Sixto A. Gonzalez

A study of the role of ion-molecule chemistry in the formation of sporadic sodium layers

Over two campaigns in 1998 and 1999, multiple sporadic sodium events were observed by the Arecibo Observatory sodium density lidar while simultaneously monitoring the plasma density using the incoherent scatter radar. In this paper, we test the theoretical explanation proposed by Cox and Plane (1998) where Na + in a plasma layer is neutralized via an ion–molecule mechanism to form a sporadic sodium layer. A particular challenge is to interpret observations made in a Eulerian frame of observation where the spatial and temporal characteristics of events cannot easily be separated. The reaction scheme in the original mechanism is modi=ed to include the reactions NaO + +N2 → Na + ·N2 +O and NaO + +O2 → Na + +O3, following the results of theoretical quantum calculations. Six unique case studies of sporadic sodium layers are presented here, and excellent agreement between simulation and observations was obtained for =ve of them. c � 2002 Published by Elsevier Science Ltd.

Detection of He+ layering in the topside ionosphere over Arecibo during equinox solar minimum conditions

We describe recent developments in and results from topside incoherent scatter radar (ISR) measurements at Arecibo, PR, emphasizing helium ion measurements. Recent improvements in the data taking modes and the data processing permit isolation of the concentrations of oxygen, helium and hydrogen ions between the F region peak and 2000 km with about 10 minute time resolution. The need for the three ion non-linear least squares fits is justified by use of the goodness of fit; a two ion fit is shown to be unacceptable. The new measurements are optimized for the rapid height variations in the solar minimum nighttime ionosphere by replacing the traditional 1 ms pulse with a 500 µs pulse. We show results from one day of a five day experiment during the Spring of 1994. For these equinox solar minimum conditions, the altitude distribution of the helium ions usually has a maximum near the O+ to H+ transition altitude (ht), forming a distinct layer, most noticeable during the night. The maximum helium ion concentrations tend to be quite low, 2 or 3 × 10³ cm−3, or 10–20% of the topside plasma at the peak of the He+ layer. The transition altitude, ht, varies from about 1200 km to 1400 km during the day to near 500 km at night, and the He+ layer follows this altitude variation. Finally we show that the location of the layer near ht and its intensification during the night can be explained using ambipolar diffusion equations.

The effects of meridional neutral winds on the O+‐H+ transition altitude over Arecibo

Improvements to the way data are taken and analyzed at the Incoherent Scatter Radar Facility at Arecibo, Puerto Rico, mean that the transition altitude. where the O + and H + ion densities are equal, can now be studied at all local times throughout the solar cycle. These data show that the diurnal variation of the O + -H + transition height is characterized by a rapid collapse following sunset, which is associated with the contraction of the plasma due to the rapidly decreasing plasma temperatures at this time. A corresponding increase in the transition altitude is seen at sunrise. The topside data also reveal that a secondary collapse, which is a nonthermal effect, is often observed at local times near midnight. Measurements taken during October 1994 and 1992 are presented and compared to results from the Sheffield University Plasmasphere Ionosphere Model (SUPIM) to study the effect of increasing solar EUV flux on the topside ionosphere. It is shown that SUPIM is able to model reasonably well the sunset and sunrise behavior of the transition altitude. In addition, the secondary collapse of the O + -H + transition altitude is also studied. It is shown that this secondary collapse may be directly correlated to the well-known midnight collapse phenomenon of the F region over Arecibo. The magnitudes of the secondary collapse under low and moderate solar activity are compared, and it is shown that the secondary collapse is less pronounced during solar minimum conditions.

Simultaneous atomic and ion layer enhancements observed in the mesopause region over Arecibo during the Coqui II Sounding Rocket Campaign

The NASA Coqui II sounding rocket campaign in Puerto Rico provided the opportunity to obtain a large number of lidar and incoherent scatter radar observations of atomic sodium and ion layers in the upper mesosphere and lower thermosphere. Sodium layer enhancements, coupled with ion layers, were frequently observed in the range of 90–105 km altitude. We found that above 97 km all of the enhanced Na layers were observed to have an associated ion layer, and below that altitude some Na enhancements could occur in their absence. Finally, we show one extraordinary case of a sporadic Na layer that grew to near its peak concentration before the associated ion layer appeared at its altitude.

Coordinated incoherent scatter radar study of the January 1997 storm

We describe many important features of the ionospheric F region as observed by the Sondrestrom, Millstone Hill, and Arecibo incoherent scatter radars (ISRs) and the Millstone Hill and Ramey Digisondes during January 6-10, 1997, with emphasis on the January 10, 1997 storm. Coordinated analysis of the data provides evidence for traveling atmospheric disturbances (TADs) and for two likely electric field penetration events linking these stations. Large and rapid changes in hmF 2 were seen at Arecibo and nearby Ramey which are related to the TADs and penetrating electric fields. Results are compared with simulations by the thermosphere-ionosphere electrodynamics general circulation model (TIEGCM), which utilizes high-latitude inputs given by the assimilative mapping of ionospheric electrodynamics (AMIE) technique. An important result ofthis study is that the TIEGCM is able to predict TADs similar to those observed. Exceptional features observed during this storm at Millstone Hill are a very large nighttime T e enhancement on January 10 and a larger decrease in NmF 2 than predicted by the TIEGCM throughout the storm period. The latter appears to be related to an underestimation of the neutral temperature by the model.

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.

The effect of electron Coulomb collisions on the incoherent scatter spectrum in the F region at Jicamarca

The fact that the incoherent backscatter spectrum narrows when the radar beam is nearly perpendicular to the magnetic field is well known and has been used at Jicamarca for more than 30 years to measure very accurate line-of-sight velocities. Recently it has become clear that these spectra are narrower than expected. We have explained this effect and also the small change to the spectral shape required at somewhat larger angles to correct the ratio of electron to ion temperature seen in some studies. Coulomb collisions affecting the motion of the electrons are responsible for the additional spectral narrowing. We have carried out very accurate simulations of electron motion resulting in incoherent scatter spectra which are qualitatively similar to spectra resulting from other types of collisions, and to those predicted in an analytic solution for the Coulomb case [Woodman, 1967]. However, we found that the spectrum of the velocity time series in the radar line of sight departs significantly from the nearly Lorentzian form expected with simple collisional models. This causes the collisional effects to extend to somewhat shorter scale lengths, or further from perpendicular to the magnetic field than expected. In order to investigate the collisional process more closely, we performed another simulation combining the effects of electron-ion collisions and a simple friction model (Langevin equation) in an adjustable combination. This one showed that the effect of electron-ion collisions alone would result in collisional effects extending several degrees farther from perpendicular to the field than when both kinds of collisions are included. Collisions affecting the speed of the electrons tend to limit the size of the effect at larger angles from perpendicular. Thus the effect of these collisions on the incoherent scatter spectrum cannot be accurately predicted from simple models but depends on the detailed physics of the collisions.

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.

Sporadic E layer observations over Arecibo using coherent and incoherent scatter radar: Assessing dynamic stability in the lower thermosphere

[1] A patchy sporadic E ionization layer was observed using the 430 MHz incoherent scatter radar at the Arecibo Radio Observatory and a new, 30 MHz coherent scatter radar imager located on St. Croix. The former measured state parameters of the layer (plasma density, temperature, and composition) while the latter observed small-scale plasma waves and the associated quasiperiodic (QP) echoes. Regularization was used to estimate horizontal vector ion drifts from Arecibo line-of-sight data. Assuming the ion temperatures and drifts to be representative of the neutral atmospheric temperatures and drifts in the mesosphere lower-thermosphere (MLT) region, we evaluated the Richardson number criterion for neutral dynamic instability in regions occupied by the layers. We find that the condition for instability was generally satisfied and thus that dynamic instability is consistent with the layer breakup. The electrodynamics of the resulting patchy layers could then give rise to the QP echoes.

Tomographic studies of aeronomic phenomena using radio and UV techniques

Tomographic characterization of ionospheric and thermospheric structures using integrated line-of-sight measurements provides a unifying paradigm for the investigation of various aeronomic phenomena. In radio tomography, measurements of the total electron content (TEC) obtained using a chain of ground receivers and a transit satellite are inverted to reconstruct a two-dimensional electron density pro;le. Similarly, prominent optically thin UV emissions, such as 911 and 1356 = A produced by radiative recombination of O + , provide the means to obtain F-region electron densities from space-based spectroscopic measurements. The existence of a number of UV sensors in orbit and in planning stage provide the means to carry out such tomographic remote sensing investigations on global scales. The inherent non-ideal acquisition geometry of such remote sensing observations, however, results in limited-angle tomographic inverse problems that are both ill-posed and ill-conditioned. Furthermore, the intrinsic presence of noise, especially in the case of UV measurements, imposes challenges on conventional reconstruction methods. To overcome these limitations, we approach the solution of these inverse problems from a regularization standpoint. In particular, we apply regularization by incorporating appropriate edge-preserving regularizing functionals that enforce piecewise smoothness of the solution. This paper describes these techniques, investigates associated inversion issues, and demonstrates their applicability through a case study. c � 2002 Published by Elsevier Science Ltd.