Showing papers on "Earth's magnetic field published in 1999"

Effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F

Abstract: We use radar observations from the Jicamarca Observatory from 1968 to 1992 to study the effects of the F region vertical plasma drift velocity on the generation and evolution of equatorial spread F The dependence of these irregularities on season, solar cycle, and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts In the early night sector, the bottomside of the F layer is almost always unstable The evolution of the unstable layer is controlled by the history of the vertical drift velocity When the drift velocities are large enough, the necessary seeding mechanisms for the generation of strong spread F always appear to be present The threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux The geomagnetic control on the generation of spread F is season, solar cycle, and longitude dependent These effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields The occurrence of early night spread F decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset The generation of late night spread F requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour These irregularities occur most often at ∼0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes The occurrence of late night spread F is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux

Global changes in intensity of the Earth's magnetic field during the past 800kyr

Abstract: Recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the Earth's past magnetic field at numerous field sites. The compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the Earth's geodynamo itself. A previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1, with the geomagnetic origin of the signal supported by an independent analysis of 10Be production made on different cores2. The persistence of long-term features in the Earth's magnetic intensity or the existence of long-term periodic changes cannot, however, be resolved in this relatively short time span. Here we present the integration of 33 records of relative palaeointensity3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19 into a composite curve spanning the past 800 kyr. We find that the intensity of the Earth's dipole field has experienced large-amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction, reflecting the emergence of non-dipole components. No stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the Earth's orbital parameters have a direct and strong influence on the geodynamo.

The terrestrial ring current: Origin, formation, and decay

Abstract: The terrestrial ring current is an electric current flowing toroidally around the Earth, centered at the equatorial plane and at altitudes of ;10,000 - 60,000 km. Changes in this current are responsible for global decreases in the Earth's surface magnetic field, which are known as geomagnetic storms. Intense geomagnetic storms have severe effects on technological systems, such as disturbances or even permanent damage to tele- communication and navigation satellites, telecommuni- cation cables, and power grids. The main carriers of the storm ring current are positive ions, with energies from ;1 keV to a few hundred keV, which are trapped by the geomagnetic field and undergo an azimuthal drift. The ring current is formed by the injection of ions originating in the solar wind and the terrestrial ionosphere. The injection process involves electric fields, associated with enhanced magnetospheric convection and/or magneto- spheric substorms. The quiescent ring current is carried mainly by protons of predominantly solar wind origin, while geospace activity tends to increase the abundance (both absolute and relative) of O 1 ions, which are of ionospheric origin. During intense magnetic storms, the O 1 abundance increases dramatically, resulting in a rapid intensification of the ring current and an O 1 dominance around storm maximum. This compositional change affects, among other processes, the decay of the ring current through the species- and energy-dependent charge exchange and wave-particle scattering loss. En- ergetic neutral atoms, products of charge exchange, en- able global imaging of the ring current and are the most promising diagnostic tool of ring current evolution. This review will cover the origin of ring current particles, their transport and acceleration, the effects of composi- tional variations in the ring current, the effects of sub- storms on ring current growth, and the dynamics of ring current decay with an emphasis on the process of charge exchange and the potential for wave scattering loss.

A doubling of the Sun's coronal magnetic field during the past 100 years

Abstract: The solar wind is an extended ionized gas of very high electrical conductivity, and therefore drags some magnetic flux out of the Sun to fill the heliosphere with a weak interplanetary magnetic field1,2. Magnetic reconnection—the merging of oppositely directed magnetic fields—between the interplanetary field and the Earth's magnetic field allows energy from the solar wind to enter the near-Earth environment. The Sun's properties, such as its luminosity, are related to its magnetic field, although the connections are still not well understood3,4. Moreover, changes in the heliospheric magnetic field have been linked with changes in total cloud cover over the Earth, which may influence global climate5. Here we show that measurements of the near-Earth interplanetary magnetic field reveal that the total magnetic flux leaving the Sun has risen by a factor of 1.4 since 1964: surrogate measurements of the interplanetary magnetic field indicate that the increase since 1901 has been by a factor of 2.3. This increase may be related to chaotic changes in the dynamo that generates the solar magnetic field. We do not yet know quantitatively how such changes will influence the global environment.

Numerical modeling of the geodynamo: Mechanisms of field generation and equilibration

Abstract: Numerical calculations of fluid dynamos powered by thermal convection in a rotating, electrically conducting spherical shell are analyzed. We find two regimes of nonreversing, strong field dynamos at Ekman number 10 -4 and Rayleigh numbers up to 11 times critical. In the strongly columnar regime, convection occurs only in the fluid exterior to the inner core tangent cylinder, in the form of narrow columnar vortices elongated parallel to the spin axis. Columnar convection contains large amounts of negative helicity in the northern hemisphere and positive helicity in the southern hemisphere and results in dynamo action above a certain Rayleigh number, through a macroscopic α 2 mechanism. These dynamos equilibrate by generating concentrated magnetic flux bundles that limit the kinetic energy of the convection columns. The dipole-dominated external field is formed by superposition of several flux bundles at middle and high latitudes. At low latitudes a pattern of reversed flux patches propagates in the retrograde direction, resulting in an apparent westward drift of the field in the equatorial region. At higher Rayleigh number we find a fully developed regime with convection inside the tangent cylinder consisting of polar upwelling and azimuthal thermal wind flows. These motions modify the dynamo by expelling poloidal flux from the poles and generating intense toroidal fields in the polar regions near the inner core. Convective dynamos in the fully developed regime exhibit characteristics that can be compared with the geomagnetic field, including concentrated flux bundles on the core-mantle boundary, polar minima in field intensity, and episodes of westward drift.

Association between Geotail plasma flows and auroral poleward boundary intensifications observed by CANOPUS photometers

Abstract: Poleward boundary intensifications are nightside geomagnetic disturbances that have an auroral signature that moves equatorward from the poleward boundary of the auroral zone. They occur repetitively, so that many individual disturbances can occur during time intervals of ∼1 hour, and they appear to be the most intense auroral disturbance at times other than the expansion phase of substorms. We have used data from three nightside conjunctions of the Geotail spacecraft in the magnetotail with the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) ground-based array in central Canada to investigate the relation between the poleward boundary intensifications and bursty plasma sheet flows and to characterize the bursty flows associated with the disturbances. We have found a distinct difference in plasma sheet dynamics between periods with, and periods without, poleward boundary intensifications. During periods with identifiable poleward boundary intensifications, the plasma sheet has considerable structure and bursty flow activity. During periods without such poleward boundary intensifications, the plasma sheet was found to be far more stable with fewer and weaker bursty flows. This is consistent with the intensifications being the result of the mapping to the ionosphere of the electric fields that give rise to bursty flows within the plasma sheet. Two different types of plasma sheet disturbance have been found to be associated with the poleward boundary intensifications. The first consists of plasma sheet flows that appear to be the result of Speiser motion of particles in a localized region of thin current sheet. The second, seen primarily in our nearest-to-the-Earth example, consists of energy-dispersed ion structures that culminate in bursts of low-energy ions and isotropic low-energy electrons and are associated with minima in magnetic field and temperature and maxima in ion density and pressure. Both types of plasma sheet disturbance are associated with localized regions of enhanced dawn-to-dusk electric fields and appear to be associated with localized enhanced reconnection. Our analysis has shown that poleward boundary intensifications are an important aspect of geomagnetic activity that is distinct from substorms. In addition to their very distinct auroral signature, we have found them to be associated with a prolonged series of ground magnetic Pi 2 pulsations and ground X component perturbations, which peak at latitudes near the ionospheric mapping of the magnetic separatrix, and with a series of magnetic Bz oscillations near synchronous orbit. Like substorms, the tail dynamics associated with the poleward boundary intensifications can apparently extend throughout the entire radial extent of the plasma sheet. Color versions of figures are available at http://www.atmos.ucla.edu/∼larry/geotail.html.

Plasma sheet access to geosynchronous orbit

Abstract: One year's worth of magnetospheric plasma analyzer data from three Los Alamos geosynchronous satellites are used for a statistical study of proton and electron fluxes at geosynchronous orbit and their dependence on local time (LT) and geomagnetic activity level as measured by Kp. When displayed as a function of LT and Kp, the fluxes exhibit distinct boundaries, which are shown to be consistent with a combination of a global pattern of particle drift through the magnetosphere and loss processes mainly due to charge exchange of the ions and auroral precipitation of the electrons. A Hamiltonian energy conservation approach combined with the (U, B, K) coordinate transformation introduced by Whipple [1978] is used to calculate the theoretical position of the separatrix between open and closed drift trajectories (Alfven layer) as a function of particle species, energy, local time, and geomagnetic activity level. The comparison of the theoretical boundaries with the observations confirms the predictions of plasma sheet access to the geosynchronous region. The analysis also provides independent statistical support for previously derived relationships between Kp and the strength of the global convection electric field.

Geomagnetic polarity transitions

Abstract: The top of Earth's liquid outer core is nearly 2900 km beneath Earth's surface, so we will never be able to observe it directly. This hot, dense, molten iron-rich body is continuously in motion and is the source of Earth's magnetic field. One of the most dynamic manifestations at Earth's surface of this fluid body is, perhaps, a reversal of the geomagnetic field. Unfortunately, the most recent polarity transition occurred at about 780 ka, so we have never observed a transition directly. It seems that a polarity transition spans many human lifetimes, so no human will ever witness the phenomenon in its entirety. Thus we are left with the tantalizing prospect that paleomagnetic records of polarity transitions may betray some of the secrets of the deep Earth. Certainly, if there are systematics in the reversal process and they can be documented, then this will reveal substantial information about the nature of the lowermost mantle and of the outer core. Despite their slowness on a human timescale, polarity transitions occur almost instantaneously on a geological timescale. This rapidity, together with limitations in the paleomagnetic recording process, prohibits a comprehensive description of any reversal transition both now and into the foreseeable future, which limits the questions that may at this stage be sensibly asked. The natural model for the geomagnetic field is a set of spherical harmonic components, and we are not able to obtain a reliable model for even the first few harmonic terms during a transition. Nevertheless, it is possible, in principle, to make statements about the harmonic character of a geomagnetic polarity transition without having a rigorous spherical harmonic description of one. For example, harmonic descriptions of recent geomagnetic polarity transitions that are purely zonal can be ruled out (a zonal harmonic does not change along a line of latitude). Gleaning information about transitions has proven to be difficult, but it does seem reasonable to draw the following conclusions with varying degrees of confidence. There appears to be a substantial decrease in the mean intensity of the dipole field during a transition to ∼25% of its usual value. The duration of an average geomagnetic polarity transition is not well known but probably lies between 1000 and 8000 years. Values outside these bounds have been reported, but we give reasons as to why such outliers are likely to be artifacts. The reversal process is probably longer than the manifestation of the reversal at Earth's surface as recorded in paleomagnetic directional data. Convection hiatus during a geomagnetic polarity transition seems unlikely, and free-decay models for reversals appear to be generally incompatible with the data. This implies that certain theorems in dynamo theory, such as Cowling's theorem, should not be invoked to explain the origin of reversals. Unfortunately, the detailed description of directional changes during transitions remains controversial. Contrary to common belief, certain low-degree nondipole fields can produce significant longitudinal confinement of virtual geomagnetic poles (VGP) during a transition. The data are currently inadequate to refute or verify claims of longitudinal dipole confinement, VGP clustering, or other systematics during polarity transitions.

Nonlinear two‐stream instabilities as an explanation for auroral bipolar wave structures

Abstract: The evolution of two counter-streaming electron beams is shown by means of 2-D kinetic simulations to lead to electron distributions and coherent localized bipolar plasma wave structures with features similar to those measured by the FAST satellite in the auroral ionosphere. Electrostatic whistler waves are generated at later times when the bipolar structures begin to lose coherence and break up in the dimension transverse to the geomagnetic field.

Solar causes of the long-term increase in geomagnetic activity

Abstract: We analyze the causes of the century-long increase in geomagnetic activity, quantified by annual means of the aa index, using observations of interplanetary space, galactic cosmic rays, the ionosphere, and the auroral electrojet, made during the last three solar cycles. The effects of changes in ionospheric conductivity, the Earth's dipole tilt, and magnetic moment are shown to be small; only changes in near-Earth interplanetary space make a significant contribution to the long-term increase in activity. We study the effects of the interplanetary medium by applying dimensional analysis to generate the optimum solar wind-magnetosphere energy coupling function, having an unprecedentedly high correlation coefficient of 0.97. Analysis of the terms of the coupling function shows that the largest contributions to the drift in activity over solar cycles 20-22 originate from rises in the average interplanetary magnetic field (IMF) strength, solar wind concentration, and speed; average IMF orientation has grown somewhat less propitious for causing geomagnetic activity. The combination of these factors explains almost all of the 39% rise in aa observed over the last three solar cycles. Whereas the IMF strength varies approximately in phase with sunspot numbers, neither its orientation nor the solar wind density shows any coherent solar cycle variation. The solar wind speed peaks strongly in the declining phase of even-numbered cycles and can be identified as the chief cause of the phase shift between the sunspot numbers and the aa index. The rise in the IMF magnitude, the largest single contributor to the drift in geomagnetic activity, is shown to be caused by a rise in the solar coronal magnetic field, consistent with a rise in the coronal source field, modeled from photospheric observations, and an observed decay in cosmic ray fluxes.

Measurements of ground-level muons at two geomagnetic locations

Abstract: We report new measurements of the muon spectra and the muon charge ratio at ground level in the momentum range from 200 MeVc to 120 GeVc for two different geomagnetic locations. Above 0.9 GeVc the absolute spectra measured in the two locations are in good agreement and are about 10% to 15% lower than previous experimental results. At lower momenta the data show latitude dependent geomagnetic effects. These observations are important for the understanding of the observed neutrino anomaly.

Initial measurements of the lunar induced magnetic dipole moment using Lunar Prospector magnetometer data

Abstract: Twenty-one orbits of Lunar Prospector magnetometer data obtained during an extended passage of the Moon through a lobe of the geomagnetic tail in April 1998 are applied to estimate the residual lunar induced magnetic dipole moment. Editing and averaging of individual orbit segments yields a negative induced moment with amplitude −2.4 ±1.6 × 1022 Gauss-cm³ per Gauss of applied field. Assuming that the induced field is caused entirely by electrical currents near the surface of a highly electrically conducting metallic core, the preferred core radius is 340±90 km. For an iron-rich composition, such a core would represent 1 to 3% of the lunar mass.

A polar vortex in the Earth's core

Abstract: Numerical dynamo models have been successful in explaining the origin of the Earth's magnetic field and its secular variation by convection in the electrically conducting fluid outer core1,2,3,4,5,6,7 An important component of the convection in the numerical dynamos are polar vortices beneath the core–mantle boundary in each hemisphere These polar vortices in the outer core have been proposed as sources for both the anomalous rotation of the inner core and the toroidal part of the geomagnetic field2,8 Here we use the observed structure of the Earth's magnetic field and its variation since 1870 to infer the existence of an anticyclonic polar vortex with a polar upwelling in the northern hemisphere of the core, consistent with the polar vortices found in numerical dynamos

Long-term drift of the coronal source magnetic flux and the total solar irradiance

Abstract: We test the method of Lockwood et al. [1999] for deriving the coronal source flux from the geomagnetic aa index and show it to be accurate to within 12% for annual means and 4.5% for averages over a sunspot cycle. Using data from four solar constant monitors during 1981-1995, we find a linear relationship between this magnetic flux and the total solar irradiance. From this correlation, we show that the 131% rise in the mean coronal source field over the interval 1901-1995 corresponds to a rise in the average total solar irradiance of {\Delta}I = 1.65 +/- 0.23 Wm^{-2}.

Ionospheric total electron content response to solar eclipses

Abstract: On October 24, 1995, and March 9, 1997, two solar eclipse events occur. It is therefore of interest to investigate how the ionosphere responded to the eclipses. Five global positioning system (GPS) ground-based receivers are specifically designed to observe large-scale ionospheric variations over the geomagnetic equatorial, equatorial anomaly crest, and midlatitude regions. Two-dimensional images of ionospheric total electron content (TEC) during the two eclipse periods are constructed. The deviations in the TEC images on eclipse days from those on reference days show that during the eclipse days the ionosphere experienced some large-scale changes. Four features of the TEC deviations, pre-ascension (PA), major depression (MD), sunset ascension (SA), and secondary depression (SD) have been observed. A detailed study shows that in geomagnetic low latitudes, PAs are possibly related to the locations of the equatorial anomaly crest. The latitudinal location, amplitude, and occurrence time of MDs suggest that the fountain effect is essential. SAs and SDs occurring in geomagnetic equatorial and low latitudes and appearing respectively before/around and after local sunset indicate that the prereversal enhancement plays an important role.

Monitoring spatial and temporal variations in the dayside plasmasphere using geomagnetic field line resonances

Abstract: It is well known that the resonant frequency of geomagnetic field lines is determined by the magnetic field and plasma density. We used cross-phase and related methods to determine the field line resonance frequency across 2.4≤

A Laboratory Model for Convection in Earth's Core Driven by a Thermally Heterogeneous Mantle.

Abstract: Thermal convection experiments in a rapidly rotating hemispherical shell suggest a model in which the convection in Earth's liquid outer core is controlled by a thermally heterogeneous mantle. Experiments show that heterogeneous boundary heating induces an eastward flow in the core, which, at a sufficiently large magnitude, develops into a large-scale spiral with a sharp front. The front separates the warm and cold regions in the core and includes a narrow jet flowing from the core-mantle boundary to the inner-core boundary. The existence of this front in the core may explain the Pacific quiet zone in the secular variation of the geomagnetic field and the longitudinally heterogeneous structure of the solid inner core.

Induction studies with satellite data

Abstract: The natural variations of the Earth's magnetic field of periods spanning from milliseconds to decades can be used to infer the conductivity-depth profile of the Earth's interior. Satellites provide a good spatial coverage of magnetic measurements, and forthcoming missions will probably allow for observations lasting several years, which helps to reduce the statistical error of the estimated response functions. Two methods are used to study the electrical conductivity of the Earth's mantle in the period range from hours to months. In the first, known as the potential method, a spherical harmonic analysis of the geomagnetic field is performed, and the Q-response, which is the transfer function between the internal (induced) and the external (inducing) expansion coefficients is determined for a specific frequency. In the second approach, known as the geomagnetic depth sounding method, the C-response, which is the transfer function between the magnetic vertical component and the horizontal derivative of the horizontal components, is determined. If one of these transfer functions is known for several frequencies, models of the electrical conductivity in the Earth's interior can be constructed. This paper reviews and discusses the possibilities for induction studies using high-precision magnetic measurements from low-altitude satellites. The different methods and various transfer functions are presented, with special emphasis on the differences in analysing data from ground stations and from satellites. The results of several induction studies with scalar satellite data (from the POGO satellites) and with vector data (from the Magsat mission) demonstrate the ability to probe the Earth's conductivity from space. However, compared to the results obtained with ground data the satellite results are much noisier, which presumably is due to the shorter time series of the satellite studies. The results of a new analysis of data from the Magsat satellite indicate higher resistivity in oceanic areas than in continental areas. However, since this holds for the whole range of periods between 2 and 20 days, this difference probably is not caused purely by differences in mantle conductivity (for which one would expect less difference for the longer periods). Further studies with data from recently launched and future satellites are needed.

An updated climatology of thermospheric neutral winds and F region ion drifts above Millstone Hill

Abstract: Incoherent scatter radar data from 63 experiments during the period 1984–1997 have been used to update earlier published climatologies of ion drifts and neutral thermospheric winds at Millstone Hill. Data are binned according to season, solar activity, geomagnetic activity, and local solar time. The derived ion drift patterns are similar to those previously published, but we now find that the main effect of the increased magnetic activity is an enhancement of the usual nighttime west-ward ion drift. A tidal decomposition of the winds from geomagnetically quiet conditions is carried out for each season/solar activity level to extract diurnal means, diurnal amplitudes and phases, and semidiurnal components. These winds are compared with previous results at Millstone Hill and the HWM-93 model. Using our expanded wind database, statistically significant semidiurnal components are now found in both summer and winter, at both solar maximum and solar minimum. Our bin-averaged results confirm earlier published findings that at all seasons the diurnal mean winds are more strongly equatorward and the diurnal amplitudes are stronger at solar cycle minimum than at solar cycle maximum. Diurnal amplitudes derived in the present work are larger than those found previously because we now use a smaller value for the O+, O collision frequency. Differences in the diurnal mean and diurnal amplitude between recent solar minimum data (1993–1997) and data from the previous solar minimum (1984–1986) are attributed to a difference in the EUV flux as inferred from Millstone Hill electron density data which one would not expect from the F10.7 index. This affects the ion drag and apparently also the relative importance of the EUV and high latitude heat sources.

Charge states of solar energetic particles using the geomagnetic cutoff technique : SAMPEX measurements in the 6 November 1997 solar particle event

Abstract: We report on the ionization states of ∼0.5–50 MeV/nucleon ions in the 6 November 1997 solar particle event using instrumentation on the SAMPEX satellite and the geomagnetic cutoff technique. Using the geomagnetic cutoffs of ∼10 MeV 4He and ∼20 MeV protons, we infer the ionization states of ∼0.5–50 MeV/nucleon C-Fe by measuring their latitude distributions. The geomagnetic cutoff method extends the measurement of ionization states beyond 10 MeV/nucleon, where charge state information is inaccessible with present electrostatic deflection techniques. In contrast to an increase of the Fe charge state observed above ∼20 MeV/nucleon in events in late 1992, we find in the 6 November 1997 event that Si & Fe charge states increased dramatically across 0.5–50 MeV/nucleon. While the origin of this newly discovered energy dependence is unknown, such significant event to event variations of solar particle charge states should be considered in models of acceleration and transport processes.

The Magnetic Field of the Earth's Lithosphere: The Satellite Perspective

Abstract: During the planning stages of the World Magnetic Survey in the early 1960s, geologists and geophysicists had no inkling that crustal magnetic anomalies might one day be observed from space. The appearance of The Magnetic Field of the Earth's Lithosphere: The Satellite Perspective is a welcome testimony to how far that science has advanced. However, although much progress has been made in understanding features discovered by the several spacecraft that have measured geomagnetic fields in low-Earth orbit, the contents of this book make it clear that many mysteries are yet to be solved.

A flux transfer event observed at the magnetopause by the Equator-S spacecraft and in the ionosphere by the CUTLASS HF radar

Abstract: Observations of a flux transfer event (FTE) have been made simultaneously by the Equator-S spacecraft near the dayside magnetopause whilst corre- sponding transient plasma flows were seen in the near- conjugate polar ionosphere by the CUTLASS Finland HF radar. Prior to the occurrence of the FTE, the magnetometer on the WIND spacecraft ~226 RE upstream of the Earth in the solar wind detected a southward turning of the interplanetary magnetic field (IMF) which is estimated to have reached the subsolar magnetopause ~77 min later. Shortly afterwards the Equator-S magnetometer observed a typical bipolar FTE signature in the magnetic field component normal to the magnetopause, just inside the magnetosphere. Almost simultaneously the CUTLASS Finland radar observed a strong transient flow in the F region plasma between 78∞ and 83∞ magnetic latitude, near the iono- spheric region predicted to map along geomagnetic field lines to the spacecraft. The flow signature (and the data set as a whole) is found to be fully consistent with the view that the FTE was formed by a burst of magneto- pause reconnection.

Balloon measurements of cosmic ray muon spectra in the atmosphere along with those of primary protons and helium nuclei over midlatitude

Abstract: We report here the measurements of the energy spectra of atmospheric muons and of the parent cosmic ray primary proton and helium nuclei in a single experiment. These were carried out using the MASS superconducting spectrometer in a balloon flight experiment in 1991. The relevance of these results to the atmospheric neutrino anomaly is emphasized. In particular, this approach allows uncertainties caused by the level of solar modulation, the geomagnetic cut-off of the primaries and possible experimental systematics, to be decoupled in the comparison of calculated fluxes of muons to measured muon fluxes. The muon observations cover the momentum and depth ranges of $0.3--40 \mathrm{GeV}/c$ and $5--886 {\mathrm{g}/\mathrm{c}\mathrm{m}}^{2}$, respectively. A comparison of these results with those obtained in a previous experiment by the same collaboration using a similar apparatus allows us to search for differences due to the different experimental conditions at low energy and to check for the overall normalization between the two measurements. The proton and helium primary measurements cover the rigidity range from 3 to 100 GV, in which both the solar modulation and the geomagnetic cut-off affect the energy spectra at low energies. From the observed low-energy helium spectrum, the geomagnetic transmission function at mid-latitude has been determined.

Observation of the East-West Anisotropy of the Atmospheric Neutrino Flux

Abstract: The east-west anisotropy, caused by the deflection of primary cosmic rays in the Earth{close_quote}s magnetic field, is observed for the first time in the flux of atmospheric neutrinos. Using a 45thinspthinspktthinspyr exposure of the Super-Kamiokande detector, 552thinspthinspe -like and 633thinspthinsp{mu} -like horizontally going events are selected in the momentum range between 400 and 3000 thinspMeV/c . The azimuthal distributions of e -like and {mu} -like events agree with the expectation from atmospheric neutrino flux calculations, verifying that the flux of atmospheric neutrinos in the GeV energy range is reasonably well modeled by calculations that account for the geomagnetic field. {copyright} {ital 1999} {ital The American Physical Society}

Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities

Abstract: . Giant pulsations are nearly monochromatic ULF-pulsations of the Earth's magnetic field with periods of about 100 s and amplitudes of up to 40 nT. For one such event ground-magnetic observations as well as simultaneous GEOS-2 magnetic and electric field data and proton flux measurements made in the geostationary orbit have been analysed. The observations of the electromagnetic field indicate the excitation of an odd-mode type fundamental field line oscillation. A clear correlation between variations of the proton flux in the energy range 30-90 keV with the giant pulsation event observed at the ground is found. Furthermore, the proton phase space density exhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce resonance instability as a possible generation mechanism, the azimuthal wave number of the pulsation wave field may be determined using a generalized resonance condition. The value determined in this way, m = - 21 ± 4, is in accord with the value m = - 27 ± 6 determined from ground-magnetic measurements. A more detailed examination of the observed ring current plasma distribution function f shows that odd-mode type eigenoscillations are expected for the case ∂f / ∂W > 0, much as observed. This result is different from previous theoretical studies as we not only consider local gradients of the distribution function in real space, but also in velocity space. It is therefore concluded that the observed giant pulsation is the result of a drift-bounce resonance instability of the ring current plasma coupling to an odd-mode fundamental standing wave. The generation of the bump-on-the-tail distribution causing ∂f / ∂W > 0 can be explained due to velocity dispersion of protons injected into the ring current. Both this velocity dispersion and the necessary substorm activity causing the injection of protons into the nightside magnetosphere are observed. Key words. Magnetospheric physics (energetic particles , trapped; MHD waves and instabilities) · Space plasma physics (wave-particle interactions).