Paola De Michelis

An average image of proton plasma pressure and of current systems in the equatorial plane derived from AMPTE/CCE‐CHEM measurements

The present study attempts to visualize the global equatorial current systems and the proton pressure in the near-Earth magnetosphere based on AMPTE/GCE-CHEM measured proton distributions, which were sorted by the AE index (“quiet”: AE < 100 nT, “active”: 100 nT < AE < 600 nT). The data were averaged over 2 years (from January 1985 to June 1987) in order to obtain the necessary spatial resolution with statistical significance. The results provide an average image of proton plasma pressure, proton plasma anisotropy, and current systems as a function of geomagnetic activity. In particular, the changes of pressure anisotropy with local time and the noon-midnight pressure asymmetry are studied and correlated to the current systems out of the equatorial plane that generate the closure circuits. Moreover, we identify and spatially locate two different current systems for the quiet period (the ring current and the inner portion of the quiet time cross-tail current) and three different current structures for the active period (the ring current, the partial ring current, and the region 2 current).

Non‐Gaussian distribution function of AE‐index fluctuations: Evidence for time intermittency

The probability distribution functions (Pdfs) of the AE-index fluctuations at different time scales have been investigated using a time series covering a period from January 01, 1978 to December 31, 1985. The Pdfs are always non Gaussian for time scales in the range 1–120 min both in quiet and disturbed periods. The scale dependence of the Pdfs indicates that AE-index is not characterized by a global time self-similarity, indicating that an intermittency phenomenon characterizes both phases. The results on Pdfs are compared with functional form, proposed by Castaing et al. [1990], to characterize intermittency phenomena in ordinary turbulent fluid flows. Moreover the relevance of these observations to the understanding of the magnetospheric dynamical configuration is pointed out.

Average terrestrial ring current derived from AMPTE/CCE-CHEM measurements

The present study attempts to analyze the average ring current density and pressure in the magnetospheric equatorial plane (as a function of geocentric distance and local time) based on the AMPTE/CCE-CHEM proton distributions according to two different AE ranges, corresponding to a low (AE<100 nT) and a moderate (100 nT<AE<600 nT) geomagnetic or auroral activity. In order to obtain a statistically significant database for a high spatial resolution, the data were averaged over 2 years. The results provide a trend of the ring current and show its general average features as a function of geomagnetic activity. In particular, the total ring current is dominated by the pressure gradient term, resulting in an eastward current in the inner magnetosphere (L<4.5) and an extended westward current at higher altitudes. The westward current is always stronger than the eastward current. Moreover, the ring current system is a function of magnetic local time, exhibiting a noon-midnight asymmetry in general, as well as a function of geomagnetic activity exhibiting an enhancement of about 20% during active times (in midnight-dusk region only). The location of the total current peak in the midnight sector is at about MLT=2300, and its changes with magnetic activity are interpreted as an effect of the particles injection from the tail during the storms/substorms. The derived current values are compared to previous model predictions and observations.

On the Earth's magnetospheric dynamics: Nonequilibrium evolution and the fluctuation theorem

[1] The Earth’s magnetosphere evolves as an out-of-equilibrium system due to the continuous coupling with the solar wind and the Earth’s ionosphere. We test the validity of the symmetries implied in the Fluctuation Theorem for the magnetospheric dynamics by investigating the long-term evolution of the Earth’s magnetospheric ring current, as monitored by the geomagnetic Dst index. We find that the symmetries implied by the Fluctuation Theorem are all verified, thus providing a proof of the existence of a steady state far from equilibrium for the Earth’s magnetosphere. A possible link between the Dst index and the entropy production rate is also proposed and discussed.

A revised forest-fire cellular automaton for the nonlinear dynamics of the Earth’s magnetotail

Abstract Recent observations and numerical simulations seem to suggest that the Earths magnetotail plasma could exist in a near-criticality configuration, and that many features of the magnetospheric response to solar wind changes could be described by avalanche models displaying self-organized criticality. Further evidences of this near-criticality dynamics have been found analyzing the statistical features of the auroral electrojet (AE) index. Here, we present a cellular automaton, based on a revised version of the well-known forest-fire model, for the nonlinear dynamics of the Earths magnetotail. This simple model, chaotically driven using a 1-d coupled map, is able to capture many of the statistical features of the magnetospheric response to solar wind changes. The results, compared with previous analyses of the AE-index features, are discussed in the framework of a near-criticality dynamics of the magnetospheric tail plasma.

Observations of high-latitude geomagnetic field fluctuations during St. Patrick’s Day storm: Swarm and SuperDARN measurements

The aim of this work is to study the properties of the magnetic field’s fluctuations produced by ionospheric and magnetospheric electric currents during the St. Patrick’s Day geomagnetic storm (17 March 2015). We analyse the scaling features of the external contribution to the horizontal geomagnetic field recorded simultaneously by the three satellites of the Swarm constellation during a period of 13 days (13–25 March 2015). We examine the different latitudinal structure of the geomagnetic field fluctuations and analyse the dynamical changes in the magnetic field scaling features during the development of the geomagnetic storm. Analysis reveals consistent patterns in the scaling properties of magnetic fluctuations and striking changes between the situation before the storm, during the main phase and recovery phase. We discuss these dynamical changes in relation to those of the overall ionospheric polar convection and potential structures as reconstructed using SuperDARN data. Our findings suggest that distinct turbulent regimes characterised the mesoscale magnetic field’s fluctuations and that some factors, which are known to influence large-scale fluctuations, have also an influence on mesoscale fluctuations. The obtained results are an example of the capability of geomagnetic field fluctuations data to provide new insights about ionospheric dynamics and ionosphere–magnetosphere coupling. At the same time, these results could open doors for development of new applications where the dynamical changes in the scaling features of the magnetic fluctuations are used as local indicators of magnetospheric conditions.

On the local Hurst exponent of geomagnetic field fluctuations: Spatial distribution for different geomagnetic activity levels

This study attempts to characterize the spatial distribution of the scaling features of the short time scale magnetic field fluctuations obtained from 45 ground-based geomagnetic observatories distributed in the Northern Hemisphere. We investigate the changes of the scaling properties of the geomagnetic field fluctuations by evaluating the local Hurst exponent and reconstruct maps of this index as a function of the geomagnetic activity level. These maps permit us to localize the different latitudinal structures responsible for disturbances and related to the ionospheric current systems. We find that the geomagnetic field fluctuations associated with the different ionospheric current systems have different scaling features, which can be evidenced by the local Hurst exponent. We also find that in general, the local Hurst exponent for quiet magnetospheric periods is higher than that for more active periods suggesting that the dynamical processes that are activated during disturbed times are responsible for changes in the nature of the geomagnetic field fluctuations.

Magnetic field fluctuation features at Swarm's altitude: A fractal approach

The European Space Agencys Swarm mission provides a qualitatively new level of observational geomagnetic data, which allows us to study the spatial features of magnetic field fluctuations, capturing their essential characteristics and at the same time establishing a correlation with the dynamics of the systems responsible for the fluctuations. Our study aims to characterize changes in the scaling properties of the geomagnetic fields spatial fluctuations by evaluating the local Hurst exponent and to construct maps of this index at the Swarms altitude (∼460 km). Since a signal with a larger Hurst exponent is more regular and less erratic than a signal with a smaller one, the maps permit us to localize spatial structures characterized by different scaling properties. This study is an example of the potential of Swarm data to give new insights into ionosphere-magnetosphere coupling; at the same time, it develops new applications where changes in statistical parameters can be used as a local indicator of overall magnetospheric-ionospheric coupling conditions.

Applying a curl-B technique to Swarm vector data to estimate nighttime F region current intensities

The innovative geometry of European Space Agency Swarm constellation opens the way for new investigations based on magnetic data. Since the knowledge of a vector field on two spherical surfaces allows calculating its curl, we propose a new technique to estimate the curl of the ionospheric magnetic field measured by Swarm satellites A and B, orbiting the Earth at two different altitudes from March to September 2014. Using this technique, we mapped the amplitude of the radial, meridional, and zonal components and of total intensity of the ionospheric current density at the satellites altitudes, i.e., the F region of the ionosphere, during two local nighttime intervals: before and after midnight. Most of the obtained results are consistent with some of the known features of nighttime F region currents; others need further investigation. The proposed technique could contribute in selecting magnetic data with minimum contamination from nighttime F region electric currents for magnetic modeling purposes.

Multifractality and punctuated equilibrium in the Earth's magnetic field polarity reversals

The Earths magnetic field has reversed its polarity hundreds of times in the Earths history. This phenomenon is generally viewed as due to a Poisson nonstationary process. Here, the geomagnetic chron time length sequence, for the past 166 Ma, is analyzed using a multifractal approach, based on ‘singularity analysis’. The existence of a multifractal structure is the signature of the occurrence of ‘punctuated equilibrium‘, which can be read as an indication of a critical configuration of the geodynamo. The results obtained are compared with those from a prototypical model of a dynamical system, introduced by Seki & Ito [1993], and are discussed in the context of fluid motions within the Earths outer liquid core.