Читать книгу Space Physics and Aeronomy, Ionosphere Dynamics and Applications - Группа авторов - Страница 61

The F‐region and topside ionospheric density is enhanced within the high‐density ionospheric structures. Convective transport of these high‐density structures into regions with enhanced precipitating particle fluxes has been suggested to be an important mechanism of generating large ion upflow fluxes (Lotko, 2007; Yau et al., 2011). Without particle precipitation, the field‐aligned plasma flows within these high‐density structures are usually downward (Ren et al., 2018; Sojka et al., 1997). When these high‐density structures convect antisunward following the convection flows to regions, such as the dayside cusp and the nightside auroral zone, intense type‐2 ion upflow fluxes (Wahlund et al., 1992), and even divergent fluxes, can form.

Observationally, a couple of fortuitous measurements showed large ion upflow fluxes on the nightside that can be related to the polar cap patches and SED plumes (Semeter et al., 2003; Tu et al., 2007; Yuan et al., 2008; Zhang et al., 2016; Zou et al., 2017b) . Using the Sondrestrom ISR, Semeter et al. (2003) reported an observation of strong ion upflow event due to drifting polar cap patch into particle precipitation at the nightside auroral poleward boundary. Combining GPS TEC and DMSP satellite observations, Yuan et al. (2008) reported large ion vertical fluxes of ~1.2 x 10¹⁴ m^‐2s^‐1 measured by DMSP satellite within the SED, when it reached the nightside polar cap boundary during the 20 November 2003 superstorm. Recently, during a strong polar cap expansion event on 1 June 2013, soft electron precipitations in the cusp region moved equatorward and crossed a preexisting SED plume, resulting in strong heating and divergent ion flows and fluxes (Zou et al., 2017b). Figure 4.7 shows the 2‐D TEC map at 0100 UT on 1 June 2013 with an evident SED plume, and the PFISR field‐aligned beam observations of the plasma density, temperature, flow, and flux within the plume. The peak upflow fluxes reached ~2 x 10¹⁴ m^‐2s^‐1 at ~600 km in this event. Using the radio plasma imager (RPI) on the IMAGE satellite, Tu et al. (2007) found that the plasma density at ~7 R_E can increase and decrease subsequently, when the SED plume extends to the polar cap and then disappears, and thus suggested that the high‐altitude density increase is due to the enhanced cleft ion fountain effect.

Figure 4.7 (a) TEC map at 0100 UT on 1 June 2013 shows the extension of SED plume into the Alaska sector. The black segments highlight the field of view of the PFISR radar. (b)–(f) PFISR field‐aligned beam observation of the density, ion temperature, electron temperature, field‐aligned velocity, and flux. Divergent plasma fluxes are seen within the SED plume when the open‐closed boundary moved across the beam

(from Zou et al., 2017b; Reproduced with permission of John Wiley and Sons).

Besides the effect of soft electron precipitation, Zhang et al. (2016b) also reported ion upflow events in the polar cap patch due to enhanced frictional heating, that is, the type‐1 ion upflow defined in Wahlund et al. (1992), using DMSP satellite. In a subsequent study based on an extended DMSP database, Ma et al. (2018) found that the highest upflow occurrence rate was associated with hot patches, which are accompanied with particle precipitation, strong convection speed, and localized FACs.

Cold plasma of ionosphere origin has indeed been observed at very high altitude. For instance, Foster et al. (2014) described in situ observations of locally enhanced cold plasma density at the ~5 Re altitude of the Van Allen Probes RBSP‐A spacecraft on magnetic field lines mapping to the point where the TOI intersected the midnight auroral oval as seen in GPS TEC imagery. Similarly, Walsh et al. (2014) reported THEMIS observations at ~12 Re altitude of enhanced cold plasma density on reconnecting dayside field lines mapping to the point where the SED plume entered the polar cap at the noontime cusp. These observations suggest that the density enhancements seen as the SED plumes and TOI at ionospheric heights could extend to very high altitudes along magnetospheric and polar cap field lines. In addition to the direct contribution of ion upflows/outflows from the cusp to the dayside reconnection site, cold plasma of the plasmaspheric plume origin has also been observed in the reconnection region (Lee et al., 2016).

The subsequent impact of the large but intermittent ion upflow/outflow fluxes associated with polar cap patches and TOI on magnetospheric dynamics is of great interest but outside the scope of this chapter and thus will not be elaborated here. Future quantitative studies using numerical models are needed to further distinguish the classical cold and the newly identified hot patches, such as whether the classical patch evolves into the hot patch under the influence of particle precipitation and FACs, or these hot patches can be produced solely by particle precipitation.