[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.

Abstract. A new technique for mapping field-aligned currents (FAC) with satellite magnetometer data has been used with Dynamics Explorer 2 measurements to produce an empirical model which maps the currents above the high-latitude ionosphere as a function of the interplanetary magnetic field (IMF), solar wind velocity, solar wind density, and dipole tilt angle. This technique uses scalar magnetic Euler potentials, derived from integrating the measured magnetic deviations in much the same way as electric potentials are derived from integrating electric fields. This method works with any configuration of two-dimensional distribution of the field-aligned current, rather than assuming that the currents are in the form of infinite sheets or belts. The radial current density is found by a surface Laplacian operator on the scalar field. The maps of the FAC produced with this new technique are more quantitative and detailed than most of the preceding statistical diagrams, and they yield much insight into how the currents vary as the IMF clock angle changes, and how the field-aligned current maps overlap the associated electric potential patterns. An optional component of the model shows changes in the currents associated with substorms, using the AL index as the controlling parameter. The most notable aspect of the substorm patterns is an increased region 0 current, which in addition to the region 2 current closes the majority of the current on the dusk side of the auroral surge. The results do not seem to agree with the traditional paradigm of the substorm current wedge closing through the ionosphere from dawn to dusk.

[1] A technique is described for predicting ground surface geomagnetic variations from measurements of the approaching interplanetary magnetic field (IMF) and solar wind. The method uses twin, empirical representations of the ionospheric electric and magnetic Euler potentials' response to the IMF drivers. The magnetic potential model, originally derived for mapping the large-scale field-aligned current structure, describes the curl-free component of the horizontal ionospheric current, also called the ''potential current.'' Using approximations that the Hall and Pedersen conductances have a fixed ratio and that there are no conductivity gradients, then the Hall current is derived from the magnetic potentials. In this case the Hall current is the same as the divergence-free ''equivalent current,'' which is used to derive the geomagnetic variations at the ground surface. The assumption that the ionospheric conductances have no gradients is avoided if the empirical model for the ionospheric electric potentials is used in addition to the magnetic potentials. In this second method the electric field provides additional information about the direction of the estimated equivalent current. Despite the approximation of a fixed conductance ratio, both calculation methods perform remarkably well for predicting the large-scale and long-period geomagnetic variations. The method that includes the electric fields has a slightly better performance, particularly in the polar cap. Corrections for the effects of currents induced underground were not applied for this demonstration. Such corrections could in principle improve the predictions, particularly for the shortperiod variations for which the effect is the greatest.

[1] Simultaneous measurements of the interplanetary magnetic field (IMF) are obtained at various locations with four spacecraft, ACE, Wind, IMP-8, and Geotail. We have devised a technique whereby the exact propagation delay time between ACE, at the L1 orbit, and each of the other three spacecraft can be derived from these measurements. This propagation delay is determined as a continuously varying function of time; when this measured delay is applied to all three components of the IMF measured by ACE, they will match the other satellites' IMF to a degree that is much better than expected. However, the actual time delays can vary by nearly an hour in either direction from the expected advection delays, and the lag times have significant changes that can occur on a timescale of a few minutes. These results are interpreted as due to the effects of tilted phase fronts that are changing orientation with time. We have used the delay measurements between multiple satellites to calculate the three-dimensional orientation and temporal variations of the phase front. The best fit phase front plane usually lies within 4 R E or less from the four-point measurements, indicating a lag resolution of a minute or less. Computer animations of the time-varying phase fronts are used to illustrate their behavior. Orientations can change on short timescales. Our findings have implications for both basic research and ''space weather'' predictions. These results give a high confidence that the same IMF that is measured near L1 will most likely impact the Earth's magnetosphere, providing ample justification for use of spacecraft data in halo orbit at L1 for monitoring the upstream solar wind prior to its interacting with the magnetosphere. However, there is strong uncertainty in the timing of the arrival of the detailed IMF structures, and these delays will need to be considered.

Nearly simultaneous measurements of auroral zone electric fields are obtained by the Dynamics Explorer spacecraft at altitudes below 900 km and above 4500 km during magnetic conjunctions. The measured electric fields are usually nearly perpendicular to the magnetic field lines. The north-south meridional electric fields are "projected" to a common altitude by a mapping function which accounts for the convergence of the magnetic field lines. When plotted as a function of invariant latitude, graphs of the projected electric fields measured by both DE 1 and DE 2 show that the large-scale electric field is the same at both altitudes, as expected. Superimposed on the large-scale fields, however, are small-scale features with wavelengths of less than 100 km which are larger in magnitude at the higher altitude. Fourier transforms of the electric fields show that the magnitudes depend on wavelength. Outside of the auroral zone the electric field spectrums are nearly identical. But within the auroral zone the high-and low-altitude electric fields have a ratio which increases with the reciprocal of the wavelength. The small-scale electric field variations are associated with field-aligned currents. These currents are measured with both a plasma instrument and magnetometer on DE 1. A Fourier transform of the east-west magnetic field component measured on the high-altitude satellite is found to be nearly identical to the Fourier transform of the north-south electric field measured on the low-altitude satellite, with a constant ratio. This ratio is proportional to the ionospheric conductivity. The experimental measurements are found to agree with a steady state theory which postulates that there are parallel potential drops associated with the variations in the perpendicular electric fields. It is assumed that there is a linear relationship between the field-aligned current and the total parallel potential drop and that the fieldaligned currents close through Pedersen currents in the ionosphere. The theory predicts that the ratio between the low-and high-altitude electric fields varies with the wavelength. Below a "critical" wavelength the electric field is not effectively transmitted to low altitudes. Owing to the good agreement between the theory and observations, it is concluded that the linear relationship between the current density and potential drop is a valid approximation. . 1/--3 , ß ß i ! ß ß ß i ß ! ß

We show that during magnetospheric substorms there is a clear tendency for the westward electrojet current to follow a characteristic curve that can be matched with an exponential equation. The substorm recovery phase has a characteristic timescale, or exponential time constant, that does not vary substantially from one substorm to the next. There is a slight trend toward shorter substorms as the peak magnitude of the electrojet current increases. This behavior can be explained by a simple model of the coupling between the currents and electric fields in the magnetotail and ionosphere. We derive an equation for the evolution of the substorm currents, including those in the magnetotail, using fundamental physical principles. There is good agreement between the measured and predicted time constants. This model is also consistent with other reported measurements of plasma flows and magnetic perturbations in the magnetotail.

[1] We have used output from the Weimer Joule heating model (2005) and the Air Force High Accuracy Satellite Drag Model (HASDM) to study the response of the thermosphere to Joule heating. Our study period of 15 January to 29 June 2001 contains a number of large and small magnetic storms during which thermospheric heating events occurred. We find that a new Joule heating model (Weimer, 2005), combined with the energy input provided by precipitating particles (NOAA/TIROS hemispheric power index), can supply more than enough energy to account for the change in total thermospheric internal and gravitational potential energy during magnetic storms. In the smaller storm heating events the energy input is about equally divided, with Joule heating only slightly dominant over particle precipitation. In the larger events, Joule heating clearly dominates. We find that the thermosphere responds globally in just 3-6 hours to an increase in energy input.

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