Technical Comments

Comment on “Tail Reconnection Triggering Substorm Onset”

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Science  12 Jun 2009:
Vol. 324, Issue 5933, pp. 1391
DOI: 10.1126/science.1167726

Abstract

Angelopoulos et al. (Research Articles, 15 August 2008, p. 931) reported that magnetic reconnection in Earth’s magnetotail triggered the onset of a magnetospheric substorm. We provide evidence that (i) near-Earth current disruption, occurring before the conventional tail reconnection signatures, triggered the onset; (ii) the observed auroral intensification and tail reconnection are not causally linked; and (iii) the onset they identified is a continuation of earlier substorm activities.

Angelopoulos et al. (1) reported that a magnetospheric substorm was initiated by reconnection in Earth’s magnetotail based on observations from NASA’s Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. Here, we reevaluate the evidence focusing on the event analyzed in the main text of (1). Figure 1A shows the relative positions of THEMIS satellites for that event. Let us first examine the conventional signatures of reconnection during this interval. By conventional signatures, we mean Earthward plasma flow with positive Bz (the north-south component of the magnetic field) in locations Earthward of the reconnection site and tailward plasma flow with negative Bz in locations tailward. Because only convective plasma flows, that is, flows perpendicular to the magnetic field, are relevant to magnetic reconnection and transport of magnetic flux to cause current disruption/dipolarization in the near-Earth region, only the convective flow components are examined here. Figure 1, B to G, shows the X component of the convective plasma flows Vx and the magnetic field elevation angles λB from probes P1, P3, and P4 near the substorm onset time on 26 February 2008 identified in (1). The inner satellites P3 and P4 detected Earthward flows ahead of tailward flows at the outer satellite P1 and before the onset (04:54:00 UT) of the THEMIS auroral electrojet index (AETH), an AE index constructed from THEMIS ground magnetic stations. Current disruption/dipolarization signifies magnetospheric current change and should be matched with ionospheric current change, that is, AE activity. If one attributes the appearance of tailward flows at P1 as reconnection onset, then it occurred after substorm disturbances close to the Earth. This time sequence is consistent with the near-Earth current disruption (CD) activated before tail reconnection further downstream, suggesting substorm onset triggered by CD and not tail reconnection. Examination on the change of λB at these satellites reveals a similar time sequence. Appreciable magnetic field configuration changes (current disruption/dipolarization) associated with the appearance of discernible plasma flows occurred first near P4 and P3, before P1.

Fig. 1

(A) Alignment of five THEMIS satellites in the magnetotail during the substorm event on 26 February 2008 using the Geocentric Solar Magnetospheric (GSM) coordinate system: P1, XGSM = –21.5 RE; P2, XGSM = –17.2 RE; P3, XGSM = –10.9 RE; P4, XGSM = –10.2 RE; P5, XGSM = –5.5 RE. (B to D) The X component of convective plasma flows at P1, P3, and P4 satellites around event onset time. The red dashed lines mark the onset of substorm activity at the satellite. (E to G) The magnetic field elevation angle at P1, P3, and P4 satellites around event onset time. (H and I) The temporal variations of the Z components of the convective plasma flow and the magnetic field at P1 around the event onset time, showing the variations of Vz and Bz for the outer satellite P1 over a longer time interval encompassing the reconnection onset signature identified in (1) as marked by the label MR (magnetic reconnection). The red vertical dashed lines mark the three onsets of Vz > 0 and Bz < 0.

The features at P1 adopted by Angelopoulos et al. (1) as the first sign of reconnection at 04:50:28 UT are Vz > 0 and Bz < 0, which are not the conventional signatures. The justification is that the plasma was moving to the neutral sheet concurrent with the negative Bz development, which is interpreted as the occurrence of reconnection Earthward of P1. However, an equally likely alternative interpretation is that these are signatures of plasma sheet thinning at P1 concurrent with an even thinner near-Earth plasma sheet, producing southward dipping of the magnetic field, as documented previously (2). Figure 1, H and I, show that there were at least two other similar changes at P1 shortly before 04:50:28 UT, namely, at 04:44:19 and 04:47:16 UT. These earlier occurrences of the same features raise doubt on the causality of tail reconnection onset and auroral intensification onset at 04:51:39 UT. It is quite likely that reconnection onset and substorm onset are not causally related in this event, that is, that they are independent phenomena. This assessment is reinforced by noting that the required communication time (96 s) is shorter than the Alfvén transit time between the P1 location and the ionosphere, ruling out the possibility of field-aligned current connecting the two phenomena. The difficulty with assuming that electrons from reconnection are responsible for the connection is that the observed electron flux presumably ejected from reconnection shown in figure 4, C and D, in (1) is far too low [< 0.02 erg/(cm2·s·sr)] by two orders of magnitude to account for the observed auroral intensity. Finally, current disruption/dipolarization associated with substorm current wedge (SCW) development occurred at P3 and P4, ~10 RE (Earth radius) Earthward of the reconnection site. This implies that the SCW would occur considerably equatorward of the brightening auroral arc, a feature that has never been seen in an isolated substorm.

The reconnection signatures presented in (1) are not compelling. Although there was a positive excursion of the Bz component at P2 around that time, an expected feature Earthward of the reconnection site, there was a persistent tailward plasma flow unaffected by the positive excursion of the Bz component. This is inconsistent with the expected plasma flow signature for reconnection. This discrepancy from the expected reconnection signatures at P2 is shown with the corresponding P1 observations (Fig. 2, A to D).

Fig. 2

(A and B) Time evolution of the total plasma flow and the magnetic field for P1 [adapted from figure 4, A and B, in (1)]. (C and D) Time evolution of the total plasma flow and the magnetic field for P2 [adapted from figure 4, F and G, in (1)]. (E to H) The X component of convective plasma flows observed by four THEMIS satellites during a 3-hour time interval encompassing the substorm interval examined in (1). (I) The corresponding official auroral indices from the Kyoto University World Data Center. Dashed line marks the onset of a substorm intensification after a substorm onset at ~04:02 UT. (J) The magnetic perturbations at the Leirvogur magnetic station: H, D, and Z components are the horizontal, declination, and vertical components, respectively. (K) A sketch to show a possible time sequence of substorm activity observed in this event and the positions of four THEMIS satellites relative to these disturbance sites. The green arrows show the convective plasma flows. Current disruption onset in the near-Earth region precedes the AETH onset on the ground by 95 s. This initial disturbance instigates current disruption in multiple sites that develop at progressively further downstream distances. Reconnection occurs in one of the current disruption sites in the midtail, preceding the AETH onset by 17 s.

Moreover, there is evidence that the substorm onset identified in (1) is simply an intensification of a substorm that started earlier. Figure 2, E to H, shows plasma flow activities at THEMIS satellites, whereas Fig. 2, I and J, show the official auroral indices AU/AL and Leirvogur magnetic activities. There were two temporally separated plasma flow activities at P1, P3, and P4, which were parts of a continuous substorm interval, as indicated by the ground magnetic activities, particularly the z component at Leirvogur. Therefore, the 04:51:39 UT onset time inferred in (1) from the Gilliam all-sky imager (ASI) was not an onset time of an isolated substorm. Intensification activity is often disconnected in longitude from the substorm onset longitude (3, 4), and Gilliam was probably not at the right local time to detect the auroral activity at substorm onset. For this earlier substorm onset, close examination indicates that the onset of convective plasma flow also started at P3 and P4 before P1 and P2.

In summary, we have pointed to several potential shortcomings of the conclusion reported by Angelopoulos et al. (1). First, based on the conventional reconnection signatures observed by THEMIS satellites, substorm disturbances occurred in the near-Earth region before the tail reconnection. Second, the unconventional signatures used in (1) to identify reconnection onset can be interpreted alternatively as plasma sheet thinning, with the inner tail being thinner than the midtail. Third, the direct connection between tail reconnection onset and auroral intensification has difficulties in establishing a credible physical link, suggesting that the two phenomena are possibly not causally related. Overall, we infer the following time history of events. Near-Earth CD triggers the substorm onset, leading to the formation of a SCW. This initial disturbance instigates CD in multiple sites that develop at progressively further downstream distances. Reconnection occurs at one of the CD sites in the midtail (Fig. 2K). This sequence is in agreement with several previous studies and the inside-out model of substorm onset (415).

References and Notes

  1. This work was supported by NSF grant ATM-0630912 and NASA grant NNX07AU74G to The Johns Hopkins University Applied Physics Laboratory.
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