Jules Aarons

Bio: Jules Aarons is an academic researcher from Boston University. The author has contributed to research in topics: Scintillation & Ionosphere. The author has an hindex of 30, co-authored 83 publications receiving 2982 citations.

The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence

Abstract: Determining the morphology of F-layer irregularities as a function of longitude in the equatorial region is vital for understanding the physics of the development of these irregularities. We aim to lay the observational basis which then can be used to test theoretical models. Theoretical models have been developed, notably in the papers by Tsunoda (1985) and by T. Maruyama and N. Matuura (1984). The question is whether the models are consistent with the morphology as we see it. According to our criteria, the data used should be confined to observations taken near the magnetic equator during quiet magnetic periods and at times within a few hours after sunset. Anomaly region scintillation data have to be used in a limited manner for studying the generation mechanism. The questions to be answered by proposed mechanisms are (1) why do the equinox months have high levels of occurrence over all longitudes and (2) why are there relatively high levels of occurrence in the Central Pacific Sector in the July–August period and in the 0–75° West Sector in the November-December period and (3) why are there very low levels of occurrence in November and December in the Central Pacific Sector and in July and August in the 0–75° West Sector. In the paper by Maruyama and Matuura, the authors have taken observations of topside soundings of spread-F. With this data set in hand, they conclude: “During the northern winter periods, there is maximum enhancement at the Atlantic longitudes of large westward geomagnetic declination and during the northern summer at the Pacific longitude of large eastward declination”. Tsunoda's conclusions from his use of scintillation data is that “scintillation activity appears to maximize at times of the year when the suset nodes occur”. The emphasis of one paper is on the maximum enhancement during the solstices and in the other paper on variations from the equinox as determined by latitude and declination. Each stresses certain characteristics of the morphology. While the two papers explain relatively different morphologies, each makes contributions. However there remain problems to be resolved before certifying a solution as to the physics explaining the longitudinal pattern of F-region irregularities. Satellitein-situ data, scintillation and spread-F observations will be reviewed. The limitation of each data set will be outlined particularly as relevant to the bias produced by the existence of thin versus extended layers of irregularities. A cartoon as to the occurrence pattern, as we see it, as a function of longitude will be shown.

Ionospheric effects of major magnetic storms during the International Space Weather Period of September and October 1999: GPS observations, VHF/UHF scintillations, and in situ density structures at middle and equatorial latitudes

Abstract: In this paper we present a study of the ionospheric effects of a halo coronal mass ejection (CME) initiated on the Sun on September 20, 1999, and causing the largest magnetic storm during this month on September 22–23, 1999, with the hourly Dst index being −167 nT at ∼2400 UT on September 22. The recurrent CME on October 18 caused an even larger magnetic storm on October 22, 1999, with Dst of −231 nT at ∼0700 UT. The ionospheric effects of these two major magnetic storms are studied through their effects on a prototype of a Global Positioning System (GPS)-based navigation system called Wide Area Augmentation System (WAAS) being developed by the Federal Aviation Administration for use in the continental United States and their impact on global VHF/UHF communication systems. It is shown that the penetration of transient magnetospheric electric fields equatorward of the shielding region at midlatitudes, which have been well-correlated in the past with rapid changes in the well-known Dst index (or through its recently available high resolution 1-min counterpart the SYM-H index), can cause large increases of total electron content (TEC), TEC fluctuations, and saturated 250-MHz scintillation, and these, in turn, may have significant impacts on WAAS. The local time of Dst changes (and not just Dst magnitude) was found to be very important for WAAS, since the largest effects on TEC are seen near dusk. The prompt penetration of these magnetospheric electric fields all the way to the magnetic equator causes augmentation or inhibition of equatorial spread F. The global ionospheric response to these storms has been obtained from ground-based TEC observations with a GPS network and space-based in situ density and electric field measurements using the Republic of China Satellite-1 (ROCSAT-I) and several Defense Meteorological Satellite Program satellites. These prompt penetration electric fields cause VHF/UHF scintillations and GPS TEC variations at low latitudes in the specific longitude sector for which the early evening period corresponds to the time of rapid Dst variations and maximum Dst phase. The effects of the delayed ionospheric disturbance dynamo and those of decreased magnetospheric convection on postmidnight irregularity generation are shown to be confined to a part of the same longitude range that actively responded to the prompt penetration of electric fields in the early evening sector.

On the coexistence of kilometer- and meter-scale irregularities in the nighttime equatorial F region

Abstract: Nighttime multifrequency scintillation and 50-MHz radar backscatter observations simultaneously performed over a nearly common ionospheric volume at the dip equator in Peru during March 1977 were used to study the relationship between the large-scale irregularities (∼0.1–1 km) giving rise to scintillations and small-scale irregularities (3 m) causing 50-MHz backscatter. It is shown that during the generation phase of equatorial irregularities in the evening hours, the kilometer- and meter-scale irregularities coexist, whereas in the later phase, approximately an hour after the onset, the meter-scale irregularities decay but the large-scale ones continue to retain their high spectral intensities. Further, multistation scintillation observations from a host of geostationary satellites as well as from the Wideband satellite indicate that eastward-drifting irregularity structures detected around midnight cause significant scintillations at UHF and L band but generally fail to give rise to appreciable backscatter. Thus, contrary to expectations, it is possible to have even L band scintillations without any plume structure on backscatter maps. This indicates that at later local time a cutoff of the spectral intensity probably occurs at some scale length between 100 and 3 m. These observational results are discussed in the context of current theories of plasma instability in the equatorial ionosphere.

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

Abstract: 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 Arecibo's L = 1.4 flux tube.

The application of GPS observations to equatorial aeronomy

Abstract: 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.

Understanding GPS : principles and applications

Abstract: Fundamentals of Satellite Navigation. GPS Systems Segments. GPS Satellite Signal Characteristics and Message Formats. Satellite Signal Acquisitions and Tracking. Effects of RF Interference on GPS Satellite Signal Receiver Tracking. Performance of Standalone GPS. Differential GPS. Integration of GPS with other Sensors. Galileo. The Russian GLONASS, Chinese Bediou, and Japanese QZSS Systems. GNSS Markets and Applications.

DARN/SUPERDARN : A global view of the dynamics of high-latitude convection

Abstract: The Dual Auroral Radar Network (DARN) is a global-scale network of HF and VHF radars capable of sensing backscatter from ionospheric irregularities in the E and F-regions of the high-latitude ionosphere. Currently, the network consists of the STARE VHF radar system in northern Scandinavia, a northern-hemisphere, longitudinal chain of HF radars that is funded to extend from Saskatoon, Canada to central Finland, and a southern-hemisphere chain that is funded to include Halley Station, SANAE and Syowa Station in Antarctica. When all of the HF radars have been completed they will operate in pairs with common viewing areas so that the Doppler information contained in the backscattered signals may be combined to yield maps of high-latitude plasma convection and the convection electric field. In this paper, the evolution of DARN and particularly the development of its SuperDARN HF radar element is discussed. The DARN/SupperDARN network is particularly suited to studies of large-scale dynamical processes in the magnetosphere-ionosphere system, such as the evolution of the global configuration of the convection electric field under changing IMF conditions and the development and global extent of large-scale MHD waves in the magnetosphere-ionosphere cavity. A description of the HF radars within SuperDARN is given along with an overview of their existing and intended locations, intended start of operations, Principal Investigators, and sponsoring agencies. Finally, the operation of the DARN experiment within ISTP/GGS, the availability of data, and the form and availability of the Key Parameter files is discussed.

Ionospheric Storms — A Review

Abstract: In this paper, our current understanding and recent advances in the study of ionospheric storms is reviewed, with emphasis on the F2-region. Ionospheric storms represent an extreme form of space weather with important effects on ground- and space-based technological systems. These phenomena are driven by highly variable solar and magnetospheric energy inputs to the Earth's upper atmosphere, which continue to provide a major difficulty for attempts now being made to simulate the detailed storm response of the coupled neutral and ionized upper atmospheric constituents using increasingly sophisticated global first principle physical models. Several major programs for coordinated theoretical and experimental study of these storms are now underway. These are beginning to bear fruit in the form of improved physical understanding and prediction of ionospheric storm effects at high, middle, and low latitude.

Radio wave scintillations in the ionosphere

Abstract: The phenomenon of scintillation of radio waves propagating through the ionosphere is reviewed in this paper. The emphasis is on propagational aspects, including both theoretical and experimental results. The review opens with a discussion of the motivation for stochastic formulation of the problem. Based on measurements from in-situ, radar, and propagation experiments, ionospheric irregularities ate found to be characterized, in general, by a power-law spectrum. While earlier measurements indicated a spectral index of about 4, there is recent evidence showing that the index may vary with the strength of the irregularity and possibly a two-component spectrum may exist with different spectral indices for large and small structures. Several scintillation theories including the Phase Screen, Rytov, and Parabolic Equation Method (PEM) are discussed next. Statistical parameters of the signal such as the average signal, scintillation index, rms phase fluctuations, correlation functions, power spectra, distributions, etc., are investigated. Effects of multiple scattering are discussed. Experimental results concerning irregularity structures and signal statics are presented. These results are compared with theoretical predictions. The agreements are shown to be satisfactory in a large measure. Next, the temporal behavior of a transionospheric radio signal is studied in terms of a two-frequency mutual coherence function and the temporal moments. Results including numerical simulations are discussed. Finally, some future efforts in ionospheric scintillation studies in the areas of transionospheric communication and space- and geophysics are recommended.