Showing papers by "Arthur D. Richmond published in 2017"

The Global-Scale Observations of the Limb and Disk (GOLD) Mission

Abstract: The Earth’s thermosphere and ionosphere constitute a dynamic system that varies daily in response to energy inputs from above and from below. This system can exhibit a significant response within an hour to changes in those inputs, as plasma and fluid processes compete to control its temperature, composition, and structure. Within this system, short wavelength solar radiation and charged particles from the magnetosphere deposit energy, and waves propagating from the lower atmosphere dissipate. Understanding the global-scale response of the thermosphere-ionosphere (T-I) system to these drivers is essential to advancing our physical understanding of coupling between the space environment and the Earth’s atmosphere. Previous missions have successfully determined how the “climate” of the T-I system responds. The Global-scale Observations of the Limb and Disk (GOLD) mission will determine how the “weather” of the T-I responds, taking the next step in understanding the coupling between the space environment and the Earth’s atmosphere. Operating in geostationary orbit, the GOLD imaging spectrograph will measure the Earth’s emissions from 132 to 162 nm. These measurements will be used image two critical variables—thermospheric temperature and composition, near 160 km—on the dayside disk at half-hour time scales. At night they will be used to image the evolution of the low latitude ionosphere in the same regions that were observed earlier during the day. Due to the geostationary orbit being used the mission observes the same hemisphere repeatedly, allowing the unambiguous separation of spatial and temporal variability over the Americas.

Magnetic Coordinate Systems

Abstract: Geospace phenomena such as the aurora, plasma motion, ionospheric currents and associated magnetic field disturbances are highly organized by Earth’s main magnetic field. This is due to the fact that the charged particles that comprise space plasma can move almost freely along magnetic field lines, but not across them. For this reason it is sensible to present such phenomena relative to Earth’s magnetic field. A large variety of magnetic coordinate systems exist, designed for different purposes and regions, ranging from the magnetopause to the ionosphere. In this paper we review the most common magnetic coordinate systems and describe how they are defined, where they are used, and how to convert between them. The definitions are presented based on the spherical harmonic expansion coefficients of the International Geomagnetic Reference Field (IGRF) and, in some of the coordinate systems, the position of the Sun which we show how to calculate from the time and date. The most detailed coordinate systems take the full IGRF into account and define magnetic latitude and longitude such that they are constant along field lines. These coordinate systems, which are useful at ionospheric altitudes, are non-orthogonal. We show how to handle vectors and vector calculus in such coordinates, and discuss how systematic errors may appear if this is not done correctly.

F-Region Dynamo Simulations at Low and Mid-Latitude

Abstract: The “ $F$ -layer dynamo” or “ $F$ -region dynamo” concept was introduced by Rishbeth (Planet. Space Sci. 19(2):263–267, 1971a; 19(3):357–369, 1971b). $F$ -region winds blow the plasma across magnetic field lines setting up transverse drifts and polarization electric fields leading to equatorial downward current during the daytime and upward current at dusk which were confirmed by satellite observations. In the daytime the $F$ -region current can close through the highly conducting $E$ -region. At night when the $E$ -region conductivity is small the $F$ -region dynamo generates polarization electric fields and is mainly responsible for the nighttime drift variations. In the evening the $F$ -region dynamo is instrumental in generating an enhanced vertical drift, the pre-reversal enhancement. The current due to the $F$ -region dynamo is larger at day than at night, but the $F$ -region dynamo contributes approximately 10–15 % to the total current at day versus approximately 50 % at night (Rishbeth in J. Atmos. Sol.-Terr. Phys. 43(56):387–392, 1981). The $F$ -region dynamo effects strongly depend on the Pedersen conductivity and therefore on the solar cycle. We will review the influence of the $F$ -region dynamo on the ionosphere in general and particularly focus on the role it plays in generating ionospheric currents and magnetic perturbations at low-earth orbiting (LEO) satellite altitudes.

Post-Storm Middle and Low-Latitude Ionospheric Electric Fields Effects

Abstract: The Earth’s upper atmosphere and ionosphere undergoes large and complex perturbations during and after geomagnetic storms. Thermospheric winds driven by enhanced energy and momentum due to geomagnetic activity generate large disturbance electric fields, plasma drifts and currents with a broad range of temporal and spatial scales from high to equatorial latitudes. This disturbance dynamo mechanism plays a fundamental role on the response of the middle and low-latitude ionosphere to geomagnetic activity. In this review, we initially describe the early evidence for the importance of this process and the first simulation study which already was able to explain its main effects on the electrodynamics of the middle and low-latitude ionosphere. We then describe the results of more recent simulations and the extensive experimental work that highlights the importance of this mechanism for ionospheric space weather studies extending to post-storms periods, and present some suggestions for future studies.

The F-Region Gravity and Pressure Gradient Current Systems: A Review

Abstract: The ionospheric gravity and pressure-gradient current systems are most prominent in the low-latitude $F$ -region due to the plasma density enhancement known as the equatorial ionization anomaly (EIA). This enhancement of plasma density which builds up during the day and lasts well into the evening supports a toroidal gravity current which flows eastward around the Earth in the $F$ -region during the daytime and evening, and eventually returns westward through the $E$ -region. The existence of pressure-gradients in the EIA region also gives rise to a poloidal diamagnetic current system, whose flow direction acts to reduce the ambient geomagnetic field inside the plasma. The gravity and pressure-gradient currents are among the weaker ionospheric sources, with current densities of a few $\mbox{nA/m}^{2}$ , however they produce clear signatures of about 5–7 nT in magnetic measurements made by low-Earth orbiting satellites. In this work, we review relevant observational and modeling studies of these two current systems and present new results from a 3D ionospheric electrodynamics model which allows us to visualize the entire flow pattern of these currents throughout the ionosphere as well as calculate their magnetic perturbations.

An application of principal component analysis to the interpretation of ionospheric current systems

Abstract: Ionospheric currents are driven by several different physical processes and exhibit complex spatial and temporal structure. Magnetic field measurements of ionospheric sources are often spatially sparse, causing significant challenges in visualizing current flow at a specific time. Standard methods of fitting equivalent current models to magnetic observations, such as line currents, spherical harmonic analysis, spherical cap harmonic analysis, and spherical elementary current systems (SECS), are often unable to capture the full spatial complexity of the currents, or require a large number of parameters which cannot be fully determined by the available data coverage. These methods rely on a set of generic basis functions which contain limited information about the geometries of the various ionospheric sources. In this study, we develop new basis functions for fitting ground and satellite measurements, which are derived from physics-based ionospheric modeling combined with principal component analysis (PCA). The physics-based modeling provides realistic current flow patterns for all of the primary ionospheric sources, including their daily and seasonal variability. The PCA technique extracts the most relevant spatial geometries of the currents from the model run into a small set of equivalent current modes. We fit these modes to magnetic measurements of the Swarm satellite mission at low and mid-latitudes and compare the resulting model with independent measurements and with the SECS approach. We find that our PCA method accurately reproduces features of the equatorial electrojet and Sq current systems with only 10 modes, and can predict ionospheric fields far from the data region.

Examining the Magnetic Signal Due To Gravity and Plasma Pressure Gradient Current With the TIE‐GCM

Abstract: Accurate magnetic field measurements at ground and Low-Earth Orbit (LEO) are crucial to describe Earth's magnetic field. One of the challenges with processing LEO magnetic field measurements to study Earthâ's magnetic field is that the satellite flies in regions of highly varying ionospheric currents which needs to be characterized accurately. The present study focuses on ionospheric current systems due to gravity and plasma pressure gradient forcing, and aims to provide guidance on the estimation of their magnetic effect at LEO altitudes with the help of numerical modeling. We assess the diamagnetic approximation which estimates the magnetic signal of the plasma pressure gradient current. The simulations indicate that the diamagnetic effect should not be removed from LEO magnetic observations without considering the gravity current effect, as this will lead to an error larger than the magnetic signal of these currents. We introduce and evaluate a method to capture the magnetic effect of the gravity driven current. The diamagnetic and gravity current approximations ignore the magnetic effect from currents set up by the induced electric field. The combined gravity and plasma pressure gradient magnetic effect tends to cancel above the F-region peak, however between approximately 300 km and the peak it exhibits a significant height and latitudinal variation with magnitudes up to 8nT. During solar minimum the combined magnetic signal is less than 1nT above 300 km. In addition to the solar cycle dependence, the magnetic signal strength varies with longitude (approximately by 50%) and season (up to 80%) at solar maximum.

Editorial: Topical Volume on Earth's Magnetic Field - Understanding Geomagnetic Sources from the Earth's Interior and its Environment

Abstract: Topical Volume on Earth's Magnetic Field-Understanding Geomagnetic Sources from the Earth's Interior and Its Environment

Relative contributions of momentum forcing and heating to high‐latitude lower thermospheric winds

Abstract: We discuss the significance of potential vorticity in the thermosphere and quantify the relative contributions of momentum forcing and heating to its total time derivative in the high-latitude lower thermosphere during the southern hemisphere summertime for negative interplanetary magnetic field (IMF) Bz conditions on the basis of numerical simulations. A term analysis of the potential vorticity equation for weak or strong southward IMF (Bz = −2.0 nT or −10.0 nT) gives the following results: the ratios of the momentum forcing term to the heating term at 142, 123, and 111 km altitudes for IMF Bz = −2.0 nT are roughly 6:1, 4:1, and 2:1, respectively, indicating that the momentum forcing term makes the larger contribution to the total time derivative of the potential vorticity, although the relative contribution of the momentum forcing weakens with descending altitude. The ratios of the momentum forcing term to the heating term at 142, 123, and 111 km altitudes for IMF Bz = −10.0 nT are roughly 3:1, 2:1, and 1:1, indicating that, at higher altitudes, the momentum forcing term makes the larger contribution to the total time derivative of the potential vorticity, but the relative contributions of momentum forcing and heating are comparable at lower altitudes. A comparison of the heating term and the momentum forcing term for IMF Bz = −2.0 nT and IMF Bz = −10.0 nT conditions indicates that the heating term increases more significantly than the momentum forcing term as IMF Bz becomes more negative.