Adriana V. R. Silva

METHOD FOR SPOT DETECTION ON SOLAR-LIKE STARS

As a planet eclipses its parent star, a dark starspot may be occulted, causing a detectable variation in the light curve. This work describes how features on the surface of a solar-like star can be studied by using planetary transits. Images of the Sun were used in order to simulate such transits, with the planet being modeled as a dark disk at various positions along its orbit. From modeling of these transits, it might be possible to infer the physical properties of the spots, such as size, intensity, position, and temperature. Recent transit observations for HD 209458 were used as tests to the model.

Correlation of Microwave and Hard X-Ray Spectral Parameters

We present the analysis of 27 solar flares with multiple peaks that were observed at hard X-ray and microwave wavelengths. A total of 57 simultaneous peaks were observed by BATSE (hard X-rays) and Owens Valley Radio Observatory (microwaves). Throughout the duration of a flare, its spectra at both wavelengths are fitted independently at all times. The hard X-ray spectra were fitted by a single power law in most cases, whereas the microwave spectra were fitted as gyrosynchrotron emission. For each individual peak, the parameters at both wavelengths (peak flux, turnover frequency, spectral indices, and delays between hard X-ray and microwave peak emission) were then compared and correlated. We have also studied impulsive and nonimpulsive bursts individually. The main results obtained were as follows. (1) In 75% of the bursts, the inferred index of the electron energy distribution of the microwave-emitting electrons, δr, is harder than that of the lower energy hard X-ray-emitting electrons, δX, on average by 0.5-2.0. This implies that there is a breakup in the energy spectra of the electrons, as is sometimes observed in the hard X-ray spectra of giant flares. (2) A soft-hard-harder spectral index temporal evolution is more commonly seen in the microwave spectra (47%) than in the hard X-ray observations (32%) and in nonimpulsive flares than in impulsive ones. (3) Delays larger than 2 s were observed between the radio and hard X-ray peaks in 65% of the bursts, with the delays decreasing as the hard X-ray energy increased. (4) Nonimpulsive flares are more microwave rich, have higher delays between their radio emission and the hard X-ray peaks, and display harder spectral indices than impulsive bursts.

MULTIPLE COMPONENTS IN THE MILLIMETER EMISSION OF A SOLAR FLARE

We analyze a small flare using imaging data at millimeter, microwave, and soft X-ray wavelengths and microwave and hard X-ray spectral observations. The remarkable aspect of this flare is evidence for the presence of MeV-energy electrons, which are responsible for the nonthermal millimeter emission, at a time when no hard X-rays from lower energy electrons are detected. This occurs during a smoothly varying phase, which is seen at radio wavelengths to last several minutes and is the brightest phase at millimeter wavelengths but is undetected in hard X-rays: it follows a brief spike of emission at flare onset, which has the more usual properties of impulsive events and features nonthermal microwave, millimeter, and hard X-ray emission. We interpret the phase that is brightest at millimeter wavelengths as being due to efficient trapping of a relatively small number of nonthermal electrons, whereas during the hard X-ray emission, trapping is much less efficient, and the decay time is much shorter at all energies, which leads to a larger ratio of hard X-ray flux to radio flux. As in many previous events studied at millimeter wavelengths, there is a discrepancy between the electron energy spectral indices inferred from the milllimeter and hard X-ray data during the impulsive phase when both are detected: again it appears that the energy spectrum at 1 MeV must be significantly flatter than at several hundred keV and below. However, there are problems in reconciling quantitatively the energy spectra for the hard X-ray-emitting and radio-emitting components: based on the most plausible parameters, the radio-emitting electrons should produce most of the hard X-rays. One solution to this contradiction is to invoke a coronal magnetic field stronger than seems likely based on the photospheric magnetic field.

A CURRENT RIBBON MODEL FOR ENERGY STORAGE AND RELEASE WITH APPLICATION TO THE FLARE OF 7 JANUARY 1992

Observations of the flare on 7 January 1992 are interpreted using a topological model of the magnetic field. The model, developed here, applies a theory of three-dimensional reconnection to the inferred magnetic field configuration for 7 January. In the model field a new bipole (∼ 1021 Mx) emerges amidst pre-existing active region flux. This emergence gives rise to two current ribbons along the boundaries (separators) separating the distinct, new and old, flux systems. Sudden reconnection across these boundary curves transfers ∼ 3 ×1020 Mx of flux from the bipole into the surrounding flux. The model also predicts the simultaneous (sympathetic) flaring of the two current ribbons. This explains the complex two-loop structure noted in previous observations of this flare. We subject the model predictions to comparisons with observations of the flare. The locations of current ribbons in the model correspond closely with those of observed soft X-ray loops. In addition the footpoints and apexes of the ribbons correspond with observed sources of microwave and hard X-ray emission. The magnitude of energy stored by the current ribbons compares favorably to the inferred energy content of accelerated electrons in the flare.

Solar atmospheric model with spicules applied to radio observation

An atmospheric model was constructed in order to reproduce quantitatively the observations at 17 GHz from Nobeyama Radio Heliograph, namely the brightness temperature at disk center (from 1.4 to 400 GHz), center-to-limb bright- ening distribution, and radius derived from 17 GHz solar maps. The two dimensional solar atmospheric model, that takes into account the curvature of the Sun, includes spicules, which physical characteristics (such as size, temperature, density, position, and inclination angle) were randomly attributed. After the interferometer instrumental response is taken into account, the results showed than an atmospheric model without spicules produces 36% of limb brightening, approximately the value observed at the solar poles. However, the inferred solar radius from the model (970 �� )w as 6 �� smaller than the mean value derived from the solar maps. An improvement of the model is made by including spicules. Results from this upgraded model showed that depending on their physical parameters, limb brightening and solar radius values are obtained in agreement with the radio observations (except for polar regions).

Rapid Submillimeter Brightenings Associated with a Large Solar Flare

We present high time resolution observations of Active Region 8910 obtained simultaneously at 212 and 405 GHz during a large Hα flare, which produced a soft X-ray class X1.1 event. Data were obtained with the new solar submillimeter telescope recently installed at the El Leoncito Observatory to explore this poorly known part of the solar emission spectrum. A small slow submillimeter enhancement (≤300 sfu) was associated to bulk emissions at X-rays, Hα, and microwaves. The event exhibited numerous submillimeter-wave 100-300 ms duration spikes, the larger ones with fluxes on the order of 220 and 500 sfu (±20%) at 212 and 405 GHz, respectively. A dramatic increase in the incidence rate of submillimeter spikes sets in as a new large loop system appears in AR 8910, and X-ray emission increases nearly 1 hr before the large flare. The brightening incidence rate (~20 per minute) correlates well with the large flare light curves at X-rays and Hα. The submillimeter spikes may be associated to microflares, waves, or quakes in flaring active regions.

Imaging the Chromospheric Evaporation of the 1994 June 30 Solar Flare

We analyze simultaneous Hα images (from the Big Bear Solar Observatory), soft and hard X-ray images and spectra (from the soft X-ray telescope [SXT], the Bragg Crystal Spectrometer [BCS], and the hard X-ray telescope [HXT] on Yohkoh), and radio time profiles (from the Owens Valley Radio Observatory) during the first 3 minutes of the 1994 June 30 flare. The strong blueshifts observed in the Ca XIX soft X-ray line are interpreted as evidence of chromospheric evaporation, with maximum up-flow velocities occurring 2 minutes prior to the hard X-ray emission peak. In this study, we search for moving sources in Hα, soft and hard X-ray images that correspond to the blueshifted component. The chromospheric evaporation in this flare is divided into two phases: an early phase with up-flow velocities of 350-450 km s-1, and a later phase (during the hard X-ray peak) characterized by velocities of 100-200 km s-1. During the first chromospheric evaporation phase, the footpoints of a loop seen in HXT low-energy maps are seen to move toward the loop-top source. No source displacement is observed in SXT images at this time. Images of the later phase of chromospheric evaporation show a change in the source morphology. The early HXT loop is no longer visible, and HXT maps during this time display the two footpoints of a new loop visible in SXT images. Now the HXT sources are stationary, and a SXT footpoint source is seen to move toward the loop top. We interpret the observed displacement of footpoint sources in HXT (early phase) and SXT (later phase) maps to be the images of the evaporating front projected onto the solar disk, while the up-flow velocities (inferred from the blueshifts) are due to the movement of the same evaporating material along the line of sight. By combining the up-flow velocities with the proper motion of the footpoint sources seen in the maps, we constructed a three-dimensional view of the magnetic loop for each chromospheric evaporation phase. The early loop is almost semicircular, with a height of 1.7 × 109 cm, whereas the later magnetic loop is more elongated (a height of 3.2 × 109 cm), with its apex closer to the footpoint where most of the evaporation took place. The implications of these magnetic configurations and the distinct evaporation phases are discussed.

The new submillimeter-wave solar telescope

A new and unique solar submillimeter telescope (SST) was installed in the El Leoncito site, Argentina Andes. It has a 1.5 m radome-enclosed cassegrain antenna, and arrays of four 212 GHz and two 405 GHz radiometers placed in the focal plane. We present a brief technical description of the system, preliminary results on its performance, the atmospheric opacity measured at the site, and the first detection of solar flare submm-wave emissions.

Temporal and angular variation of the solar limb brightening at 17 GHz

In order to better understand the atmosphere structure of the Sun, we have analyzed over 3000 daily maps of the Sun taken at 17 GHz from the Nobeyama Radioheliograph (NoRH) from 1992 through 2001, focusing on the excess brightness temperature observed near the limb. The purpose of this work is to characterize the limb brightness in two ways: (i) study the temporal variation of the intensity and radial width of polar brightening; and (ii) measure the brightness distribution along the limb as a function of position angle and compare it with data at other wavelengths throughout the solar cycle. The mean intensity of the polar regions were found to be approximately 13% and 14% above quiet Sun levels at the North and South poles, respectively. Moreover, the polar brightenings are strongly anti-correlated with solar activity (as measured by sunspot number). The radial width of the excess brightness is slightly over 1 arcmin for both polar regions. Only a small variation with the solar cycle was observed during the decline of last maximum, that is, the Southern polar brightening was found to be both wider and brighter than the Northern one for the 23rd cycle. As for the angular variation of the limb brightening, for a month during a period of minimum activity, it reaches 25% above quiet Sun levels at the poles, ∼15% near the equator, and 10% at intermediate regions. Hα images also show brightening enhancements at the polar regions for the same period. We also found a strong anti-correlation between the radio polar brightenings and the coronal holes seen in soft X-ray images from 1992 to 2001. There seems to be a strong association of the radio limb brightening at 17 GHz with faculae. The implications of these correlations are discussed.

First Images of Impulsive Millimeter Emission and Spectral Analysis of the 1994 August 18 Solar Flare

We present here the first images of impulsive millimeter emission of a flare. The flare on 1994 August 18 was simultaneously observed at millimeter (86 GHz), microwave (1-18 GHz), and soft and hard X-ray wavelengths. Images of millimeter, soft and hard X-ray emission show the same compact ( 8′′) source. Both the impulsive and the gradual phases are studied in order to determine the emission mechanisms. During the impulsive phase, the radio spectrum was obtained by combining the millimeter with simultaneous microwave emission. Fitting the nonthermal radio spectra as gyrosynchrotron radiation from a homogeneous source model with constant magnetic field yields the physical properties of the flaring source, that is, total number of electrons, power-law index of the electron energy distribution, and the nonthermal source size. These results are compared to those obtained from the hard X-ray spectra. The energy distribution of the energetic electrons inferred from the hard X-ray and radio spectra is found to follow a double power-law with slope ∼6–8 below ∼50 keV and ∼3–4 above those energies. The temporal evolution of the electron energy spectrum and its implication for the acceleration mechanism are discussed. Comparison of millimeter and soft X-ray emissions during the gradual phase implies that the millimeter emission is free-free radiation from the same hot soft X-ray emitting plasma, and further suggests that the flare source contains multiple temperatures.