Journal of Geophysical Research: Space Physics最新文献

Mars, being a small planet with a tenuous atmosphere, does not have a sharp boundary between regions dominated by solar wind plasma and planetary plasma. Instead, this transition is typically extended, allowing the interplanetary magnetic field (IMF) to penetrate into the Martian ionosphere. However, the depth of this penetration is not well understood. Using 6 years of MAVEN data, we statistically assess locations where a transition exists between the dominance of magnetic versus cold (1 eV), thermal plasma pressure to better understand the reach of the IMF. We identify the presence or absence of pressure transitions from 200 to 800 km altitude for each MAVEN orbit and find a clear transition in $��\sim �$ 55% of cases. The pressure transition locations are mapped in different coordinate systems that provide insight into the solar and planetary driving conditions that cause a detected transition region. Transitions are more likely to occur under weak-to-nominal solar wind conditions, away from strong crustal magnetic fields, near the terminator, on the dusk side of the planet compared to the dawn side, and in the negative solar wind motional electric field hemisphere. We speculate on possible causes for asymmetries that arise in the mapped locations of these pressure transitions and the effect that penetrated IMF may have on driving plasma dynamics in the Martian ionosphere.

Pc1 pulsations cover the 0.2–5 Hz frequency range with electromagnetic ion cyclotron (EMIC) waves of magnetospheric origin being generally accepted as their most important source. In the ionosphere, the initially transverse EMIC waves can couple to the compressional mode and propagate long distances in the ionospheric waveguide. By studying Pc1 waves in the topside ionosphere, we can obtain information on the spatial distribution of both the transverse (incident EMIC) and compressional waves. In the present paper, we make use of a new Swarm L2 product developed for characterizing Pc1 waves to explore the spatial distribution of these waves relative to the midlatitude ionospheric trough (MIT), which corresponds to the ionospheric footprint of the plasmapause (PP) at night. It is shown that the vast majority of Pc1 events are located inside the plasmasphere and that the spatial distributions clearly follow changes in the MIT/PP position at all levels of geomagnetic activity. In the topside ionosphere, the number of transverse Pc1 (incident EMIC) waves rapidly decreases outside the PP, while their occurrence peak is located considerably equatorward (|ΔMlat| = −5° to −15°) of the PP footprint, that is, inside the plasmasphere. On the other hand, the compressional Pc1 waves can propagate in the ionosphere outside the PP toward the poles, while in the equatorial direction there is a secondary maximum in their spatial distribution at low magnetic latitudes. Our results suggest that mode conversion taking place in the inductive ionosphere plays a crucial role in the formation of the presented spatial distributions.

Trans-ionospheric radio signals recorded on the ground exhibit random amplitude and phase fluctuations attributed to irregularities in the ionospheric electron density. Studying the ground-based measurements of trans-ionospheric radio signals can contribute to understanding plasma instability mechanisms leading to the development of ionospheric structures. In this regard, radio signals emitted from satellites, making up the Global Positioning System (GPS), and recorded by the Canadian High Arctic Ionospheric Network (CHAIN) GPS receivers, are analyzed to study the physical signatures of both amplitude and phase fluctuations. The current ionospheric scintillation paradigm posits that amplitude fluctuations arise from diffraction caused by Fresnel scale ionospheric structures, while refraction is responsible for signal phase variations. The amplitude power spectrum profile consistently displays a rollover frequency, which is not equal to the Fresnel frequency under the Taylor hypothesis. Phase screen theory is used to investigate this phenomenon further and identify an empirical relation between the rollover and Fresnel frequencies. Notably, we have found that the rollover frequency is consistently smaller than the Fresnel frequency. Furthermore, the Fresnel frequency extracted from two-component phase spectra tends to be larger than the rollover frequency. Based on our results, we have concluded that the identified Fresnel frequencies are directly linked to the ionospheric irregularities causing scintillation.

The Venusian O(¹S–¹D) 5577 Å “oxygen green line” has been an enigmatic feature of the Venusian atmosphere since its first attempted observation by the Venera spacecraft. Its first detection in 1999 and subsequent detections point to a unique auroral phenomena. However, the lack of (¹D–³P) 6300 Å “oxygen red line” emission suggests that the green line originates from deep in the ionosphere, much lower than current models predict. Here, we present 16 years of ground-based observations of the Venusian green line, comparing its behavior to the solar wind and spacecraft observations of the Venusian ionosphere. We find that all instances of green line emission occur during solar energetic particle (SEP) events, with a Matthews correlation coefficient of 0.93 between emission and the presence of SEPs. Coordinated observations between Venus Express and ground-based observatories show enhanced nightside ionospheric peak densities during the time of green line emission, with the lowest peak occurring at 115 km near local midnight. Such high density yet low altitude peaks suggest the presence of highly energetic particle precipitation. Initial modeling indicates $��\ge �$50 keV protons are needed to penetrate to such low altitudes. Comparisons of solar wind data confirm that such protons are present during all green line detections and nightside ionosphere enhancements. The association of SEP storms with green line emission and low nightside ionospheric peaks indicates that the green line is a unique global diffuse aurora, likely originating deep in the ionosphere and driven by proton precipitation, something that could be common for all non-magnetic planetary atmospheres.

Martian diffuse auroras are ultraviolet emissions spread across the nightside of Mars caused by solar energetic particles (SEP), both electrons and protons. The nightside structures of induced and crustal magnetic fields are expected to affect the diffuse auroral emission profiles caused by electrons, which is far from understood. Here we estimate magnetic field effects on emission based on a newly developed Monte Carlo model simulating collisions and electron cyclotron motions. Parameter surveys of the magnetic field intensity and dip angle (angle of magnetic field line from horizontal direction) under uniform magnetic field structure show that the effects of magnetic field dip angle on auroral altitude profiles are greater than those of magnetic field intensity. We then applied our model to the September 2017 diffuse aurora event using MAVEN SEP electron flux observations and neutral atmospheric profile from the Mars Climate Database as inputs. Comparison between horizontal and vertical magnetic field dip angle cases indicates that the horizontal dip angle case results in broader limb-integrated auroral altitude profiles than the vertical case and enhances the auroral intensity at high altitudes (>75 km). The magnetic field structure can be one of the important factors in understanding the Martian diffuse auroras.

We present a model of the interaction between energetic heliospheric ions and Pluto's induced magnetosphere. The electromagnetic fields near the dwarf planet are highly non-uniform, displaying extended signatures of pile-up and draping. While the induced magnetosphere possesses a downstream extension above 100 Pluto radii, the weak interplanetary magnetic field in the outer heliosphere leads energetic ions to gyrate on comparable length scales. We obtain the three-dimensional structure of the fields near Pluto using a hybrid model, and a particle tracing tool is applied to study the dynamics of energetic ions traveling through these fields. For multiple initial energies, we compute the ion fluxes through a plane detector downstream of Pluto. Our results are as follows: (a) Deflection by Pluto's induced magnetosphere causes highly non-uniform perturbations in the flux pattern of energetic ions at its downstream side. These patterns include regions where the fluxes are increased or reduced by up to 40%, compared to the values in uniform fields. (b) Consistent with findings from New Horizons, the modeled perturbations gradually diminish with distance downstream of the dwarf planet out to 200 Pluto radii. (c) The deflection of the energetic ions mainly occurs within regions of Pluto's induced magnetosphere where the magnetic field is significantly enhanced, thereby causing a localized reduction in gyroradii. (d) The magnitude of the depletion in flux in our steady-state model is weaker than seen by New Horizons; this may suggest that time-dependent processes in Pluto's wake (e.g., bi-ion waves) play a major role in deflecting these ions.

Observations from the Global-scale Observations of Limb and Disk (GOLD) mission have provided a new remote-sensing data source of molecular oxygen profiles in Earth's lower-to-middle thermosphere (120–200 km). GOLD O₂ observations indicate increasing densities of molecular oxygen at 170 km with rising solar activity between solar radio flux F10.7 values of 60 and ∼120 solar flux units (sfu). This is also seen in comparisons with solar extreme ultraviolet irradiance Q_EUV between 1 and ∼2.25 erg cm⁻² s⁻¹. However, the empirical Mass Spectrometer Incoherent Scatter radar 2.0 (MSIS 2.0) model overestimates O₂ densities at 170 km at these low levels of solar activity and predicts a decreasing density with increasing solar flux below ∼120 sfu. Additional data sets validate GOLD observations of O₂ and their relationship with solar activity. Accurately determining and forecasting O₂ is critical for accurately modeling plasma densities in the ionosphere and thermospheric density in the lower-to-middle thermosphere.

In this study, we compare responses of the thermosphere and ionosphere to a planet encircling dust event in 2018 (PEDE 2018) occurred at the beginning of June 2018, a rare event last observed in 2007. Using the data from NGIMS (Neutral Gas and Ion Mass Spectrometer) instrument onboard MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft, we examine wave signatures and their spectral changes of three neutral species (O, Ar, CO₂) and three ion species (O⁺, O₂⁺, CO₂⁺). Compared to low-dust period, the following salient features stand out: (a) Wave activities (amplitudes) are significantly enhanced in the neutral species, but with smaller enhancement in the ion species; (b) The apparent wavelengths in the neutrals shift and broaden from a range of 50–300 km to 80–400 km, with dominant wavelength shifting from 100 to 200 km; (c) The apparent wavelengths in the ions shift and broaden from a range of 80–300 km to 100–400 km, with dominant wavelength shifting from 150 km to 250–300 km; (d) The correlations among the wavelength distributions of various species are high with correlation coefficient above 0.57 during low-dust period and above 0.69 during PEDE 2018. These observed changes in wave signatures likely reflect filtering effects caused by changes in the middle atmosphere circulation induced by the global dust storm.

In this paper, we investigated the seasonal and geomagnetic dependence of the auroral $��E�$-region neutral winds and the tidal components between 90 and 125 km using nearly continuously sampled measurements from the Poker Flat Incoherent Scatter Radar (PFISR) from 2010 to 2019. The average winds show consistent semidiurnal oscillations between 100 and 115 km and diurnal oscillations above 115 km in all seasons with some seasonal and geomagnetic activity dependencies. In general, the semidiurnal oscillation in zonal and meridional directions is strongest in summer and weakest in winter. The diurnal oscillation is strongest in winter and weakest in spring. More details on the seasonal and geomagnetic activity dependencies are revealed in the tidal decomposition results. Tidal decomposition results show eastward mean wind below 115 km in summer, fall, and winter and westward mean wind above 115 km in all seasons. The meridional mean is northward below 115 km and southward above in all seasons. The diurnal amplitudes are small below 110 km and increase with altitude above 110 km in all seasons with larger enhancements in the meridional direction. The semidiurnal amplitudes increase with altitude below 110 km and reach a maximum at around 110 km, then decrease or keep stable (depending on the geomagnetic activity) above 110 km in both directions and all seasons. The diurnal phases shift to earlier times with the increase of geomagnetic activity but show different variations with altitudes in zonal and meridional directions. The semidiurnal phases show a downward progressing trend in both directions and in all seasons.

Intense upward electron beams were measured by the Juno JADE instrument in the northern hemisphere, low-latitude auroral zone source region. In this study we report on how these electron beams interact with plasma near and within the Jovian hectometric (HOM) emission (1 MHz 5 MHz) source region. Within the source region large upward loss cones are observed in the northern polar region at radial distances of $��\sim �$ 2Rj, magnetic latitude of $��\sim �70�°�$. Intense, narrow electron beams ( 3 keV) are then observed, but within one second wave-particle scattering is observed, filling the loss cone to energies $��>�$50 keV. These energies persist for several seconds before fading, leaving an empty loss cone again. The loss cone provides a free-energy source for HOM emission resulting from the cyclotron maser instability. We use quasilinear analysis to examine the generation of HOM and the dynamics of wave-particle interaction of the electron beams with HOM, and the generation via Landau interaction of whistler mode emission. The dynamic spectrum of the HOM emission generated by the loss-cone electrons as well as that of the low-frequency whistler-mode waves generated by the up-going electron beam can be constructed by quasilinear theory, which compare well with observation. The saturated state of the energetic electron velocity distribution function constructed via quasilinear theory also compare reasonably with observation.