[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.

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GPS and ionospheric scintillations

[1] Polar cap ionospheric measurements are important for the complete understanding of the various processes in the solar wind-magnetosphere-ionosphere system as well as for space weather applications. Currently, the polar cap region is lacking high temporal and spatial resolution ionospheric measurements because of the orbit limitations of space-based measurements and the sparse network providing ground-based measurements. Canada has a unique advantage in remedying this shortcoming because it has the most accessible landmass in the high Arctic regions, and the Canadian High Arctic Ionospheric Network (CHAIN) is designed to take advantage of Canadian geographic vantage points for a better understanding of the Sun-Earth system. CHAIN is a distributed array of ground-based radio instruments in the Canadian high Arctic. The instrument components of CHAIN are 10 high data rate Global Positioning System ionospheric scintillation and total electron content monitors and six Canadian Advanced Digital Ionosondes. Most of these instruments have been sited within the polar cap region except for two GPS reference stations at lower latitudes. This paper briefly overviews the scientific capabilities, instrument components, and deployment status of CHAIN. This paper also reports a GPS signal scintillation episode associated with a magnetospheric impulse event. More details of the CHAIN project and data can be found at http://chain.physics.unb.ca/chain.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008RS004046

Canadian High Arctic Ionospheric Network (CHAIN)

In a previous publication1 we described the results of an investigation into the spatial distribution of cosmic electromagnetic noise radiation at 5 metres wave-length. We have recently been engaged in an attempt to make a more detailed determination by using a more sensitive receiver of narrower beam-width. An interesting new feature which has emerged from these latter experiments is the occurrence of short - period irregular fluctuations which have been found to be associated with the direction of Cygnus. This region, which is a secondary peak in the cosmic noise distribution, appears to be unique in being char¬acterized by short-period variations of marked amplitude in the intensity of power flux.

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Fluctuations in cosmic radiation at radio-frequencies.

Available observations of ionospheric scintillation are analyzed to evaluate the adequacy of existing models used for the interpretation of scintillation data. The theoretical models are reviewed and the frequency and propagation geometry dependences predicted by the models are compared with the observations. The models were used to construct scintillation occurrence distribution functions which show that scintillation phenomena significantly affect the design of transionospheric radar or communication systems operating at frequencies below 1 GHz. Diversity schemes useful for mitigation of scintillation effects are considered. Mention is made of the geophysical processes thought to be responsible for scintillation.

Ionospheric scintillation

Ionospheric absorption measurements during a sunspot cycle

An investigation of the distribution of the sources of galactic radiation at 64 Mcyc./sec. is described. Methods are discussed by which the characteristics of the receiving antenna were measured and by which the magnitude of the received galactic power was estimated by comparison with a reference signal of similar amplitude characteristics. The method of analysis of results is described in detail, with emphasis on the need for allowing for the finite width of the main cone of reception of signal power and for the sensitivity of the antenna to signal sources outside the main cone. The necessary corrections are effected by a method of successive approximations, and contours of the distribution of the signal source between declinations +50 degrees and -30 degrees are derived. The results are expressed in terms of power flux and also of effective aerial temperature. Possible sources of error are discussed and an accuracy better than 1$\cdot $2 db. (30%) is expected for the regions of highest radiation intensity. Reference is made to the fluctuations observed in the radiations coming from Cygnus. The correlation of the derived distribution of galactic radiations with other astronomical data is discussed with special reference to the galactic structure. It is shown that correlation with the near galactic structure, as indicated by the general distribution of visible sources of light, is not very good, but that a more satisfactory correlation is obtained with data which are believed to be characteristic of the structure of the galaxy as a whole. The bias to the south of the galactic circle shown by radiation sources in the vicinity of the galactic centre is especially significant. Various relevant astronomical data are collected together in the accompanying diagrams. The bearing of these correlations on the question of the nature of the source of radiation is examined. It is shown that they do not clearly favour any one theory, and that neither a simple theory in terms of a distributed source in interstellar gas nor one in terms of discrete centres of radiation analogous to sunspots appears adequate to account for the observed phenomena. It is suggested that sources of both types contribute to the observed radiation and that, in general, they must be very distant and associated with the main body of the galaxy.

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An Investigation of Galactic Radiation in the Radio Spectrum

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Some Characteristics of Solar Radio Emissions

THE discovery of electromagnetic 'noise' radiations at radio frequencies was first made by K. G. Jansky1 who, in a series of measurements at about 20 Mc. s. ( 15 m.) was able to establish the close connexion between the direction of greatest intensity and the centre of the galaxy. G. Reber2 later made measurements at 160 Mc./s. ( 2 m.), and he was able, by virtue of a narrower beam, to detect a number of subsidiary peaks. He found the intensity of radiation at this higher frequency was very much less ; he also suggested a probable explanation of the radiations, namely, the interactions in encounters between positive ions and free electrons released from interstellar gases by the photoelectric action of stellar radiations. The theoretical treatment for radiation arising from such encounters was first given by H. A. Kramers3 to account for continuous radiation in X-ray spectra. The cosmic noise radiation is not observable at frequencies lower than a few megacycles per second, owing to the screening action of the ionosphere.

Cosmic Radiations at 5 Metres Wave-length

In their description of the solar radio emissions from the great sunspot of February 1946, Appleton and Hey1 pointed out that a number of sudden enhancements of intensity, observed at about 5 metres wave-length, occurred simultaneously with increases in D-layer ionization as indicated by fade-outs on long-distance radio-communication circuits. This increased ionization, which is believed to be due to enhanced solar ultra-violet radiations, strongly absorbs the radio communication waves (of the order of 20 metres wave-length) but does not produce any marked absorption of the solar radio emissions at wave-lengths of 5 metres or less.

S. J. Parsons

Papers

Fluctuations in cosmic radiation at radio-frequencies.

An Investigation of Galactic Radiation in the Radio Spectrum

Some Characteristics of Solar Radio Emissions

Cosmic Radiations at 5 Metres Wave-length

Solar and Terrestrial Radio Disturbances