Multipoint observations of substorm pre-onset flows and time sequence in the ionosphere and magnetosphere

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /2011ja017185, 2012 Multipoint observations of substorm pre-onset flows and time sequence in the ionosphere and magnetosphere Yong Shi, 1 Eftyhia Zesta, 2 L. R. Lyons, 1 X. Xing, 1 V. Angelopoulos, 3 E. Donovan, 4 M. A. McCready, 5 and C. J. Heinselman 5 Received 20 September 2011; revised 15 July 2012; accepted 23 July 2012; published 6 September [1] In this paper, we use a rare and close longitudinal conjunction of the THEMIS spacecraft with the Sondrestrom radar to present multipoint observations of important features of the time sequence of substorm pre-onset plasma flows in the ionosphere and magnetotail on March 5, We found that the onset was preceded sequentially by enhanced polar cap flows heading equatorward near the polar cap boundary, and then by tail fast flows from the mid-tail to the near-earth region. We also observed in situ fluctuations in short-period Pi2 band (30 50 s) in both the magnetic field and plasma pressure during the initial couple of min of the fast flows, occurring nearly simultaneously with the dipolarization and the fast flows and propagating earthward. Our results suggest that these fluctuations may be triggered by the fast flows, and may play an important role in the substorm onset process. Our event suggests that localized tail reconnection may be triggered by the enhanced polar cap flows, though the reconnection location cannot be unambiguously determined. Earthward fast flows are generated as a result of the tail reconnection and reach the inner magnetosphere to initiate the substorm onset. The presented case is consistent with that predicted by the Nishimura et al. (2010a) scenario. Citation: Shi, Y., E. Zesta, L. R. Lyons, X. Xing, V. Angelopoulos, E. Donovan, M. A. McCready, and C. J. Heinselman (2012), Multipoint observations of substorm pre-onset flows and time sequence in the ionosphere and magnetosphere, J. Geophys. Res., 117,, doi: /2011ja Introduction [2] Two major categories of substorm onset models have been proposed in terms of the temporal sequence of relevant activities that lead to substorm onset during the past 40 years [e.g., Lui, 1996; Baker et al., 1996], and have been under strong debate. One is commonly called the inside-out scenario [Ohtani, 2004], which starts from a near-earthregion instability and then propagates further tailward to trigger mid-tail activities, e.g., the current disruption (CD) model [Lui, 1996]. In the outside-in scenario [Ohtani, 2004], substorm onset is preceded first by mid-tail activities that later cause near-earth activity as they propagate 1 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA. 2 RVBXP, Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts, USA. 3 Institute of Geophysics and Planetary Physics and Department of Earth and Space Sciences, University of California, Los Angeles, California, USA. 4 Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada. 5 Center for Geospace Studies, SRI International, Menlo Park, California, USA. Corresponding author: Y. Shi, Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095, USA. (yongshi@atmos.ucla.edu) American Geophysical Union. All Rights Reserved /12/2011JA inward, e.g., the near-earth neutral line (NENL) model [Baker et al., 1996, and references therein]. The two fundamental magnetotail processes that are believed to be the source starting point for each of the two scenarios are CD in the near-earth region and magnetic reconnection in the midtail region, respectively. [3] The lack of key observations made it nearly impossible to prove one scenario over the other. Since the launch of the THEMIS mission in 2007, multiple works have surfaced with observational evidence for both scenarios. Studies that support the inside-out scenario were primarily based on the ionospheric observations showing that the initial auroral brightening occurs on a growth phase arc deeply equatorward of the open-closed field line boundary without simultaneous observing of any auroral activity further poleward [e.g., Donovan et al., 2006, 2008; Henderson, 2009; and relevant references therein]. In particular, Donovan et al. [2008] showed for one case that the auroral onset preceded in situ observations of the THEMIS probes as well as outward propagation of these signatures. On the other hand, taking the advantage of the radially aligned THEMIS probes, Angelopoulos et al. [2008] showed evidence of mid-tail reconnection and ensuing earthward fast flows occurring prior to the auroral substorm onset, in support of the outside-in scenario. Similar time sequences were also reported, for example, by Sergeev et al. [2008] and Lin et al. [2009], though Lin et al. [2009] attributed their results to neither the outside-in nor inside-out scenarios. 1of18

2 [4] More recently, based on the high resolution THEMIS all-sky imager (ASI) auroral data during 2008 and 2009, Nishimura et al. [2010a] found that there generally exists a repeatable time sequence showing that auroral substorm onsets are preceded sequentially by poleward boundary intensifications (PBIs) and the ensuing equatorward-moving north-south-aligned auroral structures, i.e., auroral streamers. Based on the previous suggestion that enhanced ionospheric flows near the polar cap boundary and PBI signatures are associated with localized enhancement of the reconnection rate along the nightside outer plasma sheet boundary [de la Beaujardière et al., 1991, 1994; Blanchard et al., 1996; Lyons et al., 1999; Hubert et al., 2006], and the relationship of equatorward-extending streamers with tail fast flow channels [e.g., Henderson et al., 1998; Sergeev et al.,1999; Lyons et al., 1999; Zesta et al., 2000, 2002, 2006], Nishimura et al. [2010a] suggested that substorm onset is preceded by enhanced earthward plasma flows associated with enhanced reconnection near the pre-existing open-closed field line boundary. The auroral observations of Nishimura et al. [2010a] indicated that earthward plasma flows also lead to an onset instability in the near-earth space. [5] The Nishimura et al. [2010a] scenario includes aspects of both the outside-in and inside-out scenarios, having tail reconnection first (like in the outside-in scenario) and a near-earth instability preceding substorm onsets (like the inside-out scenario). Based on the observations that the whole time sequence initiates from PBIs, Nishimura et al. [2010a] claimed that the Earthward-transported plasmas are from the open-closed field line boundary and hinted that they may be from distant reconnection. However, Nishimura et al. [2010a] did not have corresponding in situ tail observations and the tail reconnection location cannot be unambiguously determined from ionospheric observations, especially under highly stretched tail conditions prior to substorm onsets. Additionally, Nishimura et al. [2010a] did not show any tail activity tailward of the onset location following the onset. Therefore, three primary aspects remained uncertain in Nishimura et al. [2010a]: (1) the reconnection location in the tail; (2) what type is the claimed instability in the inner magnetosphere; (3) what happens after onset. [6] Follow-up works to Nishimura et al. [2010a] provide some cumulative evidence of this proposed substorm preonset time sequence. For example, Lyons et al. [2010a] used Sondrestrom Incoherent radar observations to show that there generally exists strong enhanced ionospheric flows, associated with PBIs, heading toward the polar cap boundary (and likely crossing that boundary) prior to substorm onsets, and such flow disturbances have also been seen by THEMIS spacecraft near the outer plasma sheet boundary [Angelopoulos et al., 2008, 2009; Lyons et al., 2010b]. Xing et al. [2010] showed in several cases, as well as Nishimura et al. [2010b] in one case, that tail fast flows with lower entropy occur nearly simultaneously with pre-onset auroral streamers that are close to the ionospheric foot points of the THEMIS probes. However, each work only addressed an individual aspect of the whole sequence, namely enhanced polar cap flow heading toward the polar cap boundary prior to onsets, or single point measurements of pre-onset earthward flows. The more inclusive (or complete) sequence showing pre-onset enhanced ionospheric flow near the polar cap boundary and the following earthward fast flows moving from the mid-tail to the near-earth plasma sheet has only been briefly reported by Zesta et al. [2011]. [7] In the present paper, we use a rare and close longitudinal conjunction of the THEMIS spacecraft with the Sondrestrom radar to present a multipoint, comprehensive set of observations of important features of a substorm pre-onset time sequence in both the ionosphere and magnetosphere on 5 Mar, The THEMIS probes provide the in situ flow measurements in the tail. The Sondrestrom radar is located at invariant magnetic latitude L of 74, which is close to the typical auroral poleward boundary as determined by Blanchard et al. [1995], with magnetic local midnight at 02:25 UT. The radar is used to identify the pre-onset flows near the polar cap boundary as well as the associated PBI signatures indicated by the enhancement of E region electron density. Ground all-sky imagers (ASIs) and ground magnetometer observations complement the radar and in situ observations. [8] We focus on the details of the onset timing sequence during this event, and we show evidence that the substorm onset was preceded sequentially by enhanced polar cap ionospheric convection flows that heading equatorward near the nightside polar cap boundary, and then by tail fast flows from the mid-tail to the near-earth region. Based on the known relationship of the enhanced polar cap flows near the polar cap boundary and the localized enhancement of reconnection rate along the outer plasma sheet boundary as discussed above, our results indicate that the substorm onset was preceded by localized magnetic reconnection somewhere in the tail, though we were not able to unambiguously determine whether it was a mid-tail reconnection or a distant tail one solely based on the ionospheric observations. We also observe in situ fluctuations in short-period Pi2 band (30 50 s) in both the magnetic field and pressure during the initial couple of min of the onset of the fast flows, with magnetic and thermal pressures being out of phase, occurring nearly simultaneously with the dipolarizaton and earthward fast flows. These could be possible signatures of some type of instability triggered by the fast flows that may play an important role in leading to the substorm onset. The time sequence shown in our event is consistent with that predicted by the Nishimura et al. [2010a] scenario. Our results also suggest that the whole process was very likely initiated by the enhanced polar cap flows that lead to field (or flow) perturbations that propagate tailward along the entire length of the plasma sheet boundary layer (PSBL). The perturbations, when they approach somewhere in the tail, may play an important role in triggering the localized reconnection there that is responsible for the following earthward fast flows and substorm onset. In section 2 we show ground and in situ observations sequentially, section 3 is a discussion of our results and we summarize in section Observations [9] On March 05, 2008, a weak substorm with maximum AE index of 200 nt occurred over Hudson bay around 02:00 UT [Zesta et al., 2011]. During the period of the substorm, the Sondrestrom radar was running in an equatorward scanning mode [see Lyons et al., 2010a, and references therein], which provides flow vectors every 2.5 min at locations equatorward of the radar, and the radar run was 2of18

3 Figure 1. (a) The locations of Sondrestrom radar and foot points of the five THEMIS probes at 02:00 UT as obtained from the T89 model. The magnetometers that will be used in the following analysis are also marked. The black solid line represents the magnetic local midnight and the red solid line indicates the inferred central meridian of the substorm current wedge. (b) THEMIS probe locations in X-Y plane. (c) THEMIS probe locations in X-Z plane. scheduled so that the radar location was close to the magnetic local times (MLTs) of the ionospheric footprints of the THEMIS probes. The five probes were approximately radially aligned within the central plasma sheet (CPS), and located just on the dusk side of the midnight meridian (see Figures 1b and 1c). The footprints of the five probes in the ionosphere at 02:00 UT, obtained from the T89 model [Tsyganenko, 1989], are shown in the map of Figure 1a. The locations of the radar, THEMIS all-sky imagers (ASIs) and magnetometers that will be used in the following analysis are also labeled. The black dotted lines are geographic latitude and longitude circles, while the blue solid lines are lines of constant geomagnetic latitude. The black solid line on the right-hand side of the map is the magnetic local midnight meridian, while the solid red line is the inferred central meridian of the substorm current wedge (SCW) determined from ground magnetometer observations in the later analysis. The radar was within 1 to 1.5 h of MLT from the THEMIS probes P1-P4 and was located east of the onset region that was in the vicinity of SNKQ. [10] The high temporal and spatial resolution of the THEMIS ground and in situ observations provide an opportunity to do accurate substorm timing for this case. In the following sections, we first determine the substorm onset on the ground and in the inner magnetosphere, respectively. Based on these times, a pre-onset time sequence for this case is then obtained by combining the radar and THEMIS tail observations Ground Onset [11] In this Section, we determine the substorm onset time and location using ground ASI and magnetometer data Substorm Onset Timing [12] Figure 2 shows a series of composite auroral mosaics from the THEMIS ASIs at SNKQ and GILL for the 0200: :27 UT time period of the substorm onset. The white dotted lines are the geographic latitude and longitude circles. Unfortunately, it was cloudy at the next auroral station to the east, KUUJ, which is nearest Greenland. We also include an animation of the auroral sequence with the same format as Figure 2 from 01:40 UT to 02:11 UT as auxiliary material. 1 Two major auroral intensifications can be seen at 01:45 UT and 02:01 UT in Animation S1, that are identified, respectively, as a pseudobreakup and the onset of 1 Auxiliary materials are available in the HTML. doi: / 2011JA of18

4 Figure 2. A series of composite auroral mosaics from the THEMIS ASIs at SNKQ and GILL before and after the substorm onset. a full substorm based on the poleward auroral expansion for the second, but not the first event. [13] Animation S1 shows that, before the substorm onset, there was auroral brightness continuously entering the fov of SNKQ from the east that kept moving to the west across the fov of SNKQ. Such auroral motion is the expected signature along the poleward edge of the duskward flow that lies equatorward of the Harang reversal. Such flow initially comes from the more distant plasma sheet and then turns duskward as it moves through the flow shear of the Harang reversal [Zou et al., 2009a; Nishimura et al., 2010a]. [14] In Figure 2, starting at 02:01:09 (3 s) UT (Figure 2b), auroral intensifications can be seen to the east of the center of the fov of SNKQ. This intensification expanded poleward and westward and by 02:01:42 UT (Figure 2c), the aurora had expanded to its poleward-most boundary at 70 geomagnetic latitude and reached its maximum intensity. The intensity of the aurora then decayed markedly and rapidly to nearly the pre-intensification level within tens of seconds as shown in Figure 2d. About 5 s later, at 02:02:00 UT (Figure 2e), new arc intensifications appeared to the east of the center of the fov of SNKQ. Continued decay of the 02:01:09 UT activity can still be seen in this image within the western portion of the SNKQ fov. The new intensification formed a new arc poleward of the prior intensification. Auroral breakup and poleward expansion of that arc can be seen in the following images (Figures 2f and 2g), which are typical signatures of a substorm expansion onset. [15] Figure 3 shows, from the top to bottom, the integrated auroral intensity over the entire fov of SNKQ ASI, ground north-south (H) component from magnetometers around the auroral onset location (see Figure 1 for locations), and the band-pass-filtered (10 s to 80 s) H component for those stations. The two successive intensifications seen in Figure 2 can be seen as increases of the integrated aurora intensity (Figure 3a) initiating at 02:01:15 UT and 02:02:10 UT (the two vertical lines), respectively. However, the first intensification dropped within 1 min to nearly the background value as given by the mean integrated intensity between 01:45:00 UT and 02:02:00 UT and indicated by the horizontal blue line. The exponential increase of intensity starts from the second vertical line and developed into a substorm expansion phase. Figure 3b shows a clear aurorallatitude negative bay at KUUJ at 02:02:05 UT and midlatitude positive H bay at DRBY, RMUS, and LOYS at 02:01:52 UT, respectively. These signatures are close to the time of the initiation of the major auroral intensification seen at 02:02:00 UT by SNKQ. There was a small H decrease at KUUJ at 02:01:03 UT prior to the larger drop at 02:02:05 UT, and the growth and decay of these H perturbations tracks the integrated aurora intensity seen a SNKQ (Figure 3a) quite well. The two vertical lines in Figure 3b identify the initial H decrease at KUUJ at 0201:03 UT and the onset of midlatitude positive bays at 0201:52 UT, respectively. [16] The first arc brightening and associated H perturbations at 02:01:09 UT have characteristics of a pseudobreakup as described by Nakamura et al. [1994], and the second intensification at 02:02:00 UT and the associated H decrease at KUUJ and positive H bays at lower latitudes have characteristics of the onset of a substorm. However, the interval between the two intensifications is less than one minute. Thus the two intensifications could also be viewed as two intensifications of a single substorm, with a small initial intensification being followed by the major intensification that leads to the full expansion. Our following analysis is not dependent on distinguishing between these two alternatives. [17] The onset of midlatitude Pi2 pulsations with period of s is known to be associated with substorm onset [Saito, 1969] and has often been used as such an indicator. Mende et al. [2007] reported that the Pi2 pulsation onset identified by the THEMIS ground magnetometers occurs 40 s after the auroral onset determined by the THEMIS ASI array. Milling et al. [2008] found that the onset pulsation signatures span the short-period Pi2 band (24 96 s) and the long-period Pi1 band (12 48 s), and Angelopoulos et al. [2008] used a band filter of s to identify the Pi2 onset as well as the substorm onset in order to reduce aliasing. We used a s band-pass filter to isolate the onset Pi2s shown in Figure 3c. Midlatitude Pi2 pulsations started at 02:01:42 UT (second vertical line in Figure 3c) at DRBY, LOYS and RMUS, which corresponds well with the related positive H bays at these stations and the major auroral intensification. [18] Note that the Pi2 onset at the auroral stations SNKQ and KUUJ started at 02:01:03 UT (first vertical line in 4of18

5 Figure 3. Stacked plots of (a) the integrated auroral intensity from the whole fov of SNKQ, (b) ground north-south (H) component from magnetometers around the auroral onset (please refer to Figure 1 for their locations), and (c) the band-pass-filtered (10 s to 80 s) H component of those stations. Figure 3c), which is about 1.0 min earlier than at the midlatitude stations. This time roughly corresponds to the initiation of the negative H perturbations at KUUJ and the first auroral intensification see at SNKQ. So, the higher latitude Pi2 is likely related to initiation of localized fieldaligned currents (FACs) as manifested by the first auroral intensification (Figure 2b). This is also as expected from the time delay between the Pi2 onsets at high and mid latitudes resulting from a high-latitude Pi2 precursor as proposed by Kepko et al. [2004] and is consistent with the observations of Angelopoulos et al. [2008]. We will show in later analysis that this first auroral intensification and the related ground signatures are consistent with being associated with initial B z pulses that occurred prior to the long-term dipolarization observed by the THEMIS P4 probe in the near-earth plasma sheet. [19] We summarize the different timings identified above in Table Substorm Onset Location [20] As shown in Figure 2, both arc intensifications occurred to the northeast of the center of the fov of SNKQ, and the westward traveling surge did not fully pass the central longitude of the fov of SNKQ until 02:04:27 UT (Figure 2h). Assuming that the auroral intensification is associated with the enhanced upward FAC of the substorm current wedge (SCW), this indicates that the westward electrojet did not pass the central longitude of the SNKQ fov until after 02:04:27 UT. Furthermore, based on the auroral images, the surge seems not to have reached the fov of GILL by that same time or even later (images not shown). These inferences from the images are supported by the H perturbations shown in Figure 3b. A positive H perturbation is 5of18

6 Table 1. Summary of the Timing Results During March 05, 2008 Pseudobreakup and Substorm Event Time (UT) F region enhanced flow crossing the polar cap boundary 01:43:00 First dipolarization and flow enhancement at P4 01:44:45 Auroral pseudobreakup 01:44:24 Ground magnetometer fluctuations 01:46:00 Reduction of polar cap flow (flow restricted to near 01:54:00 the separatrix) Flow enhancement at P1 01:58:17 Flow enhancement at P2 02:00:17 Flow enhancements and onsets of B z pulses at P4 02:00:30 Flow enhancements at P3 02:01:10 Long-term dipolarizations at P3 and P4 02:01:25 Negative H perturbations at KUUJ and Pi2 at KUUJ 02:01:03 and SNKQ 1st auroral intensification 02:01:09 Pi2 onset at midlatitude stations 02:01:42 Positive H bays at midlatitude stations 02:01:52 2nd auroral intensification 02:02:00 onset. This suggests that, at the onset time, the eastward edge of the SCW was likely located in the vicinity of KUUJ, and the westward edge was near the fov of SNKQ Tail Observations [23] So far, we have determined the time and location of the ground onset. We can see from Figure 1 that the foot points of the two near-earth THEMIS probes, P3 and P4, at 02:00 UT were located within the inferred SCW. Substorm onset in the inner magnetosphere is manifested by increase of the north-south component of magnetic field (B z ) that results from the formation of the SCW and is referred to as dipolarization. We will identify the tail onset using the observations at P3 and P4, which were located at ( 9.3, 6.02, 0.5) R E and ( 8.03, 6.37, 0.23) R E, respectively, around the onset time. [24] Figure 5 shows for P4, from top to bottom, the SST and ESA energy flux of ions and electrons, in GSM coordinates the three components of ion bulk velocity V x,v y and seen at SNKQ until 02:04:25. This is as expected from SNKQ being located to the southwest of the surge and underneath the enhanced eastward electrojet as discussed by Zou et al. [2009b, see Figure 14]. Note that the surge also tilted with a certain angle with respect to the north-south direction, which makes the Harang convection region also tilted that way. This puts KUUJ, located at nearly the same magnetic latitude but further east of SNKQ, to the northeast of the surge and underneath the enhanced westward electrojet, which would lead to negative H perturbation at KUUJ based on Zou et al. [2009b]. Substantial negative H perturbation did not begin at SNKQ until after 02:04:25 UT, which is 2 min later than that at KUUJ, resulting from the westward expansion of the westward electrojet. The further westward station GILL did not see any significant drop in H component, implying that the westward substorm expansion was limited. [21] To estimate the central longitude of the SCW, the ground east-west (D) and H perturbations of the magnetic field at subauroral and midlatitude stations are shown in Figure 4 in red and black, respectively, with stations arranged from top to bottom and from west to east (see Figure 1 for the station locations). Negative D perturbations initiated at the time of the major auroral intensification at all stations east of LOYS, implying that those stations are eastward of the SCW central meridian, and positive D perturbations initiated at that time at all stations west of LOYS, implying those stations are westward of the SCW central meridian. LOYS observed very weak negative D perturbations, indicating that the central meridian of the SCW was located very near but a little westward of LOYS (based on Clauer and McPherron [1974]). [22] We denote the estimated location of the central meridian by the red line in Figure 1, which is within the eastern portion of the SNKQ fov and just west of LOYS. This indicates the auroral and associated upward FAC onset very likely did not extend eastward of the fov of SNKQ at the time of the substorm onset. In addition, Figure 4 also shows that the H perturbation at the midlatitude station STJ is negative, which, based on Clauer and McPherron [1974], indicates that it was located eastward of the SCW at the Figure 4. The ground west-east (D) (black traces) and north-south (H) (red traces) component at stations lower than the onset latitudes as well as those subauroral and midlatitude stations. The black vertical line indicates the time of the substorm onset. 6of18

7 Figure 5. The stacked plots of plasma parameters observed P4. From top to bottom, the SST and ESA energy flux of ions and electrons, in GSM coordinates the three components of ion bulk velocity V x, V y and V z, the perpendicular velocity components V perp,x,v perp,y and V perp,z, the magnetic field B x and B z, the band-pass-filtered (10 to 80 s) magnetic field in filed-aligned coordinates with b z positive in field-aligned direction, b y positive eastward and b x finishing up the right-hand rule, the electric field E y and E x from the THEMIS EFI instrument in GSM coordinates, the plasma pressure, magnetic pressure and total pressure P p, P m and P tot and the band-pass-filtered (10 to 80 s) P p and P m, respectively. 7of18

8 V z, the perpendicular velocity components V perp,x,v perp,y and V perp,z, the magnetic field B x and B z, the band-passfiltered (10 to 80 s) magnetic field in field-aligned coordinates with b z positive in field-aligned direction, b y positive eastward and b x finishing up the right-hand rule, the electric field E y and E x from the THEMIS EFI instrument in GSM coordinates, the plasma pressure, magnetic pressure and total pressure P p, P m and P tot and the band-pass-filtered (10 to 80 s) P p and P m, respectively. The first vertical line at 02:00:30 UT indicates the onset of two short-term B z pulses about 39 s prior to the first auroral intensifications at 02:01:09 UT. Both the ion and electron energy flux panels show that before the pulses, P4 was located in the plasma sheet and closer to the central plasma sheet (or central current sheet, we will be using these two terms interchangeably) than to the plasma sheet boundary layer, except for a very short interval of 15 s prior to the pulses when P4 was likely closer to the boundary as indicated by a small dropout of plasma sheet electron fluxes at a few hundred ev to a few kev. Following the initial B z pulses, the long-term dipolarization started at 02:01:25 UT (the third vertical line), signifying the initiation of the SCW. Note that an E y enhancement started 30 s after the initial B z pulses at 02:01:00 UT (the second vertical line) and is associated with the start of strong earthward flows. [25] Corresponding to the initial flow enhancement (the first vertical line), we see substantial increase in the magnitude of short-period Pi2 band (30 50 s) fluctuations in the band-pass-filtered b z and b y as well as in the filtered P m and P p during the initial couple of min of the fast flows, suggesting that these fluctuations are closely related with the earthward fast flows. It is noteworthy that the magnetic and plasma pressure fluctuations are exactly in anti-phase. Note that the first two B z pulses decreased rapidly to the pre-onset level at 02:01:25 UT, and were then followed by a gradual and long-term dipolarization (the third vertical line) that lasted until 02:25:00 UT (see also Figure 10b). This process is consistent with the ground auroral variations shown in Figure 2, suggesting that the B z pulses are associated with the initial, short-lived auroral onset at 02:01:09 UT, while the ensuing long-term dipolarization is associated with the major onset at 02:02:00 UT. P4 also observed decrease of total pressure, though with some local enhancement, associated with the fast flow. [26] Figure 6 shows for P3 the same parameters as in Figure 5. The second vertical line at 02:01:10 UT indicates the onset of the earthward fast flows as well as the E y increase. The energy flux panels show that P3 was at, or close to the plasma sheet boundary for 3 min before the fast flow onset. It is not clear whether or not P3 entered the central plasma sheet because of the arrival of the fast flows, but it is very likely that P3 missed the onset signatures of the fast flows at the central current sheet as they approached its location. At 02:01:25 UT (the third vertical line), P3 observed a gradual and long-term dipolarization similar to that observed by P4, which is also timed to be associated with the major auroral onset at 02:02:00 UT. Similar to P4, P3 also observed a decrease of total pressure associated with this dipolarization. It is interesting that the magnetic field fluctuations as well as the pressure fluctuations (both magnetic and plasma) in short-period Pi2 band (with period of s) started at 02:00:25 UT (the first vertical line), which is tens of seconds prior to the onset of the fast earthward flows and the ensuing dipolarization, but at about the same time as when similar fluctuations were seen start at P4. Considering that P3 was not located in the central plasma sheet during this period, it appears that the fluctuations associated with the earthward fast flows deeper within the central plasma sheet are transmitted over a wider region ahead of the flows themselves, and do reach P3 at nearly the same time as they reach P4. The magnetic and plasma pressure fluctuations are also in anti-phase at P3. [27] Figure 7 is the same type of Figure 5 but for P2. It shows that P2 observed the onset of the earthward fast flows at 02:00:17 UT. The energy flux panels show that P2 was inside the plasma sheet for minutes before the fast flow but not near the central plasma sheet, as evidenced by the weaker energy fluxes and the magnetic pressure being much higher than the plasma pressure. With the arrival of the fast flow P2 moved in the central plasma sheet as evidenced by the enhancement in kev ion and electron fluxes and by the plasma pressure dominating the total pressure. The field and pressure fluctuations in short-period Pi2 band (30 50 s) are similar to the observations of P3 probe and started tens of seconds earlier than the fast flows, which in addition implies that P2 was not in the direct central path of the fast flow channel. So far, only at P4 both the onset of fast flow and fluctuations are coincidental. While P2 is nearer to the central current sheet than to the boundary layer, it is located at a more negative Z_gsm than P4, which could explain the presumed different location of the two probes with respect to the central path of the fast flow channel. The magnetic and plasma pressure fluctuations are in anti-phase at P2 as well. Corresponding to the flow enhancement, P2 measured a substantial increase in northward B z, and the total pressure significantly decreased after 02:01:00 UT. [28] Note that the locations of all the THEMIS probes in this event do not satisfy the requirement of entropy calculation using the formula of Wolf et al. [2006]. Since the total pressure reduced substantially, under a simple assumption of 1-D pressure balance in Z direction, the plasma pressure in the center of the plasma sheet is expected to be reduced. In addition, the B z enhancement represents a reduction of the magnetic field line stretching, which suggests a reduction of the flux tube volume. Thus, the total pressure decrease and the increase in B z demonstrate lower entropy in the central plasma sheet, which is consistent with the bubble model for flow channels proposed by Chen and Wolf [1993]. Although there was a general trend that the total pressure decreased after the onset of the fast flows, there were some local enhancements in total pressure that is more prominent at P3 and P4 than at P2. It is very likely that the deceleration of the flows may play a particular role at the near-earth probe locations. [29] Figure 8 is the same type of plot for P1, which was located in the mid-tail during the event. Earthward fast flows started at P1 at 01:58:17 UT, before and after which P1 was located near the central current sheet, evidenced by the dominance of plasma pressure and the constant energy fluxes. Between 01:56:30 and 02:01:00 UT, P1 measured a gradual decrease of the total pressure, and an increase of northward B z component, indicating that the fast flow channel measured by P1 had lower entropy than the surrounding ambient plasma sheet. After 02:01 UT and for the next few 8of18

9 Figure 6. The same as Figure 5, except that it is for P3. minutes the probe moved near or at the plasma sheet boundary layer. [30] In summary, P1 observed earthward flow enhancement first at 01:58:17 UT. This was followed by an earthward and duskward flow enhancement starting at 02:00:17 UT measured by P2 and at 02:01:10 UT at P3, respectively. P4, whose ionospheric foot point was located closest to the inferred ground onset location, observed onsets of B z pulses 9of18

10 Figure 7. The same as Figure 5, except that it is for P2. and dipolarizations and earthward flow enhancement starting at 02:00:30 UT. Both P3 and P4 observed a gradual and long-term dipolarization at 02:01:25 UT. These times are included in Table 1. THEMIS observations from all the probes consistently showed that there were fast flows consisted of reduced entropy plasma in the central plasma sheet moving earthward from the radial distance of P1, then to that of P2, and finally to that of the near-earth probes P4 and P3. 10 of 18

11 Figure 8. The same as Figure 5, except that it is for P1. [31] As shown above, it is very likely that P3 missed the onset signatures of the fast flows at the central plasma sheet as the flow approached its location. Although P2 was located in the plasma sheet at the time when the fast flow enhancement was observed at its location, it was still off the central plasma sheet. In addition, P2 was located at a more negative Zgsm than P4 and the field and pressure fluctuation started tens of seconds earlier than the flow enhancement. Furthermore, all the probes were not aligned perfectly in the radial direction as indicated by the 1 R E gap in Y direction 11 of 18

12 between the midtail probes and the near-earth ones. Thus, it is possible and likely that different probes observed different aspects of the flow channel (with only P4 observing the fast flow channel head-on), which makes an accurate timing calculation impossible. Using the peak perpendicular velocity V perp,x of km/s as measured by P2, we can roughly estimate that it would take s to go from R E of P1 to 8.0 R E of P4, assuming radially propagation and no deceleration. This timing is consistent with the time difference of 133 s between the fast flows first detected at P1 and then at P4. (see Table 1 for a summary of all the discussed times). Furthermore, as discussed above, at P4, the flow enhancement associated with enhanced convection electric field (E y ) actually started 30 s later than the initial flow enhancement, which gives an 163 s time delay from P1 to P4. [32] Next, to identify the possible source of the earthward fast flows, we take advantage of the conjunction with the Sondrestrom radar to examine what happen around the polar cap boundary before the onset and to look for evidence that the fast flows initiated near the polar cap boundary Sondrestrom Radar Observations [33] Figure 9 shows, from top to bottom, integrated auroal intensity at SNKQ, IMF B z, three velocity and two magnetic field components at the four THEMIS probes from the outermost to the innermost probe, and at the bottom two panels the E region electron density at 130 km altitude, which reflects the strength of the plasma sheet electron precipitation, and the F region convection flow vectors observed by the Sondrestrom radar. Since the Sondrestrom radar is located near the typical location of the polar cap boundary (or the nightside magnetic separatrix), it can be used to identify the polar cap boundary location based on the difference of E region electron densities within the polar cap and plasma sheet when that boundary is within the E region radar fov. de la Beaujardière et al. [1991] identified a threshold value of m 3 to distinguish the polar cap and the auroral oval region of plasma sheet precipitation, which we adopt here. Based on this value, it can be seen from the figure that the radar E region fov was mostly close to the polar cap boundary or within the polar cap until 02:05 UT, when the auroral oval appears to have expanded poleward after the substorm onset. The sharp electron density enhancement seen from 72.3 to 72.8 MLAT at 01:45 UT lies adjacent to the polar cap densities seen at higher latitudes, and is thus likely the signature of a PBI. The black bins after 01:50 UT and elsewhere in the figure are due to the electron densities being below the radar detectable threshold, and thus represent polar cap. While the densities measurements are quite noisy, we estimate that the radar E region fov was within the polar cap (i.e., the auroral oval was just south of 72 MLAT) from 01:40 to 01:58 UT, except for during the above PBI. [34] Figure 9 shows clear flow enhancements in the southeastward direction in the F region between 70.5 to 73.5 from 01:43 UT to 01:51 UT, which includes the polar cap regions seen within the E region fov. It is not possible to determine for sure whether the flows crossed the polar cap boundary and extended into the auroral oval, except during the period of the PBI. However, that the PBI was seen within the E region fov suggests that the polar cap boundary was not far equatorward of that fov even after the PBI. This suggests that, during this period, the enhanced southeastward flows were heading toward the polar cap boundary and were very likely crossing the boundary, meaning that localized enhanced reconnection was taking place. The relation of such flows with the PBI signature is also consistent with what has been seen by de la Beaujardière et al. [1994] and Lyons et al. [2010a]. Note that, even though polar cap flows significantly decreased at 0154 UT, the flows near the expected polar cap boundary continued to be elevated and headed toward the boundary (and presumably continued to cross the polar cap boundary) up through the time of the initial substorm onset (the third vertical line) within the equatorward portion of the E region fov. [35] It is noteworthy that the initial radar flow enhancement (the first vertical line) at 0143 UT (or even earlier considering the 2.5-min time resolution of the radar measurements) is accompanied by dipolarizations at both P4 and P3 at 01:44:45 UT (the second vertical line in Figure 9) and by B z fluctuations at P1 and P2 (figures not shown). There are also some large-amplitude velocity fluctuations associated with this dipolarization in the THEMIS probes and some midlatitude ground magnetometer fluctuations at 0146 UT (Figure 4). The >2 min delay between the earliest ionospheric flow enhancement and the dipolarization at P4 and P3 suggests that the polar cap flow enhancement was communicated very quickly to the inner magnetosphere and to the ground magnetometers, and was likely associated with the pseudobreakup at 01:45 UT mentioned in section We will discuss this process in more detail in the Discussion section. [36] Note that during the whole period of the polar cap flow enhancement, the IMF B z kept southward, which suggests a possible source of the polar cap flow enhancement may be the enhanced dayside reconnection The Pseudobreakup at 0145 UT [37] Figure 10a shows that auroral arc intensification initiating at 01:44:24 UT was associated with a dipolarization observed by THEMIS at that time. The panels of Figure 10a show that the strengthening and decaying of the pseudobreakup arcs are within 3 min, from 01:44:24 UT to 01:48:00 UT. Figure 10b shows the tail observations at P4 for both the pseudobreakup (first vertical line) and substorm (the second vertical line) in the same format as in Figure 5. The similarities in the time sequence of the enhanced ionospheric convection flows and tail signatures for this pseudobreakup and for the later substorm onset suggest similar triggering scenario for both events. The fundamental difference is that the pseudobreakup had no significant earthward convective flows or Ey (only velocity fluctuations), though it is possible that midtail and/or inner magnetosphere flows preceding the pseudobreakup missed the longitude locations of the THEMIS probes. This difference suggests that there may be an important role played by the strength of the intruding flows, or the location of the intruding flows in the tail, topics which could be interesting for further studies. Furthermore, radar observations of the polar cap flows showed continuous enhanced flows from 01:43 UT to 02:05 UT near the polar cap boundary and that polar cap flow changes were communicated to the inner 12 of 18

13 Figure 9. Stacked plots showing, from the top to bottom, integrated auroral intensity at SNKQ, IMF B z, three velocity components at the four THEMIS probes in GSM coordinates from the outer most probe (P1) to the inner most probe (P4), and magnetic field B x and B z components, the E region electron density at 130 km altitude, and F region convection flow vectors observed by the Sondrestrom radar. 13 of 18

14 Figure 10. (a) A series of composite auroral mosaics from the THEMIS ASIs at SNKQ and GILL before and after the pesudobreakup at 01:44 UT, and (b) the corresponding plasma sheet observations at P4 in the same format as Figure of 18

15 magnetosphere and ionosphere within a couple of minutes. Then, why do we only see one pseudo-breakup and one substorm? What conditions cooperated to make these particular polar cap flows enter into the plasma sheet and cause the two onsets? These are all causality questions that should be pursued in the future. 3. Discussion [38] In the present paper, we show observations of a sequence of polar cap flow enhancements and earthward fast flows from the midtail to near-earth region prior to a substorm onset on March 05, 2008 via a conjunction of the THEMIS spacecrafts and the Sondrestrom radar. We include the timing of a pseudobreakup that occurred 15 min before the substorm onset. We summarize the key timing results of this event in Table 1. [39] The time sequence shows that enhanced polar cap flows started as early as 18 min prior to the substorm onset, setting in motion a process that includes a pseudobreakup, continuous enhanced reconnection in the tail, and finally a burst of reconnection that was manifested with fast flows in the mid-tail that intruded to the inner magnetosphere, and followed immediately by a substorm onset. Approximately 2 3 min after the earliest time of the polar cap flows enhancement, the pseudobreakup was observed in the aurora and was accompanied by mid-tail fluctuations, inner magnetosphere dipolarizations and ground magnetometer fluctuations, suggesting that the polar cap flow enhancements were quickly communicated through the magnetosphere and down to the ionosphere. The substorm onset at 02:00:30 UT was not directly related with this very early polar cap flow enhancement. Rather, it was not until more than 15 min later, during which period the polar cap flows remained enhanced and oriented toward the polar cap boundary, that significant fast flows were observed in the midtail plasma sheet (P1) and then penetrated into the near Earth space near the time of substorm onset. This indicates that the substorm onset was related with a burst of reconnection that occurred sometime before but not far from the point when the fast flows were first seen at P1. However, it is not possible to determine this instance from our observations, nor what is the reason that the enhanced polar cap flows may have triggered the particular burst of reconnection that started the substorm sequence at that particular time. The latter is a particularly interesting subject for future research. [40] In section 2.3, we showed that ionospheric convection flows with substantial equatorward component enhanced near the polar cap boundary. As discussed in the Introduction, these ionospheric flow disturbances are known to be associated with localized enhancement of reconnection along the nightside outer plasma sheet boundary [de la Beaujardière et al., 1991, 1994; Blanchard et al., 1996; Lyons et al., 1999; Hubert et al., 2006]. Recently, Nishimura et al. [2010b] and Lyons et al. [2011] showed that PBIs and the following streamers (including those triggering substorms and those without substorms) are preceded by localized enhanced polar cap flows impinging the polar cap boundary, which suggests that those flow disturbances can potentially trigger PBIs and associated streamers. Previous theoretical modeling works have shown that fast reconnection can be driven by externally imposed plasma flow disturbances or field perturbations and then launch bursty fast flows into the outflow regions [e.g., Pei et al., 2001a, 2001b; Birn et al., 2005; Birn and Hesse, 2009]. Our results suggest that some field or flow perturbations were first caused by the enhanced polar cap flows, which extend along open polar cap field lines and encounter the PSBL along the magnetotail. The perturbations then triggered localized reconnection somewhere along the stretched thin current sheet, which is responsible for the generation of the earthward fast flows and the following pseudobreakup or substorm onsets as shown in this case. In other words, localized tail reconnection may be triggered by the flow or field perturbations caused by enhanced polar cap flows along the outer plasma sheet boundary during substorm growth phase. However, we are not able to determine unambiguously the reconnection locations in the tail solely based on these ionospheric disturbances, especially under the highly stretched thin current sheet. [41] Based on the current observations, we cannot be sure whether any of the flows observed by the radar directly connected to the flows seen by the THEMIS probes, nor can we know whether or not the enhanced tail reconnection was occurring continuously during that 22-min period or only occurred just prior to the two events we observed and how localized it was. However, since localized enhanced tail reconnection is a well-accepted source for tail fast flows, it is thus reasonable to propose that the pseudobreakup at 0145 UT was associated with the localized reconnection triggered by the enhanced ionospheric convection at 0143 UT (if not exactly at the location where they were observed by the Sondrestrom radar) and the ensuing tail perturbations, and that the substorm onset at 0200 UT was associated with localized reconnection driven by the enhanced ionospheric convection at some point before the fast tail flow were first seen at P1 at 0158 UT (if not exactly at the location where they were observed by the Sondrestrom radar). [42] As shown above, for the pseudobreakup event, we see a very fast communication of the polar cap flow disturbance through the magnetosphere and down to the ionosphere. The perturbations propagating across the PSBL to the midtail thin current sheet could be very fast (within one minute) considering the high Alfven speed there. Thus, the >2 min delay between the earliest ionospheric flow enhancements and the dipolarization at P4 and P3 suggests that the initial reconnection was earthward of 20 R E for a typical Alfven speed in the plasma sheet. We then need to take into account another 1 min of Alfvenic transit time between the inner magnetosphere and the ground. The whole timing is consistent with the ground magnetometer perturbations (see Figure 4 for ground magnetometer perturbations at 0146 UT) occurring within 3 4 min after the very earliest polar cap flow enhancement. This brief timing exercise indicates that there can be a direct causal connection between the earliest enhanced polar cap flows and the pseudobreakup. For the substorm case, it is not easy to show a similar timing because we are not able to determine exactly the onset time of the polar cap flow enhancement that is associated with the onset of tail fast flows at P1. However, the substorm pre-onset time sequence starting with the enhanced polar cap flows suggests that it was also initiated 15 of 18