Predicting Global Auroral Power:A Merging Term Plus a Viscous Term Works Best

P. T. Newell, T. Sotirelis, K. Liou, and C.-I. MengThe Johns Hopkins University Applied Physics Laboratory

Laurel, Maryland, 20723

Motivation

If thousands (or tens of thousands) of data points over multiple years are used, which, among the various coupling functions proposed, works best?

If the exercise is repeated for multiple disparate data sets, does a pattern emerge?

Popular Coupling Functions

(In ascending order of merit)

Bz

= vB2sin4(c/2)Akasofu-Perrault)

Bs

vBs (half-wave rectifier)EKL = vBTsin2(c/2) (Kan-Lee electric field)

EWAV= vBTsin4(c/2) (Wygant’s “intermediate”)

where c = arctan(By/Bz)

Approach

Ten characterizations of the magnetosphere were examined over multiyear periods at relatively high cadence (hourly)

Scores of coupling functions were tested, searching for a pattern (performance of 32 functions documented)

ab inito: No use of the time history of the target index

Data

5 Traditional (ground-based magnetometer) indices: AE, AU,

AL, Kp, and Dst

5 Space-age indices: Auroral power (Polar UVI), cusp latitude (sin(c)) and b2i (ion equatorward boundary of multi-keV ion

precipitation), both from DMSP, magnetotail inclination angle (arctan(v/h)) from GOES-8, and polar cap flux (PC) from

SuperDARN as calibrated to DMSP

Main Result

A single coupling function correlates best with 9/10 indices (11 data runs, counting multiple solar cycles)

dMP/dt = v4/3BT2/3sin8/3(c/2)

The exception is Dst. Dst correlates best (r=0.87) withp1/2dMP/dt

Meaning of dMP/dt

Dayside merging voltage is the product of three factors:

·solar wind electric field, vBT

·length of merging line, ~(BMP/BT)1/3

·% of lines which merge, sin8/3(c/2)

·Note that BMP (~v)

dMP/dt = v4/3BT2/3sin8/3(c/2)

• What is the form for the best viscous term?

• If two terms are used, is it best to used two highly performing coupling functions (both of which would be merging related) or one merging term and one viscous term?

We now know the optimized merging estimator. Here we pursue two follow up questions

Twenty Viscous Candidates

• n, v, nv, p (nv2/2), p2, p1/2, etc.

• The highest performing viscous function out of the 20 considered is:

n1/2v2

What is the optimal pair of terms?

• Is it best to use two highly performing terms (such as the Kan-Lee electric field and dMP/dt)?

• Or is it best to combine a merging term and a viscous term? And if so, is the best combination the obvious choice (best merging term plus best viscous term)?

496 Possible Combinations

• With 20 viscous-related terms and 12 merging-related terms (32 in all) there are 496 possible distinct combinations

• We evaluated the ability of all 496 combinations to predict 10 different indices (such as Kp, cusp latitude, auroral power etc) over multi-year periods at hourly cadence

Summary

• The best performing viscous term is n1/2v2

• Out of the 496 unique pairs from 32 coupling functions, the optimal pair is dMP/dt with n1/2v2

• Any reasonable merging term coupled with any reasonable viscous term does well

• dMP/dt and n1/2v2 together form a “tool kit” for model construction