High‐frequency (HF) ordinary polarized (O)‐mode waves transmitted along the Earth's geomagnetic field can excite plasma modes on both the bottomside and topside of the ionosphere, a process attributed to O‐to‐Z mode conversion. This mechanism explains how electromagnetic wave energy penetrates beyond the resonance. However, a key question is whether Z‐mode waves interacting with the resonance remain confined or escape the ionosphere via Z‐to‐O conversion. While O‐to‐Z conversion is well studied, Z‐to‐O remains less explored, especially in the presence of field‐aligned irregularities. We use two‐dimensional finite‐difference time‐domain simulations to investigate Z‐mode interaction with the resonance, with and without field‐aligned density ducts, across a range of magnetic field angles. The results show that duct‐assisted Z‐to‐O conversion is highly angle‐dependent. We compare its efficiency with that of O‐to‐Z conversion, and discuss implications for the magnetic zenith effect.

In the polar regions of the ionosphere, large-scale magnetic field-aligned irregularities serve as waveguides. These irregularities compel ordinary (O)-mode waves to travel along geomagnetic field lines as left-handed polarized (L)-mode waves, enabling them to pass through the O-mode reflection height beyond the standard radio windows. Previous experiments at the European Incoherent Scatter Scientific Association heating facility have shown that L-mode waves can excite plasma modes or escape from the ionosphere. We have developed a numerical model that adopts an arbitrary magnetic field-aligned density duct to investigate the characteristics of L-mode waves in an inhomogeneous ionosphere. The results demonstrate that L-mode waves can efficiently transmit through the plasma resonance layer of the duct to propagate beyond the O-mode reflection height. Upon reflection from their respective reflection height, they can transform into Z-mode waves, enhancing the amplitude of the electric fields at the plasma resonance. Our findings clarify how L-mode waves facilitate wave propagation beyond the O-mode reflection height and affect magnetic zenith effects.

In near-Earth space, the interplanetary magnetic field (IMF) is generally characterized by the Parker spiral configuration with a spiral angle of approximately 45° clockwise from the radial axis of the Sun. However, spacecraft observations reveal that IMF lines can exhibit non-Parker spiral configurations with either a nearly radial (outward–inward), tangential (westward–eastward), or normal (northward–southward) direction defined in radial–tangential–normal coordinates on even hourly timescales. Using the IMF and solar wind data measured by the Wind spacecraft for the period 1995–2023, we present distinctive features in the occurrence and various solar wind parameters between these non-Parker spiral IMF configuration types. Our study reveals that (1) the occurrence of non-Parker spiral IMF configuration types has the Hale cycle (∼22 yr magnetic dynamo cycle) variations with their own phases depending on the sign of the major IMF components, (2) the solar wind in the radial/tangential IMF structure is deflected eastward/westward, which corresponds to the direction opposite to/same as the rotation direction of the Sun on the plane perpendicular to the line of sight, and (3) these IMF configuration types can be characterized by magnetic field topology, which is open (for radial IMF and tangential IMF) or closed (for normal IMF). Our findings therefore provide insights into the formation mechanisms of these non-Parker spiral IMF configurations observed in near-Earth space. Moreover, our results suggest that spacecraft missions to various areas in interplanetary space are needed to better understand spatiotemporal evolutions of the solar-originated magnetic field structure in the heliosphere.

Standing poloidal Alfven waves, radial oscillations with ultralow frequency of 5–20 mHz, are excited by wave‐particle interaction in the magnetosphere. It has been a question over several decades why poloidal oscillations often persist and are not quickly converted into toroidal waves even when there is no corresponding particle source. By adopting magnetohydrodynamic simulations in the dipole model, we first show that long‐lasting poloidal waves are available if the local Alfven frequency gradient becomes negligible. Assuming that initial poloidal modes are excited by driving particles, we investigate two loss mechanisms into either toroidal modes or compressional modes. Both mechanisms enable us to derive two necessary conditions for the existence of persistent poloidal modes. It is found that these conditions are very consistent with statistical feature of observations. Our results will be useful in identifying the persistent poloidal mode in terms of the corresponding particle energy and wavenumbers.

Numerical simulations of the lunar sodium tail and its bright sodium spot are presented. As an attempt to do reverse engineering to understand the lunar sodium exosphere, we simulated velocity distributions of the sodium tail and its bright spot that emerges in the night sky around new Moon phases. In this work, three physical sodium sources (i.e., photon‐stimulated desorption [PSD], solar‐wind ion sputtering [SIS], meteor impact vaporization [MIV]), the dust anisotropy effect reported by Lunar Atmosphere and Dust Environment Explorer (LADEE), three gravitational sources (i.e., the Moon, the Earth, and the Sun), and variable solar radiation pressure effects are considered. We simulated velocity distributions of the lunar bright spot considering different source contribution ratios and their temperatures in order to find the best initial parameters which can account for the important observed recessional velocity of 12.4 km/s by Mierkiewicz et al. (2006, https://doi.org/10.1029/2006GL027650 ). We found that the upper limits of Maxwellian temperatures of 1,800 K for the PSD and 2,500 K for the MIV and that the best contribution ratio of the three major sodium sources (i.e., PSD: SIS: MIV) is found at the case of 2: 1: 3 (i.e., MIV dominant). Based on the previous brightness observations of the bright spot, the total production rate of the lunar sodium is estimated to usually vary between 0.4 and 1.3 × 10 22 /s. We introduce an interesting event in the lunar sodium tail, the Sodium Ring phenomena, which can occur transiently during the total solar eclipse due to the umbra and penumbra effects of the Earth.

Transpolar arcs (TPAs) are believed to predominantly occur under northward interplanetary magnetic field (IMF) conditions with their hemispheric asymmetry controlled by the Sun‐Earth (radial) component of the IMF. In this study, we present observations of TPAs that appear in both the northern and southern hemispheres even during a prolonged interval of radially oriented IMF. The Defense Meteorological Satellite Program (DMSP) F16 and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellites observed TPAs on the dawnside polar cap in both hemispheres (one TPA structure in the southern hemisphere and two in the northern hemisphere) during an interval of nearly earthward‐oriented IMF on October 29, 2005. The southern hemisphere TPA and one of the northern hemisphere TPAs are associated with electron and ion precipitation and mostly sunward plasma flow (with shears) relative to their surroundings. Meanwhile, the other TPA in the northern hemisphere is associated with an electron‐only precipitation and antisunward flow relative to its surroundings. Our observations indicate the following: (a) the TPA formation is not limited to northward IMF conditions; (b) the TPAs can be located on both closed field lines rooted in the polar cap of both hemispheres and open field lines connected to the northward field lines draped over one hemisphere of the magnetopause. We believe that the TPAs presented here are the result of both indirect and direct processes of solar wind energy transfer to the high‐latitude ionosphere.

Pi2 oscillations (40–150 s) in the nightside upper ionosphere are studied using magnetic field data acquired by multiple Swarm spacecraft in low‐Earth orbit and at the low‐latitude Bohyun (BOH, L = 1.3) station on 22 October 2014. Four Pi2 events were identified from the BOH data near midnight (magnetic local time = 1.5 hr), while Swarm‐A, Swarm‐B, and Swarm‐C spacecraft were orbiting in the premidnight (magnetic local time = 21–22 hr) meridian from 70° to −60° in magnetic latitude at ∼450‐ to 500‐km altitudes. Unlike previous low‐Earth orbit studies, which used a single point observation, the latitudinal structure of the amplitude and phase of ionospheric magnetic field perturbations can be determined by simultaneous multipoint observations along the latitude at a constant radial distance. We observed that the horizontal H component of BOH data is well correlated with the compressional (Bz) component of ionospheric magnetic fields when Swarm spacecraft were at |magnetic latitude|  < 30° with or without an accompanying ionospheric field perturbation in the radial (Bx) component, depending on the latitude of the spacecraft. It is found that the phase and amplitude relationship between Bx and Bz along the latitude is consistent with the model ionospheric field perturbations at 500‐km altitude, which are associated with a plasmaspheric resonance excited in a dipole numerical simulation. This indicates that the latitudinal variation of the ionospheric Pi2 pulsations in both Bx and Bz components is the consequence of the spatial mode structure in the north‐south direction of trapped fast mode waves inside the plasmasphere.

We discuss a role of the electron inertial effect on linearly polarized electromagnetic ion cyclotron (EMIC) waves at Earth. The linearly polarized EMIC waves have been previously suggested to be generated via mode conversion from the fast compressional wave at the ion‐ion hybrid (IIH) resonance. When the electron inertial effects are neglected, the wave normal angle of the mode‐converted IIH waves is 90° because the wave vector perpendicular to the magnetic field becomes infinite at the IIH resonance. When the electron inertial effect is considered, the mode‐converted IIH waves can propagate across the magnetic field lines, and the wavelength perpendicular to the magnetic field approaches the electron inertial length scale near the Buchsbaum resonance. These waves are referred to as electron inertial waves. Due to the electron inertial effect, the perpendicular wave number to the ambient magnetic field near the IIH resonance remains finite, and the wave normal angle is less than 90°. The wave normal angle where the maximum absorption occurs in a dipole magnetic field is 30–80°, which is consistent with the observed values near the magnetic equator. Therefore, the numerical results suggest that the linearly polarized EMIC wave generated via mode conversion near the IIH resonance can be detected in between the Buchsbaum and the IIH resonance frequencies, and these waves can have normal angle less than 90°.

In one model, Pi2 pulsations are driven pulse by pulse by fast mode pulses that are launched as periodic bursty bulk flows brake when they approach the Earth. We have examined this model by analyzing data from multiple spacecraft and ground magnetometers for a Pi2 pulsation event. During the event, which started at ∼2226 UT on 8 November 2014, Time History of Events and Macroscale Interactions during Substorms (THEMIS)‐D detected an ∼2‐min‐period plasma bulk flow oscillation in the near‐Earth magnetotail, while THEMIS‐E and Van Allen Probes‐B, both located on the nightside just earthward of the electron plasmapause, detected a Pi2 pulsation consisting of a 10‐mHz oscillation in the azimuthal component of the electric field and a 19‐mHz oscillation in the compressional component of the magnetic field. On the ground, magnetic field oscillations containing both frequencies were observed both on the nightside and on the dayside. The nightside observations indicated that the pulsation had a radially standing structure, which is consistent with plasmaspheric virtual resonances (PVRs) excited in a magnetohydrodynamic simulation assuming an impulsive energy source. Cross‐spectral analysis of the magnetotail flow oscillation and the Pi2 pulsation indicated low coherence between them. These results suggest that the flow oscillation contributed to the Pi2 pulsation as a broadband energy source and that only the spectral components matching the PVR frequencies were detected with well‐defined frequencies. Ionospheric currents connected to the PVRs may be responsible for the appearance of the pulsation on the dayside.

The 3 mu m spectrum of Titan contains line emission and absorption as well as a significant haze continuum. The line emission has been previously analyzed in the literature, but that analysis has not properly included the influence of haze on the line emission. We report a new analysis of the 3 mu m HCN emission spectrum using radiative transfer equations that include scattering and absorption by molecules and haze particles at altitudes lower than 500 km, where the influence of haze on the emergent spectrum becomes significant. Taking advantage of the dominance of resonant single scattering in the HCN nu(3) fundamental and of the moderate haze optical thickness of the atmosphere around 3 mu m, we adopt single dust and molecular scattering and present a formulation for the radiative transfer process. We evaluate the quantitative influence of haze scattering on the emission line intensities, and derive vertically-resolved single scattering albedos of the haze from model fits. We also present the resulting concentrations of HCN for altitudes below 500 km, where we find that the haze scattering significantly influences the retrieval of the concentrations of HCN. We conclude that the formulation we present is useful for the analysis of the HCN line emission from Titan and other similar hazy planetary or celestial objects. (C) 2018 Elsevier Ltd. All rights reserved.

Observations in space and on the ground have shown that azimuthally polarized (toroidal) magnetospheric Pc5 waves (frequency = 1.7–6.7 mHz) are more prominent on the dawnside than on the duskside, but the origin of this dawn‐dusk asymmetry is unknown. To determine the origin, we conducted a statistical analysis of toroidal Pc5 waves at the Time History of Events and Macroscale Interactions during Substorms (THEMIS) D spacecraft using ion bulk velocity measured during 2008–2014. We found that both the detection rate of narrowband Pc5 waves, which are attributed to fundamental toroidal‐mode standing Alfvén waves, and the amplitude of velocity oscillations in the fixed Pc5 band exhibit a dawn‐dusk asymmetry consistent with previous studies regardless of whether the interplanetary magnetic field is directed along the Parker spiral or orthogonal to it. These results imply that the asymmetry originates within the magnetosphere, not from a dawn‐dusk asymmetry of the Kelvin‐Helmholtz instability on the magnetopause. Based on theoretical studies published previously and a global magnetohydrodynamic simulation, we propose that the asymmetry originates from the local time dependence of the radial mass density variation and the associated gradient of the frequency of standing Alfvén waves. The THEMIS data indicate that in the L range 8–12 the average gradient is smaller in the dusk sector than in the dawn sector. This will result in weaker field line resonance (FLR), and thus weaker toroidal Pc5 waves, in the dusk sector. Density structures associated with drainage plumes may also suppress FLR in the dusk sector.