We have identified for the first time an energy-time dispersion of precipitating electron flux in a pulsating aurora patch, ranging from 6.7 keV to 580 keV, through simultaneous in-situ observations of sub-relativistic electrons of microburst precipitations and lower-energy electrons using the LAMP sounding rocket launched from the Poker Flat Research Range in Alaska. Our observations reveal that precipitating electrons with energies of 180-320 keV were observed first, followed by 250-580 keV electrons 0-30 ms later, and finally, after 500-1000 ms, 6.7-14.6 keV electrons were observed. The identified energy-time dispersion is consistent with the theoretical estimation that the relativistic electron microbursts are a high-energy tail of pulsating aurora electrons, which are caused by chorus waves propagating along the field line.

Ground-based observations of shock aurora, which is an aurora related to geomagnetic sudden commencement (SC), have mainly been limited to the dayside. On 26 February 2023, right after the SC onset at 19:24 UT, ground-based all-sky cameras and a wide-angle camera detected shock aurora at 21 MLT in the auroral zone. For this aurora, three different emissions are detected instead of the previously known two emissions: intensification of a pre-existing arc (19:25 UT), red diffuse aurora (19:28 UT), and a second, green discrete arc (19:31 UT). The relative location of these emissions differs from the dayside cases where the first and last types are not distinguished. Geomagnetic data under this aurora indicates a significant difference in the anti-sunward propagation velocity between the secondary discrete arc and the related field-aligned current.

Ionospheric convection patterns from the Super Dual Auroral Radar Network are used to determine the trajectories, transit times and decay rates of three polar cap patches from their creation in the dayside polar cap ionosphere to their end of life on the nightside. The first two polar cap patches were created within 12 minutes of each other and travelled through the dayside convection throat, before entering the nightside auroral oval after 104 and 92 minutes, respectively. When the patches approached the nightside auroral oval, an intensification in the poleward auroral boundary occurred close to their exit point, followed by a decrease in the transit velocity. The airglow decay rates of patches 1 and 2 were found to be ≈0.6% and ≈0.9% per minute, respectively. The third patch decayed completely within the polar cap and had a lifetime of only 78 minutes. After a change in drift direction, patch 3 had a radar backscatter power half-life of 4.23 minutes, which reduced to 1.80 minutes after a stagnation, indicating a variable decay rate. 28 minutes after the change in direction, and 16 minutes after stagnation, patch 3 completely disintegrated. We relate this rapid decay to increased frictional heating, which speeds up the recombination rate. Therefore, we suggest that the stagnation of a polar cap patch is a main determinant to whether or not a polar cap patch can exit through the nightside auroral oval.

In this study, by simulating the wave-particle interactions, we show that sub-relativistic/relativistic electron microbursts form the high-energy tail of pulsating aurora (PsA). Whistler-mode chorus waves that propagate along the magnetic field lines at high latitudes cause precipitation bursts of electrons with a wide energy range from a few keV (PsA) to several MeV (relativistic microbursts). The rising tone elements of chorus waves cause individual microbursts of sub-relativistic/relativistic electrons and the internal modulation of PsA with a frequency of a few Hz. The chorus bursts for a few seconds cause the microburst trains of sub-relativistic/relativistic electrons and the main pulsations of PsA. Our simulation studies demonstrate that both PsA and relativistic electron microbursts originate simultaneously from pitch angle scattering by chorus wave-particle interactions.

A specialized ground-based system has been developed for simultaneous observations of pulsating aurora (PsA) and related magnetospheric phenomena with the Arase satellite. The instrument suite is composed of 1) six 100-Hz sampling high-speed all-sky imagers (ASIs), 2) two 10-Hz sampling monochromatic ASIs observing 427.8 and 844.6 nm auroral emissions, 3) Watec Monochromatic Imagers, 4) a 20-Hz sampling magnetometer and 5) a 5-wavelength photometer. The 100-Hz ASIs were deployed in four stations in Scandinavia and two stations in Alaska, which have been used for capturing the main pulsations and quasi 3 Hz internal modulations of PsA at the same time. The 10-Hz sampling monochromatic ASIs have been operative in Tromsø, Norway with the 20-Hz magnetometer and the 5-wavelength photometer. Combination of these multiple instruments with the European Incoherent SCATter (EISCAT) radar enables us to reveal the energetics/electrodynamics behind PsA and further to detect the low-altitude ionization due to energetic electron precipitation during PsA. In particular, we intend to derive the characteristic energy of precipitating electrons during PsA by comparing the 427.8 and 844.6 nm emissions from the two monochromatic ASIs. Since the launch of the Arase satellite, the data from these instruments have been examined in comparison with the wave and particle data from the satellite in the magnetosphere. In the future, the system will be utilized not only for studies of PsA but also for other categories of aurora in close collaboration with the planned EISCAT_3D project.

An energy spectrum of electrons from 180 keV to 550 keV precipitating into the dayside polar ionosphere is observed for the first time by the HEP instrument onboard the RockSat-XN sounding rocket under geomagnetically quiet condition (AE ≤100 nT) at Andøya, Norway. The observed energy spectrum of precipitating electrons follows a power law of -4.86 and the electron flux does not vary much over the observation period (~274.4 seconds). A few minutes before the RockSat-XN observation, POES18 / MEPED observed precipitating electrons, which suggest chorus wave activities at the location close to the rocket trajectory. A ground-based VLF receiver observation at Lovozero, Russia also supports the presence of chorus waves during the rocket observation. A test-particle simulation for wave-particle interactions based on the Arase satellite data shows a similar energy spectrum of precipitating electrons, consistent with the RockSat-XN observation. These results suggest that the precipitation observed by RockSat-XN is likely to be caused by the wave-particle interactions between chorus waves and sub-relativistic electrons.

In recent years, auroral observation networks using high-sensitivity cameras have been developed in the polar regions. These networks allow us to observe dimmer auroras such as pulsating auroras (PsAs) with a high signal-to-noise ratio. We reconstructed the horizontal distribution of precipitating electrons using computed tomography with monochromatic PsA images obtained from three observation points. The three-dimensional distribution of the volume emission rate (VER) of the PsA was also reconstructed. The characteristic energy of the reconstructed precipitating electron flux ranged from 6 keV to 23 keV, and the peak altitude of the reconstructed VER ranged from 90 to 104 km. We evaluated the results using a model aurora and compared the model’s electron density with the observed electron density. The electron density was reconstructed correctly to some extent. These results suggest that the horizontal distribution of precipitating electrons associated with PsAs can be effectively reconstructed from ground-based optical observations.

This study exploits the volumetric sampling capabilities of the Resolute Bay Incoherent Scatter Radar (RISR-N) in collaboration with all-sky imagery and in-situ measurements (DMSP) to examine the interplay between cold plasma transport and auroral precipitation during a high-latitude lobe reconnection event on the dawn side. The IMF had an impulsive negative excursion in B$_z$ embedded within a prolonged period of B$_z>0$ and B$_y<0$. The combined effects of transport and magnetic stress release associated with a reconnection pulse resulted in a co-mingling of plasma patches and soft electron precipitation, creating regions of elevated electron density and temperature. Altitude profiles of ionospheric parameters extracted in the rest frame of the drifting patch showed an increase in $T_e$ above 200 km and $N_e$ below 250 km (both hallmarks of soft precipitation), while also showing small and predictable changes in $N_e$ near the F-region peak over the 34-minute duration of the event. For the first time, we identified that the simultaneous appearance of elevated $T_e$ and elevated F-region $N_e$ (i.e., a ‘hot patch’), thus providing a new formation process for hot patches. The physics-based GEMINI model was used to explore the response to the observed precipitation as a function of altitude and time. Enhancements in $N_e$ in the topside ionosphere (e.g., DMSP altitudes) are caused by upward ambipolar diffusion induced by ionospheric heating and not impact ionization. The study highlights the importance of densely distributed measurements in space and time for understanding both mesoscale and small-scale ionospheric dynamics in regions subject to complex forcing.

Pulsating auroras (PsAs) are considered to be caused by energetic (~10 keV) electron precipitations. Additionally, soft electron precipitations (~1 keV) have often been observed in PsAs. These soft electron precipitations enhance the electron density in the ionospheric F region. However, to date, the relationship between PsAs and soft electron precipitation has not been well understood. In this study, using the data taken by the European incoherent scatter radar and the auroral all-sky imager at Tromso;, we conducted two case studies to investigate, in detail, the relationship between the electron density height profile and the type of aurora. Additionally, we conducted statistical studies for 14 events to elucidate how often F region electron density enhancement occurs with a PsA. We consequently found that 76% of electron density height profiles showed a local peak in the F region, with electron temperature enhancements. It was also found that 89% of the F region peak altitudes were above the peak altitude of the ionization rate produced by electrons of characteristic energy below 100 eV. The occurrence rate of these profiles in the hourly magnetic local time (MLT) exceeded 80% in the 22–3 MLT sectors. We suggest that the electron density enhancement in the F region would have been caused by electrostatic electron cyclotron harmonic waves in the magnetosphere. Another candidate would have been polar patches that had traveled from the dayside ionosphere.

We report on the relationship between pulsating aurora and relativistic electron microburst using simultaneous observations of ground-based fast auroral imagers with the FIREBIRD-Ⅱ CubeSat for the first time. We conducted a detailed analysis of an event on October 8, 2018 and found that the occurrence of a pulsating aurora with internal modulations corresponds to the flux enhancement of electrons with energy ranging from 219.7 to 984.95 keV detected with Flight Unit 4, one of FIREBIRD’s CubeSat, with a time delay of 525 ms. Assuming that the pulsating aurora was produced by 10-keV electrons, we suggest that this time difference of 525 ms is consistent with the theory by Miyoshi et al. (2020) that a pulsating aurora and microburst occur due to the chorus waves at different latitudes along the same field line.

Pulsating aurora (PsA) is characterized by quasi-periodic intensity modulations with a period of a few to a few tens of seconds. It is caused by precipitation of energetic electrons of a few to several tens of kilo electronvolts produced by chorus waves in the magnetosphere. Precipitating electron energies of PsA have been identified in the past by sounding rocket and satellite observations, but the spatial distributions of precipitating electron energies of PsA have never been estimated. In this study, using the data from ground-based all-sky cameras at two wavelengths of 427.8 nm and 844.6 nm, we estimated the temporal and spatial variations in the precipitating electron energy of PsA. The results showed that the spatial distribution of precipitating electron energies was not uniform in the PsA patch, suggesting that the coherent spatial scale of the wave-particle interactions is smaller than each PsA patch size.

Whistler mode chorus waves scatter magnetospheric electrons and cause precipitation into the Earth’s atmosphere. Previous measurements showed that nightside chorus waves are indeed responsible for diffuse/pulsating aurora. Although chorus waves and electron precipitation have also been detected on the dayside, their link has not been illustrated (or demonstrated) in detail compared to the nightside observations. Conventional low-altitude satellite observations do not well resolve the energy range of 10–100 keV, hampering verification on resonance condition with chorus waves. In this paper we report observations of energetic electrons with energies of 30–100 keV that were made by the electron sensor installed on the NASA’s sounding rocket RockSat-XN. It was launched from the Andøya Space Center on the dayside (MLT ~ 11 h) at the L-value of ~ 7 on 13 January 2019. Transient electron precipitation was observed at ~ 50 keV with the duration of <100 s. A ground station at Kola peninsula in Russia near the rocket’s footprint observed intermittent emissions of whistler-mode waves simultaneously with the rocket observations. The energy of precipitating electrons is consistent with those derived from the quasi-linear theory of pitch angle scattering by chorus waves through cyclotron resonance, assuming a typical dayside magnetospheric electron density. Precise interaction region is discussed based on the obtained energy spectrum below 100 keV.