J.K. Hargreaves

Development of the auroral absorption substorm: Studies of pre-onset phase and sharp onset using an extensive riometer network

The development of an auroral absorption substorm has been studied using riometer measurements in the northern hemisphere. In the events studied, the onset is preceded by an absorption bay which begins to develop 1−112h before the onset. The bay may occur between L-values 3–19 and can cover as much as 150° of geomagnetic longitude, generally in the same longitudinal sector where the substorm breaks up and to the west of it. Whereas the substorm breaks up at or near the midnight meridian, the preceding bay may, in some geophysical conditions, appear in the afternoon sector. The preceding bay moves southward with a velocity between 60 and 600 ms−1, intensifying during the movement. This equatorward movement is consistent with an E × B drift in a cross-magnetotail electric field of between 0.5 and 1 mV m−1. The absorption at the onset exceeds that in the bay, and in the sector of break up the absorption shows a minimum just before the onset; to the west-of the break up the preceding bay continues its southward movement. In 14 cases studied, the sharp onset moved to the west with a velocity of 1–31 km s−1, median 6 km s−1. The onset was seen at higher L-values to the west than in the break-up sector. This applied also to the preceding bay. Whereas most onsets showed westward movement, in only about half of the cases studied was there movement towards the east. The injection area affected during the first minute of the onset was typically 1–2 L-value units, but as much as 30° of geomagnetic longitude. The onset later spread to cover 1–10 L-value units, and up to 130° of longitude. The contouring method used in the analysis of the data from the riometer is described in the Appendix.

The development of the substorm in auroral radio absorption

Abstract Observations with a meridional chain of riometers in the auroral zone show that the onset of an auroral absorption substorm is usually preceded by weak activity. This activity sometimes takes the form of a weak bay-like event, moving slowly equatorward. Its velocity (typically 200–500 m/sec) is consistent with the inward convection of energetic plasma in the magnetosphere— that is, with E × B drift in an electric field of about 1.3 mV/m. The bay intensifies as it moves, indicating energization of the plasma during convection.

THE RELATION BETWEEN 10 TO 80 keV ELECTRON PRECIPITATION OBSERVED AT GEOSYNCHRONOUS ORBIT AND AURORAL RADIO ABSORPTION OBSERVED WITH RIOMETERS

Abstract The 30 MHz absorption observed by a network of wide-beam riometers in Scandinavia is compared to the expected absorption calcvlated on the basis of simultaneously observed fluxes of precipitating electrons in the 10–80 keV energy band at geostationary distance and in magnetic coincidence with the riometer network. By choosing a reasonable altitude profile of the effective recombination coefficient, it is found to be possible to match the observed and calculated absorptions. Further calculations indicate that precipitating electrons of higher energy may be responsible for some of the observed absorption. An empirical law is given which relates the observed absorption to the energy flux of 40–80 keV electrons incident on the atmosphere.

Conjugate and closely-spaced observations of auroral radio absorption—IV: The movement of simple features

Abstract Although the time structure of auroral radio absorption, measured with riometers at high-latitude stations, is generally complex, some features are seen which have a relatively simple structure. Multiple riometer data from the magnetically-conjugate stations Great Whale River and Byrd demonstrate that many of these features show movements over a 250 km baseline. Both the velocities and the directions of movement are consistent with the substorm movements that have been detected in other auroral work, for example by visual observations or by auroral radar. The velocities range between 80 m/sec and 3.3 km/sec, the directions being predominantly westward in the evening sector and eastward in the morning. The observed behaviour is essentially the same in both hemispheres, though event-by-event comparisons show some differences of velocity. The data are applied to determine the instantaneous corresponding points in the two hemispheres; it is found that the corresponding point wanders by 130 km about its mean position. Some implications of the results are discussed.

Dynamics of auroral absorption in the midnight sector—The movement of absorption peaks in relation to the substorm onset

Abstract The movement of peaks of auroral radio absorption is studied using observations made with networks of spaced riometers in Canada and Alaska. It is found that on the average the movement of absorption peaks differs significantly from that of the onset of the A.A. substorm and in individual cases the motions of onset and subsequent peaks appear to be independent. This is taken to indicate that the time-structure of auroral absorption with periods between several minutes and an hour or two represents in general a space-time structure within the envelope of the overall substorm, involving additional mechanisms. Two classes of absorption peak are identified within the substorm. Those occurring within 10 min of the onset move more rapidly and may travel towards or away from the pole; those occurring after 30 min are of longer duration and move relatively slowly and only towards the equator. By comparison with the recovery phase of the substorm in luminous aurora it is speculated that the late peaks may indicate magnetospheric electric fields.

A modelling study of satellite beacon measurements of protonospheric replenishment

Abstract Recent satellite beacon derived measurements of the recovery of protonospheric ionization following periods of increased geomagnetic activity show that the recovery takes longer than is indicated by whistler measurements. Realistic plasmasphere models have been used to determine whether satellite beacon measurements are reliable indicators of this recovery. It is found that the recovery time of the protonospheric content is similar to that of the minimum L-value flux tube intersected by the slant raypaths. Satellite beacon results are therefore useful indicators of protonospheric recovery after a storm provided any unrepresentative diurnal variations are eliminated.

Electron content modelling: The significance of protonospheric contents

Abstract The principal advance of the ATS-6 satellite beacon experiment was the ability to deduce continuously the electron content along the entire slant path from ground-based measurements of the signal group delay. This feature has been exploited in conjunction with the more usual Faraday rotation technique to separate the total electron content into ionospheric and protonospheric components. The physical validity of the deduced quantities is investigated using a mathematical model of the plasmasphere in which integration of the time-dependent continuity and momentum equations for oxygen and hydrogen ions along selected L shells yields the ion concentrations and field-aligned fluxes. The ion concentrations are then integrated along the propagation path to various ground stations from ATS-6 to give computed values for comparison with observations. The mathematical model is used with different sets of atmospheric parameters to investigate the significance of ionospheric and protonospheric contents as derived from beacon data. The calculated electron concentrations are able to reproduce mid-latitude equinoctial electron content observations. The shape parameters τ and F can also be simulated by day, but night-time values do not match the observations well, a greater protonospheric content being required. The calculations show that the quantity N p , which is readily derived from ATS-6 observations, may be interpreted as the slant H + content above some fixed height in the case of some stations (but not others) if the plasmasphere is reasonably full. The total slant content of H + is approx. twice the value of N p , though it appears that for the Lancaster raypath a closer relationship exists between N p and the H + tube content at L = 1.8. In general, N p is most closely related to the tube content for an L value slightly greater than the minimum L intersected along the raypath.

Mid-latitude electron content modelling: the role of interhemispheric coupling

Abstract A modelling study of the electron content of the mid-latitude ionosphere and protonosphere has been carried out for solstice conditions using the mathematical model of Bailey (1983). In the model calculations coupled time-dependent O+, H+ continuity and momentum equations and O+, H+ and electron heat balance equations are solved for a magnetic shell extending over both hemispheres. The inclusion of interhemispheric flow of plasma and of heat balance has enabled us to investigate the role of interhemispheric coupling on the electron content and related shape parameters. The computed results are compared with results from slant path observations of the ATS-6 radio beacon made at Lancaster (U.K.) and Boulder, Colorado (U.S.A.). It has been found that the conjugate photoelectron heating has a major effect on the shape of the daily variation of slant slab thickness (τ) and also on the magnitude of the protonospheric content (Np). Some of the main features of τ are closely related to the sunrise and sunset times in the conjugate ionosphere. Also it is found that night-time increases in total electron content (NT) and F2 region peak electron density (Nmax) in winter are natural consequences of ionization loss at low altitudes causing an enhanced downward flow of plasma from the protonosphere which is coupled to the summer hemisphere. One other important consequence of the coupled protonosphere is that the effects on NT of the neutral air wind are not much different in winter from those in summer.

The effects of neutral air winds on the electron content of the mid-latitude ionosphere and protonosphere in summer

Abstract The effects of neutral air winds on the electron content ( N T ) and other parameters of the mid-latitude ionosphere have been modelled by means of mathematical solutions of the time-dependent continuity and momentum equations for oxygen and hydrogen ions. The geometry is chosen to represent a propagation path between a geosynchronous satellite and a ground station, and the computations are compared with results from slant path observations of the ATS-6 radio beacon made at Lancaster (U.K.) and Boulder, Colorado (U.S.A.). It is demonstrated that the electron content responds markedly to the magnitude and phase of the neutral air winds and that the effect induced by the wind on the electron content shows a consistent quantitative relationship with the wind velocity, especially during daytime. Reasonable variations in the phase and magnitude of the wind produce a range of daily electron content patterns which encompass the range of daily variations observed. The computations show that the wind gives rise to enhanced filling of the protonosphere. This shows as a depressed value of the shape factor ( F ), which by definition means that a greater fraction of the ionization is at higher altitudes. The depression of F is enhanced by a poleward wind and is suppressed or even superseded by an equatorward wind through changes of the electron density distribution with altitude.

Mid-latitude VLF emissions and the mechanism of dayside auroral particle precipitation

Abstract Simultaneous observations of VLF noise on the satellite Ariel 3 and of daytime auroral radio absorption at a high-latitude station are interpreted to show a linear relationship between the flux of auroral particles and the intensity of mid-latitude VLF emissions. The results are consistent with particle precipitation by a process of weak pitch-angle diffusion. At fluxes near the theoretical trapping limit the pitch-angle distribution becomes isotropic, as would be expected in strong diffusion, but this corresponds to a high level of radio absorption which is observed only infrequently. It is concluded that weak diffusion is the operative mechanism in most cases.