Ground Magnetic Disturbance Research Articles

Characteristics of the Polar Ionosphere Based on the Chatanika and Sondrestrom Incoherent Scatter Radars

The climatological characteristics of the polar ionospheric currents obtained from the simultaneous observations of the ionospheric electric field and conductivity are examined. For this purpose, 43 and 109 days of measurements from the Chatanika and Sondrestrom incoherent scatter radars are utilized respectively. The ionospheric current density is compared with the corresponding ground magnetic disturbance. Several interesting characteristics about the polar ionosphere are apparent from this study: (1) The sun determines largely the conductance over the Sondrestrom radar, while the nighttime conductance distribution over the Chatanika radar is significantly affected by auroral precipitation. (2) The regions of the maximum N-S electric field over the Chatanika radar are located approximately at the dawn and dusk sectors, while they tend to shift towards dayside over the Sondrestrom radar. The N-S component over Son-drestrom is slightly stronger than Chatanika. However, the E-W component over Chatanika is negligible compared to that of Sondrestrom. (3) The E-W ionospheric current flows dominantly in the night hemisphere over Chatanika, while it flows in the sunlit hemisphere over Sondrestrom. The N-S current over Chatanika flows prominently in the dawn and dusk sectors, while a strong southward current flows in the prenoon sector over Sondrestrom. (4) The assumption of infinite sheet current approximation is far from realistic, underestimating the current density by a factor of 2 or more. It is particularly serious for the higher latitude region. (5) The correlation between <TEX>${\Delta}H\;and\;J_E$</TEX> is higher than the one between <TEX>${\Delta}D\;and\;J_N$</TEX>, indicating that field-aligned current affects <TEX>${\Delta}D$</TEX>significantly.

Ground Magnetic Field Disturbance Caused by the Hall Current of Bubbles in the Earth's Plasma Sheet

AbstractThe bursty bulk flows observed by Baumjohann and Paschmann, and Angelopoulos et al. have been claimed by Chen and Wolf as bubbles in the Earth's plasma sheet. Most of the major observed characteristics of the bursty bulk flows (hereafter abbreviated as BBFs) can be interpreted naturally in terms of bubble picture. In the bubble model, bubbles carry a great percentage of the total potential drop in the ionosphere. Hereby we can estimate the Hall current around a bubble and its disturbance on the ground magnetic field. From the comparison with the characteristics of BBFs observed by the Scandinavian Magnetometer Array, a very good general agreement between the two disturbances can be seen. This implies that BBFs are very likely just the bubbles proposed by Chen and Wolf.

One-dimensional upward continuation of the ground magnetic field disturbance using spherical elementary current systems

Ionospheric equivalent currents are defined as spherical sheet currents, which reproduce the observed magnetic disturbances below the ionosphere. One way of determining these currents is to place several so called spherical elementary current systems (SECS) in the ionospheric height and to solve an inversion problem for the amplitudes of these systems. In previous studies this method has been applied to two-dimensional data sets, having both latitudinal and longitudinal spatial coverage (2D SECS method). In this paper a one-dimensional variant of this method (1D SECS) is developed. The 1D SECS method can be used even in those situations where the data set is one dimensional, e.g. with one meridionally aligned magnetometer chain. The applicability of the 1D SECS method is tested using both synthetic and real data. It is found that in real situations the errors in the 1D SECS results are 5—10% in current density profiles and ~5% in integrated currents, when compared to the results of the more accurate 2D SECS method.

Ionospheric equivalent current distributions determined with the method of spherical elementary current systems

The ground magnetic field disturbance caused by ionospheric currents can be represented by equivalent currents placed to the ionospheric plane. Equivalent currents provide valuable information about the ionospheric electrodynamics, and thus they can be used, for example, in studies of space weather, ionosphere‐magnetosphere coupling, and the magnetotelluric source effect. We derive equivalent currents by using the spherical elementary current system method. The applicability of the method for the Baltic Electromagnetic Array Research (BEAR) magnetometer array is validated by means of synthetic ionospheric current models and by investigating the goodness of the fit between the modeled and measured ground magnetic field. The applicability of the method for the sparser International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network is also proved. In addition, the combination of the elementary current system method and the complex image method, used for the calculation of the induced electromagnetic fields on ground, is introduced, and the combination of the methods is tested by using geoelectric field data from the BEAR project. Our special interest is in the effects that rapidly varying ionospheric currents have on technological conductor systems at the surface of the Earth due to geomagnetically induced currents. Comparison between equivalent currents and the time derivative vector of the horizontal magnetic field emphasizes the importance of small‐scale structures.

SUPERDARN과 GREENLAND 자력계를 이용한 전리층 전기전도도의 추정

전리층은 우주환경의 변화에 매우 중요한 역할을 하고 있다. 특히 전기전도도 분포에 관한 정보는 자기권-전리층 상호작용을 이해하는데 필수적이다. 이러한 요구에 부응해서 전기전도도를 구하려는 다양한 시도가 있었다. 본 연구에서는 SuperDARN(Super Dual Auroral Radar Network) 레이더망 중 Goose Bay 및 Stokkseyri 레이더에서 관측한 전기장과 Greenland의 서부해안에 설치된 지자기 관측소에서 동시에 얻은 지상 지자기 기록을 이용하여 전기전도도를 추정하였다. 또한 전리층을 흐르는 전류를 무한판상으로 가정하고 Biot-Savart 및 Ohm의 법칙을 적용하여 Hall 및 Pedersen 전기전도도를 추정하였다. 예상한대로 Hall 전기전도도는 오로라 제트전류대의 중심을 따라 상당히 강화됨을 알 수 있었다. 그러나 Pedersen 전기전도도는 광범위한 지역에 서 음의 값이 나타났다. 이러한 문제를 보완하기 위해서 지자기 변화 성분인 <TEX>${\Delta}D$</TEX>에 연자기력선 전류의 효과를 고려하였다. 그 결과 이전에 음으로 나타난 지역이 상당히 감소되었다. 따라서 지상 지자기 변화 자료와 레이더에서 관측된 전기장을 이용해서 전기전도도를 구하는 경우 연자기력선 전류의 효과를 고려해야 한다. The ionosphere plays an important role in the electrodynamics of space environment. In particular, the information on the ionospheric conductivity distribution is indispensable in understanding the electrodynamics of the magnetosphere and ionosphere coupling study. To meet such a requirement, several attempts have been made to estimate the conductivity distribution over the polar ionosphere. As one of such attempts we compare the ionospheric plasma convection patterns obtained from the Super Dual Auroral Radar Network (SuperDARN), from which the electric field distribution is estimated, and the simultaneously measured ground magnetic disturbance. Specifically, the electric field measured from the Goose Bay and Stokkseyri radars and magnetic disturbance data obtained from the west coast chain of Greenland are compared. In order to estimate ionospheric conductivity distribution with these information, the overhead infinite sheet current approximation is employed. As expected, the Hall conductance, height-integrated conductivity, shows a wide enhancement along the center of the auroral electrojet. However, Pedersen conductance shows negative values over a wide portion of the auroral oval region, a physically unacceptable situation. To alleviate this problem, the effect of the field-aligned current is taken into account. As a result, the region with negative Pedersen conductance disappears significantly, suggesting that the effect of the field-aligned current should be taken into account, when one wants to estimate ionospheric conductance based on ground magnetic disturbance and electric field measurements by radars.

SONDRESTROM 비간섭 산란 레이더 자료를 이용한 극지방 전리층의 기후학적 특성 연구

전리층의 전기 전도도와 전기장을 구함으로써 극지방 전리층의 기후학적 특성을 살펴보았다 이를 위해, 총 109일간의 Sondrestrom 비간섭 산란 레이더 자료를 이용하였다. 전기 전도도와 전기장을 이용하여 전리층 전류 분포를 추정하였고, 구해진 전리층 전류 밀도와 그로 인해 유발되는 지상 지자기 변화를 비교하였다. 또한 지상 지자기 변화(특히, D 성분)에 대한 연자기력선 전류의 효과도 검토되었다. Sondrestrom 상공 전리층에 대한 몇 가지 흥미로운 기후학적 특성을 본 연구로부터 알 수 있었다: (1) 주간의 전기 전도도 분포는 주로 태양 EUV복사에 의한 것이며, 야간에는 미약하다. (2) 극관 영역 전리층의 전기 전도도 분포는, 주간에는 태양 EUV복사에 의한 분포를 보이는 반면, 야간에는 Hall 및 Pedersen 전기 전도도의 시간 평균이 각각 1.6 및 1.2 siemen으로 아주 낮다. (3) 남북 성분 전기장의 최대치가 낮 영역에 나타나는 경향이 있다. 동서 성분 전기장은 Chatanika에 비해 강하다 (4) 동서 성분 전류는 낮 영역에서 강하게 흐른다. 정오 바로 전에 강한 남향 전류가 흐른다. (5) 오로라제트전류와 동시에 관측된 지상 지자기 변화 <TEX>$({{\Delta}}H)$</TEX> 사이에 높은 상관관계를 나타낸다. 하지만 무한판상을 가정한 전류가 크게 과소평가 된다. 또한 <TEX>${{\Delta}}H$</TEX>의 관계보다 더 높게 나타나며, 이것은 연자기력선 전류가 <TEX>${\Delta}7$</TEX>에 영향을 미친다는 것을 의미한다. The climatological characteristics of the polar ionosphere is examined in terms of the ionospheric conductance and electric field. For this purpose, 109 days of measurements from the Sondrestrom incoherent scatter radar are utilized. By combining these two quantities, it is possible to deduce the overhead ionospheric current distributions. The ionospheric current density thus obtained is compared with the corresponding ground magnetic disturbance. Also examined is the effect of the field-aligned current on the ground magnetic disturbance, particularly on the D component Several interesting climatological characteristics about the ionosphere over the Sonderstrom are apparent from this study. (1) The conductance distribution is mainly due to solar EUV radiation during day-time On the other hand, the conductance distribution during the night-time is very low. (2) The conductance distribution one. the polar cap region during the day-time is controlled mostly by the solar EUV radiation, while it is extremely low during night-time wish the Hall and Pedersen conductances being 1.6 and 1.2 siemen, respectively (3) The region of the maximum N-S electric field tend to locate in the dayside sector. The E-W component of the electric field is stronger than that over Chatanika (4) The E-W auroal inospheric current (J/sub E/) is more important in the sunlit hemisphere than the night hemisphere. And a strong southward current is noted in the prenoon sector (5) There is a significant correlation between the overhead ionospheric current and the simultaneously observed ground magnetic disturbance. However, the assumption for the infinite sheet current approximation is far from realistic, underestimating the current density. And the correlation between <TEX>${\Delta}H$</TEX> and J/sub E/ is higher than the one between <TEX>${\Delta}D$</TEX> and J/sub N/ , indicating that field-aligned current affects significantly <TEX>${\Delta}D$</TEX>.

Ground disturbances of the ring, magnetopause, and tail currents on the day the solar wind almost disappeared

We study ring current dynamics on a day of exceptionally low solar wind density (of ∼0.1 cm−3). Inner magnetospheric measurements of geomagnetic activity show geomagnetic quiet par excellence. On May 11, 1999, the values were |Dst| &lt; 10 nT and Kp = 0+. The field at geostationary orbit was very close to dipolar. We simulate the global evolution of the ring current ion population during this period and compare the effect of the magnetopause, ring, magnetotail, and field‐aligned currents on the Dst index. Measured H+, He+, and O+ energy and pitch angle distributions by the Magnetospheric Ion Composition Spectrometer (MICS) and HYDRA instruments on Polar on May 9 are used as initial conditions for our kinetic model. Comparing model results with Polar data on May 11, we find remarkable agreement, proving the applicability of our model for these magnetospheric conditions. We compare modeled H+ and He+ ion distributions with quiet time ring current distributions inferred from statistical studies, and we find that as a general trend, the simulation results have lower values than the statistical patterns. The ground magnetic field disturbances due to the ring and the magnetopause currents decrease quasi‐monotonically on May 11, reaching limiting magnitudes of ∼5 and ∼3 nT, respectively. These values are substantially smaller than the ∼20 nT quiet time values obtained from statistical studies.

Storm‐substorm relationship: Contribution of the tail current to Dst

The Dst index has been conventionally used as a measure of the storm intensity, which ideally assumes that the associated ground magnetic disturbance is caused by the ring current. The present study examines the contribution of the tail current to Dst, focusing on the occurrence of geosynchronous dipolarization close to the Dst minimum, in other words, the start of the storm recovery phase. The Sym‐H (referred to as Dst(Sym‐H) hereafter) index rather than the conventional Dst index is used because of its higher time resolution (1 min). For the June 1998 storm event, dipolarization started at two GOES satellites and the Geotail satellite in the near‐Earth tail when Dst(Sym‐H) reached its minimum. This result indicates that the source current was located outside of geosynchronous orbit, and therefore the recovery of Dst(Sym‐H) can be attributed to the reduction of the tail current rather than the decay of the ring current. A statistical study based on 59 storm events (79 GOES events) confirms the tendency for geosynchronous magnetic field to dipolarize at the Dst(Sym‐H) minimum. It is therefore highly likely that the Dst(Sym‐H) minimum is misidentified as the start of the ring current (storm) decay at a time when the ring current may actually be intensifying owing to substorm‐associated injection. From the magnitude of the Dst(Sym‐H) recovery during the interval of geosynchronous dipolarization, the contribution of the tail current to Dst(Sym‐H) at the Dst(Sym‐H) minimum is estimated to be 20–25%. However, the contribution of the tail current may be even larger because the tail current may not return to preintensification levels and may continue to contribute to Dst(Sym‐H) after dipolarization. The trigger of dipolarization (substorm) and the subsequent recovery of Dst(Sym‐H) tend to take place in the course of the reduction of the southward interplanetary magnetic field (IMF) BZ. It is therefore suggested that the ring current (storm) recovers after the substorm since the magnetospheric convection weakens because of weaker southward IMF BZ.

Geomagnetic disturbances: characteristics of, distinction between types, and relations to interplanetary conditions

This tutorial emphasis disturbances in auroral emissions and ionospheric currents and their relation to interplanetary conditions and the overall level of geomagnetic activity. Auroral zone disturbances are divided into three fundamentally different types: poleward boundary intensifications (PBIs), substorms, and effects of solar wind dynamic pressure enhancements. The most common type of auroral-zone disturbance is the PBI, which occurs during all levels of geomagnetic activity. PBIs have an auroral signature that often can be seen to move equatorward from the magnetic separatrix. They are typically associated with ground magnetic perturbations of few tens of nT, but perturbations can be as high as ∼500 nT . Individual PBIs are longitudinally localized, associated with the longitudinally localized flow bursts in the tail plasma sheet, and occasionally traverse essentially the entire latitudinal extent of the plasma sheet. PBIs appear to generally be the dominant type of auroral-zone disturbance during periods of enhanced magnetospheric convection, including the growth phase of substorms, convection bays, and the main phase of magnetic storms. Substorms are a far more dramatic and large scale, but far less common, disturbance than PBIs. They occur after a ≳30 min growth-phase period of enhanced convection. It is now known that at least ∼50% of substorms are associated with IMF changes that lead to a reduction in the strength of convection. However, it has not yet been shown whether or not all are most substorm onsets are caused by these types of IMF changes. Auroral activity during substorms typically initiates within a ∼1–2 h MLT sector near the equatorward boundary of the auroral oval and then expands both poleward and azimuthally. Substorms are associated with ground magnetic disturbances that range from ∼50 to ∼2000 nT , a reduction in strength of the cross-tail current, a poleward displacement of the inner edge of the plasma sheet, and a large release of plasma and magnetic field energy from the region earthward of the new inner edge of the plasma sheet. The reduction of cross-tail current is also believed to often be associated with a severance, and loss from the magnetotail, of the outer portion of the plasma sheet ( r≳25 R E). Recent studies have shown that solar wind dynamic pressure increases caused large auroral-zone disturbances during a stormtime period of strongly enhanced convection, affecting the poleward boundary, latitudinal width, and intensity of the auroral oval. Dynamic pressure increases also appear to also enhance the entire magnetospheric current system, including the magnetopause, cross-tail, region 1 field-aligned, and global ionospheric currents. Thus, in addition to PBIs, significant variations in solar wind dynamic pressure should be considered as a possibly important source of geomagnetic disturbances during periods of enhanced magnetospheric convection.

Spatial distribution of conductances and currents associated with a north-south auroral form during a multiple-substorm period

Abstract. Using the method of characteristics to invert ground-based data of the ground magnetic field disturbance and of the ionospheric electric field, we obtain spatial distributions of ionospheric conductances, currents, and field-aligned currents (FACs) associated with a north-south auroral form that drifts westwards over northern Scandinavia around 2200 UT on December 2, 1977. This auroral form is one in a sequence of such north-south structures observed by all-sky cameras, and appears 14 min after the last of several breakups during that extremely disturbed night. Our analysis shows that the ionospheric Hall conductance reaches values above 200 S in the center of the form, and upward flowing FACs of up to 25 µA/m2 are concentrated near its westward and equatorward edge. The strong upward flowing FACs are fed by an area of more distributed, but still very strong downward-flowing FACs northeastward of the auroral form. In contrast to the conductances, the electric field is only slightly affected by the passage of the form. We point out similarities and differences of our observations and results to previously reported observations and models of 'auroral fingers', 'north-south aurora', and 'auroral streamers' which are suggested to be ionospheric manifestations of bursty bulk flows in the plasma sheet.Key words. Ionosphere (auroral ionosphere; electric fields and currents) · Magnetospheric physics (magnetosphere · ionosphere interactions)

Ionospheric disturbance magnetic field continuation from the ground to the ionosphere using spherical elementary current systems

A new technique for continuation of the ground magnetic field caused by ionospheric currents to the ionosphere in spherical geometry is presented that makes use of elementary ionospheric current systems, which were introduced by Amm (1997) in extension of an earlier work by Fukushima (1976). The measured ground magnetic disturbance is expanded in terms of the ground magnetic effect of a spatial distribution of such elementary current systems. Using a matrix inversion technique, the scaling factors for each elementary current system, and therefrom the ionospheric equivalent currents are calculated. The technique can be applied to both global and local scales. Its advantages compared to the common field continuation techniques with Fourier (local scale), spherical cap (local to medium scale), or spherical (global scale) harmonic expansions are: 1) No fixed limitation of the spectral content has to be given for the whole analysis area, as it has to be done for the other techniques by truncation of a series expansion. 2) The locations of the elementary current systems can be chosen freely, such that they are most suitable with respect to the available measurement sites or the type of current system to be analysed. Results of the new technique are discussed in comparison to results of the spherical cap harmonic expansion method for a model of a Cowling channel.

Characteristics of pseudobreakups and substorms observed in the ionosphere, at the geosynchronous orbit, and in the midtail

We present a comprehensive study of a sequence of two substorms and multiple pseudobreakups using optical, magnetic and incoherent scatter radar measurements, energetic particles from two geosynchronous satellites and particle and field data from the Geotail spacecraft located at Xgsm ∼ −86 RE. Following conventional nomenclature, we classified as pseudobreakups those auroral breakups which did not exhibit significant poleward expansion (&lt; 2° magnetic latitude). Auroral intensifications following substorm breakups were also observed, and were classified separately. Pseudobreakups were found not to differ from substorm breakups in longitudinal extent (from 1.3 to 6.1 hours of magnetic local time), or in duration (from 5 to 16 minutes). In general, the ionospheric currents producing ground magnetic disturbances were more intense during substorms than pseudobreakups. We found that pseudobreakups are associated with the same magnetospheric processes as substorm breakups which involve current wedge formation, midlatitude magnetic Pi2 pulsations and energetic particle injections at the geosynchronous altitude. Moreover, pseudobreakups are associated with magnetic reconnection in the near‐Earth region, evidenced by the typical subsequent detection of a plasmoid at Geotail. This implies that the magnetotail volume influenced by a pseudobreakup is quite large in radial distance. We conclude that there is no definitive qualitative distinction between pseudobreakups and substorms but there is a continuum of states between the small pseudobreakups and large substorms.

Effects of magnetospheric precipitation and ionospheric conductivity on the ground magnetic signatures of traveling convection vortices

By using an improved TCV model (Zhu et al., 1997), a quantitative study of the effects of magnetospheric precipitation and ionospheric background conductivity on the ground magnetic signatures of traveling convection vortices (TCVs) has been conducted. In this study the localized conductivity enhancement associated with the TCVs is present and the ratio of the Hall and Pedersen conductances vary both spatially and temporally according to the hardness of the TCV precipitation. It is found that a strong conductivity enhancement associated with hard TCV precipitation can significantly distort the TCV current closure in the ionosphere and lead to ground magnetic disturbance patterns with strong asymmetry in E–W direction. The asymmetry of the ground magnetic patterns is characterized by a stronger magnetic disturbance on the side of the upward field‐aligned currents (clockwise convection cell) and a possible rotation of the whole magnetic patterns. Specifically, the modeling results predict that when the characteristic energy of the TCV precipitation is below 500 eV, the asymmetry of the ground magnetic patterns is minimal (less than 1%) and may not be detectable. When the characteristic energy of the precipitation is about 7 keV, the asymmetry of the magnetic patterns can be well above 30%. It is also found that a low ionospheric background conductivity favors the appearance of strong asymmetry in the ground magnetic patterns of the TCVs, while a high ionospheric background conductivity favors the appearance of strong ground magnetic disturbances but with less asymmetry. We concluded that the most favorable condition for the appearance of strong asymmetry in the TCV ground magnetic signatures is the condition of winter, solar minimum, and hard precipitation.

Spatial distribution of conductances and currents associated with a north-south auroral form during a multiple-substorm period

<strong class="journal-contentHeaderColor">Abstract.</strong> Using the method of characteristics to invert ground-based data of the ground magnetic field disturbance and of the ionospheric electric field, we obtain spatial distributions of ionospheric conductances, currents, and field-aligned currents (FACs) associated with a north-south auroral form that drifts westwards over northern Scandinavia around 2200 UT on December 2, 1977. This auroral form is one in a sequence of such north-south structures observed by all-sky cameras, and appears 14 min after the last of several breakups during that extremely disturbed night. Our analysis shows that the ionospheric Hall conductance reaches values above 200 S in the center of the form, and upward flowing FACs of up to 25 µA/m² are concentrated near its westward and equatorward edge. The strong upward flowing FACs are fed by an area of more distributed, but still very strong downward-flowing FACs northeastward of the auroral form. In contrast to the conductances, the electric field is only slightly affected by the passage of the form. We point out similarities and differences of our observations and results to previously reported observations and models of 'auroral fingers', 'north-south aurora', and 'auroral streamers' which are suggested to be ionospheric manifestations of bursty bulk flows in the plasma sheet.<br><br><b>Key words. </b> Ionosphere (auroral ionosphere; electric fields and currents) · Magnetospheric physics (magnetosphere · ionosphere interactions)

Analytical representation of izmem model for near-real time prediction of electromagnetic weather

Abstract In this study we describe an analytical representation of the IZMIRAN Electrodynamic Model (IZMEM) that allows easy calculation of a number of ionospheric parameters for any point above the corrected geomagnetic latitude φ ⩾ 57° over the Northern and Southern polar regions and for all seasons of a year. These are the distributions of ionospheric electric potentials, electric fields, field-aligned and ionospheric currents, horizontal ion drift velocities, Joule heating rate, and ground magnetic disturbances. Comparisons of the modeling results with the Heppner–Maynard and Foster ionospheric convection models as well as with various satellite observations are also presented and discussed. A simple, inexpensive scheme is suggested for specifications and forecast of polar ionospheric convection patterns solely from interplanetary observations made at the Lagrangian L1 point upstream in the solar wind. © 1998 Elsevier Science Ltd. All rights reserved.

An ionospheric conductance model based on ground magnetic disturbance data

An attempt is made to construct an improved ionospheric conductance model employing ground magnetic disturbance data as input. For each of the different regions in the auroral electrojets specified by different combinations of horizontal (ΔH) and vertical (ΔZ) magnetic perturbations, as well as by magnetic local time (MLT), an empirical relationship is obtained between the ionospheric conductance deduced from Chatanika radar data and magnetic disturbances from the nearby College magnetic station. The error involved in the empirical formula is generally of the order of 20–50%. However, some sectors are so poorly covered that uncertainty estimates cannot be made. The new formulas are applied to an average magnetic disturbance distribution to deduce the average conductance distribution. This is compared with a conductance model based on electron precipitation data [Hardy et al., 1987], finding good agreement in terms of the magnitude and distribution pattern. Combining our empirical relationships with the empirical formulas proposed by Robinson et al. [1987], the average energy and energy flux of precipitating electrons are also estimated. Notable similarities exist between the global distribution patterns of these and those obtained by Hardy et al. [1985]. It is proposed that the present conductance model can be used to complement more direct measurements in order to obtain the global distribution needed to study the large‐scale electrodynamics of the polar ionosphere. Several interesting characteristics about the auroral electrojet system are apparent from the empirical relationship: (1) For a given magnitude of ΔH, the electric field is relatively stronger in the eastward electrojet region than in the westward electrojet region. (2) The electric field plays a greater role in the intensification of electrojet current than the ionospheric conductance does in the poleward half of the westward electrojet, whereas the opposite trend is apparent in the equatorward half. However, no such different roles of the electric field and conductance is noticeable in the eastward electrojet region. (3) The auroral conductance enhancements tend to be largest around midnight, due to more intense particle fluxes there. (4) The mean particle energy depends on MLT but is relatively insensitive to magnetic activity.

Method of characteristics in spherical geometry applied to a Harang-discontinuity situation

Abstract. The method of characteristics for obtaining spatial distributions of ionospheric electrodynamic parameters from ground-based spatial observations of the ground magnetic disturbance and the ionospheric electric field is presented in spherical geometry. The method includes tools for separation of the external magnetic disturbance, its continuation to the ionosphere, and calculation of ionospheric equivalent currents. Based on these and the measured electric field distribution, the ionospheric Hall conductance is calculated as the primary output of the method. By estimating the Hall- to-Pedersen conductance ratio distribution, the remaining ionospheric electrodynamic parameters are inferred. The method does not assume ∇×E=0 to allow to study time-dependent situations. The application of this method to a Harang discontinuity (HD) situation on 27 October 1977, 17:39 UT, reveals the following: (1) The conductances at and north of the HD are clearly reduced as compared to the eastern electrojet region. (2) Plasma flow across the HD is observed, but almost all horizontal current is diverted into upward-flowing field-aligned currents (FACs) there. (3) The FACs connected to the Hall currents form a latitudinally aligned sheet with a magnitude peak between the electrically and magnetically defined HD, where break-up arcs are often observed. Their magnitude is larger than that of the more uniformly distributed FACs connected to the Pedersen currents. They also cause the southward shift of the magnetically defined HD with respect to the electrically defined one. (4) A tilt of the HD with respect to geomagnetic latitude as proposed by an earlier study on the same event, which used composite vector plot technique, and by statistical studies, is not observed in our single time-step analysis.Key words. Ionosphere · Electric fields and currents · Instruments and techniques · Magnetospheric physics · Current systems

Two substorm intensifications compared: Onset, expansion, and global consequences

We present observations of two sequential substorm onsets on May 15, 1996. The first event occurred during persistently negative IMF BZ, whereas the second expansion followed a northward turning of the interplanetary magnetic field (IMF). While the first onset remained localized, the second event led to a major reconfiguration of the magnetotail. The two very different events are contrasted, and it is suggested that the IMF direction controls the evolution of the expansion phase after the initial onset. Magnetic field modeling and field‐aligned mappings are used to find the high‐altitude source region of the auroral features and currents giving rise to ground magnetic disturbances: It is shown that the auroral brightening is related to processes near the inner edge of the plasma sheet but that the initial field‐aligned currents couple to the midtail region. Ground magnetograms show an abrupt, large‐scale weakening of the electrojet during the recovery phase. This event is followed by eastward drifting omega bands in a double‐oval configuration. During that period, the Geotail plasma data show oscillations at &lt;100 km/s amplitude. We argue that both these features are connected with the global tail evolution as the neutral line ceases to be active and reforms in the distant tail.

Space current around the earth obtained with Ampère’s law applied to the MAGSAT orbit and data

An application of Ampere’s law to the MAGSAT orbit and data enabled us to study the net space current (total intensity I) flowing through the plane enclosed by the satellite orbit, and its dependence on the ground magnetic disturbance revealed in the Kp- or AE-index. Even on magnetically quiet days, MAGSAT often (or sometimes persistently) detected minor or moderate disturbances in the polar regions, in particular inside the auroral oval, without increasing Kp- or AE-values. Such disturbances are attributable to field-aligned currents into or out of the ionosphere, which produce a magnetic field (of toroidal nature) detectable above the ionosphere associated with a weak magnetic field on the ground, resulting in an occasional poor correlation of I with Kp- or AE-indices. The calculated I-values on quiet days are shown to be the order of 105 A, with a small-range UT variation. During magnetic storms or substorms the I-values become one order of magnitude greater, and the net space current is always antisunward, with its intensity roughly proportional to the AE-index values. The antisunward space current under the MAGSAT level is the Pedersen current in the ionosphere, and it constitutes the return current of the westward partial ring current (flowing at a distance of several earth radii in the dusk-side magnetosphere) along with the field-aligned currents between the partial ring current and the high-latitude ionosphere in the dayside and nightside hemispheres. The antisunward ionospheric current under the MAGSAT level contributes to a noticeable enhancement in the dawn-dusk asymmetry of the H-decrease at MAGSAT level in comparison with the ground data at the developing stage of magnetic storms or substorms.

Model study of ground magnetic signatures of traveling convection vortices

We conducted a model study of ground magnetic signatures of traveling convection vortices (TCVs) that included both the ionospheric conductivity enhancement associated with the TCVs and the ground induction effect. We found that the localized conductivity enhancement can cause a significant distortion of the TCV current system and lead to a distortion of the ground magnetic disturbance patterns. The patterns of all three magnetic components are asymmetric, mainly in the E‐W direction, and the patterns of the Z component show the strongest asymmetry (20–30%). We also found that the effect of induction currents on ground magnetic signatures of the TCVs is insignificant (less than 5%). The results show that because of the presence of localized conductivity enhancements the polarity and speed of the TCVs can significantly influence the distortion features of ground magnetic patterns. The upward and downward current filaments of a TCV with a clockwise leading convection cell can wrap with each other, resulting in a rotation of the whole ground magnetic disturbance pattern. This rotation feature is most significant when the speed of the TCVs is high.