Virtual issue to honour Lennart Stenflo on his 75th birthday

I am very pleased to present this virtual issue of Physica Scripta in honour of Lennart Stenflo, celebrating his 75th birthday. The idea to produce such an issue was suggested by the editor-in-chief Suzy Lidström. I immediately thought this was an excellent idea, given Lennart's outstanding contributions to science and also my good friendship with him. Thus, Suzy and I sent out invitations to some collaborators and friends of Lennart. To our joy we got a very positive response, with many excellent contributions to choose from.

I have known Lennart since 1986 when he became the supervisor for my graduate studies and my mentor. Since then we have remained in close contact, and during the years we have worked together on many problems. There are many things that could be put forward highlighting the contributions that Lennart has made to science. To reduce the scope somewhat, I have chosen to focus on Lennart's importance for Physica Scripta here, which is still a large enough subject.

Lennart is probably the person who has worked for Physica Scripta the longest. He has been an editor for more than 32 years, and before that he was a much used referee for the journal and its predecessor 'Arkiv för Fysik'. Moreover, he has written many important publications for Physica Scripta over the years, covering several areas of plasma physics and nonlinear science.

Some examples of this include his work on stimulated scattering of large amplitude waves in the ionosphere, which he has covered in the review papers [1] and [2], describing the history of the field and his struggle to convince the scientific community of the importance of nonlinear scattering phenomena in space plasma physics. His related review in [3] covers his work on wave–wave interactions, which he had initiated already in 1965.

Another important result on circularly polarized large amplitude waves was published by Lennart in [4]. Unfortunately this interesting work has been somewhat overlooked, and often new papers are published that re-derive many of the early findings of this work.

During the years, Lennart has published many key papers on nonlinear plasma surface waves and on nonlinear Alfvén waves. These findings, as well as important contributions from other scientists, are excellently reviewed in [5] and, together with Padma Shukla, in [6]. Another review by Lennart and co-authors in [7] contains key results on Alfvén vortices.

One of the early important results by Allan Kaufman and Lennart was published in [8]. Here it was shown for the first time that supersonic solitons can exist in plasmas.

All of the above are examples from plasma physics, where Lennart has been most active. However, his interests are broad and accordingly he has made important contributions in other fields, too. One such field is atmospheric physics, especially related to acoustic gravity waves. One of his most important works in this field is published in [9], where the derivation of the Lorenz–Stenflo equations for nonlinear acoustic gravity waves is presented.

The above list may give the impression that most of Lennart's important works were published in Physica Scripta, but of course this is not true. Some papers of special interest published in other journals include his work on the nonlinear Schrödinger equation (with Nino Pereira), published in [10], his work on acoustic-solitary vortices published in [11] and his work on 'oscillons' (with Ming Yu) in [12], just to mention a few.

The published work of this focus issue will to some extent relate to the above-mentioned papers, but many more areas will be covered as the scope of Lennart's interest has been so wide.

On behalf of Physica Scripta as well as the contributors to this issue I would like to thank Lennart for the inspiration he has given us, both as a scientist, and as a colleague and close friend. We all know him as generous and supportive of his fellow physicists. We wish him the best of luck in the years to come, and are convinced that he will continue to serve as a role model for all of us.