An unusual geometry of the ionospheric signature of the cusp: implications for magnetopause merging sites
The HF radar Doppler spectral width boundary (SWB) in the cusp represents a very good proxy for the equatorward edge of cusp ion precipitation in the dayside ionosphere. For intervals where the Interplanetary Magnetic Field (IMF) has a southward component (Bz < 0), the SWB is typicall...
| Main Authors: | , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2002-01-01
|
| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/20/29/2002/angeo-20-29-2002.pdf |
| Summary: | The HF radar Doppler
spectral width boundary (SWB) in the cusp represents a very good proxy for the
equatorward edge of cusp ion precipitation in the dayside ionosphere. For
intervals where the Interplanetary Magnetic Field (IMF) has a southward
component (Bz < 0), the SWB is typically displaced poleward of the actual
location of the open-closed field line boundary (or polar cap boundary, PCB).
This is due to the poleward motion of newly-reconnected magnetic field lines
during the cusp ion travel time from the reconnection X-line to the ionosphere.
This paper presents observations of the dayside ionosphere from SuperDARN HF
radars in Antarctica during an extended interval ( ~ 12 h) of quasi-steady IMF
conditions (By ~ Bz < 0). The observations show a quasi-stationary feature
in the SWB in the morning sector close to magnetic local noon which takes the
form of a 2° poleward distortion of the boundary. We suggest that two separate
reconnection sites exist on the magnetopause at this time, as predicted by the
anti-parallel merging hypothesis for these IMF conditions. The observed cusp
geometry is a consequence of different ion travel times from the reconnection
X-lines to the southern ionosphere on either side of magnetic local noon. These
observations provide strong evidence to support the anti-parallel merging
hypothesis. This work also shows that mesoscale and small-scale structure in
the SWB cannot always be interpreted as reflecting structure in the dayside
PCB. Localised variations in the convection flow across the merging gap, or in
the ion travel time from the reconnection X-line to the ionosphere, can lead to
localised variations in the offset of the SWB from the PCB. These caveats
should also be considered when working with other proxies for the dayside PCB
which are associated with cusp particle precipitation, such as the 630 nm cusp
auroral emission.<br><br><b>Key words. </b>Ionosphere (plasma
convection) – Magnetospheric physics (magnetopause, cusp, and boundary
layers) – Space plasma physics (magnetic reconnection) |
|---|---|
| ISSN: | 0992-7689 1432-0576 |