Solar Physics and Space Environment

In collaboration with T. Kucera and A. Poland (GSFC), Andretta is extending a new diagnostic technique based on continuum absorption of hot coronal lines ($T > 10^6$~K) by relatively cool prominence material The initial work was done considering only lines observed by SOHO/CDS in the range 310--380 \AA\ and 510--640 \AA. Andretta has been working on extending this technique to the emission spectrum observed by both SOHO/CDS and SOHO/EIT in the range $\lambda>150$ \AA. Such an extension will allow a substantially more accurate determination of column densities in prominences.

Gopalswamy reviewed the X-ray and microwave signatures of coronal mass ejections (CMEs). X-ray and microwave imaging of structures associated with CMEs have provided a wealth of new information towards a better understanding of solar eruptions. The review concentrated on the recent research based on microwave imaging from the Nobeyama Radioheliograph and X-ray imaging from the {\it Yohkoh} Soft X-ray Telescope. He discussed the advances made towards understanding the near surface manifestations of CMEs best observed in X-rays and microwaves. In particular, the following issues were considered: (i) observability of CMEs in X-rays and microwaves, (ii) coronal dimming, (iii) relation between CME substructures, (iv) heating and expansion of eruptive prominences, (v) timing of flare, CME and prominence eruptions, and (vi) CME mass estimates from non-optical observations.

Goplaswamy, Yashiro (Tokyo University), Kaiser, Thompson (GSFC) and Plunkett (NRL) reported on the near-surface and outer coronal manifestations of the 1998 January 25 coronal mass ejection (CME) using white light, EUV, X-ray and hectometric radio data which reveal the three dimensional structure and long term evolution of the CME. They found that (i) the substructures of the CME (prominence core, cavity, frontal structure and the arcade formation) are clearly observed in X-ray and EUV wavelengths. (ii) The filament heats up early on and is observed as a backbone in X-rays. (iii) The filament also expands considerably as it erupts. (iv) The CME is observed through direct leading edge signature as well as through dimming process in X-rays and in EUV.

Using the observed relation between speeds of coronal mass ejections (CMEs) near the Sun and in the solar wind, Gopalswamy, Lara, Kaiser, Lepping (GSFC), Berdichevsky (ITSS) and St Cyr (CPI) estimate an "effective" acceleration acting on the CMEs. This study quantifies the qualitative results of Gosling [1997] and numerical simulations that CMEs at 1 AU with speeds closer to the solar wind. A linear relation between the global acceleration and the initial speed of the CMEs has been found. The absolute value of the acceleration is similar to the slow solar wind acceleration. This study naturally divides CMEs into fast and slow ones, the dividing line being the average solar wind speed. These results have important implications to space weather prediction models which need to incorporate this effect in estimating the CME arrival time at 1 AU. This study also shows that the arrival times of CMEs at 1 AU are significantly different from the zero acceleration case.

Gopalswamy, Kaiser, Burlaga, Thompson, Szabo (GSFC), Lara, Vourlidas (George Mason University) and Yashiro (Tokyo University) studied a set of 25 solar eruptive events associated with radio bursts in the decameter-hectometric wavelength regime. They identified the global characteristics of these events based on multiwavelength multispacecraft and ground based data. Their findings are as follows: (i) Each of the radio-rich events is associated with a white light CME; these CMEs seem to be wider and faster: On an average the width (160\deg) of the radio-rich events is much larger than the typical width of white light CMEs ($\sim$ 45\deg). There is a longitudinal dependence of measured speeds suggesting projection effect. (ii) A third of the events studied did not have metric type II bursts. In the rest of the events, the relationship between the metric and longer wavelength type II bursts is highly complex. A careful study of the three dimensional nature and long-term evolution of the CMEs and the shocks driven by them is needed to understand this relationship. (iii) Presence of a shock seems to be essential to produce an SA event; the energetic electrons seem to be accelerated in the shock front rather than from near the solar surface. (iv) For most of the shocks detected in situ, there was associated kilometric emission highlighting the importance of shock waves in producing radio emission at all heliocentric distances. (v) A significant number of shocks detected {\it in situ} (without drivers) were from limb CMEs suggesting that shocks have a greater longitudinal extent than CMEs. (vi) Spectroheliograms obtained by SOHO/EIT at 195 \AA\ had a definite signature for each of the eruptive events near the solar surface consisting of one or more of EIT waves, dimming and global enhancement; the global enhancement may be the earliest form of the CME itself.

Gopalswamy, Hanaoka (NAOJ) and Hudson (ISAS) have made first detection of coronal dimming in microwaves associated with the 1998 March 29 coronal mass ejection (CME). The dimming was observed by the Nobeyama radioheliograph at 17 GHz as a reduction in thermal free-free emission from the corona due to the displacement of coronal structures during a coronal mass ejection. The dimming was also observed in X-rays confirming the microwave observations. A low brightness feature within the white light CME could be identified that corresponds well with the microwave and X-ray dimmings. The dimming appeared drastically different in extreme ultraviolet probably due to the combined effect of coronal displacement and temperature change.

Gopalswamy, Lara and Kaiser (GSFC) compared the near-Sun and near-Earth manifestations of solar eruptions that occurred during November 1994 to June 1998. They compared CMEs, metric type II bursts and EIT waves (near the Sun) with interplanetary (IP) signatures such as magnetic clouds (MCs), IP ejecta and IP shocks. They performed a two way correlation study: (i) Look for counterparts of metric type II bursts that occurred close to the central meridian. (ii) Look for solar counterparts of IP shocks, MCs and ejecta. They used Wind and SOHO data along with metric radio burst data from ground based solar observatories. Preliminary analysis shows that (i) Most of the metric type II bursts did not have IP signatures. (ii) Most of the IP events (MC, Ejecta) did not have metric counterparts. (iii) A significant number of IP shocks without drivers were detected. In all these cases, the drivers (CMEs) were ejected transverse to the Sun-Earth line suggesting that the shocks have much larger extent than the drivers. These shocks also had good type II radio burst association.

Gopalswamy, Shibasaki (Nobeyama), Thompson, Gurman (GSFC) and DeForest (Stanford) studied the microwave enhancement and its variability in the elephant trunk coronal hole observed during the Whole Sun Month campaign (August 10-September 09, 1996) using the Nobeyama Radioheliograph images and the magnetograms and EUV images obtained simultaneously by the Michelson Doppler Imager (MDI) and the Extreme Ultraviolet Imaging Telescope (EIT) on board the SOHO spacecraft. The combined data set allowed them to understand the detailed structure of the microwave enhancement in the spatial and temporal domains. They find that the radio enhancement is closely associated with the enhanced unipolar magnetic regions in the coronal hole. When a minority polarity is present within the coronal hole, the resulting dipole is associated with a bright-point-like emission in coronal EUV lines such as the Fe XII 195 A . On the other hand, the lower temperature line emission (304 A) and the microwave enhancement are associated with the unipolar magnetic flux elements in the network. They found strong time variability of the radio enhancement over multiple time scales, consistent with the initial results obtained by SOHO instruments.

Based on the physical properties of coronal holes, Gopalswamy, Shibasaki (Nobeyama), Thompson, Gurman (GSFC) and DeForest (Stanford) attempted to constrain the region in the solar atmosphere in which the microwave enhancement originates. The brightness temperature range in which the enhancement occurs seems to be around 10,000 K. This temperature is close to the upper chromopsheric temperature. Therefore, they think that the enhancement takes place in the upper chromosphere of the coronal hole. Microwave emission from the quiet Sun is optically thick thermal bremsstrahlung from the upper chromosphere. Therefore, the optically thick layer in the coronal hole chromosphere must be hotter than the corresponding layer in the quiet chromosphere. They analyzed microwave and SOHO observations and found support for this idea. They calculated the emission measure and hence the expected optical depth of the corona in the coronal hole and found it to be negligibly small. They also invoked the result that any transition region with reasonable filling factor would produce a high brightness temperature which is never observed. Thus they conclude that the enhancement points to a difference in the upper chromosphere in the coronal hole as compared to that in the quiet Sun.

Gopalswamy and Salem (undergraduate research student from Case Western Reserve University) searched for coronal holes in the SOHO EIT data and identified 71 well defined holes during 1996 January to 1998 June. These dates correspond to the launch of SOHO and its temporary disability in June 1998. They are in the process of obtaining statistical results on area, life time, brightness temperature and variability in these holes.

Guhathakurta's recent work focuses on inferring difficult to measure coronal magnetic field strength and its topology in the corona-interplanetary medium. She along with Sittler (GSFC) has developed a two dimensional semi-empirical MHD model of the corona-interplanetary medium. This model for the first time produces an empirical heating function of the corona by using observational constraints. The model is also able to predict both qualitatively as well as quantitatively the source regions of the quasi-steady fast and slow solar wind.

Guhathakurta and Nunes are continuing to develop suitable mathematical tools for tomographic inversion of quasi-stationary white light observations of the solar corona such as coronal streamers and plumes from spacecraft measurements such as SOHO and Spartan 201 to accurately model the three-dimensional coronal density distribution.

Guhathakurta, Davila (GSFC) and Reginald (University of Delaware) took part in an expedition of the total solar eclipse of August 11, 1999. This experiment was sufficiently different from all the other eclipse observations of the past. Instead of imaging the corona they took spectroscopic observations at multiple points in the corona. The focus of this experiment was to determine the radial and latitudinal profiles of the coronal temperature and the solar wind speed.

Lara, Gopalswamy, P\'erez-Enr\'{\i}quez (UNAM), and Shibasaki (Nobeyama) studied the development of microwave polarization of a group of active regions for a period of 10 days during April, 1993 using data obtained by the Nobeyama radioheliograph. The observed sense of polarization at 17 GHz, changed with the active region position on the solar disk. This change of polarization can be explained by the mode coupling theory, according to which a weak coupling between the ordinary and extraordinary electromagnetic modes takes place when the radiation traverses a region of strong transverse magnetic field and results in a polarization reversal. Since the strength of the mode coupling depends on the physical parameters (and their gradients) of the quasi-transverse region, observations of polarization changes can be used to obtain key values magnetic field and field gradient in the active region corona. Using Yohkoh/SXT and 17 GHz intensity and polarization images of active regions, it was found that the coupling constant is typically $>$ 10$^3$ corresponding to a weak coupling regime. The mean value of the transition frequency was found to be $\sim$ 5.3 $\times$ 10$^{11}$ Hz, below which the weak coupling effect is important. In all the active regions studied, there seems to be a similarity in the position on the solar disk where the mode coupling effects become important. The polarization reversal always occurred when the active regions were farther than the 500 arc sec mark from the disk center.

Lara, Gopalswamy and DeForest (Stanford) studied the magnetic flux changes in the source regions of a large number of coronal mass ejections using the longitudinal magnetograms obtained by the Michelson Doppler Imager of the SOHO mission. Magnetic field changes associated with CMEs have been elusive for a long time. It was found that when the source region is divided into small subregions then the flux showed remarkable changes at the times of coronal mass ejections. The eruptive activity is usually centered around the interval when there is substantial change in the magnetic flux. The changes seem to be on time scale much larger than the time scale of individual eruptions. It is planned to extend this work using magnetograms of high cadence in order to identify the exact feature that results in an eruption.

Lara, Gopalswamy, and Kaiser (GSFC) are analyzing the radio spectral evolution of a number of eruptive flares in the range 50 MHz to 17 GHz. All these events involved coronal mass ejections and energetic particles. They include spatial information of the flares from Yohkoh/HXT images in determining the spectral evolution. They also study the timing of long wavelength radio bursts observed by Wind/WAVES experiment to determine all possible sources of energetic particles. They compare and discuss the relationship between the hard X-ray, radio and energetic particle spectra in order to understand the acceleration process during these eruptive events.