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\title{The {\sc Pencil Code} Newsletter}
\author{Issue 2023/3}
\date{December 15, 2023,~ $ $Revision: 1.46 $ $}
\begin{document}
\maketitle

\renewcommand{\baselinestretch}{-.4}\normalsize
\tableofcontents
\renewcommand{\baselinestretch}{1}\normalsize

\section{Highlights}

In this issue, we begin by discussing the outcome of
tests to deal with the loss of the svn bridge in GitHub.
We also have, for the first time,
%a self-presentation of all the candidates for the     %MR: not all because of Boris, couldn't get contact today
%AB: wrote now "most of"
a self-presentation of most of the candidates for the
next Pencil Code Steering Committee (PCSC).
Next, and again for the first time, we advertise the possibility
of organizing a Pencil Code User Meeting (PCUM) in
2025 and thereafter.
This newsletter is decorated with pictures
from the last PCUM in Graz.

\begin{figure}[h]\begin{center}
\includegraphics[width=\columnwidth]{church}
\end{center}\caption[]{
PCUM-23 in Graz: Excursion
}\label{church}\end{figure}

\section{GitHub's SVN service}

GitHub is closing its subversion (SVN) service by January 8, 2024.
The PCSC became active because there is a significant part of the PC community that uses SVN routinely and prefers to continue with in.
The PCSC assessed all future scenarios\footnote{\url{http://norlx65.nordita.org/~brandenb/pencil-code/PCSC/minutes/2023_11_30.txt}} and decided that there are basically two possible options:
1) drop SVN support, disregarding the needs of a part of the community, or
2) use a new server to continue SVN \& GIT support by using the \href{https://SubGit.com}{SubGit.com} software and mirror all changes back to GitHub.

The second solution allows the whole community to choose their versioning system -- and maintain GitHub's additional services, like the list of commits and contributors, the wiki, issue tracking, commenting commits, etc.
The only change for GIT users would be to change their repository URL once only from github.com to pencil-code.org with a simple command: \texttt{git remote set-url origin https://USERNAME@pencil-code.org/git/...}; see here: \url{https://pencil-code.org/download.php}. The GitHub repository would be always up-to-date but would become read-only.

\subsection{Live SubGit testing}

Members of the PCSC and the PC community have tested the SubGit setup within the ``playground'' repository. In two testing sessions, we performed stress tests like simultaneous commits from multiple people at the same time submitted to the SVN and the GIT side. We found that the SubGit software is a well-established and robust system that takes care of synchronizing the SVN and GIT repositories under all possible conditions and in a safe way.

Finally, all testers who participated in the testing phase were satisfied.
Therefore, the PC community members, PC owners, and PCSC members that participated in the SubGit testing, recommend SubGit as a possible and reliable solution to continue the SVN service. All PC owners will be informed and asked to make a decision.

This decision is needed until midnight of the 19th of December. Unfortunately, on such short notice due to GitHub's hard constraint to stop their service.

\subsection{User experience}

As previously announced through the \texttt{pencil-code-discuss@googlegroups.com}
mailing list, we tried out the new method for svn and git users during the
\textsc{Pencil Code} office hours on December 7, 2023,
and then again on December 14, 2023, 
Here some user experiences.

\paragraph{Yutong He.}
A few times I found myself kicked out of all branches but was put back on track with the auto-suggested commands.
Conflicts could arise when multiple check-ins happen at/around the same time, and need to be manually resolved
as usual.
Besides these two points, switching to \textit{playground} was straightforward for Git users.
%MR: would leave this out as it is not his experience:
%perhaps more so for SVN users according to Axel.
%AB: ok.

\paragraph{Clara Dehman.}
The December 14 test focused on generating and resolving conflicts in a collaborative coding environment using svn and Git simultaneously.

\emph{Methodology:}
%MR: would leave this para out, clear from the preceding
%AB: not sure yet.
\begin{enumerate}
\item Conflict generation: We initiated conflicts by pushing differing changes to the same line of code nearly simultaneously. Some modifications were pushed by Git users and some others were pushed by svn users.
\item Observations: In cases where another user's modification preceded, the system prompted a need to pull the latest changes before pushing new ones.
\item Conflict resolution: The resolution was done by fixing the conflicted file and selecting the desired version of the code. Then, the standard git push command was successful.
\end{enumerate}

The test was successful in demonstrating conflict generation and resolution in a mixed svn and Git environment.
Conflicts were solved as usual.

\paragraph{Axel Brandenburg.}
SVN users just need to check out a fresh copy from the new URL,
as currently described in
\url{https://pencil-code.org/download.php}, i.e.,
\texttt{svn co https://pencil-code.org/svn/playground/trunk playground}.
Everything else was just completely normal.
Conflict resolution worked as usual.

Piyali and Hongzhe successfully tested the GIT side, while Philippe tested both sides. In a final large stress-test also two more persons, Clara and Fred, committed in the same second. One after the other could finally submit their changes after updating and resolving the intentionally triggered conflicts.

\subsection{GIT detached branches}
When you encounter a situation where your repository appears to be on no specific branch, you can verify this by \texttt{git branch -a}.
To return to the master branch, use \texttt{git checkout master}. Ensure that your modifications are accurately tracked and committed before proceeding. Once everything is in order, you can execute \texttt{git push}.

\section{Steering committee election}

We currently have 20 owners; 
more than half of them have declared
themselves being available for serving on the PCSC; see
\url{http://norlx65.nordita.org/~brandenb/pencil-code/PCSC/} for the
terms of reference and the minutes of last meetings.

\begin{figure}[h]\begin{center}
\includegraphics[width=\columnwidth]{open_window}
\end{center}\caption[]{
Warm weather in Graz.
}\label{open_window}\end{figure}

We have been provided with self-descriptions 
Many of them have provided a paragraph about themselves.
of most of the eligible.

\paragraph{Sven Bingert.}
I got to know and love the \textsc{Pencil Code} when I did fluid mechanics
simulations as a dissertation topic at the Kiepenheuer Institute for
Solar Research in Freiburg. There I also got to know Wolfgang Dobler,
who introduced me to the philosophy of the code.
The simulation of the
solar corona with its extremely effective heat conduction placed high
demands on the code and led to numerous further developments. I continued
to maintain the code and was active at numerous \textsc{Pencil Code} User Meetings.
In 2014, I then moved to a scientific service and computing centre. There
I am currently a supervisor in the HPC area and several international
projects in the field of data management. I am currently also a deputy
professor in computer science at the University of G\"ottingen. The further
development and active use of the \textsc{Pencil Code} is still close to my heart.

\paragraph{Philippe Bourdin.}
My start with the \textsc{Pencil Code} (PC) was 15 years ago and I gained experience in solar
physics and also on the technical side of the code due to my demands in
high-performance computing. Since then, I introduced several mechanisms
to let the PC scale on more CPUs. In the past 2.5 years, I served as a PCSC
member and was elected as chair of the PCSC some months ago. I am very
grateful for this proof of trust and would like to candidate again as
PCSC member. My aim is to incorporate my technical understanding in the
decision making process of the PC community, continue supporting the
community as a whole, as well as being a contact person for PC users
and owners. Because the task of the PCSC is to deal with possible
personal conflicts, I like to emphasize that I address such conflicts
calmly and professionally. I look forward to supporting the community
with my experience and patience. Thank you.

\paragraph{Axel Brandenburg.}
Together with Wolfgang Dobler, we started the \textsc{Pencil Code} as a community
project from the very beginning, before anything worked -- or even compiled.
Now, 22 years later, it is a pleasure to see how the code has grown
to have an $h$-index of 37, with 37 users having done at least 37 commits
(\url{https://github.com/pencil-code/pencil-code/graphs/contributors}).
With now 20 owners, who can give check-in permissions to others, the code
can stand on its own -- even when the original roots get old.
One of my main ambitions is to facilitate maximum freedom in the sense
that \textit{everything is allowed} -- as long as it doesn't disturb anybody else.
Of course, the auto-tests don't capture everything, and there are
also a number of rules to prevent things going out of hand.
This is why we have the PCSC, so nobody is allowed to take critical
decisions alone, and bigger things are decided by the community.
The PCSC reports back to the voters and users.
For the next three years, if I'm elected, I will continue to encourage
users to make and check in their new developments for the benefit
of the whole community.

\paragraph{Piyali Chatterjee.}
I am working with the Pencil Code since 2009 and as a PCSC member since 2020. It is a
pleasure to be part of the \textsc{Pencil Code} community and see how it has grown
and stayed together. I am one of the few women developers and owners of the
code across the world, especially from south Asia. I have had the
opportunity to induct and train several PhD and project students from India
into the \textsc{Pencil Code} community. We organized the Pencil Code User Meeting
(\url{https://www.iiap.res.in/pcum2022/?q=home}) in India for the first time in
May 2022, where a few lecture modules for new users were offered
(available at
\url{https://www.youtube.com/playlist?list=PL1io4hS6YJq29NUAlDkdcHvqMddSXZ8Ko}).
My primary interest is in the application of computational
magnetohydrodynamics to study the Sun -- from the solar interior to the
corona, e.g., quiet Sun phenomena like the formation of the spicule forest as
well as eruptive phenomena like flares and coronal mass ejections.
I incorporate new and insightful modules in the \textsc{Pencil Code} to make solar MHD
simulations more realistic. The synthetic data from such simulations
can then be forward modelled to churn out observables which are directly
measured by existing space instruments like Hinode/IRIS/AIA/HMI and
hopefully VELC/SUIT on board Aditya-L1, India's first space-based solar
observatory at Lagrange point
L1.  I find that most astrophysics students in India tend to
use other codes, e.g., Pluto, AMRVAC for what they claim as ease of
documentation rather than efficiency. If elected for the second term, I
will continue to work towards expanding the \textsc{Pencil Code} community in India
while being abreast of the latest challenges and features of the code by
virtue of being part of the PCSC.

%\paragraph{Boris Dintrans.}

\begin{figure}[h]\begin{center}
\includegraphics[width=\columnwidth]{fred}
\end{center}\caption[]{
Fred Gent \& electromagnetism.
}\label{fred}\end{figure}

\paragraph{Fred Gent.}
I am a Research Fellow at Aalto University in Finland (now 50\%) working
since April 2015 within Maarit Korpi-Lagg's astroinformatics group and
also an Assistant Professor (50\%) at Nordita in Stockholm, Sweden since
1st November 2023. I have been working with the \textsc{Pencil Code}
since 2009, when making my PhD thesis at Newcastle University UK, on
supernova driven turbulence in the ISM. I have had most involvement
on the revision of the interstellar module, shock handling, Python for
the pencil code and working with HDF5. Most recently I have refined the
Runge-Kutta-Fehlberg time step method to work with third order efficiently
and quickly. I aim to work on merging the various approaches to dust and
particle modelling for applications to dust processing by SNe for my work
with Lars Mattsson at Nordita.

%\paragraph{Nils Haugen.}
%AB: he emailed me that he would not want to advertise himself.

%\paragraph{Toby Heinemann.}

\paragraph{Matthias Rheinhardt.}
Started to dive into the \textsc{Pencil Code} in 2007, first as a user, but after having joined NORDITA in
2009, also as a developer. I was and still am involved in building up what we call the ``test methods suite"
of the code, a toolbox for measuring turbulent transport coefficients. In recent years, my focus shifted to accelerating 
the code by using all compute resources on an HPC cluster node, most importantly, by exploiting the Graphical Processing Units (GPUs).
%Many of our competitor codes have already taken this turn, thus we have to keep pace to avoid judgements of ``being outdated".
%AB: new sentence
Many of our competitor codes have already taken this turn. Thus, we have to keep pace to avoid judgements of ``being outdated".
Luckily, my group ``Astroinformatics" at Aalto University in Finland, led by Professor Maarit Korpi-Lagg, is located in the Computer Science Department.
Hence, we continuously take huge benefit of having computer science students in our group, most notably the key developers of the
%{\it Astaroth} framework for GPU acceleration. These days, we are close to finishing the embedding of Astaroth into the \textsc{Pencil Code},
%AB: how about tt style
\texttt{Astaroth} framework for GPU acceleration. These days, we are close to finishing the embedding of \texttt{Astaroth} into the \textsc{Pencil Code},
%which promises us to reach a node-wise speedup of $\gtrsim 10$  for the PDE integration, with all other functionality running concurrently. ---
%AB: omit "us"?
which promises to reach a node-wise speedup of $\gtrsim 10$  for the PDE integration, with all other functionality running concurrently. ---
I have been a member of the PCSC since it was constituted, also serving as the chair for a while. Would I be re-elected, 
my commitment shall aim at keeping the code amongst the leading ones, by preserving and strengthening its exceptional versatility and convenience
as well as by guaranteeing optimum performance on forefront hardware.

\paragraph{Jennifer Schober.}
I am thrilled to put forth my candidacy as a committee member for the PCSC.
I am a group leader at the Laboratoire d'Astrophysique,
\'Ecole Polytechnique F\'ed\'erale de Lausanne (EPFL), Switzerland.
I started using the \textsc{Pencil Code} during my first
postdoctoral fellowship at Nordita in 2015 and became a code owner in 2022.
My main interest is MHD turbulence in different astrophysical systems, from
the early Universe, to galaxy clusters, and galaxies. In particular, I use
the Pencil Code to explore the characteristics of MHD dynamos, delving into
their nonlinear properties and uncovering essential details. If elected, I
am eager to collaborate with fellow committee members to ensure the Pencil
Code remains robust, adaptable, and in harmony with the evolving needs of
our community.

\begin{figure}[h]\begin{center}
\includegraphics[width=\columnwidth]{mach_etc}
\end{center}\caption[]{
Standing on the shoulders of giants.
}\label{mach_etc}\end{figure}

\section{PCUM 2024 in Barcelona}

%September 23, Monday.
%Therefore, we will start planning to have the next Pencil Code User
%Meeting  from September 23 through September 27 at
%the Institute of Space Sciences in Bellaterra (Barcelona).

At the last PCUM in Graz, it was decided that the next PCUM
will be held at the Institute of Space Sciences (ICE,
\url{https://www.ice.csic.es/}) in Barcelona.
The meeting will begin on Monday, September 23rd and will run through
Friday, September 27.

\begin{figure}[h]\begin{center}
\includegraphics[width=\columnwidth]{IMG-20231205-WA0001}
\end{center}\caption[]{
The Institute of Space Sciences in Bellaterra (Barcelona).
}\label{IMG-20231205-WA0001.jpg}\end{figure}

ICE is hosting a growing community of \textsc{Pencil Code} users, so it is a good
time to have our regular yearly meeting in this new location. This choice
will likely attract more people in the institute to know something more
about the code and, perhaps, to use it in their research.
ICE is located on the campus of Universitat Autonoma de Barcelona,
officially in the municipality of Bellaterra, which is about 50 minutes
away from downtown Barcelona by metro.
We will provide more details on the meeting in the next months, and in the
next newsletter. Meanwhile, please mark the dates on your calendar! We hope
to see many of you in Barcelona!

%\begin{figure}[h]\begin{center}
%\includegraphics[width=\columnwidth]{Screenshot_2023-12-04_About_us}
%\end{center}\caption[]{
%Entrance to the Institute of Space Sciences in Bellaterra (Barcelona).
%}\label{Screenshot_2023-12-04_About_us}\end{figure}

%\begin{figure}[h]\begin{center}
%\includegraphics[width=\columnwidth]{IMG-20231205-WA0002}
%\end{center}\caption[]{
%Meeting room 2 in Barcelona.
%}\label{IMG-20231205-WA0002.jpg}\end{figure}

\begin{figure}[h]\begin{center}
\includegraphics[width=\columnwidth]{IMG-20231205-WA0000}
\end{center}\caption[]{
One of ``our'' meeting rooms at the Space Sciences in Bellaterra.
}\label{IMG-20231205-WA0000.jpg}\end{figure}

\section{Proposals for PCUM 2025}

At the last PCUM, we discussed the possibility to apply for hosting
the PCUM in 2025 and thereafter.
Thus, please come forward with suggestions to the editors of this
newsletter.
While this implies a significant amount of work, it can also be a
%real pleasure for those who have been organizing a small meeting  %MR: unclear - "have been"?
%AB: wrote now "are"
real pleasure for those who are organizing a small meeting
for the first time.
If you are able to apply for additional funding, this would be
great, but even without that, you can do it.
The people are expected to pay their own accommodation and travel,
as well as dinners and T-shirts, if the organizers design one.
As before, the PCSC decides at the end of the last PCUM where the next   %MR: last strange
one will be.

\section{Reports from the last PCUM}

The PCUM-23 in Graz was a great success in terms of both
science dissemination and code development.
In this section, we report on a few useful developments
that were done during the last meeting.

\begin{figure}[h]\begin{center}
\includegraphics[width=\columnwidth]{group}
\end{center}\caption[]{
Webcam's view during group photo in Graz.
}\label{group}\end{figure}

\begin{figure}[h]\begin{center}
\includegraphics[width=\columnwidth]{dinner}
\end{center}\caption[]{
Conference dinner during PCUM-23.
}\label{dinner}\end{figure}

%\section{Code developments}

As usual, one finds the latest code developments on
\url{https://github.com/pencil-code/pencil-code/commits/master}.
The green and red check marks indicate whether
the travis test went through or not; see
\url{https://app.travis-ci.com/github/pencil-code/pencil-code}.

\subsection{Solving ODEs, coupled with PDEs}

One of the important new developments concerns the code's ability
to solve ODEs (time-dependent) along with PDEs -- without using the full 
3-D array for each variable.
An example is \texttt{samples/1d-tests/axionSU2back} where we use
\texttt{SPECIAL=special/axionSU2back}.
Obviously, instead of doing
\begin{verbatim}
  call farray_register_pde('axi_psi',iaxi_psi)
\end{verbatim}
for PDEs, we now do
\begin{verbatim}
  call farray_register_ode('axi_Q',iaxi_Q)
\end{verbatim}
for ODEs.
Their right-hand side is here being computed in
\texttt{subroutine dspecial\_dt\_ode}.
Note, that including ODEs is currently restricted to the {\it special} modules
as standard physics modules are not supposed to need such.
Many thanks to Matthias for having devoted his
expertise to this during the PCUM-23.
The resulting science output can be read in
\cite{2023arXiv231107557I}.

\begin{figure}[h]\begin{center}
\includegraphics[width=\columnwidth]{working}
\end{center}\caption[]{
Enjoying another beer before departure.
Code development in progress.
}\label{working}\end{figure}

\subsection{{\tt lconcurrent}}

A new switch, or at least a new name is {\tt lconcurrent}.
You may want to try out {\tt lconcurrent=F}
The code should then run somewhat slower, because it waits
until all points close to the ghost zones have been communicated
before calculating the rhs.

\subsection{{\tt lmodify}}

Another switch is {\tt lmodify}; it has been in the code
and in the manual for 19 years, but wasn't noticed by many.
It can be used to run \texttt{start.csh} without overwriting
the current snapshot, but rather to modify an existing one.
There are some additional things to be set, so please check
the manual.

%\section{Meetings}

%\subsection{Early career meeting in Salzburg}

%the Austrian Society for Astronomy and Astrophysics (ÖGAA) organizes a
%conference mainly aimed for "early career scientists" with the focus to
%foster networks among Master students, PhD students, and young
%PostDocs. Supervisors are invited to participate, too.

%We like to give you the opportunity to apply for a talk in a real
%conference environment, while you are still at an early career stage.
%Colleagues from Munich, Vienna, Innsbruck, and Graz will participate.
%As the venue is in Salzburg/Austria, it is easily reachable in Europe.

%The topics include (but are not limited to) astronomy, astrophysics,
%astroparticle physics, cosmology, solar and heliospheric physics, space
%sciences, as well as exoplanets, planets, and small bodies.

%Date: 8th and 9th of March, 2024 (noon to noon)
%Registration: now open, until 1st of December
%Fees: 10 € (for non-ÖGAA members)
%More details and registration: https://www.oegaa.at/AECC/
%(Transportation to the venue from Salzburg city will be organized.)

%Thank you for your kind consideration, on behalf of the ÖGAA board,
%Philippe Bourdin.

\section{Papers since August 2023}

As usual, we look here at new papers that make use of the {\sc Pencil Code}.
Since the last newsletter of August 21st, 7 new papers have appeared    
on the arXiv, plus 14 others, some of which were just preprints and have now been published in a journal.
We list both here, 19 altogether.
A browsable ADS list of all {\sc Pencil Code} papers can be found on:
\url{https://ui.adsabs.harvard.edu/public-libraries/iGR7N570Sy6AlhDMQRTe_A}.
If something is missing in those entries, you can also include it yourself in:
\url{https://github.com/pencil-code/pencil-code/blob/master/doc/citations/ref.bib},
or otherwise just email brandenb@nordita.org.
A compiled version of this file is available as
\url{https://github.com/pencil-code/website/blob/master/doc/citations.pdf},
where we also list a total of now 107 code comparison papers in the last
section ``Code comparison \& reference''.
Those are not included in our list below, nor among the now total number
of 699 research papers that use the {\sc Pencil Code}.

\nocite{
2024MNRAS.527.3018Z,%Zhou "Helical and non-helical large-scale dynamos in thin accretion discs"
2023MNRAS.525.5597T,%Tharakkal+ "Steady states of the Parker instability"
2023MNRAS.525.2972T,%Tharakkal "Steady states of the Parker instability: the effects of rotation"
2023NatAs...7..662W,%Warnecke "Numerical evidence for a small-scale dynamo approaching solar magnetic Prandtl numbers"
2023PhRvD.108j3029C,%Carenza+ "Magnetohydrodynamics predicts heavy-tailed distributions of axion-photon conversion"
2023SSRv..219...58K,%Kapyla+ "Simulations of solar and stellar dynamos and their theoretical interpretation"
2023PhRvD.108f3529B,%Brandenburg "Chiral magnetohydrodynamics with zero total chirality"
2023A&A...677A..46E,%Elias-Lopez+ "Vorticity and magnetic dynamo from subsonic expansion waves"
2023Entrp..25.1270B,%Brandenburg+Protiti "Electromagnetic conversion into kinetic and thermal energies"
2023BAAA...64...50H,%Hidalgo+ "Origin of magnetism in early-type stars"
2023A&A...678A...9N,%Navarrete+ "Effects of the centrifugal force on stellar dynamo simulations"
2023A&A...678A..82O,%Ortiz-Rodriguez+ "Simulations of dynamo action in slowly rotating M dwarfs: Dependence on dimensionless parameters"
2023PhRvE.108f5201M,%Mondal+Bhat "A unified treatment of mean-field dynamo and angular-momentum transport in magnetorotational instability-driven turbulence"
2023arXiv231206787S,%Sankar+ "Photospheric Lorentz force changes in eruptive and confined solar flares"
2023arXiv230812916S,%Sharma+ "Shallow relic gravitational wave spectrum with acoustic peak"
2023arXiv231109082K,%Kapyla "Effects of rotation and surface forcing on deep stellar convection zones"
2023arXiv231012855K,%K{\"a}pyl{\"a}} "Convective scale and subadiabatic layers in simulations of rotating compressible convection"
2023arXiv231008354Q,%Qazi+ "Nonlinear magnetic buoyancy instability and turbulent dynamo"
2023arXiv230704602B,%Brandenburg "Inverse cascading for initial MHD turbulence spectra between Saffman and Batchelor"
2023arXiv231107557I,%Iarygina "Backreaction of axion-SU(2) dynamics during inflation"
2023arXiv231016480V%V{\"a}is{\"a}l{\"a}}+ "Exploring the Formation of Resistive Pseudodisks with the GPU Code Astaroth"
%
%2023arXiv230705490Z,%Zhu+Shi "Helical and nonhelical (magneto-)Burgers turbulence: I. Compressibility reduction and beyond"
%2023arXiv230709385B,%Brandenburg "Relic Gravitational Waves from the Chiral Plasma Instability in the Standard Cosmological Model"
%2023arXiv230715118S,%Schober+ "Chiral magnetic anomaly and dynamos from spatial chemical potential fluctuations"
%2023arXiv230607838H,%Hackman+ "From convective stellar dynamo simulations to Zeeman-Doppler images"
%2023arXiv230607051G,%Gent+ "Transition from small-scale to large-scale dynamo in a supernova-driven, multiphase medium"
%2023arXiv230516790K,%Kapyla+ "Simulations of solar and stellar dynamos and their theoretical interpretation"
%
%2023arXiv230300911Y,%Yuvraj "On flame speed enhancement in turbulent premixed hydrogen-air flames during local flame-flame interaction"
%2023arXiv230206042Z,%Zhou+Blackman "Helical dynamo growth at modest versus extreme magnetic Reynolds numbers"
%
%2022arXiv220606566M,%Masada+Sano "Rotational Dependence of Large-scale Dynamo in Strongly-stratified Convection: What Causes It?"
%2022arXiv220509261R,%Roper Pol "Gravitational waves from MHD turbulence at the QCD phase transition as a source for Pulsar Timing Arrays"
%2021arXiv211201193C,%Candelaresi+Del Sordo "Stability of plasmas through magnetic helicity"
%2021arXiv210508287R,%Roper Pol "Gravitational radiation from MHD turbulence in the early universe"
}

\bibliography{ref}
\vspace{5mm}
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This {\sc Pencil Code} Newsletter was edited by
Axel Brandenburg {\tt <brandenb@nordita.org>}, Nordita, KTH Royal Institute of Technology and Stockholm University, SE-10691 Stockholm, Sweden;
and Matthias Rheinhardt {\tt <matthias.rheinhardt@aalto.fi>}, Department of Computer Science, Aalto University, PO Box 15400, FI-00076 Aalto, Finland.
See \url{http://www.nordita.org/~brandenb/pencil-code/newsletter} or
\url{https://github.com/pencil-code/website/tree/master/NewsLetters}
for the online version as well as back issues.
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