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1.
S.R. Prabhakaran Nayar V. Sanalkumaran Nair V.N. Radhika K. Revathy 《Solar physics》2001,201(2):405-417
The solar wind plasma exhibits many features of the solar surface passed on to the interplanetary medium as temporal variations
due to the solar rotation. The yearly average values of solar wind velocity, and geomagnetic index A
p during 1965–1999 were found to exhibit long period evolution. They were found to peak around the declining phase of each
solar cycle. While the solar wind velocity peaks around the second half of the declining phase, the IMF field strength increases
around the first half of the declining phase of each solar cycle. The power spectrum of these parameters shows peaks around
37-day, 30-day, 27-day, 13.5-day, 9-day, and 7-day periods. The temporal evolution of the power spectrum of the solar wind
plasma parameters and the geomagnetic activity index A
p are also studied in detail and presented with the help of contour graphs. These studies indicate that the strength of the
quasi-periodicities in the interplanetary medium evolves with time. 相似文献
2.
The distribution of the shocks in the heliosphere and their characteristic variations are investigated using Ulysses observations. The jumps in solar wind velocity, IMF magnitude, and proton density across the shocks and discontinuities are
evaluated and used to characterize them. The distribution of these discontinuities with respect to heliolatitude ± 80° and
with radial distance 1 to 5 AU are analyzed during solar minimum and solar maximum to understand their global behavior. It
is noticed that the jumps in solar wind parameters associated with shocks and discontinuities are more prominent during the
second orbit of Ulysses, which coincided with the maximum phase of solar activity. 相似文献
3.
The solar wind velocity near Earth shows systematic structure in and around the heliospheric current sheet. The solar wind velocity measurements at IMF sector boundary crossings at 1 AU during 1972–1977 have been used to infer the azimuthal structure of the solar wind velocity in the current sheet. We found that the solar wind velocity in the in-ecliptic portion of the current sheet varies from longitude to longitude, where it originates from the corona. Also, the yearly average value of solar wind velocity in the HCS is found to vary with the phase of the solar cycle; with a maximum value around 1974. TheK-corona brightness on the source surface corresponding to the IMF sector boundary crossings during the period of study also shows a similar but opposite pattern of variation when the data are averaged over a long period. However, this relation is not observed when we considered them individually. So, we conclude that there exists a longitudinal variation of solar wind velocity in the heliospheric current sheet. 相似文献
4.
Wavelet Analysis of solar,solar wind and geomagnetic parameters 总被引:3,自引:0,他引:3
The sunspot number, solar wind plasma, interplanetary magnetic field, and geomagnetic activity index A
p have been analyzed using a wavelet technique to look for the presence of periods and the temporal evolution of these periods. The global wavelet spectra of these parameters, which provide information about the temporal average strength of quasi periods, exhibit the presence of a variety of prominent quasi periods around 16 years, 10.6 years, 9.6 years, 5.5 years, 1.3 years, 180 days, 154 days, 27 days, and 14 days. The wavelet spectra of sunspot number during 1873–2000, geomagnetic activity index A
p during 1932–2000, and solar wind velocity and interplanetary magnetic field during 1964–2000 indicate that their spectral power evolves with time. In general, the power of the oscillations with a period of less than one year evolves rapidly with the phase of the solar cycle with their peak values changing from one cycle to the next. The temporal evolution of wavelet power in R
z, v
sw, n, B
y, B
z, |B|, and A
p for each of the prominent quasi periods is studied in detail. 相似文献
5.
The beam cyclotron instability and electron acoustic instability, driven by cross-tail current and inhomogeneity in density and magnetic field, are found to be unstable in the earth's magnetic tail region. The anomalous resistivities due to these instabilities are found to be of the order of (Ωe being the electron gyro frequency). It is also suggested that the non-linear saturation of the beam cyclotron instability may lead to conditions favourable for exciting ion acoustic instability. 相似文献
6.
Ecologically relevant estimates of seasonal variability in nitrogen uptake and allocation in two species of temperate seagrasses were obtained using in situ isotope-labelling approach. Significantly higher uptake rates of ammonium by leaves, roots and epiphytes of Amphibolis than Posidonia were observed. Overall, root uptake rates were lower than other components. Effect of season was not significant for leaves, roots or epiphytes of the two species. However, plankton uptake varied seasonally with higher rates in winter (0.98 mg N g−1 DW h−1). In contrast, nitrate uptake rates for various components were significantly affected by seasons. Uptake rates by plankton were highest ranging from 0.003 mg N g−1 DW h−1 (summer, Amphibolis) to 0.69 mg N g−1 DW h−1 (winter, Posidonia). Uptake of nitrate by roots was negligible. Biotic uptake rates for nitrate were an order of magnitude slower than ammonium, demonstrating an affinity for ammonium over nitrate as a preferred inorganic nitrogen source. Adelaide coastal waters have lost over 5000 ha of seagrasses, much of this attributed to nutrient inputs from wastewater, industrial and stormwater. Managing these inputs into future requires better understanding of the fate of nutrients, particularly biological uptake. This study attempts to quantify uptake rates of nitrogen by seagrasses. 相似文献
7.
Increased human habitation has led to a 30 to 50-fold increase in nutrient loads to the coastal waters of Adelaide, resulting in the loss of over 5000 ha of seagrass meadows. The rate of loss since the 1940s has been irregular, averaging 85 ha yr(-1), marked by a substantial peak between 1971 and 1977. A modelling approach allowed comparison of the annual input with the annual uptake rates for the different biotic components in the seagrass bed. In 2005, the estimated uptake of ammonium (465 t yr(-1)) and nitrate (3.04 t yr(-1)) by the seagrass and associated epiphytes in the Adelaide region accounted for 31% of the ammonium and <1% of the nitrate that is currently discharged into the coastal waters. Environment Improvement Programs, such as the one implemented in 1996, may reduce the total nitrogen loads to 700 t yr(-1), possibly stemming further losses and facilitating recolonisation of new seagrass. 相似文献
8.
The solar wind velocity distribution in the heliosphere is best represented using a v-map, where velocity contours are plotted in heliographic latitude-longitude coordinates. It has already been established that low-speed regions of the solar wind on the source surface correspond to the maximum bright regions of the K-corona and the neutral line of the coronal magnetic field. In this analysis, v-maps on the source surface for Carrington rotations (CRs) 1787-1795, during 1987, have been prepared using the interplanetary scintillation measurements at Research Institute of Atmospherics (RIA), Nagoya Univ., Japan. These v-maps were then used to study the time evolution of the low-speed (\leq450 km s−1) belt of the solar wind and to deduce the distribution of solar wind velocity on the heliospheric current sheet. The low-speed belt of the solar wind on the source surface was found to change from one CR to the next, implying a time evolution. Instead of a slow and systematic evolution, the pattern of distribution of solar wind changed dramatically at one particular solar rotation (CR 1792) and the distributions for the succeeding rotations were similar to this pattern. The low-speed region, in most cases, was found to be close to the solar equator and almost parallel to it. However, during some solar rotations, they were found to be organised in certain longitudes, leaving regions with longitudinal width greater than 30○ free of low-speed solar wind, i.e. these regions were occupied by solar wind with velocities greater than 450 km s−1. It is also noted from this study that the low-speed belt, in general, followed the neutral line of the coronal magnetic field, except in certain cases. The solar wind velocity on the heliospheric current sheet (HCS) varied in the range 300–585 km s−1 during the period of study, and the pattern of velocity distribution varied from rotation to rotation. 相似文献
9.
The solar wind velocity and density have been studied around the interplanetary magnetic field sector boundaries (+ to ? and ? to +) during 1965–1974, separating the data into autumnal and vernal periods. It is noticed that the solar wind velocity shows a sharp increase around the Hale type of sector boundary in both northern and southern heliosphere indicating a more favourable condition for the high speed stream after Hale type of sector boundary crossing than non-Hale boundary. 相似文献
10.
It is shown that in a heliomagnetic field the presence of a magnetic quadrupole in addition to a magnetic dipole introduces a north-south asymmetry in the heliospheric current sheet (HCS) about the heliographic equator. The dominant polarity of the interplanetary magnetic field (IMF) for the above type of current sheet reverses sign at a transition latitude
T, which lies in a heliohemisphere opposite to the one in which the HCS has more heliolatitudinal extension. The position of
T in the heliosphere and the north-south asymmetry introduced in the HCS change with the relative phase of the dipole and quadrupole components present in the solar magnetic field. The effect of the above type of asymmetric HCS in the IMF mean sector width is evaluated and the results are in agreement with the observations during the minima of solar cycle 21. 相似文献