For this study we use level 2 solar wind and IMF data provided by the ACE science team. ACE was chosen because (1) the satellite is reasonably stationary near the so-called L1 Lagrangian point, thus providing relatively uninterrupted monitoring of the solar wind conditions and (2) the epoch of the satellite best matches the period when SuperDARN provides the most coverage (see section 2.3). The time range of this study is bounded by the availability of the ACE and SuperDARN data. The earliest ACE solar wind data is from February of 1998 and, at the time of the study, SuperDARN data were available through December 2000. This study, therefore, extends from February 1998 through December 2000.
To investigate the relationship between the solar wind and ionospheric convection, we choose to average the data over periods of 10 min. It is possible that by doing so we are missing the effects of variability with shorter time scales, but we question whether variability on such a short time-scale is geoeffective to the large-scale convection. Therefore, the level 2 Magnetometer Instrument (MAG) (16-s) and Wind Electron Proton Alpha Monitor (SWEPAM) (64-s) are averaged over all 10-min periods bounded by the study time-range, and a stability criteria is applied to the averaged data to determine which periods to include in the study.
The primary reason for requiring quasi-stability of the solar wind and IMF is to minimize the effect that uncertainties inherent in determining the time delay between observation at L1 and the subsequent time of geoeffective impact in the ionosphere have on comparing the true solar wind conditions and the resulting ionospheric response. The uncertainty in timing the ionospheric response to IMF changes in the solar wind can be >10 min [e.g., Ridley et al., 1998; Collier et al., 1998; Ridley, 2000]. By requiring the solar wind to be quasi-stable for several 10-min-averaged periods, the solar wind and IMF conditions (in the averaged sense) measured at L1, when time delayed using a standard technique, are certain to be geoeffective for some, if not all, of the 10-min periods. While uncertainties remain in the predicted delay time between measurements at L1 and in the ionosphere, the predicted geoeffective conditions during quasi-stable periods are statistically more accurate. In the extreme example the solar wind and IMF are both constants, and while the time delay may still be uncertain, the geoeffective solar wind conditions are known with absolute certainty. For this study we selected periods which satisfied the quasi-steady criteria for four or more consecutive 10-min averages, or 40 min.
The definition of quasi-stability we choose for this study is
An example period selected for this study is shown in Figure 1.
The parameter depends on three solar wind quantities (IMF , IMF , and ) and uncertainty in its value depends on the uncertainties of these quantities. The ACE level 2 MAG data (IMF and ) are stated to have errors of .1 nT, and the ACE level 2 SWEPAM solar wind velocity data () are stated to have errors of 1%. Using these values, it is found that for 2 kV the uncertainty in is 4% and typically 2%. For values of 2 kV , which typically correspond to strongly northward IMF conditions with small (1 nT) IMF , the uncertainty in can be much larger. However, relatively few of the total periods in this study fall into this category as seen in Figure 2a.