Showing papers on "Earth's magnetic field published in 2006"
Jet Propulsion Laboratory1, Kyoto University2, National Institute for Space Research3, Goddard Space Flight Center4, Rutherford Appleton Laboratory5, Nagoya University6, Kanazawa University7, National Center for Atmospheric Research8, University of Alberta9, University of California, Los Angeles10, University of Bergen11, Max Planck Society12
TL;DR: In this paper, it was shown that geomagnetic storms associated with high-speed streams/CIRs will have the same initial, main, and recovery phases as those associated with ICME-related magnetic storms but that the interplanetary causes are considerably different.
Abstract: [1] Solar wind fast streams emanating from solar coronal holes cause recurrent, moderate intensity geomagnetic activity at Earth. Intense magnetic field regions called Corotating Interaction Regions or CIRs are created by the interaction of fast streams with upstream slow streams. Because of the highly oscillatory nature of the GSM magnetic field z component within CIRs, the resultant magnetic storms are typically only weak to moderate in intensity. CIR-generated magnetic storm main phases of intensity Dst < −100 nT (major storms) are rare. The elongated storm “recovery” phases which are characterized by continuous AE activity that can last for up to 27 days (a solar rotation) are caused by nonlinear Alfven waves within the high streams proper. Magnetic reconnection associated with the southward (GSM) components of the Alfven waves is the solar wind energy transfer mechanism. The acceleration of relativistic electrons occurs during these magnetic storm “recovery” phases. The magnetic reconnection associated with the Alfven waves cause continuous, shallow injections of plasma sheet plasma into the magnetosphere. The asymmetric plasma is unstable to wave (chorus and other modes) growth, a feature central to many theories of electron acceleration. It is noted that the continuous AE activity is not a series of substorm expansion phases. Arguments are also presented why these AE activity intervals are not convection bays. The auroras during these continuous AE activity intervals are less intense than substorm auroras and are global (both dayside and nightside) in nature. Owing to the continuous nature of this activity, it is possible that there is greater average energy input into the magnetosphere/ionosphere system during far declining phases of the solar cycle compared with those during solar maximum. The discontinuities and magnetic decreases (MDs) associated with interplanetary Alfven waves may be important for geomagnetic activity. In conclusion, it will be shown that geomagnetic storms associated with high-speed streams/CIRs will have the same initial, main, and “recovery” phases as those associated with ICME-related magnetic storms but that the interplanetary causes are considerably different.
585 citations
TL;DR: The Swarm mission as mentioned in this paper was selected as the 5th mission in ESA's Earth Explorer Programme in 2004 and is scheduled for launch in 2010, with two spacecraft flying sideby-side at lower altitude (450 km initial altitude), and the third one flying at higher altitude (530 km).
Abstract: The Swarm mission was selected as the 5th mission in ESA’s Earth Explorer Programme in 2004. The mission will provide the best ever survey of the geomagnetic field and its temporal evolution that will lead to new insights into the Earth system by improving our understanding of the Earth’s interior and its effect on Geospace, the vast region around the Earth where electrodynamic processes are influenced by the Earth’s magnetic field. Scheduled for launch in 2010, the mission will comprise a constellation of three satellites, with two spacecraft flying sideby- side at lower altitude (450 km initial altitude), thereby measuring the East-West gradient of the magnetic field, and the third one flying at higher altitude (530 km). High-precision and high-resolution measurements of the strength, direction and variation of the magnetic field, complemented by precise navigation, accelerometer and electric field measurements, will provide the necessary observations that are required to separate and model the various sources of the geomagnetic field. This results in a unique “view” inside the Earth from space to study the composition and processes of its interior. It also allows analysing the Sun’s influence within the Earth system. In addition practical applications in many different areas, such as space weather, radiation hazards, navigation and resource management, will benefit from the Swarm concept.
446 citations
TL;DR: Desilets et al. as discussed by the authors reported new measurements of nucleon fluxes from altitude transects in Hawaii (RC=12.8) and Bangalore, India (RC = 17.3 ) to confirm that nucleon scaling functions are energy-dependent in the range of energies at which cosmogenic nuclides are produced.
280 citations
TL;DR: In this paper, the solar activity in terms of the heliospheric modulation function was quantitatively reconstructed in high resolution throughout the Holocene by using the 10Be data from the Greenland Ice Core Project ice core as well as 14C support a high current solar activity.
Abstract: [1] Cosmogenic radionuclides extracted from ice cores hold a unique potential for reconstructing past solar activity changes beyond the direct instrumental period. Taking the geomagnetic modulation into account, the solar activity in terms of the heliospheric modulation function can quantitatively be reconstructed in high resolution throughout the Holocene. For this period our results reveal changes in heliospheric modulation of galactic cosmic rays significantly larger than the variations reconstructed on the basis of neutron monitor measurements of galactic cosmic rays for the last 50 years. Moreover, the 10Be data from the Greenland Ice Core Project ice core as well as 14C support a high current solar activity. However, although the reconstruction of solar activity on long timescales is difficult, our result suggests that the modern activity state of the Sun is not that exceptional regarding the entire Holocene. This extended solar activity record provides the basis for further detailed investigations on solar and cosmic ray physics, as well as on solar forcing of the Earth's climate whose importance is suggested by increasing paleoclimatic evidences.
268 citations
TL;DR: In this paper, the authors derived a model of the near-Earth magnetic field (up to spherical harmonic degree n = 50 for the static field, and up to n = 18 for the first time derivative) using more than 6.5 yr of high-precision geomagnetic measurements from the three satellites Orsted, CHAMP and SAC-C taken between 1999 March and 2005 December.
Abstract: SUMMARY We have derived a model of the near-Earth magnetic field (up to spherical harmonic degree n = 50 for the static field, and up to n = 18 for the first time derivative) using more than 6.5 yr of high-precision geomagnetic measurements from the three satellites Orsted, CHAMP and SAC-C taken between 1999 March and 2005 December. Our modelling approach goes in several aspects beyond that used for recent models: (i) we use different data selection criteria and allow for higher geomagnetic activity (index Kp ≤ 2o), thus we include more data than previous models; (ii) we describe the temporal variation of the core field by splines (for n ≤ 14); (iii) we take magnetometer vector data in the instrument frame and co-estimate the Euler angles that describe the transformation from the magnetometer frame to the star imager frame, avoiding the inconsistency of using vector data that have been aligned using a different (pre-existing) field model; (iv) we account for the bending of the CHAMP optical bench connecting magnetometer and star imager by estimating Euler angles in 10 day segments and (v) we co-estimate degree-1 external fields separately for every 12 hr interval. The model provides a reliable representation of the static (core and crustal) field up to spherical harmonic degree n = 40, and of the first time derivative up to n = 15.
245 citations
TL;DR: In this paper, the authors derived the global distribution of magnetic signatures of postsunset equatorial spread F (ESF) events from a continuous database covering the years 2001-2004.
Abstract: [1] Magnetic observations on board the CHAMP satellite are used for the first comprehensive study of magnetic signatures of the postsunset equatorial spread F (ESF) events. This is derived from a continuous database covering the years 2001–2004. On the basis of an extended survey, the global distribution of magnetic signatures is derived. We find a distinct seasonal/longitudinal variation of the occurrence rate of magnetic signatures that is consistent with that obtained from previous satellite observations of plasma depletions. The latitudinal distribution of the ESF magnetic signatures from CHAMP is symmetrical about the dip equator. It can be approximated by two Gaussian curves peaking at ±9.5° magnetic latitude, both exhibiting an 1σ-width of 4.5°. We further find a close relation between the occurrence frequency and the solar EUV flux. The global average of the occurrence rate is linearly proportional to solar activity attaining ∼0.1% times the F10.7 value. The response of the ESF magnetic signatures to geomagnetic activity is also investigated. However, only a weak relation between the signature occurrence rate and the Kp index is found. Using high-resolution magnetic field measurements of the ESF structures, we are able to identify very small spatial scales of spread F of only few tens of meters. The vector magnetic field observations provide experimental evidence of the electromagnetic characteristics of ESF, valuable for testing model predictions. Finally, we discuss the effect of the ESF phenomenon on magnetic field modeling efforts based on satellite data.
214 citations
TL;DR: In this article, the authors used Sheffield University Plasmasphere Ionosphere Model (SUPIM) with an intensified westward disturbed electric field in the presunrise hours, presumably due to prompt penetration from the magnetosphere, in order to study the effect of such a field in ionosphere.
Abstract: [1] The solar events that occurred at the end of October 2003 gave rise to very strong geomagnetic disturbances that peaked twice with Dst values reaching −345 nT around 0000 UT on 30 October and −400 nT around 2300 UT, on the same day. Disturbances in several ionospheric parameters were observed over Brazil. This work will focus on the ionospheric response to the initial westward prompt penetration electric field and on the strong intensification of the equatorial ionization anomaly that occurred because of the electric field polarity reversal that followed in the early morning hours of 29 October. The F layer peak height over the equator first decreased under the strong prompt penetration westward electric field, which was followed by significant height increase under eastward electric field. We have used Sheffield University Plasmasphere Ionosphere Model (SUPIM) with an intensified westward disturbed electric field in the presunrise hours, presumably due to prompt penetration from the magnetosphere, in order to study the effect of such a field in the ionosphere. The simulation results showed that prompt penetration of magnetospheric electric fields of westward polarity to the nightside equatorial region seems to be the most probable cause of the initial F layer height decreases. The intensification of the equatorial ionization anomaly and the unusual enhancement on F layer peak density, which was not modeled by the SUPIM, are explained as caused by the strong eastward electric field that followed the initial phase in combination with a highly variable disturbed meridional/transequatorial wind system as inferred from the F2 layer peak height variations. The highly dynamic wind pattern, with a short-term response (2–4 hours), is compatible with the predictions of some previous theoretical model calculations reported in the literature.
183 citations
TL;DR: Efforts to reconstruct Proterozoic supercontinents are strengthened by this demonstration of a consistently axial and dipolar geomagnetic reference frame, which itself implies stability of geodynamo processes on billion-year timescales.
Abstract: Palaeomagnetism of climatically sensitive sedimentary rock types, such as glacial deposits and evaporites, can test the uniformitarianism of ancient geomagnetic fields and palaeoclimate zones. Proterozoic glacial deposits laid down in near-equatorial palaeomagnetic latitudes can be explained by 'snowball Earth' episodes, high orbital obliquity or markedly non-uniformitarian geomagnetic fields. Here I present a global palaeomagnetic compilation of the Earth's entire basin-scale evaporite record. Magnetic inclinations are consistent with low orbital obliquity and a geocentric-axial-dipole magnetic field for most of the past two billion years, and the snowball Earth hypothesis accordingly remains the most viable model for low-latitude Proterozoic ice ages. Efforts to reconstruct Proterozoic supercontinents are strengthened by this demonstration of a consistently axial and dipolar geomagnetic reference frame, which itself implies stability of geodynamo processes on billion-year timescales.
172 citations
TL;DR: In this article, the authors used a global numerical model of the interaction of the solar wind and the interplanetary magnetic field with Earth's magnetosphere to study the formation process of Flux Transfer Events (FTEs) during strong southward IMF.
Abstract: . We use a global numerical model of the interaction of the solar wind and the interplanetary magnetic field with Earth's magnetosphere to study the formation process of Flux Transfer Events (FTEs) during strong southward IMF. We find that: (i) The model produces essentially all observational features expected for FTEs, in particular the bipolar signature of the magnetic field BN component, the correct polarity, duration, and intermittency of that bipolar signature, strong core fields and enhanced core pressure, and flow enhancements; (ii) FTEs only develop for large dipole tilt whereas in the case of no dipole tilt steady magnetic reconnection occurs at the dayside magnetopause; (iii) the basic process by which FTEs are produced is the sequential generation of new X-lines which makes dayside reconnection inherently time dependent and leads to a modified form of dual or multiple X-line reconnection; (iv) the FTE generation process in this model is not dependent on specific assumptions about microscopic processes; (v) the average period of FTEs can be explained by simple geometric arguments involving magnetosheath convection; (vi) FTEs do not develop in the model if the numerical resolution is too coarse leading to too much numerical diffusion; and (vii) FTEs for nearly southward IMF and large dipole tilt, i.e., near solstice, should only develop in the winter hemisphere, which provides a testable prediction of seasonal modulation. The semiannual modulation of intermittent FTE reconnection versus steady reconnection is also expected to modulate magnetospheric and ionospheric convection and may thus contribute to the semiannual variation of geomagnetic activity.
168 citations
TL;DR: It was found that magnetic field strength was nearly constant throughout this time, in contrast to the later period, which saw a fall in strength in patches of reverse magnetic flux in the Southern Hemisphere.
Abstract: Earth's magnetic field has decayed by about 5% per century since measurements began in 1840 Directional measurements predate those of intensity by more than 250 years, and we combined the global model of directions with paleomagnetic intensity measurements to estimate the fall in strength for this earlier period (1590 to 1840 AD) We found that magnetic field strength was nearly constant throughout this time, in contrast to the later period Extrapolating to the core surface showed that the fall in strength originated in patches of reverse magnetic flux in the Southern Hemisphere These patches were detectable by directional data alone; the pre-1840 model showed little or no evidence of them, supporting the conclusion of a steady dipole up to 1840
163 citations
Goddard Space Flight Center1, University of Maryland, College Park2, Boston College3, George Mason University4, National Oceanic and Atmospheric Administration5, Massachusetts Institute of Technology6, Los Alamos National Laboratory7, University of Alabama in Huntsville8, Royal Observatory of Belgium9, Moscow State University10
TL;DR: In this article, the authors compare the geomagnetic effects of CIR-associated storms with those of 159 other CIRs observed during 1996-2005, and find that the major storms form the extreme tail of a continuous distribution of the CIR geoeffectiveness which peaks at Dst ∼ −40 nT but is subject to a prominent seasonal variation of ∼ 40 nT which is ordered by the spring and fall equinoxes and the solar wind magnetic field direction toward or away from the Sun.
Abstract: [1] Seventy-nine major geomagnetic storms (minimum Dst ≤ −100 nT) observed in 1996 to 2004 were the focus of a “Living with a Star” Coordinated Data Analysis Workshop (CDAW) in March 2005. In nine cases, the storm driver appears to have been purely a corotating interaction region (CIR) without any contribution from coronal mass ejection-related material (interplanetary coronal mass ejections (ICMEs)). These storms were generated by structures within CIRs located both before and/or after the stream interface that included persistently southward magnetic fields for intervals of several hours. We compare their geomagnetic effects with those of 159 CIRs observed during 1996–2005. The major storms form the extreme tail of a continuous distribution of CIR geoeffectiveness which peaks at Dst ∼ −40 nT but is subject to a prominent seasonal variation of ∼40 nT which is ordered by the spring and fall equinoxes and the solar wind magnetic field direction toward or away from the Sun. The O'Brien and McPherron (2000) equations, which estimate Dst by integrating the incident solar wind electric field and incorporating a ring current loss term, largely account for the variation in storm size. They tend to underestimate the size of the larger CIR-associated storms by Dst ∼ 20 nT. This suggests that injection into the ring current may be more efficient than expected in such storms. Four of the nine major storms in 1996–2004 occurred during a period of less than three solar rotations in September to November 2002, also the time of maximum mean IMF and solar magnetic field intensity during the current solar cycle. The maximum CIR-storm strength found in our sample of events, plus additional 23 probable CIR-associated Dst ≤ −100 nT storms in 1972–1995, is (Dst = −161 nT). This is consistent with the maximum storm strength (Dst ∼ −180 nT) expected from the O'Brien and McPherron equations for the typical range of solar wind electric fields associated with CIRs. This suggests that CIRs alone are unlikely to generate geomagnetic storms that exceed these levels.
TL;DR: In this article, the effect of variations of geomagnetic activity on human cardiovascular health has been investigated, and three con- clusions have been established: adverse health effects are more pronounced at higher magnetic lat- itudes, extremely high and extremely low values seem to have adverse health effect, and a subset of the population (10-15%) is predisposed to adverse health due to geOMagnetic variations, while anthropogenic sources of electric and magnetic fields are also briefly dis- cussed.
Abstract: The possibility that conditions on the Sun and in the Earth's magneto- sphere can affect human health at the Earth's surface has been debated for many decades. This work reviews the research undertaken in the field of heliobiology, focusing on the effect of variations of geomagnetic activity on human cardiovascular health. Data from previous research are analysed for their statistical significance, resulting in support for some studies and the undermining of others. Three con- clusions are that geomagnetic effects are more pronounced at higher magnetic lat- itudes, that extremely high as well as extremely low values of geomagnetic activity seem to have adverse health effects and that a subset of the population (10-15%) is predisposed to adverse health due to geomagnetic variations. The reported health effects of anthropogenic sources of electric and magnetic fields are also briefly dis- cussed, as research performed in this area could help to explain the results from studies into natural electric and magnetic field interactions with the human
TL;DR: In this paper, the authors studied the climatology of the equatorial zonal wind in the upper thermosphere and found that the solar flux significantly influences both the daytime and nighttime winds.
Abstract: Using 3 years (2002–2004), over 16,400 orbits of measurements from the accelerometer on board the CHAMP satellite, we have studied the climatology of the equatorial zonal wind in the upper thermosphere. Several main features are noticed. The most prominent one is that the solar flux significantly influences both the daytime and nighttime winds. It overrides the geomagnetic activity effect, which is found to be rather limited to the nightside. An elevation of the solar flux level from F10.7 ? 100 × 10?22 W m?2 Hz?1 to F10.7 ? 190 × 10?22 W m?2 Hz?1 produces an eastward disturbance wind up to ?110 m s?1. This consequently enhances the nighttime eastward wind but suppresses the daytime westward wind. A seasonal variation with weaker wind (by over 50 m s?1 at night) around June solstice than in other seasons has been observed regardless of solar flux and geomagnetic activity levels. The zonal wind is eastward throughout the night except around June solstice, where it ebbs to almost zero or turns even westward in the postmidnight sector at low solar flux level. The daytime wind is found to be generally more stable than the nighttime wind, particularly unresponsive to geomagnetic activities. Predictions from the Horizontal Wind Model find good agreement with the CHAMP?observed wind at high solar flux levels during nighttime. At low solar flux levels, however, the model strongly underestimates the westward wind during morning hours by 50–120 m s?1 depending on season. The major difference between the HWM?predicted and the CHAMP?observed wind is seen in the phase of its diurnal variation. The CHAMP?observed wind turns eastward around 1200–1300 MLT instead of 1600–1700 MLT predicted by the model. Comparisons with ground FPI observations and the NCAR Thermosphere?Ionosphere?Electrodynamics General Circulation Model (TIEGCM) predictions show that the solar flux effect obtained from CHAMP is consistent with that modeled by TIEGCM. The solar flux dependence of zonal wind found here together with that of the zonal ion drift found in previous studies reflect the relative importance of the E? and F?region wind dynamo in the thermosphere?ionosphere coupling process. Furthermore, these wind measurements indicate that the Earth's atmosphere superrotates. The average superrotation speed amounts to about 22 m s?1 for a solar flux level of F10.7 ? 100 × 10?22 W m?2 Hz?1 but increases to 63 m s?1 for F10.7 ? 190 × 10?22 W m?2 Hz?1. Finally, the wind behavior presented in this study is longitudinally averaged and may differ from wind measurements at a certain longitude.
TL;DR: In this paper, the authors used the Thellier double-heating method to determine the geomagnetic field strength of the last 160 million years of the Cretaceous Normal Polarity Superchron.
Abstract: [1] The basic features of the geomagnetic reversal chronology of the last 160 million years are well established. The relationship between this history and other features of the field, however, has been elusive. The determination of past field strength (paleointensity) is especially challenging. Commonly accepted results have come from analyses of bulk samples of lava. Historic lavas have been shown to faithfully record the past field strength when analyzed using the Thellier double-heating method. Data from older lavas, however, tend to show effects of in situ and laboratory-induced alteration. Here we review an alternative approach. Single plagioclase crystals can contain minute magnetic inclusions, 50–350 nm in size, that are potential high-fidelity field recorders. Thellier experiments using plagioclase feldspars from an historic lava on Hawaii provide a benchmark for the method. Rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples. This resistance is likely related to the lack of clays. In addition, magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well-defined thermoremanent magnetization into an irresolute chemical remanent magnetization. If there is a relationship between geomagnetic reversal frequency and paleointensity, it should be best expressed during superchrons, intervals with few (or no) reversals. Thellier data sets based on single plagioclase crystals from lavas erupted during the Cretaceous Normal Polarity Superchron (∼83–120 million years ago) suggest a strong (>12 × 1022 Am2), stable field, consistent with an inverse relationship between reversal frequency and paleointensity. Superchrons may represent times when the pattern of core-mantle boundary heat flux allows the geodynamo to operate at peak efficiency, as suggested in some numerical models. Thellier data from single plagioclase crystals formed during times of moderate ( 10 reversals/million years) reversal occurrence suggest a weaker and more variable field. These paleointensity data, together with a consideration of paleomagnetic directions, suggest that geomagnetic reversals, field morphology, secular variation, and intensity are related. The linkages over tens of millions of years imply a lower mantle control on the geodynamo. On even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the Proterozoic and Archean geomagnetic field during the onset of growth of the solid inner core. Data from plagioclase crystals separated from mafic dikes, together with directional data from whole rocks, indicate a dipole-dominated field similar to that of the modern, 2.5–2.7 billion years ago. Older Archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow. Most rocks of this age have been affected by low-grade metamorphism; investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo. Absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization, anisotropy, and slow cooling. Fortunately, prior work in rock magnetism, together with advances in analytical equipment and techniques, provides a solid foundation from which these frontier issues can be approached.
TL;DR: The dipole moment is stronger today than its long time average, indicating that polarity reversal is not likely unless the current episode of moment decrease continues for a thousand years or more.
Abstract: The dipole moment of Earth's magnetic field has decreased by nearly 9% over the past 150 years and by about 30% over the past 2,000 years according to archeomagnetic measurements. Here, we explore the causes and the implications of this rapid change. Maps of the geomagnetic field on the core-mantle boundary derived from ground-based and satellite measurements reveal that most of the present episode of dipole moment decrease originates in the southern hemisphere. Weakening and equatorward advection of normal polarity magnetic field by the core flow, combined with proliferation and growth of regions where the magnetic polarity is reversed, are reducing the dipole moment on the core-mantle boundary. Growth of these reversed flux regions has occurred over the past century or longer and is associated with the expansion of the South Atlantic Anomaly, a low-intensity region in the geomagnetic field that presents a radiation hazard at satellite altitudes. We address the speculation that the present episode of dipole moment decrease is a precursor to the next geomagnetic polarity reversal. The paleomag- netic record contains a broad spectrum of dipole moment fluctuations with polarity reversals typically occurring during dipole moment lows. However, the dipole moment is stronger today than its long time average, indicating that polarity reversal is not likely unless the current episode of moment decrease continues for a thousand years or more.
TL;DR: The Potsdam Magnetic Model of Earth (POMME) as discussed by the authors is a geomagnetic field model providing an estimate of the Earth's core, crustal, magnetospheric, and induced magnetic fields.
Abstract: The Potsdam Magnetic Model of the Earth (POMME) is a geomagnetic field model providing an estimate of the Earth's core, crustal, magnetospheric, and induced magnetic fields. The internal field is represented to spherical harmonic (SH) degree 90, while the secular variation and acceleration are given to SH degree 16. Static and time-varying magnetospheric fields are parameterized in Geocentric Solar-Magnetospheric (GSM) and Solar-Magnetic (SM) coordinates and include Disturbance Storm-Time (Dst index) and Interplanetary Magnetic Field (IMF-By) dependent contributions. The model was estimated from five years of CHAMP satellite magnetic data. All measurements were corrected for ocean tidal induction and night-side ionospheric F-region currents. The model is validated using an independent model from a combined data set of Orsted and SAC-C satellite measurements. For the core field to SH degree 13, the root mean square (RMS) vector difference between the two models at the center of the model period is smaller than 4 nT at the Earth's surface. The RMS uncertainty increases to about 100 nT for the predicted field in 2010, as inferred from the difference between the two models.
TL;DR: In this article, the authors examined the field line distribution of magnetospheric electron density and mass density and found that the electron density distributions from IMAGE RPI active sounding are generally monotonic.
Abstract: [1] This paper examines the field line distribution of magnetospheric electron density and mass density. The electron density distributions from IMAGE RPI active sounding are generally monotonic. The density increases with increasing MLAT slightly faster than the dependence found from the field line dependence model of Denton et al. (2002b); in general, a power law dependence ne = ne0 (LRE/R)α with α ∼ 1 appears to be appropriate within the plasmasphere, at least for geocentric radius R > 2 RE. Our comparison to RPI data included also one field line distribution at LT = 7.4, which we fit with α = 2.5, a value typical of the plasmatrough based on previous studies. We calculated the average electron density field line distribution at low MLAT using the CRRES plasma wave data and found that the density was relatively flat near the magnetic equator with no convincing evidence for an equatorial peak. Using the average values of toroidal Afven frequencies, we calculated the mass density field line distributions and found that they were roughly monotonic for LT < 6, with α = 2 appropriate for LT = 4–5 and α = 1 appropriate for LT = 5–6. At LT = 6–8, the distribution was nonmonotonic, with a local peak in mass density at the magnetic equator. Dividing the frequency data into different groups based on activity, we found that the inferred average mass density field line dependence was insensitive to geomagnetic activity at LT = 4–6 but that at LT = 6–8, the tendency for the mass density to be peaked at the magnetic equator increased with respect to larger Alfven wave amplitude and more negative Dst. The average frequency ratios at LT = 6–8 did not change if we limited the data to cases with MLT = 8–16, for which the assumed perfect conductor boundary condition was better justified. Taken together, these results imply that heavy ions are preferentially peaked at the magnetic equator for LT = 6–8, at least during more geomagnetically active periods.
22 Aug 2006
TL;DR: The geomagnetic activity index aa can be split into two components: one associated with solar flares, prominence eruptions, and coronal mass ejections which follows the solar activity cycle and a second component associated with recurrent high speed solar wind streams which is out of phase with the solar cycle.
Abstract: The level of geomagnetic activity near the time of solar activity minimum has been shown to be a reliable indicator for the amplitude of the following solar activity maximum. The geomagnetic activity index aa can be split into two components: one associated with solar flares, prominence eruptions, and coronal mass ejections which follows the solar activity cycle and a second component associated with recurrent high speed solar wind streams which is out of phase with the solar activity cycle. This second component often peaks before solar activity minimum and has been one of the most reliable indicators for the amplitude of the following maximum. The size of the recent maximum in this second component indicates that solar activity cycle 24 will be much higher than average - similar in size to cycles 21 and 22 with a peak smoothed sunspot number of 160 plus or minus 25.
TL;DR: In this paper, it was shown that the occurrence rate of calm intervals before recurring high-speed-stream-driven storms is substantially greater than random occurrence probability would predict, even though the properties of the wind that drives calms before storms are statistically similar to the properties that drives quiet intervals during solar minimum.
Abstract: [1] Intervals of extreme geomagnetic calm just prior to recurring high-speed-stream-driven storms are noted and studied. These calm intervals may be important for preconditioning the magnetosphere for the ensuing storms. It is argued that this preconditioning causes (1) a decay of the number density of relativistic electrons in the outer radiation belt, (2) a mass loading of the convection of the middle magnetosphere during the early phase of the storms, (3) dumping of the outer electron radiation belt by the formation of a plasmaspheric drainage plume at the onset of the storm, and (4) a contribution to the inner plasma sheet and ring current by the convection of cool dense plasma into the dipole. Calm intervals tend to occur when uncompressed slow wind passes the Earth immediately prior to the CIR. Using 73 years of geomagnetic data, it is shown that the occurrence rate of calms before recurring storms is substantially greater than random occurrence probability would predict, even though the properties of the wind that drives calms before storms are statistically similar to the properties of the wind that drives quiet intervals during solar minimum. The recurrence of storms and calms are studied and it is found that both tend to recur with a 27-day period (strongest during declining phase) and that the persistence of calm recurrence (∼3 solar rotations) is greater than the persistence of storm recurrence (∼2 solar rotations). An argument is given as to why there is a tendency for calms to occur just prior to high-speed-stream-driven storms.
TL;DR: In this article, the authors developed the fundamental equations required in the reconstructions, both for isotropic pressure and, in Appendix A, for the case where the pressures parallel and perpendicular to the magnetic field are described by the double-polytropic laws.
Abstract: [1] Grad-Shafranov reconstruction is a data analysis tool for the reconstruction of two-dimensional (2-D) coherent field and flow structures from data collected as the structures move past an observing platform. To date, the method has been applied with good success to reconstruct magnetohydrostatic structures in Earth's magnetopause, in the solar wind, and in the geomagnetic tail, as the structures move past one or more observing spacecraft. However, with suitable modification, the reconstruction method can be extended to other applications, three of which are presented here: 2-D magneto-hydrodynamic structures in which dynamically important field-aligned flow is present, 2-D flow transverse to the magnetic field in the magnetospheric low-latitude boundary layer, and 2-D ordinary gasdynamic/hydrodynamic flow. We develop the fundamental equations required in the reconstructions, both for isotropic pressure and, in Appendix A, for the case where the pressures parallel and perpendicular to the magnetic field are described by the double-polytropic laws. Applications to actual or simulated data are discussed but not included in the present paper.
TL;DR: In this paper, the geoeffectiveness of CIRs, as measured by the geomagnetic Dst index, is assessed during the solar wind observational period 1964-2003.
Abstract: [1] Corotating interaction regions (CIRs) are structures formed when high-speed solar wind streams overtake slow solar wind streams as they propagate outward. These structures produce regions of enhanced density and magnetic field strength in the solar wind near the ecliptic plane. In this paper, the geoeffectiveness of CIRs, as measured by the geomagnetic Dst index, is assessed during the solar wind observational period 1964–2003. A catalogue of CIRs is constructed by consulting high-speed plasma streams (HSPS) lists present in the literature and by analyzing solar wind parameters for each HSPS event. The geoeffectiveness of CIRs is analyzed by determining the number of intense (Dst ≤ −100 nT), moderate (−100 < Dst ≤ −50 nT), or weak (−50 < Dst ≤ −30 nT) magnetic storms that followed each CIR event. Statistical distributions of CIR parameters (maximum solar wind speed, maximum convection electric field, southward magnetic field peak) and geoeffectiveness (Dst peak) are obtained. Correlation analyses of Dst index with various solar wind parameters are presented. A comparison with the geoeffectiveness of other interplanetary structures such as shocks, magnetic clouds, and sector boundaries is performed. Our results show that 33% of CIRs are followed by moderate/intense magnetic activity (Dst < −50 nT), i.e., approximately one third of the CIR events observed near Earth are geoeffective.
TL;DR: In this article, the authors reported new archeointensity results from Iranian and Syrian archeological excavations dated from the second millennium BC, and constructed a rather detailed geomagnetic field intensity variation curve for this region from 3000 BC to 0 BC.
TL;DR: In this paper, it was shown that plasmaspheric drainage plumes flowing into the dayside reconnection site mass load the reconnection rate and thereby reduce the coupling of the solar wind to the Earth's magnetosphere.
Abstract: Evidence is uncovered that plasmaspheric drainage plumes flowing into the dayside reconnection site mass load the reconnection rate and thereby reduce the coupling of the solar wind to the Earth's magnetosphere. Solar-wind/magnetosphere coupling is statistically analyzed with the 1963–2003 OMNI2 data set matched up with the AE, AU, and PCI geomagnetic indices. Times when plasmaspheric drainage plumes are flowing are discerned using multiple spacecraft with plasma detectors in geosynchronous orbit. It is found that for a given value of −vBz of the solar wind, the geomagnetic indices AE, AU, and PCI are statistically lower when plasmaspheric drainage plumes are present than when plumes are not present. This is taken as a measure of a weakened coupling of the solar wind to the Earth's magnetosphere caused by the plumes. An Addendum examines the effects of polar-cap saturation on the auroral electrojet index.
TL;DR: In this paper, the authors derived the fourth generation lithospheric field model, which is expanded to spherical harmonic degree and order 90, and fit and remove remaining external fields and polar electrojet signatures in a track-by-track scheme.
Abstract: SUMMARY The CHAMP magnetic field mission is providing highly reliable measurements from which the global lithospheric magnetic field can be determined in unprecedented resolution and accuracy. Using almost 5 yr of data, we derive our fourth generation lithospheric field model termed MF4, which is expanded to spherical harmonic degree and order 90. After subtracting from the full magnetic field observations predicted fields from an internal field model up to degree 15, an external field model up to degree two, and the predicted magnetic field signatures for the eight dominant ocean tidal constituents, we fit and remove remaining external fields and polar electrojet signatures in a track-by-track scheme. From a subset of least disturbed tracks, we estimate the MF4 model by least squares, damping ill-determined coefficients by regularization. The resulting MF4 model provides a good representation of the lithospheric field down to an altitude of about 50 km at lower latitudes, with reduced accuracy in the polar regions. Crustal features come out significantly sharper than in previous models. In particular, bands of magnetic anomalies along subduction zones become visible by satellite for the first time.
TL;DR: Appropriate shifts in the direction of orientation, which compensate for the simulated displacements, have been observed in newts, birds, sea turtles, and lobsters, and provide the strongest evidence to date for magnetic map navigation.
Abstract: The magnetic map hypothesis proposes that animals can use spatial gradients in the Earth's magnetic field to help determine geographic location. This ability would permit true navigation--reaching a goal from an entirely unfamiliar site with no goal-emanating cues to assist. It is a highly contentious hypothesis since the geomagnetic field fluctuates in time and spatial gradients may be disturbed by geological anomalies. Nevertheless, a substantial body of evidence offers support for the hypothesis. Much of the evidence has been indirect in nature, such as the identification of avian magnetoreceptor mechanisms with functional properties that are consistent with those of a putative map detector or the patterns of orientation of animals exposed to temporal and/or spatial geomagnetic anomalies. However; the most important advances have been made in conducting direct tests of the magnetic map hypothesis by exposing experienced migrants to specific geomagnetic values representing simulated displacements. Appropriate shifts in the direction of orientation, which compensate for the simulated displacements, have been observed in newts, birds, sea turtles, and lobsters, and provide the strongest evidence to date for magnetic map navigation. Careful experimental design and interpretation of orientation data will be essential in the future to determine which components of the magnetic field are used to derive geographic position.
TL;DR: In this paper, a novel type of resonant magnetic field sensor exploiting the Lorentz force and providing a frequency output is presented, where the mechanical resonator, a cantilever structure, is embedded as the frequency-determining element in an electrical oscillator.
Abstract: This paper presents a novel type of resonant magnetic field sensor exploiting the Lorentz force and providing a frequency output. The mechanical resonator, a cantilever structure, is embedded as the frequency-determining element in an electrical oscillator. By generating an electrical current proportional to the position of the cantilever, a Lorentz force acting like an additional equivalent spring is exerted on the cantilever in the presence of a magnetic field. Thus, the oscillation frequency of the system, which is a function of the resonator's equivalent spring constant, is modulated by the magnetic field to be measured. The resonant magnetic field sensor is fabricated using an industrial CMOS process, followed by a two-mask micromachining sequence to release the cantilever structure. The characterized devices show a sensitivity of 60 kHz/Tesla at their resonance frequency f0 =175 kHz and a short-term frequency stability of 0.025 Hz, which corresponds to a resolution below 1 muT. The devices can thus be used for Earth magnetic field applications, such as an electronic compass. The novel resonant magnetic field sensor benefits from an efficient continuous offset cancellation technique, which consist in evaluating the frequency difference measured with and without excitation current as output signal
TL;DR: In this article, the authors present four new records of the Iceland Basin Excursion (IBE) from rapidly deposited marine sediments in the North Atlantic Ocean, the Nordic Seas, the Gulf of Mexico, the South China Sea, and the southern Indian Ocean.
Abstract: We present four new records of the Iceland Basin Excursion (IBE) and five new records of the Laschamp Excursion (LE) obtained from rapidly deposited marine sediments in the North Atlantic Ocean, the Nordic Seas, the Gulf of Mexico, the South China Sea, and the southern Indian Ocean. Marked minima in relative paleointensity correspond with the paleomagnetic directional changes associated with all of the excursion records. The virtual geomagnetic pole (VGP) paths of the four IBE records are all similar. The VGPs move southward over Europe and Africa, reaching the southern hemisphere (three reach Antarctica), and then move to more eastern longitudes before returning northward over Australia and east Asia, describing a large counterclockwise loop. The same VGP pattern is observed in other published records. The VGP paths observed for the LE are similar to those of the IBE; however, they loop clockwise instead of counterclockwise. Despite the different sense of looping, the marked similarity among the paths for the two excursions suggests that a similar, relatively simple geometry dominated the transitional field during both the IBE and the LE. Similar dynamo mechanisms must therefore have been active in the Earth's core for both excursions. The duration of the excursions is estimated at <2,000 years, which supports the suggestion that a difference exists between the mechanisms for excursions and reversals. However, the coincidence of the longitudinal bands for VGPs associated with excursions compared to some reversal paths could also indicate an inherent link between the mechanisms for reversals and excursions.
TL;DR: In this paper, the 2-day wave event during the period from 1 December 2002 to 28 February 2003 was identified in the neutral winds by radar measurements located at four tropical stations.
Abstract: [1] Vertical coupling in the low-latitude atmosphere-ionosphere system driven by the 2-day wave in the tropical MLT region has been investigated. The problem is studied from an observational point of view. Three different types of data were analyzed in order to detect and extract the 2-day wave signals. The 2-day wave event during the period from 1 December 2002 to 28 February 2003 was identified in the neutral winds by radar measurements located at four tropical stations. The 2-day variations in the ionospheric electric currents (registered by perturbations in the geomagnetic field) and in the F-region electron densities were detected in the data from 23 magnetometer and seven ionosonde stations situated at low latitudes. Two features for each kind of wave were investigated in detail: the variation with time of the wave amplitude and the zonal wave number. The results show that the westward propagating global 2-day wave with zonal wave number 2 seen in the ionospheric currents and in F-region plasma is forced by the simultaneous 2-day wave activity in the MLT region. The main forcing agent in this atmosphere-ionosphere coupling seems to be the modulated tides, particularly the semidiurnal tide. This tide has a larger vertical wavelength than the diurnal tide and propagates well into the thermosphere. The parameter that appears to be affected, and thus drives the observed 2-day wave response of the ionosphere, is the dynamo electric field.
TL;DR: In this article, a numerical parametric study of 3D spherical dynamos is performed in order to understand the role of inertial effects in the evolution of the simulated dynamo.
Abstract: SUMMARY
A numerical parametric study of 3-D spherical dynamos is performed in order to understand the role of inertial effects in the evolution of the simulated dynamo. We vary the Prandtl (Pr) and magnetic Prandtl (Pm) numbers together, maintaining a constant ratio of thermal to magnetic diffusivities and leaving other parameters fixed. For Pr=Pm≥ 1, we find that the solution is only weakly dependent on Pr=Pm, and the principal force balance is between the magnetic, buoyancy and Coriolis forces (MAC balance). At lower values of Pr and Pm, the inertial forces begin to gain importance, and the MAC balance is disturbed. The field becomes less dipolar and weaker, with the effect that a balance between the buoyancy, Coriolis and inertial forces ensues. The low inertia, large Pr=Pm solutions resemble the geomagnetic field more closely, but there are still a few systematic differences between these solutions and the Earth's magnetic field.
TL;DR: In this article, the authors estimate the ocean magnetic signals of steady flow using a global 3-D EM numerical solution and find significant seasonal variability of ocean magnetic signal in the Indian and Western Pacific Oceans.
Abstract: Conducting ocean water, as it flows through the Earth’s magnetic field, generates secondary electric and magnetic fields. An assessment of the ocean-generated magnetic fields and their detectability may be of importance for geomagnetism and oceanography. Motivated by the clear identification of ocean tidal signatures in the CHAMP magnetic field data we estimate the ocean magnetic signals of steady flow using a global 3-D EM numerical solution. The required velocity data are from the ECCO ocean circulation experiment and alternatively from the OCCAM model for higher resolution. We assume an Earth’s conductivity model with a surface thin shell of variable conductance with a realistic 1D mantle underneath. Simulations using both models predict an amplitude range of ± 2n T atSwarm altitude (430 km). However at sea level, the higher resolution simulation predicts a higher strength of the magnetic field, as compared to the ECCO simulation. Besides the expected signatures of the global circulation patterns, we find significant seasonal variability of ocean magnetic signals in the Indian and Western Pacific Oceans. Compared to seasonal variation, interannual variations produce weaker signals.