Solar wind and seasonal influence on ionospheric currents from Swarm and CHAMP measurements

We present a new climatological model of the ionospheric current system, determined from magnetic measurements taken by the Challenging Minisatellite Payload (CHAMP) and Swarm satellites. The model describes the horizontal currents in the ionosphere, below the satellites, and the field-aligned (Birkeland) currents that connect the ionosphere with the magnetosphere. The model provides ionospheric current values at any location as continuous functions of solar wind speed, interplanetary magnetic field, dipole tilt angle, and the F10.7 index of solar flux. Geometric distortions due to variations in the Earth’s main magnetic field are taken into account, thus allowing for precise comparisons between the two hemispheres. The model is the first of its kind to describe the full 3-D electric currents and not only the field-aligned or the equivalent horizontal current. We use this capability to demonstrate a key difference between seasons: During winter, the total horizontal current is almost entirely confined to the auroral oval, for all interplanetary magnetic field orientations, where it connects upward and downward Birkeland currents. During more sunlit conditions, the horizontal current extends beyond the auroral oval and is a sum of currents connecting Birkeland currents and currents that circulate in the ionosphere. The westward electrojet is the only large-scale current structure that is persistent across seasons. Comparison with average convection maps suggests that it is comprised largely of Hall currents, which connect to Birkeland currents in the winter but not in summer.publishedVersio

The Relationship Between Cusp Region Ion Outflows and East-West Magnetic Field Fluctuations at 4,000-km Altitude

A number of interdependent conditions and processes contribute to ionospheric-origin energetic ( 10 eV to several keV) ion outflows. Due to these interdependences and the associated observational challenges, energetic ion outflows remain a poorly understood facet of atmosphere-ionosphere-magnetosphere coupling. Here we demonstrate the relationship between east-west magnetic field fluctuations ( ) and energetic outflows in the magnetosphere-ionosphere transition region. We use dayside cusp region FAST satellite observations made near apogee ( 4,180-km altitude) near fall equinox and solstices in both hemispheres to derive statistical relationships between ion upflow and spectral power as a function of spacecraft frame frequency bands between 0 and 4 Hz. Identification of ionospheric-origin energetic ion upflows is automated, and the spectral power in each frequency band is obtained via integration of power spectral density. Derived relationships are of the form for upward ion flux at 130-km altitude, with the mapped upward ion flux for a nominal spectral power nT . The highest correlation coefficients are obtained for spacecraft frame frequencies 0.1–0.5 Hz. Summer solstice and fall equinox observations yield power law indices 0.9–1.3 and correlation coefficients , while winter solstice observations yield 0.4–0.8 with . Mass spectrometer observations reveal that the oxygen/hydrogen ion composition ratio near summer solstice is much greater than the corresponding ratio near winter. These results reinforce the importance of ion composition in outflow models. If observed perturbations result from Doppler-shifted wave structures with near-zero frequencies, we show that spacecraft frame frequencies 0.1–0.5 Hz correspond to perpendicular spatial scales of several to tens of kilometers

Dynamic effects of restoring footpoint symmetry on closed magnetic field lines

Here we present an event where simultaneous global imaging of the aurora from both hemispheres reveals a large longitudinal shift of the nightside aurora of about 3 h, being the largest relative shift reported on from conjugate auroral imaging. This is interpreted as evidence of closed field lines having very asymmetric footpoints associated with the persistent positive y component of the interplanetary magnetic field before and during the event. At the same time, the Super Dual Auroral Radar Network observes the ionospheric nightside convection throat region in both hemispheres. The radar data indicate faster convection toward the dayside in the dusk cell in the Southern Hemisphere compared to its conjugate region. We interpret this as a signature of a process acting to restore symmetry of the displaced closed magnetic field lines resulting in flux tubes moving faster along the banana cell than the conjugate orange cell. The event is analyzed with emphasis on Birkeland currents (BC) associated with this restoring process, as recently described by Tenfjord et al. (2015). Using data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) during the same conditions as the presented event, the large-scale BC pattern associated with the event is presented. It shows the expected influence of the process of restoring symmetry on BCs. We therefore suggest that these observations should be recognized as being a result of the dynamic effects of restoring footpoint symmetry on closed field lines in the nightside

How the IMF By\mathit{B}_{y} Induces a Local By\mathit{B}_{y} Component During Northward IMF Bz\mathit{B}_{z} and Characteristic Timescales

We use the Lyon-Fedder-Mobarry global magnetohydrodynamics model to study the effects of the interplanetary magnetic field (IMF) $\mathit{B}_{y}$ component on the coupling between the solar wind and magnetosphere-ionosphere system when IMF $\mathit{B}_{z}$ $>$0. We describe the evolution of how a magnetospheric $\mathit{B}_{y}$ component is induced on closed field lines during these conditions. Starting from dayside lobe reconnection, the magnetic tension on newly reconnected field lines redistribute the open flux asymmetrically between the two hemispheres. This results in asymmetric magnetic energy density in the lobes. Shear flows are induced to restore equilibrium, and these flows are what effectively induces a local $\mathit{B}_{y}$ component. We show the radial dependence of the induced $\mathit{B}_{y}$ and compare the results to the induced $\mathit{B}_{y}$ during southward IMF conditions. We also show the response and reconfiguration time of the inner magnetosphere to IMF $\mathit{B}_{y}$ reversals during northward IMF $\mathit{B}_{z}$. A superposed epoch analysis of magnetic field measurements from seven Geostationary Operational Environmental Satellite spacecraft at different local times both for negative-to-positive and positive-to-negative IMF $\mathit{B}_{y}$ reversals is presented. We find that the induced $\mathit{B}_{y}$ responds within 16 min of the arrival of IMF $\mathit{B}_{y}$ at the bow shock, and it completely reconfigures within 47 min

Background removal from global auroral images: Data-driven dayglow modeling

Global images of auroras obtained by cameras on spacecraft are a key tool for studying the near-Earth environment. However, the cameras are sensitive not only to auroral emissions produced by precipitating particles, but also to dayglow emissions produced by photoelectrons induced by sunlight. Nightglow emissions and scattered sunlight can contribute to the background signal. To fully utilize such images in space science, background contamination must be removed to isolate the auroral signal. Here we outline a data-driven approach to modeling the background intensity in multiple images by formulating linear inverse problems based on B-splines and spherical harmonics. The approach is robust, flexible, and iteratively deselects outliers, such as auroral emissions. The final model is smooth across the terminator and accounts for slow temporal variations and large-scale asymmetries in the dayglow. We demonstrate the model by using the three far ultraviolet cameras on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission. The method can be applied to historical missions and is relevant for upcoming missions, such as the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) mission