Abnormal quiet days and the effect of the interplanetary magnetic field on the apparent position of the Sq focus

b
The effect of the IMF on the position of the Sq focus in summer has been discussed by Matsushita who found an apparent poleward motion of the focus on days when the IMF was directed away from the Sun compared to those when the IMF was directed towards the Sun. However, his analysis took no account of the well known phase variability in Sq(H) . It is shown here that if quiet days are separated into 'normal quiet days', NQDs (defined for a station on the poleward side of the focus as days when the minimum in H occurs between 0830 and 1330 LT) and 'abnormal quiet days', AQDs (minimum in H occurring outside the NQD range), then the apparent poleward motion of the focus only occurs on AQDs and even then only when the IMF is directed away from the Sun. This effect is found to occur in both summer and winter and is probably of magnetospheric origin.     

On the nature of abnormal quiet days in Sq(H)

Summary. It is well known that the time of occurrence of the minimum in the horizontal component of the Earth's magnetic field ( H _min) on quiet days at a mid-latitude station on the poleward side of the Sq focus shows considerable variability from day to day. This variability has previously been discussed in terms of the incidence of so-called 'abnormal quiet days'(AQD), arbitrarily defined for the station Hartland as quiet days when H _min occurred outside an interval of ± 2½ hr centred on the most common time of 1130 LT. AQDs have some interesting properties, which have been documented, but their precise nature and cause have not been elucidated. In this paper we report the results of a study of AQDs as identified at Hartland using a chain of Northern hemisphere stations situated approximately along the 0° longitude meridian and extending on both sides of the Sq focus. It is found that there are two effects on AQDs: (i) a northward component field varying in LT is superposed at all latitudes throughout the day, so reducing the amplitude of the normal Sq(H) variation at stations poleward of the focus and increasing it on the equatorward side, (ii) a southward perturbation field, of most probable magnitude 6.0 nT for the period studied, is imposed for about 4 hr at a fixed UT at all latitudes, so constituting an 'AQD event' which can lead to the occurrence of H _min at an anomalous local time for a station poleward of the focus. It is shown that the AQD event may be of large geographical extent and that it is related to the interplanetary magnetic field. All the main properties of AQD occurrences are explained, and it is suggested that much of the day-to-day variability in the amplitude and phase of the normal Sq(H) variation probably also arises from the occurrence of AQD events at times close to the diurnal turning points.     

Day-to-day variability of geomagnetic hourly amplitudes at low latitudes

A study of the variability of the amplitude of Sq at a fixed hour from one day to the next at nine stations from the dip equator to about 22° north of it has produced interesting results. The amplitude and sign of the variability change virtually randomly, making the mean practically zero. The variability occurs at all hours of the day. Its magnitudes in the components D, H and Z have the same diurnal variation, which peaks in the noon period like Sq(H) in low latitudes, and a weak seasonal variation that peaks at the June solstice (local summer). It is demonstrated that changes in the current intensities of the equatorial electrojet (EEJ) and the worldwide part of the Sq (W Sq) current layers have contrasting phases and can sometimes be in antiphase. Indeed, the changes are mostly independent. Inclusion of the magnetic element D revealed that the EEJ current system has not only an east–west but also a north–south component. The study shows that the meridional component of the EEJ current intensity evidenced at the Kodaikanal and Annamalainagar stations is an integral part of the zonal component at Trivandrum. This confirms the results of Rastogi (1996 ) and validates those of Onwumechili (1997 ). The results suggest that ionospheric conductivity mainly controls the magnitude, while the electric field and ultimately winds mainly control the phase and randomness of the day-to-day variability of the hourly amplitudes of Sq . The random component is attributed to local and/or regional atmospheric winds, probably of gravity wave origin.     

An investigation of the causes of abnormal quiet days in Sq (H)

Summary. From a study of 'abnormal quiet days' (AQDs) along the 0° meridian between 14 and 60° N, it was found (Butcher & Brown) that the minimum in H at stations on the poleward side of the Sq ( H ) focus was formed by a small negative substorm event when the normal Sq ( H ) amplitude was reduced by a superposed northward field.
In this paper we consider both the AQD event and the superposed northward field as a function of longitude and also consider in more detail the latitude variation of the superposed northward field. From such a study it is concluded that: (1) the AQD event is definitely due to a small magnetospheric substorm event; (2) the superposed northward field varies smoothly with longitude falling to zero some 110° from the longitude of its maximum amplitude; (3) the superposed northward field has a variation with geomagnetic latitude tending to zero near 20° and 70°N with a maximum near 55° N in summer and 35° N in winter; (4) there is some evidence that the effect of the IMF penetrates into the mid-latitude E-region and its effect is latitude-dependent. Although the evidence supports the suggestion that the currents responsible for the superposed northward field flow in the E-region no suggestion as to the origin of the driving force of the currents is forthcoming.     

On the latitude of the Sq(H) focus at sunspot minimum

Summary. The latitude of the Sq ( H ) focus along the 0° longitude meridian in the northern hemisphere has been determined for all the quiet days, as determined from the aa indices, for the sunspot minimum years 1963–64–65. It is shown that: (a) most of the large variability of the focus latitude is due to the effect of a superposed northward magnetic field that is present on AQDs and which tends to move the apparent focus latitude poleward in the northern hemisphere, and (b) a smaller equatorward motion is caused by the negative AQD events that occur in the 0830–1330 LT range. When these two classes of days are removed from the data set, the focus latitude is found to be completely contained within the range 36°-48° for the months March-October
with an average value of 41.5 ± 2.3, whilst in winter the range is larger with an average value of 36.7 ± 3.4. However, since the magnitude of the superposed northward field is longitude-dependent, it may be present even on days not classed as AQDs. It is shown that much of the variability in the focus latitude of the normal days along the 0° longitude meridian is caused by variations in the amplitude of the superposed northward field.     

Effect of the presence of a conducting channel between India and Sri Lanka Island on the features of the equatorial electrojet

Summary. Transient geomagnetic variations like SSCs, Bays, Sq and storm-time variations show anomalously large Z amplitudes at the three permanent magnetic observatories in India under the equatorial electrojet. Our earlier studies have shown that these anomalies cannot be explained in terms of the usual coastal effect. Another unique feature of this area is the small equatorial enhancement of all magnetic fluctuations, which still remains to be completely understood. It is also noticed that Z/H ratio at Annamalainagar is very large for night-time variations but becomes insignificant for day-time fluctuations, whereas no such large difference is seen at Trivandrum, the other coastal station. All the above features have been explained here by introducing the presence of a conducting channel in the lower crust or upper mantle between India and Sri Lanka Island. The anomalies of the equatorial electrojet in the Indian region thus do not necessitate the conducting surface of the mantle to be deeper in this area as has been suggested by earlier workers. A less conducting mantle in the Indian region otherwise could be difficult to incorporate in the present theories of mantle convection when this area is known to be tectonically active. The presence of the conducting channel is further confirmed by analogue model experiments of Papamastorakis. We further observe that the equatorial electrojet does have an associated internal part in the Indian region.     

Induced magnetic field of the equatorial electrojet at the surface of the Earth

Summary. The northward component of the induced magnetic field due to the equatorial electrojet at the Earth's surface is calculated using a more realistic local time variaton of the external field due to the electrojet than is provided for by models of the electrojet currently used in induction calculations. It is seen that appreciable induction effects can be expected about an hour before local noon for the kind of local time variation considered. Our results are in qualitative agreement with direct observations of Earth currents in the equatorial region in Nigeria. At local times when observable induction effects are present, the magnetic field due to the electrojet is necessarily three-dimensional; hence in order to obtain the internal part directly from the observed total field due to the electrojet at the Earth's surface, a three-dimensional formulation is required.     

南极中山站1991年地磁静日S_q变化

本文分析了１９９１年太阳活动峰年南极中山站地磁静日Ｓｑ变化，结果表明：１）地磁静日Ｓｑ变化叠加有很多扰动。Ｓｑ场是由Ｓ０ｑ场和Ｓｐｑ场所组成。２）Ｓ０ｑ场变化比较规则，其极昼月（夏季）的变幅比极夜月（冬季）大很多。在极昼月Ｓ０ｑ（Ｈ）变化出现有双峰图象。Ｓ０ｑ场主要是由高纬度极区电离层Ｓｑ电流体系所控制。３）Ｓｐｑ场的变化形态没有昼夜之分；它的变化强度为冬季小、夏季大。Ｓｐｑ场源主要是依赖于场向电流和电离层电导率。４）Ｓ０ｑ（Ｚ）变化比Ｓ０ｑ（Ｈ）变化要大，特别是在极夜的冬季，Ｓ０ｑ（Ｚ）的变幅比Ｓ０ｑ（Ｈ）大２／３倍。本文对北京地磁中心台磁静日资料也一并进行了分析。Ｓｑ变化主要是由北半球中、低纬电离层Ｓｑ电流体系所引起的     

Lunar semi-monthly variation in the equatorial electrojet field in India

Summary. Using an index derived from the observations of horizontal intensity at two stations in the Indian equatorial region, the characteristics of the lunar semi-monthly tide in the equatorial electrojet strength are studied. It is shown that a contamination of the lunar signal by recurrent geomagnetic disturbance is largely eliminated and that the strength of the signal vanes systematically with solar time. Comparison of amplitude between conditions of low and high solar activity indicates a difference in local time progression, while the phase change is independent of the solar activity and season. Results of seasonal subdivision of data indicate that the largest amplitude is associated with the d-season. With increased solar activity there is an increase in the d- and e-seasons and a marginal decrease in the j-season. For all the seasons, the phase progression is fairly consistent with the theoretical considerations of Stening.     

Analysis of S~p_q current systems by using corrected geomagnetic coordinates

The S~p_q equivalent current system of the quiet day geomagnetic variation in the polar region is very complicated. It is composed of several currents, such as the ionospheric dynamo current and the auroral electrojet caused by the field aligned current. S p q is unsymmetrical in both polar regions. In this paper, the S p q current systems are analyzed in the corrected geomagnetic coordinates (CGM) instead of the conventional geomagnetic coordinates (GM), and the symmetries of the S p q current in different systems are compared. Then the causes of S p q asymmetry in the GM coordinates are discussed; the effects of each component in S p q are determined.     

Analysis of geomagnetic variations in south-western Nigeria

Geomagnetic variations, observed at 11 sites in south-western Nigeria, have been analysed to derive interstation transfer functions with the site at Ile-Ife as reference station. The study involves frequencies from 1 to 6 c.p.h. The reference station Ile-Ife is 160 km northward of the continental slope off the Nigerian coastline and 400 km southward of the dip equator. The analysis has been carried out separately with selected data sections of a few hours length during daytime and during the night. Thus expressed linear relations between field components are in the case of day events of dual implication: (1) for the source field structure of the equatorial electrojet, (2) for internal conductivity conditions, including the coast effect from the Bight of Benin. Conductivity anomalies are the sole cause for an observed spatial variability of night events. A 2-D thin-sheet conductivity model has been derived taking both the source and the coast effect into consideration. This model provides a reasonably good fit between observed and computed transfer functions during day and night.     

A study on the multi-component substrom current

A new technique of eigen mode analysis, Method of Natural Orthogonal Components (MNOC) is used to analyze the ionospheric equivalent current systems obtained on the basis of magnetic data at six meridian magnetometer chains in the northern hemisphere during March 17 19, 1978. The results show that the whole current pattern for any given instant consists of a few eigen modes with different intensities. The first eigen mode exhibits a two cell current construction, characterizing the large scale magnetospheric convection and directly driven process, while the second eigen mode shows a concentrated westward electrojet at midnight sector, characterizing the substorm current wedge and the loading unloading process. The first mode consistently exists whenever during quiet periods or at substorms, and its intensity increases from the beginning of the growth phase of substorms, then quickly intensifies in the expansion phase, followed by a gradual decrease in the recovery phase. On the other hand, the intensity of the second mode remains to be near zero during both quiet time and the growth phase of substorms. Its rapid enhancement occurs in the expansion phase. These characteristics in the current patterns and the intensity variations coincide with the defined physical processes of the directly driven and loading unloading components.     

不同采收间隔双排油莎豆（Cyperus esculentus）带风场结构与防风效能的模拟研究

油莎豆（Cyperus esculentus）根系发达,具有作为防风固沙作物的巨大潜力。开展不同条带状采收方式下油莎豆带的防风固沙效益研究,对于合理利用油莎豆进行防风固沙具有重要意义。以风洞模拟为手段,分别在6 m·s^-1、8 m·s^-1、10 m·s^-1 3种风速下,对隔一带采收一带（H1）、隔一带采收两带（H2）、隔一带采收三带（H3）3种不同油莎豆采收模型在风洞不同位置的风速进行测定,并对不同模型的流场特征、风速加速率及防风效能进行分析。结果表明：（1） 3种不同油莎豆采收模型中,气流均在近床面内受油莎豆模型阻挡,风速被急剧削减形成一定面积风影区,H2与H3模型的流场结构稳定性、风影区面积均比H1模型高。（2） 随着风速增加,不同模型的防风效能均有所下降。H2与H3模型背风面在不同风速下的平均防风效能没有显著差异,但显著大于H1模型。在采收间距内,H2与H3模型防风效能均>50%,H1模型<40%。（3） 从防风固沙效益及流场稳定性来看,H2与H3模型之间没有显著差异且显著大于H1模型。但从经济效益来看,H2模型采收间距减小,经济效益降低。综合防风固沙的生态效益与获取油莎豆的经济效益来看,H3模型为推荐采收方式。本研究通过对不同油莎豆采收模式的防风固沙效益,为利用油莎豆进行防风固沙提供了指导,并为在干旱半干旱地区推广种植油莎豆,获得更大的生态经济效益提供了理论依据。     

On the anomalous features of the geomagnetic quiet-day field variations at Nagpur, India

The quiet-day geomagnetic field variation data from the recently commissioned Nagpur geomagnetic observatory, which has augmented the currently active latitudinal chain of Indian magnetic observatories, are analysed for the year 1993- The variations for diurnal frequencies (Sq) recorded at Nagpur do not follow the expected trend with latitude. This is most conspicuous in the northward horizontal ( X ) component. The anomalous behaviour at Nagpur is also seen in the diurnal harmonic amplitudes when compared with those of the neighbouring stations Alibag (south of Nagpur) and Ujjain (north of Nagpur). This behaviour is attributed to the presence of electrically conducting anomalous sources in the vicinity of Nagpur. The anomalous internal source is inferred to be located at relatively shallower depths and is highly localized.     

南极长城站冬季地磁S_q变化特征

本文利用南极长城站1987年4月至9月的地磁资料,分析了长城站地区冬季地磁S_q变化特征。分析结果表明:(1)南极长城站在初冬(4月)与冬末(9月)月份的S_q变化形态与北半球中纬度的北京地磁台的S_q变化形态基本相似,这可能是由中纬度电离层中的发电机电流所控制的。在仲冬月份(6月与7月),由于太阳紫外辐射效应减少,高纬度的地磁扰动占主导地位,反映出S_q变幅很小和以8小时以下的较短周期谐波起主要作用,(2)在初冬与冬末月份的S_q等效电流矢量,白天比黑夜大约5倍;其矢量方向在白天(08—15时)为顺时针,黑夜为反时针。在仲冬月由于电离层中电流的影响相对减弱,S_q等效电流矢量很小,白天与黑夜基本一样;其矢量分布方向与初冬和冬末的矢量方向不同,这可能是极区的电离层电流或场向电流的影响造成的。     

南极中山站极光电集流分布特征分析

本文通过 1 997年在南极中山站观测的极光和所对应的地磁场扰动 ,利用 AU、AL、AE指数描述极光电集流在南极中山站的分布规律及其强度 ,我们发现东向极光电集流强度是正值 ,西向极光电集流强度是负值 ,从绝对值来说 ,西向极光电集流强度大于东向极光电集流强度。南极中山站夏季的极光电集流强度大于冬季的极光电集流。秋夏冬季节的东向极光电集流在72 0～ 1 44 0分 (世界时 1 2～ 2 4时 )的分布规律的拟合曲线类似口朝上的抛物线形 ;西向极光电集流在 72 0～ 1 44 0分这段时间的分布规律的拟合曲线类似正弦波形。了解了极光电集流在南极中山站的分布特征 ,有利于极区电离层和磁层耦合及极光动力学的研究。     

A description of the external and internal quiet daily variation currents at North American locations for a quiet-Sun year

Summary. An order 4, degree 12 spherical harmonic analysis of the smoothed quiet geomagnetic daily variations was used to separate the external and internal geomagnetic S _q field at North American locations for the quiet-Sun year, 1965. These fields were represented by a month-by-month display of equivalent current vortex systems with dominant, pre-noon foci. The focus reached 40° latitude near the June solstice and about 30° latitude near the December solstice. The daily range of S _q current amplitudes was largest in late July to early August and smallest in mid-December. Semi-annual variations of S _q currents dominated only the equatorial region. Daily maxima in mid-latitudes, occurred mostly near local noon in December to February and about 1 hr before noon in June to mid-October. The ratio of external to internal current amplitude vxied from about 1.5 to 1.9 in the middle latitudes with both annual and semiannual changes. An error treatment indicated that the analysis reproduced the form of the surface field with a correlation coefficient of about 0.9 and the amplitudes of the surface field to about 10 per cent of the S _q daily range.     

Local time variation of geomagnetic transfer functions

The local time (LT) variation of the geomagnetic transfer function at the 32-min period was examined for observatories distributed worldwide. Two distinct variation types (types 1 and 2) were found in the real part of A. Type 1 is conspicuous at lower latitudes and its seasonal variation is small, whereas type 2 is found at higher latitudes and has its maximum in summer. These two types of LT variation are seen globally and are conspicuous when solar activity is high. The amplitudes of both types of variation vary from 0.018 to 0.078, and are independent of the mean values at each observatory. These values are relatively small, but the amplitude of Chambon la Foret is larger than the mean value, which shows that Au changes its sign in local time. The amplitudes of type 1 and 2 variations decrease and increase with geomagnetic latitude, respectively. These features suggest that they are generated by some global external fields. The most probable cause is the Sq field, although the Dp field may contribute to type 1 variation. On the other hand, for the islands of the Pacific Ocean at low latitudes, such as Honolulu and Chichijima, the type 1 variation appears not in the real but in the imaginary part of A, which suggests that currents induced in the ocean also contribute to the local time variation.     

Observation of electric currents in the Alaska oil pipeline resulting from auroral electrojet current sources

Summary. Currents in the 1.28 × 10³ km Alaska oil pipeline, induced from the ionospheric, auroral electrojet, were measured at three sites, near Fairbanks, Paxson and Valdez, Alaska, using a gradient configuration of two fluxgate magnetometers. The observed pipeline current magnitudes, which reached 50 A during times of mild geomagnetic activity, displayed a linear relationship with the electric earth potential. Using the induction relationship between the electric and magnetic fields and the typical spectral composition of the geomagnetic field at high latitudes, I obtained a spectral appearance of the current that shows a maximum in the range of 4.5- to 10-min period. Near Fairbanks the pipeline current amplitudes, I (Amperes), could be represented, approximately, by I = 0.65 B _x T ^−0.5, where B _x(nT) is the north—south geomagnetic field variation amplitude and T (min) is its apparent period. There is much less pipelines current at the sites south of Fairbanks. A previously established relationship between the local electric field and the planetary geomagnetic activity index, Ap , permitted a prediction of the pipeline current surge amplitudes in the Fairbanks region as approximately I = 5.0 Ap . Current surges larger than 500 A may be expected rather often in the Alaska pipeline during large geomagnetic storms.     

CHARACTERISTICS OF Sq VARIATION OF GEOMAGNETIC FIELD AT THE GREAT WALL STATION, ANTARCTICA IN WINTER

In this paper the characteristics of Sq variation of geomagnetic field in the region of the Chinese Great Wall Station (CGWS), Antarctica, in winter are analyzed from geomagnetic data obtained at the Geomagnetic Observatory of CGWS. The result enables us to reveal the following aspects: (1) The pattern of Sq variation at CGWS in early (Apr.) and Late winter (Sep.) is similar to that at Beijing Geomagnetic Observatory (BJO) at the middle latitude in the Northern Hemisphere. It may be controlled by the midlatitudinal ionospheric dynamo current. Amplitude of Sq variation is very small, and the harmonics in 8 hours or shorter periods in midwinter (June and July) is predominant because of the decreased effect of solar ultraviolet radiation and the dominant geomagnetic disturbance at high latitudes. (2) The vectors of Sq-equivalent current in the daytime are about five times more than that in the night. The direction of the vectors is clockwise in the daytime (08-15h) and counterclockwise in the night in earl