next up previous
Next: Lag Time Determination Up: Procedure Previous: Procedure

Solar Wind Selection Criteria

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 $\ge$40 min.

The definition of quasi-stability we choose for this study is

\vert\Delta E_{KL}\vert / E_{KL} < 7\%.
\end{displaymath} (1)

$E_{KL}$ is an expression used by Kan and Lee, [1979] interplanetary electric field and corresponds to the fastest merging rate at the subsolar magnetopause [Sonnerup, 1974] given by
E_{KL} = V B_T \sin^2(\theta/2),
\end{displaymath} (2)

where $V$ is taken as the antisunward component of the solar wind velocity, $B_T = \sqrt{B_Y^2 + B_Z^2}$, and $\theta$ is the IMF clock angle in the $(Y$- $Z)_\mathrm{GSM}$ plane, or $\theta = \cos^{-1}(B_Z/B_T)$. $\Delta E_{KL}$ is the difference between the minimum and maximum values of $E_{KL}$ during the entire $\ge$40-min period. Several other studies have used $E_{KL}$ to demonstrate a correlation between the solar wind and $\Phi_\mathsf{PC}$ [Reiff et al., 1981; Doyle and Burke, 1983; Weimer, 1995; Burke et al., 1999].

An example period selected for this study is shown in Figure 1.

Figure 1. ACE solar wind and interplanetary magnetic field (IMF) data during a 50-min period of quasi-stable conditions beginning at 1300 UT on 19 April 2000, including (a) H$^+$ density, (b) antisunward solar wind velocity, (c) IMF magnitude, (d-f) IMF $B_X$, $B_Y$, $B_Z$, (g) IMF clock angle $\theta$, (h) $B_T$, (i) an expression for the interplanetary electric field $E_{KL}$, and (j) $\Phi_\mathsf{PC}$ as determined by APL FIT. The 10-min averages and averages for the 50 min period are shown in purple and green, respectively.

Red lines in Figures 1a-1f represent the level 2 ACE H$^+$ density, antisunward solar wind velocity, IMF magnitude, and IMF $B_X$, $B_Y$, and $B_Z$ components, respectively. The quantities $\theta$, $B_T$, and $E_{KL}$ from equation (2) are shown in Figures 1g-1i, respectively. The period which satisfies equation (1) is marked by vertical dotted lines at 1300 UT and 1350 UT on 19 April 2000 in Figure 1. Between these two times, 10-min averages of each quantity are indicated by horizontal blue lines ($E_{KL}$ is only calculated as 10-min averages so it appears only in blue) and a green line indicates the average value for the entire 50 min period, $\left< E_{KL} \right> = 21.4$ kV $R_E^{-1}$. Figure 1j shows $\Phi_\mathsf{PC}$ as determined using APL FIT (see section 2.3) at 2-min and 10-min resolutions in red and blue, respectively, and the average $\Phi_\mathsf{PC}$ over the five 10-min periods in green ( $\left< \Phi_\mathsf{PC} \right> = 76.8$ kV).

Figure 2

Figure 2. Distribution of study periods in (a) $E_{KL}$ and (b) IMF $B_Z$ using 5, 7, and 10% in equation (1). The middle value of 7% was selected for this study.

shows the distribution of all the periods satisfying the quasi-stability criteria in equation (1) for three different percentages: 5%, 7%, and 10%. Figure 2a shows these distributions versus $E_{KL}$ and, for comparison, versus the IMF $B_Z$ component in Figure 2b. It can be seen that the general shape of the curves remains the same for the different percentages chosen and, thus the sampling is unbiased by the level of quasi-stability in the range 5-10%. We have selected 7% as a suitable value to use in equation (1) for this study. The choice of 7% increases the number of periods in the study from 5356 to 9464 over the 5% value while maintaining a fairly restrictive stability requirement of the solar wind.

The parameter $E_{KL}$ depends on three solar wind quantities (IMF $B_Z$, IMF $B_Y$, and $V$) and uncertainty in its value depends on the uncertainties of these quantities. The ACE level 2 MAG data (IMF $B_Z$ and $B_Y$) are stated to have errors of $<$.1 nT, and the ACE level 2 SWEPAM solar wind velocity data ($V$) are stated to have errors of $<$1%. Using these values, it is found that for $E_{KL} >$ 2 kV $R_E^{-1}$ the uncertainty in $E_{KL}$ is $<$ 4% and typically $<$ 2%. For values of $E_{KL} <$ 2 kV $R_E^{-1}$, which typically correspond to strongly northward IMF conditions with small ($< \sim$1 nT) IMF $B_Y$, the uncertainty in $E_{KL}$ can be much larger. However, relatively few of the total periods in this study fall into this category as seen in Figure 2a.

next up previous
Next: Lag Time Determination Up: Procedure Previous: Procedure

Simon Shepherd 2002-06-04