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Propagation of Solar Energetic Particles during Multiple Coronal Mass Ejection Events  

Silja Pohjolainen   Submitted: 2015-12-15 04:48

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 did not get detected near Earth even when the shock arrivals were recorded.

Authors: Pohjolainen, S., Al-Hamadani, F., Valtonen, E.
Projects: None

Publication Status: Solar Physics, accepted (Dec. 2015)
Last Modified: 2015-12-15 09:29
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Origin of wide-band IP type II bursts  

Silja Pohjolainen   Submitted: 2013-08-26 09:52

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-time-based 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.

Authors: S. Pohjolainen, H. Allawi, E. Valtonen
Projects: None

Publication Status: A&A, accepted 2013, DOI: http://dx.doi.org/10.1051/0004-6361/201220688
Last Modified: 2013-08-26 14:13
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Radio pulsating structures with coronal loop contraction  

Silja Pohjolainen   Submitted: 2012-04-17 05:16

We present a multi-wavelength study of a solar eruption event on 20 July 2004, comprising observations in Hα , EUV, soft X-rays, and in radio waves with a wide frequency range. The analysed data show both oscillatory patterns and shock wave signatures during the impulsive phase of the flare. At the same time, large-scale EUV loops located above the active region were observed to contract. Quasi-periodic pulsations with ~10 and ~15 s oscillation periods were detected both in microwave-millimeter waves and in decimeter-meter waves. Our calculations show that MHD oscillations in the large EUV loops - but not likely in the largest contracting loops - could have produced the observed periodicity in radio emission, by triggering periodic magnetic reconnection and accelerating particles. As the plasma emission in decimeter-meter waves traces the accelerated particle beams and the microwave emission shows a typical gyrosynchrotron flux spectrum (emission created by trapped electrons within the flare loop), we find that the particles responsible for the two different types of emission could have been accelerated in the same process. Radio imaging of the pulsed decimetric-metric emission and the shock-generated radio type II burst in the same wavelength range suggest a rather complex scenario for the emission processes and locations. The observed locations cannot be explained by the standard model of flare loops with an erupting plasmoid located above them, driving a shock wave at the CME front.

Authors: J. Kallunki, S. Pohjolainen
Projects: None

Publication Status: Solar Physics (TI: Advances in European Solar Physics), accepted for publication
Last Modified: 2012-04-17 13:55
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Radio emission of the Quiet Sun and Active Regions (Invited review)  

Silja Pohjolainen   Submitted: 2011-05-26 04:49

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Shibasaki K., Alissandrakis C.E., Pohjolainen S.
Projects:

Publication Status: Solar Physics, in press (June 2011), DOI: 10.1007/s11207-011-9788-4
Last Modified: 2011-06-30 06:51
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Radio Bursts Associated with Flare and Ejecta in the 13 July 2004 Event  

Silja Pohjolainen   Submitted: 2008-09-23 06:26

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.

Authors: S. Pohjolainen, K. Hori, T. Sakurai
Projects: None

Publication Status: Solar Physics, Topical Issue on Radio Physics and the Flare-CME Relationship, in press 2008
Last Modified: 2008-09-23 18:09
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CME liftoff with high-frequency fragmented type II burst emission  

Silja Pohjolainen   Submitted: 2008-09-02 05:59

Aims: Solar radio type II bursts are rarely seen at frequencies higher than a few hundred MHz. Since metric type II bursts are thought to be signatures of propagating shock waves, it is of interest 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 ejection (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, more typical type II burst appears when the shock exits the dense loop system and finally, outside the active region, the type II burst dies out when the changing geometry no longer favours the electron shock-acceleration.

Authors: S. Pohjolainen, J. Pomoell, and R. Vainio
Projects: None

Publication Status: A&A, accepted August 2008
Last Modified: 2008-09-02 07:55
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Subject will be restored when possible  

Silja Pohjolainen   Submitted: 2008-03-12 10:15

Aims. To 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 with flares. Methods. Analysis using radio imaging of these spectral features, together with images in Hα , EUV, and soft X-rays, during one particular event that occurred over 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-Alfvénic 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, complex-looking decameter-hectometer 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 for the fast-drift decimetric-metric radio source. Therefore super-Alfvénic 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 at the flanks of CMEs or from flare blast waves.

Authors: S. Pohjolainen
Projects: None

Publication Status: Astronomy & Astrophysics (Research Note), accepted February 2008
Last Modified: 2008-03-12 16:37
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Shock-related radio emission during coronal mass ejection lift-off?  

Silja Pohjolainen   Submitted: 2008-03-12 10:15

Aims. To 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 with flares. Methods. Analysis using radio imaging of these spectral features, together with images in Hα , EUV, and soft X-rays, during one particular event that occurred over 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-Alfvénic 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, complex-looking decameter-hectometer 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 for the fast-drift decimetric-metric radio source. Therefore super-Alfvénic 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 at the flanks of CMEs or from flare blast waves.

Authors: S. Pohjolainen
Projects:

Publication Status: Astronomy & Astrophysics (Research Note), Vol. 483, 2008, p. 297-300
Last Modified: 2008-09-02 06:13
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Subject will be restored when possible  

Silja Pohjolainen   Submitted: 2007-11-16 09:10

A 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 metric-hectometric 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 Wind in 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.

Authors: N.J. Lehtinen, S. Pohjolainen, K. Huttunen-Heikinmaa, R. Vainio, E. Valtonen, A.E. Hillaris
Projects: None

Publication Status: Solar Physics, November 2007, accepted
Last Modified: 2007-11-16 11:39
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Subject will be restored when possible  

Silja Pohjolainen   Submitted: 2007-11-16 09:10

A 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 metric-hectometric 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 Wind in 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.

Authors: N.J. Lehtinen, S. Pohjolainen, K. Huttunen-Heikinmaa, R. Vainio, E. Valtonen, A.E. Hillaris
Projects:

Publication Status: Solar Physics, November 2007, in press. DOI 10.1007/s11207-007-9093-4
Last Modified: 2007-11-20 03:33
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Subject will be restored when possible  

Silja Pohjolainen   Submitted: 2007-11-16 09:10

A 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 metric-hectometric 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 Wind in 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.

Authors: N.J. Lehtinen, S. Pohjolainen, K. Huttunen-Heikinmaa, R. Vainio, E. Valtonen, A.E. Hillaris
Projects:

Publication Status: Solar Physics, November 2007, in press, DOI 10.1007/s11207-007-9093-4
Last Modified: 2007-11-20 05:06
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Sources of SEP Acceleration during a Flare-CME Event  

Silja Pohjolainen   Submitted: 2007-11-16 09:10

A 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 metric-hectometric 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 Wind in 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.

Authors: N.J. Lehtinen, S. Pohjolainen, K. Huttunen-Heikinmaa, R. Vainio, E. Valtonen, A.E. Hillaris
Projects: None

Publication Status: Solar Physics, Vol. 247, 2008, p. 151-169, DOI 10.1007/s11207-007-9093-4
Last Modified: 2008-09-02 06:15
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Subject will be restored when possible  

Silja Pohjolainen   Submitted: 2007-11-16 09:09

A 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 metric-hectometric 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 Wind in 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.

Authors: N.J. Lehtinen, S. Pohjolainen, K. Huttunen-Heikinmaa, R. Vainio, E. Valtonen, A.E. Hillaris
Projects: None

Publication Status: Solar Physics, November 2007, accepted
Last Modified: 2007-11-16 09:09
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CME Propagation Characteristics from Radio Observations  

Silja Pohjolainen   Submitted: 2007-07-11 10:47

We 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 (IPS) measurements, which were available for the 7 November 2004 and 20 January 2005 events. Finally, we calculate the travel time of the interplanetary (IP) 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.

Authors: S. Pohjolainen, L. van Driel-Gesztelyi, J.L. Culhane, P.K. Manoharan, H.A. Elliott
Projects: None

Publication Status: Solar Physics Topical Issue (Sun-Earth Connection), accepted July 2007
Last Modified: 2007-07-11 12:10
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Slow halo CMEs with shock signatures  

Silja Pohjolainen   Submitted: 2005-11-22 08:44

In the solar corona shocks are formed when the speed of a disturbance exceeds the local magnetosonic speed. In the active region corona the Alfvén speed can drop down to a few hundred km s-1, 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 flanks of CMEs, or by flare (blast) waves. We study the alternative explanations for type II bursts on events where we have a slow CME, flare(s), and associated type II burst emission, to get better insight into the problem. We use multi-wavelength observations to analyse the height-time evolution of CMEs and compare it with the evolution of shock signatures in radio and EUV. Three flare-associated halo-type CME events were observed on October 30, 2004. Velocity estimates (260, 325, and 920 km s-1) from the first plane-of-the-sky CME leading front observations suggested that the first two were very slow compared to halo CMEs on the average. The CMEs were associated with flares (M4.2, X1.2, and M5.9, respectively) 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 flare starts, loop displacements and large-scale dimmings were observed in EUV. The two slow halo CMEs started as filament eruptions, but the CME velocities and/or bulk motions were affected at the times of flares. We find support to the idea that the cause of metric type II bursts in these two events is flare-related. The later CME velocity changes (acceleration around 4-5 solar radii) could also be explained by eruptions associated with later flares. The repeating homologous flare-halo CME events indicate a restoration of the same large-scale structures within 5-6 hours.

Authors: S. Pohjolainen, N.J. Lehtinen
Projects: None

Publication Status: A&A, Vol. 449, Issue 1, April I 2006, pp.359-367
Last Modified: 2007-01-18 07:17
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Early signatures of large-scale field line opening Multi-wavelength analysis of features connected with a 'halo' CME event  

Silja Pohjolainen   Submitted: 2004-12-20 06:33

A fast 'halo' type coronal mass ejection (CME) associated with a two-ribbon flare, GOES class M1.3, 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. We 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.

Authors: Pohjolainen S., Vilmer N., Khan J.I., and Hillaris A.
Projects: Soho-EIT,Soho-MDI,Soho-LASCO,Yohkoh-HXT,Yohkoh-SXT,TRACE

Publication Status: A&A, Vol. 434, Issue 1, April IV 2005, pp.329-341
Last Modified: 2005-07-30 14:37
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Repeated flaring from loop-loop interaction  

Silja Pohjolainen   Submitted: 2002-11-28 03:31

A series of solar flares was observed near the same location in NOAA active region 8996 on May 18-20, 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.

Authors: S. Pohjolainen
Projects: Soho-EIT,Soho-MDI,Yohkoh-HXT,Yohkoh-SXT

Publication Status: Solar Physics, Vol. 213, 2003, p. 319-339
Last Modified: 2004-12-22 07:59
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Prolonged millimeter-wave radio emission from a solar flare near the limb  

Silja Pohjolainen   Submitted: 2002-10-04 04:12

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 S12E75 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 (>1016 m^-3), large source dimensions (>1015 m^2), and very low temperatures (<106 K) must be assumed to explain the observed radio spectra.

Authors: S. Pohjolainen, J. Hildebrandt, M. Karlický, A. Magun, and I.M. Chertok
Projects: None

Publication Status: Astron. Astrophys., 396, 683, 2002
Last Modified: 2004-12-22 08:01
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On-the-disk Development of the Halo Coronal Mass Ejection on May 2, 1998  

Silja Pohjolainen   Submitted: 2001-04-09 21:25

A halo coronal mass ejection (CME) was observed at 15:03 UT on May 2, 1998 by the Solar and Heliospheric Observatory (SOHO) Large Angle Spectrometric Coronagraph (LASCO). The observation of the CME was preceded by a major soft X-ray flare in NOAA active region 8210, characterized by a delta spot magnetic configuration, and some activity in region 8214. A large transequatorial interconnecting loop (TIL), seen in soft X-rays, connected active region 8210 to a faint magnetic field region in the periphery of region 8214. Smaller loop systems were also connecting region 8210 to other fainter bipolar magnetic structures, the interconnecting loop (IL) east of active region 8210 being one of the most visible. We present here a multi-wavelength analysis of the large and small scale coronal structures associated with the development of the flare and of the CME, with emphasis on radio imaging data: In the early phases of the flare the radio emission sources traced the propagation paths of electrons along the TIL and along the IL, accelerated in the vicinity of active region 8210. Furthermore, jet-like flows were observed in soft X-rays and in Hα in these directions. Significantly, the TIL and IL loop systems disappeared at least partially after the CME. An EIT dimming region of similar size and shape to the SXT TIL, but noticeably offset from it, was also observed. During the `flash' phase of the flare new radio sources appeared, presenting signatures of destabilization and reconnection at discrete locations of the connecting loops. We interprete these as possible signatures of the CME lift-off on the disk. An Hα Moreton wave (blast wave) and an `EIT wave' were also observed, originating from the flaring region 8210. The signatures in radio, after the wave propagated high into the corona, include type II-like emissions in the spectra. The radio images link these emissions to fast moving sources, presumably formed at locations where the blast wave encounters magnetic structures. The opening of the CME magnetic field is revealed by the radio observations which show large and expanding moving sources overlying the later-seen EIT dimming region.

Authors: S. Pohjolainen, D. Maia, M. Pick, N. Vilmer, J.I. Khan, W. Otruba, A. Warmuth, A. Benz, C. Alissandrakis, and B.J. Thompson
Projects: Soho-EIT,Soho-MDI,Soho-LASCO,Yohkoh-HXT,Yohkoh-SXT

Publication Status: ApJ, 2001, Vol 556, pp. 421-431
Last Modified: 2004-12-22 08:04
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