It is a great pleasure to dedicate this special issue of Physica Scripta to Professor Lennart Stenflo, my mentor and close friend
for more than a quarter of a century. Lennart Stenflo is one of the prominent figures in plasma physics. Considering his many
outstanding contributions in nonlinear sciences, it was strongly felt by his friends that we honour Lennart by bringing out
these proceedings when he turns sixty (November 27, 1999). Accordingly, we sent out invitations to distinguished physicists
who collaborated with Lennart in physics and in other spheres of life, and asked them to write an article dedicated to
Lennart's Proceedings. Many colleagues responded overwhelmingly to our proposal. Due to page limitations we could
only select 36 contributions, however. They are compiled in this special issue of Physica Scripta. The executive Editor
of Physica Scripta Roger Wäppling generously supported our endeavors to celebrate this festive event. The whole-hearted
support from our colleagues clearly reflects our respect for the nice personality of Lennart, and it also demonstrates
his international leadership.
I have the pleasure of writing a few words about Lennart and his achievements in science and in other parts of life. I have
known Lennart as my mentor since, in 1971, he got the chair at the Plasma Physics Department at Umeå University (UmU).
Lennart built up an excellence in plasma physics at UmU, and trained a large number of doctoral students including myself.
Besides, the kind personality of Lennart has attracted many scientists from all over the world to visit Umei for collaborating
with him in various areas of nonlinear sciences.
Lennart has worked in many diverse fields of physics. His scientific accomplishments are too numerous to be listed here.
However, here are some highlights of his most distinguished contributions in physics:
Lennart Stenflo has derived many original results in the areas of parametric instabilities (wave–wave interactions), coherent
nonlinear structures (solitons and vortices), and chaos. Specifically, his contributions in parametric theory is, for
example, recognized by his discoveries of stimulated electromagnetic emissions from plasmas, where it is demonstrated
how the wave–wave interactions can generate radiation. His ideas have been verified experimentally in the Earth's
ionosphere and magnetosphere.
In the area of nonlinear structures, we know the Kaufman–Stenflo theory for the super-Alfvénic upper-hybrid envelope
solitons. Lennart Stenflo and his team has opened up a new field of strongly nonlinear surface plasma waves, which is
followed by many physicists around the globe. Furthermore, Lennart Stenflo presented a set of nonlinear equations
for acoustic gravity waves, which are now refered to as the Lorenz–Stenflo equations; the latter admit chaotic trajectories.
Recently, he has been working in modern astroparticle physics where our team has suggested that the neutrinos are
the building block of the universe, backed by the idea of collective neutrino plasma interactions. The strength of Lennart
Stenflo lies in the fact that he has very broad interests and feels at home with both microscopic and macroscopic nonlinear
theories in plasmas, as well as masters the physics and mathematics of fluid turbulence. Thus, many important contributions
of Lennart Stenflo have helped us in understanding the complex nonlinear phenomena in space and astrophysical plasmas, in
fluids and optics, as well as in laser produced and magnetically confined laboratory plasmas. Such a high level of Lennart's
perfection is truly admirable.
The present proceedings focus on topics which are central to Lennart's scientific interest. Thus, the authors have considered
the physics of various types of waves and their generation mechanisms, the development of turbulence and the
formation of coherent structures, particle and heat transport, neutrino-plasma coupling, as well as various aspects of
the nonlinear Schrödinger equation in plasmas, fluids, and nonlinear optics. The articles have dealt with the general picture
of the subject matter at hand and the underlying physics, as well as described the present state-of-art in the field. It was noted
that despite the diversity of the physical problems, the mathematical equations governing the particular phenomena and
their solutions remain somewhat similar. The diverse materials included in this topical issue of Physica Scripta simply
reflect the fact that Lennart's scientific knowledge is so rich, which is worth sharing.
I have had the privilege to work very closely with Lennart for such a long period of time and, therefore, have been able to
admire his fine personality and appreciate his deep dedication for science. We have also collaborated in organizing many
international conferences. On this particular occasion I also appreciate his warm feelings towards his fellow physicists
whom he always likes to support. We wish Lennart a very long and healthy life and hope that in the coming millennium
he will continue enlightening us with new physics and also show us a right path for doing high quality physics and
for appreciating each other as human beings as well.