Polar Magnetic Substorms Research Articles

The relationship between the magnetosphere and magnetospheric/auroral substorms

Abstract. On the basis of auroral and polar magnetic substorm studies, the relationship between the solar wind-magnetosphere dynamo (the DD dynamo) current and the substorm dynamo (the UL dynamo) current is studied. The characteristics of both the DD and UL currents reveal why auroral substorms consist of the three distinct phases after the input power ε is increased above 1018 erg s−1. (a) The growth phase; the magnetosphere can accumulate magnetic energy for auroral substorms, when the ionosphere cannot dissipate the power before the expansion phase. (b) The expansion phase; the magnetosphere releases the accumulated magnetic energy during the growth phase in a pulse-like manner in a few hours, because it tries to stabilize itself when the accumulated energy reaches to about 1023 erg s−1. (c) The recovery phase; the magnetosphere becomes an ordinary dissipative system after the expansion phase, because the ionosphere becomes capable of dissipating the power with the rate of 1018 ~ 1019 erg s−1. On the basis of the above conclusion, it is suggested that the magnetosphere accomplishes the pulse-like release process (resulting in spectacular auroral activities) by producing plasma instabilities in the current sheet, thus reducing the current. The resulting contraction of the magnetic field lines (expending the accumulated magnetic energy), together with break down of the "frozen-in" field condition at distances of less than 10 RE, establishes the substorm dynamo that generates an earthward electric field (Lui and Kamide, 2003; Akasofu, 2011). It is this electric field which manifests as the expansion phase. A recent satellite observation at a distance of as close as 8.1 RE by Lui (2011) seems to support strongly the occurrence of the chain of processes suggested in the above. It is hoped that although the concept presented here is very crude, it will serve in providing one way of studying the three phases of auroral substorms. In turn, a better understanding of auroral substorms will also be useful in studying the magnetosphere, because various auroral activities can be the visible guide for this endeavor.

The Growth of the Storm-Time Radiation Belt and the Magnetospheric Substorm

Surrrmary Important features of the radiation belt (or the so-called ‘ ring current’) which grows in the magnetosphere during geomagnetic storms are summarized. The growth of the belt is studied in relation to a frequent occurrence of intense magnetospheric substorms (of which manifestations in the polar upper atmosphere are the auroral and polar magnetic substorms). It is concluded that during the magnetospheric substorm, the particles responsible for the storm-time belt are produced locally in the trapping region in the evening sector, but not in the midnight sector. In 1933, after completing their study of the impact of a fully ionized gac on a magnetic dipole, Chapman & Ferraro (1933) examined the formation of a ‘ring current’ around the Earth and its stability. Their study was intended to explain a large decrease of the horizontal component (H) of the Earth’s field during the main phase of geomagnetic storms. The results appeared in the last paper of the series ‘A new theory of magnetic storms’ with the subtitle ‘Part 11-The Main Phase’. Their toroidal ring current consisted ewentially of positive ions and electrons circulating around the Earth, in which their centripetal force due to the circular motion ic balanced by the Lorentz force. The stability of such a current system was, however, questioned by AlfvCn (1955) and others. It appears now that their study was an unsuccessful one on this particular subject, but it marked an epoch in geomagnetism, since they correctly located the ‘ring current’ inside the cavity carved in the solar wind, which is now called the magnetosphere; for some earlier concepts of the ‘ring currents’ see Chapman (1952, p. 213). In examining possibilities as to how the ring current is formed, Chapman & Ferraro (ibid., p. 80) thought that positive ions near the morning side of the magnetospheric boundary tend to leap across the cavity, but drift closer to the Earth. Further, they tend also to drag the electrons from the evening side of the magnetospheric boundary and may eventually form the ring current. Although the formation of the ‘ ring current’ does not seem to be as simple as they inferred in their paper, it has now become one of the most challenging problems in magnetospheric physics. The purpose of this paper is first to review both morphological and theoretical studies of the formation of the ‘ring current’ and associated phenomena during the main phase of geomagnetic storms, particularly the auroral substorm (Sections 2 and 3). In the second part (Section 4), we try to find the clues for the process responsible for the onset of the growth of the ‘ring current ’ and magnetospheric substorms.

Auroral motions and magnetic variations associated with the onset of auroral substorms

Observations of the aurora borealis were undertaken during the period August 10–20, 1976, from Fort Smith, Northwest Territories, Canada. Two auroral substorm ‘breakup’ events which occurred in the Alberta sector during this period were photographically documented and have been studied together with the associated polar magnetic substorm events. It is found that significant westward‐directed ionospheric current flow occurs for perhaps 2 or 3 minutes prior to the appearance of the auroral breakup surge form. This current appears to lie parallel to the bright arc which forms before onset of the breakup phase. Such an observation is important relative to the timing of the onsets both of auroral and polar magnetic substorms, and also may have critical implications relative to a theoretical understanding of the sequence of events leading to the auroral substorm breakup.

A polar magnetic substorm observed in the evening sector with a two-dimensional magnetometer array

Summary. A two-dimensional array of 18 three-component magnetometers recorded magnetic fields, on 1974 August 14, associated with a polar magnetic substorm which occurred while the array was in the evening sector. An eastward ionospheric current flowed above the array for more than six hours. Perturbation fields at six representative epochs were modelled by three-dimensional magnetospheric-ionospheric current loops of Bostrdm Type 1, the fields at array stations being calculated by a method due to Kisabeth and fitted to the observed fields by iterative adjustment of parameters of the current loop geometry. Current density distribution across the current width was estimated by an inverse method due to Oldenburg. A satisfactory fit was obtained for the first epoch and very good fits for the other five. In addition to the eastward electrojet above the array, five of the six current models include a westward ionospheric current to the north of the eastward current. This northern current decayed rapidly during the first three hours, in contrast to the continuing eastward current. This difference in the lifetimes of the two electrojets, and perturbations recorded at North American observatories, support the Rostoker–Kisabeth hypothesis of continuation of the westward substorm electrojet into the evening sector, north of the eastward electrojet there. At the last five of the six epochs modelled, the eastern end of the eastward ionospheric current ended just east of the array, which recorded strong evidence of the upward field-aligned current there and located it within 5° of longitude absolutely, and its time displacements within 2°. The upward field-aligned current moved westward no more than 5° in the four hours 04.53–08.53 UT, very nearly rotating with the Earth and drifting eastward 55° relative to geomagnetic midnight. The paper concludes with some general conclusions from three near-midnight substorms studied with the array and reported previously, and the present evening-sector substorm study. Two-dimensional magnetometer arrays are suited to the detection and location of near field-aligned currents and this has led to the discovery that all four substorm current systems studied have drifted eastward, relative to geomagnetic midnight.

A relationship between polar magnetic substorms, ionospheric height rises and the occurrence of spread- F

Statistical analyses performed for ionosonde stations (at mid- and low-latitudes) in a longitude zone near the 75°W (geographic) meridian, suggest that increases in the ionospheric parameter h′ F are related to polar-magnetic-substorm onset times as recorded by a magnetogram station in the same longitude zone as the ionosonde stations. The height increases are recorded for several hours after the substorm onset times, and seem to trigger the onset of ionospheric spread- F. One individual event which was analyzed, showed a variation with latitude of the time of onset of the height increase suggesting the propagation towards the equator of an ionospheric disturbance with a speed of about 740 m s −1. On this particular occasion the onset of spread- F followed the height-rise onset at most of the stations used in the analysis. These effects were clearly discernable at the geomagnetic equator.

A Study of Two Polar Magnetic Substorms with A Two-Dimensional Magnetometer Array

Summary. The paper reports studies of the three-dimensional magnetospheric—ionospheric current systems which produced polar magnetic substorms on 1974 September 7 and September 18. The data were magnetic perturbation fields observed with a two-dimensional array of 23 three-component magnetometers located in western Canada beneath the auroral oval. In an earlier study of a substorm of September 11 (Bannister & Gough) the fields fitted calculated field for a Bostrom Type 1 current loop with field-aligned currents at east and west ends of the ionospheric segment, and with uniform current density across the width. The substorms here reported could not be modelled with uniform current density. An inverse method due to Oldenburg was therefore used to estimate current density distributions, and satisfactory fits of calculated to observed field resulted. Each substorm was modelled at six representative epochs. In general the principal ionospheric current seem by the array was westward. At four epochs of the September 7 substorm and throughout the September 18 substorm, significant eastward ionospheric current (or its equivalent in terms of the fields produced) was observed north of the westward electrojet. Northwestward bends in the ionospheric current segments were found at four epochs on September 7 and at three epochs on September 18. As in the September 11 substorm (Paper 1), these bends were either west of or close to magnetic midnight. In some cases the bends may follow the auroral oval, but in others they are sharper and may be associated with the Harang discontinuity. East of geomagnetic the ionospheric currents tend to run in a constant geomagnetic midnight latitude range. The developments of the three substorms, of September 7, 11 (Paper 1) and 18, are compared. They showed a variety of shifts in longitude, though all moved eastward relative to magnetic midnight.

Midlatitude convection electric fields and their relation to ring current development

The extension of magnetospheric convection toward low latitudes is investigated with the help of incoherent scatter measurements of E×B plasma drift velocities in the midlatitude F region above Saint‐Santin (France). The observed relations of these drifts with auroral magnetic activity, the north‐south component Bz of the interplanetary magnetic field, and the time derivative of Dst suggest that this low‐latitude extension is not systematically associated with all polar magnetic substorms, but occurs in conjunction with large (5 gammas or more) southward Bz and is closely related in intensity with the negative excursions of d(Dst)/dt. We propose that plasma injection and the resulting development of an asymmetric ring current are closely linked with the low‐latitude extension of magnetospheric convection.

Multiple‐satellite studies of magnetospheric substorms: Distinction between polar magnetic substorms and convection‐driven negative bays

It is generally believed that long intervals of intense (500–1000 γ) negative auroral zone bays are caused by magnetospheric substorm occurrences at a high repetition rate. Since individual substorm phases may then overlap in time and occur within limited local time sectors, it may be difficult to identify their characteristic signatures. In this paper we have examined ground and multiple‐satellite recordings during an ∼5‐hour interval of persistent auroral zone activity in a systematic search for substorm expansion signatures. It appears that this enduring activity started as a rather weak substorm which thereafter developed into strong magnetospheric activity driven by a continuously southward interplanetary magnetic field (IMF). During this period there were no clear mid‐latitude positive bays, and pulsation recordings from different local time sectors showed only one instance of weak Pi 2‐like variations. The associated high‐latitude equivalent ionospheric current pattern is consistent with a convection system, and we refer to this activity as the convection bay. Following the plasma sheet and ground signatures of substorm recovery early in the convection bay, there was no definite indication of substorm expansions other than this weak magnetic pulsation in any recordings during the next ∼3½ hours until a clear substorm emerged. This substorm onset, which was associated with a not particularly strong auroral zone bay, was accompanied by the full complement of substorm expansion signatures. At this time a minimum plasma sheet thickness of only a few tenths of an earth radius was observed in the region where a neutral line appears to form during substorms. Prior to this onset, the plasma sheet was relatively thick, and it seems that the neutral line was located well tailward of the Vela orbit (r ∼ 18 RE). This event and other similar observations suggest that continuous auroral zone activity during southward IMF, such as a geomagnetic storm period, may be driven by a steady state magnetospheric convection. The long absence of clear substorm onset signatures may indicate that the nonsteady enhancements of convection associated with localized substorm expansions then make only small contributions to the total convection pattern. Similarly, the ground magnetic disturbances may then be caused mainly by Sqp currents and enhanced currents due to strong particle precipitation rather than by substorm‐associated current wedges. During this quasi‐steady state the tail reconnection rate appears nearly to balance the dayside merging rate, allowing most energy extracted from the solar wind to be released continuously rather than to be first stored in the tail. It seems that a large‐scale substorm expansion may be triggered if this convection mode is perturbed, for instance, by a significant reduction of the imposed convection electric field.

Development of a polar magnetic substorm: A two-dimensional magnetometer array study

A polar magnetic substorm on 1974 September 11 was recorded by a two-dimensional array of 25 three-component magnetometers, so located that the westward ionospheric current passed over the array. The mean perturbation fields over five-minute intervals are presented at six representative epochs of the substorm, the first just before its onset and the sixth 21/2 hr later in the coda of the event. At four of the resulting ‘time frames’, the perturbation fields have been fitted, on a least-squares criterion, by calculated fields of three-dimensional current loops having downward field-aligned current at the east end of the ionospheric westward current, upward field-aligned current at the west end and closure in the magnetotail. The current density was constant across the width of each of these model currents. In three of the four time frames it proved necessary to introduce a bend to the northwest in the ionospheric current; this bend occurred within 27° (geomagnetic) west of geomagnetic midnight. An association with the Harang discontinuity is possible. The field-aligned current nearer to the array (in one time-frame the downward, east-end current, in two others the upward current at the west end) proved essential to secure any reasonable fit to the data. The ionospheric segment of the current loop moved at least 20° of longitude eastward, relative to the surface of the earth, between 07.48 and 08.58 UT. In this 70-min period the ionospheric segment moved at least 38° eastward relative to magnetic midnight.

A 12-hour case study of auroral phenomena in the midnight sector: Electrojet and precipitating particle characteristics

Airborne observations of auroral activity at midnight were conducted for a period of 9 hours by employing an ionospheric sounder and all-sky cameras. During the observation period, two substorms occurred. The first substorm was associated with a compression of the magnetosphere (as measured by Dst) and with oscillations of the earth's bow shock. At this time, auroral activity was characterized by a series of poleward and equatorward motions and by the absence of a breakup phase. Magnetic disturbances were confined to a small region in the midnight sector. The second auroral substorm displayed many of the features associated with a large-scale polar magnetic substorm. For selected times the locations of the eastward and westward electrojets were inferred from a number of high-latitude magnetic records. All-sky photographs and ionosonde data indicate that the poleward edge of the westward electrojet was bounded by a westward moving auroral surge accompanied by a strong sporadic E ionization. The equatorward electrojet boundary was less well defined by visual auroral forms. Soft particle spectrometer measurements from the Isis 2 satellite were made in conjunction with the aircraft measurements and indicate that large fluxes of field-aligned electrons gave rise to the bright auroral surge, more isotropic fluxes of less energetic electrons contributing to the auroral E layer.

Modelling of three-dimensional current systems associated with magnetospheric substorms

Summary It is now well accepted that magnetic field perturbations associated with polar magnetic substorms can be modelled by a three-dimensional equivalent current loop involving an electrojet segment in the high-latitude ionosphere linked to the outer magnetosphere by currents flowing parallel to magnetic lines of force. In this paper we present techniques for quantitative evaluation of the magnetic field perturbation at any point in space outside of the region of current flow. The technique is developed using the Biot–Savart Law and analytical expressions for the three components (H, D, Z) of the magnetic perturbation at the Earth's surface are developed. Assuming the Earth to be represented by an infinitely conducting sphere covered by a perfect insulator to some arbitrary depth, the time independent induction problem is treated and analytical expressions for H, D, and Z corrected for Earth induction are developed. The modelling technique is applied to three substorm events and the results provide an accurate and comprehensive indicator of the character and spatial extent of the inducing current system associated with magnetospheric substorms.

On the problem of simulating the Earth's induction effects in modeling polar magnetic substorms

A major problem encountered in trying to model the current system associated with a polar magnetic substorm from ground‐based magnetic observations is the difficulty of adequately evaluating the earth's induction effects. Two methods for simulating these effects are reviewed here. Method 1 simply reduces the earth to a perfect conductor and leads to very simple field equations. Method 2 considers the earth as a ‘horizontally’ layered body of finite conductivity but requires a large amount of computational time. The performances of both methods are compared when the substorm current system can be approximated by an infinitely long electrojet flowing over a flat earth. In this case it appears that for most substorm modeling problems it is sufficient to treat the earth as a perfect conductor. The depth of this perfect conductor below the earth's surface should be selected in function of the source frequency content.

A new method of mapping worldwide potential contours for ground magnetic perturbations: Equivalent ionospheric current representation

A new computer method of mapping the worldwide potential contours for ground magnetic perturbations is developed. The procedure includes the following steps: (1) the digitization of the magnetic perturbation values in (H, D) or (X, Y) coordinates by taking a suitable base level at each observatory, (2) the transformation from these coordinates into the common geomagnetic (Xm, Ym) system, (3) the interpolation of these field values to provide values at a regular network of points, and (4) the computation of the magnetic potential at each grid point to determine the associated current system in the ionosphere. This process involves the determination of the best potential field Φ from the interpolated H (Xm, Ym) values by solving the two-dimensional Poisson equation ▽²Φ = −div Hθ,λ (θ and λ are colatitude and longitude, respectively) on the basis of the iteration method. Since the equipotential contour lines can be regarded as the equivalent ionospheric current lines, the contour maps are of great use in studying spatial and time changes of the substorm current system. Several examples of the magnetic potential obtained by the new method are shown, and a comparison with hand-drawn equivalent current systems proposed in the past is made. A series of potential contours is also shown to examine how the magnetic disturbance pattern changes during polar magnetic substorms. A preliminary examination of the computed potential patterns indicates an interesting recurrent feature of the polar magnetic substorm. That is, a significant part of the equivalent current flow originating in the westward auroral electrojet flows in the evening sector at middle and low latitudes. This characteristic is quite different from most of the substorm current systems proposed in the past, in which the ‘return currents’ from the westward electrojet flow only in the midnight and morning sectors.

Ionospheric Current Structure as Determined from Ground-based Magnetometer Data

Summary The magnetic perturbations from a three-dimensional current system commonly used to describe magnetospheric substorm variations have been inverted, by using the linear inverse theory of Backus and Gilbert, to obtain estimates of the (height-integrated) ionospheric current density producing the observations. Under the assumption that electrojet intensity is not a function of longitude it has been found that displacement of stations from the central meridian yields no significant change in resolution of the current density so long as the observation sites remain within the longitudinal limits of the ionospheric current; hence a linear array can provide as much resolution as a two-dimensional one. The maximum resolution possible is shown to be ∼ 1° latitude or about one ionospheric height unit. An investigation of the dependence of resolution (of a linear array) on station spacing shows that the optimum spacing is dependent upon the number of stations and the desired standard deviation of the estimate of the current density; nevertheless, the rule of thumb that stations should be separated by 1° of latitude is shown to be a good compromise between maximizing resolution and simultaneously minimizing the number of stations. Inversion results using data from a polar magnetic substorm of 1970 June 15 indicate that the magnitude of the westward current density was at least 0·90 ± 0·15 A m−1 and that the current density cannot be approximated by a constant value over widths much greater than 1° of latitude. Published magnetic data from the equatorial region have also been inverted to obtain estimates of the current density in the equatorial electrojet.

The North American Central Plains Conductivity Anomaly

Summary An array of 41 three-component magnetometers recorded geomagnetic disturbances during August and September 1972, between latitudes 42 and 54 N and longitudes 98 and 109 W. The objective was a detailed study of the North American Central Plains (NACP) conductive body earlier discovered striking northward from the Black Hills roughly along longitude 104 W in the United States. Source fields were provided by polar magnetic substorms and by other events with incident fields more uniform over the array. Northward mapping of the conductor was carried to within 90 km of the exposed Canadian Shield of north central Saskatchewan. Between latitudes 43 N and 48 N maximum depth estimates place the conductor within the lithosphere; the currents probably flow at smaller depths, i.e. within the crust. At the Shield edge the axis of the conductor is parallel to the strike of fold belts and fault zones in metamorphic rocks, including mylonites, characteristic of intense grinding and crushing. Graphitic conductors are known in many fracture zones. The conductivity anomaly links these structural elements in the Churchill Province Shield to the metamorphic belt mapped by Lidiak in the South Dakota basement, and to the Black Hills. South of the Black Hills the conductor turns southwest to the northern end of the Southern Rockies. There is evidence which suggests, but does not demonstrate, a conductive link there between the mantle conductor under the Southern Rockies and the NACP crustal conductor. It is postulated that the linear crustal structure may be a major continental fracture zone now mapped over a total length of 1800 km, of which 300 km is exposed in the Shield. Two of three earthquakes located in southern Saskatchewan have epicentres close to the axis of the conductive body.

Time-sequence of polar magnetic substorms

The time-sequence of polar magnetic substorms is discussed to clarify some controversies on the magnetospheric substorm model including the growth phase. The main purpose of the analyses is to examine magnetic variations in the polar cap and in low latitudes. The onset of the expansion phase is confirmed to be reasonably defined by a vector change of polar-cap magnetic disturbance, a sharp intensification of the auroral electrojet disturbance and the beginning of positive ΔH disturbance in midlatitudes near midnight. It is shown that the growth phase signatures so far proposed are consistent when the onset of the expansion phase is identified from the above mentioned features.

Substorm fields in and near the auroral zone

During the last few years, the study of both temporal and spatial variations of substorm fields has rapidly expanded, mainly because of the relationships which exist between polar magnetic substorms and magnetospheric phenomena. Also during these years, proposed current systems believed to be responsible for substorm variations have evolved into complex three-dimensional systems with field-aligned and magnetospheric currents coupled to the eastward and westward electrojets. Recent model studies show that substorm variations in and near the auroral zone can easily be modelled using both two and three-dimensional current systems. In these studies, induction effects were simulated by assuming the Earth to be infinitely conducting at some depth below the surface. The use of magnetometers distributed along magnetic meridians has resulted in a better understanding of the complex current patterns making up the electrojets. For example, during the expansive phase of substorms, the westward and poleward progression of the overall westward electrojet was discovered to take place through the sequential development of a series of westward electrojets.

Infrasonic wave generation by aurora

A comparison of the characteristics of auroral infrasonic waves (AIW) in the passband from 10 to 100 sec. period is given for trans-auroral zone stations. The morphology of 2 Hz auroral infrasound observed at Kiruna, Sweden is described. An important asymmetry in the generation of AIW bow waves, with respect to poleward or equatorward motion of auroral electrojet arcs, is shown to be related to the morphology of the ionospheric E-region electric field during an auroral substorm. Auroral radar studies of AIW source arcs are presented. Incoherent scatter radar observations of electron density profiles of auroral arcs are used to describe the differences in arcs that do or do not produce AIW in an attempt to explain the asymmetry in AIW generation by supersonic auroral arcs. An infinite line-current induction model of an auroral electrojet moving transverse to its axis above a conducting earth is used to determine the relationship of an AIW to the surface magnetic perturbation produced by the electrojet. The role of the auroral electrojet in AIW production is discussed in a study of polar magnetic substorms wherein westward traveling auroral surges and the eastern ends of westward auroral electrojets were found to be strong sources of AIW. Theoretical work on the generation of AIW's is reviewed and evaluated with respect to the observed characteristics of AIW substorms. Lorentz force and Joule heat are discussed as probable source terms for the production of the acoustic energy within an auroral electrojet. The important unsolved problems in understanding auroral infrasound are outlined.

Response of the plasma sheet at ∼18REto sudden southward turnings of the interplanetary magnetic field

Simultaneous measurements of the interplanetary magnetic field (IMF) by Explorer 35 and of the magnetotail plasma by Vela satellites are examined for the period August–October 1970. Nineteen cases are found in which the IMF turns southward more than 40° (|Δθ| > 40°) in less than 10 min and a Vela satellite, operating in a high time resolution mode, is in the plasma sheet. Twelve of these cases show a large decrease of plasma pressure, or the so-called plasma sheet thinning, during the subsequent period of southward IMF. Polar magnetic substorms occur at the earth in conjunction with these twelve plasma sheet thinnings. On the other hand, seven cases show only slight plasma sheet thinnings, and there was no indication of substorms at the standard auroral zone stations. Thus it is likely that the southward IMF alone does not reduce significantly plasma pressure in the plasma sheet and that a large decrease of plasma pressure is associated with a substorm.

Phase relations between polar magnetic substorm fields at the surface of a finitely conducting Earth

Most past studies of polar electrojet morphology have not included consideration of the finite electrical conductivity of the earth. Here we present theoretical calculations based on realistic earth models and show that magnetic disturbance field components measured on the ground are expected to be offset in phase from one another by 10–20 min and from the electrojet by up to 10 min at a period of about 2 hours. These phase shifts are large enough to lead to misinterpretation in studies of spatial motions of the electrojet as well as to affect the temporal correlation between ground-based records and in situ measurements of ionospheric processes. The analysis of a standard magnetogram supports our results.