Invariant Latitude Research Articles

Solar cycle effects on parallel electric field acceleration of auroral electron beams

AbstractWe present the occurrence frequency of downgoing auroral electron beams in magnetic local time and invariant latitude, and the dependence on solar cycle, as indicated by F10.7, on whether the ionospheric foot point of the satellite is illuminated or dark, and on the energy flux carried by the electrons. As previously reported, we find that the occurrence of electron beams peaks in the premidnight local time sector and that solar illumination at the foot point (solar zenith angle) reduces both the occurrence and energy of the electron beams. The effect of solar maximum conditions (indicated by F10.7) is almost as large as the effect of the solar zenith angle. The characteristic energy of the electron beams is dependent on the energy flux carried, in addition to both solar zenith angle and F10.7. The beam energy (and therefore the parallel potential drop) is ~1.6 times higher for during solar minimum than during solar maximum for both dark and illuminated foot points. The beam energy during dark solar minimum conditions is a factor of ~3 more than during sunlit minimum conditions. The “area” covered by intense aurora is also reduced during solar maximum, for both sunlit and dark conditions. There is no evidence that the statistical results are due to the fact that acceleration via parallel electric fields moves to lower latitudes during solar maximum.

Ionospheric density perturbations recorded by DEMETER above intense thunderstorms

AbstractDEMETER (Detection of Electromagnetic Emissions Transmitted From Earthquake Regions) was a three‐axis stabilized Earth‐pointing spacecraft launched on 29 June 2004 into a low‐altitude (710 km) polar and circular orbit that was subsequently lowered to 650 km until the end of the mission in December 2010. DEMETER measured electromagnetic waves all around the Earth, except in the auroral zones (invariant latitude >65°). The frequency range for the electric field was from DC up to 3.5 MHz, and for the magnetic field, it was from a few hertz up to 20 kHz. At its altitude, the phenomena observed on the E field and B field spectrograms recorded during nighttime by the satellite in the very low frequency range are mainly dominated by whistlers. In a first step, the more intense whistlers have been searched. They correspond to the most powerful lightning strokes occurring below DEMETER. Then, it is shown that this intense lightning activity is able to perturb the electron and ion densities at the satellite altitude (up to 133%) during nighttime. These intense lightning strokes are generally associated with transient luminous events, and one event with many sprites recorded on 17 November 2006 above Europe is reported. Examining the charged particle precipitation, it is shown that this density enhancement in the high ionosphere can be related to the energetic particle precipitation induced by the strong whistlers emitted during a long‐duration thunderstorm activity at the same location.

FAST observations of solar illumination and solar cycle dependence of the acceleration of upflowing ion beams on auroral field lines

We present the magnetic local time, invariant latitude, and altitude distribution of upflowing ion beams and the dependence of their occurrence on whether the ionospheric foot point of the satellite is illuminated or dark and on solar cycle, as indicated by F10.7. The effects of illumination and solar cycle are additive, consistent with the fact that the solar EUV depends on solar cycle and that the ionospheric conditions are dependent on total flux illuminating the foot point. Consistent with previous studies, the occurrence of upflowing ion beams peaks in the premidnight local time sector, and the occurrence increases dramatically with altitude over the altitude range of FAST (~700 km to ~4000 km). Solar illumination both reduces the occurrence of ion beams observed by more than a factor of 10 and increases the altitude where the acceleration occurs so that beams are rare below ~4000 km altitude. The effect of the increased solar EUV near solar maximum is almost as large. The inferred potential drops at altitudes below 4000 km, even during dark conditions, are usually less than 1 keV; during sunlit conditions and/or high F10.7, the potentials are usually less than 500 eV. These results place constraints on the altitude of the auroral parallel potential drop and on the relative importance of ionospheric conductivity and plasma density on the acceleration processes. There is no evidence that the statistical results are due to motion of large parallel potential drops to lower latitudes near solar maximum.

The polar cliff in the morning sector of the ionosphere

Abstract. By "polar cliff" we mean the steep increase in the ionization density observed in the morning sector of the polar ionosphere. Here the properties of this remarkable feature are investigated. The data set consists of electron density and temperature measurements obtained by the Dynamics Explorer 2 satellite. Only data recorded in the Northern Hemisphere winter are considered (solar zenith angle ≥ 90°). We find that for moderately disturbed conditions, the foot of the polar cliff is located below 60° invariant latitude. Here, within about 4°, the density increases by a factor of 4, on average. The actual location of the polar cliff depends primarily on the level of geomagnetic activity, its associated density increase on geographic longitude and altitude. As to the longitudinal variations, they are attributed to asymmetries in the background ionization density at middle latitudes. Using a superposed epoch type of averaging procedure, mean latitudinal profiles of the polar cliff and the associated electron temperature changes are derived. Since these differ significantly from those derived for the afternoon/evening sector, we conclude that the subauroral ionospheric trough does not extend into the morning sector. As to the origin of the polar cliff in the morning sector, local auroral particle precipitation should play only a secondary role.

Variation of dependence of the cusp location at different altitude on the dipole tilt

Using the Cluster cusp crossings data, dependence of the cusp location at the mid-altitude on the geomagnetic dipole tilt during northward IMF is studied. The results show that the cusp center moves 0.051° Invariant Latitude (ILAT) upon the increase of 1° in the dipole tilt angle at the average altitude of 5.8 RE (Earth radius). According to the present results obtained at the altitude of the Cluster orbit and previous results obtained at other altitudes of other satellite orbits, it is found that the higher the altitude in the cusp region is, the bigger the dependence of cusp location on the dipole tilt angle will be. If the altitude increases by 1 RE in the cusp region, the dependence will increase by 0.012° ILAT upon the increase of 1° in the dipole tilt angle. Some possible physical mechanisms are discussed and it shows that the cusp location will be more sensitive to the solar wind dynamic pressure if the altitude is high.

Double cusp encounter by Cluster: double cusp or motion of the cusp?

Abstract. Modelling plasma entry in the polar cusp has been successful in reproducing ion dispersions observed in the cusp at low and mid-altitudes. The use of a realistic convection pattern, when the IMF-By is large and stable, allowed Wing et al. (2001) to predict double cusp signatures that were subsequently observed by the DMSP spacecraft. In this paper we present a cusp crossing where two cusp populations are observed, separated by a gap around 1° Invariant Latitude (ILAT) wide. Cluster 1 (C1) and Cluster 2 (C2) observed these two cusp populations with a time delay of 3 min, and about 15 and 42 min later Cluster 4 (C4) and Cluster 3 (C3) observed, respectively, a single cusp population. A peculiarity of this event is the fact that the second cusp population seen on C1 and C2 was observed at the same time as the first cusp population on C4. This would tend to suggest that the two cusp populations had spatial features similar to the double cusp. Due to the nested crossing of C1 and C2 through the gap between the two cusp populations, C2 being first to leave the cusp and last to re-enter it, these observations are difficult to be explained by two distinct cusps with a gap in between. However, since we observe the cusp in a narrow area of local time post-noon, a second cusp may have been present in the pre-noon sector but could not be observed. On the other hand, these observations are in agreement with a motion of the cusp first dawnward and then back duskward due to the effect of the IMF-By component.

Dayside auroral hiss observed at South Pole Station

We performed a statistical study of low frequency (LF) auroral hiss recorded at South Pole Station in 2004, 2005, and 2007, and very low frequency (VLF) hiss recorded in 2000–2008. As expected, most auroral hiss occurs in the pre‐midnight sector. However, there is a secondary peak in occurrence in the pre‐noon sector (1000–1530 UT; ∼ 0630–1200 magnetic local time (MLT)) and somewhat more events occur in the post‐noon sector (1530–2100 UT; ∼ 1200–1730 MLT), with a null in occurrence around noon MLT. Individual dayside events appear similar to nightside hiss, but statistically they do not extend to as high frequencies. Solar wind discontinuities or impulses on the magnetopause are not correlated with these events. All‐sky camera, photometer, magnetometer, riometer, and VLF receiver data show that dayside LF hiss almost always extends to the VLF range and is often associated with active aurora. Examination of interplanetary magnetic field (IMF), substorm conditions, and Kp/AE/QI indices at times of dayside hiss suggests differences between the pre‐noon and post‐noon events: pre‐noon events are associated with IMF By < 0, whereas post‐noon events favor Bz < 0 and show a weaker correlation with By > 0. The correlation between pre‐noon events and By < 0 may arise because under those conditions, the pattern of field‐aligned currents (FACs) shifts to later magnetic local times, causing upward FACs to be dominant during pre‐noon hours at 74°, the invariant latitude of the South Pole. Unlike pre‐noon events, post‐noon events are more often associated with substorm activity on the nightside and favor elevated Kp indices, suggesting a connection of post‐noon events to nightside activity.

Characteristics of the plasma distribution in Mercury's equatorial magnetosphere derived from MESSENGER Magnetometer observations

Localized reductions in the magnetic field associated with plasma pressure in Mercury's magnetospheric cusp and nightside plasma sheet have been routinely observed by the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft. We present a statistical analysis of near‐equatorial magnetic depressions to derive the structure of Mercury's plasma sheet pressure. Because the plasma pressure in the magnetosphere correlates with solar wind density, the pressures were normalized to a Mercury heliocentric distance of 0.39 AU. A model magnetic field was used to map observations obtained on the ascending and descending orbit nodes to the magnetic equator and revealed the presence of plasma in a toroidal section extending on the nightside from dusk to dawn. Mapping the data to invariant magnetic latitude shows that the pressure is symmetric about the magnetic equator. The average pressure normalized for heliocentric distance is 1.45 nPa and exhibits a weak, 0.05 nPa/h, dusk‐to‐dawn gradient with local time. The plasma sheet pressure can vary between successive orbits by an order of magnitude. Unlike the predictions of some global simulations of Mercury's magnetosphere, the plasma enhancements do not form a closed distribution around the planet. This difference may arise from the idealized solar wind and interplanetary magnetic field conditions used in the simulations, which maximize the size and stability of the magnetosphere, thus promoting the formation of drift paths that close around the planet. For typical plasma sheet energies, 5 keV, the first adiabatic invariant for protons fails to be conserved even within 500 km altitude at midnight, implying that stochastic processes must be considered in plasma sheet transport.

Field‐aligned distribution of the plasmaspheric electron density: An empirical model derived from the IMAGE RPI measurements

We present a newly developed empirical model of the plasma density in the plasmasphere. It is based on more than 700 density profiles along field lines derived from active sounding measurements made by the radio plasma imager on IMAGE between June 2000 and July 2005. The measurements cover all magnetic local times and vary from L = 1.6 to L = 4 spatially, with every case manually confirmed to be within the plasmasphere by studying the corresponding dynamic spectrogram. The resulting model depends not only on L‐shell but also on magnetic latitude and can be applied to specify the electron densities in the plasmasphere between 2000 km altitude and the plasmapause (the plasmapause location itself is not included in this model). It consists of two parts: the equatorial density, which falls off exponentially as a function of L‐shell; and the field‐aligned dependence on magnetic latitude and L‐shell (in the form of invariant magnetic latitude). The fluctuations of density appear to be greater than what could be explained by a possible dependence on magnetic local time or season, and the dependence on geomagnetic activity is weak and cannot be discerned. The solar cycle effect is not included because the database covers only a fraction of a solar cycle. The performance of the model is evaluated by comparison to four previously developed plasmaspheric models and is further tested against the in situ passive IMAGE RPI measurements of the upper hybrid resonance frequency. While the equatorial densities of different models are mostly within the statistical uncertainties (especially at distances greater than L = 3), the clear latitudinal dependence of the RPI model presents an improvement over previous models. The model shows that the field‐aligned density distribution can be treated neither as constant nor as a simple diffusive equilibrium distribution profile. This electron density model combined with an assumed model of the ion composition can be used to estimate the time for an Alfven wave to propagate from one hemisphere to the other, to determine the plasma frequencies along a field line, and to calculate the raypaths for high frequency waves propagating in the plasmasphere.

Statistical modeling of in situ hiss amplitudes using ground measurements

Plasmaspheric hiss is a naturally occurring extremely low frequency electromagnetic emission that is often observed within the Earth's plasmasphere. Plasmaspheric hiss plays a major role in the scattering and loss of electrons from the Earth's radiation belts, thereby contributing to the maintenance of the slot region between the inner and outer electron belt. Traditionally, in situ satellite observations have been the measurement modality of choice for studies of plasmaspheric hiss due to their ability to directly measure the hiss source region. However, satellite studies are relatively short‐lived and very few satellite receivers remain operational for an entire 11‐year solar cycle. Ground stations, in contrast, may collect multiple solar cycles' worth of data during their lifetime, yet they cannot directly measure the hiss source region. This study aims to determine the extent to which measurements of hiss at midlatitude ground stations may be used to predict the mean amplitude of in situ measurements of plasmaspheric hiss. We use coincident measurements between Palmer Station, Antarctica (L = 2.4, 50°S invariant latitude) and the THEMIS spacecraft from June 2008 through May 2010, during solar minimum. Using an autoregressive multiple regression model, we show that in the local time sector from 00 < MLT < 12, when the ionosphere above Palmer Station is in darkness and hiss is observed at Palmer, the amplitude of plasmaspheric hiss observed by the THEMIS spacecraft is 1.4 times higher than when hiss is not observed at Palmer. In the same local time sector when the ground station is in daylight and hiss is observed, the THEMIS observed amplitudes are not significantly different from those when hiss is not observed on the ground. A stronger relationship is found in the local time sector from 12 < MLT < 24 where, when Palmer is in daylight and hiss is observed, THEMIS plasmaspheric hiss amplitudes are 2 times higher compared to when hiss is not observed at Palmer. There are insufficient statistics for the 12 < MLT < 24 sector during nighttime conditions. These results suggest that hiss emissions observed at Palmer in the dusk sector are likely plasmaspheric hiss, while those observed in the dawn sector may in fact be an emission other than plasmaspheric hiss, such as either ELF hiss or dawn chorus which has originated at high L‐shells. Though these results suggest that ground measurements of plasmaspheric hiss are not likely to be a viable replacement for in situ measurements, we believe that the predictive ability of our 12 < MLT < 24 sector model may be improved by including measurements taken during geomagnetically disturbed intervals that are characteristic of solar maximum.

Effects of solar and geomagnetic activities on the sub-ionospheric very low frequency transmitter signals received by the DEMETER micro-satellite

In the framework of seismic precursor electromagnetic investigations, we analyzed the very low frequency (VLF) amplitude signals recorded by the Instrument Champ Electrique (ICE) experiment on board the DEMETER micro-satellite. The sun-synchronous orbits of the micro-satellite allowed us to cover an invariant latitude of between -65° and +65° in a time interval of about 40 min. We considered four transmitter signals emitted by stations in Europe (France, FTU, 18.3 kHz; Germany, DFY, 16.58 kHz),Asia (Japan, JP, 17.8 kHz) and Australia (Australia, NWC, 19.8 kHz). We studied the variations of these VLF signals, taking into consideration: the signal-to-noise ratio, sunspots, and the geomagnetic activity. We show that the degree of correlation in periods of high geomagnetic and solar activities is, on average, about 40%. Such effects can be fully neglected in the period of weak activity. We also find that the solar activity can have a more important effect on the VLF transmitter signal than the geomagnetic activity. Our data are combined with models where the coupling between the lithosphere, atmosphere and ionosphere is essential to explain how ionospheric disturbances scatter the VLF transmitter signal.

Plasmaspheric trough evolution under different conditions of subauroral ion drift

The statistical characteristics of the subauroral ion drift (SAID) in the ionosphere and the plasmaspheric trough evolution under different conditions of SAID were investigated in this paper, based on 566 SAID events observed by Akebono, Astrid-2, DE-2, and Freja satellites. The relationships between the latitudinal location of SAID and the Kp, AL, and Dst indices for these events were also discussed. It was found that the SAID events happened mainly at invariant latitude (ILAT) of 60.4° and magnetic local time (MLT) of 21.6 MLT and that 92.4% of the events happened when the Kp index was below 5.0, indicating a medium geomagnetic activity. The latitudinal half-width of SAID varied from 0.5° to 3.0° with a typical half-width of 1.0°. The SAID would happen at low latitudes if the geomagnetic activity was high. The effects of SAID on equatorial outer plasmasphere trough evolutions were studied with the dynamic global core plasma model (DGCPM) driven by the statistical results of SAID signatures. It was noted that locations, shapes and density of troughs vary with ILAT, MLT, latitudinal width, cross polar cap potential and lifetime of SAID events. The evolution of a trough is determined by the extent of SAID electric field penetrating into plasmasphere and not all SAID events can result in trough formations.

Statistical characterization of the American sector subauroral polarization stream using incoherent scatter radar

[1] We examine Millstone Hill incoherent scatter radar data collected over two solar cycles between 1979 and 2001 to determine average characteristics and features of storm time ion velocity and flux transport in the American sector midlatitude ionosphere. We use over 1100 radar azimuth scans identified as containing enhanced westward ion velocities associated with the subauroral polarization stream (SAPS), covering the 12–06 magnetic local time (MLT) sector and 50°–68° invariant latitude for weak to moderate disturbance levels with Dst from 50 to −200 nT. We find the magnetic latitude peak location of the SAPS flow channels decreases linearly with both Dst and MLT with a very good degree of correlation. We also examine for the first time SAPS peak westward ion fluxes, which transport material westward with magnitude between 3 × 1013 and 3 × 1014 m−2 s−1 in a manner nearly invariant to activity level. This invariance is maintained by an inverse relationship between electron density and ion velocity magnitudes with increasing Dst. Westward log ion flux and ion velocity are maximum in the dusk sector and decrease linearly with increasing MLT, smoothly varying across the dusk terminator. Finally, velocity distributions show that material in the afternoon SAPS flow is swept westward to earlier MLT values, delivering O+ flux to the cusp region. In contrast, SAPS streams in the post terminator sectors are fixed east-west in the Sun-Earth inertial frame, effectively maintaining entrained ion fluxes at the same MLT.

Large-scale aspects and temporal evolution of pulsating aurora

[1] Pulsating aurora is a common phenomenon generally believed to occur mainly in the aftermath of a substorm, where dim long-period pulsating patches appear. The study determines the temporal and spatial evolution of pulsating events using two THEMIS all-sky imager stations, at Gillam (66.18 magnetic latitude, 332.78 magnetic longitude, magnetic midnight at 0634 UT) and Fort Smith, (67.38 magnetic latitude, 306.64 magnetic longitude, magnetic midnight at 0806 UT) along roughly the same invariant latitude. Parameters have been calculated from a database of 74 pulsating aurora events from 119 days of good optical data within the period from September 2007 through March 2008 as identified with the Gillam camera. It is shown that the source region of pulsating aurora drifts or expands eastward, away from magnetic midnight, for premidnight onsets and that the spatial evolution is more complicated for postmidnight onsets, which has implications for the source mechanism. The most probable duration of a pulsating aurora event is roughly 1.5 h, while the distribution of possible event durations includes many long (several hours) events. This may suggest that pulsating aurora is not strictly a substorm recovery phase phenomenon but rather a persistent, long-lived phenomenon that may be temporarily disrupted by auroral substorms. Observations from the Gillam station show that in fact, pulsating aurora is quite common with the occurrence rate increasing to around 60% for morning hours, with 69% of pulsating aurora onsets occurring after substorm breakup.

Determination of solar cycle variations of midlatitude ELF/VLF chorus and hiss via automated signal detection

[1] An automated algorithm for detecting chorus and hiss emissions in ground-based extremely low frequency/very low frequency (ELF/VLF) wave receiver data is developed and applied to 10 years of data collected at Palmer Station, Antarctica (L = 2.4, 50°S invariant latitude). The algorithm consists of three major processing steps. First, sferics and power line hum are removed from the broadband data. Second, individual events are detected and a set of 19 scalar event parameters are determined. Finally, on the basis of the parameters, detected events are categorized by a sequential pair of neural networks as either chorus, hiss, or noise. The detector runs on a modern 8-core computer at a speed of 350x real time. Results of training indicate that the neural networks are capable of differentiating between noise and emissions with a 92% success rate and between chorus and hiss with an 84% success rate. Data collected at Palmer from May 2000 to May 2010 were processed, and yearly and seasonal trends of chorus and hiss are analyzed. Yearly occurrence rates of chorus and hiss are strongly dependent on the geomagnetic disturbance level, as measured by Kp and AE, whereas seasonal occurrence rates are more strongly dependent on variations of the day/night terminator and associated variations in ionospheric absorption.

The source region and its characteristic of pulsating aurora based on the Reimei observations

[1] Using image and particle data sets obtained from observations by the Reimei satellite, we carried out time-of-flight (TOF) analysis for 29 pulsating aurora events to understand the precise properties of pulsating auroras and the possible generation process. While the sources identified using a standard TOF model were distributed almost continuously from magnetic latitudes 50° to −20°, the sources identified using a different TOF model that takes into account whistler mode wave propagation were confined to the equatorial region up to about 15°. The latter source distribution agrees with previous statistical studies of whistler mode chorus waves. In addition, the cold plasma density of the source region and the wave frequency can be estimated from the latter TOF analysis. The estimated cold plasma densities and wave frequencies normalized by the equatorial cyclotron frequency ranged in 0.20–21.7 cm−3 and 0.22–0.65, respectively. The estimated wave frequency showed clear dependence on the invariant latitudes of the pulsating aurora source region and increased up to the frequency range of the upper band chorus as the distance from the Earth decreased (up to about 5–6 RE). These results suggest that both lower and upper band chorus wave contribute to the electron scattering of pulsating auroras, which depends on the radial distance of the source region.

A numerical simulation for the omega band formation

[1] The formation of the omega bands and torch structures in the recovery phase of a substorm is numerically simulated. The kinetic energies of auroral particles are substantially provided by nonadiabatic acceleration in the tail current sheet. The magnetic drift flux (in the adiabatic sense) of the plasma sheet ions increases with decreasing invariant latitude; it starts to increase significantly around a certain magnetic shell, which is assumed, as a first approximation, to be the interface demarcating the nonadiabatic and adiabatic regions in the tail current sheet. Region 1 field-aligned currents can be generated in the situation that this interface is inclined with respect to the direction of the average magnetic drift velocity. As long as the energy density of injected particles does not significantly change with time, the region 1 current remains stable and no electrostatic waves with long wavelengths (>100 km at the ionospheric height) grow. Since the nonadiabatic particle acceleration is assumed to weaken during the (early) recovery phase of a substorm, the flux-tube-integrated energy density of injected particles attains a latitudinal profile unstable to the hybrid Kelvin-Helmholtz/Rayleigh-Taylor instability. While injected particles with less kinetic energies are transported to magnetic shells at lower latitudes, the unstable region expands in latitude so that long-wavelength waves develop in the plasma sheet. Then omega bands similar to those observed commonly in the substorm recovery phase form. The main characteristics of the omega bands in the simulation are shown to be consistent with observations.

Simultaneous FAST and Double Star TC1 observations of broadband electrons during a storm time substorm

Broadband electrons (BBEs) exhibit remarkable electron flux enhancements over a broad energy range (0.03–30 keV) near the equatorward edge of the auroral oval during geomagnetic storms. Here, we report a BBE event observed by the Fast Auroral Snapshot (FAST) satellite at 1355–1359 UT, ∼61°–66° invariant latitudes, ∼0600 magnetic local time (MLT), and ∼3800 km altitude during a storm on 25 July 2004. The Double Star (DS) TC1 satellite was located near the magnetic equator at L = 5.7, close to the same local time as FAST. We investigate the acceleration process of BBEs from the inner magnetosphere to near the ionosphere by comparing electron data obtained by FAST and DS TC1. We also investigate both plasma and field variations in the inner magnetosphere associated with substorm onset using DS TC1 data to examine the relationship between the BBEs and the storm time substorm. Ground geomagnetic field data show a positive H‐bay at ∼1349 UT at ∼0600 MLT, indicating that a storm time substorm started just before the appearance of the BBEs. At ∼1350 UT, a tailward ion flow was observed by DS TC1. Then, DS TC1 observed a local dipolarization and a drastic ion density enhancement at ∼1351 UT, indicating that particle heating associated with the substorm was occurring in the inner magnetosphere. From ∼1352 UT, electron fluxes were isotropically enhanced at energies above ∼0.5 keV as observed by DS TC1. On the other hand, the pitch angle distribution of BBEs at the FAST altitude showed field‐aligned lower‐energy electrons below ∼0.5 keV and isotropic higher‐energy electrons above ∼0.5 keV. From these data, it was inferred that the BBEs might consist of two energy components due to the acceleration or heating of electrons at different altitudes in association with the storm time substorm.

Remote global-scale observations of intense low-altitude ENA emissions during the Halloween geomagnetic storm of 2003

[1] Remote observations of energetic neutral atoms (ENAs) emitted from low altitude at a few keV in Earth's northern and southern hemispheres during the main phase of the 29 October 2003 geomagnetic storm are presented and compared with near simultaneous in situ measurements of precipitating ions with similar energies. A simple analysis of the ENA images yields estimates of the invariant latitudes and local pitch angles of the ENAs at their emission points. The invariant latitude distribution of the ENA emission points is similar to that of the precipitating ions and peaks near 60°. The pitch-angle distributions of the ENAs at their emission points are peaked near 90° but favor the upward (escaping) direction. We interpret the ENAs as re-emissions of the precipitating ions observed in situ, after interaction with Earth's upper atmosphere.

Experimental tests of the generation mechanism of auroral medium frequency burst radio emissions

[1] Medium frequency (MF) burst is an impulsive auroral radio emission at 1.3–4.5 MHz commonly detected by ground-based instruments for a few minutes at substorm onsets. It is thought to arise from mode conversion radiation. The Dartmouth College MF radio interferometer at Toolik Field Station, Alaska (68.51 invariant latitude), measured spectra, amplitudes, and directions of arrival (DOA) of 47 MF burst events during 2006– 2007 and 49 events during 2007–2008. Statistical analysis of these events shows that they come predominantly from the south and east of Toolik, as expected because propagation conditions are more favorable poleward and westward of the active auroral arcs than equatorward or eastward during premidnight (westward moving) substorm onset activity. Case studies of a selected MF burst event on 20 November 2007 show that motions of the radio emissions qualitatively track the motions of auroral arcs simultaneously observed with all-sky camera. Case studies of DOA data of selected MF burst events on 31 January and 20 November 2007 show that higher-frequency components of MF burst arrive at higher elevation angles than lower-frequency components. Statistical studies confirm this trend. Ray-tracing analysis shows that this trend implies that sources of the higherfrequency components of the MF burst are at higher altitudes than those of the lowerfrequency components. The analysis also shows that the MF burst comes from the bottomside F region ionosphere. These observations are consistent with a mechanism of MF burst emission whereby the emissions originate from mode conversion of Langmuir or upper hybrid waves excited over a range of altitudes in the bottomside F region.