VLF goniometer observations at halley bay,Antarctica—II. Magnetospheric structure deduced from whistler observations

On 26 July 1967, a magnetically quiet day (ΣK_p = 12?) with high whistler activity at Halley Bay, it was found possible, by measurement of whistler nose-frequency and dispersion and the bearings of the whistler exit points, to make a detailed study of the magnetospheric structure associated with the whistler ducts.During the period 0509–2305 UT most of the exit points of whistlers inside the plasmasphere were situated along a strip about 100km wide passing through Halley Bay in an azimuthal direction 30°E of N between 57° and 62° invariant latitude. A mechanism which can give rise to such a well-defined locus which co-rotates with the Earth is not clear. Nevertheless, it does appear that the locus coincides with the contour of solar zenith angle 102° at 1800 UT 25 July. This was also the time of occurrence of a sub-storm and it is suggested that the magnetospheric structure was initiated by proton precipitation along the solar zenith angle 102° contour.At mid-day knee-whistlers observed outside the plasmapause had exit points which were closely aligned along an L-shell at an invariant latitude of 62.5°. They exhibited a marked variation (~ 3:1) in electron tube content over about 12° of invariant longitude and a drift of about 8 msec^?1 to lower L-shells.Throughout the period of observation the plasmapause lay about 2° polewards of the mean position found by Carpenter (1968) for moderately disturbed days.     

Characteristics of whistler ducts recorded at Gulmarg

Morphological features of whistlers recorded at low latitude ground station Gulmarg (geomag. lat., 24 26N) are studied to deduce information about ducts. The morphological characteristics of low latitude whistlers are discussed and compared with the characteristics of middle and high latitude whistlers. The maximum electron density (N _m ) at the height of the ionosphere obtained from whistler dispersion comes out to be higher than that of the background, which is in accordance with the characteristics of the whistler duct. The equivalent width of the whistler duct at the maximum height of its path is found to be close to the value obtained from satellite observations. The characteristics of whistler ducts in low latitude ionosphere are similar to those in middle and high latitude ionosphere. The width of ducts estimated from the diffuseness of the whistler track observed during magnetic storm is found to lie in the range of 50–200 km.     

An estimate of the duct life time from low latitude ground observations of whistlers at Varanasi

The observed periodicity in the whistler occurrence rate recorded at our low latitude ground station at Varanasi (geomagnetic latitude, 14°55'N) is interpreted in terms of duct life time at lowL values. Power spectrum analysis of the whistler data yields a period of about 50 min for the growth and decay of ducts. Further dispersion analysis of the whistlers has qualitatively confirmed the existence of separate ducts during the period of observations.     

Formation of whistler ducts

A theory of whistler duct formation is presented. By means of order of magnitude calculations it is shown that, when the ring current overlaps the outer plasmasphere, irregularities will cause field-aligned currents to flow, which are below the threshold sensitivity of satellite-borne magnetometers. These currents must be continuous with horizontal ionospheric currents, which produce horizontal electric fields. These fields map up to the equatorial plane and are large enough to produce flux tube interchange and hence the formation of whistler ducts in the outer plasmasphere.     

Storm-time characteristics of low-latitude whistlers

Using the results from special observations, the storm-time effects on whistler characteristics at low latitudes were examined and found to agree with previous statistical studies. A short discussion is made on the link between spread-F irregularities and magnetospheric whistler ducts. The enhanced whistler activity is explained as a consequence of the stable whistler duct region during spread-F conditions.     

Plasmasphere response to the onset of quiet magnetic conditions: Plasma convection patterns

A whistler study has been made of plasma convection within the plasmasphere during a transition from steady moderate geomagnetic activity to quiet conditions. Continuous whistler data recorded at Sanae, Antarctica (L= 3.98) for the period 0400 UT, 10 July to 0400 UT, 11 July 1973 have been analyzed in 15 min intervals.This study has revealed two distinct bulges in the plasmasphere centred on 1700 and 0100 UT. The bulges appear to result from the outward flow of plasma rather than the addition of new plasma. We tentatively interpret the late bulge at 0100 UT as being the duskside bulge of earlier studies rotated into the midnight region. In this bulge, plasma above L = 3.8 appears to convect outwards to form the bulge whereas plasma at lower L-values is relatively undisturbed. For the early bulge (1700 UT) the plasma convection pattern is similar over all observable L-values and closely reflects the shape of the estimated plasmapause in that region. Comparison of the bulges, with those obtained by Carpenter (1966) suggests that the onset of quiet conditions results in a general displacement of the bulges in an eastward direction by about 3 hr.     

Formation, Interaction and Merger of an Active Region and a Quiescent Filament Prior to Their Eruption on 19 May 2007

We report observations of the formation of two filaments?–?one active and one quiescent, and their subsequent interactions prior to eruption. The active region filament appeared on 17 May 2007, followed by the quiescent filament about 24 hours later. In the 26 hour interval preceding the eruption, which occurred at around 12:50 UT on 19 May 2007, we see the two filaments attempting to merge and filament material is repeatedly heated suggesting magnetic reconnection. The filament structure is observed to become increasingly dynamic preceding the eruption with two small hard X-ray sources seen close to the active part of the filament at around 01:38 UT on 19 May 2007 during one of the activity episodes. The final eruption on 19 May at about 12:51 UT involves a complex CME structure, a flare and a coronal wave. A magnetic cloud is observed near Earth by the STEREO-B and WIND spacecraft about 2.7 days later. Here we describe the behaviour of the two filaments in the period prior to the eruption and assess the nature of their dynamic interactions.     

Magnetic pulsation behaviour in the magnetosphere inferred from whistler mode signals

During a long series of recordings of the Doppler shift of signals from NLK, Seattle, which have propagated in ducts in the whistler mode, a number of occasions have been noted where the duct has been acted on by the electric field of micropulsation events in the Pc4–5 range. Large oscillations are produced in the Doppler shift of the received VLF signal.It is shown that the field line has an antinode of motion in the equatorial plane, and that the Doppler shift is responding almost entirely to the radial component of the duct motion. The latter enables a comparison to be made between the magnetic disturbance in the magnetosphere and that seen on the ground. Some support is given to the prediction of Hughes (1974) and Inoue (1973) that the magnetospheric disturbance vector when seen on the ground is rotated 90° by the currents induced in the ionosphere. Models of the oscillating field line enable an estimate to be made of the azimuthal component of the electric field in the equatorial plane. This is typically $\text{1}\text{mV}\text{m}$. The model also predicts the north-south magnetic field strength of the transverse standing wave at the base of the magnetosphere, and this value may be compared with that seen on the ground. Values of the order 1–2 times the ground H-component or 5–10 times the ground D-component were found.     

The formation and lifetime of whistler ducts

It is assumed that whistler ducts are formed by electric fields interchanging magnetospheric flux tubes of ionization. It is found that such ducts end several hundred kilometres above the transition level, that is usually in the altitude range of 1000–1500 km. Further, the enhancement factor is found to increase towards the equator if the background density has little latitudinal variation. Both of the above properties make such ducts ideal for trapping whistlers.The half-life of whistler ducts is estimated to be of the order of one day. During quiet times ducts decay through enhanced plasma flow into the underlying ionosphere, whereas during storm times, when the plasmasphere is depleted of ionization, large upward plasma flows reduce the enhancement factors of ducts.     

An evaluation of duct-life times from low latitude ground observations of whistlers

On certain occasions, whistler rate occurrences at Gulmarg (24°N geomagnetic) and Naini Tal (19°N geomagnetic) are found to exhibit some periodicity. Power spectrum analyses of the occurrence rates yield a dominant period of about 1 hr. It is suggested that this period is an indication of the duct-life times at low L-values. Dispersion analyses of the whistlers have qualitatively confirmed the existence of separate ducts during the period of observation. It is pointed out that power spectrum analyses may not be applicable to whistler data corresponding to high L-values.     

VLF goniometer observations at Halley Bay,Antarctica—I. The equipment and the measurement of signal bearing

A transistorized wide-band (0.5–11 kHz) VLF goniometer has been developed for the study of whistlers and ELF/VLF emissions. It consists of two crossed vertical loops from which a single loop aerial, rotating about a vertical axis at a frequency of 25 sec^?1, is synthesized electronically. During periods of high whistler activity, when the same propagation paths may be identified in successive whistler groups, it is possible to determine the bearing of the exit point of such a magnetospheric path with an error, typically, of ± 10–20°.     

Radial plasma drifts deduced from VLF whistler mode signals: A modelling study

VLF whistler mode signals have previously been used to infer radial plasma drifts in the equatorial plane of the plasmasphere and the field-aligned ionosphere-protonosphere coupling fluxes. Physical models of the plasmasphere consisting of O⁺ and H⁺ ions along dipole magnetic field lines, and including radial Ez × B drifts, are applied to a mid-latitude flux tube appropriate to whistler mode signals received at Wellington, New Zealand, from the fixed frequency VLF transmitter NLK (18.6 kHz) in Seattle, U.S.A. These models are first shown to provide a good representation of the recorded Doppler shift and group delay data. They are then used to simulate the process of deducing the drifts and fluxes from the recorded data. Provided the initial whistler mode duct latitude and the ionospheric contributions are known, the drifts at the equatorial plane can be estimated to about ± 20 ms^?1 (～10–15%), and the two hemisphere ionosphere-protonosphere coupling fluxes to about ± 10¹² m^?2 s^?1 (～40%).     

Acceleration of the solar wind by whistler waves from coronal holes

We investigate the possibility of an additional acceleration of the high speed solar wind by whistler waves propagating outward from a coronal hole. We consider a stationary, spherically symmetric model and assume a radial wind flow as well as a radial magnetic field. The energy equation consists of (a) energy transfer of the electron beam which excites the whistler waves, and (b) energy transfer of the whistler waves described by conservation of wave action density. The momentum conservation equation includes the momentum transfer of two gases (a thermal gas and an electron beam). The variation of the temperature is described by a polytropic law. The variation of solar wind velocity with the radial distance is calculated for different values of energy density of the whistler waves. It is shown that the acceleration of high speed solar wind in the coronal hole due to the whistler waves is very important. We have calculated that the solar wind velocity at the earth's orbit is about equal to 670 km/sec (for wave energy density about 10^?4 erg cm^?3 at 1.1R⊙). It is in approximate agreement with the observed values.     

Dynamics and energetics of the thermal and nonthermal components in the solar flare of January 20, 2005, based on data from hard electromagnetic radiation detectors onboard the CORONAS-F satellite

Based on data from the SONG and SPR-N multichannel hard electromagnetic radiation detectors onboard the CORONAS-F space observatory and the X-ray monitors onboard GOES satellites, we have distinguished the thermal and nonthermal components in the X-ray spectrum of an extreme solar flare on January 20, 2005. In the impulsive flare phase determined from the time of the most efficient electron and proton acceleration, we have obtained parameters of the spectra for both components and their variations in the time interval 06:43–06:54 UT. The spectral index in the energy range 0.2–2 MeV for a single-power-law spectrum of accelerated electrons is shown to have been close to 3.4 for most of the time interval under consideration. We have determined the time dependence of the lower energy cutoff in the energy spectrum of nonthermal photons E _γ0(t) at which the spectral flux densities of the thermal and nonthermal components become equal. The power deposited by accelerated electrons into the flare volume has been estimated using the thick-target model under two assumptions about the boundary energy E ₀ of the electron spectrum: (i) E ₀ is determined by E _γ0(t) and (ii) E ₀ is determined by the characteristic heated plasma energy (≈5kT (t)). The reality of the first assumption is proven by the fact that plasma cooling sets in at a time when the radiative losses begin to prevail over the power deposited by electrons only in this case. Comparison of the total energy deposited by electrons with a boundary energy E _γ0(t) with the thermal energy of the emitting plasma in the time interval under consideration has shown that the total energy deposited by accelerated electrons at the beginning of the impulsive flare phase before 06:47 UT exceeds the thermal plasma energy by a factor of 1.5–2; subsequently, these energies become approximately equal and are ～(4–5) × 10³⁰ erg under the assumption that the filling factor is 0.5–0.6.     

Solar activity dependence of trans-equatorial ion whistler

This paper describes occurrence probabilities and patterns of trans-equatorial proton (TEP), deuteron (TED) and helium (TEH) whistler from the ISIS-2 satellite in time compressed dynamic spectra. It is shown that the TEP whistlers have high occurrence probability in an active solar period, while the TED whistler has low occurrence probability. In a quiet solar period, the TEP whistler has a relatively lower occurrence probability than the TED whistler. The TEP whistler in a quiet solar period shows a strong seasonal variation. That is a higher occurrence probability in the winter than in the summer in the Northern Hemisphere. The curve of occurrence probability of the TED whistler has a valley (no occurrence) at the noon in a solar active period. The minimum occurrence probabilities, which depend on geomagnetic activity appear at about K_P = 4-5. These phenomena seem to be explained by using the bouncing surface diagram of multicomponent and inhomogeneous plasmas with various proton density. The spectral pattern of trans-equatorial ion whistlers and calculation of an approximate equation with regard to deuteron effect show that relative proton densities to electrons N_P/N_e decrease with increasing solar activity.     

Nonlinear shift of wave parameters of whistlers in the ionosphere

The expression for nonlinear shift of a wave number of a whistler wave propagating through the ionosphere has been derived and the results have been discussed. It is seen that nonlinear shift of a wave number of a whistler is significant in some physical situations. From numerical estimations it is observed that wave number shifts of a whistler for both the LCP and RCP waves become significant when the frequency of the waves are nearly equal to the ion-cyclotron frequency.     

Night-time radial plasma drifts and coupling fluxes at L = 2.3 from whistler mode measurements

A method recently reported for measuring radial drifts in the equatorial plane and ionosphere-magnetosphere coupling fluxes from the Doppler shifts and group delays on whistler mode signals is applied to VLF transmissions from station NLK on 18.6 kHz. Data from 22 nights, primarily during the months November to February, are analysed. When averaged over a time of about 90 min, drifts found are accurate to ±20 ms^?1, corresponding to an equatorial electric field accuracy of ±0.05 mVm^?1 and fluxes, to ±1.5 × 10¹² el m^?2 s^?1 (two hemisphere total). Given currently accepted values of coupling fluxes, the flux accuracy is of marginal value on individual nights, but useful information on average behaviour may be obtained.We find fluxes generally contribute less than 20% to the measured Doppler shift, most of which is therefore produced by cross-L drifts. To an accuracy of about 20% then, Doppler data alone may give information on these drifts. Doppler shift data previously accumulated over a number of years and relating to signals in ducts near L = 2.3 are re-examined. Dominating the nightly behaviour is an inward drift which reaches a maximum of ～ 100 ms^?1 as the duct ends cross the dusk terminator and an outward drift at dawn of the same magnitude which is initiated when the duct end crosses the terminator in the E or lower F-region. In some months, separate effects can be seen corresponding to sunrise at each end of the duct.During the night, there are clear differences between December and June solstice drift behaviour which are as yet unexplained.Two-hemisphere-total coupling fluxes found from the 22 nights of data are ～ 1?2 × 10¹² elm^?2s^?1. At the time of the evening maximum of inward drift the flow is to the ionosphere, but later in the night upfluxes were measured.     

中低纬度变化磁场的分析(英文)

地球变化磁场呈现复杂时空特点,这是由引起该磁场的磁层一电离层电流以及地球内部感应电流的特性决定的。为了研究变化磁场的物理成因及其在日地物理事件中的特性。首先必须将组成变化磁场的各种成分分离开来,然后逐一加以研究。 我们采用自然正交分量法对我国八个地磁台站的时均值序列进行了分析,这些台站展布在27°12′48″到49°36′的中低纬度带内,正是Sq电流体系焦点所在的纬度带。分析结果表明,由发电机过程产生的Sq电流体系是这一纬度带主要的电流体系,与磁暴环电流有关的扰动电流体系也是十分重要的电流体系,在冬季月份,它往往超过Sq程度。此外与UT有关的磁扰变化也被明显地分离出来,它的成因可能与地球磁场的偏心结构有关。这些成份的相对大小随季节变化,而且有确定的纬度分布。 我们提出了一套单台分析和多台分析的方法。考虑到自然正交分量法收效快,稳定性好,所需资料列序列短的特点,这种方法可以推广到台站使用。自然正交分量法可以从成因上分离不同成因,使它在理论研究中具有优于一般付氏分析、时序迭加等方法,可为中低纬电流系成因研究提供有用的结果。     

一个微波爆发的源区磁场和高能电子的能流估计

本文分析了１９９３年１０月２日０７：３９：４０－０７：４１：００ＵＴ时段太阳产生的一个多脉冲微波暴的观测，认为它是由多个脉冲爆发叠加在一个慢变爆发背景上组成的．根据谱分析和利用我们的日冕磁场诊断公式［１］，第一次获得了一个爆发源区的磁场强度和高能电子的信息，其主要结果是：（１）脉冲爆发分量在光薄部分的射电谱指数的平均值比慢变爆发背景的值小１，即前者的谱比后者的硬．在１９．６ＧＨｚ上的亮度温度前者比后者高６倍．（２）从脉冲爆发分量和慢变爆发背景分量推断的源区磁场平均值分别为１５８和５３１高斯，且发现在爆发期间，慢变暴源区磁场强度随时间圣马鞍形变化，在极大相的值比脉冲相和下降相低约５０％（３）产生脉冲暴分量的高能电子的柱密度ＮＬ和数密度Ｎ（＞Ｅ０）分别为慢变暴分量的４％和８％，但它们所携带的能流和发射系数要比慢变爆发分量的值高１倍和８倍！表明这两种爆发成份可能分别来自能谱不同的两群电子在不同爆发源区的辐射．