arXiv subjects

Emilia K. J. Kilpua

Publications and source records attributed to Emilia K. J. Kilpua.

24 records · Page 2Linked to original sources

arXiv2018

Forward modeling of coronal mass ejection flux ropes in the inner heliosphere with 3DCORE

Forecasting the geomagnetic effects of solar storms, known as coronal mass ejections (CMEs), is currently severely limited by our inability to predict the magnetic field configuration in the CME magnetic core and by observational effects of a single spacecraft trajectory through its 3D structure. CME magnetic flux ropes can lead to continuous forcing of the energy input to the Earth's magnetosphere by strong and steady southward-pointing magnetic fields. Here, we demonstrate in a proof-of-concept way a new approach to predict the southward field Bz in a CME flux rope. It combines a novel semi-empirical model of CME flux rope magnetic fields (3-Dimensional Coronal ROpe Ejection or 3DCORE) with solar observations and in situ magnetic field data from along the Sun-Earth line. These are provided here by the MESSENGER spacecraft for a CME event on 2013 July 9-13. 3DCORE is the first such model that contains the interplanetary propagation and evolution of a 3D flux rope magnetic field, the observation by a synthetic spacecraft and the prediction of an index of geomagnetic activity. A counterclockwise rotation of the left-handed erupting CME flux rope in the corona of 30 degree and a deflection angle of 20 degree is evident from comparison of solar and coronal observations. The calculated Dst matches reasonably the observed Dst minimum and its time evolution, but the results are highly sensitive to the CME axis orientation. We discuss assumptions and limitations of the method prototype and its potential for real time space weather forecasting and heliospheric data interpretation.

astro-ph.SR↗

arXiv2017

Determining the Intrinsic CME Flux Rope Type Using Remote-sensing Solar Disk Observations

A key aim in space weather research is to be able to use remote-sensing observations of the solar atmosphere to extend the lead time of predicting the geoeffectiveness of a coronal mass ejection (CME). In order to achieve this, the magnetic structure of the CME as it leaves the Sun must be known. In this article we address this issue by developing a method to determine the intrinsic flux rope type of a CME solely from solar disk observations. We use several well known proxies for the magnetic helicity sign, the axis orientation, and the axial magnetic field direction to predict the magnetic structure of the interplanetary flux rope. We present two case studies: the 2 June 2011 and the 14 June 2012 CMEs. Both of these events erupted from an active region and, despite having clear in situ counterparts, their eruption characteristics were relatively complex. The first event was associated with an active region filament that erupted in two stages, while for the other event the eruption originated from a relatively high coronal altitude and the source region did not feature the presence of a filament. Our magnetic helicity sign proxies include the analysis of magnetic tongues, soft X-ray and/or EUV sigmoids, coronal arcade skew, filament emission and absorption threads, and filament rotation. Since the inclination of the post-eruption arcades was not clear, we use the tilt of the polarity inversion line to determine the flux rope axis orientation, and coronal dimmings to determine the flux rope footpoints and, therefore, the direction of the axial magnetic field. The comparison of the estimated intrinsic flux rope structure to in situ observations at the Lagrangian point L1 indicated a good agreement with the predictions. Our results highlight the flux rope type determination techniques that are particularly useful for active region eruptions, where most geoeffective CMEs originate.

astro-ph.SR↗

arXiv2017

Planar magnetic structures in coronal mass ejection-driven sheath regions

Planar magnetic structures (PMSs) are periods in the solar wind during which interplanetary magnetic field vectors are nearly parallel to a single plane. One of the specific regions where PMSs have been reported are coronal mass ejection (CME)-driven sheaths. We use here an automated method to identify PMSs in 95 CME sheath regions observed in-situ by the Wind and ACE spacecraft between 1997 and 2015. The occurrence and location of the PMSs are related to various shock, sheath and CME properties. We find that PMSs are ubiquitous in CME sheaths; 85% of the studied sheath regions had PMSs with the mean duration of 6.0 hours. In about one-third of the cases the magnetic field vectors followed a single PMS plane that covered a significant part (at least 67%) of the sheath region. Our analysis gives strong support for two suggested PMS formation mechanisms: the amplification and alignment of solar wind discontinuities near the CME-driven shock and the draping of the magnetic field lines around the CME ejecta. For example, we found that the shock and PMS plane normals generally coincided for the events where the PMSs occurred near the shock (68% of the PMS plane normals near the shock were separated by less than 20° from the shock normal), while deviations were clearly larger when PMSs occurred close to the ejecta leading edge. In addition, PMSs near the shock were generally associated with lower upstream plasma beta than the cases where PMSs occurred near the leading edge of the CME. We also demonstrate that the planar parts of the sheath contain a higher amount of strongly southward magnetic field than the non-planar parts, suggesting that planar sheaths are more likely to drive magnetospheric activity.

astro-ph.SR↗

arXiv2014

Observations of an extreme storm in interplanetary space caused by successive coronal mass ejections

Space weather refers to dynamic conditions on the Sun and in the space environment of the Earth, which are often driven by solar eruptions and their subsequent interplanetary disturbances. It has been unclear how an extreme space weather storm forms and how severe it can be. Here we report and investigate an extreme event with multi-point remote-sensing and in-situ observations. The formation of the extreme storm showed striking novel features. We suggest that the in-transit interaction between two closely launched coronal mass ejections resulted in the extreme enhancement of the ejecta magnetic field observed near 1 AU at STEREO A. The fast transit to STEREO A (in only 18.6 hours), or the unusually weak deceleration of the event, was caused by the preconditioning of the upstream solar wind by an earlier solar eruption. These results provide a new view crucial to solar physics and space weather as to how an extreme space weather event can arise from a combination of solar eruptions.

physics.space-ph↗

arXiv2014

Three-dimensional evolution of flux rope CMEs and its relation to the local orientation of the heliospheric current sheet

Flux ropes (FRs) ejected from the Sun may change their geometrical orientation during their evolution which directly affects their geoeffectiveness. Therefore, it is crucial to understand how solar FRs evolve in the heliosphere to improve our space weather forecasting tools. We analyze 15 coronal mass ejections (CMEs), with clear FR signatures, observed during the decay of Solar Cycle 23 and rise of Solar Cycle 24. We estimate initial orientations of the FRs at the origin using extreme ultraviolet observations of post-eruption arcades and/or eruptive prominences. Then we reconstruct multiviewpoint coronagraph observations of the CMEs from ~2 to 30 Rs with a three-dimensional geometric representation of a FR to determine their geometrical parameters. Finally, we propagate the FRs from ~30 Rs to 1 AU through MHD-simulated background solar wind while using in-situ measurements at 1 AU of the associated magnetic cloud as a constraint for the propagation technique. These methodology allows us to estimate the FR orientation all the way from the Sun to 1 AU. We find that while the FRs deflection occurs predominantly below 30 Rs, a significant amount of deflection and rotation happens between 30 Rs and 1 AU. We compare the FR orientation to the local orientation of the heliospheric current sheet (HCS). We find that slow FRs tend to align with the streams of slow solar wind in the inner heliosphere. During the solar cycle minimum the slow solar wind channel as well as the HCS usually occupy the area in the vicinity of the solar equatorial plane, which in the past led researchers to the hypothesis that FRs align with the HCS. Our results show that exclusions from this rule are explained by interaction with the Parker-spiralled background magnetic field, which dominates over the magnetic interaction with the HCS in the inner heliosphere at least during solar minimum conditions.

physics.space-ph↗

arXiv2011

Grad-Shafranov reconstruction of magnetic clouds: overview and improvements

The Grad-Shafranov reconstruction is a method of estimating the orientation (invariant axis) and cross-section of magnetic flux ropes using the data from a single spacecraft. It can be applied to various magnetic structures such as magnetic clouds (MCs) and flux ropes embedded into the magnetopause and in the solar wind. We develop a number of improvements of this technique and show some examples of the reconstruction procedure of interplanetary coronal mass ejections (ICMEs) observed at 1 AU by the STEREO, WIND and ACE spacecraft during the minimum following the solar cycle 23. The analysis is conducted not only for ideal localized ICME events but also for non-trivial cases of magnetic clouds in fast solar wind. The Grad-Shafranov reconstruction gives reasonable results for the sample events, although it possesses certain limitations, which need to be taken into account during the interpretation of the model results.

astro-ph.IM↗