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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, A04217, doi:10.1029/2008JA013871, 2009

Magnetic latitude and local time dependence of the amplitude of geomagnetic sudden commencements Atsuki Shinbori,1 Yuji Tsuji,1 Takashi Kikuchi,1 Tohru Araki,2 and Shinichi Watari3 Received 1 November 2008; revised 14 February 2009; accepted 2 March 2009; published 28 April 2009.

[1] Statistical analysis of the main impulse (MI) amplitude of geomagnetic sudden

commencements (SCs) in a region from the middle latitudes to equator has been made using the long-term geomagnetic field data obtained from the Yap (geomagnetic latitude, q = 0.38°), Guam (q = 5.22°), Okinawa (q = 16.54°), Kakioka (q = 27.18°), Memanbetsu (q = 35.16°), and St. Paratunka (q = 45.58°) stations. The magnetic local time (MLT) dependence of SC amplitude in the middle latitudes showed magnetic field variations produced by two-cell ionospheric currents (DP 2-type currents) which are driven by the dawn-to-dusk electric field accompanying a pair of field-aligned currents (FACs). The effect of the DP 2-type currents at least expands to the low latitude (q = 16.54°). In this region, the DL part of SC produced by the enhanced Chapman-Ferraro currents can be dominant, but the DP part of SC contaminated 7% of the DL one. On the other hand, at the daytime equator between 8:00 and 16:00 (MLT), the SC amplitude is considerably enhanced with its peak amplitude of 3.24 (normalized SYM-H value) around 11:00 (MLT) due to the Cowling effect. Another interesting point is that the SC amplitude in the nighttime sector was enhanced at all the stations again, and its peak value increases with increasing magnetic latitude. This result suggests that the effect of the FACs associated with the MI phase of SC expands to the equator. Citation: Shinbori, A., Y. Tsuji, T. Kikuchi, T. Araki, and S. Watari (2009), Magnetic latitude and local time dependence of the amplitude of geomagnetic sudden commencements, J. Geophys. Res., 114, A04217, doi:10.1029/2008JA013871.

1. Introduction [2] Two-cell ionospheric currents (DP 2-type currents), which cause geomagnetic field variations in high latitudes, are produced by the large-scale convection electric fields. They are generated by two processes of solar windmagnetosphere interaction. One is the magnetic reconnection process at the dayside magnetopause between the Earth’s magnetic field and southward interplanetary magnetic field (IMF) [e.g., Dungey, 1961; Alexeev and Belenkaya, 1983]. The two-cell convection strongly depends on the magnitude of southward IMF and solar wind velocity [e.g., Dungey, 1961]. Therefore the DP 2 geomagnetic field variations caused by the dawn-to-dusk electric field have a one-to-one correspondence to the directions of northward and southward IMF [e.g., Nishida et al., 1966; Nishida, 1968]. On the other hand, when the IMF is stably directed northward, the high-latitude convection structure indicates four-cell patterns due to the lobe reconnection around the cusp [e.g., Russell, 1972; Maezawa, 1976]. 1 Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan. 2 State Oceanic Administration Key Laboratory for Polar Science, Polar Research Institute of China, Shanghai, China. 3 Applied Electromagnetic Research Center, National Institute of Information and Communications Technology, Tokyo, Japan.

Copyright 2009 by the American Geophysical Union. 0148-0227/09/2008JA013871

[3] Another is the magnetospheric compression associated with a rapid enhancement of solar wind dynamic pressure due to solar wind shocks or discontinuities [e.g., Araki, 1977, 1994; Fujita et al., 2003a; Shinbori et al., 2004, 2006]. This process manifests rapid enhancements of the Chapman-Ferraro currents in the dayside magnetopause and plasma convection in the inner magnetosphere. The Chapman-Ferraro currents rapidly enhance the H component geomagnetic field within 1 – 10 minutes in low and middle latitudes, while the plasma convection yields fieldaligned currents (FACs) resembling region-1 FACs [e.g., Araki, 1994]. In this case, the DP 2-type currents newly appear in the daytime sector of the lower latitude than twocell or four-cell convection region [e.g., Araki, 1977, 1994; Fujita et al., 2003b]. This current system produces negative and positive magnetic field variations in the morning and afternoon sectors around subauroral latitudes, respectively [Russell and Ginskey, 1995]. This phenomenon has been called geomagnetic sudden commencements (SCs). [4] Since the global simultaneous occurrence of the SC phenomena with a clear onset time and the well identified sources are the main characteristics distinguishing SCs from other magnetic field disturbances such as substorms and storms, SCs provide us with a fundamental understanding of a transient response of the magnetosphere and ionosphere to the solar wind. Since all the ionospheric and magnetospheric currents abruptly change during the compression of the magnetosphere, we can separate magnetic field components produced by each current by investigating latitude and

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SHINBORI ET AL.: MLAT AND MLT DEPENDENCE OF SC AMPLITUDE

magnetic local time (MLT) profiles of SC amplitude. Moreover, we can infer a global distribution of electric field from the magnetic field variations produced by the ionospheric current during SCs. [5] The amplitude and waveform of magnetic field variations associated with SCs strongly depend on magnetic latitude and MLT [e.g., Matsushita, 1962; Araki, 1977, 1994]. At the afternoon sector of the auroral zone, the magnetic field variations of SCs start with a preliminary reverse impulse (PRI) with a short timescale within 1 – 2 minutes followed by a positive main impulse (MI). The period of the MI phase of SCs usually continues for several to ten minutes. On the other hand, the mirror-image waveform with that in the afternoon sector appears in the morning sector of the high-latitude region [Nagata, 1952; Matsushita, 1962; Araki, 1977, 1994]. The PRI currents are generated by the dusk-to-dawn electric field carried by field-aligned currents (FACs) originated inside the magnetopause [Tamao, 1964; Kikuchi and Araki, 1979a; Araki, 1994; Fujita et al., 2003a; Chi et al., 2001, 2006; T. Kikuchi et al., Simultaneity within a few seconds of the global preliminary impulse of geomagnetic sudden commencement and its explanation by means of the Earth-ionosphere waveguide model, submitted to Journal of Geophysical Research, 2009]. The electric field which drives the ionospheric Hall and Pedersen currents is instantaneously transmitted to the low latitude and equator [e.g., Kikuchi et al., 1978; Kikuchi and Araki, 1979b; Chi et al., 2001, 2006]. The detail paradigm of physical model and interpretation of SCs has been reviewed in the papers of Araki [1994]. [6] Recently, Kikuchi et al. [2001] found a new feature of the PPI (preliminary positive impulse [Kikuchi and Araki, 1985]) occurrence which shows the PPI tends to appear in the afternoon middle latitudes, and made the model calculation of a three-dimensional current circuit including ionospheric currents and FACs. Based on the model calculation results, Kikuchi et al. [2001] proposed that the generation mechanism of the afternoon PPI is the magnetic effect of the FACs which are accompanied with the dusk-to-dawn electric fields during the PI phase of SCs. On the other hand, Sastri et al. [2006] investigated IMF Bz dependence of occurrence feature of PRI at the equator using dip equatorial stations of the Circum-pan Pacific Magnetometer Network (CPMM). Their results suggested that the PRI phenomena produced by the dusk-to-dawn electric field frequently appear in the dayside equator in the case of southward IMF condition around a raid enhancement of solar wind dynamic pressure. [7] Since the Chapman-Ferraro currents are most enhanced in the dayside magnetopause during the magnetospheric compression, the amplitude of the DL field is expected to be much larger in the daytime sector than in the nighttime one. On the other hand, an enhancement of the magnetotail currents associated with SCs produces the negative variation of the H component with the largest amplitude in the midnight sector. Therefore these magnetospheric currents are expected to produce magnetic field variations with a strong day-night asymmetry. In fact, the SC amplitude observed at the geostationary orbit (r = 6.6 Re, Re: 6375 km) shows a clear MLT dependence with its maximum and minimum values around noon and midnight, respectively [Kokubun, 1983; Kuwashima and Fukunishi,

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1985]. Shinbori et al. [2004] investigated electric and magnetic field variations in the plasmasphere associated with SCs, and showed that there is no clear MLT dependence of their amplitudes. [8] Ferraro and Unthank [1951] reported a local time dependence of the averaged MI amplitude for 55 SC and 46 SI (sudden impulse) events obtained from five lowlatitude and middle-latitude stations. The averaged amplitudes indicated a maximum around the midnight sector and a minimum around the postdawn sector, respectively. The nighttime SC amplitude is different from that on the geostationary orbit. Ferraro and Unthank [1951] also showed the equatorial enhancement of the MI amplitude of SCs from the geomagnetic field variation observed at Huancayo. Russell et al. [1992, 1994b] investigated the MI amplitude of 18 and 7 SC events for steady state northward and southward interplanetary magnetic field (IMF) conditions, respectively, in order to confirm the relationship between the SC amplitude and the dynamic pressure jump around the interplanetary shock or discontinuity. Their studies showed that the SC amplitude becomes a maximum around noon in the case of the steady state northward IMF, while the SC amplitude decreases in the daytime sector and is significantly enhanced in the nighttime sector in the case of the steady state southward IMF. Russell et al. [1994b] interpreted its characteristic variations of the southward IMF condition as a cause of enhanced region-1 (R-1) FACs associated with a strong enhancement of the dayside reconnection for the daytime variations and SC-triggered substorm effect for the nighttime variation, respectively. On the other hand, Wilson et al. [2001] analyzed an SC event occurred at 16:36 (UT) on July 8, 1991, which shows a clear reduction in the X component in the daytime sector of the middle-latitude region, while the magnetic field signature indicates a sharp positive increase of the X component in the nighttime sector. In the low-latitude region, the SC amplitude of a stepwise variation is larger in the nighttime sector than in the daytime sector. Sastri [2002] also revealed that a westward electric field penetrates at the premidnight dip equator during this SC event. [9] Araki et al. [2006] reported that the averaged SC amplitude in the low and middle latitudes tends to be significantly enhanced in the nighttime sector, based on the long-term geomagnetic field data obtained from three Japanese stations. They pointed out that the nighttime enhancement is the magnetic effect of the FACs which are accompanied with the dawn-to-dusk electric fields during the MI phase of SCs. Araki et al. [2006] also pointed out that the nighttime enhancement of SC amplitude takes place in both the cases of the northward and southward IMF conditions. However, since a local time dependence of the MI field of SCs in a whole region from middle latitudes of more than 35 degrees to equator has not been analyzed in the above study, magnetic latitude and MLT dependence of the magnetic effects of FACs in the nighttime sector remains unknown. [10] In this paper, we present statistical view of magnetic latitude and MLT dependence of the MI amplitude of SCs by analyzing a large number of SC events identified by the long-term observation data with high time resolution of 1 second or 1 minute provided from the 6 stations presented by Table 1. Based on the above statistical analysis result, we

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Table 1. Lists of Magnetogram Station, Analysis Period, and SC Events

Station St. Paratunka (PTK) Memanbetsu (MMB) Kakioka (KAK) Okinawa (OKI) Guam (GAM) Yap (YAP) All station

Magnetic Magnetic Latitude Longitude (degree) (degree) 45.58 35.16 27.18 16.54 5.22 0.38

221.13 211.00 208.50 198.41 215.64 209.21

Analysis Period

No. SC Events

2000/11 – 2007/07 1987/01 – 2008/03 1981/01 – 2008/03 1996/04 – 2007/07 1985/01 – 2005/12 1998/09 – 2008/01 2000/11 – 2005/12

1315 5851 7556 1484 5787 1443 773

try to confirm latitudinal limitation of the magnetic effects of the ionospheric DP 2-type currents produced by the polar electric field in the daytime sector and the magnetic effects of FACs at the nighttime equator during the MI phase of SCs.

2. Observation Data 2.1. Identification of SC Events [11] Using the SYM-H index [Iyemori, 1990; Iyemori and Rao, 1996] with high time resolution of 1 minute, 7556 events of SCs are identified in a long period from January 1981 to March 2008. In the present analysis, the SC events have been defined as an abrupt increase of the SYM-H value with its time variation of more than 1.5 nT/min and its amplitude of more than 5 nT. Since the SYM-H index is calculated by averaging magnetic field disturbances of the H component obtained from 6 stations which are located in the middle-latitude region with approximately equal longitudinal span, magnetic field effects of the ionospheric DP 2 type currents may be eliminated as already shown in the paper of Araki et al. [1993]. They reported that the upper envelope in the scatterplot of the Dst index depends mainly on the square root of solar wind dynamic pressure jump. Therefore we can say that our identification of SC events with the SYM-H index is a good method, based on this fact by Araki et al. [1993]. However, the above criterion of SC event possibly include other geomagnetic disturbances such as an abrupt increase of the H component geomagnetic field during the early recovery phase of geomagnetic storms or positive bay phenomena associated with the onset of substorms. These geomagnetic disturbance events except for SCs identified in the SYM-H index were excluded by checking the H component geomagnetic field data obtained from several stations in the low-latitude region or a sudden enhancement of solar wind dynamic pressure in the solar wind data observed by the IMP-8, Geotail, ACE and Wind satellites. The solar wind and IMF data were provided from the CDAWeb site (http://cdaweb.gsfc.nasa.gov/). Especially, for identification of positive bay phenomena, we checked whether Pi2 magnetic pulsations [e.g., Saito et al., 1976] are found at the same time of a sudden increase of the H component in the low-latitude region of the nighttime sector. 2.2. Station Data and Determination of SC Amplitude [12] For each SC event, the precise onset time, risetime and amplitude were identified by referring geomagnetic field variations of the H component from the rapid sampling records with the time resolution of 1 second. The geomagnetic field data are provided from 6 stations of St. Paratunka

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(PTK, 52.94N, 158.25E geographic coordinates (GR), 45.58N, 221.13E geomagnetic coordinates (GM)) Memanbetsu (MMB, 43.90N, 144.20E GR, 35.16N, 211.00E GM), Kakioka (KAK, 36.23N, 140.18E GR, 27.18N, 208.50E GM), Okinawa (OKI, 24.75N, 125.33E GR, 16.87N, 198.41E GM), Guam (GAM, 13.59N, 144.87E GR, 5.30N, 215.64E GM), and Yap (YAP, 9.49N, 138.09E GR, 0.38N, 209.21E GM) around the Japan sector from the middle latitudes to equator. The geomagnetic field data obtained at the PTK, OKI, GAM and YAP stations were provided from space whether monitoring network (SWMN) operated by NICT, while those obtained at the MMB, KAK and GAM (GUA) stations were provided from WDC-C2 in Kyoto University. In determining the precise amplitude of SCs at each station, we identified a difference between the pre-SC level and the H component value at the time when the maximum was recorded at the OKI or KAK station. Moreover, in order to minimize the standard deviation of the amplitude for each SC event, we normalized the SC amplitude obtained from each station by that in the SYMH index, which depends on the change in the square root of the solar wind dynamic pressure [Araki et al., 1993]. Therefore the normalized SC amplitude reflects the contribution of other transient current systems that develop in the magnetosphere-ionosphere current circuit as a result of sudden magnetospheric compression. Moreover, since the SC amplitude obtained from the SYM-H index corresponds to the value at the equator, we include the magnetic latitude correction for the above normalization. Table 1 presents the present analysis period and SC events at each station.

3. Statistical View of Magnetic Latitude and Local Time Dependence of SC Amplitude 3.1. MLT Dependence of SC Amplitude from Middle Latitudes to Equator [13] In this section, we first present the MLT profiles of SC amplitude normalized by the SYM-H index in a region from the middle latitudes to the equator, based on the statistical analysis using the long-term geomagnetic field observation data obtained from six stations. The data period and total number of SC events analyzed are about 5 years from November 2000 to December 2005 and 773 events, respectively, as shown in the bottom of Table 1. The statistical result is shown in Figure 1 ((a) middle latitudes, (b) low latitudes, and (c) magnetic equator). The vertical and horizontal axes indicate the SC amplitude of the H component normalized by the SYM-H index and MLT, respectively. The interval of each point on the curve of the SC amplitude is 30 minutes and the value is an averaged one calculated from SC events included in 4 hour window at the center of each MLT. In Figure 1, a common feature of MLT dependence of SC amplitude at all the stations shows that two maxima appear in the daytime and nighttime sectors, while two minima appear in the dawn and dusk sectors. Their position and value vary as a function of magnetic latitude. It is interesting that the position of the second peak around the midnight sector does not depend on magnetic latitude, but the amplitude tends to increase with increase of magnetic latitude. [14] In the middle latitude (panel a), the diurnal variations of SC amplitude in the daytime sector show a strong dawn-

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Figure 1. MLT dependence of SC amplitude in a region from middle latitude to the dip equator. The vertical and horizontal axes indicate the SC amplitude normalized by the SYM-H index and MLT, respectively. The interval of each point on the curves of the MLT variations of the SC amplitude is 30 minutes. The horizontal dashed line in each panel gives the normalized amplitude of 1.0. The MLT variations of SC amplitude show a strong dependence on magnetic latitude and two maxima and minima of SC amplitude appear at all the observation points. dusk asymmetry, whose minimum and maximum values appear around morning (8 – 9 h, MLT) and afternoon (15 – 17 h, MLT), respectively. Moreover, the distribution of the SC amplitude in these regions tends to be enhanced in the

nighttime sector (20 – 03 h, MLT), again. This dependence of the SC amplitude on MLT in the daytime sector is similar to that at the subauroral latitudes (L = 3) [Russell and Ginskey, 1995], but is quite different from that observed at

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the geostationary orbit [Kokubun, 1983; Kuwashima and Fukunishi, 1985]. The maximum amplitude at the geostationary orbit tends to appear around noon. The magnitude of the dawn-dusk asymmetry of SC amplitude in the middle latitudes tends to increase with increase of magnetic latitude. The difference between the minimum and maximum values at PTK is approximately 1.6 times larger than that at MMB. Especially, it should be noted that the minimum values at both the stations become negative in the morning sector (7 – 10 h MLT). Therefore these results imply that magnetic field variations of the H component in the middle latitudes during the MI phase of SCs are dominantly produced by ionospheric Hall currents which are generated by the enhanced dawn-to-dusk electric field due to the compression of the magnetosphere [e.g., Araki, 1994; Sastri, 2002]. The Hall currents produce negative and positive variations of the H component in the dawn and dusk sectors, respectively. [15] In the low latitude (panel b), the MLT distributions of the daytime SC amplitude show that the magnitude of the dawn-dusk asymmetry seen in the middle latitudes becomes very small, and that the peak position in the afternoon sector tends to move toward noon with decrease of magnetic latitude. Especially, this MLT distribution at OKI is almost consistent with that observed at the geostationary orbit [Kokubun, 1983; Kuwashima and Fukunishi, 1985]. In addition, the peak value around noon (11 h, MLT) is approximately 1.0; that is, the SC amplitude is almost coincident with that of the SYM-H index, whose value strongly depends on the square root of solar wind dynamic pressure jump [Araki et al., 1993]. These facts mean that the SC magnetic field disturbances in the low latitudes are mainly produced by the dayside magnetopause currents. However, it should be noted that the MLT dependence of SC amplitude in the nighttime sector shows an opposite relation to that observed at the geostationary orbit. The SC amplitude in the nighttime sector of the magnetosphere (6.6 Re) becomes significantly small and magnetic field variations associated with SCs sometimes become negative around midnight [Kuwashima and Fukunishi, 1985]. The nighttime enhancement of SC amplitude in the low latitude is almost consistent with that of the local time profile at three Japanese stations reported by Araki et al. [2006], but magnetic latitude of the OKI station is lower than that of those three Japanese stations. Therefore it is newly shown that the nighttime enhancement of SC amplitude tends to occur in lower latitude region such as OKI (q = 16.54°) in the present statistical analysis. [16] In Figure 1c, the daytime distributions of SC amplitude around the equator (YAP and GAM) show a strong enhancement with its peak amplitude of 3.4 around 11:00 (MLT), which indicates a quite different character from that seen in the low and middle latitudes. This tendency of SC amplitude at the equator is almost consistent with that reported by Sugiura [1953]. The enhancement of SC amplitude can be considered as the Cowling effect [Hirono, 1952] produced by the penetration of the dawn-to-dusk convection electric field to the equatorial ionosphere via the polar ionosphere [Kikuchi and Araki, 1979b]. It should be noted that the peak position of SC amplitude is located slightly toward the morning sector. Moreover, the daytime enhancement of SC amplitude can be seen also at GAM

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about 5 degrees apart from the equator, but the peak amplitude decreases to about a half of the value at the equator. This means that the Cowling effect weakens due to the increase of the ambient magnetic field inclination. This detail explanation will be discussed in the present paper. On the other hand, the nighttime distributions of SC amplitude show a weak enhancement with its peak position around midnight at both GAM and YAP as already described in this section. The difference between the minimum and maximum values at the dawn (4 – 5 h, MLT) and midnight (0 h, MLT) is 0.44 and 0.32 at GAM and YAP, respectively. Therefore the magnitude of the nighttime enhancement at GAM is 1.38 times larger than that at YAP. This latitude dependence of the nighttime enhancement at the equator indicates the same tendency as that in the low and middle latitudes. 3.2. Characteristics of Latitudinal Profiles of SC Amplitude [17] In the previous section, we showed the detailed feature of SC amplitude profiles as a function of MLT in the middle-latitude, low-latitude, and equatorial regions during the MI phase of SCs. As a next step, in order to confirm where each component of the magnetic effects originating from ionospheric currents, field-aligned currents and Chapman-Ferraro currents dominantly appears, we investigated the latitudinal dependence of SC amplitude in four sectors (morning, 8 h; afternoon, 16 h; noon, 12 h; midnight, 0 h). Figure 2a shows that the SC amplitude in the morning sector monotonically tends to decrease with increase of magnetic latitude. The value of SC amplitude in the middle latitudes of more than 35° is negative, while that is abruptly enhanced at the equator. On the other hand, Figure 2b shows that the SC amplitude is enhanced at both the equator and middle latitudes, and that the amplitude is the smallest at OKI. The enhancement of the SC amplitude in the afternoon sector of the middle latitudes is larger than that at the equator. These profiles shown in Figures 2a and 2b indicate that the ionospheric DP 2-type currents originating from the high latitude is more dominant around the equator (less than 5°) and in the middle latitudes (more than 27°). [18] Figures 3a and 3b show latitude profiles of SC amplitude in the noon (12 h) and midnight (0 h) sectors, respectively. The two remarkable features of the latitude profile around noon are that the SC amplitude is considerably enhanced at the equator due to an intensification of the Pedersen currents associated with the Cowling effect [Hirono, 1952] and that the amplitude in higher latitudes than OKI (q = 16.54°) is almost constant without dependence of magnetic latitude. On the other hand, the latitude profile of SC amplitude in the midnight sector (0 h, Figure 3b) shows that the amplitude is more enhanced with increase of magnetic latitude different from that around noon. Especially, the nighttime enhancement becomes remarkable in low and middle latitudes of more than 16.54°. This tendency implies that the origin of the nighttime enhancement of SC amplitude exists in high latitudes. 3.3. Latitudinal Dependence of Dawn-Dusk Asymmetry of SC Amplitude [19] In this section, we investigated the latitudinal dependence of the dawn-dusk asymmetry of SC amplitude in a

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Figure 2. Latitudinal profiles of SC amplitude in (a) the morning and (b) the afternoon sectors of 8 h and 16 h, respectively. The vertical and horizontal axes indicate the normalized SC amplitude and magnetic latitude, respectively. region from the middle latitudes to the equator in order to confirm the magnetic latitude dependence of the effects of ionospheric Hall currents on SC amplitude. The magnitude of the dawn-dusk asymmetry is defined as the asymmetry index r as the following equation, r¼

min 2 min 1 ; min 1 þ min 2

ð1Þ

where the min1 and min2 indicate the minimum values of SC amplitude at the dawn and dusk sectors, respectively. Figure 4 shows a magnetic latitude profile of the asymmetry index of SC amplitude at the six stations. In Figure 4, the asymmetry index abruptly increases at the middle latitude of more than 35°, but a slightly enhancement can be seen in the low latitude from 5.22° to 27.18°. This result represents that an effect of the ionospheric Hall currents originating from the polar ionosphere significantly appears at least up to the low latitude at KAK (q = 27.18°). The asymmetry index at OKI (q = 16.54°) gives the value of 0.142. This implies that although the DL part of SC produced by the ChapmenFerraro currents is dominant, compared to the DP part due to the ionospheric Hall currents, the DP part contaminates

7% of the DL one even at the low latitude (OKI). On the other hand, in Figure 4, the asymmetry index becomes the smallest at GAM (q = 5.22°), but slightly increases from 0.068 to 0.138 at the equator (YAP). This slight enhancement of the asymmetry index at YAP is an effect of local time dependence of equatorial electrojet currents dominantly flowing in this region.

4. Dynamic Pressure Dependence of SC Amplitude in SYM-H Index [20] The response of geomagnetic field variations to sudden changes in solar wind dynamic pressure has been well reported in many papers of Siscoe et al. [1968], Ogilvie et al. [1968], Verzariu et al. [1972], Su and Konradi [1975]. Siscoe et al. [1968] proposed that the relationship between the SC amplitude DH and the associated change in the solar wind dynamic pressure Ps is given by the following equation

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n o DH ¼ k ½Psð2Þ1=2 ½ Psð1Þ1=2 ;

ð2Þ

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Figure 3. Latitudinal profiles of SC amplitude in (a) the noon and (b) midnight sectors of 12 h and 0 h, respectively. The format of these panels is the same as that of Figure 2.

Figure 4. Latitudinal profile of asymmetry index of the minimum values of SC amplitude in the dawn and dusk sectors. The vertical and horizontal axes indicate the asymmetry index and magnetic latitude, respectively. The asymmetry index abruptly increases at the middle latitude of more than 35°. 7 of 14

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where equations (1) and (2) refer to pressures before and after the event and k is a proportional constant. The constant value ranges from 13 nT/(nPa)1/2 [Siscoe et al., 1968] to 34 nT/(nPa)1/2 [Su and Konradi, 1975]. Russell et al. [1994a] showed that the coefficient of the equation (2) has a daynight asymmetry which varies from 18.4 nT/(nPa)1/2 around

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noon to 14.9 nT/(nPa)1/2 around midnight using geomagnetic field data at 20° low latitude during northward IMF. [21] In this section, we try to confirm the relationship between the SC amplitude in the SYM-H index and changes in the square root of the associated solar wind dynamic pressure before and after the solar wind shocks or discontinuities under specific condition of IMF Bz. The motivation of this analysis is to investigate the affect of the transition currents generated under the specific condition of IMF Bz on the above relationship described by the equation (2). In this analysis, we classified four types of IMF Bz change for 773 SC events: steady state northward and southward conditions and northward and southward turning of IMF Bz using the averaged value for 10 minutes before and after a sudden enhancement of solar wind dynamic pressure. The numbers of SC events for each IMF Bz condition were 326, 266, 105 and 76 events, respectively. [22] Figures 5a – 5d show the relationship between the SC amplitude in the SYM-H index and changes in the square root of solar wind dynamic pressure for four conditions of IMF Bz: (a) steady state northward condition, (b) steady state southward condition, (c) northward turning and (d) southward turning, respectively. In all the cases of the IMF Bz condition, the SC amplitude is linearly proportional to the change in the square root of solar wind dynamic pressure. The proportional coefficient of each linear fitted line ranges from 19.6 nT/(nPa)1/2 to 20.1 nT/(nPa)1/2 with correlation coefficient of 0.7– 0.9. It should be noted that the proportional coefficient does not almost vary for all the cases of the IMF Bz condition and that the value is almost consistent with that of about 20 nT/(nPa)1/2 at the equator around noon predicted by the Tsyganenko model [Russell et al., 1994a]. This fact suggests that the SC amplitude in the SYM-H index is almost determined by the enhanced Chapman-Ferraro currents which produce the northward magnetic field parallel to the ambient magnetic field in the magnetosphere pointed out by Araki et al. [1993]. [23] Furthermore, in order to investigate the effects of the transition current system under the condition of northward turning of IMF Bz from Bz 0 to Bz > 0 proposed by Clauer et al. [2001], we analyzed the SC events occurred under the above IMF Bz condition. In the present analysis, we cannot find the SC events with just the null value of the IMF Bz before an abrupt increase of solar wind dynamic pressure. However, there exist 12 SC events with the IMF Bz near the null value of less than 0.5 nT in the events of (a) steady state northward condition and (c) northward turning of IMF Bz before the solar wind dynamic pressure enhancement. As a result, the SC amplitude is also linearly proportional to the change in the square root of solar wind dynamic pressure (not showing). The proportional coefficient of each linear fitted line ranges from 19.6 nT/(nPa)1/2 with correlation coefficient of 0.9. These parameters are Figure 5. Change in SC amplitude in the SYM-H index as a function of the square root of solar wind dynamic pressure. Figures 5a and 5b correspond to the cases of steady state northward and southward IMF conditions around the solar wind shock or discontinuities, respectively. Figures 5c and 5d correspond to the cases of northward and southward turning of IMF, respectively. R in the right bottom of each panel means the correlation coefficient.

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almost consistent with the results of the cases (a) – (d). Therefore it seems that this IMF Bz condition does not lead to a significant contribution of the transition current system to the SC amplitude.

5. Discussion 5.1. Affects of SC Amplitude in SYM-H Index Under Specific IMF Bz Condition [24] Geomagnetic field variations in low latitudes associated with a sudden enhancement of solar wind dynamic pressure are superposed by the effects of enhanced Chapman-Ferraro currents DHCF, ring currents DHRC, tail currents DHTC and other transition currents DHTR. The Chapman-Ferraro currents enhance the magnetic field intensity in all the MLT sectors on the ground, while the ring and tail currents reduce it. Therefore in the case when the significant enhancements of the ring and tail currents take place almost simultaneously with the compression of the magnetosphere, the SC amplitude in low latitudes on the ground tends to become smaller than that expected by only the Chapman-Ferraro currents. Russell et al. [1994a] interpreted a significant suppression of the nighttime SC amplitude in low latitudes as the effect of the tail currents. On the other hand, the magnetic effects due to other transition currents in the magnetosphere strongly depend on MLT and magnetic latitude [Clauer et al., 2001]. [25] Based on the above discussion, assuming that the DH (=DHCF) in equation (2) consists only of the origin of the Chapman-Ferraro currents, the observed magnetic field disturbances DHOB can be written by the following equation DHOB ¼ DH DHRC DHTC DHTR :

ð3Þ

Eliminating DH from the equations (2) and (3), we can rewrite the equation (2) as DHOB ¼

n o k ½ Psð2Þ1=2 ½ Psð1Þ1=2 ; 1þa b

ð4Þ

where a and b indicate the values of (DHRC + DHTC)/ DHCF and DH TR/DH CF, respectively. This equation suggests that the proportional coefficient depends on the magnitude of magnetic field disturbances produced by ring and tail currents and other transition currents. For example, in the case that ring and tail currents are significantly enhanced during SCs under steady state southward IMF condition, it is expected that the proportional coefficient of equation (4) tends to reduce with dependence of their magnitude. [26] The present analysis result shown in Figure 5 showed that the proportional coefficient of the linear fitted line ranges from 19.6 nT/(nPa)1/2 to 20.1 nT/(nPa)1/2 and does not almost vary under each specific IMF Bz condition: (a) steady state northward condition, (b) steady state southward condition, (c) northward turning and (d) southward turning. This implies that there are almost no affect of ring

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and tail currents. On the other hand, since there are no SC events with just the null value of the IMF Bz before the solar wind dynamic pressure enhancement in the present analysis, we could not find the contribution of the effects of the transition currents as pointed out by Clauer et al. [2001]. Therefore it can be concluded that the SC amplitude in the SYM-H index are purely or mainly produced by the Chapman-Ferraro currents described by Araki et al. [1993]. From the above discussion, by normalizing the observed SC amplitude at each station by the SYM-H value, we can derive magnetic latitude and local time dependence of magnetic field signatures originating from SC current system. 5.2. Interpretation of Magnetic Latitude and Local Time Dependence of SC Amplitude [27] Due to the arrival of an abrupt increase of solar wind dynamic pressure to the dayside magnetopause, the magnetosphere is suddenly compressed and the Chapman-Ferraro currents are enhanced. The enhanced Chapman-Ferraro currents increases the ambient magnetic field in the magnetosphere, and the information of the enhanced magnetic field propagates toward the earth as a fast-mode HM wave [e.g., Tamao, 1964; Wilken et al., 1982; Araki, 1994; Chi et al., 2001, 2006; Fujita et al., 2003a; Shinbori et al., 2004]. The amplitude of the enhanced magnetic field tends to become the maximum around noon at the geostationary orbit (r = 6.6 Re) [Kokubun, 1983; Kuwashima and Fukunishi, 1985] and low latitudes on the ground [e.g., Russell et al., 1994a]. The MLT dependence of SC amplitude at the low latitude (OKI) shows almost the similar tendency in the daytime sector between 6:00 and 18:00 (MLT) as shown in Figure 1b. This fact suggests that the DL part of SC is more dominant in low latitudes, compared to the DP one. However, it should be noted that the MLT profiles at OKI and KAK indicates the nighttime enhancement with a peak value around midnight. This signature is quite different from the results at the geostationary orbit [Kokubun, 1983; Kuwashima and Fukunishi, 1985]. We will be able to give the interpretation of this enhancement in low latitudes by considering the MI current system during SCs as described in the following sentences. [28] A few minutes after the passage of the HM waves in the magnetosphere, the field-aligned currents resembling region-1 (R-1) FACs are generated by plasma convection in the inner magnetosphere and carries a dawn-to-dusk electric field responsible for the DP (MI) field to the ionosphere [Araki, 1994; Fujita et al., 2003b]. An SC current circuit is composed of the R-1-type FACs and Pedersen currents caused by the dawn-to-dusk electric fields carried by the FACs. The electric fields generate the Hall currents which close in the ionosphere. This current system leads to the negative and positive magnetic field variations in the morning and afternoon sectors in the middle latitudes, respectively, and the equatorial enhancement associated with the intensified Pedersen currents due to the Cowling effect [Hirono, 1952] at the daytime equator [e.g., Araki, 1977, 1994; Kikuchi et al., 2001]. The local time and latitudinal profiles of SC amplitude in Figures 1 and 2 clearly show a strong effect of the DP 2-type currents in the daytime sector of the middle latitudes (MMB and PTK) during the MI phase of SCs. The latitudinal profile in the

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Figure 6. (a) A schematic diagram of a current circuit in the magnetosphere and ionosphere during the MI phase of SC. The current circuit is composed of the field-aligned currents associated with the convective plasma motion and Pedersen currents caused by the dawn-to-dusk electric fields carried by the FACs. The electric fields generate Hall currents which close in the ionosphere. (b) North-south component of magnetic fields produced by the FACs and ionospheric currents indicating thick (FACs), thin (Pedersen currents), and dotted (Hall currents) arrows. The DP (MI) fields correspond to the negative and positive total fields in the middle latitudes of the morning and evening sectors, respectively. In the nighttime sector, the DP (MI) fields indicate the positive total field in the entire region from the middle latitudes to the dip equator.

afternoon sector (Figure 2b) indicates that the DP 2-type currents expands to the low latitude at KAK (q = 27.18°) with the significant amplitude. Tsunomura [1995] and Russell and Ginskey [1995] confirmed that the SC amplitude in the morning sector of the middle latitude strongly reduces in the morning sector (04 – 12 h, MLT), based on the statistical analysis. Moreover, the asymmetry index at OKI (q = 16.54°) in Figure 3 gives the value of 0.142. This fact suggests that a slight effect of the DP 2-type currents exists in the daytime sector of the low latitudes. Thus the DP part of SC due to the ionospheric Hall current contaminates 7% of the DL part produced by the Chapmen-Ferraro currents at OKI. On the other hand, the local time distribution of SC amplitude (Figure 1c) showed that the equatorial enhancement due to the Cowling effect [Hirono, 1952] clearly appeared in the daytime sector between 8 and 16 h (MLT) at YAP and GAM. However, the equatorial enhancement at GAM decrease to about half of the value at YAP around noon. This latitude dependence indicates that the ionospheric conductivity above GAM is smaller than that above YAP. The ionospheric contribution to the SC amplitude is proportional toPionospheric current density (J), which P is given by J = E, where E is the electric field and is the height-integrated ionospheric conductivity. The

conductivity form including the Cowling effect is given by the following equation X C

¼

X P

X2 þX

þ P

XH k

tan2 I

;

ð5Þ

P P P where C, P, and H are the height-integratedP Cowling, Pedersen, and Hall conductivities, respectively. k and I indicate the height-integrated parallel conductivity to the magnetic field line and the inclination of magnetic field line at the altitude of E region of the ionosphere. The parallel conductivity is generally much larger than the Hall and Pedersen ones. Since the magnetic field inclination increases apart from the dip equator (I = 0), the parallel conductivity in the dominator of the second term on the right side in equation (5) becomes more effective and the Cowling conductivity become almost equal to the Pedersen conductivity which is smaller than the Cowling one. Therefore since the Cowling conductivity decreases due to the increase of the inclination of magnetic field line at the altitude of E region of the ionosphere, the peak value of SC amplitude at GAM tends to be smaller, compared to that at YAP. On the other hand, from the fact that the peak value of

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Figure 7. Model calculations of grand H component magnetic fields produced by field-aligned currents (FACs), ionospheric currents, and sum of them at (a) 35° geomagnetic latitude and (b) the dip equator. Both of the panels show that the daytime magnetic field perturbations during the MI phase of SC are dominantly produced by ionospheric currents, whereas the nighttime ones are mainly produced by FACs. normalized SC amplitude is around 3.24 around 11 h (MLT) at YAP, we can estimate the DP part of SC as about 70% of the SC amplitude. It is interesting that the peak location does not coincide with the maximum of ionospheric conductivity around 12 h (MLT) but is slightly toward the morning sector. This tendency is almost consistent with the results by Sugiura [1953] and suggests that the magnitude of the dawn-to-dusk electric fields in the equatorial ionosphere is the largest around 11 h (MLT) as predicted by simulation studies [Tsunomura and Araki, 1984; Tsunomura, 1998, 1999, 2000]. [29] Moreover, the type of R-1 FACs associated with the MI produce southward and northward magnetic field components in the daytime and nighttime sectors, respectively [Sastri, 2002; Araki et al., 2006]. The effects of the FACs lead to the reduction and enhancement of SC amplitude in the daytime and nighttime sectors, respectively. In the present statistical analysis, the nighttime enhancement of SC amplitude was clearly found with its peak value around midnight in a whole latitude range from the middle latitudes (PTK and MMB) to the equator (GAM and YAP) (Figure 1). The latitude dependence of the peak value showed that the SC amplitude tends to increase with increase of magnetic latitude (Figure 3b). This tendency suggests that the origin of the nighttime enhancement exists in high latitudes. There-

fore these results indicate that the effects of the R-1 type of the FACs generated during the MI phase of SCs appear in the nighttime sector from the middle latitudes to equator. This new finding of the nighttime enhancement of SC amplitude at the equator does not only lead to the expansion of the physical mechanism in the low and middle latitudes but also contributes to understanding of other magnetic field perturbations at the nighttime equator such as magnetic pulsation, DP 2 fluctuation and geomagnetic storms. [30] From the above discussion of the present analysis results, we can describe a schematic diagram of a current circuit in the magnetosphere and ionosphere during the MI phase of SCs as shown in Figure 6. In the middle latitudes, all parts of the magnetosphere-ionosphere current circuit (Figure 6a) contribute to the magnetic field perturbations on the ground associated with SC. In the morning sector, the FACs and Hall currents strongly tend to reduce the ground magnetic field, while the Pedersen currents tend to increase it during the MI phase of SC (Figure 6b). On the other hand, in the afternoon sector, the Hall and Pedersen currents strongly tend to enhance the ground magnetic field, while the FACs currents tend to reduce it during the MI phase of SC (Figure 6b). This tendency in the afternoon sector indicates an opposite sense to those in the morning sector. On the other hand, the nighttime enhancement of the SC amplitude

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takes place in all the latitude regions due to the magnetic effects produced by the R-1 type of FACs in high latitude. Moreover, the magnitude of the magnetic effects becomes larger with increase of magnetic latitude (Figure 6b). [31] In order to confirm the above physical process during the MI phase of SC, we performed numerical calculation of ground H component of magnetic fields in the middle latitude (q = 35°) and at the equator produced by the R-1 type of FACs and associated ionospheric Hall and Pedersen currents. The detail model calculation method and parameters have been described in the papers of Kikuchi et al. [2001] and Tsunomura [1999]. The results of the present numerical calculation are shown in Figure 7. The dashed and dotted lines in Figure 7 indicate magnetic field variations produced by FACs and ionospheric currents, respectively. The solid line gives the sum of the both current effects. Figure 7 shows that the magnetic effects of ionospheric currents are more prominent than those of the FACs both in the middle latitude and at the equator, while the FAC component almost contributes to the magnetic field variations on the ground. Moreover, the solid lines in each panel in Figure 7 well corresponds to the MLT profiles of SC amplitude in the middle latitudes (Figure 1a) and at the equator (Figure 1b). From the above results, we can interpret the nighttime enhancement of SC amplitude in the low and middle latitudes as the magnetic effects of the FACs. This interpretation supports the results by Sastri [2002] and Araki et al. [2006]. [32] The present results of statistical data analysis and numerical calculation lead to the other important evidence. Fukushima [1971] proposed that magnetic field disturbances with its positive sense produced by the Pedersen currents in these regions are canceled by magnetic effects with its negative sense due to the FAC for the case of infinitely homogeneous condition. Hence the Fukushima theory suggests that the magnetic field disturbances produced by the FAC cannot be detected by ground magnetometers. However, the present analysis results strongly indicate that the Fukushima theory [Fukushima, 1971] is not applicable under real conditions of the ionospheric conductivity with its inhomogeneity and a pair of the FACs. The main reason why the magnetic field disturbances produced by the Pedersen currents cannot cancel the magnetic effects of the FACs can be explained as the following process. The magnetic field disturbances due to the FACs are produced through the Biot-Savart’s law with dependence on the distance from the FACs location, while those due to the ionospheric Pedersen currents strongly depend on local ionospheric conductivity. Therefore the magnitude of magnetic field disturbances due to both the FACs and associated Pedersen currents is not always equal with its opposite sense. Especially, the nighttime enhancement of SC amplitude points out this problem of the Fukushima theory. 5.3. Future Works [33] In the present analysis, we did not investigate IMF dependence of magnetic latitude and local time distributions of SC amplitude from the middle latitudes to equator during the MI phase. Since the configuration of the magnetosphere and ionospheric convection patterns depends on the IMF condition, the local time dependence of SC amplitude may be affected by other transition currents except for the MI

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current system with correspondence to specific IMF condition pointed out by Clauer et al. [2001]. Araki et al. [2006] found that although the patterns of the diurnal variation of SC amplitude in middle latitudes do not very change for both the cases of steady state northward and southward IMF conditions, the day-night asymmetry produced by the R-1 type of FACs tends to become larger under the steady state southward IMF condition. In future, the IMF dependence of SC amplitude should be investigated, taking into account a seasonal variation of the ionospheric condition. Moreover, although we performed the numerical calculation with the steady state current systems, the calculation with timedependent transient current systems should also be done in future, depending on specific condition of the IMF. However, it is pointed out that even the numerical calculation with the steady state current systems well explains the statistical analysis results.

6. Conclusion [34] Statistical analysis of the main impulse (MI) amplitude of geomagnetic sudden commencements (SCs) in a region from the middle latitudes to equator has been made using the long-term geomagnetic field data obtained from the 6 stations presented by Table 1. The analysis results are summarized below. [35] 1. The MLT dependence of the SC amplitude in the daytime sector of the middle latitudes showed that the magnetic field variations in middle latitudes are produce by ionospheric Hall currents (DP 2-type currents) driven by the large-scale dawn-to-dusk electric field. The effects of the DP 2-type currents expanded at least to the low latitude region (KAK: q = 27.18°). [36] 2. The magnetic variations in the low latitude (OKI: q = 16.54°) with the maximum amplitude around noon (11 h, MLT) are dominantly produced by the DL part due to the Chapman-Ferraro currents. However, the DP part contaminated 7% of the DL one. [37] 3. At the daytime equator between 8:00 and 16:00 (MLT), the equatorial enhancement of normalized SC amplitude associated with the Cowling effect clearly appeared with its maximum value of 3.4 around 11:00 (MLT). The contribution of the DP part of SC at YAP was 69% of the enhanced SC amplitude around the peak. The amplitude at GAM reduces about 45% of that at the YAP station. [38] 4. The nighttime enhancement of SC amplitude appeared in the entire magnetic latitude range from the middle latitudes to equator and its peak value around midnight increases with increase of magnetic latitude significantly. The nighttime enhancement can be interpreted as the magnetic effects of the FACs. The new finding of the nighttime enhancements at the equator will lead to a contribution to understanding of other magnetic field perturbations at the nighttime equator such as magnetic pulsation, DP 2 fluctuation and geomagnetic storms. [39] Acknowledgments. We would like to thank the Institute of Cosmophysical Researches and Radio Wave Propagation (IKIR), P. Hattori at USGS/Guam Magnetic Observatory, U.S. NOAA/Weather Service Office at Yap, and Ryukyu University for their help in operating the NICT space weather monitoring magnetometers at St. Paratunka, Guam, Yap, and Okinawa, respectively. We also used magnetic field data at the Meman-

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batsu, Kakioka, and Guam stations, and ASY/SYM index was obtained from WDC for Geomagnetism, Kyoto. We acknowledge A. Lazarus, A. Szabo, and R. P. Lepping for the IMF-8 Solar Wind and MAG data; L. Frank and S. Kokubun for the Geotail CPI and MGF data; D. J. McComas and N. F. Ness for the ACE SWEPAM and MAG data; and R. Lin and R. Lepping for the Wind 3DP and MFI data provided through CDAWeb, respectively. A. Shinbori is also supported by a grant from Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists. [40] Amitava Bhattacharjee thanks J. Hanumath Sastri and another reviewer for their assistance in evaluating this paper.

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phase of the July 1991 magnetic storm, J. Geophys. Res., 106, 24,517 – 24,539.

T. Araki, SOA Key Laboratory for Polar Science, Polar Research Institute of China, Shanghai 200136, China. ([email protected])

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T. Kikuchi, A. Shinbori, and Y. Tsuji, Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 466-8601, Japan. (shinbori@stelab. nagoya-u.ac.jp) S. Watari, Applied Electromagnetic Research Center, National Institute of Information and Communications Technology, Tokyo 184-8795, Japan. ([email protected])

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