F. S. Rodrigues

Coherent and incoherent scatter radar study of the climatology and day‐to‐day variability of mean F‐region vertical drifts and equatorial spread F

We conducted a comprehensive analysis of the vertical drifts and equatorial spread F (ESF) measurements made by the Jicamarca incoherent scatter radar (ISR) between 1994 and 2013. The ISR measurements allowed us to construct not only updated climatological curves of quiet-time vertical plasma drifts but also time-versus-height maps of ESF occurrence over the past two solar cycles. These curves and maps allowed us to better relate the observed ESF occurrence patterns to features in the vertical drift curves than previously possible. We identified an excessively high occurrence of post-midnight F region irregularities during December solstice and low solar flux conditions. More importantly, we also found a high occurrence of ESF events during sudden stratospheric warming (SSW) events. We also proposed and evaluated metrics of evening enhancement of the vertical drifts and ESF occurrence, which allowed us to quantify the relationship between evening drifts and ESF development. Based on a day-to-day analysis of these metrics, we offer estimates of the minimum pre-reversal enhancement (PRE) peak (and mean PRE) values observed prior to ESF development for different solar flux and seasonal conditions. We also found that ESF irregularities can reach the altitudes at least as high as 800 km at the magnetic equator even during low solar flux conditions.

Topside equatorial ionospheric density, temperature, and composition under equinox, low solar flux conditions

We present observations of the topside ionosphere made at the Jicamarca Radio Observatory in March and September 2013, made using a full-profile analysis approach. Recent updates to the methodology employed at Jicamarca are also described. Measurements of plasma number density, electron and ion temperatures, and hydrogen and helium ion fractions up to 1500 km altitude are presented for 3 days in March and September. The main features of the observations include a sawtooth-like diurnal variation in ht, the transition height where the O+ ion fraction falls to 50%, the appearance of weak He+ layers just below ht, and a dramatic increase in plasma temperature at dawn followed by a sharp temperature depression around local noon. These features are consistent from day to day and between March and September. Coupled Ion Neutral Dynamics Investigation data from the Communication Navigation Outage Forecast System satellite are used to help validate the March Jicamarca data. The SAMI2-PE model was able to recover many of the features of the topside observations, including the morphology of the plasma density profiles and the light-ion composition. The model, forced using convection speeds and meridional thermospheric winds based on climatological averages, did not reproduce the extreme temperature changes in the topside between sunrise and noon. Some possible causes of the discrepancies are discussed.

Modulation of equatorial electrojet irregularities by atmospheric gravity waves

On 9 January 2002 and 14 November 2001, the Sao Luis 30 MHz coherent backscatter radar observed unusual daytime echoes scattered from the equatorial electrojet. The electrojet echoing layers on these days, as seen in the range time intensity maps, exhibited quasiperiodic oscillations. Time-frequency decomposition of the magnetic field perturbations ΔH, measured simultaneously by the ground-based magnetometers, also showed evidence of short-period waves. The ground-based observations were aided by measurements of the brightness temperature in the water vapor and infrared bands made by the GOES 8 satellite. The GOES 8 satellite measurements indicated evidence of deep tropospheric convection activities, which are favorable for the launch of atmospheric gravity waves (AGW) near Sao Luis. Our multitechnique investigation, combined with an analysis of the equatorial electric field and current density, indicates that AGW forcing could have been responsible, via coupling with E region electric fields, for the short-period electrojet oscillations observed over Sao Luis.

Daytime ionospheric equatorial vertical drifts during the 2008–2009 extreme solar minimum

One of the most interesting observations made by the Communication/Navigation Outage Forecasting System (C/NOFS) satellite mission was the detection of average equatorial ionospheric vertical drifts that largely differed from model predictions. C/NOFS measurements showed, in particular, downward drifts in the afternoon sector, and upward drifts around local midnight hours during the 2008 and 2009 extreme solar minimum. The unexpected behavior of the drifts has important implications for ionospheric modeling and suggests the necessity for a better understanding of the low-latitude electrodynamics. We used ground-based radar measurements to quantify the seasonal and solar flux variability of daytime equatorial drifts at lower altitudes (∼150 km) than those probed by C/NOFS (above ∼400 km). We found that average vertical drifts at 150 km altitude are in good agreement with model predictions of F region drifts and did not show the signatures of an enhanced semidiurnal pattern, as seen by C/NOFS. Comparison of the 150 km echo drifts with model predictions also shows that the increase (decrease) with height of the vertical drifts in the morning (afternoon) hours is a regular feature of the equatorial ionosphere. It occurred in all seasons and solar flux conditions between 2001 and 2011.

Equatorial zonal electric fields inferred from a 3‐D electrostatic potential model and ground‐based magnetic field measurements

[1] We present a new technique to infer quiet time zonal electric fields in the daytime equatorial ionosphere. The electric field inference technique utilizes a threedimensional (3-D) electrostatic potential model of the low-latitude ionosphere constrained by ground-based magnetic field measurements. To test this technique, inferred zonal electric fields for the Peruvian sector in Jicamarca (11.95S, 283.13E, 0.6N dip latitude) were compared with zonal electric field (vertical drift) measurements made by the Jicamarca Incoherent Scatter Radar. The comparison shows a good agreement between the inferred and measured electric fields. An example of electric field estimation for Davao (7.4N, 125.4E, 0.58S dip latitude) in the Philippines sector is also presented in this report. Inferred electric fields for Davao are in good agreement with F region vertical plasma drifts measured by drift sensors onboard the AE-E and ROCSAT-1 satellites on that longitude sector. Our results suggest that realistic estimates of quiet time zonal electric fields for the equatorial ionosphere can be obtained from the 3-D potential model whenever observatory magnetic field measurements are available. Citation: Shume, E. B., E. R. de Paula, S. Maus, D. L. Hysell, F. S. Rodrigues, and A. Bekele (2009), Equatorial zonal electric fields inferred from a 3-D electrostatic potential model and ground-based magnetic field measurements, J. Geophys. Res., 114, A06305,

AMISR‐14: Observations of equatorial spread F

A new, 14-panel Advanced Modular Incoherent Scatter Radar (AMISR-14) system was recently deployed at the Jicamarca Radio Observatory. We present results of the first coherent backscatter radar observations of equatorial spread F(ESF) irregularities made with the system. Colocation with the 50 MHz Jicamarca Unattended Long-term studies of the Ionosphere and Atmosphere (JULIA) radar allowed unique simultaneous observations of meter and submeter irregularities. Observations from both systems produced similar Range-Time-Intensity maps during bottom-type and bottomside ESF events. We were also able to use the electronic beam steering capability of AMISR-14 to “image” scattering structures in the magnetic equatorial plane and track their appearance, evolution, and decay with a much larger field of view than previously possible at Jicamarca. The results suggest zonal variations in the instability conditions leading to irregularities and demonstrate the dynamic behavior of F region scattering structures as they evolve and drift across the radar beams.

Concurrent observations at the magnetic equator of small-scale irregularities and large-scale depletions associated with equatorial spread F

In 2014 an all-sky imager (ASI) and an Advanced Modular Incoherent Scatter Radar consisting of 14 panels (AMISR-14) system were installed at the Jicamarca Radio Observatory. The ASI measures airglow depletions associated with large-scale equatorial spread F irregularities (10–500 km), while AMISR-14 detects small-scale irregularities (0.34 m). This study presents simultaneous observations of equatorial spread F (ESF) irregularities at 50–200 km scale sizes using the all-sky imager, at 3 m scale sizes using the JULIA (Jicamarca Unattended Long-term Investigations of the Ionosphere and Atmosphere) radar, and at 0.34 m scales using the AMISR-14 radar. We compare data from the three instruments on the night of 20–21 August 2014 by locating the radar scattering volume in the optical images. During this night no topside plumes were observed, and we only compare with bottomside ESF. AMISR-14 had five beams perpendicular to the magnetic field covering ~200 km in the east-west direction at 250 km altitude. Comparing the radar data with zenith ASI measurements, we found that most of the echoes occur on the western wall of the depletions with fewer echoes observed the eastern wall and center, contrary to previous comparisons of topside plumes that showed most of the echoes in the center of depleted regions. We attribute these differences to the occurrence of irregularities produced at submeter scales by the lower hybrid drift instability. Comparisons of the ASI observations with JULIA images show similar results to those found in the AMISR-14 and ASI comparison.

Spectrum sharing between WiFi and radio astronomy

The proliferation of wireless local area network also known as WiFi system has enabled easy wireless information access for consumers. However, it also causes radio frequency interference (RFI) to passive wireless systems such as radio astronomy systems (RAS), making almost impossible to get useful scientific observations around the WiFi bands. This paper proposes a new paradigm for the coexistence between WiFi and RAS. The proposed approach creates a coexistence access zone (CAZ) around the RAS site within which WiFi and RAS follow a pre-determined time-division spectrum access. Two modified WiFi medium access control (MAC) protocols are developed to embed the time-division coexistence access. Furthermore, traffic statistics based improved spectrum access is developed. Performance evaluation results show that at the cost of slight WiFi throughput reduction, RAS achieves substantial RFI-free spectrum access which were infeasible in the existing paradigm.

Multi-instrumented observations of the equatorial F -region during June solstice: large-scale wave structures and spread- F

AbstractTypical equatorial spread-F events are often said to occur during post-sunset, equinox conditions in most longitude sectors. Recent studies, however, have found an unexpected high occurrence of ionospheric F-region irregularities during June solstice, when conditions are believed to be unfavorable for the development of plasma instabilities responsible for equatorial spread-F (ESF). This study reports new results of a multi-instrumented investigation with the objective to better specify the occurrence of these atypical June solstice ESF in the American sector and better understand the conditions prior to their development. We present the first observations of June solstice ESF events over the Jicamarca Radio Observatory (11.95° S, 76.87° W, ∼ 1° dip latitude) made by a 14-panel version of the Advanced Modular Incoherent Scatter Radar system (AMISR-14). The observations were made between July 11 and August 4, 2016, under low solar flux conditions and in conjunction with dual-frequency GPS, airglow, and digisonde measurements. We found echoes occurring in the pre-, post-, and both pre- and post-midnight sectors. While at least some of these June solstice ESF events could have been attributed to disturbed electric fields, a few events also occurred during geomagnetically quiet conditions. The late appearance (22:00 LT or later) of three of the observed events, during clear-sky nights, provided a unique opportunity to investigate the equatorial bottomside F-region conditions, prior to ESF, using nighttime airglow measurements. We found that the airglow measurements (630 nm) made by a collocated all-sky camera show the occurrence of ionospheric bottomside F-region perturbations prior to the detection of ESF echoes in all three nights. The airglow fluctuations appear as early as 1 hour prior to radar echoes, grow in amplitude, and then coincide with ESF structures observed by AMISR-14 and GPS TEC measurements. They also show some of the features of the so-called large-scale wave structures (LSWS) that have been detected, previously, using other types of observations and have been suggested to be precursors of ESF. The bottomside fluctuations have zonal spacings between 300 and 500 km, are aligned with the magnetic meridian, and extend at least a few degrees in magnetic latitude.

Results of coherent backscatter radar imaging using Capon's method and measurements made by the Sao Luis radar interferometer

Interferometric radar imaging of F-region spread F irregularities is used to determine the distribution of scatterers within the radar field of view. In this study, we investigate the use of the Capons spectral method for estimating the distribution of equatorial spread F (ESF) irregularity structures observed by a small, low power coherent backscatter radar interferometer located at the equatorial site of Sao Luis (2.59°S, 44.21°W, -2.35° dip.), Brazil. To evaluate the performance of the method, we show numerical simulations for typical F-region measurement conditions. Results for the Fourier imaging method are also shown for comparison. The simulation shows that, despite the short baselines of the Sao Luis radar, the Capon technique is capable of distinguishing features with km scale sizes (in the zonal direction) at F-region heights. We also investigated the application of the Capon algorithm to actual measurements made by the Sao Luis radar, and obtained high resolution images of equatorial spread F (ESF) scattering structures. In this presentation, we will show results of the Capon images during different types of equatorial spread F events, and will compare the results with images obtained using the conventional Fourier method. As predicted by the numerical simulations, we are able to distinguish features of the scattering structures with resolution of a few km in the zonal direction. We have been able, in particular, to identify the morphology of scattering structures during bottom-type layers. Signatures of underlying deca-kilometric waves, similar to those observed by [3] and [7] were detected. The observations suggest the action of the so-called collisional shear instability.