High-latitude propagation studies using a meridional chain of LF/MF/HF receivers

For over a decade, Dartmouth College has oper- ated programmable radio receivers at multiple high-latitude sites covering the frequency range 100–5000 kHz with about a 1-s resolution. Besides detecting radio emissions of auro- ral origin, these receivers record characteristics of the iono- spheric propagation of natural and man-made signals, docu- menting well-known effects, such as the diurnal variation in the propagation characteristics of short and long waves, and also revealing more subtle effects. For example, at auroral zone sites in equinoctial conditions, the amplitudes of dis- tant transmissions on MF/HF frequencies are often enhanced by a few dB just before they fade away at dawn. The po- larization and/or direction of the arrival of ionospherically propagating signals in the lower HF range (3–5 MHz) show a consistent variation between pre-midnight, post-midnight, and pre-dawn conditions. As is well known, magnetic storms and substorms dramatically affect ionospheric propagation; data from multiple stations spanning the invariant latitude range 67–79◦ reveal spatial patterns of propagation charac- teristics associated with magnetic storms and substorms. For example, in the hours preceding many isolated substorms, fa- vorable propagation conditions occur at progressively lower latitudes as a function of time preceding the substorm onset. For some of these effects, explanations follow readily from elementary ionospheric physics, but understanding others re- quires further investigation

Structured waves near the plasma frequency observed in three auroral rocket flights

We present observations of waves at and just above the plasma frequency (<i>f<sub>pe</sub></i>) from three high frequency electric field experiments on three recent rockets launched to altitudes of 300–900 km in active aurora. The predominant observed HF waves just above <i>f<sub>pe</sub></i> are narrowband, short-lived emissions with amplitudes ranging from <1 mV/m to 20 mV/m, often associated with structured electron density. The nature of these HF waves, as determined from frequency-time spectrograms, is highly variable: in some cases, the frequency decreases monotonically with time as in the "HF-chirps" previously reported (McAdams and LaBelle, 1999), but in other cases rising frequencies are observed, or features which alternately rise and fall in frequency. They exhibit two timescales of amplitude variation: a short timescale, typically 50–100 ms, associated with individual discrete features, and a longer timescale associated with the general decrease in the amplitudes of the emissions as the rocket moves away from where the condition <i>f</i>~<i>f<sub>pe</sub></i> holds. The latter timescale ranges from 0.6 to 6.0 s, corresponding to distances of 2–7 km, assuming the phenomenon to be stationary and using the rocket velocity to convert time to distance

Combinatorial proofs of some limit formulas involving orthogonal polynomials

AbstractThe object of this paper is to prove combinatorially several (13 of them) limit formulas relating different families of hypergeometric orthogonal polynomials in Askey's chart classifying them. We first find a combinatorial model for Hahn polynomials which, as pointed out by Foata at the ICM (1983), “contains” models for Jacobi, Meixner, Krawtchouk, Laguerre and Charlier polynomials. Seven limit formulas are proved by “looking at surviving structures” when taking the limit. A simple model, T-structures, is then used to prove (using a different technique) four more limit formulas involving Meixner-Pollaczek, Krawtchouk, Laguerre, Charlier and Hermite polynomials. The theory of combinatorial octopuses (of F. Bergeron) is recalled and two more limits are demonstrated using new models of Meixner-Pollaczek, Laguerre, Gegenbauer and Hermite polynomials

Histoire postglaciaire de la végétation au lac du Diable, mont Albert, Gaspésie, Québec

Au lac du Diable (494 m d'altitude), sur le flanc oriental du mont Albert (sommet: 1154 m), la colonisation végétale postglaciaire débuta par un stade ayant pu durer quelques siècles, à physionomie comparable à celle de la toundra, caractérisé d'abord par les herbes puis par les arbustes. Les populations arborescentes s'installèrent à partir de 10 600 ans BP environ. L'aire occupée par la toundra dans la région devait alors déjà se confiner, comme de nos jours, au seul sommet du mont Albert. Les peupliers et les épinettes (surtout l'épinette blanche) immigrèrent les premiers, mais le bouleau blanc était présent dès 10 300 ans BP. Autour de 10 000 ans BP, les forêts environnantes étaient déjà dominées par les épinettes, mais vers 9500 ans BP leur nombre a fortement diminué alors qu'augmentaient les populations de mélèzes, de bouleaux blancs et surtout d'aulnes verts, et ce, jusque vers 7000 ans BP; pendant ce temps, le sapin croissait dans le secteur, mais sans abonder. Cette longue régression des épinettes fut probablement provoquée par un climat plus sec ou orageux qui favorisa une grande fréquence des feux de forêts, ce qui corrobore le modèle de Milankovitch pour le climat de l'Holocène inférieur. Après 7000 ans BP, sous un climat plus humide, les populations de sapins ont alors pu se développer sur les versants, et des communautés forestières fermées associées aux sapinières-pessières y ont dominé jusqu'à nos jours. Durant la période hypsithermique (7000 à 5000-4500 ans BP environ), à part une plus grande abondance du bouleau blanc, le paysage végétal autour du lac du Diable n'a pas été modifié par l'immigration d'arbres plus thermophiles comme le bouleau jaune ou le pin blanc. Durant l'Holocène supérieur, les populations de sapins et d'épinettes eurent tendance à augmenter et celles de bouleaux blancs ont régressé, phénomène attribué au refroidissement post-hypsithermique.The postglacial plant colonization at the Lac du Diable (alt.: 494 m), on the eastern flank of Mont Albert (summit: 1154 m), started with a tundra-like phase that lasted for some centuries, and was characterized by abundant herbs, then by shrubs. Tree populations established themselves by 10 600 years BP approximately. The area of the regional landscape occupied by tundra at that time was probably restricted to the Mont Albert summit plateau, as it is nowadays. Poplars and spruces (mainly white spruce) were the first immigrants, but white birch was present as early as 10 300 BP. Around 10 000 BP, the forests surrounding Lac du Diable were dominated by spruce. However, around 9500 years BP, their abundance was greatly reduced, whereas the number of larch, white birch and especially green alder increased until 7000 BP. In the meantime, balsam fir grew in the area but was not abundant. The long-lasting decline in spruce populations was probably caused by a dry and/or stormy summer climate that favoured frequent wildfires, these conditions corroborating the Milankovitch model for the Early Holocene climate. After 7000 BP, the balsam fir populations developed under a more humid climate and closed sprucefir forests dominated the landscape around the lake until the present. During the Hypsithermal (ca. 7000 to 5000-4500 years BP), apart from a greater abundance of white birch, the vegetational landscape around Lac du Diable remained stable, without any new species like yellow birch or white pine, for instance. During the Late Holocene, there was a tendency for population increases in spruce and balsam fir, and for a decrease in white birch, a feature linked with the post-hypsithermal cooling.Am Lac du Diable (454 m Hôhe) auf der Ostseite des Mont Albert (Spitze: 1154 m) begann die postglaziale Pflanzenansiedlung mit einer Phase, die einige Jahrhunderte gedauert haben kann und die eine der Tundra vergleichbare Physiognomik hatte, zunâchst durch Grâser, dann durch Bùsche gepràgt. Die baumartigen Bestànde siedelten sich ab ungefàhr 10 600 Jahren v.u.Z. an. Der in der Gegend von der Tundra eingenommene Platz musste damais schon wie heute allein auf den Gipfel des mont Albert begrenzt sein. Die Pappeln und Fichten (vor allem Weisstannen) siedelten sich als erste an, aber die Weissbirke war seit 10 300 Jahren v.u.Z. anzufinden. Um 10 000 Jahre v.u.Z. dominierten in den umgebenden Wâldern schon die Fichten, aber gegen 9500 Jahre v.u.Z. nahm ihre Zahl stark ab, wàhrend gleichzeitig die Bestànde von Lârchen, Weissbirken und vor allem grùnen Erlen zunahmen und das bis um 7000 Jahre v.u.Z.: wàhrend dieser Zeit nahmen Tannen in dem Gebiet zu, ohne sehr zahlreich zu sein. Dieser langsame Rùckgang der Fichten wurde môglicherweise durch ein trockeneres oder durch Gewitter gepràgtes Klima hervorgerufen, welches hâufige WaIdbrànde begùnstigte, was wiederum das Modell von Milankovitch fur das Klima im frùhen Holozàn erhàrtet. Nach 7000 Jahren v.u.Z. haben die Tannenansiedlungen sich unter einem feuchteren Klima auf den Abhàngen entwickeln kônnen, und geschlossene Waldeinheiten, die mit den Fichter-Tannenwâldern in Verbindung gebracht werden, haben bis heute dort dominiert. Wàhrend des Hypsithermal (etwa 700 bis 5000-4500 Jahre v.u.Z.) wurde die Pflanzenlandschaft um den lac du Diable abgesehen von einem vermehrten Vorkommen der Weissbirke nicht durch Ansiedlung von mehr thermophilen Bàumen wie die Gelbbirke oder die Weisskierfer verândert. Wàhrend des spàten Holozân hatten die Bestànde von Tannen und Fichten die Tendenz zuzunehmen, wàhrend die der Weissbirke zurùckgingen, ein Phânomen, das der post-hypsithermischen Abkuhlung zugeschrieben wird

Histoire Postglaciaire de la Végétation dans la Région de Mont-Saint-Pierre, Gaspésie, Québec

Les analyses pollinique et macrofossile de sédiments de deux lacs des environs de Mont-Saint-Pierre (Gaspésie), l'un dans la vallée côtière, l'autre sur le plateau, permettent de reconstituer l'histoire postglaciaire de la végétation dans deux situations physiographiques contrastées. Seul le plateau a été colonisé par une végétation initiale de type toundra (> 10 400 ans BP), pendant que la vallée était encore en partie ennoyée par la mer de Goldthwait. L'afforestation du plateau fut caractérisée d'abord par des populations de Picea sp., accompagné de Populus sp. et Larix laricina, puis par un envahissement progressif par Abies balsamea et Betula papyrifera. La baisse de la représentation pollinique de l'aulne vert (Alnus cf. crispa), maximale durant l'afforestation, paraît être un indicateur assez fidèle de la fermeture de la couverture forestière. Cette phase se termine vers 9000 ans BP sur le plateau. Le rythme de l'afforestation a été différent dans la vallée. Des taxons héliophiles s'y sont maintenus plus longtemps, ce qui peut être relié à l'activité des versants abrupts qui flanquent la vallée. Malgré des variations de l'abondance relative des arbres, la sapinière à bouleau blanc a sans doute occupé le plateau depuis 9000 ans BP environ. Par contre, la végétation de la vallée s'est passablement modifiée jusque vers 5000 à 4500 ans BP, par la migration successive de Betula alleghaniensis, de Pinus strobus, de Fraxinus nigra, dAcer saccharum et d'Ulmus americana. L'implantation progressive de ces arbres relativement thermophiles est à l'origine de la diversité du paysage actuel de la vallée. Ces reconstitutions permettent d'écarter l'hypothèse de Dansereau (1944) selon laquelle les érablières à Acer saccharum de la Gaspésie seraient des groupements hérités de l'optimum climatique de l'Holocène moyen, au cours duquel les érablières auraient été largement répandues sur le territoire. Les données montrent que ces érablières datent de l'Holocène Supérieur, la migration d'Acer saccharum ayant sans doute été freinée par des barrières topo-climatiques entraînant une discontinuité prononcée des habitats pouvant accueillir cette espèce.Pollen and macrofossil diagrams from the sediments of two lakes in the Mont-Saint-Pierre area (Gaspé, Québec) allow the reconstruction of the postglacial vegetational history on the plateau and in the coastal valley nearby. Tundra prevailed on the plateau until 10,400 years BP while the valley was partly submerged by the Goldthwait Sea. The afforestation began with the invasion by Picea sp. populations, which were associated with Populus sp. and Larix laricina, followed by progressive encroachment by Abies balsamea and Betula papyrifera. The decrease in green alder pollen, which shows maximum representation during the afforestation phase, indicates the closure of the forest canopy. The afforestation phase ended around 9000 years BP on the plateau but the timing of events was slightly different in the valley. The heliophilous plants persisted there, possibly due to active slope dynamics. The balsam firwhite birch community occupied the plateau since 9000 years BP with only small fluctuations in the relative abundance of the tree species. On the other hand, important changes took place in the valley until 5000 to 4000 years BP, when the successive immigration of Betula alleghaniensis, Pinus strobus, Fraxinus nigra, Acer saccharum and Ulmus americana occurred. This development initiated the present vegetational complex. This study clearly demonstrates that the sugar maple communities never colonized the plateau and occurred in the valley only since the Upper Holocene. This is in contradiction with Dansereau's hypothesis (1944) where the sugar maple communities in the Gaspé Peninsula are interpreted as relict stands of a previously widespread distribution of Acer saccharum during the climatic optimum of the Holocene. The late immigration of sugar maple in the area appears to be mostly due to topo-climatic barriers creating discontinuities in the habitats suitable for the species.Die Pollen-Analysen und die makrofossilen Analysen der Sedimente zweier Seen in der Umgebung des Mont-Saint-Pierre (Gaspésie), wovon einer im Kustental, der andere auf der Hochebene gelegen ist, ermôglichen die postglaziale Geschichte der Vegetation in zwei geomorphologisch kontrastiven Gebieten zu rekonstruieren. Nur die Hochebene war von einer ersten Vegetation vom Typus der Tundra (> 10 400 Jahre v.u.Z.) bewachsen, wàhrend das TaI noch zum Teil vom Goldthwait-Meer uberschwemmt war. Die Bewaldung der Hochebene war zuerst durch Picea sp. Bestànde charakterisiert, gefolgt von Populus sp. und Larix laricina, und dann durch ein progressives Uberhandnehmen von Abies balsamea und von Betula papyrifera. DasZuruckgehendes Pollen-Vorkommens der grunen Erie (Alnus cf. crispa), welche in der Bewaldungsphase ihr Hôchstvorkommen erreicht hatte, scheint ein zuverlàssiger lndikator fur die SchlieBung der Walddecke zu sein. Dièse Phase endet gegen 9000 Jahre v.u.Z. auf der Hochebene. Im TaI war der Rhythmus der Bewaldung anders. Heliophile Pflanzen konnten sich dort langer halten, was in Verbindung zur Aktivitàt der Steilabhânge, die das TaI sâumen, gebracht werden kann. Trotz der Variationen in der relativen Dichte der Baume hat der Tannenwald mit WeiRbirken zweifellos die Hochebene seit etwa 9000 Jahren v.u.Z. bedeckt. Hingegen hat sich die Vegetation des Tais ziemlich verândert bis gegen 5000 bis 4500 Jahren v.u.Z. durch die sukzessive Wanderung von Betula alleghaniensis, Pinus strobus, Fraxinus nigra, Acer saccharum und Ulmus americana. Die allmàhliche Ansiedlung dieser relativ wàrmeliebenden Baume ist der Grund fur die Vielfalt der heutigen Landschaft des TaIs. Die Daten zeigen, daB diese Ahornwâlder aus dem spâten Holozan stammen, wobei das Wandern von Acer saccharum zweifellos druch ortliche klimatische Schranken gebremst wurde, die zu einer deutlichen Diskontinuitât der Gebiete, die sich fur die Aufnahme dieser Art eignen, gefuhrt hat

High Resolution OI (630 nm) Image Measurements of Fregion Depletion Drifts During the Guara Campaign

A high performance, all‐sky, imaging system has provided data on the evolution and drift motions of F‐region depletions above the magnetic dip equator at Alcântara, Brazil, (2.3°S, 44.5°W). Monochromatic images of depletions in the OI(630 nm) nightglow were recorded on eight nights during 1‐16 October, 1994, as part of the Guará campaign. The drift motions of the depletions were typically 80–100 m/s eastward prior to local midnight and reduced to a minimum of ∼30–50 m/s in the morning hours, in accord with previous observations. However, on October 2–3 and 12–13 the depletions were observed to reverse direction for ∼60–90 min, achieving westward speeds of ∼30 m/s before the motion reverted to eastward around 0100 LT and accelerated to 35–45 m/s near dawn. Magnetic activity and other evidence suggests that these reversals in the motion of the airglow depletions probably result from reversals in the F‐region dynamo rather than from shifts in the altitude of the shear in the nighttime F‐region plasma drift

Selected Environmental Impacts of Forest Harvesting Operations with Varying Degree of Mechanization

Climate change affects forest ecosystems, impacting timber production and eco-services. Conversely, sustainable forest management has been identified as a means to help mitigate carbon dioxide emissions, a greenhouse gas and contributor to climate change, while also maximizing multiuse benefits through close-to-nature silviculture. In this study, a life cycle assessment was performed on forest harvesting operations at three research sites to provide real-world understanding of the selected environmental impacts associated with harvesting systems typical of Germany: motor-manual (chainsaw and forest tractor), semi-mechanized (single-grip harvester, chainsaw, and forwarder), and fully-mechanized (single-grip harvester and forwarder). Environmental impact categories assessed included greenhouse gas emissions, particulate matter emissions, and non-renewable energy consumption. Results from the three research sites were estimated on a machine basis. The semi-mechanized system resulted in the lowest environmental impact, the majority of which was attributed to felling and processing operations. Next, the environmental impacts were estimated for a complete rotation period and compared amongst the different harvesting systems. According to results, semi-mechanized harvesting systems had the lowest impact over the full rotation period as well as for thinning treatments when compared to motor-manual and fully-mechanized systems. The fully-mechanized system performed the best for final felling treatments. Considering variability between the research sites as well as the system boundary assessed, a diversified approach to harvesting operations may be considered, integrating semi-mechanized and fully-mechanized systems for different treatments throughout the rotation period

Selected Environmental Impacts of Forest Harvesting Operations with Varying Degree of Mechanization

Climate change affects forest ecosystems, impacting timber production and eco-services. Conversely, sustainable forest management has been identified as a means to help mitigate carbon dioxide emissions, a greenhouse gas and contributor to climate change, while also maximizing multiuse benefits through close-to-nature silviculture. In this study, a life cycle assessment was performed on forest harvesting operations at three research sites to provide real-world understanding of the selected environmental impacts associated with harvesting systems typical of Germany: motor-manual (chainsaw and forest tractor), semi-mechanized (single-grip harvester, chainsaw, and forwarder), and fully-mechanized (single-grip harvester and forwarder). Environmental impact categories assessed included greenhouse gas emissions, particulate matter emissions, and non-renewable energy consumption. Results from the three research sites were estimated on a machine basis. The semi-mechanized system resulted in the lowest environmental impact, the majority of which was attributed to felling and processing operations. Next, the environmental impacts were estimated for a complete rotation period and compared amongst the different harvesting systems. According to results, semi-mechanized harvesting systems had the lowest impact over the full rotation period as well as for thinning treatments when compared to motor-manual and fully-mechanized systems. The fully-mechanized system performed the best for final felling treatments. Considering variability between the research sites as well as the system boundary assessed, a diversified approach to harvesting operations may be considered, integrating semi-mechanized and fully-mechanized systems for different treatments throughout the rotation period