[GEM] THE GEM MESSENGER, Volume 27, Number 47

[Newsletter Editor]

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     THE GEM MESSENGER
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Volume 27, Number 47
Oct.22,2017

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Table of Contents

1. 2017 GEM Workshop Report: Tail Environment and Dynamics at Lunar Distances Focus Group

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1. 2017 GEM Workshop Report: Tail Environment and Dynamics at Lunar Distances Focus Group
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From: Chih-Ping Wang, Andrei Runov, David Sibeck, Viacheslav Merkin, and Yu Lin (cat at atmos.ucla.edu)

The Tail Environment and Dynamics at Lunar Distances FG held four sessions at the 2017 GEM summer workshop. The 1st session include presentations of recent progress on various topics of the mid-tail. The 2nd and 3rd session are a joint session with “Magnetotail Dipolarization and its Effect on the Inner Magnetosphere” and “Magnetic Reconnection in the Magnetosphere” FGs. The 4th session is a joint session with “Modeling Methods and Validation” FG on mid-tail modeling challenge.

Session 1:
Anton Artemyev presented the unique dataset gathered by two ARTEMIS spacecraft in 2010 at radial distances between lunar orbit and ~ 200 Earth radii. He identified an X-line at around ~80 Earth radii and collected statistics on hot plasma flows originating from this distant X-line. Ion spectra within these flows are well fitted by a power-law distribution with the exponential tail starting at energy ~ 2-5 keV. He estimated that the hot ion population transported toward Earth can contribute significantly to high-energy (>50keV) ion fluxes in the near-Earth magnetotail. 

Fekireselassie Beyene presented his method of estimating the total amount of magnetotail flux and showed the results from a storm time and substorm time interval. He concluded that during the substorm interval the peak flux was double the minimum flux and that during the storm interval the peak flux was less than twice the minimum flux.

Lei Cheng used the 3-D Global Hybrid simulation model (ANGIE3D) to show:(1) Alfvénic waves are generated in reconnection, propagating earthward and tailward near the plasma sheet boundary layer (PSBL). (2) Alfvénic waves propagate to the north (along the direction of B) in the Northern Hemisphere and to the south (against B) in the Southern Hemisphere in the dipole-like field region.

San Lu presented the investigation of ion temperature gradient in the Earth’s magnetotail using multi-spacecraft observations and 3-D global hybrid simulations. He demonstrated that the ion temperature ZGSM-profile is bell-shaped at different geocentric distances. Using 3-D global hybrid simulations, he showed that mapping of the XGSM-gradient of ion temperature along magnetic field lines produces such a bell- shaped profile. 

Stefan Kiehas presented investigation of midtail flows from a five year (2011–2015) statistical survey of ARTEMIS data at around 60 RE downtail. He found that a significant portion of fast flows is directed earthward (43 % (Vx > 400 km/s) to 56 % (Vx > 100 km/s)). A dawn–dusk asymmetry in the flow occurrence is seen with about 60% of tailward perpendicular flows occurring in the dusk sector. On the other hand, earthward flows are nearly symmetrically distributed over the dawn and dusk sectors. This indicates that the dawn-dusk asymmetry is more pronounced in the near-Earth region than further downtail. 

Chih-Ping Wang presented the first observational event showing the connection between an earthward moving plasma bubble in midtail at X ~ –60 RE and equatorward moving high-latitude ground magnetic bays at ~70o latitude. Enhancements of Pi2 waves were observed both within the plasma bubble and magnetic bays.

Session 2 and 3: The report for these two joint sessions is submitted by Magnetic Reconnection FG.

Session 4:
Modeling challenge for an event observed ARTEMIS in the mid-tail under prolonged N IMF from 13-14 Feb 2014. We focused on two intervals: (1) solar wind/IMF remained steady for ~4 hr. (2) IMF By changed and IMF became almost purely northward for ~1hr. Global MHD simulations of GUMICS, BATS-R-US, LFM, and OpenGGCM were conducted for this event on NASA CCMC. Additionally, higher-resolution runs were conducted for LFM (by Slava Merkin) and for OpenGGCM (by Joseph Jensen). The simulation results were compared with ARTEMIS observations in midtail and also observations in the ionosphere and ground, including flows and convection maps from SuperDARN, particle precipitations and aurora images from DMSP, field-aligned currents (FACs) from SWARM and AMPERE, and ground magnetometers.

(1) Steady solar-wind/IMF, 3-7 UT, 13 Feb 2014
ARTEMIS was in the northern lobe near the dusk flank and observed mesoscale perturbations in plasma and magnetic field. Both LFM and OpenGGCM predict mesoscale perturbations in midtail. The cross-tail configurations of GUMICS and BATS-R-US are similar to those of LFM and OpenGGCM in large scale but without mesoscale variations. The perturbations in LFM are near the flanks and likely caused by Kelvin-Helmholtz (K-H) vortices, while the perturbations in OpenGGCM are mainly in the tail current sheet associated with flapping motion. The higher-resolution OpenGGCM run does produce some flank perturbations, but it remains to be determined whether they are associated with K-H vortices. The LFM simulations (both the CCMC and high-resolution runs) qualitatively account for the mesoscale variations of Bx, n, Vx observed by ARTEMIS, and the Pc-5 perturbations observed by SuperDARN ionosphere flow velocities and ground magnetic fields. The FACs strengths from the high-resolution run are higher than the CCMC run and are closer to those of SWARM. The polar-cap size predicted by LFM is in better agreement with the aurora image from DMSP than other models.

(2) IMF By change, 18-23 UT, 14 Feb 2014 (IMF By changed from +6 to ~0 and returned to +6. IMF By was ~0 and IMF Bz ~ +4 nT from 19-21 UT)
ARTEMIS was near midnight and observed plasma sheet plasma with no significant flows, indicating that the tail plasma sheet extended beyond 60 RE. Only OpenGGCM predicts that the tail plasma sheet remained longer than 60 RE when IMF By became ~0. Both LFM and OpenGGCM predict small reverse convection cells on dayside high latitudes when IMF was predominantly northward, in agreement with the SuperDARN convection maps. The predictions of polar-cap size by OpenGGCM are in much better agreement with DMSP aurora images on the nightside than other models. OpenGGCM also predicts the formation of an elongated north-south structure of closed field-line region within the polar-cap after IMF By returned from ~0 after 21 UT,  which is in qualitative agreement with the north-south cross polar-cap arcs observed by DMSP. The model shows that this polar-cap closed field-lines structure is connected to tail. The thermal energies for the precipitating ions and electrons within the polar-cap arcs observed by DMSP were very close to those observed by ARTEMIS, supporting the OpenGGCM predictions.

In conclusion, even for this prolonged N IMF event the midtail structures and variations predicted by the four global MHD models are very different. Using higher grid resolutions can improve some of the predictions. But it remains a big challenge to identify other factors that can help further improvement.


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