bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of the magnetosheath flow in determining the motion of open flux tubes
Authors: Cooling B. M. A. , Owen C. J. , Schwartz S. J.
Published: February 2004

We have constructed a model which examines the motion of reconnected magnetic flux tubes over the surface of the magnetopause. For a given interplanetary magnetic field (IMF) we first determine the draping and strength of the magnetosheath magnetic field, flow velocity, and density over the entire surface of a paraboloid magnetopause. For a given magnetopause location we apply a test for steady state reconnection occurring between the magnetosheath field and a modeled magnetospheric field at that point. We trace the subsequent motion of these tubes along the surface of the magnetopause and into the magnetotail. Results are shown for a range of cases. The model has applications in testing various hypotheses about the location of reconnection events and, for example, IMF BY effects. In particular, it highlights the dominance of the magnetosheath flow model in determining the motion of the tubes and the necessity of sub‐Alfvénic magnetosheath flows for the occurrence of steady state reconnection poleward of the cusp during periods of northward IMF. Our model may also be used to identify likely reconnection sites on the dayside and near‐Earth nightside magnetopause and to identify possible locations for steady state reconnection.
Emission of OI(630 nm) in proton aurora
Authors: Lummerzheim D. , Galand M. , Semeter J. , Mendillo M. J. , Rees M. H. , Rich F. J.
Published: February 2004

A red aurora occurred over southern Canada and central Maine on April 11, 1997, producing a brightness of OI(630 nm) of several Kilorayleighs, which lasted for several hours. Two passes of the Defense Meteorological Satellite Program (DMSP) F12 satellite occurred during this time, and optical data were obtained from four CEDAR Optical Tomographic Imaging Facility (COTIF) sites. The DMSP F12 particle spectrometers observed proton precipitation south of the electron aurora with energy fluxes of several mW m−2. Tomographic inversion of the COTIF optical observations gives the altitude profile of emissions along a magnetic meridian. We combine all available data using an ionospheric auroral model. Our analysis shows that the model produces the observed auroral brightness from the proton precipitation alone.
Correlation of solar energetic protons and polar cap absorption
Authors: Patterson J. D. , Armstrong T. P. , Laird C. M. , Detrick D. L. , Weatherwax A. T.
Published: February 2004

This study shows the results of a model of polar cap absorption events (PCAs) using solar energetic proton flux as an input. The proton data are recorded by the Charged Particle Measurement Experiment (CPME) on board the IMP 8 satellite and are collected by the Applied Physics Laboratory at Johns Hopkins University. The IMP 8 satellite orbits the Earth at distances between 30 and 35 Earth radii, which places it in the solar energetic particle environment throughout most of its orbit. It has been shown in previous studies that these solar energetic particles have direct and immediate access to the polar atmosphere [Reid, 1970]. Our model shows that the majority of the ionization resulting from the influx of solar energetic protons occurs in the altitude range from ∼ 50–90 km. Excess ionization at these altitudes causes enhanced absorption of cosmic HF radio waves. The levels of absorption used for comparison in this study were measured directly by the riometer at South Pole station, Antarctica. The results show a very strong correlation between the incident proton flux and measured path‐integrated cosmic HF radio noise absorption for significant events, involving absorptions greater than 1.0 dB. For absorption levels lower than this it is obvious that other phenomena dominate. For HF radio waves the primary contributors to PCA are protons with energies near 20 MeV. This study extends the correlated observations of interplanetary particles and PCA throughout a 9‐year period. The close quantitative agreement between the measured and calculated values of absorption supports the validity of the assumptions and suppositions made by this model. The data also suggest a method by which the path‐integrated cosmic noise absorption may be used to probe the E and D layers of the ionosphere to determine the effective ion‐electron recombination coefficients within these regions.
Ground‐based optical observations of hydrogen emission in the auroral substorm
Authors: Deehr C. , Lummerzheim D.
Published: February 2004

A combination of all‐sky imagers (ASTV) and meridian‐scanning photometers (MSP) was used to identify the optical signature of the growth phase, onset, expansion, and recovery of 33 auroral substorms in the Alaskan sector. The discrete auroral arc that brightens at auroral substorm onset was found to be poleward of the diffuse aurora that contains the H emission by a distance of between 10 and 300 km. The average onset time was 2215 magnetic local time (MLT) and the average geographic latitude of the onset arc and the H arc was 64.6° and 63.6°, respectively. The poleward crossover of the peak H emission occurs shortly after substorm onset, for substorm intensification after 2100 MLT. The peak H emission crosses back to the equatorward position during substorm recovery between 2100 and 0100 MLT. After 0100 MLT the peak H emission remains poleward of the electron‐trapping boundary for the rest of the night. During the growth phase the peak H emission moves equatorward more quickly than does the onset arc and monotonically doubles in total intensity during the equatorward motion, in a manner quite unrelated to the fluctuations in brightness of the onset arc. In the onset arc a reduction of 427.8‐nm N2+ and 557.7‐nm [O I] emissions just prior to onset has been reported earlier and dubbed “auroral fading”. We find that the 630.0‐nm [O I] emission rises considerably relative to the others, and we conclude that the “fading” observed in the few minutes prior to onset is the ionospheric signature of a pulse of > 1010 electrons cm2s‐V−1 with energies less than 100 eV. Thus the observations reported here do not support the argument that the substorm onset begins from within the proton precipitation, but the discovery of the soft electron pulse during “auroral fading” just prior to onset does support those substorm onset theories requiring a pulse of Alfven speed electrons poleward of the electron trapping boundary and in the region of upward field‐aligned current.
The dynamics of the proton aurora in auroral breakup events
Authors: Takahashi Yukihiro , Fukunishi Hiroshi
Published: February 2004

Proton auroral dynamics during auroral breakup events are examined compared to electron auroral dynamics using monochromatic imaging and tilting‐filter photometer data obtained at Syowa station, Antarctica (corrected geomagnetic latitude: −66.23°, L = 6.15). During the period from May to September 1992, 35 typical breakup events were observed, in which 23 events showed a signature of poleward expansion of the auroral bulge, while 12 events showed a signature of the westward traveling surge. Careful analysis revealed that when an electron aurora arc is enhanced and moves poleward after substorm onset, the region of proton aurora also expands poleward, forming a sharp boundary, and that this poleward boundary always coincides with the leading edge of the electron auroral bulge. At the poleward boundary the Doppler shift of proton aurora is quite large (∼0.8 nm), while the maximum intensity of proton aurora is observed somewhat equatorward of the leading edge. These facts imply that precipitating protons with higher energies and highly field aligned pitch angles are produced at the poleward boundary through the plasma injection process in the magnetosphere. In 6 out of 35 breakup events, multiple N–S aligned electron aurora arcs (N–S auroras) appeared. An important finding is that the appearance of prominent N–S auroras in the auroral bulge results in the rapid poleward contraction of the proton aurora belt when the activity of electron aurora becomes low. It is therefore inferred that in the case accompanied by the N–S auroras the complete dipolarization of the magnetic field occurs after the recovery phase.
Observation of IMF and seasonal effects in the location of auroral substorm onset
Authors: Liou K. , Newell P. T. , Sibeck D. G. , Meng C.‐I , Brittnacher M. , Parks G.
Published: February 2004

We use Polar ultraviolet imager (UVI) and Wind observations to study the location of 648 well‐defined Northern Hemisphere auroral breakups (substorm onsets) in response to interplanetary magnetic field (IMF) orientation and season. The most likely onset location is at 2230 MLT and 67° Λm with half‐maximum widths of 3 hours of MLT and 2° Λm, respectively. The onset latitude depends primarily on IMF Bz, but also Bx: the onset latitude decreases for Bx > 0 or Bz < 0 and increases for Bx < 0 or Bz > 0. The onset longitude depends on season and IMF By. In summer, substorms tend to occur in the early evening at ∼2200 MLT, whereas in winter they tend to occur near midnight at ∼2300 MLT. The average summer‐winter difference in the onset location is ∼1 hour of MLT. Large By effects on the onset longitude occur only when Bx and By are small. Onset locations shift toward earlier local times for By > 0 and toward midnight for By < 0. The average onset local time is earliest (2200 MLT) for By > 0 in summer and latest (2330 MLT) for By < 0 in winter. These dependencies coincide with those previously reported for the evening sector ionospheric zonal flow reversal in response to IMF By and season, indicating that auroral breakups are most likely in regions of large velocity shears. A weak dependence of the MLT onset location on the IMF Bx is identified: for Bx > 0 the onset location shifts toward dusk when By > 0 but toward dawn when By < 0; the sense of this shift reverses for Bx < 0. An implication of the results is that auroral breakup is not conjugate.
Observation of the magnetospheric “sash” and its implications relative to solar‐wind/magnetospheric coupling: A multisatellite event analysis
Authors: Maynard N. C. , Savin S. , Erickson G. M. , Kawano H. , Němeček Z. , Peterson W. K. , Šafránoková J. , Sandahl I. , Scudder J. D. , Siscoe G. L. , Sonnerup B. U. Ö. , Weimer D. R. , White W. W. , Wilson G. R.
Published: February 2004

Using a unique data set from the Wind, Polar, Interball 1, Magion 4, and Defense Meteorological Satellite Program (DMSP) F11 satellites, comparisons with the Integrated Space Weather Model (ISM) have provided validation of the global structure predicted by the ISM model, which in turn has allowed us to use the model to interpret the data to further understand boundary layers and magnetospheric processes. The comparisons have shown that the magnetospheric “sash” [White et al., 1998], a region of low magnetic field discovered by the MHD modeling which extends along the high‐latitude flank of the magnetopause, is related to the turbulent boundary layer on the high‐latitude magnetopause, recently mapped by Interball 1. The sash in the data and in the model has rotational discontinuity properties, expected for a reconnection site. At some point near or behind the terminator, the sash becomes a site for reconnection of open field lines, which were previously opened by merging on the dayside. This indicates that significant reconnection in the magnetotail occurs on the flanks. Polar mapped to the high‐density extension of the sash into the tilted plasma sheet. The source of the magnetosheath plasma observed by Polar on closed field lines behind the terminator was plasma entry through the low field connection of the sash to the central plasma sheet. The Polar magnetic field line footprints in each hemisphere are moving in different directions. Above and below the tilted plasma sheet the flows in the model are consistent with the corresponding flows in the ionosphere. The turbulence in the plasma sheet allows the convection patterns from each hemisphere to adjust. The boundary layer in the equatorial plane on the flank for this interplanetary magnetic field BY condition, which is below the tilted central plasma sheet, is several RE thick and is on tailward flowing open field lines. This thick boundary layer shields the magnetopause from viscous forces and must be driven by magnetic tension. Above the plasma sheet the boundary layer is dominated by the sash, and the model indicates that the open region inside the sash is considerably thinner.
Polar cap boundary layer waves: An auroral zone phenomenon
Authors: Tsurutani Bruce T. , Arballo John K. , Galvan Carlos , Zhang Liwei Dennis , Zhou Xiao‐Yan , Lakhina Gurbax S. , Hada Tohru , Pickett Jolene S. , Gurnett Donald A.
Published: February 2004

Polar cap boundary layer waves are ELF/VLF electric and magnetic waves detected on field lines just adjacent to the polar cap. Intense waves are present at this location essentially all (96%) of the time. The wave latitude‐local time distribution is shown to be the same as that of the Feldstein auroral oval, a distribution centered at ∼75° at local noon and ∼65° at local midnight. The most intense waves are detected coincident with the strongest magnetic field gradients (field‐aligned currents). Statistically, the wave intensities are greatest near local noon (10−13 mV2 m−1 at 3 kHz) and midnight and are least near dawn and dusk (∼5 × 10−15 mV2 m−1 at 3 kHz). The noon and midnight wave intensities increase slightly when the interplanetary magnetic field is directed southward. The dawn and dusk waves appear to be controlled by the solar wind speed. Using high‐resolution data, specific frequency bands of electromagnetic whistler‐mode waves are identified: ∼200 Hz and 1‐2 and ∼5 kHz. These may correspond to previously identified “magnetic noise bursts” and “auroral hiss”, respectively. Assuming cyclotron resonant interactions, the 1‐ to 5‐kHz auroral hiss is shown to be resonant with ∼50‐eV to ∼1.0‐keV electrons. Several mechanisms, both resonant (nonlocal) and nonresonant (local), are suggested for the generation of the ∼200‐Hz electromagnetic waves. Three types of intense electric signals are present: solitary bipolar pulses (electron holes), waves at ∼4 × 102 to 6 × 103 Hz (lower hybrid waves), and narrowband waves at ∼10 kHz (electrostatic waves near the upper hybrid resonance frequency). Solitary bipolar pulse onset events have been detected for the first time. The bipolar pulses reached 2 mV m−1 peak‐to‐peak amplitudes within 3 ms. An exponential growth rate of 0.72 ms, or 0.25 fce, was determined. The previously reported “broadband nature” of the polar cap boundary layer (and low‐latitude boundary layer) waves is now postulated to be caused by a fast switching between the various electromagnetic and electrostatic modes described above. The polar cap boundary layer waves are most likely a consequence of instabilities associated with auroral zone field‐aligned currents carried by 50‐eV to 1.0‐keV electrons and protons. The currents in turn have been ascribed to be driven by the solar wind‐magnetosphere global interaction. One consequence of the presence of the waves at high altitudes is diffusion of magnetosheath plasma into the magnetosphere and magnetospheric plasma out into the magnetosheath (cross‐field diffusion, due to parasitic wave‐particle interactions). It is speculated that field‐aligned currents and similar wave modes will be detected at all planetary magnetospheres.
Plasmaspheric material on high‐latitude open field lines
Authors: Su Yi‐Jiun , Borovsky Joseph E. , Thomsen Michelle F. , Dubouloz Nicolas , Chandler Michael O. , Moore Thomas E. , Bouhram Mehdi
Published: February 2004

During periods of increased geomagnetic activity, cold and dense plasmaspheric material is observed to drain from the inner magnetosphere toward the day side magnetopause. Geosynchronous observations have shown that plasmaspheric material may participate in the dayside reconnection. However, predictions of plasmaspheric material passing through the polar region on recently opened field lines have not yet been confirmed by observations. We present evidence for the existence of such plasmaspheric material on high‐latitude open magnetic field lines based on our investigations of 20 months of Interball/Hyperboloid observations and 11 months of Polar/Thermal Ion Dynamics Experiment (TIDE) observations. In order to distinguish plasmaspheric material from the low‐energy portion of entering magnetosheath plasma from the solar wind, observed phase‐space densities are compared to modeled magnetosheath and plasmaspheric phasespace densities. The phase‐space density distribution function of magnetosheath ions is estimated from upstream solar wind parameters using the gasdynamic theory. Twenty‐one events were found in which Interball passed through open field lines on the dayside during periods of increased geomagnetic activity, two of which show evidence for the presence of plasmaspheric material. Additionally, six such events were identified from the Polar/TIDE database. Although the occurrence frequency is low, evidence of cold plasmaspheric material being transported on high‐latitude open field lines does exist.
Plasma sheet access to the inner magnetosphere
Authors: Friedel R. H. W. , Korth H. , Henderson M. G. , Thomsen M. F. , Scudder J. D.
Published: February 2004

We present here plasma data from the Polar HYDRA instrument giving comprehensive coverage of the inner magnetospheric region from L ∼ 2 outward. Data is projected to an equatorial reference plane yielding a global view of the inner extend of the plasma sheet. We determine the inner boundary for plasma sheet electrons and ions in the μ range 0.05 – 50 eV nT−1 and we compare these to the predicted Alfvén boundaries as a function of the geomagnetic activity index Kp. In general, the simple conventional drift paradigm is shown to be globally consistent with the averaged data in the inner magnetosphere, with electrons adhering better to the predicted boundaries than ions. The data are further compared to the geosynchronous slice as measured by the Los Alamos Magnetospheric Plasma Analyzer (MPA) which measures the crossing point of the Alfvén boundaries at geosynchronous altitudes with much better statistical resolution than Polar. Integral to the drift model used is an assumption about the form of the global electric field. The agreement with data validates the simple corotation and convection electric field used and shows that this model describes well the average transport for a wide range of geomagnetic activity and over a large part of the inner magnetosphere.
Plasma waves observed in the cusp turbulent boundary layer: An analysis of high time resolution wave and particle measurements from the Polar spacecraft
Authors: Pickett J. S. , Franz J. R. , Scudder J. D. , Menietti J. D. , Gurnett D. A. , Hospodarsky G. B. , Braunger R. M. , Kintner P. M. , Kŭrth W. S.
Published: February 2004

The boundary layer located in the cusp and adjacent to the magnetopause is a region that is quite turbulent and abundant with waves. The Polar spacecraft's orbit and sophisticated instrumentation are ideal for studying this region of space. Our analysis of the waveform data obtained in this turbulent boundary layer shows broadband magnetic noise extending up to a few kilohertz (but less than the electron cyclotron frequency); sinusoidal bursts (a few tenths of a second) of whistler mode waves at around a few tens of hertz, a few hundreds of hertz, and just below the electron cyclotron frequency; and bipolar pulses, interpreted as electron phase‐space holes. In addition, bursts of electron cyclotron harmonic waves are occasionally observed with magnetic components. We show evidence of broadband electrostatic bursts covering a range of ∼3 to ∼25 kHz (near but less than the plasma frequency) occurring in packets modulated at the frequency of some of the whistler mode waves. On the basis of high time resolution particle data from the Polar HYDRA instrument, we show that these bursts are consistent with generation by the resistive medium instability. The most likely source of the whistler mode waves is the magnetic reconnection site closest to the spacecraft, since the waves are observed propagating both toward and away from the Earth, are bursty, which is often the case with reconnection, and do not fit on the theoretical cold plasma dispersion relation curve.
Electrostatic electron cyclotron waves observed by the plasma wave instrument on board Polar
Authors: Menietti J. D. , Pickett J. S. , Gurnett D. A. , Scudder J. D.
Published: February 2004

We report the results of an investigation of waves observed by the Polar spacecraft at high altitudes and latitudes and at frequencies just above the cyclotron frequency. These observations are made frequently when the spacecraft is over the polar cap as well as near the dayside cusp and near the nightside auroral region, and observations are made for ratios of plasma frequency to cyclotron frequency, ƒp/ƒc ∼ 1. Using the six‐channel high‐frequency waveform receiver (HFWR) on board the spacecraft, which can provide three‐axis electric and three‐axis magnetic field measurements, we attempt to identify the wavemode of these emissions and investigate possible source mechanisms including low‐energy electron beams. We further observe electromagnetic emission associated with upper hybrid waves near and within the plasmasphere. This emission is consistent with both Z and O modes.
Observations of relativistic electron microbursts in association with VLF chorus
Authors: Lorentzen K. R. , Blake J. B. , Inan U. S. , Bortnik J.
Published: February 2004

The Solar, Anomalous and Magnetospheric Particle Explorer (SAMPEX) satellite frequently observes relativistic (> 1 MeV) electron precipitation in the radiation belts at L shells of 4–6 with bursty temporal structure lasting < 1 s. This phenomenon can occur at all local times but is most often seen between 0200 and 1000 magnetic local time. VLF chorus is also observed to occur preferentially at these same local times. Using electron observations from the SAMPEX satellite Heavy Ion Large Telescope and data from the Polar satellite plasma wave instrument, we show correlation between observations of relativistic electron microbursts and VLF chorus with frequencies <2 kHz. In addition, the duration of the individual rising frequency chorus elements is comparable to the duration of the relativistic electron microbursts. It has been speculated that relativistic electron microbursts are caused by wave‐particle interactions, which strongly scatter electrons into the loss cone for a short period. Lower‐energy electron microbursts in the range from tens to hundreds of keV have long been associated with chorus waves, since these lower‐energy electrons can resonate at the equator with whistler‐mode waves at chorus frequencies. Electrons of MeV energies do not satisfy the first‐order cyclotron resonance condition with chorus wave frequencies at the equator. However, MeV electrons may interact with chorus through higher‐order resonances or off‐equatorial interactions.
Solar wind influence on the oxygen content of ion outflow in the high‐altitude polar cap during solar minimum conditions
Authors: Elliott H. A. , Comfort R. H. , Craven P. D. , Chandler M. O. , Moore T. E.
Published: February 2004

We correlate solar wind and interplanetary magnetic field (IMF) properties with the properties of O+ and H+ during early 1996 (solar minimum) at altitudes between 5.5 and 8.9 RE geocentric using the Thermal Ion Dynamics Experiment (TIDE) on the Polar satellite. Throughout the high‐altitude polar cap we observe H+ to be more abundant than O+. O+ is found to be more abundant at lower latitudes when the solar wind speed is low (and Kp is low), while at higher solar wind speeds (and high Kp), O+ is observed across most of the polar cap. The O+ density and parallel flux are well organized by solar wind dynamic pressure, both increasing with solar wind dynamic pressure. Both the O+ density and parallel flux have positive correlations with both VswBIMF and Esw. No correlation is found between O+ density and IMF Bz, although a nonlinear relationship with IMF By is observed, possibly due to a strong linear correlation with the dynamic pressure. H+ is not as highly correlated with solar wind and IMF parameters, although H+ density and parallel flux are negatively correlated with IMF By and positively correlated with both VswBIMF and Esw. In this solar minimum data set, H+ is dominant, so that contributions of this plasma to the plasma sheet would have very low O+ to H+ ratios.
Energy content in the storm time ring current
Authors: Turner N. E. , Baker D. N. , Pulkkinen T. I. , Roeder J. L. , Fennell J. F. , Jordanova V. K.
Published: February 2004

Given the important role the ring current plays in magnetospheric energetics, it is essential to understand its strength and evolution in disturbed times. There are currently three main methods for deducing the strength of the ring current: measuring ground magnetic perturbations, measuring high‐altitude magnetic perturbations, or directly measuring ring current particles. The use of ground magnetometers is the most convenient, and many use the ground magnetometer‐derived Dst index as a proxy for the ring current. Recent work suggests, however, that a substantial portion of Dst may not be caused only by the ring current but also by local induction effects or other magnetospheric currents, so simply using the Dst index may yield inaccurate results. This study uses direct particle measurements to calculate the strength of the ring current and compares this to the measured Dst values. We investigate several magnetic storm intervals, using the Polar Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) to measure ring current ions. We then use the Dessler‐Parker‐Sckopke relation to compare this to the measured Dst. This analysis is used both to understand the general behavior of the ring current compared to Dst as well as to compare the usefulness of the Dst proxy for different types of storms. Ring current ions are shown in this analysis to contribute, on average, half of the Dst depression, with a large variation among individual events.
Energy characteristics of auroral electron precipitation: A comparison of substorms and pressure pulse related auroral activity
Authors: Chua D. , Parks G. , Brittnacher M. , Peria W. , Germany G. , Spann J. , Carlson C.
Published: February 2004

The Polar Ultraviolet Imager (UVI) observes auroral responses to incident solar wind pressure pulses and interplanetary shocks such as those associated with coronal mass ejections (CMEs). The arrival of a CME pressure pulse at the front of the magnetosphere results in highly disturbed geomagnetic conditions and a substantial increase in both dayside and nightside auroral precipitation. Our observations show a simultaneous brightening over broad areas of the dayside and nightside aurora in response to a pressure pulse, indicating that more magnetospheric regions participate as sources for auroral precipitation than during isolated substorms. We estimate the average energies of incident auroral electrons using Polar UVI images and compare the precipitation energies during pressure pulse associated events with those during isolated auroral substorms. Electron precipitation during substorms has average energies greater than 10 keV and is structured both in local time and magnetic latitude. For auroral intensifications following the arrival of a pressure pulse or interplanetary shock, electron precipitation is less spatially structured and has greater flux of lower‐energy electrons (Eave ≤ 7 keV) than during isolated substorm onsets. The average energies of the precipitating electrons inferred from UVI are consistent with those measured in situ by the Fast Auroral Snapshot (FAST) spacecraft. These observations quantify the differences between global and local auroral precipitation processes and will provide a valuable experimental check for models of sudden storm commencements and magnetospheric response to perturbations in the solar wind.
Interplanetary shock triggering of nightside geomagnetic activity: Substorms, pseudobreakups, and quiescent events
Authors: Zhou Xiaoyan , Tsurutani Bruce T.
Published: February 2004

We use Wind solar wind data and Polar UV imaging data to study the nightside magnetospheric/magnetotail responses to interplanetary shocks/pressure pulses. Of 53 interplanetary shock/pressure pulse events that occurred in 1997 and 1998 at Wind, there are 18 cases where Polar near‐midnight UV images are available. All of these 18 events are used in this study. The nightside auroral responses can be classified into three types: substorm expansion phase (SS) (or substorm further intensification) events, pseudobreakup (PB) events, and quiescent (QE) events. It is found that the solar wind preconditions determine the causes of the different auroral responses. A ∼1.5‐hour interplanetary magnetic field (IMF) Bs “precondition” (upstream of the interplanetary shock) gives good empirical results. The upstream IMF is strongly southward prior to substorm expansion phase triggerings (44% of all events), the IMF Bz is ∼0 nT for PB triggerings (39% of all events), and the IMF is purely northward for quiescent events (17%). The evidence for IMF Bs preconditioning is interpreted in terms of a plasma sheet loading mechanism. The interplanetary shock compression effects on the near‐Earth tail are discussed in light of existing substorm/PB triggering models.
Studies of ion solitary waves using simulations including hydrogen and oxygen beams
Authors: Crumley J. P. , Cattell C. A. , Lysak R. L. , Dombeck J. P.
Published: February 2004

Particle‐in‐cell simulations of solitary waves have been performed using a 2 spatial and 3 velocity dimension electrostatic code with one electron and two ion species. Data from the Fast Auroral Snapshot (FAST) and Polar spacecraft are used to provide input parameters, and on the basis of these observations, no cold plasma was included in contrast to earlier simulations. Simulations containing both oxygen and hydrogen beams are compared to simulations that contain only hydrogen to examine the effects of the oxygen on the behavior of the solitary waves. In both cases the solitary wave speeds are less than the hydrogen beam speed, and they are also greater than the oxygen beam speed for the cases including oxygen. The simulated solitary waves have spatial scales of the order of 10 λD and potential amplitudes of the order of 0.1 eϕ/kTe, which are consistent with Polar spacecraft observations in the low‐altitude auroral zone.
Statistical patterns in X‐ray and UV auroral emissions and energetic electron precipitation
Authors: Anderson P. C. , Petrinec S. M. , Liou K.
Published: February 2004

Statistical auroral images from the UV and X‐ray imagers on the Polar spacecraft were compared with each other and with statistical distributions of energetic auroral electron energy fluxes from two different energy ranges: 50 eV to 20 keV and 30 keV to 2 MeV. The individual statistical auroral ovals each had unique features absent in some or all of the other ovals; however, the differences were consistent with the known instrument energy responses, the motion of magnetospheric particles in the inner magnetosphere, and the spectral characteristics of the auroral particles. Separation of the statistical results by Kp showed little difference in the auroral distributions other than in intensity. However, separation of the X‐ray data by Dst showed significant differences, and there were indications that the UV statistical data show a similar dependence. X‐ray emissions, on average, were more intense in the evening sector for Dst > 0, associated with isolated substorm activity and substorm injections, and in the morning sector for Dst < 0, associated with geomagnetic storm activity and strong magnetospheric convection.
Penetration of auroral electric fields to the equator during a substorm
Authors: Kikuchi T. , Lühr H. , Schlegel K. , Tachihara H. , Shinohara M. , Kitamura T.‐I.
Published: February 2004

We have studied the negative magnetic bay associated with the substorm that occurred on April 20, 1993, and have found that it is markedly enhanced at the daytime dip equator, coherent with that at afternoon subauroral latitudes. The amplitude of the negative bay decreases monotonously with the latitude, but it is amplified at the dip equator by a factor of 2.5 compared to the low‐latitude negative bay. This latitudinal profile implies that in addition to the three‐dimensional current system in the magnetosphere, DP ionospheric currents originating in the polar ionosphere contribute greatly to negative bays. Penetration of the convection electric field and the effect of a shielding electric field due to Region 2 (R2) field‐aligned currents (FACs) are examined on the basis of European Incoherent Scatter (EISCAT) and International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer observations made in the afternoon sector. The northward electric field at EISCAT (66° corrected geomagnetic latitude (CGMLAT)) is well correlated with the magnetic field X component at Nurmijärvi (56° CGMLAT) during the presubstorm period, but the coherency breaks down during the substorm cycle. By assuming that the R2 FACs intensify the northward electric field at EISCAT but reduce it at Nurmijärvi, we demonstrate that the R2 FACs grow concurrently, although delay by some 17 min, with the convection electric field. Our analytical results indicate that the convection electric field decreases abruptly during the substorm and that the shielding electric field overcomes the convection electric field at around the peak of the negative bay, owing to its delayed reaction. The equatorial negative bay is thus due to an overshielding effect caused by the electric field associated with the R2 FACs.
Convection and auroral response to a southward turning of the IMF: Polar UVI, CUTLASS, and IMAGE signatures of transient magnetic flux transfer at the magnetopause
Authors: Milan S. E. , Lester M. , Cowley S. W. H. , Brittnacher M.
Published: February 2004

We present the first spacecraft‐borne imager observations of the auroral manifestation of transient magnetic flux transfer at the magnetopause. During an interval of interplanetary magnetic field Bz ≈ −10 nT, By ≈ 10 nT, and solar wind dynamic pressure and velocity Psw ≈ 5 nPa and vsw ≈ 650 km s−1, Polar Ultraviolet Imager (UVI) images show a sequence of events, each of which begins as a bifurcation of the main auroral oval in the 14 to 16 magnetic local time (MLT) sector which subsequently progresses antisunward (eastward) at 2 km s−1 toward the 19 MLT sector. The poleward portion of the bifurcation is interpreted as a poleward‐moving auroral form (PMAF) as has previously been observed by ground‐based optical instrumentation and identified as the auroral signature of flux transfer events. Ground‐based measurements of the associated plasma drift, made with the Cooperative U.K. Twin Located Auroral Sounding System (CUTLASS) Finland HF radar, show poleward (1 km s−1) and westward (1 km s−1) convection flow, consistent with the By tension force, as well as poleward‐moving regions of backscatter. International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers within the radar field of view observe poleward‐progressing, 10 min period, X component magnetic deflections, which are consistent with the effect of Hall currents associated with the plasma flow. The combined radar and optical observations suggest that the PMAFs can be 3500 km or 7 hours of MLT in length. The antisunward motion of the bifurcation of the auroral oval is interpreted as an expansion of the reconnection X line along the flank of the magnetopause.
Application of wave distribution function methods to an ELF hiss event at high latitudes
Authors: Santolík Ondřej , Parrot Michel
Published: February 2004

Multicomponent waveform data are used to calculate wave polarization and propagation characteristics, with both simple plane wave methods and more general wave distribution function (WDF) techniques. The WDF techniques allow us to estimate a continuous distribution of wave energy with respect to the wave vector directions. We describe several newly developed analysis methods, and we compare their results with existing techniques. This comparison allows us to reach a common interpretation of propagation properties and to exclude possible artifacts. The methods are applied to an electromagnetic ELF hiss event observed by the low‐altitude satellite Freja. A part of the emission can be described by simple plane wave methods. WDF techniques are, however, needed to fully characterize the wave propagation when the components of the wave magnetic field vector are not mutually coherent and the polarization degree is low. In this case we observe large peaks on the WDF shape or simultaneous propagation of downgoing and reflected upgoing waves. The WDF techniques also confirm that experimental polarization properties are consistent with their predictions by the cold plasma theory.
Solar wind electron suprathermal strength and temperature gradients: Ulysses observations
Authors: Maksimovic Milan , Gary S. Peter , Skoug Ruth M.
Published: February 2004

We use observations from the Ulysses electron spectrometer to examine global trends of the electron suprathermal population and to study, for the first time, the electron core, halo, and total temperature gradients in the fast solar wind over the poles. We use a data set covering the period from the beginning of the mission (1990, day of year 322) to approximately the first completion of Ulysses out‐of‐ecliptic orbit around the Sun (1998, day 32). This allows us to characterize very well the two states of the solar wind: the high‐speed wind, emanating from polar coronal holes, and the low‐speed streams, emanating from equatorial regions. From a classical bi‐Maxwellian (core and halo) model of electron velocity distribution functions, we define the electron suprathermal strength S as the ratio of the halo to core kinetic pressures: S=nhTh/ncTc. The fast wind has larger average values of S than the slow wind. This global correlation between S and the solar wind bulk speed is also observed on a smaller scale in the polar regions. We find that the small‐scale variations of the solar wind bulk speed in the polar regions, typically ±50 km/s around an average value of 750 km/s, are correlated with small‐scale variations of S. We present also the first observations of the electron core, halo, and total temperature gradients in time‐stationary fast solar wind periods over the poles. We examine all the previous observations in the context of simple solar wind exospheric models. We find, for instance, that the total electron temperature can be well fitted by a law of the form Te = T0 + T1 r−4/3, as predicted by the exospheric approximation.
Electron temperature and de Hoffmann‐Teller potential change across the Earth's bow shock: New results from ISEE 1
Authors: Hull A. J. , Scudder J. D. , Fitzenreiter R. J. , Ogilvie K. W. , Newbury J. A. , Russell C. T.
Published: February 2004

We present a survey of the trends between the electron temperature increase ΔTe and the de Hoffmann‐Teller frame (HTF) electrostatic potential jump ΔΦHT and their correlation with other parameters that characterize the shock transition using a new ISEE 1 database of 129 Earth bow shock crossings. A fundamental understanding of the HTF potential is central to distinguishing the reversible and irreversible changes to electron temperature across collisionless shocks. The HTF potential is estimated using three different techniques: (1) integrating the steady state, electron fluid momentum equation across the shock layer using high time resolution plasma and field data from ISEE 1, (2) using the steady state, electron fluid energy equation, and (3) using an electron polytrope approximation. We find that ΔΦHT and ΔTe are strongly and positively correlated with |Δ(mpUn2/2)|, which is in good qualitative agreement with earlier experimental surveys [Thomsen et al., 1987b; Schwartz et al., 1988] that used bow shock model normals and used the flow in the spacecraft frame. There is a strong linear organization of the ΔTe with ΔΦHT, which suggests an average effective electron polytropic index of 〈γe〉 ≈ 2. In addition, ΔTe and ΔΦHT are organized by βe, although our results may be biased by our limited sampling of shock conditions. Comparisons indicate that the differentials in the HTF potential δΦHT are proportional to the differentials in the magnetic field intensity δB across the shock, with a proportionality constant κ that is a fixed constant for a given shock crossing.
Cosmic ray survey to Antarctica and coupling functions for neutron component near solar minimum (1996– 1997): 3. Geomagnetic effects and coupling functions
Authors: Dorman L. I. , Villoresi G. , Iucci N. , Parisi M. , Tyasto M. I. , Danilova O. A. , Ptitsyna N. G.
Published: February 2004

We present the results of a ship survey conducted from Italy to Antarctica and back during the 1996–1997 solar minimum measuring the latitude variation in the neutron component of cosmic radiation at sea level. High‐energy atmospheric neutrons were detected by a 3NM‐64 and thermalized atmospheric neutrons were detected by two bare BF3counters. Discussions of the internal consistency of the data and the stability of the detectors, investigations of meteorological effects, and data corrections are presented in two companion papers. In this paper we compute updated vertical cutoff rigidities corrected for the penumbra effect, and we estimate apparent cutoff rigidities, which take into account the contribution of nonvertically incident particles to the counting rate. When comparing cosmic ray intensities observed in the same place, a small forward‐backward effect is found and explained as the effect of an asymmetric shielding structure around the monitor. Latitude dependencies (i.e., neutron intensities versus cutoff rigidity) and associated coupling functions are computed for both monitors and compared. The NM latitude dependence obtained for the 1996–1997 solar minimum is found to be almost identical to that obtained by other authors in the previous solar minimum. The absence of the so‐called “crossover” effect when comparing coupling functions of subsequent solar minima is discussed also on the basis of cosmic ray intensity changes observed by neutron monitor stations.
A model of region 1 field‐aligned currents dependent on ionospheric conductivity and solar wind parameters
Authors: Alexeev Igor I. , Belenkaya Elena S. , Clauer C. Robert
Published: February 2004

Using a paraboloid model of the magnetosphere, we compute the potential drop across open field lines as a function of the interplanetary magnetic field strength and direction as well as of solar wind velocity and density. The collisionless conductivity of the magnetosheath plasma near the magnetopause determines the efficiency of the solar wind electric field penetration into the magnetosphere. This reconnection efficiency is generally about 0.1. Thus about one tenth of the total potential produced over the magnetospheric cross section penetrates the polar cap. Knowing the potential drop across the polar cap allows us to determine the strength of the region 1 field‐aligned currents as a function of ionospheric conductivity. We compute the magnetic field disturbance produced by the region 1 field‐aligned currents by using a simple current loop in which the region 1 field‐aligned currents close through ionospheric and magnetopause currents. The region 1 field‐aligned currents decrease the magnetic field strength on the dayside of the magnetosphere, moving the cusp to lower latitudes. This corresponds to a sunward displacement of the polar cap and polar oval. The displacement of the polar oval is 8° on the dayside and about 3° on the nightside when the total strength of the region l Birkeland currents is 5 MA.
Small substorms: Solar wind input and magnetotail dynamics
Authors: Petrukovich A. A. , Baumjohann W. , Nakamura R. , Mukai T. , Troshichev O. A.
Published: February 2004

We investigated properties of 43 small magnetospheric substorms. Their general signatures were found to be consistent with the so‐called contracted oval or northern Bz substorms. Small but clear pressure changes in the tail corresponding to growth and expansion phases detected in about a half of cases testify that these substorms follow the same loading‐unloading scheme as the larger ones. However, rate of the solar wind energy accumulation in the magnetosphere was low due to azimuthal IMF orientation with dominating IMF By and small fluctuating IMF Bz. Plasma sheet signatures could be very strong and likely were localized in their cross‐tail size. Negative bays in auroral X magnetograms were of order of 100–300 nT, with maxima at Bear Island station (71°geomagnetic latitude) and in few cases were delayed after magnetotail onsets by tens of minutes. Small substorms probably differ from their larger counterparts in a way that coherency of the magnetotail reconfiguration in the inner and middle‐tail regions and across the tail is lost in smaller substorms.
Entry of plasma sheet particles into the inner magnetosphere as observed by Polar/CAMMICE
Authors: Ganushkina N. Yu. , Pulkkinen T. I. , Sergeev V. A. , Kubyshkina M. V. , Baker D. N. , Turner N. E. , Grande M. , Kellett B. , Fennell J. , Roeder J. , Sauvaud J.‐A. , Fritz T. A.
Published: February 2004

Statistical results are presented from Polar/CAMMICE measurements of events during which the plasma sheet ions have penetrated deeply into the inner magnetosphere. Owing to their characteristic structure in energy‐time spectrograms, these events are called “intense nose events.” Almost 400 observations of such structures were made during 1997. Intense nose events are shown to be more frequent in the dusk than in the dawn sector. They typically penetrate well inside L = 4, the deepest penetration having occurred around midnight and noon. The intense nose events are associated with magnetic (substorm) activity. However, even moderate activity (AE = 150–250 nT) resulted in formation of these structures. In a case study of November 3, 1997, three sequential inner magnetosphere crossings of the Polar and Interball Auroral spacecraft are shown, each of which exhibited signatures of intense nose‐like structures. Using the innermost boundary determinations from these observations, it is demonstrated that a large‐scale convective electric field alone cannot account for the inward motion of the structure. It is suggested that the intense nose structures are caused by short‐lived intense electric fields (in excess of ∼1 mV/m) in the inner tail at L=4–5.