[en] In typical observations of coronal mass ejections (CMEs), a magnetic structure of a helmet-shaped closed configuration bulges out and eventually opens up. However, a spontaneous transition between these field configurations has been regarded to be energetically impossible in force-free fields according to the Aly-Sturrock theorem. The theorem states that the maximum energy state of force-free fields with a given boundary normal field distribution is the open field. The theorem implicitly assumes the existence of the maximum energy state, which may not be taken for granted. In this study, we have constructed force-free fields containing tangential discontinuities in multiple flux systems. These force-free fields can be generated from a potential field by footpoint motions that do not conserve the boundary normal field distribution. Some of these force-free fields are found to have more magnetic energy than the corresponding open fields. The constructed force-free configurations are compared with observational features of CME-bearing active regions. Possible mechanisms of CMEs are also discussed

[en] In typical observations of coronal mass ejections (CMEs), a magnetic structure of a helmet-shaped closed configuration bulges out and eventually opens up. However, a spontaneous transition between these field configurations has been regarded to be energetically impossible in force-free fields according to the Aly-Sturrock theorem. The theorem states that the maximum energy state of force-free fields with a given boundary normal field distribution is the open field. The theorem implicitly assumes the existence of the maximum energy state, which may not be taken for granted. In this study, we have constructed force-free fields containing tangential discontinuities in multiple flux systems. These force-free fields can be generated from a potential field by footpoint motions that do not conserve the boundary normal field distribution. Some of these force-free fields are found to have more magnetic energy than the corresponding open fields. The constructed force-free configurations are compared with observational features of CME-bearing active regions. Possible mechanisms of CMEs are also discussed

[en] Complete text of publication follows. We present observations from two subsolar Cluster magnetopause crossings with southward interplanetary magnetic field and strong mirror mode fluctuations in the magnetosheath. In both events the reconnection outflow jets show strong variations on the timescale of one minute. We show that at least some of the recorded variations are truly temporal, not spatial. On the same timescale, mirror mode fluctuations appear as strong magnetic fluctuations in the magnetosheath next to the magnetopause. This suggests that mirror modes can cause the variations either through modulation of continuous reconnection or through triggering of bursty reconnection. Using a theoretical scaling law for asymmetric reconnection we show that the first of our events is compatible with the modulation hypothesis. The second event also exhibits flow reversals, pointing to patchy and bursty reconnection, but mirror modes may play a role in triggering the variations. A hybrid simulation shows formation of magnetic islands at the magnetopause, which also would be compatible with patchy reconnection. These examples illustrate that to understand magnetopause reconnection in depth, it is necessary to know the local boundary conditions in the magnetosheath, not just the IMF direction. The rather small-scale mirror mode fluctuations can strongly influence the total energy conversion at a magnetic boundary, and should be common when the solar wind, or a stellar wind, hits a magnetised celestial object.

[en] Complete text of publication follows. Based on geomagnetic data of 157 geomagnetic sites(including repeat stations and observatories) in China and calculated from IGRF10 at 39 sites out of china for 2000.0, the geomagnetic models of the total intensity (F), the declination (D) , the inclination(I) and their horizontal gradient were developed using surface Spline method. The distributions of F, D, I and their horizontal gradient in china 73-136degE 18-54degN were analyzed. The main results showed that F increased from south to north gradually and changed between 39081 and 60855 nT. D were negative in east of 100 degE and positive in west of 100 degE, which changed between -13.64 and 11.06 degree. I increased from south to north gradually and changed between 21.17 and 73.36 degree. There was a high positive value centre of the gradient of F in south-north direction in middle, which changed between 2.51 and 19.12nT/min.. The gradient of F in east-west direction were negative in east of 100 degE and positive in west of 100 degE, which changed between -10.90 and 7.33nT/min.. The gradient of D in south-north direction were negative in east of 100 degE and positive in west of 100 degE, which changed between -2.27 and 0.74 min./min.. The gradient of D in east-west direction were negative in most parts and changed between -1.23 and 0.50 min./min.. The gradient of I in south-north direction decreased from south to north gradually and changed between 0.66 and 2.55 min./min. The distribution of the gradient of I in east-west direction were like saddle and changed between -1.00 and 0.55 min./min.

[en] The force-free magnetic field equation ∇xB=αB has been solved analytically in a toroidal coordinates system for the axisymmetric torus which is antisymmetric with respect to equatorial plane without the approximation of large aspect ratios. A set of fundamental solutions are presented each of which asymptotically approaches that for the cylindrical configuration when the aspect ratio goes to infinity. With this set of fundamental solutions, the eigenvalues of the force-free magnetic field are calculated for tori of circular and elliptical cross-section. It has been shown that the antisymmetric eigenvalue increases with decreasing aspect ratio. (author)

[en] Observations have shown that in addition to a y component associated with the flaring of the tail, a net y component of the magnetic field can persist in the geomagnetic tail, roughly proportional to the interplanetary B_y. The author uses the tail equilibrium theory outlined by Birn (1987) to construct self-consistent three-dimensional models of the tail that include this effect. He finds that the net B_yN field varies only weakly along field lines and across the tail. The presence of the net B_yN in the neutral sheet implies that a part of the cross-tail current becomes field-aligned, flowing from the southern to the northern hemisphere for B_yN > 0 and from north to south for B_yN < 0. This field-aligned current is proportional to B_yN for small values. For a typical value of 2% of the lobe field strength the author finds a total current of 4.4 x 10⁵ A flowing toward one hemisphere at x = -20 R_E. This current, however is not expected to reach the Earth, because it decreases strongly along each field line away from the neutral sheet, due to a conversion into perpendicular current. The deformations of magnetic flux surfaces, implied by the models, are not consistent with an entirely closed magnetotail boundary. They suggest that in addition to traditional lobe field lines, interconnected with the solar wind field at large distances or high latitudes, there may be a class of field lines extending from the Earth into the magnetosheath region through the low latitude flanks of the tail. These field lines, which might even be associated with positive B_z at a neutral sheet crossing, effectively reduce the region of closed flux. For typical values of B_yN of 2% of the lobe field this effect is small, but for values above about 10%, not extremely unusual, it can be appreciable

[en] The guiding center Hamiltonian in the current sheet of the reversed field magnetic configuraion is presented. The unperturbed orbit and guiding center position are obtained using a simple inhomogeneous magnetic model. The contribution of a small normal magnetic component to the Hamiltonian is given by the method of Lie transformation. The result can be used to analyse the motion of charged particles in current sheet of the interplanery space, magnetotail and field-reversed pinch

[en] The current density in the vicinity of a rational surface of a force-free magnetic field subjected to an ideal perturbation is shown to be the sum of both a smooth and a delta-function distribution, which give comparable currents. The maximum perturbation to the smooth current density is comparable to a typical equilibrium current density and the width of the layer in which the current flows is shown to be proportional to the perturbation amplitude. In the standard linearized theory, the plasma displacement has an unphysical jump across the rational surface, but the full theory gives a continuous displacement.

[en] The effect of the IMF azimuthal component on magnetospheric substorm dynamics has been studied on the basis of five-minute average values of the IMF B_y and B_z components and the AL index. The results obtained from case studies and from superposed epoch analysis show the dependence of substorm dynamics on the azimuthal component: the reversal of B_y from positive to negative increases the activity with minimum delay time, while the opposite reversal either does not change or only slightly changes the activity level. This effect is more evident in winter. The reversal of the IMF vertical component from south to north after an interval of sustained southward IMF statistically gives rise to magnetic activity, too but this growth is less intense than that produced by the B_y negative turning. The role of both vertical and azimuthal IMF components must be considered in future studies of substorm triggering mechanisms. (author)

[en] We have carried out a comparative study of the evening prereversal enhancements in the equatorial F region vertical ionization drift velocities (V/sub z/) over Fortaleza (4 ⁰S, 38 ⁰W), Brazil, and Jicamarca (12 ⁰S, 77 ⁰W), Peru, two magnetic equatiorial stations in the American zone. The results show profound dissimilarities in the seasonal trends in the times and widths of the V/sub z/ prereversal peak, which reflect in the spread F characteristics as well, at the two stations. The dissimilarities are shown to be arising mainly from the difference in the magnetic field declination angles that causes differences in the conjugate E region sunset durations and, hence, in the F region polarization electric field development rates at the two stations