Dependence of SuperDARN cross polar cap potential upon the solar wind electric field and magnetopause subsolar distance

Abstract

[1] Dependence of the cross polar cap potential (CPCP) upon the interplanetary electric field (IEF), solar wind ram pressure, and magnetopause standoff distance (R MS ) are investigated by considering the Super Dual Auroral Radar Network (SuperDARN) HF radar data on the CPCP and information on the solar wind and interplanetary magnetic field from ACE satellite. Data of up to IEF $12 mV/m are available. The CPCP scatterplot versus IEF shows a linear increase between 0 and $3 mV/m. For larger IEFs, the rate of the CPCP growth decreases, and the dependence eventually saturates. If the same data are arranged in groups of R MS values, linear dependences were found for each group, but the slope of the regression line increases with R MS . The inferred linear fit lines of CPCP versus IEF for various R MS groups intersect each other at IEF $3 mV/m. This number is found to be unique because for the IEF above it (below it), the CPCP decreases (increases) with the solar wind ram pressure. This value also corresponds to the condition of the solar wind magnetic field at the subsolar point to be equal to the Earth's dipole magnetic field, as previously reported by Siscoe et al. (2002). Presented analysis suggests that the magnetopause standoff distance (the length of the merging X line) is an important factor in the CPCP saturation.

On the SuperDARN cross polar cap potential saturation effect

“…1. Khachikjan et al (2008) investigated this relationship in detail, but with the data collected during predominantly winter time. We have two objectives: first, we would like to confirm that the SuperDARN data selected with somewhat different and more restrictive criteria still show the same tendencies and, second, we go further and consider the data for winter and summer separately.…”

“…This method is deemed to be reliable, first of all, if significant amount of vectors is available for the analysis of an individual convection map. Maps that we considered had at least 200 points and, for the vast majority of cases (91% winter and 70% summer time), had more than 300 points (this threshold value was adopted by Khachikjan et al (2008) whose events have mostly been included into the present data set). Since we wanted to consider the PCN magnetic index, the data search was limited to the periods for which the PCN index was visibly showing enhancements to 5-6 and up according to the plots on the official WEB site for PCN: http:// web.dmi.dk/projects/wdcc1/pcn/pcn.html.…”

“…In pursuing this goal, useful information can be inferred from plotting the CPCP against other parameters. In terms of indices, an interesting relationship to consider is the CPCP versus the PCN index because the work by Ridley and Kihn (2004) did not show strong nonlinearity, which is in contrast with, for example, SuperDARN-based studies (Shepherd et al, 2003;Khachikjan et al, 2008). In terms of a function characterizing interaction of the solar wind and the magnetosphere, an interesting relationship to consider is the CPCP with the recently introduced by Newell et al (2007) coupling function E C .…”

“…Notably, the threshold value for the nonlinearity to begin and the CPCP plateau values vary significantly (Liemohn and Ridley, 2002;Khachikjan et al, 2008).…”

“…The effect has been demonstrated by considering E×B ion drift data from satellites crossing the polar cap (e.g., Hairston et al, Correspondence to: A. V. Koustov (sasha.koustov@usask.ca) 2003), by modeling the CPCP with AMIE technique based on magnetometer data (e.g., Russell et al, 2001;Liemohn and Ridley, 2002) and by analyzing the SuperDARN radar convection maps (e.g., Shepherd et al, 2003;Khachikjan et al, 2008). To show the saturation effect, the CPCP has been plotted against either the interplanetary electric field (IEF, Russell et al, 2001;Khachikjan et al, 2008) or the merging electric field E KL (Shepherd et al, 2003). While the first consideration characterizes the ionospheric output (CPCP) and the external driver (IEF), the second consideration takes into account the relationship between the external driver and the magnetosphere/ionosphere system.…”