Transient Oscillations Near the Dayside Open‐Closed Boundary: Evidence of Magnetopause Surface Mode?

Abstract

Geomagnetic pulsations in Pc5-6 band (~3-20 min) are persistent feature of ULF activity at dayside high latitudes. Magnetopause surface eigenmodes may be suggested as potential mechanism of these pulsations. One might expect the ground response of these modes to be near ionospheric projection of the open-closed field line boundary (OCB). Using data from instruments located at Svalbard we study transient geomagnetic response to impulsive "intrusion" of magnetosheath plasma into the dayside magnetosphere. These intrusions are triggered by modest changes of interplanetary magnetic field to southward, and observed as sudden shifts of equatorward red aurora boundary to lower latitudes and green line emission intensification. Each auroral disturbance is accompanied by burst of~1.7-2.0-mHz geomagnetic pulsations. Near-cusp latitudinal structure of ULF pulsations is compared with instant location of equatorward boundary of the red aurora, assumed to be a proxy of the OCB. Optical OCB latitude has been identified using data from the meridian scanning photometer. The latitudinal maximum of the transient geomagnetic response tends to be located near disturbed OCB proxy, within the error~1°-2°of the photometer and magnetometer methods. Recorded transient pulsations may be associated with the ground image of the magnetopause surface mode harmonic. Theoretical consideration indicates that after an initial excitation, surface large-scale mode converts into localized Alfvén oscillations and thus can exist for limited time only. Therefore, MHD surface modes in realistic inhomogeneous plasma cannot be considered in isolation, but as a combined system of modes with discrete and continuous spectra with irreversible transformation between them.

Interhemispheric Conjugacy of Concurrent Onset and Poleward Traveling Geomagnetic Responses for Throat Aurora Observed Under Quiet Solar Wind Conditions

“…Throat aurora has been suggested to correspond to a magnetopause indentation that is caused by magnetopause reconnection (Han et al, 2019). We found that all the auroras shown in Kim et al (2017), Kozyreva et al (2019), and Mende et al (2001) have clear throat aurora features. We argue that the geomagnetic responses discussed in these studies should correspond to localized magnetopause indentations that are caused by magnetopause reconnection.…”

“… d. Compare with previous studies on geomagnetic transients near local noon . Geomagnetic transients observed at high latitudes on the dayside have been extensively studied, including, MIE (Lanzerotti et al, 1990), TCV (Chaston et al, 1993; Friis‐Christensen et al, 1988; Glassmeier et al, 1989; Shi et al, 2014; Zesta, 2002), and impulsive ULF waves (Kozyreva et al, 2019; Lyatsky & Sibeck, 1997; Olson, 1986; Shen et al, 2018). MIEs have been generally accounted for geomagnetic responses to the transient variations in the IMF or solar wind pressure (Kataoka, 2003; Kawano et al, 1992; Lanzerotti et al, 1990; Lin et al, 1995; Mende et al, 1990; Sibeck & Croley, 1991).…”

“…TCVs were initially observed by ground‐based magnetometers and have been considered as the signature of flux transfer event (FTE) (Konik et al, 1994; Lanzerotti et al, 1986). However, because TCV often displays in transient ULF waveforms (Friis‐Christensen et al, 1988; Kozyreva et al, 2019; Lyatsky & Sibeck, 1997; Shi et al, 2014) and have conjugated properties (Kim et al, 2015; Murr, 2002), they have been suggested as ground signature of surface waves generated on the closed field lines. It is very difficult to distinguish the differences among the MIEs, TCVs, and impulsive ULF waves, so Zesta (2002) treated all these events as TCVs and classified them into “isolated” and “nonisolated” ones, which are suggested to be correspondent to different source mechanisms.…”

“…The auroral responses associated with geomagnetic observations of MIEs (Mende et al, 1990), TCVs (Kim et al, 2017; Murr, 2002), and impulsive ULF waves (Kozyreva et al, 2019) have also been investigated. In these studies, we noticed that all of the auroras had clear throat aurora properties, but they were just treated as transient responses to a sudden changes of the solar wind conditions (Kim et al, 2017; Kozyreva et al, 2019; Mende et al, 2001) and no attention had been paid to what particular magnetospheric processes can be reflected by these particular auroral forms. Compared with these previous studies, this paper reported some new information as follows.…”

Plasmapause surface wave oscillates the magnetosphere and diffuse aurora

“…Observations in both the topside ionosphere and the plasmasphere showed that the radial width of the plasmapause was 0.1-0.2 R E (see Supplementary Figs. 6, 7), which was much smaller than the transverse wave scale of~0.8 R E for the PSW with m-value at a geocentric distance of 4.5 R E , satisfying the requirement of the interface width in exciting surface waves 47 . The sharp discontinuity in the density led the magnetosonic wave to be undamped 24,46,48 , allowing excitation of a standing surface eigenmode, i.e., the PSW propagating sunward/westward at the plasmapause boundary and along the field lines to both the northern and southern polar ionosphere.…”