Auroral Kilometric Radiation Intensity Research Articles

Quantification of magnetosphere–ionosphere coupling timescales using mutual information: response of terrestrial radio emissions and ionospheric–magnetospheric currents

Abstract. Auroral kilometric radiation (AKR) is a terrestrial radio emission excited by the same accelerated electrons which excite auroral emissions. Although it is well correlated with auroral and geomagnetic activity, the coupling timescales between AKR and different magnetospheric or ionospheric regions have yet to be determined. Estimation of these coupling timescales is non-trivial as a result of complex, non-linear processes which rarely occur in isolation. In this study, the mutual information between AKR intensity and different geomagnetic indices is used to assess the correlation between variables. Indices are shifted to different temporal lags relative to AKR intensity, and the lag at which the variables have the most shared information is found. This lag is interpreted as the coupling timescale. The AKR source region receives the effects of a shared driver before the auroral ionosphere. Conversely, the polar ionosphere reacts to a shared driver before the AKR source region. Bow shock interplanetary magnetic field BZ is excited about 1 h before AKR enhancements. This work provides quantitatively determined temporal context to the coupling timelines at Earth. The results suggest that there is a sequence of excitation following the onset of a shared driver: first, the polar ionosphere feels the effects, followed by the AKR source region and then the auroral ionosphere.

In‐flight calibration of STEREO‐B/WAVES antenna system

The STEREO/WAVES (SWAVES) experiment on board the two STEREO spacecraft (Solar Terrestrial Relations Observatory) launched on 25 October 2006 is dedicated to the measurement of the radio spectrum at frequencies between a few kilohertz and 16 MHz. The SWAVES antenna system consists of 6 m long orthogonal monopoles designed to measure the electric component of the radio waves. With this configuration direction finding of radio sources and polarimetry (analysis of the polarization state) of incident radio waves is possible. For the evaluation of the SWAVES data the receiving properties of the antennas, distorted by the radiation coupling with the spacecraft body and other onboard devices, have to be known accurately. In the present context, these properties are described by the antenna effective length vectors. We present the results of an in‐flight calibration of the SWAVES antennas using the observations of the nonthermal terrestrial auroral kilometric radiation (AKR) during STEREO roll maneuvers in an early stage of the mission. A least squares method combined with a genetic algorithm was applied to find the effective length vectors of the STEREO Behind (STEREO‐B)/WAVES antennas in a quasi‐static frequency range (Lantenna≪λwave) which fit best to the model and observed AKR intensity profiles. The obtained results confirm the former SWAVES antenna analysis by rheometry and numerical simulations. A final set of antenna parameters is recommended as a basis for evaluations of the SWAVES data.

Auroral kilometric radiation activity during magnetically quiet periods

The behavior of auroral kilometric radiation (AKR) during magnetically quiet periods is investigated. A clear AKR onset sometimes occurs without any typical substorm signatures characterized by auroral electrojet enhancement on the ground, tail current disruption in the near‐Earth plasma sheet, and energetic particle injection from the plasma sheet. However, the onset is accompanied by auroral brightening on the contracted oval, Pi2 pulsation on the ground, and fast plasma flow in the plasma sheet. These features of quiet‐time AKR seem consistent with a contracted oval substorm, indicating that the onset of AKR is one of the elementary components of the contracted substorm. Unexpected features showing that AKR spectra do not always depend on the substorm magnitude are also presented: (1) the AKR intensity during the magnetically quiet period was sometimes comparable with the usual substorm AKR, and (2) the frequency‐bandwidth expansion of AKR at the time of onset, which is considered to be related with the vertical development of the field‐aligned potential during the substorm, was sometimes comparable between quiet‐ and disturbed‐period AKR.

Spectral variations and long-period intensity variations of auroral kilometric radiation from INTERBALL-2 satellite measurements

A statistical analysis of the auroral kilometric radiation (AKR) measurements in the POLRAD experiment on the INTERBALL-2 satellite has revealed a dependences of the size and location of the AKR generation region on geomagnetic activity: the generation region rises upward and expands with increasing magnetic disturbances. Based on our two-year measurements, we found seasonal AKR intensity variations: the AKR maximum and minimum are observed in winter and summer, respectively. The seasonal variations and the dependence of the spectrum on geomagnetic activity are assumed to have a common physical nature—the background-plasma density variations in the region of the AKR source attributable to plasma flows from the ionosphere into the magnetosphere.

Seasonal and solar cycle dynamics of the auroral kilometric radiation source region

Several year's worth of observations from the plasma wave instruments on both Magnetopause‐to‐Aurora Global Exploration (IMAGE) and Polar spacecraft are used to study the seasonal and solar cycle variations in the spectrum of auroral kilometric radiation (AKR). Only AKR observations when the spacecraft were in the Northern Hemisphere emission cones were used. The results from the seasonal analysis show significant changes in the AKR spectrum as a function of dipole tilt. The average AKR spectral peak for positive dipole tilt is ∼150 kHz but is ∼300 kHz during times of negative dipole tilt. In addition, the average emission spectrum for positive tilt is up to two orders of magnitude weaker over the 200–500 kHz frequency range when compared with the average emission spectrum for negative tilt. Assuming the cyclotron maser mechanism for AKR, these results imply that the AKR source region (the auroral density cavity) moves to higher altitudes during the summer and to lower altitudes during the winter. Using data from the DE‐1 plasma wave instrument, the magnetic local time of average AKR source region is also investigated with dipole tilt. From these observations it is found that for negative dipole tilt a broad AKR source region exists, ranging from ∼18 to ∼24 MLT, with peak emission coming from ∼20 MLT. In comparison, under positive dipole tilt the source region narrows (∼20 to ∼24 MLT) with peak emission at ∼22 MLT. Taking into account the above seasonal effect, a comparison of the average spectra from IMAGE and Polar plasma wave data also demonstrates a solar cycle effect. The average AKR spectrum at solar maximum has the same structure with dipole tilt as at solar minimum but is typically lower (by as much as 1–2 orders of magnitude). The observations presented support the concept that the expected increases in ionospheric densities (with positive dipole tilt for the Northern Hemisphere and solar EUV flux increases during solar maximum) play a significant role in magnetospheric‐ionospheric coupling by: (1) shortening the altitude range of the auroral plasma cavity, (2) confining the cavity to a smaller range of MLT and closer to midnight, and (3) decreasing the overall intensity of AKR by lessening the density depth of the auroral density cavity. The results of this study should be taken into account in future studies of using AKR as a substorm index and other statistical emission cone studies at both low and high frequencies.

Asymmetry of occurrence‐frequency and intensity of AKR between summer polar region and winter polar region sources

The statistical analyses of intensity and occurrence‐frequency of auroral kilometric radiation (AKR) observed by the plasma wave and sounder experiment (PWS) onboard the Akebono (EXOS‐D) satellite have been carried out for the seven‐year period from March 1989 to February 1996. The results show clear seasonal variations of AKR intensity and occurrence‐frequency both in the northern and southern polar regions. AKR exhibits an increase in the intensity and occurrence‐frequency of intense emissions in the winter polar regions; thus there is a distinct north‐south asymmetry of AKR. This tendency of asymmetry feature of AKR emission sources becomes more prominent in the high frequency range. These results have given strong confirmation to the GEOTAIL observation reported by Kasaba et al. [1997]. The origin of this asymmetry of AKR emissions are plausibly related with the seasonal dependence of the acceleration processes of the auroral electrons as has been suggested by Newell et al. [1996].

Auroral kilometric radiation integrated power flux as a proxy for A E

We propose to use the integrated intensity of auroral kilometric radiation from Polar measurements as a proxy for the auroral electrojet index A E in support of studies of the response of the magnetosphere and the geomagnetic tail to changes in magnetic activity. In addition to providing event timing information to understand the effects of perturbations and substorms, the resulting space-based auroral kilometric radiation index would be useful as an input to space weather efforts as an auroral activity metric.

The maser synchrotron instability in an inhomogeneous medium: Determination of the spectral intensity of auroral kilometric radiation

The auroral kilometric radiation (AKR), which emanates from high‐altitude auroral regions of the earth, is mainly emitted in cold plasma depleted regions on the right‐handed X mode. The maser synchrotron instability (MSI) is likely to be responsible for the generation of this emission, accounting for the low density of the cold plasma at the source and the small instantaneous bandwidth of AKR. As the emitted frequency lies very close to the local X mode cutoff frequency, the existence of a gradient in the earth's magnetic field dramatically influences the wave propagation at the source, and thus the resonant wave‐particle process leading to the MSI. A fully inhomogeneous treatment is therefore required. In particular, the magnetic field inhomogeneity quenches the usual nonlinear saturation mechanisms and limits the spatial extension of the source region. It becomes then plausible to determine directly the AKR intensity through a linear transfer equation, with a source term linked to the spontaneous X mode emission. Using the evaluation of the gain carried out in a previous work, we solve this linear transfer equation and determine the AKR spectral intensity. The computed AKR fluxes are consistent with those observed, provided that nonthermal features of the electronic distribution function are present at sufficient energies in the source region. It is found that no nonlinear effect occurs for most AKR events, while the saturation level linked to the trapping of resonant electrons in the wave electric field corresponds well to the maximum observed AKR spectral intensity. Finally, a scenario is proposed for the generation of AKR X mode.

The detailed intensity distribution of the AKR emission cone

Auroral kilometric radiation (AKR) observations in the northern hemisphere from 3.5 years of IMP 6 data and 4 years of Hawkeye data, at large radial distances, are combined to produce detailed intensity distributions of the AKR emission pattern below 200 kHz. All Hawkeye and IMP 6 AKR observations are normalized to a radial distance of 7 earth radii (RE) from the average source region reported by Gallagher and Gurnett (1979). The normalization is done to compensate for the expected radial variations in the intensity. Unlike previous emission pattern studies which showed the AKR frequency of occurrence above a preset power flux threshold, intensity contours of spectral power flux are presented. The AKR observations are well ordered in the source‐centered coordinates which provides further support for auroral zone generation of this emission at those altitudes. AKR is found to be emitted into a large emission cone pattern whose size decreases with frequency from the AKR spectral peak to low frequencies, consistent with previous studies. The emission cones have solid angles of 4.6, 3.3, and 2.7 steradians at 178, 100, and 56.2 kHz, respectively. Unexpected in these results is that there is less than a 10 dB variation in the illumination intensity across nearly the entire time‐averaged AKR emission patterns, at 178 and 100 kHz. In addition, simultaneous observations by Hawkeye and IMP 6 reveal that individual bursts of AKR can easily illuminate the northern emission cone uniformly. No systematic AKR intensity difference in longitude or latitude, within the cone at 178 kHz, is found during the simultaneous two spacecraft observations. This is not consistent with the hollow emission cone pattern reported by AKR at low frequencies (see Calvert, 1981c) near the source region.

A correlation between auroral kilometric radiation and field‐aligned currents

Simultaneous observations of field‐aligned currents (FAC) and auroral kilometric radiation (AKR) are compared from the polar‐orbiting satellites Triad and Hawkeye. The Triad observations were restricted to the evening‐to‐midnight local time sector (1900 to 0100 hours magnetic local time) in the northern hemisphere. This is the region in which the most intense storms of AKR are believed to originate. The Hawkeye observations were restricted to when the satellite was in the AKR ‘emission cone’ in the northern hemisphere and at radial distances ≥7 RE (earth radii) to avoid local propagation cutoff effects. A (R/7RE)² normalization to the power flux measurements of the kilometric radiation from Hawkeye is used to take into account the radial dependence of this radiation and to scale all intensity measurements so that they are independent of Hawkeye's position in the emission cone. Integrated field‐aligned current intensities from Triad are determined from the observed transverse magnetic field disturbances. There appears to be a weak correlation between the AKR intensity and the integrated current sheet intensity of field‐aligned currents. In general, as the intensity of auroral kilometric radiation increases so does the integrated auroral zone current sheet intensity increase. Statistically, the linear correlation coefficient between the log of the AKR power flux and the log of the current sheet intensity is 0.57. During weak AKR bursts (<10−18 W m−2 Hz−1), Triad always observed weak FAC’s (<0.4 A m−1), and when Triad observed large FAC's (≥0.6 A m−1), the AKR intensity from Hawkeye was moderately intense (10−15 to 10−14 W m−2 Hz−1) to intense (>10−14 W m−2 Hz−1). It is not clear from these preliminary results what the exact role is that auroral zone field‐aligned currents play in the generation or amplification of auroral kilometric radiation.

Correlations between solar wind parameters and auroral kilometric radiation intensity

The relationship between solar wind properties and the influx of energy into the nightside auroral region, as indicated by the intensity of auroral kilometric radiation, is investigated. Fluctuations, as large as three orders of magnitude and with periods related to the solar rotation period, are found in the observed intensity of auroral kilometric radiation. Correlations are made with solar wind ion density, bulk flow speed (Vsw), total IMF strength (Bsw), |Vsw × Bsw| electric field, solar wind VswBs (where Bs is the southward component of the IMF Bz), solar wind Vsw²Bsw and VswBsw², and the substorm parameter ϵ (= VswBsw²sin4(θ/2)Lo²). The best correlation is between the solar wind bulk flow speed and the intensity of auroral kilometric radiation. For the 203 daily averages examined, a linear correlation coefficient of 0.78 is found.

A correlation between auroral kilometric radiation and inverted V electron precipitation

Simultaneous observations of energetic electron precipitations and auroral kilometric radiation (AKR) were obtained from the polar orbiting satellites AE‐D and Hawkeye. The Hawkeye observations were restricted to periods when the satellite was in the AKR emission cone in the northern hemisphere and at radial distances ≳7 RE to avoid local propagation cutoff effects. In addition, the AE‐D measurements were restricted to complete passes across the auroral oval in the evening to midnight local time sector (from 20 to 01 hours magnetic local time). This is the local time region where the most intense bursts of AKR are believed to originate. A qualitative survey of AKR and electron particle precipitation shows that AKR is more closely associated with inverted V electron precipitation than with plasma sheet precipitation. Quantitatively, a good correlation is found between the AKR intensity and the peak energy of inverted V events. In addition, in the tail of the most field‐aligned portion (∼0° pitch angle) of the distribution functions of the inverted V events, systematic changes are indicated as the associated AKR intensity increases. When the AKR power flux is weak (<10−17 W/(m² Hz)), the effective tail temperature (T∥) in F(V∥) of the inverted V events is less than 1.8 × 108°K, while for T∥ greater than 1.8 × 108°K the associated AKR power flux is moderate (10−17 to 10−15 W/(m² Hz)) to intense (>10−15 W/(m² Hz)). From a determination of the simultaneous power in the inverted V events and the AKR bursts, the efficiency of converting the charge particle energy into EM radiation increases to a maximum of about 1% for the most intense AKR bursts. However, conversion efficiencies as low as 10−5% are also found. There is some evidence which suggests that the tail temperature, T∥ in F(V∥) of the inverted V events, may play an important role in the efficient generation or amplification of auroral kilometric radiation.