Showing papers on "Earth's magnetic field published in 2001"

An improved model of ionospheric electric potentials including substorm perturbations and application to the Geospace Environment Modeling November 24, 1996, event

Abstract: An improved model of ionospheric electric potentials/convection patterns is presented here. This model will produce potentials for any desired level of interplanetary magnetic field (IMF), solar wind velocity, number density, and dipole tilt angle, as well as an optional value for the AL index. The following improvements have been made: (1) Terms for solar wind electric field and dynamics pressure have been added. (2) Nonlinear changes in the potentials as the magnitude of the IMF increases, owing to the decreasing effective width of the solar wind coupling region, are now allowed. (3) The lower boundary of the potential patterns is now variable according to the conditions, rather than fixed at an arbitrary location, resulting in fewer coefficients in the spherical harmonic expansion and smoother patterns. (4) The influence of magnetospheric substorms, or nightside processes, on the potential patterns is now included as a perturbation using the AL index as an optional controlling parameter. The utility of the new substorm component is demonstrated with the event on November 24, 1996, which had been chosen for the “Geospace Environment Modeling (GEM) Substorm Challenge.” By an examination of the potential patterns over the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer chain, it is shown that the IMF alone is not sufficient to predict the electric potential patterns when there are substorms present, demonstrating the influence of the substorm “unloading” component. This model can reproduce the very complex changes that occur in the potential patterns as the IMF orientation rotates in the GSM Y-Z plane, and it is useful for scientific studies as input for other models or validation of computer simulation results. There is also a practical application of predicting “space weather” in the magnetosphere, on the basis of upstream solar wind/IMF measurements. The later application will become more effective when accurate methods become available for predicting the AL indices.

Modeling ring current proton precipitation by electromagnetic ion cyclotron waves during the May 14-16, 1997, storm

Abstract: We study mechanisms contributing to proton precipitation from the ring current during the May 14–16, 1997, geomagnetic storm. This storm was caused partly by Bz< 0 fields in the sheath region behind an interplanetary shock and partly by the magnetic cloud driving the shock. The storm was characterized by a maximum Kp=7− and a minimum Dst=−115 nT and had a distinctive two-phase decay related to the passage of the ejection at the Earth. We model the ring current development caused by adiabatic drifts and losses due to charge exchange, Coulomb collisions, wave-particle interactions, and atmospheric collisions at low altitudes. The nightside magnetospheric inflow is simulated using geosynchronous Los Alamos National Laboratory data, whereas the dayside free outflow corresponds to losses through the dayside magnetopause. We calculate the equatorial growth rate of electromagnetic ion cyclotron waves with frequencies between the oxygen and helium gyrofrequencies and their integrated wave gain as the storm progresses. The regions of maximum wave amplification compare reasonably well to satellite observations. A time-dependent global wave model is constructed, and the spatial and temporal evolution of precipitating proton fluxes during different storm phases is determined. We find that the global patterns of proton precipitation are very dynamic: located at larger L shells during prestorm conditions, moving to lower L shells as geomagnetic activity increases during storm main phase, and receding back toward larger L shells with storm recovery. However, the most intense fluxes are observed along the duskside plasmapause during the main and early recovery phase of the storm and are caused by plasma wave scattering. This study is relevant to the analysis of the anticipated new data sets from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) and Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) missions.

Global simulation of the Geospace Environment Modeling substorm challenge event

Abstract: We use a global model of Earth's magnetosphere and ionosphere to simulate the Geospace Environment Modeling (GEM) substorm challenge event of November 24, 1996. We compare our results to International Monitor for Auroral Geomagnetic Effects (IMAGE) ground magnetometer data, assimilative mapping of ionospheric electrodynamics (AMIE) polar cap potential and field aligned current patterns, Polar Visible Imaging System (VIS) estimates of the polar cap magnetic flux, GOES 8 geosynchronous magnetometer data, IMP 8 magnetometer data, and Geotail plasma and magnetic field data. We find generally good agreement between the simulation and the data. The modeled evolution of this substorm generally follows the phenomenological near-Earth neutral line model. However, reconnection in the tail is very localized, which makes establishing a causal relation between tail dynamics and auroral dynamics difficult, if not impossible. We also find that the model results critically depend on the parameterization of auroral Hall and Pedersen conductances and anomalous resistivity in the magnetosphere. For many combinations of parameters that enter these parameterizations, no substorm develops in the model, but instead the magnetosphere enters a steady convection mode. The main deviation of the model from the data is excessive convection, which leads to a strong, driven westward electrojet in the growth phase, only partial tail loading, and a reduced recovery phase. Possible remedies are a better model for auroral conductances, an improved anomalous resistivity model, and a more realistic treatment of the ring current.

DMSP observations of equatorial plasma bubbles in the topside ionosphere near solar maximum

Abstract: The Defense Meteorological Satellite Program (DMSP) flights F9 and F10 crossed postsunset local time sectors approximately 14 times per day in Sun-synchronous orbits at an altitude of ∼840 km. We have examined a large database of postsunset plasma density measurements acquired during ∼ 15,000 equatorial crossings made by DMSP F9 in 1989 and 1991 and DMSP F10 in 1991. On 2086 of these crossings equatorial plasma bubbles (EPBs) were observed as intervals of depleted and irregular plasma densities. We have analyzed these EPB events to determine their distributions with season, longitude (S/L), and levels of geomagnetic activity. The global S/L distributions of EPBs observed by the DMSP satellites are shown to be in general agreement with results from discrete ground-based measurements. That is, the seasonal variations detected at 840 km in longitude bins hosting radar/scintillation observatories appear similar to those reported from the ground. Over the Atlantic sector where EPBs occur frequently, we found good agreement with predictions of a simple model proposed by Tsunoda [1985]. In the Pacific sector the frequency of EPB occurrence is considerably lower, and poor counting statistics preclude confident predictions regarding the absolute value of seasonal variations. We suggest that relatively large equatorial magnetic fields at Flayer altitudes in the Pacific (∼0.34 G) sector more strongly inhibit the growth of the Rayleigh-Taylor instability than at Atlantic (∼0.25 G) longitudes. Contrary to general belief, we found that EPBs occurred regularly during geomagnetic storms, especially in the initial and main phases. EPB activity appears to have been suppressed from many hours to clays during storm recovery phases.

The shock-acoustic waves generated by earthquakes

Abstract: . We investigate the form and dynamics of shock-acoustic waves generated by earthquakes. We use the method for detecting and locating the sources of ionospheric impulsive disturbances, based on using data from a global network of receivers of the GPS navigation system, and require no a priori information about the place and time of the associated effects. The practical implementation of the method is illustrated by a case study of earthquake effects in Turkey (17 August and 12 November 1999), in Southern Sumatra (4 June 2000), and off the coast of Central America (13 January 2001). It was found that in all instances the time period of the ionospheric response is 180–390 s, and the amplitude exceeds, by a factor of two as a minimum, the standard deviation of background fluctuations in total electron content in this range of periods under quiet and moderate geomagnetic conditions. The elevation of the wave vector varies through a range of 20–44°, and the phase velocity (1100–1300 m/s) approaches the sound velocity at the heights of the ionospheric F-region maximum. The calculated (by neglecting refraction corrections) location of the source roughly corresponds to the earthquake epicenter. Our data are consistent with the present views that shock-acoustic waves are caused by a piston-like movement of the Earth’s surface in the zone of an earthquake epicenter. Key words. Ionosphere (ionospheric disturbances; wave propagation) – Radio science (ionospheric propagation)

Time derivative of the horizontal geomagnetic field as an activity indicator

Abstract: . Geomagnetically induced currents (GICs) in technological conductor systems are a manifestation of the ground effects of space weather. Large GICs are always associated with large values of the time derivative of the geomagnetic field, and especially with its horizontal component (dH/dt). By using the IMAGE magnetometer data from northern Europe from 1982 to 2001, we show that large dH/dt’s (exceeding 1 nT/s) primarily occur during events governed by westward ionospheric currents. However, the directional distributions of dH/dt are much more scattered than those of the simultaneous baseline subtracted horizontal variation field vector ΔH. A pronounced difference between ΔH and dH/dt takes place at about 02–06 MLT in the auroral region when dH/dt prefers an east-west orientation, whereas ΔH points to the south. The occurrence of large dH/dt has two daily maxima, one around the local magnetic midnight, and another in the morning. There is a single maximum around the midnight only at the southernmost IMAGE stations. An identical feature is observed when large GICs are considered. The yearly number of large dH/dt values in the auroral region follows quite closely the aa index, but a clear variation from year-to-year is observed in the directional distributions. The scattering of dH/dt distributions is smaller during descending phases of the sunspot cycle. Seasonal variations are also seen, especially in winter dH/dt is more concentrated to the north-south direction than at other times. The results manifest the importance of small-scale structures of ionospheric currents when GICs are considered. The distribution patterns of dH/dt cannot be explained by any simple sheet-type model of (westward) ionospheric currents, but rapidly changing north-south currents and field-aligned currents must play an important role. Key words. Geomagnetism and paleomagnetism (geomagnetic induction; rapid time variations) - Ionosphere (ionospheric disturbances)

On the solar wind control of Pc5 ULF pulsation power at mid‐latitudes: Implications for MeV electron acceleration in the outer radiation belt

Abstract: On the basis of observations made at six sites (L shell range 3.75–6.79) belonging to the International Monitor for Auroral Geomagnetic Events (IMAGE) magnetometer array, we provide a statistical study of the relationship of solar wind speed to daily ULF pulsation power for 1995, 1996, and 1997, an interval corresponding to the declining phase of solar cycle 22. We find a very strong positive correlation between solar wind speed and dayside magnetospheric pulsation power across the entire L shell range studied. In addition, we identify a number of geomagnetic storm onset days on the basis of Dst variations and demonstrate that on these days large-amplitude ULF pulsation activity can be driven during only moderate solar wind speed conditions. Removing the onset day population improves the correlation of pulsation power to solar wind speed and demonstrates how an improved solar wind speed proxy based on pulsation power can be determined by considering Dst. We also demonstrate an exponential decay of 1–10 mHz Pc5 wave power with decreasing L shell, the decay rate increasing with solar wind speed, thus indicating a stronger dependence of pulsation power on solar wind speed at higher L shells in the range L=3.75–6.79. Further, the L shell range studied is consistent with that over which storm-time enhancements of MeV electron fluxes have been observed in the outer radiation belt, and we discuss the implications of the dependence of PcS ULF wave power on both solar wind speed and L shell for mechanisms which implicate large-amplitude ULF pulsations in the acceleration of storm-time MeV energy electrons.

High Geomagnetic Intensity During the Mid-Cretaceous from Thellier Analyses of Single Plagioclase Crystals

Abstract: Recent numerical simulations have yielded the most efficient geodynamo, having the largest dipole intensity when reversal frequency is low. Reliable paleointensity data are limited but heretofore have suggested that reversal frequency and paleointensity are decoupled. We report data from 56 Thellier-Thellier experiments on plagioclase crystals separated from basalts of the Rajmahal Traps (113 to 116 million years old) of India that formed during the Cretaceous Normal Polarity Superchron. These data suggest a time-averaged paleomagnetic dipole moment of 12.5 ± 1.4 × 1022 amperes per square meter, three times greater than mean Cenozoic and Early Cretaceous–Late Jurassic dipole moments when geomagnetic reversals were frequent. This result supports a correlation between intervals of low reversal frequency and high geomagnetic field strength.

Role of the magnetosheath flow in determining the motion of open flux tubes

Abstract: We have constructed a model which examines the motion of reconnected magnetic flux tubes over the surface of the magnetopause. For a given interplanetary magnetic field (INT) we first determine the draping and strength of the magnetosheath magnetic field, flow velocity, and density over the entire surface of a paraboloid magnetopause. For a given magnetopause location we apply a test for steady state reconnection occurring between the magnetosheath field and a modeled magnetospheric field at that point. We trace the subsequent motion of these tubes along the surface of the magnetopause and into the magnetotail. Results are shown for a range of cases. The model has applications in testing various hypotheses about the location of reconnection events and, for example, IMF B-Y effects. In particular, it highlights the dominance of the magnetosheath flow model in determining the motion of the tubes and the necessity of sub-Alfvenic magnetosheath flows for the occurrence of steady state reconnection poleward of the cusp during periods of northward IMF. Our model may also be used to identify likely reconnection sites on the dayside and near-Earth nightside magnetopause and to identify possible locations for steady state reconnection.

Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere

Abstract: The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field(1-3). On adjacent geomagnetic field lines th ...

Orientation of the Magnetic Fields in Interplanetary Flux Ropes and Solar Filaments

Abstract: Coronal mass ejections (CMEs) are often associated with erupting magnetic structures or disappearing filaments. The majority of CMEs headed directly toward the Earth are observed at 1 AU as magnetic clouds—the region in the solar wind where the magnetic field strength is higher than average and there is a smooth rotation of the magnetic field vectors. The three-dimensional structure of magnetic clouds can be represented by a force-free flux rope. When CMEs reach the Earth, they may or may not cause magnetic storms, alter Earth's magnetic field, or produce the phenomena known as auroras. The geoeffectiveness of a solar CME depends on the orientation of the magnetic field in it. Two M-class solar flares erupted on 2000 February 17. The second flare occurred near a small active region, NOAA Active Region 8872. This eruption was accompanied by a halo CME. However, the February 17 CME did not trigger any magnetic activity when it arrived at the Earth. Another powerful flare, on 2000 July 14, was also associated with a halo CME, which caused the strongest geomagnetic activity of solar cycle 23. Using ACE measurements of the interplanetary magnetic fields, we study the orientation of the magnetic flux ropes in both sets of magnetic clouds and compare them with the orientation of the solar magnetic fields and disappearing filaments. We find that the direction of the axial field and helicity of the flux ropes are consistent with those of the erupted filaments. Thus, the geoeffectiveness of a CME is defined by the orientation and structure of the erupted filament and by its magnetic helicity as well. We also suggest that the geoeffectiveness of a CME can be forecasted using daily full-disk Hα and Yohkoh images and MDI magnetograms as well.

Ørsted satellite captures high‐precision geomagnetic field data

Abstract: Space-based, high-precision magnetometry is essential for understanding a variety of phenomena ranging from secular variation of the Earth's main field, through the signatures of crustal magnetism and the effects of plasma currents flowing externally to the Earth. Orsted, Denmark's first satellite, was launched on February 23, 1999 into a polar, low-Earth orbit to provide the first near-global set of high-precision geomagnetic observations since the Magsat mission of 1979–1980 (see Magsat Special Issue of Geophysical Research Letters., vol. 9, no. 4, pp. 239–379, 1982). With the new mapping of the Earth's magnetic field, the International Geomagnetic Reference Field model (IGRF), a standard model used for navigation, prospecting, and other practical purposes, has been determined with improved precision for epoch 2000 [Olsen et al., 2000a; Mandea and Langlais, 2000]. The satellite has routinely provided high-precision vector data since August 1999, and the mission is continuing well beyond its nominal 14-month lifetime into 2001.

Solar illumination as cause of the equinoctial preference for geomagnetic activity

Abstract: Geomagnetic and auroral activity vary seasonally with maxima at equinoxes, as has been known for more than a century. The cause remains under debate. The angle made by the Earth's dipole axis with the typical direction of the interplanetary magnetic field (IMF) can explain a portion (about 17%) of the effect. To explain the majority of the equinoctial effect, we suggest that geomagnetic activity peaks when the nightside auroral zones of both hemispheres are in darkness, as happens at equinox. Under such conditions, no conducting path exists in the ionosphere to complete the currents required by solar wind-magnetosphere-ionosphere coupling, and geomagnetic disturbances maximize. To test this theory, the Universal Time (UT) variation of geomagnetic activity was explored. As our model predicts, geomagnetic activity in December, measured by the Am index, evinces a deep minimum around 0300–0600 UT when the auroral oval of both hemispheres are in darkness and a maximum around 1500–1600 UT when the southern nightside oval is sunlit. In June, complementary effects are predicted and observed. Previous studies using the AE index have shown more ambiguous results. Here we show that if AE is resolved into the AU and AL components, the discrepancy disappears, with the AL component following the same pattern as does Am. We thus conclude that the intensity of global geomagnetic activity is well ordered by whether the nightside auroral oval is sunlit in one hemisphere or neither.

A 50‐degree spherical harmonic model of the magnetic field of Mars

Abstract: This paper presents a 50-degree and order spherical harmonic model of the magnetic field of Mars derived at 120-km altitude using the three orthogonal components of the vector magnetic field data acquired by the Mars Global Surveyor within the 80- to 200-km-altitude range. The downward continued vector magnetic field components to the surface of Mars delineate details of the Martian magnetic field. The strong magnetic anomalies in the southern hemisphere do not show compelling evidence for magnetic lineations similar to those associated with the seafloor spreading on Earth. The anomalies are more or less equidimensional. The magnetic signatures surrounding the impact basins Hellas, Argyre, and Isidis show that the prominent magnetic anomalies are older than the impact events. An average magnetization of ∼2 A/m is estimated for the upper crust surrounding Hellas basin. Four isolated magnetic anomalies are modeled by vertical prisms with circular cross sections of 170- to 190-km radius and 20- to 30-km thickness. The magnetization of the model prisms is 5 A/m more than that of the surroundings.

Observations of geomagnetic cutoff variations during solar energetic particle events and implications for the radiation environment at the Space Station

Abstract: Data from the polar-orbiting Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite have been used to measure the location of the geomagnetic cutoff for low-energy protons and alpha particles during several large solar energetic particle events from mid-1992 to late 1998. When fluxes are sufficiently high, the cutoff latitude can be measured up to four times per orbit, allowing the variability of the cutoff to be observed on relatively short timescales. We find significant changes in the cutoff location, often by more than 5° in less than 1 day, and these changes are well correlated with geomagnetic activity as measured by either Dst or Kp. Spacecraft in intermediate-inclination orbits such as the International Space Station (ISS) graze the geomagnetic polar cap at certain longitudes each day. Calculations show that a 5° suppression in the average geomagnetic cutoff increases by more than a factor of 2.5 the time that the ISS spends in the polar cap exposed to energetic particles. Since the Station is only vulnerable at certain longitudes, however, real-time monitoring of the cutoff location from a polar-orbiting spacecraft could be used to provide advance notice of the polar cap location and conditions, sometimes hours before the Space Station itself reaches high magnetic latitudes.

Space storms and space weather hazards

Abstract: Preface. 1. Space Storms, Ring Current and Space-Atmosphere Coupling I.A. Daglis. 2. Geomagnetic Storms as a Dominant Component of Space Weather: Classic Picture and Recent Issues Y. Kamide. 3. From the Discovery of Radiation Belts to Space Weather Perspectives J.F. Lemaire. 4. The Interplanetary Causes of Magnetic Storms, Substorms and Geomagnetic Quiet B.T. Tsurutani. 5. Interplanetary Magnetic Field Dynamics N.F. Ness. 6. Bayesian Classification of Geoeffective Solar Wind Structures J. Chen. 7. Coronal Mass Ejections at the Sun and in Interplanetary Space P.J. Cargill. 8. Measurements of Energetic Particles in the Radiation Belts B. Klecker. 9. Solar Activity Variations and Possible Effects on Climate E. Friis-Christensen. 10. Cosmic Ray and Radiation Belt Hazards for Space Missions M.I. Panasyuk. 11. Satellite Anomalies due to Space Storms D.N. Baker. 12. Space Weather Effects on Communications L.J. Lanzerotti. 13. An Introduction to Power Grid Impacts and Vulnerabilities from Space Weather J.G. Kappenman. 14. Global Magnetospheric Modelling M. Scholer. 15. The Role and Form of Modelling in Space Weather K. Papadopoulos. 16. MHD Modelling of Space Weather Drivers K. Tsinganos. 17. State of the Art in Space Weather Services and Forecasting J.A. Joselyn. 18. Space Weather: An Air Force Research Laboratory Perspectives G.P. Ginet. 19. ESA Space Weather Activities E.J. Daly. Index.

Long‐term variations in the magnetic fields of the Sun and the heliosphere: Their origin, effects, and implications

Abstract: Recent studies of the variation of geomagnetic activity over the past 140 years have quantified the “coronal source”magnetic flux Fs that leaves the solar atmosphere and enters the heliosphere and have shown that it has risen, on average, by an estimated 34% since 1963 and by 140% since 1900. This variation of open solar flux has been reproduced by Solanki et al. [2000] using a model which demonstrates how the open flux accumulates and decays, depending on the rate of flux emergence in active regions and on the length of the solar cycle. We here use a new technique to evaluate solar cycle length and find that it does vary in association with the rate of change of Fs in the way predicted. The long-term variation of the rate of flux emergence is found to be very similar in form to that in Fs, which may offer a potential explanation of why Fs appears to be a useful proxy for extrapolating solar total irradiance back in time. We also find that most of the variation of cosmic ray fluxes incident on Earth is explained by the strength of the heliospheric field (quantified by Fs) and use observations of the abundance of the isotope 10Be (produced by cosmic rays and deposited in ice sheets) to study the decrease in Fs during the Maunder minimum. The interior motions at the base of the convection zone, where the solar dynamo is probably located, have recently been revealed using the helioseismology technique and found to exhibit a 1.3-year oscillation. This periodicity is here reported in observations of the interplanetary magnetic field and geomagnetic activity but is only present after 1940. When present, it shows a strong 22-year variation, peaking near the maximum of even-numbered sunspot cycles and showing minima at the peaks of odd-numbered cycles. We discuss the implications of these long-term solar and heliospheric variations for Earth’s environment.

Deep-Earth reactor: nuclear fission, helium, and the geomagnetic field.

Abstract: Geomagnetic field reversals and changes in intensity are understandable from an energy standpoint as natural consequences of intermittent and/or variable nuclear fission chain reactions deep within the Earth. Moreover, deep-Earth production of helium, having (3)He/(4)He ratios within the range observed from deep-mantle sources, is demonstrated to be a consequence of nuclear fission. Numerical simulations of a planetary-scale geo-reactor were made by using the SCALE sequence of codes. The results clearly demonstrate that such a geo-reactor (i) would function as a fast-neutron fuel breeder reactor; (ii) could, under appropriate conditions, operate over the entire period of geologic time; and (iii) would function in such a manner as to yield variable and/or intermittent output power.

GEOSAIL: Exploring the Geomagnetic Tail Using a Small Solar Sail

Abstract: Conventional geomagnetic tail missions require a spacecraft to be injected into a long elliptical orbit to explore the spatial structure of the geomagnetic tail. However, because the elliptical orbit is inertially fixed and the geomagnetic tail is directed along the sun-Earth line, the apse line of the elliptical orbit is precisely aligned with the geomagnetic tail only once every year. To artificially precess the apse line of the elliptical orbit in a sun-synchronous manner, which would keep the spacecraft in the geomagnetic tail during the entire year, would require continuous low-thrust propulsion or periodic impulses from a high-thrust propulsion system. Both of these options require reaction mass that will ultimately limit the mission lifetime. It is demonstrated that sun-synchronous apse-line precession can be achieved using only a small, low-cost solar sail. Because solar sails do not require reaction mass, a geomagnetic tail mission can be configured that provides a continuous science return by permanently stationing a science payload within the geomagnetic tail.

Digital signal processing method and system thereof for precision orientation measurements

Abstract: The present invention provides a digital signal processing method and system thereof for producing precision platform orientation measurements and local Earth's magnetic measurements by measuring threes axes gravity acceleration digital signals by an acceleration producer, detecting Earth's magnetic field vector measurement by an Earth's magnetic field detector to achieve digital three-axes Earth's magnetic field vector signals, and producing pitch, roll, and heading angles using said three-axes gravity acceleration digital signals and said digital three-axes Earth magnetic field vector signals by a Digital Signal Processor (DSP) chipset.

Seasonal variations of the ionospheric total electron content in Asian equatorial anomaly regions

Abstract: The ionospheric total electron contents (TEC) in both northern and southern equatorial anomaly regions are examined by using the Global Positioning System (GPS) in Asian area. The TEC contour charts obtained at YMSM (25.2°N, 121.6°E; 14.0°N geomagnetic) and DGAR (7.3°S, 72.4°E; 16.2°S geomagnetic) stations in 1997, solar minimum, are investigated. It is found that the ionospheric crests manifest remarkable seasonal variations. The TEC values on both northern and southern equatorial anomaly crests yield their maximum values during the vernal and autumnal months, but the winter anomaly does not appear in the southern region. Results show that both crests are fully developed around midday in winter, postnoon in equinoxes and late afternoon in summer, and the two crests move significantly equatorward in winter but slightly poleward in summer and autumn. These phenomena can be fully explained by a combined theory of the transequatorial netural wind, the subsolar point, and the auroral equatorward wind.