M. Mendillo

The application of GPS observations to equatorial aeronomy

Routine observations of the ionospherically imposed propagation effects upon GPS satellite signals are available online from the International GPS Service for Geodynamics. With over 200 ground-based stations now reporting data, ionospheric studies ranging from the analysis of single-site observations to the full global network have demonstrated the geophysical science yield possible from this remarkable resource. In this paper we expand on the use of GPS data for comprehensive “regional studies” at low latitudes. Using the GPS observing sites in South America, we show how routine observations are processed to form reliable total electron content (TEC) values capable of describing the latitudinal, longitudinal, local time, and storm time behavior over the full span of the continent. To study the dominant F region structure at low latitudes, the Appleton anomaly, two indices are developed to assess its strength (Is) and asymmetry (Ia). TEC data at 30 s intervals are used to form phase fluctuation indices that capture plasma irregularity patterns at 15-min (fp) and hourly (Fp) time resolution. Tests of Fp at Atlantic and Pacific sector locations show them to reproduce accurately all known occurrence patterns for equatorial spread F (ESF). The use of the three indices (Is,Ia, and Fp) to formulate predictive capabilities for ESF on the basis of the enhancement or suppression of growth rate indicators was not particularly successful.

Latitude dependence of zonal plasma drifts obtained from dual‐site airglow observations

[1]xa0All-sky imagers located at Tucuman, Argentina (26.9°S, 65°W, 14.2°S dip latitude), and Arequipa, Peru (16.5°S, 71.5°W, 2.7°S dip latitude), are used to track 630 nm airglow depletion motions in the first use of multisite airglow imagers for studies of low-latitude plasma dynamics. A new image analysis technique yields a consistent determination of nighttime zonal plasma drifts from all-sky images of the depletion motions. The observed eastward plasma drifts are smaller at Arequipa than at Tucuman in the postsunset period. During the postmidnight hours, the opposite pattern occurs. These observations are interpreted using the simple plasma drift model and coupled ionosphere-electric field model of Eccles [1998a, 1998b]. The observed zonal plasma drifts result from low-latitude electrodynamics with a mix of influences from E and F region conductivities and neutral wind shears in altitude and latitude. Analysis of the observations suggests that postsunset zonal drifts near the magnetic equator (Arequipa) are strongly influenced by the E region dynamo, while the F region dynamo is the main cause of zonal drifts observed closer to the Equatorial Ionization Anomaly (Tucuman). The observed altitude-latitude behavior of the plasma drifts gives the first two-dimensional evidence for the so-called F region plasma vortexs influence at equatorial and low latitudes obtained using optical imaging techniques. With this framework a synthesis is offered for seemingly inconsistent zonal drift observations in the published literature.

Testing the thermospheric neutral wind suppression mechanism for day-to-day variability of equatorial spread F

The determination of the physical processes that cause the day-to-day variability of equatorial spread F (ESF) has long been one of the outstanding problems in terrestrial space physics. Within the context of the Rayleigh-Taylor instability model for ESF, mechanisms that either enhance or inhibit the growth of a seed perturbation offer potential sources of variability that can be tested. In this study the hypothesis that enhanced thermospheric meridional winds play a critical role in suppressing ESF is examined during the Multi-Instrumented Studies of Equatorial Thermospheric Aeronomy (MISETA) campaign of September 1998. New, high-time-resolution Fabry-Perot interferometer (FPI) observations at 6300-A nightglow made at Arequipa (Peru) provided the neutral wind measurements during the critical postsunset hours that had been sampled only sparsely in earlier morphology studies. Evidence of local ESF activity was obtained using GPS-based observations of phase fluctuations (Fp) and 6300-A all-sky optical images from the same site. Additional GPS measurements of Fp and total electron content (TEC) from Bogota (Colombia) and Santiago (Chile) were used to determine the full flux tube development of ESF plumes and to characterize the F region morphology of the interhemispheric Appleton anomaly. Correlative studies between the nightly magnitudes of the meridional winds (Um), ESF activity (Fp), and indices describing the strength (Is) and asymmetry (Ia) of the Appleton anomaly offered no convincing evidence for the wind suppression mechanism. The best available precursor for premidnight ESF appeared to be the strength of the electrodynamically driven Appleton anomaly pattern at sunset. If one assumes that the required seed perturbation for ESF onset is essentially always available, then for all practical purposes, the magnitude of the eastward electric field that causes upward drift is both the necessary and sufficient parameter to forecast ESF with reasonable success. These results reconfirm 60 years of study pointing to the dominance of electrodynamical processes in the onset and growth of plasma instabilities at low latitudes.

Toward a synthesis of equatorial spread F onset and suppression during geomagnetic storms

[1]xa0We present initial steps toward unifying our understanding of storm time equatorial spread F (ESF) by searching for the common elements in past case studies and statistical occurrence patterns. We show that the development (or inhibition) of equatorial irregularities during magnetically active periods can be understood using the AE-parameterized Fejer-Scherliess model for disturbance vertical drifts versus storm time and local time. This model takes into account the different sources of perturbation electric fields (magnetospheric and ionospheric dynamos) that ultimately drive the equatorial vertical drifts, showing prompt and delayed effects in the premidnight sector (where both generation and suppression can occur), as well as in the postmidnight period where generation dominates. The postsunset period exhibits the greatest variability for storm time ESF versus longitude, and thus we demonstrate the Fejer-Scherliess models applicability in a test case (6 April 2000) that had an AE pattern compatible with their parameterization scheme. The model successfully accounts for the pronounced longitude confinement in the observed postsunset ESF patterns. Finally, we move beyond the empirically derived relationships between geomagnetic indices and the occurrence of ESF (Aarons, 1991) into a framework of true solar-terrestrial parameters that drive such effects. Additional case studies taken from the published literature are then used to show a consistent linkage between postsunset ESF onset and the interplanetary electric field (IEF) Esw. While AE, Dst, Kp and Dst/dt indices were used in earlier studies to determine the dusk-longitude sector of disturbance electric fields, here we attribute to the IEF the main role in the determination of this longitude sector.

Seasonal dependence of MSTIDs obtained from 630.0 nm airglow imaging at Arecibo

[1]xa0All-sky imaging data of 630.0 nm airglow emissions are used to study the seasonal and solar activity dependence of medium-scale traveling ionospheric disturbances (MSTIDs) over Arecibo, Puerto Rico (18.3° N, 66.7° W, 28° N mag lat). MSTIDs are typical F-region signatures at midlatitudes, yet limited statistical results in the American sector hindered the progress in our understanding of these dynamical structures. This study compiles data from 2002 to 2007 and shows for the first time that optically-determined MSTIDs at Arecibo present a semiannual pattern with peak occurrence at both solstices. In the Japanese longitude sector, a similar pattern has been found, but one with a main peak during local summer. This paper explains the high occurrence rate during local winter at Arecibo via E-layer/F-layer coupling and inter-hemispheric coupling, thus accounting for a consistent morphology between the two longitude sectors.

Equatorial spread F‐related airglow depletions at Arecibo and conjugate observations

[1]xa0We present evidence of the incursion into the Caribbean region of airglow depletions associated with the equatorial Rayleigh-Taylor instability. Data from the Boston University all-sky imager located at Arecibo, Puerto Rico (18.3°N, 66.7°W, 28°N magnetic latitude), have been used to identify several nights with 630.0 nm airglow patterns that are typical signatures of equatorial spread F and distinctly different from the more common “airglow bands” frequently observed there. Two case studies (2 November 2002 and 26 February 2003) show the occurrence of simultaneous airglow depletions observed with another all-sky imager located at El Leoncito, Argentina (31.8°S, 69.3°W, 18°S magnetic latitude), relatively close to the Arecibo conjugate point. Supporting information is obtained from Defense Meteorological Satellite Program, ROCSAT-1, and GPS data, all of them showing the presence of strong ionospheric irregularities collocated with the airglow depletions. Mapping the circular field of view from Arecibo into the Southern Hemisphere reveals a distorted pattern due to the differences in the magnetic field characteristics in both hemispheres. This adds an interesting spatial complexity to the formulation of conjugate point observing programs in the Latin American longitude sector.

Suppression of equatorial spread F by sporadic E

[1]xa0We have examined quantitatively the influence a low-latitude, premidnight sporadic E layer might have on the daily and hourly development of equatorial spread F (ESF). In particular, we calculated changes in the flux tube–integrated Pedersen conductivity as it affects the growth rate of the Rayleigh-Taylor instability, which governs the initial development of ESF. We find that the growth rate is lowered by an order of magnitude with a density of 1 × 106 cm−3 in a slab from 115 to 120 km. Since sporadic E layers observed after dusk do not regularly reach these values, they are not a likely source of the daily variability in ESF. However, even a mild enhancement in the postsunset E region could lead to a significant suppression of ESF if it also inhibits the upward plasma drift of the prereversal enhancement, a key variable in the growth rate of the equatorial spread F instability. Thus, consistent with the nature of an instability, the second-order effect (suppressed upward drift) is more important than the first-order cause (reduced F region to E region conductivity) of inhibited ESF onset.

The current state of investigations regarding the thermospheric midnight temperature maximum (MTM)

Abstract The thermospheric midnight temperature maximum (MTM) is an upper atmospheric effect found at low latitudes. It is accompanied by an increase in pressure and a signature poleward abatement or reversal in the meridional neutral winds. The MTM exhibits a poleward propagation away from the geographic equator with two secondary maxima developing at approximately ±15° latitude. In this paper, we review early works and recent efforts regarding the MTM. Outstanding questions dealing with seasonal and longitudinal dependencies of the MTMs basic characteristics are discussed. All-sky imaging systems at Arequipa and El Leoncito observed the propagation of 6300 A airglow enhancements related to the MTM past 35°S latitude. This provides useful information on the upper latitude limit of the MTM. TIEGCM modeling efforts simulate the MTM through upward propagating semi-diurnal tides but have difficulty reproducing accurately its amplitude and occurrence time. It is suggested that the role of the terdiurnal tidal mode may be more important than previously thought. Recent comparative observation and modeling studies of MTM related 6300 A emission proved unsuccessful. We report that the amplitude of the modeled MTM was not strong enough to instigate the ‘midnight collapse’ of the F-region needed to produce the airglow signature.

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.

Zonal neutral winds at equatorial and low latitudes

Abstract Fabry–Perot interferometric (FPI) measurements of thermospheric zonal neutral winds at Arequipa, Peru ( 16.7° S , 71.5° W , −2.7° dip ), and Carmen Alto, Chile ( 23.1° S , 69.4° W , −10.2° dip ), were collected during the solar minimum periods of September–October 1996 and 1997. The data set included 39 nights from Arequipa and 14 nights for Carmen Alto, with 8 nights of simultaneous observations. Analysis of averaged results found the peak evening zonal neutral wind speed of ∼127±15 m / s eastward for the Arequipa observatory, which is located near the magnetic equator, to occur between 21:30 and 22:30 LT. In contrast, the peak evening zonal winds of ∼100±10 m / s eastward observed from Carmen Alto, which is located near the crest of the equatorial ionization anomaly (EIA), occurred ∼0.5– 1 h later. These measurements represent the first case of groundbased FPI observations of the so called equatorial temperature and wind anomaly (ETWA) over such a small latitude range in the same longitude sector. This reduction in speed of ∼20–25% at Carmen Alto relative to Arequipa is attributed to increased ion drag at Carmen Alto caused by the higher electron density within the EIA region at altitudes of 220– 300 km . Model studies were conducted using electron density and neutral atmosphere parameters form the parameterized ionospheric model (PIM) and the mass spectrometer incoherent scatter (MSIS) models, respectively, to calculate the ratio of ion–neutral collision frequencies at the two sites. We found that the increase in electron density within the EIA was sufficient to account for the observed reduction in the zonal wind. Thus, this analysis confirms the dominant role of ion drag in modulating thermospheric dynamics at equatorial latitudes. A comparison of the FPI results with the predictions by two current neutral wind models, the Horizontal Wind Model-90 and the NCAR Thermospheric Ionosphere Electrodynamics General Circulation Model (TIEGCM), reveals that neither is able to reproduce accurately the latitude dependence reported here. Model refinements for electrodynamics and improved resolution are suggested.