arXiv subjects

E. K. J. Kilpua

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

At least 37 records · Page 2Linked to original sources

arXiv2022

Cross helicity of interplanetary coronal mass ejections at 1 au

Interplanetary coronal mass ejections (ICMEs) contain magnetic field and velocity fluctuations across a wide range of scales. These fluctuations may be interpreted as Alfvénic wave packets propagating parallel or anti-parallel to the background magnetic field, with the difference in power between counter-propagating fluxes quantified by the cross helicity. We have determined the cross helicity of inertial range fluctuations at $10^{-3}-10^{-2}$ Hz in 226 ICME flux ropes and 176 ICME sheaths observed by the Wind spacecraft at 1 au during 1995-2015. The flux ropes and sheaths had mean, normalised cross helicities of 0.18 and 0.24, respectively, with positive values here indicating net anti-sunward fluxes. While still tipped towards the anti-sunward direction on average, fluxes in ICMEs tend to be more balanced than in the solar wind at 1 au, where the mean cross helicity is larger. Superposed epoch profiles show cross helicity falling sharply in the sheath and reaching a minimum inside the flux rope near the leading edge. More imbalanced, solar wind-like cross helicity was found towards the trailing edge and laterally further from the rope axis. The dependence of cross helicity on flux rope orientation and the presence of an upstream shock are considered. Potential origins of the low cross helicity in ICMEs at 1 au include balanced driving of the closed-loop flux rope at the Sun and ICME-solar wind interactions in interplanetary space. We propose that low cross helicity of fluctuations is added to the standard list of ICME signatures.

physics.space-ph↗

arXiv2022

Structure and fluctuations of a slow ICME sheath observed at 0.5 au by the Parker Solar Probe

Sheaths ahead of interplanetary coronal mass ejections (ICMEs) are turbulent heliospheric structures. Knowledge of their structure and fluctuations is important for understanding their geoeffectiveness, their role in accelerating particles, and the interaction of ICMEs with the solar wind. We studied observations from the Parker Solar Probe of a sheath observed at 0.5 au in March 2019, ahead of a slow streamer blowout CME. To examine the MHD-scale turbulent properties, we calculated fluctuation amplitudes, magnetic compressibility, partial variance of increments (PVI), cross helicity ($σ_c$), residual energy ($σ_r$), and the Jensen-Shannon permutation entropy and complexity. The sheath consisted of slow and fast flows separated by a 15-min change in magnetic sector that coincided with current sheet crossings and a velocity shear zone. Fluctuation amplitudes and PVI were greater through the sheath than upstream. Fluctuations had mostly negative $σ_r$ and positive $σ_c$ in the sheath, the latter indicating an anti-sunward sense of propagation. The velocity shear region marked an increase in temperature and specific entropy, and the faster flow behind had local patches of positive $σ_r$ as well as higher fluctuation amplitudes and PVI. Fluctuations in the preceding wind and sheath were stochastic, with the sheath fluctuations showing lower entropy and higher complexity than upstream. The two-part sheath structure likely resulted from a warp in the heliospheric current sheet (HCS) being swept up and compressed. The ejecta accelerated and heated the wind at the sheath rear, which then interacted with the slower wind ahead of the HCS warp. This caused differences in fluctuation properties across the sheath. Sheaths of slow ICMEs can thus have complex structure where fluctuation properties are not just downstream shock properties, but are generated within the sheath.

physics.space-ph↗

arXiv2021

Multi-spacecraft observations of the structure of the sheath of an interplanetary coronal mass ejection and related energetic ion enhancement

Sheaths ahead of coronal mass ejections (CMEs) are large heliospheric structures that form with CME expansion and propagation. Turbulent and compressed sheaths contribute to the acceleration of particles in the corona and in interplanetary space, but the relation of their internal structures to particle energization is still relatively little studied. In particular, the role of sheaths in accelerating particles when the shock Mach number is low is a significant open problem. This work seeks to provide new insights on the internal structure of CME sheaths with regard to energetic particle enhancements. A good opportunity to achieve this aim was provided by observations of a sheath made by radially aligned spacecraft at 0.8 and $\sim$ 1 AU (Solar Orbiter, Wind, ACE and BepiColombo) on 19-21 April 2020. The sheath was preceded by a weak shock. Energetic ion enhancements occurred at different locations within the sheath structure at Solar Orbiter and L1. Magnetic fluctuation amplitudes at inertial-range scales increased in the sheath relative to the upstream wind. However, when normalised to the local mean field, fluctuation amplitudes did not increase significantly; magnetic compressibility of fluctuation also did not increase. Various substructures were embedded within the sheath at the different spacecraft, including multiple heliospheric current sheet (HCS) crossings and a small-scale flux rope. At L1, the ion flux enhancement was associated with the HCS crossings, while at Solar Orbiter, the enhancement occurred within the rope. Substructures that are swept from the upstream solar wind and compressed in the sheath can act as particularly effective acceleration sites. A possible acceleration mechanism is betatron acceleration associated with the small-scale flux rope and the warped HCS in the sheath.

physics.space-ph↗

arXiv2021

Moving solar radio bursts and their association with coronal mass ejections

Context: Solar eruptions, such as coronal mass ejections (CMEs), are often accompanied by accelerated electrons that can in turn emit radiation at radio wavelengths. This radiation is observed as solar radio bursts. The main types of bursts associated with CMEs are type II and type IV bursts that can sometimes show movement in the direction of the CME expansion, either radially or laterally. However, the propagation of radio bursts with respect to CMEs has only been studied for individual events. Aims: Here, we perform a statistical study of 64 moving bursts with the aim to determine how often CMEs are accompanied by moving radio bursts. This is done in order to ascertain the usefulness of using radio images in estimating the early CME expansion. Methods: Using radio imaging from the Naçay Radioheliograph (NRH), we constructed a list of moving radio bursts, defined as bursts that move across the plane of sky at a single frequency. We define their association with CMEs and the properties of associated CMEs using white-light coronagraph observations. We also determine their connection to classical type II and type IV radio burst categorisation. Results: We find that just over a quarter of type II and half of type IV bursts that occurred during the NRH observing windows in Solar Cycle 24 are accompanied by moving radio emission. All but one of the moving radio bursts are associated with white--light CMEs and the majority of moving bursts (90%) are associated with wide CMEs (>60 degrees in width). In particular, all but one of the moving bursts corresponding to type IIs are associated with wide CMEs; however, and unexpectedly, the majority of type II moving bursts are associated with slow white-light CMEs (<500 km/s). On the other hand, the majority of moving type IV bursts are associated with fast CMEs (>500 km/s).

astro-ph.SR↗

arXiv2021

Turbulence/wave transmission at an ICME-driven shock observed by Solar Orbiter and Wind

Solar Orbiter observed an interplanetary coronal mass ejection (ICME) event at 0.8 AU on 2020 April 19. The ICME was also observed by Wind at 1 AU on 2020 April 20. An interplanetary shock wave was driven in front of the ICME. We focus on the transmission of the magnetic fluctuations across the shock and analyze the characteristic wave modes of solar wind turbulence near the shock observed by both spacecraft. The ICME event is characterized by a magnetic helicity based technique. The shock normal is determined by magnetic coplanarity method for Solar Orbiter and using a mixed coplanarity approach for Wind. The power spectra of magnetic field fluctuations are generated by applying both a fast Fourier transform and Morlet wavelet analysis. To understand the nature of waves observed near the shock, we use the normalized magnetic helicity as a diagnostic parameter. The wavelet reconstructed magnetic field fluctuation hodograms are used to further study the polarization properties of waves. We find that the ICME-driven shock observed by Solar Orbiter and Wind is a fast forward oblique shock with a more perpendicular shock angle at 1 AU. After the shock crossing, the magnetic field fluctuation power increases. Most of the magnetic field fluctuation power resides in the transverse fluctuations. In the vicinity of the shock, both spacecraft observe right-hand polarized waves in the spacecraft frame. The upstream wave signatures fall in a relatively broad and low-frequency band, which might be attributed to low-frequency MHD waves excited by the streaming particles. For the downstream magnetic wave activity, we find oblique kinetic Alfven waves with frequencies near the proton cyclotron frequency in the spacecraft frame. The frequency of the downstream waves increases by a factor of 7-10 due to the shock compression and the Doppler effect.

physics.space-ph↗

arXiv2020

On the occurrence of type IV solar radio bursts in the solar cycle 24 and their association with coronal mass ejections

Solar activity, in particular coronal mass ejections (CMEs), are often accompanied by bursts of radiation at metre wavelengths. Some of these bursts have a long duration and extend over a wide frequency band, namely, type IV radio bursts. However, the association of type IV bursts with coronal mass ejections is still not well understood. In this article, we perform the first statistical study of type IV solar radio bursts in the solar cycle 24. Our study includes a total of 446 type IV radio bursts that occurred during this cycle. Our results show that a clear majority, $\sim 81 \%$ of type IV bursts, were accompanied by CMEs, based on a temporal association with white-light CME observations. However, we found that only $\sim 2.2 \%$ of the CMEs are accompanied by type IV radio bursts. We categorised the type IV bursts as moving or stationary based on their spectral characteristics and found that only $\sim 18 \%$ of the total type IV bursts in this study were moving type IV bursts. Our study suggests that type IV bursts can occur with both `Fast' ($\geq 500$ km/s) and `Slow' ($< 500$ km/s), and also both `Wide' ($\geq 60^{\circ}$) and `Narrow' ($< 60^{\circ}$) CMEs. However, the moving type IV bursts in our study were mostly associated with `Fast' and `Wide' CMEs ($\sim 52 \%$), similar to type II radio bursts. Contrary to type II bursts, stationary type IV bursts have a more uniform association with all CME types.

astro-ph.SR↗

arXiv2020

On the Radial and Longitudinal Variation of a Magnetic Cloud: ACE, Wind, ARTEMIS and Juno Observations

We present observations of the same magnetic cloud made near Earth by the Advance Composition Explorer (ACE), Wind, and the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission comprising the Time History of Events and Macroscale Interactions during Substorms (THEMIS) B and THEMIS C spacecraft, and later by Juno at a distance of 1.2 AU. The spacecraft were close to radial alignment throughout the event, with a longitudinal separation of $3.6^{\circ}$ between Juno and the spacecraft near Earth. The magnetic cloud likely originated from a filament eruption on 22 October 2011 at 00:05 UT, and caused a strong geomagnetic storm at Earth commencing on 24 October. Observations of the magnetic cloud at each spacecraft have been analysed using Minimum Variance Analysis and two flux rope fitting models, Lundquist and Gold-Hoyle, to give the orientation of the flux rope axis. We explore the effect different trailing edge boundaries have on the results of each analysis method, and find a clear difference between the orientations of the flux rope axis at the near-Earth spacecraft and Juno, independent of the analysis method. The axial magnetic field strength and the radial width of the flux rope are calculated using both observations and fitting parameters and their relationship with heliocentric distance is investigated. Differences in results between the near-Earth spacecraft and Juno are attributed not only to the radial separation, but to the small longitudinal separation which resulted in a surprisingly large difference in the in situ observations between the spacecraft. This case study demonstrates the utility of Juno cruise data as a new opportunity to study magnetic clouds beyond 1 AU, and the need for caution in future radial alignment studies.

physics.space-ph↗

arXiv2020

Electron acceleration and radio emission following the early interaction of two coronal mass ejections

Context. Coronal mass ejections (CMEs) are large eruptions of magnetised plasma from the Sun that are often accompanied by solar radio bursts produced by accelerated electrons. Aims. A powerful source for accelerating electron beams are CME-driven shocks, however, there are other mechanisms capable of accelerating electrons during a CME eruption. So far, studies have relied on the traditional classification of solar radio bursts into five groups (Type I-V) based mainly on their shapes and characteristics in dynamic spectra. Here, we aim to determine the origin of moving radio bursts associated with a CME that do not fit into the present classification of the solar radio emission. Methods. By using radio imaging from the Nançay Radioheliograph, combined with observations from the Solar Dynamics Observatory, Solar and Heliospheric Observatory, and Solar Terrestrial Relations Observatory spacecraft, we investigate the moving radio bursts accompanying two subsequent CMEs on 22 May 2013. We use three-dimensional reconstructions of the two associated CME eruptions to show the possible origin of the observed radio emission. Results. We identified three moving radio bursts at unusually high altitudes in the corona that are located at the northern CME flank and move outwards synchronously with the CME. The radio bursts correspond to fine-structured emission in dynamic spectra with durations of ~1 s, and they may show forward or reverse frequency drifts. Since the CME expands closely following an earlier CME, a low coronal CME-CME interaction is likely responsible for the observed radio emission.

astro-ph.SR↗

arXiv2020

Cross Helicity of the November 2018 Magnetic Cloud Observed by the Parker Solar Probe

Magnetic clouds are large-scale transient structures in the solar wind with low plasma $β$, low-amplitude magnetic field fluctuations, and twisted field lines with both ends often connected to the Sun. Their inertial-range turbulent properties have not been examined in detail. In this Letter, we analyze the normalized cross helicity, $σ_c$, and residual energy, $σ_r$, of plasma fluctuations in the November 2018 magnetic cloud observed at 0.25 au by the Parker Solar Probe. A low value of $|σ_c|$ was present in the cloud core, indicating that wave power parallel and anti-parallel to the mean field was approximately balanced, while the cloud's outer layers displayed larger amplitude Alfvénic fluctuations with high $|σ_c|$ values and $σ_r\sim0$. These properties are discussed in terms of the cloud's solar connectivity and local interaction with the solar wind. We suggest that low $|σ_c|$ is likely a common feature of magnetic clouds given their typically closed field structure. Anti-sunward fluctuations propagating immediately upstream of the cloud had strongly negative $σ_r$ values.

physics.space-ph↗

arXiv2020

CMEs in the Heliosphere: III. A Statistical Analysis of the Kinematic Properties Derived from Stereoscopic Geometrical Modelling Techniques Applied to CMEs Detected in the Heliosphere from 2008 to 2014 by STEREO/HI-1

We present an analysis of coronal mass ejections (CMEs) observed by the Heliospheric Imagers (HIs) on board NASA's Solar Terrestrial Relations Observatory (STEREO) spacecraft. Between August 2008 and April 2014 we identify 273 CMEs that are observed simultaneously, by the HIs on both spacecraft. For each CME, we track the observed leading edge, as a function of time, from both vantage points, and apply the Stereoscopic Self-Similar Expansion (SSSE) technique to infer their propagation throughout the inner heliosphere. The technique is unable to accurately locate CMEs when their observed leading edge passes between the spacecraft, however, we are able to successfully apply the technique to 151, most of which occur once the spacecraft separation angle exceeds 180 degrees, during solar maximum. We find that using a small half-width to fit the CME can result in observed acceleration to unphysically high velocities and that using a larger half-width can fail to accurately locate the CMEs close to the Sun because the method does not account for CME over-expansion in this region. Observed velocities from SSSE are found to agree well with single-spacecraft (SSEF) analysis techniques applied to the same events. CME propagation directions derived from SSSE and SSEF analysis agree poorly because of known limitations present in the latter. This work was carried out as part of the EU FP7 HELCATS (Heliospheric Cataloguing, Analysis and Techniques Service) project (http://www.helcats-fp7.eu/).

astro-ph.SR↗

arXiv2020

Radial Evolution of Magnetic Field Fluctuations in an Interplanetary Coronal Mass Ejection Sheath

The sheaths of compressed solar wind that precede interplanetary coronal mass ejections (ICMEs) commonly display large-amplitude magnetic field fluctuations. As ICMEs propagate radially from the Sun, the properties of these fluctuations may evolve significantly. We have analyzed magnetic field fluctuations in an ICME sheath observed by MESSENGER at 0.47 au and subsequently by STEREO-B at 1.08 au while the spacecraft were close to radial alignment. Radial changes in fluctuation amplitude, compressibility, inertial-range spectral slope, permutation entropy, Jensen-Shannon complexity, and planar structuring are characterized. These changes are discussed in relation to the evolving turbulent properties of the upstream solar wind, the shock bounding the front of the sheath changing from a quasi-parallel to quasi-perpendicular geometry, and the development of complex structures in the sheath plasma.

physics.space-ph↗

arXiv2020

Three-dimensional reconstruction of multiple particle acceleration regions during a coronal mass ejection

Context. Some of the most prominent sources for particle acceleration in our Solar System are large eruptions of magnetised plasma from the Sun called coronal mass ejections (CMEs). These accelerated particles can generate radio emission through various mechanisms. Aims. CMEs are often accompanied by a variety of solar radio bursts with different shapes and characteristics in dynamic spectra. Radio bursts directly associated with CMEs often show movement in the direction of CME expansion. Here, we aim to determine the emission mechanism of multiple moving radio bursts that accompanied a flare and CME that took place on 14 June 2012. Methods. We used radio imaging from the Nancay Radioheliograph, combined with observations from the Solar Dynamics Observatory and Solar Terrestrial Relations Observatory spacecraft, to analyse these moving radio bursts in order to determine their emission mechanism and three-dimensional (3D) location with respect to the expanding CME. Results. In using a 3D representation of the particle acceleration locations in relation to the overlying coronal magnetic field and the CME propagation, for the first time, we provide evidence that these moving radio bursts originate near the CME flanks and some that are possible signatures of shock-accelerated electrons following the fast CME expansion in the low corona. Conclusions. The moving radio bursts, as well as other stationary bursts observed during the eruption, occur simultaneously with a type IV continuum in dynamic spectra, which is not usually associated with emission at the CME flanks. Our results show that moving radio bursts that could traditionally be classified as moving type IVs can represent shock signatures associated with CME flanks or plasma emission inside the CME behind its flanks, which are closely related to the lateral expansion of the CME in the low corona.

astro-ph.SR↗

arXiv2019

Extended radio emission associated with a breakout eruption from the back side of the Sun

Context. Coronal mass ejections (CMEs) on the Sun are the largest explosions in the Solar System that can drive powerful plasma shocks. The eruptions, shocks, and other processes associated to CMEs are efficient particle accelerators and the accelerated electrons in particular can produce radio bursts through the plasma emission mechanism. Aims. Coronal mass ejections and associated radio bursts have been well studied in cases where the CME originates close to the solar limb or within the frontside disc. Here, we study the radio emission associated with a CME eruption on the back side of the Sun on 22 July 2012. Methods. Using radio imaging from the Nançay Radioheliograph, spectroscopic data from the Nançay Decametric Array, and extreme-ultraviolet observations from the Solar Dynamics Observatory and Solar Terrestrial Relations Observatory spacecraft, we determine the nature of the observed radio emission as well as the location and propagation of the CME. Results. We show that the observed low-intensity radio emission corresponds to a type II radio burst or a short-duration type IV radio burst associated with a CME eruption due to breakout reconnection on the back side of the Sun, as suggested by the pre-eruptive magnetic field configuration. The radio emission consists of a large, extended structure, initially located ahead of the CME, that corresponds to various electron acceleration locations. Conclusions. The observations presented here are consistent with the breakout model of CME eruptions. The extended radio emission coincides with the location of the current sheet and quasi-separatrix boundary of the CME flux and the overlying helmet streamer and also with that of a large shock expected to form ahead of the CME in this configuration.

astro-ph.SR↗

arXiv2019

Probing the effect of cadence on the estimates of photospheric energy and helicity injections in eruptive active region NOAA AR 11158

In this work we study how the input data cadence affects the photospheric energy and helicity injection estimates in eruptive NOAA active region 11158. We sample the novel 2.25-minute vector magnetogram and Dopplergram data from the \emph{Helioseismic and Magnetic Imager} (HMI) instrument onboard the \emph{Solar Dynamics Observatory} (SDO) spacecraft to create input datasets of variable cadences ranging from 2.25 minutes to 24 hours. We employ state-of-the-art data processing, velocity and electric field inversion methods for deriving estimates of the energy and helicity injections from these datasets. We find that the electric field inversion methods that reproduce the observed magnetic field evolution through the use of Faraday's law are more stable against variable cadence: the PDFI (PTD-Doppler-FLCT-Ideal) electric field inversion method produces consistent injection estimates for cadences from 2.25 minutes up to 2 hours, implying that the photospheric processes acting on time scales below 2 hours contribute little to the injections, or that they are below the sensitivity of the input data and the PDFI method. On other hand, the electric field estimate derived from the output of DAVE4VM (Differential Affine Velocity Estimator for Vector Magnetograms), which does not fulfil Faraday's law exactly, produces significant variations in the energy and helicity injection estimates in the 2.25-minute to 2-hour cadence range. We present also a third, novel DAVE4VM-based electric field estimate, which corrects the poor inductivity of the raw DAVE4VM estimate. This method is less sensitive to the changes of cadence, but still faces significant issues for the lowest of considered cadences ($\geq$2 hours). We find several potential problems in both PDFI- and DAVE4VM-based injection estimates and conclude that the quality of both should be surveyed further in controlled environments.

astro-ph.SR↗

arXiv2019

FORESAIL-1 cubesat mission to measure radiation belt losses and demonstrate de-orbiting

Today, the near-Earth space is facing a paradigm change as the number of new spacecraft is literally sky-rocketing. Increasing numbers of small satellites threaten the sustainable use of space, as without removal, space debris will eventually make certain critical orbits unusable. A central factor affecting small spacecraft health and leading to debris is the radiation environment, which is unpredictable due to an incomplete understanding of the near-Earth radiation environment itself and its variability driven by the solar wind and outer magnetosphere. This paper presents the FORESAIL-1 nanosatellite mission, having two scientific and one technological objectives. The first scientific objective is to measure the energy and flux of energetic particle loss to the atmosphere with a representative energy and pitch angle resolution over a wide range of magnetic local times. To pave the way to novel model - in situ data comparisons, we also show preliminary results on precipitating electron fluxes obtained with the new global hybrid-Vlasov simulation Vlasiator. The second scientific objective of the FORESAIL-1 mission is to measure energetic neutral atoms (ENAs) of solar origin. The solar ENA flux has the potential to contribute importantly to the knowledge of solar eruption energy budget estimations. The technological objective is to demonstrate a satellite de-orbiting technology, and for the first time, make an orbit manoeuvre with a propellantless nanosatellite. FORESAIL-1 will demonstrate the potential for nanosatellites to make important scientific contributions as well as promote the sustainable utilisation of space by using a cost-efficient de-orbiting technology.

physics.space-ph↗

arXiv2019

Self-Similarity of ICME Flux Ropes: Observations by Radially Aligned Spacecraft in the Inner Heliosphere

Interplanetary coronal mass ejections (ICMEs) are a significant feature of the heliospheric environment and the primary cause of adverse space weather at the Earth. ICME propagation, and the evolution of ICME magnetic field structure during propagation, are still not fully understood. We analyze the magnetic field structures of 18 ICME magnetic flux ropes observed by radially aligned spacecraft in the inner heliosphere. Similarity in the underlying flux rope structures is determined through the application of a simple technique that maps the magnetic field profile from one spacecraft to the other. In many cases, the flux ropes show very strong underlying similarities at the different spacecraft. The mapping technique reveals similarities that are not readily apparent in the unmapped data and is a useful tool when determining whether magnetic field time series observed at different spacecraft are associated with the same ICME. Lundquist fitting has been applied to the flux ropes and the rope orientations have been determined; macroscale differences in the profiles at the aligned spacecraft may be ascribed to differences in flux rope orientation. Assuming that the same region of the ICME was observed by the aligned spacecraft in each case, the fitting indicates some weak tendency for the rope axes to reduce in inclination relative to the solar equatorial plane and to align with the solar east-west direction with heliocentric distance.

physics.space-ph↗

arXiv2019

Variable emission mechanism of a Type IV radio burst

Context. The Sun is an active star and the source of the largest explosions in the solar system, such as flares and coronal mass ejections (CMEs). Flares and CMEs are powerful particle accelerators that can generate radio emission through various emission mechanisms. Aims. CMEs are often accompanied by Type IV radio bursts that are observed as continuum emission in dynamic spectra at decimetric and metric wavelengths, but their emission mechanism can vary from event to event. Here, we aim to determine the emission mechanism of a complex Type IV burst that accompanied the flare and CME on 22 September 2011. Methods. We used radio imaging from the Nançay Radioheliograph, spectroscopic data from the e-Callisto network, ARTEMIS, Ondrejov, and Phoenix3 spectrometers combined with extreme-ultraviolet observations from NASA's Solar Dynamic Observatory to analyse the Type IV radio burst and determine its emission mechanism. Results. We show that the emission mechanism of the Type IV radio burst changes over time. We identified two components in the Type IV radio burst: an earlier stationary Type IV showing gyro-synchrotron behaviour, and a later moving Type IV burst covering the same frequency band. This second component has a coherent emission mechanism. Fundamental plasma emission and the electroncyclotron maser emission are further investigated as possible emission mechanisms for the generation of the moving Type IV burst. Conclusions. Type IV bursts are therefore complex radio bursts, where multiple emission mechanisms can contribute to the generation of the wide-band continuum observed in dynamic spectra. Imaging spectroscopy over a wide frequency band is necessary to determine the emission mechanisms of Type IV bursts that are observed in dynamic spectra.

astro-ph.SR↗

arXiv2018

CMEs in the Heliosphere: I. A Statistical Analysis of the Observational Properties of CMEs Detected in the Heliosphere from 2007 to 2017 by STEREO/HI-1

We present a statistical analysis of coronal mass ejections (CMEs) imaged by the Heliospheric Imager (HI) instruments aboard NASAs twin-spacecraft STEREO mission between April 2007 and August 2017 for STEREO-A and between April 2007 and September 2014 for STEREO-B. The analysis exploits a catalogue that was generated within the FP7 HELCATS project. Here, we focus on the observational characteristics of CMEs imaged in the heliosphere by the inner (HI-1) cameras. More specifically, in this paper we present distributions of the basic observational parameters - namely occurrence frequency, central position angle (PA) and PA span - derived from nearly 2000 detections of CMEs in the heliosphere by HI-1 on STEREO-A or STEREO-B from the minimum between Solar Cycles 23 and 24 to the maximum of Cycle 24; STEREO-A analysis includes a further 158 CME detections from the descending phase of Cycle 24, by which time communication with STEREO-B had been lost. We compare heliospheric CME characteristics with properties of CMEs observed at coronal altitudes, and with sunspot number. As expected, heliospheric CME rates correlate with sunspot number, and are not inconsistent with coronal rates once instrumental factors/differences in cataloguing philosophy are considered. As well as being more abundant, heliospheric CMEs, like their coronal counterparts, tend to be wider during solar maximum. Our results confirm previous coronagraph analyses suggesting that CME launch sites don't simply migrate to higher latitudes with increasing solar activity. At solar minimum, CMEs tend to be launched from equatorial latitudes while, at maximum, CMEs appear to be launched over a much wider latitude range; this has implications for understanding the CME/solar source association. Our analysis provides some supporting evidence for the systematic dragging of CMEs to lower latitude as they propagate outwards.

astro-ph.SR↗