Formation of the oxygen torus in the inner magnetosphere: Van Allen Probes observations

We study the formation process of an oxygen torus during the 12–15 November 2012 magnetic storm, using the magnetic field and plasma wave data obtained by Van Allen Probes. We estimate the local plasma mass density (ρL) and the local electron number density (neL) from the resonant frequencies of standing Alfvén waves and the upper hybrid resonance band. The average ion mass (M) can be calculated by M ∼ ρL/neL under the assumption of quasi-neutrality of plasma. During the storm recovery phase, both Probe A and Probe B observe the oxygen torus at L = 3.0–4.0 and L = 3.7–4.5, respectively, on the morning side. The oxygen torus has M = 4.5–8 amu and extends around the plasmapause that is identified at L∼3.2–3.9. We find that during the initial phase, M is 4–7 amu throughout the plasma trough and remains at ∼1 amu in the plasmasphere, implying that ionospheric O+ ions are supplied into the inner magnetosphere already in the initial phase of the magnetic storm. Numerical calculation under a decrease of the convection electric field reveals that some of thermal O+ ions distributed throughout the plasma trough are trapped within the expanded plasmasphere, whereas some of them drift around the plasmapause on the dawnside. This creates the oxygen torus spreading near the plasmapause, which is consistent with the Van Allen Probes observations. We conclude that the oxygen torus identified in this study favors the formation scenario of supplying O+ in the inner magnetosphere during the initial phase and subsequent drift during the recovery phase

Meridional Distribution of Middle-Energy Protons and Pressure-Driven Currents in the Nightside Inner Magnetosphere: Arase Observations

We examined the average meridional distribution of middle‐energy protons (10–180 keV) and pressure‐driven currents in the nightside (20–04 hr magnetic local time) ring current region during moderately disturbed times using the Arase satellite\u27s data. Because the Arase satellite has a large inclination orbit of 31°, it covers the magnetic latitude (MLAT) in the range of −40° to 40° and a radial distance of <6RE. We found that the plasma pressure decreased significantly with increasing MLAT. The plasma pressure on the same L* shell at 30° < MLAT < 40° was ∼10–60% of that at 0° < 4 MLAT < 10°, and the rate of decrease was larger on lower L* shells. The pressure anisotropy, derived as the perpendicular pressure divided by the parallel pressure minus 1, decreased with radial distance and showed a weak dependence on MLAT. The magnitude of the plasma beta at 30°<MLAT<40° was 1 or 2 orders smaller than that at 0°<MLAT<10°. The plasma pressure normalized by the value at 0°<MLAT<10° estimated from the magnetic strength and anisotropy was roughly consistent with the observed plasma pressure for L*=3.5–5.5. The azimuthal pressure‐gradient current derived from the plasma pressure was distributed over MLAT∼0–20°, while the curvature current was limited within MLAT∼0–10°. We suggest that the latitudinal dependence should be taken into account in interpretations of plasma parameters in successive orbits during magnetic storms

Arase Observation of the Source Region of Auroral Arcs and Diffuse Auroras in the Inner Magnetosphere

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

Statistical Properties of Molecular Ions in the Ring Current Observed by the Arase (ERG) Satellite

Molecular ions in the magnetosphere can be a tracer of fast ion outflows from the deep ionosphere. Statistical properties of molecular ions (O2+/NO+/N2+) in the ring current are investigated based on ion composition measurements (<180 keV/q) by medium‐energy particle experiments‐electron analyzer and low‐energy particle experiments‐ion mass analyzer instruments on board the Arase (Exploration of energization and Radiation in Geospace, ERG) satellite. The investigated period from late March to December 2017 includes 11 geomagnetic storms with the minimum Dst index less than −40 nT. The molecular ions are observed in the region of L = 2.5–6.6 and clearly identified at energies above ~12 keV during most magnetic storms. During quiet times, molecular ions are not observed. The average energy density ratio of the molecular ions to O+ is ~3%. The ratio tends to increase with the size of magnetic storms. Existence of molecular ions even during small magnetic storms suggests that the fast ion outflow from the deep ionosphere occurs frequently during geomagnetically active periods

EMIC waves converted from equatorial noise due to M/Q=2 ions in the plasmasphere: Observations from Van Allen Probes and Arase

Equatorial noise (EN) emissions are observed inside and outside the plasmapause. EN emissions are referred to as magnetosonic mode waves. Using data from Van Allen Probes and Arase, we found conversion from EN emissions to electromagnetic ion cyclotron (EMIC) waves in the plasmasphere and in the topside ionosphere. A low frequency part of EN emissions becomes EMIC waves through branch splitting of EN emissions, and the mode conversion from EN to EMIC waves occurs around the frequency of M/Q=2 (deuteron and/or alpha particles) cyclotron frequency. These processes result in plasmaspheric EMIC waves. We investigated the ion composition ratio by characteristic frequencies of EN emissions and EMIC waves and obtained ion composition ratios. We found that the maximum composition ratio of M/Q=2 ions is ~10% below 3000 km. The quantitative estimation of the ion composition will contribute to improving the plasma model of the deep plasmasphere and the topside ionosphere

Periodontal disease and atherosclerosis from the viewpoint of the relationship between community periodontal index of treatment needs and brachial-ankle pulse wave velocity

BACKGROUND: It has been suggested that periodontal disease may be an independent risk factor for the development of atherosclerosis. However, the relationship between periodontal disease and atherosclerosis has not been fully elucidated. This study aimed to assess the effects of periodontal disease on atherosclerosis. METHODS: The study design was a cross-sectional study. Subjects were 291 healthy male workers in Japan. We used the Community Periodontal Index of Treatment Needs (CPITN) score, average probing depth and gingival bleeding index (rate of bleeding gums) to assess the severity of periodontal disease. We also used the Brachial-Ankle Pulse Wave Velocity (baPWV) as the index for the development of atherosclerosis. RESULTS: The unadjusted odds ratio (OR) of atherosclerosis in relation to the CPITN score was 1.41 [95% CI: 1.16–1.73]. However, after adjustment for age, systolic blood pressure and smoking, the CPITN score had no relationship with atherosclerosis (adjusted OR: 0.91 [0.68–1.20]). CONCLUSION: Our results show no relationship between mild periodontal disease and atherosclerosis after appropriate adjustments

Penetration of MeV electrons into the mesosphere accompanying pulsating aurorae

Pulsating aurorae (PsA) are caused by the intermittent precipitations of magnetospheric electrons (energies of a few keV to a few tens of keV) through wave-particle interactions, thereby depositing most of their energy at altitudes ~ 100 km. However, the maximum energy of precipitated electrons and its impacts on the atmosphere are unknown. Herein, we report unique observations by the European Incoherent Scatter (EISCAT) radar showing electron precipitations ranging from a few hundred keV to a few MeV during a PsA associated with a weak geomagnetic storm. Simultaneously, the Arase spacecraft has observed intense whistler-mode chorus waves at the conjugate location along magnetic field lines. A computer simulation based on the EISCAT observations shows immediate catalytic ozone depletion at the mesospheric altitudes. Since PsA occurs frequently, often in daily basis, and extends its impact over large MLT areas, we anticipate that the PsA possesses a significant forcing to the mesospheric ozone chemistry in high latitudes through high energy electron precipitations. Therefore, the generation of PsA results in the depletion of mesospheric ozone through high-energy electron precipitations caused by whistler-mode chorus waves, which are similar to the well-known effect due to solar energetic protons triggered by solar flares