Solar Noise Storms Research Articles

New Insights into Type-I Solar Noise Storms from High Angular Resolution Spectroscopic Imaging with the Upgraded Giant Metrewave Radio Telescope

Type-I solar noise storms are perhaps the most commonly observed active radio emissions from the Sun at meter-wavelengths. Noise storms have a long-lived and wideband continuum background with superposed islands of much brighter narrowband and short-lived emissions, known as type-I bursts. There is a serious paucity of studies focusing on the morphology of these two types of emissions, primarily because of the belief that coronal scattering will always wash out any features at small angular scales. However, it is important to investigate their spatial structures in detail to make a spatio-temporal connection with observations at extreme-ultraviolet/X-ray bands to understand the detailed nature of these emissions. In this work, we use high angular resolution observations from the upgraded Giant Metrewave Radio Telescope to demonstrate that it is possible to detect structures with angular scales as small as ∼9″, about three times smaller than the smallest structure reported to date from noise storms. Our observations also suggest that while the individual type-I bursts are narrowband in nature, the bursts are probably caused by traveling disturbance(s) inducing magnetic reconnections at different coronal heights, and thus leading to correlated change in the morphology of the type-I bursts observed at a wide range of frequencies.

Deriving Large Coronal Magnetic Loop Parameters Using LOFAR J Burst Observations

Large coronal loops around one solar radius in altitude are an important connection between the solar wind and the low solar corona. However, their plasma properties are ill-defined, as standard X-ray and UV techniques are not suited to these low-density environments. Diagnostics from type J solar radio bursts at frequencies above 10 MHz are ideally suited to understand these coronal loops. Despite this, J-bursts are less frequently studied than their type III cousins, in part because the curvature of the coronal loop makes them unsuited for using standard coronal density models. We used LOw-Frequency-ARray (LOFAR) and Parker Solar Probe (PSP) solar radio dynamic spectrum to identify 27 type III bursts and 27 J-bursts during a solar radio noise storm observed on 10 April 2019. We found that their exciter velocities were similar, implying a common acceleration region that injects electrons along open and closed magnetic structures. We describe a novel technique to estimate the density model in coronal loops from J-burst dynamic spectra, finding typical loop apex altitudes around 1.3 solar radius. At this altitude, the average scale heights were 0.36 solar radius, the average temperature was around 1 MK, the average pressure was 0.7~text{mdyn},text{cm}^{-2}, and the average minimum magnetic field strength was 0.13 G. We discuss how these parameters compare with much smaller coronal loops.

Insights from Snapshot Spectroscopic Radio Observations of a Weak Type I Solar Noise Storm

Abstract We present a high-fidelity snapshot spectroscopic radio imaging study of a weak type I solar noise storm that took place during an otherwise exceptionally quiet time. Using high-fidelity images from the Murchison Widefield Array, we track the observed morphology of the burst source for 70 minutes and identify multiple instances where its integrated flux density and area are strongly anticorrelated with each other. The type I radio emission is believed to arise due to electron beams energized during magnetic reconnection activity. The observed anticorrelation is interpreted as evidence for presence of MHD sausage wave modes in the magnetic loops and strands along which these electron beams are propagating. Our observations suggest that the sites of these small scale reconnections are distributed along the magnetic flux tube. We hypothesize that small scale reconnections produces electron beams which quickly get collisionally damped. Hence, the plasma emission produced by them span only a narrow bandwidth and the features seen even a few mehahertz apart must arise from independent electron beams.

Discovery of Correlated Evolution in Solar Noise Storm Source Parameters: Insights on Magnetic Field Dynamics during a Microflare

Abstract A solar type-I noise storm is produced by accelerated particle beams generated at active regions undergoing magnetic field restructuring. Their intensity varies by orders of magnitude within subsecond and sub-MHz scales. But the morphological evolution of these sources is not studied at these scales due to the lack of required imaging cadence and fidelity in meterwave bands. Using data from the Murchison Widefield Array, this work explores the coevolution of size, sky-orientation, and intensity of a noise storm source associated with a weak microflare. This work presents the discovery of two correlated modes of evolution in the source parameters: a sausage like “S” mode where the source intensity and size show an anticorrelated evolution; and a torsional like “T” mode where the source size and sky-orientation show a correlated evolution. A flare mediated mode conversion is observed from “T” to “S” for the first time in these sources. These results support the idea of build up of magnetic stress energy in braided active region loops, which later become unstable causing flares and particle acceleration until they relax to a minimally braided state. The discovered mode conversion can be a future diagnostic for such events.

Spectropolarimetric Observations of Solar Noise Storms at Low Frequencies

A new high-resolution radio spectropolarimeter instrument operating in the frequency range of 15 – 85 MHz has recently been commissioned at the Radio Astronomy Field Station of the Indian Institute of Astrophysics at Gauribidanur, 100 km north of Bangalore, India. We describe the design and construction of this instrument. We present observations of a solar radio noise storm associated with Active Region (AR) 12567 in the frequency range of \({\approx}\,15\,\mbox{--}\,85~\mbox{MHz}\) during 18 and 19 July 2016, observed using this instrument in the meridian-transit mode. This is the first report that we are aware of in which both the burst and continuum properties are derived simultaneously. Spectral indices and degree of polarization of both the continuum radiation and bursts are estimated. It is found that i) Type I storm bursts have a spectral index of \({\approx}\,{+}3.5\), ii) the spectral index of the background continuum is \({\approx}\,{+}2.9\), iii) the transition frequency between Type I and Type III storms occurs at \({\approx}\,55~\mbox{MHz}\), iv) Type III bursts have an average spectral index of \({\approx}\,{-}2.7\), v) the spectral index of the Type III continuum is \({\approx}\,{-}1.6\), and vi) the degree of circular polarization of all Type I (Type III) bursts is \({\approx}\,90\%\) (\(30\%\)). The results obtained here indicate that the continuum emission is due to bursts occurring in rapid succession. We find that the derived parameters for Type I bursts are consistent with suprathermal electron acceleration theory and those of Type III favor fundamental plasma emission.

The structure of solar radio noise storms

Context. The Nançay Radioheliograph (NRH) routinely produces snapshot images of the full sun (field of view ~3 R⊙) at 6 or 10 frequencies between 150 and 450 MHz, with typical resolution 3 arcmin and time cadence 0.2 s. Combining visibilities from the NRH and from the Giant Meterwave Radio Telescope (GMRT) allows us to produce images of the sun at 236 or 327 MHz, with the same field as the NRH, a resolution as low as 20 arcsec, and a time cadence 2 s.

Length scales for September 14, 2005 solar noise storm

Source length scales are estimated for the September 14, 2005 solar noise storm from the spectral and temporal observed characteristics of the background continuum fluctuations and clusters of Type I bursts. The characteristic height of the magnetic structure where the noise storm source is located and the size of the source where Type I bursts clustering takes place were calculated. A lower limit for the height of the magnetic structure supporting the noise storm at 237MHz was estimated too.

SOLAR RADIO TYPE-I NOISE STORM MODULATED BY CORONAL MASS EJECTIONS

The first coordinated observations of an active region using ground-based radio telescopes and the Solar Terrestrial Relations Observatory (STEREO) satellites from different heliocentric longitudes were performed to study solar radio type-I noise storms. A type-I noise storm was observed between 100 and 300 MHz during a period from 2010 February 6 to 7. During this period the two STEREO satellites were located approximately 65° (ahead) and –70° (behind) from the Sun-Earth line, which is well suited to observe the earthward propagating coronal mass ejections (CMEs). The radio flux of the type-I noise storm was enhanced after the preceding CME and began to decrease before the subsequent CME. This time variation of the type-I noise storm was directly related to the change of the particle acceleration processes around its source region. Potential-field source-surface extrapolation from the Solar and Heliospheric Observatory/Michelson Doppler Imager (SOHO/MDI) magnetograms suggested that there was a multipolar magnetic system around the active region from which the CMEs occurred around the magnetic neutral line of the system. From our observational results, we suggest that the type-I noise storm was activated at a side-lobe reconnection region that was formed after eruption of the preceding CME. This magnetic structure was deformed by a loop expansion that led to the subsequent CME, which then suppressed the radio burst emission.

Acceleration of charged particles by fluctuating and steady electric fields in a X-type magnetic field

Release of stored magnetic energy via particle acceleration is a characteristic feature of astrophysical plasmas. Magnetic reconnection is one of the mechanisms for releasing energy from magnetized plasmas. Collisionless magnetic reconnection could provide both the energy release mechanism and the particle accelerator in space plasmas. Here we studied particle acceleration when fluctuating (in-time) electric fields are superposed on an static X-type magnetic field in collisionless hot solar plasma. This system is chosen to mimic the reconnective dissipation of a linear MHD disturbance. Our results are compared to particle acceleration from constant electric field superposed on an X-type magnetic field. The constant electric field configuration represents the effects of steady state magnetic reconnection. Time evolution of ion and electron distributions are obtained by numerically integrating particle trajectories. The frequencies of the electric field represent a turbulent range of waves. Depending on the frequency and amplitude of the electric field, electrons and ions are accelerated to different degrees and have energy distributions of bimodal form consisting of a lower energy part and a high energy tail. For frequencies ( ω in dimensioless units) in the range 0.5 ⩽ ω ⩽ 1.0 a substantial fraction (20%–30%) of the proton distribution is accelerated to gamma-ray producing energies. For frequencies in the range 1 ⩽ ω ⩽ 100.0 the bulk of the electron distribution is accelerated to hard X-ray producing energies. The acceleration mechanism is important for solar flares and solar noise storms but it could be applicable to all collisionless astrophysical plasmas.

A single picture for solar coronal outflows and radio noise storms

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Properties of the generation mechanism of solar noise storms

A technique for analyzing the stochastic time function by means of combined time and spectral description of this function is developed. It is shown that the modulation of radio emission in the solar atmosphere by wave processes or by inhomogeneities of a beam of excitation particles, as well as resonance oscillations of “frozen” plasma in magnetic arches of the active region, lead to duration distributions and spectra that differ from the observed characteristics of noise storms; that is, these mechanisms do not explain their origin. It was found that the quadratic distribution of the duration of noise storm bursts and its hyperbolic spectrum can be a consequence of random refraction of radio waves by inhomogeneities of turbulent coronal plasma.

Possible link between solar decimeter-wave microbursts and noise storms

A statistical study of long-lived solar microbursts (MB) at decimeter-wave frequencies has been performed for the first time. The data are obtained with the radio telescope RATAN-600 and have the form of one-dimensional scans of intensity and circular polarization with a sensitivity of about 5–10 Jy. MB fluxes and polarization degrees lie in the 0.001–0.1 s.f.u. and 10% to 100% intervals, respectively, and individual pulses have durations of one to two seconds. Microbursts can be observed in the same active region over several days. MB are compared to noise storms (NS) at meter-wave frequencies. The analysis indicates that MB are NS manifestations at decimeter-wave frequencies. The possible cause of the significant difference between the radio fluxes of MB and NS is analyzed. It is shown within the framework of a unified model of the generation of MB and NS radio waves that unlike type I bursts MB are associated with the incoherent mechanism of the generation of Langmuir waves. MB emission is, by its nature, closer to the continuous emission of noise storms, but it is pulse-like because of the high rate of pitch-angular diffusion.

Spectrum of Fluctuations of Solar Noise Storms

The calculation of the Fourier transform of noise storm (NS) fluctuations showed that the power spectrum was adequately described by the expression G(F)∼1/F. Our results rule out the possibility that NS radiation is formed from random, short-term bursts (so-called type I bursts), since the spectrum of the sum of random short fluctuations is flat, but the real NS has a hyperbolic spectrum. This spectrum is monotonic and does not contain any components that exceed the level of the statistical fluctuations (i.e., the results of observations do not reveal the presence of periodic or resonant properties of the emission source). The hyperbolic shape of the spectrum shows that the main energy of a NS is contained in the slower temporal fluctuations.

Influence of the random refraction of radio waves in the corona on the properties of solar noise storms

Scattering of radio waves on irregularities in the coronal electron density is inevitably accompanied by variations of scattered signal intensity. According to the probability theory, the scattered signal intensity distribution is described by an exponential law. The empirical intensity distribution in the majority of the observed noise storms is also adequately described by an exponential law, and this fact counts in favor of the hypothesis that the burst component of noise storms is formed as a result of the scattering of the radio radiation from quasi-stable point sources on coronal irregularities.

Particle acceleration by fluctuating electric fields at a magnetic field null point

Particle acceleration consequences from fluctuating electric fields superposed on an X-type magnetic field in collisionless solar plasma are studied. Such a system is chosen to mimic generic features of dynamic reconnection, or the reconnective dissipation of a linear disturbance. We explore numerically the consequences for charged particle distributions of fluctuating electric fields superposed on an X-type magnetic field. Particle distributions are obtained by numerically integrating individual charged particle orbits when a time varying electric field is superimposed on a static X-type neutral point. This configuration represents the effects of the passage of a generic MHD disturbance through such a system. Different frequencies of the electric field are used, representing different possible types of wave. The electric field reduces with increasing distance from the X-type neutral point as in linear dynamic magnetic reconnection. The resulting particle distributions have properties that depend on the amplitude and frequency of the electric field. In many cases a bimodal form is found. Depending on the timescale for variation of the electric field, electrons and ions may be accelerated to different degrees and often have energy distributions of different forms. Protons are accelerated to $\gamma$-ray producing energies and electrons to and above hard X-ray producing energies in timescales of 1 second. The acceleration mechanism is possibly important for solar flares and solar noise storms but is also applicable to all collisionless plasmas.

Spectral density of the burst component of solar noise storms

A spectral analysis of the radio noise storm (NS) fluctuations has shown that the power spectrum of any NS is not flat but hyperbolic and is satisfactorily described by the expression G(F) ∼ c/F. The spectrum is monotonic and contains no components exceeding the level of statistical fluctuations, i.e., the observations reveal no steady periodic or resonant properties of the emission source. Therefore, the universally accepted assumption about the NS formation from short type I bursts is in conflict with the observations, because the spectrum of the sum of short pulses is flat, while the total energy of all short bursts with durations of the order of one second in actual NSs accounts for only 3–5% of the total energy of the burst component. The remainining 95% of the energy is emitted as long-lived bursts with durations from 1–2 to 300 s. The listed NS properties are inconsistent with the hypothesis of their emission through the action of nanoflares, because the time it takes for the bulk of the energy to be released as pulses with durations >10 s exceeds considerably the lifetime of the events called nanoflares.

Energy of solar noise-storm radio bursts

Solar noise storms (NS) are analyzed by an algorithm which separates a random signal into pulses. The burst duration distribution is shown to be inversely proportional to the squared duration of bursts. The distribution ordinates are proportional to the average pulse repetition frequency, and the distribution maximum corresponds to the limiting pulse duration equal to 0.4–0.6 s. The aggregate lifetime of all short-lasting bursts is approximately equal to the aggregate lifetime of bursts of any other duration. The energy of short-lasting bursts with a duration of 0.2–0.4 s is five times smaller than the energy of longer bursts, and it constitutes only 2–5 percent of the energy of the NS burst component. The power of bursts increases as their duration changes from 0.2 to 1.2 s until it reaches some limit at a duration of 1.2–1.4 s. The power of longer bursts remains almost unchanged up to the end of the investigated duration interval (up to durations of 300 s). Solar burst chains can be some superposition of short-lasting bursts on one longer burst. Thus, the burst energy measurements do not support the widespread point of view that solar noise storms consist of short-lasting type I bursts.

The Post-Coronal Mass Ejection Solar Atmosphere and Radio Noise Storm Activity

We carried out a statistical study of solar radio noise storms whose onset was in the aftermath of coronal mass ejections (CMEs) that occurred during 1997-2004, the first half of present solar cycle 23. The work is an attempt to understand the post-CME corona through observations of noise storms since the latter are considered to be closely related to structural changes there. The radio events were taken as the starting point for our study, and details about start time and location were available for 340 of them. We imposed the following conditions to verify the association between the above two phenomena: (1) the noise storm must have occurred ≤24 hr from the onset of a CME and (2) the central position angle of the CME must be located inside an angular span of ±45° with respect to the noise storm. We found that 196/340 noise storms were associated with CMEs. More interestingly, the time interval between CME liftoff and noise storm onset in all the above cases was ≤13 hr. We suggest that this represents the upper bound of the timescale over which coronal magnetic field reorganization had taken place in the aftermath of the aforementioned 196 noise storm associated CMEs. We also found that for a particular CME, the above temporal cutoff depends on its kinetic energy. Overall, it varies inversely with the logarithm of CME kinetic energy.

Further Constraints on Electron Acceleration in Solar Noise Storms

We reexamine the energetics of nonthermal-electron acceleration in solar noise storms. A new result is obtained for the minimum nonthermal-electron number density required to produce a Langmuir-wave population of sufficient intensity to power the noise-storm emission. We combine this constraint with the stochastic electron acceleration formalism developed by Subramanian and Becker (2005) to derive a rigorous estimate for the efficiency of the overall noise-storm emission process, beginning with nonthermal-electron acceleration and culminating in the observed radiation. We also calculate separate efficiencies for the electron acceleration–Langmuir wave generation stage and the Langmuir wave–noise-storm production stage. In addition, we obtain a new theoretical estimate for the energy density of the Langmuir waves in noise-storm continuum sources.

Frequency and time profiles of metric wave isolated Type I solar noise storm bursts at high spectral and temporal resolution

Type I noise storms constitute a sizeable fraction of the active Solar radio emission component. Observations of isolated instances of such bursts, in the swept-frequency mode at metric wavelengths, have remained sparse, with several unfilled regions in the frequency coverage. Dynamic spectra of the burst radiation in the 30–130 MHz band, obtained from the recently commissioned digital High Resolution Spectrograph at the Gauribidanur Radio Observatory, have unravelled in explicit detail the temporal and spectral profiles of isolated bursts thanks to the instrument's superior frequency and time resolution. Apart from presenting details of their fundamental emission features, the time- and frequency-profile symmetry, with reference to custom-specific Gaussian distributions, has been chosen as the nodal criterion to statistically explain the state of the source regions in the vicinity of magnetic reconnections, the latent excitation agent that contributes to plasma-wave energetics, and the quenching phenomenon that causes damping of the burst emission.