[1] It has been known that the fluctuations in the interplanetary magnetic field (IMF) may be oriented in approximately planar structures that are tilted with respect to the solar wind propagation direction along the Sun-Earth line. This tilting causes the IMF propagating from a point of measurement to arrive at other locations with a timing that may be significantly different from what would be expected. The differences between expected and actual arrival times may exceed an hour, and the tilt angles and subsequent delays may have substantial changes in just a few minutes. A consequence of the tilting of phase planes is that predictions of the effects of the IMF at the Earth, on the basis of IMF measurements far upstream in the solar wind, will suffer from reduced accuracy in the timing of events. It has recently been shown how the tilt angles may be determined using multiple satellite measurements. However, since the multiple satellite technique cannot be used with real-time data from a single sentry satellite, then an alternative method is required to derive the phase front angles, which can then be used for more accurate predictions. In this paper we show that the minimum variance analysis (MVA) technique can be used to adequately determine the variable tilt of the plane of propagation. The number of points that is required to compute the variance matrix has been found to be much higher than expected, corresponding to a time period in the range of 7 to 30 min. The optimal parameters for the MVA were determined by a comparison of simultaneous IMF measurements from four satellites. With use of the optimized parameters it is shown that the MVA method performs reasonably well for predicting the actual time lags in the propagation between multiple spacecraft, as well as to the Earth. Application of this technique can correct for errors, on the order of 30 min or more, in the timing of predictions of geomagnetic effects on the ground.

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[1] We reexamine traveling convection vortices (TCVs) seen by the Magnetometer Array for Cusp and Cleft Studies on 9 November 1993. IMP-8 energetic ion observations confirm that the solar wind pressure variations previously associated with these TCVs were generated by kinetic processes within the Earth's foreshock. As expected during this interval of spiral IMF orientation, fast mode waves launched by the pressure variations first arrived in the equatorial ionosphere near dusk and propagated dawnward. We derive a model for the field-aligned currents generated by transient compressions of the magnetopause and show that it accounts for the number of TCVs seen in the prenoon ionosphere, their sense of rotation, the latitude at which they occur, and their absence in the postnoon ionosphere.

Abstract.We investigate the Northern Hemisphere Joule heating from several observational and computational sources with the purpose of calibrating a previously identified functional dependence between solar wind parameters and ionospheric total energy consumption computed from a global magnetohydrodynamic (MHD) simulation (Grand Unified Magnetosphere Ionosphere Coupling Simulation, GUMICS-4). In this paper, the calibration focuses on determining the amount and temporal characteristics of Northern Hemisphere Joule heating. Joule heating during a substorm is estimated from global observations, including electric fields provided by Super Dual Auroral Network (Super-DARN) and Pedersen conductances given by the ultraviolet (UV) and X-ray imagers on board the Polar satellite. Furthermore, Joule heating is assessed from several activity index proxies, large statistical surveys, assimilative data methods (AMIE), and the global MHD simulation GUMICS-4. We show that the temporal and spatial variation of the Joule heating computed from the GUMICS-4 simulation is consistent with observational and statistical methods. However, the different observational methods do not give a consistent estimate for the magnitude of the global Joule heating. We suggest that multiplying the GUMICS-4 total Joule heating by a factor of 10 approximates the observed Joule heating reasonably well. The lesser amount of Joule heating in GUMICS-4 is essentially caused by weaker Region 2 currents and polar cap potentials. We also show by theoretical arguments that multiplying independent measurements of averaged electric fields and Pedersen conductances yields an overestimation of Joule heating.Correspondence to: M. Palmroth (Minna.Palmroth@fmi.fi)

[1] We made a study of the characteristics of geomagnetic negative sudden impulses (SI À s) identified in the midlatitude geomagnetic SYM indices and the causative structures in the solar wind using data from the Wind and ACE spacecraft. A total of 28 SI À s with an amplitude larger than 20 nT in the H component SYM index were found over the period 1995 through 1999, with 50% of them occurring in conjunction with a positive sudden impulse, SI + (i.e., SI pair). In the SI pairs the amplitude of SI À was almost always larger than that of the preceding SI + . We attempted for the first time a classification of structures in the solar wind associated with SI À s. It is found that reverse shocks are not responsible for SI À s. Instead, SI À s are associated with varied structures such as tangential discontinuities at high-low speed stream interfaces, front boundaries of interplanetary magnetic clouds, and trailing edges of heliospheric plasma sheets. There is no preferential association of SI À s in our sample with any particular type of solar wind structure. We investigated statistically the polarization characteristics of SI À s at high latitude. The sense of the polarization in the auroral zone tended to be clockwise in the afternoon and counterclockwise in the morning. The rotational sense reversed in the polar cap. The latitudinal reversal occurred in the range from 65°to 80°. Thus the polarization distribution of SI À is not opposite to but is consistent with that of SI + . We suggest that the contribution from the longitudinal movement of a twin vortex ionospheric current system is dominant to produce the polarization of SC and SI À .