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1. What controls the occurrence of reconnection. 2

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1 1. What controls the occurrence of reconnection. 2
1. What controls the occurrence of reconnection? 2. What controls the degree of plasma heating in reconnection? => Plasma b and Alfvén speed Tai Phan (UC Berkeley) Contributors: J. Drake, J. Gosling, M. Shay, M. Swisdak, C. Haggerty, D. Larson, R. Lin, et al. Wind + THEMIS Teams

2 Reality: Not easy to trigger reconnection
Cartoon Reconnection jet 1 2 1 2 jet Reality: Not easy to trigger reconnection

3 Solar wind current sheets:
How often does reconnection occur in current sheets in the magnetosphere and solar wind? jet jet jet Solar wind current sheets: occurrence rate < 1% (Gosling, 2011) Magnetotail: occurrence rate < 5% (Angelopoulos, 1994) Magnetopause: occurrence rate ~ 50% (Paschmann et al., 1986)

4 This dependence on b was not understood theoretically
What is known observationally about the onset conditions for collisionless reconnection? Magnetotail: Thin current sheet (< 1 ion skin depth) [Sanny, 1994] Magnetopause: - Thin current sheet not enough for reconnection - Reconnection observed preferentially for low b (< 2) [Paschmann 1986] b= Pplasma / PB magnetosheath jet jet This dependence on b was not understood theoretically

5 W = li b2 b1 Db = b2-b1 Db < 2 tan(q/2) (W/li)
Swisdak et al. [2003, 2010]: Reconnection occurrence depends on b and magnetic shear in asymmetric reconnection VA W = li Reconnection allowed magnetic shear q (degrees) W no reconnection b2 b1 Diffusion region (diamagnetic drift of X-line suppresses reconnection) Db = b2-b1 density gradient scale Db < 2 tan(q/2) (W/li) Physics: Diamagnetic drift of X-line prevents reconnection if drift speed > VA Introduction

6 Testing the Swisdak prediction using solar wind reconnection events
q = 140o L jet W N b1 b2 Diffusion region wind b1 b2 0.2 0.1 Gosling et al. [2005] Db = b2-b1 = 0.1 Measure Db and magnetic shear Introduction

7 W = li b2 b1 Db = b2-b1 Db < 2 tan(q/2) (W/li) reconnection W
magnetic shear (degrees) W no reconnection b2 b1 Diffusion region (diamagnetic drift of X-line suppresses reconnection) Db = b2-b1 density gradient scale Db < 2 tan(q/2) (W/li) Introduction

8 Occurrence of solar wind reconnection vs. Db and magnetic shear
Phan et al. [ApJL, 2010] 197 reconnection events Wind ww density gradient scale Db < 2 tan(q/2) (W/li) At Db=0.1, reconnection can occur for magnetic shear down to 10o! At Db=2, reconnection requires magnetic shear >100o

9 jet bmagnetosheath bmagnetosphere
Testing the Swisdak prediction at the Earth’s magnetopause: THEMIS-D 2008 Reconnection Events magnetosheath w = li forbidden allowed w = 2 li jet Non - Reconnection Events bmagnetosheath bmagnetosphere allowed forbidden

10 Implication for Solar Probe Plus:
- The occurrence rate of solar wind reconnection closer to the Sun should be much higher than at 1 AU because b is much lower closer to the Sun b (10 Rs) = 0.1 b (1 AU) => Reconnection expected to be more common at 10 Rs than at 1 AU Similarly, more reconnection at inner planets (e.g., Mercury) than at outer planets (e.g., Saturn) (Masters et al., 2012)

11 Ion and Electron Bulk Heating in Reconnection:
Dependence on Inflow Alfven Speed Reconnection converts magnetic energy into: Kinetic energy (plasma jetting) Electron heating Ion heating VA Heating is assumed, but not always observed Diffusion region

12 Solar Wind at 1AU: not much
Amount of plasma heating different in different regions jet jet jet Solar Wind at 1AU: not much Electrons: No heating [Gosling 2007] Ions: 10s of eV heating Magnetopause: Electrons: 10s of eV Ions: 100s of eV Magnetotail: strong Electrons: keV (107 K) heating Ions: many keV

13 Solar Wind at 1AU: not much
Amount of plasma heating different in different regions jet jet jet Solar Wind at 1AU: not much Electrons: No heating [Gosling 2007] Ions: 10s of eV heating Magnetopause: Electrons: 10s of eV Ions: 100s of eV Magnetotail: strong Electrons: keV (107 K) heating Ions: many keV The degree of electron heating must depend on plasma regimes

14 THEMIS statistical survey of ion and electron heating at magnetopause
Identify reconnection exhausts Measure the amount of heating in exhaust (relative to inflow plasma) Determine the boundary conditions: magnetic field, density, b, etc… jet magnetosheath jet Goal: Determine what parameters control the degree of plasma heating Main finding: heating linearly proportional to available magnetic energy (mi VA2)

15 THEMIS statistical survey of ion and electron heating at magnetopause
Identify reconnection exhausts Measure the amount of heating in exhaust (relative to inflow plasma) Determine the boundary conditions: magnetic field, density, b, etc… jet magnetosheath jet - Electron bulk heating - Ion bulk heating

16 Electron heating (DTe) depends on inflow (magnetosheath) electron b
shocked solar wind DTe = Te, exhaust – Te, inflow 79 reconnection events VA DTe (eV) high density magnetosheath low density magnetosphere Diffusion region be, rec inflow inflow nkTe/(Brec2/2m0) b dependence: Is it B, n or T ? exhaust

17 b dependence : Is it Brec, n or Te?
Yes DTe (eV) Brec,sheath (nT) No Yes DTe (eV) DTe (eV) nsheath (cm-3) Te sheath (eV)

18 If heating depends on inflow B and n but not on Te, it must be VA , not be
DTe (eV) VA,sheath (km/s) DTe  VA,sheath1.84

19 If heating depends on inflow B and n but not on Te, it must be VA , not be
DTe (eV) low density magnetosphere high density magnetosheath VA,sheath (km/s) Diffusion region DTe  VA,sheath1.84 inflow inflow DTe  mi VA,sheath2 exhaust B2/(m0n) available magnetic energy per particle in magnetosheath

20 If heating depends on inflow B and n but not on Te, it must be VA , not be
DTe (eV) DTe (eV) VA,sheath (km/s) VA,asymmetric (km/s) DTe  VA,sheath1.84 VA,asym = [BshBsph (Bsh+Bsph)/m0(rshBsph+rsphBsh)]1/2 DTe  mi VA,sheath2 symmetric limit = B/(m0r)1/2 B2/(m0n) Cassak-Shay [2007] Swisdak-Drake [2007] Birn et al. [2010] available magnetic energy per particle in magnetosheath

21 Electron heating depends on the available magnetic energy per particle in both inflow regions
Slope= 0.017 DTe (eV) DTe (eV) m VA,asym2 (eV) = available magnetic energy VA,asym (km/s) DTe  VA,asym 2.02 DTe = mi VA,asym2 DTe~ 1.7% of available magnetic energy per particle

22 THEMIS statistical survey of ion and electron heating at magnetopause
Identify reconnection exhausts Measure the amount of heating in exhaust (relative to inflow plasma) Determine the boundary conditions: magnetic field, density, b, etc… jet jet - Electron bulk heating - Ion bulk heating

23 Ion heating also linearly dependent on available magnetic energy
Ions Electrons Slope= 0.017 Slope= 0.017 DTi (eV) DTe (eV) mi VA,asym2 (eV) mi VA,asym2 (eV) DTi = 0.13 mi VA,asym2 DTe = mi VA,asym2 DTi ~ 13% of available magnetic energy DTe~ 1.7% of available magnetic energy

24 Ion heating also linearly dependent on available magnetic energy
Ions Electrons Slope= 0.017 Slope= 0.017 DTi (eV) DTe (eV) mi VA,asym2 (eV) mi VA,asym2 (eV) DTi = 0.13 mi VA,asym2 DTe = mi VA,asym2 DTi ~ 13% of available magnetic energy DTe~ 1.7% of available magnetic energy Percentage of inflowing Poynting flux converted into ion and electron enthalpies: DTi/mi VA,asym2 = 0.13 g/(g -1) = 33% DTe/mi VA,asym2 = g/(g -1) = 4%

25 DTi = 0.13 mi VA,asym2 Solar Wind Magnetopause DTi = 0.13 mi Vjet2
Same dependence found in the Solar Wind Magnetopause Solar Wind Slope= 0.017 DTi = 0.13 mi Vjet2 DTi (eV) mi VA,asym2 (eV) DTi = 0.13 mi VA,asym2 measured reconnection jet speed ~ VA [Phan et al., 2013; 2014] [Drake et al., 2009]

26 Ion heating in solar wind reconnection events [Drake et al., 2009]
DTi = 0.13 mi Vjet2 measured reconnection jet speed ~ VA

27 Solar Wind reconnection:
Can we explain the observed differences in heating level in different regions? jet jet inflow jet Solar Wind reconnection: Electron: No heating [Gosling 2007] Ions: 10s of eV heating Magnetopause: Electrons: 10s of eV Ions: 100s of eV Magnetotail: Electron: keV heating Ions: many keV inflow VA ~ 2000 km/s inflow VA ~ 50 km/s

28 Extrapolating to magnetotail, solar wind, and solar corona
DTe = mi VA2 DTi = 0.13 mi VA2 Magnetotail inflow VA ~ 2000 km/s: DTe~ 700 eV (107 K) DTi ~ 5.6 keV 1 AU Solar wind VA ~ 50 km/s : almost no heating DTe~ 0.45 eV DTi ~ 4 eV Solar wind closer to Sun -> higher VA -> larger heating than at 1 AU Solar corona (B~ 100 G, n~ 1010 cm-3): VA ~ 2200 km/s -> DTe~ 107 K Exception: within ICME Pulupa et al. [2014]

29 Degree of ion and electron bulk heating depends on miVA2
Summary Degree of ion and electron bulk heating depends on miVA2 Implications: Strong heating in Earth’s magnetotail reconnection Little heating in solar wind reconnection at 1AU 107 K electron heating in solar corona (B~ 100 G, n~ 1010 cm-3) DTe = mi VA2 DTi = 0.13 mi VA2 Electrons: Phan et al. [GRL, 2013] Ions: Phan et al. [GRL, 2014]

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