UV Imager Research Articles

Relationship Between Substorm Auroras and Processes in the Near-Earth Magnetotail

Although the auroral substorm has been long regarded as a manifestation of the magnetospheric substorm, a direct relation of active auroras to certain magnetospheric processes is still debatable. To investigate the relationship, we combine the data of the UV imager onboard the Polar satellite with plasma and magnetic field measurements by the Geotail spacecraft. The poleward edge of the auroral bulge, as determined from the images obtained at the LHBL passband, is found to be conjugated with the region where the oppositely directed fast plasma flows observed in the near-Earth plasma sheet during substorms are generated. We conclude that the auroras forming the bulge are due to the near-Earth reconnection process. This implies that the magnetic flux through the auroral bulge is equal to the flux dissipated in the magnetotail during the substorm. Comparison of the magnetic flux through the auroral bulge with the magnetic flux accumulated in the tail lobe during the growth phase shows that these parameters have the comparable values. This is a clear evidence of the loading–unloading scheme of substorm development. It is shown that the area of the auroral bulge developing during substorm is proportional to the total (magnetic plus plasma) pressure decrease in the magnetotail. These findings stress the importance of auroral bulge observations for monitoring of substorm intensity in terms of the magnetic flux and energy dissipation.

Radio, Hard X-Ray, EUV and Optical Study of September 9, 2002 Solar Flare

The multi-wavelength analysis is performed on a flare on September 9, 2002 with data of Owens Valley Solar Arrays (OVSA), Big Bear Solar Observatory (BBSO), Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and Extreme UV Imager Telescope (EIT), and The Michelson Doppler Imager (MDI) on board of the Solar and Heliospheric Observatory (SOHO). The radio sources at 4.8 and 6.2 GHz located in the intersection of two flaring loops at 195 of SOHO/EIT respectively with two dipole magnetic fields of SOHO/MDI, in which one EIT loop was coincident with an X-ray loop of RHESSI at 12–25 keV, and two Hαbright kernels a1 and a2 of BBSO, respectively at the two footpoints of this loop; the second EIT loop connected another two Hαkernels b1 and b2 and radio sources at 7.8 and 8.2 GHz of OVSA. The maximum phase of microwave bursts was evidently later than that of hard X-ray bursts and Hαkernels a1 and a2, but consistent with that of Hαkernels b1 and b2. Moreover, the flare may be triggered by the interaction of the two flaring loops, which is suggested by the cross-correlation of radio, optical, and X-ray light curves of a common quasi-periodic oscillation in the rising phase, as well as two peaks at about 7 and 9 GHz of the microwave spectra at the peak times of the oscillation, while the bi-directional time delays at two reversal frequencies respectively at 7.8 and 9.4 GHz (similar to the peak frequencies of the microwave spectra) may indicate two reconnection sites at different coronal levels. The microwave and hard X-ray footpoint sources located in different EUV and optical loops may be explained by different magnetic field strength and the pitch angle distribution of nonthermal electrons in these two loops.

Excitation of Alfvén impulse by the anomalous resistance onset on the auroral field lines

Abstract. The onset of anomalous resistance in a layer on auroral field lines is shown to be accompanied by the excitation of an Alfvénic impulse (AI). The generated AI marks the transition of the global magnetosphere-ionosphere instability into an explosive phase with positive feedback. The spatial structure of this impulse both in space and on the ground has been described analytically and numerically under the thin layer approximation. The field-aligned currents transported by the Alfvénic impulse are concentrated near the edges of the layer with anomalous resistivity, whereas the reverse currents are spread throughout the layer. For some parameters of the layer the ionospheric attenuation of even small-scale structures is not dramatically large, so the magnetic response to the generated AI may be observed on the ground. The considered event on 3 January 1997 with magnetic-auroral intensification observed by ground magnetometers and Polar UV imager agrees with the model proposed.

The August 11th, 1999 CME

We present here a set of observations, space borne and ground based, at different wavelengths, of the solar corona at and after the time of the total solar eclipse of August 11th. It is used to consider some unusual features of the coronal dynamics related to a limb Coronal Mass Ejection (CME) observed after the total eclipse. The complementary aspect of simultaneous ground-based and space-borne observations of the corona is used to produce an accurate composite image of the White Light (W-L) corona before the CME. A high arch system (possibly a dome-like structure, with large cavities inside but without a cusp further out) which appeared on the eclipse W-L images, is suggested to be a large-scale precursor of the CME, well preceding the eruption of the top part of the brightest prominence recorded in W-L. This bright prominence is shown as a filament in absorption using the Transition Region And Corona Explorer (TRACE) images taken in different coronal lines. The analysis of the images of the Large Angle and Spectrometric Coronograph (LASCO) on board the Solar and Heliospheric Observatory (SoHO), showing the progression of the CME, is discussed in an attempt to make a connection with the surface event. A SoHO-EIT (Extreme UV Imager Telescope) image sequence details the prominence eruption and shows the sudden heating processes of the ejected parts. We found that there is no reason to assume that the huge cavity is significantly destabilised well before the eruption of the upper part of the low-lying bright twisted filament which coincides with the position of one of the legs of the high arch. Observations are still compatible with the assumption of both the break-out model and of the flux rope erupting model as a result of a shear or of an increasing poloidal magnetic flux from below. We stress the possible role of buoyancy of the giant cavity as a destabilizing factor leading to the CME, noticing that some motion of coronal material back toward the surface can be seen during at least the first phase of the CME, from both EIT and LASCO observations.

Transient reconnection in the cusp during strongly negative IMFBy

We investigate transient magnetic reconnection (flux transfer events, FTEs) in the cusp and its ionospheric signatures. We present in situ observations by the Polar spacecraft of multiple impulsive plasma injections and related magnetopause crossings in the outer stagnant cusp of the northern hemisphere. At this time the interplanetary magnetic field (IMF) was northward, dominated by strongBy(−11 nT). Plasma injections are observed in the direction of the magnetic tension force due to the IMFByand resemble the “time of flight” energy‐latitude dispersion. Associated magnetopause crossings exhibit large magnetic field rotation (>150 degrees) and strong depression of the field magnitude (down to several percent of the value in the magnetosheath). We use conjugate observations from the Greenland magnetometer network and Sondrestrom incoherent scatter radar together with the Polar UV imager to show that the transient reconnection causes a direct response of cusp auroral brightness and ionospheric convection in the direction consistent with the IMFBypolarity. The observations, in space and on the ground, are consistent with a sequence of bulges launched by transient reconnection. We present a qualitative model of the bulge structure that is consistent with most of the Polar observations during the studied interval. Conjugate ground‐based observations provide additional evidence for transient reconnection and are consistent with the ground signature of FTEs.

Direction finding of AKR sources with three orthogonal antennas

A method of the direction finding of auroral kilometric radiation (AKR) sources from board of a spinning spacecraft is presented. It uses relative power densities of radiation received on three orthogonal antennas with the assumption that the AKR is circularly polarized. The method is checked for a case of AKR source location provided by Polrad radio‐spectro‐polarimeter on board the Interball‐2 satellite. Projection of the source location along magnetic field lines coincides with the active auroral arc observed by the Polar UV imager.

Detailed analysis of a substorm event on 6 and 7 June 1989: 2. Stepwise auroral bulge evolution during expansion phase

We have analyzed in detail the auroral bulge evolution during the expansion phase of an isolated substorm, which was observed by the UV imager aboard the Akebono satellite. It was found that there were three distinct stages in the evolution. Stage 1 was characterized by rapid poleward and azimuthal (predominantly westward) expansions in a short time (about 2 min). Stage 2 was characterized by a very slow poleward and slower and continuous azimuthal expansions. There was a certain period for transition between stage 1 and stage 2, and it was characterized by a very slow poleward and rapid eastward expansions. Stage 3 started about 11 min after the onset and was characterized by a sudden reactivation of the rapid poleward and azimuthal expansions. The reactivation started around the initial onset meridian and then spread both eastward and westward. At the azimuthal front, the expansion first occurred at the lowest latitudes, spread poleward to around the highest latitudes of stage 1, and then spread further poleward after a brief interval. Hence, the local expansion also had three distinct stages similar to the global one. The ground‐based observations showed that the highest latitude of the local first stage was very close to the latitude of auroral activity that appeared near the ionospheric plasma sheet boundary layer (PSBL) region a few minutes before the onset. The further poleward expansion during the local third stage started with a significant intensification of the poleward‐most auroral activity. During the local third stage, the bright electron auroral region was bifurcated into a poleward expanding part and an equatorward moving part. The proton auroral emission coexisted in the bulge during the local first and second stages and almost disappeared soon after the bifurcation during the local third stage. Based on these observations, we discuss the evolution in the magnetosphere during the expansion phase.

Detailed analysis of a substorm event on 6 and 7 June 1989 1. Growth phase evolution of nightside auroral activities and ionospheric convection toward expansion phase onset

We have analyzed in detail the evolution of nightside auroral activities and ionospheric convection during the growth phase of an isolated substorm. The following three characteristic auroral activities were identified by meridian scanning photometers (MSP) at ground stations: (1) proton main oval, (2) fast equatorward moving (FEM) arc, and (3) near plasma sheet boundary layer (NPSBL) aurora. The proton main oval corresponds to the main part of the proton auroral region and was located at the equatorward‐most region of the electron auroral emissions and gradually moved equatorward during the growth phase. The FEM arc was a discrete auroral arc having a clear longitudinally elongated form. It appeared about 20 min before the onset on the high‐latitude side of the auroral region and moved equatorward toward the proton main oval. Its equatorward motion gradually slowed as it approached the proton main oval. As the FEM arc moved equatorward, the location of the peak velocity in the nightside return flow of the two‐cell convection also moved equatorward. The auroral breakup occurred in a localized area around the demarcation region between the two return flows, close to the latitude of the FEM arc. This was observed by the UV imager aboard the Akebono satellite. The NPSBL aurora appeared a few minutes before the onset around the poleward‐most region of the aurora and continued even after the onset. Our detailed analysis of these phenomena can give us important clues as to how the growth phase proceeds toward the expansion phase onset.

Solar wind dependence of the occurrence and motion of polar auroral arcs: A statistical study

Polar UV images from a 3‐month period in winter 1998–1999 are used for a statistical study of polar arcs. The study covers all auroral arcs that are located poleward of the northern auroral oval, and which are detectable by the UV imager. The arcs are examined with respect to their spatial and temporal behavior as well as to a possible connection to solar wind parameters using ACE satellite data. It is found that the majority of polar arcs appear during northward IMF, strong IMF magnitude, and high solar wind speed. A modified Akasofu‐Perreault epsilon parameter with a cosine function instead of a sine function (vB2 cos4(θ/2)(l02/μ0)) combines these results. It correlates well with the occurrence frequency of polar arcs for long timescales. The location of polar arcs is strongly dependent on the sign of the IMF By component. Static polar arcs occur in the Northern Hemisphere on the dawn (dusk) side of the oval for negative (positive) IMF By, whereas poleward‐moving arcs separate from the opposite side of the oval, and then move in the direction of IMF By. All polar arcs are sorted into five different categories according to their spatial structure and evolution: oval‐aligned, bending, moving, midnight, and multiple arcs. Each polar arc type occurs for a characteristic combination of solar wind parameters. IMF clock angle changes seem to have a strong influence on what type of arc occurs. Oval‐aligned arcs appear mainly during steady IMF, bending arcs after an IMF Bz sign change, and moving arcs after an IMF By sign change. For the rare midnight and multiple arc events, no characteristic IMF clock angle dependence has been found. The different types of clear polar arcs are discussed in the context of existing observational studies and transpolar arc models.

Auroral kilometric radiation source characteristics using ray tracing techniques

3‐D ray tracing to the presumed auroral kilometric radiation (AKR) source region has been performed using the input data from wave distribution function (WDF) based on the AKR waveforms recorded on board the Interball 2 satellite by the French wave experiment MEMO. Both the direction of the WDF maximum and the WDF form and angular size have been taken into account. Two instances of AKR emissions were observed on 28 January 1997 at 2037 and 2107 UT. Rays traced in R‐X mode out of the s/c point toward two different active regions on the auroral oval (as seen with Polar UV imager after projection of the source region along the magnetic field lines down to the ionosphere level). Source region apparent angular sizes based on WDF are compatible with sizes estimated from signal modulation produced by electric antenna system rotation.

Two‐dimensional structure of auroral poleward boundary intensifications

We investigate the two‐dimensional structure of auroral poleward boundary intensifications (PBIs). PBIs are a nightside auroral intensification that has been studied primarily with ground‐based meridian scanning photometers (MSPs). They have a signature that in the MSP data, appears as an increase in intensity at or near the magnetic separatrix and is often seen to extend equatorward. They are also associated with fast flows in the tail and are thus important to the dynamics of the plasma sheet. MSP data provide information about the temporal evolution of the aurora in one spatial dimension, in this case roughly along a magnetic meridian. This paper is motivated by a desire to determine the physics of PBIs that is revealed by their two‐dimensional structure. To do this, we have identified a number of PBI events in the CANOPUS Rankin Inlet and Gillam MSPs that occurred at times when high‐resolution, two‐dimensional images of the aurora over the same region were also available. The two‐dimensional images used in this study were obtained by the Freja UV imager, from October 1992 to January 1993, and by the CANOPUS Gillam all‐sky imager during the winter viewing season of 1996–1997. We find that PBIs, as observed by the MSPs, are either equatorward extending or non‐equatorward extending. Equatorward extending PBIs are either north‐south aligned structures or east‐west arcs propagating mostly equatorward, but we were not able to determine without doubt which type is the most prevalent. We suggest that equatorward extending PBIs may be the auroral footprint of two major modes of energy transfer in the plasma sheet: multiple, narrow, earthward fast‐flow channels in the plasma sheet and sequences of azimuthally broad and primarily earthward propagating phase fronts initiating near the separatrix. Nonequatorward extending PBIs are found to mostly be a series of multiple bead‐like intensifications along the poleward boundary of the aurora zone. Such PBIs may be evidence for shear instabilities at the separatrix boundary on the flanks of the magnetotail.

Excitation of twin-vortex flow in the nightside high-latitude ionosphere during an isolated substorm

Abstract. We present SuperDARN radar observations of the ionospheric flow during a well-observed high-latitude substorm which occurred during steady northward IMF conditions on 2 December 1999. These data clearly demonstrate the excitation of large-scale flow associated with the substorm expansion phase, with enhanced equatorward flows being observed in the pre-midnight local time sector of the expansion phase auroral bulge and westward electrojet, and enhanced return sunward flows being present at local times on either side, extending into the dayside sector. The flow pattern excited was thus of twin-vortex form, with foci located at either end of the substorm auroral bulge, as imaged by the Polar VIS UV imager. Estimated total transpolar voltages were ~40 kV prior to expansion phase onset, grew to ~80 kV over a ~15 min interval during the expansion phase, and then decayed to ~35 kV over ~10 min during recovery. The excitation of the large-scale flow pattern resulted in the development of magnetic disturbances which extended well outside of the region directly disturbed by the substorm, depending upon the change in the flow and the local ionospheric conductivity. It is estimated that the nightside reconnection rate averaged over the 24-min interval of the substorm was ~65– 75 kV, compared with continuing dayside reconnection rates of ~30–45 kV. The net closure of open flux during the sub-storm was thus ~0.4–0.6 × 108 Wb, representing ~15–20% of the open flux present at onset, and corresponding to an overall contraction of the open-closed field line boundary by ~1° latitude.Key words. Ionosphere (auroral ionosphere; ionosphere-magnetosphere interactions; plasma convection)

Interplanetary magnetic field control of theta aurora development

We ascertain the influence of the By component of the interplanetary magnetic field (IMF) on theta aurora evolution. During most cases where a transpolar arc is observed to move across the polar region, and form a theta aurora, there are brief (minutes) southward excursions of IMF Bz, however northward IMF is required prior to theta aurora formation. Observations show that theta aurora can form during strictly northward IMF with its motion consistent with a change in sign of IMF By. It is important to note that since transpolar arcs can persist for 20–30 min after the IMF turns southward, errors will occur in assigning instantaneous IMF conditions to “snapshots” of particular auroral patterns. We consider the entire evolution of the theta aurora and the changing IMF conditions. The influence of IMF By is best illustrated by examples which occur during steady northward IMF as compared to times when the IMF is northward on average. We show examples, provided by the Polar UV imager, when the IMF is steady northward. For one case, DMSP F13 and F14 provide in situ measurements of precipitating particles, ionospheric plasma flows and ion density. This unique data set enables us to analyze in detail the evolution of a theta aurora, in one case crossing the entire polar region. No sign change in Bz is needed for theta aurora formation.

Plasma sheet fast flows and auroral dynamics during substorm: a case study

Abstract. Interball-1 observations of a substorm development in the mid-tail on 16 December 1998 are compared with the auroral dynamics obtained from the Polar UV imager. Using these data, the relationship between plasma flow directions in the tail and the location of the auroral activation is examined. Main attention is given to tailward and earth-ward plasma flows, interpreted as signatures of a Near Earth Neutral Line (NENL). It is unambiguously shown that in the mid-plasma sheet the flows were directed tailward when the auroral bulge developed equatorward of the spacecraft ionospheric footprint. On the contrary, when active auroras moved poleward of the Interball-1 projection, earthward fast flow bursts were observed. This confirms the concept that the NENL (or flow reversal region) is the source of auroras forming the poleward edge of the auroral bulge. The observed earthward flow bursts have all typical signatures of Bursty Bulk Flows (BBFs), described by Angelopolous et al. (1992). These BBFs are related to substorm activations starting at the poleward edge of the expanded auroral bulge. We interpret the BBFs as a result of reconnection pulses occurring tail-ward of Interball-1. In addition, some non-typically observed phenomena were detected in the plasma sheet during this substorm: (i) tailward/earthward flows were superimposed on a very strong duskward flow, and (ii) wavy structures of both magnetic field and plasma density were registered. The latter observation is probably linked to the filamentary structure of the current sheet.Key words. Magnetospheric physics (auroral phenomena; plasma sheet; storms and substorms)

Cusp ion steps, field‐aligned currents and poleward moving auroral forms

We predict the field‐aligned currents around cusp ion steps produced by pulsed reconnection between the geomagnetic field and an interplanetary magnetic field (IMF) with a BY component that is large in magnitude. For BY > 0, patches of newly opened flux move westward and eastward in the Northern and Southern Hemispheres, respectively, under the influence of the magnetic curvature force. These flow directions are reversed for BY < 0. The speed of this longitudinal motion initially grows with elapsed time since reconnection, but then decays as the newly opened field lines straighten. We predict sheets of field‐aligned current on the boundaries between the patches produced by successive reconnection pulses, associated with the difference in the speeds of their longitudinal motion. For low elapsed times since reconnection, near the equatorward edge of the cusp region where the field lines are accelerating, the field‐aligned current sheets will be downward or upward in both hemispheres for positive or negative IMF BY, respectively. At larger elapsed times since reconnection, as events slow and evolve from the cusp into the mantle region, these field‐aligned current directions will be reversed. Observations by the Polar spacecraft on August 26, 1998, show the predicted upward current sheets at steps seen in the mantle region for IMF BY > 0. Mapped into the ionosphere, the steps coincide with poleward moving events seen by the CUTLASS HF radar. The mapped location of the largest step also coincides with a poleward moving arc seen by the UVI imager on Polar. We show that the arc is consistent with a region of upward field‐aligned current that has become unstable, such that a potential drop of about 1 kV formed below the spacecraft. The importance of these observations is that they confirm that the poleward moving events, as seen by the HF radar and the UV imager, are due to pulsed magnetopause reconnection. Milan et al. [2000] noted that the great longitudinal extent of these events means that the required reconnection pulses would have contributed almost all the voltage placed across the magnetosphere at this time. The observations also show that auroral arcs can form on open field lines in response to the pulsed application of voltage at the magnetopause.

Auroral precipitation power during substorms: A Polar UV Imager‐based superposed epoch analysis

The Polar UV Imager (UVI) is useful both for determining substorm onset times with precision on the order of l min and for making quantitative estimates of global power associated with auroral precipitation. Combining these capabilities, we studied 390 substorms, not necessarily isolated, using the superposed epoch analysis technique. The results quantify the phenomenology of auroral power during substorms. Precipitating auroral power rises with three distinct timescales based on distance from the average onset position. The most dramatic results are seen in the location of the auroral bulge, 2100–0000 magnetic local time (MLT). Integrated over this premidnight sector, auroral power decreases by ∼10% in the few minutes before onset. After onset, premidnight auroral power increases by a factor of 3.4 during the first 9 min. The largest increase occurs within 3 min after onset. After peaking at an average of ∼9 GW, premidnight auroral power declines at a steady 0.045 GW min−1 over ∼100 min. Less dramatic increases with longer risetimes are seen elsewhere on the nightside. Outside the LT of the bulge origin, but close enough that the bulge is likely to eventually reach it, the delay from onset to peak auroral power is ∼15–18 min, and the increase is typically in the range of a factor of 2 or 3. Finally, several hours away from the bulge, auroral power increases by less than a factor of 2, with a time delay of ∼36–45 min. Predawn, this represents the travel time of hot eastward drifting electrons in the diffuse aurora. Postdusk (1800–2100 MLT), a fairly weak (between 1.5 and 2.0 fold) increase is seen for unclear reasons. Altogether, a substorm averages 59% greater auroral power dissipation over the entire nightside in the 120 min following onset than in the preceding 120 min (152 and 96 TJ, respectively). By contrast, dayside auroral power is virtually invariant during substorms. Uncertainty in defining onset time using Polar UVI images was less than the 3‐min bin period adopted. Auroral onset is thus demonstrated to be well specified.

Substorm development as observed by Interball UV imager and 2-D magnetic array

Results of the study of two substorms from Interball auroral UV measurements and two-dimensional patterns of equivalent ionospheric currents derived from the MACCS/CANOPUS and Greenland magnetometer arrays are presented. Substorm development in 2-D equivalent ionospheric current patterns may be described in terms of the formation of two vortices in the equivalent currents: a morning vortex related to downward field-aligned current and an evening vortex related to upward field-aligned current. Poleward propagation of the magnetic disturbances during substorm expansive phase was found to be associated mainly with a poleward displacement of the morning vortex, whereas the evening vortex remained approximately at the same position. As a result, the initial quasi-azimuthal separation of the vortices was replaced by their quasi-meridional separation at substorm maximum. Interball UV images during this period showed the formation of a bright auroral border at the poleward edge of substorm auroral bulge. The auroral UV images showed also that the auroral distribution in the region between the polar border and the main auroral oval tends to have a form of bubbles or petals growing from a bright protuberant region on the equatorward boundary of the auroral oval. However, the resolution of the UV imager was not sufficient for the reliable separation of such the structures, therefore, this result should be considered as preliminary. Overlapping of the auroral UV images onto equivalent current patterns shows that the bright substorm surge was well collocated with the evening vortex whereas the poleward auroral border did not coincide with any evident feature in equivalent ionospheric currents and was located several degrees equatorward of the morning current vortex center related to downward field-aligned current. The ground-based magnetic array allowing us to obtain instantaneous patterns of equivalent ionospheric currents gives a possibility to propose a new index for substorm activity such as the magnitude of the total current between the centers of the morning and evening vortices. Such integral index would not depend on where the substorm is located and be unaffected by the migration of substorm activity poleward or equatorward.

Wideband bursts of auroral kilometric radiation and their association with UV auroral bulges

Impulsive wideband bursts of auroral kilometric radiation (AKR) observed from the Interball 2 spacecraft are characterized by a rapid rise of intensity (< l min) over a wide frequency range from 30 to 900 kHz, followed by a decay over a few minutes. Their source regions expand rapidly both upward and downward along the auroral field lines. The expansion velocity is determined from the rapid broadening of the frequency range of their leading edge (sometimes more than 100 kHz s−1). The drifts can be negative or positive. Above 500 kHz the inferred downward source expansion is sometimes as small as 2 km s−1 (at altitudes below 3000 km), much smaller than the velocity of the ion sound waves. In the medium frequency range 200 – 500 kHz (3000 – 6000 km) the expansion velocities are usually large, sometimes more than 3000 km s−1, comparable to the velocity of the Alfvén waves. In the lower frequency range, below 200 kHz (> 6000 km), the bursts show an upward source expansion, often simultaneous with the downward expansion of the same source below 3000 km. We observe a close association of the AKR bursts with the fast expansion of the auroral bulges recorded with the UV imager on Polar. It is concluded that the bursts are triggered during the rapid development of auroral substorms. The bursts show a microstructure, which consists of multiple microbursts lasting sometimes less than 6 s. They indicate that the source regions are filamented. This filamentation may be the counterpart of a filamentary nature of the currents closing the current wedge, flowing parallel to the geomagnetic field.

AKR bursts and substorm field line excitation

A new manifestation of the Auroral Kilometric Radiation is put in evidence through the observations of the POLRAD wave experiment of the INTERBALL mission, the Wide Band Bursts of the AKR, or WBB AKR. It consists in bursts of radiation with a very broad bandwidth, typically 100 – 800 kHz. The whole frequency range is excited at once or in less than few minutes. This corresponds to the excitation of long segments of auroral field lines in a time often less that one minute. The sources of the emission are stretched along the field lines between altitudes ranging from 2000 to 20000 km. The relationship of these bursts to the development of auroral bulges, in the UV spectral range, is shown by the comparison of the POLRAD wave observations and those of the UV imager of the POLAR mission. It is shown that these bursts are generated during fast expansion of the auroral bulge. An accurate timing of the burst events is made with the time evolution of the frequency integrated wave energy flux. It shows that the bursts have a rise time of few minutes which is followed by an exponential relaxation with a characteristic time of few tens of minutes. The source of the bursts first expands along the field lines then it shrinks during the relaxation phase. The bursts are triggered few minutes before the maximum intensity of the UV auroral bulge. It is also shown that the bursts actually consist of a large number of individual broad bandwidth elements, each, lasting less than few seconds.

Middle ultraviolet imager observations of the distribution of polar mesospheric clouds

Abstract Observations of the spatial structure and distribution of Polar mesospheric clouds (PMCs) in the northern and southern polar regions were made during the arctic summers of 1998–1999 and austral summer of 1997–1998 by a middle ultraviolet imager (235–263 nm). The imager look-point made multiple transpolar passes over the Arctic and Antarctic and obtained thousands of images of PMCs at latitudes poleward of 55°. The unprecedented accuracy and stability of the satellite platform (∼10 μrad) allowed investigation of both the large-scale (100 km) and small scale (∼1 km) spatial structures of PMCs. The UV imager readily observed PMCs distinct from the atmospheric backgrounds. The clouds existed in discrete, filamentary structures throughout the polar region above latitudes of ∼55°. Vertically, PMCs generally lie between altitudes of 82.0 and 83.0 km, but within this range, cloud altitudes are essentially random on transpolar scales of ∼1000 km. In some instances, the clouds cluster at common altitudes for distances of ∼100 km. The PMC altitudes do not appear correlated with latitude or local time. A diffuse signal above the clouds, possibly related to Polar Mesospheric Summer Echoes, was observed in some cases. Horizontally, PMCs exhibit structure on scales of ∼50 km to ∼1000 km that is probably related to gravity wave activity in the polar region. The horizontal periodicities of the PMCs can remain stable for at least 24 hours.