E. Zesta

Global‐scale ionospheric flow and aurora precursors of auroral substorms: Coordinated SuperDARN and IMAGE/WIC observations

We use global-scale polar cap flow vector measurements from the Super Dual Auroral Radar Network (SuperDARN) with the concurrent auroral observations from the Wideband Imaging Camera on board Imager for Magnetopause-to-Aurora Global Exploration (IMAGE/WIC) to study the polar cap flow and auroral precursors during a substorm onset on 26 December 2000. We show, for the first time, close connection between the dayside and nightside polar cap flow enhancements (with the enhanced dayside flow preceding the nightside one by several minutes) and the ensuing poleward boundary intensification (PBI)/streamer, and the later onset, forming a complete preonset sequence for a substorm onset. Our results supplement our previous study by providing further evidence that the dayside polar cap flow disturbance may be the key to initiate the whole process of a certain type of substorm by triggering reconnection somewhere in the tail via applied field (or flow) perturbations on the nightside plasma sheet boundary layer. Our results also indicate that a preexisting double oval structure is likely a favorable precondition for a certain type of substorm to be triggered by polar cap flow disturbance and the associated PBIs/streamers. On the other hand, not all our global-scale preonset auroral sequences support the recent revised onset scenario proposed by Nishimura et al. (2010a) using the all-sky imagers of the Time History of Events and Macroscale Interactions during Substorms mission. This suggests that the preceding PBI/streamer is not a sufficient condition to trigger a substorm. It may not even be a necessary condition considering the existence of various types of substorm onsets.

Response of the equatorial ionosphere to the geomagnetic DP 2 current system

The response of equatorial ionosphere to the magnetospheric origin DP 2 current system fluctuations is examined using ground-based multiinstrument observations. The interaction between the solar wind and fluctuations of the interplanetary magnetic field (IMF) Bz, penetrates nearly instantaneously to the dayside equatorial region at all longitudes and modulates the electrodynamics that governs the equatorial density distributions. In this paper, using magnetometers at high and equatorial latitudes, we demonstrate that the quasiperiodic DP 2 current system penetrates to the equator and causes the dayside equatorial electrojet (EEJ) and the independently measured ionospheric drift velocity to fluctuate coherently with the high-latitude DP 2 current as well as with the IMF Bz component. At the same time, radar observations show that the ionospheric density layers move up and down, causing the density to fluctuate up and down coherently with the EEJ and IMF Bz.

Is Diffuse Aurora Driven From Above or Below

In the diffuse aurora, magnetospheric electrons, initially precipitated from the inner plasmasheet via wave-particle interaction (WPI) processes, degrade in the atmosphere toward lower energies and produce secondary electrons via impact ionization of the neutral atmosphere. These initially precipitating electrons of magnetospheric origin can also be additionally reflected back into the magnetosphere, leading to a series of multiple reflections by the two magnetically conjugate atmospheres that can greatly impact the initially precipitating flux at the upper ionospheric boundary (700-800 km). The resultant population of secondary and primary electrons cascades toward lower energies and escape back to the magnetosphere. Escaping upward electrons traveling from the ionosphere can be trapped in the magnetosphere, as they travel inside the loss cone, via Coulomb collisions with the cold plasma, or by interactions with various plasma waves. Even though this scenario is intuitively transparent, this magnetosphere-ionosphere (MI) coupling element is not considered in any of the existing diffuse aurora research. Nevertheless, as we demonstrate in this letter, this process has the potential to dramatically affect the formation of electron precipitated fluxes in the regions of diffuse auroras.

Interhemispheric field‐aligned currents: Simulation results

We present simulation results of the 3-D magnetosphere-ionosphere current system including the Region 1 and Region 2 field-aligned currents, ionospheric currents, and interhemispheric field-aligned currents flowing between the northern and southern conjugate ionospheres in the case of asymmetry in ionospheric conductivities in two hemispheres. The model shows that the interhemispheric currents can be an important part of the global 3-D current system in high-latitude ionosphere, especially during summer-winter months, when in winter ionosphere they can be comparable and even exceed both Region 1 and Region 2 currents. An important feature of these interhemispheric currents is that they link together processes in two hemispheres, so that the currents observed in one hemisphere can provide us with information about the currents in the opposite hemisphere. Although the interhemispheric currents might play a notable role in the total 3-D current system, they have not been sufficiently studied yet. The study of the contribution from the interhemispheric currents into the total 3-D current system allows us to improve understanding and forecasting of geomagnetic, auroral, and ionospheric disturbances in two hemispheres.

The relation between transpolar potential and reconnection rates during sudden enhancement of solar wind dynamic pressure: OpenGGCM‐CTIM results

This study investigates how solar wind energy is deposited into the magnetosphere-ionosphere system during sudden enhancements of solar wind dynamic pressure (Psw), using the coupled Open Geospace General Circulation Model–Coupled Ionosphere Thermosphere Model (OpenGGCM-CTIM) 3-D global magnetosphere-ionosphere-thermosphere model. We simulate three unique events of solar wind pressure enhancements that occurred during negative, near-zero, and positive interplanetary magnetic field (IMF) Bz. Then, we examine the behavior of the dayside and nightside reconnection rates and quantify their respective contributions to cross polar cap potential (CPCP), a proxy of ionospheric plasma convection strength. The modeled CPCP increases after a Psw enhancement in all three cases, which agrees well with observations from the Defense Meteorological Satellite Program spacecraft and predictions from the assimilative mapping of ionospheric electrodynamics technique. In the OpenGGCM-CTIM model, dayside reconnection increases within 9–13u2009min of the pressure impact, while nightside reconnection intensifies about 13–25u2009min after the pressure increase. As the strong Psw compresses the dayside magnetosheath and, subsequently, the magnetotail, their magnetic fields intensify and activate stronger antiparallel reconnection on the dayside magnetopause first and near the central plasma sheet second. For southward IMF, dayside reconnection contributes to the CPCP enhancement 2–4 times more than nightside reconnection. For northward IMF, the dayside contribution weakens, and nightside reconnection contributes more to the CPCP enhancement. We find that high-latitude magnetopause reconnection during northward IMF produces sunward ionospheric plasma convection, which decreases the typical dawn-to-dusk ionosphere electric field. This results in a weaker dayside reconnection contribution to the CPCP during northward IMF.

Auroral electrojet indices in the Northern and Southern Hemispheres: A statistical comparison

The auroral electrojet (AE) index is traditionally derived from about 12 ground magnetometer observatories located around the average northern auroral oval location. The AE index calculation has only been performed with Northern Hemisphere data, because similar coverage in the Southern Hemisphere does not exist. In this study, eight southern auroral ground magnetometers and their near conjugate Northern Hemisphere counterparts are used to calculate conjugate AE indices for 274u2009days covering all four seasons from 2005 to 2010. The correlation coefficient between the northern and southern AE indices for many of the intervals is 0.65 indicating strong asymmetries between the two hemispheres. We compare our conjugate AE indices with the standard AE index and find a number of asymmetries because of station coverage gaps in the southern and northern arrays. The mean difference between the southern and northern AE indices is largest during northern summer season, and the smallest mean difference occurs in the spring. The mean differences between the southern and conjugate northern AE indices are about 31 nT with the largest differences occurring in the midnight magnetic local time (MLT) sector. We suggest that these differences may be a function of seasonal, MLT, and ionospheric effects. We also find a difference in the southern and northern AE related to UT and believe that this pertains to the distribution of the magnetometers. The fact that a difference between the southern and northern AE indices exists indicates the importance of examining geomagnetic activity in both hemispheres when considering magnetospheric phenomena.

The Future of Ground Magnetometer Arrays in Support of Space Weather Monitoring and Research

A community workshop was held in Greenbelt, Maryland, on 5–6 May 2016 to discuss recommendations for the future of ground magnetometer array research in space physics. The community reviewed findings contained in the 2016 Geospace Portfolio Review of the Geospace Section of the Division of Atmospheric and Geospace Science of the National Science Foundation and discussed the present state of ground magnetometer arrays and possible pathways for a more optimal, robust, and effective organization and scientific use of these ground arrays. This paper summarizes the report of that workshop to the National Science Foundation (Engebretson & Zesta, [Engebretson, M. J., 2017]) as well as conclusions from two follow-up meetings. It describes the current state of U.S.-funded ground magnetometer arrays and summarizes community recommendations for changes in both organizational and funding structures. It also outlines a variety of new and/or augmented regional and global data products and visualizations that can be facilitated by increased collaboration among arrays. Such products will enhance the value of ground-based magnetometer data to the communitys effort for understanding of Earths space environment and space weather effects.

Thermosphere Global Time Response to Geomagnetic Storms Caused by Coronal Mass Ejections

We investigate, for the first time with a spatial superposed epoch analysis study, the thermosphere global time response to 159 geomagnetic storms caused by coronal mass ejections (CMEs) observed in the solar wind at Earths orbit during the period of September 2001 to September 2011. The thermosphere neutral mass density is obtained from the CHAMP (CHAllenge Mini-Satellite Payload) and GRACE (Gravity Recovery Climate Experiment) spacecraft. All density measurements are intercalibrated against densities computed by the Jacchia-Bowman 2008 empirical model under the regime of very low geomagnetic activity. We explore both the effects of the pre-CME shock impact on the thermosphere and of the storm main phase onset by taking their times of occurrence as zero epoch times (CME impact and interplanetary magnetic field Bz southward turning) for each storm. We find that the shock impact produces quick and transient responses at the two high-latitude regions with minimal propagation toward lower latitudes. In both cases, thermosphere is heated in very high latitude regions within several minutes. The Bz southward turning of the storm onset has a fast heating manifestation at the two high-latitude regions, and it takes approximately 3xa0h for that heating to propagate down to equatorial latitudes and to globalize in the thermosphere. This heating propagation is presumably accomplished, at least in part, with traveling atmospheric disturbances and complex meridional wind structures. Current models use longer lag times in computing thermosphere density dynamics during storms. Our results suggest that the thermosphere response time scales are shorter and should be accordingly adjusted in thermospheric empirical models.

Major Pathways to Electron Distribution Function Formation in Regions of Diffuse Aurora

This paper discusses the major pathways of electron distribution function formation in the region of diffuse aurora. The diffuse aurora accounts for about of 75% of the auroral energy precipitating into the upper atmosphere, and its origin has been the subject of much discussion. We show that an Earthward stream of precipitating electrons initially injected from the Earth1s plasmasheet via wave-particle interactions degrades in the atmosphere toward lower energies and produces secondary electrons via impact ionization of the neutral atmosphere. These electrons of magnetospheric origin are then reflected back into the magnetosphere along closed dipolar magnetic field lines, leading to a series of reflections and consequent magnetospheric interactions that greatly augment the initially precipitating flux at the upper ionospheric boundary (700-800u2009km). To date this systematic magnetosphere-ionosphere coupling element has not been included in auroral research models, and, as we demonstrate in this article, has a dramatic effect (200-300%) on the formation of the precipitating fluxes that result in the diffuse aurora. It is shown that wave particle interaction processes that drive precipitating fluxes in the region of diffuse aurora from the magnetospheric altitudes are only the first step in the formation of electron precipitation at ionospheric altitudes, and they cannot be separated from the atmospheric “collisional machine” that redistributes and transfers their energy inside the magnetosphere-ionosphere-atmosphere coupling system.

Geomagnetically Induced Currents Caused by Interplanetary Shocks With Different Impact Angles and Speeds

The occurrence of geomagnetically induced currents (GICs) poses serious threats to modern technological infrastructure. Large GICs result from sharp variations of the geomagnetic field (dB/dt) caused by changes of large-scale magnetospheric and ionospheric currents. Intense dB/dt perturbations are known to occur often in high-latitude regions as a result of storm time substorms. Magnetospheric compressions usually caused by interplanetary shocks increase the magnetopause current leading to dB/dt perturbations more evident in midlatitude to low-latitude regions, while they increase the equatorial electrojet current leading to dB/dt perturbations in dayside equatorial regions. We investigate the effects of shock impact angles and speeds on the subsequent dB/dt perturbations with a database of 547 shocks observed at the L1 point. By adopting the threshold of dB/dt = 100 nT/min, identified as a risk factor to power systems, we find that dB/dt generally surpasses this threshold when following impacts of high-speed and nearly frontal shocks in dayside high-latitude locations. The same trend occurs at lower latitudes and for all nightside events but with fewer high-risk events. Particularly, we found nine events in equatorial locations with dB/dt > 100 nT/min. All events were caused by high-speed and nearly frontal shock impacts and were observed by stations located around noon local time. These high-risk perturbations were caused by sudden strong and symmetric magnetospheric compressions, more effectively intensifying the equatorial electrojet current, leading to sharp dB/dt perturbations. We suggest that these results may provide insights for GIC forecasting aiming at preventing degradation of power systems due to GICs.