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Click here for Norwegian version

NATURE LETTER:

Asymmetric auroral intensities in the Earth’s Northern and Southern hemispheres

In a Nature letter published July 23, 2009, Laundal and Østgaard present images of the aurora taken simultaneously in the Northern and the Southern hemispheres. These images reveal indisputable evidence that the auroras in the two hemispheres can be totally asymmetric. These findings contradict the commonly made assumption of aurora being mirror images of each other.


Link to the paper (Nature subscription required)
Link to the cover at the Nature site
Download an e-print version (limited number of downloads)

Nature podcast (skip to 17:10 for the story on asymmetric aurora):


Click here to see the animation based on data (made by Espen Madsen).


Asymmetric aurora seen on May 12, 2145 UT
The upper two images show the aurora in the North and South in magnetic coordinates. The magnetic poles are in the center. Day is up and night is down. Dusk is to the left and dawn is to the right. A bright spot is seen in the northern dawn, while a bright spot is seen in the southern dusk. In the lower panel a model presentation of the magnetic field lines connecting the two hemispheres are shown. The inset illustrate the different source regions of particles creating the two bright spots.


In this Nature letter new evidence is presented that suggests that the commonly held idea that the aurora borealis (North) and australis (South) are mirror images of each other is not always true. Aurora generally occurs simultaneously in the northern and southern polar regions at locations that are connected by geomagnetic field lines with footprints in both hemispheres (see the figure above). Such a connection predicts a symmetric temporal behaviour and spatial location. Simultaneous space-based observations of the same auroral feature in both hemispheres can provide evidence or disprove the expected conjugate behaviour.


Asymmetric aurora on May 12, 2139-2153 UT
Click here to see the animation based on data
made by Espen Madsen.


Since the beginning of the space age simultaneous global imaging from space has only been possible twice, Viking and DE1 in the mid 80s and now IMAGE and Polar. As the two spacecraft have to be in the right position at the time of observable aurora, there are very few time intervals (less than ten that have been reported) where the evolution of the global aurora can be studied in both hemispheres at the same time. In this paper we show indisputable evidence that the auroras can be totally asymmetric and contradicts the commonly made assumption of aurora being mirror images of each other. Various explanations are considered and discussed in the letter.
1) Different magnetic field strength in the two hemispheres cannot explain the differences.
2) The slowing down of the solar wind as it encounters the Earth's magnetosphere is known to create a dynamo. Due to the orientation of the interplanetary magnetic field, this solar wind dynamo will be different in the two hemispheres. However, this effect may explain one of the spots but not both.
3) The most plausible explanation is that our observations provide strong evidence for the predicted inter-hemispheric current system that is set up due to seasonal differences in conductivity in the north and south. This current system has never been observed simultaneously in the two hemispheres. Due to the lack of simultaneous auroral images covering the entire auroral oval in both hemispheres we do not know how common such totally asymmetric auroras are.

The significance of these results can also be emphasized by the following; an analysis based on data from only one hemisphere is likely to be totally inadequate to explain the data from the other hemisphere. This is important since most of our knowledge about aurora and the processes producing them is based on data from the northern hemisphere.

Authors

The authors are both from the Space Physics Group at the Dept. of Physics and Technology at the University of Bergen, Norway.

Karl Magnus Laundal is a PhD student funded through IPY_ICESTAR, Research Council of Norway, contract 176045/S30. His PhD project is focused on conjugate observations of the aurora from space and especially when we have simultaneous data from both the northern and southern hemisphere. Both temporal and spatial asymmetries are often seen in the aurora in the two hemispheres. Karl Magnus Laundal started his PhD project in 2007 and will defend his thesis in 2010.

Contact information:
email: karl.laundal [at] ift.uib.no
cell: +4791350772
Nikolai Østgaard, Professor, Principal Investigator/Project Manager of the IPY-ICESTAR project (see below) and the advisor for Karl Magus Lasundal. He is the group leader of the Space Physics Group at the University of Bergen. Conjugate imaging from space is one of his main research interests.

Contact information:
email: Nikolai.Ostgaard [at] uib.no
cell: +4747270653


If neither of the authors can be reached, please contact:
Arve Aksnes
Office: +47 555 88 153
Cell: +47 480 26 563
E-mail: Arve.Aksnes [at] mnfa.uib.no

IPY-ICESTAR

IPY-ICESTAR: International Polar Year - Interhemispheric Conjugacy Effects in Solar Terrestrial and Aeronomy Research
The study presented in this Nature Letter is a result of the IPY-ICESTAR project. This project is part of the international ICESTAR program under SCAR. Professor Østgaard is one of the co-leaders of ICESTAR. IPY-ICESTAR is the only Space Physics project that was funded through the extraordinary effort by the Research Council of Norway to support the International Polar Year. IPY-ICESTAR is funded by the Research Council of Norway through the contract 176045/S30.

Polar and IMAGE

The data used for this study are from the Polar VIS Earth Camera (PI: J. B. Sigwarth) and IMAGE Far-Ultraviolet Wide Band Camera (PI: S. B. Mende). We are indebted to the NASA IMAGE and Polar teams for the design and successful operations of the two missions. We especially thank S. B. Mende for the use of IMAGE FUV WIC data and J. B. Sigwarth for the use of Polar VIS Earth Camera data.

The material on this page can be used freely, as long as reference is made to the Nature paper, and it is stated that the data come from Polar VIS Earth (J. B. Sigwarth) and IMAGE WIC (S. B. Mende). The video must be credited Espen Madsen.

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