Refereed Conference and Journal Papers of Dr Elizabeth A Essex-Cohen in Ionospheric Physics
and Beacon Satellite Geophysics

At home in outer Melbourne in 1975 adjusting a crossed Yagi antenna monitoring ATS-1 a geostationary satellite above Hawaii.
Elizabeth Annette Essex circa 1968
Dr Elizabeth Annette Essex-Cohen   1968

Researcher in Ionospheric Physics, pioneer in GPS, early user of Satellites to study the ionosphere and plasmasphere, active user of TOPEX, pioneer of Antarctic research.
Elizabeth Annette Essex, An Investigation of Ionospheric Irregularities , University of New England, Armidale, NSW, Australia, March 1966.
Thesis Summary is online.

A Sampling of Elizabeth's Papers on Ionospheric Physics, GPS, and Remote Observation via Satellite.

In almost all her publications, Elizabeth Essex-Cohen is cited by her maiden name. In a few papers she is called E.A. Cohen, A female geophysicist never permitted by Australian authorities to travel to Macquarie Island or the Australian Antarctic Territories, she supervised research conducted on Macquarie Island and at the Casey station in Antarctica.In the later period of her research was utilising TOPEX and GPS satellites in furtherance of researching the ionosphere/plasmasphere above the region about Antarctica - notably the Weddell Sea


E. Yizengaw, P.L. Dyson, E.A. Essex†, A study of the spatial density distribution in the topside ionosphere and plasmasphere using the FedSat GPS receiver, Advances in Space Research Volume 38, Issue 11, 2006, pp 2318–2323.
† Deceased in March 2004
The spatial electron density distribution observations using the Global Positioning System (GPS) receiver onboard, the Australian research micro satellite FedSat, are being used to investigate more detailed information about the topside ionospheric and plasmaspheric density distribution than can be obtained using ground based receivers. This is because with ground based receivers, the topside ionosphere and the plasmasphere contribute only a small fraction to the total electron content and so the measurements are dominated by the higher ionospheric structure at the F2 peak. Being in a polar Low-Earth Orbit (LEO), and with a GPS Total Electron Content (TEC) sampling rate of 10 s, FedSat is capable of covering a large area of interest within short period of data sampling period which is very important to obtain realistic electron density profiles of the topside ionosphere and plasmasphere. The GPS data from FedSat can be divided into two types: GPS below-the-horizon data that is recorded when FedSat detects GPS signals that have traversed the ionosphere below FedSat’s orbiting height, GPS above-the-horizon data that is recorded when FedSat receives signals that have crossed the ionosphere above its orbiting height. FedSat’s GPS below-the-horizon data can be used to supplement ground based GPS data to provide tomographic reconstruction with better resolution. FedSat’s GPS above-the-horizon data can be used by itself to obtain the electron density distribution of the topside ionosphere and plasmasphere using tomographic reconstruction technique. For this paper we have used only above-the-horizon data to obtain the topside ionospheric electron density distribution during both geomagnetically quiet and severe storm-time conditions.
Keywords FedSat; Topside ionosphere; Plasmasphere; Tomography

2005 E. Yizengaw, P. L. Dyson, E. A. Essex, M. B. Moldwin, Comprehensive ionosphere dynamics study over the Southern Hemisphere during the severe magnetic storm on 31 March 2001 Annales Geophysicae, 2005, 23, 707-721, 2005 doi:10.5194/angeo-23-707-2005, 2005.
The effects of the 31 March 2001 severe magnetic storm on the Southern Hemisphere ionosphere have been studied using ground-based and satellite measurements. The prime goal of this comprehensive study is to track the ionospheric response from high-to-low latitude to obtain a clear understanding of storm-time ionospheric change. The study uses a combination of ionospheric Total Electron Content (TEC) obtained from GPS signal group delay and phase advance measurements, ionosonde data, and data from satellite in-situ measurements, such as the Defense Metrological Satellite Program (DMSP), TOPographic EXplorer (TOPEX), and solar wind data from the Advanced Composition Explorer (ACE). A chain of Global Positioning System (GPS) stations near the 150° E meridian has been used to give comprehensive latitude coverage extending from the cusp to the equatorial region. A tomographic inversion algorithm has been applied to the GPS TEC measurements to obtain maps of the latitudinal structure of the ionospheric during this severe magnetic storm period, enabling both the spatial and temporal response of the ionosphere to be studied. Analysis of data from several of the instruments indicates that a strong density enhancement occurred at mid-latitudes at 11:00 UT on 31 March 2001 and was followed by equatorward propagating large-scale Travelling Ionospheric Disturbances (TIDs). The tomographic reconstruction revealed important features in ionospheric structure, such as quasi-wave formations extending finger-like to higher altitudes. The most pronounced ionospheric effects of the storm occurred at high- and mid-latitudes, where strong positive disturbances occurred during the storm main phase, followed by a long lasting negative storm effect during the recovery phase. Relatively minor storm effects occurred in the equatorial region.


Endawoke Yizengaw, P. L. Dyson, and E. A. Essex, Tomographic reconstruction of the ionosphere using ground-based GPS data in the Australian region Workshop on Applications of Radio, WAR2004, [ Conference papers on Conference CD, and at ]
This paper describes the experimental procedures of tomographic imaging techniques that have been developed and used at La Trobe University. Tomographic imaging provides a powerful technique for obtaining images of the ionospheric electron density distribution, and is a relatively new technique which has promising features to supplement the most expensive ground-based vertical sounding instruments such as ionosonde and incoherent radar. The technique, which involves monitoring radio transmission from Global Positioning System (GPS) along a meridional chain of ground based receivers, has particularpotential for complementing temporalmeasurements by other observing techniques such as ionosondes. The tomographic inversion algorithm has been applied to actual GPS-based total electron content (TEC) measurements obtained during two severe magnetic storm periods (18 August 2003 and 31 March 2001). The tomographic reconstruction presented here revealed important features in ionospheric structure such as ionization troughs and quasi-wave formations. Electron density profiles obtained by the tomographic reconstruction method are in excellent agreement with profiles obtained by ionosondes at or near the GPS receiver stations, confirming the validity of the tomographic algorithm that has been developed. Geophysical interpretations of the observations are also presented.
Key words:Tomographic reconstruction, TEC, and GPS


I. Horvath and E. A. Essex The Weddell sea anomaly observed with the Topex satellite data Journal of Atmospheric and Solar-Terrestrial Physics Volume 65, Issue 6, April 2003, Pages 693–706
This paper introduces the complete image of the Weddell Sea Anomaly, observed with the over-the-ocean ionospheric total electron content (TEC) values obtained from the TOPEX satellite data with an almost unlimited coverage over the oceans, the first time according to the literature; and investigates its development. With a series of TOPEX TEC maps, this paper demonstrates the diurnal variations of both the night-time and the day-time Weddell Sea Anomaly, which appeared as a night-time TEC enhancement and as a day-time TEC depletion, during the near sunspot maximum period of 1998 and 1999 investigated. Several TOPEX passes, plotted in geomagnetic latitudes, are also presented to demonstrate the longitudinal variations of the Weddell Sea Anomaly, and also to show other ionospheric features appearing such as the southern-hemisphere mid-latitude day-time and night-time trough, the northern-hemisphere mid-latitude night-time trough and the equatorial anomaly. This paper demonstrates how large the anomaly is in reality situated west of the Faraday ionosonde station over the Bellinghausen Sea and not over the Weddell Sea that is east of Faraday. Thus the correct name should be Bellinghausen Sea Anomaly. Based upon the review paper of Dudeney and Piggott (1978), the development of the Weddell Sea Anomaly is explained with the combined effects of solar ultraviolet radiation and thermospheric neutral winds.
Weddell Sea Anomaly; Antarctica; Mid-latitude ionospheric trough; South Atlantic Magnetic Anomaly; TOPEX; TEC
Availability: Paper is downloadable for a fee here


I. Horvath and E. A. Essex Using observations from the GPS and TOPEX satellites to investigate night-time TEC enhancements at mid-latitudes in the southern hemisphere during a low sunspot number period Journal of Atmospheric and Solar-Terrestrial Physics, Volume 62, Issue 5, 15 March 2000, pp 371–391.
The state of the ionization of the upper atmosphere at low and mid latitudes in the Australian region has been studied by investigating the total electron content (TEC) obtained by a dual-frequency group path and phase path GPS technique. For the low sunspot number time period of March 1995–February 1996, one week of data centred on the Priority Regular World Day for each month have been used to investigate night-time mid-latitude peaks occurring around midnight in the Australian region. TEC from TOPEX provided additional information related to the formation of the night-time peaks. Although night-time TEC enhancements have been observed previously, there is no general agreement on their origin. From the results of the present study, the development of midnight TEC enhancements coincided with the low latitude processes occurring at around the time of vertical E×B drift velocity reversal. The TOPEX results confirmed that the upward E×B drift velocity reversal and the downward plasma flow from greater heights producing the night-time peaks at mid latitudes are triggered from a common source: the westward electric field.
Availability: Paper is downloadable for a fee.
LEO micro satellites such as the planned FedSat provide the opportunity for researchers in Space Sciences to further investigate those regions of the ionosphere for which existing equipment and techniques are either difficult or impossible to apply. In the future, constellations of microsatelites may provide a global coverage of the ionosphere previously not possible with ground based equipment.

E A Essex, P A Webb, I Horvath, C. McKinnon, N. Shilo and B. Tate
Cooperative Research Centre for Satellite Systems
Department of Physics, La Trobe University, Bundoora, Victoria 3083, Australia
Recently, with the escalating cost of large satellite missions, attention has turned to smaller satellites. Their advantages of low overall cost in construction and launch, and short time span between conception and launch has given a new impetus to the further study of the geosphere. By using a combination of space-based and ground-based receivers, it is possible to undertake new and exciting experiments directed towards furthering our knowledge of the ionosphere. Combinations of high earth orbit satellites, such as the Global Positioning System (GPS), and low earth orbit (LEO) microsatellites are providing the capability for satellite to satellite occultation experiments to reconstruct the vertical profile of the ionosphere. The topside ionospheric and plasmaspheric ionisation content may also be explored with satellite to satellite experiments. This paper describes some of the experiments proposed for future microsatellites, such as those planned for the Australian satellite FedSat.
Also available


I. Horvath and E. A. Essex Investigating Southern Hemisphere Mid-Latitude Nightime TEC Enhancements During Low Sunspot Number Period with GPS and TOPEX, Proceedings, IES 99, Ionospheric Effects Symposium, Alexandria, Virginia, May 4-6, 1999, pp 6B4-1--6B4-8
The state of the ionization of the upper atmosphere at low and mid latitudes in the Australian region has been studied by investigating total electron content (TEC) obtained by the dual-frequency group path and phase path Global Positioning System technique. For the low sunspot number time period of March 1995-February 1996, one week of data centered on the regular world days for each month have been used to investigate nighttime mid-latitude peaks occurring around midnight in the Australian region. TEC from TOPEX provided additional information related to the formation of the nighttime peaks. Although nighttime TEC enhancements have been observed previously, the origin of these large increases does not have general agreement among researchers. From the results of the present study it appears that the development of midnight TEC enhancements is a direct consequence of the low latitude processes occurring at around the time of ExB drift velocity reversal. Accordingly the midnight TEC enhancement appears to be a reverse-fountain-related formation at mid latitude and the primary source of plasma is the reverse fountain. The monthly GPS plots show a seasonal variation of nighttime TEC enhancement. TOPEX data plots provided the spatial variability of large-scale ionospheric features under different geomagnetic conditions during the season of autumnal equinox.


P. A. Webb and E. A. Essex A Global Ionospheric-Plasmasphere Model: Application to GPS, Proceedings, IES 99, Ionospheric Effects Symposium, Alexandria, Virginia, May 4-6, 1999, pp 5A1-1--5A1-8
With the recent advent of GPS satellites as a research tool in studying the ionized regions that surround the Earth a need has arisen for a simple yet accurate model for plasmasphere, the region above the ionosphere. A dynamic model based on diffuse equilibrium will be described, which is combined with the International Reference Ionosphere model to produce a global ionosphere-plasmasphere model. Some preliminary results from the model are presented and discussed.


B. S. Täte, N. M. Shilo, E. A. Essex,An Investigation of the Ionosphere in the Southern High Latitudes During Low Sunspot Numbers, Acta Geodaetica et Geophysica Hungarica March 1998, Volume 33, Issue 1, pp 83-90.
During the period of 1993–1996, the transmissions from the US Navy Navigational Satellite System (NNSS) polar orbiting satellites, received at Casey (66.28 degrees S, 110.5 degrees E), Antarctica were used to investigate the amplitude scintillations on 150 MHz and the Ionospheric Total Electron Content obtained using the Differential phase technique. The differential phase technique, measuring the phase difference between the two coherent transmissions on 150 MHz and 400 MHz from the NNSS satellites, permits only relative TEC changes to be calculated. Absolute TEC is estimated from the relative TEC using the f0F2 obtained from the digisonde records from Casey station or from GPS measurements. A JMR-1 satellite receiver system is used to obtain the phase and amplitude data, with all measurements logged on a PC. A special antenna with high gain at high elevation enabled the reception of high quality signals from the NNSS satellites. Scintillation measurements were obtained from a purpose built receiver, attached to the 24 MHz IF output of the JMR. Investigations of the ionospheric enhancements known as polar patches, and the depletions, known as polar holes were carried out. In order to locate the patches and polar holes, the TEC data are mapped in MLT magnetic co-ordinates. Comparisons to results similarly obtained from GPS satellites are also made. The results obtained are in agreement with the earlier findings. However, comparisons with models indicate large discrepancies when patches and holes are present.


E.A. Essex, P. Webb, I. Horvath. and H.A. Cohen, The Use of Microsatellites in Monitoring the Ionosphere/Plasmasphere in Proc. COSPAR Colloquium on Microsatellites as Research Tools, Ed. F.-B Hsiao, at National Cheng Hung Uniiversity Tainan, Taiwan, 15-17 December, 1997 COSPAR Colloquia Series 10, pp276-279
The advent of microsatellites with their advantages of low overall cost in construction and launch and short time span between conception and launch has given a new impetus to the further study of the geosphere. By using a combination of space based and ground based receivers it is possible to undertake new and exciting experiments directed towards furthering our knowledge of the ionosphere. Combinations of high earth orbit satellites such as GPS and low earth orbit (LEO) micro satellites are providing the capability for satellite to satellite occultation experiments to reconstruct the vertical profile of the ionosphere. The topside ionosphere and plasmasphere ionisation content may also be explored with satellite to satellite experiments. This paper will describe some of the experiments proposed for future micro satellites such as those planned for the Australian FedSat1 satellite. Model studies of some of these experiments will be discussed.

1994 H.M. Beggs*, E.A. Essex†, and D. Rasch, Antarctic polar cap total electron content observations from Casey Station, Journal of Atmospheric and Terrestrial Physics Volume 56, Issue 5, April 1994, pp 659–666.
In early 1990 a modified JMR-1 satellite receiver system was installed at Casey Station, Antarctica (g.g. 66.28°S, 110.54° E, -80.4°A, magnetic midnight 1816UT, L = 37.8), in order to monitor the differential phase between the 150 and 400 MHz signals from polar orbiting NNSS satellites. Total electron content (TEC) was calculated using the differential phase and Casey ionosonde foF2 data, and is presented here for near sunspot maximum in August 1990 and exactly one year later. The data are used to investigate long-lived ionization enhancements at invariant latitudes polewards of - 80° A, and the ‘polar hole’, a region from -70 to - 80° A on the nightside of the polar cap where reduced electron densitiy exists because of the long transport time of plasma from the dayside across the polar cap. A comparison is made between the Casey TEC data and the Utah State University Time Dependent Ionospheric Model (TDIM) which uses as variables the solar index (F 10.7), season (summer, winter or equinox), global magnetic index (Kp), IMF By direction, and universal time (UT)


M. Mallis and E. A. Essex, Diurnal and seasonal variability of the southern-hemisphere main ionospheric trough from differential- phase measurements, J. Atmos. Terr. Phys., vol. 55, pp. 1021-1037, 1993
A differential-phase technique utilizing the radio transmissions of NNSS satellites was used to make measurements of the latitudinal variations of ionospheric vertical total electron content (TEC) in the southern mid-latitude trough region from the location of Macquarie Island (a cis-auroral site; geographic coordinates 54.5°S, 154.95°E, geomagnetic coordinates 64.5 S, 177.67 E, L = 5.38) for a period of 14 months during 1987–1989. The differential-phase method provided a means of observing a relatively large expanse of ionosphere whilst centered on the cis-auroral region which was well suited for trough studies. By monitoring the two transmitted radio signals at 150 and 400 MHz from the Navy Navigation Satellite System (NNSS) polar orbiting satellites it was possible to deduce the latitudinal variation of ionospheric vertical TEC for the duration of the satellite pass. The absolute TEC was derived from Faraday-rotation and ionosonde data obtained during the same period. The main findings of this work have been the high incidence of daytime troughs for all seasons and the relative low incidence of night-time troughs. Both summer and vernal equinox seasons display a greater occurrence frequency of daytime troughs than the winter and autumnal equinox seasons. Winter-time troughs at any time are less frequent than for any other season. Comparisons with the northern-hemisphere trough display a marked difference in occurrence frequency and distribution of troughs. An attempt to explain some of these features in the light of the high-latitude convection theory is offered. Case studies are given for all seasons to highlight these findings.


G.R.A. Ellis and E. A. Essex et al , Radioastronomy through an artificial ionospheric window: Spacelab 2 observations, Advances in Space Research 8(1):63-66, 1990
Abstract Observations of the galactic background radio emission at a number of frequencies between 2.75 MHz and 0.51 MHz were made in association with the Spacelab-2 plasma depletion experiments to test the concept of making low frequency observations through an artificially created window. Following the Shuttle OMS burn at a time when foF2 was 1.99 MHz, a decrease in the maximum ionospheric electron density of approximately 30% occurred. The first observations of the radio emission at 1.704 MHz at high galactic latitudes with good angular resolution (25 degrees) were made during this event.


G.R.A. Ellis and E. A. Essex et al , Low-frequency radioastronomical observations during the Spacelab 2 plasma depletions experiment, The Australian Physicist 24:56-58, 1987

Punching a hole in the Ionosphere
Artists's sketch Spacelab in Shuttle in orbit The fact that major rocket burns can punch a short-lived hole in the ionosphere was discovered by Michael Mendillo, of Boston University. Two holes in the ionosphere were deliberated punched in 1985 by firing the thrust rockets of the Space Shuttle Challenger over Tasmania and also over Boston. Elizabeth Essex collaborated with a world-wide group of researchers in monitoring this historic experiment. Such a short-lived hole provides access on Earth and to low orbit Spacelab observers to astronomical information normally hidden by the ionosphere. Note that Spacelab was an orbiting observatory in the cargo bay of the Shuttle. Papers describing Spacelab observations during plasma depletion are listed immediately above.


M. Lambert and E. A. Essex Studies of Ionospheric Irregularities around L=4 in the Southern Hemisphere using Satellite Beacons, Proceedings, IES 99, Australian National Antarctic Research Expedition 1987, ANARE Research Notes 48, 1987, pp198-205. Webmasters Note Very large ionospheric Total Electron Content (TEC) fluctuations as seen from Macquarie Island and Beveridge (Vic) Field Stations during UTS days 263 and 264, 1984 which were correllated with dramtic auroral activity as seen at Macquarie Island Increases in the nightime TEC of up to 60% in the space of a few minutes were observed.
From this paper: Macquarie Island all-sky camera frames for the universal times: 0958, 0959, 1043 and 1044 on UT day 267, 1984 i.e., for two image pairs only one minute apart.
1986 Malcolm Lambert and E.A. Cohen, Monitoring ionospheric irregularities in the southern auroral region by means of a satellite beacon, Radio Science, Volume 21, Issue 3, May-June 1986, pp 347–350.

During 1984 total electron content (TEC) data were gathered from Melbourne, a mid-latitude station, and from Macquarie Island in the southern auroral zone, using the Faraday rotation of geostationary satellite signals. Large-scale fluctuations in the TEC due to auroral activity were observed from Macquarie Island, and some of these disturbances were seen to propagate towards Melbourne with speeds of 200 to 1,000 m/s. Large-scale rapid decreases in TEC were observed during the evening hours of about one third of autumn equinox days studied. These decreases occurred at a geomagnetic latitude of approximately 60°S and could be associated with the main ionospheric trough travelling through the ray path from the satellite to the receiving station.
1984 E.A. Cohen, The study of the effect of solar eclipses on the ionosphere based on satellite beacon observations, Radio Science Volume 19, Issue 3, pages 769–777, May-June 1984.
Satellite beacon observations during eclipses have provided much information on the behavior of the ionosphere. A combination of Faraday rotation and differential phase measurements as well as ionosonde data can provide information on the topside and bottomside ionospheric behavior during an eclipse as well as on the production rate. Recent attention has been directed toward a study of the dynamics of the F region during an eclipse and the relation between the amount of depletion in the total electron content and the percent obscuration of the sun. Of particular interest is the time delay from the maximum obscuration to the maximum depletion of the total electron content. Further modeling studies are required in order to reproduce the experimental results. Further observations are also required in order to establish whether TID's are generated following a total solar eclipse as predicted theoretically. In particular, it appears that a more sensitive technique such as differential Doppler rather than Faraday rotation is required. The total solar eclipse of June 11, 1983, with its long totality time of 4–5 min over Indonesia and Papua New Guinea will provide an ideal opportunity for further studies of these unanswered questions.
1981 E.A. Essex, M. Mendillo, J.P. Schödel, J.A. Klobuchar, A.V. da Rosa, K.C. Yeh, R.B. Fritz, F.H. Hibberd, L. Kersley, J.R. Koster, D.A. Matsoukas, Y. Nakata, and T.H. Roelofs, A global response of the total electron content of the ionosphere to the magnetic storm of 17 and 18 June 1972 Journal of Atmospheric and Terrestrial Physics, Volume 43, Issue 4, April 1981, pp 293–306
A global study is made of the response of the total electron content of the ionosphere to the sudden commencement (S.C.) geomagnetic storm of 17 June 1972. Using data from 18 sites, a comparison is made of the seasonal differences between this storm and the 17 December 1971 ionospheric storm studied on a similar global basis by Schödelet al. (1974). When storm induced total electron content enhancements occur, they generally occur within 24 h of the S.C. and they tend to be larger in the winter hemisphere; subsequent depletions are generally more severe in the summer hemisphere. The best-defined storm effect is the positive phase evening enhancements seen in the American longitude sector. The local times of the peak enhancements show a coupled seasonal/longitudinal/latitudinal pattern with latitude being the clearest ordering parameter. The magnitudes of the subsequent negative phase depletions show a longitudinal gradient from east to west.

D. W. Cornelius and E. A. Essex, HF Doppler observations associated with spread F Journal of Geophysical Research: Space Physics, Volume 84, Issue A4, 1 April 1979, pp 1361–1368.
This paper discusses HF Doppler spectrum observations at mid-latitudes when spread F was observed on local ionograms. The data indicate that Doppler broadening is related to spread F (they both commence simultaneously). The results can be simulated by using different ionospheric models. Calculations show that a rippled contour can cause HF Doppler broadening and range spreading on ionograms if the structure is traveling horizontally. Although this mechanism can lead to HF Doppler broadening, scattering at 4.5 and 7.5 MHz in an anisotropic medium from near overhead can also explain the experimental results. The observation of aspect-sensitive scatter does, however, depend critically on the statistical description of the irregularity spectrum. It is suggested that an experiment designed to measure the Doppler shift as a function of zenith angle may enable some of the current ambiguities associated with spread F to be eliminated.


D.W Cornelius and E.A Essex Observations of mid-latitude sporadic E using the HF Doppler technique, Journal of Atmospheric and Terrestrial Physics, Volume 41, Issue 5, May 1979, pps 481–499
Observations of sporadic E using the HF Doppier technique are presented. An important result is the simultaneous observation of many moving clouds in the layer. The Doppler shifts expected from various types of moving irregularities are described, together with assumptions involved in an estimate of their horizontal velocity. The reflection mechanism from non-blanketing layers relevant to the horizontal structure observed is also discussed. It appears that on some occasions partial reflection can be attributed to dense clouds of ionization seen at off vertical angles. Accurate group range data together with direction of arrival of radio waves from the sporadic E-layer are required to confirm this tentative conclusion.


E. A. Essex, Ionospheric total electron content at southern mid-latitudes during 1971–1974, Journal of Atmospheric and Terrestrial Physics, Volume 40, Issue 9, Sept. 1978, pp 1019–1024.
Measurements of the total electron content of the ionosphere carried out at Melbourne (37.47°S, 144.93°E geographic co-ordinates) using the Faraday rotation of the signals from the geostationary satellites ATS-1 and 2F-2 are presented for the period 1971–1974. The diurnal, seasonal and sunspot cycle effects of the total electron content are outlined as well as the corresponding slab thickness and hmF2 variations. These results for the declining phase of the sunspot cycle are similar to those given by Titheridge (1973a) for similar latitudes for the ascending phase of the sunspot cycle. Using two geostationary satellites, simultaneous observations of the total electron content were recorded. For a spatial separation of the 400 km sub-ionospheric point of about 1400 km, low Kp sum days showed small differences probably due to gravity waves, whereas high Kp sum days sometimes showed quite different behaviour.


E. A Essex, High to low latitude variations in the evening summer total electron content and F-region electron density - Journal of Atmospheric and Terrestrial Physics, Volume 39, Issues 9–10, September–October 1977, pp 1155–1158.
Many workers have reported evening increases in total electron content during the summer. In this paper an attempt is made to compare the latitude variations on a day to day basis using geostationary satellite data as well as ?oF2. It is found that on a day to day basis evening increases at higher latitudes are usually accompanied by large enhancements at low latitudes. Measurements of neutral air winds and plasma influx are required to determine the relative importance of these two processes.


E. A. Essex, Comparison of ionospheric gravity wave periods as measured by different experimental techniques, Journal of Atmospheric and Terrestrial Physics Volume 37, Issue 10, October 1975, pp 1349–1356
A comparison is made of ionospheric gravity wave periods as measured by several different experimental techniques. These include high frequency CW doppler, vertical incidence ionosondes, Faraday rotation measurements of total electron content and refraction of VHF signals from a geostationary satellite. These observations are found to yield an increase in power at the longer periods as the height increases, consistent with gravity wave theory.


E. A. Essex, Ray Tracing Through Realistic Ionosphere Gravity Wave Models: A Comparison with Experimental Data from Several Different Techniques, Effects of the Ionosphere on Space Physics and Communications, Proceedings of Ionospheric Effects Symposium, (Editor John M. Goodman), January 20-22, 1975, Naval Research Laboratory, Washington DC 20375, pp 1349–1356
Model ionospheres using experimentally measured parameters are constructed from a realistic medium scale ionospheric gravity wave model. The ray tracing technique is then employed to simulate the results of the original experimental measurements from the model ionospheres. The technique is found to be limited by the gravity wave model which uses only one gravity wave period in the construction of the ionospheric model whereas realistic ionospheres contain in general a spectrum of medium scale gravity waves.
Diagram showing Earth, GPS satellite orbit + Geostationary Satellite, plasmasphere and ionosphere at a particular date Significance of this paper
This paper represents a vital part of the work which Elizabeth performed during 1974 at the U.S.Air Force Geophysical Research Laborator, Hanscom Fields, near Boston. The signicant contribution she made to GPS involved her tracing of rays emitted from LEO (low earth orbiting) satellites to the Earth's surface, using the latest available geophysical models to determine path deviation, Faraday Rotation attenuation, due to ionospheric plasma and the effect of irregularities. This research, done in conjunction with USAF Senior Scientist Jack Klobuchar

  • Showed that - in absence of scintillations - ionospheric effects although significant were not of a scale as to impede reception of signals from the required minimum number of satellites from the adopted GPS Satellite Constellation.
  • Showed that size of ionospheric induced errors required compensation for ionospheric effects. Klobuchar was developing the Klobuchar's Model which is effectively broadcast from the GPS satellites.
But how to prove the reliability of her modelling for GPS satellites orbiting at 12,500 miles? Elizabeth applied her modelling effects on wireless signals from geostationary satellites, ( about are up about 22000 miles ) which are similarly affected by the ionosphere. Now Geo-stat satellites are easy to observe - each one is at fixed altitude and azimuth -- so she was able to directly test her geophysical model using published observations of various geostationary satellites - with most striking ability to model the progress of so called gravity waves (in the ionosphere). With her methods proven by data on the geostats, they could be applied with confidence to test proposed Satellite constellations for NavSta, leading to the selection of the GPS system involving 24 LEos (+ spares) of 12 hour period as shown in the accompanying diagram.
Text Searchable extract of this paper is here Later in 1975 in JATP (Journal of Atmospheric and Terrestial Physics) Elizabeth published an authoritative study of gravity waves, comparing a range of experimental datas with gravitational wave theory.

Elizabeth A. Essex*, Periodic fading of ionospheric echoes, Journal of Atmospheric and Terrestrial Physics, Volume 30, Issue 7, 1968, pp 1441–1443, IN3-IN6 doi:10.1016/S0021-9169(68)91328-7
A peculiar periodic fading of ionospheric echoes from E, E8 and F-regions is described. The fading is sometimes present on one magneto-ionic component alone.
* Now at Department of Physics, University of the West Indies, Kingston, Jamaica.


E. A. Essex* and F. H. Hibberd, Frequency and spatial correlations of fading radio echoes from the ionosphere, Journal of Atmospheric and Terrestrial Physics, Volume 30, Issue 5, 1968, pp 1019–1031.
The cross correlation of the amplitudes of diffusely reflected fading ionospheric echoes on adjacent frequencies is analysed. The correlation function is measured experimentally for a number of frequency separations for E-, Es- and F-echoes. Simultaneous measurements are made of the cross correlation at closely spaced receivers. Comparison of the correlations on adjacent frequencies and at closely spaced receivers shows that the fading arises very close to the reflection level. The maximum value of the cross correlation function between echoes on adjacent frequencies is commonly displaced from zero time. This effect is interpreted as arising from apparent vertical motions of the ionosphere.
* Now at Department of Physics, University of the West Indies, Kingston, Jamaica.