Observations of the polar cap absorption event of February 1984 by the EISCAT incoherent scatter radar

Hargreaves, J. K.; Ranta, H.; Ranta, A.; Turunen, Esa; Turunen, T.

doi:10.1016/0032-0633(87)90072-9

Cited by 35 publications

(16 citation statements)

References 15 publications

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“…Since many SPE occur during the active phase of the solar cycle, any attempt to derive or utilize an empirical relationship must be treated with utmost care due to the effects of SRE. More thorough methods, in which the observed spectrum of precipitation is used to model the absorption, are also limited by uncertainties in D-region recombination rates, although steps toward resolving these problems have been undertaken by Hargreaves et al (1987Hargreaves et al ( , 1993.…”

Section: Discussionmentioning

confidence: 99%

“…This approach has been implemented previously when using the GOES satellites to provide measurement of the incident proton flux at the top of the ionosphere (e.g. Hall et al, 1992;Hargreaves et al, 1993;Hargreaves et al, 1987, andRietveld, 1990). Similarly, IRIS is sufficiently poleward of the quiet time absolute cutoff boundary such that all but the least energetic (<1 MeV) protons will precipitate within the field of view (inferred from Leske et al, 2001).…”

Section: Geosynchronous Operational Environmental Satellites (Goes)mentioning

confidence: 99%

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On solar protons and polar cap absorption: constraints on an empirical relationship

et al. 2004

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Abstract.A large database of Solar Proton Events (SPE) from the period 1995 to 2001 is used to investigate the relationship between proton flux at geostationary orbit and Cosmic Noise Absorption (CNA) in the auroral zone. The effect of solar illumination on this relationship is studied in a statistical manner by deriving correlation coefficients of integral flux and absorption as a function of solar zenith angle limit, thus both an upper limit on the angle and the best correlated integral flux of protons are determined (energies in excess of 10 MeV). By considering the correlation of various energy ranges (from the GOES 8 differential proton flux channels) with CNA the range of proton energies for which the relationship between flux and absorption is best defined is established (15 to 44 MeV), thus confirming previous predictions about which proton energy ranges are most effective in giving rise to absorption during Polar Cap Absorption (PCA) events. An empirical relationship between the square root of the integral proton flux and the absorption, measured by the imaging riometer at Kilpisjärvi (IRIS), is determined and departures from linearity and possible causes are examined. Variations in spectral "hardness" and in collision frequency are demonstrated not to be significant causes of the observed departures from a linear relationship. Geomagnetic activity may be a significant factor in changing the relationship between the absorption and the square root of the integral proton flux, although it is concluded that the cause is likely to be more complex than a straightforward dependence on K p . It is suggested that the most significant factor might be a bias in the absorption estimates imposed by the presence of Solar Radio Emission (SRE), which is not routinely measured at the operating frequency of IRIS, making its precise effect difficult to quantify. SRE is known to be most prevalent under conditions of high solar activity, such as those that might give rise to solar proton events.

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Section: Discussionmentioning

confidence: 99%

Section: Geosynchronous Operational Environmental Satellites (Goes)mentioning

confidence: 99%

On solar protons and polar cap absorption: constraints on an empirical relationship

et al. 2004

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“…(2) Daytime, winter, the range of values over three hours near local noon (Hargreaves et al, 1987). (3) Daytime, spring, afternoon (Hargreaves et al, 1993).…”

Section: Introductionmentioning

confidence: 99%

A new method of studying the relation between ionization rates and radio-wave absorption in polar-cap absorption events

Hargreaves

2005

Ann. Geophys.

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Abstract. During polar-cap absorption events, which are caused by the incidence of energetic solar protons, the highlatitude ionospheric D region is extended down to relatively low altitudes. While the incoming proton fluxes may be monitored by satellite-borne detectors, and the resulting radiowave absorption with a ground-based riometer, the enhancement of electron density at a given altitude is less easily determined. Direct measurements by incoherent-scatter radar are infrequent and they tend to lack the necessary sensitivity at the lower levels. Computations of the electron density from the observed particle fluxes are handicapped by uncertainties in the height profile of the effective recombination coefficient. This paper describes a new approach based on finding the best-fit solution to an over-determined set of equations. The D region is treated as a set of slabs, each contributing to the total radio absorption, and the method relies on the fact that the proton spectrum varies during the event. The analysis produces a set of coefficients relating the absorption increment in the slab to the square root of the production rate, as a function of height. Values of effective recombination coefficient are also deduced over a range of heights, and these agree with previous estimates (Gledhill, 1986) to within a factor of 2. However, whereas the latter do not generally go below 60 km altitude the new determination extends the values down to 40 km.The new method provides a measurement of the height profile of the absorption in PCA events. It is shown that the slabs centred from 45 to 65 km typically account for 80% of the total daytime absorption, and that less than 1% of the total arises above 80 km or below 30 km. At night most of the absorption comes from the slabs at 75 and 80 km, with no significant contribution from slabs below 75 or above 85 km. These results would not differ significantly from estimates based on the Gledhill profiles if extrapolated downward.Correspondence to: J. K. Hargreaves (j.hargreaves@lancaster.ac.uk) Predictions based on the coefficients generated by the procedure are compared with the polar-cap absorption observed during some recent events. Typical electron-density values are derived, and the study provides an independent confirmation that the electron density and the production rate are related by a square-root law.

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“…Previous studies of the more energetic precipitation affecting the D-region using the EISCAT radar have relied on forward modelling (i.e. calculating electron densities for given fluxes) using externally defined or experimentally determined recombination-coefficient models (Hargreaves et al, 1987;Hargreaves and Devlin 1990;Bums et al, 1990), or complex ion-chemistry models (Turunen, 1993). Techniques developed for analysing riometer data with the help of the simplified ion chemistry model were first modified for use with incoherent-scatter data and have allowed the derivation of particle fluxes with some assumption of the spectral shape (exponential or power law) (Vlaskov et al, 1988;.…”

Section: Introductionmentioning

confidence: 99%

Quantitative Studies of Energetic Particle Precipitation Using Incoherent Scatter Radar.

Kirkwood

Osepian

1995

J. geomagn. geoelec

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The most direct effect of energetic particle precipitation in the upper atmosphere is electron density enhancement. Measurements of the altitude profile of electron density and its time variations can in many circumstances be used for quantitative studies of the parameters of the energetic particle flux. The inversion techniques which have been previously developed to relate typical animal particle fluxes to measurements of electron density in the E-region, and some of the results obtained, are briefly reviewed. Newly developed techniques which allow the inversion procedure to be extended to D-region electron densities and to correspondingly more energetic particles are described. Some tests of these techniques are presented and their application to substorm aurora, pulsating aurora and a polar cap absorption event are illustrated.

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Observations of the polar cap absorption event of February 1984 by the EISCAT incoherent scatter radar

Cited by 35 publications

References 15 publications

On solar protons and polar cap absorption: constraints on an empirical relationship

On solar protons and polar cap absorption: constraints on an empirical relationship

A new method of studying the relation between ionization rates and radio-wave absorption in polar-cap absorption events

Quantitative Studies of Energetic Particle Precipitation Using Incoherent Scatter Radar.

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