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Several recent studies have reported quasi-periodicities with a period between 1 and 2 years (to be called here `mid-term
quasi-periodicities') in various heliospheric parameters, like solar wind speed, interplanetary magnetic field, cosmic rays,
and geomagnetic activity. Here we study their long-term occurrence in geomagnetic activity using an extended aa index which
covers the last 15 solar cycles. We confirm their intermittent occurrence and the alternation of their dominant period between
a slightly shorter period of about 1.2–1.4 years and a slightly longer period of about 1.5–1.7 years. We find that the mid-term
quasi-periodicities were strong during two intervals of high solar activity: in the mid-19th century and since 1930. Instead,
contrary to earlier studies, we find that they were consistently weak during low solar activity from 1860s to 1920s. This
implies a long-term connection between the amplitude of mid-term quasi-periodicities and the solar dynamo strength. Since
the rotation speed at the bottom of the solar convection layer (tachocline) has recently been found to vary at a mid-term
periodicity, this suggests that the stronger the solar dynamo is, the more variable the rotation rate of the tachocline is.
We also note that the disappearance of mid-term periodicities may be used as a precursor for long intervals of very weak solar
activity, like great minima. 相似文献
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R.E. Hartle E.C. Sittler R.E. Johnson F. Crary D.T. Young D. Simpson D. Reisenfeld J.J. Berthelier J. Vilppola N. Andre 《Planetary and Space Science》2006,54(12):1211-1224
The Cassini plasma spectrometer (CAPS) instrument made measurements of Titan's plasma environment when the Cassini Orbiter flew through the moon's plasma wake October 26, 2004 (flyby TA). Initial CAPS ion and electron measurements from this encounter will be compared with measurements made by the Voyager 1 plasma science instrument (PLS). The comparisons will be used to evaluate previous interpretations and predictions of the Titan plasma environment that have been made using PLS measurements. The plasma wake trajectories of flyby TA and Voyager 1 are similar because they occurred when Titan was near Saturn's local noon. These similarities make possible direct, meaningful comparisons between the various plasma wake measurements. They lead to the following: (A) The light and heavy ions, H+and N+/O+, were observed by PLS in Saturn's magnetosphere in the vicinity of Titan while the higher mass resolution of CAPS yielded H+ and H2+as the light constituents and O+/CH4+ as the heavy ions. (B) Finite gyroradius effects were apparent in PLS and CAPS measurements of ambient O+ ions as a result of their absorption by Titan's extended atmosphere. (C) The principal pickup ions inferred from both PLS and CAPS measurements are H+, H2+, N+, CH4+ and N2+. (D) The inference that heavy pickup ions, observed by PLS, were in narrow beam distributions was empirically established by the CAPS measurements. (E) Slowing down of the ambient plasma due to pickup ion mass loading was observed by both instruments on the anti-Saturn side of Titan. (F) Strong mass loading just outside the ionotail by a heavy ion such as N2+ is apparent in PLS and CAPS measurements. (G) Except for the expected differences due to the differing trajectories, the magnitudes and structures of the electron densities and temperatures observed by both instruments are similar. The high-energy electron bite-out observed by PLS in the magnetotail is consistent with that observed by CAPS. 相似文献
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Recent helioseismic observations have found strong fluctuations at a period of about 1.3 years in the rotation speed around
the tachocline in the deep solar convection layer. Similar mid-term quasi-periodicities (MTQP; periods between 1–2 years)
are known to occur in various solar atmospheric and heliospheric parameters for centuries. Since the deep convection layer
is the expected location of the solar magnetic dynamo, its fluctuations could modulate magnetic flux generation and cause
related MTQP fluctuations at the solar surface and beyond. Accordingly, it is likely that the heliospheric MTQP periodicities
reflect similar changes in solar dynamo activity. Here we study the occurrence of the MTQP periodicities in the near and distant
heliosphere in the solar wind speed and interplanetary magnetic field observed by several satellites at 1 AU and by four interplanetary
probes (Pioneer 10 and 11 and Voyager 1 and 2) in the outer heliosphere. The overall structure of MTQP fluctuations in the different locations of the heliosphere
is very consistent, verifying the solar (not heliospheric) origin of these periodicities. We find that the mid-term periodicities
were particularly strong during solar cycle 22 and were observed at two different periods of 1.3 and 1.7 years simultaneously.
These periodicities were latitudinally organized so that the 1.3-year periodicity was found in solar wind speed at low latitudes
and the 1.7-year periodicity in IMF intensity at mid-latitudes. While all heliospheric results on the 1.3-year periodicity
are in a good agreement with helioseismic observations, the 1.7-year periodicity has so far not been detected in helioseismic
observations. This may be due to temporal changes or due to the helioseismic method where hemispherically antisymmetric fluctuations
would so far have remained hidden. In fact, there is evidence that MTQP fluctuations may occur antisymmetrically in the northern
and southern solar hemisphere. Moreover, we note that the MTQP pattern was quite different during solar cycles 21 and 22,
implying fundamental differences in solar dynamo action between the two halves of the magnetic cycle. 相似文献
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