Auroral Events Research Articles

Proton precipitation during transpolar auroral events: Observations with the IMAGE‐FUV imagers

Several transpolar auroras have been observed with the IMAGE‐Far Ultraviolet (FUV) imagers. The unique capability of the IMAGE‐FUV experiment to image the proton and electron aurora simultaneously on a global scale is used to study this phenomenon. A set of 12 transpolar aurora observed with IMAGE‐FUV is studied. We find that the dynamics of the transpolar aurora in relation with the IMF is consistent with previously published studies. The proton energy flux in the transpolar structure is determined and compared with the electron contribution. The contribution of the proton aurora inside the transpolar arc is also considered at the global scale. On the average, transpolar structure contributes ∼10% of the total precipitation. We find that the proton precipitation carries ∼5% of the energy flux of the transpolar arc. In 11 of the 12 cases of transpolar events studied, a local brightening of the auroral Lyman‐α emission is detected concurrently with the appearance of the transpolar structure. The intensification takes place at the nightside base of the transpolar arc, in the auroral oval. The duration of the intensification is variable but shorter than the lifetime of the transpolar arc. This study also suggests that the localized intensification is mainly due to an increase of the proton precipitation.

Seasonal variation of auroral electron precipitation

Global observations of nitric oxide obtained from the Student Nitric Oxide Explorer (SNOE) have been used to determine the density of nitric oxide in the lower thermosphere. A thermospheric model which takes into account all of the photochemical processes that produce and destroy nitric oxide has been used to deduce the flux of precipitating auroral electrons that produce nitric oxide. The results of this analysis allow us to infer the daily variability of precipitating electron energy deposition. The most important result of this paper is that there is a clear‐cut minimum in the electron flux at the time of the summer solstice in both hemispheres. This seasonal dependence of precipitating auroral electrons supports the idea that auroral arcs are suppressed in the solar‐illuminated hemisphere due to high ionospheric conductivity. The strength of the seasonal variations suggests that the bulk of the energy deposition may be due to discrete auroral events.

Does the aurora provide evidence for the occurrence of antiparallel magnetopause reconnection?

Antiparallel merging at the magnetopause predicts a longitudinal bifurcation of the cusp during intervals of southeast or southwest interplanetary magnetic field (IMF) orientation. In the aurora this should manifest itself as a longitudinal gap in the midday sector, bracketed by bright auroral forms on either side. With the purpose of testing this prediction, we searched for an auroral signature in our database from the arctic islands of Svalbard, focusing on strong, isolated auroral brightenings belonging to the category of events which has been previously established as a signature of transient magnetopause reconnection (flux transfer events (FTEs)). We found some very interesting events, showing clear brightenings in the eastern and western parts of the field of view (FOV), with a gap in between. In this paper we present a 1 hour long interval of auroral observations in the 1100–1400 MLT sector, showing two strong, isolated auroral events, which were separated by 20 min. The IMF was steady and directed southeast (By = 3 nT; Bz = −3 nT; clock angle = 135°), in a solar wind of low speed (370 km s−1) and low dynamic pressure (0.2 nPa). Each auroral event is characterized by an initial, strong brightening of the cusp equatorward boundary, previously referred to as equatorward boundary intensification (EBI), which is followed by a westward and poleward moving auroral form (PMAF). The main point here is that the initial brightening is longitudinally bifurcated, with near‐simultaneous auroral brightenings in the eastern (1300–1400 MLT) and western (1100–1130 MLT) parts of the all‐sky camera FOV being separated by a ∼500 km wide “gap.” The brightening event is discussed as a possible signature of transient, antiparallel reconnection, taking place at high magnetopause latitudes, while the “gap” aurora is attributed to quasi steady component reconnection in the subsolar region.

Fractal approach to description of the auroral structure

Abstract. During the last two decades the fractal geometry has become a powerful approach to different physical problems. It is also found to be useful in image processing applications. A numerical quantity that characterizes the auroral structure would be important for auroral investigations. We try to obtain the quantity on the basis of the box-counting dimension of the line of equal intensity. In this paper we present results of some tests of our procedure by simulated images. The possibilities that the approach gives us for analysis of the auroral dynamics are discussed. The auroral dynamics during several typical auroral events are considered.Key words. Ionosphere (auroral ionosphere) – Magnetospheric physics (magnetosphere-ionosphere interactions) – Space plasma physics (nonlinear phenomena)

Evening co‐rotating patches: A new type of aurora observed by high sensitivity all‐sky cameras in Alaska

We observed an interesting auroral event at Poker Flat Research Range (PFRR, MLAT ∼65.5°) using an all‐sky imaging system. This aurora, which appeared in the evening (16–19 MLT) during a geomagnetically quiet period, maintained its patch structure and position for more than 40 minutes. These patches were distinct in OI 557.7‐nm and N2+ 427.8‐nm emissions. At this time, PFRR was located at a lower latitude region than the auroral oval. An extended plasmasphere was observed a few hours beforehand with the Akebono satellite in the afternoon sector. This evidence suggests that the particles causing this aurora precipitate from the extended duskside plasmasphere. We believe that the imaging observation from the ground using our technique is an effective method for investigating dynamics around the plasmapause, a still partially‐unexplored region of space.

Multipoint measurements of field‐aligned current density in the auroral zone

In this paper we present the analysis and interpretation of a multipoint observation of magnetic field structures at the poleward edge of a premidnight auroral arc by the Enstrophy sounding rocket mission. Four Free‐Flying Magnetometers (FFMs) employing autonomous nanospacecraft technology were deployed from the main payload during the flight, and multipoint magnetic field measurements were made. Signatures consistent with both spatial and temporal interpretations were found to be present when large fluctuations in B were seen at the edge of an arc as the rocket flew into the polar cap. Reasons for the interpretation of spatial or temporal features are given and supported by a simple model of multiple payloads crossing through several moving current sheets and a study of the fine structure of this auroral event using multipoint, correlative wavelet analysis to find velocities of structures at different scale sizes. We show that the direct measurement method of current density using multipoint measurement of magnetic fields gives us a different current density than what would be inferred from a single‐point measurement and that the multipoint measurement also provides an inherent check on the validity of the measurement through a calculation of the divergence of the measured B.

Diagnostics of magnetosphere-ionosphere coupling over Indian Antarctic station Maitri, from magnetometer and riometer observations during the optical auroral event of 4–5 March 1999

A splendid display of “aurora australis” was observed over the Indian Antarctic station Maitri (geomag. lat. 62°.8 S, long. 52°.8 E; L = 4.8) on the night of 4–5 March 1999. The changes recorded by a fluxgate magnetometer, and a 30 MHz riometer operating at Maitri are interpreted in terms of currently - understood auroral physics. Interplanetary magnetic field and plasma parameters recorded by the WIND satellite indicate very high solar wind velocities (500–600 km/s), and prolonged southward B z of average value (∼ 5 nT), during the auroral event. Ion density is however an average 3–4/cm 3, and Ion pressure only 2–3 nano Pascals during the event.

A model of odd nitrogen in the aurorally dosed nighttime terrestrial thermosphere

A model of the production and loss of odd nitrogen species (mainly, N(4S), N(2D), NO, and NO+) and the enhanced vibration‐rotation band emission from NO in the infrared from the aurorally dosed terrestrial nighttime thermosphere is described. This model is assessed by analyzing the observations of the fundamental (Δv = −1, ∼5.3 μm) and overtone (Δv = −2, ∼2.7 μm) NO vibration‐rotation band limb emissions made by a Michelson interferometer and radiometer, respectively, aboard the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS‐1A) experiment on the space shuttle Discovery during an auroral event. The auroral dosing along the line of sight is inferred from the 2.7‐μm radiance profiles using a self‐consistent procedure. The dosing obtained is then used in the model to predict the spectrally resolved emission in the 5.3‐μm region. The calculated overtone radiance profiles agree with the measured ones at lower tangent altitudes, where the signal‐to‐noise ratio is large. The calculation, however, underpredicts the observed radiance, both fundamental and overtone, at higher tangent altitudes by a factor of ∼3 and gives a colder rotational temperature for the rotationally hot component of the spectrally resolved emission from nascent NO. The causes of this discrepancy and its impact on the densities of the odd N species are discussed.

Electromagnetic waves and bursty electron acceleration: implications from Freja

Abstract. Dispersive Alfvén wave activity is identified in four dayside auroral oval events measured by the Freja satellite. The events are characterized by ion injection, bursty electron precipitation below about 1 keV, transverse ion heating and broadband extremely low frequency (ELF) emissions below the lower hybrid cutoff frequency (a few kHz). Large-scale density depletions/cavities, as determined by the Langmuir probe measurements, and strong electrostatic emissions are often observed simultaneously. A correlation study has been carried out between the E and B field fluctuations below 64 Hz and 10 Hz, respectively, (the DC instruments upper threshold) and the characteristics of the precipitating electrons. This study revealed that the energisation of electrons is indeed related to the broadband ELF emissions and that the electrostatic component plays a predominant role during very active magnetospheric conditions. Furthermore, the effect of the ELF electromagnetic emissions on the larger scale field-aligned current systems has been investigated, and it is found that such an effect cannot be detected. Instead, the Alfvénic activity creates a local region of field-aligned currents. It is suggested that dispersive Alfvén waves set up these local field-aligned current regions and, in turn, trigger more electrostatic emissions during certain conditions. In these regions, ions are transversely heated, and large-scale density depletions/cavities may be created during especially active periods.Key words. Ionosphere (particle acceleraton; wave-particle interactions) Magnetospheric physics (auroral phenomena)

Attitude Estimation for a Flexible Spacecraft in an Unstable Spin

Anattitudereconstruction algorithm hasbeendeveloped fora e exiblesounding rocketwhosespin vectornutates unstablyaboutitsminorinertia axis.Theattitudeestimatesareneededforposte ightanalysisoftherocket’ sscience data. An additional goal of the work is to show that a e exible-body model can be used in a Kalman e lter or in a smoother. The attitude is estimated using a smoother whose dynamic model includes a main rigid body and a pair of elasticbooms. Boom e exure is the principal causeof nutation instability. Boom bending is modeled by a Markov process, but the laws of mechanics are used to determine its ine uence on the attitude dynamics. This smoother achieves a better attitude estimation accuracy than can be achieved using a rigid-body model. The peak attitude error is estimated to be 4 deg, and the principal error source seems to be the limited accuracy of the rocket’ s attitude sensors. HIS work deals with the poste ight attitude determination for a sounding rocket mission, the Cleft Accelerated Plasma Experimental Rocket (CAPER). CAPER was launched from the Andoya Rocket Range in Norway in January 1999. Its e ight lasted slightly longer than 1200 s and reached an apogee altitude of 1360 km. The goal of this mission was to study the behavior of the ionosphere during auroral events. Attitude information is needed to transform CAPER’ s measurements of electric e eld components into geodetic coordinates. An attitude accuracy of from 2 to 4 deg is considered acceptable for the mission. From an attitude determination standpoint, the important characteristics of the CAPER sounding rocket were as follows: Its attitude sensors were a vector sun sensor with a slit e eld of view, a e xed-head horizon crossing indicator (HCI), and a three-axis magnetometer that was mounted on a short boom. The rocket’ s attitude was passively spin stabilized with the nominal spin vector directed along its minor inertia axis. CAPER deployed several booms after exit from the atmosphere and after the e nal stage motor burn. The longest of these were two e exible 3-m booms that deployed perpendicular to the nominal spin axis and in opposite directions from each other. A schematic diagram of this cone guration appears in Fig. 1. Minor-axis spin stabilization is often used in sounding rocket experiments. Near the dawn of the space age, the Explorer-1 mission demonstrated that such a cone guration has an unstable nutation mode due to energy dissipation in the e exible booms. 1 In many sounding rocket experiments it is acceptable for the nutation amplitude to grow as long as the resultant coning half-angle does not become too large by the end of the e ight. There are two major challenges in doing poste ight attitude determination for CAPER. The e rst is the lack of rate-gyro data. This challenge dictates the use of an Eulerian dynamics model to propagate the attitude and rates between measurement sample times. The model becomes especially important toward the end of the e ight, when only magnetometer data are available. The attitude estimate in rotation about the magnetic e eld vector is based solely on dynamic model propagation during this phase, and model inaccuracy

Solar and auroral evidence for an intense recurrent geomagnetic storm during December in AD 1128

Abstract. The earliest known drawing of sunspots appears in The Chronicle of John of Worcester, which was compiled in the first half of the twelfth century. In this medieval chronicle, the Latin text describing the sunspots is accompanied by a colourful drawing, albeit idealised, which shows the apparent positions and sizes of two sunspots on the solar disk. The date of this observation of sunspots from Worcester, England is firmly established as AD 1128 December 8. Assuming that the drawing was prepared fairly carefully, the angular diameters of the two sunspots are at least about 3 arcmin and 2 arcmin in the northern and southern hemispheres, respectively. Similarly, the heliographic latitudes of both sunspots are within the approximate range of 25°–35°. About five days after this observation of sunspots on the solar disk, on the night of AD 1128 December 13, a red auroral display was observed from Songdo, Korea (the modern city of Kaesong). This auroral observation was recorded in the Koryo-sa, the official Korean chronicle of the period. In addition, five Chinese and five Korean descriptions of auroral displays were recorded in various East-Asian histories between the middle of AD 1127 and the middle of AD 1129. The ten oriental auroral records in this particular interval correspond to six distinct auroral events, which provide evidence for recurrent, though possibly intermittent, auroral activity on a timescale almost exactly equal to the synodic-solar-rotation period (approximately 27 days). The six distinct auroral events were apparently associated with two series of recurrent geomagnetic storms, both of which were sufficiently intense to produce mid-latitude auroral displays in East Asia. These ancient solar and auroral observations are interpreted in terms of present-day understanding of solar-terrestrial physics. Con-temporary ground-based and satellite measurements during the last few decades have indicated that recurrent geomagnetic storms are usually a feature of the declining phase of the solar cycle. In addition, the strength of such recurrent geomagnetic storms has been classified as moderate rather than intense. The recurrent geomagnetic storms occurring during the interval AD 1127–1129 must have been sufficiently intense to produce mid-latitude auroral displays over China and Korea, some of which appeared or extended south of the observing site. This last statement remains true even after proper allowance is made for the fact that during the twelfth century, the north geomagnetic pole was probably situated at the usual high geographic latitude, but in the same geographic longitude range as East Asia. Therefore, it may be inferred that the two series of intense recurrent geomagnetic storms occurred near a medieval maximum in the "eleven-year" solar cycle. Moreover, the overall level of solar activity appears to have been especially high at the end of the second decade of the twelfth century.b>Key words. Magnetospheric physics (auroral phenomena; storms and substorms) – Solar physics, astrophysics and astronomy (photosphere and chromosphere)

Ionosphere‐magnetosphere simulation of small‐scale structure and dynamics

This work presents first results and the numerical methods of a highly improved two‐dimensional three‐fluid simulation model of the ionosphere‐magnetosphere system. The model considers ionization and recombination, ion‐neutral friction, the Hall term in Ohm's law, and various heat sources in the energy equations. The electrodynamic response and the evolution of the collision frequencies are treated self‐consistently in the height‐resolved ionosphere. This model is the first and to our knowledge the only simulation model that can resolve the dynamic and nonlinear electromagnetic interaction between the ionosphere and the magnetosphere. The simulation is aimed at modeling fast temporal and small spatial scale ionospheric structures associated, for instance, with filamentary aurora and ionospheric heating experiments. The results presented in this paper focus on ion and electron heating by different sources, i.e., ion heating due to plasma‐neutral friction, electron heating resulting from energetic particle precipitation and by ohmic dissipation in strong field‐aligned currents. This work is motivated by a specific auroral event that was observed simultaneously with optical and radar instruments. A consistent explanation of this event is possible in the presence of ohmic heating of electrons in a strong field‐aligned electric current layer.

Estimate of auroral electron spectra, the power of ground-based multi-station optical measurements

Ground-based multi-station optical imaging has a great inherent scientific potential for investigations of the magnetosphere-ionosphere-thermosphere interactions by using inversions of the ionospheric optical signal, the aurora.Two methods for estimating characteristics of primary auroral electron spectra are compared and used to describe an auroral event. One method uses the spectral information in the images and the other method is based on the inversion of the N2+1NG 4287 Å altitude distribution. With the second method ALIS can currently give estimates of the primary electron distribution with medium time resolution (10 s).The auroral event, a passage of an eastward moving fold in a pre-existing auroral arc, is analysed and characteristics of the precipitating electrons show regions with different fluxes, e.g. a soft region that has previously been found inside the fold appears to belong to a wider region of soft precipitation that emerges as the arc activates.

Field‐aligned currents between conjugate hemispheres

Interhemispheric field‐aligned currents flowing between dayside conjugate ionospheres are calculated for different seasons. The currents are a result of redistribution of original three‐dimensional currents because of different ionospheric conductivity in dark and sunlit conjugate hemispheres. Interhemispheric currents and related patterns for the electric potential distribution are computed for a reasonable model of the ionospheric conductivity. The interhemispheric field‐aligned currents are attached to the terminator position, and they close a part of ionospheric currents, flowing in the summer high‐latitude ionosphere, through the conjugate ionosphere of the opposite hemisphere. Although the interhemispheric currents are more significant for summer/winter conditions, they can be observed for equinoctial conditions as well because of a diurnal variation in the terminator position with respect to the geomagnetic poles. Interhemispheric currents in the winter ionosphere are generated mainly by sources in the summer hemisphere. An interesting consequence of the existence of interhemispheric currents is that auroral events can be observed not in the hemisphere with incident field‐aligned currents from a magnetospheric source but in the opposite hemisphere.

On the use of photometer data to map dynamics of the magnetotail current sheet during substorm growth phase

The Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) is a large‐scale ground‐based instrument array of remote sensing equipment monitoring the high‐latitude ionosphere from the north central to the northwest portion of North America. It provides realtime coverage of auroral events such as magnetospheric substorms. In this paper the variation of magnetotail thickness is estimated during substorm growth phase by using a simple magnetic field model and data from the meridian scanning photometers in the CANOPUS array. An important parameter in the modeling process is the κ parameter, defined as the square root of the minimum curvature radius to maximum Larmor radius ratio. Through use of this parameter we are able to grossly determine the expected proton precipitation regions in the ionosphere. Two examples presented (February 9, 1995, and March 9, 1995) show that crosstail current sheet thinning from 2–0.1 RE can occur during the course of a growth phase. Within the framework of our model it is demonstrated that the magnetotail field line stretching, associated with thinning of the current sheet, is correlated with the equatorward motion of Hβ auroral emissions. Also demonstrated is the Earthward movement of the inner edge of the plasma sheet to within 6 RE.

Dynamic cusp aurora and associated pulsed reverse convection during northward interplanetary magnetic field

We report a study of ionospheric signatures of plasma entry and momentum transfer at the dayside magnetopause during northward oriented interplanetary magnetic field (IMF), combining ground observations of the dayside aurora and ionospheric ion drift (CUTLASS HF radar) with simultaneous particle precipitation data obtained from three overflights by the Defence Meteorological Satellite Program (DMSP) F12, F13 and F14 spacecraft. The observations were taken during a 37‐min long interval of strongly northward IMF (Bz=7 nT; clock angle ∼10°–15°) after a rapid northward turning. The meridan scanning photometer at the ground station recorded a long stepwise poleward retraction and latitudinal widening of the band of auroral emission in the cusp region. Thus the activity includes a series of episodes which are characterized by an initial 1–2 min poleward “step” of the auroral poleward boundary, followed by a ∼3–4 min period of relatively steady auroral latitude. The auroral events were accompanied by bursts of “reverse” two‐cell convection characterized by equatorward flow across the cusp poleward boundary. The three DMSP spacecraft, which traversed the poleward boundary of the cusp aurora from north to south, entered into a region of auroral precipitation where electrons and ions of magnetosheath origin were present, together with equatorward convection. The observations are found to be consistent with a theoretical description of a sequence of bursts of lobe reconnection involving both hemispheres. This process results in the capture of magnetosheath flux tubes and thereby closed flux is added to the dayside magnetosphere.

Sunspot and Auroral Activity During Maunder Minimum

The extremely low sunspot activity during the period of the Maunder minimum 1645–1715 was confirmed by group sunspot numbers, a new sunspot index constructed by Hoyt and Schatten (1998a,b). Neither sunspots nor auroral data time behavior indicate the presence of 11-year solar cycles as stated by Eddy (1976). The evidence for solar cycles was found in the butterfly diagram, constructed from observations made at Observatoire de Paris. After Clivier, Boriakoff, and Bounar (1998) the solar cycles were reflected also in geomagnetic activity. Results are supported by the variation of cosmogenic isotopes 10Be and 14C. The majority of the observed 14 naked-eye sunspots occurred on days when telescopic observations were not available. A part of them appeared in the years when no spot was allegedly observed. Two-ribbon flares appear in plages with only very small or no sunspots. Some of these flares are geoactive. Most aurorae (90%), which were observed during the Maunder minimum, appeared in years when no spot was observed. Auroral events as a consequence of proton flares indicate that regions with enhanced magnetic field can occur on the Sun when these regions do not produce any sunspots.

Variations, with peak emission altitude, in auroral O2 atmospheric (1, 1)/(0, 1) ratio and its relation to other auroral emissions

Spectral distributions of auroral optical emissions, peaked at distinctly different heights in the thermosphere, show significant variations, with altitude, in the O2 atmospheric (1, 1)/(0, 1) band ratio. The latter increases with height in auroras peaked between 110 and 150 km and then gradually decreases at higher altitudes. To minimize ambiguities associated with auroral height determination needed for investigating this effect, four independent height‐assessment methods are employed. The first one is based on the incoherent scatter radar (ISR) soundings of the auroral ionization profile from which the height, where precipitating particles dissipate most of their energy, can be determined. Concurrent spectroscopic observations of the thermalized rotational distributions of auroral band emissions yield the ambient air temperature, and hence an independent assessment of the height, of the thermospheric region where these emissions peak. Changes in O/O2 and O/N2 ratios with height lead to changes in the ratios of auroral emissions, from these species, peaked at different heights. Finally, changes in collision frequency with height lead to changes in the brightness of the auroral emissions, resulting from radiatively allowed transitions relative to those produced from radiatively forbidden transitions. The four methods yield comparable values for the height of the thermospheric region where emissions, from each auroral event, peak. The observed variations in O2 atmospheric (1, 1)/(0, 1) with auroral height is compared with that expected from O (1D) + O2 excitation source and quenching by O2 and O. The effects of electron impact excitation of O2(b1∑g+, v′) and high rotational levels of the P branch of O2 atmospheric (0, 0) band on O2 atmospheric (1, 1)/(0, 1) ratio are discussed. Quantitative ratios of various auroral emissions, from O, N2, and N+2, peaked at different heights, that can provide an assessment of auroral heights where these emissions peak, are listed.

Dynamic fluid‐kinetic (DyFK) modeling of auroral plasma outflow driven by soft electron precipitation and transverse ion heating

We apply a recently developed dynamic fluid‐kinetic (DyFK) model to simulate and investigate the effects of soft auroral electron precipitation and perpendicular ion heating by waves on the plasma outflow along auroral field lines. The DyFK model is constructed by coupling a fluid ionospheric model for the region from 120 to 800 km to a semikinetic treatment for topside through several RE altitude region. This approach, which is described in detail here, allows a partially self‐consistent description of the plasma transport along high‐latitude flux tubes where both low‐altitude ionospheric heating and ionization production and loss as well as high‐altitude energization and kinetic effects are incorporated and stressed. In the present work, we investigate the combined effects of the F region plasma production and electron heating by soft auroral electron precipitation and ion perpendicular wave heating at high altitudes, which produces ion conies. The auroral event simulated here involves 1.5 hours of moderate soft electron precipitation and relatively weak ion cyclotron waves along the magnetic field lines. The simulations reveal the F region electron heating and ionization by the soft electron precipitation, driving a topside O+ upflow of up to 109 cm−2 s−1 below 1000 km within 30 min after the electron precipitation is turned on. The enhanced O+ upflow plumes would be still gravitationally bound in the absence of further energization at higher altitudes. However, the synergistic effects of the increased upwelling ion supply driven by the precipitation and the wave‐driven ion heating at higher altitudes combine to enhance O+ bulk outflow by an order of magnitude above the baseline polar wind level to a net outflow flux of 108 ions cm−2 s−1 with a density of 10 ions cm−3 and bulk velocity of 12 km s−1 at 3 RE altitude. Various O+ conic velocity distributions develop within 10 min after transverse heating is initiated, and their characteristic energies saturate at approximately 10 eV for the peak wave‐induced heating rates of 10−14 ergs s−1 at 2 RE here. H+ is also affected by the increases of O+ due to H+–O+ collisional drag in the 1000 – 4000 km altitude transition region. H+ flow is much less affected by the wave heating because of the faster transit times through the high‐altitude wave heating zone and the lower H+ perpendicular heating rates which were incorporated here. The H+ bulk flow consists of a flux of 108 ions cm−2 s−1, a density of 4 ions cm−3, and a velocity of 30 km s−1 at 3 RE altitude.

A high‐latitude observation of sporadic sodium and sporadic E‐layer formation

High‐resolution radar and lidar measurements of sporadic sodium (Na) and sporadic E (E) layers were made at the Sondrestrom incoherent‐scatter radar facility on 11 December 1997. These measurements suggest a causal link between Es and Nas, supporting the proposed mechanism in which Na+ ions in the Es are neutralized to form the Nas. This Nas, by contrast, does not appear to have been formed by the presence of auroral precipitation or ionization, and, in fact, the sodium density is seen to decrease during an auroral event.