The polar cap absorption event of 19–21 March 1990: recombination coefficients, the twilight transition and the midday recovery

Hargreaves, J. K.; Shirochkov, A. V.; Farmer, A. D.

doi:10.1016/0021-9169(93)90026-u

Cited by 19 publications

(15 citation statements)

References 5 publications

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“…In fact, there is some evidence from radar data that the effective recombination coefficient may vary slowly with time (Reagan and Watt, 1976;Hargreaves et al, 1993), and such a variation would plainly reduce the accuracy of the prediction. Further, no seasonal variations have been taken into account in the present study.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

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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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Section: Conclusion and Discussionmentioning

confidence: 99%

“…(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). (4) Night, spring (Source as 3).…”

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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“…Leske et al (2001) observed a <1°shift of the CL towards approximately 22 MLT in 8-16 MeV/nucleon alpha particles detected on the SAMPEX satellite and a peak CL around 11 am magnetic equatorial time was observed in OGO4 satellite measurements for lower energy protons (Fanselow & Stone 1972). The poleward excursion around local noon has been cited as a cause of the ''mid-day recovery'' in absorption observed on mid-to high-latitude riometers (k = 60-65°) near the CL boundary (Hargreaves et al 1993;Ranta et al 1995;Uljev et al 1995) although Leinbach (1967) presented evidence to indicate that increased anisotropy of the energetic particle pitch angle distribution could be an alternative cause. A comparison of the error and correlation statistics using DRAP with the two CL models is given in Figure 6 for the seven riometers in the Churchill line and for all 94 SPEs.…”

Section: Rigidity Cut-off Latitude Modelsmentioning

confidence: 91%

“…Reagan & Watt 1976) or during the night (e.g. Hargreaves et al 1993). It is therefore beneficial to optimise PCA model parameters over similar timescales.…”

Section: Real-time Optimisation Of Model Parametersmentioning

confidence: 99%

Assimilation of real-time riometer measurements into models of 30 MHz polar cap absorption

Rogers

Honary

2015

J. Space Weather Space Clim.

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Space weather events may adversely affect high frequency (HF) radio propagation, hence the ability to provide nowcasting and forecasting of HF radio absorption is key for industries that rely on HF communications. This paper presents methods of assimilating 30 MHz radio absorption measurements into two types of ionospheric polar cap absorption (PCA) model to improve their performance as nowcasting tools. Type 1 models calculate absorption as m times the square root of the flux of solar protons above an energy threshold, E t . Measurements from 14 riometers during 94 solar proton events (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) are assimilated by optimising the day and night values of m by linear regression. Further non-linear optimisations are demonstrated in which parameters such as E t are also optimised and additional terms characterise local time and seasonal variations. These optimisations reduce RMS errors by up to 36%. Type 2 models incorporate altitude profiles of electron and neutral densities and electron temperatures. Here the scale height of the effective recombination coefficient profile in the D-region is optimised by regression. Furthermore, two published models of the rigidity cut-off latitude (CL) are assessed by comparison with riometer measurements. A small improvement in performance is observed by introducing a 3-h lag in the geomagnetic index K p in the CL models. Assimilating data from a single riometer in the polar cap reduces RMS errors below 1 dB with less than 0.2 dB bias. However, many highlatitude riometers now provide absorption measurements in near-real time and we demonstrate how these data may be assimilated by fitting a low-order spherical harmonic function to both the measurements and a PCA model with optimised parameters.

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“…A day-to-night ratio in absorption intensities of around 4-8 is often observed during PCA events (Stauning, 1996;Hargreaves et al, 1993;Ranta et al, 1995;Pietrella et al, 2002). The most plausible explanation is a drastic increase in the effective recombination rate after sundown, i.e.…”

Section: Introductionmentioning

confidence: 95%

Polar cap absorption events of November 2001 at Terra Nova Bay, Antarctica

Perrone

Alfonsi²,

Romano³

et al. 2004

Ann. Geophys.

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Abstract. Polar cap absorption (PCA) events recorded during November 2001 are investigated by observations of ionospheric absorption of a 30 MHz riometer installed at Terra Nova Bay (Antarctica), and of solar proton flux, monitored by the NOAA-GOES8 satellite in geo-synchronous orbit. During this period three solar proton events (SPE) on 4, 19 and 23 November occurred. Two of these are among the dozen most intense events since 1954 and during the current solar cycle (23rd), the event of 4 November shows the greatest proton flux at energies >10 MeV. Many factors contribute to the peak intensity of the two SPE biggest events, one is the Coronal Mass Ejection (CME) speed, other factors are the ambient population of SPE and the shock front due to the CME. During these events absorption peaks of several dB (∼20 dB) are observed at Terra Nova Bay, tens of minutes after the impact of fast halo CMEs on the geomagnetic field.Results of a cross-correlation analysis show that the first hour of absorption is mainly produced by 84-500 MeV protons in the case of the 4 November event and by 15-44 MeV protons for the event of 23 November, whereas in the entire event the contribution to the absorption is due chiefly to 4.2-82 MeV (4 November) and by 4.2-14.5 MeV (23 November). Good agreement is generally obtained between observed and calculated absorption by the empirical flux-absorption relationship for threshold energy E 0 =10 MeV. From the residuals one can argue that other factors (e.g. X-ray increases and geomagnetic disturbances) can contribute to the ionospheric absorption.

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The polar cap absorption event of 19–21 March 1990: recombination coefficients, the twilight transition and the midday recovery

Cited by 19 publications

References 5 publications

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

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

Assimilation of real-time riometer measurements into models of 30 MHz polar cap absorption

Polar cap absorption events of November 2001 at Terra Nova Bay, Antarctica

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