C. G. Giménez de Castro

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.

A solar burst with a spectral component observed only above 100 GHz during an M class flare

Context. Since the installation of submillimeter solar radio telescopes, a new spectral burst component was discovered at frequencies above 100 GHz, creating the THz burst category. In all the reported cases, the events were X-class flares and the THz component was increasing. Aims. We report for the first time an M class flare that shows a different submillimeter radio spectral component from the microwave classical burst. Two successive bursts of 2 min duration and separated by 2 min occurred in active region NOAA 10226, starting around 13:15 UT and having an M 6.8 maximum intensity in soft X-rays. Methods. Submillimeter flux density measured by the Solar Submillimeter Telescope (SST) is used, in addition to microwave total Sun patrol telescope observations. Images with Hα filters, from the Hα Solar Telescope for Argentina (HASTA), and extreme UV observations, from the Extreme-ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory (SoHO), are used to characterize the flaring region. An extensive analysis of the magnetic topology evolution is derived from the Michelson Doppler Imager (SoHO, MDI) magnetograms and used to constrain the solution space of the possible emission mechanisms. Results. The submillimeter component is only observed at 212 GHz. We have upper limits for the emission at 89.4 and 405 GHz, which are less than the observed flux density at 212 GHz. The analysis of the magnetic topology reveals a very compact and complex system of arches that reconnects at low heights, while from the soft X-ray observations we deduce that the flaring area is dense (n ∼ 10 12 cm −3 ). The reconnected arches are anchored in regions with magnetic field intensity differing by an order of magnitude. Accordingly, we conclude that the microwave emission comes from mildly relativistic electrons spiraling down along the reconnected loops. A very small portion of the accelerated electrons can reach the footpoint with the stronger magnetic field (2000 G) and produce synchrotron emission, which is observed at submillimeter frequencies. Conclusions. The finding of a submillimeter burst component in a medium-size flare indicates that the phenomenon is more universal than shown until now. The multiwavelength analysis reveals that neither positron synchrotron nor free-free emission could produce the submillimeter component, which is explained here by synchrotron of accelerated electrons in a rather complex and compact magnetic configuration.

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.

THE BEHAVIOR OF THE 17 GHz SOLAR RADIUS AND LIMB BRIGHTENING IN THE SPOTLESS MINIMUM XXIII/XXIV

The current solar minimum has surprised the entire solar community because the spotless period is presently almost 2-3 years longer than the usual minima. To better understand this, we studied the variation of the solar radius and the polar limb brightening at 17 GHz, comparing the results from the minimum at the end of cycle XXIII with those of the previous one. Daily maps obtained by the Nobeyama Radioheliograph (NoRH) from 1992 through 2010 were analyzed. Whereas the variation of the solar radius at radio frequencies indicates the heating of the solar atmosphere due to solar activity, the limb brightening intensity depends on the organization of the polar magnetic field of the Sun, including the global dipole and the features formed around it. These features are more prominent during minima periods. As a common result, researchers have observed a decrease in both radius and limb brightness intensity at 17 GHz during the present minimum when compared with the previous one. The mean solar radius is 09 ± 06 smaller and the limb brightening reduced its intensity by around 20%. Both decrements are interpreted in terms of the weaker solar chromospheric activity of the present cycle. Measurement of the radius and limb brightening at 17 GHz can be used as an alternative solar activity index and should be included in the set of parameters used to predict future cycles.

Submillimeter and X-ray observations of an X class flare

The GOES X1.5 class flare that occurred on August 30, 2002 at 1327:30 UT is one of the few events detected so far at submillimeter wavelengths. We present a detailed analysis of this flare combining radio observations from 1.5 to 212 GHz (an upper limit of the flux is also provided at 405 GHz) and X-ray. Although the observations of radio emission up to 212 GHz indicates that relativistic electrons with energies of a few MeV were accelerated, no significant hard X-ray emission was detected by RHESSI above ∼250 keV. Images at 12–20 and 50–100 keV reveal a very compact, but resolved, source of about ∼10 �� × 10 �� . EUV TRACE images show a multi-kernel structure suggesting a complex (multipolar) magnetic topology. During the peak time the radio spectrum shows an extended flatness from ∼7 to 35 GHz. Modeling the optically thin part of the radio spectrum as gyrosynchrotron emission we obtained the electron spectrum (spectral index δ, instantaneous number of emitting electrons). It is shown that in order to keep the expected X-ray emission from the same emitting electrons below the RHESSI background at 250 keV, a magnetic field above 500 G is necessary. On the other hand, the electron spectrum deduced from radio observations ≥50 GHz is harder than that deduced from ∼70–250 keV X-ray data, meaning that there must exist a breaking energy around a few hundred keV. During the decay of the impulsive phase, a hardening of the X-ray spectrum is observed which is interpreted as a hardening of the electron distribution spectrum produced by the diffusion due to Coulomb collisions of the trapped electrons in a medium with an electron density of ne ∼ 3−5 × 10 10 cm −3 .

How are the EUV and radio polar limb-brightenings correlated?

Aims. We correlate the polar limb brightening time evolution observed with pass-band filters centered at the EUV 17.1 nm (Fe ix,x) and 30.4 nm (He ii) lines with radio continuum images obtained at 17 GHz (∼1.76 cm) during solar cycle 23. Methods. We determine the limb brightening in units of the quiet Sun from daily maps at 17.1 and 30.4 nm obtained by the Extreme Ultraviolet Imager (EIT) aboard the SOHO satellite between 1997 and 2007. The limb brightness at 17 GHz is obtained from daily maps taken by the Nobeyama Radioheliograph (NoRH) since 1992. Results. The variation in the limb brightening observed at coronal heights (17.1 nm) is correlated positively with the 11 year cycle. However, the observation at chromospheric/transition region heights (17 GHz / 30.4 nm) shows a clear negative correlation with the solar cycle. Conclusions. The limb brightening measurements at 17.1 nm reproduce the emission measure clearly in the solar corona during a solar cycle in which coronal holes are constantly present at the poles during the minimum. On the other hand, the negative correlation of the polar brightening at 17 GHz and 30.4 nm with the solar cycle are shown to depend upon polar features in the lower atmosphere (chromosphere/transition region). Moreover, the polar brightening variation at these frequencies is similar to that of the photospheric faculae observed at the poles.

Pulsations at the Onset of the Great Solar Burst of 22 October 1989

The onset phase of the 22 October 1989 great solar burst was observed at 48 GHz using the multiple beam technique, which allows unambiguous flux determination irrespective of spatial angular position changes in time. We found strong quasi-periodic pulsating structures as the flux started to rise. Two significantly different time scales of ∼2.5–4.5 s and∼ 0.2–0.5 s have been observed. These pulsations might be related to magnetohydrodynamic perturbations in the active region. However the fast component also might be a signature of the acceleration and/or injection of energetic electrons.

SOLAR-T : Terahertz Photometers to Observe Solar Flare Emission on Stratospheric Balloon Flights

A new solar flare spectral component has been found with intensities increasing for larger sub-THz frequencies, spectrally separated from the well known microwaves component, bringing challenging constraints for interpretation. Higher THz frequencies observations are needed to understand the nature of the mechanisms occurring in flares. A twofrequency THz photometer system was developed to observe outside the terrestrial atmosphere on stratospheric balloons or satellites, or at exceptionally transparent ground stations. 76 mm diameter telescopes were designed to observe the whole solar disk detecting small relative changes in input temperature caused by flares at localized positions at 3 and 7 THz. Golay cell detectors are preceded by low-pass filters to suppress visible and near IR radiation, band-pass filters, and choppers. It can detect temperature variations smaller than 1 K with time resolution of a fraction of a second, corresponding to small burst intensities. The telescopes are being assembled in a thermal controlled box to which a data conditioning and acquisition unit is coupled. While all observations are stored on board, a telemetry system will forward solar activity compact data to the ground station. The experiment is planned to fly on board of long-duration stratospheric balloon flights some time in 2013-2015. One will be coupled to the GRIPS gamma-ray experiment in cooperation with University of California, Berkeley, USA. One engineering flight will be flown in the USA, and a 2 weeks flight is planned over Antarctica in southern hemisphere summer. Another long duration stratospheric balloon flight over Russia (one week) is planned in cooperation with the Lebedev Physics Institute, Moscow, in northern hemisphere summer.

The solar radius in the EUV during the cycle XXIII

Aims. We aim to determine the solar transition region and coronal radius at EUV wavelengths and its time evolution during Solar Cycle XXIII. Methods. We use daily 30.4 and 17.1 nm images obtained by the Extreme Ultraviolet Imager (EIT) aboard the SoHO satellite and derive the solar radius by fitting a circle to the limb brightness ring. Results. The weighted mean of the temporal series gives (967.

Solar Flare Observations at Submm-waves

First 405 GHz and 212 GHz solar flare observations were obtained during short campaigns while the new solar submillimeter-wave telescope (SST) was still undergoing adjustments at the CASLEO EI Leoncito observatory in the Argentina Andes. We show here preliminary results for a large X1.1 class X-ray event occurred on 2000 March 22, which exhibited a small submm-w continuum response to the slow (minutes) bulk flare emission, and numerous subsecond spikes (100-300 ms), the brightest spikes reaching about 180 and 50 s.f.u. at 405 and 212 GHz, respectively. Solar flare observations are nearly unknown in the submm-IR range of wavelengths. Few results limited to time resolution of about one minute have suggested brightness variations of 10-100 K in active regions without clear flare associations (Clark & Park 1970; Hudson 1975). The six SST beams (Kaufmann et al. 1994) are superimposed on a Kitt Peak solar magnetogram (NOAA 2000) shown in Figure 1 (left) at about the time of the event on 2000 March 22. The flare emission light-curves in compressed time scales are shown in Figure 1 (right) for different frequencies. The H-a telescope is described elsewhere (Bagala et al. 1999). The bottom plot of Figure 1 (right) shows the rate of submm-w brightest (~ 20 K) spikes incidence with time. An example of such a bright spike (labeled A in Figure 1 (right)) is shown in Figure 2 (A) in a five seconds time interval, compared to data obtained tracking a quiet solar region, near its center, in Figure 2 (B), labeled B in Figure 1 (right). The spiky incidence increases drastically after about 1730 UT approximately together with the soft X-ray level. There is a pronounced concentration of brightenings in correspondence to the bulk emissions at X-rays and H-a. Clusters of spikes seem to be added at about 1745 UT and 1820 UT coincident to flares occurring in AR