Satellite Magnetometer Research Articles

The importance of conductivity gradients in ground-based field-aligned current studies

Abstract Magnetosphere-ionosphere coupling is achieved primarily through magnetic field-aligned currents. Such currents are difficult to measure directly and are usually inferred from satellite magnetometer recordings or from ground-based measurements of the divergence of ionospheric electric fields. The latter technique requires a knowledge of the ionospheric conductance distribution. Although it is possible to obtain the ionospheric electric field distribution over large spatial areas with good temporal resolution from coherent backscatter radars, these instruments cannot measure conductivity. Since the equation for computing field-aligned currents explicitly requires the gradient in conductance to be known, the use of statistically averaged models is excluded for case studies. If a dense enough array of magnetometers is available, these data may be used in combination with radar data to produce a measured conductance distribution within the overlapping fields of view. This has been done for data obtained in northern Scandinavia. Comparing field-aligned currents, computed with and without knowing the ionospheric conductance distribution, shows that gradients in conductance can not be ignored, even for quiet geomagnetic conditions.

Field‐aligned currents in the polar cap at small IMF Bz and By inferred from SuperDARN radar observations

Routine SuperDARN observations of the ionospheric plasma convection and field‐aligned currents (FACs) in the high‐latitude ionosphere are used to study current systems established at small interplanetary magnetic field (IMF) Bz and By. By statistical averaging of available data sets we show that under this IMF condition the ionospheric convection pattern consists of two (evening and morning) convection cells that are similar in shape. The flow intensity inside the central polar cap is noticeably depressed so that plasma entering the polar cap flows around its border, predominantly along the lines of equal magnetic latitude, so that the convection cells are of a crescent‐like shape. This global pattern of plasma flow is associated with the effect of the region 0 field‐aligned currents coexisting with the region 1 and region 2 field‐aligned currents. SuperDARN observations of FACs for individual events support this conclusion. FACs were derived by analyzing the vorticity of the SuperDARN convection maps. We show that region 0 currents for small IMF Bz and By can exist in time sectors way off the magnetic noon. Thus radar observations support earlier findings from satellite magnetometer measurements of the region 0 current system at high latitudes during both the prenoon and afternoon at small IMF intensities. Because the region 0 FACs occur during small IMF intensities, it is suggested that quasi‐viscous processes play a role in their generation.

Seasonal effects in the ionosphere-thermosphere response to the precipitation and field-aligned current variations in the cusp region

Abstract. The seasonal effects in the thermosphere and ionosphere responses to the precipitating electron flux and field-aligned current variations, of the order of an hour in duration, in the summer and winter cusp regions have been investigated using the global numerical model of the Earth's upper atmosphere. Two variants of the calculations have been performed both for the IMF By < 0. In the first variant, the model input data for the summer and winter precipitating fluxes and field-aligned currents have been taken as geomagnetically symmetric and equal to those used earlier in the calculations for the equinoctial conditions. It has been found that both ionospheric and thermospheric disturbances are more intensive in the winter cusp region due to the lower conductivity of the winter polar cap ionosphere and correspondingly larger electric field variations leading to the larger Joule heating effects in the ion and neutral gas temperature, ion drag effects in the thermospheric winds and ion drift effects in the F2-region electron concentration. In the second variant, the calculations have been performed for the events of 28–29 January, 1992 when precipitations were weaker but the magnetospheric convection was stronger than in the first variant. Geomagnetically asymmetric input data for the summer and winter precipitating fluxes and field-aligned currents have been taken from the patterns derived by combining data obtained from the satellite, radar and ground magnetometer observations for these events. Calculated patterns of the ionospheric convection and thermospheric circulation have been compared with observations and it has been established that calculated patterns of the ionospheric convection for both winter and summer hemispheres are in a good agreement with the observations. Calculated patterns of the thermospheric circulation are in a good agreement with the average circulation for the Southern (summer) Hemisphere obtained from DE-2 data for IMF By < 0 but for the Northern (winter) Hemisphere there is a disagreement at high latitudes in the afternoon sector of the cusp region. At the same time, the model results for this sector agree with other DE-2 data and with the ground-based FPI data. All ionospheric and thermospheric disturbances in the second variant of the calculations are more intensive in the winter cusp region in comparison with the summer one and this seasonal difference is larger than in the first variant of the calculations, especially in the electron density and all temperature variations. The means that the seasonal effects in the cusp region are stronger in the thermospheric and ionospheric responses to the FAC variations than to the precipitation disturbances.Key words. Ionosphere (ionosphere · atmosphere interactions; ionosphere · magnetosphere interactions; ionospheric disturbances).

A three‐dimensional MHD study of solar wind mass loading processes at Venus: Effects of photoionization, electron impact ionization, and charge exchange

Ionization of hot and cold planetary oxygen ions caused by solar radiation, electron impact, and charge exchange with the solar wind protons lead to a heavy plasma population near Venus which is accelerated and incorporated into the solar wind plasma. lonization of hot and cold hydrogen also enhances the solar wind density and reduces its momentum. Charge exchange of the solar wind plasma with hot and cold planetary hydrogen removes a part of this heavy ion population. We include these processes in a three‐dimensional magnetohydrodynamic (MHD) model of the solar wind interaction with Venus and analyze their impact on the magnetic and flow field geometry around Venus. We find that photoionization is the most important of these processes if the change in the plasma composition is taken into account. The ionization processes lead to a strong deceleration of the flow around the planet, which in turn results in changes of the magnetic field. Because of the increasing mean mass and density of the plasma near the ionopause velocity gradients develop in the near terminator region, which lead to a local increase of the magnetic field. The inclusion of mass loading in our model moves the bow shock position outward, close to the location observed by the Pioneer Venus Orbiter magnetometer at solar maximum.

Observations of convection vortices in the afternoon sector using the SuperDARN HF radars

Observation of convection vortices using the new SuperDARN HF radars are presented. The velocity field derived from the overlapping fields of view of the new HF radars at Kapuskasing, Ontario, and Saskatoon, Saskatchewan, Canada, often image the portion of the convection pattern near the convection reversal boundary. Observations from near the convection reversal boundary in the afternoon sector of October 22, 1993, showed two convection vortices evolving within the field of view of both radars. The first vortex appeared at about 2120 UT and the second at about 2145 UT: 1446 MLT and 1512 MLT, respectively. The vortices were roughly 900 km in diameter and moved tailward with a velocity of about 600 m/s. At the times the vortices were observed by the radars, ground‐based magnetometers of the CANOPUS and MACCS chains show transient deflections of near 100 nT, and the GOES 6 and GOES 7 satellite magnetometers showed significant decreases in the magnetospheric magnetic field strength. Data from the Geotail satellite magnetometer lagged by an appropriate time interval indicated that there were southward turnings of the interplanetary magnetic field that coincided with the decreases of magnetospheric magnetic field strength. The observations differ in many respects from previously published vortex observations. It is theorized that the vortices were caused by the Kelvin‐Helmholtz instability at the inner edge of the low‐latitude boundary layer.

The inner magnetosheath of Venus: An analogue for Earth?

The unmagnetized planets provide examples of solar wind interactions that are free from the complications associated with magnetopause reconnection and with sensitive obstacle response to incident solar wind pressure changes. Using the Venus magnetosheath as a testbed, we search for evidence of standing slow mode “transitions” in the inner subsolar region as reported for Earth by Song et al. [1990a; 1992a; b]. Although the system at Venus is much smaller in scale, the Pioneer Venus Orbiter magnetometer data indicate that for perpendicular interplanetary magnetic field conditions the general behavior of the plasma in the magnetosheath is as expected from the simple depletion layer model. In examples of magnetic field measurements chosen for the apparently steady interplanetary conditions during the spacecraft pass, there is no clear evidence for a slow mode structure near the ionopause as might be expected on the basis of the Song et al. study. The implication is that some aspect of the Venus magnetosheath (such as its small size or the presence of local planetary ion production) makes it physically different from Earth's, that the conditions of the magnetosheath during Song's study differed significantly from those in the Venus study, or that the observations of Song et al. do not represent a steady state.

Solar cycle variation of interplanetary shocks, coronal mass ejections, and stream interactions observed at 0.7 AU

A survey of the Pioneer Venus Orbiter (PVO) magnetometer and plasma data from 1979-1980, shows that the occurrence frequency of interplanetary shocks, coronal mass ejections (CMEs) and stream interactions observed at 0.7 AU exhibits a solar cycle variation. As previously found at 1 AU, the observed number of both interplanetary shocks and CMEs peaks during solar maximum (approximately 16 and approximately 27 per year, respectively) and reaches a low during solar minimum (approximately 0 and approximately 7 per year, respectively), in phase with the variation in smoothed sunspot number. The number of stream interactions observed varies in the opposite manner, having a minimum during solar maximum (approximately 15 per year) and a maximum during solar minimum (approximately 34 per year). The percentage of CMEs and stream interactions producing interplanetary shocks also varies during the solar-cycle and exhibits interesting behavior during the declining phase. While the number of CMEs observed during this phase is decreasing, the percentage of CMEs producing interplanetary shocks reaches a maximum. Also, while the number of stream interactions observed is increasing, but has not reached maximum during the declining phase, the percentage of stream interactions producing interplanety shocks is at a maximum.

Periodic longitudinal structure of field‐aligned currents in the dawn sector: Large‐scale meandering of an auroral electrojet

The azimuthal structure of field‐aligned current (FAC) systems is one of the target subjects of the Freja satellite's magnetometer observation. In the event of May 28, 1993, the satellite observed large‐amplitude fluctuations of the latitudinal, as well as the longitudinal, magnetic field components along a postmidnight‐to‐morning orbit. By combining radar and ground magnetometer data, we inferred that the associated FAC system has a periodic structure consisting of multiple FAC sheets inclined significantly from the azimuthal (east‐west) direction and moving westward and possibly poleward. Such a structure may be interpreted in terms of the meandering of an auroral electrojet, and perhaps as the ionospheric projection of boundary waves.

Double vision: Remote sensing of a flux transfer event with ISEE 1 and 2

A flux transfer event recorded by the ISEE 1 and 2 spacecraft in the northern hemisphere near local noon is examined using analysis techniques developed recently for determination from single satellite magnetometer data of the orientation and cross‐sectional shape of a two‐dimensional disturbance moving along the magnetopause. The analysis is performed independently on the magnetic field data recorded by each satellite, enabling us to check the consistency of the results and thus to benchmark the method. The analysis is also extended to utilize the dual magnetometer measurements for determination of the event speed and size (which for a single spacecraft requires use of three‐dimensional plasma velocity data to obtain a deHoffmann‐Teller frame velocity) and for accommodating acceleration of the magnetopause normal to itself, which results in a curved spacecraft trajectory in the frame of reference moving with the disturbance. The analysis of the disturbance reveals that while its overall size is about 34,000 km in a direction parallel to the magnetopause but perpendicular to the event axis, the actual size of the bulge causing the field disturbance may have been as small as 9000 km with a dimension perpendicular to the magnetopause of the order of 1200 km. The bulge is found to be traveling at a speed of about 140 km/s toward the northern dawn quadrant of the dayside magnetopause. While plasma data from ISEE 2 provides evidence of plasma jetting, which suggests the event may be associated with reconnection, the orientation of the flux tube axis obtained from the analysis is significantly different from the expected reconnection line direction. A discussion of possible interpretations of this bulge orientation and motion is provided.

On the sources of interplanetary shocks at 0.72 AU

In order to understand the solar cycle variation of interplanetary shocks and their driving source at 0.72 AU, a survey of Pioneer Venus Orbiter (PVO) magnetometer and plasma data from 1979‐1988 has been conducted. Known shock drivers at 1.0 AU include coronal mass ejections (CMEs) and fast/slow stream interactions. In our analysis, CMEs were identified by a decrease in plasma temperature to background or below accompanied by an increase in plasma density and dynamic pressure. It was also required that the magnetic field exhibit a coherent rotation over about a day and an increase and decline in magnitude on a timescale of hours to days. Stream interactions were identified by a characteristic increase in ion temperature and velocity coincident with a decrease in density and a coincident increase in the total magnetic field magnitude. These signatures were usually preceded within 24 hours by a change in flow angle. In all, 45 shocks were identified: 36 driven by CMEs, 6 resulting from fast/slow stream interactions, and 3 with sources that could not be defined. The shocks driven by CMEs show a solar cycle variation that roughly follows the sunspot number. These shocks all have normals consistent with radial propagation of the shock fronts from the sun. In contrast, the few stream interaction related shocks show a tendency to occur later in the solar cycle and have a broader distribution of shock normals.

Old Palaeomagnetism and Future Trends

In my abstract (Runcorn, l993) I have summarized the impact that palaeomagnetism has had on studies of the Earth and the Moon and may be expected to have on Mars, when rocks are returned from this planet and detailed satellite magnetometer surveys are made from moderate heights above its surface. It is surely one of the most remarkable discoveries of science that such a weak field as that of the Earth or early Moon is faithfully recorded in rocks, though today geophysicists accept it as commonplace. It is equally remarkable that palaeomagnetism turned out to be the key technique for settling such fundamental questions of geodynamics as continental drift and sea floor spreading and the constitution and early dynamics of the Moon. Further developments in palaeomagnetism are difficult to predict but in my abstract I have produced a list of priorities with which I expect most palaeomagnetists would agree.

Reconstructing Tethered Satellite Skiprope Motion by Bandpass Filtering Magnetometer Measurements

This paper presents a unique scheme for reconstructing tethered satellite skiprope motion by ground processing satellite magnetometer measurements. The measurements are modified based on ground knowledge of the Earth’s magnetic field and passed through bandpass filters tuned to the skiprope frequency. Skiprope motion is then deduced from the steady-state outputs of the filters. Simulation results are presented which verify the scheme and show it to be robust. The concept is not just limited to tethered satellites. Indeed, it can be applied whenever there is a need to reconstruct the coning motion of a body about a known axis, given measurements of a known vector in body-fixed axes.

Magnetic fields in Venus nightside ionospheric holes: Collected Pioneer Venus Orbiter magnetometer observations

The magnetic fields detected by the Pioneer Venus Orbiter (PVO) magnetometer within the electron density depletions called “holes” in the nightside ionosphere are typically larger and more organized than the fields in the surrounding ionosphere. Moreover, they have substantial sunward/antisunward components which cause them to appear as near‐radial fields near the antisolar point. The collection of observations presented here illustrates the variety of appearances of the fields in holes. Some new results which summarize their average properties, their dependence on solar wind conditions, and their lack of geographical control are also presented. These results are potentially pertinent to the interpretation of data from the PVO entry at the end of 1992 and from the impending Mars Observer mission, which will probe the magnetic fields in the low‐altitude wake of weakly magnetized Mars.

Comparison of Echo 7 field line length measurements to magnetospheric model predictions

The Echo 7 sounding rocket experiment injected electron beams on central tail field lines near L = 6.5. Numerous injections returned to the payload as “conjugate echoes” after mirroring in the southern hemisphere. We compare field line lengths calculated from measured conjugate echo bounce times and energies to predictions made by integrating electron trajectories through various magnetospheric models: the Olson‐Pfitzer Quiet and Dynamic models and the Tsyganenko‐Usmanov model. Although Kp at launch was 3−, quiet time magnetic models best fit the echo measurements. Geosynchronous satellite magnetometer measurements near the Echo 7 field lines during the flight were best modelled by the Olson‐Pfitzer Dynamic Model and the Tsyganenko‐Usmanov model for Kp = 3. The discrepancy between the models that best fit the Echo 7 data and those that fit the satellite data was most likely due to uncertainties in the small‐scale configuration of the magnetospheric models. The field line length measured by the conjugate echoes showed some temporal variation in the magnetic field, also indicated by the satellite magnetometers. This demonstrates the utility an Echo‐style experiment could have in substorm studies.

Pioneer Venus Orbiter magnetic field and plasma observations in the Venus magnetotail

This study uses Pioneer Venus orbiter (PVO) magnetometer and plasma analyzer measurements to investigate the draped‐field tail of Venus with an emphasis on determining the magnetic field and plasma conditions within the various tail regions and their dependence upon interplanetary magnetic field (IMF) orientation. For this purpose PVO orbits during which the spacecraft's high inclination trajectory took it through the central magnetotail were identified. The criteria used to select these orbits were the existence of relatively complete magnetic field and plasma data during crossings of the magnetotail which lasted more than 3 hours. Examination of observations from four PVO tail seasons in 1981–1983 produced 12 orbits meeting these requirements. Analysis of the observations taken during those orbits indicates that the distribution of plasma within the magnetotail is highly asymmetric and controlled by the orientation of the IMF. In the plasma sheet and adjacent lobe regions downstream of the Venus hemisphere over which the solar wind motional electric field, Esw = −Vsw × Bsw, is directed away from the planet, PVO observed increasing fluxes of H+ and O+ as the spacecraft moves away from the tail axis toward the outer boundary of the tail. The greatest concentration of these O+ ions was found in the vicinity of the cross‐tail current layer downstream of this Venus hemisphere. No O+ ions were observed outside of the magnetotail based upon the magnetic field data and the definitions adopted in this study. However, only slow, <310 km/s, antisolar directed O+ is detectable by the PVO plasma analyzer and this factor may be the reason for the frequent observation of O+ in the cross‐tail current and the absence of measurable O+ outside of the magnetotail where the bulk flow speeds are significantly slower than in the distant ionosheath. Downstream of the Venus hemisphere over which the solar wind motional electric field is directed in toward the planet, PVO does not usually observe significant fluxes of E/Q = 0–8 kV ions, except sometimes directly adjacent to the outer boundary of the tail. The outer boundary of the Venus magnetotail downstream of this hemisphere is typically a well‐defined magnetopauselike current layer. The magnetotail‐ionosheath interface downstream of Venus hemisphere over which the solar wind motional electric field is outward, however, is very broad and resembles a slow mode expansion fan with field strength slowly decreasing and plasma density gradually increasing as PVO moves outward and enters the ionosheath. These results are interpreted as being due to the more efficient pick‐up of newly ionized atmospheric neutrals over the Venus hemisphere where the initial gyromotion takes the newly created ions away from the dense, lower atmosphere where they might be lost due to scattering (Cloutier et al., 1974). The implications of these findings for the formation and maintenance of the Venus magnetotail are discussed.

Magnetic mineralogy in an Archean crustal cross section: Implications for crustal magnetization

A traverse across Precambrian rocks from the Kapuskasing uplift in Ontario provided 31 samples from a crustal cross section that is now exposed at the surface. About half the samples are dominantly paramagnetic. Magnetite, accompanied in a few samples by minor amounts of pyrrhotite, is the only significant magnetic mineral in the other half; magnetic susceptibilities of these samples are constant between room temperature and about 540°C, above which temperature they drop rapidly to zero. Magnetic susceptibility does not depend on rock type, apparent original depth in the crust, or degree of metamorphism. The average induced magnetization of the most magnetic units in this section is less than 1 A/m, which is much lower than the source requirements for the deep crust deduced from aeromagnetic and satellite magnetometer surveys.

A one‐dimensional time‐dependent model of the magnetized ionosphere of Venus

The behavior and time evolution of the large‐scale magnetic fields and ionospheric plasma of the dayside Venus ionosphere were studied using a one‐dimensional model. The coupled continuity, momentum, and Maxwell's equations were solved simultaneously for three ions (O+, O2+, H+) and the magnetic field. The CO2+ ion was included photochemically. The calculated magnetic field profiles are in good agreement with observations made by the Pioneer Venus orbiter magnetometer. The magnetic field structure is found to be quasi‐steady for slow changes of the solar wind dynamic pressure. The peak of the magnetic field at 165 km is maintained by the downward convection from higher altitudes. The time scale for the decay of magnetic field by the pure one‐dimensional vertical diffusion/convection process is several hours unless the magnetic field flux is resupplied from the top of the ionosphere. Good agreement was obtained between the calculated and observed plasma densities for altitudes below 250 km including the electron density “ledge” near 190 km in magnetized ionospheres. However, the significant disagreement between the calculated and observed plasma densities at high altitudes suggests that under magnetized conditions, horizontal transport processes become important in removing the plasma and/or the magnetic field even in the subsolar region.

Upper limit on the intrinsic magnetic field of Venus

A data set of 18,000 low‐altitude nightside vector measurements from the Pioneer Venus Orbiter (PVO) magnetometer is used to establish a new upper limit on the intrinsic dipolar magnetic moment of Venus. Various techniques are used to search for coherence in the magnetic observations which might be attributable to such an intrinsic field, with negative results. A similar search is conducted for surface magnetism, that is, geographically correlated magnetic features, also with negative results. The present study represents the best limits that PVO observations can provide and is limited by the amount of low‐altitude nightside data available. We do not expect to obtain a significant amount of new low‐altitude data later in the mission. Our new upper limit to the magnetic moment is 8.4 × 1010 T m³ or about 10−5 of the terrestrial magnetic moment. This is a factor of 4 lower than the previous upper limit.

Field-aligned currents and related phenomena in the cusp

This review deals with observations of field-aligned currents (FAC) based on measurements of magnetic and electrical fields, particle fluxes (0.2–15 keV) and plasma parameters, provided by the INTERCOSMOS BULGARIA-1300 satellite. Special attention is paid to the cusp sector (10–14 MLT). The current regions inferred from the satellite magnetometer are compared with the charecteristics of the dayside oval and the cusp derived from a variety of parameters. Our topics are as follows: penetration of region 1 in the cusp; FAC reversals and plasma convection; thermal plasma structure and collocation of FAC; Associations with the IMF components etc.

Magnetic susceptibility anomalies of lithosphere beneath eastern Europe and the Middle East

Standard inversion methods for inverting aeromagnetic anomalies into magnetic susceptibility contrast in the crust are based on the assumption that the core field is constant over the region considered. This assumption, however, does not hold over regions of a few thousand kilometer extent. An inversion technique is developed which takes into account the variations in the core field, both in direction and in intensity. It can also take into account the surface topography of a magnetic layer within the lithosphere. The technique is applied to a scalar magnetic anomaly map of eastern Europe and the Middle East, derived from the magnetometer satellite (MAGSAT) data. The magnetic susceptibility contrast map thus obtained delineates the major tectonic features of the region. Small‐scale Precambrian shields are characterized by high magnetic susceptibility. The Alpine‐Himalayan belt has a distinct magnetic signature. The collision zones are low‐susceptibility regions, whereas the continental plates within the belt have high magnetic susceptibility. The well‐known iron formation of Kursk is delineated by a strong magnetic susceptibility high. The magnetometer satellite has measured the Earth’s magnetic field at two distinct local times, dawn and dusk, and provided two independent magnetic anomaly maps. The scalar magnetic anomaly map is based on the common features of the two maps. A covariance technique is developed which selects these common features on the basis of the correlation spectra of the two maps.