abstract the non-dipolar portions of earth's main magnetic field constitute substantial...

Abstract

The non-dipolar portions of Earth's main magnetic field constitute substantial differences between the geomagnetic field configurations of both hemispheres. They cause in particular different magnetic field flux densities in the opposite polar regions and different offsets of the invariant poles with respect to the rotation axis of the Earth. The offset is presently considerable larger (factor ~2) in the Southern Hemi-sphere compared to the Northern, which has substantial implications for the coupled magnetosphere-ionosphere-thermosphere system under the influence of external drivers. Recent observations have shown that the ionospheric/thermospheric response to solar wind and IMF dependent processes in the magnetosphere can be very dissimilar in the Northern and Southern Hemisphere. We present statistical studies of both the high-latitude ionospheric convection and the patterns obtained from almost a decade of measurements starting in upper thermospheric circulation 2001 of the electron drift instrument (EDI) on board the Cluster satellites and an accelerometer on board the CHAMP spacecraft, respectively. Using the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model, on the other hand, we simulated a 20-day spring equinox interval of low solar activity with both symmetric dipole and realistic (IGRF) geomagnetic field configurations to prove the importance of the hemispheric differences for the plasma and neutral wind dynamics. The survey of both the numerical simulation and the statistical observation results show some prominent asymmetries between the two hemispheres, which are likely due to the different geographic-geomagnetic offset, or even due to different patterns of geomagnetic flux densities. Plasma drift differences can partly be attributed to differing ionospheric conductivities. The forthcoming Swarm satellite mission will provide valuable observations for further detailed analyses of the North-South asymmetries of plasma convection and neutral wind dynamics.

North-South differences in Earth's high-latitude upper atmosphere dynamics: influence of solar activity and seasonal variations

Matthias Förster 1 and Ingrid Cnossen 2

1 - GFZ German Research Centre for Geosciences, Helmholtz Centre Potsdam,Telegrafenberg, D-14473 Potsdam, Germany, mfo @ gfz-potsdam.de

2 - British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, United Kingdom, inos @ bas.ac.uk

EGU 2014 Session ST 3.2 Vienna, Austria April 30, 2014

DIFFERENCES IN HIGH-LATITUDE PLASMA DRIFTS AND NEUTRAL WINDS BETWEEN THE NORTHERN AND SOUTHERN HEMISPHERE: THE ROLE OF MAGNETIC FIELD ASYMMETRY

CONCLUSIONS


CONCLUSIONS


Outline

Observational evidences

- EDI/Cluster average ion drift pattern at high magnetic latitudes

- CHAMP accelerometer neutral wind pattern at high latitudes

Are there significant differences at high latitudes between the NH and SH ?

What is the role of the geomagnetic field for M-I-T coupling processes ?

Seasonal and solar cycle dependencies?

Numerical experiment

- Global first principle model GTIM (incl. TIE GCM) of the M-I-T system

- High-latitude neutral wind and ion drift: magnitudes, variances, direction

- Neutral wind and drift parameters versus UT and IMF orientation

- Derived parameter: neutral wind vorticity


Cluster/EDI Average Convection Pattern


Wind Vector Distribution PatternCHAMP accelerometer 01 Jan 2002 - 31 Dec 2003

North Hemisphere South Hemisphere


Vorticity of Wind Vector DistributionCHAMP accelerometer 01 Jan 2002 - 31 Dec 2003

North Hemisphere South Hemisphere

cf.: M. Förster et al., Ann. Geophys., 29, 1, 181-186, 2011


North South


North South

...with Altitude Adjusted Corrected Geomagnetic Coordinates (AACGM)


Model interval: March 20 – April 08, 2008 (20 days)

Coupled M-I-TModel (CMIT)

= Lyon-Fedder- Mobarry (LFM) magnetospheric MHD code plus the TIE-GCM

Wiltberger et al., 2004,JASTP, 66, 1411-1423

Wang et al., 2004,JASTP, 66, 1425-1441

Wang et al., 2008,GRL, 35, L18105

Whole HeliosphereInterval (WHI 1)

= Carrington Rotation 2068

Bisi et al., 2011, Solar Physics, 274 (1-2)

Wiltberger et al., 2012,JASTP, 83, 39-50


Neutral wind NH / SH & model run comparisons


Ion drift NH / SH & model run comparisons


Comparison NH/SH: Neutral wind versus UT


Comparison NH/SH: Ion drift versus UT


Neutral wind variance versus UT


Neutral wind vorticity maxima versus IMF sector


Neutral wind vorticity [mHz]: Min/Max values



CONCLUSIONS


CONCLUSIONS

CONCLUSIONS

Numerical experiment for equinox conditions with two different model runs:

- Symmetric geomagnetic dipole and - Asymmetric (“real”) IGRF configuration.

The model results reveal substantial differences in the average ion drift and neutral wind parameters at high magnetic latitudes between NH and SH:

IGRF results suggest 10-15% NH > SH wind and ion drift magnitudes, but practically no differences for the symmetric dipole case;

the spatial variance of wind magnitudes is SH > NH for IGRF case only,showing a pronounced diurnal variation;

the variation of the ion drift & neutral wind versus IMF orientation shows the familiar Bz & By dependence in magnitude for both model realisations;

the average maxima & minima of wind vorticity show a ~10% difference and are in good coincidence with experimental results (CHAMP);

there is a factor ~2 difference in wind vorticity magnitudes and NH > SH differences between low (2006/8) and high (2002/3) solar activity.

See: Förster and Cnossen, JGR, Vol. 118, 5951-5966, 2013.

Follow-up studies clarify seasonal & solar activity dependences in more detail.


Average neutral wind magnitude [m/s] vs. DoY


Neutral wind direction versus UT



Neutral wind direction versus IMF sector

Solstice model intervals: December 2002 & June 2003


Neutral wind vorticity [mHz]: Min/Max values


Average model neutral wind magnitude > 80 deg


abstract the non-dipolar portions of earth's main magnetic field constitute substantial...

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