Julio Urbina

On the lack of southern hemisphere polar mesosphere summer echoes

We report VHF radar observations of the southern high-latitude mesopause region using wind profilers that were installed recently on King George Island, Antarctica, and Ushuaia, Argentina. Briefly, our observations, which were made during January and February 1993, show almost no evidence of so-called polar mesosphere summer echoes, or PMSE. Since these echoes are a predominant feature of the northern high-latitude mesosphere in summer, their absence in the southern hemisphere is both surprising and intriguing. In this paper we present evidence demonstrating the virtual absence of the echoes and demonstrate that our systems were capable of detecting them had they been present. We also outline some of the consequences of this intriguing result, which are supported by observed hemispheric differences in polar mesospheric clouds, mesospheric temperatures, upper atmospheric gravity wave activity, and mean circulation patterns.

Southern‐hemisphere PMSE: Where are they?

We report a surprising absence of PMSE (Polar Mesospheric Summer Echoes) in VHF radar observations of the southern summer high-latitude mesosphere. Our observations cover about twenty days during January-February, 1993, from King George Island, Antarctica.

50 MHz radar observations of mid‐latitude E‐region irregularities at Camp Santiago, Puerto Rico

A 50 MHz radar interferometer was used near Salinas, Puerto Rico, to probe the meter-scale E-region plasma density irregularities during two campaigns conducted in 1998. During the February–April period E-region echoing layers were primarily observed between 90 and 100 km heights. The layers were typically thin (∼1 km) and unstructured, although in several cases short period (∼90 s) layer oscillations were observed. During the June–July period E-region echoes showed more varied characteristics. In addition to low altitude layers, quasi-periodic structures with descending echoing layers were observed at altitudes above 100 km. Zonal motions detected during descending layer events were at times variable and oscillated between westward and eastward directions while the layer descent rates remained fixed.

Joint observations of sodium enhancements and field-aligned ionospheric irregularities

Resonance Lidar observations of neutral sodium and VHF coherent scatter radar observations of field-aligned 3-meter irregularities were obtained during the Coqui II rocket campaign in Puerto Rico. The Lidar, a facility instrument at the Arecibo Observatory (18.3°N, 66.8°W), and the University of Illinois Radar, located near Salinas on the south of the island, both monitored volumes near where the uplegs of the nominal rocket trajectories intersected the E-region. The Observatorys Incoherent Scatter Radar was also used to characterize the plasma layers. Preliminary investigation of the data sets has shown a potential correspondence between VHF backscatter from plasma layers and a new class of characteristic enhancements in the neutral sodium.

Coherent and incoherent scatter radar observations during intense mid‐latitude spread F

An intense mid-latitude spread-F event occurred over Puerto Rico during the night of February 17, 1998. Simultaneous observations were made with the Cornell University Portable Radar Interferometer (CUPRI) located near Isabela, PR, the University of Illinois VHF radar located at Salinas, PR, GPS receivers at Isabela and St. Croix, measuring total electron content, the Arecibo incoherent scatter radar, and the Cornell All-Sky imager located at the Arecibo Observatory. This was the first time that such a broad range of complementary instrumentation captured a mid-latitude spread-F space weather event. It was the first (and still only) time that a spread-F event over the Caribbean exhibited large Doppler shifts in the VHF spectra. This event was characterized with multiple filaments that initially produced receding Doppler velocities exceeding 300 m/s as seen by CUPRI and the Illinois radar. The Arecibo incoherent scatter radar recorded line-of-sight velocities exceeding 100 m/s that moved the F-layer peak to over 400-km altitude. Airglow images of 630.0 nm emissions from F-region heights showed depleted structures oriented southeast to northwest. The large velocities observed with the radars suggest that we caught this event in a stage of explosive development. It is interesting that the first fully documented Caribbean event occurred during a magnetically active period.

First observations of an F-region turbulent upwelling coincident with severe E-region plasma and neutral atmosphere perturbations

Abstract Highly structured electron densities in the E and F regions over Puerto Rico during the night of February 20/21, 1999 were accompanied by intense coherent VHF radar backscatter from the E region and perturbations in neutral sodium in the mesosphere. Simultaneous observations of the event were made with the VHF Cornell University Portable Radar Interferometer (CUPRI) located near Isabela, PR, the University of Illinois VHF radar located at Salinas, PR, the Arecibo incoherent scatter radar, and the sodium lidar located at the Arecibo Observatory. On this geomagnetically quiet night, regions of very different electron concentrations moved through the region. The F -region peak altitudes of the low density regions differed by about 100 km from the high-density region altitudes. The E region also exhibited an unusual enhancement with a vertical extent of about 6 km and caused intense VHF backscatter. The echoing E regions seen by both VHF radars were highly structured with multiple filaments and Doppler shifts exceeding 300 m/s (directed north and upward) some of the time. The Arecibo incoherent scatter radar recorded a large eastward component of the velocity (∼200 m/s ) during the early portion of the event, which then switched to strongly westward (peaking over 500 m/s and averaging perhaps 400 m/s for about half an hour) before returning eastward. The meridional velocity components were also variable. The Arecibo lidar showed an intense sodium layer that maintained a constant altitude until the strongest VHF echoes began. Then the layer fell 2 km over a time span of about half an hour and the lidar echoes intensified. Because (1) the timing of the events at the different locations is well correlated with the F -region drifts as measured with the Arecibo radar, and (2) because the Pedersen conductivity falls precipitiously at the start of the event, we conclude there was strong coupling between the E and F regions, perhaps even reaching the mesosphere, during this event. However, major problems may remain. How can the E -region cloud track the F -region blob when supposedly it is coupled to the neutrals? Does the neutral wind track the F -region ion velocity? We do not think so, but this is the easiest solution.

Software-defined radio: a new paradigm for integrated curriculum delivery

Software-defined radio is a rapidly developing field that is driving the development of and innovation in communications technology, and promises to significantly impact all communications sectors. Entities developing these SDR systems require a trained workforce that has been prepared with the mindset, knowledge, skills, and tools required to address both the system (breadth) and technical (depth) aspects of SDR systems. Developing SDRs necessarily involves a collection of disciplines including, but not limited to, electromagnetics, radio-frequency engineering, communications, digital signal processing, embedded systems, computer programming, and systems engineering. Whereas electrical engineering and computer science and engineering curricula at the university level may include courses in all of these areas, a students typical curriculum does not; nor does it usually involve the integration of all these topics. However, SDR can be employed as an integrative construct that facilitates systems thinking and cross-domain learning via peers. In this article, we present several significant educational efforts across six U.S. universities that have developed integrated curricula in SDR, most including a significant laboratory component.

Rainfall measurements using satellite downlink attenuation

The attenuation of a commercial satellite link signal at Ku-band is used for estimating rainfall rate along the link path. Comparisons with rainfall accumulations obtained using simultaneous radar measurements along the same path show good agreement. This method can potentially be used to complement other rainfall accumulation measurements obtained with radar and rain gauges in urban settings and areas with complex terrain.

A Blending Technique in Thermospheric Density Modeling

Uncertainty in the atmospheric density is a crucial error source for the propagation of satellites in low earth orbit (LEO). As a result, establishing accurate thermospheric neutral density models are important to predict the motion of these satellites. Unfortunately, since density data in the altitude range between 140 km and 200 km are sparse, predicting the neutral density to estimate atmospheric drag effects on the motion of satellites operating in this altitude region may cause relatively large errors. Previous study found that the JacchiaBowman model (JB2006) is the most reliable thermospheric empirical neutral density model above 200 km and the Naval Research Laboratory’s Mass Spectrometer Incoherent Scatter (NRLMSISE-00) model, whose core formulation is based on incoherent scatter radar data, and can be considered a more reliable neutral density model below approximately 140 km. We have developed a bridging technique to blend the two models between these two regions. A simple two-body model with atmospheric drag was used to compare effects of various atmospheric density models. These tests are conducted by propagating the positions of satellites orbiting between 140 and 200 km, with various ballistic coefficients, using the JB2006, the NRLMSISE-00, and the bridging technique we have developed.

Effects of the Equatorial Electrojet on FM-Based Passive Radar Systems

Passive radar systems operate by employing cooperative or noncooperative radio signals in the environment in order to sense remote targets. This particular feature of passive radar systems provides unique opportunities to expand the coverage map of conventional radars. In using passive radar systems, any undesirable obstacle between the transmitted signal and the path of the desirable target can cause obliteration on the source signal and destroy its key correlative properties. The equatorial electrojet (EEJ) can have unfavorable effects on the operation of the prospective passive radar systems, which can conceivably employ very high frequency frequency-modulated (FM) radio signals around the magnetic equator. The EEJ is a strong flow of current in the upper atmosphere around 100 km of altitude over the magnetic equator. Because of the density irregularities within, the ionospheric current is a geophysical obstacle for the deployment of passive radar systems near those latitudes. In this paper, we assess the effects of the EEJ on the operation of FM-based passive radar systems. First, we simulate the EEJ as a communication channel based on its physical properties by using Gaussian random processes. We simulate the propagation of FM signals through this communication channel and determine the changes in their correlative properties. Finally, we present the experimental data that were collected near the magnetic equator that demonstrates the malfunction of the FM-based passive radar systems due to the EEJ. These observations and numerical results show that careful considerations must be taken when implementing FM-based passive radar systems at equatorial latitudes.