2013 ARTEMIS SCIENCE NUGGETS

Identifying the magnetotail source region leading to pre-onset poleward boundary intensifications

by Y. Nishimura, UCLA AOS


Introduction

The sequence of events leading to substorms has been debated over the decades in spite of the well-documented phenomenology of the magnetotail configuration change and auroral activations. It is critical to precisely determine the event sequence using multi-point observations, and a number of studies showed that enhanced plasma flows are present prior to substorm onset. Recent detailed timing studies in conjunction with auroral observations gave further evidence that flow bursts form and propagate toward the near-Earth plasma sheet prior to substorm auroral onset, and then lead to substorm onset instability that can be identified as magnetic field fluctuations in the near-Earth plasma sheet and initial brightening of an auroral arc. Our previous investigations showed this sequence using 2D auroral observations. Their important finding is of precursor PBIs followed by streamers propagating toward the substorm onset location, strongly suggesting that enhanced plasma flows from the deep magnetotail initiate substorm onset instability in the near-Earth plasma sheet.

This precursor auroral and flow activity leads to the important question of the magnetotail source location of the precursor plasma sheet flow leading to the pre-onset PBIs and streamers and to onset. This is critical for determining the magnetotail structure associated with the precursor PBIs, and is essential for understanding the initiation of the substorm triggering sequence. Since PBIs are associated with transient flows crossing the open-closed field line boundary, the flow burst initiation process likely involves magnetotail reconnection. Two magnetotail reconnection regions, the near-Earth neutral line (NENL) and distant neutral line (DNL), have been discussed extensively and can be considered as possible source regions of flow bursts. Although a large number of studies on pre-onset flows in the plasma sheet have been performed, those studies are mostly limited to within ~30 RE downtail and thus do not distinguish if earthward flows originate from the NENL just outside the spacecraft location or from further out in the distant tail.

The ARTEMIS spacecraft provide magnetotail observations in conjunction with the THEMIS all-sky imager (ASI) array and survey beyond 45 RE, which is tailward of the nominal NENL location (~20-30 RE) but earthward of the DNL (~100 RE). The THEMIS ASIs have wide spatial coverage and high spatial and temporal resolution that are capable of detecting substorm auroral onset and pre-onset auroral activity. We use this space-ground conjunction to determine if enhanced flows associated with pre-onset PBIs and streamers originate from earthward (mid-tail source) or tailward (distant-tail source) of ARTEMIS.

Results

Figure 1 shows an ARTEMIS-ground conjunction event on 17 April 2011. The ASI snapshots on the right depict a pre-onset PBI/streamer and substorm auroral onset. The ARTEMIS-C footprint (green square, using T96 for a rough estimation) is located close to the PBI and onset meridians. The auroral keogram that covers the poleward portion of the auroral oval (Figure 1a) shows a series of PBIs and streamers that started ~30 min prior to the onset. Two of them (~0555 and 0605 UT) were moderately intense, and the second PBI was followed by a streamer propagating equatorward and led to a substorm auroral onset at ~67° MLAT at the time marked by the vertical line (0610 UT). The onset can be identified as an abrupt intensification starting from a very quiet background (Figure 1b). Although the auroral expansion and ground magnetic field perturbation are small, this event was associated with Pi2 pulsations and small positive bay (Figure 1h). This pre-onset auroral sequence, which involves a precursor PBI and streamer moving toward onset location, is consistent with the idea presented previously.

Figure 1. Auroral keograms and magnetometer data during the 17 April 2011 substorm. (Left) Keograms from the (a) FSMI, and (b) GILL ASI stations, ARTEMIS-C observations of the (c) magnetic field, (d) perpendicular flow moment, (e) E×B velocity, (f) parallel flow moment (positive parallel to the magnetic field) and (g) pressures (thermal, magnetic and dynamic), and (h) the ground magnetic field at Pinawa (PINA). (Right) Selected snapshots of THEMIS ASIs data. The blue line marks the magnetic midnight.

ARTEMIS-C was located at 46 RE downtail and near the center of the plasma sheet as can be inferred from small Bx (Figure 1c) and high Β (~10, Figure 1g) prior to the substorm onset. The plasma flow speed was initially close to zero, and then the V⊥x component increased suddenly at ~0540 UT and remained elevated for ~30 min. The E×B flow velocity varied quite similarly to the plasma flow moment, indicating that this flow burst represents plasma transport. The flow burst started almost simultaneous with the PBIs. The flow speed was not constant but (E×B)x increased twice at ~0550 and 0604 UT. These times are very close to the times of the two moderately intense PBIs mentioned above, suggesting that the flow bursts at the ARTEMIS-C location are the magnetotail counterpart of the pre-onset PBIs.

This paper identified two more supporting cases, and one of those is shown in Figure 2. The footprint of ARTEMIS-C was located near the pre-existing bright auroral form that covers the poleward portion of the auroral oval of the pre-midnight sector. The keogram in Figure 2a shows multiple intensifications on the poleward edge of this auroral form (i.e., PBIs) that extend equatorward into the Figure 2b keogram as streamers. Other than those streamers, the equatorward portion of the auroral oval was dominated by a dim diffuse auroral band at ~65° MLAT until the onset time. The onset is identified as an abrupt auroral intensification just poleward of the diffuse auroral band followed by poleward expansion. The H component of the ground magnetic field (Figure 2h) started to decrease a couple of minutes after the initial brightening. Before the onset time (0509 UT, marked by the black line), a small intensification occurred at 0506 UT and ceased quickly. We are not sure if this is the onset or separate activity such as a pseudo-breakup. However, both intensifications were preceded by the streamer at 0502 UT, so that this uncertainty of the onset time does not affect the pre-onset auroral sequence for this event.

Figure 2. 11 March 2010 substorm data. The format is the same as in Figure 1.

ARTEMIS-C was located at 52.3 RE downtail close to, but slightly to the south of, the center of the plasma sheet based on the Bx polarity. ARTEMIS-C detected an isolated flow burst of V⊥x ~ 200 km/s directed earthward during the pre-onset PBI, and the E×B flow was in the same direction as the flow moment but larger (~800 km/s), indicating that the flow moment is influenced by non-E×B effects but the direction is properly measured. This flow burst occurred simultaneous with one of the streamers (~0502 UT) and thus is likely to be the magnetotail counterpart of the streamer. The other streamers do not have associated flows at ARTEMIS-C probably because ARTEMIS was not sufficiently close to the neutral sheet (Β < 1) or because the spacecraft was located azimuthally away from the flow channels. The earthward flow direction suggests that the flow burst leading to the streamer and onset originated from further downtail of ARTEMIS.

Conclusion

We examined the magnetotail flow direction associated with pre-onset PBIs and ensuing streamers using ARTEMIS-ASI conjunctions in order to determine if substorm precursor activity originates from earthward or tailward of the ARTEMIS orbit. The substorm onsets were preceded by PBIs and streamers by ~10 min, and ARTEMIS located about 50 RE downtail detected flow bursts that can be considered as the magnetotail counterpart of the PBIs and streamers.

Those flows were directed earthward, and there was no significant tailward flow activity before the onsets. The earthward flow direction indicates that the pre-onset flow bursts in the plasma sheet and associated auroral activity originate from further downtail of ARTEMIS. Although we do not know the precise source location of the flows, the DNL is a more likely candidate of the pre-onset flow source region than is the NENL, which is generally thought to form earthward of ~30 RE. The rough estimations of the flow propagation time toward the near-Earth plasma sheet are consistent with the measured time lag between the flow and onset. Tailward flows were also found in these events but limited to the post-onset periods. The tailward flows likely originated from the NENL, and these are not related to the pre-onset PBIs or streamers but are important for the expansion phase activity.

The present study demonstrated that the distant tail plays a crucial role in leading to pre-onset PBIs/streamers and plasma sheet flows. While the distant tail property has been surveyed using ISEE-3 and Geotail, our understanding of its influence on the near-Earth is still quite limited. While the present study only discussed the flow direction from a single spacecraft, multi-spacecraft observations and modeling of the entropy profile will be needed for quantitative analyses of earthward penetration of flow bursts originating from the distant tail.

Reference

Nishimura, Y., L. R. Lyons, X. Xing, V. Angelopoulos, E. F. Donovan, S. B. Mende, J. W. Bonnell and U. Auster, Identifying the magnetotail source region leading to pre-onset poleward boundary intensifications, J. Geophys. Res., in press, DOI: 10.1002/jgra.50407.

Biographical Note

Toshi Nishimura is an Assistant Researcher in the Department of Atmospheric and Oceanic Sciences at the University of California, Los Angeles. His research interest is magnetosphere-ionosphere coupling during storms and substorms using satellites, auroral imagers and radars.

Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu