Pc3–4 ULF waves at polar latitudes

A search for Pc3–4 wave activity was performed using data from a trans-Antarctic profile of search-coil magnetometers extending from the auroral zone through cusp latitudes and deep into the polar cap. Pc3–4 pulsations were found to be a ubiquitous element of ULF wave activity in all these regions. The diurnal variations of Pc3 and Pc4 pulsations at different latitudes have been statistically examined using discrimination between wave packets (pulsations) and noise. Daily variations of the Pc3–4 wave power differ for the stations at the polar cap, cusp, and auroral latitudes, which suggests the occurrence of several channels of propagation of upstream wave energy to the ground: via the equatorial magnetosphere, cusp, and lobe/mantle. An additional maximum of Pc3 pulsations during early-morning hours in the polar cap has been detected. This maximum, possibly, is due to the proximity of the geomagnetic field lines at these hours to the exterior cusp. The statistical relation between the occurrence of Pc3–4 pulsations and interplanetary parameters has been examined by analyzing normalized distributions of wave occurrence probability. The dependences of the occurrence probability of Pc3–4 pulsations on the IMF and solar wind parameters are nearly the same at all latitudes, but remarkably different for the Pc3 and Pc4 bands. We conclude that the mechanisms of high-latitude Pc3 and Pc4 pulsations are different: Pc3 waves are generated in the foreshock upstream of the quasi-parallel bow shock, whereas the source of the Pc4 activity is related to magnetospheric activity. Hourly Pc3 power has been found to be strongly dependent on the season: the power ratio between the polar summer and winter seasons is 8. The effect of substantial suppression of the Pc3 amplitudes during the polar night is reasonably well explained by the features of Alfven wave transmission through the ionosphere. Spectral analysis of the daily energy of Pc3 and Pc4 pulsations in the polar cap revealed the occurrence of several periodicities. Periodic modulations with periods 26, 13 and 8–9 days are caused by similar periodicities in the solar wind and IMF parameters, whereas the 18-day periodicity, observed during the polar winter only, is caused, probably, by modulation of the ionospheric conductance by atmospheric planetary waves. The occurrence of the narrow-band Pc3 waves in the polar cap is a challenge to modelers, because so far no band-pass filtering mechanism on open field lines has been identified.     

2007年3月3日长时间持续Pc5 ULF波的多点联合观测分析

2007年3月3日位于磁层昏侧THEMIS的5颗卫星、同步轨道晨侧和午前的GOES 3颗卫星和地面地磁台站同时观测到了持续近4 h的Pc5 ULF波.我们用交叉小波相关分析计算脉动的传播速度,用MVA分析求解脉动的传播方向,然后结合两者的计算结果获得了Pc5相速度矢量信息.THEMIS卫星观测到Pc5具有压缩特性,且向阳传播,速度约在6~20 km/s左右,相比于磁层中阿尔芬速度(1000 km/s)较低.这些Pc5 ULF波动可能产生于磁尾或磁层内部不稳定性.GOES 3颗卫星观测到不同情况的Pc5 ULF波,极向模占主要成分,且具有波包结构,具有阿尔芬驻波特性,可能产生于K-H(Kelvin-Helmholtz)不稳定性.地面台站观测到ULF波扰动幅度随纬度升高而增强,Pc5脉动在地理纬度60°附近达到最大值, Dumont durville台站观测到的脉动与THEMIS观测到波形有很好的相似性.     

Low-Frequency Plasma Waves in the Outer Polar CUSP: A Review of Observations from Prognoz 8, Interball 1, Magion 4, and Cluster

This paper presents a review of the most interesting observations of low-frequency plasma waves together with plasma particles which were made by the Interball 1, Magion 4 and Prognoz 8 satellites in the outer polar cusp. Accelerated plasma particles, hot electron populations and very strong wave activity, particularly at low frequencies, are observed. A detailed study of the wave spectra together with the distribution function for electrons indicate the correlation between the presence of lower-hybrid waves and the population of the particles with higher energy than in the surrounding space. These experimental facts suggest that strong coupling between waves and particles is responsible for plasma heating. During polar cusp crossings by Interball 1 and Prognoz 8, FFT analysis of the wave form indicates many bursts of ULF emissions in both electric and magnetic components. These waves have highly non-stationary characteristics. To study the dynamics of changes in the spectral characteristics of the waves wavelet analysis has been used. Nonlinear interactions are studied using bispectral methods of analysis. This presentation gives the results of such an analysis for selected cusp crossings at different altitudes. An example of wave activity registered by the STAFF instrument onboard the CLUSTER spacecraft in the polar cusp is also presented.     

Statistical characteristics of the spatial distribution of Pc3-4 geomagnetic pulsations at high latitudes in the antarctic regions

The diumal variations in the parameters of Pc3 (20–60 mHz) and Pc4 (10–19 mHz) pulsations at latitudes of the dayside cusp and polar cap have been studied using data of the magnetic stations of the trans-Antarctic meridional profile for the time interval from January to March 1997 (local summer) under weakly disturbed geomagnetic conditions (AE ≤ 250 nT). The technique for estimating pulsation parameters is based on the separation of the wave packets and noise. The diumal variations in the hourly average parameters of the wave packets in the Pc3 and Pc4 bands and noise in the Pc3-4 band (10–60 mHz)—the average number of wave packets, energy of wave packets and noise, and energy of a single wave packet—turned out to be different for the stations located deep in the polar cap (Φ ～ 87°) and at the latitudes of the dayside polar cusp (Φ ～ 70°) and auroral oval (Φ ～ 66°). Several sources of pulsations caused by different channels of wave energy penetration into the magnetosphere through the dayside cusp, dayside magnetopause, and dawn flank of the magnetotail apparently exist at high latitudes.     

Central polar cap convection response to short duration southward Interplanetary Magnetic Field

Central polar cap convection changes associated with southward turnings of the Interplanetary Magnetic Field (IMF) are studied using a chain of Canadian Advanced Digital Ionosondes (CADI) in the northern polar cap. A study of 32 short duration (1 h) southward IMF transition events found a three stage response: (1) initial response to a southward transition is near simultaneous for the entire polar cap; (2) the peak of the convection speed (attributed to the maximum merging electric field) propagates poleward from the ionospheric footprint of the merging region; and (3) if the change in IMF is rapid enough, then a step in convection appears to start at the cusp and then propagates antisunward over the polar cap with the velocity of the maximum convection. On the nightside, a substorm onset is observed at about the time when the step increase in convection (associated with the rapid transition of IMF) arrives at the polar cap boundary.     

Burst regimes of long-period irregular pulsations at frequencies of 2.0–6.0 mHz and substorm activity in the nightside magnetosphere

The results of studying the simultaneous observations of burst regimes of long-period irregular pulsations at frequencies of 2.0–6.0 mHz (the series of ipcl bursts) in the region of the dayside polar cusp and magnetic field disturbances in the nightside auroral oval are presented. The data on the magnetic field at Mirny (MIR, Φ = 76.93°; Λ = 122.92°) and Yellowknife (YKC, Φ = 69.94°; Λ = 294.38°) antipodal observatories as well as the AE index values (http://www.cetp.ipsl.fr/~isgi/homepag1.htm) have been used in an analysis. It has been found out that 87% (group I) and 13% (group II) of events were registered against a back-ground of substorm activity and a quiet nightside magnetosphere, respectively. It has been revealed that several morphological characteristics of the group-I and -II ipcl bursts differ depending on the conditions in the nightside magnetosphere. It has been indicated that the intervals between peaks and the amplitudes of ipcl bursts of both types are distributed according to the exponential and power laws. The results indicate that magnetospheric plasma turbulence develops in the region where burst regimes are formed. It is assumed that the substorm processes in the magnetotail manifest themselves in plasma turbulence in the dayside cusp.     

Theoretical developments on the causes of ionospheric outflow

It is now well known that there is a substantial outflow of ionospheric plasma from the terrestrial ionosphere at high latitudes. The outflow consists of light thermal ions (H⁺, He⁺) as well as both light and heavy energized ions (H⁺, He⁺, O⁺, N⁺, NO⁺, O₂⁺, N₂⁺). The thermal ion outflows tend to be associated with the classical polar wind, while the energized ions are probably associated with either auroral energization processes or nonclassical polar wind processes. Part of the problem with identifying the exact cause of a given outflow relates to the fact that the ionosphere continuously convects into and out of the various high-latitude regions (sunlight, cusp, polar cap, nocturnal oval) and the time-constant for outflow is comparable to the convection time. Therefore, it is difficult to separate and quantify the possible outflow mechanisms. Some of these mechanisms are as follows. In sunlit regions, the photoelectrons can heat the thermal electrons and the elevated electron temperature acts to increase the polar wind outflow rate. At high altitudes, the escaping photoelectrons can also accelerate the polar wind as they drag the thermal ions with them. In the cusp and auroral oval, the precipitating magnetospheric electrons can heat the thermal electrons in a manner similar to the photoelectrons. Also, energized ions, in the form of beams and conics, can be created in association with field-aligned auroral currents and potential structures. The cusp ion beams and conics that have been convected into the polar cap can destabilize the polar wind when they pass through it at high altitudes, thereby transferring energy to the thermal ions. Additional energization mechanisms in the polar cap include Joule heating, hot magnetospheric electrons and ions, electromagnetic wave turbulence, and centrifugal acceleration.Some of these causes of ionospheric outflow will be briefly reviewed, with the emphasis on the recent simulations of polar wind dynamics in convecting flux tubes of plasma.     

Magnetic Pulsations: Their Sources and Relation to Solar Wind and Geomagnetic Activity

Ultra low frequency (ULF) waves incident on the Earth are produced by processes in the magnetosphere and solar wind. These processes produce a wide variety of ULF hydromagnetic wave types that are classified on the ground as either Pi or Pc pulsations (irregular or continuous). Waves of different frequencies and polarizations originate in different regions of the magnetosphere. The location of the projections of these regions onto the Earth depends on the solar wind dynamic pressure and magnetic field. The occurrence of various waves also depends on conditions in the solar wind and in the magnetosphere. Changes in orientation of the interplanetary magnetic field or an increase in solar wind velocity can have dramatic effects on the type of waves seen at a particular location on the Earth. Similarly, the occurrence of a magnetospheric substorm or magnetic storm will affect which waves are seen. The magnetosphere is a resonant cavity and waveguide for waves that either originate within or propagate through the system. These cavities respond to broadband sources by resonating at discrete frequencies. These cavity modes couple to field line resonances that drive currents in the ionosphere. These currents reradiate the energy as electromagnetic waves that propagate to the ground. Because these ionospheric currents are localized in latitude there are very rapid variations in wave phase at the Earth’s surface. Thus it is almost never correct to assume that plane ULF waves are incident on the Earth from outer space. The properties of ULF waves seen at the ground contain information about the processes that generate them and the regions through which they have propagated. The properties also depend on the conductivity of the Earth underneath the observer. Information about the state of the solar wind and the magnetosphere distributed by the NOAA Space Disturbance Forecast Center can be used to help predict when certain types and frequencies of waves will be observed. The study of ULF waves is a very active field of space research and much has yet to be learned about the processes that generate these waves.     

Alfven波在低纬电离层中的传播研究

Alfven波在低纬地区电离层的传播有其特殊性,一方面,低纬地区同样存在Alfven速度梯度的巨大变化,导致电离层Alfven谐振器(Ionospheric Alfven resonator, IAR)的形成;另一方面,由于在低纬地区磁倾角很小,所以剪切Alfven波在传播的过程中纬度方向跨度很大,不同纬度电离层参数将共同对其产生影响;并且,由于电离层水平分层,故磁力线与电离层不正交.本文选取双流体力学模型,在忽略场向电场的条件下,利用非正交坐标系,结合IRI07模型与MSISE00模型模拟低纬地区Alfven波的传播,得到其反射及耦合特性.结果表明,低纬地区同样存在电离层Alfven谐振现象,由耦合产生的压缩模有向磁赤道方向传播的趋势,夜间电离层状态相对于白天更适合IAR的形成,谐振频率沿磁力线L值增大单调递增.     

Numerical modelling of the thermospheric and ionospheric effects of magnetospheric processes in the cusp region

The thermospheric and ionospheric effects of the precipitating electron flux and field-aligned-current variations in the cusp have been modelled by the use of a new version of the global numerical model of the Earths upper atmosphere developed for studies of polar phenomena. The responses of the electron concentration, ion, electron and neutral temperature, thermospheric wind velocity and electric-field potential to the variations of the precipitating 0.23-keV electron flux intensity and field-aligned current density in the cusp have been calculated by solving the corresponding continuity, momentum and heat balance equations. Features of the atmospheric gravity wave generation and propagation from the cusp region after the electron precipitation and field-aligned current-density increases have been found for the cases of the motionless and moving cusp region. The magnitudes of the disturbances are noticeably larger in the case of the moving region of the precipitation. The thermospheric disturbances are generated mainly by the thermospheric heating due to the soft electron precipitation and propagate to lower latitudes as large-scale atmospheric gravity waves with the mean horizontal velocity of about 690 ms^–1. They reveal appreciable magnitudes at significant distances from the cusp region. The meridional-wind-velocity disturbance at 65° geomagnetic latitude is of the same order (100 ms^–1) as the background wind due to the solar heating, but is oppositely directed. The ionospheric disturbances have appreciable magnitudes at the geomagnetic latitudes 70°–85°. The electron-concentration and -temperature disturbances are caused mainly by the ionization and heating processes due to the precipitation, whereas the ion-temperature disturbances are influence strongly by Joule heating of the ion gas due to the electric-field disturbances in the cusp. The latter strongly influence the zonal- and meridional-wind disturbances as well via the effects of ion drag in the cusp region. The results obtained are of interest because of the location of the     

Letter to the editor The ionospheric response during an interval of Pc5 ULF wave activity

A preliminary analysis of Pc5, ULF wave activity observed with the IMAGE magnetometer array and the EISCAT UHF radar in the post midnight sector indicates that such waves can be caused by the modulation of the ionospheric conductivity as well as the wave electric field. An observed Pc5 pulsation is divided into three separate intervals based upon the EISCAT data. In the first and third, the Pc5 waves are observed only in the measured electron density between 90 and 112 km and maxima in the electron density at these altitudes are attributed to pulsed precipitation of electrons with energies up to 40 keV which result in the height integrated Hall conductivity being pulsed between 10 and 50 S. In the second interval, the Pc5 wave is observed in the F-region ion temperature, electron density and electron temperature but not in the D and E region electron densities. The analysis suggests that the wave during this interval is a coupled Alfven and compressional mode.     

Post-noon two-minute period pulsating aurora and their relationship to the dayside convection pattern

Poleward-moving auroral forms, as observed by meridian-scanning photometers, in the vicinity of the cusp region are generally assumed to be the optical signature of flux transfer events. Another class of quasi-continuous, short period (1–2 min) wave-like auroral emission has been identified, closely co-located with the convection reversal boundary in the post-noon sector, which is similar in appearance to such cusp aurora. It is suggested that these short period wave-like auroral emissions, the optical signature of boundary plasma sheet precipitation in the region 1 field-aligned current system, are associated with ULF magnetohydrodynamic wave activity, which is observed simultaneously by ground magnetometer stations. This association with ULF wave activity is strengthened by the observation of several harmonic frequencies in the pulsation spectrum, each an overtone of the fundamental standing wave resonance frequency.     

Geomagnetic pulsations and magnetic disturbances during the initial phase of a strong magnetic storm of May 15, 2005

A very strong magnetic storm of May 15, 2005, was caused by an interplanetary magnetic cloud that approached the Earths’ orbit. The sheath region of this cloud was characterized by a high solar wind density (～25–30 cm^?3) and velocity (～850 km/s) and strong variations (to ～20 nT) in the interplanetary magnetic field (IMF). It has been indicated that an atypical bay-like geomagnetic disturbance was observed during the initial phase of this storm in a large longitudinal region at high latitudes: from the morning to evening sectors of the geomagnetic local time. Increasing in amplitude, the magnetic bay rapidly propagated to the polar cap latitudes up to the geomagnetic pole. An analysis of the global space-temporal dynamics of geomagnetic pulsations in the frequency band 1–6 mHz indicated that most intense oscillations were observed in the morning sector in the region of the equivalent ionospheric current at latitudes of about 72°–76°. The wavelet structure of magnetic pulsations in the polar cap and fluctuations in IMF was generally similar to the maximum at frequencies lower than 4 mHz. This can indicate that waves directly penetrated into the polar cap from the solar wind.     

电离层Alfven谐振器对地面观测到的地磁信号的影响初步研究

本文研究了0.1~10 Hz频率范围内的ULF波从磁层到地面的传播,得到了解析解,分析了电离层Alfven谐振器、磁倾角、电离层电导率、以及波频率对地面观测到的地磁信号的影响.数值结果表明:在磁层中剪切波在竖直方向有明显的谐振结构;地面观测到的信号在IAR谐振频率出现极大值,其谐振频率随磁倾角的增大而增大;电离层电导率的变化可以改变IAR的谐振频率,并能改变波的透射,从而影响地面地磁信号的频谱.     

Geophysical phenomena during an ionospheric modification experiment at Tromsø, Norway

We present an analysis of phenomena observed by HF distance-diagnostic tools located in St. Petersburg combined with multi-instrument observation at Tromsø in the HF modified ionosphere during a magnetospheric substorm. The observed phenomena that occurred during the Tromsø heating experiment in the nightside auroral E_s region of the ionosphere depend on the phase of substorm. The heating excited small-scale field-aligned irregularities in the E region responsible for field-aligned scattering of diagnostic HF waves. The equipment used in the experiment was sensitive to electron density irregularities with wavelengths 12–15 m across the geomagnetic field lines. Analysis of the Doppler measurement data shows the appearance of quasiperiodic variations with a Doppler frequency shift, f_d and periods about 100–120 s during the heating cycle coinciding in time with the first substorm activation and initiation of the upward field-aligned currents. A relationship between wave variations in f_d and magnetic pulsations in the Y-component of the geomagnetic field at Tromsø was detected. The analysis of the magnetic field variations from the IMAGE magnetometer stations shows that ULF waves occurred, not only at Tromsø, but in the adjacent area bounded by geographical latitudes from 70.5° to 68° and longitudes from 16° to 27°. It is suggested that the ULF observed can result from superposition of the natural and heater-induced ULF waves. During the substorm expansion a strong stimulated electromagnetic emission (SEE) at the third harmonic of the downshifted maximum frequency was found. It is believed that SEE is accompanied by excitation of the VLF waves penetrating into magneto-sphere and stimulating the precipitation of the energetic electrons (10–40 keV) of about 1-min duration. This is due to a cyclotron resonant interaction of natural precipitating electrons (1–10 keV) with heater-induced whistler waves in the magnetosphere. It is reasonable to suppose that a new substorm activation, exactly above Tromsø, was closely connected with the heater-induced precipitation of energetic electrons.     

Localized structure in the cusp and high-latitude ionosphere: a modelling study

The ionospheric signature of a flux transfer event (FTE) seen in EISCAT radar data has been used as the basis for a modelling study using a new numerical model of the high-latitude ionosphere developed at the University of Sheffield, UK. The evolution of structure in the high-latitude ionosphere is investigated and examined with respect to the current views of polar patch formation and development. A localized velocity enhancement, of the type associated with FTEs, is added to the plasma as it passes through the cusp. This is found to produce a region of greatly enhanced ion temperature. The new model can provide greater detail during this event as it includes anisotropic temperature calculations for the O⁺ ions. This illustrates the uneven partitioning of the energy during an event of this type. O⁺ ion temperatures are found to become increasingly anisotropic, with the perpendicular temperature being substantially larger than the parallel component during the velocity enhancement. The enhanced temperatures lead to an increase in the recombination rate, which results in an alteration of the ion concentrations. A region of decreased O⁺ and increased molecular ion concentration develops in the cusp. The electron temperature is less enhanced than the ions. As the new model has an upper boundary of 10 000 km the topside can also be studied in great detail. Large upward fluxes are seen to transport plasma to higher altitudes, contributing to the alteration of the ion densities. Plasma is stored in the topside ionosphere and released several hours after the FTE has finished as the flux tube convects across the polar cap. This mechanism illustrates how concentration patches can be created on the dayside and be maintained into the nightside polar cap.     

ULF waves: 1997 IAGA division 3 reporter review

Highlights of studies of ULF waves from 1995 to early 1997 are presented. The subjects covered include (1) Pc 3–5 waves excited by sources in the solar wind, with emphasis on the role of the magnetospheric cavity in modifying the external source and establishing its own resonances, and the role of the plasmapause in magnetohydrodynamic wave propagation; (2) Pi 2 waves, with emphasis on the plasmaspheric resonances and possible alternative excitation by plasmasheet source waves; (3) the spatial structure of internally excited long-period waves, including a kinetic theory for radially confined ring current instability and groundbased multipoint observation of giant pulsations; (4) amplitude-modulated Pc 1–2 waves in the outer magnetosphere (Pc 1–2 bursts) and in the inner magnetosphere (structured Pc 1 waves or pearls); and (5) the source region of the quasi-periodic emissions. Theory and observations are compared, and controversial issues are highlighted. In addition, some future directions are suggested.     

Daytime quasiperiodic geomagnetic pulsations during the recovery phase of the strong magnetic storm of May 15, 2005

Intense quasimonchromatic geomagnetic pulsations with a period of ～15 min, observed on the Earth’s surface in the near-noon sector at the beginning of the recovery phase of a very strong (Dst _min = ?260 nT) magnetic storm of May 15, 2005, are analyzed. The variations were registered at auroral latitudes only in the X field component, and wave activity shifted into the postnoon sector of the polar cap an hour later; in this case pulsations were observed in the X and Y field components. Within the magnetosphere the source of magnetic pulsations could be the surface waves on the magnetopause caused by the pulse of the solar wind magnetic pressure. Geomagnetic pulsations in the polar cap, observed in phase at different latitudes, could apparently reflect quasiperiodic variations in the NBZ system of field-aligned currents. Such variations can originate due to the series of pulsed reconnections in the postnoon outer cusp at large (～20 nT) positive B _z values and large (about ?40 nT) negative values of IMF B _x .     

Energetic neutral atom imaging at low (≤10 keV) energies from Astrid: observations and simulations

Intense (10⁶ cm⁻² sr⁻¹ s⁻¹) fluxes of upflowing ENAs from the polar cap have been observed in the energy range 0.1–13 keV (hydrogen assumed) from the Astrid satellite at 1000 km altitude. If a source altitude of 400 km is assumed, the ENA emissions come from an arc-like region at magnetic latitudes 70–85° extending from dusk over to the nightside. Simulated images show that the observed emissions may be the ENA-albedo effect of the auroral ion precipitation. It is also possible that the observed emissions may originate from upward accelerated ions with cone-like pitch-angle distributions charge exchanging with the upper atmosphere.     

Space Plasma Exploration by Active Radar (SPEAR): an overview of a future radar facility

SPEAR is a new polar cap HF radar facility which is to be deployed on Svalbard. The principal capabilities of SPEAR will include the generation of artificial plasma irregularities, operation as an all-sky HF radar, the excitation of ULF waves, and remote sounding of the magnetosphere. Operation of SPEAR in conjunction with the multitude of other instruments on Svalbard, including the EISCAT Svalbard radar, and the overlap of its extensive field-of-view with that of several of the HF radars in the SuperDARN network, will enable in-depth diagnosis of many geophysical and plasma phenomena associated with the cusp region and the substorm expansion phase. Moreover, its ability to produce artificial radar aurora will provide a means for the other instruments to undertake polar cap plasma physics experiments in a controlled manner. Another potential use of the facility is in field-line tagging experiments, for coordinated ground-satellite experiments. Here the scientific objectives of SPEAR are detailed, along with the proposed technical specifications of the system.