Meridian Scanning Photometer Research Articles

Temperature and airglow brightness oscillations in the polar mesosphere and lower thermosphere

Large amplitude oscillations in airglow brightness and OH rotational temperature were observed on Dec. 31, 1993, 00 UT to Jan. 1, 1994, 18 UT over Eureka (80.0°N). The airglow brightnesses of atomic oxygen O[ 1S] emission at 5577 Å and sodium Na ( 2 P 3 2 , 1 2 ) emission at 5890 and 5896 Å were measured by a multi-channel, meridian scanning photometer. The Meinel OH (3,1) band brightness and rotational temperature were monitored by a Michelson interferometer. The period of the observed oscillations was 8.4±1 h. Supporting evidence for a non-migrating tide (zero zonal wave number) explanation of these oscillations are an observed period close to 8 h, a large vertical wavelength as derived from the airglow emissions, and wave persistence for five complete cycles. On the other hand, the small amplitude and negative phase of Krassovsky's ratio are not consistent with a tidal source for the observed oscillations. Similar 8.5±1 h variations were observed on Dec. 21, 1993. The case for a non-migrating tide as a source for these oscillations is further weakened by a 1.5 h difference in local time when these oscillations were observed to peak during the two events (which occurred 10 days apart). Explanation of the above observations in terms of an inertiogravity wave is favored by the relatively infrequent appearance of these oscillations at all altitudes monitored, the incoherency between the two events, and the fact that temperature variations lead OH airglow brightness variations, in qualitative agreement with gravity wave theory. The observed lower limit to the horizontal wavelength of the wave compares with its value predicted by the dispersion relation of inertio-gravity waves. Forced planetary waves, observed to peak in amplitude on Dec. 21, 1993, and Jan. 1, 1994, as well as the stratospheric warming of the middle of December and end of December/beginning of January, were likely driven by the same tropospheric disturbance or directly related to the source of the inertio-gravity wave observed over Eureka on Dec. 21 and Dec. 31-Jan. 1, respectively.

Simultaneous measurements of large vertical winds in the upper and lower thermosphere

High resolution vertical wind measurements of the upper and lower thermosphere were made at Poker Flat, Alaska, using a scanning Fabry-Perot spectrometer (FPS). Observations of the λ558 and λ630 nm emissions of atomic oxygen were made on 21 nights and allowed for the simultaneous determination of wind and temperature at altitudes of about 130 and 240 km, respectively. On two occasions, significant upwelling events were measured which lasted between 15 and 25 min. Peak velocities were up to 42 m/s at 130 km and 138 m/s at 240 km. Auroral activity was monitored using a meridian scanning photometer (MSP). On both occasions, the region of upwelling was located on the poleward side of the auroral oval during geomagnetically active conditions. A schematic model is used to describe an event from which the horizontal scale of the upwelling region is estimated to be less than 320 km in the lower thermosphere and less than 800 km in the upper thermosphere.

Canopus ? A ground-based instrument array for remote sensing the high latitude ionosphere during the ISTP/GGS program

Proper interpretation ofin situ satellite data requires a knowledge of the global state of the magnetosphere-ionosphere system. CANOPUS is a large-scale array of remote sensing equipment monitoring the high latitude ionosphere from the north-central to the north-west portion of North America. The array comprises thirteen magnetometers and riometers four meridian scanning photometers, a digital allsky imager and an auroral radar linked by geostationary satellite to a central receiving node in Ottawa, where the data are archived and made available in near real time to participating scientists. This paper provides a technical description of the various instruments in the CANOPUS array, and contains a summary of the key parameters which will be provided to the Central Data Handling Facility (CDHF) located at NASA/Goddard Space Flight Center, for use by the ISTP/GGS community.

Auroral latitude Pc 5 field line resonances: Quantized frequencies, spatial characteristics, and diurnal variation

On the basis of observations with the Goose Bay Radar, several stations of the the Canadian Auroral Network for the Origin of Plasmas in the Earth's Neighborhood (OPEN) Unified Study (CANOPUS) magnetometer array, and a CANOPUS meridian scanning photometer, recent work has indicated the existence of Pc 5 ULF waves with discrete and remarkably stable frequencies (1.3, 1.9, 2.7 and 3.3 mHz with <10% variation) in the local midnight and early morning (predawn) sectors. These ULF waves were interpreted as the consequence of field line resonances driven by MHD waveguide/cavity modes, possibly excited by impulsive stimulation of the magnetopause or by Kelvin‐Helmholtz instabilities in the low‐latitude boundary layer. However, the reported stability of these frequencies over long periods of time remains a puzzle, as it seems to be inconsistent with the dynamic behavior of the magnetosphere. In order to establish if these frequencies are also observed in the dayside magnetosphere, where one of the proposed source mechanisms predicts they should be seen, we have examined the temporal and spatial characteristics of Pc 5 pulsations recorded recently with the CANOPUS magnetometer array. Pure state filtering techniques were used to determine the Pc 5 signal characteristics and their temporal and spatial variation. The results show that ULF wave signals with quantized frequencies seem to be observed throughout the dayside and dusk magnetosphere. However, frequencies both higher than and between previously reported values were also seen. The difficulty in correctly determining fundamental frequencies and their stability in the presence of polychromatic, highly amplitude‐modulated signals is noted. The signal characteristics observed in the postnoon sector were found to be significantly different compared with those seen at prenoon. While local morning and noon Pc 5 revealed typical field line resonance characteristics, toroidal mode signal characteristics prevailed in the local afternoon and evening. The reason for this asymmetry is unknown. The azimuthal wave numbers were generally small and showed a linear dependence on the Pc 5 signal frequency, implying antisunward phase velocities independent of the wave frequency.

Ground and satellite observations of postdawn aurorae near the time of a sudden storm commencement

Meridian scanning photometer measurements taken in the magnetic postdawn sector at Longyearbyen, Svalbard, between 0300 and 0630 UT on December 29, 1981, are analyzed in conjunction with particle and field data retrieved during two near passes of the Dynamics Explorer 2 (DE 2) satellite. The interval included a sudden storm commencement (SSC) at 0455 UT. Pre‐SSC optical and particle measurements showed a system of arcs that are spaced at ∼1.1° intervals in magnetic latitude, embedded within the region 1 current system and span the convection reversal. The softer particle precipitation appears to have a source near the flanks of the magnetotail while the harder, more equatorward precipitation originates closer to Earth. During the SSC period the entire sky brightened, with enhanced 630.0‐nm emissions extending from the northern horizon to south of magnetic zenith; intense but spatially separated 557.7‐nm emissions dominated the southern horizon. DE 2 detected more than an order of magnitude increase and near isotropization of ring current electron fluxes, enhanced precipitation from the plasma sheet and significantly decreased auroral zone convection. Region 1/region 2 currents remained, with wavelike structures superposed. A dual timescale response to the SSC is consistent with ground and satellite measurements. On few minute travel timescales for hydromagnetic waves to pass through the system, magnetospheric particles accelerate and precipitate to increase the ionospheric conductivity. Global, field‐aligned currents change more slowly. To maintain similar field‐aligned currents with higher ionospheric conductances requires reduced electric fields. After 0520 UT the optical emissions settled into stable, but latitudinally separated, bands of 630.0‐ and 557.7‐nm emissions characteristic of cleft and plasma sheet precipitation, respectively.

Auroral Emissions at the North Magnetic Pole-A February 17, 1993 Case Study.

During the first 12 1/2 hours of February 17, 1993, 21 apparently-isolated events of discrete polar arcs were recorded in broad-band visual light by an all-sky camera and in the blue, green, and red lines by a meridian scanning photometer at Eureka, an observatory near the north magnetic pole (89° magnetic latitude). The polar arcs were located almost exclusively in the dusk sector. Most of the arcs (>60%) were faint (≤1 kR in the blue, green, or red lines). The remainder were bright arcs (1-17 kR in the green) associated with energy fluxes of 1-7 erg cm-2s-1 and characteristic energies of ∼0.6 keV. The average lifetime of an individual polar arc was 8 minutes. One of the 21 identified events of polar arcs was associated with diffuse emissions duskward of the arc. The faint (0.5 kR in the green) diffuse emissions extended from the dusk horizon of the camera up to the arc. They were associated with very soft (E0 < 0.4 keV) and low flux (0.2 erg cm-2s-1) particle precipitation. These emissions were probably the optical manifestation of particle entry phenomena reported in the past, namely the location of transpolar arcs at the poleward edge of continuous soft particle precipitation. The lifetime of the diffuse emission (40 min) was much longer than the lifetime of the polar arc (6 min) and it did not follow the duskward motion of the arc. We show a time sequence of a new phenomenon regarding sun-aligned arcs-short lived, antisunward propagating intensifications in the form of bright spots spread over a very small spatial scale along the arc.

Low-Latitude Auroras Observed at Moshiri and Rikubetsu (L=1.6) during Magnetic Storms on February 26, 27, 29, and May 10, 1992.

This paper reports latitudinal and longitudinal movements of four low-latitude auroras observed by a meridian scanning photometer and an all-sky TV camera at Moshiri and Rikubetsu (L = 1.6) in Japan during magnetic storms. It is observationally found that the low-latitude auroras occur in the region of L ∼ 2 even during moderate magnetic storms. The auroras which are characterized by 6300-A emissions of several kR are also found to take place associated with magnetospheric substorm activity during the maximum phase of magnetic storms. The auroras and associated current systems inferred from ground magnetic field fluctuations tend to expand from midnight toward both the dawnside and the duskside. It is suggested that the observed low-latitude auroras are excited by precipitating low-energy electrons originated in the plasmasphere. We discuss generation mechanisms of these electrons based on the observations.

Stereoscopic auroral intensity measurements from ground and space

Simultaneous measurements of auroral intensities from below and above the emitting layer can be used to verify imager orientation and mapping procedures for satellite auroral images, determine auroral height by stereoscopic methods, and deduce incident particle characteristics. A data reduction approach that achieves optimal spatial and temporal matching of ground and space data sets is presented. We investigate a January 25, 1987, case study of a bright auroral arc using a meridian scanning photometer and an all‐sky camera located at Rabbit Lake, Saskatchewan, and a space imager (AIRS of Polar BEAR). The ground and space latitudinal profiles of arc intensity match best for a peak deposition height of 122±7 km. Theoretical calculations indicate that the observed FUV intensities are due to electrons of average energy 3.1±1.1 keV and a total energy flux of 5.7±1.5 ergs cm−2 s−1.

A rotating, midday auroral event with northward interplanetary magnetic field

We present ground‐based observations of an isolated, daytime auroral event that was detected above Svalbard by means of an all‐sky TV camera and a multichannel, meridian scanning photometer. This 5‐min event occurred near magnetic noon, poleward of a stable cusp aurora. It emitted ∼20 kR of 557.7‐nm light, with little if any admixture of 630.0‐nm radiation. The structure rotated rapidly then underwent a sudden breakup. The spectral distribution of emitted light indicates that electrons responsible for this auroral structure were accelerated though several kilovolts between the magnetosheath and the ionosphere. Such a potential structure requires that the precipitating, arc electrons constitute a field‐aligned current out of the ionosphere. Current continuity and Ohm's law require radial electric fields within the arc. From the structure's rotational speed, we estimate that the accelerated electrons carried an upward field‐aligned current of order 100µA/m². Satellite observations of polar cap convection patterns suggest that the interplanetary magnetic field had turned northward prior to the event. The auroral structure may be explained as resulting from a transitory magnetic merging of interplanetary and lobe magnetic field lines at and/or a penetration of plasma across the high‐latitude magnetopause.

Variability of dayside convection and motions of the cusp/cleft aurora

We present measurements of the ionospheric plasma flow over the range of invariant latitudes 71–76°, observed at 10‐second resolution using both the EISCAT radars, with simultaneous observations of the 630 nm cusp/cleft aurora made by a meridian‐scanning photometer at Ny Ålesund, Svalbard. A major increase in the trans‐auroral voltage from 5 to 40 kV (associated with sunward convection in the early afternoon sector) is found to follow a southward motion of the aurora and coincide with the onset of regular transient auroral breakup events. It is shown that these observations are consistent with recent theoretical work on how ionospheric flows are excited by time‐dependent reconnection at the dayside magnetopause.

Proton aurora and substorm intensifications

Ground based measurements from the CANOPUS array of meridian scanning photometers and precipitating ion and electron data from the DMSP F9 satellite show that the electron arc which brightens to initiate substorms intensifications is formed within a region of intense proton precipitation that is well equatorward (∼4–6°) of the nightside open‐closed field line boundary. The precipitating protons are from a population that is energized via Earthward convection from the magnetotail into the dipolar region of the magnetosphere and may play an important role in the formation of the electron arcs leading to substorm intensifications on dipolelike field lines.

Substorm intensifications and field line resonances in the nightside magnetosphere

Magnetometer and HF radar data often indicate the presence of magnetohydrodynamic, field line resonances in the nightside magnetosphere. These resonances have frequencies of about 1.3, 1.9, 2.6, and 3.4 mHz and are due to cavity modes or waveguide modes which form between the magnetopause and turning points on dipolelike magnetic shells. Energy from these cavity modes tunnels to the field line resonances which are seen in the F region by the HF radar and on the ground by the magnetometers. The presence of these field line resonances gives us an excellent diagnostic tool for determining the position of the mechanism leading to the energetic electrons and field‐aligned currents associated with substorm intensifications and auroral brightening. Using data from the Canadian CANOPUS array of magnetometers, meridian scanning photometers, riometers, and bistatic auroral radars and data from the Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay in Canada, we have identified a number of intervals in which substorm intensifications occurred during times when field line resonances existed in the region of the magnetosphere where the intensification occurred. In the events that we have analyzed in detail, the ionospheric signatures of the substorm intensification began equatorward (earthward) of existing field line resonances. These observations give very strong evidence indicating that at least one component of the substorm mechanism must be active very close to the Earth, probably on dipolelike field lines in regions with trapped and quasi‐trapped energetic particles. Furthermore, the auroral intensifications started near the position of one of the equatorward resonances, indicating that the field line resonances may play a role in triggering or producing the substorm intensifications. One possible scenario is mode conversion to kinetic Alfvén waves in the resonance.

The aries auroral modelling campaign: characterization and modelling of an evening auroral arc observed from a rocket and a ground-based line of meridian scanners

An auroral arc system excited by soft electrons was studied with a combination of in situ rocket measurements and optical tomographic techniques, using data from a photometer on a horizontal, spinning rocket and a line of three meridian scanning photometers. The ground-based scanner data at 4709, 5577, 8446 and 6300 Å were successfully inverted to provide a set of volume emission rate distributions in the plane of the rocket trajectory, with a basic time resolution of 24 s. Volume emission rate profiles, derived from these distributions peaked at about 150 km for 5577 and 4709 Å, while the 8446 Å emission peaked at about 170 km with a more extended height distribution. The rocket photometer gave comparable volume emission rate distributions for the 3914 Å emission as reported in a separate paper by McDade et al. (1991, Planet. Space Sci. 39, 895). Instruments on the rocket measured the primary electron flux during the flight and, in particular, the flux precipitating into the auroral arc overflown at apogee (McEwen et al., 1991; in preparation). The local electron density and temperature were measured by probes on the rocket (Margot and McNamara (1991; Can. J. Phys. 69, 950). The electron density measurements on the downleg were modelled using ion production rate data derived from the optical results. Model calculations of the emission height profile based on the measured electron flux agree with the observed profiles. The height distribution of the N 2 + emission in the equatorward band, through which the rocket passed during the descent, was measured by both the rocket and the ground-based tomographic techniques and the results are in good agreement. Comparison of these profiles with model profiles indicates that the exciting primary spectrum may be represented by an accelerated Maxwellian or a Gaussian distribution centered at about 3 keV. This distribution is close to what would be obtained if the electron flux exciting the poleward form were accelerated by a 1–2 kV upward potential drop. The relative height profiles for the volume emission rate of the 5577 Å OI emission and the 4709 Å N 2 + emission were almost indistinguishable from each other for both the forms measured, with ratios in the range 38–50; this is equivalent to I(5577) I(4278) ratios of 8–10. The auroral intensities and intensity ratios measured in the magnetic zenith from the ground during the period before and during the rocket flight are consistent with the primary electron fluxes and height distributions measured from the rocket. Values of I(5577) I(4278) in the range 8–10 were also measured directly by the zenith ground photometers over which the arc system passed. These values are slightly higher than those reported by Gattinger and Vallance-Jones (1972) and this may possibly indicate an enhancement of the atomic oxygen concentration at the time of the flight. Such an enhancement would be consistent with our result, that the observed values of I(5577) and I(8446) are also significantly higher than those modelled on the basis of the electron flux spectrum measured at apogee.

Observations of a detached, discrete arc in association with field line resonances

Data from the Canadian Auroral Network for the OPEN Unified Study (CANOPUS) array in Canada are used to analyze the magnetic fields and auroral structures which were associated with a field line resonance which occurred near local midnight and in the early morning sector. The electric fields of this resonance, which have a frequency of about 1.95 mHz, were observed by the Johns Hopkins University, Applied Physics Laboratory HF radar at Goose Bay and had a maximum at about 70.7° invariant latitude. Effects of the field line resonance were seen in the 5577 Å, meridian scanning photometer data and the magnetometer data from the CANOPUS array. The field line resonance was accompanied by precipitating energetic electrons (energies greater than 3 keV) in a narrow auroral arc which was 3°–4° equatorward of the resonance structure seen in the radar data and approximately 1° equatorward of field lines threading the inner boundary of the proton plasma sheet. The electron precipitation in this arc was modulated at the frequency of the field line resonance. The effects of the resonance are also seen as pulsations in the magnetometer data, and the horizontal polarization of the pulsations showed a change in sense of rotation across the arc. The oscillations in the precipitating electrons might have been caused by modulation in the ELF/VLF growth rates due to the presence of the magnetohydrodynamic waves associated with the resonance, by mode conversion to kinetic Alfvén waves, or by the formation of electrostatic ion cyclotron waves due to field‐aligned currents associated with a second field line resonance collocated with the arc. The evidence presented here suggests that the modulation of ELF/VLF by magnetohydrodynamic waves on field lines near the plasmapause was the most likely cause of the auroral oscillations.

Auroral bright spot sequence near 1400 MLT: Coordinated optical and ion drift observations

Optical observations of a dayside auroral brightening sequence, by means of all‐sky TV cameras and meridian scanning photometers, have been combined with EISCAT ion drift observations within the same invariant latitude‐MLT sector. The observations were made during a January 1989 campaign by utilizing the high F region ion densities during the maximum phase of the solar cycle. The characteristic intermittent optical events, covering ∼300 km in east‐west extent, move eastward (antisunward) along the poleward boundary of the persistent background aurora at velocities of ∼1.5 km s−1 and are associated with ion flows which swing from eastward to westward, with a subsequent return to eastward, during the interval of a few minutes when there is enhanced auroral emission within the radar field of view. The breakup of discrete auroral forms occurs at the reversal (negative potential) that forms between eastward plasma flow, maximizing near the persistent arc poleward boundary, and strong transient westward flow to the south. The reported events, covering a 35 min interval around 1400 MLT, are embedded within a longer period of similar auroral activity between 0830 (1200 MLT) and 1300 UT (1600 MLT). These observations are discussed in relation to recent models of boundary layer plasma dynamics and the associated magnetosphere‐ionosphere coupling. The ionospheric events may correspond to large‐scale wave like motions of the low‐latitude boundary layer (LLBL)/plasma sheet (PS) boundary. On the basis of this interpretation the observed spot size, speed and repetition period (∼10 min) give a wavelength (the distance between spots) of ∼900 km in the present case. The events can also be explained as ionospheric signatures of newly opened flux tubes associated with reconnection bursts at the magnetopause near 1400 MLT. We also discuss these data in relation to random, patchy reconnection (as has recently been invoked to explain the presence of the sheathlike plasma on closed field lines in the LLBL). In view of the lack of IMF data, and the existing uncertainty on the location of the open‐closed field line boundary relative to the optical events, an unambiguous discrimination between the different alternatives is not easily obtained.

Auroral and plasma flow transients at magnetic noon

We present observations of a transient event in the dayside auroral ionosphere at magnetic noon. F-region plasma convection measurements were made by the EISCAT radar, operating in the beamswinging “Polar” experiment mode, and simultaneous observations of the dayside auroral emissions were made by optical meridian-scanning photometers and all-sky TV cameras at Ny Ålesund, Spitzbergen. The data were recorded on 9 January 1989, and a sequence of bursts of flow, with associated transient aurora, were observed between 08:45 and 11:00 U.T. In this paper we concentrate on an event around 09:05 U.T. because that is very close to local magnetic noon. The optical data show a transient intensification and widening (in latitude) of the cusp/cleft region, as seen in red line auroral emissions. Over an interval of about 10 min, the band of 630 nm aurora widened from about 1.5° of invariant latitude to over 5° and returned to its original width. Embedded within the widening band of 630 nm emissions were two intense, active 557.7 nm arc fragments with rays which persisted for about 2 min each. The flow data before and after the optical transient show eastward flows, with speeds increasing markedly with latitude across the band of 630 nm aurora. Strong, apparently westward, flows appeared inside the band while it was widening, but these rotated round to eastward, through northward, as the band shrunk to its original width. The observed ion temperatures verify that the flow speeds during the transient were, to a large extent, as derived using the beamswinging technique; but they also show that the flow increase initially occurred in the western azimuth only. This spatial gradient in the flow introduces ambiguity in the direction of these initial flows and they could have been north-eastward rather than westward. However, the westward direction derived by the beamswinging is consistent with the motion of the colocated and coincident active 557.7 nm arc fragment, A more stable transient 557.7 nm aurora was found close to the shear between the inferred westward flows and the persisting eastward flows to the North. Throughout the transient, northward flow was observed across the equatorward boundary of the 630 nm aurora. Interpretation of the data is made difficult by lack of IMF data, problems in distinguishing the cusp and cleft aurora and uncertainty over which field lines are open and which are closed. However, at magnetic noon there is a 50% probability that we were observing the cusp, in which case from its southerly location we infer that the IMF was southward and many features are suggestive of time-varying reconnection at a single X-line on the dayside magnetopause. This IMF orientation is also consistent with the polar rain precipitation observed simultaneously by the DMSP-F9 satellite in the southern polar cap. There is also a 25% chance that we were observing the cleft (or the mantle poleward of the cleft). In this case we infer that the IMF was northward and the transient is well explained by reconnection which is not only transient in time but occurs at various sites located randomly on the dayside magnetopause (i.e. patchy in space). Lastly, there is a 25% chance that we were observing the cusp poleward of the cleft, in which case we infer that IMF B z was near zero and the transient is explained by a mixture of the previous two interpretations.

Dayside auroral characteristics as observed from Svalbard.

Observations of dayside aurora from Svalbard recorded by multichannel meridian scanning photometers and all-sky video cameras from four days in December 1988 are presented along with magnetometer data from the Svalbard-North Norway chain of stations. These examples indicate that the midday aurora over Svalbard can be separated in different latitudinal regions, possibly as signatures of the magnetic cusp and cleft. The cusp auroral signature is characterized by stable 630.0nm emissions and weak 557.7nm emissions, although transient discrete forms may occur in this region. Within the more extended cleft, discrete auroral structures are more common. Video camera observations reveal eastward drift of 4km/s in one of these sporadic events, associated with a magnetic disturbance propagating at the same speed. The activity level of long period (several minutes) irregular magnetic pulsations increases as the dayside aurora approaches the zenith or during intensity enhancements.

Interplanetary magnetic field control of dayside auroral activity and the transfer of momentum across the dayside magnetopause

The orientation of the Interplanetary Magnetic Field (IMF) during transient bursts of ionospheric flow and auroral activity in the dayside auroral ionosphere is studied, using data from the EISCAT radar, meridian-scanning photometers, and an all-sky TV camera, in conjunction with simultaneous observations of the interplanetary medium by the IMP-8 satellite. It is found that the ionospheric flow and auroral burst events occur regularly (mean repetition period equal to 8.3 ± 0.6 min) during an initial period of about 45 min when the IMF is continuously and strongly southward in GSM coordinates, consistent with previous observations of the occurrence of transient dayside auroral activity. However, in the subsequent 1.5 h, the IMF was predominantly northward, and only made brief excursions to a southward orientation. During this period, the mean interval between events increased to 19.2 ± 1.7 min. If it is assumed that changes in the North-South component of the IMF are aligned with the IMF vector in the ecliptic plane, the delays can be estimated between such a change impinging upon IMP-8 and the response in the cleft ionosphere within the radar field-of-view. It is found that, to within the accuracy of this computed lag, each transient ionospheric event during the period of predominantly northward IMF can be associated with a brief, isolated southward excursion of the IMF, as observed by IMP-8. From this limited period of data, we therefore suggest that transient momentum exchange between the magnetosheath and the ionosphere occurs quasi-periodically when the IMF is continuously southward, with a mean period which is strikingly similar to that for Flux Transfer Events (FTEs) at the magnetopause. During periods of otherwise northward IMF, individual momentum transfer events can be triggered by brief swings to southward IMF. Hence under the latter conditions the periodicity of the events can reflect a periodicity in the IMF, but that period will always be larger than the minimum value which occurs when the IMF is strongly and continuously southward.

A transient auroral event on the dayside

On December 5, 1986, high‐latitude magnetometer stations in Greenland, as well as Iqaluit and the South Pole, showed a strong perturbation lasting for about 10 min beginning at 0930 UT in an otherwise quiet period. A pair of field aligned currents separated in the east‐west sense and moving westward (tailward) at 4–5 km/s is consistent with the data, producing a twin vortex pattern of Hall currents. Similar perturbations, but with reduced intensity, were also recorded on the afternoon side at Svalbard, Heiss Island, and several locations in northern Siberia. The perturbation was also observed with the incoherent scatter radar at Sondrestrom, these data agreeing with the twin vortex pattern. The perturbation was accompanied by auroral forms overhead at Sondrestrom that also traveled westward. Meridian scanning photometer recordings at the radar site showed the cleft, located about 3° to 5° poleward in latitude; the cleft did not move from the far northern sky for several hours, even while the disturbance was observed. Viking and Polar Bear satellites passed just before the disturbance over Greenland and DMSP encountered the disturbance near Baffin Island a few minutes later; these spacecraft observations increased our confidence in the interpretation of the data. ISEE 1/2 and IMP 8 recorded a magnetic disturbance in the solar wind, the likely cause of this event. Similar observations by others have been associated with flux transfer events. However, since the observed event occurred on closed field lines, our interpretation is quite different, it is that an impulsive penetration of solar wind plasma on an interplanetary magnetic flux tube took place through the magnetopause, ending up in the low latitude boundary layer. Some efficient mechanism is required to feed the boundary layer with the total amount observed. Other events reported in the literature may have a similar explanation.

Interpretation of complicated discrete arc structure and behavior in terms of multiple X lines

In a neutral sheet with a positive Bz, X lines and O lines occur in pairs. If it is assumed that discrete auroral arcs are the mapping along field lines of X line‐O line pairs, then much of the complicated structure and behavior of discrete arcs can be readily explained. We apply this model to representative events in nightside data from a meridian‐scanning photometer chain showing the growth, expansive, and recovery phases of two substorms, all‐sky imager data, and dayside data including Viking UV images and also to a Defense Meteorological Satellite Program photograph of auroral activity. Features explained include the appearance, brightening, fading, disappearance, dividing of one arc into two, the uniting of two arcs into one, branching of arcs, two classes of short‐lived rapidly propagating (one poleward, the other equatorward) arcs, a newly identified basic substorm intensification structure, and the reaction of two arcs to a westward traveling surge. All these features can be explained in terms of forward or reverse merging at one or more X lines. Forward merging is defined as the usual flow direction for nightside merging, and reverse merging is with the flow through the X line reversed. Which is occurring at a particular time at a given X line depends on energy considerations (that is, boundary conditions). The motion of an arc is the resultant of a poleward velocity due to flux transfer through the X line and arc and the large‐scale equatorward convection (E × B/B² drift). Magnetic islands which become entrapped in dipolar fields dissipate by fast merging with rapid motion of the island and associated arc. These explain the short‐lived rapidly propagating arcs. On the nightside these islands are a likely explanation of substorm injection events.