S. Taguchi

Global imaging of polar cap patches with dual airglow imagers

During a 2 h interval from 2240 to 2440 UT on 12 November 2012, regions of increased 630.0 nm airglow emissions were simultaneously detected by dual all-sky imagers in the polar cap, one at Longyearbyen, Norway (78.1°N, 15.5°E) and the other at Resolute Bay, Canada (74.7°N, 265.1°E). The Resolute Bay incoherent scatter radar observed clear enhancements of the F region electron density up to 1012 m−3 within these airglow structures which indicates that these are optical manifestations of polar cap patches propagating across the polar cap. During this interval of simultaneous airglow imaging, the nightside/dawnside (dayside/duskside) half of the patches was captured by the imager at Longyearbyen (Resolute Bay). This unique situation enabled us to estimate the dawn-dusk extent of the patches to be around 1500 km, which was at least 60–70% of the width of the antisunward plasma stream seen in the Super Dual Auroral Radar Network convection maps. In contrast to the large extent in the dawn-dusk direction, the noon-midnight thickness of each patch was less than 500 km. These observations demonstrate that there exists a class of patches showing cigar-shaped structures. Such patches could be produced in a wide range of local time on the dayside nearly simultaneously and spread across many hours of local time soon after their generation.

Characterization of the IMF By ‐dependent field‐aligned currents in the cleft region based on DE 2 observations

The interplanetary magnetic field (IMF) By-dependent distribution of field-aligned currents in the cleft region is studied, using the magnetic field and plasma data from 47 passes of Dynamics Explorer (DE) 2. These orbits were chosen on the conditions that cusp/cleft particles are detected and that at the same time the IMF By and Bz components satisfy the criteria |By|≥5 nT and |Bz|≤5 nT during the satellites crossing of the relevant field-aligned current region. When By is positive (negative) in addition to satisfying these conditions, there is a strong eastward (westward) magnetic perturbation caused by a pair of field-aligned current sheets, consisting of an equatorward sheet with downward (upward) current and a poleward sheet having upward (downward) current. These By-dependent field-aligned currents in the equatorward and the poleward sheets are referred to as the low-latitude cleft current (LCC) and the high-latitude cleft current (HCC), respectively. The cusp/cleft electron precipitation region and the LCC region overlap with each other to a varying degree irrespective of the sign of By. For positive (negative) By, LCC has the same direction as the morning (afternoon) region 1 current or the afternoon (morning) region 2 current. Thus an interpretation has been given in the past that the LCC region is an extension of the region 1 or region 2 current system. However, in this paper we present an alternative view that the LCC region is not an extension of the region 1 or region 2 current system and that a pair of LCC and HCC constitutes the cleft field-aligned current regime. The proposed pair of cleft field-aligned currents is explained with a qualitative model in which this pair of currents is generated on the open field lines that have just been reconnected on the dayside magnetopause. The model assumes a quasi-steady reconnection operating within certain longitudinal width extending to both sides of the stagnation point on the dayside magnetopause. The reconnected flux tubes move under the influences of the field tension and the magnetosheath flow. When the magnetosheath By is positive, the northern hemisphere field lines reconnected on the eastward side of the stagnation point are pulled toward higher latitudes, and the field lines reconnected on the westward side of the stagnation point are pulled along the dawnside magnetopause flank. The electric fields associated with these motions are present immediately inside the magnetopause (rotational discontinuity). This is the source region of LCC and HCC. The electric fields are transmitted along the field lines to the ionosphere, creating a poleward electric field and a pair of field-aligned currents when By is positive; the pair of field-aligned currents consists of a downward current at lower latitudes (LCC) and an upward current at higher latitudes (HCC). In the By negative case, the model explains the reversal of the field-aligned current direction in the LCC and HCC regions.

Multispacecraft observations of sudden impulses in the magnetotail caused by solar wind pressure discontinuities: Wind and IMP 8

Two upstream solar wind pressure discontinuities that were associated with storm sudden commencements have been examined to determine their effect on the geomagnetic tail lobe field. During the two events, occurring on March 9, 1995, and August 17, 1995, the Wind spacecraft was located in the upstream region monitoring the solar wind, and the IMP 8 spacecraft was in the geomagnetic tail lobe observing the tail response. The two events occurred during periods with northward or weak southward interplanetary magnetic field. In each case, the data suggest that the magnetic field in the tail lobe increased in magnitude directly in response to the external solar wind pressure increase. It is shown that a simple model in which a uniform magnetic field is compressed by a step function constriction accurately predicts characteristic timescales, which are of the order of a couple minutes, and the magnetic field profiles. The inferred flaring angles are consistent with model predictions, and the changes in the flaring angle across the discontinuities correspond to expectations based on changes in the subsolar magnetopause position and tail width. Overall, the results of this study indicate that the magnetotail maintains an approximate MHD equilibrium even as it responds rapidly to interplanetary pressure discontinuities.

ISTP observations of plasmoid ejection: IMP 8 and Geotail

IMP 8 and Geotail observations of traveling compression regions (TCRs) and plasmoids, respectively, are used to investigate plasmoid formation and ejection. One year of IMP 8 magnetometer measurements taken during the distant tail phase of the Geotail mission were searched for TCRs, which signal the release of plasmoids down the tail. A total of 10 such intervals were identified. Examination of the Geotail measurements showed that this spacecraft was in the magnetotail for only three of the events. However, in all three cases, clear plasmoid signatures were observed at Geotail. These plasmoids were observed at distances of X = −170 to −197 RE. The in situ plasma velocities in these plasmoids are found to exceed the time-of-flight speeds between IMP 8 and Geotail suggesting that some further acceleration may have taken place following release. The inferred lengths of these plasmoids, ∼27–40 RE, are comparable to the downtail distance of IMP 8. This indicates that TCR at IMP 8 can be caused by plasmoids forming not only earthward but also adjacent to or just tailward of the spacecraft. The closeness of IMP 8 to the point of plasmoid formation is confirmed by the small, ∼0–3 min, time delays between the TCR perturbation and substorm onset. In two of the plasmoid events, high-speed earthward plasma flows and streaming energetic particles were measured in the plasma sheet boundary layer surrounding the plasmoid along with large positive Bz at the leading edge of the plasmoid suggesting that the core of the plasmoid was “snow plowing” into flux tubes recently closed at an active distant neutral line. In summary, these unique two-point measurements clearly show plasmoid ejection near substorm onset, their rapid movement to the distant tail and their further evolution as they encounter preexisting X lines in the distant tail.

Traveling compression regions in the midtail: Fifteen years of IMP 8 observations

The characteristics of traveling compression regions (TCRs) in the midtail lobes near expansion-phase onsets have been analyzed utilizing the data set returned by the IMP 8 magnetic field investigation between 1973 and 1990. Examination of the 15 years for which we have the AL index led to the identification of 565 isolated substorm events with expansion phases for which IMP 8 was in the tail lobes and returned magnetic field measurements. For 31 of the 565 events a typical TCR variation was observed at IMP 8. These TCR perturbations have been previously studied in the ISEE 3 distant tail measurements and consist of a field compression during which BZ increases and then decreases after peaking, although BZ may not reach a positive value in many cases. For 59 of the substorms a field compression was seen at IMP 8 during which the BZ peak before the field maximum is lacking, i.e., the BZ just decreases rapidly, and the variation is unipolar. These 90 (31 plus 59) substorms occurred in the region for X=−26 to −38 RE, |Y|≤15 RE, and ZN ≤ 20 RE all in the solar wind aberrated GSM coordinate system. The “unipolar BZ compression region” has a phase of flat BZ preceding the sharp southward tilting interval. The typical timescale of this phase is 60–100 s. We interpret this flat BZ phase as a field compression created by the rapid growth of the plasmoid bulge in the Z dimension underneath IMP 8 just prior to the plasmoid being ejected tailward. The fractional occurrence frequency of the TCRs observed in our substorm database for each year was calculated by dividing the number of the substorms producing TCRs by all of the IMP 8 lobe substorm events for each year. This ratio tends to be high (maximum 41.7%) in the years of low sunspot number, and low (minimum 7.7%) in the years of high sunspot number. For years when the occurrence ratio is low, the TCRs are observed primarily within |Y| = 10 RE. When the occurrence ratio is high, the TCRs are found across a wider region in Y with a peak in the premidnight. These results suggest that the plasmoid formation is controlled by solar cycle related parameters in such a manner that the near-Earth neutral line occurs at a wider region in the dawn-dusk direction for the years of low sunspot number.

Prediction of the geomagnetic storm associated Dst index using an artificial neural network algorithm

In order to enhance the reproduction of the recovery phase Dst index of a geomagnetic storm which has been shown by previous studies to be poorly reproduced when compared with the initial and main phases, an artificial neural network with one hidden layer and error back-propagation learning has been developed. Three hourly Dst values before the minimum Dst in the main phase in addition to solar wind data of IMF southward-component Bs, the total strength Bt and the square root of the dynamic pressure,

Dual Spacecraft Observations of Lobe Magnetic Field Perturbations Before, During and after Plasmoid Release

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By-controlled convection and field-aligned currents near midnight auroral oval for northward interplanetary magnetic field

, for the minimum Dst, i.e., information on the main phase was used to train the network. Twenty carefully selected storms from 1972–1982 were used for the training, and the performance of the trained network was then tested with three storms of different Dst strengths outside the training data set. Extremely good agreement between the measured Dst and the modeled Dst has been obtained for the recovery phase. The correlation coefficient between the predicted and observed Dst is more than 0.95. The average relative variance is 0.1 or less, which means that more than 90% of the observed Dst variance is predictable in our model. Our neural network model suggests that the minimum Dst of a storm is significant in the storm recovery process.

IMP 8 observations of traveling compression regions in the mid‐tail near substorm expansion phase onset

This study examines a data set returned by IMP 8 and Geotail on January 29, 1995 during a substorm which resulted in the ejection of a plasmoid. The two spacecraft (s/c) were situated in the north lobe of the tail and both observed a traveling compression region (TCR). We show that in this instance dual s/c measurements can be used to model all three dimensions of the underlying plasmoid and to estimate its rate of expansion. For this event plasmoid dimensions of ΔX ∼18, ΔY ∼30, and ΔZ ∼10 Re are determined from the IMP 8 and Geotail observations. Furthermore, a factor of ∼2 increase in the amplitude of the TCR occurred in the 1.5 min it took to move from IMP 8 to Geotail. Modeled using conservation of magnetic flux, this increase in lobe compression implies that the underlying plasmoid was expanding at a rate of ∼140 km/s. Finally, a reconfiguration of the lobe magnetic field followed plasmoid ejection which moved magnetic flux tubes into the wake behind the plasmoid where they would become available to feed the reconnection region.

Temporal relationship between midtail traveling compression regions and substorm onset: Evidence for near‐Earth neutral line formation in the late growth phase

Using the Dynamics Explorer (DE) 2 magnetic and electric field and plasma data, By-controlled convection and field-aligned currents in the midnight sector for northward interplanetary magnetic field (IMF) are examined. The results of an analysis of the electric field data show that when IMF is stable and when its magnitude is large, a coherent By-controlled convection exists near the midnight auroral oval in the ionosphere having adequate conductivities. When By is negative, the convection consists of a westward (eastward) plasma flow at the lower latitudes and an eastward (westward) plasma flow at the higher latitudes in the midnight sector in the northern (southern) ionosphere. When By is positive, the flow directions are reversed. The distribution of the field-aligned currents associated with the By-controlled convection, in most cases, shows a three-sheet structure. In accordance with the convection the directions of the three sheets are dependent on the sign of By. The location of disappearance of the precipitating intense electrons having energies of a few keV is close to the convection reversal surface. However, the more detailed relationship between the electron precipitation boundary and the convection reversal surface depends on the case. In some cases the precipitating electrons extend beyond the convection reversal surface, and in others the poleward boundary terminates at a latitude lower than the reversal surface. Previous studies suggest that the poleward boundary of the electrons having energies of a few keV is not necessarily coincident with an open/closed boundary. Thus the open/closed boundary may be at a latitude higher than the poleward boundary of the electron precipitation, or it may be at a latitude lower than the poleward boundary of the electron precipitation. We discuss relationships between the open/closed boundary and the convection reversal surface. When as a possible choice we adopt a view that the open/closed boundary agrees with the convection reversal surface, we can explain qualitatively the configuration of the By-controlled convection on the open and close field line regions by proposing a mapping modified in accordance with IMF By.