Energy dependence of near-relativistic electron spectrum at geostationary orbit during the SEP events of 2005

In view of the renewed interest in the study of energetic particles in the outer radiation belt of the earth, we feel it will be helpful in looking for the energy dependence of the electron energy spectrum at geostationary orbit. This may give us some insight into how we can safeguard geostationary satellites from functional anomalies of the deep dielectric charging type, which are caused by charge accumulation and subsequent discharge of relativistic electrons. In this study we examine whether there is any energy dependence in relativistic electron enhancements at geosynchronous altitudes during solar energetic proton events of 2005.     

Global Pc5 wave activity observed using SuperDARN radars and ground magnetometers during an extended period of northward IMF

Observations are presented of long-lived global Pc5 ULF wave activity observed at a wide range of local times. The event was monitored in the high latitude ionosphere (∼60–80° magnetic latitude) by several SuperDARN HF radars and 5 magnetometer chains in Scandinavia, Greenland, Canada, Alaska and Russia. The event coincided with a protracted period (∼36 h) of northward interplanetary magnetic field (IMF). The study focuses on 4 h during which distinct dawn/dusk asymmetries in the wave characteristics were observed with multiple field line resonance (FLR) structures observed in the dawn flank at 1.7, 2.6, 3.3, 4.2 and 5.4 mHz and compressional oscillations in the dusk flank at 1.7 and 2.3 mHz. The data indicated an anti-sunward propagation in both the dawn and dusk flanks and a low azimuthal m number (∣m∣∼6) suggesting a generation mechanism external to the Earth's magnetosphere. A sudden increase in the solar wind dynamic pressure followed by a period of strongly northward, B_z dominated IMF, coincides with the observations and also a large increase in Pc5 wave power observed in the dawn flank. The observed enhancements in the wave activity and FLR structures are thought to be due to a Kelvin–Helmholtz driven waveguide mode. Additionally, there is no evidence that the frequencies of the FLRs are intrinsic to the solar wind. It thus seems that the frequencies were determined by the dimensions of the magnetospheric cavity.     

Multiple field line resonances: Optical,magnetic and absorption signatures

We present multi-instrument observations of ultra low frequency (ULF) wave activity from the dawn flank magnetosphere during the period 12:00–13:30 UT on the 16 December 2003. Optical, magnetic and riometer measurements from the Churchill line meridian in the Canadian sector are presented which demonstrate the presence of multiple discrete auroral arc structures accompanied by periodic magnetic and riometer absorption perturbations in the Pc5 (150–600 s) ULF band. Clear polewards propagation is demonstrated in all the instrument data sets, the magnetic signals showing most clearly the amplitude and phase characteristics consistent with discrete frequency field line resonances (FLRs) on closed field lines. Two discrete frequency field line resonant signals are apparent, at 1.8 and 3.0 mHz which resonate at approximately the same latitude. We explain this via the calculation of the Alfvén continuum, and show that both frequencies may be resonant in the same latitudinal region within instrumental resolution. The meridian scanning photometer (MSP) observations from polewards of the magnetometer determined resonant latitudes show evidence of low intensity (∼200 R) poleward moving discrete arcs related to the ULF waves. Interestingly the MSP observations demonstrate poleward phase propagation with variable rates across the field of view; faster apparent polewards phase propagation being seen at higher latitudes. We demonstrate that the complicated “braided” phase of the arcs can be explained via the precipitation resulting from the superposition of two discrete FLRs. Furthermore, we characterise the ≳25 keV energetic electron precipitation in the region of the FLRs and the arc structures via periodic D-region absorption. In this way, we link the magnetic and both soft and energetic particle precipitation signatures of FLRs together for the first time. Our results demonstrate that riometer absorption can be used to characterise FLRs, however, this is only generally possible at lower L-shells where energetic electrons in the ring current overlap with the FLR fields in the equatorial plane.     

The Role of Solar Wind Structures in the Generation of ULF Waves in the Inner Magnetosphere

The plasma of the solar wind incident upon the Earth’s magnetosphere can produce several types of geoeffective events. Among them, an important phenomenon consists of the interrelation of the magnetospheric–ionospheric current systems and the charged-particle population of the Earth’s Van Allen radiation belts. Ultra-low-frequency (ULF) waves resonantly interacting with such particles have been claimed to play a major role in the energetic particle flux changes, particularly at the outer radiation belt, which is mainly composed of electrons at relativistic energies. In this article, we use global magnetohydrodynamic simulations along with in situ and ground-based observations to evaluate the ability of two different solar wind transient (SWT) events to generate ULF (few to tens of mHz) waves in the equatorial region of the inner magnetosphere. Magnetic field and plasma data from the Advanced Composition Explorer (ACE) satellite were used to characterize these two SWT events as being a sector boundary crossing (SBC) on 24 September 2013, and an interplanetary coronal mass ejection (ICME) in conjunction with a shock on 2 October 2013. Associated with these events, the twin Van Allen Probes measured a depletion of the outer belt relativistic electron flux concurrent with magnetic and electric field power spectra consistent with ULF waves. Two ground-based observatories apart in 90^° longitude also showed evidence of ULF-wave activity for the two SWT events. Magnetohydrodynamic (MHD) simulation results show that the ULF-like oscillations in the modeled electric and magnetic fields observed during both events are a result from the SWT coupling to the magnetosphere. The analysis of the MHD simulation results together with the observations leads to the conclusion that the two SWT structures analyzed in this article can be geoeffective on different levels, with each one leading to distinct ring current intensities, but both SWTs are related to the same disturbance in the outer radiation belt, i.e. a dropout in the relativistic electron fluxes. Therefore, minor disturbances in the solar wind parameters, such as those related to an SBC, may initiate physical processes that are able to be geoeffective for the outer radiation belt.     

An investigation of the field-aligned currents associated with a large-scale ULF wave in the morning sector

Previous work by Scoffield, H.C., Yeoman, T.K., Wright, D.M., Milan, S.E., Wright, A.N., Strangeway, R.J. [2005. An investigation of the field aligned currents associated with a large scale ULF wave using data from CUTLASS and FAST. Ann. Geophys. 23, 487–498) investigated a large-scale ULF wave, occurring in the dusk sector (∼1900 MLT). The wave had a period of ∼800 s (corresponding to 1.2 mHz frequency), an azimuthal wave number of ∼7 and a full-width at half-maximum (FWHM) across the resonance of 350 km. IMAGE ground magnetometer and SuperDARN radar observations of the wave's spatial and temporal characteristics were used to parameterise a simple, two-dimensional field line resonance (FLR) model. The model-calculated field-aligned current (FAC) was compared with FACs derived from the FAST energetic particle spectra and magnetic field measurement. Here the authors use the same method to investigate the FAC structure of a second large-scale ULF wave, with a period of ∼450 s, occurring the dawn sector (∼0500 MLT) with an opposite sense background region 1–region 2 current system. This wave has a much larger longitudinal scale (m∼4.5) and a smaller latitude scale (FWHM=150 km). Unlike the dusk sector wave, which was dominated by upward FAC, FAST observations of the dawn sector wave show an interval of large-scale downward FAC of ∼1.5 μA m⁻². Downgoing magnetospheric electrons with energies of a few keV were observed, which are associated with upward FACs of ∼1 μA m⁻². For both wave studies, downward currents appear to be carried partially by upgoing electrons below the FAST energy detection threshold (5 eV), but also consist of a mixture of hotter downgoing magnetospheric electrons and upgoing ionospheric electrons of energies 30 eV–1 keV. Strong intervals of upward current show that small-scale structuring of scale ∼50 km has been imposed on the current carriers. In general, this study confirms the findings of Scoffield, H.C., Yeoman, T.K., Wright, D.M., Milan, S.E., Wright, A.N., Strangeway, R.J. [2005. An investigation of the FACs associated with a large-scale ULF wave using data from CUTLASS and FAST. Ann. Geophys. 23, 487–498).     

Effects of the magnetic field model and wave polarisation on the estimation of proton number densities in the magnetosphere using field line resonances

The cold, core plasma mass density in the Earth's magnetosphere may be deduced from the resonant behaviour of ultra-low frequency (ULF; 1–100 mHz), magnetohydrodynamic (MHD) waves. Ground-based magnetometers are the most widely used instruments for recording the signature of ULF wave activity in the magnetosphere. For a suitable model of the background magnetic field and a functional form for the variation of the proton number density with radial distance, the resonant frequencies of ULF waves provide estimates of the equatorial plasma mass density. At high latitudes, the magnetic field model becomes critical when estimating the plasma mass density from FLR data. We show that a dipole field model is generally inadequate for latitudes greater than ∼65° geomagnetic compared with models that are keyed to magnetic activity, interplanetary magnetic field and solar wind properties. Furthermore, the method often relies on the detection of the fundamental ULF resonance, which changes frequency depending on the polarisation of the oscillation. Using idealised toroidal and poloidal oscillation modes, the range of the derived densities as the ULF wave polarisation changes is of the same order as changing the density function from a constant value throughout the magnetosphere to assuming constant Alfven speed in a dipole geometry.     

On relation between mid-latitude ionospheric ionization and quasi-trapped energetic electrons during 15 December 2006 magnetic storm

We report simultaneous observations of intense fluxes of quasi-trapped energetic electrons and substantial enhancements of ionospheric electron concentration (EC) at low and middle latitudes over the Pacific region during the geomagnetic storm on 15 December 2006. Electrons with energy of tens of keV were measured at altitude of ～800–900 km by POES and DMSP satellites. Experimental data from COSMIC/FS3 satellites and global network of ground-based GPS receivers were used to determine height profiles of EC and vertical total EC, respectively. A good spatial and temporal correlation between the electron fluxes and EC enhancements was found. This fact allows us to suggest that the quasi-trapped energetic electrons can be an important source of ionospheric ionization at middle latitudes during magnetic storms.     

Storm-intensity criteria for several classes of the driving interplanetary structures

In this paper we discuss the main-phase evolution of intense magnetic storms, associated with the passage of different interplanetary magnetic structures. It is shown that their evolution, driven by intense magnetic fields in the sheath region just behind interplanetary shocks, evolves faster (implying physically different magnetospheric configurations) than that associated with intense magnetic fields in the ejecta itself and in corotating streams. The estimated ring-current injection rate for the main phase of intense magnetic storms caused by sheath fields is ～10 times greater than the estimated injection rate for N–S magnetic clouds. Based on these results, we propose storm-intensity criteria for several classes of the driving interplanetary structures. The time necessary to reach a Dst/SYM index threshold level is an important parameter for a space weather forecast.     

Comparative analysis of recurrent high-speed solar wind streams influence on the radiation environment of near-earth space in April–July 20101

This paper analyzes variations of flux of relativistic and subrelativistic electrons in the outer radiation belt of the Earth caused by the arrival of recurrent high-speed streams of solar wind during three consecutive solar rotations. The period from April to July 2010 is covered. During this time, an increase in fluxes of relativistic electrons was observed after they had reached a minimum in November 2009–January 2010. Two coronal holes of different polarity, geometry, and location relative to the solar equator were the source of high-speed solar wind streams. The relationship between the efficiency of acceleration of electrons of subrelativistic energies and the amplitude, duration of high-speed streams of solar wind and geomagnetic disturbances, as well as the wave activity in the range of 2–7 mHz, characterized by the ULF index, is confirmed. Significant increases of the flux of relativistic electrons in the outer radiation belt of the Earth were observed during the considered period with an hourly average speed of solar wind streams above 550 km/s and a duration of more than seven days. It is found that the spectrum of electrons in the Earth’s outer radiation belt over the considered period of time was softer during the observation of solar wind streams from the positive polarity coronal hole, even given the amplitude of the solar wind velocity higher than 550 km/s.     

Lunar remnant magnetic field mapping from orbital observations of mirrored electrons

Areas of lunar surface magnetic field are observed to ‘mirror’ low energy electrons present in the normal lunar space environment. The ambient electrons provide, in effect, a probe along the ambient magnetic field lines down to the lunar surface for remote sensing of the presence of surface fields. This probe, unlike direct measurement by the magnetometer, does not require low altitude or a very stable (magnetotail) ambient field to provide a mapping of regions of occurrence of such fields. Use of the on-board vector magnetometer measurements of the ambient magnetic field orientation allows accurate projection of such mapping onto the lunar surface. Preliminary maps of the lunar surface magnetic areas underlying the orbit of the ‘Particles and Fields Satellite deployed from Apollo-16’ have been generated, obtaining 40% coverage from partial data to demonstrate feasibility of the technique. As well as providing independent verification of areas such as Van de Graaff already discovered in the magnetometer data, these maps reveal many previously unreported areas of surface magnetism. The method is sensitive to fields of less than 0.1γ at the surface. Application to the full body of available PFS-1 & 2 electron data is expected to provide complete mapping of the lunar surface for areas of magnetization up to latitudes of 35–40 deg. The surface field regions observed are generally due to sources smaller than 10–50 km in size, although many individual regions are often so close together as to give much larger regions of effectively continuous mirroring. Absence of consistent mirroring by any global field places an upper limit on the size of any net lunar dipole moment of less than 10¹⁰ γ km³. Much additional information regarding the magnetic regions can be obtained by correlated analysis of both the electron return and vector magnetometer measurements at orbital altitude, the two techniques providing each other with directly complimentary measurements at the satellite and along the ambient field lines to the surface.     

Magnetic storms on Mars

Based on data from the Mars Global Surveyor magnetometer we examine periods of significantly enhanced magnetic disturbances in the martian space environment. Using almost seven years of observations during the maximum and early declining phase of the previous solar cycle the occurrence pattern and typical time profile of such periods is investigated and compared to solar wind measurements at Earth. Typical durations of the events are 20-40 h, and there is a tendency for large events to last longer, but a large spread in duration and intensity are found. The large and medium intensity events at Mars are found to occur predominantly in association with interplanetary sector boundaries, with solar wind dynamic pressure enhancements being the most likely interplanetary driver. In addition it is found that, on time scales of months to several years, the dominant cause of global variability of the magnetic field disturbance at Mars is solar wind dynamic pressure variations associated with the eccentricity of the martian orbit around the Sun.     

Storm-intensity criteria for several classes of the driving interplanetary structures

In this paper we discuss the main-phase evolution of intense magnetic storms, associated with the passage of different interplanetary magnetic structures. It is shown that their evolution, driven by intense magnetic fields in the sheath region just behind interplanetary shocks, evolves faster (implying physically different magnetospheric configurations) than that associated with intense magnetic fields in the ejecta itself and in corotating streams. The estimated ring-current injection rate for the main phase of intense magnetic storms caused by sheath fields is ∼10 times greater than the estimated injection rate for N–S magnetic clouds. Based on these results, we propose storm-intensity criteria for several classes of the driving interplanetary structures. The time necessary to reach a Dst/SYM index threshold level is an important parameter for a space weather forecast.     

Plasma boundaries at 6.6 RE as observed by GEOS-2: Large plasmapause discontinuity and injection event. Discussion of diamagnetic effects

An impulsive event observed from the nightside geostationary orbit by the GEOS-2 European satellite is analysed in detail in the case of a very quiet magnetosphere. It is characterized by a very sharp and steep plasma density gradient observed near 2230 L.T. A quite detailed picture of the situation can be sketched as a result of the GEOS-2 set of experiments. The observations roughly organize themselves as follows along the geostationary orbit: an intense and well localized d.c. electric field appeared between the outbound crossing of the plasmapause and local midnight; at the same time a sudden ULF activity arose probably indicating the presence of field-aligned currents; GEOS-2 then entered a quieter plasmasheet where clear diamagnetic effects are evidenced. These observations are consistent with a stabilization of a possible interchange instability, which would maintain the density gradient at the plasmapause. The validity of the plasma density measurements which are made through an active wave method is discussed in connection with the 2 keV mean energy of the plasmasheet particles. The macroscopic evolution equation of the plasma bulk velocity is considered. It appears that the gradients of the macroscopic drift velocity of the plasma may have a non-negligible effect rendering invalid the unsophisticated scheme of a balance between kinetic and magnetic pressures.     

Development of four magnetic storms in February 1972

Four magnetic storms were observed in February 1972, with instruments on the Explorer 45 satellite in the evening quadrant of the inner magnetosphere. The magnitude of the storms ranged from small, D_st ? ?40 γ, to moderate, D_st ? ?80 γ. During the development of the storms several substorms occurred. At the beginning of the substorms there was evidence of a partial ring current above L = 5. After the expansion phase of several substorms there was evidence of enhancement of a partial ring below L = 5. Distortions of the field in the east-west direction were observed, in conjunction with substorm expansions, that can be interpreted as due to field aligned currents flowing from the ionosphere. A substantial symmetric ring current, at L～4, developed during the largest storm. Very little additional ring current was contributed by the smallest storm. Relations between the magnetosphere inflation and ring current protons, plasmaspheric hiss, and ULF waves also measured on Explorer 45 were noted.     

Diffuse auroral precipitation in the jovian upper atmosphere and magnetospheric electron flux variability

The deposition of energetic electrons in Jupiter's upper atmosphere provides a means, via auroral observations, of monitoring electron and plasma wave activity within the magnetosphere. Not only does particle precipitation indicate a potential change in atmospheric chemistry, it allows for the study of episodic, pronounced flux enhancements in the energetic electron population. A study has been made of the effects of such electron injections into the jovian magnetosphere and of their ability to provide the source population for variations in diffuse auroral emissions. To identify the source region of precipitating auroral electrons, we have investigated the pitch-angle distributions of high-resolution Galileo Energetic Particle Detector (EPD) data that indicate strong flux levels near the loss cone. The equatorial source region of precipitating electrons has been determined from the locations of Galileo's in situ measurements by tracing magnetic field lines using the KK97 model. The primary source region for Jupiter's diffuse aurora appears to lie in the magnetic equator at 15-40 RJ, with the predominant contribution to precipitation flux (tens of ergs cm⁻² s⁻¹ sr⁻¹) stemming from <30 RJ. Variability of flux for energetic electrons in this region is also important to the irradiation of surfaces and atmospheres for the Galilean moons: Europa, Ganymede, and Callisto. The average diffuse auroral precipitation flux has been shown to vary by as much as a factor of six at a given radial location. This variability appears to be associated with electron injection events that have been identified in high-resolution Galileo EPD data. These electron flux enhancements are also associated with increased whistler-mode wave activity and magnetic field perturbations, as detected by the Galileo Plasma Wave Subsystem (PWS) and Magnetometer (MAG), respectively. Resonant interactions with the whistler-mode waves cause electron pitch-angle scattering and lead to pitch-angle isotropization and precipitation.     

Different eigenproblem models for field line resonances in cold plasma: Effect on magnetospheric density estimates

Magnetospheric plasma density can be remotely sensed through ground-based magnetometer data using a suitable model for field line resonances (FLRs) formed by standing shear Alfvén wave on closed geomagnetic field lines. The simplest type of FLR model, which is also the most relevant for magnetometer data inversion purposes, is based on solving a certain eigenvalue problem. Over the years a number of such models have been developed [Singer, H.J., Southwood, D.J., Walker, R.J., Kivelson, M.G., 1981. Alfvén wave resonances in a realistic magnetospheric magnetic field geometry. J. Geophys. Res. 86, 4589–4596; Rankin, R., Fenrich, F., Tikhonchuk, V.T., 2000. Shear Alfvén waves on stretched magnetic field lines near midnight in Earth's magnetosphere. Geophys. Res. Lett. 27, 3265–3268; Rankin, R., Kabin, K., Marchand, R., 2006. Alfvénic field line resonances in arbitrary magnetic field topology. Adv. Space Res. 38, 1720–1729]. In this paper we summarize the properties of these models and investigate the effect of using these different models on the magnetospheric density inferred from the ground-based magnetometer measurements. We also formulate a simple criterion which can be used to determine which one of these models should be used for a particular field line.     

Pc1 wave generation by sudden impulses

We have studied induction magnetometer data recorded at College, Alaska for the occurrence of ULF emissions associated with sudden impulses (SI). In this study we surveyed three years of data (1977–1979 inclusively) and found that for the 76 SI's reported in IAGA bulletins, 32 were found to be followed by ULF emissions in the College data. While the 76 SI's occurred at all local times those which were associated with ULF emissions at College peaked in occurrence near local noon. We have attempted to interpret these observations in terms of a simple model based upon a Chapman-Ferraro double-dipole model of the magnetosphere. Using this model an estimate of field compressions associated with SI's can be made and from this the increases in the temperature anisotropy and plasma beta may be estimated. This simple model predicts maximum growth rate near noon on high latitude field lines, just inside the magnetopause. Further, inspection of growth rate curves for varying plasma anisotropy and beta leads us to the conclusion that an increase in anisotropy is the primary cause of the ULF emissions observed.     

A compressional Pc4 pulsation observed by three satellites in geostationary orbit near local midnight

We describe the observation of a magnetic pulsation with a period of 55 s, recorded at geostationary orbit by three satellites (ATS 6, SMS 1 and SMS 2) in the local time sector 2100–2400. We use magnetic data from all three spacecraft and also plasma data from ATS 6. The pulsation had a large compressional magnetic component which appeared to be balanced by pressure fluctuations in the hot ring current plasma which were in antiphase with the magnetic variations. This allows the wave to be guided along a field line. From the plasma data we are also able to obtain estimates of the field line displacement and hence the electric field, which enables us to conclude that this is a second harmonic field line resonance. We find that the wave has a very short East-West (E-W) wavelength (m?100) and a westward azimuthal group velocity of about 30 km s^?1. The most probable source for this wave is a bounce resonant interaction with ring current protons. The characteristics of this wave are in many ways similar to those of giant pulsations observed on the ground. ATS 6 was near the inner edge of the ring current electrons and as the wave converted the 10 keV electron Alfvén layer back and forth across ATS 6, we were able to estimate the Alfvén layer energy gradient and obtain a value of 1 keV in 1000 km. This gradient is considerably steeper than that predicted by a steady uniform convection electric field.     

Ground-based electromagnetic studies combined with remote sensing based on Demeter mission: A way to monitor active faults and volcanoes

The identification of magnetic, electric and electromagnetic (EM) precursory signals related to volcanic activities and earthquakes is still a matter of debate. Some examples are now well established, but they are often based on a few parameters recorded on sparse equipments and with no multi-disciplinary approach. Demeter program takes into account a more complete approach of EM phenomena related to volcanic eruptions and earthquakes, by combining both ground-based and satellite EM monitoring, from direct current to several kilohertz, i.e. from ULF, ELF to VLF frequency domains.The research program stands in two parts: one is the identification of EM signals at the satellite altitude and the other consists in detailed studies in a few pilot sites on the ground. Two main test sites have been considered: La Fournaise volcano in Réunion Island and the seismogenic Corinth rift in Greece. Both sites allow for performing EM studies in a multi-disciplinary environment.La Fournaise volcano erupts on average two times a year. The self-recording Demeter EM station is composed of three modules measuring the components of the magnetic and electric fields in three different frequency domains: DC to 0.5 Hz, 0.0033-160 Hz and 8-10 kHz. Preliminary observations made during the May 2003 eruption show that electric and magnetic signals appeared before the eruption. Some signals present sharp step-like variations, with amplitudes up to several hundreds mV per km and a few hour duration, followed by periods with a higher spectral frequency content. The frequency of these signals can be of several tens of Hz.The Corinth rift is a highly seismic area, frequently affected by seismic swarms. In 2004 the region has experienced tens of earthquakes of magnitude less than 4.6. A Demeter station has been set up on the Trizonia Island along the northern mainland coast, where a 30 km long seismic gap has been identified. The station is composed of two modules recording the three components of the magnetic field and the two horizontal components of the electric field in the ULF and ELF-VLF frequency bands. The audiomagnetotelluric soundings show that the station is close to a regional conductive fault connected to the sea. The first 4 months of observation clearly show that 29 earthquakes, even of low magnitude (M?2.8), occurring at less than 140 km of distance of the station, have generated electric signals when the seismic waves have passed the EM station. For a given magnitude of the earthquake, the energy of the electric signal is independent of the distance between the focal source and the EM station, which points out local electric source mechanisms. The greater the magnitude of the earthquake, the greater is the energy of the electric signal is. The co-seismic electric signals have the same morphology as that of the passing seismic wave, and there is no noticeable time delay between the electric and the seismic signals. This simultaneity between the seismic and the electric signal is best explained by the generation of an electrokinetic effect due to the passage of the seismic wave through the seawater-saturated ground.