The paper proposes Metropolis adjusted Langevin and Hamiltonian Monte Carlo sampling methods defined on the Riemann manifold to resolve the shortcomings of existing Monte Carlo algorithms when sampling from target densities that may be high dimensional and exhibit strong correlations. The methods provide fully automated adaptation mechanisms that circumvent the costly pilot runs that are required to tune proposal densities for Metropolis–Hastings or indeed Hamiltonian Monte Carlo and Metropolis adjusted Langevin algorithms. This allows for highly efficient sampling even in very high dimensions where different scalings may be required for the transient and stationary phases of the Markov chain. The methodology proposed exploits the Riemann geometry of the parameter space of statistical models and thus automatically adapts to the local structure when simulating paths across this manifold, providing highly efficient convergence and exploration of the target density. The performance of these Riemann manifold Monte Carlo methods is rigorously assessed by performing inference on logistic regression models, log-Gaussian Cox point processes, stochastic volatility models and Bayesian estimation of dynamic systems described by non-linear differential equations. Substantial improvements in the time-normalized effective sample size are reported when compared with alternative sampling approaches. MATLAB code that is available from http://www.ucl.ac.uk/statistics/research/rmhmc allows replication of all the results reported.

/pdf/riemann-manifold-langevin-and-hamiltonian-monte-carlo-254effw0xm.pdf

Riemann manifold Langevin and Hamiltonian Monte Carlo methods

The International Reference Ionosphere (IRI) is the international standard for the specification of ionospheric densities and temperatures. It was developed and is being improved-updated by a joint working group of the International Union of Radio Science (URSI) and the Committee on Space Research (COSPAR). A new version of IRI is scheduled for release in the year 2000. This paper describes the most important changes compared to the current version of IRI: (1) an improved representation of the electron density in the region from the F peak down to the E peak including a better description of the F1 layer occurrence statistics and a more realistic description of the low-latitude bottomside thickness, (2) inclusion of a model for storm-time conditions, (3) inclusion of an ion drift model, (4) two new options for the electron density in the D region, and (5) an improved model for the topside electron temperatures. The outcome of the most recent IRI Workshops (Kuhlungsborn, 1997, and Nagoya, 1998) will be reviewed, and the status of several ongoing task force activities (e.g., efforts to improve the representation of electron and ion densities in the topside ionosphere and the inclusion of a plasmaspheric extension) will be discussed. A few typical IRI applications will be highlighted in section 6.

/pdf/international-reference-ionosphere-2000-zhazhxmwus.pdf

International Reference Ionosphere 2000

The Dual Auroral Radar Network (DARN) is a global-scale network of HF and VHF radars capable of sensing backscatter from ionospheric irregularities in the E and F-regions of the high-latitude ionosphere. Currently, the network consists of the STARE VHF radar system in northern Scandinavia, a northern-hemisphere, longitudinal chain of HF radars that is funded to extend from Saskatoon, Canada to central Finland, and a southern-hemisphere chain that is funded to include Halley Station, SANAE and Syowa Station in Antarctica. When all of the HF radars have been completed they will operate in pairs with common viewing areas so that the Doppler information contained in the backscattered signals may be combined to yield maps of high-latitude plasma convection and the convection electric field. In this paper, the evolution of DARN and particularly the development of its SuperDARN HF radar element is discussed. The DARN/SupperDARN network is particularly suited to studies of large-scale dynamical processes in the magnetosphere-ionosphere system, such as the evolution of the global configuration of the convection electric field under changing IMF conditions and the development and global extent of large-scale MHD waves in the magnetosphere-ionosphere cavity. A description of the HF radars within SuperDARN is given along with an overview of their existing and intended locations, intended start of operations, Principal Investigators, and sponsoring agencies. Finally, the operation of the DARN experiment within ISTP/GGS, the availability of data, and the form and availability of the Key Parameter files is discussed.

DARN/SUPERDARN : A global view of the dynamics of high-latitude convection

A numerical simulation study of the thermospheric winds produced by auroral heating during magnetic storms, and of their global dynamo effects, establishes the main features of the ionospheric disturbance dynamo. Driven by auroral heating, a Hadley cell is created with equatorward winds blowing above about 120 km at mid-latitudes. The transport of angular momentum by these winds produces a subrotation of the mid-latitude thermosphere or westward motion with respect to the earth. The westward winds in turn drive equatorward Pedersen currents which accumulate charge toward the equator, resulting in the generation of a poleward electric field, a westward E × B drift, and an eastward current. When realistic local time conductivity variations are simulated, the eastward mid-latitude current is found to close partly via lower latitudes, resulting in an ‘anti-Sq’ type of current vortex. Both electric field and current at low latitudes thus vary in opposition to their normal quiet-day behavior. This total pattern of disturbance winds, electric fields, and currents is superimposed upon the background quiet-day pattern. When the neutral winds are artificially confined on the nightside, the basic pattern of predominantly westward E × B plasma drifts still prevails on the nightside but no longer extends into the dayside. Considerable observational evidence exists, suggesting that the ionospheric disturbance dynamo has an appreciable influence on storm-time ionospheric electric fields at middle and low latitudes.

The ionospheric disturbance dynamo

The paper proposes Metropolis adjusted Langevin and Hamiltonian Monte Carlo sampling methods defined on the Riemann manifold to resolve the shortcomings of existing Monte Carlo algorithms when sampling from target densities that may be high dimensional and exhibit strong correlations. The methods provide fully automated adaptation mechanisms that circumvent the costly pilot runs that are required to tune proposal densities for Metropolis-Hastings or indeed Hamiltonian Monte Carlo and Metropolis adjusted Langevin algorithms. This allows for highly efficient sampling even in very high dimensions where different scalings may be required for the transient and stationary phases of the Markov chain. The methodology proposed exploits the Riemann geometry of the parameter space of statistical models and thus automatically adapts to the local structure when simulating paths across this manifold, providing highly efficient convergence and exploration of the target density. The performance of these Riemann manifold Monte Carlo methods is rigorously assessed by performing inference on logistic regression models, log-Gaussian Cox point processes, stochastic volatility models and Bayesian estimation of dynamic systems described by non-linear differential equations. Substantial improvements in the time-normalized effective sample size are reported when compared with alternative sampling approaches. MATLAB code that is available from http://www.ucl.ac.uk/statistics/research/rmhmc allows replication of all the results reported.

/pdf/ion-neutral-coupling-effects-on-low-latitude-thermospheric-1nwzknvt6l.pdf

Ion-neutral coupling effects on low-latitude thermospheric evening winds

The large-scale ionospheric convection patterns derived from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure are used to examine the relationship between global convection and the substorm expansion. It is found that, after the onset of the substorm expansion when the AL index begins to decrease sharply, an isolated convection cell evolves near midnight. As the substorm progresses, this cell expands, gradually merging with the pre-existing dusk-side convection cell. When the pre-substorm expansion convection pattern is subtracted, the residual patterns show clearly a 2-cell convection configuration which first appears in the local midnight sector after the onset of the expansion phase and gradually intensifies in spatial size as well as in magnitude during the expansion phase. Such a configuration implies that the substorm-related ionospheric convection is mainly controlled by a magnetospheric process in the tail.

Global Ionospheric Convection During Substorm Expansion

Vasyliunas (1972) showed that, for steady state lossless ion convection in the inner magnetosphere, the effects of region 2 field-aligned currents generated by the pressure gradients of the magnetospheric plasma could be represented by an equivalent magnetospheric Hall conductance Ci. We examine in the framework of a linearized fluid theory how ion losses modify the formalism introduced by Vasyliunas. The equivalent magnetospheric Hall conductance Ci is decreased and a new term Cr is added to the Σφφ component of the ionospheric conductance tensor Σ. An evaluation of this new term shows that the eastward rotation of the convection potential pattern induced by Ci is reduced and that Cr contributes to a nonnegligible part of the shielding effect. This linear formalism is compared with the nonlinear model of C. Peymirat and D. Fontaine (Numerical simulation of the magnetospheric convection including the effect of field-aligned currents and electrons precipitation, submitted to Journal of Geophysical Research, 1993), showing that the effect of the field-aligned currents on the magnetospheric convection is reproduced up to first order in the linear formalism. The linear formalism presents a simple way to include the effects of region 2 field-aligned currents into models of ionospheric electrodynamics for steady state conditions.

Modeling the ion loss effect on the generation of region 2 field‐aligned currents via equivalent magnetospheric conductances

Numerical modeling of polar ionospheric electrodynamics for July 23-24, 1983 utilizing ionospheric conductances deduced from DMSP x-ray images

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Simulation of electric field and current during the 11 June 1993 disturbance dynamo event: Comparison with the observations K.Z. Zaka, A. T. Kobea, V. Doumbia, A.D. Richmond, A. Maute, N.M. Mene, O.K. Obrou, P. Assamoi, K. Boka, J.P. Adohi, et al.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010JA016292

Arthur D. Richmond

Papers

Ion-neutral coupling effects on low-latitude thermospheric evening winds

Global Ionospheric Convection During Substorm Expansion

Modeling the ion loss effect on the generation of region 2 field‐aligned currents via equivalent magnetospheric conductances

Numerical modeling of polar ionospheric electrodynamics for July 23-24, 1983 utilizing ionospheric conductances deduced from DMSP x-ray images

Correction to “Simulation of electric field and current during the 11 June 1993 disturbance dynamo event: Comparison with the observations”