P. E. Sandholt

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 classification of dayside auroral forms and activities as a function of interplanetary magnetic field orientation

We present a classification of auroral forms in the dayside high - latitude ionosphere, based on ground observations from Svalbard. Having sorted the different auroral forms by magnetic local time (MLT) and morphological and optical spectral characteristics, we then study them as a function of the orientation of the interplanetary magnetic field (IMF). We find that the IMF clock angle θ is a good parameter with which to order the different dayside auroras. This is illustrated by two case examples covering the whole dayside: (1) the 4-hour-long passage of the sheath region of the January 10 – 11, 1997, magnetic cloud and (2) a 10-hour-long interval on January 12, 1997, during passage of the corotating stream overtaking the cloud. A variety of IMF conditions were realized. We identify the following three auroral configurations in the cusp region and the IMF clock angle regimes in which they occur: (1) In the clock angle range θ ∼ 90° the high - latitude aurora disappears, and only the low - latitude forms remain. These latter forms manifest themselves as quasiperiodic sequences of moving bands or band fragments within ∼ 73° – 78°MLAT (called poleward moving auroral forms) or quasi - steady auroral bands with east - west moving forms at low latitudes (< 73°MLAT). Strong asymmetries in auroral forms and motions are related to the east - west component (By) of the IMF. The above auroral configurations are discussed in terms of current knowledge on particle precipitation, IMF - related, field - aligned currents, and corresponding modes of solar wind - magnetosphere coupling. We find that the time history of the basic magnetopause coupling modes is manifested in the dayside aurora. We identify candidate auroral signatures of both quasi - steady and pulsed reconnection processes occurring at either low or high magnetopause latitudes. Additional auroral forms in the dawn and dusk sectors are discussed in terms of processes in a closed magnetospheric model, such as the Kelvin - Helmholtz instability.

Events of enhanced convection and related dayside auroral activity

In this paper we study the high-latitude plasma flow variations associated with a periodic (∼8 min) sequence of auroral forms moving along the polar cap boundary, which appear to be the most regularly occuring dayside auroral phenomenon under conditions of southward directed interplanetary magnetic field. Satellite data on auroral particle precipitation and ionospheric plasma drifts from DMSP F10 and F11 are combined with ground-based optical and ion flow measurements for January 7, 1992. Ionospheric flow measurements of 10-s resolution over the range of invariant latitudes from 71° to 76° were obtained by operating both the European incoherent scatter (EISCAT) UHF and VHF radars simultaneously. The optical site (Ny Alesund, Svalbard) and the EISCAT radar field of view were located in the postnoon sector during the actual observations. The West Greenland magnetometers provided information about temporal variations of high-latitude convection in the prenoon sector. Satellite observations of polar cap convection in the northern and southern hemispheres show a standard two-cell pattern consistent with a prevailing negative By component of the interplanetary magnetic field. The 630.0 nm auroral forms located poleward of the persistent cleft aurora and the flow reversal boundary in the ∼1440–1540 MLT sector were observed to coincide with magnetosheath-like particle precipitation and a secondary population of higher energy ions, and they propagated eastward/tailward at speeds comparable with the convection velocity. It is shown that these optical events were accompanied by bursts of sunward (return) flow at lower latitudes in both the morning and the afternoon sectors, consistent with a modulation of Dungey cell convection. The background level of convection was low in this case (Kp =2+). The variability of the high-latitude convection may be explained as resulting from time-varying reconnection at the magnetopause. In that case this study indicates that time variations of the reconnection rate effectively modulates ionospheric convection.

Auroral event sequences at the dayside polar cap boundary for positive and negative interplanetary magnetic field BY

The quasi-periodic sequence of moving auroral forms at the dayside polar cap boundary observed during southward directed interplanetary magnetic field (IMF) is one of the most interesting features of dayside auroral activity. Examples from the prenoon and postnoon sectors, corresponding to positive and negative IMF BY, respectively, are presented. Dual-site ground-based optical observations of one of the auroral events on January 12, 1991, are combined with “snapshot” information on the particle precipitation environment and ionospheric ion flow obtained from the polar-orbiting satellite DMSP F9. A characteristic sequence of auroral forms observed within 08-10 MLT expanded northwestward in a region of strong westward ion flow component, during intervals of strongly negative IMF BZ and positive BY. The optical events faded out near the 08 MLT meridian, within the regime of mantle precipitation (magnetosheath origin). The fading phase of the optical event traversed by the satellite along the 09 MLT meridian is characterized by structured mantle precipitation and associated ion flow shears within 72°-78° magnetic latitude (MLAT). The integrated potential drop across this event in the north-south direction, corresponding to the westward ion drift component within 72°-78° MLAT, is 25 kV. IMP 8 observations of solar wind dynamic pressure outside the bow shock do not show good correlation between pressure variations and major optical event occurrence in the present case. Negative IMF BY events observed near the noon meridian and in the postnoon sector are moving eastward, in the same direction as the ionospheric convection. The optical, particle, and ion drift observations in combination with IMF and solar wind plasma data indicate that the events represent dynamical structures of merging cell convection over the polar magnetosphere, possibly initiated by pulses of enhanced merging rate (Bn ≠ 0) at the dayside magnetopause. The fading of the optical signature and the structured ionospheric ion flow/field-aligned current at mantle latitudes are consistent with a high degree of spatial structure in the high-latitude boundary layer and of the associated solar wind-magnetosphere dynamo action (E · j < 0), provided the field line mapping implied by the present understanding of the particle precipitation characteristics is correct. The contribution from these events to the transpolar voltage (ϕpc) may be estimated on the basis of luminosity area and event repetition time (≃10 min). A wide range of spatial scales of optical events as well as their internal structure is observed. This study indicates that a nonnegligible fraction of the transpolar voltage may at times be generated by pulsed magnetopause merging. Discrete auroral activity and irregular magnetic pulsations with long periods are observed at cleft latitudes in the vicinity of the prenoon convection reversal and further north during the event sequence taking place.

Northward interplanetary magnetic field cusp aurora and high-latitude magnetopause reconnection

We present observations of two types of auroral forms located at different latitudes in the cusp region. Type 1 (south) auroras are located at ∼71° – 75° MLAT and occur during intervals of southward directed interplanetary magnetic field (IMF). Higher-latitude (77°–78°) type 2 (north) auroras are associated with northward IMF (Bz > 0). Intervals are studied which are characterized by transitions from one auroral form to the other in response to IMF switches from north (clock angle 90°) and vice versa. These observations are found to be consistent with the interpretation that type south auroras are a signature of low-latitude magnetopause reconnection and that the type north auroras are associated with high-latitude reconnection, tailward of the cusp. The latter is supported by satellite (DMSP F11) observations of particle precipitation and ionospheric convection. The simultaneous existence of type north and type south auroras is observed during intermediate IMF states (clock angle ∼60°). Observations of type north auroras may be used to study the relationship between the IMF Bx component and high-latitude reconnection. We find that type north auroras occur in the northern hemisphere for both Bx polarities. Enhanced emission intensity of the auroral green line seems to be a feature occurring predominantly during negative Bx conditions. The enhanced green line intensity is considered to be an indication of low-altitude particle acceleration in regions of strong field-aligned current intensities. Such particle accelerations (green line emission) may occur predominantly in regions of upward directed IMF By-related currents in association with the type north cusp aurora.

Cusp/cleft auroral activity in relation to solar wind dynamic pressure, interplanetary magnetic field Bz and By

Continuous optical observations of cusp/cleft auroral activities within ≈ 09-15 MLT and 70-76° magnetic latitude are studied in relation to changes in solar wind dynamic pressure and interplanetary magnetic field (IMF) variability. The observed latitudinal movements of the cusp/cleft aurora in response to IMF Bz changes may be explained as an effect of a variable magnetic field intensity in the outer dayside magnetosphere associated with the changing intensity of region 1 field-aligned currents and associated closure currents. Ground magnetic signatures related to such currents were observed in the present case (January 10, 1993). Strong, isolated enhancements in solar wind dynamic pressure (Δp/p ≥ 0.5) gave rise to equatorward shifts of the cusp/cleft aurora, characteristic auroral transients, and distinct ground magnetic signatures of enhanced convection at cleft latitudes. A sequence of auroral events of ≈ 5-10 min recurrence time, moving eastward along the poleward boundary of the persistent cusp/cleft aurora in the ≈ 10-14 MLT sector, during negative IMF Bz and By, conditions, were found to be correlated with brief pulses in solar wind dynamic pressure (0.1 < Δp/p < 0.5). Simultaneous photometer observations from Ny Alesund, Svalbard, and Danmarkshavn, Greenland, show that the events often appeared on the prenoon side (≈ 10-12 MLT), before moving into the postnoon sector in the case we study here, when IMF By < 0. In other cases, similar auroral event sequences have been observed to move westward in the prenoon sector, during intervals of positive By. Thus a strong prenoon/postnoon asymmetry of event occurence and motion pattern related to the IMF By polarity is observed. We find that this category of auroral event sequence is stimulated bursts of electron precipitation that originate from magnetosheath plasma that has accessed the dayside magnetosphere in the noon or near-noon sector, possibly at high latitudes, partly governed by the IMF orientation as well as by solar wind dynamic pressure pulses.

Simultaneous observations of the cusp in optical, DMSP and HF radar data

A favourable conjunction of HF coherent radar backscatter, meridian scanning photometer data and an overflight of the DMSP F13 spacecraft has enabled the study of the ionospheric signature of the cusp with these three important techniques simultaneously. Strong HF backscatter power, poleward-moving red line auroral forms and latitude-dispersed ion precipitation features are all observed to be collocated. The precipitation of ions in the 0.1–2 keV energy range is found to be very closely associated with the production of the F region irregularities detected by the HF radar.

Dayside auroral configurations: Responses to southward and northward rotations of the interplanetary magnetic field

We report specific types of daytime auroral activities occurring in response to certain rotations of the interplanetary magnetic field (IMF). The IMF rotations we focus on consist of a sharp southward turning from an initial northward orientation, followed by slower rotations back to an intermediate state, which is dominated by the By component (small Bz). When the IMF turned from north to south, the day side auroral configuration changed from one typical of northward conditions to one typical of southward conditions. The type 2 cusp aurora in the north (∼78°–79°magnetic latitude) was replaced by activation of a sequence of moving auroral forms (type 1 cusp aurora) at lower latitudes, each form appearing at the equatorward boundary of the previous one, resulting in ∼100- to 200-km total displacements of the auroral equatorward boundary in the cusp region during intervals of ∼5- to 10-min duration. This auroral activity is accompanied by enhancement of ionospheric convection and onset of magnetic bays on the ground. Since southward rotations of the IMF are expected to give rise to enhancements of the magnetopause reconnection rate, we conclude that these auroral/convection events represent distinct ionospheric signatures of such reconnection rate enhancements. When the IMF turned partially northward again, into an intermediate state, the aurora responded as expected, i.e., the equatorward auroral displacement was halted or reversed, and the type 2 aurora reappeared. The coexistence of the types 1 and 2 cusp auroras in the intermediate state is interpreted in terms of the simultaneous occurrence of reconnections at low and high magnetopause latitudes. The dynamical configurations of particle precipitation and ionospheric convection in the cusp region in such cases are discussed.

A statistical study of flux transfer event signatures in the dayside aurora: The IMF By ‐related prenoon‐postnoon symmetry

Theoretical and experimental studies suggest four main classes of magnetopause/boundary layer processes to explain how particles, momentum and energy transport from the solar wind into the magnetosphere-ionosphere system. Assuming that the IMF B y -related prenoon-postnoon asymmetry of plasma convection is specific to the magnetic merging coupling mode, we determine statistically the local time distribution of transient auroral events as a function of the IMF B y polarity. This is based on concurrent observations of the interplanetary magnetic field components B z and B y and optical auroral observations of cusp/cleft activities within approximately 1000-1400 magnetic local time (MLT) and 71°-81° magnetic latitudes. Intervals of negative IMF B z and nonzero B y are selected for this study. Earlier case studies have shown that transient auroral events moving eastward or westward in the midday sector, depending on the IMF B y polarity, are candidate ionospheric signature of magnetopause flux transfer events (FTEs). An example of this auroral event sequence, illustrating some main characteristics of the expected ionospheric footprints of FTEs, is presented. The statistical study reported here shows an asymmetric prenoon-postnoon auroral occurrence distribution which depends on IMF B y , consistent with the predictions of the reconnection model within a 95% level of confidence. This result confirms the previously existing evidence in favor of an FTE-related interpretation of the actual auroral phenomenon.

Implications of the altitude of transient 630-nm dayside auroral emissions

The altitude from which transient 630-nm (“red line”) light is emitted in transient dayside auroral breakup events is discussed. Theoretically, the emissions should normally originate from approximately 250 to 550 km. Because the luminosity in dayside breakup events moves in a way that is consistent with newly opened field lines, they have been interpreted as the ionospheric signatures of transient reconnection at the dayside magnetopause. For this model the importance of these events for convection can be assessed from the rate of change of their area. The area derived from analysis of images from an all-sky camera and meridian scans from a photometer, however, depends on the square of the assumed emission altitude. From field line mapping, it is shown for both a westward and an eastward moving event, that the main 557.7-nm emission comes from the edge of the 630 nm transient, where a flux transfer event model would place the upward field-aligned current (on the poleward and equatorward edge, respectively). The observing geometry for the two cases presented is such that this is true, irrespective of the 630-nm emission altitude. From comparisons with the European incoherent scatter radar data for the westward (interplanetary magnetic field By > 0) event on January 12, 1988, the 630-nm emission appears to emanate from an altitude of 250 km, and to be accompanied by some 557.7-nm “green-line” emission. However, for a large, eastward moving event observed on January 9, 1989, there is evidence that the emission altitude is considerably greater and, in this case, the only 557.7-nm emission is that on the equatorward edge of the event, consistent with a higher altitude 630-nm excitation source. Assuming an emission altitude of 250 km for this event yields a reconnection voltage of >50 kV during the reconnection burst but a contribution to the convection voltage of >15 kV. However, from the motion of the event we infer that the luminosity peaks at an altitude in the range of 400 and 500 km, and for the top of this range the reconnection and average convection voltages would be increased to >200 kV and >60 kV, respectively. (These are all minimum estimates because the event extends in longitude beyond the field-of-view of the camera). Hence the higher-emission altitude has a highly significant implication, namely that the reconnection bursts which cause the dayside breakup events could explain most of the voltage placed across the magnetosphere and polar cap by the solar wind flow. Analysis of the plasma density and temperatures during the event on January 9, 1989, predicts the required thermal excitation of significant 630-nm intensities at altitudes of 400-500 km.