Preliminary Impulse Research Articles

Response of ionospheric electric fields at mid‐low latitudes during sudden commencements

AbstractUsing in situ observations from the Republic of China Satellite‐1 spacecraft, we investigated the time response and local time dependence of the ionospheric electric field at mid‐low latitudes associated with geomagnetic sudden commencements (SCs) that occurred from 1999 to 2004. We found that the ionospheric electric field variation associated with SCs instantaneously responds to the preliminary impulse (PI) signature on the ground regardless of spacecraft local time. Our statistical analysis also supports the global instant transmission of electric field from the polar region. In contrast, the peak time detected in the ionospheric electric field is earlier than that of the equatorial geomagnetic field (~20 s before in the PI phase). Based on the ground‐ionosphere waveguide model, this time lag can be attributed to the latitudinal difference of ionospheric conductivity. However, the local time distribution of the initial excursion of ionospheric electric field shows that dusk‐to‐dawn ionospheric electric fields develop during the PI phase. Moreover, the westward electric field in the ionosphere, which produces the preliminary reverse impulse of the geomagnetic field on the dayside feature, appears at 18–22 h LT where the ionospheric conductivity beyond the duskside terminator (18 h LT) is lower than on the dayside. The result of a magnetohydrodynamic simulation for an ideal SC shows that the electric potential distribution is asymmetric with respect to the noon‐midnight meridian. This produces the local time distribution of ionospheric electric fields similar to the observed result, which can be explained by the divergence of the Hall current under nonuniform ionospheric conductivity.

The chain response of the magnetospheric and ground magnetic field to interplanetary shocks

AbstractIn response to interplanetary (IP) shocks, magnetic field may decrease/increase (negative/positive response) in nightside magnetosphere, while at high latitudes on the ground it has two‐phase bipolar variations: preliminary impulse and main impulse (MI). Using global MHD simulations, we investigate the linkage between the MI phase variation on the ground and the magnetospheric negative response to an IP shock. It is revealed that although the two phenomena occur at largely separated locations, they are physically related and form a response chain. The velocity disturbances near the flanks of the magnetopause cause the magnetic field to decrease, resulting in a dynamo which thus powers the transient field‐aligned currents (FACs). These FACs further generate a pair of ionospheric current vortex, leading to MI variations on the ground. Therefore, we report here the intrinsic physically related chain response of the magnetospheric and ground magnetic field to IP shocks, and thus link the magnetospheric sudden impulse (SI) and ground SI together.

Transmission line model for the near‐instantaneous transmission of the ionospheric electric field and currents to the equator

AbstractThe simultaneous onset of the preliminary impulse (PI) of the geomagnetic sudden commencement at high latitude and dayside dip equator is explained by means of the TM0 mode waves propagating at the speed of light in the Earth‐ionosphere waveguide (EIW) [Kikuchi et al., 1978]. A couple of issues remain to be addressed in the EIW model: (1) How is the TM0 mode wave excited by the field‐aligned currents (FACs) in the polar region? (2) How are the quasi‐steady ionospheric currents achieved by the TM0 mode waves? (3) How simultaneous or delayed are the onset and peak of the equatorial PI with respect to the high‐latitude PI? To address these issues, we examine the TEM (TM0) mode wave propagation in the finite‐length transmission lines replacing the pair of FACs (magnetosphere‐ionosphere (MI) transmission line) and the Earth‐ionosphere waveguide (ionosphere‐ground (IG) transmission line). The issue (1) is addressed by showing that a fraction of the TEM mode wave is transmitted from the MI to IG transmission lines through the polar ionosphere. To address the issues (2) and (3), we examine the properties of the finite‐length IG transmission line with finite ionospheric conductivity. It is shown that the ionospheric currents start to grow instantaneously and continue to grow gradually with time constants of 1–10 s depending on the ionospheric conductivity. The MIG transmission line enables us to explain the instantaneous onset and delayed peak time of the equatorial PI and quick electric field response of the low‐latitude ionosphere and inner magnetosphere.

Optical and SuperDARN radar observations of dusksideshock aurora over Zhongshan Station

We present observations of a duskside shock aurora occurred on 21 April 2001 by the SuperDARN radar at SyowaStation and the all-sky camera at Zhongshan Station (ZHS) in Antarctica when the radar was operated in fast-scan mode coveringthe ZHS region. With the two independent data sets, we examine ionospheric plasma convection and aurora arising from a suddenimpulse (SI) event associated with an interplanetary shock. During the transient shock compression, the aurora was quiescentwithout any optical emission at the preliminary impulse of the SI. About 7 min later, a new thin auroral arc with brighter emissionsand a lifetime of ~14 min expanded westward from the region above ZHS during the main impulse of the SI. SuperDARN radarline-of-sight measurements showed periodical oscillation in the flow direction with ultra-low-frequency waves having a period of~8 min during the shock compression. We suggest that downward field-aligned current during the preliminary impulse stage of theSI was the main driver of the first plasma flow reversal, and the subsequent new discrete auroral arc may be associated withfield-aligned acceleration in the region of the main impulse related upward field-aligned currents. The ground magnetometer observationssuggest that the oscillation of the ionospheric convection on the duskside was associated with field line resonance activity.

Evolution of negative SI‐induced ionospheric flows observed by SuperDARN King Salmon HF radar

The spatial evolution of vortex‐like flow structures induced by a negative sudden impulse (SI−) is studied on the basis of SuperDARN King Salmon HF radar (KSR) with other ground and satellite data. A large dip in the solar wind density induced a fairly large SI− with a SYM‐H amplitude of ∼40 nT. The SI‐induced ionospheric flow signatures in the evening sector (MLT ∼ 19 h) were observed by KSR as a westward flow associated with the preliminary impulse (PI) followed by a more intense eastward flow with the main impulse (MI) in the sub‐auroral region of the magnetic latitude ∼60–70 deg, consistent with the local ground magnetic field observations. Following the first PI‐MI flow sequence, KSR saw a second and possibly third sequence of flow variation which were much smaller in flow amplitude than the first pair but showed qualitatively very similar flow variations and latitudinal/longitudinal propagation characteristics. These observations can be interpreted as aftershocks of the first PI‐MI; the same sequence of vortices and field‐aligned currents were generated and then drifted anti‐sunward with the same mechanism, namely the pumping motion of the dayside magnetosphere. These results are qualitatively consistent with predictions suggested by recent numerical simulations.

A numerical simulation of a negative solar wind impulse: Revisited

Response of the magnetosphere‐ionosphere system to a negative impulse of the solar wind dynamic pressure (the negative SI) is studied again with foci of the following three points; multiple convection oscillations, mirror‐image relationship between the negative and positive SIs, and appearance of the overshielding potential. When the negative impulse impinges on the magnetopause, the Region 1 (R1)‐type field‐aligned current (FAC) and R2‐type FAC appears alternatively in the dayside polar ionosphere (∼70°, ∼10 hLT and 14 hLT). These ionospheric current systems shift nightward and poleward. This alternative appearance of FACs invokes positive and negative ionospheric potential patterns switching alternatively. It is revealed that the negative SI is accompanied with multiple convection oscillations repeating more than the positive SI. We also notice that the magnetospheric current system producing the preliminary impulse (PI) of the intensive negative SI is a mirror‐image of that of the positive SI. The multiple convection oscillations and the mirror‐image relationship were not discussed by Fujita et al. (2004) who studied a moderate‐amplitude negative SI. In addition, the R2‐type FACs induced by the negative and positive SIs tend to yield the overshielding electric potential in the ionosphere. The shielding potential invoked by the SIs has shorter duration than that for the northward turn of the interplanetary magnetic field. The duration is longer for the negative SI than for the positive SI.

Direct measurements of the Poynting flux associated with convection electric fields in the magnetosphere

Observations of Poynting fluxes associated with onset of convection electric fields are essential for understanding of electromagnetic energy transport from the solar wind toward the magnetosphere leading to changes in the convection electric field, which is one of the most fundamental parameters in the magnetosphere‐ionosphere coupled system. We present Cluster multispacecraft observations of Poynting fluxes associated with abrupt changes in large‐scale electric fields during sudden commencements and southward turning of the interplanetary magnetic field (IMF). The Cluster spacecraft detected Poynting fluxes dominated by the field‐aligned upward component during the preliminary impulse of sudden commencements and in the initial period after southward turning of the IMF. The upward Poynting flux indicates existence of Alfvén waves transporting electromagnetic energy from the ionosphere toward the magnetosphere leading to magnetospheric convection changes. The waveguide model and global magnetohydrodynamic (MHD) simulation calculating evolution of the Poynting flux following solar wind pressure enhancements also show upward Poynting fluxes propagating from the ionosphere toward the magnetosphere faster than the propagation of compressional waves. We conclude that the ionosphere acts as a channel to transmit electromagnetic energy supplied as field‐aligned currents toward a wide region in the magnetosphere‐ionosphere system instantaneously, leading to changes in magnetospheric convection electric fields.

Anomalous occurrence features of the preliminary impulse of geomagnetic sudden commencement in the South Atlantic Anomaly region

Occurrence features of the preliminary impulse (PI) of geomagnetic sudden commencement (SC) both in the Pacific Ocean and South Atlantic Anomaly (SAA) regions were investigated using the long‐term magnetic field data obtained from the Circum‐pan Pacific Magnetometer Network (CPMN) and NICT Space Weather Monitoring (NSWM) magnetometer networks. The low‐latitude preliminary reverse impulse (PRI) at Okinawa (OKI: dip latitude = 37.97°) in the Pacific Ocean region appeared in all the magnetic local time (MLT) sectors with the peak occurrence rate of 40% around noon. On the other hand, the PRI occurrence rate at Santa Maria (SMA: dip latitude = −34.35°) near the center of the SAA region showed a significant enhancement in the daytime sector (0800–1600 h, MLT) with the peak value of 80%, which resembles the occurrence feature of the equatorial PRI. Moreover, the PRI amplitude around noon at SMA was about 3.0 times larger than that at OKI. From the calculation of the ionospheric conductivity derived from the IRI‐2007 and NRLMSISE‐00 models, it is shown that the height‐integrated conductivity was more enhanced in the SAA region (SMA), where the ambient magnetic field intensity is weak, compared with that in the Pacific Ocean region (OKI). Therefore, the anomalous increase of the PRI occurrence and amplitude is caused by the significant enhancement of the ionospheric conductivity due to the weakness of the ambient magnetic field intensity in the SAA region.

Geomagnetic disturbances on ground associated with particle precipitation during SC

Abstract. We have examined several cases of magnetosphere compression by solar wind pressure pulses using a set of instruments located in the noon sector of auroral zone. We have found that the increase in riometric absorption (sudden commencement absorption, SCA) occurred simultaneously with the beginning of negative or positive magnetic variations and broadband enhancement of magnetic activity in the frequency range above 0.1 Hz. Since magnetic variations were observed before the step-like increase of magnetic field at equatorial station (main impulse, MI), the negative declinations resembled the so-called preliminary impulse, PI. In this paper a mechanism for the generation of PI is introduced whereby PI's generation is linked to SCA – associated precipitation and the local enhancement of ionospheric conductivity leading to the reconstruction of the ionospheric current system prior to MI. Calculation showed that PI polarity depends on orientation of the background electric field and location of the observation point relative to ionospheric irregularity. For one case of direct measurements of electric field in the place where the ionospheric irregularity was present, the sign of calculated disturbance corresponded to the observed one. High-resolution measurements on IRIS facility and meridional chain of the induction magnetometers are utilized for the accurate timing of the impact of solar wind irregularity on the magnetopause.

Geomagnetic sudden commencement at the dawn-time dip equator

Abstract Waveform of geomagnetic sudden commencement (SC) on the ground depends on both latitude and local time (LT). Using high-resolution (1 s) geomagnetic field measurements obtained at Jicamarca (JIC, MAGLAT = 0.0°) from 1998 to 2005, we examined the waveforms of 188 SC events and found that some of the SCs observed at nighttime (approx. 22-05 LT) and dawn time (approx. 05-07 LT) had a common observational feature, i.e., a positive impulse always appeared at the beginning of the main impulse of the SC, which led to these events show preliminary positive impulse (PPI) or ‘Stepwise’ structure in the waveforms. In particular, the occurrence rate of the PPI/Stepwise events at the dawn time was clearly higher than that at the nighttime. This is the first complete report of this observational feature for the SCs observed at the dip equator. Based on a comparison of our observations with the previously proposed model calculation of Kikuchi et al. (2001), we suggest that the PPI/Stepwise events observed at the dawn-time dip equator reflect the effects of the ionospheric currents (ICs) and field-aligned currents (FACs). We therefore argue that this observational result provides evidence for the validity of the model calculation.

Effects of the induction electric field on ionospheric current systems driven by field‐aligned currents of magnetospheric origin

The effects of induction electric field on ionospheric current systems driven by field‐aligned currents of magnetospheric origin were studied, and a new method for estimating the equivalent current function representing the system was developed. It was found that a current system with a period of less than 4 min is significantly affected by the induction field and tends to be symmetric with noon, probably because the field overcomes the electrostatic field that creates the asymmetry. This result is consistent with the observed result for the preliminary impulse of storm commencement in which the local time variation of its appearance is symmetric with noon.

Non-traditional high-energy processes for disintegration and exposure of finely disseminated mineral complexes

For the first time the modern non-traditional processes for treatment of rebellious noble metal-bearing mineral ore, aimed at improving disintegration efficiency for finely dispersed mineral complexes are analyzed. Advantages and perspectives of a new high-efficient, energy-saving and ecologically safe process of the nanosecond powerful electromagnetic impulse (PEMI) effect, are proved. The said process provides a stable increase in the valuable component recovery at lower energy consumption and production costs of the finite product at the stage of dressing rebellious gold-bearing ores and concentrated products. The possible mechanisms of mineral disintegration under PEMI effect are considered, experimental data, cited in the paper, prove high efficiency of the preliminary impulse treatment, thus, the increase in the gold recovery at the cyanidation stage amounted to 4–12 % from rebellious ores, 10–30 % from gravity concentrates, 5–45 % from flotation concentrates, 30–80 % from oil flotation tailings.

Playing with fire: European terracotta models, 1740-1840

European sculptors of the Neoclassical period often modeled their works in clay before producing finished pieces in marble. These terracotta models are often more animated and revealing than the finished marbles and provide extraordinary insights into the creative process. This book offers the first comprehensive overview of Neoclassical terracotta models by European artists, featuring the works of Pajou, Houdon, and Canova, among many others. Leading scholars analyse modeling at every stage, from the brusque preliminary impulse to the highly finished statuette that is a work of art in its own right. They also comment on related issues: the model's role in the statuary of great men and in tomb sculpture; the sculptors' depictions of arcadia and the loves of the gods; the centering of activity in Rome, site of the collections of antiquities; and much more. Some 135 terracotta models are presented and reproduced, and short biographies of the artists are also included. This book is the catalogue for an exhibition that will be on view at the Musee du Louvre, Paris (September 15, 2003 to January 5, 2004); The Metropolitan Museum of Art (January 26 to April 23, 2004); and the Nationalmuseum, Stockholm (May 6 to August 29, 2004).

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: 1. Generation of the field‐aligned current associated with the preliminary impulse

The magnetospheric response to a solar wind impulse, a geomagnetic sudden commencement, is studied using an MHD model of the coupled solar wind‐magnetosphere‐ionosphere system. This paper discusses propagation of the first signal launched by the impulse and generation of the field‐aligned current that causes the ground magnetic signal detected as the preliminary impulse (PI). It is revealed that the PI current is first excited as an enhanced Chapman‐Ferraro current in the magnetopause and next turns to the magnetosphere along the wavefront of the compressional signal launched by the impulse. It is finally converted to a field‐aligned current via mode coupling due to plasma nonuniformity. The current in the wavefront region is an inertia current. We present a quantitative model of the PI model presented by Araki [1994] by using a numerical simulation.

Effect of the orientation of interplanetary shock on the geomagnetic sudden commencement

The rise time of the geomagnetic sudden commencement (SC) in low latitudes generally lies within a range from 2 to 10 min. An SC observed on 15 December 1995 was remarkable because it had an extremely long rise time of ∼30 min, although as seen by Wind, the interplanetary shock responsible for the SC was sharp as usual. The amplitude of the SC was consistent with the estimated value experimentally deduced from the increase in the solar wind dynamic pressure, implying that a complete compression of the magnetosphere occurred. However, the in situ measurement of the magnetotail by Geotail exhibited this event as an unusually gradual compression compared with a typical SC. Even this gradual compression of the magnetosphere produced a preliminary impulse on the ground that was lengthy, small, but detectable at auroral latitudes. The multisatellite observations of the interplanetary shock and the SC signals indicate that the shock normal was highly inclined duskward. Thus the orientation of the interplanetary shock or discontinuity is important in determining the SC field observed on the ground. Even if the local measurement of the solar wind dynamic pressure at a spacecraft appears to be rapid, a tilted pressure front could require many minutes to compress the forward part of the magnetosphere, producing a long rise time. Thus we can not predict the speed of the magnetospheric responses on the basis of the real‐time solar wind observations without knowing the orientation of the causative shock/discontinuity.

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.

Preliminary Impulse in Stoic Psychology

Preliminary Impulse in Stoic Psychology

A multipoint determination of the propagation velocity of a sudden commencement across the polar ionosphere

We use magnetic field and riometer data from ground observatories in both the Arctic and Antarctic regions to characterize the high‐latitude propagation of a sudden storm commencement (SC) that occurred at 0901 UT February 21, 1994. (1) High time resolution magnetic field data from both hemispheres indicate extremely rapid propagation of the initial part of the SC signal at high latitudes. An initial inflection point was observed in the data from dawn sector stations in both polar caps nearly simultaneously (Δt <2 s) but ∼3 s earlier in the southern hemisphere. (2) Data from the Magnetometer Array for Cusp and Cleft Studies (MACCS) in Arctic Canada, with stations from 0130 to 0600 magnetic local time, indicate dispersive propagation of the preliminary impulse (PI) at speeds decreasing from ∼150 to ∼50 km/s, directly away from a source near or slightly poleward of the cusp, rather than along the auroral oval. (3) Plots of two‐dimensional, 20‐s resolution equivalent convection vectors in the Northern Hemisphere reveal the imposition and rapid propagation and decay of a short‐lived large‐scale flow pattern opposite to the normal dawn sector flow, followed by an intensification of the original pattern. However, the perturbed flows were not dominated by the localized, tailward propagating vortices predicted in some models of SC events. In both hemispheres, observations of the PI are consistent with the temporary imposition of a field‐aligned current pair antisymmetric about local noon in the polar ionosphere and with horizontal fast mode propagation of a pulse through the ionosphere. (4) Riometer signatures do not match the magnetic variations in either time or intensity, and they propagate with a much lower velocity (∼1 km/s). We infer that the initial magnetic signatures are generated by the arrival of Alfvén waves and associated Birkeland currents at the near‐cusp ionosphere, while the riometer signatures at these high latitudes are generated predominantly by interplanetary particles associated with the causative coronal mass ejection.

Occurrence Characteristics of the Preliminary Impulse of Geomagnetic Sudden Commencement Detected at Middle and Low Latitudes.

The occurrence frequency of preliminary impulse (PI) of geomagnetic sudden commencements was statistically studied using the geomagnetic records of H-component at Fredericksburg (FRD; 49.0° geomagnetic latitude) and San Juan (SJG; 29.2°). In the daytime, positive PI predominantly occurs before noon and negative PI in the afternoon. These features, which are the same as those of high-latitude PI, indicate that high-latitude ionospheric currents responsible for PI extends toward middle and low latitudes. Almost all PIs detected in the nighttime are negative at FRD though the occurrence rate is much smaller than in the daytime. The diurnal variation of the occurrence rates of PI shows a drastic seasonal change at SIG, which is probably caused by a seasonal variation of the ionosphere. No correlation between the occurrence rates and the solar activity was found at either observatories. Ionospheric currents which are driven by a pair of field-aligned currents were numerically calculated on the assumption of the stationary state, and the geomagnetic field variation caused by the three-dimensional currents is simulated. Although the symmetric distribution of the field-aligned currents with respect to the noon-midnight meridian has been assumed, the calculated diurnal variation of magnetic field exhibits a severe deflection from the symmetry in middle and low latitudes.