2016 ARTEMIS SCIENCE NUGGETS
Investigation of triggering of poleward moving auroral forms using satellite-imager coordinated observations
by Boyi Wang
Poleward Moving Auroral Forms (PMAFs), which are considered as an ionospheric signature of transient dayside reconnection and FTEs, are strongly dependent on the orientation of the interplanetary magnetic field (IMF). Mixed reports have been given on correlations between IMF Bz and PMAFs. Previous case studies indicate that IMF southward turnings are correlated with PMAFs. In contrast, events without appreciable changes in IMF have also been reported, indicating spontaneous triggering of PMAFs. And there has been no report about how often PMAFs are triggered by IMF Bz changes. Thus, it is important and necessary to statistically study triggering of PMAFs. To study this, accurate measurements of the solar wind and foreshock phenomena in conjunction with imaging are important. It is necessary to measure IMF structures near the dayside bow shock to examine triggering of PMAFs. Time History of Events and Macroscale Interaction During Substorms (THEMIS) B and C flew close to the bow shock in 2008, 2009 and 2011, and their 2 and 4-day orbital period are favorable for conjunctions with the Antarctic ground-based all sky imagers.
|Figure 1. Figure 1 Time series of (a) magnetic field observed by THEMIS B, (b) magnetic field observed by THEMIS C, (c) OMNI IMF, (d) keogram made from red line emission, (e) background subtracted keogram made from red line emission, (f) average luminosity from 73° to 84° south magnetic latitude using original keogram (panel d), (g) the luminosity of background subtracted keogram (panel e) along the dashed line shown in panel e on June 5, 2008. (h-n) show the time series of the same parameters as (a-g) except on Aug 19, 2008. Magenta dashed lines in panel (a-c) and (h-j) indicate southward turnings of the IMF and Magenta dashed lines in the keograms show PMAFs. The solid line in panel d and k shows the poleward boundary of auroral oval. The dashed lines in panels d and e are chosen to present luminosity profile shown in f and g, respectively. The dashed lines in panels k and l are chosen to present luminosity profile shown in m and n, respectively.|
The first case is from 1550 to 1800 UT on June 5, 2008. Two PMAFs are identified as two brightening structures that propagated poleward at ~1614 UT and 1647 UT by magenta dashed lines in Figure 1e. These two PMAFs appeared within ~20 min after the two southward turnings of the IMF that identified in both THEMIS and OMNI data. The relatively good correspondence between the IMF Bz and PMAFs indicates that the southward turnings of the IMF may be associated with those PMAFs.
The second event is from 1150 to 1330 UT on August 19, 2008. Three southward turnings are identified by THEMIS B and THEMIS C (marked by magenta dashed lines in Figure 1h, 1i). In contrast, OMNI data show substantially different IMF variations and only one southward turning is identified during this period. The differences between OMNI and THEMIS suggest that the IMF near the bow shock measured at THEMIS was different substantially from the IMF around the L1 point because of localized structures that missed the L1 point or time evolution of solar wind structures during their propagation.
Superposed Epoch Analysis
Figure 2a shows superposed epoch analysis of PMAF intensity. Figures 2b-2d show superposed epoch analyses of IMF ΔBz calculated from the IMF Bz data obtained from THEMIS B, THEMIS C and OMNI, respectively. Zero epoch time is also the onset time of each PMAF. The color, dashed lines and solid lines show constant percentiles.
|Figure 2. Figure 2 (a) shows superposed epoch analysis of the luminosity difference from the background luminosity. (b-d) show superposed epoch analyses of ΔBz calculated from IMF Bz obtained from THEMIS B, THEMIS C and OMNI, respectively. The zero epoch times are PMAF onset times. The color, dashed and solid lines show the percentages of total cases.|
Figure 2a shows that intensities of all the PMAFs increase over 20 luminosity units after the onset time and last for more than 5 minutes, satisfying our criteria of selecting PMAFs. Figure 2b and 2c show that for THEMIS-B and C, ~70% of events show a reduction of the IMF Bz relative to the reference level (Bz at -20—-15 min) and the decreasing trend of the IMF Bz occur after ~-8 mins. On the other hand, in Figure 2d, the superposed epoch analysis of IMF ΔBz obtained from OMNI shows that the Bz reduction magnitude was smaller and only ~40% of cases have a reduction of IMF Bz. Such differences in percentages between THEMIS and L1 measurements (OMNI) indicate that the IMF far away from the bow shock could be quite different from the IMF near the bow shock. If a solar wind monitor is far from the bow shock, the satellite may miss IMF changes that trigger PMAFs, and those PMAFs would be classified as spontaneous events. Thus, it is important to have a solar wind monitor near the bow shock for examining IMF triggering of PMAFs. Based on our analysis, at least 10% more PMAFS would be classified as associated with IMF southward turnings by using THEMIS B/C than by using the OMNI data.
In summary, we examined how often solar wind parameter changes play a role in triggering PMAFs, by comparing aurora and solar wind measurements from the AGO AP1 imager and THEMIS. The superposed epoch analysis based on 60 conjunction cases in 2008, 2009 and 2011 shows that~70% of events are preceded by IMF southward turnings by ~8 minutes, indicating their important role in the triggering of PMAFs. However, the superposed epoch analysis of the IMF Bz obtained from OMNI data shows that only 40% of PMAFs onsets occur ~8 mins after IMF southward turnings. This indicates that the measurements at the L1 point may miss small-scale structures that reached the subsolar bow shock or that the solar wind may evolve between the two measurements. Those PMAFs that were identified as spontaneous without correlation with solar wind changes by OMNI are found to be associated with IMF southward turnings by THEMIS measurements. Thus, measurements near the bow shock are critically important for evaluating triggering of PMAFs. In the paper, we also show that fore shock phenomena and other effects may also play a role in triggering PMAFs.
ReferencesWang, B., Y. Nishimura, Y. Zou, L. R. Lyons, V. Angelopoulos, H. Frey, and S. Mende (2016), Investigation of triggering of poleward moving auroral forms using satellite-imager coordinated observations, J. Geophys. Res. Space Physics, 121, 10,929–10,941, doi:10.1002/2016JA023128.
Boyi Wang is a PhD student in space physics at the University of California, Los Angeles. She is studying dayside aurora (PMAFs and diffuse aurora) and the related magnetospheric signatures.
Please send comments/suggestions to Emmanuel Masongsong / emasongsong @ igpp.ucla.edu