Low‐latitude boundary layer near noon: An open field line model

Abstract

We propose that many features of the cusp and low‐latitude boundary layer (LLBL) observed near noon MLT can be explained by interpreting the LLBL as being on open lines with an inner boundary at the separatrix between open and closed magnetic field lines. This interpretation places the poleward boundary of the LLBL and equatorward boundary of the cusp along the field line that bifurcates at the cusp neutral point. The interpretation accounts for the abrupt boundary of magnetosheath particles at the inner edge of the LLBL, a feature that is inconsistent with LLBL formation by diffusion onto closed field lines, and for the distribution of magnetosheath particles appearing more as one continuous region than as two distinct regions across the noon cusp/LLBL boundary. Furthermore, we can explain the existence of energetic radiation belt electrons and protons with differing pitch angle distributions within the LLBL and their abrupt cutoff at the poleward boundary of the LLBL. By modeling the LLBL and cusp region quantitatively, we can account for a hemispherical difference in the location of the equatorial boundary of the cusp that is observed to be dependent on the dipole tilt angle but not on the interplanetary magnetic field (IMF) x component. We also find important variations and hemispherical differences in that the size of the LLBL that should depend strongly upon the x component of the IMF. This prediction is observationally testable. Finally, we find that when the IMF is strongly northward, the LLBL may include a narrow region adjacent to the magnetopause where field lines are detached (i.e., have both ends connected to the IMF).

Dayside isotropic precipitation of energetic protons

“…Our model computations con®rmed that a week magnetic ®eld in the outer cusp necessarily should contribute to the regular scattering of energetic protons on closed ®eld lines in $1±2°wide zone near the cusp meridian. In a quantitative way, and using existing empirical models, this con®rms the previous results of Lyons et al (1994) and Alem and Delcourt (1995). Our results also extend previous ®ndings by specifying the Fig.…”

“…First, there is a weak magnetic ®eld in the high-latitude high-altitude cusp region near the magnetopause (referred to hereafter as the Ôouter cusp'), where the magnetosheath plasma¯ows along the magnetopause surface and does not require substantial magnetospheric magnetic ®eld to balance the impulse carried by plasma. The eectiveness of ion scattering in that region was recently con®rmed in trajectory computations for lower energy (<10 keV) protons (Alem and Delcourt, 1995), it was also discussed by Lyons et al (1994). The authors of both papers also presented the examples of observations to show that isotropic precipitation of energetic protons extends equatorwards of the equatorward boundary of cusp-like precipitation, and argue that it lies on closed ®eld lines.…”

Coordinated Cluster and ground-based instrument observations of transient changes in the magnetopause boundary layer during an interval of predominantly northward IMF: relation to reconnection pulses and FTE signatures

“…This evolution is seen in full along the flow streamlines in the steady state case and thus may sometimes be seen if the satellite follows the flow streamline quite closely (Onsager et al, 1993;. Thus, several authors have argued that much of the low-altitude LLBL precipitation must be on open field lines (Lockwood and Smith, 1993;Lyons et al, 1994;Moen et al, 1996;Fuselier et al, 1991Fuselier et al, , 1992Fuselier et al, , 1999. Other authors, while accepting that this is true when reconnection is taking place, now argue that there is also a closed LLBL at low-altitudes nearer dawn and dusk (Newell and Meng, 1997).…”

“…The origin of this layer is one of the major unanswered questions in magnetospheric physics and a key unknown in this regard is the topology of the LLBL field lines: it is interesting to note that roughly half of the papers cited above interpret the LLBL in terms of closed field lines, and the other half in terms of open field lines. There are three main classes of theory of LLBL formation (see review by Sibeck et al, 1999): (1) magnetosheath plasma is injected by some process (such as wavedriven diffusion) onto closed field lines that are already populated with magnetospheric plasma (Drakou, 1994;Lotko and Sonnerup, 1995;Treumann et al, 1991Treumann et al, , 1995Winske et al, 1995); (2) The plasma mixture arises on newly opened field lines along which magnetosheath plasma has flowed into the magnetosphere but magnetospheric plasma has yet to escape, either due to time-of-flight considerations (Lockwood and Smith, 1993;Lockwood, 1997a, b;Fuselier et al, 1999;Onsager and Lockwood, 1997), or ion reflection at the reconnection layer Alfvén waves (Cowley, 1982; or because a magnetic bottle still exists on open field lines (Daly and Fritz, 1982;Scholer et al, 1982a;Cowley and Lewis, 1990;Lyons et al, 1994); (3) The field lines of the LLBL had been open, allowing for the magnetosheath plasma to enter, but have subsequently been reclosed by re-reconnection (Nishida, 1989;Song and Russell, 1992;Richard et al, 1994). In both (2) and (3), gradient and curvature drift across the open-closed boundary may sometimes help to replenish magnetospheric plasma that has been lost when it flowed across the magnetopause along open field lines.…”

A multi-spacecraft survey of magnetic field line draping in the dayside magnetosheath

“…Waves on the magnetopause surface, such as Kelvin-Helmholtz waves, may result in departures from perfect draping, particularly in the equatorial flank regions, while magnetic reconnection and the consequent motion of flux tubes across the magnetopause can create a disordered magnetic field structure over any part of the magnetopause, particularly for southward IMF. In general, the magnetosphere is open and always reconnecting (Lyons et al, 1994), so any oncoming IMF phase front will typically encounter a reconnecting magnetopause rather than a quasi-dipole. The effects of these surface phenomena on the magnetosheath field immediately upstream, close to the magnetopause, are not clear.…”