Showing papers on "Earth's magnetic field published in 2004"

Transport of solar wind into Earth's magnetosphere through rolled-up Kelvin–Helmholtz vortices

Abstract: Establishing the mechanisms by which the solar wind enters Earth's magnetosphere is one of the biggest goals of magnetospheric physics, as it forms the basis of space weather phenomena such as magnetic storms and aurorae1. It is generally believed that magnetic reconnection is the dominant process, especially during southward solar-wind magnetic field conditions when the solar-wind and geomagnetic fields are antiparallel at the low-latitude magnetopause2. But the plasma content in the outer magnetosphere increases during northward solar-wind magnetic field conditions3,4, contrary to expectation if reconnection is dominant. Here we show that during northward solar-wind magnetic field conditions—in the absence of active reconnection at low latitudes—there is a solar-wind transport mechanism associated with the nonlinear phase of the Kelvin–Helmholtz instability5. This can supply plasma sources for various space weather phenomena.

A Simplified Statistical Model for the Geomagnetic Field and the Detection of Shallow Bias in Paleomagnetic Inclinations: was the Ancient Magnetic Field Dipolar?

Abstract: The assumption that the time-averaged geomagnetic field closely approximates that of a geocentric axial dipole (GAD) is valid for at least the last 5 million years and most paleomagnetic studies make this implicit assumption. Inclination anomalies observed in several recent studies have called the essential GAD nature of the ancient geomagnetic field into question, calling on large (up to 20%) contributions of the axial octupolar term to the geocentric axial dipole in the spherical harmonic expansion to explain shallow inclinations for even the Miocene. In this paper, we develop a simplified statistical model for paleosecular variation (PSV) of the geomagnetic field that can be used to predict paleomagnetic observables. The model predicts that virtual geomagnetic pole (VGP) distributions are circularly symmetric, implying that the associated directions are not, particularly at lower latitudes. Elongation of directions is North-South and varies smoothly as a function of latitude (and inclination). We use the model to characterize distributions expected from PSV to distinguish between directional anomalies resulting from sedimentary inclination error and from non-zero non-dipole terms, in particular a persistent axial octupole term. We develop methodologies to correct the shallow bias resulting from sedimentary inclination error. Application to a study of Oligo-Miocene redbeds in central Asia confirms that the reported discrepancies from a GAD field in this region are most probably due to sedimentary inclination error rather than a non-GAD field geometry or undetected crustal shortening. Although non-GAD fields can be imagined that explain the data equally well, the principle of least astonishment requires us to consider plausible mechanisms such as sedimentary inclination error as the cause of persistent shallow bias before resorting to the very expensive option of throwing out the GAD hypothesis.

Resonance effects indicate a radical-pair mechanism for avian magnetic compass

Abstract: Migratory birds are known to use the geomagnetic field as a source of compass information. There are two competing hypotheses for the primary process underlying the avian magnetic compass, one involving magnetite, the other a magnetically sensitive chemical reaction. Here we show that oscillating magnetic fields disrupt the magnetic orientation behaviour of migratory birds. Robins were disoriented when exposed to a vertically aligned broadband (0.1-10 MHz) or a single-frequency (7-MHz) field in addition to the geomagnetic field. Moreover, in the 7-MHz oscillating field, this effect depended on the angle between the oscillating and the geomagnetic fields. The birds exhibited seasonally appropriate migratory orientation when the oscillating field was parallel to the geomagnetic field, but were disoriented when it was presented at a 24 degrees or 48 degrees angle. These results are consistent with a resonance effect on singlet-triplet transitions and suggest a magnetic compass based on a radical-pair mechanism.

Extending comprehensive models of the Earth's magnetic field with Ørsted and CHAMP data

Abstract: SUMMARY A new model of the quiet-time, near-Earth magnetic field has been derived using a comprehensive approach, which includes not only POGO and Magsat satellite data, but also data from the Orsted and CHAMP satellites. The resulting model shows great improvement over its predecessors in terms of completeness of sources, time span and noise reduction in parameters. With its well separated fields and extended time domain of 1960 to mid-2002, the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997. Because all sources are coestimated in a comprehensive approach, intriguing north–south features typically filtered out with other methods are being discovered in the lithospheric representation of the model, such as the S Atlantic spreading ridge and Andean subduction zone lineations. In addition, this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection. The F-region currents, through which the satellites pass, are now treated as lying within meridional planes, as opposed to being purely radial. Results are consistent with those found previously for Magsat, but an analysis at Orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite. Besides the model, a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset. This has proven very useful in diagnosing the cause of peculiar artefacts in the Magsat vector data, which seem to suggest the presence of a small misalignment bias in the vector magnetometer.

Strength of the geomagnetic field in the Cretaceous Normal Superchron: New data from submarine basaltic glass of the Troodos Ophiolite

Abstract: [1] We present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the Troodos Ophiolite ( similar to 92 Ma), formed during the Cretaceous Normal Superchron (CNS), a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all. Monte Carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength. Our data suggest a dipole strength equivalent to the present field or nearly twice the post-CNS average. The mean and standard deviation of the dipole moment (81 +/- 43 ZAm(2); Z = 10(21)) from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from DSDP Site 522. The new data set for the CNS suggests a picture of a strong and stable field during the period of time when it stopped reversing. Moreover, the similarity of the CNS data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability.

Modeling a space weather event from the Sun to the Earth: CME generation and interplanetary propagation

Abstract: [1] We present a three-dimensional (3-D) numerical ideal magnetohydrodynamics (MHD) model describing the time-dependent expulsion of a coronal mass ejection (CME) from the solar corona propagating to 1 astronomical unit (AU) The simulations are performed using the Block Adaptive Tree Solar-Wind Roe Upwind Scheme (BATS-R-US) code We begin by developing a global steady-state model of the corona that possesses high-latitude coronal holes and a helmet streamer structure with a current sheet at the equator The Archimedean spiral topology of the interplanetary magnetic field is reproduced along with fast and slow speed solar wind Within this model system, we drive a CME to erupt by the introduction of a Gibson-Low magnetic flux rope that is anchored at both ends in the photosphere and embedded in the helmet streamer in an initial state of force imbalance The flux rope rapidly expands and is ejected from the corona with maximum speeds in excess of 1000 km/s Physics-based adaptive mesh refinement (AMR) allows us to capture the structure of the CME focused on a particular Sun-Earth line with high spatial resolution given to the bow shock ahead of the flux rope as well as to the current sheet behind The CME produces a large magnetic cloud at 1 AU (>100 R⊙) in which Bz undergoes a full rotation from north to south with an amplitude of 20 nT In a companion paper, we find that the CME is very effective in generating strong geomagnetic activity at the Earth in two ways First, through the strong sustained southward Bz (lasting more than 10 hours) and, second, by a pressure increase associated with the CME-driven shock that compresses the magnetosphere

On the propagation of bubbles in the geomagnetic tail

Abstract: . Using three-dimensional magnetohydrodynamic simulations, we investigate the propagation of low-entropy magnetic flux tubes ("bubbles") in the magnetotail. Our simulations address fundamental properties of the propagation and dynamics of such flux tubes rather than the actual formation process. We find that the early evolution, after a sudden reduction of pressure and entropy on a localized flux tube, is governed by re-establishing the balance of the total pressure in the dawn-dusk and north-south directions through compression on a time scale less than about 20s for the typical magnetotail. The compression returns the equatorial pressure to its original unperturbed value, due to the fact that the magnetic field contributes only little to the total pressure, while farther away from the equatorial plane the magnetic field compression dominates. As a consequence the pressure is no longer constant along a flux tube. The subsequent evolution is characterized by earthward propagation at speeds of the order of 200-400km/s, depending on the initial amount of depletion and the cross-tail extent of a bubble. Simple acceleration without depletion does not lead to significant earthward propagation. It hence seems that both the entropy reduction and the plasma acceleration play an important role in the generation of fast plasma flows and their propagation into the near tail. Earthward moving bubbles are found to be associated with field-aligned current systems, directed earthward on the dawnward edge and tailward on the duskward edge. This is consistent with current systems attributed to observed bursty bulk flows and their auroral effects. Key words. Magnetospheric physics (magnetospheric configuration and dynamics; magnetotail; plasma sheet)nguage:

Geomagnetic conjugate observations of medium-scale traveling ionospheric disturbances at midlatitude using all-sky airglow imagers

Abstract: [1] We report for the first time simultaneous observations of medium-scale traveling ionospheric disturbances (MSTIDs) at geomagnetic conjugate points in both hemispheres, using two all-sky airglow imagers at midlatitudes. A 630-nm all-sky CCD imager at Sata, Japan, detected MSTIDs with a wavefront elongated from NW to SE on the night of August 9, 2002. During this event, MSTIDs with a wavefront elongated from SW to NE were observed at the geomagnetic conjugate point, Darwin, Australia. To investigate geomagnetic conjugacy of the MSTID structures, the Darwin images were mapped The MSTID structures mapped from Darwin to its magnetic conjugate points along the geomagnetic field lines (B) coincide closely with those in the Sata images. This result suggests that polarization electric field (Ep) plays an important role in the generation of MSTIDs. Ep maps along B and moves the F region plasma upward or downward by E × B drifts, causing plasma density perturbations with structures mirrored in the northern and southern hemispheres.

Extremely high speed solar wind: 29–30 October 2003

Abstract: [1] On 29-30 October 2003 the Solar Wind Electron Proton Alpha Monitor (SWEPAM) instrument on the Advanced Composition Explorer (ACE) spacecraft measured solar wind speeds in excess of 1850 km/s, some of the highest speeds ever directly measured in the solar wind. These speeds were observed following two large coronal mass ejection (CME) driven shocks. Surprisingly, despite the unusually high speeds, many of the other solar wind parameters were not particularly unusual in comparison with other large transient events. The magnetic field reached -68 nT, a large but not unprecedented value. The proton temperatures were significantly higher than typical for a CME in the solar wind at 1 AU (>10 7 K), but the proton densities were moderate, leading to low to moderate proton beta. The solar wind dynamic pressure was not unusual for large events but, when coupled with the large negative B z , was sufficient to cause intense geomagnetic disturbances.

Power requirement of the geodynamo from ohmic losses in numerical and laboratory dynamos

Abstract: In the Earth's fluid outer core, a dynamo process converts thermal and gravitational energy into magnetic energy. The power needed to sustain the geomagnetic field is set by the ohmic losses (dissipation due to electrical resistance). Recent estimates of ohmic losses cover a wide range, from 0.1 to 3.5 TW, or roughly 0.3-10% of the Earth's surface heat flow. The energy requirement of the dynamo puts constraints on the thermal budget and evolution of the core through Earth's history. Here we use a set of numerical dynamo models to derive scaling relations between the core's characteristic dissipation time and the core's magnetic and hydrodynamic Reynolds numbers--dimensionless numbers that measure the ratio of advective transport to magnetic and viscous diffusion, respectively. The ohmic dissipation of the Karlsruhe dynamo experiment supports a simple dependence on the magnetic Reynolds number alone, indicating that flow turbulence in the experiment and in the Earth's core has little influence on its characteristic dissipation time. We use these results to predict moderate ohmic dissipation in the range of 0.2-0.5 TW, which removes the need for strong radioactive heating in the core and allows the age of the solid inner core to exceed 2.5 billion years.

Testing loss mechanisms capable of rapidly depleting relativistic electron flux in the Earth's outer radiation belt

Abstract: [1] We investigate how relativistic electrons are lost from the Earth's magnetosphere in order to better understand the dynamic variability of the radiation belts. We identify 52 events where the >2 MeV electron flux at geostationary orbit decreases rapidly and use a superposed epoch analysis of multispacecraft data to characterize the accompanying solar wind and geomagnetic conditions and examine the relevance of potential loss mechanisms. The results show that the flux decrease events follow a common sequence. The electron flux is reduced first in the dusk sector concurrent with the stretching of the magnetic field to a more tail-like configuration. The extreme stretching at dusk is caused by the formation of a partial ring current driven by changing solar wind conditions. We investigate three possible causes of the ensuing flux decrease: adiabatic electron motion in response to the changing magnetic field topology, drift out the magnetopause boundary, and precipitation into the atmosphere. The analysis reveals that the flux depletion is likely due to enhanced precipitation into the atmosphere, but the exact cause of the enhanced precipitation is still uncertain.

Dependence of the duration of geomagnetic polarity reversals on site latitude.

Abstract: An important constraint on the processes governing the geodynamo—the flow in the outer core responsible for generating Earth's magnetic field—is the duration of geomagnetic polarity reversals; that is, how long it takes for Earth's magnetic field to reverse1. It is generally accepted that Earth's magnetic field strength drops to low levels during polarity reversals, and the field direction progresses through a 180° change while the field is weak1. The time it takes for this process to happen, however, remains uncertain, with estimates ranging from a few thousand up to 28,000 years. Here I present an analysis of the available sediment records of the four most recent polarity reversals. These records yield an average estimate of about 7,000 years for the time it takes for the directional change to occur. The variation about this mean duration is not random, but instead varies with site latitude, with shorter durations observed at low-latitude sites, and longer durations observed at mid- to high-latitude sites. Such variation of duration with site latitude is predicted by simple geometrical reversal models, in which non-dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction, and provides a constraint on numerical dynamo models.

Solar and interplanetary sources of major geomagnetic storms during 1996–2002

Abstract: [1] During the 7-year period of the current solar cycle, 64 geoeffective coronal mass ejections (CMEs) were found to produce major geomagnetic storms (DST < � 100 nT) at the Earth. In this paper we examine solar and interplanetary properties of these geoeffective coronal mass ejections (CMEs). The observations reveal that full-halo CMEs are potential sources of intense geomagnetic activity at the Earth. However, not all fullhalo CMEs give rise to major geomagnetic storms, which complicates the task of space weather forecasting. We examine solar origins of the geoeffective CMEs and their interplanetary effects, namely, solar wind speed, interplanetary shocks, and the southward component of the interplanetary magnetic field, in order to investigate the relationship between the solar and interplanetary parameters. In particular, the present study aims at ascertaining solar parameters that govern important interplanetary parameters responsible for producing major geomagnetic storms. Our investigation shows that fast full-halo CMEs associated with strong flares and originating from a favorable location, i.e., close to the central meridian and low and middle latitudes, are the most potential candidates for producing strong ram pressure at the Earth’s magnetosphere and hence intense geomagnetic storms. The results also show that the intensity of geomagnetic storms depends most strongly on the southward component of the interplanetary magnetic field, followed by the initial speed of the CME and the ram pressure. INDEX TERMS: 7513 Solar Physics, Astrophysics, and Astronomy: Coronal mass ejections; 2784 Magnetospheric Physics: Solar wind/ magnetosphere interactions; 2788 Magnetospheric Physics: Storms and substorms; 2139 Interplanetary Physics: Interplanetary shocks; KEYWORDS: CME, halo CMEs, IP shocks, solar wind, geomagnetic storms, DST index

Auroral ion acceleration in dispersive Alfvén waves

Abstract: [1] Observations from the FAST satellite are used to create a model for dispersive Alfven waves above the auroral oval. Using this model, it is shown how these waves may accelerate ionospheric ions transverse to the geomagnetic field and cause ion outflow. The model waves grow from ionospheric conductivity variations due to auroral electron precipitation and resonate in the cavity between the ionosphere and the peak in the Alfven speed that occurs at altitudes of ∼1 Earth radius (Re). By tracing ions in the model wave field, it is demonstrated that for transverse wave amplitudes (E⊥) satisfying E⊥/Bo Ωi/k⊥ the ion motion may become stochastic allowing acceleration to energies exceeding that prescribed by λ⊥. The transversely accelerated ions in both the coherent and stochastic cases flow upward from the ionosphere under the influence of the mirror force to altitudes of 1 Earth radii over timescales as small as a few seconds to minutes with energies in the keV range. Ions accelerated by these means may account for the intense outflowing ion fluxes observed in Alfven waves above the auroral oval.

A phenomenological study of the long-term cosmic ray modulation, 850-1958 AD

Abstract: [1] The modulation of the galactic cosmic radiation over the past 1150 years is investigated using 10Be data from Greenland and the South Pole. For this purpose, we introduce the use of 22-year averages to study the long-term modulation. After allowance for secular changes in the geomagnetic dipole, it is shown that the 22-year mean intensity of the galactic cosmic radiation (GCR) in the vicinity of 1–2 GeV/nucleon returned to approximately the same high level at the widely separated times of the Oort (1050 AD), Spoerer (1420–1540), and the latter portion of the Maunder (1645–1715) periods of low solar activity. In terms of the modulation potential, ϕ, this asymptotic intensity corresponds to a mean residual modulation of ∼84 MV. The GCR intensity was significantly less during the Wolf (∼1320) and Dalton (1810) minima, and ϕ ∼ 200 MV. The higher temporal resolution data from Greenland shows that there were large 11-year and other fluctuations superimposed upon these high intensities during the Spoerer and Maunder minima (Δϕ ≈ 200–300 MV), indicating the continued presence of a substantial and time-dependent heliomagnetic field. Throughout the Spoerer minimum, the GCR intensity repeatedly returned to a condition of very low modulation, indicating that the cosmic ray spectrum incident on the Earth approached the level of the local interstellar spectrum. These results imply the continued presence of either (or both) (1) the normal cyclic variation of the heliospheric current sheet and/or (2) a cyclic variation of the diffusion coefficients throughout these periods of low solar activity. The data indicate that the modulation (i.e., depression) of the cosmic ray intensity during the instrumental era (1933–present) has been one of the greatest in the past 1150 years. Further, approximately the same low value has been attained on five previous widely separated occasions since 850 AD, and we speculate that the heliospheric magnetic field has reached an asymptotic limit at those times. The 10Be data exhibit a previously unrecognized feature, which we have named “the precipitous decrease,” in which the 1–2 GeV/nucleon intensity decreased by ∼40–45% between 1700 and 1739 corresponding to Δϕ > 500 MV, at a time of low but increasing solar activity. A lower cosmic ray intensity than that attained in 1739 was not observed again until after 1950, at a time of high solar activity. These features and the large 11-year modulation events during the Spoerer and Maunder minima indicate that the long-term variations in the GCR intensity are poorly related to sunspot number during epochs of low solar activity. It is shown that there is better agreement between the variations in the 10Be data, and the changes in the open solar magnetic flux predicted by the Schrijver et al. [2002] and Solanki et al. [2002] models based on historic sunspot numbers. In particular, they both exhibit characteristics consistent with the precipitous decrease in the 10Be data, although the amplitudes are smaller than implied by the 10Be data.

Middle latitude LF (40 kHz) phase variations associated with earthquakes for quiet and disturbed geomagnetic conditions

Abstract: The phase (P) and amplitude (A) anomalies in subionospheric LF signal (40 kHz) along the path Japan–Kamchatka of 2300 km have been studied for the data observed by means of a digital OminiPAL receiver for 2 years. The empirical model of background P and A daily variations for quiet and disturbed geomagnetic conditions in the absence of seismic activity is developed. We pay special attention to the P and A features during large magnetic storms. A sensitivity threshold of LF signal to deforming influence of the geomagnetic and seismic factors is defined. Two cases of bay-like behavior of LF phase and amplitude in nighttime are described as a clear earthquake precursor of LF signal. We have found from the statistical study that LF signal effect is observed only for earthquakes with M⩾5.5 and we discuss the possible mechanisms of the effect.

Morphology of the ring current derived from magnetic field observations

Abstract: . Our examination of the 20 years of magnetospheric magnetic field data from ISEE, AMPTE/CCE and Polar missions has allowed us to quantify how the ring current flows and closes in the magnetosphere at a variety of disturbance levels. Using intercalibrated magnetic field data from the three spacecraft, we are able to construct the statistical magnetic field maps and derive 3-dimensional current density by the simple device of taking the curl of the statistically determined magnetic field. The results show that there are two ring currents, an inner one that flows eastward at ~3 RE and a main westward ring current at ~4–7 RE for all levels of geomagnetic disturbances. In general, the in-situ observations show that the ring current varies as the Dst index decreases, as we would expect it to change. An unexpected result is how asymmetric it is in local time. Some current clearly circles the magnetosphere but much of the energetic plasma stays in the night hemisphere. These energetic particles appear not to be able to readily convect into the dayside magnetosphere. During quiet times, the symmetric and partial ring currents are similar in strength (~0.5MA) and the peak of the westward ring current is close to local midnight. It is the partial ring current that exhibits most drastic intensification as the level of disturbances increases. Under the condition of moderate magnetic storms, the total partial ring current reaches ~3MA, whereas the total symmetric ring current is ~1MA. Thus, the partial ring current contributes dominantly to the decrease in the Dst index. As the ring current strengthens the peak of the partial ring current shifts duskward to the pre-midnight sector. The partial ring current is closed by a meridional current system through the ionosphere, mainly the field-aligned current, which maximizes at local times near the dawn and dusk. The closure currents flow in the sense of region-2 field-aligned currents, downward into the ionosphere near the dusk and upward out of the ionosphere near the dawn. Key words. Magnetospheric physics (current systems; storms and substorms; magnetospheric configuration and dynamics)

Noon-time equatorial electrojet: Its spatial features as determined by the CHAMP satellite

Abstract: [1] New observations obtained by the satellite CHAMP reveal a detailed picture of the noon-time equatorial electrojet (EEJ). The low orbit of CHAMP and its high-precision magnetometers reveal the spatial structure of the EEJ with unprecedented accuracy. Data from more than two and a half years have been used to investigate average features and also the global characteristics of the EEJ. Rather than interpreting the magnetic signatures, we determined the horizontal current distribution by using a very general current model (series of line currents). This makes the results independent of satellite altitude and ambient field geometry. The procedure for determining the structure of the electrojet is fully automated, giving an objective response. Some of the spatial features of the noon-time EEJ are as follows: The electrojet current peaks right at the dip equator. There is no deviation from it either on a seasonal basis or with longitude. The width of the EEJ (≈4° in latitude) at half the peak value of the current density profile is for a given longitude fairly constant, independent of the amplitude. Return currents north and south of the eastward current are a common feature of the EEJ. They peak at latitudes some 5° away from the dip equator. The intensity of the EEJ varies strongly from day to day. The average peak current density exhibits a clear dependence on longitude. Peaks show up over South America and Indonesia. The average current density follows closely the monthly mean of the solar flux index, F10.7. The total EEJ eastward current is about three times as strong as the return current. The total current and the peak current density are related to each other by a power law. This suggests that the longitude dependence of the EEJ intensity can be explained by the varying cross-sectional area of the Cowling channel.

Fast computation of the geoelectric field using the method of elementary current systems and planar Earth models

Abstract: . The method of spherical elementary current systems provides an accurate modelling of the horizontal component of the geomagnetic variation field. The interpolated magnetic field is used as input to calculate the horizontal geoelectric field. We use planar layered (1-D) models of the Earth's conductivity, and assume that the electric field is related to the local magnetic field by the plane wave surface impedance. There are locations in which the conductivity structure can be approximated by a 1-D model, as demonstrated with the measurements of the Baltic Electromagnetic Array Research project. To calculate geomagnetically induced currents (GIC), we need the spatially integrated electric field typically in a length scale of 100km. We show that then the spatial variation of the electric field can be neglected if we use the measured or interpolated magnetic field at the site of interest. In other words, even the simple plane wave model is fairly accurate for GIC purposes. Investigating GIC in the Finnish high-voltage power system and in the natural gas pipeline, we find a good agreement between modelled and measured values, with relative errors less than 30% for large GIC values. Key words. Geomagnetism and paleomagnetism (geomagnetic induction; rapid time variations) – Ionosphere (electric field and currents)

Magnetic storms in October 2003

Abstract: Preliminary results of an analysis of satellite and ground-based measurements during extremely strong magnetic storms at the end of October 2003 are presented, including some numerical modeling. The geosynchronous satellites Ekspress-A2and Ekspress-A3, and the low-altitude polar satellites Coronas-F and Meteor-3M carried out measurements of charged particles (electrons, protons, and ions) of solar and magnetospheric origin in a wide energy range. Disturbances of the geomagnetic field caused by extremely high activity on the Sun were studied at more than twenty magnetic stations from Lovozero (Murmansk region) to Tixie (Sakha-Yakutia). Unique data on the dynamics of the ionosphere, riometric absorption, geomagnetic pulsations, and aurora observations at mid-latitudes are obtained.

ULF geomagnetic field measurements in Japan and some recent results associated with Iwateken Nairiku Hokubu earthquake in 1998

Abstract: Previous studies on Spitak, Loma Prieta, and Guam earthquakes indicate that large earthquakes were accompanied by preceding magnetic anomalies. In order to confirm these facts and to investigate ULF phenomena in details, a network of ULF magnetometers has been installed in Japan. Network observations including small arrays have been carried out. Also, the geomagnetic data observed at Matsukawa station associated with Iwateken Nairiku Hokubu earthquake (M6.1, September 3, 1998, depth 10 km) are presented. 4.5 years data have been analyzed and the obtained result is discussed. The variation of spectral density ratio between the horizontal and vertical components (polarization) exhibits an anomalous behavior two weeks before the earthquake. This is a unique change discovered from the rather long-term analysis, which suggests that this anomalous change might be a possible signal associated with the earthquake preparation phase.

A statistical study of the geoeffectiveness of magnetic clouds during high solar activity years

Abstract: [1] Using the Dst value corrected for the effects of magnetopause currents (Dst*) and solar wind magnetic field and plasma data from 1 January 1998 to 30 April 2002, during elevated solar conditions, we have statistically examined the relationship of 271 storms (Dst* ≤ −30 nT) to 104 magnetic clouds. It is found that most of the magnetic clouds result in geomagnetic storms, but only about 30% of storms are due to magnetic clouds. A storm can be driven by a cloud's various regions or their combinations with dissimilar occurrence percentages. These percentages change as a function of geomagnetic activity levels as well. It is found that the leading field is the most geoeffective region and the sheath region is equally effective at causing magnetic storms during solar maximum (42%) compared to solar minimum (43%) as a percentage of magnetic cloud-induced storms. The occurrence percentage of intense storms caused by clouds is 72%, which is much higher than the ∼20% occurrence percentage of smaller storms caused by clouds. It is also found that “unipolar Bz” and “bipolar Bz” clouds have different geoeffectiveness percentages, depending on the Bz orientation. The long-known control of magnetic activity mainly by southward Bz is supported by the results of this study. It is also shown that multistep development storms can result not only from both the combinations of sheath and cloud fields but also from different fields within a cloud. A new name, quasi-cloud, is proposed for those cloud-like solar wind structures which show evidence of relatively organized field rotations.

Interpretation of Cluster data on chorus emissions using the backward wave oscillator model

Abstract: The measurements of chorus emissions by four closely separated Cluster spacecraft provide important information concerning the chorus generation mechanism. They confirm such properties of the wave source as their strong localization near the equatorial cross section of a magnetic flux tube, an almost parallel average wave-vector direction with respect to the geomagnetic field, and an energy flux direction pointing outward from the generation region. Inside this region, Cluster discovered strong temporal and spatial variations in the amplitude with correlation scale lengths of the order of 100 km across the magnetic flux. The wave electric field reached 30 mV/m, and the maximum growth and damping rates are of the order of a few hundreds of s−1. These and other properties of the detected chorus emissions are discussed here in relation with the backward wave oscillator mechanism. According to this mechanism, a succession of whistler wave packets is generated in a small near-equatorial region with temporal an...

Dependence of plasmaspheric morphology on the electric field description during the recovery phase of the 17 April 2002 magnetic storm

Abstract: [1] A comparison of how well three different electric field models can predict the storm time plasmapause shape is conducted. The magnetic storm of 17 April 2002 is selected for this event, and plasmapause locations are extracted from images from the EUV instrument on the Imager for Magnetopause-to-Aurora Geomagnetic Effects (IMAGE) satellite throughout the main phase and recovery phase of the event. The three electric field descriptions are as follows: the modified McIlwain E5D analytical formula, the Weimer statistical compilation from low-Earth orbit satellite data, and a self-consistent Poisson equation solution for the subauroral potential pattern. It is found that all of the models have certain strengths and weaknesses in predicting the plasmapause location during this storm. The modified McIlwain model did well on the nightside but not on the dayside because the electric fields near noon are too small (analogous to too large of a conductance in the subauroral dayside ionosphere). The Weimer model did well overall, but the resulting plasmapause is usually smaller than the observed one because the electric fields are a bit too strong in the inner magnetosphere (perhaps because of an ionosphere-magnetosphere mapping problem). The self-consistent model is also quite good in general, but the plasmapause in the postmidnight sector was always inward of the observed one. This is because of too low a conductance at the location of the field-aligned currents that close the partial ring current. It is concluded that the latter two models provide a sufficient description of the storm time development of the plasmaspheric morphology during this storm, with the self-consistent model being the best choice. Another conclusion is that plasmapause locations extracted from EUV images should be compared with peak density gradients from model results rather than with any one isocontour of the cold plasma density itself.

A detailed study of the polarity reversal mechanism in a numerical dynamo model

Abstract: [1] We analyze the mechanism of magnetic polarity reversals in a three- dimensional numerical dynamo model A dynamo driven by compositional convection in a rotating spherical fluid shell with a solid, electrically conducting inner core exhibits regular reversals of its dominantly axial dipole magnetic field at Rayleigh number Ra = 300, Ekman number E = 001, Prandtl number Pr = 1 and Roberts number q = 20 The fluid motions that sustain the field include (1) azimuthal jets which generate toroidal magnetic field; (2) high-latitude, helical convective plumes which generate poloidal magnetic field; and (3) meridional circulation which transports the magnetic field Inverse poloidal field is produced locally in the convective plumes Outcrops of reversed field create inverse magnetic flux spots on the core-mantle boundary above the plumes that are precursors to the reversal The dipole polarity change as seen from the surface occurs when the reversed magnetic flux is distributed over the core-mantle boundary by the meridional circulation In our model, the reversed flux is transported from south to north and the transitional field has a strong quadrupole component The duration of the dipole transition is the meridional transport time, and corresponds to a few thousand years in the Earth's core The duration of the stable polarity epochs depends on several effects, including the strengths of the sources of normal and reversed poloidal field in the plumes, flux transport, and flux diffusion Comparable reversal periods are found in an equivalent kinematic dynamo model with steady velocities and without Lorentz forces, confirming that these reversals are not triggered by changes in the flow and are primarily magnetic induction effects

Frequencies of standing Alfvén wave harmonics and their implication for plasma mass distribution along geomagnetic field lines: Statistical analysis of CRRES data

Abstract: [1] The relationship among the frequencies of the harmonics of standing Alfven waves depends on the variation of plasma mass density along the geomagnetic field line. This in turn means that observed standing wave frequencies may be used to infer the mass density variation, which is difficult to measure with particle instruments on spacecraft. Determination of the density variation is important in understanding mass transport processes in the ionosphere-magnetosphere system and also in improving magnetospheric diagnostic techniques using ULF waves. We investigate the frequencies of multiharmonic toroidal standing Alfven waves detected in the electric and magnetic fields measured by the Combined Release and Radiation Effects Satellite (CRRES). The data cover the entire CRRES mission period from July 1990 to October 1991. Using a semi-automated procedure, we identify over 4000 samples of the fundamental toroidal frequency (f1), which are often accompanied by the second (f2) and third (f3) harmonics. Most (∼3000) fundamental frequency samples are taken at dipole L shells from 4 to 8 and at magnetic local time (MLT) from 1200 to 1800, and we perform statistical analyses of the frequencies in this L-MLT domain. The most frequently observed ratios are f2/f1 ∼ 2.5 and f3/f1 ∼ 4.0 for 4 ≤ L < 6 and f2/f1 ∼ 2.8 and f3/f1 ∼ 4.3 for 6 ≤ L < 7. These observations are compared with the theoretical ratios obtained for the density variation of the form ρ = ρeq(LRE/R)α, where ρeq is the equatorial mass density, L is the magnetic shell parameter, R is geocentric distance to the field line, and the power law density index α is a free parameter. We find that α ∼ 0.5 fits the average observed frequency ratios at 4 ≤ L < 6, consistent with a diffusive equilibrium solution. No single value of α fits the average observed frequency ratios at 6 ≤ L < 7. In that case, theoretical solutions indicate that the mass density is locally peaked at the equator; that is, the mass density decreases as one moves off-equator, then increases again toward the ionosphere. Combined with the results of recent studies of electron density (which have not found such a peak in density at the magnetic equator), this indicates that heavy ions are preferentially concentrated at the magnetic equator.