Dependence of substorm occurrence probability on the interplanetary magnetic field and on the size of the auroral oval
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Kamide, Y.↗
Engineering topics
Akasofu, S.-I.
Publications and source records attributed to Akasofu, S.-I..
Interplanetary magnetic field and magnetospheric substorms
The interplanetary magnetic field (IMF) changes and the associated responses of the magnetosphere on November 1, 1972, are examined. IMF Bz changes consisted of a sudden southward turning, a slow northward turning, and a subsequent steady northward sense. Magnetospheric substorms occurred throughout this period.
Akasofu, S.-I.↗
Systematic study of plasma flow during plasma sheet thinnings
On the basis of a study of Imp 6 measurements of plasma flow, it is concluded that there is no clear indication of a predominance of tailward plasma flow beyond about X = -15 R sub E in the midnight sector of the plasma sheet during the expansive phase of a substorm. In fact, it is shown statistically that sunward plasma flow is more frequently observed in the midnight sector within about 30 R sub E from the earth than in any other direction during plasma sheet thinning at the substorm expansion. This result supports the conclusion that there is no definite evidence for the formation of a reconnection neutral line in the near-earth plasma sheet during most substorms.
Lui, A. T. Y.↗
The response of the dayside aurora to sharp northward and southward transitions of the interplanetary magnetic field and to magnetospheric substorms
Latitudinal shifts of the discrete dayside aurora are examined with respect to variations in the interplanetary magnetic field (IMF) and to magnetospheric substorms. Within 10-15 min after steplike southward (northward) transitions in the IMF the dayside auroral oval moves equatorward (poleward). In at least some cases the auroral shift is similar to an exponential relaxation in latitude from an initial to a final steady state value; for these cases the average exponential time constant is estimated to be 17 min. Substorm features in the dayside aurora include (1) an equatorward shift by 1-3 deg of the equatorward discrete auroral boundary, (2) a brightening of the discrete aurora near substorm onset, (3) the formation of multiple auroral bands, and (4) poleward motion of short-lived individual auroral arcs occurring nearly coincident in time with the equatorward boundary shifts, the average poleward velocity of these arcs being approximately 800 m/s.
Horwitz, J. L.↗
Auroral kilometric radiation as an indicator of auroral magnetic disturbances
Satellite low-frequency radio measurements have shown that auroral kilometric radiation, an intense radio emission from the earth's auroral regions, is closely associated with auroral and magnetic disturbances. In this paper a detailed investigation of this relationship is presented, using the auroral electrojet (AE) index as an indicator of auroral magnetic disturbances and radio measurements from the Imp 6 spacecraft. This study indicates that the mean power flux of the 178-kHz radiation tends to be proportional to the 1.2 power of (AE) for AE more than 100 gamma and, with less certainty, to the square of (AE) for AE less than 100 gamma. The correlation coefficient between log AE and the logarithm of the power flux is 0.514. Occasionally, a kilometric radiation event is detected which is not detected by the ground magnetometer stations, even though an auroral substorm is in progress. This study shows that the remote detection of kilometric radio emissions from the earth can be used as a reasonably reliable indicator of auroral substorm activity.
Voots, G. R.↗
Field-aligned currents and ionospheric electric fields
It is shown that the observed distribution of the ionospheric electric field can be deduced from an equation combining Ohm's law with the current continuity equation by using the 'observed' distribution of field-aligned currents as the boundary condition for two models of the ionosphere. The first model has one conductive annular ring representing the quiet-time auroral precipitation belt; the second has two conductive annular rings that simulate the discrete and diffuse auroral regions. An analysis is performed to determine how well the electric-field distribution can be reproduced. The results indicate that the first model reproduces the Sq(p)-type distribution, the second model reproduces reasonably well a substorm-type potential and ionospheric current patterns together with the Harang discontinuity, and that the distribution of field-aligned currents is the same for both models.
Yasuhara, F.↗
Physics of magnetospheric substorms
Magnetosphere substorm physics are updated in the monograph. Major topics include: (1) open magnetosphere and the auroral oval; (2) auroras and auroral particles; (3) plasma distribution in the magnetosphere; (4) magnetosphere responses to interplanetary disturbances; (5) magnetospheric substorms and magnetotail phenomena; (6) magnetospheric currents, plasma injection, plasmasphere disturbances; and (7) magnetospheric substorms and solar-terrestrial relations. Other topics covered include: open field lines and the open magnetosphere, field-aligned currents, auroral particles and atmospheric emissions, plasma mantle, plasma sheet, radiation belts, magnetic flux transfer to the magnetotail, polar cap phenomena, substorm parameters, thinning of the plasma sheet, auroral electrojets, diurnal variations and dawn-dusk asymmetry of particle distributions, and instabilities.
Akasofu, S.-I.↗
Simultaneous observations of discrete and diffuse auroras by the Isis 2 satellite and airborne instruments
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Deehr, C. S.↗
Correlated observations of several auroral substorms on February 17, 1971
The purpose of this study is to correlate in detail auroral activity with the corresponding disturbances in the magnetotail. The auroral data were recorded by optical instruments aboard an airplane flying over the Arctic Ocean along the Alaska meridian and by the Alaska meridian chain of all-sky cameras. The corresponding magnetotail observations were made by various instruments on Vela 6A and Imp 5; the interplanetary magnetic field was monitored concurrently by Explorer 35. Three successive substorms were observed on February 17, 1971. Each substorm was readily identified by the classical auroral and magnetic signatures which accompanied its onset. The observed variations of plasma and magnetic field in the magnetotail were consistent with the idea that a neutral line formed in the range X sub SM between -12 and -18 R sub E at the onset of each substorm expansive phase and then moved tailward past X sub SM = -18 R sub E some tens of minutes afterward. The Z component of the tail magnetic field decreased rather steadily for a period of 1-2 1/2 hours after each substorm and until the onset of the next expansive phase, reaching a minimum value just before each onset.
Hones, E. W., Jr.↗
Dependence of the latitude of the cleft on the interplanetary magnetic field and substorm activity
The latitudinal motion of the cleft (the polar cusp) associated with the southward interplanetary magnetic field (IMF) and substorm activity is examined. The cleft location is identified on the basis of the location of midday auroras and of electron precipitation by the OGO 4 and ISIS 1 satellites. It is found that the IMF and substorm activity control independently the latitude of the cleft and that they can shift the cleft location by 3 or 4 deg under average conditions.
Kamide, Y.↗
Solar cycle review /General aspects/
Several topics are discussed in this review of the solar cycle. The basic solar-interplanetary magnetic structure is considered, and the high speed solar wind streams and their sunspot cycle variation are described, with attention to the sunspot cycle variation, the high speed solar wind streams and geomagnetic disturbances, and the causes of changes of the north-south component of the interplanetary magnetic field. Solar storms, solar flares, and magnetic bubbles are examined with attention directed to the solar cycle variation of transient polar activities and the interplanetary disturbances associated with solar storms. Some features of cosmic rays, the 11-year cycle variation and the Forbush decreases, are considered. Attention is also directed to the terrestrial atmosphere and to the outer planets and their relation to the solar cycle.
Akasofu, S.-I.↗
The latitudinal morphology of 10-eV to 10-keV electron fluxes during magnetically quiet and disturbed times in the 2100-0300 MLT sector
Electron and proton data from Isis 1 and 2 have been used to examine the change in the latitudinal morphology of auroral particle fluxes as a function of substorm time in the dipole magnetic local time period from 2100 to 0300. Clear and repeatable systematics in the latitudinal morphology were observed during the various phases of a substorm, allowing the substorm phase to be identified on the basis of particle data alone. Based on this study, a new phenomenological substorm model is presented which relates particle precipitation from various parts of the magnetotail to auroral oval morphology.
Winningham, J. D.↗
Incoherent scatter radar observations of westward electric fields. II
In this paper we describe the results of a series of geomagnetic meridian plane radar incoherent scatter plasma transport measurements. From such data the eastward (zonal) component of the electric field can be deduced. The results show that the electric field has an eastward component whenever the discrete auroras are northward of the region of the measurement. The zonal field component turns westward as the discrete auroras move equatorward of the region of the measurement. Thus, there is an eastward electric field boundary near the equatorward limit of the discrete aurora, which is the poleward boundary of the diffuse aurora during undisturbed periods. During a particularly disturbed period, we also observed three pronounced substorm-related enhancements of the westward-directed zonal field. For midnight and morning sector substorms, the enhancements preceded the substorm onset times by 20-30 min.
Rino, C. L.↗
Electric and magnetic field observations during a substorm on February 24, 1970
Description of a series of electric field measurements obtained from the Injun 5 satellite and simultaneous magnetic disturbances observed in the interplanetary medium and on the ground during a magnetic substorm. The substorm analyzed took place on Feb. 24, 1970. Prior to the onset of the substorm, a greatly enhanced antisunward plasma flow was observed over the polar cap. The enhanced plasma flow occurred about 30 minutes after a switch in the direction of the interplanetary magnetic field from northward to southward. The electric fields across the polar cap immediately before and during the substorm were essentially unchanged, and it is thus indicated that an enhancement in the ionospheric conductivity rather than the electric field must be responsible for the large increase in the auroral electrojet current during the substorm.
Gurnett, D. A.↗
On the cause of northward magnetic field along the negative X axis during magnetospheric substorms
The magnetic fields produced by a three-dimensional current system, consisting of a flow into the morning part of the auroral oval along tail-like field lines, along the auroral oval, and out from the evening part of the oval along tail-like field lines, are computed. It is demonstrated that the major parts of the well-known 'positive bay' in low latitudes on the earth's surface, the positive H variation at the synchronous distance, and the positive Bz variation along the magnetotail during magnetospheric substorms can be caused by the proposed current system.
Kamide, Y.↗
The aurora and the magnetosphere - The Chapman Memorial Lecture
Review of recent progress in magnetospheric physics, in particular, in understanding the magnetospheric substorm. It is shown that a number of magnetospheric phenomena can now be understood by viewing the solar wind-magnetosphere interaction as an MHD dynamo; auroral phenomena are powered by the dynamo. Also, magnetospheric responses to variations of the north-south and east-west components of the interplanetary magnetic field have been identified. The magnetospheric substorm is entirely different from the responses of the magnetosphere to the southward component of the interplanetary magnetic field. It may be associated with the formation of a neutral line within the plasma sheet and with an enhanced reconnection along the line. A number of substorm-associated phenomena can be understood by noting that the new neutral line formation is caused by a short-circuiting of a part of the magnetotail current.
Akasofu, S.-I.↗
A model of the open magnetosphere
The Chapman-Ferraro image method is extended to construct an idealized model of the open magnetosphere that responds to a change of the interplanetary field direction as well as to a change of the field magnitude or of the solar wind momentum flux. The magnetopause of the present model is an infinite plane surface having a normal field component distribution that is consistent with the merging theory. An upper limit on the inward displacement of the magnetopause following a southward turning of the interplanetary field is obtained. The results are in fair agreement with a single event reported by Aubry et al. (1971). The model determines the field configuration and the total magnetic flux connecting the magnetosphere to interplanetary space.
Kan, J. R.↗
High latitude equivalent current systems during extremely quiet times
The magnetic perturbation patterns in the polar cap and auroral zone regions are obtained for extremely quiet days using two different techniques. It is shown that the form of the equivalent current flow pattern is extremely sensitive to the level of quietness, and that even so-called quiet days are at times disturbed by substorm activity. Certain characteristic equivalent flow not typically observed during substorms is noted in the polar cap, and this flow appears to be associated with effects of polar cap perturbations discussed by Svalgaard (1973). A region of equatorward flow at high latitudes near the dawn meridian, appears to be Hall current driven by an eastward electric field. The dayside sub-auroral zone is dominated by the Sq-current system, while the nightside shows no significant current flow in the absence of substorm activity.
Rostoker, G.↗