Auroral Radio Absorption Research Articles

Intermittency on simultaneous observations of riometer at several Antarctic locations

It is well known that auroral radio wave absorption, as measured by riometers, consists of periods of relative quiescence which are interrupted by short bursts of activity. Such patterns in activity are observed in systems ranging from the stock market to turbulence, i.e. they exhibit intermittency. In the case of the auroral absorption it has also been found that intermittency strongly depends on the magnetic local time, being largest in the night-time sector. This can be interpreted as indicating that the precipitating particles responsible of the absorption exhibit intermittency, especially near the substorm eye, where the level of turbulence increases. Here, we analyse simultaneous observations of riometer absorption at seven Antarctic locations, to determine whether they exhibit intermittency. We determine the Probability Distribution Functions of the fluctuations of riometer absorption for absorption events larger than 0.1dB, as well as those for the time-intervals between absorption events. Observations are for locations within the austral auroral absorption zone and on the polar cap. It is found that the parameters of a power law used to describe the calculated PDFs are consistent with the formation of coherent structures being more frequent within the auroral zone, as a manifestation of intermittency.

Variability and spatial fine structure of precipitating and trapped medium‐energy electron fluxes in the noon sector

The relationships between the precipitating and trapped components of magnetospheric electron flux for energy ranges exceeding 30 and 100 keV have been investigated using data from polar orbiting satellites, the study being restricted to a limited geographic region at auroral latitudes in the noon sector. The electron flux of these energies is the cause of auroral radio absorption. The data are analyzed at two levels of detail. Variations between different passes are studied using their median values, and variations within passes are derived from individual data points at 2 s intervals, equivalent to about 10 km in distance. Several types of behavior are recognized. Basically, the ratio of precipitating to trapped flux at energies exceeding 30 keV varies in proportion to the trapped flux, though there is a limiting upper value where the two components are approximately equal. The precipitating flux never exceeds the trapped flux by any significant amount. These types appear to be consistent with weak and strong pitch angle scatterings, respectively. The precipitation at >100 keV varies somewhat with the >100 keV trapped flux but more strongly with the >30 keV component, consistent with scattering by chorus waves produced by electrons less energetic than those being scattered. Comparison between the two energy ranges shows that the precipitating component is always softer than the trapped. The detailed relationship between the precipitating and trapped components varies from pass to pass by an amount related to the east‐west component of the interplanetary magnetic field. Superimposed on the above behavior are large reductions of precipitation, spatial rather than temporal in nature, during which the trapped flux remains virtually unchanged. These reductions appear to be due to structures some tens of kilometers across, perhaps related to “ducts” within the magnetosphere. Some theoretical considerations based on the Kennel and Petscheck theory of scattering are given in an Appendix.

Properties of auroral radio absorption patches observed in the morning sector using imaging riometer and incoherent-scatter radar

The investigation concerns the size and motion of spatial fine structure in auroral radio absorption as observed by imaging riometer. Electron density profiles in the D and E regions determined by simultaneous incoherent-scatter radar were inverted to provide an estimate of the energy spectrum of the incoming electrons, and it is shown that the gradient-curvature drift at these energies is not consistent with the observed motion of the absorption fine structure, being too large by a factor of 5 or 6. However, a comparison with F-region drifts determined by the SuperDARN radar network supports a hypothesis that the cause lies in moving structures within the magnetosphere.

Comparison of Antarctic riometer radio wave absorption and THEMIS mission energetic electron fluxes

Simultaneous observations of in situ plasma properties in the tail of the Earth’s magnetosphere and of ground based instruments, lying on the same geomagnetic field lines, have recently proved to yield significant new results. In most cases magnetosphere ionosphere interactions during the night-time northern hemisphere conditions are studied. Here, observations of energetic electrons in the tail of the Earth’s magnetosphere made by the THEMIS mission satellites are compared with auroral radio wave absorption determined by riometers in the Antarctic for sunlit conditions. Days for which satellites and riometers are connected by the same geomagnetic field line are selected using a geomagnetic field model. The six days analysed show clear associations between fluxes and absorptions in some cases. However, these do not necessarily correspond to conjugacy intervals. Hours of positive associations are 1.65 times those for negative associations, all hours and days considered (1.42–3.6 on five days and 0.58 on the other day). These computations are assumed appropriate since the footprints of the satellites used approximately follow corrected geomagnetic parallels for all six days studied. The use of a finer parameterization of geomagnetic models to determine conjugacy may be needed.

Auroral radio absorption: The prediction question

Abstract The paper summarises key facts and figures about the phenomenon of auroral radio absorption which are relevant to the problem of predicting its occurrence and intensity. The regions most affected are well known in terms of magnetic latitude (or L-value) and local time. The greater difficulty comes from its highly variable character. Exact predictions are never likely to be possible, but statistical forecasts, which would give the likelihood that some critical level will be reached or exceeded, may well be possible. At the present time it is suggested that they might be based on magnetic indices, since their prediction is already well established. Direct measurements of the solar wind, now available in real time, offer another possibility, perhaps to be considered in parallel. The substorm character of the absorption might also be applied, using the observed duration of absorption events and their known dynamics over the Earth’s surface, as an early warning. A comprehensive reference list is included.

Intermittency of the turbulent processes in the Earth's magnetosphere detected from the ground-based measurements

Turbulent processes in the Earth's magnetosphere are reflected in the dynamical behavior of the geomagnetic indices and other parameters determined from ground based observations. Intermittent properties of one minute Polar Cap (PC) index and auroral radio wave absorption are studied using 1995-2000 data sets. It was found that the probability distribution functions (PDFs) of both PC-index and absorption fluctuations display a strong non-Gaussian shape. This indicates that they are not characterized by a global time self-similarity but rather exhibit intermittency, as previously reported for solar wind velocity and auroral electrojet index values. In the case of the auroral absorption it was also found that intermittency strongly depends on the magnetic local time, being largest in the nighttime sector. This shows that the acceleration of precipitating particles is intermittent, especially near the substorm eye, where the level of turbulence increases. Application of the Local Intermittency Measure (LIM) technique confirms the aforementioned results to a better precision.

Auroral radio emission and absorption of medium frequency radio waves observed in Iceland

Abstract In order to study the generation and propagation processes of MF auroral radio emissions (referred to as auroral roar and MF burst) in the polar ionosphere, an Auroral Radio Spectrograph (ARS) system was installed at Husafell station in Iceland (invariant latitude: 65.3°). Data analysis of man-made transmissions also provides useful information for the ionosphere study as well as an investigation of auroral radio emissions since the propagation character of MF radio waves changes depending on electron-neutral collisions in the bottomside ionosphere. Thus, ionospheric absorption is examined in comparison with the solar zenith angle and auroral phenomena. The results indicate that the ARS data can be used to detect ionization occurring at distant regions. In late 2006, the ARS detected one auroral roar and twoMF bursts, which were identified as left-handed polarized waves. Results of data analysis, including other auroral data and particle spectra observed by the DMSP satellite, suggest that the MF bursts are generated by electrons with an average energy of several keV associated with auroral breakup. On the other hand, the auroral roar is generated as upper hybrid waves by relatively low-energy electrons over the observation site and propagates downward, being converted into L-O mode electromagnetic waves.

Seasonal variations in the incidence of auroral radio absorption events at very high latitude, and the influence of the magnetotail

Abstract. A statistical analysis has been made of the incidence of auroral radio absorption events at South Pole, and of its dependence on basic geophysical parameters such as season, time of day, and magnetic activity level. It is found that at low and moderate levels of activity the incidence of events in the winter season is at least twice that in the summer. However, at high activity no events at all occurred during the local summer night, which appears to be explicable as the effect of the magnetotail and the consequent distortion of the magnetosphere when the southern polar region is tilted strongly towards the Sun. Previous results from even higher latitudes show the effect in an even more exaggerated form, in that both the day and night periods of absorption activity exhibit strong seasonal variations.

Unusual strong quasi-monochromatic ground Pc5 geomagnetic pulsations in the recovery phase of November 2003 superstorm

Abstract. Unusually large-amplitude morning Pc5 magnetic pulsations during the recovery phase of the huge magnetic storm in November 2003 have been studied by using ground-based multi-point observations. Two main spectral Pc5 enhancements were observed: at f≈2 mHz, which featured slowly increasing frequency with decreasing latitude, and at f≈3 mHz, which was latitude independent. The Pc5 pulsations were observed at wide range of latitudes (more than 10°) with the same very strong amplitude (up to 500nT) and with the same polarization. Only the 3-mHz peak was clearly seen in the spectra of pulsating auroral radio absorption, as observed by the Finnish riometer chain. Short and localized bursts of PiB (f~50–100 mHz) magnetic pulsations and simultaneous short bursts of energetic electron precipitation were observed in the morning sector, as well. The beginning of the large-amplitude morning Pc5 activity occurred simultaneously with a substorm onset in the evening and midnight sectors. However, the spectra of pulsations in the morning and evening sectors were different. They were compared with spectra of IMF and solar wind parameters, measured by ACE spacecraft. The similarity between the spectra of morning Pc5 and IMF By was found, but the spectra of evening Pi3 pulsations were similar to the spectra of solar wind density variations. The Pc5 and PiB pulsations, as well as bursts of the auroral radio absorption, suddenly disappeared, when the solar wind density abruptly dropped. We suppose that the ~2-mHz Pc5 geomagnetic pulsations could be attributed to field line resonance (FLR), however, the 3-mHz oscillations were apparently non-resonance origin. Keywords. Magnetospheric physics (MHD waves and instabilities; Solar wind-magnetosphere interaction; Storms and substorms)

Temporal fine structure of nighttime spike events in auroral radio absorption, studied by a wavelet method

The auroral absorption spike event, occurring generally at the onset of a substorm, is distinguished by its high intensity and short duration. This paper reports the presence of a fine structure within the spike event. Analysis of selected examples using the Morlet wavelet shows that within the 1 – 2 min duration of the spike are significant modulations with periodicities in the bands 15 – 60 s (67 – 16 mHz), and 5 – 10 s (200 – 100 mHz), the former being the stronger. The slower fluctuations can amount to more than 10% of the absorption, and they were observed in every example (seven out of nine) in which the spike was moving poleward. They were absent in the other two cases, when the spike moved equatorward. In the examples studied, the 15 – 60 s absorption pulsations were accompanied by magnetic micropulsations of impulsive type (Pi) having a periodity that was similar or harmonically related. The connection is only close while the spike event is moving. Consideration of the details suggests that both the magnetic and the absorption pulsations are related to the acceleration process at substorm onset, the flux of energetic particles into the auroral zone producing the radio absorption being modulated with, though not by, the geomagnetic field variations. The 5 – 10 s pulsations, which are considerably weaker, appeared in both the absorption and the magnetic records, but in this case with no obvious connection between them.

A study of substorm-associated nightside spike events in auroral absorption using imaging riometers at South Pole and Kilpisjärvi

Abstract The short-duration ‘spike’ events which are a common feature of substorm-associated auroral radio absorption in the midnight sector are observed both at Kilpisjarvi (69.05 °N, 20.79 °E, L = 5.9) in the auroral zone and at the much higher latitude of the South Pole (90.00 °S, L = 13). These events have been known for many years, but it is only recently, since the evolution of the imaging riometer, that it has become possible to measure their size, shape and movements. It is found that the spike events are remarkably similar at the two latitudes studied. They are usually elliptical in shape with the major axis generally along rather than across the L shells; median dimensions are 167 km by 74 km at the South Pole, and 190 km by 80 km at Kilpisjarvi. It may be significant that in each case the perturbed region of the ionosphere maps to an almost circular region at the magnetospheric equatorial plane, and that the total magnetic flux included within the event is similar at each latitude. The velocities of the events are variable in the range of several 100 m/s to 2 or 3 km/s; the direction of motion tends to be poleward at the beginning of a precipitation event, and is often equatorward towards the end. The east-west component has not shown any consistency of direction at Kilpisjarvi, though its magnitude may be as large as the north-south one. It is shown that the true duration of a spike event is only 1–2 min. The effect of the spike event's limited extent on wide-beam measurements is investigated, and it is shown that a typical wide-beam riometer underestimates the absorption by a factor of 2 to 3 for spikes occurring at the onset of a substorm. This does not necessarily apply to the later phases of the substorm when the precipitation is likely to be more widespread. The slowly moving absorption bay which may precede the intense precipitation at substorm onset has been detected over Kilpisjarvi. It is identified as an arc-like feature extending east-west across the entire field of view, but containing structure. Its typical north-south extent is 60–100 km, and its equatorward speed is a little over 100m/s.

Dynamics of auroral precipitation in the morning sector observed with the imaging riometer at South Pole station: evidence of co-rotating precipitation regions

The movement of auroral radio absorption events over distances up to 300 km has been studied for several intense substorms using the imaging riometer at South Pole station. In the morning sector, two kinds of motion have been identified. Individual events move through the field-of-view at speeds generally in the range 400–900 m/s, and there is a tendency for successive absorption peaks to maximize at an almost constant longitude for several hours. The gradient-curvature drift of electrons with energies of order 25 keV may contribute to the rapid motions observed, though their tendency not to follow the path of constant invariant latitude suggests that other factors may be involved. Several mechanisms that might account for co-rotating precipitation regions have been examined, but none has been found to be entirely satisfactory. The most plausible hypothesis is that they are a consequence of irregularities in the cold plasma in the magnetosphere.

Space‐time structure of auroral radio absorption events observed with the imaging riometer at south pole

An imaging riometer system comprising 49 independent beams has been operating at South Pole station since January 1988. A study of intense, short‐duration events from the premidnight sector has defined their typical shape as elliptical, with axial ratio 2.3 oriented along the local L shell. The space‐time evolution shows rapid intensifications of the moving absorption patches.

On the prediction of auroral radio absorption on the equatorial side of the absorption zone

Observations of auroral radio absorption made in Finland over the years 1972–1983 have been compiled for use in absorption predictions. The data are presented as values of Q(1), the probability of at least 1 dB absorption being recorded by a 30 MHz riometer, and the results are compared with the predictions of an existing model regarding the latitudinal variation, the influence of solar and magnetic activity indices, and the pattern of daily variation. New formulae for basing absorption predictions on the A p index are proposed.

Eiscat observations of the ionospheric D-region during auroral radio absorption events

Electron densities in the D-region have been observed with EISCAT during energetic electron precipitation events. Sample results are presented which demonstrate the value of the technique in studying variations of electron density with fine temporal and spatial resolution. Different types of absorption event can be characterized in terms of the changes in the incoming electron spectrum inferred from profiles of electron density. We contrast the D-region behaviour of night- and day-time events in terms of precipitating spectrum and absorption profile. A softening of the electron spectrum during the course of a morning event is clearly seen.

High resolution observations of D-region by EISCAT and their comparison to riometer measurements

High resolution electron density profiles were obtained with the incoherent scatter radar EISCAT at Ramfjordmoen over the height range 70–130 km during a period of auroral radio absorption events. The experiment, which was specially designed for D-region study, was carried out with a vertical radar beam and was based on a Barker-coded multipulse scheme with four frequency channels. The achieved height and time resolutions in electron density profiles were 750 m and 15 s respectively. The absorption values calculated from these electron density data show good temporal and spatial correspondence with absorption values obtained at local and nearby riometer stations.

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.

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.

Auroral radio absorption and the westward travelling surge

Measurements from a network of riometers during the passage of an auroral westward travelling surge are presented. These show that the energetic electron precipitation producing the radio absorption expands in an almost identical fashion to the softer precipitation associated with the visible surge, but. it is delayed by about two minutes with respect to the surge. The delay is interpreted as a hardening of the precipitating electron spectrum as the surge goes by. Simultaneous observations of electrons at synchronous orbit are shown to support this conclusion.

Loss cone fluxes and pitch angle diffusion at the equatorial plane during auroral radio absorption events

The flux and pitch angle distribution of energetic electrons near the loss cone have been investigated over the energy range 15–300 keV, using measurements on the geosynchronous satellite GEOS-2 at the times of auroral radio absorption events detected by riometers in Scandinavia. It is shown that conditions of strong pitch angle diffusion apply only during the most intense absorption events ( 6 dB at 30 MHz) which are relatively infrequent. During most events the loss cone is partially depleted, with the degree of depletion increasing as the absorption becomes weaker. The variation of the pitch angle diffusion coefficient with the observed radio absorption is estimated. A consequence of loss cone depletion is a tendency to overestimate the smaller events when computing the radio absorption from flux measurements in the 0°–5° range of detector pointing angles. An empirical law is derived which enables the computation of radio absorption consistent with measurements. D-region recombination laws are discussed and limits are set on the height profile of the effective recombination coefficient.