bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of solar wind variations on the midlatitude ionosphere
Authors: Huang Chao‐Song , Foster J. C. , Erickson P. J.
Published: October 2002

It is known that the solar wind has significant effect on the magnetospheric convection and that magnetospheric electric fields can penetrate into the midlatitude and low‐latitude ionosphere. In this paper, we present observations of midlatitude ionospheric plasma perturbations during two events. In one event, the interplanetary magnetic field (IMF) oscillated between southward and northward with a period of ∼2 hours, while the solar wind pressure was relatively stable. In the other event the solar wind dynamic pressure oscillated with a period of ∼1 hour, while the IMF remained strongly northward and stable. In both events the Millstone Hill incoherent scatter radar detected quasiperiodic perturbations in all F region plasma parameters (electron density, ion velocity, electron temperature, and ion temperature) in the midlatitude dayside ionosphere. The midlatitude ionospheric perturbations were well correlated with the IMF or solar wind pressure variations. Near the F peak altitude the electron density could be increased or decreased by ∼30% within 30–70 min. We suggest that the midlatitude ionospheric electron density perturbations were caused by the penetration of magnetospheric electric fields which were controlled or modulated by the oscillations in the IMF or solar wind pressure.
Effects of the solar wind termination shock on the modulation of Jovian and galactic electrons in the heliosphere
Authors: Potgieter M. S. , Ferreira S. E. S.
Published: October 2002

A drift‐modulation model with a solar wind termination shock (TS), including a Jovian electron source, was used to study the effects of this shock on electron spectra and radial intensity profiles. With electron measurements available up to ∼70 AU at ∼16 MeV from Pioneer 10, we compared computations with these observations using new diffusion coefficients and a new local interstellar electron spectrum (LIS). Satisfying the constraints imposed on the diffusion tensor by the observations, the effects of varying the location of the heliospheric TS and the location of the heliospheric boundary were investigated and illustrated. We find that Jovian and especially low‐energy galactic electrons can be reaccelerated effectively at the TS. Reasonable compatibility between the model and the measurements is produced with the shock radius present at ∼90 AU and the outer boundary radius at 120–130 AU. The intensity of the Jovian electrons is too low at the TS to contribute meaningfully to the TS‐accelerated and combined galactic and Jovian electron intensity at all energies. The shock acceleration results in moderate but still significant changes on the combined galactic and Jovian electron radial intensity profiled at 16 MeV with respect to nonshock solutions. A change in the radial gradient from positive upstream to negative downstream close to the shock (within ∼5 AU) is caused. These effects can be enlarged by decreasing the shock radius while keeping the outer boundary radius fixed or, more effectively, by moving the outer boundary outward while keeping the shock radius fixed. Between ∼100 MeV and ∼2 GeV the effects of the TS on modulated spectra are large and quite significant, even at Earth. The higher the LIS is made, the more noticeable the shock's effect on the 16 MeV radial profile (gradients) becomes. If the distance between the shock and outer heliospheric boundary was kept constant, the impact of the TS on the 16 MeV radial profile remains qualitatively unchanged, regardless of the position of the TS. In conclusion we consider the location of the outer boundary with respect to the shock location as more significant to electron modulation than the actual location of the termination shock. This model and approach confirm that a considerable amount of modulation can take place in the outer heliosphere, especially in the region between the TS and the modulation boundary, depending on the energies considered and the value of the LIS.
Flare index variability in the ascending branch of solar cycle 23
Authors: Özgüç A. , Ataç T. , Rybák J.
Published: October 2002

A brief description and the results of the determination of the flare index as a measure of solar activity on the ascending branch of cycle 23 up to 31 December 2000 are presented. The patterns of similar activity indices that arise under different physical conditions during the rising activity phase are compared with the flare index. All studied solar indices rise more slowly in the current cycle than the last one, except the total solar irradiance. The intermediate‐term periodicities in the daily flare index data were studied using the Fourier transform, and it was found that the 35‐, 62‐, 116‐, 198‐, and 276‐day periodicities are in operation during the ascending branch of cycle 23. Contrary to the previous three cycles, 155‐, 73‐, or 51‐day periodicities were not detected by the Fourier transform in this branch of the cycle. The wavelet transform results show that the occurrence of flare index power is highly intermittent in time, and the most pronounced power peaks were found to be present at 35 days (the temporal locations at 1998.3, 1999.9, and 2000.5 years), 116 days (the temporal interval 1999.7–2000.4 years) and 276 days period (interval roughly from 1999.5 until the end of the flare index data). Comparison of the Fourier and wavelet transform results has clarified the importance of different periodicities, whether they are (62, 198 days) or are not (35, 116, 276 days) the harmonics of the basic ones, as well as the temporal location of their occurrence.
Variable time delays in the propagation of the interplanetary magnetic field
Authors: Weimer D. R. , Ober D. M. , Maynard N. C. , Burke W. J. , Collier M. R. , McComas D. J. , Ness N. F. , Smith C. W.
Published: October 2002

Simultaneous measurements of the interplanetary magnetic field (IMF) are obtained at various locations with four spacecraft, ACE, Wind, IMP‐8, and Geotail. We have devised a technique whereby the exact propagation delay time between ACE, at the L1 orbit, and each of the other three spacecraft can be derived from these measurements. This propagation delay is determined as a continuously varying function of time; when this measured delay is applied to all three components of the IMF measured by ACE, they will match the other satellites' IMF to a degree that is much better than expected. However, the actual time delays can vary by nearly an hour in either direction from the expected advection delays, and the lag times have significant changes that can occur on a timescale of a few minutes. These results are interpreted as due to the effects of tilted phase fronts that are changing orientation with time. We have used the delay measurements between multiple satellites to calculate the three‐dimensional orientation and temporal variations of the phase front. The best fit phase front plane usually lies within 4 RE or less from the four‐point measurements, indicating a lag resolution of a minute or less. Computer animations of the time‐varying phase fronts are used to illustrate their behavior. Orientations can change on short timescales. Our findings have implications for both basic research and “space weather” predictions. These results give a high confidence that the same IMF that is measured near L1 will most likely impact the Earth's magnetosphere, providing ample justification for use of spacecraft data in halo orbit at L1 for monitoring the upstream solar wind prior to its interacting with the magnetosphere. However, there is strong uncertainty in the timing of the arrival of the detailed IMF structures, and these delays will need to be considered.
Interplanetary magnetic field control of theta aurora development
Authors: Cumnock J. A. , Sharber J. R. , Heelis R. A. , Blomberg L. G. , Germany G. A. , Spann J. F. , Coley W. R.
Published: October 2002

We ascertain the influence of the By component of the interplanetary magnetic field (IMF) on theta aurora evolution. During most cases where a transpolar arc is observed to move across the polar region, and form a theta aurora, there are brief (minutes) southward excursions of IMF Bz, however northward IMF is required prior to theta aurora formation. Observations show that theta aurora can form during strictly northward IMF with its motion consistent with a change in sign of IMF By. It is important to note that since transpolar arcs can persist for 20–30 min after the IMF turns southward, errors will occur in assigning instantaneous IMF conditions to “snapshots” of particular auroral patterns. We consider the entire evolution of the theta aurora and the changing IMF conditions. The influence of IMF By is best illustrated by examples which occur during steady northward IMF as compared to times when the IMF is northward on average. We show examples, provided by the Polar UV imager, when the IMF is steady northward. For one case, DMSP F13 and F14 provide in situ measurements of precipitating particles, ionospheric plasma flows and ion density. This unique data set enables us to analyze in detail the evolution of a theta aurora, in one case crossing the entire polar region. No sign change in Bz is needed for theta aurora formation.
High‐latitude thermospheric vertical wind activity from Dynamics Explorer 2 Wind and Temperature Spectrometer observations: Indications of a source region for polar cap gravity waves
Authors: Innis J. L. , Conde M.
Published: October 2002

The large‐scale distribution of thermospheric vertical wind activity, from ∼250 to 650 km altitude, was studied using observations from the Wind and Temperature Spectrometer on the Dynamics Explorer 2 satellite. We calculated the vertical velocity standard deviation, σ(Vz), within a sliding window of width 120 s, corresponding to an along‐track distance of ∼900 km. Maps of σ(Vz) in local magnetic time and invariant latitude reveal a high‐latitude region of enhanced activity that largely fills the polar cap, maximizing in the midnight‐dawn sector. Separating the data by altitude suggests that most of the vertical wind energy present at 250–450 km is dissipated within a few hundred vertical kilometers. Northern and Southern Hemisphere high‐latitude σ(Vz) fields were found to be very similar, indicating no significant hemispheric differences. No strong correlation was found between solar illumination and σ(Vz) at high latitudes, although the dayside vertical wind activity may be slightly reduced compared to night and twilight intervals. However, a clear relationship to geomagnetic activity, as measured by the AE index, was found; vertical wind activity increases with increasing AE. We interpret these results as a possible signature of polar cap gravity waves with sources in or near the midnight‐dawn auroral oval. We use the probability density functions of σ(Vz), separated by AE, to infer the temporal characteristics of the wave source and consequently to provide preliminary estimates of the production rates of polar cap gravity waves.
Electron‐acoustic solitary waves via vortex electron distribution
Authors: Mamun A. A. , Shukla P. K.
Published: October 2002

We consider the nonlinear propagation of electron‐acoustic waves in a plasma composed of a cold electron fluid, hot electrons obeying a trapped/vortex‐like distribution, and stationary ions. The properties of small but finite amplitude electron‐acoustic solitary waves (EASWs) are studied by employing the reductive perturbation technique, whereas those of arbitrary amplitude EASWs are investigated by means of the pseudopotential approach. It is found that the present plasma model supports EASWs having a positive potential, which corresponds to a hole (hump) in the cold (hot) electron number density. It is also shown that as the trapped electron temperature increases, the amplitude of the EASWs increases but their width decreases. The application of our results in interpreting the salient features of the broadband electrostatic noise in the dayside auroral zone is discussed.
Determination of geometrical and kinematical properties of halo coronal mass ejections using the cone model
Authors: Zhao X. P. , Plunkett S. P. , Liu W.
Published: October 2002

Many broadside coronal mass ejections (CMEs) propagate almost radially beyond the first couple of solar radii, and their angular widths remain nearly constant while propagating through the corona. Assuming that these characteristics hold true for halo CMEs that originate far from solar limbs, some useful geometric and kinematic properties of halo CMEs may be reproduced using a simple geometrical model of a CME as a cone. The cone model uses three free parameters, characterizing the angular width and the central position of the halo CME. These geometric properties can be determined by matching the observed halos at a series of times with the modeled halos for a series of radial distances. The kinematic properties, the radial velocity and acceleration, of the halo CME can also be determined on the basis of the series of times and radial distances. These properties are important for predicting the geoeffectiveness of a halo CME and cannot be observed directly with currently available instrumentation. As a test, the geometric and kinematic properties of the 12 May 1997 halo CME have been inferred using the cone model. This shows that the cone model does provide a new way of testing our understanding of halo CMEs, though there are limitations for some halo CMEs.
Changes of the outer to inner heliospheric intensity ratios of >70 MeV galactic cosmic rays as a function of solar magnetic polarity
Authors: Webber W. R. , Lockwood J. A.
Published: October 2002

We have examined the temporal history of the outer to inner heliospheric intensity ratios of >70 MeV cosmic rays using the Voyager, Pioneer, and IMP spacecraft during the time period from 1984 to 2001. This ratio effectively measures the changes in the integral radial cosmic ray gradient between the outer heliosphere and the Earth. In the time period from 1984 to 1990, when the solar magnetic polarity was negative, this outer to inner heliospheric ratio out to ∼50 AU changed temporally in such a way that an average radial gradient ∼2%/AU near the ecliptic plane was maintained throughout this entire period. After 1990, when the solar magnetic polarity became positive, this average ratio began to slowly decrease, as measured at all three spacecraft throughout the heliosphere out to 50–70 AU, reaching a minimum value corresponding to an average 0.5%/AU radial gradient in 1996. In 1997–1998, in conjunction with the first large transient decreases in the new 11‐year solar cycle, the intensity ratios between the outer heliosphere spacecraft and the Earth began to increase rapidly. By the time the Bastille Day transient in 2000 at the Earth had propagated to V2 and V1, which were now between 65 and 82 AU, the outer to inner heliospheric intensity ratios had increased from a value ∼1.4 in 1997 to between 3.0 and 4.0, respectively, at V2 and V1 in 2001. Therefore this time period represents a transition back to a negative solar magnetic polarity and to the reestablishment of a ∼2%/AU average radial gradient similar to that of the previous negative polarity cycle in the heliosphere, but now observed out to ∼80 AU in 2001. This particularly simple pattern for the temporal changes in the average radial gradient in alternate solar magnetic polarity cycles can thus be followed through almost a complete 22‐year solar magnetic cycle.
Minimum energy state and minimum angle rotation of the magnetic field in a current sheet with sheared magnetic field
Authors: Miura Akira
Published: October 2002

In order to answer why and how “the minimum angle rotation of the magnetic field” is realized in a current sheet with a sheared magnetic field, Taylor's helicity constraint, which is valid for low‐β plasmas, is applied to a one‐dimensional planar current sheet with a sheared magnetic field. A single constant helicity is defined for the total rectangular volume surrounding the current sheet and is shown to be gauge‐invariant. The minimization of the magnetic energy with the constraint of the constant total helicity shows that the field is described by the constant α force‐free equation and that the current sheet is a special class of tangential discontinuities with a constant field strength, or a “perpendicular rotational discontinuity.” The total rotational angle of the magnetic field across the current sheet is proportional to the ratio of the total magnetic energy/helicity in the force‐free state. It is proposed that among an infinite number of force‐free states the current sheet relaxes into a unique force‐free state with the absolute minimum ratio of energy/helicity and thus into the absolute minimum energy state for a given constant helicity. Therefore, in the relaxed state the total rotational angle of the magnetic field across the current sheet is minimum and less than 180°. Although the present study of the relaxed state is applicable only to a tangential discontinuity, a qualitative resemblance of the model prediction with observations in situ and simulations of quasi‐perpendicular rotational discontinuities suggests that the observed minimum angle rotation of the magnetic field in a current sheet with a sheared magnetic field is an emergence of plasma relaxation or self‐organization in space plasmas.
Relativistic electron drift shell splitting
Authors: Selesnick R. S. , Blake J. B.
Published: October 2002

A survey of equatorial outer radiation belt relativistic electron pitch angle distributions measured on the Polar satellite shows anisotropies that are consistent with the effects of drift shell splitting. Simulations based on the observed radial intensity gradients and on drift shell calculations in a magnetospheric field model show that for low geomagnetic activity levels, the measured average anisotropies are similar to those predicted. With increasing geomagnetic activity the anisotropies do not change as much as is predicted, particularly on the nightside where the pitch angle distributions have local minima in the direction perpendicular to the magnetic field. At a given location and activity level there is substantial variability in the measured anisotropies about their long‐term averages.
Small‐scale field‐aligned plasmaspheric density structures inferred from the Radio Plasma Imager on IMAGE
Authors: Carpenter D. L. , Spasojević M. A. , Bell T. F. , Inan U. S. , Reinisch B. W. , Galkin I. A. , Benson R. F. , Green J. L. , Fung S. F. , Boardsen S. A.
Published: October 2002

Among the objectives of the Radio Plasma Imager (RPI) on IMAGE is the observation of the Earth's plasmasphere from the satellite's polar orbit, with apogee ≈8 RE geocentric distance and perigee near 1200 km altitude. This objective is here pursued by (1) remote sounding from high‐altitude regions outside the main plasmasphere, (2) sounding within the plasmasphere, and (3) in situ passive measurements of natural wave activity. During sounding of the plasmasphere from both outside and inside the plasmapause, RPI echoes that follow non‐field‐aligned ray paths are usually not the discrete traces on range‐versus‐frequency records (plasmagrams) that are predicted by ray‐tracing simulations in smooth magnetospheric density models. Instead, such RPI echoes exhibit various amounts of spreading, from ≈0.5 RE to ≈2 RE in virtual range (range assuming free‐space speed of light propagation). The range spreading is attributed to scattering from, partial reflection from, and propagation along geomagnetic field‐aligned electron density irregularities. There exists a substantial body of theoretical work on mechanisms that might explain the appearance of such irregularities both within and beyond the plasmasphere. That the spread‐producing irregularities are field‐aligned is suggested by the efficiency with which RPI excites discrete echoes that propagate along the geomagnetic field, sometimes into both hemispheres. The spatial distributions and scale sizes of the spread‐producing irregularities remain to be investigated. The RPI echo data, however, coupled with earlier evidence from topside sounders and whistler mode instruments, suggest that they can have cross‐field scale sizes within a range from ≈200 m to over 10 km and electron densities within ≈10% of background. RPI is found capable of detecting plasmapause locations from distances of ≈3 RE or more. When minimal signal integration is used, the location and range of density values of a steep plasmapause can be determined from distances of order 1 RE, and echoes can at times be returned from points extending inward from the plasmapause to locations where the electron density reaches ≈3000 cm−3, which is usually at L < 3.
Features of global geomagnetic Sq field from 1980 to 1990
Authors: Takeda M.
Published: October 2002

Seasonal and year‐to‐year variations of geomagnetic Sq field were examined using the global equivalent current system for one solar cycle from 1980 to 1990. It was confirmed that the Sq current intensity is about half in solar minimum of that in solar maximum and its year‐to‐year variation is relatively smooth for each month. At 0200 UT, vortices in the winter and summer hemispheres shift to the afternoon and morning side, respectively, but with phase delay by about 1 or 2 months. On the contrary, at 1400 UT the northern vortex is located in the morning side of the southern vortex in most months of all years, and seasonal variation of the location is not so clear. As for the morphology of the current system, it was found that there are only two types in the shape of the Sq current system at 0200 UT. That is, it shows “northern summer type” from April to September and “northern winter type” from October to March, and there is no “equinox type,” and it clearly differs in March from that in September. This feature is similar to that of the seasonal variation type of the tidal winds in the upper mesosphere.
Total electron and proton energy input during auroral substorms: Remote sensing with IMAGE‐FUV
Authors: Hubert B. , Gérard J.‐C. , Evans D. S. , Meurant M. , Mende S. B. , Frey H. U. , Immel T. J.
Published: October 2002

The IMAGE satellite carries three FUV imagers observing N2 LBH, O I 1356 Å, and HI Lyman α emissions in the polar aurora. These simultaneous observations are used to characterize the precipitating electron and proton energy fluxes. The proton energy flux is derived from the Lyman α measurements on the basis of efficiency curves calculated with a Monte Carlo simulation of the proton aurora. The resulting proton contribution to the N2 LBH and O I 1356 Å emissions is calculated and subtracted to obtain the electron contribution in the other two channels. These two quantities are used to determine the precipitating electron average energy and energy flux. The proton and electron energy fluxes are integrated over the hemisphere to obtain the rate of auroral energy dissipation (hemispheric power) carried by the protons and electrons separately. The time development of the proton and electron aurora during four winter time events is examined. Although the onsets of the proton and electron aurora coincide in time and space, the time of the peak of energy dissipation and the recovery time are often found to differ. The fractional energy flux carried by the protons is highest during quiet periods and reaches a minimum during the most active phase of the substorms. This result is in agreement with the dependence of the fractional proton hemispheric power on magnetic activity measured by NOAA 15. The hemispheric power deduced from the FUV images is compared to the NOAA‐deduced values and found to be in reasonable agreement. Sources of uncertainties in the determination of the hemispheric power are discussed on the basis of several sensitivity tests. In particular, it is found that the most critical factor is the assumption made on the energy of the auroral protons if this energy is <25 keV.
Equatorial spread F found in 5577 Å and 6300 Å airglow observations from Hawaii
Authors: Kim Yong Ha , Hong Seung Soo , Weinberg J. L.
Published: October 2002

We have searched for F region ionospheric disturbances in a database of airglow observations from Mt. Haleakala, Hawaii. The airglow scan observations were made with narrowband filters of OI 5577 Å and OI 6300 Å for 164 nights in the period of April 1965 through October 1968. The OI 6300 emissions are produced mostly via recombination of O2+ in the F region, whereas the OI 5577 emissions are generated both in the E and F regions. The database has unique significance, since it covers more than three years of observations in the 1960s when there was no other program that systematically monitored ionospheric airglows. We find disturbances in the OI airglow brightness, from which we constructed projected maps of airglow distributions at the F region altitude. The projected maps clearly show wavelike disturbances whose crests are mostly aligned along a north–south direction, structures seen even in the raw scan data. The disturbances were found exclusively in the southerly azimuth toward the equator, implying that they are one of the equatorial ionospheric irregularities, known as equatorial spread F (ESF). The ESF disturbances occurred 19 nights out of a total of 161 nights of azimuth‐scanning observations with a maximum in September. The disturbances seem to arise regardless of geomagnetic activities. The seasonal variation of the occurrence is consistent with recent analyses of satellite data in which ESF is suggested to be caused by gravity waves from the troposphere, where they are generated by strong convection in intertropical convergence zone.
Polar‐Interball coordinated observations of plasma and magnetic field characteristics in the regions of the northern and southern distant cusps
Authors: Dubinin E. , Skalsky A. , Song P. , Savin S. , Kozyra J. , Moore T. E. , Russell C. T. , Chandler M. O. , Fedorov A. , Avanov L. , Sauvaud J.‐A. , Friedel R. H. W.
Published: October 2002

The structure of the high‐latitude magnetosphere near the polar cusps is studied by using coordinated observations from the Polar and Interball satellites. While Polar surveyed the northern polar cusp region, Interball sampled the high‐altitude region in the vicinity of the southern cusp in similar local times. A “magnetic turbulent region” that is distinct from the magnetosphere and magnetosheath and consists of recurrent small‐scale events is observed in both hemispheres. The small‐scale events often contain a steep ramp followed by a wave train. The events are characterized by a steep increase in the ion energy followed by dispersed velocity filter features. Kinetic and fluid characteristics of ions and a test of the Walen relation show that the events are associated with reconnection processes. S‐shaped polarization of the magnetic field variations is a characteristic feature in many cases indicating the existence of intermediate shocks. The change in the low‐energy cutoff of injected ions indicates that the reconnection site is about a few Earth radii from Polar. Reflected ions on the reconnected magnetospheric field lines, which are not predicted by simple reconnection scenarios, are also observed. Coexistence of transmitted and reflected ions gives rise to a “stagnant” plasma on the magnetospheric field lines. Characteristics of ions and electrons strongly support the suggestion that the field lines threading the plasma in these regions, which extend from the magnetosheath to the magnetosphere, are connected, at least at one end, with the Earth. In other words, the boundary layer is located, at least partly, on reconnected field lines. In contrast, a turbulent region in the indentation of the cusps has been thought to be in the magnetosheath and not to be connected with the Earth. Although the events observed by both satellites have a certain similarity, the probability of conjugated events being recorded by both satellites is small. It is believed that the visible similarity between the observations from the two hemispheres is due to structures that are characteristic features of a single transient reconnection event.
A study of the dusk convection cell's response to an IMF southward turning
Authors: Nishitani Nozomu , Ogawa Tadahiko , Sato Natsuo , Yamagishi Hisao , Pinnock Mike , Villain Jean‐Paul , Sofko George , Troshichev Oleg
Published: October 2002

One example of the response of ionospheric convection and the polar cap boundary to a sudden change in the interplanetary magnetic field (IMF) orientation has been studied by using ground magnetometers, the Super Dual Auroral Radar Network (SuperDARN), and Defense Meteorological Satellite Program (DMSP) particle detectors when the IMF suddenly changed from northward (+6 nT) to strongly southward (−19 nT) at 1716 UT on 5 September 1995. The Bz component was fairly constant for ∼2 hours before and ∼25 min after the sudden IMF change. The convection flow changed almost simultaneously over a global extent. This initial change of the convection pattern can be characterized by a sudden formation of a large flow vortex in the afternoon sector. This agrees with the earlier findings by Ruohoniemi and Greenwald [1998] and Ridley et al. [1998]. On the other hand, the response of the polar cap boundary (or its proxy) is more complicated. The Saskatoon radar, located in the late morning sector, observed an equatorward shift of the cusp scatter region simultaneously with the initial response of the convection flows. The DMSP particle data also showed a simultaneous equatorward expansion of the auroral oval in the 2100 magnetic local time (MLT) sector. The radar and particle data indicate the immediate equatorward expansion of the precipitation regions in the noon and premidnight sectors. About 10–20 min after the initial change, there were changes observed in the dusk region, namely, an equatorward expansion of the current reversal boundary observed by the Greenland magnetometer chain in the dusk sector between 1740 and 1750 UT and an equatorward expansion of the convection reversal boundary detected by the Stokkseyri, Halley, and Syowa radars. The delayed responses were observed 18‐8 min before a substorm onset was recorded at midlatitude stations at 1756 UT. These observations indicate that there were two kinds of ionospheric responses to the southward turning of the IMF; the first response is the formation of the convection vortex and the equatorward shift of the polar cap boundary at noon and at ∼2100 MLT, and the second response is the equatorward expansion of the convection reversal boundary in the dusk sector. We make the case that the first response is associated with the propagation of magnetosonic waves and that the second response is consistent with the Cowley and Lockwood [1992] picture of the redistribution of the newly created open flux in the polar cap region.
High‐latitude irregularities of the magnetospheric electric field and their relation to solar wind and geomagnetic conditions
Authors: Golovchanskaya I. V. , Maltsev Y. P. , Ostapenko A. A.
Published: October 2002

Having processed high‐resolution electric field measurements of the low‐altitude spacecraft Dynamic Explorer 2 in order to reveal and study signatures of rapid magnetospheric streams, we found that along with a regular large‐scale electric field there exists an irregular one with amplitudes of 50–200 mV m−1 and transverse scale sizes varying from several to a hundred kilometers. This component behaves quite differently compared to the regular field: In particular, it is most common and of highest intensity under strong northward Bz interplanetary magnetic field (IMF) and low AE and Dst conditions. Thus low average electric field (as expected and observed at quiet times) does not mean low variance. Large‐amplitude electric field fluctuations typically occur at or around the convection reversal boundary, expanding to the central polar region or shifting to the equator depending on Bz IMF direction. They are accompanied by large variations of the magnetic field component, with one‐to‐one correspondence of peaks and valleys of electric and magnetic field oscillations. The field‐aligned currents inferred are on the order of 10–100 A km−2, which is large enough to be associated with discrete arcs.
Reply
Authors: Turner N. E. , Pulkkinen T. I. , Baker D. N. , McPherron R. L.
Published: October 2002

Turner et al. [2000] analyzed the contribution of cross‐tail currents to the Dst index. In order to estimate this contribution we used modified versions of the Tsyganenko models which had been adjusted to match spacecraft data in the tail, and we isolated a tail region and calculated its influence. We concluded that the tail currents were responsible for around 25% of the Dst response during moderately disturbed times. Maltsev and Ostapenko [2002] conclude that our estimate was low by a factor of 2, owing to that fact that we neglected dayside currents and that the model we used systematically underestimates the cross‐tail current system. We appreciate their insightful analysis of our work, but we disagree with their conclusions. The models we used were modified to match spacecraft data in the tail, so we do not feel they underestimate the tail currents, and we consider the tail currents to be primarily located in the magnetotail, so we feel our decision to neglect dayside currents was justified. Additionally, we feel that some of the discrepancies between our results and theirs are due to different definitions of tail and ring currents and our decisions on whether to include the induced ground current contribution in our estimates of the tail current contribution to Dst. Here we respond briefly to their arguments and conclude that we still find the approximate magnitude of the tail current contribution to Dst to be around 25%. Additionally, Maltsev and Ostapenko include their own analysis of the tail current contribution to Dst, but we will limit our response to those comments which directly relate to our work.
Modeling the ring current magnetic field during storms
Authors: Ganushkina N. Y. , Pulkkinen T. I. , Kubyshkina M. V. , Singer H. J. , Russell C. T.
Published: October 2002

We present a new model for the inner magnetosphere ring current to get a realistic representation of the magnetic field during storm times. We use a bean‐shaped current system, which has cross section that is close to the observed distribution of trapped particles in the inner magnetosphere. The model is symmetric both longitudinally and in the north‐south direction, and the current density in the radial direction varies as a Gaussian. The latitudinal distribution of the current density is specified by an “anisotropy index,” which is zero for a particle distribution that is isotropic along field lines. Increasing the anisotropy index gives a particle distribution concentrated closer to the equator. We use this method to model the magnetic field evolution during two geomagnetic storms: one on May 2, 1998, when Dst reached −80 nT, and the other on May 15, 1997, when it reached −120 nT. The ring current in the Tsyganenko (T89) magnetic field was replaced by our new ring current representation, and the model free parameters are specified using observations from GOES and Polar satellites and Dst measurements for each time step separately. We discuss the field configuration changes during the storm, and we evaluate the capability of our modeling technique to represent the large‐scale magnetospheric configuration during storm periods.
Seasonal variations of high‐latitude field‐aligned currents inferred from Ørsted and Magsat observations
Authors: Christiansen F. , Papitashvili V. O. , Neubert T.
Published: October 2002

In this paper we report on field‐aligned currents inferred from high‐precision three‐component geomagnetic field observations made on board the Danish satellite Ørsted over polar regions. Because of a slow drift in local time of the satellite orbit through the “noon‐midnight” sector, we were able to study the seasonal dependence of the dynamic properties of the dayside and nightside field‐aligned current systems over the Northern and Southern Hemispheres. We find an average over‐the‐pole distance between dayside and nightside currents of 32° during summer but 37° during winter and 36° during equinox. The decrease in the size of the summer polar cap is caused by a shift of both daytime and nighttime current systems to higher magnetic latitudes. For comparison, the dawn‐dusk cross‐polar distance of the Region 1/Region 2 field‐aligned currents has been determined from high‐precision data observed by Magsat, a satellite flown in 1979–1980 in a “dawn‐dusk” orbit. The latter results show that the dawn‐dusk distance between R1/R2 currents exhibits little seasonal dependence and amounts to ∼34° for all seasons in both polar caps. The seasonal dependence is confirmed for the high‐latitude field‐aligned intensities; they are larger by a factor of 1.5–1.8 in the sunlit (summer) polar cap in comparison with the winter hemisphere. Our results suggest that the R1/R2 and dayside field‐aligned currents are well balanced between the pairs of downward/upward currents for all seasons as well as between hemispheres during equinox. We were not able to confirm results reported in earlier studies that the net currents tend to increase with an enhancement of ionospheric conductivity caused by the solar illumination or substorm activity.
Neutral wind influence on the electrodynamic coupling between the ionosphere and the magnetosphere
Authors: Peymirat C. , Richmond A. D. , Roble R. G.
Published: October 2002

The influence of the ionospheric wind dynamo on the steady state electrodynamic interaction between the ionosphere and the inner magnetosphere is evaluated with the Magnetosphere‐Thermosphere‐Ionosphere Electrodynamics General Circulation Model (MTIE‐GCM) of Peymirat et al. [1998]. Two types of interaction between the solar wind–magnetosphere (SWM) dynamo and the ionospheric dynamo are considered: either the SWM dynamo is a voltage generator imposing a fixed electric potential in the polar cap or it is a hybrid current/voltage generator such that the electric potential in the polar cap results from the combined action of the SWM and ionospheric dynamos. The following results are found. (1) The dynamo effect of winds that are accelerated by the high‐latitude ion convection increases the meridional electric field in the auroral zone but reduces the potential variation around the Auroral Zone Equatorial boundary (AZEQ) and the plasma pressure in the nightside magnetosphere close to the Earth, corresponding to an enhancement of the shielding effect produced by Region 2 field‐aligned currents. The magnitudes of the wind‐induced effects are on the order of 10% for the shielding potential and 20% for the plasma pressure reduction. (2) The wind‐induced reduction of magnetospheric plasma pressure near the Earth is ∼4 times larger for a doubling of the plasma source density in the tail. (3) The relative influence of winds on shielding is similar for polar cap potential drops of 30 kV and 70 kV. (4) When the wind is allowed to influence the polar cap potential, it increases the potential drop along the polar cap boundary by ∼10% but does not modify the net influence on the shielding potential along AZEQ. (5) The influence of the neutral winds set up by the high‐latitude convection is restricted to the auroral zone.
VHF coherent radar signals from the E region ionosphere and the relationship to electron drift velocity and ion acoustic velocity
Authors: Nielsen E. , del Pozo C. F. , Williams P. J. S.
Published: October 2002

The Scandinavian Twin Auroral Radar Experiment (STARE) coherent radar system measures the Doppler shifts caused by ∼1 m plasma waves in the high‐latitude E region ionosphere. These Doppler velocities are here related to the electron drift velocity and ion acoustic velocity derived from measurements with the incoherent radar system European Incoherent Scatter (EISCAT). The Doppler velocity is limited in magnitude to near the ion acoustic velocity in the plasma. For large flow angles θ, i.e., the angle between the radar line of sight and the electron drift velocity, the Doppler shifts are equal to the component of the electron drift velocity on the line of sight. For θ ∼ 40° the Doppler velocity is equal to the ion acoustic velocity at 105‐km altitude, and for decreasing flow angle the Doppler velocity increases. For 0° < θ < 60° the variation with flow angle can be described as cosαθ, where the α decreases from 0.8 to 0.2 with an increase in drift speed from ∼400 to 1600 ms−1. The ratio of the line‐of‐sight velocity for θ ∼ 0° to the ion acoustic velocity decreases from 1.2 at low velocities to 1.05 at large velocities. The systematic variations of the Doppler shifts with drift speed and flow angle make it possible, in principle, to recover the electron velocity from the coherent radar measurements. The observations are used to illustrate how well the recovery is possible in practice.
Evidence for kinetic Alfvén waves and parallel electron energization at 4–6 RE altitudes in the plasma sheet boundary layer
Authors: Wygant J. R. , Keiling A. , Cattell C. A. , Lysak R. L. , Temerin M. , Mozer F. S. , Kletzing C. A. , Scudder J. D. , Streltsov V. , Lotko W. , Russell C. T.
Published: October 2002

We present evidence based on measurements from the Polar spacecraft for the existence of small‐scale, large‐amplitude kinetic Alfvén waves/spikes at the plasma sheet boundary layer (PSBL) at altitudes of 4–6 RE. These structures coincide with larger‐scale Alfvénic waves that carry a large net Poynting flux along magnetic field lines toward the Earth. Both structures are typically observed in the PSBL but have also been observed deeper in the plasma sheet. The small‐scale spikes have electric field amplitudes up to 300 mV m−1 and associated magnetic field variations between 0.5 and 5 nT. Previous analysis has shown that the larger‐scale Alfvén waves have periods of ∼20–60 s and carry enough Poynting flux to explain the generation of the most intense auroral structures observed in the Polar Ultraviolet Imager data set. In this paper it is shown that the smaller‐scale waves have durations in the spacecraft frame of 250 ms to 1 s (but may have shorter time durations since the Nyquist frequency of the magnetic field experiment is ∼4 Hz.). The characteristic ratio of the amplitudes of the electric to magnetic field fluctuations is strong evidence that the waves are kinetic Alfvén waves with scale sizes perpendicular to the magnetic field on the order of 20–120 km (with an electron inertial length c/ωpe∼10 km and an ion gyroradius ∼20 km). Theoretical analysis of the observed spikes suggests that these waves should be very efficient at accelerating electrons parallel to the magnetic field. Simultaneously measured electron velocity space distribution functions from the Polar Hydra instrument include parallel electron heating features and earthward electron beams, indicating strong parallel energization. The characteristic parallel energy is on the order of ∼1 keV, consistent with estimates of the parallel ∫ Edl associated with small‐scale kinetic Alfvén wave structures. The energy flux in the electron “beams” is ∼0.7 ergs cm−2 s−1. These observations suggest that the small‐scale kinetic Alfvén waves are generated from the larger‐scale Alfvén waves through one or more of a variety of mechanisms that have been proposed to result in the filamentation of large‐amplitude Alfvén waves. The observations presented herein provide strong evidence that in addition to the auroral particle energization processes known to occur at altitudes between 0.5 and 2 RE, there are important heating and acceleration mechanisms operating at these higher altitudes in the plasma sheet.
Exponential relaxation of optical emissions in sprites
Authors: Barrington‐Leigh Christopher P. , Pasko Victor P. , Inan Umran S.
Published: October 2002

The optical emissions in a large number of bright sprites observed over one storm in 1998 exhibit a relaxation that is closely exponential in time. This feature was unexpected but might be explained by the presence of quasi‐constant electric fields over times of several milliseconds, in which case the optical relaxation would be a direct indication of the exponentially changing electron density. The relaxation rates for sprites appear to have an upper bound that is consistent with the dissociative electron attachment rates expected at sprite altitudes. The experimental results are consistent with existing large‐scale electrodynamic models of sprites as well as with the streamer mechanism as the underlying physical process for sprite ionization.
A physics‐based study of the Dst‐AL relationship
Authors: Shen Chao , Liu Zhenxing , Kamei Toyohisa
Published: October 2002

Understanding the storm‐substorm relationship is an important problem in magnetospheric dynamics. In the past this question has been explored using statistical techniques and empirical relationships involving the Dst and AE/AL indices. In this research we have developed a relationship between AL and Dst based on the physics of the process. We argue that the magnetospheric electric field causes both the westward auroral electrojet and the particle injection into the ring current region. The adiabatic acceleration of ring current particles and the thermal properties of the ring current source have been included in the calculations. The injection function deduced is found to be a function of AL index and the solar wind dynamical pressure Pdyn. For AL> AL0 (∼−500 nT) the injection function is proportional to AL [Pdyn (t − td)]0.33with a time lag td≈ 45 min; while for large AL < AL0(∼−500 nT) the injection function is proportional to −|AL|1.7 [Pdyn(t − td)]0.33. This result is consistent with previous statistical results. We also find that the ring current decay time is a function of AL. Finally, we obtain an algorithm which predicts Dst from ALand the solar wind dynamical pressure Pdyn. The correlation coefficients between Dst predicted by the algorithm and the observed Dst index are 0.84 and 0.80 for the years 1998 and 1999, respectively.