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Keywords:

  • magnetopause;
  • dayside magnetosphere

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. GMC Identification
  5. 3. Multiple GMCs
  6. 4. Application of the Magnetopause Models
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

[1] On 29–31 October 2003, numerous geosynchronous magnetopause crossings (GMCs) were identified using magnetic field data from two GOES satellites and plasma data from four Los Alamos National Laboratory (LANL) satellites. We can distinguish four long-lasting intervals, when geosynchronous satellites observed GMCs in a wide range of local time: at ∼0600–0900 UT 29 October; from ∼1000 UT 29 October to 0400 UT 30 October; from ∼1700 UT 30 October to ∼0800 UT 31 October, and at ∼1100–1300 UT 31 October. During a part of those intervals the GMCs occurred in the dawn and dusk sectors under northward interplanetary magnetic field (IMF) that indicates to magnetospheric compression by extremely high solar wind pressure. We found that at 0400–1000 UT 31 October the compression was accompanied with large-amplitude Pc5 pulsation, which can be attributed to global magnetospheric mode (cavity resonance). Multiple GMCs were revealed for the time interval of Pc5 pulsation occasion. An amplitude of the magnetopause oscillation in noon sector was estimated of about 0.26∼0.6 RE. An application of the magnetopause models enabled us studying the magnetopause dawn–dusk asymmetry, which was revealed on the main phase and in maximum of two great geomagnetic storms on 29 and 30 October. We shown that the asymmetry can be formally represented as a shifting of the dayside magnetopause toward the dusk on average distance of 0.2∼0.3 RE. Besides, in some cases the asymmetry was larger and required a shifting of about 0.4 RE. It was shown that the magnitude of the dawn–dusk asymmetry is related to the internal geomagnetic disturbances rather than to the external conditions in the IMF.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. GMC Identification
  5. 3. Multiple GMCs
  6. 4. Application of the Magnetopause Models
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

[2] Geosynchronous magnetopause crossings (GMC) are caused by strongly disturbed interplanetary conditions, which are characterized by high solar wind (SW) pressure and/or strong southward component of the interplanetary magnetic field (IMF). Numerous studies of the GMCs [e.g., Dmitriev et al., 2004, and references therein] show that duration of the magnetosheath interval, when geosynchronous satellite is located in the magnetosheath, can achieve up to 4 hours. There are a few cases when multiple GMCs were observed by various geosynchronous satellites within several consequent days: 7–9 February 1986 [Rufenach et al., 1989], 9–13 June 1991 [McComas et al., 1994], and 14–16 July 2000.

[3] The time interval of 29–31 October 2003 (Halloween event) is characterized by extremely strong solar, heliospheric, and geomagnetic disturbances [Lopez et al., 2004; Veselovsky et al., 2004]. Two great solar events at ∼1000 UT 28 October and ∼2040 UT 29 October ejected extremely fast coronal mass ejections (CMEs) with propagation velocity of more than 1800 km/s. The fast CMEs generate strong foreword shocks in the upstream solar wind. The CME-driven shocks pushed the Earth magnetosphere at ∼0600 UT 29 October and ∼2000 UT 30 October, respectively (see Figure 1), and triggered sudden increases of the H-SYM index (tens of nT) that are associated with strong solar wind pressure enhancements [Burton et al., 1975]. Great geomagnetic storms that developed on 29–30 October indicate a very large southward IMF BZ component accompanying the CMEs. Unfortunately, experimental data of solar wind plasma provided for this event by upstream monitors Geotail, Solar and Heliospheric Observatory (SOHO), and ACE are ambiguous [Dmitriev et al., 2005]. Hence the solar wind conditions can be analyzed only indirectly.

image

Figure 1. Extremely strong geomagnetic disturbances on 29–31 October 2003 are revealed as very large variations of the Dst and H-SYM indices of low-latitude geomagnetic activity (gray histogram and black line, respectively).

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[4] Geosynchronous magnetopause crossings demonstrate a number of unusual properties, which are associated with dynamics of the strongly disturbed magnetosphere. One of them is a dawn-dusk asymmetry [e.g., Dmitriev et al., 2004, and references therein]. Statistically, the asymmetry is represented as an increasing function of negative IMF BZ [Kuznetsov and Suvorova, 1998; Suvorova et al., 2005]. On the other hand, Dmitriev et al. [2004] show that the dawn-dusk asymmetry should be attributed to magnetospheric phenomena such as asymmetrical ring current and/or magnetopause erosion, which is operating more intensively in the prenoon sector. In this sense the negative IMF BZ should be considered as only “associating” parameter, which is required for erosion of the dayside magnetopause or for intensification of the asymmetrical ring current. A magnitude of the dawn-dusk asymmetry is determined only approximately and ambiguously on the base of model approaches. A magnetopause model by Kuznetsov and Suvorova [1998] describes the asymmetry as a magnetopause shifting toward the dusk at ∼2 RE for strong southward IMF. Introducing an asymmetry in Chao et al.'s [2002] magnetopause model, a 15° rotation of the magnetopause nose point toward dawn is required [Dmitriev et al., 2004].

[5] Previous empirical magnetopause studies [Sibeck, 1995; Dmitriev and Suvorova, 2000a, 2000b] and MHD simulations [Elsen and Winglee, 1997] revealed two interesting effects for dayside magnetopause under strongly disturbed solar wind conditions. The first one is that the southward IMF turnings are less effective in moving the magnetopause under the high solar wind pressure. The second effect is that the influence of solar wind pressure to the magnetopause diminishes when the IMF BZ is strongly southward. Moreover, recent experimental studies show that geosynchronous magnetopause crossings do not observed when SW pressure is under some critical value. This experimental fact is explained by so-called “BZ-influence saturation,” one of the newest magnetospheric effects revealed in studies of the GMCs [Kuznetsov and Suvorova, 1996, 1998; Shue et al., 1998; Suvorova et al., 1999; Dmitriev and Suvorova, 2000a, 2000b; Yang et al., 2003; Suvorova et al., 2005]. This effect is consistent in limitation of the IMF BZ influence to the magnetopause, such that for a given SW pressure the magnetopause does not approach the Earth in response to the negative IMF BZ increasing above a threshold value. Comprehensive analysis of the GMCs [Suvorova et al., 2005] permits revealing the minimum SW conditions required for the GMCs:

  • equation image

where Psw is the total solar wind pressure, which usually is very close to the dynamic pressure Pd, excepting cases of relatively large magnetic and/or thermal pressures. From this expression one can simply find that even under very strong negative IMF BZ the GMCs should not be observed when the SW pressure is less than Psw = 4.8 nPa (Pd ∼ 4.6 nPa) because the IMF BZ influence saturates at BZ ∼ −20 nT. For large positive IMF BZ the SW pressure should be more than Psw = 21 nPa to push the magnetopause inside the geosynchronous orbit.

[6] One more effect associated with magnetospheric compression is ULF Pc5 pulsations (periods ranging from a few minutes to 10 min), which at large distances can be observed as multiple magnetopause crossings. The Pc5 pulsations are associated with two effects of different nature. The first one is the solar wind pressure pulse effect [Song, 1994; Motoba et al., 2003], which causes quasi-periodic oscillations of the dayside magnetosphere. The other effect is global oscillations associated with a global magnetospheric mode or cavity resonance, which is “triggered” by enhanced SW pressure [Korotova and Sibeck, 1994; Kleimenova et al., 1996, 2000; Huang et al., 2003]. An important property of the global magnetospheric oscillations is their independence from the current solar wind conditions. Moreover, multiple magnetopause crossings caused by the global oscillations might be observed even under weaker SW conditions than required for nonoscillating magnetopause [Suvorova et al., 2005].

[7] In the present study we demonstrate the unusual dynamics of the strongly compressed magnetopause on 29–31 October 2003 and compare it with predictions of a few different magnetopause models. GMC identification by GOES and LANL satellites is performed in section 2. Section 3 is devoted to determination of sources for multiple CMCs events. An application of the magnetopause models for GMCs is presented in section 4, and finally section 5 is a summary.

2. GMC Identification

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. GMC Identification
  5. 3. Multiple GMCs
  6. 4. Application of the Magnetopause Models
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

[8] To identify the GMCs, we use magnetic field data from the GOES satellites and plasma data from the LANL satellites (see Table 1). A method of identification is described by Suvorova et al. [2005]. Because the high-resolution (∼1 min) data of solar wind plasma is not available on 29–31 October 2003, we use a simplified method for estimation of the solar wind propagation from an upstream monitor to geosynchronous orbit (timing). An example of the GMC identification using GOES-10 magnetic data at 1630–1830 UT 29 October 2003 is presented in Figure 2. At that time, GOES-10 is moving in the prenoon sector from ∼0730 LT to ∼0930 LT. Within time intervals 1701–1719 UT and 1739–1810 UT and since 1822 UT the GOES-10 is located in the magnetosheath and observes variations of the Bz and By magnetic field components that are very close to variations of the IMF, which is observed by Geotail just in front of the magnetosphere (XGSE ∼ 20, YGSE ∼ 13, ZGSE ∼ 0). This fact testifies correct choice of the timing dT = 0 min. Significant increases of the geomagnetic field H at geosynchronous orbit (up to 200 nT) and large positive variations of H-SYM index indicate that at 1645–1815 UT the GMCs are mainly caused by the SW pressure enhancements.

image

Figure 2. Geosynchronous magnetopause crossings (GMC) identification using GOES-10 magnetic measurements and interplanetary magnetic field (IMF) data from the Geotail upstream monitor at 1630–1830 UT on 29 October 2003. The figure represents, from top to bottom, the H-SYM index; the Hp component and total magnetic field H measured by GOES (solid and dotted lines, respectively); the Bz, By, and Bx magnetic field components in GSM measured by GOES-10 and Geotail (solid and dotted lines, respectively); and latitude (in degrees) and local time (in hours) of GOES-10 in GSM coordinate system. The Bz and By components measured by GOES are divided on 10 and Bx component is divided on 5. Vertical dotted and dashed lines indicate magnetosheath entrances and exits, respectively. The GMCs are mainly caused by the solar wind pressure enhancements.

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Table 1. Data Used in the Studya
ExperimentTitleParametersTime Res.Data Source
  • a

    MAG, Magnetometer instrument; SWEPAM, Solar Wind Electron, Proton, and Alpha Monitor; MGF, Magnetic field instrument; CPI, Comprehensive Plasma Instrument; MPA, multiple-particle analyzer; IMF, interplanetary magnetic field; SW, solar wind.

ACE MAGACIMF16 shttp://www.srl.caltech.edu/ACE/
ACE SWEPAM SW plasma30 minR. Skoug (personal communication, 2004)
Geotail MGFGEIMF1 minhttp://cdaweb.gsfc.nasa.gov/
Geotail CPI SW plasma1 minhttp://cdaweb.gsfc.nasa.gov/
GOES-10 MAGG0Magnetic field1 minhttp://cdaweb.gsfc.nasa.gov/
GOES-12 MAGG2Magnetic field1 minhttp://cdaweb.gsfc.nasa.gov/
1990-095 MPAL0Plasma1.5 minhttp://cdaweb.gsfc.nasa.gov/
1991-080 MPAL1Plasma1.5 minhttp://cdaweb.gsfc.nasa.gov/
1994-084 MPAL4Plasma1.5 minhttp://cdaweb.gsfc.nasa.gov/
LANL-97A MPAL7Plasma1.5 minhttp://cdaweb.gsfc.nasa.gov/

[9] Identification of the GMCs using LANL plasma data is more complicated because of the high level of noise, which originated from very intensive fluxes of solar energetic particles on 28–31 October 2003 that are penetrating into the plasma spectrometers. Because of the noise, the initial LANL plasma data should be carefully cleaned. An example of the ion and electron spectrograms derived from LANL 1991-080 measurements in the prenoon sector at 1600–2200 UT on 29 October 2003 is presented in Figure 3. From ∼1830 UT to ∼2130 UT the LANL observed spectra proper to the magnetosheath. This fact is supported by integral distributions of the electron phase space density constructed at several times during the interval considered (see Figure 4). As derived earlier [Montgomery et al., 1970; Scudder et al., 1973; Feldman et al., 1982; Feldman, 1985], the magnetosheath spectra show a clear flat top out to some break point in energy range from tens eV to few keV. The break-point location depends on the solar wind velocity. At 1903–1911 UT and at 1935–1952 the LANL satellite returns to the magnetosphere and observes spectra with a more rounded appearance (not shown).

image

Figure 3. (top) Ion and (bottom) electron spectrograms detected by Los Alamos National Laboratory (LANL) 1991-080 at 1600–2200 UT on 29 October 2003. From ∼1830 UT to ∼2130 UT the LANL is located in the magnetosheath, excepting a few short-time magnetosphere entrances at ∼1905 UT and ∼1940 UT.

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image

Figure 4. Cuts through the electron phase space density distribution measured by LANL 1991-080 at several times during the interval shown in Figure 3. The magnetosheath spectra show a clear flat top out to some breakpoint in energy range from tens eV to few keV, depending on the solar wind velocity.

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[10] For routine identification of the GMCs in the LANL plasma data we use a method proposed by Suvorova et al. [2005], which does not require the full plasma spectra. The method is based on the density/temperature ratios of ions (RI) and electrons (RE) calculated from the multiple-particle analyzer (MPA) LANL key parameters. Typically, the ratios RI and RE increase in the magnetosheath up to 100 and 200, respectively. On 29–31 October 2003 the noisy LANL data caused the RI and RE not to increase significantly in the magnetosheath, and thus the threshold ratios for the magnetosheath are estimated at approximately RI = 10 and RE = 20.

[11] Figure 5 demonstrates an example of the GMC identification based on the LANL 1991-080 plasma data at 1800–2000 UT 29 October 2003. The GMCs observed by GOES-12 are also presented for comparison. The upstream monitor Geotail is located just in front of the dayside magnetosphere (XGSE ∼ 20, YGSE ∼ 13, ZGSE ∼ 0). LANL 1991-080 is moving from dawn sector toward noon (from 0700 LT to 0900 LT) and GOES-12 is moving in the afternoon sector from 1300 LT to 1500 LT. At 1811 UT GOES-12 is leaving the magnetosheath due to the SW pressure decreasing, which is revealed from negative variation of the H-SYM index accompanied with positive IMF BZ at 1800–1815 UT. Three GMCs at 1817–1821 UT might be caused mainly by fast variations of the SW pressure. The RI and RE ratios increase significantly (>10 times) at 1832 UT that indicates to LANL 1991-080 entrances to the magnetosheath or at least the low-latitude boundary layer (LLBL). At that time GOES-12 is located in the magnetosheath, where variations of By and Bz magnetic field components measured by GOES correspond well to the IMF variations measured by Geotail. The timing in the present case is determined as dT = 1 min. A time delay between observations of the GMCs on GOES-12 and LANL 1991-080 can be explained by gradual increase of SW pressure, which is sufficient for the GMC observed by GOES-12 at ∼1330 LT and then becomes strong enough for the GMC observed by LANL 1991-080 at 0730 LT. Note that at 1830–1945 UT the IMF Bz is strong negative and the RI and RE ratios covary with the H-SYM. This means that GMCs are mainly driven by the SW pressure variation. When the pressure decreases at 1902–1916 UT and at 1935–1953 UT, the LANL 1991-080 returns to the magnetosphere and the RI and RE ratios drop down. At the same time, GOES-12 entrances to the LLBL and observes magnetopause current layer field, which is different from the magnetic field in the magnetosheath and magnetosphere. A GMC at 1953 UT is observed simultaneously by GOES-12 (at ∼1500 LT) and by LANL 1991-080 (at ∼0900 LT), when the plasma ratios increase up to RI ∼ 10 and RE > 20. Hence we have shown a good correspondence of the RI and RE increases with the appearance of magnetosheath features in the ion and electron spectra detected by LANL. Moreover, we demonstrate GMCs observed simultaneously by the LANL and GOES satellites. That testifies clearly that the GMC identification method based on the LANL plasma data is robust on 29–31 October 2003.

image

Figure 5. GMC identification using electron ratios RE and ion ratios RI calculated from LANL 1991-080 plasma measurements at 1800–2000 UT on 29 October 2003. For reference the GMCs identified from GOES-12 magnetic measurements are shown. The figure represents, from top to bottom, the H-SYM index; the RI (solid line) and RE (dotted line) ratios calculated from the LANL plasma measurements; the Hp component (parallel to satellite spin axis) and total magnetic field H measured by GOES (solid and dotted lines, respectively); the Bz, By, and Bx magnetic field components in GSM measured by the GOES and Geotail (solid and dotted lines respectively); and the geographic local time of the LANL and GSM local time of the GOES (solid and dotted lines, respectively). The Bz and By components measured by GOES are divided on 10 and Bx component is divided on 5. Vertical dotted and dashed lines indicate the magnetosheath entrances and exits, respectively. The GMCs appear in the LANL plasma data as significant increases of the RI and RE (up to ∼10 and >20, respectively). After 1830 UT the GMCs are caused by solar wind pressure pulses.

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[12] Results of the GMC identification are presented in Figure 6. We can distinguish three long time intervals when at least two geosynchronous satellites are located in the magnetosheath at different local time: ∼0600–0900 UT 29 October; from ∼1000 UT 29 October to 0400 UT 30 October; and from ∼1700 UT 30 October to ∼0800 UT 31 October. During these intervals we can find several GMCs in the dawn and dusk sectors. Some of the GMCs are occurred under northward IMF that shows extremely high SW pressure accompanying the GMCs. It is important to note a long-lasting magnetosheath interval at ∼1900–2300 UT, 30 October that is observed during about 4 hours by GOES–10 and LANL 1991-080. The IMF turning northward at ∼2200 UT indicates to significant compression of the magnetosphere at that time.

image

Figure 6. Geomagnetic and solar wind disturbances on 29–31 October 2003 (from top to bottom): great geomagnetic storms in H-SYM index; IMF Bz measured by ACE; magnetosheath intervals (black thick lines), low-latitude boundary layer (LLBL), or plasma sheet (gray thick lines) and multiple GMCs (gray thick lines restricted by crosses) identified using geosynchronous satellites GOES and LANL. Long-lasting magnetosheath intervals testify to extremely strong compression of the dayside magnetosphere.

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3. Multiple GMCs

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. GMC Identification
  5. 3. Multiple GMCs
  6. 4. Application of the Magnetopause Models
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

[13] In Figure 6 we indicate two time intervals of multiple magnetopause crossings observed by LANL 1994-084 (geomagnetic longitude (GLON) = 146°) and LANL-97A (GLON = 104°) at 0000–0400 UT 30 October and at 0400–1000 UT 31 October 2003. The multiple GMCs can be caused by various reasons, such as fast variations of the solar wind pressure or global geomagnetic pulsations, which are not controlled directly by the solar wind conditions. In order to determine a nature of the multiple GMCs, we analyze geomagnetic field fluctuations at various magnetospheric sites. We use 1-min magnetic data from GOES-10 (GLON = −133.5°), GOES-12 (GLON = −71.3°), ground-based stations Memambetsu (geomagnetic latitude (GLAT) = 43.91°, GLON = 144.19°), Leirvogur (GLAT = 64.18°, GLON = −21.7°) and the H-SYM index of the low-latitude geomagnetic activity. Local time of the geosynchronous satellites and magnetic stations for considering time intervals is presented in Figure 7. One can see that the observations cover practically whole local time range and the stations Memambetsu and Leirvogur are located in the same quadrants as the LANL and GOES satellites, respectively.

image

Figure 7. Local time of geosynchronous satellites (indicated by solid lines) LANL-97A (L7), 1994-084 (L4), GOES-10 (G0), GOES-12 (G2), and magnetic stations (indicated by dotted lines) Memambetsu (MMB) and Leirvogur (LRV) at (a) 0000–0400 UT 30 October and (b) at 0400–1000 UT 31 October 2003.

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[14] Multiple GMCs and corresponding solar wind and magnetospheric fluctuations at 0000–0400 UT 30 October 2003 are shown in Figure 8. LANL 1994-084 observes multiple GMCs in the vicinity of local noon at ∼0030 UT, 0100–0140 UT, and probably at ∼0230–0300 UT. Simultaneous increases of the RE and RI ratios up to the magnetosheath threshold values indicate the magnetopause approaches to the Earth such that the satellite entrances to the LLBL and further into the magnetosheath. At 0250–0350 UT the multiple GMCs are observed in the prenoon sector by LANL-97A. The plasma observations (ratio RI) of two satellites at 0200–0400 UT demonstrate pretty high cross-correlation up to r ∼ 0.5 with the time shift τ = 0 min. Note that we calculate the cross-correlations within ∼1-hour time intervals in order to eliminate long-duration trends in the time profiles. Hence we can conclude that the prenoon and postnoon sectors of the magnetopause oscillate in phase with the same period of ∼5 min. Such Pc5-type waves are not very regular, but they appear to have high amplitudes at times, judging from the separation that occurs between O+ and H+ in the E/q spectrum (not shown). This separation occurs when the plasma has a large E × B drift in the wave electric field; the same drift velocity gives 16 times the E/q for O+ compared with H+. However, the overall quasi-steady convection was also quite strong, so the east-west sloshing of cold plasma that is often observed in Pc5 events was seen here as mostly a modulation in the cold ions coming from the west (i.e., in the eastward looking direction).

image

Figure 8. Solar wind and magnetospheric fluctuations accompanying multiple GMCs at 0000–0400 UT 30 October 2003. The figure represents, from top to bottom, IMF Bz in GSM; electron ratio RE and ion ratio RI calculated from the LANL 1994-084 (dotted line) and LANL-97A (solid lines) plasma data; total magnetic field measured by GOES-12 (dotted line) and GOES-10 (solid line); variations of H component of the geomagnetic field measured on stations Memambetsu (dotted line) and Leirvogur (solid line); and H-SYM index of the low-latitude geomagnetic activity (solid line) and solar wind dynamic pressure calculated from the ACE SWEPAM data (dotted line, right axis). The variation of H component on the station Memambetsu is multiplied on 10. Horizontal dashed lines in the second and third panels indicate thresholds for the magnetosheath. The multiple GMCs and corresponding magnetospheric oscillations are probably caused by the solar wind pressure pulses.

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[15] The Pc5-type waves observed by the LANL 1994-084 satellite correlate very well (r ∼ 0.7, τ = 0 min) with variations of the geomagnetic field at the station Memambetsu, which is located at the same local time. The latter has a high correlation (r ∼ 0.8, τ = 0 min) with the H-SYM index. Hence the dayside magnetopause approaches to the Earth are accompanied with increases of the low-latitude geomagnetic field. However, there is no correlation with the magnetic field measured by the GOES satellites in the dusk and night sectors as well as with high-latitude magnetic field fluctuations observed at Leirvogur magnetic station in 2-hour vicinity of the midnight. Note that until ∼0300 UT the nightside geomagnetic activity is mainly induced by strong southward IMF BZ.

[16] Finally, we have found high correlations for the quasi-periodical 5-min oscillations of the magnetopause and magnetosphere in noon region, including prenoon and postnoon sectors, and no correlations at dusk and night sectors. In addition at the bottom panel of Figure 8 one can see that long-time variations of the H-SYM index follow the SW dynamic pressure calculated from the ACE SWEPAM data. This fact leads us to assume that the geomagnetic fluctuations and the multiple GMCs could be caused by the solar wind pressure pulses. A pattern of the Pc-5 geomagnetic pulsation initiated by the pressure pulses is described in detail in several studies [Sibeck et al., 1989; Song, 1994; Motoba et al., 2003]. In that case the dayside magnetopause location and geomagnetic field strength in the vicinity of noon are driven by quasi-periodic enhancements of the SW pressure, while the magnetospheric fluctuations at the flanks are associated with Kelvin-Helmholtz waves, which propagate from the dayside toward the magnetotail [Song, 1994]. Note that on 29–31 October 2003 the solar wind speed was extremely high (>1000 km/s). Such a high speed is the main driver of the Kelvin-Helmholtz instability on the magnetopause [e.g., Engebretson et al., 1998, and references therein]. However, it is impossible to make a firm conclusion about the nature of the Pc-5 pulsations on 30 October 2003 without high time resolution solar wind plasma data, which are not available for the given period.

[17] Figure 9 represents another time interval with multiple GMCs at 0400–1000 UT 31 October 2003. A long-lasting magnetosheath interval at ∼0530–0710 UT is caused by significant enhancement of the SW pressure, which is accompanied by large positive variation of the H-SYM index. At 0400–1000 UT the geomagnetic field demonstrates giant pulsations with amplitude of tens of nT and period of ∼6 to 8 min. One can see a prominent coherence in the geomagnetic pulsations and magnetopause oscillations at different local time. The cross-correlation between the LANL-97A plasma measurements (ratios RI and RE) in vicinity of noon and LANL 1994-084 measurements in the postnoon sector is better than r = 0.4 (τ = 0 min), excepting the long-lasting magnetosheath interval. Note that the moderate correlation coefficients for the LANL plasma parameters RI and RE are mainly due to their strong nonlinear dependence on the magnetopause location: inside the magnetosphere (magnetosheath) their large (small) values change slightly, while sharp increases (decreases) correspond to magnetosheath entrances (exits). In a case of such nonlinear response, more important parameter is the time delay, which should be equal to 0 min for coherent oscillations. Variations of the RI and RE ratios correlate well (r > 0.5, τ = 0 min) with the low-latitude geomagnetic field represented by H-SYM index. The correlation vanishes when the LANL satellites entrance to the magnetosheath. At 0400–0530 the LANL 1994-084 plasma data in the postnoon sector and the GOES-10 magnetic data in the dusk sector demonstrate pretty high cross-correlation r ∼ 0.5 (τ = 0 min). The high-altitude geomagnetic activity measured at Leirvogur magnetic station in the dawn sector correlates (r = 0.4 ∼ 0.6, τ = 0 min) with low-latitude magnetic fluctuations detected at Memambetsu in the postnoon sector. Note that the latter have very high correlation (r = 0.95, τ = 0 min) with the H-SYM index. There is no correlation with the GOES magnetic measurements performed in vicinity of midnight, when the satellites pass through the plasmasheath.

image

Figure 9. The same as in Figure 8 but for time interval 0400–1000 UT 31 October 2003. The multiple GMCs are associated with high-amplitude global mode magnetospheric pulsations.

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[18] Hence in the present case the coherent (i.e., time delay τ = 0 min) Pc-5 geomagnetic pulsations are observed at different local times and latitudes both at the Earth surface and in the space and they correlate with the dayside magnetopause oscillations. The Pc-5 pulsations are accompanied with fast variations of the SW pressure. The pressure variations have relatively small amplitude (only a few nPa), which is apparently insufficient for the direct driving of high-amplitude geomagnetic oscillations. Hence the fast weak variations of the solar wind pressure may only play role of an initiator of the giant Pc-5 pulsation. It is interesting to note a double-frequency oscillation (period of about 3∼4 min) observed in the postmidnight and dawn sectors by GOES-12 and at magnetic station Leirvogur that could indicate an additional high-frequency source of magnetic oscillations in this longitudinal region. However, 1-min time resolution does not permit detail consideration of the so fast oscillations.

[19] A pattern of the Pc-5 pulsation at 0400–1000 UT 31 October is very similar to the magnetospheric oscillations of global magnetospheric mode or cavity resonance [Korotova and Sibeck, 1994; Huang et al., 2003]. Note that the previous studies of the global mode were based mainly on the analysis of the ground magnetic field oscillations observed at different latitudes from the equatorial to auroral region. In the present study we demonstrate very good correlation between the global geomagnetic oscillations observed at different longitudes and altitudes from the Earth surface to the magnetopause. The coherence is additional evidence that the oscillations on 31 October should be attributed rather to a global mode than to local ionospheric or inner magnetospheric disturbances. Note that convincing proof of the global magnetospheric mode requires data with higher time resolution (say seconds), which permits distinguishing between standing and propagating waves. However, the presence of giant geomagnetic pulsations with amplitude of tens nT under relatively week solar wind pressure fluctuations of a few nPa testifies against compression waves, which are generated by the pressure fluctuations and propagate from the outer to inner magnetosphere.

[20] We can use the GMCs observed simultaneously by LANL 1994-084 and LANL-97A in order to estimate the amplitude of the magnetopause oscillations on 31 October (see Table 2). A magnetopause model by Kuznetsov and Suvorova [1998] (hereafter KS98 model) and a modified version of Chao et al.'s [2002] model [Yang et al., 2003] (hereafter Ch03 model) is applied for calculation of the solar wind pressures Pd1 and Pd2 required for the GMCs, which are observed at ∼0500 UT, respectively, by LANL-97A at 1213 LT and LANL 1994-084 at 1500 LT satellites. The pressure ratio is used to determine the ratio of distances to the magnetopause at given LT:

  • equation image
Table 2. Amplitude of the Global Magnetospheric Oscillations at 500 UT 31 October 2003
 Pd1, nPa L7Pd2, nPa L4ΔR, RE
1213 LTa1500 LTa
  • a

    Local time in GSM.

KS9824.230.40.26
CH0341.670.60.6

[21] Hence when LANL 1994-084 observes the GMC at 1500 LT the distance to the magnetopause R2 at 1213 LT should be smaller. The difference between R1 and R2 determines the amplitude of the magnetopause oscillation:

  • equation image

[22] We subscribe the distance R1 = 6.6 to the GMC observed by LANL-97A at 1213 LT. The magnetopause distance R2 is obtained for local time 1500 LT using equation (2), where the pressure ratio is calculated from the magnetopause models. The KS98 and Ch03 models provide different estimations for the amplitude ΔR, and, thus, we obtain a range for the amplitude of oscillations ΔR = 0.26∼0.6 RE. Hence during global mode magnetospheric pulsation, the magnetosphere size varies significantly: up to 5∼10% in the noon region.

4. Application of the Magnetopause Models

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. GMC Identification
  5. 3. Multiple GMCs
  6. 4. Application of the Magnetopause Models
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

[23] Because of discontinuous and ambiguous solar wind plasma data on 29–31 October 2003 [Skoug et al., 2004; Dmitriev et al., 2005], magnetopause models can not be applied and tested for the considered interval. Instead we use the magnetopause models KS98, Ch03, and a model by Shue et al. [1998] (hereafter Sh98 model) for estimation of the solar wind dynamic pressure, which is required for a GMC at given direction and for given IMF Bz. By this way, we shall study the magnetopause dawn-dusk asymmetry.

[24] From the magnetosheath intervals we obtain the lower level of dynamic pressure. In other words, when geosynchronous satellite is located in the magnetosheath, the actual pressure might be higher than the model prediction, and vice versa, from magnetosphere intervals (when a satellite is located inside the magnetosphere) the upper level of the Pd is obtained. Calculation of the upper and lower dynamic pressures for each time moment permits to construct a “corridor” of possible values of the SW dynamic pressure. In calculations based on several geosynchronous satellites we take the maximum of lower pressures and the minimum of upper pressures within a time window because of difference in time resolution and in moments of the measurements for different satellites. In the present study we choose 5-min width of the time window.

[25] Figure 10 represents the SW dynamic pressure predicted from the KS98, Sh98, and Ch03 magnetopause models. Note that the Sh98 and Ch03 models take into account helium contribution of its average value of 4% into the dynamic pressure. Because in the 29–31 October event the He contribution is unknown, the pressure calculated from the Sh98 and Ch03 models is divided by 1.16 in order to obtain a “pure” dynamic pressure of the solar wind protons. As a rule, the largest and smallest pressures are obtained for the Ch03 and KS98 models, respectively. The difference between the models is owing to two factors. First, in the subsolar point the KS98 model always predicts smaller SW dynamic pressures. Second, contrary to the symmetrical Sh98 and Ch03 models, the KS98 model represents asymmetrical magnetopause shape relative to the Sun-Earth line.

image

Figure 10. Solar wind dynamic pressure calculated for the magnetosheath and magnetosphere intervals using the ACE data on interplanetary magnetic field (IMF) and different magnetopause models (top) Ch03, (middle) Sh98, and (bottom) KS98. The upper and lower levels of the estimated pressure are indicated by gray and black circles, respectively.

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[26] Comparison of the model predictions in the GSM equatorial plane is shown in Figure 11. The magnetopause axis for the KS98 model is shifted toward the dusk of dY = 2 RE for strong southward IMF (Bz ∼ −20 nT) such that the SW pressure required for GMC in the afternoon sector is much larger than in the prenoon sector. The magnetopause shifting permits a prediction of the magnetosheath intervals in the prenoon sector while the postnoon part of geosynchronous orbit is still located in the magnetosphere.

image

Figure 11. Magnetopause cross section in the GSM equatorial plane predicted by models KS98 (dotted line), Sh98 (dashed line), and Ch03 (thick solid line) for strongly disturbed solar wind conditions (Pd = 10 nPa and Bz = −20 nT). Geosynchronous orbit is indicated by thin line. The asymmetrical model by KS98 predicts the magnetopause, which is shifted on dY = 2 RE toward the dusk. Owing to the shift the magnetosheath occupies geosynchronous orbit at ∼0900 LT, while at ∼1500 LT the geosynchronous orbit is located in the magnetosphere.

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[27] Several cases of the magnetopause dawn-dusk asymmetry can be found in Figure 6. For example, on 29 October at about 1830 UT the LANL 1991-080 (L1) observes MS interval at ∼0730 LT, while the 1990-095 (L0) does not observe any GMC at ∼1600 LT. The difference of the local time shifts from noon, 4.5 hours toward the dawn for the 1991-080 and 4 hours toward the dusk for the 1990-095, indicates to a duskward skewing of the dayside magnetopause. Otherwise the MS interval should be observed by 1990-095, which is located closer to noon than 1991-080. In other words, the revealing of duskward magnetopause skewing is based on two conditions. First, one geosynchronous satellite is situated in the magnetosheath in the prenoon sector, while in the postnoon sector another geosynchronous satellite is situated inside the magnetosphere. Second, distance of the latter satellite to noon is the same or smaller than for the former satellite. In a case of dawnward skewing the conditions are reversed relative to noon.

[28] Note that we can tell nothing about the dawn-dusk asymmetry if the above mentioned conditions are not satisfied. As an example we can consider a case of GMCs, which are observed under very large negative IMF Bz ∼ −20 nT at 2330 UT on 29 October by LANL satellites 1994-084 (at ∼0800 LT), 1991-080 (at ∼1130 LT), and GOES-10 (at ∼1330 LT). At that time GOES-12 is located at ∼1800 LT and it does not observe any GMCs. As one can see, the GOES-10 is located closer to noon in the postnoon sector than 1994-084 in the prenoon sector. However, the GOES-10 is situated in the magnetosheath. The observations show us that the solar wind disturbance is strong enough to push the magnetopause inside geosynchronous orbit at regions, which are shifted from noon up to 4 hours. However, at the same time the strength of disturbance is insufficient to push the magnetopause deeper to provide a GMC at flanks (at 1800 LT). Using only the four above-indicated satellites, we cannot tell anything definitely about the magnetopause skewing because both the dawnward and duskward skewing are equally possible as well as the case of symmetrical magnetopause.

[29] We should emphasize that Figure 6 does not exhibit any case of the dawnward magnetopause skewing. On the other hand, the prominent duskward skewing is easily revealed at ∼2300 UT 29 October for the satellites 1994-084 (L4) and GOES-10 (G0); at ∼0015 UT 30 October for 1994-084 (L4) and 1991-080 (L1); and at ∼2300 UT 30 October for 1994-084 (L4) and GOES-10 (G0). It is important to note that the dawn-dusk asymmetry is observed only on the main phase or in maximum of geomagnetic storms, which are accompanied mainly with a southward IMF.

[30] Figure 12 represents a case of the duskward magnetopause skewing in the maximum of great geomagnetic storm, when the IMF BZ was positive at 2314–2340 UT 30 October 2003. At that time the GOES-10 was located in the postnoon magnetosphere (1427–1453 LT), in which the satellite entered from the magnetosheath at 2240 UT. In the magnetosheath, the GOES-10 observed practically the same variations of the IMF components Bz and By as the ACE in the interplanetary medium. We determine the time lag dT = 24 from the best cross-correlation between the GOES-10 and ACE magnetic measurements. The lag is very close to the time of ∼21 min, which is required for direct solar wind propagation from the ACE to GOES-10 with velocity Vx ∼ −1157 km/s measured by ACE [Skoug et al., 2004]. The LANL 1994-084 was going in the dawn sector toward noon. Until 2214 UT it was located inside the magnetosphere where the ratios RI and RE are very small (∼0.1 and 10, respectively). Magnetosheath intervals were observed at 2214–2224 UT and 2233–2240 UT when the ratios became relatively large: RI ∼ 10 and RE ∼ 100. Since 2314 UT the LANL 1994-084 entranced into LLBL, where the ratios RI and RE are slightly smaller than in the magnetosheath but much larger than in the magnetosphere. At 2314–2340 UT the local time of the LANL 1994-084 increases from 0857 LT to 0923 LT.

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Figure 12. GMC identification using LANL 1994-084 plasma measurements and GOES-10 magnetic measurements at 2200–2400 UT on 30 October 2003. The figure represents, from top to bottom, the H-SYM index; the ion ratio (RI) (solid line) and electron ratio (RE) (dotted line) calculated from the LANL plasma measurements; the Hp component (parallel to satellite spin axis) and total magnetic field H measured by GOES (solid and dotted lines, respectively); the Bz, By, and Bx magnetic field components in GSM measured by GOES and ACE (solid and dotted lines respectively); the geographic local time of the LANL and GSM local time of the GOES (solid and dotted lines, respectively). The Bz and By components measured by GOES are divided on 10 and Bx component is divided on 5. Vertical dotted and dashed lines indicate the LANL magnetosheath or LLBL entrances and exits, respectively. A duskward skewing of the magnetopause is seen under positive Bz at 2315–2335 UT.

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[31] The duskward skewing of the magnetopause can be found in Figure 12 within 17 min from 2314 UT to 2331 UT, when the local time shift from noon for the LANL 1994-084 is larger than for the GOES-10. At that time the nonradial components of the solar wind velocity are very small: Vx = 1221 km/s, Vy = −54 km/s, and Vz = 13 km/s (R. Skoug, personal communication, 2004). Such a slow nonradial flow causes a little aberration (∼2°) of the nose point toward the dusk that is equivalent to the dawnward skewing, which is opposite to the observations. At 2320–2340 UT the IMF (Bx ∼ 20 nT, By ∼ 20 nT, Bz ∼ 20 nT) is practically perpendicular to the Parker spiral that should cause an additional duskward rotation of the nose point [e.g., Walters, 1964]. Hence under given solar wind plasma and IMF conditions the magnetosphere skewing should be rather dawnward than duskward; that is not the case. This fact means that the IMF orientation along the Parker spiral as well as the southward Bz play a minor role in the magnetopause dawn-dusk asymmetry, while the major role belongs to the internal magnetospheric sources, such as strong asymmetrical ring current [Dmitriev et al., 2004]. In the present case the duskward skewing of the dayside magnetosphere can be interpreted as a manifestation of the very strong ring current (Dst ∼ −400 nT), which should have a remnant asymmetry within ∼1 hour after the IMF Bz reversal from the south to north.

[32] The magnetosphere duskward skewing can be revealed also in Figure 10 as “contradictory” model predictions, when the lower pressure is larger than the upper one. A part of those cases is due to strong and fast IMF BZ variations about zero within a 5-min interval, such that the maximum pressure required for magnetosheath interval appears larger than the minimum pressure required for magnetosphere interval. However those variations take place only in few cases: at ∼0600–0900 UT 29 October, ∼1600–1800 UT 30 October, and at 0000–0200 UT 31 October. The other cases are associated with incorrect prediction of the asymmetrical magnetopause shape. In this case the symmetrical magnetopause models (Sh98 and Ch03) predict that the maximum Pd required for GMC in the prenoon sector is smaller than the minimum Pd required for the magnetosphere interval in the postnoon sector.

[33] Figure 13 demonstrates this effect clearly for the Ch03 model; at 1835 UT, 2240–2300 UT on 29 October and at 0015–0030 UT on 30 October the black circles corresponding to the lower pressures are located above the gray circles corresponding to the upper pressures. The KS98 model provides contradictory predictions more often because of overestimation of the dawn-dusk magnetopause asymmetry; the model predicts too high solar wind pressures for GMCs in the afternoon sector in comparison with the prenoon sector. However, at 2240–2300 UT on 29 October and at 0015–0030 UT on 30 October the KS98 model predicts correctly the magnetopause dawn-dusk asymmetry, while the predictions of the Sh98 and Ch03 models are contradictory.

image

Figure 13. Detail comparison of the solar wind dynamic pressure Pd estimated from Ch03 and KS98 models during time interval from 1800 UT 29 October to 0100 UT 30 October 2003. The figure represents, from top to bottom, the H-SYM index; the IMF clock (solid line) and azimuth (dotted line) angles; the IMF Bz in GSM; the upper (gray circles) and lower (black circles) Pd calculated from Ch03 and KS98 models, respectively; and the magnetosheath intervals (black thick lines) observed by geosynchronous satellites. The IMF orientation aligned with the Parker spiral (azimuth angle of –45°) is indicated by gray dashed line on the second panel. Cases when the upper Pd is under the lower Pd calculated from the symmetrical Ch03 model are attributed to the magnetopause dawn-dusk asymmetry. The same cases for the asymmetrical KS98 model testify to the model overestimation of the asymmetry.

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[34] Statistically, the KS98 model's contradictory predictions during the Halloween event can be minimized simply by decreasing the duskward magnetopause shifting from the initial dY = 2 RE to dY = 0.2∼0.3 RE for strong negative IMF Bz. However in some cases accompanied even with moderate negative Bz ∼ −10 nT (at ∼0015 UT 30 October) the magnetopause dawn-dusk asymmetry is so large that the shifting of more than dY = 0.4 RE is required. Hence the IMF Bz influences indirectly to the magnetopause dawn-dusk asymmetry. In Figure 13 one can see that the asymmetry is accompanied with different IMF azimuth angles. In other words the IMF is not necessarily aligned with the Parker spiral (azimuth angle of −45°) to provide the magnetopause asymmetry. This fact supports results of the previous statistical studies of the magnetopause dawn-dusk asymmetry under strongly disturbed conditions.

5. Summary

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. GMC Identification
  5. 3. Multiple GMCs
  6. 4. Application of the Magnetopause Models
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

[35] During extremely disturbed interplanetary and geomagnetic conditions on 29–31 October 2003 the dayside magnetosphere size is very small such that the dayside magnetopause is located inside geosynchronous orbit. The Halloween event is one of a few unique events in which continuous magnetosheath intervals were observed during up to 4 hours and numerous GMCs are occurred within 3 consequent days. The GOES magnetic data and LANL plasma data enable us to find at least two long-lasting intervals (>12 hours) during 29–31 October when numerous GMCs are observed in a wide range of local time from 0700 LT to 1600 LT. However, GMC identification using LANL plasma data is difficult and sometime ambiguous due to a radiation background, which is originated from very intensive fluxes of solar energetic particles. Owing to the “white noise” associated with the radiation background in the LANL spectrograms, a careful data clearing and verification is required before a numerical analysis.

[36] Strong compression of the magnetosphere is accompanied sometimes with large-amplitude Pc5 pulsations. We showed that during the compression at 0000–0400 UT on 30 October the pattern of magnetospheric oscillations resembles the Pc5-type pulsations, which are initiated by the solar wind pressure pulses accompanying with Kelvin-Helmholtz waves on the magnetopause. At 0400–1000 UT 31 October we reveal the global magnetospheric mode pulsation. The quasi-harmonic coherent oscillations of the geomagnetic field with period of 4–6 min are observed at different longitudinal and latitudinal locations and they are accompanied with multiple GMCs. The Halloween event provides a unique opportunity to estimate the amplitude of the global mode oscillations for strongly compressed dayside magnetosphere of about 0.3∼0.6 RE in the vicinity of noon.

[37] Application of the KS98, Sh98, and Ch03 magnetopause models for the observed GMCs permits studying the magnetopause dawn-dusk asymmetry. Our method is based on observations of at least two geosynchronous satellites that is similar to a method suggested by McComas et al. [1993, 1994]. However, in the present study we apply the empirical magnetopause models for numerical estimation of the magnitude of asymmetry. The asymmetry is represented as a shift of the dayside magnetopause toward the dusk on dY = 0.2∼0.3 RE in average, which is much smaller than the shift reported previously by Kuznetsov and Suvorova [1998]. However, the shift varies from the case to case and can exceed dY = 0.4 RE. Note that geometrically such a shift is equivalent to dawnward rotation of the magnetopause nose point of several degrees that does not contradict the statistical results by Dmitriev et al. [2004]. In the present study we found that the magnetopause asymmetry is observed during either the main phase or maximum of strong geomagnetic storms, which were accompanied with both southward and northward IMF. Hence the dawn-dusk magnetopause asymmetry should be related rather to the magnetospheric storm-time conditions than to the IMF orientation or negative Bz. This fact supports the previous suggestions of an internal source of the magnetopause dawn-dusk asymmetry such as asymmetrical ring current.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. GMC Identification
  5. 3. Multiple GMCs
  6. 4. Application of the Magnetopause Models
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

[38] The authors thank NASA/GSFC ISTP for providing data from the Geotail, ACE, GOES, and LANL satellites. We thank T. Nagai from Tokyo Institute of Technology Earth and Planetary Sciences for providing the Geotail magnetic data, L. A. Frank from the University of Iowa for providing the Geotail plasma data, C. W. Smith from the University of New Hampshire for providing the ACE magnetic data, and R. Skoug from Los Alamos National Laboratory for providing the ACE plasma data. We also thank NASA and NOAA for providing the GOES magnetic data, Los Alamos National Laboratory for providing the LANL plasma data, and Kyoto World Data Center for Geomagnetism for providing the Dst and ASY/SYM indices. This work was supported by grant NSC92-2811-M-008-021.

[39] Lou-Chuang Lee thanks David G. Sibeck and another reviewer for their assistance in evaluating this paper.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. GMC Identification
  5. 3. Multiple GMCs
  6. 4. Application of the Magnetopause Models
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. GMC Identification
  5. 3. Multiple GMCs
  6. 4. Application of the Magnetopause Models
  7. 5. Summary
  8. Acknowledgments
  9. References
  10. Supporting Information
FilenameFormatSizeDescription
jgra17529-sup-0001-t01.txtplain text document1KTab-delimited Table 1.
jgra17529-sup-0002-t02.txtplain text document0KTab-delimited Table 2.

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