Auroral Absorption Research Articles

D-region signatures of substorm growth phase and onset observed by EISCAT

High resolution electron density measurements by EISCAT during the pre-onset phase and onset of an auroral absorption substorm are used to investigate the characteristics of electron precipitation during these substorm phases. The development of the pre-onset phase is the result of a uniform increase of electron fluxes with energies of a few tens of keV, with no particularly hard component. The absorption spike observed at substorm onset contains fine structure when investigated at 10 s resolution, indicating a rapid hardening of the precipitating spectrum at the onset.

An experimental investigation of skywave sea-state radar techniques at a latitude near the Auroral Zone

In 1980 the Communications Research Centre (CRC) undertook a limited experimental program to investigate the feasibility of measuring sea-state characteristics off the east coast of Canada by means of high-frequency skywave radar. The propagation path, from the radar receiver at Ottawa, Ontario, to the observation area east of Newfoundland and Labrador, was tangent to the auroral zone, at times within and at best only a few degrees of latitude south of the auroral absorption zone. The experiment was conducted on a number of occasions between October 1980 and April 1982, for a period of about 6 h at midday on each occasion. Several agencies were involved: CRC personnel designed and conducted the experiment, using the Sampled Aperture Receiving Array (SARA) facility, and carried out the bulk of the analysis; transmissions were provided from Ava, New York, by the Rome Air Development Center (RADC); the Centre for Cold Ocean Resources Engineering (C-CORE) at Memorial University of Newfoundland, St. John's, assembled ground-truth maps, operated a transponder, and developed an algorithm for automatic derivation of wave direction. CRC analysis techniques relied heavily upon computer-aided manual selection and interpretation of the Doppler spectra. Early attempts to construct automatic algorithms for the extraction of wave-height statistics encountered intractable problems, although the C-CORE wave-direction algorithm did meet with some success. Under quiet ionospheric conditions it was found that wave heights sometimes could be measured with acceptable accuracy, although accurate measurement was difficult when wave heights were low (2 m). Wave direction was more easily derived; it was usually observable even when wave heights could not be measured. The project demonstrated that a skywave radar could function at northern latitudes, but the limitations of the experimental facilities left practicability still in question. Considerable improvement would be expected, however, if unlimited freedom of choice of operating frequency were permitted, if the transmitter signal were focused to increase the power density in the target area, and if the receiving array were more comprehensive, i.e., designed specifically for the task.

Post-onset development of the auroral absorption substorm in relation to magnetospheric electrons

Abstract In a previous paper we described the behaviour of the energetic electron precipitation at the onset of an auroral absorption substorm, on the basis of the dynamics of visible aurora and radio absorption in relation to electron measurements by the satellite GEOS-2. We now report an extension ofthat study to the post-onset activity of the same substorm. Clear differences emerge when comparing both the direction of propagation and the temporal development of the electron spectrum during the subsequent activity, with those at the onset. The velocities of the absorbing regions in the ionosphere are observed to have equatorward and eastward components, as opposed to the westward propagation at the onset. The maximum intensity of radio absorption during the substorm main phase was associated with more intense fluxes of high energy electrons than at the onset, but with the superposition of a component softening with time, in direct contrast to the rapid hardening seen at the onset. The softening spectrum during the time of maximum absorption is interpreted as the arrival of a gradient-curvature drifting population, and at this time the flux increases at small pitch angles were confined to the range 45–180 keV, and the isotropy of the electron pitch angle distribution was maximum.

Influence of the solar wind on auroral absorption events at very high geomagnetic latitudes.

We investigate numerous unusual appearances of auroral absorption events measured by riometers located at the center of the Southern polar cap. We have shown that such nighttime (in MLT) events take place during the expansion phase of magnetospheric substorms and are closely associated with both high velocity (V≥600km s-1) solar wind fluxes and strong negative Bz values. The polar ionosphere behaviour is also studied. A model for the propagation of the westward travelling surge was recently developed by ROTHWELL et al. (1984). This model seems to be an appropriate tool for describing the events if one takes into account the solar wind velocity.We believe that the daytime polar cap auroral absorption events which are not as numerous and intense as their nighttime counterparts have a different origin.

GEOS-2 observations of energetic electrons in the morning sector during auroral radio absorption events

The temporal development of two auroral absorption events in the morning sector is compared with simultaneous observations of electrons from the satellite GEOS-2, utilising the good energy resolution over the range 15–300 ke V, to show that the electrons effective in contributing to the observed radio absorption are confined to the range 30–130 keV. By far the most important are those below 80 keV, and as a geophysical monitor the riometer may be considered an efficient indicator of electron fluxes of energy typically of 60–70 keV. The ionospheric effects of the precipitated fluxes are predicted and the results used to discuss the validity of the model atmosphere and of the profiles of effective recombination coefficient and specific absorption. Integration of the calculated profiles of incremental radio absorption yields total estimates within 30% of the observed intensities. The absorbing layer maximises at altitudes of 85–90 km and has a typical half-height of 25 km. It is shown further that the electron flux characteristics are consistent with gradient-curvature drift from a particle source in the midnight sector.

The day to night absorption ratio in auroral and subauroral zone riometer measurements during auroral absorption

The day to night ratio of auroral absorption has been studied using data from auroral and subauroral latitudes and by application of different kinds of statistical analyses. Ratios between 0.5 and 3.0 are obtained, depending on the criteria applied to the selection of data. Previous studies obtained similar ratios, but reached different conclusions about the effective solar control of auroral absorption. It is concluded here that evidence of solar control of the day to night ratio of auroral absorption, or the lack thereof, cannot be extracted by these statistical analyses.

The relationship of auroral absorption to the electron energy distribution of the plasma

An auroral absorption event in the D-region of the atmosphere has been studied by simultaneous measurements of electron temperature, electron density and hyperthermal electrons with a Langmuir probe, and of radio absorption coefficient by 30 MHz riometers. The absorption of the radio waves cannot be explained only by the enhancement of the electron density but requires that the electron collision frequency v be increased above its normal value by the presence of a high energy tail in the electron distribution function. A model is used to determine the characteristics of the hyperthermal electrons in order to evaluate their contribution to the collision frequency and to the absorption coefficient. Good agreement is found between theoretical and experimental values.

Auroral radio absorption as an indicator of magnetospheric electrons and of conditions in the disturbed auroral D-region

In a previous paper we demonstrated a method by which the auroral radio absorption measured with a riometer can be predicted from energetic electron measurements at geosynchronous orbit. The present paper enquires to what extent the process can be inverted: what levels of magnetospheric electron flux, and of D-region production rate, electron density and incremental absorption, are predicted by a given measurement of radio absorption and what reliance can be placed on such predictions? Using data from 45 precipitation features recorded with riometers in Scandinavia and at geosynchronous orbit with GEOS-2, it is shown that electron fluxes in the ranges 20–40,40–80 and 80–160 keV increase with increasing absorption and can be predicted to better than 50% for absorption events of 2 dB or greater. Electrons above 160 keV show little or no correlation with absorption. D-region production rates and electron densities can be predicted to within factors of 2 and √2, respectively. It is more difficult to specify the height of the absorbing region because of uncertainly in the profile of the effective recombination coefficient. Having regard to other data, an α eff profile is proposed which satisfies rocket and incoherent scatter data as well as the present calculations. It is shown that any day-night variation in auroral absorption is associated with a change of spectrum rather than a change of recombination coefficient.

Development of an auroral absorption substorm: Studies of substorm related absorption events in the afternoon-early evening sector

We discuss the effects in ionospheric absorption of particle precipitation observed in the afternoon-early evening sector during substorms with onset in the midnight sector. All events considered here occurred during magnetically disturbed periods, K p > 3. For many of the substorm events a smooth southward moving absorption bay is seen in the midnight and evening sectors about 1 h preceeding the onset. The magnetic pulsation activity is low during this preceding bay. After substorm onset near magnetic midnight the precipitation region may expand with a sharp onset at the front towards the West in spatially confined regions at high and low L-values separately with about equal velocities. The observations are consistent with a model of westward expansion of the energetic electron precipitation in two regions, aligned parallel to the auroral oval, at high and low L-values of about L ⩾ 6 and L ⩽ 4.8. The westward expanding absorption activity correlates well with local magnetic variations. In magnetic pulsations PiB events are seen at high latitudes simultaneously with the westward moving onsets while at low latitudes IPDP pulsations are observed during the active part of the absorption events. Later in the substorm event a slowly varying absorption event (SVA) is sometimes observed at the lower L-values, L ∼ 3–4.

Auroral riometer absorptions and the F-region disturbances observed over a wide range of latitudes

Standard riometer data from a southern auroral station were compared with ionograms obtained at five stations positioned from sub-auroral to equatorial latitudes. The rapid onset in riometer absorption, during intense substorm activities in an equinoctial period, was associated with a sequential propagation of ionospheric disturbances deduced from the F-region parameters h′F and range spread- F. The time shift between absorption maxima and extrapolated commencement times of the disturbances was consistent with the presence of large-scale travelling ionospheric disturbances (TIDs), propagating equatorwards with velocities lying typically in the range 600–900 m s −1, and with a median velocity of 720 m s −1. It is suggested that the onset of TIDs is associated with high-energy particle precipitation, manifested by the occurrence of auroral absorption events. Similarity of absorption increases at the southern and northern conjugate points, found from a previous riometer study, would indicate that large-scale TIDs are simultaneously generated in both hemispheres.

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 1−1 1 2 h 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.

Night‐time enhancements of ionospheric electron content at L = 4 during substorms

Observations of the ATS‐6 radio beacon from Fairbanks show structured enhancements of the night‐time electron content during substorms. The probability of their occurrence increases with the level of magnetic disturbance, and they are most likely to be observed when the daily sum of K indices exceeds 20 in winter or 30 in summer. There is a remarkably close coincidence between these electron‐content enhancements and auroral absorption events detected by a riometer, though the relative magnitudes vary with the time of day. Electron density profiles measured with the Chatanika incoherent‐scatter radar confirm that the electron density at 90 km is enhanced during auroral absorption events. However, D and E‐region ionization is insufficient to account for the observed enhancement of the electron content, and an F‐region effect is a more likely cause. During one such event the F‐region critical frequency was 8 MHz.

Dynamics and spatial scale of auroral absorption spikes associated with the substorm expansion phase

The dynamics and spatial scale of auroral absorption events, named ‘spikes,’ have been investigated with a new multi‐narrow‐beam riometer experiment. Measurements of the components of the ionospheric electric field perpendicular to the geomagnetic field at the time of a spike were made with the auroral radar system Stare. An absorption spike is caused by a ribbon‐shaped enhancement in the ionospheric electron density causing maximum absorption at an altitude near 90 km. The velocity of a ribbon is generally poleward directed and of magnitude between 300 and 3000 m/s. The half width at half maximum and the length of a ribbon are characterized by dimensions of 10 to 50 km and >400 km, respectively. The electric field within a ribbon is so weak that its poleward motion cannot be explaned as E × B drift in the ionosphere. Equatorward of the ribbon the meridional component of the electric field points equatorward. The height of maximum absorption suggests an effective mean energy of the precipitating electrons of about 30 keV. Finally, we find that these events typically occurred during dusk to midnight in magnetic local time. Events which occurred at early local times were associated with high values of Kp.

Lower ionosphere ion production, density and composition in an auroral absorption event

A payload instrumented to measure energetic particle precipitation, ionization density and composition of ions and neutral constituents in the D- and lower E-region was launched on 21 February 1976 from Esrange into an auroral absorption event. Energetic particles were measured in the spectral range 40 to 800 keV. The reported ion composition measurements were made during rocket ascent in the height range 60–104.6 km. Water cluster ions dominated below 71km above which height NO + was the most abundant ionic species. Negative ions were found below a very sharp ledge in concentration located at 74 km. The importance of the formation of these on the loss rate and recombination life time of positive cluster ions and hence on the positive ion composition is discussed. From the measured ion composition a height profile of nitric oxide is derived showing a concentration between 7 × 10 8cm −3 and 3 × 10 8cm −3 in the height range from 80 to 100 km.

Properties of spike events in auroral radio absorption

This paper considers the spatial extent and the spectral index of spike events in auroral radio absorption observed with riometers. On the evidence of narrow‐ and wide‐beam observations at more than one radio frequency and satellite particle data, it is shown that the typical spike event extends for only tens of kilometers in the ionosphere, compared with several hundred kilometers for auroral absorption in general. The spectral index is found to be close to 2.0, as for the general absorption, implying that the spike absorption is not produced at an unusually low altitude. It is concluded that for spike events the electron precipitation is very intense, is not unusually energetic, and is spatially restricted.

Variations in ionospheric currents and electric fields in association with absorption spikes during the substorm expansion phase

Small‐scale auroral absorption events (‘spikes’) have been associated with the onset of an auroral substorm. In this paper we present detailed observations of ionospheric currents, electric fields, and aurora at the time of two spikes observed on March 1, 1977. The distribution of electric fields in the E layer around the time of occurrence of such absorption events has been investigated using the Stare auroral radar system. We find that the poleward motion of a substorm‐activated arc cannot be explained by E × B drift in the ionosphere. We furthermore find a predominantly southward electric field equatorward of the active arc, while it can be either northward or southward pointing poleward of the arc. Thus a poleward expanding arc leaves a westward current in its wake. We also detect an increase in the westward field component south of the arc. The only region where the westward field dominates is within an auroral structure which moves westward near the beginning of one of the substorms investigated. An example is shown of the reduction of the electric field over a large area including that of the riometer site, lasting for a few minutes shortly after the occurrence of an absorption spike.

On the apparent equatorward propagation of auroral substorm absorption events at low auroral latitudes

Auroral substorm absorption events observed at two stations. Val d'Or, P.Q. and Ottawa, Ont., located on the lower latitude side of the auroral zone (around L = 4) show a consistent tendency for an apparent equatorward propagation of the events. The observed velocities of propagation do not show any local time dependence or other trends expected from a pure gradient B drift of substorm electrons from the midnight precipitation region to the day side. Though a limited number of events are analysed the results indicate the possible influence of strong electric fields on the drift velocities of the substorm electrons.

The onset of an auroral absorption substorm

In this report the features of the breakup of an auroral absorption substorm are discussed. In the longitudinal sector where the substorm breaks up, a sharp onset is preceded by weak absorption which intensifies as it moves equatorward. The maximum value of absorption after the onset depends on the intensification of the equatorward‐moving absorption preceding the breakup. On the basis of the studied substorms it seems that before the onset the weak absorption decreases having a minimum just before the onset and that at longitudes near the breakup a sharp onset can be seen without any preceding absorption, or the minimum of the absorption preceding the onset lasts longer.

On the poleward expansion of ionospheric absorption regions triggered by sudden commencements of geomagnetic storms

A review of riometer data from the south pole (74.2° invariant latitude) for a 7‐year period (1967–1973) indicates the occurrence of sc‐triggered poleward expansions of auroral absorption regions. For 15 such events the average AE index in the hour before the sc's was twice as large as that for 30 sc's that did not trigger substorm events at auroral latitudes. This result indicates that the triggering of substorms involves the enhancement of processes already in progress by the compression of the magnetosphere.

Observations of long-period pulsations in electron precipitation at 75° magnetic latitude

X ray pulsations with a period of ∼150 s were observed in the predawn hours during a balloon flight from Sondre Stromfjord, Greenland (L = 14.5). Simultaneous riometer observations of auroral absorption at the south pole showed absorption pulsations with a period of ∼300 s. These results suggest surface wave instabilities with different temporal features on portions of the magnetopause above and below the magnetic equator.