Aims. We study the emergence of a toroidal flux tube into the solar atmosphere and its interaction with a pre-existing field of an active region. We investigate the emission of jets as a result of repeated reconnection events between colliding magnetic fields. Methods. We perform 3D simulations by solving the time-dependent, resistive MHD equations in a highly stratified atmosphere. Results. A small active region field is constructed by the emergence of a toroidal magnetic flux tube. A current structure is build up and reconnection sets in when new emerging flux comes into contact with the ambient field of the active region. The topology of the magnetic field around the current structure is drastically modified during reconnection. The modification results in a formation of new magnetic systems that eventually collide and reconnect. We find that reconnection jets are taking place in successive recurrent phases in directions perpendicular to each other, while in each phase they release magnetic energy and hot plasma into the solar atmosphere. After a series of recurrent appearance of jets, the system approaches an equilibrium where the efficiency of the reconnection is substantially reduced. We deduce that the emergence of new magnetic flux introduces a perturbation to the active region field, which in turn causes reconnection between neighboring magnetic fields and the release of the trapped energy in the form of jet-like emissions. This is the first time that self-consistent recurrency of jets in active regions is shown in a three-dimensional experiment of magnetic flux emergence.

During the interval 2012 March 7-11 the geospace experienced a barrage of intense space weather phenomena including the second largest geomagnetic storm of solar cycle 24 so far. Significant ultra-low-frequency wave enhancements and relativistic-electron dropouts in the radiation belts, as well as strong energetic-electron injection events in the magnetosphere were observed. These phenomena were ultimately associated with two ultra-fast (>2000 km s −1 ) coronal mass ejections (CMEs), linked to two X-class flares launched on early 2012 March 7. Given that both powerful events originated from solar active region NOAA 11429 and their onsets were separated by less than an hour, the analysis of the two events and the determination of solar causes and geospace effects are rather challenging. Using satellite data from a flotilla of solar, heliospheric and magnetospheric missions a synergistic Sun-to-Earth study of diverse observational solar, interplanetary and magnetospheric data sets was performed. It was found that only the second CME was Earth-directed. Using a novel method, we estimated its near-Sun magnetic field at 13 R e to be in the range [0.01, 0.16] G. Steep radial fall-offs of the near-Sun CME magnetic field are required to match the magnetic fields of the corresponding interplanetary CME (ICME) at 1 AU. Perturbed upstream solar-wind conditions, as resulting from the shock associated with the Earth-directed CME, offer a decent description of its kinematics. The magnetospheric compression caused by the arrival at 1 AU of the shock associated with the ICME was a key factor for radiation-belt dynamics.

The orbits of charged particles (electrons and protons), in a Harris-type 3D field topology of a reconnecting current sheet (RCS), are analyzed by dynamical systems methods. The focus is on values of the magnetic and electric fields relevant to RCSs in the solar atmosphere. First, a perturbative form of the equations of motion is used to determine the stability perpendicularly to the plane of reconnection, which is crucial in the efficiency of the RCS as an accelerator. The problem is shown to correspond to a case of "parametric resonance". The orbits are then studied with the complete form of the equations of motion. These can be reduced to a two degrees of freedom Hamiltonian nonlinear system by exploiting the existence of an additional integral of motion besides the energy. The orbits are studied analytically by normal form theory. Regular and chaotic orbits are identified by the use of appropriate Poincaré surfaces of section. The kinetic energy gain for escaping particles is calculated as a function of the initial conditions of injection of an orbit in the sheet. Formulae relating the kinetic energy gain to the physical parameters of the sheet and the initial conditions of the orbits are given both for electrons and protons.

Aims. We present a spectroscopic analysis of the pre-eruptive configuration of active region NOAA 11429, prior to two very fast coronal mass ejections (CMEs) on March 7, 2012 that are associated with this active region. We study the thermal components and the dynamics associated with the ejected flux ropes. Methods. Using differential emission measure (DEM) analysis of Hinode/EIS and SDO/AIA observations, we identify the emission components of both the flux rope and the host active region. We then follow the time evolution of the flux rope emission components by using AIA observations. The plasma density and the Doppler and non-thermal velocities associated with the flux ropes are also calculated from the EIS data. Results. The eastern and western parts of the active region, in which the two different fast CMEs originated during two X-class flares, were studied separately. In both regions we identified an emission component in the temperature range of log T = 6.8−7.1 associated with the presence of flux ropes. The time evolution of the eastern region showed an increase in the mean DEM in this temperature range by an order of magnitude, 5 h prior to the first CME. This was associated with a gradual rise and heating of the flux rope as manifested by blue-shifts and increased non-thermal velocities in Ca xv 200.97 Å, respectively. An overall upward motion of the flux ropes was measured (relative blue-shifts of ∼12 km s −1 ). The measured electron density was found to be 4 × 10 9 −2 × 10 10 cm −3 (using the ratio of Ca xv 181.90 Å over Ca xv 200.97 Å). We compare our findings with other works on the same AR to provide a unified picture of its evolution.

Abstract. Using high quality VLA observations, we detected for the first time spatially resolved oscillations in the microwave total intensity (I) and circular polarization (V ) emission of a sunspot-associated gyroresonance (g-r) source. Oscillations were detected at 8.5 and 5 GHz during several time intervals of our 10-hour-long dataset. The oscillations are intermittent: they start suddenly and are damped somehow more gradually. Despite their transient nature when they are observed they show significant positional, amplitude and phase stability. The spatial distribution of intensity variations is patchy and the location of the patches of strong oscillatory power is not the same at both frequencies. The strongest oscillations are associated with a small region where the 8.5 GHz emission comes from the second harmonic of the gyrofrequency while distinct peaks of weaker oscillatory power appear close to the outer boundaries of the 8.5 and 5 GHz g-r sources, where the emissions come from the third harmonic of the gyrofrequency. Overall, the 5 GHz oscillations are weaker than the 8.5 GHz oscillations (the rms amplitudes of the I oscillations are 1.3-2.5 ×10 4 K and 0.2-1.5 ×10 5 K, respectively). At both frequencies the oscillations have periods in the three-minute range: the power spectra show two prominent peaks at 6.25-6.45 mHz and 4.49-5.47 mHz. Our models show that the microwave oscillations are caused by variations of the location of the third and/or second harmonic surfaces with respect to the base of the chromosphere-corona transition region (TR), i.e. either the magnetic field strength or/and the height of the base of the TR oscillates. The best-fit model to the observed microwave oscillations can be derived from photospheric magnetic field strength oscillations with an rms amplitude of 40 G or oscillations of the height of the base of the TR with an rms amplitude of 25 km. Furthermore, small variations of the orientation of the magnetic field vector yield radio oscillations consistent with the observed oscillations.