Introduction
The Earth's magnetosphere is a cavity formed in the solar wind due to the
exclusion of the ionized gas (plasma) of the solar wind by the terrestrial
magnetic field.
Ultra-low-frequency (ULF) waves occur virtually everywhere within the
magnetosphere and at any time. They are a response to changes in the
magnetosphere, and are thus evidence of its dynamic behaviour. The designation
ULF usually refers to waves with frequencies less than 1 Hz. Waves
with frequencies in the mHz range have scale sizes comparable to the
size of the magnetosphere and are therefore strongly affected by the magnetospheric
structure.
The first observation of ULF waves was published by Balfour Stewart in 1861 using
magnetographs at Kew Observatory. He was describing pulsations of the
geomagnetic field, with an amplitude of roughly 0.2 % of the background field, and
frequencies in the range between 30 mHz and 3 mHz. Such events became known as
geomagnetic micropulsations. Stewart suggested
that sunspots are the primary cause of geomagnetic disturbances, but that the
direct effects result from currents flowing in the surface of the Earth
and in the - at that time hypothetical - electrically conducting upper layer
of the atmosphere, today known as ionosphere. Indeed, although many observations
of such waves were made in the succeeding decades, and apparently different
types of micropulsations were classified morphologically (according
to frequency, amplitude, occurrence time, etc.), little basic advance in
understanding the physical mechanism of pulsations was made until the
mid-twentieth century.
In 1942, Alfven proposed the existence of transverse
electromagnetic-hydrodynamic waves in a magnetized plasma. Dungey then
suggested in 1954 that the theory of generalized Alfven waves, also known as
magnetohydrodynamic (MHD) waves, could be used to explain the observed
properties of geomagnetic micropulsations. In the succeeding decades,
observations using ground-based magnetometers, auroral radars, and satellite
measurements of magnetic fields, electric fields and particle properties,
have led to a flowering of the theory of MHD waves in magnetospheric plasmas,
confirming Dungey's original proposals and extending them greatly.
Our research group is working on questions about ULF-pulsations in the magnetospheres of our solar
system and about excitation of ULF-pulsations. We are currently working on the following topics:
- Comparative studies of field-line resonances in the magnetospheres of Mercury, Earth, Jupiter, Saturn, and
Ganymed
- Magnetosphere-ionosphere-coupling by means of low-frequency waves
- Drift-bounce resonance instabilities
Literature
- Glassmeier, K.H., S. Buchert, U. Motschmann, A. Korth, A. Pedersen,
- Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current
instabilities,
- Ann. Geophys., 17, 338-350, 1999.
- Glassmeier, K.H., C. Othmer, R. Cramm, M. Stellmacher, M. Engebretson,
- Magnetospheric field line resonances: A comparative planetology approach,
- Surv. Geophys., 20, 61-109, 1999.
- Vogt, J., G. Haerendel, K.H. Glassmeier,
- A model for the reflection of Alfven waves at the source region of the Birkeland current system: the Tau generator,
- J. Geophys. Res., 104, 269-278, 1999.
- Othmer, C. , K.H. Glassmeier, and R. Cramm,
- Concerning field line resonances in Mercury's magnetosphere,
J. Geophys. Res., 104, 10.369-10.378, 1999.
- Cramm, R., K.H. Glassmeier, M. Stellmacher, C. Othmer,
- Evidence for resonant mode coupling in Saturn's magnetosphere,
J. Geophys. Res., 103,11.951-11.960, 1998.
- Glassmeier, K.H.,
- Currents in Mercury's magnetosphere, Proc. AGU Chapman Conference on Magnetospheric Currents,
in press, Washington, 2000.
- Cramm, R., K.H. Glassmeier, C. Othmer, K.H. Fornacon, H.U.Auster, W. Baumjohann,
E. Georgescu,
- A case study of a radially polarized Pc4 event observed by the
Equator-S satellite, Ann. Geophys., in press, 2000.
Related links:
Oulu Space Physics Textbook