D. R. Weimer’s research while affiliated with Virginia Tech and other places

What is this page?


This page lists works of an author who doesn't have a ResearchGate profile or hasn't added the works to their profile yet. It is automatically generated from public (personal) data to further our legitimate goal of comprehensive and accurate scientific recordkeeping. If you are this author and want this page removed, please let us know.

Publications (142)


Substorms and Solar Eclipses: A Mutual Information Based Study
  • Article
  • Full-text available

December 2023

·

67 Reads

S. E. Coyle

·

J. B. H. Baker

·

·

[...]

·

D. R. Weimer

Solar eclipses present a rare glimpse into the impact of ionospheric electrodynamics on the magnetosphere independent of other well studied seasonal influences. Despite decades of study, we still do not have a complete description of the conditions for geomagnetic substorm onset. We present herein a mutual information based study of previously published substorm onsets and the past two decades of eclipses which indicates the likelihood of co‐occurrence is greater than random chance. A plausible interpretation for this relation suggests that the abrupt fluctuations in ionospheric conductivity during an eclipse may influence the magnetospheric preconditions of substorm initiation. While the mechanism remains unclear, this study presents strong evidence of a link between substorm onset and solar eclipses.

Download

Symmetric magnetic fields observed at conjugate locations on 16 November 2017. The blue lines show the magnetic fields measured in the Northern Hemisphere at the sites indicated with the blue labels on the left side, and the red lines show the magnetic fields measured in the Southern Hemisphere at the sites indicated with the red labels on the left side. The signs of the eastward and vertical components have been reversed for the southern sites. Site locations are shown in Table 1.
Explanation for eastward and vertical sign reversals. (a) Northward ΔB underneath a westward electrojet is negative in both hemispheres. (b) Eastward ΔB positioned underneath poleward directed electrojet have opposite signs at the conjugate points. (c) Vertical ΔB underneath downward field alined currents (FAC) also have opposite signs.
Unequal magnetic fields observed at conjugate locations on 3 December 2016. The blue lines show the magnetic fields measured in the Northern Hemisphere at the sites indicated with the blue labels on the left side, and the red lines show the magnetic fields measured in the Southern Hemisphere at the sites indicated with the red labels on the left side. The signs of the eastward and vertical components have been reversed for the southern sites. Site locations are shown in Table 1.
Interplanetary Magnetic Field (IMF) measurements taken on the Advanced Composition Explorer satellite, 3 December 2016. From bottom to top: The Z component of the IMF, drawn in brown (sienna). The Y component of the IMF, colored turquoise. The ‐X component of the solar wind velocity (purple). At the top, the green line shows the propagation delay, in minutes, from the point of measurement to the Earth.
Y and Z components of the Interplanetary Magnetic Field (IMF) and the X (Northward) component of ΔB at four conjugate locations, from 7:00 to 19:00 UT on 3 December 2016. The upper two rows show the Y and Z components of the IMF, colored turquoise and brown respectively, shifted in time to the bow shock using flat‐plane convection delays. (about 78 min). The other four graphs show the Northward component of ΔB at the four most poleward PENGUIn sites (drawn in red) and their Northern counterparts (blue). The thin vertical lines mark three times that are referenced in Figure 6.

+5

Magnetic Field Observations on Interhemispheric Conjugate Chains

September 2023

·

44 Reads

A chain of magnetometers has been placed in Antarctica for comparisons with magnetic field measurements taken in the Northern Hemisphere. The locations were chosen to be on magnetic field lines that connect to magnetometers on the western coast of Greenland, despite the difficulty of reaching and working at such remote locations. We report on some basic comparisons of the similarities and differences in the conjugate measurements. Our results presented here confirm that the conjugate sites do have very similar (symmetric) magnetic perturbations in a handful of cases, as expected. Sign reversals are required for two components in order to obtain this agreement, which is not commonly known. More frequently, a strong Y component of the Interplanetary Magnetic Field (IMF) breaks the symmetry, as well as the unequal conductivities in the opposite hemispheres, as shown in two examples. In one event the IMF Y component reversed signs twice within 2 hours, while the magnetometer chains were approaching local noon. This switch provided an opportunity to observe the effects at the conjugate locations and to measure time lags. It was found that the magnetic fields at the most poleward sites started to respond to the sudden IMF reversals 20 min after the IMF reaches the bow shock, a measure of the time it takes for the electromagnetic signal to travel to the magnetopause, and then along magnetic field lines to the polar ionospheres. An additional 9–14 min is required for the magnetic perturbations to complete their transition.




Testing the electrodynamic method to derive height-integrated ionospheric conductances

January 2021

·

96 Reads

·

9 Citations

We have used empirical models for electric potentials and the magnetic fields both in space and on the ground to obtain maps of the height-integrated Pedersen and Hall ionospheric conductivities at high latitudes. This calculation required use of both “curl-free” and “divergence-free” components of the ionospheric currents, with the former obtained from magnetic fields that are used in a model of the field-aligned currents. The second component is from the equivalent current, usually associated with Hall currents, derived from the ground-level magnetic field. Conductances were calculated for varying combinations of the interplanetary magnetic field (IMF) magnitude and orientation angle, as well as the dipole tilt angle. The results show that reversing the sign of the Y component of the IMF produces substantially different conductivity patterns. The Hall conductivities are largest on the dawn side in the upward, Region 2 field-aligned currents. Low electric field strengths in the Harang discontinuity lead to inconclusive results near midnight. Calculations of the Joule heating, obtained from the electric field and both components of the ionospheric current, are compared with the Poynting flux in space. The maps show some differences, while their integrated totals match to within 1 %. Some of the Poynting flux that enters the polar cap is dissipated as Joule heating within the auroral ovals, where the conductivity is greater.


Testing the Electrodynamic Method to Derive Height-Integrated Ionospheric Conductances

August 2020

·

27 Reads

We have used empirical models for electric potentials and the magnetic fields in both space and on the ground to obtain maps of the height-integrated Pedersen and Hall ionospheric conductivities at high latitudes. This calculation required use of both "curl-free" and "divergence-free" components of the ionospheric currents, with the former obtained from magnetic fields that are used in a model of the field-aligned currents. The second component is from the equivalent current, usually associated with Hall currents, derived from the ground-level magnetic field. Conductances were calculated for varying combinations of the Interplanetary magnetic field (IMF) magnitude and orientation angle, as well as the dipole tilt angle. The results show that reversing the sign of the Y component of the IMF produces substantially different conductivity patterns. The Hall conductivities are largest on the dawn side in the upward, Region 2 field-aligned currents. Low electric field strengths in the Harang discontinuity lead to inconclusive results near midnight. Calculations of the Joule heating, obtained from the electric field and both components of the ionospheric current, are compared with the Poynting flux in space. The maps show some differences, while their integrated totals match to within 1 %. Some of the Poynting flux that enters the polar cap is dissipated as Joule heating within the auroral ovals, where the conductivity is greater.


Atmospheric Escape Processes and Planetary Atmospheric Evolution

July 2020

·

473 Reads

·

123 Citations

The habitability of the surface of any planet is determined by a complex evolution of its interior, surface, and atmosphere. The electromagnetic and particle radiation of stars drive thermal, chemical, and physical alteration of planetary atmospheres, including escape. Many known extrasolar planets experience vastly different stellar environments than those in our solar system: It is crucial to understand the broad range of processes that lead to atmospheric escape and evolution under a wide range of conditions if we are to assess the habitability of worlds around other stars. One problem encountered between the planetary and the astrophysics communities is a lack of common language for describing escape processes. Each community has customary approximations that may be questioned by the other, such as the hypothesis of H‐dominated thermosphere for astrophysicists or the Sun‐like nature of the stars for planetary scientists. Since exoplanets are becoming one of the main targets for the detection of life, a common set of definitions and hypotheses are required. We review the different escape mechanisms proposed for the evolution of planetary and exoplanetary atmospheres. We propose a common definition for the different escape mechanisms, and we show the important parameters to take into account when evaluating the escape at a planet in time. We show that the paradigm of the magnetic field as an atmospheric shield should be changed and that recent work on the history of Xenon in Earth's atmosphere gives an elegant explanation to its enrichment in heavier isotopes: the so‐called Xenon paradox.


Improving Neutral Density Predictions Using Exospheric Temperatures Calculated on a Geodesic, Polyhedral Grid

January 2020

·

242 Reads

·

26 Citations

A new model of exospheric temperatures has been developed, with the objective of predicting global values with greater spatial and temporal accuracy. From these temperatures, the neutral densities in the thermosphere can be calculated, through use of the Naval Research Laboratory Mass Spectrometer and Incoherent Scatter radar Extended (NRLMSISE‐00) model. The exospheric temperature model is derived from measurements of the neutral densities on several satellites. These data were sorted into triangular cells on a geodesic grid, based on location. Prediction equations are derived for each grid cell using least error fits. Several versions of the model equations have been tested, using parameters such as the date, time, solar radiation, and nitric oxide emissions, as measured with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. Accuracy is improved with the addition of the total Poynting flux flowing into the polar regions, from an empirical model that uses the solar wind velocity and interplanetary magnetic field. Given such inputs, the model can produce global maps of the exospheric temperature. These maps show variations in the polar regions that are strongly modulated by the time of day, due to the daily rotation of the magnetic poles. For convenience the new model is referred to with the acronym EXTEMPLAR (EXospheric TEMperatures on a PoLyhedrAl gRid). Neutral densities computed from the EXTEMPLAR‐NRLMSISE‐00 models combined are found to produce very good results when compared with measured values.


A Third Generation Field‐Aligned Current Model

January 2020

·

83 Reads

·

14 Citations

A new empirical model of field‐aligned currents in the Earth's ionosphere has been developed. This model is derived using magnetometer data from the CHAallenging Minisatellite Payload, Ø rsted, and Swarm satellite missions, which has created a database that spans more than 15 years. These data have been associated with solar wind conditions using the Advanced Composition Explorer satellite, as well as the F10.7, S10.7, M10.7, and Y10.7 solar indices. With the wealth of data and associated driving conditions, this model has been developed to reproduce field‐aligned current maps of the ionosphere based on solar wind electric field, interplanetary magnetic field clock angle, dipole tilt angle, solar index, and geographic hemisphere. This model was constructed using a series of spherical cap harmonic analysis fits based on small selections of the overall database. The coefficients of these fits were then used to develop a model that would reproduce these coefficients based on the previously described driving conditions. One of the most notable improvements demonstrated by this model is the ability to show distinct current regions in the ionosphere, particularly with respect to Region 0 currents during northward Bz and highly positive or negative By.


Empirical Modeling of the Geomagnetic Field for GIC Predictions

September 2019

·

35 Reads

·

3 Citations

Empirical models can be used to predict the geomagnetic perturbations that cause Geomagnetic Induced Currents (GIC). Using examples from a model developed at Virginia Tech, the advantages and weaknesses of empirical models are discussed. The primary advantages are low computational cost and high speeds, which makes such models useful for predictions of when and where troublesome GIC may occur. In comparison to other model types, the Virginia Tech model excels in some types of metrics and poorly in others. While the low‐frequency variations are predicted well, the chaotic, high‐frequency fluctuations and substorm perturbations are not. To illustrate these strengths and weaknesses, examples from three events are shown. Two strategies are discussed for improving model performance. One method is to superpose high‐frequency fluctuations that have spectral characteristics appropriate for the given IMF conditions, based on an analysis of the data. Another technique is adding evolving, substorm patterns at appropriate times. While these strategies would improve model skills on indices and metric evaluations, adding the high‐frequency components might not be as useful for exact calculations of GIC within a specific network of conductors. The coherency of rapid field variations over long distances needs to be known.


Citations (58)


... Both groups developed global simulation models for the ionosphere and magnetosphere using advanced numerical methods [106,107]. These models became known as the Lyon-Fedder-Mobarry (LFM) [6,108] and Integrated Space Weather Model (ISM) [109]. ...

Reference:

Adaptive Global Magnetohydrodynamic Simulations
MHD Simulation of Magnetospheric Transport at the Mesoscale
  • Citing Chapter
  • March 2013

... Heelis and Maute (2020), Sarris (2019), and Richmond (2021) have all recently pointed out aspects of energy transfer from the solar wind via Joule heating that remain poorly understood. One of the most significant of these is ionospheric conductivity, which is central to understanding magnetosphere-ionosphere coupling and is a required input for many empirical and numerical models but is, as Weimer and Edwards (2021) have stated, arguably one of the least measured and estimated parameters. More generally, there are overall far fewer estimates of quantities that are central in describing magnetosphere-ionosphere-thermosphere (MIT) coupling in the Southern Hemisphere (SH) relative to the Northern Hemisphere (NH). ...

Testing the electrodynamic method to derive height-integrated ionospheric conductances

... In specific regions of Mercury's magnetosphere, the sodium ion density can be comparable to the proton density and potentially has a significant influence on Mercury's magnetospheric dynamics (Gershman et al., 2014). Since the heavy ions in planetary magnetospheres have a profound impact on atmospheric escape and global magnetospheric dynamics (Gronoff et al., 2020;Kronberg et al., 2014), investigating the behavior of heavy ions in Mercury's magnetosphere can potentially provide insights into the similar dynamics of other planetary magnetospheres. ...

Atmospheric Escape Processes and Planetary Atmospheric Evolution

... The Colaba observatory was also at prenoon, LT = 11, when it observed the extraordinary H depression during the 1859 Carrington storm. It is widely known that IMF B Y causes the LT asymmetry of the dayside FAC distribution (e.g., Burch et al., 1985;Edwards et al., 2020;Weimer, 2001). For the February 1958 and July 1959 storms, however, no IMF measurement is available. ...

A Third Generation Field‐Aligned Current Model

... Variations in neutral density lead to dynamic drag forces on low Earth orbit (LEO) satellites flying through the thermosphere, which in turn causes orbital track changes (Berger et al., 2020). Given that the physics-based general circulation models (GCMs) are generally considered the most powerful tool in understanding the complicated three-dimensional time-dependent behavior of Earth's atmosphere, numerous efforts have been made to improve the thermospheric quantities such as neutral temperature, density, composition, and winds through the assimilation of observations into physics-based GCMs (e.g., Cantrall et al., 2019;Cierpik et al., 2003;Codrescu et al., 2004Codrescu et al., , 2022Lomidze & Scherliess, 2015;Matsuo et al., 2013;Mehta et al., 2019;Mlynczak et al., 2018;Ruan et al., 2018;Sutton, 2018;Weimer et al., 2020). Matsuo et al. (2013) presented an application of ensemble Kalman filtering (EnKF) to a GCM of the thermosphere and ionosphere by ingesting the electron density of COSMIC/FORMOSAT-3 and the neutral mass density of the Challenging Mini-Satellite Payload (CHAMP) satellite, and showed that assimilation of the electron density profiles and the in situ neutral mass density observations into the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) is capable of improving the neutral density specification. ...

Improving Neutral Density Predictions Using Exospheric Temperatures Calculated on a Geodesic, Polyhedral Grid

... the "ionospheric equivalent current function" (Kamide et al., 1981;Richmond and Kamide, 1988). A detailed description of the process is provided by Weimer (2019), which includes the separation of the magnetic effects into their internal and external sources. The magnetic fields are calculated from the gradient of a scalar potential. ...

Derivation of Hemispheric Ionospheric Current Functions From Ground‐Level Magnetic Fields

... The calculation method of anti-ballistic coefficient B is shown in Equation (4). In the formula, C is the damping coefficient of the satellite, A is the effective area of the satellite, and m is the satellite's mass [26,27]: ...

Correlations Between the Thermosphere's Semi-Annual Density Variations and Infrared Emissions Measured with the SABER Instrument

... In the past 30 years covering 22-24th solar cycle, several well-known superstorm events have been investigated on the matter of cause-and-effect from their solar origins to the ionosphere-thermosphere (IT) system. These unique storms have provoked a worldwide interest in the scientific community as they created extremely enhanced electromagnetic fields and particle environments that behave differently than predicted by conventional theory (Fuller-Rowell et al., 2018;Mannucci & Tsurutani, 2018;Riley et al., 2018). Now it is widely recognized that the energy dissipated in the upper atmosphere during a superstorm event is extremely large, which would undoubtedly cause drastic modification in global electrodynamic climatology, thermospheric wind, and neutral composition, leading to great changes in the ionosphere. ...

How Might the Thermosphere and Ionosphere React to an Extreme Space Weather Event?
  • Citing Chapter
  • January 2018

... Contrary to simulation results, satellite observations spanning over a decade suggest that the FAC typically demonstrates a linear increase with IEF (Weimer et al., 2017), even for IEF surpassing levels of 3-6 mV/m, where transpolar potential and FAC saturation are anticipated in previous studies (e.g., McPherron et al., 2018;Siscoe et al., 2004). Consequently, the behavior of the R1 current system under significant IEF diverges notably from theoretical predictions. ...

Linear Response of Field‐Aligned Currents to the Interplanetary Electric Field

... The storm period was close to the September equinox, which also guarantees almost identical illumination by the Sun in the two hemispheres of the Earth. As a result, symmetry between both hemispheres in the ionospheric conductivity and in the Earth's current systems is created (Hartinger et al. 2017;Timoçin et al., 2018). Under these circumstances, we have a simpler physical system for studying. ...

Associating ground magnetometer observations with current or voltage generators: Magnetometer current/voltage generator