Reply to comment by G. A. Abel, M. P. Freeman, and G. Chisham on “Location of the reconnection line for northward interplanetary magnetic field”

Abstract

[1] In their comment, Abel et al. [2005] express concern about the conclusions presented in the study on the location of the reconnection line for northward interplanetary magnetic field (IMF) by Trattner et al. [2004]. This study concluded that both reconnection scenarios, antiparallel and component reconnection, occur simultaneously [see also Nishida et al., 1998; Onsager et al., 2001]. During northward IMF conditions, the location of the high-latitude antiparallel reconnection region depends on the IMF By component, and its local time extent is limited. A reconnection line formed in the antiparallel region at high latitudes will not be limited to this region but will extend into regions with smaller magnetic shear angles, forming long reconnection lines extending over several hours in MLT. Therefore, being magnetically connected to an antiparallel or component reconnection site depends mainly on the location of the observing satellite. [2] The method used by Trattner et al. [2004] is based on calculating the distance to the reconnection site from lowvelocity cutoffs of the downward precipitating and the mirrored magnetosheath distributions in the cusp [see Onsager et al., 1991; Fuselier et al., 2000]. The calculated distance was traced back along the geomagnetic field, starting at the position of the Polar satellites, by using the semiempirical Tsyganenko 1996 (T96) magnetic field model [Tsyganenko, 1995]. The IMF direction and geomagnetic field direction at the location of the reconnection site was used to calculate the magnetic shear angle for the reconnection site. For this purpose, the IMF observed upstream of the bow shock by the Wind spacecraft, had to be draped around the magnetopause using the model by Cooling et al. [2001], while again the T96 magnetic field model at the reconnection location was used for the geomagnetic field direction. [3] In their comment, Abel et al. [2005] rebinned the events of Trattner et al. [2004] and showed that events for component reconnection seems to mainly occurring during positive IMF Bx conditions while antiparallel reconnection events are occurring during negative IMF Bx conditions. Abel et al. [2005] suggest that a possible explanation lies in how IMF field lines drape around the magnetopause. In particular, they suggest an increased likelihood of departure from the model orientation for an increased distance from the point of first contact. For negative IMF Bx conditions, the IMF will contact the magnetopause first in the northern hemisphere at high latitudes and close to the observations of the Polar satellite discussed by Trattner et al. [2004]. For positive IMF Bx conditions the IMF will contact the magnetopause first in the southern hemisphere and draping these field lines around the magnetopause to the northern hemisphere might cause large deviations from the upstream clock angle, causing some of the component events to be consistent with antiparallel reconnection. While Abel et al. [2005] put forth an interesting scenario, we do not think the data support the conclusions.