High Magnetic Latitudes Research Articles

First U.S.‐China joint ground‐based Fabry‐Perot interferometer observations of longitudinal variations in the thermospheric winds

AbstractFor the first time, three Fabry‐Perot interferometers from the U.S. (Boulder, 40°N, 105°W, 49°N magnetic latitude (MLAT)) and China (Xinglong: 40°N,115°E, 34°N, MLAT; Kelan: 39°N, 112°E, 33°N MLAT) were used to examine the longitudinal variations in the thermospheric winds due to the geomagnetic latitude differences between the American and Asian sectors. During a case of quiet geomagnetic condition, the meridional winds were very similar at the U.S. and Chinese stations. The meridional winds at Boulder reached most equatorward winds after midnight, whereas in China, the largest equatorward winds were found near midnight. The Boulder zonal winds turned westward earlier in the morning hours and had larger diurnal variations because of its higher magnetic latitude. During the case of low geomagnetic activity (Kp~2), the meridional winds were still similar in the U.S. and in China. Boulder zonal winds had much larger diurnal variation compared to the quiet condition (Kp~1). Thermosphere‐ionosphere‐electrodynamics general circulation model simulations show a very good agreement with observation for the meridional winds. The simulated zonal winds exhibit noticeable differences with observations, but the general tendencies in longitudinal variations with a larger diurnal variation near the auroral oval are correct. Simulations showed that the ion drift is not directly responsible for the longitudinal variations in the winds. The pressure gradient had more direct effect on the longitudinal changes in the winds. The simulation results also showed larger diurnal variations at higher geomagnetic latitudes due to the auroral oval heating. Nonmigrating tides were not observed in the two cases in October 2012.

An Improved Model of the Jovian Magnetosphere

The Jovian magnetosphere is modulated by the solar wind and centrifugal force. The configuration of the magnetic field in the previous model of the magnetosphere including the centrifugal force is consistent with the observations at low magnetic latitude (Λ < 50°), while there is a substantial difference between the results of the model and the observations at high magnetic latitude (Λ ≥ 50°), especially in the distant magnetotail. Based on the previous model, a new configuration of the Jovian magnetosphere in the night side is suggested by a three-step transformation in this study. The new magnetosphere obtained by the transformation method is flattened in the z-direction and stretched in the x-direction in distant magnetotail, which agree with general knowledge.

Current‐voltage relation for the Saturnian system

Saturn's magnetosphere is populated by plasma created from neutrals ejected by the moon Enceladus. These neutrals are ionized and picked up by the planetary magnetic field requiring large amounts of angular momentum to be transferred from Saturn's upper atmosphere to the magnetospheric plasma. The resulting upward currents that supply this angular momentum are associated with electrons, which travel toward the planetary atmosphere. At high magnetic latitudes along the flux tube, parallel electric fields may develop to enhance the field‐aligned current density flowing between the two regions. We show that, similar to the Jovian system, the current‐voltage relation in the Saturnian system must be evaluated at the top of the acceleration region, which occurs at ~1.5 RS along the magnetic field line as measured from the center of the planet. Owing to the large abundance of protons in the Saturnian system, cold electrons carry the majority of the field‐aligned current for net potential drops less than 500 V. For the flux tube intersecting the equatorial plane at 4 RS, field‐aligned potentials of 50–130 V are consistent with the energy fluxes inferred from the Enceladus emission. In the middle magnetosphere, field‐aligned potentials of ∼1.5 kV produce ionospheric electron energy fluxes of 0.3 mW/m2 when hot electrons comprise 0.3% of the magnetospheric electron population.

Jupiter's aurora in ultraviolet and infrared: Simultaneous observations with the Hubble Space Telescope and the NASA Infrared Telescope Facility

We compare Jupiter's northern auroral emissions in infrared (IR) and ultraviolet (UV) using ground‐based IR observations from the NASA Infrared Telescope Facility and UV observations from Hubble Space Telescope on 16 December 2000, the only date for which simultaneous observations in the two wavelength regions exist. We use polar projections and longitudinal brightness cuts to compare the IR ( ions) and UV (H2, H, and Lyman‐alpha) aurorae, consisting of the main auroral emission, emission regions both poleward and equatorward of the main emission, and those associated with the Io footprint and its extended tail. We demonstrate that (1) the IR main emission and the equatorward diffuse emissions are generally good proxies for the UV and vice versa, (2) the spatial distribution and temporal behavior of UV and IR emissions within the main emission, at high magnetic latitudes, differ substantially, (3) UV and IR emissions associated with the Io interaction appear at the Io footprint and along an extended (downstream) tail but differ in relative brightness. While the UV aurora is excited directly, the IR aurora is a thermal emission, its intensity depends on both the number density of the ions and the temperature. Three main factors may contribute to the observed morphological differences of the simultaneous emissions in the two wavelengths, namely ion transport, local heating, and the energy of the precipitating electrons. We estimate the ion transport distances, based on the ion lifetime and suggest that ion transport cannot account for large‐scale morphological differences between the UV and IR emissions. We propose that neutral gas heating by particle precipitation and Joule heating locally enhances the emission with no UV counterpart. Additionally, we estimate that local temperature variations are reflected in the IR emission with a time lag of several hours with respect to the UV. Finally, high precipitating electron energies exceeding a certain value might lead to chemical loss of the low altitude ions, suppress the lower IR emitting layers, and contribute to the observed differences of the emissions between the two wavelength regimes.

The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind–magnetosphere‐ionosphere coupling

The angle μ between the geomagnetic dipole axis and the geocentric solar magnetospheric (GSM) zaxis, sometimes called the “dipole tilt,” varies as a function of UT and season. Observations have shown that the cross‐polar cap potential tends to maximize near the equinoxes, when on averageμ= 0, with smaller values observed near the solstices. This is similar to the well‐known semiannual variation in geomagnetic activity. We use numerical model simulations to investigate the role of two possible mechanisms that may be responsible for the influence ofμon the magnetosphere‐ionosphere system: variations in the coupling efficiency between the solar wind and the magnetosphere and variations in the ionospheric conductance over the polar caps. Under southward interplanetary magnetic field (IMF) conditions, variations in ionospheric conductance at high magnetic latitudes are responsible for 10–30% of the variations in the cross‐polar cap potential associated withμ, but variations in solar wind–magnetosphere coupling are more important and responsible for 70–90%. Variations in viscous processes contribute slightly to this, but variations in the reconnection rate with μare the dominant cause. The variation in the reconnection rate is primarily the result of a variation in the length of the section of the separator line along which relatively strong reconnection occurs. Changes in solar wind–magnetosphere coupling also affect the field‐aligned currents, but these are influenced as well by variations in the conductance associated with variations inμ, more so than the cross‐polar cap potential. This may be the case for geomagnetic activity too.

How changes in the tilt angle of the geomagnetic dipole affect the coupled magnetosphere‐ionosphere‐thermosphere system

The orientation of the Earth's magnetic field has changed dramatically during the geological past. We have investigated the effects of changes in dipole tilt angle on the magnetosphere, ionosphere, and thermosphere, using the Coupled Magnetosphere‐Ionosphere‐Thermosphere (CMIT) model. The dipole tilt angle modulates the efficiency of solar wind‐magnetosphere coupling, by influencing the diurnal variation in the angleμ between the dipole axis and the GSM z axis. This influences how much Joule heating occurs at high magnetic latitudes. The dipole tilt angle also controls the geographic distribution of the Joule heating, as it determines the geographic latitude of the magnetic poles. Changes in the amount and distribution of Joule heating with tilt an`gle produce further changes in temperature and neutral winds. The latter affect the O/N2 ratio, which in turn modifies the peak electron density of the F2 layer, NmF2. All these effects are most important when the Interplanetary Magnetic Field (IMF) is southward, while being almost negligible under northward IMF. However, a change in dipole tilt also changes the inclination of the magnetic field, which affects the vertical component of ionospheric plasma diffusion along the magnetic field, regardless of the IMF direction. Changes in vertical plasma diffusion are responsible for ∼2/3 of the changes in NmF2 and most of the low to midlatitude changes in hmF2under southward IMF and for most of the changes in both variables under northward IMF. Thermal contraction may be responsible for high‐latitude decreases in hmF2 with increasing tilt angle under southward IMF.

Effects of the dipole tilt and northward and duskward IMF on dayside magnetic reconnection in a global MHD simulation

By using a three‐dimensional global magnetohydrodynamic simulation of the Earth's magnetosphere, we have studied whether antiparallel or subsolar reconnection better describes magnetospheric dynamics at the dayside magnetopause for the case of northward and duskward interplanetary magnetic field (IMF) and a positive dipole tilt. The simulation shows that the reconnection dominantly occurs at high latitudes where antiparallel field regions appear in the northern dusk sector and in the southern dawn sector. For the northward and duskward IMF case of 30° dipole tilt, the reconnection region in the dusk sector moves tailward at high magnetic latitudes in the Northern Hemisphere along the magnetopause further than that for the southward IMF case. Both the convective electric field and the resistive electric field are peaked in the region where the field lines are antiparallel. Moreover the perpendicular and parallel components of the current in the antiparallel field region are greater than those of the current in the subsolar and magnetic equator regions. The field‐aligned current, in the Southern (winter) Hemisphere on the dawn region is stronger than that in the Northern (summer) Hemisphere on the dusk region because the reconnection site shifts toward the tail in the winter hemisphere, which is just opposite to the case of southward IMF studied by Park et al. (2006). In the Northern (summer) Hemisphere, a three‐cell pattern appears in the ionosphere because the reconnection site shifts tailward and duskward from the cusp. In the Southern (winter) Hemisphere, the three‐cell pattern is distorted and has a weaker cross‐polar cap potential than that in the Northern Hemisphere due to the effects of the positive dipole tilt.

GPS slant total electron content accuracy using the single layer model under different geomagnetic regions and ionospheric conditions

The use of observations from the Global Positioning System (GPS) has significantly impacted the study of the ionosphere. As it is widely known, dual-frequency GPS observations can provide very precise estimation of the slant Total Electron Content (sTEC—the linear integral of the electron density along a ray-path) and that the precision level is bounded by the carrier-phase noise and multi-path effects on both frequencies. Despite its precision, GPS sTEC estimations can be systematically affected by errors in the estimation of the satellites and receivers by Inter-Frequency Biases (IFB) that are simultaneously determined with the sTEC. Thus, the ultimate accuracy of the GPS sTEC estimation is determined by the errors with which the IFBs are estimated. This contribution attempts to assess the accuracy of IFBs estimation techniques based on the single layer model for different ionospheric regions (low, mid and high magnetic latitude); different seasons (summer and winter solstices and spring and autumn equinoxes); different solar activity levels (high and low); and different geomagnetic conditions (quiet and very disturbed). The followed strategy relies upon the generation of a synthetic data set free of IFB, multi-path, measurement noise and of any other error source. Therefore, when a data set with such properties is used as the input of the IFB estimation algorithms, any deviation from zero on the estimated IFBs should be taken as indications of the errors introduced by the estimation technique. The truthfulness of this assessment work is warranted by the fact that the synthetic data sets resemble, as realistically as possible, the different conditions that may happen in the real ionosphere. The results of this work show that during the high solar activity period the accuracy for the estimated sTEC is approximately of ±10 TECu for the low geomagnetic region and of ±2.2 TECu for the mid-latitude. During low solar activity the accuracy can be assumed to be in the order of ±2 TECu. For the geomagnetic high-disturbed period, the results show that the accuracy is degraded for those stations located over the region where the storm has the strongest impact, but for those stations over regions where the storm has a moderate effect, the accuracy is comparable to that obtained in the quiet period.

Scintillations effects on satellite to Earth links for telecommunication and navigation purposes

Radio wave scintillations are rapid fluctuations in both amplitude and phase of signals propagating through the atmosphere. GPS signals can be affected by these disturbances which can lead to a complete loss of lock when the electron density strongly fluctuates around the background ionization level at small spatial scales. This paper will present recent improvements to the theoretical Global Ionospheric Scintillation Model (GISM), particularly tailored for satellite based navigation systems such GPS coupled with Satellite Based Augmentation System (SBAS). This model has been improved in order to take into account GPS constellation, signals, and receiver response to ionospheric scintillation environments. A new modelling technique, able to describe the scintillation derived modifications of transionospheric propagating fields is shown. Results from GPS derived experimental measurements performed at high and low magnetic latitudes will show preliminary assessments of the scintillation impact on real receivers and system operations. Nevertheless, comparisons between theoretical scintillation models, such as WBMOD and GISM, with GPS derived experimental data will be shown.

Two‐dimensional hybrid code simulation of electromagnetic ion cyclotron waves in a dipole magnetic field

Proton temperature anisotropy (T &gt; 1) with sufficiently high will drive the electromagnetic ion cyclotron (EMIC) instability. A two‐dimensional hybrid code is employed to simulate the EMIC waves in a dipole magnetic field. We initialize the electron‐proton plasma such that the MHD equilibrium J × B = − · is satisfied for the parallel force, while it is only satisfied along the magnetic equator for the perpendicular force. A high temperature ratio T = 9 and low plasma beta β = 0.1 are used to lessen the effects of nonequilibrium off the equator. The plasma is more unstable to the EMIC waves along the magnetic equator where the temperature ratio is high and the plasma β is low. There, waves are generated with dominant left‐hand polarization with the expected EMIC wave frequencies based on linear theory. These waves propagate along the magnetic field toward the ionosphere; and the inhomogeneity of the magnetic field causes the waves in the low L shell region to travel faster than those in the high L shell, so that the wavefronts turn oblique to the local magnetic field. When the wave vector turns perpendicular to the magnetic field at high magnetic latitudes, the initially left‐hand waves become linearly polarized. The linear polarization of the EMIC waves is consistent with some observations and with ray tracing results for reflected waves. We also introduce resistive layers to damp the waves near the boundaries to reduce the waves that are reflected by the boundary condition. The power spectra indicate that there is a radial propagation of wave energy with a group speed equal to about 0.1 times the equatorial Alfvén speed. Additionally, we find that there is a radial coherence length equal to about 10 associated with the EMIC wave structure.

Oblique lower band chorus waves: Time shifts between discrete elements observed by the Cluster spacecraft

We report observations of remarkable frequency differences and time shifts between the corresponding elements of lower band chorus recorded by the four Cluster spacecraft at low magnetic latitudes on 20 January 2004. The Poynting flux measurements made by the Spatio‐Temporal Analysis of Field Fluctuations (STAFF) instrument confirm that the chorus source is located in the magnetic equatorial plane. Surprisingly, the spacecraft located closer to the equator systematically received the corresponding chorus elements later than the spacecraft located at higher magnetic latitudes. Owing to the orbit, the spacecraft located closer to the equator were at lower L shells. The time shifts and frequency differences depended almost linearly on the perpendicular distance between the various spacecraft. Analysis of data from both the wideband data and STAFF instruments shows that chorus emissions were generated with highly oblique angles of propagation. We discuss several properties of the chorus source that could lead to these observations. We show that the sources moving across the magnetic field lines are reasonably consistent with these observations. We propose that the transverse motion of the chorus sources is a consequence of a feedback between the oblique waves and counter streaming electrons during nonlinear cyclotron interactions, and we provide an estimate of the transverse velocity of the source.

Magnetosphere-associated storms and the autonomous storms in the ionosphere–plasmasphere environment

Global GPS-derived ionosphere maps (GIM) of total electron content (TEC) were transformed into magnetic latitude (MLAT) versus magnetic local time (MLT) frame. TEC enhancement or depletion marked by W index show dominant electron content depressions and the ionosphere–plasmasphere storms increasing by nighttime, at high magnetic latitudes and over the crests of equatorial anomaly. Based on W maps, the planetary W p index was produced and used for derivation of a catalogue of more than 140 TEC storms during 1999–2009. In total 33 space weather intense storms and 35 moderate storms are revealed with four series of indices (AE, A p, D st and W p) but more than half W p storms were either partially overlapping in time with magnetic storm or observed autonomously under non-storm magnetosphere conditions. Relation between an annual number of intense D st storms and W p storms has been used for their prediction towards the peak of the forthcoming 24th solar cycle.

Plasmasphere effects for GPS TEC measurements in North America

Plasmasphere effects on total electron content (TEC) measurements conducted using Global Positioning System (GPS) receivers are typically neglected for the North American region, because of the relatively high magnetic latitudes there, but model and measurement cases for this region are presented here to demonstrate the magnitude of the effects for GPS TEC measurements away from vertical and for the associated equivalent vertical TEC values. For high solar flux conditions, the effects of high, distant electron content can range up to 25 TEC units for equivalent vertical TEC, or up to 65 TEC units for slant TEC determinations derived from vertical TEC measurements. The effects of the plasmasphere for TEC calibrations conducted in North America include systematic overestimation of the equivalent vertical TEC and excessive latitudinal gradients, especially at night. These may not be readily evident in the calibration results, and some alternatives for addressing these circumstances are considered.

Missing data and the accuracy of magnetic-observatory hour means

Abstract. Analysis is made of the accuracy of magnetic-observatory hourly means constructed from definitive minute data having missing values (gaps). Bootstrap sampling from different data-gap distributions is used to estimate average errors on hourly means as a function of the number of missing data. Absolute and relative error results are calculated for horizontal-intensity, declination, and vertical-component data collected at high, medium, and low magnetic latitudes. For 90% complete coverage (10% missing data), average (RMS) absolute errors on hourly means are generally less than errors permitted by Intermagnet for minute data. As a rule of thumb, the average relative error for hourly means with 10% missing minute data is approximately equal to 10% of the hourly standard deviation of the source minute data.

Stormtime dynamics of the global thermosphere and equatorial ionosphere

Abstract. During magnetic storms the development of equatorial plasma bubbles (EPBs) and distributions of thermospheric densities are strongly influenced by the histories of imposed magnetospheric electric (εM) fields. Periods of intense EPB activity driven by penetration εM fields in the main phase are followed by their worldwide absence during recovery. A new method is applied to estimate global thermospheric energy (Eth) budgets from orbit-averaged densities measured by accelerometers on polar-orbiting satellites. During the main phase of storms Eth increases as long as the stormtime εM operates, then exponentially decays toward quiet-time values during early recovery. Some fraction of the energy deposited at high magnetic latitudes during the main phase propagates into the subauroral ionosphere-thermosphere where it affects chemical and azimuthal-wind dynamics well into recovery. We suggest a scenario wherein fossils of main phase activity inhibit full restoration of quiet-time dayside dynamos and pre-reversal enhancements of upward plasma drifts near dusk denying bottomside irregularities sufficient time to grow into EPBs.

고위도 하부 열권 바람의 소용돌이도와 발산 분석: 행성간 자기장(IMF)에 대한 의존도

이 연구에서는 고위도 하부 열권 역학을 좌우하는 물리적 과정을 이해하기 위하여, 상이한 행성간 자기장(Interplanetary Magnetic Field, IMF)에 따른 남반구 고위도 하부 열권에서의 소용돌이도(vorticity)와 발산(divergence)을 분석하였다. 이 연구를 위하여 미국립대기연구소(National Center for Atmospheric Research, NCAR)의 열권-이온권 전기역학적 대순환 모델(Thermo sphere-Ionosphere Electrodynamic General Circulation Model, TIEGCM)을 이용하였다. 소용돌이도와 발산 분석은 전체 수평 바람장을 최종적으로 결정하는 운동량원(momentum source)을 밝히는데 도움을 주는 근원적인 흐름을 파악할 수 있게 해주며, 운동량원들의 상대적인 강도를 분석해내는데 좋은 도구가 된다. 고위도 하부 열권의 평균 바람장은 태양 복사와 쥴가열에 의해 유발되는 발산운동 보다는 이온대류에 의해 유발되는 회전운동에 의해 주로 지배된다는 것이 확인되었다. <TEX>$IMF{\neq}0$</TEX>와 IMF=0인 경우의 고위도 열권 하부에서의 소용돌이도 차이(difference vorticity)가 모든 IMF 조건에서 발산장 차이(difference divergence)에 비해 훨씬 더 크게 나타났다. 이는 IMF가 발산적인 흐름보다 회전적인 흐름에 더 강하게 영향을 끼치며, 나아가 IMF가 발산운동을 유발하는 에너지 유입보다는 회전운동을 유발하는 운동량 유입에 더 강하게 영향을 끼침을 의미한다. IMF의 방향에 따라 고위도 하부 열권에서의 소용돌이도 차이의 양상이 매우 달랐다 <TEX>$B_y$</TEX>가 음일 때는 지자기 위도 <TEX>$-70^{\circ}$</TEX> 이상의 고위도에서 극을 중심으로 양의 소용돌이도 차이가 나타나고 <TEX>$B_y$</TEX>가 양일 때는 음의 소용돌이도 차이가 나타났다. <TEX>$B_z$</TEX>가 음인 경우 소용돌이도 차이가 저녁 영역에는 양이고 새벽 영역에는 음이며, <TEX>$B_z$</TEX>가 양인 경우에는 반대의 분포를 보였다. <TEX>$B_z$</TEX>가 음일 때가 양일 때보다 소용돌이도 차이가 더 큰 것으로 확인되었는데, 이는 IMF <TEX>$B_z$</TEX>가 남쪽으로 향할 때가 북쪽으로 향할 때보다 고위도 이온권의 이온대류를 더 강화시켜 열권 중성대기의 회전적인 흐름을 더 강하게 유발시킴을 의미한다. To better understand the physical processes that control the high-latitude lower thermospheric dynamics, we analyze the divergence and vorticity of the high-latitude neutral wind field in the lower thermosphere during the southern summertime for different IMF conditions. For this study the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (NCAR-TIEG CM) is used. The analysis of the large-scale vorticity and divergence provides basic understanding flow configurations to help elucidate the momentum sources that ulti-mately determine the total wind field in the lower polar thermosphere and provides insight into the relative strengths of the different sources of momentum responsible for driving winds. The mean neutral wind pattern in the high-latitude lower thermosphere is dominated by rotational flow, imparted primarily through the ion drag force, rather than by divergent flow, imparted primarily through Joule and solar heating. The difference vorticity, obtained by subtracting values with zero IMF from those with non-zero IMF, in the high-latitude lower thermosphere is much larger than the difference divergence for all IMF conditions, indicating that a larger response of the thermospheric wind system to enhancement in the momentum input generating the rotational motion with elevated IMF than the corresponding energy input generating the divergent motion. the difference vorticity in the high-latitude lower thermosphere depends on the direction of the IMF. The difference vorticity for negative and positive <TEX>$B_y$</TEX> shows positive and negative, respectively, at higher magnetic latitudes than <TEX>$-70^{\circ}$</TEX>. For negative <TEX>$B_z$</TEX>, the difference vorticities have positive in the dusk sector and negative in the dawn sector. The difference vorticities for positive <TEX>$B_z$</TEX> have opposite sign. Negative IMF <TEX>$B_z$</TEX> has a stronger effect on the vorticity than does positive <TEX>$B_z$</TEX>.

High-latitude plasma convection during Northward IMF as derived from in-situ magnetospheric Cluster EDI measurements

Abstract. In this study, we investigate statistical, systematic variations of the high-latitude convection cell structure during northward IMF. Using 1-min-averages of Cluster/EDI electron drift observations above the Northern and Southern polar cap areas for six and a half years (February 2001 till July 2007), and mapping the spatially distributed measurements to a common reference plane at ionospheric level in a magnetic latitude/MLT grid, we obtained regular drift patterns according to the various IMF conditions. We focus on the particular conditions during northward IMF, where lobe cells at magnetic latitudes &gt;80° with opposite (sunward) convection over the central polar cap are a permanent feature in addition to the main convection cells at lower latitudes. They are due to reconnection processes at the magnetopause boundary poleward of the cusp regions. Mapped EDI data have a particular good coverage within the central part of the polar cap, so that these patterns and their dependence on various solar wind conditions are well verified in a statistical sense. On average, 4-cell convection pattern are shown as regular structures during periods of nearly northward IMF with the tendency of a small shift toward negative clock angles. The positions of these high-latitude convection foci are within 79° to 85° magnetic latitude and 09:00–15:00 MLT. The MLT positions are approximately symmetric ±2 h about 11:30 MLT, i.e. slightly offset from midday toward prenoon hours, while the maximum (minimum) potential of the high-latitude cells is at higher magnetic latitudes near their maximum potential difference at ≈−10° to −15° clock angle for the North (South) Hemisphere. With increasing clock angle distances from ≈IMFBz+, a gradual transition occurs from the 4-cell pattern via a 3-cell to the common 2-cell convection pattern, in the course of which one of the medium-scale high-latitude dayside cells diminishes and disappears while the other intensifies and merges with the opposite main cell of the same polarity to form the large "round-shaped" convection cell when approaching a well-known IMFBy-dominated configuration. Opposite scenarios with interchanged roles of the respective cells occur for the opposite turning of the clock angle and at the Southern Hemisphere. The high-latitude dayside cells become more pronounced with increasing magnitude of the IMF vector.

Probability distributions of electron precipitation at high magnetic latitudes

We have integrated more than 600 million energetic electron spectra measured by the Special Sensor for Precipitating Particles, version 4 (SSJ4) sensor on nine Defense Meteorological Satellite Program (DMSP) spacecraft to obtain total number fluxes (Jtot [#/cm2 s sr]) and energy fluxes (JEtot [keV/cm2 s sr]). These quantities were first separated into bins of 1° in magnetic latitude (MLat), 1 h in magnetic local time (MLT) and unit steps of Kp and then into 26 × 26 logarithmically‐spaced matrices covering the ranges 104 to 1010.25 for JEtot and Jtot and 10−3 to 103.25 for average energy EAVE. Joint probability densities were then calculated as ratios of the number of samples within a matrix element to the total samples in each MLat‐MLT‐Kp bin. Our analysis shows that (1) in all bins probabilities for detecting any of the three parameters were lognormally distributed, and (2) in most bins distributions were multimodal. Measured multimodal probability distributions are well represented as superpositions of contributions from as many as four lognormal populations. These distributions have inherent positive skews with significant separations between their mean and most probable values. Average values of JEtot and EAVE now widely used to model distributions of auroral conductance at different levels of Kp are verified in the large DMSP data set. However, the lognormal and multimodal characters of their realizations in nature indicate that probabilities of detecting them at a given time in a MLat‐MLT‐Kp bin never exceed 10%.

A Stabilization Procedure For The Transformation Of Magnetic Data

Magnetic anomalies are as a result of the summation of magnetic effects produced by spatial variation of magnetic polarization. Consequently, transformations which attenuate the unwanted components of the observed anomalies have been of great interest in processing of magnetic data. Transforming data into the frequency domain has a problem caused by imperfections introduced in the data spectrum by aliasing and Gibb=s effect, which are caused by the discretization and truncation of data respectively. Stable transformation such as upward continuation, integration and reduction to the pole at high magnetic latitudes either preserves or attenuates the amplitude of these imperfection in the data spectrum. Whereas unstable transformation such as downward continuation, reduction to the pole at low latitudes and first vertical derivatives, greatly enhances the amplitude of the spectrum imperfections so that any direct application of such transformation (without stabilization) will produces poor result. Using equivalent source technique usually produce computations too large for computers to handle and the choice of a good stabilizing factor can be arbitrary. In this paper comparison is first made between the conventional filtering technique and the equivalent source technique using theoretical data and secondly a quantitative method is developed by using an algorithm which uses the correlation coefficient between successive pairs of the transformed maps. IFE Journal of Science Vol. 9 (1) 2007 pp. 77-86

Significant depletions of the ionospheric plasma density at middle latitudes: A possible signature of equatorial spread F bubbles near the plasmapause

Plasma density irregularities in the equatorial F region ionosphere are generally called equatorial spread F (ESF). Large‐scale plasma bubbles associated with ESF processes are generated in the bottomside F region and penetrate the F peak to the topside F region. It is still not fully understood how high ESF bubbles can reach. In this paper, we present the measurements of deep depletions of the ionospheric plasma density at 840‐km altitude with the Defense Meteorological Satellite Program (DMSP) satellites in the evening sector on 29 October 2003. The plasma depletions occurred after an interval of strong southward interplanetary magnetic field (IMF). A prominent feature of the observations is that the plasma depletions occurred at very high magnetic latitudes (MLAT), as well as at equatorial latitudes. The equatorial depletions were identified as plasma bubbles. The depletions at middle latitudes (20°–46° MLAT) were separated from the conventional ionospheric midlatitude trough and identified as the extension of ESF flux tubes into these latitudes. It implies that the plasma bubble, which caused the emptied flux tube at 46° MLAT, reached the altitude of 6800 km over the magnetic equator. The observations may represent the highest altitude/latitude of plasma bubbles that has ever been reported in the literature. For the plasma depletions at even higher latitudes (47°–49° MLAT) near the subauroral polarization stream (SAPS)/midlatitude trough, there are two candidate generation mechanisms. One is the extension of ESF flux tubes into these latitudes, implying that the plasma bubbles reached the altitude of 8400 km over the magnetic equator. Because the SAPS/midlatitude trough corresponds to the plasmapause at the plasmaspheric heights, the observations suggest that ESF plasma bubbles may have reached the plasmapause. Another mechanism is a field‐aligned, low‐density structure between the plasmasphere and ionosphere, and the plasma structure is generated through the strong electric field in the subauroral polarization stream region.