共查询到20条相似文献，搜索用时 31 毫秒
1.
《Planetary and Space Science》1987,35(10):1301-1316
The magnetic field vector residuals observed from the Magsat satellite have been used to obtain the dependence of the polar cap boundary and the current system on IMF for quiet and mildly disturbed conditions (Kp ⩽ 3 +). The study has been carried out for the summer months in the Southern Hemisphere. “Shear reversals” (SRs) in vector residuals indicative of the infinite current sheet approximation of the field-aligned currents (FACs) indicate roughly the polar cap boundary or the poleward boundary of the plasma sheet. This is also the poleward edge of the region 1 FACs. The SR is defined to occur at the latitude where the vector goes to minimum and changes direction by approximately 180°.It is found that SRs mainly occur when the interplanetary magnetic field (IMF) has a southward-directed Bz- component and in the latitude range of about 70°–80°. SRs in the dusk sector occur predominantly when the azimuthal component By is positive and in the dawn sector when By is negative, irrespective of the sign of Bz These results agree with the known merging process of IMF with magnetopause field lines. When SRs occur on both dawn and dusk sectors, the residuals over the entire polar cap are nearly uniform in direction and magnitude, indicating negligible polar currents. Similar behaviour is observed during highly disturbed conditions usually associated with large negative values of Bz.Forty-one Magsat orbits with such SRs are quantitatively modelled for preliminary case studies of the resulting current distribution. It is found that SRs, in the plane perpendicular to the geomagnetic field, for the current vectors and the magnetic vector residuals (perturbations relative to the unperturbed field) occur at almost the same latitudes. The electrojet intensities range from 1.2 × 104 to 6.5 × 105 A (amperes). A preliminary classification of polar cap boundary crossings characterized by vector rotations rather than SRs also shows that they tend to occur mainly for negative Bz. 相似文献
2.
This paper describes a DMSP data set of 150 auroral images during magnetically quiet times which have been analyzed in corrected geomagnetic local time and latitudinal coordinates and fit to offset circles. The fit parameters R (circle radius) and (X, Y) (center location) have been compared to the hourly interplanetary magnetic field (IMF) prior to the time of the satellite scan of the aurora. The results for variation of R with Bz, agree with previous works and generally show about a 1° increase of R with increase of southward Bz by 1 nT. The location of the circle center also has a clear statistical shift in the Southern Hemisphere with IMF By such that the southern polar cap moves towards dusk (dawn) with By > (By < 0). 相似文献
3.
After reviewing the basic characteristics of the polar cap arcs, it is suggested that their appearance can be explained if the open region splits into two, one located in the dawn sector and the other in the dusk sector. It is suggested that a distinct splitting occurs temporarily when an IMF tangential discontinuity passes by the magnetosphere and the sign of the IMF By component changes at the discontinuity, provided that the IMF Bz component is positive on both sides. As a result, the dawn or the dusk side of the polar region will be connected to either the front side or the hind side of the discontinuity, depending on the sign of the By component across the discontinuity. As the dynamo process is expected to operate in each of the two open regions (as is the case in the single open region), it is reasonable to infer that a sheet of plasma and of field-aligned currents forms in the region between the two open regions, resulting in the polar cap arcs across the polar region. The four-cell convection pattern may also appear. A model of the magnetosphere is constructed to demonstrate such a possibility. 相似文献
4.
The geometry of the open flux area in the polar region is computed by superposing a uniform interplanetary magnetic field (IMF) with various orientation angles to a model of the magnetosphere. It is confirmed that the IMF By component is as important as the Bz component in “opening” the magnetosphere. It is also shown that the computed area of open field lines is remarkably similar to the observed ones which were determined by using the entry of solar electrons. In particular, when the IMF vector is confined in the X-Z-plane and the Bz component has a large positive value, the open area becomes crescent-shaped, coinciding approximately with the cusp region. 相似文献
5.
A. V. Mordvinov 《Solar physics》2007,246(2):445-456
A comparative analysis of solar and heliospheric magnetic fields in terms of their cumulative sums reveals cyclic and long-term
changes that appear as a magnetic flux imbalance and alternations of dominant magnetic polarities. The global magnetic flux
imbalance of the Sun manifests itself in the solar mean magnetic field (SMMF) signal. The north – south asymmetry of solar
activity and the quadrupole mode of the solar magnetic field contribute the most to the observed magnetic flux imbalance.
The polarity asymmetry exhibits the Hale magnetic cycle in both the radial and azimuthal components of the interplanetary
magnetic field (IMF). Analysis of the cumulative sums of the IMF components clearly reveals cyclic changes in the IMF geometry.
The accumulated deviations in the IMF spiral angle from its nominal value also demonstrate long-term changes resulting from
a slow increase of the solar wind speed over 1965 – 2006. A predominance of the positive IMF B
z
with a significant linear trend in its cumulative signal is interpreted as a manifestation of the relic magnetic field of
the Sun. Long-term changes in the IMF B
z
are revealed. They demonstrate decadal changes owing to the 11/22-year solar cycle. Long-duration time intervals with a dominant
negative B
z
component were found in temporal patterns of the cumulative sum of the IMF B
z
. 相似文献
6.
《Planetary and Space Science》2007,55(6):792-808
Observations are presented of long-lived global Pc5 ULF wave activity observed at a wide range of local times. The event was monitored in the high latitude ionosphere (∼60–80° magnetic latitude) by several SuperDARN HF radars and 5 magnetometer chains in Scandinavia, Greenland, Canada, Alaska and Russia. The event coincided with a protracted period (∼36 h) of northward interplanetary magnetic field (IMF). The study focuses on 4 h during which distinct dawn/dusk asymmetries in the wave characteristics were observed with multiple field line resonance (FLR) structures observed in the dawn flank at 1.7, 2.6, 3.3, 4.2 and 5.4 mHz and compressional oscillations in the dusk flank at 1.7 and 2.3 mHz. The data indicated an anti-sunward propagation in both the dawn and dusk flanks and a low azimuthal m number (∣m∣∼6) suggesting a generation mechanism external to the Earth's magnetosphere. A sudden increase in the solar wind dynamic pressure followed by a period of strongly northward, Bz dominated IMF, coincides with the observations and also a large increase in Pc5 wave power observed in the dawn flank. The observed enhancements in the wave activity and FLR structures are thought to be due to a Kelvin–Helmholtz driven waveguide mode. Additionally, there is no evidence that the frequencies of the FLRs are intrinsic to the solar wind. It thus seems that the frequencies were determined by the dimensions of the magnetospheric cavity. 相似文献
7.
The behavioural features of the IMF Bz component for different solar wind velocity regimes have been studied. The study revealed a significant difference in variations of the Bz component between high-speed and low-speed regimes. Formation mechanisms for the IMF meridional component as well as the relationship of Bz with dynamical properties of the large-scale magnetic fields on the Sun are discussed. 相似文献
8.
The geometry of the open field line region in the polar region is computed for a variety of the interplanetary magnetic field (IMF) orientation. The open field line region can be identified as the area bounded by the auroral oval, namely the polar cap. The polar cap geometry varies considerably with the orientation of the IMF and magnitude, particularly when the IMF Bz component is positive and large. The corresponding exit points of the open field lines on the magnetopause are also examined. The results will be a useful guide in interpreting various upper atmospheric phenomena in the highest latitude region of the Earth and also in observing chemical releases outside the magnetopause. 相似文献
9.
The interplanetary magnetic field (IMF) changes and the associated responses of the magnetosphere on November 1, 1972, are examined. IMF Bz changes consisted of a sudden southward turning, a slow northward turning, and a subsequent steady northward sense. Magnetospheric substorms occurred throughout this period. 相似文献
10.
The influence of the Bz and By polarity of the IMF on the location of the dayside regions of precipitating, low energy electrons recorded by the DMSP F-2 satellite, is investigated. The average differential electron flux was determined for 2 months during local summer. It is found that the spatial distributions are similar for electrons in the range from 50 to 183 eV. The region of maximum intensity for Kp? 2+ over the Southern Hemisphere is located on the opposite side of noon from the Northern Hemisphere. The current intensity carried by precipitating electrons in the cusp region agrees with that measured by the TRIAD magnetometer. When the IMF is northward a marked asymmetry of the low energy electron precipitation between positive and negative By cases is observed. For positive By the maximum electron flux occurs between 0800 and 1200 M.L.T.and ?76 and ?83° M.L.A.T.and for negativeBythe region occurs between 1200 and 1500 M.L.T. and ?79 and ?82° M.LAT. The dynamical variations associated with substorm activity when the IMF is southward obscure the expected By effect. 相似文献
11.
This paper reports the study concerning the latitudinal dispalacement of the auroral oval as a function of the northward orientation of the Bz-component IMF and the relation between southward Bz and the auroral dynamics in the night sector. 相似文献
12.
F. Primdahl 《Planetary and Space Science》1985,33(11):1327-1330
Recent observations of partial penetration of the IMF By into the magnetosphere (Fairfield, 1979; Cowley and Hughes, 1983) are shown to agree with the idea of a magnetopause current Ky, induced by IMF By (Primdahl and Spangslev, 1983). The slow decay of Ky, caused by Joule heat losses in the cusp ionospheres is responsible for the appearance (on the average) of a fraction of IMF By inside the magnetosphere, and this also explains the large statistical scatter of the data. A decay time constant of 4–5 days is derived from the average fraction of IMF By observed inside the magnetosphere in good agreement with the 7-day time constant previously proposed. 相似文献
13.
We study the interplanetary features and concomitant geomagnetic activity of the two high-speed streams (HSSs) selected by the Whole Heliosphere Interval (WHI) campaign participants: 20 March to 16 April 2008 in Carrington rotation (CR) 2068. This interval was chosen to perform a comprehensive study of HSSs and their geoeffectiveness during this ??deep?? solar minimum. The two HSSs within the interval were characterized by fast solar-wind speeds (peak values >?600 km?s?1) containing large-amplitude Alfvénic fluctuations, as is typical of HSSs during normal solar minima. However, the interplanetary magnetic field (IMF) magnitude [B o] was exceptionally low (??3??C?5 nT) during these HSSs, leading to lower than usual IMF B z values. The first HSS (HSS1) had favorable IMF polarity for geomagnetic activity (negative during northern Spring). The average AE and Dst for the HSS1 proper (HSS1P) were +?258 nT and ??21 nT, respectively. The second HSS (HSS2) had a positive sector IMF polarity, one that is less favorable for geomagnetic activity. The AE and Dst index averages were +?188 nT and ??7 nT, both lower than corresponding numbers for the first event, as expected. The HSS1P geomagnetic activity is comparable to, and the HSS2P geomagnetic activity lower than, corresponding observations for the previous minimum (1996). Both events?? geomagnetic activities are lower than HSS events previously studied in the declining phase (in 2003). In general, V sw was faster for the HSSs in 2008 compared to 1996. The southward IMF B z was lower in the former. The product of these two parameters [V sw and IMF B z ] comprises the solar-wind electric field, which is most directly associated with the energy input into the magnetosphere during the HSS intervals. Thus the combined effects led to the solar wind energy input in 2008 being slightly less than that in 1996. A detailed analysis of magnetic-field variances and Alfvénicity is performed to explore the characteristics of Alfvén waves (a central element in the geoeffectiveness of HSSs) during the WHI. The B z variances in the proto-CIR (PCIR) were ???30 nT2 and <?10 nT2 in the high speed streams proper. 相似文献
14.
The horizontal electric field has been measured with balloons over the Pacific Ocean near the Sanriku Coast in Japan. By comparing the electric-field data obtained during magnetically disturbed periods, 16–17 October 1973, 6–7 October 1975 and 3–4 October 1977, with IMF Bz, auroral zone AU and AL, equatorial Dst and , mid-latitude magnetic fields (H, D, Z at Kakioka), and the ionospheric electron density (?0F2 at Kokubunji), it is found that the observed electric fields of about 9 mVm?1 made the clockwise rotation during the growth and recovery stages of the magnetospheric substorms. Relations between high and middle latitude ionospheres and between the magnetosphere and the ionosphere are discussed in relation to the origin and propagation of these electric fields. 相似文献
15.
Ten years data set is used to separate the influence of IMF Bz-component and solar wind speed on the dawn-dusk component of magnetic variations in the summer polar cap. The reference level was chosen from most quiet periods of winter solstices (small polar cap and auroral zone conductivity) to exclude the inner source component. The linear regression analysis was then used to calculate the PC variation response to Bz under different ranges of solar wind speed. As a result, taking into account the value of polar cap conductivity and effects of induced currents, the response of dawn-dusk electric field component to Bz and V was obtained and the potential difference across the polar cap was estimated to be for Bz ? + 1γ. The results give a proof for simultaneous operation in the magnetosphere of two electric field generation mechanisms, related to the boundary layer processes and magnetic field reconnection. The above-mentioned functional form was shown to correlate effectively with AE index (R = 0.73). 相似文献
16.
We have studied the dayside magnetosphere structure and its Kp, AE and IMF-dependence using the magnetic data from IMP and HEOS satellites obtained during 1966–1972. An analysis of the field line configurations has been done on the basis of results of a least squares fitting of the model coefficients to the data subsets. The plots of the magnetopause subsolar point distance and of the polar cusp latitude vs Kp and AE have been obtained. A detailed study of the model field distribution has revealed a substantial difference in the polar cusp field line geometry between the cases of weak and strong geomagnetic activity. We find that this results in a considerable longitudinal extension of the isointensity contours of particle precipitation at ionospheric heights during disturbed periods with Kp ? 3 or AE ? 300 nT. The same effect has been detected for the data subsets corresponding to the IMF Bz < 0. In contrast, at quiet times the precipitation isolines are much closer to circles. We conclude therefore that the cleft-like structure of polar cusps pertains only to active periods and can be explained by a magnetic effect of enhanced Birkeland currents. 相似文献
17.
A well established correlation exists between the IMF By and the cusp field-aligned and horizontal currents (Wilhjelm et al., 1978). The northern and southern cusp currents may be parts of one large scale current system (D'Angelo, 1980) flowing mainly at the magnetopause and driven by the z-component of the solar wind electric field. Primdahl and Spangslev (1981) suggested that the large scale current system seems to shield out the IMF By from the interior of the magnetosphere. This paper proposes that the currents are induced by the change of sign of By at the IMF sector boundary crossings, and argues that the time constant for decay of the currents may well be one week or larger. The percentage errors in inferring the IMF sector polarity from the Godhavn H magnetogram increases with increasing time since the last sector boundary crossing. This is in accordance with a steady decay of the induced currents. Finally experimental tests are proposed to demonstrate the feasability of and possibly distinguish between the mechanisms. 相似文献
18.
S.W.H. Cowley 《Planetary and Space Science》1981,29(8):809-818
The origin of magnetospheric asymmetry effects associated with the equatorial plane component of the interplanetary magnetic field (IMF) is discussed in terms of the forces exerted on open flux tubes mapping into the solar wind. It is argued that the downstream relaxation of the magnetosphere under the action of these stresses towards a state of reduced stress is such as to allow, in effect, partial penetration of this field component (both Bx and By in magnetospheric coordinates) into the magnetosphere. Many of the associated phenomena are therefore qualitatively described by the ‘dipole plus uniform IMF’ model, since this represents the idealization of exactly zero electromagnetic stress and hence gives a lowest order picture of the effects which result from magnetospheric relaxation toward that state. This is true of IMF By-associated effects which are well documented experimentally and which form a rich and consistent set of phenomena which have received considerable attention over the past decade. It is argued here that exactly corresponding phenomena are expected to be associated with the IMF Bx field as well, but because of the differing field direction these will take differing and usually less obvious forms than the similar effects associated with By. The suggested partial penetration of the IMF Bx field should be directly testable in the dipolar field region of the magnetosphere, but in the tail North-South displacements of the current sheet (and possibly magnetopause) are expected to occur instead. Some evidence of the latter displacements are presented. The other major IMF Bx effect should be noon-midnight displacements of the polar cap, such as have been recently reported. Little IMF Bx effect on auroral zone flows is anticipated, by contrast with the dawn-dusk asymmetries in this flow associated with IMF By. 相似文献
19.
We studied the relationship between the power-law exponent γ on the rigidity R of the spectrum of galactic cosmic-ray (GCR) intensity variation (δD(R)/D(R)∝R ?γ ) and the exponents ν y and ν z of the power spectral density (PSD) of the B y and B z components of the interplanetary magnetic field (IMF) turbulence (PSD～f ?ν , where f is the frequency). We used the data from neutron monitors and IMF for the period of 1968?–?2002. The exponents ν y and ν z were calculated in the frequency interval Δf=f 2?f 1=3×10?6 Hz of the resonant frequencies (f 1=1×10?6 Hz, f 2=4×10?6 Hz) that are responsible for the scattering of GCR particles with the rigidity range detected by neutron monitors. We found clear inverse correlations between γ and ν y or ν z when the time variations of the resonant frequencies were derived from in situ measurements of the solar wind velocity U sw and IMF strength B during 1968?–?2002. We argue that these inverse relations are a fundamental feature in the GCR modulation that is not restricted to the analyzed years of 1968?–?2002. 相似文献
20.
Global images of the Earth's inner magnetosphere and its response to the coronal mass ejection (CME) on the 15 July 2000 were
obtained by the IMAGE spacecraft. The images were taken in energetic neutral atoms (ENA) by the High-Energy Neutral Atom (HENA)
imager. ENAs are produced by charge exchange between the hot ion population of the magnetosphere and the cold neutral hydrogen
geocorona. The ENA images show how plasma is injected into the nightside magnetosphere as the interplanetary magnetic field
(IMF) turns strongly southward. As the IMF B
z increases and the storm intensity decreases, the ENA images show that the ring current becomes closed and symmetric as IMF
B
z reaches positive values. 相似文献