S. Pohjolainen

CME Propagation Characteristics from Radio Observations

AbstractnWe explore the relationship among three coronal mass ejections (CMEs), observed on 28 October 2003, 7 November 2004, and 20 January 2005, the type II burst-associated shock waves in the corona and solar wind, as well as the arrival of their related shock waves and magnetic clouds at 1 AU. Using six different coronal/interplanetary density models, we calculate the speeds of shocks from the frequency drifts observed in metric and decametric radio wave data. We compare these speeds with the velocity of the CMEs as observed in the plane-of-the-sky white-light observations and calculated with a cone model for the 7 November 2004 event. We then follow the propagation of the ejecta using Interplanetary Scintillation measurements, which were available for the 7 November 2004 and 20 January 2005 events. Finally, we calculate the travel time of the interplanetary shocks between the Sun and Earth and discuss the velocities obtained from the different data. This study highlights the difficulties in making velocity estimates that cover the full CME propagation time.n

CME liftoff with high-frequency fragmented type II burst emission

Aims. Solar radio type II bursts are rarely seen at frequencies hig her than a few hundred MHz. Since metric type II bursts are thought to be signatures of propagating shock waves, it is of interes t to know how these shocks, and the type II bursts, are formed. In particular, how are high-frequency, fragmented type II bursts created? Are there differences in shock acceleration or in the surrounding medium that could explain the differences to the “typical” metric type IIs? Methods. We analyse one unusual metric type II event in detail, with comparison to white-light, EUV, and X-ray observations. As the radio event was associated with a flare and a coronal mass ejec tion (CME), we investigate their connection. We then utilize numerical MHD simulations to study the shock structure induced by an erupting CME in a model corona including dense loops. Results. Our simulations show that the fragmented part of the type II burst can be formed when a coronal shock driven by a mass ejection passes through a system of dense loops overlying the active region. To produce fragmented emission, the conditions for plasma emission have to be more favourable inside the loop than in the interloop area. The obvious hypothesis, consistent with our simulation model, is that the shock strength decreases significantly in the space between the denser loops. The later, mo re typical type II burst appears when the shock exits the dense loop system and finally, outside the active region, the type II burst dies ou t when the changing geometry no longer favours the electron shock-acceleration.

Early signatures of large-scale field line opening. Multi-wavelength analysis of features connected with a "halo" CME event

A fast halo-type coronal mass ejection (CME) associated with a two-ribbon flare, GOES class M 1.3, was observed on February 8, 2000. Soft X-ray and EUV images revealed several loop ejections and one wave-like moving front that started from a remote location, away from the flare core region. A radio type-II burst was observed near the trajectory of the moving soft X-ray front, although association with the CME itself cannot be ruled out. Large-scale dimmings were observed in EUV and soft X-rays, both in the form of disappearing transequatorial loops. We can pinpoint the time and the location of the first large-scale field-line opening by tracing the electron propagation paths above the active region and along the transequatorial loop system, in which large-scale mass depletion later took place. The immediate start of a type-IV burst (interpreted as an upward moving structure) which was located over a soft X-ray dimming region, confirms that the CME had lifted off .W e compare these signatures with those of another halo CME event observed on May 2, 1998, and discuss the possible connections with the magnetic breakout model.

Shock-related radio emission during coronal mass ejection lift-off? (Research Note)

Aims. We identify the source of fast-drifting decimetric-metric radio emission that is sometimes observed prior to the so-called flare continuum emission. Fast-drift structures and continuum bursts are also observed in association with coronal mass ejections (CMEs), not only flares. Methods. We analyse radio spectral features and images acquired at radio, Hα, EUV, and soft X-ray wavelengths, during an event close to the solar limb on 2 June 2003. Results. The fast-drifting decimetric-metric radio burst corresponds to a moving, wide emission front in the radio images, which is normally interpreted as a signature of a propagating shock wave. A decimetric-metric type II burst where only the second harmonic lane is visible could explain the observations. After long-lasting activity in the active region, the hot and dense loops could be absorbing or suppressing emission at the fundamental plasma frequency. The observed burst speed suggests a super-Alfvenic velocity for the burst driver. The expanding and opening loops, associated with the flare and the early phase of CME lift-off, could be driving the shock. Alternatively, an instantaneous but fast loop expansion could initiate a freely propagating shock wave. The later, complexlooking decametre-hectometre wave type III bursts indicate the existence of a propagating shock, although no interplanetary type II burst was observed during the event. The data does not support CME bow shock or a shock at the flanks of the CME as the origin of the fast-drift decimetric-metric radio source. Therefore super-Alfvenic loop expansion is the best candidate for the initiation of the shock wave, and this result challenges the current view of metric/coronal shocks originating either in the flanks of CMEs or from flare blast waves.

Sources of SEP Acceleration during a Flare – CME Event

AbstractnA high-speed, halo-type coronal mass ejection (CME), associated with a GOES M4.6 soft X-ray flare in NOAA AR 0180 at S12W29 and an EIT wave and dimming, occurred on 9 November 2002. A complex radio event was observed during the same period. It included narrow-band fluctuations and frequency-drifting features in the metric wavelength range, type III burst groups at metricu2009–u2009hectometric wavelengths, and an interplanetary type II radio burst, which was visible in the dynamic radio spectrum below 14 MHz. To study the association of the recorded solar energetic particle (SEP) populations with the propagating CME and flaring, we perform a multi-wavelength analysis using radio spectral and imaging observations combined with white-light, EUV, hard X-ray, and magnetogram data. Velocity dispersion analysis of the particle distributions (SOHO and Windin situ observations) provides estimates for the release times of electrons and protons. Our analysis indicates that proton acceleration was delayed compared to the electrons. The dynamics of the interplanetary type II burst identify the burst source as a bow shock created by the fast CME. The type III burst groups, with start times close to the estimated electron-release times, trace electron beams travelling along open field lines into the interplanetary space. The type III bursts seem to encounter a steep density gradient as they overtake the type II shock front, resulting in an abrupt change in the frequency drift rate of the type III burst emission. Our study presents evidence in support of a scenario in which electrons are accelerated low in the corona behind the CME shock front, while protons are accelerated later, possibly at the CME bow shock high in the corona.n

Slow halo CMEs with shock signatures

Context. In the solar corona, shocks are formed when the speed of a disturbance exceeds the local magnetosonic speed. In the active region corona the Alfv· speed can drop to a few hundred km/s, but globally it is much higher. There has been a long debate on whether the shocks responsible for type II bursts are created by bow shocks in front of coronal mass ejections (CMEs), shocks in the anks of CMEs, or by are (blast) waves. Aims. We study the alternative explanations for type II bursts in events where we have a slow CME, are(s), and associated type II burst emission. Methods. We use multi-wavelength observations to analyse the heightntime evolution of CMEs and compare it with the evolution of shock signatures in radio and EUV. Results. Three are-associated halo-type CME events were observed on October 30, 2004. Velocity estimates (260, 325, and 920 km/s) from the rst plane-of-the-sky CME leading front observations suggested that the rst two were very slow compared to halo CMEs, on average. The CMEs were associated with ares (M4.2, X1.2, and M5.9) and each event was also associated with coronal (metric) type II emission that is known to be a signature of a propagating shock front. After the are starts, loop displacements and large-scale dimmings were observed in EUV. The two slow halo CMEs started as lament eruptions, but the CME velocities and/or bulk motions were a ected at the times of ares. We nd support for the idea that the cause of metric type II bursts in these two events is are-related. The later CME velocity changes (acceleration around 4-5 solar radii) could also be explained by eruptions associated with later ares. The repeating homologous arenhalo CME events indicate a restoration of the same large-scale structures within 5-6 hours.

Non-thermal processes associated with rising structures and waves during a “halo” type CME

We analyse structures and events connected with a flare-associated “halo” type coronal mass ejection (CME) observed on Decemberxa018,xa02000. A GOESxa0C7.0xa0class X-ray flare started at 11:02xa0UT in NOAA Active Region 9269, located at N14xa0E03. Yohkoh SXT observed slowly rising soft X-ray loops already some 5xa0min before flare start. H α images show a two-ribbon flare, remote brightenings, and a partly disappearing filament near the active region. A metric radio precursor was observed to start at 11:06:30xa0UT, simultaneously with impulsive emission in hard X-rays and microwaves. The frequency-drifting precursor envelope was superposed with J- and reverse drift bursts. The radio bursts traced large-scale soft X-ray loop structures aboutxa0160u2009000xa0km away from the flare core, and hard X-ray emission was observed at the ends of some of these loops. The precursor emission points to a rising structure where electron acceleration takes place. Later on, a radio typexa0II burst (signature of a propagating shock, driven either by an ejecta or a blast wave) and an EITxa0wave were observed. We conclude that possible sources for the rising structure and accelerator of electron beams are (1) large-scale loops that connect the flare core region and the precursor site in the close vicinity of two separate rising filaments, and (2) a growing shock that accelerates electrons along closed field lines until the multipolar field is opened and the CMExa0is lifted off. As neither X-ray nor EUVxa0ejecta could be observed whether in the direction of the typexa0II burst or near the radio precursor, we find some support for the shock wave scenario.

Solar particle events with helium-over-hydrogen enhancement in the energy range up to 100 MeV nucl−1

Flux measurements of solar energetic particles (SEPs) in the ERNE instrument onboard SOHO indicate that the abundance of 4He-nuclei compared to protons in the energy range up to 100xa0MeVxa0nucl−1 was exceptionally high during the particle events on 27 May 1998 and 28 December 1999. The 4He/p ratio stayed between 0.15–0.50 for more than ten hours. There was also a prolonged enhancement in helium-3, 3He/4H ≈1%. Observations of EIT and LASCO on board SOHO confirm that the originators of both SEP events were western eruptions, flares and coronal mass ejections (CMEs). The onset of the SEP release took place close to the maximum of flares which were probably triggered by the rising CMEs. The observations suggest that the SEP events were started with the flare-(pre)accelerated particles, but impact of the CME-associated shocks might explain the continuation and modification of the helium and proton fluxes well after the flare production. These observations support the idea that the helium enhancements in the CME-associated events reflect the availability of seed particles that originate previously in flares.

Repeated Flaring from Loop–Loop Interaction

A series of solar flares was observed near the same location in NOAA active region 8996 on 18–20 May 2000. A detailed analysis of one of these flares is presented where the emitting structures in soft and hard X-rays, EUV, Hα, and radio at centimeter wavelengths are compared. Hard X-rays and radio emission were observed at two separate loop footpoints, while soft X-rays and EUV emission were observed mainly above the nearby positive polarity region. The flare was confined although the observed type III bursts at the time of the flare maximum indicate that some field lines were open to the corona. No flux emergence was evident but moving magnetic features were observed around the sunspot region and within the positive polarity (plage) region. We suggest that the flaring was due to loop–loop interactions over the positive polarity region, where accelerated electrons gained access to the two separate loop systems. The repeated radio flaring at the footpoint of one loop was visible because of the strong magnetic fields near the large sunspot region while at the footpoint of the other loop the electrons could precipitate and emit in hard X-rays. The simultaneous emission and fluctuations in radio and X-rays – in two different loop ends – further support the idea of a single acceleration site at the loop intersection.