Arthur D. Richmond

Bio: Arthur D. Richmond is an academic researcher from National Center for Atmospheric Research. The author has contributed to research in topics: Thermosphere & Ionosphere. The author has an hindex of 67, co-authored 262 publications receiving 15605 citations. Previous affiliations of Arthur D. Richmond include University of California, Los Angeles & High Altitude Observatory.

Electrodynamics of the equatorial evening ionosphere: 1. Importance of winds in different regions

Abstract: The importance of winds at different altitudes and latitudes for the electrodynamics of the low-latitude evening ionosphere is examined with a model of the global coupled ionosphere-thermosphere system. The model reproduces the main observed features of the evening equatorial plasma vortex and the prereversal enhancement (PRE) of the vertical drift. The electrodynamics is driven primarily by the zonal wind forced by the diurnally varying zonal pressure-gradient force. The zonal wind lags the zonal pressure-gradient force owing to inertia. When ion drag is important, the time lag of the wind behind the pressure gradient force is shortened, and the high-altitude evening wind turns eastward earlier than the wind at lower altitudes, where ion drag is less important. Therefore, a vertical shear of the zonal wind tends to develop at altitudes around the transition between small and large ion drag at the bottom of the F region. This wind shear is closely associated with the vertical shear in the zonal convection velocity that is part of the evening plasma vortex. Unlike previous studies, we find that the winds driving the PRE lie mainly on field lines with apexes above the peak of the equatorial F layer, field lines that extend in magnetic latitude out to nearly 30° and encompass the entire evening equatorial ionization anomaly region. Contrary to previous suggestions, the westward convection in the bottomside of the evening plasma vortex is found to weaken, rather than strengthen, the PRE. Daytime winds have relatively little influence on the low-latitude evening electrodynamics.

Changes in the Earth's magnetic field over the past century: Effects on the ionosphere-thermosphere system and solar quiet (Sq) magnetic variation

Abstract: [1] We investigated the contribution of changes in the Earth's magnetic field to long-term trends in the ionosphere, thermosphere, and solar quiet (Sq) magnetic variation using the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model. Simulations with the magnetic fields of 1908, 1958, and 2008 were done. The strongest differences occurred between ~40°S–40°N and ~100°W–50°E, which we refer to as the Atlantic region. The height and critical frequency of the F2 layer peak, hmF2 and foF2, changed due to changes in the vertical E × B drift and the vertical components of diffusion and transport by neutral winds along the magnetic field. Changes in electron density resulted in changes in electron temperature of the opposite sign, which in turn produced small corresponding changes in ion temperature. Changes in neutral temperature were not statistically significant. Strong changes in the daily amplitude of the Sq variation occurred at low magnetic latitudes due to the northward movement of the magnetic equator and the westward drift of the magnetic field. The simulated changes in hmF2, foF2, and Sq amplitude translate into typical trends of ±1 km/decade (night) to ±3 km/decade (day), −0.1 to +0.05 MHz/decade, and ±5 to ±10 nT/century, respectively. These are mostly comparable in magnitude to observed trends in the Atlantic region. The simulated Atlantic region trends in hmF2 and foF2 are ~2.5 times larger than the estimated effect of enhanced greenhouse gases on hmF2 and foF2. The secular variation of the Earth's magnetic field may therefore be the dominant cause of trends in the Atlantic region ionosphere.

Optimal interpolation analysis of high-latitude ionospheric electrodynamics using empirical orthogonal functions: Estimation of dominant modes of variability and temporal scales of large-scale electric fields

Abstract: [1] In this paper the optimal interpolation method, in conjunction with empirical orthogonal function (EOF) bases and the maximum likelihood method for online error covariance parameter estimation, is successfully implemented for the objective analysis of large-scale high-latitude ionospheric electrodynamic variables. This study demonstrates how this methodology can be used to extract information about the temporal and spatial coherence of the large-scale electric field for a magnetic cloud event on 10–11 January 1997. Compared with the temporal persistence of the interplanetary magnetic field (IMF) and solar wind parameters, the timescale of the spatially coherent part of the electric field, on the spatial scale of the EOFs, is shorter. The principal components of the high-latitude electric field are analyzed during two periods when either the IMF BY or BZ component was relatively steady while the other component varied. The first principal component, not surprisingly, generally reflects the change in the ionospheric convection pattern predicted by IMF-dependent empirical models and thus tends to represent changes in the convection that are directly driven by solar wind–magnetosphere interactions. However, it is the second principal component that is more strongly correlated with the westward auroral electrojet, suggestive of a link to substorm phenomena.

An analysis of the momentum forcing in the high-latitude lower thermosphere

Abstract: [1] We analyze the dynamics of the high-latitude thermsopheric wind system below 170 km for negative IMF Bz by using a fully nonlinear model with a realistic distribution of the forcing. A transition of the forcing patterns and their relative contribution to the high-latitude lower thermospheric wind system occurs around 123 km under various conditions, weak or strong IMF, summer or winter. Winds around and above 123 km are sustained by the gradient-wind balance among divergent/convergent pressure gradient, Coriolis, and horizontal momentum advection (mainly centrifugal) accelerations. Below 123 km winds are maintained by the approximate balance of divergent/convergent pressure gradient, Coriolis, and Hall ion drag accelerations through modified geostrophy. The dominant contribution to the wind tendency (time rate of change) is the rotational component of the ion drag acceleration. The wind tendency above 123 km tends to resemble rotational Pedersen ion drag acceleration well, which reflects a rotated pattern of the E × B velocity. Near and below 123 km the wind tendency is also affected by the rotational component of the Hall ion drag acceleration whose pattern no longer closely resembles the pattern of the E × B velocity, and the wind pattern can differ significantly from that well above 123 km. Simulations for different strengths of the IMF and different seasons indicate that largely divergent/convergent Coriolis and horizontal momentum advection accelerations tend approximately to balance with the horizontal pressure gradient (as well as with divergent/convergent ion drag at lower altitudes) under various conditions. As the forcing increases the radius of curvature of the strong winds also tends to increase, so that the centrifugal acceleration does not increase quadratically with the maximum wind speed, and the tendency for a rough balance between the Coriolis and horizontal momentum advection accelerations in the duskside vortex above 123 km is maintained.

Seasonal and longitudinal variations of the solar quiet (Sq) current system during solar minimum determined by CHAMP satellite magnetic field observations

Abstract: [1] Vector magnetometer observations from the Challenging Minisatellite Payload (CHAMP) satellite are used to determine the solar quiet (Sq) current system during the recent solar minimum. Observations from 2006 to 2008 are combined, and after removal of a main field model and accounting for field-aligned currents, the longitudinal and seasonal variation of the Sq currents are determined through the method of spherical harmonic analysis. Comparison with Sq currents derived from ground-based magnetometers in the African/European longitude sector reveals similar amplitudes and seasonal variations, indicating that the CHAMP observations can reliably determine the Sq current system. The seasonal variation is consistent with prior observations during solar minimum conditions and in the Northern Hemisphere exhibits a primarily annual variation with peak currents during local summer. The seasonal variation in the Southern Hemisphere is characterized by a semiannual variation with the maxima occurring around the equinoxes. Significant longitudinal variations are also observed, and they display a seasonal variability. During Northern Hemisphere summer, the predominant feature at local noon is a wave number 1 variation in longitude. During the remainder of the year, a wave 3 longitudinal structure is observed at this local time. The longitudinal variations are considered to be due to a combination of the orientation and strength of the geomagnetic field as well as the tidal winds in the lower thermosphere. Variations in tidal winds due to nonmigrating tides may influence the dynamo-generated electric fields and currents, resulting in the observed longitudinal variations of the Sq current function.

Riemann manifold Langevin and Hamiltonian Monte Carlo methods

Abstract: 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.

International Reference Ionosphere 2000

Abstract: 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.

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

Abstract: 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.

The ionospheric disturbance dynamo

Abstract: 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.

Riemann manifold Langevin and Hamiltonian Monte Carlo methods

Abstract: 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.