Prof Róbert von FáySiebenbürgen
Research Topics
Solar Physics
Studies of linear and nonlinear resonant MHD wave heating under
solar atmospheric conditions.
My major new results are:
 derivation of the generalised governing equation for resonant slow
MHD waves in isotropic and anisotropic steady plasmas;
 obtaining jump conditions to connect solutions across a resonant layer.
These universal connection formulae play a crucial role, similar to the
RankineHugoniot relations of nonlinear gas dynamics;
 proof of the validity of the universal character of the jump conditions
in dissipative MHD and in dilute plasmas;
 making successful applications of resonant MHD wave theory (e.g. sunspots,
magnetic canopy, fibrils, coronal loops and magnetosphere);
 developing the tools of global coronal seismology.
Studies of nanoscale reconnection MHD heating under solar
atmospheric conditions.
My key new results are:
 deriving the observational signatures of microscale reconnection;
 proposing a unified model for explosive events and blinkers;
 studying the processes of random energy deposition in magnetic flux tubes.
Space Weather/Space Plasma Physics
Studies of resonant flow instabilities in MHD involve the generalisation
of stability theory when resonance occurs in a steady plasma. This theory has been
extended to dilute plasmas, which more realistically describes the magnetosphere,
heliosphere or space weather.
New achievements are:
 establishing for the first time, the conditions when resonance
can cause MHD flow instability far below the KelvinHelmholtz threshold;
 derivation of the linear and nonlinear governing equation for dilute
plasmas;
 establishing propagation windows for linear modes.
Computational Magnetohydrodynamics (CMHD)
Modelling of explosive events (EEs), blinkers, nanoflares and solar
tornadoes. EEs, blinkers and nanoflares are the smallest scale phenomena observable
with the latest very highresolution satellite techniques and are believed to be the
basic building blocks of atmospheric heating. My joint groundbased (Tenerife) and
satellite (SOHO, Yohkoh, TRACE) observation campaign was one out of the very few
European projects supported by ESA/NASA SOHO Science Planning (11/1998).
New results are:
 resolving the controversial 'chromospheric downflow problem';
 observing indirect consequences of MHD wave propagation;
 using reconnection driven resonant MHD wave heating to derive a
first unified theory combining magnetic reconnection and MHD wave heating;
 developing a versatile software package to convert computational results into
directly observable quantities taking account of nonequilibrium ionisation;
 observing tornadoes on the Sun (this was lucky given that there is about
chance in a million of success!). Gave numerous TV, radio and journal interviews on
solar tornadoes.
Helioseismology
Helioseismology provides one of the most precise measurements in astronomy
and yields information about the internal structure of the Sun. Since there is a
significant discrepancy between theoretical predictions and satellite measurements I am
revisiting the models of solar internal f/p/gmodes to include the combined
effects of an atmospheric magnetic field, temperature and steady state changes
during a solar cycle.
Novel and very exciting achievements are:
 deriving the dispersion relation and propagation windows for
magnetoacousticgravity surface waves in steady plasmas;
 establishing the effect of differential rotation and meridional flow
on the solar p/fmodes;
 predicting the cyclic changes of the p/fmodes during a full solar
cycle;
 reconciliation of the results of the revisited theoretical model with observations
justified my original proposal.
Stability of MHD shear flows
Stability of open shear flows is of fundamental importance in geophysics
and astrophysics. Examples of such flows include plasma flows in the vicinity of the
magnetopauses of the Earth and planets, the boundaries between fast and slow streams of
the solar wind, the flow in the vicinity of the heliopause, flows in the interaction
regions of colliding stellar winds in binary stellar systems, and astrophysical jets.
To study the stability of a shear flow with respect to perturbations finite in space we
have to solve an initialvalue problem. Then two scenarios are possible. In the
first scenario the initial finite perturbation exponentially grows at any spatial position.
Such a type of instability is called absolute. In the second scenario the initial
perturbation also grows exponentially, but it is swept away by the flow from any finite
region so fast that it decays at any fixed spatial position. Such a type of instability is
called convective. The classification of absolute and convective instability is important
for the understanding of the physical processes in geophysical and astrophysical plasmas,
and for the interpretation of new satellite (SOHO, TRACE, RHESSI) observational data. In
spite of it fundamental importance there is little attention paid to this problem in MHD.
My novel and very exciting achievements are:
 derived the threshold of dissipative instability of MHD tangential discontinuities
which is below than the KelvinHelmholtz treshold;
 the critical velocity of the dissipative instability depends on the ratio of
the dissipative coeffcients at the two sides of the discontinuity and be anywhere
between the negative energy wave threshold and the KHI threshold;
 solving a classical problem of the absolute and convective instability of a
tangential discontinuity in a compressible fluid;
 establishing the effect of compressibility and dissipation on the threshold of absolute
and convective instabilitity in open shear flows.
