Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userConstraints on the acceleration region of type III radio bursts from decimetric radio spikes and faint X-ray bursts
76
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We study the release of energy during the gradual phase of a flare, characterized by faint bursts of non-thermal hard X-ray (HXR) emission associated with decimetric radio spikes and type III radio bursts starting at high frequencies and extending to the heliosphere. We characterize the site of electron acceleration in the corona and study the radial evolution of radio source sizes in the high corona. Imaging and spectroscopy of the HXR emission with Fermi and RHESSI provide a diagnostic of the accelerated electrons in the corona as well as a lower limit on the height of the acceleration region. Radio observations in the decimetric range with the ORFEES spectrograph provide radio diagnostics close to the acceleration region. Radio spectro-imaging with LOFAR in the meter range provide the evolution of the radio source sizes with their distance from the Sun, in the high corona. Non-thermal HXR bursts and radio spikes are well correlated on short timescales. The spectral index of non-thermal HXR emitting electrons is -4 and their number is about $2times 10^{33}$ electrons/s. The density of the acceleration region is constrained between $1-5 times 10^9$ cm$^{-3}$. Electrons accelerated upward rapidly become unstable to Langmuir wave production, leading to high starting frequencies of the type III radio bursts, and the elongation of the radio beam at its source is between 0.5 and 11.4 Mm. The radio source sizes and their gradient observed with LOFAR are larger than the expected size and gradient of the size of the electron beam, assuming it follows the expansion of the magnetic flux tubes. These observations support the idea that the fragmentation of the radio emission into spikes is linked to the fragmentation of the acceleration process itself. The combination of HXR and radio diagnostics in the corona provides strong constrains on the site of electron acceleration.
rate research
Read More
The Sun is an active source of radio emission which is often associated with the acceleration of electrons arising from processes such as solar flares and coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), numerous solar S bursts (where S stands for short) and storms of Type III radio bursts have been observed, that are not directly relates to flares and CMEs. Here, we expand our understanding on the spectral characteristic of these two different types of radio bursts based on observations from the Low Frequency Array (LOFAR). On 9 July 2013, over 3000 solar S bursts accompanied by over 800 Type III radio bursts were observed over a time period of ~8 hours. The characteristics of Type III radio bursts are consistent to previous studies, while S bursts show narrow bandwidths, durations and drift rates of about 1/2 the drift rate of Type III bursts. Type III bursts and solar S bursts occur in a region in the corona where plasma emission is the dominant emission mechanism as determined by data constrained density and magnetic field models.
Type III bursts and hard X-rays are both produced by flare energetic electron beams. The link between both emissions has been investigated in many previous studies, but no statistical studies have compared both coronal and interplanetary type III bursts with X-ray flares. Using coronal radio events above 100 MHz exclusively from type III bursts, we revisited long-standing questions: Do all coronal type III bursts have X-ray counterparts. What correlation, if any, occurs between radio and X-ray intensities. What X-ray and radio signatures above 100 MHz occur in connection with interplanetary type III bursts below 14 MHz. We analysed data from 2002 to 2011 starting with coronal type III bursts above 100 MHz. We used RHESSI X-ray data greater than 6 keV to make a list of 321 events that have associated type III bursts and X-ray flares, encompassing at least 28 percent of the initial sample of type III events. We examined the timings, intensities, associated GOES class, and any interplanetary radio signature. For our 321 events, the X-ray emission at 6 keV usually lasted longer than type III burst groups at frequencies greater than 100 MHz. A weak correlation was found between the type III radio flux at frequencies below 327 MHz and the X-ray intensity at 25-50 keV, with an absence of events at high X-ray intensity and low type III radio flux. Interplanetary type III bursts less than 14 MHz were observed for 54 percent of the events, increasing when events were observed with 25-50 keV X-rays. A stronger interplanetary association was present when 25-50 keV count rates were above 250 counts per second or 170 MHz fluxes were greater than 1000 SFU, relating to more energetic electrons above 25 keV and events where magnetic flux tubes extend into the high corona. On average type III bursts increase in flux with decreasing frequency, the rate varies from event to event.
Type III radio bursts are intense radio emissions triggered by beams of energetic electrons often associated with solar flares. These exciter beams propagate outwards from the Sun along an open magnetic field line in the corona and in the interplanetary (IP) medium. We performed a statistical survey of 29 simple and isolated IP type III bursts observed by STEREO/Waves instruments between January 2013 and September 2014. We investigated their time-frequency profiles in order to derive the speed and acceleration of exciter electron beams. We show these beams noticeably decelerate in the IP medium. Obtained speeds range from $sim$ 0.02c up to $sim$ 0.35c depending on initial assumptions. It corresponds to electron energies between tens of eV and hundreds of keV, and in order to explain the characteristic energies or speeds of type III electrons ($sim 0.1$c) observed simultaneously with Langmuir waves at 1 au, the emission of type III bursts near the peak should be predominately at double plasma frequency. Derived properties of electron beams can be used as input parameters for computer simulations of interactions between the beam and the plasma in the IP medium.
We analyze radio bursts observed in events with interacting/non-interacting CMEs that produced major SEPs (Ip $>$ 10 MeV) fromApril 1997 to December 2014.We compare properties of meter (m), deca-hectometer (DH) type II as well as DH type III bursts, and time lags for interacting-CME-associated (IC) events and non-interacting-CME-associated (NIC) events. About 70% of radio emissions were observed in events of both types from meters to kilometers. We found high correlations between the drift rates and mid-frequencies of type II radio bursts calculated as the mean geometric between their starting and ending frequencies for both NIC and IC-associated events (Correlation coefficient textit{R}$^{2}$ = 0.98, power-law index $varepsilon$ = 1.68 $pm $ 0.16 and textit{R}$^{2}$ = 0.93, $varepsilon$ = 1.64 $pm $ 0.19 respectively).We also found a correlation between the frequency drift rates of DH type II bursts and space speeds of CMEs in NIC-associated events. The absence of such correlation for IC-associated events confirms that the shock speeds changed in CME--CME interactions. For the events with western source locations, the mean peak intensity of SEPs in IC-associated events is four times larger than that in NIC-associated SEP events. From the mean time lags between the start times of SEP events and the start of m, DH type II, and DH type III radio bursts, we inferred that particle enhancements in NIC-associated SEP events occurred earlier than in IC-associated SEP events. The difference between NIC events and IC events in the mean values of parameters of type II and type III bursts is statistically insignificant.
There is a wide consensus that the ubiquitous presence of magnetic reconnection events and the associated impulsive heating (nanoflares) is a strong candidate for solving the solar coronal heating problem. Whether nanoflares accelerate particles to high energies like full-sized flares is unknown. We investigate this question by studying the type III radio bursts that the nanoflares may produce on closed loops. The characteristic frequency-drifts that type III bursts exhibit can be detected using a novel application of the time-lag technique developed by Viall & Klimchuk (2012) even when there are multiple overlapping bursts. We present a simple numerical model that simulates the expected radio emission from nanoflares in an active region (AR), which we use to test and calibrate the technique. We find that in the case of closed loops the frequency spectrum of type III bursts is expected to be extremely steep such that significant emission is produced at a given frequency only for a rather narrow range of loop lengths. We also find that the signature of bursts in the time-lag signal diminishes as: (1)the variety of participating loops within that range increases; (2)the occurrence rate of bursts increases; (3) the duration of bursts increases; and (4) the brightness of the bursts decreases relative to noise. In addition, our model suggests a possible origin of type I bursts as a natural consequence of type III emission in a closed-loop geometry.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
