# Presentation: 191824

## Electron-Ion Equilibration in Collisionless Shocks

-    By jump conditions, Te << Ti at the
-      shock front, but collisions, viscosity rapidly
-      establish Te = Ti  (e.g., terrestrial shocks...)

-   ISM: n ~ 1 - 100 cm -3, VSH ~ 100 -
-    10,000 km s -1

-   1 A.U.  <  L mfp  <  300 pc ; _collisionless_

-     MHD waves, turbulence assume the role of
-      collisions in shock transition
-    Shock front thickness  ~  RLarmor (i)  ( mi Vi / Zi e B)

- (STREAM  K.E.)    (PLASMA INSTABILITIES)  (WAVES)  (HEATING)

## Properties of Collisionless Shocks

-   Ions have most of flow energy   plasma waves resonant mostly with ions,
-     so that Te ≤ Ti ; heating anisotropic (T||  T)
-  Wave heating is _nonthermal_      fe (V) ,  fi (V) *non-Maxwellian* close to the
-    shock front (e.g., flat-topped electron distributions in solar wind; Feldman
-    et al. 1983)
-  Degree of collisionless heating sensitive to shock parameters (VS ,  B-n , )
-    *quasi-**** shock structures very different from quasi-|| ones; *_complicated;_  a
-    key problem in plasma physics (Kennel 1985)
-   If g0  (Te/Tp)0 , then   me/mp ≤ g0 ≤ 1
-   If g0 < 1, Te and Tp evolve downstream via Coulomb collisions; equilibration time
- 			teq(e-p)   Te 3/2 / ne ln e		(Spitzer 1978)

## Measuring g0 in ISM Shocks

-   Optical, UV, X-ray spectroscopy Of non-radiative shocks are best probes of g0
-  - Optical: collisional excitation of cold H I    narrow Balmer lnes
-                   excitation of charge-exchange (hot) H I   broad Balmer lines
-    	 (Chevalier, Kirshner & Raymond 1980)

- - *Use broad FWHM to bracket*
*        **range of shock speeds*
* **Use I**b **/ I**n** to obtain **g**0 **, f**H+*
*  **(Ghavamian 1999; Ghavamian*
*  **et al. 2001,2002)*

|  |   NE 
Cygnus |  RCW
    86 | Tycho’s
   SNR | SN 1006 |
| --- | --- | --- | --- | --- |
|    Vshock 
(km s -1) |    300 - 
   400 |   600 -
  650 |   1950 -
  2300 |    2900 |
|       g0 | 0.7 - 1.0 | 0.4 - 0.5 |   ≤ 0.1 |  ≤ 0.07 |

## g0  in ISM Shocks (cont’d)

-    Combined X-ray and Optical in DEM L 71
-      (Rakowski et al. 2003):
-    - Measure broad FWHM to constrain
-      Tave ( 3/16 mp Vsh 2 / k)
-    - Measure evolution of Te behind blast wave
-      by fitting X-ray spectra from radially spaced
-       extraction regions
-   -   Shock speeds probed here vary from
-       500 km s-1 (overlap with RCW 86) to
-       1000 km s-1 (not sampled before)
-     -  Independent confirmation from Ib/In not
-        possible: contamination from CR/fast
-        neutral precursor?  (Ghavamian et al.
-       2003)

- more evidence g0, Vsh
-   are _anticorrelated_

## Ion-Ion equil. in ISM Shocks

- Raymond et al. (2003)

-   Simultaneously probe electron-ion and ion-ion equil.
-     in collisionless shocks via UV spectroscopy:
- -  First, constrain shock speed via modeling of Balmer-
-    dom. Emission  (if present) or proper motion studies
-     (_if_ D is known)
- -  Second, measure widths of ion UV lines
-       *OVI  ****1032,1038, Ly **** (broad) **** (FUSE/HST/HUT)*
*      **C IV ****1548, 1550   N V ****1238,1243,  He II ****1640 ***
*	**	**(HST/HUT)*
-     equil: vi1 = (m2/m1)1/2 vi2              unequil.: vi1 = vi2
-    Trace the evolution of postshock emissivity ***  * get Te

-  Widths of C, O, He lines in SN 1006 indicate negligible
-   ion-ion equilibration (Laming et al. 1996)

-  Observations of NE Cygnus Loop indicate Tp / To < 2.5
-    (Raymond et al. 2003)

-  Collisionless heating effective for ALL species in
-    slow (< 350 km s-1)shocks: a clue to the mechanism?

## Collisionless Heating Mechanisms

-   Solar wind shocks: Earth’s bow shock, interplanetary shocks occur in n ~ 1 cm-3
-     plasmas (similar to ISM)
-   Response of the plasma characterized by Magnetosonic Mach number:
- 			MA   Vsh /  (CS2 + VA2)1/2
-   Difference: SW fully ionized, T ~ 105 K    1.5 < MA < 25 (young SNRs: MA  ~ 100s)
-   _Anticorrelation between __g__0__ and M__A__ seen in Balmer-dom. SNRs is also seen in solar_
-    _wind shocks __(Schwartz et al. 1988)_
-    Quasi-  shocks in solar wind:
-   - Some cold inflowing ions reflected from shock pass upstream
-   - Counter-motion believed to cause two-stream instability   plasma waves
-   - Plasma waves overrun by shock; amplified; decay by scattering,_ heating ions_
-   - Less and less efficient at high MA, as shock front becomes more turbulent
-      		*Relevant to **g**0** -V**sh** relation in Balmer SNRs?*

## Equilibration in the Cas A Forward Shock

-   No Balmer-dominated shocks; no Tp  constraint
-    Median expansion rate of forward shock:
-     R / R  ≈  0.002 yr-1   Vbw ~ 5030 km s-1
-     (Delaney & Rudnick 2003) for D = 3.4 kpc
-    Similar angular scales to DEM L71, *but spatial*
*     **scale 15 times finer*
-    1 Ms on Cas A     4000 cts in a 3”  1”
-      strip of outer rim
-  -  follow variation in thermal component behind
-     selected whisp to fit for Te  at shock front;
-     *combine with V**bw** to constrain T**e**/T**p*

- 4 - 6 keV continuum
- (Gotthelf et al. 2001)

- (Gotthelf et al. 2001)

- .

## Questions...

-   How does electron-ion equilibration inject particles into the cosmic ray acceleration
-     process?
-   Broad component profiles in Balmer-dominated shocks appear to be *Maxwellian;*
-    What does this imply about plasma processes at collisionless shock fronts?
-   - Can we probe non-Maxwellian electron distributions in Cas A?
-   Quasi-|| vs. quasi-  shocks: Does the higher CR acceleration efficiency in quasi-
-     shocks also mean that g0 ~ 1 in these shocks?
-  -  Can radio polarization + X-ray studies of Cas A shed light on this issue?
-  -  Can we measure what effect CR acceleration energy losses play on equilibration
-      in Cas A?
-   What is the more fundamental parameter for tracing equilibration: MA or Vshock ?
-    Are g0 vs. Vshock relations for Balmer-dominated shocks also true for shocks
-     in fully ionized gas?  Does ion-neutral drag make a difference?