2:00pm - 2:15pm
Examining the Current and Magnetic Perturbations at LEO Altitude Due to Plasma Pressure Gradient and Gravity Forces
National Center for Atmospheric Research, United States of America
In the last several decades Low Earth Orbit (LEO) satellites have provided extensive magnetic measurements to advance our understanding of the complex ionospheric current system. Interpreting the measured magnetic field and its associated current is still challenging since the measured magnetic field reflects the influence of various current sources. At high latitude ionospheric currents are mainly driven by the magnetosphere-ionosphere coupling through ion convection and field-aligned current. At mid- and low latitude the collisional interaction between thermospheric winds and ions is important and sets up electric fields and currents. Smaller low latitude current is produced by gravity and plasma pressure gradient forces. Although the latter currents are weak they can produce persistent 5-7 nT perturbations at LEO altitudes in regions of enhanced plasma as in the equatorial anomaly region, and its effect can last into the evening. The effect of the plasma pressure gradient force on the magnetic perturbations can be estimated with the diamagnetic approximation but the gravity force contribution is difficult to estimate. Knowledge about the characteristics of these currents and their associated magnetic perturbation is important for studies analyzing day and evening low latitude LEO magnetic perturbations.
In this presentation we will introduce a numerical model capable of calculating the 3D ionospheric current system and its associated magnetic perturbation at LEO altitude. We apply this model to examine the current system and the magnetic perturbations associated with gravity and plasma pressure gradient forces with respect to local time, season and solar cycle. We will evaluate the diamagnetic approximation by comparing with the magnetic perturbation derived from considering the simulated 3D current system. We will discuss the value of the simulation results with respect to studies based on LEO magnetic field measurements.
2:15pm - 2:30pm
Separating Earth's Internal and External Field in SWARM Observations with the Help of Global Magnetosphere - Ionosphere - Thermosphere Simulations
University of New Hampshire, United States of America
We use Open Geospace General Circulation Model (OpenGGCM) simulations to predict magnetic field perturbations at Low Earth Orbiting (LEO) satellites such as SWARM, at high latitudes. The simulations allow us to separate three different major contributions to the observed perturbations, i.e., the perturbations caused by currents in the outer magnetosphere, field-aligned currents (FACs), and the currents flowing in the ionosphere. We find that at an altitude of 500 km the strongest contribution comes from FACs, followed by the perturbations caused by the ionospheric currents, while the magnetospheric currents make only a minor contribution. The high latitude perturbations do not average out over extended quiet time periods. There are significant variations in the patterns; however, on a large scale, the basic shape of the pattern remains stable. Thus, without explicitly removing the perturbations from the data, any spherical harmonics fit is expected to incur a bias. Although the predicted OpenGGCM perturbations do not compare particularly well with SWARM data, the simulations reproduce the overall pattern. However, they may still be useful to reduce the bias of the ensemble and produce better global spherical harmonic fits, by producing an ensemble whose external field contributions average out. Since this paper only scratches the surface of the role that models of the external field can play in producing unbiased internal field models, much progress is still possible, for example by improving the external model, investigating larger ensembles, and by considering data from geomagnetically disturbed times.
2:30pm - 2:45pm
An Analysis of Ionospheric Versus Oceanic Tidal Magnetic Signals
1CIRES/University of Colorado Boulder, United States of America; 2High Altitude Observatory, National Center for Atmospheric Research, United States of America; 3Institute of Geophysics, ETH Zürich, Switzerland
Motional induction in the ocean by lunar tides has long been observed by both land and satellite measurements of magnetic fields. Recent progress has been made using satellite-detected oceanic tidal magnetic signals to perform inversions for lithospheric and upper mantle conductivity (Grayver et al., 2016; Schnepf et al., 2015). The main benefits of using tidal signals for electromagnetic (EM) sensing are 1) that the source galvanically interacts with the underlying lithosphere and mantle (opposed to being inductively coupled, as is the case in traditional magnetotelluric studies), and 2) because the global tidal signal is a grid of sources, a small number of frequency components are needed for probing conductivity. In fact, using only one tidal mode (i.e. one frequency) can still provide a good picture of oceanic lithospheric and upper mantle conductivity.
The use of M2 magnetic signals for regional 3-D inversions of conductivity calls for a more accurate separation of ocean and ionospheric signals for two reasons. First, the persistence of weak ionospheric M2 signals in the nighttime magnetic data is measurable. Second, limiting the analysis to nighttime data when jointly inverting with submarine cable voltage data cannot separate the ionospheric and ocean signals. To better constrain this error, we have conducted a global analysis of the tidal signals at geomagnetic observatories and compared the observations with predictions by physics-based models of the ionospheric and oceanic M2 signals. Our study focuses on the recent deep solar minimum (May 28-August 28, 2009), when the magnetic disturbance was minimum.
Our global analysis used hourly data from 63 non-polar geomagnetic observatories. We directly fit for the M2 amplitudes using the robust least-squares method of Schnepf et al. (2014) for the northward (X), eastward (Y), downward (Z) and total field (F) components.
Studies that have used tidal EM signals for EM sensing relied on the downward component (eg., Grayver et al., 2016) and our results suggest that the ionospheric M2 component is weakest for this component—in fact, across ocean regions this predicted signal is generally under 0.1 nT. However, the horizontal signal is much larger. The northward component of the signal approaches 2 nT near the equatorial electrojet and polar electrojet, whereas the eastward component is broadly strong in the summer hemisphere with its largest amplitudes near the northern geomagnetic pole.
The results of our 1x1 degree forward modeling agree well with the observed estimated signals. Using data from 64 stations, we determined the chi-squared value for 63 degrees of freedom and found that for the total scalar field it is 37, whereas for the downward component it is 39, for the northward component it is 17, and for the eastward component it is 9. The agreement between the physics-based model predictions and the observations is very encouraging for EM sensing applications as we will be able to separate ionospheric and oceanic lunar magnetic signals.
2:45pm - 3:00pm
The lithospheric Magnetic Field Measured by the Swarm Satellite Constellation
1Laboratoire de Planétologie et de Géodynamique de Nantes, France; 2Institut de Physique du Globe de Paris, France
The Swarm constellation of satellites was launched in November 2013 and has since then delivered high quality scalar and vector magnetic field measurements. A consortium of several research institutions was selected by the European Space Agency (ESA) to provide a number of scientific products which are made available to the scientific community. Models describing the lithospheric magnetic field are now produced on a yearly basis thanks to a continuous effort in improving the software design and taking explicit advantage of the Swarm satellite orbits and configuration.
In this presentation, we will report on the activities carried out to analyze and process the magnetic field measurements and to assess the third update of the lithospheric magnetic field model. Current limitations and difficulties with respect to source field separation and signal to noise ratio will be discussed particularly in view of the possible evolutions of the Swarm constellation geometry and orbit in the forthcoming years.
3:00pm - 3:15pm
Hemispherical Differences in the Ionospheric Current System as Seen by Swarm and CHAMP
1University in Bergen, Birkeland Centre for Space Science, Norway; 2DTU Space, Technical University of Denmark, Kongens Lyngby, Denmark
We present an empirical model of the global ionospheric current system, with emphasis on the polar regions. The model is based on magnetic field measurements from the Swarm and CHAMP spacecraft, after removal of a model main magnetic field. The magnetic field measurements are used to fit spherical harmonic potentials associated with Birkeland currents and horizontal equivalent ionospheric currents. The combination of these potentials can be used to calculate the true horizontal current, without any assumption about ionospheric conductivity. We parametrize the spherical harmonic coefficients in terms of sunlight conditions, solar wind speed, the F10.7 index, and the interplanetary magnetic field. Global spherical harmonics are used, so that the currents in both hemispheres are modeled simultaneously. The differences in main magnetic field between hemispheres are taken into account by use of magnetic apex coordinates. Comparisons between hemispheres can thus be made approximately independent of hemispheric, longitudinal, and temporal variations in the Earth's main magnetic field. We use the model to challenge previous observations that more currents flow through the Northern hemisphere.
3:15pm - 3:30pm
Identifying Intervals of Spatio-Temporally Invariant Field-Aligned Currents from Swarm: Assessing the Validity of Single Spacecraft Methods
1UCL Mullard Space Science Laboratory, United Kingdom; 2University of Alberta, Canada
Field-aligned currents (FACs) are a fundamental component of coupled solar-wind-magnetosphere-ionosphere systems. By assuming that FACs can be approximated by spatially and temporally invariant infinite current sheets, single-spacecraft magnetic field measurements can be used to estimate the currents flowing in space. By combining data from multiple spacecraft on similar orbits, these stationarity assumptions can be tested. In this Technical Report, we present a new technique that combines cross-correlation and linear fitting of multiple spacecraft measurements to determine the reliability of the FAC estimates. We show that this technique can identify those intervals in which the currents estimated from single spacecraft techniques are both well-correlated and have similar amplitudes, thus meeting the spatial and temporal stationarity requirements. Using data from ESA’s Swarm mission from 2014 and 2015, we show that larger scale currents (>450 km) are well correlated and have a one-to-one fit up to 50% of the time, whereas small scale (<50 km) currents show similar amplitudes only ~1% of the time despite there being a good correlation 18% of the time. It is thus imperative to examine both the correlation and amplitude of the calculated FACs in order to assess both the validity of the underlying assumptions and hence ultimately the reliability of such single spacecraft FAC estimates.