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1.
The Builth Volcanic Series of Llanvirnian age in Llanelwedd Quarries, mid-Wales, carries three components of natural remanent magnetisation. Component P, regarded as primary, is a thermochemical remanence directed at D = 181.7°, I = +54.5°, α95 = 4.4° relative to bedding. Component S is a secondary component with in situ D = 178.7°, I = ?6.7°, α95 = 5.4° and is believed to be a low-temperature chemical remanence (CRM) of Permo-Carboniferous age. Component R is directed close to the present geomagnetic field and is believed to be a recent viscous remanence (VRM).The results are of interest for three reasons. First, they are an unusually good example of multi-component NRM analysis, the three components being so clearly discriminated by thermal demagnetisation because they have almost completely separate blocking temperature ranges. Second, they provide evidence of a Permo-Carboniferous event (possibly a mild thermal or hydrothermal pulse promoting CRM acquisition) some 40 km north of the Hercynian orogenic front. Third, they illustrate very clearly the importance of detailed demagnetisation: this work revises the pole position for these rocks by ～ 10° and removes an obstacle to the palaeomagnetic recognition of the ～ 1000 km wide Iapetus Ocean cutting Britain in Ordovician time. 相似文献
2.
D. J. Mullan 《Solar physics》1989,121(1-2):239-259
Although progress has been made in understanding certain aspects of the physics of solar and stellar flares, there are a number of topics which, in the author's opinion, still pose a problem. We summarize these topics here. 相似文献
3.
We propose that bubbles of matter ejected from magnetic reconnection sites in polar plumes drive the solar wind in coronal holes. 相似文献
4.
Mullan Donal Swindles Graeme Patterson Tim Galloway Jennifer Macumber Andrew Falck Hendrik Crossley Laura Chen Jie Pisaric Michael 《Theoretical and Applied Climatology》2017,129(3-4):1089-1108
Theoretical and Applied Climatology - Climate models project that the northern high latitudes will warm at a rate in excess of the global mean. This will pose severe problems for Arctic and... 相似文献
5.
D. J. Mullan James MacDonald 《Monthly notices of the Royal Astronomical Society》2005,356(3):1139-1148
Spruit has shown that an astrophysical dynamo can operate in the non-convective material of a differentially rotating star as a result of a particular instability in the magnetic field (the Tayler instability). By assuming that the dynamo operates in a state of marginal instability, Spruit has obtained formulae which predict the equilibrium strengths of azimuthal and radial field components in terms of local physical quantities. Here, we apply Spruit's formulae to our previously published models of rotating massive stars in order to estimate Tayler dynamo field strengths. There are no free parameters in Spruit's formulae. In our models of 10- and 50-M⊙ stars on the zero-age main sequence, we find internal azimuthal fields of up to 1 MG, and internal radial components of a few kG. Evolved models contain weaker fields. In order to obtain estimates of the field strength at the stellar surface, we examine the conditions under which the Tayler dynamo fields are subject to magnetic buoyancy. We find that conditions for Tayler instability overlap with those for buoyancy at intermediate to high magnetic latitudes. This suggests that fields emerge at the surface of a massive star between magnetic latitudes of about 45° and the poles. We attempt to estimate the strength of the field which emerges at the surface of a massive star. Although these estimates are very rough, we find that the surface field strengths overlap with values which have been reported recently for line-of-sight fields in several O and B stars. 相似文献
6.
We have adapted the ZEUS code to model magnetic interactions in partially ionized gas. When two regions of opposite polarity come into contact with each other, ions drifting in response to the Lorentz force fall into the minimum of the magnetic field, and then the drifting ions force the neutrals to take part in the flow. Because of the finite time required for ion-atom collisions to occur, the gas which emerges from the interaction site has an ion/atom ratio which may be altered relative to that in the ambient medium. In order to model this effect, we adapt the Zeus code to a two-step iterative process involving a cycle between the hydrodynamic (HD) and the magnetohydrodynamic (MHD) versions of the code. The ion and atom fluids are coupled by collisions. Our simulations show that in chromospheric conditions, outflowing gas exhibits enhancements in ion/atom ratios which may be as large as a factor of 10 or more. The magnitude of the enhancements is determined by two key ratios which enter into the problem: the degree of ionization (ni/na), and the plasma parameter. We show that, in the context of the mechanism we propose here, the amplitude of the ion/atom enhancements in the solar chromosphere is subject to a remarkable self-regulation because the ion density ni is almost invariant over the height range of interest to us. Our results are relevant in the context of the Sun, where the coronal abundances of elements with low first ionization potential (FIP) are systematically enhanced in certain magnetic structures. Although data for stars other than the Sun are sparse, we point out that our results are also useful for interpreting the available stellar data. 相似文献
7.
R. I. C. C. Francis A. B. Mullan J. A. Renwick 《Theoretical and Applied Climatology》2003,75(1-2):1-14
Summary ?We evaluate United Kingdom Meteorological Office (UKMO) one-month ensemble forecasts of mean sea-level pressure (MSLP) in
the southern hemisphere (SH) to 60° S, with a special focus on their utility near New Zealand (NZ). There are 105 9-member
ensembles, at approximately two-week intervals, between 1995 and 1999. Each forecast is averaged over two successive 15-day
periods and verified against the NCEP/NCAR reanalysis data set. Compared to climatology, the skill of the ensemble mean is
slightly positive in days 1–15, and slightly negative in days 16–30. Skill near NZ is slightly lower than the SH averages.
For SH-scale circulation patterns (as seen in the first few principal components), skill is greater than for most individual
grid points, but is still negligible or negative in days 16–30. Moderate skill-spread correlations (ρ ≈−0.5) were found for
some skill scores. The way that skill varies with season and the Southern Oscillation Index is consistent with other research
but not statistically significant for this small data set. Probabilistic forecasts of low and high pressures have skill similar
to that of the ensemble mean. The ensemble spread is generally too small, in that the analysis lies within the ensemble less
often than the theoretically optimum value of 80% of the time. Measured as a fraction of the natural variability, the spread
increases substantially with time and latitude: it is less than 0.5 near the equator in days 1–15, and takes values near 1
only at higher latitudes during days 16–30. The initial sequential structure of the ensembles (a consequence of the use of
time lags in their genesis) is still apparent in days 1–15 but has disappeared by days 16–30. Three potential alternatives
to the ensemble mean were all found to have less skill than it.
Received June 17, 2001; revised July 4, 2002; accepted November 22, 2002
Published online March 17, 2003 相似文献
8.
Shifting nutrient limitation and eutrophication effects in marsh vegetation across estuarine salinity gradients 总被引：1，自引：0，他引：1
Caitlin Mullan Crain 《Estuaries and Coasts》2007,30(1):26-34
In light of widespread coastal eutrophication, identifying which nutrients limit vegetation and the community consequences
when limitation is relaxed is critical to maintaining the health of estuarine marshes. Studies in temperate salt marshes have
generally identified nitrogen (N) as the primary limiting nutrient for marsh vegetation, but the limiting nutrient in low
salinity tidal marshes is unknown. I use a 3-yr nutrient addition experiment in mid elevation,Spartina patens dominated marshes that vary in salinity along two estuaries in southern Maine to examine variation in nutrient effects. Nutrient
limitation shifted across estuarine salinity gradients; salt and brackish marsh vegetation was N limited, while oligohaline
marsh vegetation was co-limited by N and phosphorus (P). Plant tissue analysis ofS. patens showed plants in the highest salinity marshes had the greatest percent N, despite N limitation, suggesting that N limitation
in salt marshes is partially driven by a high demand for N to aid in salinity tolerance. Fertilization had little effect on
species composition in monospecificS. patents stands of salt and brackish marshes, but N+P treatments in species-rich oligohaline marshes significantly altered community
composition, favoring dominance by high aboveground producing plants. Eutrophication by both N and P has the potential to
greatly reduce the characteristic high diversity of oligohaline marshes. Inputs of both nutrients in coastal watersheds must
be managed to protect the diversity and functioning of the full range of estuarine marshes. 相似文献
9.
10.
The phenomenon of MHD wave refraction is useful in interpreting the properties of the magnetic fluctuations in certain parcels
of solar wind. In the physics of MHD wave refraction, variations in the Alfvén speed VAlf play a dominant role. Here, we compile statistics of the 1-min averages of VAlf at the location of the ACE spacecraft during its first 5 years of operation. We find that monthly distributions of VAlf are close to log-normal, with standard deviations σV as small as 0.11 in the logarithm. Variations in the monthly mean VAlf are correlated significantly with sunspot number. We also compile monthly distributions of the plasma β parameter. The distributions
of both VAlf and β are significantly narrower than they would be if the various solar wind parameters were statistically independent.
In the Tp–VAlf plane, we find a zone of avoidance at low VAlf: for VAlf ≤10 – 15 km/s, there are no samples in the 1-min data that are cooler than Tp = 10 000 – 15 000 K. This feature can be understood in the context of MHD wave refraction, although other explanations are
also possible. 相似文献