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Observations of Low Frequency Waves During FAST-Geotail Conjunctions

K. Sigsbee and C. A. Cattell


FAST and the Dynamics of the Auroral Zone

The Fast Auroral Snapshot(FAST) satellite was launched on August 21, 1996 by a winged Pegasus XL rocket released from an L-1011 jet aircraft. The FAST satellite was inserted into a polar orbit with an altitude range of 350 to 4200 km (220 to 2,600 miles) above Earth in order to study the dynamics of the aurora borealis or northern lights. FAST carries instruments to measure energetic electrons and ions, electric fields, and magnetic fields above Earth's auroral zone.


Physics of the Magnetotail

It is believed that many of the physical processes that produce the aurora in Earth's ionosphere are coupled to the Earth's magnetotail, a region of space behind the night side of Earth where the solar wind stretches the Earth's magnetic field out into a long tail. In 1992, the Geotail satellite, a joint mission of the Japanese Institute of Space and Astronautical Science (ISAS) and NASA, was launched to study the dynamics of the Earth's magnetotail from the near-Earth region (8 Earth radii (Re) from the Earth) to the distant tail (200 Re). Geotail is a part of the International Solar-Terrestrial Physics Program.


FAST - Geotail Conjunctions

By performing analysis of near-simultaneous data from FAST and Geotail, we hope to gain a better understanding of how processes in the magnetotail couple to the auroral zones. Using a model of Earth's magnetic field, the positions of FAST and Geotail in space can be traced along magnetic field lines to magnetic footpoints at an altitude of 100 km (60 miles) in Earth's ionosphere. When the geomagnetic latitudes and longitudes of the FAST and Geotail footpoints pass within 10 degrees of one another, the connection between processes observed in the magnetotail by Geotail and auroral phenomena observed by FAST can be studied. Figure 1 shows a map of the FAST and Geotail magnetic footprints during the conjunction which occurred on January 18, 1997 between 03:00 and 08:00 UT [Sigsbee et al., 1998].




Figure 1: FAST and Geotail Footpoints

Click on the image to see the full size version.

Low Frequency Waves and Field Line Resonances

One way to examine coupling between the magnetotail and auroral zone is to study the low frequency waves observed by FAST and Geotail during conjunctions. The electric and magnetic fields from FAST and from Geotail were analyzed with the continuous Morlet wavelet transform. Although the continuous wavelet transform is computationally slower than traditional Fourier transform methods, the wavelet transform is better suited to analyzing non-stationary signals with sudden variations, such as those associated with magnetotail and auroral processes. Figures 2 and 3 show magnetic field data from FAST and Geotail and the corresponding wavelet scalegrams during the January 18, 1997 conjunction. Wave power at frequencies consistent with field line resonances was observed by FAST during the nightside auroral zone pass on orbit 1620. Waves at similar frequencies were observed by Geotail at a distance of 30 Earth radii in the magnetotail. This is a significant result because it has long been believed that field line resonances and MHD waves which propagate towards Earth can be excited by reconfigurations of the magnetotail.


Future Work

Ongoing studies of the waves observed by Geotail and FAST will involve a more careful examination of wave polarizations and the travel time of the waves between the magnetotail and auroral zone. Conjunctions with the Polar satellite and various ground stations will also be considered.


Acknowledgments - This work was supported by NASA Grants NAG5-3596 for FAST and NAS5-2718 for Geotail.

Click on the images to see the full size version.

Figure 2: Scalegram of Geotail BX GSE


Figure 3: Scalegram of FAST Magnetic Field Data



References

Sigsbee, K., et al., FAST - Geotail correlative studies of magnetosphere - ionosphere coupling in the nightside magnetosphere, Geophys. Res. Lett., 25, 1998.


Related Science Nuggets

Geotail Studies of Low Frequency Waves in the Near Magnetotail Using the Wavelet Transform - K. Sigsbee and C. A. Cattell

A Global Reconfiguration of the Magnetosphere Observed by Geotail and Polar - K. Sigsbee and C. A. Cattell

Relationship of FAST Data to Auroral Activity Observed by the Polar Ultraviolet Imager - K. Sigsbee and C. A. Cattell


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