Paul Cassak West Virginia University CEDAR/GEM Student Day Tutorial June 19, 2016

… the tale of magnetic reconnection …

Thanks to - Robert and LoisImage courtesy of NASA

What Is Reconnection?

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What Is Magnetic Reconnection?

• “Breaking” of magnetic field linesin a plasma

• Crucial for magnetospheric dynamics – Generates Dungey cycle

• Major component of space weather – See review in Cassak, Space

Weather, 14, 186, 2016

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Computer animation with BATS-R-US using NASA’s CCMC resources

Alfvén On Reconnection

• H. Alfvén, Keynote Address, Proceedings from NASA Workshop on Double Layers, Huntsville AL, March 17-19, 1986 – Referring to “magnetic merging”:

• “I was naïve enough to believe that such a pseudo-science would die by itself in the scientific community, and I concentrated my work on more pleasant problems.  To my great surprise the opposite has occurred: ‘merging’ pseudo-science seems to be increasingly powerful.  Magnetospheric physics and solar wind physics today are no doubt in a chaotic state, and a major reason for this is that part of the published papers are science and part pseudo-science, perhaps even with a majority in the latter group."   

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Why Reconnection Is Weird - I

• Conventional wisdom: magnetic field lines cannot “end”

– Justification: Gauss’ law (no magnetic monopoles)

– One would think they cannot “break”

• Conventional wisdom is actually wrong:magnetic field lines can end!

– The crux - Gauss’ law is a statement about the magnetic field at a fixed time • Gauss’ law does not say anything about the magnetic field time evolution

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r ·B = 0

Why Reconnection Is Weird - II• Reconnection is not allowed in ideal-MHD

(no dissipation) – Frozen-in theorem (Alfvén)

• Magnetic field lines in ideal plasma retain their identity • they can’t break • two plasma elements connected by a field line

at one time are connected at all other times

– Field line motion • Magnetic field and bulk plasma

move together • Can attribute unique velocity to

magnetic field line; same as bulk plasma speed

– Alfvén didn’t like reconnection because it was described using notion of field line motion in a region where ideal-MHD is not valid 6

Why Reconnection Is Natural

• Consider two opposing magnets in vacuum being brought together – It looks like magnetic field lines are breaking!?!

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Magnetic Reconnection• The breaking of field lines in vacuum is accompanied by an

electric field (it is required from Ampère-Maxwell and Faraday’s laws!)

• Reconnection cannot happen in an ideal (MHD) plasma because one cannot produce an electric field (the plasma is a superconductor)

• However, the ideal limit is never perfect – Dissipation occurs at small scales, allows electric fields, magnetic fields can break

• Canonical picture - a component of the magnetic field changes direction (due to Dungey, 1953)

• How reconnection works – Reconnection allows field lines to break – Creates magnetic field lines that are strongly bent,

they sling the top and bottom – The plasma moves with the field lines, plasma ejected to the top and bottom – Removal of plasma makes more want to come in from the left and right

• Brings more magnetic field in left and right, they break, and the process continues – Reconnection is a self-reinforcing process!

E+ v ⇥B = 0

Dungey, 1953

Why Reconnection Is Special

• Converts energy in stretched magnetic fields to energy of the plasma (kinetic and thermal)

–Very efficient! Almost all the magnetic energy in oppositely directed fields is converted

• Magnetic energy beforehand: B2 / 8π • Kinetic energy afterward: rho v2 / 2

– Implies v ~ cA, the Alfvén speed (Parker, 1957)

• Often happens explosively! –Reconnection is slower than the Alfvén time

• Magnetic energy is able to be stored, then reconnection releases it

• It is dynamically significant –Small-scale effect alters dynamics on huge scales –Reconnection heats the plasma and accelerates particles

Reconnection in the Magnetosphere• For IMF pointing (due) southward:

– Field lines at the dayside are oppositely directed and reconnect – Get stretched by the solar wind

• Stretching the field costs energy – Field lines in the nightside (the “magnetotail”)

are again oppositely directed • They reconnect again, releasing the stored energy

• For IMF pointing (due) northward – No reconnection near the subsolar point – Like a rubber band around a rock,

the IMF “drapes” around the magnetosphere – Draped field is anti-parallel to the terrestrial

field near the polar cusps, reconnection happens there – Not much stretching of the field lines occurs,

very little energy transferred to the magnetosphere

• The IMF direction, due to reconnection, is a keydetermining factor of solar wind-magnetospheric coupling

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Dungey, 1961

Evidence for Magnetospheric Reconnection

• Ramification of reconnection is convection pattern of magnetic fields at the polar caps (Dungey, 1961)

– These flows had been observed with ground based magnetometers

• Early indirect evidence of reconnection – IMF direction correlated with auroral activity (Fairfield and Cahill, 1966) – Southward IMF implies magnetopause erosion (Aubry et al., 1970) – Energetic particles from the Sun reach polar caps first (Fennell, 1973)

• Early direct evidence (measurements from NASAs ISEE mission) – Plasma accelerates where field changes direction (Paschmann et al., 1979) – Magnetic field normal to magnetosphere edge (Sonnerup et al., 1981)

• Modern direct evidence (satellites flying through reconnection) – Is now common, clean example from NASAs Polar satellite shown here

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Dungey,1961

Mozer et al., 2002

Where Else Reconnection is Seen• Solar corona

– Solar flares/coronal mass ejections – Prominence eruptions – Coronal jets, Ellerman bombs

• Magnetically confined fusion devices – Major disruptions – Sawtooth crash

• Astrophysical contexts – Stellar flares – Soft Gamma Repeaters (SGRs) on magnetars? – Accretion disks – Dynamo (creation of large scale magnetic field) – Turbulence

12SGR 1E 1547.0-5408, from NASA’s Swift

http://www.ipp.mpg.de/ippcms/eng/pr/exptypen/tokamak/index.html

http://tfy.tkk.fi/fusion/

Magnetic Reconnection

• Reconnection is a “grand challenge problem” in plasma physics –An ISI search in June 2012 found >9,100 papers from 1957-2012

• >500 papers per year garnering >17,000 citations as of 2012

Reconnection Publications Reconnection Citations600

30010,000

5,000

1975

1992

2012

1975

1992

2012

15,000

1958

1957

Vocabulary of Reconnection

• There are linear and non-linear phases of reconnection – Linear phase = tearing – Nonlinear phase = reconnection (or merging)

• In principle, it can be resistive or “collisionless” – In the magnetosphere, classical (Spitzer) collisions cannot play a role

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“Outflow region” or “downstream”

“Inflow region” or “upstream”

“jet” or “exhaust”

“diffusion region” or “dissipation region”

“guide field” - out-of-plane component of magnetic field

“reconnecting field” - oppositely directed field

“reconnected field” - vertical field after reconnection

“field reversal” or “neutral line” - where reconnecting field goes through 0

“X-line” or “reconnection line” or “separator” - point where field lines break, reconnecting field is zero “reconnection electric field E” - out-of-plane electric field which allows field lines to break

“reconnection rate” - rate per-unit-out-of-plane distance that flux is reconnected, equal to E

Describing Reconnection

• Can describe using fluid (MHD, Hall-MHD, two-fluid) or kinetic (Vlasov-Maxwell or Boltzmann-Maxwell) models

• Key consideration - what sets the electric field allowing reconnection - Write electron equation of motion, solve it for electric field (generalized

Ohm’s law - in cgs)

- GEM Reconnection challenge (Birn et al., 2001) showed that the Hall effect is important for collisionless reconnection

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Hall effect

Electron inertia

Divergence of electron

pressure tensor

ResistivityConvection

E+v ⇥B

c= ⌘J+

J⇥B

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Open Questions - General

• Energy storage/release and reconnection onset • How charged particles are

accelerated by reconnection • How energy is (irreversibly) converted to heat • The dissipation mechanism

(what breaks the frozen-in condition) • The rate of reconnection, what controls it,

and why • The nature of 3D reconnection • What stops reconnection • How to include kinetic description in global modeling

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Open Questions - Dayside• How does dayside reconnection proceed?

- Where does it occur for arbitrary solar wind conditions? - Are there differences between 2D theory and the 3D world? - Are there unique kinetic signatures of the dissipation region? - How do local asymmetries, magnetosheath flow, and guide fields

(including diamagnetic effects) affect local properties of reconnection? • Local diamagnetic effects and flow can shut reconnection off (locally)

• Bursty vs. steady? - What are the properties and generation mechanism(s) of flux transfer events? - How important a role do they play in coupling and mass transport? - What are the ionospheric signatures of flux transfer events?

• How does reconnection contribute to solar wind-magnetospheric coupling?

- Can local changes in the plasma environment alter the global reconnection rate (local vs. global)?

- How do local changes in the reconnection rate impact geomagnetic indices?

- What is the role of reconnection in mass entry into the magnetosphere (in comparison with Kelvin-Helmholtz)?

- What are the meso- and macro-scale ramifications of dayside reconnection? 17

KOMAR ET AL.: TRACING MAGNETIC SEPARATORS

Figure 5. Results of the present algorithm in the MHD simulation with !IMF = 90ı looking (a) earthwardand (b) at an oblique angle for perspective. Magnetic nulls are enclosed by (purple) spheres, and Earthis the (green) sphere at the origin (to scale). The last closed magnetic field lines in the ecliptic plane aredisplayed in red, and adjacent half-closed topologies are displayed in gray.

[29] The x coordinates of the nulls displayed in Figure 6ado not follow the same trend as in vacuum superposition.The nulls in vacuum superposition are in the dawn-duskplane (x = 0) for all !IMF as there is no Bx component of theIMF [see equation (8)]. In the MHD simulations, the nullsare near x = 0 for small clock angles but migrate toward thenightside as !IMF increases toward 150ı. Interestingly, thistrend is broken for !IMF = 165ı which has a null with a +xcoordinate.

[30] One might suggest the migration of the nulls to thenightside results from the draping of the IMF over the mag-netosphere. Draping causes the IMF to be oriented sunwardin the southern hemisphere and tailward in the northernhemisphere for northward IMF, with the opposite being truefor southward IMF. However, this effect would make thenulls migrate opposite to the observed direction, so drap-ing cannot explain the migration of the nulls’ x coordinate.We conclude that there is no simple explanation of the trend

Figure 6. Plots of magnetic null (a) x, (b) y, and (c) z coor-dinates as functions of IMF clock angle !IMF. Solid linesdisplay vacuum superposition prediction from equations (8)to (10), and asterisks are the coordinates of nulls in the MHDsimulations.

in the x coordinate of the nulls, but this is not surpris-ing since null locations are dependent on the shape of themagnetopause, which has a multi-parameter dependence onupstream solar wind conditions [Lu et al., 2011; Liu et al.,2012].

4.2. The Magnetic Separators[31] The separator tracing method described in section 2

is used to trace the dayside separators for the MHD simu-lations. We start from the magnetic nulls described in theprevious section and use hemispheres with radii of RHS =1 RE to trace the separators.

[32] Care must be taken in tracing the separator for !IMF =180ı due to the infinite number of nulls in the eclipticplane. We start by centering a hemisphere at the subso-lar point rNull = (7.87, 0.00, 0.00) RE and the hemisphereis discretized into the same N! ! N" grid as described insection 2.1. The hemisphere’s coordinates span longitude0ı " " " 180ı and latitude –90ı " # " 90ı. The cho-sen longitude range only traces the portion of the separatorduskward of the subsolar point. The algorithm iterativelymarches in the ecliptic plane until it no longer detects amerging location, ending at r = (2.93, 9.33, 0.00) RE. Thedawnward portion of the separator is traced likewise by forc-ing the hemisphere to have a longitude range of –180ı "" " 0ı, ending at r = (2.93, –9.33, 0.00) RE. The resultingseparator is stitched together with the subsolar point as thecenter of each portion.

[33] An example of a traced separator is shown inFigure 5, with the blue spheres denoting the intersectionof the separator with the hemispheres form the iterativetechnique described in section 2.1. For perspective, the lastclosed field lines in the ecliptic plane are shown in red, andthe adjacent half-closed field lines are shown in gray.4.2.1. Comparison With the Last Closed Field Line

[34] The last closed field line on the Sun-Earth line hasbeen used to approximate the magnetic separator since itclosely approaches both magnetic nulls (northward IMF:Dorelli et al. [2007]; southward and northward IMF: Hu

5003

Komar et al., 2013

Russell and Elphic, 1979 Sandel et al., 2003

Open Questions - Nightside• What causes reconnection onset?

– Electron or ion tearing? – First sign of disruption or the result of other (MHD-scale) instabilities? – How does process depend on the normal (dipolar) magnetic field?

• What is the physics of transient reconnection events such as bursty bulk flows, dipolarization fronts, and entropy bubbles?

– Dependence on magnetospheric conditions? – Their role in energy and mass transport? – How do they expand and spread as a function of time?

• How are energetic particles and thermal energy gains caused by reconnection? – What are the dominant mechanisms - reconnection electric fields,

slingshot effect, secondary islands, …? – What thermalizes plasma and what is temperature gain for given

upstream conditions?

• What is the physics of reconnection at the kinetic scale and how does it couple to the magnetosphere at macro-scales?

– Role of the extended electron diffusion region, pressure anisotropies, etc. – How do particulars of nightside reconnection impact modeling

of substorm injections? 18Wiltberger et al., 2000

Sitnov et al., 2009

NASA’s Magnetospheric Multi-Scale (MMS) Mission

• NASA mission made up of four coupled satellites – Designed to study the dissipation

scales of reconnection process – Successful launch on

March 12, 2015 from KSC – Fly in tetrahedron

separated by ~10 km • Not unlike Cluster mission, but

spacecraft separation is smaller • Must take data much faster to

resolve smallest scales – ~100 times faster than Cluster – Incredible engineering feat

• Results already coming in (Burch et al., Science, 2016;>65 papers in GRL) 19

Courtesy of NASA’s MMS

Sample of MMS Data

• From Burch et al., Science, 2016 • Summary talk Thursday at 9:15

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First release: 12 May 2016 www.sciencemag.org (Page numbers not final at time of first release) 15

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First release: 12 May 2016 www.sciencemag.org (Page numbers not final at time of first release) 14

Fig. 3. Summary data for two magnetopause crossings of MMS2 on 2015 October 16. The crossings are shown by the vertical blue dashed lines. Boundary-normal coordinates (L, M, N) are used with N normal to the boundary and away from the Earth, L perpendicular to N and in the plane of reconnection (nearly along the magnetospheric magnetic-field direction), and M normal to the L, N plane (generally westward). These directions were determined from a minimum variance analysis of the magnetic field data between 13:05:40 and 13:06:09 UT. The (x, y, z) GSE components of the L, M and N axes are: L = (0.3665, –0.1201, 0.9226) GSE, M = (0.5694, –0.7553, –0.3245) GSE, and N = (0.7358, 0.6443, –0.2084) GSE. Panel data include: (A) magnetic-field vectors, (B) energy-time spectrogram of ion energy flux, (C) energy-time spectrogram of electron energy flux, (D) total plasma density, (E) ion flow velocity vectors, (F) magnitudes of electron and ion convection velocities, (G) current computed from velocity moments of ions and electrons, (H) current computed from ∇ × B, (I) parallel and perpendicular (to B) electron temperatures, and (J) electric-field vectors. In the very low-density region to the left of the first vertical blue dashed line spacecraft charging effects on plasma moment calculations may affect the data. The diagram to the right is the result of a numerical plasma simulation (Movie 1) using parameters from the magnetopause crossing centered on 13:07 UT. Spatial coordinates in the diagram are shown both in km and in ion diffusion lengths, L(di). Color scale indicates JM current density.

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This Week and Summary• Reconnection sessions this week

– Monday 6/20, PM1 - 1:30pm-3:30pm • Individual session - kinetic physics in day- and night-side reconnection; presentations on MMS results; scene setting

talks by Allison Jaynes, John Dorelli, and Slava Merkin

– Monday 6/20, PM2 - 4:00pm-6:00pm • Joint with Mesoscale Auroral and Polar Cap Dynamics and Substorms focus groups - Ionospheric signatures of

magnetopause reconnection, dayside-nightside coupling, role of the ionosphere in transient reconnection processes

– Tuesday 6/21, PM1 - 1:30pm-3:30pm • Joint with Dayside Kinetic and Transient Phenomena focus groups - magnetopause reconnection, flux-transfer

events, Kelvin-Helmholtz instability, magnetopause waves, and other boundary layer processes

– Friday 6/24, PM1 - 1:30pm-3:30pm • Individual session - unfinished business from Monday, computational efforts, open forum, incorporation of kinetic

modeling into CCMC, topics of interest for next year, and the potential future of the reconnection focus group

• “The Tale of Magnetic Reconnection”21