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First detection of metric emission from a solar surge  

Alexander Hillaris   Submitted: 2022-03-03 01:37

We report the first detection of metric radio emission from a surge, observed with the Nanšay Radioheliograph (NRH), STEREO and other instruments. The emission was observed during the late phase of the M9 complex event SOL2010-02-012T11:25:00, described in a previous publication and was associated with a secondary energy release, also observed in STEREO 304┼ images: there was no detectable soft X-ray emission. Triangulation of the STEREO images allowed the identification of the surge with NRH sources near the central meridian. The radio emission of the surge occurred in two phases and consisted of two sources, one located near the base of the surge, apparently at or near the site of energy release, and another in the upper part of the surge; these were best visible in the frequency range of 445.0 to about 300MHz, whereas a spectral component of different nature was observed at lower frequencies. Sub-second time variations were detected in both sources during both phases, with 0.2-0.3s a delay of the upper source with respect to the lower, suggesting superluminal velocities. This effect can be explained if the emission of the upper source was due to scattering of radiation from the source at the base of the surge. In addition, the radio emission showed signs of pulsations and spikes. We discuss possible emission mechanisms for the slow time variability component of the lower radio source. Gyrosynchrotron emission reproduced fairly well the characteristics of the observed total intensity spectrum at the start of the second phase of the event, but failed to reproduce the high degree of the observed circular polarization as well the spectra at other instances. On the other hand, type IV-like plasma emission from the fundamental could explain the high polarization and the fine structure in the dynamic spectrum; moreover, it gives projected radio source positions on the plane of the sky, as seen from STEREO-A, near the base of the surge. Taking everything into consideration, we suggest type IV-like plasma emission with a low intensity gyrosynchrotron component as the most plausible mechanism.

Authors: C. E. Alissandrakis, S. Patsourakos, A. Nindos, C. Bouratzis and A. Hillaris
Projects: None

Publication Status: AA/2022/43169, accepted for publication in Astronomy & Astrophysics
Last Modified: 2022-03-04 12:39
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High-resolution observations with ARTEMIS-JLS and the NRH. III. Spectroscopy and imaging of fiber bursts  

Alexander Hillaris   Submitted: 2022-02-03 11:32

Aims. We study the characteristics of intermediate drift bursts (fibers) embedded in a large type-IV event. Methods. We used high-sensitivity, low-noise dynamic spectra obtained with the acousto-optic analyzer (SAO) of the ARTEMIS-JLS solar radiospectrograph, in conjunction with high time-resolution images from the Nanšay radioheliograph (NRH) and extreme ultraviolet (EUV) images from the Transition Region and Coronal Explorer (TRACE) to study fiber bursts during the large solar event of July 14, 2000. We computed both 2D and 1D images and applied high pass time filtering to the images and the dynamic spectrum in order to enhance the fiber-associated emission. For the study of the background continuum emission we used images averaged over several seconds. Results. Practically all fibers visible in the SAO dynamic spectra are identifiable in the NRH images. Fibers were first detected after the primary energy release in a moving type-IV event, probably associated with the rapid eastward expansion of the flare and the post-flare loop arcade. We found that fibers appeared as a modulation of the continuum intensity with a root mean square value of the order of 10%. Both the fibers and the continuum were strongly circularly polarized in the ordinary mode sense, indicating plasma emission at the fundamental. We detected a number of discrete fiber emission sources along two parallel stripes of ∼300 Mm in length, apparently segments of large-scale loops encompassing both the EUV loops and the CME-associated flux rope. We found cases of multiple fiber emissions appearing at slightly different positions and times; their consecutive appearance can give the impression of apparent motion with supra-luminal velocities. Images of individual fibers were very similar at 432.0 and 327.0 MHz. From the position shift of the sources and the time delays at low and high frequencies, we estimated the exciter speed and the frequency scale length along the loops for a well-observed group of fibers; we obtained consistent values from imaging and spectral data, supporting the whistler origin of the fiber emission. Finally we found that fibers in emission and fibers in absorption are very similar, confirming that they are manifestations of the same wave train.

Authors: C. E. Alissandrakis, C. Bouratzis, A. Hillaris
Projects: None

Publication Status: Published: A&A 627, A133 (2019)
Last Modified: 2022-02-03 11:54
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Detection of spike-like structures near the front of type-II bursts  

Alexander Hillaris   Submitted: 2022-02-03 11:29

Aims. We examine high time resolution dynamic spectra for fine structures in type II solar radio bursts Methods. We used data obtained with the acousto-optic spectrograph receiver of the Artemis-JLS (ARTEMIS-IV) solar radio spectrograph in the 450ľ270 MHz range at 10 ms cadence and identified more than 600 short, narrowband features. Their characteristics, such as instantaneous relative bandwidth and total duration were measured and compared with those of spikes embedded in type IV emissions. Results. Type II associated spikes occur mostly in chains inside or close to the slowly drifting type II emission. These spikes coexist with herringbone and pulsating structures. Their average duration is 96 ms and their average relative bandwidth 1.7%. These properties are not different from those of type IV embedded spikes. It is therefore possible that they are signatures of small-scale reconnection along the type II shock front.

Authors: S. Armatas, C. Bouratzis, A. Hillaris, C. E. Alissandrakis, P. Preka-Papadema, X. Moussas, E. Mitsakou, P. Tsitsipis, A. Kontogeorgos
Projects: None

Publication Status: published A&A 624, A76 (2019)
Last Modified: 2022-02-03 11:54
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High-resolution observations with ARTEMIS/JLS and the NRH: IV. Imaging spectroscopy of spike-like structures near the front of type-II bursts  

Alexander Hillaris   Submitted: 2022-02-03 11:25

Context. Narrowband bursts (spikes) are very small duration and bandwidth bursts which appear on dynamic spectra from microwave to decametric frequencies. They are believed to be manifestations of small-scale energy release through magnetic reconnection. Aims. We study the position of the spike-like structures relative to the front of type-II bursts and their role in the burst emission. Methods. We used high-sensitivity, low-noise dynamic spectra obtained with the acousto-optic analyzer (SAO) of the ARTEMIS-JLS solar radiospectrograph, in conjunction with high-time-resolution images from the Nanšay Radioheliograph (NRH) in order to study spike-like bursts near the front of a type-II radio burst recorded at the west limb during the November 3, 2003 extreme solar event. The spike-like emission in the dynamic spectrum was enhanced by means of high-pass-time filtering. Results. We identified a number of spikes in the NRH images. Due to the lower temporal resolution of the NRH, multiple spikes detected in the dynamic spectrum appeared as single structures in the images. These spikes had an average size of  20000 and their observed brightness temperature was 1.4 to 5:6  109 K, providing a significant contribution to the emission of the type-II burst front. At variance with a previous study on the type-IV associated spikes, we found no systematic displacement between the spike emission and the emission between spikes. At 327.0 MHz, the type II emission was located about 0.3 R above the pre-existing continuum emission, which, in turn, was located 0.1 R above the western limb. Conclusions. This study, combined with our previous results, indicates that the spike-like chains aligned along the type II burst MHD shock front are not a perturbation of the type II emission, as in the case of type IV spikes, but a manifestation of the type II emission itself. The preponderance of these chains, together with the lack of isolated structures or irregular clusters, points towards some form of small-scale magnetic reconnection, organized along the type-II propagating front.

Authors: S. Armatas, C. Bouratzis, A. Hillaris, C. E. Alissandrakis, P. Preka-Papadema, A. Kontogeorgos, P. Tsitsipis, X. Moussas
Projects: None

Publication Status: Accepted for Publication in Astron. /& Astrophys 10 January 2022
Last Modified: 2022-02-03 11:54
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A Wavelet Based Approach to Solar-Terrestrial Coupling  

Alexander Hillaris   Submitted: 2019-06-10 06:33

Transient and recurrent solar activity drive geomagnetic disturbances; these are quantified (amongst others) by DST , AE indices time-series. Transient disturbances are related to the Interplanetary Coronal Mass Ejections (ICMEs) while recurrent disturbances are related to corotating interaction regions (CIR). We study the relationship of the geomagnetic disturbances to the solar wind drivers within solar cycle 23 where the drivers are represented by ICMEs and CIRs occurrence rate and compared to the DST and AE as follows: terms with common periodicity in both the geomagnetic disturbances and the solar drivers are, firstly, detected using continuous wavelet transform (CWT). Then, common power and phase coherence of these periodic terms are calculated from the cross-wavelet spectra (XWT) and wavelet-coherence (WTC) respectively. In time-scales of ≈27 days our results indicate an anti-correlation of the effects of ICMEs and CIRs on the geomagnetic disturbances. The former modulates the DST and AE time series during the cycle maximum the latter during periods of reduced solar activity. The phase relationship of these modulation is highly non-linear. Only the annual frequency component of the ICMEs is phase-locked with DST and AE. In time-scales of ≈1.3-1.7 years the CIR seem to be the dominant driver for both geomagnetic indices throughout the whole solar cycle 23.

Authors: Ch. Katsavrias, A. Hillaris, P. Preka--Papadema
Projects: None

Publication Status: Advances in Space Research, Volume 57, Issue 10, p. 2234-2244.
Last Modified: 2019-06-12 12:17
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Interplanetary Type IV Bursts  

Alexander Hillaris   Submitted: 2019-06-10 06:31

We study the characteristics of moving type IV radio bursts that extend to hectometric wavelengths (interplanetary type IV or type {IV}{IP} bursts) and their relationship with energetic phenomena on the Sun. Our dataset comprises 48 interplanetary type IV bursts observed with the Radio and Plasma Wave Investigation (WAVES) instrument onboard Wind in the 13.825 MHz - 20 kHz frequency range. The dynamic spectra of the Radio Solar Telescope Network (RSTN), the Nançay Decametric Array (DAM), the Appareil de Routine pour le Traitement et l' Enregistrement Magnetique de l' Information Spectral (ARTEMIS-IV), the Culgoora, Hiraso, and the Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN) Radio Spectrographs were used to track the evolution of the events in the low corona. These were supplemented with soft X-ray (SXR) flux-measurements from the Geostationary Operational Environmental Satellite (GOES) and coronal mass ejections (CME) data from the Large Angle and Spectroscopic Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO). Positional information of the coronal bursts was obtained by the Nançay Radioheliograph (NRH). We examined the relationship of the type IV events with coronal radio bursts, CMEs, and SXR flares. The majority of the events (45) were characterized as compact, their duration was on average 106 minutes. This type of events was, mostly, associated with M- and X-class flares (40 out of 45) and fast CMEs, 32 of these events had CMEs faster than 1000 km s-1. Furthermore, in 43 compact events the CME was possibly subjected to reduced aerodynamic drag as it was propagating in the wake of a previous CME. A minority (three) of long-lived type {IV}{IP} bursts was detected, with durations from 960 minutes to 115 hours. These events are referred to as extended or long duration and appear to replenish their energetic electron content, possibly from electrons escaping from the corresponding coronal type IV bursts. The latter were found to persist on the disk, for tens of hours to days. Prominent among them was the unusual interplanetary type IV burst of 18 - 23 May 2002, which is the longest event in the Wind/WAVES catalog. The three extended events were typically accompanied by a number of flares, of GOES class C in their majority, and of CMEs, many of which were slow and narrow.

Authors: Alexander Hillaris, Constantine Bouratzis, Alexander Nindos
Projects: None

Publication Status: Solar Physics, Volume 291, Issue 7, pp.2049-2069
Last Modified: 2019-06-12 12:18
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High resolution observations with Artemis-IV and the NRH. I. Type IV associated narrow-band bursts  

Alexander Hillaris   Submitted: 2019-06-10 06:29

Context. Narrow-band bursts appear on dynamic spectra from microwave to decametric frequencies as fine structures with very small duration and bandwidth. They are believed to be manifestations of small scale energy release through magnetic reconnection. Aims: We analyzed 27 metric type IV events with embedded narrow-band bursts, which were observed by the ARTEMIS-IV radio spectrograph from 30 June 1999 to 1 August 2010. We examined the morphological characteristics of isolated narrow-band structures (mostly spikes) and groups or chains of structures. Methods: The events were recorded with the SAO high resolution (10 ms cadence) receiver of ARTEMIS-IV in the 270-450 MHz range. We measured the duration, spectral width, and frequency drift of ~12 000 individual narrow-band bursts, groups, and chains. Spike sources were imaged with the Nançay radioheliograph (NRH) for the event of 21 April 2003. Results: The mean duration of individual bursts at fixed frequency was ~100 ms, while the instantaneous relative bandwidth was ~2%. Some bursts had measurable frequency drift, either positive or negative. Quite often spikes appeared in chains, which were closely spaced in time (column chains) or in frequency (row chains). Column chains had frequency drifts similar to type-IIId bursts, while most of the row chains exhibited negative frequently drifts with a rate close to that of fiber bursts. From the analysis of NRH data, we found that spikes were superimposed on a larger, slowly varying, background component. They were polarized in the same sense as the background source, with a slightly higher degree of polarization of ~65%, and their size was about 60% of their size in total intensity. Conclusions: The duration and bandwidth distributions did not show any clear separation in groups. Some chains tended to assume the form of zebra, lace stripes, fiber bursts, or bursts of the type-III family, suggesting that such bursts might be resolved in spikes when viewed with high resolution. The NRH data indicate that the spikes are not fluctuations of the background, but represent additional emission such as what would be expected from small-scale reconnection.

Authors: C. Bouratzis, A. Hillaris, C.E. Alissandrakis, P. Preka-Papadema, X. Moussas, C. Caroubalos, P. Tsitsipis, A. Kontogeorgos
Projects: None

Publication Status: Astronomy & Astrophysics, Volume 586, id.A29, 11 pp.
Last Modified: 2019-06-12 12:19
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Detection of spike-like structures near the front of type-II burstsA  

Alexander Hillaris   Submitted: 2019-06-10 06:26

Aims: We examine high time resolution dynamic spectra for fine structures in type II solar radio bursts Methods: We used data obtained with the acousto-optic spectrograph receiver of the Artemis-JLS (ARTEMIS-IV) solar radio spectrograph in the 450-270 MHz range at 10 ms cadence and identified more than 600 short, narrowband features. Their characteristics, such as instantaneous relative bandwidth and total duration were measured and compared with those of spikes embedded in type IV emissions. Results: Type II associated spikes occur mostly in chains inside or close to the slowly drifting type II emission. These spikes coexist with herringbone and pulsating structures. Their average duration is 96 ms and their average relative bandwidth 1.7%. These properties are not different from those of type IV embedded spikes. It is therefore possible that they are signatures of small-scale reconnection along the type II shock front.

Authors: S. Armatas, C. Bouratzis, A. Hillaris, C.E. Alissandrakis, P. Preka-Papadema, X. Moussas, E. Mitsakou, P. Tsitsipis, A. Kontogeorgos
Projects: None

Publication Status: Astronomy & Astrophysics, Volume 624, id.A76, 6 pp.
Last Modified: 2019-06-12 12:20
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High resolution observations with Artemis-JLS, (II) Type IV associated intermediate drift bursts  

Alexander Hillaris   Submitted: 2019-06-10 06:24

Aims: We examined the characteristics of isolated intermediate drift bursts and their morphologies on dynamic spectra, in particular the positioning of emission and absorption ridges. Furthermore we studied the repetition rate of fiber groups. These were compared with a model in order to determine the conditions under which the intermediate drift bursts appear and exhibit the above characteristics. Methods: We analyzed sixteen metric type IV events with embedded intermediate drift bursts, observed with the Artemis-JLS radio spectrograph from July 1999 to July 2005 plus an event on 1st August 2010. The events were recorded with the SAO high resolution (10 ms cadence) receiver in the 270-450 MHz range with a frequency resolution of 1.4 MHz. We developed cross- and autocorrelation techniques to measure the duration, spectral width, and frequency drift of fiber bursts in 47 intermediate drift bursts (IMD) groups embedded within the continuum of the sixteen events mentioned above. We also developed a semi-automatic algorithm to track fibers on dynamic spectra. Results: The mean duration of individual fiber bursts at fixed frequency was δt ≈ 300 ms, while the instantaneous relative bandwidth was fw/f ≈ 0.90% and the total frequency extent was ∆ftot ≈ 35 MHz. The recorded intermediate drift bursts had frequency drift, positive or negative, with average values of df/fdt equal to -0.027 and 0.024 s-1 respectively. Quite often the fibers appeared in groups; the burst repetition rate within groups was, on average, ̃0.98 s. We distinguish six morphological groups of fibers, based on the relative position of the emission and absorption ridges. These included fibers with emission or absorption ridges only, fibers with the absorption ridge at lower or higher frequency than the emission, or with two absorption ridges above and below the emission. There were also some fibers for which two emission ridges were separated by an absorption ridge. Some additional complex groups within our data set were not easy to classify. A number of borderline cases of fibers with very high drift rate (̃0.30 s-1) or very narrow total bandwidth (̃8 MHz) were recorded; among them there was a group of rope-like fibers characterized by fast repetition rate and relatively narrow total frequency extent. We found that the whistler hypothesis leads to reasonable magnetic field values (̃4.6 G), while the Alfvén-wave hypothesis requires much higher field. From the variation of the drift rate with time we estimated the ratio of the whistler to the cyclotron frequency, x, to be in the range of 0.3-0.6, varying by ̃0.05-0.1 in individual fibers; the same analysis gives an average value of the frequency scale along the loop of ̃220 Mm. Finally, we present empirical relations between fiber burst parameters and discuss their possible origin.

Authors: C. Bouratzis, A. Hillaris, C.E. Alissandrakis, P. Preka-Papadema, X. Moussas, C. Caroubalos, P. Tsitsipis, A. Kontogeorgos
Projects: None

Publication Status: Astronomy & Astrophysics, Volume 625, id.A58, 15 pp.
Last Modified: 2019-06-12 12:20
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Radio Observations of the January 20, 2005 X-Class Event  

Alexander Hillaris   Submitted: 2019-06-08 09:12

We present a multi-frequency and multi-instrument study of the 20 January 2005 event. We focus mainly on the complex radio signatures and their association with the active phenomena taking place: flares, CMEs, particle acceleration and magnetic restructuring. As a variety of energetic particle accelerators and sources of radio bursts are present, in the flare-ejecta combination, we investigate their relative importance in the progress of this event. The dynamic spectra of {Artemis-IV-Wind/Waves-Hiras with 2000 MHz-20 kHz frequency coverage, were used to track the evolution of the event from the low corona to the interplanetary space; these were supplemented with SXR, HXR and gamma-ray recordings. The observations were compared with the expected radio signatures and energetic-particle populations envisaged by the {Standard Flare-CME model and the reconnection outflow termination shock model. A proper combination of these mechanisms seems to provide an adequate model for the interpretation of the observational data.

Authors: C. Bouratzis, P. Preka-Papadema, A. Hillaris, P. Tsitsipis, A. Kontogeorgos, V. G. Kurt X. Moussas
Projects: None

Publication Status: Sol Phys (2010) 267: 343. https://doi.org/10.1007/s11207-010-9648-7
Last Modified: 2019-06-12 12:23
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Fine Structure of Metric Type-IV Radio Bursts Observed with the ARTEMIS-IV Radio Spectrograph: Association with Flares and Coronal Mass Ejections  

Alexander Hillaris   Submitted: 2019-06-08 05:44

Fine structures embedded in type-IV burst continua may be used as diagnostics of the magnetic field restructuring and the corresponding energy release associated with the low corona development of flare/CME events. A catalog of 36 type-IV bursts observed with the SAO receiver of the ARTEMIS-IV solar radio-spectrograph in the 450-270 MHz range at high cadence (0.01 sec) was compiled; the fine structures were classified into five basic classes with two or more sub-classes each. The time of fine structure emission was compared with the injection of energetic electrons as evidenced by HXR and microwave emission, the SXR light-curves and the CME onset time. Our results indicate a very good temporal association between energy release episodes and pulsations, spikes, narrow-band bursts of the type-III family and zebra bursts. Of the remaining categories, the featureless broadband continuum starts near the time of the first energy release, between the CME onset and the SXR peak, but extends for several tens of minutes after that, covering almost the full extent of the flare-CME event. The intermediate drift bursts, fibers in their majority, mostly follow the first energy release but have a wider distribution, compared to other fine structures.

Authors: C. Bouratzis, A. Hillaris, C. E. Alissandrakis, P. Preka-Papadema, X. Moussas, C. Caroubalos, P. Tsitsipis, A. Kontogeorgos
Projects: None

Publication Status: Solar Physics, Volume 290, Issue 1, pp.219-286, 01/2015
Last Modified: 2019-06-12 12:23
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Metric radio bursts and fine structures observed on 17 January, 2005  

Alexander Hillaris   Submitted: 2014-06-10 14:09

A complex radio event was observed on January 17, 2005 with the radio-spectrograph ARTEMIS-IV, operating at Thermopylae, Greece; it was associated with an X3.8 SXR flare and two fast Halo CMEs in close succession. We present dynamic spectra of this event; the high time resolution (1/100 s) of the data in the 450-270 MHz range, makes possible the detection and analysis of the fine structure which this major radio event exhibits. The fine structure was found to match, almost, the comprehensive Ondřejov; Catalogue which it refers to the spectral range 0.8-2 GHz, yet seems to produce similar fine structure with the metric range.

Authors: Bouratzis, C.; Preka-Papadema, P.; Moussas, X.; Alissandrakis, C.; Hillaris, A.
Projects: None

Publication Status: Advances in Space Research, Volume 43, Issue 4, p. 605-611.
Last Modified: 2014-06-11 09:54
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Space storm measurements of the July 2005 solar extreme events from the low corona to the Earth  

Alexander Hillaris   Submitted: 2014-06-10 14:07

The Athens Neutron Monitor Data Processing (ANMODAP) Center recorded an unusual Forbush decrease with a sharp enhancement of cosmic ray intensity right after the main phase of the Forbush decrease on 16 July 2005, followed by a second decrease within less than 12 h. This exceptional event is neither a ground level enhancement nor a geomagnetic effect in cosmic rays. It rather appears as the effect of a special structure of interplanetary disturbances originating from a group of coronal mass ejections (CMEs) in the 13-14 July 2005 period. The initiation of the CMEs was accompanied by type IV radio bursts and intense solar flares (SFs) on the west solar limb (AR 786); this group of energetic phenomena appears under the label of Solar Extreme Events of July 2005. We study the characteristics of these events using combined data from Earth (the ARTEMIS IV radioheliograph, the Athens Neutron Monitor (ANMODAP)), space (WIND/WAVES) and data archives. We propose an interpretation of the unusual Forbush profile in terms of a magnetic structure and a succession of interplanetary shocks interacting with the magnetosphere.

Authors: Caroubalos, C.; Preka-Papadema, P.; Mavromichalaki, H.; Moussas, X.; Papaioannou, A.; Mitsakou, E.; Hillaris, A.
Projects: None

Publication Status: Advances in Space Research, Volume 43, Issue 4, p. 600-604.
Last Modified: 2014-06-11 09:54
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The improved ARTEMIS IV multichannel solar radio spectrograph of the University of Athens  

Alexander Hillaris   Submitted: 2014-06-10 14:03

We present the improved solar radio spectrograph of the University of Athens operating at the Thermopylae Satellite Telecommunication Station. Observations now cover the frequency range from 20 to 650 MHz. The spectrograph has a 7-meter moving parabola fed by a log-periodic antenna for 100 650 MHz and a stationary inverted V fat dipole antenna for the 20 100 MHz range. Two receivers are operating in parallel, one swept frequency for the whole range (10 spectrums/sec, 630 channels/spectrum) and one acousto-optical receiver for the range 270 to 450 MHz (100 spectrums/sec, 128 channels/spectrum). The data acquisition system consists of two PCs (equipped with 12 bit, 225 ksamples/sec ADC, one for each receiver). Sensitivity is about 3 SFU and 30 SFU in the 20 100 MHz and 100 650 MHz range respectively. The daily operation is fully automated: receiving universal time from a GPS, pointing the antenna to the sun, system calibration, starting and stopping the observations at preset times, data acquisition, and archiving on DVD. We can also control the whole system through modem or Internet. The instrument can be used either by itself or in conjunction with other instruments to study the onset and evolution of solar radio bursts and associated interplanetary phenomena.

Authors: Kontogeorgos, A.; Tsitsipis, P.; Caroubalos, C.; Moussas, X.; Preka-Papadema, P.; Hilaris, A.; Petoussis, V.; Bouratzis, C.; Bougeret, J.-L.; Alissandrakis, C. E.; Dumas, G.
Projects: None

Publication Status: Experimental Astronomy, Volume 21, Issue 1, pp.41-55
Last Modified: 2014-06-11 09:54
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Solar flares with and without SOHO/LASCO coronal mass ejections and type II shocks  

Alexander Hillaris   Submitted: 2014-06-10 14:01

We analyse of a set of radio rich (accompanied by type IV or II bursts) solar flares and their association with SOHO/LASCO Coronal Mass Ejections in the period 1998 2000. The intensity, impulsiveness and energetics of these events are investigated. We find that, on the average, flares associated both with type IIs and CMEs are more impulsive and more energetic than flares associated with type IIs only (without CME reported), as well as flares accompanied by type IV continua but not type II shocks. From the last two classes, flares with type II bursts (without CMEs reported) are the shortest in duration and the most impulsive.

Authors: Hillaris, A.; Petousis, V.; Mitsakou, E.; Vassiliou, C.; Moussas, X.; Polygiannakis, J.; Preka-Papadema, P.; Caroubalos, C.; Alissandrakis, C.; Tsitsipis, P.; Kontogeorgos, A.; Bougeret, J.-L.; Dumas, G.
Projects: None

Publication Status: Advances in Space Research, Volume 38, Issue 5, p. 1007-1010.
Last Modified: 2014-06-11 09:54
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Type II Radio Emission and Solar Particle Observations  

Alexander Hillaris   Submitted: 2014-06-10 13:59

The 28 October 2003 flare gave us the unique opportunity to compare the acceleration time of high-energy protons with the escaping time of those particles which have been measured onboard spacecraft and by neutron monitors network as GLE event. High-energy emission time scale and shock wave height and velocity time dependencies were also studied.

Authors: Kuznetsov, S. N.; Kurt, V. G.; Yushkov, B. Yu.; Myagkova, I. N.; Kudela, K.; Belov, A. V.; Caroubalos, C.; Hilaris, A.; Mavromichalaki, H.; Moussas, X.; Preka-Papadema, P.
Projects: None

Publication Status: International Journal of Modern Physics A, Volume 20, Issue 29, pp. 6705-6707 (2005).
Last Modified: 2014-06-11 09:54
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ARTEMIS IV Radio Observations of the 14 July 2000 Large Solar Event  

Alexander Hillaris   Submitted: 2014-06-10 13:57

In this report we present a complex metric burst, associated with the 14 July 2000 major solar event, recorded by the ARTEMIS-IV radio spectrograph at Thermopylae. Additional space-borne and Earth-bound observational data are used, in order to identify and analyze the diverse, yet associated, processes during this event. The emission at metric wavelengths consisted of broad-band continua including a moving and a stationary type IV, impulsive bursts and pulsating structures. The principal release of energetic electrons in the corona was 15 20 min after the start of the flare, in a period when the flare emission spread rapidly eastwards and a hard X-ray peak occurred. Backward extrapolation of the CME also puts its origin in the same time interval, however, the uncertainty of the extrapolation does not allow us to associate the CME with any particular radio or X-ray signature. Finally, we present high time and spectral resolution observations of pulsations and fiber bursts, together with a preliminary statistical analysis.

Authors: Caroubalos, C.; Alissandrakis, C. E.; Hillaris, A.; Nindos, A.; Tsitsipis, P.; Moussas, X.; Bougeret, J.-L.; Bouratzis, K.; Dumas, G.; Kanellakis, G.; Kontogeorgos, A.; Maroulis, D.; Patavalis, N.; Perche, C.; Polygiannakis, J.; Preka-Papadema, P.
Projects: None

Publication Status: Solar Physics, v. 204, Issue 1/2, p. 165-177 (2001).
Last Modified: 2014-06-11 09:54
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Radio Observations of the 20 January 2005 X-class Flare  

Alexander Hillaris   Submitted: 2014-06-10 13:55

We present a multi-frequency and multi-instrument study of the 20 January 2005 event. We focus mainly on the complex radio signatures and their association with the active phenomena taking place: flares, CMEs, particle acceleration, and magnetic restructuring. As a variety of energetic-particle accelerators and sources of radio bursts are present, in the flare - ejecta combination, we investigate their relative importance in the progress of this event. The dynamic spectra of ARTEMIS-IV - Wind/ Waves - HiRAS, with 2000 MHz - 20 kHz frequency coverage, were used to track the evolution of the event from the low corona to the interplanetary space; these were supplemented with SXR, HXR, and γ-ray recordings. The observations were compared with the expected radio signatures and energetic-particle populations envisaged by the Standard Flare - CME model and the reconnection outflow termination shock model. A proper combination of these mechanisms seems to provide an adequate model for the interpretation of the observational data.

Authors: Bouratzis, C.; Preka-Papadema, P.; Hillaris, A.; Tsitsipis, P.; Kontogeorgos, A.; Kurt, V. G.; Moussas, X..
Projects: None

Publication Status: Solar Physics, Volume 267, Issue 2, pp.343-359
Last Modified: 2014-06-11 09:54
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On the relationship of shock waves to flares and coronal mass ejections  

Alexander Hillaris   Submitted: 2014-06-10 13:52

Context. Metric type II bursts are the most direct diagnostic of shock waves in the solar corona. Aims: There are two main competing views about the origin of coronal shocks: that they originate in either blast waves ignited by the pressure pulse of a flare or piston-driven shocks due to coronal mass ejections (CMEs). We studied three well-observed type II bursts in an attempt to place tighter constraints on their origins. Methods: The type II bursts were observed by the ARTEMIS radio spectrograph and imaged by the Nançay Radioheliograph (NRH) at least at two frequencies. To take advantage of projection effects, we selected events that occurred away from disk center. Results: In all events, both flares and CMEs were observed. In the first event, the speed of the shock was about 4200 km s-1, while the speed of the CME was about 850 km s-1. This discrepancy ruled out the CME as the primary shock driver. The CME may have played a role in the ignition of another shock that occurred just after the high speed one. A CME driver was excluded from the second event as well because the CMEs that appeared in the coronagraph data were not synchronized with the type II burst. In the third event, the kinematics of the CME which was determined by combining EUV and white light data was broadly consistent with the kinematics of the type II burst, and, therefore, the shock was probably CME-driven. Conclusions: Our study demonstrates the diversity of conditions that may lead to the generation of coronal shocks.

Authors: Nindos, A.; Alissandrakis, C. E.; Hillaris, A.; Preka-Papadema, P.
Projects: None

Publication Status: Astronomy & Astrophysics, Volume 531, id.A31, 12 pp
Last Modified: 2014-06-11 09:55
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The 17 January 2005 Complex Solar Radio Event Associated with Interacting Fast Coronal Mass Ejections  

Alexander Hillaris   Submitted: 2014-06-10 13:49

On 17 January 2005 two fast coronal mass ejections were recorded in close succession during two distinct episodes of a 3B/X3.8 flare. Both were accompanied by metre-to-kilometre type-III groups tracing energetic electrons that escape into the interplanetary space and by decametre-to-hectometre type-II bursts attributed to CME-driven shock waves. A peculiar type-III burst group was observed below 600 kHz 1.5 hours after the second type-III group. It occurred without any simultaneous activity at higher frequencies, around the time when the two CMEs were expected to interact. We associate this emission with the interaction of the CMEs at heliocentric distances of about 25 R ⊙. Near-relativistic electrons observed by the EPAM experiment onboard ACE near 1 AU revealed successive particle releases that can be associated with the two flare/CME events and the low-frequency type-III burst at the time of CME interaction. We compare the pros and cons of shock acceleration and acceleration in the course of magnetic reconnection for the escaping electron beams revealed by the type-III bursts and for the electrons measured in situ.

Authors: Hillaris, A.; Malandraki, O.; Klein, K.-L.; Preka-Papadema, P.; Moussas, X.; Bouratzis, C.; Mitsakou, E.; Tsitsipis, P.; Kontogeorgos, A.
Projects: None

Publication Status: Solar Physics, Volume 273, Issue 2, pp.493-509
Last Modified: 2014-06-11 09:54
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Abstracts by Author
First detection of metric emission from a solar surge
High-resolution observations with ARTEMIS-JLS and the NRH. III. Spectroscopy and imaging of fiber bursts
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Metric radio bursts and fine structures observed on 17 January, 2005
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Solar flares with and without SOHO/LASCO coronal mass ejections and type II shocks
Type II Radio Emission and Solar Particle Observations
ARTEMIS IV Radio Observations of the 14 July 2000 Large Solar Event
Radio Observations of the 20 January 2005 X-class Flare
On the relationship of shock waves to flares and coronal mass ejections
The 17 January 2005 Complex Solar Radio Event Associated with Interacting Fast Coronal Mass Ejections
The relativistic solar particle event of 2005 January 20: prompt and delayed particle acceleration
Fine Structure of Metric Type-IV Radio Bursts Observed with the ARTEMIS-IV Radio Spectrograph: Association with Flares and Coronal Mass Ejections
CME Expansion as the Driver of Metric Type II Shock Emission as Revealed by Self-consistent Analysis of High-Cadence EUV Images and Radio Spectrograms

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