Crystal engineering--the planning and construction of crystalline supramolecular architectures from modular building blocks--permits the rational design of functional molecular materials that exhibit technologically useful behaviour such as conductivity and superconductivity, ferromagnetism and nonlinear optical properties. Because the presence of two cooperative properties in the same crystal lattice might result in new physical phenomena and novel applications, a particularly attractive goal is the design of molecular materials with two properties that are difficult or impossible to combine in a conventional inorganic solid with a continuous lattice. A promising strategy for creating this type of 'bi-functionality' targets hybrid organic/inorganic crystals comprising two functional sub-lattices exhibiting distinct properties. In this way, the organic pi-electron donor bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and its derivatives, which form the basis of most known molecular conductors and superconductors, have been combined with molecular magnetic anions, yielding predominantly materials with conventional semiconducting or conducting properties, but also systems that are both superconducting and paramagnetic. But interesting bulk magnetic properties fail to develop, owing to the discrete nature of the inorganic anions. Another strategy for achieving cooperative magnetism involves insertion of functional bulky cations into a polymeric magnetic anion, such as the bimetallic oxalato complex [MnIICrIII(C2O4)3]-, but only insoluble powders have been obtained in most cases. Here we report the synthesis of single crystals formed by infinite sheets of this magnetic coordination polymer interleaved with layers of conducting BEDT-TTF cations, and show that this molecule-based compound displays ferromagnetism and metallic conductivity.

Coexistence of ferromagnetism and metallic conductivity in a molecule-based layered compound

The observational evidence on the strength of the coronal magnetic field above active regions is reviewed. Recent advances in observations and plasma theory are used to determine which data are the more reliable and to revise some earlier estimates of field strength. The results from the different techniques are found to be in general agreement, and the relation 279-01, 1.02 ≲ R/R⊙ ≲ 10 is consistent with all the data to within a factor of about 3.

Coronal magnetic fields.

The most striking feature of the Sun's magnetic field is its cyclic behaviour. The number of sunspots, which are dark regions of strong magnetic field on the Sun's surface, varies with a period of about 11 years. Superposed on this cycle are secular changes that occur on timescales of centuries and events like the Maunder minimum in the second half of the seventeenth century, when there were very few sunspots. A part of the Sun's magnetic field reaches out from the surface into interplanetary space, and it was recently discovered that the average strength of this interplanetary field has doubled in the past 100 years. There has hitherto been no clear explanation for this doubling. Here we present a model describing the long-term evolution of the Sun's large-scale magnetic field, which reproduces the doubling of the interplanetary field. The model indicates that there is a direct connection between the length of the sunspot cycle and the secular variations.

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Evolution of the Sun's large-scale magnetic field since the Maunder minimum.

Ephemeral active regions attain maximum development within 1 day or less of their initial appearance and are typically observed for 1–2 days. They appear mostly as small bipolar regions having a typical dimension of about 30000 km and a maximum total flux of the order of 1020 Mx. The ephemeral regions generally do not produce sunspots and flares, though they are identified in Hα as small active centers.

Ephemeral active regions

In order to test different models of coronal heating, we have investigated how the magnetic field strength of coronal flux tubes depends on the end-to-end length of the tube. Using photospheric magnetograms from both observed and idealized active regions, we computed potential, linear force-free, and magnetostatic extrapolation models. For each model, we then determined the average coronal field strength, B, in approximately 1000 individual flux tubes with regularly spaced footpoints. Scatter plots of B versus length, L, are characterized by a flat section for small L and a steeply declining section for large L. They are well described by a function of the form log = C1 + C2 log L + C3/2 log(L2 + S2), where C2 ≈ 0, -3 ≤ C3 ≤ -1, and 40 ≤ S ≤ 240 Mm is related to the characteristic size of the active region. There is a tendency for the magnitude of C3 to decrease as the magnetic complexity of the region increases. The average magnetic energy in a flux tube, B2, exhibits a similar behavior, with only C3 being significantly different. For flux tubes of intermediate length, 50 ≤ L ≤ 300 Mm, corresponding to the soft X-ray loops in a study by Klimchuk & Porter (1995), we find a universal scaling law of the form Lδ, where δ = -0.88 ± 0.3. By combining this with the Klimchuk & Porter result that the heating rate scales as L-2, we can test different models of coronal heating. We find that models involving the gradual stressing of the magnetic field, by slow footpoint motions, are in generally better agreement with the observational constraints than are wave heating models. We conclude, however, that the theoretical models must be more fully developed and the observational uncertainties must be reduced before any definitive statements about specific heating mechanisms can be made.

https://iopscience.iop.org/article/10.1086/308398/pdf

Magnetic Field and Plasma Scaling Laws: Their Implications for Coronal Heating Models

A solar magnetograph employing as detectors a pair of self-scanning 512-element integrated diode arrays is described. Coupled to a 1.5-m telescope, photospheric flux as small as 5(106) maxwells is detected, corresponding in intensity to ΔI/I = 3(10−4) at λ0.8688 μm. Measured photometric properties of the diode array are given, including MTF as a function of wavelength, dark current as a function of temperature, completeness of readout, optical and electronic fixed-pattern noise. An integrating preamplifier is presented that achieves a measured noise, when connected to the array, equivalent to 950 electrons at the input for a bandwidth of 3(105) Hz. These data provide a basis for an evaluation of the detector performance at low light levels beyond the needs of the magnetograph. Operated at near liquid nitrogen temperature, the noise and cooling characteristics indicate the detector has promise as a low light level sensor.

Solar magnetograph employing integrated diode arrays.

A major new solar-research telescope conceived and built during a time of budget restraint is described. The observation of magnetic and velocity (circulation) field structure on a synoptic basis and with diffraction-limited resolution is the aim. New optical features include the use of oversize mirrors and windows to avoid thermal edge effects and the placement of the coelostat feed outside the vacuum, mainly for economy. The site selected has prevailing winds that clear thermals from these mirrors. Test data in the form of the system MTF and optical transmission, together with examples of full disk magnetograms and photoheliograms, show present performance capability. Measured MTF indicates a response of 0.2 at 1 sec of arc (whereas diffraction-limited response would be about 0.8). System transmission, including the accompanying spectrograph, is only 2-3% (wavelength 0.44-1.1 microns). Thus, both the optical quality and efficiency are subject to improvement.

Kitt Peak 60-cm vacuum telescope

A time sequence of longitudinal magnetograms of two active regions, McMath Regions 9281 and 9760, have indicated magnetic field changes occurring in localized areas with time scales of the order of hours. We believe the observed field changes are evolutionary in nature, rather than related to the occurrence of small flares. Three examples of evolutionary magnetic changes are discussed.

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C. Slaughter

Papers

Solar magnetograph employing integrated diode arrays.

Kitt Peak 60-cm vacuum telescope

Observations of Magnetic Field Changes in Active Regions

Photospheric magnetic fields on March 7, 1970.

Apparent solar oscillations and fluctuations in the earth's atmosphere.