VLF, magnetic bay and Pi2 substorm signatures at auroral and midlatitude ground stations

A superposed epoch analysis of 100–300 substorms is performed to determine the median size and shape of the substorm-associated VLF chorus, magnetic bay, and Pi2 pulsation burst observed at the near-auroral Halley research station, Antarctica, and at the midlatitude Faraday station at three different local times (2230, 2330, 0130 MLT). The spatial and temporal properties of the magnetic bay signatures are compared with the University of York implementation of the Kisabeth–Rostoker substorm current wedge (SCW) model and the Weimer pulse model, respectively. These constitute the best analytical models of the substorm to date. It is shown that the polarities and relative amplitudes of the observed magnetic bays in the H, D, and Z components at Halley at midnight MLT and at Faraday in the premidnight sector are consistent with the York model for a SCW 3 hours wide in MLT with its westward electrojet at 67°S magnetic latitude. In particular the little-discussed Z component of the bay agrees with the model and is shown to be the clearest substorm signature of the three components, especially at midlatitude. The midnight and postmidnight bays are similar to the premidnight case but progressively smaller and cannot be fully reconciled with the model. The shape of the H and Z bays at Halley and the D bays at Faraday fit a normalized Weimer pulse well, with Weimer's 2 h−1 recovery rate, but the other components do not. The D component at Halley and H at Faraday do fit the Weimer pulse shape but with a faster recovery rate of 4 h−1. It is proposed that this is due to the effect of a decaying current in the SCW combining with the geometrical effect of changing SCW configuration and position relative to the observing station. The Z component at Faraday recovers more slowly than the 2 h−1 Weimer prediction; we cannot explain this. Secondary bays at Halley and Faraday show a clear tendency to recur after 2 hours. Inflection points just prior to onset at Halley and Faraday are argued to be related to reduced convection associated with northward turning of the IMF. The median substorm signature at Halley in the Pi2 frequency band (7–25 mHz) is well correlated with the bay structure, showing that it is part of a broader band, possibly turbulent, spectrum in the substorm-dependent DP2 current. There is evidence of a minor additional narrow band component occurring at substorm onset. This is the dominant signal at Faraday which shows the classic midlatitude substorm signature, a short Pi2 pulsation burst at onset, that decreases progressively in intensity with increasing local time, implying a source region biased to the evening side or else preferred propagation to the east from a near-midnight source

The electric field response to the growth phase and expansion phase onset of a small isolated substorm

International audienceWe capitalise on the very large field of view of the Halley HF radar to provide a comprehensive description of the electric field response to the substorm growth phase and expansion phase onset of a relatively simple isolated substorm ( |AL| 12 h) of magnetic quiescence, such that prior to the start of the growth phase, the apparent latitudinal motion of the radar backscatter returns is consistent with the variation in latitude of the quiet-time auroral oval with magnetic local time. The growth phase is characterised by an increasing, superimposed equatorward motion of the equatorward edge of the radar backscatter as the auroral oval expands. Within this backscatter region, there is a poleward gradient in the Doppler spectral width, which we believe to correspond to latitudinal structure in auroral emissions and magnetospheric precipitation. During the growth phase the ionospheric convection is dominated by a relatively smooth large-scale flow pattern consistent with the expanding DP2 (convection) auroral electrojets. Immediately prior to substorm onset the ionospheric convection observed by the radar in the midnight sector has a predominantly equatorward flow component. At substorm onset a dramatic change occurs and a poleward flow component prevails. The timing and location are quite remarkable. The timing of the flow change is within one minute of the dispersionless injection observed at geostationary orbit and the Pi2 magnetic signature on the ground. The location shows that this sudden change in flow is due to the effect of the upward field aligned current of the substorm current wedge imposed directly within the Halley radar field of view

Magnetic local time variation and scaling of poleward auroral boundary dynamics

The balance of dayside and nightside reconnection processes within the Earth's magnetosphere, and its effect on the amount of open magnetic flux threading the ionosphere is well understood in terms of the expanding-contracting polar cap model. However, the nature and character of the consequential fluctuations in the polar cap boundary are poorly understood. By using the poleward auroral luminosity boundary (PALB), as measured by the FUV instrument of the IMAGE spacecraft, as a proxy for the polar cap boundary we have studied the motion of this boundary for more than two years across the complete range of magnetic local time. Our results show that the dayside PALB dynamics are broadly self-similar on timescales of 12 minutes to 6 hours and appear to be monofractal. Similarity with the characteristics of solar wind and interplanetary magnetic field (IMF) variability suggest that this dayside monofractal behaviour is predominantly inherited from the solar wind via the reconnection process. The nightside PALB dynamics exhibit scale-free behaviour at intermediate timescales (12-90 minutes) and appear to be multifractal. We propose that this character is a result of the intermittent multifractal structure of magnetotail reconnection

A statistical model of vorticity in the polar ionosphere and implications for extreme values

Measurements of vorticity in the Earth’s ionosphere enable the characterisation of turbulent structure in the ionospheric plasma flow and how it varies spatially in relation to large-scale magnetic field-aligned current (FAC) systems. We have determined the spatial variation of the probability density function (PDF) of ionospheric vorticity measurements made by the Super Dual Auroral Radar Network (SuperDARN) across the northern polar ionosphere for the 6-year interval 2000-2005 inclusive. These PDFs are highly leptokurtic, with heavy tails, and are well-modelled by Tsallis q-exponential probability distributions. The parameters of the model q-exponential distributions have been determined using maximum likelihood estimation (MLE), resulting in a statistical model of ionospheric vorticity that covers the polar ionosphere. The spatial variation of the model parameters is highly variable, with the shape and scale of the model distributions varying systematically in relation to the well-known FAC regions, showing that FACs have a major influence on the character of ionospheric plasma vorticity. From the model distributions we estimate the probability of observing extreme vorticity values with the SuperDARN radars (beyond thresholds of 5, 10, 20, and 40 mHz) across the northern polar ionosphere

Refractive elastic scattering of carbon and oxygen nuclei: The mean field analysis and Airy structures

The experimental data on the $^{16}$O$+^{12}$C and $^{18}$O$+^{12}$C elastic scatterings and their optical model analysis are presented. Detailed and complete elastic angular distributions have been measured at the Strasbourg Vivitron accelerator at several energies covering the energy range between 5 and 10 MeV per nucleon. The elastic scattering angular distributions show the usual diffraction pattern and also, at larger angles, refractive effects in the form of nuclear rainbow and associated Airy structures. The optical model analysis unambiguously shows the evolution of the refractive scattering pattern. The observed structure, namely the Airy minima, can be consistently described by a nucleus-nucleus potential with a deep real part and a weakly absorptive imaginary part. The difference in absorption in the two systems is explained by an increased imaginary (mostly surface) part of the potential in the $^{18}$O$+^{12}$C system. The relation between the obtained potentials and those reported for the symmetrical $^{16}$O$+^{16}$O and $^{12}$C$+^{12}$C systems is drawn.Comment: 10 pages, 9 figures, Phys. rev. C in pres

Data‐driven basis functions for SuperDARN ionospheric plasma flow characterisation and prediction

The archive of plasma velocity measurements from the Super Dual Auroral Radar Network (SuperDARN) provides a rich dataset for investigation of magnetosphere-ionosphere-thermosphere coupling. However, systematic gaps in this archive exist in space, time, and radar look-direction. These gaps are generally infilled using climatological averages, spatially smoothed models, or a priori relationships determined from solar wind drivers. We describe a new technique for infilling the data gaps in the SuperDARN archive which requires no external information and is based solely on the SuperDARN measurements. We also avoid the use of climatological averaging or spatial smoothing when computing the infill. In this regard, our approach captures the true variability in the SuperDARN measurements. Our technique is based on data-interpolating Empirical Orthogonal Function (EOF) analysis. This method discovers from the SuperDARN data a series of dynamical modes of plasma velocity variation. We compute the modes of a sample month of northern hemisphere winter data, and investigate these in terms of solar wind driving. We find that the By component of the Interplanetary Magnetic Field (IMF) dominates the variability of the plasma velocity. The IMF Bz component is the dominant driver for the background mean field, and a series of non-leading modes which describe the two-cell convection variability, and the substorm. We recommend our new technique for reanalysis investigations of polar-scale plasma drift phenomena, particularly those with rapid temporal fluctuations and an indirect relationship to the solar wind

An empirical orthogonal function reanalysis of the northern polar external and induced magnetic field during solar cycle 23

We apply the method of data-interpolating Empirical Orthogonal Functions (EOFs) to ground-based magnetic vector data from the SuperMAG archive to produce a series of month-length reanalyses of the surface external and induced magnetic field (SEIMF) in 110,000 km2 equal-area bins over the entire northern polar region at 5-minute cadence over solar cycle 23, from 1997.0 to 2009.0. Each EOF re-analysis also decomposes the measured SEIMF variation into a hierarchy of spatio-temporal patterns which are ordered by their contribution to the monthly magnetic field variance. We find that the leading EOF patterns can each be (subjectively) interpreted as well-known SEIMF systems or their equivalent current systems. The relationship of the equivalent currents to the true current flow is not investigated. We track the leading SEIMF or equivalent current systems of similar type by inter-monthly spatial correlation, and apply graph theory to (objectively) group their appearance and relative importance throughout a solar cycle, revealing seasonal and solar cycle variation. In this way, we identify the spatiotemporal patterns which maximally contribute to SEIMF variability over a solar cycle. We propose this combination of EOF and graph theory as a powerful method for objectively defining and investigating the structure and variability of the SEIMF or their equivalent ionospheric currents for use in both geomagnetism and space weather applications. It is demonstrated here on solar cycle 23, but is extendable to any epoch with sufficient data coverage

The influence of sudden commencements on the rate of change of the surface horizontal magnetic field in the United Kingdom

Sudden commencements (SCs) are rapid increases in the northward component of the surface geomagnetic field, related to sharp increases in the dynamic pressure of the solar wind. Large rates of change of the geomagnetic field can induce damaging currents in ground power networks. In this work, the effect of SCs on the (one minute) rate of change of the surface magnetic field (R) at three UK stations is investigated. The distributions of R during SCs are shifted to higher values than the data set as a whole. Rates of change greater than 10 nTmin‐1 are 30‐100 times more likely during SCs, though less than 8% of the most extreme R (≥ 99.99th percentile) are observed during SCs. SCs may also precede geomagnetic storms, another potential source of large R. We find that the probability of observing large R is greatly enhanced for three days following an SC. In the 24 hours following an SC it is 10 times more likely than at any given time to observe rates of change between 10 and several hundred nTmin‐1. Additionally, between 90 and 94% of data (depending on station) above the 99.97th percentile is recorded within three days of an SC. All values of R ≥ 200 nTmin‐1 in the UK have been observed within three days of an SC. These results suggest that accurately predicting sudden commencements is critically important to identify intervals during which power networks at similar geomagnetic latitudes to the UK are at risk from large GICs

Testing the SOC hypothesis for the magnetosphere

As noted by Chang, the hypothesis of Self-Organised Criticality provides a theoretical framework in which the low dimensionality seen in magnetospheric indices can be combined with the scaling seen in their power spectra and the recently-observed plasma bursty bulk flows. As such, it has considerable appeal, describing the aspects of the magnetospheric fuelling:storage:release cycle which are generic to slowly-driven, interaction-dominated, thresholded systems rather than unique to the magnetosphere. In consequence, several recent numerical "sandpile" algorithms have been used with a view to comparison with magnetospheric observables. However, demonstration of SOC in the magnetosphere will require further work in the definition of a set of observable properties which are the unique "fingerprint" of SOC. This is because, for example, a scale-free power spectrum admits several possible explanations other than SOC. A more subtle problem is important for both simulations and data analysis when dealing with multiscale and hence broadband phenomena such as SOC. This is that finite length systems such as the magnetosphere or magnetotail will by definition give information over a small range of orders of magnitude, and so scaling will tend to be narrowband. Here we develop a simple framework in which previous descriptions of magnetospheric dynamics can be described and contrasted. We then review existing observations which are indicative of SOC, and ask if they are sufficient to demonstrate it unambiguously, and if not, what new observations need to be made?Comment: 29 pages, 0 figures. Based on invited talk at Spring American Geophysical Union Meeting, 1999. Journal of Atmospheric and Solar Terrestrial Physics, in pres

Pair densities at contact in the quantum electron gas

The value of the pair distribution function g(r) at contact (r = 0) in a quantum electron gas is determined by the scattering events between pairs of electrons with antiparallel spins. The theoretical results for g(0) as a function of the coupling strength r_s in the paramagnetic electron gas in dimensionality D=2 and 3, that have been obtained from the solution of the two-body scattering problem with a variety of effective scattering potentials embodying many-body effects, are compared with the results of many-body calculations in the ladder approximation and with quantum Monte Carlo data.Comment: 7 pages, 2 figure