same horizontal periodicity was evident in the radar plumes and large-scale irregularities that emerged later in the event. We surmise that horizontal periodicity in bottom-type layer irregularity distribution is indicative of large-scale horizontal waves in the bottomside F-region that may serve as seed waves for large-scale Rayleigh Taylor instabilities. Key words. Ionosphere (equatorial ionosphere; ionospheric irregularties; plasma waves and instabilities)

We present preliminary radar results obtained with a new method of spectrum estimation of moderately overspread equatorial spread F (ESF) echoes over Jicamarca. This new method is similar to the classical pulse-to-pulse methods but with nonuniform pulse spacings, i.e., using aperiodic transmitter pulses, allowing us to avoid range and frequency aliasing, at the cost of extra clutter at some ranges. We also present different ways of dealing with the extra clutter, depending on the spectral characteristics of the clutter. We have identified two kinds of clutter: ground clutter (due to mountains and transmitter pulses) and ionospheric clutter (due to ESF and equatorial electrojet echoes). Preliminary radar results are very encouraging, and ways of improving them are proposed based on Monte Carlo simulations. Particularly, we propose alternating the phases of the uncoded aperiodic pulses using random binary phase codes to flatten the spectrum of the ionospheric clutter. The theoretical implications of the newly estimated spectra, particularly from topside echoes, are presented and discussed in a companion paper.

Common-volume observations of sporadic E-layers made on 14-15 June 2002 with the Arecibo incoherent scatter radar and a 30MHz coherent scatter radar imager located on St. Croix are described. Operating in dual-beam mode, the Arecibo radar detected a slowly descending sporadic E-layer accompanied by a series of dense E-region plasma clouds at a time when the coherent scatter radar was detecting quasi-periodic (QP) echoes. Using coherent radar imaging, we collocate the sources of the coherent scatter with the plasma clouds observed by Arecibo. In addition to patchy, polarized scattering regions drifting through the radar illuminated volume, which have been observed in previous imaging experiments, the 30MHz radar also detected large-scale electrostatic waves in the E-region over Puerto Rico, with a wavelength of about 30km and a period of about 10min, propagating to the southwest. Both the intensity and the Doppler shifts of the coherent echoes were modulated by the wave.

The National Center for Atmospheric Research thermosphere=ionosphere=electrodynamic general circulation model (TIEGCM) is one of the few models that self-consistently solves the coupled equations for the neutral atmosphere and ionosphere. Timely questions are how well the TIEGCM currently simulates the low-latitude ionosphere and what modi9cations might bring about better predictions. Comparisons between data obtained in and around Jicamarca, Peru, near the magnetic equator, and simulations with the TIEGCM indicate good progress has been made but reveal some serious discrepancies. Good-to-excellent agreement is obtained for electron densities, electron and ion temperatures, and nmax. The agreement is fair to poor for hmax, zonal drifts, the 630 nm oxygen nightglow, and the horizontal neutral winds. The most important discrepancy is in the simulated neutral temperature, which is at least 100 K too cold relative to Fabry–Perot interferometer observations. Increasing the EUV ?uxes in the model to improve prediction of the model temperature also improves representation of airglow observations and of the ionosphere, for which the model typically underrepresents the electron densities. The disparity in neutral temperature is also present in comparisons with the empirical model MSIS which represents the largest database of thermospheric temperature measurements. Since the neutral and ionized atmospheres are tightly coupled at low latitudes, simultaneous measurements of neutral and ion parameters, preferably over an extended time period, would be invaluable to further the understanding of the region. Better knowledge of the EUV ?uxes and the high altitude O + ?uxes may also help

Jicamarca unattended long term investigations of the ionosphere and atmosphere radar observations of equatorial spread F (ESF) plasma irregularities made between August 1996 and April 2000 are analyzed statistically. Interpretation of the data is simplified by adopting a taxonomy of echo types which distinguishes between bottom-type, bottomside, topside, and post-midnight irregularities. The data reveal patterns in the occurrence of ESF in the Peruvian sector that are functions of season, solar flux, and geomagnetic activity. We confirm earlier work by Fejer et al. (J. Geophys. Res. 104 (1999) 19,859) showing that the quiet-time climatology of the irregularities is strongly influenced by the climatology of the zonal ionospheric electric field. Under magnetically quiet conditions, increasing solar flux implies greater pre-reversal enhancement amplitudes and, consequently, irregularity appearances at earlier times, higher initial altitudes, and higher peak altitudes. Since the post-reversal westward background electric field also grows stronger with increasing solar flux, spread F events also decay earlier in solar maximum than in solar minimum. Variation in ESF occurrence during geomagnetically active periods is consistent with systematic variations in the electric field associated with the disturbance dynamo and prompt penetration described by Fejer and Scherliess (J. Geophys. Res. 102 (1997) 24,047) and Scherliess and Fejer (J. Geophys. Res. 102 (1997) 24,037). Quiet-time variability in the zonal electric field contributes significantly to variability in ESF occurrence. However, no correlation is found between the occurrence of strong ESF and the time history of the zonal electric field prior to sunset.

Coherent radar imaging (CRI) is used in an attempt to overcome the angular resolution limitation of conventional single-station radars and is used to image the horizontal structure inside the resolution volume. This recently developed technique has been successfully applied to radar observations of the ionosphere as well as the lower atmosphere. However, no statistical analysis of the robustness of the various techniques has been presented to date. In this work, three CRI techniques are reviewed: Fourier-based, Capon's, and maximum entropy (MaxEnt) methods. The Fourier-based method is the simplest of the three algorithms but has inherent resolution limitations. Although quite different in nature and performance, both Capon's and MaxEnt methods can be posed as constrained optimization problems. A statistical comparison of performance of the three CRI techniques, using various receiver configurations and two distinct cases of scattering structure, is made using simulated data. The results show that the MaxEnt method exhibits the best performance in the case of aspect-sensitive scattering with a broad characteristic. In the localized scattering case, however, Capon's method shows superior performance for signals with high signal-to-noise ratio (SNR), but MaxEnt method outperforms all methods for low SNR. In general, both Capon's and MaxEnt methods are able to reproduce the gross characteristics of the scattering media under observation.

A unified picture of plasma irregularities in equatorial spread F is developed from the analysis of satellite, sounding rocket, and coherent scatter radar observations. The coherent scatter data are analyzed using a new in-beam radar imaging technique that permits direct comparison between radar data, in situ data, and computer simulations of the irregularities. Three varieties of irregularities, all produced by ionospheric interchange instabilities, are found to occur. Thin bottom-type layers are composed of waves with primary transverse wavelengths less than about 1 km and with significant parallel wavenumbers. These exist on magnetic flux tubes controlled by the E region dynamo and drift westward in the postsunset ionosphere. A nonlocal analysis is used to calculate their linear growth rate. When the F region dynamo takes control of the flux tube, bottomside irregularities can emerge. These are more robust irregularities with longer primary wavelengths and which exhibit greater vertical development. Nonlinear analyses explain the appearance of steepened structures in rocket observations and solitary waves in satellite observations of bottomside layers. The one-dimensional spectra of these irregularities obey power laws but are anisotropic and have variable spectral indices and spectral breaks. Very strong polarization electric fields can eject large regions of deeply depleted plasma through the F peak and form topside irregularities. Theoretical calculations supported by satellite data show that ion inertia may become important for topside irregularities. The one-dimensional spectra of irregularities in the inertial regime obey a k −5/3 power law, but strong plasma inhomogeneity implies that Kolmogorov weak turbulence is not the explanation. Topside depletions are shown to bifurcate and also to pinch off from the bottomside.

The 30-MHz radar observations of coherent scatter from midlatitude E region plasma irregularities are presented and interpreted. Our analysis indicates that quasiperiodic (QP) echoes can be interpreted as quasi-point target scatterers drifting through the radar beam at approximately constant altitude. Interferometry data collected with multiple baselines show that the patchy scatterers are not organized along wave fronts but instead form drift and decay independently of one another. An analysis of the intensity of the echoes versus range reveals that the morphology of the QP echo striations is consistent with expectations for drifting field-aligned irregularities with finite aspect angle sensitivity. An analysis of the effects of refraction, aspect sensitivity, and scattering geometry shows that the plasma irregularities in question reside at altitudes between 95 and 120 km. Type I echoes observed on one occasion imply the existence of strong zonal polarization electric fields with magnitudes of about 20 mV/m. A numerical model of the E region current system demonstrates how such electric fields can be generated by elongated, patchy sporadic E layers electrically coupled to the F region dynamo.

Estimates of the zonal ionospheric electric field have been made on the basis of JULIA radar observations of the equatorial electrojet at Jicamarca. Two observing techniques were used and their merits compared. One technique (here referred to as the “oblique” technique) made use of a new, small antenna array with a broad beam in the equatorial plane. Electric fields were estimated from the Doppler shifts of type II radar echoes at low elevation angles as a function of range in the manner of Balsley [1969a]. For the other technique the main Jicamarca antenna array was used, and interferometry was employed to measure altitude profiles of the phase speeds of intermediate-scale primary gradient drift waves in the electrojet directly overhead. Electric fields were inferred from these profiles following a modeling approach similar to the one described by Murthy and Ravindran [1994], Zonal ionospheric electric field measurements comparable to seasonal average incoherent scatter radar measurements were obtained using both methods. However, the interferometric observations reveal the presence of intense waves and jets in the zonal neutral wind similar to what has been seen in recent chemical release sounding rocket experiments. Zonal winds mainly control the zonal phase speeds of the primary gradient drift waves in the late afternoon and evening. Although the prereversal enhancement of the zonal electric field is evident in data from the oblique technique, there is no evidence of it in measurements derived from the interferometric technique.

Results of Doppler radar observations are discussed of HF scattering from heater induced inhomogeneities in an underdense ionospheric plasma. The quasiperiodic variations shown by the Doppler frequency shift are associated with the E×H plasma drift in the electric field of geomagnetic pulsations. Combined with a theoretical model of hydromagnetic wave propagation, with allowance for an arbitrary dip angle of the geomagnetic field and arbitrary angle of incidence, the Doppler shift measurements at several points along the geofield tube permit estimating height-integrated ionospheric conductivities

A new interferometric imaging technique has been used at Jicamarca to study the morphology and dynamics of plasma irregularities occurring during spread F conditions. The technique produces two-dimensional, in-beam images of the coherent scatter from these irregularities, instantaneously mapping the structure of the underlying plasma instabilities in the equatorial plane. We present sequences of images depicting backscatter from bottom-type layers, bottomside layers, and topside plumes drifting and rising through the ionospheric regions illuminated by the Jicamarca radar. The high spatial and temporal resolution of the images permits us to observe dynamical phenomena missed by conventional fixed- and steered-beam techniques. The new images show evidence of intermediate- and large-scale plasma depletions bifurcating and pinching off from the bottomside as they ascend. Analytic models are presented to describe these dynamical effects. The images also differentiate between bottom-type and bottomside spread F layers, the former being shown to have much smaller scale primary waves than the latter. Ample evidence of secondary plasma instabilities appear throughout the images. The consequences of these secondary instabilities and of shear flow are discussed.

This paper presents coherent scatter radar observations of plasma irregularities in the equatorial F region ionosphere made with the JULIA (Jicamarca unattended long-term studies of the ionosphere and atmosphere) radar and discusses model equations and simulations pertinent to their interpretation. The data set shows that bottom-type and bottomside scattering layers are prevalent throughout solar minimum equinox. Bottom-type layers are undifferentiated and relatively weak layers confined to a narrow range of altitudes falling within regions of the ionosphere driven by the E region dynamo. These layers are often precursors to large-scale radar plumes as well as to bottomside layers, which are broader, more structured, and more intense layers than the bottom-type variety. Bottomside layers are the source of intermediate-scale plasma depletions that can ascend and penetrate through to the topside. Fluid simulations of the collisional interchange instability suggest that dynamo theory explains why one kind of layer emerges at a given time and altitude.

Coherent scatter radar imaging observations have been made of a bottomside spread F layer at Jicamarca. Whereas conventional range-time-intensity (RTI) images cannot resolve the primary waves in the layers, the in-beam images show that the layers are composed of vertical chains of patches of scatterers drifting through the radar scattering volume. These chains outline and delineate primary collisional interchange instability waves with wavelengths of a few km. Different phases of the primary waves ascend at different rates, and the Doppler spectra of the radar backscatter represent superpositions of spectral lines from different patches on the various phases of the primary waves. A computer simulation of the fluid instability reproduces many features of the radar observations, including the patchy appearance of the backscatter from small-scale, linearly damped waves excited by the nonlinear instability. Some of the patches come into existence by means of a secondary gradient drift instability, leading them to be arranged quasi-periodically in the vertical direction. It is shown that the coherent scatter Doppler frequency spectrum may give a poor representation of the line-of-sight velocity distribution, particularly when a small scattering volume is sampled for a short time.

Multiple baseline interferometric imaging of a large-scale topside spread F depletion has been performed at the Jicamarca Radio Observatory near Lima, Perú. A new imaging technique makes it possible to examine the detailed structure of the scatter from field-aligned irregularities in and around the depletion. A new antenna at Jicamarca, physically separated from the main antenna array, provided very long interferometry baselines up to -94 A long for the observations. High-resolution images of coherent backscatter from title radar plume were computed írom tite interferometry data using tite maximum entropy metltod. These images show that scattering regions with small Doppler velocities lay mainly along the boundary of the depleted region. Meanwhile, regions with high Doppler velocities were located within the depletion itselí and could be seen convecting upward through the depleted channel. Por pares

Radar interferometry with multiple collinear antenna baselines has been performed at Jicamarca, near Lima, Peru, and high-resolution images of plasma irregularities in equatorial spread F have been constructed from the data. The images represent the brightness distribution of the radar backscatter, which is the Fourier transform of the visibility or spatial cross correlation of the scattered radar signal on the ground. The visibility data are noisy and incompletely sampled, and we have used the maximum entropy method to perform the Fourier transform, making optimal use of the data available. Using the current antenna configuration at Jicamarca, the radar imaging technique is able to resolve structures within the radar beam that are just a few kilometers wide. By animating sequences of images, we can observe the evolution of ionospheric irregularities without confusing spatial and temporal variations.

GPS satellites broadcast at two frequencies (Ll of 1575.42 MHz and L2 of 1227.6 MHz). The dispersive property of the ionosphere is frequently used to correct positional measurements for ionospheric effects. Independent measurements at the two frequencies can also be combined to form a relative ionospheric delay and a measure of the total electron content (TEC) which is uncertain by an additive constant. In a previous paper (Musman et al., 1990) estimates of this offset were utilized in constructing models of the time history of the equivalent zenith delay at Westford, Massachusetts. An ionospheric model composed of uniform shells whose electron density changes slowly in a typical diurnal pattern would produce relative ionospheric delays with a simple u-shaped or j-shaped curve. Most of the change in delay would be a result of changes in geometry between the observer and the satellite. Departures from a simple pattern are indicative of ionospheric disturbances and the influence of the protonosphere. From GPS data alone, it is ambiguous whether these disturbances are due to spatial structures, temporal changes, or some combination of the two. Equatorial spread F (ESF) refers to a variety of equatorial ionospheric disturbances, some of which are associated with rising plasma plumes having low electron density and a high degree of turbulence. This phenomenon occurs primarily between local sunset and local midnight at sites within about 15° of the magnetic equator. In some seasons, disturbances can occur during two out of three evenings, while at other times it can be much quieter. GPS observations at Kwajalein (9°N latitude) reported here for August 14, 1990, show severe ionospheric disruption. Two independent and simultaneous sets of radar observations confirm the presence of ESF and reveal quite a bit about the spatial and temporal conditions which affect the system. GPS observations on August 15, 1990, when no ESF was present are much quieter. We find that tens of minute variations in the TEC correspond to the motion of large scale features across the GPS field of view. More severe GPS effects are seen to be collocated with turbulent low density plumes which rise rapidly to high altitudes and drift west to east across the GPS line of sight. Severe disruption can occur in moderately sophisticated GPS systems during such events, at least near solar maximum.

An irregular sporadic E ionization layer was observed by the Arecibo incoherent scatter radar and a coherent scatter radar imager located on St. Croix during the passage of an MSTID observed by the Boston University all-sky camera in 630 nm imagery. The MSTID in question was not very intense and was barely detectable in the vertical F region plasma drifts measured by Arecibo. The intensity of the coherent scatter from small-scale irregularities in the sporadic E layer appeared nonetheless to be modulated by the MSTID and was strongest in the F region airglow crests, mapped along magnetic field lines to the E region volume being observed. The coherent scatter Doppler shifts were highly correlated with altitude displacements in the sporadic E layer, and the sign of the correlation was controlled by the sign of the background zonal electric field. The MSTID did not appear to modulate the morphology of the sporadic E layer irregularities themselves, which took the form of convective rolls and which drifted with the ambient neutral wind.

[1] In order to study nighttime medium-scale traveling ionospheric disturbances (MSTIDs), we have developed a new midlatitude ionosphere electrodynamics coupling model (MIECO) which can model the coupling process between the E and F regions with dipole magnetic field lines. Using the new model, MSTID structure is reproduced from random perturbation on an Es layer by the coupled Perkins and sporadic-E (Es)-layer instabilities in a wide latitudinal range. A typical wavelength of ∼150 km, larger amplitude, and smaller MSTID's tilt angles at lower latitudes are consistent with observations. It is shown that the polarization process in the E region driven by neutral winds is essentially important for the full development of MSTIDs as well as the seeding of NW–SE perturbation in the F region.

[1] Plasma density structures and associated irregularities in the nighttime midlatitude ionosphere are frequently observed as frontal structures elongated from northwest to southeast (NW-SE) in the Northern Hemisphere. The frontal structures and the coupling process between the E and F regions are studied with a three-dimensional numerical model, which can simulate two instability mechanisms: Perkins instability in the F-region and sporadic-E (Es)-layer instability in the E region. The fastest growth of the coupled instability occurs when the unstable conditions on NW-SE perturbation are satisfied in both regions. The perturbation of F-region integrated conductivity grows much faster than the isolated Perkins instability. The meridional component of a rotational wind shear blows an existing Es layer southward, and the F-region structure follows the E-region drift velocity. The NW-SE structure in the E region can be formed from random perturbation regardless of the F-region condition. When the F region is unstable on the NW-SE perturbation, however, the NW-SE structure is formed in both regions with a common scale length. We conclude that (1) the Es-layer instability plays a major role in seeding NW-SE structure in the F region, and the Perkins instability is required to amplify its perturbation; (2) the rotational wind shear in the E region produces southwestward phase propagation of the NW-SE structure in both the E and F regions; and (3) the coupling process has a significant effect on the scale of the Es-layer perturbation rather than the growth rate of the Es-layer instability.

Incoherent scatter data from a hybrid long-pulse/double-pulse experiment at Jicamarca are analyzed using a full-profile analysis similar to the one implemented by Holt et al. (1992). In this case, plasma density, electron and ion temperatures, and light ion composition profiles in the topside are estimated simultaneously. Full-profile analysis is crucial at Jicamarca, since the long correlation time of the incoherent scatter signal at 50 MHz invalidates conventional gated analysis. Results for a 24 h interval in April of 2006 are presented, covering altitudes through 1600 km with 10 min time resolution, and compared with results from the NRL ionospheric model SAMI2. The analysis provides the first comprehensive assessment of ionospheric conditions over Jicamarca at sunrise as well as the first 24-h record of helium ion layers. Possible refinements to the experiment and the algorithm are discussed.

[1] A three-dimensional numerical simulation in the nighttime midlatitude ionosphere is developed for the first time and applied to the Perkins instability in the midlatitude F region. Growth of the Perkins instability is successfully reproduced under nighttime condition, and the numerical results basically agree with a linear theory and previous two-dimensional numerical studies. Northwest-southeast (NW-SE) alignment of density perturbations is generated from random seeding by applying a southeastward neutral wind. The perturbations are dominant at altitudes of 200–300 km where a steep density gradient exists, which is consistent with the altitude of 630-nm airglow emission that often shows NW-SE alignment. Further simulation in terms of the E-F coupling can be done in the near future.

[1] High frequency (HF) radars are used to detect ionospheric irregularities, meteor trails, and moving targets. The Precision Expandable Radar Calibration Sphere (PERCS) is a simple radar target in space to help determine the operational parameters of ground HF radars. PERCS will have a known radar cross section that is independent of observation direction within 0.5 dB. The PERCS satellite can be launched in a stowed configuration that has about 1 m in diameter. After launch, the PERCS will expand to a diameter of almost 10 m. Upon expansion, a stable wire frame is formed to act as a radar scatter target in the form of a polyhedral sphere. The simplest version of the sphere has 60 vertices (V60) that are joined to 90 rigid segments. Each segment is hinged so that the PERCS can be folded into a compact package for launch. Analysis of the V60 wire frame with a 10 m diameter shows that the radar cross section (RCS) is nearly independent of viewing angle up to 30 MHz. Another design with 240 vertices produces even better performance. Radar systems will be calibrated using the radar echo data and the precise knowledge of the target RCS, position, and velocity. The PERCS can reflect radar signals from natural targets such as field aligned and current driven irregularities not presently accessible from ground-based radars. The wire frame structure has several advantages over a metalized spheroid “balloon” with (1) much less drag, (2) larger radar cross section, and (3) lower fabrication cost.

Cornell University supported the Air Force Research Laboratory C/NOFS mission through a combination of experiment development, data acquisition and processing, simulation, and modeling of ionospheric processes pertinent to forecasting so-called equatorial spread F phenomena. A new radar mode capable of providing simultaneous plasma density and drift measurements in the equatorial ionosphere has been developed and implemented at the Jicamarca Radio Observatory. The new mode will be crucial for C/NOFS calibration/validation as well as for ongoing campaign Support. Data acquired in this and other modes have been taken at Jicamarca in support of DMSP and COSMIC satellite overflights. Processed parameters have been posted and distributed to the C/NOFS science team through the Madrigal distributed data server. These data can also be used to test and initialize the AFRL forecast models. Additional experimental runs will be performed throughout the actual C/NOFS mission. Three specific investigations were carried out during the Cornell award period An improved ionospheric irregularity forecast strategy based on an evolving understanding of a counter streaming plasma instability process has been formulated. The new strategy should outperform traditional ones based solely on Rayleigh Taylor-type instability mechanisms. A new long-pulse incoherent scatter radar data processing scheme has been developed for improved measurement of plasma density, temperature, and composition profiles in the topside ionosphere. The new mode will be particularly useful right after C/NOFS launch, which will occur near solar minimum. A new means of inverting satellite radio occultation data has been developed and demonstrated. The method reflects artifacts that would otherwise obscure the E and valley regions and will increase the usefulness of beacon data from C/NOFS.

[1] Electron densities retrieved from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) measurements are compared with those measured by incoherent scatter radars (ISR) and ionosondes in this paper. These results show that electron density profiles retrieved from COSMIC RO data are in agreement with the ISR and ionosonde measurements. The ionospheric characteristics (NmF2 and hmF2) derived from the COSMIC satellites are also compared with those calculated by the latest International Reference Ionosphere model (IRI-2001) and the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (NCAR-TIEGCM). The comparison of the magnitude of the COSMIC NmF2 data with those calculated by the IRI model and the TIEGCM is good. However, features such as the north-south asymmetry and longitudinal variation of the equatorial anomaly that are seen in the COSMIC data and the TIEGCM simulations are not fully present in the IRI model. On the other hand, the TIEGCM produces a stronger winter anomaly than that seen in either the COSMIC data or the IRI model.

[1] Aperture synthesis radar imaging has been used to investigate coherent backscatter from ionospheric plasma irregularities at Jicamarca and elsewhere for several years. Phenomena of interest include equatorial spread F, 150-km echoes, the equatorial electrojet, range-spread meteor trails, and mesospheric echoes. The sought-after images are related to spaced-receiver data mathematically through an integral transform, but direct inversion is generally impractical or suboptimal. We instead turn to statistical inverse theory, endeavoring to utilize fully all available information in the data inversion. The imaging algorithm used at Jicamarca is based on an implementation of the MaxEnt method developed for radio astronomy. Its strategy is to limit the space of candidate images to those that are positive definite, consistent with data to the degree required by experimental confidence limits; smooth (in some sense); and most representative of the class of possible solutions. The algorithm was improved recently by (1) incorporating the antenna radiation pattern in the prior probability and (2) estimating and including the full error covariance matrix in the constraints. The revised algorithm is evaluated using new 28-baseline electrojet data from Jicamarca.

[1] Coherent backscatter from the auroral electrojet was observed by a 30 MHz imaging radar in Anchorage during the Joint Observations of Upper Latitude Electrodynamics (JOULE) campaign conducted from the Poker Flat Research Range in the spring of 2003. The observations were made at the same time that ionospheric electric fields and plasma number densities were measured in situ by instruments on sounding rockets. Neutral wind profiles were also measured during the campaign from triangulation of chemiluminescent trails from rocket releases. Aperture synthesis radar imaging techniques permit the sorting of the coherent backscatter into small azimuth and range bins and the determination of the scattering altitude. Individual Doppler spectra could thereby be unambiguously associated with in situ electric field measurements in the same small volume. We find that the Doppler shifts of the auroral echoes correspond to the ion acoustic speed times the cosine of the flow angle, where the former is predicted according to an empirical wave heating law. Type I echoes are only observed for very small flow angles regardless of the convection speed.

[1] A new 30 MHz coherent scatter ionospheric radar has been operating at the equatorial station at Sao Luis (2.33°S, 44°W, dip latitude 1.3°S), Brazil, since December 2000. This VHF radar has a peak power of only 8 kW but uses long coded pulses and a high PRF with coherent integration to achieve good sensitivity. Two side-by-side square antenna arrays composed of 16 5-element Yagi antennas directed vertically are used for transmission and reception. This radar measures the backscattered signals from E and F region ionospheric irregularities. In the standard operational mode, the irregularity intensity, as well as the vertical and zonal velocities using Doppler analysis and interferometry, respectively, are determined. We initially present a brief description of the radar system, signal characteristics and data processing, followed by some of the initial observations. Electrojet echoes ranged from about 94 to 108 km in altitude with the strongest echoes coming from about 104 km and with an uplift to about 110 km occurring in the late afternoon. Echoes from the valley region (150 km echoes) were strong, quasi-periodic with periods of about 10 to 15 minutes, and had the necklace shape observed at others sites. F region bottom-type, bottomside, and topside (plumes) spread F layers were observed at night. The large-scale topside F region plumes, moving eastward and upward, reached altitudes of about 1,400 km and were preceded by bottom-type layers around 400 km altitude that were moving westward. The characteristics of the echoes were similar to those observed by the JULIA radar at Jicamaca, Peru. However, some differences in the behavior of the echoes between the two sites were noted.

[1] A collisional shear instability in a magnetized plasma is described and evaluated. The instability is related to electrostatic Kelvin Helmholtz but operates in inhomogeneous plasmas in the collisional regime. Boundary value analysis predicts that the linear growth rate for the instability could be comparable to that of the collisional interchange instability in the equatorial F region ionosphere under ideal conditions. An initial value simulation of a nonlinear model of the instability run under realistic conditions produces growing waves with a relatively long growth time (50 min) and with an initial wavelength of about 30 km. The simulation results are consistent with recent radar observations showing large-scale plasma waves in the bottomside equatorial ionosphere at sunset prior to the onset of spread F conditions. The role of shear instability in preconditioning the F region for interchange instabilities to occur after sunset is discussed.

The relaxation behavior of phase observables for different particle diffusion models is found to establish a ground for radioscience interpretations of coherent backscatter spectra. The characteristic function for a random walk process at twice the incident radiation wave number is associated with the complex amplitude of the scattered field from a medium containing refractive index fluctuations. The phase relaxation function can be connected to the evolution of the characteristic function and may describe the average regression of the scattered field from a spontaneous fluctuation undergoing turbulent mixing. This connection holds when we assume that the stochastic description of particle movements based on a diffusion model is valid. The phase relaxation function, when identified as the generalized susceptibility function of the fluctuation dissipation theorem, is related to the spectral density of the scattered field from steady-state fluctuations.