bluuebunny/crossref_metadata_2025 huggingface dataset:
An empirical dynamic equation for energetic electrons at geosynchronous orbit
Authors: O'Brien T. P. , McPherron R. L.
Published: May 2003

We develop an empirical dynamic equation for the hourly evolution of 1.8–3.5 MeV energetic electron flux at local noon at geosynchronous altitude. We use a genetic algorithm combined with a neural network to identify the magnetospheric quantities that can be used to model the time evolution of hourly energetic electron fluxes. We then apply a statistical technique to analyze how these quantities modify the evolution of energetic electron flux through phase space. We fit this evolution to an analytical dynamic equation. We show that the ring current index Dst and a Pc 5 ULF wave power index together can produce a meaningful model of the noon energetic electron flux. We also use this model to demonstrate how the magnetosphere would respond to a hypothetical increase in ULF wave power without a storm in Dst. We demonstrate that while the model electrons do respond to any enhancement in ULF wave power, their response is much larger in the presence of a magnetic storm with sustained ULF wave power.
Creation and destruction of transitory coronal holes and their fast solar wind streams
Authors: Neugebauer M. , Liewer P. C.
Published: May 2003

A qualitative model is proposed whereby foot point exchange diffusion of open field lines from a solar active region may have led to the formation of a coronal hole and a corotating stream of fast solar wind. The solar wind speed changed from fast back to slow when the coronal hole was later disrupted by the emergence of a new active region. A corollary of this model is that a corotating interplanetary field line may have both fast and slow solar wind at different places along its length.
Origins of Jupiter's main oval auroral emissions
Authors: Cowley S. W. H. , Bunce E. J. , Nichols J. D.
Published: May 2003

We review recent theoretical progress in understanding the origins of the jovian “main auroral oval” emissions, based on the hypothesis that they are connected with the magnetosphere‐ionosphere coupling current circuit which maintains plasma corotation in the middle magnetosphere. After the development of the basic theory, we compare results obtained with differing models of the background magnetic field, showing that an appropriate choice is crucial in determining the strength, width, and location of the precipitating auroral electron energy flux associated with the region of upward field‐aligned current. Specific comparison is made between results for a dipole field and an empirically based current sheet model, in which the angular velocity profile of the plasma is calculated self‐consistently using Hill‐Pontius theory. We also develop simple approximation regimes appropriate to small and large equatorial distances, showing that the currents depend on the mass outflow rate of iogenic plasma and not on the effective ionospheric Pedersen conductivity in the small‐distance regime, and vice versa at large distances. We also discuss the expected anticorrelation of the auroral emission intensity with solar wind dynamic pressure and present steady state results representing differing degrees of magnetospheric compression. Significant modulation of the emission is anticipated on this basis.
Sporadic auroras
Authors: Silverman S. M.
Published: May 2003

A neglected phenomenon, that of localized, sporadic auroral occurrence at relatively low latitudes during periods of quiet to moderate magnetic activity, is described and discussed. Some 54 cases of such occurrences are tabulated, all, with one exception, from the United States at times from 1880 to 1940.
Quiet and perturbed ionospheric representation according to the electron content from GPS signals
Authors: Brunini Claudio , Van Zele María Andrea , Meza Amalia , Gende Mauricio
Published: May 2003

Signals from Global Positioning System (GPS) satellites received at the surface of the Earth have passed through the terrestrial atmosphere and are therefore affected by refraction in the ionosphere. A large number of permanent dual frequencies GPS tracking stations have been built up in the last years and their data have proved to be suitable to study the ionosphere. The main goal of this paper is to assess the capability of these observations to continuously and routinely monitor the ionosphere at a global scale. The interest was focused on retrieving the coefficients of a spherical harmonics expansion that describe the global distribution of the vertical total electron content (VTEC) from the GPS signal. To test our results, we compare them with other GPS‐derived results, with VTEC values computed with the International Reference Ionosphere (IRI‐95) model, and with direct VTEC determinations provided by Topex‐Poseidon satellite. A large set of global VTEC maps with a time resolution of 2 hours was used to describe the evolution of this ionospheric variable at quiet geomagnetic periods during the year 1997. The outstanding features of the VTEC during the 15 May 1997 geomagnetic storm have also been studied. The results show that our method is able to identify large‐scale features and seasonal variations of the VTEC in quiet conditions, as well as its variations during a large geomagnetic storm.
Longitude variability of the solar semidiurnal tide in the lower thermosphere through assimilation of ground‐ and space‐based wind measurements
Authors: Cierpik K. M. , Forbes J. M. , Miyahara S. , Miyoshi Y. , Fahrutdinova A. , Jacobi C. , Manson A. , Meek C. , Mitchell N. J. , Portnyagin Y.
Published: May 2003

Wind measurements from the Upper Atmosphere Research Satellite (UARS) and model output from the Middle Atmosphere General Circulation Model (GCM) at Kyushu University are used to investigate the nature of nonmigrating semidiurnal tides between 50–55°N using combined space‐based (SBM) and ground‐based (GBM) wind measurements at 95 km. The GCM is used to create a mock database to test the effects of various sampling scenarios, data gaps, and relative weighting between SBM and GBM, on retrieval of the longitude structure of the semidiurnal tide. SB sampling is based upon orbital characteristics of UARS. GB sampling corresponds to hourly radar measurements from Saskatoon (52°N, 107°W), Sheffield (53°N, 4°W), Collm (52°N, 15°E), Obninsk (55°N, 37°E), and Kazan (56°N, 49°E). Results are presented for the month of August when semidiurnal amplitudes are large and sampling by UARS instruments is good. By compositing over a 5–10 day “fit span,” it is found that the combination of temporal coverage by GB radars and spatial sampling by the satellite is sufficient to allow reasonable recovery of the zonal wave number s = 1, 2, 3 components of the semidiurnal tide. Over significantly longer fit spans, the contributions of GBM become less critical. Using actual UARS and GBM during 1–20 August 1993, the semidiurnal amplitude of eastward wind is found to vary from a minimum value (12 ms−1) at 20°E, to a maximum of 45 ms−1 near 160°E, and a secondary maximum (29 ms−1) at 300°E. The zonal wave number components corresponding to this longitude variation in the semidiurnal tide are 7.7 ± 1.9 ms−1, 19.8 ± 1.5 ms−1 and 13.0 ± 1.3 ms−1 for s = 1, 2, 3 (westward), respectively where ±1−σ uncertainties are indicated. These results are in reasonable agreement with those simulated within the Kyushu GCM. However, there is roughly a four‐ to five‐hour phase offset between the phases recovered from the observational data and from the Kyushu GCM, possibly connected with strong model phase gradients in this atmospheric regime.
High‐latitude dayside energetic precipitation and IMF BZ rotations
Authors: Østgaard N. , Detrick D. L. , Rosenberg T. J. , Vondrak R. R. , Frey H. U. , Mende S. B. , Håland S. E. , Stadsnes J.
Published: May 2003

X‐ray images from space and optical and riometer data from the ground are used to examine two discrete high‐latitude dayside arcs. The dayside auroral oval is usually associated with soft precipitation of ≤1 keV electrons but the observed X‐ray features and riometer data indicate more energetic precipitation as the X‐rays are produced by electrons with energies >2.5 keV and cosmic radio absorption is associated with ∼10 keV electron precipitation. Both the X‐ray images from space as well as the ground based optical and riometer data show two energetic precipitation events appearing at high latitudes and subsequently moving equatorward at ∼0.5 km/s. The average energy of the electrons is estimated to be 4–8 keV, and the energy deposition is 6–10 mW/m2. Owing to the high latitudes it is unlikely that these electrons were energized on the nightside and adiabatically drifted to the dayside. Instead, we think that the high‐latitude events are controlled by the solar wind. The precipitation appears at ∼77–79° magnetic latitude and coincides with a northward turning of the interplanetary field. Owing to the uncertainties in determining the exact location of open‐closed boundary we interpret the events to be caused by either KH instabilities at the inner edge of the LLBL or by lobe reconnection. In the latter case the arcs are produced by electrons accelerated by parallel electric fields resulting from converging horizontal electric fields associated with convection enhancements due to lobe reconnection at the front surface of the magnetotail. These flux tubes map to the poleward edge of the cusp. The equatorward movement is attributed to erosion of field lines as the interplanetary field turns southward with associated dayside reconnection on flux tubes with footpoints at lower latitudes.
Simultaneous images of the northern and southern auroras from the Polar spacecraft: An auroral substorm
Authors: Frank L. A. , Sigwarth J. B.
Published: May 2003

The first simultaneous images of the auroras in the Northern and Southern Hemispheres with a single camera have been gained with the Earth Camera onboard the Polar spacecraft with sufficient angular resolution to provide maps of the intensities. An auroral substorm was fortuitously observed on 1 November 2001 when the spacecraft was favorably positioned to observe both auroras. The auroral intensities during the onset and the early expansive phase were mapped into corrected geomagnetic coordinates. The onset brightening at far‐ultraviolet wavelengths was first detected in the southern auroras and subsequently seen in the northern auroras about 1 min later on L‐shells of about 5 RE. During the early phases of the auroral expansive phase, the southern aurora continued to be brighter. The unequal auroral intensities in the two hemispheres suggest that there is strong coupling of the ionosphere and magnetosphere. The mapping of the auroras revealed that there is a local time difference between the onset positions of about 40 min, with the southern auroras positioned to the east relative to those in the north. This displacement was also similarly detected in the quiet auroral regions, which were not participants in the auroral substorm. Such displacements may be useful in developing improvements in the accuracy of global magnetic models that are important for determination of the position of the in situ particle and field measurements in the magnetosphere. The greater brightness of the northern quiet time auroras by tens of percent as first reported by aircraft observations at visible wavelengths in both hemispheres has been verified by the more global view from the Polar spacecraft.
Solar 3He‐rich events and energy spectra of particles accelerated in two stages
Authors: Zhang T. X.
Published: May 2003

Energy spectra of solar 3He‐rich events are theoretically and parametrically investigated according to the two‐stage acceleration model, which was proposed by Zhang [1995, 1999], and self‐consistently explained various aspects of acceleration and enhancement including electrons, 3He, and heavy ions. First, an expression for the energy spectra of energetic particles is derived from the two‐stage acceleration model. Then, it is shown that all species of particles approximately have power law form spectra. The spectral index (denoted by γ) is determined by the exponent of power spectrum of magnetic fluctuations (ν). Given a spectral index that is consistent with observations, differential fluxes (or intensities) are plotted versus energies for electrons, 3He, and some typical heavy ions: carbon (C), oxygen (O), silicon (Si), and iron (Fe). The results obtained from this model very closely approximate measurements shown by Reames et al. [1985] and Mason et al. [1994]. Furthermore, the magnetic fluctuations and particle heating can significantly affect the differential fluxes of solar energetic particles and may cause the energy spectra to be complicated and different from event to event.
Bursty bulk flow intrusion to the inner plasma sheet as inferred from auroral observations
Authors: Kauristie K. , Sergeev V. A. , Amm O. , Kubyshkina M. V. , Jussila J. , Donovan E. , Liou K.
Published: May 2003

The physics of the interaction of bursty bulk flows (BBFs) with the near‐Earth plasma is not yet known in detail. We address this issue with comprehensive observations made with the MIRACLE network during a well‐documented streamer event. In this case an equatorward intruding auroral streamer inclined from northwest (NW) to southeast (SE) reached the region of the proton oval of 486 nm (Hβ) luminosity. This Hβ band is assumed to map poleward of the ≥30 keV proton isotropic boundary, which is at geocentric distances ∼7–8 RE according to the appropriate magnetic field models. A significant enhancement (30%) in the Hβ luminosity was recorded just at the time and in the location where the equatorward end of the streamer reached the Hβ band. This enhancement implies a corresponding pressure enhancement in the equatorial magnetosphere. The simultaneous poleward shift of the Hβ emission suggests that also a magnetic field dipolarization took place in the same region. An NW‐aligned electrojet (300–400 km in width) accompanied the equatorward development of the streamer (with streamer locating at the dusk flank of this electrojet), and it produced a sharp negative magnetic bay (∼300 nT) resembling a substorm onset. Both auroral and equivalent current observations suggest that the plasma jet was not completely dissolved or stopped during this interaction, but rather diverted in the azimuthal direction. Equivalent currents reconstructed from ground magnetic field observations indicate that a R1‐type field‐aligned current system (upward current ∼0.5 MA) accompanied the streamer development. However, our observations do not show any signatures of a R2‐type current system expected to build up during the flow braking and diversion according to previously published simulation studies.
Effect of solar wind pressure pulses on the size and strength of the auroral oval
Authors: Boudouridis A. , Zesta E. , Lyons R. , Anderson P. C. , Lummerzheim D.
Published: May 2003

It has recently been found that solar wind dynamic pressure changes can dramatically affect the precipitation of magnetospheric particles on the high‐latitude ionosphere. We have examined the effect of large solar wind dynamic pressure increases on the location, size, and intensity of the auroral oval using particle precipitation data from Defense Meteorological Satellite Program (DMSP) spacecraft. Three events have been selected for study during the time period after 1997 when four DMSP spacecraft (F11–F14) were simultaneously operational. Interplanetary magnetic field (IMF) orientation is different from event to event. For each event, we determine equatorward and poleward boundaries of the auroral oval before and after an increase in solar wind pressure. Also, using measured integral fluxes, we construct precipitating particle energy input maps for the auroral oval. All cases studied show a significant change of the auroral oval location, size, and intensity in response to the solar wind pressure pulse. Most prominent are an increase of the auroral zone width and a decrease of the polar cap size when the solar wind dynamic pressure increases under steady southward IMF conditions. An increase in total precipitating particle energy flux is also observed. A smaller response is seen when the IMF Bz has a simultaneous northward turning and when it is nearly zero before the pressure enhancement. Our results also point to significant differences between the auroral precipitation response to solar wind pressure changes and its response to isolated substorms, the former inducing a global auroral reaction while the latter is related to more localized premidnight disturbances. Auroral UV observations from the Polar spacecraft during our events are found to give results consistent with the results we get from the precipitating particle observations.
Substorm expansion phase: Observations from Geotail, Polar and IMAGE network
Authors: Perraut Sylvaine , Le Contel Olivier , Roux Alain , Parks G. , Chua D. , Hoshino M. , Mukai T. , Nagai T.
Published: May 2003

We describe the signature of a substorm detected in the midtail while Geotail was located close to the midnight meridian. At the same time, the UVI imager on board Polar identifies a bulge which develops at low latitude and rapidly expands towards the north, east, and west, corresponding to the expansion phase. The magnetograms of the IMAGE network are consistent with these observations; during the expansion phase they give evidence for a northward expansion of the magnetic perturbation. Mapping indicates that the Geotail footprint is located north of the initial bulge and south of the high‐latitude oval. During the expansion phase, Geotail is located in the center of the neutral sheet and detects an ion flow velocity, perpendicular to Bo and directed tailward while Bz changes from positive to negative. During the recovery phase, Geotail, which is not at the center of the sheet anymore, detects an ion velocity directed earthward but essentially field aligned, while Bz is positive and the high‐latitude auroral structure is located north of Geotail footprint. The radial component of the velocity is always dominant. We interpret these observations as evidence for a tailward moving dipolarization front that first destroys the inner part of the thin current sheet (TCS) formed during the growth phase. This dipolarization/current disruption starts in the near‐Earth plasma sheet and expands tailward. This “erosion” produces a negative Bz component at the earthward edge of the TCS. For large enough distances this contribution can eventually be dominant, thereby producing a negative Bz. In this interpretation the formation of an X‐point/X‐line is the consequence of the erosion of the currents in near‐Earth tail. Present observations give evidence for an association between increases in the ion velocity and small‐scale Alfvenic fluctuations. The plasma sheet electrons are heated via interaction with the waves. This heating is preferentially along the direction of the magnetic field. Large ion flow velocities coincide with wave observations.
Auroral boundary correlations between UVI and DMSP
Authors: Carbary J. F. , Sotirelis T. , Newell P. T. , Meng C.‐I.
Published: May 2003

Equatorward and poleward auroral boundaries from latitude profiles of Polar Ultraviolet Imager (UVI) auroral images were correlated with over 23,000 boundaries derived from 2 years of Defense Meteorological Satellite Program (DMSP) electron precipitation data. Latitude differences between DMSP and UVI boundaries were averaged into 1‐hour and 3‐hour sectors of magnetic local time (MLT). The statistical distributions of these differences generally peak near zero but have Gaussian‐like shapes with widths of about 3°–4° in magnetic latitude. The mean values of these offsets exhibit systematic trends in MLT, with the largest disagreement near 05 MLT for the poleward boundaries and near noon for the equatorward boundaries. The mean offsets were fit to second‐order harmonic expansions, which approximately “calibrate” image boundaries with respect to the precipitating electron boundaries. The harmonic fits suggest that the images can yield approximate precipitation boundaries for such purposes as estimating the area of the polar cap.
Precipitation and nonlinear effects in geomagnetic field line resonances
Authors: Prakash M. , Rankin R. , Tikhonchuk V. T.
Published: May 2003

The structure of auroral arcs sustained by field line resonances (FLRs) is determined using a model that describes the interplay between ionospheric feedback, nonlinear, and dispersive effects in a curvilinear geomagnetic topology. The model includes modulation of Pedersen conductance by hundreds of eV electrons that precipitate in the ionosphere through the action of shear Alfvén wave field‐aligned currents (FACs). The competition between ionospheric feedback dissipation, wave dispersion, and nonlinearity results in large‐amplitude, long‐period oscillations of the FAC, and in emission of slow‐moving small‐scale secondary auroral arcs and density perturbations. Using observed values of nightside conductivities and realistic topology of geomagnetic field lines, we obtain FLRs with frequencies in the range of a few mHz, spatial scales up to several km near the ionosphere, and FAC amplitudes extending to tens of μA/m2. Our model explains the excitation of structured auroral arcs in regions of low ionospheric conductance.
A comparison of IMP 8 observed bow shock positions with model predictions
Authors: Merka J. , Szabo A. , Narock T. W. , King J. H. , Paularena K. I. , Richardson J. D.
Published: May 2003

Over 12 years of IMP 8, data was searched for observed bow shock crossings. Out of the total 4562 crossings found, we used the 2293 unambiguous bow shocks for which upstream interplanetary magnetic field and solar wind parameters were available to study selected bow shock models under normal and unusual solar wind conditions. The chosen models were F79, NS91, FR94, FR94c, CL95, and P95 [Formisano, 1979; Němeček and Šafránková, 1991; Farris and Russell, 1994; Cairns and Lyon, 1995; Peredo et al., 1995]. This statistical study investigates these models' reliability not only for average solar wind plasma and interplanetary magnetic field (IMF) conditions but also for unusual conditions and as a result points out some deficiencies of these models. Statistically, the predictions of F79 and the phenomenological and MHD models FR94, FR94c, and CL95 are the most accurate, with F79 giving a slightly better result. The P95 model predicts standoff distances which are too large by ∼20%. For large values of the IMF and its components, all models except NS91 underestimate the bow shock distance. Furthermore, the models underestimate the bow shock distance when the upstream Mach numbers are low (≲5). The models also do not properly reflect changes in the relative orientation of the IMF and solar wind velocity vectors. An independent evaluation of the dawn and dusk sectors suggests an asymmetry in the bow shock shape and/or a different reaction of the flanks to solar wind deviations from a radial flow. Taking the upstream parameters from a distant solar wind monitor (the Wind spacecraft) resulted in the models predicting the shock farther away from the Earth, which is likely a result of the spacecraft separation perpendicular to the solar wind flow, or of calibrational differences of the plasma density measurements by the spacecraft.
Characterizing the long‐period ULF response to magnetic storms
Authors: Posch J. L. , Engebretson M. J. , Pilipenko V. A. , Hughes W. J. , Russell C. T. , Lanzerotti L. J.
Published: May 2003

This study presents an analysis of long‐period ULF wave power observed at 44 ground magnetometer stations in the western hemisphere, at latitudes from the equator to the polar caps, during the magnetic storm intervals of May 1997, March 1998, May 1998, September 1998, and October 1998 identified by the GEM research community. Comparison of the long‐period pulsations (in the Pc5 frequency range) observed on the ground to the solar wind velocity and solar wind pressure observed by the WIND satellite confirms previous results that high solar wind velocity correlates well with increased Pc5 power. Although some variability was evident among the five storms, solar wind pressure and variability in velocity or pressure exhibited considerably weaker correlations. Each storm exhibited narrow‐band Pc5 wave activity during the recovery phase, but only in the dawn‐noon sector at each site. During the main phase, and at all other local times during the recovery phase, wave activity was broadband. The strongest Pc5 power was observed at frequencies above 2 mHz at stations in the auroral zone and poleward and below 2 mHz at stations at middle and low latitudes. Comparison to published time profiles of energetic electrons at geosynchronous orbit suggests that if ULF wave activity is responsible for the initial energization of these electrons, broadband activity rather than narrowband (Pc5) waves must play the dominant role. Our data cannot rule out, however, the importance of narrowband waves in providing additional energization during the recovery phase.
Observations of the Sun's magnetic field during the recent solar maximum
Authors: Sanderson T. R. , Appourchaux T. , Hoeksema J. T. , Harvey K. L.
Published: May 2003

We present new observations and analyses of the Sun's magnetic field and coronal holes. Using magnetic field observations from the Wilcox Solar Observatory, we present a simple means whereby the tilt angle of the current sheet can be calculated. We use a data set covering the last 26 years, which shows for the first time how the dipole component rotated once during a full 22‐year solar cycle. We show how this influenced the current sheet. At solar minimum, the Sun's coronal magnetic field was essentially dipolar and aligned parallel to the spin axis. As a result, the heliospheric current sheet was flat and had very little warp. Around solar maximum, the dipole was perpendicular to the spin axis, and the ratio of quadrupole to dipole strength was high for much of the time. This meant that the current sheet was tilted and highly warped, and reached up to high latitudes. Surprisingly, there were also times close to solar maximum when the quadrupole/dipole ratio was low, and the current sheet was relatively flat, but still highly inclined. We apply for the first time to solar magnetic data a method, which quantitatively analyses the quadrupole component of the magnetic field. From the terms of the expansion of the observed photospheric magnetic field, we compute the position of the poles of the magnetic field. We combine for the first time over an extended period of time magnetic field data from the Wilcox Solar Observatory with coronal hole positions taken from the National Solar Observatory/Kitt Peak. We find that the position of the coronal holes followed the motion of the poles of the magnetic field as the poles moved over the surface of the Sun and that the polar coronal holes broke up into groups of smaller like‐polarity holes as the poles approached the midlatitude regions and the quadrupole became more important. We discuss the implications for energetic particle observations at Ulysses.
Persistence of the Gleissberg 88‐year solar cycle over the last ∼12,000 years: Evidence from cosmogenic isotopes
Authors: Peristykh Alexei N. , Damon Paul E.
Published: May 2003

Among other longer‐than‐22‐year periods in Fourier spectra of various solar–terrestrial records, the 88‐year cycle is unique, because it can be directly linked to the cyclic activity of sunspot formation. Variations of amplitude as well as of period of the Schwabe 11‐year cycle of sunspot activity have actually been known for a long time and a ca. 80‐year cycle was detected in those variations. Manifestations of such secular periodic processes were reported in a broad variety of solar, solar–terrestrial, and terrestrial climatic phenomena. Confirmation of the existence of the Gleissberg cycle in long solar–terrestrial records as well as the question of its stability is of great significance for solar dynamo theories. For that perspective, we examined the longest detailed cosmogenic isotope record—INTCAL98 calibration record of atmospheric 14C abundance. The most detailed precisely dated part of the record extends back to ∼11,854 years B.P. During this whole period, the Gleissberg cycle in 14C concentration has a period of 87.8 years and an average amplitude of ∼1‰ (in Δ14C units). Spectral analysis indicates in frequency domain by sidebands of the combination tones at periods of ≈91.5 ± 0.1 and ≈84.6 ± 0.1 years that the amplitude of the Gleissberg cycle appears to be modulated by other long‐term quasiperiodic process of timescale ∼2000 years. This is confirmed directly in time domain by bandpass filtering and time–frequency analysis of the record. Also, there is additional evidence in the frequency domain for the modulation of the Gleissberg cycle by other millennial scale processes. Attempts have been made to explain 20th century global warming exclusively by the component of irradiance variation associated with the Gleissberg cycle. These attempts fail, because they require unacceptably great solar forcing and are incompatible with the paleoclimatic records.
Numerical experiments on wave propagation towards a 3D null point due to rotational motions
Authors: Galsgaard K. , Priest E. R. , Titov V. S.
Published: May 2003

We describe 3D resistive magnetohydrodynamic (MHD) numerical experiments at a null point driven by the rotation of magnetic field lines near the spine of the null. When field lines around the spine are rotated, a twist wave propagates toward the null along field lines, satisfying a Klein–Gordon equation. While the helical Alfvén wave spreads out as the null is approached, a fast‐mode wave focuses on the null and wraps around it. Only a weak diffusion of the twisted field line structure is found to take place.
On the role of ion temperature anisotropy in the growth and propagation of shear‐modified ion‐acoustic waves
Authors: Teodorescu C. , Koepke M. E. , Reynolds E. W.
Published: May 2003

Oblique ion‐acoustic waves, excited by the combination of magnetic field‐aligned (parallel) electron drift and sheared parallel ion flow, are investigated in magnetized laboratory plasma that is characterized by ion temperature anisotropy. Direct measurements of the parallel and perpendicular ion temperatures, parallel and perpendicular ion drift velocities, electron temperature and parallel electron drift velocity, parallel and perpendicular wave vector components, and mode frequency and growth rate are used to elucidate the shear‐modified ion‐acoustic (SMIA) instability mechanism and document an observed correlation between ion temperature anisotropy and wave propagation angle. Experimental measurements show that anisotropy significantly influences the growth rate and propagation angle of oblique ion‐acoustic waves. These results support the ion‐acoustic wave interpretation of broadband waves in the auroral energization region where shear and anisotropy are known to exist and may have ramifications for many space plasmas in which anisotropy exists in the electron temperature or ion temperature.
Kinetic theory of the Alfvén wave acceleration of auroral electrons
Authors: Lysak Robert L. , Song Yan
Published: May 2003

Recent observations have indicated that in addition to the classical “inverted‐V” type electron acceleration, auroral electrons often have a field‐aligned distribution that is broad in energy and sometimes shows time dispersion indicating acceleration at various altitudes up the field line. Such acceleration is not consistent with a purely electrostatic potential drop and suggests a wave heating of auroral electrons. Alfvén waves have been observed on auroral field lines carrying sufficient Poynting flux to provide energy for such acceleration. Calculations based on the linear kinetic theory of Alfvén waves indicate that Landau damping of these waves can efficiently convert this Poynting flux into field‐aligned acceleration of electrons. At high altitudes along auroral field lines that map into the plasma sheet boundary layer (PSBL), the plasma gradients are relatively weak and the local kinetic theory can describe this wave–particle interaction. At lower altitudes, the gradient in the Alfvén speed becomes significant, and a nonlocal description must be used. A nonlocal theory based on a simplified model of the ionospheric Alfvén resonator (IAR) is presented. For a given field‐aligned current (FAC), the efficiency of the wave–particle interaction increases with the ratio of the thermal velocity of the electrons to the Alfvén speed at high altitudes. These calculations indicate that wave acceleration of electrons should occur at and above the altitude where the quasi‐static potential drops form.
IMAGE FUV and in situ FAST particle observations of substorm aurorae
Authors: Mende S. B. , Carlson C. W. , Frey H. U. , Immel T. J. , Gérard J.‐C.
Published: May 2003

Images from the IMAGE Wide‐band Imaging Camera (WIC) and Spectrographic Imager (SI) channels SI‐12 and SI‐13 were compared to in situ data taken by FAST during several substorms. FAST spacecraft observations have shown that the high latitude auroral ionosphere has several distinct regions. Intense auroras are seen in regions of upward directed quasi static electric fields and of Alfven wave accelerated superthermal electrons. In two of the cases presented, the satellite passed through an active poleward propagating substorm surge on its duskward flank. Both show that the superthermal wave accelerated component to be on the polar cap boundary of the surge, and that it could be distinguished from quasi static field “inverted V” precipitation which occurred at the more equatorward parts of the auroral oval. In one of these cases, the surge was accompanied by intense ion outflow. Three cases showed the FAST satellite passing through the substorm aurora at midnight or the dawn side outside of the surge and the wave accelerated electrons were less clearly separated from the inverted V type precipitation. The wave accelerated electrons were seen to be part of very short‐lived transient events, i.e., bursts. The region of auroral forms, associated with Alfven waves exhibit relatively soft auroral precipitation with very intense electron fluxes. By comparing the intensity of the WIC and SI‐13 channels of IMAGE FUV, it is possible sometimes to distinguish optically between the two types of regions. Global imaging of the two regions would allow the separation of quasi‐static plasma convection regions containing inverted V‐s from regions of Alfven wave driven electrons signifying substorm related magnetic field dynamics.
Properties of small‐scale Alfvén waves and accelerated electrons from FAST
Authors: Chaston C. C. , Bonnell J. W. , Carlson C. W. , McFadden J. P. , Ergun R. E. , Strangeway R. J.
Published: May 2003

The FAST spacecraft observes Alfvénic fluctuations in field quantities throughout the auroral oval. In this report, we show preliminary results from a statistical study of Alfvén waves with transverse scales comparable to the electron skin‐depth observed over the frequency range from 0.2 to ∼20 Hz in the spacecraft frame. This study shows that these waves are most commonly found in the magnetospheric cusps and at the polar cap boundary of the auroral oval in the premidnight sector where the most intense events are observed. It is shown that the wave Poynting flux is well correlated with the energy flux of field‐aligned upgoing and downgoing electrons. It is also shown that over the FAST altitude range (350–4175 km), the wave Poynting flux decreases with decreasing altitude, most likely due to losses through electron acceleration. It is demonstrated, however, through the use of simulations and observations that most of the electron acceleration occurs at altitudes above FAST between 1 and 2 Earth radii (RE), with the most energetic electrons arriving from an altitude of ∼2–3 RE for statistically determined density and composition profiles.
Excitation of tall auroral rays by ohmic heating in field‐aligned current filaments at F region heights
Authors: Otto A. , Lummerzheim D. , Zhu H. , Lie‐Svendsen Ø. , Rees M. H. , Lanchester B. S.
Published: May 2003

The formation of tall red rays in the ionosphere has been a longstanding unresolved problem of auroral physics. These rays are pencil‐like structures which can extend from 150 km at their base to as high as 600 km. At these heights it is very difficult to deposit sufficient power in order to account for the luminosity of tall rays. This work examines ohmic heating by collisional processes in strong field‐aligned current sheets to account for visible tall rays. The mechanism is demonstrated by two‐dimensional simulation in a fully self‐consistent treatment of the ionosphere and coupling to the magnetosphere. We find that a filamentary current density of about 600 μAm−2 over about ten seconds can pump sufficient energy into the ambient oxygen atoms to produce visible auroral red rays. The ohmic heating leads to an electron temperature in excess of 10,000 K in the upper F‐region.
Seasonal and diurnal variation of geomagnetic activity: Revised Dst versus external drivers
Authors: Häkkinen Lasse V. T. , Pulkkinen Tuija I. , Pirjola Risto J. , Nevanlinna Heikki , Tanskanen Eija I. , Turner Niescja E.
Published: May 2003

Daily and seasonal variability of long time series of magnetometer data from Dst stations is examined. Each station separately shows a local minimum of horizontal magnetic component near 18 local time (LT) and weakest activity near 06 LT. The stations were found to have different baselines such that the average levels of activity differed by about 10 nT. This effect was corrected for by introducing a new “base method” for the elimination of the secular variation. This changed the seasonal variability of the Dst index by about 3 nT. The hemispheric differences between the annual variation (larger activity during local winter and autumn solstice) were demonstrated and eliminated from the Dst index by addition of two Southern Hemisphere stations to a new index termed Dst6. Three external drivers of geomagnetic activity were considered: the heliographic latitude, the equinoctial effect, and the Russell–McPherron effect. Using the newly created Dst6 index, it is demonstrated that these three effects account for only about 50% of the daily and seasonal variability of the index. It is not clear what drives the other 50% of the daily and seasonal variability, but it is suggested that the station distribution may play a role.
Twenty‐three cycles of changing open solar magnetic flux
Authors: Lockwood M.
Published: May 2003

This paper presents a comparison of various estimates of the open solar flux, deduced from measurements of the interplanetary magnetic field, from the aa geomagnetic index and from photospheric magnetic field observations. The first two of these estimates are made using the Ulysses discovery that the radial heliospheric field is approximately independent of heliographic latitude, the third makes use of the potential‐field source surface method to map the total flux through the photosphere to the open flux at the top of the corona. The uncertainties associated with using the Ulysses result are 5%, but the effects of the assumptions of the potential field source surface method are harder to evaluate. Nevertheless, the three methods give similar results for the last three solar cycles when the data sets overlap. In 11‐year running means, all three methods reveal that 1987 marked a significant peak in the long‐term variation of the open solar flux. This peak is close to the solar minimum between sunspot cycles 21 and 22, and consequently the mean open flux (averaged from minimum to minimum) is similar for these two cycles. However, this similarity between cycles 21 and 22 in no way implies that the open flux is constant. The long‐term variation shows that these cycles are fundamentally different in that the average open flux was rising during cycle 21 (from consistently lower values in cycle 20 and toward the peak in 1987) but was falling during cycle 22 (toward consistently lower values in cycle 23). The estimates from the geomagnetic aa index are unique as they extend from 1842 onwards (using the Helsinki extension). This variation gives strong anticorrelations, with very high statistical significance levels, with cosmic ray fluxes and with the abundances of the cosmogenic isotopes that they produce. Thus observations of photospheric magnetic fields, of cosmic ray fluxes, and of cosmogenic isotope abundances all support the long‐term drifts in open solar flux reported by Lockwood et al. [1999a, 1999b].
Damping of large‐scale traveling ionospheric disturbances detected with GPS networks during the geomagnetic storm
Authors: Tsugawa T. , Saito A. , Otsuka Y. , Yamamoto M.
Published: May 2003

Large‐scale traveling ionospheric disturbances (LSTIDs) during the geomagnetic storm on 22 September 1999 were studied using total electron content (TEC) data from the GPS Earth Observation Network (GEONET) in Japan, International GPS Service (IGS), and Continuously Operating Reference Stations (CORS) in the United States. The damping rates of the LSTIDs were precisely derived in several local time sectors and were found to depend on values of the background TEC. This indicates that the dominant physical mechanism of the LSTIDs' damping is the ion‐drag effect by the background ionosphere. The high‐resolution TEC data from GEONET revealed that two successive LSTIDs were damped significantly as they traveled equatorward in the dawn sector. The ratio of the perturbation component of TEC to the background component (ΔI/I0) decreased exponentially with the damping rate of 0.89/1000 km and 0.77/1000 km. We studied also the amplitude of ΔI/I0 at high latitudes using IGS data and found that the damping rates of LSTIDs at high latitudes tended to be smaller than those at midlatitudes. Global TEC observations during this geomagnetic storm by the IGS and CORS networks detected that several LSTIDs propagated also equatorward in the afternoon sector and in the night sector. The LSTIDs in the afternoon sector were most damped with the damping rate of 1.04/1000 km, which corresponds to the e‐folding length of 961 km. The damping rate of LSTIDs in the night sector was found to be small. The LSTIDs had a tendency to be damped rapidly in the regions where background TEC was large. This dependence of the damping rate on latitude and local time indicates that this intense damping of LSTIDs was caused mainly by the ion‐drag effect that is proportional to the ion collision frequency. The relation between the damping rates and the background TEC derived from the observation are consistent with those estimated with a theoretical calculation of the gravity wave damping by the ion‐drag effect. The worldwide distribution of GPS receivers enabled us to estimate the longitudinal extent of these LSTIDs. The zonal width of LSTIDs in the dawn sector was narrower than 45° in longitude (2,900 km) around 55°N and wider than 20° in longitude (1,800 km) around 40°N. These widths were narrower than those reported in previous studies.
Double layers in expanding plasmas and their relevance to the auroral plasma processes
Authors: Singh Nagendra , Khazanov George
Published: May 2003

When a dense plasma consisting of a cold and a sufficiently warm electron population expands, a rarefaction shock forms [Bezzerides et al., 1978]. In the expansion of the polar wind in the magnetosphere, it has been previously shown that when a sufficiently warm electron population also exists, in addition to the usual cold ionospheric one, a discontinuity forms in the electrostatic potential distribution along the magnetic field lines [Barakat and Schunk, 1984]. Despite the lack of spatial resolution and the assumption of quasi‐neutrality in the polar wind models, such discontinuities have been called double layers (DLs). Recently similar discontinuities have been invoked to partly explain the auroral acceleration of electrons and ions in the upward current region [Ergun et al., 2000]. By means of one‐dimensional Vlasov simulations of expanding plasmas, for the first time we make here the connection between (1) the rarefaction shocks, (2) the discontinuities in the potential distributions, and (3) DLs. We show that when plasmas expand from opposite directions into a deep density cavity with a potential drop across it and when the plasma on the high‐potential side contains hot and cold electron populations, the temporal evolution of the potential and the plasma distribution generates evolving multiple double layers with an extended density cavity between them. One of the DLs is the rarefaction‐shock (RFS) and it forms by the reflections of the cold electrons coming from the high‐potential side; it supports a part of the potential drop approximately determined by the hot electron temperature. The other DLs evolve from charge separations arising either from reflection of ions coming from the low‐potential side or stemming from plasma instabilities; they support the rest of the potential drop. The instabilities forming these additional double layers involve electron‐ion (e‐i) Buneman or ion‐ion (i‐i) two‐stream interactions. The electron‐electron two‐stream interactions on the high‐potential side of the RFS generate electron‐acoustic waves, which evolve into electron phase‐space holes. The ion population originating from the low‐potential side and trapped by the RFS is energized by the e‐i and i‐i instabilities and it eventually precipitates into the high‐potential plasma along with an electron beam. Applications of these findings to the auroral plasma physics are discussed.
Stormtime energy transfer in global MHD simulation
Authors: Palmroth M. , Pulkkinen T. I. , Janhunen P. , Wu C.‐C.
Published: May 2003

We use a three‐dimensional global magnetohydrodynamic (MHD) simulation code to examine the energy flow from the solar wind to the magnetosphere. We simulate a major magnetic storm, which occurred on 6–7 April 2000. During this disturbed period the energy input to the magnetosphere was highly enhanced. For the energy transfer calculation a method for identifying the magnetopause surface from the simulation is developed. We calculate the total energy flux component normal to the magnetopause surface, thus giving the energy flux transferred from the solar wind to the magnetosphere. With this method we identify the locations on the magnetopause surface where significant energy transfer takes place during the storm evolution. During the main phase the energy is transferred from the plane parallel and antiparallel to the interplanetary magnetic field (IMF) clock angle Sunward of XGSE > −10 RE. During the recovery phase most of the energy is transferred in the low‐latitude equatorial sectors Sunward of the dawn‐dusk terminator. We discuss the possible explanations to the observed energy transfer locations. We also compare the time evolution of the total transferred energy to the time evolution of the empirical ϵ parameter calculated from the solar wind parameters. During the main phase the total transferred energy in the simulation is well correlated with ϵ, although it is about four times larger. During the recovery phase the total transferred energy and ϵ are not well correlated, and their ratio is much larger than during the main phase. Finally, we discuss limitations of the developed method, which is based on calculating fluxes through surfaces using surface integrals.