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A 9 day periodic oscillation in solar wind properties, geomagnetic activity, and upper atmosphere has been reported for the year 2005. To understand the energy transfer processes from the high‐speed solar wind streams into the upper atmosphere, we examined Joule heating and hemispheric power (HP) from the assimilative mapping of ionospheric electrodynamics (AMIE) outputs for 2005. There are clear 9 day period variations in all AMIE outputs, and the 9 day periodic oscillation in the global integrated Joule heating is presented for the first time. The band‐pass filter centered at 9 day period shows that both Joule heating and HP variations are correlated very well to the neutral density variation. It indicates that the energy transfer process into the upper atmosphere associated with high‐speed solar wind streams is a combination of Joule heating and particle precipitation, while Joule heating plays a dominant role. The sensitivities of Joule heating and HP to the solar wind speed are close to 0.40 and 0.15 GW/(km/s), respectively. Key Points Nine‐day periodic oscillation in global integrated Joule heating has been reported The sensitivities of Joule heating and HP to solar wind speed Correlation of Joule heating and HP variation to the solar wind speed change

High‐speed solar wind streams cause recurrent geomagnetic activity and ionospheric disturbances. In this study, we analyze the equatorial ionospheric ion drift measured by the Defense Meteorological Satellite Program (DMSP) satellites near dusk when high‐speed solar wind streams with a period of 13.5 days occurred during January–April 2007. A well‐defined quantitative correlation between the solar wind velocity and the equatorial ionospheric ion drift is identified for the first time. The plasma drift in the dusk equatorial ionosphere induced by high‐speed solar wind streams is eastward in the zonal direction and downward in the vertical direction at the altitude of DMSP orbit (∼840 km) during this low solar activity period (January–April 2007). The zonal component of the equatorial ionospheric ion drift is inversely correlated with the vertical component. The ionospheric ion zonal drift varies, on average, from −40 to 40 m s−1 when the solar wind velocity varies from 300 to 700 km s−1 over a 13.5 day period, and the ion vertical drift varies from 10 to −10 m s−1. The quantitative correlations between the solar wind velocity and ionospheric ion drift and between the vertical and zonal components of the ion drift velocity are important for understanding the equatorial ionospheric electrodynamics associated with high‐speed solar wind streams and for space weather prediction. Key Points Quantitative relationship between ionospheric ion drift and solar wind found Observations of ionospheric response to high‐speed solar wind streams made New correlation between the two components of ionospheric ion drift found

When the solar wind speed falls below the local Alfvén speed, the magnetotail transforms into an Alfvén wing configuration. A Grid Agnostic Magnetohydrodynamics for Extended Research Applications (GAMERA) simulation of Earth's magnetosphere using solar wind parameters from the 24 April 2023 sub‐Alfvénic interval is examined to reveal modifications of Dungey‐type magnetotail reconnection during sustained sub‐Alfvénic solar wind. The simulation shows new magnetospheric flux is generated via reconnection between polar cap field lines from the northern and southern hemisphere, similar to Dungey‐type magnetotail reconnection between lobe field lines mapping to opposite hemispheres. The key feature setting the Alfvén wing reconnection apart from the typical Dungey‐type is that the majority of new magnetospheric flux is added to the polar cap at local times 1–3 (21‐23) in the northern (southern) hemisphere. During most of the sub‐Alfvénic interval, reconnection mapping to midnight in the polar cap generates relatively little new magnetospheric flux. Plain Language Summary Similar to how a shock wave forms around a supersonic plane, the supersonic plasma emanating from the sun forms a shock wave around Earth. However, the speed of sound through the plasma depends on different parameters that vary substantially based on the origin and evolution of solar material flowing into interplanetary space. In some coronal mass ejections, the characteristics of the plasma are such that the flow is sub‐sonic, leaving the magnetosphere in a unique state. Determining whether there are any space weather impacts associated with the sub‐sonic flow has been difficult due to lack of observations, but a recent event has ignited interest. This study examines the global structure and dynamics of the magnetosphere in a simulation representative of the sub‐sonic flow interval of the April 2023 geomagnetic storm. Key Points On 24 April 2023, Earth's magnetosphere experienced an interval of sustained sub‐Alfvénic solar wind driving Sub‐Alfvénic driving suppresses typical Dungey‐type magnetotail reconnection but polar cap expansion is still limited Global simulations have strong Earthward flows localized ∼10 RE tailward of theterminator, where most new magnetospheric flux is generated

The solar wind, representing one of the most impacting phenomena in the circum-terrestrial space, constitutes one of the several manifestations of the magnetic activity of the Sun. With the aim of shedding light on the scales beyond the rotational period of the Sun (i.e., Space Climate scales), this study investigates the phase relationship of a solar activity physical proxy, the Ca II K index, with solar wind properties measured near the Earth, over the whole space era (last five solar cycles). Using a powerful tool such as the Hilbert–Huang transform, we investigate the dependence of their phase coherence on the obtained time scale components. Phase coherence at the same time scales is found between all the components and is also preserved between adjacent components with time scales ≳ 2 yrs. Finally, given the availability of the intrinsic modes of oscillation, we explore how the relationship of Ca II K index with solar wind parameters depends on the time scale considered. According to our results, we hypothesize the presence of a bifurcation in the phase-space Ca II K index vs. solar wind speed (dynamic pressure), where the time scale seems to act as a bifurcation parameter. This concept may be pivotal for unraveling the complex interplay between solar activity and solar wind, bearing implications from the prediction and the interpretation point of view in Space Climate studies.

Seasonal features of geomagnetic activity and their solar-wind–interplanetary drivers are studied using more than five solar cycles of geomagnetic activity and solar wind observations. This study involves a total of 1296 geomagnetic storms of varying intensity identified using the Dst index from January 1963 to December 2019, a total of 75 863 substorms identified from the SuperMAG AL/SML index from January 1976 to December 2019 and a total of 145 high-intensity long-duration continuous auroral electrojet (AE) activity (HILDCAA) events identified using the AE index from January 1975 to December 2017. The occurrence rates of the substorms and geomagnetic storms, including moderate (-50nT≥Dst>-100nT) and intense (-100nT≥Dst>-250nT) storms, exhibit a significant semi-annual variation (periodicity ∼6 months), while the super storms (Dst≤-250 nT) and HILDCAAs do not exhibit any clear seasonal feature. The geomagnetic activity indices Dst and ap exhibit a semi-annual variation, while AE exhibits an annual variation (periodicity ∼1 year). The annual and semi-annual variations are attributed to the annual variation of the solar wind speed Vsw and the semi-annual variation of the coupling function VBs (where V = Vsw, and Bs is the southward component of the interplanetary magnetic field), respectively. We present a detailed analysis of the annual and semi-annual variations and their dependencies on the solar activity cycles separated as the odd, even, weak and strong solar cycles.

We report the first observation of the plasmasphere's periodic response to solar wind high‐speed streams (HSS) during the declining phase of Solar Cycle 23, based on plasmapause location data from the IMAGE and THEMIS satellites. In both 2005 and 2008, the daily variability of the plasmapause exhibits a strong anti‐correlation with solar wind speed, oscillating coherently at specific timescales. A similar anti‐correlated variation is identified in the latitude of the midlatitude ionospheric trough (MIT) minimum, derived from electron density measurements by the DMSP F16 satellite. Periodogram analysis reveals a distinct 9‐day periodicity in 2005, and both 9‐ and 13.5‐day periodicities in 2008 across all parameters. These findings provide direct evidence of magnetospheric modulation by recurring solar wind drivers and establish a clear connection between the plasmasphere and the midlatitude ionosphere under periodic solar forcing.

Neutron monitor counting rates and solar wind velocity, interplanetary magnetic field, sunspot number and total solar irradiance measurements from 2013 to 2018 corresponding to the end of solar maximum and the decreasing phase of the Solar Cycle 24 have been used. The main objective is to check whether the periodicities observed in the cosmic rays are affected by the magnetic rigidity or the height at which the neutron monitors are placed. A Global Neutron Monitor (GNM) has been defined as representative of the neutron monitor global network. This GNM is constructed by averaging the counting rates of a set of selected neutron monitors. The selection process is based on the combination of three new data quality criteria, which are applied to neutron monitors in the Neutron Monitor Data Base giving a final pool of 22 selected neutron monitors. Morlet wavelet analysis is applied to the GNM and the selected solar activity parameters to find the common periodicities. Short-term periodicities of 13.5, 27, 48, 92, 132 and 298 days have been observed in cosmic ray intensity. A clear inverse relationship between rigidity and spectral power has been obtained for the 13.5-, 48-, 92-, 132-day periods. A not so clear but still observed direct relationship between the height of the neutron monitors and the spectral power for the 48-, 92-, 132-day periods has been also found. The periodicity of 92 days is the one which shows the highest dependence with rigidity cutoff and height. As far as we know, this is the first time that these dependencies are reported. We think that these observations could be explained by assuming some cosmic ray intensity energy dependence in such periodicities and a competitive effect between rigidity and height.

We consider the problem of forecasting the solar wind speed using not only well-known magnetic field data sets, such as the Wilcox Solar Observatory (WSO) and the Global Oscillations Network Group (GONG) but others, such as the Infrared Magnetograph (IRmag) at the National Astronomical Observatory of Japan and the Solar Telescope for Operative Prediction (STOP) in Russia. We use these observations to study Carrington rotation (CR) 2164 (21 May – 17 June 2015). Our initial calculations are based on the Wang-Sheeley-Arge (WSA) model and include determining the coronal magnetic field using the potential field source surface (PFSS) approximation. The speed of the ambient solar wind near the Sun is calculated using an empirical equation that considers the flux tube expansion factor (FTEF) and the distance of the flux tube footpoint from the coronal hole boundary (DCHB) at the photospheric level. The solar wind bulk speed at the Earth’s orbit is calculated using the Heliospheric Upwind eXtrapolation (HUX) model. It is shown that the discrepancies in the speed values from four different data sets could reach ≈ 200 km s −1 , which is significant. We compare our predictions with in situ data from the Advance Composition Explorer (ACE) and demonstrate that a better coincidence between calculated and empirical results, accounting for the magnetic field strength in coronal holes, can be achieved.

Interstellar pickup ions are an ubiquitous and thermodynamically important component of the solar wind plasma in the heliosphere. These PUIs are born from the ionization of the interstellar neutral gas, consisting of hydrogen, helium, and trace amounts of heavier elements, in the solar wind as the heliosphere moves through the local interstellar medium. As cold interstellar neutral atoms become ionized, they form an energetic ring beam distribution comoving with the solar wind. Subsequent scattering in pitch angle by intrinsic and self-generated turbulence and their advection with the radially expanding solar wind leads to the formation of a filled-shell PUI distribution, whose density and pressure relative to the thermal solar wind ions grows with distance from the Sun. This paper reviews the history of in situ measurements of interstellar PUIs in the heliosphere. Starting with the first detection in the 1980s, interstellar PUIs were identified by their highly nonthermal distribution with a cutoff at twice the solar wind speed. Measurements of the PUI distribution shell cutoff and the He focusing cone, a downwind region of increased density formed by the solar gravity, have helped characterize the properties of the interstellar gas from near-Earth vantage points. The preferential heating of interstellar PUIs compared to the core solar wind has become evident in the existence of suprathermal PUI tails, the nonadiabatic cooling index of the PUI distribution, and PUIs’ mediation of interplanetary shocks. Unlike the Voyager and Pioneer spacecraft, New Horizon’s Solar Wind Around Pluto (SWAP) instrument is taking the only direct measurements of interstellar PUIs in the outer heliosphere, currently out to ∼ 47 au from the Sun or halfway to the heliospheric termination shock.

On 21 April 2023, a significant M1.7 solar flare erupted from Active Region 13283, accompanied by a filament eruption and a full-halo Coronal Mass Ejection, which reached Earth on 23 April, triggering a severe geomagnetic storm, with Kp reaching 8 (G4) and Dst plummeting to −212 nT together with a sharply distinguished long-lasting negative double-dip behavior of the z -component of the interplanetary magnetic field. This event led to remarkable auroral displays, even at mid-latitudes in Europe. The flare-induced filament eruption caused distinct intensity dimming in the solar corona, observed in specific EUV wavelengths. We observed the dimming region growing at its fastest rate before the flare reached its peak of intensity. Notably, the proximity of the flare to a large southern coronal hole influenced the expansion and propagation of the coronal mass ejection toward Earth, probably impacting the solar wind speed and density. Additionally, we observed a sudden expansion of the coronal hole during the flare, leading us to speculating that the adjacent flare may have further stimulated the flow of solar-wind particles along the open magnetic-field lines. In accordance with the severe Dst-index disturbance, we also report changes in the potential of the pipeline of an Italian energy infrastructure company with respect to the surrounding soil as well as double-dip variation in the H-component of the terrestial magnetic field observed locally (reminiscent to what reported in Dst-index and IMF B z ) temporal profiles, confirming the effects of the geomagnetic storm at Italy mid-latitudes. Several solar radio events have been observed too. Therefore this study provides insights into the dynamic solar phenomena and their potential geomagnetic implications.