A scheme for finding the front boundary of an interplanetary magnetic cloud

• Main Authors: R. P. Lepping, T. W. Narock, C.-C. Wu
• Format: Article
• Language: English
• Published: Copernicus Publications 2009-03-01
• Series: Annales Geophysicae
• Online Access: https://www.ann-geophys.net/27/1295/2009/angeo-27-1295-2009.pdf
• ISSN: 0992-7689; 1432-0576

We develop a scheme for finding a "refined" front boundary-time (<I>t_B</I>*) of an interplanetary magnetic cloud (MC) based on criteria that depend on the possible existence of any one or more of four specific solar wind features. The features that the program looks for, within &plusmn;2 h (i.e., the initial uncertainty interval) of a preliminarily estimated front boundary time, are: (1) a sufficiently large directional discontinuity in the interplanetary magnetic field (IMF), (2) a significant proton plasma beta (&beta;_<I>P</I>) drop, (3) a significant proton temperature drop, and (4) a marked increase in the IMF's intensity. Also we examine to see if the "MC-side" of the boundary has a MC-like value of &beta;_<I>P</I>. The scheme was tested using 5, 10, 15, and 20 min averages of the relevant physical quantities from WIND data, in order to find the optimum average to use. The 5 min average, initially based on analysis of <I>N</I>=26 carefully chosen MCs, turned out to be marginally the best average to use for our purposes. Other criteria, besides the four described above, such as the existence of a magnetic hole, plasma speed change, and/or field fluctuation level change, were examined and dismissed as not reliable enough, or usually associated with physical quantities that change too slowly around the boundary to be useful. The preliminarily estimated front boundary time, <I>t_B</I>, and its initial &plusmn;2-h uncertainty interval are determined by either an automatic MC identification scheme or by visual inspection. The boundary-scheme was developed specifically for aiding in forecasting the strength and timing of a geomagnetic storm due to the passage of a MC in real-time, but can be used in post ground-data collection for imposing consistency when choosing front boundaries of MCs. This scheme has been extensively tested, first using 81 bona fide MCs, collected over about 8.6 years of WIND data (at 1 AU), and also by using 122 MC-like structures as defined by Lepping et al. (2005) over about the same period. Final statistical testing of the 81 MCs to see how close the refined boundary-time <I>t_B</I>* lies with respect to a preliminary time <I>t_B</I>(VI) was carried out, i.e., to find &Delta;<I>t</I>₁=(<I>t_B</I>*–<I>t_B</I>(VI)), for the full set of MCs, where <I>t_B</I>(VI) is usually a very accurate time previously determined from visual inspection, This testing showed that 59 &Delta;<I>t</I>₁s (i.e., 73%) lie within &plusmn;30 min, 71 &Delta;<I>t</I>₁s (i.e., 88%) lie within &plusmn;45 min, and only 5 cases lie outside a |&Delta;<I>t</I>₁| of 1.0 h, which is only 6% of the full 81, and these 6% would be considered unsatisfactory. Since MC parameter fitting is usually done on the basis of 30 or 60 min averages, these results seem quite satisfactory. The program for this front boundary estimation scheme is located at the Website: <a href="http://wind.nasa.gov/mc/boundary.php" target="_blank">http://wind.nasa.gov/mc/boundary.php</a>.