Marek Vandas

A new method is developed for the determination of the global pattern of closure currents in the ionosphere from data on field aligned currents (FACs) and modification of the International Geomagnetic Reference Field (IGRF) due to compression on the magnetopause. The study of ionospheric closure currents is important because they contribute to Joule heating and the ionosphere‐thermosphere coupling. The problem is solved in several steps. First, the Euler potentials for the magnetic field above the ionosphere are calculated. Next, the Euler potentials for the current density of FACs are determined. The magnetic field induced by FACs is calculated with these potentials. Finally, the closure currents in the base of the ionosphere are derived from the condition that FACs do not penetrate the base of the ionosphere.

Context. A magnetized plasma cloud acts as a magnetic obstacle to an external plasma flow. We used an analytical approach to examine forces acting on a spherical static obstacle, which can be applied to the kinematic analysis of the obstacle’s motion. Aims. We aimed to determine the magnetic field in the sheath region around a spherical obstacle for cases when the background magnetic field is perpendicular, parallel, or at an arbitrary angle to the plasma flow direction. Additionally, we calculated the diagmagnetic force exerted by the magnetic field on the obstacle. Methods. We applied a potential field method, under the conditions that the normal component of the magnetic field is conserved on the bow shock and is zero at the obstacle. We analytically determined the modification of the background magnetic field by a supersonic obstacle. Results were compared with the field modification by a slow, subsonic obstacle. Uniform and nonuniform background magnetic fields were considered and the diamagnetic force acting on an obstacle was calculated. Results. The diamagnetic force on the slow obstacle in the ambient field is zero, while the supersonic obstacle experiences a decelerating diamagnetic force in the uniform background field. The magnetic field with a gradient produces an accelerating diamagnetic force on a subsonic obstacle, pushing it away from a stronger magnetic field. The diamagnetic force on the supersonic obstacle from the nonuniform background field is much weaker.

Magnetic field measurements in the Earth's magnetosheath are compared with four analytical models, namely Kobel and Flückiger (1994), (https://doi.org/10.1029/94ja01778), Romashets and Vandas (2019), (https://doi.org/10.1029/2018ja026006), Vandas and Romashets (2019), (https://doi.org/10.1016/j.pss.2019.07.007), and Romashets et al. (2010), (https://doi.org/10.1016/j.jastp.2010.10.010). The bow shock and the magnetopause are modeled as paraboloids in all models except that of Vandas and Romashets (2019), (https://doi.org/10.1016/j.pss.2019.07.007), in which they are assumed to be spheroids. 34 magnetosheath crossings by spacecraft are analyzed in detail and it is concluded that the model by Kobel and Flückiger (1994), (https://doi.org/10.1029/94ja01778) performs the best.

Context. Shock waves in the solar corona are associated with solar flares and coronal mass ejections (CMEs). Type II solar bursts are radio signatures of shock waves in the solar corona. They are driven by solar flares or CMEs. Despite extensive studies, the intricate spectral patterns observed in type II solar bursts occasionally pose new challenges for the theory of electron acceleration in shocks. Aims. We study a newly identified feature in type II solar bursts called spectral cleaving. This feature is characterized by the actual branching of a type II radio emission lane in radio spectral data. Methods. We analyzed the type II burst exhibiting spectral cleaving in high-fidelity dynamic spectra obtained using the URAN-2 radio telescope (8.25–33 MHz; Poltava region, Ukraine) on 2011 February 14. The high-resolution spectrograms were examined to ascertain its spectral morphology. Results. Our research represents the first recognition of spectral cleaving as a peculiarity of type II bursts that is yet to be classified. This effect occurs due to the shift (or migration) of radio source(s) along a shock front, which in turn is caused by changes in the magnetic field orientation ahead of the propagating shock front. Conclusions. The spectral cleaving observed in solar type II bursts reveals a distinct phenomenon that indicates complex interactions between shock waves and magnetic fields in the solar corona. This discovery enhances our understanding of the mechanisms behind solar radio emissions and emphasizes the need for further observational studies to verify these findings.

Romashets and Vandas (2024) derived a method for the determination of Euler potentials at a spherical surface and applied it to the geomagnetic field. Here, we apply it to find Euler potentials at the source surface. A regular mesh defined by Euler potentials divides the source surface to surface elements with the same magnetic flux. By tracing magnetic-field lines away from the source surface, Euler potentials can be extended into the heliosphere.

Finding the magnetic flux mapping in the ionosphere is very important. It would not only divide the surface into the elements with the same flux, but also indicate locations of conjugated points. It is important for studies of field aligned currents and bouncing of energetic charged particles and their precipitation. The existing methods involve numerical magnetic field lines tracing in the entire volume of the magnetosphere or numerical integration along assumed contour lines of the Euler potentials on the surface of the ionosphere. It is possible to determine the mapping with these methods near the magnetic equator, but not on middle latitudes and near and inside the polar caps. Our approach is to search for the Euler potentials as a sum of basic functions with their coefficients. Each basic function is a product of a sine or cosine of longitude multiplied by m and the Legendre polynomial of the colatitude angle cosine and of the order n. Maxima of m and n in this calculation were set to 13. The difference between the radial component from the cross product of the Euler potentials gradients and from International Geomagnetic Reference Field is less than 0.01 percent. We discuss the possibility of using orthogonal coordinates defined on the sphere's surface, which remain finite functions of θ and φ everywhere except for the vicinities of the North and South poles. The issues with numerical errors accumulated on long tracing are avoided when using this approach.

Euler potentials are found for the magnetic field in the inner magnetosphere, consisting of the Earth's dipole field, the Dungey term, ring current (RC), and field‐aligned current (FAC). The magnetic field in this approximation is axially symmetric. One Euler potential for the dipole, Dungey term, and RC is the same, while that for the magnetic field induced by FAC is different and calculated in this work under the condition that all four field contributions have the same second Euler potential. The Euler potentials are determined for quiet, severe, and superstorm geomagnetic conditions. The restriction of the model is its axial symmetry, so changes of the location and intensity of the currents with azimuthal angle are not explicitly present. On the other hand, these parameters can be changed in the model, and one can study local distributions.

Euler potentials of the Earth's dipole magnetic field with field‐aligned currents are found. The results generalize ones obtained earlier for systems of plane currents parallel to an ambient uniform magnetic field. A real current system corresponding to an observed increase of the magnetic field measured by a low‐altitude spacecraft is used in this modeling. Parameters of charged particle trajectories, such as length and period, are determined for typical and strongly disturbed conditions. A particle distribution function is calculated.

Observations of the magnetic field for two passages through the Earth's magnetosheath are used for comparison with two simple models of the magnetosheath field that describe it as a potential field. We apply the Kobel and Flückiger (1994, https://doi.org/10.1029/94JA01778) model and the Romashets and Vandas (2019, https://doi.org/10.1029/2006JA012072) model complemented with the Jelínek et al. (2012, https://doi.org/10.1029/2011JA017252) empirical determination of the bow shock and magnetopause shapes and positions. The models yield a satisfactory match to observations when the positions and shapes of the bow shock and the magnetopause are controlled by the instantaneous solar wind upstream dynamic pressure and the instantaneous upstream interplanetary magnetic field vector is taken into account.

Context. Recent investigations indicate that the magnetic field configuration in interplanetary flux ropes is in contrast with the common magnetic field models that are used to fit them, namely constant-alpha force-free fields, whose twist increases without limits toward the flux-rope boundary. Therefore, magnetic field configurations with a constant twist are now being employed in fits. Aims. Real flux ropes have varying twist. Therefore, analytical magnetic field configurations with prescribed twist distributions are searched for in cylindrical geometry. Methods. Equations for the field solenoidality and for the force-free condition are solved for case when a twist profile is prescribed. Results. A model of a force-free magnetic field configuration with an arbitrarily given twist distribution in a cylinder and its relative helicity per unit length are presented. It is applied to a core-envelope model recently suggested in studies of twist in magnetic clouds.