Auroral absorption of HF radio waves in the ionosphere: A review of results from the first decade of riometry

Hargreaves, J. K.

doi:10.1109/proc.1969.7275

Cited by 110 publications

(111 citation statements)

References 147 publications

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“…The fit gives k=2.4×10 −4 dB cm s 1/2 . This result appears to be reasonably consistent with the empirical relations quoted in Table 1 of Hargreaves (1969) when the difference in riometer frequency (30 MHz versus 38.2 MHz) and minimum energy (40 keV versus 25 keV) is accounted for.…”

Section: The Methods Testedsupporting

confidence: 87%

“…In this system, all points on the same field line have the same latitude and longitude. The 557.7 nm and CNA images were transformed into AACGM co-ordinates assuming a typical altitude for the 557.7 nm emission of 100 km (Kosch et al, 1998b) and a typical altitude for the absorbing layer of 90 km (Hargreaves, 1969). The images were then linearly interpolated to the coordinates of the field-line previously defined.…”

Section: Instrumentationmentioning

confidence: 99%

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Comparison of methods to determine auroral ionospheric conductances using ground-based optical and riometer data

2008

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Abstract.Ground-based images of auroral optical emissions and cosmic radio noise absorption provide information on particle precipitation which enhances ionospheric conductances. Knowledge of this conductance field is important to understand the current systems associated with auroral features. Three methods of using ground-based optical and riometer data to estimate ionospheric conductances in the aurora are compared to conductances derived from incoherent scatter radar measurements. It is shown that a method using the 557.7 nm emission intensity alone gives the best results for the Pedersen conductance whilst a method using both this intensity and cosmic noise absorption is best for the Hall conductance. A method using cosmic noise absorption alone gives reasonable performance for the Hall conductance and the Hall/Pedersen conductance ratio, but performs poorly for the Pedersen conductance. It also appears to underestimate the Hall conductance significantly during times when softer precipitation is present, for example in discrete auroral arcs. There is some indication that the methods do not degrade noticeably for angles up to ∼20 • off magnetic zenith.

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Section: The Methods Testedsupporting

confidence: 87%

Section: Instrumentationmentioning

confidence: 99%

Comparison of methods to determine auroral ionospheric conductances using ground-based optical and riometer data

2008

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“…As frequency can be considered to be a scaling factor for absorption magnitude (e.g. Hargreaves, 1969), all absorption data have been scaled to a frequency of 30 MHz. The scaling was performed according to Eq.…”

Section: Observations Of Particle Precipitationmentioning

confidence: 99%

The driving mechanisms of particle precipitation during the moderate geomagnetic storm of 7 January 2005

et al. 2007

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Abstract. The arrival of an interplanetary coronal mass ejection (ICME) triggered a sudden storm commencement (SSC) at ∼09:22 UT on the 7 January 2005. The ICME followed a quiet period in the solar wind and interplanetary magnetic field (IMF). We present global scale observations of energetic electron precipitation during the moderate geomagnetic storm driven by the ICME. Energetic electron precipitation is inferred from increases in cosmic noise absorption (CNA) recorded by stations in the Global Riometer Array (GLO-RIA). No evidence of CNA was observed during the first four hours of passage of the ICME or following the sudden commencement (SC) of the storm. This is consistent with the findings of Osepian and Kirkwood (2004) that SCs will only trigger precipitation during periods of geomagnetic activity or when the magnetic perturbation in the magnetosphere is substantial. CNA was only observed following enhanced coupling between the IMF and the magnetosphere, resulting from southward oriented IMF. Precipitation was observed due to substorm activity, as a result of the initial injection and particles drifting from the injection region. During the recovery phase of the storm, when substorm activity diminished, precipitation due to density driven increases in the solar wind dynamic pressure (P dyn ) were identified. A number of increases in P dyn were shown to drive sudden impulses (SIs) in the geomagnetic field. While many of these SIs appear coincident with CNA, SIs without CNA were also observed. During this period, the threshold of geomagnetic activity required for SC driven precipitation was exceeded. This implies that solar wind density driven SIs occurring during storm recovery can drive a different response in particle precipitation to typical SCs.

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“…The data from those riometers that do not operate at 30 MHz have been corrected using the simplified assumption that the CNA is inversely proportional to the square of the operating frequency (e.g. Hargreaves, 1969). Table 1 provides a complete list of the riometers stations and their AACGM (altitude adjusted corrected geomagnetic) coordinates (Baker and Wing, 1989).…”

Section: Instrumentationmentioning

confidence: 99%

Energetic electron precipitation during sawtooth injections

Kavanagh

Donovan

et al. 2007

Ann. Geophys.

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Abstract. We present simultaneous riometer observations of cosmic noise absorption in the nightside and dawn-noon sectors during sawtooth particle injections during 18 April 2002. Energetic electron precipitation (>30 keV) is a feature of magnetospheric substorms and cosmic radio noise absorption acts as a proxy for qualitatively measuring this precipitation. This event provides an opportunity to compare the absorption that accompanies periodic electron injections with the accepted paradigm of substorm-related absorption. We consider whether the absorption is consistent with the premise that these injections are quasi-periodic substorms and study the effects of sustained activity on the level of precipitation. Four consecutive electron injection events have been identified from the LANL (Los Alamos National Laboratory) geosynchronous data; the first two showing that additional activity can occur within the 2-4 h sawtooth periodicity. The first three events have accompanying absorption on the nightside that demonstrate good agreement with the expected pattern of substorm-absorption: discrete spike events with poleward motion at the onset followed by equatorward moving structures and more diffuse absorption, correlated with optical observations. Dayside absorption is linked to gradient-curvature drifting electrons observed at geostationary orbit and it is shown that low fluxes can lead to a lack of absorption as precipitation is suppressed; precipitation begins when the drifting electron flux surpasses some critical level following continuous injections of electrons from the magnetotail. In addition it is shown that the apparent motion of absorption determined from an azimuthal chain of riometers exhibits an acceleration that may be indicative of an energisation of the drifting electron population.

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Auroral absorption of HF radio waves in the ionosphere: A review of results from the first decade of riometry

Cited by 110 publications

References 147 publications

Comparison of methods to determine auroral ionospheric conductances using ground-based optical and riometer data

Comparison of methods to determine auroral ionospheric conductances using ground-based optical and riometer data

The driving mechanisms of particle precipitation during the moderate geomagnetic storm of 7 January 2005

Energetic electron precipitation during sawtooth injections

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