Herbert E. Whitney

Bio: Herbert E. Whitney is an academic researcher. The author has contributed to research in topics: Scintillation & Interplanetary scintillation. The author has an hindex of 6, co-authored 11 publications receiving 891 citations.

Global morphology of ionospheric scintillations

Abstract: Starting with post World War II studies of fading of radio star sources and continuing with fading of satellite signals of Sputnik, vast quantities of data have built up on the effect of ionospheric irregularities on signals from beyond the F layer. The review attempts to organize the available amplitude and phase scintillation data into equatorial, middle-, and high-latitude morphologies. The effect of magnetic activity, solar sunspot cycle, and time of day is shown for each of these three latitudinal sectors. The effect of the very high levels of solar flux during the past sunspot maximum of 1979-1981 is stressed. During these years unusually high levels of scintillation were noted near the peak of the Appleton equatorial anomaly (∼ ±15° away from the magnetic equator) as well as over polar latitudes. New data on phase fluctuations are summarized for the auroral zone with its sheet-like irregularity structure. One model is now available which will yield amplitude and phase predictions for varying sites and solar conditions. Other models, more limited in their output and use, are also available. The models are outlined with their limitations and data bases noted. New advances in morphology and in understanding the physics of irregularity development in the equatorial and auroral regions have taken place. Questions and unknowns in morphology and in the physics of irregularity development remain. These include the origin of the seeding sources of equatorial irregularities, the physics of development of auroral irregularity patches, and the morphology of F-layer irregularities at middle latitudes.

Microwave equatorial scintillation intensity during solar maximum

Abstract: A comparison of scintillation levels at 1.5 GHz made from the Appleton anomaly region of the magnetic equator and from the region close to the magnetic equator (termed the electrojet latitudes) showed increased F region irregularity intensity over the anomaly region during years of high sunspot number. Peak to peak fading greater than 27 dB was noted from Ascension Island (through a dip latitude of 17°) in the anomaly region while only 7–9 dB from Natal, Brazil, and Huancayo, Peru, were noted, the last two paths being close to the magnetic equator. The hypothesis advanced is that the dominant factor responsible for the intense gigahertz scintillation is the traversal of the propagation path through the anomaly region. During years of high sunspot numbers the high levels of ΔN constituting the F region irregularity structure are due to (1) very high electron density in the anomaly region (compared to the electrojet region) and (2) the late appearance of these high electron densities (to 2200 local time) in the anomaly region. The patches or plumes of irregularities seen in the postsunset time period then produce high ΔN; scintillation excursions are proportional to this parameter. The postulation of vertical irregularity sheets in the patches was examined to determine the possibility of this being an important factor in the difference between electrojet and anomaly scintillation levels. Older gigahertz data from the sunspot maximum years 1969–1970 were reanalyzed, and more recent observations from other studies were also reviewed. It was found that through the anomaly region, high scintillation indices were noted at a variety of azimuths of the propagation path rather than just along a path closely aligned with the magnetic meridian. A more complete evaluation of the geometrical factor, which must be of considerable importance in determining the absolute value of the scintillation intensity, awaits further observations.

Estimation of the Cumulative Amplitude Probability Distribution Function of Ionospheric Scintillations

Abstract: The fading characteristics of ionospheric amplitude scintillations can be described by a cumulative amplitude probability distribution function (cdf). The cdf expresses the probability (percentage of time) that the signal amplitude will equal or exceed a given amplitude. Distributions of amplitude variations are made with the use of ionospheric scintillations observed on beacon signals from synchronous satellites transmitting at 136 MHz. The resulting distributions are divided into six groups corresponding to ranges of the scintillation index, the predominant measure in scintillation studies. The model distributions are then combined with the occurrence of scintillations in various index ranges to produce cumulative amplitude probability distributions. These have been done for long-term observations made at Hamilton, Massachusetts, Narssarssuaq, Greenland, and Huancayo, Peru. The results allow engineers to determine margins necessary for communication and navigation systems. Individual 15-min distributions have been compared to the theoretical distributions obtained by Nakagami [1960] in his m-distribution method of characterizing amplitude scintillation and were found to be in good agreement. The m parameter is shown to be a measure of the frequency dependence of scintillations and can be used to determine a spectral index for interpolating the amplitude distributions to other frequencies of interest.

The scintillation boundary

Abstract: The high-latitude transition or boundary between high and low amplitude fluctuations of satellite radio beacon signals has been examined as a function of time. The boundary of the scintillation region reaches its lowest latitude (54° geomagnetic north) between 1930 and 2230 local time. The most northerly boundary latitude of approximately 80° geomagnetic north occurs during the period of 1030–1330 local standard time. General agreement is demonstrated between the observations of Transit 4 at 54 MHz at a subauroral site, Explorer 22 at 40 MHz at three sites, and the main zone of discrete auroral events and precipitation of low-energy particles as shown by Piddington (1965) and Hartz and Brice (1967). High amplitude scintillations poleward of the auroral oval and soft electron precipitation in this latitude range show similar characteristics.

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.

Global morphology of ionospheric scintillations

Abstract: Starting with post World War II studies of fading of radio star sources and continuing with fading of satellite signals of Sputnik, vast quantities of data have built up on the effect of ionospheric irregularities on signals from beyond the F layer. The review attempts to organize the available amplitude and phase scintillation data into equatorial, middle-, and high-latitude morphologies. The effect of magnetic activity, solar sunspot cycle, and time of day is shown for each of these three latitudinal sectors. The effect of the very high levels of solar flux during the past sunspot maximum of 1979-1981 is stressed. During these years unusually high levels of scintillation were noted near the peak of the Appleton equatorial anomaly (∼ ±15° away from the magnetic equator) as well as over polar latitudes. New data on phase fluctuations are summarized for the auroral zone with its sheet-like irregularity structure. One model is now available which will yield amplitude and phase predictions for varying sites and solar conditions. Other models, more limited in their output and use, are also available. The models are outlined with their limitations and data bases noted. New advances in morphology and in understanding the physics of irregularity development in the equatorial and auroral regions have taken place. Questions and unknowns in morphology and in the physics of irregularity development remain. These include the origin of the seeding sources of equatorial irregularities, the physics of development of auroral irregularity patches, and the morphology of F-layer irregularities at middle latitudes.

GPS and ionospheric scintillations

Abstract: [1] Ionospheric scintillations are one of the earliest known effects of space weather. Caused by ionization density irregularities, scintillating signals change phase unexpectedly and vary rapidly in amplitude. GPS signals are vulnerable to ionospheric irregularities and scintillate with amplitude variations exceeding 20 dB. GPS is a weak signal system and scintillations can interrupt or degrade GPS receiver operation. For individual signals, interruption is caused by fading of the in-phase and quadrature signals, making the determination of phase by a tracking loop impossible. Degradation occurs when phase scintillations introduce ranging errors or when loss of tracking and failure to acquire signals increases the dilution of precision. GPS scintillations occur most often near the magnetic equator during solar maximum, but they can occur anywhere on Earth during any phase of the solar cycle. In this article we review the subject of GPS and ionospheric scintillations for scientists interested in space weather and engineers interested in the impact of scintillations on GPS receiver design and use.

Penetration of magnetosheath plasma to low altitudes through the dayside magnetospheric cusps

Abstract: Daytime high-latitude fluxes of low-energy ( 107 cm−2 ster−1 sec−1 with typical energy fluxes in the range 0.01 to 0.1 ergs cm−2 ster−1 sec−1. It is concluded that solar wind plasma can penetrate to low altitudes through the high-latitude cusp in the magnetopause, which is often referred to as the neutral point. This flux is related to a number of geophysical phenomena, including magnetospheric surface currents, daytime auroras, VLF and LF emissions, ionospheric irregularities, and geomagnetic fluctuations.

Control of the seasonal and longitudinal occurrence of equatorial scintillations by the longitudinal gradient in integrated E region Pedersen conductivity

Abstract: An enigma of equatorial research has been the observed seasonal and longitudinal occurrence patterns of equatorial scintillations (and range-type spread F). We resolve this problem by showing that the seasonal maxima in scintillation activity coincide with the times of year when the solar terminator is most nearly aligned with the geomagnetic flux tubes. That is, occurrence of plasma density irregularities responsible for scintillations is most likely when the integrated E-region Pedersen conductivity is changing most rapidly. Hence the hitherto puzzling seasonal pattern of scintillation activity, at a given longitude, becomes a simple deterministic function of the magnetic declination and geographic latitude of the magnetic dip equator. This demonstrated relationship is consistent with equatorial irregularity generation by the collisional Rayleigh-Taylor instability and irregularity growth enhancement by the current convective and (wind-driven) gradient drift instabilities. Some discrepancies in this relationship, however, have been found in scintillation data obtained at lower radio frequencies (below, say, 300 MHz) that suggest the presence of other irregularity-influencing processes. The role of field-aligned currents, associated with the longitudinal gradient in integrated E-region Pedersen conductivity produced at the solar terminator, in equatorial irregularity generation via the current convective instability has not been discussed previously.