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Auroral arcs and diffuse auroras are common phenomena at high latitudes, though characteristics of their source plasma and fields have not been well understood. We report the first observation of fields and particles including their pitch‐angle distributions in the source region of auroral arcs and diffuse auroras, using data from the Arase satellite at L ~ 6.0–6.5. The auroral arcs appeared and expanded both poleward and equatorward at local midnight from ~0308 UT on 11 September 2018 at Nain (magnetic latitude: 66°), Canada, during the expansion phase of a substorm, while diffuse auroras covered the whole sky after 0348 UT. The top part of auroral arcs was characterized by purple/blue emissions. Bidirectional field‐aligned electrons with structured energy‐time spectra were observed in the source region of auroral arcs, while source electrons became isotropic and less structured in the diffuse auroral region afterwards. We suggest that structured bidirectional electrons at energies below a few keV were caused by upward field‐aligned potential differences (upward electric field along geomagnetic field) reaching high altitudes (~30,000 km) above Arase. The bidirectional electrons above a few keV were probably caused by Fermi acceleration associated with the observed field dipolarization. Strong electric‐field fluctuations and earthward Poynting flux were observed at the arc crossing and are probably also caused by the field dipolarization. The ions showed time‐pitch‐angle dispersion caused by mirror reflection. These results indicate a clear contrast between auroral arcs and diffuse auroras in terms of source plasma and fields and generation mechanisms of auroral arcs in the inner magnetosphere. Plain Language Summary: Auroral arcs or curtains are a familiar phenomenon seen at high latitudes, and aurora is also present in diffuse form. In near‐Earth space, plasmas and electric and magnetic fields are known to have interactions that cause the auroral phenomena, but a direct link between measurements in space and the corresponding auroras near the surface has been hard to make. The Arase satellite carries advanced plasma and field instruments allowing detailed study of conditions in space, so campaign observations were carried out at Nain, Canada, in September 2018, at a time when it was expected to be connected to the region by following magnetic field lines. Both discrete and diffuse auroras were observed from the ground, while in space electrons and ions were observed, whose characteristics changed depending on whether the spacecraft was lined up with discrete auroras, or later, with diffuse auroras. The auroras and particles changed with time due to a "substorm," which made the auroras brighten while making the particles more energetic. The observations suggest mechanisms by which the particles may be energized during substorms, with direct measurements in the source region. Key Points: This is the first observation of the source plasma features of auroral arcs and diffuse auroras in the inner magnetosphere by AraseUnique purple/blue auroral arcs appeared in association with an arrival of corotating interaction region (CIR) in the solar windDuring the arc crossing, field‐aligned electrons, time‐pitch‐angle dispersion of ions, and strong electric field variations were observed [ABSTRACT FROM AUTHOR]

We present the first and direct comparison between magnetospheric plasma waves and polar mesosphere winter echoes (PMWE) simultaneously observed by the conjugate observation with Arase satellite and high‐power atmospheric radars in both hemispheres, namely, the Program of the Antarctic Syowa Mesosphere, Stratosphere, and Troposphere/Incoherent Scatter Radar at Syowa Station (SYO; −69.00°S, 39.58°E), Antarctica, and the Middle Atmosphere Alomar Radar System at Andøya (AND; 69.30°N, 16.04°E), Norway. The PMWE were observed during 03–07 UT on 21 March 2017, just after the arrival of corotating interaction region in front of high‐speed solar wind stream. An isolated substorm occurred at 04 UT during this interval. Electromagnetic ion cyclotron (EMIC) waves and whistler mode chorus waves were simultaneously observed near the magnetic equator and showed similar temporal variations to that of the PMWE. These results indicate that chorus waves as well as EMIC waves are drivers of precipitation of energetic electrons, including relativistic electrons, which make PMWE detectable at 55‐ to 80‐km altitude. Cosmic noise absorption measured with a 38.2‐MHz imaging riometer and low‐altitude echoes at 55–70 km measured with an medium‐frequency radar at SYO also support the relativistic electron precipitation. We suggest a possible scenario in which the various phenomena observed in near‐Earth space, such as magnetospheric plasma waves (EMIC waves and chorus waves), pulsating auroras, cosmic noise absorption, and PMWE, can be explained by the interaction between the high‐speed solar wind containing corotating interaction regions and the magnetosphere. Key Points: EMIC waves and chorus waves in the magnetosphere and PMWE in the mesosphere were observed simultaneously by the conjugate observationPMWE were detected by the PANSY and MAARSY radars in both Northern and Southern Hemispheres during the equinox periodThe temporal variation of the chorus wave power was quite similar to those of PMWE powers observed in both hemispheres [ABSTRACT FROM AUTHOR]

Abstract: The Arase satellite observed clear dipolarization signatures at r~4.3–4.6 RE, GMLAT~16°–18°, and MLT~5.5–5.7 hr around 15:00 UT on 27 March 2017 when Dst~−70 nT. Strong magnetic field fluctuations were embedded and their characteristic frequency was close to the local gyrofrequency of O+ ions. After the dipolarization, O+ flux was enhanced at ≤15 keV, while H+ flux showed no clear variations. These observations provide evidence for the direct supply of O+ ions from the ionosphere. There were no clear signatures for the nonadiabatic local acceleration of O+ ions. We consider that a bump‐on‐tail structure in the energy spectrum around 30–50 keV due to a combination of charge exchange loss and drift motion of ions masks the nonadiabatic acceleration. Occurrence of the magnetic field dipolarization at dawn, which is far from the well‐known premidnight occurrence peak, may be due to an eastward skewing of partial ring current during the storm main phase. [ABSTRACT FROM AUTHOR]

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 intensityTaking advantage of high sampling rate of the imager, we estimated the energy of precipitating electronsThe energy of precipitating electrons was reasonable compared with the cyclotron resonance energy of each wave [ABSTRACT FROM AUTHOR]