Nestor Aponte

Caribbean Ionosphere Campaign, year one: Airglow and plasma observations during two intense mid‐latitude spread‐F events

A series of campaigns has been carried out in the Caribbean over a one-year period to study intense mid-latitude spread-F events using a cluster of diversified instrumentation. These events are relatively rare but a number of them have now been captured and will be discussed in this and several companion papers. This paper focuses on 630 nm airglow images obtained by the Cornell All-Sky Imager for two of the more spectacular cases that began on February 17, 1998 and February 17, 1999. In the latter case, and for the first time, a poleward surge of depletion/enhancement airglow zones was captured by radar as well as an airglow imager. In the former case structures grew in place overhead and produced strong VHF F-region backscatter as observed by the CUPRI and University of Illinois radars; the other event, exactly one year later, did not result in detectable 3-m backscatter. The two data sets show quantitatively that the low airglow region is elevated in height and depleted in plasma density and Pedersen conductivity. We suggest an enhanced eastward electric field inside the low conductivity zone may be responsible for the surge. The data also suggest small scale turbulence can only be observed in developing structures.

Brightening of 630.0 nm equatorial spread‐F airglow depletions

[1] Observations from the Boston University all-sky imaging system at Arecibo, Puerto Rico (18.3°N, 66.7°W, 28°N mag), show an unusual behavior of nighttime 630.0-nm airglow depletions. Associated with equatorial spread-F (ESF), these structures move eastward before reversing their motion and become airglow enhancements. Few other cases have been found, all during December solstices. For the case study presented here, data from the Arecibo incoherent scatter radar and the Republic of China Scientific Satellite (ROCSAT-1) provide supporting information. The radar shows that around local midnight the background zonal and meridional plasma motions reverse to westward and southward, respectively. ROCSAT-1 shows enhanced ion density, i.e., a low-latitude plasma blob, above the bright feature recorded by the all-sky imager, indicating a possible connection between both phenomena. Drifts parallel to the magnetic field are observed only in the region where the enhancement occurs. One possible interpretation of this change in the brightness of the depleted structure involves the influence of northward meridional winds and a reversal in the zonal drift motion, most likely caused by a zonal wind reversal.

On the electrical structure of airglow depletion/height layer bands over Arecibo

Using a combination of airglow images and incoherent scatter radar data, we have explored the electrical structure of the airglow depleted, height layer bands over a mid-latitude site. We find a reproducible electrical signature in both components of the electric field in all events studied. The most pronounced feature is a large northward/upward electric field in the heart of the structure. The latter is identical to the radially outward field reported for mid-latitude conjugate electric fields [Saito et al., 1995], found to trace the poleward edge of the equatorial anomaly. We favorably compare the Arecibo drift to a typical satellite event. These electric fields may reflect a nonlinear evolved state of the Perkins instability or some, as yet, unexplained coupling between atmospheric motion and the plasma embedded in it. We show here that the F-region Pedersen conductivity is much lower in these structures than outside and suggest that this is related to a polarization electric field inside the structure.

Ionospheric topography maps using multiple-wavelength all-sky images

We outline a technique to create three-dimensional topographic maps of the Earths ionosphere. Using all-sky images at 630.0 and 777.4 nm taken with the Cornell All-Sky Imager (CASI) while located at the Arecibo Observatory, we can estimate the maximum density (Nm) and the height (Hm) of the F layer over a 1000 × 1000 km area. This is possible because to first order, the 777.4 nm emission, produced by radiative recombination, is proportional to the integral of the square of the plasma density, whereas the 630.0 nm line, produced by charge exchange and dissociative recombination, depends on both plasma height and density. Using the neutral atmosphere given by the Mass Spectrometer Incoherent Scatter (MSIS-86) model and electron densities from the international reference ionosphere 1995 (IRI-95) model, we show that the estimates given by these maps are good to within 5% of the values used as input into the models. These errors are slightly larger (10%) when extreme gradients in the height of the F layer are present. We apply our technique to two different nights in 1999. In one example these maps show a steeply rising ridge of ionization stretching equatorward of the Caribbean site, punctuated by a series of parallel ridges and valleys. We compare these observations with previous work at Arecibo during very high magnetic activity. In our case we find no evidence for particle precipitation and agree with Sahai et al. [1981a] that spatial variations may have affected the earlier study. Another example shows the Appleton anomaly much farther north than normal. Instability processes are indicated in the former case, while physical mechanisms associated with a magnetic storm are explored in the latter case.

Horizontal parameters of daytime thermospheric gravity waves and E region neutral winds over Puerto Rico

We report on the electron density perturbation amplitudes, periods (up to 60 min), horizontal and vertical wavelengths, phase speeds, and propagation directions of daytime traveling ionospheric disturbances (TIDs) from 115 to 300 km altitude using dual-beam experiments at the Arecibo Observatory (AO), Puerto Rico. As in previous studies, we find a near continuum of waves above the AO. While the TIDs propagate in nearly all directions except purely westward, we find that most propagate southward southeastward. We find that TID amplitudes increase nearly exponentially with increasing period, although with a much smaller slope for periods >30 min. TID amplitudes peak on the bottomside of the F region. Typical vertical wavelengths increase from less than 50 km at low altitudes to ∼100–300 km. Horizontal wavelengths increase from ∼70–100 km to ∼150–500 km over the same altitude range. Median vertical wavelengths, horizontal wavelengths, and periods increase with altitude up to z∼ 100–150 m/s. We also measure the E region horizontal neutral winds and find that they peak at ∼150 m/s near z∼105 km in the middle of the day. Wave phase speeds are in general greater than these ambient winds. In addition, by tracing individual wave packets vertically in altitude, we find that a packets vertical wavelength generally peaks near the altitude where its inferred ion velocity amplitude is maximum. The vertical wavelength generally decreases above this altitude, an observation that is consistent with gravity wave packet theory.

Advection of the equatorial anomaly over Arecibo by small-storm related disturbance dynamo electric fields

During highly disturbed geomagnetic periods, both the Arecibo radar and the Ramey ionosonde have recorded impressive nighttime ionospheric enhancements in which the peak electron density exceeded 1 × 1012 m−3 and the F2 peak height went above 400 km. In the past it has been suggested that these events could be caused by either a downward plasmaspheric flux that increases the density in the F region or by an eastward electric field that pushes the equatorial anomaly poleward to the Caribbean sector. On February 17–18, 1999 the Arecibo radar made observations during an event in which the electron density again rose to daytime values near midnight. For this event, the peaks in density were observed predominantly southeast of Arecibo while the ions sustained a northward-eastward motion due to an eastward-southward storm dynamo electric field. TEC maps from GPS for this night confirmed that the density enhancements were due to a poleward expansion of the equatorial anomaly.

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.

High‐resolution electron temperature measurements using the plasma line asymmetry

[1] In this paper, we present the first results of a new technique for measuring the electron temperature in the daytime ionosphere using the Arecibo incoherent scatter radar (ISR). The technique utilizes the plasma line component of the incoherent scatter spectrum. The difference in the up- and down-shifted plasma line frequencies is related to the density and temperature of the ionosphere, as well as more minor effects resulting from photoelectrons, currents, and other sources. The shift is very small (the order of 1 kHz in a plasma line frequency of several MHz) but can be measured quite accurately with the coded long pulse plasma line technique. We compare the results to ion line measurements of the electron temperature, and the two independent techniques show good agreement. In addition to providing a measure of the electron temperature that is independent of the ion line, the approach allows for a sensitive test of kinetic plasma theory including a magnetic field, gives us the ability to study photoelectron populations and electron currents, and will allow us to constrain ion line fits in the bottomside (and possibly topside) regions to more accurately fit for composition. Citation: Nicolls, M. J., M. P. Sulzer, N. Aponte,

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.

Corection of the Jicamarca electron-ion temperature ratio problem: Verifying the effect of electron Coulomb collisions on the incoherent scatter spectrum

Ever since the first attempts to fit Jicamarca autocorrelation function (ACF) measurements in the 1970s using a full nonlinear least squares analysis, an apparent electron-ion temperature ratio below unity has been deduced for a large portion of the F region data. The cause of this unexpected and geophysically unreasonable result has been a mystery until recently, when Sulzer and Gonzalez [1999] (herein SG) explained how electron Coulomb collisions can distort, or narrow, the incoherent backscatter spectrum, and that for this narrowing to be observable two conditions must be met. First, the radar k vector must lie in a small range near perpendicular to the magnetic field, and second, the radar wavelength must be sufficiently long. Both of these conditions are true at Jicamarca. The accurate calculations from the SG theory are now available in a compact library, which we have incorporated into an incoherent scatter least squares fitting code. Using this code, we have reduced Jicamarca ACF data taken with the Faraday double-pulse mode, and find that the SG theory correctly interprets the ACF data from Jicamarca, thereby solving the longstanding Te/Ti ratio problem and thus allowing accurate electron and ion temperature measurements.