Comparison of the Drag Model, ENLIL Model and Observations for

the July 12, 2012 CME+Shock Event

Phillip HessJie Zhang, Dusan Odstrcil

George Mason University Fairfax, VA 22032ISEST Workshop

Hvar, Croatia06/19/2013

Purpose

To use prior events for which we have multiple data sets to constrain the physics of the Aerodynamic Drag Model to make CME and shock arrival predictions based only on initial conditions as accurate as possible

Provide a comparison between observations, analytic and numeric models for a well observed CME event

Snapshot of CME at 17:54 UT from three viewpoints fit with GCS Model (Thernisien 2006, 2009)

ENLIL Data courtesy D. Odstrcil

ENLIL Data courtesy D. Odstrcil

ENLIL Data courtesy D. Odstrcil

ENLIL Data courtesy D. Odstrcil

Aerodynamic Drag Modelr (t )= 1

γln [1+γ (v 0−v sw) t ]+v sw t+r 0

Where γ is the drag parameter, v0 is the initial CME speed, v

sw is the ambient solar wind

speed, r0 is the initial CME speed (Vrsnak et al 2012)

Where cd is the drag coefficient, A

is the CME cross-section, ρ

sw is the ambient solar

wind density, ρ is the initial CME density, L is the CME thickness, ρ

sw0 is the initial

ambient solar wind density, and ρ0 is the initial CME density, ρ

sw1AU is the ambient solar

wind at 1AU, and ρ1AU

is the CME density at 1 AU.

The first three steps are described in detail in Vrsnak et al, the fourth and fifth were obtained by making three assumptions: (1) That L (CME thickness) could be approximated by r. This is essentially a spherical assumption. (2) That ρ

sw ~ 1/r2 and (3)

That ρ0 ~ 1/r3

Or, to make fitting simpler γ can be treated as a constant

γ=cd A ρswM +M v

=cd A ρsw

V ( ρ+ρsw2

)=

cd

L( ρρsw

+ 12)=

cd

r (ρ0

ρsw0 r+ 1

2)=

cdρ1AU

ρsw1AU+ r

2

Comparison of ParametersIn-Situ ENLIL Drag Model

FittingDrag Model with varying γ

1 AU Driver Velocity (km/s)

487.7 533.1 492.5 477.8

1 AU Shock Velocity (km/s)

608.3 707.5 621.4 614.4

Constant γ Max γ Min γ Avg. γ

Driver 2.78e-8 5.07e-8 2.36e-8 3.23e-8

Shock 2.02e-8 2.95e-8 1.26e-8 1.79e-8

For both fronts, ENLIL over estimates the velocity by 10-15% but the drag model does well, whether the data points are being fit to data with a constant γ or γ varies to

provide the best comparison with in-situ arrival

For both fronts, the constant γ falls between the min and max γ but the average value of the varying γ is above the constant γ

Conclusions

If more events are studied, a unique CD

can be determined for each event based on initial knowledge, improving the accuracy of predictions

For this predictive scheme to work, the best way of determining the initial CME velocity and density and the ambient solar wind density and velocity will have to be determined

By comparing, observations to analytical and numerical models, we can understand more about all three