Reply to Tsurutani et al.'s comment on "Storming the Bastille: the effect of electric fields on the ionospheric F-layer" by Rishbeth et al. (2010)

Abstract

Despite the opening paragraph of the commentary by Tsurutani et al. (2013), it is not clear to which “fundamentally incorrect statements” in Rishbeth et al. (2010) they are referring. The commentary makes three points and we will discuss these in the order in which they appear in Tsurutani et al. (2013). First is a discussion of how total electron content (TEC) enhancements to 200 or more units may appear in the expanded crests of the anomaly, which are located near 25– 30 magnetic latitude. Tsurutani et al. (2013) point out that these enhancements result from the presence of an outward meridional E×B drift originating from a prompt penetration electric field. Rishbeth et al. (2010) agree and cite previous work by Tsurutani et al. (2004), Yin et al. (2004), Basu et al. (2007) and Lin et al. (2005, 2009), all of which show that the appropriate TEC enhancements arise in the presence of enhanced outward meridional E × B drifts. In their comment, Tsurutani et al. (2013) state, “The main point we wish to make from this figure is that the CHAMP data show that the EIAs become displaced from their normal location (∼ ±10) poleward with increasing time.” This point is well recognized and appears in Rishbeth et al. (2010) by citing previous work, such as “. . . Tsurutani et al. (2004) describe a so-called super-fountain effect that creates a poleward displacement of the equatorial anomaly peaks in the presence of enhanced outward meridional E × B drifts. . . ” We conclude that no fundamentally incorrect statements appear in Rishbeth et al. (2010) on this topic. Tsurutani et al. (2013) move forward with their commentary by describing computer simulations of the effects of enhanced meridional E × B drifts. Much of this work is a recapitulation of previously published work, but it appears that the point is to show that TEC peaks appear at 25 to 30 magnetic latitude in such simulations. It may also be appropriate to point out that equatorward meridional winds can also influence the vertical drift motion and associated TEC enhancements at middle latitudes (Lin et al., 2005), and that such winds have been shown to be significant during the particular event simulated by Tsurutani et al. (2013) for their commentary (Basu et al., 2005). Tsurutani et al. (2013) present these simulations to address the question of where the plasma originates and, in this regard, the authors make conclusions that can be easily tested. It is valuable to recall that Rishbeth et al. (2010) state that “. . . TEC enhancements at latitudes beyond 25 degrees cannot generally be attributed to transport from the equator.” The work of Tsurutani et al. (2013) assumes a zonal electric field of 4 mV m−1, which generously translates to a upward drift of 130 m s−1, for a duration of 2 h. If we assume a peak plasma density near 350-km altitude, then a flux tube through this location at 25 magnetic latitude has an apex height above 1800 km. For plasma at the equator, at 350km altitude, moving under the influence of an E ×B drift of 130 m s−1, it will take over 3 h to arrive at 25 magnetic latitude. In their comment Tsurutani et al. (2013) perhaps obfuscate the statement above made by Rishbeth et al. (2010) by later invoking transport from ±20. If 20 is the location that they wish to identify as the plasma source for an enhancem nt at 25, there would be no argument. What is potentially misleading is the statement by Tsurutani et al. (2013) that “Plasma originally from latitudes lower than where the