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We present a three-dimensional reduced magnetohydrodynamics model to depict turbulence induced by Alfvén waves within the slow solar wind originating from an open magnetic field positioned in a coronal hole near the solar equator. The nonlinear interactions between outward- and inward-propagating waves result in turbulence. As part of this investigation, we examine the conditions prevailing in an equatorial coronal hole using the interplanetary scintillation (IPS) observations. From IPS observations, we measure the magnetic field and velocity, both input parameters of our Alfvén wave turbulence model. We show the conditions of equatorial corona, a contributor to the slow solar wind, can generate Alfvén wave turbulence that accelerates and energizes the slow wind.

The origin of nonradial solar wind flows and their effect on space weather are poorly understood. Here we present a detailed investigation of 12 nonradial solar wind events during solar cycles 23–24, covering the period 1995–2017. In all these events the azimuthal flow angles of the solar wind exceed 6° as measured at the L1 Lagrangian point of the Sun–Earth system, for periods of 24 hr. In addition, all the events were selected during periods when coronal mass ejections (CMEs) and/or corotating interaction regions (CIRs) were absent. For most of the events, the near-Earth solar wind density was <5 cm−3 for periods exceeding 24 hr, similar to the well-known “solar wind disappearance events” wherein near-Earth solar wind densities dropped by two orders of magnitude for periods exceeding 24 hr. The solar source regions determined for all the cases were found to be associated with active region–coronal hole (AR–CH) pairs located around the central meridian. Further, the dynamical evolution of the source regions, studied using both the Extreme-ultraviolet Imaging Telescope and the Michelson Doppler Imager, showed a clear reduction in the CH area accompanied by the emergence of new magnetic flux regions. This dynamic evolution in the AR–CH source regions eventually disturbed the stable CH configurations, thereby giving rise to the extremely nonradial solar wind outflows. We discuss, based on our results, a possible causative mechanism for the origin of these highly nonradial flows that were not associated with either CMEs or CIRs.

We examined solar photospheric magnetic fields for the solar cycles 21–25, covering the period 1975–2024. The unsigned photospheric magnetic fields at low latitudes (0°–45°), known as solar toroidal fields, showed a solar cycle variation with the total field strength being stronger during the maximum of cycle 25 than the maximum of cycle 24. However, the unsigned field strength of photospheric magnetic fields at high latitudes (45°–78°), known as solar polar fields, has shown a significant steady decline since around the mid-1990s. The unsigned field strength of solar polar fields, after an increase during 2015–2020, has declined again until 2024, continuing the declining trend for a long 30 yr. Also, we examined the solar wind microturbulence levels in the inner heliosphere (0.2–0.8 au), using interplanetary scintillation observations at 327 MHz, covering the period 1983–2022, that steadily declined since the mid-1990s and continued until 2022, synchronously with the solar polar fields. We found that the floor level in both solar toroidal fields and solar wind magnetic fields was reduced during the minimum of cycle 23 and recovered back during the minimum of cycle 24. In addition, a hemispherically asymmetric solar polar reversal was evident in the signed (axial) solar polar fields during cycles 21–25, with the reversal in cycle 25 for the northern hemisphere already completed but the same for the southern hemisphere yet to be completed. We discuss the implications of the long declining trend and other anomalies in solar cycle activity.

The extended minimum of Solar Cycle 23, the extremely quiet solar-wind conditions prevailing and the mini-maximum of Solar Cycle 24 drew global attention and many authors have since attempted to predict the amplitude of the upcoming Solar Cycle 25, which is predicted to be the third successive weak cycle; it is a unique opportunity to probe the Sun during such quiet periods. Earlier work has established a steady decline, over two decades, in solar photospheric fields at latitudes above 45 ∘ and a similar decline in solar-wind micro-turbulence levels as measured by interplanetary scintillation (IPS) observations. However, the relation between the photospheric magnetic fields and those in the low corona/solar-wind are not straightforward. Therefore, in the present article, we have used potential-field source-surface (PFSS) extrapolations to deduce global magnetic fields using synoptic magnetograms observed with National Solar Observatory (NSO), Kitt Peak, USA (NSO/KP) and Solar Optical Long-term Investigation of the Sun (NSO/SOLIS) instruments during 1975 – 2018. Furthermore, we have measured the normalized scintillation index [ m ] using the IPS observations carried out at the Institute of Space–Earth Environment Research (ISEE), Japan during 1983 – 2017. From these observations, we have found that, since the mid-1990s, the magnetic field over different latitudes at 2.5 R ⊙ and 10 R ⊙ (extrapolated using the PFSS method) has decreased by ≈ 11.3 – 22.2 % . In phase with the declining magnetic fields, the quantity m also declined by ≈ 23.6 % . These observations emphasize the inter-relationship among the global magnetic field and various turbulence parameters in the solar corona and solar-wind.

The 3D structure of the solar wind and its evolution in time are needed for heliospheric modeling and interpretation of energetic neutral atoms observations. We present a model to retrieve the solar wind structure in heliolatitude and time using all available and complementary data sources. We determine the heliolatitude structure of solar wind speed on a yearly time grid over the past 1.5 solar cycles based on remote-sensing observations of interplanetary scintillations, in situ out-of-ecliptic measurements from Ulysses , and in situ in-ecliptic measurements from the OMNI 2 database. Since in situ out-of-ecliptic information on the solar wind density structure is not available apart from the Ulysses data, we derive correlation formulae between the solar wind speed and density and use the information on the solar wind speed from interplanetary scintillation observations to retrieve the 3D structure of the solar wind density. With the variations of solar wind density and speed in time and heliolatitude available, we calculate variations in solar wind flux, dynamic pressure, and charge-exchange rate in the approximation of stationary H atoms.

We examined the roles of Notch1 signaling and its cross-talk with other signaling pathways, including p53 and phosphatidylinositol-3-kinase (PI3K)/Akt, in cadmium-induced cellular damage in HK-2 human renal proximal tubular epithelial cells. Following exposure to cadmium chloride (CdCl 2 ), the level of Notch intracellular domain (NICD), the cleaved form of the Notch1 receptor, was increased and accumulated in the nuclear fraction. Knockdown of Notch1 with siRNA or treatment with the γ -secretase inhibitor, DAPT ( N -[ N -(3,5-difluorophenacetyl- L -alanyl)]-S-phenylglycine t-butyl ester), prevented CdCl 2 -induced morphological change of HK-2 cells and reduction of cell viability. Knockdown of Jagged1 or Jagged2, the ligands of the Notch1 receptor, partially suppressed cadmium cytotoxicity. Inhibition of p53 activity with pifithrin- α or inhibition of PI3K with LY294002 suppressed CdCl 2 -induced cellular damage and elevation of Notch1-NICD. In addition, treatment with the epidermal growth factor receptor (EGFR) inhibitor, AG1478, and the insulin-like growth factor-1 receptor inhibitor, PPP, suppressed both Notch1-NICD accumulation and Akt phosphorylation in HK-2 cells exposed to CdCl 2 . However, knockdown of Notch1 did not affect CdCl 2 -induced p53 accumulation and phosphorylation but suppressed phosphorylation of EGFR, Akt, and p70 S6 kinase. Depletion of Notch1 suppressed CdCl 2 -induced reduction of E-cadherin expression and elevation of Snail expression. Furthermore, treatment with SB216763, an inhibitor of glycogen synthase kinase-3, suppressed the potency of LY294002 treatment to reduce Snail expression in HK-2 cells exposed to CdCl 2 . Knockdown of Snail with siRNA partially prevented HK-2 cells from CdCl 2 -induced reduction of E-cadherin expression and cellular damage. These results suggest that cadmium exposure induces the activation of Notch1 signaling in renal proximal tubular cells with cooperative activation by the p53 and PI3K/Akt signaling pathways; the resultant expression of Snail, a repressor of E-cadherin expression, might lead to cellular damage by decreasing cell–cell adhesion.

Interplanetary scintillation (IPS) observations made in the Cycle 23/24 minimum using the Solar‐Terrestrial Environment Laboratory (STEL) multi‐station system indicated that during intervals the solar wind had a significantly non‐dipolar structure that consisted of fast wind components at the poles and the equator and slower wind components in between. The solar wind structure revealed from the IPS observations was consistent with a marked increase in the occurrence of fast winds observed in situ near the earth. The poleward boundary of the slow wind region observed during this minimum was ±30 north and south. In addition, our IPS observations revealed that the organization of the 3‐dimensional solar wind was highly variable during 2007–2008. These features greatly differ from those observed during the previous minima. This fact may be attributed to the weak magnetic field intensity at the poles during the Cycle 23/24 minimum.

We report kinematic properties of slow interplanetary coronal mass ejections (ICMEs) identified by SOHO/LASCO, interplanetary scintillation, and in situ observations and propose a modified equation for the ICME motion. We identified seven ICMEs between 2010 and 2011 and compared them with 39 events reported in our previous work. We examined 15 fast ( V SOHO − V bg >500 km s −1 ), 25 moderate (0 km s −1 ≤ V SOHO − V bg ≤500 km s −1 ), and 6 slow ( V SOHO − V bg <0 km s −1 ) ICMEs, where V SOHO and V bg are the initial speed of ICMEs and the speed of the background solar wind. For slow ICMEs, we found the following results: i) They accelerate toward the speed of the background solar wind during their propagation and reach their final speed by 0.34±0.03 AU. ii) The acceleration ends when they reach 479±126 km s −1 ; this is close to the typical speed of the solar wind during the period of this study. iii) When γ 1 and γ 2 are assumed to be constants, a quadratic equation for the acceleration a =− γ 2 ( V − V bg )| V − V bg | is more appropriate than a linear one a =− γ 1 ( V − V bg ), where V is the propagation speed of ICMEs, while the latter gives a smaller χ 2 value than the former. For the motion of the fast and moderate ICMEs, we found a modified drag equation a =−2.07×10 −12 ( V − V bg )| V − V bg |−4.84×10 −6 ( V − V bg ). From the viewpoint of fluid dynamics, we interpret this equation as indicating that ICMEs with 0 km s −1 ≤ V − V bg ≤2300 km s −1 are controlled mainly by the hydrodynamic Stokes drag force, while the aerodynamic drag force is a predominant factor for the propagation of ICME with V − V bg >2300 km s −1 .

Extensive interplanetary scintillation (IPS) observations at 327 MHz obtained between 1983 and 2009 clearly show a steady and significant drop in the turbulence levels in the entire inner heliosphere starting from around ∼1995. We believe that this large‐scale IPS signature, in the inner heliosphere, coupled with the fact that solar polar fields have also been declining since ∼1995, provide a consistent result showing that the buildup to the deepest minimum in 100 years actually began more than a decade earlier. Key Points Shows near steady decrease of scintillation/microturbulence in inner heliosphere Decrease of scintillation shown to be correlated with the polar magnetic fields Continuous IPS monitoring of heliosphere is very useful for future predictions

We report the results of a multi-instrument, multi-technique, coordinated study of the solar eruptive event of 13 May 2005. We discuss the resultant Earth-directed (halo) coronal mass ejection (CME), and the effects on the terrestrial space environment and upper Earth atmosphere. The interplanetary CME (ICME) impacted the Earth’s magnetosphere and caused the most-intense geomagnetic storm of 2005 with a Disturbed Storm Time ( Dst ) index reaching −263 nT at its peak. The terrestrial environment responded to the storm on a global scale. We have combined observations and measurements from coronal and interplanetary remote-sensing instruments, interplanetary and near-Earth in-situ measurements, remote-sensing observations and in-situ measurements of the terrestrial magnetosphere and ionosphere, along with coronal and heliospheric modelling. These analyses are used to trace the origin, development, propagation, terrestrial impact, and subsequent consequences of this event to obtain the most comprehensive view of a geo-effective solar eruption to date. This particular event is also part of a NASA-sponsored Living With a Star (LWS) study and an on-going US NSF-sponsored Solar, Heliospheric, and INterplanetary Environment (SHINE) community investigation.