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Types of Microwave Quasi-Periodic Pulsations in Single Flaring Loops  

Victor Melnikov   Submitted: 2010-12-18 03:03

Quasi-periodic pulsations (QPP) of microwave emission generated in single flaring loops observed with the Nobeyama Radioheliograph (NoRH) and Nobeyama Radio Polarimeters (NoRP) are studied. Specific features of the time profiles, i.e. the visible presence or absence of QPPs, are not accounted for in the selection. The time evolution of the periods of the QPPs is examined using wavelet and correlation analyses. In ten out of twelve considered events, at least one or more significant spectral components with periods from 5 ? 60 s have been found. The quality of the oscillations is rather low: Q= πN, where N is the number of cycles, mostly varies in the range 12 to 40, with an average of 25.We suggest that the detected QPPs can be classified into four types: i) those with stable mean periods (e.g. of 15 ? 20 s or 8 ? 9 s, the prevailing type); ii) those with spectral drift to shorter periods (mostly in the rise phase of the microwave emission); iii) those with drift to longer periods (mostly in the decay phase); iv) those with multiple periods showing an X-shaped drift (e.g. in the range from 20 ? 40 s in the rise phase).

Authors: E.G. Kupriyanova, V.F. Melnikov, V.M. Nakariakov, K. Shibasaki
Projects: None

Publication Status: Solar Phys (2010) 267: 329?342
Last Modified: 2010-12-19 11:02
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Dynamics of the flaring loop system of 2005 August 22 observed in microwaves and hard X-rays  

Victor Melnikov   Submitted: 2010-12-09 00:03

We studied the spatial dynamics of the flaring loop in the 2005 August 22 event using microwave (NoRH) and hard X-ray (RHESSI) observations together with complementary data from SOHO/MDI, SMART at Hida, SOHO/EIT, and TRACE. We have found that (1) the pre-flare morphology of the active region exhibits a strongly sheared arcade seen in Hα and the J-shape filament seen in EUV; (2) energy release and high-energy electron acceleration occur in a sequence along the extensive arcade; (3) the shear angle and the parallel (to the magnetic neutral line) component of the footpoint (FP) distance steadily decrease during the flare process; (4) the radio loop shrinks in length and height during the first emission peak, and later it grows; after the fourth peak the simultaneous descending of the brightest loop and formation of a new microwave loop at a higher altitude occur; (5) the hard X-ray coronal source is located higher than the microwave loop apex and shows faster upward motion; (6) the first peak on microwave time profiles is present in both the loop top and FP regions. However, the emission peaks that follow are present only in the FP regions. We conclude that after the first emission peak the acceleration site is located over the flaring arcade and particles are accelerated along magnetic field lines. We make use of the collapsing magnetic trap model to understand some observational effects.

Authors: V. E. Reznikova, V. F. Melnikov, H. Ji, and K. Shibasaki
Projects: RHESSI

Publication Status: ApJ. 2010, V.724, P.171-181
Last Modified: 2010-12-09 11:18
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Quasi-Periodic Pulsations in Solar Flares  

Victor Melnikov   Submitted: 2009-07-26 06:48

Quasi-periodic pulsations (QPP) are a common feature of flaring energy releases in the solar atmosphere, observed in all bands, from radio to hard X-ray. In this review we concentrate on QPP with the periods longer than one second. Physical mechanisms responsible for the generation of long QPP split into two groups: ?load/unload? mechanisms and MHD oscillations. Load/unload mechanisms are repetitive regimes of flaring energy releases by magnetic reconnection or by other means. MHD oscillations can affect all elements of the flaring emission generation: triggering of reconnection and modulation of its rate, acceleration and dynamics of non-thermal electrons, and physical conditions in the emitting plasmas. In the case of MHD oscillations, the periodicity of QPP is determined either by the presence of some resonances, e.g. standing modes of plasma structures, or by wave dispersion. Periods and other parameters of QPP are linked with properties of flaring plasmas and their morphology. Observational investigation of the QPP generation mechanisms based upon the use of spatial information, broadband spectral coverage and multi-periodicity is discussed.

Authors: Nakariakov, V. M. and Melnikov, V. F.
Projects: CORONAS-F/SPIRIT,Hinode/XRT,RHESSI,SoHO-MDI,SoHO-SUMER,TRACE,Yohkoh-HXT

Publication Status: Space Science Reviews (accepted), Online First
Last Modified: 2009-07-27 10:48
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Peak Frequency Dynamics in Solar Microwave Bursts  

Victor Melnikov   Submitted: 2009-07-26 06:36

We analyze the dynamics of the broadband frequency spectrum of 338 microwave bursts observed in the years 2001-2002 with the Owens Valley Solar Array. A subset of 38 strong microwave bursts that show a single spectral maximum are studied in detail. Our main goal is to study changes in spectral peak frequency u_pk with time. We show that, for a majority of these simple bursts, the peak frequency shows a high positive correlation with flux density - it increases on the rise phase in ~83% of 24 bursts where it could be cleanly measured, and decreases immediately after the peak time in ~62% of 34 bursts. This behavior is in qualitative agreement with theoretical expectations based on gyrosynchrotron self-absorption. However, for a significant number of events (30-36%) the peak frequency variation is much smaller than expected from self-absorption, or may be entirely absent. The observed temporal behavior of u_pk is compared with a simple model of gyrosynchrotron radio emission. We show that the anomalous behavior is well accounted for by the effects of Razin suppression, and further show how an analysis of the temporal evolution of u_pk can be used to uniquely determine the relative importance of self-absorption and Razin suppression in a given burst. The analysis technique provides a new, quantitative diagnostic for the gyrosynchrotron component of solar microwave bursts. Applying this analysis technique to our sample of bursts, we find that in most of the bursts (60%) the spectral dynamics of u_pk around the time of peak flux density is caused by self-absorption. On the other hand, for a significant number of events (~70%), the Razin effect may play the dominant role in defining the spectral peak and dynamics of u_pk, especially on the early rise phase and late decay phase of the bursts.

Authors: Melnikov V.F., Gary D.E., and Nita G.M.
Projects: None

Publication Status: Solar Physics, V.253, pp.43-73. Erratum corrected.
Last Modified: 2009-07-27 10:48
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2002 August 24 Limb Flare Loop: Dynamics of Microwave Brightness Distribution  

Victor Melnikov   Submitted: 2009-06-22 20:33

High-resolution radio observation of Nobeyama Radioheliograph at 17 and 34 GHz allowed studying the dynamics of microwave brightness distribution along the giant limb flaring loop in the event of 2002 August 24. It is found that on the rising phase of the radio burst the brightness distribution was highly asymmetric, with a strong maximum near the southern footpoint (SFP) and much weaker brightness enhancements near the loop top (LT) and northern footpoint. On the decay phase, the LT gradually became most bright. The similar dynamics of brightness distribution are shown to happen for all major temporal subpeaks of the burst. Results of our diagnostics show two important properties: (1) the number density of mildly relativistic electrons in the LT is much higher than near the footpoints (FPs) during rise, maximum and decay of each major peak; and (2) the ratio of the electron number densities in the LT and an FP increases from the maximum to decay phase. Model simulations with making use of the nonstationary Fokker-Planck equation have allowed us to find the model explaining the major properties of the microwave brightness distribution and dynamics. The model is characterized by a compact source of electrons located near the center of an asymmetric magnetic loop; the source is nonstationary, long lasting, and injecting high-energy electrons with the pitch-angle distribution mostly directed toward the SFP but also having a very weak isotropic component. This easily explains the observed brightness asymmetry. The observed dynamics comes due to two reasons: faster precipitation of electrons having their mirror points near the ends of the magnetic trap, and relatively faster decay of the lower energy electrons responsible for the gyrosynchrotron emission near the FPs with higher magnetic field.

Authors: Reznikova, V. E., Melnikov, V. F., Shibasaki, K., Gorbikov, S. P., Pyatakov, N. P., Myagkova, I. N., Ji, H.
Projects: RHESSI,TRACE

Publication Status: ApJ 2009, V.697, P.735-746
Last Modified: 2009-06-23 07:30
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Thermal Fronts in Flaring Magnetic Loops  

Victor Melnikov   Submitted: 2009-06-22 20:20

We examine the existence of thermal fronts in solar flaring magnetic loops. In the past, two opposite conclusions have been made from the analysis of the evolution of a system in which hot electrons are injected into a coronal magnetic loop. One, made on the basis of analytical solutions, claims that this leads to the formation of ion-acoustic turbulence and subsequently a thermal front develops, while the opposing view obtained later on the basis of detailed numerical modeling shows no evidence of thermal fronts. In our study a one dimensional Vlasov solver is implemented to solve the distribution functions for each particle species f_a(r, v, t ). The numerical simulations are electrostatic since we examine motions along the field lines only. The results presented here show that earlier numerical results only showed the absence of thermal fronts because of the restricted size of the initial hot-electron region. If larger hot regions are simulated then thermal fronts do indeed form. This dependence on the system size is also explained. These results therefore confirm some of the theory presented by Levin and Melnikov and Brown et al. (i.e., the hot electrons are confined). The net effect of thermal fronts is that the electron energy is confined (m_i/m_e)^1/2 times longer than the estimate based on the free streaming of hot electrons.

Authors: Arber, T. D., Melnikov, V. F.
Projects: Hinode/XRT,RHESSI

Publication Status: ApJ 2009, V.690, P.238-243
Last Modified: 2009-06-23 07:30
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Abstracts by Author
Types of Microwave Quasi-Periodic Pulsations in Single Flaring Loops
Dynamics of the flaring loop system of 2005 August 22 observed in microwaves and hard X-rays
Quasi-Periodic Pulsations in Solar Flares
Peak Frequency Dynamics in Solar Microwave Bursts
2002 August 24 Limb Flare Loop: Dynamics of Microwave Brightness Distribution
Thermal Fronts in Flaring Magnetic Loops

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