Polar Ionospheric X-ray Imaging Experiment Research Articles

Thermospheric nitric oxide at higher latitudes: Model calculations with auroral energy input

[1] The nitric oxide (NO) density in the lower thermosphere has been calculated by a photochemical model for NOx and compared with measured NO densities from Student Nitric Oxide Explorer (SNOE). At higher latitudes the most important contributor for NO density increases is energetic electron precipitation. The electron energy is estimated in two ways, from auroral ultraviolet (UV) and X-ray measurements obtained from Ultraviolet Imager (UVI) and Polar Ionospheric X-ray Imaging Experiment (PIXIE) on board the Polar satellite and from ground magnetometer measurements. For the time intervals when the Polar satellite was not above the northern hemisphere, a parameterization of the electron energy flux from ground magnetic measurements was used. This parameterization was based on data from the SuperMAG database compared to UVI/PIXIE derived electron energy fluxes. The negative perturbation in the northward ground magnetic component is found to be linearly related to the precipitating electron energy flux. The 4-day period studied is from 30 April (day 120) until 4 May 1998, where the onset of a geomagnetic storm occurred 2 May (day 122). The results of the comparisons show an overall larger modeled nitric oxide density at auroral latitudes than what was measured by SNOE. The largest discrepancies were for the day of the storm onset, when the background atmosphere was more distorted by Joule heating. The next day the agreement between the model and the observations was far better, which might be due to less amount of Joule heating this day.

Characteristics of night-time absorption spike events

Abstract. Sudden increases in cosmic radio noise absorption, known as spike events, have been identified as signatures of substorms in the previous studies. Using data from the IRIS (Imaging Riometer for Ionospheric Studies) at Kilpisjärvi, Finland (L~6) more than 450 night-time spike events between 1994 and 2003 have been identified. Spike events fall into four distinct categories based on their structure and the background magnetic activity as indicated by a local westward electrojet (IL index) derived from the IMAGE (International Monitor for Auroral Geomagnetic Effects) magnetometer network as well as Pi2 magnetic pulsations from SAMNET (The UK Sub-Auroral Magnetometer Network). Classifying the types of absorption spikes allows for identification of phenomena such as multiple onsets and pseudobreakups from riometer data. In addition we have studied the statistical variation of absorption spikes and their sub-classes. This includes examining the magnetic local time (MLT) distribution and the seasonal and solar-cycle variation in spike occurrence. Those that seem to represent substorm onsets show a decidedly different MLT variation to those isolated spikes that represent pseudobreakups. The occurrence of spikes during different levels of geomagnetic activity is examined using the Kp index. Wavelet analysis has been used for studying the temporal structure of spikes; also the direction of motion of spike events and localisation of spikes are presented for all events and each sub-class and results are compared with previous studies. Statistical studies are supported with X-ray images of aurora from PIXIE (The Polar Ionospheric X-ray Imaging Experiment) when available.

Height profiles of the ionospheric electron density derived using space‐based remote sensing of UV and X ray emissions and EISCAT radar data: A ground‐truth experiment

In this study we have derived height profiles of the ionospheric electron density Ne using remote sensing of UV and X‐ray emissions from the Polar satellite and EISCAT radar data. The latter technique gives the most accurate determination of Ne providing a means to ground‐truthing the satellite imaging measurements. The UV‐emission data are taken from the Ultraviolet Imager (UVI) on Polar, while the X‐ray data are measured by the Polar Ionospheric X‐ray Imaging Experiment (PIXIE). As UVI yields a far better resolution in time and space than PIXIE, our primary approach involves UVI and EISCAT data. For a substorm event occurring on 24 March 1998, we derive Ne−UVI profiles valid in the E region above ∼105 km. By comparing with simultaneous Ne−EISCAT values, we find that the two techniques match fairly well in many cases. The altitude of maximum electron density is usually below 110 km. A few cases reveal Ne−EISCAT maxima in the upper E region (130–150 km), indicating a very soft precipitating electron energy spectrum. During such conditions, we observe the largest discrepancies between the Ne−UVI and Ne−EISCAT profiles. This may reflect the difficulty of obtaining proper energy characteristics from UV emissions, when the mean electron energy is less than ∼2 keV. A recalculation of these Ne−UVI values has been performed, requiring that the altitudes of the Ne−UVI maximum must match the altitudes of the Ne−EISCAT maximum. The results reveal a much better agreement between the two data sets, suggesting that UVI is measuring about the same energy flux as EISCAT. Even though the modified Ne−UVI values deviate strongly from the old Ne−UVI profiles, the effects on the Pedersen conductance, ΣP, are insignificant. Also, we find that ΣP−UVI are within ±30% of ΣP−EISCAT for 15 of 18 cases, suggesting that remote sensing of UV‐emissions provide a fairly reliable tool to monitor the Pedersen conductance. We have investigated a second approach by including PIXIE X‐ray data to derive Ne−UVI + PIXIE values valid in the whole E region and upper D region. Despite the coarse PIXIE resolution, we observe a fairly good match with the Ne−EISCAT profiles. By calculating the Hall and Pedersen conductances, ΣH and ΣP, we find that the values derived from satellite imaging measurements are within ±25% of the EISCAT values for all four cases, supporting the space‐based remote sensing technique to investigate the ionospheric electrodynamics. The results presented in this study suggest that the procedures developed to derive Ne values from the satellite imaging measurements are reliable. We also find that the Ne−UVI and Ne−UVI+PIXIE values on average are slightly larger (5 and 13%) than the Ne−EISCAT values. These discrepancies may be caused by the difference in resolution between the satellite remote sensing data and the radar data, as smoothing of discrete precipitation may result in an overestimation of Ne.

Energetic electron precipitation and the NO abundance in the upper atmosphere: A direct comparison during a geomagnetic storm

Nitric oxide (NO) densities at heights between 96 and 150 km in the Earth's upper atmosphere are directly compared with the energy deposition from precipitating energetic electrons. The comparisons are done for the beginning of a geomagnetic storm event on 2 May 1998. The electron energy is derived from X‐ray bremsstrahlung observations from the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on board the Polar spacecraft. Measurements of the NO density are performed by the Student Nitric Oxide Explorer (SNOE) on the dayside by measuring airglow spectral features of the NO γ‐band. Since a significant part of the electron precipitation takes place during the night, and considering the long lifetime of NO, we have accumulated the X‐ray data in geographical boxes. This enables us to follow the development of the total energy deposition over a specific area during the night and morning hours. In agreement with theoretical predictions we find an increase in NO at higher latitudes due to electron precipitation. At 106 km altitude, which is found to be the average altitude of the peak values of both NO intensity and precipitating electron energy, ∼83% of the NO density is produced by electron precipitation. At this altitude we find that the electron precipitation results in the production of ∼8 NO molecules per keV deposited energy. The comparison of the data also shows effects of a horizontal neutral wind. Above 100 km the peak in NO density is displaced equatorward of the peak in electron energy deposition.

Comparisons of thermospheric high‐latitude nitric oxide observations from SNOE and global auroral X‐ray bremsstrahlung observations from PIXIE

Three years (March 1998 through March 2001) of nitric oxide (NO) observations in the Northern Hemispheric thermosphere (90–170 km) as made by the Student Nitric Oxide Explorer (SNOE) spacecraft are compared in a broad statistical analysis with the daily‐averaged northern auroral bremsstrahlung x‐ray observations, which are taken to be a good proxy for the population of precipitating energetic electrons. The latter are made by the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on board the NASA GGS Polar spacecraft. A modest correlation between these two data sets is found, indicating that about 20–40% of the variation in the number density of thermospheric nitric oxide at high latitudes is caused by variations in the precipitation of energetic particles from the magnetosphere into the auroral regions of the ionosphere. Time delays between the two data sets are examined in this study, as well as the altitude profile of the nitric oxide observations. Differences in the response and recovery times in the two data sets are also carefully considered along with hemispheric asymmetries as a function of season, leading to stronger correlations which are then discussed in terms of the properties of the data sets used.

The dependence of AKR production on the intensity and energy spectra of auroral bremsstrahlung

Our limited knowledge of the energy dependence of electron precipitation associated with auroral kilometric radiation (AKR) production is improved with the use of bremsstrahlung x‐ray measurements. Simultaneous observations of bremsstrahlung x‐rays above 2 keV from the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on the POLAR satellite are compared with the frequency distributions of AKR waves measured with the Plasma Wave Instrument (PWI) on the GEOTAIL satellite. Various AKR characteristics such as the low‐ and high‐ frequency cutoffs and the frequencies at peak intensity are compared with various x‐ray parameters such as the magnetic local times of emission, the total intensities, and the spectral shapes. In particular, it is found that higher fluxes of x‐rays occur when the various cutoffs are at lower frequency. Also, higher AKR cutoff frequencies are associated with higher e‐folding x‐ray energy spectra.

Examination of the storm/substorm relationship using global auroral X‐ray images

Global measurements by the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) instrument on board NASA's Polar spacecraft of the auroral X‐ray emissions associated with substorms show significant differences between isolated substorms and those occurring during geomagnetic storms. During isolated substorms at substorm onset the emissions are initially seen in the premidnight‐to‐midnight sector and then spread toward dawn. The emissions extend beyond 12 magnetic local time (MLT) with the most intense emissions occurring in the postdawn sector. The timescales for the appearance of X‐ray emissions in the various morningside MLT sectors are consistent with the drift times of electrons, of energies that produce the photons observed by PIXIE, under the influence of a magnetospheric electric field inferred from ionospheric ion drift measurements. During substorm recovery, emissions die away gradually at increasing MLT values with the last of the emissions in the noon sector disappearing several hours (∼3–4) after substorm onset. During the stormtime substorms, onset‐associated emissions are seen in the premidnight‐to‐midnight regions and spread toward dawn as in the isolated substorms. However, the emissions do not reach much beyond ∼9–10 MLT and very intense emissions are seen in the predawn MLT sectors. These intense predawn emissions are seen throughout the stormtime periods and are associated with significantly enhanced magnetospheric convection. There are brief reductions in intensity in the morningside emissions shortly after substorm onset consistent with a brief reduction in the cross‐tail electric field, followed by intensification again in the predawn sector on a timescale consonant with drifting electrons under the influence of a magnetospheric electric field. We conclude that the differences between the temporal evolution and morphology of the auroral X‐ray emissions during isolated and stormtime subtorms are the result of pitch angle scattering mechanisms whose MLT distribution and intensity are dependent on the strength of the magnetospheric electric field.

A relation between the energy deposition by electron precipitation and geomagnetic indices during substorms

Observations from the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) and the Ultraviolet Imager (UVI) on board the Polar satellite have been used to derive the total energy dissipation (UA) in the Northern Hemisphere by electron precipitation in the energy range from 100 eV to 100 keV. Comparing with geomagnetic indices, we find that during substorms, UA is linearly related to the quick look AEQL1/2 and ALQL1/2 indices. The best correlation (0.86) is found between the energy flux above 10 keV and the ALQL index, which reflects that the energetic electron precipitation modulates the westward electrojet intensity by affecting the Hall conductance. The AUQL index which reflects the eastward electrojet intensity shows poor correlation with UA, either for soft or energetic electrons. This is consistent with an electric field dominance in the dusk sector and a minor role for auroral conductance in the eastward electrojet. On the basis of ionospheric electrodynamics, we argue that a nonlinear relation between UA and AE (and AL) is more appropriate than a linear relation. We show that the linear relations reported by others do not fit our data set and that they provide UA values which are too low. For the total electron energy flux (0.1–100 keV) we find the best fit to be UA[GW] = 4.4 ALQL1/2 − 7.6, with a correlation coefficient of 0.83, which is slightly better than that for the AEQL index (0.77). By time‐integrating the UA derived from ALQL, we obtain estimates of the total deposited energy during substorms within ±20% of the time‐integrated UA derived from UV and X rays.

Energy analysis of substorms based on remote sensing techniques, solar wind measurements, and geomagnetic indices

Observations from the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) and the Ultraviolet Imager (UVI) on board the Polar satellite have been used to examine the energy deposition in the Northern Hemisphere by precipitating electrons for seven substorms during 1997. By combining the results from these two remote sensing techniques we derive the 5 min average electron energy distributions from 100 eV to 100 keV with a spatial resolution of ∼700 km. During growth phase we find that most of the energy is carried by electrons below 10 keV. The study shows that the maximum intensity of the substorm is mostly defined by the flux of electrons below 10 keV. In contrast, the electrons above 10 keV show a greater intensification at substorm onset and are more confined azimuthally during the expansion phase. By using the most appropriate parameterized methods to estimate the energy increase of the ring current (UR) and the Joule heating rate in both hemispheres (UJ) and by adding the rate of energy deposition in both hemispheres by precipitating electrons (UA), we estimate the total energy dissipation rate during substorms (UT). Our estimate of UA is a factor 2–4 larger than that reported in earlier studies. We find that the contributions to the total time‐integrated energy dissipation over the duration of the substorm, W(UT), from W(UR), W(UJ), and W(UA) on average are 15%, 56%, and 29%. Comparing W(UT) and the time‐integrated ϵ parameter, W(ϵ), which approximates the solar wind input due to dayside reconnection, we find that the ϵ parameter does not always provide enough energy into the magnetosphere to balance W(UT). An additional energy transfer mechanism is needed to balance the energy budget. We find that a viscous interaction that transfers 0.17% of the solar wind kinetic energy in addition to the energy transfer due to dayside reconnection, estimated by the ϵ parameter, is sufficient to balance the total energy dissipation UT.

Relationship between X‐Ray, ultraviolet, and kilometric radiation in the auroral region

An investigation is made of the simultaneous emission of X‐ray and UV photons as a function of time and location in the auroral oval, and comparisons are made of their time variations with the intensity variations of Auroral Kilometric Radiation (AKR). The X rays were measured with the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) instrument on the Polar satellite, and ultraviolet emissions were acquired with the UVI instrument also on the Polar satellite. AKR was measured using the Plasma Wave Instrument (PWI) carried on the Geotail satellite. For the events studied the time profiles of AKR were compared with those of electron precipitation into the auroral atmosphere as measured by the emission of X rays and ultraviolet radiation. The latter emissions exhibited fluctuations which on a scale of minutes to tens of minutes were coherent over several hours of magnetic local time (MLT). The polar region images of X‐ray and UV emissions were usually very similar. The cross‐correlation coefficients between X‐ray and ultraviolet emissions were as high as 0.8 over a wide range of MLT, indicating that the time variations of these different emissions were similar, although the UV observations were limited to the nightside of the Earth and to the field‐of‐view of the UVI. The correlation coefficient between AKR and precipitation as a function of the MLT of the precipitation often exceeded 0.8, usually near midnight, but was >0.5 over several hours of MLT. This widespread correspondence of AKR and precipitation fluctuations is traced to the large‐scale coherence of precipitation fluctuations around the auroral oval.

Global X‐ray observations of magnetospheric convection‐driven auroral disturbances

Global observations of the auroral x‐ray emissions during three large magnetic storms in 1998 are presented from the Polar Ionospheric X‐ray Imaging Experiment (PIXIE). The cross‐polar‐cap potential drop (ΔΦPC) remained large (>100 kV) throughout most of the stormtime periods, and most of the x‐ray emissions observed were associated with enhanced earthward convection and dawnward drift of electrons caused by substantial cross‐tail electric fields. Large increases in ΔΦPC drove impulsive disturbances termed convection enhancements that showed signatures inconsistent with substorm activity, including substantial auroral emissions in the morning sector and little activity in the premidnight sector.

Dayside/nightside auroral X ray emission differences‐Implications for ionospheric conductance

In this study we examine the occurrence probability of intense X ray emissions (>2 keV) from the auroral ionosphere in daylight and in darkness. The Polar Ionospheric X ray Imaging Experiment (PIXIE) on board the NASA/GGS Polar spacecraft remotely images the Earth's ionosphere from high altitude (apogee of about 9 Earth radii). This instrument is capable of observing the entire northern auroral region at once, and is normally unaffected by scattered sunlight; resulting in an unbiased study of differences in auroral emissions between daylight and darkness. The differences in the occurrence probability between daylight and darkness are interpreted as being due to differences in the local ionospheric conductivity, as described in previous studies using in situ particle populations and lower energy ionospheric emissions. There is also a clear dependence upon season which is in accordance with the Russell‐McPherron effect. Under similar geomagnetic conditions, comparisons of occurrence probability separated by whether each region is in daylight or in darkness illustrate that intense auroral emissions are consistently more likely when a given region is in darkness than when the same region is in daylight.

Cause of the localized maximum of X‐ray emission in the morning sector: A comparison with electron measurements

The Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on board the Polar satellite has provided the first global scale views of the patterns of electron precipitation through imaging of the atmospheric X‐ray bremsstrahlung. While other remote sensing techniques like ultraviolet (UV) and visible imaging sense emissions that are dominantly produced by low‐energy electrons (<10 keV), the PIXIE X‐ray images used in this study respond to electrons of energy above ∼3 keV. From a statistical study of global X‐ray emission a localized maximum in the morning sector, delayed with respect to substorm onset, is found to be a common feature during substorms. The time delay of this morning precipitation relative to substorm onset strongly indicates that this localized maximum is caused by electrons injected in the midnight sector drifting into a region in the dawnside magnetosphere where some mechanism effectively scatters the electrons into the loss cone. In this study we have examined two isolated substorms that occurred on July 31, 1997, and September 4, 1997, to investigate these features in more detail. PIXIE images are used to examine the global structures of the two events. Particle measurements from several low‐altitude satellites, NOAA and DMSP, have provided fine structure information of the spatial and spectral development of the morning precipitation. We find good spatial correlation between X‐ray emission and electron measurements for both these events. Comparison of measured electron spectra and electron spectra derived from X‐ray measurements by PIXIE are also presented and correlates fairly well. The technique for deriving electron characteristics from the X‐ray measurements are described and discussed. We find that both the electron spectra and the X‐ray spectra can be represented by double or single exponentials. The electron spectra measured in the early stage of the localized morning maximum of X‐ray emission strongly indicate that the scattering of >2–10 keV electrons by waveparticle interaction into the loss cone is the main mechanism for this precipitation.

Studies of X-ray observations from PIXIE

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) on board the NASA/GGS POLAR spacecraft has been making observations of ionospheric X-ray emissions from the vantage of space for more than 3 years. A wide variety of observations have been made by PIXIE, which are detailed in this work. These include the local time distribution of the auroral X-ray intensity as well as the dependence of auroral X-rays on geomagnetic activity and solar wind magnetic field conditions. The auroral X-rays are produced as energetic electrons within the magnetosphere precipitate and are stopped in the ionosphere. Comparisons of the X-ray auroral intensity with other instrument observations have been made, which enables us to distinguish between temporal and spatial processes. In addition, several other X-ray features (not of an auroral nature) have been observed by PIXIE, and are described.

Global X-ray emission during an isolated substorm — a case study

The polar ionospheric X-ray imaging experiment (PIXIE) and the UV imager (UVI) onboard the Polar satellite have provided the first simultaneous global scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung and the auroral UV emissions. While the UV images in the Lyman–Birge–Hopfield-long band used in this study respond to the total electron energy flux which is usually dominated by low-energy electrons (<10 keV), the PIXIE images of X-ray bremsstrahlung above ∼2.7 keV respond to electrons of energy above ∼3 keV. Comparison of precipitation features seen by UVI and PIXIE provides information on essentially complementary energy ranges of the precipitating electrons. In this study an isolated substorm is examined using data from PIXIE, UVI, ground-based measurements, and in situ measurements from high- and low-altitude satellites to obtain information about the global characteristics during the event. Results from a statistical study of isolated substorms, which has reported a significant difference in the patterns of energetic electron precipitation compared to the less energetic precipitation are confirmed. A localized maximum of electron precipitation in the morning sector delayed with respect to substorm onset is clearly seen in the X-ray aurora, and the time delay of this morning precipitation relative to substorm onset strongly indicates that this intensification is caused by electrons injected in the midnight sector drifting into a region in the dawnside magnetosphere where some mechanism effectively scatter the electrons into the loss cone. In this study, we also present the results from two low-altitude satellite passes through the region of the localized maximum of X-ray emission in the morning sector. Measured X rays are compared with X-ray fluxes calculated from the electron spectral measurements. By fitting the electron spectra by a sum of two exponentials we obtain fairly good agreement between calculated and directly measured X-ray flux profiles.

Global storm time auroral X‐ray morphology and timing and comparison with UV measurements

The Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on NASA's Polar spacecraft provides the first global images of the auroral oval in X‐rays and allows very accurate measurements of the timing of geomagnetic disturbances to a degree of temporal resolution not available from previous imagers due to its photon counting characteristics. On October 19, 1998, a magnetic cloud associated with a CME encountered the Earth's magnetopause near 0500 UT, generating a magnetic storm that reached a minimum value in Dst of −139 nT. The z component of the interplanetary magnetic field (IMF) (Bz) remained remarkably steady for the first 10 hours of the storm as did the solar wind particle pressure. The PIXIE and UVI instruments on the Polar spacecraft were both imaging the auroral oval from 0800 to 1800 UT; six distinct impulsive auroral enhancements were observed by the imagers during this time period. Global imaging combined with geosynchronous particle observations allowed classification of the geomagnetic disturbances associated with the events. Only two of the events were classified as substorms; one was classified as a poleward boundary intensification, one was a convection bay, and one was a pseudobreakup. A sixth event occurred after a dramatic northward turning of the IMF at the end of the 10‐hour Bz south period but was very weak and transient. The effects of the northward turning were counteracted by a simultaneous increase in the By component of the IMF. The first sign of significant substorm activity occurred over 8 hours after the cloud encountered the Earth and was not associated with any change in the solar wind magnetic field or particle pressure. The cross polar cap potential remained large (&gt; 100 kV), and most of the X‐ray emissions observed were associated with enhanced earthward convection caused by large cross‐tail electric fields; 50% were collected from the 0000 – 0600 magnetic local time (MLT) sector.

Magnetospheric and ionospheric response to a substorm: Geotail HEP‐LD and Polar PIXIE observations

The High Energy Particle ‐ Low energy particle Detector experiment (HEP‐LD) on board the Geotail spacecraft and the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on board the Polar satellite have been used to examine a substorm event. On December 10, 1996, around 1700 UT, a substorm event with two onsets took place. The event occurred during a weak magnetic storm that started on December 9. Several of the classical substorm features were observed during the event: reconnection and neutral‐line formation in the near‐Earth geomagnetic tail, injection of energetic particles at geosynchronous orbit, and particle precipitation into the ionosphere. Magnetic field line mapping of the energetic precipitation area into the geomagnetic tail shows that the substorm development on ground is closely correlated with topological changes in the near‐Earth tail. In the first onset, mainly soft electrons are involved, with only a transient energetic precipitation delayed relative to the onset. The second onset about 30 min later includes both soft and energetic electrons. The source regions of both onsets are found to be located near the earthward edge of the plasma sheet, while the source region of the transient energetic precipitation during the first onset is in the distant tail. Magnetic reconnection occurs sporadically and burst‐like before the onsets. Both onsets appear to be triggered by northward turning of the interplanetary magnetic field. The study also demonstrates that the concept of pseudobreakups should be used with care unless global observations are available.

Statistical X ray auroral emissions – PIXIE observations

The Polar Ionospheric X ray Imaging Experiment (PIXIE) on board the NASA/GGS POLAR spacecraft has been making observations of X rays emitted simultaneously over the entire auroral zone since March, 1996. PIXIE remotely senses the bremsstrahlung produced by the loss of multi‐keV electrons from the magnetosphere into the ionosphere. In this study we examine twenty‐six months (April 1996 – July 1998) of X ray observations of the northern auroral region from the PIXIE instrument. X ray flux is obtained from the PIXIE observations, which allows us to produce for the first time synoptic auroral maps representing the precipitation of energetic electrons, as a function of geomagnetic activity.

Global‐scale electron precipitation features seen in UV and X rays during substorms

The Polar Ionospheric X‐ray Imaging Experiment (PIXIE) and the ultraviolet imager (UVI) onboard the Polar satellite have provided the first simultaneous global‐scale views of the patterns of electron precipitation through imaging of the atmospheric X‐ray bremsstrahlung and the auroral ultraviolet (UV) emissions. While the UV images respond to the total electron energy flux, which is usually dominated by electron energies below 10 keV, the PIXIE, 9.9–19.7 keV X‐ray images used in this study respond only to electrons of energy above 10 keV. Previous studies by ground‐based, balloon, and space observations have indicated that the patterns of energetic electron precipitation differ significantly from those found in the visible and the UV auroral oval. Because of the lack of global imaging of the energetic electron precipitation, one has not been able to establish a complete picture. In this study the development of the electron precipitation during the different phases of magnetospheric substorms is examined. Comparisons are made between the precipitation patterns of the high‐energy (PIXIE) and low‐energy (UVI) electron populations, correlated with ground‐based observations and geosynchronous satellite data. We focus on one specific common feature in the energetic precipitation seen in almost every isolated substorm observed by PIXIE during 1996 and which differs significantly from what is seen in the UV images. Delayed relative to substorm onsets, we observe a localized maximum of X‐ray emission at 5–9 magnetic local time. By identifying the location of the injection region and determining the substorm onset time it is found that this maximum most probably is caused by electrons injected in the midnight sector drifting (i.e., gradient and curvature drift) into a region in the dawnside magnetosphere where some mechanism effectively scatters the electrons into the loss cone.

The position of the auroral energetic electron precipitation region obtained from PIXIE global X-ray observations

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) on NASA's POLAR spacecraft has been making global observations of X-rays from the Earth's auroral regions since March, 1996. PIXIE is a remote-sensing global monitor of multi-keV electrons which have precipitated into the ionosphere from the magnetosphere. This study examines nine months (May, 1997 - January, 1998) of X-ray observations from the PIXIE instrument of the northern auroral oval. X-ray flux is obtained from the PIXIE observations, allowing us to determine a synoptic auroral precipitation pattern of energetic particles. In addition, we examine the mean magnetic latitude at ionospheric altitude of the detected X-rays as a function of geomagnetic indices.