Showing papers on "Dipole model of the Earth's magnetic field published in 2001"

Separator current sheets: Generic features in minimum-energy magnetic fields subject to flux constraints

Abstract: Equations are found for force-free magnetic equilibria in the “coronal half-space” z>0, subject to boundary conditions on the normal magnetic field at z=0. The distribution of normal field is assumed to be composed of NS isolated unipolar source regions of arbitrary shape, arranged arbitrarily on the plane. The equilibria are found by minimizing the magnetic energy subject to constraints on the total flux interconnecting pairs of source regions. For NS source regions interconnected in ND ways, there are Nc=ND−NS+1 distinct constraints on the field. Minimization subject to these constraints leads to an Nc-dimensional space of equilibria, for given boundary data. All field-lines connecting source regions are current-free, but the equilibrium contains Nc current-sheets lying at certain interfaces. In a two-dimensional magnetic field current sheets occur at points topologically equivalent to X-type neutral points in the potential field. In a three-dimensional field current sheets occur at points topologically...

Local anisotropy in incompressible magnetohydrodynamic turbulence

Abstract: It is a well known fact that in the presence of a dc applied magnetic field, magnetohydrodynamic (MHD) turbulence develops spectral anisotropy from isotropic initial conditions. Typically, the reduced spectrum is steeper in the direction of the magnetic field than it is in any transverse direction. One might expect that a dc field is not essential, and it is the local mean field that is responsible. To address this issue, three-dimensional MHD pseudo-spectral incompressible relaxation simulations are performed, and structure functions computed according to whether the separation is parallel to, or transverse to, the local mean magnetic field. Correlation lengths are longer in the locally averaged magnetic field direction than in any perpendicular direction, even when the global mean magnetic field is zero. Local anisotropy is observed to be stronger in regions of strong magnetic field. A general definition of anisotropy angles and a methodology to study local anisotropy are proposed.

Day the solar wind almost disappeared: Magnetic field fluctuations, wave refraction and dissipation

Abstract: On May 11, 1999, the Advanced Composition Explorer (ACE) spacecraft observed a rarefied parcel of solar wind. This has come to be known as “The Day the Solar Wind Disappeared.” Little if any change is seen in the large-scale interplanetary magnetic field during this time, but the magnetic field fluctuations are depressed and significantly more transverse to the mean field. The high Alfven speed resulting from the constant field intensity and low ion density enhances wave refraction, and we examine this as a possible explanation for the fluctuation properties. The solar wind possesses a very low proton β, thereby separating the cyclotron and ion inertial length scales and permitting a test of possible dissipation dynamics. We find that the test favors the ion inertial scale theories.

The Role of Nonlinear Alfvén Waves in Shear Formation during Solar Magnetic Flux Emergence

Abstract: Observations show that magnetic flux is constantly emerging at the solar photosphere to expand into the corona. Magnetic buoyancy is essential in bringing the magnetic field from the solar interior to the surface and beyond. In the simulations of buoyancy instabilities reported here, it has been discovered that nonlinear Alfven waves may play an important and dramatic role in the rise of magnetic flux. For initial states, our two-and-a-half-dimensional (2.5D), time-dependent simulations capitalize on the availability of a family of two-dimensional (2D) analytical solutions of isothermal magnetostatic atmospheres threaded by a layer of sheared undulating magnetic field. The magnetic field supports the weight of the atmosphere in an unstable configuration that sets the stage for the demonstration of magnetic flux emergence in a stratified atmosphere. When the system is perturbed, magnetic loops buoyantly rise from the flux layer and shearing motions are found to naturally arise in conjunction with mixed-mode (interchange and undulating) instabilities. The shearing motions take the form of large-amplitude shear Alfven waves that are driven by a component of the magnetic tension force pointing in the invariant direction. The waves are significant in that they transport magnetic flux from the shear layer into the ascending magnetic loops, causing them to become greatly inflated. The presence of such shear Alfven waves in magnetic loops rising through the photosphere provides an explanation for the impulsive shearing motions observed in newly emerged bipolar active regions.

Structure of small-scale magnetic fields in the kinematic dynamo theory

Abstract: A weak fluctuating magnetic field embedded into a a turbulent conducting medium grows exponentially while its characteristic scale decays. In the interstellar medium and protogalactic plasmas, the magnetic Prandtl number is very large, so a broad spectrum of growing magnetic fluctuations is excited at small (subviscous) scales. The condition for the onset of nonlinear back reaction depends on the structure of the field lines. We study the statistical correlations that are set up in the field pattern and show that the magnetic-field lines possess a folding structure, where most of the scale decrease is due to the field variation across itself (rapid transverse direction reversals), while the scale of the field variation along itself stays approximately constant. Specifically, we find that, though both the magnetic energy and the mean-square curvature of the field lines grow exponentially, the field strength and the field-line curvature are anticorrelated, i.e., the curved field is relatively weak, while the growing field is relatively flat. The detailed analysis of the statistics of the curvature shows that it possesses a stationary limiting distribution with the bulk located at the values of curvature comparable to the characteristic wave number of the velocity field and a power tail extending to large values of curvature where it is eventually cut off by the resistive regularization. The regions of large curvature, therefore, occupy only a small fraction of the total volume of the system. Our theoretical results are corroborated by direct numerical simulations. The implication of the folding effect is that the advent of the Lorentz back reaction occurs when the magnetic energy approaches that of the smallest turbulent eddies. Our results also directly apply to the problem of statistical geometry of the material lines in a random flow.

Large-scale magnetic field and sunspot cycles

Abstract: H magnetic synoptic charts of the Sun are processed for 1915 - 1999 and the spherical harmonics are calculated. It is shown that the polarity distribution of the magnetic field on H charts is similar to the polarity distribution of the Stanford magnetic field observations during 1975 - 1999. The index of activity of the large-scale magnetic field A.t/, representing the sum of the intensities of dipole and octupole components, is introduced. It is shown that the cycle of the large-scale magnetic field of the Sun precedes on the average by 5.5 years the sunspot activity cycle, W.t/. This means that the weak large-scale magnetic fields of the Sun do not result from decay and diffusion of strong fields from active regions as it is supposed in all modern theories of the solar cycle. On the basis of the new data the intensity of the current solar cycle 23 is predicted and some aspects of the theory of the solar cycle are discussed. The origin and role of the large-scale magnetic field in the organization of the general solar magnetism and its connection with sunspot activity is a key question for understanding the 22-yr cycle of magnetic activity. Conspicuous features of solar activity are the cyclic occurrences in the solar atmosphere of pairs (or groups) of sunspots with different polarity and with strong fields up to 5 kG. The polarity of these pairs has different orientation in both hemispheres and after a minimum of activity they reverse polarity. The large-scale magnetic field regions of the Sun are another remarkable magnetic manifestation. It changes polarity too, but at a maximumof sunspot activity. Many modern theories of a solar cycle consider the weak large-scale magnetic field as the result of breakup of strong fields of active regions and their drift to poles, i.e., as a secondary product of the activity of strong magnetic fields. The underlying idea is that the supergranular motions, differential rotation, and merid- ional flow transform the magnetic fields of active regions to form the large-scale field patterns (Babcock, 1961; Leighton, 1964; DeVore, Sheeley, and Boris, 1984; Wang, Nash, and Sheeley, 1989). However, many questions, e.g., concerning the duration of the solar cycle and magnetic field reversals of the Sun, remain open.

Properties of large electric fields in the plasma sheet at 4–7 R E measured with Polar

Abstract: Measurements from the Polar satellite provide evidence for large electric field structures in the plasma sheet at geocentric distances of 4–7 RE- These structures had amplitudes perpendicular to the ambient magnetic field that can exceed 100 m V m−1 (6 s averaged). Two years (from May 1, 1996, to April 30, 1998) of electric field data (EZ component, approximately along GSE z) were surveyed. The distribution in invariant latitude (ILAT) and magnetic local time (MLT) of large perpendicular electric field events (defined as ≥20 mV m−1 for a 6-s average) delineates the statistical auroral oval with the majority of events occurring in the nightside centered around midnight and a smaller concentration around 1500 MLT. The magnitude-versus-altitude distribution of the electric fields between 4 and 7 RE in the nightside could be explained by models which assume either shear Alfven waves propagating into regions of larger background magnetic fields or electrostatic structures being mapped quasi-statically along equipotential magnetic field lines. In addition, this survey yielded 24 very large amplitude events with |E⊥| ≥100 mV m−1 (6 s averaged), all of which occurred in the nightside. In the spacecraft frame, the electric field structures occurred on timescales ranging from 10 to 60 s. About 85% of these events occurred in the vicinity of the outer boundary of the plasma sheet; the rest occurred in the central plasma sheet. The polarity of the electric fields was dominantly perpendicular to the nominal plasma sheet boundary. For a large fraction of events (≤50%) the ratios of electric and magnetic fields in the period range from 10 to 60 s were consistent with Alfven waves. Large Poynting flux (up to 2.5 ergs cm−2s−1) dominantly directed downward along the background magnetic field was associated with 21 events. All 24 events occurred during geomagnetic disturbances such as magnetic substorms. A conjugate study with ground stations for 14 events (out of the 24 events) showed that these structures occurred during times of rapid changes in the H component (or X component) of magnetometer data. For most events this time corresponded to the expansion phase; two events occurred during a quick recovery of the negative H bay signature. Thus there is evidence that large electromagnetic energy transfer processes in the plasma sheet occur during the most dynamic phase of geomagnetic disturbances. From the statistical analysis it was found that Polar observed events larger than 100 mV m−1 (50 mV m−1) in the plasma sheet between 2100 and 0300 MLT with a 2–4% (15%) probability per crossing. These probabilities will be compared to the probability of substorm occurrence during Polar plasma sheet crossings.

Dawn/dusk asymmetry in particles of solar wind origin within the magnetosphere

Abstract: Solar wind/magnetosheath plasma in the mag- netosphere can be identified using a component that has a higher charge state, lower density and, at least soon after their entry into the magnetosphere, lower energy than plasma from a terrestrial source. We survey here observations taken over 3 years of He 2+ ions made by the Magnetospheric Ion Composition Sensor (MICS) of the Charge and Mass Magne- tospheric Ion Composition Experiment (CAMMICE) instru- ment aboard POLAR. The occurrence probability of these solar wind ions is then plotted as a function of Magnetic Lo- cal Time (MLT) and invariant latitude (3) for various energy ranges. For all energies observed by MICS (1.8-21.4 keV) and all solar wind conditions, the occurrence probabilities peaked around the cusp region and along the dawn flank. The solar wind conditions were filtered to see if this dawnward asymmetry is controlled by the Svalgaard-Mansurov effect (and so depends on the BY component of the interplanetary magnetic field, IMF) or by Fermi acceleration of He 2+ at the bow shock (and so depends on the IMF ratio BX/BY ). It is shown that the asymmetry remained persistently on the dawn flank, suggesting it was not due to effects associated with direct entry into the magnetosphere. This asymmetry, with enhanced fluxes on the dawn flank, persisted for lower energy ions (below a "cross-over" energy of about 23 keV) but reversed sense to give higher fluxes on the dusk flank at higher energies. This can be explained by the competing ef- fects of gradient/curvature drifts and the convection electric field on ions that are convecting sunward on re-closed field lines. The lower-energy He 2+ ions E ◊ B drift dawnwards as they move earthward, whereas the higher energy ions cur- vature/gradient drift towards dusk. The convection electric field in the tail is weaker for northward IMF. Ions then need less energy to drift to the dusk flank, so that the cross-over energy, at which the asymmetry changes sense, is reduced.

Topology of Magnetic Field and Coronal Heating in Solar Active Regions

Abstract: The photospheric vector magnetic fields, Hα and soft X-ray images of AR 7321 were simultaneously observed with the Solar Flare Telescope at Mitaka and the Soft X-ray Telescope of Yohkoh on October 26,1992, when there was no important activity in this region Taking the observed photospheric vector magnetic fields as the boundary condition, 3D magnetic fields above the photosphere were computed with a new numerical technique Then quasi-separatrix layers (QSLs), ie, regions where 3D magnetic reconnection takes place, were determined in the computed 3D magnetic fields Since Yohkoh data and Mitaka data were obtained in well-arranged time sequences during the day, the evolution of 3D fields, Ha features and soft X-ray features in this region can be studied in detail Through a comparison among the 3D magnetic fields, Hα features and soft X-ray features, the following results have been obtained: (a) Hα plages are associated with the portions of QSLs in the chromosphere; (b) diffuse coronal features (DCFs) and bright coronal features (BCFs) are morphologically confined by the coronal linkage of the field lines related to the QSLs; (c) BCFs are associated with a part of the magnetic field lines related to the QSLs These results suggest that as the likely places where energy release may occur by 3D magnetic reconnection, QSLs play an important role in the chromospheric and coronal heating in this active region

Dynamic model of the magnetosphere: Case study for January 9–12, 1997

Abstract: The dynamics of the magnetospheric current systems are studied in the course of the specific magnetospheric disturbance on January 9–12, 1997, caused by the interaction of the Earth's magnetosphere with a dense solar wind plasma cloud. To estimate the contribution of the different sources of the magnetospheric magnetic field to the disturbance ground measured, a dynamic paraboloid model of the magnetosphere is used. The model input parameters are defined by the solar wind density and velocity, by the strength and direction of the interplanetary magnetic field, and by the auroral AL index. The total energy of the ring current particles is calculated from the energy balance equation, where the injection function is determined by the value of the solar wind electric field. New analytical relations describing the dynamics of the different magnetospheric magnetic field sources dependent on the model input parameters are obtained. The analysis of the magnetic disturbances during the January 9–12, 1997, event shows that in the course of the main phase of the magnetic storm the contribution of the ring current, the currents on the magnetopause, and the currents in the magnetotail are approximately equal to each other by an order of magnitude. Nevertheless, in some periods one of the current systems becomes dominant. For example, an intense Dst positive enhancement (up to +50 nT) in the course of the magnetic storm recovery phase in the first hours on January 11, 1997, is associated with a significant increase of the currents on the magnetopause, while the ring current and the magnetotail current remain at a quiet level. A comparison of the calculated Dst variation with measurements indicates good agreement. The root mean square deviation is ∼ 8.7 nT in the course of the storm.

Electric Current and Magnetic Shear in Solar Active Regions

Abstract: The present Letter is aimed at the study of properties of the current and magnetic shear in solar active regions. The distribution of the magnetic shear and current provides information on the nonpotentiality of solar flare-producing regions. We propose a rate factor R = shear magnitude/helicity magnitude, which is an index of the deviation from the chiralical magnetic field. As an example, NOAA Active Region 6659 has been analyzed, which was a large δ magnetic configuration in 1991 June. It is found that the shear and gradient of the magnetic field are important and reflect a part of the electric current in solar active regions, which is notable in the vicinity of the magnetic neutral line, while the total current in the active region is mainly contributed by the helicity; i.e., it relates mainly with force-free equilibrium.

Field line diffusion in solar wind magnetic turbulence and energetic particle propagation across heliographic latitudes

Abstract: The transport of energetic particles in the heliosphere is strongly influenced by the magnetohydrodynamic turbulence found in the solar wind. This turbulence causes a magnetic field line random walk, which can explain the Ulysses observations at high heliographic latitudes of particles accelerated at corotating interaction regions (CIRs). A three-dimensional model of magnetic turbulence allows us to evaluate a nonquasi-linear magnetic field line diffusion coefficient, even in the case of anisotropic turbulence, as observed in the solar wind. The excursion in latitude of a magnetic field line starting from a CIR located at a large heliospheric distance r0 ∼ 8–10 AU, and at the limiting latitude for the direct observation of CIRs, ϑ ∼ 35°, is evaluated with a Monte Carlo simulation. In the calculations the random “force” terms are proportional to the square root of the diffusion coefficient in each direction. Considering that the correlation length in the latitudinal direction is larger than the one in the direction perpendicular to the mean magnetic field, we find that the magnetic field lines travel faster in the latitudinal direction. The implications for energetic particle transport are discussed and compared to the observation of energetic particle events at high solar latitudes by the Ulysses spacecraft. Both ion observations and electron observations can be explained by field line random walk.

Collisionless model of the solar wind in a spiral magnetic field

Abstract: We present a kinetic collisionless model of the solar wind generalized to take into account the spiral structure of the interplanetary magnetic field. This model, which also includes Kappa velocity distributions, calculates self-consistently the electric potential profile and derives the solar wind speed and the temperatures of the medium. We study how the inclusion of the spiral geometry changes the plasma parameters compared to the case of a radial magnetic field. Whereas the interplanetary electric potential, the wind density and bulk speed are not significantly changed, we show that the electron and proton temperatures are modified; in particular, we find a decrease of the proton temperature and of its anisotropy, and an increase of the electron temperature. We discuss these results and the validity of the model.

Simple estimation of equivalent magnetic dipole moment to characterize ELF magnetic fields generated by electric appliances incorporating harmonics

Abstract: A simple method of quantifying the ELF (extremely low frequency) magnetic field distribution around electric appliances, which takes the harmonics into account, is newly proposed. The proposed method involves: (1) a simple estimation of the position of an equivalent magnetic dipole moment inside an appliance, using two magnetic field meters; (2) identification of the amplitude of the dipole moment magnetic-field measurements at certain points; and (3) calculation of the magnetic field distribution around the appliance using the estimated dipole moment. In this method, the dipole moment vector is assumed to be a similar value by allowing an uncertainty of 6 dB in the estimated magnetic field, which enables easy estimation of the dipole moment. In addition, the frequency characteristics of the magnetic field are taken into account by considering the harmonic components in the magnetic field waveform. The proposed method was applied to 13 types of appliances, and their equivalent magnetic dipole moments and harmonic components were determined. The results revealed that the proposed method is applicable to many electric appliances. The conditions required for the adoption of the method were also clarified.

Transient electromagnetic field of a vertical magnetic dipole on a two-layer conducting Earth

Abstract: The transient field-frequency domain of magnetic dipole in a two-layered conducting Earth has been studied. It is obtained as a summation of waveguide modes plus contributions from branch cuts in the complex-plane of the longitudinal wave number. Results can be used to evaluate numerical solutions of more complicated modeling algorithms.

Electrodynamics of the poleward auroral border observed by Polar during a substorm on April 22, 1998

Abstract: Observations from Polar during a substorm on April 22, 1998, are used to specify electrodynamic characteristics of the high-latitude auroral boundary on the nightside. Polar was moving equatorward near invariant latitude 72°, 2305 magnetic local time as it crossed the auroral boundary near the end of the substorm's expansion phase. This boundary was marked by severe east-west plasma flow shears, a reversal of the in-track electric field component, and multiple field-aligned currents. Harmonizing ground measurements with auroral images and in situ particle and field data from Polar reveals five electrodynamic features of the boundary. (1) A 20-min delay occurred between substorm onset and when the total magnetic flux in the polar cap began to decrease. This represents the time that elapsed before reconnection of open lobe flux began along a near-Earth X-line. (2) The reconnection electric field at the ionospheric projection of the X-line ranged between 20 and 70 mV m−1. Reconnection was intermittent, turning on and off at different locations. (3) Electric and magnetic field structures observed by Polar suggest that Alfven waves propagating along the auroral boundary carried a double-layer current. Downward Poynting flux was observed at the poleward auroral boundary associated with these currents. (4) Magnetic and electric field oscillations with periods of ∼90 s were detected on open field lines beginning ∼4 min before Polar entered the auroral oval. Oscillations with similar frequencies were observed both on the ground near Polar's magnetic footprint and at geosynchronous orbit. This indicates that the oscillations represent a large-scale phenomenon occurring over a large portion of the nightside magnetosphere. Coupling on open field lines derives from fringing fields associated with ionospheric closure of DP 1 currents. (5) Upward flowing hydrogen and oxygen ions were detected at and equatorward of the auroral boundary. Perpendicularly accelerated O+ ions detected in the immediate vicinity of the boundary can be explained by direct acceleration by the ambient electric field perpendicular to the local magnetic field. Equatorward of the boundary, O+ distributions were typical of ion conics.

Thermospheric vertical wind activity maps derived from Dynamics Explorer‐2 WATS observations

Abstract: Large-scale distribution of thermospheric vertical wind activity was studied using observations from the Wind and Temperature Spectrometer on the Dynamics Explorer-2 satellite. We calculated the vertical velocity standard deviation, σ(Vz), within a sliding window of width 120 seconds, corresponding to an along-track distance of ∼900 km. Maps of σ(Vz) in local magnetic time and invariant latitude reveal a high-latitude region of enhanced activity that largely fills the polar cap. Activity appears to be a maximum in the midnight-dawn sector of the cap, possibly at the nominal auroral oval latitude. We provisionally interpret our results as evidence of polar cap gravity waves with sources in or near the midnight-dawn auroral oval. Waves propagating poleward effectively fill the cap with vertical wind oscillations. Equatorward propagating waves move approximately parallel to the local thermospheric wind, resulting in the possibility of critical-layer dissipation.

Earth's Magnetic Field: Ocean Current Contributions to Vertical Profiles in Deep Oceans

Abstract: SUMMARY The Earth’s main magnetic field, arising in the core, should, in the ocean, have a welldefined uniform gradient with depth. Superimposed upon this uniform gradient may be magnetic signals due to a variety of sources. These include crustal magnetization, the transient fluctuations arising external to the Earth and causing secondary induced fields within it; and, the focus of the present paper, magnetic signals arising from the motional induction of seawater moving in the steady main magnetic field of Earth. There are circumstances where theory predicts such motionally-induced magnetic fields to be of order 10 2 nT, and to vary with depth in a way which is directly related to the velocity profile. Exploratory soundings of the magnetic field with depth have been made in the oceans around Australia, both to test these predictions, and to investigate the practicability of measuring such profiles. The magnetic field parameter observed has been that of the ‘total field’, which should sense the component of the ocean velocity which lies in the magnetic meridian. The magnetometer has been lowered by cable from a ship and also operated free-fall to the seafloor (and return). The observations appear both to confirm the theoretical gradient of the main field where there is no ocean current and, where ocean currents exist, to give evidence of their profiles resolved in the direction of magnetic north. In particular, observations taken in an eddy of the East Australian Current show the correct contrast in sign for north and south flowing streams.

Nuclear matter in intense magnetic field and weak processes

Abstract: We study the effect of magnetic field on the dominant neutrino emission processes in neutron stars. The processes are first calculated for the case when the magnetic field does not exceed the critical value to confine electrons to the lowest Landau state. We consider here magnetic fields up to ~1019 Gauss. We find that fields of strength 1015-1016 Gauss have significant effect on the neutronization process, for the direct URCA process the effect of the magnetic field becomes significant only at ~1019 Gauss. In order to estimate the effect we derive the composition of cold nuclear matter at high densities and in beta equilibrium, a situation appropriate for neutron stars. The hadronic interactions are incorporated through the exchange of scalar and vector mesons in the framework of relativistic mean field theory. In addition the effects of anomalous magnetic moments of nucleons are also considered.

An Estimate of Drift Effects in Various Models of the Heliospheric Magnetic Field

Abstract: We use a comparison with other models to estimate how significant drifts will be in the Fisk model of the heliospheric magnetic field (HMF). We also show that drifts are still be present in an idealized field model for the period shortly after the solar polarity reversal.

Soft X-ray Solar Activities Associated with Interplanetary Magnetic Flux Ropes

Abstract: We studied the soft X-ray solar events that could be associated with the interplanetary magnetic flux ropes observed by the WIND satellite during 1995 through 1998. The timings of the launches of the magnetic flux ropes from the Sun were estimated using flux rope speeds derived by the fitting of a cylindrical model. In the reasonable time window, soft X-ray solar signatures were found in approximately 70% of the flux ropes. Parameters (e.g., axis direction, strength of magnetic field, radius, and helicity) of the magnetic flux ropes obtained by the model fitting were compared with the characteristics of the corresponding soft X-ray events observed by Yohkoh. According to the result of the comparison, the magnetic flux ropes with strong magnetic fields or high speeds were observed in association with higher soft X-ray solar activities.

Magnetic dipole model of a permanent magnet based device

Abstract: The present paper describes the electromagnetic field of a permanent magnet based device comprising parts that have a rotational movement. The equivalence between the field of a permanent magnet and a magnetic dipole constitutes the main assumption. The real device is thus modelled by a hypothetical structure made of magnetic dipoles. To calculate the magnetic and the electric field, the Hertz vector potential approach was used. The magnetic dipole modelling shows a resultant magnetic field which is rotating 15 times faster than its field source, it underlines ways by which the anisotropy of the magnetic field can be greatly enhanced, and it succeeds in providing concise computational formulae for the field and the energy flow within the field that can be related to results of a more traditional approach, such as the finite element method.

On the role of charge exchange in the formation of the Martian magnetic pileup boundary

Abstract: In the magnetic pileup boundary (MPB) of Mars a steep increase in the magnitude of the magnetic field is observed in the data from the magnetometer on board Mars Global Surveyor, and the growth of the field is more precipitous than common solar wind-ionosphere models predict. Analysis of both Venus and Mars magnetic field data has strongly implied that plasma pressure is exchanged for magnetic pressure in the magnetic pileup field. To explain this, effects of the Martian exosphere must be included. Then charge exchange between the shock-compressed solar wind protons, which dominate the thermal pressure, and neutral exospheric atoms (mainly H and O) should play a crucial role in the formation of the MPB, as is also inherently required by the conservation of momentum flux. To test this idea, a model of the solar wind interaction with the Martian upper atmosphere, with charge exchange included, is constructed to reproduce the observed magnetic field of the MPB of Mars. The simulation results show that charge exchange is able to account for the sudden buildup of the magnetic field in the Martian MPB, with the implication that the same mechanism produces the magnetic field increase at Venus and comets.