bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of the electric field fine structure associated with the westward traveling surge and large‐scale auroral spirals
Authors: Marklund G. T. , Karlsson T. , Blomberg L. G. , Lindqvist P.‐A. , Fälthammar C.‐G. , Johnson M. L. , Murphree J. S. , Andersson L. , Eliasson L. , Opgenoorth H. J. , Zanetti L. J.
Published: February 2004

The characteristics of the fine scale electric field associated with the westward traveling surge and large‐scale auroral spirals and surges are investigated using high‐resolution electric field, magnetic field, particle and UV imager observations from four eveningside auroral oval crossings by the Freja satellite. Three of the crossings were associated with signatures of auroral substorms and one crossing went directly through the head of a surge close in time and space to substorm onset. Three passes were adjacent to auroral spiral formations, one poleward of and one equatorward of such forms and one through the multiple arc region near the front of an extended region of auroral activity. The ambient electric field was found to intensify in the direction toward the spiral head (or the center of the auroral activity region) over a region comparable to the size of the visible auroral forms. These results confirm previous findings that the spiral or surge head is associated with negative space charge and an intense upward field‐aligned current. The fourth pass, directly through the surge head reveals a very complicated structure of the surge region. Narrowly structured, intense (up to 700 mV/m) and mostly converging electric fields associated with intense electron precipitation (of both high and medium energies) and balanced field‐aligned currents (up to 30 μA/m2) are seen near the edge of the surge head and adjacent to auroral structures in the wake. These narrow regions are embedded within more extended regions of intense high‐energy electron precipitation but very weak electric fields and field‐aligned currents. According to some existing models of the surge, a pronounced westward electric field component and a southward polarisation electric field is expected within the entire high‐conductivity region but evidence in support of this was not found in the data. Rather, these suggest that a significant part of the upward surge current is closed by distributed downward field‐aligned currents from the near surroundings. The Freja electric field is typically seen to intensify at the edges of or in‐between bright auroral structures and to decrease within the arcs similar to what is observed in the ionosphere. The surge electric field is, however, much more intense than previously observed or anticipated at these altitudes with characteristics rather similar to those observed in the auroral acceleration region. Since the particle data indicate that most of the acceleration takes place above Freja altitudes, it seems as if Freja traversed the lower part of the auroral acceleration region associated with the surge.
Northward interplanetary magnetic field cusp aurora and high‐latitude magnetopause reconnection
Authors: Øieroset M. , Sandholt P. E. , Denig W. F. , Cowley S. W. H.
Published: February 2004

We present observations of two types of auroral forms located at different latitudes in the cusp region. Type 1 (south) auroras are located at ∼71° – 75° MLAT and occur during intervals of southward directed interplanetary magnetic field (IMF). Higher‐latitude (77°–78°) type 2 (north) auroras are associated with northward IMF (Bz > 0). Intervals are studied which are characterized by transitions from one auroral form to the other in response to IMF switches from north (clock angle <50°) to south (clock angle >90°) and vice versa. These observations are found to be consistent with the interpretation that type south auroras are a signature of low‐latitude magnetopause reconnection and that the type north auroras are associated with high‐latitude reconnection, tailward of the cusp. The latter is supported by satellite (DMSP F11) observations of particle precipitation and ionospheric convection. The simultaneous existence of type north and type south auroras is observed during intermediate IMF states (clock angle ∼60°). Observations of type north auroras may be used to study the relationship between the IMF Bx component and high‐latitude reconnection. We find that type north auroras occur in the northern hemisphere for both Bx polarities. Enhanced emission intensity of the auroral green line seems to be a feature occurring predominantly during negative Bx conditions. The enhanced green line intensity is considered to be an indication of low‐altitude particle acceleration in regions of strong field‐aligned current intensities. Such particle accelerations (green line emission) may occur predominantly in regions of upward directed IMF By‐related currents in association with the type north cusp aurora.
Solar wind proton deposition into the Martian atmosphere
Authors: Brecht Stephen H.
Published: February 2004

The direct impact of solar wind H+ with the planet Mars is calculated using a three‐dimensional hybrid particle code. The simulation results show a strong dependence on solar wind velocity and interplanetary magnetic field angle with the solar wind velocity vector. The energy fluxes calculated approach the solar EUV heating rates from photoelectrons and are found to be asymmetric. The heating is also found on the nightside of the planet due to the large ion gyroradii of the incoming solar wind protons. The percentage of solar wind protons lost to the surface of simulation ranged from 3% to 28% depending on the ram presssure of the solar wind. Finally, these results support earlier suggestions that the solar wind may be a significant source of He in the atmosphere of Mars.
Modeling the F layer during specific geomagnetic storms
Authors: Codrescu M. V. , Fuller‐Rowell T. J. , Kutiev I. S.
Published: February 2004

Important progress has been made recently in developing an understanding of the effects of geomagnetic storms in the thermosphere and ionosphere. Numerical simulations of theoretical storms with the coupled thermosphere ionosphere model (CTIM) have provided a better understanding of the dynamics of the upper atmosphere and have also permitted the identification of the processes responsible for global storm effects at highlatitude and midlatitude. The theory developed based on the model simulations can explain most of the apparent coherence of local time and seasonal dependencies and the apparent randomness in the longitudinal response of the global ionosphere, uncovered through statistical analysis of storm observations. A true test of the model and the theory is their ability to predict the large‐scale distribution of storm effects for specific storms. In this paper, CTIM simulation results for the December 7–9, 1982, period are presented. We compare model results with DE 2 temperature and plasma density data. We also compare modeled electron densities with ionosonde data from several sectors in both hemispheres. The global characteristics of the response are reproduced by the model, and we are able to explain the pronounced longitude differences in the summer hemisphere. The Australian sector passes through midnight during the main driven phase of the storm and experiences the largest energy input and the largest neutral composition changes. The deepest ionospheric negative phase seen in ionosonde data is over Australia and is consistent with this interpretation. Given the large uncertainties in our knowledge of the magnitude and spatial distribution of energy input during a particular storm, predicting local changes is still a challenge.
Two‐dimensional saturation of broadband electron acoustic waves in the foreshock region
Authors: Schriver D. , Sotnikov V. , Ashour‐Abdalla M. , Fiala V.
Published: February 2004

To analyze the saturation of beam‐driven electron acoustic waves that can occur in the electron foreshock region, fully self‐consistent two‐dimensional electrostatic simulations have been carried out. This has been done for the case of a negative energy beam mode that is excited when the beam velocity is lower than the thermal velocity of the background plasma. This mode can be excited well away from the electron foreshock boundary upstream from the Earth. The nonlinear saturation of the instability in the two‐dimensional case is due to spreading of the beam velocity and damping of the initially negative energy mode by the beam particles. Although the saturated wave energy contained in the waves propagating along the beam direction is dominant, a significant amount of wave energy appears in the direction propagating oblique to the beam causing perpendicular heating of beam particles. The electron acoustic wave growth in the simulations is in agreement with linear theory and previous theoretical predictions. An interesting unexpected result was the additional generation of low‐frequency part of the electron acoustic wave spectrum propagating in the direction opposite to that of the main spectrum of the electron acoustic waves. These backward propagating waves were found only in the wave spectrum transverse (oblique) to the beam direction.
Dynamics of energetic ions and electrons between L = 2.5 and L = 7 during magnetic storms
Authors: Korth A. , Friedel R. H. W.
Published: February 2004

The variation of energetic electron and ion fluxes with L values between 2.5 and 7 as measured by two magnetic spectrometers (medium electrons A and B, MEA and MEB) on CRRES will be presented for one intense and two weak magnetic storms, paying particular attention to the rate of accompanying substorms. Electrons in the energy range 45–602 keV and ions in the range 77–703 keV will be examined. The data are presented in cuts through L versus time plots which are ideal to demonstrate the global aspect and the long‐term variations of a magnetic storm in the outer radiation belt. The time resolution is half an orbital period of CRRES (5.5 hours). Substantial decreases and enhancements of electron and ion intensity are seen at the beginning and recovery of intense magnetic storms (Dst < −100 nT). Electrons and ions are shifted to lower L values during these storms and the slot region narrows for ions. The slot region can disappear completely for electrons during the recovery of the storm. For small magnetic storms (Dst about −30 nT), which seem to be caused by a series of substorms, the ring current does not move to lower L values and stays at L values of about 5.5 but weakens, even though Dst decreases. This is attributed to the strong substorm current systems. Knowledge of the past history of the radiation belts is required to separate “true” storm time depressions of Dst from substorm‐related effects.
Three spacecraft observations of the geomagnetic tail during moderately disturbed conditions: Global perspective
Authors: Zhou X.‐Y. , Russell C. T. , Mitchell D. G.
Published: February 2004

On April 22, 1979, from 0840 to 1050 UT, conditions varied from quiet to disturbed both on the ground and in the geomagnetotail corresponding to the northward and southward interplanetary magnetic field (IMF) conditions, respectively. In this interval, ISEE 3 monitored the solar wind, 206 RE upstream and 82 RE to the dawnside of the Earth. Very near the center of the tail, ISEE 1 and ISEE 2 were 17 and 16 RE behind the Earth. IMP 8 was also located in the tail at about 35 and 6 RE above the expected position of the current sheet and 16 RE to dawnside of the Earth. The combination of the three spacecraft monitoring the tail in key locations, the strong northward IMF switching to southward, and the excellent coverage of the midnight sector by the North American magnetometer chains make this a unique and ideal case to study. A global perspective of the observations of these four spacecraft, observations of two synchronous spacecraft, GOES 2 and GOES 3, and of ground‐based magnetometers is presented herein. For IMF northward, a flapping wave in the tail is seen to propagate away from the Earth at about 95 km/s, while simultaneous plasma convection in the center of the tail was usually in the reverse direction, toward the Earth. A dipolarization of the tail field was observed by the ISEE pair near the current sheet center, and it moved toward the Earth at close to the speed of the convecting plasma. The current sheet was significantly twisted at the position of IMP 8 under the influence of the IMF By conditions, even though Bz was northward. For IMF southward, a reconnection point appeared to form inside the location of ISEE, and then it moved down the tail rapidly. The onset of this activity at the ISEE pair was simultaneous with the onset of ground activity. Plasma convection was mainly away from the Earth. The current sheet became bent at 16 RE, and the bend appeared to move down the tail some time after the plasma flow was earthward. Highly structured magnetic fields were observed at the ISEE pair, the configuration of which is difficult to discern with only two spacecraft. A substorm that developed rapidly after the arrival of southward IMF, accompanied by an enhanced electrojet at L values normally well inside synchronous orbit, had little effect at synchronous orbit at 0100 LT. However, the near‐tail region was immediately affected. Nevertheless, despite the strong southward fields observed in the tail and the strong flows, the energetic electron data indicate that the ISEE spacecraft remained on closed field lines for the entire period under study. The extensive documentation of the input and output of this event should make it an excellent candidate for computer simulation.
Geotail waveform observations of broadband/narrowband electrostatic noise in the distant tail
Authors: Kojima H. , Matsumoto H. , Chikuba S. , Horiyama S. , Ashour‐Abdalla M. , Anderson R. R.
Published: February 2004

Broadband electrostatic noise (BEN) and narrowband electrostatic noise (NEN) are common wave activities in the plasma sheet boundary and the tail lobe regions, respectively. Similar wave emissions can be observed in the magnetosheath region. We demonstrate the nature of these waves based on the waveform observations by the plasma wave instrument on board the Geotail spacecraft. The above observed waveforms are divided into two types of classifications. The BEN type emissions observed in the plasma sheet boundary and magnetosheath consist of a series of isolated bipolar pulses. They are termed after their waveforms as plasma sheet boundary layer electrostatic solitary waves (PSBL ESW) and magnetosheath electrostatic solitary waves (MS ESW). On the other hand, the waveforms of the NEN type emissions are quasi‐monochromatic. They are termed as lobe electrostatic quasi‐monochromatic waves (lobe EQMW) and magnetosheath electrostatic quasi‐monochromatic waves (MS EQMW). The waveform observations with the high time resolution show that one of the common features of these waves is the burstiness. The burstiness means that their amplitudes or frequencies rapidly change of the order of a few milliseconds to a few hundreds of milliseconds. Further, we show that the PSBL ESW, lobe EQMW, MS ESW, and MS EQMW are parallel propagating waves relative to the ambient magnetic field. The similarities of the ESW and EQMW in the magnetosheath and magnetotail suggest the possibility that these waves are generated by the same generation mechanism.
A study of Pc 3 coherence at cusp latitudes
Authors: Olson John V. , Szuberla Curt A. L.
Published: February 2004

Previous studies have taken data from multi‐instrument arrays, both ground‐based and in situ, to determine the spatiotemporal characteristics of Pc 3 pulsations (22–100 mHz) at cusp latitudes. Correlations of Pc 3 frequency band magnetometer, photometer, and riometer data have been cited as evidence of a local (ionospheric), rather than a distant (magnetospheric), source for these cusp pulsations. Electron precipitation modulated at Pc 3 frequencies is thought to be the source mechanism for the unstructured portion of the Pc 3 spectrum; such precipitation can modify local ionospheric conductivities, allowing modulated currents to flow at like frequencies. In this paper we undertake a statistical analysis of induction coil magnetometer records taken at Cape Parry and Sachs Harbor, Canada, in May–June 1985. Comparing the distribution functions of interstation X–X and Y–Y polarization and coherence estimates, we arrive at a simple model to estimate an upper bound for the coherence length, O(200 km), of Pc 3 source regions near the cusp. Such a determination is an important constraint in identifying the source of Pc 3 frequency band modulations of cusp latitude electron precipitation.
Observation and analysis of lower hybrid solitary structures as rotating eigenmodes
Authors: Pinçon J. L. , Kintner P. M. , Schuck P. W. , Seyler C. E.
Published: February 2004

Lower hybrid solitary structures (LHSS) are observed to be composed of wave modes rotating in the right‐handed sense about the geomagnetic field. The data analyzed were measured at altitudes near 800 km in the auroral ionosphere by the plasma wave interferometer aboard the AMICIST rocket. Clear evidence of these modes is obtained by an estimation of the local frequency‐wavenumber spectrum derived from wavelet analysis of the electric field. This evidence demonstrates that the phase velocity direction reverses as the payload traverses the structure, implying that the structure is composed of rotating electric fields. These observations are consistent with the result of three‐dimensional numerical simulations investigating lower hybrid wave behavior in the presence of a density cavity. This suggests that the observed characteristics of LHSS may be explained by the excitation of localized lower hybrid eigenmodes and the scattering of background VLH hiss from plasma density depletions.
Three‐dimensional MHD simulation of the April 14, 1994, interplanetary coronal mass ejection and its propagation to Earth and Ulysses
Authors: Dryer M. , Wu C.‐C. , Smith Z. K.
Published: February 2004

A three‐dimensional (3‐D), time‐dependent, magnetohydrodynamic (MHD) model is used to simulate the interplanetary propagation of a disturbance that started in the low corona via the destabilization of a southern hemisphere helmet‐streamer on April 14, 1994. A severe geomagnetic storm occurred at Earth, and a forward‐reverse shock structure was detected at Ulysses (3.2 AU) at E30°S60°. The model is initiated at 18Rs (where Rs is the solar radius, 6.95 × 105 km) within the supersonic and super‐Alfvenic region of the solar wind; hence no consideration is given to the disturbance's evolution from ∼ 1Rs to 18Rs. We refer to the interplanetary disturbance as an interplanetary/coronal mass ejection (ICME) to indicate that it may be relevant to what has been referred to in the literature as a CME but is not related to the flux rope or magnetic cloud that has also received much attention.We find that the 3‐D MHD model, with a simple pressure pulse (suggested by Yohkoh soft Xray observations), provides a satisfactory comparison with the SSC timing at Earth and the Ulysses observations.
Predicting solar wind structures and their geoeffectiveness
Authors: Chen James , Cargill Peter J. , Palmadesso Peter J.
Published: February 2004

This paper describes a feature‐based pattern‐recognition technique that utilizes real‐time solar wind measurements to identify and predict the occurrence of solar wind structures that can cause geomagnetic storms. The technique is based on the knowledge that storms are caused by solar wind events with certain identifiable features, the two most important ones being (1) extended periods and (2) large amplitudes of southward interplanetary magnetic field (Bz < 0). Using the measured solar wind properties available at the current time t the profiles of the north‐south magnetic field component, Bz(t) , and the east–west component, Bz(t) , are estimated for the solar wind that has yet to arrive, where t is the future time. On the basis of the estimated Bz and By profiles the occurrence, onset time, duration, and severity of impending storms are predicted. It is shown that the occurrence and durations of large storms exceeding a predetermined threshold can be predicted accurately. Successful predictions can be made after examining the initial ∼20% of a geoeffective (i.e., storm‐causing) structure. For long‐duration southward Bz events such as magnetic clouds the advance prediction time can be from several hours to in excess of 10 hours. The technique is tested using 5 months of historical solar wind data. We develop a procedure to extend the capability of the technique by incorporating additional features. Reduction of potential “false alarms” and “misses” resulting from such extensions is discussed.
Geomagnetic storm predictions from solar wind data with the use of dynamic neural networks
Authors: Wu Jian‐Guo , Lundstedt Henrik
Published: February 2004

Dynamic neural networks have been shown as an encouraging alternative to traditional approaches for nonlinear temporal predictions. We use partially recurrent neural networks to study solar wind‐magnetosphere coupling by predicting geomagnetic storms. The solar wind and Dst data used in this study are selected from the period 1963 to 1992. Statistical cross‐correlation analyses and neural networks are applied to finding the best coupling functions. It is found that the results from both studies are consistent and that the coupling functions P1/3VBs, P1/2VBs, V2Bs, VBs, and VBz are well suited for predicting geomagnetic storms. Also, the relative importance of solar wind parameters is investigated in detail, aimed at finding the optimal combinations of solar wind parameters for predicting geomagnetic storms. The two basic combinations giving accurate prediction are Bs, n, V and Bz, n, V. Addition of B, F, or By can further improve predictions. We find that the best combination outperforms the best coupling functions in terms of prediction accuracy. Geomagnetic storms are very accurately predicted 1–2 hours in advance from the solar wind alone. The accurate predictions 1–2 hours ahead might well imply that the internal dynamic magnetospheric processes in forming geomagnetic storms occur on a timescale of about 1 hour. The predictions 3–5 hours ahead are useful in practical operation according to their acceptable accuracy. The prediction goodness is decreased with increasing prediction time. We consider this as a result of the fact that the internal magnetospheric dynamics limits the short‐term prediction accuracy. The short‐term predictability of geomagnetic storms and the validity of the results from cross‐correlation analyses are discussed.
Nonbouncing Pc 1 wave bursts
Authors: Mursula K. , Rasinkangas R. , Bösinger T. , Erlandson R. E. , Lindqvist P.‐A.
Published: February 2004

On April 11, 1986, at about 0600 UT a long Pc 1 wave event of the hydromagnetic chorus type started on the ground, as registered by the Finnish pulsation magnetometer network. The main pulsation band at about 0.3 Hz was observed for several hours. Soon after start, this band smoothly extended to higher frequencies, forming another separate wave band which finally reached up to 0.5 Hz. During the event the Viking satellite was on its southbound pass over Scandinavia, close to the MLT sector of the ground network. From 0650 until 0657 UT, Viking observed a chain of Pc 1 bursts with increasing frequency. The strongest bursts could be grouped into two separate wave regions whose properties differed slightly. The higher‐latitude region had a frequency of 0.3 Hz, well in agreement with the main Pc 1 band on the ground. The lower‐latitude region contained the highest frequencies observed on the ground at about 0.5 Hz. The latitudinal extent of both wave regions was about 0.5°. They had slightly different normalized frequencies, Alfvén velocities, and repetition periods. Most interestingly, the repetition periods of both wave sources were too short for the bursts to be due to a wave packet bouncing between the two hemispheres. The results give new information about the high‐latitude Pc 1 waves, showing that they can consist of separate repetitive but nonbouncing bursts. We suggest that the long‐held bouncing wave packet hypothesis is generally incorrect and discuss two alternative models where the burst structure is formed at the equatorial source region of the waves.
Global positioning system phase fluctuations at auroral latitudes
Authors: Aarons J.
Published: February 2004

Using observations of a diversity of propagation paths of Global Positioning System satellites from each of 11 high‐latitude stations, it was possible to determine the behavior pattern for the development of phase fluctuations in the auroral and subauroral irregularity regions. The coverage comprised a large range of longitudes and latitudes with all data measuring the same parameter, i.e., phase fluctuations. Starting with characteristics of the irregularity development on magnetically quiet days, it was noted that the start and stop times of phase fluctuations correlated with the entry and exit into the irregularity oval. Maximum occurrence of phase fluctuation took place near magnetic midnight. During magnetic storms the irregularity oval expands equatorward and poleward and phase fluctuations increase in intensity. Using the time difference of the development of irregularities between stations considerably separated in longitude, the conclusion was reached that during a magnetic storm there is a long time feeding of the turbulent energy that develops irregularities in the oval. While the geographic position of the station relevant to the oval is important during storms, the dynamics of each storm modifies the simple behavior shown during quiet times. Observations at corrected geomagnetic latitudes >80° indicate that phase scintillation intensities were lower than those in the oval during both quiet and disturbed conditions in years of low solar flux. Since the technique yields a measure of total irregularity intensity for the total propagation path, it was not possible solely from these data to determine the altitude of the irregularities. In case studies it was found that during magnetically quiet periods when irregularities are noted on propagation paths in the irregularity oval, F layer critical frequencies and thicknesses were greater than those of the E layer. During magnetic storm periods, E layer critical frequencies are for various periods of time greater than those of the F layer. During storms, there are probably contributions from both layers with different mechanisms for the development of irregularities operating as a function of time, storm development, and latitude.
Toward forecasting space weather in the heliosphere
Authors: Sibeck D. G. , Richardson J. D.
Published: February 2004

We cross correlate years of solar wind ion plasma speed and entropy observations at various sites throughout the heliosphere to determine the conditions under which one spacecraft can be successfully used as a monitor for other spacecraft. Peak correlation coefficients occur when two spacecraft lie radially aligned, although coefficients are generally enhanced during an ∼2– to 3‐month period surrounding alignment. Within the inner heliosphere, the prevalence of corotating solar wind streams can enable useful predictions even when the monitoring spacecraft lies on the other side of the Sun. Similarly, observations at Earth can be useful in predicting conditions in the outer heliosphere to the orbit of Jupiter or even Saturn during periods of near‐radial alignment. Observations at Earth cannot be used to successfully predict conditions beyond this distance without considering steepening on the leading edge of streams or stream interactions.
Energetics during the main phase of geomagnetic superstorms
Authors: Mac‐Mahon R. Monreal , Gonzalez W. D.
Published: February 2004

The study of energy transfer between the different regions of the solar wind ‐ magnetosphere ‐ ionosphere system is a fundamental issue in Solar‐Terrestrial Physics. In this work we have studied the degradation of the solar wind energy budget through the solar wind ‐ magnetosphere coupling down to the ring current injection and the auroral ionospheric dissipation during the main phase of magnetic superstorms (Dst < −240 nT). The interplanetary magnetic field, density, temperature, and solar wind velocity measurements collected by the ISEE 3 satellite, and the total energy flux of high‐latitude precipitating particles collected by the NOAA 6 satellite were used for this study. The solar wind energy budget was determined from the kinetic energy flux of the particles in the interplanetary medium. The energy transfer from the solar wind into the magnetosphere was estimated through a dayside magnetopause ram pressure corrected version of the Perrault and Akasofu ε function. The energy injection into the ring current was estimated under the DPS theorem restriction and introducing the decay parameter τ in the evolution equation as a continuous function of the Dst index. The energy dissipation estimate in the auroral ionosphere via Joule heating in one hemisphere was computed using ionospheric and interplanetary data through a new method. Previous statistical and case studies for substorms have shown that the total energy dissipated as Joule heating is roughly twice that of the ring current injection. Our results show that the energy dissipation via Joule heating in the auroral ionosphere is about half of the ring current energy injection during superstorms.
Evolution of the global aurora during positive IMF Bz and varying IMF By conditions
Authors: Cumnock J. A. , Sharber J. R. , Heelis R. A. , Hairston M. R. , Craven J. D.
Published: February 2004

The DE 1 imaging instrumentation provides a full view of the entire auroral oval every 12 min for several hours during each orbit. We examined five examples of global evolution of the aurora that occurred during the northern hemisphere winter of 1981‐1982 when the z component of the interplanetary magnetic field was positive and the y component was changing sign. Evolution of an expanded auroral emission region into a theta aurora appears to require a change in the sign of By during northward interplanetary magnetic field (IMF). Theta aurora are formed both from expanded duskside emission regions (By changes from positive to negative) and dawnside emission regions (By changes from negative to positive), however the dawnside‐originating and duskside‐originating evolutions are not mirror images. The persistence of a theta aurora after its formation suggests that there may be no clear relationship between the theta aurora pattern and the instantaneous configuration of the IMF.
Generalized Rayleigh‐Taylor instability in the presence of time‐dependent equilibrium
Authors: Basu B.
Published: February 2004

Plasma instability under the combined influence of the gravity and an eastward electric field, commonly referred to as the generalized Rayleigh‐Taylor instability, is considered for a time‐dependent equilibrium situation. In the nighttime equatorial ionosphere the time‐dependent equilibrium situation arises because of the vertically upward E0 × B0 drift of the plasma in conjunction with the altitude‐dependent recombination process and the collisional diffusion process. After determining the time‐dependent equilibrium density and, in particular, the inverse density gradient scale length L−1, which determines the growth rate of the instability, the stability of small‐amplitude perturbations is analyzed. The general solution of the problem, where the effects of all of the above‐mentioned processes are included simultaneously, requires numerical analysis. In this paper the effects are studied in limiting situations for which useful analytic solutions can be obtained. The effect of diffusion on L−1 is studied by neglecting both the upward plasma drift and the altitude variation of the recombination frequency νR, and it is verified that the effect is negligible for typical values of the ionospheric parameters. The effects of the other two processes on L−1 are studied by neglecting diffusion. The effect of the altitude variation of νR on the linear growth of the perturbations is studied by adopting the so‐called local approximation. It is found that the value of L−1 and hence the value of the growth rate are enhanced by the altitude variation of νR. The enhancements rapidly increase with time to large values at lower altitudes and to significant values at higher altitudes when compared with the values for the spatially uniform νR case. Consequently, the time evolution of the instability and, more importantly, the level of fluctuations at saturation will be significantly affected by the enhancements. The nonlocal aspect of the instability in the upward drifting plasma is studied by neglecting, for the sake of obtaining a closed form analytic solution, both the altitude dependence of νR and the thermal effects. It is shown that to a very good approximation, the unstable modes are localized in the vertical direction with localization distance ∝λ1/2, where λ is the wavelength of the mode, and that the localized mode, while it grows in time, drifts vertically upward with the same speed as the upward drifting plasma. In view of the result that the altitude variation of νR significantly enhances the local growth of the perturbations, it should be retained in the nonlocal analysis; in which case, the appropriate differential equations have to be solved numerically.
Ground‐based signatures for the nightside polar cap boundary
Authors: Doe R. A. , Vickrey J. F. , Weber E. J. , Gallagher H. A. , Mende S. B.
Published: February 2004

This work describes an application of incoherent scatter (IS) radar and all‐sky imaging techniques to the identification of signatures for the nightside auroral zone/polar cap boundary. This identification is made by estimating the characteristic energy of precipitating electrons across all latitudes measured by the IS radar. The primary method used to discern the boundary is based on an algorithm wherein the detailed shape of E region ionization profiles is mapped to the required number flux from a library of monoenergetic ionization profiles. This procedure results in a reconstructed energy distribution for the precipitating electrons from which a characteristic energy is derived. The latitudinal gradient of the characteristic energy is subsequently used to identify the boundary. Monochromatic all‐sky imagers are used to establish the time history of the boundary and estimate characteristic energy (at magnetic zenith). Additional plasma signatures such as auroral ionospheric cavities and localized F region Te enhancements are shown to provide contextual clues for the location of the boundary. Three in‐depth case study periods are used to qualify the various boundary identification methods. A detailed study of 30 IS radar scans in the geomagnetic meridian is used to conclude that a latitudinal gradient in characteristic energy of −5 to −11 keV per degree, when measured adjacent to an extended region of low‐energy precipitation, can locate the boundary with a latitudinal precision of 0.2°.
Estimation of the O+, O collision frequency from coincident radar and Fabry‐Perot observations at Millstone Hill
Authors: Buonsanto M. J. , Sipler D. P. , Davenport G. B. , Holt J. M.
Published: February 2004

The formula for the O+, O momentum transfer collision frequency has been uncertain due to a discrepancy between results of theoretical calculations and some of the joint radar/optical studies. The former suggest a multiplicative factor equal to 1.2–1.3 times the formula derived by Dalgarno [1964] and Banks [1966], while the latter suggest a multiplicative factor F = 1.7 [Salah, 1993]. We present results of a new analysis of data from 30 nights of coincident incoherent scatter radar (ISR) and Fabry‐Perot interferometer (FPI) experiments conducted at Millstone Hill between 1988 and 1992. The O+, O collision frequency is estimated from FPI measurements of the horizontal neutral wind in the magnetic meridian, ISR measurements of the ion drift velocity parallel to the Earth's magnetic field and other data at the calculated height of peak 630 nm emission, and the mass spectrometer and incoherent scatter 86 model. A complete error analysis is carried out for each derived value of F. This allows us to carry out Monte Carlo simulations which confirm that random errors lead to an increase in the mean value of F and which provide us with an unbiased result, F = 1.15 ± 0.2. However, this result was obtained from an analysis which neglected vertical neutral winds, about which we have little information. The most likely effect of these winds would be an increase in the value of F, so that our best estimate from this study is F = 1.4 ± 0.3, which is consistent with theoretical calculations.
A self‐consistent determination of the heliospheric termination shock structure in the presence of pickup, anomalous, and galactic cosmic ray protons
Authors: le Roux J. A. , Fichtner H.
Published: February 2004

A self‐consistent time‐dependent model is used to study the modification of the heliospheric termination shock in the upwind direction under the influence of three suprathermal particle populations, namely pickup, anomalous and galactic cosmic ray protons. By ensuring that the resulting modulated cosmic ray proton spectra are consistent with those observed by the Voyager 2 and Pioneer 10 spacecraft during the solar activity minimum in 1987, two alternative modifications of the shock can be identified. While the first is characterized by a low injection efficiency of pickup protons into the process of diffusive shock acceleration and mainly determined by those particles, the second, resulting from a high injection efficiency, is clearly dominated by anomalous cosmic rays.
Long‐duration whistler waves in the magnetosheath: Wave characteristics and the possible source region
Authors: Matsui H. , Hayashi K. , Kokubun S. , Mukai T. , Yamamoto T. , Tsuruda K. , Saito Y. , Okada T.
Published: February 2004

A type of whistler waves termed “long‐duration whistler waves” (LDWW) in ELF range (1 ∼ 64 Hz) in the magnetosheath is studied according to the wave characteristics and generation based on data observed by the search coil magnetometer onboard the Geotail satellite. LDWW are band‐limited emissions near the lower hybrid frequency typically lasting several tens of minutes. Orientations of wave normal are determined from waveform data of vector magnetic field with an assumption of plane waves and are observed to be fairly well organized in a certain duration of LDWW. Ambiguity in propagation directions is removed by comparison of the phase relation between magnetic components and one component of electric field. The propagation vectors of LDWW are statistically aligned along the “Parker spiral” but are primarily reversed in the dusksides and in the dawnsides of the magnetosheath; that is, one is sunward, and the other is antisunward. This outstanding asymmetry strongly suggests that the bow shock region is the common source region with waves propagating away from the bow shock along the draped magnetic field in the magnetosheath. Electrons composing a type of distribution function in flat‐topped shape are concurrently observed during a series of LDWW events and are likely to yield a favorite condition for LDWW propagating a long path free from attenuation caused by wave particle interactions.
A comparison of cosmic ray intensities near the Earth at the sunspot minima in 1976 and 1987 and during 1995 and 1996
Authors: Lockwood J. A. , Webber W. R.
Published: February 2004

The cosmic ray proton and helium intensities measured on the IMP spacecraft near the Earth from ∼ 45 MeV to several hundred MeV in April 1996 indicate that the intensities have recovered to almost the same maximum values they had at the last sunspot minimum. The 1996 intensities near the Earth are an even closer match to those observed two 11‐year cycles earlier in 1976. This confirming evidence of a 22‐year periodicity in the low‐energy cosmic ray intensities and spectral shapes is attributed to the effects of the difference in the entry of cosmic rays into the heliosphere as well as their different drift motion inside the heliosphere as a consequence of the 22‐year periodicity in the solar magnetic field. This observation thus suggests that drifts play a significant role in the cosmic ray modulation process.
On the generation and propagation of auroral electromagnetic ion cyclotron waves
Authors: Lund E. J. , LaBelle J.
Published: February 2004

On a recent sounding rocket flight, electromagnetic ELF waves below the proton gyrofrequency were detected in an auroral arc. These waves would be resonant with ƒcH+ or ƒcO+ in the auroral acceleration region; the latter waves are associated with modulated parallel electron fluxes. Following previous papers, we use homogeneous linear theory to calculate temporal and convective growth rates for electron beam‐driven electromagnetic ion cyclotron (EMIC) waves in various plasmas with H+, He+, and O+ ions, although the inhomogeneity in the source region makes this approximation questionable. We find that all three EMIC modes are unstable with growth rates inversely proportional to the mass of the ion associated with the mode; maximum convective growth occurs at frequencies of 0.8–1.0Ωi. However, the growth rate of the O+ EMIC mode is too low to account for the observations unless relatively large beam densities are used, implying that nonlinear or inhomogeneous plasma effects must play an important role in the instability. We also assess the propagation of these EMIC waves in a cold plasma. Our ray tracing calculations show that propagation effects can explain both the localization of the O+ waves and the absence of He+ waves at lower altitudes. We also show that the wider latitudinal spread and low power spectral density of the H+ waves can also be explained by propagation effects, but the H+ rays which can reach the ionosphere are not the rays for which the beam‐driven instability produces the highest growth rates in a homogeneous plasma.
Optical interferometric studies of the nighttime equatorial thermosphere: Enhanced temperatures and zonal wind gradients
Authors: Meriwether J. W. , Biondi M. A. , Herrero F. A. , Fesen C. G. , Hallenback D. C.
Published: February 2004

Fabry‐Perot interferometric observations at 630 nm of equatorial thermospheric winds and temperatures in the four cardinal directions and zenith from Arequipa, Peru, during local winter for moderate and high solar fluxes showed elevated temperatures over the Andes Mountains that persisted through the night. The difference in temperature between east and west observations was typically ∼100 to 200 K for moderate flux values and as high as 400 K at solar maximum. Correlated with these localized heating regions were differences in the zonal thermospheric wind of 50 to 70 m/s for observations to the west and to the east of the Arequipa observatory. Also noted in these periods for the region over the Andes was the increased variance of the temperature values above the measurement error. These effects of increased variability and localized heating were not observed at solar minimum. The lack of a significant local time dependence in the diurnal variation of the temperature enhancements suggests that the origin of the heating cannot be related to the coupling of the electrodynamics of the ionosphere to the thermosphere. Instead the hypothesis is advanced that gravity wave energy from the surface penetrates into the thermosphere, where viscous dissipation causes the heating. Such wave activity would also explain the increased variability of the temperatures for the thermosphere regions over mountainous terrain.
Large‐m waves generated by small‐m field line resonances via the nonlinear Kelvin‐Helmholtz instability
Authors: Allan W. , Wright Andrew N.
Published: February 2004

Recently, ultralow‐frequency waves with large azimuthal wavenumber (large m) have been observed on similar L shells and with the same (or similar) frequencies as small‐m field line resonances (FLRs). The large‐m waves appeared to the west of the small‐m FLRs and had westward phase propagation while the small‐m FLRs had tailward phase propagation. We propose an extension to an earlier waveguide model to explain these observations. We suggest that small‐m tailward propagating waveguide modes drive the small‐m FLRs. Phase mixing within these FLRs allows the development of the nonlinear Kelvin‐Helmholtz (K‐H) instability near the resonant field lines. Phase‐mixing scale lengths are limited by ionospheric dissipation, and we show that realistic ionospheric Pedersen conductivities result in the dominance of a single zero‐frequency K‐H wave in each small‐m FLR region having m consistent with observation of the large‐m disturbances. K‐H growth rates are significant, but not large enough to disrupt the small‐m FLRs. We propose that unstable ion distributions amplify the seed K‐H waves as the ions drift westward. This leads to observable large‐m drift waves at or beyond the westward limits of the small‐m FLR regions.
Transverse cosmic ray gradients in the heliosphere and the solar diurnal anisotropy
Authors: Ahluwalia H. S. , Dorman L. I.
Published: February 2004

We describe a new scheme for computing the asymmetric part of the cosmic ray particle density gradient. The method makes use of the ecliptic components (Ar, Aϕ) of the cosmic ray solar diurnal anisotropy observed with a detector on spinning Earth and the interplanetary magnetic field sector structure in the heliosphere. We apply this methodology, to compute the magnitudes and directions of the asymmetric (Gθa) particle density gradient in the heliosphere for selected intervals of time during the 1965 to 1993 period. Data from six detectors of the global network are used for this purpose. Their median rigidity of response (Rm) covers the following range: 10 GV ≤ Rm ≤ 300 GV of the galactic cosmic ray (GCR) spectrum. We find that Gθa is well behaved over an intermediate rigidity range (10 GV ≤ Rm ≤, 67 GV) where it is inversely proportional to the rigidity (R) of the GCR protons. At higher rigidities (R ≳ 100 GV), it may fall off more steeply and even reverse sign at large (≳ 1 AU) off‐ecliptic distances. The direction of Gθa changes consistently immediately after each observed solar polar field reversal. Sudden, conspicuous, short‐lived (∼ 1 year) reversals of Gθa are observed during the declining phases of solar activity cycles 20(1974) and 21(1984). They may be caused by a sudden burst of solar activity late in the cycle. We have determined Gθa values from Deep River neutron monitor data (Rm = 16 GV) for the 1965 to 1993 period. Extreme computed values range from (3.0 ± 0.8)%/AU in 1968 to (−2.4 ± 0.8)%/AU in 1987.