Stormtime substorm onsets: occurrence and flow channel triggering

Our determination of whether a given substorm onset is consistent with the Nishimura et al. (2010a) scenario is based on whether a streamer is detected approaching the substorm onset location during a period of time ending at the first detection of onset. Since it is the incoming channel of low-entropy plasma adjacent to the streamer that is critical for the onset, and the flow channels are typically ~ 100 km in width (Gallardo-Lacourt et al. 2014b), the streamer is expected to reach near (within ~ 1°) of the onset latitude. Also, the streamer can be a little to the east of the actual onset due to the westward magnetic drift of low-entropy plasma when it approaches the growth phase arc as seen in Rice Convection Model simulations (Yang et al. 2014). We do not consider the 20 of 80 identified substorm onsets where the initial onset brightening was either obscured (by clouds, trees, or moon) or was just outside the FOV of the imagers.

The example in Fig. 1 and movie Additional file 1 shows a clear example of the type streamer triggering that we commonly see. In this case, the oval was a few degrees in latitude thick and had activity along and near the auroral poleward boundary during the substorm growth phase. The equatorward boundary of oval is identified in each image prior to and at onset via the equatorward boundary of a band of diffuse emissions, this boundary moving equatorward during the growth phase. An arc can be seen to have formed along the auroral poleward boundary (identified in the 0504:33 UT panel of Fig. 1) ~ 7 min before onset, extending across ~ 4 h of MLT. The arc was not strictly east–west, tilting equatorward from west to east. This equatorward tilt then increased with time, the eastern portion of the arc moving equatorward within the oval and approaching the oval equatorward boundary. We call such tilted arcs “tilted streamers,” designating them as streamers because they extend equatorward a few degrees in latitude. Similar tilted streamers were identified in Zesta et al. (2006) and shown to map to the tail as primarily radial structures that traverse a large radial extent of the tail, consistent with them being streamers associated with earthward moving tail flow bursts.

As the streamer tilted equatorward toward the diffuse emission band along the equatorward boundary of the oval [this band likely being proton aurora due to precipitation of particles from the partial ring current (e.g. Zou et al. 2009)], onset occurred at 0511:21 UT (identified in the 0511:54 panel of Fig. 1). The onset initiated just to the west of, and ~ 1° of equatorward of, the eastern edge of the tilted streamer. The onset activity then extended westward, consistent with the expected azimuthal drift of low-entropy plasma brought earthward by the streamer associated flow burst, and then expanded poleward.

Figure 3 shows examples of events that are consistent with onset triggering by tilted streamers that are not as ideal as the event in Fig. 1. In the 10 November 2006 example (auroral movie for this event, with images every 3 s, is given in Additional file 4), a relatively short arc first formed along the auroral poleward boundary (a “poleward boundary intensification”). The arc then tilted equatorward as a tilted streamer leading to onset at 0932 UT just equatorward of the eastern edge of the tilted streamer as the streamers edge reached ~ 1° poleward of the onset location. While the onset was just to the east of the bright light of the moon (which shows in the images from all three ASIs used for the ASI mosaic), the streamer and the onset can be clearly seen.

The events in Fig. 3 on 13 October 2012 (3 s auroral movie in Additional file 5) are the closest together in time of our identified onsets. The activity following the 0739 UT onset was localized in longitude to < 1 h of MLT and was followed by the 0744 UT onset that expanded after the activity for the 0739 UT onset had faded away. While the auroral activity with the first onset has all the characteristics of a substorm and expanded poleward by a substantial ~ 3° in latitude, it did not reach the auroral poleward boundary and would thus be formally classified as a pseudo-breakup. The streamer identified prior to the first onset tilted equatorward to almost the onset location as can be seen in 0738:33 image in Fig. 3. A new tilted streamer first became visible at ~ 0742 UT just poleward and east of the expansion phase activity of the first onset, and its eastern edge then tilted equatorward to near the location of the second onset.

Of the 60 onsets in Table 1 that were observed well by the ASIs, we identified 45 for which a tilted streamer appeared to trigger the onset as in the examples in Figs. 1 and 3. For the 14 of the remaining, we identified what we call a “Harang streamer” with its leading edge moving to very near the onset location as illustrated by the example on 2 March 2011 in Fig. 4 and the 3 s movie in Additional file 6. (For one case, onset was clearly observed by we could not determine if it were a tilted or Harang streamers due to tree FOV interference.) In the 2 March 2011 example, a brightening developed at ~ 0510 UT at the westward edge of an arc along the auroral poleward boundary. This PBI quickly developed a Harang reversal shaped streamer. This Harang streamer then expanded with time with its equatorward portion moving equatorward by ~ 5° in latitude until it became very near the diffuse auroral band in the equatorward portion of the oval and led to a substorm onset. Substorm expansion followed, the expansion phase activity appearing to be continually fed by the new, intruding plasma flowing adjacent to the auroral streamer. (While the expansion phase activity for this event became broad in azimuthal coverage, ~ 3 h in MLT, and persisted for ~ 8–10 min, the activity never reached the auroral poleward boundary, so it would be formally classified as a pseudo-breakup.)

The example in Fig. 4 shows that Harang streamers roughly following the large-scale ionospheric flow pattern around the Harang flow shear (e.g., Gkioulidou et al. 2009) of the duskside convection cell and have been identified by Nishimura et al. (2010a) as a type of streamer that can trigger substorm onsets. Our criteria for distinguishing Harang streamers from tilted streamers were based on visually identifying a turning of the streamer to the west before its apparent triggering of an onset. Despite our categorizing of the triggering streamers as either tilted or Harang, the poleward portion of all our identified streamers moved toward the east as they titled equatorward in their poleward portion. Such a tilting is commonly seen for streamers (Zesta et al. 2002, 2006). It is interesting that we detected a higher percentage of substorm onsets that appear to have been triggering by Harang streamers during HSS storms (19%) than during CME storms (12%), though it is not readily clear whether this difference is significant.

In this paper, we have tested the Nishimura et al. (2010a) scenario for substorm onset triggering by the flow bursts in the plasma sheet that are often manifested in the ionosphere as auroral streamers. We have taken advantage of the broad coverage of the ionospheric mapping of the plasma sheet offered by the THEMIS ASI network to detect substorm onsets and pre-onset streamers, and chosen substorms during geomagnetic storms because of the brighter than normal auroral emissions that occur during storms. We selected storm main phases (9 CME related and 9 HSS related) having good viewing over North America with the THEMIS ASI array.

We observed a streamer heading to near the substorm onset location for all 60 onsets in Table 1 that we identified and were observed well by the ASIs. This is consistent with substorm onsets being very often triggered by the intrusion of plasma with lower entropy than the surrounding plasma to the onset region. However, we note that our auroral observations do not give conclusive proof that the large majority of substorm onsets are triggered by the streamer scenario, since we are not able to directly observe the intrusion of the lower entropy plasma and thus cannot prove that all the streamers we observed were associated with intrusion of lower entropy plasma to the onset region. Direct tests of the intrusion of new plasma to the onset region can be performed using spacecraft (Xing et al. 2010), though sparse coverage of the plasma sheet by spacecraft that makes opportunities for such a test is very limited. Radar observations can also be used, and can allow for direct detection of the ionospheric mapping of flow bursts to the onset region (Nishimura et al. 2014).

It is important that the poleward portion of all the identified streamers tilted equatorward as they extended eastward, consistent with them being guided by the large-scale flow poleward of the Harang reversal. The majority are what we have referred to here as tilted streamers, where they continue to extend eastward as their eastern tip moves equatorward to near the substorm onset location. However, there were 14 of the 60 cases, identified as Harang streamers, where the streamer discernibly turned toward the west poleward of its reaching to near the onset latitude, indicating flow around the Harang reversal. Whether or not tilted streamers are as common during non-stormtimes is not addressable by the current study.

By selecting substorms during both CME and HSS storms, we have been able to check substorm onset occurrence rates during each type of storm. We observed substantially less substorm onsets for CME storms than for HSS storms, this difference being a factor of four between the four CME events having the smoothest southward IMF Bz during the period of good ASI viewing and the four HSS storms with the best combination of good ASI viewing and fluctuating IMF 9 (Fig. 2). This result is inconsistent with Liou et al. (2017). A plausible reason for this discrepancy is their using only the AL index to identify substorms. Specifically, they identified substorms as sharp drops in the SuperMAG AL index by an average amount of 100 nT that persist for 25 min. It is apparent from the SuperMAG AL index for our CME events in Fig. 2 that there are more such drops that are not associated with identified auroral substorms than drops that are associated. We attribute this to the common stormtime occurrence of aurora streamers, which are associated with substantial ground magnetic depressions that are localized to the longitudes of the streamers. This was found to be very common in the detailed study of the 17 March 2013 storm main phase (Lyons et al. 2016), and streamers without substorms have been found to give very clear substorm-like signatures in ground magnetic field observations (Lyons et al. 2013). It would be interesting to check this possibility for the main phases considered here using the THEMIS ASIs and the longitudinally distributed magnetic field measurements from individual stations available from SuperMAG. Additionally, our results from the four CME events with the most steady, strong southward IMF (Fig. 2) are not consistent with the ~ 2–4 h repetition of substorms that has been suggested for moderate to strong southward IMF conditions. Instead, our results are consistent with the findings of Tsurutani et al. (2003) that substorms occurrence is low during such conditions. Our results also support that inferences of Kim et al. (2008) that substorms occur frequently during HSS period due to the highly fluctuating IMF.

This paper has dealt only with the substorm onset process, and our results give no information on what controls the ensuing expansion phase development. Some or our onsets (10 of 80, as indicated in Table 1) led only to a pseudo breaks, some expanded only to the preexisting poleward boundary of the oval, while others expanded poleward (some far poleward) into the preexisting polar cap. Also, some onsets lead to expansion phase activity that was quite localized in longitude, whereas the expansion phase activity of others traversed several hours of nightside MLT. Additionally, expansion phase activity after onset continued for a wide range of time periods, ranging for several minutes to tens of minutes. Our results suggest that the onset process may be similar for this variety of ensuing substorm expansions, which would make it interesting to consider what controls the substorm expansion characteristics.