Browsing by Author "Weimer, Daniel R."

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    Anomalously low geomagnetic energy inputs during 2008 solar minimum
    Deng, Yue; Huang, Yanshi; Solomon, Stan; Qian, Liying; Knipp, Delores; Weimer, Daniel R.; Wang, Jing-Song (American Geophysical Union, 2012-09-01)
    The record-low thermospheric density during the last solar minimum has been reported and it has been mainly explained as the consequence of the anomalously low solar extreme ultraviolet (EUV) irradiance. In this study, we examined the variation of the energy budget to the Earth's upper atmosphere during last solar cycle from both solar EUV irradiance and geomagnetic energy, including Joule heating and particle precipitation. The globally integrated solar EUV power was calculated from the EUV flux model for aeronomic calculations (EUVAC) driven by the MgII index. The annal average of solar power in 2008 was 33 GW lower than that in 1996. The decrease of the globally integrated geomagnetic energy from 1996 to 2008 was close to 29 GW including 13 GW for Joule heating from Weimer (2005b) and 16 GW for particle precipitation from NOAA Polar-Orbiting Environmental Satellites (POES) measurements. Although the estimate of the solar EUV power and geomagnetic energy vary from model to model, the reduction of the geomagnetic energy was comparable to the solar EUV power. The Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM) simulations indicate that the solar irradiance and geomagnetic energy variations account for 3/4 and 1/4 of the total neutral density decrease in 2008, respectively.
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    Comparison of a Neutral Density Model With the SET HASDM Density Database
    Weimer, Daniel R.; Tobiska, W. Kent; Mehta, Piyush M.; Licata, R. J.; Drob, Douglas P.; Yoshii, Jean (American Geophysical Union, 2021-12)
    The EXospheric TEMperatures on a PoLyhedrAl gRid (EXTEMPLAR) method predicts the neutral densities in the thermosphere. The performance of this model has been evaluated through a comparison with the Air Force High Accuracy Satellite Drag Model (HASDM). The Space Environment Technologies (SET) HASDM database that was used for this test spans the 20 years 2000 through 2019, containing densities at 3 hr time intervals at 25 km altitude steps, and a spatial resolution of 10 degrees latitude by 15 degrees longitude. The upgraded EXTEMPLAR that was tested uses the newer Naval Research Laboratory MSIS 2.0 model to convert global exospheric temperature values to neutral density as a function of altitude. The revision also incorporated time delays that varied as a function of location, between the total Poynting flux in the polar regions and the exospheric temperature response. The density values from both models were integrated on spherical shells at altitudes ranging from 200 to 800 km. These sums were compared as a function of time. The results show an excellent agreement at temporal scales ranging from hours to years. The EXTEMPLAR model performs best at altitudes of 400 km and above, where geomagnetic storms produce the largest relative changes in neutral density. In addition to providing an effective method to compare models that have very different spatial resolutions, the use of density totals at various altitudes presents a useful illustration of how the thermosphere behaves at different altitudes, on time scales ranging from hours to complete solar cycles.
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    Conjugate observations of electromagnetic ion cyclotron waves associated with traveling convection vortex events
    Kim, Hyomin; Clauer, C. Robert; Gerrard, Andrew J.; Engebretson, Mark J.; Hartinger, Michael D.; Lessard, Marc R.; Matzka, Juergen; Sibeck, David G.; Singer, Howard J.; Stolle, Claudia; Weimer, Daniel R.; Xu, Zhonghua (2017-07)
    We report on simultaneous observations of electromagnetic ion cyclotron (EMIC) waves associated with traveling convection vortex (TCV) events caused by transient solar wind dynamic pressure (P-d) impulse events. The Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft located near the magnetopause observed radial fluctuations of the magnetopause, and the GOES spacecraft measured sudden compressions of the magnetosphere in response to sudden increases in Pd. During the transient events, EMIC waves were observed by interhemispheric conjugate ground-based magnetometer arrays as well as the GOES spacecraft. The spectral structures of the waves appear to be well correlated with the fluctuating motion of the magnetopause, showing compression-associated wave generation. In addition, the wave features are remarkably similar in conjugate hemispheres in terms of bandwidth, quasiperiodic wave power modulation, and polarization. Proton precipitation was also observed by the DMSP spacecraft during the wave events, from which the wave source region is estimated to be 72 degrees-74 degrees in magnetic latitude, consistent with the TCV center. The confluence of space-borne and ground instruments including the interhemispheric, high-latitude, fluxgate/induction coil magnetometer array allows us to constrain the EMIC source region while also confirming the relationship between EMIC waves and the TCV current system.
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    Correlations Between the Thermosphere's Semiannual Density Variations and Infrared Emissions Measured With the SABER Instrument
    Weimer, Daniel R.; Mlynczak, M. G.; Emmert, J. T.; Doornbos, E.; Sutton, E. K.; Hunt, L. A. (2018-10)
    This paper presents measurements of the amplitudes and timings of the combined, annual, and semiannual variations of thermospheric neutral density, and a comparison of these density variations with measurements of the infrared emissions from carbon dioxide and nitric oxide in the thermosphere. The density values were obtained from measurements of the atmospheric drag experienced by the Challenging Minisatellite Payload, Gravity Recovery and Climate ExperimentA, Gravity field and Ocean Circulation Explorer, and three Swarm satellites, while the optical emissions were measured with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. These data span a time period of 16years. A database containing global average densities that were derived from the orbits of about 5,000 objects (Emmert, 2009, https://doi.org/10.1029/2009JA014102, 2015b, https://doi.org/10.1002/2015JA021047) was employed for calibrating these density data. A comparison with the NRLMSISE-00 model was used to derive measurements of how much the density changes over time due to these seasonal variations. It is found that the seasonal density oscillations have significant variations in amplitude and timing. In order to test the practicality of using optical emissions as a monitoring tool, the SABER data were fit to the measured variations. Even the most simple fit that used only filtered carbon dioxide emissions had good correlations with the measured oscillations. However, the density oscillations were also well predicted by a simple Fourier series, contrary to original expectations. Nevertheless, measurements of the optical emissions from the thermosphere are expected to have a role in future understanding and prediction of the semiannual variations. Plain Language Summary The uppermost atmosphere, known as the thermosphere, undergoes oscillations in the density of the neutral atoms and molecules, producing two peaks and valleys in the density in each year. The timing of of these "semiannual" variations or oscillations, as well as their amplitudes, tends to vary. Their unpredictability makes it harder to accurately model the amount of drag experienced by orbiting satellites. It had been noticed that the infrared light emitted by carbon dioxide molecules in the thermosphere has a tendency to follow the semiannual oscillations. Such emissions have been measured by an instrument on a NASA satellite for the past 16years. We have compared these emissions with the variations in the semiannual oscillations that were derived from measurements of the drag seen by six different satellites flown by both NASA and the European Space Agency during the same time period, though not at the same time. The results of the comparison show how well the infrared emissions match the density oscillations, due to changes in both the composition and temperature of the thermosphere that influence both. Results show that further study will be needed to be able to accurately predict the density oscillations.
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    Derivation of Hemispheric Ionospheric Current Functions From Ground-Level Magnetic Fields
    Weimer, Daniel R. (2019-04)
    The horizontal currents in the high-latitude ionosphere are the primary driver of the magnetic field perturbations that are observed at the surface of the Earth. These currents and their ground effects are an important aspect of the magnetosphere-ionosphere coupling process. This paper discusses the method of inversion that uses spherical harmonic potential function, in which magnetic field measurements on the ground can be used to derive maps of the "ionospheric equivalent currents," a mathematical representation of the horizontal currents flowing on a thin shell. It is shown that the use of both internal telluric and external current sources is required when fitting the spherical harmonic series; otherwise, the ionospheric currents will be overestimated. Furthermore, the inversion needs to compensate for magnetic effects of the magnetospheric ring current; otherwise, this current is projected onto the ionosphere. The amplification of the surface horizontal magnetic field and the suppression of the vertical magnetic field are demonstrated. The equivalent currents may be useful for estimating the ionospheric conductivity values. Additionally, these currents can be compared with the results from simulation models as a means of validation. Plain Language Summary Currents in the high-latitude ionosphere produce changes in the magnetic field at the surface of the Earth. This paper discusses a technique that uses measurements of these changes in the magnetic field to solve the problem of deriving maps of the currents flowing in the ionosphere. While the first description of this method dates back to the 1940s, this obscure practice can now be more useful with the more recent availability of globally distributed magnetic field measurements. The details of this particular "inversion" technique are described. It is shown that for greatest accuracy, the mirror image currents that occur beneath the Earth's surface need to be considered, as well as the currents that are actually located in the magnetosphere, far above the ionosphere. This result is useful in the study of the interaction between the solar wind and the Earth's magnetosphere, and the resulting currents.
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    Empirical Ionospheric Models: The Road To Conductivity
    Edwards, Thomas Raymond (Virginia Tech, 2019-04-15)
    The Earth's polar ionosphere is a highly dynamic region of the upper atmosphere, and acts as the closure of the greater magnetospheric current system. This region plays host to many electrodynamic effects that impact terrestrial systems, such as power grids, railroads, and pipelines. These effects are fundamentally related to the currents, electric fields, and conductivity present in the polar ionosphere. Understanding and predicting the electrodynamics of this region is vital to being able to determine the physical impacts on terrestrial systems and provide predictions to government and commercial entities. Empirical models play a key role in the research and forecasting of the solar wind and interplanetary magnetic field's impact on the polar ionosphere, and is an active area of development and research. Recent interest in polar ionospheric conductivity has led to a community-wide campaign to develop our understanding of this portion of the electrodynamic system. Characterizing the interactions between the solar wind and the polar ionosphere is a difficult task, as the region of interest is highly data starved in many respects. In particular, satellite based data products are scarce due to being costly and logistically difficult. Recent advancements in data sources (such as the Swarm and CHAMP satellite missions) as well as continued research into the physical relationships between solar wind and interplanetary magnetic field drivers have provided the opportunity to develop new, novel tools to study this region of interest. In this dissertation, two polar ionosphere models are described in Chapters 3 and 4, along with the original research that their construction has produced in Chapter 1. These two models are combined to provide a foundation for future research in this area, which is described in Chapter 5.
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    An empirical model of ground-level geomagnetic perturbations
    Weimer, Daniel R. (American Geophysical Union, 2013-03-01)
    A new empirical model for predicting ground-level geomagnetic perturbations has been developed. This model is based on global measurements of the magnetic field at multiple stations in the Northern Hemisphere collected over an 8 year period, along with the simultaneous measurements of the interplanetary magnetic field (IMF). Variations in ionospheric conductivity are implicitly contained in the measurements used in the model's construction, including the solar F-10.7 index. Provided with the IMF, solar wind velocity, dipole tilt angle (for season), and F-10.7 index, this model computes all three vector components of the magnetic perturbations at specified locations. The model results are consistent with the corresponding maps of the ionospheric electric potential. Interestingly, maps of the vertical component have patterns that resemble maps of the overhead, ionospheric field-aligned currents. Comparisons of model calculations with measurements at different locations show very good results, particularly at low frequencies. There are random variations at higher frequencies that are not reproduced well with the model, but they tend to occur in proportion to the predicted levels. This model could be useful for providing regional forecasts of geomagnetic activity with an approximately 1 h lead time. Citation: Weimer, D. R. (2013), An empirical model of ground-level geomagnetic perturbations, Space Weather, 11, 107-120, doi:10.1002/swe.20030
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    Global Variations in the Time Delays Between Polar Ionospheric Heating and the Neutral Density Response
    Weimer, Daniel R.; Mehta, Piyush M.; Licata, R. J.; Tobiska, W. K. (American Geophysical Union, 2023-04)
    We present results from a study of the time lags between changes in the energy flow into the polar regions and the response of the thermosphere to the heating. Measurements of the neutral density from the Challenging Mini-satellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) missions are used, along with calculations of the total Poynting flux entering the poles. During two major geomagnetic storms in 2003, these data show increased densities are first seen on the dayside edge of the auroral ovals after a surge in the energy input. At lower latitudes, the densities reach their peak values on the dayside earlier than on the night side. A puzzling response seen in the CHAMP measurements during the November 2003 storm was that the density at a fixed location near the "Harang discontinuity" remained at unusually low levels during three sequential orbit passes, while elsewhere the density increased. The entire database of measurements from the CHAMP and GRACE missions were used to derive maps of the density time lags across the globe. The maps show a large gradient between short and long time delays between 60 degrees and 30 degrees geographic latitude. They confirm the findings from the two storm periods, that near the equator, the density on the dayside responds earlier than on the nightside. The time lags are longest near 18-20 hr local time. The time lag maps could be applied to improve the accuracy of empirical thermosphere models, and developers of numerical models may find these results useful for comparisons with their calculations. The interaction of the solar wind with the Earth's magnetosphere causes varying levels of heating in the ionosphere. This heating is produced by auroral currents at high latitudes, which in tur n causes the density of the upper atmosphere to change. A topic of importance is to determine how rapidly the density can increase at different locations around the globe following a surge in the heating, which can be calculated from measurements of the solar wind velocity and embedded magnetic fields. This study used measurements of the atmospheric density on two satellite missions known as Challenging Mini-satellite Payload and Gravity Recovery and Climate Experiment. The results show that the density increases first near the poles, and much longer at lower latitudes, as expected. The time lags between changes in the energy input and the density response have been determined for the first time on a global scale. Maps of the time lags are derived. Near the equator the lags are shorter near local noon, and longer before local midnight. The time lag maps can used to improve empirical and numerical models of the thermosphere. More accurate models are needed for more precise predictions of the drag that satellites will encounter, and the subsequent changes in their orbits.
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    High correlations between temperature and nitric oxide in the thermosphere
    Weimer, Daniel R.; Mlynczak, M. G.; Hunt, L. A.; Tobiska, W. K. (American Geophysical Union, 2015-07-01)
    Obtaining accurate predictions of the neutral density in the thermosphere has been a long-standing problem. During geomagnetic storms the auroral heating in the polar ionospheres quickly raises the temperature of the thermosphere, resulting in higher neutral densities that exert a greater drag force on objects in low Earth orbit. Rapid increases and decreases in the temperature and density may occur within a couple days. A key parameter in the thermosphere is the total amount of nitric oxide (NO). The production of NO is accelerated by the auroral heating, and since NO is an efficient radiator of thermal energy, higher concentrations of this molecule accelerate the rate at which the thermosphere cools. This paper describes an improved technique that calculates changes in the global temperature of the thermosphere. Starting from an empirical model of the Poynting flux into the ionosphere, a set of differential equations derives the minimum, global value of the exospheric temperature, which can be used in a neutral density model to calculate the global values. The relative variations in NO content are used to obtain more accurate cooling rates. Comparisons with the global rate of NO emissions that are measured with the Sounding of the Atmosphere using Broadband Emission Radiometry instrument show that there is very good agreement with the predicted values. The NO emissions correlate highly with the total auroral heating that has been integrated over time. We also show that the NO emissions are highly correlated with thermospheric temperature, as well as indices of solar extreme ultraviolet radiation.
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    Improved Neutral Density Predictions Through Machine Learning Enabled Exospheric Temperature Model
    Licata, Richard J.; Mehta, Piyush M.; Weimer, Daniel R.; Tobiska, W. Kent (American Geophysical Union, 2021-12)
    The community has leveraged satellite accelerometer data sets in previous years to estimate neutral mass density and exospheric temperatures. We utilize derived temperature data and optimize a nonlinear machine-learned (ML) regression model to improve upon the performance of the linear EXospheric TEMPeratures on a PoLyhedrAl gRid (EXTEMPLAR) model. The newly developed EXTEMPLAR-ML model allows for exospheric temperature predictions at any location with one model and provides performance improvements over its predecessor. We achieve reductions in mean absolute error of 2 K on an independent test set while providing similar error standard deviation values. Comparing the performance of both EXTEMPLAR models and the Naval Research Laboratory Mass Spectrometer and Incoherent Scatter radar Extended model (NRLMSISE-00) across different solar and geomagnetic activity levels shows that EXTEMPLAR-ML has the lowest mean absolute error across 80% of conditions tested. A study for spatial errors demonstrated that at all grid locations, EXTEMPLAR-ML has the lowest mean absolute error for over 60% of the polyhedral grid cells on the test set. Like EXTEMPLAR, our model's outputs can be utilized by NRLMSISE-00 (exclusively) to more closely match satellite accelerometer-derived densities. We conducted 10 case studies where we compare the accelerometer-derived temperature and density estimates from four satellites to NRLMSISE-00, EXTEMPLAR, and EXTEMPALR-ML during major storm periods. These comparisons show that EXTEMPLAR-ML generally has the best performance of the three models during storms. We use principal component analysis on EXTEMPLAR-ML outputs to verify the physical response of the model to its drivers.
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    Improving Neutral Density Predictions Using Exospheric Temperatures Calculated on a Geodesic, Polyhedral Grid
    Weimer, Daniel R.; Mehta, P. M.; Tobiska, W. K.; Doornbos, E.; Mlynczak, M. G.; Drob, Douglas P.; Emmert, J. T. (2019-12-10)
    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.
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    Interhemispheric Asymmetries in the Ground Magnetic Response to Interplanetary Shocks: The Role of Shock Impact Angle
    Xu, Z.; Hartinger, Michael D.; Oliveira, Denny M.; Coyle, Shane; Clauer, C. Robert; Weimer, Daniel R.; Edwards, T. R. (2020-03)
    Interplanetary (IP) shocks drive magnetosphere-ionosphere (MI) current systems that in turn are associated with ground magnetic perturbations. Recent work has shown that IP shock impact angle plays a significant role in controlling the subsequent geomagnetic activity and magnetic perturbations; for example, highly inclined shocks drive asymmetric MI responses due to interhemispherical asymmetric magnetospheric compressions, while almost head-on shocks drive more symmetric MI responses. However, there are few observations confirming that inclined shocks drive such asymmetries in the high-latitude ground magnetic response. We use data from a chain of Antarctic magnetometers, combined with magnetically conjugate stations on the west coast of Greenland, to test these model predictions (Oliveira & Raeder, 2015, https://doi.org/10.1002/2015JA021147; Oliveira, 2017, https://doi.org/10.1007/s13538-016-0472-x). We calculate the time derivative of the magnetic field (partial derivative B/partial derivative t) in each hemisphere separately. Next, we examine the ratio of Northern to Southern Hemisphere partial derivative B/partial derivative t intensities and the time differences between the maximum. partial derivative B/partial derivative t immediately following the impact of IP shocks. We order these results according to shock impact angles obtained from a recently published database with over 500 events and discuss how shock impact angles affect north-south hemisphere asymmetries in the ground magnetic response. We find that the hemisphere the shock strikes first usually has (1) the first response in partial derivative B/partial derivative t and (2) the most intense response in partial derivative B/partial derivative t. Additionally, we show that highly inclined shocks can generate high-latitude ground magnetic responses that differ significantly from predictions based on models that assume symmetric driving conditions.
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    Low latitude thermospheric responses to magnetic storms
    Earle, Gregory D.; Davidson, R. L.; Heelis, R. A.; Coley, W. R.; Weimer, Daniel R.; Makela, J. J.; Fisher, D. J.; Gerrard, Andrew J.; Meriwether, J. (American Geophysical Union, 2013-06-01)
    Thermospheric density and neutral velocity perturbations associated with three magnetic storms in the autumn season of 2011 are examined using data from the neutral wind meter (NWM) on the Communication/Navigation Outage Forecast System (C/NOFS) satellite. The data from perigee passes near 400km altitude show marked increases in neutral density during the storms and associated increases in horizontal neutral flow speeds. These thermospheric responses are characterized by enhanced meridional neutral flows with peak perturbation amplitudes near 100m/s and relative neutral density enhancements ranging from 50-100%. The increases in the neutral density and meridional flow velocity at equatorial latitudes occur about 5-7h after the initial perturbations are observed in the z component of the interplanetary magnetic field (IMF), and they persist for 20-30h. The perturbations in the neutral density are in good agreement with temperature increases predicted by an empirical model that has been validated using data from the CHAMP and Gravity Recovery and Climate Experiment missions, with a maximum lag time of similar to 1-1.5h between the model temperature increases and the observed density perturbations. The model temperatures are in excellent agreement with ground-based low-latitude temperature measurements during the storms. Ground-based wind measurements during one of the storms provide additional data for comparison with the perturbation wind amplitudes measured aboard the satellite.
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    MSIS-UQ: Calibrated and Enhanced NRLMSIS 2.0 Model With Uncertainty Quantification
    Licata, Richard J.; Mehta, Piyush M.; Weimer, Daniel R.; Tobiska, W. Kent; Yoshii, Jean (American Geophysical Union, 2022-11)
    The Mass Spectrometer and Incoherent Scatter radar (MSIS) model family has been developed and improved since the early 1970's. The most recent version of MSIS is the Naval Research Laboratory (NRL) MSIS 2.0 empirical atmospheric model. NRLMSIS 2.0 provides species density, mass density, and temperature estimates as function of location and space weather conditions. MSIS models have long been a popular choice of thermosphere model in the research and operations community alike, but-like many models-does not provide uncertainty estimates. In this work, we develop an exospheric temperature model based in machine learning that can be used with NRLMSIS 2.0 to calibrate it relative to high-fidelity satellite density estimates directly through the exospheric temperature parameter. Instead of providing point estimates, our model (called MSIS-UQ) outputs a distribution which is assessed using a metric called the calibration error score. We show that MSIS-UQ debiases NRLMSIS 2.0 resulting in reduced differences between model and satellite density of 25% and is 11% closer to satellite density than the Space Force's High Accuracy Satellite Drag Model. We also show the model's uncertainty estimation capabilities by generating altitude profiles for species density, mass density, and temperature. This explicitly demonstrates how exospheric temperature probabilities affect density and temperature profiles within NRLMSIS 2.0. Another study displays improved post-storm overcooling capabilities relative to NRLMSIS 2.0 alone, enhancing the phenomena that it can capture.
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    Testing the electrodynamic method to derive height-integrated ionospheric conductances
    Weimer, Daniel R.; Edwards, Thom (2021-01-14)
    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 "divergencefree" 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.
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    Wavelet Based ULF Pulsation Index for Studying Conjugate ULF Pulsation at High Latitudes and Its Applications to Space Weather
    Xu, Zhonghua; Kim, H.; Clauer, C. Robert; Weimer, Daniel R.; Deshpande, K. (2016-12-14)
    A wavelet-based index is described in this study and applied to present geomagnetic Ultra Low Frequency (ULF) pulsations observed in Antarctica and their magnetically conjugate locations in West Greenland. The index is effective for identification of pulsation events in the Pc4-5 frequency range, which is related to the Geomagnetically Induced Currents (GICs) shown by many researchers, and measures important characteristics of ULF pulsations in both the temporal and frequency domains. We discuss how the wavelet indices can be used to monitor geomagnetic pulsations in both hemispheres simultaneously. The wavelet analysis shows valuable information for GIC- related studies, including the spectrum, correlation, and magnitude of the geomagnetic pulsations. The comparison between conjugate locations reveals the similarities and differences of ULF pulsations in both hemispheres. Also, since the Greenland chains are located near the coastal area, while the Antarctic chains are over thousands meters of the ice-sheet on the East Antarctic plateau, inter-hemispheric comparisons of vertical magnetic field perturbations can be used to reveal how sensitive ULF pulsations are to ground conductivity.