solar flare hard x-ray spikes observed by rhessi: a statistical study jianxia cheng jiong qiu,...
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Solar flare hard X-ray spikes observed by RHESSI: a statistical study
Jianxia Cheng Jiong Qiu, Mingde Ding, and Haimin Wang
outline
Introduction Observations and data analysis Properties of HXR spikes summary
1.Introduction
Solar flare emission at sub-second was reported in hard X-ray in 70s and 80s (van Beek et al. 1974,1976; Hoyng et al. 1976; de Jager & de Jonger 1978; Kiplinger et al. 1983; 1984, 1989).
Kiplinger et al. (1983) found 53 out of 3000 flares produce spikes as short as 45 ms.
These energetic flare bursts on short timescales are believed to be nonthermal in nature, and their temporal and spectral properties place constraints on the physical nature of the source.
Several mechanisms: dynamic magnetic reconnection (Kliem et al., 2000); nonthermal electron injections.
Peak energy differences: time of flight; trap model (the collisional timescale increases with the particle energy).
It
Islow
Ir
1-7 represents 7 different energy bands
25-100 keV
An example: different criteria to define a spike
Event distribution
sample flares
Sample flares are steeper than the spike-associated flares, suggesting that more intensive flares have in general a greater chance to produce spikes.
HXR spikes can occur in both impulsive and gradual events.
spk Time difference between spike peak time and flare start time normalized to the flare rise time
spk distribution
70% of spikes around the
peak times of the associated
flare
Spikes are most probably nonthermal in nature
Most of spikes discovered in 40-60 and 60-100 keV, 20% as high as 100-300 keV.
All these statistical results indicate that spikes are small scale energetic events produced during the most energetic stage of flares. In particular, they tend to occur during the rise phase of intensive flares. On the other hand, impulsive and gradual flares have an equal chance to produce spikes.
3.Properties of HXR spikes
Spikes duration is range from 0.2-2s with mean value of about 1s. It is independent of energy bands. Symmetric rise and decay phase, this is different from flares.
Nearly all spikes have harder spectral than their underlying components. This is agree with the general scenario that flare HXR emission exhibits a harder spectrum at emission peaks than at valley.
Time lags between 60-100 keV and 25-40 kev
The majority of events exhibit time lags shorter than 0.5s. Mean time lag is about 0.8s and -0.74s.
High energy delay
Low energy delay
High energy delay
Low energy delay
About or more than 2/3 events are low energy delayed. On average, high energy delayed spikes have a harder count spectrum than low energy delayed events.
summary Both impulsive and long duration flares can produce HXR spikes with nearly
equal production rates. Flares with high peak count rates are more productive in HXR spikes.
Almost all spikes occur in the rise phase of the flares, and a large percentage, up to 70%, of spikes are produced at or about the flare peak times.
The mean duration of spikes is about 0.9−1.0 s, independent of photon energies. The rise and decay times of spikes are shown to be almost the same. This differs from ordinary flares that usually have a longer decay phase dominated by thermal emission
Most of the spikes can be detected in very high energy bands up to 100−300 keV. The HXR spectra of spikes are harder than those of the underlying slow-varying components. This fact implies the nonthermal origin of spikes.
Evident energy-dependent time lags are present in a fraction of spikes, indicative of time-of-flight or Coulomb collision effects. It is also shown that, on average, spikes lagging in high energy emissions have harder spectra than spikes exhibiting lags in low-energy emissions.
Thank you
These numbers are significantly greater than Poisson distribution
The negative occurrence is nearly ½ of positive or less
Most of our spikes detected are real signals.
25-100keVS1
S2
Time cadence 125 ms
With spike
Without spike
Define a spike:
sigslow ntItItI )()()(r5,4,3sign swsmt 8,4
Spike/flare number variations with different criteria