Convection dynamics and driving mechanism of a small substorm during dominantly IMF By+, Bz+ conditions

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L08803, doi: /2003gl018878, 2004 Convection dynamics and driving mechanism of a small substorm during dominantly IMF By+, Bz+ conditions Jun Liang, 1 G. J. Sofko, 1 E. F. Donovan, 2 M. Watanabe, 1 and R. A. Greenwald 3 Received 20 October 2003; revised 27 February 2004; accepted 11 March 2004; published 16 April [1] Ground-based optical, magnetic and radar measurements detected a small substorm on October 9, Solar wind observations on GEOTAIL revealed a prolonged dominant Bz+ and steady By+ interplanetary magnetic field (IMF) prior to the substorm onset, except for a southward excursion at UT, and a square-wave IMF Bx-By structure at UT. We find that the IMF southward excursion led to the dayside convection enhancement and energy transport into the magnetosphere. When the dayside convection decreased, two pseudobreakups occurred as the consequence of the release of magnetospheric energy into the ionosphere. The substorm onset was associated with the IMF Bx-By structure in directly driven fashion. There was also a Stage-2 expansion which was internally driven within the magnetotail. INDEX TERMS: 2407 Ionosphere: Auroral ionosphere (2704); 2463 Ionosphere: Plasma convection; 2744 Magnetospheric Physics: Magnetotail; 2784 Magnetospheric Physics: Solar wind/magnetosphere interactions; 2788 Magnetospheric Physics: Storms and substorms. Citation: Liang, J., G. J. Sofko, E. F. Donovan, M. Watanabe, and R. A. Greenwald (2004), Convection dynamics and driving mechanism of a small substorm during dominantly IMF By+, Bz+ conditions, Geophys. Res. Lett., 31, L08803, doi: /2003gl Introduction [2] The energy of substorms usually originates from the solar wind. The physical processes involved in dissipating this energy were divided into loading-unloading and directly driven [Rostoker et al., 1987]. The loadingunloading process is widely accepted to be the classical scenario of a substorm, in which the growth phase is defined as a prolonged period (>30 min) of energy storage in the magnetotail, and the expansive phase (EP) marks the abrupt energy release into the nightside auroral ionosphere. The directly driven process is manifested by cross-correlation studies that the auroral electrojet indices correlate well with the solar wind energy input by short time delay (20 min) [e.g., Bargatze et al., 1985]. The loading-unloading and directly driven processes have different timescales, but both play important roles in substorm dynamics. Correspondingly, the substorm current pattern at high latitudes 1 Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Saskatchewan, Canada. 2 Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada. 3 Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA. Copyright 2004 by the American Geophysical Union /04/2003GL018878$05.00 also contains two components; the first is related to the 2-cell global convection pattern which is usually directly controlled by the solar wind, and the second one is related to the substorm current wedge (SCW) system which is associated with the release of previously stored energy in the magnetotail [Kamide and Kokubun., 1996]. The latter component is usually accompanied with the emergence and enhancement of a small near-midnight counterclockwise convection vortex during the substorm EP in addition to the preexisting 2-cell pattern [Lu et al., 1998; Kamide et al., 1994]. [3] It is the purpose of this letter to describe a particular substorm event on October 9, 2000 and its associated ionospheric convection. The substorm was small in magnitude but showed very well-defined magnetic, optical auroral, and radar signatures as observed by the CANOPUS (Canadian Auroral Network for the Open Program Unified Study) magnetometer array and the meridian scanning photometers (MSP), the NORSTAR (NORthern Solar Terrestrial ARray) all-sky imager, and the SuperDARN (Super Dual Auroral Radar Network), respectively. There were initially two weak auroral and magnetic activations starting at 0732 UT and 0743 UT which were pseudobreakups, while the EP onset occurred at 0752 UT. Beginning at 0802 UT there was a second auroral brightening with the expected characteristics of a Stage-2 expansion [Erickson et al., 2000]. The first two pseudobreakups, the EP onset, and the Stage-2 expansion are found to be related to loading-unloading, directly driven, and internal magnetotail processes, respectively. 2. Solar Wind, Optical, Magnetic and Radar Observations for October 9, 2000 Event [4] Solar wind parameters from GEOTAIL observation for this event are shown in Figure 1. GEOTAIL measured a predominantly positive IMF Bz during UT, except for a 10-min short negative Bz excursion. The ion dynamic pressure of the solar wind was stable at 2 npa. There are two candidates possibly responsible for the pseudobreakups and substorm onset on Oct., 9, 2000, namely the southward IMF excursion at UT, and the subsequent Bx-By structure seen as a squarewave signature at UT when the IMF Bz component was very small. The square-wave change, in which Bx became less negative while By increased, caused a +By-dominated solar wind, which could have made the antiparallel merging conditions easier to satisfy in the dawn flank magnetopause in the region south of the ecliptic plane. The bottom panel gives the northern polar cap (PCN) magnetic activity index, which was obtained from magnetometer site at Thule (85.4 MLAT) [Troshichev et al., 1988], and the merging solar wind electric field L of5

2 Figure 1. Solar wind observations at GEOTAIL on October 9, The first three panels give the ion dynamic pressure and the GSM components of IMF observed by SWA and MGF instruments, respectively. The GSM position of the satellite at the start of each UT hour is labeled at the top of the plot in unit of Earth s Radius (Re). The bottom panel gives the merging solar wind electric field (dotted line) and the PCN index (solid line). qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi defined by E sw = V x B 2 y þ B2 z sin 2 (q/2), where V x, B y and B z are the solar wind velocity, GSM Y and Z components of IMF, respectively, and q = arctan (B y /B z ). Arrows indicate the peak-to-peak correspondences between the two data sets with min lags. [5] Figure 2 shows the magnetometer records of the X-component (geomagnetic north) from six CANOPUS sites. Their magnetic latitudes (MLAT), longitudes (MLON) and magnetic local time (MLT) midnight are also indicated. The top panel gives Gillam MSP 630 nm emissions. Figure 2 shows that there is very little magnetic activity during UT, as expected under such predominantly IMF Bz+ conditions. After 0732 UT there were three Pi2 bursts as observed at the mid-latitude Pinawa station. Accompanying the three Pi2 bursts were magnetic deviations and auroral intensifications as shown by the MSP observations. During the first two Pi2 bursts at 0732 UT and 0743 UT, there were only weak magnetic and optical auroral activations which were classified as pseudobreakups. The third Pi2 burst starting at 0752 UT was accompanied with a 100 nt negative bay at Fort Churchill and a strong increase in the MSP intensity. The third magnetic and auroral activation differed from the previous two not only in the magnitude of disturbances but also in the consequent spatial coverage and thus was defined as a substorm EP instead of a third pseudobreakup. About 10 min later a second auroral brightening occurred in conjunction with a second magnetic bay and Pi2 pulsations at Eskimo Point. Rostoker [1968] found that during a substorm the magnetic bay disturbances often occur in a two-stage way. Erickson et al. [2000] also found that the initial substorm expansion after the onset is usually short-lived (10 min) and followed by a Stage-2 expansion (Stage-2 EP). The observation of the second magnetic bay and auroral brightening in our event is consistent with such features. The start times of the first pseudobreakup, the EP onset, and the Stage-2 EP are indicated by P1, S1, and S2 respectively in Figure 2. For the first pseudobreakup a short-lived positive excursion in the X-component magnetic record was visible at Rabbit Lake, indicating the passage of westward traveling surge over the station. The second pseudobreakup and ensuing EP also both featured the surge form at Fort Smith. During the whole event sequence the activations did not extend westward to the GOES-10 footprint (23 MLT) because no magnetic dipolarization effect was seen from its observation (not plotted). [6] The whole sequence of auroral activations was observed by the NORSTAR all-sky imager (NASI) at Rankin Inlet. There was also good scatter from all the eight northern SuperDARN radars with 2-min resolution. Figure 3 shows six frames of combined NASI 630 nm images and global SuperDARN convections based upon the potential map procedure of Ruohoniemi and Baker [1998]. The IMF parameters required in the model are determined from GEOTAIL observations with 22-min delay. Figure 3a shows that, at UT strong flows (1 km/s) were seen on the dayside at 80 MLAT, implying enhanced reconnection at the dayside magnetopause, but near midnight the convection as well as the optical emission was very weak. Although the dusk convection cell may subject to uncertainty due to the relatively sparse radar observations on that side, a well-defined contracted dawn cell, as expected under IMF By+ conditions, was easily identified on the morning side. At UT, the dayside convection Figure 2. Magnetometer records of X-component (geomagnetic north) for 6 CANOPUS stations with scale 20 nt/ div. For the Eskimo Point and Pinawa stations the Pi2 pulsations are superposed in gray lines with scale 1 nt/div. The local midnight time of each station is indicated by an arrow. The top panel shows the Gillam MSP nm intensities in gray scale. Three characteristic times, P1, S1, and S2, are indicated by dashed lines. 2of5

3 Figure 3. Six frames of the global SuperDARN convections (with electrical potential contours overlaid) and NASI nm emissions. Dotted circles from outside towards the center represent 60, 70, and 80 MLAT respectively. The colorscale for the convection velocity, grayscale for the NASI emission intensity, the cross-pc potential and IMF parameters, are shown at the upper-left, lower-right, and upper-right corners of each frame, respectively. diminished, but the near-midnight flows at high-latitude (75 MLAT, presumed to be on open field lines because of the darkness of 630 nm emissions) are somewhat intensified. The first pseudobreakup began at 0732 UT, which marked the start of the optical and magnetic activations of the whole event. In Figure 3c an auroral patch was first seen at 0.4 MLT, MLAT. Northeast of the patch there was a small counterclockwise convection flow vortex centered at 1MLT,72 MLAT. The nightside flows at lower latitude (70 MLAT) had become obviously stronger. By UT, the initial auroral patch had intensified and expanded both westward and eastward. The nightside flows continued to grow and a single large postmidnight convection cell with focus around 2.3 MLT, 71 MLAT was established. At UT, just after the EP onset, strong flows were seen again on the dayside, while the nightside convection cell was relatively unchanged in position compared to the previous frame but strongly intensified. Significant auroral brightening and poleward expansion to 71.5 MLAT were clearly seen. At UT, during the Stage-2 EP, the intensified auroral luminosity expanded eastward to 2.7 MLT. Enhanced flows were mostly on the nightside instead of on the dayside. During the whole event the premidnight convection remained weak. [7] The variations of cross-polar cap (PC) potential and the intensity of the brightest pixel within the NASI field of view (FoV) at each exposure time during UT are given in Figure 4. Comparing Figure 4 to Figure 1, the cross-pc potential and the PCN index both showed a peak at UT which was very likely the consequence of the IMF southward excursion, while the other mutual peak at is associated with the IMF Bx-By structure. That second major cross-pc potential peak led the Stage-1 EP auroral brightening by 2 4 min. During UT there was a third strong enhancement of cross-pc potential which occurred almost simultaneously with the Stage-2 auroral brightening, but no counterpart is seen in the PCN Figure 4. The cross-pc potential (solid line with dots representing the mid-time of each scan) derived from global SuperDARN convection measurements and the maximum emission intensity (dotted line) within the NASI FoV. 3of5

4 index. Also, no major peak in the solar wind electric field which might contribute to this enhancement can be found. It is likely to be associated with an internal magnetotail process. 3. Discussion [8] The IMF southward excursion led to enhanced coupling between the solar wind and magnetosphere as indicated by the strong increases of the PCN index and cross-pc potential after 0706 UT. When the solar windmagnetosphere coupling diminished as shown by the significant drops of both PCN index and cross-pc potential from 0718 UT to 0730 UT, there were some increases of nightside polar cap convection shown in Figure 3b which may hint the energy storage in the tail lobe, but the low plasma sheet convection and auroral activation indicated that no significant energy had penetrated into the near-earth plasma sheet and diverted into the auroral ionosphere at that stage. Accompanying the first pseudobreakup at 0732 UT the nightside flows were enhanced. A small counterclockwise convection vortex appeared with focus around 1 MLT, while the previous 2-cell pattern persisted on the dawn and dusk sectors. The increase of nightside convection and the appearance of the small nearmidnight counterclockwise cell after substorm EP onset have been previously reported by Lu et al. [1998] and Kamide et al. [1994], who postulated that they were signatures of the magnetotail energy release process. Kamide and Kokubun [1996] proposed that during the substorm EP such a near-midnight cell is added to the preexisting 2-cell global convection pattern, representing an unloading versus a directly driven component of the auroral electrojet. These signatures were clearly shown by the SuperDARN measurement in Figure 3c. However, this unloading process starting at 0732 UT did not evolve into a full-scale EP but only led to two successive pseudobreakups, likely due to the insufficiency of previously-stored energy in the tail and/or unfavorable ionospheric conditions [Koskinen et al., 1993]. [9] After its emergence the near-midnight convection vortex progressed eastward. Surprisingly, by the time 0748 UT it had merged with the morning cell on the flank to form a large single postmidnight cell centered at 2.3 MLT. The distinction between the two components of the auroral electrojet became vague then. The unified convection cell had expanded to lower latitude, implying that energy could readily penetrate into the near-earth plasma sheet, and its focus was located more or less directly east of the intensified auroras. The 630 nm auroras are caused by soft electron precipitation associated with upward field-aligned currents (FACs), while the counterclockwise convection cell represents downward FACs. Thus a SCW-like current system appears to have already begun to form by UT, just before the effect of IMF Bx-By structure began to impact on the ionosphere and caused sharp rises of the cross-pc potential and PCN index after 0750 UT. This current system tended to divert the cross-tail current and thus might assist in the current disruption process in case there was a third disturbance in the near-earth plasma sheet. Furthermore, the first two auroral activations resulted in a considerable increase of the nightside ionospheric conductance. The third magnetospheric disturbance would be greatly aided by the new energy input from the enhanced solar wind activation because the magnetotail/ionosphere had been so preconditioned that the incoming energy could be readily diverted and dissipated in the nightside ionosphere. Therefore the disturbance responded to the IMF Bx-By structure in directly driven fashion and led to the substorm EP onset at 0752 UT. Comparing Figures 3e to 3d we see that the postmidnight convection cell did not change much in shape but only strongly intensified in magnitude, indicating there was no extra unloading component appearing for this particular substorm EP. Such a directly driven EP onset is uncommon. Koskinen et al. [1993] reported a pseudobreakup which lagged the IMF southward turning at the magnetopause by 20 min. In our event the lag between the EP onset and the IMF Bx-By structure at the magnetopause is only 16 min. This is almost the least time required for the near-earth tail to respond to the upstream solar wind variations [Bargatze et al., 1985]. We suggest that such a short time delay would not have been possible without the preconditioning of the magnetosphere/ionosphere to facilitate the SCW formation as in our event. [10] For the convection enhancement associated with the Stage-2 EP, we have stated it is likely to be associated with an internal magnetotail process. We notice in Figure 3f that the flows through the bright auroras were almost purely equatorward, substantially different from previous patterns. This can be explained by the Stage-2 EP dynamics proposed by Erickson et al. [2000], as follows. After the Stage-1 EP the initial current disruption at the near-earth plasma sheet produced a tailward-traveling rarefaction wave that caused further thinning of the mid-tail plasma sheet and led to the formation of a neutral line. The reconnection generated the earthward flow bursts that showed up as the observed band of equatorward ionospheric flows, and resulted in the Stage-2 EP by reconnection-associated SCW mechanisms [e.g., Baker et al., 1996]. [11] Acknowledgments. We acknowledge the national funding agencies of USA, Canada, U.K., and France that support the northern SuperDARN radars. CANOPUS is funded by the Canadian Space Agency (CSA). NORSTAR is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the CSA. GEOTAIL magnetic field data were provided by S. Kokubun through DARTS at the Institute of Space and Astronautical Science in Japan. References Baker, D. N., et al. (1996), Neutral line model of substorms: Past results and present view, J. Geophys. Res., 101, 12,975 13,010. Bargatze, L. F., et al. (1985), Magnetospheric impulse response for many levels of geomagnetic activity, J. Geophys. Res., 90, Erickson, G. M., et al. (2000), Electromagnetics of substorm onsets in the near-geosynchronous plasma sheet, J. Geophys. Res., 105, 25,265 25,290. Kamide, Y., and S. Kokubun (1996), Two-component auroral electrojet: Importance for substorm studies, J. Geophys. Res., 101, 13,027 13,046. Kamide, Y., et al. (1994), Ground-based studies of ionospheric convection associated with substorm expansion, J. Geophys. Res., 99, 19,451 19,466. Koskinen, H. E. J., et al. 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5 Substorms (ICS-4), edited by S. Kokubun and Y. Kamide, pp , Kluwer Acad., Norwell, Mass. Rostoker, G. (1968), The macrostructure of geomagnetic bays, J. Geophys. Res., 73, Rostoker, G., et al. (1987), The roles of direct input of energy from the solar wind and unloading of stored magnetotail energy in driven magnetospheric substorms, Space Sci. Rev., 46, Ruohoniemi, J. M., and K. B. Baker (1998), Large-scale imaging of high-latitude convection with Super Dual Auroral Radar Network HF radar observations, J. Geophys. Res., 103, 20,797 20,811. Troshichev, O. A., et al. (1988), Magnetic activity in the Polar Cap A new index, Planet. Space Sci., 36, E. F. Donovan, Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4. R. A. Greenwald, Applied Physics Laboratory, Johns Hopkins University, Johns Hopkins Road, Laurel, MD , USA. J. Liang, G. J. Sofko, and M. Watanabe, Institute of Space and Atmospheric Studies, University of Saskatchewan, 116 Science Place, Saskatoon, Saskatchewan, Canada SK S7N 5E2. (liang@dansas.usask.ca) 5of5