Geomagnetic Field Measurements Research Articles

Mesospheric planetary wave signatures in the equatorial electrojet

Using five years (1994–1998) of simultaneous and collocated measurements of horizontal winds associated with mesosphere and lower thermosphere (MLT) region and geomagnetic field measurements in the Indian geomagnetic dip equatorial region, an extensive experimental study has been made on the influences of mesospheric planetary waves on the equatorial electrojet (EEJ). The winds are measured using medium‐frequency (MF) radar (1.98 MHz) located at Tirunelveli. The EEJ strength is determined by measuring the geomagnetic field strength at both the geomagnetic dip and off‐dip equatorial stations, Trivandrum and Alibag, India. By noting the simultaneous time evolution of spectral signal (Morlet wavelet analysis) in both the EEJ strength and mesospheric winds at 88 km, it is found that mesospheric planetary waves with periodicities of 2–25 days often do propagate up to the ionospheric dynamo region and have their influences on EEJ strength. Sometimes only one of the parameters shows strong spectral signal in this periodicity range. It is also observed that often the diurnal oscillation in EEJ strength and mesospheric winds exhibits coherent variations in amplitude when the signature of mesospheric planetary‐scale oscillations is found in the EEJ strength. Also, it is found that variations in high‐energy solar radiations (F10.7 is used as a proxy) can influence the EEJ strength at planetary timescales. This leads to an interesting question. What actually causes the vertical coupling between ionosphere and atmosphere through upward propagating planetary waves to be operative only during some (and not all) of the episodes?

Fast Particle Filtering for Attitude and Angular-Rate Estimation from Vector Observations

Extending and consolidating the recently introduced quaternion particle filter for a spacecraft’s attitude estimation and its companion, the angular-rate particlefilter, this paper presents a novel algorithm for the estimation of both a spacecraft’s attitude and angular rate from vector observations. Belonging to the class of Monte Carlo sequential methods, the new estimator is a particle filter that uses approximate numerical representation techniques for performing the otherwise exact time propagation and measurement update of potentially non-Gaussian probability density functions in inherently nonlinear systems. The paper develops thefilter and its implementation in the case of a low-Earth-orbit spacecraft, acquiring noisy Geomagnetic field measurements via a three-axis magnetometer. The new estimator copeswith the curse of dimensionality related to the particle filtering technique by introducing innovative procedures that permit a significant reduction in the number of particles. This renders the new estimator computationally efficient and enables its implementationwith a remarkably small number of particles (relative to the dimension of the state). The results of a simulation study demonstrate the viability and robustness of the new filter and its fast convergence rate.

A Comparative Analysis of Ground, Magnetospheric and Interplanetary Observations of Long Period Magnetic Oscillations

Continuous measurements of the geomagnetic field variations at ground stations are important to investigate several aspects of magnetospheric dynamics related to variations in the solar wind conditions which, ultimately, originate from the Sun. We present a comparative analysis of geomagnetic field measurements at several ground stations with simultaneous magnetospheric and interplanetary observations in order to understand the origin and characteristics of the observed fluctuations. The results suggest that long period geomagnetic field fluctuations can be directly driven by solar wind density fluctuations at the same frequencies via the modulation of the magnetopause current. We also discuss the possible occurrence of additional contributions related with cavity/waveguide resonances of the entire magnetosphere as well as those of resonance processes of the geomagnetic field lines.

Coupling of Pi2 wave energy in the inner magnetosphere as inferred from low-latitude ground observations

Taking advantage of high time-resolution (1 s) geomagnetic field measurements obtained at low-latitude stations, we investigated frequency differences of Pi2 pulsations between the dayside and the nightside. Firstly, we examined two Pi2 cases globally observed by multiple ground stations and found that the dominant frequency peaks at the dayside showed latitudinal dependence, i.e., the higher (lower) frequency peak was predominant at lower (higher) latitude. We also noticed that the dominant Pi2 frequency on the nightside was apparently higher than that on the dayside. We argue that the multiple frequency peaks observed on the ground are harmonics of a plasmaspheric cavity resonance mode (CRM) and that the latitudinal dependence of the dominant frequency peaks may result from the energy coupling between the CRM and field line resonance (FLR) occurred in the near Earth space. We also argue that the frequency difference between the dayside and the nightside could be caused by the reason that a higher harmonic was observed in the nightside plasmasphere but was not effectively observed in the dayside. In addition, we statistically examined 829 Pi2 events that were simultaneously recorded at Kakioka (KAK, Maglat=27.2°) and Jicamarca (JIC, Maglat=0.0°), the two stations separated by ∼10 h in local time (LT) and one of them (JIC) is located at the dip equator. We found that the Pi2 frequency observed at KAK on the nightside was higher than that observed at JIC on the dayside on average. After investigating the occurrence histogram of the frequency difference (Δf=fJIC?fKAK) for the events simultaneously observed at KAK and JIC, we found that close to half of the events had the identical frequency (|Δf|<1.0 mHz). The statistical results are consistent with the facts reflected by case studies.

Variations in the chorus source location deduced from fluctuations of the ambient magnetic field: Comparison of Cluster data and the backward wave oscillator model

We study the motion of the source region of magnetospheric chorus emissions using multipoint measurements of VLF wave emissions and geomagnetic field onboard the Cluster spacecraft. The geomagnetic field data are matched to a parameterized model of the local magnetic field, and the spatiotemporal dynamics of the magnetic field are obtained on this basis. The wave data from the Wide Band Data instrument are used to obtain the power spectral density and number of chorus elements. Comparison of these data shows that the chorus remains related to the magnetic field minimum, while the position of this minimum can vary rather strongly during periods of enhanced geomagnetic activity. These results support the backward wave oscillator (BWO) model of chorus emissions, which attributes chorus generation to an absolute instability of whistler mode waves in the presence of a step‐like velocity distribution of energetic electrons. Such an instability takes place in a small vicinity of the local “magnetic equator” of a magnetic field line. Quantitative agreement between the data and the model is demonstrated by variation of the statistical chorus characteristics with changing of the deduced BWO parameters.

Geomagnetic sudden commencement at the dawn-time dip equator

Abstract Waveform of geomagnetic sudden commencement (SC) on the ground depends on both latitude and local time (LT). Using high-resolution (1 s) geomagnetic field measurements obtained at Jicamarca (JIC, MAGLAT = 0.0°) from 1998 to 2005, we examined the waveforms of 188 SC events and found that some of the SCs observed at nighttime (approx. 22-05 LT) and dawn time (approx. 05-07 LT) had a common observational feature, i.e., a positive impulse always appeared at the beginning of the main impulse of the SC, which led to these events show preliminary positive impulse (PPI) or ‘Stepwise’ structure in the waveforms. In particular, the occurrence rate of the PPI/Stepwise events at the dawn time was clearly higher than that at the nighttime. This is the first complete report of this observational feature for the SCs observed at the dip equator. Based on a comparison of our observations with the previously proposed model calculation of Kikuchi et al. (2001), we suggest that the PPI/Stepwise events observed at the dawn-time dip equator reflect the effects of the ionospheric currents (ICs) and field-aligned currents (FACs). We therefore argue that this observational result provides evidence for the validity of the model calculation.

Multispacecraft and ground‐based observations of substorm timing and activations: Two case studies

Two case studies are performed to investigate substorm timing and activations based on Double Star TC1, Cluster, Polar, IMAGE, LANL geostationary satellites and ground‐based geomagnetic field measurements. In both events, an earthward flow associated with plasma sheet thinning is measured by Cluster 8–10 min ahead of the auroral breakup. A couple of minutes after the breakup, either TC1 at ∼X‐10 RE first detects plasma sheet expansion and then the LANL satellites near the midnight measure energetic electron injections at geostationary orbit or the LANL satellites first measure the electron injections and then TC1 detects the plasma sheet expansion. More than about 20 min later, Cluster at X∼16 RE and Polar (at higher latitude) successively observe plasma sheet expansion. The open magnetic flux of the polar cap, Ψ, is found to continually increase during the early substorm phase and then to rapidly fall when the IMF turns northward. When Ψ reaches its minimum value, bright and broad auroral activities start to decrease. Tailward progression of the magnetic dipolarization and a poleward expansion of auroral bulges are shown to closely map to one another. These results suggest that substorm activations start in the midtail before ground onset and then move earthward, which leads to an expansion onset in the near‐Earth tail around X∼ ‐(8–9) RE. After onset, the activations progress both earthward and tailward. Substorm onset is possibly related to plasma sheet reconnection of close field lines, while tail lobe reconnection of open field lines release more energy to support the full expansion of the substorm. In a fully developed expansion phase, an initial dipolarization in the near‐Earth may eventually evolve to enable disruption of the cross‐tail current over a wide region of the magnetotail.

Rapid and sensitive magnetometer surveys of large areas using SQUIDs – the measurement system and its application to the Niederzimmern Neolithic double‐ring ditch exploration

AbstractA geomagnetic field measurement system for the detection of archaeological signatures in the subsoil is presented based on the superconducting quantum interference device (SQUID). The system provides fast mapping of large areas with high magnetic field gradient resolution as well as lateral precision. The acquired data are geographically referenced and also the altitude profile is given. The properties of the system were tested intensively at the large Neolithic double‐ring ditch enclosure of Niederzimmern near Weimar, Germany. Differences of the signal acquisition compared with caesium magnetometers are discussed. In the Niederzimmern double‐ring ditch enclosure, with an area of 27 ha, archaeological patterns were found only near the gates. These SQUID measurements, together with accompanying excavations, provide a complex picture of the double‐ring ditch enclosure, dated about 5600 years old. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Robust Spacecraft Angular Rate Estimation from Vector Observations Using Interlaced Particle Filtering

A novel algorithm is presented for the estimation of the spacecraft angular rate from vector observations. Belonging to the class of Monte Carlo sequential methods, the new estimator is a particle filter that uses approximate numerical representation techniques for performing the otherwise exact time propagation and measurement update of potentially non-Gaussian probability density functions in inherently nonlinear systems. This paper develops the filter and its implementation in the case of a low Earth orbit spacecraft, acquiring noisy geomagnetic field measurements via a three-axis magnetometer. Because the effective measurement noise in this case is time correlated, a special procedure is developed to account for that correlation in the particle filter implementation. The new estimator copes with the absence of an exact inertia tensor by employing a secondary particle filter that computes a maximum-likelihood estimate of the tensor of inertia, thus avoiding the need to expand the primary filter's state. This renders the new estimator highly efficient and enables its implementation with a remarkably small number of particles. The results of a simulation study are presented, in which the new filter is compared to a recently presented conventional extended Kalman filter. The comparison demonstrates the viability and robustness of the new algorithm and its fast convergence rate.

A LTS-SQUID System for Archaeological Prospection and Its Practical Test in Peru

We present a new geomagnetic field measurement system for the detection of archaeological signatures in the soil. The system provides a unique fast mapping of large areas with high magnetic field gradient resolution as well as lateral precision. The data acquired by the device are geographically referenced and suitable for embedding in a geographic information system (GIS).

Orbit Determination Using the Geomagnetic Field Measurement via theUnscented Kalman Filter

OrbitdeterminationofspacecraftusingonlymagnetometermeasurementsviatheunscentedKalman filter(UKF) is presented. An algorithm was formulated by adopting the UKF and an adequate dynamic model developed for processing geomagnetic field measurements. The paper consists of the analysis of force and estimation models, the dependencyofposition accuracyonorbittype,andmeasurement errors,as wellas acomparisonof theUKFandthe extended Kalman filter (EKF). Finally, the developed algorithm used actual magnetometer flight data from the Magnetic Field Satellite (MAGSAT). The results obtained from the MAGSAT data demonstrate that the achieved position error was approximately 2 km. The UKF performs similar to the EKF in position accuracy for a sampling interval of less than 20 s. In contrast, for a sampling interval of 40 s, the EKF yields lower position accuracy than the UKF.Inparticular,thedecomposedpositionerrorsarenotbiasedbecausetheUKFisnotaffectedbylinearizationof the measurement function. This improvement makes the magnetometer-based orbit determination method more robust and reliable as a real-time orbit determination system for small satellites that require moderate position accuracy, and also as a backup orbit determination system for large satellites. Nomenclature adrag = acceleration due to atmospheric drag, m=s 2 ageo = acceleration due to geopotential, m=s 2 aSRP = acceleration due to the solar radiation pressure, m=s 2 asun=moon = acceleration due to the sun and moon’s gravity, m=s 2 Br, B� , B� = the Earth magnetic field vector components, nT B � = inverse of the ballistic coefficient, m 2 =kg e = eccentricity

ULF fluctuations of the geomagnetic field and ionospheric sounding measurements at low latitudes during the first CAWSES campaign

Abstract. We present an analysis of ULF geomagnetic field fluctuations at low latitudes during the first CAWSES campaign (29 March-3 April 2004). During the whole campaign, mainly in the prenoon sector, a moderate Pc3-4 pulsation activity is observed, clearly related to interplanetary upstream waves. On 3 April, in correspondence to the Earth's arrival of a coronal mass ejection, two SIs are observed whose waveforms are indicative of a contribution of the high-latitude ionospheric currents to the low-latitude ground field. During the following geomagnetic storm, low frequency (Pc5) waves are observed at discrete frequencies. Their correspondence with the same frequencies detected in the radial components of the interplanetary magnetic field and solar wind speed suggests that Alfvénic solar wind fluctuations may act as direct drivers of magnetospheric fluctuations. A cross-phase analysis, using different pairs of stations, is also presented for identifying field line resonant frequencies and monitoring changes in plasmaspheric mass density. Lastly, an analysis of ionospheric vertical soundings, measured at the Rome ionosonde station (41.8° N, 12.5° E), and vertical TEC measurements deduced from GPS signals within an European network shows the relation between the ULF resonances in the inner magnetosphere and thermal plasma density variations during geomagnetically quiet conditions, in contrast to various storm phases at the end of the CAWSES campaign.

Attitude Estimation from Vector Observations Using a Genetic-Algorithm-Embedded Quaternion Particle Filter

A novel algorithm is presented for the estimation of spacecraft attitude quaternion from vector observations in gyro-equipped spacecraft. The new estimator is a particle filter that uses approximate numerical representation techniques for performing the otherwise exact time propagation and measurement update of potentially non-Gaussian probability density functions in inherently nonlinear systems. The new method can be applied using various kinds of vector observations. In this paper, the case of a low-Earth-orbit spacecraft, acquiring noisy geomagnetic field measurements via a three-axis magnetometer, is considered. A genetic algorithm is used to estimate the gyro bias parameters, avoiding the need to augment the particle filter's state and rendering the estimator computationally efficient. Contrary to conventional filters, which address the quaternion's unit norm constraint via special (mostly ad hoc) techniques, the new filter maintains this constraint naturally. An extensive simulation study is used to compare the new filter to three extended Kalman filters and to the unscented Kalman filter in Gaussian and non-Gaussian scenarios. The new algorithm is shown to be robust with respect to initial conditions and to possess a fast convergence rate. An evaluation of the Cramer-Rao estimation error lower bound demonstrates the filter's asymptotic statistical efficiency and optimality.

ULF geomagnetic pulsations in the southern polar cap: Simultaneous measurements near the cusp and the geomagnetic pole

During November 2003, a 1‐week test campaign of ULF geomagnetic field measurements was conducted at the new Italian/French base of Concordia (Dome C, Antarctic plateau), close to the geomagnetic pole, at a corrected geomagnetic latitude of ∼89°S. An analysis of these measurements is presented, together with a comparison with simultaneous measurements conducted at the Italian “Mario Zucchelli” base, at Terra Nova Bay, also located in the polar cap, but at lower latitude (∼80°S). The variable solar wind and interplanetary magnetic field conditions made it possible to monitor the Pc 3–4 ULF pulsation activity under different magnetospheric conditions. For quiet conditions, the pulsation power does not show evidence for a diurnal modulation at Dome C, while at Terra Nova Bay it maximizes around local magnetic noon. This result can be interpreted in terms of the different latitude of the two stations which might be linked to different magnetospheric regions: Dome C deep in the polar cap, and Terra Nova Bay approaching the cusp around noon. Conversely, during a strong geomagnetic storm, the wave activity at the two stations becomes very similar, and major ground manifestations are driven by strong solar wind pressure pulses. The clear dependence of the pulsation frequency with the interplanetary magnetic field strength suggests that upstream waves can be the source of pulsations not only at Terra Nova Bay but also at Dome C, in the deep polar cap. We also find a clear relationship between the pulsation power and the solar wind speed which is more pronounced at Dome C, where cusp‐related phenomena are not present.

Gyroless Attitude Estimation of Sun-Pointing Satellites Using Magnetometers

Deterministic attitude estimation methods based on vector observations are used for the coarse-attitude calculation of the sun-pointing satellites. The objective of the deterministic attitude estimation is to get the direction cosine matrix without considering the stochastic noises. The existing deterministic attitude estimation methods require two complete (three-axis) vector observations corresponding to two known reference vectors. This letter suggests a new deterministic attitude estimation method using only geomagnetic-field measurements on two axes for sun-pointing satellites in the sun-acquisition mode.

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

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

Attitude and orbit determination for small satellite using magnetometer measurement

The measurement of geomagnetic field can provide a reliable and economical basis for attitude and orbit information of low earth orbiting satellite. Because the earth's magnetic field is a function of position, and its measurement on the orbit are fully observable, orbit estimation can be obtained using extend Kalman filter (EKF) algorithm. With the assistant of angle velocity information from gyro measurement, attitude estimation can also be obtained. At the same time, gyro drift rate estimation is a part of the filter output. Although orbit and attitude determination are independent of each other, the filter can give the orbit and attitude estimation at the same time. The results of the numerical test show that a signal EKF can estimate both orbit and attitude by using magnetometer and gyro measurement only. The accuracy, usually is sufficient for low earth orbiting satellites.

Spacecraft Attitude Rate Estimation from Geomagnetic Field Measurements

Methods are developed for estimating the rotation rate of a spacecraft using only measured magnetic e eld data. The goal is to provide rate information for use in applications such as detumbling, nutation damping, and momentum management without using gyroscopes. Two algorithms are developed, a deterministic algorithm and an extended Kalman e lter. Both algorithms employ the magnetic e eld direction kinematics equation and Euler’ s equation for attitude motion of a rigid body with momentum wheels. Neither algorithm requires a model of the Earth’ s magnetic e eld. The deterministic algorithm solves a nonlinear least-squares problem for the unknown angular momentum component along the magnetic e eld direction. The extended Kalman e lter estimates the attitude rate vector, corrections to e ve of the six inertia matrix elements, and two error states of the measured magnetic e eld direction. It uses an initial rate estimate from the deterministic algorithm to avoid divergence. The algorithms have been tested using data from a spinning sounding rocket. They achieve initial accuracies in the range 2‐7 deg/s when the rocket spins at about 80 deg/s, and their accuracies improve to 1 ‐2 deg/s after the spin rate decays to 20 deg/s. These results indicate a lower bound on the ratio of the error to the nominal spin rate, which suggests that dynamic modeling error is the dominant source of uncertainty. I. Introduction M ANY spacecraft need estimates of their rotation rates. The spin rate may be used to apply a control that stops tumbling, to manage the total system angular momentum, to aid a star tracker, or as part of an attitude determination system. The most common method of determining attitude rate is by direct rate-gyro measurement. This paper develops two attitude rate estimation methods that rely on Earth magnetic e eld measurements rather than on rate-gyro

The temporal and spatial variations of low frequency geomagnetic pulsations at polar cusp and cap latitudes

Geomagnetic field measurements at two Antarctic stations are compared during two weeks in the local summer (January 1-15, 1992). Low frequency (0.6-6 mHz) pulsations are observed at each station near local magnetic noon. The same wave packets appear in some cases also at the other station, although with a significant attenuation, more clearly in the morning sector; the waves show a near noon reversal of the polarization sense from counter-clockwise in the morning to clockwise in the afternoon indicating a westward and an eastward propagation, respectively.

Deflection of paleomagnetic directions due to magnetization of the underlying terrain

In the summers of 1989 and 1991 we made 344 near‐ground level measurements of the ambient geomagnetic field above recent basalts on the island of Hawaii using a three‐component fluxgate magnetometer. We studied 12 surface features, including a lava pond, lava channels, long tilted blocks, smooth sloping surfaces, two fissures, and a deep U‐shaped road cut. We observed substantial differences (up to 20°) between the observed and expected (International Geomagnetic Reference Field, IGRF) magnetic field directions over these features except those composed of shelly pahoehoe and a flat (horizontally) thin lava pond. We also observed inclinations that were systematically shallower than the IGRF field by up to 5°. We show that these shallower inclinations can be explained by the magnetization of the regionally sloping surface of the southern side of the island. We found that all of the observed inclination deflections can be explained by simple two‐dimensional models which assume uniform induced and remanent magnetization parameters in the local terrain. Our observations imply that the inclination deflections cannot be corrected without a complete knowledge of the preexisting terrain and the remanence in the underlying flows upon which the lavas cooled. Since this information is rarely available, it is difficult or impossible to discriminate between dispersion of paleomagnetic directions caused by the magnetic terrain effect and dispersion due to other factors such as paleosecular variation (PSV). We therefore conclude that PSV dispersion parameters cannot be accurately determined from paleomagnetic measurements on highly magnetic volcanic flows. We also suggest that some of the geomagnetic excursions inferred from measurements on volcanic rocks may be at least in part due to the magnetic terrain effect. It is unnecessary to invoke ad hoc mechanisms such as clastic, block, or crustal rotations, distortion of the top crust, or flow deformation to explain the large between‐site dispersions or inclination anomalies observed in many of the paleomagnetic data from volcanic rocks. Our observations also bring into question the general reliability of paleomagnetic pole positions inferred from volcanic rocks, as a systematic inclination deflection due to local and regional slopes and irregular terrain, such as those we observed, would lead to a corresponding error in. the inferred paleolatitude. The magnetic terrain effects also offer alternative explanations for anomalous paleomagnetically inferred plate motions.