Cusp Aurora Research Articles

Transpolar Arcs Are Not Always Cusp‐Aligned: Evidence of HiLDA‐Aligned Arcs

AbstractTranspolar arcs (TPAs) are often cusp‐aligned. Especially when multiple TPAs appear simultaneously, they join at the auroral signature of the cusp. Here we investigate the dayside connection point of TPAs using Defense Meteorological Satellite Program measurements and identify three cases where the tip of the TPA ends in a localized brightening. One is a typical cusp spot with a TPA attached. The cusp appears just poleward of the oval with a near circular shape. In the second case, multiple cusp spots are observed over a 3 MLT wide region, each connected to a TPA. In the third case, the brightening at the tip of a TPA is identified as high‐latitude dayside aurora (HiLDA). Cusp aurora appears between the HiLDA and the duskside oval. Plasma flows and particle characteristics indicate a lobe origin of the HiLDA. Our results indicate a more complicated connection between TPAs and dayside aurora than previously anticipated.

Multiple Satellite Observations of the High‐Latitude Cusp Aurora During Northward IMF Conditions

AbstractCusp auroras poleward of the typical auroral oval are ascribed to high‐latitude lobe reconnection when the Interplanetary Magnetic Field (IMF) Bz is predominantly northward. In this study, we further investigate the ionospheric characteristics of a unique high‐latitude cusp region employing multiple satellite observations. A cusp aurora event with wide spatial spread was observed in the postnoon polar cap region. It was found to be associated with northward IMF Bz and positive By components. The cusp aurora was located from 68° to 86° in magnetic latitude and within 15–17 hr in magnetic local time. This broad coverage in the polar cap indicates direct precipitating particles from the magnetosheath. Particle energy is different between the equatorward and poleward edges of the cusp aurora. The precipitating ions at the equatorward side maintain magnetosheath particle characteristics as expected, while ions with higher energies occurred in the poleward side. Further, the poleward edge of the cusp aurora was nearly situated in the center of a convection shear and was associated with an upward field‐aligned current. These observations suggest a lobe cell circulation, hence we attribute the formation of the cusp aurora to the high‐latitude lobe reconnection. Simultaneous observations in the southern hemisphere indicate the absence of cusp aurora. The auroral presence only in the northern hemisphere is probably due to the combination of large dipole tilt angle and positive IMF Bz, which facilitates the lobe reconnection.

A Comparative Study on the Factors Controlling the Cusp Auroral Intensity Between the Northern and Southern Hemispheres

AbstractThe previous study has indicated that the interplanetary magnetic field (IMF) By and solar wind flow speed (Vsw) have crucial effects on the cusp auroral intensity in the Northern Hemisphere. However, it is not clear how these factors influence the cusp aurora in the Southern Hemisphere. Using DMSP/SUSSI observations from 2004 to 2017 during non‐storm periods, we compared the factors affecting the intensity of cusp aurora between the Northern and Southern Hemispheres by dividing the observations into weak‐ and intense‐emission events. We found that the auroral intensity in the southern cusp: (a) has a positive correlation with solar wind power input, (b) is statistically weak under southward IMF conditions, and (c) is critically controlled by the IMF By and Vsw. These results are essentially consistent with those obtained in the Northern Hemisphere. However, the IMF By effect shows apparent interhemispheric asymmetry, that is, the IMF By effect reaches the maximum at 2 nT in the Northern Hemisphere, but at −2 nT in the Southern Hemisphere. Further analysis shows that the 2‐nT deviations disappear after dividing all events by season and the IMF By distributions of intense‐emission events show clear seasonal difference. Based on these results, we suggest that the dependence of the cusp auroral intensity on the IMF By is not only controlled by −Vsw × By field, but also affected by season.

Suppression of Ionospheric Irregularity Due to Auroral Particle Impact

AbstractThe VISIONS‐2 35.039 sounding rocket was launched from Ny‐Ålesund, Svalbard, on 7 December 2018 at 11:06 UT, and traveled overhead of the cusp aurora. The payload reached an apogee of 806.6 km and provided measurements of the electric field (E) and electron density (Ne) with a high sampling rate of 6,250 Hz. The high‐sampling‐data make it possible to estimate the horizontal structure of E and Ne on scales ranging from meters to kilometers scale. The horizontal variation in the electron density and electric field (ΔNe/Ne and ΔE) and integrated power of Ne and E (ΣPNe and ΣPE) for 1–10, 10–100, and 100–1,000 Hz range were derived. The derived values were compared with the 557.7 and 630.0 nm emission intensities obtained from an all‐sky camera installed in Ny‐Ålesund projected at the footprint of the rocket, which was calculated by tracing the Earth's magnetic field line from the rocket altitude to the emission layer. ΔNe/Ne, ΔE, ΣPNe, and ΣPE increased with the 630.0 nm emission intensity. Of particular interest is the lower level irregularity was observed when the 557.7 nm emission intensity exceeded 4.5 kR compared with other time interval. This may suggest that particle impact ionization created sufficient Pedersen conductance in the E‐region to short the F‐region current. This short‐circuit seemed to cause the suppression of the irregularities in the F‐region and lower level irregularities were also observed in the rocket altitude.

The Critical Factor in Controlling the Auroral Intensity in the Cusp Region as Revealed by a Statistical Study on Midday Gap and Non‐Gap Events

AbstractTaking advantage of the high spatial‐resolution and global coverage of Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager observations, we investigated the critical interplanetary factors in controlling the cusp auroral emission by dividing the midday auroras into the gap (weak emission) and non‐gap (intense emission) events. Although the cusp auroral intensity is essentially determined by a parameter related to the IMF (interplanetary magnetic field) direction, IMF magnitude (|By|), and solar wind speed (V), we found that the cusp aurora is statistically weak during the southward IMF but intense when the V and IMF |By| are greater. Further, we confirmed that even with V > 600 km/s, the intense‐aurora event still shows a minimum occurrence near the IMF |By| = 0. However, when the IMF |By| is greater, the V becomes less significant for the intense‐aurora occurrence. These results demonstrate that the IMF |By| is critical in controlling the cusp auroral intensity, most likely by producing an electric field which is given as .

The correspondence between dayside long-period geomagnetic pulsations and the open-closed field line boundary

Long-period pulsations in the nominal Pc5-6 band (periods about 3–15 min) have been known to be a persistent feature of dayside high latitudes. A mixture of broadband Irregular Pulsations at Cusp Latitudes (IPCL) and narrowband P≿5 waves is often observed. The mechanism and origin of IPCL have not been firmly established yet. Magnetopause surface eigenmodes were suggested as a potential source of high-latitude ULF waves with frequencies less than 2 mHz. A ground response to these modes is expected to be beneath the ionospheric projection of the open-closed field line boundary (OCB). To unambiguously resolve a possible association of IPCL with the magnetopause surface modes, multi-instrument observation data from Svalbard have been analyzed. We examine the latitudinal structure of high-latitude pulsations in the Pc5-6 band recorded by magnetometers covering near-cusp latitudes. This structure is compared with an instant location of the equatorward boundary of the cusp aurora, assumed to be a proxy of the OCB. The optical OCB latitude has been identified by an automatic algorithm, using data from the meridian scanning photometer at Longyearbyen, Svalbard. The comparison has shown that the latitudinal maximum of the broadband IPCL maximizes about 2°–3° deeper in the magnetosphere than the OCB optical proxy. Therefore, these pulsations cannot be associated with the ground image of the magnetopause surface modes. It is likely that an essentially non-dipole geometry of field lines and a high variability of the magnetopause region may suppress the excitation efficiency. The obtained result imposes important limitations on possible mechanisms of high-latitude dayside ULF variations.

Identifying the evolution of Southern Hemisphere poleward moving auroral forms (PMAFs) in the context of plasma convection and magnetic reconnection

AbstractIn this study we examine the dynamic evolution of Southern Hemisphere auroral precipitation patterns, specifically poleward moving auroral forms (PMAFs). The spatial, structural, and temporal characteristics of these events have been linked to underlying magnetopause‐boundary layer flux transfer events which are responsible for the generation of these ionospheric transient forms. We present five case studies of Southern Hemisphere PMAFs using optical data for 630.0 nm and 427.8 nm wavelengths from the US Automatic Geophysical Observatory Network in Antarctica. Emphasis is put on the unique capabilities of two‐dimensional optical data when it comes to studying PMAFs in the context of pulsed magnetic reconnection. In a detailed examination of three PMAFs recorded in 2007, we have found that their motion closely follows the underlying plasma convection patterns as determined from backscatter radar data, which in turn are governed by the interplanetary magnetic field (IMF) configuration. Of particular interest are periods when the positive IMF By component causes an asymmetric convection pattern which is biased toward the dawn region. These cases are consistent with the expected antisymmetric nature of the northern and southern hemisphere convection. In addition, we present two “pseudo” PMAFs which are actually independent auroral arcs executing poleward motions and occurring when the IMF Bz was positive. Using two‐dimensional images instead of one‐dimensional keograms or meridian scanning photometer recordings, we were able to distinguish these arcs from PMAFs. The misidentification of these arcs as PMAFs would cause inconsistencies with the theories of cusp aurora dynamics and magnetic reconnection, and some previous studies may have included such non‐PMAF events.

Dynamic properties of throat aurora revealed by simultaneous ground and satellite observations

AbstractThroat aurora is defined as south‐north aligned auroral arcs equatorward of the dayside cusp aurora and was suggested to be the results of cold magnetospheric plasma interaction with magnetopause reconnection, but its observational properties have not yet been well established. In this paper we carefully examine a sequence of throat auroras observed over Svalbard on 27 December 2003. Observations from 630.0 nm [OI] show that poleward moving throat auroras frequently show brightening followed by dimming in auroral intensity, and sometimes, the brightening throat aurora is a precursor of poleward moving auroral forms. Simultaneous all‐sky images and HF radar backscatter observations along geomagnetic meridian show that the throat aurora brightening is drifting with ionospheric E × B convection and is colocated with enhanced spectral width poleward of the convection reversal boundary (CRB), while its dimming tends to be in the vicinity of the CRB. This leads us to propose that the throat aurora brightening and dimming may be on the open and closed field lines, respectively. Particle data from NOAA 16 confirm that the dimming throat aurora is associated with precipitation of magnetosheath‐like particles mixed with magnetospheric ions (>30 keV), which is characteristic of the low‐latitude boundary layer. For particle data from Defense Meteorological Satellite Program F16, we notice that the dimming throat aurora is associated with lower fluxes of magnetosheath‐like electrons accompanied with magnetospheric electrons, which is most likely on closed field lines. The dynamic properties of throat aurora presented in this paper are thus important to understand its generation mechanisms.

Horizontal profile of a moving red line cusp aurora

AbstractAuroral emission at a wavelength of 630.0 nm (red line) has a long radiative time. In this study, we show how this long radiative time controls the horizontal extent of a moving mesoscale aurora, which is typical of the cusp. Using high time resolution (4 s) observations by an all‐sky imager at Longyearbyen, Svalbard, and observations by the European Incoherent Scatter Svalbard Radar (ESR), we examined the auroral emission enhancements obtained in the cusp during an interval of southward interplanetary magnetic field on 27 November 2011. Simultaneous observations from the all‐sky imager and the ESR clearly show the manner in which auroral emission regions passed through the radar's field of view. When the front edge of the moving auroral structure intersected the radar's field of view, the ESR electron temperature was enhanced. A few minutes later, the ESR electron temperature dropped to the background level, indicating that the mesoscale electron precipitation region shifted away from the radar's field of view. At that time, the auroral emission in the radar's field of view decreased but still had adequate intensities. These results provide evidence demonstrating that moving cusp auroral emission occurs behind the electron precipitation region as well as inside that region. We interpreted this feature semiquantitatively by using the equation of continuity of the density of the excited atomic oxygen. Our model indicates that the maximum intensity in the moving auroral structure occurs at a point along the trailing edge of the electron precipitation region and that the velocity of that region is important for determining how the 630.0 nm aurora emissions extend horizontally.

GPS scintillations associated with cusp dynamics and polar cap patches

This paper investigates the relative scintillation level associated with cusp dynamics (including precipitation, flow shears, etc.) with and without the formation of polar cap patches around the cusp inflow region by the EISCAT Svalbard radar (ESR) and two GPS scintillation receivers. A series of polar cap patches were observed by the ESR between 8:40 and 10:20 UT on December 3, 2011. The polar cap patches combined with the auroral dynamics were associated with a significantly higher GPS phase scintillation level (up to 0.6 rad) than those observed for the other two alternatives, i.e., cusp dynamics without polar cap patches, and polar cap patches without cusp aurora. The cusp auroral dynamics without plasma patches were indeed related to GPS phase scintillations at a moderate level (up to 0.3 rad). The polar cap patches away from the active cusp were associated with sporadic and moderate GPS phase scintillations (up to 0.2 rad). The main conclusion is that the worst global navigation satellite system space weather events on the dayside occur when polar cap patches enter the polar cap and are subject to particle precipitation and flow shears, which is analogous to the nightside when polar cap patches exit the polar cap and enter the auroral oval.

Pulsations of the polar cusp aurora at Saturn

AbstractThe magnetospheric cusp is a region connecting the interplanetary environment to the ionosphere and enabling solar wind particles to reach the ionosphere. We report the detection of several isolated high‐latitude auroral emissions with the Ultraviolet Imaging Spectrograph of the Cassini spacecraft. We suggest that these auroral spots, located in the dawn‐to‐noon sector and poleward of the main emission, are the ionospheric signatures of the magnetospheric cusp, in agreement with some previous observations with the Hubble Space Telescope. The high‐latitude cusp auroral signature has been associated with high‐latitude lobe reconnection in the presence of a southward interplanetary magnetic field. The occurrence rate of the polar cusp aurora suggests that lobe reconnection is frequent at Saturn. Several auroral imaging sequences reveal a quasiperiodic brightening of the polar cusp aurora with a period in the range of 60 to 70 min. Similar pulsations in the energetic electron fluxes and in the azimuthal component of the magnetic field are simultaneously observed by Cassini instruments, suggesting the presence of field‐aligned currents. Pulsed dayside magnetopause reconnection is a likely common triggering process for the cusp auroral brightenings at Saturn and the quasiperiodic pulsations in the high‐latitude energetic electron fluxes.

Source of the dayside cusp aurora.

Monochromatic all‐sky imagers at South Pole and other Antarctic stations of the Automatic Geophysical Observatory chain recorded the aurora in the region where the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites crossed the dayside magnetopause. In several cases the magnetic field lines threading the satellites when mapped to the atmosphere were inside the imagers' field of view. From the THEMIS magnetic field and the plasma density measurements, we were able to locate the position of the magnetopause crossings and map it to the ionosphere using the Tsyganenko‐96 field model. Field line mapping is reasonably accurate on the dayside subsolar region where the field is strong, almost dipolar even though compressed. From these coordinated observations, we were able to prove that the dayside cusp aurora of high 630 nm brightness is on open field lines, and it is therefore direct precipitation from the magnetosheath. The cusp aurora contained significant highly structured N2 + 427.8 nm emission. The THEMIS measurements of the magnetosheath particle energy and density taken just outside the magnetopause compared to the intensity of the structured N2 + 427.8 nm emissions showed that the precipitating magnetosheath particles had to be accelerated. The most likely electron acceleration mechanism is by dispersive Alfvén waves propagating along the field line. Wave‐accelerated suprathermal electrons were seen by FAST and DMSP. The 427.8 nm wavelength channel also shows the presence of a lower latitude hard‐electron precipitation zone originating inside the magnetosphere.

DE-1 versus Cluster spectral density observations: the effect on ion outflow above the polar cap

<p>Wave-particle interaction is a very important mechanism in describing the outflow of ions at high latitudes and high altitudes. Quasi-linear perpendicular velocity diffusion coefficients are used to describe the effect of wave-particle interactions, therefore it is essential to determine the correct diffusion coefficients that must be used to model the outflow of ions. In this study a Monte Carlo method is used to assess the role of different diffusion coefficients for O+ and H+ ions at high altitudes above the polar cap. Two different sets of diffusion coefficients obtained from Barghouthi [1997]; Barghouthi et al. [1998] and Nilsson et al. [2013] are used. Barghouthi [1997]; Barghouthi et al. [1998] used spectral density measurements from Dynamic Explorer 1 spacecraft (DE-1) observations to calculate the diffusion coefficients, while Nilsson et al. [2013] used spectral density measurements from the Cluster spacecraft to obtain the diffusion coefficients. It was found that diffusion coefficients from Barghouthi [1997]; Barghouthi et al. [1998] in the cusp (aurora) and central polar cap (polar wind) respectively, describe well the observations of ion outflow at altitudes lower than 5 RE, but yield unreasonably high parallel velocities and temperatures at higher altitudes. Also diffusion coefficients from Cluster spectral density measurements produce reasonable results for high altitudes and unreasonably low parallel velocities and temperatures for the low altitude region. Therefore it is suggested that a combination of these diffusion coefficients is used where the diffusion coefficients given by Barghouthi [1997]; Barghouthi et al. [1998] are used at low altitudes and the diffusion coefficients obtained from Cluster measurements are used at high altitudes.</p>

On the collocation of the cusp aurora and the GPS phase scintillation: A statistical study

AbstractThe climatology map of the GPS phase scintillation identifies two regions of high scintillation occurrences: around magnetic noon and around magnetic midnight. The scintillation occurrence rate is higher around noon, while the scintillation level is stronger around magnetic midnight. This paper focuses on the dayside scintillation region. In order to resolve the role of the cusp auroral processes in the production of irregularities, we put the GPS phase scintillation in the context of the observed auroral morphology. Results show that the occurrence rate of the GPS phase scintillation is highest inside the auroral cusp, regardless of the scintillation strength and the interplanetary magnetic field (IMF). On average, the scintillation occurrence rate in the cusp region is about 5 times as high as in the region immediately poleward of it. The scintillation occurrence rate is higher when the IMF Bz is negative. When partitioning the scintillation data by the IMF By, the distribution of the scintillation occurrence rate around magnetic noon is similar to that of the poleward moving auroral form (PMAF): there is a higher occurrence rate at earlier (later) magnetic local time when the IMF By is positive (negative). This indicates that the irregularities which give rise to scintillations follow the IMF By‐controlled east‐west motion of the aurora and plasma. Furthermore, the scintillation occurrence rate is higher when IMF By is positive when the cusp is shifted toward the post noon sector where it may get easier access to the higher density plasma. This suggests that the combined auroral activities (e.g., PMAF) and the density of the intake solar EUV ionized plasma are crucial for the production of scintillations.

Three-dimensional imaging of the plasma parameters of a moving cusp aurora

During a period of negative IMF BZ on 13 January 2013, an all-sky imager at Longyearbyen, Svalbard observed a mesoscale aurora moving towards the east-northeast in the cusp, passing through the field of view of the EISCAT Svalbard Radar (ESR) elevation scan. The elevation scans that were being performed at that time have a horizontal coverage of approximately 300km, at an altitude of 300km. The plasma data obtained from the elevation scans and the 630-nm aurora emission data from the all-sky imager have shown that ion temperature enhanced 50–60s earlier than electron density, and that the maximum auroral intensity in the ESR's field of view occurred about 40s after the electron density enhancement. On the basis of these results we have constructed three-dimensional images of elevated ion temperatures and enhanced electron density associated with the mesoscale moving cusp aurora. The three-dimensional image shows that the enhancement of the ion temperature is prominent in the horizontal area of ~160km×~80km below an altitude of ~300km, and that this volume forms on the forward side of the enhanced electron density region. We interpret these configurations as being a result of a mesoscale twin-cell convection, which is embedded in the background flow such that the symmetrical axis of the twin-cell is inclined from the background flow direction by several tens of degrees. Our method for visualizing three-dimensional features such as these could be an effective approach to understanding the mesoscale dynamics of the cusp, which is usually located in latitudes that are difficult for the currently-operated radars that permit three-dimensional, simultaneous measurements to investigate.

Investigating the particle precipitation of a moving cusp aurora using simultaneous observations from the ground and space

Using observations of a moving cusp aurora from a high-sensitivity all-sky imager at Longyearbyen, Svalbard, and in situ observations of the precipitating particles from a spacecraft that flew over the aurora, we examined the particle precipitation features in the early and final stages of the moving cusp aurora. We focused on two auroral structures created near noon, separated by approximately 3 min, during a southwestward interplanetary magnetic field (IMF) condition on 17 December 2012. The second auroral structure occurred when the IMF turned further southward. Immediately after the appearance of the latter structure, the two auroral structures were adjacently situated, and the DMSP F18 spacecraft passed through these regions. A detailed comparison of the data from particle spectrometers onboard the spacecraft and the 630 nm aurora image data demonstrates that the ion precipitation in the young cusp aurora (i.e., second auroral structure) had a high energy flux, whereas that in the old cusp aurora (i.e., first auroral structure) had a very low energy flux. For the electron precipitation, the features in both regions were found to be very similar; the energy flux at approximately 100 eV often exceeded 1 × 109 eV cm−2 s−1 sr−1 eV−1 in both regions. This indicates that the electron precipitation in the moving cusp aurora is maintained at a high flux level over a certain interval from its starting time. Thus, we suggest that the electron precipitation flux in the moving cusp aurora is controlled by a mechanism independent of the ion precipitation.

A model based method for obtaining the open/closed field line boundary from the cusp auroral 6300 Å[OI] red line

Ground based optical instruments are invaluable tools for studies of processes associated with the cusps and auroral morphology. In this work we present a method for obtaining the magnetic latitude of the open/closed field line boundary (OCB) from the cusp 6300 Å[OI] auroral red line using a meridian scanning photometer. The method which is based on a pre‐defined reference cusp aurora produced by the GLOW model is examined with respect to uncertainties, and we describe how a set of equations describing the error is constructed. The method is applicable to data from optical instruments located at high latitude observation sites such as Svalbard and Antarctica. Equations describing both errors and the mapping altitude for transforming the OCB from instrument centered coordinates to magnetic latitude for instrumentation located in Svalbard (Longyearbyen) are presented. Further, by applying the GLOW model we present results illustrating the great variability in the altitude profile of the atomic oxygen 6300 Å[OI] red line emission in the cusp. A simple calculation showing how a poleward neutral wind will change the latitudinal shape of the cusp aurora is also performed.

A magnetohydrodynamic simulation study of Kronian field‐aligned currents and auroras

Magnetohydrodynamic simulations of the interaction of Saturn's magnetosphere with the solar wind indicate that Kelvin‐Helmholtz (K‐H) waves can form on the dayside magnetopause when the interplanetary magnetic field (IMF) is northward. Dayside magnetic reconnection occurs at Saturn for northward IMF, and the combination of K‐H waves and reconnection caused enhanced vorticity in Saturn's magnetosphere. We have used a very high resolution version (Δx = 0.1 RS) of our simulation code to study the consequences of the vortices and reconnection for the generation of field‐aligned currents (FACs) and auroras in Saturn's ionosphere. We found three bands of alternating FACs toward and away from the dawn side of the ionosphere and two sets on the dusk side. The K‐H waves generated a series of toward and away currents along the dayside magnetopause. In the ionosphere they appear as a series of spots of up and down currents. The K‐H field‐aligned currents are adjacent to nearly continuous currents located from 16:00 LT, to around past midnight, and to about 07:00 LT. The largest currents densities (j∥ > 5 × 10−8 A m−2) are found at the highest latitudes. They map to the magnetopause and to the near‐Saturn tail region. We used the away current density and the Knight relationship to estimate the energy flux related to discrete auroras and obtained ∼1 mW m−2 in the region with the strongest currents. This gives approximately 7 GW for the auroral UV emitted power. We found a region of enhanced thermal energy flux in the region where cusp auroras are observed.

On the relationship between flux transfer events, temperature enhancements, and ion upflow events in the cusp ionosphere

[1] A transit of the dayside aurora across the field-of-view of the EISCAT Svalbard Radar occurred on 20 December 1998. This offered an excellent opportunity to study the spatial structure of the cusp/cleft aurora using meridian scanning photometer and incoherent scatter radar. We were able to identify distinct regions of upflow driven by ion heating (type 1) and upflow driven by electron heating (type 2) around poleward moving auroral forms, a transient auroral feature tied to flux transfer events. A quiet period before the auroral transit allowed us to estimate a neutral temperature profile, which enabled calculation of the ion-neutral relative wind. We found evidence for purely ion heating-driven upflow equatorward of the cusp auroral boundary, and for electron heating-driven upflow near the equatorward auroral boundary. The greatest upflow occurred near the center of the cusp aurora when both ion and electron temperatures were enhanced. The observed upflows were greater than expected from ambipolar diffusion alone, suggesting that ion-neutral frictional heating did contribute to upflow events in most cases. The great variability observed in ion temperature indicates that the ion flow was greatly structured within the aurora. Type 1–2 upflows may be considered as spatial structures of active cusp. Upflows are observed at various times in their evolution, and one upflow event, estimated to be 5–10 minutes old, showed a lifting of the F region of some 100 km, indicating a hybrid of type 1 and type 2.

Simultaneous observations of reconnection pulses at Cluster and their effects on the cusp aurora observed at the Chinese Yellow River Station

Simultaneous observations of reconnection pulses at Cluster and their effects on the cusp aurora observed at the Chinese Yellow River Station