View source for FG: Transient Phenomena at the Magnetopause and Bow Shock and Their Ground Signatures

===Chairs===

Hui Zhang, University of Alaska Fairbanks (hzhang@gi.alaska.edu) 

Q.-G. Zong, University of Massachusetts Lowell (qgzong@gmail.com)

Michael Ruohoniemi, Virginia Polytechnic and State University (mikeruo@vt.edu)

David Murr, Augsburg College (murrdl@augsburg.edu)

===Description===
The "Transient Phenomena at the Magnetopause and Bow Shock and Their Ground Signatures" focus group will employ both observations and simulations to investigate the transient phenomena at the magnetopause and bow shock and their ground signatures. The goal of this focus group is to provide a fundamental physical understanding of the transient phenomena at the magnetopause and bow shock including magnetic reconnection, FTEs, and Hot Flow Anomalies. This focus group encourages participation from communities interested in spacecraft observations (e.g., THEMIS and Cluster), ground-based observations (all-sky camera, radar, magnetometer), and global simulations. Coordinated multi-point observations are especially encouraged. This focus group is unique in the sense that it connects phenomena in regions ranging from the distant solar wind, bow shock, magnetosheath, and magnetosphere, all the way down to the ionosphere. Thus it will attract participation from a broad community including CEDAR and SHINE who do not normally interact.

===Comparison of Transient Phenomena at the Bow Shock===
{| border="1" Class="wikitable"
|-
!!! HFAs !! SHFAs !! Foreshock Bubbles !! Foreshock Cavities !! Foreshock Cavitons!!Foreshock compressional boundary	!!Density Holes	!!SLAMs
|-
| '''Depletion in the density and magnetic field strength'''  || Yes || Yes || Yes || Yes || Yes || Yes on the turbulent side || Yes || Yes 
|-
| '''Compressions at edges'''||	Yes	||Yes	 ||Only on the upstream edge	||Yes||	Yes	||Yes	||Yes	||Yes

|-
| '''Presence of energetic (>30 keV) particles'''	||Yes||	Yes	||Yes ||	Yes	||Yes	 ||No	 ||Yes	||No
|-
|'''Significant flow deflection''' ||	Yes||	Yes	||Yes ||	No	||No	||No	||Yes	||No
|-
|'''Significant plasma heating'''	||Yes	||Yes	||Yes	||Modest||	No	||No	||Yes	 ||Yes
|-
|'''Associated with an IMF discontinuity'''	||Yes	||No		||Yes	||Sometimes||	No	||No	||Yes	||No
|-
|'''Duration'''	||Minutes||	Minutes	||Minutes	||Minutes	||Minutes	||Minutes	||Seconds||~10 s
|-
|'''Scale size'''||	A few RE	||A few RE	||Up to 10 RE	||A few RE	||~ RE||	~ RE	||Ion gyroradius	||Ion gyroradius
|-
|'''Generation Mechanisms'''	||Interaction of IMF discontinuities with the bow shock	||Interaction of foreshock cavitons with the bowshock	
||Kinetic interactions between suprathermal, backstreaming ions and incident solar wind plasma with embedded IMF discontinuities that move through and alter the ion foreshock.	
||Antisunward-moving slabs of magnetic field lines connected to the bow shock that are sandwiched between broader regions of magnetic field lines that remain unconnected to the bow shock. 	
||Nonlinear evolution of ULF waves ||	Backstreaming ions result in increased pressure within the foreshock region leading to its expansion against the pristine solar wind and the generation of FCB.	
||Possibly due to backstreaming particles interacting with the original solar wind	||Nonlinear wave steepening
|}

Several transient kinetic phenomena have been reported upstream from the Earth's bow shock including hot flow anomalies (HFAs), spontaneous hot flow anomalies (SHFAs), foreshock bubbles (FBs), foreshock cavities, foreshock cavitons, foreshock compressional boundary, density holes, and Short, Large-Amplitude Magnetic structures (SLAMs). The kinetic processes associated with these phenomena modify the solar wind just prior to its interaction with the Earth's magnetosphere. 

'''Hot Flow Anomalies (HFAs)'''

HFAs are marked by greatly heated plasmas and substantial flow deflections, with durations of a few minutes [e.g., Schwartz et al., 1985; Schwartz, 1995; Thomsen et al., 1986; Lucek et al., 2004; Facsko et al., 2008; Zhang et al., 2010; Wang, Zong, and Zhang, 2012]. HFAs are thought to be produced by the interaction of certain types of upstream discontinuities with the bow shock [Thomas et al., 1991; Wang, Zong, and Zhang, 2013]. The ions reflected from the bow shock are energized and trapped in the vicinity of the discontinuities when the motional electric field points toward the discontinuity. 

'''Spontaneous Hot Flow Anomalies (SHFAs)'''

Description: SHFAs are similar to HFAs, though SHFAs occur independent of any discontinuity in the pristine, upstream magnetic fields.  They form due to processes internal to the quasi-parallel foreshock, and display all of the same core and compression region characteristics as HFAs.
Some Refs: Zhang et al [Under review, JGR]; Omidi et al. [JGR, 2013]

'''Foreshock Bubbles (FBs)'''

Description: FBs can form when energetic foreshock ions upstream of quasi-parallel planetary bow shocks interact with rotational discontinuities in the pristine, upstream IMF.  Unlike HFAs, FBs form independent of any connection between the discontinuity and the bow shock.  FBs form just upstream of the responsible discontinuity and move anti-sunward with it.  As an FB impinges upon a planetary bow shock and sweeps up more and more energetic ions in the foreshock, it grows in time, potentially reaching sizes on the same order as Earth's entire dayside magnetosphere.  Due to the building concentration of suprathermal ions in their cores, FBs exhibit very high core temperatures, resulting in the expulsion of thermal plasma, which drops the core density and field strength.  Core fields are highly distorted, and within the core, there are very strong and sometimes even sunward bulk flow deflections.   Compression regions form around the edges of the core, and provided sufficient conditions, the upstream compression regions can evolve into fast magnetosonic shocks (i.e., if the difference between the upstream bulk velocity and the rate at which the FB grows back into the upstream plasma approaches and exceeds the fast magnetosonic speed).  FBs should be particularly efficient particle accelerators since they involve two converging shocks (i.e., that of the FB at the upstream edge and the bow shock itself) and increased wave activity in their cores.  When a FB impacts the bow shock and magnetosphere, it should first result in expansion of the system into the tenuous core followed by a sudden and strong compression of the system due to the upstream shock/compression region.  FBs can result in extreme magnetopause deformations and global magnetosphere-ionosphere activity.  
Some Refs: Omidi et al. [JGR, 2010]; Turner et al. [JGR, 2013]

'''Foreshock Cavities'''

Foreshock cavities, although more common, are less prominent than the HFAs in the sense that the solar wind distributions within the cavities show little evidence of heating or significant flow deflection although a second, suprathermal population is present. Foreshock cavities can be identified based on enhanced magnetic field strengths and densities bounding regions of depressed magnetic field strength and density containing a suprathermal ion component [e.g., Sibeck et al., 2002, 2004; Schwartz et al., 2006; Billingham et al., 2011]. 
Formation mechanism: Antisunward-moving slabs of magnetic field lines connected to the bow shock that are sandwiched between broader regions of magnetic field lines that remain unconnected to the bow shock. The slabs are filled with reflected and energized particles from the bow shock. The corresponding pressure enhancement causes the cavities to expand, depressing the internal densities and magnetic fields but enhancing these parameters at the expanding edge [Schwartz et al., 2006]. 

'''Foreshock Cavitons'''

Foreshock cavitons are about an RE in size. Their cores exhibit drops in density and magnetic field, while their outer edges show plasma and magnetic field enhancements. They form as a result of the nonlinear evolution of two types of waves: the parallel propagating sinusoidal waves and the highly oblique, linearly polarized, fast magnetosonic waves [Lin, 2003; Lin and Wang, 2005, Omidi, 2007; Blanco-Cano et al., 2009, 2011]. Foreshock cavitons are embedded in regions with ULF activity, which is in contrast to the isolated character of foreshock cavities.

'''Foreshock Compressional Boundary'''

The foreshock compressional boundary (FCB) [Sibeck et al., 2008; Omidi et al., 2009] forms in the ion foreshock and is associated with enhanced densities and magnetic field strengths. Backstreaming ions result in increased pressure within the foreshock region leading to its expansion against the pristine solar wind and the generation of FCB. Although the FCB itself is associated with increases in the magnetic field strength and density, these quantities are reduced on the turbulent side of the FCB as compared to the pristine solar wind. FCBs may be a steady state feature, but observed transiently because of slight changes in the IMF orientation. 

'''Density Holes'''

Similar to HFAs, density holes display significant bulk flow deflections and are filled with heated plasma. Density holes are accompanied by similarly shaped magnetic holes. They have enhanced densities and compressed magnetic field at one or both edges. Density holes have durations of ~18 seconds, which are much shorter than that of HFAs, and scale sizes of an ion gyroradius, which are much smaller than those of HFAs [Parks et al., 2006]. Density holes are possibly formed by backstreaming particles interacting with the original solar wind [Parks et al., 2006].

'''Short, large-amplitude magnetic structures (SLAMS)'''

SLAMS (Short, Large-Amplitude, Magnetic Structures) are large amplitude (the magnetic field magnitude is enhanced over the undisturbed field by at least a factor of 2.5) magnetic structures with durations of the order of 10 s [Schwartz, 1991; Lucek et al., 2002]. They grow rapidly (~seconds) out of ULF waves in the foreshock region.  See also: Wilson III et al. [JGR, 2013]

===HFA Events (from THEMIS)===

14-Jul-2008/21:44:15 UT

14-Jul-2008/21:45 UT

14-Jul-2008/21:47:10 UT

14-Jul-2008/21:49:10 UT

14-Jul-2008/22:28:30 UT

15-Jul-2008/01:31:30

15-Jul-2008/01:32:30

15-Jul-2008/01:40:30

15-Jul-2008/05:13:30

15-Jul-2008/08:13

15-Jul-2008/08:14:15

15-Jul-2008/09:14:30

15-Jul-2008/11:59

15-Jul-2008/12:11

15-Jul-2008/12:46

15-Jul-2008/13:05:30

15-Jul-2008/13:06:30

23-Jul-2008/06:42:30

23-Jul-2008/09:24

23-Jul-2008/10:06:15

23-Jul-2008/10:08:30

04-Aug-2008/01:31

04-Aug-2008/02:06

04-Aug-2008/02:35:45

08-Aug-2008/02:50:30

08-Aug-2008/04:30

11-Aug-2008/06:16:45

11-Aug-2008/11:31

12-Aug-2008/00:40:30

12-Aug-2008/01:05:45

12-Aug-2008/02:57

15-Aug-2008/04:25:30

15-Aug-2008/04:37

15-Aug-2008/16:52:30

18-Aug-2008/22:26

18-Aug-2008/22:46

19-Aug-2008/01:12:30

19-Aug-2008/12:50

19-Aug-2008/22:37:45

22-Aug-2008/21:31:30

22-Aug-2008/22:19:30

08-Sep-2008/08:21

08-Sep-2008/09:37:30

08-Sep-2008/10:58:30

08-Sep-2008/15:23:15

08-Sep-2008/20:16

08-Sep-2008/20:18

09-Sep-2008/18:03:30

09-Sep-2008/19:27

09-Sep-2008/19:30:30

12-Sep-2008/00:22

12-Sep-2008/04:00:30

12-Sep-2008/04:15

16-Sep-2008/00:05

04-Oct-2008/13:19

05-Oct-2008/21:36

05-Oct-2008/21:38:30

06-Oct-2008/07:31:30

06-Oct-2008/10:19:30

09-Oct-2008/23:31

11-Oct-2008/20:45:30

11-Oct-2008/20:50

===Foreshock Bubble Events (from THEMIS) ===
14-Jul-2008/21:51:30 UT

14-Jul-2008/21:55 UT

14-Jul-2008/21:56:20 UT

14-Jul-2008/21:58 UT

14-Jul-2008/22:04 UT

14-Jul-2008/22:20 UT

14-Jul-2008/22:37:30 UT

15-Jul-2008/00:09

15-Jul-2008/00:20

15-Jul-2008/00:26

15-Jul-2008/01:48

11-Aug-2008/18:30

11-Aug-2008/18:50

19-Aug-2008/03:04

19-Aug-2008/04:08:45

19-Aug-2008/13:25

19-Aug-2008/13:42:30

19-Aug-2008/13:45

04-Sep-2008/17:35

08-Sep-2008/20:20-20:24

16-Sep-2008/17:46:30

09-Oct-2008/19:22:30

09-Oct-2008/19:24

13-Oct-2008/23:34