Maria Virginia Alves

Dynamics of coronal mass ejections in the interplanetary medium

Context. Coronal mass ejections (CMEs) are large plasma structures expelled from the low corona to the interplanetary space with a wide range of speeds. In the interplanetary medium CMEs suffer changes in their speeds because of interaction with the ambient solar wind. Aims. To understand the interplanetary CME (ICME) dynamics, we analyze the interaction between these structures and the ambient solar wind (SW), approaching the problem from the hydrodynamic point of view. Methods. We assume that the dynamics of the system is dominated by two kinds of drag-force dependence on speed (U), as ∼U and ∼U 2 . Furthermore, we propose a model that takes variations of the ICME radius (R) and SW density (ρsw) into account as a function of the distance (x )a sR(x) = x 0.78 and ρsw(x) = 1/x 2 , respectively. Then, we solve the equation of motion and present exact solutions Results. Considering CME speeds measured at a few solar radii and at one AU, we were able to constrain the values of the constants (viscosity and drag coefficient) for the linear (U) and quadratic (U 2 ) speed dependences, which seems to reproduce the ICME – SW system well. We found different solutions in which the concavity of the curves of the ICME speed profile changes, depending on the dominant factor, either the ICME radius or the SW density. Conclusions. This work shows that the macroscopic ICME propagation may be described by the hydrodynamic theory and that it is possible to find analytical solutions for the ICME-SW interaction.

Nonlinear generation of the fundamental radiation of interplanetary type III radio bursts

A new generation mechanism of interplanetary type III radio bursts at the fundamental electron plasma frequency is discussed. It is shown that the electromagnetic oscillating two-stream instability, driven by two oppositely propagating Langmuir waves, can account for the experimental observations. In particular, the major difficulties encountered by the previously considered electromagnetic decay instability are removed. 19 references.

On the relation between DC current locations and an EUV bright point: A case study

Context. Motion of the photospheric plasma forces the footpoints of magnetic flux tubes to move. This can give rise to electric currents in the solar atmosphere. The dissipation of these electric currents and the consequent heating of the solar plasma may be responsible for the formation of Extreme-UltraViolet (EUV) and X-ray bright points. Earlier bright point models usually consider either the emergence or the canceling of photospheric magnetic features as being responsible for reconnection causing the bright point. Aims. We investigate the consequences of different patterns of horizontal photospheric plasma motion for the generation of electric currents in the solar atmosphere and locate them with respect to an observed EUV bright point. The goal is to find out whether these currents might be responsible for the heating of bright points. Methods. To perform this study we use a “data driven” three dimensional magnetohydrodynamic model. The model solves an appropriate set of magnetohydrodynamic equations and uses, as initial condition, the magnetic field extrapolated from the line-of-sight component of the photospheric magnetic field observed by MDI/SoHO and the height-stratified, equilibrium density and temperature of the solar corona. We apply different patterns of horizontal photospheric plasma motion, derived from the temporal evolution of the photospheric magnetic structures in the course of the bright point lifetime, as boundary conditions of the model. Results. All applied patterns of horizontal photospheric plasma motion (shearing, convergence and fragmentation) lead to the formation of electric currents in the chromosphere, transition region and corona. Currents do not develop everywhere in the region where the motion is applied but in specific places where the magnetic field connectivity changes significantly. An important result is that the position where the electric currents develop is independent of the motion pattern used as boundary condition of the model. A comparison with data obtained by TRACE in the 1550 A channel and by the EIT in the 195 A channel shows that the region where the strongest current concentrations are formed coincides with the region where the EUV bright point appears.

Signatures of two distinct driving mechanisms in the evolution of coronal mass ejections in the lower corona

[1] We present a comparison between two simulations of coronal mass ejections (CMEs), in the lower corona, driven by different flux rope mechanisms presented in the literature. Both mechanisms represent different magnetic field configurations regarding the amount of twist of the magnetic field lines and different initial energies. They are used as a “proof of concept” to explore how different initialization mechanisms can be distinguished from each other in the lower corona. The simulations are performed using the Space Weather Modeling Framework (SWMF) during solar minimum conditions with a steady state solar wind obtained through an empirical approach to mimic the physical processes driving the solar wind. Although the two CMEs possess different initial energies (differing by an order of magnitude) and magnetic configurations, �� ��

Statistical analysis of 26 yr of observations of decametric radio emissions from Jupiter

Jupiter is a complex and at the same time very powerful radio source in the decameter wavelength range. The emission is anisotropic, intrinsically variable at millisecond to hour timescales, and also modulated by various external processes at much longer periods, ranging from ~10 h to months or years (including Jovian day and year, solar activity and solar wind variations, and for ground-based observations, terrestrial day and year). As a consequence, long-term observations and their statistical study have proved to be necessary for disentangling and understanding the observed phenomena. We have built a database from the available 26 yr of systematic, daily observations conducted at the Nancay Decameter Array and recorded in digital format. This database contains all observed Jovian decametric emissions, classified with respect to the time-frequency morphology, their dominant circular polarization, and maximum frequency. We present the results of the first statistical analysis of this database. We confirm the earlier classification of Jovian decameter emissions in Io-A, -A′, -B, -C, -D and non-Io-A, -B, -C types, but we also introduce new emission types (Io-A′′ and Io-B′) and precise and characterize the non-Io-D type. We determine the contours of all emission types in the CML− Φ Io plane (Central Meridian Longitude in Jupiter’s System III coordinates versus Io Phase), provide representative examples of their typical time-frequency patterns, and the distribution of emission’s maximum frequency as a function of Λ Io (Io’s Longitude). Finally, we present a statistical analysis of the distributions of the occurrence rate, duration, intensity and polarization for each emission type. non-Io-DAM appears to be related to small-scale, possibly bursty auroral structures.

CROSS-FIELD DIFFUSION OF ENERGETIC (100 keV to 2 MeV) PROTONS IN INTERPLANETARY SPACE

Magnetic field magnitude decreases (MDs) are observed in several regions of the interplanetary medium. In this paper, we characterize MDs observed by the Ulysses spacecraft instrumentation over the solar south pole by using magnetic field data to obtain the empirical size, magnetic field MD, and frequency of occurrence distribution functions. The interaction of energetic (100 keV to 2 MeV) protons with these MDs is investigated. Charged particle and MD interactions can be described by a geometrical model allowing the calculation of the guiding center shift after each interaction. Using the distribution functions for the MD characteristics, Monte Carlo simulations are used to obtain the cross-field diffusion coefficients as a function of particle kinetic energy. It is found that the protons under consideration cross-field diffuse at a rate of up to ≈11% of the Bohm rate. The same method used in this paper can be applied to other space regions where MDs are observed, once their local features are well known.

A theory of the fundamental plasma emission of type-III solar radio bursts

Results from plasma wave experiments in spacecraft give support to nonlinear interactions involving Langmuir waves, electromagnetic waves and ion-acoustic waves in association with type III solar radio bursts. In this paper we present a theory of the fundamental plasma emission of type-III solar radio bursts. Starting from the generalized Zakharov equations, considering the pump wave as a pair of oppositely propagating Langmuir waves with different amplitudes, and the excitation of electromagnetic and induced Langmuir waves, we obtain a general dispersion relation for the coupled waves. We numerically solve the general dispersion relation using the pump wave amplitude and plasma parameters as observed in the interplanetary medium. We compare our results with previous models. We find that the stability properties depend on the pump wave numbers and on the ratio of wave amplitude between the forward and backward pump wave. The inclusion of a second pump wave allows the simultaneous generation of up and down converted electromagnetic waves. The presence of a second pump with different amplitude from the first one brings a region of convective instability not present when amplitudes are the same.

Auroral precipitating energy during long magnetic storms

The power energy input carried by precipitating electrons into the auroral zone is an important parameter for understanding the solar wind-magnetosphere energy transfer processes and magnetic storms triggering. Some magnetic storms present a peculiar long recovery phase, lasting for many days or even weeks, which can be associated with the intense and long-duration auroral activity named HILDCAA (High Intensity Long Duration Continuous AE Activity). The auroral energy input during HILDCAAs has been pointed out as an essential key issue although there have been very few quantitative studies on this topic. In the present work, we have estimated the auroral electron precipitating energy during the events of long (LRP) and short (SRP) storm recovery phase. The energy has been calculated from the images produced by the Ultraviolet Imager (UVI) on board the Polar satellite. In order to obtain accurate energy values, we developed a dayglow estimate method to remove solar contamination from the UVI images, before calculating the energy. We compared the UVI estimate to the Hemispheric Power (HP), to the empirical power obtained from the AE index and to the solar wind input power. Our results showed that the UVI electron precipitating power for the LRP events presented a quasi-periodic fluctuation, which has been confirmed by the other estimates. We found that the LRP events are a consequence of a directly driven system, where there is no long-term energy storage in the magnetosphere, and the auroral electrojets during these events are directly affected by the electron precipitating power.

O vento solar e a atividade geomagnética

Nowadays, there are a lot of doubts about solar wind generation and most of what is known comes from observations. This is because its generation is too complex and the non-existence of observations in the regions where the wind is accelerated. However, near the Earth its features are well known and always measured. Solar wind interaction with the Earth magnetosphere leads to the formation of several magnetospheric regions. If the conditions are favorable such interaction leads also to geomagnetic activity. Studying geomagnetic activity variations caused by solar wind conditions is a main topic in space physics. For practical and technological aplications such variations can, for instance, damage ground and onbord satellites sistems because of the enhancement in atmospheric currents and levels of radiation, making this kind of research so important. This paper presents a brief revision about some of the main effects of solar wind and Earths magnetosphere interaction.

Ondas de choque não colisionais no espaço interplanetário

An introduction to the collisionless shock waves propagating through the interplanetary space is presented. Basic concepts related to the propagation of information in ordinary gases and plasmas are reviewed. The wave modes in plasma and their steepening to shock waves are discussed. The shocks found in interplanetary space - planetary bow shocks, transient shocks driven by interplanetary coronal mass ejections and recurrent shocks due to the interaction of high-speed and low-speed solar wind streams are also discussed. Shocks can also be classified according to their propagation direction - away or toward the sun (forward and reverse shocks) and according if they are fast or slow mode magnetosonic waves steepened (fast and slow shocks). Finally, an example of calculated parameters for a transient interplanetary shock observed in solar wind near earth and its magnetospheric effects is presented.