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Subsections

ESMN research

ESMN stands for “European Solar Magnetism Network”.  The name specified the ESMN research topic: solar magnetism in all its facets.

Solar magnetism is one of the great challenges of astrophysics.  The intricate structure of the sun's magnetic fields, the solar activity cycle and the solar influence on the heliosphere represent major quests of (astro-)physics which bear directly on the human environment.  The solar magnetic fields are generated by enigmatic dynamo processes in the solar interior, are organised into the highly complex patterns of solar activity observed in the solar photosphere, dominate the structure of the outer solar atmosphere (chromosphere, transition region, corona), regulate the solar wind, affect the extended heliosphere including near-earth space weather, and underlie the solar variability which influences life on Earth through climate modulation.

The ESMN went for the roots of space weather and coronal plasma behaviour by integrating complementary European efforts to chart and understand the structure and dynamics of solar surface magnetism, the patterns by which it betrays subsurface dynamo properties, and the electrodynamical coupling to the outer solar atmosphere.  At the solar surface, the magnetic fields gain dominance over the gas pressure so that the surface field configurations control what happens further out.  The ESMN therefore concentrated on joint observing and analysis programs with optical solar telescopes (solar surface and lower atmosphere) and space telescopes (outer atmosphere), with considerable interpretative support from theory.

The ESMN studied the structure and dynamics of solar surface fields, the topology and evolution of solar active regions, and the electrodynamical coupling between the solar interior, photosphere, and outer atmosphere by perfecting solar magnetometry, organising joint multi-telescope observing campaigns, and analysing the results through numerical inversions and simulations.

In practice, the ESMN exploited advanced European technology - five solar telescopes at the Canary Islands and ESA's SOHO mission in space - to gain understanding of the roots of solar magnetism.

Overview

The ESMN goal was to gain basic insight in the roots of solar magnetism by establishing the structure and dynamics of magnetic fields at the solar surface, charting the patterns that constrain the solar dynamo, and identifying the magnetic coupling between the different solar regimes from the interior to the corona.  The ESMN science objectives were:
(a) structure and dynamics of solar surface fields;

(b) topology and evolution of solar active regions;

(c) magnetic coupling between the solar interior, photosphere, and outer atmosphere.

The ESMN partners combined effort and expertise in a coordinated attack with the following implementation objectives:

(d) perfection of magnetometry instrumentation and methodology;

(e) solar magnetometry through multi-telescope observing campaigns;

(f) interpretation through numerical inversions and simulations.

Structure and dynamics of solar surface fields.

Solar surface magnetism consists of a remarkable hierarchy of discrete strong-field structures with highly dynamic patterning (as may be appreciated by playing the vivid speckle movies available on the DOT website.  The basic entity consists of the tiny flux tubes.  They constitute an important astrophysical paradigm, pertinent also to accretion disks and other faraway objects but directly observable only on the sun thanks to the advent of speckle image restoration and adaptive optics wavefront correction.  They are arranged into a magnetic network pattern by convective surface flows and occur at larger density in solar plage (faculae) which is`< an important modulation ingredient in the solar irradiance.  Detailed examination of the dynamical network configuration at the solar surface may also resolve the ongoing debate in what measure continuous micro- or nano-flaring contributes steady heating to the corona.

It is highly probable that much weaker fields permeate the internetwork areas between flux tube clusters, but internetwork fields were not yet diagnosed convincingly at the ESMN start.  They may constitute a weak-field dynamo which may play an important role in setting solar variability.

The larger elements in the strong-field magnetic hierarchy (pores, umbrae, larger spots with penumbrae, and fully-developed active regions) also pose a rich variety of astrophysics research issues and contribute solar modulation input.  For example, sunspot oscillations provide important lessons in magnetohydrodynamics.  In particular, concerted observation and numerical simulation of wave modes have established how weak shocks travel up in the umbral chromosphere as umbral flashes and how running penumbral waves persist into the corona in the outer sunspot reaches.

Prominences are enigmatic cool condensations in the hot corona that are sustained and thermally isolated by highly complex magnetic fields.  Advanced polarimetry including Hanle diagnostics permitted mapping the field configuration with unprecedented angular and temporal resolution and to establish the magnetohydrodynamical prominence structuring and instability mechanisms.  These are also of interest to terrestrial studies of magnetic confinement in Tokamaks and related plasma fusion experiments.

Solar eruptions accelerate particles to relativistic speed and unleash powerful coronal mass ejections into space, producing significant effects in the near-earth environment.  Multi-telescope observations combining magnetometry at high angular and temporal resolution with space diagnostics including HESSI X-ray mapping managed to catch flares and prominence eruptions while solar activity remained high during the ESMN years, and charted these events in diagnostic detail at different atmospheric levels.

Topology and evolution of solar active regions.

Active regions bring much large-scale magnetic flux to the solar surface, are the seat of eruptive activity, and provide key insights into the ill-understood dynamo processes that produce the solar activity cycle.

Bipolar active regions (simple sunspot pairs connected by coronal loops) are the building blocks of the larger and more complex active regions which produce eruptive activity when their topology favours formation of current sheets and occurrence of magnetic reconnection.  Flares and prominence eruptions draw their energy from sudden relaxation of the magnetic field, but the trigger mechanism remains unknown.  The precursor geometry can be identified through comparison of the before-and-after coronal field topology derived from high-resolution surface magnetometry, with extrapolative identification of magnetic nulls, separators, separatrices and quasi-separatrix layers where reconnection may occur, and with special emphasis on the topological role of field helicity.

Active regions result from the emergence of flux tube systems that rise from the base of the convection zone through buoyancy influenced by the Coriolis force, magnetic tension, drag, vorticity and other effects.  The emergence and disappearance of active regions and their preferential grouping in activity nests over long time scales betray solar dynamo properties that also contribute to the erratic cyclical modulation of solar activity.  The observational strategy to understand this behaviour was to combine solar surface magnetometry with sufficient resolution, field size and duration with numerical dynamo modelling.  MDI on SOHO was the key long-sequence instrument while shorter-duration studies of individual active regions at higher resolution will shed light on active region emergence and decay.

Magnetic coupling between the solar interior, photosphere and outer atmosphere.

The solar surface is not only the layer where the bulk of the solar radiation leaves our star and where the dynamo patterns generated in the interior are directly observable, but it is also the layer where gas pressure gives in to magnetic pressure.  The dynamics of the photosphere with its granulation and turbulent wave excitation is hydrodynamically controlled outside active regions, but the coronal topology and dynamics are governed by magnetism.  Since the coronal fields are rooted in the photosphere (no magnetic monopoles) their changing configurations are dictated by photospheric foot point motions: there is magnetic coupling between the regimes.  It is also likely that MHD disturbances excited in the low atmosphere contribute substantially to coronal heating and the generation of the solar wind.

Just above the photosphere the flux tubes merge into the chromospheric network.  It contributes most of the solar variability in the ultraviolet but the network heating process has not yet been identified.  Likely candidates are various types of magnetohydrodynamic waves and dissipation of magnetic shear and stress induced by the perpetual displacements of the photospheric tubes. Mapping time-dependent flux tube topology and analysing flux tube dynamics is a high-priority quest for high-resolution magnetometry.

Yet higher up, the magnetic network expands into magnetic canopies above which all gas motions are constrained by the ambient field.  Canopy geometries, canopy dynamics (including wave mode conversions from acoustic and internal gravity to Alfvén waves) and canopy linkages across network cells were key research topics for concerted photosphere-chromosphere imaging during the ESMN years.

At coronal levels, the basic building block is the coronal loop, delineating slender field configurations at specific temperature that become visible through density contrasts.  The connection between the basic photospheric and coronal field structures is highly enigmatic-how do tubes turn into loops? Observations in H alpha and EUV lines indicate a plethora of low-lying finely-scaled structures with rapid large-amplitude dynamical changes, making clear that traditional assumptions as hydrostatic equilibrium and axial symmetry (“plane-parallel layers” with a “transition region”) are now replaced by much more sophisticated magnetohydrodynamical modelling and interpretation based on multi-diagnostic ground- and space-based data gathering.

Interest in the electrodynamical coupling between the low and high solar atmosphere transcends solar physics because via the solar wind, cosmic ray modulation, and coronal mass ejections it extends to the near-earth environment and the terrestrial climate.

Canary Island Telescopes

A Europe-wide effort (by JOSO = Joint Organisation for Solar Observations) has established the Canary Islands as a “most-favoured” region for solar observing.  A quintet of advanced solar telescopes on the Canary Islands constitutes the ESMN's “capital” in cutting-edge telescope technology.  Each telescope represents represent the state of the art in key techniques.  Collectively, they represent a formidable and unsurpassed facility for solar research, in particular solar surface magnetometry.  The five telescopes are highly complementary, so that multi-telescope co-observing was a major ESMN strategy.

They are, respectively:

Space missions

SOHO (Solar and Heliospheric Observatory) remained the flagship of spacebased solar physics durng the ESMN years.  SOHO keeps operating very successfully while orbiting the first Lagrangian point of the sun-earth system.  The US TRACE mission adds ultraviolet imaging, often in concert with SOHO's Michelson Doppler Imager (MDI) and groundbased magnetometry.  The Japanese-led Hinode mission was launched in 2006 and put the first Stokes vector spectropolarimeter in space.  Subsequently, the Solar Orbiter mission accepted by ESA will put a solar magnetograph as close as 0.2 AU to the sun at steadily increasing out-of-ecliptic latitude.  These major space projects effectively confirm the importance of the ESMN research topic and represent an important career perspective for the young ESMN researchers.

Spatial resolution enhancement

The atmospheric seeing, the bad influence of the earth's atmosphere on solar image sharpness, is a limiting factor to image quality even at the Canary Island sites.  Improvements are possible through phase diversity plus speckle restoration and through the application of adaptive optics.  These techniques are presently coming of age.  By advancing these techniques, ESMN collaborations effectively enhanced the frequency of top-resolution data gathering at the Canaries by as much as a factor of three to four, a capital increase in telescope efficiency.

Stokes magnetometry

The state of the art in Stokes vector spectropolarimetry (measuring all four Stokes parameters I, Q, U and V through complex polarisation diagnostics and calibration procedures) is currently reached most prominently by the TIP (Tenerife Infrared Polarimeter) installed by the La Laguna at the German VTT telescope.  The further development of Stokes spectropolarimetry was a large-priority activity in many ESMN teams.

In additon, the La laguna, Meudon and Arcetri teams together were world leader in the theory and calibration of polarised radiative transfer, with much ESMN-linked effort on inversion methodology. These programs strengthen the diagnostic value of solar magnetometry because Stokes vector spectropolarimetry requires sophisticated spectral-line modelling to derive the magnetic field vector amplitude and orientation.  The huge amounts of data that the new instruments generate are handled efficiently by artificial neural networks as developed by the Potsdam team.

Polarised radiative transfer

The ESMN also encompasses efforts in the theory and numerical modelling of polarised radiative transfer, including new inversion methodology.  These programs strengthen the diagnostic value of solar magnetometry.  Stokes vector magnetographs require sophisticated spectral-line modelling to derive the magnetic field vector amplitude and orientation.  The new magnetographs require such elaborate methods for optimum data interpretation.

Numerical modelling

Direct confrontation of observations with concerted numerical modelling has proven to be a particularly fruitful solar physics venue.  ESMN collaborations led by the group at Oslo used (and use) actual data as simulation input in order to enable direct comparison with diverse observational diagnostics including spectral sequences from Canary Island telescopes and from SOHO.  The same forward-modeling-plus-reproduction approach was taken by the Budapest team with regards to active region patterning and by the Potsdam group in simulating magneto-atmospheric waves in sunspot atmospheres.

Task division

The ESMN program divided the research objectives over detailed tasks in which the partner teams collaborated in various combinations according to their expertise. By and large the topic distribution was as follows:

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