Vasili Lobzin

Direct Radio Probing and Interpretation of the Sun's Plasma Density Profile

The Suns electron number density profile ne (r) is vital for solar physics but not well measured or understood within a few solar radii RS . Here, a new technique extracts ne (r) directly from coronal type III radio bursts for 40 ≤ f ≤ 180 MHz. Unexpectedly, wind-like regions with ne (r – RS )–2 are quite common below 2RS , and coronal type IIIs often have closely linear 1/f – t spectra. The profile ne (r – RS )–2 is consistent with the radio data and simulations and is interpreted in terms of conical flow from localized sources (e.g., UV funnels) close to the photosphere. It is consistent with solar wind acceleration occurring for 2 ≤ r/RS ≤ 10.

AUTOMATIC RECOGNITION OF CORONAL TYPE II RADIO BURSTS: THE AUTOMATED RADIO BURST IDENTIFICATION SYSTEM METHOD AND FIRST OBSERVATIONS

Major space weather events such as solar flares and coronal mass ejections are usually accompanied by solar radio bursts, which can potentially be used for real-time space weather forecasts. Type II radio bursts are produced near the local plasma frequency and its harmonic by fast electrons accelerated by a shock wave moving through the corona and solar wind with a typical speed of ~1000 km s?1. The coronal bursts have dynamic spectra with frequency gradually falling with time and durations of several minutes. This Letter presents a new method developed to detect type II coronal radio bursts automatically and describes its implementation in an extended Automated Radio Burst Identification System (ARBIS 2). Preliminary tests of the method with spectra obtained in 2002 show that the performance of the current implementation is quite high, ~80%, while the probability of false positives is reasonably low, with one false positive per 100-200?hr for high solar activity and less than one false event per 10000?hr for low solar activity periods. The first automatically detected coronal type II radio burst is also presented.

The solar type II radio bursts of 7 March 2012: Detailed simulation analyses

Type II solar radio bursts are often indicators for impending space weather events at Earth. They are consequences of shock waves driven by coronal mass ejections (CMEs) that move outward from the Sun. We simulate such type II radio bursts by combining elaborate three-dimensional (3-D) magnetohydrodynamic (MHD) predictions of realistic CMEs near the Sun with an analytic kinetic radiation theory developed recently. The simulation approach includes the reconstruction of initial solar magnetic fields, the dimensioning of the initial flux rope of the CME with STEREO spacecraft data, and the launch of the CME into an empirical data-driven corona and solar wind. In this paper, we simulate a complicated double CME event (a very fast CME followed by a slower CME without interaction) and the related coronal and interplanetary type II radio bursts that occurred on 7 March 2012. We extend our previous work to show harmonic and interplanetary emission as well as the simulations surprising ability (for these events at least) for predicting emission for two closely spaced CMEs leaving the same active region. We demonstrate that the theory predicts well the observed fundamental and harmonic emission from ∼20 MHz to 50 kHz or from the high corona to near 1 AU. Specifically, the theory predicts flux, frequency, and time variations that are consistent with the presence or absence of observed type II emissions when interfering emissions are absent and are not inconsistent with observations when interfering type III bursts are present. The predicted and observed type II emission is predominantly fundamental for these two events. Harmonic emission occurs for the second CME only for a short time interval, when an extended shock has developed that can drive flank emission. The coronal and interplanetary emission follow closely hyperbolic lines in frequency-time space, consisting of a succession of islands of emission with varying intensity. The islands develop due to competition between the shock moving through varying coronal and solar wind magnetic field structures (e.g., loops and streamers), growth of the driven radio source due to the spherical expansion of the shock, and movement of the active radio sources from the shocks nose to its flanks.

Evidence for Wind-like Regions, Acceleration of Shocks in the Deep Corona, and Relevance of 1/f Dynamic Spectra to Coronal Type II Bursts

Type II radio bursts are produced near the local plasma frequency fp and near 2fp by shocks moving through the corona and solar wind. In the present Letter eight well-defined coronal type II radio bursts (30-300 MHz) are analyzed. Three results are presented. First, it is found that the dependence of the central frequency on time can be fitted to a power-law model, f ∝ (t − t0)−α, with 0.6 ≤ α ≤ 1.3. Assuming a constant shock velocity, these results provide evidence that the density profile ne(r) in the type II source regions closely resembles the solar wind, with ne(r) ∝ r−2. One possible interpretation is that the solar wind starts within a few solar radii of the photosphere, most probably within 1 solar radius. Another relies on a gasdynamic Whitham analysis and demonstrates a possibility for blast shocks to accelerate, thereby reducing apparent power-law indices to solar wind-like values. Second, for the events considered it is found that radio burst emission in the form of 1/f versus t dynamic spectra closely follows straight lines. In future this will allow much more objective identification of type II bursts in solar radio data and plausibly real-time correlation with coronagraph and other solar radar. Third, it is demonstrated that 1/f versus t dynamic spectra can provide direct evidence for acceleration of the shock deep in the corona, thus complementing coronagraph studies.

RIEGER-TYPE PERIODICITY IN THE OCCURRENCE OF SOLAR TYPE III RADIO BURSTS

This Letter presents the first observations of a Rieger-type periodicity with the period of days in the occurrence rate of solar coronal type III radio bursts. The periodicity was detected during the time interval from 2000 June 22 to 2003 December 31. This interval partially contains the maximum and the declining phase of solar cycle 23. The radio spectra were provided by the Learmonth Solar Radio Observatory in Western Australia, part of the USAF Radio Solar Telescope Network.

EVIDENCE FOR GENTLY SLOPING PLASMA DENSITY PROFILES IN THE DEEP CORONA: TYPE III OBSERVATIONS

Type III radio bursts are produced near the local electron plasma frequency fp and near its harmonic 2fp by fast electrons ejected from the solar active regions and moving through the corona and solar wind. The coronal bursts have dynamic spectra with frequency rapidly falling with time, the typical duration being about 1-3 s. In the present paper, 37 well-defined coronal type III radio bursts (25-450 MHz) are analyzed. The results obtained substantiate an earlier statement that the dependence of the central frequency of the emission on time can be fitted to a power-law model, f(t) ∝ (t – t 0)–α, where α can be as low as 1. In the case of negligible plasma acceleration and conical flow, it means that the electron number density within about 1 solar radius above the photosphere will decrease as r –2, like in the solar wind. For the data set chosen, the index α varies in the range from 0.2 to 7 or bigger, with mean and median values of 1.2 and 0.5, respectively. A surprisingly large fraction of events, 84%, have α ≤ 1.2. These results provide strong evidence that in the type III source regions the electron number density scales as n(r) ∝ (r – r 0)–β, with minimum, mean, and median β = 2α of 0.4, 2.4, and 1.0, respectively. Hence, the typical density profiles are more gently sloping than those given by existing empirical coronal models. Several events are found with a wind-like dependence of burst frequency on time. Smaller power-law indices could result from the effects of non-conical geometry of the plasma flow tubes, deceleration of coronal plasma, and/or the curvature of the magnetic field lines. The last effect is shown to be too weak to explain such low power-law indices. A strong tendency is found for bursts from the same group to have similar power-law indices, thereby favoring the hypothesis that they are usually produced by the same source region.

SOLAR CYCLE VARIATIONS OF THE OCCURRENCE OF CORONAL TYPE III RADIO BURSTS AND A NEW SOLAR ACTIVITY INDEX

This Letter presents the results of studies of solar cycle variations of the occurrence rate of coronal type III radio bursts. The radio spectra are provided by the Learmonth Solar Radio Observatory (Western Australia), part of the USAF Radio Solar Telescope Network (RSTN). It is found that the occurrence rate of type III bursts strongly correlates with solar activity. However, the profiles for the smoothed type III burst occurrence rate differ considerably from those for the sunspot number, 10.7 cm solar radio flux, and solar flare index. The type III burst occurrence rate (T3BOR) is proposed as a new index of solar activity. T3BOR provides complementary information about solar activity and should be useful in different studies including solar cycle predictions and searches for different periodicities in solar activity. This index can be estimated from daily results of the Automated Radio Burst Identification System. Access to data from other RSTN sites will allow processing 24 hr radio spectra in near-real time and estimating true daily values of this index. It is also shown that coronal type III bursts can even occur when there are no visible sunspots on the Sun. However, no evidence is found that the bursts are not associated with active regions. It is also concluded that the type III burst productivity of active regions exhibits solar cycle variations.

Low Altitude Solar Magnetic Reconnection, Type III Solar Radio Bursts, and X-ray Emissions.

Type III solar radio bursts are the Sun’s most intense and frequent nonthermal radio emissions. They involve two critical problems in astrophysics, plasma physics, and space physics: how collective processes produce nonthermal radiation and how magnetic reconnection occurs and changes magnetic energy into kinetic energy. Here magnetic reconnection events are identified definitively in Solar Dynamics Observatory UV-EUV data, with strong upward and downward pairs of jets, current sheets, and cusp-like geometries on top of time-varying magnetic loops, and strong outflows along pairs of open magnetic field lines. Type III bursts imaged by the Murchison Widefield Array and detected by the Learmonth radiospectrograph and STEREO B spacecraft are demonstrated to be in very good temporal and spatial coincidence with specific reconnection events and with bursts of X-rays detected by the RHESSI spacecraft. The reconnection sites are low, near heights of 5–10 Mm. These images and event timings provide the long-desired direct evidence that semi-relativistic electrons energized in magnetic reconnection regions produce type III radio bursts. Not all the observed reconnection events produce X-ray events or coronal or interplanetary type III bursts; thus different special conditions exist for electrons leaving reconnection regions to produce observable radio, EUV, UV, and X-ray bursts.

Automatic recognition of complex magnetic regions on the Sun in SDO magnetogram images and prediction of flares: Techniques and results for the revised flare prediction program Flarecast

In the present paper, solar magnetograms provided by the Helioseismic and Magnetic Imager (HMI) on-board Solar Dynamics Observatory (SDO) spacecraft are used to identify active regions automatically by thresholding the line-of-sight component of the solar magnetic field. The flare potential of the regions is predicted by locating potential active regions with strong-gradient polarity inversion lines (SPILs) and estimating 18 physically relevant parameters of these regions. In particular, parameters of interest include the sum of north-south gradients, sum of east-west gradients, length of SPIL, and total integrated magnetic flux. For deterministic prediction of flares, analysis for thresholding of single parameters and different combinations, which include up to 4 parameters, are presented and compared. If the false alarm rate does not exceed 10% (20%), the probabilities for correct prediction of X-ray flares of class M and greater, M5 and greater, and X in the 24 h window are 71% (86%), 84% (96%), and 94% (100%), respectively. These probabilities are for the best 4-parameter technique found. A technique for probabilistic forecasting was also developed. These deterministic and probabilistic techniques will be implemented in a revised version of the flare warning program, Flarecast, which will be operational in the Australian Space Forecast Centre.

Automatic recognition of type III solar radio bursts in STEREO/WAVES data for onboard real‐time and archived data processing

Type III radio bursts are produced near the local electron plasma frequency and/or near its harmonic by fast electrons ejected from the solar active regions and moving through the corona and solar wind. These bursts have dynamic spectra with frequency rapidly falling with time. This paper presents two new methods developed to detect type III bursts automatically in the data from High Frequency Receiver (HFR) of the STEREO/WAVES radio instrument onboard the STEREO spacecraft. The first technique is applicable to the low-frequency band (HFR-1: 125 kHz to 1.975 MHz) only. This technique can possibly be implemented in onboard satellite software aimed at preliminary detection of bursts and identification of time intervals with relatively high solar activity. In the second technique the bursts are detected in both the low-frequency band and the high-frequency band (HFR-2: 2.025 MHz to 16.025 MHz), with the computational burden being higher by 1 order of magnitude as compared with that for the first technique. Preliminary tests of the method show that the performance of the first technique is quite high, PdL=72%±3%. The performance of the second technique is considerably higher, PdL+H=81%±1%, while the number of false alarms does not exceed 10% for one daily spectrum.