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1.
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   

2.
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   

3.
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   

4.
An empirical formula has been constructed using the results of correlative analyses to determine in what form the AL index, as a measure of the intensity of the westward auroral electrojet, depends on interplanetary parameters. The formula thus obtained shows that AL is mainly determined by BsV2 where Bs is the southward component of the IMF and V is the solar wind velocity, and is modulated in a characteristic way by the combined effect of the east-west component of the IMF and the tilt angle of the Earth's dipole axis toward the Sun-Earth line. In contrast, effects of the solar wind density and the IMF variability were found to be insignificant.Implications of the empirical formula are discussed mainly in relation to the problem of the location in the dayside magnetosphere of the region where the reconnection process to initiate the substorm takes place.  相似文献   

5.
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   

6.
Observed magnetospheric asymmetries which occur in response to the y-component of the IMF are discussed in terms of the open model of the magnetosphere. The torque which the IMF exerts on the magnetosphere about the Earth-Sun axis results in asymmetric addition of open flux tubes to the tail lobes about the noon-midnight meridian. In response an IMF-associated By field appears across the tail lobes. The ratio between internal and external By fields will generally be same as the ratio between internal and external electric fields. If the tail flux asymmetry is related to an asymmetric distribution of the field normal to the tail magnetopause then an asymmetry in tail lobe electric field and plasma populations will immediately result, as observed. If the flux asymmetry is associated with a twist in the tail then the By field will appear but not necessary the electric field and plasma asymmetries. Generally both effects may occur together. Simple open tail lobe models are derived which demonstrate the asymmetry effects. These represent more physically satisfactory models of the tail and its plasma populations than available hitherto, but they remain somewhat unrealistic in a number of respects. Finally, it is shown that the observed asymmetry effects on closed (auroral zone) field lines may be at least qualitatively accounted for if the cross-magnetosphere IMF-associated By field pervades not only the open but also the closed field line regime, as may be generally expected.  相似文献   

7.
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   

8.
Auroral boundary variations and the interplanetary magnetic field   总被引：1，自引：0，他引：1  
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   

9.
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   

10.
One year of magnetic field data from the geostationary spacecraft ATS 6 have been analysed for effects associated with the equatorial plane components of the interplanetary magnetic field (IMF). It is shown that perturbation fields in the Y (dawn to dusk) direction appear in association with the Y component of the IMF, in agreement with previous theoretical suggestions. On average a fraction 0.28 ± 0.02 of the IMF Y field appears at geostationary orbit, such that the average ATS 6 By field is 1.9 ± 0.4 nT larger when IMF By is positive than when it is negative. The perturbation field magnitudes are also found to depend strongly on local time, however, with largest effects appearing in the midnight and dawn quadrants, where the average perturbation fields are nearly half the simultaneous IMF Bv. field. At noon this fraction drops to one fifth, and no average effect occurs in the dusk quadrant. Both the daily mean perturbation fields and the diurnal modulation are also found to depend upon the level of magnetic disturbance as measured by KP, or equivalently upon IMF Bz, and upon season of the year. Overall stronger daily mean perturbation fields occur when KP is low or when IMF Bz is positive, than when KP is high or when IMF Bz is negative. This effect is not linear, however, and there is also a trend in the data towards increasing perturbation fields with IMF Bz negative and decreasing. On dividing the data according to season, increasingly strong daily mean effects are found in the order winter, summer and equinox for both quiet and disturbed magnetospheres. Diurnal modulations of the perturbation field magnitudes for low KP (IMF Bz > 0) take the form of large amplitude quasi-sinusoidal variations about mean values which are very marked in the equinox data, are present to a lesser degree during summer and are absent during winter conditions. When Kp is high (IMF Bz < 0) significant deviations from mean perturbation field values occur generally only during nightside hours and little seasonal dependence is evident. Finally, it is shown that the highest correlation between the IMF data and the ATS 6 perturbation fields occurs with zero time delay between the two data sets, showing that a prompt response to IMF conditions occurs at geostationary orbit within the 1 h time resolution available in this study. Although many details of the above ATS 6 response remain to be understood, these results overall demonstrate in a very direct manner the magnetically “open” nature of the Earth's magnetosphere.  相似文献   

11.
We describe a simple method for determining the time at which the meridian of a sub-auroral magnetic observatory crosses that of the Harang discontinuity—the separation of the eastward and westward electrojets which flow in the evening and morning sectors of the auroral oval. We then consider how this time, determined from examination of magnetograms from sub-auroral observatories varies with the dawn-dusk (y) component of the Interplanetary Magnetic Field. We find that the time at which the Harang discontinuity is identified in the Northern Hemisphere is earlier for By > 0 than the occasions when By < 0, and that the converse is observed in the Southern Hemisphere. Also we suggest that there is no significant seasonal variation in the relationship between the time of the discontinuity and By. The sense of the azimuthal shift of the auroral electrojet currents with changes in By is consistent with the theory of Cowley (1981). However, the magnitude of the observed shifts is approximately an order of magnitude greater than the theoretical predictions. We suggest that this difference between observation and theory arises from the use of a dipole magnetic field model at auroral zone latitudes in the theoretical estimation of azimuthal displacement.  相似文献   

12.
The characteristics of latitudinal angles of solar wind flow (θv) observed near earth have been studied during the period 1973-2003. The average magnitude of θv shows distinct enhancements during the declining and maximum phases of the sunspot cycles. A close association of Bz component of IMF in the GSE system and the orientation of meridional flows in the solar wind is found which depends on the IMF sector polarity. This effect has been studied in typical geomagnetic storm periods. The occurrence of non-radial flows is also found to exhibit heliolatitudinal dependence during the years 1975 and 1985 as a characteristic feature of non-radial solar wind expansion from polar coronal holes.  相似文献   

13.
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   

14.
The pressure corrected hourly data from the global network of cosmic ray detectors, measurements of the interplanetary magnetic field (IMF) intensity (B) at Earth’s orbit and its components B x , B y , B z (in the geocentric solar ecliptic coordinates) are used to conduct a comprehensive study of the galactic cosmic ray (GCR) intensity fluctuations caused by the halo coronal mass ejection of 13 May 2005. Distinct differences exist in GCR timelines recorded by neutron monitors (NMs) and multidirectional muon telescopes (MTs), the latter respond to the high rigidity portion of the GCR differential rigidity spectrum. The Forbush decrease (FD) onset in MTs is delayed (～5 h) with respect to the onset of a geomagnetic storm sudden commencement (SSC) and a large pre-increase is present in MT data before, during, and after the SSC onset, of unknown origin. The rigidity spectrum, for a range of GCR rigidities (≤200 GV), is a power law in rigidity (R) with a negative exponent (γ=?1.05) at GCR minimum intensity, leading us to infer that the quasi-linear theory of modulation is inconsistent with observations at high rigidities (>1 GV); the results support the force field theory of modulation. At present, we do not have a comprehensive model for the FD explaining quantitatively all the observational features but we present a preliminary model listing physical processes that may contribute to a FD timeline. We explored the connections between different phases of the FD and the power spectra of IMF components but did not find a sustained relationship.  相似文献   

15.
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   

16.
An expression for the magnetopause reconnection power based on the dawn-dusk component of the reconnection electric field, that reduces to the substorm parameter ? for the limit that involves equal geomagnetic (BG) and magnetosheath (BM) magnetic field amplitudes at the magnetopause, is contrasted with the expression based on the whole reeonnection electric field vector (Gonzalez, 1973). The correlation examples of this report show that this (more general) expression for the reconnection power seems to correlate with the empirical dissipation parameter UT (Akasofu, 1981), with slightly better correlation coefficients than those obtained from similar correlations between the parameter ? and UT Thus, these (better) correlations show up for the more familiar values of the ratio BG/BM > 1. Nevertheless, the (expected) relatively small difference that seems to exist between these correlation coefficients suggests that, for practical purposes, the parameter ? could be used as well (instead of the more general expression) in similar correlation studies due to its simpler format. On the other hand, studies that refer mainly to the difference in the magnitudes of ? and of the more general expression are expected to give results with less negligible differences.  相似文献   

17.
The distribution of By in the geomagnetic tail associated with a net cross-tail magnetic flux, recently experimentally discovered, is here investigated within the framework of two-dimensional but non-planar field adiabatic time-independent equilbria. It is found that the flux distribution is controlled by the pressure anisotropy of the plasma, By being enhanced at the current sheet centre relative to that in the lobes for P∥>P⊥ and vice-versa for P⊥>P∥. For P⊥>P∥ a broad region of depressed field strength is found across the centre plane of the current sheet, terminated at its outer boundaries by spikes in the perpendicular current, across which By and Bx are “switched on” and rapidly increase towards their values in the low-β lobes. For P∥>P⊥ a thin high-current density layer forms at the sheet centre if the marginal firehose condition is approached, across which the Bx field reverses by rotation at nearly constant magnitude about the z-axis. The field magnitude in this thin layer depends upon the pressure anisotropy, such that the plasma remains just firehose stable within it, and may approach an appreciable fraction of the lobe field strength even for moderate anisotropies. Such structures have been observed in the geomagnetic tail, but do not appear to be a common feature of the quiet-time plasmasheet, where the field strength at the centre plane can reach small values with little obvious enhancement of By. In terms of the present model these observations require that either P⊥>P∥ in the quiet-time tail or that the plasma is within one or two per cent of isotropy if P∥>P⊥. These results then indicate that the production of plasma pressure anisotropy during adiabatic inward transport towards the Earth, which is generally expected to lead to P∥>P⊥ and its destruction by either macroscopic or microscopic processes, requires further study.  相似文献   

18.
The variation of the geomagnetic activity index Ap at the IMF sector boundaries (+ to ? and ? to +) has been studied for three solar cycles, separating data into vernal and autumnal equinoxes. It was found that a reported increase in Ap as an effect of a Hale boundary can be better attributed to the occurrence of a negative IMF Bz component in the geocentric solar magnetospheric coordinate system and to the occurrence of high speed solar wind streams.  相似文献   

19.
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 Δ?(kV) ≈ 6(V300)2 ? 9Bz(γ) 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   

20.
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   

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