bluuebunny/crossref_metadata_2025 huggingface dataset:
Geomagnetic field line resonances at low latitudes: Pulsation event study of 16 August 1993
Authors: Vellante M. , De Lauretis M. , Förster M. , Lepidi S. , Zieger B. , Villante U. , Pilipenko V. A. , Zolesi B.
Published: October 2002

The analysis of a pulsation event recorded at a gradient installation in central Italy (L = 1.55) and at a higher latitude station (Nagycenk, Hungary, L = 1.88) is presented. The event occurred on 16 August 1993 during the main phase of a geomagnetic storm. Field line resonance signatures have been identified at the gradient installation by using different spectral techniques, comprising standard methods, such as interstation amplitude ratio, phase difference, and single station H/D amplitude ratio, and more advanced amplitude‐phase methods. The capacity of determining the local resonant frequency fr by the different techniques is compared; in particular, we find that the H/D amplitude ratio fails to provide in some cases an estimate of fr. A clear early morning decrease of fr both at L = 1.55 and at L = 1.88 has been found, confirming previous experimental and theoretical results. The estimated fr values at L = 1.55 are significantly higher than those usually reported at these latitudes, suggesting peculiar conditions of the ionosphere‐plasmasphere system during this event. The local time variation of the toroidal mode ULF pulsation eigenfrequencies has been numerically calculated at both latitudes using the standard empirical neutral gas model MSIS–86 and the empirical neutral wind model HWM90. Further numerical simulation runs involved a particular disturbed neutral gas composition which is appropriate for stormy periods and an improved meridional wind estimated as best approximation to the observed hmF2 values. The calculated eigenfrequencies agree with the experimentally observed values at L = 1.88 but are smaller than the values observed in the early morning hours at L = 1.55. The effect of the E × B vertical drift is suggested to be the cause of the discrepancy. Possible signatures of the 2nd harmonic at L = 1.55, never reported so far at such low latitudes, have been found. We estimated a harmonic ratio f2/f1 ∼ 1.7–1.8 in agreement with theoretical models. A high‐frequency resolution analysis at the gradient installation confirms previous suggestions of the existence of a discrete spectrum of field line resonances driven by some cavity/waveguide modes.
Latitudinal structure of outer Io plasma torus
Authors: Moncuquet Michel , Bagenal Fran , Meyer‐Vernet Nicole
Published: October 2002

We present a model of the latitudinal structure of the Io plasma torus (IPT), which is able to explain Ulysses results and to reconcile several in situ data sets. Basically, the observed temperature inversion and the polytropic law are due to “velocity filtration” of particles having non‐Maxwellian velocity distributions. This mechanism acts as a high‐pass filter for particle energies if the particles are confined in an attractive monotonic potential well. These conditions are met in the IPT, where the attractive potential is due to the centrifugal force that confines plasma ions since the plasma is corotating with Jupiter, whereas electrons are confined by an ambipolar electric field preserving electric neutrality, and the electron velocity distribution is known to have a suprathermal tail. The suprathermal electron population has a velocity distribution that decreases with increasing energy as a power law, as is frequently observed in space plasmas, and the velocity distribution can be conveniently modeled with a “kappa” function [Meyer‐Vernet et al., 1995]. Adopting such a kappa distribution for the electrons and for all ion species detected in the torus and including temperature anisotropy, we construct a collisionless kinetic model based on the so‐called “bi‐kappa distributions” to calculate the latitudinal structure. Following Bagenal [1994], we adopt the nearly equatorial data set from Voyager 1 to represent empirically the radial structure. The model reconciles the Voyager 1 and 2 and Ulysses observations and demonstrates that these data sets possess similar latitudinal and radial variations of the IPT densities and temperatures. This model also generates a radial ion temperature profile past ∼7.5 Jovian radii, which is compatible with a quasiadiabatic radial temperature decrease at the torus equator.
Time‐resolved statistical analysis of nonlinear electrostatic fluctuationsin the ionospheric E region
Authors: Larsen Y. , Hanssen A. , Krane B. , Pécseli H. L. , Trulsen J.
Published: October 2002

Low‐frequency electrostatic fluctuations detectedby the instruments on the Rocket and Scatter Experiment (ROSE) rockets inthe E region over northern Scandinavia are analyzed.It is demonstrated that the time‐resolved statistical properties of the fluctuationsdeviate significantly from those associated with a Gaussian process. The characteristicsof the fluctuations are analyzed with particular attention to non‐Gaussianeffects and phase‐coherent mode couplings of electrostatic fluctuations. Non‐Gaussianeffects are analyzed by means of the kernel estimator for probability densityfor short data segments. Short‐time phase‐coherent effects are analyzed andquantified by means of the squared wavelet bicoherence, which has desirablestatistical properties in the frequency regions of interest. The results arecompared with similar results from rocket data obtained over SøndreStrømfjord in Greenland. The results show a remarkable similarity,indicating that the observed phase coherence is a robust feature of the saturatedstage of the E region instabilities.
Vibrationally excited nitrogen and oxygen and the origin of negative ionospheric storms
Authors: Prölss Gerd W. , Werner Stefan
Published: October 2002

How important are vibrationally excited molecular nitrogen and oxygen for the negative phase of ionospheric storms? In order to answer this question we performed a numerical simulation of an ionospheric storm which occurred in summer during a period of high solar activity. We find that during such conditions, vibrationally excited N2and O2 are important and cannot be neglected. The negative phase of the ionospheric storm, however, can almost entirely be explained by the observed perturbations of the neutral gas composition.
Formation of reflected electron bursts by the nonstationarity and nonuniformity of a collisionless shock front
Authors: Lembège B. , Savoini P.
Published: October 2002

The impact on electron dynamics of the nonstationarity and nonuniformity of a quasi‐perpendicular supercritical collisionless planar shock is analyzed by means of a two‐dimensional full‐particle electromagnetic simulation code. Trajectories of preselected self‐consistent electrons (in contrast with the test particles approach) have been analyzed in order to determine the reflection conditions. Four different classes of reflected electrons have been identified according to the time these spent within the shock front itself before being reinjected into the upstream region. However, the shock front is revealed to be strongly nonstationary and nonuniform (front rippling). Different sources of shock front nonstationarity are shown to take place over different time and spatial scales. The longer time scale is due to the shock front reformation associated to the dynamics of reflected ions. The smaller time scale is due to the propagation of front rippling along the shock front. One key result is that electrons hitting the shock front are not reflected uniformly in time; instead, bursts of energetic reflected electrons are formed by the shock front reformation. In addition, electrons are not reflected uniformly in space, but packs of reflected electrons are formed along the rippled shock front. Comparison of two‐dimensional with one‐dimensional simulation results evidences that nonuniformity (rippling) of the shock front contributes to diffuse electron bursty patterns both in real and velocities space. Persistent (one‐dimensional) and more diffuse (two‐dimensional) electron bursts have different impacts on upstream wave emission, which are also discussed.
Effects of induced currents on Dst and on magnetic variations at midlatitude stations
Authors: Häkkinen Lasse V. T. , Pulkkinen Tuija I. , Nevanlinna Heikki , Pirjola Risto J. , Tanskanen Eija I.
Published: October 2002

Magnetic variations observed at Earth's surface are primarily caused by magnetospheric and ionospheric currents and secondarily affected by currents induced within Earth. For studies of space processes it is necessary to separate the internal contribution from the external one. In this paper we consider the Dst index which reflects the properties of the ring current. A spherical harmonic analysis is applied, using the axisymmetric assumption, to make the separation of magnetic data to external and internal parts. By examining 12 storms in 1997 and 1998, our results show that during the storm main phase the internal contribution to Dst is roughly 30%, after which it decreases to about 20% during the recovery phase. This is supported by an approximate model calculation of the induction in Earth. We also consider H variations at the four Dst observatories (Honolulu, San Juan, Hermanus, Kakioka) separately and at a typical continent station (Boulder) for comparison. It is seen that Kakioka systematically has the largest internal contribution during the storm main phase, while Hermanus has only a very small internal part at that time. The three other stations are closer to the ideal case (i.e., the internal part is roughly 1/3). As the anomalous behavior at Kakioka is thus opposite to that at Hermanus, their effects approximately average out in the computation of Dst. The differences between the stations are obviously due to differences in local ground conductivity structures. This conclusion is supported by investigating the Parkinson induction vectors which are larger at Kakioka and Hermanus than at the other observatories.
Return current region aurora: E∥, jz, particle energization, and broadband ELF wave activity
Authors: Lynch K. A. , Bonnell J. W. , Carlson C. W. , Peria W. J.
Published: October 2002

In this paper we consider ion heating in downward current, downward electric field, premidnight auroral regions. We show a case study, a test particle model, and a statistical study. The case study and statistical study use electron, ion, wave, and field‐aligned current data and include wave spectra and ion pitch angle distributions. The ion pitch angle distribution functions are used to calculate, through the momentum balance equation, a self‐consistent local parallel electric field. The test particle model shows ion pitch angle distributions and moments resulting from a “pressure‐cooker” arrangement of magnetic and electric fields. This simple model can reproduce the ion pitch angle distributions and the moments of these distributions as seen in the statistical database; the parallel electric field calculated from these moments is consistent with the imposed electric field profile. The statistical database uses data from 23 premidnight auroral Fast Auroral Snapshot crossings. The statistical study shows that the ion to electron energy ratio varies from 1 to 10; the broadband ELF wave power is controlled by the local value of jz · E∥; the relationship between jz and E∥ is controlled by ni; and the wave power near the cyclotron frequency is sufficient to provide the observed ion energies for oxygen in all cases and the observed ion energies for hydrogen for events near the poleward edge. The strong correlation between the wave power and jz · E∥ validates the use of the ion distributions as a probe of the local E∥. As expected from theoretical arguments, the current‐voltage relationship in these regions is seen to be controlled by the local ionospheric density. Allowing for suitable range in values of the density, the model can be applied to both strong return‐current regions within the auroral cavity and weak‐field regions at the poleward boundary.
Effects of Alfvén wave reflection on tail pressure release and auroral acceleration
Authors: Blixt Erik Mårten , Vogt Joachim
Published: October 2002

When plasma in the near‐Earth geomagnetic tail convect sunward, it passes the Earth at close distance. The highly conducting ionosphere exerts a drag on the plasma flow and yields a shear in the magnetic field. In the slow‐flow approximation this shear must be balanced by pressure gradients in the equatorial plane, and observations indicate the presence of such, that is, azimuthal pressure gradients pointing toward local midnight in the inner magnetosphere. We investigate the role of parallel potential drops in the acceleration region in this context, using a steady one‐dimensional convection model aligned with the azimuthal direction around the Earth. Parallel potential drops provide a means to decouple the magnetosphere from the ionosphere; thus they reduce ionospheric drag, and initiate a process whereby azimuthal pressure gradients in the equatorial plane are relaxed. By taking into account the reflection of Alfvén waves on the auroral acceleration region, we find that the decoupling becomes dependent on the transverse scale of the acceleration region. Being faster at small scales (∼20 km), while at scales comparable to the width of the auroral oval, it is much slower, and the coupling becomes perfect. We find that it takes ∼30 min to “normalize” the pressure at midnight after it had been doubled. The model also allows for estimates of the maximum pressure gradient present right before the decoupling process is initiated, and values up to 10 times higher than normal are found. Further more, a linear relationship between field‐aligned currents/pressure gradient and the parallel potential drop is derived. At smaller scales (∼20 km) the proportionality constant becomes similar to the Lyons‐Evans‐Lundin constant used in linear current voltage relations.
Plasma sheet electromagnetic power generation and its dissipation along auroral field lines
Authors: Angelopoulos V. , Chapman J. A. , Mozer F. S. , Scudder J. D. , Russell C. T. , Tsuruda K. , Mukai T. , Hughes T. J. , Yumoto K.
Published: October 2002

During a fortuitous meridional conjunction of Polar and Geotail at the nightside magnetosphere throughout the course of a geomagnetic substorm, measurements of Poynting flux indicate that most of the electromagnetic energy flux density that is radiated in the form of waves at the location of Geotail at ∼18REis dissipated before it reaches Polar at ∼5RE, i.e., above the auroral acceleration region. While the Poynting flux measured at Polar (and to a greater extent at Geotail) is more than sufficient to account for particle acceleration below the satellite, it still represents a small portion of the earthward directed particle energy flux density measured at Geotail. If even a small portion of the bursty bulk flow energy couples to Alfven waves, it would be energetically sufficient to account for the expected auroral energy deposition during substorms. Power dissipation via kinetic Alfven waves along auroral field lines represents a viable mechanism by which localized reconnection flows can slow down. This may explain why fast earthward flows reported at midtail (>30RE) distances can exist with no near‐Earth counterpart and why any putative candidates of an ionospherically reflected flow burst pulse in the tail have very small amplitudes.
A numerical simulation of the Pi2 pulsations associated with the substorm current wedge
Authors: Fujita S. , Nakata H. , Itonaga M. , Yoshikawa A. , Mizuta T.
Published: October 2002

The present paper deals with the transient behavior of MHD perturbations in the inner magnetosphere induced by an impulsive localized eastward current (source current) as a model of Pi2 pulsations in the magnetosphere. The magnetospheric model consists of a dipole magnetic field, plasmasphere, ionosphere with Pedersen conductivity, and a free outer boundary. The source current is an impulsive magnetospheric current at the onset of the substorm current wedge and is distributed around the equatorial plane of L = 10 with ±2 hour longitudinal extent around midnight. The numerical results allow us to track variation in the expected Pi2 pulsation signals in both local time and L. The poloidal‐mode wave exhibits plasmasphere virtual resonance, resulting in large amplitudes around midnight, weakening toward dayside. The toroidal‐mode wave is excited as a field line resonance immediately after the wave front of the poloidal‐mode wave crosses regions where the radial gradient of VA is steep. The toroidal‐mode wave has largest amplitude at the local time of the east/west edge of the source current. The duration of this wave is ∼5 min. In the middle plasmasphere where the radial gradient of the VA is smaller, the poloidal‐mode wave tends to predominate over the toroidal‐mode wave. These numerical results are consistent with satellite observations, in so far as the day‐night asymmetry of Pi2 pulsations and the observation of transient toroidal waves.
Plasma entry across the distant tail magnetopause 2. Comparison between MHD theory and observation
Authors: Hasegawa H. , Maezawa K. , Mukai T. , Saito Y.
Published: October 2002

A total of 1263 lobe magnetopause crossings were identified utilizing the plasma and magnetic field data obtained by the GEOTAIL spacecraft at large downtail distances (−100 RE > X > −210 RE). On the basis of these crossing events, we compare the observed variations in the plasma and magnetic field parameters with the ones predicted from the MHD framework and study the structure of the tail boundary layer as well as the nature of the solar wind plasma entry. The above crossing events have been classified into three (open, closed, and ambiguous) types based on the jump condition for the MHD discontinuities [Hasegawa et al., 2002]. For the open‐type boundaries, the changes in the plasma flow velocity agree well with the Walén relation, i.e., the tangential stress balance relation expected for a rotational discontinuity (RD), at the external interface of the magnetopause transition region. In the lobe/mantle region inside the interface, however, the velocity changes deviate from the Walén relation and the plasma and field parameters exhibit a smooth variation from the values characteristic to the magnetosheath to those characteristic to the core lobe. This behavior is in good agreement with the prediction that a slow‐mode expansion fan is formed in the tail lobe inside the RD magnetopause. Our comparison suggests that the magnetosheath plasma is decelerated and rarefied mainly across the expansion fan rather than across the RD, as it is transferred to the inner part of the tail lobe.
Plasma entry across the distant tail magnetopause 1. Global properties and IMF dependence
Authors: Hasegawa H. , Maezawa K. , Mukai T. , Saito Y.
Published: October 2002

As many as 1800 magnetopause crossings were identified utilizing the plasma and magnetic field data obtained by the GEOTAIL spacecraft in the distant (−100 RE > X > −210 RE) tail region. We classified the magnetopause crossings into three (open, closed, and ambiguous) types based on the jump conditions for the MHD discontinuities and statistically investigated the properties of these boundaries. The most conspicuous finding is that for a considerable percentage (∼25%) of the lobe‐magnetosheath crossings, the magnetotail boundary is locally open for northward as well as southward IMF polarities, indicating that magnetic reconnection frequently occurs regardless of the polarity of the IMF Bz. Our statistics confirm that the magnetic field has a finite normal component for the locally open boundaries but a negligibly small one for the closed boundaries. The field lines threading the open boundary are directed inward (outward) in the Northern (Southern) Hemisphere, supporting the view that such field lines are connected to the Earth. The jump in the plasma flow velocity at the open boundaries agrees well in magnitude and direction with the ones expected for the rotational discontinuity. The flow direction is directed into the magnetotail, which is again consistent with the model of the rotational discontinuity. The spatial distribution of the open boundaries suggests that the locally open portions of the magnetotail boundary migrate toward high latitudes for southward IMF cases and toward low latitudes (toward the plasma sheet) for northward IMF cases. These results indicate that the solar wind plasma is effectively transferred to the lobe/mantle region through the open portion of the boundary whose location is controlled by the direction of the IMF.
Correlation of Alfvén wave Poynting flux in the plasma sheet at 4–7 RE with ionospheric electron energy flux
Authors: Keiling A. , Wygant J. R. , Cattell C. , Peria W. , Parks G. , Temerin M. , Mozer F. S. , Russell C. T. , Kletzing C. A.
Published: October 2002

A comparison of Poynting flux in the plasma sheet at geocentric distances of 4–7 RE to the energy flux of magnetically conjugate precipitating electrons at 100‐km altitude is presented. We have investigated 40 plasma sheet crossings by the Polar satellite, including both cases with large in situ values of Poynting flux (∼1 ergs cm−2 s−1) and cases with low values (≤0.1 ergs cm−2 s−1). The values correspond to ∼125 and ∼12 ergs cm−2 s−1, respectively, when mapped along converging magnetic field lines to 100 km. The north‐south component of the electric field and the east‐west component of the magnetic field were the primary source of the Poynting flux. On the basis of the phase relationship and ratio of E and B, the majority of Poynting flux events were identified as Alfvén waves. The Poynting flux measured at high altitudes by Polar was correlated with the intensity of the conjugate auroral emission in the ultraviolet frequency range, which can be used to estimate energy deposition due to precipitating electron beams. The electron energy flux during times of intense Poynting flux in the plasma sheet exceeded 20 ergs cm−2 s−1. In the absence of strong Poynting flux in the plasma sheet, electron precipitation was small (≤5 ergs cm−2 s−1). The mapped Poynting flux was in almost all events larger by a factor of 1–10 than the ionospheric electron energy flux. These results show that Alfvénic Poynting flux in the midtail region is associated with and capable of powering localized regions of magnetically conjugate auroral emissions. Furthermore, the large Poynting flux events observed at the outer edge of the plasma sheet were conjugate to the poleward border of the active auroral regions, giving further evidence that at least some of the discrete aurora connects to the plasma sheet boundary layer.
Possible hydrodynamic waves in the topside ionospheres of Mars and Venus
Authors: Wang J.‐S. , Nielsen E.
Published: October 2002

The dispersion relation for hydrodynamic waves in an ionosphere with at most a weak magnetic field shows that gravity waves as well as hydrodynamic hybrid waves may be excited in the topside ionosphere of Mars and Venus owing to fluctuations in the solar wind pressure. The gravity wave, which propagates horizontally with a frequency equal to the buoyancy frequency, belongs to the classic branches of acoustic‐gravity wave (AGW) mode. The hybrid waves result from coupling between two different hydrodynamic wave modes. One of these modes is the AGW mode, and the other is excited independent of gravity but dependent on the presence of horizontal gradients in the background plasma pressure and density. The latter mode propagates horizontally but can also propagate vertically if there is a vertical gradient in the horizontal velocity. This new mode is therefore called background gradient wave (BGW). The hybrid waves will cause fluctuations with a wavelength of tens of kilometers in the vertical plasma altitude profiles when they propagate vertically. The period of the waves will be of the order around 103 ∼ 104 s. Further properties of possible AGW‐BGW waves in Mars' and Venus' ionospheres are given. Radio occultation observations at Mars and Venus show electron density fluctuations in the high‐altitude ionosphere. The fluctuations are mainly noise, but they may in part be caused by hydrodynamic wave activity. To verify wave activity, more detailed measurements are required and may be obtained with the low‐frequency radar planned for the Mars Express mission.
A statistical study of wave properties and electron density at 1700 km in the auroral region
Authors: Hamrin M. , Norqvist P. , André M. , Eriksson A. I.
Published: October 2002

We present a comprehensive overview of the electron density and six different wave types in the range ∼1 Hz to ∼1 MHz, and we investigate their occurrence, average wave frequency and amplitude as a function of location, Kp index, and solar illumination. Twenty‐one months of Freja observations from the Northern Hemisphere obtained at ∼1700 km altitude and invariant latitudes 40°–75° are used. We find that waves around the lower hybrid frequency occur in one low‐latitude dayside band and one high‐latitude nightside band. The latter band correlates with precipitating auroral electrons and coexists with electromagnetic ion cyclotron (EMIC) waves. This indicates the importance of energetic electrons for the wave generation. Both broadband ELF waves and broadband high‐frequency whistler mode waves are found at high latitudes, but whistler mode emissions are most common in regions of high electron densities on the dayside, while broadband ELF waves are found where the density is reduced on the nightside. Moreover, the average density in the presence of broadband ELF waves is more reduced when the ionosphere is dark than when it is sunlit. However, broadband whistler mode waves, Langmuir waves, and waves with an upper cutoff just below the proton gyrofrequency coincide with density enhancements when the ionosphere is dark. Ion heating correlated with auroral electrons coexists with EMIC waves and the high‐latitude band of waves around the lower hybrid frequency. Furthermore, ion heating not correlated with downgoing electrons coexists with broadband ELF waves.
Motion and origin of noon high‐latitude poleward moving auroral arcs on closed magnetic field lines
Authors: Kozlovsky Alexander , Kangas Jorma
Published: October 2002

We present observations of the near‐noon high‐latitude poleward moving auroral arcs, which show essential differences from well‐known poleward moving auroral forms associated with reconnection and flux transfer events. We suggest that the observed arcs are a specific class of dayside auroras referred to in the paper as poleward moving auroral arcs. The arcs were investigated together with the ionospheric plasma flows derived from the European Incoherent Scatter VHF measurements over Svalbard. The auroral arc motion was monitored at 1000–1300 magnetic local time by the all‐sky camera on 9 December 1998. It has been found that the noon auroral arcs move poleward at the velocity of the order of 150–350 m s−1, and this velocity does not show any dependence on the velocity of the ionospheric plasma convection along the same direction. The arcs appear at 10–20 min after strong changes (up to 3 km s−1) in the east‐west plasma flow, which resulted from variations in the interplanetary magnetic field. Ionospheric plasma drift and ground magnetometer data show the location of the auroras equatorward of the convection reversal. Spectrums of the northward electric field variations measured by the radar demonstrate clear peaks corresponding to the magnetic field line eigenfrequency oscillations with amplitude on the order of 10 mV m−1. The period of the oscillations increases from 7 min at 74.3° magnetic latitude (MLAT) to 14 min at 76.4° MLAT, and corresponding spectral characteristics were observed in the magnetic field measured on the ground. The observed features allow us to suggest that the noon auroral arcs arise on closed magnetic field lines as a result of interference between Alfvén field line eigenmode toroidal oscillations on different L shells. The field line resonance oscillations at the near‐cusp L shells are excited by the Alfvén impulse associated with the convection disturbance following after variations in interplanetary parameters.
Outer zone relativistic electron acceleration associated with substorm‐enhanced whistler mode chorus
Authors: Meredith Nigel P. , Horne Richard B. , Iles Roger H. A. , Thorne Richard M. , Heynderickx Daniel , Anderson Roger R.
Published: October 2002

We present plasma wave and particle data from the CRRES satellite during three case studies to investigate the viability of a local stochastic electron acceleration mechanism to relativistic energies driven by resonant interactions with whistler mode chorus. We first consider a strong geomagnetic storm that contains prolonged substorm activity during its 3‐day recovery phase. The recovery phase is characterized by electron injections at subrelativistic energies, enhanced whistler mode chorus amplitudes, and a gradual increase in the flux of relativistic electrons (E > 1 MeV) over the entire outer zone, with fluxes exceeding the prestorm level by an order of magnitude in the region 3.5 < L < 4.5. We next consider a strong geomagnetic storm that contains very little substorm activity during its 3‐day recovery phase. Here the recovery phase is characterized by a lack of sustained electron injections at subrelativistic energies, a low level of chorus amplitudes, and a net reduction in the flux of relativistic electrons in the outer zone. Finally, we examine a period of prolonged substorm activity in the absence of a significant storm signature, as measured by Dst. This period is characterized by electron injections at subrelativistic energies, enhanced chorus amplitudes, and a gradual increase in the flux of relativistic electrons in the region 4 < L < 6.5. These results suggest that the gradual acceleration of electrons to relativistic energies seen on a timescale of days during geomagnetic storms can be effective only when there are periods of prolonged substorm activity following the main phase of the geomagnetic storm. Furthermore, gradual electron acceleration to relativistic energies can be obtained during periods of prolonged substorm activity in the absence of a significant storm signature as indicated by Dst. The case studies show that the acceleration mechanism is confined to the region outside of the plasmapause and occurs in the presence of enhanced chorus waves. These results suggest that a local acceleration mechanism involving the energization of a seed population of electrons with energies of the order of a few hundred keV to relativistic energies by wave‐particle interactions involving whistler mode chorus contributes to the reformation of the relativistic outer zone population following prolonged substorm activity.
Pressure balance across magnetic flux ropes in the ionosphere of Venus
Authors: Ledvina S. A. , Nunes D. C. , Cravens T. E. , L. Tinker J.
Published: October 2002

The planet Venus has a dense atmosphere, and hence an ionosphere, and does not possess an intrinsic magnetic field. The solar wind interacts directly with the atmosphere and ionosphere of Venus. The Pioneer Venus Orbiter (PVO) observed many small‐scale magnetic rope‐like structures within the ionosphere. We examine thermal pressure variations for ∼2000 ropes for a wide range of altitudes and solar zenith angles. We demonstrate that flux ropes are statistically force‐free at low altitudes, meaning that a decrease of thermal pressure variation across them is not needed to compensate for the magnetic pressure in these ropes. However, at higher altitudes, there is a statistical tendency for ropes to display a thermal pressure variation across them, indicating that these ropes are not fully force‐free.
Field‐aligned ion motions in the E and F regions
Authors: Fujii R. , Oyama S. , Buchert S. C. , Nozawa S. , Matuura N.
Published: October 2002

Characteristics of field‐aligned (FA) ion motions in the E and F region (below F peak) ionosphere have been determined based on an analysis of European incoherent scatter (EISCAT) Svalbard and Kiruna‐Sodankylä‐Tromsø (KST) data. Sporadic/burst upward FA ion motions are observed in southward electric field enhancement regions or in the postmidnight region, while relatively stable downward FA ion motions are seen in northward electric field enhancement regions or in other regions. Long‐term (diurnal) variations of these upward and downward FA ions are likely driven by the large‐scale from day‐to‐night thermospheric wind. However, each short‐lived upward FA flow has neither one‐to‐one correspondence to an enhancement (or depletion) of the electric field nor that of electron density. The driving mechanism of the short‐lived upward flow cannot thus be understood yet, although atmospheric gravity waves can be one of the possible mechanisms to create the short‐term upward FA ion motions in the southward electric field region or in the postmidnight region.
Substorm energy budget during low and high solar activity: 1997 and 1999 compared
Authors: Tanskanen E. , Pulkkinen T. I. , Koskinen H. E. J. , Slavin J. A.
Published: October 2002

Substorms during the years 1997 and 1999 are analyzed to examine substorm energy budget just after solar minimum and prior to solar maximum. The energy input from the solar wind into the magnetosphere is estimated in terms of the time integral of Akasofu's epsilon parameter computed from Wind and ACE observations. The ionospheric Joule heating dissipation is estimated using the local electrojet index IL derived from the IMAGE magnetometer array of the MIRACLE ground‐based network in the Scandinavian sector. In total, 839 substorms from the midnight sector have been investigated to show that on average the Northern Hemisphere Joule heating accounts for ∼30% of solar wind energy input during 1997 and 1999 substorms. We found that during the active year 1999, there were 26% more substorm events, they were 15% more intense, and they were located at lower latitudes than during 1997. Isolated and stormtime substorms were also examined separately. Mean intensity of isolated substorms was about −350 nT, whereas it was about −670 nT for stormtime events. This study confirms our previous results (for 1997 only) that the amount of Joule dissipation depends on the energy input during the substorm expansion phase. Furthermore, the correlation is best for substorms recorded in the postmidnight sector, indicating that the energy budget and substorms size are largely controlled by processes driven directly by the solar wind.
Investigating wave data from the FAST satellite by reconstructing the wave distribution function
Authors: Stenberg G. , Oscarsson T. , André M. , Chaston C. C.
Published: October 2002

We study wave measurements made by the FAST satellite at 4100‐km altitude in the auroral region. Three electric and three magnetic wave field components are available, and wave emissions appear close to the local proton gyrofrequency (187 Hz) and at its harmonics. The particle observations reveal an electron beam as well as counterstreaming protons and oxygen ions. The plasma density is very low. We investigate the linear wave mode structure using a five‐component bi‐Maxwellian plasma model. The streaming electrons cause an instability at large perpendicular wave vectors. Using cross spectra and autospectra computed from the data, we reconstruct the wave distribution function (WDF). The results show that the observations are consistent with a linear description of the plasma. We find a good agreement between the reconstructed wave energy distribution, and the instability and our analysis gives a wavelength of ∼2.5 km. The reconstructed WDF fits into a simple geometrical picture, where the satellite is passing through the outer edge of a region with precipitating electrons.
A modeling study of the nighttime equatorial E region behavior during magnetospheric substorms and storms
Authors: Kim V. P. , Hegai V. V. , Lal M.
Published: October 2002

The results of a modeling study of the variations of altitude ion density distribution in the nighttime E region over the magnetic equator during strong magnetospheric substorms and storms due to perturbations of the zonal electric field Ey are presented. The ion density N is calculated by solving the time‐dependent continuity equation for ion density by assuming the time variations of Ey, which are fitted to some of the observed ones. It is shown that during severe magnetospheric disturbances, the altitude structure of the nighttime equatorial E region is substantially modified, with the largest effects in the lower part of E region (z < 105 km), where a deep trough in ion density is formed instead of the quiet E region maximum. The ion density in the trough can be ∼1/6 times the undisturbed value of N. The E region maximum NmE can be lifted up by ∼30 km, being ∼2× decreased as compared with the quiet time value of NmE. In the course of the magnetospheric disturbance the E region is bifurcated into two layers.
Global Pc5 caused by a DP 2–type ionospheric current system
Authors: Motoba T. , Kikuchi T. , Lühr H. , Tachihara H. , Kitamura T.‐I. , Hayashi K. , Okuzawa T.
Published: October 2002

A global Pc5 geomagnetic pulsation (period of ∼6 min) was observed coherently from the auroral to equatorial latitudes in the local time sector of 0700–2100 LT during 1834–1900 UT on 21 April 1993. The Pc5 at the dayside dip equator (Sao Luiz, Brazil, and Ancon, Peru) was characterized by an equatorial enhancement with an enhancement ratio of ∼4. In the afternoon sector (1200–1600 magnetic local time) the Pc5 at the auroral latitude was coherent with that at the dip equator within a time resolution of 3 s. The Pc5 amplitudes decreased sharply away from the auroral zone but were enhanced in the dayside dip equator, in a manner that resembled the latitudinal profile of a DP 2 magnetic fluctuation (period of ∼40 min) studied by Kikuchi et al. [1996] except for the enhanced amplitude in the auroral zone. In the dayside auroral oval the Pc5 fluctuations in the magnetic X/H component were reversed in phase between the morning and afternoon sectors. Additionally, the magnetic Y/D component fluctuations around noon were correlated with the dayside equatorial Pc5. At most dayside middle‐/low‐latitude stations the Pc5 fluctuations in the H component, which are regarded as evidence for magnetospheric compressions, were strong. However, near the dawn terminator the D component was dominant rather than the H component, suggesting the influences of ionospheric currents originating in the polar region. There was no significant azimuthal phase propagation away from noon for the auroral to equatorial Pc5 signals. The global observational results suggest that the dawn‐dusk electric field in the polar ionosphere accompanying a pair of field‐aligned currents extends instantaneously to the equatorial ionosphere and completes a DP 2–type ionospheric current system responsible for the global coherent Pc5.
A study of black aurora from aircraft‐based optical observations and plasma measurements on FAST
Authors: Peticolas L. M. , Hallinan T. J. , Stenbaek‐Nielsen H. C. , Bonnell J. W. , Carlson C. W.
Published: October 2002

Black aurora was observed on 30 January 1998 in a narrow‐field camera forty seconds before and a minute after the magnetic footprint of the FAST satellite passed the magnetic zenith of the camera. Electron energy flux measured by FAST provided strong evidence that FAST passed over black aurora. This region was characterized by small‐amplitude wave and DC electric fields (<10 mV m−1) and small upward current densities (∼0.2μA m−2). The electron energy flux dropouts associated with black aurora were associated with spatially restricted regions in which the strong pitch angle diffusion is suppressed at energies greater than ∼2 keV. These dropouts were surrounded by hot (∼1.8 keV) plasma sheet electrons exhibiting strong pitch angle diffusion at all measured energies. Analyzing the electron spectra from six different nights of optical and satellite conjugate data demonstrates a surprising prevalence of electron distributions that showed the suppression of strong pitch angle diffusion at energies above ∼2 keV. What made the electron signature on the night of the black aurora observations unique were the large spatial regions of strong pitch angle diffusion at all energies (corresponding to the diffuse aurora) with only very narrow spatial regions of the partially empty loss cones (corresponding to the black aurora). We suggest that the pitch angle diffusion surrounding the black aurora is due to electron cyclotron waves (ECH) strongly scattering electrons into the loss cone at energies less than ∼2 keV and oblique, upper band whistler mode chorus waves scattering electrons at all measured energies in the plasma sheet. We further suggest that the scattering by the chorus waves is suppressed in localized regions leading to the black auroral features observed.
Structure of the Earth's lower ionosphere observed by GPS/MET radio occultation
Authors: Hocke K. , Igarashi K.
Published: October 2002

The application of the radio occultation technique to the lower ionosphere is described and analyzed by means of the international reference ionosphere (IRI) and the GPS/Meteorology (GPS/MET) experiment. Electron density profiles are derived from total electron content (TEC) measurements along radio links between GPS satellites and a GPS receiver on board the low Earth orbit satellite Microlab‐1 during radio occultations. The retrieval method is based on Abel inversion and requires only spherical symmetry for the lower ionosphere. The profiles are sorted for geographic latitude, noon, and midnight in June–July 1995 and February 1997. By means of a sliding window average, meridional slices of electron density are calculated from the sorted profiles. GPS/MET slices of electron density agree better with corresponding IRI slices for noon than for midnight. This is mainly due to the variable nature of particle precipitation from the magnetosphere, which is a significant ionization source, in particular for the lower ionosphere at high latitudes around midnight. Depending on ionospheric conditions, the applied retrieval method seems to be limited to the recovery of absolute electron density values exceeding 109–1010 m−3. This is possibly due to incomplete separation of E and F region contributions to the refraction of the GPS signal. Around noon, GPS/MET and IRI observe/predict a similar base height of the E region as well as similar absolute values of electron density in the E region. The radio occultation data also permit retrieval of meridional slices of average plasma irregularities (vertical scales <7 km) of the lower ionosphere at noon, midnight, winter, and summer.
Antisunward net Birkeland current system deduced from the Oersted satellite observation
Authors: Yamashita S. , Iyemori T. , Nakano S. , Kamei T. , Araki T.
Published: October 2002

From the precise magnetic field measurements by the Oersted satellite at low altitudes over various local times, a distinct spatial variation at middle and low latitudes was detected in the eastward component residuals (ΔBϕ) after the subtraction of a geomagnetic main field model. The average ΔBϕ decreases on the dayside as latitude increases, and it increases on the nightside. That is, the latitudinal structure of the ΔBϕ has a negative trend in the northward direction on the dayside and a positive trend on the nightside. The slope of the latitudinal profile of ΔBϕ is steep on both dayside and nightside under geomagnetically disturbed condition and has a high correlation with geomagnetic activity index such as the ap. These results strongly suggest the existence of a day‐night net Birkeland current system that flows into the polar ionosphere on the dayside and flows out on the nightside. These net currents seem to distribute in wide local time range, and the center of the net currents is estimated to be around 1000 LT on the dayside and around 2200 LT on the nightside, which is essentially consistent with previous theoretical predictions and ground geomagnetic observations.
Proton aurora in the cusp
Authors: Frey H. U. , Mende S. B. , Immel T. J. , Fuselier S. A. , Claflin E. S. , Gérard J.‐C. , Hubert B.
Published: October 2002

Frequently, the Far Ultraviolet Instrument (FUV) on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft observes intense ultraviolet emission from a localized dayside region poleward of the general auroral oval location. This emission is especially distinct in the Doppler‐shifted emission of hydrogen atoms produced by precipitating protons. We interpret this as a direct signature of proton precipitation into the cusp after reconnection of magnetospheric lobe field lines. This cusp signature appears only when the interplanetary magnetic field (IMF) has a positive northward Bz component. However, the intensity of the precipitation, and hence the intensity of UV emission, is not controlled by the magnitude of Bz but rather by the solar wind dynamic pressure. A statistical analysis of 18 cases observed in summer and fall 2000 shows good correlation between the UV intensity and the dynamic pressure and between the location in local time and the IMF By component. A quantitative analysis of observations from all three FUV subinstruments allows for an estimate of proton and electron energy fluxes during these times. In general, these estimates agree with results from in situ measurements by spacecraft and show that during these times, protons may contribute significantly to the overall energy deposition into the cusp.
The roles of static and dynamic electric fields in the auroral acceleration region
Authors: Bryant Duncan A.
Published: October 2002

Charged particles and electric fields measured by the Fast Auroral Snapshot satellite (FAST) are being widely interpreted as final vindication of the theory that the electrons responsible for the brightest and most highly structured forms of aurora gain their energy by being accelerated in static electric fields. However, measurements of electrostatic waves from the same satellite also give further impetus to the opposing theory that the electrons are energized by the dynamic electric fields of plasma waves. Consideration of the different capabilities of static and dynamic electric fields leads to a model of the auroral electron acceleration region incorporating acceleration by both types of fields. An experimental test of the model is suggested.
Constructing the magnetospheric model including pressure measurements
Authors: Kubyshkina M. V. , Sergeev V. A. , Dubyagin S. V. , Wing S. , Newell P. T. , Baumjohann W. , Liu A. T. Y.
Published: October 2002

We modeled the magnetotail configuration during a highly disturbed period on 19 April 1985 based on data from three magnetospheric spacecraft (Active Magnetospheric Tracer Explorers (AMPTE)/IRM, AMPTE/CCE, and ISEE 1) and proton precipitation measured by DMSP spacecraft. The goal was to test the procedure of including the plasma pressure as input data when constructing the magnetotail magnetic field model. On the basis of AMPTE/IRM data we confirm statistically a high degree of pressure isotropy at 8–17 RE distances in the tail plasma sheet, which allows computation of the plasma pressure from magnetospheric magnetic field model and easy mapping of the pressure along the corresponding flux tubes. The usage of pressure data not only increases the amount of data for event‐oriented modeling but also imposes the nonlocal constraints on the configuration and mapping between magnetosphere and ionosphere to stabilize the solution of inverse problem and get it to be more realistic. Observational and modeling results indicate large systematic deviations of observed configuration from predictions of standard models (T89 or T96) [Tsyganenko, 1989, 1995] and reveal highly variable configurations, justifying a need for the development of event‐oriented modeling. Indications of localized B minimum and current density maximum at around 12 RE have been obtained in the model with best data coverage, but precipitation from this region showed no signatures of discrete structures. The structured precipitation has been mostly seen in poleward half of the auroral zone precipitation, which is mapped to rather distant parts (>30 RE) of the plasma sheet.