Main Impulse Research Articles

On the nature of response of dayside equatorial geomagnetic H-field to sudden magnetospheric compressions

A step-like increase in the solar wind ram pressure causes a sudden compression of the magnetosphere which manifests as a sudden commencement (SC) in the geomagnetic field. SC manifests in two basic forms in the geomagnetic H-field of the dayside dip equatorial region, namely, as a positive impulse termed SC(+), and as a positive impulse preceded by a sharp negative impulse termed, SC(−, +) or SC *. Unlike at high latitudes, the factor(s) that determine this bi-modal response of the equatorial geomagnetic H-field to sudden magnetospheric compressions are not known. As an exploratory step in identifying the causative factor(s), we have performed a statistical study of the characteristics of the daytime (06-18 IST) SCs recorded at the equatorial station, Kodaikanal (10.25N, 77.5E, dipole latitude 0.6N) and the low latitude station, Alibag (18.6N, 72.9E, dipole latitude 9.5N) over the period 1957–2002. A total of 304 SCs have been analyzed out of which 99 (32.6 per cent) were of SC * type. We find that the average value of the amplitude of the positive main impulse (MI) in H-component of SC * is higher than that of the conventional SC(+). This difference in the MI amplitude is statistically significant and persists even when the analysis is restricted to the interval of SC * occurrence, namely, 0730–1730 IST. Such a behavior is not seen in the amplitude of the MI at Alibag, away from the influence of the equatorial electrojet. It is found for the first time that a linear relationship exists between the amplitudes of the preliminary reverse impulse (PRI) and MI in SC * events. Our study of the H-field response at dayside dip equatorial stations of the Circum-pan Pacific Magnetometer Network (CPMN) to sudden magnetospheric compressions under different IMF orientation (northward and southward), showed that the impact of an impulse in solar wind dynamic pressure under southward IMF is associated with SC * in both the events considered. On the other hand, under northward IMF the dynamic pressure increase consistently led to SC(+) in the two events studied. The statistical and case study results imply that, when compared to SC(+), the contribution of ionospheric currents of polar origin gain prominence when SC * is excited at dayside dip equator, and the orientation of IMF Bz may be one of the (if not the only one) deterministic factors underlying the bi-modal response of the dayside equatorial H-field to a sudden increase in solar wind dynamic pressure.

Nighttime enhancement of the amplitude of geomagnetic sudden commencements and its dependence on IMF-Bz

Abstract We present a statistical study of the diurnal variation of the occurrence frequency of geomagnetic sudden commencements (SCs) observed at Kakioka (geomagnetic latitude, θ = 27.4°). SCs with an H-component amplitude (ΔH) larger than 40 nT occur more frequently in the nighttime than the daytime, while those with smaller amplitudes (ΔH < 39 nT) occur more frequently in the daytime. Three large amplitude SCs (ΔH = 85, 117 and 145 nT at Kakioka) were analyzed in detail. All three exhibited larger amplitudes during the nighttime at all low latitudes except those near the dayside equator. A statistical study reveals that the averaged amplitudes are slightly larger in the daytime at Alibag (θ = 10.2°) but considerably larger in the nighttime at three higherlatitude Japanese stations, Kanoya (θ = 21.9°), Kakioka and Memambetsu (θ = 35.4°). Case studies of two moderate amplitude SCs which occurred at the same UT indicate that nighttime SC amplitudes at low latitudes are slightly (considerably) larger than daytime amplitudes when the interplanetary magnetic field (IMF) points northward (southward). We suggest that the diurnal variation of SC amplitudes can be explained by a combination of field aligned and resultant ionospheric currents produced during the main impulse of SCs.

An impulse voltage calibrator of low output impedance

A calibrator that produces full lightning impulses and chopped lightning impulses with variable chopping time is described. The calibrator uses series-connected power metal-oxide field-effect transistors as the main impulse switch and insulated gate bipolar transistors (IGBTs) as the chopping switch, and its output impedance is low to drive a termination impedance of 50 /spl Omega/. The performance of the calibrator is verified by comparing the simulated calibrator output parameters based on the calibrator component values and output parameters calculated from measured output waveforms using a calibrated digitizer.

Explosive Propositions: Artists React to the Atomic Age

Argument“How should a modern artist react to the atomic age?” Time magazine posed this question in 1952 to open a review of an exhibition of paintings inspired by the “explosion of the atomic bomb” and by the “discovery of nuclear energy.” The energetic paintings of the Italian Spatial Movement were, according to Time, “almost as explosive as the bomb itself.” “Explosiveness” was a defining feature of much 1950s art, whose main impulse, gestural abstraction, has previously been understood as the urgent expression of the artist's subjectivity. This paper argues that explosiveness in art can also be seen as an “expression” of the realities of the nuclear era. Postwar artists were ambivalent about the explosive forces, both liberating and devastating, that lay within the atom. Toward the end of the 1950s, no longer content merely to represent atomic disintegration, some artists went so far as to propose the outright self-destruction of the work of art as the only fitting means of expression in the atomic age.One must – and this is not an exaggeration – keep in mind that we are living in the atomic age, where everything material and physical could disappear from one day to another, to be replaced by nothing but the ultimate abstraction imaginable.– Yves Klein 1958 (Klein 1973, 10)

A numerical simulation of a negative sudden impulse

A numerical experiment of magnetospheric response to a negative pressure impulse in the solar wind is carried out by using a MHD model of the solar wind-magnetosphere-ionosphere coupled system. The numerical simulation confirms mirror-image relationship of the ionospheric and magnetospheric signatures between the negative and positive impulses, which has been suggested by previous observations. The plasma processes associated with the negative impulse are again divided into the three phases—the preliminary impulse phase, and the first and second main impulse phases in terms of the ionosphere-magnetosphere coupling. The SC transient cell convection in the second main impulse phase is related to the Region 2 current in the case of the negative impulse. In the last, we discuss a possible model for the auroral brightening at the onset of the negative impulse.

A numerical simulation of the geomagnetic sudden commencement: 2. Plasma processes in the main impulse

A geomagnetic sudden commencement (SC) is studied numerically based on a model of buffeting the magnetosphere by a solar wind density impulse. This paper treats two successive current systems in the main impulse (MI) phase. The two current systems have different current generating mechanisms. The first current generator appears behind the wavefront of a compressional disturbance launched by the impulse. The inertia current of the compressional mode is generated by free energy due to deceleration of plasma flows. A field‐aligned current (FAC) is excited through mode conversion from the compressional wave in a VA gradient region. The magnetospheric flows and the ionospheric flows are not connected self‐consistently to each other. The second generator is located in the tailward side of the cusp. It is the same as the generator of the region 1 current system. The current generated there is connected with the FAC with the region 1 sense via a diamagnetic current flowing around an isolated enhancement of pressure in the nightside equatorial magnetosphere. The pressure enhancement is induced through compression of the magnetospheric flank due to the solar wind impulse. In this period, plasma convection vortices appear both in the magnetosphere and in the ionosphere, which are correspondent to each other. This is a peculiar convection confined within the magnetosphere (the SC transient cell convection). This convection is driven though compression of the magnetospheric flank due to the solar wind impulse.

Archaeological Reflexivity and the "Local" Voice

There have recently been a number of attempts to develop reflexive field methods in archaeology (eg Andrews et al 2000, Bender et al 1997, Chadwick 1998, Dowdall and Parrish 2003, Faulkner 2002, Fotiadis 1993, Gero 1996, Hodder 1999a, 2000, Lucas 2001, Politis 2001). It might be argued that this turn to the reflexive in archaeology is ironic. After all, socio-cultural anthropology has recently seen a sustained critique of the concept of reflexive ethnographic method (Lynch 2000, Salzman, 2002, Robertson 2002). At the very least, the archaeological move might seem delayed, given what Robertson (2002) describes as a 20-year history of reflexive discussion in anthropology (Clifford and Marcus 1986, Gupta and Ferguson 1997) and given the indications of even earlier beginnings (Robertson 2002). I wish to argue at the start of this paper, however, that the development of reflexive field methods in archaeology is neither delayed nor ironic. Rather it results from specific issues and problems which are of a rather different nature from those found in ethnography. Archaeology as a discipline grew in the 18th and 19th centuries as an integral part of the projects of nationalism and colonialism (Trigger 1984). For many European countries, for example, the archaeological past still has a self-evident relationship with the state. The protection of ancient monuments is a function of national governments, however much local and diverse voices might be raised against them. A closely related issue is that the distant past in many parts of the world may have no present communities which can stake a direct claim on it. There is no one today, for example, who can speak for, or represent the interests, of the Beaker people of the 3rd millennium be in Europe, and the same is true for countless other cultural groupings identified by archaeologists in the deep past. A reflexivity that derives from the fieldworker's interaction with 'other', 'indigenous' voices of 'informants' is less likely to emerge in archaeology. It is precisely when the past is claimed by present communities that a reflexivity has been forced on archaeology. By reflexivity here, I mean initially the recognition and incorporation of multiple stakeholder groups, and the self-critical awareness of one's archaeological truth claims as historical and contingent. Post-colonial processes, global interactions, and the massive rise in the destruction of archaeological sites and monuments around the world have together created an awareness of divergent opinions about how the past should be managed. While there have been parallel intellectual debates in archaeology over the last 20 years (Shanks and Tilley 1987), the main impulse towards reflexive concerns has been the increased use of the past in identity formation and land-rights claims (Layton 1989; Gathercole and Lowenthal 1989, Kohl and Fawcett 1995. For a recent review see Meskell 2002a). While reburial issues in the United States have led to some objectivist retrenchment, they have also led to greater consultation (in the Native American Graves Protection and Repatriation Act and Section 106 of the National Historic Preservation Act) and to anti-objectivist calls for the full integration of oral histories and indigenous knowledge (eg Anyon et al 1996; Stoffle et al 2001, Watkins 2000). The materiality and monumentality of the archaeological past mean that archaeological sites and monuments are often central to the construction of the national and colonial memory and counter-memory (eg Abu el-Haj 1998, Rowlands 1993; Meskell 2002b). The resulting conflicts over ownership, guardianship and interpretation have often been very public. The moves towards reflexivity, as defined above, have proceeded in the increasingly ethically-conscious halls of the academy, but also in local, national and international heritage management committees. Indeed, it has been the world of heritage management that has often been in the forefront of the development of guidelines which lead towards collaboration and multiple perspectives. …

Penetration electric fields at the nightside dip equator associated with the main impulse of the storm sudden commencement of 8 July 1991

The geomagnetic storm sudden commencement (ssc) of 8 July 1991 was reported [Wilson et al., 2001] to be characterized by a reduction (enhancement) of X component at midlatitudes in the noon (midnight) sector in the 1‐hour period after its start at 1636 UT. This distinctive feature is seen even after accounting for the effects of the Chapman‐Ferraro current by subtracting the step‐like increase of X/H component at low‐latitude stations on the same magnetic meridian from the midlatitude data. We present evidence to show that over the same 1‐hour period after the start of the ssc on 8 July 1991 an eastward electric field disturbance (peak value ≈1.2 mV/m) grew up and decayed at the premidnight dip equator. The eastward electric field evidenced at the nightside dip equator is interpreted as the signature of the penetration of the dawn‐to‐dusk electric field associated with an enhancement of region 1 field‐aligned currents (FACs) driven by the solar wind. The negative disturbance in the H/X component at midlatitudes in the noon sector is explained as the magnetic effect of FACs that carry the large‐scale electric fields from the magnetosphere to the polar ionosphere as well as the disturbance of polar origin‐type 2 ionospheric currents excited by the large‐scale electric field.

Field‐aligned current effects on midlatitude geomagnetic sudden commencements

The geomagnetic sudden commencement (SC) on February 18, 1999, was preceded by a preliminary positive impulse (PPI) at noon (1146 LT) mid‐latitudes (34.9° and 26.9° geomagnetic latitude (GML)), and by a preliminary reverse impulse (PRI) near the dip equator (−0.3° and 4.9° GML) in the same local‐time sector. By assuming that the step‐like SC at a lower latitude (14.5° GML) was entirely caused by the Chapman‐Ferraro currents, we subtracted this magnetic field from the SC at midlatitudes and equatorial latitudes, to identify the magnetic fields caused by the field‐aligned currents (FACs) and ionospheric currents. We found that the PPI was composed of a positive impulse (true‐PPI) with a timescale of less than l min and a succeeding negative impulse (several minutes), with their amplitudes decreasing with decreasing latitudes. The true‐PPI occurred simultaneously with the equatorial PRI, and the succeeding negative impulse occurred with the DP 2‐type ionospheric current component of the main impulse (MI) of the SC (DP (MI)). Analysis of 46 well‐defined PPI events showed that the afternoon PPIs occurred exclusively in winter, while there was no significant seasonal dependence in the morning PPIs. None of the afternoon PPIs could be explained by the conventional SC model based on the Chapman‐Ferraro currents and the DP 2‐type ionospheric currents. We apply the Biot‐Savart law to a three‐dimensional current circuit including FACs to interpret the afternoon PPIs. Model calculations assuming a seasonal asymmetry in the ionospheric conductivity indicate that the FACs played a predominant role at midlatitudes in the winter hemisphere, while the ionospheric currents played a predominant role in the summer hemisphere. It is concluded that the true‐PPIs and succeeding negative impulses were dominated by the magnetic effects of the FACs that carry the electric fields responsible for the PRIs and DP (MIs), respectively.

A case study test of Araki's physical model of geomagnetic sudden commencement

This paper tests the Araki [1994] computational model of the Earth‐ionosphere system during geomagnetic sudden commencement (sc). In particular, we test the model's ability to predict the signs of the preliminary and main impulses, given the latitude and the magnetic local time (MLT), using a case study of an sc which occurred at 0949 UT on November 22, 1997. Data from a global network of magnetometer stations and from satellites are used. Model predictions compare well with the case study data at high latitudes (above ∼ 72°N Altitude Adjusted Corrected Geomagnetic Coordinates (AACGM)), less well for lower latitudes, particularly on the nightside. In addition, the position of the footprints of the field‐aligned currents (FACs) associated with the sc ground signature varies with MLT, contrary to the model. Data from satellites in polar and geostationary orbits suggest that the FACs for both the preliminary and the main impulses map to gradients in magnetospheric plasma concentration, such as the outer radiation belt and the plasmapause.

Preliminary impulse of the geomagnetic storm sudden commencement of November 18, 1993

The characteristics of the geomagnetic sudden commencement (SC) that occurred at 1211:30 UT on November 18, 1993, following an interval of prolonged geomagnetic quietness are studied using high time resolution data of several magnetometer networks. We present the first results concerning the behavior of the preliminary reverse impulse (PRI) of the H component of the SC near the dip equator from simultaneous observations in different longitude (local time) sectors. It is found that the preliminary reverse impulse appeared only in the prenoon (0912 LT) sector very close to the magnetic equator (dip 0.6°). At locations farther away (dip 6.0°–7.2°) but on the same meridian, the preliminary reverse impulse diminished in amplitude and led to a delayed onset of the main impulse (MI) of the SC. The preliminary reverse impulse is not apparent at locations close to the magnetic equator (dip 1.2°–2.7°) in either the afternoon (1300 LT) or the near‐dusk (1740 LT) sectors. What is seen instead here is an unambiguous reduction in the rate of increase of H component coincident with the preliminary reverse impulse in the forenoon sector. HF Doppler radar measurements of F layer vertical plasma drift close to the magnetic equator (dip 4.9°) near the dusk sector showed the SC‐related disturbance to be a decrease in ambient upward drift with considerable temporal structure, which indicates the imposition of a westward electric field. The preliminary reverse impulse is seen in the subauroral to polar region (MLAT 56.8°–76°) on the afternoon side simultaneous with that near the prenoon dip equator and with a conspicuous increase in amplitude and duration with latitude over MLAT range 66°–76°. Theoretical calculations suggest that the global current system set up by a pair of field‐aligned currents at 80° latitude and shifted to morningside (centers at 1300 and 0300 LT) could, in general, account for the observed behavior of the preliminary reverse impulse, except in the dip equatorial region near dusk.

'Young India: A Bengal Eclogue': Or Meat-eating, Race, and Reform in a Colonial Poem

This essay uses a poem written by an Englishman in early nineteenth-century Calcutta, 'Young India: A Bengal Eclogue', to examine the contexts of reform, revision, and writing in Bengal in the 1820s and 1830s. The reform movement known as Young Bengal has been strangely bypassed in postcolonial narratives; yet this was a historical and epistemological moment that defined the shape of modern India. Radical and yet imitative, the movement incorporated paradoxes that continue to inhabit Indian public life. Certainly its main impulses found a continuum in the nationalist movement; Gandhi, typically, subverted one of its principles of heresy, the consumption of beef in order to overthrow the hefty English, into a mantra of vegetarianism. Its imitative element was instantly material for satire; Bengalis themselves lampooned the follies of Young Bengal mercilessly in various media, as did Parker in his poem. Also implicated in the politics of meat-eating in the nineteenth century was the construction of the manly Englishman in contrast to the Indian male whose effeminacy was invariably linked to his education, as demonstrated in many of Kipling's works. The essay therefore also explores the deterioration in race relations from the time of one Anglo-Indian poet, Henry Meredith Parker, to another, Rudyard Kipling, showing how an acquired intimacy with the colonized gave way to a deliberate new imperialism permeated with a prejudice and ignorance that served to exclude Europeans from many areas of Indian life.

AID'S CRISIS OF LEGITIMACY: CURRENT PROPOSALS AND FUTURE PROSPECTS

Aid to Africa is less threatened by the current foreign policy and fiscal pressures than by a complex crisis of legitimacy regarding the enterprise. Despite official rhetoric to the contrary, most aid insiders are disappointed by the current achievements of aid. The current malaise has resulted in a considerable ferment of thinking about how to make aid more effective. This article describes four sets of proposals that have been advanced. These include proposals to make aid more selective, as well as to adopt a more need-based approach, which includes proposals to promote government ownership, and to focus aid on non-governmental actors in the private and voluntary sectors. It is argued that each of these proposals is desirable, but that together they are probably incompatible, certainly difficult for the donors to implement and unlikely to address the real problems that have plagued aid and African development these last thirty-five years. The real challenge, not addressed by most current reform proposals, is to change how donors and recipients manage aid, so that it contributes to a sound development strategy. The article concludes that the main impulse for reform must come from African countries themselves, rather than from the donor countries. RUMOURS OF THE DEATH of foreign aid to Africa are much exaggerated. True, the volume of foreign aid from the West to Africa has been declining since 1994.1 After three decades of uninterrupted growth in the volume of aid going to Africa, any decline would have set off alarm bells. Yet, in 1997, sub-Saharan Africa still received over US$ 16 billion in official development assistance (ODA). Given the previous growth, even if aid declined by 5 percent a year for a decade, it would still be greater in 2007 than it was in the mid-1980s. It is thus very premature to speak of 'an end of aid to Africa'. Under any plausible scenario of the policy-making process in the developed countries, aid will continue to mark the African landscape for the foreseeable future. There are several well known reasons for the observed recent decline. First, the end of ie Cold War has changed foreign policy priorities in the West and weakened the claims of African states for international resources. The author teaches in the Department of Political Science, Michigan State University. 1. Unless otherwise indicated, the aid data reported in this article come from the Development Assistance Committee (DAC) of the Organization for Economic Cooperation and Development (OECD), as presented in its annual report on the aid efforts of its member states.

Regulating labour in a colonial context: Maritime labour legislation in the Australian colonies, 1788–1850

The development of separate laws in the Australian colonies, regulatory responses to the emergence of a free labour market in particular, has received limited attention from historians. The punitive and oppressive character of early labour laws has been explained largely in terms of the ‘convict heritage’. However, an examination of maritime labour law shows that the convict experience provides only a partial explanation. The main impulse for these laws, and their deviation from imperial statutes, was mercantile and labour market factors. Most notably, shipowners and merchants influenced colonial authorities, which were themselves keenly aware of the colonies ‘ dependence on shipping. This article demonstrates the need to include a wider array of interest groups in explanations of the legal and political culture of convict and settler societies before 1850.

Characteristics of geomagnetic sudden commencement observed in middle and low latitudes

Characteristics of geomagnetic sudden commencement (SC) observed in middle and low latitudes are discussed on the basis of data analyses of magnetic variations and a numerical calculation. From the data analyses, it is found that a negative impulse is usually superposed on the main impulse of SC in H component just after its onset, at the stations in middle to high latitudes in the local time range from the morning to the early afternoon. The superposition of the negative impulse causes the decrease of SC amplitude in H component with increasing latitude in this area. The occurrence of the negative impulse does not seem to be dependent on geomagnetic activity. Examining a case study and a numerical analysis, it is suggested that this negative impulse is a signature of the magnetic variation due to the polar-originated ionospheric current system responsible for the main impulse of SC (DPMI). The discussion includes the possible interpretation of PPI introduced by Kikuchi and Araki (1985) and the availability of D component magnetic data in middle latitude forarough estimationof the magnetosphere-ionosphere coupling process.

Macroeconomic analysis of the czech economy 1996

This article consist of analysis of all main indicators of Czech economy in 1996. They are: GDP and economic growth focused on economic dynamism; Disproportionality in the development of the main components of aggregate demand and supply and productive efficiency of the domestic economy; Productivity focused on trends in productivity of labour in the national economy and selected sectors and finally on total factor productivity (capital and labour); External economic relation focused on balance of trade deficit and trends in export and import and also on the trends in the use of net import of goods and services and trade deficit financing - capital inflow and balance of payments; Public Finance; Monetary Policy; Inflation focused on general trends and two main impulses: demand and cost.

North/South Asymmetry of sc/si Magnetic Variations Observed along the 210.DEG. Magnetic Meridian.

Magnetic field data from the 210° magnetic meridian chain of stations have been analyzed to examine which component of the DL and DP fields dominates main impulses of sc and si observed on the ground. The DL and DP fields correspond to transfer processes from the magnetopause through the magnetosphere to the Earth's surface and through the polar ionosphere into the equatorial ionosphere, respectively. Northern and Southern Hemisphere asymmetry of sc and si disturbances appears at low and middle latitudes. Amplitude ranges of the sc and si main impulses observed at low-latitude conjugate pairs (L = 2. and 1.6) in the summer hemisphere are about twice as wide as those in the winter hemisphere. The equatorial enhancement of sc and si main impulses can be seen at Yap island (o = -0.3 °) during the daytime at 0600-1800 LT. These observational facts can be interpreted by invoking an asymmetry in the Northern and Southern Hemisphere twin-vortex-type ionospheric currents driven by the DP field, i.e., by invoking enhanced ionospheric conductivities in the summer hemisphere. The instantaneous penetration of sc and si-associated DP fields through the ionosphere from the polar region to the equator must play an important role in the energy transfer of sc and si disturbances from the solar-wind region to the magnetic equator.

The CME Event of February 21, 1994: Response of the Magnetic Field at the Earth's Surface.

On February 21, 1994, a large coronal mass ejection (CME) passed by the Earth. In the solar wind, the CME front was noted by very large changes in the solar wind dynamic pressure, flow direction, and magnetic field, as measured by several spacecraft. At the Earth's surface, many ground stations also recorded the passage of this CME, as a storm sudden commencement onset. We examine here several interesting features of the surface magnetic field response to this sudden commencement onset. In particular, we use the magnetometer records to examine the propagation time of the initial impulse along the Earth's surface. We also study in detail the initial amplitude change at several stations, and note the variation in the response as a function of magnetic latitude. In addition, we crudely estimate the relative contributions of the Chapman-Ferraro and the ionospheric current system to the surface response. Also of interest is the polarization and period of Psc oscillations superposed on the main impulse at several stations, including polarization reversals as a function of magnetic latitude. Corresponding ionospheric currents at stations along the 210° MM chain are also computed, and very large and sudden changes in the ionospheric current flow patterns are reported during the initial minutes of the sudden commencement onset. These and other results are interpreted in terms of previous models, paradigms, and statistical results.

Fractional model equation for anomalous diffusion

In recent years the phenomenon of anomalous diffusion has attracted more and more attention. One of the main impulses was initiated by de Gennes' idea of the “ant in the labyrinth”. Several authors presented asymptotic probability density functions for the location of a random walker on a fractal object. As this density function and the time dependence of its second moment are now well established, a modified diffusion equation providing the correct result is formulated. The parameters of this fractional partial differential equation are uniquely determined by the fractal Hausdorff dimension of the underlying object and the anomalous diffusion exponent. The presented equation reduces exactly to the ordinary isotropic diffusion equation by appropriate choice of the parameters. A closed form solution is given in terms of Fox's H-function. In the asymptotic case a “halved” diffusion equation can be established. Furthermore, the differences to equations considered previously are discussed.

Penetration of polar electric fields to the nightside dip equator at times of geomagnetic sudden commencements

Measurements of Doppler Velocity, VD of F ‐ region reflections at normal incidence over Kodaikanal (dip 3°N, 77° 28′E) are used to study the nature of perturbations in F ‐ region vertical plasma drift, Vz associated with the geomagnetic sudden commencements (sc) on July 8, 1991, and January 1, 1992. Both the events which occurred in a narrow time window, 1630‐1700 UT (2200‐2230 IST) were of sc* type at middle‐ and high‐ latitude stations in the afternoon sector. At Kodaikanal, which is on the nightside, the sc of January 1, 1992, is characterized by a double‐step main impulse (MI) in H ‐ component (the structure of the sc on July 8 could not be ascertained from normal run magnetograms because of the large amplitude and very small rise time of the sc). It is found that the usual downward motion of F ‐ region plasma during the premidnight hours at Kodaikanal suddenly ceased (and even reversed to upward in one event) for ∼1 min coincident with the preliminary impulse (PI) and was immediately enhanced in association with the MI of the sc*. This pattern which is consistently seen in the two events implies that an eastward electric field prevails near the nightside dip equator at the time of the first impulse of double‐step MI there and the PI of sc* at high latitudes. Our Doppler velocity observations constitute the first and direct experimental evidence of vertical plasma motions due to the sc ‐ associated electric fields in the nighttime dip equatorial ionosphere. They substantiate the view based on theory (Kikuchi and Araki, 1979) and ground‐based magnetic observations (Araki et al, 1985) that the dusk‐to‐dawn electric field imposed on the polar ionosphere with the onset of PI of sc* is instantaneously transmitted to the dip equator on the nightside as on the dayside.