[GEM] THE GEM MESSENGER, Volume 26, Number 40

Newsletter Editor editor at igpp.ucla.edu
Thu Sep 29 09:08:09 PDT 2016


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     THE GEM MESSENGER
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Volume 26, Number 40
Sep.29,2016

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Table of Contents

1. 2016 GEM-CEDAR Workshop Report: Transient Phenomena at the Magnetopause and Bow Shock and Their Ground Signatures Focus Group

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1. 2016 GEM-CEDAR Workshop Report: Transient Phenomena at the Magnetopause and Bow Shock and Their Ground Signatures Focus Group
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From: Hui Zhang, Q.-G. Zong, Michael Ruohoniemi, and David Murr (hzhang14 at alaska.edu)

The "Transient Phenomena at the Magnetopause and Bow Shock and Their Ground Signatures" focus group held three sessions with 25 presentations covering the following research areas: 1. Kinetic and transient processes in the foreshock, bow shock, and magnetosheath 2. Dayside magnetopause processes and transport 3. Magnetospheric signatures of dayside transients 

1. Kinetic and transient processes in the foreshock, bow shock, and magnetosheath
joint session with the “Dayside Kinetic Processes in Global Solar Wind-Magnetosphere Interaction” Focus Group

Kinetic effects throughout the dayside magnetosphere are driven to a large extent by an array of local and external transient phenomena. The purpose of this joint session was to determine how results from these focus groups can be combined to understand these effects on the dayside system from a global perspective.

Terry Liu showed THEMIS observations of a new ion and electron foreshock upstream of a foreshock bubble's shock. Foreshock bubble's shock could be an additional accelerator and a particle source for the parent shock acceleration. Sanni Hoilijoki and Heli Hietala presented, on behalf of Yann Pfau-Kempf, recent results obtained with the hybrid-Vlasov model Vlasiator. Magnetosheath perturbations are found to deform the bow shock so that transient foreshock-like field-aligned ion beams form, a scenario supported by Geotail observations. Heli Hietala presented ARTEMIS observations of ULF wave growth in the foreshock at lunar distances. The growth rate obtained from the two spacecraft measurements, as well as the other properties of the waves, match well the results of a dispersion solver that uses the observed ion beam distribution as an input. Andrey Samsonov presented a method for incorporating kinetic foreshock effects into a global MHD model. They simulated four events with very distant subsolar magnetopause crossings that occurred during nearly radial IMF intervals lasting from one to several hours. They changed the solar wind boundary conditions for a global model assuming that the density and velocity in the foreshock cavity decrease to ~60 % and ~94 % of the respective ambient solar wind values during intervals with small IMF cone angles. Christina Chu presented a hot flow anomaly (HFA) analog simulated in BATS-R-US and observations of how it affected the magnetosphere. This work will be used to analyze ground signatures observed with HFAs. Hui Zhang presented a statistical study to determine what kinds of discontinuities are more efficient to generate HFAs. Their results show that magnetic field on at least one side of the interplanetary discontinuities has to be connected to the bow shock in order to form HFAs. Discontinuities with large magnetic shear angles are more efficient to form HFAs. Current sheets with thickness from 1000 km to about 3162 km are more efficient to form HFAs. HFAs are more likely to form when the reflected flow from the bow shock is along the discontinuity.

2. Dayside magnetopause processes and transport

The “Transient phenomena at the Magnetopause and Bow Shock and their Ground Signatures” focus group jointed with the “Magnetic Reconnection in the Magnetosphere” and “Dayside Kinetic Processes in Global Solar Wind-Magnetosphere Interactions” FGs in the afternoon of Tuesday (06/21/2016). This joint session had talks on observation and modeling with an emphasis on global magnetospheric aspects of reconnection.  Sun-Hee Lee showed that (1) the inverse dispersions of energetic ions were observed by MMS/EIS in the magnetosheath just outside the magnetopause and the observed ion structure can be explained as the effect of a transient solar wind dynamic pressure pulse, and (2) using combined ground radar and MMS/EIS observations, they estimated a longitudinal extent of 1.5 R_E for the reconnection line. Naritoshi Kitamura reported that the extension of the Geotail mission until March 2019 was approved for the coordinated observations with the MMS spacecraft. A conjunction event between Geotail and MMS on 18 November 2015 showed that the magnetopause reconnection line shifts toward the winter hemisphere for southward IMF.  Richard Denton used magnetic and particle data from MMS to find the motion of the MMS spacecraft through the reconnection structure described in the Burch et al. Science paper. 

Chih-Ping Wang showed that during a prolonged (~5 hr) northward IMF interval with very steady SW/IMF conditions, ARTEMIS at X = 60 R_E near the dusk magnetopause boundary layer observed quasi-periodic (7-10 min) perturbations in the plasma and magnetic field propagating tailward with a spatial scale of ~8 R_E in the X direction. Simulation of this event with LFM model shows that K-H waves are formed in the near-Earth flanks and propagate to the mid-tail, which qualitatively explain the observed perturbations. Cong Zhao used magnetometer and fast plasma instrument measurement from four MMS spacecraft to calculate the gradient of magnetic and plasma pressure as well as the curvature force. The force analysis shows that the magnetopause is in force balance and reveals multiple sub-layers exist in the magnetopause.  Maimaitirebike Maimaiti showed a case study when RISR-N was located in the noon sector and directly measured reverse convection in the dayside throat region while the IMF was transitioning from strong positive B_y to strong positive B_z. Time-lagged correlation analysis reveals that the IMF B_y influence acted on a lag time which was 10 minutes faster than that of the B_z component, and this was attributed to the occurrence of magnetic merging at two different magnetopause sites as determined by favored merging geometries for the two components of the IMF.

On the theoretical side, Sanni Hoilijoki showed that reconnection rates at the dayside magnetopause in a global hybrid-Vlasov simulation correlate well with the analytical model by Cassak and Shay [2007]. In addition, their results indicate that magnetosheath waves affect the reconnection rate.  Xuanye Ma showed that magnetic reconnection with a super-critical perpendicular sheared flow forms an expanding outflow region to maintain the total pressure balance, and violates the Walen relation. Plausible observational signatures in the outflow region include decreased density and pressure and increased magnetic field strength. Sasha Ukhorskiy showed that, for the first time, the high-resolution LFM global MHD model was coupled with a symplectic test-particle code and used to investigate the role of the Kelvin-Helmholtz (KH) instability in the magnetopause losses of energetic hydrogen and oxygen ions. They showed that the KH substantially increases the loss rates for both ion species at the dusk as well as the dawn magnetopause flanks and that after the magnetopause crossing and prior to the escape into the interplanetary space, energetic oxygen remains in the magnetosheath much longer than hydrogen, which is consistent with recent MMS observations.  Andrey Samsonov calculated magnetopause positions for stationary cases with northward and southward IMF orientations using a set of empirical and global MHD models. The differences in positions of selected reference points between northward and southward cases characterize the strength of MI currents, but their exact meaning is still not understood.  Kris Maynard used OpenGGCM to show evidence that reconnection happens at two simultaneous X-lines during FTE formation. They quantified the reconnection rate using the quasi-potential.  

3. Magnetospheric signatures of dayside transients

This session was joint with the “Dayside Kinetic Processes in Global Solar Wind-Magnetosphere Interactions” and “ULF wave Modeling, Effects, and Applications” FGs. There were 8 presentations, and throughout the session discussion topics included (1) preferred driving conditions/magnetopause perturbations for triggering different magnetospheric signatures and (2) modeling the ULF response to localized magnetopause indentations.

Slava Merkin presented results from the effort coupling the high-resolution version of the LFM global magnetosphere with Sasha Ukhorskiy’s test particle simulation. The work was done primarily by Kareem Sorathia at JHU/APL and showed that losses of magnetospheric energetic particles (100 keV protons and O+ ions) at the magnetospheric flanks were enhanced by the well-developed Kelvin-Hemholtz instability. De-sheng Han discussed throat aurora, using statistical analysis to show that auroral features relate to scales of ~3RE in the equatorial plane and are the ionospheric signatures of the interaction of cold magnetospheric ions with dayside magnetopause reconnection. This implies that throat aurora may provide important information on studying the interaction of cold magnetospheric plasma with magnetopause reconnection. Boyi Wang discussed the driving mechanisms of poleward moving auroral forms (PMAFs) with coordinated all sky imager and satellite observations, showing a strong statistical relationship with southward turnings of the IMF (72%), with a response time of ~8 minutes. Boyi Wang also discussed the dayside auroral response on closed field lines to an IMF discontinuity, using multiple satellites in the dayside magnetosphere, magnetosheath, and solar wind. They associated the IMF discontinuity with a localized, propagating magnetopause compression, brightening/azimuthal propagation of dayside diffuse aurora, and localized magnetospheric ULF waves with large amplitudes. Michael Hartinger discussed how the high-latitude ground magnetic response to an interplanetary shock depends strongly on the local ionospheric conductivity; inter-hemispheric comparisons from recently deployed Antarctic AAL-PIP magnetometers, Greenland magnetometers, and global MHD simulations show the response varies rapidly with location relative to the auroral oval. Hui Zhang presented HFA generated Pc3 ULF waves observed by multiple spacecraft and ground magnetometers. The ULF waves are standing Alfvén waves. The wave power of poloidal mode is stronger than that of toroidal mode. The Pc3 ULF waves were observed at dawn, noon and dusk sectors, indicating the magnetospheric response to the HFA is global. The goal of the work presented by Heli Hietala is to determine impact rates of magnetosheath high speed jets and their properties at the magnetopause, which can then be used as input to global magentospheric models. The high speed jets are related to kinetic foreshock processes, and drive significant local increases in dynamic pressure and ULF fluctuations at the magnetopause. The jets occur preferentially in radial IMF conditions, happening at rates as large as 9/hour with typical perpendicular scales of 1.34 RE. Alexa Halford spoke about BARREL observations of a solar energetic electron event. There were ULF oscillations observed with precipitation and it is yet unclear if this is due to the movement of the open closed boundary or processes within the magnetosheath as these same oscillations were not observed in the solar wind.


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