Coronal Holes and Solar Wind High-Speed Streams: II. Forecasting the Geomagnetic Effects     

Bojan Vršnak  Manuela Temmer  Astrid M. Veronig 《Solar physics》2007,240(2):331-346

We present a simple method of forecasting the geomagnetic storms caused by high-speed streams (HSSs) in the solar wind. The method is based on the empirical correlation between the coronal hole area/position and the value of the Dst index, which is established in a period of low interplanetary coronal mass ejection (ICME) activity. On average, the highest geomagnetic activity, i.e., the minimum in Dst, occurs four days after a low-latitude coronal hole (CH) crosses the central meridian. The amplitude of the Dst dip is correlated with the CH area and depends on the magnetic polarity of the CH due to the Russell – McPherron effect. The Dst variation may be predicted by employing the expression Dst(t)=(−65±25×cos λ)[A(t ^*)]^0.5, where A(t ^*) is the fractional CH area measured in the central-meridian slice [−10°,10°] of the solar disc, λ is the ecliptic longitude of the Earth, ± stands for positive/negative CH polarity, and t−t ^*=4 days. In periods of low ICME activity, the proposed expression provides forecasting of the amplitude of the HSS-associated Dst dip to an accuracy of ≈30%. However, the time of occurrence of the Dst minimum cannot be predicted to better than ±2 days, and consequently, the overall mean relative difference between the observed and calculated daily values of Dst ranges around 50%.  相似文献   

An overview of the 1995 SWARM shallow-water internal wave acousticscattering experiment     

Apel  J.R. Badiey  M. Ching-Sang Chiu Finette  S. Headrick  R. Kemp  J. Lynch  J.F. Newhall  A. Orr  M.H. Pasewark  B.H. Tielbuerger  D. Turgut  A. Von Der Heydt  K. Wolf  S. 《Oceanic Engineering, IEEE Journal of》1997,22(3):465-500

  相似文献   

Moraine development at a small High‐Arctic valley glacier: Rieperbreen,Svalbard     

Astrid Lys  Ida Lnne 《第四纪科学杂志》2001,16(6):519-529

Ice‐cored lateral and frontal moraine complexes, formed at the margin of the small, land‐based Rieperbreen glacier, central Svalbard, have been investigated through field observations and interpretations of aerial photographs (1936, 1961 and 1990). The main focus has been on the stratigraphical and dynamic development of these moraines as well as the disintegration processes. The glacier has been wasting down since the ‘Little Ice Age’ (LIA) maximum, and between 1936 and 1990 the glacier surface was lowered by 50–60 m and the front retreated by approximately 900 m. As the glacier wasted, three moraine ridges developed at the front, mainly as melting out of sediments from debris‐rich foliation and debris‐bands formed when the glacier was polythermal, probably during the LIA maximum. The disintegration of the moraines is dominated by wastage of buried ice, sediment gravity‐flows, meltwater activity and some frost weathering. A transverse glacier profile with a northward sloping surface has developed owing to the higher insolation along the south‐facing ice margin. This asymmetric geometry also strongly affects the supraglacial drainage pattern. Lateral moraines have formed along both sides of the glacier, although the insolation aspect of the glacier has resulted in the development of a moraine 60 m high along its northern margin. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

In memoriam: Klaus Keil, 1934–2022     

Astrid Holzheid 《Chemie der Erde / Geochemistry》2022,82(2):125885

  相似文献   

How accurately are climatological characteristics and surface water and energy balances represented for the Colombian Caribbean Catchment Basin?     

Isabel Hoyos  Astrid Baquero-Bernal  Stefan Hagemann 《Climate Dynamics》2013,41(5-6):1269-1290

In Colombia, the access to climate related observational data is restricted and their quantity is limited. But information about the current climate is fundamental for studies on present and future climate changes and their impacts. In this respect, this information is especially important over the Colombian Caribbean Catchment Basin (CCCB) that comprises over 80 % of the population of Colombia and produces about 85 % of its GDP. Consequently, an ensemble of several datasets has been evaluated and compared with respect to their capability to represent the climate over the CCCB. The comparison includes observations, reconstructed data (CPC, Delaware), reanalyses (ERA-40, NCEP/NCAR), and simulated data produced with the regional climate model REMO. The capabilities to represent the average annual state, the seasonal cycle, and the interannual variability are investigated. The analyses focus on surface air temperature and precipitation as well as on surface water and energy balances. On one hand the CCCB characteristics poses some difficulties to the datasets as the CCCB includes a mountainous region with three mountain ranges, where the dynamical core of models and model parameterizations can fail. On the other hand, it has the most dense network of stations, with the longest records, in the country. The results can be summarised as follows: all of the datasets demonstrate a cold bias in the average temperature of CCCB. However, the variability of the average temperature of CCCB is most poorly represented by the NCEP/NCAR dataset. The average precipitation in CCCB is overestimated by all datasets. For the ERA-40, NCEP/NCAR, and REMO datasets, the amplitude of the annual cycle is extremely high. The variability of the average precipitation in CCCB is better represented by the reconstructed data of CPC and Delaware, as well as by NCEP/NCAR. Regarding the capability to represent the spatial behaviour of CCCB, temperature is better represented by Delaware and REMO, while precipitation is better represented by Delaware. Among the three datasets that permit an analysis of surface water and energy balances (REMO, ERA-40, and NCEP/NCAR), REMO best demonstrates the closure property of the surface water balance within the basin, while NCEP/NCAR does not demonstrate this property well. The three datasets represent the energy balance fairly well, although some inconsistencies were found in the individual balance components for NCEP/NCAR.  相似文献   

Fourth International Conference on Fluvial Sedimentology     

Anthony Orr Clarke 《The Professional geographer》1990,42(2):236-237

  相似文献   

Seasonality change and the third millennium BP rainforest crisis in southern Cameroon (Central Africa)     

Alfred Ngomanda  Katharina Neumann  Astrid Schweizer  Jean Maley 《Quaternary Research》2009,71(3):307-318

The third millennium BP crisis of the central African rainforest is not sufficiently understood. The low resolution of most pollen profiles and a large plateau of the calibration curve aggravate the exact dating of the event, and its causal climatic parameters are debated. We present a high-resolution pollen profile from the swamp site Nyabessan in the southern Cameroonian rainforest, covering the period 3100-2300 cal yr BP. Between 3100 and 2500 cal yr BP, the climate was favourable for a regional evergreen forest with Caesalpiniaceae and Lophira and a local Raphia swamp forest. Around 2500/2400 cal yr BP, a significant decrease of mature forest and swamp forest taxa and an increase of pioneers indicate that the rain forest was seriously disturbed and replaced by secondary formations. The dominance of Trema orientalis, a pioneer well adapted to seasonal desiccation, points to a much more accentuated seasonality after 2500 cal yr BP, which seems to be linked to a southwards shift of the ITCZ during the northern hemisphere winter months. We propose that the rain forest crisis between 2500 and 2200 cal BP created favourable conditions for farming and paved the way for a major expansion of Bantu speaking populations.  相似文献   

A catalogue of M51 type galaxy associations     

Ari Jokimäki  Harley Orr  David G. Russell 《Astrophysics and Space Science》2008,315(1-4):249-283

A catalog of 232 apparently interacting galaxy pairs of the M51 class is presented. Catalog members were identified from visual inspection of multi-band images in the IRSA archive. The major findings in the compilation of this catalog are (1) A surprisingly low number of the main galaxies in M51 systems are early type spirals and barred spirals. (2) Over 70% of the main galaxies in M51 systems are 2-armed spirals. (3) Some systems that were classified as M51 types in previous studies are not M51 types as defined in this catalog. There were a number of systems previously classified as M51 systems for which the “companion” is identified as an HII region within the main galaxy or foreground star within the Milky Way. (4) It was found that only 18% of the M51 type companions have redshift measurements in the literature. There is a significant need for spectroscopic study of the companions in order to improve the value of the catalog as a sample for studying the effects of M51 type interaction on galaxy dynamics, morphology, and star formation. Further spectroscopy will also help constrain the statistics of possible chance projections between foreground and background galaxies in the catalog. The catalog also contains over 430 additional systems which are classified as “possible M51” systems. The reasons for classifying certain systems as possible M51 systems are discussed.  相似文献   

Late Pleistocene stratigraphy and sedimentary environments of the Severnaya Dvina‐Vychegda region in northwestern Russia     

Astrid Lyså  Eiliv Larsen  Jan‐Pieter Buylaert  Ola Fredin  Maria A. Jensen  Denis Kuznetsov  Andrew S. Murray  Dmitry A. Subetto  Aurelien van Welden 《Boreas: An International Journal of Quaternary Research》2014,43(4):759-779

The Late Pleistocene stratigraphy from the Severnaya Dvina‐Vychegda region of northwestern Russia is revised based on investigations of new localities, revisiting earlier localities, introduction of about 110 new OSL dates and burial depth corrections of earlier published OSL dates, in addition to six new radiocarbon dates. Most of the OSL samples studied here are from fluvial and subaquaeous sediments, which we found to be well bleached. Six chronostratigraphical units and their sedimentary environment are described, with the oldest unit consisting of pre‐Eemian glacial beds. For the first time, Early Weichselian sediments are documented from the region and a fluvial environment with some vegetation and permafrost conditions is suggested to have persisted from the end of the Eemian until at least about 92 ka ago. The period in which a Middle Weichselian White Sea Lake could have existed is constrained to 67?62 ka, but as the lake level never reached the thresholds of the drainage basin, the lake probably existed only for a short interval within this time‐span. Blocking and reversal of fluvial drainage started again around 21?20 ka ago when the Fennoscandian Ice Sheet advanced into the area, reaching its maximum 17?15 ka ago. At that time, an ice‐dammed lake reached its maximum water level, which was around 135 m above present sea level. Drainage of the lake started shortly after 15 ka ago, and the lake was emptied within 700 years. Severe periglacial conditions, with permafrost and aeolian activity, prevailed in the area until about 10.7 ka.  相似文献   

Morphology of riffle–pool sequences in the River Severn,England     

Paul A. Carling  Harriet G. Orr 《地球表面变化过程与地形》2000,25(4):369-384

Despite the occurrence of riffle–pool sequences in many rivers there are few data concerning riffle–pool unit morphology. Of many criteria proposed to identify riffle–pool units, only three methods can be regarded as objective and robust. These are the ‘zero‐crossing’, the ‘spectral analysis’ and the ‘control‐point’ methods. In this paper statistics are developed using the first two of these methods to describe the streamwise morphology of 275 riffles and 285 pools which form a continuous 32·1 km reach of the bed of the River Severn in Shropshire, England. Yalin's theoretical relationship between the average riffle:pool unit length ( λ _p) and channel width ( W ), λ _p = 3 W , applies to the River Severn. Reach‐average riffle height ( H ) is a constant proportion of bankfull depth ( h ); typically H ≅ 0·16 h . Riffle height is a positive function of riffle length. Pool depth is a positive function of pool length. However, both riffle length and pool length increase more rapidly than the bed‐level amplitude, such that long riffles or pools are relatively ‘flat’. As channel gradient reduces, bedforms flatten and become more asymmetric as riffle stoss sides and the proximal slope of pools lengthen at the expense of riffle lee sides and pool distal slopes. The statistical relationships between riffle steepness (H/L) and water depth are similar to those for equilibrium subaqueous dunes. The Severn data are consistent with Yalin's theoretical analysis relating riffle bedform length (L _r) to water depth, i.e. L _r = α2π h, wherein α ≅ 1 for steep near‐equilibrium bedforms but α ≅ 2 to 3 as the relative depth decreases and riffles become long, low features. Theoretical consideration and turbulence data indicate that the frequency of coherent turbulent‐flow structures associated with the riffle–pool mixing length in the Severn should be of the order of 50 to 100 s. The morphological similarity of the steepest River Severn riffles with dunes raises intriguing questions with respect to self‐similar, convergent organization of periodic alluvial bedforms and to bedform dynamic classification particularly. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献