Krishna Khanal

One of the key mechanisms proposed for coronal heating is dissipation of quasi-two-dimensional (2D) turbulence generated above the photosphere by continual mixing and convective shuffling of open and closed magnetic field lines within the so-called magnetic carpet. This class of heating mechanism is modeled within the Nearly Incompressible—Magnetohydrodynamics (NI-MHD) framework and has previously been shown to supply sufficient energy to drive the solar wind to supersonic and super-Alfvénic speeds and heat the corona to temperatures exceeding one million kelvin. In addition to turbulent heating, the overall energy budget of the low corona and the chromospheric-coronal transition region is strongly influenced by loss mechanisms associated with thermal conduction and radiative cooling, which must therefore be included in realistic models. To that end, we present solutions to a coupled system of NI-MHD turbulence transport equations and solar wind evolution equations that self-consistently incorporates the effects of collisional thermal conduction and radiative cooling. Results show that an interplay between turbulence dissipation and heat-loss mechanisms can result in the expected solar-wind driving as well as coronal heating. The predicted density and thermal conduction flux at the inner boundary, as well as the overall wind speed, are broadly consistent with representative ranges of observed coronal and solar wind parameters. The model results are sensitive to the ratio of von Kármán–Taylor constant to correlation length, as well as to the assumed expansion factor of the associated flux tube. This sensitivity is explored through a parametric survey.

Low-frequency turbulence in the solar chromosphere remains poorly understood. We address (1) the sources of low-frequency turbulence that potentially heat the chromosphere, and (2) how turbulence is transported and dissipated throughout the chromosphere and lower corona. We use particle-in-cell simulations to investigate mixed polarity magnetic fields corresponding to emergent magnetic carpet field in coronal holes or quiet Sun regions for strong (imbalanced) and weak (balanced) guide magnetic fields. The initial mixed polarity magnetic field transitions rapidly to a turbulent state dominated by advected small-scale nonlinear structures, with a minority slab turbulence population, and the emergent field is largely annihilated. Turbulence is anisotropic for an imbalanced magnetic field and more isotropic for balanced cases. We develop a transport model for turbulence advected and dissipated throughout the chromosphere by randomly distributed energy-containing scale dynamical flows described by log-normal statistics. We compute the expectations for the total energy per unit volume 〈 y 〉( h ) J m ⁻³ , the Elsässer specific energy 〈 〈 Z ∞ 2 〉 〉 ( h ) m ² s ⁻² , the heating rate 〈 H ̇ 〉 ( h ) J m ⁻³ s ⁻¹ , and the correlation length 〈 λ 〉( h ) in kilometers as functions of height h above the photosphere. Turbulent energy is injected into the low corona by a random “patchwork” of sites across the transition region surface. The expected energy injection rates 〈 S ̇ 〉 J m ⁻² s ⁻¹ for the chromosphere and at the base of the corona exceed the estimated energy requirements needed to heat both the chromosphere and corona. Similarly, we show that spicules can be heated gradually with increasing height by entrained magnetic carpet and photospheric turbulence.

The temporal variability of magnetopause reconnection is an important aspect of solar wind magnetosphere coupling. Even under stable solar wind driving, reconnection can be triggered, modulated, or suppressed because of magnetic field and plasma conditions near the magnetopause boundary. We analyze a unique event in which a THEMIS satellite crosses the subsolar magnetopause three times within a 5 min interval in the presence of a cold‐ion population on the magnetospheric side of the boundary. During the first crossing, the satellite detects reconnection outflow and a D‐ shaped ion velocity distribution earthward from the boundary, indicating an active reconnection. The signatures disappear during the second crossing when the magnetospheric cold‐ion density increases significantly and reappear during the third crossing when the magnetospheric density drops to a level comparable to that of the first crossing. The solar wind and magnetosheath conditions do not change much during the interval. The magnetospheric population is evidently associated with a plasmaspheric plume with considerable variation in density. According to the theory of mass loading, the presence of such a plume population results in the local Alfvén speed at the second crossing being 40% smaller compared to the first and third crossings. However, the theory itself does not suggest suppression. We discuss possible suppression mechanisms considering the additional effects of the prevailing solar wind and local magnetopause conditions.

Magnetopause reconnection is the dominant mechanism for transporting solar wind energy and momentum into the magnetosphere‐ionosphere system. Magnetopause reconnection can occur along X‐lines of variable extent in the direction perpendicular to the reconnection plane. Identifying the spatial extent of X‐lines using satellite observations has critical limitations. However, we can infer the azimuthal extent of the X‐lines by probing the ionospheric signature of reconnection, the antisunward flow channels across the ionospheric Open‐Closed Field Line Boundary (OCB). We study 39 dayside magnetopause reconnection events using conjugate in situ and ionospheric observations to investigate the variability and controlling factors of the spatial extent of reconnection. We use spacecraft data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) to identify in situ reconnection events. The width of the antisunward flow channels across the OCB is measured using the concurrent measurements from Super Dual Auroral Radar Network (SuperDARN). Also, the X‐line lengths are estimated by tracing the magnetic field lines from the ionospheric flow boundaries to the magnetopause. The solar wind driving conditions upstream of the bow shock are studied using solar wind monitors located at the L1 point. Results show that the magnetopause reconnection X‐lines can extend from a few Earth Radii (RE) to at least 22 RE in the GSM‐Y direction. Furthermore, the magnetopause reconnection tends to be spatially limited during high solar wind speed conditions.

High‐intensity long‐duration continuous auroral electrojet activities (HILDCAAs) are recognized as the continuous (duration >2 days) auroral electrojet activities (AE > 1,000 nT) caused by interplanetary Alfvén waves driving magnetic reconnection at the Earth's magnetopause. This paper focuses on the study of geoeffectiveness of HILDCAA on the basis of associated geomagnetically induced currents (GICs) and pipe‐to‐soil voltage (PSV) observed in an underground pipeline at Finnish Natural Gas Station, Mäntsälä. In this study, 113 HILDCAAs with different interplanetary sources (72 corotating interaction region‐storm preceded, 29 interplanetary coronal mass ejection‐storm preceded, and 12 nonstorm) have been selected and their possible contributions in underground pipeline corrosion have been quantitatively compared by assessing GIC/PSV profiles for each of these events. In addition, the spectral characteristics of GIC during these events have been studied using continuous wavelet transforms. The Morlet wavelet has been applied to 10‐s modeled GIC data corresponding to each event to explore the main band structures and the periodicities found in GIC spectrum. GIC/PSV modeling is based on plane wave method and distributed source transmission line analogy between pipelines and electric circuits. HILDCAAs are found to drive a small‐amplitude, but continuous, fluctuation in GIC throughout the event duration of several days. This makes the cumulative effect of HILDCAAs in pipeline corrosion noteworthy. The spectral analysis shows that GIC possesses both short‐term, as well as continuous, power distribution with different periodicities. CIR‐preceded HILDCAAs are found to be more geoeffective while taking associated GIC and PSV into account. Possible physical explanations supporting the results have been presented.

Polar cap potential (PCV) is an important parameter used for determining what kind of interaction takes place between solar wind and magnetosphere. Highly energetic particles from Sun driven by solar wind constantly bombard with Earth's magnetosphere–ionosphere system that results into a phenomenon like auroras, and major geomagnetic disturbances. Solar wind electron deposition determines the magnitude of field‐aligned current (FAC) and ultimately leads to PCV variation. Several studies found that increase in magnitude of IMF‐B_z causes an electric field of cross magnetosphere to increase, and it leads to increase in magnitude of ionospheric cross‐polar cap potential (PCV). Moreover, PCV was found to be a linear function of V_sw. In this research, we aim to study how field‐aligned current (FAC), for example, region 1 current and PCV, is related during different forms of geomagnetic disturbances. In all events, FAC and PCV are found to have corresponding fluctuations—especially at times of significant variation of IMF‐B_z (negative B_z interval) following the linearity of equation suggested by Moon in Moon (2012, https://doi.org/10.5140/JASS.2012.29.3.259). We found one‐to‐one correspondence between FAC and PCV. We did CWT analysis and found that FAC and PCV have more or less same spectral behaviors for each event considered. The cross‐correlation analysis shows a high and positive correlation between FAC and PCV at 0‐min time lag for all geomagnetic activity. The CWT analysis clearly supports the result of cross correlation between FAC and PCV. We found that FAC and V_sw, FAC‐B, and FAC and AE are also positively correlated with high‐correlation coefficient at lag 0 min for all geomagnetic storm. However, FAC‐B_z, FAC‐B_y, and FAC‐SYM (H) have varying correlation in different events. For a particular storm and substorm, the parameters B_z and B_y may not necessarily be varied with FAC in regular sequence but IMF (B) always show positive correlation with FAC for all geomagnetic activity. This paper presents a clear relation between FAC and PCV. This result will help to identify some of the outstanding issues in determining the causal mechanism of PCV variation, a crucial thing to understanding the coupling between the solar wind and M‐I system.