fractal reconnection at the earth’s magnetopause and associated ionospheric convection. gary abel,...

Fractal reconnection at the Earth’s magnetopause and associated

ionospheric convection.

Gary Abel, Iain Coleman, Mervyn Freeman and Gareth Chisham

British Antarctic Survey

MRT Workshop 9th -10th August 2004

A quote from the classic Dungey [1961] paper

“A steady laminar flow will be assumed here for simplicity, but it should be noted that large variations of the field were detected by Pioneer I” … “The connection between the neutral points (reconnection) and the auroras (convection) is obvious in this model, but it remains to study … the effect of turbulence.” Dungey [PRL 1961].

• We commonly model dayside reconnection in terms of a uniform IMF phase front draped across the magnetopause by a laminar hydrodynamic flow.

• Such models naturally give rise to large-scale, spatially coherent reconnection structures on the magnetopause and in ionospheric convection.

• The draped IMF is in reality highly disordered, and not well described by laminar models.

• What is the implication of this at the magnetopause and in the ionosphere?

Introduction

Antiparallel Reconnection with Perfect Draping

The clock angle of the magnetosheath B-field is everywhere equal to the upstream IMF clock angle of 135°. The antiparallel reconnection regions

are shown in black

Fractal anti-parallel reconnection

• Magnetosheath is noisy• Difference between upstream measured clock angle and sheath

clock angle has standard deviation of around 40º

But!!!

Fractal anti-parallel reconnection

Perfect Draping White (gaussian) Noise Draping

• Adding significant amounts of gaussian noise breaks down the structure of the reconnection regions.

• This is because gaussian noise is spatially uncorrelated.

• It may be more reasonable to model the “noisy” part of noisy draping as fractal turbulence, rather than gaussian white noise.

• In the following slides, a random fractal of power spectrum –5/3 is added to the magnetosheath field, such that the standard deviation of the clock angle is similar to that in the gaussian case.

The Trouble with Gaussian Noise

Fractal anti-parallel reconnection

Perfect Draping Red Noise Draping

Gaussian (White) Noise

Fractal Noise

Time Series of Reconnection Sites with Fractal Turbulence

• Fractal turbulence is correlated on large spatial scales.

• Thus, it preserves the antiparallel reconnection structures of the laminar model to far greater extent than gaussian noise can manage.

• However, further structure is introduced to magnetopause reconnection sites.

• Is this structure reflected in ionospheric convection?

The Importance of Fractal Turbulence

Investigate spatial structuring of ionospheric flows using structure function analysis

• Structure function analysis using SuperDARN radar.

• Radar measure LOS component of convection velocity.

• Use Halley meridional beam – ranges 10 to 65.

• 1997 – 2001.

• Calculate <|v(r+l)-vr|m>, m=1,2,3, l=1,2,… 55

• Precondition data so we only include fluctuations of ±1

Poleward of Open/Closed Field Line Boundary -

Nightside

Slope = 0.81

Slope = 0.57

Slope = 0.30

)(P ss lvl

Summary and Conclusions

• M-I system is a complex system driven by a complex driver.

• In order to fully understand the system we must understand the fractal nature.

• Our simple model suggests that a fractal solar wind can give rise to structured reconnection on the magnetopause while maintaining spatially large scale features.

• Observations in the ionosphere show the fluctuations in convection velocity are consistent with such a model.