We investigate the origin of the fine structure of the energy spectrum of precipitating electrons for the pulsating aurora (PsA) observed by the low‐altitude Reimei satellite. The Reimei satellite ...achieved simultaneous observations of the optical images and precipitating electrons of the PsA from satellite altitude (~620 km) with resolution of 40 ms. The main modulation of precipitation, with a few seconds, and the internal modulations, with a few hertz, that are embedded inside the main modulations are identified above ~3 keV. Moreover, stable precipitations at ~1 keV are found for the PsA. A “precipitation gap” is discovered between two energy bands. We identify the origin of the fine structure of the energy spectrum for the precipitating electrons using the computer simulation on the wave‐particle interaction between electrons and chorus waves. The lower band chorus (LBC) bursts cause the main modulation of energetic electrons, and the generation and collapse of the LBC bursts determines on‐off switching of the PsA. A train of rising tone elements embedded in the LBC bursts drives the internal modulations. A close set of upper band chorus (UBC) waves causes the stable precipitations at ~1 keV. We show that a wave power gap around the half gyrofrequency at the equatorial plane in the magnetosphere between LBC and UBC reduces the loss rate of electrons at the intermediate energy range, forming a gap of precipitating electrons in the ionosphere.
Key Points
Fine structure of energy spectrum of pulsating aurora electrons
Two different populations coexisted in the precipitating electrons
Simulation reproduces the fine structure of the energy spectrum
A thermospheric wind data set from a Fabry‐Perot interferometer (630 nm) and the ion velocity from a Dynasonde in Tromsø, Norway, was analyzed for nine winter seasons to study the dynamics of the ...thermosphere and F‐region ionosphere at an auroral latitude. This study focused on bifurcation in the zonal component of the neutral wind and ion velocity at midnight and its dependence on the Y component of the interplanetary magnetic field (IMF). Ionospheric plasma convection patterns are evidently imprinted on the thermospheric wind variations as aspects of the westward and eastward accelerations at dusk and late morning, respectively. The zonal wind bifurcates immediately before midnight for IMF By < 0, but for By > 0, it inverts gradually into the postmidnight sector. Neutral wind streams, originating from higher latitudes, may result in the dependence because of anti‐sunward plasma flow distorted in the polar cap.
Plain Language Summary
The ionosphere is partially ionized plasma, but the particle minority of ions plays an important role in controlling dynamics of the thermosphere. Particle collision is the fundamental process for momentum transfer from ionospheric ions to thermospheric neutral particles. The ionospheric plasma flow pattern at high latitudes depends on the direction of the interplanetary magnetic field (IMF), and the pattern may be projected on the thermospheric wind. However, the dependence is not yet well understood. This study derived statistical experimental features regarding the dependence of the thermospheric wind, analyzing data from an optical interferometer (Fabry‐Perot interferometer) and a radio wave technique (Dynasonde) in Tromsø, Norway. The wind pattern around midnight is different from the ionospheric plasma convection, in accordance with the IMF direction. The zonal wind bifurcates immediately before midnight for IMF By < 0, but for By > 0, it inverts gradually into the postmidnight sector. Neutral wind streams, originating from higher latitudes, may cause the dependence because of anti‐sunward plasma flow distortion in the polar cap. In summary, this study concludes that the zonal wind bifurcation at auroral latitudes is caused by the ion velocity bifurcation, and that advection from the polar cap region affects the wind response time to the ion velocity bifurcation.
Key Points
The thermospheric wind from a Fabry‐Perot interferometer (630 nm) and the ionospheric plasma velocity from a Dynasonde were compared
The zonal wind bifurcates immediately before midnight for interplanetary magnetic field By < 0, but for By > 0, it inverts gradually into the postmidnight sector
The wind bifurcation signature is different from the ion velocity bifurcation, probably due to advection from the polar cap region
We report observations of a stellar occultation by Pluto on 2019 July 17. A single-chord high-speed (time resolution = 2 s) photometry dataset was obtained with a CMOS camera mounted on the Tohoku ...University 60 cm telescope (Haleakala, Hawaii). The occultation light curve is satisfactorily fitted to an existing atmospheric model of Pluto. We find the lowest pressure value at a reference radius of
r
= 1215 km among those reported after 2012. These reports indicate a possible rapid (approximately 21
−5
+4
% of the previous value) pressure drop between 2016, which is the latest reported estimate, and 2019. However, this drop is detected at a 2.4
σ
level only and still requires confirmation from future observations. If real, this trend is opposite from the monotonic increase of Pluto’s atmospheric pressure reported by previous studies. The observed decrease trend is possibly caused by ongoing N
2
condensation processes in the Sputnik Planitia glacier associated with an orbitally driven decline of solar insolation, as predicted by previous theoretical models. However, the observed amplitude of the pressure decrease is larger than the model predictions.
We have identified for the first time an energy‐time dispersion of precipitating electron flux in a pulsating aurora patch, ranging from 6.7 to 580 keV, through simultaneous in‐situ observations of ...sub‐relativistic electrons of microburst precipitations and lower‐energy electrons using the Loss through Auroral Microburst Pulsation 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 500–1,000 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.
Plain Language Summary
Microbursts, which are bursts of high energy electrons, and pulsating auroras, which periodically blink and caused by the precipitation of low energy electrons, are observed in the Earth's polar ionosphere. The detection time differences of the electrons associated with microbursts and pulsating auroras were detected by a sounding rocket. A possible mechanism for the generation of these precipitations is the interaction of electrons with a particular type of wave, known as “chorus,” which propagates along geomagnetic lines. The observed energy‐time dispersion of the precipitating electrons is quantitatively consistent with theories of electron precipitation based on this interaction.
Key Points
A sounding rocket observed simultaneously precipitating sub‐relativistic electron microbursts and pulsating auroral electrons
250–580 keV electron precipitations were detected 0–30 ms after 180–320 keV electron precipitations in a single auroral patch
The energy dispersion of observed electrons is consistent with the theory that they are due to chorus waves propagating to higher latitudes
In this study, by simulating the wave‐particle interactions, we show that subrelativistic/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 kiloelectron volts (PsA) to several megaelectron volts (relativistic microbursts). The rising tone elements of chorus waves cause individual microbursts of subrelativistic/relativistic electrons and the internal modulation of PsA with a frequency of a few hertz. The chorus bursts for a few seconds cause the microburst trains of subrelativistic/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 along the field line.
Plain Language Summary
Pulsating aurora electron and relativistic electron microbursts are precipitation bursts of electrons from the magnetosphere to the thermosphere and the mesosphere with energies ranging from a few kiloelectron volts to tens of kiloelectron volts and subrelativistic/relativistic, respectively. Our computer simulation shows that pulsating aurora electron (low energy) and relativistic electron microbursts (relativistic energy) are the same product of chorus wave‐particle interactions, and relativistic electron microbursts are high‐energy tail of pulsating aurora electrons. The relativistic electron microbursts contribute to significant loss of the outer belt electrons, and our results suggest that the pulsating aurora activity can be often used as a proxy of the radiation belt flux variations.
Key Points
We demonstrate that subrelativistic/relativistic electron microbursts are the high‐energy tail of pulsating aurora electrons
Our simulation studies demonstrate that both pulsating aurora and relativistic electron microbursts originate simultaneously
Pulsating aurora electron and relativistic electron microbursts are the same product of chorus wave‐particle interactions
Pulsating auroras (PsAs) are considered to be caused by energetic (>a few keV) electron precipitation. Additionally, soft electron precipitation (<a few keV) has often been observed in PsAs. This ...soft electron precipitation enhances 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 Tromsø, 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.
Key Points
A total of 76% of the electron density height profiles during pulsating auroras had a local enhancement in the ionospheric F region
The occurrence rate of these profiles exceeded 80% in 22–3 magnetic local time
A total of 89% of the F region peak altitudes were above the peak altitude of the ionization rate produced by 100 eV electrons
Pulsating auroras (PsAs) are thought to be generated by precipitating electrons scattered by lower‐band chorus (LBC) waves near the magnetic equator. One‐to‐one correlation between the LBC intensity ...and the PsA intensity has been reported. Electrostatic electron cyclotron harmonic (ECH) waves can also scatter electrons. However, direct correlation between ECH and PsA has not been reported yet. In this study, using a coordinated Exploration of energization and Radiation in Geospace (Arase) satellite and ground‐based imager observation, we report that not only LBC but also ECH have correlation with PsA. We estimated the precipitating electron energy by assuming that the time lag when the cross‐correlation coefficient became the highest was travel time of electrons from the modulation region. We found that the estimated energies show reasonable values as the cyclotron resonance energy of each wave.
Plain Language Summary
Pulsating auroras (PsAs), which have quasiperiodic on‐off switching emission, are caused by the intermittent electron precipitation from the magnetosphere. Such electrons are precipitated by wave‐particle interactions. The candidate waves to interact with electrons are lower‐band chorus (LBC) and electrostatic electron cyclotron harmonic (ECH) waves. One‐to‐one correspondence between the LBC wave intensity and the PsA intensity has been reported by previous studies. However, the correlation between ECH and PsA has not been reported yet. In this study, using a coordinated Exploration of energization and Radiation in Geospace (Arase) satellite and ground‐based all‐sky imager observation, we report that not only LBC but also ECH waves have correlation with PsAs.
Key Points
The lower‐band chorus and electrostatic electron cyclotron harmonic wave intensities had correlation with the pulsating auroral intensity
Taking advantage of high sampling rate of the imager, we estimated the energy of precipitating electrons
The energy of precipitating electrons was reasonable compared with the cyclotron resonance energy of each wave
Jupiter's sodium nebula, which originates from Io's volcanic gas, shows variations in its brightness due to the volcanic activity on Io. Imaging observation of D-line brightness in the sodium nebula ...was performed from 2013 through 2015 in a conjunction with the HISAKI mission. The D-line brightness of the sodium nebula had been stably faint and dim until January 2015, but it showed a distinct enhancement from February through March, 2015. The brightness increased by three times during this enhancement. Details in variations of Jupiter's sodium nebula are shown in this paper.
Inner magnetospheric electrons are precipitated in the ionosphere via pitch‐angle (PA) scattering by lower band chorus (LBC), upper band chorus (UBC), and electrostatic electron cyclotron harmonic ...(ECH) waves. However, the PA scattering efficiency of low‐energy electrons (0.1–10 keV) has not been investigated via in situ observations because of difficulties in flux measurements within loss cones at the magnetosphere. In this study, we demonstrate that LBC, UBC, and ECH waves contribute to PA scattering of electrons at different energy ranges using the Arase (ERG) satellite observation data and successively detected the moderate loss cone filling, that is, approaching strong diffusion. Approaching strong diffusion by LBC, UBC, and ECH waves occurred at ∼2–20 keV, ∼1–10 keV, and ∼0.1–2 keV, respectively. The occurrence rate of the approaching strong diffusion by high‐amplitude LBC (>50 pT), UBC (>20 pT), and ECH (>10 mV/m) waves, respectively, reached ∼70%, ∼40%, and ∼30% higher than that without simultaneous wave activity. The energy range in which the occurrence rate was high agreed with the range where the PA diffusion rate of each wave exceeded the strong diffusion level based on the quasilinear theory.
Key Points
The pitch‐angle scattering efficiencies by plasma waves are statistically investigated using in situ observations
Lower band chorus waves caused approaching strong diffusion with the highest occurrence rate in the energy range of ∼2–20 keV
Electrostatic electron cyclotron harmonic waves could contribute approaching strong diffusion in the ∼0.1–1 keV energy range
The innermost Galilean satellite, Io, supplies a large amount of volcanic gasses to the Jovian magnetosphere. The fast rotation of Jupiter and the outward transport of ionized gasses are responsible ...for forming a huge and rotationally dominant magnetosphere. The plasma supply from the satellite has a key role in the characterization of the Jovian magnetosphere. In fact, significant variations of the plasma population in the inner magnetosphere caused by the volcanic eruptions in Io were found in early 2015, using a continuous data set of the Io plasma torus obtained from an extreme ultraviolet spectroscope onboard the Hisaki satellite. The time evolution of the Io plasma torus radial distribution showed that the outward transport of plasma through 8 RJ from Jupiter was enhanced for approximately 2 months (from the end of January to the beginning of April 2015). Intense short‐lived auroral brightenings––which represent transient energy releases in the outer part of the magnetosphere—occurred frequently during this period. The short‐lived auroral brightenings accompanied well‐defined sporadic enhancements of the ion brightness in the plasma torus, indicating a rapid inward transport of energy from the outer part of the magnetosphere and the resultant enhancement of hot electron population in the inner magnetosphere. This evidently shows that the change in a plasma source in the inner magnetosphere affects a large‐scale radial circulation of mass and energy in a rotationally dominant magnetosphere.
Plain Language Summary
We present the first continuous and long‐term monitoring of both ultraviolet aurora activity and ionized gas around Jupiter obtained by the Earth‐orbiting spectroscope satellite, Hisaki. The innermost Galilean satellite, Io, is the volcanically most active body in our solar system. The volcanic gasses are ionized in the magnetosphere, the region manipulated by the planetary magnetic field, and obtain angular momentum from Jupiter's fast rotation through the magnetic field connecting with Jupiter. When Io's volcanic activity increased in early 2015, Hisaki observed that the Jovian magnetosphere was filled with iogenic ionized gasses for over 2 months and Jupiter's powerful auroral breakups occurred very frequently. This is contradictory to the terrestrial magnetosphere in which the aurora breakup occurs as a result of the solar wind‐energy penetration into the magnetosphere. Although Io occupies only a very small region in the vast Jovian magnetosphere, it releases significant amounts of material around the space near Jupiter, extracts energy from Jupiter's rotation, and affects activation of the powerful aurora of the giant planet.
Key Points
Evolution of Io plasma torus radial distribution caused by volcanic eruptions in Io was observed in early 2015
Outward plasma transport from the Io plasma torus through 8 RJ from Jupiter enhanced for approximately 2 months
An inner magnetosphere plasma source is shown to affect large‐scale mass/energy radial circulation in rotationally dominant magnetosphere
PDF
