Characteristic time scale auroral electrojet data (original) (raw)
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Characteristic time scale of auroral electrojet data
Geophysical Research Letters, 1994
The structure run, on of the AE time series shows that the AE time series is self-affine such that the scaling exponent changes at the time scale of approximately 113 (ñ9) minutes. Autocorrelation fimction is shown to have scaling properties similar to those of the structure function. From this result it can be deduced that the time scale at which the scaling properties of the AE data change should equal the typical autocorrelation time of these data. We find the typical autocorrelation time of the AE data is 118 (+9) minutes. The characteristic time scale of the AE data appears as a spectral break in their power spectrum at a period of about twice the autocorrelation time.
We analyze the Auroral Electrojet (AE) index data from the period of Solar minimum and maximum, with respect to their predictability and intermittency. Neural networks are employed to predict the behavior of the AE- data for the different intermittent intervals, as well as recurrence plots where these intervals are visualized. We also compute the multifractal singularity spectrum as additional evidence for the existence of intermittency, and show that this spectrum is independent of the Solar activity.
Scaling behavior of auroral luminosity fluctuations observed by Auroral Large Imaging System (ALIS)
Journal of Geophysical Research, 2008
1] Auroral images obtained by the Auroral Large Imaging System, an optical facility with extremely low noise level and large dynamical range, have been examined in order to study the scaling properties of spatial variations in auroral luminosity. The images under examination were obtained during substorm conditions and contained active auroral forms of different types. The field of view was ±17°from magnetic zenith, which allowed the investigation of auroral fluctuations at scales from 6.4 to 51.2 km. The narrow field of view is assumed to decrease the contamination from aspect angle broadening which leads to distortions of scaling characteristics in the horizontal plane. We analyzed the logscale diagrams constructed by discrete wavelet transform of the data as well as the standard deviation, kurtosis, and probability density functions of the fluctuations. It is found that the data series for substorm auroral forms often reveal closeness to self-similarity, the observed scaling exponent varying between 0.6 and 0.7. Auroral fluctuations in different emission lines exhibit a very similar scaling behavior. A comparison is performed with fluctuations in an electric field observed by the Dynamics Explorer 2 satellite under substorm conditions. It is shown that the intermittency features are more pronounced for substorm electric fields than for auroral fluctuations.
Observed Auroral Electric Field Modulation In The Equatorial Electrojet
7th International Congress of the Brazilian Geophysical Society, 2001
We will present Range Time Intensity (RTI) maps and H component of the Earth magnetic field from magnetometer data to show an observed auroral electric field modulation in the equatorial electrojet during specific disturbed period of December 1999. The RTI maps were obtained from data of the RESCO radar placed in São Luís (2.3° S; 44.2° W; dip:-0.5°), locate at the dip equator. The magnetometer data are from in Vassouras (22.4° S, 43.7° W, dip:-34°) and Jicamarca (12° S, 76.9° W, dip: 1°). We use also the auroral geomagnetic indexes AU and AL to characterize auroral activity and the Dst index as an equivalent equatorial magnetic disturbance index. We start summarizing some important features of the equatorial electrojet. In this paper, we present the technique applied to process our data, to be followed by a discussion of some interesting events observed, and our conclusions.
Aspect angle dependence of irregularity phase velocities in the auroral electrojet
Geophysical Research Letters, 1980
Long term averaged values of the System Parameters electron irregularity drift velocity in the auroral electrojet obtained from VHF radar studies at Our data have been gathered using the 50 MHz Siple, Antarctica, show that the magnitude of the auroral radar operated at Siple Station, Antarcobserved drifts is strongly range dependent. This tica (75ø55'S, 83ø55'W geographic) during the feature is explainable in terms of the dispersion eighteen-month period Jan. 1977-Nov. 1978. for the irregularity generation process, 1 shows a plan view of the Siple system, and which shows that the observed irregularity phase gives the normal operating parameters. velocity is strongly dependent on the angle be-
Annales Geophysicae, 2011
We investigate time evolution of scaling index α A that characterizes auroral luminosity fluctuations at the beginning of substorm expansion. With the use of UVI images from the Polar satellite, it is shown that α A typically varies from values less than unity to ∼1.5, increasing with breakup progress. Similar scaling features were previously reported for fluctuations at smaller scales from all-sky TV observations. If this signature is interpreted in terms of non-linear interactions between scales, it means that the power of smallscale fluctuations is transferred with time to larger scales, a kind of the inverse cascade. Scaling behavior in the aurora during substorm activity is compared with that in the fieldaligned currents simulated numerically in the model of nonlinear interactions of Alfvénic coherent structures, according to the Chang et al. (2004) scenario. This scenario also suggests an inverse cascade, manifesting in clustering of smallscale field-aligned current filaments of the same polarity and formation of "coarse-grained" structures of field-aligned currents.
On the threshold electric field for the ∼1-m auroral irregularity appearance
Journal of Geophysical Research, 1989
We study the electric field threshold for 1-m auroral electrojet irregularity excitation, using data from the special Finnish EISCAT experiment held on November 17, 1983. We find that at times irregularities can arise when the ionospheric electric field is 5-15 mV/m, i.e., well below the Farley-Buneman instability threshold. The electron density fluctuations derived here are increased when the E field becomes more than 15 mV/m. As a result the dependence of electron density fluctuation amplitude on electric fields has a kind of a "soft" E field threshold. Possible reasons for such a dependence are discussed. We show that unresolved ionospheric microstructure in the radar scattering volume can cause this "threshold spreading." radar auroral backscatter can also be observed if the scattering volume is inhomogeneous, which we will call microstructure. Then the auroral backscatter can be detected if there are electric field patches in the scattering volume which will support the FB instability [Siren et al., 1977]. Publications by Greenwald [1979b], Nielsen et al. [1982], and Andr• [1983] have shown that even the 1-m-irregularities can be observed for low irregularity drift velocities, of 100-200 m/s or less. If we suggest that the irregularity drift velocity is close to the E x B electron drift, the E fields for such events are around 5-10 mV/m [Greenwald et al., 1978; Nielsen and Schlegel, 1985]. The appearance of 1-m irregularity in the case of extremely low irregularity drift velocities can possibly be attributed to a strong ionospheric neutral wind. This neutral wind could support the GD and FB turbulence development, as well as low drift velocity irregularities [Gershman, 1974; Nielsen et al., 1982].
Annals of Geophysics, 2019
This work aims at showing how analysis techniques, designed to highlight the multi-scale structure of a signal, may be of help in describing fluctuations of the ionospheric medium. In particular, the technique used here is the ALIF (Adaptive Local Iterative Filter). We have applied ALIF to the characterisation of plasma irregularities in the equatorward boundary of the auroral oval. The data used are from the LEO satellite DEMETER (Detection of ElectroMagnetic Emissions Transmitted from Earthquake Regions) while it crosses the Equatorward boundary of the oval. The time series analysed are those from the electric field instrument ICE (Instrument de Champ Electrique) on board of DEMETER, and the identification of the equatorward boundary in the time series of in situ data is based on the comparison between the satellite trajectory and the picture of the auroral oval from the simultaneous auroral imaging by the DMSP satellite. In detail, we present an analysis of the multi-scale features and dynamics of ionospheric plasma along the DEMETER trajectory in different regions, which seem to be characterized by plasma irregularities of different nature. A clear change of the multi-scale nature of plasma fluctuations is observed in the correspondence of the irregularities originated by particle precipitation in the equatorward boundary of the auroral oval.
Rank ordering multifractal analysis of the auroral electrojet index
Nonlinear Processes in Geophysics, 2011
In the second half of the 90s interest grew on the complex features of the magnetospheric dynamics in response to solar wind changes. An important series of papers were published on the occurrence of chaos, turbulence and complexity. Among them, particularly interesting was the study of the bursty and fractal/multifractal character of the high latitude energy release during geomagnetic storms, which was evidenced by analyzing the features of the Auroral Electrojet (AE) indices. Recently, the multifractal features of the small time-scale increments of AE-indices have been criticized in favor of a more simple fractal behavior. This is particularly true for the scaling features of the probability density functions (PDFs) of the AE index increments. Here, after a brief review of the nature of the fractal/multifractal features of the magnetospheric response to solar wind changes, we investigate the multifractal nature of the scaling features of the AE index increments PDFs using the Rank Ordering Multifractal Analysis (ROMA) technique. The ROMA results clearly demonstrate the existence of a hierarchy of scaling indices, depending on the increment amplitude, for the data collapsing of PDFs relative to increments at different time scales. Our results confirm the previous results by and the more recent results by Rypdal and .
New Features of Time Domain Electric-Field Structures in the Auroral Acceleration Region
Physical Review Letters, 1997
The Polar Satellite carries the first three-axis electric field detector flown in the magnetosphere. Its direct measurement of electric field components perpendicular and parallel to the local magnetic field has revealed new classes and features of electric field structures associated with the plasma acceleration that produces discrete auroras and that populates the magnetosphere with plasma of ionospheric origin. These structures, associated with the hydrogen ion cyclotron mode, include very large solitary waves, spiky field structures, wave envelopes of parallel electric fields, and very large amplitude, nonlinear, coherent ion cyclotron waves. [S0031-9007(97)03793-9] PACS numbers: 94.30. Kq, 52.35.Fp, 52.35.Hr, 94.30.Tz