Michael Mendillo

Patterns of F2-layer variability

The ionosphere displays variations on a wide range of time-scales, ranging from operational time-scales of hours and days up to solar cycles and longer. We use ionosonde data from thirteen stations to study the day-to-day variability of the peak F2-layer electron density, NmF2, which we use to define quantitative descriptions of variability versus local time, season and solar cycle. On average, for years of medium solar activity (solar decimetric flux approximately 140 units), the daily fluctuations of NmF2 have a standard deviation of 20% by day and 33% by night. We examine and discuss the patterns of behaviour of ionospheric and geomagnetic variability, in particular the equinoctial peaks. For further analysis we concentrate on one typical midlatitude station, Slough. We find that the standard deviations of day-to-day and night-to-night values of Slough NmF2 at first increase with increasing length of the dataset, become fairly constant at lengths of 10–20 days and then increase further (especially at equinox) because of seasonal changes. We found some evidence of two-day waves, but they do not appear to be a major feature of Sloughs F2 layer. Putting together the geomagnetic and ionospheric data, and taking account of the day-to-day variability of solar and geomagnetic parameters, we find that a large part of F2-layer variability is linked to that of geomagnetic activity, and attribute the rest to ‘meteorological’ sources at lower levels in the atmosphere. We suggest that the greater variability at night is due to enhanced auroral energy input, and to the lack of the strong photochemical control of the F2-layer that exists by day.

Onset conditions for equatorial spread F

The problem of day-to-day variability in the occurrence of equatorial spread F (ESF) is addressed using multidiagnostic observations and semiempirical modeling. The observational results are derived from a two-night case study of ESF onset conditions observed at Kwajalein Atoll (Marshall Islands) using the ALTAIR incoherent scatter radar and all-sky optical imaging techniques. The major difference between nights when ESF instabilities did not occur (August 14, 1988) and did occur (August 15, 1988) in the Kwajalein sector was that the northern meridional gradient of 6300-A airglow was reduced on the night of limited ESF activity. Modeling results suggest that this unusual airglow pattern is due to equatorward neutral winds. Previous researchers have shown that transequatorial thermospheric winds can exert a control over ESF seasonal and longitudinal occurrence patterns by inhibiting Rayleigh-Taylor instability growth rates. We present evidence to suggest that this picture can be extended to far shorter time scales, namely, that “surges” in transequatorial winds acting over characteristic times of a few hours to a day can result in a stabilizing influence upon irregularity growth rates. The seemingly capricious nature of ESF onset may thus be controlled, in part, by the inherent variability of low-latitude thermospheric winds.

Investigations of thermospheric-ionospheric dynamics with 6300-Å images from the Arecibo Observatory

Pilot observations were conducted at Arecibo, Puerto Rico, using an all-sky, image-intensified CCD camera system in conjunction with radar, ionosonde, and Global Positioning System (GPS) diagnostic systems during the periods January 19–28, 1993, and February 21 to August 22, 1995. These represent the first use of campaign mode operations of an imager at Arecibo for extended periods of F region observations. The January 1993 period (the so-called “10-day run”) yielded a rich data set of gravity wave signatures, perhaps the first case of direct imaging of thermospheric wave train properties in the F region. The 6-month 1995 campaign revealed two additional optical signatures of F region dynamics. A brightness wave in 6300 A passing rapidly through the field of view (FOV) has been linked to meridional winds driven by the midnight temperature maximum (MTM) pressure bulge. On May 3, 1995, during a period of geomagnetic activity, a 6300-A airglow depletion pattern entered the Arecibo FOV. Such effects represent the optical signatures of equatorial spread F instabilities that rise above the equator to heights near 2500 km, thereby affecting Arecibos L = 1.4 flux tube.

Modelling F2-layer seasonal trends and day-to-day variability driven by coupling with the lower atmosphere

This paper presents results from the TIME-GCM-CCM3 thermosphere–ionosphere–lower atmosphere flux-coupled model, and investigates how well the model simulates known F2-layer day/night and seasonal behaviour and patterns of day-to-day variability at seven ionosonde stations. Of the many possible contributors to F2-layer variability, the present work includes only the influence of ‘meteorological’ disturbances transmitted from lower levels in the atmosphere, solar and geomagnetic conditions being held at constant levels throughout a model year. In comparison to ionosonde data, TIME-GCM-CCM3 models the peak electron density (NmF2) quite well, except for overemphasizing the daytime summer/winter anomaly in both hemispheres and seriously underestimating night NmF2 in summer. The peak height hmF2 is satisfactorily modelled by day, except that the model does not reproduce its observed semiannual variation. Nighttime values of hmF2 are much too low, thus causing low model values of night NmF2. Comparison of the variations of NmF2 and the neutral [O/N2] ratio supports the idea that both annual and semiannual variations of F2-layer electron density are largely caused by changes of neutral composition, which in turn are driven by the global thermospheric circulation. Finally, the paper describes and discusses the characteristics of the F2-layer response to the imposed ‘meteorological’ disturbances. The ionospheric response is evaluated as the standard deviations of five ionospheric parameters for each station within 11-day blocks of data. At any one station, the patterns of variability show some coherence between different parameters, such as peak electron density and the neutral atomic/molecular ratio. Coherence between stations is found only between the closest pairs, some 2500 km apart, which is presumably related to the scale size of the ‘meteorological’ disturbances. The F2-layer day-to-day variability appears to be related more to variations in winds than to variations of thermospheric composition.

Electrodynamics of midlatitude spread F: 1. Observations of unstable, gravity wave‐induced ionospheric electric fields at tropical latitudes

In part 1 of our series exploring the role of electrical forces in midlatitude spread F, we present observations of an elect.rolyiia.iuically driven traveling ionospheric disturbance which passed over Arecibo observatory between 22 and 24 AST on.January 26, 1993. The total electric potential differences driving the wave were of the order of 1 kV. Our analysis indicates that this disturbance is the result of a midlatitude F region plasma instability seeded by a therniospheric gravity wave. Two novel measurements, in addition to typical incoherent scatter observations, were crucial to this determinatiou: tie use of G300 A airglow images front the coupling, energetics, and dynamics of atmospheric regions (CEDAR) all-sky imager to track the two-dimensional, mesoscale dynamics of the disturbance and the rise of a portable ionosonde to simultaneonsly measure the field line integrated ionospheric conductivity in the conjugate hemisphere. we have also determined that this disturbance, like several previously observed midlatitude disturbances, is consistent with our theoretical knowledge of the basic instability of the midlatitude ionosphere described originally by Perkins [1973].

GPS phase fluctuations in the equatorial region during sunspot minimum

GPS multisite and multisatellite observations of phase fluctuations are used to characterize irregularity development in the equatorial region. We use 30-s samples of total electron content obtained from phase differences between 1.2 and 1.6-GHz signals as gathered by the International GPS Service for Geodynamics (IGS). These observations from a number of ground receivers allowed for simultaneous studies over a range of latitudes and longitudes in the region near and distant from the magnetic equator. Several storm periods were analyzed in the time period from November 3,1993, to October 2,1995. By examining phase fluctuations at latitudes near the magnetic equator, in the anomaly region, and poleward of the anomaly region, it was found that very high altitudes were reached by plumes of irregularities during seven magnetic storms in solar minimum years. These also occurred during periods of magnetic quiet. The data set allowed the determination of effective altitudes of equatorial irregularities. During low solar flux years, there was a plethora of different altitude regimes from thin layers to altitudes over 2000 km.

Effects of Solar Flares on the Ionosphere of Mars

All planetary atmospheres respond to the enhanced x-rays and ultraviolet (UV) light emitted from the Sun during a flare. Yet only on Earth are observations so continuous that the consequences of these essentially unpredictable events can be measured reliably. Here, we report observations of solar flares, causing up to 200% enhancements to the ionosphere of Mars, as recorded by the Mars Global Surveyor in April 2001. Modeling the altitude dependence of these effects requires that relative enhancements in the soft x-ray fluxes far exceed those in the UV.

GPS phase fluctuations in the equatorial region during the MISETA 1994 campaign

In this paper we present the first coordinated use of Global Positioning System (GPS) multisite and multisatellite observations with ground based radar and optical diagnostics to investigate equatorial irregularity patterns. Thirty second samples of total electron content (TEC) obtained from GPS phase differences between 1.2- and 1.6-GHz signals are used to study phase fluctuations at several stations. Comparisons were made with various types of ground measurements during the multi-instrument studies of the equatorial thermosphere aeronomy (MISETA) period. Depletions of 6300A airglow emission from Arequipa, Peru, correlated with phase fluctuations recorded at the same site. Phase fluctuations at Arequipa occurred at the times when the Jicamarca radar backscatter returns from plumes were noted but were also seen on other nights when there were no radar returns from plumes. Levels of phase fluctuations noted at Arequipa varied considerably on nights when only thin layers of irregularities were observed by the Jicamarca radar. Differences of ionospheric conditions between the two sites, separated by only 5.5° geographic longitude, may account for the different behavior patterns of irregularities noted. Similar differences in the general behavior pattern of phase fluctuations were shown when data from Arequipa and Fortaleza, Brazil, were compared. These stations, 33° apart, but at the same dip latitude had different patterns for some days. During a magnetic storm, a very high altitude plume was observed by the radar and by phase fluctuations noted at Santiago at 18° dip latitude. This correlation of high plume altitude during some periods of magnetic activity was validated by additional examples of phase fluctuations from three other magnetic storms in the solar minimum years of 1994 and 1995.

OI 630 nm imaging observations of equatorial plasma depletions at 16° S dip latitude

Abstract Equatorial ionospheric irregularities in the F-layer have been the subject of intensive experimental and theoretical investigations during recent years. The class or irregularities which continues to receive much attention is characterized by large scale plasma depletions, generally referred to as ionospheric plumes and bubbles. The OI 630.0 nm F-region night-glow emissions arising from recombination processes can be used to observe the dynamics of transequatorial ionospheric plasma bubbles and smaller scale plasma irregularities. In a collaborative project between the Center for Space Physics of Boston University and Brazils National Institute for Space Research (INPE), an all-sky imaging system was operated at Cachoeira Paulista (22.7° S, 45.0° W, dip latitude 15.8° S), between March 1987 and October 1991. In addition to the imager, photometer and VHP polarimeter observations were conducted at Cachoeira Paulista, with ionospheric soundings carried out at both C. Paulista and Fortaleza, the latter at 3.9° S, 38.4° W, 3.7° S dip latitude. For this longitude, the observed seasonal variation of the airglow depletions shows a maximum from October through March and a very low occurrence of airglow depletions from April through September. This long series of OI 630.0 nm imaging observations has permitted us to determine that when there are extended plumes, the altitudes affected over the magnetic equator often exceed 1500 km and probably exceed 2500 km at times, the maximum projection that can be seen from Cachoeira Paulista. This holds true even during years of low solar flux.

Discovery of the distant lunar sodium tail and its enhancement following the Leonid Meteor Shower of 1998

Night-time measurements using a bare CCD all-sky imaging system have detected the presence of an extensive region of neutral sodium emission (589.1 nm) in the direction of the anti-solar/lunar points. The emission was observed to occur during the nights of 21–22 August and 18–20 November, 1998 UT, centered on the new Moon period. The Moon is the most likely source of the neutral sodium, making this the first detection of the lunar sodium tail out to a distance of hundreds of lunar radii. The greater brightness of the emission feature on 19 November is attributed to the Leonid meteor shower which peaked on 17 November, 1998, less than two days before new Moon.