|
Zou, S., L. R. Lyons, M. J. Nicolls, C. J. Heinselman, and S. B. Mende (2009), Nightside ionospheric electrodynamics associated with substorms: PFISR and THEMIS ASI observations, J. Geophys. Res., 114, A12301, doi:10.1029/2009JA014259.
Introduction
Substorms are a major geomagnetic disturbance, during which energy stored in the magnetotail during the growth phase is released in an abrupt manner after the expansion onset. Dramatic changes occur both in the magnetosphere and in the high latitude ionosphere during substorms. In this paper, we focus on observations of ionospheric electrodynamics during the substorm expansion phase using data from the newly available Poker Flat Incoherent Scatter Radar (PFISR) and THEMIS all-sky imagers (ASIs).
Observations
In the first section, we focus on the region west of substorm onset. Figure 1a displays selected auroral images taken by the imager located at Fort Yukon for substorm onset at 0836 UT on Nov. 14, 2007. A well-defined westward traveling surge (WTS) can be seen in the bottom left panel about six minutes after the onset. Figure 1b shows corresponding observations from PFISR and Alaska magnetometers for this event. When the WTS presented within the radar field-of-view (fov), westward flows equatorward of it increased significantly and the E-region electron density also increased in the region of strongest velocity gradient. This observation indicates that the poleward Petersen currents contributed at least partially to the local closure of the upward field-aligned current (FAC) associated with the WTS. These poleward Pedersen currents should be closed by the downward Region 2 FAC further equatorward of them.
Figure 1.
Auroral images, nightside convection flow direction, raw electron densities, and magnetic perturbations. |
Click here to enlarge the image. |
Figure 1(a) shows selected auroral images obtained by the ASI at Fort Yukon on 14 November 2007, with PFISR beams projected. The geomagnetic coordinates are also shown as white dotted lines. The PFISR radar was located at ~21.6 MLT around this time. Figure 1(b) (panels a–c) show nightside convection flow direction, magnitude, and vector measured by PFISR on 14 November 2007 as a function of magnetic latitude and universal time; (panels d–f) show raw electron densities with no correction for Te/Ti or Debye length effects measured by beam 12, 10, and 13 (upward along magnetic field) as a function of universal time. Altitude is indicated on the left Y-axis and magnetic latitude is indicated on the right Y-axis. (panel g) shows magnetic perturbations of H (red), D (blue) and Z (green) components recorded by four Alaska magnetometers that are roughly at the same local time as a function of universal time. The vertical magenta line indicates the substorm onset time based on auroral images.
In the second section, we focus on the region east of substorm onset. Figure 2a displays selected auroral images taken by imagers located at Fort Yukon and White for substorm onsets at 1302 UT and 1314 UT on Feb. 9, 2008. No discrete auroral arc brightening was observed over Alaska until 1336 UT, in the late expansion phase. Figure 2b shows observations from PFISR and Alaska magnetometers for this event. After the second onset, a peak in the eastward flows can be seen within the center of the radar fov (panel b and c). The E-region electron density depleted dramatically just poleward of the flow peak (panel d) in association with the formation of downward FAC, while it increased just equatorward of it (panels e and f), related with the enhancement of upward Region 2 FAC. This observation indicates that the downward FAC of the substorm current wedge (SCW) formed at onset feeds the more equatorward upward FAC in the Region 2 region via equatorward Pedersen currents. The density depletion ceased after the intrusion of a bright discrete auroral arc at the eastern boundary of the auroral bulge at 1336 UT.
Figure 2.
The same data as in Figure 1, but for the substorm onset at ~0650 UT on 9 February 2008. |
Click here to enlarge the image. |
In the third section, we focus on the region at substorm onset location. Figure 3a displays selected auroral images taken by the imager located at Fort Yukon for substorm onset at 0947 UT on March 27, 2007. The breakup arc occurred right within the center of the radar fov. Figure 3b shows observations from PFISR and Alaska magnetometers for this event. A clear Harang reversal can be seen in panel c, marked by a magenta segment, which formed during the growth phase. The location of the breakup arc, represented by the star, was in the center of the Harang reversal. The occurrence of the weak and homogeneous E-region electron density increase, caused by almost pure proton precipitations (highlighted by a red box in panel f), and the subauroral polarization streams (SAPS) flows (panel b and c) slightly equatorward of the onset latitude imply that the onset location in the equatorial plane was quite close to the inner boundary of the electron plasma sheet.
Figure 3.
The same data as in Figure 1, but for the substorm onset at ~0947 UT on 27 March 2007. 2008. |
Click here to enlarge the image. |
Summary and Conclusions
1. Weak and homogeneous ionization confined to the E region (~100-150 km) are frequently observed during substorm growth phase.
This is evidence for precipitating energetic protons, supported by POES satellite observations. The region where almost pure proton precipitation
is observed is usually in the evening sector and equatorward of the equatorial boundary of electron precipitations.
2. West of onset, westward flows near the equatorward portion of the PFISR fov increase in response to the passage of the WTS poleward of them.
Meanwhile, significant E-region ionization increases occur poleward of the strongest westward flow enhancement and within the WTS. This observation
indicates that the poleward Pedersen currents contribute at least partially to the current closure of the upward FAC associated with the WTS.
3. Eastward of onset, an enhanced eastward flow channel is observed and accompanied by E-region ionization depletion just poleward of the peak flows,
indicating the formation of downward FAC of the SCW at onset. On the other hand, ionization further increases equatorward of the peak flow and
equatorward of the downward FAC, indicating strengthening of the Region 2 upward FAC. This, together with the observed flow enhancement, suggests
that the downward FAC formed at onset feeds the more equatorward upward FAC in the Region 2 region via equatorward Pedersen currents.
4. Enhanced eastward flows start to decrease when the eastern edge of the auroral bulge intrudes into the radar fov from the west, the intrusion
being associated with ionization enhancement.
5. For events occurring within the radar meridian, 10 out of 11 events were found to be associated with the Harang reversal. In particular,
five out of the six events for which the onset occurred within the radar fov show that the breakup arc was located within the center of the Harang
reversal. Occasionally, almost pure proton precipitation was observed slightly equatorward of the breakup arc before onset.
A 2-D picture of the evolution of ionospheric electrodynamics was generated by synthesizing observations from the three categories, providing
a schematic description of the important relationship between Region 2 and substorm expansion electrodynamics.
Biographical Note Shasha Zou is a research fellow in the Department of Atmospheric, Oceanic and Space Sciences at University of Michigan, Ann Arbor. Her research interests focus on the magnetosphere-ionosphere coupling electrodynamics during substorms.