QSAS Index deHoffmann-Teller Analysis

Contents:

Overview

DeHoffmann Teller (HT) analysis basically consists of calculating a frame of reference in which the electric field vanishes. The velocity of this frame can under certain conditions be regarded as the motion speed of a rigid plasma structure, e.g., a plasma bubble or a plasma boundary like e.g., the magnetopause.

An optional plasma density may be supplied. The Alfven velocity is then calculated. The velocity relative to the deHoffmann-Teller velocity may then be compared with the Alfven speed. A component-by-component plot of these two quantities - a Walen plot - may give an indication of reconnection. If the two quantities are nearly proportional, it may be an indication of reconnection.

DeHoffmann-Teller analysis is described in Chapter 9 of www.issi.unibe.ch:issi_sr.html.



Input data

Required inputs are a magnetic field vector time series and a plasma velocity time series. The latter is used to calculate the electric field through E = -V x B.

Time tags are taken from input slot fro V (or E), and the B-field and other inputs are joined to these time tags.

Optional inputs :
An optional plasma density may be supplied to calculate the Alfven velocity.
If the desnity is supplied, optional temperatures Tperp and Tpar may be supplied. The Alfven velocity will then be corrected for pressure anisotropies.
There may be cases where the pressure anisotropy prevents calculation of a Alfven velocity. A NaN (not a number) will be the inserted in the Alfven velocity. Leave slot empty if N/A.
An optional normal unit normal, e.g., a boundary normal may also be supplied - the HT velocity along this normal is then calculated. The nomal may be a QSAS object or can be typed directly in the in put slot. Ignore the red background in the latter case. Leave slot empty if N/A.



Outputs

Output quantities are:

- E_HT - vector time series with the Hoffmann-Teller electric field
- E_C - vector time series with the convective electric field (E= V x B)
- V_VHT - vector time series with the rest velocity, i.e. (V(i) - V_HT)
- V_Alfven - vector time series with the Alfven velocity. Only valid if density is inputted

In addition, the each time interval, number of samples, the complete HT velocity, and accelleration is printed in the PlugIn output window. If a boundary normal is supplied in the Normal slot, the velocity and acceleration along this normal is also printed.

Slope and correlation coefficients are returned as metadata in the E_HT and V_ALFVEN outputs.



Bugs/Caveats

No bugs known. If alpha (pressure anisotropy) results in an imaginary correction, NaN may be returned as Alfven velocity.


History

None known.

2005-01-30 : corrected bug in pressure correction (Pperp and Ppar were swapped).

2005-05-13 : catch singularity in 1-alpha correction.

2014-06-04 : DVOS variant for QSAS 3.x.

2016-10-10 : removed above singularity catch - NaN returned if sqrt(1-alpha) becomes imaginary.

2016-11-16 : slope and correlation coefficients returned in metadata.


SEH, 15 Nov, 2016