Raffaella D'Amicis

Solar Energetic Particle Modulations Associated with Coherent Magnetic Structures

In situ observations of solar energetic particles (SEPs) often show rapid variations of their intensity profile, affecting all energies simultaneously, without time dispersion. A previously proposed interpretation suggests that these modulations are directly related to the presence of magnetic structures with a different magnetic topology. However, no compelling evidence of local changes in magnetic field or in plasma parameters during SEP modulations has been reported. In this paper, we performed a detailed analysis of SEP events and we found several signatures in the local magnetic field and/or plasma parameters associated with SEP modulations. The study of magnetic helicity allowed us to identify magnetic boundaries, associated with variations of plasma parameters, which are thought to represent the borders between adjacent magnetic flux tubes. It is found that SEP dispersionless modulations are generally associated with such magnetic boundaries. Consequently, we support the idea that SEP modulations are observed when the spacecraft passes through magnetic flux tubes, filled or devoid of SEPs, which are alternatively connected and not connected with the flare site. In other cases, we found SEP dropouts associated with large-scale magnetic holes. A possible generation mechanism suggests that these holes are formed in the high solar corona as a consequence of magnetic reconnection. This reconnection process modifies the magnetic field topology, and therefore, these holes can be magnetically isolated from the surrounding plasma and could also explain their association with SEP dropouts.

EVIDENCE FOR NONLINEAR DEVELOPMENT OF MAGNETOHYDRODYNAMIC SCALE INTERMITTENCY IN THE INNER HELIOSPHERE

The formation of coherent structures in turbulence is a signature of a developing cascade and therefore might be observable by analyzing inner heliospheric solar wind turbulence. To test this idea, data from the Helios 2 mission, for six streams of solar wind at different heliocentric distances and of different velocities, were subjected to statistical analysis using the partial variance of increments (PVI) approach. We see a clear increase of the PVI distribution function versus solar wind age for higher PVI cutoff, indicating development of non-Gaussian coherent structures. The plausibility of this interpretation is confirmed by a similar behavior observed in two-dimensional magnetohydrodynamics simulation data at corresponding dimensionless nonlinear times.

Multi Element Telescope for Imaging and Spectroscopy (METIS) coronagraph for the Solar Orbiter mission

METIS, the “Multi Element Telescope for Imaging and Spectroscopy”, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission to be launched in 2017. The unique profile of this mission will allow 1) a close approach to the Sun (up to 0.28 A.U.) thus leading to a significant improvement in spatial resolution; 2) quasi co-rotation with the Sun, resulting in observations that nearly freeze for several days the large-scale outer corona in the plane of the sky and 3) unprecedented out-of-ecliptic view of the solar corona. This paper describes the experiment concept and the observational tools required to achieve the science drivers of METIS. METIS will be capable of obtaining for the first time: • simultaneous imaging of the full corona in polarized visible-light (590-650 nm) and narrow-band ultraviolet HI Lyman α (121.6 nm); • monochromatic imaging of the full corona in the extreme ultraviolet He II Lyman α (30.4 nm); • spectrographic observations of the HI and He II Ly α in corona. These measurements will allow a complete characterization of the three most important plasma components of the corona and the solar wind, that is, electrons, hydrogen, and helium. This presentation gives an overview of the METIS imaging and spectroscopic observational capabilities to carry out such measurements.

STATISTICS OF DENSITY FLUCTUATIONS DURING THE TRANSITION FROM THE OUTER SOLAR CORONA TO THE INTERPLANETARY SPACE

This paper investigates the evolution of the plasma density fluctuations of the fast and slow solar wind from the solar corona into the interplanetary space. The study is performed by comparing the low-frequency spectra and the phase correlation of the proton density oscillations, measured in the inner heliosphere with the Helios 2 in situ instrumentation, with those due to the large-scale density perturbations observed with UVCS/SOHO in the outer corona. We find that the characteristics of density fluctuations of the fast solar wind are maintained in the transition from the outer corona to the inner heliosphere, thus suggesting a coronal imprint for the heliospheric large-scale 1/f 2 noise spectrum. In contrast, a quick dynamical evolution is observed in the slow wind, which, starting from large-scale fluctuations with strong phase correlations in the outer corona, gives rise to a Kolmogorov-like spectrum and an accumulation of density structures at small scales at 0.3 AU. This can be explained in the framework of nearly incompressible turbulence.

RADIAL EVOLUTION OF THE INTERMITTENCY OF DENSITY FLUCTUATIONS IN THE FAST SOLAR WIND

We study the radial evolution of the intermittency of density fluctuations in the fast solar wind. The study is performed by analyzing the plasma density measurements provided by Helios 2 in the inner heliosphere between 0.3 and 0.9 AU. The analysis is carried out by means of a complete set of diagnostic tools, including the flatness factor at different timescales to estimate intermittency, the Kolmogorov-Smirnov test to estimate the degree of intermittency, and the Fourier transform to estimate the power spectral densities of these fluctuations. Density fluctuations within the fast wind are rather intermittent and their level of intermittency, together with the amplitude of intermittent events, decreases with the distance from the Sun, at odds with the intermittency of both magnetic field and all other plasma parameters. Furthermore, the intermittent events are strongly correlated, exhibiting temporal clustering. This indicates that the mechanism underlying their generation departs from a time-varying Poisson process. A remarkable, qualitative similarity with the behavior of plasma density fluctuations obtained from a numerical study of the nonlinear evolution of parametric instability in the solar wind supports the idea that this mechanism has an important role in governing density fluctuations in the inner heliosphere.

PERSISTENT AND SELF-SIMILAR LARGE-SCALE DENSITY FLUCTUATIONS IN THE SOLAR CORONA

Density fluctuations of the low and midlatitude solar corona plasma are analyzed during the recent solar minimum period. Long time series of the intensity of the neutral hydrogen Lyα, 1216 A, line have been observed with the UltraViolet Coronagraph Spectrometer/Solar and Heliospheric Observatory at 1.7 R ☉, in low-latitude streamers and in regions where the slow solar wind is accelerated. Their frequency composition is investigated by using three different techniques, namely the Fourier, the Hurst, and the phase coherence analyses. The Fourier analysis reveals the existence of low-frequency f –α power spectra in the range from ~3 × 10–6 Hz to ~10–4 Hz, corresponding to periods from a few hours to a few days. The coronal density fluctuations are dominated by discontinuities separating structures with a minimum characteristic timescale of about 3 hr and a corresponding spatial scale of about 3 × 104 km. The nonlinear analysis technique based on the structure functions shows that for large timescales the coronal density fluctuations are statistically self-affine and give rise to an average Hurst exponent H = 0.654 ± 0.008. This indicates that the process underlying the variability of the corona and the slow wind at coronal level is a persistent mechanism, generating correlations among the plasma density fluctuations. Finally, the analysis based on the phase coherence index shows a high degree of phase synchronization of the coronal density variations for large timescales, which shows that the solar corona is dominated by phase coherent structures. The results of the analysis suggest a coupling of the variability of the solar corona and the photospheric dynamics induced by the convection at supergranular scale.

Wavelet Analysis as a Tool to Localize Magnetic and Cross-helicity Events in the Solar Wind

In this paper, we adopt the use of the wavelet transform as a new tool to investigate the time behavior at different scales of reduced magnetic helicity, cross-helicity, and residual energy in space plasmas. The main goal is a better characterization of the fluctuations in which interplanetary flux ropes are embedded. This kind of information is still missing in the present literature, and our tool can represent the basis for a new treatment of in situ measurements of this kind of event. There is a debate about the origins of small-scale flux ropes. It has been suggested that they are formed through magnetic reconnection in the solar wind, such as across the heliospheric current sheet. On the other hand, it has also been suggested that they are formed in the corona, similar to magnetic clouds. Thus, it looks like that there are two populations, one originating in the solar wind via magnetic reconnection across the current sheet in the inner heliosphere and the other originating in the corona. Small-scale flux ropes might be the remnants of the streamer belt blobs formed from disconnection; however, a one-to-one observation of a blob and a small-scale flux rope in the solar wind has yet to be found. Within this panorama of possibilities, this new technique appears to be very promising in investigating the origins of these objects advected by the solar wind.

Numerical simulations of coronal hole‐associated neutral solar wind as expected at the Solar Orbiter position

[1] Neutral hydrogen is indicative of the behavior of the main solar wind component formed by protons out to at least 5 R ⊙ . In fact, beyond this distance, the characteristic time for charge exchange between hydrogen atoms and protons becomes larger than the coronal expansion timescale, causing the neutrals to decouple from the charged solar wind. The mean free path of the neutral component rapidly increases with the radial distance so that neutrals generated at heliocentric distances >24 R ⊙ fly unperturbed and eventually are detected by Solar Orbiter (perihelion at approximately 48 R ⊙ ), since their mean free path is long enough to let neutrals reach the neutral solar wind detector. However, the computation of the differential flux shows that the bulk of the flux detected at the Solar Orbiter vantage point mainly comes from about 9 R ⊙ . Neutrals retain information on the three-dimensional distribution of hydrogen at the level where they are generated as the proton velocity distribution is frozen within the generated neutrals and transferred up to the Solar Orbiter position. In the present study, we report our preliminary results from our simulation of the neutral solar wind distribution as predicted at the Solar Orbiter position and considering the evolution of a coronal hole-emerging solar wind whose major parameters are estimated by the Solar and Heliospheric Observatory (SOHO) Ultraviolet Coronagraph Spectrometer (UVCS) experiment. The synergy between corona remote sensing and in situ neutral particle observations will enable us to infer the degree of anisotropy, if any, in the neutral and charged coronal hydrogen close to the Sun.

Multi Element Telescope for Imaging and Spectroscopy (METIS) coronagraph for the solar Orbiter mission

METIS, the “Multi Element Telescope for Imaging and Spectroscopy”, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission to be launched in 2017. The unique profile of this mission will allow 1) a close approach to the Sun (up to 0.28 A.U.) thus leading to a significant improvement in spatial resolution; 2) quasi co-rotation with the Sun, resulting in observations that nearly freeze for several days the large-scale outer corona in the plane of the sky and 3) unprecedented out-of-ecliptic view of the solar corona. This paper describes the experiment concept and the observational tools required to achieve the science drivers of METIS. METIS will be capable of obtaining for the first time: • simultaneous imaging of the full corona in polarized visible-light (590-650 nm) and narrow-band ultraviolet HI Lyman α (121.6 nm); • monochromatic imaging of the full corona in the extreme ultraviolet He II Lyman α (30.4 nm); • spectrographic observations of the HI and He II Ly α in corona. These measurements will allow a complete characterization of the three most important plasma components of the corona and the solar wind, that is, electrons, hydrogen, and helium. This presentation gives an overview of the METIS imaging and spectroscopic observational capabilities to carry out such measurements.