arXiv subjects

X. Moussas

Publications and source records attributed to X. Moussas.

At least 19 recordsLinked to original sources

arXiv2022

High-resolution observations with ARTEMIS/JLS and the NRH: IV. Imaging spectroscopy of spike-like structures near the front of type-II bursts

Narrowband bursts (spikes) appear on dynamic spectra from microwave to decametric frequencies. They are believed to be manifestations of small-scale energy release through magnetic reconnection. We study the position of the spike-like structures relative to the front of type-II bursts and their role in the burst emission. We used high-sensitivity, low-noise dynamic spectra obtained with the acousto-optic analyzer (SAO) of the ARTEMIS-JLS radiospectrograph, in conjunction with images from the Nançay Radioheliograph (NRH) in order to study spike-like bursts near the front of a type-II radio burst 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. 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 ~200" and their observed brightness temperature was 1.4-5.6x10^9K, 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 RSUN above the pre-existing continuum emission, which, was located 0.1 RSUN above the western limb. This study 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.

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arXiv2019

High resolution observations with Artemis--JLS, (II) Type IV associated intermediate drift bursts

Aims. We examined the characteristics of isolated intermediate drift bursts (IMDs) and their morphologies on dynamic spectra, in particular the positioning of emission and absorption ridges and the repetition rate of fiber groups. These were compared with a model in order to determine the conditions under which the IMDs appear and exhibit the above characteristics. Methods. We analyzed sixteen metric type IV events with embedded IMDs. The events were recorded with the Artemis-JLS/SAO high resolution (10 ms cadence) receiver in the 270-450MHz 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 IMD groups. We also developed a semi-automatic algorithm to track fibers on dynamic spectra. Results. 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 or with two emission ridges were separated by an absorption ridge. Some borderline cases of IMDs with very high drift rate (~0.30 s-1) or very narrow total bandwidth (~8 MHz) were recorded; among them a group of rope-like IMDs with fast repetition rate and relatively narrow total frequency extent. The whistler hypothesis leads to reasonable magnetic field (~4.6 G), but the Alfven-wave hypothesis requires much higher field. We estimated the ratio of the whistler to the cyclotron frequency, x, to be ~ 0.3 to 0.6 and the average frequency scale along the loop of ~220Mm. We present empirical relations between fiber burst parameters and discuss their possible origin.

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arXiv2019

Detection of spike-like structures near the front of type-II bursts

Aims. We examine high time resolution dynamic spectra for fine structures in type II solar radio bursts Methods. We used data obtained with the (SAO) 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.

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arXiv2015

High resolution observations with Artemis-IV and the NRH. I. Type IV associated narrow-band bursts

Narrow band bursts appear on dynamic spectra from microwave to decametric frequencies as fine structures with very small duration and bandwidth. They are thought to mark small scale magnetic reconnection. We analyzed 27 metric type-IV events with narrow band bursts observed by the ARTEMIS-IV radiospectrograph in 30/6/1999-1/8/2010. We examined the morphological characteristics of isolated narrow-band bursts and groups or chains of spikes. The events were recorded with the SAO (10 ms cadence) receiver of ARTEMIS-IV in the 270-450 MHz range. We measured the duration, spectral width, and frequency drift of ~12000 individual narrow-band bursts, groups, and chains. Spike sources were imaged with the NRH for the event of 21 April 2003. 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, positive or negative. 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 IIId-bursts; most of the row chains exhibited negative drifts similar to fiber bursts. From the 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 ~60% of their size in total intensity. The duration and bandwidth distributions did not show any clear separation in groups. Some chains tended to assume the form of zebra, lace stripes, fibers, 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.

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arXiv2015

Properties of solar energetic particle events inferred from their associated radio emission

We study selected properties of Solar Energetic Particle (SEP) events as inferred from their associated radio emissions. We used a catalogue of 115 SEP events that consists of entries of proton intensity enhancements at one AU, with complete coverage over solar cycle 23, based on high-energy (~68 MeV) protons from SOHO/ERNE and we calculated the proton release time at the Sun using velocity dispersion analysis (VDA). After an initial rejection of cases with unrealistic VDA path lengths, we assembled composite radio spectra for the remaining events using data from ground-based and space-borne radio-spectrographs. For every event we registered the associated radio emissions and we divided the events in groups according to their associated radio emissions. The proton release was found to be most often accompanied by both type III and II radio bursts, but a good association percentage was also registered in cases accompanied by type IIIs only. The worst association was found for the cases with type II only association. These radio association percentages support the idea that both flare- and shock-resident particle release processes are observed in high-energy proton events. In cases of type III-associated events we extended our study to the timings between the type III radio emission, the proton release, and the electron release as inferred from VDA based on Wind/3DP 20-646 keV data. Typically, the protons are released after the start of the associated type III bursts and simultaneously or before the release of energetic electrons. For the cases with type II radio association we found that the distribution of the proton release heights had a maximum at ~2.5 Rs. Most (69%) of the flares associated to our SEP events were located at the western hemisphere, with a peak within the well-connected region of 50-60 deg western longitude.

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arXiv2014

Fine Structure of Metric Type-IV Radio Bursts Observed with the ARTEMIS-IV Radio Spectrograph: Association with Flares and Coronal Mass Ejections

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.

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arXiv2013

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

On 13 June 2010, an eruptive event occurred near the solar limb. It included a small filament eruption and the onset of a relatively narrow coronal mass ejection (CME) surrounded by an extreme ultraviolet wave front recorded by the Solar Dynamics Observatory's (SDO) Atmospheric Imaging Assembly (AIA) at high cadence. The ejection was accompanied by a GOES M1.0 soft X-ray flare and a Type-II radio burst; high-resolution dynamic spectra of the latter were obtained by the ARTEMIS IV radio spectrograph. The combined observations enabled a study of the evolution of the ejecta and the EUV wavefront and its relationship with the coronal shock manifesting itself as metric Type-II burst. By introducing a novel technique, which deduces a proxy of the EUV compression ratio from AIA imaging data and compares it with the compression ratio deduced from the band-split of the Type-II metric radio burst, we are able to infer the potential source locations of the radio emission of the shock on that AIA images. Our results indicate that the expansion of the CME ejecta is the source for both EUV and radio shock emissions. Early in the CME expansion phase, the Type-II burst seems to originate in the sheath region between the EUV bubble and the EUV shock front in both radial and lateral directions. This suggests that both the nose and the flanks of the expanding bubble could have driven the shock.

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arXiv2012

Wavelet Analysis on Solar Wind Parameters and Geomagnetic Indices

The sun as an oscillator produces frequencies which propagate in the heliosphere, via solar wind, to the terrestrial magnetosphere. We searched for those frequencies in the parameters of the near Earth solar plasma and the geomagnetic indices for the past four solar cycles. The solar wind parameters used in this work are the interplanetary magnetic field, plasma beta, Alfven Mach number, solar wind speed, plasma temperature, plasma pressure, plasma density and the geomagnetic indices DST, AE, Ap and Kp. We found out that each parameter of the solar wind exhibit certain periodicities which differentiate in each cycle. Our results indicate intermittent periodicities in our data, some of them shared between the solar wind parameters and geomagnetic indices.

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arXiv2011

The 17 January 2005 Complex Solar Radio Event Associated with Interacting Fast Coronal Mass Ejections

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\odot. 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.

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arXiv2010

Spatial & Temporal Characteristics of Ha flares during the period 1975-2002 (comparison with SXR flares)

Although the energetic phenomena of the Sun (flares, coronal mass injections etc.) exhibit intermittent stochastic behavior in their rate of occurrence, they are well correlated to the variations of the solar cycle. In this work we study the spatial and temporal characteristics of transient solar activity in an attempt to statistically interpret the evolution of these phenomena through the solar cycle, in terms of the self-organized criticality theory.

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arXiv2010

Type II and IV radio bursts in the active period October-November 2003

In this report we present the Type II and IV radio bursts observed and analyzed by the radio spectrograph ARTEMIS IV1, in the 650-20MHz frequency range, during the active period October-November 2003. These bursts exhibit very rich fine structures such fibers, pulsations and zebra patterns which is associated with certain characteristics of the associated solar flares and CMEs.

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arXiv2010

Radio Bursts in the Active Period January 2005

We present complex radio bursts recorded by the radiospectrograph ARTEMIS-IV in the active period of January 2005. The wide spectral coverage of this recorder, in the 650-20 MHz range, permits an analysis of the radio bursts from the base of the Solar Corona to 2 Solar Radii; it thus facilitates the association of radio activity with other types of solar energetic phenomena. Furthermore the ARTEMIS-IV1, high time resolution (1/100 sec) in the 450-270 MHz range, makes possible the detection and analysis of the fine structure which most of the major radio events exhibit.

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arXiv2010

A Study of Halo Coronal Mass Ejections and Related Flare and Radio Burst Observations in Solar Cycle 23

We present a statistical study of dynamical and kinetic characteristics of CMEs which show temporal and spatial association with flares and type II radio bursts or complex radio events of type II bursts and type IV continua. This study is based on a set of earth-directed full halo CMEs occurring during the present solar cycle, with data from the Large Angle Spectrometric Coronagraphs (LASCO) and Extreme-Ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory (SOHO) mission and the Magnetic Fields Investigation (MFI) and 3-D Plasma and Energetic Particle Analyzer Investigation experiment on board the WIND spacecraft.

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arXiv2010

Type II Shocks Characteristics: Comparison with associated CMEs and Flares

A number of metric (100-650 MHz) typeII bursts was recorded by the ARTEMIS-IV radiospectrograph in the 1998-2000 period; the sample includes both CME driven shocks and shocks originating from flare blasts. We study their characteristics in comparison with characteristics of associated CMEs and flares.

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arXiv2010

Ten Years of the Solar Radiospectrograph ARTEMIS-IV

The Solar Radiospectrograph of the University of Athens (ARTEMIS-IV) is in operation at the Thermopylae Satellite Communication Station since 1996. The observations extend from the base of the Solar Corona (650 MHz) to about 2 Solar Radii (20 MHz) with time resolution 1/10-1/100 sec. The instruments recordings, being in the form of dynamic spectra, measure radio flux as a function of height in the corona; our observations are combined with spatial data from the Nancay Radioheliograph whenever the need for 3D positional information arises. The ARTEMIS-IV contribution in the study of solar radio bursts is two fold- Firstly, in investigating new spectral characteristics since its high sampling rate facilitates the study of fine structures in radio events. On the other hand it is used in studying the association of solar bursts with interplanetary phenomena because of its extended frequency range which is, furthermore, complementary to the range of the WIND/WAVES receivers and the observations may be readily combined. This reports serves as a brief account of this operation. Joint observations with STEREO/WAVES and LOFAR low frequency receivers are envisaged in the future.

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arXiv2010

Space Storm Measurements of 17 and 21 April 2002 Forbush Effects from Artemis-IV Solar Radio-Spectrograph, Athens Neutron Monitor Station and Coronas-F Satellite

In this report we present two complex eruptive solar events and the associated Cosmic Ray effects (Forbush decrease). We use combined recordings from a number of Earthbound Receivers, Space Experiments and data archives (such as the ARTEMIS-IV Radio spectrograph, the Athens NEUTRON MONITOR, the LASCO CME Lists, the SONG of the {CORONAS-F} satellite, etc.). The influence of solar transients on the interplanetary medium conditions and the cosmic ray flux is analysed and discussed. The observed time sequence of events of this time period indicates that the initiation of CMEs is closely related to the appearance of type II and IV radio bursts and strong solar flares. Their effects extend from the lower corona to the near Earth vicinity affecting Cosmic Ray measurements and space weather. As regards the Forbush decrease our data indicate significant amplification at the presence of a MHD shock.

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arXiv2010

The improved ARTEMIS IV multichannel solar radio spectrograph of the University of Athens

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.

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arXiv2010

Solar flares with and without SOHO/LASCO coronal mass ejections and type II shocks

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.

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