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Ionospheric current contribution to the main impulse of a negative sudden impulse
© Vichare et al.; licensee Springer. 2014
Received: 6 December 2013
Accepted: 29 July 2014
Published: 18 August 2014
The geomagnetic field response to a moderate-amplitude negative sudden impulse (SI−) that occurred on 14 May 2009 at 10:30 UT was examined at 97 geomagnetic observatories situated all over the globe. The response signature contains a contribution from magnetospheric as well as ionospheric currents. The main impulse (MI) is defined as the maximum depression in the observed geomagnetic field. It is observed that for low-to-high latitudes, the amplitude of the MI is larger in the afternoon to post-dusk sector than in the dawn-noon sector, indicating asymmetry in the MI amplitude. We estimated the contribution at various observatories due to the Chapman-Ferraro magnetopause currents using the Tsyganenko model (T01) and subtracted this from the observed MI amplitude to obtain the contribution due to ionospheric currents. It is found that the ionospheric currents contribute significantly to the MI amplitude of moderate SI− even at low-to-mid latitudes and that the contribution is in the same direction as that from the magnetopause currents near dusk and in the opposite direction near dawn. The equivalent current vectors reveal a clockwise (anticlockwise) ionospheric current loop in the afternoon (morning) sector during the MI of the negative pressure impulse. This evidences an ionospheric twin-cell-vortex current system (DP2) due to field-aligned currents (FACs) associated with the dusk-to-dawn convection electric field during the MI of an SI−. We also estimated the magnetic field variation due to prompt penetration electric fields, which is found to be very small at low latitudes in the present case. The studied SI− is not associated with shock, and hence no preliminary reverse impulse was evident. In addition, the summer hemisphere reveals larger MI amplitudes than the winter hemisphere, indicating once again the role of ionospheric currents.
A sudden enhancement/drop in solar wind dynamic pressure causes the sudden compression/expansion of the magnetosphere, forming a positive/negative sudden impulse (SI) in the geomagnetic field (Nishida and Jacobs 1962). The signatures of these impulses in the ground magnetic field are quite complex due to the contributions from several current systems flowing in the magnetosphere and ionosphere. Using rapid-run magnetograms, it has been observed that the geomagnetic response signature of an SI is preceded by an initial rapid variation. The broad features of the geomagnetic signature include a first rapid response, also known as a preliminary impulse (PI) with a time scale of a few seconds to a few minutes, followed by a main impulse (MI). Araki (1977) described the geomagnetic signature of SIs in terms of a preliminary impulse of polar origin (DPpi), a main impulse dominating at low latitude (DLmi), and a main impulse of polar origin (DPmi). The PI is further categorized into positive and negative PIs. If the PI is negative (positive) during enhanced (reduced) solar dynamic pressure, then it is known as a preliminary reversed impulse (PRI). The DL field is a stepwise change in the geomagnetic field, and the DP field is a bipolar type. In this paper, we define the MI of a negative SI as the maximum reduction in the total field containing DPpi, DLmi, and DPmi parts.
Over the past six decades, several studies have been dedicated to understanding the characteristics of SIs (Sugiura 1953; Nagata and Abe 1955; Matsushita 1962; Sano 1962; Tamao 1964; Nishida et al. 1966; Rastogi and Sastri 1974; Araki et al. 1985, 2006; Araki and Nagano 1988; Araki 1994; Sastri et al. 1995; Yamada et al. 1997; Takeuchi et al. 2000, 2002a; Villante and Di Giuseppe 2004; Huang et al. 2008; Shinbori et al. 2009; Wang et al. 2009). Based on the magnetic field observations on ground, an equivalent ionospheric current system for the PI has been deduced (Araki et al. 1985; Araki 1994), which is similar to the DP2 current system (Nishida et al. 1966). This current system consists of twin-vortex type ionospheric currents generated by field-aligned currents (FACs) that give a bipolar signature on the ground (Araki 1994). The MI signature mainly comprises contribution due to currents flowing at the magnetopause and transient currents generated in the ionosphere and magnetosphere. Rastogi and Sastri (1974) found that the equatorial enhancement is larger for a PRI than for an MI, highlighting the stronger role of ionospheric conductivity in a PRI than that during an MI. Attempts have been made to study the local time (LT) variation of MI amplitude; for example, Shinbori et al. (2009) studied this variation statistically. However, not many studies have been carried out to investigate the LT variation for an individual SI. Gaining an understanding of this is important, as the results may differ for various SI events because the geomagnetic field response to SIs strongly depends on factors such as the orientation and magnitude of the interplanetary magnetic field (Wing and Sibeck 1997; Lee and Lyons 2004), orientation of shocks/discontinuities (Takeuchi et al. 2002b; Wang et al. 2006), the value of pressure before disturbance, and the change in pressure amplitude (Borodkova et al. 2006). Therefore, any statistical studies performed without consideration of these aspects may be unable to provide a true picture of the LT variation, which indicates the importance of case studies.
It has been estimated that the time required for a plasma front to sweep past the geomagnetic field is not more than 30 s, but the observed rise time of SIs is a few minutes. According to the model presented by Dessler et al. (1960), a negative correlation between SI amplitude and rise time can be expected because the larger the distance of the SI source from the observer, the longer the rise time and the smaller the amplitude due to attenuation. The time duration of SI depends on the following: the time taken by the shock front or discontinuity to sweep the geoeffective distance along the magnetosphere, the thickness of the shock front or the discontinuity in the solar wind, inertia of the magnetospheric plasma, and the broadening of the wave front during the passage through the magnetosphere (Nishida 1966). Wang et al. (2006) statistically investigated the effect of the interplanetary shock strength and orientation on SI rise time and found that oblique shocks result in a longer rise time of SIs.
Nishida and Jacobs (1962) noticed a similarity between positive and negative SIs and claimed that any theory of positive impulse (SI+) should be able to explain the phenomenon of negative impulse (SI−) in an opposite sense. In support of this, several studies investigating geomagnetic field response to the negative solar dynamic pressure pulses reported that the response is opposite to that of the positive impulse (Rastogi and Sastri 1974; Araki and Nagano 1988; Sastri et al. 1995; Takeuchi et al. 2000), which indicated that negative SIs can be explained by the physical model used for positive impulses but with reversed current systems. The numerical simulation of a moderate amplitude SI− using solar wind-magnetosphere-ionosphere MHD model by Fujita et al. (2004) showed that the SI− is basically a mirror image of the SI+. However, Fujita et al. (2012) later revisited the simulations with larger amplitude SI− and concluded that SI−and SI+ are not in a mirror image relation. They listed a few differences between SI− and SI+, such as the overshielding of the convection electric field occurring in the MI phase of a negative SI event and in the PI phase of SI+, but the duration of the shielding for SI+ being shorter than that for the SI−. Using SuperDARN data, Hori et al. (2012) observed differences in the ionospheric flow during SI− and SI+, and Takeuchi et al. (2000, 2002a) also reported that the polarization distribution of SI− is not opposite to that of SI+. Thus, various studies carried out during the last decade have pointed out differences between positive and negatives SIs, and have demonstrated that an SI− is not simply mirror image of an SI+.
Tamao (1964) proposed the mechanism of an equivalent current system for a PRI, based on the three-dimensional propagation of hydromagnetic waves in cold plasma, whereas Kikuchi and Araki (1979) proposed the Earth-ionosphere waveguide TM0 mode as a mechanism of propagation of polar electric fields to the equator and applied this to the PRI occurrence at the equator. Tamao (1964) model suggested that the direct source of the PRI is an ionospheric Hall-current system caused by the incidence of mixed transverse hydromagnetic waves (pure transverse plus converted transverse), and Chi et al. (2001) observed a very good agreement between the observed rise time of the PRI and the rise time calculated using Tamao’s (1964) model. However, according to Kikuchi and Araki (2002), Tamao’s (1964) model cannot be applied at low latitudes because the rotational electric fields of the converted transverse mode do not generate the Hall currents that close in the ionosphere. They also pointed out that the Hall currents accompany the Pedersen currents in a collision-dominated ionosphere, and therefore the energy dissipated by the Pedersen currents could not be supplied by the converted transverse mode. Therefore, although both models predict and explain a few features of an SI, it appears that a more detailed investigation is required to resolve the issues described above.
This paper presents a case study of a single SI− event using all available ground-based geomagnetic observatories and geosynchronous satellite observations to enable an understanding of the characteristics of the geomagnetic field response to the negative solar wind pressure impulse. This paper is presented as follows: firstly in ‘Database’ section, we introduce the event of a negative solar wind pressure impulse along with other solar wind and interplanetary parameters. ‘Observation’ section presents the response to the SI− at geosynchronous satellites and at various ground observatories, and ‘Results and discussion’ section describes the characteristics of the SI−, such as the contribution due to sudden changes in IMF, local time variation of the MI amplitude and duration, equivalent ionospheric currents during the MI phase, latitudinal variation of MI amplitude, hemispheric asymmetry, and the geomagnetic response after attaining maximum depression. Finally, ‘Conclusions’ section discusses and summarizes the findings of the present investigation.
List of geomagnetic observatories
Geographic Lat (°)
Geographic Long (°E)
Geomagnetic Lat (°)
Geographic Lat (°)
Geographic Long (°E)
Geomagnetic Lat (°)
A statistical study by Takeuchi et al. (2002a) reported that 50% of SI− events occur in tandem with SI+ events. They found that interplanetary reversed shocks are not a primary cause of SI− events. Instead, SI− events are produced by rapid decreases in the solar wind dynamic pressure associated with a variety of interplanetary structures such as tangential discontinuities at high-low speed stream interfaces within co-rotating interaction regions (CIRs), the front boundaries of magnetic clouds, the rear boundaries of non-compressive density enhancements (NCDEs), the front boundary of a small-scale plasma hole, and earthward propagating shocks from the rear side of a solar flare (Takeuchi et al. 2002b; Rastogi et al. 2010). Takeuchi et al. (2002a) identified a pair of SI−-SI+ associated with a small-scale plasma hole embedded in a CIR, close to a heliospheric current sheet (HCS) and parallel to the Parker spiral direction.
In the present case, the southward component of the interplanetary magnetic field (IMF Bz) suddenly turned northward during a negative impulse and then remained constant until a positive enhancement of the SW density took place, where IMF Bz rapidly turned southward. In addition, the negative and positive impulses were accompanied by sudden turnings of the X and Y components of the IMF. The magnetic pressure of SW remained high between SI− and SI+, with discernible discontinuities at the edges. The interplanetary structure seems, therefore, to be bounded by tangential discontinuities. These characteristics of the IMF and the SW parameters may indicate that a cavity with a low-particle density was impinging on the Earth's magnetosphere. The scale length of the discontinuity appeared to be about 5.6 × 106 km. Using magnetic co-planarity, the orientation of the discontinuity (the angle between the discontinuity normal and the GSE X-axis) is estimated (Schwartz 1998) and was found to be parallel to the Parker spiral at 1 AU.
The bottom plot in Figure 1 depicts the SYM-H index that indicates a sudden drop at approximately 12:00 UT associated with a negative pressure pulse and an increase at 16:30 UT corresponding to a positive impulse. The MI signature during the SI− is shown between the two vertical lines in the SYM-H plot. The amplitude of the negative impulse in the SYM-H is around −27 nT. The onset of the negative pulse starts at approximately 11:50 UT and attains a minimum after a time interval of approximately 11 min. Note that the fall time of 11 min on the ground is much longer than that seen by the ACE satellite (approximately three times as long). There is also a small increase seen in the SYM-H index immediately after the negative impulse. Note that a small increase appeared in the SW dynamic pressure just after the minima at 10:44 UT, but this increase appears to be very small compared to the corresponding increase seen in the SYM-H index. However, it should be noted that just after the decrease, the SW density data was not available for a few minutes, and hence it was not possible to obtain accurate information pertaining to the SW density variation during that time period. A positive impulse is seen in SYM-H at 16:30 UT with amplitude of around 10 nT and a rise time of approximately 10 min. It is also observed that 3-h kp values during the interval did not rise above the value of 3.
Two features can be observed that occurred before the onset of the event at all three satellites: a gradual increase in the magnetic field and wavy structures. The fluctuating structures at GOES-12 and GOES-10 (both located in the nearby MLT zone in the morning hours) are similar, while those of GOES-11 (3:00 MLT) shows damped fluctuations, indicating daytime enhancement of the wavy structures. These observations can be attributed to the fluctuations riding on the steady increase of the solar wind dynamic pressure observed prior to the SI− impulse (Figure 1).
As the ground magnetic observatory at Huancayo lies on the same meridian and equatorial plane as the GOES-12 satellite, we compare the magnetic field responses at these two locations. The amplitude of the MI at the HUA station is larger (approximately −50 nT) than at GOES-12 (approximately −19 nT), and the amplitude ratio of the satellite to the ground is 0.34, which is moderately smaller than that recorded by Kokubun (1983) (0:55 at 07:00 MLT). The fall time during a negative impulse at GOES-12 is approximately 6 min, which is longer than the SW density impulse duration (4 min) and shorter than that on the ground (approximately 11 min in SYM-H).
Also note that after the minimum, the geomagnetic field shows a positive variation in general and then tries to attain steady state. This variation after the maximum depression is also different at different locations and local time sectors.
Geomagnetic latitudinal belt between 40°N and 50°N
The D-variations (expressed in nanotesla) in the four lowest plots in the left-side panel with MLTs between 03:30 and 06:41 show an initial increase after the onset time, and thereafter a decrease. At 07:52 MLT, the southern hemispheric station PST shows a very small D-variation, and the stations beyond this MLT show a positive type of MI signature in the D component. Few stations show an initial very small decrease followed by an increase. The signature of the main impulse in the D component is observed to be negative after midnight to near dawn and positive from afternoon to post-dusk.Response waveforms in the H and D components show noticeable variations after the main impulse signature. The geomagnetic response signature in the H component shows a positive variation in general after achieving the minimum and then tries to attain a quasi-steady state. We call the first variation occurring immediately after the minimum the ‘after maximum deviation’ (AMD) variation. In actual fact, one more oscillation appears at a few stations after this variation, but it has a very weak amplitude. Similarly in the D component, a signature opposite to the main impulse is observed just after the maximum deviation. To define the various phases of the response waveform, we indicate points d1, d2, d3, h1, h2, and h3 in the topmost plots of Figure 4. The points indicated by a subscript 1 show the onset time, which is also indicated by a vertical dashed line, and is assumed to be same for all the plots. Points with subscripts 2 and 3 indicate the end of the MI and AMD, respectively.
The amplitude of the MI is defined as the maximum deviation of the magnetic field from its pre-SI value. Hence, the MI amplitude in the H component is the variation in the H component from the onset time to the time where H attains the minimum value (the H-variation roughly attains the minimum at approximately 11 to 13 min after the onset time). If the times at points h1, h2, and h3 are th1, th2, and th3, respectively, then the quantity defined by (H(th2) − H(th1)) represents the MI amplitudes in H. Similarly, if the times at points d1, d2, and d3 are td1, td2, and td3 respectively, then the quantities defined by (D(td2) − D(td1)) represent the MI amplitudes in the D component. The amplitude difference between points 2 and 3 gives the amplitude of the AMD, i.e., (H(th3) − H(th2)) and (D(td3) − D(td2)). The MI amplitude in the H-component is minimum (approximately −7 nT) near dawn and thereafter continues to increase throughout the afternoon sector, reaching a maximum (approximately −47 nT) near dusk (at 18:54 MLT).
Low-latitude region (between 10° and 40° geomagnetic latitude)
At low latitudes, the geomagnetic field response to the solar wind pressure change is often step-like due to the Chapman-Ferraro magnetopause currents and a propagating compressional wave front. Geomagnetic variations at low latitudes can be considered as remote to field-aligned, auroral ionospheric, and equatorial electrojet currents (Araki 1977, 1994). It has been found that a geomagnetic SI at low latitudes is proportional to the change of the square root of the solar wind pressure. Siscoe et al. (1968), Russell et al. (1994), and Takeuchi et al. (2002a) reported that the slope of the regression line is consistent during SI+ and SI− events.
where MIdawn (= −11 nT) and MIdusk (= −33 nT) are the minimum and maximum values of MI amplitudes at dawn and dusk respectively, which gives value of γ equal to 0.5 in the present case. The dawn-dusk asymmetry index computed by Shinbori et al. (2009) for low latitudes up to 27° is around 0.2 and suddenly increases to approximately 1.2 at 35° latitude (Figure four in Shinbori et al. (2009)), and the average over 10° to 40° latitude is around 0.6, which compares well with our case study.
Equatorial magnetic observatories
The amplitudes of MI at HUA, ASC, AAE, TIR, and GUA are 50, 23, 40, 23, and 23 nT, respectively. It is interesting to note that the amplitude of the MI at ASC, which is near magnetic noon (at 11:00 MLT), has an amplitude of much less than that at 07:00 MLT and at 14:40 MLT. However, this can be justified through the dip latitude of ASC, which is quite high (approximately 25° S), and hence it is located quite far from the dip equator and experiences very small ionospheric conductivity compared to the equatorial Cowling conductivity. While comparing the observations in a narrow belt of the equatorial zone, it is necessary to bear in mind that the Cowling conductivity gradients in the vicinity of the magnetic equator are very sharp, resulting in large amplitude differences within a few degrees of latitude. The geomagnetic latitude of HUA is 2.1°N, which is closer to the geomagnetic equator than AAE (5.36°N GMlat). Shinbori et al. (2009) found that the daytime enhancement of the normalized SI amplitude at 5° away from the magnetic equator (GAM) has a peak amplitude of less than half the value of that is found close to the magnetic equator (YAP). From their Figure one, at 07:00 MLT, YAP had a normalized amplitude of 1.5, which was comparable with that at near noon at GAM. It is therefore quite possible that the present observation of the larger MI amplitude at 07:00 MLT could be due to the fact that HUA is located closer to the geomagnetic equator than AAE. Thus, in view of the magnetic latitudes of the observatories, the differences in the MI amplitudes could be due to the sharp latitudinal gradients of Cowling conductivity in a narrow belt of the equatorial region. Further, the dip latitudes of HUA and TIR are almost same (both situated close to the dip equator) and are located near dawn and dusk respectively. A comparison of the MI amplitude at these two locations reveals larger MI amplitude at the near-dawn station, HUA, than that at the near-dusk station, TIR.
Results and discussion
Contribution due to sudden changes in the IMF
MLT dependence of ionospheric current contribution in the MI amplitude
Equivalent current system during the MI
Local time dependence of the time duration of the MI
Latitudinal variation of MI amplitude
Magnetic field variations after maximum deviation
As mentioned in ‘Ground-based observations’ section, most of the geomagnetic response profiles show variations after attaining a peak in the negative amplitude. We measure the amplitude of this variation wherever the variation is clearly detectable and refer to this as the 'after maximum deviation' (AMD). In ‘Conclusions’ section, we shall discuss the waveform of the total magnetic field obtained from the superimposed magnetopause and ionospheric currents, which clearly demonstrates the existence of an AMD, and hence it is part of the SI response. However, it can be noticed from Figure 1 that after the main decrease of SW pressure, there was a small enhancement in the ACE data. The SW pressure was 0.1 nPa at the end of the main decrease but then reached 0.22 nPa after a few minutes. The estimated magnetic field variation associated with this pressure change is approximately 2 to 3 nT on the ground, which is very small compared to the observed amplitudes of the AMD variations. It is also important to note that the AMD signature is associated with almost steady interplanetary conditions, with a steady northward IMF, and hence there is no contribution from overshielding or penetration electric fields. It can therefore be assumed that the AMD variations are not associated with the external driver.
This paper investigates the geomagnetic field responses to a negative pressure impulse of moderate amplitude, which can be useful for the understanding of existing theories. The geomagnetic field response to the negative pressure impulse essentially produces a negative MI in the horizontal component of the geomagnetic field, which is sometimes preceded by a strong positive PRI (Araki and Nagano 1988). Careful inspection of the geomagnetic field response at 97 ground observatories during the presently studied SI− event reveals that not a single ground station showed a clear intense positive variation before the MI. Some of the stations showed a very small positive variation before the MI, but the higher resolution data of 1-s sampling interval did not show any enhanced rapid positive variation, and hence we could not clearly identify the PRI signature for the studied event. According to present understanding, the PRI is due to the DPpi system, which is produced by the polarization electric field associated with the shock, mapped to high latitudes (Vestine and Kern 1962; Tamao 1964; Araki 1994). During the present SI− case, the velocity of the solar wind changed only by approximately 10 km/s, and no shock was evident; therefore, the absence of a PRI signature during the present case can be justified.
The magnetospheric response to the present SI− case recorded by GOES is a clear MI type, the amplitude of the MI increases during daytime, and the ratio of the MI amplitude at a geosynchronous height to that at the ground in the near-dawn sector is less than 1, which is consistent with Kokubun (1983). The MI duration is found to be longer at the ground than at a geosynchronous height, and this could be attributed to the larger distance of the SI source from the ground (the compressional wave requires a longer time to cross a larger path in the magnetosphere to reach the ground). In addition to this, due to inertia of the Earth's environment, the magnetosphere and ionosphere require extra time to reconfigure in response to applied sudden changes (Bhaskar and Vichare 2013). For ground observations, the magnetospheric and ionospheric reconfiguration times contribute to the observed MI duration, while at the altitude of GOES, only the magnetospheric reconfiguration time contributes. Also, as suggested by Nishida (1966), a broadening of the wavefront while passing from a geosynchronous height to the ground could be the cause of the observed longer duration of the MI.
The amplitude of the MI is found to vary with the MLT. In the geomagnetic latitude range between 60°S and 65°N, the MI amplitude is largest in the afternoon to post-dusk and smallest near dawn, except in the equatorial region. The MI of sudden impulse contains contributions from the Chapman-Ferraro currents flowing at the magnetopause, the ionospheric Hall and Pedersen currents, and FACs. The contribution from the magnetopause current is symmetrical about noon (Figure 8b) and hence has the same magnitude and direction in the morning and afternoon. The contributions from FACs are also symmetrical about local noon, flipping direction between local day and night (Araki et al. 2006; Shinbori et al. 2009), and therefore the morning and afternoon sectors would not experience any difference in the MI amplitude due to FACs. Moreover, contributions due to the Pedersen currents are in the opposite direction to those of the FACs during the daytime (Shinbori et al. 2009), and hence the net contribution from these two current systems would be very small during the daytime. We also estimated the contribution due to sudden northward turning of the IMF during the present SI− impulse and found this to be very small (approximately −2 nT) at low latitudes. In order to understand the source of the dawn-dusk asymmetry observed in the MI amplitude, we estimated the contribution from the magnetopause currents in the magnetic field recorded by the ground observatories using the Tsyganenko model (T01) and subtracted this value from the observed MI amplitude to obtain the ionospheric contribution in the MI signature. We found that the magnetic field signature due to the ionospheric currents has large negative amplitudes in the afternoon-to-dusk sector and very small negative (or sometimes positive) variations near dawn. This indicates that the ionospheric contribution near dusk is in the same direction as that due to the Chapman-Ferraro current and that the variations are in the opposite directions near dawn. This hypothesis is further substantiated by plotting the equivalent current system derived using H and D components of the residual field, which reveals a two-cell ionospheric current system where the direction of the current appears to be counterclockwise in the morning sector and clockwise in the afternoon sector. The explanation for the existence of this twin-cell ionospheric current system is given below.
Due to the impact of the solar plasma, the convection electric field is set up in the magnetosphere and is transmitted to the polar ionosphere along the magnetic lines of force (Araki 1994). According to Araki and Nagano (1988), during the MI of the negative impulse, the FACs flow into the auroral ionosphere at dusk and out of the ionosphere near dawn, in response to the dusk-to-dawn electric field. The Hall currents generated by the FACs set up twin-vortex current loops at ionospheric heights, each centering near dawn and dusk. The downward FAC sets up a clockwise current vortex, and the upward FAC creates a counterclockwise vortex in the ionosphere, which would result in a positive variation in the horizontal component of the magnetic field near-dawn, on ground (between the center of the current loop and the equator), and a negative variation near dusk. Therefore, the magnetic field due to the twin-cell ionospheric current at near dusk would add to that due to the magnetopause currents and would cause an enhancement of the MI amplitude. Similarly, it would result in the reduction of the MI amplitude in the near dawn. It is possible that the effect of the twin-vortex type ionospheric current generated by the FAC (DP2 Hall currents) extends towards the low-to-mid latitudes (Kikuchi et al. 2001; Araki et al. 2006). However, its magnitude decreases with decreasing latitude, and hence the shape and amplitude of the H waveform are strongly dependent on the latitude and local time. Thus, the observations presented in this paper indicate that the contribution to the MI amplitude observed on the ground due to the ionospheric two-cell convection (DP2) currents driven by the FACs is significant, even for the moderate amplitude SI−, and confirms the existence of a dusk-to-dawn electric field during the MI of the negative solar pressure impulse. Furthermore, we have shown that the amplitude of the MI depicts hemispheric asymmetry with higher values in the summer hemisphere than in the winter hemisphere, which once again prove the importance of the ionospheric conductivity and hence the contribution from ionospheric currents in the MI amplitude.
A comparison of the MI amplitude at two near-equatorial stations located near-dawn and near-dusk reveals a larger MI amplitude at the near-dawn station, HUA (in the American sector), than at the near-dusk station, TIR (in the Indian sector). If it is assumed that the effect of twin current loops can extend even at equatorial latitudes, then near dawn, the effect should be opposite to that due to the MI and result in a reduction of the amplitude. However, present observations do not appear to be consistent with this supposition.
An interesting feature noticed is that the variation in the magnetic field after the MI shows a very good anti-correlation with the MI amplitudes in both the H and D components. Since at a given station, MI and AMD experience the same ionospheric conductivity, it can be considered that the same enhancing factor is acting on these two variations, and therefore the proportionality exist between the two. In addition, the superimposed magnetic field variations depicted in Figure 14 show variations after the maximum deflection, particularly in the afternoon sector. Thus, the AMD variations are part of the SI variations and can provide information about the DPmi system. The amplitude of the AMD depends on the strength of the DPmi, the time of onset of the DPmi system, the duration of the DPmi, and the strength and duration of DL. In the near future, we plan to study the LT and latitudinal dependence of this variation, which would assist in understanding the DL and DPmi systems more precisely. Very recently, a few studies have investigated the ionospheric flow patterns after the occurrence of an MI. The multiple undulating structures in ionospheric flow variations and in ground magnetic variations have been noted through observational and simulation studies (Yu and Ridley 2011; Hori et al. 2012; Fujita et al. 2012). Several mechanisms (such as the flow vortex chain in the magnetopause region due to the rebound of the magnetopause, multiple ionospheric convection, and compressional wave bouncing between the ionosphere and the dayside magnetopause) have been sought to explain these oscillations. However, it is not possible to gain a clear picture until further observational studies have been performed.
The first author is previously known as Geeta Jadhav.
We express our sincere thanks to Professor T. Araki for important discussions. The results presented in this paper rely on the data collected at magnetic observatories. We therefore thank the national institutes that support the observatories and INTERMAGNET for promoting high standards of magnetic observatory practice (http://www.intermagnet.org). We would also like to give special thanks to the observatories providing 1-s data, ACE Science Center, GOES satellite data center, and WDC-C2, Kyoto, for providing the data.
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