High Magnetic Latitudes Research Articles

Ulysses observations of auroral hiss at high Jovian latitudes

During the Ulysses flyby of Jupiter, a whistler‐mode emission was periodically detected by the Unified Radio and Plasma wave (URAP) experiment during intervals when the spacecraft extended to high magnetic latitudes. The signal was detected between the local electron plasma frequency and lower hybrid resonance and appears as a funnel‐shaped structure on frequency‐versus‐time spectrograms; these characteristics are very reminiscent of whistler‐mode auroral hiss observed at high latitudes at Earth. Ray tracing of the emission occurrences suggests the emission source is on magnetic field lines extending out to at least 65 RJ. This location associates the emission with the boundary between open and closed field lines — not the Io torus. The emission radiates about 107 W of power. Consequently, the auroral input power derived from the solar wind to drive the emission is believed to be 1010‐12 W (or about 1% of the energy associated with Io torus electrical processes).

Observations of terrestrial far UV emissions by the Faust Telescope

The FAUST telescope was developed to provide precise photometry of extended astronomical objects in the 1400–1800 Å wavelength interval, and has a field of view of 7.6° and an angular resolution of 2 arc minutes. It was carried into orbit by the space shuttle Atlantis in March 1992 as part of the ATLAS‐1 mission. During this flight, on several occasions it was pointed at the Earth to provide an estimate of the terrestrial nightglow, auroral and shuttleglow emissions. Our preliminary findings on these non‐astronomical measurements are that auroral precipitation activity dominates in the high magnetic latitude, and that outside the high magnetic latitude region we consistently see diffuse emission of .005 to .02 Rayleighs/Å, slowly varying over the nightside. We explore interpretations of this emission as terrestrial nightglow and shuttleglow, and find a possible explanation as NO chemiluminescent nightglow.

Radio emission from chemically peculiar stars

In five VLA observing runs the initial survey of radio emission from magnetic Bp-Ap stars by Drake et al. is extended to include a total of 16 sources detected at 6 cm out of 61 observed, giving a detection rate of 26 percent. Of these stars, three are also detected at 2 cm, four at 3.6 cm, and five at 20 cm. The 11 new stars detected as radio sources have spectral types B5-A0 and are He-weak and Si-strong. No classical (SrCrEu-type) Ap stars have yet been detected. The 16 detected sources show a wide range of radio luminosities with the early-B He-S stars on average 20 times more radio luminous than the late-B He-W stars and 1000 times more luminous than Theta Aurigae. Multifrequency observations indicate flat spectra in all cases. Four stars have a detectable degree of circular polarization at one or more frequencies. It is argued that the radio-emitting CP (chemically peculiar) stars form a distinct class of radio stars that differs from both the hot star wind sources and the active late-type stars. The observed properties of radio emission from these stars may be understood in terms of optically thick gyrosynchrotron emission from a nonthermal distribution of electrons produced in a current sheet far from the star. In this model the electrons travel along magnetic fields to smaller radii and higher magnetic latitudes where they mirror and radiate microwave radiation.

Solar cycle dependence of spacecraft charging in low Earth orbit

Recent experimental evidence has shown that Defense Meteorological Satellite Program (DMSP) polar orbiting spacecraft at 840 km can develop electric potentials as severe as −1430 V while at high magnetic latitudes. To explore this charging region, an analysis of DMSP F6, F7, F8, and F9 satellite precipitating particle and ambient plasma measurements taken during periods of high, medium, and low solar flux is performed. One hundred eighty‐four charging events ranging from −46 to −1430 V are identified, and an extreme solar cycle dependence is found as charging is most frequent and severe during solar minimum. Satellite measurements and time‐dependent ionospheric model (TDIM) output are used to determine the cause of the solar cycle dependence and to characterize the environments which generate and inhibit these potentials. The electron precipitation associated with various DMSP charging levels is analyzed; it is suggested that precipitating electrons as low as 2 to 3 keV may contribute to charging though higher‐energy electrons make greater contributions. Secondary electron production due to incident electrons below 1 keV is shown to inhibit charging. The energetic electron fluxes shown to generate charging do not vary significantly over the solar cycle. Instead, DMSP ambient plasma data and TDIM generated results identify a variation in plasma density over 1 or more orders of magnitude as the cause of the solar cycle dependence, and an ambient plasma density of less than 104 cm−3 is found necessary for significant negative charging (≥100 V) to occur.

Prediction of the Ulysses Jovian hectometric observations

The Ulysses unified radio and plasma wave (URAP) experiment is well suited to observe the Jovian hectometric radio emission (HOM) and so complement the observations made by the low‐frequency band receiver on Voyager. When the spacecraft encounters Jupiter, in February 1992, observations will be possible from considerably higher magnetic latitudes, in both hemispheres, than those accessible to Voyager. It should also be possible to determine the source location, as URAP can measure the direction of the incoming waves and all four Stokes parameters. Based upon the HOM beaming and source model of Ladreiter and Leblanc (1990a,b), the following predictions can be made: (1) During the inbound trajectory, Ulysses will, at first, be close to the Jovian equatorial plane when equal amounts of right‐handed (RH) and left‐handed (LH) circularly polarized HOM should be observed from the northern hemisphere and the southern hemisphere, respectively. During the three days immediately before closest approach, the spacecraft will move to a higher northerly magnetic latitude where it will be outside the main HOM emission beam so that a relatively weak RH polarized signal should be observed. (2) At closest approach, Ulysses will be very close to the low‐frequency HOM sources and will cross the HOM shadow zone, which extends out to about 20 Jovian radii (RJ) close to the magnetic equatorial plane. (3) During the outbound trajectory, the spacecraft will be at a fairly high southerly magnetic latitude and outside the main HOM emission beam, so that a relatively weak LH polarized signal should be observed.

Emission characteristics and source location of the smooth Neptunian kilometric radiation

The observations of the planetary radio astronomy (PRA) experiment aboard Voyager 2 reveal the existence of smooth and bursty radio emission. Both recur in a rather regular pattern with a 16.1‐hour period (the Neptunian spin period). We describe the phenomenology of the smooth component in terms of frequency, polarization, and occurrence in magnetic longitude and latitude. The existence of both right‐handed and left‐handed polarized emissions is consistent with two sources (one in each hemisphere) which radiate independently in the TUX mode. Because Voyager passed Neptune at less than 5000 km from the surface at high northern magnetic latitudes, the radio sources were occulted by the planet near the encounter. We have taken advantage of this occultation to locate the northern hemisphere sources by calculating the radio horizon (based on the offset tilted dipole (OTD2) model) for two spacecraft positions close to the encounter. We find that the northern source is located at high magnetic latitudes δm > 40°. By using a geometrical beaming model which assumes emission in a hollow cone pattern we fit the observed PRA intensity profile. The best fit is obtained for a radio source at L=6, thus confirming δm > 40°. The longitudinal extent of the source is at least 180°, from −90° to +90° magnetic longitude. To locate the southern sources we use the large excursions of Voyager 2 to high southern magnetic latitudes. The source is found to be located also at high magnetic latitudes but is possibly more limited in longitude than the northern source. We estimate the uncertainty in source location due to the very limited knowledge of the magnetic field near the Neptunian surface, and we show that the current OTD2 model is satisfactory for the source locations for the lowest observed frequencies; however, an angular uncertainty of about 20° remains for sources in the northern hemisphere. The observed pattern of the smooth emission is strongly frequency dependent, which is in agreement with the azimuthal asymmetries of the magnetic field as predicted by the OTD2 model.

Electron signatures of satellite sweeping in the magnetosphere of Uranus

The Voyager 2 Cosmic Ray System found large‐scale macrosignatures of satellite sweeping for MeV electrons near the orbits of the satellites Miranda, Ariel, and Umbriel in the magnetosphere of Uranus. Due to the large magnetic inclinations of satellite orbits at Uranus, sweeping rates vary along the orbits with the McIlwain L parameter. However, no evidence was found, where expected, for fresh sweeping signatures at such positions. Although the maximal electron intensity occurs near Voyager 2's minimum L (4.67) as predicted by the Q3 field model, the intensity minima in the macrosignatures show large outward displacements (≤0.5 RU) from minimum‐L positions of the associated satellites. These radial displacements increased with measured electron energy and at higher magnetic latitudes. Pitch angle distributions are generally more anisotropic outside the macrosignatures and more isotropic within, as determined from comparison of inbound and outbound intensity profiles at different latitudes. These anisotropy measurements provide the basis for latitudinal flux extrapolation, which when coupled with power law scaling of spectral distributions allow the calculation of phase space density profiles. The latter show local minima in the macrosignatures and are indicative of distributed electron sources in the inner magnetosphere and/or nonadiabatic transport processes such as pitch angle scattering and magnetospheric recirculation. Preliminary diffusion coefficients with values DLL ∼ 10−7–10−6 RS² and radial dependence DLL ∼ L3–L4 have been estimated for the macrosignatures. The low‐order L dependence of DLL is consistent with diffusion driven by ionospheric dynamo. However, quantitative modeling of radial and pitch angle diffusion is required to assess the formative processes for the macrosignatures before more physically meaningful transport parameters can be determined.

Neptune's cosmic ray cutoff

The Voyager 2 cosmic ray experiment observed the signature of a magnetic cutoff of cosmic ray protons immediately after the closest approach to Neptune. The cutoff signature shows that the simultaneous observation of a trapped particle dropout could not have occurred at high magnetic latitudes, but is likely to be a drift shadow of the planet caused by the large offset of the particle drift shells. The OTD2 dipole model of Neptune's magnetic field predicts a drift shadow at approximately the correct times. In addition, the cutoff signature is similar to that predicted by the OTD2 model, although the model is not accurate throughout the observation period. The similarity is consistent with small estimated corrections to the predicted signature based on the locally observed magnetic field values, indicating that the L shell values derived from the model are approximately valid after the closest approach time.

Magnetic Orientation Of Migratory Wheatears (Oenanthe Oenanthe) In Sweden And Greenland

ABSTRACT Orientation experiments were performed with wheatears (Oenanthe oenanthe) subjected to artificially manipulated magnetic fields, in Sweden and Western Greenland, during the autumn migration period. The objective was to compare responses by birds exposed to widely different geomagnetic conditions and, specifically, to clarify if birds are able to use magnetic cues for orientation at high geomagnetic latitudes, as in Western Greenland. Orientation experiments were run under clear sunset skies and under simulated total overcast. Clear-sky tests did not reveal any clearcut orientation responses by wheatears in deflected and vertical magnetic fields. There was a tendency, however, for previous experience of the relationship between geomagnetic cues and visual information to affect the birds’ orientation in a vertical magnetic field. Under simulated overcast, the birds closely followed a 90° shift in magnetic direction in both study areas, and both samples failed to exhibit statistically significant mean directions when tested in vertical magnetic fields. The results clearly demonstrate that wheatears possess a magnetic compass. Furthermore, the birds are able to detect and use local geomagnetic information even at high magnetic latitudes in Western Greenland, notwithstanding the steep inclination (+81°) and large declination (—46°). A persistent attraction towards magnetic northwesterly headings, under both clear and overcast skies, is not consistent with migratory directions according to ringing recoveries and warrants further investigation.

Energetic charged particle angular distributions near (r ≤ 2 RN) and over the pole of Neptune

Energetic ion ( > 28 keV) and electron ( > 22 keV) pitch angle distributions very close to (r ≤ 2RN) and over the north planetary pole of Neptune are presented using data from the Low Energy Charged Particle (LECP) and magnetometer (MAG) Experiments on the Voyager 2 spacecraft. The particle data are temporally structured and spectrally soft; a similarity with Earth‐like auroral signatures has previously been noted. However, the pitch angle distributions (showing trapped distributions at high magnetic latitudes) do not support an Earth‐like auroral interpretation, and alternative explanations for the temporal dynamics must be sought. Between r ∼ 1.6 and 2.0 RN and in the vicinity of the magnetic equator, the higher energy ion and electron pitch angle distributions (E ≥ 80 keV) display dramatic “bite‐outs” at ∼90°. This bite‐out feature could be caused by interactions with the newly discovered ring 1989N3R.

Optical spectrum variations of the helium-strong star HD 64740

High signal-to-noise coude Reticon spectra of the helium-strong star HD 64740 have been obtained for several rotational phases, in two spectral regions (3980-4160A and 4400-4580A) at the Canada-France-Hawaii telescope. These data, and previously published Zeeman analyzer observations, have been used to determine tentative abundance and magnetic field geometries of the star. Profile modeling has been performed with a line synthesis program that incorporates the effects of an assumed magnetic field and abundance distribution into the calculation of the line profiles. Helium appears to be overabundant in two spots near the magnetic poles of HD 64740. Silicon and magnesium show evidence for a high magnetic latitude band of enhanced abundances. Nitrogen and oxygen have roughly uniform abundances. The model suggests an inclination of 36 deg for the star's rotation axis, and a magnetic obliquity of approximately 78 deg. No constraint can be obtained on the surface magnetic field of HD 64740 with the current data. Additional observations are recommended. 66 refs.

Optical auroral conjugacy: Viking UV Imager—South Pole Station ground data

High time resolution satellite UV images of the northern auroral oval are compared with South Pole station keogram camera and all‐sky camera observations to determine the displacement of the South Pole conjugate point from its location calculated using the international geomagnetic reference field magnetic field model. The results presented extend the study of optical conjugacy to high magnetic latitudes in a solstice period, and to sunlit auroral events. Three high‐latitude events were analyzed: a poleward expansion, an arc poleward of the midnight oval, and a substantial movement in the afternoon auroral oval. They were determined to occur on closed field lines. A range of displacement for the South Pole conjugate point of 5.3° in latitude and 1.9 hours in MLT was measured.

Ecological causes and consequences of bird orientation

An advanced orientation capability offers possibilities for birds to optimize movement patterns in a wide variety of ecological situations. The adaptive significance of different patterns of angular dispersion and of orientation responses to topography and sociality are elucidated. The orientation capacity is characterized by flexibility, exemplified by reorientation promoting safety and restoration of fat reserves during migration. There are also limitations to the orientation process, leading to costs of migration through mis- or disorientation, and to constraints on the evolution of routes and timing of migratory flights. Young migrants may acquire an erroneous compass sense, and misorient several thousands of kilometers off their normal course. Widespread and dense fog of long duration causes disorientation and mortality among land birds migrating over the sea. Orientational constraints in the evolution of migration routes may be most easily disclosed at high geographic and magnetic latitudes. Here the birds are faced with special difficulties in using their celestial as well as their magnetic compasses. The sun compass could be used for great circle orientation, but observed spring flight trajectories of high arctic shorebirds and geese seem to conform with rhumbline routes.

The influence of centrifugal forces on the B field structure of an axially symmetric equilibrium magnetosphere

We investigate a stationary model of a rotating, axially symmetric “pole‐on” magnetosphere in MHD force balance. In this model both the planet's rotational and dipole axes are aligned with the magnetotail axis, which is the axis of symmetry in a cylindrical (r, ϕ, z) coordinate system. For mathematical convenience the magnetosphere is confined on the nightside by a cylindrical magnetopause with constant radius. On the sunward side, the magnetosphere is closed by an appropriate image dipole. Inside the magnetospheric cavity we assume isotropic thermal plasma pressure. We assume further that, in general, planetary rotation leads to differentially rotating magnetotail field lines causing field‐aligned Birkeland currents and a corresponding toroidal magnetic Bϕ component which leads to twisted magnetotail field lines. We calculate the deformation of magnetotail field lines under the influence of both thermal plasma pressure and centrifugal forces. We present “linear” (analytic) solutions to the Grad‐Shafranov equation which include the centrifugal force term. In the linear model, two free physical parameters, k and ω, measure the plasma thermal pressure and the ratio between plasma rotational and thermal energy densities, respectively. Low ω and high k values indicate the plasma‐dominated case. Conversely, low k and high ω values indicate the rotation‐dominated case. One limiting case, k = ω = 0, generates a simple vacuum magnetic field of a dipole confined within the magnetospheric cavity. The nonrotational magnetosphere with hot thermal plasma leads to a field configuration without a toroidal Bϕ component and without field‐aligned Birkeland currents. The other extreme, namely, a rapidly rotating magnetosphere with cold plasma, leads to a configuration in which the plasma must be confined within a thin disk in a plane where the radial magnetic field component Br vanishes locally. Utmost stretched magnetotail configurations can be achieved by increasing either the plasma thermal pressure or the rotation frequency of the magnetosphere, or both. In the rotation‐dominated case we found the plasma sheet thinner and located closer to the magnetopause than in the purely plasma‐dominated case. This implies that hypothetical polar aurorae, under the influence of magnetospheric rotation, appear at higher magnetic latitudes on both the dayside and the nightside. This also implies that the size of the polar oval shrinks under the influence of magnetospheric rotation. A “pole‐on” magnetosphere can be expected at Uranus in the year 2014 twice during a Uranian day.

Magnetospheric cavity modes and field-line resonances: The effect of radial mass density variation

The relationship between magnetospheric cavity modes and field-line resonances (FLRs) is examined as a function of the radial plasma mass density distribution in the dayside magnetosphere. Using a numerical model of wave coupling we find that, for monotonie mass densities of the form L − q , the radial position and amplitude of cavity-mode driven FLRs increases as q decreases. This makes the afternoon sector of Magnetic Local Time the most favourable for the detection of such resonances, and suggests that FLRs observed at high magnetic latitudes are not driven by global cavity modes. We also find that the maximum damping of the fundamental cavity mode by coupling to a FLR appears to occur for q ≅ 4 and m ≅ 3, where m is the azimuthai wave number. Using models for morning and afternoon mass densities which include a small plasmapause at L ≈ 4, the structure of wave fields is examined in detail. We find that the base of the plasmapause should be a favoured position for the occurrence of long-period hydromagnetic wave activity. In addition to double FLRs which have been described previously, quasi-FLRs should be present at the plasmapause base. These occur when a cavity mode period (or the period of any other monochromatic driving wave) is less than the minimum field-line eigenperiod at the plasmapause base. Field-lines are driven off-resonance, but a relatively large amplitude peak still results when the driving period is close to the minimum eigenperiod. The distinguishing feature of a quasi-FLR is that there is no polarization reversal across the amplitude peak, and the polarization is not linear at the peak. It is suggested that amplitude peaks with no polarization reversal across the peak may be indicators of the plasmapause position in ground-based observations.

Thermospheric dynamics during November 21–22, 1981: Dynamics Explorer measurements and thermospheric general circulation model predictions

Time‐dependent aurora and magnetospheric convection parameterizations have been derived from solar wind and aurora particle data for November 21–22, 1981. These parameterizations are used to drive the auroral and magnetospheric convection models that are embedded in the National Center for Atmospheric Research thermospheric general circulation model (TGCM). The TGCM has been used to calculate the time‐dependent global circulation, temperature and compositional structures for the geomagnetic moderately disturbed period. The results of the TGCM calculation are compared with measurements made by the Dynamics Explorer 2 satellite along its orbital path during several passes over the northern and southern hemisphere polar caps. TGCM input parameterizations of auroral particle precipitation and ion drift generally agree with satellite measurements of these inputs for the period. There is also good agreement between TGCM‐predicted neutral winds and DE 2 neutral wind observations showing the dominant influence of magnetospheric convection on the high‐latitude circulation. On the eveningside of the auroral oval there is a transition from a magnetospheric convection‐driven wind pattern at high magnetic latitudes to one that is driven principally by solar EUV and UV heating at middle and low latitudes. This transition is consistent with the measured latitudinal falloff of electric field strength. The comparison of experimental measurements and time‐dependent model calculations demonstrates that the great natural variability that is observed in the large‐scale structure and dynamics in the thermosphere can be interpreted successfully in terms of variations in the thermospheric forcing mechanisms that are incorporated in the TGCM.

Electrostatic waves in the magnetosphere of Uranus

During its encounter with Uranus the plasma wave receiver on Voyager 2 observed electrostatic waves similar in many respects to those observed in other planetary magnetospheres. The most prominent type observed was the Bernstein mode emissions between harmonics of the electron cyclotron frequency. As is the case at other planets, the most intense Bernstein waves were observed near the magnetic equator of the planet, even though the tilt of the Uranian magnetic moment with respect to the rotational axis is very large. A small offset in the location of these electrostatic waves from the equator predicted by the offset, tilted dipole magnetic field model suggests some warping of the magnetic equator due to ring currents or external currents flowing on the magnetopause. Other examples of electrostatic Bernstein waves were observed closer to the planet and at higher magnetic latitudes. The energy of resonant electrons is calculated to be a few hundred electron volts, and measurements of electrons with this energy indicate the critical flux required to drive the Bernstein mode is available. The existence of the Bernstein modes near the upper hybrid resonance frequency leads to estimates of the electron density at several locations within the Uranian magnetosphere, and these compare well with densities measured by the plasma science investigation. In addition to the Bernstein modes, a number of highly sporadic emissions were observed in the vicinity of the Miranda L shell. While the absolute determination of the mode of these waves is uncertain, it is likely that some are electrostatic modes. Since this region of the Uranian magnetosphere is very perturbed and interesting, we shall attempt to identify possible modes associated with the waves.

Beaming of Uranian nightside kilometric radio emission and inferred source location

Taking advantage of the large range of magnetic latitudes explored by Voyager 2, due to the 60° tilt of the Uranian dipole field with respect to the rotation axis of the planet, we study the beaming pattern of the broadband kilometric radio emission detected over Uranus' nightside. Using the simple physical assumptions of an emission close to the local gyrofrequency and in a time‐stationary average lobe (i.e., assuming that the 10 min‐averaged intensity received does not vary with time for a given observing geometry), we constrain the radiation source to a high south magnetic latitude region (∼80°, or L∼40) magnetically connected to the magnetospheric tail. We discuss the different source locations derived by other authors from different analyses. We determine then the beaming pattern of the main components (smooth and bursty) of the Uranian nightside radio emission and their variations with frequency. It appears to be a hollow cone whose axis is tangent to the magnetic field in the source region, with a large aperture (semi‐apical angle ∼50°) and with an angular thickness (at −3 dB) increasing with decreasing frequency from about 30° at 750 kHz to more than 55° below ∼350 kHz; the bursty emission occurs about 90° away from the magnetic field. The motion of the spacecraft relative to the radiation lobe explains most of the observed emission characteristics. The origin of the bursty emission and its possible link to the smooth component are discussed. The results are compared with the present experimental and theoretical knowledge of planetary radio emission beaming.

On the conjugacy of the aurora: High and low latitudes

On December 5, 1983 the polar orbiting satellites NOAA‐7 and DMSP‐F6, while in opposite hemispheres, each crossed a narrow structure of auroral electron precipitation at nearly the same Universal Time (UT) and at the same high magnetic latitude local time location. The Operational Line Scanner (OLS) image taken by DMSP‐F6 of the northern polar region showed that this enhancement was associated with a sun‐aligned arc (SAA) which extended poleward from the midnight sector auroral zone and passed across the polar cap very near to the magnetic pole. The DMSP‐F6 plasma measurements showed particle precipitation at background levels in the regions adjacent to the SAA suggesting that those magnetic field lines connected to the interplanetary medium via the northern tail lobe. NOAA‐7 observations show that the analogous precipitation structure in the southern hemisphere was embedded in a region of low intensity ion and electron precipitation which had the characteristics of the plasma sheet population. Comparisons between the boundaries of the lower latitude auroral zone precipitation and the electron distributions at those boundaries were also made. The results of these comparisons together with those made for the high latitude SAA argue for a high degree of inter‐hemisphere conjugacy in the electron precipitation over a wide range of invariant magnetic latitudes.

Reduction to the pole as an inverse problem and its application to low‐latitude anomalies

Traditionally, reduction to the pole has been accomplished either by space‐ or wavenumber‐domain filtering. In the two‐dimensional case, this procedure is stable regardless of the latitude, as long as the source strike is not parallel to the horizontal projection of the geomagnetic field. In the three‐dimensional case, however, reduction‐to‐the‐pole filtering is stable only at high magnetic latitudes. At latitudes lower than 15 degrees, it is of no practical use due to a sharply increasing instability toward the magnetic equator. The three‐dimensional instability of this filtering technique is demonstrated, and the reduction‐to‐the‐pole problem is formulated in the context of a general linear inverse problem. As a result, stable solutions are found by using well‐known stabilizing procedures developed for the inverse linear problem. The distribution of magnetization of an equivalent layer of doublets that reproduces the observed data is computed. The magnetic doublets are parallel to the magnetization direction which is assumed constant throughout the sources. The magnetic field reduced to the pole is then obtained by changing the inclinations of the geomagnetic field and the doublets to 90 degrees and recalculating the total field. The usefulness and limitations of the method at low magnetic latitudes are assessed using theoretical data. The effects of noise and anomaly truncation are also investigated for both high and low latitudes. In all cases, application of the proposed method produced meaningful results regardless of the latitude. The method is applied to field data from two different low‐latitude anomalies. The first anomaly is due to a seamount in the Gulf of Guinea with reversed magnetization. The geomagnetic field at this location is about −23 degrees. The second anomaly is an intrabasement anomaly from Parnaiba Basin, Brazil, where the magnetization is assumed to be induced by a geomagnetic field with −1.4 degree inclination. The results obtained confirm that the proposed method produces stable, meaningful, reduced‐to‐the‐pole maps.