Showing papers on "Earth's magnetic field published in 2005"

Modeling the dynamics of the inner magnetosphere during strong geomagnetic storms

Abstract: [1] This work builds on and extends our previous effort (Tsyganenko et al, 2003) to develop a dynamical model of the storm-time geomagnetic field in the inner magnetosphere, using space magnetometer data taken during 37 major events in 1996–2000 and concurrent observations of the solar wind and interplanetary magnetic field (IMF) The essence of the approach is to derive from the data the temporal variation of all major current systems contributing to the distant geomagnetic field during the entire storm cycle, using a simple model of their growth and decay Each principal source of the external magnetic field (magnetopause, cross-tail current sheet, axisymmetric and partial ring currents, and Birkeland current systems) is driven by a separate variable, calculated as a time integral of a combination of geoeffective parameters NλVβBsγ, where N, V, and Bs are the solar wind density, speed, and the magnitude of the southward component of the IMF, respectively In this approach we assume that each source has its individual relaxation timescale and residual quiet-time strength, and its partial contribution to the total field depends on the entire history of the external driving of the magnetosphere during a storm In addition, the magnitudes of the principal field sources were assumed to saturate during extremely large storms with abnormally strong external driving All the parameters of the model field sources, including their magnitudes, geometrical characteristics, solar wind/IMF driving functions, decay timescales, and saturation thresholds, were treated as free variables, and their values were derived from the data As an independent consistency test, we calculated the expected Dst variation on the basis of the model output at Earth's surface and compared it with the actual observed Dst A good agreement (cumulative correlation coefficient R = 092) was found, in spite of the fact that ∼90% of the spacecraft data used in the fitting were taken at synchronous orbit and beyond, while only 37% of those data came from distances 25 ≤ R ≤ 4 RE The obtained results demonstrate the possibility to develop a truly dynamical model of the magnetic field, based on magnetospheric and interplanetary data and allowing one to reproduce and forecast the entire process of a geomagnetic storm, as it unfolds in time and space

Modeling the Sun’s Magnetic Field and Irradiance since 1713

Abstract: We use a flux transport model to simulate the evolution of the Sun's total and open magnetic flux over the last 26 solar cycles (1713-1996). Polar field reversals are maintained by varying the meridional flow speed between 11 and 20 m s-1, with the poleward-directed surface flow being slower during low-amplitude cycles. If the strengths of the active regions are fixed but their numbers are taken to be proportional to the cycle amplitude, the open flux is found to scale approximately as the square root of the cycle amplitude. However, the scaling becomes linear if the number of active regions per cycle is fixed but their average strength is taken to be proportional to the cycle amplitude. Even with the inclusion of a secularly varying ephemeral region background, the increase in the total photospheric flux between the Maunder minimum and the end of solar cycle 21 is at most ~one-third of its minimum-to-maximum variation during the latter cycle. The simulations are compared with geomagnetic activity and cosmogenic isotope records and are used to derive a new reconstruction of total solar irradiance (TSI). The increase in cycle-averaged TSI since the Maunder minimum is estimated to be ~1 W m-2. Because the diffusive decay rate accelerates as the average spacing between active regions decreases, the photospheric magnetic flux and facular brightness grow more slowly than the sunspot number and TSI saturates during the highest amplitude cycles.

Dayside global ionospheric response to the major interplanetary events of October 29–30, 2003 “Halloween Storms”

Abstract: We demonstrate extreme ionospheric response to the large interplanetary electric fields during the "Halloween" storms that occurred on October 29 and 30, 2003. Within a few (2 - 5) hours of the time when the enhanced interplanetary electric field impinged on the magnetopause, dayside total electron content increases of approx.40% and approx.250% are observed for the October 29 and 30 events, respectively. During the Oct 30 event, approx.900% increases in electron content above the CHAMP satellite (approx.400 km altitude) were observed at mid-latitudes (+/-30 degrees geomagnetic). The geomagnetic storm-time phenomenon of prompt penetration electric fields is a possible contributing cause of these electron content increases, producing dayside ionospheric uplift combined with equatorial plasma diffusion along magnetic field lines to higher latitudes, creating a "daytime super-fountain" effect.

Geomagnetic dipole strength and reversal rate over the past two million years

Abstract: Independent records of relative magnetic palaeointensity from sediment cores in different areas of the world can be stacked together to extract the evolution of the geomagnetic dipole moment1,2 and thus provide information regarding the processes governing the geodynamo. So far, this procedure has been limited to the past 800,000 years (800 kyr; ref. 3), which does not include any geomagnetic reversals. Here we present a composite curve that shows the evolution of the dipole moment during the past two million years. This reconstruction is in good agreement with the absolute dipole moments derived from volcanic lavas, which were used for calibration. We show that, at least during this period, the time-averaged field was higher during periods without reversals but the amplitude of the short-term oscillations remained the same. As a consequence, few intervals of very low intensity, and thus fewer instabilities, are expected during periods with a strong average dipole moment, whereas more excursions and reversals are expected during periods of weak field intensity. We also observe that the axial dipole begins to decay 60–80 kyr before reversals, but rebuilds itself in the opposite direction in only a few thousand years.

Dayside global ionospheric response to the major interplanetary events of October 29-30, 2003 Halloween Storms : Violent sun-earth connection events of October-November 2003

Abstract: We demonstrate extreme ionospheric response to the large interplanetary electric fields during the Halloween storms that occurred on October 29 and 30, 2003 Within a few (2-5) hours of the time when the enhanced interplanetary electric field impinged on the magnetopause, dayside total electron content increases of ∼40% and ∼250% are observed for the October 29 and 30 events, respectively During the Oct 30 event, ∼900% increases in electron content above the CHAMP satellite (∼400 km altitude) were observed at mid-latitudes (±30 degrees geomagnetic) The geomagnetic storm-time phenomenon of prompt penetration electric fields is a possible contributing cause of these electron content increases, producing dayside ionospheric uplift combined with equatorial plasma diffusion along magnetic field lines to higher latitudes, creating a daytime super-fountain effect

Timescale for MeV electron microburst loss during geomagnetic storms

Abstract: [1] Energetic electrons in the outer radiation belt can resonate with intense bursts of whistler-mode chorus emission leading to microburst precipitation into the atmosphere. The timescale for removal of outer zone MeV electrons during the main phase of the October 1998 magnetic storm has been computed by comparing the rate of microburst loss observed on SAMPEX with trapped flux levels observed on Polar. Effective lifetimes are comparable to a day and are relatively independent of L shell. The lifetimes have also been evaluated by theoretical calculations based on quasi-linear scattering by field-aligned waves. Agreement with the observations requires average wide-band wave amplitudes comparable to 100 pT, which is consistent with the intensity of chorus emissions observed under active conditions. MeV electron scattering is most efficient during first-order cyclotron resonance with chorus emissions at geomagnetic latitudes above 30 degrees. Consequently, the zone of MeV microbursts tends to maximize in the prenoon (0400–1200 MLT) sector, since nightside chorus is more strongly confined to the equator.

Continuous geomagnetic field models for the past 7 millennia: 2. CALS7K

Abstract: [1] We present two continuous global geomagnetic field models for recent millennia: CALS3K.2, covering the past 3000 years, and CALS7K.2, covering 7000 years from 5000 BC to 1950 AD. The models were determined by regularized least squares inversion of archeomagnetic and paleomagnetic data using spherical harmonics in space and cubic B splines in time. They are derived from a greatly increased number of paleomagnetic directional data, compared to previous efforts, and for the first time a significant amount of archeointensity data is used in this kind of global model, allowing the determination of evolution of geomagnetic dipole strength. While data accuracy and dating uncertainties remain a limitation, reliable low-resolution global models can be obtained. The results agree well with previous results from virtual axial dipole moment (VADM) studies from archeomagnetic intensity data apart from a systematic offset in strength. A comparison of model predictions with the previous 3000 year model, CALS3K.1, gives general agreement but also some significant differences particularly for the early epochs. The new models suggest that the prominent two northern hemisphere flux lobes are more stationary than CALS3K.1 implied, extending considerably the time span of stationary flux lobes observed in historical models. Between 5000 BC and 2000 BC there are time intervals of weak dipole moment where dipole power is exceeded by low-degree nondipole power at the core-mantle boundary. Model coefficients and evaluation code can be obtained from the EarthRef Digital Archive (ERDA) together with animations and snapshots plots for every 100 years at http://www.earthref.org. Detailed URLs for the different material are listed in Appendix A.

Longitudinal structure of the equatorial anomaly in the nighttime ionosphere observed by IMAGE/FUV

Abstract: [1] The Far Ultraviolet Imager (FUV) on board the IMAGE satellite provides an instantaneous global view of the OI 135.6-nm nightglow with 2 min time resolution. Because the OI 135.6-nm emission from the nighttime ionosphere is determined by the line-of-sight integrated plasma density, the nightglow images are useful for studying the nighttime low-latitude ionosphere globally. With the IMAGE/FUV 135.6-nm observations from March to June 2002, we have examined the global characteristics of the nighttime equatorial anomaly (EA) by constructing a constant local time map (LT map), in which pixels within an assigned local time range are extracted from the IMAGE/FUV nightglow images obtained over an observation period of 3 days or more and are put together to compose a global distribution map of emission intensities at that local time. These LT maps show that the development of the EA has a significant longitudinal structure, in which peaks and dips of the crest emission intensity and the crest latitude have about 90° longitudinal separation in the longitude range from 0° to 250°. Although there is not enough data over the American sector, this result suggests that the EA longitudinal structure has a prominent zonal component of the wave number 4. The observed longitudinal structure of the nighttime EA could not be fully explained by factors such as the empirical electric field and neutral wind models, the geomagnetic declination angle, or the displacement of the geomagnetic equator from the geographic equator. To explain the observed longitudinal structure of the EA, in particular, the wave number 4 feature, we may need to consider other forcing, for example, nonmigrating tide originated from the lower atmosphere.

A Geologic Time Scale 2004: The geomagnetic polarity time scale

Abstract: The patterns of marine magnetic anomalies for the Late Cretaceous through Neogene (C-sequence) and Late Jurassic through Early Cretaceous (M-sequence) have been calibrated by magnetostratigraphic studies to biostratigraphy, cyclostratigraphy, and a few radiometrically dated levels. The geomagnetic polarity time scale for the past 160 myr has been constructed by fitting these constraints and a selected model for spreading rates. The status of the geomagnetic polarity time scale for each geological period is summarized in Chapters 11–22 as appropriate. PRINCIPLES Magnetic field reversals and magnetostratigraphy The principal goal of magnetostratigraphy is to document and calibrate the global geomagnetic polarity sequence in stratified rocks and to apply this geomagnetic polarity time scale for high-resolution correlation of marine magnetic anomalies and of polarity zones in other sections. The basis of magnetostratigraphy is the retention by rocks of a magnetic imprint acquired in the geomagnetic field that existed when the sedimentary rock was deposited or the igneous rock underwent cooling. The imprint most useful for paleomagnetic directions and magnetostratigraphy is recorded by particles of iron oxide minerals. Most of the material in this chapter is updated from summaries in Harland et al . (1990) and Ogg (1995). Excellent reviews are given in Opdyke and Channell (1996) for magnetostratigraphy and McElhinny and McFadden (2000) for general paleomagnetism.

Global thermospheric neutral density and wind response to the severe 2003 geomagnetic storms from CHAMP accelerometer data

Abstract: [1] Measurements of atmospheric density near 410 km from the STAR accelerometer on the CHAMP satellite are used to illustrate the spatial-temporal dependence of the thermospheric response to the severe solar storms occurring during 29 October to 1 November 2003. This interval includes periods of elevated magnetic activity with KP values of 5–9, as well as undisturbed intervals that serve to define quiet time baseline densities. Measurements are available from −87° to +87° latitude during both day and night at local times near 1300 and 0100 hours, respectively. During times of maximum geomagnetic activity for this study, density measurements exhibit enhancements of 200–300%. Northern Hemisphere daytime responses are much larger than in the Southern Hemisphere; the origins of this effect are unknown. Nighttime density disturbances more readily propagate to equatorial latitudes, possibly facilitated by the predominant equatorward flow in both hemispheres due to the diurnal tides driven by in situ EUV heating. The CHAMP density measurements are compared with density predictions from the NRL-MSISe00 empirical density model and demonstrate some model shortcomings. Measurements of cross-track accelerations provide the opportunity to estimate zonal winds from the equator to about ±60° latitude, transitioning to a measure of purely meridional winds at the turning point of the orbit near ±87° latitude. A periodic variation in cross-track winds with an apparent period of 24 hours appears at high latitudes and exhibits similar amplitudes and temporal-latitudinal structures to the empirical HWM-93 wind model when projected into the cross-track direction. This periodicity is due to the displacement of geomagnetic and geographic coordinates. At low latitudes, CHAMP and HWM-93 both yield westward winds of order 100 ms−1 during midday under quiet magnetic conditions; however, during severely disturbed periods the HWM-93 winds generally show a greater westward intensification (to 250 ms−1) than the CHAMP measurements. At night, CHAMP winds are near zero under quiet conditions whereas HWM-93 indicates eastward winds of order 50–100 ms−1. Under disturbed conditions the CHAMP winds shift to westward values of order 200 to 250 ms−1, while HMW-93 values do not exceed about 50 ms−1 in the westward direction. The physical origins of the observed effects are difficult to isolate, and unequivocal interpretation will require sophisticated numerical modeling taking into account self-consistent interactions between the neutral winds, drifts, and ionization densities.

Properties and geoeffectiveness of magnetic clouds in the rising, maximum and early declining phases of solar cycle 23

Abstract: The magnetic structure and geomagnetic response of 73 magnetic clouds (MC) observed by the WIND and ACE satellites in solar cycle 23 are examined. The results have been compared with the surveys from the previous solar cy- cles. The preselected candidate MC events were investigated using the minimum variance analysis to determine if they have a flux-rope structure and to obtain the estimation for the axial orientation ( C, C ). Depending on the calculated inclination relative to the ecliptic we divided MCs into "bipo- lar" ( C 45 ). The number of ob- served MCs was largest in the early rising phase, although the halo CME rate was still low. It is likely that near solar maximum we did not identify all MCs at 1 AU, as they were crossed far from the axis or they had interacted strongly with the ambient solar wind or with other CMEs. The occurrence rate of MCs at 1 AU is also modified by the migration of the filament sites on the Sun towards the poles near solar maxi- mum and by the deflection of CMEs towards the equator due to the fast solar wind flow from large polar coronal holes near solar minimum. In the rising phase nearly all bipolar MCs were associated with the rotation of the magnetic field from the south at the leading edge to the north at the trailing edge. The results for solar cycles 21-22 showed that the direction of the magnetic field in the leading portion of the MC starts to reverse at solar maximum. At solar maximum and in the declining phase (2000-2003) we observed several MCs with the rotation from the north to the south. We observed unipo- lar (i.e. highly inclined) MCs frequently during the whole investigated period. For solar cycles 21-22 the majority of MCs identified in the rising phase were bipolar while in the declining phase most MCs were unipolar. The geomagnetic response of a given MC depends greatly on its magnetic structure and the orientation of the sheath fields. For each event we distinguished the effect of the sheath fields and the MC fields. All unipolar MCs with magnetic field southward at the axis were geoeffective (Dst< 50 nT) while those with the field pointing northward did not cause magnetic storms at all. About half of the all identified MCs were not geoffective or the sheath fields preceding the MC caused the storm. MCs caused more intense magnetic storms (Dst< 100 nT) than moderate magnetic storms ( 50 nT Dst 100 nT).

Earth Observation with CHAMP Results from Three Years in Orbit

Abstract: Orbit and Earth Gravity Field.- Ice Mass Balance and Antarctic Gravity Change: Satellite and Terrestrial Perspectives.- Gravity Model TUM-2Sp Based on the Energy Balance Approach and Kinematic CHAMP Orbits.- On the Contribution of CHAMP to Temporal Gravity Field Variation Studies.- Earth Gravity Field and Seasonal Variability from CHAMP.- Comparison of Superconducting Gravimeter and CHAMP Satellite Derived Temporal Gravity Variations.- Improvements in Arctic Gravity and Geoid from CHAMP and GRACE: An Evaluation.- Evaluation of Gravity Data by EIGEN-2 (CHAMP-only) Model in China.- Energy Balance Relations for Validation of Gravity Field Models and Orbit Determinations Applied to the CHAMP Mission.- Evaluation of Terrestrial Gravity Data by Independent Global Gravity Field Models.- Recent Developments in CHAMP Orbit Determination at GFZ.- On Calibrating the CHAMP On-Board Accelerometer and Attitude Quaternion Processing.- Evaluation of the CHAMP Accelerometer on Two Years of Mission.- A New Method to Detect and Estimate CHAMP Clock Bias Change Cycle Slip.- Comparison of Different Stochastic Orbit Modeling Techniques.- Determination of Non-Conservative Accelerations from Orbit Analysis.- CHAMP and Resonances.- CHAMP Gravity Field Solutions and Geophysical Constraint Studies.- Application of Eigenvalue Decomposition in the Parallel Computation of a CHAMP 100x100 Gravity Field.- Time-Variable Gravity Seen by Satellite Missions: On its Sampling and its Parametrization.- Gravity Field Recovery by Analysis of Short Arcs of CHAMP.- Statistical Assessment of CHAMP Data and Models Using the Energy Balance Approach.- Multiscale Geopotential Solutions from CHAMP Orbits and Accelerometry.- Multiscale Modeling from EIGEN-1S, EIGEN-2, EIGEN-GRACE01S, UCPH2002_0.5, EGM96.- A Comparison of Various Procedures for Global Gravity Field Recovery from CHAMP Orbits.- Precise Orbit Determination for CHAMP Using an Efficient Kinematic and Reduced-Dynamic Procedure.- On Bias and Scale and Thrust Factors for CHAMP Accelerometry.- CHAMP Accelerometer Preprocessing at GeoForschungsZentrum Potsdam.- CHAMP Clock Characterization Revisited.- How Baltic Sea Water Mass Variations Mask the Postglacial Rebound Signal in CHAMP and GRACE Gravity Field Solutions.- The Impact of the New CHAMP and GRACE Gravity Models on the Measurement of the General Relativistic Lense-Thirring Effect.- Recovery of Isostatic Topography over North America from Topographic and CHAMP Gravity Correlations.- Dynamic Topography as Reflected in the Global Gravity Field.- Impact of the CHAMP Mission on Estimating the Mean Sea Surface.- Improved Estimates of the Oceanic Circulation Using the CHAMP Geoid.- Contemporary Changes in the Geoid About Greenland: Predictions Relevant to Gravity Space Missions.- Mantle Viscosity and S-Wave to Density Conversion Profiles using CHAMP Geoid Data.- Regional Geoid Undulations from CHAMP, Represented by Locally Supported Basis Functions.- Earth Magnetic Field.- Ionospheric Plasma Effects for Geomagnetic LEO Missions at Mid- and Low-Latitudes.- Interpretation of CHAMP Crustal Field Anomaly Maps Using Geographical Information System (GIS) Technique.- Magnetic Crustal Thickness in Greenland from CHAMP and Orsted Data.- CHAMP Magnetic Anomalies of the Antarctic Crust.- Magnetic Petrology Database for Interpretation Satellite Magnetic Anomalies.- Balloon Geomagnetic Survey at Stratospheric Altitudes.- Effect of Varying Crustal Thickness on CHAMP Geopotential Data.- Reliability of CHAMP Anomaly Continuations.- Introducing POMME, the POtsdam Magnetic Model of the Earth.- Alternative Parameterisations of the External Magnetic Field and its Induced Counterpart for 2001 and 2002 Using Orsted, Champ and Observatory Data.- New Insight into Secular Variation Between MAGSAT and CHAMP/ORSTED.- Time Structure of the 1991 Magnetic Jerk in the Core-Mantle Boundary Zone by Inverting Global Magnetic Data Supported by Satellite Measurements.- Use of Champ Magnetic Data to Improve the Antarctic Geomagnetic Reference Model.- Secular Variation of the Geomagnetic Field from Satellite Data.- The Spectrum of the Magnetic Secular Variation.- Geomagnetic Induction Modeling Based on CHAMP Magnetic Vector Data.- Electromagnetic Induction by Sq Ionospheric Currents in a Heterogeneous Earth: Modeling Using Ground-based and Satellite Measurements.- Wavelet Analysis of CHAMP Flux Gate Magnetometer Data.- Modelling the Ocean Effect of Geomagnetic Storms at Ground and Satellite Altitude.- 3-D Modelling of the Magnetic Fields Due to Ocean Tidal Flow.- The Enhancement of the Thermospheric Density During the Sept. 25-26, 2001 Magnetic Storm.- On the Modelling of Field-Aligned Currents from Magnetic Observations by Polar Orbiting Satellites.- The Low-Altitude Cusp: Multi-Point Observations During the February 2002 SIRCUS Campaign.- Detection of Intense Fine-Scale Field-Aligned Current Structures in the Cusp Region.- A Comparative Study of Geomagnetic Pi2 Pulsations Observed by CHAMP and on the Ground.- ULF Wave Magnetic Measurements by CHAMP Satellite and SEGMA Ground Magnetometer Array: Case Study of July 6, 2002.- Classes of the Equatorial Electrojet.- The ESPERIA Project: a Mission to Investigate the near-Earth Space.- Status of the CHAMP ME Data Processing.- Neutral Atmosphere and Ionosphere.- Atmospheric and Ocean Sensing with GNSS.- Amplitude Variations in CHAMP Radio Occultation Signal as an Indicator of the Ionospheric Activity.- About the Potential of GPS Radio Occultation Measurements for Exploring the Ionosphere.- Validation of GPS Ionospheric Radio Occultation results onboard CHAMP by Vertical Sounding Observations in Europe.- Ionospheric Tomography with GPS Data from CHAMP and SAC-C.- Topside Plasma Scale Height Modelling Based on CHAMP Measurements: First Results.- Differential Code Bias of GPS Receivers in Low Earth Orbit: An Assessment for CHAMP and SAC-C.- Ionosphere/Plasmasphere Imaging Based on GPS Navigation Measurements from CHAMP and SAC-C.- Three-Dimensional Monitoring of the Polar Ionosphere with Ground- and Space-Based GPS.- Comparison of Electron Density Profiles from CHAMP Data with NeQuick Model.- Model for Short-term Atmospheric Density Variations.- Atmospheric Profiling with CHAMP: Status of the Operational Data Analysis, Validation of the Recent Data Products and Future Prospects.- Simulated Temperature and Water Vapor Retrieval from Bending Angles and Refractivity Measurements using an Optimal Estimation Approach.- An Analysis of the Lower Tropospheric Refractivity Bias by Heuristic Sliding Spectral Methods.- Diffractive Integrals for Bistatic Remote Sensing Using GPS Signals.- Canonical Transform Methods for Analysis of Radio Occultations.- GPS Radio Occultation with CHAMP: Comparison of Atmospheric Profiles from GFZ Potsdam and IGAM Graz.- Evaluation of Stratospheric Radio Occultation Retrieval Using Data from CHAMP, MIPAS, GOMOS, and ECMWF Analysis Fields.- Derivation of the Water Vapor Content from the GNSS Radio Occultation Observations.- Processing of CHAMP Radio Occultation Data Using GRAS SAF Software.- Gravity Wave "Portrait" Reconstructed by Radio Holographic Analysis of the Amplitude of GPS Radio Occultation Signals.- Global Analysis of Stratospheric Gravity Wave Activity Using CHAMP Radio Occultation Temperatures.- Tropical Tropopause Characteristics from CHAMP.- Comparisons of MIPAS/ENVISAT and GPS-RO/CHAMP Temperatures.- Comparison of GPS/SAC-C and MIPAS/ENVISAT Temperature Profiles and Its Possible Implementation for EOS MLS Observations.- Structure and Variability of the Tropopause Obtained from CHAMP Radio Occultation Temperature Profiles.- An Assessment of an Ionospheric GPS Data Assimilation Process.- The Continuous Wavelet Transform, a Valuable Analysis Tool to Detect Atmospheric and Ionospheric Signatures in GPS Radio Occultation Phase Delay Data.- The CHAMP Atmospheric Processing System for Radio Occultation Measurements.- Potential Contribution of CHAMP Occultation to Pressure Field Improvement for Gravity Recovery.- Analysis of Gravity Wave Variability from SAC-C and CHAMP Occultation Profiles between June 2001 and March 2003.- The CHAMPCLIM Project: An Overview.

Global distribution of the thermospheric total mass density derived from CHAMP

Abstract: [1] A global distribution of the thermospheric total mass density at 400 km altitude is derived from the high-accuracy accelerometer on board the CHAMP satellite with good temporal and spatial coverage. It shows two interesting features. One is the anomalous distribution at low latitudes. Instead of maximizing at the dayside equator, the thermospheric density shows maxima at about 20°–25° geomagnetic latitude on both sides of the equator between 10 and 20 magnetic local time. This latitudinal distribution resembles fairly well the equatorial ionization anomaly, thus indicating strong magnetic control of the thermospheric mass density via ionosphere-thermosphere coupling. The thermospheric density shows a secondary maximum at the nightside equator shortly before midnight, in reminiscence of the well-known thermospheric midnight temperature maximum. Another feature to notice is that the thermospheric density is highly structured at high latitudes, with localized density enhancements possibly related to Joule/particle heating. This structure is, however, different from the cellular structure recognized by the National Center for Atmospheric Research thermosphere general circulation model at altitudes of 120–300 km. This indicates that the exact cellular structure may not necessarily extend to 400 km. A comparison between observations and the Mass Spectrometer Incoherent Scatter 1990 (MSIS90) model predictions shows that although the model describes the general structure of the observed density reasonably well, it misses the double peaks at low latitudes completely. This causes an underestimation of the total mass density by 15–20% in the crest region. At high latitudes an underestimation of 20–30% occurs in the midnight sector and the cusp region. Outside these localized areas the agreement between observations and predictions is quite good, with only ∼5% difference on average under quiet geomagnetic conditions.

Magnetic compass orientation of migratory birds in the presence of a 1.315 MHz oscillating field

Abstract: The radical pair model of magnetoreception predicts that magnetic compass orientation can be disrupted by high frequency magnetic fields in the Megahertz range. European robins, Erithacus rubecula, were tested under monochromatic 565 nm green light in 1.315 MHz fields of 0.48 μT during spring and autumn migration, with 1.315 MHz being the frequency that matches the energetic splitting induced by the local geomagnetic field. The birds’ responses depended on the alignment of the oscillating field with respect to the static geomagnetic field: when the 1.315 MHz field was aligned parallel with the field lines, birds significantly preferred northerly directions in spring and southerly directions in autumn. These preferences reflect normal migratory orientation, with the variance slightly increased compared to control tests in the geomagnetic field alone or to tests in a 7.0 MHz field. However, in the 1.315 MHz field aligned at a 24° angle to the field lines, the birds were disoriented in both seasons, indicating that the high frequency field interfered with magnetoreception. These finding are in agreement with theoretical predictions and support the assumption of a radical-pair mechanism underlying the processes mediating magnetic compass information in birds.

An overview of the impulsive geomagnetic field disturbances and power grid impacts associated with the violent Sun‐Earth connection events of 29–31 October 2003 and a comparative evaluation with other contemporary storms

Abstract: [1] An overview is provided of the geomagnetic storms and impacts on electric power grids associated with the violent Sun-Earth events of October 2003 During the period from 29 to 31 October 2003, two large geomagnetic storms were observed, as measured by periods of high Kp, Ap, and Dst indices In fact, these storms had Ap rankings of 6th and 16th all time This ranking would suggest that the October 2003 storms would be significant with regard to geomagnetically induced currents (GIC) in power grids However, the resulting geomagnetic storms were much lower in delta B and dB/dt intensity than other historically large geomagnetic storms A variety of geomagnetic storm processes drove observed GIC For example, ground observations indicated the presence of large dB/dt pulsations and GIC at North American midlatitude locations on 29 October 2003 that may be due to unusually intense Kelvin-Helmholtz shearing Sustained disturbance conditions at low-latitude and equatorial latitude locations that are likely linked to ring current intensifications may be the source of sustained GIC at these locations and the cause of large power transformer failures Comparative evaluations will be provided for the 29–31 October 2003 storms and other important and contemporary storms, such as those observed on 13–14 March 1989, 13–14 July 1982, and 15–16 July 2001 Rather than an index-based evaluation method, the comparative evaluations presented in this paper are based on comparisons of storm morphology This approach provides a more meaningful comparison of geomagnetic field disturbance dynamics that are important to characterize large GIC threats to power grid infrastructures

High‐altitude cusp flow dependence on IMF orientation: A 3‐year Cluster statistical study

Abstract: [1] We report on the statistical properties of the plasma flows measured by the Cluster spacecraft in the high-altitude cusp region of the Northern Hemisphere as a function of the interplanetary magnetic field (IMF) orientation, with selected clock angle intervals. The technique uses a magnetic field model, taking into account the actual solar wind conditions and level of geomagnetic activity, in order to model the magnetopause and cusp displacements as a function of these conditions. The distributions of the magnetic field vector show a clear consistency with the IMF clock angle intervals chosen and demonstrate that the technique used here fixes the positions of the cusp boundaries adequately. The antisunward convection observed in the exterior cusp suggests that this region is statistically quite convective under southward IMF, while for northward IMF the region appears more stagnant. The presence of large parallel (downward) flows at the equatorward edge of the cusp shows that plasma penetration occurs preferentially at the dayside low-latitude magnetopause for southward IMF conditions; in contrast, under northward IMF the results are suggestive of plasma penetration from the poleward edge of the cusp, combined with a substantial sunward convection, but no flows are observed at all at the dayside boundary with the plasma sheet. The comparison of the measured flow speed with the Alfven speed suggests that the magnetosheath adjacent to the external boundary is more sub-Alfvenic, even for high magnetic latitudes, under northward IMF than under southward IMF. This result is consistent with the preference for the plasma depletion layer to develop under such conditions. The transverse plasma convection in the exterior cusp appears to be controlled by the IMF BY component as well; for dawnward (duskward) IMF orientations the convection is preferentially directed toward dusk (dawn). These results are interpreted as strong arguments in favor of the cusp being structured, at large scales, by the occurrence of magnetic reconnection at the high-latitude magnetopause for northward IMF and at the low-latitude magnetopause for southward IMF.

The IDV index: Its derivation and use in inferring long‐term variations of the interplanetary magnetic field strength

Abstract: [1] On the basis of a consideration of Bartels' historical u index of geomagnetic activity, we devise an equivalent index that we refer to as the interdiurnal variability (IDV). The IDV index has the interesting and useful property of being highly correlated with the strength of the interplanetary magnetic field (B; R2 = 0.75) and essentially unaffected by the solar wind speed (V; R2 = 0.01) as measured by spacecraft. This enables us to obtain the variation of B from 1872 to the present, providing an independent check on previously reported results for the evolution of this parameter. We find that solar cycle average B increased by ∼25% from the 1900s to the 1950s and has been lower since. If predictions for a small solar cycle 24 bear out, solar cycle average B will return to levels of ∼100 years ago during the coming cycle(s).

Solar source of the largest geomagnetic storm of cycle 23

Abstract: [1] The largest geomagnetic storm of solar cycle 23 occurred on 2003 November 20 with a Dst index of −472 nT, due to a coronal mass ejection (CME) from active region 0501. The CME near the Sun had a sky-plane speed of ∼1660 km/s, but the associated magnetic cloud (MC) arrived with a speed of only 730 km/s. The MC at 1 AU (ACE Observations) had a high magnetic field (∼56 nT) and high inclination to the ecliptic plane. The southward component of the MC's magnetic field was made up almost entirely of its axial field because of its east-south-west (ESW) chirality. We suggest that the southward pointing strong axial field of the MC reconnected with Earth's front-side magnetic field, resulting in the largest storm of the solar cycle 23.

Evolution and rotation of large-scale photospheric magnetic fields of the Sun during cycles 21–23 - Periodicities, north-south asymmetries and r-mode signatures

Abstract: We present the results of an extensive time series analysis of longitudinally-averaged synoptic maps, recorded at the National Solar Observatory (NSO/Kitt Peak) from 1975 to 2003, and provide evidence for a multitude of quasi-periodic oscillations in the photospheric magnetic field of the Sun. In the low frequency range, we have located the sources of the 3.6 yr, 1.8 yr, and 1.5 yr periodicities that were previously detected in the north-south asymmetry of the unsigned photospheric flux (Knaack et al. 2004, A&A, 418, L17). In addition, quasi-periodicities around 2.6 yr and 1.3 yr have been found. The 1.3 yr period is most likely related to large-scale magnetic surges toward the poles and appeared in both hemispheres at intermediate latitudes ∼30°-55° during the maxima of all three cycles 21-23, being particularly pronounced during cycle 22. Periods near 1.3 yr have recently been reported in the rotation rate at the base of the convection zone (Howe et al. 2000, Science, 287, 2456), in the interplanetary magnetic field and geomagnetic activity (Lockwood 2001, J. Geophys. Res., 106, 16021) and in sunspot data (Krivova & Solanki 2002, A&A, 394, 701). In the intermediate frequency range, we have found a series of quasi-periodicities of 349-307 d, 282 ± 4 d, 249-232 d, 222-209 d, 177 ± 2 d, 158-151 d, 129-124 d and 103-100 d, which are in good agreement with period estimates for Rossby-type waves and occurred predominantly in the southern hemisphere. We provide evidence that the best known of these periodicities, the Rieger period around 155 d, appeared in the magnetic flux not only during cycle 21 but also during cycle 22, likely even during cycle 23. The high frequency range, which covers the solar rotation periods, shows a dominant (synodic) 28.1 ± 0.1 d periodicity in the southern hemisphere during cycles 21 and 22. A periodicity around 25.0-25.5 d occurred in the south during all three cycles. The large-scale magnetic field of the northern hemisphere showed dominant rotation periods at 26.9 ± 0.1 d during cycle 21, at 28.3-29.0 d during cycle 22 and at 26.4 ± 0.1 d during cycle 23.

Structural and temporal requirements for geomagnetic field reversal deduced from lava flows

Abstract: Reversals of the Earth's magnetic field reflect changes in the geodynamo—flow within the outer core—that generates the field. Constraining core processes or mantle properties that induce or modulate reversals requires knowing the timing and morphology of field changes that precede and accompany these reversals1,2,3,4. But the short duration of transitional field states and fragmentary nature of even the best palaeomagnetic records make it difficult to provide a timeline for the reversal process1,5. 40Ar/39Ar dating of lavas on Tahiti, long thought to record the primary part of the most recent ‘Matuyama–Brunhes’ reversal, gives an age of 795 ± 7 kyr, indistinguishable from that of lavas in Chile and La Palma that record a transition in the Earth's magnetic field, but older than the accepted age for the reversal. Only the ‘transitional’ lavas on Maui and one from La Palma (dated at 776 ± 2 kyr), agree with the astronomical age for the reversal. Here we propose that the older lavas record the onset of a geodynamo process, which only on occasion would result in polarity change. This initial instability, associated with the first of two decreases in field intensity, began ∼18 kyr before the actual polarity switch. These data support the claim6 that complete reversals require a significant period for magnetic flux to escape from the solid inner core and sufficiently weaken its stabilizing effect7.

Theoretical effects of geomagnetic activity on low‐latitude ionospheric electric fields

Abstract: [1] The influence of geomagnetic activity on middle- and low-latitude thermospheric winds and ionospheric electric fields is investigated using model results from the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model. Model runs are made for different levels of geomagnetic activity. Model results show that the equatorward ionospheric currents produced by disturbance winds develop positive charge accumulation at low latitudes that maximizes in the premidnight sector. The local time of maximum electric potential perturbation depends significantly on universal time so that the local time of reversal of the equatorial zonal perturbation electric field varies with longitude by 2 to 3 hours, depending on the intensity of geomagnetic activity. The westward perturbation electric field in the postsunset period indicates that stronger geomagnetic activity will produce a lower driven height of the evening F region. After geomagnetic activity ceases, model results show that the zonal disturbance winds can last for many days in the postrecovery period, while the meridional disturbance winds decay more rapidly. The long-lasting zonal winds, through the Pedersen currents they drive, help maintain meridional disturbance potential drops that decay much more slowly than the zonal disturbance potential drops after the activity ceases.

Predicting surface geomagnetic variations using ionospheric electrodynamic models

Abstract: [1] A technique is described for predicting ground surface geomagnetic variations from measurements of the approaching interplanetary magnetic field (IMF) and solar wind. The method uses twin, empirical representations of the ionospheric electric and magnetic Euler potentials' response to the IMF drivers. The magnetic potential model, originally derived for mapping the large-scale field-aligned current structure, describes the curl-free component of the horizontal ionospheric current, also called the “potential current.” Using approximations that the Hall and Pedersen conductances have a fixed ratio and that there are no conductivity gradients, then the Hall current is derived from the magnetic potentials. In this case the Hall current is the same as the divergence-free “equivalent current,” which is used to derive the geomagnetic variations at the ground surface. The assumption that the ionospheric conductances have no gradients is avoided if the empirical model for the ionospheric electric potentials is used in addition to the magnetic potentials. In this second method the electric field provides additional information about the direction of the estimated equivalent current. Despite the approximation of a fixed conductance ratio, both calculation methods perform remarkably well for predicting the large-scale and long-period geomagnetic variations. The method that includes the electric fields has a slightly better performance, particularly in the polar cap. Corrections for the effects of currents induced underground were not applied for this demonstration. Such corrections could in principle improve the predictions, particularly for the short-period variations for which the effect is the greatest.