Silja Pohjolainen

Interplanetary flux rope ejected from an X-ray bright point - The smallest magnetic cloud source-region ever observed

Using multi-instrument and multi-wavelength observations (SOHO/MDI and EIT, TRACE and Yohkoh/SXT), as well as computing the coronal magnetic field of a tiny bipole combined with modelling of Wind in situ data, we provide evidences for the smallest event ever observed which links a sigmoid eruption to an interplanetary magnetic cloud (MC). The tiny bipole, which was observed very close to the solar disc centre, had a factor one hundred less flux than a classical active region (AR). In the corona it had a sigmoidal structure, observed mainly in EUV, and we found a very high level of non- potentiality in the modelled magnetic field, 10 times higher than we have ever found in any AR. From May 11, 1998, and until its disappearance, the sigmoid underwent three intense impulsive events. The largest of these events had extended EUV dimmings and a cusp. The Wind spacecraft detected 4.5 days later one of the smallest MC ever identified (about a factor one hundred times less magnetic flux in the axial component than that of an average MC). The link between this last eruption and the interplanetary magnetic cloud is supported by several pieces of evidence: good timing, same coronal loop and MC orientation, same magnetic field direction and magnetic helicity sign in the coronal loops and in the MC. We further quantify this link by estimating the magnetic flux (measured in the dimming regions and in the MC) and the magnetic helicity (pre- to post-event change in the solar corona and helicity content of the MC). Within the uncertainties, both magnetic fluxes and helicities are in reasonable agreement, which brings further evidences of their link. These observations show that the ejections of tiny magnetic flux ropes are indeed possible and put new constraints on CME models.

Modeling solar near-relativistic electron events - Insights into solar injection and interplanetary transport conditions

Context. Solar near-relativistic electrons (>30 keV) are observed as discrete events in the inner heliosphere following different types of solar transient activity. Several mechanisms have been proposed for the production of these electrons. One candidate is related to solar flare activity. Other candidates include shocks driven by fast coronal mass ejections (CMEs) or processes of magnetic reconnection in the aftermath of CMEs. Aims. We study eleven near-relativistic (NR) electron events observed by the Advanced Composition Explorer (ACE) between 1998 and 2005 with the aim of estimating the roles played by solar flares, CME-driven shocks, and processes of magnetic restructuring in the aftermath of the CMEs in the injection of NR electrons. The main goal is to infer the underlying injection profile from particle observations at 1 AU, as well as the interplanetary transport conditions. Methods. We used Monte Carlo simulations to model the transport of particles along the interplanetary magnetic field. By taking the angular response of the LEFS60 telescope of the EPAM instrument onboard ACE into account, we were able to deconvolve the transport effects from the observed intensities, and thus infer the solar injection profile. Results. In this set of events, we have identified two types of injection episodes: short ( 1 h). Short injection episodes seem to be associated with the flare processes and/or the reconnection phenomena in the aftermath of the CME, while time-extended episodes seem to be consistent with injection from CME-driven shocks. Conclusions. We find that there is no single scenario that operates in all the events. The interplanetary propagation of NR electrons can occur both under strong scattering and under almost scatter-free propagation conditions and several injection phases (related to flares and/or CMEs) are possible.

Large-scale patterns on the Sun observed in the millimetric wavelength range

The nature and behaviour of large-scale patterns on the solar surface, indicated by the areas of brightness-temperature depressions in the millimetric wavelength range, is studied. A large sample of 346 individual, low-temperature regions (LTRs) was employed to provide reliable statistical evidence. An association of 99% was found between the locations of LTRs and the large-scale magnetic field inversion lines, and 60% of the LTRs were associated with the inversion line filaments. A tentative physical association with filaments is reconsidered, and one particularly well-observed case is presented. The heights of the perturbers causing brightness-temperature depressions are discussed. The long-term evolution of the latitudinal distribution of LTRs is presented in a butterfly diagram. Two belts of low-temperature regions outline the active region belts, shifting with them towards the equator during the solar activity cycle. The low-temperature region belts of the forthcoming cycle appear already at the maximum of the actual cycle at latitudes of about 55 °. The superpositions of the temperature minima distributions in the synoptic maps show patterns appearing as ‘giant cells’ and compatible with indications inferred from magnetographic data. The reliability of the inferred cells is considered, and a statistical analysis reveals a negligible probability for an accidental distribution appearing in the form of giant cells.

Origin of wide-band IP type II bursts

Context. Different types of interplanetary (IP) type II bursts have been observed, where the more usual ones show narrow-band and patchy emissions, sometimes with harmonics, and which at intervals may disappear completely from the dynamic spectrum. The more unusual bursts are wide-band and diffuse, show no patches or breaks or harmonic emission, and often have long durations. Type II bursts are thought to be plasma emission, caused by propagating shock waves, but a synchrotron-emitting source has also been proposed as the origin for the wide-band type IIs. Aims. Our aim is to find out where the wide-band IP type II bursts originate and what is their connection to particle acceleration. Methods. We analyzed in detail 25 solar events that produced well-separated, wide-band IP type II bursts in 2001–2011. Their associations to flares, coronal mass ejections (CMEs), and solar energetic particle events (SEPs) were investigated. Results. Of the 25 bursts, 18 were estimated to have heights corresponding to the CME leading fronts, suggesting that they were created by bow shocks ahead of the CMEs. However, seven events were found in which the burst heights were significantly lower and which showed a different type of height-time evolution. Almost all the analyzed wide-band type II bursts were associated with very high-speed CMEs, originating from different parts of the solar hemisphere. In terms of SEP associations, many of the SEP events were weak, had poor connectivity due to the eastern limb source location, or were masked by previous events. Some of the events had precursors in specific energy ranges. These properties and conditions affected the intensity-time profiles and made the injection-timebased associations with the type II bursts difficult to interpret. In several cases where the SEP injection times could be determined, the radio dynamic spectra showed other features (in addition to the wide-band type II bursts) that could be signatures of shock fronts. Conclusions. We conclude that in most cases (in 18 out of 25 events) the wide-band IP type II bursts can be plasma emission, formed at or just above the CME leading edge. The results for the remaining seven events might suggest the possibility of a synchrotron source. These events, however, occurred during periods of high solar activity, and coronal conditions affecting the results of the burst height calculations cannot be ruled out. The observed wide and diffuse emission bands may also indicate specific CME leading edge structures and special shock conditions.

Radio Bursts Associated with Flare and Ejecta in the 13 July 2004 Event

We investigate coronal transients associated with a GOES M6.7 class flare and a coronal mass ejection (CME) on 13 July 2004. During the rising phase of the flare, a filament eruption, loop expansion, a Moreton wave, and an ejecta were observed. An EIT wave was detected later on. The main features in the radio dynamic spectrum were a frequency-drifting continuum and two type II bursts. Our analysis shows that if the first type II burst was formed in the low corona, the burst heights and speed are close to the projected distances and speed of the Moreton wave (a chromospheric shock wave signature). The frequency-drifting radio continuum, starting above 1 GHz, was formed almost two minutes prior to any shock features becoming visible, and a fast-expanding piston (visible as the continuum) could have launched another shock wave. A possible scenario is that a flare blast overtook the earlier transient and ignited the first type II burst. The second type II burst may have been formed by the same shock, but only if the shock was propagating at a constant speed. This interpretation also requires that the shock-producing regions were located at different parts of the propagating structure or that the shock was passing through regions with highly different atmospheric densities. This complex event, with a multitude of radio features and transients at other wavelengths, presents evidence for both blast-wave-related and CME-related radio emissions.

Prolonged millimeter-wave radio emission from a solar flare near the limb

We present a multi-wavelength analysis of a gradual radio flare on June 27, 1993 which showed emission at millimeter waves long after the soft X-ray flux had peaked. The radio flare located at S12 E75 was associated with a GOES class M3.6 flare that lasted for more than one hour and hard X-ray emission during the rising phase of the soft X-ray/radio emission. The maximum radio flux density at 35 GHz was 60 sfu, but the calculated thermal bremsstrahlung flux from the GOES soft X-rays was less than half of that. The possible explanations for this prolonged millimeter wave emission could be accelerated high-energy electrons gyrating along the field-lines (nonthermal gyrosynchrotron emission) or thermal bremsstrahlung from evaporating chromospheric warm and dense plasma (cool enough to go undetected by GOES), or a mixture of these. Our model calculations show that even an inhomogeneous source containing both kinds of particles would not be able to produce such a spectral shape. A second source with extremely high electron densities (>10 1 6 m - 3 ), large source dimensions (>10 1 5 m 2 ), and very low temperatures (< 10 6 K) must be assumed to explain the observed radio spectra.

Microflaring of a solar bright point

A 50 x 50 arcsec region near the solar disc center containing a bright point (BP) was observed with the SUMER-spectrograph of the SOHO observatory. The data consist of two hours observation of four far-UV emission lines formed between 2 x 10 4 -6 x 10 5 K, with 2 arcsec spatial, 2.8 min temporal and 4 km s - 1 spectral resolution. A striking feature was the strong microflaring of the major persistent BP (with size 8 x 8 arcsec) and the appearance of several short lived transients. The microflaring of each individual 2 x 2 arcsec pixel inside the main BP was coherent, indicating strong interaction of the possible sub arc sec building blocks (magnetic flux tubes). Using the emission measure at 10 5 K as an indicator of the loop foot point area and magnetic filling factor, we suggest 10 per cent filling factor for the BP observed. This is similar to that on the average surface of a medium-active solar type star.

Propagation of Solar Energetic Particles During Multiple Coronal Mass Ejection Events

We study solar energetic particle (SEP) events during multiple solar eruptions. The analysed sequences, on 24 – 26 November 2000, 9 – 13 April 2001, and 22 – 25 August 2005, consisted of halo-type coronal mass ejections (CMEs) that originated from the same active region and were associated with intense flares, EUV waves, and interplanetary (IP) radio type II and type III bursts. The first two solar events in each of these sequences showed SEP enhancements near Earth, but the third in the row did not. We observed that in these latter events the type III radio bursts were stopped at much higher frequencies than in the earlier events, indicating that the bursts did not reach the typical plasma density levels near Earth. To explain the missing third SEP event in each sequence, we suggest that the earlier-launched CMEs and the CME-driven shocks either reduced the seed particle population and thus led to inefficient particle acceleration, or that the earlier-launched CMEs and shocks changed the propagation paths or prevented the propagation of both the electron beams and SEPs, so that they were not detected near Earth even when the shock arrivals were recorded.

Origin of two extreme solar particle events

We performed an analysis of high-energy particle emission from the Sun in two extreme solar particle events observed even with ground-based neutron monitors (NMs). We model particle transport and interactions from near-Sun source through the solar wind and the Earth’s magnetosphere and atmosphere in order to make a deep analysis of the events. The time profile of the proton source at the Sun is deduced and compared with observed electromagnetic emissions. Several complementary to each other data sets are studied jointly with the broadband dynamic radio spectra, EUV images as well as other data available for both events. We find a common scenario for both eruptions, including the flare’s dual impulsive phase, the coronal mass ejection (CME)-launch-associated burst and the late low-frequency type III radio bursts at the time of the relativistic proton injection into the interplanetary medium. The analysis supports the idea that the two considered events start with emission of relativistic protons previously accelerated during the flare and CME launch, then trapped in large-scale magnetic loops and later released by the expanding CME.

Observations of solar energetic particle events during multiple coronal mass ejections

We investigate associations of solar energetic particle events with multiple solar eruptions incorporating both coronal mass ejections (CMEs) and intense flares. Searching through the time period from 1996 to the end of 2013 we found three series of eruptions with start times occurring in a time window of less than two days and consisting of at least three fast and wide CMEs from the same active region and associated with intense X-ray flares and clear type II emissions. The selected events, on 24 November 2000, 9-11 April 2001, and 22-23 August 2005, were all halo CMEs associated with Xor M-class flares. In all cases, clear type III bursts and interplanetary type II radio emissions were observed, indicative that the CMEs were driving interplanetary shocks. The first two CMEs and flares in each group of triple eruptions were associated with large solar energetic particle events up to high (∼100 MeV) proton energies, while the third one in each case was not associated with an observable enhancement of proton intensity. We investigate the possible solar and interplanetary causes for the absence of solar protons at ∼1 AU during the third eruptions.