Context. Interstellar Boundary Explorer images have revealed a globally distributed flux of energetic neutral atoms (ENAs) at similar to 0.2-6 keV while Cassini observed an ENA belt at 5-55 keV likely originating from the inner heliosheath (IHS) protons via charge exchange with penetrating interstellar neutrals. Such ENAs are considered to reflect solar wind variations to some extent. Aims. We explore ENA flux sensitivity in the IHS to solar wind changes at Carrington rotation (CR) resolution and quantify its dependence on IHS ion distributions. Methods. We utilized three models for ion distributions designed to respond to solar wind changes upstream of the termination shock (TS), from which the corresponding variations in ENA fluxes were computed. All three ion models employ a regularized kappa distribution for solar wind protons. The models differ in the treatment of transmitted pickup ions (PUIs) and reflected PUIs with different combinations of regularized kappa and filled-shell distributions. Results. Our ENA estimates reveal the potential for substantial flux change rates between adjacent CR times, often exceeding several tens of percent, a feature not recognized in previous studies. Such rapid variations in ENA flux levels exhibit a correlation with concurrent fluctuations in solar wind speed and density upstream of the TS. However, the specific characteristics of these ENA changes are contingent on the ion distribution model and the energy considered. Most notably, employing the filled-shell distribution for transmitted PUIs induces noticeable alterations in ENA flux near their cutoff energy (similar to 0.5-1.5 keV), responding promptly to rapid variations in solar wind bulk speed. Furthermore, the inclusion of reflected PUIs is critical in the high-energy regime (> similar to 10 keV), which is typically associated with coronal hole fast streams, where ENA fluxes exhibit strong correlations with changes in the solar wind bulk speed and dynamic pressure. Conclusions. The results underscore the importance of precise PUI information in the IHS for accurate ENA estimation during swift solar wind changes.

Loss of relativistic electrons with energies above 0.5 MeV through the magnetopause, commonly referred to as magnetopause shadowing, has been extensively investigated using both simulations and observations. However, direct and systematic observational evidence remains limited. In this study, we extend a previously developed magnetopause model by incorporating inward magnetopause motion, which generates induced electric fields within the boundary layer. These fields are amplified during compression and further enhanced by magnetic field strengthening on the magnetospheric side. Relativistic electrons (0.5-a few MeV) are influenced by the induced fields primarily through gyroradius reduction caused by energy loss and through drift toward the magnetosphere, both of which inhibit escape. Stronger electric fields extend the energy range of confined electrons toward higher energies. Depending on the local field geometry, the induced electric field can either accelerate or decelerate electrons. In contrast, ultra-relativistic electrons (several MeV), with gyroradii comparable to the magnetopause thickness, are less affected and more likely to escape directly. When a finite magnetic field component normal to the magnetopause is present, electrons across a broad energy range (0.5-10 MeV) can escape efficiently, as this component guides their motion across the boundary and reduces sensitivity to the induced field. In such cases, energy variations still occur but become less significant with increasing normal component strength, owing to shorter residence times within the magnetopause.

In this paper, we conducted an analysis of the heliospheric current sheets (HCSs) in the heliosheath (HS), utilizing observations by Voyager 2 between 2008 and 2018. Employing rigorous criteria, we identified a total of 34 HCSs that indicate significant changes in magnetic polarity. These occurrences were more prevalent during solar maximum periods when the HCS expanded to higher latitudes, coinciding with the spacecraft positioned at an average latitude of -31 degrees from the solar equator. We determined certain features of the large-scale structures of the identified HCSs. Most importantly, employing two distinct methods indicates that the thickness of the HCSs within the HS ranges from similar to 0.003 to similar to 0.4 au with an average thickness of similar to 0.03 to similar to 0.1 au, depending on methods of event selection and fitting techniques. This thickness surpasses that known near 1 au or other heliospheric distances. It is also notably thicker than the typical proton inertial length, implying unfavorable conditions for magnetic reconnection. Additionally, our analysis reveals a frequent tilt of HCS planes relative to the solar equatorial plane by a varying angle up to several tens of degrees, likely implying a common occurrence of a warped structure of the HCS within the HS. Longitudinally, the HCS planes closely align with the Parker spiral field direction expected in the HS. Finally, for a large fraction of the identified HCS events, the HCS planes are likely characterized by a rotational discontinuity. These findings are valid within the limits of the 1 hr resolution data used in this study.

Exohiss waves below 0.1 electron cyclotron frequency ( f ce ) are structureless whistler‐mode emissions typically observed in the plasmatrough. Plasmaspheric hiss may possibly propagate from the plasmasphere into the plasmatrough and evolve into exohiss waves. We investigated the relationship of exohiss occurrence and characteristics with plasmaspheric hiss occurrence and solar wind parameters, analyzing Van Allen Probe observations from 1 October 2012 to 28 February 2018. Exohiss waves observed in the plasmatrough occurred more frequently on the dayside than the nightside, which was consistent with the plasmaspheric hiss distribution in the plasmasphere. Exohiss occurrence gradually increased up to ∼4 hr after hiss measurements and showed a magnetic local time dependence on the plasmaspheric hiss amplitude. We also determined the relative contribution of each solar wind parameter to exohiss distribution as based on exohiss measurements made 0–4 hr after plasmaspheric hiss measurements. A stronger southward interplanetary magnetic field (IMF) B Z limited the region of exohiss occurrence to the prenoon sector, again consistent with the distribution of plasmaspheric hiss. Prenoon exohiss was also observed for stronger dynamic pressure ( P SW ), but the plasmaspheric hiss appeared in the postnoon sector. This discrepancy indicated that prenoon exohiss is locally excited rather than a product of plasmaspheric hiss leakage. In addition, during enhanced solar wind conditions with southward IMF B Z or higher P SW , the intensity of the lower‐band chorus was enhanced even below 0.1 f ce, corresponding to the frequency range of the exohiss, implying that the nightside exohiss may be related to the evolution of low‐frequency chorus waves.

Magnetosonic waves, or equatorial noise, are whistler-mode emissions distributed near the Earth's magnetic equator between proton cyclotron frequency and lower hybrid resonance frequency. Their origin and characteristics inside and outside the plasmasphere have been investigated due to their potential role in scattering energetic electrons and protons. However, their characteristics in plasmaspheric plumes remained undocumented. This study, for the first time, statistically investigated magnetosonic waves in the plasmaspheric plumes based on the entire mission period (2012-2019) of Van Allen Probes A and B. Results showed that the occurrence rate of magnetosonic waves in plumes was 25%, with an average amplitude and wave normal angles of 44 pT and 84 degrees-88 degrees, respectively. Increased geomagnetic activity enhanced its amplitude and ratio of plasma frequency to electron cyclotron frequency. Approximately 78% of selected magnetosonic wave events were simultaneously observed with plasmaspheric hiss that is most effective in scattering electrons in the plume.

It is suggested that magnetosonic waves (also known as equatorial noise) can scatter radiation belt electrons in the Earth’s magnetosphere. Therefore, it is important to understand the global distribution of these waves between the proton cyclotron frequency and the lower hybrid resonance frequency. In this study, we developed an empirical model for estimating the global distribution of magnetosonic wave amplitudes and wave normal angles. The model is based on the entire mission period (approximately 2012–2019) of observations of Van Allen Probes A and B as a function of the distance from the Earth (denoted by L*), magnetic local time (MLT), magnetic latitude (λ), and geomagnetic activity (denoted by the Kp index). In previous studies the wave distribution inside and outside the plasmasphere were separately investigated and modeled. Our model, on the other hand, identifies the wave distribution along with the ambient plasma environment—defined by the ratio of the plasma frequency (fpe) to the electron cyclotron frequency (fce)—without separately determining the wave distribution according to the plasmapause location. The model results show that, as Kp increases, the dayside wave amplitude in the equatorial region intensifies. It thereby propagates the intense region towards the wider MLT and inward to L* < 4. In contrast, the fpe/fce ratio decreases with increasing Kp for all regions. Nevertheless, the decreasing aspect differs between regions above and below L* = 4. This finding implies that the particle energy and pitch angle that magnetosonic waves can effectively scatter vary depending on the locations and geomagnetic activity. Our model agrees with the statistically observed wave distribution and ambient plasma environment with a coefficient of determination of > 0.9. The model is valid in all MLTs, 2 ≤ L* < 6, |λ| < 20°, and Kp ≤ 6.

Small-scale magnetic flux ropes (SMFRs) are observed more frequently than larger-scale magnetic flux ropes (e.g., magnetic clouds) in interplanetary space. We selected 235 SMFRs by applying cylindrical linear force-free fitting to 20-year observations of the Wind satellite, which meets the criteria of low beta, low temperature, an enhanced magnetic field, and a rotation feature. By examining the pitch angle distribution of suprathermal electrons for these events, we found that approximately 45.1% of the SMFRs were accompanied by unidirectional beams (strahl). A much smaller percentage of SMFRs (∼10.7%) were associated with bidirectional beams. We also found a small percentage (∼7.2%) of (sunward) conic distributions during SMFR events. Last, the remaining ∼37.0% of SMFRs were associated with complex electron distributions. The unidirectional beams and most of the conics (together corresponding to ∼50% of the total 235 SMFRs) imply open-field SMFRs with only one end connected to the Sun. For ∼37.7% of the unidirectional beam SMFRs, the local IMF field polarity was orthogonal or inverted (possibly due to interchange reconnection). Based on the solar wind conditions around the bidirectional beams, we suggest that more than half of the bidirectional beams were not necessarily closed-field-line SMFRs.

In this paper, we present observations of the Space Radiation Detectors (SRDs) onboard the Next Generation Small Satellite-1 (NEXTSat-1) satellite. The SRDs, which are a part of the Instruments for the study of Stable/Storm-time Space (ISSS), consist of the Medium-Energy Particle Detector (MEPD) and the High-Energy Particle Detector (HEPD). The MEPD can detect electrons, ions, and neutrals with energies ranging from 20 to 400 keV, and the HEPD can detect electrons over an energy range from 0.35 to 2 MeV. In this paper, we report an event where particle flux enhancements due to substorm injections are clearly identified in the MEPD A observations at energies of tens of keV. Additionally, we report a specific example observation of the electron distributions over a wide energy range in which we identify electron spatial distributions with energies of tens to hundreds of keV from the MEPD and with energy ranging up to a few MeV from the HEPD in the slot region and outer radiation belts. In addition, for an ~1.5-year period, we confirm that the HEPD successfully observed the well-known outer radiation belt electron flux distributions and their variations in time and L shell in a way consistent with the geomagnetic disturbance levels. Last, we find that the inner edge of the outer radiation belt is mostly coincident with the plasmapause locations in L, somewhat more consistent at subrelativistic energies than at relativistic energies. Based on these example events, we conclude that the SRD observations are of reliable quality, so they are useful for understanding the dynamics of the inner magnetosphere, including substorms and radiation belt variations.

In this paper, an operational Dst index prediction model is developed by combining empirical and Artificial Neural Network (ANN) models. ANN algorithms are widely used to predict space weather conditions. While they require a large amount of data for machine learning, large-scale geomagnetic storms have not occurred sufficiently for the last 20 years, Advanced Composition Explorer (ACE) and Deep Space Climate Observatory (DSCOVR) mission operation period. Conversely, the empirical models are based on numerical equations derived from human intuition and are therefore applicable to extrapolate for large storms. In this study, we distinguish between Coronal Mass Ejection (CME) driven and Corotating Interaction Region (CIR) driven storms, estimate the minimum Dst values, and derive an equation for describing the recovery phase. The combined Korea Astronomy and Space Science Institute (KASI) Dst Prediction (KDP) model achieved better performance contrasted to ANN model only. This model could be used practically for space weather operation by extending prediction time to 24 h and updating the model output every hour.

AbstractElectromagnetic ion cyclotron (EMIC) waves can cause relativistic electron scattering and atmospheric precipitation, primarily via cyclotron resonant interactions in the Earth's radiation belts. However, the conventional quasilinear resonance theory suggests that the cyclotron resonance condition is not satisfied for 90° pitch angle (PA) electrons, which constitute the majority of electrons in the outer radiation belt, such that scattering mainly affects low‐PA electrons. In contrast to this theory, using test particle calculations, we demonstrate that even exactly 90° PA electrons can be significantly scattered by large‐amplitude EMIC waves. The finite wave force results in the parallel transport of 90° PA electrons away from the equator, corresponding to intrinsically nonresonant scattering. This can lead to parallel velocity that meets cyclotron resonance conditions as local PA deviates from 90°. Different types of resonance are identified depending on the wave normal angle, that is, first‐ and second‐order resonances for parallel and oblique waves, respectively.

We report observations of dynamically unstable strong wind shear (Richardson number < 0.25) capable of inducing Kelvin‐Helmholtz instability in the polar mesospheric summer echoes (PMSE) layer (80–90 km) using very high frequency radar measurements in Kiruna (67.8°N, 20.4°E), Sweden, in 2006. The unstable strong wind shear can play an important role in producing PMSE by inducing turbulence and adiabatic cooling. We find that the strong shears take up 64% of the observed wind profiles and are frequently composed of systematically single‐shear/multishear (layer) structures, which gradually or abruptly vary in wind directions, so‐called wind shifts, through heights at intervals of 4–8 km. The strong shear rate normalized by PMSE counts has a good correlation (R = 0.7) in day‐to‐day variation with energetic electron (>30 keV) precipitations that were related to high‐speed solar wind streams. The observations of strong wind shear can be supported by satellite‐measured temperature modulations matching with the peaks of the first three high‐speed solar wind stream events. This study suggests that PMSE production is closely associated with the strong shear that is in turn linked to the effects of energetic electron precipitation.

We present global statistical models of both wave amplitude and wave normal angle (WNA) of plasmaspheric hiss using Van Allen Probe‐A observations. They utilize the time history of solar wind parameters, that is, interplanetary magnetic field B Z and solar wind speed, and the AE index for each measurement of hiss waves as inputs. The solar wind parameter‐based model generally results in higher performance than using only the AE index as an input. Both observations and model results reveal a clear dependence of hiss wave distribution on the magnetic local time (MLT): Higher amplitudes with field‐aligned (<30 o ) WNAs occur more frequently on the dayside than on the nightside. Such a tendency does not depend on magnetic latitude (MLAT), but slightly larger WNAs with a relatively low amplitude frequently appear for larger MLAT (>10 o ). We also examine how significantly the electron loss rates in the slot region can be changed by incorporating the model output of hiss waves into a diffusive transport simulation. Simulation results show that during a typical timescale (roughly a couple of days) of a corotating interaction region‐driven storm, the nightside hiss waves with larger WNA (>30 o ) do not contribute to the electron loss in the slot region due to their low amplitude and large WNA, while dayside hiss with WNAs less than 30 o and comparatively higher amplitudes leads to a fast drop in flux, especially for electrons of a few hundred keV.

Intense turbulence is frequently observed in the polar mesosphere summer echoes (PMSE) from the data of very high frequency radar operating at Esrange, Sweden. The turbulence can be estimated from the turbulent energy dissipation rate ($\epsilon $) by considering aspect sensitivity. We find that variation in turbulence at altitude 82–86 km is in a good correlation with enhanced geomagnetic disturbances and precipitating energetic electrons into the mesosphere induced by high-speed solar wind streams. In addition, intense turbulence (a few tens of mW/kg) frequently occurs in the common volume with large plasma/neutral horizontal speeds ($\geq 150~\text{m}\,\text{s}^{-1}$) at 82–90 km altitudes. The large velocities are ready to form wind shear/shift. Therefore, we suggest that the summer mesospheric turbulence is to a significant extent accompanied by large plasma/neutral velocities and wind shear in the mesosphere, in turn linked to solar wind energy input during geomagnetic disturbances.

Plasmaspheric hiss waves commonly observed in high‐density regions in the Earth's magnetosphere are known to be one of the main contributors to the loss of radiation belt electrons. There has been a lot of effort to investigate the distributions of hiss waves in the plasmasphere, while relatively little attention has been given to those in the plasmaspheric plume. In this study, we present for the first time a statistical analysis of the occurrence and the spatial distribution of wave amplitudes and wave normal angles for hiss waves in plumes using Van Allen Probes observations during the period of October 2012 to December 2016. Statistical results show that a wide range of hiss wave amplitudes in plumes from a few picotesla to >100 pT is observed, but a modest (<20 pT) wave amplitude is more commonly observed regardless of geomagnetic activity in both the midnight‐to‐dawn and dusk sector. By contrast, stronger amplitude hiss occurs preferentially during geomagnetically active times in the dusk sector. The wave normal angles are distributed over a broad range from 0° to 90° with a bimodal distribution: a quasi‐field‐aligned population (<20°) with an occurrence rate of <60% and an oblique one (>50°) with a relative low occurrence rate of ≲ 20%. Therefore, from a statistical point of view, we confirm that the hiss intensity (a few tens of picotesla) and field‐aligned hiss wave adopted in previous simulation studies are a reasonable assumption but stress that the activity dependence of the wave amplitude should be considered.

By analyzing a data set from the European Incoherent SCATter (EISCAT) Very High Frequency (VHF) radar at Tromsø, we find that both radar reflectivity and upward ion velocity in a polar mesospheric summer echo (PMSE) layer simultaneously increased at the commencement of a local geomagnetic disturbance, which occurred at midnight on 9 July 2013. The onset of the upward velocity was followed by periodic repetition of ~5 min during the initial 30‐min stage, and then at later stage the vertical velocity oscillated with ~7‐ and ~20‐min periodicities at 85‐ to 90‐km altitudes. The ~5‐min periodicity is close to the buoyancy period, and the ~7‐ and ~20 min periodicities are consistent with gravity waves, thus suggesting that gravity waves can be generated by the effects of the geomagnetic disturbance. On the other hand, the variation of PMSE intensity (85–90 km) was in phase with fluctuations of electron densities (90–110 km) with ~12‐ and ~13‐min periodicities at the initial and later stages, respectively. The initial creation of PMSE can be attributed to both the sudden onset of particle precipitation and ice particles produced by adiabatic cooling during the rapid updraft, as detected by large upward velocity. Our periodogram analysis suggests that variations of PMSE intensity seem to follow the same periods with E region electron density, which is moduled by energetic electron precipitation, while vertical velocity oscillates at atmospheric gravity wave periods.

The ratio of the proton ring velocity ( V R ) to the local Alfven speed ( V A ), in addition to proton ring distributions, plays a key factor in the excitation of magnetosonic waves at frequencies between the proton cyclotron frequency f cp and the lower hybrid resonance frequency f LHR in the Earth's magnetosphere. Here we investigate whether there is a statistically significant relationship between occurrences of proton rings and magnetosonic waves both outside and inside the plasmapause using particle and wave data from Van Allen Probe‐A during the time period of October 2012 to December 2015. We also perform a statistical survey of the ratio of the ring energy ( E R , corresponding to V R ) to the Alfven energy ( E A , corresponding to V A ) to determine the favorable conditions under which magnetosonic waves in each of two frequency bands ( f cp < f ≤ 0.5 f LHR and 0.5 f LHR < f < f LHR ) can be excited. The results show that the magnetosonic waves in both frequency bands occur around the postnoon (12–18 magnetic local time, MLT) sector outside the plasmapause when E R is comparable to or lower than E A , and those in lower‐frequency bands ( f cp < f ≤ 0.5 f LHR ) occur around the postnoon sector inside the plasmapause when E R / E A > ~9. However, there is one discrepancy between occurrences of proton rings and magnetosonic waves in low‐frequency bands around the prenoon sector (6–12 MLT) outside the plasmapause, which suggests either that the waves may have propagated during active time from the postnoon sector after being excited during quiet time, or they may have locally excited in the prenoon sector during active time.

The Van Allen Probes (VAPs) are the only modern National Aeronautics and Space Administration (NASA) spacecraft broadcasting real‐time data on the Earth's radiation belts for space weather operations. Since 2012, the Korea Astronomy and Space Science Institute (KASI) has contributed to the receipt of these data via a 7 m satellite‐tracking antenna and used these beacon data for space weather operations. An approximately 15 min period is required from measurement to acquisition of Level‐1 data. In this paper, we demonstrate the use of VAP data for monitoring space weather conditions at geostationary orbit (GEO) by highlighting the Saint Patrick's Day storm of 2015. During that storm, Probe‐A observed a significant increase in the relativistic electron flux at 3 R E . Those electrons diffused outward resulting in a large increase of the electron flux >2 MeV at GEO, which potentially threatened satellite operations. Based on this study, we conclude that the combination of VAP data and National Oceanic and Atmospheric Administration‐Geostationary Operational Environmental Satellite (NOAA‐GOES) data can provide improved space environment information to geostationary satellite operators. In addition, the findings obtained indicate that more data‐receiving sites would be necessary and data connections improved if this or a similar system were to be used as an operational data service.

We present the dependence of the magnetosonic wave amplitudes both outside and inside the plasmapause on the solar wind and AE index using Van Allen Probe‐A spacecraft during the time period of 1 October 2012 to 31 December 2015, based on a correlation and regression analysis. Solar wind parameters considered are the southward interplanetary magnetic field (IMF B S ), solar wind number density ( N SW ), and bulk speed ( V SW ). We find that the wave amplitudes outside (inside) the plasmapause are well correlated with the preceding AE , IMF B S , and N SW with time delays, each corresponding to 2–3 h (3–4 h), 4–5 h (3–4 h), and 2–3 h (8–9 h), while the correlation with V SW is ambiguous both inside and outside the plasmapause. As measured by the correlation coefficient, the IMF B S is the most influential solar wind parameter that affects the dayside wave amplitudes both outside and inside the plasmapause, while N SW contributes to enhancing the duskside waves outside the plasmapause. The AE effect on wave amplitudes is comparable to that of IMF B S . More interestingly, regression with time histories of the solar wind parameters and the AE index preceding the wave measurements outside the plasmapause shows significant dependence on the IMF B S , N SW , and AE : the region of peak coefficients is changed with time delay for IMF B S and AE , while isolated peaks around duskside remain gradually decrease with time for N SW . In addition, the regression with magnetosonic waves inside the plasmapause shows high coefficients around prenoon sector with preceding IMF B S and V SW .

The auroral green-line emission at 557.7nm wavelength as arising from the atomic oxygen O(S-1 -> D-1) transition typically peaks at an altitude of similar to 100km specifically in the nightside oval, induced by auroral electrons within an energy range of similar to 100eV-30keV. Intense aurora is known as being suppressed by sunlight in summer daytime but usually occurs in low electrical background conductivity. However, in the present study in summer (July) sunlit condition, enhancements of O(S-1) emission rates observed by using the Wind Imaging Interferometer/UARS were frequently observed at low altitudes below 90km, where ice particles are created initially as subvisible and detected as polar mesosphere summer echoes, emerging to be an optical phenomenon of polar mesospheric clouds. The intense O(S-1) emission occurring in summer exceeds those occurring in the daytime in other seasons both in occurrence and in intensity, frequently accompanied by occurrences of supersonic neutral velocity (300-1500ms(-1)). In the mesosphere, ion motion is controlled by electric field and the momentum is transferred to neutrals. The intense O(S-1) emission is well associated with high-energy electron precipitation as observed during an event of high-speed solar wind streams. Meanwhile, since the minimum occurrences of O(S-1) emission and supersonic velocity are maintained even in the low precipitation flux, the mechanism responsible is not only related to high-energy electron precipitation but also presumably to the local conditions, including the composition of meteoric-charged ice particles and charge separation expected in extremely low temperatures (<150K).