Showing papers in "Solar Physics in 2002"

The RHESSI Imaging Concept

Abstract: The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observes solar hard X-rays and gamma-rays from 3 keV to 17 MeV with spatial resolution as high as 2.3 arc sec. Instead of focusing optics, imaging is based on nine rotating modulation collimators that time-modulate the incident flux as the spacecraft rotates. Starting from the arrival time of individual photons, ground-based software then uses the modulated signals to reconstruct images of the source. The purpose of this paper is to convey both an intuitive feel and the mathematical basis for this imaging process. Following a review of the relevant hardware, the imaging principles and the basic back-projection method are described, along with their relation to Fourier transforms. Several specific algorithms (Clean, MEM, Pixons and Forward-Fitting) applicable to RHESSI imaging are briefly described. The characteristic strengths and weaknesses of this type of imaging are summarized.

Transverse Oscillations in Coronal Loops Observed with TRACE – II. Measurements of Geometric and Physical Parameters

Abstract: We measure geometric and physical parameters oftransverse oscillations in 26 coronal loops, out of the 17 events described in Paper I by Schrijver, Aschwanden, and Title (2002). These events, lasting from 7 to 90 min, have been recorded with the Transition Region and Coronal Explorer (TRACE) in the 171 and 195 A wavelength bands with a characteristic angular resolution of 1", with time cadences of 15–75 seconds. We estimate the unprojected loop (half) length L and orientation of the loop plane, based on a best-fit of a circular geometry. Then we measure the amplitude A(t) of transverse oscillations at the loop position with the largest amplitude. We decompose the time series of the transverse loop motion into an oscillating component A osc(t) and a slowly-varying trend A trend(t). We find oscillation periods in the range of P=2–33 min, transverse amplitudes of A=100–8800 km, loop half lengths of L=37 000–291 000 km, and decay times of t d=3.2–21 min. We estimate a lower limit of the loop densities to be in the range of n loop=0.13–1.7×109 cm−3. The oscillations show (1) strong deviations from periodic pulses, (2) spatially asymmetric oscillation amplitudes along the loops, and (3) nonlinear transverse motions of the centroid of the oscillation amplitude. From these properties we conclude that most of the oscillating loops do not fit the simple model of kink eigen-mode oscillations, but rather manifest flare-induced impulsively generated MHD waves, which propagate forth and back in the loops and decay quickly by wave leakage or damping. In contrast to earlier work we find that the observed damping times are compatible with estimates of wave leakage through the footpoints, for chromospheric density scale heights of ≈400–2400 km. We conclude that transverse oscillations are most likely excited in loops that (1) are located near magnetic nullpoints or separator lines, and (2) are hit by a sufficiently fast exciter. These two conditions may explain the relative rarity of detected loop oscillations. We show that coronal seismology based on measurements of oscillating loop properties is challenging due to the uncertainties in estimating various loop parameters. We find that a more accurate determination of loop densities and magnetic fields, as well as advanced numerical modeling of oscillating loops, are necessary conditions for true coronal seismology.

The RHESSI Spectrometer

Abstract: RHESSI observes solar photons over three orders of magnitude in energy (3 keV to 17 MeV) with a single instrument: a set of nine cryogenically cooled coaxial germanium detectors. With their extremely high energy resolution, RHESSI can resolve the line shape of every known solar gamma-ray line except the neutron capture line at 2.223 MeV. High resolution also allows clean separation of thermal and non-thermal hard X-rays and the accurate measurement of even extremely steep power-law spectra. Detector segmentation, fast signal processing, and two sets of movable attenuators allow RHESSI to make high-quality spectra and images of flares across seven orders of magnitude in intensity. Here we describe the configuration and operation of the RHESSI spectrometer, show early results on in-flight performance, and discuss the principles of spectroscopic data analysis used by the RHESSI software.

Group Sunspot Numbers: Sunspot Cycle Characteristics

Abstract: We examine the "Group" sunspot numbers constructed by Hoyt and Schatten to determine their utility in characterizing the solar activity cycle. We compare smoothed monthly Group Sunspot Numbers to Zurich (International) Sunspot Numbers, 10.7-cm radio flux, and total sunspot area. We find that the Zurich numbers follow the 10.7-cm radio flux and total sunspot area measurements slightly better than the Group numbers. We examine several significant characteristics of the sunspot cycle using both Group numbers and Zurich numbers. We find that the "Waldmeier Effect" - the anti-correlation between cycle amplitude and the elapsed time between minimum and maximum of a cycle - is much more apparent in the Zurich numbers. The "Amplitude-Period Effect" the anti-correlation between cycle amplitude and the length of the previous cycle from minimum to minimum - is also much more apparent in the Zurich numbers. The "Amplitude-Minimum Effect" - the correlation between cycle amplitude and the activity level at the previous (onset) minimum is equally apparent in both the Zurich numbers and the Group numbers. The "Even-Odd Effect" - in which odd-numbered cycles are larger than their even-numbered precursors - is somewhat stronger in the Group numbers but with a tighter relationship in the Zurich numbers. The "Secular Trend" - the increase in cycle amplitudes since the Maunder Minimum - is much stronger in Group numbers. After removing this trend we find little evidence for multi-cycle periodicities like the 80 year Gleissberg cycle or the two- and three-cycle periodicities. We also find little evidence for a correlation between the amplitude of a cycle and its period or for a bimodal distribution of cycle periods. We conclude that the Group numbers are most useful for extending the sunspot cycle data further back in time and thereby adding more cycles and improving the statistics. However, the Zurich numbers are more useful for characterizing the on-going levels of solar activity.

Long-period cycles of the sun's activity recorded in direct solar data and proxies

Abstract: Different records of solar activity (Wolf and group sunspot number, data on cosmogenic isotopes, historic data) were analyzed by means of modern statistical methods, including one espe- cially developed for this purpose It was confirmed that two long-term variations in solar activity - the cycles of Gleissberg and Suess - can be distinguished at least during the last millennium The results also show that the century-type cycle of Gleissberg has a wide frequency band with a double structure consisting of 50 - 80 years and 90 - 140 year periodicities The structure of the Suess cycle is less complex showing a variation with a period of 170 - 260 years Strong variability in Gleissberg and Suess frequency bands was found in northern hemisphere temperature multiproxy that confirms the existence of a long-term relationship between solar activity and terrestial climate

Transverse oscillations in coronal loops observed with TRACE – I. An Overview of Events, Movies, and a Discussion of Common Properties and Required Conditions

Abstract: We study transverse loop oscillations triggered by 17flares and filament destabilizations; only 2 such cases have been reported in the literature until now. Oscillation periods are estimated to range over a factor of ∼15, with most values between 2 and 7 min. The oscillations are excited by filament destabilizations or flares (in 6% of the 255 flares inspected, ranging from about C3 to X2). There is no clear dependence of oscillation amplitude on flare magnitude. Oscillations occur in loops that close within an active region, or in loops that connect an active region to a neighboring region or to a patch of strong flux in the quiet Sun. Some magnetic configurations are particularly prone to exhibit oscillations: two active regions showed two, and one region even three, distinct intervals with loop oscillations. The loop oscillations are not a resonance that builds up: oscillations in loops that are excited along their entire length are likely to be near the fundamental resonance mode because of that excitation profile, but asymmetrically excited oscillations clearly show propagating waves that are damped too quickly to build up a resonance, and some cases show multiple frequencies. We discuss evidence that all oscillating loops lie near magnetic separatrices that outline the large-scale topology of the field. All magnetic configurations are more complicated than a simple bipolar region, involving mixed-polarities in the interior or vicinity of the region; this may reflect that the exciting eruptions occur only in such environments, but this polarity mixing likely also introduces the large-scale separatrices that are involved. Often the oscillations occur in conjunction with gradual adjustments in loop positions in response to the triggering event. We discuss the observations in the context of two models: (a) transverse waves in coronal loops that act as wave guides and (b) strong sensitivity to changes in the field sources for field lines near separatrices. Properties that favor model b are (1) the involvement of loops at or near separatrices that outline the large-scale topology of the field, (2) the combined occurrence of oscillations and loop translations, (3) the small period spread and similar decay time scale in a set of oscillating loops in one well-observed event, and (4) the existence of loops oscillating in antiphase with footpoints close together in two cases. All other properties are compatible with either model, except the fact that almost all of the oscillations start away from the triggering event, suggestive of an outward-pushing exciting wave more in line with model a. The spread in periods from event to event suggests that the oscillations may reflect the properties of some driver mechanism that is related to the flare or mass ejection.

Polar Coronal Holes During Cycles 22 and 23

Abstract: The National Solar Observatory/Kitt Peak synoptic rotation maps of the magnetic field and of the equivalent width of the He i 1083 nm line are used to identify and measure polar coronal holes from September 1989 to the present This period covers the entire lifetime of the northern and southern polar holes present during cycles 22 and 23 and includes the disappearance of the previous southern polar coronal hole in 1990 and and formation of the new northern polar hole in 2001 From this sample of polar hole observations, we found that polar coronal holes evolve from high-latitude (∼ 60° ) isolated holes The isolated pre-polar holes form in the follower of the remnants of old active region fields just before the polar magnetic fields complete their reversal during the maximum phase of a cycle, and expand to cover the poles within 3 solar rotations after the reversal of the polar fields During the initial 12–14 years, the polar holes are asymmetric about the pole and frequently have lobes extending into the active region latitudes During this period, the area and magnetic flux of the polar holes increase rapidly The surface areas, and in one case the net magnetic flux, reach an initial brief maximum within a few months Following this initial phase, the areas (and in one case magnetic flux) decrease and then increase more slowly reaching their maxima during the cycle minimum Over much of the lifetime of the measured polar holes, the area of the southern polar hole was smaller than the northern hole and had a significantly higher magnetic flux density Both polar holes had essentially the same amount of magnetic flux at the time of cycle minimum The decline in area and magnetic flux begins with the first new cycle regions with the holes disappearing about 11–18 years before the polar fields complete their reversal The lifetime of the two polar coronal holes observed in their entirety during cycles 22 and 23 was 87 years for the northern polar hole and 83 years for the southern polar hole

Active-Region Monitoring and Flare Forecasting – I. Data Processing and First Results

Abstract: This paper discusses a near real-time approach to solar active-region monitoring and flare prediction using the Big Bear Solar Observatory Active Region Monitor (ARM). Every hour, ARM reads, calibrates, and analyses a variety of data including: full-disk Hα images from the Global Hα Network; EUV, continuum, and magnetogram data from the Solar and Heliospheric Observatory (SOHO); and full-disk magnetograms from the Global Oscillation Network Group (GONG). For the first time, magnetic gradient maps derived from GONG longitudinal magnetograms are now available on-line and are found to be a useful diagnostic of flare activity. ARM also includes a variety of active-region properties from the National Oceanic and Atmospheric Administration's Space Environment Center, such as up-to-date active-region positions, GOES 5-min X-ray data, and flare-to-region identifications. Furthermore, we have developed a Flare Prediction System which estimates the probability for each region to produce C-, M-, or X-class flares based on nearly eight years of NOAA data from cycle 22. This, in addition to BBSO's daily solar activity reports, has proven a useful resource for activity forecasting.

OSCILLATIONS IN QUIESCENT SOLAR PROMINENCES OBSERVATIONS AND THEORY – (Invited Review)

Abstract: An extensive observational background about the existence of oscillations in quiescent solar prominences has been gathered during the last twenty years. From these observations, information about different oscillatory parameters such as period, wavelength, phase speed, damping time, etc., has been obtained. This observational background, combined with a growing number of theoretical studies about magneto-hydrodynamic waves in prominences, should allow the development of prominence seismology which, following helioseismology's approach, seeks to infer the internal structure and properties of solar prominences. The most recent observational and theoretical developments on prominence oscillations are reviewed here, with an emphasis on the aspects suitable to develop an observation versus theory feedback, but also pointing out key topics which should be the subject of future research for a further advancement of this field.

The magnetic helicity budget of a cme-prolific active region

Abstract: Coronal mass ejections (CMEs) are thought to be the way by which the solar corona expels accumulated magnetic helicity which is injected into the corona via several methods. DeVore (2000) suggests that a significant quantity is injected by the action of differential rotation, however Demoulin et al. (2002b), based on the study of a simple bipolar active region, show that this may not be the case. This paper studies the magnetic helicity evolution in an active region (NOAA 8100) in which the main photospheric polarities rotate around each other during five Carrington rotations. As a result of this changing orientation of the bipole, the helicity injection by differential rotation is not a monotonic function of time. Instead, it experiences a maximum and even a change of sign. In this particular active region, both differential rotation and localized shearing motions are actually depleting the coronal helicity instead of building it. During this period of five solar rotations, a high number of CMEs (35 observed, 65 estimated) erupted from the active region and the helicity carried away has been calculated, assuming that each can be modeled by a twisted flux rope. It is found that the helicity injected by differential rotation (≈−7×1042 Mx2) into the active region cannot provide the amount of helicity ejected via CMEs, which is a factor 5 to 46 larger and of the opposite sign. Instead, it is proposed that the ejected helicity is provided by the twist in the sub-photospheric part of the magnetic flux tube forming the active region.

LONGITUDINAL INTENSITY OSCILLATIONS IN CORONAL LOOPS OBSERVED WITH TRACE II. Discussion of Measured Parameters

Abstract: In this paper, we give a detailed discussion of the parameters of longitudinal oscillations in coronal loops, described in Paper I We found a surprising absence of correlations between the measured variables, with the exception of a relation between the estimated damping length and the period of the intensity variations Only for 2 out of the 38 cases presented in Paper I did we find a significant perturbation in the 195 A TRACE data The loops supporting the propagating disturbances were typically stable, quiescent loops and the total luminosity of the analyzed structures generally varied by no more than 10% The observed density oscillations are unlikely to be flare-driven and are probably caused by an underlying driver exciting the loop footpoints It was demonstrated that the rapid damping of the perturbations could not simply be explained as a consequence of the decreasing intensity along the loops However, we found that (slightly enhanced) thermal conduction alone could account for the observed damping lengths and wavelengths, and, additionally, explain the correlation between propagation period and damping length

Spectral and Spatial Variations of Flare Hard X-ray Footpoints

Abstract: In a sample of strong RHESSI M-class flares we have made a study of the relationship between the ‘hardness’ of the HXR spectrum and the intensity in the 30–50 keV energy range. In all events we find clear evidence for a ‘soft—hard—soft’ pattern of correlation between hardness and flux, on time scales as short as 10 s. We investigate whether or not this pattern is intrinsic to the acceleration mechanism. The RHESSI images in this energy range are dominated by footpoint brightenings, and we have searched for a correlation between footpoint separation velocity and spectral hardness, to be compared qualitatively with theoretical flare models. We find quite systematic footpoint motions, and also note that episodes in which footpoint separation varies rapidly often correspond with episodes of significant change in the flare spectral index, though not as the simplest flare models would predict. We report also on one of our events, on 14 March 2002, which exhibits highly sheared HXR footpoint ribbons extending over a scale of 100 arc sec. For this flare we find a correlation between footpoint motion and hard X-ray flux.

RHESSI and Trace Observations of the 21 April 2002 X1.5 Flare

Abstract: Observations of the X1.5 flare on 21 April 2002 are reviewed using the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Transition Region and Coronal Explorer (TRACE). The major findings are as follows: (1) The 3–25 keV X-rays started < 4 min before the EUV (195 A) emission suggesting that the initial energy release heated plasma directly to ≳20 MK, well above the 1.6 MK needed to produce the Fexu (195 A) line. (2) Using coaligned 12–25 keV RHESSI and TRACE images, further evidence is found for the existence of hot (15–20 MK) plasma in the 195 A passband. This hot, diffuse emission is attributed to the presence of the Fe xxiv (192 A) line within the TRACE 195 A passband. (3) The 12–25 keV source centroid moves away from the limb with an apparent velocity of ~ 9.9 km s−1, slowing to ~ 1.7 km s−1 after 3 hours, its final altitude being ~ 140 Mm after ~ 12 hours. This suggests that the energy release site moves to higher altitudes in agreement with classical flare models. (4) The 50–100 keV emission correlates well with EUV flare ribbons, suggesting thick-target interactions at the footpoints of the magnetic arcade. The 50–100 keV time profile matches the time derivative of the GOES light curve (Neupert effect), which suggests that the same electrons that produced the thick-target hard X-ray emission also heat the plasma seen in soft X-rays. (5) X-ray footpoint emission has an E−3 spectrum down to ~ 10 keV suggesting a lower electron cutoff energy than previously thought. (6) The hard X-ray (25–200 keV) peaks have FWHM durations of ~ 1 min suggesting a more gradual energy release process than expected. (7) The TRACE images reveal a bright symmetric front propagating away from the main flare site at speeds of ≥ 120 km s−1. This may be associated with the fast CME observed several minutes later by LASCO. (8) Dark sinuous lanes are observed in the TRACE images that extend almost radially from the post-flare loop system. This ‘fan of spines’ becomes visible well into the decay phase of the flare and shows evidence for both lateral and downward motions.

The Magnetic Helicity Injected by Shearing Motions

Abstract: Photospheric shearing motions are one of the possible ways to inject magnetic helicity into the corona. We explore their efficiency as a function of their particular properties and those of the magnetic field configuration. Based on the work of M. A. Berger, we separate the helicity injection into two terms: twist and writhe. For shearing motions concentrated between the centers of two magnetic polarities the helicity injected by twist and writhe add up, while for spatially more extended shearing motions, such as differential rotation, twist and writhe helicity have opposite signs and partially cancel. This implies that the amount of injected helicity can change in sign with time even if the shear velocity is time independent. We confirm the amount of helicity injected by differential rotation in a bipole in the two particular cases studied by DeVore (2000), and further explore the parameter space on which this injection depends. For a given latitude, tilt and magnetic flux, the generation of helicity is slightly more efficient in young active regions than in decayed ones (up to a factor 2). The helicity injection is mostly affected by the tilt of the AR with respect to the solar equator. The total helicity injected by shearing motions, with both spatial and temporal coherence, is at most equivalent to that of a twisted flux tube having the same magnetic flux and a number of turns of 0.3. In the solar case, where the motions have not such global coherence, the injection of helicity is expected to be much smaller, while for differential rotation this maximum value reduces to 0.2 turns. We conclude that shearing motions are a relatively inefficient way to bring magnetic helicity into the corona (compared to the helicity carried by a significantly twisted flux tube).

Active-Region Filaments and X-ray Sigmoids

Abstract: We use Yohkoh soft X-ray telescope data and Hα full-disk observations to study the evolution of chromospheric filaments and coronal sigmoids in 6 active regions in association with coronal mass ejections (CMEs). In two cases, CMEs are directly observed by the SOHO/LASCO C2 coronagraph. In four cases, other observations (magnetic clouds, geomagnetic storms, sigmoid-arcade evolution) are used as CME indicators. Prior to eruption, each active region shows a bright coronal sigmoidal loop and underlying Hα filament. The sigmoid activates, erupts and gets replaced by a cusp, or an arcade. In contrast, the Hα filament shows no significant changes in association with sigmoid eruption and CME. We explain these observations in a framework of the classical two-ribbon flare model.

Chromospheric Height and Density Measurements in a Solar Flare Observed with RHESSI – II. Data Analysis

Abstract: We present an analysis of hard X-ray imaging observations from one of the first solar flares observed with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft, launched on 5 February 2002. The data were obtained from the 22 February 2002, 11:06 UT flare, which occurred close to the northwest limb. Thanks to the high energy resolution of the germanium-cooled hard X-ray detectors on RHESSI we can measure the flare source positions with a high accuracy as a function of energy. Using a forward-fitting algorithm for image reconstruction, we find a systematic decrease in the altitudes of the source centroids z(e) as a function of increasing hard X-ray energy e, as expected in the thick-target bremsstrahlung model of Brown. The altitude of hard X-ray emission as a function of photon energy e can be characterized by a power-law function in the e=15–50 keV energy range, viz., z(e)≈2.3(e/20 keV)−1.3 Mm. Based on a purely collisional 1-D thick-target model, this height dependence can be inverted into a chromospheric density model n(z), as derived in Paper I, which follows the power-law function n e(z)=1.25×1013(z/1 Mm)−2.5 cm−3. This density is comparable with models based on optical/UV spectrometry in the chromospheric height range of h≲1000 km, suggesting that the collisional thick-target model is a reasonable first approximation to hard X-ray footpoint sources. At h≈1000–2500 km, the hard X-ray based density model, however, is more consistent with the `spicular extended-chromosphere model' inferred from radio sub-mm observations, than with standard models based on hydrostatic equilibrium. At coronal heights, h≈2.5–12.4 Mm, the average flare loop density inferred from RHESSI is comparable with values from hydrodynamic simulations of flare chromospheric evaporation, soft X-ray, and radio-based measurements, but below the upper limits set by filling-factor insensitive iron line pairs.

Soho Contribution to Prominence Science

Abstract: We present the main current issues concerning prominence studies We recall the large range of plasma parameters found in prominences which makes the work of the MHD modeler more difficult We also summarize the capabilities of the SOHO instrumentation We present and discuss the most recent SOHO results concerning the determination of temperature, densities, and velocities We put some emphasis on the different morphologies observed, the diagnostic capabilities of the Lyman lines profiles when accompanied by improved non-LTE modeling, and the information gathered from the first prominence oscillations measured from space We also make an account of eruptive prominences We finally discuss what could be done with present and future SOHO data to improve our understanding of prominences

Moving Dipolar Features in an Emerging Flux Region

Abstract: On 25 January, 2000, we observed active region NOAA 8844 with the Flare Genesis Experiment (FGE), a balloon-borne observatory with an 80-cm solar telescope FGE was equipped with a vector polarimeter and a tunable Fabry–Perot narrow-band filter It recorded time series of filtergrams, vector magnetograms and Dopplergrams at the Ca i 61222 A line, and Hα filtergrams with a cadence between 25 and 75 min At the time of the observations, NOAA 8844 was located at approximately 5° N 30° W The region was growing rapidly; new magnetic flux was constantly emerging in three supergranules near its center We report on the structure and behavior of peculiar moving dipolar features (MDFs) in the emerging flux, and we describe in detail how the FGE data were analyzed In longitudinal magnetograms, the MDFs appeared to be small dipoles flowing into sunspots and supergranule boundaries Previously, dipolar moving magnetic features (MMFs) have only been observed flowing out from sunspots The FGE vector magnetograms show that the MDFs occurred in a region with nearly horizontal fields, the MDFs being distinguished as undulations in these fields We identify the MDFs as stitches where the emerging flux ropes were still tied to the photosphere by trapped mass We present a U-loop model that accounts for their unusual structure and behavior, as well as showing how emerging flux sheds entrained mass

The subsurface radial gradient of solar angular velocity from MDI f -mode observations

Abstract: We report quantitative analysis of the radial gradient of solar angular velocity at depths down to about 15 Mm below the solar surface for latitudes up to 75° using the Michelson Doppler Imager (MDI) observations of surface gravity waves (fmodes) from the Solar and Heliospheric Observatory (SOHO) A negative outward gradient of around −400 nHz/R ⊙, equivalent to a logarithmic gradient of the rotation frequency with respect to radius which is very close to −1, is found to be remarkably constant between the equator and 30° latitude Above 30° it decreases in absolute magnitude to a very small value at around 50° At higher latitudes the gradient may reverse its sign: if so, this reversal takes place in a thin layer extending only 5 Mm beneath the visible surface, as evidenced by the most superficial modes (with degrees l>250) The signature of the torsional oscillations is seen in this layer, but no other significant temporal variations of the gradient and value of the rotation rate there are found

The evolution of the sun’s open magnetic flux: II. full solar cycle simulations

Abstract: In this paper the origin and evolution of the Sun's open magnetic flux is considered by conducting magnetic flux transport simulations over many solar cycles. The simulations include the effects of differential rotation, meridional flow and supergranular diffusion on the radial magnetic field at the surface of the Sun as new magnetic bipoles emerge and are transported poleward. In each cycle the emergence of roughly 2100 bipoles is considered. The net open flux produced by the surface distribution is calculated by constructing potential coronal fields with a source surface from the surface distribution at regular intervals. In the simulations the net open magnetic flux closely follows the total dipole component at the source surface and evolves independently from the surface flux. The behaviour of the open flux is highly dependent on meridional flow and many observed features are reproduced by the model. However, when meridional flow is present at observed values the maximum value of the open flux occurs at cycle minimum when the polar caps it helps produce are the strongest. This is inconsistent with observations by Lockwood, Stamper and Wild (1999) and Wang, Sheeley, and Lean (2000) who find the open flux peaking 1–2 years after cycle maximum. Only in unrealistic simulations where meridional flow is much smaller than diffusion does a maximum in open flux consistent with observations occur. It is therefore deduced that there is no realistic parameter range of the flux transport variables that can produce the correct magnitude variation in open flux under the present approximations. As a result the present standard model does not contain the correct physics to describe the evolution of the Sun's open magnetic flux over an entire solar cycle. Future possible improvements in modeling are suggested.

Time Evolution of low-Frequency Periodicities in Cosmic ray Intensity

Abstract: The long-time series of daily means of cosmic-ray intensity observed by four neutron monitors at different cutoff rigidities (Calgary, Climax, Lomnický Stit and Huancayo/Haleakala) were analyzed by means of the wavelet transform method in the period range ∼ 60 to ∼ 1000 days. The contributions of the time evolution of three quasi-periodic cosmic-ray signals (∼ 150 d, ∼ 1.3 yr and ∼ 1.7 yr) to the global one are obtained. While the ∼ 1.7-yr quasi-periodicity, the most remarkable one in the studied interval, strongly contributes to the cosmic ray intensity profile of solar cycle 21 (particularly in 1982), the ∼ 1.3-yr one, which is better correlated with the same periodicity of the interplanetary magnetic field strength, is present as a characteristic feature for the decreasing phases of the cycles 20 and 22. Transitions between these quasi-periodicities are seen in the wavelet power spectra plots. Obtained results support the claimed difference in the solar activity evolution during odd and even solar activity cycles.

Understanding Solar Flare Waiting-Time Distributions

Abstract: The observed distribution of waiting times Δt between X-ray solar flares of greater than C1 class listed in the Geostationary Operational Environmental Satellite (GOES) catalog exhibits a power-law tail ∼(Δt)γ for large waiting times (Δt>10 hours). It is shown that the power-law index γ varies with the solar cycle. For the minimum phase of the cycle the index is γ=−1.4±0.1, and for the maximum phase of the cycle the index is −3.2±0.2. For all years 1975–2001, the index is −2.2±0.1. We present a simple theory to account for the observed waiting-time distributions in terms of a Poisson process with a time-varying rate λ(t). A common approximation of slow variation of the rate with respect to a waiting time is examined, and found to be valid for the GOES catalog events. Subject to this approximation the observed waiting-time distribution is determined by f(λ), the time distribution of the rate λ. If f(λ) has a power-law form ∼λα for low rates, the waiting time-distribution is predicted to have a power-law tail ∼(Δt)−(3+α) (α>−3). Distributions f(λ) are constructed from the GOES data. For the entire catalog a power-law index α=−0.9±0.1 is found in the time distribution of rates for low rates (λ<0.1 hours −1). For the maximum and minimum phases power-law indices α=−0.1±0.5 and α=−1.7±0.2, respectively, are observed. Hence, the Poisson theory together with the observed time distributions of the rate predict power-law tails in the waiting-time distributions with indices −2.2±0.1 (1975–2001), −2.9±0.5 (maximum phase) and −1.3±0.2 (minimum phase), consistent with the observations. These results suggest that the flaring rate varies in an intrinsically different way at solar maximum by comparison with solar minimum. The implications of these results for a recent model for flare statistics (Craig, 2001) and more generally for our understanding of the flare process are discussed.

The topology of a mixed-polarity potential field, and inferences for the heating of the quiet solar corona

Abstract: We study the statistical properties of the connectivity of the corona over the quiet Sun by analyzing the potential magnetic field above the central area of source planes sprinkled randomly with some 300 magnetic monopoles each. We find that the field is generally more complex than one might infer from a study of the field within the source plane alone, or from a study of the 3D field around a small number of sources. Whereas a given source most commonly connects to only its nearest neighbors, it may connect to up to several dozen sources; only a weak trend relates the source strength and the number of connections. The connections between pairs of sources define volumes, or domains, of connectivity. Domains that have a finite cross section with the source plane are enclosed by surfaces that contain a pair of null points. In contrast, most of the bounding surfaces of domains that lie above the source plane appear not to contain null points. We argue that the above findings imply (i) that we should expect at best a weak correlation between coronal brightness and the flux in an underlying flux concentration, and (ii) that the low-lying chromospheric field lines (such as are observable in Hα) provide information on source connections that are largely complementary to those traced by the higher-reaching coronal field lines (observable in the extreme ultraviolet). We compare sample TRACE and SOHO/MDI observations of the quiet corona and photosphere with our finding that the number density of null points within the source plane closely matches that of the sources; because we find essentially no foci of coronal brightening away from significant photospheric magnetic flux concentrations, we conclude that coronal heating at such null points does not contribute significantly to the overall heating. We argue that the divergence of field lines towards multiple sources restricts the propagation of braids and twists, so that any coronal heating that is associated with the dissipation of braids induced by footpoint shuffling in mixed-polarity network is likely (a) to occur predominantly low in the corona, and (b) to be relatively more efficient in quiet Sun than in active regions for a given field strength and loop length.

The Orientational Relaxation of Bipolar Active Regions

Abstract: In the mean, bipolar active regions are oriented nearly toroidally, according to Hale's polarity law, with a latitude-dependent tilt known as Joy's Law. The tilt angles of individual active regions deviate from this mean behavior and change over time. It has been found that on average the change is toward the mean angle at a rate characteristic of 4.37 days (Howard, 1996). We show that this orientational relaxation is consistent with the standard model of flux tube emergence from a deep dynamo layer. Under this scenario Joy's law results from the Coriolis effect on the rising flux tube (D'Silva and Choudhuri, 1993), and departures from it result from turbulent buffeting of the tubes (Longcope and Fisher, 1996). We show that relaxation toward Joy's angle occurs because the turbulent perturbations relax on shorter time scales than the perturbations from the Coriolis force. The turbulent perturbations relax more rapidly because they are localized to the topmost portion of the convection zone while the Coriolis perturbations are more widely distributed. If a fully-developed active region remains connected to the strong toroidal magnetic field at the base of the convection zone, its tilt will eventually disappear, leaving it aligned perfectly toroidally. On the other hand, if the flux becomes disconnected from the toroidal field the bipole will assume a tilt indicative of the location of disconnection. We compare models which are connected and disconnected from the toroidal field. Only those disconnected at points very deep in the convection zone are consistent with observed time scale of orientational relaxation.

Multiple magnetic clouds in interplanetary space

Abstract: An interplanetary magnetic cloud (MC) is usually considered the byproduct of a coronal mass ejection (CME). Due to the frequent occurrence of CMEs, multiple magnetic clouds (multi-MCs), in which one MC catches up with another, should be a relatively common phenomenon. A simple flux rope model is used to get the primary magnetic field features of multi-MCs. Results indicate that the magnetic field configuration of multi-MCs mainly depends on the magnetic field characteristics of each member of multi-MCs. It may be entirely different in another situation. Moreover, we fit the data from the Wind spacecraft by using this model. Comparing the model with the observations, we verify the existence of multi-MCs, and propose some suggestions for further work.

High-resolution solar spectroscopy with TESOS - Upgrade from a double to a triple system

Abstract: We present the characteristics and demonstrate the performance of the Triple Etalon SOlar Spectrometer (TESOS) operated at the German Vacuum Tower Telescope (VTT) on Tenerife. The Fabry–Perot interferometer TESOS is ideally suited for precise measurements of photospheric and chromospheric motion. Installed in 1997 and equipped with two etalons, TESOS has recently been completed with a third etalon and upgraded with two high-speed, backside-illuminated CCD cameras. The image scale of 0.089 arc sec pixel−1 is adapted to the resolution of the telescope. The improved system enables frame rates up to 5 frames per second. The spectral resolution of 300 000 allows for spectral diagnostics of weak photospheric lines, including individual CH-lines within the G-band at 430.6 nm.

Spectroscopic Observation of Coronal Waves

Abstract: A time sequence over 80 min of coronal green-line spectra was obtained with a corona- graph at the Norikura Solar Observatory. Doppler velocities, line intensities, and line widths were derived through fitting a single Gaussian to the observed line profiles. Coronal waves have been clearly detected in the Doppler velocity data. The Fourier analysis shows powers in a 1–3 mHz range, and in higher frequencies (5–7 mHz) at localized regions. The propagation speed of the waves was estimated by correlation analysis. The line intensity and line width did not show clear oscillations, but their phase relationship with the Doppler velocity indicates propagating waves rather than standing waves. The existence of Alfven waves whose speed is 500 km s−1 or faster is possible but inconclusive, while the existence of slower waves (of the order of 100 km s−1, possibly sound waves) is evident. The energy carried by the detected sound waves is far smaller than the required heat input rate to the quiet corona.

Development of an Automatic Filament Disappearance Detection System

Abstract: This paper presents an efficient and automatic method for detecting filament disappearances. This method was applied to the Big Bear Solar Observatory's (BBSO) full-disk Hα images. The initial step is to detect the filaments in the solar image, then determine if they are growing, stable or disappearing. If a disappearing filament is found, the solar community can be automatically alerted in near real time. This system is proven to be accurate and fast. In addition, three statistical studies of the appearance and disappearance of all filaments in 1999 are presented.

Hard X-ray Microflares down to 3 keV

Abstract: The excellent sensitivity, spectral and spatial resolution, and energy coverage down to 3 keV provided by the Reuven Ramaty High-Energy Solar Spectroscopic Imager mission (RHESSI) allows for the first time the detailed study of the locations and the spectra of solar microflares down to 3 keV. During a one-hour quiet interval (GOES soft X-ray level around B6) on 2 May, 1:40–2:40 UT, at least 7 microflares occurred with the largest peaking at A6 GOES level. The microflares are found to come from 4 different active regions including one behind the west limb. At 7′′ resolution, some events show elongated sources, while others are unresolved point sources. In the impulsive phase of the microflares, the spectra can generally be fitted better with a thermal model plus power law above ∼ 6–7 keV than with a thermal only. The decay phase sometimes can be fitted with a thermal only, but in some events, power-law emission is detected late in the event indicating particle acceleration after the thermal peak of the event. The behind-the-limb microflare shows thermal emissions only, suggesting that the non-thermal power law emission originates lower, in footpoints that are occulted. The power-law fits extend to below 7 keV with exponents between −5 and −8, and imply a total non-thermal electron energy content between 1026–1027 erg. Except for the fact that the power-law indices are steeper than what is generally found in regular flares, the investigated microflares show characteristics similar to large flares. Since the total energy in non-thermal electrons is very sensitive to the value of the power law and the energy cutoff, these observations will give us better estimates of the total energy input into the corona. (Note that color versions of figures are on the accompanying CD-ROM.)

Wavelet Analysis of solar, solar wind and geomagnetic parameters

Abstract: The sunspot number, solar wind plasma, interplanetary magnetic field, and geomagnetic activity index Ap have been analyzed using a wavelet technique to look for the presence of periods and the temporal evolution of these periods. The global wavelet spectra of these parameters, which provide information about the temporal average strength of quasi periods, exhibit the presence of a variety of prominent quasi periods around 16 years, 10.6 years, 9.6 years, 5.5 years, 1.3 years, 180 days, 154 days, 27 days, and 14 days. The wavelet spectra of sunspot number during 1873–2000, geomagnetic activity index Ap during 1932–2000, and solar wind velocity and interplanetary magnetic field during 1964–2000 indicate that their spectral power evolves with time. In general, the power of the oscillations with a period of less than one year evolves rapidly with the phase of the solar cycle with their peak values changing from one cycle to the next. The temporal evolution of wavelet power in Rz, vsw, n, By, Bz, |B|, and Ap for each of the prominent quasi periods is studied in detail.