Preflare Phase Research Articles

Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA

Abstract NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has observed several small microflares in its initial solar pointings. In this paper, we present the first joint observation of a microflare with NuSTAR and Hinode/XRT on 2015 April 29 at ∼11:29 UT. This microflare shows the heating of material to several million Kelvin, observed in soft X-rays with Hinode/XRT, and was faintly visible in the extreme ultraviolet with SDO/AIA. For three of the four NuSTAR observations of this region (pre-flare, decay, and post-flare phases), the spectrum is well fitted by a single thermal model of 3.2–3.5 MK, but the spectrum during the impulsive phase shows additional emission up to 10 MK, emission equivalent to the A0.1 GOES class. We recover the differential emission measure (DEM) using SDO/AIA, Hinode/XRT, and NuSTAR, giving unprecedented coverage in temperature. We find that the pre-flare DEM peaks at ∼3 MK and falls off sharply by 5 MK; but during the microflare’s impulsive phase, the emission above 3 MK is brighter and extends to 10 MK, giving a heating rate of about erg s−1. As the NuSTAR spectrum is purely thermal, we determined upper limits on the possible non-thermal bremsstrahlung emission. We find that for the accelerated electrons to be the source of heating, a power-law spectrum of with a low-energy cutoff keV is required. In summary, this first NuSTAR microflare strongly resembles much more powerful flares.

Quasi-oscillatory dynamics observed in ascending phase of the flare on March 6, 2012

Context. The dynamics of the flaring loops in active region (AR) 11429 are studied. The observed dynamics consist of several evolution stages of the flaring loop system during both the ascending and descending phases of the registered M-class flare. The dynamical properties can also be classified by different types of magnetic reconnection, related plasma ejection and aperiodic flows, quasi-periodic oscillatory motions, and rapid temperature and density changes, among others. The focus of the present paper is on a specific time interval during the ascending (pre-flare) phase. Aims. The goal is to understand the quasi-periodic behavior in both space and time of the magnetic loop structures during the considered time interval. Methods.We have studied the characteristic location, motion, and periodicity properties of the flaring loops by examining space-time diagrams and intensity variation analysis along the coronal magnetic loops using AIA intensity and HMI magnetogram images (from the Solar Dynamics Observatory(SDO)). Results. We detected bright plasma blobs along the coronal loop during the ascending phase of the solar flare, the intensity variations of which clearly show quasi-periodic behavior. We also determined the periods of these oscillations. Conclusions. Two different interpretations are presented for the observed dynamics. Firstly, the oscillations are interpreted as the manifestation of non-fundamental harmonics of longitudinal standing acoustic oscillations driven by the thermodynamically nonequilibrium background (with time variable density and temperature). The second possible interpretation we provide is that the observed bright blobs could be a signature of a strongly twisted coronal loop that is kink unstable.

VERY LONG-PERIOD PULSATIONS BEFORE THE ONSET OF SOLAR FLARES

ABSTRACT Solar flares are the most powerful explosions occurring in the solar system, which may lead to disastrous space weather events and impact various aspects of our Earth. It remains a big challenge in modern astrophysics to understand the origin of solar flares and predict their onset. Based on the analysis of soft X-ray emission observed by the Geostationary Operational Environmental Satellite, this work reports a new discovery of very long-periodic pulsations occurring in the preflare phase before the onset of solar flares (preflare-VLPs). These pulsations typically have periods of 8–30 min and last for about 1–2 hr. They are possibly generated from LRC oscillations of plasma loops where electric current dominates the physical process during magnetic energy accumulation in the source region. Preflare-VLPs provide essential information for understanding the triggering mechanism and origin of solar flares, and may be a convenient precursory indicator to help us respond to solar explosions and the corresponding disastrous space weather events.

IMPULSIVITY PARAMETER FOR SOLAR FLARES

ABSTRACT Three phases are typically observed during solar flares: the preflare, impulsive, and decay phases. During the impulsive phase, it is believed that the electrons and other particles are accelerated after the stored energy in the magnetic field is released by reconnection. The impulsivity of a solar flare is a quantifiable property that shows how quickly this initial energy release occurs. It is measured via the impulsivity parameter, which we define as the inverse of the overall duration of the impulsive phase. We take the latter as the raw width of the most prominent nonthermal emission of the flare. We computed this observable over a work sample of 48 M-class events that occurred during the current Solar Cycle 24 by using three different methods. The first method takes into account all of the nonthermal flare emission and gives very accurate results, while the other two just cover fixed energy intervals (30–40 keV and 25–50 keV) and are useful for fast calculations. We propose an alternative way to classify solar flares according to their impulsivity parameter values, defining three different types of impulsivity, namely, high, medium, and low. This system of classification is independent of the manner used to calculated the impulsivity parameter. Lastly, we show the relevance of this tool as a discriminator of different HXR generation processes.

THE ROLE OF ERUPTING SIGMOID IN TRIGGERING A FLARE WITH PARALLEL AND LARGE-SCALE QUASI-CIRCULAR RIBBONS

In this paper, we present observations and analysis of an interesting sigmoid formation, eruption and the associated flare that occurred on 2014 April 18 using multi-wavelength data sets. We discuss the possible role of the sigmoid eruption in triggering the flare, which consists of two different set of ribbons: parallel ribbons as well as a large-scale quasi-circular ribbon. Several observational evidence and nonlinear force-free field extrapolation results show the existence of a large-scale fan-spine type magnetic configuration with a sigmoid lying under a section of the fan dome. The event can be explained with the following two phases. During the pre-flare phase, we observed the formation and appearance of sigmoid via tether-cutting reconnection between the two sets of sheared fields under the fan dome. The second, main flare phase, features the eruption of the sigmoid, the subsequent flare with parallel ribbons, and a quasi-circular ribbon. We propose the following multi-stage successive reconnections scenario for the main flare. First, tether-cutting reconnection is responsible for the formation and the eruption of the sigmoid structure. Second, the reconnection occurred in the wake of the erupting sigmoid produces the parallel flare ribbons on the both sides of the circular polarity inversion line. Third, the null-type reconnection higher in the corona, possibly triggered by the erupting sigmoid, leads to the formation of a large quasi-circular ribbon. For the first time we suggest a mechanism for this type of flare consisting of a double set of ribbons triggered by an erupting sigmoid in a large scale fan-spine type magnetic configuration.

LARGE-SCALE CONTRACTION AND SUBSEQUENT DISRUPTION OF CORONAL LOOPS DURING VARIOUS PHASES OF THE M6.2 FLARE ASSOCIATED WITH THE CONFINED FLUX ROPE ERUPTION

We present a detailed multi-wavelength study of the M6.2 flare which was associated with a confined eruption of a prominence using TRACE, RHESSI, and NoRH observations. The pre-flare phase of this event is characterized by spectacular large-scale contraction of overlying extreme ultraviolet (EUV) coronal loops during which the loop system was subjected to an altitude decrease of ~20 Mm for an extended span of ~30 min. This contraction phase is accompanied by sequential EUV brightenings associated with hard X-ray (HXR) (up to 25 keV) and microwave (MW) sources from low-lying loops in the core of the flaring region which together with X-ray spectra indicate strong localized heating in the source region before the filament activation and associated M-class flare. With the onset of the impulsive phase of the M6.2 flare, we detect HXR and MW sources that exhibit intricate temporal and spatial evolution in relation with the fast rise of the prominence. Following the flare maximum, the filament eruption slowed down and subsequently confined within the large overlying active region loops; the event did not lead to a coronal mass ejection (CME). During the confinement process of the erupting prominence, we detect MW emission from the extended coronal region with multiple emission centroids which likely represent emission from hot blobs of plasma formed after the collapse of the expanding flux rope and entailing prominence material. RHESSI observations reveal high plasma temperature (~30 MK) and substantial non-thermal characteristics with electron spectral index (~5) during the impulsive phase of the flare. The time-evolution of thermal energy exhibits a good correspondence with the variations in cumulative non-thermal energy which suggest that the energy of accelerated particles efficiently converted to hot flare plasma implying an effective validation of the Neupert effect.

IMAGING AND SPECTROSCOPIC DIAGNOSTICS ON THE FORMATION OF TWO MAGNETIC FLUX ROPES REVEALED BYSDO/AIA ANDIRIS

Helical magnetic flux rope (MFR) is a fundamental structure of coronal mass ejections (CMEs) and has been discovered recently to exist as a sigmoidal channel structure prior to its eruption in the EUV high-temperature passbands of the Atmospheric Imaging Assembly (AIA). However, when and where the MFR is built up are still elusive. In this paper, we investigate two MFRs (MFR1 and MFR2) in detail, whose eruptions produced two energetic solar flares and CMEs on 2014 April 18 and 2014 September 10, respectively. The AIA EUV images reveal that for a long time prior to their eruption, both MFR1 and MFR2 are under formation, which is probably through magnetic reconnection between two groups of sheared arcades driven by the shearing and converging flows in the photosphere near the polarity inversion line. At the footpoints of the MFR1, the Interface Region Imaging Spectrograph Si iv, C ii, and Mg ii lines exhibit weak to moderate redshifts and a non-thermal broadening in the pre-flare phase. However, a relatively large blueshift and an extremely strong non-thermal broadening are found at the formation site of the MFR2. These spectral features consolidate the proposition that the reconnection plays an important role in the formation of MFRs. For the MFR1, the reconnection outflow may propagate along its legs, penetrating into the transition region and the chromosphere at the footpoints. For the MFR2, the reconnection probably takes place in the lower atmosphere and results in the strong blueshift and non-thermal broadening for the Mg ii, C ii, and Si iv lines.

MAGNETIC AND DYNAMICAL PHOTOSPHERIC DISTURBANCES OBSERVED DURING AN M3.2 SOLAR FLARE

This letter reports on a set of full-Stokes spectropolarimetric observations in the near infrared He I 10830 A spectral region covering the pre-, flare, and post-flare phases of an M3.2 class solar flare. The flare originated on 2013 May 17 and belonged to active region NOAA 11748. We detected strong He I 10830 A emission in the flare. The red component of the He I triplet peaks at an intensity ratio to the continuum of about 1.86. During the flare, He I Stokes V is substantially larger and appears reversed compared to the usually larger Si I Stokes V profile. The photospheric Si I inversions of the four Stokes profiles reveal the following: (1) the magnetic field strength in the photosphere decreases or is even absent during the flare phase, as compared to the pre-flare phase. However, this decrease is not permanent. After the flare the magnetic field recovers its pre-flare configuration in a short time (i.e., in 30 minutes after the flare). (2) In the photosphere, the line-of-sight velocities show a regular granular up- and down-flow pattern before the flare erupts. During the flare, upflows (blueshifts) dominate the area where the flare is produced. Evaporation rates of ~ $10^{-3}$ and ~ $10^{-4}$ g cm$^{-2}$ s$^{-1}$ have been derived in the deep and high photosphere, respectively, capable of increasing the chromospheric density by a factor of two in about 400 seconds.

Dynamics of Microwave Sources Associated with the Neutral Line and the Magnetic-Field Parameters of Sunspots as a Factor in Predicting Large Flares

We studied the evolution of five active regions (ARs) where strong X-class flares occurred in 2011 – 2012 (NOAA ARs 11158, 11166, 11263, 11283 and 11520). Our study focuses on the pre-flare phase of these ARs (during a few days before the flare) to reveal features in the microwave radiation and magnetic-field characteristics of the ARs that indicate that a powerful flare is about to take place. One well-developed AR (NOAA AR 11654) that did not produce large flares was also studied. We used daily multiwavelength spectral-polarization solar observations in the range of 1.65 – 6.0 cm made with the RATAN-600 radio telescope and data obtained by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI). Whenever X-class flares occurred, we found that a new compact microwave source developed that was associated with the neutral line of the photospheric magnetic field (neutral line associated source – NLS) above the place with highest gradient of magnetic field. In some cases this became predominant in radio emission of the AR one to two days before a large flare. No such source was detected in AR without a powerful flare. An analysis of magnetic-field characteristics of sunspots (based on SDO/HMI data) showed that the large X-flares we investigated occurred in ARs with high magnetic flux (∼ 1022 Mx) and with an increasing magnetic-field gradient. We first identified the positions of the developing microwave source (NLS) with the location of the large flare that was registered in the AR one to two days later. Radio characteristics and dynamics of NLS detected before large flares possibly reflect the place in the corona where the energy for flaring is stored. Thus, an early detection of a rapidly developing microwave source NLS and an increasing magnetic-field gradient can be used as a factor in predicting large flares.

MAGNETIC FIELD RESTRUCTURING ASSOCIATED WITH TWO SUCCESSIVE SOLAR ERUPTIONS

We examine two successive flare eruptions (X5.4 and X1.3) on 2012 March 7 in the NOAA active region 11429 and investigate the magnetic field reconfiguration associated with the two eruptions. Using an advanced non-linear force-free field (NLFFF) extrapolation method based on the SDO/HMI vector magnetograms, we obtain a stepwise decrease in the magnetic free energy during the eruptions, which is roughly 20%-30% of the energy of the pre-flare phase. We also calculate the magnetic helicity, and suggest that the changes of the sign of the helicity injection rate might be associated with the eruptions. Through the investigation of the magnetic field evolution, we find that the appearance of the "implosion" phenomenon has a strong relationship with the occurrence of the first X-class flare. Meanwhile, the magnetic field changes of the successive eruptions with implosion and without implosion were well observed.

WHERE IS THE CHROMOSPHERIC RESPONSE TO CONDUCTIVE ENERGY INPUT FROM A HOT PRE-FLARE CORONAL LOOP?

Before the onset of a flare is observed in hard X-rays there is often a prolonged pre-flare or pre-heating phase with no detectable hard X-ray emission but pronounced soft X-ray emission suggesting that energy is being released and deposited into the corona and chromosphere already at this stage. This work analyses the temporal evolution of coronal source heating and the chromospheric response during this pre-heating phase to investigate the origin and nature of early energy release and transport during a solar flare. Simultaneous X-ray, EUV, and microwave observations of a well observed flare with a prolonged pre-heating phase are analysed to study the time evolution of the thermal emission and to determine the onset of particle acceleration. During the 20 minutes duration of the pre-heating phase we find no hint of accelerated electrons, neither in hard X-rays nor in microwave emission. However, the total energy budget during the pre-heating phase suggests that energy must be supplied to the flaring loop to sustain the observed temperature and emission measure. Under the assumption of this energy being transported toward the chromosphere via thermal conduction, significant energy deposition at the chromosphere is expected. However, no detectable increase of the emission in the AIA wavelength channels sensitive to chromospheric temperatures is observed. The observations suggest energy release and deposition in the flaring loop before the onset of particle acceleration, yet a model in which energy is conducted to the chromosphere and subsequent heating of the chromosphere is not supported by the observations.

Giant events in the 23rd solar cycle: Common and specific features

Giant X-ray events (>X17) in the 23rd solar cycle (October 28 and November 4, 2003, and September 7, 2005) are considered, which were almost completely observed in hard X rays (>150 keV) from the INTEGRAL satellite and partly from the RHESSI satellite. These events are compared with two events completely observed from the RHESSI satellite (X8.2 on November 2, 2003, and X7.1 on January 20, 2005). Time profiles of the plasma temperature, calculated from the GOES data on soft X rays during these events, have similar structures. This allowed us to choose a zero time point in each event and compare the dynamics of their evolution. Nonthermal radiation began approximately 10 min before the zero time point (preflare phase). During about 20 min after the zero point, bursts of nonthermal radiation were observed in all the three events, which pointed to several episodes of electron acceleration with variable spectra and plasma heating with different efficiency. It is shown that giant events are characterized by a large emission measure and a relatively low temperature of the flare plasma but not intensive acceleration processes. Observations of γ radiation from the decay of π0-mesons show that protons are accelerated to relativistic energies 4 min after the chosen zero point.

STUDY ON THE TRIGGERING PROCESS OF SOLAR FLARES BASED ONHINODE/SOT OBSERVATIONS

We investigated four major solar flare events that occurred in active regions NOAA 10930 (December 13 and 14, 2006) and NOAA 11158 (February 13 and 15, 2011) by using data observed by the Solar Optical Telescope (SOT) onboard the Hinode satellite. To reveal the trigger mechanism of solar flares, we analyzed the spatio-temporal correlation between the detailed magnetic field structure and the emission image of the Ca H line at the central part of flaring regions for several hours prior to the onset of flares. We observed in all the flare events that the magnetic shear angle in the flaring regions exceeded 70 degrees, as well as that characteristic magnetic disturbances developed at the centers of flaring regions in the pre-flare phase. These magnetic disturbances can be classified into two groups depending on the structure of their magnetic polarity inversion lines; the so-called "Opposite-Polarity" and "Reversed-Shear" magnetic field recently proposed by our group, although the magnetic disturbance in one event of the four samples is too subtle to clearly recognize the detailed structure. The result suggests that some major solar flares are triggered by rather small magnetic disturbances. We also show that the critical size of the flare-trigger field varies among flare events and briefly discuss how the flare-trigger process depends on the evolution of active regions.

KINETIC MODELING OF PARTICLE ACCELERATION IN A SOLAR NULL-POINT RECONNECTION REGION

The primary focus of this paper is on the particle acceleration mechanism in solar coronal three-dimensional reconnection null-point regions. Starting from a potential field extrapolation of a Solar and Heliospheric Observatory (SOHO) magnetogram taken on 2002 November 16, we first performed magnetohydrodynamics (MHD) simulations with horizontal motions observed by SOHO applied to the photospheric boundary of the computational box. After a build-up of electric current in the fan-plane of the null-point, a sub-section of the evolved MHD data was used as initial and boundary conditions for a kinetic particle-in-cell model of the plasma. We find that sub-relativistic electron acceleration is mainly driven by a systematic electric field in the current sheet. A non-thermal population of electrons with a power-law distribution in energy forms in the simulated pre-flare phase, featuring a power-law index of about -1.78. This work provides a first step towards bridging the gap between macroscopic scales on the order of hundreds of Mm and kinetic scales on the order of cm in the solar corona, and explains how to achieve such a cross-scale coupling by utilizing either physical modifications or (equivalent) modifications of the constants of nature. With their exceptionally high resolution - up to 135 billion particles and 3.5 billion grid cells of size 17.5 km - these simulations offer a new opportunity to study particle acceleration in solar-like settings.

High Energy Emission from Magnetar Due to Giant Flare

We propose a theoretical model for magnetar giant flare to explain the flaring activity on 2004 December 27 from SGR1806-20 comprehensively. A global rearrangement is expected by the magnetic reconnection that requires explaining the giant SGR flares. In this paper we propose two regions of flares: preflare on the surface of magnetar and main burst at a distance of light cylinder radius. Acquiring the maximum potential drop on the magnetar surface, adopting space charge limited flow model, and using magnetic field B ≈ 1015 G, the luminosities of flare energies release for the preflare phase and main burst phase are found to be in the order of 1041 erg·sˉ1 and 1044 erg·sˉ1 respectively, conforming to magnetar burst energy and flare temperature is determined by considering black body radiation.

ON THE ROLE OF ROTATING SUNSPOTS IN THE ACTIVITY OF SOLAR ACTIVE REGION NOAA 11158

We study the role of rotating sunspots in relation to the evolution of various physical parameters characterizing the non-potentiality of the active region NOAA 11158 and its eruptive events using the magnetic field data from the Helioseismic and Magnetic Imager (HMI) and multi-wavelength observations from the Atmospheric Imaging Assembly (AIA) on board Solar Dynamics Observatory (SDO). From the evolutionary study of HMI intensity and AIA channels, it is observed that the AR consists of two major rotating sunspots one connected to flare-prone region and another with CME. The constructed space-time intensity maps reveal that the sunspots exhibited peak rotation rates coinciding with the occurrence of the major eruptive events. Further, temporal profiles of twist parameters, viz., average shear angle, $\alpha_{\rm av}$, $\alpha_{\rm best}$, derived from HMI vector magnetograms and the rate of helicity injection, obtained from the horizontal flux motions of HMI line-of-sight magnetograms, corresponded well with the rotational profile of the sunspot in CME-prone region, giving predominant evidence of rotational motion to cause magnetic non-potentiality. Moreover, mean value of free-energy from the Virial theorem calculated at the photospheric level shows clear step down decrease at the on set time of the flares revealing unambiguous evidence of energy release, intermittently that is stored by flux emergence and/or motions in pre-flare phases. Additionally, distribution of helicity injection is homogeneous in CME prone region while it is not and often changes sign in flare-prone region. This study provides clear picture that both proper and rotational motions of the observed fluxes played significant role to enhance the magnetic non-potentiality of the AR, leading to favorable conditions for the observed transient activity.

Unusual Emissions at Various Energies Prior to the Impulsive Phase of the Large Solar Flare and Coronal Mass Ejection of 4 November 2003

The GOES X28 flare of 4 November 2003 was the largest ever recorded in its class. It produced the first evidence for two spectrally separated emission components, one at microwaves and the other in the THz range of frequencies. We analyzed the pre-flare phase of this large flare, twenty minutes before the onset of the major impulsive burst. This period is characterized by unusual activity in X-rays, sub-THz frequencies, Hα, and microwaves. The CME onset occurred before the onset of the large burst by about 6 min. It was preceded by pulsations of 3 – 5 s periods at sub-THz frequencies together with X-ray and microwave enhancements. The sub-THz pulsations faded out as impulsive bursts were detected at 100 – 300 keV and 7 GHz, close to the time of the first Hα brightening and the CME onset. The activities detected prior to and at the CME onset were located nearly 2 arcmin south of the following large flare, suggesting they were separate events. This unusual activity brings new clues to understanding the complex energy buildup mechanisms prior to the CME onset, occurring at a distinct location and well before the major flare that exploded afterwards.

Quasi-periodic Oscillations of Solar Active Regions in Connection with Their Flare Activity – NoRH Observations

The sunspot-associated sources at the frequency of 17 GHz give information on plasma parameters in the regions of magnetic field about B=2000 G at the level of the chromosphere-corona transition region. The observations of short period (from one to ten minutes) oscillations in sunspots reflect propagation of magnetohydrodynamic (MHD) waves in the magnetic flux tubes of the sunspots. We investigate the oscillation parameters in active regions in connection with their flare activity. We confirm the existence of a link between the oscillation spectrum and flare activity. We find differences in the oscillations between pre-flare and post-flare phases. In particular, we demonstrate a case of powerful three-minute oscillations that start just before the burst. This event is similar to the cases of the precursors investigated by Sych et al. (Astron. Astrophys. 505, 791, 2009). We also found well-defined eight-minute oscillations of microwave emission from sunspot. We interpret our observations in terms of a relationship between MHD waves propagating from sunspots and flare processes.

OBSERVATIONAL PREDICTION OF HIGH MAGNETIC REYNOLDS NUMBER PRE-FLARE RECONNECTION EVENTS: AN APPLICATION OF NITTA'S SELF-SIMILAR RECONNECTION MODEL

We applied the evolutionary model of magnetic reconnection to simple pre-flare reconnection events driven by flux emergence as the first step in inspecting the realizability of the reconnection events predicted by this model. Previous works paid scant attention to the dependence of the magnetic Reynolds number (R*em) on reconnection events. We aim to clarify how the pre-flare phase of reconnection events in the high R*em range that is frequently encountered in astrophysical applications is observed. We clarify that (1) the time variation of the emission measure distribution strongly depends on R*em, (2) the expected light curve for sufficiently low R*em shows a long lifetime property while that for high R*em shows an impulsive property, and (3) in the case of recurrent small reconnection events on the same loop, the released magnetic energy scale is inversely correlated to the rear-end speed of the moving bright point along the loop. Note that other reconnection models cannot totally explain integration of these properties. If evidence of phenomena with these properties can be detected from, e.g., the Hinode observation, it strongly supports the validity of the self-similar reconnection model.

Observations of the December 6, 2006 solar flare: Electron acceleration and plasma heating

The flare plasma temperature calculated from GOES-11 (1.5–12.4 and 3.1–24.8 keV) data is compared with the solar nonthermal fluxes in various energy ranges in the December 6, 2006 event. Particle acceleration and plasma heating episodes took place in the pre-flare and impulsive phases; a hard (ACS SPI > 150 keV) X-ray emission was observed 5 min before the onset of the GOES X-ray flare and was not accompanied by a temperature rise. A close correlation has been found between the flare plasma temperature and the hard X-ray intensity. The temperature delayed by 0.4 min turned out to be directly proportional to the logarithm of the ACS SPI count rate within the first 3 min of the impulsive phase. This shows that the accelerated electrons responsible for the X-ray emission were the main plasma heating source in the pre-flare and impulsive phases. The correlation between the temperature and the hard X-ray intensity disappears after the observation of a resonance peak at a frequency of 245 MHz. Significant electron fluxes may no longer be able to effectively heat the expanding plasma when its density in the interaction region reaches ∼109 cm−3. The observations of the July 23, 2002 and December 5, 2006 events confirm the trends found.