arXiv subjects

Emilia Kilpua

Publications and source records attributed to Emilia Kilpua.

26 records · Page 2Linked to original sources

arXiv2021

A possible link between turbulence and plasma heating

Numerical simulations and experimental results have shown that current sheets formation in space plasmas can be associated with enhanced vorticity. Also, in simulations the generation of such structures is associated with strong plasma heating. Here, we compare four-point measurements in the terrestrial magnetosheath turbulence from the Multiscale magnetospheric mission (MMS) of the flow vorticity and the magnetic field curlometer versus their corresponding one-point proxies PVI(V) and PVI(B) based on the Partial Variance of Increments (PVI) method. We show that the one-point proxies are sufficiently precise in identifying not only the generic features of the current sheets and vortices statistically, but also their appearance in groups associated with plasma heating. The method has been further applied to the region of the turbulent sheath of an interplanetary coronal mass ejection (ICME) observed at L1 by WIND spacecraft. We observe current sheets and vorticity associated heating in larger groups (blobs), which so far have not been considered in the literature on turbulent data analysis. The blobs represent extended spatial regions of activity with enhanced regional correlations between the occurrence of conditioned currents and vorticity, which at the same time are also correlated with enhanced temperatures. This heating mechanism is substantially different from the plasma heating in the vicinity of the ICME shock, where plasma beta is strongly fluctuating and there is no vorticity. The proposed method describes a new pathway for linking the plasma heating and plasma turbulence, and it is relevant to in-situ observations when only single spacecraft measurements are available.

physics.plasm-ph↗

arXiv2021

The spheromak tilting and how it affects modelling coronal mass ejections

Spheromak type flux ropes are increasingly used for modelling coronal mass ejections (CMEs). Many models aim in accurately reconstructing the magnetic field topology of CMEs, considering its importance in assessing their impact on modern technology and human activities in space and on ground. However, so far there is little discussion about how the details of the magnetic structure of a spheromak affect its evolution through the ambient field in the modelling domain, and what impact this has on the accuracy of magnetic field topology predictions. If the spheromak has its axis of symmetry (geometric axis) at an angle with respect to the direction of the ambient field, then the spheromak starts rotating so that its symmetry axis finally aligns with the ambient field. When using the spheromak in space weather forecasting models this tilting can happen already during insertion and significantly affects the results. In this paper we highlight this issue previously not examined in the field of space weather and we estimate the angle by which the spheromak rotates under different conditions. To do this we generated simple purely radial ambient magnetic field topologies (weak/strong positive/negative) and inserted spheromaks with varying initial speed and tilt, and magnetic helicity sign. We employ different physical and geometric criteria to locate the magnetic centre of mass and axis of symmetry of the spheromak. We confirm that spheromaks rotate in all investigated conditions and their direction and angle of rotation depend on the spheromak's initial properties and ambient magnetic field strength and orientation.

astro-ph.SR↗

arXiv2021

Uncovering Erosion Effects on Magnetic Flux Rope Twist

Magnetic clouds (MCs) are transient structures containing large-scale magnetic flux ropes from solar eruptions. The twist of magnetic field lines around the rope axis reveals information about flux rope formation processes and geoeffectivity. During propagation, MC flux ropes may erode via reconnection with the ambient solar wind. Any erosion reduces the magnetic flux and helicity of the ropes, and changes their cross-sectional twist profiles. This study relates twist profiles in MC flux ropes observed at 1 AU to the amount of erosion undergone by the MCs in interplanetary space. The twist profiles of two well-identified MC flux ropes associated with the clear appearance of post eruption arcades in the solar corona are analysed. To infer the amount of erosion, the magnetic flux content of the ropes in the solar atmosphere is estimated, and compared to estimates at 1 AU. The first MC shows a monotonically decreasing twist from the axis to periphery, while the second displays high twist at the axis, rising twist near the edges, and lower twist in between. The first MC displays a larger reduction in magnetic flux between the Sun and 1 AU, suggesting more erosion, than that seen in the second MC. In the second cloud, rising twist at the rope edges may have been due to an envelope of overlying coronal field lines with relatively high twist, formed by reconnection beneath the erupting flux rope in the low corona. This high-twist envelope remained almost intact from the Sun to 1 AU due to the low erosion levels. In contrast, the high-twist envelope of the first cloud may have been entirely peeled away via erosion by the time it reaches 1 AU.

astro-ph.SR↗

arXiv2020

Using radio triangulation to understand the origin of two subsequent type II radio bursts

Context: Eruptive events such as coronal mass ejections (CMEs) and flares accelerate particles and generate shock waves which can arrive at Earth and can disturb the magnetosphere. Understanding the association between CMEs and CME-driven shocks is therefore highly important for space weather studies. Aims: We present a study of the CME/flare event associated with two type II bursts observed on September 27, 2012. The aim of the study is to understand the relationship between the observed CME and the two distinct shock wave signatures. Methods: The multi-wavelength study of the eruptive event (CME/flare) was complemented with radio triangulation of the associated radio emission and modelling of the CME and the shock wave employing MHD simulations. Results: We found that, although temporal association between the type II bursts and the CME is good, the low-frequency type II(LF-type II) burst occurs significantly higher in the corona than the CME and its relationship to the CME is not straightforward. The analysis of the EIT wave (coronal bright front) shows the fastest wave component to be in the southeast quadrant of the Sun. This is also the quadrant in which the source positions of the LF-type II were found to be located, probably resulting from the interaction between the shock wave and a streamer. Conclusions: The relationship between the CME/flare event and the shock wave signatures is discussed using the temporal association,as well as the spatial information of the radio emission. Further, we discuss the importance and possible effects of the frequently non-radial propagation of the shock wave.

astro-ph.SR↗

arXiv2019

Particle Telescope aboard FORESAIL-1: simulated performance

The Particle Telescope (PATE) of FORESAIL-1 mission is described. FORESAIL-1 is a CubeSat mission to polar Low Earth Orbit. Its scientific objectives are to characterize electron precipitation from the radiation belts and to observe energetic neutral atoms (ENAs) originating from the Sun during the strongest solar flares. For that purpose, the 3-unit CubeSat carries a particle telescope that measures energetic electrons in the nominal energy range of 80--800 keV in seven energy channels and energetic protons at 0.3--10 MeV in ten channels. In addition, particles penetrating the whole telescope at higher energies will be measured in three channels: one $>$800 keV electron channel, two integral proton channels at $>$10 MeV energies. The instrument contains two telescopes at right angles to each other, one measuring along the spin axis of the spacecraft and one perpendicular to it. During a spin period (nominally 15 s), the rotating telescope will, thus, deliver angular distributions of protons and electrons, at 11.25-degree clock-angle resolution, which enables one to accurately determine the pitch-angle distribution and separate the trapped and precipitating particles. During the last part of the mission, the rotation axis will be accurately pointed toward the Sun, enabling the measurement of the energetic hydrogen from that direction. Using the geomagnetic field as a filter and comparing the rates observed by the two telescopes, the instrument can observe the solar ENA flux for events similar to the only one so far observed in December 2006. We present the Geant4-simulated energy and angular response functions of the telescope and assess its sensitivity showing that they are adequate to address the scientific objectives of the mission.

astro-ph.IM↗

arXiv2019

Reconstructing coronal hole areas with EUHFORIA and adapted WSA model: optimising the model parameters

The adopted WSA model embedded in EUHFORIA (EUropean Heliospheric FORecasting Information Asset) is compared to EUV observations. According to the standard paradigm coronal holes are sources of open flux thus we use remote sensing EUV observations and \textsc{catch} (Collection of Analysis Tools for Coronal Holes) to extract CH areas and compare them to the open flux areas modelled by EUHFORIA. From the adopted WSA model we employ only the Potential Field Source Surface (PFSS) model for the inner corona and the Schatten Current Sheet (SCS) model for the outer (PFSS+SCS). The height, $R_{\rm ss}$, of the outer boundary of the PFSS, known as the source surface, and the height, $R_{\rm i}$, of the inner boundary of the SCS are important parameters affecting the modelled CH areas. We investigate the impact the two model parameters can have in the modelled results. We vary $R_{\rm ss}$ within the interval [1.4, 3.2]$R_{\rm \odot}$ with a step of 0.1$R_{\rm \odot}$, and $R_{\rm i}$ within the interval [1.3, 2.8]$R_{\rm \odot}$ with the same step, and the condition that $R_{\rm i}<R_{\rm ss}$. This way we have a set of 184 initial parameters to the model and we assess the model results for all these possible height pairs. We conclude that the default heights used so far fail in modelling accurately CH areas and lower heights need to be considered.

astro-ph.SR↗

arXiv2017

The temporal and spatial scales of density structures released in the slow solar wind during solar activity maximum

In a recent study, we took advantage of a highly tilted coronal neutral sheet to show that density structures, extending radially over several solar radii (Rs), are released in the forming slow solar wind approximately 4-5 Rs above the solar surface (Sanchez-Diaz et al. 2017). We related the signatures of this formation process to intermittent magnetic reconnection occurring continuously above helmet streamers. We now exploit the heliospheric imagery from the Solar Terrestrial Relation Observatory (STEREO) to map the spatial and temporal distribution of the ejected structures. We demonstrate that streamers experience quasi-periodic bursts of activity with the simultaneous outpouring of small transients over a large range of latitudes in the corona. This cyclic activity leads to the emergence of well-defined and broad structures. Derivation of the trajectories and kinematic properties of the individual small transients that make up these large-scale structures confirms their association with the forming Slow Solar Wind (SSW). We find that these transients are released, on average, every 19.5 hours, simultaneously at all latitudes with a typical radial size of 12 Rs. Their spatial distribution, release rate and three-dimensional extent are used to estimate the contribution of this cyclic activity to the mass flux carried outward by the SSW. Our results suggest that, in interplanetary space, the global structure of the heliospheric current sheet is dominated by a succession of blobs and associated flux ropes. We demonstrated this with an example event using STEREO-A in-situ measurements.

astro-ph.SR↗

arXiv2016

Observational evidence for the associated formation of blobs and raining inflows in the solar Corona

The origin of the slow solar wind is still a topic of much debate. The continual emergence of small transient structures from helmet streamers is thought to constitute one of the main sources of the slow wind. Determining the height at which these transients are released is an important factor in determining the conditions under which the slow solar wind forms. To this end, we have carried out a multipoint analysis of small transient structures released from a north-south tilted helmet streamer into the slow solar wind over a broad range of position angles during Carrington Rotation 2137. Combining the remote-sensing observations taken by the Solar-TErrestrial RElations Observatory (STEREO) mission with coronagraphic observations from the Solar and Heliospheric Observatory (SoHO) spacecraft, we show that the release of such small transient structures (often called blobs), which subsequently move away from the Sun, is associated with the concomitant formation of transient structures collapsing back towards the Sun; the latter have been referred to by previous authors as raining inflows. This is the first direct association between outflowing blobs and raining inflows, which locates the formation of blobs above the helmet streamers and gives strong support that the blobs are released by magnetic reconnection.

astro-ph.SR↗