Signatures of storm sudden commencements in geomagnetic H, Y and Z fields at Indian observatories during 1958–1992

The work describes an intensive study of storm sudden commencement (SSC) impulses in horizontal (H), eastward (Y) and vertical (Z) fields at four Indian geomagnetic observatories between 1958–1992. The midday maximum of δH has been shown to exist even at the low-latitude station Alibag which is outside the equatorial electrojet belt, suggesting that SSC is associated with an eastward electric field at equatorial and low latitudes. The impulses in Y field are shown to be linearly and inversely related to δH at Annamalainagar and Alibag. The average SC disturbance vector is shown to be about 10–20°W of the geomagnetic meridian. The local time variation of the angle is more westerly during dusk hours in summer and around dawn in the winter months. This clearly suggests an effect of the orientation of shock front plane of the solar plasma with respect to the geomagnetic meridian. The δZ at δC have a positive impulse as in δH. The ratio of δZ/δH are abnormally large exceeding 1.0 in most of the cases at Trivandrum. The latitudinal variation of δZ shows a tendency towards a minimum over the equator during the nighttime hours. These effects are explained as (1) resulting from the electromagnetic induction effects due to the equatorial electrojet current in the subsurface conducting layers between India and Sri Lanka, due to channelling of ocean currents through the Palk Strait and (2) due to the concentration of induced currents over extended latitude zones towards the conducting graben between India and Sri Lanka just south of Trivandrum.     

Meridional equatorial electrojet current in the American sector

Huancayo is the only equatorial electrojet station where the daytime increase of horizontal geomagnetic field (H) is associated with a simultaneous increase of eastward geomagnetic field (Y). It is shown that during the counter electrojet period when H is negative, Y also becomes negative. Thus, the diurnal variation of Y at equatorial latitudes is suggested to be a constituent part of the equatorial electrojet current system. Solar flares are known to increase the H field at an equatorial station during normal electrojet conditions (nej). At Huancayo, situated north of the magnetic equator, the solar flare effect, during nej, consists of positive impulses in H and Y and negative impulse in Z field. During counter electrojet periods (cej), a solar flare produces a negative impulse in H and Y and a positive impulse in Z at Huancayo. It is concluded that both the zonal and meridional components of the equatorial electrojet in American longitudes, as in Indian longitudes, flows in the same, E region of the ionosphere.     

对流电场、场向电流和极光区电集流变化的地磁响应

对流电场、场向电流和极光区电集流是磁层一电离层耦合的主要物理过程．它们的演化发展时间分别为几分钟至半小时的量级．本文用１００°Ｅ和３００°Ｅ的两个地磁经度链附近各１１个台站的１ｍｉｎ均值地磁Ｈ和Ｚ分量资料,分析了１９９４年４月１６-１７日磁暴期间磁层耦合过程对极光区和中低纬区电离层扰动的地磁特征．强磁暴开始时,台站所处的地方时位置不同,则观测到的电离层和地磁响应也完全不同．这是磁层对流和一、二区场向电流共同作用的结果．一般说,扰时极光区的西向电集流变化更为强烈．随着耦合的发展,极光区范围会向南北扩展,电集流中心带则向低纬侧移动．在中低纬区,二区场向电流的建立能屏蔽一区场向电流所产生的扰动,并引起反向的电流及地磁变化．由此,中低纬区夜间有可能出现短时间的东向电场,又可通过ＥＸＢ的垂直向上漂移作用抬升Ｆ层等离子体,并发生同一经度链附近的多站电离层ｈ＇Ｆ同时突增现象．另一方面,磁赤道附近的台站则更多地受内磁层赤道环电流和电离层赤道电集流的影响．     

Lunar and luni-solar variations of the geomagnetic field in the Indian region

Summary Lunar and luni-solar geomagnetic components have been computed upto four harmonics for low latitude station Alibag, outside equatorial electrojet belt, and the equatorial electrojet stations Annamalainagar, Kodaikanal and Trivandrum in the south Indian region. The computations are confined to data of very high solar activity period 1958–61. Amplitudes of lunar semidiurnal component (L ₂), in the horizontal intensity (H), undergo an equatorial enhancement. Phase difference of 2 hrs is noticed inL ₂ (H) between nonelectrojet and electrojet stations. In the vertical intensity (Z), L ₂ is maximum ine andj-seasons at Trivandrum, close to the magnetic equator. Ind-season, however, maximumL ₂ (Z) occurs at Annamalainagar (dip 5°.4N). The phase difference between the electrojet and nonelectrojet stations observed inL ₂ (H) is not noticed inL ₂ (Z). The differential vertical upward drift motion of charged particles may explain the observed phase difference inL ₂ (H). Seasonal variations in amplitudes and times of maxima are noticed at all the stations inL ₂ (H) andL ₂ (Z). Similar variation is also noticed at Alibag inL ₂ of declination (D).     

The features and a possible mechanism of semiannual variation in the peak electron density of the low latitude F2 layer

Ionospheric data observed in 30 stations located in 3 longitude sectors (East Asia/Australia Sector, Europe/Africa Sector and America/East Pacific Ocean Sector) during 1974–1986 are used to analyse the characteristics of semiannual variation in the peak electron density of F2 layer (NmF2). The results indicate that the semiannual variation of NmF2 mainly presents in daytime. In nighttime, except in the region of geomagnetic equator between the two crests of ionospheric equatorial anomaly, NmF2 has no obvious semiannual variation. In the high latitude region, only in solar maxima years and in daytime, there are obvious semiannual variations of NmF2. The amplitude distribution of the semiannual variation of daytime NmF2 with latitude has a “double-humped structure”, which is very similar to the ionospheric equatorial anomaly. There is asymmetry between the Southern and the Northern Hemispheres of the profile of the amplitude of semiannual variation of NmF2 and longitudinal difference. A new possible mechanism of semiannual variation of NmF2 is put forward in this paper. The semiannual variation of the diurnal tide in the lower thermosphere induces the semiannual variation of the amplitude of the equatorial electrojet. This causes the semiannual variation of the amplitude of ionospheric equatorial anomaly through fountain effect. This process induces the semiannual variation of the low latitude NmF2.     

Solar flare effect on the geomagnetic field and ionosphere

This paper studies the ionospheric and geomagnetic response to an X6.2 solar flare recorded at 14:30 UT on December 13, 2001, in quiet geomagnetic conditions which allow the variations in the geomagnetic field and ionosphere measurements to be easily related to the solar flare radiation.By using measurements from the global positioning system (GPS) and geomagnetic observatories, the temporal evolution of ionospheric total electron content variation, vTEC_V, and geomagnetic field variations, δB, as well as their rates of variation, were obtained around the subsolar point at different solar zenith angles. The enhancement of both parameters was recorded one to three minutes later than the Geostationary Operational Environmental Satellite (GOES) programme recording; such delay tends to depend on the latitude, longitude, and solar zenith angle of the observatory's observations.The vTEC_V is related to the local time and the δB to the intensity and position of the ionospheric currents.The vTEC_V′s maximum value is always recorded later than the maximum values reached by δB and the X-ray intensity. The maximum δB is larger in the local morning than in the afternoon.The rates of vTEC_V and δB have two maximum values at the same time as the maximum values recorded by Hα (for each ribbon).This work shows the quantitative and qualitative relations between a solar flare and the ionospheric and geomagnetic variations that it produces.     

Extended main phase of some sudden commencement great geomagnetic storms with double SSCs

Hourly equatorial Dst (H) values for a few sudden commencement great geomagnetic storms recorded during the solar cycle 22 are plotted for 72 h of storm time and critically examined. Magnetic records taken at selected low latitude Indian stations are also scrutinised for finer details like SSCs, SIs and other fluctuations. Unusually prolonged main phases lasting more than 20 h characterize the two great storms of 13 March 1989 and 24 March 1991. A second SSC/SI pair, occurring some hours after the first main SSC, has also been identified in these storms. Only the great storm of 28 October 1991, with two SSCs and a main phase duration of 21 h, could be studied in conjunction with simultaneous interplanetary data, including Bz changes. Double negative Bz changes correlate well with the extended and enhanced main phase of this storm. Successive magnetic clouds preceded by interplanetary shock waves could generate such great magnetic storms in association with southward IMF changes.     

电离层对流和极光区电集流的地磁链观测

本文采用31个高纬地磁台站资料考察1997年5月15日一次中等磁暴期间极光区电集流和电离层对流的空间分布和时间变化;其中20站处于纬度60°N～80°N之间的西半球,而另11站是偶极磁经度约为120°E的欧洲IMAGE地磁站链.对此纬度链和经度链上各站1 min精度地磁资料的综合分析结果表明,极光区电集流中心的相对强度及其纬度位置是随世界时和地方时区不断变化的.电集流中心所处位置的变化可能是其中心的南北移动造成的,也可能是中心带与磁纬圈间的相互倾斜所致.另一方面,电离层对流形态和晨昏对流圈的经向跨度及其两端的位置是基本不变的.有关结论得到同期的非相干散射雷达EISCAT观测的证实和补充.     

Poleward expansion of the westward electrojet depending on the solar wind and IMF parameters

The effect of the interplanetary parameters on the latitudinal position of the substorm westward electrojet is studied in the work. The data from the IMAGE chain of magnetic stations and POLAR and WIND satellites for the period close to the solar activity minimum (1995–1996) and for the period of the solar activity maximum (2000) have been used for this purpose. It has been indicated that the electrojet poleward edge reaches, on average, higher latitudes at a higher solar wind velocity and at a larger (B _s ) IMF southward component. It has been indicated that the average latitude of the westward electrojet center increases with increasing solar wind velocity and decreases with increasing IMF southward component, as a result of which the electrojet center is, specifically, not observed at high geomagnetic latitudes at large values of the IMF southward component.     

Disturbance daily variation of electrojet current at Indian longitude sector

Using the hourly mean data of the horizontal (H) and vertical (Z) components of the geomagnetic field at the set of nine observatories in India, it has been found that the Disturbance Daily Variation (SD) of H shows a prominent midday depression over the magnetic equator of the H field and a midday increase of Z field at stations near the northern fringe of the equatorial electrojet belt. The magnetic disturbance introduces a westward electric field over the equatorial region, causing a band of westward ionospheric current over the magnetic equator during the day time hours. The latitudinal extent of the disturbance time counter electrojet current seems to be larger than that of quiet time normal electrojet current. This suggests a systematic westward electric field superposed on the normal Sq field at low latitude ionosphere during disturbed periods, the source of which has to be clearly defined. Further correlative data analysis is required to isolate these sources of the disturbed equatorial electric fields.     

Geomagnetic storm effects at low latitudes

The geomagnetic horizontal (H) field from the chain of nine observatories in India are used to study the storm-time and disturbance daily variations. The peak decrease in storm-time variation in H showed significant enhancements at the equatorial electrojet stations over and above the normally expected decrease due to the ring current effects corrected for geomagnetic latitudes. The disturbance daily variation of H at equatorial stations showed a large decrease around midday hours over and above the usual dawn-maximum and dusk-minimum seen at any mid-latitude stations around the world. These slow and persistent additional decreases of H of disturbance daily variation at equatorial latitudes could be the effect of a westward electric field due to the Disturbance Ionospheric dynamo coupled with abnormally large electrical conductivities in the E region over the equator.     

Ionospheric and Solar Wind Variation during Magnetic Storm Onset and Main Phase at Low- and Mid-latitudes

The relationship between the F2-layer critical frequency and solar wind parameters during magnetic storm sudden commencement (SSC) and main phase periods for intense (IS) and very intense (VIS) class of storms is investigated. The analysis covers low- and mid-latitude stations. The effects of ionospheric storm during SSC period is insignificant compared to the main phase, but can trigger the latter. The main phase is characterized by severe negative storm effect at both latitudes during VIS periods while it is latitudinal symmetric for IS observations. The IS reveal positive/negative storm phase in the low-/mid-latitudes, respectively. Ionization density effect is more prominent during VIS events, and is attributed to large energetic particle and solar activity input into the earth magnetosphere. However, ionospheric effect is more significant at the low-latitude than at the mid-latitude. Lastly, ionospheric storm effect during a geomagnetic storm may be related to the combinational effect of interplanetary and geomagnetic parameters and internal ionospheric effect, not necessarily the solar wind alone.     

Geomagnetic signatures of sudden ionospheric disturbances during extreme solar radiation events

We performed a comparative study of geomagnetic variations, which are associated with sudden ionospheric disturbances (SIDs) caused by great X-class solar flares on July 14, 2000 (Bastille flare) and on October 28, 2003 (Halloween flare). Intense fluxes of solar X-rays and EUV radiation as well as solar energetic particles (SEP) were considered as sources of abundant ionization of the ionosphere and upper atmosphere. Flare-initiated SIDs are revealed as transient geomagnetic variations, which are generated by enhanced electric currents flowing mainly in the bottom-side ionosphere. Those so-called solar flare effects (SFEs) were studied by using of geomagnetic data from INTERMAGNET worldwide network of ground-based magnetometers. In subsolar region the SFE is mainly controlled by the flare X-rays and/or EUV radiation. We found that in the Halloween flare the contribution of X-rays was comparable with the EUV, but in the Bastille flare the EUV flux was dominant. The ionization at high latitudes is generated by the SEP, which energy flux is comparable and even exceeds the solar electromagnetic radiation in that region. It was shown that in the Halloween event the pattern of SFE is formed by a two-vortex current system, which is similar to the quiet day Sq current system. However, during the Bastille flare, the pattern of induced currents is quite different: the northern vortex shifts westward and southern vortex shifts eastward such that the electroject is substantially tilted relative to the geomagnetic equator. From numerical estimations we found that at middle latitudes the SEP-initiated geomagnetic effect becomes comparable with the effects of solar electromagnetic radiation. It was also shown that the SEP contribute to the SFE in the nightside hemisphere. The revealed features of the SEP impact to the ionosphere were found in a good agreement with the theory of energetic particle penetration to the bottom-side magnetosphere.     

2015年3月磁暴期间中国中低纬地区电离层变化分析

2015年3月17日爆发了本太阳活动周最大的地磁暴,Dst指数达到-233 nT.本文利用电离层测高仪f_。F_2和h_mF_2、北斗同步卫星(BDSGEO)TEC以及GPS电离层闪烁S4指数对此次磁暴期间中国中低纬地区(北京、武汉、邵阳和三亚)的电离层变化进行分析,并对此次磁暴所引发电离层暴的可能机制进行了探讨.磁暴期间,中低纬电离层暴整体表现为正相暴之后长时间强的负相暴.3月17日白天中纬正相暴为风场抬升电离层所致,而驼峰区及低纬地区正相暴由东向穿透电场所引起;3月18日白天长时间的强负相暴为西向扰动发电机电场和成分扰动所引起;3月17和18日夜间的负相暴可能是日落东向电场受到抑制以及赤道向风场对扩散的抑制导致驼峰向赤道压缩所致,同时被抑制的日落东向电场强度不足以触发产生赤道扩展F,导致低纬三亚和邵阳夜间电离层闪烁在磁暴期间受到完全抑制.这是我们首次基于北斗同步卫星TEC组网观测开展的电离层暴研究.     

电离层电场的半年变化对F2区峰值电子浓度的影响

利用一个电离层理论模式,模拟了太阳活动低年、地磁宁静情况下,中低纬和赤道地区电离层F2区峰值电子浓度(NmF2)的半年变化规律,重点讨论了电离层电场对NmF2半年变化的影响.模拟结果表明,当输入的电场没有周年和半年变化时,磁赤道地区电离层NmF2本身就具有一定的半年变化特征,而在稍高的纬度上,NmF2半年变化的强度较弱.当输入的电场具有一定的半年变化时,电离层NmF2的半年变化强度有明显的改变,且这种改变随地方时和地磁纬度不同有明显的差别.在地磁赤道附近的电离层赤道槽地区,从上午到午夜的时间内,具有半年变化的电场对电离层NmF2半年变化的强度是减弱的作用,在其他的时间内,电场对电离层NmF2半年变化强度是加强的作用.而在稍高纬度的电离层驼峰地区,情况明显不同.从上午一直到翌日日出前,具有半年变化的电场对电离层NmF2半年变化的幅度都是加强的作用.在其他的时间内,电场对电离层NmF2半年变化的幅度是减弱的作用.同时,研究表明电离层电场对NmF2半年变化的作用和“赤道喷泉”现象强烈相关.     

Induction of auroral zone electric currents within the Alaska pipeline

The Alaskar pipeline is a highly conducting anomaly extending 800 miles (1300 km) from about 62° to 69° geomagnetic latitude beneath the most active regions of the ionospheric electrojet current. The spectral behavior of the magnetic field from this current was analyzed using data from standard geomagnetic observatories to establish the predictable patterns of temporal and spatial changes for field pulsation periods between 5 min and 4 hr. Such behavior is presented in a series of tables, graphs and formulae. Using 2- and 3-layer models of the conducting earth, the induced electric fields associated with the geomagnetic changes were established. From the direct relationship of the current to the geomagnetic field variation patterns one can infer counterpart temporal and spatial characteristics of the pipeline current. The relationship of the field amplitudes to geomagnetic activity indices,A _p, and the established occurrence of various levels ofA _p over several solar cycles were employed to show that about half of the time the induced currents in the pipe would be under 1 A for the maximum response oscillatory periods near 1 hr. Such currents should be of minimal consequence in corrosion effects for even a section of the pipeline unprotected by sacrificial electrodes. Of greater interest was the result that the extreme surges of current should reach over one-hundred amperes in the pipeline during high activity.     

Midday reversal of equatorial ionospheric electric field

A comparative study of the geomagnetic and ionospheric data at equatorial and low-latitude stations in India over the 20 year period 1956–1975 is described. The reversal of the electric field in the ionosphere over the magnetic equator during the midday hours indicated by the disappearance of the equatorial sporadic E region echoes on the ionograms is a rare phenomenon occurring on about 1% of time. Most of these events are associated with geomagnetically active periods. By comparing the simultaneous geomagnetic H field at Kodaikanal and at Alibag during the geomagnetic storms it is shown that ring current decreases are observed at both stations. However, an additional westward electric field is superimposed in the ionosphere during the main phase of the storm which can be strong enough to temporarily reverse the normally eastward electric field in the dayside ionosphere. It is suggested that these electric fields associated with the V × Bz electric fields originate at the magnetopause due to the interaction of the solar wind and the interplanetary magnetic field.     

Nighttime enhancements in ionospheric electron content: seasonal and solar cycle variation

Various characteristics of anomalous nighttime enhancement in ionospheric electron content (IEC) at Lunping (14.08°N geomagnetic), a station near the crest of the equatorial anomaly, have been presented by considering the IEC data for the 21st solar cycle. Out of a total of 1053 enhancements, 354 occur in pre-midnight and 699 occur in post-midnight hours, which indicates an overall dominance of post-midnight events at Lunping. The occurrence is more frequent during summer, less during the equinox and least during winter months. All the characteristics of the enhancements have seasonal dependencies and they reach their maximum values during summer months. The occurrence of the pre-midnight events show positive and post-midnight events show negative correlation with solar activity. The results have been discussed and compared with those at low-latitude stations in India and Hawaii and at the mid-latitude station, Tokyo.     

Long-period geomagnetic pulsations as solar flare precursors

We compare long-period pulsations of the horizontal component of the geomagnetic field at intervals that precede extreme solar flares. To this end, we use the wavelet–skeleton technique to process the geomagnetic field disturbances recorded at magnetic stations over a wide geographical range. The synchronization times of wavelet–skeleton spectral distributions of long-period pulsations of geomagnetic oscillations over all magnetic stations are shown as normalized histograms. A few days before an intense solar flare, the histograms show extremes. This means that these extremes can be regarded as flare precursors. The same technique is used to analyze the parameters of near-Earth space. The histograms obtained in this case are free of the aforementioned extrema and, therefore, cannot point to an upcoming flare. The goal of this study is to construct a correlation–spectral method for the short-term prediction of solar flare activity.     

f0F2 半年变化的特征和一种可能机制

利用东亚和澳大利亚扇区不同地磁纬度台站和两个极区台站1974-1986 年电离层垂直探测数据, 分析了f₀F₂ 半年变化的主要特征. 提出了产生f₀F₂ 半年变化的一种新的物理机制: 低热层日潮的半年变化, 引起赤道电急流幅度的半年变化, 通过 喷泉效应! 使得赤道电离层异常幅度产生半年变化, 从而形成f₀F₂ 的半年变化.