Säm Krucker

An Observational Overview of Solar Flares

We present an overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era. Following an introductory discussion and overview of the status of observational capabilities, the article is split into topical sections which deal with different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections. We also discuss flare soft X-ray spectroscopy and the energetics of the process. The emphasis is to describe the observations from multiple points of view, while bearing in mind the models that link them to each other and to theory. The present theoretical and observational understanding of solar flares is far from complete, so we conclude with a brief discussion of models, and a list of missing but important observations.

On the Origin of Impulsive Electron Events Observed at 1 AU

A statistical survey of 12 impulsive electron events detected at energies down below 1 keV and 58 events detected above 25 keV observed at 1 AU by the 3-D Plasma and Energetic Particles experiment on the Wind spacecraft is presented. Timing analysis of the velocity dispersion reveals two different kinds of electron events: (1) events released from the Sun at the onset of a radio type III burst, which suggest that these electrons are part of the population producing the type III radio emission; and (2) events in which the electrons are released up to half an hour later than the onset of the type III burst. These electrons therefore may be produced by a different acceleration mechanism than the population producing the radio emission. Both types of behavior can be observed during the same impulsive electron event at different energies, but most events show the same timing at all energies. At lower energies ( 25 keV), events not related to type III bursts are more numerous (41 of 58). However, events of both classes are observed below 1 keV. Impulsive electron events not related to type III radio bursts are observed to be proton rich, with an order-of-magnitude lower electron-to-proton ratio than events related to type III bursts. For roughly 3/4 of the events not related to type III bursts, large-scale coronal transient waves, also called EIT waves or coronal Moreton waves, are observed by the Extreme Ultraviolet Imaging Telescope (EIT) on board SOHO. Temporal and spatial correlations together with hydromagnetic simulations show that at least some impulsive electron events are more likely related to the propagating Moreton wave than to the flare phenomenon itself.

Energy Distribution of Heating Processes in the Quiet Solar Corona

We have determined the variations in the emission measure of the solar corona using EUV Imaging Telescope/Solar and Heliospheric Observatory observations of iron lines in a quiet region of the Sun. The emission measure is found to vary significantly in at least 85% of all the pixels within 42 minutes. The variations are interpreted as heating events that bring chromospheric material above the one million degree threshold of the observed lines and that cool the coronal plasma below that limit. A method to assess heating events has been developed. The thermal energy input by such microflares is calculated from the observed increases in emission measure and the derived temperature. Heating events have been found in the range from 8 ? 10 -->24 to 1.6 ? 10 -->26 ergs. The energy input by ?3 ? events of the emission measure increase the amounts to about 16% of the average radiated power of the coronal plasma in the quiet corona. The frequency distribution of microflares is an approximate power law of the form f(E)=f -->0E-?, with a power-law index ? between 2.3 and 2.6. Since the low-energy cutoff is due to sensitivity limitations and the power-law index is steeper than 2, these observations demonstrate the possibility that microflares dominate the energy input into the quiet corona. The observed power law would have to continue to about 3 ? 10 -->23 ergs in order to match the observed minimum heating requirement.

ENERGETIC PARTICLE OBSERVATIONS DURING THE 2000 JULY 14 SOLAR EVENT

Data from nine high-latitude neutron monitors are used to deduce the intensity-time and anisotropytime pro—les and pitch-angle distributions of energetic protons near Earth during the major solar event on 2000 July 14 (also known as the Bastille Day event). In addition, particle and magnetic —eld measurements from W ind, the Advanced Composition Explorer, and the Solar and Heliospheric Observatory (SOHO) are used in the analysis. The observations are —tted with good agreement between two independent numerical models of interplanetary transport. The rapid decrease of anisotropy from a high initial value cannot be explained by a simple model of interplanetary transport. Hence, we invoke a barrier or magnetic bottleneck consistent with an observed magnetic disturbance from an earlier coronal mass ejec

Hard X-Ray Source Motions in the 2002 July 23 Gamma-Ray Flare

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is used to study the hard X-ray (HXR) source motions of the 2002 July 23 γ-ray flare. Above 30 keV, at least three HXR sources are observed during the impulsive phase that can be identified with footpoints of coronal magnetic loops that form an arcade. On the northern ribbon of this arcade, a source is seen that moves systematically along the ribbon for more than 10 minutes. On the other ribbon, at least two sources are seen that do not seem to move systematically for more than half a minute, with different sources dominating at different times. The northern source motions are fast during times of strong HXR flux but almost absent during periods with low HXR emission. This is consistent with magnetic reconnection if a higher rate of reconnection of field lines (resulting in a higher footpoint speed) produces more energetic electrons per unit time and therefore more HXR emission. The absence of footpoint motion in one ribbon is inconsistent with simple reconnection models but can be explained if the magnetic configuration there is more complex.

Hard X-Ray Emissions from Partially Occulted Solar Flares

Observations of solar flares partially occulted by the solar limb provide diagnostics of coronal hard X-ray (HXR) emissions in the absence of generally much brighter emissions from footpoints of flare loops. In this paper, a statistical survey of 55 partially occulted flares observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) is presented, revealing the existence of two different components of coronal HXR emissions. Below ~15 keV thermal HXR emission with a gradual time profile is generally dominant, while at higher energies an additional component is seen in 50 out of 55 events. This additional component shows faster time variations in the order of tens of seconds and is most prominent during the rise of the thermal emission. A comparison of the centroid positions of these two emissions shows that they are most often cospatial within ~2000 km, although for a few events clear separations are observed as well. The spectra of the high-energy component show a rather steep (soft) power law with indices mostly between ~4 and ~6. Thin target emission in the corona from flare-accelerated electrons is discussed as a possible origin of the fast time variation component.

Energy Distribution of Microevents in the Quiet Solar Corona

Recent imaging observations of EUV line emissions have shown evidence for frequent flarelike events in a majority of the pixels in quiet regions of the solar corona. The changes in coronal emission measure indicate impulsive heating of new material to coronal temperatures. These heating or evaporation events are candidate signatures of ‘‘ nanoflares ’’ or ‘‘ microflares ’’ proposed to interpret the high temperature as well as the very existence of the corona. The energy distribution of these microevents reported in the literature differ widely, and so do the estimates of their total energy input into the corona. Here we analyze the assumptions of the different methods, compare them by using the same data set, and discuss their results. We also estimate the different forms of energy input and output, keeping in mind that the observed brightenings are most likely secondary phenomena. A rough estimate of the energy input observed by EIT on the SOHO satellite is of the order of 10% of the total radiative output in the same region. It is considerably smaller for the two reported TRACE observations. The discrepancy can be explained by flare selection and different thresholds for flare detection. There is agreement on the slope and the absolute value of the distribution if the same methods are used and a numerical error is corrected. The extrapolation of the power law to unobserved energies that are many orders of magnitude smaller remains questionable. Nevertheless, these microevents and unresolved smaller events are currently the best source of information on the heating process of the corona. Subject headings: Sun: activity — Sun: corona — Sun: flares — Sun: X-rays, gamma rays

RHESSI Hard X-Ray Imaging Spectroscopy of the Large Gamma-Ray Flare of 2002 July 23

We present Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) hard X-ray images in different energy bands for the large X-class flare of 2002 July 23; these images are used to construct spatially resolved hard X-ray spectra for each of four prominent features: a bright, soft source high in the corona, two localized, hard footpoints in opposite polarity magnetic regions that show highly correlated flux and spectral variations in time, and a third footpoint bounded by the other three sources. The power-law spectral indices of the two correlated footpoints differ by ~0.3-0.4, which may be the result of differing column densities from the electron source.

X-Ray Network Flares of the Quiet Sun

Temporal variations in the soft X-ray (SXR) emission and the radio emission above the solar magnetic network of the quiet corona are investigated using Yohkoh SXR images with deep exposure and VLA observations in the centimeter radio range. The SXR data show several brightenings, with an extrapolated occurrence probability of one brightening per 3 seconds on the total solar surface. During the roughly 10 minutes of enhanced flux, total radiative losses of the observed plasma are around 1025 ergs per event. These events are more than an order of magnitude smaller than previously reported X-ray bright points or active region transient brightenings. For all of the four SXR events with simultaneous radio observations, a corresponding radio source correlating in space and time can be found. There are several similarities between solar flares and the SXR/radio events presented in this paper. (1) Variations in temperature and emission measure during the SXR enhancements are consistent with evaporation of cooler material from the transition region and the chromosphere. (2) The ratio of the total energies radiated in SXR and radio frequencies is similar to that observed in flares. (3) At least one radio event shows a degree of polarization as high as 35%. (4) In three out of four substructures the centimeter radio emission peaks several tens of seconds earlier than in the SXR emission. (5) The associated radio emission tends to be more structured and to have faster rise times. These events thus appear to be flare-like and are called network flares.

Two Classes of Solar Proton Events Derived from Onset Time Analysis

We analyze onset times for 26 solar energetic (from 30 keV to 6 MeV) proton events that exhibit clear velocity dispersion, as observed by the three-dimensional Plasma and Energetic Particles experiment on the Wind spacecraft. Assuming that the particles are injected simultaneously at all energies and travel the same path length, we find two classes of proton events: (1) for the 18 class 1 events, the derived path lengths are between 1.1 and 1.3 AU, indicating that the first arriving protons travel essentially scatter-free; and (2) the eight class 2 events show longer path lengths around 2 AU. For all proton events, the observed temporally related electron events all have electron path lengths around 1.1-1.3 AU. Relative to the electron injection time at the Sun, the protons of the first class are injected ~0.5-2 hr later. Assuming these particles are accelerated by the associated coronal mass ejection (CME) shock, the protons at all energies between 0.03 and 6 MeV appear to be accelerated (or released) simultaneously high in the corona, roughly ~1-10 R☉ above the electrons. The pitch-angle distributions are observed to be similar for both classes of events, making it unlikely that propagation effects are responsible for the longer path length of class 2 events. The late proton onset times at 1 AU of class 2 are therefore more likely explained by a successively later solar release (or escape) of protons at successive lower energies. Assuming again acceleration at the CME shock, the release (or escape) of the protons of class 2 events appears to depend on energy and occur at a higher altitude for lower energies, with the most energetic protons possibly released simultaneously with the electrons.