Acetylenic coupling is currently experiencing some of the most intensive study of its long history. Rigid and sterically undemanding di- and oligoacetylene moieties, which are frequently encountered in natural products, are finding increasing application as key structural elements in synthetic receptors for molecular recognition. Interesting electronic and optical properties of extensively pi-conjugated systems have spurred research into new linear oligoalkynes and acetylenic carbon allotropes. The synthetic challenges associated with these efforts have in turn spawned new methods. While classical Glaser conditions are still frequently used for homocoupling, the demand for increasingly selective heterocoupling conditions has provided the focus of research over the past decades. These efforts have undoubtedly been hampered by a relatively poor mechanistic understanding of these processes. More recently, palladium-catalyzed coupling methods have led to improvements in both the selectivity and reliability of acetylenic homo- and heterocouplings and paved the way for their application to ever more complicated systems. The variety of acetylenic coupling protocols, the current mechanistic understanding, and their application in natural product and targeted synthesis are discussed comprehensively for the first time in this review, with an emphasis on the most recently developed methods, and their application to the synthesis of complex macromolecular structures.

Ground‐based optical and digital ionosonde measurements were conducted at Thule, Greenland to measure ionospheric structure and dynamics in the nighttime polar cap F layer. These observations showed the existence of large‐scale (800–1000 km) plasma patches drifting in the antisunward direction during a moderately disturbed (Kp ≥ 4) period. Simultaneous Dynamics Explorer (DE‐B) low‐altitude plasma instrument (LAPI) measurements show that these patches with peak densities of ∼106 el cm−3 are not locally produced by structured particle precipitation. The LAPI measurements show a uniform precipitation of polar rain electrons over the polar cap. The combined measurements provide a comprehensive description of patch structure and dynamics. They are produced near or equatorward of the dayside auroral zone and convect across the polar cap in the antisunward direction. Gradients within the large scale, drifting patches are subject to structuring by convective instabilities. UHF scintillation and spaced receiver measurements are used to map the resulting irregularity distribution within the patches.

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We address the question regarding the two‐dimensional shape of equatorial plasma bubbles in the plane transverse to the geomagnetic field. By comparing the east‐west spatial relationship of ion‐density depletions measured in‐situ by the Atmospheric Explorer E (AE‐E) satellite to backscatter plumes measured by the ALTAIR radar, we show that plasma bubbles are vertically elongated depletions that extend upward from the bottom side of the F layer, in the form of tilted wedges, rather than more isotropically shaped but isolated structures. The shape of plasma bubbles is inferred from (1) ion density depletions that exceeded 99% in the ‘neck’ regions of plumes and (2) the eastward drift velocities of the plumes. The expected electrodynamics of vertically elongated plasma bubbles are consistent with the observations of large eastward drift velocities of plumes that are comparable to F region plasma drift measurements made at Jicamarca and to F region neutral wind measurements made at Kwajalein. The results also reveal that the west wall of large‐scale altitude modulations of the bottomside F layer that produces the primary plumes and bubbles becomes structured, and evolves with the generation of secondary plumes and bubbles.

A multifrequency (ten spectral lines between VHF and S band) coherent radio beacon is presently transmitting continuously from a 1000‐km, high‐inclination orbit for the purpose of characterizing the transionospheric communication channel. Its high phase‐reference frequency (2891 MHz) permits direct observation of complex‐signal scintillation, and its very stable, sun‐synchronous orbit allows repeated pre‐midnight observations at low latitudes and near‐midnight observations at auroral latitudes. We present here early results of the observations; salient points include the following. First, most of the data are consistent with phase‐screen modeling of the production of ionospheric scintillation, including an ƒ−2 frequency dependence for phase variance. Second, propagation theories invoking weak, single scatter seldom are adequate, because even moderate intensity scintillation usually is accompanied by phase fluctuations comparable to or greater than a radian. Third, under conditions producing GHz scintillation (near the geomagnetic equator), lower frequencies show marked diffraction effects, including breakdown of the simple ƒ−2 behavior of phase variance and loss of signal coherenceacross a band as narrow as 11.5 MHz at UHF.