Y. Kamide

Two‐step development of geomagnetic storms

Using the Dst index, more than 1200 geomagnetic storms, from weak to intense, spanning over three solar cycles have been examined statistically. Interplanetary magnetic field (IMF) and solar wind data have also been used in the study. It is found that for more than 50% of intense magnetic storms, the main phase undergoes a two-step growth in the ring current. That is, before the ring current has decayed significantly to the prestorm level, anew major particle injection occurs, leading to a further development of the ring current, and making Dst decrease a second time. Thus intense magnetic storms may often be the result of two closely spaced moderate storms. The corresponding signature in the interplanetary medium is the arrival of double-structured southward IMF at the magnetosphere.

A fresh perspective of the substorm current system and its dynamo

[1] The common view for the substorm current system is a current wedge involving principally an azimuthal current system. We first evaluate briefly previous studies that provide strong evidence for the dominance of a meridional current system (MCS) in substorms. Based on these clues, we propose the dynamo for the MCS to be a kinetic current disruption process such that dipolarization is achieved by magnetic field line slippage, thus producing the dynamo action of a tailward directed current with an earthward directed electric field. The total current strength is estimated to be ∼ 10 MA for an average substorm size, consistent with that obtained from the KRM algorithm.

“Fine structure” of the storm-substorm relationship: Ion injections during DST decrease

Abstract Following a long period of consensus on the storm-substorm relationship, a dispute on this topic has emerged in recent years. The importance of substorms for the buildup of the terrestrial ring current, which is the major element of magnetic storms, has been questioned in several studies. This paper is an effort to assess the “fine structure” of the storm-substorm relationship, by investigating the correlation between the changes in Dst and the substorm-associated O + enhancements in the inner magnetosphere during the storm main phase. For this purpose we use energetic ion measurements from the Magnetospheric Ion Composition Spectrometer (MICS) on board the Combined Release and Radiation Effects Satellite (CRRES), and the newly produced high-resolution (5-min) Dst index for the intense storm of June 5, 1991. Substorm signatures from both MICS measurements and ground magnetometers correlate well with changes in the Dst decrease rate. This implies a significant influence of substorm occurrence on storm dynamics.

Interplanetary Causes of Great and Superintense Magnetic Storms

Abstract We examine possible interplanetary mechanisms for the creation of the largest magnetic storms at the Earth. We consider the effects of interplanetary shock events on magnetic cloud and sheath plasma. We also examine the effects of a combination of a long-duration southward sheath magnetic field, followed by a magnetic cloud BS event. Examination of profiles of the most intense storms from 1957 to the present indicate that the latter (double IMF BZ events) is an important cause of superintense DST events.

Climatological characteristics of the auroral ionosphere in terms of electric field and ionospheric conductance

The contributions of the north-south component of the electric field and the Hall conductance to the auroral electrojet are examined separately. For this purpose, 52 days of measurements from the Chatanika incoherent scatter radar, which was located near one of the standard AE stations, College, are utilized. A number of interesting characteristics of the auroral electrojet system and auroral electrojet indices are noted: (1) The electric field distribution along the auroral region is roughly symmetric with respect to the 1100-2300 magnetic local time meridian. (2) The electric field, particularly the southward component, becomes a dominant feature along auroral latitudes with increasing magnetic activity. (3) The Hall conductance distribution in the postnoon sector is mainly determined by the Sun, thus making the eastward electrojet and the AU index dependent upon season. On the other hand, the Hall conductance associated with the major part of the westward electrojet in the midnight-postmidnight sector is controlled by precipitating electrons. (4) Since the Hall conductance of solar origin in the postnoon sector can be estimated, it would be possible to monitor electric field enhancements contributing to the eastward electrojet. By assuming the same electric field, except for the sign being applied to the westward electrojet, the AL index can be used to estimate the contribution of the Hall conductance associated with particle precipitation. This is a clear indication that the two indices, AU and AL, are governed by different physical processes. Thus it is recommended to use the two indices separately, rather than the combined AE, in monitoring the auroral electrojet system. (5) The Harang discontinuity seems to be a boundary separating the region of precipitating energetic particles on its northeast side from that of soft particles on its southwest side.

Short-duration convection bays and localized interplanetary magnetic field structures on November 28, 1995

We present ground-based, plasma sheet, and magnetosheath observations of two subsequent short-duration (10–20 min) increases of the postmidnight westward electrojet on November 28, 1995. Appearing as though small (150–200 nT) substorms, they were not accompanied by any substorm expansion onset signatures. Auroral breakup, worldwide Pi2 pulsations, and the corresponding plasma sheet activity, such as fast flows, current disruption, and plasmoid generation, were all observed only at the recovery of the second electrojet increase. These convection bays were associated with the equatorward expansion of the auroras and simultaneous magnetic variations in the polar cap and middle latitudes. Growth phase signatures of the lobe field increase and tailward stretching of magnetic field were also observed in the plasma sheet. Bursty bulk flows in the plasma sheet seem to be quenched at the onset of first convection bay and did not resume until the auroral breakup which concluded the second convection bay. A point of interest of this event was the “incomplete” convection/current system with a well-developed dawn vortex in the absence of well-defined dusk vortex; instead, a complicated transient activity dominated over the afternoon-dusk local time sector. We interpret this asymmetry either in terms of the magnetopause encounter with the edge of the solar wind driver, i.e., strong southward IMF, which hits only the dawn part of the magnetosphere, or with an extremely slant interplanetary discontinuity. This unique configuration was inferred from observations of uncorrelated strong southward Bz events by the Wind and IMP 8 spacecraft in the dusk and dawn magnetosheath, respectively, as well as from the directional analysis of the interplanetary discontinuities which form the edges of these structures. We suggest that interaction of the magnetosphere with very slant solar wind discontinuities may bring various specific features to magnetospheric and ionospheric dynamics that have not been reported.

The Storm‐Substorm Relationship: Current Understanding and Outlook

The intensification of the ring current during a geospace storm has been one of the key issues in space physics. Substorms have been considered responsible for bringing in particles from the magnetotail, which get trapped on closed drift paths to form a symmetrical ring current. It is now recognized that the ring current develops dominantly from a sustained enhancement of the convection electric field. The magnetic perturbations observed during a storm main phase are then due to a partial ring current, which closes in part through the ionosphere and in part through the magnetopause. An enhanced convection electric field moves the plasma Earth-ward, thus energizing it, and when this field is reduced, the particles become trapped and a symmetric ring current is formed. Substorms, however, are always accompanied by the injection of energetic particles and their contribution to the storm-time ring current is a matter of current debate. Considering the electrodynamic nature of the interaction between different regions of the magnetosphere and the dominantly global nature of its dynamics, storms and substorms are not expected to just co-exist, but the ways in which they influence each other are not clear yet. The Chapman Conference at Lonavala (2001) saw the cementing of a new paradigm for the ring current and the storm-substorm relationship. The accumulating evidence against the substorms being the main constituents of storm main phase and the recognition of the dominant role of partial ring current led to a consensus (Lonavala consensus) marking a turning point in the understanding of the storm-substorm relationship.

Substorm changes of the electrodynamic quantities in the polar ionosphere: CDAW 9

On the basis of ground magnetometer data from 75 northern hemisphere stations and the ionospheric conductivity distribution estimated from Viking satellite observations of auroral images, various electrodynamic quantities in the polar ionosphere are calculated for the April 1, 1986, Coordinated Data Analysis Workshop (CDAW) 9 substorm. Since the Scandinavia and Russia chains of magnetometers were located in the premidnight-midnight sector during this interval and the estimated conductivity distribution is instantaneous, our data set provides us with a unique opportunity to examine some long-standing problems associated with the substorm expansion onset. Several important findings of this study are summarized as follows: (1) Before the expansion onset of the substorm, intensifications of ionospheric currents or the cross-polar cap potential are very weak in this particular example. Both quantities begin to increase notably only with the initiation of the substorm expansion onset. (2) The intensified westward electrojet flows along the poleward half of the enhanced ionospheric conductivity belt in the midnight sector during the expansion phase, while its equatorward half is occupied by a weak eastward electrojet. (3) The Joule heating rate and the energy input rate of auroral particles are quite comparable preceding the expansion onset. During the expansion phase of the substorm, however, Joule heating shows a marked intensification, but the latter increases only moderately, indicating that the Joule dissipation is more effective than auroral particle energy input during substorm times. (4) The Hall currents are not completely divergence-free. The corresponding field-aligned currents show highly localized structures during the maximum epoch of the substorm, with the upward current being located in the region of the steepest conductivity gradient on the poleward side of the westward electrojet in the midnight sector. This is indirect evidence that the so-called imperfect Cowling channel is effective behind the westward traveling surge.

Convection in the distant magnetotail under extremely quiet and weakly disturbed conditions

Measurements of the magnetic field and low-energy plasma by the Geotail spacecraft have been used to study magnetospheric convection in the distant tail at X = −(79 – 200)RE under extremely quiet and weakly disturbed conditions. The analysis was carried out separately for the tail lobes and the plasma sheet; these regions were identified by plasma and magnetic field parameters. It is shown that plasmas in the northern and southern tail lobes move tailward along magnetic field lines at the same time as magnetic field lines converge towards the plasma sheet. The main processes occurring in the distant tail under extremely quiet conditions (northward interplanetary magnetic field) and weakly disturbed conditions (southward IMF) are very similar, excluding the passage of high-speed tailward plasmoids, which are obviously generated in the near-Earth plasma sheet. Magnetic turbulence, that is, fluctuations of the magnetic field occurring against the background of a totally northward field (BZ>0), consistent with intense oscillations in the plasma flow VX component, is typical of all conditions. It is concluded that the stretched and antiparallel field lines of the northern and southern tail lobes are reconnected in the plasma sheet. Tail lobe plasma constantly enters the plasma sheet, where the lobe magnetic field energy converts into the kinetic energy of the plasma producing the magnetic turbulence. It seems likely that the direction (northward or southward) of the IMF is not necessary for tail formation, and processes in the near tail (substorm activity) does not strongly influence reconnection in the distant tail.

The Interplanetary Causes of Magnetic Storms, HILDCAAs and Viscous Interaction

Abstract A review of the interplanetary causes of geomagnetic activity is presented. Intense southward interplanetary magnetic fields in the sheath region ahead of fast interplanetary manifestations of solar CMEs (ICMEs), and the intrinsically high B Z fields of magnetic clouds within ICMEs, are the two most predominant causes of major storms with D ST ≤−100 nT. This is true during solar maximum when ICMEs dominate the interplanetary medium and also during the declining phase of the solar cycle when corotating streams and proto-corotating interaction regions (PCIRs) are the dominant large scale structures. PCIRs are high magnetic field regions caused by the interaction of coronal hole high-speed streams with the upstream slow speed streams. PCIRs cause only moderate to weak magnetic storms (rarely storms with D ST 〈-−100 nT) because of the highly variable B z structure within them. It is thought that the B z fluctuations within the PCIR are compressed high-speed stream Alfven waves. The B z fluctuations associated with nonlinear Alfven waves within the high-speed streams cause continuous auroral activity called HILDCAAs. These HILDCAA events lead to annual AE averages that are sometimes higher during the solar cycle descending phase (such as in 1974) than during solar maximum (1979 or 1981). We quantify an upper limit of the efficiency of viscous interaction energy input into the magnetosphere: 1 to 3 × 10 −3 of the solar wind ram energy. This is in contrast to an efficiency of 5 to 10 × 10 −2 for magnetic reconnection during substorms and magnetic storms. Finally, a specific mechanism of viscous interaction is explored: low latitude boundary layer (LLBL) resonant wave-particle interactions. The waves are sufficiently intense to cross-field diffuse magnetosheath plasma onto closed field lines to create the LLBL. Pitch angle scattering will lead to auroral energy deposition of ∼ 1 erg cm −2 s − , sufficient for the creation of the dayside aurora.